THE D´ORBIGNY ANGRITE: EVIDENCES FOR AND AGAINST A FINAL THERMAL EVENT AT ~1000ºC.

VARELA M.E.; HWANG S.L.; SHEN P; CHU, H-T; YUI T.F.; IIZUKA Y.

Houston

Conferencia; Lunar and Planetary Science Conference; 2014

NASA

Angrites are a small group of rocks as old as the solar system, as indicated by their Pb-Pb ages (e.g, LEW Cliff 86010: 4558.55 ± 0.15 Ma and D?Orbigny: 4563.36 ± 0.34.) [1-2]. The genesis of these rocks is controversial and no consensus has yet been reached. The mineralogical composition of an-grites is unusual. All phases are usually out of equilib-rium. Ferroan, aluminous and titaniferous augite coex-ist with anorthite, Mg-rich olivine, Ca, Fe-rich olivine, kirschsteinite, a variety of oxides, phosphates, sulfides and some Ni, Fe metal [e.g., 3-6]. On the base of their textures, angrites are divided into two subgroups: coarse-grained (?slowly-cooled? angrites, e.g., Angra dos Reis, LEW 86010) and fine-grained (?quenched? angrites, e.g., Asuka-881371, D?Orbigny) [7] as a re-sult of different cooling rates. In the case of D´Orbigny the study of the crystal structure of druse clinopyroxene [8] help to quantify some of the parameters related with the last thermal history of this rock. The random distribution of Fe2+ and Mg2+ over the M1 and M2 sites suggests a closure temperature of 1000 ºC. However, as pointed out by [8], the high closure temperature (1000 ºC) that indi-cates an extremely fast cooling rate [9] of clinopyrox-ene is in contradiction with its grain size (up to 2 mm), perfect prismatic shape and homogeneous chemical composition, implying a slow growth with enough time for chemical homogenization. Here we present some new evidences ?based on SEM and TEM observations- indicating that D´Orbigny was exposed to late high temperature events. The time du-ration of such thermal events however remain contro-versial.