Water in the nominally anhydrous minerals of the upper mantle : analytical and experimental developments

Abstract

The identification of 'water' (i.e., Off, H+, H20) as a trace element at defect sites in nominally anhydrous minerals (NAMs) has drastically changed the concept of water storage in the mantle and crust. Water, even at ppm level, is demonstrated to have dramatic effects on propagation of seismic waves, conductivity and rheology of minerals. It has been revealed, experimentally and in natural rocks, that water is incorporated 111 NAMs by different substitution mechanisms; however, the concentration of water in NAMs and in the different substitutions in particular, has remained, to this point, undiscovered. This is due to the lack of a general infrared calibration that can provide reliable and easily obtainable quantitative results. The reason for this lack of an effective method is twofold: 1) it was difficult to determine total absorbance, and 2) there was no general calibration factor that could relate the total absorbance to the absolute concentration of water. In my thesis a new infrared method has been developed for estimating the total absorbance in anisotropic minerals with unpolarized light, thus making analysis much easier. The total polarized absorbance is three times the average unpolarized absorbance, if the measurements are taken from randomly oriented mineral sections. The absolute and independent water concentration has been determined by secondary ion mass spectrometry (SIMS), analysing ¹⁶0H/³⁰Si ratios. The SIMS has been calibrated against synthetic glasses and minerals with a wide range of chemical compositions, in order to consider possible matrix effects that may corrupt the results. The total absorbances in some selected minerals have been cal ibrated against the SIMS concentrations to determine calibration factors for the different substitution mechanisms in ol (and opx). Through this approach, it has been revealed that the only available calibration factor for olivine cannot be applied to all substitutions. In fact, the different substitution mechanisms have considerably different calibration factors: [Si] = 0.572±0.041; [Ti] = 0.182±0.067; [trivalent] = 0.178±0.049 and [Mg] = 0.03±0.03. This development has the potential to provide a more accurate and general tool for quantitative infrared spectroscopy of water in ol. The different substitution mechanisms were studied experimentally by conducting a series of experiments in chemically simple systems at different pressures and temperah1res to monitor the effect of physico-chemical parameters on the incorporation of water. It is demonstrated that the buffering mechanism (chemistry) seems to play at least as important role as pressure and temperature. The solubility of water m both olivine and orthopyroxene increases with increasing pressure, while temperature alone enhances the incorporation of water in orthopyroxene. There is a slight inverse temperature effect for olivine. The substitution mechanism of water, in orthopyroxene and olivine, does not change with pressure and temperature if the buffering mechanism remains the same. The major substitution mechanisms observed in natural NAMs have also been reproduced in chemically complex experimental systems, near the solidus, using a fe1ti le Jherzolite composition with olivine, orthopyroxene and clinopyroxene layers in various arrangements. The substitution mechanism (i.e., position of absorption peaks) of water in NAMs does not change over the studied P-T range and was similar to those discovered in mantle xenoliths and other previous experiments. The near-equilibrium nature of the experiments allowed me to study the partitioning of water among NAMs and estimate the average storage capacity of the mantle. Show more