¹Elishevah van Kooten,²Larissa Cavalcante,³Daniel Wielandt,³Martin Bizzarro
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1111/maps.13459]
¹Institut de Physique du Globe de Paris, Université de Paris, CNRS, UMR 7154, 1 rue Jussieu, 75238 Paris, France
²Institute of Chemistry, University of São Paulo, 03178 São Paulo, Brazil
³Centre for Star and Planet Formation and Natural History Museum of Denmark, University of Copenhagen, DK‐1350 Copenhagen, Denmark
Published by arrangement with John Wiley & Sons

CM meteorites are dominant members of carbonaceous chondrites (CCs), which evidently accreted in a region separated from the terrestrial planets. These chondrites are key in determining the accretion regions of solar system materials, since in Mg and Cr isotope space, they intersect between what are identified as inner and outer solar system reservoirs. In this model, the outer reservoir is represented by metal‐rich carbonaceous chondrites (MRCCs), including CR chondrites. An important question remains whether the barrier between MRCCs and CCs was a temporal or spatial one. CM chondrites and chondrules are used here to identify the nature of the barrier as well as the timescale of chondrite parent body accretion. We find based on high precision Mg and Cr isotope data of seven CM chondrites and 12 chondrules, that accretion in the CM chondrite reservoir was continuous lasting <3 Myr and showing late accretion of MRCC‐like material reflected by the anomalous CM chondrite Bells. We further argue that although MRCCs likely accreted later than CM chondrites, CR chondrules must have initially formed from a reservoir spatially separated from CM chondrules. Finally, we hypothesize on the nature of the spatial barrier separating these reservoirs.

¹Max Collinet,¹Timothy L.Grove
Geochimcia et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2020.03.005]
¹Massachusetts Institute of Technology, Earth, Atmospheric and Planetary Sciences department, 77 Massachusetts avenue, 02139, MA, USA
Copyright Elsevier

We present the results of melting experiments on a suite of carbonaceous and ordinary chondritic compositions (CV, CM, CI, H and LL) performed at low pressure (0.1 to 13.1 MPa) and over a range of oxygen fugacity (log fO₂ – (log fO₂)_IW = -2.5 to -1 and +0.8, IW being the iron-wustite buffer). These experiments constrain the composition of partial melts (F = 5-25 wt.%) of chondritic planetesimals. Most experiments (IW -2.5 to -1) were conducted in Molybdenum-Hafnium Carbide pressure vessels, which prevented the loss of alkali elements from the melt. The results show that all planetesimals not significantly depleted in moderately volatile elements relative to the sun’s photosphere (e.g. CI, H and LL compositions) produced low-degree melts (<15 wt.%) rich in SiO₂, Al₂O₃ and alkali elements, regardless of the fO₂. Despite their high apparent viscosities (10^4-5 Pa.s), such low-density partial melts (2400-2500 kg/m³) were mobilized and, upon crystallizing, formed rocks containing up to 80 vol.% of plagioclase An_10-30 (i.e. oligoclase) such as the trachyandesite achondrites Graves Nunataks 06128/9, Northwest Africa 6698 and 11575, the Almahata Sitta clast ALM-A, as well as smaller “albitic clasts” in polymict ureilites and “alkali-silica-rich” inclusions in non-magmatic iron meteorites. We suggest that silica- and alkali- rich melts were widespread in small bodies of the early solar system but that much evidence was erased by subsequent stages of melting and planetary accretion and differentiation.

¹Max Collinet,¹Timothy L.Grove
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2020.03.004]
¹Massachusetts Institute of Technology, Earth, Atmospheric and Planetary Sciences department, 77 Massachusetts avenue, 02139, MA, USA
Copyright Elsevier

Acapulcoites-lodranites, ureilites, brachinites, brachinite-like achondrites and winonaites are the main groups of primitive achondrites. They are variably depleted in incompatible lithophile elements (Al, Na, K and rare earth elements) and siderophile/chalcophile elements relative to chondrites and are interpreted as the residual mantle of planetesimals from which silicate melts and sulfide/metal melts were extracted. We use a series of melting experiments conducted with various chondritic compositions (CV, CM, CI, H and LL) to constrain the oxygen fugacity (fO2), the temperature, extent of melting and the initial bulk composition of the parent bodies of primitive achondrites. They melted at different and variable fO2: ΔIW -0.5/-1.0 for brachinites, ΔIW -1.3/-2.5 for ureilites, ΔIW -1.6/-2.7 for acapulcoites/lodranites and ΔIW -2.5/-3.0 for winonaites (with ΔIW = log fO2 – (log fO2)IW; IW being the iron-wustite buffer). Those main groups of primitive achondrites, which have nucleosynthetic anomalies characteristic of the “non-carbonaceous” reservoir and the inner solar system, were not initially depleted in Na2O and K2O relative to the sun’s photosphere. This suggests, in accordance with the enrichment in the heavy isotopes of Zn, Rb and K in eucrites, that the depletion of moderately volatile elements in planetesimals that melted to a larger extent (e.g. Vesta, the angrite parent body) resulted from evaporative losses during partial melting. The depletion of moderately volatile elements in terrestrial planets is likely inherited from partial melting and differentiation of small planetary bodies rather than from the incomplete complete condensation of the solar nebula.