Possible fluid–rock interactions on differentiated asteroids recorded in eucritic meteorites

Abstract

The eucritic meteorites are basaltic rocks that originate from the upper part of the crust of some small bodies as exemplified possibly by asteroid 4-Vesta. A few eucrites appear to have been modified by different degrees of a late stage alteration process that caused significant variations in mineralogy. Three distinct alteration stages are identified: (1) Fe-enrichment along the cracks that cross cut the pyroxene crystals ("Fe-metasomatism"); secondary olivine and minute amounts of troilite are found only occasionally in cracks at this stage; (2) deposits of Fe-rich olivine (Fa 64-86 ) and minor amounts of troilite are frequent inside the cracks; sporadic secondary Ca-rich plagioclase (An 97-98 ) is associated with the fayalitic olivine; (3) at this stage, the Fe-enrichment of the pyroxene is accompanied by a marked Al-depletion; moreover, secondary Ca-rich plagioclase is more frequent and partly fills some cracks or rims of the primary plagioclase crystals. The composition of the secondary phases on one hand, the lack of incompatible trace element enrichment in the metasomatized pyroxenes on the other hand, rule out a silicate melt as the metasomatic agent. Although no hydrous phase has been yet identified in the studied samples, aqueous fluids are plausible candidates for explaining the deposits of ferroan olivine and anorthitic plagioclase inside the fractures of the studied unequilibrated eucrites.

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

“…Harzburgitic and orthopyroxenitic diogenites contain from ∼64 to The field for primary pyroxenes in unmetamorphosed basaltic clasts from polymict eucrite Y-75001 is taken from Barrat et al (2011). Representative tie lines between low-Ca pyroxene hosts and high-Ca pyroxene exsolutions are shown in panels a-c. Data plotted are from Table S2.…”

“…Mittlefehldt and Lindstrom, 1993;Takeda et al, 1994). Ferroan olivine is also present in late-stage veins transecting pyroxenes in some basaltic eucrites (e.g., Barrat et al, 2011); these are discussed in Section 8.…”

“…In contrast, Barrat et al (2011), documented a variety of compositional and mineralogical changes crosscutting pyroxenes in several eucrites, which they concluded were cause by interaction with latestage aqueous fluids. The veins included simple Fe-enrichment in Pasamonte, Fe-enrichment coupled with formation of fine-grained ferroan olivine and sometimes very calcic plagioclase (An 97-99 ) in several eucrites, and this same assemblage but on wider scale and with development of Al-depletions .…”

Iron isotope fractionation in planetary crusts

“…The origin of these phases is a matter of debate. Barrat et al (2011) have proposed that this Feenrichment is the result of a metasomatic event. The nature of the metasomatic agent is not clear, but could have been an aqueous fluid.…”

DETECTION OF WIDESPREAD HYDRATED MATERIALS ON VESTA BY THE VIR IMAGING SPECTROMETER ON BOARD THE DAWN MISSION

“…HEDs formed under moderately reducing conditions and have low concentrations of volatile elements (Mittlefehldt et al 1998). The HED parent body is thought to be volatile-depleted, as evidence for hydrous alteration is rarely found in HEDs (Mittlefehldt & Lindstrom 1997;Treiman et al 2004;Barrat et al 2011). Apatite, a volatilebearing mineral, is a minor constituent of eucrites and very rare in diogenites.…”