1:45 PM - 2:00 PM
[SMP29-01] Pb-isotope of Fe-Ni alloy associated with serpentinite: Geochemical constraints on the fluid-mediated hyper-reducing environment in mantle wedge
Keywords:Fe-Ni alloy, serpentinization, Pb isotope composition, reducing fluid, mantle wedge
In order to understand those fluid-mediated hyper-reducing environments in serpentinizing mantle wedge of an active convergent plate margin, we revisited so-called ‘josephinite' —an unusual ‘metallic rock' composed mainly of Fe-Ni alloy (awaruite)— found in the Josephine Ophiolite, by engaging in microtextural and geochemical analyses.
Microtextural analyses using high-resolution FE-SEM and FE-EPMA found the presence of Ni-As mineral within aggregates of Fe-Ni alloy; the Ni-As mineral also occurs as discrete grains in ‘josephinite'-hosting serpentinite, suggesting that Ni-As mineral acted as a precursor seed before the crystallization of Fe-Ni alloy.
Since each leachate obtained by stepwise leaching of a ‘josephinite' pebble confirmed compositional homogeneity, this allows to calculate weighted mean values that give much precise Pb isotope compositions of aliquots of seventeen leaching steps determined by ID-TIMS; the values yielded 207Pb/204Pb = 18.3378 ± 0.0016, 206Pb/204Pb = 15.5693 ± 0.0015, and 208Pb/204Pb = 38.0879 ± 0.0044. These newly obtained high precision Pb isotope compositions revealed that the ‘josephinite'-forming fluids have a minor GLOSS-like sedimentary component. The presence of Ni arsenide also supports the infiltration of arsenic-bearing external fluids derived from sedimentary rocks.
Considering the geological context of the Josephine Ophiolite, the ‘josephinite’-forming fluids might have been derived from the top layer of subducting sediments. The lack of high-pressure metamorphic rocks such as blueschist in ‘josephinite'-hosted serpentinite implies that the ‘josephinite' formation might have occurred at a relatively shallow level of serpentinizing mantle wedge.
The absence of carbonate minerals further postulates that redox agent of the ‘josephinite' forming hyper-reducing environment was H2 evolved by the serpentinization rather than CH4 that is commonly preserved in blueschist and eclogites. This fact would speculate a widespread hyper-reducing environment caused by H2 fluids in a shallow level of serpentinizing mantle wedge.