EXPERIMENTAL DETERMINATIONS OF NON-TRADITIONAL STABLE ISOTOPE FRACTIONATION

Experimentally-determined stable isotope fractionations provide a context for interpreting disparities in isotope ratios between the bulk silicate Earth (BSE) and primitive solar system bodies represented by meteorites. Isotopes of Mg, Si, Fe, and Ni have been targeted for study thus far.

With help from Bob Newton, the authors have developed a variant of the three-isotope method (developed earlier at the University of Chicago) for investigating equilibrium isotope exchange at high temperatures and pressures. In this approach, the advance towards isotope exchange equilibrium between mineral phases is followed using an isotope tracer for the element of interest. Experimental results for Fe, Si, and Ni thus far establish the factors that control fractionation among mineral phases of interest. These include coordination number as well as oxidation state.

As an example, we consider Mg and Si, lithophiles with comparable volatilities. These elements experienced similar processing in the solar protoplanetary disk, but unlike Si, Mg is not appreciably soluble in Fe-Ni alloy. Evidence is mounting that the BSE is nearly indistinguishable from chondrites in ²⁵Mg/²⁴Mg (< 0.1 ‰) while (³⁰Si/²⁸Si)_BSE > (³⁰Si/²⁸Si)_chondrites by at least ~0.1 to 0.2 ‰. Experiments and complementary studies of Enstatite clan meteorites establish the fractionation of Si isotopes between metal and silicate. The Δ³⁰Si (sil/met) at temperatures of core-mantle equilibration (~3000K) is consistent with an increase in fO₂ of ~ 1 to 2 log units in the lower mantle following initial differentiation based on Δ³⁰Si (BSE/chondrite) and the implied Si concentrations in the core.

Experiments show that Ni and Fe exhibit less metal-silicate fractionation than Si. The three-isotope method is proving to be invaluable for exploring equilibrium isotope partitioning under deep crustal and mantle conditions.