Episodic fluid flow in an eclogite-facies shear zone: Insights from Li isotope zoning in garnet
Episodic fluid flow in an eclogite-facies shear zone: Insights from Li isotope zoning in garnet
复制标题
榴辉岩相剪切带中的幕式流体流动：石榴石中锂同位素分带的见解
作者：
W. F. Hoover;S. Penniston‐Dorland;L. Baumgartner;A. Bouvier;B. Dragovic;M. Locatelli;S. Angiboust;P. Agard
间歇性流体超压和逃逸被认为是许多俯冲带地震现象的原因或结果，但在挖掘出的高压变质岩中很难对其进行约束。对石榴石中锂同位素的现场测量揭示了蒙维索蛇绿岩（西阿尔卑斯山）中目前暴露的俯冲剪切带中幕式流体迁移的证据。通过二次离子质谱 (SIMS) 对来自榴辉岩块和相关交代反应果皮的石榴石进行了分析。所有分析的石榴石都保留了 δ7Li 的核心-边缘分区和地幔中大的负 δ7Li 偏移 (NE)。这些偏差不能用分析过程中的仪器质量分馏、平衡分馏或石榴石中锂的晶内扩散来解释。相反，NEs是通过在孔隙流体中大量扩散过程中锂同位素的动力学分馏而产生的，并且分馏的同位素成分通过同扩散生长并入石榴石中。与负 δ7Li 相关的不平衡石榴石生长结构支持了这一解释，并表明交代作用推动了石榴石的快速生长。确定了四个不同的 NE，要求在相邻剪切区内传输至少四个流体脉冲。榴辉岩相沿俯冲剪切带的幕式流体输送的证据支持依赖于循环流体超压和逃逸的中深度地震活动模型。
Episodic fluid overpressure and escape is invoked as a cause or consequence of many subduction-zone seismic phenomena but can be challenging to constrain in exhumed high-pressure metamorphic rocks. In situ measurements of lithium isotopes in garnet reveal evidence of episodic fluid transport in a subduction shear zone now exposed in the Monviso ophiolite (Western Alps). Garnet from an eclogite block and associated metasomatic reaction rind was analyzed by secondary ion mass spectrometry (SIMS). All analyzed garnet preserves core-rim zoning in δ7Li and large negative δ7Li excursions (NEs) in mantles. These excursions cannot be explained by instrumental mass fractionation during analysis, equilibrium fractionation, or intracrystalline diffusion of Li within garnet. Instead, NEs were produced by kinetic fractionation of Li isotopes during bulk diffusion through a pore fluid, and the fractionated isotopic compositions were incorporated into garnet by syn-diffusion growth. Disequilibrium garnet growth textures associated with negative δ7Li support this interpretation and suggest metasomatism drove rapid garnet growth. Four distinct NEs were identified requiring that at least four pulses of fluid were transported within the adjacent shear zone. This evidence of episodic fluid transport along a subduction shear zone at eclogite facies supports models of intermediate-depth seismicity that rely on cyclic fluid overpressure and escape.