OH-defects in detrital quartz grains: Potential for application as tool for provenance analysis and overview over crustal average

Abstract

OH-defects of 95 detrital quartz grains from 4 localities in North-west Germany (2 North Sea beach sands, one Triassic sandstone, and one Carboniferous sandstone) were studied with infrared (IR) microscopy. By applying novel analytical strategies, the water contribution of fluid and mineral inclusions was minimised and the amount of water incorporated as OH-point defects was quantified. The defect water concentration in all studied quartz grains ranges between 0 and 50wt.ppm H2O with a mean value around 10wt.ppm. Interestingly, grains from the investigated sandstones exhibit in average nearly three times higher defect water concentrations (18wt.ppm) than the grains from the North Sea (6.5wt.ppm). Quartz grains with extreme undulose extinction always exhibit low defect water contents and water-rich grains usually show small undulosity, but also grains with low defect water and low undulosities are common. IR spectra of the detrital quartz grains were compared to reference spectra from samples of known localities and rock types in order to identify potential sources from which the quartz grains were sampled. Most detrital quartz grains exhibit IR signature typical for granites (showing an Al-specific band at 3378cm−1) and regional metamorphic rocks, but also absorption bands typical for pegmatites and hydrothermal quartz (showing a Li-specific band at 3480cm−1) are observed. In contrast, IR signatures typical for high-pressure origin (i.e., hydrogarnet substitution with an absorption band at 3585cm−1) and for tourmaline-bearing rocks (showing a B-specific band at 3595cm−1) are subordinate to insignificant. In view of the large scatter of defect water between individual quartz grains the strategy presented here offers an option to estimate the average defect water content of quartz in the Earth's crust.