TRACE-ELEMENT MODELING OF AQUEOUS FLUID - PERIDOTITE INTERACTION IN THE MANTLE WEDGE OF SUBDUCTION ZONES

Recently measured partition coefficients for Rb, Th, U, Nb, La (Ce), P b, Sr, Sm, Zr, and Y between Iherzolite assemblage minerals and H2O-ri ch fluid (Ayers et al. 1997; Brenan et al. 1995a,b) are used in a two- component local equilibrium model to assess the effects of interaction between slab-derived aqueous fluids and wedge Iherzolite on the trace element and isotopic composition of island are basalts (IAB). The mod el includes four steps representing chemical processes, with each proc ess represented by one equation with one adjustable parameter, in whic h aqueous fluid: (1) separates from eclogite in the subducted slab (Ra yleigh distillation, mass fraction of fluid released F-fluid); (2) asc ends through the mantle wedge in isolated packets, exchanging elements and isotopes with depleted Iherzolite (zone refining, the rock/fluid mass ratio n); (3) mixes with depleted Iherzolite (physical mixing, th e mass fraction of fluid in the mixture X-fluid); (4) induces melting to form primitive IAB (batch melting, mass fraction of melt F-melt). T he amount of mantle Iherzolite processed by the fluid in step (2) dete rmines its isotopic and trace element signature and the relative contr ibutions of slab and wedge to primitive IAB. Assuming an average deple ted Iherzolite composition and mineralogy (70% olivine, 26% orthopyrox ene, 3% clinopyroxene and 1% ilmenite) and using nonlinear regression to adjust parameter values to obtain an optimal fit to the average com position of IAB (McCulloch and Gamble 1991) yields values of F-fluid = 0.20, n = 26, X-fluid = 0.17, and F-melt = 0.15, with r(2) = 0.995 an d the average relative error in trace element concentration = 6%. The average composition of IAB can also effectively be modeled with no con tribution from the slab other than H2O (i.e., skip model step I): n = 27, X-fluid = 0.21, F-melt = 0.17, with r(2) = 0.992. By the time the fluid reaches the IAB source, exchange with depleted wedge Iherzolite reduces the Sr-87/Sr-86 ratio isotopic composition to near-mantle valu es and the slab contribution to < 50% for all but the most incompatibl e elements (e.g., Pb). The IAB may retain the slab signature for eleme nts such as B and Be that are highly incompatible and that have very l ow concentrations in the depleted mantle wedge. The relatively high eq uilibrium D-mineral/fluid values measured by Ayers et al. (1997), Bren an et al. (1995a) and Stalder et al. (1998) suggest that large amounts of fluid (> 5 wt%) must be added to Iherzolite in the IAB source. Dec reasing X-fluid below 0.05 causes model results to have unacceptably h igh levels of error and petrologically unreasonable values of F-melt. That H2O contents of IAB are generally <6 wt% suggests that not all of the H2O that metasomatizes the IAB source remains in the source to di ssolve in the subsequently formed melt. Modeling of the compositions o f specific primitive IAB from oceanic settings with low sediment input and depleted mantle wedges (Tonga, Marianas) shows a generally lower level of fluid-wedge interaction (low n), and therefore a larger slab component in primitive IAB.