TEMPERATURE-CHEMICAL POTENTIAL GRADIENTS REFLECTED IN VERY LOW-GRADE METAMORPHIC MINERAL ASSEMBLAGES IN VOLCANIC ROCKS

Very low-grade metamorphism (VLGM; i.e., below 150 ºC) of silicic tuffs and basaltic lavas results in a series of depth-controlled zones characterized by one or more secondary minerals (typically zeolites, mafic phyllosilicates, silica phases, carbonates, and feldspars). The presence of metastable phases (i.e., opal and chalcedony) as well as phase rule analysis of assemblages comprising the boundaries of the mineral zones preclude their formation as progressions of equilibrium assemblages with increasing temperature and pressure. The Rule of Cornu (1908), which states that mineral assemblages become progressively less hydrous with increasing temperature, is often used to explain VLGM of volcanic rocks. Chemographic analysis suggests that the components SiO₂ and CaAl₂O₄ in silicic tuffs and basaltic lavas, respectively, are more appropriate descriptive variables for VLGM. Copious hydrochemical data gathered from volcanic aquifers and geothermal systems has established that the chemical potentials of SiO₂ and CaAl₂O₄ (as reflected by aqueous activities of SiO₂ and the activity product Ca⁺² × AlO₂^-, respectively) vary systematically with temperature in these systems. Phase diagrams relating the stabilities of zeolites and albite to temperature and the activity of silica successfully predict the progression of mineral zones observed in VLGM of silicic tuffs that is consistent with the thermal distribution of silica polymorphs. Mineral assemblage zones formed during VLGM of basaltic lavas suggest that these zones are not controlled by silica activity (as free silica phases are typically not present below ~100 ºC), but are consistent with a regular trend across a phase diagram depicting their stability as a function of CaAl₂O₄ chemical potential and temperature. Both phase diagrams permit evaluation of the degree of metastable phase relations in these systems and accurately predict temporal parageneses observed in natural samples that result from decreases in these chemical potentials. Univariant equilibrium controls mineral zone boundaries above ~100 ºC in silicic tuffs and basalts as activities of aqueous silica approach equilibrium with quartz.