Diogenites as polymict breccias composed of orthopyroxenite and harzburgite

Abstract

Abstract-A few relatively unbrecciated olivine-rich diogenites consist of an equilibrium assemblage of olivine and magnesian orthopyroxene (harzburgite). More common diogenites with smaller amounts of olivine are breccias containing two distinct orthopyroxenes-one magnesian and one ferroan. These diogenites are mixtures of a harzburgite lithology that is more magnesian, with the ''normal'' orthopyroxenite lithology that is ferroan and may contain small amounts of plagioclase. Both lithologies likely formed by fractional crystallization in multiple plutons emplaced within the crust of asteroid 4 Vesta. Minor element trends in orthopyroxenes indicate that these plutons exhibited a range of compositions. We propose a revised taxonomy for the HED (howardites, eucrites, and diogenites) suite where all ultramafic samples are referred to as diogenites. Within this group, the prefixes dunitic, harzburgitic, and orthopyroxenitic are used to distinguish diogenites consisting of more than or equal to 90% olivine, olivine + orthopyroxene, and more than or equal to 90% orthopyroxene, respectively. The prefix polymict is used to describe brecciated mixtures of any of these rock types. The recognition that olivine is a significant phase in some diogenites is consistent with spectral interpretations of olivine in a deeply excavated crater on Vesta, and has important implications for the bulk composition and petrogenesis of that body.

Origin, Internal Structure and Evolution of 4 Vesta

“…The diogenites are coarse-grained cumulates that formed in fractionally-crystallized magmas (Bowman et al 1997); the existence of olivine-bearing diogenites (Beck and McSween 2010) indicate that these magmas did not crystallize orthopyroxene alone. The diogenites constitute the deepest component of the layered crust model (Takeda 1997).…”

The origin of Vesta’s crust: Insights from spectroscopy of the Vestoids

“…Asteroid differentiation is far from being completely understood and, while the majority of models favor the formation of a dunite mantle (e.g. Mason, 1962;Ruzicka et al, 1997), there are others that have suggested alternative asteroidal mantle materials such as harzburgite (Righter and Drake, 1997;Beck and McSween, 2010), high-Ca pyroxene rich cumulates , and even a relict layer of highly metamorphosed chondritic material (McCoy et al, 2006).…”

“…There are two main mechanisms suggested for the formation of ferroan diogenites: that they formed from different parental melts relative to the more typical Mg-rich diogenites (Yamagchi et al, 2009); they are produced late in a sequence of fractional crystallization within individual magma chambers (Beck and McSween, 2010). Both of these formation mechanisms are possible within the partial melting model for Vesta.…”

Joint Anomaly Detection and Spectral Unmixing for Planetary Hyperspectral Images

“…This region was chosen because Olivine, a mineral indicative of deep magmatic processes, was first detected in this area [55]. This mineral is never found alone in meteorites coming from Vesta-4 [56], [57], implying the use of an algorithm able to decompose spectra as a combination of mixtures of materials.…”