Open File Report OF-AR-

New Mexico Bureau of Geology and Mineral ResourcesA division of

New Mexico Institute of Mining and Technology

40Ar/39Ar Geochronology Results

Prepared By:

New Mexico Bureau of Geology, Socorro, NM 87801

Prepared For:

Initially prepared as:

SOCORRO 2011

-1-

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

40Ar/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

-2-

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 K-

rich 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).

-3-

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.

Table 1. Summary of 40Ar/39Ar results and analytical methods

ageSample Lab # Irradiation mineral analysis steps/analyses Age ±2σ MSWD8297-Mancos 58726 NM-221 phlogogpite furnace step-heat q 12 24.94 0.11 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 40Ar/39Ar 37Ar/39Ar 36Ar/39Ar 39ArK K/Ca 40Ar* 39Ar Age ±1σ

(°C) (x 10-3) (x 10-15 mol) (%) (%) (Ma) (Ma)

8297-Mancos, Phlogopite, 8.37 mg, J=0.0008356±0.07%, D=1.0014±0.001, NM-221L, Lab#=58726-01Xi A 650 208.7 0.3975 708.3 0.428 1.3 -0.2 0.3 -0.78 2.14Xi B 750 35.71 1.947 102.8 0.797 0.26 15.3 1.0 8.25 0.73X C 850 58.66 0.2754 141.4 0.938 1.9 28.8 1.7 25.29 0.80X D 920 45.22 0.0175 99.10 3.480 29.2 35.2 4.5 23.86 0.37

E 1000 23.11 0.0092 20.16 14.32 55.3 74.2 15.9 25.67 0.10F 1075 19.04 0.0071 6.709 22.84 72.0 89.6 34.1 25.53 0.06G 1110 18.74 0.0049 6.474 15.78 104.1 89.8 46.7 25.19 0.07H 1180 18.40 0.0059 5.985 21.17 86.5 90.4 63.6 24.91 0.06I 1210 17.73 0.0027 3.829 19.17 190.6 93.6 78.9 24.84 0.06J 1250 17.35 0.0041 2.059 21.82 125.2 96.5 96.3 25.07 0.05K 1300 17.36 0.0197 2.180 4.138 25.9 96.3 99.6 25.02 0.09

X L 1680 28.18 0.0481 42.63 0.483 10.6 55.3 100.0 23.35 0.68Integrated age ± 2σ n=12 125.4 23.0 K2O=6.89% 24.94 0.11Plateau ± 2σ steps E-K n=7 MSWD=19.43 119.238 104.0±106.7 95.1 25.12 0.21Isochron±2σ steps C-L n=10 MSWD=16.06 40Ar/36Ar= 297.9±15.6 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/aCorrection 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

0 0.005 0.010 0.015 0.020 0.025 0.030 0.035 0.040 0.045 0.050 0.055 0.0600

0.0005

0.0010

0.0015

0.0020

0.0025

0.0030

0.0035A

B

C

D

E

F GH

IJ

K

L

0

50

100

0

100

200

300

0 10 20 30 40 50 60 70 80 90 10020

22

24

26

28

30

D920

E1000

F1075

G1110

H1180

I1210 J

1250K

1300

L1680

Integrated Age = 24.94 ± 0.11 Ma

24.97 ± 0.12 Ma* (MSWD = 3.80)

Cumulative %39Ar Released

%40

Ar*

App

aren

t Age

(Ma)

K/C

a36

Ar/40

Ar

36Ar/40Ar

Isochron Age = 25.10 ± 0.28 Ma40Ar/36Ar Intercept = 297.9 ± 15.6MSWD = 16

Steps C-L

8297-Mancos Pholgopite

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.