Mineral composition of mid-ocean-ridge basalts from ODP Leg 187 sites@en

The chemical compositions of olivine, plagioclase, pyroxene, and spinel in lavas collected during Ocean Drilling Program Leg 187 in the Australian Antarctic Discordance, Southeast Indian Ridge (41°-46°S, 126°-135°E) were analyzed, and modeling of the theoretical equilibrium petrogenetic conditions between olivine and melt was conducted. The cores of larger olivine phenocrysts, particularly in the isotopic Indian-type mid-ocean-ridge basalt (MORB), are not equilibrated with melt compositions and are considered to be xenocrystic. Larger plagioclase phenocrysts with compositionally reversed zonation are also xenocrystic. The compositions of primary magma were calculated using a \"maximum olivine fractionation\" model for primitive MORB that should fractionate only olivine. Olivine compositions equilibrated with calculated primary magma and compositions of calculated primary magma suggest that (1) isotopic Pacific-type MORB is more fractionated than Indian-type MORB, (2) Pacific-type MORB was produced by higher degrees of partial melting than Indian-type MORB, and (3) primary magma for Indian-type MORB was segregated from mantle at 10 kbar (~30 km depth), whereas that for Pacific-type MORB was segregated at 15 kbar (~45 km depth).

对东南印度洋中脊（41°S~46°S，126°E~135°E）澳大利亚南极不整合带大洋钻探计划（Ocean Drilling Program）第187航次采集的熔岩中的橄榄石、斜长石、辉石与尖晶石的化学成分进行了分析，并开展了橄榄石与熔体之间理论平衡成岩条件的模拟研究。较大的橄榄石斑晶——尤其是同位素特征为印度洋型洋中脊玄武岩（mid-ocean-ridge basalt, MORB）的样品中的橄榄石斑晶——的核部并未与熔体成分达到平衡，被认为是捕虏晶。具有成分反环带的较大斜长石斑晶同样属于捕虏晶。针对仅发生橄榄石分异的原始洋中脊玄武岩，采用“橄榄石最大分异”模型计算了原生岩浆的化学成分。通过与计算得到的原生岩浆达到平衡的橄榄石成分以及计算得到的原生岩浆成分，可得到以下三点结论：（1）同位素特征为太平洋型的洋中脊玄武岩比印度洋型洋中脊玄武岩分异程度更高；（2）太平洋型洋中脊玄武岩的部分熔融程度高于印度洋型洋中脊玄武岩；（3）印度洋型洋中脊玄武岩的原生岩浆是在10千巴（约30千米深度）的条件下从地幔中分离而出，而太平洋型洋中脊玄武岩的原生岩浆则是在15千巴（约45千米深度）的条件下分离而出。