Fig. 1.

(A and B) Photomicrographs of 89321 coesite eclogite from the Papua New Guinea UHP terrane. Cross-polarized light at 10× magnification. Abbreviations are garnet (Grt), omphacite (Omp), phengite (Ph), and rutile (Rt). Coesite (Coe) occurs as an inclusion in omphacite. (C) Peak pressure–temperature–time constraints for 89321 coesite eclogite (star). Thermobarometry constraints for coesite eclogite are indicated by dark gray boxes (22). Timing of peak metamorphism based on ⁴⁰Ar/³⁹Ar phengite (this study), Lu-Hf garnet (27), and U-Pb zircon (31) ages. Apatite fission track ages (0.6 Ma) from host gneiss at the coesite locality constrain the timing of exhumation to shallow crustal levels (27). Abbreviations for metamorphic facies are blueschist (BS), greenschist (GS), amphibolite (AM), eclogite (EC), and granulite (GR); mineral abbreviations are albite (Ab), jadeite (Jd), and quartz (Qtz). The light gray path shows the range in pressure–temperature conditions for the UHP terrane (33). The dashed blue line indicates subduction zone geotherm. The solidus for water-saturated crustal rocks and dehydration melting of phengite (36, 52) provide maximum temperature limits for the pressure–temperature–time path followed by coesite eclogite during exhumation to the surface.