超临界地质流体与翡翠成因

Supercritical Geofluids and Genesis of Jadeite Jade

  • 摘要: 超临界地质流体是在高温高压下硅酸盐矿物(岩石)-H2O体系中形成的化学组成介于富水流体和岩浆熔体之间的中间状态流体。它们具有很强的溶解能力、迁移能力和反应能力,是地球内部物质循环的重要载体和介质。传统的初始翡翠(硬玉岩)的区域变质成因、岩浆成因和交代成因学说,都有相应的实验反应、岩相学或岩理学依据及矿物形成顺序、矿物组合及其空间分布等方面存在的问题。流体沉淀(Fluid precipitate)和热液交代(Metasomatic replacement)成因理论阐述了翡翠中矿物的两种形成方式。在硬玉、绿辉石等矿物颗粒及其集合体之中,普遍观察到的流体熔融包裹体等现象, 揭示了早期的翡翠(硬玉岩)是从流体与熔融体混溶的系统中结晶形成的。在地球科学领域超临界地质流体研究进展的基础上,本文结合对翡翠研究的进展,提出了翡翠超临界地质流体成因的认识:俯冲板片脱水并在合适的温度压力条件下形成的超临界地质流体,携带其所溶解的成矿物质,因温度压力等条件的变化,在地幔楔裂隙或断裂中经结晶充填及之后叠加的交代作用形成了早期的翡翠。提出这一成因认识,有典型的流体熔融包裹体、强烈的溶蚀残余、早期的充填结构与中晚期的交代结构、高场强元素富集、丰富乃至复杂的矿物(或固相物质)组合、含有指示矿物的高压-超高压脉体等系列的证据链。早期翡翠形成之前的岩浆作用产物铬铁矿被溶蚀或较晚阶段被交代为翡翠的绿色提供了Cr3+的来源;早期翡翠形成之后,还经历了超临界地质流体分溶后的熔体作用和流体作用(包括结晶作用和交代作用)、变形变质作用和外生作用等多期次地质作用。翡翠的成因理论体系的建立,有必要在重视超临界地质流体作用阶段的基础上,重新进行深入研究和梳理。

     

    Abstract: Supercritical geofluids are intermediate fluids formed in silicate mineral (rock)-H2O systems under high temperature and high pressure, whose chemical compositions are between aqueous fluids and magmatic melts. They have strong dissolution, migration and reaction abilities, and are important carriers and mediums for material circulation in the earth. The traditional theories of regional metamorphic origin, magmatic origin and metasomatic origin of initial jadeite jade (jadeitite) have their respective theoretical bases such as experimental reaction, petrography and petrogenesis, but also have some problems in mineral formation sequence, mineral assemblages and their spatial distribution. The original theory of fluid precipitate and metasomatic replacement has proposed two modes in which constituent minerals of jadeite jade are formed. Among mineral particles of jadeite and omphacite and their aggregates, frequently observed phenomena such as fluid-melt inclusions reveal that the initial jadeite jades (jadeitites) were crystallized from fluid and melt miscible systems. On the basis of the research progress on supercritical geofluids in the field of earth science, combined with the research progress on jadeite jade, this article introduces a formation model of jadeite jade from a supercritical geofluid system: The supercritical geofluid formed in the subduction plate dehydrated and under appropriate temperature and pressure conditions carried the dissolved materials; due to the changes of temperature and pressure and other conditions, the early jadeite jade was formed in the mantle wedge fissure or fracture through crystallization filling and then superimposed metasomatism. The formation model is proposed based on a series of evidence chains such as typical fluid-melt inclusions, strong dissolution remnants, early filling textures and mid-late metasomatic textures, enrichment of high field strength elements, rich and even complex mineral (or solid substance) assemblages, and high-pressure to ultra-high-pressure veins containing indicator minerals. Chromites, the product of magmatism before the formation of early jadeite jade, were dissolved or replaced (during a later stage), provided the source of Cr3+ for the green color of jadeite jade. After the formation in the early period, jadeite jades also experienced multi-period geological processes, such as melt action and fluid action (including crystallization and metasomatism) after the separation of supercritical geofluids, deformation, metamorphism and exogenetic action. In order to establish the theoretical system of jadeite jade formation, it is necessary to conduct further research and review on the basis of paying attention to the action period of supercritical geofluids.

     

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