Fast exhumation of the ultrahigh-pressure Alpe Arami garnet peridotite (Central Alps, Switzerland): constraints from geospeedometry and thermal modelling

Previous studies suggest that the metamorphic evolution of the ultrahigh‐pressure garnet peridotite from Alpe Arami was characterized by rapid subduction to a depth of c. 180 km with partial chemical equilibration at c. 5.9 Gpa/1180 °C and an initial stage of near‐isothermal decompression followed by enhanced cooling. In this study, average cooling rates were constrained by diffusion modelling on retrograde Fe–Mg zonation profiles across garnet porphyroclasts. Considering the effects of temperature, pressure and garnet bulk composition on the Fe–Mg interdiffusion coefficient, cooling rates of 380–1600 °C Myr^?1 for the interval from 1180 to 800 °C were obtained. Similar or even higher average cooling rates resulted from thermal modelling, whereby the characteristics of the calculated temperature‐time path depend on the shape and size of the hot peridotite body and the boundary conditions of the cooling process. The very high cooling rates obtained from both geospeedometry and thermal modelling imply extremely fast exhumation rates of c. 15 mm yr^?1 or more. These results agree with the range of exhumation rates (16–50 mm yr^?1) deduced from geochronological results. It is suggested that the Alpe Arami peridotite passively returned towards the surface as part of a buoyant sliver, caused as a consequence of slab breakoff.     

Reconstruction of the exhumation path of the Alpe Arami'ql garnet-peridotite body from depths exceeding 160 km

Chemical disequilibrium exists between all phases of the Alpe Arami garnet-peridotite body (Ticino, Switzerland) which hampers the evaluation of P–T  conditions of origin, yet disequilibrium offers the inherent possibility to derive a P–T–t path for this mantle slice. We tried to tackle this problem by carrying out new mineral analyses and taking diffusion rates and bulk-rock compositional effects into consideration. Peak metamorphic conditions from mineral core compositions were estimated as 1120±50 °C/5±0.2 GPa. These values are significantly higher than previously published results and were determined from a combination of the O'Neill & Wood (1979) Fe/Mg garnet–olivine exchange thermometer and the Al-in-orthopyroxene barometer (Brey & Köhler, 1990), and are supported by the Ca/Cr ratios in garnet, which are in accord with these conditions. Details of the exhumation path were derived from (1) rim compositions of minerals that yield a first retrograde stage of 720±50 °C/2±0.25 GPa (2) a spinel lherzolite assemblage in narrow shear zones (tectonic phase F0', after Möckel, 1969) which documents a second retrograde stage at 500–600 °C/0.8–1.5 GPa. The Ca content in olivine (Köhler & Brey, 1989) can be used to evaluate further P–T  conditions along the retrograde path. We measured very low values (30–40  ppm Ca) in the cores of olivine and a remarkable increase towards the rim (120  ppm). The low core values may reflect an equilibrium stage during the main Alpine metamorphism. The increasing values towards the olivine rims probably represent a late-stage heating event. The initial cooling rates for the peridotite body are between 2700 and 5100 °C Ma⁻¹, depending on which diffusion data are used.     

Silica precipitates in omphacite from eclogite at Alpe Arami, Switzerland: evidence of deep subduction

Observations of oriented SiO₂ precipitates in omphacite from eclogite with tholeiitic basalt protolith bordering the Alpe Arami garnet peridotite massif, Ticino, Switzerland, and petrological studies of the eclogitic mineral assemblages, suggest that this rock was subjected to higher‐pressure metamorphism than previously realized. We employed various calibrations of the Fe²⁺ ? Mg exchange thermometer and calculations of equilibria with thermodynamic data, considering the calcium–Tschermak's component (CaAl₂SiO₆), of garnet‐pyroxene pairs. From these calculations, it is concluded that the eclogitic lenses have recorded at least four stages of mineral growth corresponding to the following: Stage I (prograde) c. 2.4 GPa; 700 °C; Stage IIa (maximum recorded grade) c. 7.0 GPa; 1100 °C; Stage IIb (retrograde) c. 3.7 GPa; 900 °C; Stage III (retrograde) c. 2.1 GPa; 750 °C. Because of the preservation of Stage I, a relatively rapid subduction and exhumation of Alpe Arami eclogite is suggested. The exhumation path of the eclogitic rock is in good agreement with most exhumation paths inferred for the Alpe Arami garnet lherzolite proposed previously by several authors based upon a variety of different observations, although the eclogite and peridotite exhumation paths may diverge at depths greater than 120 km.     

Formation of kyanite–quartz veins of the Alpe Sponda,Central Alps,Switzerland: implications for Al transport during regional metamorphism

In this study, we have investigated the formation of quartz–kyanite veins of the Alpe Sponda, Central Alps, Switzerland. We have integrated field observations, fluid inclusion and stable isotope data and combined this with numerical geochemical modeling to constrain the chemical processes of aluminum transport and deposition. The estimated P–T conditions of the quartz–kyanite veins, based on conventional geothermometry (garnet–biotite, white mica solvus and quartz–kyanite oxygen isotope thermometry) and fluid inclusion data, are 550 ± 30°C at 5.0 ± 0.5 kbar. Geochemical modeling involved construction of aqueous species predominance diagrams, calculation of kyanite and quartz solubility, and reaction–path simulations. The results of the modeling demonstrate that (1) for the given chemical composition of the vein-forming fluids mixed Al–Si aqueous species are dominant in transporting Al, and that (2) fluid cooling along a small temperature gradient coupled with a pH decrease is able to explain the precipitation of the quartz–kyanite assemblages in the proportions that are observed in the Alpe Sponda veins. We conclude that sufficient amounts of Al can be transported in typical medium- to high-grade regional metamorphic fluids and that immobile behavior of Al is not very likely in advection–dominanted fluid–rock systems in the upper and middle crust.