¹R. J. Windmill, ¹I. A. Franchi, ¹X. Zhao, ¹R. C. Greenwood, ¹M. Anand
Meteoritics & Planetary Science (in Press) Open Source Link to Article [DOI: 10.1111/maps.70210]
¹Planetary and Space Sciences, School of Physical Sciences, The Open University, Milton Keynes, UK
Published by arrangement with John Wiley & Sons

The light element distribution in planetary cores and the processes driving core evolution in rocky planets are poorly understood. Magmatic iron meteorites are samples from the cores of ancient embryonic planetesimals and therefore provide a window into the processes governing core evolution. We performed high precision oxygen isotope analyses on chromite from IIIAB iron meteorites to investigate the oxygen isotopic evolution across a protoplanetary core using laser-assisted fluorination. We identify three unexpected and hitherto unreported discrete isotopic subgroups within the IIIAB chemical group and discuss possible causes for their existence. The most likely explanation is that they may be sampling multiple parent bodies, either completely unrelated or mixed during an impact. This would have significant implications for the use of the chemical classification scheme for iron meteorites as well as models for IIIAB core evolution. Second, that they may be evidence that oxygen mobility across the core was controlled by diffusion. If this is the case, they may represent homogenized melt pools in a wider core context, recording oxygen diffusion into a planetary core, which could help explain the density deficit observed in Earth’s core. Third, we discuss whether core rain out through a heterogeneous IIIAB mantle and inefficient mixing in the core could explain the isotopic results. Finally, we compare these IIIAB oxygen isotopic signatures to published data for main group pallasite minerals and conclude that the meteorite groups cannot be from a common parent body, answering a long-standing question in meteoritical science.

I

¹Yasuaki Tsuruoka, ^1,5Hideto Yoshida, ^2,6Yuki Inoue, ²Daiki Yamamoto, ³Hiroyuki Kagi, ⁴Akira Miyake, ¹Shogo Tachibana
Meteoritics & Planetary Science (in Press) Open Source Link to Article [DOI: 10.1111/maps.70213]

¹Department of Earth and Planetary Science, The University of Tokyo, Bunkyo-ku, Tokyo, Japan
²Department of Earth and Planetary Sciences, Kyushu University, Nishi-ku, Fukuoka, Japan
³Geochemical Research Center, The University of Tokyo, Bunkyo-ku, Tokyo, Japan
⁴Department of Geology and Mineralogy, Kyoto University, Sakyo-ku, Kyoto, Japan
⁵The Kiso Observatory, Institute of Astronomy, The University of Tokyo, Kiso-machi, Nagano, Japan
⁶Atmosphere and Ocean Research Institute, The University of Tokyo, Kashiwa-shi, Chiba, Japan
Published by arrangement with John Wiley & Sons

Dmisteinbergite is a metastable polymorph of CaAl₂Si₂O₈, which has been found in calcium−aluminum-rich inclusions (CAIs) in chondrites, although anorthite is the predominant CaAl₂Si₂O₈ phase in natural type B CAIs. The occurrence/absence of igneous dmisteinbergite would put tighter constraints on the thermal history of CAIs. However, little attention has been paid for crystallization of dmisteinbergite in CAI melts even though its kinetically controlled crystallization (i.e., crystallization from a supercooled melt) has been proposed. In this study, we conducted dynamic crystallization experiments on CAI analogs in the presence of low-pressure hydrogen gas (1–10 Pa) to simulate the formation of igneous CAIs and to understand crystallization behavior of CaAl₂Si₂O₈ phases. Dmisteinbergite was found in samples cooled at rates of 1–20 °C h⁻¹, while anorthite was found in samples cooled at 5 °C h⁻¹, suggesting that the slow cooling (likely slower than on the order of 1 °C h⁻¹) might promote the formation of anorthite. Because the slow cooling rates during the final stage of crystallization of CAIs likely reflect conditions approaching the background temperature of CAI-forming regions, CaAl₂Si₂O₈ phases in type B CAIs could constrain the thermal history and formation environment of those regions.

Dmisteinbergite is a metastable polymorph of CaAl₂Si₂O₈, which has been found in calcium−aluminum-rich inclusions (CAIs) in chondrites, although anorthite is the predominant CaAl₂Si₂O₈ phase in natural type B CAIs. The occurrence/absence of igneous dmisteinbergite would put tighter constraints on the thermal history of CAIs. However, little attention has been paid for crystallization of dmisteinbergite in CAI melts even though its kinetically controlled crystallization (i.e., crystallization from a supercooled melt) has been proposed. In this study, we conducted dynamic crystallization experiments on CAI analogs in the presence of low-pressure hydrogen gas (1–10 Pa) to simulate the formation of igneous CAIs and to understand crystallization behavior of CaAl₂Si₂O₈ phases. Dmisteinbergite was found in samples cooled at rates of 1–20 °C h⁻¹, while anorthite was found in samples cooled at 5 °C h⁻¹, suggesting that the slow cooling (likely slower than on the order of 1 °C h⁻¹) might promote the formation of anorthite. Because the slow cooling rates during the final stage of crystallization of CAIs likely reflect conditions approaching the background temperature of CAI-forming regions, CaAl₂Si₂O₈ phases in type B CAIs could constrain the thermal history and formation environment of those regions.