Petrology of basalts from DSDP Hole 83-504B

Basalts from Hole 504B, Leg 83, exhibit remarkable uniformity in major and trace element composition throughout the 1075.5 m of basement drilled. The majority of the basalts, Group D', have unusual compositions relative to normal (Type I) mid-ocean ridge basalts (MORB). These basalts have relatively high mg values (0.60-0.70) and CaO abundances (11.7-13.7%; Ca/Al = 0.78-0.89), but exhibit a marked depletion in compatible trace elements (Cr and Ni); moderately incompatible trace elements (Zr, Y, Ti, etc.); and highly incompatible trace elements (Nb, LREE, etc.). Petrographic and compositional data indicate that most of these basalts are evolved, having fractionated significant amounts of plagioclase, olivine, and clinopyroxene. Melting experiments on similar basalt compositions from the upper portion of Hole 504B (Leg 70; Autio and Rhodes, 1983) indicate that the basalts are co-saturated with olivine and plagioclase and often clinopyroxene on the 1-atm. liquidus. […]

第83航次504B钻孔所获玄武岩，在已钻探的1075.5米基底地层中，主量元素与微量元素组成均表现出显著的均一性。其中绝大多数归入D'组的玄武岩，相较于常规（I型）洋中脊玄武岩（mid-ocean ridge basalt, MORB）具有独特的地球化学特征。这类玄武岩具有较高的镁值（0.60~0.70）与氧化钙（CaO）含量（11.7%~13.7%；钙铝比Ca/Al为0.78~0.89），但相容微量元素（铬Cr、镍Ni）显著亏损；中等不相容微量元素（锆Zr、钇Y、钛Ti等）以及强不相容微量元素（铌Nb、轻稀土元素（Light Rare Earth Element, LREE）等）均呈亏损状态。岩石学与地球化学组成数据显示，该类玄武岩大多经历了演化分异过程，发生了斜长石、橄榄石与单斜辉石的显著分离结晶作用。针对504B钻孔上部层位（第70航次；Autio与Rhodes，1983）中类似玄武岩组成开展的熔融实验表明，该类玄武岩在1个标准大气压的液相线条件下，可与橄榄石、斜长石及常见单斜辉石达到共饱和状态。[...]