Probing the evolution of primitive achondrite parent bodies: insights from LA-ICP-MS analysis of silicate minerals

Ureilites and acapulcoite-lodranite clan (ALC) meteorites are attractive targets to study the processes of early planetary differentiation, in which an unsorted mass of primitive meteoritic material evolves into a body with a silicate mantle and metallic core. These achondrites record the earliest stages of planetary differentiation: ALC meteorites are products of thermal metamorphism and low degree melt extraction from a primitive asteroid, and main group ureilites represent ultramafic mantle restites after extraction of metal and basaltic melts. As such, constraining the geochemistry of these ultramafic achondrite meteorites refines our understanding of the first critical stages of the planetary evolution.

Here, we present trace and major element contents in the major silicate minerals (olivine, augite, orthopyroxene and plagioclase) of 10 ureilites and 7 ALC meteorites measured in situ using LA-ICP-MS. The Fe/Mg/Mn relations within both meteorite groups confirm the absence of any igneous fractionation processes and Fe loss/gain, while high-Ca pyroxene in ALC suggests that these minerals are affected by low degree partial melting. The Cr distribution between olivine and pigenonite in ureilites, when interpreted as a geothermometer, [1] yields equilibration temperature ranges that are in good agreement with previous results for pyroxene. REE mass fractions of the ALC silicates reflect the complex nature of partial melting and migration of basaltic melts on the parent body scale. The mass fraction of moderately volatile elements, such as Cr, Mn, Zn, Pb, Cu are found to be depleted in silicate minerals compared to the chondritic reservoir, likely as a result of evaporation/condensation processes, either (i) due to incomplete condensation from the nebula, or (ii) due to evaporative losses from the parent body by e.g. catastrophic impacts. At the same time, the distribution of Zn between olivine, pyroxene and plagioclase minerals suggests that Zn can be fractionated during partial melting of the primitive ultramafic lithologies.

Combined, major and trace element systematics of silicate minerals in ALC and ureilites enable to reconstruct the meteorite petrogenesis complimentary to the partial melting histories of the parent body, described based on their petrology and mineralogy. Importantly, the element compositions do not correlate with the oxygen isotopic signatures, which are thought to record nebular processes. The ability to measure trace element compositions in separate minerals in situ adds an extra dimension in understanding complex processes during partial melting and cooling of asteroidal parent bodies.