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.