Parent body histories recorded in oxidized chondrite sulfides: implications for core formation

Core formation within highly oxidized, sulfur-rich parent bodies that lack metal is poorly understood. Under some conditions, it may be possible to crystallize a metallic core from sulfide melts. These materials could be identified among iron meteorites if they retain the distinct siderophile trace element characteristics of their parent sulfide melts. We analyzed sulfides in an R chondrite across a range of metamorphic conditions to observe the distribution of siderophile elements up to the onset of differentiation. While some elements partition similarly into sulfides as they would into metals, several others show distinct affinities for pentlandite, pyrrhotite, oxides, and/or silicates. If FeNi metal were to crystallize from a pentlandite-rich melt, its composition could likely be distinguished by elevated Ni content and depleted W, Mo, and Pt trace element concentrations. Introduction: In chondritic meteorites, oxidation state and sulfur content play a critical role in establishing the mineralogy of core forming components (metals and sulfides). This is evident when comparing the Rumuruti chondrites (RCs) and ordinary chondrites (OCs), which otherwise share many similar petrologic and isotopic characteristics [e.g., 1]. However, RCs formed under more oxidizing and more sulfur-rich conditions [2]. As a consequence, RCs do not contain FeNi metal above trace abundances [3], and their Fe-Ni-S systems are instead dominated by sulfides pentlandite and pyrrhotite. The observation that oxidized primitive materials, including RCs and many carbonaceous chondrites, lack appreciable metal content raises two questions: (a) Can core formation occur on highly oxidized, sulfur-rich parent bodies that initially lack metal? (b) How could core components of such parent bodies be identified among iron meteorites? Sulfide phases are among the first to melt in chondritic mineralogies [4], and evidence for the segregation of pentlandite melt from silicate residue is preserved in oxidized primitive achondrites [5]. Once removed, liquid pentlandite (FeNiS) can subsequently precipitate Ni-rich metal [6], providing a pathway for metallic core formation despite an initial lack of metal. Siderophile elements with high affinities for FeNi metal are the most common tools used in the classification of iron meteorites [e.g., 7]. Despite a lack of FeNi metal, RCs still contain chondritic abundances of siderophile elements [8], which are largely held by sulfides [5]. If the siderophile trace element systematics among sulfides can be characterized, then predictions can be made regarding diagnostic features for iron meteorites that crystallized from sulfide melts. We have measured the distribution of siderophile elements among RC sulfides across multiple petrologic grades to assess their origins and distribution during metamorphism up to the point of incipient melting. This data may be used to estimate the trace element characteristics of sulfide melts, to provide diagnostic tools for identifying iron meteorites that crystallized from sulfide melts, and to establish a priori parameterization for modelling core formation on oxidized, sulfur-rich parent bodies. Methods: R chondrite Northwest Africa (NWA) 11304 is a polymict breccia composed of petrologic types R3-6, providing an ideal opportunity to observe the sequence of metamorphic processes within a single sample. Sections of NWA 11304 were analyzed for major element chemistry of sulfides via EMPA. In situ trace element measurements for sulfides were collected using LA-ICP-MS. Metamorphic classifications of clasts within NWA 11304 are based on silicate major