Neotethys closure history of Anatolia: insights from 40Ar–39Ar geochronology and P–T estimation in high pressure metasedimentary rocks

The multiple high-pressure (HP), low-temperature (LT) metamorphic units of Western and Central Anatolia offer a great opportunity to investigate the subduction- and continental accretion-related evolution of the eastern limb of the long-lived Aegean subduction system. Recent reports of the HP–LT index mineral Fe-Mg-carpholite in three metasedimentary units of the Gondwana-derived Anatolide–Tauride continental block (namely the Afyon Zone, the Ören Unit and the southern Menderes Massif) suggest a more complicated scenario than the single-continental accretion model generally put forward in previous studies. This study presents the first isotopic dates (white mica 40Ar–39Ar geochronology), and where possible are combined with P–T estimates (chlorite thermometry, phengite barometry, multi-equilibrium thermobarometry), on carpholite-bearing rocks from these three HP–LT metasedimentary units. It is shown that, in the Afyon Zone, carpholite-bearing assemblages were retrogressed through greenschist-facies conditions at c. 67–62 Ma. Early retrograde stages in the Ören Unit are dated to 63–59 Ma. In the Kurudere–Nebiler Unit (HP Mesozoic cover of the southern Menderes Massif), HP retrograde stages are dated to c. 45 Ma, and post-collisional cooling to c. 26 Ma. These new results support that the Ören Unit represents the westernmost continuation of the Afyon Zone, whereas the Kurudere–Nebiler Unit correlates with the Cycladic Blueschist Unit of the Aegean Domain. In Western Anatolia, three successive HP–LT metamorphic belts thus formed: the northernmost Tavşanlı Zone (c. 88–82 Ma), the Ören–Afyon Zone (between 70 and 65 Ma), and the Kurudere–Nebiler Unit (c. 52–45 Ma). The southward younging trend of the HP–LT metamorphism from the upper and internal to the deeper and more external structural units, as in the Aegean Domain, points to the persistence of subduction in Western Anatolia between 93–90 and c. 35 Ma. After the accretion of the Menderes–Tauride terrane, in Eocene times, subduction stopped, leading to continental collision and associated Barrovian-type metamorphism. Because, by contrast, the Aegean subduction did remain active due to slab roll-back and trench migration, the eastern limb (below Southwestern Anatolia) of the Hellenic slab was dramatically curved and consequently teared. It therefore is suggested that the possibility for subduction to continue after the accretion of buoyant (e.g. continental) terranes probably depends much on palaeogeography.

Solute transport in crystalline rocks at Äspö — I: Geological basis and model calibration

Water-conducting faults and fractures were studied in the granite-hosted A¨ spo¨ Hard Rock Laboratory (SE Sweden). On a scale of decametres and larger, steeply dipping faults dominate and contain a variety of different fault rocks (mylonites, cataclasites, fault gouges). On a smaller scale, somewhat less regular fracture patterns were found. Conceptual models of the fault and fracture geometries and of the properties of rock types adjacent to fractures were derived and used as input for the modelling of in situ dipole tracer tests that were conducted in the framework of the Tracer Retention Understanding Experiment (TRUE-1) on a scale of metres. After the identification of all relevant transport and retardation processes, blind predictions of the breakthroughs of conservative to moderately sorbing tracers were calculated and then compared with the experimental data. This paper provides the geological basis and model calibration, while the predictive and inverse modelling work is the topic of the companion paper [J. Contam. Hydrol. 61 (2003) 175]. The TRUE-1 experimental volume is highly fractured and contains the same types of fault rocks and alterations as on the decametric scale. The experimental flow field was modelled on the basis of a 2D-streamtube formalism with an underlying homogeneous and isotropic transmissivity field. Tracer transport was modelled using the dual porosity medium approach, which is linked to the flow model by the flow porosity. Given the substantial pumping rates in the extraction borehole, the transport domain has a maximum width of a few centimetres only. It is concluded that both the uncertainty with regard to the length of individual fractures and the detailed geometry of the network along the flowpath between injection and extraction boreholes are not critical because flow is largely one-dimensional, whether through a single fracture or a network. Process identification and model calibration were based on a single uranine breakthrough (test PDT3), which clearly showed that matrix diffusion had to be included in the model even over the short experimental time scales, evidenced by a characteristic shape of the trailing edge of the breakthrough curve. Using the geological information and therefore considering limited matrix diffusion into a thin fault gouge horizon resulted in a good fit to the experiment. On the other hand, fresh granite was found not to interact noticeably with the tracers over the time scales of the experiments. While fracture-filling gouge materials are very efficient in retarding tracers over short periods of time (hours–days), their volume is very small and, with time progressing, retardation will be dominated by altered wall rock and, finally, by fresh granite. In such rocks, both porosity (and therefore the effective diffusion coefficient) and sorption Kds are more than one order of magnitude smaller compared to fault gouge, thus indicating that long-term retardation is expected to occur but to be less pronounced.

Solute transport in crystalline rocks at Äspö — II: Blind predictions, inverse modelling and lessons learnt from test STT1

Based on the results from detailed structural and petrological characterisation and on up-scaled laboratory values for sorption and diffusion, blind predictions were made for the STT1 dipole tracer test performed in the Swedish A¨ spo¨ Hard Rock Laboratory. The tracers used were nonsorbing, such as uranine and tritiated water, weakly sorbing 22Na+, 85Sr2 +, 47Ca2 +and more strongly sorbing 86Rb+, 133Ba2 +, 137Cs+. Our model consists of two parts: (1) a flow part based on a 2D-streamtube formalism accounting for the natural background flow field and with an underlying homogeneous and isotropic transmissivity field and (2) a transport part in terms of the dual porosity medium approach which is linked to the flow part by the flow porosity. The calibration of the model was done using the data from one single uranine breakthrough (PDT3). The study clearly showed that matrix diffusion into a highly porous material, fault gouge, had to be included in our model evidenced by the characteristic shape of the breakthrough curve and in line with geological observations. After the disclosure of the measurements, it turned out that, in spite of the simplicity of our model, the prediction for the nonsorbing and weakly sorbing tracers was fairly good. The blind prediction for the more strongly sorbing tracers was in general less accurate. The reason for the good predictions is deemed to be the result of the choice of a model structure strongly based on geological observation. The breakthrough curves were inversely modelled to determine in situ values for the transport parameters and to draw consequences on the model structure applied. For good fits, only one additional fracture family in contact with cataclasite had to be taken into account, but no new transport mechanisms had to be invoked. The in situ values for the effective diffusion coefficient for fault gouge are a factor of 2–15 larger than the laboratory data. For cataclasite, both data sets have values comparable to laboratory data. The extracted Kd values for the weakly sorbing tracers are larger than Swedish laboratory data by a factor of 25–60, but agree within a factor of 3–5 for the more strongly sorbing nuclides. The reason for the inconsistency concerning Kds is the use of fresh granite in the laboratory studies, whereas tracers in the field experiments interact only with fracture fault gouge and to a lesser extent with cataclasite both being mineralogically very different (e.g. clay-bearing) from the intact wall rock.

Assessing Initial Conditions for Chloride Transport Across Low- permeability Argillaceous Rocks, Wellenberg, Switzerland

Information about fluid evolution and solute transport in a low-permeability metamorphic rock sequence has been obtained by comparing chloride concentrations and chlorine isotope ratios of pore water, groundwater, and fluid inclusions. The similarity of d37Cl values in fluid inclusions and groundwater suggests a closed-system evolution during the metamorphic overprint, and signatures established at this time appear to form the initial conditions for chloride transport after exhumation of the rock sequence.

The Yanque prospect (Peru): from polymetallic Zn-Pb mineralization to a nonsulfide deposit

The Yanque nonsulfide Pb-Zn deposit (inferred resources 12.5 Mt @ 3.7% Pb and @ 3.5% Zn) is located in the Andahuaylas-Yauri ore province (Cuzco, southern Peru). The deposit occurs within a base metal mineralized district, centered on the medium-sized Dolores porphyry copper. A thorough geological, mineralogical and geochemical study has carried out in order to define: the relationships between the Dolores Cu-porphyry ore and the Yanque Zn-Pb polymetallic mineralization, and the characteristics of the economic nonsulfide concentrations. Both sedimentary and igneous rocks constitute the backbone of the Yanque-Dolores area. The sedimentary lithologies belong to the Soraya, Mara and Ferrobamba Fms. (upper Jurassic-middle Cretaceous). The Yanque orebody is hosted by the Mara Fm., which prevailingly consists of a siliciclastic sedimentary breccia. The original sulfide mineralization consisted of galena, pyrite and sphalerite. The host rock has been affected by a strong hydrothermal alteration, characterized by prevailing sericite/illite, as in the typical porphyry-related phyllic-argillic alteration stage, and by minor kaolinite, dolomite and quartz. Minor element geochemistry, characterized by Sb, As, Mn, Ag and locally also by Cu, points to magmatic-hydrothermal related mineralizing fluids. The Pb isotopic compositions from Dolores and Yanque sulfides are similar, and are typical of the Tertiary magmatically-derived ores in this part of Peru. The hydrothermally altered rocks at Yanque have the same Pb isotopic compositions as the sulfides, thus confirming the hypothesis that the Yanque primary Zn-Pb mineralization may have been produced by hydrothermal circulation related to the emplacement of the Dolores Cu-porphyry, as it is the case of other porphyry Cu systems associated with polymetallic mineralization elsewhere. However, no simple genetic model for the mineralization involving just one fluid circulation episode is able to explain the data. The Yanque economic nonsulfide ore association consists of sauconite, hemimorphite, smithsonite and cerussite, which result from the weathering and alteration of the original sulfide mineralization. Zinc is allocated mainly in sauconite (Zn-smectite), rather than in carbonates: a factor strictly related to the prevailing siliciclastic character of the host rock. Distinctive features of the Yanque orebody are the comparable ore grades for both Pb and Zn (3.5% Zn and 3.7% Pb), and the inverse supergene chemical zoning. In fact, contrary to other supergene ores of this type, zinc prevails in the top zone of the Yanque deposit, whereas lead content increases with depth. Considering the different mobility of the two metals in solution, it may be assumed that most of the primary zinc that was the source for the Yanque nonsulfides was originally located far from the position occupied by the galena mineralization, whose remnants have been observed on site. Zinc sulfides may have been originally contained in the eroded rock volumes that surrounded the actual deposit: the zinc-rich solutions have possibly migrated through the siliciclastic Mara Fm. and precipitated the nonsulfide minerals by porosity filling and replacement processes. In this sense, the Yanque secondary Zn-Pb deposit could be considered as a special type of “Exotic” mineralization.

Revisiting the Nd-142 deficits in the 1.48 Ga Khariar alkaline rocks, India

The 146Sm–142Nd system plays a central role in tracing the silicate differentiation of the Earth prior to 4.1 Ga. After this time, given its initial abundance, the 146Sm can be considered to be extinct. Upadhyay et al. (2009) reported unexpected negative 142Nd anomalies in 1.48 Ga rocks of the Khariar nepheline syenite complex (India) and inferred that an early enriched, low-Sm/Nd reservoir must have contributed to the mantle source rocks of the Khariar complex. As 146Sm had been effectively extinct for about 2.6 billion years before the crystallisation of the Khariar samples, this Nd signature should have remained isolated from the convective mantle for at least that long. It was thus suggested that the source rock of Khariar samples had been sequestered in the lithospheric root of the Indian craton. Using a different chemical separation method, and a different Thermal Ionization Mass Spectrometry (TIMS) analysis protocol, the present study attempted to replicate these negative 142Nd anomalies, but none were found. To determine which data set is correct, we investigated three possible sources of bias between them: imperfect cancellation of Faraday collector efficiencies during multidynamic TIMS analysis, rapid sample fractionation between the sequential measurement of 146Nd/144Nd and 142Nd/144Nd, and non-exponential law behaviour resulting from so-called “domain mixing.” Incomplete cancellation of collector efficiencies was found unlikely to cause resolvable biases at the estimated level of variation among collector efficiencies. Even in the case of highly variable efficiency and resolvable biases, there is no reason to suspect that they would reproducibly affect only four rocks out of 10 analysed by Upadhyay et al. (2009). Although domain mixing may explain apparent “reverse” fractionation trends observed in some TIMS analyses, it cannot be the cause of the apparent negative anomalies in the study of Upadhyay et al. (2009). It was determined that rapid mass fractionation during the course of a multidynamic TIMS analysis can bias all measured Nd ratios. After applying an approximate correction for this effect, only one rock from Upadhyay et al. (2009) retained an apparent negative 142Nd anomaly. This, in conjunction with our new, anomaly-free data set measured at fractionation rates too low to cause bias, leads to the conclusion that the anomalies reported by Upadhyay et al. (2009) are a subtle and reproducible analytical artefact. The absence of negative 142Nd anomalies in these rocks relaxes the need for a mechanism (other than crust formation) that can isolate a Nd reservoir from the convective mantle for billions of years.

Precambrian to Paleozoic zircon record in the Siviez-Mischabel basement (western Swiss Alps)

U–Pb zircon analyses from three meta-igneous and two metasedimentary rocks from the Siviez-Mischabel nappe in the western Swiss Alps are presented, and are used to derive an evolutionary history spanning from Paleoarchean crustal growth to Permian magmatism. The oldest components are preserved in zircons from metasedimentary albitic schists. The oldest zircon core in these schists is 3.4 Ga old. Detrital zircons reveal episodes of crustal growth in the Neoarchean (2.7–2.5 Ga), Paleoproterozoic (2.2–1.9 Ma) and Neoproterozoic (800–550 Ma, Pan-African event). The maximum age of deposition for the metasedimentary rocks is given by the youngest detrital zircons within both metasedimentary samples dated at ~490 Ma (Cambrian-Ordovician boundary). This is in the age range of two granitoid samples dated at 505 ± 4 and 482 ± 7 Ma, and indicates sedimentation and magmatism in an extensional setting preceding an Ordovician orogeny. The third felsic meta-igneous rock gives a Permian age of intrusion, and is part of a long-lasting Variscan to post-Variscan magmatic activity. The zircons record only minor disturbance of the U–Pb system during the Alpine orogeny.

Matrix Porewater In Crystalline Rocks: Extraction and Analysis

The chemical and isotopic characterization of porewater residing in the inter- and intragranular pore space of the low-permeability rock matrix is an important component with respect to the site characterization and safety assessment of potential host rocks for a radioactive waste disposal. The chemical and isotopic composition of porewater in such low permeability rocks has to be derived by indirect extraction techniques applied to naturally saturated rock material. In most of such indirect extraction techniques – especially in case of rocks of a porosity below about 2 vol.% – the original porewater concentrations are diluted and need to be back-calculated to in-situ concentrations. This requires a well-defined value for the connected porosity – accessible to different solutes under in-situ conditions. The derivation of such porosity values, as well as solute concentrations, is subject to various perturbations during drilling, core sampling, storage and experiments in the laboratory. The present study aims to demonstrate the feasibility of a variety of these techniques to charac-terize porewater and solute transport in crystalline rocks. The methods, which have been de-veloped during multiple porewater studies in crystalline environments, were applied on four core samples from the deep borehole DH-GAP04, drilled in the Kangerlussuaq area, Southwest Greenland, as part of the joint NWMO–Posiva–SKB Greenland Analogue Project (GAP). Potential artefacts that can influence the estimation of in situ porewater chemistry and isotopes, as well as their controls, are described in detail in this report, using specific examples from borehole DH-GAP04

Magnetic anisotropy in clinopyroxene and orthopyroxene single crystals

Pyroxenes constitute an important component in mafic igneous and metamorphic rocks. They often possess a prismatic habit, and their long axis, the crystallographic c axis, helps define a lineation in a textured rock. Anisotropy of magnetic susceptibility (AMS) serves as a fabric indicator in igneous and metamorphic rocks. If a rock’s AMS is carried by pyroxenes, it can be related to their crystallographic preferred orientation and degree of alignment. This requires knowing the intrinsic AMS of pyroxene single crystals. This study provides a comprehensive low-field and high-field AMS investigation of chemically diverse orthopyroxene and clinopyroxene crystals in relation to crystal structure, chemical composition, oxidation state of Fe, and the possible presence of ferromagnetic inclusions. The paramagnetic anisotropy, extracted from high-field data, shows clear relationships to crystallographic directions and Fe concentration both in clinopyroxene and orthopyroxene. In the diopside-augite series, the intermediate susceptibility is parallel to b, and the maximum is at 45° to the c axis. In aegirine, the intermediate axis remains parallel to b, while the maximum susceptibility is parallel to c. The AMS of spodumene depends on Fe concentration. In enstatite, the maximum susceptibility aligns with c and the minimum with b, and in the case of hypersthene, the maximum susceptibility is normal to the exsolution lamellae. Magnetite inclusions within augite possess a ferromagnetic anisotropy with consistent orientation of the principal susceptibilities, which dominates the low-field anisotropy. These results provide better understanding of magnetic anisotropy in pyroxenes and form a solid basis for interpretation of magnetic fabrics in pyroxene-bearing rocks.

Predictive modelling of mineral scaling, corrosion and the performance of solute geothermometers in a granitoid-hosted, enhanced geothermal system

Experience is lacking with mineral scaling and corrosion in enhanced geothermal systems (EGS) in which surface water is circulated through hydraulically stimulated crystalline rocks. As an aid in designing EGS projects we have conducted multicomponent reactive-transport simulations to predict the likely characteristics of scales and corrosion that may form when exploiting heat from granitoid reservoir rocks at ∼200 °C and 5 km depth. The specifications of an EGS project at Basel, Switzerland, are used to constrain the model. The main water–rock reactions in the reservoir during hydraulic stimulation and the subsequent doublet operation were identified in a separate paper (Alt-Epping et al., 2013b). Here we use the computed composition of the reservoir fluid to (1) predict mineral scaling in the injection and production wells, (2) evaluate methods of chemical geothermometry and (3) identify geochemical indicators of incipient corrosion. The envisaged heat extraction scheme ensures that even if the reservoir fluid is in equilibrium with quartz, cooling of the fluid will not induce saturation with respect to amorphous silica, thus eliminating the risk of silica scaling. However, the ascending fluid attains saturation with respect to crystalline aluminosilicates such as albite, microcline and chlorite, and possibly with respect to amorphous aluminosilicates. If no silica-bearing minerals precipitate upon ascent, reservoir temperatures can be predicted by classical formulations of silica geothermometry. In contrast, Na/K concentration ratios in the production fluid reflect steady-state conditions in the reservoir rather than albite–microcline equilibrium. Thus, even though igneous orthoclase is abundant in the reservoir and albite precipitates as a secondary phase, Na/K geothermometers fail to yield accurate temperatures. Anhydrite, which is present in fractures in the Basel reservoir, is predicted to dissolve during operation. This may lead to precipitation of pyrite and, at high exposure of anhydrite to the circulating fluid, of hematite scaling in the geothermal installation. In general, incipient corrosion of the casing can be detected at the production wellhead through an increase in H2(aq) and the enhanced precipitation of Fe-bearing aluminosilicates. The appearance of magnetite in scales indicates high corrosion rates.

P-T-t estimation of syn-kinematic strain in low-grade quartz-feldspar bearing rocks using thermodynamic modeling and 40Ar/39Ar dating techniques: example of the Plan-de-Phasy shear zone unit (Briançonnais Zone, Western Alps)

Pressure–Temperature–time (P–T–t) estimates of the syn-kinematic strain at the peak-pressure conditions reached during shallow underthrusting of the Briançonnais Zone in the Alpine subduction zone was made by thermodynamic modelling and 40Ar/39Ar dating in the Plan-de-Phasy unit (SE of the Pelvoux Massif, Western Alps). The dated phengite minerals crystallized syn-kinematically in a shear zone indicating top-to-the-N motion. By combining X-ray mapping with multi-equilibrium calculations, we estimate the phengite crystallization conditions at 270 ± 50 °C and 8.1 ± 2 kbar at an age of 45.9 ± 1.1 Ma. Combining this P–T–t estimate with data from the literature allows us to constrain the timing and geometry of Alpine continental subduction. We propose that the Briançonnais units were scalped on top of the slab during ongoing continental subduction and exhumed continuously until collision.

Microscale mapping of alteration conditions and potential biosignatures in basaltic-ultramafic rocks on early Earth and beyond

Subseafloor environments preserved in Archean greenstone belts provide an analogue for investigating potential subsurface habitats on Mars. The c. 3.5-3.4 Ga pillow lava metabasalts of the mid-Archean Barberton greenstone belt, South Africa, have been argued to contain the earliest evidence for microbial subseafloor life. This includes candidate trace fossils in the form of titanite microtextures, and sulfur isotopic signatures of pyrite preserved in metabasaltic glass of the c. 3.472 Ga Hooggenoeg Formation. It has been contended that similar microtextures in altered martian basalts may represent potential extraterrestrial biosignatures of microbe-fluid-rock interaction. But despite numerous studies describing these putative early traces of life, a detailed metamorphic characterization of the microtextures and their host alteration conditions in the ancient pillow lava metabasites is lacking. Here, we present a new nondestructive technique with which to study the in situ metamorphic alteration conditions associated with potential biosignatures in mafic-ultramafic rocks of the Hooggenoeg Formation. Our approach combines quantitative microscale compositional mapping by electron microprobe with inverse thermodynamic modeling to derive low-temperature chlorite crystallization conditions. We found that the titanite microtextures formed under subgreenschist to greenschist facies conditions. Two chlorite temperature groups were identified in the maps surrounding the titanite microtextures and record peak metamorphic conditions at 315 ± 40°C (XFe3+(chlorite) = 25-34%) and lower-temperature chlorite veins/microdomains at T = 210 ± 40°C (lower XFe3+(chlorite) = 40-45%). These results provide the first metamorphic constraints in textural context on the Barberton titanite microtextures and thereby improve our understanding of the local preservation conditions of these potential biosignatures. We suggest that this approach may prove to be an important tool in future studies to assess the biogenicity of these earliest candidate traces of life on Earth. Furthermore, we propose that this mapping approach could also be used to investigate altered mafic-ultramafic extraterrestrial samples containing candidate biosignatures.