18 Northwest Africa (NWA) 4255 is a meteorite found in the region of Tindouf (southwestern 19 Algeria), classified as brecciated olivine diogenite. Based on textural observations and 20 orthopyroxene compositions, two different lithologies were determined: harzburgitic and 21 orthopyroxenitic. The orthopyroxenitic lithology contains orthopyroxene (Mg no. 73.99-22 75.68) and spinel (Cr no. 83.09-85.11, Mg no. 15.57-22.45). On the other hand, the 23 harzburgitic lithology contains orthopyroxene (Mg no. 74.54-77.14) and olivine (Mg no. 24 70.94-72.57). The iron metal and the sulfides (Troilites) of this sample are present in both 25 lithologies and are low in Ni (Ni < 0.1wt%). The Fe/Mn ratio of orthopyroxenes ranges from 26 22.28 to 32.64 and show a large overlap between both lithologies. Lowest ratios are unusual; 27 they are below the defined field for diogenites and olivine diogenites. Δ17O values are − 28 0.234 ± 0.003 (1σ) and confirm that the NWA 4255 originated from 4Vesta. The results of 29 this study show that there is a genetic linkage between the two lithologies of NWA 4255 and 30 correspond to in situ crystallization processes. This olivine diogenite reflects transition 31 between two major magmatic processes in 4Vesta. The magma ocean of 4Vesta crystallized at 32 equilibrium, allowing the formation of a dunitic and harzburgitic mantle. This late lithology is 33 linked to the peritectic reaction between the olivines formed and the evolved liquid. Our 34 sample then reflects this crucial step of separating this mantle from the residual liquid. This 35 melt evolving on the peritectic allowed the formation of the observed harzburgitic assemblage 36 and then evolves out from the peritectic reaction to proceed to a fractional crystallization 37 process involving the formation of orthopyroxenite. 38 39 Introduction 40