Boron in chondritic meteorites

Abstract

Alpha-track images (ATI), prepared by etching neutron-irradiated cellulose nitrate films attached to polished thin sections of meteorites, provide useful qualitative maps of boron and lithium distribution. However, extreme care must be taken to avoid contamination during preparation, particularly by boron in diamond polishing paste. Boron and lithium are virtually absent from chondrules, metal and sulphide grains in carbonaceous and ordinary chondrites. Matrix material, by contrast, contains sufficient boron to account for the 0.2–1.0 ppm boron usually present. Many Antarctic meteorites contain secondary Fe-Si-rich weathering products: ion microprobe analysis showed significant enrichment of boron and chlorine in one meteorite, probably attributable to effects of sea-spray aerosols. Fifteen new analyses of 11 Antarctic chondrites, gave boron contents in the range 0.24–1.0 ppm, with a mean value of 0.55 ppm. The set comprised three carbonaceous chondrites and eight ordinary chondrites, with no obvious difference in boron content between the two groups. Types 3 and 4 ordinary chondrites show a wider range in values of B 10 6Si than previously found. Using literature abundances for other elements, the present data do not confirm that boron resembles sulphur more closely than sodium in depletion patterns from Cl chondrite abundances.