A regional gradient in the composition of metamorphic fluids in pelitic schist, Pecos Baldy, New Mexico

Abstract

Two metamorphic isograds cut across graphitic schist near Pecos Baldy, New Mexico. The southern isograd marks the first coexistence of staurolite with biotite, whereas the northern isograd marks the first coexistence of andalusite with biotite. The isograds do not record changes in temperature or pressure. Instead, they record a regional gradient in the composition of the metamorphic fluid phase. Ortega Quartzite, which contains primary hematite, lies immediately north of the graphitic schist. Mineral compositions within the schist change gradually toward the quartzite, reflecting gradients in$\mu _{{\text{O}}_{\text{2}} }$ and$\mu _{{\text{H}}_{\text{2}} {\text{O}}}$. The chemical potential gradients, locally as high as 72 cal/m in$\mu _{{\text{O}}_{\text{2}} }$ and 9 cal/m in$\mu _{{\text{H}}_{\text{2}} {\text{O}}}$, controlled the positions of the two mapped isograds. The staurolite-biotite isograd records where$X_{{\text{H}}_{\text{2}} {\text{O}}}$ fell below 0.80, at$f_{{\text{O}}_{\text{2}} }$ near 10⁻²³ bars; the andalusite-biotite isograd records where$X_{{\text{H}}_{\text{2}} {\text{O}}}$ fell below 0.25, at$f_{{\text{O}}_{\text{2}} }$ near 10⁻²² bars. Dehydration and oxidation were coupled by graphite-fluid equilibrium.

The chemical potential gradients apparently formed during metamorphism, as graphite in schist reacted with hematite in quartzite. Local oxidation of graphite formed CO₂ which triggered dehydration reactions along the schistquartzite contact. This process created a C-O-H fluid which infiltrated into overlying rocks. Upward infiltration, local fluid-rock equilibration and additional infiltration proceeded until the composition of the infiltrating fluid evolved to that in equilibrium with the infiltrated rock. This point occurs very close to the staurolite-biotite isograd. Pelitic rocks structurally above this isograd show no petrographic evidence of infiltration, even though calculations indicate that volumetric fluid/rock ratios may have exceeded 15 and thin, rare calc-silicate beds show extensive K-metasomatism and quartz veining.