Serpentinization of oceanic peridotites: 2. Kinetics and processes of San Carlos olivine hydrothermal alteration

Abstract

[1] The kinetics of the reaction (Mg,Fe)-olivine + H 2 O → serpentine + magnetite + brucite + H 2 were investigated at 500 bars in the 250-350 C range using natural olivine (San Carlos; Fo 91 ) with grain sizes between 1 and 150 mm and for run durations up to 514 d. The amount of magnetite produced, which directly relates to reaction progress, was accurately monitored using up to 24 time-resolved magnetic measurements per experiment. Eighty percent of serpentinization was achieved after 60 d for olivine grain sizes of 5-15 mm and after 500 d for grain sizes of 50-63 mm. Serpentinization kinetics were found to be inversely proportional to the geometrical surface area of the starting olivine grains. They were one or two orders of magnitude slower than serpentinization kinetics commonly used for modeling serpentinization-related processes. The nature of the serpentine mineral product depended on the initial olivine grain size (IGS); for IGS in the 5-150 mm range lizardite formed, and olivine dissolution was the rate-limiting process. At IGS below 5 mm, chrysotile crystallized instead of lizardite, and the relationship between olivine surface area and reaction kinetics no longer held. We infer that for such small olivine grain sizes dissolution is no longer the rate-limiting process. Serpentinization in our experiments was associated with the creation of new reactive surface area according to two cooperative processes: etch pits formation associated with dissolution and grain fracturing for IGS above 20 mm. Interestingly, fractures and etch pits with similar geometry and sizes were also observed for residual olivine (with a typical grain size of 50 mm) in serpentinized peridotite samples from the Southwest Indian Ridge. This suggests that the processes governing olivine serpentinization kinetics in our experiments are similar to those prevailing in natural systems. We therefore suggest that the new kinetic data set that we present here, which encompasses a range of olivine grain sizes and reaction temperatures, is relevant to the serpentinization of olivine in the oceanic crust insofar as water is available.

Abiotic methane formation during experimental serpentinization of olivine

“…These experiments were performed using methods very similar to those of this study, except that the reaction vessel was periodically rotated during heating. After heating for [22][23][24][25][26][27][28][29][30][31][32][33][34][35][36] (Table 1). However, isotopic labeling indicated that the CH 4 in our experiments was not derived from reduction of dissolved inorganic carbon but instead, was derived entirely from background sources (i.e., composed of 99 mol % 12 CH 4 ).…”

“…The experiments were conducted over a temperature range of 200°C to 320°C. Experiments were not extended to lower temperatures, because the rates of serpentinization were found to be too slow below 200°C to be readily observable on laboratory timescales (29,30).…”

Sculpting of Rocks by Reactive Fluids

“…Without fracturing in concert with carbonation, the solid volume increase and pore filling processes associated with these reactions would decrease flow rates and thus the rate of carbon sequestration. As argued by Kelemen et al (2011), there is every reason to believe that carbonate growth in partly serpentinised peridotites near the surface in Oman would be able to drive fracturing by a mechanism similar to the serpentinisation driven fracturing first proposed by MacDonald and Fyfe (1985) and confirmed by many subsequent studies, including the recent experiments of Malvoisin et al (2012).…”

“…Interestingly, the experiments of Malvosin et al (2012) show the same sigmoidal shape predicted by our model. Fracturing of olivine grains is also observed in the serpentinisation experiments and Malvoisin et al (2012) stressed the role of new surface area production by etch pit formation and fracturing. The causal connection between these two processes was proposed by Plümper et al (2012;Fig.…”

“…Recently, Malvoisin et al (2012) reported the first experimental results on serpentinisation of peridotite, where the reaction progress was followed in detail over time by measuring the amount of magnetite produced. Experiments ran for up to 500 days for variable initial grain sizes, allowing the reaction progress to be followed to ~80% conversion to serpentine (chrysotile).…”