Chemistry of gabbroic rocks from ODP Hole 118-735B

Gabbroic rocks and their late differentiates recovered at Site 735 represent 500 m of oceanic layer 3. The original cooling of a mid-ocean ridge magma chamber, its penetration by ductile shear zones and late intrusives, and the subsequent penetration of seawater through a network of cracks and into highly permeable magmatic hydrofracture horizons are recorded in the metamorphic stratigraphy of the core. Ductile shear zones are characterized by extensive dynamic recrystallization of primary phases, beginning in the granulite facies and continuing into the lower amphibolite facies. Increasing availability of seawater during dynamic recrystallization is reflected in depletions in 18O, increasing abundance of amphibole of variable composition and metamorphic plagioclase of intermediate composition, and more complete coronitic or pseudomorphous static replacement of magmatic minerals. Downcore correlation of synkinematic assemblages, bulk-rock oxygen isotopic compositions, and vein abundance suggest that seawater is introduced into the crust by way of small cracks and veins that mark the end of the ductile phase of deformation. This "deformation-enhanced" metamorphism dominates the upper 180 and the lower 100 m of the core. In the lower 300 m of the core, mineral assemblages of greenschist and zeolite facies are abundant within or adjacent to brecciated zones. Leucocratic veins found in these zones and adjacent host rock contain diopside, sodic plagioclase, epidote, chlorite, analcime, thomsonite, natrolite, albite, quartz, actinolite, sphene, brookite, and sulfides. The presence of zircon, Cl-apatite, sodic plagioclase, sulfides, and diopside in leucocratic veins having local magmatic textures suggests that some of the veins originated from late magmas or from hydrothermal fluids exsolved from such magmas that were subsequently replaced by (seawater-derived) hydrothermal assemblages. The frequent association of these late magmatic intrusive rocks within the brecciated zones suggests that they are both artifacts of magmatic hydrofracture. Such catastrophic fracture and hydrothermal circulation could produce episodic venting of hydrothermal fluids as well as the incorporation of a magmatically derived hydrothermal component. The enhanced permeability of the brecciated zones produced lower temperature assemblages because of larger volumes of seawater that penetrated the crust. The last fractures were sealed either by these hydrothermal minerals or by late carbonate-smectite veins, resulting in the observed low permeability of the core.从735站位（Site 735）采获的辉长岩（Gabbroic rocks）及其晚期分异产物，代表了500米厚的大洋层3（oceanic layer 3）。岩芯的变质地层记录了洋中脊岩浆房（mid-ocean ridge magma chamber）的初始冷却过程、韧性剪切带（ductile shear zones）与晚期侵入体（late intrusives）的穿切作用，以及后续海水通过裂隙网络渗透至高渗透性岩浆水力破裂层（highly permeable magmatic hydrofracture horizons）的过程。韧性剪切带以原生矿物相的广泛动态重结晶（dynamic recrystallization）为特征，该过程始于麻粒岩相（granulite facies），并延续至低阶角闪岩相（lower amphibolite facies）。动态重结晶过程中海水供给量的增加，体现为全岩δ¹⁸O亏损、成分多变的角闪石（amphibole）与中等成分变质斜长石（plagioclase）丰度升高，以及岩浆矿物（magmatic minerals）被更彻底的冠状或假象静态交代（coronitic or pseudomorphous static replacement）。对同变形矿物组合（synkinematic assemblages）、全岩氧同位素组成（bulk-rock oxygen isotopic compositions）与脉体丰度（vein abundance）的岩芯向下对比表明，海水通过标记韧性变形阶段结束的小型裂隙与脉体进入地壳。这种“变形增强型变质作用”（deformation-enhanced metamorphism）主导了岩芯上部180米与下部100米的地层。 在岩芯下部300米中，绿片岩相（greenschist facies）与沸石相（zeolite facies）的矿物组合大量发育于角砾岩化带（brecciated zones）内部或其邻区。这些带内及相邻围岩中的浅色脉体（leucocratic veins）含有透辉石（diopside）、钠质斜长石（sodic plagioclase）、绿帘石（epidote）、绿泥石（chlorite）、方沸石（analcime）、杆沸石（thomsonite）、钠沸石（natrolite）、钠长石（albite）、石英（quartz）、阳起石（actinolite）、榍石（sphene）、板钛矿（brookite）与硫化物（sulfides）。部分浅色脉体具有局部岩浆结构（magmatic textures），且含有锆石（zircon）、氯磷灰石（Cl-apatite）、钠质斜长石、硫化物与透辉石，这表明部分脉体源自晚期岩浆，或是由这类岩浆出溶的热液流体（hydrothermal fluids）形成，后续又被（海水来源的）热液矿物组合交代。这类晚期岩浆侵入岩常与角砾岩化带伴生，说明二者均为岩浆水力破裂的产物。这类灾难性破裂（catastrophic fracture）与热液循环（hydrothermal circulation）既可引发热液流体的间歇性喷溢（episodic venting），也可引入岩浆源热液组分（magmatically derived hydrothermal component）。角砾岩化带的渗透性（permeability）增强使得更多海水渗透进入地壳，从而形成了低温矿物组合。最后一期裂隙被这些热液矿物或晚期碳酸盐-蒙脱石脉（carbonate-smectite veins）封堵，最终形成了观测到的岩芯低渗透性特征。