New Constraints on the P-T Evolution of the Alpe Arami Garnet Peridotite Body (Central Alps, Switzerland)

The metamorphic evolution of the garnet peridotite body of AlpeArami, Central Alps, is a matter of current controversy. Inthis paper, the inter- and intragrain distribution of majorand trace elements obtained by electron and ion probe microanalysesis used to better constrain the P–T evolution of thisperidotite. Using the compositions of homogeneous porphyroclastcores, peak metamorphic conditions of 1180 ± 40°Cand 5·9 ± 0·3 GPa are estimated, basedon consistent results from the application of several independentthermometers (Fe–Mg exchange between garnet, pyroxenesand olivine, Ni exchange between garnet and olivine, Co andNi exchange between orthopyroxene and clinopyroxene), the Al-in-orthopyroxenebarometer and the Ca–Cr systematics of garnet. Orthopyroxeneand clinopyroxene porphyroclasts are, however, not in equilibriumwith respect to some elements with low diffusivities, such asCa, Ti, Cr, V and Sc. This disequilibrium appears to be themain cause for the lower P–T values suggested by someof the previous workers. On the other hand, there is no evidencefor an ultradeep (>200 km) origin of the Alpe Arami bodyas postulated recently. Chemical zonation profiles across mineralgrains suggest that during retrograde evolution a near-isothermaldecompression was followed by accelerated cooling. KEY WORDS: Alpe Arami; Central Alps; garnet peridotite; ultrahigh-pressure metamorphism; geothermobarometry; secondary ion mass spectrometry (SIMS)