氯在岩浆演化过程中的地球化学性质初探

地质过程中,氯可以影响很多元素的性质。本文利用前人的实验数据,模拟计算不同压力和初始含水量等条件下,氯在岩浆和共存富水流体相中浓度随岩浆结晶分异的变化。结果显示,在岩浆演化之初,氯在各种压力下都基本留在岩浆中,表现出高度不相容元素的特点。对于结晶分异晚期的岩浆体系,氯的行为受压力的影响较大。在低压下,氯在岩浆/富水流体之间的分配系数较高,水是否达到饱和对氯在岩浆中的含量变化影响不大。随着压力的升高,该系数迅速降低。在中等压力下,岩浆中水含量一旦达到饱和,大量的氯进入流体相,形成高盐度的流体,氯在岩浆中的含量即迅速降低。随着岩浆分异的进行,共存的流体中的氯含量也随之降低。在更高的压力下,水无法达到饱和,氯始终表现为高度不相容的特点。氯的上述性质可以很好地解释它在很多结晶分异程度较低的火山岩玻璃(如MORB和OIB)中表现为高度不相容的特点,也可以帮助理解氯在侵入岩和喷出岩中性质的差别,以及“花岗岩大岩体不成矿,小岩体成大矿”的现象。

A Preliminary Study on the Geochemical Behavior of Chlorine in the Magma Evolution

Chlorine is an important element, which can dramatically influence the geochemical behavior of many other elements in geological processes. In this study, the behaviour of Cl in magmas and co-magmatic fluids under different pressures, initial water contents during fractional crystallization of magma, has been modelled based on previously published experimental data. Our modelling results show that the behaviour of Cl is controlled by pressure, initial water contents and the degree of magma fractionation. At the early stage of magma fractionation, Cl is a highly incompatible element under essentially all pressures. By contrast, Cl is sensitive to pressure at the late stage of magma fractionation. Water saturation has little effect on the behaviour of Cl at low pressure, because of its high magma/water distribution coefficient. At moderately high pressure, Cl is scavenged into co-magmatic fluid phase to form high salinity fluids when water saturation is achieved. Consequently, Cl concentration in magmas is dreased sharply. The salinity of the co-magmatic fluids is also decreased as the fractional crystallization of magma continues. However, at much higher pressure, no co-magmatic fluid was formed as there is no saturated water, Cl stays in the magma due to its high incompatibility. These behaviours of Cl can be applied to explain the high incompatibility of Cl in low fractional crystallized volcanic glass (e.g. MORB and OIB). They are helpful for understanding the difference between the Cl property in intrusive rocks and that in eruptive rocks, and the phenomenon that no mineralization occurred in large bodies of granite but good mineralization occurred in small bodies of granite.