Open File Report OF-AR-17
New Mexico Bureau of Geology and Mineral Resources
A division of
New Mexico Institute of Mining and Technology
40
Ar/39Ar Geochronology Results
from a Cinder Cone from the Cutter
Quadrangle and from Two Dikes from
the Alivio Quadrangle, New Mexico
Prepared By:
Richard P. Esser and William C. McIntosh
New Mexico Bureau of Geology, Socorro, NM 87801
Prepared For:
Dr. William R. Seager
New Mexico State University, Las Cruces, NM 88003-8001
Data Repository for:
Initially prepared as:
NM Bureau of Geology and Mineral
Resources
NM Geochronology Research
Laboratory Internal Report
Memoir 49
NMGRL-IR 158A
April 4, 2003
SOCORRO 2003
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
Bill Richardson, Governor of New Mexico
Michael J. Davis, Superintendent of Public Instruction
Appointed
Ann Murphy Daily, President, 1999–2004, Santa Fe
Randall E. Horn, Secretary/Treasurer, 1997–2003, Albuquerque
Sidney M. Gutierrez, 2001–2007, Albuquerque
Anthony L. Montoya, Jr., 2001–2003, Socorro
Robert E. Taylor, 1997–2003, Silver City
NEW MEXICO GEOCHRONOLOGY RESEARCH LABORATORY STAFF
WILLIAM MCINTOSH, Geochronologist
MATT HEIZLER, Geochronologist
LISA PETERS, Argon Laboratory Technician
RICHARD ESSER, Argon Laboratory Technician
BUREAU STAFF
BRUCE D. ALLEN, Field Geologist
RUBEN ARCHULETA, Metallurgical Lab. Technician II
SANDRA H. AZEVEDO, Cartographer II
ALBERT BACA, Lead Maintenance Carpenter
JAMES M. BARKER, Associate Director for Operations,
Senior Industrial Minerals Geologist
PAUL W. BAUER, Associate Director for Government Liaison,
Senior Geologist, Manager of Geologic Mapping Program
LYNN A. BRANDVOLD, Senior Chemist
BRIAN S. BRISTER, Petroleum Geologist
RON BROADHEAD, Associate Director for Industry Liaison,
Principal Senior Petroleum Geologist
RITA CASE, Administrative Secretary II (Alb. Office)
STEVEN M. CATHER, Senior Field Geologist
RICHARD CHAMBERLIN, Senior Field Geologist
SEAN D. CONNELL, Albuquerque Office Manager, Field Geologist
RUBEN A. CRESPIN, Manager, Fleet/General Services
JEANNE DEARDORFF, Assistant Editor
NELIA W. DUNBAR, Analytical Geochemist
RICHARD ESSER, Senior Lab. Associate
ROBERT W. EVELETH, Senior Mining Engineer
KARL FRISCH, GIS Technician
PATRICIA L. FRISCH, Assistant Curator of Mineral Museum
LEO O. GABALDON, Cartographer II
NANCY S. GILSON, Editor
KATHRYN E. GLESENER, Senior Cartographer/Manager,
Cartography Section
DEBBIE GOERING, Business Office Coordinator
TERRY GONZALES, Information Specialist
IBRAHIM GUNDILER, Senior Extractive Metallurgist
LYNN HEIZLER, Senior Lab. Associate
MATT HEIZLER, Assistant Director for Laboratories, Geochronologist
LYNNE HEMENWAY, Geologic Information Center Coordinator
GRETCHEN K. HOFFMAN, Senior Coal Geologist
GLEN JONES, Assistant Director for Computer/Internet Services
THOMAS J. KAUS, Cartographer I
PHILIP KYLE, Professor, Geochemistry
SUSIE KYLE, Administrative Secretary I
LEWIS A. LAND, Hydrogeologist
ANNABELLE LOPEZ, Petroleum Information Coordinator
THERESA LOPEZ, Administrative Secretary I
DAVID W. LOVE, Principal Senior Environmental Geologist
JANE A. CALVERT LOVE, Managing Editor
VIRGIL W. LUETH, Assistant Director for Public Outreach,
Mineralogist/Economic Geologist, Curator of Mineral Museum
MARK MANSELL, GIS Specialist
DAVID MCCRAW, Senior Geologic Lab. Associate
WILLIAM MCINTOSH, Senior Volcanologist
CHRISTOPHER G. MCKEE, X-ray Facility Manager
VIRGINIA T. MCLEMORE, Minerals Outreach Liaison,
Senior Economic Geologist
PATRICIA JACKSON PAUL, Geologic Lab. Associate
LISA PETERS, Senior Lab. Associate
L. GREER PRICE, Senior Geologist/Chief Editor
ADAM S. READ, Senior Geological Lab. Associate
BEN REBACH, Cartographer II
WILLIAM D. RAATZ, Petroleum Geologist
MARSHALL A. REITER, Principal Senior Geophysicist
JOHN SIGDA, Geohydrologist
GREGORY SANCHEZ, Mechanic-Carpenter Helper
TERRY THOMAS, ICP–MS Manager
FRANK TITUS, Senior Outreach Hydrologist
LORETTA TOBIN, Executive Secretary
AMY TRIVITT-KRACKE, Petroleum Computer Specialist
JUDY M. VAIZA, Assistant Director for Finance
MANUEL J. VASQUEZ, Mechanic II
SUSAN J. WELCH, Manager, Geologic Extension Service
MAUREEN WILKS, Geologic Librarian, Manager of Publication Sales
EMERITUS
GEORGE S. AUSTIN, Emeritus Senior Industrial Minerals Geologist
CHARLES E. CHAPIN, Emeritus Director/State Geologist
JOHN W. HAWLEY, Emeritus Senior Environmental Geologist
JACQUES R. RENAULT, Emeritus Senior Geologist
SAMUEL THOMPSON III, Emeritus Senior Petroleum Geologist
ROBERT H. WEBER, Emeritus Senior Geologist
Plus research associates, graduate students, and undergraduate assistants.
Introduction
Three volcanic rocks from southern New Mexico were submitted for 4 0Ar/3 9Ar
dating by Dr. William Seager. Groundmass concentrate was prepared from the WS-2 and
WS-3 samples. No dateable material was found in sample WS-1.
40
Ar/39Ar Analytical Methods and Results
The groundmass concentrate samples were analyzed by the furnace incremental
heating age spectrum 4 0Ar/3 9Ar method. Abbreviated analytical methods for the furnace
sample is given in Table 1. Details of the overall operation of the New Mexico
Geochronology Research Laboratory are provided in the Appendix. Figures 1 and 2 show
the age spectra and inverse isochrons yielded by the groundmass concentrates.
Groundmass concentrate WS-2 (Figure 1) yields a moderately discordant age
spectrum. The initial six heating steps for WS-2 are concordant at approximately 2 Ma
(step C has been excluded because the furnace failed to reach the desired temperature).
Step H (1250°C) follows with almost 70%% of the total 3 9ArK released and an apparently
anomalously old age of 7.15 Ma. Step I (the fusion step) has only a minor amount of the
total potassium and is older still at 15.70 Ma. Radiogenic yields for WS-2 are very low for
the majority of the age spectrum (<6%), but increase to 9% and 16% for the final two
heating steps. The K/Ca ratios are variable, ranging from less than 0.05 to 1.5. A plateau
assigned to steps D through G yields a weighted mean age of 2.04±0.36 Ma (29.3% of the
39
ArK released). The inverse isochron results for WS-2 are analytically indistinguishable
from the spectrum weighted mean age and atmosphere.
The age spectrum for the WS-3 groundmass concentrate (Figure 2) is highly
discordant with ages ranging from 18.2 Ma to 27.1 Ma. The lower temperature heating
steps for WS-3 yield the youngest ages while the highest temperature steps yield the oldest
ages. The intermediate temperature steps (steps C through F) present a gradient from the
youngest ages to the oldest ages. The radiogenic yields are consistent throughout much of
the age spectrum at about 50-55%. The K/Ca ratios are high (4.1) for the earliest gas
released, but then steadily decrease through the rest of the heating steps to less than 0.1. A
weighted mean for the five highest temperature heating steps yields an age of 26.80±0.68
Ma (52.6% of the 3 9ArK released). The inverse isochron results for WS-3 are analytically
indistinguishable from the spectrum weighted mean age and atmosphere.
-1-
Discussion
For the WS-2 groundmass concentrate sample, the spectrum weighted mean age
(2.04±0.36 Ma) is inferred to be the best estimate of the age of the lava emplacement.
However, the final two, anomalously old, heating steps of the WS-2 age spectrum indicate
that excess argon or contamination may be a problem with this sample. The excess argon
may be contained within a relatively high temperature (>1100°C), low potassium (K/Ca<0.5)
mineral phase, such as pyroxene or olivine. An additional problem with this sample is the
very low radiogenic yields (<6%) that combine with the young age and low potassium
content of the sample to yield a relatively imprecise age. Also, because of the low potassium
content and the low radiogenic yields, the weighted mean age may be more susceptible to
the influence of excess argon. Therefore, the weighted mean age of 2.04±0.36 Ma should
be considered a maximum age.
The age spectrum for the WS-3 groundmass concentrate sample is more difficult to
interpret. While the overall shape of the spectrum is indicative of a ~26 Ma sample that has
undergone 4 0Ar* loss because of alteration/hydration, the radiogenic yields are higher and
more consistent than would be expected. The alternative to the 4 0Ar* loss scenario is an
~18 Ma sample that has been contaminated by excess argon or xenocrysts. However, the
inverse isochron contains no evidence of an excess argon component. Because the shape of
the age spectrum is more similar to other samples known to have alteration and/or hydration
products, it is our estimate that the weighted mean from steps E to 1 (26.80±0.68 Ma)
yields the best estimate of the eruption age of WS-3. However, argon loss may have
influenced some or all of the heating steps included in the weighted mean age so that it
should be considered a minimum age. This hypothesis is strengthen by the apparent field
relationship between the WS-3 dike and the Vicks Peak Tuff. According to the field
description provided by Dr. Seager, the WS-3 dike crosscuts several units below the Vicks
Peak Tuff, but does not intersect the Vicks Peak Tuff itself. The WS-3 dike may also
merge into a basaltic andesite flow immediately below the Vicks Peak Tuff. If this last
statement is true, then the stratigraphically lower WS-3 sample must be older than the Vicks
Peak Tuff (28.56±0.08 Ma; McIntosh et al., 1990). This indicates that the WS-3
groundmass concentrate has lost enough 4 0Ar* through alteration/hydration to decrease its
40
Ar/3 9Ar apparent age by at least 2 million years.
-2-
References Cited
Deino, A., and Potts, R., 1990. Single-Crystal 4 0Ar/3 9Ar dating of the Olorgesailie
Formation, Southern Kenya Rift, J. Geophys. Res., 95, 8453-8470.
Mahon, K.I., 1996. The New “York” regression: Application of an improved statistical
method to geochemistry, International Geology Review, 38, 293-303.
McDougall, I., and Harrison, T.M., 1988, Geochronology and thermochronology by the
40
Ar/3 9Ar method: New York Oxford [Oxfordshire], Oxford University Press ;
Clarendon Press, xi, 212 p.
McIntosh, W.C., Sutter, J.F., and Chapin, C.E., 1990, High-Precision 4 0Ar/3 9Ar sanidine
geochronology of ignimbrites in the Mogollon-Datil volcanic field, southwestern New
Mexico: Bulletin of Volcanology, v. 52, p. 584-601.
Samson, S.D., and, Alexander, E.C., Jr., 1987. Calibration of the interlaboratory 4 0Ar/3 9Ar
dating standard, Mmhb-1, Chem. Geol., 66, 27-34.
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.
-3-
Table 1. 40Ar/39Ar analytical methods used for the groundmass concentrate samples.
Sample preparation and irradiation:
Geological samples provided by Dr. William Seager.
Groundmass concentrates were prepared using standard separation techniques (crushing, sieving, franzing and hand-picking).
Samples were packaged and irradiated in machined Al discs for 7 hours in D-3 position, Nuclear Science Center, College Station, TX.
Neutron flux monitor Fish Canyon Tuff sanidine (FC-1). Assigned age = 27.84 Ma (Deino and Potts, 1990)
relative to Mmhb-1 at 520.4 Ma (Samson and Alexander, 1987).
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 7 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 2.82x10-16 moles/pA.
Total system blank and background for the furnace averaged 4400, 57.0, 1.7, 6.8, 14.1 x 10-18 moles
at masses 40, 39, 38, 37, and 36, respectively for temperatures <1300°C.
J-factors determined to a precision of ± 0.1% by CO2 laser-fusion of 4 single crystals from each of 6 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 = 0.0002±0.0003; (36Ar/37Ar)Ca = 0.00028±0.000005; and (39Ar/37Ar)Ca = 0.0007±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).
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 final errors reported at ±2s, unless otherwise noted.
Table 2. 40Ar/39Ar analytical data.
ID
Power
(Watts)
40
Ar/39Ar
37
Ar/39Ar
36
Ar/39Ar
-3
(x 10 )
39
ArK
-15
(x 10
mol)
K/Ca
40
Ar*
(%)
Ar
Age
(%)
(Ma)
(Ma)
319
4.8
1.5
2.10
1.41
2.27
1.68
7.15
15.70
5.76
210
1.4
4.8
0.28
0.38
0.20
0.39
0.37
0.52
0.82
39
WS-2, 102.76 mg groundmass concentrate, J=0.000702±0.11%, NM-133, Lab#=51961-01
# A
575 39874
6.529 134012.6
0.030
0.078
0.7
0.0
# B
650
236.8
0.3365
788.6
3.17
1.5
1.6
3.8
# C
700
88.61
12.62
299.5
0.026
0.040
1.3
3.9
D
750
42.63
0.4542
138.8
5.87
1.1
3.9
10.9
E
825
55.07
0.6295
182.8
3.14
0.81
2.0
14.7
F
925
30.74
0.7553
98.19
6.04
0.68
5.8
21.9
G
1025
67.92
0.8848
225.6
9.37
0.58
1.9
33.1
# H
1200
62.15
2.388
191.9
54.7
0.21
9.1
98.7
# I
1600
76.09
28.37
224.4
1.05
0.018
16.0 100.0
Integrated age ± 2s
n=9
83.4
K2O=0.44 %
Plateau ± 2s
steps D-G
n=4
MSWD=1.68
24.4
0.76
29.3
40
36
Ar/
Ar=297±2
Isochron±2s
n=6
MSWD=2.3
#
#
#
#
WS-3, 14.09 mg groundmass concentrate, J=0.0007019±0.11%, NM-133, Lab#=51962-01
A
575 1739.3
0.1244
5813.4
0.116
4.1
1.2
1.4
B
650
26.36
0.2445
40.31
2.07
2.1
54.9
26.2
C
700
31.93
0.4006
45.22
0.649
1.3
58.3
34.0
D
750
37.10
0.7190
57.57
1.11
0.71
54.3
47.4
E
825
40.00
0.9025
63.17
1.26
0.57
53.5
62.6
F
925
41.87
1.022
68.18
1.24
0.50
52.1
77.5
G
1025
44.38
1.435
82.20
0.717
0.36
45.5
86.2
H
1200
50.97
7.282
105.2
0.610
0.070
40.2
93.5
I
1600
67.11
7.792
156.8
0.540
0.065
31.9 100.0
Integrated age ± 2s
n=9
8.31
K2O=0.32 %
Plateau ± 2s
steps E-I
n=5
MSWD=6.32
4.37
0.38
52.6
40
Ar/36Ar=296±3
Isochron±2s
n=7
MSWD=11
±1s
2.04
0.36
1.82
0.43
27
18.23
23.42
25.34
26.92
27.43
25.44
25.92
27.12
24.17
10
0.15
0.27
0.22
0.22
0.24
0.37
0.42
0.49
0.53
26.80
0.68
26.32
1.01
Notes:
Isotopic ratios corrected for blank, radioactive decay, and mass discrimination, not corrected for interferring reactions.
Ages calculated ralative to FC-1 Fish Canyon Tuff sanidine interlaboratory standard at 27.84 Ma.
Errors quoted for individual analyses include analytical error only, without interferring reaction or J uncertainties.
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.
Plateau and integrated ages incorporate uncertainties in interferring reaction corrections and J factors.
Decay constants and isotopic abundances after Steiger and Jaeger (1977).
# symbol preceding sample ID denotes analyses excluded from plateau age calculations.
Discrimination = 1.0069 ± 0.001
Correction factors:
(39Ar/37Ar)Ca = 0.00089 ± 3e-05
(36Ar/37Ar)Ca = 0.00028 ± 1.1e-05
(38Ar/39Ar)K = 0.01077
(40Ar/39Ar)K = 0.0002 ± 0.0003
80
40
0
10
1
0.1
20
K/Ca
% Radiogenic
L# 51961: WS-2, 102.76 mg groundmass concentrate
0.01
Apparent Age (Ma)
15
2.04 ± 0.36 Ma
10
(MSWD = 1.68)
H
5
1250
B
0
F
D
800
E
975
G
1075
875
Integrated Age = 5.76 ± 0.82 Ma
-5
0
10
20
30
40
50
60
70
80
90
100
Cumulative 39ArK Released
0.0034 A
BE
F
0.0032 GD
H
0.003
I
0.0028
0.0034
A
G E
B
0.0033
D
0.0032
F
H
0.0031
0.0026
0.003
0.0024
0.0029
0.0022
0.002
36Ar/40Ar
I
0.0028
0.0027
0.0018
0.0026
0.0016
0.0025
0
0.01
0.02
0.03
0.04
0.05
0.06
0.07
0.08
0.0014
0.0012
0.001
0.0008
Isochron age = 1.82 ± 0.43 Ma
40Ar/36Ar Intercept = 297 ± 2
MSWD = 2.3, n = 6
0.0006
0.0004
0.0002
0
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
39Ar/40Ar
Figure 1. 40Ar/39Ar age spectrum and inverse isochron for the WS-2 groundmass concentrate. The weighted
mean of steps D through G (2.04 ± 0.36 Ma) is the preferred age of this sample. All errors are two-sigma.
80
40
0
10
1
0.1
40
K/Ca
% Radiogenic
L# 51962: WS-3, 14.09 mg groundmass concentrate
0.01
35
26.80 ± 0.68 Ma* (MSWD = 6.32)
Apparent Age (Ma)
30
25
C
750
20
F
975
E
875
D
800
G
1075
I
H
1650
1250
B
700
15
10
5
Integrated Age = 24.17 ± 0.53 Ma
0
0
10
20
30
40
50
60
70
80
90
100
Cumulative 39ArK Released
0.0034
A
0.0032
0.003
0.0028
0.0026
0.0024
I
0.0022
H
36Ar/40Ar
0.002
G
0.0018
0.0016
D
F
B
E
0.0014
C
0.0012
0.001
0.0008
Isochron age = 26.32 ± 1.01 Ma
0.0006
40Ar/36Ar
Intercept = 296 ± 3
MSWD = 11, n = 7
0.0004
0.0002
0
0
0.005
0.01
0.015
0.02
0.025
0.03
0.035
0.04
0.045
0.05
0.055
0.06
39Ar/40Ar
Figure 2. 40Ar/39Ar age spectrum and inverse isochron for the WS-3 groundmass concentrate. The weighted
mean of steps E through I (26.80 ± 0.68 Ma) is the preferred age of this sample. All errors are two-sigma.