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

The kinetic of H extraction-incorporation in diopside single-crystals (Ca (sub 0.97) Na (sub 0.02) Cr (sub 0.01) Mg (sub 0.97) Fe (sub 0.036) Si (sub 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 + or - 24) kJ/mol/RT], with log D ₀ (in m ² /s) = -6.7+ or -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] (super *) 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+ or -16) kJ/mol/RT], with log D ₀ (in m ² /s) = -3.4+ or -0.8. Self-diffusion along [010] follows the diffusion law DH = D ₀ exp[-(143+ or -33) kJ/mol/RT], with log D ₀ (in m ² /s) = -5.0+ or -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 (super 3+) +O (super 2-) +1/2H ₂ (g) = Fe (super 2+) +OH (super -) 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.