Martin Suttle1,2, Matthew Genge1,2 & Sara Russell2, +447748716692, mds10@ic.ac.uk.

1. Dept. of Earth Sciences, Royal School Of Mines, Imperial College London, Prince Consort Road, South Kensington, London, UK, SW7 2BP

2. Dept. of Mineralogy, The Natural History Museum, Cromwell Road, South Kensington, London, UK, SW7 5BD

A Microchondrule-bearing Micrometeorite

[1] Nesvorný, et al., 2003. The Astrophysical Journal, 591:486-497. [2] Nesvorný, et al., 2010. The Astrophysical Journal, 713:816-836. [3] Suavet, et al., 2010. Earth and Planetary Science Letters, 293:313-320.[4] Suttle et al., 2017. Geochimica et Cosmochimica Acta, 206:112-136.[5] Ebel, et al., 2012. Meteoritics & Planetary Science, 47:585-593.

8. References

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7. ConclusionsI. Intact chondrules are exceedingly rare among

micrometeorites (<1%)[10]. This study reports thefirst occurrence of a microchondrule within amicrometeorite.

II. A diverse mix of phases including a high-temperature refractory CAI, primitive LICEsilicates, low-temperature aqueous alterationproducts (phyllosilicates and Fe-sulphides) and amicrochondrule droplet formed in an impactplume are present within this micrometeorite.The complex geological history of the hostasteroid, from accretion to parent bodyevolution, is therefore, recorded.

1. Introduction

2. Overview

4. Anhydrous silicates

5. Element distribution

6. Microchondrule

3. IR Spectroscopy

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The Earth receives a continuous flux of cosmic dust derived from young disrupted asteroids and sublimating short period comets[1,2]. Occasionally unique micrometeoriteswith distinct petrographies are reported[3]. These samples expand the inventory of asteroid parent bodies and provide further clues to the formation and evolution of thesolar system. In this study we report the discovery of an unusual dehydroxylated fine-grained micrometeorite containing a devitrified microchondrule droplet. Thisspherule inclusion contains a volatile-rich composition indicating formation in an energetic high density plume.

Particle CP94-050-182 (78x108μm) is a fine-grained micrometeorite,surrounded by a partial magnetite rim. The internal mineralogy is aheterogeneous mix of anhydrous silicates suspended in a fine-grained, porousgroundmass. The presence of randomly orientated dehydration cracks androunded submicron vesicles indicates that thermal decomposition of low-temperature phases occurred during atmospheric entry. Dispersed micron-scale Mn-bearing chromite spinels (MnCr2O4) are also abundant within thematrix. A single spherical object (<10μm diameter) composed of devitrifiedglass is located near the micrometeorite’s perimeter.

A global mid-IR spectrum ofCP94-050-182 reveals an olivinesignature, indicating that theparticle’s matrix is primarilycomposed of (nano-)crystallineanhydrous silicates.However, dehydration cracksrequire the former presence ofhydrated phyllosilicates prior toatmospheric entry.This particle has, therefore,experienced high-temperatures(~700-1200°C) recrystallizationduring atmospheric entry[4].

Element mapping pin-points distinct phaseswithin a complex micrometeorite’s matrix.

Here, the condensation of Cr-rich silicates(olivine & pyroxenes, see no.4) is expectedto suppress the simultaneous growth of Cr-bearing spinels[6], which are also foundwithin the groundmass. The refractoryphases in this micrometeorite are,therefore, incompatible with a simplisticcondensation scenario and, instead requiresaccretion of materials from (at least) 2geochemically distinct reservoirs or acomplex cooling history.

Additionally, a ghost CAI is present,highlighted by the presence of hotspots inthe Ca, Al and Ti maps. These phases arealso rare among micrometeorites.

The small spherule inclusion isinterpreted as a glassymicrochondrule on the basis of thehighly spherical morphology and achemical composition similar todroplet microchondrules in LL3.4chondrite Manych[7]. The highabundance of volatiles requires highdust densities in order to retainvolatile components[8,9], high peaktemperatures to destroy crystalnuclei and quench cooling toprevent crystallization. Theseproperties, combined with the smallparticle size may indicate formationin an impact event. Finally,devitrification of the host glass mostlikely occurred during atmosphericentry heating.

Anhydrous silicates remain unaffected by entry heating, and preserve primitive compositions, similar to thelow-Fe-Cr-enriched (LICE) and low-Fe-Mn-enriched (LIME) silicates found in cometary IDPs, Stardust missionsamples and some carbonaceous chondrites[5]. These anhydrous silicates form by high-temperaturecondensation (~1200K) under reducing conditions from a gas of solar composition and at high dustdensitites.

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[6] Sugiura, et al., 2009. Meteoritics & Planetary Science, 44:559-572. [7] Dodd, 1978. Earth and Planetary Science Letters, 40:71-82[8] Alexander and Grossman, 2005. Meteoritics & Planetary Science, 40:541-556.[9] Fedkin and Grossman, L., 2013. Geochimica et Cosmochimica Acta, 112:226-250.[10] Taylor et al., 2012. Meteoritics and Planetary Science, 47:550-564.