OH Defects in Quartz as Petrological Indicator and Provenance Tool

Pressure, temperature and chemical system are the predominant factors controlling trace element incorporation (e.g., H, Li, Al, B) in quartz during growth. In particular, hydrogen plays an important role as charge balancer for metal ions (M³⁺) in the substitution Si⁴⁺=M³⁺+H⁺ by forming infrared-active OH-groups with the oxygen ions of the crystal lattice (e.g., AlOH). Characteristic absorption bands in the IR spectra can be quantified and assigned to specific defects. The natural high abundance of quartz in combination with the dependence on crystallisation conditions makes the OH defect content in quartz a potential tool for deciphering differentiation trends in plutonic bodies and a provenance tool for distinguishing different quartz suites in siliciclastic material.

To shed light on recent advances in our understanding of OH defect incorporation in quartz, we present combined results from three different approaches: (1) piston cylinder and internally heated pressure vessel synthesis experiments were performed in a water saturated granitic system at 1 - 5 kbar and 700 - 950 °C. Quartz crystals analysed by IR spectroscopy reveal a narrow pressure window around 4.5 kbar with increased LiOH content that can potentially be used as an indicator for crystallisation depths. (2) Natural quartz crystals from different sample depths from borehole profiles of two variscan granitic bodies were analysed. Both granites show a large variation in total OH defect content and speciation in the roof region, and more uniform contents towards the bottom of the respective body. In one case, the transition towards an underlying, slightly older granite is discernible by a drop in OH defect content. (3) Detrital quartz grains from sandstones and siliciclastic sediments from different regions and ages were analysed systematically. Results reveal that the OH defect inventory shows significant internal variations from sample to sample in all investigated sedimentary successions and thus may be used as a tool to identify changes in the source region.

In summary, OH defects in quartz monitor parts of their geological history, and the systematic investigation can be applied as an analytical tool to study both igneous and sedimentary processes.