The Alpe Arami story: Triumph of data over prejudice

Abstract

The Alpe Arami garnet peridotite of the Southern Swiss Alps is associated with eclogites and included within quartzofeldspathic gneisses. Controversy has swirled around the depth of origin of this massif since the 1970s when application of the newly-developed technique of thermobarometry suggested a depth of last equilibration of greater than 120 km. Such controversy accelerated in 1996 when we reported microstructural evidence of extensive precipitation of ilmenite and spinel from oli- vine and proposed a much greater depth of origin. Subsequent experiments showed that it was possible to dissolve the observed amount of TiO2 in olivine, but only at depths in excess of 300 km, agreeing with the earlier proposal. In 1999 we added new, independent, evidence concerning exsolution of high-pressure clinoenstatite from diopside that in-and-of-itself required a depth of origin in excess of 250 km. Subsequently, we also added evidence from the surrounding eclogites of very high pressures and experimental evidence that the pyroxenes included in the amoeboid garnets of this rock had ex- solved from a majoritic parent at perhaps even greater pressures. In refutation of the first two of these observations, suggestions were made that (i) we had made a serious error in our estimate of how much ilmenite was present in olivine (and therefore how much TiO2 had been dissolved in olivine); (ii) the il- menite had not been exsolved from olivine but former titanian clinohumite had been present and broke down to yield the ilmenite; (iii) the pyroxene exsolved from diopside had been high-temperature cli- noenstatite. In all three of these cases, the alternatives offered were claimed to be accommodated at low pressures. Here we review the essence of this controversy and show that the only scenario that can ex- plain all of the data is the one that we originally proposed; indeed, the more recent data have strongly supported that interpretation and pushed the minimum origin of the massif to depths approaching 400 km.