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.