Slab components in the Mariana arc: Light, heavy and novel

The isotopic composition of Li promised huge potential as a tracer of slab components in arcs. Altered oceanic crust and most sediments entering the subduction zone are heavy, fractional dehydration processes should only accentuate this signature in fluids released from the slab and arc lavas are elementally enriched in Li. Surprisingly, very few arcs show any indication of a heavy Li isotope signature. We have analysed several arcs in detail and find no evidence for a heavy slab component. Yet this negative result is important as it indicates considerable exchange between slab-derived fluids and the mantle wedge, likely aided by the high diffusivity of Li and the large Li reservoir represented by mantle olivine. Interacton of slab derived components with the mantle is an important consideration in accounting for the notable U-series disequilibria seen in island arc lavas and the Marianas in particular. A key observation is that the fluid component, linked by Pb isotope measurements to the mafic oceanic crust, has extreme (226Ra-230Th) and 238U-230Th excesses coupled with 235U-231Pa deficits. This ensemble of features can be modelled as a result of interaction of a slab component with the mantle at low porosities, although the magnitude of the (226Ra-230Th) excesses is not readily explained. Alternatively, residual accessory phases present during slab-dehydration may well have a key but currently poorly constrained role in shaping the U-series disequilibria of the fluid component. Such a process has been plausibly invoked to control many element budgets and likely plays a major role for the actinides, but currently there is dearth of information on the partitioning of Pa in key phases such as monazite or allanite. To this already heady cocktail of light stable and heavy, radiogenic isotopes, the future offers novel insights from transition element isotope measurements. Initial data in Mariana lavas show a range in δ97Mo/95Mo, with fluid rich samples some 0.3 heavier than sediment rich counterparts. Since pelagic sediments are typically heavier than Fe/Mn crusts on altered mafic crust, this sense of difference in Mo isotopes is intriguing and may attractively be related to the process of element transport