Mössbauer Study of Serpentine Minerals in the Ultramafic Body of Tehuitzingo, Southern Mexico

Serpentine ``polymorph'' minerals (chrysotile, lizardite, and antigorite) are hydrous Mg-Fe silicates that commonly form serpentine rock (serpentinite) by hydration of olivine-pyroxene peridotites from the mantle of the Earth. During the complex geologic history of orogenic belts, the redox and hydration state of the mantle changes, and olivine and pyroxenes are replaced by serpentine group minerals during tectonic deformation and uplift. Unfortunately, modern microanalysis of minerals by electron probe does not distinguish the oxidation state of iron, and it has to be assumed or estimated by precise methods, such as Mössbauer spectrometry. The studied samples were collected in the Xayacatlán Formation of the Tehuitzingo area, State of Puebla, within the Paleozoic Acatlán Complex. The original mantle peridotite was completely converted to serpentinite, with secondary crystallization of Fe-Mg oxides, calcsilicates, and carbonates. The three serpentine ``polymorphs'' were identified in the studied samples, although with a clear predominance of the high-temperature member antigorite, which was preliminary determined by optical petrography, X-ray diffraction, electron probe, and scanning electron microscopy. Microprobe total iron content in most specimens was <2%, but some were higher. Principal substitutions involving ferrous and ferric iron in serpentine minerals occur in the octahedral site, but minor and infrequent Fe⁺³ substitution at the tetrahedral site may also occur according to some Mössbauer studies. This paper studied the iron valence state and its position in serpentine minerals of the Tehuitzingo ultramafic body using Mössbauer techniques. The analyses in most samples detected one doublet, compatible with Fe⁺² in octahedral coordination, but only two specimens displayed two doublets corresponding to Fe⁺² and Fe⁺³ in octahedral coordination. Doublets corresponding to Fe⁺³ in tetrahedral sites were not found. The parameters obtained for all the Fe⁺² doublets are similar (QS=2.76+/-0.08 mm/sec, and IS=1.12+/-0.01), whereas the ratio Fe⁺³/Fe⁺² (0-0.34) has a strong tendency for iron to be in a divalent state. This rather low oxidation ratio of the Tehuitzingo antigorite serpentinite reflects a highly reduced state of the body, compatible with the relatively high-temperature (>600°C) regional metamorphism that serpentinized the studied rock.