Effect of Metamorphism on the Point Defects in Quartz: Characterization using Different Spectroscopic Techniques

The present study examines the effect of metamorphism on point defects in quartz. A granite sample with crystallization age of ~460 Ma (Albesti granite, Romania) and its metamorphic equivalent were used for the analysis. This sill-like granitoid occurs close to a ductile shear zone (locally named Bughea shear zone) of presumed Variscan age; the granitoid is exposed as relatively undeformed away from the shear zone as well as highly strained into a mylonitic fabric, when caught into the shear zone. Mineralogical differences were also observed, the metamorphic sample exhibiting lower quartz content and reduced grain strength. The point defects in quartz were characterized using thermoluminescence (TL), optically stimulated luminescence (OSL), electron spin resonance (ESR), scanning electron microscopy coupled with cathodoluminescence (SEM-CL) and Raman spectroscopy.

ESR data showed the presence of E’ (an unpaired electron at an oxygen vacancy site (≡Si·), Al-related defects ([AlO₄]⁰) and peroxy (≡Si-O-O·) in both the samples with increase of these centres in the metamorphic sample, especially in the case of the latter. TL measurements showed four peaks (110, 160, 280, 380 °C) in both samples, though TL intensity was 40% lower in the metamorphic quartz. The OSL decay curves were dominated by fast component for un-deformed sample and the OSL intensity of the metamorphic quartz was approximately 60% lower than that of un-deformed granite. The OSL dose-response curve (DRC) is well represented by a sum of two saturating exponential functions. The DRC of metamorphic granite exhibited higher uncertainties, due to its low signal. The DRC shapes before and after heating were nearly identical for both samples. SEM-CL analysis showed emission in blue (~450 nm) and red region (~650 nm; attributed to NBOHC (≡Si–O·)). The CL emission of metamorphic quartz was 65% lower than that of un-deformed quartz. Raman spectroscopy showed narrowing of quartz bands in the region of 100 to 500 cm^⁻¹, reflecting the shortening of the O-Si-O bond (463 cm^⁻¹) and lattice compression (125 and 204 cm^⁻¹) during metamorphism. The increase at the 1030 cm^⁻¹ peaks indicates the high aluminium-to-silicon ratio which concords with the ESR measurements.

These findings provide new insights into mineralogy and quartz point defect dynamics under metamorphic conditions, with implications for geological processes. At the conference, detailed results and their implications will be presented.

Acknowledgement: This research is funded by European Research Council ERC grant PROGRESS-CoG “Reading provenance from ubiquitous quartz: understanding the changes occurring in its lattice defects in its journey in time and space by physical methods”