The Effects of Fe3 and Mn3 on Aluminum Silicate Phase Relations in North-Central New Mexico, U.S.A.

Aluminum silicate triple-point occurrences are common in metamorphic rocks of northern New Mexico. The three polymorphs show extensive solid solution, with Fe and Mn substituting for Al. Mineral textures, the spatial distribution of phases, and the systematic partitioning of Fe and Mn indicate that the kyanite-andalusite-sillimanite assemblages crystallized in equilibrium. The compositions of minerals in the three-phase assemblage vary across the study area, recording regional variations in the pressures and temperatures of metamorphism. The highest-pressure rocks, exposed at Rio Mora, contain kyanite at higher elevations and sillimanite at lower elevations. A sub-horizontal isograd separates the two minerals. Kyanite and sillimanite have nearly identical Fe content which varies systematically with XFe2O3 in hematite or ilmenite. Andalusite occurs only along a single manganiferous layer, in crystals rich in MnAlSiO5 and saturated in FeAlSiO5. Triple-point assemblages can be found wherever the folded manganiferous layer crosses the unfolded kyanite-sillimanite isograd. The Truchas Range, preserving slightly lower pressures of metamorphism, shows kyanite-andalusite-sillimanite in rocks with titaniferous hematite. Andalusite is enriched in Fe relative to kyanite and sillimanite, but no polymorphs contain Mn. Rocks with lower XFe2O3 in hematite have kyanite and sillimanite without andalusite, whereas rocks with pure hematite contain only andalusite. The shallowest erosional levels are preserved in the western Picuris Range where the three polymorphs occur as pure minerals in ilmenite-bearing rocks. Hematitic samples contain only andalusite which shows extensive solid solution of both Fe and Mn. The assemblage kyanite-andalusite-sillimanite is not invariant. Iron and manganese both add degrees of freedom. These transition metals have stabilized the three-phase assemblage, in apparent chemical equilibrium, across a P-T interval of 500-540 °C, 3·8-4·6 kb in rocks from New Mexico. The saturation level of FeAlSiO5 in andalusite does not vary with Mn content but does vary with pressure and temperature. Calculations indicate that a 2-3 kb decrease in pressure or a 25-50 °C increase in temperature results in a 1 mole per cent increase in XFeAlSiO5 in iron-saturated andalusite.