HIGH-PRECISION SILICON ISOTOPE RATIO MEASUREMENTS OF EARTH AND ENSTATITIC METEORITES AND IMPLICATIONS FOR Si ISOTOPE FRACTIONATION DURING CORE FORMATION

Introduction: Core formation processes in planets may impart stable isotope ratio signatures on the bulk silicate Earth due to partitioning between the metallic core and silicate [1]. Enstatite chondrites (E-chondrites) are the only primitive meteorite group with stable oxygen isotope compositions similar to Earth [2]. The Echondrites also possess a metal phase with substantial amounts of Si. We have begun a program of analyses aimed at searching for Si isotope fractionation between metal and silicate in E-chondrites and related rocks with the goal of testing the suggestion that there is substantial Si isotope fractionation between these phases during planet formation. Silicon has long been suggested as a probable light element in Earth’s Fe-Ni core [3]. New studies of silicon isotopic compositions (Si/Si) of the silicate phases of Earth and E-chondrites [1, 4] produced conflicting results as to their similarity or dissimilarity. Differences observed in Si/Si between mantle rocks and meteorites in general have been attributed to fractionation between core and mantle (metal and silicate) [1]. A recent experimental study of Si isotope fractionation between silicate and metal phases suggests a 2 ‰ Si/Si fractionation between Si in silicate and Si in metal at high pressure and temperature [5]. If such a fractionation is generally applicable [1], even at lower pressures for example, then the E-chondrites, with their abundant Si in metal, provide the opportunity to test the assertion that there is a strong Si isotope fractionation between metal and silicate in rocks. Our first Si isotopic data for Earth’s mantle and Emeteorite silicate suggest that there is no substantial difference between them, contradicting the results of Georg et al. [1], but supporting those of Fitoussi et al. [4]. Samples and Methods: Silicon Isotopes. To date we have Si-isotope data on San Carlos (SC) olivine and the silicate phase of the Mt. Egerton meteorite (USNM 3272). Mt. Egerton is an enstatitic meteorite formed from E-chondrites; it mainly consists of coarse enstatite and metal containing 2.06 wt.% Si. Silicon was extracted from silicate using dilute HF [6], and from iron metal by dissolution in concentrated HCl to form a silica gel [7] that was then dissolved in dilute HF. These digestions were further diluted with MilliQ water by a factor of 250 prior to purification. Silicon was purified for isotopic analysis using a cation exchange resin [8]. Silicon isotope ratio measurements were made with a ThermoFinnigan Neptune multiplecollector inductively coupled plasma-source mass spectrometer (MC-ICPMS). Corrections for instrumental mass bias were performed by sample-standard bracketing. Internal precision for the measurements is ±0.06 ‰ (2se) or better based on replicate standard analyses. The external reproducibility of our in-house standard is similar to the internal precision. Silicon isotopic analyses of more meteorites, including the metaland silicate phases of some chondrites, are in progress, and results will be reported at the conference. Oxygen Isotopes. High-precision infrared laserheating fluorination with an analytical precision of ~0.02 ‰ is essential for the differentiation of close or overlapping ΔO ranges. We obtained oxygen isotope data for 7 E-chondrites, for San Carlos (SC) olivine and other Earth materials by dual inlet gas-source mass spectrometry (ThermoFinnigan DeltaPlus). All isotope results presented here are averages from duplicate and triplicate sample aliquots, and averages from multiple mass spectrometrical analyses.