Serpentine, Iron-rich Phyllosilicates and Fayalite Produced by Hydration and Mg Depletion of Peridotite, Duluth Complex, Minnesota, USA

Abstract

Serpentine formed from Fe-rich igneous forsterite (Fa 38–43 ) in two samples of peridotite from the Layered Series of the Duluth igneous complex, Minnesota, is enriched to at least 30% of the Fe end-member. In addition to lizardite, chrysotile, and clinochlore, electron microprobe and transmission electron microscope study reveals the presence of metamorphic fayalite (Fa 53–80 ) and Fe-rich phyllosilicates such as magnesian hisingerite, low-Al ferrosaponite, and chamosite. These phyllosilicates are physically mixed on a nano-scale (in part interlayered) with the serpentine. Chlorite and serpentine compositions tend to mirror the bimodal frequency of Fa% in olivine, suggesting a measure of local FeMg –1 exchange equilibrium. A trioctahedral, ferrian ferrosaponite is believed to have replaced the hisingerite, and a more Si-rich ferrosaponite has filled pores in chrysotile and locally replaced it. Both varieties of ferrosaponite are attended unevenly by the presence of up to 1 wt % Na 2 O and K 2 O. Mixtures of lizardite with hisingerite show a gradual increase in Ca toward the latter up to 0·08 atoms per formula unit (1·5 wt % CaO). The presence of 10 and 13 wt % MgO in hisingerite indicates at least 25 and 34 mol % solid solution of Mg-serpentine. Both saponite and hisingerite probably contribute to the yellow color of alteration in thin section. The abundance of Fe-rich phyllosilicates (X Fe = 0·5–0·9) and magnetite in the phyllosilicate microveins argues for a loss of Mg from the rock accompanying the hydrous alteration. This mass change accounts for the growth of magnesian fayalite as a unique product of a low-temperature hydration reaction that ordinarily only consumes olivine. The removal of Mg is inferred to have taken place initially at low fO 2 and temperature very roughly 300°C, under the action of an infiltrating acidic, aqueous fluid. This fluid was probably influenced by nearby concentrations of Cu–Ni sulfides, or alternatively it was derived from underlying graphitic, sulfidic metapelites. Mg loss accompanying serpentinization is not exclusively a feature of hydrospheric alteration; it can instead be induced by a hydrothermal fluid of hitherto unrecognized composition. Growth of hisingerite and ferrosaponite (and its partial oxidation) is inferred to have taken place at a late stage under cooler and more oxidizing conditions. Similar MgFe-phyllosilicates and ferroan olivine are found in terrestrial ore deposits, on the ancient (Noachian) cratered surface of Mars, and in Martian meteorites (nakhlites). Other layered ultramafic–mafic intrusions, zoned intrusions, and ophiolites with Cu–Ni sulfide concentrations may conceivably be found locally to show the same style of serpentinization with major Mg depletion.