1,721,433 research outputs found
Dolomite discloses a hidden history of subducting slabs
No full textDolomite and magnesite are the major carbon reservoirs in the subducted oceanic lithosphere. Compositional complexities in dolomite and magnesite solid solutions are often overlooked, but normal and oscillatory zoning in dolomite from mafic eclogites of Tianshan (China) demonstrates that prograde pressure-temperature histories and interactions with coexisting mixed fluids can be recorded in carbonates. Thermodynamic modeling and comparison with experimental results warn against a simplistic approach to carbonate-bearing assemblages and show that magnesite occurrence is not an unambiguous evidence for ultrahigh-pressure metamorphism
Melting carbonated epidote eclogites : carbonatites from subducting slabs
Full text linkCurrent knowledge on the solidus temperature for carbonated eclogites suggests that carbonatitic liquids should not form from a subducted oceanic lithosphere at sub-arc depth. However, the oceanic crust includes a range of gabbroic rocks, altered on rifts and transforms, with large amounts of anorthite-rich plagioclase forming epidote on metamorphism. Epidote disappearance with pressure depends on the normative anorthite content of the bulk composition; we therefore expect that altered gabbros might display a much wider pressure range where epidote persists, potentially affecting the solidus relationships. A set of experimental data up to 4.6 GPa, and 1000 degrees C, including new syntheses on mafic eclogites with 36.8 % normative anorthite, is discussed to unravel the effect of variable bulk and volatile compositions in model eclogites, enriched in the normative anorthite component (An(37) and An(45)). Experiments are performed in piston cylinder and multianvil machines. Garnet, clinopyroxene, and coesite form in all syntheses. Lawsonite was found to persist at 3.7 GPa, 750 degrees C, with both dolomite and magnesite; at 3.8 GPa, 775-800 degrees C, fluid-saturated conditions, epidote coexists with kyanite, dolomite, and magnesite. The anhydrous assemblage garnet, omphacite, aragonite, and kyanite is found at 4.2 GPa, 850 degrees C. At 900 degrees C, a silicate glass of granitoid composition, a carbonatitic precipitate, and Na-carbonate are observed. Precipitates are interpreted as evidence of hydrous carbonatitic liquids at run conditions; these liquids produced are richer in Ca compared to experimental carbonatites from anhydrous experiments, consistently with the dramatic role of H2O in depressing the solidus temperature for CaCO3. The fluid-absent melting of the assemblage epidote + dolomite, enlarged in its pressure stability for An-rich gabbros, is expected to promote the generation of carbonatitic liquids. The subsolidus breakdown of epidote in the presence of carbonates at depths exceeding 120 km provides a major source of C-O-H volatiles at sub-arc depth. In warm subduction zones, the possibility of extracting carbonatitic liquids from a variety of gabbroic rocks and epidosites offers new scenarios on the metasomatic processes in the lithospheric wedge of subduction zones and a new mechanism for recycling carbon
Carbon mobilized at shallow depths in subduction zones by carbonatitic liquids
Full text linkMore than half a gigaton of CO 2 is subducted into Earth's interior each year. At least 40% of this CO 2 is returned to the atmosphere by arc volcanism. Processes that are known to release carbon from subducting slabs - decarbonation or carbonate dissolution in fluids - can account for only a portion of the CO 2 released at arc volcanoes. Carbonatitic liquids may form from the subducting crust, but are thought to form only at very high temperatures. Melting of carbonated rocks could restrict the subduction of carbon into the deeper Earth. However, the behaviour of such rock types in subduction zones is unclear. Here I use laboratory experiments to show that calcium-rich hydrous carbonatitic liquids can form at temperatures as low as 870 to 900 °C, which corresponds to shallow depths of just 120 km beneath subduction zone arcs, in warm thermal regimes. I find that water strongly depresses the solidus for hydrous carbonate gabbro and limestone rocks, creating carbonatitic liquids that efficiently scavenge volatile elements, calcium and silicon, from the slab. These extremely mobile and reactive liquids are expected to percolate into the mantle wedge, and create a CO 2 source for subduction zone magmatism. Carbonatitic liquids thus provide a potentially significant pathway for carbon recycling at shallow depths beneath arcs
Modelling metamorphic rocks in complex systems : present-day developments in high pressure experimental petrology
No full textMetamorphic processes at subduction zones are strictly related to the transfer and release of water via dehydration reactions from the subducting slab toward the mantle wedge. Results and present-day developments in high pressure experimental petrology in the laboratory of Milano are presented here, dealing with lithologies which are expected to be subducted, i.e. mainly mafics and ultramafics. Particular emphasis will be given to hydrate-bearing assemblages. Experiments performed in mafic rocks at pressures of up to 5 GPa show that phase relationships are controlled by amphibole s.s. to 2.5 GPa and by epidote group minerals and lawsonite at higher pressure. Phase relationships point out to strong sensitivity of assemblages from continuous reactions and therefore from bulk composition considered. In ultramafics, at temperatures between 680 and 800°C, amphibole and chlorite are the major hydrous phases able to transport water deep into the Earth's Mantle. At pressure > 4.8 GPa, a hydrous silicate with a 10Å phase structure forms at the expense of chlorite, providing a «carrier» by which water might be transported down to 200 km depth. The interaction between alkali-rich fluids and mantle peridotites lead to the occurrence of potassic hydrous phases such as phlogopite. Experiments on K-peridotite suggest a pressure dependent relevance of a «tale» component in phlogopites
High pressure behaviour of hydrates and carbonates: experimental constraints in mafic and ultramafic systems
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The EU and US Global Human Rights Sanction Regimes: Useful Complementary Instruments to Advance Protection of Universal Values? A Legal Appraisal, E. Fahey (a cura di), Routledge Handbook on Transatlantic Relations
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Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Magmatic epidote
No full textEpidote was first recognized as a magmatic mineral in the alpine Bergell tonalite by Cornelius (1915). Field observations and microscopic textures let Cornelius to conclude “... the only possibility is, that epidote is a primary mineral in our tonalite, crystallizing early from the magma, i.e., before (in part also contemporaneous with) biotite” (translated from German, Cornelius (1915), p.170). This knowledge disappeared and for the following 70 years, epidote and zoisite were categorized as metamorphic minerals. The petrologic significance of magmatic epidote was then rediscovered when Zen and Hammarstrom (1984) identified epidote as an important magmatic constituent of intermediate calc-alkaline intrusives in plutons of the North American Cordillera. Zen and Hammarstrom (1984) also suggested that epidote indicates a minimum intrusive pressure of about 0.5 to 0.6 GPa. Subsequently, magmatic epidote was described from many granodioritic to tonalitic plutons, but also from monzogranite (e.g., Leterrier 1972), dikes of dacitic composition (Evans and Vance 1987), and orbicular diorite (Owen 1991, 1992). Furthermore, epidote was not only recognized in crystallizing plutons or dikes but also in high pressure migmatites and pegmatites derived from eclogites (Nicollet et al. 1979; Franz and Smelik 1995).
The role of epidote during magmatic crystallization is relatively well understood, and crystallization temperatures and sequences involving epidote in intermediate magmas (granodiorite-tonalite-trondhjemite, TTG) are experimentally determined and confirmed from natural intrusives. In contrast, little attention is directed towards the inverse process, i.e., melting of epidote bearing lithologies. Epidote and zoisite are omnipresent in eclogite of intermediate temperature (Enami et al. 2004) and denominates three subfacies (i.e., epidote- blueschist, epidote-amphibolite, and epidote-eclogite facies).
Indeed the epidote-amphibolite facies intersects the wet granite solidus near 0.5 GPa at 680°C, defining the pressure above which epidote may be present during melting processes. Experiments on natural compositions have confirmed that epidote and zoisite are stable above the wet granite solidus in the pressure range 0.5 to 3.0 GPa (Poli and Schmidt 1995; Poli and Schmidt 2004), and thus they are involved in partial melting processes. Unfortunately, it is difficult to recognize the participation of epidote during partial melting in nature, as epidote is one of the first phases to ‘melt out’. On the other hand, it is exactly the relatively narrow temperature interval of epidote + melt, which makes epidote a significant provider for H2O during fluid-absent melting (Vielzeuf and Schmidt 2001).
In this chapter we use the term “epidote” or “epidote minerals” in a general sense for all minerals of the epidote group including zoisite, and “epidotess” for the monoclinic solid solution between Ca2Al3Si3O12(OH) and Ca2Al2Fe3+Si3O12 (OH) (“ps”). Solid solutions with significantly more than one Fe per formula unit have not been reported as magmatic epidote. “Zoisite” is used only to specifically designate the orthorhombic polymorph. The review is limited to epidote with relatively low REE contents. The stability and role of allanite, a common early accessory mineral in granitoid intrusions, is discussed by Gieré and Sorensen (2004). We first review natural occurrences of magmatic epidote starting with criteria to identify a magmatic origin of epidote. Our compilation of magmatic epidote occurrences focuses on the oddities, i.e., the <5% of magmatic epidote which are not part of the widespread “epidote in TTG” plutons. We then review experimentally determined phase relations of epidote minerals in coexistence with melt. This includes melting and crystallization reactions as well as the bulk composition effect on the magmatic occurrence. The factors influencing the variation in “minimum pressure” indicated by magmatic epidote in intrusions receive particular intention. Finally we investigate the role of epidote during fluid-absent melting processes
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