Serpentinization of oceanic peridotites: 1. A high‐sensitivity method to monitor magnetite production in hydrothermal experiments

Abstract

[1] A new method using the magnetic properties of magnetite, Fe 3 O 4 , was developed to monitor experimental serpentinization. The saturation remanent magnetization signal (Jrs) was measured during the course of experiments designed to react San Carlos olivine, (Mg 0.91 , Fe 0.09 ) 2 SiO 4 , with water at 250 to 350°C and 500 bars. At the end of the experiments, the ratio with saturation magnetization (Jrs/Js ratio) allowed to convert each successive Jrs measurement into an in situ amount of magnetite produced by the serpentinization reaction. Water weight loss was also measured on the end product to determine the final degree of serpentinization. The application of this procedure to a series of experiments performed at 300°C/500 bars for various run duration (9 to 514 days) and starting olivine grain size (1 to 150 mm) shows a linear relationship between magnetite production and reaction progress. This relationship can be safely transposed to other experimental conditions using thermochemical modeling and/or the Fe content of the product phases. We show that this high-sensitivity magnetic method is a powerful tool to precisely monitor serpentinization kinetics in Fe-bearing systems. It represents, in addition, a new indirect mean for monitoring the production of hydrogen which is bound to magnetite production rate.Citation: Malvoisin, B., J. Carlut, and F. Brunet (2012), Serpentinization of oceanic peridotites: 1. A high-sensitivity method to monitor magnetite production in hydrothermal experiments,

Experimental constraints on fluid-rock reactions during incipient serpentinization of harzburgite

“…To get an independent estimate for the extent of alteration, we employed a 381 linear relationship between saturation magnetization (Js) and extent of 382 serpentinization (S): Js = 8.979 x 10 -4 x S (Malvoisin et al, 2012b). Using this 383 relationship, the increase in the measured magnetization saturation during the 384 experiment, Jsend -Isinitial = 3.397 x 10 -5 Am 2 suggests that 3.8 % of the protolith 385 underwent serpentinization.…”

“…A 579 comparison of the H2 concentrations measured by Berndt et al (1996) and Klein and 580 McCollom (2013) suggests that the release rate of H2 is strongly influenced by grain 581 size (surface area) of the starting materials (cf. Malvoisin et al, 2012b). A common 582 feature of some experiments is that aqueous H2 concentrations increase rapidly 583 shortly after the beginning of an experiment, as primary mineral surfaces are 584 directly exposed to the interacting fluid.…”

Evolution of Fe redox state in serpentine during subduction

“…The saturation magnetization of the serpentinite depends linearly on concentration of magnetite; this calculation is made using a proportionality factor of 92 A m 2 /kg between Js and the nominal weight of magnetite (Malvoisin et al, 2012;O'Reilly, 1984).…”

Eight good reasons why the uppermost mantle could be magnetic

“…In mantle peridotites, magnetite is commonly thought to form as a secondary phase through serpentinization at various depths: in the mantle (e.g., Facer et al, 2009), at intermediate depths (e.g., Deschamps et al, 2010), or near the surface (e.g., Toft et al, 1990;Malvoisin et al, 2012).…”