Suprasubduction zone (SSZ) ophiolites of the northern Appalachians (eastern North America) have provided key constraints on the fundamental tectonic processes responsible for the evolution of the Appalachian orogen. The central and southern Appalachians, which extend from southern New York to Alabama (USA), also contain numerous ultramafic-mafic bodies that have been interpreted as ophiolite fragments; however, this interpretation is a matter of debate, with the origin(s) of such occurrences also attributed to layered intrusions. These disparate proposed origins, alongside the range of possible magmatic affinities, have varied potential implications for the magmatic and tectonic evolution of the central and southern Appalachian orogen and its relationship with the northern Appalachian orogen. We present the results of field observations, petrography, bulk-rock geochemistry, and spinel mineral chemistry for ultramafic portions of the Baltimore Mafic Complex, which refers to a series of ultramafic-mafic bodies that are discontinuously exposed in Maryland and southern Pennsylvania (USA). Our data indicate that the Baltimore Mafic Complex comprises SSZ ophiolite fragments. The Soldiers Delight Ultramafite displays geochemical characteristics-including highly depleted bulk-rock trace element patterns and high Cr# of spinel-characteristic of subduction-related mantle peridotites and serpentinites. The Hollofield Ultramafite likely represents the "layered ultramafics" that form the Moho. Interpretation of the Baltimore Mafic Complex as an Iapetus Ocean-derived SSZ ophiolite in the central Appalachian orogen raises the possibility that a broadly coeval suite of ophiolites is preserved along thousands of kilometers of orogenic strike.

Yellowstone Lake hydrothermal vent systems have been studied using ROV assets to better understand the chemical and mineralogical evolution of the sublacustrine sediments through which the hot spring fluids discharge to the lake floor. Here we focus on the deposits/alteration and coexisting vent fluid chemistry associated with the Deep Hole on the lake floor, east of Stevenson Island. Remote in its location, at 120 m below the lake surface, this region in the northeast portion of Yellowstone Lake is associated with numerous hydrothermal vents and hot springs, providing evidence of high-temperature fluid-mineral interaction and phase separation phenomena. Vapor-dominated hydrothermal fluids issuing from Deep Hole vents attain temperatures in excess of 150 degrees C and are enriched in magmatically derived H2S and CO2. Upon mixing with lake water in the root zone of the hydrothermally active vents, the dissolved gases render the mixed fluid, both acidic and reducing, effectively transforming diatomaceous sediment, with detritally sourced Al and Fe components, to an alteration assemblage dominated by kaolinite, pyrite, and lesser boehmite. These alteration processes have been modeled by computer based simulations, coupling fluid flow and mineral dissolution kinetics, to provide insight on the temporal evolution of the vent system. Results predict rapid dissolution of amorphous silica. The magnitude and rate of silica loss, facilitated by the continuous influx of acidic source fluids, yields an increasingly silica poor alteration mineral sequence with time, characterized by quartz, followed by kaolinite and ultimately boehmite. These data are consistent with the observed decrease in SiO2/Al2O3 ratio of the vent deposits with increasing abundance of trace immobile elements, suggesting significant mass loss with reaction progress. Pyrite is predicted to form from sulfidation of magnetite, with noteworthy decrease in magnetic intensity, as measured for hydrothermally altered sediment in the near-field vent environment. Moreover, hydrogen isotope compositional data for kaolinite, together with delta D vent fluid data, suggest temperatures in keeping with the high temperatures measured for the vent deposits and discharging fluid, while supporting the potential use of kaolinite as a geothermometer. The predicted and observed transformation of silica-rich protolith to kaolinite, boehmite, and pyrite underscores the large scale dissolution and removal of silica, with possible implications for the temporal evolution of vent deposits on the lake floor in the Stevenson Island Deep-Hole region. Published by Elsevier B.V.

Hydrogen isotope ratios (D/H) measured on geological samples are used to trace the pathways of water in magmatic and hydrothermal systems. Interpreting respective isotope data relies thereby on the theoretical and empirical constraints on hydrogen isotope fractionation. This study revisits a recently discovered hydrogen isotope fractionation effect between alkali-rich and alkali-poor regions within hydrous alkali silicate melts (Wang et al., 2015). In a series of experiments conducted at variable P-T-X-H2O,X- D2O conditions, we studied this intramolecular isotope effect by H-1 and H-2 solid-state nuclear magnetic resonance (NMR) spectroscopy on quenched hydrous sodium tetrasilicate melts (Na(2)Os4SiO(2) with 1:1 H2O and D2O).

A series of experiments was performed to constrain the chemical and isotopic evolution of insoluble organic material (IOM) during hydrothermal alteration at temperatures ranging from 250 degrees C to 450 degrees C at 50 MPa. Experiments involved IOM that was extracted from the Murchison (CM2) meteorite or synthesized by aqueous carbonization of dextrose. Flash (dry) pyrolysis experiments at 400 - 1000 degrees C were also conducted with Murchison IOM to distinguish between the effects of hydrothermal and thermal degradation. Extended reaction times (up to 3905 h) were employed to establish D/H equilibria between IOM and H2O. The H isotope compositions of the H2O used in the experiments ranged from delta D= -447 parts per thousand to 3259 parts per thousand. Results revealed that the extent of the IOM H isotope evolution strongly depends on the delta D composition of the coexisting H2O with minimal temperature effects. The empirical relationship that describes the isotope exchange between IOM and H2O is as follows:

We report first principles density functional calculations of the Born effective charges and electronic dielectric tensors for the relaxor PMN (PbMg1/3Nb2/3O3). Visualization of the Born charge tensors as "charge ellipsoids" have provided microscopic insights on the factors governing piezoelectric enhancements with polarization rotation. Several 15 and 30-atom ferroelectric and antiferroelectric supercells of PMN involving 1:2 and 1:1 chemical ordering have been studied. A cascading set of ferroelectric phonon instabilities lead to several low symmetry monoclinic structures. We find a ground state with a 15-atom unit cell with 1:2 chemical ordering along [111] with a monoclinic C2 structure. (c) 2006 Elsevier B.V. All rights reserved.

We compute the c/a lattice strain versus temperature for nonmagnetic hcp iron at high pressures using both first-principles linear response quasiharmonic calculations based on the full potential linear-muffin-tin-orbital (LMTO) method and the particle-in-cell (PIC) model for the vibrational partition function using a tight-binding total-energy method. The tight-binding model shows excellent agreement with the all-electron LMTO method. When the hcp structure is stable, the calculated geometric-mean frequency and the Helmholtz free energy of epsilon-Fe from PIC and the linear response lattice dynamics agree very well, as does the axial ratio as a function of temperature and pressure. On-site anharmonicity proves to be small up to the melting temperature, and PIC gives a good estimate of its sign and magnitude. At low pressures, epsilon-Fe becomes dynamically unstable at large c/a ratios, and the PIC model might fail where the structure approaches lattice instability. The PIC approximation describes well the vibrational behavior away from the instability, and thus is a reasonable approach to compute high temperature properties of materials. Our results show significant differences from earlier PIC studies, which gave much larger axial ratio increases with increasing temperature, or reported large differences between PIC and lattice dynamics results.