QUARTZ EXSOLUTION IN CLINOPYROXENE IS NOT PROOF OF ULTRA-HIGH PRESSURES: EVIDENCE FROM PHASE EQUILIBRIA AND ECLOGITE FROM THE EASTERN BLUE RIDGE, SOUTHERN APPALACHIANS, USA

Oriented quartz rods in clinopyroxene have become widely used as one of the diagnostic indicators of ultra-high pressure (UHP) metamorphism. Exsolved quartz is taken as evidence of decompression of a non-stochiometric Ca – Eskola pyroxene (Ca_0.5[]_0.5AlSi₂O₆) by the following reaction: 2Ca_0.5[]_0.5AlSi₂O₆ (CaEs)=CaAl₂SiO₆ (CaTs) + 3SiO₂. CaEs pyroxene is presumed to be stable only at UHP conditions based on experimental work. Reintegration of quartz rods with host pyroxene to create a CaEs-bearing pyroxene composition has been used by workers to claim pressures of ≥ 25 kbar. Reintegration is often based on transmitted light optics, which may lead to overestimation of quartz and incomplete identification of exsolved minerals. Electron images are less likely to overestimate quartz, and in some cases show the presence of amphibole associated with exsolved quartz. Eclogite from the Eastern Blue Ridge, North Carolina contains clinopyroxene (Jd₂₀CaTs₅Ac₅CaEs₀Di₆₅Hd₅) with oriented needles of quartz and calcic amphibole (pargasite – tschermakite - edenite) that appear to have exsolved together. The quartz + amphibole exsolutions are surrounded by 1-5 μm zoned haloes (~Jd₁₀CaTs₁₀Ac₅CaEs₀Di₇₀Hd₅). The modes of quartz (1.8–5.6 vol. %), amphibole (3.3–12.1 vol. %) and clinopyroxene haloes (3.1–11.4 vol. %) were determined using BSE images, and reintegrated with the host clinopyroxene. Reintegrated pyroxene compositions were nearly identical to the analyzed host pyroxene with no CaEs component. CaEs pyroxene has been repeatedly synthesized at UHP conditions in a number of studies. However, examination of the phase equilibria cited as evidence for CaEs stability only at conditions of ≥ 25 kbar shows that clinopyroxene with 10 mol % CaEs is stable well within the quartz field, and provides a pressure minimum similar to the albite=jadeite + quartz barometer. The presence of a dilute CaEs component in clinopyroxene does not require UHP conditions. Exsolution of quartz needles with or without amphibole does not require a CaEs component in the original pyroxene. Such exsolution is present in the Blue Ridge eclogite that has a well-constrained P-T path and does not contain coesite or other evidence for UHP metamorphism.