Martin Suttle 1,2 , Matthew Genge 1,2 & Sara Russell 2 , +447748716692, [email protected]. 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 0.0 0.5 1.0 1.5 2.0 2.5 3.0 0 10 20 MnO wt% FeO wt% 0.0 0.5 1.0 1.5 2.0 2.5 3.0 0 5 10 15 20 Cr 2 O 5 wt% FeO wt% 7. Conclusions I. Intact chondrules are exceedingly rare among micrometeorites (<1%) [10] . This study reports the first occurrence of a microchondrule within a micrometeorite. II. A diverse mix of phases including a high- temperature refractory CAI, primitive LICE silicates, low-temperature aqueous alteration products (phyllosilicates and Fe-sulphides) and a microchondrule droplet formed in an impact plume are present within this micrometeorite. The complex geological history of the host asteroid, from accretion to parent body evolution, is therefore, recorded. 1. Introduction 2. Overview 4. Anhydrous silicates 5. Element distribution 6. Microchondrule 3. IR Spectroscopy 8 9 10 11 12 13 Reflectance (arbitary units) Wavelength (μm) The Earth receives a continuous flux of cosmic dust derived from young disrupted asteroids and sublimating short period comets [1,2] . Occasionally unique micrometeorites with distinct petrographies are reported [3] . These samples expand the inventory of asteroid parent bodies and provide further clues to the formation and evolution of the solar system. In this study we report the discovery of an unusual dehydroxylated fine-grained micrometeorite containing a devitrified microchondrule droplet. This spherule 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 a heterogeneous mix of anhydrous silicates suspended in a fine-grained, porous groundmass. The presence of randomly orientated dehydration cracks and rounded submicron vesicles indicates that thermal decomposition of low- temperature phases occurred during atmospheric entry. Dispersed micron- scale Mn-bearing chromite spinels (MnCr 2 O 4 ) are also abundant within the matrix. A single spherical object (<10μm diameter) composed of devitrified glass is located near the micrometeorite’s perimeter. A global mid-IR spectrum of CP94-050-182 reveals an olivine signature, indicating that the particle’s matrix is primarily composed of (nano-)crystalline anhydrous silicates. However, dehydration cracks require the former presence of hydrated phyllosilicates prior to atmospheric entry. This particle has, therefore, experienced high-temperatures (~700-1200°C) recrystallization during atmospheric entry [4] . Element mapping pin-points distinct phases within a complex micrometeorite’s matrix. Here, the condensation of Cr-rich silicates (olivine & pyroxenes, see no.4) is expected to suppress the simultaneous growth of Cr- bearing spinels [6] , which are also found within the groundmass. The refractory phases in this micrometeorite are, therefore, incompatible with a simplistic condensation scenario and, instead requires accretion of materials from (at least) 2 geochemically distinct reservoirs or a complex cooling history. Additionally, a ghost CAI is present, highlighted by the presence of hotspots in the Ca, Al and Ti maps. These phases are also rare among micrometeorites. The small spherule inclusion is interpreted as a glassy microchondrule on the basis of the highly spherical morphology and a chemical composition similar to droplet microchondrules in LL3.4 chondrite Manych [7] . The high abundance of volatiles requires high dust densities in order to retain volatile components [8,9] , high peak temperatures to destroy crystal nuclei and quench cooling to prevent crystallization. These properties, combined with the small particle size may indicate formation in an impact event. Finally, devitrification of the host glass most likely occurred during atmospheric entry heating. Anhydrous silicates remain unaffected by entry heating, and preserve primitive compositions, similar to the low-Fe-Cr-enriched (LICE) and low-Fe-Mn-enriched (LIME) silicates found in cometary IDPs, Stardust mission samples and some carbonaceous chondrites [5] . These anhydrous silicates form by high-temperature condensation (~1200K) under reducing conditions from a gas of solar composition and at high dust densitites. 0.1 1.0 10.0 Al Ca Si Mg Fe Cr Mn K Na Cl Zn S O Abundance (Normalised to CI values) [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.