Accretionary mixing of a eucrite impactor and the regolith of the L chondrite parent body

1,2Brendt C. Hyde,1Kimberly T. Tait,2Desmond E. Moser,3Douglas Rumble III,1,4Michelle S. Thompson
Meteoritics & Planetary Science (in Press) Link to Article []
1Department of Natural History, Royal Ontario Museum, Toronto, ON, Canada, M5S 2C6
2Department of Earth Sciences, University of Western Ontario, London, ON, Canada, N6A 5B7
3Carnegie Institution of Washington, Washington, DC, 20015 USA
4Department of Earth, Atmospheric and Planetary Sciences, Purdue University, West Lafayette, IN, 47907 USA
Published by arrangement with John Wiley & Sons

Northwest Africa (NWA) 869 is the largest sample of chondritic regolith breccia, making it an ideal source for research on accretionary processes and primordial chemical mixing. One such process can be seen in detail through the first identification of a eucrite impactor clast in an L chondrite breccia. The ~7 mm diameter clast has oxygen isotope compositions (Δ17O = −0.240, −0.258‰) and pigeonite and augite compositions typical for eucrites, but with high areal abundance of silica (9.5%) and ilmenite (1.5%). The rim around the clast is a mixture of breccia and igneous phases, the latter due to either impactor‐triggered melting or later metamorphism. The rim has an oxygen isotope composition falling on a mixing line between known eucrite and L chondrite compositions (Δ17O = 0.326‰) and, coincidentally, on the Mars fractionation line. Pyroxene grains from the melt component in the rim have compositions that fall on a mixing line between the average eucrite pyroxene composition and equilibrated L chondrite composition. The margins of chondritic olivine crystal clasts in the rim are enriched in Fe as a result of diffusion from the Fe‐rich melt and suggest cooling on the scale of hours. The textures and chemical mixing observed provide evidence for an unconsolidated L chondrite target material, differing from the current state of NWA 869 material. The heterogeneity of oxygen isotope and chemical signatures at this small length scale serve as a cautionary note when extrapolating from small volumes of materials to deduce planetesimal source characteristics.


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