The Hampton Tuff, High Lava Plains, Oregon : Implications for Westward Migrating Silicic Volcanism

The Hampton Tuff is a 3.9 ± .02 Ma (2σ) ignimbrite sheet from the High Lava Plains of central Oregon. The majority of known outcrops exist to the north, within ~22 mi (~35 km) of the Frederick Butte Volcanic Center, the proposed source of the tuff. Thickness of the tuff is typically ~8 m (~25 ft), but varies from 6.5 m (20 ft) up to ~46 m (150 ft). In nearly all cases, these are minimum thicknesses as the bases of outcrops are rarely exposed and the tops are eroded. Assuming a constant thickness of 8 m and a defined area of 2418 km², the estimated eruption volume is ~20 km³, dense rock equivalent (DRE). Welding facies varies from nonwelded with pumice to densely welded with fiamme. Westward migration of bimodal volcanism of the High Lava Plains consists mainly of rhyolitic domes, ignimbrites and ash-flow sheets, and is associated with widespread tholeiitic basalt. Ignimbrites represent the dominant volume of erupted high-silica rhyolite in this region and include the Devine Canyon Tuff (9.7 Ma), the Prater Creek Tuff (8.4 Ma), and the Rattlesnake Tuff (7.1 Ma); each of which represents 100-300 km³ of erupted material. A westward younging trend of rhyolitic volcanism of the High Lava Plains represents a coarse mirror of the northeast trending Yellowstone hotspot track from ~12 Ma to present. The Hampton Tuff is the youngest and westernmost tuff of the mapped ignimbrites within this westward younging trend. Although less voluminous than other High Lava Plains ignimbrites, the Hampton Tuff bears the high-iron signature (up to 3.9 wt% FeO*) that is characteristic of regional rhyolites and sparse dacites (up to 6.4 wt% FeO*). Microprobe analysis of glass shards from the Hampton Tuff indicates at least four compositional clusters with distinct ranges of silica that vary inversely with iron content. Although silica content of rhyolite compositions range from 73-77.5 wt% SiO₂ only ~25% of analyses are high-silica rhyolite (>75 wt% SiO₂), suggesting that the magma chamber is an example of "arrested development" of a magmatic system that could evolve more voluminous high-silica rhyolite like the Rattlesnake Tuff, given the opportunity to stage in the crust and enough thermal input. Field mapping and new ⁴⁰Ar/³⁹Ar age dates correlate units previously mapped as the Tuff of Espeland Draw by Johnson (1998), the ash-flow tuff west of Hampton Butte (Walker, 1970) and the Hampton Tuff of Iademarco (2009); also included are outcrops previously considered to be the Buckaroo Lake tuff by Streck (unpublished data). Field mapping also leads to the exclusion of several outcrops near Wagontire Mountain thought to be the Hampton Tuff and (or) the Tuff of Buckaroo Lake (MacLean, 1994). Major element analysis of the Buckaroo Lake tuff (6.85 Ma), sample HP-91-9, (Jordan, et al., 2004), confirms that the Buckaroo Lake tuff and the Hampton Tuff have distinct geochemical compositions. Presented herein is the discovery of a previously unknown ignimbrite of the High Lava Plains. The ignimbrite is informally named the Potato Lake tuff. It has an age of 5.13 ± 0.02 Ma (2σ) and has a chemical composition that is distinct from the Hampton Tuff, as well as the Buckaroo Lake tuff. Chemical analysis, outcrop descriptions, and age data are presented

How Graphene Is Cut upon Oxidation?

Our first principles calculations reveal that oxidative cut of graphene is realized by forming epoxy and then carbonyl pairs. Direct forming carbonyl pair to tear graphene up from an edge position is not favorable in energy. This atomic picture is valuable for developing effective graphene manipulation means. The proposed epoxy pairs may be related to some long puzzling experimental observations on graphene oxide

Combinatorial–computational–chemoinformatics (C3) approach to finding and analyzing low-energy tautomers

Finding the most stable tautomer or a set of low-energy tautomers of molecules is critical in many aspects of molecular modelling or virtual screening experiments. Enumeration of low-energy tautomers of neutral molecules in the gas-phase or typical solvents can be performed by applying available organic chemistry knowledge. This kind of enumeration is implemented in a number of software packages and it is relatively reliable. However, in esoteric cases such as charged molecules in uncommon, non-aqueous solvents there is simply not enough available knowledge to make reliable predictions of low energy tautomers. Over the last few years we have been developing an approach to address the latter problem and we successfully applied it to discover the most stable anionic tautomers of nucleic acid bases that might be involved in the process of DNA damage by low-energy electrons and in charge transfer through DNA. The approach involves three steps: (1) combinatorial generation of a library of tautomers, (2) energy-based screening of the library using electronic structure methods, and (3) analysis of the information generated in step (2). In steps 1–3 we employ combinatorial, computational and chemoinformatics techniques, respectively. Therefore, this hybrid approach is named “Combinatorial*Computational*Chemoinformatics”, or just abbreviated as C3 (or C-cube) approach. This article summarizes our developments and most interesting methodological aspects of the C3 approach. It can serve as an example how to identify the most stable tautomers of molecular systems for which common chemical knowledge had not been sufficient to make definite predictions

On the Chemical Origin of the Gap Bowing in (GaAs)1−xGe2x Alloys: A Combined DFT–QSGW Study

Motivated by the research and analysis of new materials for photovoltaics and by the possibility of tailoring their optical properties for improved solar energy conversion, we have focused our attention on the (GaAs)1−xGe2x series of alloys. We have investigated the structural properties of some (GaAs)1−xGe2x compounds within the local-density approximation to density-functional theory, and their optical properties within the Quasiparticle Self-consistent GW approximation. The QSGW results confirm the experimental evidence of asymmetric bandgap bowing. It is explained in terms of violations of the octet rule, as well as in terms of the order–disorder phase transition

Surface and interstitial transition barriers in rutile (110) surface growth

We present calculated surface and interstitial transition barriers for Ti, O, O-2, TiO, and TiO2 atoms and clusters at the rutile (110) surface. Defect structures involving these small clusters, including adcluster and interstitial binding sites, were calculated by energy minimization using density-functional theory (DFT). Transition energies between these defect sites were calculated using the NEB method. Additionally, a modified SMB-Q charge equilibration empirical potential and a fixed-charge empirical potential were used for a comparison of the transition energy barriers. Barriers of 1.2-3.5 eV were found for all studied small cluster transitions upon the surface except for transitions involving O-2. By contrast, the O-2 diffusion barriers along the [001] direction upon the surface are only 0.13 eV. The QEq charge equilibration model gave mixed agreement with the DFT calculations, with the barriers ranging between 0.8 and 5.8 eV