Open File Report OF-AR-36
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 Mancos 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 647
May 19, 0200
SOCORRO 2011
Introduction
A sample (8297- Mancos) of a minette dyke from Mancos Canyon in Mesa Verde
National Park was submitted by Paul Carrara. A previous sample from a dike in Navajo
Canyon, about 20 Km to the west of where 8297-Mancos was collected was dated at the
NMGRL and yielded an age of 25.65±0.08 Ma. Phlogopite from 8297-Mancos was
separated and analyzed by the 40Ar/39Ar
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 mineral separates and
monitors (Fish Canyon tuff sanidine, 28.02, Renne et al., 1998) were loaded into
aluminum discs and irradiated for 7 hours at the Nuclear Science Center in College
Station, Texas.
The phlogopite 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.
8297-Mancos
Weighted Mean Age=24.97±0.12 Ma
n/ntotal =4/12
MSWD=3.80
8297-Mancos yielded a somewhat disturbed age spectrum (Figure 1a). The initial
4.5% of the39Ar released yielded anomalously young apparent ages (-0.78 Ma-23.86 Ma)
followed by a rise in apparent age to 25.67 Ma. The apparent ages then decrease to 25.19
Ma over the following 42.2 % of the spectrum. The radiogenic yields (35.2 to 89.8%) and
K/Ca values (29.2-104.1) rise over this portion of the age spectrum. The remainder of the
age spectrum is nearly concordant and a weighted mean age of 24.97±0.12 Ma is
calculated from steps H to K excluding the final 0.4% of the 39Ar released. The K/Ca
values are somewhat oscillatory (86.5 to 190.6 and down to 10.6) over this portion of the
spectrum while the radiogenic yields increase steadily to 96.3% radiogenic. Inverse
-1-
isochron analysis of steps C-L yield an isochron age of 25.10±0.28 Ma with a 40Ar/36Ar
intercept of 297.9±15.6 (Figure 1b).
Discussion
We assign the weighted mean age (24.97±0.12 Ma) calculated from steps H-K as
the intrusion age of this minette dike. The discordance displayed in this age spectrum is
possibly the result of minor alteration, minor 39Ar recoil (redistribution of 39Ar from Krich to K-poor alteration phases), or slight excess Ar (40Ar/36Ar >295.5). The fact that the
integrated age (the age usually chosen as the best eruption age in samples suspected of
having undergone 39Ar recoil), the isochron age and the weighted mean age all agree
within the quoted 2 sigma error, suggests that the amount of recoil or excess is minor.
We feel confident that the assigned age (24.97±0.12 Ma) is an accurate intrusion age of
this minette. We note that 8297-Mancos is ~0.5 Ma younger than the previously dated
minette from a nearby canyon (25.65±0.08 Ma).
-2-
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. Chem. Geol., v. 145, p.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
8297-Mancos
Lab #
58726
Irradiation
NM-221
mineral
phlogogpite
age
analysis
steps/analyses
furnace step-heat q
12
Age
24.94
±2σ
0.11
MSWD
23.07
Sample preparation and irradiation:
Minerals separated with standard heavy liquid, Franz Magnetic and hand-picking techniques.
Samples in NM-221 irradiated in a machined Aluminum tray for 7 hours in D-3 position, Nuclear Science Center, College Station, TX.
Neutron flux monitor Fish Canyon Tuff sanidine (FC-2). Assigned age = 28.02 Ma (Renne et al, 1998).
Instrumentation:
Analyses performed on a Mass Analyzer Products 215-50 mass spectrometer on line with automated all-metal extraction system.
Phlogopite step-heated, using a Mo double-vacuum resistance furnace.
Analytical parameters:
Electron multiplier sensitivity averaged 6.88 x 10-17 moles/pA for furnance analyses and 6.21e-17 for laser analyses.
1.73 x 10-18 moles at masses 40, 39, 38, 37 and 36, respectively for the furnace analyses.
Total system blank and background averaged 166, 0.96, 0.24, 0.74, 8.6 x 10-18 moles at masses 40, 39, 38, 37 and 36, respectively for the furnace analyses.
J-factors determined by CO2 laser-fusion of 6 single crystals from each of 10 radial positions around the irradiation tray.
Decay constants and isotopic abundances after Steiger and Jäger (1977).
Table 2. 40Ar/39Ar analytical data.
ID
Power
40
Ar/39Ar
37
Ar/39Ar
36
Ar/39Ar
-3
(°C)
(x 10 )
39
(x 10
ArK
-15
mol)
K/Ca
40
Ar*
(%)
39
Ar
(%)
8297-Mancos, Phlogopite, 8.37 mg, J=0.0008356±0.07%, D=1.0014±0.001, NM-221L, Lab#=58726-01
A
650
208.7
0.3975
708.3
0.428
1.3
-0.2
0.3
B
750
35.71
1.947
102.8
0.797
0.26
15.3
1.0
C
850
58.66
0.2754
141.4
0.938
1.9
28.8
1.7
D
920
45.22
0.0175
99.10
3.480
29.2
35.2
4.5
E
1000
23.11
0.0092
20.16
14.32
55.3
74.2
15.9
F
1075
19.04
0.0071
6.709
22.84
72.0
89.6
34.1
G
1110
18.74
0.0049
6.474
15.78
104.1
89.8
46.7
H
1180
18.40
0.0059
5.985
21.17
86.5
90.4
63.6
I
1210
17.73
0.0027
3.829
19.17
190.6
93.6
78.9
J
1250
17.35
0.0041
2.059
21.82
125.2
96.5
96.3
K
1300
17.36
0.0197
2.180
4.138
25.9
96.3
99.6
X L
1680
28.18
0.0481
42.63
0.483
10.6
55.3
100.0
Integrated age ± 2σ
n=12
125.4
23.0
K2O=6.89%
Xi
Xi
X
X
Plateau ± 2σ steps E-K
n=7
MSWD=19.43 119.238
Isochron±2σ steps C-L
n=10
MSWD=16.06
104.0±106.7
95.1
40
Ar/36Ar= 297.9±15.6
Age
(Ma)
±1σ
(Ma)
-0.78
8.25
25.29
23.86
25.67
25.53
25.19
24.91
24.84
25.07
25.02
23.35
24.94
2.14
0.73
0.80
0.37
0.10
0.06
0.07
0.06
0.06
0.05
0.09
0.68
0.11
25.12
0.21
25.10
0.28
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.
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 = 0.00068 ± 5e-05
(36Ar/37Ar)Ca = 0.00028 ± 2e-05
(38Ar/39Ar)K = 0.0125
(40Ar/39Ar)K = 0 ± 0.0004
8297-Mancos Pholgopite
%40Ar*
100
50
0
300
100
Apparent Age (Ma)
30
0
28
24.97 ± 0.12 Ma* (MSWD = 3.80)
26
1000
E
24
1075
F
1110
G
1180
H
1250
J
1210
I
1300
K
920
D
22
1680
L
Integrated Age = 24.94 ± 0.11 Ma
20
0
10
20
30
40
Cumulative
0.0035
50
60
%39Ar
70
80
90
100
Released
A
0.0030
B
0.0025
C
D
L
0.0015
36
Ar/40Ar
0.0020
0.0010
E
Steps C-L
Isochron Age = 25.10 ± 0.28 Ma
40 36
Ar/ Ar Intercept = 297.9 ± 15.6
MSWD = 16
0.0005
0
0
0.005
0.010
0.015
0.020
FG
0.025
0.030
36
40
0.035
0.040
0.045
0.050
H
0.055
IK
J
0.060
Ar/ Ar
Figure 1. Age spectrum (1a) and isochron (1b) for phlogopite 8297-Mancos.
Points shown in purple not included in weighted mean age calculation.
All errors quoted at 2 sigma.
K/Ca
200