INDIUM AND TIN IN BASALTS, SULFIDES, AND THE MANTLE

The geochemistry of In and Sn are poorly understood, in part, because of difficulties in obtaining accurate concentrations for these element s in geological materials. Furthermore, In/Sn ratios in sulfides could be sufficiently high to facilitate the use of In-115 - Sn-115 geochro nology, if the separation and precise measurement techniques were avai lable. In this paper we describe methods for the separation of In and Sn from silicates and sulfides. Indium can be measured by thermal ioni zation mass spectrometry (TIMS) at very high sensitivity(>l3%). Howeve r, its mass fractionation is difficult to correct reliably. Tin is mor e difficult to measure by TIMS because of its higher ionization potent ial. Both elements can be measured effectively using the new technique of MC-ICPMS, since the ionization efficiency is extremely high, molec ular interferences are negligible, and mass fractionation in spiked In and Sn can be corrected by monitoring the mass bias in admired Pd and Sb, respectively. Using these techniques, it is demonstrated that In and Sn concentrations can be measured reliably for silicates and sulfi des. Indium and Sn data for international silicate rock standards are in excellent agreement with recommended values. The Sn/Sm ratios deter mined for ocean island basalts (OIB) are within the same range as thos e recently reported, where Sn was measured by spark source mass spectr ometry. Indium is very uniform in OIB and behaves as a slightly incomp atible trace element, comparable in bulk distribution coefficient to t he heavy rare earths or Y. In/Y in OIB is very uniform, averaging 0.00 28 +/- 0.0005 (1 sigma), but is weakly related with Pb/Ce, implying th at these ratios may be partly controlled by sulfide at small degrees o f partial melting. The similarity in average In/Y between OIB, N-MORB (0.0025) and the continental crust (0.0025), together with the similar ity in Sn/Sm in MORB, OIB, and continental crust contrasts with chalco phile/lithophile and siderophile/lithophile element ratios such as Pb/ Ce and W/Ba, which are high in the continental crust because of decoup ling in the subduction environment. The overall behavior of both In an d Sn within the silicate Earth is dominated by lithophile affinity. Th e primitive mantle is estimated to have In/Y = 0.003 +/- 0.001, both h igher and lower than previous estimates and corresponding to an In con centration of 14 ppb. Ignoring any In that may have been partitioned i nto the core, the corresponding total Earth concentration of >10 ppb c orresponds to <85% depletion relative to CI chondrites. This is less d epleted than anticipated by at least a factor of 2, given the supposed volatility of In based on assumed condensation temperatures and deple tions in volatile lithophile elements. There is no evidence that In ha s been segregated into the Earth's core. This can be explained if, dur ing the earlier stages of accretion, under reducing conditions, In was too volatile to be transferred into the core. During the later stages of accretion, conditions may have been relatively oxidising such that In behaved as a lithophile element with higher condensation temperatu re rather than as a volatile chalcophile element. Hence, the In/Y rati o of Earth's primitive mantle may be representative of the mixture of volatile depleted and undepleted material that accreted in the inner s olar system. SNC meteorites have a similar range of In/Y to the silica te Earth, suggesting Mars accreted from a similar mixture of material already depleted in In, and presumably other moderately volatile eleme nts. In contrast, the In/Y ratio in lunar basalts ranges through four orders of magnitude from silicate Earth values in lunar soils to extre mely In-depleted compositions. This is unlikely to be caused by hetero geneous distributions of extreme In depletion on the Moon as a result of volatile depletion. Rather, the more reducing conditions appear to result in In behaving as a relatively compatible trace element during lunar melting and differentiation. Although their behavior on Earth is strongly lithophile, In and Sn are sometimes enriched in sulfides and In/Sn can be sufficiently high in some sphalerite, chalcopyite, and t etrahedrite that the predicted Sn-115 excess caused by decay of In-115 in ancient sulfide deposits should be measurable with MC-ICPMS.