Hydrogen in diopside: Diffusion, kinetics of extraction-incorporation, andsolubility

The kinetic of H extraction-incorporation in diopside single-crystals (Ca0. 97Na0.02Cr0.01Mg0.97 Fe0.036Si1.99O6) deduced by monitoring OH infrared abs orption 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). Th e 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] an d [100]* at 1 atm total pressure is two orders of magnitude faster than H u ptake 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] follow s 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 uptak e. The kinetics of extraction incorporation of H in this diopside follows t he reaction Fe3+ + O2- + 1/2H(2) (g) = Fe2+ + OH- and are not rate limited by the mobility of protons but more probably by the mobility of electron ho les connected with the Fe oxidation-reduction process. The results suggest that the kinetics of H uptake in clinopyroxenes will increase with increasi ng Fe content until it is rate controlled by the kinetics of H self-diffusi on. We predict a rate for H exchange in diopside appropriate to the upper m antle almost as fast as H exchange in olivine. The insensitivity of H solub ility on temperature and P-o2 for samples recovered from low-temperature co nditions (below 1273 K) and/or rapidly quenched samples let us suggest the use of OH concentration measurements in diopside as a potential pH(2) senso r.