SOUTHWESTERN LIMITS OF INDIAN-OCEAN RIDGE MANTLE AND THE ORIGIN OF LOW PB-206 PB-204 MIDOCEAN RIDGE BASALT - ISOTOPE SYSTEMATICS OF THE CENTRAL SOUTHWEST INDIAN RIDGE (17-DEGREES-E-50-DEGREES-E)

Basalts from the Southwest Indian Ridge reflect a gradual, irregular isotopic transition in the MORB (mid-ocean ridge basalt) source mantle between typical Indian Ocean-type compositions on the east and Atlantic-like ones on the west. A probable southwestern limit to the huge Indian Ocean isotopic domain is indicated by incompatible-element-depleted MORBs from 17-degrees to 26-degrees-E, which possess essentially North Atlantic- or Pacific-type signatures. Superimposed on the regional along-axis gradient are at least three localized types of isotopically distinct, incompatible-element-enriched basalts. One characterizes the ridge between 36-degrees and 39-degrees-E, directly north of the proposed Marion hotspot, and appears to be caused by mixing between hotspot and high epsilon(Nd), normal MORB mantle; oceanic island products of the hotspot itself exhibit a very restricted range of isotopic values (e.g., Pb-206/Pb-204 = 18.5-18.6) which are more MORB-like than those of other Indian Ocean islands. Between 39-degrees and 41-degrees-E, high Ba/Nb lavas with unusually low Pb-206/Pb-204 (16.87-17.44) and epsilon(Nd) (-4 to +3) are dominant; these compositions are not only unlike those of the Marion (or any other) hotspot but also are unique among MORBs globally. Incompatible-element-enriched lavas in the vicinity of the Indomed Fracture Zone (approximately 46-degrees-E) differ isotopically from those at 39-degrees-41-degrees-E, 36-degrees-39-degrees-E, and both the Marion and Crozet hotspots. Thus, no simple model of ridgeward flow of plume mantle can explain the presence or distribution of all the incompatible-element-enriched MORBs on the central Southwest Indian Ridge. The upper mantle at 39-degrees-41-degrees-E, in particular, may contain stranded continental lithosphere, thermally eroded from Indo-Madagascar in the middle Cretaceous. Alternatively, the composition of the Marion hotspot must be grossly heterogeneous in space and/or time, and one of its intrinsic components must have substantially lower Pb-206/Pb-204 than yet measured for any hotspot. The origin of the broadly similar but much less extreme isotopic signatures of MORBs throughout most of the Indian Ocean could be related to the initiation of the Marion, Kerguelen, and Crozet hotspots, which together may have formed a more than 4400-km-long band of juxtaposed plume heads beneath the nearly stationary lithosphere of prebreakup Gondwana.