A Preliminary Study of Magmatic Volatiles

Introduction: Angrites are a rare group of achondrites notable for their critical silica undersaturation, lack of shock metamorphic features and ancient crystallisation ages [1-3]. They are sub-divided into plutonic or basaltic angrites on the basis of their inferred petrogenesis. Similar studies to those already carried out on HEDs and shergottites [4, 5] could be used to shed light on magmatic processes on the angrite parent body (APB). Here we present results from carbon and nitrogen stepped combustion experiments on the plutonic angrites Angra dos Reis (AdoR) and NWA 2999, and the basaltic angrite D’Orbigny. Results are summarized below. Magmatic volatiles are released once the silicate matrix starts to become plastic, usually between 800-1000°C; depending on the CRE age of the specimen, it is possible for this to be mixed with a spallogenic component, the effects of which can be removed following correction. We have not yet applied this correction to the data but expect it to have little effect on either carbon or nitrogen abundance, and to reduce isotopic composition by < 2‰. We also discount NWA 2999 from further consideration, because it contained large amounts of terrestrial organic carbon which swamped all other potential carbon-bearing components.