Geochemical evidence for the origin of some ultramafic inclusions from Victorian basanites

1970, Phys. Earth Planet. Interiors 3, 302—308. North-HollandPublishing Company, Amsterdam

GEOCHEMICAL EVIDENCE FOR THE ORIGIN OF SOME ULTRAMAFIC INCLUSIONS FROM VICTORIAN BASANITES

J. D. KLEEMAN and J. A. COOPER Department of Geophysics and Geochemistry, Australian National University, Canberra

The ultramafic inclusions occurring in some undersaturated Victorian basanites are shown to be accidental xenoliths. U, Th and K abundances and lead isotope measurements in inclusions and hosts, and uranium distribution studies in the phases of the inclusions all indicate that there is no genetic relationship between the ultramafics and the host basanites. The xenoliths are thought

to be fragments ofan inhomogeneous upper mantle of peridotitic composition. The lead isotope data on the lherzolites are consistent with a two-stage differentiation process from a primeval source, and the uranium concentration (mean 0.3 p.p.m.) in the primary clinopyroxene suggests that this mineral may have this uranium abundance in a peridotitic upper mantle.

1. Introduction

these results indicate that this glass is not chemically related to the basanite.

Ultramafic inclusions have widespread distribution in undersaturated basalts, and the abundance and dis- 3. Uranium distribution studies tribution of trace elements and their isotopes is an important consideration when discussing their origin and 3. 1. Uranium distribution in minerals and glasses history. This paper summarizes works already in press The same inclusions used in the U, Th and K study, as separate papers: KLEEMAN et al. (1969), COOPER and and three mote in addition, were studied for uranium GREEN (1969). For complete references, the reader is distribution using fission-track analysis. The fissionreferred to those papers. tracks were registered in Lexan plastic prints, and their densities compared to those produced simultaneously 2. Previous investigations in a standard by the same thermal neutron dose. The GREEN et a!. (1968) investigated the U, Th and K results are summarized in table 1. abundances of six inclusions and six basanite host The lherzolites have the typical four-phase primary rocks. The lherzolites have consistently higher Th/U assemblage of olivine, orthopyroxene, clinopyroxene ratios when compared to their hosts, and they found and spine!. In addition one has apparently primary much larger distinctions between them in the K/Th and phlogopite, another has hornblende, and two have priK/U ratios. Both these indexes showed much lower mary apatite. They range from dunitic, with little other values in the lherzolites compared with the basanites. than olivine, to lherzolitic, and rich in clinopyroxene. Although they considered the possibility of a very dis- In five of the inclusions the primary clinopyroxene (I) tinctive partition relationship between inclusion and has a high uranium content (mean 0.30 p.p.m. U), and host, the authors favoured the interpretation that the in most cases this is partially melted to a modified inclusions were accidental xenoliths preserving their phase (cpx Ia) with lower U content, which has the own geochemical characteristics. It was noted that 1 % same optic orientation, but slightly different chemistry. contamination by introduction of basanite liquid would This readjustment takes place in bands across grains, more than double the K content of the inclusions, and or on rims, and it is easily recognized by its more turbid produce element ratios halfway between those of in- nature, caused by the presence of glass blebs. Where clusions and hosts. This is particularly important be- the clinopyroxene is in contact with spinel, there is a cause two of the inclusions contained patches of glass: partial melting reaction involving them, and a liquid 302

303

ORIGIN OF SOME ULTRAMAFIC INCLUSIONS TABLE

1

Uranium abundance in phases of lherzolite inclusions Sample no.

Total rock

2640

0.0030

2728

0.0097

2642 2604 2700 2669 2659 2683 2638 Error

0.0180 0.0212 0.1136 0.0400 n.a. n.a. n.a. —

Clinopyroxene I Ia II

—

0.0057 0.042 0.35 0.28 0.37 0.33 0.16 —

3%

Apatite i Ii

Olivine I

Orthopyroxene I

Spinel I

Other

Phlogopite 0.0005

—

—

—

—

0.0002

0.0007

0.0002

—

—

—

—

0.0002

0.0006

0.0005

—

—

—

—

—

—

—

—

0.047 0.037 0.035 0.016 0.013 10%

38.4

_* —

0.014 0.012 0.016 20%

—

—

—

—

—

—

—

32.1 3%

5.8 6%

0.0005 0.0007 0.0006 0.0005 0.0007 0.0005 0.0002 20—40%

0.0031 0.0049 0.0035

0.0039 0.0022 0.0005 —

0.0002 0.0006 0.0007 0.0003 0.0005 0.0005

—

Hornblende 0.0005 — — — — —

—

15—40%

*

Insufficient resolution due to small size.

t

Only 2640 and 2638 included contact zone in fission-track specimen.

Glass at reaction sites

25—40%

—

100%

Interior glass veinlets

—

—

—

1.08—1.25

— 0.36 0.68 0.12—0.70 1.44 0.76—1.32 2.45—3.90 1.73—3.84 2.19—4.26 0.29—2.22 0.72—2.14 0.41—0.79 2.26—6.06 2.26—6.06 8—13% 11—20%

Glass veinlets near contactt 0.22—0.52 —

— — — — — —

1.85—3.70 11—20%

All figures p.p.m. uranium.

is produced, now preserved as a glass. In this glass there are euhedra of olivine II, spine! II, plagioclase, apatite II and clinopyroxene II, and this secondary clinopyroxene hasa lower uranium content again (mean 0.014 p.p.m. U). In all but one of the inclusions there is no continuity between glass formed at these partial reaction sites, and the glassy groundmass of the host basanite. Such continuity would have been immediately obvious from the plastic print, due to the relatively high uranium content (0.4—6.0 p.p.m. U) of the glass. Inclusion 2638 did, however, have physical continuity between glass formed internally, and the groundmass of the host, although there is a gradual change in chemical content. This inelusion also contained abundant apatite, and where the glass is in contact with it, the turbid primary (I) phase TABLE

has a clear, recrystallised rim, with euhedral outlines against the glass. This rim of apatite IL has much lower uranium content (5.8 p.p.m.) than the primary phase (32.1 p.p.m. U). 3.2. Crystal—liquid partitions Where clinopyroxene crystals are growing out of the glass, and where the apatite rims are clear, and have euhedral outlines against the glass, it can be assumed that there is at least local equilibrium between the glass (liquid) and these two phases. On this basis, partition coefficients have been calculated between the uranium contents of the glass and crystals at that point. The results of these calculations are shown in table 2. The U content of the glass is between 100 and 250 times as concentrated as the clinopyroxene crystallites growing 2

Uranium partition between coexisting phases and liquids Sample no. 2604 2700 2669 2659 2683 26381 2638ii

Glass veinlets/cpx Ia 16—28 46—106 8—63 37—72 — —

Glass patches/cpxll

Glass patches/Ap II

—

—

—

—

—

—

240—253—270 97—101—105 194—224—250 —

— —

0.50—0.84—1.45 0.86—0.99—1.24

All figures are ratios between the uranium contents of the minerals indicated.

Cpx I/Opx I Cpx I/O! I 71 80 95 150 320

500 470 740 470 320

—

—

—

—

304

J. D.

KLEEMAN

AND J. A. COOPER

out of it, and between 0.5 and 1.2 that of the apatite tween liquid and source material. This conclusion folrims. The partition between glass veinlets and recrys- lows if it is assumed that any liquid formed by melting tallizing clinopyroxene (Ia) is considered to be dynamic, is completely removed from the source material. Ifonly and so does not enter the present argument. part of the liquid generated in such lherzolite material These partitions apply to lower pressure conditions escaped, leaving some trapped, then on later cooling when compared to the primary assemblage, which was the melting process would be essentially reversed, and formed at pressures greater than 8 kb, but provided the partially extracted material would revert to an asthere are only slight compositional changes, the effect semblage with lower abundance of “basalt” compoof moderate pressures, of the order of 10 to 15 kb, nents, including U, and clinopyroxene. This clinoshould be slight. It is considered that there have been pyroxene would have a uranium content similar to the no major changes in the oxidation state between high pre-melting assemblage, but would be present in lower and low pressure conditions, especially since the modal amounts, compared to the primary assemblage. Fe~~/Fe~ + + ratios are high in both inclusions and hosts. 4. Lead isotope measurements These partition coefficients are now used to calculate the uranium content of a hypothetical liquid that could 4. 1. Measurements have been in equilibrium with the primary clinopyrThe isotopic composition of lead has been measured oxene I and apatite I. These calculations derive that in a suite of inclusions and hosts from the western this liquid would have had 28—35 p.p.m. U to have Victorian basanites. The vacuum volatilization technibeen in equilibrium with apatite with 35 p.p.m. U, and que makes it possible to extract lead from large quantibetween 30—75 p.p.m. U to have been in equilibrium ties of sample, and up to 300 grams were used. Despite with clinopyroxene containing 0.30 p.p.m. U. These this, only seven out of twenty lherzolite samples gave levels are a factor of ten to a hundred times higher than sufficient lead for isotopic measurements. The lherzolite those observed in basalts, even the very undersaturated lead content ranged from 0.3 p.p.m. to less than 0.01 nephelinitic magmas. It is therefore concluded that p.p.m., based on volatilization yield. Very low lead these inclusions have not formed as an accumulate content correlates with very low uranium abundance. from the magma in which they now occur, or in any The basanite lead content was much higher: 2—5 p.p.m. other natural basaltic magma. This data also implies The results are presented in tables 3 and 4, and that the lherzolite material could not have acted as plotted on figs. I and 2. The diagram insets show the source rock for the present host, or for a basaltic basanite results after isotopic fractionation has been magma, provided that equilibrium was maintained be- removed by double spiking. It is not possible to do TABLE

3

Lead isotope data and lead content of Lherzolites which yielded sufficient lead for isotopic analysis (samples not double spiked) Sample no.

2639 Mt. Leura 2642 Mt. Leura 2669 Mt. Shadwell 2903 Mt. Noorat 2904 Mt. Noorat 2697 Mt. Noorat 2700 Mt. Noorat

Pb extracted (p.p.m.) (pg)

206/204

206/207

206/208

207/204

208/204

Load composition

0.3

60

18.42

1.170

0.4708

15.73

39.11

oxalate

0.2

40

18.12

1.155

0.4694

15.68

38.60

oxalate

0.2

50

17.87

1.147

0.4685

15.58

38.14

oxalate

0.09

25

16.70

1.084

0.4594

15.40

36.35

sulphide

0.13

35

16.23

1.049

0.4499

15.47

36.08

sulphide

0.25

100

18.50

1.173

0.4720

15.77

39.20

oxalate

16.6

1.086

0.461

15.3

36.1

sulphide (multiplier)

0.2

15

305

ORIGIN OF SOME ULTRAMAFIC INCLUSIONS TABLE

4

Lead isotope data and approximate lead content of host basanite. Lower figures displaced to the right are figures corrected through double spiking using NBS 981 standard lead as reference calibrator. Sample no. 2642 Mt. Leura 2679 Mt. Shadwell 2693 Mt. Noorat 2740 Mt. Gambier 2909 (JC45) Mt. Schank

Pb (p.p.m.) 5

206/204 18.65k

206/207 1.1837

18.538 2

18.499

2

l8.72~

2

l8.52~

3.5

18.492

206/208 0.4764

1.1874 1.1837

18.433

1.l85~

18.517

0.4793

1.1903

18.384

this in the case of the inclusions, due to the smaller amounts of lead extracted from the samples. Most of the discussion is based on the un-normalized data.

15.558

0.4773

1.1777

38.51 39.17

l5.51~ 15.747

0.4761

38.46 39.40

l5.73~

0.4733

38.68 38.73

15.545

0.4808

l.l83~ 1.1743

39.15

15.824

0.4736

208/204

15.612 15.628

0.4754

1.1774 18.347

15.757 0.4793

0.4776

1.1835

207/204

38.44 39.07

15.609

38.61

and their mineralogical characteristics place their source within the upper mantle.

4.2. Discussion

5. Lead isotope evolution in the lherzolites If is assumed that these inclusions are accidental

The basanite points fall in a small group with similar ratios to modern lead, but the lherzolites are different, all being B type anomalous, forming a sub-linear pattern with various degrees of deficiency of 206Pb. If the lherzolite material was genetically related to the basanites, then each group must have the same lead isotope ratios. From this it can be said that the lherzolites are not residual material after the extraction ofthe basanite host magma, nor are they crystal differentiates from that magma. They are therefore accidental xenoliths,

xenoliths of an inhomogeneous upper mantle, some useful discussion follows regarding the processes which gave rise to the present spread of ratios. There are two reference lines on fig. 1. One is the lead growth curve for a /L value of 9.0 (23 8U/2 °4Pb).The straight dotted line is the line of best fit drawn through a large number of world-wide volcanics. It is not a regression line through the small number of points plotted from the present study, although it is not in disharmony with them.

/

LHERZOLITE

::~

__

206

Pb/204 Pb

Fig. 1. 206Pb/204Pb—207Pb/204Pb plot of unspiked lead isotope ratios of whole rock lherzolite and host basanite samples. Also in plot is p 8.99 growth curve and meteoritic zero isochron passing through the Primordial Point 9.56, 10.42. The inset shows the basanite values corrected for fractionation by double spiking calibrated to N.B.S. 981 value.

306

3. D. KLEEMAN AND J. A. COOPER

40

/LHERZOLITE • BASANITE

20 8

Pb

/

38

36

16

17

206 Fig. 2.

18

19

Pb/ 204Pb

208Pb/204Pb—206Pb/204Pb plot of unspiked lead isotope ratios of whole rock lherzolite and host basanite samples. The inset shows the basanite values corrected for fractionation by double spiking calibrated to N.B.S. 981 value.

A linear distribution of the lherzolite results could be explained in three ways: 1. example A modern of leadcontent, from twoand endone members. For onemixing of basanite poorer in radiogenic lead than the lherzolites. This seems unlikely from the U, Th and K results. 2. All the lherzolite samples hadearlier, similarand Pb the isotopic compositions about 2 b.y. ago or present spread is due to different proportions ofradiogenic lead added to each by different U/Pb environments, all apparently less than or equal to that of the basanites. 3. The lherzolites formed later from some other material which had this history. Physically this means that all the lherzolite material formed at this time in the past as a mass with the same isotope ratios, or an older lherzolite mass or its parent had its lead isotopes homogenized about 2 b.y. ago. The data do not exclude the possibility that some of the samples were formed at subsequent times, or that the lherzolites were residual or accumulates from earlier basaltic magmas. The U, Th contents and uranium distribution data do not support the latter alternatives. Fig. 2 demonstrates that the lherzolite lead has vary8Pb deficiency proportionate to 206pb ing degrees This of 20 indicates that the lherzolite material deficiency. has a limited range ofTh/U ratios despite large fluctuations in U/Pb ratio. The lead isotope composition of the lherzolites could

be satisfied by a two stage differentiation process from a primeval mantle source. A knowledge of t 1, the(i.e. time 238U/204Pb ~ ofdifferentiation, would enable the values) of each stage of each sampJe to be calculated. If it is assumed that the 232Th/238U ratio of the first stage (k 1 value) is equal of 3.9, 232Th/23 8U to forthe theprimordial next stage,value can also be then k2, thefor each sample (GAST, 1969). Table 5 shows calculated the calculated jz 1, #2 and k2 values if t1 is assumed to be 1.5, 2.0, 2.5 and 3.0 b.y.. All samples require that #2 is lower than t~ifor all values of t1. This suggests loss of a component of higher /2 value at this time. lt is considered that the small range ofcalculated j~(8.6—9.1) justifies the use of an invariant primordial k1 value to calculate approximate k2 values. Since it has already been concluded that the lherzolite has not passed through a modern chemical fractionation, the calculated k2 values can be compared with the direct measurements of Th/U (= 3.7—5.0) reported by GREEN et a!. (1968). If t1 equals 2.0 or 2.5 b.y. there is a much more satisfactory agreement than the older or younger selections of t~for the group as a whole, although some individual samples are not restricted by this range. 6. Uranium distribution in the upper mantle The uranium distribution data are consistent with a hypothesis that the inclusions are accidental xenoliths of inhomogeneous upper mantle material, and the lead

ORIGIN OF SOME ULTRAMAFIC INCLUSIONS TABLE

307

there may be locally an additional 0.07 p.p.m. avail-

5

Calculated p~,Pz and k 2 values for lherzolite lead isotope composition assuming a two stage history, 4 different values for t1, k1 = 3.9, and the promordial lead isotope ratios of MURTHY and PA1-rERsON (1962), and following GAST (1969)

rom eat ow consi erations. 6.2. Basalt genesis

Sample

t1 (b.y.)

P1

112

2639 2642 2669 2903 2904 2697 2700

15

8.9 8.9 8.7 8.6 8.8 9.0 8.4

8.3 7.2 7.1 2.6 0.2 8.3

5.4 5.3 4.7 4.3 26.0 5.4

2.8

3.4

2639 2642 2669 2903 2904 2697 2700

20

89 8:9 8.7 8.6 8.9 9.0 8.5

8 5 7.7 7.2 4.1 2.4 8.5

5.0 4.9 4.7 4.0 5.2 5.0

4.2

~.6

2639 2642 2669 2903 2904 2697 2700

2 5

8.9 8.9 8.8 8.8 91 9:0

4.7 4.6 4.5 3.9 45 4.8

8.6

8.5 7.9 7.5 5.0 36 8:5 5.o

3.7

tinue to enter the liquid. Using the pyrolite model as a basis for a model for uranium distribution in the upper mantle, as described

2639 2642 2669 2903

30

90 9:0 8.9 9.0

86 8:0 7.7 5.6

46 4.5 4.3 3.9

above, the uranium abundances of basalts can be calculated from the degrees of melting inferred from major element and phase relationship studies. The results of these calculations are consistent with observed abun-

9.0 8.9

8:6

4.6

dances in basalts.

5.6

3.7

2697 2700

k2

able, if apatite is present. These estimates compare very favourably with those

isotope data suggest that all of these samples have been reacted geochemically in the past. This implies that they are not primordial upper mantle material, and it is doubtful if such primordial mantle material still exists, 6. 1. Uranium abundance in the upper mantle A major point of this aspect of study is that there is an important amount of uranium in an essential phase of the lherzolite inclusions; clinopyroxene would account for 15—20% of the modal content of a pyroxene pyrolite zone in the upper mantle. Ifthis clinopyroxene had a mean abundance of 0.3 p.p.m. U, then this alone would account for 0.045—0.060 p.p.m. in such a rock. The possibility that there may also be up to around 0.2% modal apatite with about 35 p.p.m. U means

The primary assemblages of at least six of the inclusions have the potential for producing basalt liquid of normal uranium content, and therefore a model upper mantle based on them would be very suitable from the point of view of the geochemistry of uranium. The first infinitesimal drop of liquid formed by partial melting would necessary be of very high uranium content, since it would be in equilibrium with the primary clinopyroxene. However the observed clinopyroxene/liquid .

.

.

partition coefficients imply that for slightly higher degrees ofmelting, the uranium is rapidly partitioned into the liquid, producing liquids (nepheline normative on other grounds) with high U content, but with the U content decreasing with higher degrees of melting, as low-U clinopyroxene (Ia), enstatite and olivine con-

7. Conclusion The primary conclusion from the three studies summarized in this paper, is that the lherzolite inclusions from the newer volcanics of western Victoria are not accumulates from their host basamtes, and they have not acted as source rock for the generation of that magma in which they now occur. The inclusions are interpreted as accidental xenoliths of an inhomogeneous peridotitic upper mantle. As such, speculations on the evolution of the lead isotopes indicate that they were geochemically fractionated at about 2.0—2.5 b.y.. A model for the uranium distribution in a peridotitic upper mantle, based on the highuranium primary assemblages in the lherzolites, is found to satisfy predictions from heat-flow considerations, and provide a suitable source rock from which to extract basalt magmas of typical uranium content.

308

J. D. KLEEMAN AND J. A. COOPER

Acknowledgements These studies were initiated at the suggestion of Dr. D. H. Green, who also criticised this manuscript. The cost of neutron irradiations for the uranium distribution studies was met by a grant from the Australian Institute of Nuclear Science and Engineering.

References COOPER J. A. and D. H. GREEN (1969) Earth Planet. Sci. Letters

6, 69. GA5T, P. W. (1969) Earth Planet. Sci. Letters 5, 353. GREEN, D. H., J. W. MORGAN and K. S. HEIER (1968) Earth Planet. Sci. Letters 4, 155.

KLEEMAN I. D., D. H. GREEN and J. F. LOVERING (1969) Earth

Planet. Sci. Letters 5, 449.

MURTHY, V. R. and C. C. PATTERSON (1962) J. Geophys. Res.

67, 1161.