Journal of Petrology, Volume 39, Issue 7: July 1 1998.

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

J. J. MAHONEY¹, R. FREI², M. L. G. TEJADA¹, X. X. MO³, P. T. LEAT⁴ AND T. F. NÄGLER⁵¹SCHOOL OF OCEAN AND EARTH SCIENCE AND TECHNOLOGY, UNIVERSITY OF HAWAII, HONOLULU, HI 96822, USA ²GEOLOGISK INSTITUT, KOBENHAVNS UNIVERSITET, KOBENHAVN, DENMARK ³GRADUATE SCHOOL, CHINA UNIVERSITY OF GEOSCIENCES, BEIJING 100 083, CHINA ⁴BRITISH ANTARCTIC SURVEY, HIGH CROSS, MADINGLEY ROAD, CAMBRIDGE CB3 0ET, UK ⁵MINERALOGISCH-PETROGRAPHISCHES INSTITUT, UNIVERSITÄT BERN, ERLACHSTR. 9A, CH-3012 BERN, SWITZERLAND

RECEIVED JUNE 9, 1997; REVISED TYPESCRIPT ACCEPTED FEBRUARY 3, 1998

The isotopic difference between modern Indian Ocean and Pacific or North Atlantic Ocean ridge mantle (e.g. variably lower ²⁰⁶Pb/²⁰⁴Pb for a given [epsilon]_Ndand 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 [epsilon]_Nd(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 [epsilon]_Nd units for basalts in the 60-80 Ma range vs 15 [epsilon]_Nd 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.

Keywords: mantle geochemistry;old Indian Ocean;Tethyan crust

Pages 1285-1306

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