The tectonic origins of the Ninetyeast Ridge (NER) will be investigated from geochrnonological/isotopic (Ar/Ar) measurements and from geochemical (Sr, Nd, Hf, and Pb isotopes) analyses of basalt samples collected from 7 DSDP and ODP drill sites, and from 23 dredged samples collected from NER in 2007. These analyses will address the origins of hotspots as the NER and if they are sourced beneath the ridge in the deep mantle or more shallow in the mid-upper mantle or sublithosphere. The data generated from this project will help advance geodynamical models of hotspots.
Broader impacts: The broader impacts were viewed as limited in scope but will provide educational benefit and training for undergraduate and graduate students, and a JOI-sponsored middle school teacher.
Volcanoes occur in three different environments: Regions of plate divergence, such as spreading ridges on the ocean floor, such as the Mid-Atlantic Ridge, regions of plate convergence, such as the Aleutian Islands, and the Cascade volcanoes in northwest USA; and a third type, intraplate volcanoes that are not usually associated with plate boundaries, such as those forming the Hawaiian Islands,. In this research effort we studied lavas forming a 5000 km long north-south chain of volcanoes, the Ninetyeast Ridge, on the eastern Indian Ocean seafloor. This volcanic chain is formed of intraplate volcanoes. Although now below sea level the volcanoes forming this ridge were oceanic islands. Our geochemical studies of lavas collected by drilling and dredging on the Ninetyeast Ridge led to two major conclusions: (1) The eruption ages, determined by Ar geochronology, increase systematically from south (43 million years) to north (77 million years). This systematic age progression showed that the Ninetyeast Ridge formed as the Indian plate moved rapidly northward over a relatively stationary source of magma, commonly designated a hotspot. In this case, the Kerguelen hotspot, now forming the Kerguelen Archipelago and Heard Island in the southeast Indian Ocean. (2) The geochemical characteristics of the lavas forming the Ninetyeast Ridge region require that the magmas were derived from compositionally heterogeneous sources. This conclusion is consistent with results for other oceanic island volcanoes, e.g., Hawaiian lavas. A new result arising from our study is that a garnet-bearing residue of partial melting was an important source component for Ninetyeast Ridge lavas, but this melting process preceded the hotspot melting process by 200 to 300 million years. The geochemical studies at MIT involved a graduate student and three undergraduate students, a research scientist and a professor. Also part of the geochemical team were a graduate student and professor from the University of British Columbia and a research scientist at Harvard University. Scientists from Texas A&M University, India and Russia pursued complementary geophysical research.
