Tracing recycled Li in the mantle: insights into mantle heterogeneities

Li isotope measurements (the ratio of the stable isotopes 7 and 6, expressed as d7Li) have a fundamental importance in the study of crust-mantle material cycling processes, due to the strong Li isotopic contrast between materials of mantle provenance (low d7Li 3 - 5%, and [Li] ~ 4 ppm) and those with near surface histories, such as altered oceanic crust (mean d7Li ~8%, and [Li] ~ 10 ppm) (Brenan, and Shaw, 1998, Seyfried and Chan, 1998, Chan and Teagle, 2001). Mantle containing recycled material should therefore have a heavy Li isotope composition. However, recent work has suggested that effect of recycling Li could be more complex than simply increasing the d7Li of the mantle. It has been proposed that dehydration during subduction fractionates the Li isotopic composition of the slab. Fluids carry heavy Li into the mantle wedge, leaving the deep recycled residue isotopically light (Tomascak, 2002, Zack et al, 2003). Thus isotopically heavy material is returned to the shallow mantle and light material deep recycled. It might be anticipated that a shallow MORB reservoir should hence evolve to higher d7Li than a deep OIB reservoir. A key to using Li isotopes as a tracer for recycled material is being able to measure Li isotopes more precisely than before, in order to distinguish the small-scale natural variations. This has been made possible with recent developments in plasma ionisation multi-collector mass spectrometry (PIMMS). The Thermo Finnigan Neptune, PIMMS shows excellent stability and sensitivity for Li. Using rapid sample-standard bracketing, Li isotopes can be measured with a ñ0.2% or better reproducibility, a factor of four better than most reported techniques. d7Li results from Iceland, Azores, La Palma (Canaries), show a limited range in Li isotopes d7Li 3-4.5, with similar values to a suite of MORB we have analysed (Elliot et al, 2002). However results from Pacific islands Mangaia, Rurutu, and Tubuai, which have a HIMU mantle signature, tend to show heavier d7Li up to 5.7%. These analyses were made on fresh, hand picked olivines to avoid potential alteration in these ~10My samples. HIMU OIB have frequently been proposed to be derived from sources with a large component of deep recycled, mafic oceanic crust, which recent work suggests should have a light Li isotope signature. We therefore propose that the heavy d7Li results from enriched mantle wedge, which contains the heavy d7Li residue from the slab dehydration event (Zack et al, 2003), and not the slab itself as previously thought.