The origin of unequilibrated EH chondrites – Constraints from in situ analysis of Si isotopes, major and trace elements in silicates and metal

Abstract

Various lines of evidence suggest that material isotopically similar to enstatite chondrites may have accreted to the terrestrial planets. However, the enrichment of light Si isotopes in bulk enstatite chondrites is not easy to reconcile with the heavy Si isotopic composition of the Bulk Silicate Earth (BSE). To investigate the origin of the light Si isotopic composition of enstatite chondrites, we have obtained in-situ Si isotope data and simultaneously major-and trace element data in silicate and metal phases of chondrules, a metal-troilite spherule, and matrix from the enstatite chondrites Sahara 97072 (EH3) and Indarch (EH4) using laser ablation split stream-ICP-MS, which combines femto-second LA-MC-ICP-MS and Quadrupole-ICP-MS. Silicates in chondrules show variations in δ 30 Si (‰ variations of 30 Si/ 28 Si relative to NBS-28) ranges from-1.06 ± 0.13‰ (2 S.E.) to-0.38 ± 0.11‰. δ 30 Si in matrix silicates ranges from-0.96 ± 0.18‰ to-0.22 ± 0.12‰. The δ 30 Si-value of silicate phases varies independently of Mg/Si, ruling out simple equilibrium condensation from nebular gas. Some silicates in both enstatite chondrites have δ 30 Si-values like CI chondrites, whereas Si in other silicates is isotopically lighter, suggesting that the precursor materials of EH chondrites were already depleted in heavy Si isotopes. The metal phases in the matrix show average δ 30 Si of-6.0 ± 0.6‰. In spite of different metamorphic grades, the fractionation of Si isotopes between matrix metal and silicates in Sahara 97072 and Indarch shows no systematic differences, and thus no re-equilibration of Si 2 isotopes occurred between silicates and metal at metamorphic temperatures below 900 K. The δ 30 Si-value of metal from a metal-troilite spherule from Sahara 97072 (-8.24 ± 0.12‰) is lower than that of matrix metals. These differences were likely inherited from different formation environments of matrix-and spherule metal. If metal formation occurred under equilibrium conditions, then matrix metal may have formed at higher temperatures than the MTS metal. or at similar temperatures but slightly lower oxygen fugacities, or the MTS metal equilibrated with gas or silicates which were not incorporated into EH chondrites because they were lost from the EH chondrite formation region. Alternatively, the differences in δ 30 Si of different metals could also reflect variable kinetic isotope fractionation during the formation of metal and exsolution of perryite. The considerably lower δ 30 Si-values of bulk EH chondrites compared to CI-and other chondrites partly reflects the presence of Si bearing metal with isotopically light Si and partly silicates with isotopically light Si. The latter indicate loss of a heavy Si-rich silicate component from the EH3 formation region, presumably together with refractory elements. Although the Si isotopic composition of bulk EH chondrites precludes that these represent major building material of the Earth, the combination of complementary heavy Si isotope-and refractory element-enriched reduced materials and ca...

Subsolar Al/Si and Mg/Si ratios of non-carbonaceous chondrites reveal planetesimal formation during early condensation in the protoplanetary disk

“…In some sense, with the first refractory-rich planetesimals we have identified the hidden reservoir of objects with the same isotope composition as the ECs but different chemical composition, that was postulated by Dauphas (2017) to explain the Earth. The opposite mass-dependent isotopic fractionation of Si in the Earth and ECs may be a natural consequence of the formation of the first condensates, as discussed by Kadlag et al (2019).…”

Early solar irradiation as a source of the inner solar system chromium isotopic heterogeneity

“…53 Mn-53 Cr Systematics of Physically Separated Components and Leachates Chromium in enstatite chondrites is mainly hosted in sulfides like daubréelite (Ramdohr, 1963) and troilite (Weyrauch et al, 2018;Zhang et al, 1995). In other sulfides as well as in silicates and metal, it can be a minor or trace element (Kadlag et al, 2019;Mason, 1966). MTSs from Sahara 97072 have lower ε 53 Cr and ε 54 Cr compared to all other fractions (Fig.…”

“…The ε 54 Cr variations among different inner solar system objects and among different components of chondrites can be explained by the incorporation of dust grains and pebbles with variable degrees of irradiation. Some of these components (e.g., MTS) may have formed in different environments (Kadlag et al, 2019) compared to the main components of the EH chondrite parent body. The lower ε 53 Cr and ε 54 Cr in the MTS fraction from an EC suggest shielding of bigger metal grains, possibly in an early formed planetary body, that was disrupted during collision and partly re-accreted.…”

“…Blank contributions (<1 ng) are negligible compared to the total amount of Cr in the sample aliquots. The Cr separation procedure is described in Kadlag et al (2019).…”

“…Accretion of variably irradiated material, including more strongly and weakly irradiated material than EC, is a possibility. Several studies have shown that a predominant EC component is inconsistent with the chemical and stable Si isotope composition of the Earth (Fitoussi & Bourdon, 2012;Kadlag et al, 2019;Palme & O'Neill, 2014;and references therein). However, since the true irradiated endmember compositions in Earth and Mars are currently unknown, mixing proportions are poorly constrained.…”