EARLY SOLAR SYSTEM IGNEOUS PROCESSES AS REVEALED BY HED PHOTOMICROGRAPHY

Meteorites belonging to the HED clan are so closely related that they likely originate from a common parent body. Multiple lines of evidence suggest that this body is 4 Vesta. Some non-cumulate eucrites exhibit sufficiently unique O isotope ratios to be sourced from other asteroidal parent bodies; the Vestan members of the howardite-eucrite-diogenite clan display a variety of igneous textures and products from the solar system’s very earliest epochs.

Following accretion, planet-wide melting and differentiation are the fundamental processes that lead to igneous activity on planetary bodies. Many observations suggest that Vesta is one of the few surviving members of an early population of continent-sized protoplanets that melted completely. The inferred Fe-Ni metallic core spans approximately 40% of Vesta’s interior, and strongly supports the presence of a global magma ocean on Vesta within a few million years of CAI formation 4.567 billion years ago. Cooling and solidification of a magma ocean remains a poorly understood planetary process; the HED clan supplies us with a variety of samples from an early protoplanet that experienced just such a transition and has since remained relatively intact.

Within this narrow window of early solar system history, we see many of the primary igneous processes that characterize the larger terrestrial planets: fractionation of a magma ocean into a metallic core, mafic mantle and basaltic crust. The upper mantle and lower crust are represented by olivine and orthopyroxene cumulates (diogenites); residual basaltic magma crystallization (cumulate eucrites); extrusion in multiple basalt flows (non-cumulate eucrites), as well as extreme brecciation and lithologic mixing due to later impacts (howardites). Additional photomicrographs of unique igneous textures due to as yet unidentified processes will be presented, subject to audience discussion and interpretation.