Metamorphic CO2 Production in Collisional Orogens: Petrological Constraints from Phase Diagram Modeling of Himalayan, Scapolite-bearing, Calc-silicate Rocks in the NKC(FIMAS(T)-HC system
Metamorphic CO2 Production in Collisional Orogens: Petrological Constraints from Phase Diagram Modeling of Himalayan, Scapolite-bearing, Calc-silicate Rocks in the NKC(FIMAS(T)-HC system
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DOI：
10.1093/petrology/egx005
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发表时间：
2017-01-01
影响因子：
3.9
通讯作者：
Mosca, Pietro
中科院分区：
文献类型：
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作者：
Groppo, Chiara;Rolfo, Franco;Mosca, Pietro
对来自碰撞造山带的变质二氧化碳通量进行可靠的定量估计是我们了解深层碳循环的基础，但它仍然远远没有受到限制。主要的不确定性包括对变质二氧化碳产生过程的性质和通过这些反应可能释放的二氧化碳量的了解不足。以往的变质脱碳反应研究主要采用简化模型体系中端元间的简单模型反应。然而，钙硅酸盐岩石的完全定量建模需要对具有6个以上组分的非常复杂的系统进行研究。此外，在以往的研究中很少包括石长石固溶体，尽管这种矿物通常是钙硅酸盐岩石的主要成分。本研究的重点是(1)在含角长石的钙硅酸盐岩石中产生二氧化碳的过程，(2)讨论适合理解和量化这些过程的方法方法。在此框架下，考虑了Na20 K2O Ca0)(Fe0) Mg0 A1203 Si02 (Ti02)H20 0O2 INKC(F)MAS(T)HCJ体系的相关系和脱挥发反应，并应用于喜马拉雅地区高品位斜辉石+方解石k长石方解石斜长石+黝帘石钙硅酸盐岩石。利用(1)等压T - X（CO2）相图剖面和拟剖面，以及(2)混合挥发性P - T相图投影，研究了涉及角长石和斜长石固溶体的NKC(F)MAS(T)-HC平衡。这种相图方法使我们能够识别以前从未识别过的含角长石、产生二氧化碳、不变（即等压不变）平衡，如果不考虑计算中的Na - Ca固溶体，就无法检测到这些平衡。结果表明，所研究的钙硅酸盐岩石在进阶变质过程中表现为一个近封闭的内部缓冲体系，所观察到的大部分关键微结构对应于等压不变或不变组合。在这样一个近乎封闭的系统中，流体主要是在等压不变点的渐进加热过程中产生的，在那里矿物模式也发生了突变。所提出的相图方法进一步允许定量估计在这些等压不变点产生的流体的数量和组成。在这种钙硅酸盐岩石类型的渐进变质作用过程中，平均每1000厘米(3)反应岩石产生2.5 mol CO2 （110 g）。喜马拉雅造山带中也有大量类似的含钙硅酸钙岩石，这表明这种类型的钙硅酸钙岩石可能在喜马拉雅变质作用中产生了大量的富co2流体。在整个造山带的尺度上对这些量的初步估计表明，总变质CO2产量为（2-7）× 10(17) mol，对应于（1-3）× 10(10) Mt CO2。综合超过20myr（即最大的进变质持续时间）
A reliable quantitative estimate of the metamorphic CO2 flux from collisional orogens is fundamental to our understanding of the deep carbon cycle, but it is still far from being constrained. Among major uncertainties are the poor knowledge of the nature of metamorphic CO2-producing processes and the amount of CO2 potentially released through these reactions. Previous studies of metamorphic decarbonation reactions in metacarbonate rocks mainly used simple model reactions between end-members in simplified model systems. However, fully quantitative modelling of calcsilicate rocks requires an investigation of very complex systems with more than six components. Moreover, scapolite solid solution has been rarely included in previous studies, although this mineral is often a major constituent of calc-silicate rocks. This study focuses on (1) the CO2-producing processes occurring in scapolite-bearing calc-silicate rocks and (2) the discussion of a methodological approach suitable to understand and quantify these processes. In this framework, phase relations and devolatilization reactions in the Na20 K2O Ca0)(Fe0) Mg0 A1203 Si02 (Ti02)H20 0O2 INKC(F)MAS(T)HCJ system are considered, with application to high-grade clinopyroxene + calcite K-feldspar scapolite plagioclase+ zoisite calc-silicate rocks from the Himalaya. The NKC(F)MAS(T)-HC equilibria involving scapolite and plagioclase solid solutions are investigated using (1) isobaric T X(CO2) phase diagram sections and pseudosections and (2) mixed-volatile P T phase diagram projections. This phase diagram approach allowed us to identify scapolite-bearing, CO2-producing, univariant (i.e. isobaric invariant) equilibria that have never been recognized before, and that could not be detected without considering Na Ca solid solutions in the calculations. It is demonstrated that the investigated calc-silicate rocks behaved as a nearly closed, internally buffered, system during prograde metamorphism and that most of the observed key microstructures correspond to isobaric univariant or invariant assemblages. In such a nearly closed system, the fluid was mostly produced during prograde heating at the isobaric invariant points, where abrupt changes in mineral modes also occurred. The proposed phase diagram approach further allows quantitative estimation of the amount and composition of the fluid produced at such isobaric invariant points. On average, 2,5 mol of CO2 (110 g) per 1000 cm(3) of reacting rock were produced during prograde metamorphism of this calc-silicate rock-type. Because similar scapolitebearing calc-silicate rocks are abundant in the Himalayan orogen, it is suggested that this calc-silicate rock-type might have produced large amounts of CO2-rich fluids during Himalayan metamorphism. A preliminary estimate of these amounts at the scale of the whole orogen suggests a total metamorphic CO2 production of (2-7) x 10(17) mol, corresponding to (1-3) x 10(10) Mt of CO2. Integrated over 20 Myr (i.e. the maximum duration of prograde metamorphism), the