DIFFERENTIATION OF THE UREILITE PARENT ASTEROID

The processes of planetary differentiation driven by decay of short-lived ²⁶Al in the interiors of small asteroids in the first few million years of solar system history differed significantly from those on larger bodies. Studies of primitive achondrites and several new “ungrouped achondrites” suggest that efficient extraction of low-degree, Si- and alkali-rich melts from the mantles of such asteroids would have been common, leading to formation of andesitic or trachyandesitic, rather than basaltic, crusts.

One such case may have been the parent body of the ureilite meteorites (UPB). Ureilites are ultramafic (Fo 75-95 olivine + pyroxene) achondrites that represent the residual mantle of a partially differentiated, carbon-rich asteroid. Several lines of evidence, including unequilibrated oxygen isotopes, experiments, and physical modelling, suggest that melt extraction on the UPB must have been a near-fractional (incremental), rather than batch, process. This hypothesis can be tested with the compositions of the extracted melts (i.e., ureilitic crustal rocks).

Although there are no whole meteorites that represent ureilitic crustal rocks, polymict ureilites (samples of rubble piles formed after disruption of the UPB) contain feldspathic clasts (microns to millimeters) that could be remnants of the crust. The most common lithology identified from these clasts, the “albitic lithology,” consists of plagioclase of An_0-30, FeO-rich pigeonite, augite, phosphates, Fe-Ti oxides, and incompatible element-rich glass. A few larger clasts (up to 26 g), from the Almahata Sitta (AhS) polymict ureilite (e.g., the ALM-A trachyandesite) have been interpreted as more representative samples of this lithology. Its “evolved” composition supports fractional melt extraction.

We will discuss two new AhS clasts that paint a more complex picture. MS-MU-012 is the first known plagioclase-bearing (An₆₇), yet still ultramafic, ureilite. AhS 3005 is an andesite that resembles the albitic lithology in places but also contains a more primitive assemblage of An₅₅ plagioclase, olivine and orthopyroxene. We use petrologic and trace element data to evaluate whether these clasts represent late incremental melts, or rather basaltic batch melts from which the albitic lithology could have been derived by fractional crystallization.