The cosmic ray exposure history and pre-atmospheric mass of Almahata Sitta

Kees Welten (1), Marc Caffee (2), Matthias Meier (3) and Rainer Wieler (3), (1) Space Sciences Laboratory, UC Berkeley, CA, USA; (2) PRIME Lab, Purdue University, IN, USA; (3) Isotope Geology and Mineral Resources, ETH Zčrich, Switzerland.

We measured cosmogenic 10Be and noble gases in two fragments (#4 and #47) of the Almahata Sitta ureilite. The high concentrations of 10Be (22.4 and 24.0 dpm/kg) are only consistent with irradiation in the outer 20-40 cm of a large object (R = 2 m) if we assume an average density of <2.3 g/cm3 as well as high C contents (2-4 wt%). Based on these assumptions, the cosmogenic noble gas concentrations yield a cosmic-ray exposure (CRE) age of 15-20 Myr. Although a previous 2╣ exposure of Almahata Sitta on the surface of the Ureilite Parent Body can not be excluded yet, the high 10Be concentrations indicate a minimum 4╣ exposure of ~5 Myr. Additional noble gas and radionuclide measurements are in progress to further constrain the CRE history and pre-atmospheric mass of Almahata Sitta.