ALPE ARAMI - A PERIDOTITE MASSIF FROM THE MANTLE TRANSITION ZONE

Petrologic discoveries made over the last ten years have shown that in regions of continent-continent collision, rocks of the continental cr ust can be subducted to much greater depths than previously considered reasonable. Documentation of such extreme subduction has come from di scovery of diamond, coesite and other high pressure phases; in many ca ses these phases, metastable at the Earth's surface, are only preserve d as inclusions in strong, refractory minerals that either have a broa d pressure range of stability or exhibit very sluggish kinetics of rea ction to low pressure forms. The Alpe Arami peridotite of the Swiss Al ps displays extensive exsolution of FeTiO3 rods in the oldest generati on of olivine. The shape, orientation and abundance of the titanate ro ds provide strong indication that the phase originally exsolved was th e orthorhombic perovskite phase stable only at pressures greater than 10 GPa (300 km depth) at mantle temperatures. We show here that the di slocation substructure of the oldest generation of olivine is younger than the titanate rods and similar to that observed in peridotites the world over and in experiments; the slip systems represented are incap able of producing the unique and unexplained lattice preferred orienta tion (LPO) displayed by this generation of olivine. We also have condu cted preliminary experiments to investigate the maximum solubility of FeTiO3 in olivine. Our results suggest that the solubility of TiO2 imp lied by the abundance of titanate precipitates may be impossible under any conditions of olivine stability. On the other hand, the measured solubility in wadsleyite (beta-olivine) under the conditions of our ex periments is comparable to that inferred for Alpe Arami olivine. This latter observation combined with the determination that the titanate r ods and LPO of this generation of olivine are the oldest features yet identified in these rocks, leads us to speculate that this massif has been brought to the Earth's surface from within the mantle transition zone, at depths of 410-660 km. The only mechanism by which we can envi sion this to have been accomplished is for the Lepontine gneisses that now surround the massif to have been subducted to great depth followi ng collision of Africa and Europe, and to have picked up the peridotit e on their way back to the surface by buoyant upwelling.