Carbonation and decarbonation of eclogites: the role of garnet
Carbonation and decarbonation of eclogites: the role of garnet
复制标题
榴辉岩的碳化和脱碳：石榴石的作用
DOI：
10.1007/s004100050515
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发表时间：
1999
影响因子：
3.5
通讯作者：
R. Luth
中科院分区：
文献类型：
--
作者：
R. Knoche;R. Sweeney;R. Luth
摘要碳酸盐岩是地幔中原生碳和俯冲碳的重要载体。此外，碳酸盐与硅酸盐和还原碳（金刚石或石墨）的共存，允许对地幔的氧化状态进行限制。碳酸盐-硅酸盐-蒸汽反应控制碳酸盐+硅酸盐组合如何随着压力的增加而从含碳蒸汽+硅酸盐组合形成。在含橄榄石的岩石中，如橄榄岩，被认为是上地幔中的主要岩石类型，最低压力的碳酸盐形成反应涉及橄榄石（±单斜辉石）与CO2反应（例如，Wyllie等人，1983年）。榴辉岩中主要矿物组合为绿辉石单斜辉石+石榴子石，不含橄榄石。因此，必须寻找替代的碳酸盐形成反应。Luth（1995）通过反应白云石+2柯石英=透辉石+2 CO2对单斜辉石的碳酸化进行了实验研究。本文通过在多顶锤装置上测定5 ~ 11 GPa压力范围内3菱镁矿+蓝晶石+2柯石英=镁铝榴石+3CO_2反应的位置，探讨了石榴石与CO_2反应的另一种可能性。在温度≥1200 °C时，榴辉岩的碳酸化作用随着压力的增加而开始，首先是通过与单斜辉石的反应进行的，因为在给定的温度下，焦岩碳酸化反应的压力比透辉石碳酸化反应的压力高。将石榴石的镁铝榴石成分和单斜辉石的透辉石成分稀释到适合地幔榴辉岩的水平并不改变这一结论。在较低的温度下，适合于“冷”板，有可能匡威的情况将保持，通过与石榴石反应进行初始碳酸化，但这种可能性有待实验证实。榴辉岩在“正常地幔”地热条件下通过压力降低而发生的脱碳酸作用，就像在地幔对流圈的上升翼中一样，将由单斜辉石+ CO2的形成所控制。在比该反应更高的压力下，菱镁矿与蓝晶石和柯石英反应分解产生的任何CO2将与单斜辉石反应生成白云石+柯石英。CO2从榴辉岩释放到地幔橄榄岩中，将在亚固相线条件下形成碳酸盐，如果温度高于橄榄岩-CO2固相线，则会产生碳酸盐熔体。
Abstract Carbonates are potentially significant hosts for primordial and subducted carbon in the Earth's mantle. In addition, the coexistence of carbonate with silicates and reduced carbon (diamond or graphite), allows constraints to be placed on the oxidation state of the mantle. Carbonate-silicate-vapor reactions control how carbonate + silicate assemblages may form from carbon-bearing vapor + silicate assemblages with increasing pressure. In olivine-bearing rocks such as peridotite, considered the dominant rock type in the upper mantle, the lowest-pressure carbonate-forming reactions involve olivine (±clinopyroxene) reacting with CO2 (e.g., Wyllie et al. 1983). In eclogitic rocks, the essential mineral assemblage is omphacitic clinopyroxene + garnet, without olivine. Therefore, alternative carbonate-forming reactions must be sought. The carbonation of clinopyroxene via the reaction dolomite + 2 coesite = diopside + 2 CO2 was studied experimentally by Luth (1995). The alternative possibility that garnet reacts with CO2 is explored here by determining the location of the reaction 3 magnesite + kyanite + 2 coesite = pyrope + 3 CO2 between 5 and 11 GPa in multi-anvil apparatus. At the temperatures ≥1200 °C, carbonation of eclogitic rocks with increasing pressure will proceed initially by reaction with clinopyroxene, because the pyrope-carbonation reaction lies at higher pressures for a given temperature than does the diopside-carbonation reaction. Diluting the pyrope component of garnet and the diopside component of clinopyroxene to levels appropriate for mantle eclogites does not change this conclusion. At lower temperatures, appropriate for “cold” slabs, it is possible that the converse situation will hold, with initial carbonation proceeding via reaction with garnet, but this possibility awaits experimental confirmation. Decarbonation of an eclogite under “normal mantle” geothermal conditions by a decrease in pressure, as in an ascending limb of a mantle convection cell, would be governed by the formation of clinopyroxene + CO2. At higher pressure than this reaction, any CO2 produced by the breakdown of magnesite reacting with kyanite and coesite would react with clinopyroxene to produce dolomite + coesite. Release of CO2 from eclogite into mantle peridotite would form carbonate at sub-solidus conditions and produce a dolomitic carbonate melt if temperatures are above the peridotite-CO2 solidus.