Isotope ratios of basalts from ODP Leg 115 holes (Table 1)

Strontium, neodymium, and lead isotope ratios are reported for 13 Leg 115 basalts as well as 3 basalts from Texaco drill hole SM-1 on the Mascarene Plateau. The 87Sr/86Sr ratios and eNd range from 0.70330 to 0.70439 and 5.5 to 7.4, respectively, although 87Sr/86Sr ratios higher than 0.70383 are found only in SM-1 basalts. The high 87Sr/86Sr values are thought to reflect seawater Sr in secondary phases, although all samples were strongly leached in HC1 before analysis. 206Pb/204Pb ratios range from 18.53 to 18.80, and sho high 207Pb/204Pb and 208Pb/204Pb ratios relative to 206Pb/204Pb ratios, typical of Indian Ocean mid-ocean ridge (MORB) and oceanic-island basalts (OIB). Isotopic compositions of Leg 115 basalts generally fall between fields for MORB and Reunion Island basalts, consistent with the conclusion drawn from geochronological studies that Deccan flood basalt volcanism, the Chagos-Laccadive Ridge, and the Mascarene Plateau are all products of the Reunion mantle plume. Isotopic compositions of magmas produced by this plume have varied systematically with time in the direction of less "depleted," less MORB-like isotopic signatures. This compositional change has been accompanied by a decrease in eruption rate. We interpret Deccan volcanism as the voluminous beginning of the plume. Reduced entrainment of asthenosphere following melting of the plume head resulted in less MORB-like isotope ratios in magmas and a decrease in eruptive activity with time.

本数据集报道了马斯卡林海台（Mascarene Plateau）德士古（Texaco）SM-1钻孔的3件玄武岩，以及13件第115航次玄武岩的锶、钕与铅同位素比值。其87Sr/86Sr比值与εNd值分别介于0.70330~0.70439与5.5~7.4之间；仅在SM-1钻孔玄武岩中发现87Sr/86Sr比值高于0.70383的样品。尽管所有样品在分析前均经浓盐酸（HCl）充分淋滤，但高87Sr/86Sr比值被认为反映了次生矿物相中海水锶的混入。206Pb/204Pb比值介于18.53~18.80之间，且相对于206Pb/204Pb比值，207Pb/204Pb与208Pb/204Pb比值偏高，该特征与印度洋洋中脊玄武岩（MORB, mid-ocean ridge basalt）及洋岛玄武岩（OIB, oceanic-island basalt）的典型同位素特征一致。第115航次玄武岩的同位素组成总体上介于洋中脊玄武岩与留尼旺岛玄武岩的同位素值域之间，这与地质年代学研究得出的结论相符：德干洪流玄武岩火山作用、查戈斯-拉克代夫海岭与马斯卡林海台均为留尼旺地幔柱的产物。该留尼旺地幔柱形成的岩浆同位素组成随时间呈系统性演化，逐渐向“亏损程度更低”、更偏离洋中脊玄武岩型同位素信号的方向转变。这一成分演化过程伴随有喷发速率的下降。我们将德干火山作用视作该地幔柱活动的巨量初始阶段。地幔柱头部熔融后，软流圈的夹带混入作用减弱，使得岩浆的同位素比值更偏离洋中脊玄武岩型特征，且喷发活动随时间逐步衰减。