39Ar age spectra and the thermal history of the region

Earth and Planetary Science Letters, 55 (1981) 123-149 Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands

123

[2]

Excess 4°mrin metamorphic rocks from Broken Hill, New South Wales: implications for 4°Ar/39Ar age spectra and the thermal history of the region T. Mark Harrison 1and Ian McDougall Research School of Earth Sciences, Australian National University, Canberra, A.C.T. (Australia) Received January 21, 1981 Revised version received April 27, 1981

`mAr/39Ar age spectrum analyses of samples from Broken Hill, New South Wales, indicate that the region has experienced a complex thermal history following high-grade metamorphism, 1660 Ma ago. The terrain cooled slowly ( ~ 3 ° C Ma - t ) until about 1570 Ma ago, when the temperature fell below about 500°C. Following granitoid emplacement ~ 1500 Ma ago, the region remained relatively cold until affected by a thermal pulse 520±40 Ma ago, causing temperatures to rise to ~ 3 5 0 ° C in some places. During this event, accumulated 4°Ar was released from minerals causing a significant Ar partial pressure to develop. Laboratory Ar solubility data combined with the `mAr/agAr age spectra gives a local estimate of this partial pressure of ~ 10 -4 atm. The region finally cooled below 100°C about 280 Ma ago. 4°Ar/39Ar age spectrum analyses of hornblende, plagioclase and clinopyroxene containing excess 4°Ar are characterized by saddle-shaped age spectra. Detailed analysis of plagioclase samples reveals a complex diffusion behaviour, which is controlled by exsolution structures. This effect, in conjunction with the presumed different lattice occupancy of excess `mAr with respect to radiogenic 'mAr, appears to be responsible for the saddle-shaped age spectra.

1. Introduction

The Broken Hill orebody, the largest known Pb-Zn deposit in the world [1], is contained in the high-grade metamorphic rocks of the Willyama Complex [2] in western New South Wales. The Broken Hill Block [3] comprises regional metamorphic and subordinate igneous rocks of the Willyama Complex which crop out over a triangular region about 110 km from north to south and about 70 km along the base (Fig. 1), centered on the city of Broken hill. The Broken Hill Block is composed dominantly of metasedimentary rocks along with lesser amounts of metavolcanics, granitoid, mafic and ultramafic rocks [3]. Oblique exposure of the teri Present address: Department of Terrestrial Magnetism, 5241 Broad Branch Road, N.W., Washington, D.C. 20015, U.S.A.

rain which was affected by a regional prograde metamorphism of the low- to intermediatepressure/high-temperature type has produced an apparent progressive series of metamorphic zones (Fig. 1) from andalusite-muscovite in the north, to two-pyroxene granulites in the south [4,46,47]. These rocks have experienced a very complex tectonic history. Recent analyses [5,48] have proposed three major deformations, each of which has caused folding as well as the development of planar schistosity. The first two phases appear to be synchronous with the highest grade of metamorphism with the third generation of folding occurring slightly later. Subsequent to these deformations, intrusion of granitoids of the Mundi Mundi type occurred. In addition, the region has been cut by well-defined shear zones [6] where retrograde mineral assemblages are strongly developed. The last recognisable igneous in the block is the intru-

0012-821X/81/0000-0000/$02.50 © 1981 Elsevier Scientific Publishing company

124

sion of ultramafic bodies, some of which cross-cut shear zones [3]. Folded late Proterozoic sediments unconformably overly the Willyama Complex, evidence of a tectonic event not registered by the basement Willyama rocks, unless reflected in movement along the shear zones. Most of the previous geochronological work done in the block has focussed on the high-grade metamorphic rocks which enclose the Broken Hill orebodies. By combining both his own R b / S r whole rock data with that of Pidgeon [7], Shaw [8] calculated the age of the high-grade metamorphic event in these "mine sequence" rocks to be 1660 ___ 10 Ma. (Ages are calculated throughout this study using the decay constants and isotope abundances recommended by Steiger and J/iger [9]. Uncertainties quoted are the level of 1 a.) Using the 2°Tpb/2°6pb whole rock isochron approach on comparable rocks, Reynolds [10] obtained an age of 1660 ~ 16 Ma, equivalent to that of Shaw [8], firmly establishing the time of the culmination of metamorphism. R b / S r measurements on whole rock and muscovite separates from Mundi Mundi type granitoids gave what was interpreted to be an intrusion age of 1490___20 Ma [7]. Muscovite pegmatites from various locations yield an average R b / S r age of 1494 _.+ 25 Ma [7], suggesting that they were emplaced essentially synchronously with the Mundi Mundi adamellites. Muscovite K-Ar ages measured on most rock types throughout the block gave ages from 1381 Ma down to 482 Ma [11,12], probably reflecting both partial radiogenic 4°Ar(4°Ar*) loss in primary samples, and the age of mineral growth in the youngest samples. This last point is supported by a R b / S r isochron age of 484 Ma on retrograde muscovites from the "mine sequence" [7]. With the exception of two anomalously old samples, the 19 biotite K-Ar measurements of Richards and Pidgeon [11] gave ages of 508-4-44 Ma. Results of 14 biotite samples analysed by the R b / S r method yield an isochron age of 520 ~ 35 Ma [7], confirming the existence of a thermal event at ,-, 520 Ma. A K-Ar study of mineral separates from mafic rocks in the high-grade zone was undertaken in the early 1960's by I. McDougall, on samples supplied

by R.A. Binns, in the hope of defining the age of prograde metamorphism. However, the calculated K-Ar ages of some plagioclase and pyroxene separates exceeded the age of the earth and hornblendes ranged up to 2700 Ma. It was apparent that the effects of excess 4°Ar had obscured the primary age information, and the study remained incomplete. As the 4°Ar/39Ar age spectrum technique is capable of resolving excess 4°mr gradients in hornblendes [18], then application of this technique to minerals from Broken Hill could possibly clarify three unresolved problems. These are: (1) what are the 4°Ar* closure ages of hornblende, plagioclase and pyroxene in this terrain; (2) at what time(s) was the excess 4°Ar component introduced; and (3) how variable was the concentration of excess 4°Ar throughout the region? This study reports conventional K-Ar and 4°Ar/39Ar age spectrum data, including the earlier unpublished analyses of I. McDougall, on mineral separates from mafic high-grade metamorphic rocks, an andalusite schist, a Mundi Mundi granitoid, and an ultramafic intrusion. From these analyses we obtain: the 4°Ar* retention ages of hornblende and plagioclase, the timing of the introduction of excess 4°Ar, the quantity of excess 4°Ar introduced and estimates of the partition function of Ar between hornblende and plagioclase. Using the limited laboratory data available for the solubility of Ar in these phases, estimates of the variation of Ar partial pressure (PAr) within the marie granulites have been calculated.

2. Experimental methods Mineral separates of hornblende, plagioclase, pyroxene, biotite, muscovite and apatite were obtained using heavy liquid and magnetic separator techniques. Final purification of hornblende from pyroxene concentrates was affected using a vibrating watchglass off which pyroxenes tend to move, leaving the hornblende impurity behind. Purity of most separates was better than 99%, though hornblende and pyroxene may contain up to 3% of the other phase. All separates used for 4°Ar/39Ar age spectrum analysis were sized between 0.25 and

125

TABLE 1 K-Ar dating results Sample identification

Material

K (wt.%)

40Ar (×10

m mol g - I )

100 4°Ar* / total a°Ar

(%) 79-154 79-154 79-156 79-171 79-171 79-173 79-173 79-173 79-173 79-174 79-174 79-458 79-458 79-459 79-459 79-459 79-461 79-461 79-461 79-462 79-462 79-462 79-462 80-233 G A 608 GA 610 GA 610 GA 610 GA 679 GA 679 GA 680 G A 680 GA 680 GA 681 G A 682 GA 683 G A 684 G A 685 GA 685 GA 907 GA 907 GA 907 G A 907 GA 907A GA 1192 GA 1303

hbl plag musc hbl plag hbl plag cpx mt hbl plag hbl plag hbl plag cpx hbl plag cpx hbl plag- 1 plag-2 cpx hbl bi hbl cpx plag bi musc hbl opx epx hbl cpx hbl hbl hbl opx hbl opx cpx plag hbl bi bi

0.525 0.210 8.960 0.285 0.436 i 0.316 0.0301 i 0.0303 i 0.0326 i 0.450 0.0776 i 0.117 0.0171 i 0.113 0.00980 0.00975 0.112 0.00755 0.00873 0.223 0.0230 0.0241 0.00915 0.331 7.689 0.201 0.012 0.020 4.639 7.694 0.213 0.016 0.011 0.328 0.030 0.500 0.468 0.105 0.007 0.097 0.008 0.010 0.020 0.099 7.740 7.867

J K determined by isotope dilution.

19.06 9.274 333.9 13.06 2.702 17.42 3.848 7.566 6.70 20.59 2.965 5.680 3.451 6.119 4.195 0.937 I 5.927 6.900 1.103 10.30 2.148 2.218 0.4374 3.775 81.91 9.990 2.633 7.785 74.25 306.6 10.34 0.4465 1.985 7.321 2.445 14.21 27.56 11.41 3.526 5.464 0.7411 0.6884 3.924 5.750 83.71 85.90

99.5 95.5 99.8 99.0 93.6 97.1 96.8 69.3 95.4 99.6 95.6 81.4 83.6 97.9 93.3 88.2 97.2 92.7 89.4 93.5 93.5

48.3 72.2 96.6 92.1 94.5 82.1 91.7 98.1 99.4 92.3 52.7 80.7 89.7 79.6 94.6 98.0 88.0 80.2 88.2 60.5 63.6 87.5 91.7 98.4 99. I

Calculated age ± 1 s.d. (Ma) 1390.0± 13.0 1589.0± 48.0 1416.0± 13.0 1628.0± 46.0 1971.0± 18.0 1834.0± 37 2935.0± 26.0 2961.0-+ 40.0 3653.0 + 40.0 1626.0± 44.0 1440.0± 22.0 1691.0± 15.0 3624.0± 33.0 1813.0± 56.0 4848.0 ± 97.0 2534.0± 51.0 1787.0± 38.0 6150.0± 62.0 2918.0 ± 44.0 1637.0± 26.0 2495.0 ± 22.0 2475.0± 22.0 1674.0± 50.0 560.6± 7.0 529.3 ___ 9.0 1714.0± 27.0 3756.0-+ 71.0 4689.0± 87.0 746.2___ 13.0 1482.0± 25.0 1691.0± 29.0 1151.0_+116.0 3450.0 ± 350.0 972.1 ± 22.0 2314.0± 43.0 1167.0± 19.0 1690.0± 28.0 2703.0± 68.0 5123.0± 98.0 1858.0± 192.0 2484.0 ± 85.0 2099.0±218.0 3580.0 ± 276.0 1894.0± 47.0 535.8± 19.0 540.0__ + 20.0

Excess 4°Ar ( × 10-1° mol g - I )

7.1 0.64 2.3 3.6 3.5 6.4 0.98 2.2 0.58 3.3 1.2 4.1 1.1 6.8 0.58 1.9 2.0

1.2 8.7

1.1

2.8 6.8 1.2

3.9 1.4

126

0.15 mm in diameter except for GA 683 hornblende which was sized to between 0.15 and 0.09 mm in diameter. Argon extractions and analyses were performed using the standard high-vacuum fusion method and mass spectrometer techniques [18]. Potassium was measured for all samples on an IL 433 flame photometer using a sample solution containing a sodium-lithium buffer. Samples containing less than 0.1% K and others that gave poor agreement between triplicate flame photometer results were analysed by isotope dilution using a 41K-enriched spike. Results of K-Ar measurements are given in Table 1. Irradiation of samples for 4°Ar/39Ar dating was performed in the core of the TRIGA reactor, U.S. Geological Survey, Denver, Colorado. The fast neutron flux in this position is about 3.2 × 1013 n c m - 2 s - I at 1 MW power and the fast to thermal flux ratio is 1.17 [13]. The samples were placed in flat-bottomed, 5-cm-long quartz tubes which were filled to a height not exceeding 0.5 cm, and irradiated in a sample assembly containing four levels. Each level contains one central position radially surrounded by six geometricallly equivalent positions. Flux monitors were placed in the central position and in one outside position on each level. Samples and flux monitors were then irradiated for 100 MW hours equivalent to a fast neutron dose of about 1.1 × 1019 n cm -2. Intralaboratory standard 77-600 hornblende (414.2 ___3.7 Ma) was used as the flux monitor. As expected, the measured 4°Ar/39Ar ratio of the flux monitors on different levels were significantly different (Table 2) from one another because of vertical fluence gradients. However, with the exception of the first level, a significant difference was also found between flux monitors on the same level, an TABLE 2 4°Ar/39Ar ratios of flux monitor Irradiation level

Outside position

Central position

1 2 3 4

12.202 (0.08%) 11.277 (0.07%) 11.890 (0.119~) 14.195 (0.099~)

12.191 (0.11%) 11.518 (0.11%) 12.113 (0.11%) 14.409 (0.10%)

effect not previously observed [13], indicating the existence of a measurable horizontal gradient. The average difference in 4°Ar/39Ar ratio of the two monitors is 1.8%, with the centrally placed monitor consistently yielding the higher ratio. As individual flux monitor ratios can be duplicated to better than 0.3%, the difference observed is real and indicates that the central position is receiving a lower fast neutron flux. It was felt that the outer flux monitor for each of the levels exhibiting this difference was the best neutron flux integrator for the unknown samples, and was accordingly used in the age calculations. The average 37Ar/39Ar ratio of all eight flux monitors is 16.12 (±0.3%). This value, together with the K / C a ratio of 0.0373 ~ 0.0001 for 77-600 hornblende yields an expression for the K / C a ratio of the unknowns as: K / C a = 0.601 × (39Ar/a7Ar)u~nown

(1)

To determine correction factors for interfering isotopes produced by nuclear reactions during irradiation, particularly in view of the unfavourable K / C a ratios of many of the samples, CaF2 and K 2 S O 4 samples were included in the irradiation package. Unfortunately, the quartz tube containing the KESO4 leaked water inside, making the salt unusable. As the majority of the irradiated samples have very high 4°Ar/39Ar ratios, a precise (4°Ar/39Ar)K correction is not critical to calculated ages. We have used (4°Ar/39Ar)K =6.0>( 10 -3 which is the average of three determinations made i n TRIGA by Dalrymple et al. [13]. Measurements of the irradiated CaF2 yield correction factors for (36Ar/37Ar)Ca = 2.31 × 10-4 and (39Ar/37Ar)ca = 5.81 × 10-4. Results of blank corrected 4°Ar/39Ar analyses are given in Table 3. Techniques used in dating apatite by the fission track method are the same as those described by Naeser [14]. The single apatite analysis was obtained using the external detector method, and the age calculated using a decay constant for 238U fission of 7.03 × 10 -17 a -1. The result is given in Table 4.

127

3. Results and discussion

Conventional K-Ar analyses of 46 samples of hornblende, plagioclase, pyroxene, biotite, muscovite and magnetite are given in Table 1, and sample locations are shown in Fig. 1. Three biotite analyses (GA 608, GA 1192, GA 1303) give ages of 535 ± 5 Ma, consistent with the early Palaeozoic resetting event defined by Evernden and Richards [15] and Richards and Pidgeon [11] of 508 ± 44 Ma. A fourth biotite (GA 679) from the northern end of the Broken Hill Block (Fig. 1) gives an age of 746 Ma. However, the low potassium content of this sample (4.6%) indicates a high degree of alteration, complicating an interpreta-

Fig. 1. Map of the Broken Hill Block showing locations of samples taken for K-At analysis. The two insets show the distribution of samples from the mafic granulites near the North Broken Hill mine (fight) and at Black Bluff (left). The bodies identified by crosses are granitoids of the Mundi Mundi type. The isograds shown are taken from Phillips [4].

tion invoking excess 4°Ar. The coexisting muscovite from this sample yields an age of 1482 Ma which is essentially identical to both the R b / S r muscovite ages from pegmatites and the R b / S r whole rock and muscovite ages for the Mundi Mundi granitoids [7]. A K-Ar muscovite age for the Mundi Mundi granite (79-156) gives a somewhat younger result of 1416 Ma; this muscovite subsequently was analysed by the 4°Ar/39Ar age spectrum method. Results of this experiment are given in Table 3 and shown in Fig. 2. The age spectrum is somewhat complex but exhibits a plateau segment over the last 50% of 39At release at 1507___7 Ma. The R b / S r whole rock and muscovite model ages for the Mundi Mundi granite are virtually concordant at 1490 -~ 20 Ma and 1495 ___20 Ma, respectively [7], indicating rapid cooling after intrusion. For the K-Ar muscovite system to begin accumulating 4°Ar* essentially consequent on emplacement requires that the ambient temperature be lower than about 350°C, the inferred closure temperature of 4°Ar* in muscovite [16]. Hornblende from an ultramafic intrusion (80233) which cuts across a shear zone (Fig. 1) gives a K~Ar age of 561 Ma (Table 1). For reasons that will become clear later, this age can neither be an artifact of 4°Ar* loss nor excess 4°Ar introduction. Although the result is preliminary, it seems probable that the numerous related ultramafic bodies were intruded roughly synchronously with the early Palaeozoic thermal event and perhaps played a role in providing rapid heat transfer high into the crust. The remaining samples are all from mafic granulite grade metamorphic rocks and can be divided conveniently into two groups based on K-Ar age. All samples taken from south of the city of Broken Hill (Fig. 1) yield hornblende and pyroxene ages in excess of 1600 Ma (79-171, 79-173, 79-174, 79-458, 79-459, 79-461, 79-462, GA 610, GA 680, GA 681, GA 682, GA 685, GA 907). In contrast, samples 79-154, GA 681 and GA 683 taken from north of the city (Fig. 1) give K-Ar hornblende ages of 1390, 972 and 1167 Ma, respectively. For the present, the discussion will focus on these three samples. The difference in hornblende ages between these two districts is

128

undoubtedly related to both different thermal histories and local variations in the ambient pressure of regionally degassed 4°Ar*. In the hope of obtaining a measure of the relative magnetides of

these two effects, 4°Ar/39Ar age spectrum analyses were carried out on 79-154 and GA 683 hornblendes. The release pattern for 79-154 hornblende,

TABLE3 ~Ar/39Aragespect~m results Step Temperature (°C)

40

39 Ar/

79-458 h o r n b l e n d e TF 5 370 470 570 670 770 820 880 910 940 950 990 i000 i000 i000 1000 I010 1020 FUSE

90.37 222.3 410.6 703.6 557.7 300.2 206.5 ' ~59.3 94.69 80.84 77.41 76.69 80.77 81.23 81.37 82.82 83.06 84.64 92.95

79-458 p l a g i o c l a s e TF s 330 430 460 700 850 880 910 940 980 1020 1050 1090 1120 1150 1200 1230 1250 FUSE

377.2 439.1 409.0 308.2 26.77 18.63 17.31 33.36 32.03 37.72 54.94 83.38 89.96 141.5 250.5 459.5 653.7 639.4 861.5

79-459 h o r n b l e n d e TF s 270 350 410 470 530 590 650 700 740 790 840 880 890 900 910 930 970 1000 1000 I010 1020 1050 1060 1090 FUSE

79.82 651.7 468.3 489.4 856.8 671.3 469.8 366.7 285.9 201.5 182.7 192.5 104.0 80.14 70.52 65.58 68.53 68.39 73.26 76.15 79.96 81.39 85.41 107.5 114.2 168.3

Ar

1 37Ar 39Ar ~ 3 * / 6Ar/39Ar

39At * K (xl0 "14 mole|)

Cum~atlve ~'Ar (%)

40 At* 40At t o t a l (%)

40Ar/39ArK

Apparent Age (Ma)

Standard ~ dev. (Ma)

(J = 0.018182) 32.68 12.20 14.01 24.91 36.66 44.04 40.11 32.84 32.24 32.25 32.53 32.55 32.28 32.24 32.37 32.43 32.45 32.49 38.50

0.03108 0.2420 0.6645 2.576 2.777 0.7737 0.1634 0.05551 0.2609 0.01316 0.01189 0.01321 0.01024 0.01020 0.009982 0.01369 0.01271 0.01042 0.03636

~

52.8 0.831 0.430 0.355 0.600 1.88 2.45 10.6 27.3 66.9 39.1 51.9 129. 67.0 16.9 125. 7.68 71.4 25.6

0.156 0.236 0.303 0.416 0.768 1.23 3.22 8.33 20.9 28.2 37.9 62.3 74.9 78.0 80.4 81.8 95.2 100.0

92.1 64.7 49.6

85.027 152.66 216.95

1686. 2397. 2883~

ii. 50. 76.

24.4 76.5 90.6 93.8 97.7 98.0 97.6 98.7 98.8 98.4 97.7 96.7 98.8 90.9

76.533 164.74 147.95 90.880 80.661 77.573 77.407 81.471 81.949 82.171 82.539 83.080 85.382 86.766

- 6 1573. 2499. 2355. 1760. 1629. 1587. 1585. 1640. 1646. 1649. 1655. 1661. 1690. 1708.

28. 14. 13. ii. Ii. 11. ii. 11. 11. ii. Ii. ii. ii. ll.

5.21 16.5 20.5 25.2 35.4 42.5 46.7 51.5 55.2 59.4 63.0 67.3 71.9 77.0 81.0 84.6 86.1 i00.0

79.8 90.7 91.7 90.3 64.7 71.7 74.8 80.9 81.7 76.1 81.6 87.6 88.7 87.8 91.8 91.9 92.1 90.7 93.2

376.62 405.94 411.90 312.96 23.645 17.302 18.840 38.859 36.954 40.011 59.827 95.897 103.93 157.41 301.47 556.78 797.09 772.50 1052.0

3999. 4120. 4143. 3704. 747.3 575.3 618.5 1103. 1063. 1128. 1502. 2030. 2130. 2680. 3646. 4639. 5244. 5191. 5721.

16. 18. 17. 17. 24. 14. 18. 15. ii. 44. 13. 19. 16. 17. 16. 18. 18. 21. 17.

0.062 0.149 0.175 0.208 0.241 0.280 0.410 0.552 0.751 0.995 2.10 9.12 i1.8 16.6 "17.7 20.0 32.3 76.8 84.5 87.5 90.4 99.0 99.6 99.7 100.0

95.8 28.3 70.1 56.0 69.6 72.1 67.8 70.9 57.0 63.3 71.3 84.8 94.2 92.3 97.5 92.5 93.2 98.2 98.7 98.2 96.2 97.0 97.6 98.4 79.3 54.3

80.042 210.43 374.79 3~5.99 729.75 629.76 444.83 308.39 195.07 151.70 151.14 171.51 101.01 77.653 71.501 66.602 67.567 69.138 74.163 77.265 80.599 82.696 86.438 113.40 123.11 107.02

1787. 3059. 3949. 4038. 5050. 4802. 4226. 3641. 2948." 2593. 2588. 2764. 2062. 1753. 1662. 1586. 1601. 1626. 1702. 1747. 1795. 1824. 1876. 2207. 2313. 2134.

Ii. 192. 56. 123. 143. 119. 61. 32. 54. 35. 42. 15. 12. 12. ii. 17. 12. ii. Ii. ii. 12. 12. 12. 16. 29. 21.

(J = 0.021706) 341.7 20.17 148.7 170.6 323.6 391.1 417.8 416.5 401.2 370.5 356.5 351.4 353.7 334.0 395.4 407.1 414.6 412.3 406.2

0.3338 0.1329 0.1448 0.1284 0.1004 0.1081 0.1068 0.1094 0.1051 0.1070 0.1077 0.1051 0.1067 0.1267 0.1533 0.2104 0.2601 0.2710 0.2893

73.2 3.10 6.75 2.35 2.79 6.07 4.27 2.48 2.86 2.23 2.50 2.11 2.56 2.74 3.07 2.35 2.14 0.896 8.30

(J = 0.021151) 42.11 12.17 12.38 28.90 49.12 91.35 156.7 114.0 99.81 86.65 63.08 42.43 39.49 39.35 39.28 39.60 39.50 39.32 39.64 39.92 40.49 40.62 49.82 126.9 134.6 89.43

0.01557 1.501 0.3284 0.3451 0.5118 0.2748 0.2577 0.2929 0.3686 0.2144 0.1401 0.09514 0.02713 0.02350 0.01127 0.01085 0.01762 0.01186 0.01185 0.01149 0.01359 0.01555 0.01634 0.03778 0.03336 0.2468

58.7 0.227 0.316 0.0677 0.120 0.122 0.0677 0.474 0.517 0.728 0.891 4.02 25.6 9.62 17.6 4.11 8.37 44.8 162. 28.4 10.8 10.5 31.7 1.86 0.638 0.0102

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133

TABLE 3 (continued)

Step Temperature (°C)

40Ar/39Ar I 37Ar/39Ar 2 36Ar/39Ar ~ (xl0 -4 )

79-156 m u s c o v i t e TF S 320 420 490 560 600 620 630 660 690 730 730 730 770 790 800 820 830 850 890 910 960 970 1010 10207 1070 1090 FUSE

0.006723 0.1686 0.1391 0.02695 0.02312 0.005314 0.01114 0.008788 0.009668 0.0007992 0.002093 0.009231 0.00993 0.02304 0.006428 0.01068 0.008419 0.01271 0.007964 0.009126 0.003183 0.002305 0.001914 0.01244 0.1013 0.2454 0.4382

l Corrected

for line b l a n k

2 Corrected

for d e c a y of 37Ar. by manometric

Uncertainties s TF = total 6 Negative 7 Gas

Cumulative 39At (%)

40At* 40At total (%)

40Ar/39ArK

Apparent Age (Ma)

Standard ~

1406. 1054. 1036. 1005. 952.1 987.0 1036. 1078. 1358. 1361. 1290. 1240. 1283. 1346. 1407. 1456. 1483. 1501. 1511. 1514. 1512. 1500. 1497. 1511.

9.8 15. 18. 9.9 8.0 8.5 8.1 8.4 9.6 9.6 17. 9.0 9.2 9.5 9.9 10. I0. I0. 10. 14. 10. i0. 10. 10.

1536. 1536. 1316.

ii. 15. 23.

0ev.

(Ma)

(J = 0.022892)

54.02 48.08 39.07 34.28 31.29 32.37 34.76 36.30 49.34 49.23 45.78 43.21 45.36 48.43 51.60 54.47 55.75 56.73 57.28 57.62 57.34 56.62 56.57 57.40 65.11 69.73 70.43

3 Obtained

* 39 A r K (xl0 -14 moles)

quoted

fusion

83.73 454.2 175.9 58.07 30.91 18.61 28.91 19.14 8.865 0.9847 4.665 0.6972 0.2817 0.2297 0.1603 7.156 0.6305 0.7127 0.7056 5.202 0.1235 0.03322 2.805 3.821 218.2 374.8 796.7

(i x 10 -14 mol

measurement

745. 3.03 6.85 14.5 37.9 83.7 70.6 43.1 218. 723. 638. 231. 147. 112. 146. 161. 141. 215. 184. 228. 389. 455. 397. 310. ~lO. 7.92 4.63 3.10

0.061 0.199 0.492 1.25 2.94 4.36 5.23 9.63 24.2 37.1 41.7 44.7 47.0 49.9 53.1 55.9 60.2 63.9 68.5 76.3 85.5 93.5 99.6 99.7 99.8 99.9 100.0

95.4 67.5 83.6 93.1 96.3 97.9 97.1 97.8 99.4 99.9 99.7 99.9 99.9 99.8 99.8 99.6 i00. 100. 100. 99.7 100. I00. 99.8 99.8 . 88.4 81.6 63.7

.

51.54S 34.664 33.882 32.558 30.372 31.818 33.904 35.727 49.075 49.199 45.636 43.182 45.348 48.420 51.593 54.258 55.729 56.709 57.259 57.463 57.327 56.612 56.483 57.282 . 58.670 58.675 46.927

.

40Ar).

of gas c a l i b r a t e d

using

38At spike.

at the level of one s t a n d a r d error.

split.

ages c a l c u l a t e d .

fraction

lost d u e to v a c u u m

leak.

Concentration

estimated

shown in Fig. 3 conforms well to a model of diffusion loss [17] for the first two-thirds of the gas evolved, rising smoothly from an apparent outgassing age of about 960 Ma to an age of 1493 Ma before falling over the last 30% of gas release to an age of about 1370 Ma. The first five or so lowtemperature steps, corxesponding to about 1% of the total 39Ar, defines a profile characteristic of very minor absorption of excess 4°Ar. The diffusion loss segment indicates about 15% 4°Ar* loss at or since 950 Ma. A key to the unusual behaviour of the last third of gas release may be found in the K / C a ratios calculated form the

from total

f u s i o n age and c o n c e n t r a t i o n

data.

39Ar/37Ar ratio of each step. During the asymptotic rise in apparent age from 1400 Ma to the peak value of 1493 Ma, the K / C a remains constant over 11 steps at 0.0683 __+0.0002, indicating only one phase is contributing to the desorption of 39At, and thus 4°Ar*. However, as the age begins to decrease, so too does the K / C a decrease to a value of between 0.0004 and 0.0444. This indicates that at the point at which the age begins to decrease, a regime begins to outgas that contains a significantly lower K / C a ratio than the rest of the sample, possibly due to recoil effects or exsolutioncontrolled concentrations of Ca (see section 6).

TABLE 4 Apatite fission track dating results Sample

Material

nX 1015 (n cm - 2 )

Ps (cm - 2 )

Pi (cm-2)

N

r

Age I (Ma -+- 1 o)

79-156

apatite

2.14

6 . 6 8 x 106 (1440) 2

2 . 9 7 × l06 (640) 2

7

0.924

282 -4- 14

n = neutron dose; Ps = spontaneous track density; Pl =induced track density; N = number of grains counted; r = correlation coefficient. I ~F=7.03×10-17 a-l. 2 Brackets show number of tracks counted.

134

17t

I

,

I

,

I

,

,

,

,

,71.61.5t

16

1.5 ~ d ~ r ~ 1.4 1.5" Ga 1.2" I.I

6 F t--

I

!

I

I

I

I

I

I

|

GA 6 8 5 hornblende

1.5Ga

t21.17

1.0-~-

muscovite

0 ,9 -

0"80

' 2_'0 ' 4~0 ' 6b ' 8'0 ' I00 percent 39Ar released

08 0

20 percent

40 3~r

60

80

I00

released

Fig. 2. 4°AE//a9Ar. age spectrum of 79-156 muscovite from a Mundi Mundi granitoid. Note the plateau segment over the last 50% of gas release at 1507 Ma.

Fig. 4. 4°Ar/39Ar age spectrum for GA 683 hornblende. The complexity of the spectrum may in part be due to the fine grain size used in this analysis (see text).

The coexisting plagioclase from this sample gives a K-Ar age of 1609 Ma. It is unreasonable that a feldspar should give an older age than hornblende unless it has been contaminated by excess 4°Ar: It will be shown in later discussions that this is indeed the case. The 40./~/39Ar age spectrum of GA 683 (Fig. 4) is even more complex than 79-154 in that it does not conform at all to models of diffusive redistri-

bution of 4°Ar. This may be a result of the finer grain size used for the analysis (0.15-0.09 mm), which may have obscured the original 4°Ar distribution. Alternatively, the spectrum may be a combination of an excess 4°Ar profile imposed on a plateau at about 1100 Ma along with an effect similar to that just described for 79-154 hornblende. In any case, this spectrum does not supply any unambiguous age information. The difference in K-Ar hornblende ages of 79-154, GA 681 and GA 683 may be due to differing diffusion radii for 4°Ar, different temperature histories, or a combination of the two. It is worth noting that both the age spectra for 79-154 hornblende and 79-156 muscovite show no significant indication of having responded to the 520-Ma event which reset the biotite K / A r and R b / S r systems. However, the possibility of the muscovite sample being slightly outgassed during this time and later contaminated by a small amount of excess 4°Ar which obscures the --~ 520-Ma outgassing age cannot be ruled out. Either way, this information provides a useful constraint on the maximum temperature of that event. In order that the biotite K / A r and R b / S r systems be reset, a temperature somewhat in excess of ,~ 300°C [16] is thought to be required. However, had the temperature far exceeded about 350°C, the age spectrnm of 79-154 muscovite would not be expected

1.7

I

I

I

I

|

I

I

I

I

1.6-

1.51.4 1.3

Ga

t.zI.I

1.0 0.9 Q8

79-154 hornblende

0

' 4'0 ' 6b ' 8b percent 39At released

2b

I

100

Fig. 3. 4°Ar/39Ar age spectrum of 79-154 hornblende. The decrease in age over the last 30% of gas release is correlated with a change in the K / C a ratio (see text).

135

to still retain the crystallization age of about 1500 Ma. Results of K-Ar analyses of minerals from south of the city of Broken Hill are significantly different from those just discussed form the north. Calculated age of hornblendes, plagioclases and pyroxenes range from 1627 to 2703 Ma, 1488 to 6705 Ma, and 1981 to 5123 Ma, respectively. Clearly these samples have been contaminated by excess 4°Ar during some geological event or events, but these data do not in themselves provide any useful information regarding this effect. I n the hope of elucidating the age of 4°Ar* retention in minerals and thus the quantity of excess 4°At in the various phases, an extensive program of 4°Ar//39Ar age spectrum measurements was made on coexisting hornblende, plagioclase and pyroxene from two granulite bodies, one adjacent to the North Broken Hill mine and the other at Black Bluff (Fig. 1). Both these bodies are interpreted as being metamorphosed marie volcanics [3]. For the present, the discussion will focus on the marie granulite near the North Broken Hill mine. This rock occurs (Fig. 1) in a steeply dipping, 15-m-wide, east-west-trending layer which outcrops for about 400 m along strike. The rock contains hornblende, bytownite (An7v), clinopyroxene, orthopyroxene (minor) and an opaque mineral. The layer is bounded to the north and south by quartz-feldspar-biotite gneiss. Three samples were taken in the marie granulite perpendicular to the contact with the gneiss (Fig. 1); 79-173 comes from 1 m away from the south contact, 79-171 comes from {he middle of the section, and 79-174 was taken 4 m away from the north contact. An '~Ar//39Ar age spectrum analysis of 79-171 hornblende (Fig. 5) shows a complex release pattern, typical of a mineral contaminated on its periphery by excess ~Ar [18]. However, there are differences in detail from previously published age spectra on hornblende known to contain excess 4°Ar. Initially high ages decrease rapidly onto a six-step plateau at 1577 __+3 Ma in the middle portion of release, corresponding to about 20% of the gas evolved. These six increments are statistically indistinguishable [19] from one another, but distinctly different to the adjoining steps. Over the

0

I

2

2

hor0b,ende

1.7¸

1.6

I

1.5 0

20 40 60 80 percent 3'Ar releosed

I00

Fig. 5. 4°Ar/39Ar age spectrum for 79-171 hornblende from the mafic granulite near the North Broken Hill mine which contains a steep, diffusion-controlled gradient of excess 4°Ar near the margins of the crystals. The central portion of gas release yields a plateau age of 1577 Ma.

remaining 40% of gas release, the ages begin to rise reaching geologically unreasonable ages in excess of 1660 Ma. Isochron treatment (Fig. 7) of the plateau segment [20} yields an age of 1573 + 5 (MSWD = 1.44) and an initial 4°Ar/36Ar ratio of 3 4 8 _ 58 (Fig. 6), indicating no uniformly held excess 4°At component. The age indicated by the plateau and isochron analyses likely represents the i

I

I

i

i

i

/

I

i

79-171 hornblende/

4,OAr/,,Ar

x I0~

2I

0

/R~= 548±58

o

,

(MSWD=I.44)

4

8

3SAr/~Smr x io z Fig. 6. ]sochron plot of the plateau segment of 79-171 hornblende age spectrum. The good fit to a line corresponding to an age of 1573 Ma as well as the essentially atmospheric intercept, indicates that this portion of gas release has not been affected by excess 4°At.

136

,,,,,

o,

,

,

1 h°rnblenjU Go

a.7-t 1.6 1.5

0

,

percen~39Arreleased

z'o

'

'

6'o

'

8'o

' Ioo

Fig. 7. 40Af/39AI" "age spectrum for 79-173 hornblende from the mafic granulite neat the North Broken Hill mine. Note the higher initial age than for 79-171 hornblende and the correspondingly older minimum age of about 1660 Ma.

time at which 4°Ar* diffusion effectively ceased from this sample owing to a regional decline in temperature below about 500°C [18]. The plateau portion from this sample exhibits a constant K / C a of 0.0379 4-0.0002 which decreases to 0.0311 at 99% of 39Ar release and drops still further over the last one percent to a value of 0.005. The behaviour of the age spectrum over this segment is opposite to that observed in 79-154 hornblende where the ages did not rise but instead decreased by about 10%. It is possible that there is a class of lattice sites which have experienced the near grain boundary effects of 4°Ar* loss or excess 4°Ar introduction, but for reasons developed later, do not degas until very high temperatures are reached. Should this occur, the high-temperature release segment of a sample peripherically contaminated by excess 4°Ar would give ages significantly older than the plateau value, yielding a saddle-shaped age spectrum. For a sample that had experienced 4°Ar* loss (e.g., 79-154 hornblende), the opposite behaviour would be expected. This correlation between the high- and low-age regimes is true of both 79-154 and 79-171 hornblendes as well as the remaining five hornblendes analysed and seems to be typical of meta-

morphic amphiboles containing non-uniform distributions of 4°Ar*. For these samples, an estimate of the age of 4°Ar* retention will only be obtainable from the central portion of the age spectrum. If the 4°Ar* retention age of 79-171 hornblende is taken to be 1575 Ma, then the amount of excess 4°Ar present in this sample can be calculated to be 6.4 X 10 -~l mol g-~ (Table 1). It is clear from the age spectrum (Fig. 5) that the sample did not equilibrate with the ambient PAr but instead contains only a fractional amount of this gas in a diffusion controlled profile near the grain boundaries. This is the result of the sluggish rate of 4°Ar diffusion at relatively low temperatures. Even though the samples have not equilibrated with the ambient PAr, in order that there be any excess 4°Ar within the grains requires that the edge of the crystals be in equilibrium with the excess 4°Ar partial pressure. Thus, extrapolation of the observed diffusion gradients to zero percent 39Ar release should give an estimate of the concentration of 4°Ar in the samples at equilibrium. This extrapolation from the age spectrum of 79-171 hornblende yields an age of 4 Ga which corresponds to an apparent concentration of excess 4°Ar of ,-~6.0X10 -9 mol g-~ (Table5). The 4°Ar/36Ar composition of this gas phase can be estimated from this step to be > 1500. Results of 4°mr/39Ar age spectra analyses of hornblende and plagioclase form 79-173, the sample closest to the gneiss contact, are shown in Figs. 7 and 14. The hornblende spectrum is similar in shape to that of 79-171 hornblende (Fig. 5), but the amount of excess 4°Ar present is substantially greater, about 3.6 X 10- ~0 mol g-]. The portion of the age spectra comparable to the plateau segment of 79-171 hornblende does not yield a plateau but instead decreases to a minimum at 1664 Ma before rising again. Although the poor resolution of this analysis has probably obscured a lower age portion present, it is thought that as the concentration of excess 4°Ar increases, the probability of resolving an unaffected region becomes more remote. This observation is borne out in all subsequent analyses. The concentration of excess 4°Ar that would be present at equilibrium in this sample is estimated from the age spectrum to be 1.6X 10 -s tool g-~

137 TABLE 5 Calculated "equilibrium" excess 4°Ar concentrations Sample

Material

Indicated age (Ga)

K ~ (wt.%)

Apparent excess 4°Ar 2 ( × 10 - ~0 mol g - 1)

79-173 79-173 79-173 79-171 79-458 79-458 79-459 79-459 79-461 79-461 79-462 79-462

mt plag hbl hbl hbl plag hbl plag hbl plag hbl plag

~ 3.7 5.8 5.3 4.0 ~3.5 4.2 5.5 7.0 5.5 7.3 3.3 3.5

0.0326 0.0303 0.316 0.285 0.117 0.0171 0.113 0.0105 0.112 0.00873 0.223 0.023

~ 6.4 22.0 163.0 60.0 ~ 17.0 4.8 66.0 15.0 65.0 15.0 27.0 4.0

i Values from Table 1. 2 Calculated on the basis of 4°Ar retention ages for plagiocalse and hornblende of 520 Ma and 1570 Ma, respectively.

(Table 5) and the 4°Ar/36Ar ratio of this phase to be > 7000. 4°Ar/39Ar analysis of the coexisting plagioclase also yields a saddle-shaped age spectrum (Fig. 14). The significance of this result along with the other four plagioclase age spectra measured is discussed at length in a later section. The second mafic granulite layer chosen for detailed study is at Black Bluff (Fig. 1). This rock is well exposed, accessible and was described in some detail by Binns [21] in his petrological study of the region. The minerals present in the four samples investigated (79-458, 79-459, 79-461, 79462) are hornblende, bytownite ( A n 7 4 ) , clinopyroxene, orthopyroxene and ilmenite. Sampling was mainly parallel to the layer although different samples occur at varying distances from the steeply dipping contact with the encompassing quartzfeldspar-biotite gneiss. Sample Ga 685 (Table 1) comes from the western outcrop at Black Bluff but a precise location is not known. Age spectra of the four hornblende samples (Figs. 8, 9, 10, 11) from Black Bluff are comparable in shape and detail to those described from the mafic granulite near the North Broken Hill mine. Once again, the resolution of a plateau segment is inversely proportional to the amount of excess 4°Ar present. The sample least affected by excess 4°Ar, 79-462, contains a poorly defined plateau

segment at 1570__+ 17 Ma (Fig. 11). This age is indistinguishable from the 1575-Ma age obtained from 79-171 hornblende giving some support to this being the time of 4°Ar* retention in hornblende in this region. In order of increasing excess 4°Ar contamination, 78-458 (Fig. 8) yields a minimum in its age spectrum over two steps of 1 5 8 6 ± 8 Ma, 79-459 (Fig. 9) decreases to a

0 2 2 I

J O ~

hornblende

Ga

1.6

1

0

.

5 ~ 20 40 60 80 percenl' 3~Arreleosed

I00

Fig. 8. 4°Ar/39Ar age spectrum for 79-458 hornblende from the mafic granulite at Black Bluff. The third and fourth steps of the age spectrum yield negative ages, thought to be the result of migration of reactor produced 36At (see text for discussion).

138

024

68 I

I

I

0 1 2

I

3

h7ogrr~ble£~

Ga

Ga

1.7'

1.7 ¸

1.6,5

o

~ - -

I

1.6

I

I

I

zb 4'0 8'0 percent 39Arreleased

i

ioo

1.5

0

20 40 60 80 percent 39Arreleased

I00

Fig. 9. 4°Ar/J9Ar age spectrum for 79-459 hornblende from the mafic granulite at Black Bluff.

Fig. 11. 4°Ar/39Ar age spectrum for 79-462 hornblende from the mafic granulite at Black Bluff.

minimum at 1586 Ma, and 79-461 (Fig. 10) decreases to 1606 Ma before rising again. In the latter case, the resolution in the spectrum over the segment where a plateau might be expected is poor and is probably a contributory cause for the somewhat older minimum age. In all cases, the rise in apparent age following the plateau of minimum age corresponds to a significant decrease in the K / C a ratio. This rise in age becomes more pro-

nounced the greater the amount of excess 4°Ar present, these data are consistent with an age of 4°Ar* retention in hornblende of 1575 Ma, indicated by the plateau and isochron analysis of 79-171 hornblende (Figs. 5 and 6). Assuming a closure age of 1575 Ma, the concentrations of excess 4°At in these samples have been calculated and are given in Table 1. The apparent equilibrium concentrations of excess 4°Ar obtained for these samples by the method already described have been calculated and are given in Table 5. An estimate of the temperature required to produce the excess 4°At gradients observed in the hornblendes can be calculated from the age spectra. it can be seen in Figs. 5 and 7-11 that the bulk of the excess 4°Ar is contained within the first 0.5% of 39Ar release. For spherical geometry and an average grain radius of 0.01 cm, this corresponds to an average penetration distance of Ar into the hornblende of about 0.2 #m. Combining the well-known relationship for calculation of approximate transport distances, £ = ( D t ) ~/2, with the Arrhenius equation, D = D o e x p ( - E / R T ) , where D is the diffusion coefficient; the average diffusion distance of excess 4°Ar near the grain surface, £ = 2 × 10 -5 cm; estimated duration of the thermal event, t = 10 Ma; activation energy, E = 63.3 kcal mol -I [18]; frequency factor, Do =

024

3dL_

II

hornblende

Ga2

:iii 1.5

0

J 20 40 60 80 percent 3'Ar released

I00

Fig. I0. 4°~r/39~ age spectrum for 79-461 hornblende from the mafic granulite at Black Bluff.

139 I

0.022 cm2 s -t [18]; gas constant, R = 1.987 cal m o l - ] deg- 1; gives a temperature estimate for this event of:

5-

(2)

4,

Tel ~ ~

-E/R

ln(~2/t.Do)

~ 350°C

This temperature is consistent with the estimate obtained form considerations of 4°Ar* and 87Sr* retention in micas. Before proceeding, a brief mention of an unusual aspect of the age spectrum for 79-458 hornblende (Fig. 8) is in order. The initial two gas fractions analysed, which yield unreasonably high apparent ages (2.4 and 2.9 Ga), are followed by two steps with measured 4°Ar/36Ar ratios of 273 ± 2 and 190 ± 1, significantly lower than atmospheric Ar. It is difficult to explain this effect without invoking a rather improbable mechanism. The likeliest of these is that 36Ar produced in the reactor by a (n,na) reaction of 4°Ca has migrated relative to 37mr produced by a (n,a) reaction on 4°Ca, with the result that the (36Ar/37Ar)ca correction factor used for those two steps did not properly account for all the reactor produced 36Ar, giving rise to very low 4°Ar/a6Ar ratios. Two age spectra of clinopyroxene separates from 79-459 and 79-462 (Figs. 12 and 13) have complex release patterns with the oldest ages occurring late in the gas release and accompanied by K / C a ratios that vary by a factor of 80-120 over the pattern. These results are broadly similar to an analysis reported by Lanphere and Dalrymple [22] for an igneous pyroxene (L3) known to contain excess 4°Ar. The age spectrum for that sample decreased from an initially high age of 417 Ma to a minimum which approaches the known crystallisation age, before rising to an age in excess of 4 Ga. In the case of L3, as with 79-459, the K / C a ratio decreased by more than two orders of magnitude during the course of the gas release. The implication we draw from these data is that the excess 4°Ar is being held dominantly in Ca related sites which only begin to outgas at high temperatures. In detail, the two samples analysed in this study have less regular spectra than L3. The ages tend to rise and fall twice before the final climb to the maximum ages which exceed 4 Ga. For the sample containing the lower concentration of excess 4°Ar, 79-462, two ages are associated with the

I

I

I

I

I

I

I

I

79-459

clinopyroxene

Go

2

I

o

!

2.O

i

4'o

1

do

I

8b

i

ioo

percent 3~r released

Fig. 12. ~Ar/39Ar age spectrum for 79-459 clinopyroxene from the mafic granulite at Black Bluff.

minima in the spectra, ,-~ 500 Ma and 1490 Ma. The --~ 500-Ma age may possibly be real and related to the known event at that time and the 1490-Ma age may be a maximum estimate of the 4°Ar closure age of pyroxene in this terrain. Age spectra have been determined on four plagioclase samples form Black Bluff and one sample from the granulite near the North Broken Hill mine. Results of these analyses are shown

I

I

I

I

I

I

I

I

!

er

4

79-462 clinopyroxene

5 Go 2

0

0

zo

40

6'0 ' 8'o 'lOC

percent 39Arreleased Fig. 13. 4°Ar/39Ar age spectrum for 79-462 clinopyroxene from the mafic granulite at Black Bluff.

140

together in Fig. 14 for purposes of comparison; calculated K / C a ratios for those samples are shown in Fig. 15. All samples exhibit the saddleshape spectra characteristic of feldspars which contain excess 4°Ar [22]. From their 4°Ar/39Ar age spectrum studies of minerals known to contain excess 4°Ar, Lanphere and Dalrymple [22] concluded that the minimum in the age spectrum approached but did not reach the known geological age. This probably depends on the resolution of the spectrum and the amount of excess 4°Ar present. Inspection of Fig. 14 reveals a relationship between the degree of contamination of excess 4°Ar and the minimum age in the spectrum. Although an age spectrum is not the ideal format to present these kinds of data, as variations in potassium content between samples will also affect the relative magnitude of the apparent excess age, the amount of excess 4°Ar in the samples (Table 1) shown in Fig. 14 is essentially equivalent to the ascending order shown by the identification number, with 79-461 containing 3.6 times more excess 4°Ar than 79-462. For the three samples containing the least amount of excess 4°Ar ( < 3.5 X 10 -l° real g-i). 79-173, 79-458, and 79-462, the minimum ages recorded in the release patterns are 599 Ma, 575 Ma and 648 Ma, respectively. Minimum ages in the spectra of 79-459 and 79-461 are 1.2 I

10

98-

I

I

I

I

I

I

BROKENHILL plagioclases

I

I

i"

F~ [~/79.459

7

Ga 5 4

Jr~l

2

_

I

o

I

2b

I

4b

I

6b

I

8'o

percent 39Arreleased

I

ioo

Fig. 1 4 . 4°Al"/39Ar age spectra for plagioclases from the mafic granulites near the North Broken Hill mine and at Black Bluff. All age spectra are characterized by a saddle-shape.

I0 "i

I

i

I

I

I

I

I

I

I

BROKENHILL plogioclases I04 1

K/Ca:

79-173

I0-3= 79-459

~

10

0

m

-

4

6

,

' 20 ' 40 ' 6'0 ' 8b ' IOO

percent~PArreleased

Fig. 15. Plot of K / C a ratios of plagioclase against the percent of 39Ar released. The high initial values indicate that a K-rich phase is degassing.

and 2.2 Ga, respectively, in keeping with the much higher excess 4°Ar content of 79-461 (Table 1). The minimum age recorded of 575 Ma is both a maximum age for the closure of plagioclase feldspars to 4°Ar* loss as well as a maximum age for the introduction of excess 4°Ar. Support for the first point is taken from the study of Richards and Pidgeon [11] who measured a plagioclase separate from the mine sequence and obtained a K-Ar age of 528 Ma. This plagioclase is very K-rich making it rather insensitive to the effects of excess 4°Ar. We are left to conclude that heating which culminated about 520 Ma ago, was sufficient to cause plagioclase 'to lose any accumulated 4°Ar*. In the absence of any recorded event since that time, it follows that 4°Ar was reintroduced into plagioclase almost immediately, as the regional outgassing of K-feldspars and biotite began to generate a significant PAr Evidence which indicates that the excess 4°Ar present in other minerals also was introduced at this time includes the coherence of the coexisting hornblende/plagioclase Ar partition coefficients (see section 5) as well as the accord between the calculated ambient temperature during contamination of the hornb-

141

lende (equation (2)) and the estimate of the peak temperature of the --~ 520-Ma event (section 4) of about 350°C. With this information, concentrations of excess 4°Ar in these plagioclases have been calculated and are given in Table 1. It is less obvious that the initial ages in the plagioclase age spectra represent equilibrium with the ambient PAr as the gas released at high temperature often shows even higher apparent ages. The discussion will continue on the basis that the initial release reflects near equilibrium; additional support for this assumption will be developed in a later section.

Rb/Sr

BOO-

BROKEN HILL BLOCK

Pb/P t.r

6OO r

°C 400

K/Ar &

~ Rb/Sr 200

t.//

bi

ap f.

O

. . . .

0

.----

I

05

'

"

'

•

I

IO

'

'

'

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I

1.5

. . . .

2D

Go 4. Thermal history of the Broken Hill Block

Sufficient temperature information has been obtained from the various geochronological systems to warrant a presentation of the inferred thermal history of the Broken Hill region. As the terrain probably has experienced local temperature variations, the single temperature-time curve shown in Fig. 16 will necessarily be somewhat schematic, but the salient points are representative of the entire region. Briefly summarising, culmination of high-grade metamorphism associated with temperatures up to 800°C [23] occurred about 1660 Ma ago, but the regional temperature remained above --~ 500°C until about 1575 Ma, indicated by some of the 4°Ar/39Ar age spectra data from hornblende near Broken Hill. Additional evidence for this protracted cooling is available from a 2°7pb/2°6pb apatite isochron from the mine sequence which gives an age of 1570 ___ 10 Ma (B. Gulson, personal communication). Very high temperatures (~> 500°C) would be required during the period between 1660 and 1570 Ma to maintain open system behaviour in the apatites. The temperatures in the northern part of the region 1500 Ma ago were less than ~ 350°C in order that the Mundi Mundi muscovite began accumulating 4°Ar* essentially consequent on emplacement. A well-established thermal event about 520 Ma ago is constrained to a maximum temperature in the Broken Hill Block of between --~ 300 and 350°C. An apatite fission track age indicates that the region cooled below --~ 100°C about 280 Ma ago (Table 4).

Fig. 16. Schematic thermal history of the Broken Hill Block. The temperature-time points are discussed at length in the text.

5. Excess Ar

It has been argued already that the high initial ages in the release patterns of both hornblende and .plagioclase can be translated into a concentration of excess 4°Ar. Concentrations for those sampies analysed by the 4°Ar/39Ar spectrum method are given in Table 5, and can be used to estimate the partition coefficient of Ar between hornblende and plagioclase. Excluding the hornblendeplagioclase pair from 79-458 on the basis that the migration of reactor-produced 3tAr has probably diluted an originally older age present in the hornblende age spectrum, these data give an Ar partition coefficient between hornblende and plagioclase of 6 ± 2. However, if the partition coefficient is estimated simply from the concentrations of excess 4°Ar in hornblende and plagioclase (Table 1), a misleading value of about 0.3 results. This illustrates the problem of obtaining partition data from conventional K-Ar results (e.g. [24]). Because the rate of diffusion of Ar varies significantly between these phases, it is unlikely that the concentration of excess 4°Ar found in minerals represents an equilibrium condition but more probably reflects the relative rates of a°Ar transport within the different phases (Table 5). For the apparent equilibrium concentrations of excess 4°Ar to be of any use in estimating ambient

142

Ar pressures, some additional knowledge is needed that may be obtained from laboratory experiments. The information required is the gas/solid partition behaviour of Ar for the various phases examined in this study as a function of temperature. There is little of this information available, and most of it is shown in Fig, 17 on a distribution coefficient versus reciprocal temperature plot. The best defined data is that for magnetite [25] grown in an Ar environment at temperatures between 450 and 700°K. Lancet and Anders [25] observed that Henry's Law was obeyed up to 10-2 atm for Ar before saturation occurred. The apparent heat of solution calculated from the data was - 15.3 ___0.3 kcal mol-i, the negative sign indicating that the mixing of Ar and magnetite is exothermic. The assumption that these data represent equilibrium is reasonable, although it is known for other materials that during growth, disequilibrium amounts of environmental substances (e.g., water in quartz) may be incorporated into the structure. The remaining data are shown in the expanded portion of Fig. 17 along with the extrapolated curve for the magnetite data. The numerals next to

102 5-

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08

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\

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~

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so,o e ~ iooo, ~spo \ . .'. , '.... , . . . . IO 0.5

0

IO~,OK Fig. 17. P l o t o f d i s t r i b u t i o n c o e f f i c i e n t o f A r b e t w e e n v a r i o u s m i n e r a l s a n d v a p o u r as a f u n c t i o n o f r e c i p r o c a l a b s o l u t e t e m perature.

each datum point represents the duration of the heating experiment in hours. If equilibrium is reached during the run, then regardless of the duration of the experiment, the calculated distribution coefficient should be the same. However, if equilibrium had not been achieved, then the longer the experiment duration (under constant temperature and pressure conditions), the greater would be the value of the apparent distribution coefficient calculated. In most cases, the time-dependent nature of the apparent distribution coefficient [26,27] reveals that the minerals were not in equilibrium with the Ar atmosphere. Single analyses of Gerling et al. [28] and Nikolayev et al. [29] make a similar assessment difficult, but as the run durations in these experiments were all less than one hour, it is reasonable to assume that equilibrium was not attained. Also shown in Fig. 17 is the datum point for Ar in an enstatite melt [30]. In order to assess the possible applicability of the albite/Ar partition data of Laughlin and Yoder [26] to the bytownite samples of this study, three experiments were undertaken. Aliquants of 79-459 plagloclase (An 74) were placed in a sealed furnace through which Ar flowed at 1 atm pressure. After treatment, the samples were analysed for Ar concentration using conventional extraction and mass spectrometer methods. Two samples were heated at 1000°C for periods of 76 and 144 hours. The third sample was run at 1200°C for 24 hours. Results of the two experiments at 1000°C yield calculated 4°Ar distribution coefficients of 4.7 and 8.9 × 10 -5 cm3 STP g - ] atm - t for the 76- and 144-hour experiments, respectively, indicating that equilibrium had not been reached. The run at 1200°C gave a calculated distribution coefficient of 7.7 × 10 -3 cm3 STP g - l atm-i. The data obtained at 1000°C is comparable to that of Laughlin and Yoder [26], suggesting similar solubility a n d / o r kinetic behaviour of albite and bytownite. The distribution coefficient calculated for the 1000°C, 338-hour run [26] of 2.26 × 10 -4 cm3 STP g - I arm-i is possibly a close minimum estimate of the true value. The run at 1200°C may also be close to equilibrium as a result of the increased transport rate accompanying the higher temperature. These two data plot within a factor of two of the extrapolation of the magnetite solubility curve,

143

suggesting that these two phases may have similar partition behaviour. Some evidence for this observation can be found from examining the relative concentrations of excess 4°Ar in coexisting magnetite and plagioclase from sample 79-173 (Table 1). Although we have no direct evidence that this magnetite achieved equilibrium with the ambient PAr, the 4°Ar/36Ar ratio of the gas released from the magnetite is essentially identical to that from the initial release of the plagioclase (Table 3), which requires that the magnetite has absorbed enough Ar during the -~ 520-Ma event to dominate over any pre-existing gas. The calculated equilibrium concentration in the plagioclase of 5.0)< 10 -5 cm3 STP g - t (2.2× 10 -9 mol g - i ) is about 3 times greater than the total excess 4°Ar found in 79-173 magnetites (Table 1) of 1.4 × 10-5 cm3 STP g - l (6.4× 10 -1° mol g - l ) . By themselves, these data indicate that plagioclase and magnetite have roughly comparable solubilities for Ar at ,-~ 350°C. Should the magnetite have only absorbed one-tenth of its equilibrium concentration, an unlikely possibility, the two phases would still contain similar amounts within a factor of ten of each other. This similarity is not too surprising in that the excess Ar is possibly contained in broadly similar sites within the respective lattices. There are three principal trapping sites for Ar in solids: structural holes, edge dislocations, and lattice vacancies [25]. Lancet and Anders [25] concluded that for magnetite, structural holes were too small to contain an Ar atom, and edge dislocations too few, especially in annealed material, to account for the abundances observed. However, anion vacancies were both abundant and of sufficient size to hold At. The principal trapping site for Ar in plagioclases is probably also anion vacancies, for the following reasons. Structural holes in feldspars are not large enough to contain an Ar atom and edge dislocations are probably too few, although it is difficult to rigourously assess this latter effect. This leaves cation and anion vacancies to consider. The diffusion of oxygen, and thus oxygen vacancies, in feldspars [31] is much more rapid than the diffusion of large cations [32] at temperatures below --, 460°C [33]. At the low temperatures believed to prevail ( ~ 350°C) during excess Ar in-

troduction at Broken Hill, it is likely that relatively rapid diffusion via anion vacancies was the dominant mechanism by which Ar was introduced. In contrast, the indicated equilibrium concentration of excess Ar in the coexisting hornblende of sample 79-173 (Table 5) is 3.6 × 10 -4 cm s STP g-1 (1.6 × 10 -8 mol g - l ) , about 8 times the amount recorded by the plagioclase. This result is not unexpected in that structural holes in amphiboles [34] are large enough to hold Ar and are vastly more numerous than anion vacancies at ,-~ 350°C, giving hornblende a greater intrinsic capacity to absorb excess Ar. By taking the distribution coefficient for Ar in both magnetite and plagioclase at 350°C to be ,~ 0.2 (see Fig. 17), an approximate distribution coefficient of -~ 1.2 can be calculated for hornblende using the average hornblende using the average hornblende/plagioclase partition coefficient of 6 -4- 2, calculated from Table 5. Using these two distribution coefficients, estimates of ambient Ar pressures can be calculated directly from the apparent hornblende and plagioclase equilibrium concentrations in Table 5. The environmental pressures calculated in this manner are shown in Fig. 18 plotted as PAr versus the distance of the sample away from the nearest contact with the quartz-feldspar-biotite gneiss. The general agreement between the Ar pressures calculated from coexisting hornblende and plagioclase indicates the consistency of the calculated distribution coefficient for Ar between these two phases. The absolute value for the Ar pressures indicated in Fig. 18 may be in error by as much as an order of magnitude as a result of simplifying assumptions, but all data will be systematically affected. It is dearly seen in Fig. 18 that the Ar pressure recorded by our samples decreases in a regular fashion with increasing distance from the granulite-gneiss contact. This result indicates that the source of excess Ar was, not unexpectedly, the encompassing quartz-feldspar"biotite gneiss, and that the granulite did not equilibrate with the surrounding PAr, nut instead contains a concentration gradient across the layer, indicating a relatively low permeability. Calculated Ar pressures are low enough to ensure that saturation has not occurred in any of the phases.

144

.

~r x

\o I.

•

minerals in the gneiss to the granulite be only 0.3% efficient. Had the excess 4°Ar partial pressure remained high in the quartz-feldspar-biotite gneiss until after the regional temperature fell below about 200°C, the constituent minerals of the gneiss would contain similar amounts of excess 4°Ar to that documented in the mafic granulites. However, the calculated age of a biotite with such a concentration of excess 4°Ar would only be increased on the average by about 3%. This is a possible explanation of the excessive scatter of the K / A r biotite ages in the region [11].

BLACKBWFF

.oRT. M . E

I0-4

\

(bars) 2 .

•

79-461 ~ 7 9 - 459 • 79-171

79-462 79-458 C

~

0

~

5 I0 15 2_'0 25 distance from contact (m)

Fig. 18. Apparent Ar partial pressure (PAr) calculated from coexisting hornblende-plagioclase pairs in both the mafic granulite near the North BrokenHill mine (open symbols) and at Black Bluff (filled symbols), plotted against distance from the nearest contact with the quartz-feldspar-biotite gneiss. These data indicate that the excess 4°Ar originated outside the mafic granulites and only partially permeated inwards.

The indicated Ar pressure adjacent to both granulite layers (Fig. 18) is --~ 3 × 10-4 atm, suggesting a fairly constant regional value. We can estimate the possible magnitude of the Ar pressure derived from the quartz-feldspar-biotite gneiss during the -~ 520-Ma event f r o m the following considerations. After about 1 Ga of 4°Ar* accumulation, a gneiss with 5% K would contain about 10 -8 mol g - i of 4°Ar*, mostly within biotite and K-feldspar. Instantaneous outgassing of this 4°At* into a rock with a porosity of 1% would produce a pressure of: P = n R T / V ~ 0.1 atm

(3)

where n / V = 2 . 8 × 10 -3 tool 1-1, gas constant, R = 0.08205 1 atm tool -l d e g -1, and temperature, T = 623°K. This value of .~ 10 - 1 atm is probably an overestimate of the maximum Ar pressure attained, as outgassing probably took place over an extended period of time during which loss of 4°Ar from the system is likely to have occurred. Our estimate of --~ 3 × 10-4 atm (Fig. 18) as the effective Ar pressure in the gneiss during this period would require that the transfer of gas between

6. The problem of saddle-shaped age spectra

Of the 4°Ar/39Ar age spectrum analyses presented here, the appearance of a saddle-shaped age spectrum is best developed in plagioclase. If we can understand why the 4°Ar/39Ar systematics of this mineral respond to excess 4°Ar in this way, then it seems likely that this may provide the mechanism responsible for this behaviour in other phases as well. A plot of the variation in K / C a ratio of plagioclases during gas release is shown in Fig. 15. All samples exhibit the same trend with initially high K / C a values which decrease rapidly by more than an order of magnitude to lower values at about 30% of gas release, then remain relatively constant for the remainder of the extraction. It is reasonable to believe that this indicates preferential outgassing of the K-derived isotope (39Ar) rather than inhibited release of the Ca-derived isotope (37At). To obtain clarification of this issue, diffusion coefficients were calculated from both the release of 39Ar and 37At during the step-wise extraction experiments. Ideally, the behaviour of such data on an Arrhenius-type plot should give some indication of the nature of the lattice position in which the Ar resides. To do this, the assumption was made that outgassing of these isotopes was occurring form infinite sheets, with a half-width l, on the basis that the incoherent boundaries between exsolution lamellae would be likely to control the effective diffusion radius. Two equations ((4) [35]; (5) [36]) were used for calculat-

145

ing diffusion coefficients from the step-wise extraction data: o

l--7=

.~rt/2.f

/t

for0~
(4)

D_ 4 In (l-f) 12 ~2. t -8for 0.45 ~
(5)

where D is the diffusion coefficient, l is the diffusion half-width, f is the fractional loss and t is the duration of heating, usually 45 minutes. The stepwise extraction data was accommodated in these equations by finding the difference in D / I 2 between that calculated for f at step n and the D / I 2 calculated at step n - 1. The residual in this subtraction is the D / I 2 for step n. Data for both 39mr and 3TAr obtained in this manner for plagioclases 79-459 and 79-462 are shown in Fig. 19 as - l o g D / I 2 versus reciprocal absolute temperature. Data from a K-derived isotope will also reflect the behaviour of Na sites as the alkalies are contained in equivalent lattice positions. Results from both samples show very similar behaviour. The initial

I

[

I

I

I

I

I

I

I

I

4-\

# 6-

•

~

1

C:3 7 o

10"

,2~,oo0 ~ o f

?,0o

4?0 c'cl I

I

I

0.6 07 {38 09 ID 1.1 12 1.3 1.4 15 1.6

IG~/OK Fig. 19. Arrhenius plot of calculated D/I 2 values from the gas release data of 79-459 and 79-462plagioclasesversus reciprocal absolute temperature. Other plagioclase samples give similar results. See text for a detailed explanation of these data.

,~ 35% of 39Ar release plots on a line corresponding to an activation energy ( E ) of ,~ 15 kcal m o l - ] . Between --~ 35 and --~ 50% of gas release, the D / I 2 values drop nearly an order of magnitude before climbing again along a line proportional to an activation energy of 35 kcal m o l - ] . The 37Ar behaves in a similar manner with the exception of the first ,~ 35% of gas released for which data plot on a line with an activation energy also of --~ 35 kcal mol - l , but D / I 2 values are displaced by nearly 3 orders of magnitude from the line defined by the 39Ar data in the early stages of outgassing. These results seem surprising in that they indicate either that isotopes of Ar have significantly different diffusion rates, or that plagioclase contains more than one phase. The answer appears to lie with the second possibility. The structure of plagioclase for nearly all compositions is controlled by some kind of solvus. In the case of bytownite (ANT0 to An9o), the relevant mechanism is Huttenlocher exsolution (see Smith [37] for a review). The model for Huttenlocher development is the growth of two distinct phases which form alternating lamellae; a homogeneous anorthite-rich (,~ Ana8 ) phase, and a complex, modulated albiteanorthite phase termed "e"-plagioclase [37]. This latter material is characterized in plagioclase with a bulk composition of An75 , by alternating albite and anorthite lamellae with a wavelength of about 50 ,~, the anorthite regions being about 3 times the width of the albite zones. Each couple is separated from the next by what is termed an anti-phase boundary [37], seen by Wenk [38] as a surface of large-scale diffusion. Ignoring the possible effects of recoil [39,40] for the moment, this model appears to accommodate our observations from the calculated diffusion data (Fig. 19) of three distinct diffusion regimes: (1) a high diffusion probability site for K (Na) with an E of ,-~ 15 kcal mol-1, (2) a lower diffusion probability site for Ca with an E of ~ 35 kcal mol -~, and (3) a site containing both Ca and K (Na) parallel to the line defined by the early Ca release of ,-~ 35 kcal mol - l , but with D / I 2 values lower by two orders of magnitude at any temperature. These three regimes would roughly correspond to (1) the albite-like "e"-plagioclase domain (Ans), (2) the anorthite-like "e"-plagioclase domain

146 (An80), and (3) the thicker transitional anorthite (--~ Anas ) lamellae. As regions (2) and (3) are of essentially the same composition [41,42], the equivalence in E is not unexpected and the difference of a factor of 100 in D / I 2 values (Fig. 19) can reasonably be attributed to lamellae widths which differ by a factor of 10. The lower E in the albite region may be both a result of the proximity to anti-phase boundaries and the lower bond strength of Na with respect to Ca. To test these predictions, a sample of 79-459 plagioclase was prepared from a thin section by thinning under ion bombardment for examination in JEOL 100-CX transmission electron microscope. The existence of both "e"-plagioclase and transitional anorthite structures in this sample can be seen from an electron diffraction pattern. In addition to "a", "b" and diffuse "c" reflections indicative of transitional anorthite, the pattern also shows "e" and "f" reflections, diagnostic of "e"-plagioclase in bytownite. Direct imaging of these fea-

tures is shown in Fig. 20. The "e"-plagioclase can clearly be seen in Fig. 20 with super-lattice fringes averaging about 60 ,~. The transitional anorthite averages about 600 ,~ giving a difference in radii between the two anorthite-like phases of about a factor of 10, essentially that observed from the calculated diffusion data (Fig. 19). However, other regions of the crystal show less regularity in the thickness of the transitional anorthite down to average widths of about 150 ,~. Presumably the factor of 10 is a bulk average value. This general confirmation provides us with an insight into an unusual aspect of the diffusion behaviour seen in Fig. 19. A transition occurs between the low-temperature outgassing regimes for Ar generated from K and Ca and their similar high-temperature behaviour, where the slopes become positive, which in this diagram indicates that the diffusivity is decreasing with increasing temperature. What this probably reflect is that regimes (1) and (2) have outgassed completely by 1000°C, and the apparent positive slope is an

Fig. 20 Electronmicrograph ( × 66,000) showingboth the modulated"e"-plagioclasesuperlattice fringesand the coarser transitional anorthite. The two structures differ in radius by a factor of 10.

147

artifact of the mixture of nearly outgassed low retention sites with only partially outgassed sites of high retentivity. It is interesting to note that data from an 3TAr diffusion study in anorthite by Fechtig et al. [43] plots on the extension of our regime (2), that of the anorthite-like "e"-plagioclase domains. Although the relative amount of "e"-plagioclase and transitional anorthite seen in Fig. 20 of -,~ 2 : 1 does not reflect the abundances indicated by the diffusion data (Fig. 19) of -~ 1:2, it is expected that recoil [39,40] of both reactor produced isotopes would cause significant redistribution, resulting in some 39Ar depletion in the "e"-plagioclase regions. The information we have gathered allows us to return to the problem of saddle-shaped age spectra in plagioclase with some insight as to its nature. The initially high ages which decrease to a minimum by ,~ 35% of 39mr release (Fig. 14) likely indicates an excess *oAr diffusion gradient preserved in the "e"-plagioclase, with which this portion of outgassing has been associated. The resolution of a geologically meaningful minimum age is then a function of the degree of contamination of this phase. The second release of excess 4°Ar paradoxically occurs in the latter stages of release, reaching a maximum value close to the end of gas evolution. This portion of gas is related to the coarser, transitional anorthite lamellae, and the mechanism responsible for this behaviour can be speculated. The *oAr* and reactor-produced 39Ar all originate with varying recoil energies from large cation sites, and should be expected to occupy, or have access to, essentially identical diffusion sites in plagioclase. A prediction form this interpretation is that undisturbed plagioclases should yield flat age spectra. Dalrymple and Lanphere [44] have measured two samples of geologically undisturbed calcic plagioclases and found them both to possess ideal release patterns. However, it has been argued that a significant proportion of excess 4°Ar is held in anion vacancies due to the relatively rapid diffusion of those defects with respect to large cation vacancies at low temperatures [31-33]. As a result of the lowtemperature dependence of diffusion of anion

vacancies [31], they will be much more retentive of *oAr than the large cation sites at the higher extraction temperatures. If this is the case, then step-wide outgassing of the transitional anorthite lamellae would result in a disproportionate release of 39mr early with respect to the excess *oAr, presumed to be confined to a steep gradient near the edge of these features. As the 39Ar becomes depleted, the excess *oAr begins to outgas causing progressively higher apparent 4°Ar/39Ar ratios, giving the impression on an age spectrum diagram of a high concentration of excess *oAr in the center of those regions. The implication of this model is that minerals with complex exsolution behaviour may give rise to anomalous age spectra if contaminated by excess 4°Ar. Certainly this is the case for pyroxenes [22] which are well known to exsolve in both igneous and metamorphic environments. Perhaps this offers an explanation for the relatively anomalous behaviour of the metamorphic hornblendes of this study compared to igneous hornblendes containing excess *OAr [18] in that igneous hornblendes are less likely to contain well-developed exsolution features due to the sluggish kinetics of amphibole unmixing [45].

7. Conclusions

The principal conclusions of this study are as follows: (1) The Broken Hill Block has experienced a complex thermal history since high-grade metamorphism 1660 Ma ago. After a slow cooling during the first 100 Ma (--~ 3°C Ma-]), the terrain remained well below 300°C until ,~ 520 Ma when the region experienced a thermal pulse reaching temperatures of ,~ 350°C. Intrusion of ultramafic bodies occurred about 560 Ma ago and the region cooled below ~, 100°C about 280 Ma ago. (2) Excess *oAr was incorporated into minerals during the --~ 520-Ma event at a temperature of about 350°C. In conjunction with solubility data, the excess *oAr profiles in hornblendes and plagioclases from granulite layers give an internally consistent estimate of the ambient PAr of ,-~ 3 X 10-4 atm.

148

(3) The saddle-shaped age spectra clearly developed in plagioclases apparently results from the combination of exsolution features and the presumed different lattice occupancy of excess 4°Ar with respect to 4°Ar*, 39Ar, and 3TAr.

Acknowledgements We thank Don Rusling of the U.S. Geological Survey TRIGA reactor for performing the irradiation and John Fitzgerald for the transmission electron micrographs of plagioclase. An excellent introduction to the geology of Broken Hill was provided by Bill Laing, Bon Brown and Jim Stroud. Ray Binns supplied many of the samples used in this study. We thank Robyn Maier for technical assistance and Gall Stewart for typing the manuscript.

References 1 C. Bauchau, Essal de typologie quantitative des gisements de plombe et de zinc avec la r6partition de l'argent, Bull. B.R.G.M. 2 (1971) 1-72. 2 D. Mawson, Geological investigations of the Broken Hill area, Roy. Soc. S. Aust., Mem. 2 (1912) 211-319. 3 B.P.J. Stevens, W.J. Stroud, I.L. Willis, G.M. Bradley, R.E. Brown and R.G. Barnes, A guide to the stratigraphy and mineralization in the Broken Hill Block, N.S.W., Geol. Surv. N.S.W. Publ. 062 (1979) 170 pp. 4 g.N. Phillips, Broken Hill--a Precambrian hotspot?--A comment, Precambrian Res. 4 (1977) 215-220. 5 R.A. Glen, W.P. Laing, A.J. Parker and R.W.R. Rutland, Tectonic relationships between the Proterozoic Gawler and Willyama Orogenic Domains, Australia, J. Geol. Soc. Aust. 24 (1977) 125-t50. 6 P-,.H. Vernon, The Willyama Complex, J. Geol. Soc. Aust. 16 (1969) 20-55. 7 R.T. Pidgeon, A rubidium-strontium geochronological study of the Willyama Complex, Broken Hill, Australia, J. Petrol. 8 (1967) 283-324. 8 S.E. Shaw, Rb-Sr isotopic studies of the mine sequence rocks at Broken Hill, Monogr. Australas. Inst. Min. Metal1. (1969) 185-198. 9 R.H. Steiger and E. J~tger, Subcommission on Geochronology: convention on the use of decay constants in geo- and cosmochronology, Earth Planet. Sci. Lett. 36 (1977) 359362. I0 P.H. Reynolds, A U-Th-Pb lead isotope study of rocks and ores from Broken Hill, Australia, Earth Planet. Sci. Lett. 12 (1971) 215-223.

11 J.R. Richards and R.T. Pidgeon, Some age measurements on micas from Broken Hill, Australia, J. Geol. SOc. Aust. l0 (1963) 243-260. 12 R.A. Binns and J.A. Miller, Potassium-argon age determinations on some rocks from the Broken Hill region of New South Wales, Nature 199 (1963) 274-275. 13 G.B. Dalrymple, E.C. Alexander, M.A. Lanphere and G.P. Kraker, Irradiation of samples for 4°Ar/39Ar dating using the Geological Survey TRIGA reactor, U.S. Geol. Surv. Prof. Paper (in press). 14 C.W. Naeser, Fission track dating, U.S. Geol. Surv. OpenFile Rep. 76-190 (revised, 1978). 15 J.F. Evernden and J.R. Richards, Potassium-argon ages at Broken Hill, Australia, Nature 192 (1961) 446. 16 G.A. Wagner, G.M. Reimer and E. J~iger, Cooling ages derived by apatite fission track, mica Rb-Sr and K-Ar dating: the uplift and cooling history of the central Alps, Univ. Padua Mem. XXX (1977) 27 pp. 17 G. Turner, The distribution of potassium and argon in chondrites, in: Origin and Distribution of the Elements, L.H. Ahrens, ed. (Pergamon, Oxford, 1968) 387-398. 18 T.M. Harrison and I. McDougall, Investigations of an intrusive contact, northwest Nelson, New Zealand, II. Diffusion of radiogenic and excess 4°At in hornblende revealed by 4°Ar/39Ar age spectrum analyses, Geochim. Cosmoclaim. Acta 44 (1980) 2005-2020. 19 G.B. Dalrymple and M.A. Lanphere, Potassium-Argon Dating: Principles, Techniques, and Applications to Geochronology (Freeman, San Francisco, Calif., 1969) 258 pp. 20 D. York, Least squares fitting on a straight line with correlated errors, Earth Planet. Sci. Lett. 5 (1969) 320-324. 21 R.A. Binns, Zones of progressive regional metamorphism in the Willyama Complex, Broken Hill district, New South Wales, J. Geol. SOc. Aust. 11 (1964) 283-330. 22 M. Lanphere and G.B. Dalrymple, Identification of excess 4°At by the ~Ar/39Ar age spectrum technique, Earth Planet. Sci. Lett. 32 (1976) 141-148. 23 G.N. Phillips and V.J. Wall, Regional metamorphism of a sediment-rich sequence under conditions of a high geothermal gradient: the Willyama Complex, Broken Hill, in: The Crust and Upper Mantle of Southeast Australia, Bur. Miner. Resour., Rec. 1972/2 (1979) 66-67 (abstract). 24 E.H. Hebeda, N.A.I.M. Boelrijk, H.W.A. Priem, E.A. Th. Verdurmen and R.H. Verschure, Excess radiogenic Ar and undisturbed Rb-Sr systems in basic intrusives subjected to Alpine metamorphism in southeastern Spain, Earth Planet. SOl. Lett. 47 (1980) 81-90. 25 M.S. Lancet and E. Anders, Solubilities of noble gases in magnetite: implications for planetary gases in meteorites, Geochim. Cosmochim. Acta 37 (1973) 1371-1388. 26 A.W. Laughlin and H.S. Yoder, Introduction and diffusion of argon into natural albite, Annu. Rep. Director, Geophys. Lab., Carnegie Inst. Washington 1969-1970 (1971) 145148. 27 D.W. Schwartzman and B.J. Giletti, Argon diffusion and absorption of pyroxenes from the Stillwater Complex, Montana, Contrib. Mineral. Petrol. 60 (1977) 143-159.

149 28 E.K. Gerling, D.Yu. Pushkarev and N.V. Kotov, Behaviour of certain minerals when heated under argon pressure, Proc. U.S.S.R. Acad. Sci. Geol. Ser. 11 (1965) 3-13. 29 S.D. Nikolayev, K.G. Knorre and E.B. Lebedev, Absorption of argon by pyroxenes at elevated temperatures, Geochim. Int. 9 (1972) 777-780. 30 T. kirsten, Incorporation of rare gases in solidifying enstatite melts, J. Geophys. Res. 73 (1968) 2807-2810. 31 B.J. Giletti, M.P. Semet and R.A. Yund, Studies in diffusion, III. Oxygen in feldspars: an ion microprobe determination, Geochim. Cosmochim. Acta 42 (1978) 45-57. 32 K.A. Foland, Alkali diffusion in orthoclase, in: Geochemical Transport and Kinetics, A.W. Hofmann et al., eds., Carnegie Inst. Washington Publ. 634 (1974) 77-98. 33 S.R. Hart, Diffusion compensation in natural silicates, Geochim. Cosmochim. Acta 45 (1981) 279-291. 34 W.A. Deer, R.A. Howie and J. Zussman, An Introduction to the Rock-Forming Minerals (Longmans, Green and Co., London, 1966) 528 pp. 35 W. Jost, Diffusion in Solids, Liquids, and Gases (Academic Press, New York, N.Y., 1960) 558 pp. 36 J. Crank, The Mathematics of Diffusion (Oxford University Press, Oxford, 1975) 2nd ed., 414 pp. 37 J.V. Smith, Feldspar Minerals (Springer-Verlag, Berlin, 1974) 3 volumes. 38 H.-R. Wenk, An albite-anorthite assemblage in low-grade amphibolite facies rocks, Am. Mineral. 54 (1979) 12941299. 39 G. Turner and P.H. Cadogan, Possible effects of 39Ar recoil in 4°Ar-39Ar dating of lunar samples, Proc. 5th Lunar Sci. Conf., Geochim. Cosmochim. Acta 2, Suppl. 5 (1974) 16011615.

40 J.C. Huneke and S.P. Smith, The realities of recoil: 39Ar recoil out of small grains and anomalous age patterns in 39Ar-4°Ar dating, Proc. 7th Lunar Sci. Conf., Geochim. Cosmochim. Acta 2, Suppl. 7 (1976) 1987-2608. 41 N. Morimoto, M. Kitamura and Y. Nakajama, Antiphase relations in superstructures of the e-plagioclase, Proc. Jpn. Acad. 51 (1975) 729-732. 42 T.L. Grove, Structural characterization of labradoritebytownite plagioclase from volcanic, plutonic, and metamorphic environments, Contrib. Mineral. Petrol. 64 (1977) 273- 302. 43 H. Fechtig, W. Gentner and J. Zahringer, Argonbestimmungen an Kaliummineralien, VII. Diffusionsveduste yon Argon in Mineralien und ihre Auswirkung auf die KaliumArgon-Altersbestimmung, Geochim. Cosmochim. Acta 19 (1960) 70- 79. 44 G.B. Dalrymple and M.A. Lanphere, 4°Ar/39Ar age spectra of some undisturbed terrestrial samples, Geochim. Cosmochim. Acta 38 (1974) 715-738. 45 M.F. Gittos, G.W. Lorimer and P.E. Champness, The phase distributions in some exsolved amphiboles, in: Electron Microscopy in Mineralogy (Springer-Verlag, Berlin, 1976) 238- 247. 46 G.J. Corbett and G.N. Phillips, Regional retrograde metamorphism of a high grade terrain: the Willyama Complex, Broken Hill, Australia, Lithos 14 (1981) 59-73. 47 G.N. Phillips, Water activity changes across an amphibolite-granulite fades transition, Broken Hill, Australia, Contrib. Mineral. Petrol. 75 (1980) 377-386. 48 R.W. Marjoribanks, R.W.R. Rutland, R.A. Glen and W.P. Laing, The structure and tectonic evolution of the Broken Hill region, Australia, Precambrian Res. 13 (1980-) 209-240.