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.

This content is PDF only. Please click on the PDF icon to access.

First Page Preview

First page PDF preview
You do not have access to this content, please speak to your institutional administrator if you feel you should have access.