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 southern Indian Ocean, exhibit the largest range (e.g., 87Sr/86Sr=0.7047-0.7079) of isotopic compositions yet observed on a single oceanic island. Isotopic compositions are well correlated and are accompanied by systematic changes in incompatible trace element ratios, particularly those involving Nb. These variations are interpreted as resulting from mixing between two components. One is characterized by high 87Sr/86Sr, low 206Pb/204Pb and 143Nd/144Nd ratios, and negative Nb and Eu anomalies, and is derived ultimately from the upper continental crust. The other has lower 87Sr/86Sr, and higher 206Pb/204Pb and 143Nd/144Nd ratios, and lacks the depletions in Nb and Eu. Two possible compositions are considered for the low-87Sr/86Sr component of the source. The first is at the low-87Sr/86Sr end of the Heard Island data array, represented most closely by lavas from the Laurens Peninsula. However, trace element variations suggest that these lavas might not be representive of the Heard plume. The second is close to the low-87Sr/86Sr end of the isotopic array for lavas from the main volcano. In this case a lithospheric mantle origin is suggested for the Laurens Peninsula lavas. The relationships between isotopic data, major element compositions, and incompatible trace element ratios indicate that the continent-derived material is probably present in the mantle source, where it makes a maximum contribution of <4 wt.% for all but one Heard Island sample. However, if the Kerguelen Plateau is a submerged continental block, shallow-level contamination 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 consistent with sampling of a regional reservoir in which the same two components exist in variable proportions (generally 1-5 wt.% of the continent-derived component). The distinctive isotopic compositions of Indian Ocean MORB are consistent with mixing of a similar component into an Atlantic-or Pacific-like MORB mantle source. The relatively unradiogenic 206Pb/204Pb isotopic compositions of these 'enriched' Indian Ocean mantle components are unlike any present-day marine sediments and indicate that their source has had 238U/204Pb ratios (μ) much lower than typical upper continental crust for > 1 Ga. These ages pre-date the formation of Gondwana (∼600-∼130 Ma) and therefore do not support sediment subduction beneath Gondwana as the cause of enrichment in the sub-Indian Ocean mantle. We propose that the enrichment of Indian Ocean OIB sources was due to subduction of upper-crustal material beneath a Proterozoic precursor of Gondwana at ∼ 1-2 Ga. The enrichment of the Indian Ocean MORB sources could have had a similar origin, or could have been derived from sub-continental lithospheric mantle returned to the asthenospheric mantle, perhaps during the break-up of Gondwana (200-130 Ma). © 1994 Oxford University Press