¹Gary R. Huss (>10)
Meteoritics & Planetary Science (in Press) Open Access Link to Article [DOI: 10.1111/maps.70231]
¹Hawai‘i Institute of Geophysics and Planetology, School of Ocean and Earth Science and Technology, University of Hawai‘i at Mānoa, Honolulu, Hawaii, USA

Published by arrangement with John Wiley & Sons

We report on the petrography and oxygen isotopic compositions from in situ analyses of carbonates (Ca-carbonate, dolomite, and Fe-rich magnesite), magnetite, and apatite in particles from asteroid Bennu. Using oxygen isotope thermometry of Ca-carbonate, dolomite, and magnetite, we estimated crystallization temperatures and the oxygen isotopic compositions of the aqueous fluids from which they precipitated. The Ca-carbonate–magnetite pairs formed at lower temperatures (−5 to 20°C) than the dolomite–magnetite pairs (5–89°C); the uncertainties are ±10–16°C. The inferred fluid compositions for Ca-carbonate–magnetite pairs are δ¹⁸O = 0.6‰–9‰ and Δ¹⁷O = 1.8‰–2.7‰, whereas for dolomite–magnetite pairs, they are δ¹⁸O = −2.2‰ to 7.6‰ and Δ¹⁷O = −0.1‰ to 0.9‰. On a three-isotope oxygen diagram (δ¹⁷O vs. δ¹⁸O), 11 out of 13 of our inferred aqueous fluid compositions plot along a single line with a slope of ~0.8, potentially indicating precipitation from a single isotopically evolving fluid. Two dolomite–magnetite pairs appear to be outliers and may have formed from fluids with different oxygen isotopic compositions. The oxygen isotope data presented herein overlap with those reported from samples of asteroid Ryugu, supporting a possible close genetic relationship between these asteroids.

¹Matteo Del Rio, ^2,3Luigi Folco, ^2,3Matteo Masotta, ¹Ana Černok, ^2,3Enrico Mugnaioli
Meteoritics & Planetary Science (in Press)
Open Access Link to Article [DOI: 10.1111/maps.70238]
¹Dipartimento di Matematica, Informatica e Geoscienze, Università di Trieste, Trieste, Italy
²Dipartimento di Scienze della Terra, Università di Pisa, Pisa, Italy
³Center for the Instrument Sharing of the University of Pisa (CISUP), Pisa, Italy

Published by arrangement with John Wiley & Sons

Microkrystites are glassy impact spherules. They contain primary crystallites carrying information on the physical and chemical conditions prevailing during vaporization, melting, and ejection in large impact cratering events. We report on the geochemistry and mineralogy down to nanometer scale of the first microkrystite (ODP1144A,14) from the Australasian tektite/microtektite strewn field. It is a dark-gray, opaque, 160 × 210 μm prolate spheroid, recovered from the microtektite layer in the Ocean Drilling Program Hole 1144A in the South China Sea. The microkrystite shows a micro-glomeroporphyritic texture consisting of clusters of skeletal ferropseudobrookite (Fe²⁺_0.7,Fe³⁺_0.3)_Σ=1(Ti_1.7Fe³⁺_0.3)_Σ=2O₅ crystals (~70 area%) set in a silicate glassy matrix. The latter features nanoscale Fe-Ti liquid-immiscibility textures and Si-rich inclusions composed of quartz, lechatelierite, and shock-produced coesite. Coesite confirms an impact origin of the microkrystite. Major and trace element compositions of the glassy matrix and of the spherules accreted onto the particle surface match those of Australasian microtektites, firmly linking ODP1144A,14 to the Australasian strewn field. Vesicles and partially digested lechatelierite inclusions indicate formation as an impact-melt droplet rather than a vapor condensate. We suggest that the non-silicate part of the ODP1144A,14 microkrystite derived from an immiscible Fe–Ti-rich melt batch (about the size of a droplet) produced by impact melting at T > 1400°C of target Fe-Ti oxides (commonly found in the shocked target ejecta fragments associated with microtektites in the same stratigraphic horizon) that could not fully homogenize with the bulk silicate microtektite precursor melt under the transient disequilibrium conditions that are characteristic of impact melting and fragmentation during high-velocity ejection. We suggest that the involvement of target Fe-Ti oxides during impact melting may have contributed to the Cr-rich, Ni-poor terrestrial mafic component observed in some Australasian tektites and microtektites. This provides further support to earlier assessment of the dual origin, dominantly chondritic and lesser terrestrial, of the mafic component in Australasian tektites and microtektites.