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This review focuses on the solubility, origin, abundance, and degassing of carbon dioxide (CO 2 ) in magma±hydrothermal systems, with applications for those workers interested in intrusion-related deposits of gold and other metals. The solubility of CO 2 increases with pressure and magma alkalinity. Its solubility is low relative to that of H 2 O, so that ¯uids exsolved deep in the crust tend to have high CO 2 /H 2 O compared with ¯uids evolved closer to the surface. Similarly, CO 2 /H 2 O will typically decrease during progressive decompression-or crystallization-induced degassing. The temperature dependence of solubility is a function of the speciation of CO 2 , 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 CO 2 to mantle magmas. Graphite, diamond, and a free CO 2 -bearing ¯uid may be the primary carbon-bearing phases in other mantle source regions. Growing evidence suggests that most CO 2 is 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 ¯uid 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 CO 2 is 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 CO 2bearing vapor does not hinder, and in fact may promote, the ascent and eruption of the host magma. Carbonic ¯uids are poorly miscible with aqueous ¯uids, particularly at high temperature and low pressure, so that the presence of CO 2 can 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 CO 2 may indirectly aid the process of metallogenesis by inducing phase separation.

International audienceContinental intraplate volcanoes, such as Erebus volcano, Antarctica, are associated with extensional tectonics, mantle upwelling and high heat flow. Typically, erupted magmas are alkaline and rich in volatiles (especially CO2), inherited from low degrees of partial melting of mantle sources. We examine the degassing of the magmatic system at Erebus volcano using melt inclusion data and high temporal resolution open-path Fourier transform infrared (FTIR) spectroscopic measurements of gas emissions from the active lava lake. Remarkably different gas signatures are associated with passive and explosive gas emissions, representative of volatile contents and redox conditions that reveal contrasting shallow and deep degassing sources. We show that this unexpected degassing signature provides a unique probe for magma differentiation and transfer of CO2-rich oxidised fluids from the mantle to the surface, and evaluate how these processes operate in time and space. Extensive crystallisation driven by CO2 fluxing is responsible for isobaric fractionation of parental basanite magmas close to their source depth. Magma deeper than 4 kbar equilibrates under vapour-buffered conditions. At shallower depths, CO2-rich fluids accumulate and are then released either via convection-driven, open-system gas loss or as closed-system slugs that ascend and result in Strombolian eruptions in the lava lake. The open-system gases have a reduced state (below the QFM buffer) whereas the closed-system gases preserve their deep oxidised signatures (close to the NNO buffer)