Peridotites Related to 4vesta: Deep Crustal Igneous Cumulates and Mantle
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چکیده
In advance of the arrival of the DAWN spacecraft at 4Vesta in 2011, it is useful to reassess our knowledge of the constitution and probable evolution of this ancient differentiated asteroid. It has been recognized for some time that the eucrite, diogenite and howardite meteorites likely represent samples from 4Vesta or from related Vestoids based upon the good match of reflectance spectra of these bodies to those of eucrites [1]. Oxygen isotopic compositions for eucrites, diogenites and howardites [2] exhibit more dispersion than for terrestrial samples and angrites, which may imply a shorter-lived period of parent body differentiation. The likelihood that mesosiderites also derive from this same body is strongly supported by (a) similar oxygen isotopic compositions [3], (b) overlapping Fe/Mn ratios in constituent olivines and pyroxenes, (c) similar calcic plagioclase compositions, (d) silica polymorph in both, and (e) eucritic and diogenitic clasts in some mesosiderites (e.g., NWA 2907) and vice versa. Diogenite Diversity: Newly discovered diogenites from Northwest Africa have expanded our knowledge of the ranges in modal mineralogy and mineral compositions beyond those reported previously [4]. For example, Northwest Africa 5405 contains ~10 vol. % olivine (Fa28.5, FeO/MnO = 49.4) in addition to major orthopyroxene (Fs23.8Wo2.1, FeO/MnO = 26.8) and accessory clinopyroxene, calcic plagioclase, chromite [Cr/(Cr+Al) = 0.675], troilite and kamacite. Northwest Africa 5312 is even more olivineand troilite-rich, and is composed of 46 vol.% orthopyroxene (Fs20.6Wo1.5, FeO/MnO = 31), 24 vol.% olivine (Fa22.1, FeO/MnO = 48), 18 vol.% troilite, 6 vol.% merrillite, chromite [Cr/(Cr+Al) = 0.73] and taenite. The molar ferrosilite content in orthopyroxene for 32 diogenites with less than 25 vol.% olivine (most with 0-5 vol.%) ranges from 0.14 to 0.31 with a median of 0.24 (see Figure 1). This and the variations in major and accessory modal mineralogy support the hypothesis that diogenites represent a series of orthopyroxeneolivine-(plagioclase) cumulates from “basaltic” liquids ranging widely in Mg/(Mg+Fe) ratio (utilizing orthopyroxene/liquid KD Fe/Mg values of [5]). Vesta Peridotites: Spectral evidence [6] suggests that the interior of 4Vesta contains significant olivine. There are 8 different diogenitic specimens that contain more than 40 vol.% olivine, including ALHA 77256, NWA 1459, GRA 98108, EETA 79002, NWA 4223, NWA 1877 and MIL 07001 [e.g., 7]. These chromiteDiogenites
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