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[PPS07-14] Shock features of Cr-Ti spinel in Martian meteorites
Keywords:Martian meteorite, chromite, shock metamorphism
Martian meteorites experience varying degrees of shock metamorphism during their launching from Mars. In this study, we focus on the shock metamorphic microstructure of Cr-Ti spinel to constrain the conditions for shock metamorphism of Martian meteorites. We have previously reported the presence of lamellae composed of chenmingite in the Asuka 12325 (A 12325) meteorite, even at a distance from the shock melt vein (SMV), and the formation conditions may depend on the host chemical composition and shock pressure. In this study, we attempted to further investigate the formation conditions of the lamellae by comparing and observing Cr-Ti spinels in Martian meteorites with several degrees of shock metamorphism.
Thin sections of five Martian meteorites A 12325, Northwest Africa (NWA) 1110, NWA 12241, NWA 13366 and Amgala 001 were prepared. Each sample was observed by polarising microscopy and scanning electron microscopy (SEM JEOL JSM-7001F), followed by micro-area sectioning by focused ion beam (FIB Helios NanoLab G3 CX) and TEM-STEM observation by transmission electron microscopy (TEM JEM-2100F) at Kyoto University.
The shock metamorphic microstructure of NWA 13366 indicates the effects of strong residual heat, such as the absence of high-pressure minerals in the SMV and the overall browning of olivine. In four meteorites other than NWA 13366, high-pressure minerals such as ringwoodite and garnet were observed around the SMV. Crystalline plagioclase remained in A 12325 and NWA 12241, while all other meteorites were fully maskelynitized. From these results, the degree of shock metamorphism [e.g., Hu and Sharp 2022] was estimated to be strongest for NWA 13366 (~50 GPa), moderate for NWA 1110 and Amgala 001 (~30 GPa) and weaker for A 12325 and NWA 12241 (~17-22 GPa).
In NWA 12241, a small amount of lamellar microstructure was observed in the Ti-rich spinel, similar to A 12325. The selected area electron diffraction (SAED) patterns of the lamellae were indexed by chenmingite. The orientation relationships between chenmingite and chromite were observed; [-111]chr//[-101]chen and [0-11]chr//[010]chen. Some SAED patterns in the lamellae of NWA 12241 suggest a doubly periodic superstructure in the b-axis direction of chenmingite. In NWA 1110 and Amgala 001, lamellae were observed even in Ti-poor and Cr-rich spinel cores. The chromite particles of Amgala 001 also had Fe-rich rims, in which fine Ti-rich exsolution lamellae were present. STEM-EDS analysis showed the lamellae in the spinel core as chenmingite with no compositional differences from the host. On the other hand, the SAED pattern of the Fe-rich rims remained as a spinel structure. In NWA 13366, no lamellar microstructure was observed, regardless of the composition of the Cr-Ti spinel.
Our results suggest that chenmingite, a low-temperature, high-pressure phase of chromite, may be widespread in Martian meteorites. High temperature may not be necessary for phase transitions to chenmingite, possibly attained by slight deformation without diffusion. The required conditions for a phase transition may be ~17-22 GPa and <250 °C in Ti-rich regions and ~30 GPa and <500 °C in Cr-rich regions. It is also clear that the phase transition depends on the composition of Ti-Cr-Fe, as the phase transition is less likely to occur with increasing Fe components. On the other hand, no chenmingite lamellae were found in the meteorite with high residual heat, suggesting that the lamellae are easily back-transformed by heat.
The high-pressure phase of chromite may reflect shock metamorphic conditions closer to peak pressure due to weaker kinetic constraints on phase transitions like maskelynite. This feature could be useful for estimating the shock metamorphism of extraterrestrial materials with small sample volumes. The relationship between chemical composition and the temperature and pressure conditions for lamella formation should be clarified more quantitatively.
Thin sections of five Martian meteorites A 12325, Northwest Africa (NWA) 1110, NWA 12241, NWA 13366 and Amgala 001 were prepared. Each sample was observed by polarising microscopy and scanning electron microscopy (SEM JEOL JSM-7001F), followed by micro-area sectioning by focused ion beam (FIB Helios NanoLab G3 CX) and TEM-STEM observation by transmission electron microscopy (TEM JEM-2100F) at Kyoto University.
The shock metamorphic microstructure of NWA 13366 indicates the effects of strong residual heat, such as the absence of high-pressure minerals in the SMV and the overall browning of olivine. In four meteorites other than NWA 13366, high-pressure minerals such as ringwoodite and garnet were observed around the SMV. Crystalline plagioclase remained in A 12325 and NWA 12241, while all other meteorites were fully maskelynitized. From these results, the degree of shock metamorphism [e.g., Hu and Sharp 2022] was estimated to be strongest for NWA 13366 (~50 GPa), moderate for NWA 1110 and Amgala 001 (~30 GPa) and weaker for A 12325 and NWA 12241 (~17-22 GPa).
In NWA 12241, a small amount of lamellar microstructure was observed in the Ti-rich spinel, similar to A 12325. The selected area electron diffraction (SAED) patterns of the lamellae were indexed by chenmingite. The orientation relationships between chenmingite and chromite were observed; [-111]chr//[-101]chen and [0-11]chr//[010]chen. Some SAED patterns in the lamellae of NWA 12241 suggest a doubly periodic superstructure in the b-axis direction of chenmingite. In NWA 1110 and Amgala 001, lamellae were observed even in Ti-poor and Cr-rich spinel cores. The chromite particles of Amgala 001 also had Fe-rich rims, in which fine Ti-rich exsolution lamellae were present. STEM-EDS analysis showed the lamellae in the spinel core as chenmingite with no compositional differences from the host. On the other hand, the SAED pattern of the Fe-rich rims remained as a spinel structure. In NWA 13366, no lamellar microstructure was observed, regardless of the composition of the Cr-Ti spinel.
Our results suggest that chenmingite, a low-temperature, high-pressure phase of chromite, may be widespread in Martian meteorites. High temperature may not be necessary for phase transitions to chenmingite, possibly attained by slight deformation without diffusion. The required conditions for a phase transition may be ~17-22 GPa and <250 °C in Ti-rich regions and ~30 GPa and <500 °C in Cr-rich regions. It is also clear that the phase transition depends on the composition of Ti-Cr-Fe, as the phase transition is less likely to occur with increasing Fe components. On the other hand, no chenmingite lamellae were found in the meteorite with high residual heat, suggesting that the lamellae are easily back-transformed by heat.
The high-pressure phase of chromite may reflect shock metamorphic conditions closer to peak pressure due to weaker kinetic constraints on phase transitions like maskelynite. This feature could be useful for estimating the shock metamorphism of extraterrestrial materials with small sample volumes. The relationship between chemical composition and the temperature and pressure conditions for lamella formation should be clarified more quantitatively.