MAGNETITE VARIABILITY IN ANTIGORITE SERPENTINITES FROM GUATEMALA
A critical aspect of peridotite hydration to form serpentine involves model redox reactions that create magnetite as well as brucite (e.g., Frost and Beard, 2007). Magnetite formation results in part because the serpentine preferentially partitions Mg relative to Fe2+ compared to olivine or orthopyroxene, so excess iron derived mainly from olivine must form an iron-rich brucite (amakinite component – Fe2+(OH)2) or, via redox reactions, magnetite, involving reactant oxygen or product H2 (or CH4 if a carbon source is present). Our rocks are brucite absent, although magnesite and/or dolomite can be present (Mg#Carb > Mg#Atg). A possible solution could involve Fe3+-rich antigorite, however, antigorite composition stoichiometries do not indicate (permit) the presence of ferric iron.
Consequently, a likely interpretation is that the magnetite-free rocks did once contain magnetite produced by serpentinization but continued reaction in the presence of a fluid that permitted the back reaction of iron into serpentine, retaining the whole rock Mg# but requiring a rebalancing of fO2. This process was exacerbated by shearing, enhancing permeability, for fluid flow and by long-term storage at elevated P & T in these mantle wedge antigoritites. Nonetheless, the rocks appear to have been a relatively closed system for major oxides.