Chemical, isotope and mineral composition of hydrothermally altered upper oceanic crust drilled at ODP Site 129-801C

ODP Hole 801C penetrates >400 m into 170-Ma oceanic basement formed at a fast-spreading ridge. Most basalts are slightly (10-20%) recrystallized to saponite, calcite, minor celadonite and iron oxyhydroxides, and trace pyrite. Temperatures estimated from oxygen isotope data for secondary minerals are 5-100°C, increasing downward. At the earliest stage, dark celadonitic alteration halos formed along fractures and celadonite, and quartz and chalcedony formed in veins from low-temperature (<100°C) hydrothermal fluids. Iron oxyhydroxides subsequently formed in alteration halos along fractures where seawater circulated, and saponite and pyrite developed in the host rock and in zones of restricted seawater flow under more reducing conditions. Chemical changes include variably elevated K, Rb, Cs, and H2O; local increases in FeT, Ba, Th, and U; and local losses of Mg and Ni. Secondary carbonate veins have 87Sr/86Sr = 0.706337 - 0.707046, and a negative correlation with d18O results from seawater-basalt interaction. Carbonates could have formed at any time since the formation of Site 801 crust. Variable d13C values (-11.2? to 2.9?) reflect the incorporation of oxidized organic carbon from intercalated sediments and changes in the d13C of seawater over time. Compared to other oceanic basements, a major difference at Site 801 is the presence of two hydrothermal silica-iron deposits that formed from low-temperature hydrothermal fluids at the spreading axis. Basalts associated with these horizons are intensely altered (60-100%) to phyllosilicates, calcite, K-feldspar, and titanite; and exhibit large increases in K, Rb, Cs, Ba, H2O, and CO2, and losses of FeT, Mn, Mg, Ca, Na, and Sr. These effects may be common in crust formed at fast-spreading rates, but are not ubiquitous. A second important difference is that the abundance of brown oxidation halos along fractures at Site 801 is an order of magnitude less than at some other sites (2% vs. 20-30%). Relatively smooth basement topography (<100 m) and high sedimentation rate (8 m/Ma) probably restricted the access of oxygenated seawater. Basement lithostratigraphy and early low-temperature hydrothermal alteration and mineral precipitation in fractures at the spreading axis controlled permeability and limited later flow of oxygenated seawater to restricted depth intervals.大洋钻探计划（Ocean Drilling Program, ODP）801C钻孔穿透了逾400米的170百万年（170 Ma）洋壳基底，该基底形成于快速扩张脊。多数玄武岩发生了轻度（10%~20%）重结晶，转化为皂石（saponite）、方解石（calcite）、少量绿鳞石（celadonite）以及羟基氧化铁（iron oxyhydroxides），并伴生微量黄铁矿（pyrite）。根据次生矿物的氧同位素数据估算的形成温度介于5℃至100℃之间，且随深度增加而升高。在蚀变初期，沿裂隙及绿鳞石分布区形成暗色绿鳞石蚀变晕；同时低温（<100℃）热液流体在脉体中结晶出石英（quartz）与玉髓（chalcedony）。随后，在海水循环流通的裂隙蚀变晕中形成羟基氧化铁；在还原性更强的环境下，寄主岩石及海水流通受限的区域则发育皂石与黄铁矿。化学组分变化表现为：钾（K）、铷（Rb）、铯（Cs）及水（H₂O）含量普遍升高；局部区域全铁（Fe_T）、钡（Ba）、钍（Th）及铀（U）含量增加，而镁（Mg）与镍（Ni）含量出现局部亏损。 次生碳酸盐脉的⁸⁷Sr/⁸⁶Sr比值介于0.706337至0.707046之间，其与δ¹⁸O值呈负相关关系，这一特征由海水与玄武岩的相互作用所致。碳酸盐矿物可在801站位洋壳形成后的任意时期形成。碳同位素δ¹³C值变化范围为-11.2‰至2.9‰，其反映了夹层沉积物中氧化态有机碳的混入，以及海水δ¹³C组成随时间的演化。 与其他洋壳基底相比，801站位的主要差异在于存在两处由扩张轴处低温热液流体形成的热液硅铁沉积体。赋存于上述层位的玄武岩发生了强烈蚀变（蚀变程度达60%~100%），转化为层状硅酸盐（phyllosilicates）、方解石、钾长石（K-feldspar）及榍石（titanite）；其化学组分表现为K、Rb、Cs、Ba、H₂O及CO₂含量大幅升高，而全铁（Fe_T）、锰（Mn）、Mg、Ca、钠（Na）及锶（Sr）含量出现亏损。这类蚀变效应在快速扩张形成的洋壳中或较为普遍，但并非普遍存在。第二个显著差异在于，801站位沿裂隙发育的棕色氧化晕的丰度比其他部分站位低一个数量级（占比2% vs. 20%~30%）。相对平缓的基底地形（起伏<100 m）及较高的沉积速率（8 m/Ma）可能限制了含氧海水的侵入。基底岩石地层学特征、早期低温热液蚀变作用，以及扩张轴处裂隙内的矿物沉淀作用共同控制了岩层渗透性，并限制了含氧海水后续仅能在有限的深度区间内流动。