Osmium isotopes in geology

hzternotiona~ Jo‘ournnlof Mass S”ectromefry

297

and Zon Physics

ELsevierPublishing Company. Amsterdam. Printed in the Netherlands.

OSMIUM

ISOTOPES

IN GEOLOGY

G. H. RILEY*

SonthwestCenterfor Adcanced Stud&s, P.O. Box 30365, Dallas, Tex. 75.230 (U.S.A.) 5. E. DELONG Department of Geology,

The University of Texas, Austin, Tex. 78712 (U.S.A.)

(Received December 12tl1, 1969)

ABSTRACT The &kcay, “‘Re --, ls’Os , has a sufficiently long half-life to have applications in geology for both dating and natural isotopic tracer studies. Tmprovements in analytical techniques are urgently needed to explore these applications and to establish the detailed geochemical relationships of rhenium and osmium with greater certainty. Despite the dearth of published data, some trends are clear: (i) Rhenium is enriched into the crust relative to osmium. (ii) Geochemical correlations between rhenium and molybdenum and between osmium and nickel seem plausible. (iii) There is sufficient rhenium in the mantle to modify ?he isotopic abundance of 18’ OS in mantle derivatives, by a time dependent term. (iv) A mantk Os/Re ratio comparable to that in chondritic meteorites satisfies common osmium isotopic abundances. INTRODUCTION

During the past two decades, isotopic measurements applied to problems in the earth sciences have expanded rapidly in scope and in the degree of instrumental sophistk~tion. In particular, geologic investigations based 011long-iived radioactive isotopes with sui‘able parent and daughter ekrnents have yielded a wealth of information on geoIogic age, metamorphic history, and petrogenesis’*. Data relevant to all three of these important facets of isotope geology may be attained by application of a single decay scheme in certain circumstances. The most fruitful work of this sort has resuited from investigations using the decay schemes “Kb -+ “Sr, 238U -, 2G6Pb, and 23sU -+ “‘Pb, These systems * Present addres: Kemwxtt Explorations (Australii) Pty; Ltd., 190 Hay St. East, Pcrtb, Westem

Australia,#OO.

znt- 3. MLss Spectn7m. zcn Pkys., 4

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G. H. RILEY,

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have been particuIarIy successful because of the wide dispersal of the parent isotope in geologic materials and because of the geochemical incoherence of the parent and daughter elements. Thus, mineral phases enriched in either rubidium or uranium are usuahy substantially depleted in non-radiogenic strontium or lead- On a larger de, the crustai enrichment ofrubidium is paralleled by a depletion of this element with respect to strontium in t5e earth’s mantle. The utility of the above systems is well documented in the literature and their value as geochronologic indicators is self-evident. There remains a tremendous number of terrestrial geologic problems suitable for the application of current techniques at their present leve! of development. Nevertheless, there is a great challenge and real need to improve techniques’9~26 related to commonly used decay schemes and to examine the feasibility of using suitable, alternate decay schemes involving other naturally occurring isotopes. transition and to present We wish to draw attention to the 183Re 4 “‘0s information which suggests that osmium isotope research will provide valuable information on the chemico-physical structure of the earth. Hopefully, these comments will motivate researchers to undertake the necessary development work to formalize techniques suitable for the analysis of osmium isotopes in various geologic materials_

“‘I&

+

187f3S SYSTEMATIC5

The rare element rhenium has two naturally occurring odd-Z, cdd-A isotopes The noMe metal osmium, has seven naturally occurring, stable isotopes_ The isotopic abundance of the nuclides of these two metals is summarized below2’_ Nuciide

% Abundance

18’Re

37.07 62-93

?87Re 1840~

0.018

1s60S 1870S

1.59 I.64 13.3 -16.1 26.4 41 .o

188

OS

lS90S 1900s ‘g20S

The major isotope of rhenium is radioactive and decays to 1870~ by fl emission’* with a half-life cf 3.3 x lOro years’l_ The common abundance of the

interfering isotope is low ( 1870~, 1.64 o/,) thus enhancing the applicability of the decay scheme to rocks with only moderate enrichments of rhenium/osmium. IM- J. Mass Spectrom. Ion Phys.,

4 (1970) 297-304

OSMIUM

ISOTOPES

IN

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GEOLOGY

The total decay energy of the transition is extra-ordinarily low’* so that the &spectrum is heavily weighted by low-momenta particles. This feature has caused grave difficulties in establishing a precise half-life by counting techniques. However, comparative age determinations of rhenium-bearing minerals have resulted in a geologically calibrated half-life whose uncertainty is suEcientiy small to make it useful in many applications.

GEOCHE&fISTRY

The Iow abundances and the difficulty of analysis of rhenium and csmium have not been conducive to determinations of these elements in common rock types. Until the use of neutron activation techniques”’ during the past several years, knowledge of the geochemical distribution of rhenium and osmium was restricted to observations of a few mineral species in which these elements were extraordinarily enriched_ Rhenium is a widely dispersed element that forms no minecal in which it is a major constituent (except the recently discovered, and rare mineral, dzhezkazganite, containing 43 o/o Re, which occurs as microscopic grains in copper orestO). High concentrations are observed in some varieties of molybdenite deposits. Apart from moIybdenite, the concentration of rhenium does not exceed 1 p-p-min other mine&s except for slight enrichments in a few cupriferous ore bodies3s4. Osmium is also rare in crustal rocks. The naturally occurring alloy, osmiridrum, is composed of the metallic elements osmium and iridium and contains traces of the other platinoid elements. In addition to natural alioys, osmium occurs as sub-microscopic grains of complex platinoid su1phides2s. All known occurrences arc inT+ariabIy associated with uItrama& rocks or aIIuvia1 deposits derived from ultramatics8. Although the total number of observations is small, some rather broad generalizations can be proposed. Riienium is demonstrably enriched in molybdenite and is most often associated with silicic rocks. The platinoids, incIuding osmium, are contained in uiframafic mantIe derivatives and one may infer a geochemical coherence with the other Group-8 elements (Fe, Co, and Ni). If substantiated by further measurements, such a gross partitioning between parent and daughter elements is, of course, an extremely useful property. The apparent coherence between rhenium-molybdenum and osmium-nicke1 wf1l be further cieve!opcd here. Recent whole-rock determinations of very low concentrations of rhenium and osmium by neutron activation have supplied a source of data for comparison of these elements with molybdenum and nickel concentrations. Table 1 cdntains representative data of the concentrations of these four e1ements in various rock types and meteorites. The available data give inadequate coverage of the entire geochemical environment and the accuracy of some determinations may be.quesInt- J. Mass Spzctrom

bn Phys., 4 (i970)

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G. H. RILEY,

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Sample

Re

Cbondrites Tron meteorites W-l (Diabase) AGV-1 (An&site) BCR-1 (Basalt) PCC-1 (eridotite) DTS-I (Dunite) G-l (Granite)

* Minimum

OS

.S_ E_ DELONG

Ni

MO

(p-p. 6.)

Ref.

(p.p.6.)

Ref.

(p.p.m.)

Ref.

@p.m.)

56.9 505 0.42 0.34 G.34 0.046 0.013 0.56

17 6, 11 I6 16 16 i6 16 16

662 6,857 0.26 so.02 10.03 5.9 1.27 SO.06

I,17 1I,23 I6 ?6 16 16 I6 16

1.6 10.4 0.5 3.7 3.9 5.5 6.6 7

12

27 5 2 2 2 2 5

16,400 13 85,900 13 78 5 17.8 2 15.0 2 2,430 2 2,330 2 1.5 5

values quoted.

tionable. Nevertheless, they are useful enough to provide some insight into correlations of the four elements. Theoretical descriptions of solid-liquid equilibria based on the logarithmic distibution !awf4 may be extended to the concentration ratios of the four elements of interest here. An equation relating the Ni/Os and Mo!Re ratios may be written as

where K=

k,i --ko,

k .Mcb -kmThe terms k,i,ka, khfo,and kRo are Nernst distribution coefficients’. The superscript “0” indicates the cuncentration ratio in the originai Iiquid system. A similar equation may be written to relate the Re/Os and MO/N ratios.

Fig. 1. Coherent pair ratios show two distinct linear treads lat. I. Ma& .Sp&rro~!~ 10s Pkjs, .-

4 (1970)

297-304

on a logarithmic

Ref.

OSMIUM

Ref 0s

o.os60 0.0736 1.6 *17 +2s 7.8 x lo- .3 0.010 *9.3

ISOTOPES

IN

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GEOLOGY

MofNi

M/OS

MolRe

9.8 x 10-S 1.21 x 1o-4 6.4~10-~ 0.208 0.260

2.48 x 101 I.25 x IO4 3.0 x 105

28.1 20.6 1.19 x 103 1.09 x loa 4.6 x lo3 1.2x 105 5.1 x lo5 1.25x lo4

‘8.9 x 105 *5.0x lo5 4.1 x 105 1.83x 106 *2.5x lo*

2.26x 1O-3 2.8 x10-3 4.67

If we use the data iisted in Table I and plot the log of the ratio of each coherent pair (i.e., log N/OS vs. log Mo/Re) as shown in Fig. 1, two distmct linear trends appear to be present. The same situation (with the same samples linearly related) arises in a plot of the non-coherent ratios (Fig. 2).

-4

-3

-2

-1

0

I

2

Log,,Mo/x. Fig. 2. The iogari*chmicdistribution law is maintaixd

for non-coherent pair ratios.

It is not clear that the logarirh.mic distribution law should be applicable to such a disparate suite of samples, so that definitive interpretation of these linear trends is difficult. The details of the geochemical relationships suggested by these plots may, in fact, prove to be spurious when more osmium concentration data become available. Nonetheless, with the extant data, coherence betwezm molybdenum and rhenium and between nickel and osmium is a reasonable working hypothesis. Even without the support of these intriguing correlations, it is evident that concentrations of rhenium and osmium are inversely related in the geologic environmenP.

Ink 3. Mass Spectrom. fan P&s., 4 (1970) 297-304

302

G. H. RILEY,

S. E. DELONG

A rhenium-free environment wiii have an unchanging “‘0s isotopic abundance. While it is cIear that magmatic processes have enriched the crust in rhenium relative to the mantIe (Fig. 2), there is no evidence of complete depletion of rhenium in the mantle system- An estimate of the Re/Os ratio may be calculated from observations of a secular radiogenic increase in the “‘OS isotopic abundance in primary mantle-deri\.ed materiais. Very few comparisons of the natural isotopic composition of osmium have ever been made because of analytical di&ulties and limited availability of osmiumrich samples. Published data are reproduced here in Table 2. Descriptions and TABLE 2 XP--l-VW ISOTOPIC

COMPOSITION

OF

OSMIL??

Sample

Source

Crude pIarinum Crude pfatinum cbniridhm Osmiddium asmiridium Osmiridium OSIllilidiUm Iron meteorite

Alaska South America UEIIS

AxstraIia Austrrilia south Africa KWwatersrand Initial ratio

I.082

~0.009

I.033 iO.006 1.023 po.009 1.086i0.010 1.014~0.012 0.890~0.008 0.882&0.007 0.830

AI1 data from ref. II.

precise localities of the terrestrial samples are unspecified in the originai publication. However, as the samples were either osmiridium or platinum concentrates, it is reasonable to ass?Jmethat they are cognate with ultramafic mantle material!. By examining information on commercial platinoid ore deposits in the geographic source area given for each sample, an estimate of their respective ages can be made. It is possible, then, by assuming that the mantle is a well-mixed reservoir to plot

mEE?fEFm

mEs?zm-

fIfJ&’

Fig. 3. The growth of upper -tJe ra70s/1860s ratio shown as a function of time. Oveqlotted an the diagram, are mode1 growth lines of vzuious mantJe compositions (data from Table I).

Pyrolite 1 correspondsto a composition of dunite:basaIt= 3:l; pyrolite 2 is peridotite:basaIt= 3:l. ht.

J. Mass Spec:rom. Ion Phys.., 4 (1970) 297-304

OSMIUM

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the evoi?ltion of 1870s in terms of the ratio 1s70s/‘860s for the &mantleenvironment. Figure 3 shows that the 1870s/1860s ratio has evoIved from a meteoritic value of 0.83 to an extrapolated value of about 1.07 in present-day mantle material, i.e., a change of about 6.3 ‘A per 10’ years. (To emphasize the magnitude of this change, the 87Srj86Sr ratio evolving from the meteoritic vaIuelg of 0.698976 at the same growth rate would have a present-day ratio of about 0.901. In fact, recent mantle-derived volcanic roclcs have uniformly iow 87Sr/86Srratios” in the range 0.701-0.705, corresponding to a growth rate of 0.126 % per 10’ years.) Interpretation of Fig_ 3 could be slightly complicated by the a-decay of “*Pt to 1860s. However, unless the half-life is substantially shorter than 1012 years or unless the Pt/Os ratio of the mantle is much greater than that observed in meteorites, this decay would cause no significant change in the conclusions made here. The intriguing possibility remains that, with a very major improvement in instrumentation, measurements of the coupled Re-Pt-0s isotopic systems in uftramafic rocks couId yield information on the long-term chemical interaction of the earth’s mantle and core. These striking variations in the 18’Os isotopic abundance lead to speculations on the Os/Re ratio in the mantle in the context of various compositional models. Tf the upper mantle is essentially a peridot-ite-ciunite mixture with an OsjRe ratio of about iO0, the rate of change of 1870s/1860s would be about 0.7 % per 10’ years. h mantle-model based on chondritic analyses (average Os/Re ratio of 11.6) produces an 1870s/1860s growth rate cf about 7 ‘;‘o per 10’ years, and is therefore compatible with the observed ratios in platinoid ores (Fig. 3). The pyroiite model requires a mantle composition of one part basalt plus three parts of ar, ultramafic rock component. The paucity of Re-Os data pertinent to such mixtures prec!udes any definitive statement concerning their 1870s/‘860s evolution. Nevertheless, two pyrolite modelsg*22 (based on the1 imited data in Table 1) are shown in Fig. 3 in order to expIore their compatibility with the isctopic data. Future analyses will aid in resolving these differences and will afford a more reliable comparison with the chondritic model.

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