LOOKING AT OCEAN ISLANDS FROM THE BOTTOM: THE OSMIUM ISOTOPIC COMPOSITION OF SAVAI’I AND TUBUAI’I (SAMOA) MANTLE XENOLITHS

Ocean islands are one of the best probes of Earth’s deep mantle. Hawaii, Earth's major hotspot, has been a key scientific laboratory for the productive career of Fred Frey of MIT. In early important work, Frey used its basalts to understand mantle source enrichment and melting, inverting trace elements through melting models [1]. Even earlier, Frey analyzed Hawaiian mantle xenoliths for the rare earth element partitioning between clinopyroxene and garnet and its relationship to Hawaiian lavas [2] showing how valuable xenoliths are in ocean island geochemical studies.

A suite of 25 spinel peridotites from the islands of Savai’i and Tubuai’i hosted in alkali basalts were analyzed for their osmium isotopic compositions, and major and trace elements. The goal is to examine their mantle source compositions for domains that record ancient depletion events that were recycled along with a component of continental and oceanic crust present in the host basalts [3]. In Samoan xenoliths, Re abundances are typically low (0.0003 to 0.035 ppb), but Re abundances are higher in the Tubaui xenoliths (0.030 to 0.354 ppb). Os abundances are also typically low (<1ppb) but range up to values exceeding 5 ppb in both suites. Osmium isotopic compositions are typical of the convecting mantle (¹⁸⁷Os/¹⁸⁸Os = 0.1240 to 0.1304), but several xenoliths from Samoa have isotopic compositions <0.120 while just one xenolith from Tubuai'i has such low isotopic compositions. The xenoliths are thought to originate in the oceanic lithospheric mantle. The ¹⁸⁷Os/¹⁸⁸Os therefore records preexisting heterogeneities in the upper mantle “locked into” the lithosphere at the mid-ocean ridge, and that may have been modified by the metasomatic processes associated with the upwelling plume [4]. Carbonatite signatures are clearly recorded in the trace element signatures of the xenoliths, and the carbonatite metasomatism may have modified the Os-isotopic composition of the xenoliths so that they record slightly more radiogenic values. The unradiogenic Os component in the xenoliths may relate to subcontinental lithospheric mantle or residues of ancient depletion events that are preserved in the convecting mantle.

[1] Clague and Frey, 1982, J Pet 23, 447. [2] Reid and Frey, 1971, JGR 76, 1184. [3] Jackson et al., 2007, Nature 448, 684. [4] Hauri et al., 1993, Nature 365, 221.