MAGNETITE VARIABILITY IN ANTIGORITE SERPENTINITES FROM GUATEMALA

A conspicuous aspect of most serpentinites is an abundance of magnetite. In a comprehensive study of serpentinites from ophiolitic and tectonic mélange origin in central Guatemala, a highly variable amount of magnetite/spinel has been found in antigorite-dominant serpentinites, from a common peppering of oxide to essentially none. Whole rock analyses show Mg# [100*Mg/(Mg+Fe) atomic] varies from ~88 to 92, whereas Mg# of average antigorite varies from as high as 97 in magnetite-rich rocks to essentially the same value for both rock and serpentine, ~90, in “oxide-free” rocks. In addition, the near oxide-free rocks are typically highly sheared, with little-to-no remnant protolith textures.

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.