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Zeolithe in Kristallingesteinen
The goal of the thesis is to study the occurrences and formation of zeolites hosted in crystalline basement rocks. The low-grade fissure mineral assemblages including zeolites are the key to the appreciation of water-rock interaction in hydrothermal and geothermal systems at relatively low temperatures (< 250 °C) located in granites and gneisses of the crystalline basement. Extensive work is done on zeolite occurrences in sedimentary rocks often pyroclastic origin and volcanic rocks, whereas elements necessary for zeolite formation derive from primary glass of from feldspar. In contrast the formation of zeolites in granites and gneisses is poorly studied and no systematic of evaluation and spatial distribution are carried out either chemical studies on zeolites or formation consideration are done.
Therefore a systematic evaluation of zeolites in the Central Swiss Alps is presented. Ca-zeolites occur in various assemblages in late fissures and fractures in granites and gneisses. The systematic study of zeolite samples showed that the majority of finds originate from three regions particularity rich in zeolite-bearing fissures: (1) in the central and eastern part of the Aar- and Gotthard Massif, including the Gotthard road tunnel and the Gotthard-NEAT tunnel, (2) Gibelsbach/Fiesch, in a fissure breccia between Aar Massif and Permian sediments, and (3) in Penninic gneisses of the Simano nappe at Arvigo (Val Calanca). The excavation of tunnels in the Aar- and Gotthard massif give an excellent overview of zeolite frequency in Alpine fissures, whereas 32 % (Gotthard NEAT) and 18 % (Gotthard road tunnel) of all fissures are filled with zeolites. The number of different zeolites is limited to 6 species: laumontite, stilbite and scolecite are abundant and common, whereas heulandite, chabazite and epistilbite occur occasionally. Ca is the dominant extra-framework cations, with minor K and Na. Heulandite and chabazite additionally contain Sr up to 29 and 10 mole%, respectively. Na and K content of zeolites tends to increase during growth as a result of systematic changes in fluid composition and/or temperature. The K enrichment of stilbite found in surface outcrops compare to stilbite in the subsurface may indicate late cation exchange during interaction with surface water. Texture data, relative age sequences derived from fissure assemblages and equilibrium calculations shows that the Ca-dominated zeolites precipitated from fluid with decreasing temperature in the order (old to young = hot to cold): scolecite, laumontite, heulandite, chabazite and stilbite. The components necessary for zeolite formation are derived from dissolving primary granite and gneiss minerals. The nature of these minerals depends on the metamorphic history of the host rock. Zeolites in the Aar Massif derived from the dissolution of epidote or calcite and albite that were originally formed during Alpine greenschist metamorphism. Whereas albitization of plagioclase in higher grade rocks releases the necessary components for zeolite formation, a process that is accompanied by a distinct porosity increase. Zeolite fissures occur in the zone where fluid inclusions in earlier formed quartz contain H2O dominated fluids. This is consistent with equilibrium calculations that predict a low CO2 tolerance of zeolite assemblages particularly at low temperature. Pressure decrease along the uplift and exhumation can increase zeolite stability. The major zeolite forming reaction consumes calcite and albite; it increases pH and the total of dissolved solids. The produced Na2CO3 waters are in accord with reported deep groundwater (thermal water) in the continental crust, which are typically oversaturated with respect to Ca-zeolites.
A detailed local study of the mineralogical, chemical and petrological evolution of crystalline basement rocks in Arvigo was performed to assess information about the evolution of fluid-rock interaction during uplift of the Alpine orogen. The Arvigo fissures contain the assemblage epidote, prehnite, chlorite and various species of zeolites. Fluid rock interaction takes place along a retrograde exhumation path which is characterized with decreasing temperature by: (1) coexisting prehnite/epidote, that reveals temperature conditions of 330 – 380 °C, (2) chlorite formation at temperature of 333 ± 32 °C and (3) formation of zeolites < 250 °C. The formation of secondary minerals is related to the hydrothermal replacement reaction during albitization and chloritization that releases components for the formation of Ca-Al silicates and form a distinct reaction front. The fluid-rock interaction is associated with a depletion of Al2O3, SiO2, CaO, Fe2O3 and K2O in the altered wall rock. The reaction is associated with an increase in porosity up to 14.2 ± 2.2 %, caused by the volume decrease during albitization and the removal of chlorite. The propagation of the sharp reaction front through the gneiss matrix occurred via a dissolution-reprecipitation mechanism. Zeolite formation is tied to the plagioclase alteration reaction in the rock matrix, which releases components for zeolite formation to a CO2-poor, alkaline aqueous fluid.
A combined study of 40Ar/39Ar age dating, apatite fission track and chemical characterization of tunnel and surface samples are present to carry out the position of low-temperature water-rock interaction in respect to the Alpine history. Apatite FT analysis yields an exhumation rate of 0.45 mm a-1, a cooling rate of 13 °C Ma-1 and a geothermal gradient of 28 °C km-1. Combining these with the 40Ar/39Ar plateau age for apophyllite of ∼2 Ma, a minimum formation temperature and depth of 70 °C and 2800 m, respectively can be assumed. Temperature-time evolution of fissures in the Aar Massif and thermodynamic mineral evolution indicate that laumontite were formed between 7 and 2 Ma before present at temperatures between 150 and 70 °C.
Das Ziel dieser Arbeit ist das Verständnis über das Auftreten und die Bildung von Zeolithen in Kristallingesteinen am Beispiel der Zentralen Schweizer Alpen, welche im Gegensatz zu dem Vorkommen von Zeolithen in Sedimenten und Vulkaniten nur spärlich erforscht sind. Dabei sind Zeolithe und niedriggradige Mineralvergesellschaftungen in Graniten und Gneisen der Schlüssel zum Verständnis der Wasser-Gesteins Wechselwirkung in hydrothermalen Systemen (< 250 °C).
|SWD-Schlagwörter:||Mineralogie , Geochemie , Zeolith , Schweiz , Alpen , Metamorphose , Schweiz / Neue Eisenbahn-Alpentransversale|
|Freie Schlagwörter (deutsch):||Wasser/Gestein-Wechselwirkung , Laumontit|
|Freie Schlagwörter (englisch):||fluid-rock interaction , laumontite , low-temperature metamorphism|
|Institut:||Institut für Mineralogie, Petrologie und Geochemie|
|Fakultät:||Fakultät für Chemie, Pharmazie und Geowissenschaften|
|Erstgutachter:||Bucher, Kurt (Prof. Dr.)|
|Tag der mündlichen Prüfung:||06.07.2009|