Rapid fluid infiltration and permeability enhancement during middle-lower crustal fracturing: Evidence from amphibolite-granulite-facies fluid-rock reaction zones, Sor Rondane Mountains, East Antarctica
Rapid fluid infiltration and permeability enhancement during middle-lower crustal fracturing: Evidence from amphibolite-granulite-facies fluid-rock reaction zones, Sor Rondane Mountains, East Antarctica
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中下地壳破裂过程中流体的快速渗透和渗透率增强：来自东南极洲索龙达山脉角闪岩-麻粒岩相流体-岩石反应带的证据
DOI：
10.1016/j.lithos.2020.105521
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发表时间：
2020
期刊：
影响因子：
3.5
通讯作者：
Noriyoshi Tsuchiya
中科院分区：
文献类型：
--
作者：
Diana Mindaleva;Masaoki Uno;Fumiko Higashino;Takayoshi Nagaya;Atsushi Okamoto;Noriyoshi Tsuchiya
渗透率是流体在地壳中渗透的关键控制因素。然而，对地壳渗透率的定量地质限制是有限的，特别是在其时间演化方面。本文基于流体渗透和地壳破裂的变质作用，对中下地壳渗透率演化进行了约束。研究了东南极洲Sør Rondane Mountains Mefjell地区的基性麻粒岩和正闪角闪片岩（opx-hbl片岩）样品。毫米级角闪岩相反应带沿裂缝发育。在基性麻粒岩中划分出两个带：(i)反应带（1 ~ 2mm厚）；（ii）基性麻粒岩寄主岩。opx-hbl片岩样品可划分为3个带：(i)放线石-明矾岩带（厚度1.4 mm）；（ii）放线石-正辉石带（1.6 mm厚）；(3)寄主岩。这些区域从矿物学、反应结构和微量元素剖面上都很明显。基性麻粒岩的p - t条件为0.55 GPa和620℃，opx-hbl片岩的p - t条件为0.3 GPa和450℃。磷灰石颗粒中的氯浓度从裂隙向寄主岩方向逐渐降低。氯浓度谱表明，两种样品的氯输运过程均以平流输运为主，扩散输运较少。根据这些结果，基性麻粒岩和opx-hbl片岩的流体入渗时间尺度分别为~8 h和~10 h。根据反应区的水活度估算，反应区的水压力梯度为2 ~ 15 MPa/mm。围岩渗透率为10−21 ~ 10−23，裂缝渗透率为10−8 ~ 10−9m2。结果表明，低渗透（10−21-10−23m2）寄主岩石的流体通量有限，导致含氯流体快速入渗（~10 h）。低渗透介质导致流体积聚和进一步压裂。空间平均渗透率增加了几个数量级（10−10 - 10−16m2），流体压力降低。寄主岩石和裂缝之间的渗透率对比表明，在与地球物理观测相当的时间尺度上，地壳压裂增加了渗透率。与以往研究的平均长期渗透率（Myr）估算值（例如10−18m2）相比，裂缝-反应带系统的渗透率在中下地壳（10−21-10−23至10−10 - 10−16m2）中波动较大。
Permeability is a key control on fluid infiltration in the crust. However, quantitative geological constraints on crustal permeability are limited, particularly with regards to its temporal evolution. Here we constrain the permeability evolution in the middle–lower crust, based on metamorphic processes associated with fluid infiltration and crustal fracturing. We investigated mafic granulite and orthopyroxene–hornblende schist (opx–hbl schist) samples from Mefjell, Sør Rondane Mountains, East Antarctica. Millimetre-scale amphibolite-facies reaction zones occur along fractures in these rocks. In the mafic granulite, two zones were identified: (i) reaction zones (1–2 mm thick); and (ii) mafic granulite host rock. The opx–hbl schist sample can be divided into the following three zones: (i) actinolite–cummingtonite zones (1.4 mm thick); (ii) actinolite–orthopyroxene zones (1.6 mm thick); and (iii) host rock. These zones are evident from the modal mineralogy, reaction textures, and trace element profiles. TheP–Tconditions of fluid infiltration are estimated to be 0.55 GPa and 620 °C for the mafic granulite, and 0.3 GPa and 450 °C for the opx–hbl schist, respectively. Chlorine concentrations in apatite grains show a gradual decrease from the fractures towards the host rocks. Chlorine concentration profiles suggest that the dominant processes of chlorine transport were advection with minor diffusion for both samples. Based on these results, the timescales of fluid infiltration are constrained to be ~8 h for the mafic granulite and ~10 h for the opx–hbl schist. The pressure gradient across the reaction zones was estimated from the H2O activity in the reaction zones to be 2–15 MPa/mm. The permeability of the host rock and fractures were estimated to be 10−21–10−23and 10−8–10−9m2, respectively. Our results show that rapid infiltration of Cl-bearing fluids (~10 h) occurred due to a limited fluid flux from low-permeability (10−21–10−23m2) host rocks. The low-permeability media led to fluid accumulation and further fracturing. The spatio-averaged permeability then increased by more than several orders of magnitude (10−10–10−16m2) and the fluid pressure decreased. The contrasting permeability between the host rocks and fractures reveals permeability enhancements associated with crustal fracturing on timescales comparable to geophysical observations. Compared with average long-term (Myr) permeability estimates from previous studies (e.g., 10−18m2), the permeability obtained from the fracture–reaction zone systems shows large fluctuations in the middle–lower crust (10−21–10−23to 10−10–10−16m2).