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Deformation-enhanced diagenesis and bacterial proliferation in the Nankai accretionary prism
Solid Earth ( IF 3.4 ) Pub Date : 2021-05-12 , DOI: 10.5194/se-2021-54
Vincent Famin , Hugues Raimbourg , Muriel Andréani , Anne-Marie Boullier

Abstract. Understanding diagenetic reactions in accreted sediments is critical for establishing the balance of fluid sources and sinks in accretionary prisms, which is in turn important for assessing the fluid pressure field and the ability for faults to host seismic slip. For this reason, we studied diagenetic reactions in deformation bands (shear zones and veins) within deep mud sediments from the Nankai accretionary prism (SW Japan) drilled at site C0001 during IODP Expedition 315, by means of microscopic observation, X-ray diffraction, and major-trace element analyses. Deformation bands are not only more compacted than the host sediment, but are also enriched in framboidal pyrite, as observed under microscopy and confirmed by chalcophile element enrichments (Fe, S, Cu, As, Sb, Pb). In tandem, clays in deformation bands undergo a destabilization of smectite or illite/smectite mixed layers, and/or a slight crystallization of illite, which is matched by a correlated increase in B and Li compared to the host sediment. The two diagenetic reactions of sulfide precipitation and clay transformation are both explained by a combined action of sulfate-reducing and methanogen bacteria, which strongly suggests an increased activity of anaerobic microbial communities localized in deformation bands. This local bacterial proliferation was possibly enhanced by the liberation of hydrogen from strained phyllosilicates. We suggest that the proliferation of anoxic bacteria, boosted by deformation, may participate in the pore water freshening observed at depth in accretionary prisms. Deformation-enhanced metabolic reactions may also explain the illitization observed in major faults of accretionary prisms. Care is therefore needed before interpreting illitization, and other diagenetic reactions as well, as evidence of shear heating, as these might be biogenic instead of thermogenic.

中文翻译:

南开增生棱镜中形变的成岩作用和细菌增殖

摘要。理解沉积物中的成岩反应对于建立增生棱柱体中的流体源和汇的平衡至关重要,而这反过来对于评估流体压力场和断层承载地震滑动的能力也很重要。因此,我们通过显微镜观察,X射线衍射,和主要痕量元素分析。变形带不仅比宿主沉积物更致密,而且还富含黄菊黄铁矿,如在显微镜下观察到的,并通过亲硫元素富集(Fe,S,Cu,As,Sb,Pb)得到证实。串联起来 变形带中的粘土经历了蒙脱石或伊利石/蒙脱石混合层的失稳,和/或伊利石的轻微结晶,这与硼和锂相对于基质沉积物的相关增加相匹配。硫化物沉淀和粘土转变的两个成岩反应都可以通过硫酸盐还原菌和产甲烷菌的联合作用来解释,这强烈表明位于变形带的厌氧微生物群落的活性增加。这种局部细菌的增殖可能是由于从应变的层状硅酸盐中释放出氢而增强的。我们建议通过变形促进缺氧细菌的增殖,可能参与在增生棱镜深处观察到的孔隙水清新。变形增强的代谢反应也可以解释在增生棱镜的主要断层中观察到的未熟化。因此,在解释非法化和其他成岩反应之前,还需要格外小心,以作为剪切加热的证据,因为这些可能是生物成因的,而不是热成因的。
更新日期:2021-05-12
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