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 crust can be subducted to much greater depths than previously considered reasonable. Documentation of such extreme subduction has come from discovery of diamond, coesite and other high pressure phases; in many cases these phases, metastable at the Earth's surface, are only preserved as inclusions in strong, refractory minerals that either have a broad pressure range of stability or exhibit very sluggish kinetics of reaction to low pressure forms. The Alpe Arami peridotite of the Swiss Alps displays extensive exsolution of FeTiO3 rods in the oldest generation of olivine. The shape, orientation and abundance of the titanate rods provide strong indication that the phase originally exsolved was the orthorhombic perovskite phase stable only at pressures greater than 10 GPa (300 km depth) at mantle temperatures. We show here that the dislocation 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 incapable of producing the unique and unexplained lattice preferred orientation (LPO) displayed by this generation of olivine. We also have conducted preliminary experiments to investigate the maximum solubility of FeTiO3 in olivine. Our results suggest that the solubility of TiO2 implied 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 experiments is comparable to that inferred for Alpe Arami olivine. This latter observation combined with the determination that the titanate rods 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 envision this to have been accomplished is for the Lepontine gneisses that now surround the massif to have been subducted to great depth following collision of Africa and Europe, and to have picked up the peridotite on their way back to the surface by buoyant upwelling.