Rb-Sr isotope geochemistry of lherzolites and their constituent minerals from Victoria, Australia

Earth and Planetary Science Letters, 28 (1975) 69-78 ©Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands

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Rb-Sr ISOTOPE GEOCHEMISTRY OF LHERZOLITES AND THEIR CONSTITUENT MINERALS FROM VICTORIA, A U S T R A L I A A.D.M. BURWELL Department o f Science, Oxford Polytechnic, tteadington, OxJbrd (Great Britain]

Received July 11, 1975 Revised version received August 20, 1975

Major element data and Rb, Sr and s~ Sr/S~ Sr analyses for seven spinel lherzolite xenoliths and their Recent host basalt from Victoria, Australia, are presented. The exotic nature of the xenoliths is indicated by a wide spread in B7Sr/a~ Sr values (0.7035-0.7076) compared with the basalt (0.7041). Five of the lherzolites provide evidence of a thermal event in the mantle 650 m.y. ago. Equilibration temperatures calculated from the compositions of the lherzolite phases (ca. 1050°C) apparently relate to this event. Estimates of the local geothermal gradient suggest temperatures of less than 700°C in the source region before eruption of the lherzolites. Isotopic analyses of the lherzolite minerals show that orthopyroxene contains more radiogenic Sr than coexisting olivine and clinopyroxene in three of the xenoliths. The s7 Sr/Sn Sr relationships between clinopyroxene and orthopyroxene suggest that internal isotopic disequilibrium has existed in the source region for up to 550 m.y.

1. Introduction Spinel lherzolites, which occur as xenoliths in basalt, have been extensively studied because their restricted range o f mineralogy and composition on a worldwide basis suggests that they are o f fundamental importance in studies o f the mantle. The origin o f spinel lherzolite xenoliths has been the subject o f considerable discussion. O'Hara [1,2] and O'Hara and Mercy [3] have regarded the majority o f spinel lherzolite xenoliths as being genetically related to their host basalts, i.e. they are cumulates formed from the ascending magma. However, there is now a considerable b o d y o f isotopic evidence which suggests that these xenoliths are exotic relative to their host basalt. Various workers [ 4 - 7 ] have demonstrated an exotic relationship on the basis o f Sr isotope distribution. The majority o f spinel lherzolite xenoliths would therefore appear to be primary or, more probably, depleted mantle which has been accidentally incorporated in an ascending magma.

Recent work on the isotopic composition of minera] phases separated from lherzolite xenoliths has produced the somewhat surprising result that the phases have, in some cases, been in a state o f isotopic disequilibrium within the mantle [ 8 - 1 0 ] . Allsop et a]. [11] and Barrett [12] have demonstrated disequilibrium existing between the minerals o f xenoliths in kimberlite. Isotopic disequilibrium between phases may be informative in at least two ways: (1) isotopic differences between phases, if maintained above solidus temperatures, would explain variations in the initial isotopic compositions o f mantle-derived melts [ 1 3 15] ; and (2) isotopic disequilibrium may generate mineral isochrons which provide information on the timing o f events within the mantle [ 10]. In this paper, analytical data will be presented which demonstrate partial internal isotopic disequilibrium for some o f the lherzolites. By using major element analyses o f constituent phases, equilibration temperatures o f the lherzolites are cal-

70 culated and estimates of the range of temperatures over which disequilibrium has been maintained are made.

2. Geological setting and sample details The spinel lherzolites were selected from a suite of xenoliths occurring in a basanite of Recent age (0.4 m.y., K - A r age, D.C. Rex) which forms part of the Newer Volcanic Series at Mt. Leura, Victoria, Australia. The xenoliths were between 10 and 15 cm in length and apparently free of alteration although several were traversed by thin basalt veins. They contained olivine (70-75%), enstatite (15-20%), chrome diopside (5-10%), and spinel (less than 2%). There was little variation in modal content from one xenolith to another. No secondary phases appeared to be present in the majority of the xenoliths although the pyroxenes in SL5 had been recrystallised to form a granular texture interstitial to a primary olivine mosaic. Lherzolite xenoliths from this area have been extensively studied. Various aspects of the K, Th, U and Pb isotope geochemistry have been investigated [ 1 6 - 1 8 ] . A comprehensive study of the major, trace and rare earth element distributions has been undertaken by Frey and Green [19], whilst Dasch and Green [20] have studied Sr isotope variations amongst the xenoliths and host basalts. All these workers reach the consensus that the lherzolites are fragments of inhomogeneous mantle which have been accidentally incorporated in the parent magma of the host basalt.

3. Analytical techniques Considerable care was taken during processing of samples to avoid contamination. Marginal portions of the xenoliths and those pieces visibly contaminated by basalt veining were discarded after hydraulic splitting. The remainder was rinsed in distilled water before further crushing and quartering for whole rock analysis. Average sample size for whole rock analysis was 250 g. Mineral separation was effected by the use of

magnetic methods, Clerici heavy liquids and, finally, hand-picking. Mineral samples were warmed several times in distilled-deionised water and then in quartzdistilled water before further processing. At least 1 g of each mineral was separated. Major element analyses of basalt and lherzolite whole rocks were performed by a Philips 1212 XRF spectrometer. Major element analyses of the mineral separates were carried out on an AEI electron microprobe. During preparation of samples for mass spectrometric analysis, considerable care was taken to minimise contamination. All reagents were redistilled: HF and HC104 in a two-bottle F.E.P. still [21] ; H2 O, HC1 and HNO3 in a quartz sub-boiling still [22]. Initial digestion of samples was performed in P.T.F.E. capsules contained within Monel steel jackets [23]. Subsequent evaporations were done under P.T.F.E. hoods through which filtered air was passed. Sr and Rb concentrations were determined by isotope dilution using 99.8% 848r spike and 99.1% a7 Rb spike. Purification of samples for Rb analysis was effected by using a zirconium phosphate ion exchange medium [24]. Samples for Sr analysis which contained high Mg contents were passed initially through a 75-cm 3 column of Dowex 50W-X8. The evaporated residue from this column was then passed through a 12-cm a column of Dowex resin. This method produced small, readily analysed residues. Overall blank levels were 2.5-3.0 ng Rb and 5 - 6 ng Sr for the whole rocks. Improved reagent purity resulted in blank levels of 0.8-1.0 ng Rb and 0.7-1.4 ng Sr for the mineral separates. Sr blank contributions are negligible (less than 1%) and no corrections for blank to Sr data have been made. Corrections of up to 5% and 15% have been made to whole rock and mineral Rb data respectively. Rb, Sr and 87Sr/86Sr analyses were made on a modified AEI MS5 with 30 cm radius, 90 ° sector analyser tube and Faraday collector. Data processing was performed by an on-line Digico Micro 16-V computer. Samples were loaded on single Ta filaments and stabilised with 0.1-1.0 M Ha PO4. Typical beam intensities were 10-12 A (Rb) and 0.3-1.0 × 10-n A

(Sr). Reproducibility was checked periodically by analysis of the Eimer and Amend standard which

71 TABLE 1 Major element analyses of seven spinel lherzolites and one basalt (whole rocks) SLI

SL2

SL3

SL4

SL5

SL6

SL7

B1 Basalt

SiO2 TiO2 A1203 Fe2 03 FeO MnO MgO CaO K20 Na20 P20s

41.8 0.01 1.17 1.81 6.82 0.14 46.1 1.03 0.01 n.d. 0.01

42.1 0.07 1.89 1.44 6.97 0.13 43.8 2.07 0.01 n.d. 0.03

41.3 0.04 0.77 1.61 6.74 0.13 47.3 0.54 0.01 n.d. 0.02

41.1 0.06 1.23 3.10 10.4 0.17 42.1 0.86 0.00 n.d. 0.04

40.2 0.02 0.77 2.00 6.75 0.13 48.4 0.62 0.01 n.d. 0.01

41.2 0.11 2.62 1.57 7.47 0.14 42.5 2.81 0.00 n.d. 0.02

42.2 0.12 2.96 1.91 7.58 0.14 40.5 2.55 0.00 n.d. 0.03

43.0 3.05 12.9 2.96 9.40 0.17 9.53 8.41 2.22 4.06 1.81

Total

98.90

98.51

98.46

99.06

98.91

98.64

97.99

97.51

SL1-SL7: spinel lherzolite whole rocks; BI: host basalt. Analyses by XRF except:.(1) FeO by metavanadate method; (2) Na20 by flame photometry.

gave an overall weighted mean of 0.70814 + 0.00002 (20). All quoted STSr/S6Sr values are normalised to STSr/S6Sr = 8.3752, but are n o t adjusted to Eimer and Amend SVSr/86Sr = 0.7080.

4. Results In Table 1 major element data for the host basanite and the seven whole rock spinel lherzolites are presented. The host basalt is typical of inclusion-bearing types, being of alkaline affinity and strongly undersaturated (12% normative nepheline). The lherzolites

show considerable variation in terms of CaO and A12 0 3 , but all may be termed depleted relative to pyrolite [25]. SL4 appears to be anomalous because of its high EFeO/MgO ratio. The limited number of major element analyses of minerals (Table 2) show that the clinopyroxenes are of diopside or endiopside type, the compositions of the orthopyroxenes fall in the range e n s t a t i t e bronzite, and the olivines are highly magnesian (FosT-Fo92, Table 3). Rb, Sr and STSr/86 Sr analyses of the basalt and lherzolite whole rocks are presented in Table 4. The basalt has a 878r/S68r ratio of 0.7041 whilst the lherzolites fall in the range 0 . 7 0 3 5 - 0 . 7 0 7 6 . Analyses

TABLE 2 Majorelementanalysesoflherzolite minerals SL3

SL4

SL6

cpx

opx

ol

cpx

opx

ol

SiO2 A1203 FeO MgO CaO Na20

55.0 3.94 2.47 15.8 20.3 1.80

58.3 2.53 5.18 32.8 0.89 0.15

42.3 0.00 8.20 49.4 0.06 0.00

53.1 4.80 3.59 15.3 18.7 1.76

56.5 2.88 8.76 32.0 0.89 0.16

40.8 0.06 14.5 47.06 0.07 0.00

39.5 0.04 10.0 49.2 0.10 n.d.

Total

99.31

99.85

99.96

97.25

101.19

102.49

98.84

SL3 and SL4 analyses by electron microprobe (analyst T. Padfield), SL6 data by XRF (analyst A. Gray).

ol

72 TABLE 3 Atomic formulae of lherzolite minerals SL3

SL4

cpx

opx

ol

Si

1.99

1.99

1.03

AI Fe II Ca Mg Na

0.17 0.07 0.78 0.85 0.13

0.10 0.15 0.03 1.67 0.01

0.17 1.78 -

Fo (mole %)

-

-

91.5

o f m i n e r a l s s e p a r a t e d f r o m these l h e r z o l i t e s ( T a b l e 5) s h o w t h a t co-existing c l i n o p y r o x e n e a n d o r t h o p y r o x e n e f r o m f o u r o f t h e x e n o l i t h s have i n d i s t i n g u i s h able 87Sr/86 Sr ratios. In t h e r e m a i n i n g t h r e e n o d u l e s t h e f o l l o w i n g r e l a t i o n s h i p s b e t w e e n t h e m i n e r a l s exist: 87 Sr/ 86 Srcp x

87Sr/86Srol • 87Sr/86Sropx

T h e m i n e r a l i s o t o p e d a t a are s u m m a r i s e d in Fig. 1.

SL6

cpx

opx

1.96 0.21 0.11 0.74 0.84 0.13

1.94 0.12 0.25 0.03 1.64 0.01

-

-

ol 1.01 0.26 1.71 86.7

Sr (ppm)

Leggo a n d H u t c h i n s o n [4] were a m o n g s t the first w o r k e r s t o d e m o n s t r a t e significantly d i f f e r e n t 87 Sr/ s6 Sr ratios b e t w e e n h o s t basalts and t h e i r e n c l o s e d x e n o l i t h s f r o m t h e Massif C e n t r a l , F r a n c e . Subseq u e n t l y , t h e i r findings h a v e b e e n e n d o r s e d b y w o r k

Rb/Sr

87Rb/86Sr

87Sr/S6Sr

Abs. std. error (2a)

SL1

0.19 0.23

17.5 18.2

0.0107 0.0126

0.0190 0.0212

0.7076 0.7072

0.0002 0.0004

SL2

0.48 0.44

25.1 24.9

0.0190 0.0178

0.0552 0.0515

0.7043

0.0002

SL3

0.29 0.27

9.36 9.05

0.0308 0.0295

0.0892 0.0855

0.7045

0.0002

SL4

0.22

6.61 6.66

0.0328

0.0952

0.7047

0.0002

SL5

0.44

8.92 9.08

0.0494

0.1435

0.7052

0.0003

SL6

0.19

19.4 20.0

0.0098

0.0283

0.7041

0.0002

SL7

0.26

15.2 15.7

0.0170

0.0492

0.7035

0.0002

0.0456

0.1322

0.7042 0.7041 0.7041

0.0001 0.0001 0.0001

B1 (Basalt)

44.6 44.7

980 975 983

89.7

5.1. Basalt-lherzofite relationship

Whole rock Rb, Sr and 87Sr/86Sr analyses Rb (ppm)

0.98 0.21 1.82 -

5. Discussion

TABLE 4

Sample

ol

73 TABLE 5 Rb, Sr and 87Sr/86 analyses of minerals separated from the spinel lherzolites Sample

Mineral

Rb (ppm)

Sr(ppm)

Rb/Sr

87Rb/86Sr

STSr/86Sr

Abs. std. error(2o)

SL1

cpx

0.059

356

0.0002

0.0005

0.0002 0.0001 0.0003 0.0004

opx

0.047

1.64

0.0287

0.0833

0.7084 0.7084 0.7085 0.7082

SL2

cpx opx

0.17 0.13

251 1.98

0.0007 0.0631

0.0020 0.1833

0.7047 0.7046

0.0002 0.0003

SL3

cpx opx ol

0.072 0.096 0.12

163 2.01 1.96

0.0004 0.0478 0.0570

0.0013 0.1376 0.1736

0.7041 0.7049 0.7044

0.0002 0.0002 0.0004

SL4

cpx opx

0.16 0.062

258 1.32

0.0006 0.0470

0.0017 0.1367

ol

0069 0.059

0.0632

0.1837

0.7043 0.7053 0.7053 0.7045

0.0001 0.0006 0.0013 0.0001

cpx

0.41

0.0024

0.0070

0.0001 0.0002 0.0003

SL5

1.03 0.995 169

opx

0.28

5.56

0.0509

0.1472

0.7055 0.7054 0.7055

SL6

cpx opx ol

0.045 0.10 0.035 0.042

140 0.942 0.421 0.424

0.0003 0.1093 0.0911

0.0009 0.3155 0.2633

0.7040 0.7050 0.7040

0.0001 0.0004 0.0004

SL7

cpx opx

0.83 0.069

110 1.28

0.0076 0.0539

0.0220 0.1563

0.7036 0.7036 0.7034

0.0002 0.0009 0.0008

on suites of xenoliths from other areas [ 5 - 7 ] . The interpretation placed on these results has been that the xenoliths are exotic to the host basalt, i.e. there is no genetic relationship between the two. In some cases this interpretation is confirmed by complemen-

SL7

~

key -

c:clinopyroxene

o

× : orthopyroxene

-c-

o: olivine

SL6

SL5 ---oSL4_c -

-c-

x-

__.._ow

SL3 -c-

, 0.703

----n--

S L 2 - - x....¢ -- _ ! i 0.704 0.705

! 0.706

i 0.707

SLI --x---~_ I 0.708

Fig. 1. Variation in s7Sr/S6 Sr amongst minerals separated from ltlerzolites SL1-SL7. Error bars +-20.

i

0.709

tary work; e.g. the results of Dasch and Green's [20] work are supported by rare earth data [19] and U - P b work [ 1 6 - 1 8 ] . Six of the xenoliths in this study have 875r/86Sr ratios in the range 0 . 7 0 3 5 - 0 . 7 0 5 2 . Two of the xenoliths have 87Sr/86Sr values indistinguishable from that of the host basalt and could therefore either be parental to, or cumulates from, the basalt. All six xenoliths have 87Sr/86Sr ratios which fall within the range of values determined by Dasch and Green [20] for basalts from this area (87Sr/86Sr = 0 . 7 0 3 8 0.7045 + 0.0002). On the basis of isotopic evidence, six of the lherzolites could have been genetically related to P l i o c e n e - R e c e n t basalts erupted in Victoria. The remaining xenolith would be either a fragment of isotopically anomalous mantle or is an inclusion which has been contaminated by radiogenically enriched Sr. The hypothesis outlined above is untenable, however, when the implications of Fig. 2 are considered.

74 0.708. ,

L~

~r23 0.707_ 0.706.

0.705. 'SL3 0.704

,,~'~SL6

+BI

"~'SL7 0.703

0

01,

012 Rb8~/Sr 86

Fig. 2. A plot of a7 Sr/a6 Sr against s7 Rb/86 Sr for the lherzolite whole rocks (SL1 - S L 7 ) and the basalt whole rock (B1). The line represents an age of 650 -+ 125 m.y. Intercept on s7 Sr/8~ Sr axis is 0.7038 -+ 0.0001 (decay constant h = 1.39 X 10-1° yr-1). Error bars +-la.

Fig. 2 is a plot of STSr/a6Sr vs. 87Rb/86Sr for the basalt and lherzolite whole rocks. It shows that five of the lherzolites define an isochron with an age of 550 -+ 125 m.y. There are reasons for supposing that this age has real significance (see section 5.4). It is thought to date the separation of a melt from the mantle during the late Precambrian/early Palaeozoic. It is unlikely that the lherzolites defining the isochron could have been involved in a further melting episode during the Pliocene-Recent because such an event would have destroyed the linear relationship between aTSr/a6Sr and 87Rb/a6Sr.

5.2. Mineral in ter-relationships Amongst the lherzolites of this study, four contain clinopyroxenes which are isotopically indistinguishable from their associated orthopyroxenes. The remaining three lherzolites contain orthopyroxenes with a significantly higher proportion of radiogenic Sr than their associated clinopyroxenes and olivines (Fig. 1). Internal disequilibrium within lherzolites has been described by several workers [8-10,20]. (Paul [6] has also described internal disequilibrium but this was not reproduced when the same samples were re-

analysed by the'procedures described here [26] ). In most of these studies, the possibility that disequilibrium was caused by late-stage contamination cannot be excluded. Contamination may be effected either by a process of bulk addition of Sr (and Rb) to the sample, or through ionic exchange [27]. Rb and Sr concentration data for the orthopyroxenes suggest that these minerals at least have not been affected by bulk contamination. Rb and Sr probably behave as incompatible elements in orthopyroxene. Consequently, when orthopyroxene is in equilibrium with a melt, if the mineral lattice shows no preference for either Rb or Sr, the ratio (Rb/Sr)q~x should be approximately equal to (Rb/Sr)mel t. The range of(Rb/Sr)q~x values (excluding the anomalous sample SL1) is 0.05-0.11. This range corresponds to Rb/Sr values typically found in alkali basalts. The orthopyroxenes therefore retain Rb and Sr contents essentially the same as those existing at the time of their last equilibration with a melt. Bulk contamination by a source of Sr external to the enclosing basalt therefore seems unlikely. The wide spread in STSr/86Sr values also precludes contamination from the enclosing basalt. A further test of contamination is a plot of Sr concentration against s 75r/86Sr" Such plots are difficult to interpret because of unknown variables such as initial 87 Sr/86 Sr ratios of samples, nature and duration of contamination process, etc. No regular trend is observed in such a plot for the orthopyroxenes. A strong positive correlation between Sr and s 75r/86Sr is present amongst the clinopyroxenes. However, there is a broad correlation between Sr and Rb/Sr for four of the clinopyroxene samples (SL2, SL3, SL4, and SL6). The correlation between Sr and 875r/86Sr is therefore expected if, as the whole rock data imply, the clinopyroxenes have remained separate systems for 650 m.y. The internal isotopic disequilibrium within three of the samples therefore appears to be a mantle-derived feature. The whole rock data suggest a mantle event 650 m.y. ago when isotopes were presumably homogenised between mineral phases. At varying times after this event (see section 5.4) diffusion between orthopyroxene and the surrounding phases became ineffective resulting in growth of s7 Sr/86 Sr in orthopyroxene at a faster rate than in the clinopyroxeneolivine system. The data imply isotopic diffusion

75 rates are greater in olivine than in orthopyroxene within the mantle. The data of Dasch and Green [20] apparently conflict with that of the present study since olivine from one of their lherzolites is considerably more radiogenic than the coexisting orthopyroxene. However, they attribute the high 87Sr/a6Sr ratio in the olivine to equilibration with a Rb-rich phase (e.g. phlogopite) at a late stage. This implies that olivine has a greater tendency to equilibrate isotopically than enstatite. The absence of any high-Rb accessory phase in the lherzolites of this study may have resulted in the equilibration of olivine with a radiogenically depleted mineral, i.e. clinopyroxene. Although no high-Rb accessory phases appear to be present, comparison of Rb data in Tables 4 and 5 shows that the whole rocks contain more Rb than can be accounted for by the constituent minerals. This discrepancy may be due to concentration of Rb along grain boundaries [20]. The internal isotopic equilibrium observed in four of the lherzolites may be a primary, mantle-derived feature or may be the result of a heating effect by the host basalt. The pyroxenes in SL5 have a granular texture which may reflect recrystallisation, and therefore isotopic homogenisation, in response to changing pressure and temperature conditions during ascent from the mantle.

solubility of diopside and enstatite. The lherzolites SL3 and SL4 yield temperatures of 1055°C and 1065°C respectively. The pyroxenes from Victorian lherzolites analysed by Frey and Green [19] last equilibrated at temperatures in the range 10501160 °C. As a check on these results, temperatures have been calculated by using the geothermometer of Powell and Powell [29]. This method utilises the F e Mg exchange reaction between olivine and calciumrich pyroxene. It gives a temperature to which a small correction for pressure effect must be made. The lherzolites SL3 and SL4 produce the following relationship: T= 1015 + 5.4P (T = temperature in °C, P = pressure correction factor, °C, kbar-j ). The lherzolites analysed by Frey and Green [19] produce a very similar equation: r = 1010 + 5.3 P

.~ 40

SHIELD GEOTHERM

~

30

5. 3. Temperature estimates and their significance Isotopic disequilibrium between minerals in the mantle may be used to derive ages from mantle material (section 5.4) and to explain isotopic variation amongst basalts. O'Nions and Pankhurst [15] have derived models which explain secular and regional variations in initial 875r/86Sr amongst basalts in terms of differing degrees of melting of mantle which is in a state of internal isotopic disequilibrium. Implicit in their model is the assumption that inter-mineral disequilibrium can be partially maintained at lherzolite super-solidus temperatures. In this section, an attempt is made to estimate the range of temperatures over which internal disequilibrium has been maintained in the lherzolites from Victoria. The geothermometer devised by Wood and Banno [28] predicts temperatures from the degree of mutual

OCEANIC OTHERM

20.

I0

QT\ / \ AB

0 600

800

1000

1200

T°C. Fig. 3. Pressure-temperature relationships in the lherzolite source region. The P - T line for the lherzolites SL3 and SL4, determined by the method of Powell and Powell [29 ], is shown together with the vapour-saturated liquidii for quartztholeiite (QT) and alkali basalt (AB) [30] and the spinel lherzolite-garnet lherzolite transition curve [ 36 ]. The geotherms are taken from Clark and Ringwood [37].

76 Some limit may be placed on the value of P by determining the intersection of this P - T line on the spinel lherzolite/garnet lherzolite boundary (Fig. 3). The intersection falls at approximately 18 kbars, corresponding to a maximum equilibration temperature of approximately 1100°C. The equilibration temperatures are considerably lower than the dry liquidus temperature of alkali basalt, even at 1 bar, and they therefore reinforce the suggestion that the lherzolites are not genetically related to the host basalt. However, Fig. 3 shows that the lherzolites last equilibrated at temperatures above the vapour-saturated liquidus for quartz tholeiite [30]. It has been suggested that lherzolites may form as cumulates from hypersthene-normative magmas [2]. It is therefore possible that the Victorian lherzolites were precipitated from a tholeiitic magma at depths of less than 60 km. The whole rock isotopic data suggest that this event occurred approximately 650 m.y. ago. During progressive cooling of the mantle after this event, it might be expected that the compositions of the primary phases would constantly re-adjust in response to changing temperature until the time of eruption of the lherzolites (0.4 m.y. ago). However, internal isotopic disequilibrium in some of the lherzolites shows that migration of isotopes between orthopyroxene and clinopyroxene-olivine has not occurred for up to about 550 m.y. Consequently, migration of major elements between minerals must have been very limited during this time. Temperatures calculated from the geothermometers do not therefore relate to temperatures in the source region for the majority of the period 650-0.4 m.y. An alternative approach to temperature estimation is through heat flow data. Jaeger [31 ] shows that heat flow is significantly higher in southeast Australia than in shield areas. Assuming that the geothermal gradient beneath southeast Australia is intermediate to shield and oceanic values, it is possible to estimate the temperatures of the lherzolites before their eruption. Taking a limiting value of 60 km for the field of stability of spinel lherzolite produces temperatures of less than 700°C in the source region prior to eruption. The rate of cooling of the mantle in the period 650 m.y. ago to the present day is unknown. However, if the lherzolites were maintained at tempera-

tures of less than 700°C for any considerable time, it is not surprising that the rate of migration of isotopes was low enough to generate inter-mineral disequilibrium. It is also apparent that the existence of internal isotopic disequilibrium in mantle-derived material is no test of the existence of disequilibrium effects at lherzolite super-solidus temperatures. 5.4. Mantle dates Several previous studies of mantle-derived rocks have yielded "ages". Allsop et al. [11] and Barrett [12] have derived a spread of ages from co-existing mineral pairs in kimberlites which they interpret as marking melting events which left eclogitic and peridotitic residua. A number of studies have produced ages which fall within the range 500-700 m.y. Roe [32], working on alpine-type ultramafics from New Caledonia, obtained an age of 730 m.y., whilst a suite of spinel lherzolites analysed by Paul [6] yielded an age of 550 +- 200 m.y. Both these ages, which considerably predate the emplacement of the ultramafic material, were considered to mark a thermal event within the mantle. Recently, Stueber and Ikramuddin [10] have found that a lherzolite from Antarctica yielded a mineral age of 610 -+ 110 m.y. and a date of 1270 -+ 230 m.y. was obtained from a lherzolite from New Mexico. The present study has shown that the mantle beneath southeast Australia underwent a thermal event approximately 650 m.y. ago, possibly involving the separation of a melt from residual lherzolite. Although isotopic migration on a large scale was limited after this event, migration between minerals continued to varying degrees. Co-existing clinopyroxene-orthopyroxene pairs yield "dates" of approximately 550 m.y. (SL4), 400 m.y. (SL3), and 200 m.y. (SL6). These dates may mark the times at which isotopic closure of orthopyroxene occurred. A mineral age of approximately 700 m.y. (clinopyroxene-orthopyroxene) has been derived by Dasch and Green [20] for a therzolite from Mr. Noorat, Victoria. It would appear, from the coincidence of ca. 700 m.y. ages from Victoria, that there is real significance in this date. During the late Precambrian to early Palaeozoic, southeast Australia was an area

77 o f plate convergence [33]. The resultant geosyncline includes sediments and volcanics o f Cambrian age [34]. Thus the ages o f ca. 700 m . y . o b t a i n e d f r o m mantle-derived material in this area m a y be ~irectly related to s y n c h r o n o u s events in the overlying crust. A l t h o u g h the data are limited, there does appear to be a cluster o f mantle dates at, around 6 0 0 - 7 0 0 m . y . Evidence o f unusual activity within the mantle at this time m a y be f o u n d in the rapid migration o f G o n d w a n a l a n d relative to the magnetic pole [35] and possibly in the d e v e l o p m e n t o f major orogenies (the pan-African/Australian m e t a m o r p h i s m ) .

Acknowledgements

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The a u t h o r gratefully acknowledges the advice and e n c o u r a g e m e n t o f Prof. P.G. Harris and Drs. M.H. D o d s o n and R.A. Cliff. Dr. R.J. Pankhurst kindly read the manuscript and p r o v i d e d m a n y useful criticisms. Mr. T. Padfield and Mr. D.C. R e x are to be t h a n k e d for providing electron m i c r o p r o b e and K - A r analyses respectively. The author is grateful to N.E.R.C. for the allocation o f a Research S t u d e n t s h i p which enabled the analytical w o r k to be d o n e at the University o f Leeds.

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References

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