TRACE-ELEMENT BEHAVIOR IN HYDROTHERMAL EXPERIMENTS - IMPLICATIONS FORFLUID PROCESSES AT SHALLOW DEPTHS IN SUBDUCTION ZONES

Chemical evaluation of fluids affected during progressive water-sedime nt interactions provides critical information regarding the role of sl ab dehydration and/or crustal recycling in subduction zones. To place some constraints on geochemical processes during sediment subduction, reactions between decollement sediments and synthetic NaCl-CaCl2 solut ions at 25-350 degrees C and 800 bar were monitored in laboratory hydr othermal experiments using an autoclave apparatus. This is the first a ttempt in a single set of experiments to investigate the relative mobi lities of many subduction zone volatiles and trace elements but, becau se of difficulties in conducting hydrothermal experiments on sediments at high P-T conditions, the experiments could only be designed for a shallow (similar to 10 km) depth. The experimental results demonstrate mobilization of volatiles (B and NH4) and incompatible elements (As, Be, Cs, Li, Pb, Rb) in hydrothermal fluids at relatively low temperatu res (similar to 300 degrees C). In addition, a limited fractionation o f light from heavy rare earth elements (REEs) occurs under hydrotherma l conditions. On the other hand, the high field strength elements (HFS Es) Cr, Hf, Nb, Ta, Ti, and Zr are not mobile in the reacted fluids. T he observed behavior of volatiles and trace elements in hydrothermal f luids is similar to the observed enrichment in As, B, Cs, Li, Pb, Rb, and light REEs and depletion in HFSEs in arc magmas relative to magmas derived directly from the upper mantle. Thus, our work suggests a lin k between relative mobilities of trace elements in hydrothermal fluids and deep are magma generation in subduction zones. The experimental r esults are highly consistent with the proposal that the addition of su bduction zone hydrous fluids to the subarc mantle, which has been depl eted by previous melting events, can produce the unique characteristic s of arc magmas. Moreover, the results suggest that deeply subducted s ediments may no longer have the composition necessary to generate the other distinct characteristics, such as the B-delta(11)B and B-Be-10 s ystematics, of arc lavas. Finally, the mobilization of B, Cs, Pb, and light REEs relative to heavy REEs in the hydrothermal fluids fractiona te the ratios of B/Be, B/Nb, Cs/Rb, Pb/Ce, La/Ba and LREE/HREE, which behave conservatively during normal magmatic processes. These results demonstrate that the composition of slab-derived fluids has great impl ications for the recycling of elements; not only in arc magmas but als o in mantle plumes.