火成岩中的超临界流体晶及其研究意义

流行火成岩理论中，岩浆被默认为自然熔体，因而火成岩中的矿物晶体都形成于熔体的结晶作用，可称为熔体晶。许多证据表明，火成岩中也可以含有从超临界流体析出的晶体，被称为超临界流体晶(文中简称为流体晶)。根据三个典型实例分析，流体晶既可以从超临界流体直接析出，类似于从热液析出晶体的过程；也可以由超临界流体浓缩产生的熔体结晶形成。不管是哪一种晶出方式，流体晶产生的前提都是岩浆达到流体过饱和态；而满足这一前提的基本条件则是透岩浆流体过程和岩浆快速上升。结合前人关于熔体流体平衡条件的研究进展，以及熔体黏度对挥发分含量和岩浆上升速度对熔体黏度的依赖，发现透岩浆流体过程与岩浆上升过程之间具有耦合关系，这种关系可以用来阐明岩浆系统行为非线性变化的原因。流体晶的研究具有重要意义，可用于：(1)提供一种研究岩浆系统流体条件的新途径；(2)揭示岩浆系统偏离理想态的程度；(3)为反演岩浆系统动力学过程提供新的约束；(4)为识别致矿侵入体和评估侵入体的找矿潜力提供新的矿物学标志；(5)理解火成岩理论中一些长期得不到解决的问题，如岩浆中挥发分溶解度问题、冻结岩浆的活化问题、岩浆成矿专属性问题。

Supercritical fluid crystals in igneous rocks and the implications

In the prevailing igneous theory, the term magma is acquiescently considered synonymous to natural melt. Consequently, all mineral crystals in an igneous rock are crystallized from melt and can be named as melt crystals. However, many observations imply that crystals crystallized from supercritical fluid phases also occur in igneous rocks and can be classified as supercritical fluid crystals or simply fluid crystals. In this paper, based on the analysis of three typical instances, we proposed that fluid crystals not only can form directly in a supercritical fluid phase, but also can form in the melt phase generated by the supercritical fluid through concentration. By all accounts, the precondition for fluid crystals is that the magmatic fluid attains to the supersaturation state. In order to satisfy this fundamental condition, the transmagmatic fluid process and/or rapid ascent of the magma are required. Based on previous investigations of melt-fluid equilibrium condition, and considering the dependencies of melt viscosity on volatile content and of magma ascent rate on magma viscosity, we found a coupling relationship between transmagmatic fluid process and magma ascent. This relation can be used to explain the nonlinear behavior of a magma system. Thus, it is very important to study fluid crystals as (1) it provides a new approach to reconstruct the fluid condition of a magma system; (2) it reveals the deviation level of a magma system from the ideal state; (3) it adds a new constraint to the inversion of dynamic processes of a magmatic system; (4) it provides new mineralogical indicators for recognizing ore-causal intrusions and assessing ore producing potentials; and (5) it provides solutions to some long-standing ambiguous issues in the prevailing igneous theory, such as solubility of magmatic volatile components, reactivation of frozen magma and specificity of magmatic metallogenesis.