Tracing the Indian Ocean Mantle Domain Through Time: Isotopic Results from Old West Indian, East Tethyan, and South Pacific Seafloor

摘要

The isotopic difference between modern Indian Ocean and Pacific or North Atlantic Ocean ridge mantle (e.g. variably lower 206Pb/204Pb for a given εNd and 208Pb/204Pb) could reflect processes that occurred within a few tens of millions of years preceding the initial breakup of Gondwana. Alternatively, the Indian Ocean isotopic signature could be a much more ancient upper-mantle feature inherited from the asthenosphere of the eastern Tethyan Ocean, which formerly occupied much of the present Indian Ocean region. Age-corrected Nd, Pb, and Sr isotopic data for 46-150 Ma seafloor lavas from sites in the western Indian Ocean and ocean-ridge-type Tethyan ophiolites (Masirah, Yarlung-Zangpo) reveal the presence of both Indian-Ocean-type compositions and essentially Pacific-North Atlantic-type signatures. In comparison, Jurassic South Pacific ridge basalts from Alexander Island, Antarctica, possess normal Pacific-North Atlantic-type isotopic ratios. Despite the very sparse sampling of old seafloor, the age-corrected εNd(t) values of the old Indian Ocean basalts cover a greater range than seen for the much more thoroughly sampled present-day spreading axes and islands within the Indian Ocean (e.g. 18 εNd units for basalts in the 60-80 Ma range vs 15 εNd units for 0-10 Ma ones). The implications of these results are that the upper mantle in the Indian Ocean region is becoming increasingly well mixed through time, and that the Indian Ocean mantle domain may not greatly pre-date the age of earliest spreading in the Indian Ocean