^1,2Ritesh Kumar Mishra,³Kuljeet Kaur Marhas,⁴Justin Ibrahim Simon,⁵Yves Marrocchi,⁵Johan Villeneuve
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.70062]
¹Independent Researcher, Dhawalpur, India
²Veer Kunwar Singh University, Ara, India
³Planetary Sciences Division, Physical Research Laboratory, Ahmedabad, India
⁴Astromaterials Research and Exploration Science Division, NASA-Johnson Space Center, Houston, Texas, USA
⁵Centre de Recherches Pétrographiques et Géochimiques, Nancy, France
Published by arrangement with John Wiley & Sons

Ordinary, enstatite, and Rumuruti type have the lowest abundance of refractory inclusions amongst chondritic meteorites. Calcium-aluminum-rich inclusions (CAIs) within these are hallmarked by a relatively small average diameter of ~45 μm (size range 4–382 μm). One CAI, one amoeboid olivine aggregate (AOA), one spinel-bearing chondrule, and two aluminum-rich chondrules from Semarkona (LL3.00) along with one CAI each from Allan Hills (ALHA) 81251 (LL3.2) and Chainpur (LL3.4) were identified following an extensive search. These objects were studied for their petrography, mineral chemistry, relative (26Al) chronology, and three oxygen isotopic compositions. The initial 26Al/27Al ratio of (4.96 ± 0.14) × 10−5 (2σ) in a type A CAI in Chainpur, the largest size (1500 × 1200 μm) found so far in the noncarbonaceous (ordinary) chondrites, forming in an 16O-rich early solar system reservoir (Δ17O = −24‰) is consistent with previous studies. The Chainpur CAI 1 has a Wark–Lovering rim, the first reported case within the noncarbonaceous chondrites. The hibonite–pyroxene spherule in ALHA81251 (CAI 1) is the first reported case of a hibonite–pyroxene spherule in the ordinary chondrites of these rare objects (~12 known so far) within meteorites. The hibonite–pyroxene spherule in ALHA81251 has a low abundance of 26Al/27Al ratio of (1.2 ± 0.6) × 10−5 with Δ17O of ~ −14.5‰ ± 2.0‰. An olivine-phyric Al-rich chondrule in Semarkona (Ch 54) formed at ~0.9 Ma with Δ17O of ~0‰, while Semarkona (Ch 44) formed in a relatively 16O-rich reservoir with Δ17O of ~ −2.0‰. The spinel-bearing chondrule in Semarkona (Ch 205) shows no resolved excess in Δ26Mg and has a planetary-like oxygen isotopic composition. Oxygen isotope composition and 26Al-26Mg relative chronology of these objects confirm their origin and evolution under cosmochemical conditions similar to their “typical” carbonaceous kindred and extend the knowledge of the cosmochemical environment in the early solar system.

¹Takumi Miura,²Hidenori Terasaki,^2,3Hyu Takaki,²Kotaro Kobayashi,⁴Geoffrey David Bromiley,²Takashi Yoshino
Meteoritics & Planetary Science (in Press) Open Access Link to Article [https://doi.org/10.1111/maps.70068]
¹Department of Earth and Space Science, Osaka University, Osaka, Japan
²Department of Earth Sciences, Okayama University, Okayama, Japan
³Institute for Planetary Materials, Okayama University, Tottori, Japan
⁴School of Geosciences, The University of Edinburgh, Edinburgh, UK
Published by Arrangement with John Wiley & Sons

During precursor stages of planet formation, many planetesimals and planetary embryos are considered to have differentiated, forming an iron-alloy core and silicate mantle. Percolation of liquid iron-alloy in solid silicates is one of the major possible differentiation processes in these small bodies. Based on the dihedral angles between Fe-S melts and olivine, a criterion for determining whether melt can percolate through a solid, it has been reported that Fe-S melt can percolate through olivine matrices below 3 GPa in an oxidized environment. However, the dihedral angle between Fe-S melts and orthopyroxene (opx), the second most abundant mineral in the mantles of small bodies, has not yet been determined. In this study, high-pressure and high-temperature experiments were conducted under the conditions of planetesimal and planetary embryo interiors, 0.5–5.0 GPa, to determine the effect of pressure on the dihedral angle between Fe-S melts and opx. Dihedral angles tend to increase with pressure, although the pressure dependence is markedly reduced above 4 GPa. The dihedral angle is below the percolation threshold of 60° at pressures below 1.0–1.5 GPa, indicating that percolative core formation is possible in opx-rich interiors of bodies where internal pressures are lower than 1.0–1.5 GPa. The oxygen content of Fe-S melt decreases with increasing pressure. High oxygen contents in Fe-S melt reduce interfacial tension between Fe-S melt and opx, resulting in reduced dihedral angles at low pressure. Combined with previous results for dihedral angle variation of the olivine/Fe-S system, percolative core formation possibly occurs throughout bodies up to a radius of 1340 km for an olivine-dominated mantle, and up to 770 km for an opx-dominated mantle, in the case of S-rich cores segregating under relatively oxidizing conditions. For mantles of small bodies in which abundant olivine and opx coexist, the mineral with the largest volume fraction and/or smallest grain size will allow formation of interconnected mineral channels, and, therefore, the wetting property of this mineral determines the wettability of the melt, that is, controls core formation.