Mineral/Fluid/Melt Trace Element Partitioning Data -- The Search for Signatures to Subduction Processes

Trevor H. Green, John Adam and Geoffrey T. Nichols, GEMOC, Macquarie

Determination of trace element partitioning behaviour (D values) at high pressure between fluid/melt/minerals for major mantle and slab minerals and rutile may provide crucial constraints on the possible role of rival metasomatic agents (aqueous fluid, high-SiO2 hydrous melt, carbonatitic melt causing the distinctive trace element characteristics of convergent plate boundary magmas (CPBM). Fluid/melt partitioning data for compositions ranging from basalt (to andesite) to trondhjemite show that aqueous fluids do not generally favour trace elements relative to melt, except for Rb and Pb. Fluid coexisting with the trondhjemitic melt records the highest D values and it is noteworthy that the fluid major element composition approaches that of the melt. Significant changes to trace element ratios indicated by this data include an increase in Rb/La, U/Th and possibly Pb/Sr.

In general, mineral/fluid Ds are higher than mineral/melt values, but noteworthy exceptions occur. Thus for clinopyroxene (cpx) mineral/fluid Ds for Pb and Ba are similar to mineral/melt values. Cpx/fluid fractionates U/Th more strongly than cpx/melt. Cpx/silicate and cpx/carbonate melt Ds appear similar, except for Zr and Hf, which are fractionated in opposite direction. For amphibole (amph) Rb and Pb mineral/fluid and mineral/melt Ds are close in value, but Rb/Ba behaviour is distinctly different for amph/fluid (<1) and each melt (>1 for carbonate melt, ~1 for silicate melt). Also Nb/Ta is fractionated more by amph/fluid and amph/carbonate melt than by amph/silicate melt, and HFSE/REE is higher for amph/carbonate pairs than for either amph/silicate melt or amph/fluid pairs. For garnet (gt) mineral/fluid Ds for Ba and Sr are lower than mineral/melt values, and U/Nb and Pb/Sr will decrease in fluids but will increase in melts through gt fractionation. Gt/silicate or carbonate melt Ds show very similar behaviour. Although only a small number of rutile/fluid and melt D values are available, the very high D values are striking, so that a small volume of rutile may have a major effect on trace elements. Rutile/fluid fractionates Nb/Ta in the opposite direction to rutile/melt and to cpx, amph or gt/melt. Thus rutile/fluid fractionation will show a decrease in Nb/Ta compared with an increase in Nb/Ta for all the mineral/melt fractionating cases. Also rutile/fluid DU >> DTh', that is opposite to cpx/fluid, but similar to (though much higher than) gt/fluid.

The cpx, amph, gt/fluid or melt D data indicate that for potential fluids or melts that could affect the peridotitic mantle wedge source region for CPBM, relatively lower Rb/Ba and HFSE/REE in the CPBM point to a carbonatitic melt modifying role, whereas higher U/Th and Rb/Ba and lower U/Nb suggest a fluid role. If rutile/fluid partitioning behaviour exerts an important control on the geochemistry of the CPBM source region, then derived magmas may have Nb/Ta < model mantle, whereas if rutile/melt control (together with cpx, amph or gt) is more significant than Nb/Ta will be > model mantle. Thus recently obtained Nb/Ta data for CPBM of 11 to 33 may reflect this contrasting rutile/fluid or melt Nb and Ta partitioning behaviour. An important corollary is that a model continental crustal value of Nb/Ta ~ 11 suggests that any contribution to continental crust from CPBM should come from magmas derived from a rutile/fluid affected source region. Additional evidence for a fluid rather than a silicate melt role may come from assessment of Rb/La, U/Th and Pb/Sr relative to Nb/Ta. The fluid/melt partitioning data summarized here suggest that a negative correlation of these ratios would confirm fluid involvement. However, it is crucial to verify that rutile/fluid and rutile/melt Nb, Ta partitioning data show contrasting behaviour.

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