[S-IT19] 地球深部科学

[SIT19-P05] ブリッジマナイトの高温非晶質化速度

*西 真之¹、金子 晃大²、扇谷 碩²、出倉 春彦²、柿澤 翔³、河口 彰吾³、小林 慎太郎³、境家 達弘¹、近藤 忠¹ (1.大阪大学理学研究科宇宙地球科学専攻、2.愛媛大学地球深部ダイナミクス研究センター、3.高輝度光科学研究センター )

キーワード：ブリッジマナイト、隕石、非晶質化

High-pressure minerals found in shocked meteorites provide pressure–temperature–time information that can be used to constrain the impact histories of their parent bodies. The occurrence of bridgmanite in shocked meteorites suggests that the peak pressure during the impact process reaches at least ~23 GPa at which bridgmanite is thermodynamically stable. Although most high-pressure phases (e.g., coesite, stishovite, pyrope, ringwoodite, and akimotoite) are kinetically stable below ~800 K at ambient pressure, bridgmanite is amorphized at temperatures above ~400 K. Considering that post-shock temperatures above 400 K are maintained for a long duration in the L6 chondrite, the presence of natural bridgmanite in our environments is unexpected. Despite its importance in the shock histories of meteorites and mantle mineralogy, it remains unclear how bridgmanite is preserved.
To gain insights into the enigmatic existence of bridgmanite in shocked meteorite, we carried out time-resolved high-temperature XRD measurements on synthetic bridgmanite with a composition of MgSiO₃ at the synchrotron radiation facilities of SPring-8 (BL02B2). Our results show that the significant volume expansion due to the amorphization induces static stress that can reach up to ~0.5 GPa, which prevents the progress of the amorphization. This time-insensitive amorphization kinetics may have enabled the preservation of bridgmanite in the shocked meteorite that fell on Earth. Also, because natural bridgmanite grains can be distinguished from amorphous ones in the backscattered electron image, the amorphous/crystal fraction can be used to estimate the remnant post-shock temperature. Accordingly, if 10% of bridgmanite remains crystalline, the post-shock temperature conditions can be estimated to be ~600 K, irrespective of the time duration.