Open File Report OF-AR-19
New Mexico Bureau of Geology and Mineral Resources
A division of
New Mexico Institute of Mining and Technology
40
Ar/39Ar Geochronology Results
from the Bay of Los Angeles, Baja,
Mexico
Prepared By:
Richard P. Esser and William C. McIntosh
New Mexico Bureau of Geology, Socorro, NM 87801
Prepared For:
Dr. Gary Axen
Dept. of Earth and Space Science, University of California, Los Angeles, CA 90095
Initially prepared as:
NM Geochronology Research
Laboratory Internal Report
NMGRL-IR 293 & 309
March 27, 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
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Principal Senior Petroleum Geologist
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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
Sixteen samples from Baja, Mexico were submitted for 4 0Ar/3 9Ar dating by Drs.
Gary Axen and Mary Kairouz of the University of California, Los Angeles. Four additional
samples from students of Dr. Axen were also submitted. Table 1 shows the entire sample
list from this project, the unit the sample is from and the material that was dated in this
study. The 4 0Ar/3 9Ar dates will be used to constrain the earliest and subsequent normal
faulting events in the Bay of Los Angeles, Baja.
Table 1. Sample List
Sample
21-34-1
21-37-3A
21-37-3B
ddf
21-37-3C
21-37-3D
ddf
21-30-1
21-33-1
21-26-3
21-30-2
21-57-1A
21-48-2
21-26-4B
21-45-2
21-41-2
21-43-1
21-9-1
22-47-1
36-108
22-72-1
1-49-4D
Unit
Material Dated
Tsvb
opB
opB
opB
opB
oB
oB
oB
hBA
hBA
lig
lig
woB
woB
oB
woB
oB
~Tsvb
oB
??
??
??
??
Hornblende
Groundmass Concentrate
None
None
Groundmass Concentrate
Groundmass Concentrate
None
None
None
Hornblende
Biotite
None
Groundmass Concentrate
None
None
Hornblende
None
Groundmass Concentrate
None
Groundmass Concentrate
40
Ar/39Ar Analytical Methods and Results
Samples were prepared for the 4 0Ar/3 9Ar dating by using standard minerals
separation techniques. Hand specimens were crushed and sieved to approximately 48-60
mesh (~275 mm). Biotite and hornblende phenocrysts were hand-picked from crushed
samples. For the groundmass concentrate samples, highly magnetic and non-potassium
bearing phenocryst phases (e.g. olivine) were removed by the Franz magnetic separator and
by hand-picking. Several additional samples were also sent that were not analyzed (see
Table 1). These unanalyzed samples were regarded as inferior alternates to the dated
samples, and therefore were not irradiated.
-1-
The biotite separate was analyzed using the CO2 laser. Abbreviated analytical
methods for the biotite are given in Table 2; the analytical results are in Table 3. The
hornblendes and groundmass concentrates were analyzed using the resistance furnace.
Abbreviated analytical methods for the furnace samples are given in Table 2 while the
analytical results are given in Table 4. Details of the overall operation of the New Mexico
Geochronology Research Laboratory are provided in the Appendix.
The 21-48-2 biotite sample was analyzed using the single-crystal step-heating (two
heating steps) method. The first (low power) of the two heating step was used to reduce
atmospheric 4 0Ar adhering onto the surface of the mica. The remaining heating step (high
power) then fused the grain for maximum signal size. Figure 1 shows the age distribution
of the laser step-heated single-crystal biotites. The ages range from –14 Ma to 31.2 Ma
with a gaussian population at 14.37±0.12 Ma (B steps only). The radiogenic yields range
from essentially zero to 89.8%. The K/Ca values are highly variable, ranging from 0.42 to
80.1. In virtually every instance, the fusion steps (step B) yield higher K/Ca values, higher
radiogenic yields and more precise apparent ages.
The three hornblende samples each yielded flat to slightly discordant age spectra.
The age spectrum for sample 21-9-1 (Figure 2) is flat over 90% of the 3 9ArK released,
yielding a plateau from steps F to K with an age of 24.54±0.31 Ma (93.4% of the
cumulative 3 9ArK released; MSWD=2.37). The radiogenic yields are initially low (steps A
through E) but then increase to greater than 60% for the remainder of the age spectrum.
The K/Ca ratios for the age spectrum range from 0.047 to 0.17, but are consistent at about
0.05 for the bulk of the 3 9ArK released. Hornblende sample 21-34-1 yields a similarly
shaped age spectrum (Figure 3). The K/Ca ratios do not vary significantly from those
values observed for the 21-9-1 hornblende samples, but the radiogenic yields are
approximately 10-15% higher for 21-34-1. The 21-34-1 age spectrum yields a weighted
mean age from steps F to K of 24.19±0.14 Ma (88.4% of the cumulative 3 9ArK released;
MSWD=6.92). Hornblende 21-57-1A (Figure 4) yields an age spectrum similar to both
21-9-1 and 21-34-1. The 21-57-1A hornblende has the lowest radiogenic yields, only
reaching 53%, but it also has the highest K/Ca values (0.01 to 0.26). The weighted mean of
steps F through K yields an age of 19.99±0.52 Ma (92.1% of the cumulative 3 9ArK
released; MSWD=4.83).
The groundmass concentrate from sample 36-108 yielded a flat age spectrum
(Figure 5) for greater than 95% of the 3 9ArK released. The radiogenic yields range from
zero at the lowest temperature steps to 40.7% for the 1200°C step. The K/Ca ratios are
highly variable, ranging from 0.044 to 8.7, with the highest values at the lowest temperature
-2-
steps. The weighted mean of the flattest steps on the age spectrum yield an apparent age of
19.43±0.47 Ma (98.4% of the 3 9ArK released; MSWD=1.71).
The groundmass concentrate from sample 1-49-4D yielded a flat age spectrum
(Figure 6) for greater than 90% of the 3 9ArK released. The only discordance in the age
spectrum is the anomalously old age for the fusion step. The radiogenic yields range from
zero at the lowest temperature steps to 69.9%. Unlike the 36-108 sample, the K/Ca ratios
for 1-49-4D are consistent, ranging only from 0.017 to 0.52. The weighted mean age of
steps D through H is 20.85±0.22 Ma (90.8% of the 3 9ArK released; MSWD=2.14).
Groundmass concentrate 21-30-1 was analyzed twice. Both analyses of the 21-30-1
sample yield nearly identical age spectra (Figures7 and 8). The first analysis (Figure 7) has
radiogenic yields that range from 0.7 to 52.9% and K/Ca ratios that range from 0.029 to
0.83. The weighted mean age of steps B to H is 11.88±0.19 Ma (93.9% of the cumulative
39
ArK released; MSWD=0.96). The second analysis (Figure 8) has radiogenic yields that
range from 3.4 to 49.9% and K/Ca ratios that range from 0.030 to 0.68. The weighted
mean age of steps B to H is 11.59±0.20 Ma (93.8% of the cumulative 3 9ArK released;
MSWD=1.09). The weighted mean ages for the two analyses of groundmass concentrate
21-30-1 overlap at 2s uncertainty.
The 21-37-3A groundmass concentrate yields a flat age spectrum (Figure 9) for
greater than 90% of the 3 9ArK released. The radiogenic yields for 21-37-3A only reach
26%. The K/Ca values range from 0.13 to 0.73. The initial heating step is younger than the
subsequent heating steps, while the final two heating steps are slightly older. The weighted
mean age of steps B through H is 12.86±0.34 Ma (93.2% of the cumulative 3 9ArK released;
MSWD=0.45).
The 21-37-3D age spectrum (Figure 10) is flat for ~90% of the 3 9ArK released, but
the final two heating steps also show an increase in apparent age. The radiogenic yields for
21-37-3D range from 4.1 to 69.8%, but are highest for the intermediate temperature steps.
The K/Ca ratios range from 0.11 at the highest temperature steps to 2.1 for the intermediate
temperature steps. 87.1% of the age spectrum (steps B through G) yields a weighted mean
age of 13.30±0.14 Ma (MSWD=3.62).
The final groundmass concentrate (21-45-2; Figure 11) yields the most discordant
age spectrum. Approximately 60% of the age spectrum is very flat with ages between 17.0
and 18.72 Ma. However, the final two heating steps yield ages of 24.8 and 20.7 Ma. The
Radiogenic yields are among the lowest for this sample suite, ranging from 3.5% to 20.8%.
K/Ca ratios range from 0.086 to 0.79. The weighted mean of the flattest portion of the age
-3-
spectrum (steps B through G) yields an age of 18.10±0.60 Ma (59.8% of the cumulative
39
ArK released; MSWD=0.73).
For all of the samples analyzed using the age spectrum method, the inverse isochron
plots (shown below each samples age spectrum plot and also in the analytical data table) do
not show indications of significant excess 4 0Ar. In each, the inverse isochron age agrees
with the respective age spectrum weighted mean age. Also, the 4 0Ar/3 6Ar ratio derived from
the isochron agrees statistically with the present-day atmospheric values (295.5).
Discussion
The 21-48-2 biotite yields a gaussian population (14.37±0.12 Ma) that is
interpreted to be the age of the ignimbrite deposit. The initial heating steps on each biotite
crystal predominantly degassed atmospheric argon from the surface of the mica. This is
supported by the low radiogenic yields, the lack of significant 3 9ArK and the large
uncertainties on the ages. Conversely, the fusion steps yield significantly more radiogenic
40
Ar, moles 3 9ArK, and more precise ages. Despite this sample being “lithic rich”, there
does not appear to be xenocrysts within the analyzed 21-48-2 biotite population.
For each of the three hornblende samples (21-9-1, 21-34-1 and 21-57-1A), the
spectrum weighed mean age is interpreted to be the most accurate age for the tuff or lava
emplacement. Although all three of the hornblende age spectra exhibit minor age anomalies
in the first ~10% of 3 9ArK released, the higher temperature steps (>1000°C) are relatively
isochronous. The lowest temperature argon released from each sample is a combination of
radiogenic 4 0Ar and abundant atmospheric 4 0Ar probably released from minor alteration
products (clay), rather than the hornblende lattice. Small quantities of alteration products,
such as those present on these hornblende samples are common and are usually completely
degassed at the lowest temperature steps and therefore do not significantly affect the rest of
the age spectrum. Although anomalously old ages are commonly observed in the highest
temperature heating steps of many hornblende age spectra, and are attributed to excess
argon-containing mineral inclusions (e.g. zircon), there does not appear to be significant
excess argon present in the three hornblendes analyzed in this study.
With the exception of sample 21-45-2, all of the other groundmass concentrate
samples yield straightforward age spectra where weighted mean ages of the flattest portions
are inferred to record the lava eruption age. In all cases, the initial heating steps are
predominantly low radiogenic argon degassed from the surface of the groundmass
concentrate and/or minor alteration products (e.g. clay) and are thus excluded from the
weighted mean age. Although the later, higher temperature heating steps for many of the
-4-
age spectra are lower in radiogenic 4 0Ar than would be expected from mid-Miocene or older
basalts, argon loss from alteration does not appear to be a significant factor in the weighted
mean ages. The anomalously old apparent ages yielded by the highest temperature steps of
the 21-45-2 groundmass concentrate most likely result from small amounts of excess argon
(4 0ArE). Any 4 0ArE present within a sample is indistinguishable from radiogenic argon
(4 0Ar*), thereby making the apparent 4 0Ar/3 9Ar ratio higher and apparent age older.
Although an inverse isochron is in many cases capable of correcting for 4 0ArE, the inverse
isochron results for 21-45-2 are not indicative of significant quantities of excess argon as its
age is not statistically distinguishable from the age spectrum results. Therefore, the age
spectrum weighted mean age for 21-45-2 is also considered the most accurate assessment
of the age of this sample. In the case of the two 21-30-1 aliquots, because both weighted
mean results were analytically indistinguishable, a weighted mean of the two ages is
assumed to yield the most accurate age for this sample. 11.74±0.29 Ma is the preferred age
for 21-30-1.
Table 5 shows a summary of the 4 0Ar/3 9Ar apparent ages yielded by this study;
Figure 6 shows a graphical summary. The cover letter provided with the samples by Dr.
Gary Axen indicates that several of the samples are constrained by stratigraphic
relationships. One stated relationship indicates that sample 21-45-2 (unit woB) is from the
top of unit oB (sample 21-30-1), which would make 21-45-2 younger than 21-30-1. The
40
Ar/3 9Ar results contradict this field relationship: the weighted mean age for 21-45-2 is
18.10±0.60 Ma and for 21-30-1 is 11.74±0.29 Ma. Although groundmass concentrate 2145-2 is the only sample from this suite that contains any noteworthy excess 4 0Ar and also
has the largest analytical error, we do not feel that it contains enough excess 4 0Ar to increase
the apparent age by nearly 7 million years. We are unable to explain the apparent
discrepancy between the 21-45-2 weighted mean age and the 21-30-1 weighted mean age.
Another stratigraphic relationship stated in the cover letter is that sample 21-37-3D is the
“highest flow”. It is unclear if this means that 21-37-3D is the highest and thus youngest
flow in the sample suite or just within the opB unit. Because the weighted mean 4 0Ar/3 9Ar
age for 21-37-3D is analytically indistinguishable from the other opB sample, 21-37-3A, we
cannot confirm or deny that 21-37-3D is the highest flow within opB. However, sample 2137-3D is older than 21-30-1 by approximately 1.5 million years. As was the case for the
21-45-2 sample, we cannot attribute this age discrepancy to excess argon or analytical
problems. Additional stratigraphic relationships are not apparent in the cover letter
provided.
-5-
Table 5. Summary of 4 0Ar/3 9Ar apparent ages.
Sample
Age±2s Error (Ma)
21-48-2 biotite
21-9-1 hornblende
21-34-1 hornblende
ddf
21-57-1A hornblende
36-018 groundmass concentrate
1-49-4d groundmass concentrate
21-30-1 #1 groundmass concentrate
21-30-1 #2 groundmass concentrate
Weighted Mean
21-37-3A groundmass concentrate
21-37-3D groundmass concentrate
21-45-2 groundmass concentrate
-6-
14.37±0.12
24.54±0.31
24.14±0.35
19.99±0.52
19.43±0.47
20.85±0.22
11.88±0.19
11.59±0.20
11.74±0.29
12.86±0.34
13.30±0.14
18.10±0.60
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.
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.
-7-
Table 2. 40Ar/39Ar analytical methods used for the biotite, groundmass concentrate and feldspar samples heated in the resistance
furnace and the CO2 laser.
Sample preparation and irradiation:
Geological samples provided by Drs. Gary Axen and Mary Kairouz.
Sanidine, hornblende and groundmass concentrates were prepared using standard mineral separation
techniques (crushing, sieving, franzing, density liquids 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.
Sanidine crystals were fused by a 50 watt Synrad CO2 laser.
Furnace samples step-heated in Mo double-vacuum resistance furnace. Heating duration 7 to 10 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 for the furnace samples averaged 2.36x10-16 moles/pA.
Total system blank and background for the furnace averaged 1230, 7.1, 0.7, 7.8, 4.0 x 10-18 moles
at masses 40, 39, 38, 37, and 36, respectively for temperatures <1300°C.
Electron multiplier sensitivity for the laser samples averaged 1.3x10-16 moles/pA.
Total system blank and background for the furnace averaged 726, 2.2, 0.9, 3.7, 3.3 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 4 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 3. 40Ar/39Ar analytical data.
ID
40
Ar/39Ar
37
Ar/39Ar
36
Ar/39Ar
-3
(x 10 )
#
#
#
#
#
#
#
#
#
#
39
ArK
-15
(x 10
K/Ca
mol)
21-48-2 bi, E6:154, single crystal biotite, J=0.0007048±0.10%, D=1.00717±0.00172, NM-154,
07A
235.9
1.223
836.2
0.038
0.42
05A
407.7
0.0242
1368.5
0.524
21.1
03A
65.84
0.0795
204.9
0.052
6.4
10A 1864.5
-0.0056
6282.0
0.301
03B
46.52
0.0122
121.2
1.101
41.7
05B
15.01
0.0326
12.68
2.821
15.6
02B
24.23
0.0102
43.81
1.595
50.2
01B
22.58
0.0269
38.20
3.161
18.9
07B
12.58
0.0324
4.365
1.550
15.7
08B
18.27
0.0657
22.99
0.643
7.8
04B
14.70
0.0064
10.88
0.751
80.1
10B
20.95
0.0191
31.94
1.745
26.7
06B
20.79
0.0158
31.34
1.834
32.3
01A 1339.2
0.0329
4493.1
0.621
15.5
09B
19.25
0.0120
25.71
1.105
42.6
02A
775.5
0.0086
2578.4
0.234
59.1
04A
80.89
-0.1899
209.2
0.011
08A
572.1
-0.8237
1868.2
0.110
09A 1801.8
0.0145
6018.1
0.138
35.1
06A 2061.0
0.0189
6891.0
0.265
27.0
Mean age ± 2s
n=10
MSWD=0.83
33.2 ±43.0
40
Age
(%)
(Ma)
(Ma)
-14.2
4.2
6.7
10.3
13.56
14.27
14.29
14.30
14.31
14.54
14.55
14.58
14.60
14.6
14.75
17.2
24.0
25.3
29.6
31.2
14.37
6.0
4.8
3.4
18.4
0.64
0.13
0.27
0.21
0.10
0.37
0.26
0.25
0.20
12.7
0.27
7.6
10.8
6.6
18.4
19.4
0.12
Ar*
±1s
Lab#=53323
-4.7
0.8
8.1
0.4
23.0
75.1
46.6
50.0
89.8
62.8
78.1
54.9
55.5
0.9
60.5
1.8
23.5
3.5
1.3
1.2
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.
Mean age is weighted mean age of Taylor (1982). Mean age error is weighted error
of the mean (Taylor, 1982), multiplied by the root of the MSWD where MSWD>1, and also
incorporates uncertainty in J factors and irradiation correction uncertainties.
Decay constants and isotopic abundances after Steiger and Jaeger (1977).
# symbol preceding sample ID denotes analyses excluded from mean age calculations.
Discrimination = 1.00717 ± 0.00172
Correction factors:
(39Ar/37Ar)Ca = 0.0007 ± 2e-05
(36Ar/37Ar)Ca = 0.00028 ± 5e-06
(38Ar/39Ar)K = 0.01077
(40Ar/39Ar)K = 0.0002 ± 0.0003
Table 4. 40Ar/39Ar analytical data for samples analyzed in the resistance furnace.
ID
Temp.
40
Ar/39Ar
37
Ar/39Ar
36
Ar/39Ar
-3
(°C)
(x 10 )
ArK
-15
(x 10
n=11
21-34-1, 27.16 mg hornblende,
# A
700
386.0
# B
750
173.8
# C
850
189.7
# D
950
197.9
#† E
990
36.57
F
1020
24.41
G
1040
23.47
H
1070
21.25
I
1110
21.64
J
1150
20.98
K
1250
21.40
# L
1675
288.5
Integrated age ± 2s
Plateau ± 2s
Isochron±2s
40
Ar*
(%)
mol)
21-9-1, 25.01 mg hornblende, J=0.0007039±0.10%, D=1.00717±0.00172, NM-154,
# A
700 1354.0
7.064
4462.1
0.387
# B
750
209.1
4.766
635.2
0.145
# C
850
288.4
5.962
892.5
0.150
# D
950
236.4
8.436
752.7
0.134
#† E
990
62.89
9.668
162.4
0.221
F
1020
28.73
10.17
37.42
0.76
G
1040
32.51
10.68
47.37
1.10
H
1070
26.03
10.53
25.55
9.8
I
1110
24.24
10.81
21.49
0.90
J
1150
25.21
10.51
22.58
1.28
K
1250
25.16
10.95
21.83
1.76
# L
1675
175.4
29.39
540.5
0.072
Integrated age ± 2s
n=12
16.7
Plateau ± 2s
steps F-K
n=6
MSWD=2.37
15.6
Isochron±2s
K/Ca
39
Ar
Age
(%)
(Ma)
(Ma)
45
27.6
31.9
18.6
20.0
23.53
24.64
24.59
23.87
24.66
24.93
23.3
25.01
13
3.4
4.5
3.8
1.8
0.46
0.36
0.14
0.33
0.27
0.25
5.8
0.91
24.54
0.31
24.47
0.12
13.5
22.1
27.0
23.2
21.70
24.36
23.62
24.16
24.72
23.82
24.78
42
23.61
4.0
2.6
3.2
2.9
0.56
0.24
0.14
0.10
0.43
0.26
0.16
10
0.60
39
Lab#=53322-01
0.072
2.7
2.3
0.11
10.4
3.2
0.086
8.7
4.1
0.060
6.2
4.9
0.053
25.0
6.2
0.050
64.4
10.7
0.048
59.7
17.4
0.048
74.3
75.9
0.047
77.5
81.3
0.049
77.0
89.0
0.047
78.0
99.6
0.017
10.3 100.0
K2O=0.36 %
0.048
Ar/36Ar=298±3
93.4
40
MSWD=1.9
±1s
J=0.0007033±0.10%, D=1.00717±0.00172, NM-154, Lab#=53321-01
steps F-K
7.413
2.204
3.734
5.174
8.047
9.209
8.988
8.928
9.559
10.11
9.652
66.65
n=12
1272.2
529.8
570.6
609.3
68.11
20.18
18.93
9.937
9.987
10.34
9.005
886.4
n=6
MSWD=6.92
n=11
MSWD=4.6
0.95
0.314
0.249
0.358
0.71
1.82
4.81
9.5
0.637
1.11
2.00
0.035
22.5
19.9
0.069
2.8
4.2
0.23
10.1
5.6
0.14
11.3
6.7
0.099
9.2
8.3
0.063
46.8
11.4
0.055
78.7
19.5
0.057
79.3
40.9
0.057
89.7
83.2
0.053
90.0
86.0
0.050
89.4
91.0
0.053
91.3
99.8
0.008
11.1 100.0
K2O=0.45 %
0.056
Ar/36Ar=292±3
40
88.4
24.14
0.35
24.19
0.14
ID
Temp.
40
Ar/39Ar
37
Ar/39Ar
(x 10 )
21-57-1A, 28.86 mg hornblende,
# A
700
570.1
# B
750
125.2
# C
850
200.3
# D
950
146.0
#† E
990
75.63
F
1020
32.93
G
1040
36.26
H
1070
55.62
I
1110
29.68
J
1150
33.28
K
1250
32.39
# L
1675
89.26
Integrated age ± 2s
Isochron±2s
Ar/39Ar
-3
(°C)
Plateau ± 2s
36
steps F-K
K/Ca
39
ArK
-15
(x 10
40
Ar*
(%)
mol)
Ar
Age
(%)
(Ma)
(Ma)
20.0
21.5
20.8
17.0
16.4
20.88
20.73
21.59
19.80
19.48
19.71
22.4
20.14
5.8
1.9
2.6
2.1
1.4
0.44
0.46
0.46
0.18
0.31
0.25
3.3
0.74
19.99
0.52
19.81
0.40
-2159
-13.6
-4
15.8
16.8
18.0
18.65
19.67
19.50
15.0
1889
9.5
37
3.1
1.7
1.1
0.54
0.23
0.35
3.9
39
±1s
J=0.0007033±0.10%, D=1.00717±0.00172, NM-154, Lab#=53320-01
2.724
1.971
2.094
2.545
3.528
4.765
5.463
5.304
4.963
5.244
7.271
38.91
n=12
1876.5
366.7
622.5
449.2
213.1
56.93
68.84
132.0
48.92
62.05
59.05
254.6
n=6
MSWD=4.83
n=11
MSWD=2.8
0.87
0.503
0.78
0.71
0.465
1.24
1.50
8.5
20.6
2.93
5.73
0.134
43.9
40.4
0.19
2.8
2.0
0.26
13.6
3.1
0.24
8.2
4.9
0.20
9.2
6.5
0.14
17.1
7.6
0.11
50.1
10.4
0.093
45.1
13.8
0.096
30.7
33.1
0.10
52.7
80.0
0.097
46.2
86.6
0.070
48.0
99.7
0.013
19.3 100.0
K2O=0.83 %
0.10
Ar/36Ar=298±3
92.1
40
36-108, 30.80 mg groundmass concentrate J=0.0016348±0.10%, D=1.00552±0.00107, NM-156, Lab#=53491-01
# A
575 #######
-17.9211 100783.7
0.339
-1.5
0.3
# B
650
536.5
-3.0328
1830.1
1.64
-0.9
1.6
# C
700
492.4
#######
1635.7
0.036
-0.3
1.6
D
750
174.2
0.0585
571.4
3.77
8.7
3.1
4.7
E
825
113.1
0.8521
363.7
9.5
0.60
5.1
12.3
F
925
73.21
1.220
227.4
17.7
0.42
8.4
26.7
G
1025
31.19
1.045
84.34
16.7
0.49
20.4
40.1
H
1200
16.47
1.404
33.45
39.5
0.36
40.7
72.1
I
1600
21.02
11.53
52.04
34.6
0.044
31.4 100.0
Integrated age ± 2s
n=9
123.7
K2O=0.94 %
Plateau ± 2s
steps D-I
n=6
MSWD=1.71
121.7
0.57
98.4
40
Ar/36Ar=292±2
Isochron±2s
n=9
MSWD=0.5
1-49-4d, 29.71 mg groundmass concentrate, J=0.001634±0.10%, D=1.00552±0.00107, NM-156, Lab#=53490-01
# A
575 7220.1
1.730
24730.3
0.378
0.29
-1.2
0.3
# B
650
183.0
2.002
603.6
1.93
0.25
2.6
1.6
#† C
700
38.90
1.734
114.7
0.405
0.29
13.3
1.8
D
750
28.94
1.644
74.43
8.1
0.31
24.5
7.4
E
825
18.69
1.478
40.26
19.3
0.35
37.0
20.5
F
925
17.19
1.486
34.11
27.3
0.34
42.1
39.0
G
1025
14.42
1.208
24.89
19.9
0.42
49.7
52.5
H
1200
10.16
0.9889
10.63
59.3
0.52
69.9
92.6
#† I
1600
14.24
29.82
29.16
10.9
0.017
56.8 100.0
Integrated age ± 2s
n=9
147.5
K2O=1.17 %
Plateau ± 2s
steps D-H
n=5
MSWD=2.14
133.9
0.43
90.8
40
36
Ar/
Ar=293±2
Isochron±2s
n=7
MSWD=2.2
19.43
0.47
20.05
0.24
-279
14.1
15.2
20.78
20.30
21.23
21.03
20.813
24.19
20.29
130
3.4
2.9
0.47
0.26
0.21
0.21
0.093
0.30
0.83
20.85
0.22
21.00
0.14
ID
Temp.
(°C)
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)
3.5
12.15
11.21
12.65
11.99
11.94
11.86
11.81
12.1
11.51
3.6
0.47
0.44
0.46
0.50
0.24
0.14
0.19
1.2
0.74
39
21-30-1, 22.51 mg groundmass concentrate, J=0.0007053±0.10%, D=1.00717±0.00172, NM-154, Lab#=53332-01
# A
600
394.8
2.038
1327.2
1.94
0.25
0.7
4.8
B
700
51.92
1.597
143.8
4.79
0.32
18.4
16.6
C
750
36.90
1.059
95.29
2.98
0.48
23.9
24.0
D
800
44.88
1.143
118.4
2.99
0.45
22.2
31.3
E
875
41.40
1.195
108.5
3.11
0.43
22.8
39.0
F
975
20.98
0.6133
39.33
4.35
0.83
44.8
49.7
G
1075
17.67
0.8143
28.40
7.4
0.63
52.9
68.1
H
1250
23.10
10.97
49.96
12.4
0.047
40.0
98.7
# I
1680
75.00
17.68
227.0
0.539
0.029
12.5 100.0
Integrated age ± 2s
n=9
40.5
K2O=0.98 %
Plateau ± 2s
steps B-H
n=7
MSWD=0.96
38.0
0.38
93.9
40
Ar/36Ar=294±3
Isochron±2s
n=9
MSWD=1.4
21-30-1 #2, 23.28 mg groundmass concentrate, J=0.0007081±0.10%, D=1.00717±0.00172, NM-154, Lab#=53336-01
# A
600
415.3
2.040
1358.5
1.98
0.25
3.4
4.8
B
700
53.80
1.727
154.0
4.58
0.30
15.7
15.9
C
750
36.07
1.097
91.87
2.94
0.46
25.0
23.0
D
800
42.81
1.174
112.7
3.05
0.43
22.4
30.4
E
875
44.24
1.405
118.1
3.40
0.36
21.4
38.7
F
975
21.94
0.7537
43.94
5.00
0.68
41.1
50.8
G
1075
18.12
0.9105
30.98
6.8
0.56
49.9
67.2
H
1250
23.47
11.17
51.69
12.9
0.046
38.9
98.5
# I
1680
69.46
16.94
207.3
0.602
0.030
13.8 100.0
Integrated age ± 2s
n=9
41.3
K2O=0.96 %
Plateau ± 2s
steps B-H
n=7
MSWD=1.09
38.7
0.34
93.8
40
36
Ar/
Ar=298±3
Isochron±2s
n=9
MSWD=1.0
21-37-3A, 23.50 mg groundmass concentrate, J=0.0007071±0.10%, D=1.00717±0.00172, NM-154, Lab#=53335-01
# A
600
200.5
0.7258
656.7
2.87
0.70
3.3
4.3
B
700
55.98
0.9737
156.2
8.1
0.52
17.7
16.6
C
750
44.44
0.7612
116.1
5.39
0.67
22.9
24.7
D
800
48.04
0.6946
128.0
6.7
0.73
21.4
34.9
E
875
51.95
0.8282
141.0
8.5
0.62
19.9
47.7
F
975
37.65
1.448
94.31
9.0
0.35
26.3
61.3
G
1075
51.22
2.469
140.4
4.08
0.21
19.4
67.5
H
1250
82.26
2.738
243.2
19.8
0.19
12.9
97.5
# I
1680
83.05
3.934
243.8
1.63
0.13
13.6 100.0
Integrated age ± 2s
n=9
66.1
K2O=1.53 %
Plateau ± 2s
steps B-H
n=7
MSWD=0.45
61.6
0.42
93.2
40
Ar/36Ar=294±4
Isochron±2s
n=9
MSWD=1.4
±1s
11.88
0.19
11.98
0.20
17.8
10.74
11.48
12.22
12.05
11.48
11.52
11.70
12.4
11.89
4.0
0.54
0.44
0.41
0.51
0.21
0.16
0.19
1.1
0.77
11.59
0.20
11.42
0.16
8.3
12.59
12.96
13.06
13.17
12.60
12.65
13.51
14.4
12.9
2.0
0.55
0.40
0.49
0.45
0.31
0.48
0.69
1.0
1.0
12.86
0.34
13.1
0.8
ID
Temp.
(°C)
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)
9.8
13.51
13.441
13.387
13.230
12.94
13.23
14.41
15.91
13.31
1.8
0.18
0.093
0.088
0.075
0.13
0.24
0.30
0.61
0.30
13.30
0.14
13.26
0.08
13.5
18.72
18.39
17.24
18.40
17.77
17.0
24.8
20.7
20.3
3.1
0.59
0.66
0.74
0.80
0.72
1.3
1.9
1.8
2.4
18.10
0.60
17.6
1.6
39
21-37-3D, 21.68 mg groundmass concentrate, J=0.0007043±0.10%, D=1.00717±0.00172, NM-154, Lab#=53333-01
# A
600
191.0
0.3856
620.1
2.19
1.3
4.1
1.5
B
700
23.09
0.4555
42.14
13.2
1.1
46.2
10.4
C
750
15.65
0.3264
17.14
12.9
1.6
67.8
19.1
D
800
14.65
0.2464
13.87
17.8
2.1
72.2
31.1
E
875
14.96
0.2525
15.33
27.4
2.0
69.8
49.6
F
975
21.26
0.2583
37.43
44.4
2.0
48.1
79.5
G
1075
29.66
0.4571
65.16
13.5
1.1
35.2
88.5
# H
1250
40.50
3.724
99.67
15.5
0.14
28.0
99.0
# I
1680
55.71
4.859
147.5
1.56
0.11
22.5 100.0
Integrated age ± 2s
n=9
148.4
K2O=3.73 %
Plateau ± 2s
steps B-G
n=6
MSWD=2.62
129.2
1.8
87.1
40
36
Ar/
Ar=298±3
Isochron±2s
n=9
MSWD=6.1
21-45-2, 22.03 mg groundmass concentrate, J=0.0007052±0.10%, D=1.00717±0.00172, NM-154, Lab#=53334-01
# A
600
306.4
0.6423
1000.8
1.84
0.79
3.5
2.5
B
700
71.20
0.7929
191.1
8.4
0.64
20.8
13.9
C
750
72.49
0.8541
196.4
5.26
0.60
20.0
21.1
D
800
81.35
0.9287
229.5
5.63
0.55
16.7
28.7
E
875
91.51
1.154
260.9
6.19
0.44
15.9
37.1
F
975
86.61
1.114
245.9
10.6
0.46
16.2
51.6
G
1075
155.7
1.340
481.9
7.9
0.38
8.6
62.2
# H
1250
235.3
2.470
730.7
26.0
0.21
8.3
97.5
# I
1680
187.2
5.966
580.0
1.82
0.086
8.7 100.0
Integrated age ± 2s
n=9
73.6
K2O=1.82 %
Plateau ± 2s
steps B-G
n=6
MSWD=0.73
44.0
0.51
59.8
40
36
Ar/
Ar=298±3
Isochron±2s
n=9
MSWD=2.6
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.
† symbol preceding sample ID denotes analyses excluded from plateau age calculations.
Discrimination = 1.00552 ± 0.00107
Correction factors:
(39Ar/37Ar)Ca = 0.0007 ± 2e-05
(36Ar/37Ar)Ca = 0.00028 ± 5e-06
(38Ar/39Ar)K = 0.01077
(40Ar/39Ar)K = 0.0002 ± 0.0003
±1s
3.0
100
50
0
100
10
1
Relative Probability
Initial heating steps
14.37 ± 0.12 Ma
(MSWD = 0.83)
Fusion steps
0
5
10
15
20
25
30
35
Apparent Age (Ma)
Figure 1. 40Ar/39Ar age probability distribution diagram (ideogram) the 21-48-2 biotite sample. The preferred age
of this sample is the weighted mean of the fusion steps (14.37 ± 0.12 Ma). All errors are two-sigma..
K/Ca
% Radiogenic
1.0
Moles 39ArK
L#53323: 21-48-2, single-crystal biotite
80
40
0
1
0.1
45
K/Ca
% Radiogenic
L# 53322: 21-9-1, 25.01 mg hornblende
0.01
40
Apparent Age (Ma)
35
30
24.54 ± 0.31 Ma* (MSWD = 2.37)
25
H
1120
G
1090
F
20
K
1300
J
I
1200
1160
15
10
D
5
Integrated Age = 25.01 ± 0.91 Ma
0
0
10
20
30
40
50
60
70
80
90
0.045
0.05
100
Cumulative 39ArK Released
0.004
0.0035
A
D
L
0.003
C
B
36Ar/40Ar
E
0.0025
0.002
0.0015
G
F
0.001
H
Isochron age = 24.47 ± 0.12 Ma
40Ar/36Ar Intercept = 298 ± 3
MSWD = 1.9, n = 11
0.0005
J I
K
0
0
0.005
0.01
0.015
0.02
0.025
0.03
0.035
0.04
0.055
39Ar/40Ar
Figure 2. 40Ar/39Ar age spectrum and inverse isochron for the 21-9-1 hornblende sample. The preferred age of this
sample is the weighted mean of the spectrum steps F through K (24.54 ± 0.31 Ma). All errors are at two-sigma.
80
40
0
1
0.1
40
K/Ca
% Radiogenic
L# 53321: 21-34-1, 27.16 mg hornblende
0.01
35
Apparent Age (Ma)
30
24.14 ± 0.35 Ma* (MSWD = 6.92)
25
20
E
F
1070
H
1120
G
1090
I
K
J
1300
1200
D
15
10
5
A
Integrated Age = 23.61 ± 0.60 Ma
0
0
10
20
30
40
50
Cumulative
60
39Ar
K
70
80
90
100
Released
0.004
0.0035
A
DB
0.003
L C
36Ar/40Ar
0.0025
0.002
E
0.0015
0.001
G
Isochron age = 24.19 ± 0.14 Ma
40Ar/36Ar Intercept = 292 ± 3
MSWD = 4.6, n = 11
0.0005
0
0
0.005
0.01
0.015
0.02
F
H
I K
0.025
0.03
0.035
0.04
0.045
J
0.05
0.055
39Ar/40Ar
40Ar/39Ar
Figure 3.
age spectrum and inverse isochron for the 21-34-1 hornblende sample. The preferred age of this
sample is the weighted mean of the spectrum steps F through K (24.14 ± 0.35 Ma). All errors are at two-sigma.
80
40
0
1
0.1
K/Ca
% Radiogenic
L# 53320:, 21-57-1A, 28.86 mg hornblende
0.01
40
35
Apparent Age (Ma)
30
25
19.99 ± 0.52 Ma* (MSWD = 4.83.)
20
F G
1090
15
H
1120
I
1160
J
1200
K
1300
L
10
5
Integrated Age = 20.14 ± 0.74 Ma
0
0
10
20
30
40
50
Cumulative
0.0034
39Ar
K
60
70
80
90
100
Released
A
0.0032
C
0.003
D
B
E
0.0028
L
0.0026
0.0024
H
36Ar/40Ar
0.0022
0.002
J
0.0018
G
0.0016
K
F
I
0.0014
0.0012
0.001
0.0008
Isochron age = 19.81 ± 0.40 Ma
40Ar/36Ar Intercept = 298 ± 3
MSWD = 2.8, n = 11
0.0006
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 0.065 0.07
39Ar/40Ar
Figure 4. 40Ar/39Ar age spectrum and inverse isochron for the 21-57-1A hornblende sample. The preferred age of
this sample is the weighted mean of the spectrum steps F through K (19.99 ± 0.52 Ma). All errors are at twosigma.
80
40
0
1
0.1
40
K/Ca
% Radiogenic
L# 53491: 36-108, 30.80 mg groundmass concentrate
0.01
19.43 ± 0.47 Ma (MSWD = 1.71)
35
Apparent Age (Ma)
30
25
20
15
E
875
10
D
H
1250
G
1075
F
975
I
1650
5
0
-5
Integrated Age = 15.0 ± 3.9 Ma
-10
0
10
20
30
40
50
Cumulative
39Ar
K
60
70
80
90
100
Released
0.004
0.0035 A B
D
C
E
F
0.003
G
36Ar/40Ar
0.0025
I
H
0.002
0.0015
0.001
Isochron age = 20.05 ± 0.24 Ma
40Ar/36Ar Intercept = 292 ± 2
MSWD = 0.5, n = 9
0.0005
0
0
0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09
0.1
0.11 0.12 0.13 0.14 0.15
39Ar/40Ar
Figure 5. 40Ar/39Ar age spectrum and inverse isochron for the 36-108 groundmass concentrate sample. The preferred age of this sample is the weighted mean of the spectrum steps D through I (19.43 ± 0.47 Ma). All errors are
at two-sigma.
80
40
0
1
0.1
40
K/Ca
% Radiogenic
L# 53490: 1-49-4d, 29.71 mg groundmass concentrate
0.01
35
Apparent Age (Ma)
30
25
20.85 ± 0.22 Ma* (MSWD = 2.14)
20
F
975
D
800
15
E
875
G
1075
I
1650
H
1250
10
5
0
-5
Integrated Age = 20.29 ± 0.83 Ma
-10
0
10
20
30
40
50
60
70
80
90
100
Cumulative 39ArK Released
0.004
0.0035 A
B
C
0.003
D
36Ar/40Ar
0.0025
E
0.002
F
G
0.0015
I
H
0.001
Isochron age = 21.00 ± 0.14 Ma
40Ar/36Ar Intercept = 293 ± 2
MSWD = 2.2, n = 7
0.0005
0
0
0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09
0.1
0.11 0.12 0.13 0.14 0.15
39Ar/40Ar
Figure 6. 40Ar/39Ar age spectrum and inverse isochron for the 1-49-4d groundmass concentrate sample. The preferred age of this sample is the weighted mean of the spectrum steps D through H (20.85 ± 0.22 Ma). All errors are at
two-sigma.
80
40
0
1
0.1
K/Ca
% Radiogenic
L# 53332-01, 21-30-1, 22.51 mg groundmass concentrate
0.01
25
Apparent Age (Ma)
20
11.88 ± 0.19 Ma* (MSWD = 0.96)
15
B
750
10
D
850
C
800
G
1125
F
1025
E
925
H
1300
5
0
A
-5
Integrated Age = 11.51 ± 0.74 Ma
-10
0
10
20
30
40
50
60
Cumulative
39Ar
K
70
Released
80
90
100
0.004
0.0035
A
I
0.003
E
B
D
36Ar/40Ar
0.0025
C
H
0.002
F
G
0.0015
0.001
Isochron age = 11.98 ± 0.20 Ma
40Ar/36Ar Intercept = 294 ± 3
MSWD = 1.4, n = 9
0.0005
0
0
0.01
0.02
0.03
0.04
0.05
0.06
0.07
0.08
0.09
0.1
0.11
39Ar/40Ar
Figure 7. 40Ar/39Ar age spectrum and inverse isochron for the 21-30-1 groundmass concentrate sample. The preferred age of this sample is the weighted mean of the spectrum steps B through H (11.88 ± 0.19 Ma). All errors are at
two-sigma.
80
40
0
10
1
0.1
40
K/Ca
% Radiogenic
L# 53336-01, 21-30-1 #2, 23.28 mg groundmass concentrate
0.01
35
Apparent Age (Ma)
30
25
20
11.59 ± 0.20 Ma* (MSWD = 1.09)
15
10
A
C
800
B
750
5
D
850
F
1025
E
925
H
1300
G
1125
I
0
-5
Integrated Age = 11.89 ± 0.77 Ma
-10
0
10
20
30
40
50
Cumulative
39Ar
K
60
70
80
90
100
Released
0.004
0.0035
A
I
0.003
E
0.0025
36Ar/40Ar
B
D
C
H
0.002
F
G
0.0015
0.001
Isochron age = 11.42 ± 0.16 Ma
40Ar/36Ar Intercept = 298 ± 3
MSWD = 1.0, n = 9
0.0005
0
0
0.01
0.02
0.03
0.04
0.05
0.06
0.07
0.08
0.09
0.1
0.11
0.12
39Ar/40Ar
Figure 8. 40Ar/39Ar age spectrum and inverse isochron for the second 21-30-1 groundmass concentrate sample. The
preferred age of this sample is the weighted mean of the spectrum steps B through H (11.59 ± 0.20 Ma). All errors are
at two-sigma.
80
40
0
10
1
0.1
25
K/Ca
% Radiogenic
L# 53335: 21-37-3A, 23.50 mg groundmass concentrate
0.01
20
Apparent Age (Ma)
12.86 ± 0.34 Ma* (MSWD = 0.45)
15
C
800
B
750
10
D
850
E
925
F
1025
I
H
1300
G
1125
5
A
0
-5
Integrated Age = 12.9 ± 1.0 Ma
-10
0
10
20
30
40
50
60
70
80
90
100
0.07
0.08
0.09
0.1
Cumulative 39ArK Released
0.004
0.0035
A
H
0.003
I
36Ar/40Ar
0.0025
BG
C
E
D
F
0.002
0.0015
0.001
Isochron age = 13.1 ± 0.8 Ma
40Ar/36Ar Intercept = 294 ± 4
MSWD = 1.4, n = 9
0.0005
0
0
0.01
0.02
0.03
0.04
0.05
0.06
39Ar/40Ar
Figure 9. 40Ar/39Ar age spectrum and inverse isochron for the 21-37-3A groundmass concentrate sample. The preferred age of this sample is the weighted mean of the spectrum steps B through H (12.86 ± 0.34 Ma). All errors are at
two-sigma..
80
40
0
10
1
0.1
K/Ca
% Radiogenic
L# 53333: 21-37-3D, 21.68 mg groundmass concentrate
0.01
25
Apparent Age (Ma)
20
13.30 ± 0.14 Ma* (MSWD = 2.62)
15
C
800
B
750
10
D
850
E
925
G
1125
F
1025
H
1300
5
0
-5
Integrated Age = 13.31 ± 0.30 Ma
-10
0
10
20
30
40
50
60
70
80
90
100
0.08
0.09
0.1
Cumulative 39ArK Released
0.004
0.0035
A
0.003
I
0.0025
36Ar/40Ar
H
G
0.002
B
F
0.0015
C
0.001
Isochron age = 13.26 ± 0.08 Ma
40Ar/36Ar Intercept = 298 ± 3
MSWD = 6.1, n = 9
0.0005
0
E
D
0
0.01
0.02
0.03
0.04
0.05
0.06
0.07
39Ar/40Ar
Figure 10. 40Ar/39Ar age spectrum and inverse isochron for the 21-37-3D groundmass concentrate sample. The preferred age of this sample is the weighted mean of the spectrum steps B through G (13.30±0.14 Ma). All errors are at
two-sigma.
80
40
0
10
1
0.1
40
0.01
35
Apparent Age (Ma)
30
18.10 ± 0.6 Ma* (MSWD = 0.73)
25
20
B
750
15
C
800
10
D
850
E
925
H
1300
F
1025
G
1125
5
0
-5
Integrated Age = 20.3 ± 2.4 Ma
-10
0
10
20
30
40
50
60
Cumulative
39Ar
K
Released
70
80
90
100
0.004
0.0035
A
I
H G
0.003
EF D
C
B
36Ar/40Ar
0.0025
0.002
0.0015
0.001
Isochron age = 17.6 ± 1.6 Ma
40Ar/36Ar Intercept = 298 ± 3
MSWD = 2.6, n = 9
0.0005
0
0
0.01
0.02
0.03
0.04
0.05
0.06
0.07
0.08
39Ar/40Ar
Figure 11. 40Ar/39Ar age spectrum and inverse isochron for the 21-45-2 groundmass concentrate sample. The
preferred age of this sample is the weighted mean of the spectrum steps B through G (18.10 ± 0.60 Ma). All
errors are at two-sigma .
K/Ca
% Radiogenic
L# 53334: 21-45-2, 22.03 mg groundmass concentrate
Biotite
Hornblende
Groundmass concentrate
21-48-2
14.37 ± 0.12 Ma
21-9-1
24.54 ± 0.31 Ma
21-34-1
24.14 ± 0.35 Ma
21-57-1A
19.99 ± 0.52 Ma
36-108
19.43 ± 0.47 Ma
1-49-4d
20.85 ± 0.22 Ma
21-30-1 #1
11.88 ± 0.19 Ma
21-30-1 #2
11.59 ± 0.20 Ma
21-37-3A
12.82 ± 0.35 Ma
21-37-3D
13.30 ± 0.14 Ma
21-45-2
18.10 ± 0.60 Ma
10
12
14
16
18
20
22
24
26
Weighted Mean Age ± 2 sigma (Ma)
Figure 12. Summary plot of the 40Ar/39Ar ages yielded by this study. All of
the errors are two-sigma.