IGPP Seminar Series

Lunar rock-rain: Diverse silicate impact-vapor condensates in an Apollo-14 breccia

by Prof. Paul Warren


An Apollo 14 breccia pebble has been found to contain diverse silicate impact-vapor conden-sates, as quenched-melt spheroids, mostly far less than 5 m across; and as quenched-textured clasts up to 200 m. Their defining trait is extreme depletion in the refractory oxides Al2O3 (gen-erally 0.3-1 wt%) and CaO; implausible for igneous or impact-melt materials. Bulk compositions are nearly pure SiO2+MgO+FeO, but nonetheless diverse, with mg from 7 to 84 mol%, and FeO/SiO2 (wt.) from 0.002 to 0.67. Bulk compositions and textures of many spheroids suggest that liquid immiscibility occurred, implying a post-condensation T of ~1680C. The largest clast (200 m) consists mainly of thousands of relict condensate spheroids. Aggregation of such large clasts probably took place in mushy puddles. The scarcity of this distinctive material among lunar impactites is enigmatic, because models predict that for every 10 kg of impact melt, 1 kg of im-pact vapor should form. Possibly the impact vapor of large lunar cratering events tends to mostly condense and settle even before it escapes clear of surrounding melt.
Tuesday, 16 January 2007
3853 Slichter Hall
Refreshments at 3:45 PM
Lecture at 4:00 PM