NEW MEXICO PERSPECTIVES ON THE AL2SIO5 TRIPLE POINT: PAST, PRESENT AND FUTURE

Polymorphs of a simple chemical system, Al₂SiO₅, stand as one of the anchors of phase equilibria in metamorphic petrology. Aluminous bulk compositions from northern New Mexico preserve a number of localities with two or three coexisting Al-silicate polymorphs. One key field observation from the Truchas Peaks area is the systematic spatial distribution of the kyanite-sillimanite-andalusite (K-S-A) isograds. Grambling (1981) noted that the occurrence of K, S, and A were closely related to elevation, and that the Al₂SiO₅ isograds were established following regional, contractional deformation. Based on field and petrographic observations Grambling (1981) proposed the equilibrium occurrence of coexisting K-S-A for the Truchas Peaks area. Grambling and Williams (1985) documented the coexistence of the polymorphs across multiple mountain ranges in northern New Mexico and documented the effect of Fe³⁺ and Mn³⁺ substitution on the stability of coexisting polymorphs. Notably, andalusite could accommodate significant amounts of Fe³⁺ and Mn³⁺ expanding its stability field relative to sillimanite and kyanite. Subsequently, polymetamorphic models with kyanite and possibly sillimanite growth at ca. 1650 Ma followed by andalusite and sillimanite growth at ca. 1400 Ma were used to explain the occurrence of three coexisting aluminum silicate minerals in some regions. More recently U-Th-total-Pb dating and U-Pb isotopic analyses of matrix monazite and monazite inclusions with kyanite, sillimanite and andalusite yield ages of 1436 Ma to about 1400 Ma, inconsistent with polymetamorphic models. Reaction textures show a K-S-A reaction sequence that may represent a near equilibrium occurrence of the polymorphs. In the future we anticipate that the study and interpretation of the Al₂SiO₅ polymorphs will remain key in the study of reaction kinetics, petrochronology and tectonic petrology and the special NM localities will be an essential data point.

Grambling, J. A. (1981). Kyanite, andalusite, sillimanite, and related mineral assemblages in the Truchas region, New Mexico. American Mineralogist 66, 702–722.

Grambling, J. A. & Williams, M. L. (1985). The effects of Fe³⁺ and Mn³⁺ on aluminum silicate phase relations in north-central New Mexico, USA. Journal of Petrology 26, 324–354.