Origin and evolution of the ureilite parent magmas: Multi-stage igneous activity on a large parent body

Abstract

Analyses of olivine and pigeonite cores (interpreted as cumulate crystals) in eight low-shock ureilites show well-defined correlations between Fe/ X (where X is one of the minor elements Mn, Cr, Ca, Al, Ti, P, or Ni) and Fe/ Mg ratios. For the lithophile minor elements these trends are linear, with positive slope, and pass through or near the origin, indicating various degrees of FeO reduction of the parent magmas. The trends shown by P and Ni are consistent with this interpretation, and, additionally, require equilibrium crystallization of 20–27 mole % metal. Graphite was most likely the reducing agent, which implies that the ureilites equilibrated at different pressures, because the graphite fO 2 buffer is pressure dependent. There is no evidence for fractionation of olivine or pryroxene in any of these trends. The Ca, Ti and Al trends show significant scatter, indicating that there must have been multiple parent magmas, with very similar Mg/Mn and Mg Cr ratios, but different Mg Al , Mg Ca and Mg Ti ratios. CaO/Al 2O 3 ratios vary by a factor of ~3 among the eight magmas, and are superchondritic (~ 14–32, molar), similar to lunar mare basalts. We propose the following model for generation and crystallization of ureilite parent magmas (UPM). Chondritic material undergoes 10–25% partial melting, leaving a plagioclase-depleted residue. The magma or magmas form a differentiated crust on the ureilite parent body (UPB), with mafic cumulates and plagioclase cumulates spatially separated. Later remelting (<10%) of the mafic cumulates produced the UPM. Siderophile elements indicate that the source region of the UPM had not experienced segregation of metal such as would occur during core formation. In the source region, the pressure must have been at least 0.5–1 kb, because only at pressures this high does the graphite buffer allow iron oxide to be the stable phase relative to metallic iron. The graphite-bearing UPM were emplaced at various levels in the crust, where they were reduced to various extents, dependent on depth. Ureilites are early olivine-pigeonite cumulates from these magmas. Compositions of interstitial, low-Ca pyroxenes indicate that the trapped interstitial liquids from which they crystallized had lower Mn Mg , Cr Mg , Ca Mg , Al Mg and Na Mg ratios than the UPM, and indicate an abrupt change in liquid composition coincident with impact excavation of ureilites. This interstitial material may be derived by mixing a shock melt of nearby olivine-pigeonite cumulates with residual primary liquid. Our model predicts that the UPB had a differentiated crust, did not have a core, and was at least 235 km in radius. The lack of a ureilite-like body among spectrally analyzed asteroids suggests that either the spectral signature of the UPB is not that of ureilites themselves (perhaps of carbon-free feldspathic material instead), that the UPB is not intact, or that ureilites did not originate in the asteroid belt.