金丸 礼

Abstract Silica polymorphs in meteorites provide critical constraints on crystallization processes associated with thermal activity in the early solar system. A detailed investigation of silica polymorphs in eucrites (the largest group of achondrites) using cathodoluminescence imaging and laser‐Raman spectroscopy revealed significant variations in the relative abundance of silica polymorphs. Based on these variations, the eucrites were divided into four “Si‐groups” according to their dominant silica phase: Si‐0 (cristobalite‐dominant eucrites), Si‐I (quartz‐dominant eucrites), Si‐II (quartz and tridymite‐dominant eucrites), and Si‐III (tridymite‐dominant eucrites). In studied eucrites, tridymite and cristobalite form lathy euhedral shapes, while quartz is anhedral, coexistent with opaques and phosphates, suggesting that silica polymorphs were crystallized from different stages and formation processes. We propose a new model that explains the formation pathways of silica minerals in eucrites and accounts for the distinct formation histories represented by each Si‐group: tridymite crystallizes from alkali‐rich immiscible melts (starting at ≥ ~1060°C), cristobalite crystallizes from quenched melts (~1060°C), and quartz crystallizes from extremely differentiated melts and/or by solid‐state transformation from tridymite and cristobalite through interactions with sulfur‐rich vapor below ~1025°C. This model explains the occurrences of silica polymorphs in eucrites without requiring secondary heating or shock processes.

Abstract Silica polymorphs occur under various pressures and temperature conditions, and their characteristics can be used to better understand the complex metamorphic history of planetary materials. Here, we conducted isothermal heating experiments of silica polymorphs in basaltic eucrites to assess their formation and stability. We revealed that each silica polymorph exhibits different metamorphic responses: (1) Quartz recrystallizes into cristobalite when heated at ≥ 1040 °C. (2) Monoclinic (MC) tridymite recrystallizes into no other polymorphs when heated at ≤ 1070 °C. (3) Silica glass recrystallizes into quartz when heated at 900–1010 °C, and recrystallize into cristobalite when heated at ≥ 1040 °C. These results suggest that MC tridymite in eucrites does not recrystallize into other polymorphs during the reheating events, nor does it recrystallize from other silica phases below the solidus temperature of eucrite (~ 1060 °C). Additionally, we found that pseudo-orthorhombic (PO) tridymite crystallizes from quenched melts in the samples heated at ≥ 1070 °C. Previously, cristobalite has been considered as the initial silica phase, which crystallizes from eucritic magma. Our findings suggest that the first crystallizing silica minerals may not always be cristobalite. These require a reconsideration of the formation process of silica minerals in eucrites.

We performed a cathodoluminescence (CL) study of Ca-rich plagioclase (An_85-86Ab₁₄Or_<1) in Stillwater gabbronorite experimentally shocked at 20.1, 29.8, and ∼41 GPa, for characterization of the shock effects. Chroma CL image of unshocked plagioclase showed the homogeneous red CL emission. In contrast, experimentally shocked plagioclase showed the heterogeneous CL emission colors in red and blue. The Raman spectra analysis identified that the red and blue portions correspond to plagioclase and maskelynite, respectively. In our observation, plagioclase experimentally shocked at 20 GPa was partially converted into maskelynite. At 30 GPa, most of plagioclase were converted into maskelynite. At 40 GPa, plagioclase was fully converted into maskelynite. Our observations of Ca-rich plagioclase indicated that the maskelynization starts at a slightly lower pressure and completes at a higher pressure than those in the previous studies (∼24 GPa and ∼28 GPa, respectively). These pressure differences may be due to the high sensitivity of CL, which allows for the detection of small (a few µm in size) and rare phases that may have been overlooked in the traditional methods. The CL spectra of plagioclase showed a continuous change with increasing shock pressure. Hence, the CL imaging method using plagioclase and maskelynite is found to be very effective to estimate precisely shock pressure. In particular, there was a marked decrease in the CL intensity of Mn²⁺ and Fe³⁺ centers. Furthermore, the shock-induced center around the UV region was observed in experimentally shocked plagioclase and maskelynite. These CL features reflect the destruction of the framework structure to varying extents depending upon shock pressure. Combined with the FTIR analysis in the present study, the transition of plagioclase to maskelynite was clearly illustrated in spectra. The reflectivity decreased continuously with increasing shock pressures during maskelynization. Additionally, the absorption at ∼8.6 µm observed in plagioclase was absent in maskelynite. This feature can be used as a diagnostic feature to characterize plagioclase and maskelynite by FTIR. The combination of detailed petrology using CL and FTIR spectra provides valuable insights into the shock scale for achondrites and planetary materials rich in shock-experienced plagioclase.