Isotopic communication across the water – rock interface: Preservation in solids and signatures in fluids

Jennifer L Druhan,Nicole Fernandez,O Chudaev,Y Kharaka,R Harmon,R Millot,O Shouakar-Stash

doi:10.1051/e3sconf/20199812004

Jennifer L Druhan, Nicole Fernandez + Show 5 more

Open Access

https://doi.org/10.1051/e3sconf/20199812004

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Abstract

Water – rock interactions are characteristically associated with isotopic fractionation between phases, yet their remains significant uncertainty in the degree to which these isotope ratios record the conditions of mineral formation or equilibration, and thus their fidelity as proxy records. Here, we present a novel generic modeling framework allowing sub-grid scale tracking of the isotopic signatures recorded in newly formed minerals over the timescale of precipitation and continued isotopic exchange between phases. We demonstrate that the surface area of the solid phase strongly influences the extent to which isotopes reflect the conditions of formation through time.

Highlights

Weathering reactions occur in systems that are out of equilibrium
In an attempt to accomplish this calculation in a logical and self-consistent manner, we develop a feature of the CrunchTope software which enables us to track a running average of the isotopic mole fractions of solid phase most recently precipitated from the fluid through time (Druhan et al, 2013)
Our baseline condition illustrates a rapid decrease in solute concentrations through time (Fig. 1A), reaching bulk chemical equilibrium on the order of 6 hours after the start of the simulated precipitation from an oversaturated solution

Summary

Introduction

Weathering reactions occur in systems that are out of equilibrium. Commonly, these transformations are limited by some time-dependent maximum rate of mass transfer that keeps them from reestablishing equilibrium instantaneously [1]. There is significant uncertainty associated with the extent to which these signatures at the time of mineral formation are later altered or erased as a result of continued water-rock interaction [8] To this end, many recent studies have focused on the isotopic composition of newly formed solids and the coevolving fluid phase. We report an update to this approach in which the window of time used for this calculated running average has been reframed as a user specified input parameter, such that we are capable of constraining the component of mineral volume in communication with the surrounding fluid for a given system We have developed this model as a means of directly constraining the extent to which fluids impart mixed kinetic and equilibrium signatures and alter those signatures through time in open systems [16]. This model is applied to explore a suite of parameter space, constraining the subset of environmental conditions under which a generic AB mineral records equilibrium vs. kinetic signatures, and the timescale over which these signatures are subsequently lost

Model Framework

Parameter Space

Results and Discussion