1Ai-Cheng Zhang, 1Tian-Ran Du, 1Lang Zhang, 1Xiao-Wen Liu, 2Li-Ping Qin
Meteoritics & Planetary Science (in Press)
Link to Article [DOI: 10.1111/maps.70260]
1State Key Laboratory of Critical Earth Material Cycling and Mineral Deposits, School of Earth Sciences and Engineering, Nanjing University, Nanjing, China
2State Key Laboratory of Lithospheric and Environmental Coevolution, School of Earth and Space Sciences, University of Science and Technology of China, Hefei, China
Published by arrangement with John Wiley & Sons
Late veneer hypothesis involves a process whereby material with a generally bulk chondritic composition is added to the silicate shell of a differentiated planetary body, subsequent to core formation. Understanding the origin and extent of the putative late veneer is critical for understanding the late stages of planetary accretion and evolution. The late veneer concept was originally proposed based on the absolute and relative abundances of highly siderophile elements in whole-rock samples of Earth’s mantle, and supported by data for several other planetary bodies. No supporting mineralogical evidence, however, has been reported. Here, we report the discovery of an Ir,W,Ru,Rh-rich metal grain in early-formed unbrecciated cumulate eucrite Northwest Africa 8326. We suggest that the W-rich nature of this grain could not be a result of eucrite parent melt evolution, or due to exsolution from a pre-existing sulfide phase during cooling or S degassing. Instead, it is best explained with the addition of a chondrite-related component. The survival of this W-rich metal grain during high-temperature melting processes could be related to its high melting point. The W-rich grain in NWA 8326 represents the first mineralogical evidence, supporting the late veneer hypothesis. Refractory metal grains in unbrecciated achondrites like that reported here potentially have important impacts on the highly siderophile element patterns and isotopic modal ages of their host rocks.
