¹Max Collinet,¹Timothy L. Grove
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13471]
¹Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, 02139 Massachusetts, USA
Published by arrangement with John Wiley & Sons

Ureilites are carbon‐rich ultramafic achondrites that have been heated above the silicate solidus, do not contain plagioclase, and represent the melting residues of an unknown planetesimal (i.e., the ureilite parent body, UPB). Melting residues identical to pigeonite‐olivine ureilites (representing 80% of ureilites) have been produced in batch melting experiments of chondritic materials not depleted in alkali elements relative to the Sun’s photosphere (e.g., CI, H, LL chondrites), but only in a relatively narrow range of temperature (1120 ºC–1180 ºC). However, many ureilites are thought to have formed at higher temperature (1200 ºC–1280 ºC). New experiments, described in this study, show that pigeonite can persist at higher temperature (up to 1280 ºC) when CI and LL chondrites are melted incrementally and while partial melts are progressively extracted. The melt productivity decreases dramatically after the exhaustion of plagioclase with only 5–9 wt% melt being generated between 1120 ºC and 1280 ºC. The relative proportion of pyroxene and olivine in experiments is compared to 12 ureilites, analyzed for this study, together with ureilites described in the literature to constrain the initial Mg/Si ratio of the UPB (0.98–1.05). Experiments are also used to develop a new thermometer based on the partitioning of Cr between olivine and low‐Ca pyroxene that is applicable to all ureilites. The equilibration temperature of ureilites increases with decreasing Al₂O₃ and Wo contents of pyroxene and decreasing bulk REE concentrations. The UPB melted incrementally, at different fO₂, and did not cool significantly (0 ºC–30 ºC) prior to its disruption. It remained isotopically heterogenous, but the initial concentration of major elements (SiO₂, MgO, CaO, Al₂O₃, alkali elements) was similar in the different mantle reservoirs.

¹Connor D.Hilton, ¹Richard J.Walker
Earth and Planetary Science Letters 540, 116248 Link to Article [https://doi.org/10.1016/j.epsl.2020.116248]
¹Department of Geology, University of Maryland, College Park, MD 20742, USA
Copyright Elsevier

The origin of the IAB main group (MG) iron meteorites is explored through consideration of 182W isotopic compositions, thermal modeling of 26Al decay, and mass independent (nucleosynthetic) Mo isotopic compositions of planetesimals formed in the noncarbonaceous (NC) protosolar isotopic reservoir. A refined 182W model age for the meteorites Campo del Cielo, Canyon Diablo, and Nantan suggests that the IAB-MG parent body underwent some form of metal-silicate segregation as early as 5.3 ± 0.4 Myr after calcium-aluminum rich inclusion (CAI) formation or as late as 13.8 ± 1.4 Myr after CAI formation. If melting of the IAB-MG occurred prior to 7 Myr after CAI formation, it was likely driven by 26Al decay for a parent body radius >40 km. Otherwise, additional heat from impact is required for melting metal this late in Solar System history. If melting was partially or wholly the result of internal heating, a thermal model of 26Al decay heat production constrains the accretion age of the IAB-MG parent body to ∼1.7 ± 0.4 Myr after CAI formation. If melting was, instead, dominantly caused by impact heating, thermal modeling suggests the parent body accreted more than 2 Myr after CAI formation. Comparison of Mo mass independent isotopic compositions of the IAB-MG to other NC bodies with constrained accretion ages suggests that the Mo isotopic composition of the NC reservoir changed with time, and that the IAB-MG parent body accreted between 2 to 3 Myr after CAI formation, thus requiring an origin by impact. The relationship between nucleosynthetic Mo isotopic compositions and accretion ages of planetesimals from the NC reservoir suggests that isotopic heterogeneity developed from either addition of s-process material to, or removal of coupled r-/p-process material from the NC reservoir.