Fluid-Metasedimentary Rock Interactions Within Subduction Zone Mélange: The Secret to Trace Element Enrichments in Arc Magmas?

Fluid alteration of deeply-subducted metasedimentary rocks on the island of Syros, Greece, was investigated using geochemical mass balance analysis to evaluate potential sources for trace elements in arc magmas. Concentrations of large ion lithophile elements (LILE; e.g., Ba, K, Rb, Cs, Ca, Sr), U, and Pb are generally elevated in arc magmas relative to more refractory elements like Ti, Th, Hf, Nb, and Zr. This geochemical ``fingerprint'' is considered to be a strong indicator of fluid addition to arc magma source regions. However, the source for trace elements and the fluids that transport them are controversial because experiments and field observations suggest that metamorphic dehydration of subducted mafic, ultramafic, and/or sedimentary rock reservoirs alone is unable to liberate and transport the required elements to the mantle wedge. New petrological and geochemical data from contacts between subducted metasedimentary rock and meta-ultramafic mélange matrix on Syros reveal that multiple sources may be required. We propose that dehydration fluids from subducting slabs infiltrate and equilibrate with heterogeneous meta-ultramafic/metamafic rocks in mélange zones near the slab-mantle interface. When these fluids gain access to subducted metasedimentary rock they can preferentially leach LILE, U, and Pb, without sediment melting, due mostly to the breakdown of phengite and epidote. Migration of these trace element-enriched fluids into the sub-arc mantle wedge could directly trigger partial melting and produce the elevated Ba/Th, Sr/Th, Pb/Th, U/Th and radiogenic Sr that are characteristic geochemical features of arc magmatism. Our findings provide a mechanism for the geochemical decoupling of Th from U and Pb consistent with the U-Th isotopic disequilibrium observed in many arcs. Infiltration at shallower depths (e.g., Syros) would produce metasomatized mantle that could be carried deeper by wedge corner flow to undergo partial melting when it reached sub-arc regions.