Geochemistry of Cr-spinels from ODP Leg 127/128 basalts@en

Cr-rich spinels obtained from drill cores in the margins of heavily altered basaltic sills and flows contain a wealth of petrological information about the host magmas that crystallized them. These host igneous units, cored during Leg 127 of the Ocean Drilling Program in the Japan Sea, underwent extensive hydrothermal alteration that makes petrogenetic interpretations difficult. Most spinels in these samples exhibit minor or no zoning and are similar in composition to others reported from mid-ocean ridge basalt and backarc basin basalt tholeiites. Most show a range in Cr/(Cr + Al) of 0.26 to 0.62, a range in Mg/(Mg + Fe**2+) of 0.53 to 0.68, low TiO2 (<0.7%), and low Fe**3+/(Cr + Al + Fe**3+) (<0.1), which is consistent with crystallization from a tholeiitic liquid at a magmatic oxygen fugacity 2 log units or more below the nickel-nickel oxide oxygen buffer. By recasting the composition of these spinels in terms of an ideal reciprocal solution model and by deriving a Delta G(ex) for spinel-liquid Mg-Fe**2+ exchange, direct determination of magmatic Mg/(Mg + Fe**2+) is possible. The compositions of these Leg 127 spinels clearly show that some units underwent syn- and post-eruption enrichment in Mg/(Mg + Fe**2+) during hydrothermal alteration and diagenesis. This study proves that Cr-spinel is a unique and powerful tool for deciphering the original magmatic characteristics of heavily altered basalts.富铬尖晶石（Cr-rich spinels）采自日本海大洋钻探计划（Ocean Drilling Program）第127航次所钻取的、经强烈蚀变的玄武岩床与岩流边缘的钻孔岩心，蕴含着其寄主结晶岩浆的丰富岩石学信息。本次研究涉及的寄主火成岩单元均取自该航次的钻探岩心，经受过广泛的热液蚀变作用，这使得岩石成因解释难度大幅提升。该批样品中的绝大多数尖晶石仅发育微弱的成分环带或无环带，其化学成分与洋中脊玄武岩及弧后盆地玄武岩拉斑玄武岩中已报道的尖晶石较为相似。多数尖晶石的Cr/(Cr+Al)比值介于0.26至0.62之间，Mg/(Mg+Fe²+)比值介于0.53至0.68之间，二氧化钛（TiO₂）含量低于0.7%，且Fe³+/(Cr+Al+Fe³+)比值低于0.1，这与在比镍-镍氧化物氧缓冲剂低2个对数单位以上的岩浆氧逸度条件下，从拉斑质熔体中结晶的特征相一致。通过采用理想互溶溶液模型重构这批尖晶石的化学成分，并推导尖晶石-熔体间Mg-Fe²+交换反应的过剩吉布斯自由能（ΔG^ex），便可直接测定岩浆的Mg/(Mg+Fe²+)比值。大洋钻探计划第127航次所获尖晶石的化学成分清晰表明，部分寄主火成岩单元在热液蚀变及成岩作用过程中，发生了喷发同期与喷发后的Mg/(Mg+Fe²+)富集作用。本研究证实，富铬尖晶石是解译强烈蚀变玄武岩原始岩浆特征的独特且高效的研究工具。