Hafnium Isotope Constraints on the Longevity and Distribution of the Indian Ocean Mantle Signature

Hf isotope and trace element concentration data are presented for a suite of oceanic crustal basalts from 20 deep sea drilling sites in the Indian Ocean, located on 55 Ma to 155 Ma oceanic crust. Sr, Nd and Pb isotope data for these samples have been presented previously by other authors (Weis and Frey, JGR, 1996; Mahoney et al., J. Petrol., 1998; Zhang et al., J.Petrol., 2005). Old Indian Ocean (OIO) crustal basalts measured thus far span a range in epsilon Hf (eHf) values of +5.5 to +21.3 (corresponding eNd range = +2.7 to +10.4). Similar to most Indian Ocean MORB, nearly all of the of the OIO basalts lie on or above the Nd-Hf isotope "mantle array" line (i.e., the regression through all oceanic basalt Nd-Hf isotope data), with the most extreme samples lying 7.7 eHf units above and 1.7 eHf units below this line (these vertical departures are termed delta epsilon Hf or DeHf). Apart from samples that are products of enriched sources (i.e., eNd < +6), DeHf correlates with initial 87Sr/86Sr, Ba/Nb and Ce/Pb. Similar correlations are observed in Indian Ocean MORB and have been interpreted as mixing trends between a N. Atlantic/Pacific-type MORB source and an "Indian Ocean component" of continental origin (i.e., either subduction-modified mantle lithosphere from orogenic belts or lower continental crust; Janney et al., J. Petrol., in press, Hanan et al., Nature, 2004). Thus, our data indicate that distinct "Indian Ocean-type" Nd-Hf isotopic signatures have been present in the upper mantle beneath most of the Indian Ocean basin for at least the last 100 Myr. Samples of Tethyan oceanic crust (> 150 Ma) from Sites 261 and 765 lie near the mantle array in Nd-Hf isotope space (-2<DeHf<+2), largely overlapping with the field for Pacific/N. Atlantic MORB. Thus, they express at most a minor contribution from a high-DeHf Indian Ocean-type source component. This preliminary finding contrasts with Sr-Nd-Pb isotopic evidence requiring a major contribution from an Indian Ocean-type source for nearly all Tethyan oceanic crust, including that from Sites 261 and 765 (Zhang et al., J. Petrol., 2005). Further Hf isotope measurements of Tethyan ocean crust and ophiolite samples should help to clarify the time at which the Nd-Hf isotope signature of the Indian Ocean upper mantle developed.