Petrology, mineralogy, and geochemistry of the olivine diogenite NWA 4255: new insights into the magmatic evolution of asteroid 4 Vesta

摘要

Northwest Africa (NWA) 4255 is a meteorite found in the region of Tindouf (southwestern Algeria), classified as brecciated olivine diogenite. Based on textural observations and orthopyroxene compositions, two different lithologies were determined: harzburgitic and orthopyroxenitic. The orthopyroxenitic lithology contains orthopyroxene (Mg no. 73.99-75.68) and spinel (Cr no. 83.09-85.11, Mg no. 15.57-22.45). On the other hand, the harzburgitic lithology contains orthopyroxene (Mg no. 74.54-77.14) and olivine (Mg no. 70.94-72.57). The iron metal and the sulfides (Troilites) of this sample are present in both lithologies and are low in Ni (Ni<0.1wt%). The Fe/Mn ratio of orthopyroxenes ranges from 22.28 to 32.64 and show a large overlap between both lithologies. Lowest ratios are unusual; they are below the defined field for diogenites and olivine diogenites. O-17 values are -0.234 +/- 0.003 (1 sigma) and confirm that the NWA 4255 originated from 4Vesta. The results of this study show that there is a genetic linkage between the two lithologies of NWA 4255 and correspond to in situ crystallization processes. This olivine diogenite reflects transition between two major magmatic processes in 4Vesta. The magma ocean of 4Vesta crystallized at equilibrium, allowing the formation of a dunitic and harzburgitic mantle. This late lithology is linked to the peritectic reaction between the olivines formed and the evolved liquid. Our sample then reflects this crucial step of separating this mantle from the residual liquid. This melt evolving on the peritectic allowed the formation of the observed harzburgitic assemblage and then evolves out from the peritectic reaction to proceed to a fractional crystallization process involving the formation of orthopyroxenite.