The basis for chromite-melt diffusion chronometry in an oxybaro-geospeedometric context

Abstract:

When primitive mantle-derived magmas enter the crust and cool, the crystallization of ferro-magnesian silicate minerals such as olivine drive melt compositions into a relatively oxidized state, as Fe³⁺ is incompatible in such silicates. If chromite crystallizes prior to or concomitantly with olivine in the primitive melt system and if the melt remains siliceous enough to ensure chromite stability during magmatic differentiation, then the composition of chromite in equilibrium with residual melts will become more ferric as melt Fe³⁺/ΣFe increases.

Thus, early crystallized chromite grains entrained in a differentiated melt will be in chemical disequilibrium with respect to Fe³⁺ concentration. Diffusive equilibration of Fe³⁺ progresses via the exchange of trivalent Cr and/or Al in chromite with Fe³⁺ in the melt.

Therefore, one explanation for chromite crystal zonation with respect to Fe, Al, and Cr is that the diffusion of Fe³⁺ into chromite failed to completely re-equilibrate chromite with its host magma prior to eruption. Such features should be contextualized by examination of Fe²⁺-Mg equilibria in order to assess for mineral (both oxide and silicate)-melt equilibrium of this subcomposition. In the case where microlitic silicates like pyroxene are in chemical equilibrium (with respect to Fe²⁺-Mg) with both zoned chromite and melt phases, then the silicate (i) crystallized after the chromite to alter melt Fe³⁺/ΣFe, (ii) the zonation of the chromite grain likely reflects Fe³⁺ heterogeneity and (iii) the profiles of Fe, Al, and Cr in the zoned chromite may be interpreted in a diffusion-chronometric context.

Further work on the diffusivities of these elements in spinel-structured oxides is required to better constrain timescale information regarding changes in melt oxidation state and temperature during magma ascent and/or eruption based on chromite zonation. We will provide natural examples of these processes.