动态高压下斜长石的熔融和玻璃化研究

Melting and vitrification of plagioclase under dynamic high pressures.

斜长石在冲击波产生的动态高温高压作用下较易熔融和淬火为非晶态物质,其矿物学名为熔长石。在随州陨石冲击变质特征的研究中,发现橄榄石、辉石等矿物除出现微裂隙外均保存完好,而多数斜长石颗粒则已熔融和玻璃化。可区分出两种形态的熔长石,一种是保持了原有矿物颗粒外形的‘继形熔长石’,另一种则以熔池状和脉状体形式产出的‘改形熔长石’。两种熔长石的光学和物理性质相似,它们的化学成分与晶质斜长石也基本相同,说明斜长石在高压下熔融时没有与周围物质发生明显的组分交换、在吉林陨石的人工冲击实验产物中,斜长石是最先熔融的硅酸盐矿物,也多以填隙的他形颗粒或脉状体和熔池的形式产出。经与其它陨石的静态高压淬火实验研究结果的比较,推定继形和改形熔长石的形成条件为≈20GPa和≤1100℃。本研究结果对了解地幔橄榄岩和俯冲的大洋板片部分熔融作用,以及地球火山岩玻璃和超基性岩中铬尖晶石的玻璃相岩浆包襄体物理化学性质有一定参考价值。

Under the shock-induced high pressures and temperatures, olivine and pyroxene in the Suizhou 16 ehondrite kept intact, but plagioclase experienced melting and quenching, and transformed into maskelynite, a melted dense plagioclase glass. Two occurrence types of maskelynite can be observed in this meteorite. One is the thetomorphic maskelynite, an in-situ melted glass with reserved grain morphology of the host plagioclase. Another is the allomorphic maskelynite, that fills fractures and cracks in other nearby silicate minerals, forming melt veinlets or melt pockets. Both two glasses show similar optical and physical properties. EPMA results show that there is no obvious diffusion of chemical elements between the melt glass and the surrounding minerals or trapped mineral fragments. The shock-loading experiments on the Jilin meteorite show that plagioclase is the first molten mineral among silicate minerals in meteorite, and the formed maskelynite also occurs in the form of melt veinlets or melt pockets. According to the results of high- pressure melting and quenching experiments on some other meteorites, it has been estimated that the formation condition for both thetomorphic and allomorphic maskelynites in Suizhou is - 20GPa and ≤ 1100°C. The results of this study are of some significance for understanding of partial melting of the mantle＇s peridotite and the subducted oceanic slab, as well as that of formation of the volcanic glasses and the characteristics of glassy melt inclusions in Cr-spinels in the ultramafic rocks.