(Table 1) Sulfur contents and isotope composition in rocks and minerals from DSDP/ODP Hole 504B

DSDP Hole 504B is the only hole in oceanic crust to penetrate through the volcanic section and into hydrothermally altered sheeted dikes. We have carried out petrologic and sulfur isotopic analyses of sulfide and sulfate minerals and whole rocks from the core in order to place constraints on the geochemistry of sulfur during hydrothermal alteration of ocean crust.
The nearly 600 m-thick pillow section has lost sulfur to seawater and has net d34S = -1.8 per mil due to degassing of SO2 during crystallization and subsequent low temperature interaction with seawater. Hydrothermally altered rocks in the 200 m-thick transition zone are enriched in S and 34S (4300 ppm and +3.0 +/-1.2 per mil, respectively), whereas the more than 500 m of sheeted dikes contain 720 ppm S with d34S = +0.6 +/-1.4 per mil. These data are consistent with the presence of predominantly basaltic sulfur in hydrothermal fluids deep in the crust: following precipitation of anhydrite during seawater recharge, small amounts of seawater sulfate were reduced at temperatures >250°C through conversion of igneous pyrrhotite to secondary pyrite and minor oxidation of ferrous iron in the crust. The S- and 34S-enrichments of the transition zone are the results of seawater sulfate reduction and sulfide deposition during subsurface mixing between upwelling hot (up to 350°C) hydrothermal fluids and seawater. Seawater sulfate was probably reduced through oxidation of ferrous iron in hydrothermal fluids and in the transition zone rocks.
Alteration of the upper crust resulted in loss of basaltic sulfur to seawater, fixation of minor seawater sulfur in the crust and redistribution of magmatic sulfur within the crust. This caused net increases in sulfur content and d34S of the upper 1.8 km of the oceanic crust.

深海钻探计划（Deep Sea Drilling Project, DSDP）504B钻孔是全球首个在洋壳中穿透火山岩段并进入热液蚀变席状岩墙的钻孔。本研究对该钻孔岩芯中的硫化物、硫酸盐矿物及全岩样品开展了岩石学与硫同位素分析，以期约束洋壳热液蚀变过程中的硫地球化学行为。

厚度近600米的枕状熔岩段因岩浆结晶过程中SO₂脱气及后续与海水的低温相互作用，发生了硫向海水的流失，其整体硫同位素比值δ³⁴S为-1.8‰。厚度200米的过渡带热液蚀变岩石的硫含量与δ³⁴S值均显著富集，分别达4300 ppm与+3.0±1.2‰；而席状岩墙段（厚度超500米）的硫含量为720 ppm，δ³⁴S为+0.6±1.4‰。上述数据与洋壳深部热液流体以玄武质硫为主的特征相符：在海水补给过程中硬石膏沉淀后，少量海水硫酸盐在温度高于250℃的条件下被还原，具体机制为火成磁黄铁矿转化为次生黄铁矿，同时地壳内的亚铁发生轻微氧化。过渡带的硫与硫同位素富集，是上升热液流体（最高温度达350℃）与海水在地下混合过程中，海水硫酸盐还原及硫化物沉淀的结果。海水硫酸盐的还原可能通过热液流体及过渡带岩石中的亚铁氧化作用实现。

上洋壳的蚀变作用导致玄武质硫流失进入海水，少量海水硫在洋壳中被固定，同时岩浆硫在洋壳内部发生再分配。这一过程使得洋壳上部1.8 km范围内的硫含量与δ³⁴S值整体升高。