In-situ high-pressure and high-temperature spectroscopic studies of phengite in ultrahigh-pressure eclogite: implications for water transport during ultra-deep continental subduction

Abstract

Pressure and temperature responses of natural phengite [K_0.93Na_0.03(Al_1.46Mg_0.45Fe_0.09)(Si_3.59Al_0.39Ti_0.02)O₁₀(OH_1.94F_0.06)] in ultrahigh-pressure eclogite from the main hole of the China Continental Scientific Drilling Project (CCSD), the Dabie-Sulu orogenic belt have been studied using in-situ high-pressure mid-infrared and high-temperature Raman spectroscopic measurements up to ~ 20 GPa and 800 °C, respectively. Linear positive pressure dependences were observed for the infrared absorption bands associated with the aluminosilicate vibrations up to ~ 19 GPa, indicating the steady compression of the structure framework. The frequencies of the O–H stretching doublet bands, initially at 3601 and 3626 cm⁻¹, displayed linear downshifts up to 16.6 GPa at − 2.02 and − 2.72 cm⁻¹/GPa, respectively, implying high stability of the hydroxyl groups under compression. In the high-temperature Raman spectra, the bands initially centered at 265, 420, 703 cm⁻¹, and the O–H stretching modes at 3620 cm⁻¹ exhibited modest linear negative shifts with increasing temperature up to 800 °C. Comparisons between experimental results of the present study and those of the previous studies make it plausible that phengite with a higher Si content, i.e., a higher tetrahedral Si/Al ratio, would have higher stabilities under both high pressure and high temperature, and is likely to transport water to greater depths during subduction processes.