Major-oxides and trace-elements at DSDP Hole 78-543A

Four chemically distinct basalts were cored in 44 m of basement penetration at Deep Sea Drilling Project Site 543, in Upper Cretaceous crust just seaward of the deformation front of the Barbados Ridge and north of the Tiburon Rise. All four types are moderately fractionated abyssal tholeiites. The four types have different magnetic inclinations, all of reversed polarity, suggesting eruption at different times which recorded secular variation of the earth's magnetic field. Extensive replacement of Plagioclase by K-feldspar has occurred at the top of the basalts, giving analyses with K2O contents up to 5 %. The earliest stages of alteration were dominantly oxidative, resulting in fractures lined with celadonite and dioctahedral smectite, and pervasive replacement of olivine and most intersertal glass with iron hydroxides and green clay minerals. Latef, non-oxidative alteration resulted in formation of olive-green clays and pyrite veins in a portion of the rocks. Basalts affected by this alteration actually lost K2O (to abundances lower than in adjacent fresh basalt glasses), and gained MgO (to abundances higher than in the glasses). Finally, fractures and interpillow voids were lined with calcite, sealing in much fresh glass. Oxygen-isotope measurements on the calcite indicate that this occurred at 12 to 25C. Either altering fluids were warm or the basalts had become buried with a considerable thickness of sediments, such that temperatures increased until a conductive thermal gradient was established, when the veining occurred.在位于巴巴多斯海脊变形前锋外侧、蒂布朗隆起北侧的上白垩统地壳中，深海钻探计划（Deep Sea Drilling Project）543站位的44米基底取芯段内，采集到4种化学特征截然不同的玄武岩。该4类玄武岩均为中等分异的深海拉斑玄武岩，且各自具有不同的反向极性磁倾角，表明它们是在不同时期喷发的，记录了地球磁场的长期变化。 玄武岩顶部发生了广泛的钾长石交代斜长石作用，使得样品的氧化钾（K₂O）含量最高可达5%。早期蚀变以氧化作用为主，表现为裂缝被绿鳞石和二八面体蒙脱石充填，同时橄榄石与大部分粒间玻璃被普遍置换为氢氧化铁与绿色粘土矿物。后期非氧化蚀变则使部分岩石中形成橄榄绿色粘土与黄铁矿脉。受此类蚀变影响的玄武岩，其氧化钾含量反而降低至低于相邻新鲜玄武岩玻璃的水平，而氧化镁（MgO）含量则升高至高于玻璃的水平。最终，裂缝与枕状熔岩间的空隙被方解石充填，封存了大量新鲜玻璃。对方解石开展的氧同位素测量结果显示，该过程发生于12~25℃的温度区间，这表明要么蚀变流体温度较高，要么玄武岩被巨厚沉积物掩埋，随着温度升高形成传导热梯度，进而发生了脉体充填作用。