Measurement of Volume Change and Mass Transfer During Serpentinization: Insights From the Oman Drilling Project
Measurement of Volume Change and Mass Transfer During Serpentinization: Insights From the Oman Drilling Project
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DOI：
10.1029/2019jb018877
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发表时间：
2020-05
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影响因子：
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通讯作者：
B. Malvoisin;Chang Zhang;O. Müntener;L. Baumgartner;P. Kelemen
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作者：
B. Malvoisin;Chang Zhang;O. Müntener;L. Baumgartner;P. Kelemen
蛇纹岩作用对地幔岩石圈物理化学性质的演化起着关键作用。蛇纹石化反应的速度由流体的输送控制，而流体的输送本身取决于反应过程中的体积变化。元素迁移可以强烈地改变体积变化的大小和符号。在这里，我们测量了阿曼钻井项目期间在深处采集的蛇纹岩中垂直于蛇纹状矿脉的固体体积变化和元素迁移。样品被平行于蛇纹石主脉网络的蛇纹石/水镁石混合物广泛取代(反应程度为80%)。蛇纹石和水镁石的镁含量指示在100℃以下与少量流体发生反应。流动微量元素(Na、Ca、Sr、Rb和Ba)的浓度垂直于主脉呈下降趋势。原生橄榄石含有单斜辉石/磁铁矿混合辉石的平行片状矿物。纳米尺度的断层扫描显示，这些包裹体在蛇纹化过程中不会发生反应，但会破裂和移位。我们使用这些惰性标记物来测量59%到74%的正体积变化，这接近封闭系统中反应预期的52%。化学数据表明，在反应过程中，除水的添加外，主要元素组成没有变化。在蛇纹石和水镁石中仍保留着Al、Ti、V、Sc和Cr中的初始橄榄石分带。因此，蛇纹石化可以是一种局部置换过程，在此过程中，固体体积在微米级均匀增加，水相物质的传输受到限制。
Serpentinization plays a key role on the evolution of the physicochemical properties of the mantle lithosphere. The rate of serpentinization reactions is controlled by the transport of fluid, which itself depends on volume change during reaction. Element transfer can strongly modify the magnitude and sign of volume change. Here, we measure solid volume change and element transport perpendicular to a serpentine vein in a serpentinized dunite collected at depth during the Oman Drilling Project. The sample is extensively replaced (extent of reaction > 80%) by a serpentine/brucite mixture parallel to a main serpentine vein network. The Mg content of serpentine and brucite indicates reaction with a small amount of fluid at temperatures below 100°C. Concentrations of fluid‐mobile trace elements (Na, Ca, Sr, Rb, and Ba) decrease perpendicular to the main vein. Primary olivine contains parallel platelets of a clinopyroxene/magnetite symplectite. Tomography at the nanoscale reveals that these inclusions do not react during serpentinization but are cracked and displaced. We use these inert markers to measure a 59% to 74% positive volume change that is close to the 52% expected for reaction in a closed system. Chemical data indicate no change in major element composition during reaction except for the addition of water. The initial olivine zoning in Al, Ti, V, Sc, and Cr is still preserved in serpentine and brucite. Serpentinization can thus be a local replacement process during which the solid volume homogeneously increases at the micrometer scale and the transport of aqueous species is limited.