Fluid-rock interactions provide a very important control for the trace element budgets in subduction zones. Most subduction zone fluids are derived from the slab, and rise into the overlying mantle wedge, while interacting with various host rocks. We carried out a geochemical and fluid inclusion study in high pressure subduction zone complexes on Catalina Island (California) and Syros (Greece) to study chemical fractionation during fluid transport.
Most metabasites in subduction zone complexes are enclosed in a "melange" that also contains sedimentary and/or ultramafic rocks. Those melanges often contain
actinolite/tremolite - chlorite/talc - carbonate assemblages (henceforth referred to as 'actinolitite') formed at relatively low temperatures and high pressures, similar to their
counterparts known from high-temperature metamorphic reactions on calcareous rocks. Whole-rock geochemistry of actinolitite inclusions in metabasite rocks from the Catalina and from Syros indicates that they have interacted with large volumes of fluids. Easy movable elements, like the LILE, are extensively depleted down to about 5-20% of typical mantle rock inventories. The fluids have all but eliminated the original trace element signatures, prohibiting a reconstruction of their origin. Major element inventories, however, are compatible with a MORB-origin and large inventories of Cr (1800-2400 ppm) and Ni (1200-1800 ppm) suggest a mafic/ultramafic origin.
The metamorphic assemblages studied show a low grade overprint, whereby actinolitite formation did not exceed 5 kb, probably in presence of an H2O-CO2 fluid. CO2 rich fluids are required because otherwise unreasonably high pressures need to be invoked. However, fluid inclusions found in quartz veins near the actinolitites of Catalina Island contain no CO2, suggesting that these fluids probably represent a late stage overprint.
U-Pb abundance and isotope data of actinolitite bulk rocks and mineral separates suggest 206Pb/204Pb = 38.4, consistent with a sedimentary origin and m's of ~1. However, apparent late stage fractionation of U and Pb in the suggests open system behavior, possibly during late stage emplacement processes.
Our first microthermobarometry data of fluid inclusions in quartz veins suggest a relatively low-grade (i.e., late) origin of these veins. These fluid inclusions were last equilibrated with saline hydrous fluids (5-15 wt% NaCl). Fluid inclusions in the highest-grade rocks appear to be older on basis of extensive decrepitation. They were also formed from saline hydrous fluids (8-15 wt% NaCl), but Raman spectra indicate considerable amounts of N2 and CH4 (in a 66%-34% proportion).
Phase equilibria, major and trace element geochemistry of actinolitites suggest that they were formed during an early phase of metasomatism. This period of fluid interaction was characterized by probably localized transport of very large volumes of CO2-rich fluids. Even though there may have been some intermediate stages of fluid transfer, there is evidence for three later stages of chemical transport by fluid interaction. The oldest fluid amongst those is characterized by N2 and CH4 components, followed by a nitrogen and methane free fluid. The latest chemical exchange is indicated by the disturbances in U/Pb isotope relationships. Thus, fluid motion through subduction zone complexes is very complex, and meaningful mass balances can only be established after a detailed analysis of all phases of metasomatism.