Open File Report OF-AR-35
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 Navajo Canyon Dike in Mesa Verde
National Park
Prepared By:
Lisa Peters
New Mexico Bureau of Geology, Socorro, NM 87801
Prepared For:
Paul Carrara
Us Geological Survey, Denver, CO
Initially prepared as:
Data Repository for:
NM Geochronology Research
Laboratory Internal Report
NM Bureau of Geology and Mineral
Resources
NMGRL-IR 578
March 16, 2007
SOCORRO 2011
Introduction
A sample from a minette dike in Navajo Canyon, Mesa Verde National Park was
submitted by Paul Carrara of the USGS. Several of these dikes are found in the park and
have been assumed to be related to the Shiprock complex (~30 Ma). Biotite was
separated from the submitted dike sample (6126) and analyzed by the 40Ar/39Ar method.
40
Ar/39Ar Analytical Methods and Results
The minette was crushed, sieved and cleaned with distilled water and the biotite
was removed by hand-picking under a binocular microscope. The sample was loaded
into an aluminum disc and irradiated for 15 hours at the GSTR TRIGA nuclear reactor at
the US Geological Survey in Denver, Colorado.
The biotite mineral separate was analyzed with the furnace incremental heating
age spectrum method. Abbreviated analytical methods for the dated sample are given in
Table 1, and details of the overall operation of the New Mexico Geochronology Research
Laboratory is provided in the Appendix. The age results are summarized in Table 1 and
the argon isotopic data are given in Table 2.
6126
Weighted Mean Age=25.65±0.08 Ma
n/ntotal =5/11
MSWD=0.75
Sample 6126 yielded a slightly disturbed age spectrum with increasing apparent
ages (10.51 to 25.89 Ma) over the initial 33.4% of the 39Ar released (Figure 1a).
Increasing radiogenic yields (4 to 94%) and K/Ca values (0.43 to 69) are correlated with
these increasing apparent ages. A small drop in apparent age from 25.89 Ma to 25.66 Ma
is revealed between heating steps E and F. The age’s radiogenic yields remain fairly
constant over the next 65.3% of the age spectrum from which a plateau age of 25.65±0.08
Ma is calculated. The K/Ca values reveal a steady decline from 69.9 to 10.9 for the
plateau steps. The final heating step reveals a rise in apparent age to 28.00 Ma and a drop
in both radiogenic yield (58.9%) and K/Ca (7.4). Inverse isochron analysis (Figure 1b) of
-1-
steps C-J reveals an atmospheric 40Ar/36Ar intercept (294.2±19.0) and an isochron age of
25.71±0.18 Ma that agrees within error to the weighted mean age. The isochron MSWD
of 4.2 suggests scatter that is not solely explained by analytical uncertainty.
Discussion
We assign the weighted mean age (25.65±0.08 Ma) calculated from steps F-J as
the intrusion age of this minette dike. Overall the age spectrum exhibits only minor
complexity which suggests to us that the assigned plateau age is an accurate intrusion
age. Further support for this interpretation is afforded by the agreement of the total gas
and plateau ages.
-2-
References Cited
Renne, P.R., Owens, T.L., DePaolo, D.J., Swisher, C.C., Deino, A.L., and Darner, D.B.,
1998. Intercalibration of standards, absolute ages and uncertainties in 40Ar/39Ar
dating, Chem. Geol., 145, 117-152.
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.
-3-
Table 1. Summary of 40Ar/39Ar results and analytical methods
Sample
6126
Lab # Irradiation
56142 NM-194
mineral
biotite
age
analysis
furnace step-heat
steps
5
Age
25.65
±2σ
0.08
MSWD
0.75
Sample preparation and irradiation:
Biotite separated with hand-picking techniques.
Sample was loaded into a machined Al disc and irradiated for 15 hours at the GSTR TRIGA nuclear reactor, US Geological Survey Denver, Colorado.
Neutron flux monitor Fish Canyon Tuff sanidine (FC-2). 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.
Flux monitor fused by a 50 watt Synrad CO2 laser.
Reactive gases removed during a 2 minute reaction with 2 SAES GP-50 getters, 1 operated at ~450°C and
1 at 20°C. Gas also exposed to a W filament operated at ~2000°C and a cold finger operated at -140°C.
Biotite step-heated for 8 minutes, using a Mo double-vacuum resistance furnace.
Reactive gases removed during furnace analysis by reaction with 3 SAES GP-50 getters, 2 operated at ~450°C and
1 at 20°C. Gas also exposed to a W filiment operated at ~2000°C.
Analytical parameters:
Electron multiplier sensitivity averaged 2.88 x 10-16 moles/pA.
Total system blank and background for laser analyses averaged 2680, 11.6, 2.11, 39.3, 14.2 x 10-18 moles at masses 40, 39, 38, 37 and 36, respectively.
J-factors determined to a precision of ± 0.11% by CO2 laser-fusion of 10 single crystals from each of 10 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 = 0.01±0.002; (36Ar/37Ar)Ca = 0.000276±0.000001; and (39Ar/37Ar)Ca = 0.000676±0.000005.
Table 2. 40Ar/39Ar analytical data.
ID
Temp
40
Ar/39Ar
37
Ar/39Ar
36
Ar/39Ar
-3
(°C)
(x 10 )
39
(x 10
ArK
-15
K/Ca
6126, Biotite, 5.35 mg, J=0.0036236±0.11%, D=1.002±0.001, NM-194E, Lab#=56142-01
A
650
40.27
1.175
131.1
2.37
0.43
B
750
8.665
11.77
20.43
6.38
0.043
C
850
5.420
0.1240
5.069
32.5
4.1
D
920
4.556
0.0145
1.952
100.9
35.2
E
1000
4.248
0.0074
0.8447
276.9
68.6
F
1075
4.158
0.0073
0.6653
336.2
69.9
G
1110
4.210
0.0078
0.8432
135.3
65.5
H
1180
4.103
0.0090
0.4616
282.5
56.9
I
1210
4.084
0.0200
0.4822
57.7
25.5
J
1250
4.378
0.0467
1.441
7.54
10.9
Xi K
1680
7.336
0.0686
10.20
16.8
7.4
Integrated age ± 2σ
n=11
1255.1
6.8
Plateau ± 2σ steps F-J
n=5
MSWD=0.75
Isochron±2σ steps C-J
n=8
MSWD=4.16
819.1
Ar*
(%)
mol)
Xi
Xi
X
X
X
40
4.0
41.5
72.5
87.3
94.1
95.3
94.1
96.7
96.5
90.3
58.9
39
Ar
(%)
0.2
0.7
3.3
11.3
33.4
60.2
71.0
93.5
98.1
98.7
100.0
K2O=24.87%
61.0 ±52.2
65.3
40
Ar/36Ar=294.2±19.0
Age
(Ma)
±1σ
(Ma)
10.51
23.52
25.46
25.77
25.89
25.66
25.65
25.69
25.53
25.62
28.00
25.70
2.90
0.71
0.15
0.07
0.05
0.04
0.06
0.04
0.08
0.38
0.31
0.10
25.65
0.08
25.71
0.09
Notes:
Isotopic ratios corrected for blank, radioactive decay, and mass discrimination, not corrected for interfering reactions.
Errors quoted for individual analyses include analytical error only, without interfering reaction or J uncertainties.
Integrated age calculated by summing isotopic measurements of all steps.
Integrated age error calculated by quadratically combining 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 error is weighted error of Taylor (1982).
Decay constants and isotopic abundances after Steiger and Jäger (1977).
X symbol preceding sample ID denotes analyses excluded from plateau age calculations.
i symbol preceding sample ID denotes analyses excluded from isochron age calculations.
Weight percent K2O calculated from 39Ar signal, sample weight, and instrument sensitivity.
Ages calculated relative to FC-2 Fish Canyon Tuff sanidine interlaboratory standard at 28.02 Ma
Decay Constant (LambdaK (total)) = 5.543e-10/a
Correction factors:
(39Ar/37Ar)Ca = 0.00067 ± 3e-05
(36Ar/37Ar)Ca = 0.00028 ± 1e-05
(38Ar/39Ar)K = 0.01077
(40Ar/39Ar)K = 0.01 ± 0.002
% Radiogenic
6126 Biotite
1a.
100
50
0
20
0
27.0
25.65 ± 0.08 Ma* (MSWD = 0.75)
Apparent Age Ma
26.5
26.0
25.5
25.0
1000
E
920
D
750
850
C
1075
F
1180
H
1110
G
1210
I
1250
J
24.5
24.0
23.5
Integrated Age = 25.70 ± 0.10 Ma
23.0
0
10
20
30
40
50
60
70
80
90
100
Cumulative %39Ar Released
1b.
0.0040
0.0035
A
0.0030
36Ar/40Ar
0.0025
0.0020
B
0.0015
K
0.0010
C
Steps C-J
Isochron age = 25.71 ± 0.18 Ma
40Ar/36Ar Intercept = 294.2 ± 19.0
0.0005
D
J
EG
FH
I
MSWD = 4.2, n = 8
0
0
0.04 0.06
0.10
0.14
0.18
0.22
0.26
39Ar/40Ar
Figure 1. Age spectrum, K/Ca and radiogenic yield diagrams (1a)
and isochron (1b) for 6126 biotite. All errors quoted at two sigma.
K/Ca
80
60
40