Mass-independent and mass-dependent Cr isotopic composition of the Rumuruti (R) chondrites: Implication for their origin and their significance for planet formation

Ke Zhu, Frédéric Moynier, Martin Schiller, Conel M. O’D. Alexander, Jean-Alix Barrat, Addi Bischoff, Martin Bizzarro

Geochimica et Cosmochimica Acta
In Press, Journal Pre-proof, Available online 15 October 2020

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“Chromium (Cr) isotopes play an important role in cosmochemistry and planetary science, because they are powerful tools for dating (53Mn-53Cr short-lived chronometry), tracing (54Cr nucleosynthetic anomalies) the origins of the materials, and studying the processes involved in volatile element fractionation and planetary differentiation (Cr stable isotopic fractionation). The foundation for using Cr isotopes is to precisely know the compositions of the various chondritic reservoirs. However, the Cr isotope composition of Rumuruti (R) chondrites remains unknown. Here, we report high-precision mass-independent (average 2SE uncertainty of ∼0.02 and ∼0.06 for ε53Cr and ε54Cr, respectively; ε indicates 10,000 deviation) and mass-dependent (uncertainty of average 0.03 ‰ for δ53Cr; δ indicates 1,000 deviation) Cr isotope data for 12 bulk R chondrites of petrologic types 3-6 (included R chondrite breccias), and one R chondrite-like clast (MS-CH) in the Almahata Sitta polymict ureilite. All the R chondrites show homogeneous bulk ε54Cr values, -0.06 ± 0.08 (2SD), similar only to those of the Earth-Moon system and enstatite chondrites. These first ε54Cr data for R chondrites provide significant addition to the ε54Cr-Δ17O diagram, and position them as a potential endmember for planetary precursors. The R chondrites possess a higher 55Mn/52Cr of 0.68 ± 0.04 and higher ε53Cr values 0.23 ± 0.05 (2SD) relative to most of other chondrite groups. This likely results from the lower (e.g. than ordinary and enstatite chondrites) chondrule abundance in R chondrites. The stable Cr isotope composition of R chondrites is homogeneous with a δ53Cr = -0.12 ± 0.03 ‰ (2SD). Combined with previous data of other groups of chondrites, we show that the stable Cr isotopic composition of all the chondrites is homogeneous with δ53Cr of -0.12 ± 0.04 ‰ (2SD, N = 40) and is independent of the petrologic type and redox conditions. The lack of mass-dependent fractionation between all groups of chondrites suggests that the average chondrite δ53Cr value is also representative for the initial composition all differentiated planets in the Solar System. Finally, the MS-CH clast in Almahata Sitta has a Cr isotopic composition (ε53Cr = 0.18 ± 0.04, ε54Cr = -0.16 ± 0.07, and δ53Cr = -0.11 ± 0.05 ‰) that is consistent (within error) with it being an R chondrite-like clast.”