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.