AGE DETEMINATIONS OF MASSACHUSETTS GRANITES
FROM RADI0(ImIC LEAD IN ZIRCON
by
GEORGE ROGER WEBBER
B.Sc., Queen's University, 1949
MoSc.,
Moester University, 1952
SUBMITTED IN PARTIAL FULFILLMENT
ssp
OF THE REQUIREMENTS FOR THE
-INDGREN
DEGREE OF DOCTOR OF
PHILOSOPHY
at the
INSTITUTE OF TECHIOLOGY
MASSACHUSETTS
J
Signature of Author
1955
DetrTment oft Geo
rg nd GeophySloe
0aY
16s 1955
Certified by____________________
Thesis Supervisor
rman$
nae Co dtee
par
on Graduate Students
AGE DETEMINATIONS OF MASSACHUSETTS GRANITES
FROM RADIOGENIC LEAD IN ZIRCON
by
George Roger Webber
Submitted to the Department of Geology and
Geophysics on My 16, 1955, in partial fulfillment of the
requirements for the degree of Doctor of Philosophy.
ABSTRACT
Radiogenic lead in zircon was used to determine
the ages of fourteen granite samples from eastern
Massachusetts and two samples from Maine. Zircon from
the granites was analysed for lead with an optical spectrograph and activity was measured with a proportional alpha
counter. A gamma ray scintillation spectrometer was used
to obtain the ratio of uranium to thorium in the samples.
Zircon from each rock sample was split into
three fractions according to slight differences in
magnetism wherever there was a sufficient amount. Age
determination results on the various fractions are given
in the following table:
Ages in Millions of Years
from Different Magnetic
Splits of Zircon
Most
InterLeast
Magnetic mediate Magnetic
Granite
448
504
495
Quincy Peabody
460
42
559
type Cae Ann
415
granite
438
Milford
granite
678
769
58It ~
1
581
662
632
Dedham
granodiorite
Of
It
712
580
736
Mica diorite near Fitchburg
gneiss
granite
Northbridge
I
i
t
Chelmsford granite
Calais, Maine
LaOke irtv
"
* appears anomalously high
54140*--
338
57
76
756
713
382
392
976
-
-
477
-829
-0-
455
1---- -MMONNINININO
The Calais granite from Maine which is believed
to be Devonian is the only granite of the above group for
which there is good field evidence of age., The age of
756 million years appears much too old for a Devonian rock.
Further investigation Is needed to find the cause of this
discrepancy.
Both lead and activity values are higher in the
in the nonmagnetic zircon from the same rock.
than
magnetic
due to a tendency for iron and uranium to
probably
is
This
the zircon structure at the sam stage
in
be concentrated
of crystallization. The ages from the different fractions
of zircon agree fairly well, with the exception of one
sample of Northbridge granite gneiss which gives ages of
338, 392 and 976 million years.
Ages for the Quincy granite are the most selfconsistent. It is a distinctive rock type and contains
more zircon than the other granites. It would, therefore,
be a good rock for more detailed age work.
Thesis Supervisor:
Title:
Patrick K. Hurley
Professor of Geology
Thesis Supervisor: William H. Dennen
Title: Assistant Professor
of Geology
ACKNOWLEDGMiMTS
I would like to thank my thesis advisors,
Professor L. U. Ahrens (thesis advisor until his
departure from M.I.T. in 1953), Professor W. H. Dennen
and Professor P. M. Hurley for their guidance in the
course of the thesis investigation.
Thanks also to
Professor H. W. Fairbairn, who prov,ded instruction
and assistance in separatory techniqaes.
George Shumway and Mark Smith, former graduate
students at M.I.T., did earlier work on the problem of
analysing zircon for lead. Their notes were of great
assistance.
Dr. W. H. Pinson, Jr. contributed helpful
discussion of analytical problems.
Many others in the
Department of Geology and Geophysics provided discussion
on various phases of the work.
I wish to thank my wife, Joan, for typing the
thesis and assisting with some of the routine p.rocedures.
This work was sponsored by D.I.C. Project 6618,
Navy Contract N5ori-07830 and D.I.C. Project 6617, Navy
Contract N5ori-07829.
TABLE OF CONTENTS
.0.0..0
ABSTRACT.....*****....................*..****
2
TS..........................................
4
INTRODUCTION.............................................
8
COLLECTION AND LOCATION OF SAMPLES.......................
9
GEOLOGICAL EVIDENCE FOR AGE..............................
13
ACENOWLEDGMLB1
1.
Massachusetts Granites.................
.
(a) Relation to fossiliferous strata.............
13
14
(b) Intrusive relationships between the granites. 15
(c) Indirect estimates of ag....................
2.
15
Maine Grnt...................16
EXPERIMENTAL PROCEDURES AND RESULTS......................
17
1. Separation of Zircon from the Rock................
17
2. Measurement of the Activity of the Zircon and
Determination of the UfrhRatio...................
19
3.
21
Spectrographic Lead Analyais......................
(a) Sadr
(b)
4.
21
...................
Sample preparation.........................
23
(c) Arcing conditions............................
24
(d) Calculation ofresults.......................
25
Computation of Ages...............
....
30
....
35
DISCUSSION.................o.....
.
....
.g..............
35
1. Consistency of Age Resuts...................
2. Absolute Ag......................
......
36
3.
Variation of Magnetism of Zircon and Its
Relation to Zircon Composition...................
4.
Relation of Zircon Age to the Age of Its
Source
Rock.....................................
CONCLUSIONS AND RECOMMENDATIONS.........................
40
43
45
APPENDIX
PRECAUTIONS AGAINST CONTAMINATION.......................
51
SEPARATION OF ZIRCON............**...........*...*.**
53
ARCING
TESTS.........................................
56
PRECISION OF FILLING ELECTRODES.........................
60
ARDS......................................
62
ANALYTICAL PROCEDURE FOR LEAD DETERMINATION.............
65
CORRECTION FOR VARIATION OF INTENSITIES WITH TIME.......
74
NONUNIFORMITY OF SLIT ILLUMINATION......................
78
RECOMMENDATIONS.........................................
81
BIBLIOGRAPHY FOR APPENDX...............................
83
BIO0RAPHY...............................................
84
INTERNAL STAND
TABLES
Table
1
Page
2
Emerson's Classification of Massachusetts
Granites........................................
Activity Neasurements...........................
13
22
3
Analysis of Ceylon Zircons......................
30
4
Results of Spectrographic Lead Determination....
32
5
Age Determination Results......................
33
FIGURES
Page
1
Location of Samples.............................
12
2-7
Addition Curves for Determination of Lead
Content of Zircon 1644..........................
28
8
Lead Working Curve..............................
29
9
Age DeterminationB..............................
34
10
Bismuth as an Internal Standard.................
64
11
Indium as an Internal Standard..................
64
12
Result of Weighing Electrodes...................
64
13
14
Acetylene Tetrabromide Separation quipment.....
Exterior of Grating Spectrograph.. ............
72
15
16
Grating - Showing Masking....................... 72
Hilger Microphotometer.......................... 72
17
Sample Calibration Curves....................... 73
18
Variation of Intensities from Different
Photographic Plates............................
77
Density Variation Along Lead Line...............
80
19
72
INTRODUCTION
The determination of the age of a number of
granites from eastern Massachusetts was undertaken as part
of a program of age determination in the Department of
Geology and Geophysics at Mtassachusetts Institute of
Technology.
Radiogenic lead in zircon was used to determine
the ages. This method was originated by E. S. Larsen, Jr.,
of the U. S. Geological Survey, and his co-workers (Larsen,
Keevil and Harrison, 1952; Larsen, Waring and Berman, 1953).
In accordance with this method, zircon from the granites was
analysed for lead by means of an optical spectrograph by the
writer and the activity was measured with an alpha counter
by P. N. Hurley, Professor of Geology at Matssachusetts
Institute of Technology. Assuming that the lead in zircon
is nearly all radiogenic, the age of the specimen is given
by the relationship T = A where T a age in millions of
years, Pb = concentration of lead in parts per million,
A
= activity in alphas per milligram-hour, C = a constant
whose value is dependent on the relative amount of activity
due to uranium or thorium.
COLLECTION AND LOCATION OF SPLES
Samples of granite, ranging in weight from 50 to
100 lbs., were collected from a number of quarries (described
by Dale, 1923) in eastern Massachusetts. Their location is
indicated in Figure 1. In general, the aim was to collect
samples from at least two quarries of each quarry area
sampled.
In two cases it was necessary to take samples from
non-quarry locations.
The samples were carefully chosen as
representing average-looking, fresh-appearing granite (with
the exception of sample R3107 which was obtained from a
small outcrop and was considerably weathered).
In addition to these samples, five samples of
granite were used which had been collected by H. W. Fairbairn,
Professor of Geology at
assachusetts Institute of Technology.
Three of these were from eastern Massachusetts (locations
shown in Figure 1) and two were from Maine.
The types of granite,locations and references to
descriptions of the quarries are given below 3
R3004 - Hornblende-augite granite from the Linehan quarry,
about three miles west-southwest of Peabody, Mass.
(Dale, 1923, p. 287).
R3005 - Hornblende granite from the Flat Ledge quarry,
about half a mile north-northwest of Rockport,
Mass. (Dale, 1923, p. 29 4).
R3006 - Hornblende granite from the Blood Ledge quarry,
about two miles west-northweat of Rockport, Mass.
(Dale, 1923, p. 300).
R3011 - Mica diorite from the Leavitt quarry, two miles west
of Leominster, Mss. (Dale, 1923, p. 353).
R3012 - Biotite granite from the Norerose quarry, about tc
miles northeast of ilford, Aass. (Dale, 1923,
p. 348).
R3013 - Biotite granite gneiss from the Blanchard quarry,
about one and a half miles west-northwest of Uxbridge
station, Mass. (Dale, 1923, p. 352).
R3014 - Hornblende granite from t1he Curry quarry, about twc
and a half miles east-sousheast of Wrentham station,
Mass. (Dale, 1923, p. 314).
R3105 - Granite from outcrop on Tigh Rock, 1.7 miles east
of Wrentham, Mass.
R3106 - Biotite granite from th'v West quarry, one and three
quarter miles north-northeast of Milford, Mass. (Dale,
1923, p. 348).
R3107 - Granite gneiss from ati outcrop at the north end of
Whitinaville, Mass.
R3108 - Muscovite-biotite grinite gneiss from the Fletcher
quarry, one and one quarter miles northwest of West
Chelmsford station, Mass. (Dale, 1923, p. 309).
10
R1982 - Coarse gray granodiorite from road out on Route 1
in Dedham, Mshs., north of Route 128 (collected by
H. W. Fairbairn).
R3051 - Granodiorite on Route 1, three miles north of
B. N. 188 which is located at the north end of
North Attleboro, Mafss, (collected by H. W. Fairbairn).
R3052 - Granodiorite on Route 1, 8.2 miles north of B. N. 188
which is at the north end of North Attleboro, Mass.
(collected by H. W. Fairbairn).
R3063 - Granite on Hollingsworth and Whitney lumber road,
about six miles west of Lake Parlin on Route 201 in
Maine (collected by H. W. Fairbairn).
R3078 - Granite on Route I, seven miles south of Calais,
Maine (collected by H. W. Fairbairn).
11
QUATERNARY
GRANITE.ETC
(POST-PENNSYLVAN IAN)
Ct'vONIAN On
GNANITECtTC.
CARGONIIIOuS (PME-PENN(5IDIMENTAN
SY LVANIAN)
AND VOLCANIC)
CAMSRIAN
(I0sIIEtROuS
STUATA)
*K, CRETACCOUS AND MIOCENE STRATA
GNUSS AND
SENCATW QUATERNARY
LOCATION OF SAMPLES
(Base map and geology
from LaForge, 1932)
Figure 1
12
GNUISS AND
SCHIST
SCH1ST
(PARTLY PRI-C.AM- (MAINLY PtBRIAN.PARTLY
CAMSRIAN)
PALE OZ IC)
GEOLOGICAL EVIDENCE FOR AGE
1. Massachusetts Granites
The geological classification of the Massachusetts
granites, as given by B. K. Emerson in his report on the
Geology of Massachusetts (Emerson, 1917), is shown below:
Sample
No.
R3108
f(Erson)
R3011
Fitchburg granite
Late Carboniferous or
Post-Carboniferous
R3004
R3005
Quincy granite
Early Carboniferous
Name of Granite
Ayer granite (Chelmsford)
"3
"t
Age
(Eerson)Late Carboniferous or
Pobt-Carboniferous
i
t
R3006
R3012
R3106
R3014
R3105
R1982
Milford granite
it
Devonian?
I
I"
Dedhatm granodiorite
"1
Devonian?
i
"
i
R3051
I
R3052
R3013
R3107
"1
Northbridge granite gneiss
i"
"f
I
Table 1
13
Archean?
I
In Figure 1 which shows the location of these
samples on a map by LaForge (1932), the above rock types
are divided into two age groups, Pre-Pennsylvanian and
Post-Pennsylvanian.
(a)
Relation to fossiliferous strata
Unfortunately there is a notable lack of fossils
in sediments associated with Massachusetts granite.
Cambrian fossiliferous strata are found near Quincy (about
10 miles southeast of central Boston) and at Hoppin Hill
(about 30 miles southwest of central Boston)(see Figure 1).
At Quincy, the Middle Cambrian Braintree slate is intruded
by Quincy granite, a distinctive rook similar petrographically to the Quincy granites at Peabody and Cape Ann, and
correlated with them by field workers.
From this evidence
it is probable that the Cape Ann and Peabody rocks are
post-Middle Cambrian.
The relationship of the Dedham
granodiorite to the Cambrian is more controversial.
Rock
which has been mapped as Dedham varies considerably petrographically, and it is not unlikely that very different
intrusives are inoluded in the classification.
At Hoppin
Hill, near the Massachusetts-Rhode Island border, the
relationship between Lower Cambrian Hoppin slate and adjacent
granite (classified as Dedham) has been a subject of
considerable controversy. Warren and Powers (1914) contended
that the granite is younger than the sediments. For a
summary of the problem and additional references, see Dowse
(1950), who concludes that the granite is older than the
14
sediments and therefore Precambrian in age.
Quincy
granite in Rhode Island is overlain by basal strata of
the Carboniferous Narragansett Basin (3aerson, 1917,
p. 188).
(b)
Intrusive relationships between the granites
In areas where Quincy-type granite and Dedham-
type granite occur, it has been reported that the Quincy
is intruded into the Dedham (Emerson, 1917, pp. 187, 188).
In Rhode Island, Quincy granite is intrusive into Milford
granite (3nerson, 1917, p. 188).
Emerson states that the
Milford granite is intrusive into the Northbridge granite
gneiss (Emerson, 1917, p. 155).
(c) Indirect estimates of age
Emerson assigned the Northbridge granite gneiss
to the Precambrian but had no direct proof of this.
The. mica diorite (Dale's classification) from the
Leavitt quarry is on the eastern border of an area mapped
by Enerson as Fitchburg granite.
The Fitchburg pluton has
been tentatively assigned to Late Devonian by N. P. Billings
(Billings, Rodgers and Thompson, 1952).
The Chelmsford granite is of controversial origin
and is discussed in some detail by Currier and Jahns (1952).
Jahns assigns it to late Paleozoic.
The Quincy granite has been correlated with the
White Mountain magma series of New Hampshire by some writers
15
(Williams and Billings, 1938) on the basis of its alkaline
nature.
The White
ountain magma series has been dated as
probably Mississippian by Billings (Billings, Rodgers and
Thompson, 1952).
Age determinations of the Conway granite
of the White Mountain magma series made by Larsen, Gottfried,
Waring and others, are tabulated by Faul (1954, p. 267).
They range from 201 to 255 million years (average - 235
million years).
2.
Maine Granite
The granite at Calais, Maine, from which sample
R3078 came, is very well dated from field evidence,
according to J. B. Thompson, Jr., Professor of Geology at
Harvard University (personal communication).
The granite
is overlain by, and contributes pebbles to, the fossiliferous
Perry Formation (Upper Devonian).
It is intrusive into the
fossiliferous Eastport Formation, which is late Silurian in
age.
Thus the granite was probably intruded during the early
Devonian.
For a description of these formations see Bastin
and Williams (1914), and Smith and White (1905).
The granite from near Lake Parlin in Maine has
been mapped as older than adjacent sediments which are
classified as Silurian or Devonian (Hurley and Thompson,
1950).
16
EXPERIMENTAL PROCEDURES AND RESULTS
The experimental prlocedure for determination of
the age of a rock from radiorenic lead in zircon may be
divided into four steps:
1.
Separation of zircon from the rock.
2.
Measurement of the activity of the zircon and
determination of the*U/RIh ratio.
3.
Spectrographic lead analysis.
4* Computation of ages.
1.
Separation of Zircon from the Rock
The granite collected in the field was first
broken up on a steel plate with a sledge hammer.
The size
o.' the fragments was further decreased with a rock splitter
and then the sample was comminuted successively in a small
This ground product was then
jaw crusher and disc grinder.i
screened to obtain a size fraction of -60 and +200 mesh.
FIfteen to thirty pounds of this size fraction was obtained
from each sample.
A combination of magnetic separation by use of a
Frantz isodynamic separator, and heavy liquids separation
17
using acetylene tetrabromide, methylene iodide and clerici
solution was then used to separate zircon, which is
comparatively nonmagnetic and heavy. The zircon was treated
with hot nitric acid to remove pyrite in the concentrate.
Impurities were handpicked from the zircon.
H. W. Fairbairn had noted in previous work that
the zircon concentrate could be separated into various
fractions according to its varying, weak magnetic properties,
and that these fractions had widely different activities.
It was therefore considered desirable to see what effect this
would have on age determinations. Accordingly, wherever a
sufficient amount of zircon was obtained, it was split into
three fractions according to its magnetic properties, by
means of the Frantz isodynamic separator.
fractions are listed below.
The various
The separations were made by
using a full field of 1.4 amperes and varying the inclination
of the separator. The fraction that is magnetic at 50 is the
most magnetic, and the fraction that is nonmagnetic at 20 is
the least magnetic.
R3004A - Magnetic at 50.
B - Nonmagnetic at 50 , magnetic at 20 .
C - Nonmagnetic at 20.
R3005A - Magnetic at 54.
B - Nonmagnetic at 50, magnetic at 20.
C - Nonmagnetic at 20.
R3006A - Magnetic at 20.
B - Nonmagnetic at 20.
R3011A - Nonmagnetic at 20.
18
R3012A - Magnetic at 20.
B - Nonmagnetic at 20.
R3013A - Magnetic at 50
B - Nonmagnetic at 54, magnetic at 24.
C - Nonmagnetic at 20.
R3014B - Nonmagnetic at 20.
R3105A - Magnetic at 50
B - Nonmagnetic at 50 , magnetic at 20 .
C - Nonmagnetic at 20.
R3106A - Magnetic at 50*
B - Nomagnetic at 54, magnetic at 24.
C - Nonmagnetic at 20.
R3107A - Magnetic at 50.
B - Nonmagnetic at 54, magnetic at 24
C - Nonmagnetic at 20.
R3108A - Nonmagnetic at 20.
Zircon from the granites collected by H. W. Fairbairn
was separated by him, ground to -400 mesh size, and split into
two fractions, magnetic and nonmagnetic at 20 on the Frantz
isodynamio separator.
Only the magnetic fraction was aval
-
able at the time the lead analyses were made, so that the
zircons R1982, R3051, R3052, R3063 and R3078 which were
analysed were all magnetic at 20.
2.
Measurement of the Activity of the Zircon and Determination
of the Uf.h Ratio
The activity measurements were made by P. K. HurLey
19
iho has contributed the following information:
The equivalent uranium was determined by thick
source alpha counting in a proportional counter.
The
instrument was a Nuclear Measurements Corporation low
background proportional alpha counter with 2% geometry,
continuously flushed with argon. The counter characteristics were well known from several years of previous
operation. The alpha count from a standard source
prepared by the National Bureau of Standards could be
duplicated by the instrument to within a fraction of a
per cent over a fairly broad plateau in collection voltage.
A solid source absorption factor for zircon was
calculated by the method described by Nogami and Hurley
(1948), on the assumption that alpha particles would be
counted that emerged from the source with an air range
exceeding 0.5 cm.
This factor was calculated to be 0.493;
that is, the number of alpha particles per sq. cm. per hour
multiplied by 0.493 gives a value for the radium equivalent
in the zircon in units to 10-12 gmse/gm.
It was found
experimentally that this factor was 11% too low for this
instrument, probably reflecting the fact that the
instrument's threshold was set so as to exclude ion
collections from alphas with a substantially greater
residual range than 0.5 cm.
Standard zircon samples
measured by other laboratories were found to be in close
agreement with their established values when this corrected
absorption factor was used.
20
A method of radiometric determination of uranium
and thorium developed by P, K. Hurley utilizing a gamma
ray scintillation spectrometer was used to obtain the ratio
of uranium and thorium in the samples,
Thorium produces
lead at only 1/3 the rate of uranium, so that the age ratios
in the zircons are dominantly Pb2 0 6/j
238
ratios.
This means
that the radium equivalent in units of 10-12 ps/Sm Multiplied by a factor of 1.04 is numerically equal to the number
of alpha particles per milligram per hour within the zircon,
which is sometimes referred to as the Activity Index,
The
low proportion of thorium contribution in the age ratio also
establishes the constant (C) to be used in the age formula,
T = C
, where T a age in millions of years, Pb = concent-
ration of lead in parts per million, and A = activity in
alphas per milligram-hour.
The average value for C was 2590.
Table 2 shows the results of the activity
measurements.
3.
Spectrographic Lead Analysis
(a) standards
For age determination work it is necessary that
the lead determinations be as accurate as possible as well
as precise.
Replicate chemical and spectrochemical results
from a single laboratory may be precise (i.e., reproducible)
but differ from equally precise analyses of the same material
from another laboratory (Fairbairn and others, 1951).
The problem of accuracy rests largely on the
21
Table 2
Ativy
Measurents
aMl No.
Activity
in alphs_ er millM-hour)
R3004A
4115
R3004B
R3005C
190
185
1105
900
560
R3006A
395
R3006B
280
R3011A
R3012A
535
435
365
R3004C
R3005A
R3005B
R3012B
925
R3013A
R3013B
R3013C
R3014UB
R3105A
R3105B
R3105C
R3106A
R3106B
R3106C
R3107A
R3107B
R3107C
R3108A
R1982
R3052
760
330
525
675
680
460
885
770
570
720
605
375
830
237
630
R3051
500
R3063
R3078
476
920
22
standards used for comparison with the unknowns. If the
standards do not behave in the same manner as the unknowns,
the results may be reproducible but not accurate.
Ideally,
the standards for these particular analyses would be zircons
similar to the unknowns and containing known amounts of
natural, radiogenic lead (i.e., not artificially added).
The standards actually used in these analyses
were made up by adding varying amounts of lead to a zircon
which was available in considerable quantity (zircon R1644
from North Carolina beach sand).
During the course of
experimentation two batches of standards were made up.
In
one, the lead was added as Pb0 2 , in the other, lead was
added in the form of a Pb-Ba glass (Bureau of Standards
Standard Sample No. 89) containing 17.5% PbO.
(b)
Samlie preparation
The zircon unknowns and standards were ground to
a fine powder in an agate mortar, weighed, and mixed with
NaCl so that the resultant mixture contained 80% zircon
and 20% NaCI by weight.
The sodium chloride was added to
improve the arcing qualities of zircon.
The zircon mixture was packed tightly in the
cavity of a 1/8 inch graphite electrode which held about
12 mg. of material (weighing electrodes and applying a weight
factor showed no detectable improvement in precision).
23
(a)
Aring conditions
Grating spectrograph - Wadsworth mount, 21 foot instrument,
with a dispersion of 2.54 R/m.
External optics - short focus spherical lens focussed on slit.
Slit height - 10 mm.
Slit width - .05 mm.
Step sector.
Wavelength region - 3850 R to 4450 A.
Current - 3 amp.
Line voltage - 220 volts.
Lead line used - Pb 4057.820 A.
Electrodes - 1/8 inch National Carbon Co. special graphite
electrodes.
Anode - the lower electrode--contained the sample in
a cavity 1/20 inch in diamter and 5/32 inch deep.
Cathode - the upper electrode--sharpened with a penel1
sharpener.
Plates - Kodak 103-0.
Arcing tima - 30 seconds.
Two electrodes of material were used for each spectrum (i.e..
two arcings were superimposed)*
Seven spectra were
obtained per plate.
Development - 4-1/2 minutes in Kodak D-19 at 200 C t 10.
Fixing - 15 minutes in Kodak Acid Fixer.
Washing - at least 1/2 hour in running water.
24
(d) Calculation of results
The basic principle of quantitative spectrographic
analysis is that I w K.C or log I = log K + log C, where I
is the intensity of line emission, C is the concentration
of the analysis element, and K is a constant.
Intensity may be calculated from the degree of
blackening of the line on the plate.
This involves calibration
of plates to determine reaction of photographic emulsion to
changes in light intensity. The method used here was a selfcalibrating method (Ahrens, 1950, p. 136) including a background correction.
When this relationship is determined,
standard curves of I versus C may be constructed from
measurement of the darkness of the lead lines produced by
samples containing known concentrations of lead.
The lead
content of unknowns may then be read directly from the curves.
The zircon used as a standard base contained a
certain amount of lead before any was added.
It was there-
fore necessary to determine this concentration.
To do this,
the addition method was used (Ahrens, 1950, p. 135).
Standards
consisting of the zircon base (zircon R1644) with varying known
concentrations of added lead were arced, and the resultant
intensity values were plotted versus concentration of added
lead. A curve was fitted to these values. During the course
of experimentation several sets of these addition plots were
made using slightly different procedures (Figures 2 to 7).
When these curves are extrapolated back to zero intensity, the
distance of the intercept on the concentration axis from zero
25
per cent added lead represents the concentration of lead
in the zircon before addition. The final value for lead
in zircon R1644 was calculated by averaging the results
of several addition plots.
The values obtained from the
various plots were 47, 45, 46 and 43 parts per million
PbO (Figures 2, 3, 4, 6), the average value being 45 parts
per million PbO.
Figures 5 and 7 were not used in this
calculation because they represent different plots of the
data in Figure 6 as will be explained below.
Theoretically these curves should be straight
lines,, but nonuniformity of illumination of the slit in
the optical set up used in the arcing procedure introduces
curvature to the lines.
One of the addition curves
(Figure 5) was recalculated to correct for this nonuniform
illumination. This resulted in the curve shown in Figure 7,
and this corrected curve is essentially a straight line in
Curvature at the higher
the lower concentration ranges.
concentration range is possibly due to self-absorption
produced in the arc when atoms in the outer portions of the
arc absorb the energy produced by lead atoms in the inner
portion of the arc and thus decrease the total light emitted.
The standards used for comparison with the unknowns
were the artificial Pb-Ba glass standards which were arced
during the same timn period as the unknowns. Each photographic plate contained three spectra (3 duplicate exposures)
of standards and four spectra of the unknowns. The
standard
with no added lead appears on all the plates, and standards
26
with added lead appear on various plates throughout the
series.
It was thought, before the samples were run, that
an inter-plate shift in intensity values might be detected,
and that the unknowns on each plate could then be determined
relative to the standard values on the same plate.
However,
as it turned out, the reproducibility of the standards did
not warrant this shift correction from plate to plate.
There was a general tendency, however, for later intensity
values in the series to be lower than earlier values.
Plates used during the series had the same emulsion number
but were from four different boxes in succession.
In order
to minimize the effect of the time shift, mean intensity
values for zircon R160 were calculated for each box of
plates.
All intensity values of standards and unknowns
were then shifted the appropriate amount (dependent on the
box of plates from which the values came) to convert them
to the level of one box.
The original and corrected values
of the standards are shown in the addition plots in Figures
5 and 6 respectively.
The corrected values are shown in
the log-log plot of Figure 8.
Thus Figure 8 is the curve
actually used in calculation of the final results.
As a check on the accuracy of the lead determination,
three Ceylon zircons, which had been analysed for lead by
C. L. Waring, of the U. S. Geological Survey, were analysed
and excellent checks were obtained.
in Table 3.
27
The results are shown
28
LEAD WORKING CURVE
0
-
- -
-
-
-
-
-;--
-
5
*------
0
100
200
PARTS PER MILLION PbO
Figure 8
29
500
1000
Table 3
Analysis of Ceylon Zircons
Parts Per Million Pb
Sample No.
Waring (U.S.GS.)
Webber
R3028
80
79
R3036
115
113
R3053
88
81
The results of the lead analysis of the
Massachusetts and Maine granites are shown in Table 4.
4. Computation of Ages
The computation of age by a lead method depends
on the decay of parent atoms U238, U235 and Th232 to
produce daughter atoms Pb206 Pb207 and Pb208 respectively.
Knowing the rates of deoay, we can determine the time at
which decay in a closed system began if we can measure the
amount of parent and daughter atoms at the present time.
In the age determination method used here, a
measure of the present amount of parent is obtained from
the activity measurements (since each parent emits alpha
particles at a constant rate) and a measure of the amount
of daughter is obtained from the spectrographic lead
measurements.,
The age is calculated from the formula
T m C.Pb
-of
where T is the age in millions of years, Pb is the lead
30
content in parts per million, A is the activity in alphas
per milligram-hour, and C is
a constant whose value is
dependent on the U/fh ratio (C = 2590 in this case).
Use of this method is based on the assumptions
that lead content at the time of formation of the zircon
was low enough not to introduce much error and that the
zircon is resistant to chemical alteration.
reported consistent results using the method.
e
Larsen has
For a
of the difficulties involved in lead age measure-
ments, see Paul (1954, pp. 282-300).
The results of the age dalculation are given in
Table 5 and shown graphically in Figure 9.
31
Table 4
Results of Spectrograhie Lead Determination
Sample
No.
Results of Replicate
Analyses in pm PbO
R3004A
R3004B
R3004C
R3005A
R3005B
R3005C
R3006A
R3006B
R3011A
R3012A
R3012B
R3013A
R3013B
R3013C
R3014B
R3015A
R3105B
R3105C
R3106A
R3106B
R3106C
R3107A
R3107B
R3107C
R3108A
R1982
R3052
R3051
R3063
R3078
(86, 83, 88, 77)? 88, 90
(43, 38, 39.5, 39), 40.5, 39
(33.5, 28, 36.6, 40), 34.6, 34*5
244, 300, 228
(150, 153, 164, 157), 158, 160
83, 130
(60.5, 60, 54, 83), 72, 79
(51.5, 52, 51), 50
(89, 86, 80), 86
159, 123, 135
117, 89, 102
(120, 135, 121, 118), 163, 120, 135
(118, 155, 127, 105), 120, 122
(130, 134, 138)
(122, 133, 123), 135
200
163, 165
140, 150, 132
202, 222, 218
218, 235, 182
(150, 165, 133), 133, 171
130, 142, 140
(139, 120, 100, 121), 118
71
286
520, 554
124, 118, 119
103, 90, 108
137, 144
291, 290, 290
* Values bracketed together are from
the same photographic plate
32
Average
Concentration
in 2pm PbO
85.3
39.8
34.5
257
157
107
68.1
51.1
85.2
139
103
130
124
134
128
200
164
141
214
212
150
137
120
71
286
537
120
100
141
290
Table 5
Ae Determination Results
Ages in Millions of Years from
Different Maxnetic Solits of Zircon
Granite
Quincy
type
granite
Peabody
Cape Ann
Milford granite
Dedham11 granodiorite
~
I
Sample
No.
Magnetic
R3004
A*
A
R3005
R3006
R3012
R3106
R3014
R3105
R3052
Most
A A58
12
R3051
Chelmsford granite
Calais, Maine
LaKe Pt1;i "f
*
'
R3011
R3013
R3107
R3108
R3078
R3063
B*
A
A
338
457
756
Least
Magnetic
C* 448
B
A
A
B
504
20
415
76-9
662
B
B
580
C
480
5440**
R1982
Mica diorite near Fitchburg
granite
gneiss
Northbridge
"
I
it
495
559
Intermediate
392
C
460
B
678
C
B
A
C
C
A
713
Letters refer to sample numbers of different magnetic splits of zircon
Appears anomalously high
438
6
0
736
382
976
455
829
FITCHBURG
AYER
R3011
R3108
II I I I I
11111 I
QUINCY
MILFORD
R3006
R3005
R3004
R3106
R3012
R3051
R1982
R3105
R3014
R3052
R3013
DEDHAM
NORTHBRIDGE
R3107
R3063
MAINE
R 3078
I
I
I
I
L:~i
.1
I
I...'
Li:
I
I I I II II
lI11111
I
L~I
L
AGE IN 10 YEARS
9
55
o
0
g
I
I
-
I
F,)
I
U)
I
-
-
DISCUSSION
1.
Consistency of Ag Results
Although the lead measurements and activities
are different in the different magnetic fractions of
zircon from the same granite, the ages agree fairly well.
A notable exception is the value of 976 million years for
sample R3013C as compared to 338 and 392 million years for
R3013A and R3013B respectively.
The ages of granites which have been mapped as
the same rock type are boxed in Figure 9. Where these
specimens were close together geographically or are
distinctive rock types (e.g., the Quincy granites) they
are boxed with a solid line.
Where the relationship is
less certain, a dotted line was used.
It is evident that
the results are generally self-consistent.
this are found in the Dedham series.
R1982 is anomalously old.
Exceptions to
The age from zircon
The two samples of Dedham
granodiorite from near Wrentham (R3105 and R3014) give
similar ages but samples R3051 and R3052 appear to be
younger.
Unfortunately only one split of zircon was
available from each of R1982, R3051, and R3052.
35
The relative ages for the Fitchburg, Quincy
(Cape Ann and Peabody). Milford, and Dedham (samples
R3014 and R3105 near Wrentham) agree with the relative
order given by Emerson (Table 1).
However, the one
sample of zircon from Ayer granite (R3108) gives an age
greater than the average of any of the other groups,
whereas Emerson classed it as younger than the Quincy
granites.
The Northbridge granite gneiss, which he
classed as Archean, gives values ranging from less than
the Fitchburg sample to the sam age as the Quincy
granites, with one anomalously high value., Two samples
of Dedham granodiorite (R3051 and R3052) give ages
similar to those of the Quincy granites.
The Maine granites give an age about the sam
as the Milford granites and the Dedham granodiorite near
Wrentham (samples R3014 and R3105).
2.
AbsoluteAe
The only granite of those analysed which is well
dated from geological field evidence is the granite from
Calais, Maine.
According to field evidence, this granite
is early Devonian in age.
The value obtained from zircon
analysis is 756 million years.
This is much older than our
present geological time scale would estimate for Lower
Devonian (estimates are around 300 to 350 million years).
So far only one split of zircon from the Calais granite has
been analysed.
36
ONMMUMMONOMM11.
The fact that zircon ages for the Quincy
granites average about 460 million years, whereas similar
granites in New Hampshire have given an age of 201 to 255
million years and are believed to be Mississippian from
geological field evidence, is suspiciously suggestive when
viewed in conjunction with the Calais granite data.
It is
possible that there is a systematic error in the results
given here.
Excessively high age values could result from
high lead values or low activity values.
In the lead
determinations, precautions were taken to avoid systematic
errors. A zircon base was used for the standards to avoid
matrix differences; standards were arced throughout the
period of analysis of the unknowns; analysis of three
samples of Ceylon zircon gave values which agreed very well
with the results obtained by C. L. Waring.
The only
apparent possibility of an error in the lead analysis is
that the zircon unknowns may have behaved very differently
from the standard zircon and the Ceylon zircons in the
spectrographic procedure.
P. M. Burley reports that close agreement has
been obtained in interlaboratory checks of the activity of
standard zircon samples.
The possibility that lead contamination was
introduced in the laboratory is always present in
individual cases. Great care was taken to avoid this
possibility.
The zircon from the Calais granite gave quite
37
a high lead result, so that the contamination needed to
produce an error of the magnitude of a factor of two would
be quite large. None of the low concentration zircon
standards which were arced in large quantities showed
variations which would suggest such a degree of contamination.
Some of these low standards were subjected to all the
preparation treatment that the unknown zircon received and
showed no detectable contamination.
The analysis of
nonmagnetic zircon from the Calais granite is highly desirable
to test the consistency of the high age value and to eliminate
the possibility of a random contamination error.
Impurities present in the zircon as a result of
incomplete separation of the zircon could cause random
errors. These impurities would have to be very high in lead.
The possibility of this would be checked by analysing the
nonmagnetic zircon fraction of the Calais zircon. If age
results are consistent, it is not likely that impurities
caused the age discrepancy.
In this method of age determination it is assumed
that the lead in the zircon structure is all radiogenic.
If there was a large concentration of nonradiogenic lead
in the zircon, the ages obtained would be too great.
In
general, the consistent age results of different magnetic
fractions of zircorb which contain varying amounts of lead,
suggest that the lead is radiogenic.
In particular, the
Calais granite zircon contains an apparent concentration of
about 270 parts per million lead, whereas analyses of granite
38
as a whole generally show only about 15 to 20 parts per
million lead.
It does not seem reasonable that zircon,
generally considered an unfavorable host for nonradiogenic
lead, should be so excessively enriched in it in comparison
with granite as a whole.
Loss of uranium or thorium from zircon would
result in high ages.
This loss could be due either to
treatment of zircon in the separation procedure or to
natural leaching processes.
It is generally considered
that zircon, being a mineral resistant to chemical attack
with a dense crystal structure, will not be very susceptible
to leaching of elements held within it. It is, however,
possible that some of the zircons of odd composition are
not so impervious.
Composition of zircon is quite variable
and may produce differences in susceptibility to leaching
which would make zircon reliable for age determination work
in some cases and not. reliable in other cases.
More work
on leaching susceptibility of zircon is needed.
It might
be thought that the nitric acid treatment of zircon crystal s
could remove some of the radioactive elements from the near
surface area and thus lower the activities. However, grinding
of the zircon and rechecking activities, although it did
reveal differences, did not indicate large differences, nor
were the values consistently raised or lowered. Independent
values of activity obtained from the gamma ray seintillatlon
spectrometer agreed with results from the alpha counter so
that a surface leaching phenomenon is not likely to be
39
the
cause of lowered activities.
Larsen and his co-workers report consistent age
determinations and have not reported any evidence of major
discrepancies.
On this account we must view with caution
the alternative of ascribing differences of analytical ages
from geological field age determinations to fundamental
differences between a zircon age and the age of intrusion o f
granites. This is,however, a possibility and may hold true
in certain cases.
3.
Variation of Magnetism of Zircon and Its Relation to
Zircon Composition
Tables 2 and 4 show that both the lead measurements
and the activities tend to be higher in the more magnetic
fractions of zircon.
The following discussion examines
possible causes for this.
Spectrographic plates containing Massachusetts
zircon show that iron and manganese are considerably more
abundant in magnetic fractions than in nonmagnetic fractions
of the zircon. This could be the result of magnetic
inclusions or of differences in the composition of the zircon
itself. The fact that the magnetism and activity are approximately correlated suggests that the magnetism is not the
result of the presence of foreign minerals with the zircon
unless they are rich in uranium or thorium. According to
P. M. Hurley, the activity of the Massachusetts zircons is
almost entirely due to uranium. E. S. Larsen, Jr., and
George Phair (Faul, 1954) report that most common rock40
forming minerals contain low amounts of uranium compared
to the amount of uranium in zircon.
It does not, there-
fore, seem likely that any of the common rock-forming
minerals could be the cause of the magnetic properties
of the zircon.
Inclusions of certain rather rare minerals that
can contain large amounts of uranium might cause variability
in magnetic properties.
These include uranium minerals
themselves and accessory minerals such as monazite and
xenotime.
A mineral like xenotime may be present as the
result of exsolution from the zircon structure.
R. C. Shields,
a graduate student at Massachusetts Institute of Technology,
in the course of an unpublished investigation of the yttrium
content of zircon (1955), analysed 15 zircons (not the
Massachusetts zircons) and found that 13 of these contained
from 1 to 4.5% Y203 . In two samples in which yttrium was
not detectable, there is doubt that they are zircon. A
correlation between magnetic fractions and yttrium content
was not apparent.
Differences in magnetism in the zircon may be due
to different compositions of zircons which have crystallized
at different times or under slightly different environmental
conditions. In connection with this possibility, the
following information on variation of zircon from different
environments is pertinent.
E. S. Larsen, Jr., and his co-workers report that
metamict zircon and nonmetamict zircon from the same rock
41
may differ in radioactivity by as much as tenfold, and
that a similar difference in radioactivity imay be found
from zone to zone in a single zircon crystal (Larsen et
al., 1953).
They also found that, in the California
batholith, zircon from the more basic rock types has much
lower activities than zircon from granites (Paul, 1954,
p. 84).
V. M. Goldschmidt (1954, p. 563) predicted that
uranium and thorium would be concentrated in the latest
fractions (outermost zones) of zircon crystals from igneous
rocks when they are formed in a simple single sequence of
crystallization.
Goldschmidt also pointed out the difference
in resistance of different zircons to hydrothermal alteration.
Altered varieties are rich (up to several per cent) in hafnium,
yttrium earth metals, thorium, uranium, phosphorus, niobium,
beryllium, and water of hydration (Goldschmidt, 1954, p. 425).
Goldschmidt ascribed destruction of the zircon structure to
the presence of radioactive elements, as have recent workers
such as P. M. Hurley and H. W. Fairbairn (Hurley and Fairbairn,
1953), and H. D. Holland and his co-workers (Holland, Schulz
and Bass, 1953).
Zircon from beach sand is the stable type that has
been able to resist the effects of weathering. Goldschmidt
noted that it was the variety which is low in hafnium
(Goldschmidt, 1954, p. 425).
The writer has noticed (from
the lead analysis plates) that the beach sand zircon from
North Carolina (1644),
which was used as a standard base in
42
the lead analysis, is notably lower in iron and manganese
than zircon from the Massachusetts granites and has a
higher concentration of titanium, vanadium and chromium.
Titanium, vanadium and chromium generally are concentrated
in minerals at earlier stages of crystallization than are
the elements iron and manganese.
This suggests that the
magnetism (which is related to iron content) and the
activity (which affects the stability of the zircon) are
due to the tendency for uranium and iron to be concentrated
in the zircon structure at the same comparatively late
stage of crystallization. Local variations in supply of
these elements and fluctuations in environmental conditions
would tend to complicate the situation and give rise to
reversals in the zoning of zircon crystals.
4.
Relation of Zircon Ag to the Age of Its Source Rock
Assuming for the moment that a true age can be
determined for zircon, the relationship of this age to the
age of the rock (granitic and related rocks in this case)
from which the zircon was obtained must still be established.
There are many uncertainties in this problem and the remarks
here will be confined to an outline of the possibilities.
Generally, the field geologist is interested in
the time of intrusion in the case of intrusive rocks. This
is the time that is commonly thought of in connection with
age determinations. More precisely, age determinations are
immediately concerned with the determination of age of
43
particular minerals (in this case, zircon),
The age of
the zircon may represent the time of intrusion but there
are other possibilities.
There may be a considerable delay between the
time that a magma starts to crystallize and the time at
which it is emplaced. Thus the zircon age might be greater
than the age of intrusion.
If a granitic rock was formed by alteration of a
sedimentary rock, it is possible that old zircon present in
the sedimentary rock could be preserved and give an age a
great deal older than the time of formation of the granite.
Another possibility is that a period of metamorphism could
introduce or reconstitute zircon crystals to give an age
considerably younger than the age of emplacement of the host
rock.
The proof of the meaning of a zircon age lies in
comparison with other methods of age determination and in
detailed investigation of zircon from different environments.
Results which appear to be anomalous should not be
discarded.
It is possible that they are reflections of the
complicated possibilities of zircon formation. For instar e,
the anomalously high age value in sample R3013C as compared
to R3013A and R3013B might be an indication of two ages of
zircon.
CONCLUSIONS AND REC01000DATIONS
The age results from different magnetic
fractions of zircon from the same granite agree fairly
well, as do the ages of granites which have been tuapped
as the same rock type.
There are a few exception to
this rule.
The relative ages of the rocks agree with
field evidence insofar as the Fitchburg granite appears to
be younger than the Quincy granite, which in turn appears
to be younger than the Milford granite and two samples of
Dedham granodiorite.
about the same age.
The latter two groups appear to be
Results which are different from
conclusions arrived at from field evidence are:
four out
of five values for the Northbridge granite gneiss indicate
that it is the same age as or younger than the Quincy
granites; two samples classed as Dedham granodiorite appear
to be about the same age as the Quincy granite; the one
sample of Ayer granite (Chelmsford) gives an age older than
all the other rocks except for one of the Dedham rocks.
The value of 756 million years obtained for a
granite from Calais, Maine, which is probably early Devonian
45
on the basis of good field evidence, suggests that the
absolute values obtained in this investigation may be
too high. This is also suggested by the fact that zircon
ages for Quincy granites average about 460 million years,
which is about twice the age obtained by other workers for
similar rocks in New Hampshire.
Further work is needed to
determine the possibility of a systematic error.
would involve:
This
(1) the analysis of another zircon fraction
from the Calais granite to check the consistency of the high
result, (2) interlaboratory checks on lead and activity
measurements.
.
It would be highly desirable to improve precision
of lead measurement, and further work in this direction is
recomnended.
On a longer range basis, it would be advisable to
determine ages on rocks from areas sampled by other workers.
The White Mountain magma series of New Hampshire would be
excellent for this purpose.
A direct comparison of White
Mountain rocks with Quincy granite in one laboratory should
be enlightening. The Quincy granite is a good rock for
further work because of its high content of zircon.
The co-variation of activity, lead content and
magnetism of zircon is probably related to a tendency for
iron and uranium to be concentrated in the zircon structure
at the same stage of crystallization.
Composition of zircon
varies considerably and may produce differences in susceptibility to leaching which would make zircon reliable for age
46
determination work in som cases and not reliable in other
cases.
It is recommended that laboratory investigation be
done on susceptibility of various zircons to leaching.
Since the composition of zircon depends largely on its
environment during formation, it may be possible to establish
some correlation between rock type and reliability of zircon
ages from these rocks.
Zircon ages determined for intrusive rocks may
not always represent the age of intrusion of the rock.
For
this reason, apparently anomalous results should not be
disregarded but should be rechecked,
It is therefore
recommended that more samples of Dedham granodiorite and
Northbridge granite gneiss be obtained to check the
apparently inconsistent results.
47
'BIBLIOGRAPHY
Ahrens, L. H. (1950) Spectrochemical Analysis, AddisonWesley Press, Inc., Cambridge, Mass.
Bastin, E. S., and Williams, H. S. (1914) U. S.
Geological Survey Geological Atlas, folio 192.
Billings, Marland P., Rodgers John, and Thompson,
James B., Jr. (19525 Geology of the Appalachian
Highlands of East-Central New York, Southern
Vermont, and Southern New Hampshire; Guidebook
for Field Trips in New England, sponsored by the
Geological Society of America, pp. 23-33.
Currier, L. W., and Jahns, R. H. (1952) Geology of the
"Chelmsford Granite" Area; Guidebook for Field
Trips in New England, sponsored by the Geological
Society of Aerica, pp. 105-117.
Dale, T. Nelson (1923) The Commercial Granites of New
England, U. S. Geological Survey Bulletin 738.
Dowse, A. M. (1950) New Evidence on the Cambrian Contact
at Hoppin Hill, North Attleboro, Mass., American
Journal of Science, vol. 248, pp. 95-99.
Emrson, B. K. (1917) Geology of Massachusetts and Rhode
Island, U. S. Geological Survey Bulletin 597.
Fairbairn, H. W., Schlecht, W. G., Stevens, R. E.,
Dennen, W. H., Ahrens, L. H., and Chayes, F.
(1951) A Cooperative Investigation of Precision
and Accuracy in Chemical, Spectrochemical and
Modal Analysis of Silicate Rocks, U. S.
Geological Survey Bulletin 980.
Faul, Henry (1954) Nuclear Geology, John Wiley and Sons,
Inc., New York.
Goldschmidt, V. N. (1954) Geochemistry, Oxford University
Press, London.
Holland, H. D., Schuls, D. A., and Bass, N. N. (1953) The
Effect of Nuclear Radiation on the Structure of
Zircon (abstract), Trans. Am. Geophys. Union,
vol. 34, p. 342.
48
Hurley, P. M., and Fairbairn, H. W. (1953)
Radiation
Damage in Zircons: A Possible Age Method
Bull. Geol. Soc. Amer., vol. 64, pp. 659474.
Hurley, P. M1., and Thompson, J. B. (1950) Airborne
Magnetometer and Geological Reconnaissance
Survey in Northwestern Maine, Bull. Geol. Soc.
Amer., vol. 61, pp. 835-842.
LaForge, L. (1932) Geology of the Boston Area, Mass.,
U. S. Geological Survey Bulletin 839.
Larsen, E. S., Jr., Keevil, N. B., and Harrison, H. C.
(1952) Method for Determining the Age of
Igneous Rocks, Using the Accessory Minerals,
Bull. Geol. Soc. Amer., vol. 63, pp. 1045-1052.
Larsen, E. S., Jr., Waring, C. L., and Berman, J. (1953)
Zoned Zircon from Oklahoma, Am. Mineralogist,
vol. 38, pp. 1118-1125.
Nogami, H. H., and Hurley, P. M. (1948) The Absorption
Factor in Counting Alpha Rays from Thick
Mineral Sources, -Trans., Amer. Geophysical
Union, vol. 29, No. 3, pp. 335-340.
Smith, 0. 0., and White, D,. (1905) The Geology of the
Perry Basin in Southeastern Maine, U. S.
Geological Survey, Prof. Paper 35.
Warren, C. H., and Powers, Sidney (1914) Geology of the
Diamond Hill-Cumberland District in Rhode Island,
Mass., Bull. Geol.
Soc. Amer., vol. 25, p. 460.
Williams, Charles R., and Billings, Marland P. (1938)
Petrology and Structure of the Franconia
Quadrangle, New Hamshire, Bull. Geol. Soc. Amer.,
Vol. 49, p. 1011-1 L.
49
50
PRECAUTIONS AGAINST CONTAMINATION
Care was taken at all stages of the procedures
to avoid introduction of lead contamination. Rock
pebbles low in lead were passed through the crushing and
grinding apparatus and analysed spectrographically to
test for lead. No contamination was detected. No
materials which were liable to contain any appreciable
content of lead were permitted to be used in grinding
and crushing apparatus.
An agate mortar and pestle used
for final grinding and a glass spatula were reserved
specifically for these purposes. They were cleaned with
dilute nitric acid between samples.
A small plastic
tamping device was used to avoid contact with metal in
the final stage of filling electrodes. The sodium chloride
flux was tested on the spectrograph and showed no detectable
lead.
Clerici solution is known to carry appreciable
concentrations of lead, so it was necessary to test its
effect on zircon. Crystals of zircon R1644D which had
previously had no contact with clerici solution were soaked
in it for one hour and then rinsed very lightly with water
51
and acetone.
Some of this clerici treated zircon was mixed
with NaCI (80% zircon, 20% NaCl) and packed in four
electrodes.
Four other electrodes were also prepared
containing zircon Rl6"D which had never been treated
with clerici solution. These electrodes were arced (two
electrodes per exposure) on the same plate. No
contamination effect was evident.
52
SEPARATION OF ZIRCON
After the granite had been crushed and a
screened fraction (-60 mesh +200 mesh) obtained, zircon
was separated from other minerals by a combination of
magnetic and heavy liquid separation. In general the
sequence of operations was as follows:
1. The Frantz isodynamic separator was used
in vertical position with full field.
2. The nonmagnetic fraction from step one was
split into two fractions, sink and float, using acetylene
'tetrabromide. This was done by means of apparatus devised
by H. W. Fairbairn and shown in Figure 13.
It consists
of two large stainless steel beakers supported one above
the other on an aluminum framework. The sandy material to
be separated was mixed with acetylene tetrabromide in a
separate beaker and poured into the upper beaker.
A
propeller-type stirrer kept the sandy material in suspension.
This mixture was allowed to drain gradually into the lower
beaker, which had been filled with acetylene tetrabromide,
by way of a valve-controlled tube. The sink then collected
in the bottom of the lower beaker and the float was allowed
to overflow, by way of a lip around the top of the beaker,
53
into a large fritted glass funnel. Acetylene tetrabromide
was recovered in a large flask under the funnel.
The float
was washed with acetone to recover more of the heavy
liquid.
The sink was recovered from the bottom of the lower
steel beaker and washed with acetone.
Sink from the acetylene tetrabromide was
passed through the Frantz isodynamic separator with an
3.
inclination of 150 and current of 0.4 amperes.
4.
The nonmagnetic fraction was treated with
boiling nitric acid (removes apatite and pyrite principally).
5.
The acid-treated material was split into sink
and float fractions using methylene iodide.
Small separatory
funnels were used at this stage.
6.
The sink was separated magnetically success-
ively at settings of 0.7, 1.2 and 1.4 amperes--all at 150.
This was done to recover different fractional concentratio ns
of other heavy minerals.
7.
The nonmagnetic fraction from step six was
separated into two fractions, sink and float, in clerici
solution. The sink at this stage was nearly all zircon as
shown by examination with a binocular microscope.
8.
The aircon was separated into three fractions,
where possible:
a. magnetic at 1.4 amperes, 5*
00M~f0
b. nonmagnetic at 1.4 amperes, 54 but magnetic at
1.4 amperes, 20
c. nonmagnetic at 1.4 amperes, 20
54
9.
Impurities were removed by handpicking.
Variations of the above procedure, and repetition
of some phases, were used to suit the individual samples.
55
ARCING TESTS
Electrodes
Tests of burning qualities of zircon in carbon
and graphite electrodes were made.
The most satisfactory
burn was achieved with 1/8 inch graphite electrodes.
There
is less wandering of the arc when carbon is used but
zirconium sparked erratically in the early stages.
Line
The Pb 2833
f
and Pb 4057 R lines were examined
in both the grating and prism spectrographs (using both
quartz and glass optics).
The Pb 4057 line showed the best
sensitivity and had least background in all cases. The
grating spectrograph produced a lower background, which made
it appear preferable to the prism spectrograph.
Other
considerations favoring use of the grating are that the
prism instrument has given some focussing trouble from time
to time and that the wide dispersion of the grating (about
2.54 R per mm.) lessens the chance of interference from
neighboring lines.
56
Grain Size
A smoother burn is obtained and the alkali
phase of arcing is longer when zircon is finely ground.
Tim
It was found necessary to stop the arc before
zirconium made its appearance as there is some interference
with the lead line, and erratic values result.
In early
experiments an arcing time of 35 seconds was used, but
when the unknowns were run, early flashing of zirconium
necessitated shortening the arcing time to 30 seconds.
Flux
A sodium chloride flux was used throughout these
experiments (mixture 80% zircon, 20% sodium chloride).
The presence of sodium chloride lowers the temperature of
the are and suppresses the less volatile elements (e.g.,
zirconium).
It also suppresses the CN emission which would
otherwise interfere with Pb 4057 9.
57
STANDARDS
The standards used for construction of the
addition curves (Figures 2 to 7) were made up in two
separate sets.
The first set of standards was made
by adding Pb0 2 to zircon R1644A.
A mixture containing
1% PbO2 was prepared and diluted with successive
additions of zircon R6L644A to obtain a series of standards
containing additions of 0.1, 0.0287, 0.00823, 0.00260,
0.000745 and 0.000214% Pb0 2 . These standards were used
in construction of the curves in Figures 2 and 3.
The second set of standards was made by adding
Bureau of Standards Pb-Ba glass containing 17.5% PbO to
zircon R1644A.
Two mixes were independently prepared
containing 0.1%added PbO and 0.01%added PbO.
Some of
each of these mixes were mixed again to obtain a standard
containing 0.03%added PbO.
Some of the 0.01%added PbO
mix was also further diluted with zircon R1644A to contain
0.003% added PbO. This was the set of standards used in
the construction of curves in Figures 4 to 7.
These
standards were used during the arcing of the unknowns.
The supply of zircon R1644A that had originally
58
been separated from beach sand was rather low so a new
supply was separated.
This zircon was designated R164D.
59
PRECISIONa07 F3LING
LECTRODES
In order to check how uniformly electrodes were
being filled, a group of ten were weighed before and
after being tilled with zircon R16WA. The zircon R1644A
used in the test was ground in six batches to test the
effect of grinding on uniformity or filling. Those values
that are bracketed together represent fillings from the
same grinding operation.
1.
11.6 ag.
2.
11.5 mg.
am
M
I
-
am
=
3.
12.1 mg.
4.
12.2 mg.
5.
11.9 Mg.
am
00
-Wa an
11.8 mg.
6.
go
00
am to
a
M
so
7.
12.7 mg.
8.
12.7 mg.
am 4W
aM
d
-Wa
60
a&
9.
10.
12.5 M9.
12.1 mg.
These results indicate that electrodes can be
filled quite precisely when the material is from the same
batch of ground material, but that variation is greater
when the material is ground separately.
These same electrodes were arced individually.
The intensity values obtained are shown along with the
weights in Figure 12.
(Note that these are single
electrode exposures and are therefore not directly
comparable with results obtained from the superimposed
electrode exposures).
The variation in arcing is much
greater than the variation in weighing and there is no
clear correlation between the two. It was decided not to
weigh the electrodes in the actual analyses because the
possible increase in precision did not appear great enough
to justify the extra time.* During the filling of electrodes
small amounts of graphite are sometimes rubbed or broken off
the electrodes, so that some variation would be introduced
in the weighing process.
Weighing of the electrodes may, however, be
desirable if an improvement in precision is attempted.
Unless samples which are run in replicate are from different
batches of grinding, there is a danger of introducing
systematic errors in the means.
Also, if the zircon sampi es
differ greatly in specific gravity, an error may be introduced
if no weighing is made.
61
INTERNAL STANDARDS
At early stages of the experiments, indium was
used as an internal standard; however, the working curve
using the intensity of the lead line alone did not differ
greatly from that using the lead-indium intensity ratio.
It was found that when a sample was arced several times on
the saw plate, the intensity values for the lead line
were generally closer together than the relative intensity
values (IPVIn) (Figure 11).
It was therefore decided to
use the Ib values, and no internal standard was used in
the analyses presented here.
Bismuth is frequently used as an internal standard
when Pb 2833 is used for lead analysis. Pb 2833 was not
used In these analyses mainly because it is not as sensitive
as Pb 4057. However, two plates were run on the Hilger
spectrograph to see how effectively the Bi 2898 line corrects
for variations in the Pb 2833 line. Conditions were as
follows:
Sample - R1644A with added Bi.
Quartz optics.
Right end of plate - 3500
62
.
Arcing time - 35 seconds.
1/8 inch graphite electrodes.
103-0 plates.
The results are shown in Figure 10.
(Notes
these were single electrode exposures).
The internal standard does not appear to have
been very effective under these conditions.
It was thought that when the unknown zircons
were arced, the values from plate to plate could be
shifted according to the values of standards on the same
plate but, as it turned out, the reproducibility of the
standards on the individual plates was not good enough
to warrant this shift.
It may be, then, that an internal
standard method, even if less reliable on a single plate,
would be preferable to the method that was used in these
analyses.
The problem is a matter of degree of efficiency
of the standard and would have to be tested.
It is highly
recomended that further work on internal standards be
done.
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.1
ANALYTICAL PROCEDURE FOR- LEAD DETERMINATION
Spectogahic Procedure
The following procedure was used when the unknowns
were analysed.
Earlier tests Were done under generally
similar but somewhat different conditions.
Grating spectrograph.
Masking - 2-13/16" of grating only exposed (Figure 15).
External optics - short focus lens focussed on slit
(Figure 14).
Actually the arc stand was not in the
correct position for sharp focus on the slit but was
somewhat (16 mm.) behind this position.
This should
make no difference to analytical results except that
the sensitivity is reduced.
It would be preferable
in future to have the arc stand in position for sharp
focus on the slit to give higher sensitivity.
Wavelength setting - center 4150 R.
Focus - 11.65.
Plate - 103-0 - placed in the center position of the plateholder (which is capable of holding three plates).
New boxes of plates were kept in a refrigerator in
their original protective wrappings until several
65
hours before use.
They were then removed from the
refrigerator and not returned to the refrigerator.
Excessive moisture my cause greater damage to
photographic plates than slight excessive heat, so
it was felt that the plates should not be returned
to the moist atmosphere of the refrigerator without
their protective wrappings.
Slit height - 10 sm.
Slit width - .05 wu.
Anode excitation.
Current - 3 amps.
Voltage - 220 volts.
Step sector - The sector was adjusted so that seven steps
were obtained.
In the first step the light was
unsectored, in the others it was reduced respectively
by 1/2, 1/4, 1/8, 1/16, 1/32, 1/64.
The actual
setting was made by turning on the arc so that light
was cast on the slit.
The height of the sector was
then adjusted so that the shadow of the bottom of the
last step of the sector to be used was lined up with
the bottom of the slit.
The lower electrodes (anode) were 1/8 inch
diameter National Carbon Co. special graphite electrodes
about 3/4 inch long with a cavity of 1/20 inch diamter
and 5/32 inch depth.
The drill used for cutting these
electrodes was reserved specifically for this purpose.
The upper electrodes were 1/8 inch special
66
graphite electrodes about 1-1/4 inches long.
sharpened in a pencil sharpener.
These were
It is particularly
necessary that the upper electrode be sharp; otherwise
excessive wandering of the arc will result.
The initial electrode separation was eleven mma.
The position of the lower electrode was adjusted before
arcing by sighting from behind the electrodes through the
short focus lens and lining up lower electrodes with the
image of the top of an empty lens holder ring which was
mounted on the step sector stand.
The arc was started by bringing the upper
electrode down to touch the lower electrode.
The upper
electrode was then raised quickly so that the inverted
image of its glowing tip was projected on the bottom of
the ring on the sector stand.
Light from the initial
contact was prevented from reaching the slit by shielding
the arc with a hand until the separation was made.
As
soon as the electrodes were separated, the timing clock
This whole starting operation took a
fraction of a second. The upper electrode was continually
adjusted to keep it in position and the lower electrode
was started.
was allowed to burn away. The arc was turned off at the
end of thirty seconds because irregular sparking of the
arc started after this time.
Two samples of the same material were arced before
racking down the plate.
In other words, a double exposure
was made for each spectrum appearing on the plates.
67
The photographic plates were developed in
total darkness in D-19 (full strength) for 4-1/2 minutes,
rinsed lightly in water, then fixed for 15 minutes in
Kodak Acid Fixer.
They were then washed in running water
for at least half an hour.
After a light rinse in
distilled water they were sponged gently to remove
excessive moisture and allowed to dry.
During the drying
they were covered with a folded paper towel (not in contact
with the plate) to prevent accumulation of dust.
After drying, the plates were labelled and the
position of the lead line marked with an inked dot.
The plates were measured in a Hilger Microphotometer (Figure 16).
The microphotometer was allowed to warm
up for at least half an hour before using.
Details of the
measurement are as follows:
The plate was placed emulsion side up in the microphotometer plate holder.
The image of the plate on the micro-
photometer screen was focussed so that the lead line was
sharp and granularity of the plate was evident.
The
microphotometer slit height and width were set at 10 and
8 scale divisions respectively.
The image of the lead
line was brought parallel to the slit by adjustment of
the plate position.
The zero scale setting was made with
the photocell closed to light.
The 50 scale setting
(maximum scale) was made with the cell open by scanning
unexposed sections of the plate from top to bottom of the
plate in the vertical zone near the lead line position and
68
continually adjusting the scale back to 50 whenever
readings went off scale. The zero and 50 scale settings
were not changed during the course of the reading of the
plate.
The lead lines were measured by scanning the
center of each measurable step.
Background readings were
taken on the darkest step on both sides of the lead line.
Because the background varies along the plate, readings
were made by scanning the portion of the plate within a
fixed distance on each side of the plate and taking the
readings on each side which represented the lightest
portions of the plate.
If a certain position on the
plate near the lead line had been chosen (an alternative
method of reading the background) it would not give valid
results if a line were to appear at that position in some
sample that differed slightly in composition from the
standards.
A quick re-reading of one step on each line
was made to check the possibility of drift of the scale.
No appreciable shift was observed. Microphotometer error
is small (about I or 2% standard deviation) compared with
other errors in most spectrographic work.
Various workers
(including the writer) in the Cabot Spectrographic Laboratory
have noted periods when the microphotometer scale wandered
erratically.
This does not happen very frequently but it
emphasizes the need for rechecking values.
The instrument
should not be used during one of these periods.
The
erratic behavior my be due to interference from outside
69
power units which are in operation in the same building.
Calibration curves (for the conversion of
microphotometer readings to intensities) were constructed
for each plate using a slight modification of the selfcalibrating method described by Ahrens (1950, p. 136).
d
Instead of using the function - where do a 50 (the
reading on an unexposed section of a plate) and d = the
microphotometer reading on a line, the Seidel function
3- .1was used (Ahrens, 1954, pp. 11 and 12). The purpose
of this is to extend linearity of the curve to low values.
A sample calibration curve is shown in Figure 17. Partial
curves were constructed for each line and the average
slope of the lines was computed for each plate (in practically all cases this average slope turned out to be 690).
To obtain the intensity value for a given lead line a
do
-r -1 value, measured on a step which gave a value lying
between 1 and 10, was plotted on the appropriate step
(see Figure 17) and then projected up slope (using the
average slope for that plate) to the intensity scale
where the intensity value was obtained. Similarly the
-g -1 value for background was converted to intensity and
subtracted from intensity of the line to give the final
intensity value.
Note that the intensity scale is arbitrary
in its position but gives the same relative results as long
as it is always fixed in the same position. There are no
absolute units to the intensity scale. The values are
purely relative.
70
Intensity values were converted to concentrations
by means of a working curve which is a plot of intensity
versus concentration.
The working curve was constructed
from standards as described in the section on "Corrections
for Variation of Intensities with Time."
71
now-- -- ---
-
.r-
-
Acetylene Tetrabromide
Separation Fquipment
Exterior of
Grating Spectrograph
Figure 13
Figure 14
Grating - Showing
Masking
Hilger
Microphotometer
Figure 15
Figure 16
72
.|
ElillFhIBIT1111
lilki
.Man
.LIJ
L
1
I
i:
g.
Figure 1.7
73
CORRECTION FOR VARIATION OF INTENSITIES WITH TIM
When the intensity values of standards and
unknowns were calculated, it became evident that there was
a variation with time over the period of 71 days during
which analyses were made.
Intensity values for the same
samples tended to be lower in later plates than in earlier
plates.
This is illustrated in Figure 18 (uncorrected
values) which shows intensity values of the standards and
unknowns plotted in chronological order according to the
plate on which they were recorded.
Where more than one
exposure of the sane sample was recorded on one plate, the
average value is plotted.
Unfortunately, variation of the standard zircon
R1644 was too great within an individual plate to justify
shifting unknown values on the same plate to fit fluctuations
in the standards.
Photographic plates used in the analyses
came from four different boxes.
It is possible that the time
shift was the result of differences between plates in the
different boxes.
Whatever the reason, there is an apparent
shift with time and values obtained for unknowns should be
more valid if compared with standards arced within a narrow
74
period of time.
The limit to the narrowness of this
time period is that there must be enough standard arced
It
within that period to establish a good reference.
was decided to use the four photographic plate boxes as
divisions within which results would be compared directly.
There were not enough values of all concentrations
of the standards to asks the boxes completely independent
of each other, so the intensity values were all shifted to
the level of one box.
This was done by first computing
the average value of zircon R1164
for each box.
To do this
it was necessary to compute R1644A in terms of Ri644D.
These two standards were run on two plates (a total of seven
values each) and the average intensity of R1644A was found
to be 1.21 times that of R1644D on one plate and 1.26 times
that of R1644D on the other plate (average value - 1.23).
All values of R1644A were divided by 1.23 to convert them
to hypothetical values of R1644D.
The average values of
Rl644D (including hypothetical R1644D values) were
calculated for each box and then the factors necessary to
convert each of these averages to the average for box
number four were computed. Each intensity value of unknowns
and standards was then multiplied by the factor for its
particular box.
The test of this correction factor was
taken to be its effect on the standards containing added
lead. These values were not used in computation of the
factors and so should be valid tests of the correction.
The precision of these standards tends to be improved. The
75
effect of the correction on the values obtained on
different plates is shown in Figure 18 which shows
uncorrected and corrected values. Where corrected values
were not plotted, they are the same as uncorrected values.
Corrected values of the standards are shown in
the form of working curves in Figure 6 (linear coordinates)
and Figure 8 (log-log coordinates).
The latter curve was
the one used for determination of the unknowns.
Standard deviations of intensity values for the
standards before and after correction for plate boxes are
shown below.
PerCent Standard Deviation
Before
Correction
After
Correction
R16AD
20.4
16.9
R1644A
15.88
12.63
8.32
9.35
plus .003% added PbO
plus .01% added PbO
18.02
10.67
884
9.6
7.42
10.3
plus .03% added PbO
plus .1% added PbO
These values give an approximate idea of the
precision of the method in terms of intensity.
76
VARIATION OF INTENSITIES
FROM
DIFFERENT PHOTOGRAPHIC PLATES
IlkI
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UNCORMECTED
comNErCT
0
Figure 18
77
NONWIPORMITY OF STI
The steep slope of the call.bration curve
(Figure 17) is due to nonuniform distribution of light
along the slit of the spectrograph. This, in turn, is
due to variation of intensity of light emitted from
various parts of the arc.
Several unsectored exposures
were made to study the intensity variation from top to
bottom of the lead line.
do
-
It was found that the function
I decreased gradually from one end of the line (the
anode end) to the other (see Figure 19).
In other
words, the Pb 4057 wavelength was emitted more strongly
near the anode.
The effect of this, when a step sector is used,
is that each successive sectored portion of the slit
receives somewhat less than half as much light as the
preceding one, going from top to bottom of the slit (anode
to cathode of the inverted image of the arc).
This
results in a steeper calibration curve.
In order to check the effect of nonuniform slit
illumination on the working curve, the
de
-1 values for
the standards used in arcing the unknowns were corrected
78
for this nonuniform illumination effect.
The corrected
calibration curve obtained had a slope or 640 as compared
to 690 in the uncorrected case.
The working curve was
recalculated and gave the plot shown in Figure 7 using
linear coordinates. This curve shows an approximately
straight line relationship at lower concentration values
as compared to the curvature of the other plots and thus
conforms to the theory that intensity varies directly with
concentration.
In the higher concentration range the curve
flattens out, probably due to self-absorption.
This
corrected curve was not used to determine concentration
of lead in unknowns but only to check the cause for the
shapes of the addition curves.
The straight line relation-
ship in the lower concentration range tends to indicate
that the added lead is behaving in the sam manner as the
residual lead; otherwise there would be' a deviation from
a straight line in going from the region where residual
lead is dominant to that in which added lead is dominant,
79
DENSITY VARIATION
ALONG LEAD LINE_
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-
RECOMMENDATIONS
Improvement in precision of the lead measurement
is desirable.
This would make possible a single plate
analysis system similar to the one used by C. L. Waring,
of the U. S. Geological Survey (Waring and Worthing,
1953), by which a sample is run on a single plate with
standards with similar lead concentration.
The following
steps are recommended:
1.
Change the external optics on the spectro-
graph so that uniform illumination of the slit is obtained.
This may be done by means of two cylindrical lenses.
The
method is described by Harrison, Lord and Loofbourow (1948,
p. 129).
It will be necessary to ascertain whether or not
the resultant loss in sensitivity is too great.
2.
Experiment with a change in the amount, and
possibly with the type, of flux.
Specifically, it would
be advisable to try the arcing conditions used by Waring
and Worthing (1953).
By using an increased amount of flux
it may be possible to arc past completion of the lead
without the erratic behaviour of the arc toward the end of
the arcing time.
Again, the effect on sensitivity must be
observed.
81
3. Try internal standardization again. Change
in fluxing conditions, with the possibility of arcing to
completion of lead and the internal standard line, may
improve the effect of the internal standard.
It is possible that spectrographic analyses may
give a systematic error because of the difference in
behavior between the standards with artificially added
lead and the unenowns.
This could be cheelked by use of
a standard and unknown which gave similar, high results
by the spectrographic method.
They could both be analysed
by a dithizone colorimetric technique (Sandell, 1950,
pp. 388-407) to see whether they would give similar
results.
The factors which would cause a systematic
analytical error in spectrographic work would not do this
in the colorimetric analysis, although other systematic
errors might exist,
W. H. Pinson, Jr., Research Associate
in the Department of Geology and Geophysics at Massachusetts
Institute of Technology, has suggested using a colorimetric
method to analyse natural zircons which could then be used
as standards for spectrographic analysis.
It should be
remembered that even natural zircons may behave differently
from one another in the arc.
82
BIBLIOGRAPY FOR APPENDIX
Akrens, L. H. (1950) Spectrochemical Analysis, AddisonWesley Press, Inc., Cambridge, Mase
Ahrens, L. H. (1954) Quantitative Spectrochemical
Analysis of Silicates, Pergamon Press, London.
Harrison, G. R., Lord, R. C., and Lootbourow, J. R. (1948)
Practical Spectroscopy,
Prentice-Hall, New York.
Waring, C. L., and Worthing, Helen (1953) A Spectrographic Method for Determining Trace Amounts
of Lead in Zircon and Other Minerals, Am.
Mineralogist, vol. 38, p. 827.
83
BIOGRAPHY
George Roger Webber - born November 2, 1926, in
Toronto, Ontario, Canada.
Education
Primary and secondary school in Hamilton, Ontario. Senior
matriculation, Westdale Secondary School, June 1945.
McMaster University, Hamilton. Engineering year,
September 1945 to June 1946.
Queen's University, Kingston, September 1946 to June 1949.
Bachelor of Science Degree in mineralogy and geology,
June 1949.
McMaster University, Hamilton, September 1950 to June 1952.
Part time assistant in optical mineralogy, October 1951 to
April 1952. Master of Science degree in geology, September
1952. Thesis: "Spectrochemical Analysis of the White
Mountain Magma Series and Some Finnish Granites."
Massachusetts Institute of Technology, Cambridge, September
1952 to June 1955. Part time Teaching Assistant in
mineralogy, October 1952 to January 1953. Part time
Research Assistant in the Cabot Spectrographic Laboratory,
February 1953 to May 1954 and October 1954 to May 1955.
Geological Field Experience
Ontario Department of Mines, Thunder Bay District, Ontarlo,
May to September 1947 and May to September 1948.
Jalore Mining Co. Ltd., Michipicoten District, Ontario,
May to November 1949 and April to September 1950.
Iron Ore Co. of Canada, Labrador, June to September 1951
and June to September 1952.
Nindamar Metals Corp. Ltd., Stirling, Nova Scotia, June to
September 1954.