TIMESCALES AND RATES OF SUBDUCTION CHANNEL PROCESSES: INSIGHTS FROM THE (U)HP TERRANE OF SOUTHEAST PAPUA NEW GUINEA

Documenting pressure-temperature-time histories of rocks subducted to mantle depths and returned to the surface is particularly challenging because protoliths (re)crystallize throughout their evolution and mineral assemblages rarely chemically and isotopically equilibrate. Bruce Watson’s legacy of pioneering research on the redistribution of chemical elements and their isotopes in the solid Earth has inspired multimethod thermochronologic and trace element thermobarometric studies that reveal the timescales of metamorphic and magmatic processes in the (ultra)high pressure ((U)HP) terrane of Papua New Guinea (PNG). The timing of UHP metamorphism is 7–8 Ma, known from concordant ages obtained on cogenetic metamorphic zircon (U-Pb SIMS), garnet (Lu-Hf), and phengite (⁴⁰Ar/³⁹Ar) from coesite eclogite. Both phengite and omphacite yielded atmospheric ³⁸Ar/³⁶Ar and ²⁰Ne/²²Ne, trapped during crystallization at mantle depths. There is no evidence that phengite contains excess Ar, an interpretation often used to dismiss the validity of ⁴⁰Ar/³⁹Ar data from (U)HP terranes. Zircon U-Pb and trace element SIMS depth profiles from the coesite eclogite host gneiss document intragrain age and geochemical heterogeneities indicating chemical and isotopic disequilibrium during zircon growth following (ca. 4 m.y.) UHP metamorphism. Many aspects of Bruce Watson’s legacy remain to be applied to the (U)HP PNG terrane. For example, modelling the rates of zircon dissolution and growth during subduction and exhumation can constrain transport through the rock cycle. Additionally, partial melts crystallized during rapid (cm/yr) exhumation provide targets to assess cold magma storage and transport. These highlighted results, together with future research targets, indicate that the (U)HP terrane is a natural laboratory to determine the flux of recycled materials, including atmospheric gases, that are subducted to, and exhumed from mantle depths.