Abstract
This study explores the petrology of five giant (>400 μm) hydrated fine-grained micrometeorites from the Transantarctic Mountain (TAM) micrometeorite collection. For the first time, the extent and mechanisms of aqueous alteration in unmelted cosmic dust are evaluated and quantified. We use a range of criteria, previously defined for use on hydrated chondrites, including phyllosilicate fraction, matrix geochemistry and micro textures. Collectively, these micrometeorites represent ∼2.22 mm2 of intensely altered hydrated chondritic matrix (with petrologic subtypes of <1.2 in the scheme of Howard et al. (2015)) and reveal a range of alteration styles. Two particles are found to contain pseudomorphic chondrules with thick fine-grained rims, while another micrometeorite contains several aqueously altered CAIs. Their outlines range from well-defined to indistinct, demonstrating that the advanced stages of aqueous alteration progressively remove evidence of coarse-grained components. The remaining two micrometeorites entirely lack coarse-grained components but are similarly altered. Thus, the combined chondrule-to-matrix ratio among these giant micrometeorites is extremely low (6.45 area%), and significantly below the average ratio found in typical CM or CR chondrites (∼20%, Weisberg et al., 2006). Our findings are consistent with previous analyses from smaller Antarctic micrometeorites, which suggest that chondrules (and CAIs) derived from hydrated carbonaceous chondrite parent bodies are underrepresented among the micrometeorite flux, even when considering contributions from coarse-grained micrometeorites. Therefore, to explain the relative paucity of anhydrous material, we propose that the flux of fine-grained micrometeorites is primarily derived from intensely aqueously altered, primitive C-type asteroids, which have lost the majority of their refractory coarse-grained components by replacement with secondary phyllosilicate minerals.
Highlights
The interaction between liquid water and solid nebular condensates is termed aqueous alteration and represents a critical process of the early solar system’s evolution
Our findings are consistent with previous analyses from smaller Antarctic micrometeorites, which suggest that chondrules derived from hydrated carbonaceous chondrite parent bodies are underrepresented among the micrometeorite flux, even when considering contributions from coarse-grained micrometeorites
To explain the relative paucity of anhydrous material, we propose that the flux of fine-grained micrometeorites is primarily derived from intensely aqueously altered, primitive C-type asteroids, which have lost the majority of their refractory coarse-grained components by replacement with secondary phyllosilicate minerals
Summary
The interaction between liquid water and solid nebular condensates is termed aqueous alteration and represents a critical process of the early solar system’s evolution. Most primitive extraterrestrial materials, including ordinary chondrites (Doyle et al, 2015), carbonaceous chondrites (Rubin et al, 2007; Harju et al, 2014; King et al, 2015), micrometeorites (Genge et al, 1997; Suttle et al, 2017a), interplanetary dust particles (Rietmeijer, 1991) and cometary dust (Zolensky et al, 2006; Noguchi et al, 2017) show evidence of aqueous alteration This is most advanced, and most extensively studied, among the hydrated carbonaceous chondrite group (containing the CM and CI chondrites as well as a significant fraction [~70%, Harju et al, 2014] of the CR chondrites). These meteorites are dominated by secondary minerals, formed during alteration and contain between 2 and 20wt% water (Tonui et al, 2003; Rubin et al, 2007), primarily this is structural water held inside hydrated phyllosilicate minerals Both the CM and CI chondrites are dominated by a complex, mixed assemblage of interlocking Fe and Mg-phyllosilicate (Tomeoka and Buseck, 1985; Browning et al, 1996), representing multiple generations of secondary mineral growth (Elmaleh et al., 2015; Lee et al, 2012; 2013). They contain accessory Fe-Mg-Ca carbonates, minor Fe-oxides and dispersed hydrated Fe-Ni sulfides (Weisberg et al, 2006; Howard et al, 2009; 2015; King et al, 2015)
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