Beryllium sediment subduction

This chapter focuses on subduction zone processes and their implications for mantle composition. It examines subduction contributions to the shallow mantle that may be left behind in the wedge following arc magma genesis, as well as the changing composition of the slab as it is processed beneath the fore-arc, volcanic front and rear arc on its way to the deep mantle. Much of this chapter uses boron and the beryllium isotopes as index tracers boron, because it appears to be completely recycled in volcanic arcs with little to none subducted into the deep mantle, and cosmogenic e, with a 1.5 Ma half-life, because it uniquely tracks the contribution from the subducted sediments. 

MORE melting and dehydration in serpentinized peridotite was illustrated in Section 3.17.5.4. Here, an example of the effects on trace-element transfer as a function of fluid rock ratios is illustrated. The layered structure of the oceanic lithosphere may cause the fluid rock ratio in the sedimentary layer to be greater than one (as fluids from the underlying mafic and peridotitic layers must rise through the sediments). Thus, some of the trace elements (e.g., beryllium, thorium) commonly considered to be only efficiently mobilized by melts could also be quite effectively mobilized by fluids, if the entire subducted lithosphere is considered. 

Recently, an apparent contradiction was put forward to argue for melting of sediments contemporaneously with dehydration of MORE. It was estimated that >30-40% of the subducted beryllium and thorium, which are strongly enriched in sediments, are recycled into the mantle and extracted to the surface via arc volcanism (Johnson and Plank, 1999, and references therein). At the same time, boron, which is strongly enriched in altered MORE, and uranium appear to be effectively recycled into arc magmas by fluids. Eased on bulk partition... 

The role of subducted sediment is particularly well documented for selected, high quality data on arc basalts in which the high Th/La component has Th/La identical to that in the subducting sediment column (Plank, 2003). Very efficient recycling of subducted thorium, together with subducted °Be (present only in surficial sediments), has also been taken as evidence for transport of sediment-derived thorium and beryllium in a partial melt, rather than an aqueous fluid (e.g., Johnson and Plank (1999), Kelemen et al. (1995a) but apparently in disagreement with Morris (see Chapter 2.11) and Schmidt and Poli (see Chapter 3.17). 

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