Osmium Isotopes in the Convecting Mantle:
Constraints on Magma Transport, Slab Recycling,
and Core-Mantle Interaction

Erik H. Hauri DTM, 5241 Broad Branch Rd., NW, Washington, DC 20015, USA


The systematics of the Re-Os isotope system in basalts and peridotites from oceanic hotspot regions are beginning to provide a well-constrained view of the degree to which hotspot magmas interact with the oceanic lithosphere during ascent. New Os isotope data for shield-stage lavas from Hawaiian volcanoes, combined with numerical modeling of diffusive and reactive melt transport, indicate only a minor contribution from the depleted MORB mantle (DMM) during 95% of Hawaiian volcano construction. The excellent correlation of Os isotopes with other isotopes indicates that "chromatographic" element fractionations during melt transport are not important; otherwise Os (D=10) would be completely decoupled from the incompatible isotopic tracers (D=0.01=0.1). Due to the absence of "chromatographic" isotope signatures in these basalts, the minor DMM component is probably not derived from the oceanic mantle lithosphere, but rather is added (by simple mixing) as melts of the DMM asthenosphere surrounding the Hawaiian plume. This minor DMM influence is most apparent in "Kea Trend" volcanoes, which probably sample the periphery of the Hawaiian plume. The full range of isotopic variability is observed in "Loa Trend" volcanoes located closer to the plume axis. In these volcanoes, the DMM component is almost non-existent and the variability is attributed to heterogeneity within the Hawaiian plume. Isotopic data (including Os) for Loa Trend volcanoes is correlated with major and trace element signatures, and indicate bulk compositional heterogeneity in the Hawaiian mantle which is consistent with the presence of a substantial amount (tens of percent) of a recycled oceanic crustal component.

The new results at Hawaii have a general applicability to other hotspots. Evidence of "chromatographic" melt migration is present in the trace element and isotopic data for mantle xenoliths from hotspots; however, the amount of mass exchanged during this process is insufficient to influence significantly the chemistry of shield-stage basalts (even for compatible elements like Ni and Os). The Os isotope signature of magma-lithosphere interaction is observed only in the small-volume, late-stage alkalic lavas (often containing mantle xenoliths) and in highly-differentiated lavas (crustal contamination). As a result, primitive shield-stage hotspot magmas from oceanic regions faithfully record the chemical variability present within mantle plumes, and the consistently high Os isotope ratios measured in such lavas suggest the presence of a ubiquitous component of recycled oceanic crust (and corresponding major element variability) within modern mantle plumes.

Rhenium and osmium, by virtue of their siderophile behavior, also have the potential to place constraints on mass exchange between the mantle and core, as suggested by Walker and co-workers to explain the radiogenic nature of OIB compared to DMM. During partial melting and fractional crystallization, rhenium behaves as a moderately incompatible element, similar to HREE and Ti. Re/Ti and Re/HREE ratios are consistently lower in OIB compared to MORB and are uncorrelated with Os isotopes. These observations are opposite to those expected from core exchange with OIB mantle and isolation of the core from DMM. Further tests are underway, but the behavior of Re suggests that core-mantle exchange is limited to <1%.