1,721,050 research outputs found
Salt-rich aqueous fluids formed during eclogitization of metabasites in the Alpine continental crust (Austroalpine Mt. Emilius unit, Italian western Alps)
No full texthe metabasite bodies of the Mt. Emilius continental unit (western Italian Alps) underwent a stage of Alpine eclogite-facies metamorphism (1.1–1.3 GPa and 450–550°C) accompanied by polyphase deformation and recrystallization. The metabasites consist of two main rock types: (1) eclogites (omphacite+garnet+glaucophane+epidote+phengite/paragonite) preserving no relics of their precursors; (2) eclogitized granulites, i.e. rocks whose incomplete eclogitic recrystallization (clinopyroxene II+garnet II+epidote+amphibole+chlorite) allowed survival of textural and mineralogical relics of pre-Alpine granulitic assemblages (clinopyroxene I+garnet I+plagioclase+amphibole). In this latter case the pre-Alpine granulites were converted to eclogites as the result of infiltration of aqueous fluids during eclogitization. In both eclogites and eclogitized granulites hydrated high-pressure foliations are cut by eclogitic metamorphic veins. The bulk rock chemistry of the metabasites influenced the compositions of both the vein- and rock-forming clinopyroxenes and the compositional correlation between the vein- and rock-forming clinopyroxenes indicates that the syn-eclogitic fluids have re-equilibrated with the metabasite hosts. The predominant vein minerals (omphacite, epidote and garnet) contain primary high-salinity fluid inclusions. The fluids consist of two-phase (liquid+vapour) and of multiphase (liquid+vapour+salt+additional quartz) salty aqueous inclusions containing NaCl, CaCl2 and MgCl2 as the main chloride species. The vein inclusions show a salinity range from 17 to 45 wt.% salts in eclogites and from 20 to 50 wt.% salts in eclogitized granulites. In contrast, fluid inclusions in matrix minerals of the eclogitized granulites contain primary two-phase inclusions displaying a salinity range between 10 and 25 wt.% salts. The marked difference in fluid salinities recorded by the inclusions in the eclogitic veins and those in the partially re-equilibrated eclogitized granulites is interpreted in terms of progressive hydration during eclogitization of granulite-facies rocks, which caused an increase in the salt content of the residual inclusion fluids
Crust-to-mantle fluid cycling, the origin of dolomite-peridotite and implications for carbonatite sources: the Ulten Zone case.
No full textDolomite-bearing orogenic garnet peridotites witness fluid-mediated carbon recycling in a mantle wedge (Ulten Zone, Eastern Alps, Italy).
No full textWe document the presence of dolomite ± apatite in orogenic peridotites from the Ulten Zone (UZ, Italian Alps), the remnants of a Variscan mantle wedge tectonically coupled with eclogitized continental crust. These dolomite peridotites are associated with dominant carbonate-free amphibole peridotites, which formed in response to infiltration of aqueous subduction fluids lost by the associated crustal rocks during high-pressure (HP) metamorphism and retrogression. Dolomite-free and dolomite-bearing peridotites share the same metamorphic evolution, from garnet- (HP) to spinel-facies (low-pressure, LP) conditions. Dolomite and the texturally coexisting phases display equilibrium redistribution of rare earth elements and of incompatible trace elements during HP and LP metamorphism; clinopyroxene and amphiboles from carbonate-free and carbonate-bearing peridotites have quite similar compositions. These features indicate that the UZ mantle rocks equilibrated with the same metasomatic agents: aqueous CO2-bearing fluids enriched in incompatible elements released by the crust. The P– T crystallization conditions of the dolomite peridotites (outside the field of carbonatite melt + amphibole peridotite coexistence), a lack of textures indicating quench of carbonic melts, a lack of increase in modal clinopyroxene by reaction with such melts and the observed amphibole increase at the expense of clinopyroxene, all suggest that dolomite formation was assisted by aqueous CO2-bearing fluids. A comparison of the trace element compositions of carbonates and amphiboles from the UZ peridotites and from peridotites metasomatized by carbonatite and/or carbon-bearing silicate melts does not help to unambiguously discriminate between the different agents (fluids or melts). The few observed differences (lower trace element contents in the fluid-related dolomite) may ultimately depend on the solute content of the metasomatic agent (CO2-bearing fluid versus carbonatite melt). This study provides strong evidence that C–O–H subduction fluids can produce ‘carbonatite-like’ assemblages in mantle rocks, thus being effective C carriers from the slab to the mantle wedge at relatively low P– T. If transported beyond the carbonate and amphibole solidus by further subduction, dolomite-bearing garnet + amphibole peridotites like the ones from Ulten can become sources of carbonatite and/or C-bearing silicate melts in the mantle wedge
The water and fluid-mobile element cycles during serpentinite subduction : a review
Full text linkThe key role of serpentinites in the global cycles of volatiles, halogens and fluid-mobile elements in oceans and in subduction zones is now ascertained by many studies quantifying their element budgets and the composition of fluids they release during subduction. Geochemical tracers (e.g. B, As, Sb; stable B and radiogenic Sr and Pb isotopes) have also been employed to trace the provenance of serpentinites (slab or forearc mantle?) accreted to the plate interface of fossil subduction zones. In turn, this helps defining the tectonic processes, seismicity and mass transfer attending rock burial and exhumation within subduction zones. The results suggest that the sole use of geochemical data is insufficient to track the origin of subduction-zone serpentinites and the timing of serpentinization, whether oceanic or subduction-related. Integrated multidisciplinary studies of ophiolitic serpentinites show that pristine, oceanic, geochemical imprints (e.g. high11B, marine Sr isotopes, low As + Sb) become reset towards more radiogenic Sr, lower11B, and higher As + Sb via metasomatic exchange with crust-derived fluids during subduction accretion to the plate interface.The dehydration fluids released by serpentinite dehydration at various subduction stages and still preserved in these rocks as inclusions, carry significant amounts of halogens and fluid-mobile elements. The key compositional similarities of antigorite-breakdown fluids from different localities (Betic Cordillera, Spain; Central Alps, Switzerland) indicate that rocks record comparable subduction processes. We individuate the fluid-mediated exchange with sedimentary and/or crustal reservoirs during subduction as the key mechanism for geochemical hybridization of serpentinite. The antigorite dehydration fluids produced by hybrid serpentinites have high Cs, Rb, Ba, B, Pb, As, Sb and Li overlapping those of the arc lavas and representing the mixed serpentinite–sediment (crustal) component released to arcs. This helps discriminating the mass transfer processes responsible for supra-subduction mantle metasomatism and arc magmatism. The studied plate-interface hybrid serpentinites are also proxies of forearc mantle metasomatized by slab fluids. Based on the above observations, we propose that the mass transfer from slabs to plate interface and/or forearc mantle and the subsequent down-drag of this altered mantle to subarc depths potentially is a major process operating in subduction zones.The nominally anhydrous olivine, orhopyroxene, clinopyroxene and garnet produced by serpentinite dehydration host appreciable amounts of halogens and fluid-mobile elements that can be recycled in the deep mantle beyond arcs. Involvement of de-serpentinized residues in lower mantle metasomatism begins to be increasingly recognized by studies of ocean island basalts (OIB) and of B-bearing blue diamonds and by the isotopic serpentinite compositions presented here
Fluid-mobile elements in serpentinites: Constraints on serpentinisation environments and element cycling in subduction zones.
No full textFluid-mobile element (FME) systematics in serpentinites are key to unravel the environments of mantle rock hydration, dehydration, and element recycling in subduction zones. Here we compile serpentinite geochemical data and, for the first time, report discriminative FME enrichment trends for mid ocean ridge vs. forearc serpentinisation by applying alkali element-U ratios. Characteristic element fractionations are thereby governed by redox-dependent differential U mobility at mid ocean ridges and in forearcs, and by high Cs input in forearcs due to fluids equilibrated with sediments. Simple modelling reproduces the observed enrichment trends in serpentinites that range over several orders of magnitude. From these systematics, first constraints on potentially discriminative fractionation trends for unconventional fluid tracers such as B, As, and Sb can be deduced. Prominent W enrichments that correlate with FMEs suggest significant W mobility in low-temperature serpentinising environments. Application of the alkali element-U systematics to the subducted metaperidotites of Erro Tobbio (recording initial brucite + antigorite breakdown during subduction) and Almirez (recording final antigorite breakdown) reveal that pre-subduction FME enrichment signatures are retained in progressively subducted hydrous mantle rocks to beyond subarc levels. Associated dehydration veins and fluid inclusions reveal subordinate alkali element-U fractionation trends during dehydration. Subducted hydrous mantle rocks therefore may introduce characteristic element signatures and thus contribute towards mantle geochemical heterogeneities
Melt- versus fluid-induced metasomatism in spinel to garnet wedge peridotites (Ulten Zone, Eastern Italian Alps): clues from trace element and Li abundances
No full textPeak Metamorphic Conditions and Retrograde P-T Paths in the Eastern Wilson Terrane and the Dessent Unit (Northern Victoria Land, Antarctica): New Constraints to Tectonic Model for the Wilson Terrane and the Wilson Terrane - Bowers Terrane Boundary
No full textThe fate of B, Cl and Li in the subducted oceanic mantle and in the antigorite breakdown fluids
No full text- …
