Magmatism, serpentinization and life: Insights through drilling the Atlantis Massif (IODP Expedition 357)
Magmatism, serpentinization and life: Insights through drilling the Atlantis Massif (IODP Expedition 357)
复制标题
DOI：
10.1016/j.lithos.2018.09.012
复制
发表时间：
2018-12
期刊：
影响因子：
3.5
通讯作者：
G. Früh-Green;B. Orcutt;S. Rouméjon;M. Lilley;Y. Morono;C. Cotterill;S. Green;J. Escartín;
中科院分区：
文献类型：
--
作者：
G. Früh-Green;B. Orcutt;S. Rouméjon;M. Lilley;Y. Morono;C. Cotterill;S. Green;J. Escartín;
IODP第357次考察使用了两个海底钻机，在亚特兰蒂斯地块海洋岩心复合体（大西洋中脊30°N）的9个地点取了17个浅孔的岩心。这次考察的目的是调查沿沿着一个主要拆离断层带上升到海底的高度蚀变的超镁铁质和镁铁质序列的浅层地下的蛇纹石化过程和微生物活动。岩芯回收量超过57米，钻孔穿透深度从海底以下1.3米到16.4米不等，一个钻孔的岩芯回收量高达总穿透量的75%。岩心显示出高度不均匀的岩石类型和蚀变，这些岩石类型和蚀变与大量岩石化学变化有关，反映了拆离断层带内岩浆作用、流体-岩石相互作用和质量转移的多个阶段。超镁铁岩以普遍的蛇纹化方辉橄榄岩为主，间有蛇纹化纯橄榄岩和少量辉石岩脉;辉长岩以熔融体和脉状产出。白云岩侵入体和玄武质岩石代表了最新的岩浆活动。基性岩的比例在体积上小于先前通过在IODP站点U1309钻探亚特兰蒂斯地块的中央圆顶而回收的基性岩的数量。这表明，在地幔橄榄岩中的熔融积累在一个复杂的拆离断层带的脊变换的交点和/或构造换位的岩石类型的不同模式。岩心显示出以滑石-角闪石-辉长岩叠印为主的高度蛇纹化和交代蚀变。交代作用在超镁铁质和镁铁质区域（辉长岩和/或粗玄岩侵入体）之间的接触处最为普遍，并指出在沿着拆离断层折返过程中，有通道的流体流动和二氧化硅流动。蛇纹岩中的镁铁质透镜体的存在及其对机械强度较弱的滑石、蛇纹石和辉长岩的蚀变可能对拆离断层带的形成也至关重要，并可能有助于上地幔橄榄岩/辉长岩序列的持续去顶。还开发了海底钻探的新技术，以便对环境进行地球化学和微生物表征。Anin situsensor包和水采样系统记录溶解甲烷，氧气，pH值，氧化还原电位（Eh）和温度的实时变化，并在钻井和钻井后的底层水采样。这些参数的系统偏移以及钻井后流体中氢和甲烷浓度的升高为所有地点的活动蛇纹石化提供了证据。此外，还向钻井液中注入了化学示踪剂，以进行污染测试，并在一些地点成功地部署了钻孔塞系统，以供今后进行流体取样。国际大洋钻探项目第357次考察的一项主要成就是沿着东西向剖面获取微生物样本，这将使人们更好地了解随着超镁铁岩和镁铁岩的蚀变和在海底的就位，微生物群落如何演变。严格的采样处理协议允许非常低的微生物细胞检测的限制，我们的研究结果表明，亚特兰蒂斯地块地下含有相对较低密度的微生物生命。
IODP Expedition 357 used two seabed drills to core 17 shallow holes at 9 sites across Atlantis Massif ocean core complex (Mid-Atlantic Ridge 30°N). The goals of this expedition were to investigate serpentinization processes and microbial activity in the shallow subsurface of highly altered ultramafic and mafic sequences that have been uplifted to the seafloor along a major detachment fault zone. More than 57 m of core were recovered, with borehole penetration ranging from 1.3 to 16.4 meters below seafloor, and core recovery as high as 75% of total penetration in one borehole. The cores show highly heterogeneous rock types and alteration associated with changes in bulk rock chemistry that reflect multiple phases of magmatism, fluid-rock interaction and mass transfer within the detachment fault zone. Recovered ultramafic rocks are dominated by pervasively serpentinized harzburgite with intervals of serpentinized dunite and minor pyroxenite veins; gabbroic rocks occur as melt impregnations and veins. Dolerite intrusions and basaltic rocks represent the latest magmatic activity. The proportion of mafic rocks is volumetrically less than the amount of mafic rocks recovered previously by drilling the central dome of Atlantis Massif at IODP Site U1309. This suggests a different mode of melt accumulation in the mantle peridotites at the ridge-transform intersection and/or a tectonic transposition of rock types within a complex detachment fault zone. The cores revealed a high degree of serpentinization and metasomatic alteration dominated by talc-amphibole-chlorite overprinting. Metasomatism is most prevalent at contacts between ultramafic and mafic domains (gabbroic and/or doleritic intrusions) and points to channeled fluid flow and silica mobility during exhumation along the detachment fault. The presence of the mafic lenses within the serpentinites and their alteration to mechanically weak talc, serpentine and chlorite may also be critical in the development of the detachment fault zone and may aid in continued unroofing of the upper mantle peridotite/gabbro sequences.New technologies were also developed for the seabed drills to enable biogeochemical and microbiological characterization of the environment. Anin situsensor package and water sampling system recorded real-time variations in dissolved methane, oxygen, pH, oxidation reduction potential (Eh), and temperature and during drilling and sampled bottom water after drilling. Systematic excursions in these parameters together with elevated hydrogen and methane concentrations in post-drilling fluids provide evidence for active serpentinization at all sites. In addition, chemical tracers were delivered into the drilling fluids for contamination testing, and a borehole plug system was successfully deployed at some sites for future fluid sampling. A major achievement of IODP Expedition 357 was to obtain microbiological samples along a west–east profile, which will provide a better understanding of how microbial communities evolve as ultramafic and mafic rocks are altered and emplaced on the seafloor. Strict sampling handling protocols allowed for very low limits of microbial cell detection, and our results show that the Atlantis Massif subsurface contains a relatively low density of microbial life.