Fluid mantle

The diffusion of U and Th within a solid is, in general, very slow due to their large size and charge (Van Orman et al. 1998). Even at mantle temperatures, it is expected that a solid will not fully equilibrate with the surrounding phases (fluid, melt or other solid phases) if solid diffusion controls the equilibration. As yet, there have been no direct determinations of diffusion coefficients for any other decay chain element. 

U-series disequilibria are powerful for studying mantle melting because secular equilibrium between all members of the chain should exist for any parcel of solid mantle upwelling from depth. Prior to formation of any fluid phase (early formed melt or volatile rich fluid), upwelling material will have been solid for times that are much longer than... 

Hauri EH, Whitehead JA, Hart SR, (1994) Fluid dynamic and geochemical aspects of entrainment in mantle plumes. J Geophys Res 99 24275-24300... 

In summary, many recent studies have argued for three components in most arc lavas depleted mantle, basalt-derived fluid, and sediment (in bulk, via fluid, or via melt)... 

Several authors have suggested that the composition of subduction zone fluids is likely to change by chromatographic interaction during their passage through the mantle... 

Three lines of evidence suggest that the sediment component is added to the arc mantle before the principal fluid component that appears in arc magmas. First, the mechanical mixing of sediment and peridotite observed in outcrops of subduction... 

In summary, there is permissive evidence that the sedimentary component is added to the mantle wedge before the fluid addition and melting that leads to arc magmatism. 

Therefore, since both Ba and Sr are fluid mobile, the large Ra-excesses at high Ba/Th and Sr/Th ratios are inferred to result from fluid addition to the mantle wedge. At the other end of the arrays in Figure 10 it is notable that the intercept for any arc is with the Ba/Th, not ( Ra/ °Th) axis. This contrasts strikingly with MORB, for example. 

In summary, there is now convincing evidence that Ra is added to the mantle wedge source of arcs by fluids and that the most recent of these additions must have occurred less than 8 kyr ago. The evidence depends on the magnitude of ( Ra/ °Th) and the quality of its correlation with subduction parameters. The evidence is most robust for oceanic arcs and it usually hinges on data for a few volcanoes, usually the ones with the lowest Th concentrations. 

A second possibility is that the inclined U-Th arrays reflect addition of fluids containing some Th (Fig. 8c) and Pa from a source with higher U/Th than the mantle wedge (e.g., Yokoyama et al. 2002). As discussed earlier, U is significantly more fluid-mobile than Pa or Th, but the absolute magnitude remains to be confirmed. Bourdon et al. (2003) used a plot of ( Pa/ U) versus Nb/U ratios for the Tonga lavas to show that a... 

Figure 13. Model curves for continuous fluid addition to a MORB-type mantle source in (a) a °Th-isochron diagram, and (b) a ( Ra/ °Th) versus diagram. The time constant for...

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