Hydrogen in diopside; diffusion, kinetics of extraction-incorporation, and solubility

Abstract

The kinetic of H extraction-incorporation in diopside single-crystals (Ca 0.97 Na 0.02 Cr 0.01 Mg 0.97 Fe 0.036 Si 1.99 O 6 ) deduced by monitoring OH infrared absorption bands for samples heated from 973 to 1273 K at 0.1 atm and 1 atm of pH 2 , is independent of crystallographic orientation, P O₂ , and pH 2 . The diffusion law is D = D 0 exp[ -(126 ± 24) kJ/mol/RT], with log D 0 (in m 2 /s) = -6.7 ±1.1. Hydrogen self-diffusion obtained from H-D exchange in the same diopside samples over 873-1173 K, and along directions [001] and [100]* at 1 atm total pressure is two orders of magnitude faster than H uptake and follows the diffusion law D H = D 0 exp[ -(149+16) kJ/mol/RT], with log D 0 (in m 2 /s) = - 3.4 ± 0.8. Self-diffusion along [010] follows the diffusion law D H = D 0 exp[ -(143 + 33) kJ/mol/RT], with log D 0 (in m 2 /s) = -5.0 + 1.7 and is one order of magnitude faster than H uptake. The kinetics of extraction incorporation of H in this diopside follows the reaction Fe 3+ + O 2- + 1/2H 2 (g) = Fe 2+ + OH - and are not rate limited by the mobility of protons but more probably by the mobility of electron holes connected with the Fe oxidation-reduction process. The results suggest that the kinetics of H uptake in clinopyroxenes will increase with increasing Fe content until it is rate controlled by the kinetics of H self-diffusion. We predict a rate for H exchange in diopside appropriate to the upper mantle almost as fast as H exchange in olivine. The insensitivity of H solubility on temperature and P O₂ for samples recovered from low-temperature conditions (below 1273 K) and/or rapidly quenched samples let us suggest the use of OH concentration measurements in diopside as a potential pH 2 sensor.