Garnet re‐equilibration by coupled dissolution–reprecipitation: evidence from textural,major element and oxygen isotope zoning of ‘cloudy’ garnet

The analysis of texture, major element and oxygen isotope compositions of cloudy garnet crystals from a metapelite sampled on Ikaria Island (Greece) is used to assess the model of growth and re‐equilibration of these garnet crystals and to reconstruct the pressure–temperature–fluid history of the sample. Garnet crystals show complex textural and chemical zoning. Garnet cores (100–200 μm) are devoid of fluid inclusions. They are characterized by growth zoning demonstrated by a bell‐shaped profile of spessartine component (7–3 mol.%), an increase in grossular from 14 to 22 mol.% and δ¹⁸O values between 9.5 ± 0.3‰ and 10.4 ± 0.2‰. Garnet inner rims (90–130 μm) are fluid inclusion‐rich and show a decreasing grossular component from 22 to 5 mol.%. The trend of the spessartine component observed in the inner rim allows two domains to be distinguished. In contrast to domain I, where the spessartine content shows the same trend as in the core, the spessartine content of domain II increases outwards from 2 to 14 mol.%. The δ¹⁸O values decrease towards the margins of the crystals to a lowest value of 7.4 ± 0.2‰. The outer rims (<10 μm) are devoid of fluid inclusions and have the same chemical composition as the outermost part of domain II of the inner rim. Garnet crystals underwent a four‐stage history. Stage 1: garnet growth during the prograde path in a closed system for oxygen. Garnet cores are remnants of this growth stage. Stage 2: garnet re‐equilibration by coupled dissolution–reprecipitation at the temperature peak (630 < T < 650 °C). This causes the creation of porosity as the coupled dissolution–reprecipitation process allows chemical (Ca) and isotopic (O) exchange between garnet inner rims and the matrix. The formation of the outer rim is related to the closure of porosity. Stage 3: garnet mode decreases during the early retrograde path, but garnet is still a stable phase. The resulting garnet composition is characterized by an increasing Mn content in the inner rim’s domain II caused by intracrystalline diffusion. Stage 4: dissolution of garnet during the late retrograde path as garnet is not a stable phase anymore. This last stage forms corroded garnet. This study shows that coupled dissolution–reprecipitation is a possible re‐equilibration process for garnet in metamorphic rocks and that intra‐mineral porosity is an efficient pathway for chemical and isotopic exchange between garnet and the matrix, even for otherwise slow diffusing elements.     

Spectroscopic evidence for protostellar infall

The observational evidence for infall associated with star formation is discussed. Whilst spectral energy distributions of young protostellar objects are consistent with infall, the best direct evidence comes from millimetre and sub-millimetre spectral line observations. Considerations of the formation of the line profiles and the chemical effects of gas-grain interactions suggest that there is only a very short ‘window’ in the evolutionary track of a protostellar object during which infall is directly observable. This may explain why so few infall candidates have been detected. It is argued that self-consistent models of the dynamical and chemical evolution of collapsing cores, coupled to multiple high resolution line observations, will provide definitive evidence for the presence of infall in these objects.     

Structural control and origin of volcanism in the Taupo volcanic zone,New Zealand

Taupor volcanic zone (TVZ) is the currently active volcanic arc and back-arc basin of the Taupo-Hikurangi arc-trench system, North Island, New Zealand. The volcanic arc is best developed at the southern (Tongariro volcanic centre) end of the TVZ, while on the eastern side of the TVZ it is represented mainly by dacite volcanoes, and in the Bay of Plenty andesite/dacite volcanoes occur on either side of the Whakatane graben. The back-arc basin is best developed in the central part of the TVZ and comprises bimodal rhyolite and high-alumina basalt volcanism. Widespread ignimbrite eruptions have occurred from this area in the past 0.6 Ma. Normal faults occur in both arc and back-arc basin. They are generally steeply dipping (>40°) and strike between 040° and 080°. In the back-arc basin, fault zones are en echelon and have the same trend as alignments of rhyolite domes and basalt vents. Open fissures have formed during historic earthquakes along some of the faults, and geodetic measurements on the north side of Lake Taupo suggest extension of 14±4 mm/year. In the Bay of Plenty and M_L=6.3 earthquake occurred on 2 March 1987. Modelling of known structure in the area together with data derived from this earthquake suggests block faulting with faults dipping 45°±10° NW and a similar dip is suggested by seismic profiling of faults offshore of the Bay of Plenty where extension is estimated to be 5±2 mm/year. To the east of the TVZ, the North Island shear belt (NISB) is a zone of reverse-dextral, strike-slip faults, the surface expression of which terminates at the eastern end of the TVZ. On the opposite side of the TVZ in the offshore western Bay of Plenty and on line with the NISB is the Mayor Island fault belt. If the two fault belts were once continuous, as seems likely, strike-slip faults probably extend through the basement of the TVZ. When extension associated with the arc and back-arc basin is combined with these strike-slip faults, the resulting transtension provides a suitable tectonic environment for caldera formation and voluminous ignimbrite eruptions in the back-arc basin. The types of volcano in the TVZ are considered to be related to the source of magma and overlying crustal structure. Lavas of the arc are probably formed by a multistage process involving (1) subsolidus slab dehydration, (2) anatexis of the mantle wedge, (3) fractionation and minor crustal assimilation and (4) magma mixing. High-alumina basalts of the back-arc basin may be derived by partial melting of peridotite at the top of the mantle wedge, while rhyolitic magmas are thought to come from partial melting of lavas and subvolcanic reservoirs associated with the southern end of the Coromandel volcanic zone. Extreme thinning associated with transtension in the back-arc basin will favour the eruption of large-volume, gas-rich ignimbrites accompanied by caldera formation.     

Field Investigation of a New Recharge Approach for ASR Projects in Near‐Surface Aquifers

Aquifer storage and recovery (ASR) is the artificial recharge and temporary storage of water in an aquifer when water is abundant, and recovery of all or a portion of that water when it is needed. One key limiting factor that still hinders the effectiveness of ASR is the high costs of constructing, maintaining, and operating the artificial recharge systems. Here we investigate a new recharge method for ASR in near‐surface unconsolidated aquifers that uses small‐diameter, low‐cost wells installed with direct‐push (DP) technology. The effectiveness of a DP well for ASR recharge is compared with that of a surface infiltration basin at a field site in north‐central Kansas. The performance of the surface basin was poor at the site due to the presence of a shallow continuous clay layer, identified with DP profiling methods, that constrained the downward movement of infiltrated water and significantly reduced the basin recharge capacity. The DP well penetrated through this clay layer and was able to recharge water by gravity alone at a much higher rate. Most importantly, the costs of the DP well, including both the construction and land costs, were only a small fraction of those for the infiltration basin. This low‐cost approach could significantly expand the applicability of ASR as a water resources management tool to entities with limited fiscal resources, such as many small municipalities and rural communities. The results of this investigation demonstrate the great potential of DP wells as a new recharge option for ASR projects in near‐surface unconsolidated aquifers.     

Long-term variations of annual and growing season rainfalls in Nigeria

Summary Evidence for changes in the annual and growing season rainfall series for the period 1919 to 1985 in Nigeria are examined on a regional basis, using power-spectral and lowpass filter techniques, and the Mann-Kendall rank statistic. Four regions, the Coastal Zone, the Guinea-Savanna Zone, the Midland area and the Sahel, are used in the investigation of rainfall variation from south to north across the country.Quasi-periodic oscillations in the annual and growing season rainfall series are found to be concentrated in four spectral bands: 2.0–2.4, 2.7–2.9, 3.2–3.6 and 5.6–6.3 years. The spatial coherence of the fluctuations in annual and growing season rainfall is found to be limited to Nigeria south of 11 degrees north latitude. Evidence also emerges of a progressive decline in annual and growing season rainfall for northern Nigeria, north of nine degrees north latitude, for the period 1939–1985.With 5 Figures     

Changes to sediment sources following wildfire in a forested upland catchment,southeastern Australia

Few investigations link post‐fire changes to sediment sources and erosion processes with sediment yield response at the catchment scale. This linkage is essential if downstream impacts on sediment transport after fire are to be understood in the context of fire effects across different forest environments. In this study, we quantify changing source contributions to fine sediment (<63 µm) exported from a eucalypt forest catchment (136 ha) burnt by wildfire. The study catchment is one of a pair of research catchments located in the East Kiewa River valley in southeastern Australia that have been the subject of a research program investigating wildfire effects on runoff, erosion, and catchment sediment/nutrient exports. This previous research provided the opportunity to couple insights gained from a range of measurement techniques with the application of fallout radionuclides ¹³⁷Cs and ²¹⁰Pb_ex to trace sediment sources. It was found that hillslope surface erosion dominated exports throughout the 3·5‐year post‐fire measurement period. During this time there was a pronounced decline in the proportional surface contribution from close to 100% in the first six months to 58% in the fourth year after fire. Over the study period, hillslope surface sources accounted for 93% of the fine sediment yield from the burnt catchment. The largest decline in the hillslope contribution occurred between the first and second years after fire, which corresponded with the previously reported large decline in sediment yield, breakdown of water repellency in burnt soils, substantial reduction in hillslope erodibility, and rapid surface vegetation recovery. Coupling the information on sediment sources with hillslope process measurements indicated that only a small proportion of slopes contributed sediment to the catchment outlet, with material derived from near‐channel areas dominating the post‐fire catchment sediment yield response. Copyright © 2011 John Wiley & Sons, Ltd.     

Influence of afforestation on water regime in Jizera Catchments,Czech Republic

This paper studies the influence of afforestation on the water regime in two catchments in the Jizera Mountains that are similar in size and altitude but have different afforestation pattern. In this paper a range of different modelling tools is used to establish whether the differences in catchment water regime can be quantified and attributed to differences in catchment characteristics. Frequency analysis of low and high flows and a number and duration of flows over a threshold value are used to look for the differences in flow regime in both catchments. Low flow conditions are modelled using the Wittenberg nonlinear store approach. A rainfall-runoff process is modelled using a Data Based Mechanistic approach. The results indicate that the differences in the catchment response to external climatic factors outweigh the influence of land use apart from the low flows, where the changes in the response might be attributed to afforestation.     

Conditioning of coefficient matrices of Ordinary Kriging

The solution of a set of linear equations is central to Ordinary Kriging. Computers are commonly applied because of the amount of data and work involved. There has, until recently, been little attention devoted toward the conditioning of kriging matrices. This article considers implications of conditioning upon numerical stability, instead of on robustness which has been the main focus of past work. The effect of properties of the stationary covariance matrix on the conditioning of the kriging matrix is discussed. The relationship between the covariance and autocorrelation functions allows some conclusions about the conditioning of covariance matrices, based on past work in deconvolution. The conditioning of some coefficient matrices of stationary kriging, defined in terms of either the semivariogram or the covariance, is examined.     

Role of coal type and rank on methane sorption characteristics of Bowen Basin, Australia coals

The effect of coal composition, particularly the organic fraction, upon gas sorption has been investigated for Bowen Basin and Sydney Basin, Australia coals. Maceral composition influences on gas retention and release were investigated using isorank pairs of hand-picked bright and dull coal in the rank range of high volatile bituminous (0.78% R_{o max}) to anthracite (3.01% R_{o max}). Adsorption isotherm results of dry coals indicated that Langmuir volume (V_L) for bright and dull coal types followed discrete, second-order polynomial trends with increasing rank. Bright coals had a minimum V_L at 1.72% R_{o max} and dull coals had a minimum V_L at 1.17% R_{o max}. At low rank, V_L was greater in bright coal by about 10 cm³/g, but as rank increased, the bright and dull trends converged and crossed at 1.65% R_{o max}. At ranks higher than 1.65% R_{o max}, both bright and dull coals followed similar trends. These competing trends mean that the importance of maceral composition on V_L varies according to rank. In high volatile bituminous coals, increases in vitrinite content are associated with increases in adsorption capacity. At ranks higher than medium to low volatile bituminous, changes in maceral composition may exert relatively little influence on adsorption capacity. The Langmuir pressure (P_L) showed a strong relationship of decreasing P_L with increasing rank, which was not related to coal type. It is suggested that the observed trend is related to a decrease in the heterogeneity of the pore surfaces, and subsequent increased coverage by the adsorbate, as coal rank increases. Desorption rate studies on crushed samples show that dull coals desorb more rapidly than bright coals and that desorption rate is also a function of rank. Coals of lower rank have higher effective diffusivities. Mineral matter was found to have no influence on desorption rate of these finely crushed samples. The evolution of the coal pore structure with changing rank is implicated in diffusion rate differences.