Constraints on Cooling of the Lower Ocean Crust From Epidote Veins in the Wadi Gideah Section, Oman Ophiolite
Constraints on Cooling of the Lower Ocean Crust From Epidote Veins in the Wadi Gideah Section, Oman Ophiolite
复制标题
阿曼蛇绿岩干河道吉迪亚剖面绿帘石脉对下洋地壳冷却的制约
DOI：
10.1029/2018gc007679
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发表时间：
2018
期刊：
影响因子：
3.7
通讯作者：
Bach W
中科院分区：
文献类型：
--
作者：
Bieseler B;Diehl A;Jöns N;Lucassen F;Bach W
海洋地壳中高温热液蚀变的时间、深度和程度是了解下海洋地壳在吸积后如何冷却的关键。绿帘石脉是在下地壳Wadi Gideah剖面和Wadi Amadhi席状岩墙根部区域的强烈蚀变空间重复区采集的。87 Sr/86 Sr比值范围较窄，介于0.70429至0.70512之间，而δ 18 OSMOW中的O同位素组成介于−0.7‰至+4.9‰之间。这些组成表明水岩比在1至5之间，地层温度在300至450 °C范围内。在Wadi Gideah剖面地壳最低3 km处，一部分样品的流体包裹体捕获温度从338 °C线性增加到465 °C，Wadi Amadhi基底席状岩墙样品的记录温度为340 °C。盐度是均匀的，并分散在海水值附近（3.2 ± 0.2重量%）。一个模型，其中冷却到2公里（即，熔融透镜的深度）之后，下地壳缓慢离轴热液冷却至1百万年，预测下地壳的热梯度与1百万年至2.5百万年之间观察到的趋势相匹配。我们认为，绿帘石脉形成于离轴热液系统中，这些系统在离轴50至100公里的范围内到达地壳底部。这种深环流为开采热量提供了一种有效的机制，这些热量从洋中脊年轻侧翼的地壳中逸出。
Timing, depth, and extent of high‐temperature hydrothermal alteration in the ocean crust is key to understanding how lower oceanic crust is cooled after accretion. Epidote veins were collected in spatially recurring zones of intense alteration of the lower crustal Wadi Gideah section and the root zone of sheeted dykes at Wadi Amadhi.87Sr/86Sr ratios feature a narrow range from 0.70429 to 0.70512, while O isotope compositions vary between −0.7‰ and +4.9‰ in δ18OSMOW. These compositions indicate water‐rock ratios between 1 and 5 and formation temperatures in the range of 300 to 450 °C. Fluid inclusion entrapment temperatures for a subset of samples linearly increase from 338 °C to 465 °C in lowermost 3 km of crust of the Wadi Gideah section and record 340 °C for the Wadi Amadhi basal sheeted dike sample. Salinities are uniform throughout and scatter closely around seawater values (3.2 ± 0.2 wt%). A model in which cooling down to 2 km (i.e., the depth of the melt lens) is followed by slow off‐axis hydrothermal cooling of the lower crust to 1 Myr predicts a thermal gradient for the lower crust that matches this observed trend for ages between 1 and 2.5 Myr. We suggest that the epidote veins formed in off‐axial hydrothermal systems that reach the base of the crust within 50‐ to 100‐km off axis. This deep circulation provides an efficient mechanism for mining heat that escapes the crust in the young flanks of mid‐ocean ridges.
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