Preliminary 40ar 39ar Geochronology Of Igneous Intrusions From Uvalde County Texas Defining A More Precise Eruption History For The Southern Balcones Volcanic Province

"One of the world's great karstic aquifer systems, the Edwards aquifer system supplies water for more than 2 million people and for agricultural, municipal, industrial, and recreational uses. This volume reviews the current state of knowledge, current and emerging challenges to wise use of the aquifer system, and some technologies that must be adopted to address these challenges"--

Proceedings of the Fourth International Conference on Large Meteorite Impacts and Planetary Evolution held at the Vredefort Dome, South Africa, in Aug. 2008.

"Extending from Colorado, USA, on the north to the state of Chihuahua, Mexico, on the south, the Rio Grande rift divides the Colorado Plateau on the west from the interior of the North American craton on the east. This volume focuses on the Rio Grande rift's upper crustal basins and is organized geographically with study areas progressing from north to south. Nineteen chapters cover a variety of topics, including sedimentation history, rift basin geometries and the influence of older structure on rift basin evolution, faulting and strain transfer within and among basins, relations of magmatism to rift tectonism, and basin hydrogeology"--Provided by publisher.

Large silicic magma reservoirs grow incrementally over 10,000-100,000 years. However, the accumulation and destabilization of eruptible rhyolite can occur in only months to centuries. Explosive rhyolite eruptions are a significant hazard. However, their rarity limits the opportunity to link geophysical monitoring signals to precursory magmatic processes. Consequently, petrologic reconstructions provide essential context for interpreting unrest at active silicic systems. I apply coupled compositional and temporal information at two rhyolitic systems to interrogate the magmatic processes that build shallow magma systems, the dynamics that precede rhyolitic eruptions, and link petrologic and geophysical observations. The Bishop Tuff (BT), an architype example of a zoned rhyolitic ignimbrite, was deposited during the Long Valley caldera-forming eruption. I present 40Ar/39Ar incremental heating dates of individual BT sanidine that support a revised, more precise eruption age (764.8 ± 0.3 ka), demonstrate that this technique mitigates the age dispersion common in sanidine fusion datasets, and propose that rejuvenation of the solidified margins of the magma reservoir yields sanidine that record pre-eruption 40Ar/39Ar dates. The Laguna del Maule volcanic field (LdM), located in the southern Andes, hosts an exceptional concentration of rhyolite erupted since the last glacial retreat and currently is in a state of remarkable volcanic unrest. I present coupled compositional and geochronological data for whole rocks and minerals to investigate the recent flare-up of rhyolite volcanism and ongoing unrest at LdM. The shallow accumulation of the modern LdM magma system began at c. 40 ka. Zircon compositions show two chemically distinct magma reservoirs accumulated simultaneously and fed rhyolite eruptions during separate periods; the first between c. 23 and 19 ka and the second beginning at 14.5 ka and increasing in frequency and spatial distribution into the late Holocene. The extraction and eruption of crystal-poor rhyolite during the late Holocene occurred over only decades. Exsolved volatiles delivered by recurring magma recharge enhanced melt extraction rates and pressurized the resulting crystal-poor magma bodies. This process provides a mechanism to resolve apparently incongruent geophysical and petrologic observations and suggests the ongoing unrest could herald a future, potentially explosive, rhyolite eruption at LdM.