Experimentally Determined Conditions in the Fish Canyon Tuff, Colorado, Magma Chamber

Abstract

The Fish Canyon Tuff, Colorado, forms one of the largest (3000 km3 known silicic eruptions in Earth history. The tuff is a homogeneous quartz latite consisting of 40% phenocrysts (plagioclase, sanidine, biotite, hornblende, quartz, magnetite, apatite, sphene, and ilmenite) in equilibrium with a highly evolved rhyolitic melt now represented by the matrix glass. Melt inclusions trapped in hornblende and quartz phenocrysts are identical to the newly analyzed matrix glass composition indicating that hornblende and quartz crystallized from a highly evolved magma that subsequently experienced little change. This study presents experimental phase equilibrium data which are used to deduce the conditions (P, T, fO2, fH2O, etc.) in the Fish Canyon magma chamber prior to eruption. These new data indicate that sanidine and quartz are not liquidus phases until 780°C temperatures are achieved, consistent with Fe-Ti oxide geothermometry which implies that the magmatic temperature prior to eruption was 760±30°C. Natural Fe-Ti oxide pairs also suggest that log fO2 was -12.4 (intermediate between the Ni-NiO and MnO-Mn3O4 oxygen buffers) in the magma chamber. This fO2.102 is supported by the experimentally determined variations in hornblende and melt Mg-numbers as functions of fO2 A new geobarometer based on the aluminum content of hornblendes in equilibrium with the magmatic assemblage hornblende, biotite, plagioclase, quartz, sanidine, sphene, ilmenite or magnetite, and melt is calibrated experimentally, and yields pressures accurate to ±0.5 kb. Total pressure in the Fish Canyon magma chamber is inferred to have been 2.4 kb (equivalent to a depth of 7.9 km) based on the Al-content of natural Fish Canyon hornblendes and this new calibration. This depth is much shallower than has been proposed previously for the Fish Canyon Tuff. Variations in experimental glass (melt) composition indicate that the magma was water-undersaturated prior to eruption. XH2O in the fluid phase that may have coexisted with the Fish Canyon magma is estimated to have been 0.5 by comparing the An-content of natural plagioclases to experimental plagioclases synthesized at different XH2O and Ptotals. This ratio corresponds to about 5 wt.% water in the melt at depth. The matrix glass chemistry is reproduced experimentally under these conditions: 760°C, 2.4 kb, XH2O=0.5, and log fo2=NNO+2 log units. The fugacity of SO2 (91 b) is calculated from the coexistence of pyrrhotite and magnetite. Maximum CO2 fugacity (2520 b) is inferred assuming the magma was volatile saturated at 2.4 kb.