981 resultados para Rocks, Carbonate
Resumo:
Time-space relations of extension and volcanism place critical constraints on models of Basin and Range extensional processes. This paper addresses such relations in a 130-km-wide transect in the eastern Great Basin, bounded on the east by the Ely Springs Range and on the west by the Grant and Quinn Canyon ranges. Stratigraphic and structural data, combined with 40Ar/39Ar isotopic ages of volcanic rocks, document a protracted but distinctly episodic extensional history. Field relations indicate four periods of faulting. Only one of these periods was synchronous with nearby volcanic activity, which implies that volcanism and faulting need not be associated closely in space and time. Based on published dates and the analyses reported here, the periods of extension were (1) prevolcanic (pre-32 Ma), (2) early synvolcanic (30 to 27 Ma), (3) immediately postvolcanic (about 16 to 14 Ma), and (4) Pliocene to Quaternary. The break between the second and third periods is distinct. The minimum gap between the first two periods is 2 Ma, but the separation may be much larger. Temporal separation of the last two periods is only suggested by the stratigraphic record and cannot be rigorously demonstrated with present data. The three younger periods of faulting apparently occurred across the entire transect. The oldest period is recognized only at the eastern end of the transect, but appears to correlate about 150 km northward along strike with extension in the Northern Snake Range-Kern Mountains area. Therefore the oldest period also is regional in extent, but affected a different area than that affected by younger periods. This relation suggests that distinct extensional structures and master detachment faults were active at different times. The correlation of deformation periods of a few million years duration across the Railroad Valley-Pioche transect suggests that the scale of active extensional domains in the Great Basin may be greater than 100 km across strike.
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In the Mt. Olympos region of northeastern Greece, continental margin strata and basement rocks were subducted and metamorphosed under blueschist facies conditions, and thrust over carbonate platform strata during Alpine orogenesis. Subsequent exposure of the subducted basement rocks by normal faulting has allowed an integrated study of the timing of metamorphism, its relationship to deformation, and the thermal history of the subducted terrane. Alpine low-grade metamorphic assemblages occur at four structural levels. Three thrust sheets composed of Paleozoic granitic basement and Mesozoic metasedimentary cover were thrust over Mesozoic carbonate rocks and Eocene flysch; thrusting and metamorphism occurred first in the highest thrust sheets and progressed downward as units were imbricated from NE to SW. 40Ar/39Ar spectra from hornblende, white mica, and biotite samples indicate that the upper two units preserve evidence of four distinct thermal events: (1) 293–302 Ma crystallization of granites, with cooling from >550°C to <325°C by 284 Ma; (2) 98–100 Ma greenschist to blueschist-greenschist transition facies metamorphism (T∼350–500°C) and imbrication of continental thrust sheets; (3) 53–61 Ma blueschist facies metamorphism and deformation of the basement and continental margin units at T<350–400°C; (4) 36–40 Ma thrusting of blueschists over the carbonate platform, and metamorphism at T∼200–350°C. Only the Eocene and younger events affected the lower two structural packages. A fifth event, indicated by diffusive loss profiles in microcline spectra, reflects the beginning of uplift and cooling to T<100–150°C at 16–23 Ma, associated with normal faulting which continued until Quaternary time. Incomplete resetting of mica ages in all units constrains the temperature of metamorphism during continental subduction to T≤350°C, the closure temperature for Ar in muscovite. The diffusive loss profiles in micas and K-feldspars enable us to “see through” the younger events to older events in the high-T parts of the release spectra. Micas grown during earlier metamorphic events lost relatively small amounts of Ar during subsequent high pressure-low temperature metamorphism. Release spectra from phengites grown during Eocene metamorphism and deformation record the ages of the Ar-loss events. Alpine deformation in northern Greece occurred over a long time span (∼90 Ma), and involved subduction and episodic imbrication of continental basement before, during, and after the collision of the Apulian and Eurasian plates. Syn-subduction uplift and cooling probably combined with intermittently higher cooling rates during extensional events to preserve the blueschist facies mineral assemblages as they were exhumed from depths of >20 km. Extension in the Olympos region was synchronous with extension in the Mesohellenic trough and the Aegean back-arc, and concurrent with westward-progressing shortening in the external Hellenides.
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Electron microprobe data are presented for chevkinite-group minerals from granulite-facies rocks and associated pegmatities of the Napier Complex and Mawson Station charnockite in East Antarctica and from the Eastern Ghats, South India. Their compositions conform to the general formula for this group, viz. A(4)BC(2)D(2)Si(4)O(22) where, in the analysed specimens A = (rare-earth elements (REE), Ca, Y, Th), B = Fe(2+) Mg, C = (Al, Mg, Ti, Fe(2+), Fe(3+), Zr) and D = Ti and plot within the perrierite field oftlic total Fe (as FeO) (wt.%) vs. CaO (wt.%) discriminator diagram of Macdonald and Belkin (2002). In contrast to most chevkinite-group minerals, the A site shows unusual enrichment in the MREE and HREE relative to the LREE and Ca. In one sample from the Napier Complex, Y is the dominant cation among the total REE + Y in the A site, the first reported case of Y-dominance in the chevkinite group. The minerals include the most Al-rich yet reported in the chevkinite group (<= 9.15 wt.% Al(2)O(3)), sufficient to fill the C site in two samples. Conversely, the amount of Ti in these samples does not fill the D site. and, thus, some of the Al could be making up the deficiency at D, a situation not previously reported in the chevkinite group. Fe abudances are low, requiring Mg to occupy up to 45% of the B site. The chevkinite-group minerals analysed originated from three distinct parageneses: (1) pegmatites containing hornblende and orthopyroxene or garnet; (2) orthopyroxene-bearing gneiss and granulite; (3) highly aluminous paragneisses in which the associated minerals are relatively magnesian or aluminous. Chevkinite-group minerals from the first two parageneses have relatively high FeO content and low MgO and Al(2)O(3) contents; their compositions plot in the field for mafic and intermediate igneous rocks. In contrast, chevkinite-group minerals from the third paragenesis are notably more aluminous and have greater Mg/Fe ratios.
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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.
Resumo:
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
Resumo:
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.
Resumo:
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.
Resumo:
High-pressure mechanical squeezing was applied to sample pore waters from a sequence of highly indurated and overconsolidated sedimentary rocks in a drillcore from a deep borehole in NE Switzerland. The rocks are generally rich in clay minerals (28–71 wt.%), with low water contents of 3.5–5.6 wt.%, resulting in extremely low hydraulic conductivities of 10− 14–10− 13 m/s. First pore-water samples could generally be taken at 200 MPa, and further aliquots were obtained at 300, 400 and 500 MPa. Chemical and isotopic compositions of squeezed waters evolve with increasing pressure. Decreasing concentrations of Cl−, Br−, Na+ and K+ are explained by ion filtration due to the collapse of the pore space during squeezing. Increasing concentrations of Ca2 + and Mg2 + are considered to be a consequence of pressure-dependent solubilities of carbonate minerals in combination with sorption/desorption reactions. The pressure dependence was studied by model calculations considering equilibrium with carbonate minerals and the exchanger population on clay surfaces, and the trends observed in the experiments could be confirmed. The compositions of the squeezed waters were compared with results of independent methods, such as aqueous extraction and in-situ sampling of ground and pore waters. On this basis, it is concluded that the chemical and isotopic composition of pore water squeezed at the lowest pressure of 200 MPa closely represents that of the in-situ pore water. The feasibility of sampling pore waters with water contents down to 3.5 wt.% and possibly less opens new perspectives for studies targeted at palaeo-hydrogeological investigations using pore-water compositions in aquitards as geochemical archives.
Resumo:
The new mineral species vapnikite, Ca3UO6, was found in larnite pyrometamorphic rocks of the Hatrurim Formation at Jabel Harmun in the Judean desert, Palestinian Autonomy, Israel. Vapnikite is an analogue of the synthetic ordered double-perovskite β-Ca3UO6 and is isostructural with the natural fluorperovskite – cryolite Na3AlF6. Vapnikite Ca3UO6 (P21/n,Z = 2, a = 5.739(1), b = 5.951(1), c = 8.312(1) Å, β = 90.4(1)°, V = 283.9(1) Å3) forms yellow-brown xenomorphic grains with a strong vitreous lustre. Small grains up to 20 – 30 m m in size are wedged between larnite, brownmillerite and ye'elimite. Vapnikite has irregular fracture, cleavage and parting were not observed. The calculated density is 5.322 g cm–3, the microhardness is VHN25 = 534 kg mm–2 (mean of seven measurements) corresponding to the hardness of ~5 on the Mohs scale. The crystal structure of vapnikite Ca3UO6 differs from that of its synthetic analogue β-Ca3UO6 by having a larger degree of Ca, U disorder. Vapnikite formed at the high-temperature retrograde stage of pyrometamorphism when larnite rocks were altered by fluids/melts of high alkalinity.
Resumo:
Geological site characterisation programmes typically rely on drill cores for direct information on subsurface rocks. However, porosity, transport properties and porewater composition measured on drill cores can deviate from in-situ values due to two main artefacts caused by drilling and sample recovery: (1) mechanical disruption that increases porosity and (2) contamination of the porewater by drilling fluid. We investigated the effect and magnitude of these perturbations on large drill core samples (12–20 cm long, 5 cmdiameter) of high-grade, granitic gneisses obtained from 350 to 600 m depth in a borehole on Olkiluoto Island (SW Finland). The drilling fluid was traced with sodium–iodide. By combining out-diffusion experiments, gravimetry, UV-microscopy and iodide mass balance calculations, we successfully quantified the magnitudes of the artefacts: 2–6% increase in porosity relative to the bulk connected porosity and 0.9 to 8.9 vol.% contamination by drilling fluid. The spatial distribution of the drilling-induced perturbations was revealed by numerical simulations of 2D diffusion matched to the experimental data. This showed that the rims of the samples have a mechanically disrupted zone 0.04 to 0.22 cm wide, characterised by faster transport properties compared to the undisturbed centre (1.8 to 7.7 times higher pore diffusion coefficient). Chemical contamination was shown to affect an even wider zone in all samples, ranging from 0.15 to 0.60 cm, inwhich iodide enrichmentwas up to 180 mg/kgwater, compared to 0.5 mg/kgwater in the uncontaminated centre. For all samples in the present case study, it turned out that the magnitude of the artefacts caused by drilling and sample recovery is so small that no correction is required for their effects. Therefore, the standard laboratory measurements of porosity, transport properties and porewater composition can be taken as valid in-situ estimates. However, it is clear that the magnitudes strongly depend on site- and drilling-specific factors and therefore our results cannot be transferred simply to other locations. We recommend the approach presented in this study as a route to obtain reliable values in future drilling campaigns aimed at characterising in-situ bedrock properties.
Resumo:
The 39Ar-40Ar technique is often used to date the metamorphic evolution of basement rocks. The present review article examines systematic aspects of the K-Ar decay system in different mineral chronometers frequently found in mono- and polymetamorphic basements (amphibole, biotite, muscovite/phengite, K-feldspar). A key observation is that the measured dissolution rate of silicates in aqueous fluids is many orders of magnitude faster, and has a much lower activation energy, than the rate of Fickian diffusion of Ar. The effects of this inequality are patchy age zonations, very much like those observed in many U-Pb chronometers, unaccompanied by intra-crystalline bell¬shaped Ar loss profiles. Recognizing the importance of the respective rate constants in field situations leads to re-evaluating the ages and the interpretive paradigms in classic examples such as the Central Alpine "Lepontine" amphibolite event and the Western Alpine eclogitic event.
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The thorium and rare-earth element (Th-REE) deposit at Morro do Ferro formed under supergene lateritic weathering conditions. The ore body consists of shallow NW-SE elongated argillaceous lenses that extend from the top of the hill downwards along its south-eastern slope. The deposit is capped by a network of magnetite layers which protected the underlying highly weathered, argillaceous host rock from excessive erosion. The surrounding country rocks comprise a sequence of subvolcanic phonolite intrusions that have been strongly altered by hydrothermal and supergene processes. From petrological, mineralogical and geochemical studies, and mass balance calculations, it is inferred that the highly weathered host rock was originally carbonatitic in composition, initially enriched in Th and REEs compared to the surrounding silicate rocks. The intrusion of the carbonatite caused fenitic alteration in the surrounding phonolites, consisting of early potassic alteration followed by a vein-type Th-REE mineralization with associated fluorite, carbonate, pyrite and zircon. Subsequent weathering has completely decomposed the carbonatite forming a residual supergene enrichment of Th and REEs. Initial weathering of the carbonatite has created a chemical environment that might have been conductive to carbonate and phosphate complexing of the REEs in groundwaters. This may have appreciably restricted the dissolution of primary REE phases. Strongly oxidic weathering has resulted in a fractionation between Ce and the other light rare earth elements (LREEs). Ce3+ is oxidized to Ce4+ and retained together with Th by secondary mineral formation (cerianite, thorianite), and by adsorption on poorly crystalline iron- and aluminium-hydroxides. In contrast, the trivalent LREEs are retained to a lesser degree and are thus more available for secondary mineral formation (Nd-lanthanite) and adsorption at greater depths down the weathering column. Seasonally controlled fluctuations of recharge waters into the weathering column may help to explain the observed repetition of Th-Ce enriched zones underlain by trivalent LREE enriched zones.
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Five test runs were performed to assess possible bias when performing the loss on ignition (LOI) method to estimate organic matter and carbonate content of lake sediments. An accurate and stable weight loss was achieved after 2 h of burning pure CaCO3 at 950 °C, whereas LOI of pure graphite at 530 °C showed a direct relation to sample size and exposure time, with only 40-70% of the possible weight loss reached after 2 h of exposure and smaller samples losing weight faster than larger ones. Experiments with a standardised lake sediment revealed a strong initial weight loss at 550 °C, but samples continued to lose weight at a slow rate at exposure of up to 64 h, which was likely the effect of loss of volatile salts, structural water of clay minerals or metal oxides, or of inorganic carbon after the initial burning of organic matter. A further test-run revealed that at 550 °C samples in the centre of the furnace lost more weight than marginal samples. At 950 °C this pattern was still apparent but the differences became negligible. Again, LOI was dependent on sample size. An analytical LOI quality control experiment including ten different laboratories was carried out using each laboratory's own LOI procedure as well as a standardised LOI procedure to analyse three different sediments. The range of LOI values between laboratories measured at 550 °C was generally larger when each laboratory used its own method than when using the standard method. This was similar for 950 °C, although the range of values tended to be smaller. The within-laboratory range of LOI measurements for a given sediment was generally small. Comparisons of the results of the individual and the standardised method suggest that there is a laboratory-specific pattern in the results, probably due to differences in laboratory equipment and/or handling that could not be eliminated by standardising the LOI procedure. Factors such as sample size, exposure time, position of samples in the furnace and the laboratory measuring affected LOI results, with LOI at 550 °C being more susceptible to these factors than LOI at 950 °C. We, therefore, recommend analysts to be consistent in the LOI method used in relation to the ignition temperatures, exposure times, and the sample size and to include information on these three parameters when referring to the method.