Thermal characteristics of fossil fluids from the Philippine Sea slab:
Insights from fluid inclusions and thermochronology
Abstract
For quantitative understanding of thermal features of fossil fluid
activity derived from the Philippine Sea slab, we applied
fluid-inclusion and thermochronometric analyses to hydrothermal veins
and their host rocks outcropping in the Hongu area in southwestern
Japan. Although hydrothermal events at ~150{degree
sign}C and ~200{degree sign}C were identified by
fluid-inclusion analyses of quartz veins, no thermal anomaly was found
associated with the veins’ host rocks. Cooling ages showed no variation
as a function of distance from the veins. Using zircon, we determined
U-Pb ages of 77.3-66.9 Ma in the youngest population, fission-track
pooled ages of 34.1-24.0 Ma, and (U-Th)/He single-grain ages of 23.6-8.7
Ma. Apatite yielded pooled fission-track ages of 12.0-9.0 Ma. All these
ages can be explained by fluid flow that occurred either (1) before
~10 Ma at a depth where ambient temperature is higher
than closure temperature of the apatite fission-track system (90{degree
sign}C-120{degree sign}C, equivalent to ~3-km depth)
or (2) after ~10 Ma but of such duration that is too
short to have annealed fission tracks in apatite, which process requires
~10 yr at ~150{degree sign}C or as
short as a few months at ~200{degree sign}C. Apatite
fission-track ages of ~10 Ma might reflect regional
mountain uplift and exhumation related to rapid subduction of the
Philippine Sea slab and associated with clockwise rotation of the
Southwest Japan Arc.