Fracture-related hydrothermal alteration of metagranitic rock and associated changes in mineralogy, geochemistry and degree of oxidation: a case study at Forsmark, central Sweden

Red-staining of rocks due to fluid–rock interaction during hydrothermal circulation in fractures is a common feature in crystalline sequences. In this study, red-stained metagranitic rock adjacent to fractures in Forsmark, central Sweden, has been studied with emphasis on the mineral reactions and associated element mobility occurring during the alteration. The main mineral reactions associated with the hydrothermal alteration are an almost complete saussuritization of plagioclase accompanied by total chloritization of biotite. Magnetite has been partly replaced by hematite whereas quartz and K-feldspar were relatively unaffected by the hydrothermal alteration. We show that redistribution of elements on the whole rock scale was very limited and is mainly manifested by enrichment of Na₂O and volatiles and depletion of CaO, FeO and SiO₂ in the red-stained rock. However, on the microscale, element redistribution was more extensive, with both intragranular and intergranular migration of e.g. Ca, K, Na, Al, Si, Fe, Ba, Cs, Rb, Sr, Ti and REEs. The altered rock shows a shift towards higher total oxidation factors, but the change is smaller than 1σ and the red-staining of the rock is due to hematite dissemination rather than a significant oxidation of the rock. An increase in the connected porosity is also observed in the altered rock.     

A conceptual approach for assessing the impact of climate change on groundwater and related surface waters in cold regions (Finland)

A literature review of the impacts of anticipated climate change on unconfined aquifers is presented, along with a conceptual framework for evaluating the complex responses of surface and subsurface hydrology to climate variables in cold regions. The framework offers a way to conceptualize how changes in one component of the system may impact another by delineating the relationships among climate drivers, hydrological responses, and groundwater responses in a straight-forward manner. The model is elaborated in the context of shallow unconfined aquifers in the boreal environment of Finland. In cold conditions, climate change is expected to reduce snow cover and soil frost and increase winter floods. The annual surface water level maximum will occur earlier in spring, and water levels will decrease in summer due to higher evapotranspiration rates. The maximum recharge and groundwater level are expected to occur earlier in the year. Lower groundwater levels are expected in summer due to higher evapotranspiration rates. The flow regimes between shallow unconfined aquifers and surface water may change, affecting water quantity and quality in the surface and groundwater systems.     

Quartz Cement in Middle Jurassic Reservoir Sandstones in North Sea. A Review. Part Ⅰ: Occurrence and Character

Quartz cementation is a very important diagenetic event and main porosity-occluding factor in the reservoir sandstones in middle Jurassic i n North Sea. Petrological studies indicate the quartz cementation is limited in shallower burial depth and increases significantly at depth around 3.5 - 4 km. T he amount of quartz cement increases with burial depth from an average of approx imately 4%-5 vol% at 2.5 km to 17%-20 vol% at 4 km. Highest abundances of quartz cement exceeds 28% which has been found in the Ness Formation. Variations in qu artz cementation between different sandstone facies is minimal for any one forma tion, with exception of the generally lower quantities present in the heterolith ic sandstone. Homogenisation temperatures of fluid inclusion in quartz overgrowt h from the North Sea reservoirs indicate that few quartz cement occur below the temperature 70 ℃～80 ℃, the onset of large volume quartz cement starts at the temperatures higher than 90 ℃ which suggests the quartz cementation is temperat ure - dependent during progressing diagenesis of sandstones.     

Assessing parameter identifiability for multiple performance criteria to constrain model parameters

ABSTRACT

Reliable simulations of hydrological models require that model parameters are precisely identified. In constraining model parameters to small ranges, high parameter identifiability is achieved. In this study, it is investigated how precisely model parameters can be constrained in relation to a set of contrasting performance criteria. For this, model simulations with identical parameter samplings are carried out with a hydrological model (SWAT) applied to three contrasting catchments in Germany (lowland, mid-range mountains, alpine regions). Ten performance criteria including statistical metrics and signature measures are calculated for each model simulation. Based on the parameter identifiability that is computed separately for each performance criterion, model parameters are constrained to smaller ranges individually for each catchment. An iterative repetition of model simulations with successively constrained parameter ranges leads to more precise parameter identifiability and improves model performance. Based on these results, a more consistent handling of model parameters is achieved for model calibration.     

Hengill,a high temperature thermal area in Iceland

The results of geological, geochemical and isotopic investigations in the Hengill thermal area in Iceland are presented. This area may be regarded as typical of the Icelandic high temperature areas. The Hengill area is mainly built up of pillow lavas and hyaloclastites, which were piled up in subglacial eruptions. Its western half is traversed by a very active NE-SW trending fault zone about 5 km broad, within which are several eruptive fissures of postglacial age. The volcanic rocks are basalts of various kinds, but minor occurrences of intermediate and rhyolitic rocks are also found. Solfataras and fumaroles occur mainly at higher altitude. Within the fault zone they have a linear distribution along faults. Hot water springs are found in the lowlands on the southern border of the thermal area. The concentrations of chlorine, tritium and deuterium in the water indicate that the hot springs on the lowland are fed by a deep lying aquifer which receives water from a recharge area situated in the western branch of the active volcanic zone upto 70 km to the northeast. The hot springs at higher altitude are the outlets of a secondary circulation system fed by local rain water. This water derives its heat partly from the heat content of the rocks penetrated and partly by steam and gas boiled off from the deeper lying aquifer. Preliminary results from other high temperature areas in Iceland indicate the presence of similar hydrological systems.     

The distribution of trace metals in the western North Atlantic off Nova Scotia

The distributions of Fe, Mn, Zn, Cu, Ni, Cd and Co have been determined in a section across the Scotian Shelf into the Atlantic Slope water. Significant differences in concentration exist for most of the trace metals between the four water masses in the section. Depletions of trace metal concentration in the highly productive Atlantic Slope water relative to the underlying Central Atlantic water are thought to be due to biological activity.The distributions of Fe and Mn are strongly related to the distribution of suspended particulate matter. The concentrations of Fe and Mn, extracted from the suspended matter on the Scotian Shelf, are considerably higher than those in the non-detrital fraction of the underlying sediments. This suggests that post-depositional changes cause the loss of both elements from the non-detrital fraction of the particles. Whereas Mn shows major nearshore increases in concentration related to continental runoff, nearshore Fe concentrations are largely controlled by particulate matter distribution. Continental runoff does not appear to have much influence upon the distributions of the other trace metals.     

Trace element partitioning and melt structure: An experimental study at 1 atm pressure

Diopside-melt and forsterite-melt rare earth (REE) and Ni partition coefficients have been determined as a function of bulk compositions of the melt. Available Raman spectroscopic data have been used to determine the structures of the melts coexisting with diopside and forsterite. The compositional dependence of the partition coefficients is then related to the structural changes of the melt.The melts in all experiments have a ratio of nonbridging oxygens to tetrahedral cations ($\text{NBO}\text{T}$) between 1 and 0. The quenched melts consist of structural units that have, on the average, 2 (chain), 1 (sheet) and 0 (three-dimensional network) nonbridging oxygens per tetrahedral cation. The proportions of these structural units in the melts, as well as the overall $\text{NBO}\text{T}$, change as a function of the bulk composition of the melt.It has been found that Ce, Sm, Tm and Ni crystal-liquid partition coefficients ($\text{K}{}^{\mathrm{crystal?liq}}{}_{\mathrm{i}}\text{=}\text{C}{}^{\mathrm{crystal}}{}_{\mathrm{i}}\text{C}{}^{\mathrm{liq}}{}_{\mathrm{i}}$) decrease linearly with increasing $\text{NBO}\text{T}$. The values of the individual REE crystal-liquid trace element partition coefficients have different functional relations to $\text{NBO}\text{T}$, so that the degree of light REE enrichment of the melts would depend on their $\text{NBO}\text{T}$.The solution mechanisms of minor oxides such as CO₂, H₂O, TiO₂, P₂O₅ and Fe₂O₃ in silicate melts are known. These data have been recast as changes of $\text{NBO}\text{T}$ of the melts with regard to the type of oxide and its concentration in the melt. From such data the dependence of crystal-liquid partition coefficients on concentration and type of minor oxide in melt solution has been calculated.