Geochemistry of ODP Leg 115 basalts

Melting-phase relations at high pressures and Sr-Nd isotopic compositions are reported for basalts collected from the western Indian Ocean during Ocean Drilling Program Leg 115. Based on the concentrations of high-field-strength elements, we have subdivided the basalts into eight groups. A tholeiitic primary magma estimated using an olivine maximum fractionation model is representative of depleted lavas. This melt is in equilibrium with lherzolite minerals at 1.3 GPa and 1330°C under dry conditions. Also, an alkaline primary magma, representative of enriched lavas, is not saturated with orthopyroxene under dry conditions, but it is saturated with lherzolite minerals under CO2-saturated conditions at 1.7 GPa and 1350°C. These results imply that the tholeiitic magmas were segregated from mantle diapirs at shallower levels than the alkaline magmas. The highest 143Nd/144Nd value is obtained for the most depleted tholeiitic basalts, and the lowest value corresponds to the enriched alkaline basalt. The Sr isotopes of the basalts range from 0.70378 to 0.70449 and are inversely correlated with the Nd isotopic values. The present experimental and geochemical data suggest that depleted mantle material is underlain by the enriched material in the upper mantle beneath the region.

本研究报道了大洋钻探计划（Ocean Drilling Program）第115航次采自西印度洋的玄武岩的高压熔融相关系以及锶-钕（Sr-Nd）同位素组成。基于高场强元素（high-field-strength elements）的浓度，我们将这批玄武岩划分为8个组群。通过橄榄石最大分馏模型（olivine maximum fractionation model）估算得到的拉斑玄武质原生岩浆，可代表亏损型熔岩。该熔体在干态条件下、1.3 GPa与1330℃时与二辉橄榄岩（lherzolite）矿物达到平衡。此外，代表富集型熔岩的碱性原生岩浆在干态条件下未达到斜方辉石（orthopyroxene）饱和，但在1.7 GPa、1350℃的二氧化碳饱和条件下可与二辉橄榄岩矿物达到饱和。上述结果表明，拉斑玄武质岩浆相较于碱性岩浆，是在更浅的地幔底辟（mantle diapirs）环境中发生分离的。亏损程度最高的拉斑玄武质玄武岩测得最高的143Nd/144Nd比值，而最低比值则对应富集型碱性玄武岩。该批玄武岩的锶同位素比值范围为0.70378至0.70449，且与钕同位素数值呈负相关。本研究的实验与地球化学数据表明，研究区域下方的上地幔中，亏损地幔物质之下伏有富集地幔物质。