Tin in mantle-derived rocks: Constraints on Earth evolution

Abstract

We have analyzed tin together with other trace elements in a variety of mantle-derived rocks by isotope dilution-spark source mass spectrometry. Tin concentrations in oceanic basalts (mid-ocean ridge basalts, or MORB; oceanic island basalts, or OIB; island arc volcanics, or IAV) vary from 0.4 to 6 ppm. Tin is a moderately siderophile element which behaves like the moderately incompatible and lithophile element Sm during igneous processes in the mantle. The Sn Sm ratio is therefore similar in various reservoirs of the silicate portion of the Earth at the present day. To investigate possible secular variations of siderophile/chalcophile element abundances, we have also analyzed komatiites and basalts with ages ranging from Archean to Tertiary. Rocks having ages up to 3.4 Ga yield Sn Sm ratios comparable to those of the oceanic basalts. These results indicate that core growth during the last 3.4 Ga was negligibly small. In addition, the Sn Sm ratios in different mantle reservoirs are identical within the uncertainty of the data scatter. This indicates that tin was homogenized within the mantle after core formation. We conclude that the value of Sn Sm = 0.32 found in mantlederived rocks is representative of the primitive mantle because the estimated continental-crustal abundances of Sn and Sm yield a similar Sn Sm ratio. This results in Sn = 0.12 ± 0.01 ppm for the primitive mantle abundance, in agreement with that determined from fertile peridotite xenoliths (0.15 ± 0.02 ppm). To determine an absolute depletion factor for the volatile siderophile element tin in the primitive mantle, we have also analyzed four carbonaceous chondrites, including the CI chondrites Orgueil and Ivuna. We obtain a CI-value of tin of 1.62 ± 0.03 ppm. Tin is depleted in the Earth's primitive mantle by a factor of 33 ± 3; the volatility corrected depletion factor is similar to other moderately siderophile elements ( Newsom, 1990).