Microstructure and geochemical signatures of metasedimentary origin pseudotachylyte: Implications for fluid activity during paleoseismicity
Microstructure and geochemical signatures of metasedimentary origin pseudotachylyte: Implications for fluid activity during paleoseismicity
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DOI：
10.1016/j.tecto.2018.07.010
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发表时间：
2018-10
期刊：
影响因子：
2.9
通讯作者：
Yoshihiro Nakamura;T. Toyoshima;M. Satish-Kumar
中科院分区：
文献类型：
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作者：
Yoshihiro Nakamura;T. Toyoshima;M. Satish-Kumar
假速水石的氧化还原态提供了与古地震活动期间流体活动相关的物理化学性质的有价值的信息。在这项研究中，我们使用流体缓冲矿物（例如石墨和其他次生矿物）评估了假速水石中流体的行为。在日本北海道的日高变质带中发现了两种类型的假速水石； Pst I 和 Pst II。 Pst I 由层状硅酸盐分解产生的磁黄铁矿和钾长石组成，而 Pst II 则由石英和长石以及层状硅酸盐分解产生的重结晶黑云母微晶石、钾长石和斜长石组成。页硅酸盐的选择性摩擦熔融表现为大离子亲石元素（例如 Ba、Sr、Rb、Cs、B 和 K）浓度的显着变化。此外，与钛矿和羟基磷灰石相关的石墨通常分布在 Pst II 基体内。与从原岩中提取的变质石墨（δ13C=-25.1±0.8‰，n=15）、碎裂石、超碎裂石和Pst I相比，石墨表现出不同的结晶度、形态和较高的碳稳定同位素组成（δ13C=-21.9±2.3‰，n=16） (δ13C=−25.2±0.8‰，n=15)。两种假速石的微观结构和地球化学特征均受与硫化物矿物和石墨共存的层状硅酸盐的脱水-氧化还原反应控制。具体地，Pst II中富含13 C的石墨的形成可以通过流体沉积石墨与来自H 2 O-CH 4 流体的含水矿物的共沉淀和/或H 2 O-CH 4 流体从残余石墨的脱气来推断。我们的数据表明，可以产生还原性的含碳流体，然后通过变沉积岩的摩擦熔融将其溶解到假速成岩熔体中。通过同震滑移产生含碳流体可能是锁定在地壳中的耐火石墨再流动过程中被忽视的过程。
The redox state of pseudotachylyte provides valuable information on the physicochemical properties related to fluid activity during paleoseismicity. In this study, we evaluate the behavior of fluids in pseudotachylytes using fluid buffering minerals such as graphite and other secondary minerals. Two types of pseudotachylyte were identified in the Hidaka metamorphic belt, Hokkaido, Japan; Pst I and Pst II. Pst I is composed of pyrrhotite and K-feldspar generated by the breakdown of phyllosilicates, whereas Pst II includes recrystallized biotite microlites, K-feldspar, and plagioclase due to the breakdown of quartz and feldspars, as well as phyllosilicates. The selective frictional melting of phyllosilicates is indicated by significant changes in the concentrations of large ion lithophile elements (e.g., Ba, Sr, Rb, Cs, B, and K). In addition, the graphite associated with titanite and hydroxyapatite is often distributed within the Pst II matrix. The graphite exhibits different crystallinity, morphology, and higher carbon stable isotopic compositions (δ13C = −21.9 ± 2.3‰,n= 16), when compared with metamorphic graphite extracted from the protolith (δ13C = −25.1 ± 0.8‰,n= 15), cataclasite, ultracataclasite, and Pst I (δ13C = −25.2 ± 0.8‰,n= 15). The microstructure and geochemical signatures in both pseudotachylytes are controlled by dehydration–redox reaction of phyllosilicates coexisting with sulfide minerals and graphite. In particular, the formation of13C enriched graphite in the Pst II can be deduced by co-precipitation of fluid-deposited graphite with hydrous minerals from H2O-CH4fluids and/or degassing of H2O-CH4fluids from residual graphite. Our data demonstrated that the carbonic fluids of a reducing nature could be produced, and then dissolved into the pseudotachylyte melt by frictional melting of metasedimentary rocks. The production of carbonic fluids by coseismic slip may be an overlooked process in the remobilization of refractory graphite locked in the crust.