Geochemistry of fluids from Earth’s deepest ridge-crest hot-springs: Piccard hydrothermal field, Mid-Cayman Rise
Geochemistry of fluids from Earth’s deepest ridge-crest hot-springs: Piccard hydrothermal field, Mid-Cayman Rise
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DOI：
10.1016/j.gca.2018.01.021
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发表时间：
2018-05
影响因子：
5
通讯作者：
Jill M. McDermott;S. Sylva;S. Ono;C. German;J. Seewald
中科院分区：
文献类型：
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作者：
Jill M. McDermott;S. Sylva;S. Ono;C. German;J. Seewald
皮卡德热液区位于超缓慢扩张的开曼群岛中部隆起的玄武岩基底中，是目前已知最深的海底温泉（4 957 - 4 987米）。由于其深度很大，皮卡德遗址是研究极压对海底喷口流体形成的影响的绝佳自然系统。为了研究岩石成分和深循环条件对流体化学的作用，在2012年和2013年收集的样品中检查了Piccard高温喷口流体中有机、无机和溶解挥发性物质的丰度和同位素组成。然而，来自无机化学（Cl、SiO2、Ca、Br、Fe、Cu、Mn）的几条证据支持在地下高得多的温度下形成流体。由于系统的深度很大，可以达到这些可能超过500 °C的高温。我们的数据表明，一个单一的根深蒂固的源流体饲料高温通风口在整个皮卡德领域。高温Piccard流体H2丰度（19.9 mM）甚至高于在许多超镁铁质影响的系统中观察到的丰度，例如Rainbow（16 mM）和Von Damm热液场（18.2 mM）。然而，在Piccard的情况下，这些极高的H2丰度可以产生从流体玄武岩反应发生在非常高的temperature.Magmatic和热源的碳在高温黑烟喷口的描述。溶解的CO2可能是岩浆成因的，CH 4可能来自热成因的来源和非生物CH 4的矿物托管的流体包裹体的淋滤组合，和CO丰度处于平衡的水煤气变换反应。长链正构烷烃（C2 H6、C3 H8、n-C4 H10、i-C4 H10）可能来自原始海水来源的溶解和颗粒有机碳的热蚀变、高温排气外围微生物生态系统的夹带和/或非生物地幔来源。在354 °C的测量温度下，Beebe Woods流体中溶解的NaHCOOH与通过H2还原NaHCO 2的非生物生产的热力学平衡一致。在温度相对较高的398 °C Beebe Vent流体中，缺乏NaHCOOH，这表明了这种平衡的温度敏感性。丰富的玄武岩海底露头和喷口场的轴向位置，沿着多条地球化学证据，支持极高温度的流体-岩石反应，基性岩基质是Piccard流体化学的主要控制因素。这些结果扩大了已知的喷口流体成分的多样性，与支持微生物生活在现代和古代海洋的影响。
Hosted in basaltic substrate on the ultra-slow spreading Mid-Cayman Rise, the Piccard hydrothermal field is the deepest currently known seafloor hot-spring (4957–4987 m). Due to its great depth, the Piccard site is an excellent natural system for investigating the influence of extreme pressure on the formation of submarine vent fluids. To investigate the role of rock composition and deep circulation conditions on fluid chemistry, the abundance and isotopic composition of organic, inorganic, and dissolved volatile species in high temperature vent fluids at Piccard were examined in samples collected in 2012 and 2013.Fluids from the Beebe Vents and Beebe Woods black smokers vent at a maximum temperature of 398 °C at the seafloor, however several lines of evidence derived from inorganic chemistry (Cl, SiO2, Ca, Br, Fe, Cu, Mn) support fluid formation at much higher temperatures in the subsurface. These high temperatures, potentially in excess of 500 °C, are attainable due to the great depth of the system. Our data indicate that a single deep-rooted source fluid feeds high temperature vents across the entire Piccard field. High temperature Piccard fluid H2abundances (19.9 mM) are even higher than those observed in many ultramafic-influenced systems, such as the Rainbow (16 mM) and the Von Damm hydrothermal fields (18.2 mM). In the case of Piccard, however, these extremely high H2abundances can be generated from fluid-basalt reaction occurring at very high temperatures.Magmatic and thermogenic sources of carbon in the high temperature black smoker vents are described. Dissolved ΣCO2is likely of magmatic origin, CH4may originate from a combination of thermogenic sources and leaching of abiotic CH4from mineral-hosted fluid inclusions, and CO abundances are at equilibrium with the water–gas shift reaction. Longer-chained n-alkanes (C2H6, C3H8,n-C4H10,i-C4H10) may derive from thermal alteration of dissolved and particulate organic carbon sourced from the original seawater source, entrainment of microbial ecosystems peripheral to high temperature venting, and/or abiotic mantle sources. Dissolved ΣHCOOH in the Beebe Woods fluid is consistent with thermodynamic equilibrium for abiotic production via ΣCO2reduction with H2at 354 °C measured temperature. A lack of ΣHCOOH in the relatively higher temperature 398 °C Beebe Vent fluids demonstrates the temperature sensitivity of this equilibrium.Abundant basaltic seafloor outcrops and the axial location of the vent field, along with multiple lines of geochemical evidence, support extremely high temperature fluid-rock reaction with mafic substrate as the dominant control on Piccard fluid chemistry. These results expand the known diversity of vent fluid composition, with implications for supporting microbiological life in both the modern and ancient ocean.