40Ar/39Ar Thermochronologic Constraints on the Tectonothermal Evolution of the Northern East Humboldt Range Metamorphic Core Complex

The northern East Humboldt Range (NEHR) of northeastern Nevada exposes a suite of complexly deformed migmatitic, upper amphibolite-facies rocks in the footwall of the Ruby Mountains-East Humboldt Range (RM-EHR) detachment fault. New 40Ar/39Ar data on hornblende, muscovite, biotite, and potassium feldspar help constrain the kinematic and thermal evolution of this terrain during Tertiary extensional exhumation. Hornblende samples from relatively high structural levels yield discordant age spectra that suggest initial cooling during early Tertiary time (63–49 Ma). When coupled with petrological constraints indicating a strongly decompressional P-T-t path above 550°C, the hornblende data suggest that exhumation of the RM-EHR may have initiated in early Tertiary time, approximately coincident with the initial phases of unroofing in the Wood Hills immediately to the east and with the end of thrusting in the late Mesozoic to early Tertiary Sevier orogenic belt of eastern Nevada and western Utah. This temporal coincidence suggests that gravitational collapse of tectonically thickened crust in the internal zone of the Sevier belt could have driven the initial phases of unroofing. Thermal history during the final stage of exhumation of the NEHR is constrained by discordant hornblende cooling ages of 36-29 Ma from deep structural levels and biotite, muscovite, and potassium feldspar cooling ages of 27-21 Ma from a range of structural levels. Comparison of muscovite, biotite, and potassium feldspar cooling ages with previously published fission-track cooling ages implies very rapid cooling rates at temperatures below the closure temperature for muscovite (270°–350°C), but time gaps of \u3e 7 m.y. between hornblende and mica cooling ages suggest that cooling at higher temperatures was more gradual. In addition, comparison of mica cooling ages with previously published fission-track apatite cooling ages suggests pronounced thermal gradients between the NEHR and adjacent areas during latest Oligocene to earliest Miocene time. Such thermal gradients could be readily explained if the RM-EHR detachment fault dipped \u3e 30° between the 300°C and 100°C isotherms. Finally, biotite cooling ages increase southward through the East Humboldt Range, compatible with northward extrapolation of a previously recognized pattern of WNW-younging biotite cooling ages from the Ruby Mountains. A simple model involving the propagation of footwall uplift in the direction of tectonic transport beneath an initially listric normal fault can explain the principle features of the Oligocène to Miocene thermochronologic data set for the RM-EHR

Shock-induced deformation features in terrestrial peridot and lunar dunite

Single crystals of terrestrial olivine (Fo90) were experimentally shock-loaded along [010] to peak pressures 280, 330, and 440 kbar and the resulting deformation features were compared to those present in olivine from lunar dunite 72415. Recovered fragments were examined on the Universal stage to determine the orientation of the planar fractures. With increasing pressure the percentage of pinacoids ({100}, {010}, and {001}) and prisms ({hkO}, {hOl}, and {Okl}), decreases (~40 to ~25%), whereas the percentage of bi pyramids {hkl} increases (~20-50% ). The complexity of the distribution of bipyramids also increases with increasing pressure. Other shock-induced deformation features, including varying degrees of recrystallization, are found to depend on pressure as observed by others. Lunar dunite 72415 was examined and found to contain olivine with well-developed shock deformation features. The relative proportion of pinacoid, prism, and bipyramid planar fractures measured for olivine from 72415 indicates that this rock appears to have undergone shock pressure in the range 330-440 kbar. It displays a preponderance of bipyramid fractures along (2 h = k) planes which have not previously been reported in laboratory or naturally shocked samples. If this dunite was brought to the surface of the moon as a result of excavation of an Imbrium event-sized impact crater, the shock-pressure range experienced by the sample and the results of cratering calculations suggest that it could have originated no deeper than 50-150 km

Tab. 1: Trace-element concentrations in ore samples collected from mercury mines and deposits in southwestern Alaska

A belt of small but numerous mercury deposits extends for about 500 km in the Kuskokwim River region of southwestern Alaska. The southwestern Alaska mercury belt is part of widespread mercury deposits of the circum Pacific region that are similar to other mercury deposits throughout the world because they are epithermal with formation temperatures of about 200 °C, the ore is dominantly cinnabar with Hg-Sb-As±Au geochemistry, and mineralized forms include vein, vein breccias, stockworks, replacements, and disseminations. The southwestern Alaska mercury belt has produced about 1400 t of mercury, which is small on an international scale. However, additional mercury deposits are likely to be discovered because the terrain is topographically low with significant vegetation cover. Anomalous concentrations of gold in cinnabar ore suggest that gold deposits are possible in higher temperature environments below some of the Alaska mercury deposits. We correlate mineralization of the southwestern Alaska mercury deposits with Late Cretaceous and early Tertiary igneous activity. Our 40Ar/39Ar ages of 70 ±3 Ma from hydrothermal sericites in the mercury deposits indicate a temporal association of igneous activity and mineralization. Furthermore, we suggest that our geological ancl geochemical data from the mercury deposits indicate that ore fluids were generated primarily in surrounding sedimentary wall rocks when they were cut by Late Cretaceous and early Tertiary intrusions. In our ore genesis model, igneous activity provided the heat to initiate dehydration reactions and expel fluids from hydrous minerals and formational waters in the surrounding sedimentary wall rocks, causing thermal convection and hydrothermal fluid flow through permeable rocks and along fractures and faults. Our isotopic data from sulfide and alteration minerals of the mercury deposits indicate that ore fluids were derived from multiple sources, with most ore fluids originating from the sedimentary wall rocks

Ratiometric QD-FRET Sensing of Aqueous H₂S in Vitro

We report a platform for the ratiometric fluorescent sensing of endogenously generated gaseous transmitter H₂S in its aqueous form (bisulfide or hydrogen sulfide anion) based on the alteration of Förster resonance energy transfer from an emissive semiconductor quantum dot (QD) donor to a dithiol-linked organic dye acceptor. The disulfide bridge between the two chromophores is cleaved upon exposure to bisulfide, resulting in termination of FRET as the dye diffuses away from the QD. This results in enhanced QD emission and dye quenching. The resulting ratiometric response can be correlated quantitatively to the concentration of bisulfide and was found to have a detection limit as low as 1.36 ± 0.03 μM. The potential for use in biological applications was demonstrated by measuring the response of the QD-based FRET sensor microinjected into live HeLa cells upon extracellular exposure to bisulfide. The methodology used here is built upon a highly multifunctional platform that offers numerous advantages, such as low detection limit, enhanced photochemical stability, and sensing ability within a biological milieu

In Vitro Detection of Hypoxia Using a Ratiometric Quantum Dot-Based Oxygen Sensor

A quantum-dot based ratiometric fluorescent oxygen probe for the detection of hypoxia in live cells is reported. The system is composed of a water-soluble near-infrared emissive quantum dot conjugated to perylene dye. The response to the oxygen concentration is investigated using enzymatic oxygen scavenging in water, while in vitro studies were performed with HeLa cells incubated under varying O₂ levels. In both cases a significant enhancement in dye/QD emission intensity ratio was observed in the deoxygenated environment, demonstrating the possible use of this probe for cancer research