On silica activity and serpentinization
On silica activity and serpentinization
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DOI：
10.1093/petrology/egm021
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发表时间：
2007-07-01
影响因子：
3.9
通讯作者：
Beard, James S.
中科院分区：
文献类型：
--
作者：
Frost, B. Ronald;Beard, James S.
蛇纹岩是普通地壳岩石中硅活性最低的。在蛇纹石化前沿，存在橄榄石、蛇纹石和水镁石，二氧化硅活性（相对于石英）在10（-2.5）到10（-5）之间，取决于温度。本文认为，这种低硅活性是产生蛇纹岩化异常地球化学环境特征的关键性质。从蛇纹石的Fe3Si2O5(OH)(4)组分中提取二氧化硅驱动磁铁矿的形成，产生极低的还原条件，部分蛇纹石化橄榄岩中含有稀有铁合金。在低温度下，透辉石的不一致溶解形成Ca2+、蛇纹石和二氧化硅变得越来越有利，产生了蛇纹石的碱性流体特征。这些流体与邻近岩石的相互作用产生了罗纹岩，我们认为脱硅也是罗纹岩形成过程的一部分。低硅活性也解释了蛇纹岩或蛇纹岩邻近岩石中低硅矿物的存在，如水辉石、水辉石、硬玉、一水硬石和刚玉。二氧化硅活性随温度降低而降低的趋势表明，蛇纹岩中某些矿物的存在可以作为蛇纹岩化温度的指示物。随着温度的降低，它们包括透辉石、顺长石和一水硬石。由于蛇纹岩+水镁石组合标志着大多数蛇纹岩中二氧化硅活性最低，水镁石的存在和分布对于解释导致任何给定蛇纹岩水化的过程至关重要，而水镁石通常是蛇纹岩中的一个隐相。
Serpentinites have the lowest silica activity of common crustal rocks. At the serpentinization front, where olivine, serpentine, and brucite are present, silica activities (relative to quartz) are of the order of 10(-2.5) to 10(-5), depending on the temperature. Here we argue that this low silica activity is the critical property that produces the unusual geochemical environments characteristic of serpentinization. The formation of magnetite is driven by the extraction of silica from the Fe3Si2O5(OH)(4) component of serpentine, producing extremely reducing conditions as evinced by the rare iron alloys that partially serpentinized peridotites contain. The incongruent dissolution of diopside to form Ca2+, serpentine, and silica becomes increasingly favored at lower T, producing the alkalic fluids characteristic of serpentinites. The interaction of these fluids with adjacent rocks produces rodingites, and we argue that desilication is also part of the rodingite-forming process. The low silica activity also explains the occurrence of low-silica minerals such as hydrogrossular, andradite, jadeite, diaspore, and corundum in serpentinites or rocks adjacent to serpentinites. The tendency for silica activity to decrease with decreasing temperature means that the presence of certain minerals in serpentinites can be used as indicators of the temperature of serpentinization. These include, with decreasing temperature, diopside, andradite and diaspore. Because the assemblage serpentine + brucite marks the lowest silica activity reached in most serpentinites, the presence and distribution of brucite, which commonly is a cryptic phase in serpentinites, is critical to interpreting the processes that lead to the hydration of any given serpentinite.