Publication Date:
2022-05-25
Description:
Author Posting. © The Author(s), 2014. This is the author's version of the work. It is posted here by permission of Elsevier for personal use, not for redistribution. The definitive version was published in Lithos 227 (2015): 1-20, doi:10.1016/j.lithos.2015.03.015.
Description:
Serpentine seamounts located on the outer half of the pervasively fractured Mariana
forearc provide an excellent window into the forearc devolatilization processes, which
can strongly influence the cycling of volatiles and trace elements in subduction
zones. Serpentinized ultramafic clasts recovered from an active mud volcano in the
Mariana forearc reveal microstructures, mineral assemblages and compositions that
are indicative of a complex polyphase alteration history. Petrologic phase relations
and oxygen isotopes suggest that ultramafic clasts were serpentinized at
temperatures below 200 °C. Several successive serpe ntinization events represented
by different vein generations with distinct trace element contents can be recognized.
Measured Rb/Cs ratios are fairly uniform ranging between 1 and 10, which is
consistent with Cs mobilization from sediments at lower temperatures and lends
further credence to the low-temperature conditions proposed in models of the thermal structure in forearc settings. Late veins show lower fluid mobile element
(FME) concentrations than early veins, suggesting a deacreasing influence of fluid
discharge from sediments on the composition of the serpentinizing fluids. The
continuous microfabric and mineral chemical evolution observed in the ultramafic
clasts may have implications as to the origin and nature of the serpentinizing fluids.
We hypothesize that opal and smectite dehydration produce quartz-saturated fluids
with high FME contents and Rb/Cs between 1 and 4 that cause the early pervasive
serpentinization. The partially serpentinized material may then be eroded from the
basal plane of the suprasubduction mantle wedge. Serpentinization continued but the
interacting fluids did not carry the slab-flux signature, either because FME were no
longer released from the slab, or due to an en route loss of FMEs. Late chrysotile
veins that document the increased access of fluids in a now fluid-dominated regime
are characterized by reduced trace element contents with a slightly increased Rb/Cs
ratio near 10. This lack of geochemical slab signatures consistently displayed in all
late serpentinization stages may indicate that the slab-derived fluids have been
completely reset (i.e. the FME excesses were removed) by continued water-rock
reaction within the subduction channel. The final stage of diapiric rise of matrix and
clasts in the conduits is characterized by brucite-dominated alteration of the clasts
from the clast rim inward (independent of the intra-clast fabric relations), which
corresponds to re-equilibration with alkaline, low-silica activity fluids in the rising mud.
Description:
This study was funded through a grant of the DFG to
WB (BA 1605/5-1).
Keywords:
Serpentinization
;
Polyphase alteration
;
Mud volcano
;
Fluid mobile elements recycling
;
Hydrated mantle wedge
;
Forearc peridotites
;
Subduction zone
Repository Name:
Woods Hole Open Access Server
Type:
Preprint
Format:
application/pdf
Permalink
