Subduction zones mass balance

With realistic estimates of the volatile output from the mantle, particularly the mantle wedge, it is possible to assess the state of volatile mass balance for the mantle—comparing inputs via subduction zones with outputs via arc, mid-ocean ridge, and (possibly) plume-related magmatism. Understanding the volatile systematics of the mantle is a key component in defining its structure and evolutionary history. 

Figure 9 Examples of models proposed for the chemical structure of the terrestrial mantle, (a) Whole mantle convection with depletion of the entire mantle. Some subducted slabs pass through the transition zone to the coremantle boundary. Plumes arise from both the core-mantle boundary and the transition zone. This model is not in agreement with isotopic and chemical mass balances, (b) Two-layer mantle convection, with the depleted mantle above the 660 km transition zone and the lower mantle retaining a primitive composition, (c) Blob model mantle where regions of more primitive mantle are preserved within a variously depleted and enriched lower mantle, (d) Chemically layered mantle with lower third above the core comprising a heterogeneous mixture of enriched (mafic slabs) and more primitive mantle components, and the upper two-thirds of the mantle is depleted in incompatible elements (see text) (after Albarede and van der Hilst, 1999).

Using average MORE or the range of compositions of oceanic basalts (e.g., Hofmann, 1988 Chapter 3.13 and http //petdb.ldeo.columbia.edu Lehnert et al., 2000), the fluxes derived here can be applied to determine the average compositions of oceanic crust that is subducted and recycled into the mantle. These compositions thus influence the composition of subduction zone magmas (see Chapter 3.18) and bear on the chemical mass balance of the mantle. 

In addition to supplying volatiles that are lost via arc-related volcanism, the subducting slab may also contribute volatiles to both the back-arc and fore-arc regions. To complete a realistic mass balance for subduction zones, therefore, it is essential to quantify volatile fluxes at the back-arc and fore-arc. As we discuss below, both fluxes are severely underconstrained at present. 

The volatile output and composition at fore-arcs and back arcs remains poorly constrained. Currently, we are unaware of any representative volatile flux measurements for fore-arcs. These measurements are critical to arrive at an accurate volatile mass balance of subduction zones, and to evaluate if volatiles are transferred beyond this region to the zone of magma generation and/or to the deeper mantle. 

Better constraints are required for the input parameters (particularly for the noble gases and nitrogen). Additional information on the volatile composition of both oceanic sediments and crustal basement is needed to improve estimates of mass balance at subduction zones. 

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