Disparate relationships between pH and goethite-water 18O/16O and D/H fractionation factors

Abstract

For goethites synthesized at low (∼1–2) and high (∼13–14) extremes of pH, the values of 1000ln18αgt-w at ca. 25 °C are, respectively, +6.2(±0.6) and + 0.9(±0.6). Note that 18αgt-w = (18O/16O)gt/(18O/16O)w and gt = goethite (α-FeOOH); w = liquid water. A steady-state, flux-balance model indicates that this pH-related difference in values of 1000ln18αgt-w arises because of an effectively irreversible oxygen flux associated with aqueous Fe(III) hydroxide adsorption on, and incorporation in, goethite during growth at crystal surfaces. The model predicts that the transition from a low-pH to a high-pH value of 1000ln18αgt-w is relatively abrupt. At 22 to 25 °C, extant data indicate that the transition occurs in the interval of pH from about 6 to 8. Consequently, uncertainty about the pH of ancient waters could be a factor in interpretations of goethite δ18O values. However, an experimentally determined equation for the temperature dependence of 1000ln18α(LpH) has been applied with apparent success to natural goethites that formed in the presence of meteoric waters (18α(LpH) = low-pH 18αgt-w). This suggests that those ancient waters may have been acidic (pH < 6) in the immediate environments of goethite crystallization.Published values of D/H fractionation between synthetic goethite and water (Dαgt-w) show no systematic effect of either pH or temperature (T) over a range of pH from 1 to 14 and a range of T from 22 °C to 62 °C. Within these ranges, the average value of Dαgt-w is 0.905(±0.005) – a value which seems to characterize the D/H fractionation observed in natural goethites. If hydrogen in transitional, chemisorbed Fe(III) hydroxides – represented here as Fe(OH)3A – rapidly exchanges D/H with ambient water during crystal growth, there is a model-based expectation that Dαgt-w will not be affected by pH. This postulated D/H exchange would cease when Fe(OH)3A irreversibly transitions to FeOOH during crystallization – i.e., when Fe, O, and H are incorporated in the structure of the growing crystal.Thus, the flux balance model suggests that the observed dichotomy in the effects of pH on 18αgt-w and Dαgt-w is a result of distinctly different exchangeabilities of 18O/16O and D/H in transitional Fe(III) hydroxides that are chemisorbed on growing surfaces of goethite crystals. Observed values of 18αgt-w and Dαgt-w may represent steady-state – but not necessarily thermodynamic equilibrium – distributions of isotopes between H2O and the oxygen or hydrogen in the interiors of crystallizing goethites. Nevertheless, these steady-state values of Dαgt-w and 18αgt-w are generally reproducible and systematically relate δ18O and δD values of goethite to environmental variables.