Ultrahigh-pressure metamorphic rocks from the Chinese Continental Scientific Drilling Project: II Oxygen isotope and fluid inclusion distributions through vertical sections

In order to reconstruct the formation and exhumation mechanisms of UHP metamorphic terrains, the Chinese Continental Scientific Drilling Program (CCSD) has been carried out in Donghai of the Dabie-Sulu ultrahigh-pressure (UHP) metamorphic belt, East China. Eclogite, gneiss, amphibolite (retrograded from eclogite), ultramafic rocks, and minor schist and quartzite have been drilled. Aiming to reveal the fluid behaviour in a vertical sequence of an UHP slab, we investigated fluid inclusion and oxygen isotope characteristics of selected drillcores from the main hole and the pilot-holes PP2 and ZK 703 of the CCSD. More than 540 laser-ablation oxygen isotope analyses on garnet, omphacite, quartz, kyanite, amphibole, phengite, rutile, epidote, amphibole, plagioclase, and biotite from various rocks in the depth range of 0–3,000 m (mainly eclogite and gneiss) show that the investigated rocks can be divided into two groups: ¹⁸O-depleted rocks (as low as δ¹⁸O = −7.4‰ for garnet) indicate interaction with cold climate meteoric waters, whereas ¹⁸O-normal rocks (with bulk δ¹⁸O > +5.6‰) have preserved the O-isotopic compositions of their protoliths. Meteoric water/rock interaction has reached depths of at least 2,700 m. Oxygen isotope equilibrium has generally been achieved. Isotopic compositions of mineral phases are homogeneous on a mm to cm scale regardless of lithology, but heterogeneous on the scale of a few metres. Oxygen isotope distributions in the vertical sections favour an “in situ” origin of the UHP metamorphic rocks. The very negative δ¹⁸O eclogites usually have higher hydroxyl-mineral contents than the normal δ¹⁸O rocks, indicating higher water content during UHP metamorphism. Fluid inclusion data suggest that rocks with depleted ¹⁸O compositions have had different fluid histories compared to those with normal δ¹⁸O values. Rocks with depleted ¹⁸O mainly have primary medium-to-high salinity inclusions in omphacite, kyanite and quartz, and abundant secondary low-salinity or pure water inclusions in quartz, indicating a high-salinity-brine-dominated fluid system during peak UHP metamorphism; no carbonic inclusions have been identified in these rocks. By contrast, primary very high-density CO₂ inclusions are commonly found in the rocks with normal δ¹⁸O values. These observations suggest that fluid and oxygen isotope composition of minerals are related and reflect variable degrees of alterations of the Dabie-Sulu UHP metamorphic rocks.     

Fluid-controlled quartz recovery in granulite as revealed by cathodoluminescence and trace element analysis (Bamble sector,Norway)

Dispersed quartz veinlets in Proterozoic enderbitic charnockite from Tromøy, Bamble sector (southern Norway), with Ti-concentrations of mostly 80–180 ppm, produce bright blue cathodoluminescence (CL), against contrasting micro-textures of secondary pure quartz with low CL intensity. The textures comprise grain boundary alteration, healed micro-fractures, patches of secondary quartz, and euhedral quartz nuclei. The secondary quartz locally makes up 50% of the total quartz. Trace element analysis has been done by electron-probe micro-analysis (EPMA), laser-induced coupled plasma mass spectrometry (LA-ICPMS), and additional electron-paramagnetic resonance (EPR) spectroscopy. These studies testify systematic trace element reduction and extensive quartz recovery during retrograde metamorphism. We assume healing of a part of the aqueous fluid inclusions during a late thermal event, possible correlated with Paleozoic magmatic activity in the Oslo-Rift. During final cooling, aqueous fluids were largely retrapped in patches of secondary quartz associated with healed fractures, whereas carbonic inclusions (without secondary quartz) may have survived retrograde metamorphism. The variable but generally high salinity of fluid inclusions is assumed to be in part the result of quartz recovery by which ion concentrations were increased in the percentage range.Editorial Responsibility: J. Touret     

The recreational use of beaches and consequences for the development of new trends in management: the case of the beaches of the Metropolitan Region of Barcelona (Catalonia, Spain)

This article offers some reflections on beach management, on the basis of the results of a survey on the recreational use of beaches and beach users' perception, which was carried out in the Metropolitan Region of Barcelona in 1992. The first part outlines the situation of the beaches, recent metropolitan beach policy and the organization of administrative bodies responsible for beach management. Survey objectives and methods are also explained briefly. In the second part, the principal results of the survey are presented and different beaches are compared in an attempt to analyse the types of users, the uses they make and the perception they have of the beaches. The conclusions identify new emerging social needs which demonstrate that new trends in beach management are also needed. Practical recommendations, related to this emerging perception of beaches as natural systems, are suggested.     

Tectonically controlled fluid flow and water-assisted melting in the middle crust: An example from the Central Alps

Melting triggered by influx of a free aqueous fluid in the continental crust has commonly been inferred, but the source of water in such contexts remains a matter of debate. We focus on the Tertiary migmatites in the Southern Steep Belt of the Central Alps (Switzerland) to discuss the petrology, structures and geodynamic setting of water-assisted melting. These migmatites comprise various structural types (e.g. metatexites, diatexites, melt in shear zones), which reflect variable leucosome fractions. The melting event itself as well as the variable melt fractions are related to the amount of aqueous fluids. At a given P and T, melt-fractions in rocks of minimum melt composition correlate with the amount of infiltrated aqueous fluids. In more granodioritic systems the water distributes between melt and newly crystallizing hydrous phases such as amphibole, such that the melt fraction correlates with the contents of H₂O, Al, and Ca in the system. Phase-equilibrium modelling indicates that the stabilization of amphibole leads to slightly lower melt fractions than in a granitic system at the same P, T and bulk water content. Phase-equilibrium models further indicate that in the Alpine migmatite belt: (1) several wt.% water (fluid:rock ratio of  1:30) are necessary to produce the inferred melt fraction; (2) the activity of H₂O in the fluid is high; and (3) spatially associated metapelites are unlikely as a source for the required aqueous fluids.

We present a tectonic scenario for the southern margin of the Central Alps, to which these migmatites are confined, and we propose that water was produced from dehydration reactions in metapelites in the Southern Alps. We model fluid production rates at the time of melting and demonstrate that the resulting fluid flow pattern is mainly controlled by the differences in permeability between the fluid source region and melting region. The proposed model requires strong gradients in temperature and permeability for the two tectonic blocks. This is consistent with the scenario involving indenter tectonics at the boundary between the Central and the Southern Alps in Oligocene times.     

From subduction to collision: Thermal overprint of HP/LT meta-sediments in the north-eastern Lepontine Dome (Swiss Alps) and consequences regarding the tectono-metamorphic evolution of the Alpine orogenic wedge

The Cenozoic-age metamorphic structure of the Alps consists of a throughgoing pressure-dominated belt (blueschists and eclogites) that strikes parallel to the orogen and was later truncated by two thermal domes characterised by Barrow-type metamorphism (Lepontine dome and Tauern window). This study documents for the first time that relics of Fe-Mg carpholite occur also within meta-sedimentary units that are part of the north-eastern Lepontine structural and metamorphic dome, where so far exclusively Barrovian assemblages were found. They occur in meta-sediments of both Valais Oceanderived Lower Penninic Bündnerschiefer and structurally lower Europe-derived Sub-Penninic cover nappes and slices. These high-pressure units were subsequently overprinted by a thermal event, as is documented by the growth of new minerals typical for Barrovian metamorphism.We present evidence for a two-stage metamorphic evolution in the northern part of the Lepontine dome: (1) Early subduction-related syn-D1 (Safien phase) HP/LT metamorphism under blueschist facies conditions (350–400 °C and 1.2–1.4 GPa) was immediately followed by “cold” isothermal (or cooling) decompression during D2 nappe-stacking (Ferrera phase). (2) Collisionrelated Barrovian overprint (500–570 °C and 0.5–0.8 GPa) postdates the D3 nappe-refolding event (Domleschg phase) and represents a late heating pulse, separated by D2 and D3 from the D1 high-pressure event. It occurred before and/or during the initial stages of D4 (Chiéra phase) representing a second nappe-refolding event.In discussing possible heat sources for the late Barrow-type heating pulse it is argued that heat release from radioactive decay of accreted material may play an important role in contributing much to heat production. Based on the field evidence, we conclude that heat transfer was essentially conductive during these latest stages of the thermal evolution.     

Salinity‐induced increase of the hydraulic conductivity in the hyporheic zone of coastal wetlands

In coastal zones globally, salinization is rapidly taking place due to the combined effects of sea level rise, land subsidence, altered hydrology, and climate change. Although increased salinity levels are known to have a great impact on both biogeochemical and hydrological processes in aquatic sediments, only few studies have included both types of processes and their potential interactions. In the present paper, we used a controlled 3‐year experimental mesocosm approach to test salinity induced interactions and discuss mechanisms explaining the observed hydrological changes. Surface water salinity was experimentally increased from 14 to 140 mmol Cl per L (0.9 and 9 PSU) by adding sea salt which increased pore water salinity but also increased sulfate reduction rates, leading to higher sulfide, and lower methane concentrations. By analyzing slug test data with different slug test analysis methods, we were able to show that hydraulic conductivity of the hyporheic zone increased 2.8 times by salinization. Based on our hydrological and biogeochemical measurements, we conclude that the combination of pore dilation and decreased methane production rates were major controls on the observed increase in hydraulic conductivity. The slug test analysis method comparison allowed to conclude that the adjusted Bouwer and Rice method results in the most reliable estimate of the hydraulic conductivity for hyporheic zones. Our work shows that both physical and biogeochemical processes are vital to explain and predict hydrological changes related to the salinization of hyporheic zones in coastal wetlands and provides a robust methodological approach for doing so.     

Pleistocene mammals from the southern Brazilian continental shelf

Fossils of terrestrial mammals preserved in submarine environment have been recorded in several places around the world. In Brazil such fossils are rather abundant in the southernmost portion of the coast, associated to fossiliferous concentrations at depths up to 10 m. Here is presented a review of such occurrences and the first record of fossils in deeper areas of the continental shelf. The fossils encompass several groups of both extinct and extant mammals, and exhibit several distinct taphonomic features, related to the marine environment. Those from the inner continental shelf are removed and transported from the submarine deposits to the coast during storm events, thus forming large konzentrat-lagerstätte on the beach, called “Concheiros”. The only fossils from deeper zones of the shelf known so far are a portion of a skull, a left humerus and of a femur of Toxodon sp. and a lower right molar of a Stegomastodon waringi, all collected by fishermen at depths around 20 m. The presence of fossils at great depths and distances from the present coastline, without signs of abrasion and far from areas of fluvial discharges does indicate that these remains have not been transported from the continent to the shelf, but have been preserved directly on the area that today correspond to the continental shelf. These remains indicate the existence of large fossiliferous deposits that have developed during periods of sea-level lowstand (glacial maxima) and have been submerged and reworked by the sea-level rise at the end of the last glaciation.     

Protracted fluid-induced melting during Barrovian metamorphism in the Central Alps

The timing and dynamics of fluid-induced melting in the typical Barrovian sequence of the Central Alps has been investigated using zircon chronology and trace element composition. Multiple zircon domains in leucosomes and country rocks yield U–Pb ages spanning from ~32 to 22 Ma. The zircon formed during Alpine melting can be distinguished from the inherited and detrital cores on the basis of their age, Th/U (<0.1) and trace element composition. Ti-in-zircon thermometry indicates crystallization temperatures around 620–700°C. Their composition allows discriminating between (1) zircon formation in the presence of early garnet, (2) zircon in equilibrium with abundant L-MREE-rich accessory phases (allanite, titanite and apatite) typical of metatonalites, and (3) zircon formed during melting of metasediments in feldspar-dominated assemblages. The distribution of zircon overgrowths and ages indicate that repeated melting events occurred within a single Barrovian metamorphic cycle at roughly constant temperature; that in the country rocks zircon formation was limited to the initial stages of melting, whereas further melting concentrated in the segregated leucosomes; that melting occurred at different times in samples a few meters apart because of the local rock composition and localized influx of the fluids; and that leucosomes were repeatedly melted when fluids became available. The geochronological data force a revision of the temperature–time path of the migmatite belt in the Central Alps. Protracted melting over 10 My followed the fast exhumation of Alpine eclogites contained within the same region and preceded fast cooling in the order of 100°C/Ma to upper crustal levels.     

Automobile Emissions of Acetonitrile: Assessment of its Contribution to the Global Source

Based on an estimated global fuel consumption of 2.57 × 10¹⁵g(C) y^–1 and the assumption thatthe fossil fuel burned in Austria is globallyrepresentative, an upper limit of 0.021 (+150%, –50%)Tg y^–1 for global CH₃CN emission dueto fossil fuel burning was obtained from the relativeenhancement of the concentrations of toluene, benzene,and acetonitrile (methyl cyanide) during strong,short-term traffic pollution. This is less than 6% ofthe total global budget of CH₃CN, which is dominatedby an emission rate of 0.8 Tg y^–1 from biomassburning.