Parent body depth–pressure–temperature relationships and the style of the ureilite anatexis

Abstract

Abstract-New analyses of mafic silicates from 14 ureilite meteorites further constrain a strong correlation (Singletary and Grove 2003) between olivine-core Fo ratio and the temperature of equilibration (T E ) recorded by the composition of pigeonite. This correlation may be compared with relationships implied by various postulated combinations of Fo and pressure P in models for ureilite genesis by a putative process of anatectic (depth-linked, P-controlled) smelting. In such models, any combination of Fo and P together fixes the temperature of smelting. Agreement between the observed correlation and these models is poor. The anatectic smelting model also carries implausible implications for the depth range at which ureilites of a given composition (Fo) form. Actual ureilites (and polymict ureilite clasts: Downes et al. 2008) show a distribution strongly skewed toward the low-Fo end of the compositional range, with approximately 58% in the range Fo 76-81 . In contrast, the P-controlled smelting model implies that the Fo 76-81 region is a small fraction of the volume of the parent body: not more than 3.2%, in a model consistent with the Fo-T E observations; and even ignoring the Fo-T E evidence not more than 11% (percentages cited require optimal assumptions concerning the size of the parent body). This region also must occur deep within the body, where no straightforward model would imply a strong bias in the impact-driven sampling process. The ureilites did not derive preponderantly from one atypical ''largest offspring'' disruption survivor, because cooling history evidence shows that after the disruption (whose efficiency was increased by gas jetting), all of the known ureilites cooled in bodies that were tiny (mass of order 10 )9 ) in comparison with the precursor body. The Ca ⁄ Al ratio of the ureilite starting matter cannot be 2.5 times chondritic, as has been suggested, unless the part of the body from which ureilites come is at most 50% of the whole body. Published variants of the anatectic, P-controlled smelting model have the ureilites coming from a region that is >50 vol% of their parent body; and to invoke a larger body would have the drawback of implying that the Fo 76-81 spike represents an even smaller fraction of the parent body's interior. The ureilites' moderate depletions in incompatible elements are difficult to reconcile with a fractional fusion model. It is not plausible that melt formed grossly out of equilibrium with the medium-sized ureilite crystals. The alternative to pressurecontrolled smelting, i.e., a model of gasless or near-gasless anatexis, has very different implications for the size and evolution of the original parent body. To yield internal pressures prohibitive of smelting in even the shallowest and most ferroan portion of its anatectic mantle, the body would have to be larger than roughly 690 km in diameter. A 400 km body would have approximately 12 vol% of the interior (or 13 vol% of the interior apart from the thermal ''skin'' that never undergoes anatexis) prone, if both...

Stable isotopes and the noncarbonaceous derivation of ureilites, in common with nearly all differentiated planetary materials

“…5 and EA-1). Moreover, as discussed by and Warren (2010), there are numerous reasons to doubt that major smelting occurred during the ureilite anatexis. For example, one line of evidence can be quickly described: The abundant Fe-metal that the smelting model inevitably forms is consistently lacking in the ureilites, and yet they retain highly siderophile elements (e.g., Ir, Os) that inevitably partition into Fe-metal, in near-chondritic proportions Rankenburg et al, 2007).…”

Origin and history of ureilitic material in the solar system: The view from asteroid 2008 TC3 and the Almahata Sitta meteorite

“…This type of investigation was begun by Michel et al. (), but that work considered only solid parent asteroids, whereas the UPB is thought to have been partly molten at the time of catastrophic disruption (Warren and Kallemeyn ; Warren ; Wilson and Goodrich ). Therefore, we have begun an investigation into catastrophic disruption and reassembly of asteroids in various stages of melting (Michel et al.…”

Origin of mass‐independent oxygen isotope variation among ureilites: Clues from chondrites and primitive achondrites

“…Criticisms of the smelting model have been provided by Warren (), and countered to some extent by Goodrich et al. ().…”