Mineralogical and experimental evidence for very deep exhumation from subduction zones

In 1996, we proposed that the Alpe Arami Iherzolite, Switzerland, contains evidence within it implying origin at a depth of greater than 300 km. Sugge stion of such extraordinary depth of exhumation of these rocks has been con troversial. Principal amongst the original evidence was a very high concent ration of oxide precipitates in olivine. From the abundance, morphology, cr ystallography, and topotaxy of these oxides, it was argued that the inferre d very high solubility of highly-charged cations (principally Ti and Cr) re presented a previously unrecognized mantle environment and that pressures i n 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 concentr ations of TiO2 we originally inferred (> 0.6 wt%). We answer the question i n the affirmative for P greater than or equal to 10 GPa, consistent with ou r hypothesis. In addition to these experimental results, we also have disco vered 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 crystallo graphic observations strongly suggest that the precipitating phase was high -pressure clinoenstatite, thereby providing independent evidence of a minim um depth of origin of this massif of 250 km. To those observations, we add here exsolution of SiO2 from omphacite of Alpe Arami eclogite. Discoveries similarly implying very great depth of exhumation of mantle rocks and/or su bduction to and return from such depths are now known from other continenta l collision terranes. In particular, recent discovery of exsolution of pyro xenes from garnets in the peridotites of the Western Gneiss terrane of Norw ay provides unambiguous evidence of a very deep origin for these peridotite s (>185-200 km), and discovery of microdiamonds in metasediments of the Erz gebirge of Saxony adds to the growing list of continental terranes exhumed from > 100 km. Realization that such deep subduction and exhumation have oc curred 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 p rocess is an important question for future research. (C) 2000 Elsevier Scie nce Ltd. All rights reserved.