Spinel oxides as petrogenetic indicator minerals : providing a basis for ascent chronometry and geohygrometry using experimental petrology and high-precision in situ analysis : a dissertation submitted in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Earth Science at Massey University, Manawatū campus, New Zealand
The spinel-structured oxides are near-ubiquitously observed accessory phases in ultramafic-felsic igneous rocks. Their status as so-called “petrogenetic indicator” minerals reflects the sensitivity of their composition to changes in the thermodynamic state of magmatic systems. Analyses of their compositions has thus elucidated a wealth of insight into the states and evolution of magmatic systems in general. This thesis builds upon this foundation by illustrating that ionic diffusion within chromian spinel is also highly sensitive to oxygen fugacity. Using the results of internally heated pressure vessel experiments, I show that increasing oxygen fugacity is correlated with higher Fe³⁺ mobility within the spinel structure at the expense of Al over Cr, while Fe²⁺-Mg systematics follow established empirical relationships. Cr mobility remains slow under highly oxidizing conditions but is still expected to diffuse and approach an equilibrium composition over longer timescales. These observations can be utilized to infer ascent rates for chromite-bearing mafic igneous rocks. Using sparse mineral data from the Troodos Ophiolite, model results indicate that not only are chromites out of equilibrium with residual melts but that they were disequilibrated less than 200 years prior to eruption. Another potential use for spinel minerals is as sensitive geohygrometers. However, routine spectroscopic methods cannot be used for these minerals due to the effect that abundant Fe has on the detection limit of OH during analysis. Instead, ion microprobe methods were utilized and demonstrate that H detection is achievable for titanomagnetite. While chromian spinels do not appear to hold any H, titanomagnetites produced during IHPV experiments and those sourced from natural samples contain an intrinsic H signal intensity that indicates H is soluble to trace concentrations. These results suggest that titanomagnetite should be systematically studied through experimental methods in order to identify how H is incorporated. Thus, the “petrogenetic indicator” status of both chromian and Fe-rich spinel minerals may be expanded into the fields of diffusion chronometry and geohygrometry, respectively.
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