Cathodoluminescence zoning and minor elements in forsterites from the Murchison (C2) and Allende (C3V) carbonaceous chondrites

Abstract

Although Mg-rich olivine occurs as isolated grains and in aggregates of carbonaceous meteorites¹, its chemical features have generally been neglected in favour of the more esoteric Ca/Al-rich inclusions with their challenging mineralogy and isotope systematics. Because cathodoluminescence has proved useful in revealing textures and compositional varieties for some minerals in meteorites, we have now applied the technique to look for textural features of olivine in carbonaceous meteorites relevant to the unresolved dispute over its origin, whether from a vapour or a liquid^2–9. Our cathodoluminescence photographs of forsterite grains in Murchison (C2) and Allende (C3) meteorites reveal a blue core (inclusion-free) with planar boundaries to a red or dark rim (inclusions of glass, metal and silicate). We have also performed high-precision electron microprobe analyses revealing in these forsterites unusually large amounts of the ‘minor’ elements Al, Ti and Ca in the blue cores (and sharp chemical changes at the blue/red boundary), suggesting formation by crystallization at high temperatures from a source rich in these metals. Following conclusions drawn from previous analyses of olivine in meteorites (achondrites¹⁰, Niger (I) C2⁷, Belgica 7904 (C2)¹¹), we believe the minor element signature should be able to characterize olivines in micrometeorites and in deep-sea particles (see accompanying paper¹²).