1,721,115 research outputs found
Provenance of detrital rutiles from the Jurassic sandstones in the Central Sakarya Zone, NW Turkey: U-Pb ages and trace element geochemistry
Full text linkThis provenance study focuses on detrital rutile grains from Jurassic sandstones of the Bayırköy Formation in the central Sakarya Zone. Cr and Nb concentrations of detrital rutile grains in the Jurassic sandstones vary from 18 to 6855 μg/g and 70–13440 μg/g, respectively. Source area discrimination based on the Cr-Nb concentrations shows that 79 % of the detrital rutile grains originated from metapelitic and 21 % from metamafic rocks. The calculated rutile formation temperatures vary from 471 to 798 ℃ with an average temperature of 635 ℃ at P=10 kbar. Zr-in-rutile thermometer gives overlapping temperatures for all detrital rutile grains from both the metapelitic and metamafic sources. This demonstrates that most of the detrital rutiles sourced from metapelitic and metamafic rocks underwent similar metamorphic conditions and have similar metamorphic history. The U-Pb rutile dating yielded ages for the detrital rutiles in the time range of 346 to 319 Ma, which gives the age of metamorphism for the potential source rocks. Trace element compositions, Zr-in-rutile thermometer and U-Pb rutile geochronology show that detrital rutile grains were predominantly derived from early Carboniferous rocks that underwent metamorphism in amphibolite-facies conditions. Amphibolite-facies rocks of the Sarıcakaya Massif in the central Sakarya Zone seem to be the primary source lithologies for the detrital rutiles in the Jurassic Bayırköy Formation as it comprises previously-mentioned source lithologies and has a close geographic position to the area studied. Carboniferous Variscan metamorphism was followed by emplacement of numerous post-collisional granitoids in the central Sakarya Zone. © 2020 Elsevier Gmb
Temperature dependence of Zr in rutile: empirical calibration of a rutile thermometer
No full textRutile is an important carrier of high field strength elements (HFSE; Zr, Nb, Mo, Sn, Sb, Hf, Ta, W). Its Zr content is buffered in systems with quartz and zircon as coexisting phases. The effects of temperature ( T) and pressure ( P) on the Zr content in rutile have been empirically calibrated in this study by analysing rutile - quartz - zircon assemblages of 31 metamorphic rocks spanning a T range from 430 to 1,100 degreesC. Electron microprobe measurements show that Zr concentrations in rutile vary from 30 to 8,400 ppm across this temperature interval, correlating closely with metamorphic grade. The following thermometer has been formulated based on the maximum Zr contents of rutile included in garnet and pyroxene: T (in degreesC) = 127.8 x ln (Zr in ppm) - 10 No pressure dependence was observed. An uncertainty in absolute T of +/- 50 degreesC is inherited from T estimates of the natural samples used. A close approach to equilibrium of Zr distribution between zircon and rutile is suggested based on the high degree of reproducability of Zr contents in rutiles from different rock types from the same locality. At a given locality, the calculated range in T is mostly +/- 10 degreesC, indicating the geological and analytical precision of the rutile thermometer. Possible applications of this new geothermometer are discussed covering the fields of ultrahigh temperature (UHT) granulites, sedimentary provenance studies and metamorphic field gradients
Rutile geochemistry and its potential use in quantitative provenance studies
No full textRutile is among the most stable detrital minerals in sedimentary systems. Information contained in rutile is therefore of prime importance, especially in the study of mature sediments, where most diagnostic minerals are no longer stable. In contrast to zircon, rutile provides information about the last metamorphic cycle as rutile is not stable at greenschist facies conditions. Several known geochemical characteristics of rutile can be used to retrace provenance. The lithology of source rocks can be determined using Nb and Cr contents in rutile, because the most important source rocks for rutile, metapelites and metabasites, imprint a distinct Nb and Cr signature in rutiles. Since Zr in rutile, coexisting with zircon and quartz, is extremely temperature dependent, this relationship can be used as a geothermometer. Metapelites always contain zircon and quartz, thus the Nb and Cr signatures of metapelites indicate rutiles that can be used for thermometry. The result is effectively a single-mineral geothermometer, which is to our knowledge the first of its kind in provenance studies. Several other trace elements are variably enriched in rutile, but the processes creating these variations are so far not understood. In a case study, Al, Si, V, Cr, Fe, Zr, Nb and W contents in rutiles were obtained by electron microprobe from three sediment samples from Upstate New York. A Pleistocene glacial sand, whose source was granulite-facies rocks of the southern Adirondacks, has detrital rutile geochemical signatures which are consistent with the local Geology; a predominantly metapelitic source with a minor metabasitic contribution. Calculated temperatures for the metapelitic rutiles from the glacial sand are consistent with a predominantly granulite-facies source. The two other samples are from Paleozoic elastic wedges deposited in the foreland of the Taconian and Acadian orogenies. Here several geochemical patterns of detrital rutiles are comparable to rutiles derived from the Adirondacks, implying that rutiles eroded from the Taconian and Acadian orogens were originally derived from similar high grade gneiss terranes, like those found in the Adirondacks. The preferred tectonic scenario calls for an accretionary wedge where eroded Grenville province sediments accumulated, which were later recycled during the Taconian and Acadian orogenies. (C) 2004 Elsevier B.V All rights reserved
A recipe for the use of rutile in sedimentary provenance analysis
No full textRutile has received considerable attention in the last decade as a valuable petrogenetic indicator mineral. Based on both new and previously published data, we carve out advantages and pitfalls regarding TiO2-minerals in sedimentary provenance analysis. This results in a recipe for the use of rutile in provenance studies. The main points are: Rutile geochemistry from different grain size fractions does not differ systematically, and hence rutiles should be extracted from the fraction containing the most rutile grains (usually 63-200 mu m). Similarly, different magnetic susceptibility of rutile does not systematically imply different trace element composition. Before interpretation of TiO2-mineral data, it is important to determine the polymorph type. Rutile, anatase and brookite appear to differ systematically in trace element composition. As an alternative to Raman spectroscopy, chemical classification according to Nb, Cr, Sn, Fe, V, and Zr concentrations can be applied. For rutile, a new host lithology discrimination scheme based on Cr-Nb systematics is introduced (x = 5( ) (Nb [ppm] - 500) - Cr [ppm]), which leads to better classification results than previously published discrimination methods. According to this equation, metamafic rutiles have negative values of x, while metapelitic rutiles have positive values. Evaluation of the growth temperature calculations of metamorphic rutile after different authors shows that the equations given by Tomkins et al. (2007) should be applied to both metamafic and metapelitic rutiles. Although there is a pressure effect on the Zr incorporation in rutile, the pressure range for most rutiles of 5-15 kbar introduces an uncertainty in calculated temperature of no more than +/- 35 degrees C. The distribution of calculated temperatures from detrital rutiles is crucial; only well-defined temperature populations should be used for thermometry interpretation. (c) 2012 Elsevier B.V. All rights reserved.Deutsche Forschungsgemeinschaft [EY 23/10-2, ZA 285/4-2
Cs-Rb-Ba systematics in phengite and amphibole: an assessment of fluid mobility at 2.0 GPa in eclogites from Trescolmen, Central Alps
No full textEclogites from Trescolmen that contain abundant hydrous minerals (phengite, amphibole, paragonite, zoisite, talc, apatite) show petrographic evidence for fluid infiltration under conditions of 2.0 to 1.8 GPa, 650 degreesC. Large ion lithophile elements (LILE, e.g. Cs, Rb, Ba and Sr) were analysed by in-situ techniques in all eclogite mineral phases in order to characterize the behaviour of fluid-mobile elements at high pressure. In-situ analysis of carefully-chosen metamorphic assemblages circumvents the problem of partial late-stage alteration, which can severely influence the calculated element budgets of whole-rock samples. Phengite is the dominant host for Cs, Rb, and Ba in both eclogite and adjacent garnet mica schist samples, and incorporates > 90% of the budgets of these elements in whole rocks. LILE contents of phengites in phengite-rich rocks are likely to record the Cs/Rb and Ba/Rb ratios of their host rock protoliths. The LILE patterns of eclogite are consistent with protoliths derived from basalt that underwent seafloor alteration, whereas those of mica schist are almost identical to average upper continental crust. In contrast, LILE patterns of eclogite samples that lack phengite, but do contain amphibole, are unlike any plausible protolith, but are identical to those of amphibole in phengite-bearing samples. This observation points to homogenization of the LILE in different lithologies, which we correlate with petrographic evidence for fluid infiltration. Because phengite in garnet mica schist has a strong capacity to buffer the fluid with respect to Cs, Rb, and Ba, homogenization of amphiboles is best explained by fluid infiltration from the surrounding metapelites into eclogite bodies, implying at least metre-scale fluid mobility. The amphibole homogenization can be most easily modelled by a pervasive open-system fluid flux through the eclogites, possibly facilitated by ductile deformation during the early stages of uplift. Simple calculations give minimum fluid-rock ratios of similar to0.001 to 0.004. Demonstration of the mobility of very small volumes of fluid through eclogite is an important prerequisite of many subduction zone models that try to explain across-are variations in trace element geochemistry. The low fluid-rock ratios from this study are not in contrast with oxygen isotope heterogeneities reported from other eclogite localities. Fluid mobile elements such as Cs, Rb and Ba are more sensitive indicators of small volume, fluid-rock interaction and are therefore potentially valuable for understanding fluid infiltration processes in systems where oxygen isotope shifts are not large enough to be detectable
Cs-Rb-Ba systematics in phengite and amphibole: an assessment of fluid mobility at 2.0 GPa in eclogites from Trescolmen, Central Alps
No full textEclogites from Trescolmen that contain abundant hydrous minerals (phengite, amphibole, paragonite, zoisite, talc, apatite) show petrographic evidence for fluid infiltration under conditions of 2.0 to 1.8 GPa, 650 degreesC. Large ion lithophile elements (LILE, e.g. Cs, Rb, Ba and Sr) were analysed by in-situ techniques in all eclogite mineral phases in order to characterize the behaviour of fluid-mobile elements at high pressure. In-situ analysis of carefully-chosen metamorphic assemblages circumvents the problem of partial late-stage alteration, which can severely influence the calculated element budgets of whole-rock samples. Phengite is the dominant host for Cs, Rb, and Ba in both eclogite and adjacent garnet mica schist samples, and incorporates > 90% of the budgets of these elements in whole rocks. LILE contents of phengites in phengite-rich rocks are likely to record the Cs/Rb and Ba/Rb ratios of their host rock protoliths. The LILE patterns of eclogite are consistent with protoliths derived from basalt that underwent seafloor alteration, whereas those of mica schist are almost identical to average upper continental crust. In contrast, LILE patterns of eclogite samples that lack phengite, but do contain amphibole, are unlike any plausible protolith, but are identical to those of amphibole in phengite-bearing samples. This observation points to homogenization of the LILE in different lithologies, which we correlate with petrographic evidence for fluid infiltration. Because phengite in garnet mica schist has a strong capacity to buffer the fluid with respect to Cs, Rb, and Ba, homogenization of amphiboles is best explained by fluid infiltration from the surrounding metapelites into eclogite bodies, implying at least metre-scale fluid mobility. The amphibole homogenization can be most easily modelled by a pervasive open-system fluid flux through the eclogites, possibly facilitated by ductile deformation during the early stages of uplift. Simple calculations give minimum fluid-rock ratios of similar to0.001 to 0.004. Demonstration of the mobility of very small volumes of fluid through eclogite is an important prerequisite of many subduction zone models that try to explain across-are variations in trace element geochemistry. The low fluid-rock ratios from this study are not in contrast with oxygen isotope heterogeneities reported from other eclogite localities. Fluid mobile elements such as Cs, Rb and Ba are more sensitive indicators of small volume, fluid-rock interaction and are therefore potentially valuable for understanding fluid infiltration processes in systems where oxygen isotope shifts are not large enough to be detectable
Cold subduction of oceanic crust: Implications from a lawsonite eclogite from the Dominican Republic
No full textLawsonite eclogite is a rare rock type that has been described from only five natural occurrences. In contrast, laboratory experiments and thermal models predict that lawsonite eclogite should be widespread in subducted oceanic crust deeper than 1.5 GPa. Here we report a new lawsonite eclogite find from the Dominican Republic that provides constraints on the conditions of subducted crust and on its return to the surface. In this sample, lawsonite coexisting with omphacite occurs as both inclusions in garnet and as porphyroblasts, the latter being partly replaced at their margins by epidote and zoisite. Peak pressure conditions estimated from lawsonite-phengite-omphacite-garnet assemblages were ca 1.6 GPa at a temperature of 360degreesC, implying formation under a geotherm of ca. 8degreesC/km. Peak temperature conditions of 410-450degreesC were in the zoisite eclogite field, suggesting that the sample crossed from the stability field of lawsonite eclogite into that of zoisite eclogite as a result of increasing temperature. A comparison with other reported occurrences indicates that most lawsonite eclogite exhumed at the Earth's surface formed in accretionary wedges. The rarity of lawsonite eclogite at the Earth's surface may be principally due to two factors: (i) that in 'normal' subduction settings lawsonite eclogite enters the subduction factory and hence is usually not exhumed; and (ii) that in accretionary wedge settings, where the PT path leaves the stability field of lawsonite eclogite due to heating, lawsonite eclogite is only preserved if the exhumation path is constrained to a narrow window where the terminal stability of lawsonite is not crossed
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