Carbon dioxide in magmas and implications for hydrothermal systems
Carbon dioxide in magmas and implications for hydrothermal systems
复制标题
DOI：
10.1007/s001260100185
复制
发表时间：
2001-09
影响因子：
4.8
通讯作者：
J. Lowenstern
中科院分区：
文献类型：
--
作者：
J. Lowenstern
本文主要介绍岩浆-热液系统中二氧化碳的溶解度、来源、丰度和脱气作用，并为那些对与侵入岩有关的金和其他金属矿床感兴趣的工作者提供应用。CO2的溶解度随压力和岩浆碱度的增加而增加。它的溶解度相对于H2O较低，因此与靠近地表的流体相比，地壳深处的流体往往具有较高的CO2/H2O。类似地，CO2/H2O通常在渐进减压或结晶诱导的脱气期间降低。溶解度的温度依赖性是CO2形态的函数，CO2以分子形式溶解在流纹中（逆行温度溶解度），但在玄武岩中以溶解的碳酸盐基团存在（逆温度溶解度）。菱镁矿和白云石在相对广泛的地幔条件下是稳定的，但在固相线上方熔融，从而为地幔岩浆提供CO2。石墨、金刚石和含游离CO2流体可能是其他地幔源区的主要含碳相。越来越多的证据表明，大多数CO2是通过俯冲洋壳及其上覆沉积物毯的再循环而贡献给弧岩浆的。在地壳中岩浆与围岩的相互作用过程中，额外的碳可以被添加到岩浆中。对侵入和喷出火成岩中流体和熔体包裹体的研究提供了大量证据，证明许多岩浆在中地壳（10-15 km）深处是饱和的，CO2是出溶蒸汽的一个重要部分。玄武岩浆和某些岩浆都是这种情况。在大多数情况下，含CO2蒸气的存在并不妨碍，事实上可能促进寄主岩浆的上升和喷发。碳质流体与水相流体混溶性差，特别是在高温和低压下，因此CO2的存在可以在岩浆挥发相和热液体系中诱导不混溶性。由于包括金在内的一些金属在气相中比共存的液体更易挥发，因此CO2的存在可能通过诱导相分离间接帮助成矿过程。
This review focuses on the solubility, origin, abundance, and degassing of carbon dioxide (CO2) in magma–hydrothermal systems, with applications for those workers interested in intrusion-related deposits of gold and other metals. The solubility of CO2increases with pressure and magma alkalinity. Its solubility is low relative to that of H2O, so that fluids exsolved deep in the crust tend to have high CO2/H2O compared with fluids evolved closer to the surface. Similarly, CO2/H2O will typically decrease during progressive decompression- or crystallization-induced degassing. The temperature dependence of solubility is a function of the speciation of CO2, which dissolves in molecular form in rhyolites (retrograde temperature solubility), but exists as dissolved carbonate groups in basalts (prograde). Magnesite and dolomite are stable under a relatively wide range of mantle conditions, but melt just above the solidus, thereby contributing CO2to mantle magmas. Graphite, diamond, and a free CO2-bearing fluid may be the primary carbon-bearing phases in other mantle source regions. Growing evidence suggests that most CO2is contributed to arc magmas via recycling of subducted oceanic crust and its overlying sediment blanket. Additional carbon can be added to magmas during magma–wallrock interactions in the crust. Studies of fluid and melt inclusions from intrusive and extrusive igneous rocks yield ample evidence that many magmas are vapor saturated as deep as the mid crust (10–15 km) and that CO2is an appreciable part of the exsolved vapor. Such is the case in both basaltic and some silicic magmas. Under most conditions, the presence of a CO2-bearing vapor does not hinder, and in fact may promote, the ascent and eruption of the host magma. Carbonic fluids are poorly miscible with aqueous fluids, particularly at high temperature and low pressure, so that the presence of CO2can induce immiscibility both within the magmatic volatile phase and in hydrothermal systems. Because some metals, including gold, can be more volatile in vapor phases than coexisting liquids, the presence of CO2may indirectly aid the process of metallogenesis by inducing phase separation.