Open File Report OF-AR-34
New Mexico Bureau of Geology and Mineral Resources
A division of
New Mexico Institute of Mining and Technology
40
Ar/39Ar Geochronology Results
from volcanic rocks McMurdo Sound
area, Antarctica
Prepared By:
William C. McIntosh and Richard P. Esser
New Mexico Bureau of Geology, Socorro, NM 87801
Prepared For:
Tim Paulsen
University of Wisconsin, Oshkosh
Terry Wilson
Ohio State University
Initially prepared as:
NM Geochronology Research
Laboratory Internal Report
NMGRL-IR 459
November 4, 2005
SOCORRO 2007
NEW MEXICO BUREAU OF GEOLOGY AND MINERAL RESOURCES
Peter A. Scholle, Director and State Geologist
a division of
NEW MEXICO INSTITUTE OF MINING AND TECHNOLOGY
Daniel H. López, President
BOARD OF REGENTS
Ex Officio
Bil l Rich ardson, Governor of New Mexico
Veronica C. Garcia, Secretary of Education
Appointed
Jerry A. Armijo, President, 2003–2009, Socorro
Rich ard N. Carpenter, Secretary/Treasurer, 2003–2009, Santa Fe
Anne Murphy Da ily, 2005–2011, Santa Fe
S idney M. Gutierrez, 2001–2007, Albuquerque
Michael J. Gorospe, 2005–2006, Socorro
NEW MEXICO GEOCHRONOLOGY RESEARCH LABORATORY STAFF
W ILLIAM M C I NTOS H , Geochronologist
M ATT HEIZ LER , Geochronologist
LISA PETER S , Argon Laboratory Technician
R IC H A R D E S SER , Argon Laboratory Technician
BUREAU STAFF
B R UC E D. ALLE N , Field Geologist
RU BE N A R C HULETA , Metallurgical Lab. Technician II
VALE NTI N A AVR AMIDI, Business Office Manager
S A N D R A H. A ZEVEDO, Cartographer II
ALBER T B AC A , Lead Maintenance Carpenter
J AMES M. B AR KE R , Associate Director for Operations,
Senior Industrial Minerals Geologist
PAUL W. B AUER , Associate Director for Government Liaison,
Senior Geologist
D OUG B LA N D , Economic Geologist
RO N B R OAD HEAD , Associate Director for Industry Liaison,
Principal Senior Petroleum Geologist
R ITA C ASE, Administrative Secretary II (Alb. Office)
S TEVE N M. C AT HE R , Senior Field Geologist
R IC H A R D C HAM BE R LI N , Senior Field Geologist
S EA N D. C O N N ELL, Albuquerque Office Manager, Field Geologist
RU BE N A. C R ESPI N , Manager, Fleet/General Services
G I NA D ’AM BOS IO , Editor
N ELIA W. D U N B A R , Assistant Director for Laboratories, Geochemist
R IC H A R D E S SER , Senior Lab. Associate
RO BE R T W. E VELET H , Senior Mining Engineer
B O N N IE F R E Y , Chemistry Lab Manager
PATR IC IA L. F R IS C H , Assistant Curator of Mineral Museum
LEO O. G A BALDO N , Cartographer II
N A N C Y S. G ILSO N , Editor
KAT H R Y N E. G LESE NER , Senior Cartographer/Manager,
Cartography Section
I B R A H IM G U N DILER , Senior Extractive Metallurgist
L Y N N H EI ZLER , Senior Lab. Associate
M ATT HEIZ LER , Geochronologist
L Y N NE H EME N W A Y , Geologic Information Center Coordinator
G R ETC HE N K. H OFFMA N , Senior Coal Geologist
S TEVE H OOK , Petroleum Geologist
PEGG Y J O H N S TO N , Senior Hydrogelogist
G LE N J O NES , Assistant Director for Computer/Internet Services
T HOMAS J. K AUS , Cartographer II
P HILIP K YLE, Professor, Geochemistry
LEWIS A. LA ND , Hydrogeologist
A N NA BELLE LOPEZ , Petroleum Information Coordinator
D AVID W. L OVE, Principal Senior Environmental Geologist
J A NE A. C ALVER T LOVE, Managing Editor
V IR G IL W. LUET H , Assistant Director for Public Outreach,
Mineralogist/Economic Geologist, Curator of Mineral Museum
M A R K M A N S ELL, GIS Specialist
D AVID M C C R A W , Senior Geologic Lab. Associate
W ILLIAM M C I NTOS H , Senior Volcanologist
C H R IS TOP HE R G. M C K EE, X-ray Facility Manager
V IR G I N IA T. M C L EMOR E, Minerals Outreach Liaison,
Senior Economic Geologist
LISA PETER S , Senior Lab. Associate
L. G REER P R ICE , Senior Geologist/Chief Editor
G EOFF RA WLI NG , Field Geologist
ADAM S. READ , Senior Geological Lab. Associate
M A R S HALL A. REITER , Principal Senior Geophysicist
G R EG OR Y S A N C HE Z , Mechanic-Carpenter Helper
M I KE S MIT H , Database Technician
M I KE T IMMO N S , Manager, Geologic Mapping Program
LORETTA T O B I N , AdministrativeServices Coordinator
AMY T R IVITT -K R AC KE , Petroleum Computer Specialist
S U S IE U L B R IC HT , Executive Secretary
M A N UEL J. V ASQUEZ , Mechanic II
S TAC EY W AG NE R , State Mapping
S U S A N J. W ELC H , Manager, Geologic Extension Service
J E N N I FER W HITEIS , GIS Technician
M AU R EE N W ILKS , Geologic Librarian, Manager of Publication Sales
R Y A N W OOD , Administrative Secretary I
EMERITUS
G EOR GE S. A USTI N , Emeritus Senior Industrial Minerals Geologist
L Y N N R LE B R A N DVOLD , Emeritus Senior Chemist
C H A R LES E. C HAPI N , Emeritus Director/State Geologist
J O H N W. H A WLE Y , Emeritus Senior Environmental Geologist
S A MUEL T H OMPSO N III, Emeritus Senior Petroleum Geologist
RO BE R T H. WE BE R , Emeritus Senior Geologist
Plus research associates, graduate students, and undergraduate assistants.
40
Ar/39Ar geochronology results from volcanic rocks
from McMurdo Sound area, Antarctica
NMGRL Internal Report IR-459
By William C. McIntosh and Richard Esser
Prepared for
Tim Paulsen, University of Wisconsin, Oshkosh
Terry Wilson, Ohio State University
November 4, 2005
Introduction
Twenty-seven samples of lavas and pyroclastic material from the McMurdo
Sound area, Antarctica, were submitted by Tim Paulsen and Terri Wilson. The goal of
dating these rocks is to constrain the ages of volcanic vents and linear arrays of vents to
help evaluate the ages of tectonic events including fault offsets.
40
Ar/39Ar Analytical Methods and Results
Groundmass concentrates were prepared from all 27 samples using crushing,
sieving, magnetic separation, dilute HCl and ultrasonic cleaning, and hand picking to
remove K-poor phenocrysts where possible. During the process of preparing groundmass
concentrates, we noted that the majority of the samples were far more glass-rich than is
ideally desirable for 40Ar/39Ar dating. More ideal holocrystalline samples were apparently
not obtainable in the field, probably in part due to lack of erosion of sampled volcanic
units. After preparation, groundmass concentrates and monitor minerals (Fish Canyon
Tuff sanidine, 28.02 Ma, Renne et al., 1998) were loaded into machined aluminum discs
and irradiated for seven hours at the Texas A & M reactor. Aliquots of groundmass
concentrate, typically 60 – 200 mg in size, were analyzed by the furnace incrementalheating age-spectrum method, using nine to ten heating steps ranging from 650°C to
1600°C. Abbreviated analytical methods for the dated samples are given in Table 1, and
details of the overall operation of the New Mexico Geochronology Research Laboratory
are provided in the Appendix.
Age spectra and isochrons plots for the 27 analyses are presented in detail in
Figures 2 through 28 (Figures are arranged in order of sample number and are keyed in
Table 1). Table 1 and Figure 1 summarize preferred ages (either weighted-mean or
isochron ages). Table 2 presents the complete analytical data set. Analytical
uncertainties in the ages of individual heating steps are quoted at the ±1 sigma level, all
other uncertainties are quoted at ±2 sigma.
The quality of results from this study varies widely among the 27 analyses. To
ease presentation and discussion, the results are divided into four types, on the basis of
the character age spectrum and isochron, and the radiogenic 40Ar yields of the analytical
steps. Radiogenic yield is percent of measured 40Ar ascribed to radiogenic decay, as
opposed to trapped 40Ar of atmospheric origin. Anomalously low radiogenic 40Ar yields
from many of the analyses in this study reduce the precision and accuracy of the age
determinations, as compared to typical alkali basaltic samples of similar age dated in our
laboratory in other studies. The criteria for the four types is as follows:
Type 1 (n = 5 of 27 analyses) moderately high radiogenic yields, relatively flat age
spectra
Type 2 (n = 10) moderate to low radiogenic yields, fairly well-defined spectra or
isochrons
Type 3 (n = 7) moderate to low radiogenic yields, poorly defined spectra and isochrons
Type 4 (n = 5) low radiogenic yield, very poorly defined spectra and isochrons
Type 1 - moderately high radiogenic yield, relatively flat age spectra
Five of the 27 step-heating analyses yielded relatively flat age spectra (Figures
keyed in Table 1), which provide the most precise and accurate age determinations from
this study. With the exception of initial low-precision, low-radiogenic-yield steps, most
of the individual steps from these five Type 1 analyses are relatively precise, with 1σ
uncertainties typically ranging from ± 0.02 Ma to ±0.1 Ma (Table 2). Radiogenic yields
vary in proportion with age, as expected (given that radiogenic 40Ar increases with time),
but these yields are generally higher than those of Types 2, 3, and 4 analyses, reaching
20% to 90% for the middle-temperature steps of the age spectra (Figures are keyed in
Table 1 and analytical details are in Table 2). K/Ca ratios, calculated from the measured
ratio of 39ArK and 37ArCa , show behavior typical of multimineralic groundmass
concentrates: i.e. K/Ca is highest for low temperature steps where K-rich phases are
degassing, and lowest for high temperature steps where K-poor phases such as pyroxene
dominate degassing. Three of the Type 1 samples (20-00,
11-29-02-6, and 12-3-1-01) have mean K/Ca ratios ranging 6.2 to 8.7, values that are
significantly higher than typical basalts (K/Ca generally < 1). These three samples
apparently have evolved compositions such as such as trachyte or phonolite. The mean
K/Ca ratios of the remaining two Type 1 samples (12-5-01-2 and 12-18-1-02) are <1.0,
consistent with basaltic compositions. Weighted-mean ages for the five Type 1 samples
were calculated for the broad, flat central portions of these spectra, wherein the ages of
individual steps agreed or nearly agreed at the 2 sigma level. The segments selected for
weighted-mean age calculations range from 64 to 100% of the total 39Ar released, and are
closely analogous to the age “plateaus” defined by numerous other workers, although
strict arbitrary plateau criteria were not used in the current study.
Weighted-mean ages for the five Type 1 analyses range from 1.32 ± 0.05 Ma to
4.92 ± 0.02 Ma. Four of the five weighted-mean age errors are less than ±0.05 Ma, and
the fifth is still relatively precise ±0.11 Ma. Calculated ages and errors tend to be
insensitive to which specific steps are used for their calculation, in keeping with the flat
nature of the age spectra. MSWD values for weighted-mean-age segments of Type 1
analyses range from 0.2 to 4.4. The higher MSWD values reflect some scatter of
individual steps outside of analytical uncertainty. As expected for flat age spectra with a
range of radiogenic yields, data from the five Type 1 analyses form well correlated linear
arrays on isochron plots, which yield 40Ar/36Ar intercepts near the atmospheric ratio
(295.5) and intercept ages that agree within error with the weighted-mean ages (Table 2,
Figures keyed in Table 1).
Two of the five Type 1 analyses (20-00, 11-29-02-6) have integrated ages
indistinguishable from the weighted-mean and isochron ages, indicating no excess 40Ar.
(The integrated age is age calculated by summing Ar isotope measurements from all
heating steps, thereby being equivalent a total fusion analysis and nearly equivalent to a
conventional K-Ar age determination.)
The other three Type 1 analyses (12-3-1-01, 12-5-01-2 and 12-18-1-02) have
anomalously old initial heating steps, causing the integrated ages to be older than the
weighted-mean or isochron ages. These anomalously old initial ages probably indicate
some excess 40Ar degassing from matrix glass in the initial heating step, although the
isochrons for these analyses do not indicate significant excess 40Ar in subsequent steps.
Large amounts of atmospheric 40Ar in these initial heating steps complicate their
interpretation.
Type 2 – moderate to low radiogenic yield, fairly well-defined spectra or isochrons
Ten of the 27 step-heating analyses yielded moderately disturbed age spectra,
although the precision of individual steps is comparable to Type 1 analyses. Radiogenic
yields for these Type 2 analyses are similar to those of Type 1 analyses, with most
ranging from 29% to 83%, with the exception of the three youngest samples which have
significantly lower yields (samples 6-00, 12-11-1-05, and 12-19-01-7, 40Ar * = 2-5%.
K/Ca spectra for all Type 2 analyses fall during the course of the analyses, consistent
with degassing dominated by glass and feldspar at low temperature, with increasing
contributions of pyroxene, olivine, and other K-poor phases at high temperatures. This
behavior and the low bulk K/Ca ratios (<0.5) are consistent with basaltic compositions
for Type 2 samples.
The precision, shape, and degree of discordance of the age spectra vary among the
ten Type 2 analyses. In general, the precision of the ages of individual heating steps for
Type 2 sample analyses is worse than that of Type 1 analyses, with the most precise steps
typically in the ±0.05 to ± 0.2 Ma range. Most analyses have several discordant steps at
the low and/or high temperature ends of the spectra. Many of the discordant steps are
anomalously old, suggesting the presence of excess 40Ar trapped within the rock at the
time of eruption. For each of the ten Type 2 analyses, weighted-mean ages were
calculated for the relatively flat central portions of these spectra, selecting individual
steps that agreed or nearly agreed at the 2 sigma level. On average, MSWD values for
the Type 2 weighted-mean age spectra segments are higher than those of Type 1
analyses, rangin9 from 0.7 to 16.0. In addition, Type 2 weighted-mean ages,
uncertainties, and MSWD values are more sensitive to which heating steps are selected
than results from Type 1 samples. Many of the Type 2 analyses have integrated ages
significantly older than the weighted mean ages. The older integrated ages are a
consequence of the anomalously old low- and high-temperature steps indicative of excess
40
Ar.
Plotting the data on isochron diagrams gives further support for excess 40Ar in
some Type 2 samples. Four of ten Type 2 analyses (6-00, 13-00, 12-9-01-1, and 12-1101-5) have isochron plots with 40Ar/36Ar intercept values (304±3, 300±2, 307±5, and
330±14 respectively) that are significantly greater than atmospheric argon (295.5).
Similar levels of modest homogeneous excess 40Ar, with 40Ar/36Ar intercept values in the
305 to 315 range, are common in Antarctic basalts from the Royal Society Range area,
and probably reflect eruption through thick continental crust. Intercept ages calculated
from isochron plots of Type 2 analyses vary from sample to sample in how they compare
to weighted-mean ages. For the four samples with high 40Ar/36Ar intercept values, the
isochron intercept ages are somewhat younger than the weighted-mean ages, which is an
expected consequence if excess 40Ar is present in the samples. As was the case for Type
2 weighted-mean ages, the 40Ar/36Ar intercept values, the intercept ages, and intercept age
uncertainties for Type 2 samples are all sensitive to the choice of steps included or
excluded from the isochron. In general, steps were selected to minimize MSWD values.
For each Type 2 analysis, it was necessary to select either the weighted-mean age
or the isochron age as the preferred age interpreted as most representative of the eruption
age. For the ten Type 2 analyses, three weighted-mean ages and seven isochron ages
were selected as preferred ages. Preferred ages for Type 2 samples range from
effectively zero (-0.02 ± 0.02 Ma) to 11.44 ± 0.16 Ma. Uncertainties for Type 2
preferred ages range from ± 0.02 Ma to ± 0.35 Ma, significantly larger than Type 1 age
uncertainties, largely reflecting the lower precision and greater scatter of individual
analytical steps of Type 2 analyses.
Type 3 – moderate to low radiogenic yield, poorly defined spectra and isochrons
Eight of the 27 step-heating analyses yielded imprecise individual analytical steps
with low radiogenic yields, and considerably disturbed age spectra, in comparison to
Type 1 and Type 2 analyses. Radiogenic yields for these eight Type 3 analyses range
from 2% to 29%. Yields are proportional to age, as expected, but are anomalously low
especially for the older samples. K/Ca ratios for Type 3 analyses are typical for basaltic
groundmass.
Seven of the eight Type 3 analyses (3-00, 11-00, 12-1-1-04, 12-3-01-3, 12-3-01-4,
12-19-01-3) have age spectra similar in shape to Type 2 analyses, but more discordant
and less precise (note that most of the Type 3 age spectra are plotted with larger age
scales than Type 1 and 2 spectra). Precision on apparent ages of individual steps is
generally poor, typically ±0.15 Ma or worse. The lowest temperature and, in some
cases, the highest temperature steps tend to have near-zero radiogenic yields and
discordant, imprecise, commonly anomalously old ages. The central portions of the age
spectra of these seven analyses are flatter than the low- and high-temperature ends of the
spectra, but even these relatively flat central parts are sufficiently imprecise and
discordant that their weighted-mean ages have low precision (±0.13 Ma to ±0.39 Ma).
MSWD values for these segments of the age spectra range from 1.1 to 5.9. Isochrons for
these seven Type 3 analyses have 40Ar/36Ar intercepts that range from atmospheric to
slightly elevated. Some of the isochron intercept ages agree with or overlap the
weighted-mean ages and others do not. Several of the isochron ages are less precise than
the weighted-mean ages, as a result of the uniformly low radiogenic yields and
consequent lack of well defined linear arrays on the isochron plots. In general, both
weighted-mean and isochron ages and their uncertainties tend to be sensitive which
heating steps are included and which excluded from calculations. Likewise, the choice of
weighted-mean versus isochron age for the preferred age of Type 3 analyses can make a
significant difference to the interpreted erupted age of the sample.
The eighth Type 3 analysis (12-11-01-04) yielded more precise individual step
ages (±0.04 Ma) than the other seven Type 3 analyses, but the age spectrum has an
unusual shape. Initial low-temperature steps have ages near 0.5 Ma, and then decline
with increasing temperature to a quasi-flat Type of high-temperature steps with a
weighted-mean age of 0.28 ± 0.08 Ma. Radiogenic yield for all steps are quite low,
ranging from 2.1 to 7.5 %. The isochron for this analysis has a 40Ar/36Ar intercept that is
imprecise (316 ± 81), but suggestive of excess 40Ar. The isochron intercept age for this
analysis is near zero (-00.5 ± 0.06 Ma), which is significantly younger than the weightedmean age of the youngest, high-temperature portion of the age spectrum.
As shown in Table 1, weighted-mean ages were selected were selected as
preferred ages for two of the seven Type 3 analyses, and isochron ages were selected for
the remaining five analyses. In general, isochron ages were preferred over weightedmean ages, because of lower MSWD values, and because of indications of excess 40Ar
and the high potential of even small amounts of excess 40Ar to influence the apparent ages
of these young, low-radiogenic-yield samples. Preferred ages of Type 3 samples range
from near zero (-0.27 ± 0.22 Ma and -0.05 ± 0.06 Ma) to 4.88 ± 0.68 Ma.
Type 4 - low radiogenic yield, very poorly defined spectra and isochrons
Four of the 27 analyses yielded age spectra that have low radiogenic yields, very
imprecise individual steps, and strongly disturbed age spectra. Radiogenic yields are very
low (maximum yields 2% to 13%) and the precision values for the ages of the most
precise individual steps range from ±0.15 Ma to ±0.6 Ma. The age spectra of these four
analyses are quite disturbed, with highly discordant low-temperature and high
temperature steps. For three of the four Type 4 analyses (11-19-01-7, Cone 54, Cone 8889), weighted-mean ages were calculated for the quasi-flat central segments of the
spectra, representing 18 to 62 % of the 39Ar released. No weighted-mean age was
calculated from the highly disturbed age spectrum of sample 12-10-1-6. Isochron plots
results from the four Type 4 analyses have elevated 40Ar/36Ar intercepts (300 ± 8 to
306±16) suggesting modest amounts of excess 40Ar. Preferred ages for the four Type 4
analyses include one weighted-mean age and three isochron ages. The preferred ages
range from -0.29 ± 0.036 to 7.4 ± 2.1 Ma. The large uncertainties result from a
combination of low radiogenic yields, imprecise ages of individual steps, and discordance
between steps.
Discussion
In general, the results from this study are somewhat lower in precision and
accuracy than expected from alkali basaltic rocks such as those in the McMurdo Sound
region. The difference probably lies in the glass content of the samples. Higher glass
content, typical of samples collected from the near exterior of lava flows or from
pyroclastic deposits, leads to hydration and incorporation of atmospheric argon into the
matrix glass. This adversely affects 40Ar/39Ar results in a number of ways, including
reduced radiogenic yields, lowered precision of analytical steps, and disturbed age
spectra, in part due to enhanced 39Ar recoil effects. Compared to holocrystalline samples
from lava flow interiors, glassy lava samples may also suffer from larger effects of excess
40
Ar, retained at the time of extrusion because of rapid cooling. The high glass content of
the samples in this study may be due preferred sampling of near-vent facies of littleeroded volcanic cones selected to identify the location of faults or tectonic features of
interest. That said, although the data set is imperfect, it may be perfectly useful for
bracketing the timing of tectonic events, depending on what level of temporal resolution
is required.
The highest quality ages are those from the five Type 1 analyses. Radiogenic
yields from these analyses are high enough to allow relatively precise analyses of
individual steps. Age spectra are flat, and weighted-mean and isochron ages agree
closely, so the preferred ages (summarized in Table 1) are insensitive to how the data is
treated (i.e. weighted-mean versus isochron approach) and to the choice of which
analytical steps to accept or exclude. We interpret the preferred ages of Type 1 analyses
to be accurate determinations of the eruption ages of the sampled units.
The preferred ages of the ten Type 2 analyses are less precise than those of Type 1
analyses, and should be interpreted with some caution. These preferred ages are more
model dependent than Type 1 ages, being more sensitive to choice of weighted-mean
versus isochron interpretations, and to choices of which analytical steps to exclude. We
interpret the Type 2 ages as being reasonably accurate measurements of eruption ages,
but caution that actual eruption ages may be outside of the quoted analytical
uncertainties. An additional complicating factor for Type 2 ages is that some show clear
evidence of containing moderate amounts of excess 40Ar, as indicated by anomalously old
low- or high-temperature heating steps, integrated ages significantly older than weightedmean or isochron ages, and elevated 40Ar/36Ar isochron intercepts (typically 305-315).
We have observed similar effects of excess 40Ar in other McMurdo Sound basalts,
particularly those erupted through thick crust in the Royal Society Range area. Using
isochron ages for these rocks should compensate for the presence of excess 40Ar, but this
does call for more caution in interpretation compared to results from simpler excess
excess-40Ar-free lavas. For example, some workers decline to use rocks with excess 40Ar
for high precision time scale work (e.g. Singer et al, 1996).
For all of the reasons stated above, additional caution is recommended for
interpretations based on the seven Type 3 ages, and great caution is suggested if Type 4
ages are used at all.
All of the ages determined in this study were generated blind without reference to
sample location or stratigraphy. If the geologic context of the samples permits, the
accuracy of the preferred ages could be tested to some extent by comparing multiple
samples from the same eruptive event, or by comparing the order of age determinations
to stratigraphic relationships established by fieldwork.
Figure 1 shows two compilations of all twenty-seven ages determinations
generated in this study, one sorted into analysis Types 1 to 4, and the other unsorted. The
sorted compilation displays the progressively increasing uncertainties from Type 1 to
Type 4 analyses. Ages range from effectively zero to 11.44 ± 0.16 Ma. All but one of
the near-zero-age analyses are Type 3 or Type 4 and thus should be interpreted with
caution.
References Cited
Renne, P. R., C. C. Swisher, A. L. Deino, D. B. Karner, T. L. Owens, and D. J. Depaolo,
1998a, Intercalibration Of Standards: Absolute Ages and Uncertainties In 40Ar/39Ar
Dating: Chemical Geology, v. 145, p. 117-152.
Samson, S.D., and, Alexander, E.C., Jr., 1987. Calibration of the interlaboratory 40Ar/39Ar
dating standard, Mmhb-1, Chem. Geol., 66, 27-34.
Singer, B.S., and Pringle, M.S., 1996, Age and duration of the Matuyama-Brunes
geomagnetic polarity reversal from 40Ar/39Ar incremental heating analyses of lavas,
Earth and Planetary Science Letters, 139, 47-61.
Steiger, R.H., and Jäger, E., 1977. Subcommission on geochronology: Convention on
the use of decay constants in geo- and cosmochronology. Earth and Planet. Sci.
Lett., 36, 359-362.
Taylor, J.R., 1982. An Introduction to Error Analysis: The Study of Uncertainties in
Physical Measurements,. Univ. Sci. Books, Mill Valley, Calif., 270 p.
York, D., 1969. Least squares fitting of a straight line with correlated errors, Earth and
Planet. Sci. Lett., 5, 320-324.
Table 1. Summary of 40Ar/39Ar results from groundmass concentrates from McMurdo Sound volcanic rock samples.
Preferred Age
Irrad
analysis
n
%39Ar
MSWD
NM-182F
NM-182E
NM-182E
NM-182G
NM-182E
Plateau
Plateau
Plateau
Plateau
Plateau
8
9
6
5
6
93.1
100.0
67.9
70.5
64.0
3.3
1.8
1.4
4.4
0.2
294
295
297
281
294
±
±
±
±
±
12
3
5
10
4
8.7
6.2
6.7
0.4
0.6
4.92
4.01
1.43
4.10
1.32
±
±
±
±
±
0.02
0.02
0.02
0.11
0.05
Type 2 - moderate to low yield, fair spectrum/isochron
6-00
3
5
55298-01 NM-185C
7-00
4
3
55290-01 NM-185A
13-00
6
55
55156-01 NM-182G
17-00
7
83
55145-01 NM-182E
11-29-01-2
9
29
55146-01 NM-182E
12-1-01-1
12
59
55150-01 NM-182F
12-1-01-2
13
62
55157-01 NM-182G
12-9-01-1
19
29
55304-01 NM-185D
12-11-01-5
22
31
55292-01 NM-185B
12-19-01-7
26
2
55302-01 NM-185C
Isochron
Plateau
Isochron
Plateau
Isochron
Plateau
Isochron
Isochron
Isochron
Isochron
7
6
5
4
6
6
8
5
5
5
1.3
1.3
0.7
8.5
5.9
16.0
6.5
2.4
5.2
1.8
304
296
300
254
295
291
300
307
330
298
±
±
±
±
±
±
±
±
±
±
3
4
2
51
6
11
9
5
14
8
1.0
0.2
1.0
0.5
1.2
0.5
2.5
0.4
1.8
0.4
0.17
0.31
11.44
5.01
1.91
8.32
8.32
3.16
1.53
-0.02
±
±
±
±
±
±
±
±
±
±
0.10
0.08
0.16
0.08
0.19
0.35
0.25
0.22
0.23
0.02
Type 3 - moderate to low yield, poor spectrum/isochron
3-00
2
14
55288-01 NM-185A
11-00
5
29
55153-01 NM-182F
12-1-01-4
14
26
55155-01 NM-182F
12-3-01-3
16
3
55158-01 NM-182G
12-3-01-4A
17
4
55151-01 NM-182F
12-11-01-4
21
8
55294-02 NM-185B
12-19-01-3
24
2
55286-01 NM-185A
Plateau
Plateau
Isochron
Isochron
Isochron
Isochron
Isochron
8
4
7
6
6
5
6
2.7
1.1
4.5
0.9
0.8
7.2
0.7
297
287
299
300
301
316
303
±
±
±
±
±
±
±
5
10
6
3
2
81
3
0.2
0.2
0.2
0.4
0.4
0.1
0.7
3.50
4.60
4.88
0.23
0.07
-0.05
-0.27
±
±
±
±
±
±
±
0.20
0.18
0.68
0.22
0.08
0.06
0.22
Type 4 - low yield, very poor spectrum/isochron
11-29-01-7
10
13
55154-01
12-10-01-6
20
9
55296-02
12-19-01-5
25
6
55308-01
Cone 56
27
2
55306-01
Cone 88/89
28
2
55300-01
Isochron
Isochron
Isochron
Plateau
Isochron
6
7
6
5
4
13.0
1.1
0.5
6.1
1.9
306
304
298
302
300
±
±
±
±
±
13
4
3
7
8
0.8
0.2
0.5
0.3
0.6
2.90
7.40
0.06
0.41
-0.29
±
±
±
±
±
1.40
2.10
0.08
0.39
0.36
Sample
Figure
Max %40Ar*
L#
Type 1 - moderate yield, flat age spectrum
20-00
8
91
55152-01
11-29-02-6
11
65
55147-01
12-3-01-2
15
41
55148-01
12-5-01-2
18
52
55159-01
12-18-01-2
23
21
55149-01
NM-182F
NM-185B
NM-185D
NM-185D
NM-185C
67.3
62.6
58.7
87.3
49.4
61.8
40
Ar/36Ar ± 2σ
K/Ca Age(Ma) ± 2σ
Notes:
Results are divided into Types based on age spectra and isochrons, as detailed in body of the report. Figure is figure number in report.
Max %40Ar* is maximum radiogenic yield, excluding inprecise initial and final heating steps. L# is laboratory number. Irrad is irradiation tray.
n is number of heating steps used in weighted-mean of isochron age. %39Ar is percent of total 39Ar used in weighted-mean age calculation. 40Ar/36Ar is isochron intercept.
Sample preparation and irradiation:
Samples provided by Tim paulsen and Terry Wilson.
Groundmass concentrates were prepared using standard separation techniques (crushing, sieving, franzing and hand-picking).
Concentrates were loaded into a machined Al discs and irradiated for 7 hours in the D-3 position, Nuclear Science Center, College Station, TX.
Neutron flux monitor Fish Canyon Tuff sanidine (FC-1). Assigned age = 28.02 Ma (Renne et al., 1998)
Instrumentation:
Mass Analyzer Products 215-50 mass spectrometer on line with automated all-metal extraction system.
Samples step-heated in Mo double-vacuum resistance furnace. Heating duration 9 minutes.
Reactive gases removed by reaction with 3 SAES GP-50 getters, 2 operated at ~450°C and
1 at 20°C, together with a W filiment operated at ~2000°C.
Analytical parameters:
Electron multiplier sensitivity averaged 2.92x10-16 moles/pA.
Total system blank and background for the furnace averaged 1596, 15.9, 1.6, 40.3, 6.6 x 10-18 moles
at masses 40, 39, 38, 37, and 36, respectively for temperatures <1300°C.
J-factors determined to a precision of ± 0.2% by CO2 laser-fusion of 4 single crystals from each of
ten radial positions around the irradiation tray.
Correction factors for interfering nuclear reactions were determined using K-glass and CaF2 and are as follows:
(40Ar/39Ar)K = 0±0.0004; (36Ar/37Ar)Ca = 0.000289±0.000005; and (39Ar/37Ar)Ca = 0.00068±0.00002
Age calculations:
Weighted mean age calculated by weighting each age analysis by the inverse of the variance.
Weighted mean error calculated using the method of (Taylor, 1982), multiplied by suqare root of MSWD if MSWD > 1.
Total gas ages and errors calculated by weighting individual steps by the fraction of 39Ar released.
Isochron ages, 40Ar/36Ari and MSWD values calculated from regression results obtained by the methods of York (1969).
Decay constants and isotopic abundances after Steiger and Jäger (1977).
All weighted-mean and isochron age errors reported at ±2σ.
12-18-01-2
12-3-01-2
12-19-01-7
6-00
7-00
12-11-01-5
11-29-01-2
12-19-01-3
12-11-01-4
12-3-01-4A
12-3-01-3
Type 1 analyses
11-29-02-6
12-5-01-2
20-00
12-9-01-1
A)
Type 2 analyses
17-00
12-1-01-1
12-1-01-2
13-00
Type 3 analyses
3-00
Cone 88/89
12-19-01-5
Cone 56
11-00
12-1-01-4
Type 4 analyses
11-29-01-7
0
2
4
12-10-01-6
6
Age (Ma)
8
10
Cone 88/89
B)
12-19-01-3
Type 1 to 4 analyses
12-11-01-4
12-19-01-7
12
12-19-01-5
12-3-01-4A
6-00
12-3-01-3
7-00
Cone 56
12-18-01-2
12-3-01-2
12-11-01-5
11-29-01-2
11-29-01-7
12-9-01-1
3-00
11-29-02-6
12-5-01-2
11-00
12-1-01-4
20-00
17-00
12-10-01-6
12-1-01-1
12-1-01-2
0
2
4
6
Age (Ma)
8
13-00
10
12
Figure 1. Summary of preferred ages. A) divided into analysis Types 1 through 4,
B) undivided.
1
100
50
0
1
0.1
K/Ca
% Radiogenic
L# 55288: 3-00, 219.42 mg Groundmass Concentrate
0.01
14
Apparent Age (Ma)
12
10
650
A
8
3.50 ± 0.20 Ma
(MSWD = 2.7)
6
4
865
775 D
C
750
B
2
920
1105
E 1000 G
F
1200
H
I
J
0
Integrated Age = 5.02 ± 0.85 Ma
-2
0
10
20
30
40
50
60
70
80
90
100
Cumulative % 39ArK Released
0.0035
J
IG H
A
A JIG
H F
0.0030
E
0.0030
F
E
D
C
B
D C
B
0.0020
36Ar/40Ar
0.0025
0.0010
0.0020
0
0.0015
0
0.0010
0.002
0.004
0.006
0.008
Isochron age = 3.34 ± 0.62 Ma
40Ar/36Ar
Intercept = 297 ± 5
MSWD = 3.3, n = 6
0.0005
0
0
0.01
0.02
0.03
0.04
0.05
0.06
0.07
39Ar/40Ar
Figure 2. Sample 3-00, age spectrum and isochron diagram.
2
100
50
0
10
1
0.1
5
K/Ca
% Radiogenic
L# 55298: 6-00, 223.01 mg Groundmass Concentrate
0.01
Apparent Age (Ma)
4
3
0.53 ± 0.09 Ma
J
(MSWD = 2.0)
2
650
A
1
0
775
C
750
B
920
E
860
D
1105
F G
1190 1305
I
H
-1
Integrated Age = 1.03 ± 0.28 Ma
-2
0
10
20
30
40
50
60
70
80
90
100
Cumulative % 39ArK Released
0.0035
I
J
J H
IAGB
F
CE D
A
B
H
G F
0.0030
C E
D
0.0030
0.0020
36Ar/40Ar
0.0025
0.0010
0.0020
0
0
0.004
0.008
0.012
0.016
0.0015
0.0010
Isochron age = 0.17 ± 0.10 Ma
40Ar/36Ar
Intercept = 304 ± 3
MSWD = 1.3, n = 7
0.0005
0
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
1.1
1.2
39Ar/40Ar
√ Figure 3. Sample 6-00, age spectrum and isochron diagram.
3
100
50
0
1
0.1
0.01
5
Apparent Age (Ma)
K/Ca
% Radiogenic
L# 55290: 7-00, 220.18 mg Groundmass Concentrate
4
3
0.31 ± 0.08 Ma
(MSWD = 1.3)
J
2
1
0
755
B
650
775
C
-1 A
850
D
1200
H
1105
1005 G
F
920
E
I
Integrated Age = 0.71 ± 0.39 Ma
-2
0
10
20
30
40
50
60
70
80
90
100
Cumulative % 39ArK Released
0.0035
I
AH
GE C
JF B
D
A
F G
J
0.0030
I
E
H
B
C
D
0.0030
0.0020
36Ar/40Ar
0.0025
0.0010
0.0020
0
0
0.008
0.016
0.024
0.0015
0.0010
Isochron age = 0.28 ± 0.15 Ma
40Ar/36Ar
Intercept = 296 ± 4
MSWD = 1.2, n = 6
0.0005
0
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
1.1
1.2
39Ar/40Ar
Figure 4. Sample 7-00, age spectrum and isochron diagram.
4
100
50
0
100
10
Apparent Age (Ma)
12
K/Ca
% Radiogenic
L# 55153, 11-00, Groundmass Concentrate, 80.09 mg
1
10
4.6 ± 0.2 Ma
(MSWD = 1.1)
8
1115
G
6
1005
F
4
930
775 870
E
D
C
Integrated Age = 13.1 ± 3.0 Ma
755
B
2
0
10
20
30
40
Cumulative
50
60
%39Ar
K
70
1275
H
80
90
100
0.40
0.45
0.50
Released
0.0035
A
0.0030
H
B
G
D
C
0.0025
36Ar/40Ar
F
E
0.0020
I
0.0015
0.0010
Isochron age = 5.2 ± 0.6 Ma
40Ar/36Ar
Intercept = 287 ± 10
MSWD = 0.1, n = 4
0.0005
0
0
0.05
0.10
0.15
0.20
0.25
0.30
0.35
39Ar/40Ar
Figure 5. Sample 11-00, age spectrum and isochron diagram.
5
100
50
0
1
K/Ca
% Radiogenic
L# 55156, 13-00, Groundmass Concentrate, 75.66 mg
Apparent Age (Ma)
20
0.1
875
D
18
11.70 ± 0.23 Ma
(MSWD = 4.9)
16
14
775
C
12
1015
F
935
E
10
1275
H
1115
G
1725
I
50
8
30
Integrated Age = 25.0 ± 2.7 Ma
6
10
0
10
20
30
40
50
60
70
Cumulative %39ArK Released
80
-10
0.0035
B
C
A
D
0.0030
E
F
0.0025
I
36Ar/40Ar
H
0.0020
0.0015
G
0.0010
Isochron age = 11.44 ± 0.16 Ma
40Ar/36Ar
Intercept = 300 ± 2
MSWD = 0.7, n = 5
0.0005
0
0
0.02
0.04
0.06
0.08
0.10
0.12
0.14
0.16
0.18
0.20
39Ar/40Ar
Figure 6. Sample 13-00, age spectrum and isochron diagram.
6
100
50
0
1
0.1
0.01
Apparent Age (Ma)
8
7
5.01 ± 0.08 Ma
6
(MSWD = 8.5)
5
755
B
4
870
D
775
C
930
E
1010
F
1115
G
3
1285
H
I
Integrated Age = 5.39 ± 0.46 Ma
2
0
10
20
30
40
50
60
70
80
90
100
0.40
0.45
0.50
Cumulative %39ArK Released
0.0040
0.0035
A
0.0030
I
36Ar/40Ar
0.0025
H
G
0.0020
0.0015
F
C
0.0010
Isochron age = 5.19 ± 0.25 Ma
40Ar/36Ar Intercept = 254 ± 51
MSWD = 10.0, n = 5
0.0005
B
E
D
0
0
0.05
0.10
0.15
0.20
0.25
0.30
0.35
39Ar/40Ar
Figure 7. Sample 17-00, age spectrum and isochron diagram.
K/Ca
% Radiogenic
L# 55145, 17-00, Groundmass Concentrate, 78.84 mg
100
50
0
100
10
Apparent Age (Ma)
8
K/Ca
% Radiogenic
L# 55152, 20-00, Groundmass Concentrate, 78.41 mg
1
7
4.92 ± 0.02 Ma
(MSWD = 3.3)
6
650
A
5
755
B
4
870
D
775
C
930
E
1110
G
1005
F
1275 1725
H
I
3
Integrated Age = 4.87 ± 0.08 Ma
2
0
10
20
30
40
Cumulative
50
60
%39Ar
K
70
80
90
100
0.40
0.45
0.50
Released
0.0035
A
0.0030
36Ar/40Ar
0.0025
0.0020
0.0015
I
0.0010
B
H
Isochron age = 4.93 ± 0.05 Ma
40Ar/36Ar
0.0005
C
G
Intercept = 294 ± 12
F
D
MSWD = 3.8, n = 8
E
0
0
0.05
0.10
0.15
0.20
0.25
0.30
0.35
39Ar/40Ar
Figure 8. Sample 20-00, age spectrum and isochron diagram.
8
100
50
0
10
1
0.1
Apparent Age (Ma)
8
K/Ca
% Radiogenic
L# 55146: 11-29-01-2, Groundmass Concentrate, 78.50 mg
0.01
A
6
1.92 ± 0.05 Ma
(MSWD = 0.1)
4
1275
H
2
760
B
775
C
870
D
E
1105
G
F
I
0
Integrated Age = 3.01 ± 0.45 Ma
-2
0
10
20
30
40
Cumulative
0.0035
50
60
%39Ar
70
K
Released
0.7
0.8
80
90
100
A
G
I
H
0.0030
F
E
B
D
0.0025
36Ar/40Ar
C
0.0020
0.0015
0.0010
Isochron age = 1.91 ± 0.19 Ma
40Ar/36Ar
Intercept = 295 ± 6
MSWD = 5.9, n = 6
0.0005
0
0
0.1
0.2
0.3
0.4
0.5
0.6
0.9
1.0
1.1
1.2
39Ar/40Ar
Figure 9. Sample 11-29-01-2, age spectrum and isochron diagram.
9
100
50
0
1
8
0.1
Apparent Age (Ma)
K/Ca
% Radiogenic
L# 55154, 11-29-01-7, Groundmass Concentrate, 80.84 mg
3.64 ± 0.20 Ma
7
(MSWD = 0.4)
6
1280
H
5
4
1115
G
760
B
3
C
D
900
E 1010
F
Integrated Age = 5.73 ± 0.68 Ma
2
0
10
20
30
40
50
60
%39Ar
K
Cumulative
0.0035
A
0.0030
A
GF
0.0030
H
E
B
D
70
80
90
100
Released
G F
E D
C
H
B
C
0.0020
I
0.0025
36Ar/40Ar
0.0010
0.0020
0
0.0015
0
0.04
0.08
0.12
0.16
0.20
0.24
I
0.0010
Isochron age = 2.9 ± 1.4 Ma
40Ar/36Ar
Intercept = 306 ± 13
MSWD = 13.0, n = 7
0.0005
0
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
39Ar/40Ar
Figure 10. Sample 11-29-01-07, age spectrum and isochron diagram.
10
100
50
0
10
1
0.1
Apparent Age (Ma)
8
7
4.01 ± 0.02 Ma
(MSWD = 1.8)
6
5
4
755
B
655
A
3
870
D
775
C
930 1015
F
E
1115
G
H I
Integrated Age = 3.94 ± 0.10 Ma
2
0
10
20
30
40
Cumulative
50
60
%39Ar
K
70
80
90
100
Released
0.0035
A
0.0030
H
I
G
36Ar/40Ar
0.0025
0.0020
F
E
0.0015
D
C
B
0.0010
Isochron age = 4.02 ± 0.04 Ma
40Ar/36Ar Intercept = 295 ± 3
MSWD = 1.7, n = 8
0.0005
0
0
0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50 0.55 0.60
39Ar/40Ar
Figure 11. Sample 11-29-02-6, age spectrum and isochron diagram.
K/Ca
% Radiogenic
L# 55147, 11-29-02-6, Groundmass Concentrate, 78.01 mg
100
50
0
1
0.1
Apparent Age (Ma)
18
K/Ca
% Radiogenic
L# 55150, 12-1-01-1, Groundmass Concentrate, 77.26 mg
0.01
16
14
650
A
12
8.32 ± 0.35 Ma
(MSWD = 16.0)
10
8
6
870
D
775
C
755
B
1275
H
925
1110
E 1005 G
F
4
Integrated Age = 9.88 ± 0.83 Ma
2
0
10
20
30
40
Cumulative
50
60
%39Ar
K
70
80
90
100
Released
0.0035
A
0.0030
G
I
H
36Ar/40Ar
0.0025
0.0020
C
F
0.0015
E
D
0.0010
Isochron age = 8.60 ± 0.36 Ma
40Ar/36Ar
Intercept = 291 ± 11
MSWD = 3.3, n = 4
0.0005
0
0
0.04
0.08
0.12
0.16
0.20
0.24
0.28
39Ar/40Ar
Figure 12. Sample 12-1-01-1, age spectrum and isochron diagram.
12
100
50
0
10
1
0.1
12
Apparent Age (Ma)
K/Ca
% Radiogenic
L# 55157, 12-1-01-2, Groundmass Concentrate, 78.98 mg
11
8.44 ± 0.10 Ma
(MSWD = 6.6)
10
9
8
760
B
775
C
880
D
1015
F
930
E
1275
H
1115
G
1725
I
7
A
6
5
Integrated Age = 8.31 ± 0.17 Ma
0
10
20
30
40
Cumulative
50
60
%39Ar
K
70
80
90
100
Released
0.0035
A
0.0030
0.0025
36Ar/40Ar
H
I
B
0.0020
E
D
G
0.0015
C
F
0.0010
Isochron age = 8.32 ± 0.25 Ma
40Ar/36Ar
0.0005
Intercept = 300 ± 9
MSWD = 6.5, n = 8
0
0
0.02 0.04 0.06 0.08 0.10 0.12 0.14 0.16 0.18 0.20 0.22 0.24
39Ar/40Ar
Figure 13. Sample 12-1-01-2, age spectrum and isochron diagram.
13
100
50
0
1
0.1
Apparent Age (Ma)
12
K/Ca
% Radiogenic
L# 55155, 12-1-01-4, Groundmass Concentrate, 76.90 mg
0.01
5.16 ± 0.38 Ma
10
(MSWD = 4.6)
8
6
4
780
C
760
B
870
D
935
E
1010
F
1275
H
1110
G
2
0
Integrated Age = 5.3 ± 1.4 Ma
A
-2
0
10
20
30
40
Cumulative
0.0035
50
60
%39Ar
K
70
80
90
100
0.40
0.45
Released
A
I
H
0.0030
G
B
C
F
D
E
36Ar/40Ar
0.0025
0.0020
0.0015
0.0010
Isochron age = 4.88 ± 0.68 Ma
40Ar/36Ar
0.0005
Intercept = 299 ± 6
MSWD = 4.5, n = 7
0
0
0.05
0.10
0.15
0.20
0.25
0.30
0.35
39Ar/40Ar
Figure 14. Sample 12-1-01-4, age spectrum and isochron diagram.
14
100
50
0
100
10
Apparent Age (Ma)
6
K/Ca
% Radiogenic
L# 55148, 12-3-01-2, Groundmass Concentrate, 77.72 mg
1
A
5
4
1.43 ± 0.02 Ma
(MSWD = 1.4)
3
2
755
B
1
855
D
775
C
935
E
1275
H
1115
G
1015
F
1725
I
0
-1
Integrated Age = 1.62 ± 0.14 Ma
-2
0
10
20
30
40
50
60
70
80
90
100
Cumulative %39ArK Released
0.0035
A
0.0030
H
I
G
0.0025
B
F
36Ar/40Ar
C
E
0.0020
D
0.0015
0.0010
Isochron age = 1.41 ± 0.08 Ma
40Ar/36Ar Intercept = 297 ± 5
MSWD = 1.6, n = 6
0.0005
0
0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
39Ar/40Ar
Figure 15 Sample 12-3-01-2, age spectrum and isochron diagram.
15
100
50
0
1
0.1
0.01
10
Apparent Age (Ma)
K/Ca
% Radiogenic
L# 55158, 12-3-01-3, Groundmass Concentrate, 77.43 mg
8
0.54 ± 0.21 Ma
(MSWD = 1.1)
6
1725
I
4
1275
H
2
0
-2
765
B C 880 E 1010
D
F
0
10
20
1115
G
30
Integrated Age = 3.6 ± 1.3 Ma
40
Cumulative
0.0035
50
60
%39Ar
K
70
80
90
100
Released
CF
D
I G
AB E
H
0.0030
G
A
0.0030
I
D
F
B
H
C
E
0.0025
36Ar/40Ar
0.0020
0.0020
0.0010
0.0015
0
0
0.02
0.04
0.06
0.08
0.10
0.0010
Isochron age = 0.23 ± 0.22 Ma
40Ar/36Ar
0.0005
Intercept = 300 ± 3
MSWD = 0.9, n = 6
0
0
1
2
3
4
5
6
7
8
9
39Ar/40Ar
Figure 16. Sample 12-3-01-3, age spectrum and isochron diagram.
16
100
50
0
1
0.1
0.01
12
Apparent Age (Ma)
K/Ca
% Radiogenic
L# 55151, 12-3-01-4A, Groundmass Concentrate, 79.44 mg
10
0.91 ± 0.49 Ma
8
(MSWD = 5.7)
1275
H
6
4
2
755
B
0
775
C
855
D
925
E
-2
-4
1110
G
1005
F
I
Integrated Age = 7.4 ± 2.2 Ma
0
10
20
30
40
50
60
70
80
90
100
Cumulative %39ArK Released
0.0035
I
C
A H B G
0.0030
I DF
BE
A G
HC
36Ar/40Ar
0.0030
F
D
E
0.0020
0.0025
0.0010
0.0020
0
0
0.02
0.04
0.06
0.08
0.10
0.0015
0.0010
Isochron age = 0.07 ± 0.08 Ma
40Ar/36Ar
Intercept = 301 ± 2
MSWD = 0.8, n = 6
0.0005
0
0
4
8
12
16
20
24
28
39Ar/40Ar
Figure 17. Sample 12-03-01-4A, age spectrum and isochron diagram.
17
100
50
0
1
0.1
0.01
10
Apparent Age (Ma)
K/Ca
% Radiogenic
L# 55159, 12-5-01-2, Groundmass Concentrate, 78.57 mg
8
4.10 ± 0.11 Ma
(MSWD = 4.4)
6
4
755
B
935 1010
E
F
875
D
775
C
1275
H
1125
G
2
Integrated Age = 11.1 ± 1.9 Ma
0
0
10
20
30
40
50
60
70
80
90
100
Cumulative %39ArK Released
0.0035
A
0.0030
H
G
0.0025
36Ar/40Ar
I
F
0.0020
C
D
B
E
0.0015
0.0010
Isochron age = 4.31 ± 0.20 Ma
40Ar/39Ar
Intercept = 281 ± 10
MSWD = 6.2, n = 7
0.0005
0
0
0.05
0.10
0.15
0.20
0.25
0.30
0.35
0.40
0.45
0.50
0.55
0.60
39Ar/40Ar
Figure 18. Sample 12-5-01-2, age spectrum and isochron diagram.
18
100
50
0
1
0.1
K/Ca
% Radiogenic
L# 55304: 12-9-01-1, 211.15 mg Groundmass Concentrate
0.01
8
Apparent Age (Ma)
7
3.53 ± 0.10 Ma
6
(MSWD = 2.8)
5
4
750
B
3
H
775
C
865
D
920
E
1005
F
1300
I
J
1105
G
2
1
Integrated Age = 5.49 ± 0.81 Ma
0
0
10
20
30
40
50
60
70
80
90
100
Cumulative % 39ArK Released
0.0035
J
A
0.0030
J
I
H
G
A
IH
0.0030
B
C
G
F
D
B
E
0.0020
C
D
0.0025
F
0.0010
36Ar/40Ar
E
0.0020
0
0
0.004
0.008
0.012
0.016
0.0015
0.0010
Isochron age = 3.16 ± 0.22 Ma
40Ar/36Ar Intercept = 307 ± 5
MSWD = 2.4, n=5
0.0005
0
0
0.01
0.02
0.03
0.04
0.05
0.06
0.07
39Ar/40Ar
Figure 19 Sample 12-9-01-1, age spectrum and isochron diagram.
19
100
50
0
1
0.1
0.01
30
Apparent Age (Ma)
K/Ca
% Radiogenic
L# 55296: 12-10-01-6, 219.22 mg Groundmass Concentrate
25
20
1725
I
15
1105
G
750
B
10
775
860
C
D
Integrated Age = 16.4 ± 2.3 Ma
5
0
10
20
30
920
E
40
1205
H
1005
F
50
60
70
80
90
100
Cumulative % 39ArK Released
0.0035
A
A
G
C
G
H
C
F
J
E
I D
BF
0.0030
H E
B
0.0030
J
D
I
0.0020
0.0025
36Ar/40Ar
0.0010
0.0020
0
0
0.0004
0.0008
0.0012
0.0016
0.0015
0.0010
Isochron age = 7.4 ± 2.1 Ma
40Ar/36Ar
0.0005
Intercept = 304 ± 4
MSWD = 1.1, n = 7
0
0
0.004
0.008
0.012
0.016
0.020
0.024
0.028
39Ar/40Ar
Figure 20. Sample 12-10-01-6, age spectrum and isochron diagram.
20
L# 55294: 12-11-01-4, 216.63 mg Groundmass Concentrate
100
50
0
1
0.1
3
0.01
2
0.28 ± 0.08 Ma
(MSWD = 2.4)
1
750
A
0
750775
B C
860
D
920
E
1725
J
12001300
H
I
1005 1105
F
G
-1
Integrated Age = 0.43 ± 0.07 Ma
-2
0.0035
J
I
H
G
A
0.0030
E
C
B
F
D
0.0025
0.0020
0.0015
Isochron age = -0.05 ± 0.06 Ma
0.0010
40Ar/36Ar
0.0005
Intercept = 316 ± 81
MSWD = 7.2, n = 5
0
0
0.005
0.010
0.015
0.020
0.025
0.030
0.035
0.040
Figure 21. Sample 12-11-01-4, age spectrum and isochron diagram.
21
100
50
0
10
1
0.1
K/Ca
% Radiogenic
L# 55292: 12-11-01-5, 230.56 mg Groundmass Concentrate
0.01
Apparent Age (Ma)
6
2.10 ± 0.05 Ma
(MSWD = 2.2)
4
I
650
A
2
750
B
860
D
775
C
920
E
F
G
1200
H
0
Integrated Age = 2.46 ± 0.13 Ma
-2
0
10
20
30
40
50
60
70
80
90
100
Cumulative % 39ArK Released
0.0035
I
J
0.0030
H
A G
F
0.0025
E
B
D
36Ar/40Ar
C
0.0020
0.0015
0.0010
Isochron age = 1.53 ± 0.23 Ma
40Ar/36Ar
0.0005
Intercept = 330 ± 14
MSWD = 5.2, n = 5
0
0
0.02
0.04
0.06
0.08
0.10
0.12
0.14
39Ar/40Ar
Figure 22. Sample 12-11-01-5, age spectrum and isochron diagram.
22
100
50
0
1
0.1
6
K/Ca
% Radiogenic
L# 55149, 12-18-01-2, Groundmass Concentrate, 81.30 mg
0.01
Apparent Age (Ma)
5
4
1.32 ± 0.05 Ma
(MSWD = 0.2)
3
2
1
755
B
0
870
D
775
C
930 1010
E
F
1275
H
1115
G
I
-1
Integrated Age = 3.15 ± 0.59 Ma
-2
0
10
20
30
40
50
60
70
80
90
100
Cumulative %39ArK Released
0.0035
I
A
G
H
F
0.0030
E
B
D
C
36Ar/40Ar
0.0025
0.0020
0.0015
0.0010
Isochron age = 1.35 ± 0.10 Ma
40Ar/36Ar Intercept = 294 ± 4
MSWD = 0.1, n = 6
0.0005
0
0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
39Ar/40Ar
Figure 23. Sample 12-18-01-2, age spectrum and isochron diagram.
23
100
50
0
1
0.1
Apparent Age (Ma)
5
K/Ca
% Radiogenic
L# 55286: 12-19-01-3, 220.97 mg Groundmass Concentrate
0.01
4
0.29 ± 0.13 Ma
3
J
(MSWD = 1.9)
2
1
0
750
B
-1
-2
I
1105
G
865
D
775
C
920
E
1200
H
1005
F
Integrated Age = 1.50 ± 0.50 Ma
0
10
20
30
40
50
60
70
80
90
100
Cumulative % 39ArK Released
0.0035
G
I
A
F
C
H
E
J
D
B
0.0030
36Ar/40Ar
0.0025
0.0020
0.0015
Isochron age = -0.27 ± 0.22 Ma
0.0010
40Ar/36Ar
0.0005
Intercept = 303 ± 3
MSWD = 0.7, n = 6
0
0
0.002
0.004
0.006
0.008
0.010
0.012
0.014
39Ar/40Ar
Figure 24. Sample 12-19-01-3, age spectrum and isochron diagram.
24
100
50
0
1
0.1
K/Ca
% Radiogenic
L# 55308: 12-19-01-5, 205.57 mg Groundmass Concentrate
0.01
Apparent Age (Ma)
15
0.33 ± 0.13 Ma
10
(MSWD = 1.2)
1190
H
1300
I
5
0
750
B
775 865 9201005
C
D
E F
1105
G
-5
J
Integrated Age = 8.8 ± 1.6 Ma
0
10
20
30
40
50
60
70
80
90
100
Cumulative % 39ArK Released
0.0035
JF E
BG
D
AC
I H
0.0030
E
G
J
A
H I
B
C
F
0.006
0.008
D
0.0030
0.0020
0.0025
36Ar/40Ar
0.0010
0.0020
0
0
0.002
0.004
0.010
0.0015
0.0010
Isochron age = 0.06 ± 0.08 Ma
40Ar/36Ar
0.0005
Intercept = 298 ± 3
MSWD = 0.5, n = 6
0
0
0.5
1.0
1.5
2.0
2.5
3.0
39Ar/40Ar
Figure 25 Sample 12-19-01-5, age spectrum and isochron diagram.
25
L# 55302: 12-19-01-7, 227.53 mg Groundmass Concentrate
100
50
0
1
0.1
5
0.01
4
650
A
3
J
0.09 ± 0.08 Ma
(MSWD = 1.5)
2
1
0
-1
1190
H
865 920 1005 1100
G
775 D
F
E
C
745
B
1305
I
Integrated Age = 2.10 ± 0.79 Ma
-2
0.0035
J
I
A
H
F
C
E
G
D
B
0.0030
0.0025
0.0020
0.0015
0.0010
Isochron age = -0.02 ± 0.02 Ma
40Ar/36Ar
Intercept = 298 ± 8
MSWD = 1.8, n = 5
0.0005
0
0
0.004
0.008
0.012
0.016
0.020
0.024
Figure 26. Sample 12-19-01-7, age spectrum and isochron diagram.
26
100
50
0
1
0.1
0.01
5
Apparent Age (Ma)
K/Ca
% Radiogenic
L# 55306: Cone 56, 211.73 mg Groundmass Concentrate
4
0.41 ± 0.39 Ma
3
(MSWD = 6.1)
2
1
0
860
D
740
B
920
E
775
C
-1
1005
F
1190
H
1105
G
1300
I
Integrated Age = 7.9 ± 1.6 Ma
0
10
20
30
40
50
60
70
80
90
100
0.007
0.008
Cumulative % 39ArK Released
0.0035
C
A
0.0030
E
G
H
B
I
F
D
J
36Ar/40Ar
0.0025
0.0020
0.0015
0.0010
Isochron age = -0.50 ± 0.39 Ma
40Ar/36Ar
Intercept = 302 ± 7
MSWD = 3.2, n = 8
0.0005
0
0
0.001
0.002
0.003
0.004
0.005
0.006
39Ar/40Ar
Figure 27. Sample Cone 56, age spectrum and isochron diagram.
27
100
50
0
1
0.1
Apparent Age (Ma)
5
K/Ca
% Radiogenic
L# 55300: Cone 88/89, 217.55 mg Groundmass Concentrate
0.01
4
0.07 ± 0.15 Ma
3
(MSWD = 1.7)
2
1
750
B
0
-1
-2
775
C
920
E
865
D
1005
F
1310
I
1190
H
1105
G
Integrated Age = 7.0 ± 1.5 Ma
0
10
20
30
40
50
60
70
80
90
100
Cumulative % 39ArK Released
0.0035
H
A
0.0030
B
F
C
G
I
D
E
J
36Ar/40Ar
0.0025
0.0020
0.0015
Isochron age = -0.29 ± 0.36 Ma
0.0010
40Ar/36Ar
0.0005
Intercept = 300 ± 8
MSWD = 1.9, n = 4
0
0
0.001 0.002 0.003 0.004 0.005 0.006 0.007 0.008 0.009 0.010 0.011
39Ar/40Ar
Figure 28. Sample Cone 88/89, age spectrum and isochron diagram.
28
Table 2. 40Ar/39Ar analytical data.
ID
Power
40
Ar/39Ar
37
Ar/39Ar
36
Ar/39Ar
-3
(Watts)
(x 10 )
39
(x 10
K/Ca
ArK
-15
40
(%)
mol)
3-00, Groundmass Concentrate, 219.42 mg, J=0.0001168±0.09%, D=1.0063±0.001, NM-185A,
†∆ A
650 2121.9
0.8312
6945.1
9.45
0.61
B
725 119.5
1.262
349.2
17.7
0.40
C
775 131.1
1.373
393.9
1.41
0.37
D
825 163.1
1.641
493.1
7.37
0.31
E
900 198.0
2.158
611.2
4.43
0.24
F
1000 244.4
2.654
779.1
4.92
0.19
G
1100 422.3
3.743
1367.2
7.06
0.14
H
1200 384.1
6.525
1235.2
27.6
0.078
∆ I
1300 448.1
41.85
1470.2
1.23
0.012
†∆ J
1725 453.9
30.09
1500.5
0.938
0.017
Integrated age ± 2σ
n=10
82.1
Plateau ± 2σ
steps B-I
Isochron±2σ
n=8
MSWD=2.7
n=6
MSWD=3.3
71.7
40
steps B-E
Isochron±2σ
n=4
MSWD=2.0
n=7
MSWD=1.3
55.7
40
Isochron±2σ
steps B-G
n=6
MSWD=1.3
n=5
MSWD=1.2
42.2
40
Age
(%)
(Ma)
(Ma)
14.6
3.468
3.11
3.70
3.72
3.03
3.92
4.15
3.74
2.82
5.02
2.1
0.100
0.16
0.14
0.18
0.23
0.37
0.34
0.44
0.51
0.85
3.50
0.20
3.34
0.62
3.3
11.5
13.8
33.0
11.3
34.8
10.8
43.7
8.9
49.1
5.9
55.1
4.4
63.7
5.1
97.4
3.8
98.9
2.9 100.0
K2O=1.23%
87.3
Lab#=55298-01
2.3
11.3
3.1
43.9
4.2
48.2
4.4
67.5
4.6
77.6
3.3
82.0
3.3
88.3
3.1
92.0
3.7
99.5
0.3 100.0
K2O=1.23%
66.3
3.40
0.456
0.613
0.486
0.644
1.45
1.62
0.87
1.48
0.6
1.03
0.61
0.062
0.089
0.051
0.066
0.21
0.22
0.15
0.17
1.1
0.28
0.53
0.09
0.17
0.10
4.5
0.378
0.236
0.338
0.209
0.35
0.72
0.95
0.51
1.70
0.71
2.5
0.065
0.080
0.063
0.076
0.23
0.25
0.15
0.25
0.37
0.39
0.31
0.08
0.28
0.15
114
4.45
4.69
4.56
22
0.13
0.29
0.20
Lab#=55290-01
0.8
4.2
2.6
35.0
2.9
37.1
3.3
49.7
1.5
59.4
0.7
66.6
1.4
71.5
2.7
98.2
1.7
99.3
3.5 100.0
K2O=0.94%
0.22 ±0.18
Ar/36Ar=296±4
11-00, Groundmass Concentrate, 80.09 mg, J=0.001115±0.07%, D=1.0063±0.001, NM-182F,
†∆ A
650 2618.8
1.022
8664.7
4.10
0.50
B
725
13.84
2.395
40.01
17.6
0.21
C
775
9.454
2.422
24.79
2.88
0.21
D
825
13.26
2.878
38.01
7.4
0.18
±1σ
Lab#=55288-01
0.96 ±0.61
Ar/36Ar=304±3
7-00, Groundmass Concentrate, 220.18 mg, J=0.0001168±0.09%, D=1.0063±0.001, NM-185A,
†∆ A
650 2751.0
1.437
9237.6
2.63
0.36
B
725
69.87
1.490
230.8
19.3
0.34
C
775
38.73
1.732
127.8
1.27
0.29
D
825
48.89
2.310
160.7
7.93
0.22
E
900
66.71
2.445
223.1
6.10
0.21
F
1000 236.2
2.740
794.4
4.48
0.19
G
1100 236.5
6.674
790.6
3.10
0.076
†∆ H
1200 166.5
15.11
552.7
16.7
0.034
†∆ I
1300 137.2
44.31
469.3
0.680
0.012
†∆ J
1725 229.7
28.33
758.8
0.439
0.018
Integrated age ± 2σ
n=10
62.7
Plateau ± 2σ
Ar
39
0.22 ±0.28
Ar/36Ar=297±5
6-00, Groundmass Concentrate, 223.01 mg, J=0.0001175±0.15%, D=1.0063±0.001, NM-185C,
†∆ A
650 697.0
0.7645
2304.6
9.54
0.67
B
725
69.00
0.6357
226.4
27.4
0.80
C
775
68.02
0.4062
220.5
3.55
1.3
D
825
51.61
0.4401
167.0
16.3
1.2
E
900
66.08
0.8409
213.6
8.50
0.61
† F
1000 209.9
1.882
687.8
3.64
0.27
† G
1100 230.3
3.045
754.4
5.36
0.17
†∆ H
1185 132.3
14.51
438.2
3.05
0.035
†∆ I
1300 183.8
42.51
611.2
6.36
0.012
†∆ J
1725 925.6
27.84
3131.0
0.398
0.018
Integrated age ± 2σ
n=10
84.1
Plateau ± 2σ
Ar*
67.3
Lab#=55153-01
2.2
16.0
24.6
17.1
6.2
32.6
37.0
48.1
Table 2. 40Ar/39Ar analytical data.
ID
Power
40
Ar/39Ar
37
Ar/39Ar
(x 10 )
E
900
8.322
F
1000
10.32
G
1100
18.72
H
1275
30.10
I
1725
21.72
Integrated age ± 2σ
Plateau ± 2σ
steps B-E
Isochron±2σ
†∆
†∆
†∆
†∆
Ar/39Ar
-3
(Watts)
†∆
†∆
†∆
†∆
36
4.086
4.730
5.669
46.93
9.510
n=9
21.22
26.33
52.11
104.5
41.70
n=4
MSWD=1.1
n=4
MSWD=0.1
39
(x 10
K/Ca
ArK
-15
40
(%)
mol)
5.01
4.20
4.20
16.0
5.20
66.6
0.12
0.11
0.090
0.011
0.054
32.9
40
steps E-I
Isochron±2σ
n=5
n=5
MSWD=4.9
332.0
40
MSWD=0.7
steps B-E
Isochron±2σ
n=4
n=5
MSWD=8.5
119.5
40
MSWD=10.0
steps B-I
n=8
MSWD=3.3
1039.0
(%)
(Ma)
(Ma)
4.81
5.91
7.63
6.45
20.51
13.1
0.16
0.24
0.29
0.44
0.25
3.0
4.6
0.2
5.2
0.6
1107
39.7
16.3
21.7
12.43
12.17
11.567
12.00
11.78
25.0
89
4.0
1.4
1.1
0.36
0.19
0.062
0.14
0.23
2.7
11.70
0.23
11.44
0.16
36.3
4.943
4.972
5.092
5.091
4.713
3.825
4.36
4.96
5.39
8.5
0.020
0.054
0.025
0.044
0.058
0.093
0.13
0.52
0.46
5.01
0.08
5.19
0.25
6.21
4.910
4.887
4.943
4.962
4.889
4.911
4.960
4.931
4.87
0.46
0.017
0.017
0.013
0.015
0.015
0.016
0.022
0.022
0.08
4.92
0.02
0.19
Ar/36Ar=287±10
49.4
5.3
4.3
4.9
8.0
13.1
22.8
55.4
29.5
26.2
0.6
10.7
12.2
30.7
39.5
52.3
66.7
98.1
100.0
69.3
Lab#=55145-01
1.8
78.3
67.3
83.3
78.7
64.3
30.5
27.1
12.4
2.4
31.7
39.7
55.9
65.1
72.5
80.0
99.2
100.0
0.47
Ar/36Ar=254±51
62.6
8.7
±1σ
Lab#=55156-01
0.44
Ar/36Ar=300±2
20-00, Groundmass Concentrate, 78.41 mg, J=0.001123±0.07%, D=1.0063±0.001, NM-182F,
†∆ A
650
55.73
0.0583
178.2
76.5
8.8
B
725
3.479
0.0471
3.576
308.4
10.8
C
775
3.068
0.0455
2.220
125.0
11.2
D
825
2.761
0.0477
1.087
148.7
10.7
E
900
2.692
0.0514
0.8246
80.3
9.9
F
1000
2.776
0.0596
1.234
107.1
8.6
G
1100
3.067
0.1283
2.201
95.6
4.0
H
1275
3.483
0.1537
3.537
94.3
3.3
I
1725
3.552
0.1261
3.811
79.6
4.0
Integrated age ± 2σ
n=9
1115.5
Plateau ± 2σ
Age
55.6
61.9
68.2
92.2
100.0
17-00, Groundmass Concentrate, 78.84 mg, J=0.0011102±0.07%, D=1.0063±0.001, NM-182E,
†∆ A
650 1034.3
0.8855
3438.4
4.67
0.58
B
725
3.156
0.8736
2.566
55.9
0.58
C
775
3.693
1.321
4.463
15.1
0.39
D
825
3.054
1.384
2.116
30.9
0.37
E
900
3.230
1.436
2.728
17.5
0.36
† F
1000
3.658
1.653
4.876
14.1
0.31
†∆ G
1100
6.262
2.825
15.52
14.3
0.18
†∆ H
1275
7.954
15.40
23.93
36.6
0.033
†∆ I
1725
19.65
27.27
65.91
1.62
0.019
Integrated age ± 2σ
n=9
190.9
Plateau ± 2σ
Ar
39
28.7
28.4
20.2
10.3
46.9
13-00, Groundmass Concentrate, 75.66 mg, J=0.0011103±0.11%, D=1.0063±0.001, NM-182G,
A
650 #######
1.754
N.A.
2.89
0.29
B
725 465.5
0.5905
1507.8
48.3
0.86
C
775 166.5
0.5167
535.9
7.1
0.99
D
825 136.5
0.7177
425.5
88.8
0.71
E
900
47.60
0.7405
140.2
42.3
0.69
F
1000
26.67
0.6172
69.79
61.1
0.83
G
1100
10.45
0.8586
16.00
69.0
0.59
H
1275
20.34
3.095
49.39
150.7
0.16
I
1725
22.50
3.136
57.08
8.9
0.16
Integrated age ± 2σ
n=9
479.1
Plateau ± 2σ
Ar*
Lab#=55152-01
5.5
69.7
78.7
88.5
91.1
87.0
79.1
70.4
68.6
6.9
34.5
45.7
59.0
66.2
75.8
84.4
92.9
100.0
93.1
Table 2. 40Ar/39Ar analytical data.
ID
Power
40
Ar/39Ar
37
Ar/39Ar
36
Ar/39Ar
-3
(Watts)
(x 10 )
Isochron±2σ
n=8
39
(x 10
K/Ca
ArK
-15
Ar*
(%)
mol)
40
MSWD=3.8
40
Ar
Age
(%)
(Ma)
39
Ar/36Ar=294±12
4.93
11-29-01-2, Groundmass Concentrate, 78.50 mg, J=0.0011254±0.07%, D=1.0063±0.001, NM-182E, Lab#=55146-01
†∆ A
650 499.1
0.5156
1667.8
8.5
0.99
1.3
1.9
12.8
B
725
5.168
0.3755
14.38
141.6
1.4
18.4
32.6
1.929
C
775
3.249
0.4008
7.927
30.3
1.3
28.9
39.2
1.906
D
825
4.085
0.5341
10.77
54.6
0.96
23.1
51.1
1.919
E
900
8.416
0.7177
25.40
8.2
0.71
11.5
52.9
1.97
† F
1000
10.08
0.9041
32.03
15.9
0.56
6.9
56.3
1.40
† G
1100
23.51
1.355
76.07
38.0
0.38
4.8
64.6
2.31
†∆ H
1275
41.08
3.666
132.3
160.5
0.14
5.6
99.5
4.64
†∆ I
1725
62.80
9.074
210.4
2.40
0.056
2.2 100.0
2.83
Integrated age ± 2σ
n=9
460.0
3.01
Plateau ± 2σ
steps B-E
Isochron±2σ
n=4
MSWD=0.1
n=6
MSWD=5.9
234.7
40
1.23
Ar/36Ar=295±6
51.0
steps C-F
Isochron±2σ
n=4
MSWD=0.4
n=6
MSWD=13.0
11-29-02-6, Groundmass Concentrate, 78.01 mg,
∆ A
650
57.57
0.0615
B
725
3.105
0.0548
C
775
3.044
0.0670
D
825
3.148
0.0864
E
900
3.877
0.1233
F
1000
4.383
0.1545
G
1100
7.751
0.4581
H
1275
12.03
1.423
I
1725
10.32
0.9789
Integrated age ± 2σ
n=9
Plateau ± 2σ
Isochron±2σ
steps A-I
n=9
n=8
12-1-01-1, Groundmass Concentrate, 77.26 mg,
†∆ A
650 300.2
0.6114
B
725
12.26
0.8134
C
775
10.65
0.9266
D
825
7.346
1.135
88.9
40
0.79
Ar/36Ar=306±13
18.5
(Ma)
0.05
4.2
0.042
0.047
0.043
0.17
0.11
0.21
0.35
0.78
0.45
1.92
0.05
1.91
0.19
11-29-01-7, Groundmass Concentrate, 80.84 mg, J=0.0011099±0.07%, D=1.0063±0.001, NM-182F, Lab#=55154-01
†∆ A
650 1053.8
0.5137
3489.4
5.03
0.99
2.2
1.0
44.9
† B
725
19.01
0.5310
56.99
28.8
0.96
11.6
7.0
4.42
C
775
13.83
0.5485
41.02
4.76
0.93
12.7
8.0
3.51
D
825
16.59
0.6084
50.25
23.0
0.84
10.8
12.8
3.59
E
900
21.85
0.6693
67.65
30.5
0.76
8.8
19.2
3.84
F
1000
28.84
0.6781
91.65
30.5
0.75
6.3
25.6
3.63
† G
1100
45.56
0.8511
145.4
206.1
0.60
5.9
68.6
5.34
† H
1275
28.20
4.076
86.58
142.7
0.13
10.5
98.3
5.93
†∆ I
1725
21.32
1.446
31.61
8.1
0.35
56.8 100.0
24.10
Integrated age ± 2σ
n=9
479.5
5.73
Plateau ± 2σ
±1σ
8.7
0.16
0.23
0.16
0.21
0.25
0.39
0.24
0.18
0.68
3.64
0.20
2.9
1.4
5.7
42.1
50.4
67.7
73.1
80.9
92.1
97.4
100.0
4.86
4.016
3.971
4.036
3.964
4.014
3.912
3.98
4.052
3.94
0.47
0.015
0.020
0.017
0.033
0.034
0.058
0.10
0.096
0.10
100.0
4.01
0.02
4.02
0.04
J=0.0011213±0.10%, D=1.0063±0.001, NM-182E, Lab#=55147-01
186.7
3.799
3.669
3.919
6.518
8.154
19.81
34.43
28.41
55.2
348.4
79.7
166.3
51.6
75.1
107.6
50.8
24.9
959.7
MSWD=1.8
959.7
8.3
9.3
7.6
5.9
4.1
3.3
1.1
0.36
0.52
40
MSWD=1.7
4.2
64.0
64.6
63.4
50.6
45.3
25.0
16.4
19.4
6.2
Ar/36Ar=295±3
J=0.0011114±0.07%, D=1.0063±0.001, NM-182F, Lab#=55150-01
983.3
27.72
21.79
10.69
23.2
44.3
9.2
21.5
0.83
0.63
0.55
0.45
3.2
33.7
40.3
58.3
12.3
35.8
40.7
52.1
19.4
8.279
8.58
8.571
2.5
0.097
0.17
0.066
Table 2. 40Ar/39Ar analytical data.
ID
Power
40
Ar/39Ar
37
Ar/39Ar
(x 10 )
E
900
7.026
F
1000
7.524
G
1100
39.35
H
1275
32.13
I
1725
86.26
Integrated age ± 2σ
Plateau ± 2σ
steps B-G
Isochron±2σ
1.225
1.831
3.358
12.63
30.44
n=9
steps B-I
Isochron±2σ
MSWD=16.0
n=4
MSWD=3.3
n=8
n=8
12-1-01-4, Groundmass Concentrate, 76.90 mg,
†∆ A
650 616.0
1.937
B
725
26.01
2.614
C
775
26.73
2.342
D
825
15.95
2.272
E
900
9.732
2.795
F
1000
14.61
3.078
G
1100
39.74
4.307
† H
1275
54.75
29.92
†∆ I
1725 143.7
27.69
Integrated age ± 2σ
n=9
Plateau ± 2σ
steps B-G
Isochron±2σ
n=6
n=7
12-3-01-2, Groundmass Concentrate, 77.72 mg,
†∆ A
650 215.9
0.0588
B
725
2.684
0.0479
C
775
2.246
0.0540
D
825
1.754
0.0630
E
900
1.910
0.0739
F
1000
2.483
0.0960
G
1100
4.148
0.4679
†∆ H
1275
6.586
0.4139
†∆ I
1725
5.471
0.3653
Integrated age ± 2σ
n=9
Plateau ± 2σ
steps B-G
10.09
13.60
119.7
96.24
245.0
n=6
12-1-01-2, Groundmass Concentrate, 78.98 mg,
†∆ A
650 440.7
0.0771
B
725
11.28
0.0903
C
775
7.326
0.1437
D
825
8.895
0.1805
E
900
8.402
0.1557
F
1000
6.787
0.2274
G
1100
8.244
0.5408
H
1275
12.25
1.636
I
1725
12.40
2.066
Integrated age ± 2σ
n=9
Plateau ± 2σ
Ar/39Ar
-3
(Watts)
∆
∆
†∆
†∆
36
n=6
39
(x 10
K/Ca
ArK
-15
40
Ar*
(%)
mol)
10.8
8.9
16.0
53.8
0.90
188.4
0.42
0.28
0.15
0.040
0.017
110.6
40
Ar
Age
(%)
(Ma)
(Ma)
8.30
7.32
8.54
9.56
33.2
9.88
0.11
0.13
0.34
0.27
1.7
0.83
8.32
0.35
8.60
0.36
14.0
8.403
8.360
8.730
8.411
8.405
8.295
8.611
8.50
8.31
3.7
0.070
0.059
0.053
0.050
0.037
0.050
0.079
0.12
0.17
8.44
0.10
8.32
0.25
6.4
4.98
6.47
5.38
5.05
4.77
4.93
6.38
2.5
5.3
5.2
0.24
0.33
0.18
0.16
0.20
0.39
0.53
1.8
1.4
5.16
0.38
4.88
0.68
6.7
1.440
1.458
1.438
1.429
1.384
1.484
1.789
1.615
1.62
1.8
0.022
0.033
0.018
0.020
0.023
0.037
0.053
0.067
0.14
1.43
0.02
39
59.0
48.6
10.8
14.7
19.0
57.8
62.5
71.0
99.5
100.0
0.47
Ar/36Ar=291±11
58.7
±1σ
J=0.001102±0.11%, D=1.0063±0.001, NM-182G, Lab#=55157-01
1467.6
23.87
10.57
15.26
14.13
8.694
13.90
27.25
28.05
8.2
157.4
19.8
127.7
79.2
124.8
102.8
179.7
17.0
816.7
MSWD=6.6
808.5
6.6
5.7
3.6
2.8
3.3
2.2
0.94
0.31
0.25
40
MSWD=6.5
1.6
37.5
57.5
49.5
50.5
62.4
50.7
35.4
34.5
2.5
Ar/36Ar=300±9
1.0
20.3
22.7
38.3
48.0
63.3
75.9
97.9
100.0
99.0
J=0.0011231±0.08%, D=1.0063±0.001, NM-182F, Lab#=55155-01
2074.6
80.43
80.29
45.61
25.29
42.33
127.5
183.2
489.9
8.3
13.7
4.32
14.1
7.9
8.4
10.4
18.7
1.04
86.9
MSWD=4.6
58.9
0.26
0.20
0.22
0.22
0.18
0.17
0.12
0.017
0.018
40
MSWD=4.5
0.5
9.4
12.0
16.7
25.6
16.1
6.1
5.6
0.8
0.18
Ar/36Ar=299±6
9.6
25.3
30.2
46.5
55.6
65.3
77.3
98.8
100.0
67.8
J=0.0011088±0.09%, D=1.0063±0.001, NM-182E, Lab#=55148-01
719.4
6.661
5.150
3.522
4.068
6.088
11.66
19.38
15.88
23.0
178.8
37.2
90.1
50.3
125.4
137.3
247.2
22.1
911.5
8.7
10.7
9.4
8.1
6.9
5.3
1.1
1.2
1.4
MSWD=1.4
619.2
6.7
1.5
26.8
32.5
41.0
37.4
27.9
17.9
13.6
14.8
2.5
22.1
26.2
36.1
41.6
55.4
70.4
97.6
100.0
67.9
Table 2. 40Ar/39Ar analytical data.
ID
Power
40
Ar/39Ar
37
Ar/39Ar
Ar/39Ar
-3
(Watts)
(x 10 )
Isochron±2σ
n=6
12-3-01-3, Groundmass Concentrate, 77.43 mg,
†∆ A
650 414.5
0.6011
B
725
26.78
0.8626
C
775
18.31
0.9929
D
825
28.33
1.046
E
900
13.32
1.113
F
1000
16.73
1.163
G
1100
63.70
1.374
†∆ H
1275
71.05
6.705
†∆ I
1725
47.60
35.88
Integrated age ± 2σ
n=9
Plateau ± 2σ
36
steps B-G
Isochron±2σ
39
(x 10
K/Ca
ArK
-15
Ar*
(%)
mol)
40
MSWD=1.6
40
Ar
Age
(%)
(Ma)
39
Ar/36Ar=297±5
steps B-G
Isochron±2σ
0.08
18.8
0.78
0.64
0.44
0.57
0.24
1.32
4.56
0.6
3.6
3.6
0.25
0.29
0.26
0.18
0.20
0.53
0.63
1.3
1.3
0.54
0.21
0.23
0.22
J=0.0011152±0.12%, D=1.0063±0.001, NM-182G, Lab#=55158-01
1371.1
89.57
61.15
95.41
44.43
56.55
213.7
234.7
170.1
19.4
14.5
5.41
20.2
13.3
19.4
75.1
131.7
1.11
300.1
n=6
MSWD=1.1
148.0
n=6
MSWD=0.9
0.85
0.59
0.51
0.49
0.46
0.44
0.37
0.076
0.014
40
2.3
1.4
1.7
0.8
2.1
0.7
1.0
3.2
0.7
0.43
Ar/36Ar=300±3
6.4
11.3
13.1
19.8
24.3
30.7
55.8
99.6
100.0
49.3
n=6
MSWD=5.7
n=6
MSWD=0.8
157.9
40
0.36
Ar/36Ar=301±2
68.8
12-5-01-2, Groundmass Concentrate, 78.57 mg, J=0.0011137±0.12%, D=1.0063±0.001, NM-182G, Lab#=55159-01
†∆ A
650 3300.3
1.141
10797.1
4.16
0.45
3.3
3.4
B
725
4.198
1.008
7.423
40.4
0.51
49.7
36.0
C
775
4.549
1.406
9.006
7.8
0.36
44.1
42.2
D
825
3.789
1.772
6.615
21.1
0.29
52.3
59.3
E
900
3.942
2.076
6.963
8.9
0.25
52.2
66.4
F
1000
4.978
2.121
10.87
9.2
0.24
39.0
73.8
† G
1100
10.18
3.956
30.40
10.6
0.13
15.0
82.4
† H
1275
14.63
35.33
53.52
20.1
0.014
11.9
98.6
†∆ I
1725
35.13
18.82
86.20
1.78
0.027
31.9 100.0
Integrated age ± 2σ
n=9
123.9
Plateau ± 2σ
Isochron±2σ
steps B-F
n=5
n=7
(Ma)
1.41
12-3-01-4A, Groundmass Concentrate, 79.44 mg, J=0.0011133±0.08%, D=1.0063±0.001, NM-182F, Lab#=55151-01
†∆ A
650 3463.8
1.851
11409.3
4.79
0.28
2.7
2.1
177
B
725
73.50
1.645
244.6
40.9
0.31
1.9
19.9
2.74
C
775
68.28
1.246
227.5
15.8
0.41
1.7
26.8
2.30
D
825
23.31
1.417
77.52
31.6
0.36
2.2
40.5
1.04
E
900
18.52
1.634
61.56
15.3
0.31
2.5
47.2
0.94
F
1000
15.91
1.250
53.45
19.9
0.41
1.3
55.9
0.43
G
1100
56.19
1.324
187.2
34.4
0.39
1.8
70.8
1.99
†∆ H
1275 118.9
10.91
390.2
65.2
0.047
3.8
99.2
9.00
†∆ I
1725 158.4
15.32
535.1
1.75
0.033
1.0 100.0
3.2
Integrated age ± 2σ
n=9
229.7
7.4
Plateau ± 2σ
±1σ
MSWD=4.4
MSWD=6.2
127.7
40
0.39
Ar/36Ar=281±10
70.5
12-9-01-1, Groundmass Concentrate, 211.15 mg, J=0.0001182±0.10%, D=1.0063±0.001, NM-185D, Lab#=55304-01
†∆ A
650
-2.751
42471.4
2.10
0.19
2.7
1.9
† B
725 226.8
1.238
694.1
11.9
0.41
9.6
12.6
† C
775 136.2
0.9793
393.9
5.47
0.52
14.6
17.5
D
825
76.12
0.9153
199.2
13.2
0.56
22.8
29.4
29
0.61
0.58
0.21
0.21
0.17
0.47
0.99
1.8
2.2
0.91
0.49
0.07
0.08
208
4.193
4.03
3.980
4.133
3.904
3.08
3.58
22.69
11.1
25
0.035
0.11
0.049
0.091
0.099
0.14
0.27
0.54
1.9
4.10
0.11
4.31
0.20
72.2
4.65
4.23
3.696
12.5
0.20
0.13
0.067
Table 2. 40Ar/39Ar analytical data.
ID
Power
40
Ar/39Ar
37
Ar/39Ar
36
-3
(Watts)
∆
†∆
†∆
†∆
(x 10 )
E
900
56.79
F
1000
66.80
G
1100 254.3
H
1185 450.2
I
1300 427.8
J
1750 438.9
Integrated age ± 2σ
Plateau ± 2σ
steps D-G
Isochron±2σ
†∆
†
†
†
†
†
†
†
†∆
†∆
∆
∆
∆
†∆
39
(x 10
K/Ca
ArK
-15
136.6
171.7
806.2
1441.2
1368.4
1413.2
14.4
13.8
16.2
5.76
27.2
1.24
111.2
n=4
MSWD=2.8
57.5
0.48
0.34
0.36
0.16
0.10
0.038
40
MSWD=2.4
40
Ar*
(%)
mol)
1.065
1.501
1.421
3.168
5.096
13.31
n=10
n=5
Ar
Age
(%)
(Ma)
(Ma)
3.520
3.451
3.46
5.24
5.11
4.82
5.49
0.046
0.053
0.23
0.41
0.38
0.47
0.81
3.53
0.10
3.16
0.22
39
29.1
42.4
24.2
54.7
6.4
69.3
5.5
74.4
5.6
98.9
5.1 100.0
K2O=1.71%
0.21
Ar/ Ar=307±5
51.7
36
±1σ
12-10-01-6, Groundmass Concentrate, 219.22 mg, J=0.0001173±0.10%, D=1.0063±0.001, NM-185B, Lab#=55296-02
A
650
-1.795
36166.8
2.92
0.28
3.4
3.6
77.4
B
725 1191.2
1.414
3791.3
20.7
0.36
6.0
29.4
15.0
C
775 1044.5
1.674
3329.4
2.12
0.30
5.8
32.0
12.83
D
825 624.1
1.966
1925.5
12.1
0.26
8.9
47.0
11.68
E
900 716.0
2.030
2240.0
9.37
0.25
7.6
58.6
11.46
F
1000 807.9
2.117
2555.9
6.37
0.24
6.5
66.5
11.16
G
1100 1815.9
3.014
5835.5
11.2
0.17
5.1
80.4
19.3
H
1200 765.0
9.766
2411.6
11.5
0.052
7.0
94.7
11.29
I
1300 703.1
33.60
2073.5
3.72
0.015
13.3
99.3
20.08
J
1725 713.6
49.48
2063.3
0.583
0.010
15.2 100.0
23.53
Integrated age ± 2σ
n=10
80.6
K2O=1.20%
16.4
11.3
1.1
0.94
0.52
0.60
0.69
1.7
0.65
0.56
0.84
2.3
Plateau ± 2σ
N.A.
N.A.
7.4
2.1
0.488
0.536
0.443
0.412
0.307
0.220
0.206
0.386
0.62
1.40
0.43
0.033
0.054
0.062
0.034
0.043
0.048
0.059
0.062
0.20
0.31
0.07
0.28
0.08
-0.05
0.06
n=0
Isochron±2σ
†∆
†
†
†
Ar/39Ar
n=7
MSWD=N.A.
Isochron±2σ
steps E-I
40
MSWD=1.1
12-11-01-4, Groundmass Concentrate, 216.63 mg,
A
650
36.60
0.9941
B
725
33.55
1.839
C
775
33.89
2.307
D
825
28.41
2.782
E
900
32.03
3.279
F
1000
29.43
2.866
G
1100
43.03
3.958
H
1200
58.36
13.56
I
1300 137.8
16.57
J
1725 229.0
11.61
Integrated age ± 2σ
n=10
Plateau ± 2σ
N.A.
N.A.
J=0.0001179±0.10%, D=1.0063±0.001, NM-185B, Lab#=55294-02
116.4
105.5
108.3
90.39
104.5
96.93
143.5
195.3
461.3
756.2
n=5
MSWD=2.4
n=5
MSWD=7.2
12-11-01-5, Groundmass Concentrate, 230.56 mg,
†∆ A
650 185.3
0.4083
B
725
33.07
0.2742
C
775
31.37
0.2338
D
825
34.99
0.3623
† E
900
47.02
0.6727
† F
1000
70.86
0.9362
†∆ G
1100 154.8
3.389
†∆ H
1200 140.1
16.36
0.43 ±0.21
Ar/36Ar=304±4
31.3
3.66
1.73
8.09
5.94
4.45
4.42
7.86
0.888
0.632
69.0
0.51
0.28
0.22
0.18
0.16
0.18
0.13
0.038
0.031
0.044
23.6
40
6.3
45.4
7.5
50.7
6.1
53.2
6.8
64.9
4.5
73.6
3.5
80.0
2.2
86.4
3.1
97.8
2.1
99.1
2.9 100.0
K2O=1.04%
0.11 ±0.14
Ar/36Ar=316±81
34.1
J=0.0001169±0.11%, D=1.0063±0.001, NM-185B, Lab#=55292-01
579.9
78.68
72.96
84.10
118.6
194.7
482.2
449.5
26.3
48.8
6.61
25.7
11.5
5.69
4.59
6.05
1.2
1.9
2.2
1.4
0.76
0.54
0.15
0.031
7.5
29.8
31.3
29.1
25.6
18.9
8.2
6.2
18.9
54.1
58.9
77.4
85.6
89.7
93.0
97.4
2.94
2.075
2.073
2.144
2.538
2.827
2.67
1.85
0.15
0.024
0.034
0.027
0.039
0.063
0.15
0.14
Table 2. 40Ar/39Ar analytical data.
ID
Power
40
Ar/39Ar
37
Ar/39Ar
Ar/39Ar
-3
(Watts)
(x 10 )
†∆ I
1300 379.8
†∆ J
1725 1246.3
Integrated age ± 2σ
Plateau ± 2σ
36
steps B-D
Isochron±2σ
39
(x 10
1219.7
3869.7
3.15
0.451
138.8
n=3
MSWD=2.2
81.2
0.016
0.031
40
MSWD=5.2
40
Ar*
(%)
mol)
31.32
16.25
n=10
n=5
K/Ca
ArK
-15
Ar
Age
(%)
(Ma)
(Ma)
4.74
22.1
2.46
0.35
1.2
0.13
2.10
0.05
1.53
0.23
39
5.8
99.7
8.4 100.0
K2O=1.98%
1.82 ±0.78
Ar/36Ar=330±14
58.5
12-18-01-2, Groundmass Concentrate, 81.30 mg, J=0.0011232±0.06%, D=1.0063±0.001, NM-182E, Lab#=55149-01
†∆ A
650 308.7
0.6059
1009.2
21.8
0.84
3.4
9.0
21.2
B
725
3.826
0.7771
10.96
58.0
0.66
17.0
33.1
1.322
C
775
3.293
0.7428
9.067
8.6
0.69
20.5
36.7
1.368
D
825
3.351
0.8100
9.362
35.0
0.63
19.4
51.3
1.321
E
900
4.616
0.9056
13.67
11.4
0.56
14.1
56.0
1.323
F
1000
7.301
1.032
22.76
16.9
0.49
9.1
63.0
1.34
G
1100
14.62
1.635
47.91
24.1
0.31
4.1
73.0
1.20
†∆ H
1275
18.70
8.212
62.56
62.8
0.062
4.8
99.1
1.81
†∆ I
1725
36.09
8.698
122.8
2.28
0.059
1.5 100.0
1.07
Integrated age ± 2σ
n=9
240.9
3.15
Plateau ± 2σ
steps B-G
Isochron±2σ
n=6
n=6
MSWD=0.2
154.1
40
MSWD=0.1
0.57
Ar/36Ar=294±4
64.0
steps B-F
Isochron±2σ
n=5
n=6
MSWD=1.9
65.3
40
MSWD=0.7
0.65 ±0.11
Ar/36Ar=303±3
57.3
0.05
1.35
0.10
Isochron±2σ
steps B-G
n=6
MSWD=1.2
n=6
MSWD=0.5
61.7
40
0.46 ±0.12
Ar/36Ar=298±3
48.2
4.6
0.093
0.14
0.098
0.14
0.086
0.40
0.26
0.30
0.34
0.50
0.29
0.13
-0.27
0.22
12-19-01-5, Groundmass Concentrate, 205.57 mg, J=0.0001188±0.08%, D=1.0063±0.001, NM-185D, Lab#=55308-01
†∆ A
650
-2.275
39003.6
3.19
0.22
4.9
2.5
122.1
B
725 281.7
1.069
945.5
14.0
0.48
0.8
13.4
0.51
C
775 161.4
0.9121
538.6
4.95
0.56
1.4
17.3
0.50
D
825
111.1
1.030
371.3
12.3
0.50
1.3
26.9
0.31
E
900
111.4
1.279
374.6
5.03
0.40
0.7
30.9
0.17
F
1000 120.3
1.220
401.9
6.93
0.42
1.3
36.3
0.35
G
1100 484.2
1.234
1623.1
18.4
0.41
1.0
50.7
1.01
†∆ H
1185 1072.0
2.372
3423.8
25.9
0.22
5.6
70.9
12.9
†∆ I
1300 814.8
7.815
2605.6
36.6
0.065
5.6
99.6
9.78
†∆ J
1750 1881.6
18.46
6353.0
0.576
0.028
0.3 100.0
1.3
Integrated age ± 2σ
n=10
127.9
K2O=2.01%
8.8
Plateau ± 2σ
2.6
0.044
0.084
0.043
0.092
0.10
0.14
0.17
0.57
0.59
1.32
12-19-01-3, Groundmass Concentrate, 220.97 mg, J=0.0001159±0.09%, D=1.0063±0.001, NM-185A, Lab#=55286-01
†∆ A
650 5003.0
1.536
16367.5
1.99
0.33
3.3
1.7
34.5
B
725 105.7
0.7594
353.2
27.2
0.67
1.3
25.6
0.289
C
775 110.1
0.7186
367.0
1.43
0.71
1.6
26.9
0.36
D
825 110.4
0.7538
367.3
17.6
0.68
1.7
42.4
0.399
E
900 147.7
0.8875
492.3
8.08
0.57
1.6
49.5
0.49
F
1000
94.54
0.8496
318.3
10.9
0.60
0.6
59.1
0.117
† G
1100 442.6
1.228
1467.3
22.9
0.42
2.1
79.2
1.90
†∆ H
1200 288.1
7.796
953.5
21.3
0.065
2.4
97.9
1.47
†∆ I
1300 268.7
33.63
877.3
1.77
0.015
4.6
99.5
2.64
†∆ J
1725 255.1
19.02
830.7
0.609
0.027
4.4 100.0
2.37
Integrated age ± 2σ
n=10
113.9
K2O=1.71%
1.50
Plateau ± 2σ
±1σ
12.2
0.26
0.16
0.10
0.11
0.12
0.45
1.1
0.89
1.8
1.6
0.33
0.13
0.06
0.08
Table 2. 40Ar/39Ar analytical data.
ID
Power
40
Ar/39Ar
37
Ar/39Ar
36
Ar/39Ar
-3
(Watts)
(x 10 )
39
(x 10
K/Ca
ArK
-15
40
Ar*
(%)
mol)
Ar
Age
(%)
(Ma)
39
12-19-01-7, Groundmass Concentrate, 227.53 mg, J=0.0001182±0.09%, D=1.0063±0.001, NM-185C, Lab#=55302-01
†∆ A
650 1132.5
0.8520
3728.0
30.0
0.60
2.7
28.2
6.6
B
725
51.48
0.8689
173.5
22.2
0.59
0.5
49.0
0.060
C
775
55.20
1.399
189.4
0.976
0.36
-1.2
49.9
-0.14
D
825
59.78
1.211
199.8
6.87
0.42
1.4
56.4
0.182
E
900
81.07
1.638
274.1
7.33
0.31
0.3
63.2
0.046
F
1000
97.99
1.671
329.3
7.60
0.31
0.9
70.4
0.18
†∆ G
1100 169.6
2.451
567.2
8.55
0.21
1.3
78.4
0.48
†∆ H
1185 196.8
6.508
654.9
16.2
0.078
1.9
93.7
0.82
†∆ I
1300 164.9
27.41
556.5
6.53
0.019
1.7
99.8
0.60
†∆ J
1750 559.1
44.46
1886.0
0.203
0.011
1.0 100.0
1.23
Integrated age ± 2σ
n=10
106.4
K2O=1.52%
2.10
Plateau ± 2σ
steps B-F
Isochron±2σ
n=5
MSWD=1.5
n=5
MSWD=1.8
44.9
40
0.40 ±0.23
Ar/36Ar=298±8
42.2
Cone 56, Groundmass Concentrate, 211.73 mg, J=0.0001183±0.10%, D=1.0063±0.001, NM-185D, Lab#=55306-01
A
650 5831.5
0.8155 18572.8
10.6
0.63
5.9
10.0
B
725 306.7
0.8129
1024.3
11.8
0.63
1.3
21.1
C
775 332.4
0.7726
1121.6
0.768
0.66
0.3
21.9
D
825 197.3
0.7632
654.3
16.9
0.67
2.1
37.9
E
900 157.0
0.9266
530.2
9.90
0.55
0.2
47.2
F
1000 157.4
0.9935
529.3
10.6
0.51
0.7
57.2
G
1100 200.5
1.328
673.7
16.9
0.38
0.8
73.2
H
1185 267.2
5.133
890.2
18.6
0.099
1.7
90.8
I
1300 231.2
26.46
772.2
9.40
0.019
2.3
99.7
†∆ J
1750 519.5
37.76
1553.8
0.288
0.014
12.2 100.0
Integrated age ± 2σ
n=10
105.7
K2O=1.62%
†∆
†
†
†
Plateau ± 2σ
steps E-I
Isochron±2σ
n=5
MSWD=6.1
n=8
MSWD=3.2
65.4
40
0.31 ±0.48
Ar/36Ar=302±7
61.8
Isochron±2σ
steps C-G
n=5
MSWD=1.7
n=4
MSWD=1.9
56.1
40
0.60 ±0.39
Ar/36Ar=300±8
46.0
(Ma)
1.2
0.049
0.14
0.065
0.097
0.11
0.16
0.18
0.16
0.98
0.79
0.09
0.08
-0.02
0.02
71.9
0.87
0.21
0.86
0.08
0.23
0.33
0.97
1.15
13.87
7.9
6.6
0.27
0.39
0.18
0.15
0.15
0.18
0.23
0.22
0.86
1.6
0.41
0.39
-0.50
0.39
Cone 88/89, Groundmass Concentrate, 217.55 mg, J=0.0001181±0.13%, D=1.0063±0.001, NM-185C, Lab#=55300-01
†∆ A
650 5633.0
0.9333 18042.7
12.5
0.55
5.4
10.2
63.2
†∆ B
725 211.1
0.6970
703.0
21.2
0.73
1.6
27.6
0.72
C
775 186.0
0.6392
628.7
2.11
0.80
0.1
29.3
0.05
D
825 155.0
0.6871
521.2
12.8
0.74
0.7
39.8
0.23
E
900 112.9
0.8512
379.4
12.0
0.60
0.8
49.6
0.19
F
1000
95.25
0.9427
324.1
15.9
0.54
-0.5
62.6
-0.092
∆ G
1100 209.7
1.713
705.9
13.3
0.30
0.6
73.5
0.26
†∆ H
1185 219.2
5.593
729.2
25.4
0.091
1.9
94.4
0.89
†∆ I
1300 195.7
26.03
643.2
6.42
0.020
4.0
99.6
1.70
†∆ J
1725 303.5
31.80
919.8
0.442
0.016
11.3 100.0
7.49
Integrated age ± 2σ
n=10
122.0
K2O=1.82%
7.0
Plateau ± 2σ
±1σ
6.1
0.19
0.22
0.14
0.11
0.087
0.19
0.19
0.18
0.45
1.5
0.07
0.15
-0.29
0.36
Notes:
Isotopic ratios corrected for blank, radioactive decay, and mass discrimination, not corrected for interferring reactions.
Errors quoted for individual analyses include analytical error only, without interferring reaction or J uncertainties.
Table 2. 40Ar/39Ar analytical data.
ID
Power
(Watts)
40
Ar/39Ar
37
Ar/39Ar
36
Ar/39Ar
-3
(x 10 )
39
(x 10
ArK
-15
mol)
K/Ca
40
Ar*
(%)
Ar
Age
(%)
(Ma)
39
Integrated age is volume-weighted mean of all steps.
Integrated age calculated by recombining isotopic measurements of all steps.
Integrated age error calculated by recombining errors of isotopic measurements of all steps.
Plateau age is inverse-variance-weighted mean of selected steps.
Plateau age error is inverse-variance-weighted mean error (Taylor, 1982) times root MSWD where MSWD>1.
Decay constants and isotopic abundances after Steiger and Jaeger (1977).
† symbol preceding sample ID denotes analyses excluded from plateau age calculations.
∆ symbol preceding sample ID denotes analyses excluded from inverse isochron calculations.
Ages calculated relative to FC-2 Fish Canyon Tuff sanidine interlaboratory standard at 28.02 Ma
Decay Constant (LambdaK (total)) = 5.543e-10
Discrimination (a.m.u.) = 1.0063 ± 0.001
Correction factors:
(39Ar/37Ar)Ca = 0.00068 ± 2e-05
(36Ar/37Ar)Ca = 0.000289 ± 5e-06
(38Ar/39Ar)K = 0.0131
(40Ar/39Ar)K = 0 ± 0.0004
±1σ
(Ma)