OXYGEN ISOTOPIC DEPLETION IN SEDIMENTARY ROCK SEQUENCES: FLUID FLUX IN ACCRECTIONARY WEDGE COMPLEXES AND THE ROLE OF REVERSED GEOTHERMS

It has long been recognized that there are notable depletions in ¹⁸O contents of some siliciclastic sedimentary rock sequences with increasing metamorphic grade. Theoretically, progressive dehydration reactions should increase the ¹⁸O of these rocks, albeit slightly, based on water-mineral oxygen isotope partitioning equilibria (T dependant). Given normal metamorphic gradients, these infiltrating fluids should evolve to isotopically lighter compositions while at the same time enriching the sedimentary rocks in ¹⁸O, which is the opposite to the ¹⁸O depletion trends typically observed. Recent models typically invoke hydrothermal circulation systems with magmatic heat sources driving convection, which allows isotopically lighter and cooler water (meteoric) to be entrained into these systems, accounting for the low ¹⁸O.  However, this is generally inconsistent the permeability structure and evidence of syn-deformational prograde metamorphic assemblages present.

 

An alternate model involves progressive dehydration reactions within a region of inverted geotherms to account for the very light oxygen isotopic signatures of the infiltrating metamorphic fluid.  Within large sedimentary accretionary wedge complexes (shallow subduction), internal radioactive heating, magmatic heating across the buttress, and mid-wedge level frictional heating occurs (Barrovian-like). The continuous subduction of hydrated ocean crust and sediments undergoing progressive dehydration reactions during underplating provides a continual source of isotopically light, low- to moderate-T fluids (low silica) that are heated during upward egression through the deforming wedge complex. Typically, these fluids would be slightly less that seawater salinities due to their dehydration and pore space reduction origin consistent with observation from fluid inclusions in quartz vein systems in these environments. These large magnitude of fluid volumes are required in order to account for the high degree of ¹⁸O isotopic depletions. Even with these high fluid/rock, heterogeneous isotopic re-equilibration is typical and chemical modifications (pressure solution fabrics) are common as most of the fluid flux would be focussed along fabrics and shear zones, some of which host orogenic-style gold mineralization.