How do secondary iron enrichments form within basaltic eucrites? An experimental approach

Abstract

Some basaltic eucrites and basaltic lithologies in howardites derived from the asteroid 4 Vesta exhibit unusual secondary veinlet textures consisting mostly of fayalitic olivine and Fe‐enrichments within pyroxenes. Recent studies discussed the formation of these Fe‐rich phases either by interaction with a vapor and/or liquid phase (metasomatism), or by a high‐temperature melting process. We therefore performed a series of heating and hydrothermal experiments with liquids of different compositions on natural pyroxene crystals (augite and orthopyroxene) to evaluate these contrasting hypotheses. The results of the heating experiments show that incongruent melting of pyroxenes at about 1070 °C causes textures that are very similar to those observed in the meteorites. We conclude that a part of the natural secondary veins might be explained by heating processes at similar temperatures. The hydrothermal experiments with aqueous liquids of different Fe‐enriched compositions clearly indicate ion exchange reactions resulting in partial Fe‐enrichments of the pyroxene. Interestingly, these Fe‐enrichments occurred independent of the Fe content of the liquid, which can be explained by an internal origin of Fe from the pyroxenes. In one hydrothermal experiment of augite with Fe‐oxalate solution, deposition of fayalitic olivine was observed. From our experimental observations, we conclude that aqueous liquids are plausible candidates for explaining the deposition of Fe‐enrichments and fayalitic olivine inside the fractures of pyroxene. However, we cannot rule out a high‐temperature melting process slightly above the peritectic point of pyroxene to explain a fraction of observed secondary Fe‐enrichments.

Malotas (b), a new eucrite from an old fall

“…The highly porous nature of silica in the silica-troilite pockets (Fig. 11) could point to a vapor-driven alteration, in agreement with some previous studies (e.g., Rombeck et al, 2021).…”

“…Therefore, the occurrence of Fe-rich veins in unmetamorphosed pyroxenes (e.g., Vollmer et al, 2020) supports a latestage metasomatism scenario, probably caused by a degassing magma ocean (Sarafian et al, 2017). Recently, Rombeck et al (2021) performed a series of heating and hydrothermal experiments with liquids of different Fe-enriched compositions. These led them to conclude that the metasomatic reactions of aqueous liquids with basaltic eucrite minerals could explain the presence of veinlet-like textures with Fe-enrichments and/or fayalitic olivine grains inside the fractures of pyroxene.…”

“…The lack of Fe‐enrichment in Opx rims as well as of fayalitic olivine veins inside the fractures of pyroxene may point toward the absence of aqueous fluids that could have served as metasomatic media regardless of whether the fluid contained Fe or not, as shown by the experimental work of Rombeck et al. (2021).…”

“…Consistent with this observation, the disparity in TiO 2 and especially in Ti/REE, is not related to the grain size but to the fact that Opx rims are more abundant in the CG lithology, and therefore, the Ti and P are more abundant. Thus, in addition to Mg and Si, the metasomatic fluid was carrying P, Ti, and Fe (e.g., element mobilization by fluids; Rombeck et al., 2021), although Fe was not sufficiently abundant in the fluid to produce Fe‐rich rims in the Opx nor to give rise to fayalitic olivine veins.…”

“…The host pyroxene have acted as a major host for Mg and Fe first and, therefore, the system could get oversaturated in Si, Ca, Al, and other available elements leading to the formation of Pl veins and silica polymorphs but insufficient to form fayalitic olivine. The lack of Feenrichment in Opx rims as well as of fayalitic olivine veins inside the fractures of pyroxene may point toward the absence of aqueous fluids that could have served as metasomatic media regardless of whether the fluid contained Fe or not, as shown by the experimental work of Rombeck et al (2021).…”