GEOCHEMISTRY OF HEARD-ISLAND (SOUTHERN INDIAN-OCEAN) - CHARACTERIZATION OF AN ENRICHED MANTLE COMPONENT AND IMPLICATIONS FOR ENRICHMENT OF THE SUB-INDIAN OCEAN MANTLE

Lavas from Heard Island, located on the Kerguelen Plateau in the south ern Indian Ocean, exhibit the largest range (e.g., Sr-87/Sr-86 = 0.704 7-0.7079) of isotopic compositions yet observed on a single oceanic is land. Isotopic compositions are well correlated and are accompanied by systematic changes in incompatible trace element ratios, particularly those involving Nb. These variations are interpreted in incompatible trace element ratios, particularly those involving Nb. These variation s are interpreted as resulting from mixing between two components. One is characterized by high Sr-87/Sr-86, low Pb-206/Pb-204 and Nd-143/Nd -144 ratios, and negative Nb and Eu anomalies, and is derived ultimate ly from the upper continental crust. The other has lower Sr-87/Sr-86, and higher Pb-206/Pb-204 and Nd-143/N-144 ratios, and lacks the deplet ions in Nb and Eu. Two possible compositions are considered for the lo w-Sr-87/Sr-86 component of the source. The first is at the low-Sr-87/S r-86 end of the Heard Island data array, represented most closely by l avas from the Laurens Peninsula. However, trace element variations sug gest that these lavas might not be representive of the Heard plume. Th e second is close to the low-Sr-87/Sr-86 end of the isotopic array for lavas from the main volcano. In this case a lithospheric mantle origi n is suggested for the Laurens Peninsula lavas. The relationships betw een isotopic data, major element compositions, and incompatible trace element ratios indicate that the continent-derived material is probabl y present in the mantle source, where it makes a maximum contribution of < 4 wt.% for all but one Heard Island sample. However, if the Kergu elen Plateau is a submerged continental block, shallow-level contamina tion cannot be ruled out. The binary mixing model developed to explain the Heard Island geochemical variations is extended to include other Indian Ocean oceanic island and mid-ocean ridge basalts (OIB and MORB) . We show that isotopic compositions of Indian Ocean OIB are consisten t with sampling of a regional reservior in which the same two componen ts exist in variable proportions (generally 1-5 wt.% of the continent- derived component). The distinctive isotopic compositions of Indian Oc ean MORB are consistent with mixing of a similar component into an Atl antic- or Pacific-like MORB mantle source. The relatively unradiogenic Pb-206/Pb-204 isotopic compositions of these 'enriched' Indian Ocean mantle components are unlike any present-day marine sediments and indi cate that their source has had U-238/Pb-204 ratios (mu) much lower tha n typical upper continental crust for > 1 Ga. These ages pre-date the formation of Gondwana (approximately 600-approximately 130 Ma) and the refore do not support sediment subduction beneath Gondwana as the caus e of enrichment in the sub-Indian Ocean mantle. We propose that the en richment of Indian Ocean OIB sources was due to subduction of upper-cr ustal material beneath a Proterozoic precursor of Gondwana at approxim ately 1-2 Ga. The enrichment of the Indian Ocean MORB sources could ha ve had a similar origin, or could have been derived from sub-continent al lithospheric mantle returned to the asthenospheric mantle, perhaps during the break-up of Gondwana (200-130 Ma).