Авторы: WELCH S., BEARD B., JOHNSON C.

2003 г.

Geochim.Cosmochim.Acta

    Equilibrium and kinetic Fe isotope fractionation between aqueous ferrous and ferric species measured over a range of chloride concentrations (0, 11, 110 mM Cl
) and at two temperatures (0 and 22°C) indicate that Fe isotope fractionation is a function of temperature, but independent of chloride contents over the range studied. Using 57Fe-enriched tracer experiments the kinetics of isotopic exchange can be fit by a second-order rate equation, or a first-order equation with respect to both ferrous and ferric iron. The exchange is rapid at 22°C, 60–80% complete within 5 seconds, whereas at 0°C, exchange rates are about an order of magnitude slower. Isotopic exchange rates vary with chloride contents, where ferrous-ferric isotope exchange rates were 25 to 40% slower in the 11 mM HCl solution compared to the 0 mM Cl
(10 mM HNO3) solutions; isotope exchange rates are comparable in the 0 and 110 mM Cl
solutions. The average measured equilibrium isotope fractionations, Fe(III)-Fe(II), in 0, 11, and 111 mM Cl
solutions at 22°C are identical within experimental error at 2.760.09, 2.870.22, and 2.760.06 ‰, respectively. This is very similar to the value measured by Johnson et al. (2002a) in dilute HCl solutions. At 0°C, the average measured Fe(III)-Fe(II) fractionations are 3.250.38, 3.510.14 and 3.560.16‰for 0, 11, and 111 mM Cl
solutions. Assessment of the effects of partial re-equilibration on isotope fractionation during species separation suggests that the measured isotope fractionations are on average too low by 0.20‰and 0.13‰for the 22°C and 0°C experiments, respectively. Using corrected fractionation factors, we can define the temperature dependence of the isotope fractionation from 0°C to 22°C as: (see formula in PDF) where the isotopic fractionation is independent of Cl
contents over the range used in these experiments. These results confirm that the Fe(III)-Fe(II) fractionation is approximately half that predicted from spectroscopic data, and suggests that, at least in moderate Cl
contents, the isotopic fractionation is relatively insensitive to Fe-Cl speciation.

Файл: 531.pdf