接触变质带中的脱碳作用

Decarbonation in contact metamorphism

地球大气中的碳含量控制着地球长期(百万年尺度)和短期气候。最近的一些工作表明,地质历史中温室气候的时期和高强度的岩浆作用之间可能存在时间上的重合。从而提出假设,即火山和侵入岩与地壳碳酸盐岩之间相互作用释放的二氧化碳可能在调节外生系统的长期碳收支方面发挥了重要作用。例如,在Pangea超大陆裂解的过程中,特提斯体系大陆弧与白垩纪环太平洋弧的活动时间上重合。这些剧烈的大陆弧活动导致的接触变质作用释放的CO_(2)可能有助于形成白垩纪的温暖气候。除了这些宏观假设之外,接触变质作用产生的碳通量仍有待岩石学研究来解决。岩浆的加热和流体的渗透会促使围岩发生脱碳反应。富含H_(2)O的流体渗透会显著降低CO_(2)或其它含碳组分的活度,并有效地从系统中移除反应产物,从而促使通过发生更多的脱碳反应来恢复化学平衡。因此,渗透流体的通量和模式,无论是岩浆还是天水来源,对研究碳通量至关重要。在本文中,我们回顾了岩浆侵入周围的热量和流体传输以及碳酸盐岩和碳质变泥质岩组合的脱碳反应。对现代火山弧的观察表明,如果岩浆与上覆板块的碳酸盐岩地层相互作用,则会产生大量的碳通量。白垩纪大陆弧中丰富的矽卡岩和钙硅酸盐地体为量化从露头到造山尺度的碳通量提供了多样的野外研究机会。最后,本文概述了未来研究将要探索或解决的技术问题,包括流体来源和流动模式、含盐流体的相平衡以及脱碳的时间尺度等。

An essential control of both long-term(>My)and short-term climate is the amount of carbon in the Earth’s atmosphere.Recent work has suggested a possible temporal coincidence between greenhouse intervals and enhanced magmatism,motivating the hypothesis that CO_(2) emissions from the volcanics and plutonism intersecting crustal carbonates may play a strong role in modulating the long-term carbon budget of exogenic system.For example,during the dispersal of Pangea,the initiation of continental arcs in the Neo-Tethys regime overlapped the culmination of circum-Pacific arcs in the Cretaceous.The pulses of CO_(2) released during periods of vigorous continental arc activity potentially contributed to the Cretaceous Thermal Maximum.In addition to these big-picture hypotheses,many questions concerning the carbon fluxes from contact metamorphism remain to be solved by petrological studies.Decarbonation reactions take place in response to magmatic heating as well as fluid infiltration.The infiltrating H_(2)O-rich fluid significantly lowers the activity of CO_(2) or other carbon species and effectively remove the reaction product from the system.Thus,the flux and pattern of infiltrating fluid,either magmatic or meteoric,is critical to the carbon flux.In this contribution,we review the heat and fluid transport around a magmatic intrusion and the decarbonation chemistry of metacarbonate and carbonaceous metapelite assemblages.Observations on modern arc volcanics suggest that the volcanic carbon fluxes are much higher if the magma interacts with the carbonate sequence on the overriding place.Abundant skarn and calc-silicate formations in the Cretaceous continental arcs provide diverse and ample field targets for quantifying carbon fluxes from outcrop to orogenic scales.In the end,we outline the technical questions that future studies are to explore or address,including the fluid origin and flow pattern,phase equilibria with saline fluid,and the timescales of decarbonation.