The volatile content of Vesta: Clues from apatite in eucrites

Abstract

Abstract-Apatite was analyzed by electron microprobe in 3 cumulate and 10 basaltic eucrites. Eucritic apatite is fluorine-rich with minor chlorine and hydroxyl (calculated by difference). We confirmed the hydroxyl content by measuring hydroxyl directly in apatites from three representative eucrites using secondary ionization mass spectroscopy. Overall, most eucritic apatites resemble fluorine-rich lunar mare apatites, but intriguing OH-and Cl-rich apatites suggest a role for water and/or hydrothermal fluids in the Vestan interior or on other related differentiated asteroids. Most late-stage apatite found in mesostasis has little hydroxyl or chlorine and is thought to have crystallized from a degassed magma; however, several apatites exhibit atypical compositions and/or textural characteristics. For example, the isotopically anomalous basaltic eucrite Pasamonte has apatite in the mesostasis with significant OH. Apatites in Juvinas also have significant OH and occur as veinlets crosscutting silicates. Euhedral apatites in the Moore County cumulate eucrite occur as inclusions in pyroxene and are also hydroxyl-rich (0.62 wt% OH). The OH was confirmed by SIMS analysis and this apatite clearly points to the presence of water, at least locally, in the Vestan interior. Portions of Elephant Moraine (EET) 90020 have large and abundant apatites, which may be the product of apatite accumulation in a zone of melt-rock reaction. Relatively chlorine-rich apatites occur in basaltic eucrite Graves Nunataks (GRA) 98098 (approximately 1 wt% Cl). Particularly striking is the compositional similarity between apatite in GRA 98098 and apatites in lunar KREEP, which may indicate the presence of residual magmas from an asteroid-wide magma ocean on Vesta.

Asteroid (4) Vesta: I. The howardite-eucrite-diogenite (HED) clan of meteorites

“…In contrast, Mittlefehldt and Lindstrom (2003) concluded that the depletion of light REE but not of other highly incompatible elements (Hf, Ta) reflected subsolidus REE exchange between the Ca-phosphates in the coarse-and fine-grained lithologies coupled with non-representative sampling of this heterogeneous rock. Rare-earth-element-rich merrillite is present only in the finegrained lithology (Sarafian et al, 2013;Yamaguchi et al, 2001). Several other basaltic eucrites with granulitic texture (Agoult, A-87272, DaG 945, and NWA 2362) have REE patterns with some similarities to that of EET 90020, and these have again been interpreted to be restites (Yamaguchi et al, 2009).…”

“…Eucrites are generally considered to be volatile-poor igneous rocks, but the apatites in them contain volatile elements in the structural X site. Sarafian et al (2013) have shown that apatite grains in basaltic and cumulate eucrites have mostly F in the X sites, but some grains in Juvinas contain up to about 25% OH in the X sites. The apatite grains in eucrites are poor in Cl, with <3% of the X site filled by it.…”

“…The apatite grains in eucrites are poor in Cl, with <3% of the X site filled by it. However, unbrecciated, metamorphosed basaltic eucrite GRA 98098 contains apatite grains with ∼11% of the X sites filled by Cl (Sarafian et al, 2013).…”

Overview of the composition of asteroid 4 Vesta: Constraints from the Dawn spacecraft mission and HEDs

“…New chemical analyses of howardite and eucrite polymict breccias ) define the proportions of petrologic components that comprise Vesta's regolith and crust. Analyses of bound water in apatite crystals in some eucrites (Sarafian et al 2013) reveal that Vesta's magmas were not completely dry, as has been commonly assumed. And laboratory spectra of olivinebearing diogenites do not show the diagnostic features of olivine , probably explaining Dawn's lack of olivine detection in Vesta's huge Rheasilvia basin.…”

Phosphate Rocks

“…The volatile components indicate the presence of apatite in eucrites. Eucritic apatite is fluorine rich with minor chlorine and hydroxyl (calculated by difference) [106], [107], [108].…”