Magnesium and chromium isotope evidence for initial melting by radioactive decay of 26Al and late stage impact-melting

1Elishevah M.M.E. van Kooten, , 1Martin Schiller, 1Martin Bizzarro
Geochimica et Cosmochimica Acta (in Press) Link to Article [http://dx.doi.org/10.1016/j.gca.2017.03.033]
1Centre for Star and Planet Formation and Natural History Museum of Denmark, University of Copenhagen, DK-1350 Copenhagen, Denmark
Copyright Elsevier

Polymict ureilites are meteoritic breccias that provide insights into the differentiation history of the ureilite parent body.We have sampled a total of 24 clasts from the polymict ureilite Dar al Gani 319, representing a variety of lithologies such as mantle residues, cumulates and crustal fragments that are genetically related to monomict ureilites.In addition, we sampled four non-indigenous dark clasts and two chondrule-containing clasts from the same meteorite.We report on the petrology and the bulk mass-dependent and mass-independent magnesium and chromium isotope systematics of these clasts.The DaG 319 polymict ureilite consists predominantly of clasts related to Main Group ureilite residues (MG clasts) with varying Mg#s (0.74-0.91), as well as a significant fraction of olivine-orthopyroxene clasts related to Hughes Type ureilites (HT clasts) with consistently high Mg#s (∼∼0.89).In addition, DaG 319 contains less abundant feldspathic clasts that are thought to represent melts derived from the ureilite mantle.A significant mass-dependent Mg-isotope fractionation totaling ΔΔμμ2525Mg = ∼∼450 ppm was found between isotopically light feldspathic clasts (μμ2525Mg = –305±±25 to 15±±12 ppm), MG clasts (μμ2525Mg = –23±±51 ppm) and HT clasts (μμ2525Mg = 157±±21 ppm).We suggest that this isotopic offset is the result of equilibrium isotope fractionation during melting in the presence of an isotopically light magnesite component.We propose Mg-carbonates to be stable in the upper ureilite mantle, and pure carbon phases such as graphite to be stable at higher pressures.This is consistent with HT clasts lacking carbon-related phases, whereas MG clasts contain abundant carbon.The timing of differentiation events for the ureilitic clasts are constrained by high precision 5353Mn-5353Cr systematics and 2626Al-2626Mg model ages.We show that a dichotomy of ages exist between the differentiation of main group ureilite residues and HT cumulates rapidly after CAI formation and later remelting of cumulates with corresponding feldspathic melts, at 3.8±±1.3 Myr after CAI formation.Assuming an initial 2626Al/2727Al abundance[(2626Al/2727Al)0 = View the MathML source1.33-0.18+0.21×× 10−5] similar to the angrite parent body, the early melting event is best explained by heat production from 2626Al whereas the late event is more likely caused by a major impact. Variations in 5454Cr between MG clasts and HT clasts agree with a carbonaceous chondrite impactor onto the ureilite parent body. This impactor may be represented by abundant dark clasts found in polymict ureilites, which have μμ26Mg∗26Mg∗ and μμ5454Cr signatures similar to CI chondrites. Similar volatile-rich dark clasts found in other meteorite breccias provide insights into the timing of volatile influx to the accretion region of the terrestrial planets.

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