Variation of mineralogy and organic material during the early stages of aqueous activity recorded in Antarctic micrometeorites

Abstract

Micrometeorites (MMs) recovered from surface snow near the Dome Fuji Station, Antarctica are almost free from terrestrial weathering and contain very primitive materials, and are suitable for investigation of the evolution and interaction of inorganic and organic materials in the early solar system. We carried out a comprehensive study on seven porous and fluffy MMs [four Chondritic porous (CP) MMs and three fluffy fine-grained (Fluffy Fg) MMs] and one fine-grained type 1 (Fg C1) MM for comparison with scanning electron microscope, transmission electron microscope, X-ray absorption near-edge structure analysis, and secondary ion mass spectrometer.They show a variety of early aqueous activities. Four out of the seven CP MMs contain glass with embedded metal and sulfide (GEMS) and enstatite whiskers/platelets and do not have hydrated minerals. Despite the same mineralogy, organic chemistry of the CP MMs shows diversity. Two of them contain considerable amounts of organic materials with high carboxyl functionality, and one of them contains nitrile (C≡N) and/or nitrogen heterocyclic groups with D and 15 N enrichments, suggesting formation in the molecular cloud or a very low temperature region of the outer solar system. Another two CP MMs are poorer in organic materials than the above-mentioned MMs. Organic material in one of them is richer in aromatic C than the CP MMs mentioned above, being indistinguishable from those of hydrated carbonaceous chondrites. In addition, bulk chemical compositions of GEMS in the latter organic poor CP MMs are more homogeneous and have higher Fe/(Si+Mg+Fe) ratios than those of GEMS in the former organic-rich CP MMs. Functional group of the organic materials and amorphous silicate in GEMS in the organic-poor CP MMs may have transformed in the earliest stage of aqueous alteration, which did not form hydrated minerals.Three Fluffy Fg MMs contain abundant phyllosilicates, showing a clear evidence of aqueous alteration. Phyllosilicates in thee MMs are richer in Fe than those in hydrated IDPs, typical fine-grained hydrated (Fg C1) MMs, and hydrated carbonaceous chondrites. One of the Fluffy Fg MMs contains amorphous silicate, which is richer in Fe than GEMS and contains little or no nanophase Fe metal but contains Fe sulfide. Because the chemical compositions of the amorphous silicate are within the compositional field of GEMS in CP IDPs, the amorphous silicate may be alteration products of GEMS. The entire compositional field of GEMS in the CP MMs and the amorphous silicate in the Fluffy Fg MM matches that of the previously reported total compositional range of GEMS in IDPs.One Fluffy Fg MM contains Mg-rich phyllosilicate along with Fe-rich phyllosilicate and Mg-Fe carbonate. Mg-rich phyllosilicate and Mg-Fe carbonate may have been formed through the reaction of Fe-rich phyllosilicate, Mg-rich olivine and pyroxene, and water with C-bearing chemical species.These data indicate that CP MMs and Fluffy Fg MMs recovered from Antarctic surface snow contain materials that throw a light on the earl...

A microchondrule‐bearing micrometeorite and comparison with microchondrules in CM chondrites

“…Thus, a cometary parent body is another possible source, as they also contain LIME/LICE silicates, microchondrules (Noguchi et al. ), and minerals with exotic or extreme chemical compositions.…”

“…in Noguchi et al. ). In contrast, the prevalence and petrology of microchondrules in hydrated chondrites remains largely unstudied.…”

“…), and containing glass with embedded metal sulfides (GEMS) and primitive nebular condensates (Noguchi et al. ) are currently associated with distant cometary parent bodies, formed beyond the nitrogen snow line (Dartois et al. , ).…”

“…; Noguchi et al. ; Yabuta et al. ) and provide a compelling example of how extraterrestrial dust has expanded our collective inventory of solar system materials.…”

“…Carbon-rich micrometeorites, characterized by extreme deuterium excesses (Duprat et al 2010), and containing glass with embedded metal sulfides (GEMS) and primitive nebular condensates are currently associated with distant cometary parent bodies, formed beyond the nitrogen snow line (Dartois et al 2013(Dartois et al , 2018. These particles are termed ultracarbonaceous Antarctic micrometeorites (UCAMMs) (Dobricǎ et al 2009;Duprat et al 2010;Imae et al 2013;Noguchi et al 2017;Yabuta et al 2017) and provide a compelling example of how extraterrestrial dust has expanded our collective inventory of solar system materials. Likewise, unique micrometeorites with asteroidal affinities are also known; these include a fragment of basaltic crust derived from a differentiated protoplanet ); a polycrystalline, fine-grained micrometeorite exhibiting evidence of flash heating and quench cooling while in space (Noguchi et al 2013); and an unusually Ni-rich, oxidized, and irradiated giant micrometeorite with possible affinities to the CK chondrite class (Cordier et al 2018).…”