New Ar/Ar ages from the Ninetyeast Ridge, Indian Ocean: Beginning of a robust Indo-Atlantic hotspot reference frame

The Indo-Atlantic hotspot reference frame is one of the pillars of current plate reconstruction models. Most of the geodynamic literature (including the description of this Union session) assumes that this reference frame is relatively well-determined, at least for the last 80 Ma. Geochronological data sets -- principally Ar/Ar analyses of seamount and ocean island basalt -- are critical inputs to plate motion models. However, the quality of the existing Indo-Atlantic hotspot data sets, although sufficient for demonstrating general age progressive volcanism, is insufficient for robust determinations of absolute plate motion in general and tests of fixed hotspot frames in particular. Most of these Ar/Ar age determinations were made over 20 years ago; significantly better quality age data are now possible with the current generation of rare gas mass spectrometers and improved sample preparation techniques. Only after a significant number of new analyses on existing sample collections, and the collection of new sample suites where more detailed geographic coverage proves necessary, will we be able to accurately characterize the Indo-Atlantic hotspot reference frame, even for the last 80 Ma. Full development and testing of geodynamic models dependent on well-determined hotspot reference frame(s) -- fixed or moving -- will not be possible until this task is complete. Our research group is focusing on the Indian Ocean hotspot system, starting with the 5000 km long Ninetyeast Ridge (NER). Early work on NER basement cores recovered during DSDP Legs 22 and 26 and ODP Leg 121 showed generally age progressive volcanism, from greater than 80 Ma at Site 758 in the north to less than 40 Ma at Site 254 in the south. We have determined reliable Ar/Ar ages based on internally consistent incremental- heating age spectra from at least three aliquots of at least two distinct samples of volcanic basement from five of the six sites re-examined. The precision of our results is better than 0.5 m.y. at each site, i.e., less than the life span of typical hotspot shield volcanoes. Our new Ar/Ar ages show a remarkably linear age progression, from 77 Ma at Site 758 in the north to 42 Ma at Site 254 in the south. Most significantly, the overall rate of volcanic propagation along the ridge is 35 percent faster than previously suggested, necessitating a significant re- evaluation of current plate and hotspot motion models. The spatial resolution of the existing drill sites (6 sites along a 5000 km long ridge) is insufficient to test for likely spreading ridge jumps and microplate capture events, which also must be taken into account when deriving accurate models. However, we have recently recovered basalt from 23 of 33 new dredge sites along 3500 km of the NER, with a geographic distribution specifically designed to enable such tests. Further tests of hotspot models for the Indian hotspot reference frame as a whole will require a similar re-analysis of other volcanic lineaments, especially the Laccadives-Chagos Ridge.