Experimental evaluation of the Coesite-Quartz phase boundary at low-temperature & high-pressure conditions

Coesite was first synthesized in the early 1950s, then discovered in nature in 1960, and first applied as an index mineral for pressure-temperature (P-T) metamorphic condition and tectonic setting in 1984 with applications in the Dora Maira Massif (Italy) and the Western Gneiss Region (Norway). Until today, several experimental studies have focused on the calibration of the coesite-quartz phase boundary, mostly at high-temperature (HT) and high-pressure (HP) conditions. Among the various studies, there is a general agreement with only small differences at HT-HP conditions. For example, at 1200 °C the coesite-quartz phase boundary ranges between 31 to 33.5 kbar. However, differences in the HT are amplified in extrapolations to lower temperatures that are relevant to several recent coesite discoveries. At 550 °C the coesite-quartz phase boundary ranges between 23 to 28 kbar, providing a wide spread of P conditions.

The increasing number of coesite discoveries from different terranes in recent years highlights the importance of precisely locating the coesite-quartz phase boundary at low-temperature (LT). This study aims to validate the coesite-quartz phase boundary at LT (550-750 °C) and HP (28-30 kbar). The chosen P-T conditions are typical for coesite crystallization in subduction zone settings. The experiments were conducted in end-loaded piston-cylinder apparatuses using 12.7-mm diameter experimental assemblies composed of MgO filler pieces, graphite heater tubes, borosilicate glass insulators, and NaCl. Silver capsules, with volumes varying from 20 to 15 mm³, were filled with amorphous SiO₂ powder and deionized H₂O (≈2:1 ratio). Ten experiments were successfully performed. The experiments were conducted along the 550 °C, 650 °C and 750 °C isotherms at 28, 29, and 30 kbar for 48-72 hours. In summary, at 750 °C, 100 % coesite formed at 30 kbar, and 100% quartz was obtained at 29 kbar. At 650 °C, we synthesized 100 % coesite at 30 kbar, 100 % quartz at 28 kbar, and both large crystals of coesite (>200 μm) and small crystals of quartz (<100 μm) at 29 kbar. At 550 °C, experiments resulted in 100 % of small coesite crystals (<70 μm) at 30 kbar, 100 % quartz crystals at 28 kbar, and both coesite and quartz at 29 kbar. Moreover, a reversal experiment was conducted from quartz to coesite. Quartz was crystallized experimentally at 700 °C and 10 kbar for 24 hours. Then, it was loaded in a new experiment at 650 °C and 30 kbar, resulting in a complete recrystallization of coesite; no quartz was left at the end of the experiment.

These results suggest that the coesite-quartz phase boundary at relatively low temperature may occur at higher pressure than previously extrapolated. The ongoing reaction reversals experiments from coesite stability field to quartz will better constrain the character of coesite-quartz phase boundary at LT-HP conditions typical for subduction zone settings, where coesite likely crystallized in natural rocks. Acknowledgements: This work was funded by the National Science Centre (Poland) through the project 2021/43/D/ST10/02305 to K. Kośmińska.