SILICA-RICH ORTHOPYROXENITE IN THE BOVEDY CHONDRITE

A large (>4.5 x 7 x 4 mm), igneous-textured clast in the Bovedy (L3) c hondrite is notable for its high bulk SiO2 content (almost-equal-to 57 .5 wt%). The clast consists of normally zoned orthopyroxene (83.8 vol %), tridymite (6.2 %), an intergrowth of feldspar (5.8 %) and sodic gl ass (3.1 %), pigeonite (1.0 %), and small amounts of chromite (0.2 %), augite, and Fe,Ni-metal; it is best described as a silica-rich orthop yroxenite. The oxygen-isotopic composition of the clast is similar, bu t not identical, to Bovedy and other ordinary chondrites. The clast ha s a superchondritic Si/Mg ratio, but has Mg/(Mg + Fe) and Fe/Mn ratios that are similar to ordinary chondrite silicate. The closest chemical analogues to the clast are radial-pyroxene chondrules, diogenites, py roxene-silica objects in ordinary chondrites, and silicates in the IIE iron meteorite Weekeroo Station. The clast crystallized from a silice ous melt that cooled fast enough to prevent complete attainment of equ ilibrium but slow enough to allow nearly complete crystallization. The texture, form, size and composition of the clast suggest that it is a n igneous differentiate from an asteroid or planetesimal that formed i n the vicinity of ordinary chondrites. The melt probably cooled in the near-surface region of the parent object. It appears that in the sour ce region of the clast, metallic and silicate partial melt were largel y-to-completely lost during a relatively low degree of melting, and th at during a higher degree of melting, olivine and low-Ca pyroxene sepa rated from the remaining liquid, which ultimately solidified to form t he clast. While these fractionation steps could not have all occurred at the same temperature, they could have been accomplished in a single melting episode, possibly as a result of heating by radionuclides or by electromagnetic induction. Fractionated magmas can also account for other Si-rich objects in chondrites.