PSYCHOLOGY OF A CHANGING PARADIGM: ULTRA-HIGH-PRESSURE METAMORPHISM

The evolution of Ultra High Pressure Metamorphism (UHPM) is both a scientific and human story – of application of the scientific method to verify new advances but also of emotional resistance to new ideas. Before the paradigm of plate tectonics, the concept of isostasy had the effect of conceptually isolating crust from mantle except for breaches by volcanism. Geosyncline theory envisioned crustal materials to be essentially isolated from the mantle. In the 1960s, however, Gary Ernst discovered in the laboratory that glaucophane is stable only at low T and (for that time) high P, with implication that blueschists go down 10s of km at geologically rapid rates and immediately return to the surface. No mechanism was evident for such rapid burial and exhumation; the concept of tectonic overpressure was created to explain away the laboratory data. The advent of plate tectonics provided a simple mechanism for rapid burial and exhumation and tectonic overpressure was experimentally discredited as an alternative. Nevertheless, this false concept is invoked to this day when uncomfortably high pressures are implied by observations. Similarly, in the 1970s when Gary Ernst obtained a pressure of 4 GPa for the Alpe Arami peridotite using geobarometery, the dominant response was belief that refinement of the technique would reduce the apparent depth - but all later P determinations but one have yielded greater depths.

Real progress in acceptance of UHPM began with the discovery of coesite in Dora Maira and Norway, but not without extreme resistance, especially in Norway. Similarly, discovery of metamorphic microdiamonds continues to be rejected by many until confirmed in thin section and microstructural evidence of the former presence of mineral compositions not stable at low P continues to be greatly disputed except in cases where experimental evidence predated discovery of the relevant microstructure(s). This, despite the fact that the method has long been used to reconstruct high-T compositions of, for example, feldspars and pyroxenes.

Nevertheless, progress in this field has accelerated since the mid-1990s, establishing that deep subduction of continental material has accompanied continental collision multiple times in multiple places. Models are evolving that explain this phenomenon in the context of plate tectonics.