Flow directions in ash-flow tuffs: a comparison of geological and magnetic susceptibility measurements, Tshirege member (upper Bandelier Tuff), Valles caldera, New Mexico, USA

This study concludes that the elongation axis (K ₁) of the ellipsoid of anisotropic magnetic susceptibility (AMS) is a suitable proxy for flow axis in ashflow tuffs. 153 oriented samples (176 specimens) were studied from 18 sites in the 1.1 Ma Tshirege member of the Bandelier Tuff. These sites are distributed around the Valles caldera at distances of 5–25 km outside of the rim.K ₁ axes correlate well with postulated radial flow axes at 13 sites.K ₁ also agrees with measured geological flow indicators, mainly imbricated larger clasts, at 7 sites. At 2 of the 5 sites where significant disagreement is seen between theoretical radial flow directions and measuredK ₁ axes, theK ₁ axes correspond well with geological flow indicators, indicating that the divergence of flow from the predicted radial flow pattern is real. Two major topographic buttresses are suggested as the cause of flow divergence for the Tshirege ash flows: the San Pedro buttress northwest of the caldera, and the San Miguel buttress in the southeast. In situK ₁ axes plunge about 7° toward the source at two-thirds of the sites; therefore the plunge ofK ₁ is a plausible in situ indicator for thedirection of flow. Multiple flow zones in sections of several meters thickness indicate changes of flow direction that are both rapid and large during ash-flow emplacement. These observations raisre the question of how best to represent mean flow directions in ash-flow sheets: by eigenvector methods, by vector-sum methods, or by modes. A method for measuring imbrication of larger clasts using apparent dips in vertical joints is outlined. Imbrication, determined in this way at one-third of the sites, dips toward the source, i.e., up-flow. The minimum (K ₃) axis of the AMS ellipsoid correlates with the flow foliation rather than with the larger clast imbrication. The flow axes of ash flows correspond with theK ₁ axes, not with the declination ofK ₃ axes as suggested by some authors. Initial dip of the sampled ash flows is not large and does not affect the paleomagnetic remanence direction, which is reversed with a mean ofD=173.5°,I=-38.4°, ₉₅=3.4°N=18. This mean is not different at the 95% confidence level from that of earlier workers. The mean pole, at 098.0°E, 74.8°N,A ₉₅=3.3°,N=18, is about 15° far-sided relative to the expected time-averaged geomagnetic pole, suggesting a history of emplacement too short to adequately average secular variation.     

Tourmaline mineralization in the Barberton greenstone belt,South Africa: early Archean metasomatism by evaporite-derived boron

Tourmaline-rich rocks are common in the lowgrade, interior portions of the Barberton greenstone belt of South Africa, where shallow-marine sediments and underlying altered basaltic and komatiitic lavas contain up to 50% tourmaline. The presence of tourmaline-bearing rip-up clasts, intraformational tourmalinite pebbles, and tourmaline-coated grains indicates that boron mineralization was a low-temperature, surficial process. The association of these lithologies with stromatolites, evaporites, and shallow-water sedimentary structures and the virtual absence of tourmaline in correlative deep-water facies rocks in the greenstone bels strengthens this model.Five tourmaline-bearing lithologic groups (basalts, komatiites, evaporite-bearing sediments, stromatolitic sediments, and quartz veins) are distinguished based on field, petrographic, and geochemical criteria. Individual tourmaline crystals within these lithologies show internal chemical and textural variations that reflect continued growth through intervals of change in bulk-rock and fluid composition accompanying one or more metasomatic events. Large single-crystal variations exist in Fe/Mg, Al/Fe, and alkali-site vacancies. A wide range in tourmaline composition exists in rocks altered from similar protoliths, but tourmalines in sediments and lavas have similar compositional variations. Boron-isotope analysis of the tourmalines suggest that the boron enrichment in these rocks has a major marine evaporitic component. Sediments with gypsum pseudomorphs and lavas altered at low temperatures by shallow-level brines have the highest ¹¹B values (+2.2 to-1.9); lower ¹¹B values of late quartz veins (-3.7 to-5.7) reflect intermediate temperature, hydrothermal remobilization of evaporitic boron. The ¹¹B values of tourmaline-rich stromatolitic sediments (-9.8 and-10.5) are consistent with two-stage boron enrichment, in which earlier marine evaporitic boron was hydrothermally remobilized and vented in shallow-marine or subaerial sites, mineralizing algal stromatolites. The stromatolite-forming algae preferentially may have lived near the sites of hydrothermal discharge in Archean times.     

Dissolution/precipitation kinetics of boehmite and gibbsite: Application of a pH-relaxation technique to study near-equilibrium rates

The dissolution and precipitation rates of boehmite, AlOOH, at 100.3 °C and limited precipitation kinetics of gibbsite, Al(OH)₃, at 50.0 °C were measured in neutral to basic solutions at 0.1 molal ionic strength (NaCl + NaOH + NaAl(OH)₄) near-equilibrium using a pH-jump technique with a hydrogen-electrode concentration cell. This approach allowed relatively rapid reactions to be studied from under- and over-saturation by continuous in situ pH monitoring after addition of basic or acidic titrant, respectively, to a pre-equilibrated, well-stirred suspension of the solid powder. The magnitude of each perturbation was kept small to maintain near-equilibrium conditions. For the case of boehmite, multiple pH-jumps at different starting pHs from over- and under-saturated solutions gave the same observed, first order rate constant consistent with the simple or elementary reaction: .

This relaxation technique allowed us to apply a steady-state approximation to the change in aluminum concentration within the overall principle of detailed balancing and gave a resulting mean rate constant, (2.2 ± 0.3) × 10⁻⁵ kg m⁻² s⁻¹, corresponding to a 1σ uncertainty of 15%, in good agreement with those obtained from the traditional approach of considering the rate of reaction as a function of saturation index. Using the more traditional treatment, all dissolution and precipitation data for boehmite at 100.3 °C were found to follow closely the simple rate expression:

R_net,boehmite=10^-5.485{m_OH^-}{1-exp(ΔG_r/RT)}, with R_net in units of mol m⁻² s⁻¹. This is consistent with Transition State Theory for a reversible elementary reaction that is first order in OH⁻ concentration involving a single critical activated complex. The relationship applies over the experimental ΔG_r range of 0.4–5.5 kJ mol⁻¹ for precipitation and −0.1 to −1.9 kJ mol⁻¹ for dissolution, and the pH_m ≡ −log(mH⁺) range of 6–9.6. The gibbsite precipitation data at 50 °C could also be treated adequately with the same model:R_net,gibbsite=10^-5.86{m_OH^-}{1-exp(ΔG_r/RT)}, over a more limited experimental range of ΔG_r (0.7–3.7 kJ mol⁻¹) and pH_m (8.2–9.7).     

Geohazards surveys in the North Carolina upper continental margin

Geophysical surveys on the upper continental slope and shelf-break area off North Carolina confirm previous suggestions that this highly dissected region is characterized by deposition of sediment swept from the adjacent shelf. Seismic reflection profiling, echo sounding and side-scan sonar studies failed to reveal conclusive evidence for slumping or other gravity processes which have been described for similar areas to the north. Outcrops appear to be lacking and the upper slope is a region of sediment accumulation and/or periodic bypassing.     

Tourmaline in Proterozoic Massive Sulfide Deposits from Rajasthan, India

We have analyzed the chemical composition and boron isotope composition of tourmaline from tourmalinites, granite and a quartz-tourmaline vein from the Deri ore zone and from a pegmatitic band in the Rampura-Agucha ore body. These two Proterozoic massive sulfide deposits occur in the Aravalli-Delhi orogenic belt, Rajasthan, northwest India. Tourmaline from stratiform tourmalinites closely associated with the massive sulfides in the Deri deposit have preserved their original chemical compositions despite regional and thermal metamorphism in the area. These tourmalines have low Fe/(Fe + Mg) ratios (0.19–0.30; mean 0.26) that suggest formation close to the sediment-sea water interface. The δ¹¹B values (−15.5 and −16.4‰) are compatible with boron derived from leaching of argillaceous sediments and/or felsic volcanics underlying the original massive sulfide deposit during its formation. Boron isotope compositions measured in tourmaline from a post-ore granite and quartz-tourmaline vein in the Deri deposit indicate that boron in these tourmalines was derived from the tourmalinites produced during ore formation. The boron isotope systematics of a coarse brown tourmaline crystal from a pegmatitic band on the hanging wall contact of the Rampura-Agucha deposit indicate that 45 ± 25% of the boron within the original tourmaline was lost during upper amphibolite facies regional metamorphism. Received: 3 April 1996 / Accepted: 11 April 1996     

Mitigating the Effects of Climate Change on the Water Resources of the Columbia River Basin

The potential effects of climate change on the hydrology and water resources of the Columbia River Basin (CRB) were evaluated using simulations from the U.S. Department of Energy and National Center for Atmospheric Research Parallel Climate Model (DOE/NCAR PCM). This study focuses on three climate projections for the 21st century based on a `business as usual' (BAU) global emissions scenario, evaluated with respect to a control climate scenario based on static 1995 emissions. Time-varying monthly PCM temperature and precipitation changes were statistically downscaled and temporally disaggregated to produce daily forcings that drove a macro-scale hydrologic simulation model of the Columbia River basin at 1/4-degree spatial resolution. For comparison with the direct statistical downscaling approach, a dynamical downscaling approach using a regional climate model (RCM) was also used to derive hydrologic model forcings for 20-year subsets from the PCM control climate (1995–2015) scenario and from the three BAU climate(2040–2060) projections. The statistically downscaled PCM scenario results were assessed for three analysis periods (denoted Periods 1–3: 2010–2039,2040–2069, 2070–2098) in which changes in annual average temperature were +0.5,+1.3 and +2.1 °C, respectively, while critical winter season precipitation changes were –3, +5 and +1 percent. For RCM, the predicted temperature change for the 2040–2060 period was +1.2 °C and the average winter precipitation change was –3 percent, relative to the RCM controlclimate. Due to the modest changes in winter precipitation, temperature changes dominated the simulated hydrologic effects by reducing winter snow accumulation, thus shifting summer streamflow to the winter. The hydrologic changes caused increased competition for reservoir storage between firm hydropower and instream flow targets developed pursuant to the Endangered Species Act listing of Columbia River salmonids. We examined several alternative reservoir operating policies designed to mitigate reservoir system performance losses. In general, the combination of earlier reservoir refill with greater storage allocations for instream flow targets mitigated some of the negative impacts to flow, but only with significant losses in firm hydropower production (ranging from –9 percent in Period1 to –35 percent for RCM). Simulated hydropower revenue changes were lessthan 5 percent for all scenarios, however, primarily due to small changes inannual runoff.