PETROLOGIC IMPLICATIONS OF FERROUS AND FERRIC IRON IN SERPENTINES

Equilibrium Mg/Fe²⁺ partitioning between metamorphic olivine and antigorite provides a starting point for understanding the processes of serpentinization. Partitioning plots show: (1) the presence of ferric iron in low-Fe antigorite, (2) a preference for Mg in antigorite relative to that in MgFe-olivine, and (3) a dependence of lnK_D on the X_Fe of olivine. In the context of Mg/Fe²⁺ exchange equilibrium, antigorite can be taken to serve as a proxy for lizardite in pseudomorphic serpentinites, thereby rationalizing the typically high Mg-content of the serpentine minerals in altered mantle peridotites. The production of magnetite, Fe-Ni-Co alloys, and hydrogen is then a consequence of mass-balance in an oxygen-conserved hydration reaction. Microprobe analyses of many antigorites suggest contents of roughly 0.06 apfu of ferric iron (formula unit of 4.82 cations). By contrast, much greater relative amounts of ferric iron are present in some (especially yellow-colored) lizardites, in which case X_{total Fe} of lizardite can resemble that in host olivine and orthopyroxene, and magnetite may be absent. Ferric iron enters lizardite by means of a ferri-Tschermaks component and/or a vacancy-balanced substitution on the M site. These substitutions, like that of Al, will extend lizardite’s field of stability considerably.

Corrected for Fe³⁺ in antigorite, the dependence of lnK_D on the X_Fe of olivine (dlnK_D/dX_Fe^OL slope = 3.3, cf. 2.5 for higher T olivine-cummingtonite pairs) is a manifestation of the strong non-ideality of olivine at low temperature. It is consistent with a symmetrical Margules interaction parameter of around 10 kJ, as suggested by Sack & Ghiorso (1989) and Ghiorso et al. (1995). As a result, the Fe²⁺ content of serpentine can be expected to show little increase in metaperidotites with primary olivine in the near-solvus range Fa_20-80. This has implications for the production of magnetite and hydrogen in non-depleted mantle and crustal magmatic peridotites, including perhaps extra-terrestrial planetary peridotites.