Atmospheric Ar and Ne trapped in coesite eclogite during Late Miocene (U)HP metamorphism: implications for the recycling of noble gases in subduction zones

Several isotopic methods, including ⁴⁰Ar/³⁹Ar dating of phengite, have been used to determine the timing and duration of (U)HP metamorphism. However, in some (U)HP terranes phengite ⁴⁰Ar/³⁹Ar data , has yielded anomalously old ages interpreted to result from the presence of extraneous Ar (i.e., either inherited or excess Ar). We analyzed Ar and Ne extracted from phengite and omphacite from coesite eclogite in the Papua New Guinea (U)HP terrane to 1) assess the reliability of ⁴⁰Ar/³⁹Ar phengite ages to record the timing of (U)HP metamorphism in the youngest (U)HP terrane on Earth, and 2) to assess the non-radiogenic trapped Ar and Ne compositions in minerals that crystallized during subduction zone metamorphism. Step heat experiments on irradiated phengite yielded a ⁴⁰Ar/³⁹Ar weighted mean age of 8.31 +/- 0.32 Ma (2σ) corresponding to ~88% ³⁹Ar released. These results are concordant with previously published ²³⁸U/²⁰⁶Pb zircon ages, and nearly concordant with a Lu-Hf garnet isochron age, both obtained on the same sample. Results suggest that phengite reliably records the timing of peak (U)HP metamorphism and that excess ⁴⁰Ar is not present in this coesite eclogite. Step heat experiments on irradiated phengite and pyroxene yielded ³⁸Ar/³⁶Ar above atmospheric values (>0.1885). These higher ³⁸Ar/³⁶Ar ratios from outgassed irradiated samples results from reactor-produced ³⁸Ar_Cl likely due to the presence of Cl-derived from fluid inclusions (i.e., via the nuclear reaction ³⁷Cl(n,γ)³⁸Cl(β)³⁸Ar). The high temperature release of ³⁸Ar_Cl may result from smaller fluid inclusions (<1-2 μm). To further investigate the composition of non-radiogenic trapped Ar and Ne in coesite eclogite, step heat experiments were performed on multiple unirradiated splits of phengite and omphacite. Both minerals yielded atmospheric ³⁸Ar/³⁶Ar, including for high temperature (>1400°C) steps. The abundance of radiogenic ⁴⁰Ar corresponds to the respective [K] and ~8 Ma age of minerals also suggesting the absence of excess ⁴⁰Ar in these samples. Omphacite outgassed at high temperature (>1400°C) also yielded atmospheric ²⁰Ne/²²Ne. Results indicate that atmospheric Ar and Ne were trapped when minerals crystallized at ~8 Ma during (U)HP metamorphism. The survival of trapped atmospheric Ar and Ne in minerals formed during (U)HP metamorphism supports models that call for recycling of noble gases from the atmosphere back into the mantle at subduction zones.