O,H, C isotope study of rocks from the KTB pilot hole: crustal profile and constraints on fluid evolution

The pilot hole of the Continental Deep Borehole (KTB) drilling project is located in the Bavarian Oberpfalz at the western margin of the Bohemian Massif. The 4-km deep borehole penetrated various paragneisses and minor orthogneisses with intercalations of amphibolites and metagabbros. The different lithologies have systematically different whole-rock oxygen isotope values and give little evidence for large scale water-rock interaction. Minor fluid interaction is well documented during retrograde metamorphism by non-equilibrium fractionations between refractory minerals (quartz, garnet and hornblende) and altered minerals (chlorite/biolite and feldspar). Ubiquitous vein mineralisation indicates fluid-induced retrogression at temperatures between 150°C and 400°C. The D values of hydroxylbearing minerals are very uniform in all lithologic units. The calculated hydrogen isotope composition of the fluid in equilibrium with matrix and vein minerals increases from -45 for metabasic rocks, to -20 for gneisses, to about -5 for vein minerals. The oxygen isotope composition of the fluid has been buffered by the rock and decreases with decreasing temperature because of increasing fractionations at low temperatures and low water-rock ratios. Modern fluids sampled from open cavities within the borehole have isotopic compositions that suggest a continuous fluid evolution during retrogression in a closed system. The ¹³C values of calcite and graphite also indicate closed system mixing processes.     

Hydrothermal alteration of Variscan granites,southern Schwarzwald,Federal Republic of Germany

Hercynian S-type granites from the southeastern Schwarzwald granite series represent cogenetic biotite-and two-mica granites. Oxygen- and hydrogen-isotope data show that hydrothermal alteration invoking isotopically light surface waters resulted in a drastic reduction in ¹⁸O and D and pronounced disequilibrium between the minerals. Effective water-rock ratios are calculated to be high, about 0.8 vol units. A shift in the¹⁸O/¹⁶O and the chemical composition of the fluid due to water-rock interaction is continuously traced from pure H₂O with meteoric isotopic character in the deep-seated biotite granites to slightly saline water with rock-equilibrated isotopic composition in the two-mica granites at a shallower level. Substantial retrograde hydrometamorphism in the temperature range 500° to 200° C resulted mineralogically in high-temperature chloritization of biotite, and low-temperature muscovitization as well as feldspar alteration, respectively. Another result of the re-equilibration of cations is strong disturbance of the Rb–Sr system which affects measured ages and initial⁸⁷Sr/⁸⁶Sr values. Hydrothermal differentiation and alteration probably overlap to a very large extent magmatic differentiation processes.     

Temperature–composition–time (T–X–t) data from authigenic K-feldspar: An integrated methodology for dating fluid flow events

Integration of fluid inclusion analysis with high spatial resolution Ar–Ar dating of K-feldspar cements has been used to resolve and reconstruct palaeo-fluid flow. Fluid inclusion analysis allows discrimination of distinct cement phases, thereby identifying discrete episodes of fluid flow. Ar–Ar dating of the same cements via high spatial resolution laserprobe establishes absolute age constraints on the framework previously constructed. Integration of these two datasets yields temperature–composition–time data.     

Structural architecture of a highly oblique divergent plate boundary segment

The Reykjanes Peninsula in southwest Iceland is a highly oblique spreading segment of the Mid-Atlantic Ridge oriented about 30° from the direction of absolute plate motion. We present a complete and spatially accurate map of fractures for the Reykjanes Peninsula with a level of detail previously unattained. Our map reveals a variability in the pattern of normal, oblique- and strike-slip faults and open fractures which reflects both temporal and spatial strain partitioning within the plate boundary zone. Fracture density varies across the length and width of the peninsula, with density maxima at the ends and at the northern margin of the zone of volcanic activity. Fractures with similar strike cluster into distinct structural domains which can be related to patterns of faulting predicted for oblique extension and to their spatial distribution with respect to volcanic fissure swarms. Additional structural complexity on the Reykjanes Peninsula can be reconciled with magmatic periodicity and associated temporal strain partitioning implied by GPS data, as well as locally perturbed stress fields. Individual faults show variable slip histories, indicating that they may be active during both magmatic and amagmatic periods associated with different strain fields.     

Évolution de l'onde semi-diurne M2 de la marée à Brest de 1846 à 2005

The work of searching, recovering and quality control of ancient sea-level measurements at Brest is presented. This work enables us to complete a study carried out by Cartwright in 1972, which showed a decrease in the tidal M2 semi-diurnal amplitude of 1% per century. After including these ancient data, as well as the last four decades of observations in the analysis, our results show an increase of the amplitude of M2 after 1960 and a decrease before 1880, suggesting a long-period oscillation rather than a steady secular trend. To cite this article: N. Pouvreau et al., C. R. Geoscience 338 (2006).     

Monazite behaviour and age significance in poly-metamorphic high-grade terrains: A case study from the western Musgrave Block, central Australia

Integrated, in situ textural, chemical and electron microprobe age analysis of monazite grains in a migmatitic metapelitic gneiss from the western Musgrave Block, central Australia has identified evidence for multiple events of growth and recrystallisation during poly-metamorphism in the Mesoproterozoic. Garnet + sillimanite-bearing metapelite underwent partial melting and segregation to palaeosome and leucosome during metamorphism between 1330 and 1296 Ma, with monazite grains in leucosome recording crystallisation at 1300 Ma. Monazite breakdown during melting is inferred to have occurred in the palaeosome. During a subsequent granulite facies event at 1200 Ma, deformation and metamorphism of leucosome and palaeosome resulted in partial disturbance of ages and potential minor growth on 1300 Ma monazite in leucosome. Growth of new, high-Y (+HREE) monazite in palaeosome domains occurred during garnet breakdown in the presence of sillimanite to cordierite and spinel, as a result of post-peak isothermal decompression. Diffusive enrichment of resorbed garnet rims in Y + HREE suggests garnet breakdown occurred slower than volume diffusion of REE. Monazite in both palaeosome and leucosome were subsequently partially to penetratively recrystallised during a retrogression event that is suggested to have occurred at 1150–1130 Ma. The intensity of recrystallisation and disturbance of ages appears linked to proximity to retrogressed garnet porphyroblasts and their occurrence in the relatively reactive or ‘fertile’ local environments provided by the palaeosome/mesosome volumes, which caused localised changes in retrogressive fluids towards compositions more aggressive to monazite. Like reaction textures, it is apparent that domainal equilibrium and reaction may control or at least strongly influence monazite REE and U–Th–Pb chemistry and hence ages.     

Formation of refractory inclusions by evaporation of condensate precursors

Berman’s (1983) activity-composition model for CaO-MgO-Al₂O₃-SiO₂ liquids is used to calculate the change in bulk chemical and isotopic composition during simultaneous cooling, evaporation, and crystallization of droplets having the compositions of reasonable condensate precursors of Types A and B refractory inclusions in CV3 chondrites. The degree of evaporation of MgO and SiO₂, calculated to be faithfully recorded in chemical and isotopic zoning of individual melilite crystals, is directly proportional to evaporation rate, which is a sensitive function of P_H2, and inversely proportional to the droplet radius and cooling rate. When the precursors are partially melted in pure hydrogen at peak temperatures in the vicinity of the initial crystallization temperature of melilite, their bulk chemical compositions evolve into the composition fields of refractory inclusions, mass-fractionated isotopic compositions of Mg, Si, and O are produced that are in the range of the isotopic compositions of natural inclusions, and melilite zoning profiles result that are similar to those observed in real inclusions. For droplets of radius 0.25 cm evaporating at P_H2 = 10⁻⁶ bar, precursors containing 8 to 13 wt.% MgO and 20 to 23% SiO₂ evolve into objects similar to compact Type A inclusions at cooling rates of 2 to 12 K/h, depending on the precise starting composition. Precursors containing 13 to 14 wt.% MgO and 23 to 26% SiO₂ evolve into objects with the characteristics of Type B1 inclusions at cooling rates of 1.5 to 3 K/h. The relatively SiO₂-poor members of the Type B2 group can be produced from precursors containing 14 to 16 wt.% MgO and 27 to 33% SiO₂ at cooling rates of <1 K/h. Type B2’s containing 27 to 35 wt.% SiO₂ and <12% MgO require precursors with higher SiO₂/MgO ratios at MgO > 15% than are found on any condensation curve. The characteristics of fluffy Type A inclusions, including their reversely zoned melilite, can only be understood in the context of this model if they contain relict melilite.     

In situ measurement of Re-Os isotopes in mantle sulfides by laser ablation multicollector-inductively coupled plasma mass spectrometry: analytical methods and preliminary results

A method has been developed for the in situ determination of Re-Os isotopes in single grains of sulfides in mantle-derived peridotites using a laser ablation microprobe attached to a multicollector-induced coupled plasma mass spectrometer (MC-ICPMS). High-precision Os isotope analysis by MC-ICPMS is demonstrated by the measurement of interlaboratory Os standards. Evaluation of mass bias correction procedures shows that the exponential law provides the best fit to the Os isotope data and that the ratio of the mass fractionation coefficients for Re and Os remains constant for the range of typical instrument operating conditions. This relationship enables the accurate and precise correction of the isobaric interference of ¹⁸⁷Re on ¹⁸⁷Os for ¹⁸⁷Re/¹⁸⁸Os values up to 1.6.Results are presented for single sulfide inclusions in olivine macrocryts from kimberlites in the Siberian and Slave Cratons, and sulfides enclosed in silicates and interstitial to silicates in peridotite xenoliths from the Slave Craton and Massif Central, France. Enclosed sulfides larger than 50 μm in diameter and with Os contents ≥40 ppm give ¹⁸⁷Os/¹⁸⁸Os ratios with a precision of 0.1% (2 SE), which is equivalent to N-TIMS whole-rock data. Interstitial sulfides typically have lower Os (10 to 30 ppm) and give analyses with lower precision (∼1 to 2%) but still provide valuable information on the movement of Os within the lithosphere. The sulfide inclusions in silicates preserve significantly less radiogenic Os isotopic compositions than interstitial sulfides and accordingly produce significantly older and more realistic Re-Os age information. Whole-rock Os isotope compositions reflect the proportions of different generations of enclosed and interstitial sulfides; this calls into question the significance of many published “depletion ages.”     

Trace Element Analysis of NIST SRM 614 and 616 Glass Reference Materials by Laser Ablation Microprobe-Inductively Coupled Plasma-Mass Spectrometry

Fifty elements in NIST SRM 614 and 616 glass reference materials were determined by laser ablation microprobe-inductively coupled plasma-mass spectrometry (LAM-ICP-MS). The values determined for NIST SRM 614 agreed well with the NIST-certified and information values (mean relative difference ± 3.6%), except for B, Sc and Sb. The values determined for NIST SRM 616 agreed with the NIST-certified and information values within a mean relative difference of ± 1.5%, except for B, Sc and Ga. In addition, at an 80 μm sampling scale, NIST SRM 614 and 616 glass discs were homogeneous for trace elements within the observed precisions of 5 and 15% (mean), respectively. Detection limits were in the range 0.01 - 0.3 μg g⁻¹ for elements of lower mass numbers (amu < 80) and 1 - 10 ng g⁻¹ for heavy elements (amu > 80). Detection at the sub ng g⁻¹ level is possible for most of the heavy elements by using an ablation pit size larger than 10 0 μm.     

Using Spatial Data to Explore Landholder Awareness and Concern about Dryland Salinity

This paper draws upon research in Victoria's Goulburn Broken Catchment exploring landholder responses to dryland salinity. It has been assumed that part of the explanation for limited adoption of recommended practices was that landholders were unaware of the extent of dryland salinity. Socio-economic data from a mail survey was combined in a Geographic Information System (GIS) with other layers, including the location of discharge sites and depth to saline ground water. Comparisons of expert maps and landholder identified salinity sites suggested that landholders in the upper catchment had excellent knowledge of the current extent of salinity on their property. At the same time, the expert maps failed to predict half of the saline-affected sites identified by landholders. It seems that the extensive community education effort undertaken in this region had been successful in raising salinity awareness. Our research also highlighted that most landholders were not concerned about the impacts of dryland salinity and appear to believe they can 'live with salt'. This is an important issue because the small, diffused amounts of salt exported from these properties in the upper catchment are/will have a substantial impact downstream. The research methodology and findings have important policy and management implications, and these are discussed.