Mineralogy, mineral chemistry, and redox equilibria in ten aggregate particles from asteroid Bennu

Abstract

Examination of 10 Bennu aggregate particles has revealed the presence of many phases which taken together can provide constraints on the oxygen fugacity (fO₂) of Bennu samples. Phyllosilicates (saponite and serpentine), carbonates, oxides (magnetite, chromite), sulfides (pyrrhotite, pentlandite), phosphate (hydroxyapatite, Na-Mg-phosphate), and phosphides (schreibersite, andreyivanovite) all occur in most Bennu particles. The Bennu samples have experienced a high degree of aqueous alteration leaving only <1% of the original mineralogy unaltered. However, both the precursor anhydrous and alteration phases can present different constraints on fO₂. Precursor phases include olivine, pyroxene, spinel, hibonite, chromite, phosphide, very rare Fe-Ni metal, apatite, and possibly MgS and MnS, all of which are typically <25 μm in size. Alteration phases include phyllosilicates, carbonates, magnetite, sulfides, sulfates, phosphates, chlorides, and fluorides. Detailed calculations of fO₂ rely on having quantitative electron microprobe analyses of the phases involved in equilibria amongst both the precursor and alteration phases. In general, the absence of Fe-Ni metal, coupled with the stability of the Fe₃O₄ component in chromite, places a lower limit on the fO₂. Concomitantly, the absence of Fe sulfates places an upper limit on fO₂. Altogether, the textures, mineral compositions, and calculations suggest that some components in the Bennu samples (chondrules, inclusions) may have originally equilibrated at fO₂ well below the iron-wüstite buffer but then experienced higher fO₂ near or higher than the fayalite-magnetite-quartz (FMQ) buffer during aqueous alteration that produced coarser grained oxidized assemblages.