Mineralogical and experimental evidence for very deep exhumation from subduction zones

Abstract

In 1996, we proposed that the Alpe Arami lherzolite, Switzerland, contains evidence within it implying origin at a depth of greater than 300 km. Suggestion of such extraordinary depth of exhumation of these rocks has been controversial. Principal amongst the original evidence was a very high concentration of oxide precipitates in olivine. From the abundance, morphology, crystallography, and topotaxy of these oxides, it was argued that the inferred very high solubility of highly-charged cations (principally Ti and Cr) represented a previously unrecognized mantle environment and that pressures in excess of 10 GPa were the most likely explanation of the observations. We have now successfully completed high-pressure experiments to test whether there are any conditions under which olivine can dissolve the high concentrations of TiO 2 we originally inferred (>0.6 wt%). We answer the question in the affirmative for P≥10 GPa, consistent with our hypothesis. In addition to these experimental results, we also have discovered in the same rocks exsolution lamellae of clinoenstatite in diopside. These lamellae contain antiphase domains which indicate that the originally-precipitating phase was a C2/c pyroxene; additional geologic and crystallographic observations strongly suggest that the precipitating phase was high-pressure clinoenstatite, thereby providing independent evidence of a minimum depth of origin of this massif of 250 km. To those observations, we add here exsolution of SiO 2 from omphacite of Alpe Arami eclogite. Discoveries similarly implying very great depth of exhumation of mantle rocks and/or subduction to and return from such depths are now known from other continental collision terranes. In particular, recent discovery of exsolution of pyroxenes from garnets in the peridotites of the Western Gneiss terrane of Norway provides unambiguous evidence of a very deep origin for these peridotites (>185–200 km), and discovery of microdiamonds in metasediments of the Erzgebirge of Saxony adds to the growing list of continental terranes exhumed from >100 km. Realization that such deep subduction and exhumation have occurred multiple times spanning the entire Phanerozoic strongly suggest that this phenomenon is a normal process of continental collision rather than a bizarre curiosity. Whether such exhumation is a single- or multiple-step process is an important question for future research.