Trace elements in quartz: a tool for sediment provenance

International audience

English. Quartz is ubiquitous within continental crust and can virtually be found within all rock types (plutonic, metamorphic and sedimentary). During erosion, weathering and sedimentation processes, it has a very high preservation potential and is often used to trace sediments production and transport dynamics. The QUARTZ project (French ANR funding) aims to assess the potential of quartz as a quantitative tracer for sediment sourcing river dynamics by combining conventional characterization method such as Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS), Laser-Induced Breakdown Spectroscopy (LIBS), and Cathodo-Luminescence (CL), with dosimetric methods, such as Electronic Spin Resonance (ESR) and Optically Stimulated Luminescence (OSL) which are more classically used for Quaternary sediment dating.This study focuses on the Strengbach catchment (ca. 40 km²) draining a low-elevation mountain range located in easternmost France (the Vosges Mountains). Samples representative of the catchment main bedrocks units were collected and prepared mechanically and chemically to extract quartz grains. These grains were mounted on thin-sections for LA-ICP-MS, LIBS and CL analyses. Whole-rock thin sections were also prepared for comparison. Quartz trace elements were analyzed in all samples using LA-ICP-MS and LIBS. Specific thin-sections (100-µm thick) and a high energy LA-ICP-MS laser setup were used to prevent quartz tearing under laser ablation (193 nm Excimer laser). LIBS imaging analyses were carried out using an Ablatom microLIBS equipment allowing micrometric chemical analysis on a macroscopic scale (surface area of several square centimeters). LIBS is an excellent complement to LA-ICP-MS having a good sensitivity to light elements sometimes difficult or impossible to quantify using mass spectrometry (e.g. Li, B, Na, Mg, P, O). Trace elements concentrations obtained by LA-ICP-MS and LIBS techniques were compared to CL, ESR and OSL signals. This allowed to identify specific quartz signatures, depending on rock type (gneiss, granite, or sandstone) and quartz origin (i.e. magmatic, metamorphic, recrystallized, hydrothermal or sedimentary). These chemical signatures are later to be compared to the measured quartz signatures of the Strengbach river sediments. Further comparison between whole-rock samples and separated grains permitted to assess the impact of the sample preparation on the quartz trace element signatures and to identify quartz populations likely to be more represented in the alluvial sediments produced by the different bedrock lithologies. Finally, ongoing analyses point out the complex contribution of trace elements to OSL, ESR and CL signals.