Research Interests

Alpine stilbite: Fluid induced mineral composition adjustments during exhumation

Research Collaboratores:
Prof. Dr. Kurt Bucher, Institute for Geosciences Freiburg University, Germany

Stilbite is locally present as a very late mineral on fractures and fissures of granitic basement in the Central Swiss Alps. Stilbite samples from the Gotthard rail base tunnel provide evidence that they originally formed as a K-absent variety at depth. However, all stilbite samples from surface outcrops above the tunnel display significant potassium concentrations. Interestingly, water from fractures in the tunnel (at 50˚C) is oversaturated with respect to stilbite and essentially potassium-free whereas waters from high-Alpine brooks above the tunnel (and at other high-Alpine areas) have unusually high K/Na ratios.
The data suggest that stilbite that may actively form on tunnel fissures as a K-absent variety by precipitation from water. The stilbite fracture coatings one formed were gradually exhumed and uplifted and finally reached the today’s erosion surface about 2000 m above the tunnel. However, the stilbite reaches the erosion surface as a K-rich variety as a result of interaction of the original low-K stilbite with surface water and near-surface groundwater.
This leads to the conclusion that minerals once formed at depth may significantly change their composition once they reach the ground water zone on their way to the erosion surface. In the case of the stilbites, if surface outcrops would have been the only source of samples and data, the K-rich composition could have been mistaken for the composition of the mineral when it formed, which is not the case. Late stage compositional readjustments may be difficult to discern in samples from surface outcrops. The provided data that show that original mineral compositions may be adjusted by late stage water-rock interaction in a highly selective way.

Bucher K. & Weisenberger T.B. (2013) Fluid induced mineral composition adjustments during exhumation: The case of Alpine stilbite. Contributions to Mineralogy and Petrology, 166 (5). 1489-1503. doi: 10.1007/s00410-013-0939-5 (pdf)


Zeolites formation in crystalline basement rocks (Switzerland)

Research Collaboratores:
Prof. Dr. Kurt Bucher, Institute for Geosciences Freiburg University, Germany


Six different Ca-zeolite minerals occur widespread in various assemblages in late fissures and fractures in granites and gneisses of the Swiss Alps. The zeolites formed as a result of water-rock interaction at relatively low temperatures (< 250°C) in the continental upper crust. The zeolites typically overgrow earlier minerals of the fissure assemblages, but zeolites also occur as monomineralic fissure fillings. They represent the youngest fissure minerals formed during uplift and exhumation of the Alpine orogen. A 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 Massifs, (2) Gibelsbach/Fiesch, in a fissure breccia located at the boundary of Aar Massif and Permian sedimentary rocks, and (3) in Penninic gneisses of the Simano nappe at Arvigo (Val Calanca).
Rail and road tunnel construction across the Aar- and Gotthard massif provided excellent data on zeolite frequency in Alpine fissures. It was found that 32 % (Gotthard NEAT rail base tunnel, Amsteg section) and 18% (Gotthard road tunnel) of all studied fissures are filled with zeolites. The number of different zeolites is limited to six species: laumontite, stilbite and scolecite are abundant and common, whereas heulandite, chabazite and epistilbite occur occasionally. Calcium is the dominant extra-framework cation, with minor K and Na. Heulandite and chabazite contain Sr up to 29 and 10 mol % extra-framework cations, respectively. Na and K contents in zeolites tend to increase during growth as a result of changes in fluid composition and/or temperature. The K enrichment of stilbite found in surface outcrops compared to subsurface samples may indicate late stage cation exchange with surface water. Texture data, relative age sequences derived from fissure assemblages and equilibrium calculations show 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 necessary components for zeolite formation are derived from dissolving primary granite and gneiss minerals. The nature of these minerals depends, among other factors, on the metamorphic history of the host rock. Zeolites in the Aar Massif derived from the dissolution of epidote, secondary calcite and albite that were originally formed during Alpine greenschist metamorphism from primary granite and gneiss assemblages. Zeolite fissures occur in areas of H2O dominated fluids. This is consistent with equilibrium calculations that predict a low CO2 tolerance of zeolite assemblages, particularly at low temperature.

Reference:
Weisenberger T. & Bucher K. (2010) Zeolites in fissures of granites and gneisses of the Central Alps. Journal of Metamorphic Geology, 28 (8), 825-847.  doi:10.1111/j.1525-1314.2010.00895.x (pdf)

Mass tranfer, mineral evolution and pororsity  generation during fluid-rock interaction in crysalline basement rock

Research Collaboratores:
Prof. Dr. Kurt Bucher, Institute for Geosciences Freiburg University, Germany


Low-grade mineral assemblages are the key to the appreciation of water-rock interaction in hydrothermal and geothermal systems located in granites and gneisses. Zeolite formation is an important process in rocks of the continental crust. It takes place at temperatures below 250°C under hydrothermal conditions. A detailed 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.

Reference:
Weisenberger T. & Bucher K. (2011) Mass transfer and porosity evolution during low temperature water-rock interaction in gneisses of the Simano nappe: Arvigo, Val Calanca, Swiss Alps. Contributions to Mineralogy and Petrology. 162 (1), 61-81. doi: 10.1007/s00410-010-0583-2 (pdf)

Timing of low-temperature mineral formation during exhumation and cooling in the Central Alps, Switzerland

Research Collaboratores:
Prof. Dr. Kurt Bucher, Institute for Geosciences Freiburg University, Germany
PD Dr. Meinert Rahn, Eidgenössisches Nuklearsicherheitsinspektorat ENS, Switzerland
Roelant vn der Lelij & Dr. Richard Spikings, Department of Mineralogy, Université de Genève, Switzerland


The mineral assemblage quartz, laumontite and apophyllite-(KF) occur in a fissure within the Southern Aar granite located in the Aar Massif (Switzerland). They were formed during exhumation of the Alpine orogen and laumontite and apophyllite marks the latest fissure minerals in the Central Alps. A combined study of 40Ar/39Ar age dating, apatite fission track (FT) 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.
Elements for laumontite formation derived during dissolution of primary minerals. Changes of laumontite chemistry could be an effect of temperature drop or a change in fluid chemistry that would be supported by later apophyllite formation.

Reference:
Weisenberger T.B., Rahn M. van der Lelji R., Spikings R. & Bucher K.  (2012) Timing of low-temperature mineral formation during exhumation and cooling in the Central Alps, Switzerland. Earth and Planetary Science Letters. 327-328, 1-8, doi: 10.1016/j.epsl.2012.01.007 (pdf)