Chemistry and fluid inclusions of the sheeted dike comples of ODP Holes 137-504B and 140-504B

Fluid inclusions in variably altered diabase recovered from Ocean Drilling Program Legs 137 and 140 at Hole 504B, Costa Rica Rift, exhibit fluid salinities up to 3.7 times that of seawater values (11.7 wt% NaCl equivalent) and exhibit uncorrected homogenization temperatures of 125°C to 202°C. The liquid-dominated inclusions commonly are entrapped in zones of secondary plagioclase and may be primary in origin. Fluid salinities are similar to compositions of fluids venting on the seafloor (0.4-7.0 wt% NaCl) and overlap with those measured in metabasalt samples recovered from near the Kane Fracture Zone on the Mid-Atlantic Ridge and from the Troodos ophiolite, Cyprus. The salinity variations may reflect hydration reactions involving formation of secondary mineral assemblages under rock-dominated conditions, which modify the ionic strength of hydrothermal fluids by consuming or liberating water and chloride ion. Rare CO2-CH4-bearing inclusions, subjacent to zones where talc after olivine becomes an important secondary mineral phase (1700 mbsf), may have formed due to local interaction of seawater and olivine at low water to rock ratios. Corrected average fluid inclusion homogenization temperatures exhibit a gradient from 159°C at a depth of 1370 mbsf to 183°C at a depth of 1992 mbsf and are in apparent equilibrium with the present conductive downhole temperatures. These data indicate that fluid inclusions may be used to estimate downhole temperatures if logging data are unavailable. The compositional and thermal evolution of the diabase-hosted fluids may reflect late-stage, off-axis circulation and conductive heating of compositionally modified seawater in the sheeted dike complex at Hole 504B.

采自哥斯达黎加裂谷504B钻孔、大洋钻探计划（Ocean Drilling Program, ODP）第137与140航次所获蚀变程度不均一的辉绿岩中的流体包裹体（fluid inclusions），其流体盐度最高可达海水盐度的3.7倍（即11.7重量百分比氯化钠当量，wt% NaCl equivalent），未校正均一温度（homogenization temperatures）介于125℃至202℃之间。此类液相占优型流体包裹体通常赋存于次生斜长石带中，可能为原生成因。该流体盐度与海底喷溢流体的组分（0.4~7.0 wt% NaCl）相近，且与大西洋中脊（Mid-Atlantic Ridge）凯恩断裂带（Kane Fracture Zone）附近及塞浦路斯特罗多斯蛇绿岩套（Troodos ophiolite）中获取的变玄武岩（metabasalt）样品测得的盐度值重合。盐度的变化可能反映了岩石主导体系下形成次生矿物组合的水化反应，此类反应通过消耗或释放水与氯离子，改变热液流体的离子强度。罕见的含CO₂-CH₄流体包裹体分布于橄榄石蚀变为滑石成为主要次生矿物相的下方区域（海底以下1700米，meters below seafloor, mbsf），其可能形成于低水岩比条件下海水与橄榄石的局部相互作用。校正后的流体包裹体平均均一温度呈现出从1370mbsf处的159℃到1992mbsf处的183℃的温度梯度，且与当前传导型井下温度表观平衡。这些数据表明，在缺乏测井数据的情况下，可通过流体包裹体估算井下温度。赋存于辉绿岩中的流体的组分与热演化过程，可能反映了504B钻孔席状岩墙杂岩中经组分改造的海水的后期轴外循环与传导加热作用。