Rock pressure vs. fluid pressure as a controlling influence on mineral stability: an example from New Mexico

A graphite-absent sequence of quartzite and schist on the north flank of the Picuris Range in north-central New Mexico provides a natural laboratory to test the thesis of Bruton and Helgeson that fluid pressure (P_f) is the effective pressure on solid phases during metamorphism. We have studied the Hondo Canyon and Section 8 areas of this range in detail. The presence of chloritoid + kyanite in the Ortega quartzite and staurolite in the adjacent Rinconada schists can best be explained by bulk compositional effects. A difference in P_f of 200 ± 100 bars between the two rock types can best be explained by a contrast of permeability between the quartzites and mica schists, because the preferred orientation of minerals and planar grain boundaries in the micaceous rocks favor greater and more rapid fluid flow. P_f builds up to values closer to lithostatic pressure in the quartzites than in the mica-rich rocks. In fluid-present systems during low- and medium-grade metamorphism generally, the effective pressure on solid minerals appears to be fluid pressure rather than rock pressure. -from Authors