The petrology of seafloor rodingites: Insights from geochemical reaction path modeling
The petrology of seafloor rodingites: Insights from geochemical reaction path modeling
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DOI：
10.1016/j.lithos.2008.10.022
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发表时间：
2009-09-01
期刊：
影响因子：
3.5
通讯作者：
Klein, Frieder
中科院分区：
文献类型：
--
作者：
Bach, Wolfgang;Klein, Frieder
应用热力学反应路径模型来深入了解镁铁质/超镁铁质边界处罗金石的形成。这些模型的建立是为了研究当流体从经历蛇纹石化的橄榄岩移动到辉长岩体时流体-岩石平衡的变化。在 200°C、300°C 和 400°C 下，沿着蛇纹石化流体与辉长岩反应程度增加的方向研究了相关系。预计在 200°C 和 300°C 时会形成罗辉长岩（钙铝榴石 + 透辉石绿泥石）的典型相组合，但仅在流体基本上不受与辉长岩反应影响的区域（即接触面附近）形成。与超镁铁质岩石或邻近充满蛇纹石化流体的裂缝。随着流体更容易受到与辉长岩反应的影响，葡萄石或绿帘石取代了石榴石，透闪石取代了单斜辉石。一旦流体化学性质通过与辉长岩的反应而完全重置，预测的组合就是典型的绿片岩相：钠长石斜长石、阳铁矿、绿泥石和绿帘石。模型中预测的这些转变与在不同环境下的天然罗丹石中观察到的转变非常相似。因此，我们的模型结果支持了这样的假设：罗丁石在蛇纹石化过程中形成，并且仅在存在受蛇纹石化反应控制的流体的区域中形成。我们的计算进一步表明，海底罗丁岩内矿物组合的形成和空间矿物学变异主要是由孔隙流体中质子和含水二氧化硅的陡峭活动梯度驱动的。 Ca 的传质可能是通过 CaOH+ 物质的扩散实现的，预计这会在镁铁质-超镁铁质边界上显示出非常陡峭的浓度梯度。由于透辉石和石榴石在低 SiO2 活度下具有很高的稳定性，罗鼎石起到了 Ca 陷阱的作用。我们的模型计算进一步预测了辉长岩和单斜辉石岩前驱岩在 200 摄氏度和 300 摄氏度下会形成透辉石，这表明透辉岩脉的形成不需要 800 摄氏度左右的非常高的温度。 (C) 2008 Elsevier B.V. 保留所有权利。
Thermodynamic reaction path models were applied to provide insights into the formation of rodingites at mafic/ultramafic boundaries. The models were set up to investigate the shifts in fluid-rock equilibria as fluids move from peridotite undergoing serpentinization into a gabbroic body. Phase relations were investigated in the direction of increasing extent of reaction of the serpentinization fluid with gabbro at 200 degrees C, 300 degrees C, and 400 degrees C. Phase assemblages typical of rodingite (grossular + diopside chlorite) are predicted to form at 200 degrees C and 300 degrees C, but only in areas where the fluid is essentially unaffected by reactions with gabbro, i.e., near the contact with ultramafic rock or adjacent to a fissure filled with serpentinization fluid. As the fluid becomes more affected by reactions with the gabbro, prehnite or epidote-ss replace garnet and tremolite replaces clinopyroxene. Once the fluid chemistry is completely reset by reactions with gabbro, the predicted assemblage is typical of greenschist facies: albitic plagioclase, actinofite, chlorite, and epidotess. These transitions predicted in the model are very similar to what is observed in natural rodingites from different settings. Our model results hence support the hypothesis that rodingites form during serpentinization and only in areas where fluids controlled by serpentinization reactions are present. Our calculations further indicate that the formation of mineral assemblages and spatial mineralogical variability within rodingites from the ocean floor is mainly driven by steep activity gradients in protons and aqueous silica in pore fluids. Mass transfer of Ca is likely by diffusion of the CaOH+ species, which is predicted to show a very steep concentration gradient across a mafic-ultramafic boundary. Rodingites act as Ca traps because of the great stability of diopside and garnet at low SiO2 activities.Our model calculations further predict the formation of diopsidite from gabbroic and clinopyroxenite precursor rocks at 200 degrees C and 300 degrees C, suggesting that very high temperatures on the order of 800 degrees C are not required in the formation of diopsidite veins. (C) 2008 Elsevier B.V. All rights reserved.