RESUMEN
Over the past decades, there has been increasing evidence for the prehistoric use of rock crystal in mountainous environments, including craft specialisation and long-distance exchange. Yet there are only a few known sites where the mineral was quarried in sustainable quantities. One of them is situated near Fiescheralp in the Upper Valais (Switzerland) and dates to the Early Mesolithic and a final stage of the Neolithic. Here we present the first petrographic characterisation of a prehistoric rock crystal mine in the Swiss Alps, involving a combination of different methods. The article provides a detailed description of the fluid inclusions within the quartz crystals and an overview over the related mineral paragenesis. This gives interesting new insights into the formation of the analysed fissure and allows comparing rock crystal artefacts found in other archaeological sites to this particular source. The results form the basis for further investigations concerning the circulation and distribution of the raw material in the past.
RESUMEN
Fluid assisted Alpine fissure-vein and cleft formation starts at prograde, peak or retrograde metamorphic conditions of 450-550 °C and 0.3-0.6 GPa and below, commonly at conditions of ductile to brittle rock deformation. Early-formed fissures become overprinted by subsequent deformation, locally leading to a reorientation. Deformation that follows fissure formation initiates a cycle of dissolution, dissolution/reprecipitation or new growth of fissure minerals enclosing fluid inclusions. Although fissures in upper greenschist and amphibolite facies rocks predominantly form under retrograde metamorphic conditions, this work confirms that the carbon dioxide fluid zone correlates with regions of highest grade Alpine metamorphism, suggesting carbon dioxide production by prograde devolatilization reactions and rock-buffering of the fissure-filling fluid. For this reason, fluid composition zones systematically change in metamorphosed and exhumed nappe stacks from diagenetic to amphibolite facies metamorphic rocks from saline fluids dominated by higher hydrocarbons, methane, water and carbon dioxide. Open fissures are in most cases oriented roughly perpendicular to the foliation and lineation of the host rock. The type of fluid constrains the habit of the very frequently crystallizing quartz crystals. Open fissures also form in association with more localized strike-slip faults and are oriented perpendicular to the faults. The combination of fissure orientation, fissure quartz fluid inclusion and fissure monazite-(Ce) (hereafter monazite) Th-Pb ages shows that fissure formation occurred episodically (1) during the Cretaceous (eo-Alpine) deformation cycle in association with exhumation of the Austroalpine Koralpe-Saualpe region (~ 90 Ma) and subsequent extensional movements in association with the formation of the Gosau basins (~ 90-70 Ma), (2) during rapid exhumation of high-pressure overprinted Briançonnais and Piemontais units (36-30 Ma), (3) during unroofing of the Tauern and Lepontine metamorphic domes, during emplacement and reverse faulting of the external Massifs (25-12 Ma; except Argentera) and due to local dextral strike-slip faulting in association with the opening of the Ligurian sea, and (4) during the development of a young, widespread network of ductile to brittle strike-slip faults (12-5 Ma). SUPPLEMENTARY INFORMATION: The online version contains supplementary material available at 10.1186/s00015-021-00391-9.
RESUMEN
Quantitative analysis of gases by Raman spectroscopy is based on relative Raman scattering cross sections (RRSCS) and the evolution of different spectral parameters (peak position, peak area, peak intensity, etc.). However, most of the calibration data were established at low pressure (low density) and without evaluating the effect of the composition. Using these data may lead to considerable errors, especially when applied to gas mixtures at high pressure as found in natural fluid inclusions. The aim of this study is to reevaluate the RRSCS of CO2 and to establish new calibration data based on the variation of CO2 Fermi diad splitting as a function of pressure (density) and composition over a pressure range of 5-600 bar at 22 and 32 °C. A high-pressure optical cell system (HPOC) and a heating-cooling stage were used for Raman in situ analyses at controlled PTX conditions. Our experimental results show that the RRSCS of CO2 varies slightly with pressure but can be considered constant over the studied pressure range. It can be used to measure the proportion of CO2 in gas mixtures with an uncertainty of about ±0.5 mol%. Different polynomial equations were provided to calculate pressure and density of CO2-N2 gas mixtures with an uncertainty of ±20 bar or 0.01 g·cm-3. A comparison of PVTX properties of natural CO2-N2 fluid inclusions hosted in quartz from the Central Alps (Switzerland) obtained by Raman measurement and as derived from phase transition temperatures by microthermometry experiments shows comparable values.