Geochemistry and petrogenesis of basalts from DSDP Leg 92 holes@en

Basalts recovered on DSDP Leg 92 include all the major basalt types so far recovered from the ocean crust of the eastern Pacific. Basalts from Holes 597, 597A, 597B, 597C, and 599B are tholeiites exhibiting all the mineralogical and geochemical characteristics of N-type mid-ocean ridge basalts (MORB). Fragments of ferrobasalts and alkali basalts were also obtained, however, from Holes 60IB and 602B, respectively. Hole 597C, which penetrated 91 m into basement and is the deepest hole so far drilled in fast-spreading crust, yielded basalts that can be divided into three major lithologic units. The lowest unit, Unit III, contains modal olivine and comprises basalts which, at about 8 to 10% MgO, are as basic as any sampled from fast-spreading crust. The middle unit, Unit II, is the most evolved; its basalts are olivine free and contain between 6 and 7.5% MgO. The upper unit, Unit I, is intermediate in composition between Units II and III; it is characterized by both modal olivine and glomerocrysts made up of plagioclase and rare olivine. Unit I is probably a massive flow, whereas Units II and III may be massive flows or sills. The basalts appear to have undergone three stages of alteration (\"deuteric,\" \"relatively reducing,\" and \"oxidizing\"), the intensity of alteration decreasing markedly downcore. Hole 597B, at 26.4 m of basement penetration the only other \"deep\" hole, contains just one lithologic unit, which closely resembles Unit I of Hole 597C. […]深海钻探计划（Deep Sea Drilling Project, DSDP）第92航次回收的玄武岩，涵盖了迄今从东太平洋洋壳获取的全部主要玄武岩类型。597、597A、597B、597C及599B钻孔所获玄武岩均为拉斑玄武岩，具备N型洋中脊玄武岩（Mid-Ocean Ridge Basalt, MORB）的全部矿物学与地球化学特征。不过，研究人员分别从60IB孔与602B孔采获了铁质玄武岩和碱性玄武岩岩块。597C孔钻进洋壳基底深度达91米，是迄今在快速扩张洋壳中完成的最深钻井，其产出的玄武岩可划分为3个主要岩性单元。最低部单元（单元III）含橄榄石实际矿物，所对应玄武岩的MgO含量约为8%~10%，与快速扩张洋壳中采样的任何玄武岩同样基性。中部单元（单元II）演化程度最高，其玄武岩不含橄榄石，MgO含量介于6%~7.5%之间。上部单元（单元I）的成分介于单元II与单元III之间，以同时含有橄榄石实际矿物以及由斜长石和稀少橄榄石组成的聚斑晶为特征。单元I大概率为块状熔岩流，而单元II与单元III则可能为块状熔岩流或岩床。该批玄武岩似乎经历了三期蚀变作用："自变质"、"相对还原"与"氧化"，蚀变强度随岩心深度增加显著降低。597B孔是另一个"深"钻孔，其洋壳基底钻进深度为26.4米，仅包含一个岩性单元，该单元与597C孔的单元I极为相似。[…]