OH-defects in quartz

Quartz is the second most abundant mineral in the Earths crust and an important constituent of many rock types. Primarily, quartz often crystallizes in water-bearing granitic systems, where the nominally anhydrous quartz incorporates water as defects in the crystal lattice. As the environmental conditions (e.g., pressure, temperature) change, the defects tend to adjust to the new conditions and, therefore, can be regarded as monitor/archive for the equilibration conditions. Although the water incorporation at elevated pressures (corresponding to 15-75 km depth) has partly been studied, detailed investigations at more relevant pressures (corresponding to 3-15 km depth) are hitherto missing and their performance is planned for this project. Different types of defects and their abundance will be characterised by structural (spectroscopic) and chemical analysis methods. In this respect, the chemical characterisation is of enormous importance, since traces of some specific metals foster the generation of defects. Results from the experimental part of the project will be compared with quartz grains from two important rock types (granites and sandstones). Granitic bodies can reach a considerable size (up to many kilometres in all dimensions). In the course of the crystallisation its composition is modified, and with this the defects in the quartz crystals. Theses changes follow patterns that mirror the evolution of the granitic melt and potentially define characteristic trends. When the weathered material of granitic bodies is transported away by gravity, wind and water, quartz exhibits the highest chemical and mechanical resistivity amongst the abundant minerals, and, therefore, becomes one of the most abundant minerals in sedimentary rocks. Thus, with help of the knowledge gained from the experimental work and the findings from granites, the investigation of defects in quartz grains from sandstones can be applied as novel tool for provenance analysis.

In contrast to other rock forming minerals, quartz has a rather simple chemical formula (SiO2) and a low chemical variability. However, quartz is able to incorporate traces of protons (H+) and metal cations (such as Al3+) depending on the formation conditions. The incorporated protons form together with the oxygen of the quartz lattice so called OH defects. In this project, this OH incorporation behaviour was studied under controlled conditions (similar to those in the Earth's crust) and natural samples from different geological context were analyzed. The experimental study showed no systematic trend of total OH incorporation between 1 and 5 kbar (corresponding to 3 - 15 km depth, a typical range of formation conditions for granites) with average values equivalent to 200 wt ppm water. Interestingly, this value is similar to the most OH-rich natural quartz crystals from hydrothermal origin and from sedimentary archives, but about 20 times higher than average quartz from the Earth's crust. Quartz from young (Late Paleozoic, approximately 300 Ma) granites from Central Europe cluster around 20-30 wt ppm (with some extremes up to 100 wt ppm), while average quartz from Scandinavia (Proterozoic, approximately 1800 Ma) contain only 3 wt ppm water. The difference between young and old samples can be explained by a continuous loss during low-grade thermal overprint over long time, and the mismatch between quartz from young granites and the most OH-rich samples (including those from the experiments) by a difference in water activity during formation. Systematic analysis of large igneous bodies further revealed interesting differentiation trends that mimic the variation in trace element contents throughout a section of the intrusions. Siliciclastic samples show significant variations with respect to the OH defect inventory in quartz grains and may be used as tool to identify changes in the source region. Europe is an excellent example to test the potential of quartz as tool for provenance analysis due to the clear regional contrast, where comparably low average OH defect contents in quartz from the old basement of Scandinavia meet sources from young granites from Central Europe with much higher OH defect contents. OH defect distributions allow further to estimate mixing relations between different sources in recent river systems that may turn out to be more robust than estimates based on less abundant minerals. In the particular case of the Elbe river system a smaller Scandinavian (glacial) contribution is suggested than previously thought. In Japanese river systems and offshore sediments, a general increase of OH defect contents in quartz from East to West is observed. There is no temporal evolution between 15 Ma and 6 Ma, but a shift towards lower values at 1 Ma that parallels a change in plate motions by that time.