The application of clumped isotopes (Δ47) in carbonate minerals as a sensitive temperature proxy in paleo-environments depends on a well-constrained clumped isotope fractionation for the necessary ...step of phosphoric acid digestion of the carbonate mineral to produce CO2. Published estimates for clumped isotope fractionations vary, and the effect of different carbonate mineralogies is still under debate. Differences in the sample preparation design and sample digestion temperatures are potential sources for varying acid fractionations and could be a source for discrepant Δ47-temperature calibrations observed in different laboratories. To evaluate the clumped isotope acid fractionation at 70°C and simultaneously account for a potential cation effect we analyzed a set of eight carbonate minerals (calcite, aragonite, dolomite and magnesite) that were driven to a stochastic isotope distribution by heating them to temperatures of 1000°C. Our study reveals significant cation- and mineral-specific differences for the Δ47 acid fractionation of carbonate minerals digested at 70°C or 100°C. The Δ47 acid fractionation at 70°C for calcite is 0.197±0.002 ‰, for aragonite 0.172±0.003 ‰, whereas dolomite has a significantly larger acid fractionation of 0.226±0.002 ‰. For magnesite digested at 100°C we observed a Δ47 acid fractionation of 0.218±0.020 ‰. Projected to an acid digestion at 25°C, our acid fractionation for calcite of 0.260 ‰ is statistically indistinguishable from existing studies. We further show that the Δ47 of the calcite standards ETH-1 and ETH-2 of 0.265 ‰ and 0.267 ‰, respectively, are in the range of the determined acid fractionation projected to 25°C suggesting that they have an identical and near stochastic isotope distribution. The observed differences in the Δ47 acid fractionation between calcite and aragonite (ΔΔ47=−0.025 ‰) and between calcite and dolomite (ΔΔ47=−0.029 ‰) does not correlate with the phosphoric acid fractionation of oxygen isotopes, but rather depends on the radius of the cation as well as on the mineral structure. Our results reveal that the acid fractionation of dolomite at 70°C is significantly distinct from the one of calcite, but at 90°C the two are within error of each other due to the different acid fractionation temperature dependence of calcite and dolomite. Thus it is necessary to use a mineral-specific Δ47 acid fractionation factor for dolomite to avoid differences in the final Δ47 signal from dolomites digested at 90°C and dolomites digested at lower temperatures. Similar effects may apply also to other carbonates such as magnesite and siderite. However, their mineral specific Δ47 acid fractionation at digestion temperatures around 90°C might be also similar to the one of calcite so that potential differences could be within the range of the analytical error.
•Absolute Δ47 acid fractionation of calcite, aragonite and dolomite at 70°C•Cation effect on Δ47 acid fractionation•Dolomite specific Δ47 acid fractionation temperature dependence•Overlap of the Δ47 acid fractionations of calcite and dolomite digested at 90°C
Petrographic classifications of sand and sandstone proposed more than half a century ago are still in use, although they were formulated at a time when depositional and post-depositional sedimentary ...processes were poorly understood, and before the relationships between tectonics and sedimentation could be interpreted in modern plate-tectonic terms. As a consequence, too many scientific articles and technical reports are still encumbered with obsolete concepts, graphical tools, and ambiguous terminology that make sediment descriptions awkward and misleading. A renovation that treasures the legacy of the pioneers is required.
The descriptive petrographic classification of sand and sandstone proposed in this paper is based on the quasi-universally used Gazzi-Dickinson point-counting method, and simply translates into words ternary compositions of quartz, feldspar, and lithic fragments without introducing any new names. The classic QFL plot is subdivided into 15 fields - labelled by adjectives introduced long ago by K.A.W. Crook and endorsed by W.R. Dickinson and more recently by G.J. Weltje - which reflect relative abundances of the three main framework components (provided they exceed 10%QFL). According to standard use, the less abundant component goes first, the more abundant last (e.g., litho-feldspatho-quartzose composition translates into Q > F > L > 10%QFL). For lithic-rich sand and sandstone, information on the prevailing rock fragment type can be added by an additional free adjective (e.g., metamorphiclastic, carbonaticlastic), as proposed long ago by R.V. Ingersoll. For lithic-poor feldspatho-quartzose and quartzose sand and sandstone, further formal subdivisions are proposed based on the Q/F ratio, thus reaching a total of 18 compositional fields overall. Modern sand known to be derived from different source rocks and found in major world's rivers, deserts, and deep-sea fans fits in the pigeonholes defined by the relative abundance of quartz, feldspar, and lithic fragments.
The aim of this classification is to restore directness in sandstone petrology, and to avoid ambiguities generated in the past by making reference to badly defined archetypes, such as greywacke or arkose, thus confusing petrographic composition with subjective considerations about plate-tectonic setting, texture, hydraulic behaviour, mechanical durability, or chemical durability in the illusion that a classification could be genetic at the same time as descriptive.
The stable oxygen isotope composition of orthophosphate (δ18OPO4) is a widely used (paleo)temperature indicator and more recently, a useful tracer of phosphorus-cycling. In natural aqueous systems ...(e.g., oceans, rivers, soil/ground water) the largest reactive phosphorus pool is dissolved inorganic phosphate. Here, we present a new experimentally-determined equation for thermodynamic equilibrium O-isotope fractionations between dissolved phosphate and water, catalyzed by the enzyme inorganic pyrophosphatase (PPase) between 3 and 37°C;1000lnα(PO4-H2O)=14.43(±0.39)1000/T(K)-26.54(±1.33)r2=0.99The new equation is slightly offset by +0.5 to +0.7‰ from recent empirically-determined fractionations based on biogenic apatite, with both based on modern cf-irms TC/EA analysis of Ag3PO4. Dissolved phosphate–water fractionations are offset by +0.9 to +2.3‰ from the earlier empirical relation for biogenic phosphate–water fractionation determined using fluorination of BiPO4. The equation presented here is thus, specific to equilibrium fractions between dissolved phosphate and water and appropriate for use in recent/future oxygen isotope studies of dissolved phosphate using similar cf-irms TC/EA analytical methods.
Granitoids are a major component of the continental crust. They play a pivotal role in its evolution, either by adding new material (continental growth), or by reworking older continental crust. ...These two roles correspond to two main ways of forming granitic magmas, either by partial melting of pre-existing crustal rocks yielding granitic melts directly, or by fractionation of mantle-derived mafic to intermediate magmas. Both models represent endmembers, or paradigms that have shaped the way the geological community envisions granitoids, their occurrence, features, formation and meaning for crustal evolution and differentiation of the whole planet.
In this paper, we expose the two competing paradigms and their implications. We explore the evidence on which each model is based, and how each school of thought articulates a comprehensive view of granitic magmatism based on field geological, petrological, geochemical (including isotopes) and physical constraints; and how, in turn, each view shapes the thinking on crustal growth and evolution, and the interpretation of proxies such as age and Hf isotopic patterns in detrital zircon databases. We emphasize that both schools of thought build a different, but internally consistent view based on a large body of evidence, and we propose that each of them is, or has been, relevant to some portions of the Earth. Thus, the key question is not so much “which” model applies, but “where, when and to which extent”.
•Granitoids may form either by melting of older crust or by fractionation of basaltic magmas.•These two views lead to different models for crustal growth and evolution.•Both models probably apply in different places, times and proportions on the Earth.
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The Izu‐Bonin‐Mariana (IBM) fore arc preserves igneous rock assemblages that formed during subduction initiation circa 52 Ma. International Ocean Discovery Program (IODP) Expedition 352 cored four ...sites in the fore arc near the Ogasawara Plateau in order to document the magmatic response to subduction initiation and the physical, petrologic, and chemical stratigraphy of a nascent subduction zone. Two of these sites (U1440 and U1441) are underlain by fore‐arc basalt (FAB). FABs have mid‐ocean ridge basalt (MORB)‐like compositions, however, FAB are consistently lower in the high‐field strength elements (TiO2, P2O5, Zr) and Ni compared to MORB, with Na2O at the low end of the MORB field and FeO* at the high end. Almost all FABs are light rare earth element depleted, with low total REE, and have low ratios of highly incompatible to less incompatible elements (Ti/V, Zr/Y, Ce/Yb, and Zr/Sm) relative to MORB. Chemostratigraphic trends in Hole U1440B are consistent with the uppermost lavas forming off axis, whereas the lower lavas formed beneath a spreading center axis. Axial magma of U1440B becomes more fractionated upsection; overlying off‐axis magmas return to more primitive compositions. Melt models require a two‐stage process, with early garnet field melts extracted prior to later spinel field melts, with up to 23% melting to form the most depleted compositions. Mantle equilibration temperatures are higher than normal MORB (1,400 °C–1,480 °C) at relatively low pressures (1–2 GPa), which may reflect an influence of the Manus plume during subduction initiation. Our data support previous models of FAB origin by decompression melting but imply a source more depleted than normal MORB source mantle.
Plain Language Summary
This projects looks at how subduction zones form and evolve before island arc volcanism becomes established. Subduction zones are important because they are the primary sites for recycling chemically enriched crustal materials and because they form some of Earth's most important ore deposits. We drilled two deep core holes on the inner trench wall of the Izu‐Bonin subduction zone to recover samples from its earliest history, before formation of Izu‐Bonin island arc volcanoes. Samples from these cores were analyzed chemically to establish how lava compositions varied through time and to understand the processes that control their chemistry. We found a diverse set of lavas, with chemical compositions that are low in elements that are normally enriched in arc lavas. Calculations also show that these lavas were hotter than normal mid‐ocean ridge lavas. We found that the first lavas in a new subduction zone form by the upwelling of hot material from deeper in the Earth, which partially melts as it rises toward the surface. Small amounts of melt formed in equilibrium with garnet deeper in the Earth, while later melts formed in equilibrium with spinel at shallower depths in the Earth. The high extent of melting required, and the high calculated temperatures of these lavas, suggests the involvement of the Manus hot spot, which was located above the newly formed subduction zone some 52 million years ago. The lavas found in these drill holes are similar to lavas found in ocean crust that has been emplaced into mountain belts throughout the world and supports the proposition that in most areas, on‐land ocean crust formed above subduction zones, not at mid‐ocean ridges.
Key Points
Fore‐arc basalts (FABs) formed by decompression melting in response to subduction initiation and differ from mid‐ocean ridge basalts
FABs form by two‐stage melting, with early garnet field melts extracted prior to spinel field melting, resulting in LREE/HREE depletion
Highly depleted FAB may reflect both an older depletion event and higher ambient temperatures related to the Manus plume
On the contemporary Earth, distinct plate tectonic settings are characterized by differences in heat flow that are recorded in metamorphic rocks as differences in apparent thermal gradients. In this ...study we compile thermal gradients defined as temperature/pressure (T/P) at the metamorphic peak and ages of metamorphism (defined as the timing of the metamorphic peak) for 456 localities from the Eoarchean to Cenozoic Eras to test the null hypothesis that thermal gradients of metamorphism through time did not vary outside of the range expected for each of these distinct plate tectonic settings. Based on thermal gradients, metamorphic rocks are classified into three natural groups: high dT/dP >775°C/GPa, mean ∼1110°C/GPa (n = 199) rates, intermediate dT/dP 775-375°C/GPa, mean ∼575°C/GPa (n = 127), and low dT/dP <375°C/GPa, mean ∼255°C/GPa (n = 130) metamorphism. Plots of T, P, and T/P against age demonstrate the widespread occurrence of two contrasting types of metamorphism-high dT/dP and intermediate dT/dP-in the rock record by the Neoarchean, the widespread occurrence of low dT/dP metamorphism in the rock record by the end of the Neoproterozoic, and a maximum in the thermal gradients for high dT/dP metamorphism during the period 2.3 to 0.85 Ga. These observations falsify the null hypothesis and support the alternative hypothesis that changes in thermal gradients evident in the metamorphic rock record were related to changes in geodynamic regime. Based on the observed secular changes, we postulate that the Earth has evolved through three geodynamic cycles since the Mesoarchean and has just entered a fourth. Cycle I began with the widespread appearance of paired metamorphism in the rock record, which was coeval with the amalgamation of widely dispersed blocks of protocontinental lithosphere into supercratons, and was terminated by the progressive fragmentation of the supercratons into protocontinents during the Siderian-Rhyacian (2.5 to 2.05 Ga). Cycle II commenced with the progressive reamalgamation of these protocontinents into the supercontinent Columbia and extended until the breakup of the supercontinent Rodinia in the Tonian (1.0 to 0.72 Ga). Thermal gradients of high dT/dP metamorphism rose around 2.3 Ga leading to a thermal maximum in the mid-Mesoproterozoic, reflecting insulation of the mantle beneath the quasi-integral continental lithosphere of Columbia, prior to the geographical reorganization of Columbia into Rodinia. This cycle coincides with the age span of most anorogenic magmatism on Earth and a scarcity of passive margins in the geological record. Intriguingly, the volume of preserved continental crust of Mesoproterozoic age is low relative to the Paleoproterozoic and Neoproterozoic Eras. These features are consistent with a relatively stable association of continental lithosphere between the assembly of Columbia and the breakup of Rodinia. The transition to Cycle III during the Tonian is marked by a steep decline in the thermal gradients of high dT/dP metamorphism to their lowest value and the appearance of low dT/dP metamorphism in the rock record. Again, thermal gradients for high dT/dP metamorphism show a rise to a peak at the end of the Variscides during the formation of Pangea, before another steep decline associated with the breakup of Pangea and the start of a fourth cycle at ca. 0.175 Ga. Although the mechanism by which subduction started and plate boundaries evolved remains uncertain, based on the widespread record of paired metamorphism in the Neoarchean we posit that plate tectonics was established globally during the late Mesoarchean. During the Neoproterozoic there was a change to deep subduction and colder thermal gradients, features characteristic of the modern plate tectonic regime.
Silindirle Sıkıştırılmış Betonlar (SSB) için literatürde çeşitli sıkıştırma ekipmanları ve işlemleri üzerine çalışmalar gerçekleştirilmiş; ancak, optimum metodolojinin belirlemesi adına bir uzlaşmaya ...varılamamıştır. En yaygın sıkıştırma yöntemleri; vibrasyon masası, vibrasyon çekici, modifiye proktor ve yoğurmalı sıkıştırıcı olarak sıralanabilir. Bu çalışma kapsamında dört farklı sıkıştırma metodunun SSB'nin fiziksel özellikleri, mekanik performansı ve boşluk yapısı üzerindeki etkisini araştırmak amacıyla farklı çimento dozajı, farklı su içeriği ve agrega gradasyonundan oluşan SSB karışımları söz konusu dört sıkıştırma yöntemi ile üretilmiştir. SSB karışımlarının gerçek saha koşulları altındaki performanslarının belirlenebilmesi adına geçirimli gözenek boşluğu hacmi, su emme kapasitesi, sertleşmiş haldeki yoğunluğu, basınç dayanımı ve yarmada-çekme dayanımı gibi temel özellikleri incelenmiştir. Temel malzeme performansı ortaya konan farklı sıkıştırma ekipmanı ve teknikleri kullanılarak üretilen ve farklı kompozisyonlara sahip SSB karışımların boşluk karakterizasyonu mikroyapısal düzeyde detaylı olarak incelenmiştir. Bu tezin esas amacı olan SSB karışımların boşluk karakterizasyonunun çimento dozu, su miktarı, agrega boyutu ve sıkıştırma yöntemiyle ilişkisini görmek için 12× büyütmeye sahip yüksek-çözünürlüklü petrografik mikroskop vasıtasıyla boşluklar karakterize edilmiştir. Boşlukların özelliklerini belirlemek ve boşluk miktarını ölçmek için SSB karışımlarından alınan çok sayıda 50x60 mm boyutlu ince kesit örneklerinin mikroskop görüntüleri 1.25x0.04 mm lens vasıtasıyla alınmış ve görüntü-analiz yazılımı ile analiz edilmiştir. Üretilen SSB karışımları için tüm parametreler göz önünde bulundurulduğunda sıkıştırılabilirlik açısından en iyi performansı yoğurmalı sıkıştırıcı yöntemi ile üretilen numuneler sergilemiştir. Diğer yöntemlere göre daha az yaygın olan yoğurmalı sıkıştırıcı yöntemi, yoğurma işlemi sırasında saha koşullarını yansıtma hususunda en başarılı yöntem olarak kabul edilmiştir. Bununla birlikte yoğurma enerjisinin artışı sıkıştırılabilirliği ilk aşamada arttırırken, yoğurma sayısının 75 olması durumunda 60 yoğurmaya göre sonuç çok değişmemektedir. Vibrasyon çekici ve modifiye proktor, yoğurmalı sıkıştırıcının ardından en iyi performansları sergilerken, vibrasyon masası özellikle düşük su içeriklerinde sıkıştırılabilirlik açısından zayıf performanslar sergilemiştir.
A novel high pressure column flow reactor was used to investigate the evolution of solute chemistry along a 2.3m flow path during pure water- and CO2-charged water–basaltic glass interaction ...experiments at 22 and 50°C and 10−5.7 to 22bars partial pressure of CO2. Experimental results and geochemical modelling showed the pH of injected pure water evolved rapidly from 6.7 to 9–9.5 and most of the iron released to the fluid phase was subsequently consumed by secondary minerals, similar to natural meteoric water–basalt systems. In contrast to natural systems, however, the aqueous aluminium concentration remained relatively high along the entire flow path. The aqueous fluid was undersaturated with respect to basaltic glass and carbonate minerals, but supersaturated with respect to zeolites, clays, and Fe hydroxides. As CO2-charged water replaced the alkaline fluid within the column, the fluid briefly became supersaturated with respect to siderite. Basaltic glass dissolution in the column reactor, however, was insufficient to overcome the pH buffer capacity of CO2-charged water. The pH of this CO2-charged water rose from an initial 3.4 to only 4.5 in the column reactor. This acidic reactive fluid was undersaturated with respect to carbonate minerals but supersaturated with respect to clays and Fe hydroxides at 22°C, and with respect to clays and Al hydroxides at 50°C. Basaltic glass dissolution in the CO2-charged water was closer to stoichiometry than in pure water. The mobility and aqueous concentration of several metals increased significantly with the addition of CO2 to the inlet fluid, and some metals, including Mn, Cr, Al, and As exceeded the allowable drinking water limits. Iron became mobile and the aqueous Fe2+/Fe3+ ratio increased along the flow path. Although carbonate minerals did not precipitate in the column reactor in response to CO2-charged water–basaltic glass interaction, once this fluid exited the reactor, carbonates precipitated as the fluid degassed at the outlet. Substantial differences were found between the results of geochemical modelling calculations and the observed chemical evolution of the fluids during the experiments. These differences underscore the need to improve the models before they can be used to predict with confidence the fate and consequences of carbon dioxide injected into the subsurface.
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