Steady-state volcanism, paleoeffusion rates, and magma system volume inferred from plagioclase crystal size distributions in mafic lavas: Dome Mountain, Nevada

The Dome Mountain ( DM) lavas, located in the moat of the Timber Mountain caldera of the SW Nevada volcanic field, cap a thick sequence of welded tuffs, massive and bedded tuffs, and mafic and rhyolitic lava flows. These 10.9-m.y.-old flows comprise a pile of some twenty flows with a total thickness of about 300 m. Individual flows range in thickness from 3 to 20 m and are characterized by a basal vesicular zone, a massive central region and an upper vesicular region containing, in most cases, cooling fractures. The flows dip gently away from the summit of DM, a prominent topographic feature of the region with an elevation of 2065 m. Silica contents range from 48.8 wt.% at the base of the pile to 59.4 wt.% at the top, although most of the flows are andesitic. Crystal size distributions (CSDs) for plagioclase in these comagmatic lavas have been determined to provide information on magma storage times. These times, when coupled with phase equilibria, can be used to infer cooling rates, which are indicative of the relative dimensions of magmatic bodies and the vigor of various magmatic cooling processes. Most of the CSDs show no sign of crystal fractionation or accumulation. Assuming a growth rate of 10−10 cm/s, calculated mean crystal residence times for most lavas cluster in the range of 1.5 to 4 years. Two of the lower lavas have residence times in the range of 4 to 9 years. The residence times correlate inversely with nucleation density and broadly correlate inversely with silica content: high-residence-time, low-silica lavas occur at the base of the pile, whereas low-residence-time, high-silica lavas occur at the top of the pile. One interpretation of the residence time data is that the lavas are from an open system of more or less constant residence time but varying spatially in system locations; e.g., a magma chamber in which steady-state recharge and eruption may have been reached. Alternatively, these lavas may represent small samples, closely spaced in time, of a slowly cooling, large batch or reservoir of magma. The greater residence times associated with the lower flows may indicate the increased amount of time necessary to form the conduit of the magma-volcano system by the oldest magma or simply that part of the system with the highest liquidus temperature. A minimum volume of 1.4 to 2.5 km3 for the magma chamber is estimated for the DM system using the estimated residence times in a simplified cooling model of a sheet of aspect ratio R. The actual system volume depends on R, which is strictly unknown, and thus the actual system volume may be much larger (e.g., ~ 200 km3). A mean paleoeffusion rate of 0.05 km3/yr is obtained for the lavas by assuming that the residence time of each lava corresponds to the amount of time elapsed since the eruption of the underlying flow.