THE TERRESTRIAL LI ISOTOPE CYCLE: LIGHT-WEIGHT CONSTRAINTS ON MANTLE CONVECTION

Abstract

The two stable isotopes of Li are significantly fractionated by exchange reactions with clays near the Earth’s surface. The isotopic legacy of this process provides a robust tracer of surface material that is returned (recycled) to the mantle. Altered oceanic crust has a heavy Li isotopic composition (high 7Li/6Li). Heterogeneous distribution of subducted, altered oceanic crust in the mantle will result in variations in Li isotope ratios. A rapidly accumulating dataset of Li isotope analyses on mantle-derived materials indeed indicates a significant range in Li isotope ratios. This observation provides powerful evidence for the widespread distribution of recycled material in the convecting mantle. There is substantial overlap in Li isotopic compositions of ocean island basalts (OIB) and mid-ocean ridge basalts (MORB). Some OIB, however, have slightly heavier Li compositions than typical, depleted MORB. At face value this suggests a larger contribution of recycled oceanic crust in the sources of some OIB than in the upper mantle. Yet recent evidence implies that heavy Li is lost from the slab at subduction zones and the recycled residual crust is left with an isotopically light signature. Extremely light Li isotope ratios are observed in some continental mantle xenoliths but not in OIB proposed to contain large proportions of recycled crust. Studies of subduction zone lavas imply the heavy Li isotope signature of altered oceanic crust is transferred to the mantle above the subducting plate. Thus cycling of sub-arc mantle may be an important process in forming chemical heterogeneities sampled by OIB. Inferences from initial Li isotope results are thus notably different from those from the longer established but more equivocal radiogenic