Disturbed 40Ar39Ar spectra from hornblendes: Thermal loss or contamination

Chemical Geology (Isotope Geoscience Section), 103 ( 1993 ) 271-281

271

Elsevier Science Publishers B.V., Amsterdam [PD]

Disturbed 4°Ar-39Ar spectra from hornblendes: Thermal loss or contamination? D.C. Rex, P.G. Guise and J.-A. Wartho Department of Earth Sciences, The University of Leeds, Leeds, LS2 9.1T. UK ( Received January 28, 1992; revised and accepted June 22, 1992 )

ABSTRACT Rex, D.C., Guise, P.G. and Wartho, J.-A., 1993. Disturbed 4°Ar-39Ar spectra from hornblendes: Thermal loss or contamination? Chem. Geol. ( lsot. Geosci. Sect. ), 103:271-281. Accurately weighed aliquots of the ~ 500-Ma-old interlaboratory standard hornblende MMHb-1 were mixed with known quantities of a ~ 100 Ma-old Himalayan biotite and subjected to normal 4°Ar/39Ar incremental heating analysis. The addition of the biotite created monotonically increasing apparent ages, similar to profiles interpreted as showing Ar diffusive loss in the literature. The release of Ar from biotite dominates the low-temperature portion of the spectra and lowers the observed age. Lower C a / K ratios in the first increments are consistent with contamination, but do not distinguish the composition of the contaminant phase. A large number of published amphibole "diffusive loss" profiles may, in fact, be due to contamination by biotite or other phases which release Ar at low extraction temperatures.

I. Introduction

Igneous hornblendes generally yield 4 ° A r / 39Ar age spectra which are flat. Several are used as reference standards, for example, MMHb-1 (Alexander et al., 1978; Samson and Alexander, 1987); Hb3gr (Turner et al., 1971 ); and BH-6 (Wang et al., 1990). In contrast, metamorphic hornblendes often have "disturbed" age spectra which can vary greatly in shape, ranging from those interpreted as showing Ar loss (Harrison and McDougall, 1980) to those proposed to have excess radiogenic Ar (Berger and York, 1981 ) passing through some with flat spectra (Treloar et al., 1989). Turner et al. (1966) interpreted disturbed 4 ° A r / 3 9 A r age profiles from meteorite samples using a theoretical analysis which assumed episodic Ar loss by diffusion. In this Correspondence to: D.C. Rex, Department of Earth Sciences, The University of Leeds, Leeds, LS2 9JT, UK.

0009-2541/93/$06.00

analysis the first increments should define the age of disturbance which caused the diffusive loss; subsequent steps should increase in age, and where the total 4 ° A r loss is < 2 0 % , will reach the inferred age of the meteorite. Harrison and McDougall (1980) subsequently applied Turner's theory to interpret age spectra from overprinted terrestrial metamorphic hornblendes as Ar loss profiles, assuming a uniform diffusion size. Recently Gaber et al. (1988), Lee et al. ( 1991 ), and Wartho et al. ( 1991 ) have shown that Ar release from hornblende is controlled by phase changes and not by volume diffusion, suggesting that natural "frozen" diffusion profiles in hornblendes cannot be readily identified by the 4°Ar/39Ar dating method, confirming previous suggestions of Gerling et al. (1966) and Hanson et al. (1975). Berger (1975), studying thermal overprinting in the contact zone of the Eldora stock, Colorado, U.S.A., also interpreted 4°Ar/39Ar spectra in

© 1993 Elsevier Science Publishers B.V. All rights reserved.

272

D.C. REX ET AL.

contamination, which could be identified from low Ca/K ratios in the low-temperature steps. This approach had been used previously by Pankhurst et al. (1973) to infer biotite contamination in hornblendes which contained

terms of phase changes during step heating, and suggested that all data below 900°C from a hornblende age spectrum should be ignored. He also argued that hornblende spectra with this type of profile could be explained by biotite

m

r~ LO

I

1[

15

lo

10

5

5 I

c

t

I

0

0R16 Hnb. run 1246 200C

960

lO00

1050

Hnb

DRI6

1125

2000

988

I010

PJn 1247

1085

1180

920

150C,

i500

c~

,,m

lO00

ir}O3

# 50C

500

( 20

40 tTum %

3g

60 Ar

80

i00

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20

40 Oum ~

60 ~9 S ~

80

108

Fig. 1. Duplicate 4°Ar-39Ar age and C a / K spectra for hornblende DRI6. Temperatures ( ° C ) o f major gas release steps shown above boxes. TABLEI 4°Ar-39Ar data for hornblende DR16 Temper-

Volume ( 10 "9 cm 3 STP)

Ca/K

4°Ar*/39K

ature (°C)

39K

37Ca

% Atm. 4°Ar

Age (Ma)

Error (2a)

% 39Ar

27.8 0.2 1.8 0.4 1.1 0.7 0.5 0.5 0.3 0.6

839.3 495.4 593.9 673.3 1556 1757 1830 1822 1871 1743

13.0 32.8 52.0 26.2 8.3 14.3 13.9 13.4 15.8 8.0

3.5 2.0 1.4 1.9 10.6 12.5 31.2 11.2 18.4 7.3

38C1

DR16 hornblende, run 1246, 0.04263 g, J=0.006588 ( + 1.0%): 710 780 825 880 920 960 1000 1050 1125 1320

0.474 0.268 0.185 0.252 1.441 1.696 4.231 1.520 2.497 0.992

0.253 0.164 0.293 0.729 8.191 10.33 25.16 8.942 15.00 5.763

0.001 0.007 0.003 0.003 0.016 0.020 0.049 0.015 0.030 0.011

1.061 1.216 3.149 5.760 11.31 12.12 11.83 11.71 11.96 11.56

89.91 47.97 59.17 68.67 207.8 250.1 266.8 265.0 276.5 247.1

Integrated age 1718 + 23.2 Ma, K=0.69 wt%, 4°Ar*=768-10-7 cm 3 g-i

273

DISTURBED 4°Ar-39ArSPECTRA FROM HORNBLENDES TABLE 1

(continued)

Temperature

V o l u m e ( 109 cm 3 STP )

(°c)

39K

Ca/K

37Ca

4°Ar*/39K

% Atm. 4°Ar

Age (Ma)

Error

37.7 6.8 5.0 1.0 0.1 0.3 0.2 0.0 0.3

791.2 571.9 645.2 1495 1798 1833 1805 1891 1895

11.3 7.9 8.4 2.8 4.5 2.2 1.8 3.3 2.0

% 39Ar

(2or)

38C1

DRI6 hornblende, run 1247, weight =0.05464 g, J = 0 , 0 0 6 2 7 6 ( _+_1.0%): 700 845 920 960 985 1010 1085 t120 1335

0.441 0.573 0.485 1.462 3.537 3.339 2.123 2.100 2.421

0.240 0.579 1.420 8.503 21.31 19,63 12,30 12,61 14.70

0.009 0.004 0.003 0.018 0.049 0.050 0.027 0.027 0.037

1.081 2.013 5.822 11.57 11.99 11.70 11.53 11.95 12.08

87.71 59.43 68.50 205.6 272.4 280.8 273.9 295.1 296.2

2.7 3.5 2.9 8.9 21.5 20.3 12.9 12.7 14.7

Integrated age 1730_+ 2.4 Ma, K = 0.69 wt%, 4°Ar* = 773- 10-7 cm 3 g-~

o 40

040

030

030 LJ

020

0.?0

0 10

010 ~

-

I

t

I

DR16 Bto. run 124B

DE15 B i e

150

~un 1249

150

~v

ioo

%

c~
1oo


50

50

.

.

.

.

J

?0

I

J

40 60 Cum % 39 AP

~

-

BO

_

_

_

100

20

40 Cure ~

60 39 Ar"

80

100

Fig. 2. Duplicate 4°Ar-39Ar age and Ca/K spectra for biotite DRI6. excess radiogenic Ar. The use of C a / K ratios has been criticised by McDougall and Harrison (1988) on the grounds that hornblendes commonly show anomalous C a / K ratios in the low-temperature steps of a spectrum. Our interest in this problem arose from analysis of a hornblende (Table 1; Fig. 1 ) and biotite (Table 2; Fig. 2) from sample DR16,

an amphibolite dyke cutting the main gneissic foliation within the Besham Group of orthogneisses from the Indian plate of N.W. Pakistan (Treloar and Rex, 1990). The gneisses were affected by a thermal event at ~ 1800 Ma, imbricated with metasediments of early Paleozoic age, and re-metamorphosed during the Himalayan orogeny. Amphibole ages from

274

D.C. REX ET AL.

TABLE 2 4°Ar-39Ar data for biotite DRI6 Temperature

Volume ( 109 cm 3 STP)

(°c)

39K

Ca/K

37Ca

4°Ar*/39K

% Atm. 4°Ar

Age (Ma)

Error (2a)

% 39Ar

8.90 9.71 9.73 9.11 10.61 13.90 11.51 10.54 12.66

67.8 10.5 10.5 14.2 9.0 5.7 6.8 8.5 47.2

99.3 108.1 108.3 101.6 117.8 152.8 127.4 117.0 139.7

0.5 0.4 0.4 0.4 0.5 0.5 0.3 1.2 16.6

16.7 26.1 13.0 13.7 11.5 9.5 7.5 1.6 0.2

80.9 19.6 8.5 12.6 16.1 9.9 8.6 9.0 1.8

90.4 104.3 105.9 103.6 97.9 119.0 140.1 117.9 330.7

1.6 0.6 0.9 0.6 0.6 0.6 0.9 0.6 3.7

10.4 25.9 12.8 10.3 12.1 12.8 9.0 6.3 0.5

38C1

DRI6 biotite, run 1248, 0.03505 g, J = 0 . 0 0 6 3 5 8 ( + 1.0%): 675 770 850 920 965 I000 1070 1170 1360

18.55 28.99 14.46 15.24 12.73 10.54 8.29 1.80 0.276

0.258 0.344 0.138 0.201 0.499 1.359 1.042 0.916 1.093

0.066 0.093 0.045 0.050 0.039 0.036 0.029 0.006 0.007

0.028 0.024 0.019 0.026 0.078 0.256 0.250 1.015 7.880

Integrated age l 12.8 _ 2.2 Ma, K = 7.1 wt%, 4°Ar* = 321.2- 10-7 cm 3 g

DRI6 biotite, run 1249, 0.03300 g, J = 0 . 0 0 6 1 6 3 ( + 1.0%): 645 720 765 850 920 980 1040 1120 1325

11.06 27.63 13.63 10.98 12.95 13.65 9.60 6.75 0.58

0.182 0.293 0.158 0.106 0.303 0.781 1.029 1.219 0.490

0.039 0.091 0.041 0.038 0.042 0.042 0.033 0.023 0.002

0.033 0.021 0.023 0.019 0.047 0.114 0.213 0.359 1.669

8.33 9.66 9.81 9.59 9.05 11.06 13.10 10.96 32.64

Integrated age 109.5 + 2.1 Ma, K = 7.5 wt%, 4°Ar* = 328.8.10-7 cm 3 g-i TABLE 3

4°Ar-39Ardata for hornblende M M H b - I Temperature

Volume (10 -9 cm 3 STP)

(oc)

39K

37Ca

Ca/K

4OAr. / 39K

% Atm. a°Ar

Age (Ma)

E~or (2a)

% 39At

57.8 14.2 3.6 6.4 1.0 0.4 0.5 0.1 2.3 7.2

566.0 457.5 496.3 488.8 519.0 519.8 517.7 516.8 304.6 1790.5

64.3 67.6 67.7 15.9 1.9 1.0 0.9 2.0 31.3 22.4

0.6 0.4 0.5 1.6 12.8 28.9 32.5 19.8 1.3 1.6

38C1

MMHb-1 hornblende, run 1215, 0.02128 g, J = 0 . 0 0 6 3 7 7 ( + 1.0%): 675 750 850 920 960 1000 1070 1170 1250 1350

0.087 0.064 0.080 0.243 1.965 4.452 4.998 3.052 0.203 0.250

0.143 0.052 0.092 0.414 4.454 10.61 12.31 7.741 0.689 1.132

0.006 0.001 0.001 0.005 0.021 0.048 0.054 0.031 0.004 0.007

3.269 1.605 2.277 3.387 4.511 4.744 4.901 5.048 6.740 9.019

57.79 45.27 49.66 48.80 52.26 52.36 52.12 52.02 28.84 16.40

Integrated age 510.1 __9.0 Ma, K = 1.618 wt%, 4°Ar* = 370.7.10-7 cm 3 g-1

275

DISTURBED a°Ar-~gArSPECTRA FROM HORNBLENDES TABLE 3

(continued)

Temperature

Volume ( 10 -9 cm 3 STP)

(°c)

39K

37Ca

M M H b - I hornblende, run 710 750 850 920 960 1000 1045 1145 1240 1325

0.102 0.038 0.082 0.213 1.686 3.791 3.431 3.810 0.195 0.133

Ca/K

4°Ar*/39K

% Atm. 4°Ar

Age (Ma)

Error (2o')

% 39Ar

65.3 0.3 7.6 4.9 1.5 0.7 0.4 2.3 0.1 7.7

557.5 500.9 569.5 530.4 510.1 509.7 510.3 515.4 600.9 770.1

68.2 71.5 32.7 15.0 2.6 0.8 0.9 1.1 18.9 33.2

0,8 0,3 0,6 1.6 12.5 28.1 25.5 28.3 1.4 1.0

38C1

1216, 0.01935 g, J = 0 . 0 0 6 0 0 0 ( + 1.0%):

0.093 0.081 0.087 0.359 3.759 9.053 8.364 9.666 0.523 0.418

0.008 0.002 0.001 0.003 0.022 0.056 0.050 0.057 0.003 0.002

1.808 4.267 2.098 3.357 4.437 4.752 4.851 5.049 5.341 6.252

60.35 53.34 61.87 56.96 54.46 54.42 54.48 55.11 65.87 88.75

Integrated age 516.7 + 9i I Ma, K = 1.66 wt%, 4°Ar= 385.0.10-7 cm 3 g-i

I c_j

tO

5 i

ij

_

i

I

MMHb I I Pun 1 2 1 5

"un

MMHD-L

~21~

i

% 500 I= 400

'10 0

i

200

C0

I

i

I

! ±

0

0

?0

40 60 Cum % 39 AP

_

80

_

J

0 1oo

i

20

L

40

~

J

60

Cum % 39 AP

80

J

JO~J

Fig. 3. Duplicate 4°Ar-39Ar age and C a / K spectra for hornblende MMHb-1.

several of these dykes preserve the Proterozoic age whereas the biotites show a great variation, with ages ranging from 74 to 1640 Ma. Some of these hornblendes give age spectra which would be potentially ascribed to Ar loss follow-

ing the Turner model. We decided, however, to examine systematically the possibility that biotite contamination was the controlling factor, by subjecting artificial mixtures of hornblende and biotite to 39Ar-4°Ar analysis.

D.C. REX ET AL.

276 TABLE4 4°Ar-39Ardata ~ r M M H b - I mixed with biotite, sample DRI6 Temperature

(oc)

Volume ( 10 -9 cm 3 STP) 39K

37Ca

Ca/K

4°Ar*/39K

% Atm. 4°Ar

Age (Ma)

Error (2a)

% 39Ar

69.5 45.0 8.4 5.3 1.5 0.6 0.6 0.7 21.0 18.9

327.1 236.0 298.9 363.0 495.5 513.8 515.1 518.9 484.0 413.0

21.9 13.4 19.6 6.4 1.7 0.8 0.7 0.9 11.0 19.1

1.0 1.4 1.2 1.5 12.2 28.3 24.6 27.7 1.2 0.8

92.1 77.2 16.0 14.1 1.9 0.6 0.9 3.2 56.5 71.2

495.5 166.2 143.3 188.8 448.8 497.9 511.1 513.2 472.0 437.0

4.23 5.5 2.5 5.0 1.2 0.7 0.7 0.9 15.1 16.1

0.0 2.5 3.8 2.8 12.3 28.5 24.8 23.9 0.8 0.7

76.6 14.7 7.5 5.6 1.0 1.1 0.5 0.7 4.2 33.4

128.5 128.4 158.7 243.0 426.3 488.7 512.1 515.5 412.8 277.2

3.1 3.2 4.4 1.9 1.1 1.0 1.0 0.8 32.6 11.3

5.0 5.7 5.1 7.3 10.6 19.1 27.8 18.1 0.5 0.8

38C1

MMHb-1 (0.8% biotite), run 1256, 0.04396 g, J = 0 . 0 0 6 5 3 8 ( _+ 1.0%): 650 745 830 910 965 1000 1050 1150 1245 1335

0.314 0.472 0.406 0.506 3.997 9.271 8.058 9.097 0.397 0.271

0.187 0.066 0.078 0.296 8.102 21.74 19.49 22.53 1.030 0.579

0.009 0.003 0.005 0.004 0.033 0.079 0.074 0.085 0.004 0.003

1.186 0.280 0.384 1.162 4.034 4.667 4.814 4.930 5.169 4.250

30.40 21.38 27.56 34.09 48.35 50.39 50.55 50.98 47.06 39.35

Integrated age 502.0 + 8.8 Ma, K = 1.63 wt%, 4°Ar* = 366.0.10-7 cm 3 g-i M M H b - I (3.3% biotite), run 1257, 0.06123 g, J=0.006551 ( + 1.0%): 650 665 745 840 965 1000 1055 1155 1260 1370

0.002 1.167 1.800 1.333 5.853 13.54 11.80 11.36 0.374 0.346

0.023 0.223 0.121 0.149 10.41 30.15 28.19 27.79 0.936 0.710

0.000 0.013 0.008 0.004 0.046 0.116 0.106 0.105 0.005 0.003

25.45 0.380 0.133 0.222 3.540 4.432 4.752 4.869 4.977 4.083

48.25 14.73 12.62 16.84 43.11 48.51 49.99 50.23 45.65 41.84

Integrated age 470.5 _+8.3 Ma, K = 1.69 wt%, 4°Ar* = 353.4.10-7 cm 3 g~ MMHb-1 (6.9% biotite), run 1255, 0.04280 g, J = 0 . 0 0 6 1 6 3 ( _+ 1.0%): 700 755 855 930 960 1000 1070 1165 1240 1340

1.818 2.044 1.831 2.641 3.831 6.893 10.02 6.552 0.179 0.298

0.150 0.084 0.159 1.498 6.385 14.75 23.75 16.15 0.531 1.107

0.014 0.009 0.007 0.018 0.044 0.097 0.148 0.099 0.005 0.008

0.165 0.082 0.172 1.129 3.317 4.259 4.716 4.906 5.912 7.384

11.98 11.97 14.92 23.39 43.25 50.48 53.26 53.67 41.72 26.95

Integrated age 423.4 + 7.6 Ma, K = 1.95 wt%, 4°Ar* = 362.1.10-7 cm 3 g~

2. Materials analysed Mixtures were prepared by adding biotite from amphibolite D R 1 6 to standard horn-

blende MMHb-I. The 4 ° A r - 3 9 A r analysis for the D R 1 6 biotite gives an integrated age of ~ 110 Ma but the spectrum (Table 2; Fig. 2) is discordant ranging from 90 to 150 Ma. How-

DISTURBED4°Ar-39ArSPECTRAFROM HORNBLENDES

277

8

6 (_}

4

2 I

I

I

1

MMHb-1 + 0 . 8 % Run 1 2 5 6

I

0

8i0.

I

MMHb-I + Bun 1257

I

I

3.3% 8 i o .

600

600

1000

965

1050

1150

1000

1055

1155

g65 &

400

400

&

o

D 200

=

200

I

0 0

20

I

I

I

40 60 Cure % 39 AP

1

0

80

o

iO0

I

20

I

40 60 Cure X 39 Ar

I

BO

100

8

,'R,

6 4

2 I

l

o

1

I

MMHb-1 + 6.9~ Bio.

Run 1255 60o 1070

I000

tt65

960 400

200

o

I

2o

I

I

40 60 Cum % 39 Ar

I

BO

100

Fig. 4. a°Ar-39Ar age and C a / K spectra for M M H b - I mixed with increasing amounts of biotite DRI6. Temperatures ( ° C ) of major gas release steps shown above boxes.

278

ever, more than 60% of the 39Ar released comes from the 100-120-Ma steps. MMHb-I is an igneous hornblende extracted from a 520-Ma syenite (Alexander et al., 1978 ) and is used as a neutron fluence monitor by several laboratories including Leeds. The spectrum (Table 3; Fig. 3 ) is flat apart from the small amounts of gas at the extreme limits; these account for only 3% and 2.5% of the gas at the low and high temperature limits, respectively. 3. Results

Three mixtures, with biotite concentrations of 0.8%, 3.3% and 6.9% by weight were analysed. Results are given in Table 4 and Fig. 4. Analytical methods are given in the Appendix. A similar step heating schedule was used in all runs.

4. Discussion

The spectra produced from the artificially mixed hornblende (MMHb-1) and biotite (DR16) bear a striking similarity to "diffusive loss" profiles, for example the spectra produced by Harrison and McDougall (1980) from the Rameka Gabbro hornblendes, New Zealand. We quote this study because McDougall and Harrison ( 1988 ) use it as a prime example of Ar diffusive loss; there are numerous other examples in the literature. The similarity includes not only the trend in apparent age but also the systematic C a / K variations. Increasing biotite content clearly controls the low-temperature release profiles. The main difference is that the mixed M M H b - I - D R 1 6 samples give age spectra and C a / K ratios which rise to the plateaux appropriate for pure MMHb-I. However, higher temperatures are required to reach these plateaux as the amount of biotite in the mixture increases (Fig. 4). Using the K and Ar contents of the "pure" constituents in the same weight proportions as in the mixtures we have calculated the following "integrated" ages: 509, 486 and 409 Ma,

D.C. REX ET AL.

which are in reasonable agreement with the experimental results (Table 4). From the analyses of the biotite D R I 6 (Table 2 ) it can be seen that > 40% of the 39Arhas been evolved by ~ 750 ° C and 90% by 1050 ° C, whereas for the pure MMHb-1 hornblende (Table 3) only 1% of the 39Ar has been released by 750°C and 70% at 1050°C. For this reason the biotite-"contaminated" MMHb-1 age spectra show young ages and low C a / K ratios in the low-temperature steps and increase monotonically to the plateau age of 515 Ma in the high-temperature steps with corresponding high C a / K ratios. (N.B. The age of MMHb-I is not consistent with the 520-Ma figure normally quoted because of the use of three standards to calibrate the fluence, see the Appendix. ) The C a / K ratios which increase for biotite and hornblende mixtures clearly reflect early biotite release followed by the hornblende release at higher temperatures. Thus the C a / K ratios seem to strongly support the contamination explanation for many examples in the literature. Brereton ( 1972 ) and Berger ( 1975 ) noted the difficulties which could arise in using these ratios for identifying mineral species contaminating a hornblende sample. To be certain that a sample is pure, mineral separates must not only be checked under a binocular microscope but also should be examined with a scanning electron microscope (SEM) for the presence of inclusions of other mineral species. MMHb-I hornblende was found to contain sphene lamellae and minor inclusions of biotite, apatite and K-feldspar by Lee et al. (1991). Similarly, our analysis of polished grain mounts of DR16 hornblende revealed numerous minute inclusions, including two phases of biotite. Thus, the examination confirms that this is a case of biotite contamination and not diffusive loss from the hornblende. Our results show that biotite contamination can produce "hornblende" spectra with patterns that have elsewhere been interpreted in terms of diffusive Ar loss profiles. Low-tern-

279

DISTURBED ~Ar-39Ar SPECTRA FROM HORNBLENDES

perature release increments correspond to low C a / K ratios, quite different from the high ratios characteristic of hornblende from the temperature steps above 1000°C. This supports Berger's (1975) suggestion that steps below 900 °C should be disregarded from age calculations if the C a / K ratios are different from the steps above 900°C. Ross and Sharp (1988) also conclude that age spectra from hornblendes with white mica intergrowths resemble those which have been attributed to volume diffusion; they go on to suggest that ages associated with the early, low C a / K ratios could be cooling ages for the mica inclusions. From our mixing experiments, however, it can be seen that the biotite age is not well defined until the mixture contains 6.9 wt% biotite. At lower concentrations, the low-temperature hornblende Ar masks the mica age, to an extent which depends on the relative ages of the two mineral components. Our explanation of the apparent Ar loss spectra in the literature (e.g., Harrison and McDougall, 1980) is that they can be explained by biotite contamination (Wartho, 1991 ). Such an explanation is consistent with C a / K patterns and the observation of hornblende reactions during heating (Gaber et al., 1988; Wartho et al., 1991 ).

5. Conclusions Hornblende spectra which have been interpreted using simple volume diffusion models must be re-evaluated with emphasis on potential contamination of the hornblende, as our experimental work has demonstrated. However, plateau ages assumed from these spectra are unlikely to be seriously affected. The C a / K ratios should be examined because variations may indicate contamination, but they may not permit identification of the contaminating mineral species. (It should be noted that extended periods between irradiation and analysis of samples may lead to diffi-

culties in accurately measuring C a / K ratios due to decay of 37Ar. ) All samples should be examined under SEM, especially if any conclusions are to be drawn from the low-temperature steps of the age spectrum.

Acknowledgements We wish to thank M.H. Dodson for his comments and assistance in the interpretation of the data. T. Oddy for his careful mineral preparation, G.E. Lloyd for SEM analyses and A.C. Barnicoat for commenting on the manuscript. We thank K.A. Foland and G.S. Odin for valuable reviews of the manuscript.

Appendix - -

4°Ar-39Ar

analytical details

Samples for 4°Ar-39Ar analysis were individually weighed, wrapped in high-purity A1 foil and loaded into Spectrosil ® phials. After evacuation to ~ 10 -2 Torr, the phials were sealed and packed into an A1 canister. Irradiation was carried out at the Petten facility in The Netherlands. A fast neutron dose of ~ 5- 10 ~8 n cm -2 was given and monitored by the inclusion of ten aliquots of hornblende standard F Y I 2 a (Roddick, 1983), four aliquots of M M H b - I (Alexander et al., 1978) and ten aliquots of HB3gr (Turner et al., 1971 ). When calculating the neutron fluence (J) for each standard position we use the 4°Ar*/a°K ratios quoted in Roddick (1983) to provide interlaboratory comparison (however, we have noted some inconsistency between these accepted values). Fluence variation over the length of the canister was of the order of 15%, similar to that found by other users of the facility. The large number of standards spaced along the canister allowed calibration offluence to 1%. Errors quoted on the integrated ages take account of this uncertainty but the individual step ages do not. Ar was extracted from each sample in a double vacuum, resistance-heated furnace, developed in Leeds after the ideas of Professor G. Turner (pets. commun., 1982 ) and Staudacher et al. (1978). Samples were loaded into the arms of a glass storage tree above the furnace and the entire system baked overnight at 125 ° C under vacuum. Following further degassing of the furnace (to 1350°C) and the getters, a sample was dropped into the crucible and step heating commenced. The temperature of the furnace was monitored with an infra-red optical pyrometer and is estimated to be accurate to _+25 °C with reproducibility of + 5 °C. The furnace was allowed to cool for 10 min after

280 each 30-min heating step and the evolved gas purified over two successive getters (mixtures of T i - Z r metal shavings and Ti sponge) heated to 800°C and then allowed to cool. The gas was then transferred to a small volume inlet section by adsorption on charcoal at liquid nitrogen temperature prior to admission to the mass spectrometer. Ar isotope analyses were performed using a MAP 2 1 5 ® mass spectrometer with Nier-type source, Faraday collector and magnet peak jumping under computer control. Ion beams were detected by a Cary 4 0 1 M ® electrometer with 10 Itresistor, digitized with a Solartron 7 0 6 0 ® voltmeter and stored on computer disc for subsequent processing. Measured mass spectrometer peak intensities were corrected for the following: Amplifier response and non-linearity; linear extrapolation to gas inlet time; spectrometer mass discrimination; and radioactive decay of 39Ar and 37Ar. Interfering isotopes from neutron reactions on K and Ca, corrections used are those given in Roddick (1983) because no significant variation in pure salts irradiated at this facility compared to the Herald reactor have been found. Atmospheric Ar extraction blanks are mainly dominated by the contribution from the AI sample packet ranging from 1- 10- 8 cm 3 4OAr STP at 700 ° C when the AI melts through 8" 10- lo at 900°C up to 3- 10 - 9 at 1350°C. The mass spectrometer discrimination and sensitivity were monitored by analysing atmospheric Ar from a pipette system. The measured atmospheric 4°Ar/36Ar (typically 298.0) changes with filament life but is determined at any time with a precision of + 0.12% (2a). The sensitivity (typically 0.25 mA cm -3 STP) also changes with time but is known to 0.3% (2g). Replicate analyses indicate that the concentration of radiogenic 4°Ar is determined to + 1.5% (2a) for a single analysis. The 4°Ar/39Ar ratio, age and errors for each gas fraction were calculated using formulae similar to those given by Dalrymple and Lanphere ( 1971 ). Duplicate analyses of sample gas fraction indicate a precision of + 0.2% (2a) in measured 4°Ar/ 39Ar beams greater than 0.8.10 -9 cm 3 STP. Errors in these ratios were evaluated by numerical differentiation of the equation used to determine the isotope ratios and quadratically, propagating the errors in the measured ratios. All errors are quoted at the 2~rlevel unless otherwise stated. Ages are calculated using the constants recommended by Steiger and J~iger (1977).

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D.C. REX ET AL.

4°Ar/39Ar dating experiments. Geochim. Cosmochim. Acta, 45:795-811. Brereton, N.R., 1972. A reappraisal of the 4°Ar/39Ar stepwise degassing technique. Geophys. J.R. Astron. Soc., 27: 449-478. Dalrymple, G.B. and Lanphere M.A., 1971. 4°Ar/39Ar technique of K - A r dating: a comparison with the conventional technique. Earth Planet. Sei. Lett.. 12: 300315. Gaber, L.J., Foland, K.A. and Cobato, C.E., 1988. On the significance of argon release from biotite and amphibole during 4°Ar/39Ar vacuum heating. Geochim. Cosmochim. Acta, 52: 2457-2465. Gerling, E.K., Petrov, B.V. and Kol'tsova, T.V., 1966. A comparative study of the activation energy of argon liberation and dehydration energy in amphiboles and biotites. Geochem. Int., 3: 295-305. Hanson, G.N., Simmons, K.R. and Bence, A.E., 1975. 4°Ar/39Ar age spectrum ages for biotite, hornblende and muscovite in a contact metamorphic zone. Geochim. Cosmochim. Acta, 39:1269-1277. Harrison, T.M. and McDougall, I., 1980. Investigations of an intrusive contact, NW Nelson, New Zealand, II. Diffusion of radiogenic and excess 4°Ar in hornblende revealed by 4°Ar/39Ar age spectrum analysis. Geochim. Cosmochim. Acta, 44: 2005-2020. Lee, J.K.W., Onstott, T.C., Cashman, K.V., Cumbest, R.J. and Johnson, D., 1991. A critical evaluation of the 4°Ar/39Ar incremental heating of hornblende. Geology, 19: 872-876. McDougall, I. and Harrison, T.M., 1988. Geochronology and Thermochronology by the 4°Ar/39Ar Method. Oxford Monographs on Geology and Geophysics, No. 9, Oxford University Press, Oxford, 212 pp. Pankhurst, R.J., Moorbath, S., Rex, D.C. and Turner, G., 1973. Mineral age patterns in ca. 3700 my old rocks from West Greenland. Earth Planet. Sci. Lett., 20:157170. Roddick, J.C., 1983. High precision intercalibration of 4°Ar-39Ar standards. Geochim. Cosmochim. Acta, 47: 887-898. Ross, J.A. and Sharp, W.D., 1988. The effects of subblocking tempertature metamorphism on the K / A r systematics of hornblendes: 4°Ar/39Ar dating of polymetamorphic garnet amphibolite from the Franciscan Complex, California. Contrib. Mineral. Petrol., 100: 213-221. Samson, S.D. and Alexander, Jr., E.C., 1987. Calibration of the Interlaboratory 4°Ar/39Ar dating standard MMhb-1. Chem. Geol. (Isot. Geosci. Sect.), 66: 2734. Staudacher, Th., Jessberger, E.K., D6flinger, D. and Kiko, J., 1978. A refined ultrahigh-vacuum fur.nace for rare gas analysis. J. Phys., Sect. E: Sci. Instrum., 11: 781784. Steiger, R.H. and J/iger, E., 1977. Subcommission on

DISTURBED 4°Ar-39Ar SPECTRA FROM HORNBLENDES

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Turner, G., Huneke, J.C., Podosek, F.A. and Wasserburg, G.J., 1971. 4°Ar-a9Ar ages and cosmic ray exposure ages of Apollo 14 samples. Earth Planet. Sci. Lett., 12: 19-35. Wang, S., Cordani, U. and Kawashita, K., 1990. BSP-1 hornblende, a flux monitor proposed for 4°Ar-39Ar dating on Precambrian rocks. Bull. Liais. Int. Geol. Correl. Prog. Proj. 196-Int. Union Geol. Sci. Subcomm. Geochronol., 8: 31-32. Wartho, J.-A., 199 I. Argon isotope systematics and mineralogy of metamorphic hornblendes from the Karakoram. Ph.D. Thesis, The University of Leeds, Leeds, 255 pp. Wartho, J.-A., Dodson, M.H., Rex, D.C. and Guise, P.G., 1991. Mechanisms of argon release from Himalayan metamorphic hornblende. Am. Mineral., 76: 14461448.