Mantle rocks uplifted to crustal levels: Diffusion profiles in minerals ofspinel-plagioclase lherzolites from Tallante, Spain

Xenoliths containing plagioclase and Al-rich spinel have been found in lher zolites from Spain. The presence of these two Al-rich minerals permits us t o estimate the pressure of mineral equilibria and helps to decipher the the rmal history of mantle rocks. In this article, we describe the chemical com position of minerals from these mantle-derived peridotites with special emp hasis on zoning in minerals and its interpretation in the framework of diff usion models. Mineral geothermobarometry and modeling of diffusion profiles permit us to outline the general features of the thermal history of the ro cks as follows: nearly adiabatic ascent of a relatively hot (ca. 1200 degre esC) diapir (from the asthenosphere?) through the depth range of the stabil ity of spinel lherzolites with a velocity of 10 mm/y or less over 3-4 Ma or more, During this time, concentrations of slowly diffusing components (Al, Cr) became uniform in pyroxene grains and reached relatively high levels. After the protolith was intruded into the continental crust at a depth of c a. 15 km or even less, it started to cool owing to heat losses by conductio n until the temperature decreased to 900-850 degreesC, after which it was a ffected by a magmatic event that brought the Iherzolite nodules to the surf ace. The stage of thermal relaxation from ca. 1200 to ca. 850-900 degreesC could have taken approximately 1 Ma as suggested by the modeling of Al diff usion haloes in pyroxene grains. The results of this work indicate that the diapir sampled by the investigated nodules was characterized by unusually high kinetic mechanical energy, which led to emplacement at shallow depth ( ca, 15 km or even less) and resulted in the nearly complete breakdown of th e continental lithosphere.