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 inthe southern Indian Ocean, exhibit the largest range (e.g.,⁸⁷Sr/⁸⁶Sr=0.7047–0.7079) of isotopic compositions yetobserved on a single oceanic island. Isotopic compositions arewell correlated and are accompanied by systematic changes inincompatible trace element ratios, particularly those involvingNb. These variations are interpreted as resulting from mixingbetween two components. One is characterized by high ⁸⁷Sr/⁸⁶Sr,low ²⁰⁶Pb/²⁰⁴Pb and ¹⁴³Nd/¹⁴⁴Nd ratios, and negative Nb andEu anomalies, and is derived ultimately from the upper continentalcrust. The other has lower ⁸⁷Sr/⁸⁶Sr, and higher ²⁰⁶Pb/²⁰⁴Pband ¹⁴³Nd/¹⁴⁴Nd ratios, and lacks the depletions in Nb and Eu.Two possible compositions are considered for the low-⁸⁷Sr/⁸⁶Srcomponent of the source. The first is at the low-⁸⁷Sr/⁸⁶Sr endof the Heard Island data array, represented most closely bylavas from the Laurens Peninsula. However, trace element variationssuggest that these lavas might not be representive of the Heardplume. The second is close to the low-⁸⁷Sr/⁸⁶Sr end of the isotopicarray for lavas from the main volcano. In this case a lithosphericmantle 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-derivedmaterial is probably present in the mantle source, where itmakes a maximum contribution of <4 wt.% for all but one HeardIsland sample. However, if the Kerguelen Plateau is a submergedcontinental block, shallow-level contamination cannot be ruledout. The binary mixing model developed to explain the Heard Islandgeochemical variations is extended to include other Indian Oceanoceanic island and mid-ocean ridge basalts (OIB and MORB). Weshow that isotopic compositions of Indian Ocean OIB are consistentwith sampling of a regional reservoir in which the same twocomponents exist in variable proportions (generally 1–5wt.% of the continent-derived component). The distinctive isotopiccompositions of Indian Ocean MORB are consistent with mixingof a similar component into an Atlantic-or Pacific-like MORBmantle source. The relatively unradiogenic ²⁰⁶Pb/²⁰⁴Pb isotopiccompositions of these ‘enriched’ Indian Ocean mantlecomponents are unlike any present-day marine sediments and indicatethat their source has had ²³⁸U/²⁰⁴Pb ratios (µ) much lowerthan typical upper continental crust for > 1 Ga. These agespre-date the formation of Gondwana (600-130 Ma) and thereforedo not support sediment subduction beneath Gondwana as the causeof enrichment in the sub-Indian Ocean mantle. We propose thatthe enrichment of Indian Ocean OIB sources was due to subductionof upper-crustal material beneath a Proterozoic precursor ofGondwana at 1–2 Ga. The enrichment of the Indian OceanMORB sources could have had a similar origin, or could havebeen derived from sub-continental lithospheric mantle returnedto the asthenospheric mantle, perhaps during the break-up ofGondwana (200–130 Ma).