OH point defects in quartz – a review

Abstract

Research results of the past 6 decades on the incorporation of OH point defects in quartz are summarised and evaluated in terms of their application to natural samples and processes, and a link between experimental petrology, natural archives, and model calculations is made. A strong focus is put on recent studies on quartz as a rock-forming mineral, as a geochemical and/or petrological tracer, and as a tool for provenance analysis in sediments and sedimentary rocks. The most relevant defects for natural specimens are generated by coupled substitution involving mono- and trivalent cations, the most prominent being Li+, Al3+, and B3+. OH incorporation is rather a function of the availability of trace metals and water than of pressure and temperature, though temperature indirectly influences the incorporation by the solubility of trace metals in the fluid. Pressure has a negative influence on the formation of OH defects, so the most pure quartzes are probably formed in the deep crust close to the quartz/coesite transition. Natural quartz grains from the Earth's crust have on average 10 wt ppm (weight parts per million) water (5 wt ppm median), but grains with OH defect contents corresponding to up 250 wt ppm water have been discovered in sedimentary archives, matching the concentration of quartz from high-pressure experiments <4 kbar under water-saturated conditions in granitic systems. A rough division into three classes is suggested: (1) grains with pristine igneous and/or hydrothermal origin, (2) mildly thermally annealed grains, and (3) strongly dehydrated grains. While samples derived from the currently exposed Scandinavian Shield are dominated by the third class, considerable contributions of the first two classes are found in the younger rock systems in Central Europe. OH defect contents may be used to estimate mixing ratios for sediments with different sources, provided that a sufficiently large data set exists and that the different sources can be clearly distinguished by their OH inventory. Furthermore, metamorphic overprint leads to a higher degree of equilibration of OH defects between individual grains and may thus be used as a geothermometer. Finally, OH defect retention in quartz allows for estimating timescales of volcanic processes.