Armalcolite in crustal paragneiss xenoliths, central Mexico

Abstract

Aluminous armalcolite has been found in two sillimanite-bearing xenoliths that were recently exhumed from the lower crust of central Mexico. The ranges of compositions are (Fe5.1s-0.66Mgo.lS-0.2SAlo.lS-0.lS V6.66-o.lOFe6.6o-o.o6Titt4-l.s6)Os and (Fefi1o-o.S1 MgO.lS-0.29Alo.16-0.19V6.62-0.06Fe6.1o-o.s1 Ti1.19-1.71)Os. The occurrence of armalcolite is unusual in crustal paragneisses because most terrestrial armalcolite occurs in volcanic rocks that are derived from partial melting in the Earth's mantle. Textures suggest that armalcolite is a late product formed by the reaction rutile + ilmenitess = armalcolitess during rapid transport of the xenoliths to the surface. Phase equilibria in the system MgO-FeO-Fe203- Ti02, which indicate that armalcolite is stable in the crust at 900-1200 °C, are consistent with this interpretation. Thermodynamic properties are estimated for oxides in the system MgO-FeO-Fe203Ti02 to constrain activity-composition relations for armalcolite and conditions of formation. Activity coefficients calculated for armalcolite range from 0.27 to 1.36, depending on the ilmenite model used, at temperatures between 1000 and 1300 °C. Depth of formation of armalcolite in the crust is not well constrained. Thermodynamic calculations at 800-1200°C for the compositions observed indicate that the armalcolite in one xenolith would have been in equilibrium with rutile at values of 102 between the hematite + magnetite buffer (HM) and the fayalite + magnetite + quartz (FMQ) buffer, and that armalcolite in the other xenolith would have been in equilibrium with rutile and ilmenite at values of 102 between FMQ and two log units below the FMQ buffer.