Ultramafic clasts from the South Chamorro serpentine mud volcano reveal a polyphase serpentinization history of the Mariana forearc mantle

摘要: Abstract Serpentine seamounts located on the outer half of pervasively fractured Mariana forearc provide an excellent window into devolatilization processes, which can strongly influence cycling volatiles and trace elements in subduction zones. Serpentinized ultramafic clasts recovered from active mud volcano reveal microstructures, mineral assemblages compositions that are indicative a complex polyphase alteration history. Petrologic phase relations oxygen isotopes suggest were serpentinized at temperatures below 200 °C. Several successive serpentinization events represented by different vein generations with distinct element contents be recognized. Measured situ Rb/Cs ratios fairly uniform ranging between 1 10, is consistent Cs mobilization sediments lower lends further credence to low-temperature conditions proposed models thermal structure settings. Late veins show fluid mobile (FME) concentrations than early veins, suggesting decreasing discharge subducting slab composition serpentinizing fluids. The continuous microfabric chemical evolution observed may have implications as origin nature We hypothesize opal smectite dehydration produce quartz-saturated fluids high FME 4 cause pervasive serpentinization. partially material then eroded basal plane suprasubduction mantle wedge. Serpentinization continued but interacting did not carry pronounced sedimentary signature, either because FMEs no longer released slab, or due en route loss FMEs. chrysotile document increased access now fluid-dominated regime characterized reduced slightly ratio near 10. This lack sediment-dominated geochemical signatures consistently displayed all late stages indicate sediment-derived been completely reset (i.e. excesses removed) water–rock reaction within channel. final stage buoyant rise matrix conduits brucite-dominated clast rim inward (independent intra-clast fabric relations), corresponds re-equilibration alkaline, low-silica activity rising mud.