Isotope and trace element variations in lavas from Raivavae and Rapa, Cook–Austral islands: constraints on the nature of HIMU- and EM-mantle and the origin of mid-plate volcanism in French Polynesia

We have examined the major and trace element and Sr–Nd–Hf–Pb–Os isotope variations in two suites of lavas from the islands of Raivavae and Rapa from the Cook–Austral chain to better constrain the nature and origin of the source(s) of these lavas. Combined with data from other Cook–Austral islands and seamounts, the Raivavae and Rapa samples reveal the presence of three distinct components involved in the genesis of these lavas, a HIMU component, a depleted (or DM) component, and an “enriched” (EM) component. Northern Cook–Australs lavas (those north of the Austral Fracture Zone (AFZ)) falling along the “Macdonald” pseudo-age progression reflect mixing of the HIMU and DM components, whereas lavas south of the AFZ reflect mixing of the DM and EM components. depletions in potassium and other fluid soluble elements in HIMU lavas suggest that the HIMU source contains ancient recycled (dehydrated) oceanic crust. The EM lavas have systematically less radiogenic Os-isotopes than the HIMU lavas. Therefore, the enriched or EM component cannot be generated through the addition of pelagic or terrigenous sediment to the HIMU source. Instead, the EM component in the southern Cook–Australs may represent recycled subduction-modified sub-arc mantle. High 208Pb/204Pb values in the DM component relative to East Pacific Rise mid-ocean ridge basalts (MORB) preclude derivation of this component from assimilated lithospheric mantle. The Sr–Nd–Pb isotope characteristics of the DM component resemble Pacific alkalic seamount lavas, and may derive from easily fusible components (e.g., pyroxenite veins) dispersed in the Pacific upper mantle. ivavae and Rapa lavas define a Hf–Nd isotope correlation that is considerably steeper than the mantle Hf–Nd isotope array. The unusually low εHf values (for a given εNd value) characteristic of the HIMU source are consistent with the presence of recycled oceanic crust. However, the presence of low εHf values in the Rapa EM lavas reveals that anomalously low εHf values are not restricted to the HIMU source. Regardless of the ultimate origin of the Raivavae and Rapa lavas, the comparatively low εHf values in these lavas indicate that multiple low-εHf components are present in the mantle. The addition of these components may help resolve the apparent terrestrial Hf–Nd mass-imbalance with respect to estimates of the bulk earth Nd- and Hf-isotope composition. s and seamounts from the Cook–Austral chain do not follow a clear age progression as predicted by the standard plume model for the origin of ocean island basalts (OIB). Previous models for the origin of the Cook–Austral chain have appealed to the existence of multiple small plumes. However, these models are implausible because they do not explain how several deep-seated plumes become aligned with each other in the direction of plate motion. We propose that volcanism along the Cook–Austral chain (and perhaps in much of French Polynesia) is generated through self-perpetuating melting anomalies that preferentially sample enriched components in a marble-cake mantle. These melting anomalies preferentially form or strengthen where lithospheric boundary conditions are favorable, such as at pre-existing transform faults or where the lithosphere has been thinned by a previous period of volcanism.