Mineralogical alteration of a type A CAI from Allende CV3 chondrite: Formation of secondary dmisteinbergite and its phase transition to anorthite
Mineralogical alteration of a type A CAI from Allende CV3 chondrite: Formation of secondary dmisteinbergite and its phase transition to anorthite
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阿连德 CV3 球粒陨石中 A 型 CAI 的矿物学蚀变：次生辉钙石的形成及其向钙长石的相变
DOI：
10.1111/maps.13961
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发表时间：
2023
期刊：
影响因子：
--
通讯作者：
Zolensky M. E.
中科院分区：
文献类型：
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作者：
Enokido;Y.;Nakamura;T.;Matsumoto M.;Miyake;A.;Shibuya;T.;Park;C.;Zolensky M. E.
在阿连德CV 3球粒陨石中的致密A型富钙铝包体（CAI）中发现了六方晶型的CaAl 2Si 2 O 8。扫描电镜和透射电镜观察表明，钙铝榴石始终与钙铝榴石接触，钙铝榴石与黄长石接触。此外，钙长石与钙长石之间存在晶体学关系。根据结构和晶体学证据，以下矿物学蚀变过程被认为发生在CAI。(1)黄长石被钙铝榴石取代。钙铝榴石中高密度的囊泡表明，水钙铝榴石可能是主要的蚀变相和脱水后期变质。(2)Dmisteinbertal由（水）钙铝榴石通过与富硅流体反应形成。(3)纳米级的矿物是在矿物中形成的。(4)Dmisteinbertite转化为钙长石。(5)钙长石和钙长石均蚀变为霞石。(6)Hydrogrossular脱水至grossular。CAI中的钙铝榴石、钙长石、霞石等矿物似乎是在阿连德母体中发生交代作用的过程中形成的。根据过去的实验研究，除了氢钙铝榴石脱水之外，这些反应可能在中等温度（200-250°C）下在高pH值流体（pH 13-14）中发生。流体组成的阶段性变化似乎发生在反应（2）、（4）和（5）之前，因为这些反应在下一个反应开始之前没有完成。反应（5）和（6）的发生需要更高的温度。我们对CAI的观察表明，随着阿连德母小行星温度的升高，它经历了多次交代作用。
Dmisteinbergite, a hexagonal form of CaAl2Si2O8, was found in a compact type A Ca‐Al‐rich inclusion (CAI) in the Allende CV3 chondrite. Scanning and transmission electron microscopic observations show that dmisteinbergite was always in contact with grossular and grossular was in contact with melilite. In addition, there is a crystallographic relationship between dmisteinbergite and anorthite. Based on the textural and crystallographic evidence, the following mineralogical alteration processes are proposed to have occurred in the CAI. (1) Melilite was replaced by grossular. High densities of vesicles in the grossular indicate that hydrogrossular might have been the primary alteration phase and dehydrated by later metamorphism. (2) Dmisteinbergite formed from (hydro)grossular through a reaction with Si‐rich fluid. (3) Nano‐sized minerals are formed within dmisteinbergite. (4) Dmisteinbergite was transformed to anorthite. (5) Both anorthite and dmisteinbergite were altered to nepheline. (6) Hydrogrossular was dehydrated to grossular. (Hydro)grossular, dmisteinbergite, anorthite, and nepheline in the CAI seem to have formed in the course of metasomatism that occurred in the Allende parent body. Except for the hydrogrossular dehydration, these reactions could have occurred at moderate temperature (200–250°C) in high pH fluids (pH 13–14) according to past experimental studies. Episodic changes in fluid composition seem to have occurred before reactions (2), (4), and (5), because these reactions were not completed before the next reaction started. Higher temperature is required for reactions (5) and (6) to occur. Our observation of the CAI suggests that it experienced multiple episodes of metasomatism as temperatures were rising in the Allende parent asteroid.