Philip 'Flip' Froelich

Research

The Fluid Geochemistry of Ridge Subduction:
Chile Triple Junction (ODP Leg-141)

The primary objectives of this project were to establish the fluid and mineral geochemical mechanisms and fluid circulation pathways and timescales responsible for the unique authigenic mineralizations in Site 863 of Leg-141 of the Ocean Drilling Project. This section, drilled in the Chile Triple

isolated sample photo Junction ridge-trench-continent collision zone, recovered a vertically-tilted late Pleistocene sand-siltstone unit at the toe of the accretionary prism directly above the recently-subducted (less than 50 kyr) Chile Ridge axis. The vertical sequence of both contemporary (on-going) and relict (last 50,000 years or so) geochemical reactions related to hydrothermal pulses penetrating upward from the over-ridden ridge axis have produced a classic stratified zeolite facies. This reaction sequence coincides with the observed depth-stratified (temperature gradient) and time-dependent (paragenetic sequence order) occurrences of zeolite facies minerals in the section: smectite & carbonate cements, forams replaced with silica (zeolites), and discrete K- and Ca-zeolites. We have captured the major, minor, trace element and isotope chemistry of the pore fluids, and have separated and identified the sequences of authigenic minerals precipitated during and shortly after hydrothermal pulses. A simple pore fluid vertical reaction-diffusion model suggests that in situ diagenesis today is sufficient to explain these fluid-mineral compositions. However, the basal fluids are highly-evolved alkaline (pH 10.5) CaCl2 solutions rich in Li & B and very depleted in Mg, K, & Na, suggesting a recent basaltic (hydrothermal) source. Organic matter in the section is immature and of marine origin, but the presence of thermogenic volatile hydrocarbons in the basal section require migration from landward below the decollement, perhaps during collision and tilting 20-100 kyr ago.

The primary data set consists of high resolution pore water profiles for major and minor cations (Na, K, Ca, Mg, SO4, Cl, B, F, Alk, pH), nutrients (P, NH4, Si), trace elements (Li, Mn, Cu, Ba, Rb, Cs, Mo, As, Co, U, As, V, Cd, Sr, and Br). REE analyses were unsuccessful (concentrations too low). Isotopic analyses include Sr-87/86, O-18/16, He-3/He-4, and Li-6/7 analyses of all pore fluids. A new method was developed during this project for the determination of lithium isotopes. Boron isotope analyses were unsuccessful. Separation and identification by XRD of fine grained authigenic mineral phases and their vertical zonations included: calcite, smectites (K- and MgFe-bearing), zeolites (heulandite, clioptilolite, analcime, mordenite and their related isomorphs).

Based on this data set and the Sawyer et al. thermal model for Site 863 plus the kinematic constraints from the tectonic model of the timing of collision with the ridge axis and tilting of this section, we have developed a hydrogeological model to constrain the water and chemical fluxes, the timing and the extent of authigenic mineral formation. This then provides one estimate of the fluid and geochemical fluxes associated with ridge subduction, a critical and as-yet poorly understood component of the global impact of fluid migration for mass and heat transfer through the Cenozoic.

ship collage