Hydrogen diffusion and storage mechanisms in diopside

Abstract: Hydrogen is a widespread trace element in many nominally anhydrous minerals (minerals without water or hydroxyl ions in their structural formula) from the Earth's crust and mantle. The hydrogen is normally incorporated in the form of hydroxyl ions and can be regarded as structurally bound water. The most important minerals of the upper mantle: olivine, orthopyroxene, clinopyroxene and garnet, all contain small but significant amounts of hydrogen. This means that the upper mantle has the capacity to store the equivalent of several world oceans. To know how much water there is in the Earth's interior is important knowledge, as small differences in mantle water content influences models of mantle dynamics. The mantle plays an important role in the hydrological cycle as water in oceanic crust and sediments is subducted at converging plate boundaries and again released through volcanic eruptions during millions of years. Detailed knowledge of the amount of water that is retained within the mantle for longer periods of time (~109 years) is still lacking.Investigating the water content of the mantle is a task shared between the fields of mineralogy, petrology, geophysics, and theoretical physics (i.e. atomistic models).My approach as a mineralogist has been to investigate in detail the mechanisms that are responsible for water incorporation in nominally anhydrous upper mantle minerals, with a special emphasis on the pyroxenes as they can carry substantial amounts of water, up to 1300 ppm H2O. The fundamental questions here are how much of the original xenolith water is lost during transport to the surface and if the spectroscopic features measured in the minerals are representative for mantle conditions.The redox reaction: Fe2+ + OH- ↔ Fe3+ + O2- + ½H2, which is relatively fast, is thought to be the dominant hydrogen exchange reaction in many minerals (Ingrin & Skogby, 2000). The reaction is fast enough to suggest that water in nominally anhydrous minerals equilibrates with the transporting magma and related fluids during ascent to the surface. Nevertheless, several studies show systematic variations in water content with geological environment (Bell & Rossman, 1992; Peslier et al., 2002), implying a complex relationship between host mineral, mantle source region, magma type and eruption style. This thesis is focused on the dehydration-hydration mechanisms in diopside, the most common variety of clinopyroxene in the upper mantle. The approach has been to study the kinetics and temperature dependence of the reactions controlling hydrogen diffusion in synthetic Fe-poor diopside.Other reactions are likely to be obscured by the iron redox reaction if measured in natural mantle diopside containing several wt% FeO. Therefore, synthetic diopside with very low amounts (0.7 wt% FeO) of iron had to be used in order to measure the influence and co-dependence of the iron redox reaction with other possible reactions.The experiments were carried out by stepwise heating of the samples in both air and hydrogen. After each step, OH-absorbance was measured using Fourier Transform Infrared spectroscopy, and the relative amounts of ferric and ferrous iron was monitored by Mössbauer spectroscopy. When comparing the amounts of ferric iron and hydrogen (in atoms per formula unit), there is considerable deviation from the ideal 1:1 relationship expected from the iron redox reaction. The dehydration process is reversible to a great extent and re-hydration continues even after all iron is reduced.The results of this study show that other reactions apart from the iron redox reaction are active, and that they are significantly slower. If these slower reactions are active in mantle diopsides, there is a possibility that they may preserve signatures from the mantle source region.