Thermochronological evolution of calcite formation at the potential Yucca Mountain repository site, Nevada: Part 2 fluid inclusion analyses and UPb dating

The presence of two-phase fluid inclusions in thin secondary mineral crusts at the potential Yucca Mountain nuclear waste repository has raised questions regarding the origin, timing, and temperature of past fluid flow through the repository horizon. The geologically recent passage of fluids with high temperatures would call into question the suitability of the site for the storage of high level nuclear waste. This study determined the thermal history of fluid flow through the site using fluid inclusion analyses and constrained the timing of thermal fluids by dating silica minerals spatially associated with the fluid inclusions using U-Pb techniques. Results provide a detailed time-temperature history of fluid migration through primary and secondary pore spaces during the past 8 to 9 million years. One hundred and fifty-five samples were collected in the unsaturated zone from the C-shaped Exploratory Studies Facility (ESF), the ECRB cross drift which crosses the potential repository horizon, and exploratory alcoves. Detailed petrographic and paragenetic studies indicated that the oldest secondary minerals consisted of heterogeneously distributed calcite with lesser chalcedony, quartz, opal, and fluorite. The oldest secondary minerals were overgrown by intermediate bladed calcite. The youngest secondary minerals include chemically distinct Mgenriched, growth-zoned sparry calcite (MGSC) and intergrown U-enriched opal. Fluid inclusion petrography indicated that 50 % of the samples (n = 78) contained fluid inclusion assemblages with two-phase fluid inclusions, and that assemblages of liquid-only fluid inclusions represent \u3e 96% of all fluid inclusions within the secondary minerals. Assemblages of two-phase inclusions also contain liquid-only inclusions that did not nucleate a vapor-bubble owing to formation at relatively low temperatures. Virtually all two-phase fluid inclusions occur in paragenetically old calcite; rare two-phase inclusion assemblages were observed in old fluorite (n = 3) and quartz (n = 2). Rare two-phase fluid inclusions were observed in early-intermediate calcite; sparse, irregularly shaped liquid-only inclusions form the only fluid inclusion assemblages observed in late-intermediate minerals and young MGSC. Homogenization temperatures for calcite across the site are generally 45 - 60 °C, but higher temperatures reaching 83 °C were recorded in the north portal and ramp of the ESF and cooler temperatures of ~ 35 - 45 °C were recorded in the intensely fracture zone. Samples from lithophysal cavities in the ESF and ECRB contain multiple populations of two-phase inclusions. Inclusion temperatures are highest in early calcite (\u3e 45 °C) and cooler in paragenetically younger early calcite, indicating cooling with time. The cooler temperatures coincide with temperatures recorded in the intensely fractured zone and indicate that secondary minerals in the intensely fractured zone began to precipitate later than secondary minerals in other locations. Freezing point depressions determined for inclusions range from -0.2 to -1.6 °C indicating trapping of a low salinity fluid. A small number of fluid inclusions in fluorite and quartz were identified and evaluated. Four inclusions in these minerals homogenized at temperatures higher than those recorded for calcite (91 ° - 95 °C) . Two approaches were used to constrain the timing of thermal fluids at Yucca Mountain. First, the age of MGSC was determined, and it provides a minimum age for fluids with elevated temperatures owing to the presence of only liquid-only inclusions in MGSC. Results indicate that MGSC began to precipitate across the site between 2.90 ± 0.06 Ma and about 1.95 ± 0.06 Ma, and MGSC has continued to precipitate to within the last half million years. These ages constrain fluids with elevated temperatures to have accessed the site more than about 2.90 Ma. Second, more precise temporal constraints were determined for samples in which datable opal or chalcedony occur in the intermediate or older parts of the mineral crusts, or are spatially related to 2-phase fluid inclusions. Such ages indicate that two-phase fluid inclusions are older than 5.32 ± 0.02 Ma, and that fluids with elevated temperatures were present at Yucca Mountain before this time. Results from this study are consistent with a model of descending meteoric water that infiltrated the cooling tuff sequence, became heated, and precipitated secondary minerals within the vadose zone. Fluid inclusions indicate that fluids with elevated temperatures were present during the early history of Yucca Mountain. Sparse, liquid-only fluid inclusions in late intermediate to young calcite indicate that secondary minerals were precipitated from low temperature fluids during the past 5 million years. This study demonstrates that the hypothesis of geologically recent upwelling hydrothermal fluids is untenable and should not disqualify Yucca Mountain as a potential nuclear waste storage site

Geochemistry of pyrite and whole rock samples from the Getchell Carlin–type gold deposit, Humboldt County, Nevada

Carlin-type gold deposits were not recognized as a new major type of gold deposit until the 1960’s. The Getchell deposit, which is now known to be a Carlin-type gold deposit, was discovered in 1934 (Joralemon, 1951). This deposit is located in north central Nevada (Fig. 1). Much is known about the physical characteristics of Carlin-type gold deposits (Cline et al., 2005). An unusual characteristic of these deposits is that free gold is generally not present. Instead, gold occurs as sub-micrometer particles in the mineral pyrite or marcasite. The marcasite and pyrite that are gold bearing commonly occur as rims on gold-free pyrite or marcasite cores. These rims typically contain, in addition to gold, elevated arsenic, antimony, thallium, mercury, and copper. We have samples of ore from the Getchell deposit that contain metals that are not typically present in these deposits. For example, silver, which is typically very minor, is as abundant as gold. It is currently unclear if this silver is in the pyrite rims with the gold and other hydrothermally transported metals, or if the silver is in other minerals in the rock. If this is the case, this would indicate another source for the silver. In this study we are conducting petrographic examinations and chemical analyses of pyrites to quantify gold and other metals. First we used the microscope to look at polished thin sections of the samples. Pyrite crystals were identified that had visible rims giving us a location to analyze for gold and other trace metals. Second, we are using an electron microprobe to quantitatively determine elements and their abundances in pyrite and marcasite cores and rims. We expect to see a suite of elements typical of the Carlin-type systems, such as: Au, Sb, Hg, As, Cu, and Tl in the pyrites. Our study is determining the chemistry of four types of pyrite that have been identified, to locate gold, silver, and other trace metals. Our analyses will indicate whether or not silver occurs with the other Carlin metals, or if it is alternatively present in other minerals in the rock

Thermochronological evolution of calcite formation at the proposed Yucca Mountain repository site, Nevada: Part 1, secondary mineral paragenesis and geochemistry

In the near future a decision will be made as to whether or not Yucca Mountain, 90 miles northwest of Las Vegas, Nevada is a suitable site for a permanent, underground, high level nuclear waste repository. A major factor in determining the suitability of Yucca Mountain as a repository is the potential for the site to be flooded by water during the regulatory lifetime. The current study was undertaken to examine the past fluid history at the site, to gain a better understanding of the possibility of flooding in the near geologic future. To estimate the past fluid flux into the repository horizon, research has focused on secondary minerals that precipitated in open space in lithophysal cavities, fractures, and breccias in the host Miocene tuffs. U.S. Geological Survey researchers concluded that secondary minerals formed from descending surficial meteoric fluids in a vadose environment. State of Nevada scientists observed 2-phase fluid inclusions with homogenization temperatures of 35 to 85 °C in secondary minerals and concluded that these minerals formed in the phreatic environment from upwelling hydrothermal fluids. They further concluded that upwelling hydrothermal fluids repeatedly invaded the site, have invaded the site in the recent geologic past, and could do so again making Yucca Mountain an unsafe site for high level nuclear waste storage. These studies did not constrain the timing of incursion of the fluids with elevated temperatures or the extent of this fluid flux across the site. This report provides the geologic context for subsequent fluid inclusion and geochronological studies (Wilson et al., 2002) that identified the temperature and extent of the fluid incursion and placed absolute temporal constraints on the fluid history at Yucca Mountain. Here we describe a detailed paragenetic study that determined the depositional history of secondary minerals at Yucca Mountain. One hundred and fifty-five samples of secondary minerals were collected from lithophysal cavities, fractures, and breccias at Yucca Mountain. Extensive petrography, paragenetic studies, and microprobe mapping indicate that early secondary minerals were heterogeneously distributed across the site and consist of variable amounts of calcite, opal, chalcedony, fluorite, and quartz. Early calcite contained variable trace amounts of Mg (up to 1.3 wt. %). Intermediate minerals consist of mainly calcite, often in bladed habits, with minor opal, and chalcedony and quartz. These minerals contain no diagnostic trace element variations. The latest secondary minerals deposited across the site consist of sparry calcite and minor intergrown opal. This sparry calcite exhibits fine (~ 50 u.m) Mg-enriched and depleted growth zones and is chemically distinct from all other calcite. Mg-enriched growth-zoned sparry calcite (MGSC) contains up to ~ 1.0 wt. % Mg and has been identified in \u3e 65 % of the samples collected from across the site. MGSC and associated opal are always the paragenetically youngest minerals; where MGSC is not present, young secondary minerals did not precipitate. Calcite exhibits ranges for 513C from -8.5 %o to 9.5 %o, and for 818O from 5.2 %o to 22.1 %o. Samples exhibit generally consistent trends of decreasing C and increasing O isotopic compositions from paragenetically older to younger calcite. C and O isotope signatures for MGSC are between 16 %o and 20 %o for 618O and -3 %o and -8.5 %o for 513C. However, signatures for the various stages are not unique and are not diagnostic in correlating secondary mineral stages across the site. Early calcite is generally more luminescent than later calcite, but luminescence was not sufficiently consistent to aid in constraining the paragenetic sequence. LA-ICP-MS analyses indicate that higher levels of U, Th, and Sr are locally present in MGSC compared to paragenetically early calcite, however, this variation is not present in all samples. An important observation is that 90% of primary and secondary open space in the tuffs at Yucca Mountain contains no secondary mineral record. Where secondary minerals are present, the older secondary mineral record is heterogeneous across the site. However, MGSC, which forms the youngest part of the secondary mineral record, is present in a majority of samples and exhibits a more homogeneous distribution across the site. Secondary mineral abundances and textures indicate that secondary minerals precipitated in a vadose environment. The observed features are not consistent with secondary mineral precipitation in a phreatic environment saturated with aqueous fluids. Growth zoning in the outermost MGSC is consistent with formation from discontinuous influx of small fluid volumes with variable Mg content from surficial fluids that percolated downwards. Fluctuations in the Mg content in MGSC may be related to climate changes that occurred in the last few million years

Transient Gamma Ray Spectrometer Measurements of Gamma-Ray Lines from Novae. II. Constraining the Galactic Nova Rate from a Survey of the Southern Sky during 1995-1997

The good energy resolution (3--4 keV FWHM) of the Transient Gamma Ray Spectrometer (TGRS) on board the WIND spacecraft makes it sensitive to Doppler-shifted outbursts of 511 keV electron-positron annihilation radiation, the reason being that the Doppler shift causes the cosmic line to be slightly offset from a strong instrumental background 511 keV line at rest, which is ubiquitous in space environments. Such a cosmic line (blueshifted) is predicted to arise in classical novae due to the annihilation of positrons from $\beta$-decay on a timescale of a few hours in an expanding envelope. A further advantage of TGRS - its broad field of view, containing the entire southern ecliptic hemisphere - has enabled us to make a virtually complete and unbiased 3-year search for classical novae at distances up to ~1 kpc. We present negative results of this search, and estimate its implications for the highly-uncertain Galactic classical nova rate and for future space missions.Comment: 22 pp. + 3 fig

Lensing at cosmological scales: a test of higher dimensional gravity

Recent developments in gravitational lensing astronomy have paved the way to genuine mappings of the gravitational potential at cosmological scales. We stress that comparing these data with traditional large scale structure surveys will provide us with a test of gravity at such scales. These constraints could be of great importance in the framework of higher dimensional cosmological models.Comment: 4 pages, latex, 3 figure

Localisation of massive fermions on the brane

We construct an explicit model to describe fermions confined on a four dimensional brane embedded in a five dimensional anti-de Sitter spacetime. We extend previous works to accommodate massive bound states on the brane and exhibit the transverse structure of the fermionic fields. We estimate analytically and calculate numerically the fermion mass spectrum on the brane, which we show to be discrete. The confinement life-time of the bound states is evaluated, and it is shown that existing constraints can be made compatible with the existence of massive fermions trapped on the brane for durations much longer than the age of the Universe.Comment: 20 pages, LaTeX-RevTex, 15 figures, submitted to PR

Activation of AMPA Receptors in the Suprachiasmatic Nucleus Phase-Shifts the Mouse Circadian Clock In Vivo and In Vitro

The glutamatergic neurotransmission in the suprachiasmatic nucleus (SCN) plays a central role in the entrainment of the circadian rhythms to environmental light-dark cycles. Although the glutamatergic effect operating via NMDAR (N-methyl D-aspartate receptor) is well elucidated, much less is known about a role of AMPAR (α-amino-3-hydroxy-5-methylisoxazole-4-propionic acid receptor) in circadian entrainment. Here we show that, in the mouse SCN, GluR2 and GluR4 AMPAR subtypes are abundantly expressed in the retinorecipient area. In vivo microinjection of AMPA in the SCN during the early subjective night phase-delays the behavioral rhythm. In the organotypic SCN slice culture, AMPA application induces phase-dependent phase-shifts of core-clock gene transcription rhythms. These data demonstrate that activation of AMPAR is capable of phase-shifting the circadian clock both in vivo and in vitro, and are consistent with the hypothesis that activation of AMPA receptors is a critical step in the transmission of photic information to the SCN

Carotenoid accumulation during tomato fruit ripening is modulated by the auxin-ethylene balance

Background : Tomato fruit ripening is controlled by ethylene and is characterized by a shift in color from green to red, a strong accumulation of lycopene, and a decrease in β-xanthophylls and chlorophylls. The role of other hormones, such as auxin, has been less studied. Auxin is retarding the fruit ripening. In tomato, there is no study of the carotenoid content and related transcript after treatment with auxin. Results : We followed the effects of application of various hormone-like substances to “Mature-Green” fruits. Application of an ethylene precursor (ACC) or of an auxin antagonist (PCIB) to tomato fruits accelerated the color shift, the accumulation of lycopene, α-, β-, and δ-carotenes and the disappearance of β-xanthophylls and chlorophyll b. By contrast, application of auxin (IAA) delayed the color shift, the lycopene accumulation and the decrease of chlorophyll a. Combined application of IAA + ACC led to an intermediate phenotype. The levels of transcripts coding for carotenoid biosynthesis enzymes, for the ripening regulator Rin, for chlorophyllase, and the levels of ethylene and abscisic acid (ABA) were monitored in the treated fruits. Correlation network analyses suggest that ABA, may also be a key regulator of several responses to auxin and ethylene treatments. Conclusions : The results suggest that IAA retards tomato ripening by affecting a set of (i) key regulators, such as Rin, ethylene and ABA, and (ii) key effectors, such as genes for lycopene and β-xanthophyll biosynthesis and for chlorophyll degradation

α,β-D-Constrained Nucleic Acids Are Strong Terminators of Thermostable DNA Polymerases in Polymerase Chain Reaction

(SC5′, RP) α,β-D- Constrained Nucleic Acids (CNA) are dinucleotide building blocks that can feature either B-type torsional angle values or non-canonical values, depending on their 5′C and P absolute stereochemistry. These CNA are modified neither on the nucleobase nor on the sugar structure and therefore represent a new class of nucleotide with specific chemical and structural characteristics. They promote marked bending in a single stranded DNA so as to preorganize it into a loop-like structure, and they have been shown to induce rigidity within oligonucleotides. Following their synthesis, studies performed on CNA have only focused on the constraints that this family of nucleotides introduced into DNA. On the assumption that bending in a DNA template may produce a terminator structure, we investigated whether CNA could be used as a new strong terminator of polymerization in PCR. We therefore assessed the efficiency of CNA as a terminator in PCR, using triethylene glycol phosphate units as a control. Analyses were performed by denaturing gel electrophoresis and several PCR products were further analysed by sequencing. The results showed that the incorporation of only one CNA was always skipped by the polymerases tested. On the other hand, two CNA units always stopped proofreading polymerases, such as Pfu DNA polymerase, as expected for a strong replication terminator. Non-proofreading enzymes, e.g. Taq DNA polymerase, did not recognize this modification as a strong terminator although it was predominantly stopped by this structure. In conclusion, this first functional use of CNA units shows that these modified nucleotides can be used as novel polymerization terminators of proofreading polymerases. Furthermore, our results lead us to propose that CNA and their derivatives could be useful tools for investigating the behaviour of different classes of polymerases

Persistence of viral reservoirs in multiple tissues after antiretroviral therapy suppression in a macaque RT-SHIV model

Although antiretroviral therapy (ART) can suppress HIV-1 replication sufficiently to eliminate measurable plasma viremia, infected cells remain and ensure viral recrudescence after discontinuation of ART. We used a macaque model of HIV-1/AIDS to evaluate the location of infected cells during ART. Twelve macaques were infected with RT-SHIVmne, a SIV containing HIV-1 reverse transcriptase, conferring sensitivity to non-nucleoside reverse transcriptase inhibitors (NNRTIs). Ten to fourteen weeks post-infection, 6 animals were treated with 3 or 4 antiretroviral drugs for 17-20 weeks; 6 control animals remained untreated. Viral DNA (vDNA) and RNA (vRNA) were measured in peripheral blood mononuclear cells (PBMC) and at necropsy in multiple tissues by quantitative PCR and RT-PCR. The majority of virally infected cells were located in lymphoid tissues with variable levels in the gastrointestinal tract of both treated and untreated animals. Tissue viral DNA levels correlated with week 1 plasma viremia, suggesting that tissues that harbor proviral DNA are established within the first week of infection. PBMC vDNA levels did not correlate with plasma viremia or tissue levels of vDNA. vRNA levels were high in lymphoid and gastrointestinal tissues of the untreated animals; animals on ART had little vRNA expressed in tissues and virus could not be cultured from lymph node resting CD4+ cells after 17-20 weeks on ART, indicating little or no ongoing viral replication. Strategies for eradication of HIV-1 will need to target residual virus in ART suppressed individuals, which may not be accurately reflected by frequencies of infected cells in blood. © 2013 Kline et al