INDIUM AND TIN IN BASALTS, SULFIDES, AND THE MANTLE

Abstract

The geochemistry of In and Sn are poorly understood, in part, because of difficulties in obtaining accurate concentrations for these elements in geological materials. Furthermore, In/Sn ratios in sulfides could be sufficiently high to facilitate the use of 115In- 115Sn geochronology, if the separation and precise measurement techniques were available. In this paper we describe methods for the separation of In and Sn from silicates and sulfides. Indium can be measured by thermal ionization mass spectrometry (TIMS) at very high sensitivity (>13%). However, its mass fractionation is difficult to correct reliably. Tin is more difficult to measure by TIMS because of its higher ionization potential. Both elements can be measured effectively using the new technique of MC-ICPMS, since the ionization efficiency is extremely high, molecular interferences are negligible, and mass fractionation in spiked In and Sn can be corrected by monitoring the mass bias in admixed Pd and Sb, respectively. Using these techniques, it is demonstrated that In and Sn concentrations can be measured reliably for silicates and sulfides. Indium and Sn data for international silicate rock standards are in excellent agreement with recommended values. The Sn/Sm ratios determined for ocean island basalts (OIB) are within the same range as those recently reported, where Sn was measured by spark source mass spectrometry. Indium is very uniform in OIB and behaves as a slightly incompatible trace element, comparable in bulk distribution coefficient to the heavy rare earths or Y. In/Y in OIB is very uniform, averaging 0.0028 +/- 0.0005 (1σ), but is weakly related with Pb/Ce, implying that these ratios may be partly controlled by sulfide at small degrees of partial melting. The similarity in average In/Y between OIB, N-MORB (0.0025) and the continental crust (0.0025), together with the similarity in Sn/Sm in MORB, OIB, and continental crust contrasts with chalcophile/lithophile and siderophile/lithophile element ratios such as Pb/Ce and W/Ba, which are high in the continental crust because of decoupling in the subduction environment. The overall behavior of both In and Sn within the silicate Earth is dominated by lithophile affinity. The primitive mantle is estimated to have In/Y = 0.003 +/- 0.001, both higher and lower than previous estimates and corresponding to an In concentration of 14 ppb. Ignoring any In that may have been partitioned into the core, the corresponding total Earth concentration of >10 ppb corresponds to <85% depletion relative to CI chondrites. This is less depleted than anticipated by at least a factor of 2, given the supposed volatility of In based on assumed condensation temperatures and depletions in volatile lithophile elements. There is no evidence that In has been segregated into the Earth's core. This can be explained if, during 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 temperature rather than as a volatile chalcophile element. Hence, the In/Y ratio of Earth's primitive mantle may be representative of the mixture of volatile depleted and undepleted material that accreted in the inner solar system. SNC meteorites have a similar range of In/Y to the silicate Earth, suggesting Mars accreted from a similar mixture of material already depleted in In, and presumably other moderately volatile elements. In contrast, the In/Y ratio in lunar basalts ranges through four orders of magnitude from silicate Earth values in lunar soils to extremely In-depleted compositions. This is unlikely to be caused by heterogeneous 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 tetrahedrite that the predicted 115Sn excess caused by decay of 115In in ancient sulfide deposits should be measurable with MC-ICPMS.