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• W2020238915 endingPage "227" @default.
• W2020238915 startingPage "199" @default.
• W2020238915 abstract "The Northwest Africa 5738 eucrite contains a record of unprecedented geochemical complexity for a sample from the HED asteroid. It originated with a uniquely evolved (Stannern Trend) primary igneous composition, combining ultra-high bulk incompatible element and Na2O concentrations with a relatively low mg. Its bulk oxygen-isotopic composition (Δ′17O = −0.27‰), as well as its trace element composition (e.g., Ga/Al), confirm other evidence for classification as a eucrite. Pyroxene mg equilibration, exsolution and “cloudy” inclusions, all reflect a typical eucritic degree of thermal metamorphism. The rock contains an unprecedented array of microscopic fluid-metasomatic vein deposits. Most common are curvy microveins within pyroxene, which consist dominantly of Ca-plagioclase (typically An95, in stark contrast with the rock’s An68–78 primary-igneous plagioclase), with Fe-olivine (Fo14) and Cr-spinel as additional major constituents. Likely related to these microveins are small masses of intergrown Ca-plagioclase (again roughly An95) and silica (or high-Si glass). Analyses of the microvein Cr-spinels show stoichiometry implying a significant Fe3+ content (Fe2O3 0.7–2.3 wt.%), and fO2 up to roughly IW+3; clearly elevated in comparison to the normal HED fO2 of about IW−1. The fO2 results show an anticorrelation with equilibration T (and with Mg/Fe), which suggests the parent fluid system became more oxidizing as it cooled. NWA 5738 also contains apparent secondary iron metal. The Fe-metals are very pure, with Ni consistently below an EPMA detection limit of ∼0.01 wt.%. The vein-like shapes of roughly 1/3 of the largest Fe-metals suggest origin by deposition from a fluid. The role of pyroxene exsolution as template for a denticular (sawtooth) Fe-metal edge shape, and the survival of Fo14 olivine in a rock with abundant silica and a far higher bulk mg, suggest that the most intense thermal metamorphism occurred no later than the secondary alteration. Near-complete lack of spatial association suggests that the Fe-metals formed during a distinct time period from the curvy microveins. The immediate cause of Fe-metal deposition was most plausibly (or anyway, least implausibly) an abrupt downshift in the fluid fO2. Considering the extremely evolved bulk composition, the fluid(s) may have been largely deuteric. However, more likely the main source of fluid was a nearby buried mass of volatile-rich impactor matter, such as carbonaceous chondrite, that hit the asteroid at low enough velocity to remain mostly intact. We further speculate that the abrupt drop in fluid fO2 may have been caused by a process of carbon-fueled “smelting” (cf. ureilites), triggered by an impact-effected shift of the carbonaceous material to a changed environment, with higher T and/or lower P. These and other recent eucrite results point to a need for greater scrutiny regarding the absence of comparable alteration-veining in rocks from the lunar highland crust, a mysterious lack in view of recent evidence for abundant lunar water." @default.
• W2020238915 created "2016-06-24" @default.
• W2020238915 creator A5014312676 @default.
• W2020238915 creator A5034637836 @default.
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• W2020238915 creator A5045053808 @default.
• W2020238915 creator A5061945619 @default.
• W2020238915 creator A5066981120 @default.
• W2020238915 date "2014-09-01" @default.
• W2020238915 modified "2023-10-18" @default.
• W2020238915 title "Northwest Africa 5738: Multistage fluid-driven secondary alteration in an extraordinarily evolved eucrite" @default.
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