Contrasted effect of aluminum on the serpentinization rate of olivine and orthopyroxene under hydrothermal conditions

Abstract

Olivine and pyroxene are the major minerals of ultramafic rocks. The hydrothermal alteration of these rocks leads to the serpentinization reaction that mainly forms serpentine and variable amounts of talc, brucite and magnetite, as well as hydrogen. The serpentinization kinetics of pyroxene under hydrothermal conditions has been very little studied in comparison with olivine, and both have been evaluated experimentally only in simple aqueous fluids. Here, we evaluate the effect of aluminum on the serpentinization rate of olivine and orthopyroxene at 200°C, 340°C and 200MPa to simulate natural hydrothermal conditions. We used low-pressure diamond-anvil cells (lp-DAC) and time-resolved X-ray diffraction to monitor in situ the progress of the serpentinization reaction in four experiments. We also performed two long-lasting additional experiments with orthopyroxene for six days at 340°C and 200MPa, for which in situ monitoring was not possible. At 340°C in presence of Al, olivine conversion into lizardite is extremely fast (half-time reaction t1/2=7h) while orthopyroxene did not react much even after 6days (11%). In contrast to olivine, orthopyroxene conversion to serpentine was faster without Al (48% in 6days). Magnetite was also observed in the run with olivine only at 340°C. In experiments run with orthopyroxene only, we observed the exclusive formation of proto-serpentine instead of lizardite. We propose that the contrasted effect of Al on the serpentinization rate of olivine and orthopyroxene results from the complexation of Al in the solution that reacts differently with the different mineral surfaces during their dissolution. The positively charged olivine surface allows the adsorption of the dominant negatively charged Al(OH)4− complex, while the neutral surface of orthopyroxene does not. This adsorption process could facilitate both the dissolution of olivine and the nucleation-growth of an Al-enriched lizardite.