On the source of diogenites and olivine diogenites: Compositional diversity from variable fO2

Jennifer T. Mitchell, Andrew G. Tomkins

Geochimica et Cosmochimica Acta
In Press, Accepted Manuscript, Available online 23 May 2019


“The genesis of diogenites and olivine-bearing diogenites has been debated for decades, with current models favouring formation via either mineral settling in a homogeneous magma ocean, or as late stage intrusions into the crust of asteroid 4 Vesta. Using pMELTS, both equilibrium and fractional crystallisation modelling was conducted on a large range of melt compositions generated by varied extent of batch melt extraction from 11 bulk Vesta starting compositions at a range of fO2 conditions to simulate the magma ocean concept. The resulting mineral compositions were compared with those of 200 diogenite meteorites in an attempt to resolve this debate. Models that involve < 20% initial partial melt cannot produce orthopyroxenites. Orthopyroxenitic diogenites have compositional ranges from En53-En82, whereas ‘olivine diogenites’ show less compositional diversity with orthopyroxenes ranging from En71-En76. Olivine-bearing diogenites are therefore not the most magnesian samples, which contradicts expected crystallisation trends expected from a single homogeneous source. The orthopyroxene compositions produced by models that use fO2 previously suggested for Vesta of ΔIW -2.05 are too magnesian, and the extent of source partial melting used in the models has negligible effect on this result. Modelling using different initial oxygen fugacity conditions produces a large range of pyroxene compositions that better match the range seen in diogenites, with models ranging from ΔIW fO2 -1.6 to -1.2 producing the best fit. These results thus imply that the diogenites crystallised from a variety of magmas sourced from a region of heterogeneous oxygen fugacity. This variation can be explained by metasomatism of a homogenous source region by fO2-modifying sulfidation reactions. The model orthopyroxene compositions are displaced with regards to Wo from natural diogenites; this can be explained by a delayed genesis model whereby a Ca-poor diogenite source developed in response to the melt extraction necessary for formation of a eucritic crust. Our models suggest that diogenites were derived from a series of magma chambers in the Vestan crust.”