40Ar-39Ar plateau and isochron ages from the Ninetyeast and Broken Ridges (Table 1)

Concordant plateau and isochron ages are obtained from 40Ar-39Ar incremental heating experiments on volcanic rocks recovered by drilling at three Leg 121 sites along the Ninetyeast Ridge and two dredge locations on the southern scarp of the Broken Ridge, eastern Indian Ocean. The new data confirm a northerly increase in the age of volcanism along the Ninetyeast Ridge, from 38 to 82 Ma; this lineament links current hotspot volcanism near the Kerguelen islands with the Rajmahal flood basalt eruptions at M0 time (117 ± 1 Ma). The Broken Ridge was formed over the same hotspot at 88-89 Ma, but later experienced rift-related volcanism in Paleocene time (63 Ma). The geometry and distribution of ages along these prominent volcanic ridges and the Mascarene-Chagos-Laccadive-Maldive ridge system in the western Indian Ocean are most compatible with plate motions over fixed hotspots near Kerguelen and Reunion islands, respectively.

本研究通过对东印度洋九十度海岭（Ninetyeast Ridge）121航次三个钻探站位以及破碎海岭（Broken Ridge）南部陡坡两处拖网采样获取的火山岩开展40Ar-39Ar增量加热实验（40Ar-39Ar incremental heating experiments），获得了一致的坪年龄与等时年龄。新数据证实，九十度海岭的火山活动年龄随纬度向北递增，范围为38 Ma至82 Ma；该线状构造将凯尔盖朗群岛（Kerguelen Islands）附近现今的热点火山活动，与M0期（117±1 Ma）的拉贾马哈洪水玄武岩（Rajmahal flood basalt）喷发事件联系起来。破碎海岭形成于约88~89 Ma的同一热点区域，后续在古新世（Paleocene）发生了与裂谷作用相关的火山活动。上述典型火山海岭以及西印度洋马斯克林-查戈斯-拉克沙德维-马尔代夫海岭（Mascarene-Chagos-Laccadive-Maldive ridge system）的几何形态与年龄分布特征，分别与凯尔盖朗与留尼汪群岛（Reunion Islands）附近固定热点之上的板块运动模式最为吻合。