Pyrrhotite and Pentlandite in Ll3 to Ll6 Chondrites: Determining Compositional and Microstructural Indicators of Formation Conditions

Introduction: The compositions, textures, and crystal structures of sulfides can be used to constrain oxygen fugacity, aqueous, thermal, and cooling history [e.g., 1–5]. The most abundant sulfides in extraterrestrial samples are the pyrrhotite group [(Fe,Ni,Co,Cr)1– xS], which can occur with pentlandite [(Fe,Ni,Co,Cr)9– xS8]. The pyrrhotite group sulfides are largely nonstoichiometric and have a range of compositions (0<x<0.125) and distinct crystal structures (polytypes). The stoichiometric end members are 2C (troilite; FeS, hexagonal) and 4C (Fe7S8, monoclinic) pyrrhotite. There are also non-integral NC-pyrrhotites with intermediate compositions with 0<x<0.125 (all hexagonal); which includes the integral 5C (Fe9S10), 6C (Fe11S12), and 11C (Fe10S11) pyrrhotites [e.g., 6–8]. Intergrowths of NC-pyrrhotites with 2C or 4C pyrrhotite are common in terrestrial assemblages [6]. Geothermometry of pyrrhotite-pentlandite intergrowths in meteorites shows that most formed via primary cooling from high temperature or thermal metamorphism [e.g., 9–11]. Sulfides in the LL4 to LL6 chondrites equilibrated between 600 and 500°C, consistent with formation during cooling after thermal metamorphism [11]. Moreover, because sulfides are present in both asteroids and meteorites, their comparison could yield valuable insights. Analyses of Hayabusa particles have identified asteroid 25143 Itokawa as LL5–6 chondrite material [e.g., 12,13], thermally metamorphosed between ~780 and 840°C [12]. Sulfides were observed in Hayabusa particles [12], and may record additional information on the formation conditions of asteroid Itokawa. Our goal is to determine the origins and formation conditions of sulfides in LL3 to LL6 chondrites, so that we can compare them with Hayabusa sulfides and understand their origins in future studies. Samples and Analytical Procedures: Sulfides consisting of pyrrhotite and pentlandite in Semarkona USNM1805-17 (LL3.00), Soko-Banja USNM3078-1 (LL4), Siena USNM3070-3 (LL5), and Saint-Séverin USNM2608-3 (LL6) were selected for analysis. Their chemical compositions were studied with the Smithsonian Institution JEOL 8900 Superprobe electron probe microanalyzer (EPMA, [11]) and the University of Arizona (UAz) Cameca SX-100 EPMA. X-ray element maps and high-resolution images of a sulfide assemblage from each meteorite was obtained with the FEI Helios NanoLab 660 focused-ion-beam scanningelectron microscope (FIB-SEM) at UAz and the JEOL JXA-8530F Hyperprobe EPMA at Arizona State University (ASU). The FIB-SEM was also used to extract ~10 × 5 μm sections transecting the pyrrhotitepentlandite interfaces within sulfide grains from each meteorite, which were thinned to electron transparency (<100 nm) using methods of [14]. FIB sections were then analyzed using the 200 keV aberration-corrected Hitachi HF5000 scanning transmission electron microscope (TEM) at UAz.