Opaque minerals in chondrules and fine‐grained chondrule rims in the Bishunpur (LL3.1) chondrite

Abstract

Abstract-We present a detailed petrographic and electron microprobe study of metal grains and related opaque minerals in the chondrule interiors and rims of the Bishunpur (LL3.1) ordinary chondrite. There are distinct differences between metal grains that are completely encased in chondrule interiors and those that have some portion of their surface exposed outside of the chondrule boundary, even though the two types of metal grains can be separated by only a few microns. Metal grains in chondrule interiors exhibit minor alteration in the form of oxidized P-, Cr-, and Si-bearing minerals. Metal grains at chondrule boundaries and in chondrule rims are extensively altered into troilite and fayalite. The results of this study suggest that many metal grains in Bishunpur reacted with a type-I chondrule melt and incorporated significant amounts of P, Cr, and Si. As the system cooled, some metal oxidation occurred in the chondrule interior, producing metal-associated phosphate, chromite, and silica. Metal that migrated to chondrule boundaries experienced extensive corrosion as a result of exposure to the external atmosphere present during chondrule formation. It appears that chondrulederived metal and its corrosion products were incorporated into the fine-grained rims that surround many type-I chondrules, contributing to their Fe-rich compositions. We propose that these finegrained rims formed by a combination of corrosion of metal expelled from the chondrule interior and accretion of fine-grained mineral fragments and microchondrules.

Early aqueous activity on the ordinary and carbonaceous chondrite parent bodies recorded by fayalite

“…The oxygen-isotope systematics of fayalite and magnetite in type 3 ordinary and carbonaceous chondrites indicate that these minerals are in isotopic disequilibrium with the chondrule olivines, precluding a high-temperature origin of fayalite and magnetite during chondrule formation 24 . Instead, the large differences in d 18 O values between the coexisting fayalite and magnetite (up to 10% in A-881317 and up to 8% in MAC 88107) are consistent with a low-temperature origin of these minerals in equilibrium with an aqueous solution 25,26 .…”

Oxygen, silicon, and Mn-Cr isotopes of fayalite in the Kaba oxidized CV3 chondrite: Constraints for its formation history

“…Extremely FeO-rich olivine (ϳFa 96 ) occurs as fine-grained rims and lacy networks around and within silica grains in some unequilibrated ordinary chondrites (Brigham et al, 1986;Krot and Wasson, 1993;Wood and Holmberg, 1994). Fayalite was also reported as corrosion products rimming metal grains in the Bishunpur (LL3.1) chondrite (Lauretta et al, 2001;Lauretta and Buseck, 2003). This fayalitic olivine is usually of secondary origin, forming through reactions between preexisting phases and gas or fluid.…”

Oxidation of Minor Elements from an Iron–Nickel–Chromium–Cobalt–Phosphorus Alloy in 17.3% CO2–H2 Gas Mixtures at 700–1000 °C

“…The elements Cr and P are the least noble and they occur in primitive meteorites as oxide inclusions in the alloy and as rims surrounding metal grains [6,7]. Magnetite is frequently observed in rims of chondritic metal grains, and as such this type of feature is thought to be nebular in origin.…”