1,2Bastien Soens,3,4Stepan M.Chernonozhkin,3Claudia González de Vega,3Frank Vanhaecke,5Matthias van Ginneken,1Philippe Claeys,1Steven Goderis
Geochimica et Cosmochimica Acta (in Press) Link to Article []
1Analytical-, Environmental-, and Geo-Chemistry, Vrije Universiteit Brussel, Pleinlaan 2, BE-1050 Brussels, Belgium
2Laboratoire G-Time, Université Libre de Bruxelles 50, Av. F.D. Roosevelt CP 160/02, BE-1050 Brussels, Belgium
3Department of Chemistry, Ghent University, Krijgslaan 218 Building S12, BE-9000 Gent, Belgium
4Chair of General and Analytical Chemistry, Research group – Isotope ratio analysis, Montanuniversität Leoben, Franz Josef-Straße 18, 8700 Leoben, Austria1
5Centre for Astrophysics and Planetary Science, University of Kent, CT2 7NZ, Canterbury, Kent, United Kingdom
Copyright Elsevier

Achondritic micrometeorites represent one of the rarest (ca. 0.5–2.1%) particle types among Antarctic micrometeorite collections. Here, we present major, trace element and oxygen isotope compositions on five vitreous, achondritic cosmic spherules (341–526 µm in size) recovered from the Widerøefjellet sedimentary trap in the Sør Rondane Mountains (SRMs) of East Antarctica. We also present the first iron isotope data for four of these achondritic cosmic spherules. The particles were initially identified based on the atomic concentrations of Fe-Mg-Mn and their distribution in Fe/Mg versus Fe/Mn space, spanning a relatively wide range in Fe/Mg ratios (ca. 0.48–1.72). The Fe/Mn ratios cover a more restricted range (22.4–31.7), comparable to or slightly below the values measured for howardite-eucrite-diogenite (HED) and martian meteorites. One particle (WF1801-AC3) displays an elevated Fe/Mn ratio of ∼78, comparable to the values determined for lunar rocks. The negative correlation observed between the CaO+Al2O3 contents and the Fe/Si ratios of achondritic spherules reflects both the mineralogy of the precursor materials, as well as the extent of volatilization experienced during atmospheric entry heating. This trend suggests that the primary mineralogy of precursor materials may have been compositionally similar to basaltic achondrites. Based on their distribution in Ca/Si versus Al/Si space, we argue that the majority of achondritic cosmic spherules predominantly sample pyroxene- and/or plagioclase-rich (i.e., basaltic) precursor bodies. Such precursor mineralogy is also inferred from their rare earth element (REE) patterns, which show resemblances to fine-grained basaltic eucrites or Type 1 achondritic spherules (n = 3 – av. REEN = 11.2–15.5, (La/Yb)N = 0.93–1.21), pigeonite-rich equilibrated eucrite precursors or Type 2 achondritic spherules (n = 1 – av. REEN = 27.9, (La/Yb)N = 0.10), and possibly Ca-phosphates from (primitive) achondritic bodies (n = 1 – av. REEN = 58.8, (La/Yb)N = 1.59). This is clearly demonstrated for particle WF1801AC-1, which was likely inherited from a fine-grained eucritic precursor body. The pre-atmospheric oxygen isotope composition was reconstructed through compensation of mass-dependent fractionation processes as well as mixing with atmospheric oxygen, using iron isotope data. Two particles (WF1801AC-2, WF1801-AC4) display corrected oxygen isotope compositions (δ18O = 3.7–4.4‰) largely consistent with HED meteorites and may thus originate from HED-like parent bodies. The corrected oxygen isotope compositions (δ18O = 12.6–12.8‰) of the remaining particles (WF1801-AC3, WF1801-AC5) do not correspond to known meteorite fields and may represent two distinct types of unknown achondritic parent bodies or residual atmospheric entry effects. Finally, the abundance (ca. 0.5%) of achondritic cosmic spherules within the Widerøefjellet sedimentary trap is comparable to that observed in the South Pole Water Well (SPWW – ca. 0.5%), Novaya Zemlya glacier (ca. 0.45%) and Transantarctic Mountain (TAM) (ca. 2.1%) collections, confirming their overall rarity in micrometeorite collections. Unambiguous evidence for micrometeorites from the Moon or Mars remains absent from collections to date.


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