GAS SATURATION AND EVOLUTION OF VOLATILE AND LIGHT LITHOPHILE ELEMENTS IN THE BANDELIER MAGMA CHAMBER BETWEEN 2 CALDERA-FORMING ERUPTIONS

We have studied the distribution of light and volatile elements (Li, B e, B, H2O, F, Cl) in matrix glasses and melt inclusions in quartz and sanidine phenocrysts from tephras of the Cerro Toledo Rhyolite, which were erupted from the Bandelier magma chamber between two caldera-form ing eruptions at 1.608 Ma and 1.225 Ma. Since the small-volume tephras of the Cerro Toledo Rhyolite record chemical evolution of the upper p arts of the magma chamber during this 383-kyr period, we can track vol atile evolution in the magma during its differentiation, specifically, the development of a separate fluid phase from the magma, the partiti oning of volatile and light elements between this fluid and the magma, and the timing of the second caldera eruption at 1.225 Ma. The melt i nclusion data reveal that Li, Be, and B were progressively enriched wi th time at the top of the magma chamber, showing a similar three fold enrichment. By contrast, F, Cl, and H2O show more variable behavior. F was enriched 3.6 times in the magma and Cl increased 2.5 times, while water concentrations remained constant. Evidence for a separate fluid phase includes (1) comparatively small magmatic enrichment factors fo r Cl and H2O, elements which are known to partition into fluid, (2) de cline of the Cl/Be and Cl/B ratios in melt inclusions with time, indic ating preferential loss of Cl, and (3) maximum Cl contents of 2900-300 0 ppm in melt inclusions, which are concentrations at or close to the saturation limit for Cl in a two-phase gas-saturated magma. The lack o f temporal H2O enrichment in the magma suggests that it became gas sat urated at an early stage. The Cl fluid-magma partition coefficient was initially low, due to the presence of a low-salinity single-phase vap or. The fluid is calculated to have increased its salinity from 0.76 w t % equivalent NaCl to 3.1 wt % NaCl by crystallization and Cl enrichm ent in the magma. Consequently, the fluid was transformed to two immis cible phases. As a result, the Cl partition coefficient increased, and the Cl content in the magma was buffered at 2900-3000 ppm. Overpressu res in the magma chamber have been modeled as a function of crystalliz ation, total pressure and depth, H2O solubility in rhyolite, resurgenc e, crystallization contraction, and initial fluid mass. For the large amounts of crystallization (40-70 %) required by the geochemical relat ions, unrealistically large overpressures (> 75 MPa) are indicated, un less (1) the magma existed as a foam or (2) the chamber was able to de gas passively, and thus buffer the buildup of pressure. To maintain re alistic overpressures in the magma chamber (< 50 MPa), an equilibrium was required between gas buildup by crystallization and volatile loss by passive degassing. The change from a one-phase vapor to a two-phase immiscible fluid before the Upper Bandelier Tuff eruption may have pr omoted partial or complete sealing of the magmatic-hydrothermal system , permitting significant overpressures to develop. The result could ha ve been a cataclysmic caldera-forming eruption.