汇聚板块边缘岩浆中金属和氯的地球化学性质研究

总结了铜(Cu)、金(Au)、铼(Re)和氯(Cl)在汇聚板块边缘岩浆中的性质。在岛弧型的火山岩岩浆演化的早期,Cu、Au和Re均表现为中度不相容元素,含量随SiO2含量的增加而增加。在SiO2质量分数为58%时,多数岛弧型火山岩中Au、Cu的含量会突然大幅度下降。这一变化与铁和钛的变化是耦合的,铁和钛均由不相容元素变为相容元素,显示钛磁铁矿开始结晶了。进一步的研究表明,钛磁铁矿的结晶使硫酸根被还原为氢硫酸根,后者与Au、Cu形成氢硫酸根络合物,被萃取到流体相中,从而形成成矿流体。这一过程可以很好地解释Au、Cu矿床广泛分布于汇聚板块边缘的现象。与Au、Cu相反,Re的含量在SiO2质量分数为60%时才开始下降,而且是缓慢下降。这是因为Re通常比Au、Cu更亲石。此外,Re还具有强烈的挥发性。氯在东Manus岩浆中表现为高度不相容的特点。氯的性质主要受压力、初始水含量和岩浆演化分异程度的控制。计算结果显示,由于MORB和OIB含水量低,分异演化程度低,氯在上述岩浆中表现为高度不相容的特点。相比之下,氯在岛弧岩浆中的性质就复杂得多。随着水含量和岩浆房深度的不同,氯的性质可以从相容变到高度不相容。

The geochemical behaviors of some metals and chlorine during the evolution of convergent margin magmas

The behaviors of Cu, Au, Re and Cl in subduction-related volcanic glasses are reviewed in this paper. In the early stage of magma evolution, Cu, Au and Re increase with increasing SiO2 content. In many cases, Cu and Au drop suddenly at SiO2 content of ~58 wt%, concurrent with a switch in the behavior of Ti and Fe from concentration increase to decrease as SiO2 rises. This relates to the crystallization of titanomagnetite, which reduces sulphate to sulphide, forming Au-Cu hydrosulphide complexes, and thus Au and Cu are extracted into comagmatic fluids, which can plausibly explain the genetic links between Au-Cu ore deposits and convergent margin magmas. By contrast, Re starts to decrease at SiO2 contents of ~60 wt% showing a continuous decrease, instead of rapid drop as Cu and Au do. The solubility of Re in magmas increases with increasing oxygen fugacity, therefore the observed Re behavior is controlled by oxygen fugacity bufferring during titanomagnetite crystallization. Rhenium is also highly volatile, as suggested by considerably higher Re concentrations in undegassed magmas. Chlorine is highly incompatible during the whole evolution of the eastern Manus basin magmas. Modelling shows that the behavior of Cl in magmas is strongly influenced by pressure, initial H2O content, and the degree of magmatic fractionation. Accordingly, Cl is usually highly incompatible in MORB and OIB, because of low H2O contents and relatively low degrees of fractional crystallization. The behavior of Cl in arc magmas is more complicated, ranging from highly incompatible to compatible depending on H2O contents and depths of magma chambers.