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Fluid-inclusion and stable-isotope evidence for fluid infiltration and veining during metamorphism in marbles and metapelites
No full textSyn-metamorphic quartz + carbonate veins within lower-amphibolite-facies marbles and greenschist-facies metapelites of the O'Kane Canyon (northern Victoria Land, Antarctica) are characterised by fluids of different composition: H2O-CO2-CH4 mixtures (34-44 H2O, 50-61 CO2, 3-6 CH4, in mole %) (Type A), or H2O-CO2-N-2 fluids (35-43 H2O 48-54 CO2, 9-11 N-2, in mole %) (Type B). In metapelites, quartz + carbonate veins contain H2O-CO2-CH4 (Type C) fluids, which have a composition 30-68 H2O, 29-64 CO2, 2-6 CH4, in mole %) similar to Type A fluids in marbles. Isochores for both Type A and Type B fluids intersect the estimated peak conditions of 500 degrees C and 3-3.5 kbar, indicating that fluids with different compositions were trapped in the marbles during metamorphic veining. In greenschist-facies metapelites, isochores for Type C fluids cross the metamorphic P-T conditions at 2 kbar and 400 degrees C. Overall isochore distribution indicates early cooling (500-400 degrees C) without significant uplift. For the N-2-bearing Type B fluid inclusions, the delta(13)C value of the CO2 is 2.5 parts per thousand PDB and the delta D of H2O is 0 parts per thousand. The delta(18)O of host vein-quartz is 20.3 parts per thousand, and delta(13)C Of calcite within the host vein and in marble are both -1.5 parts per thousand. Tn contrast, the delta(13)C values of carbonate veins containing Type A H2O-CO2-CH4 fluids are lighter (-5.3 parts per thousand) than in the host marble (-2.9 to -1.5 parts per thousand). Type A fluid inclusions have light delta(13)C CO2 values (0.1 parts per thousand) and delta D H2O values (-84 parts per thousand). Type C inclusions also have light delta(13)C CO2 values (-3.1 parts per thousand), and variable delta D H2O (-30 and 98 parts per thousand). Fluid-inclusion and stable-isotope studies indicate that the analysed quartz-calcite veins formed by local processes in a similar structural context. The veins preserved distinct fluid-inclusion and isotopic compositions, indicating diverse sources of fluids: internally derived metamorphic H2O-CO2-N-2 mixtures (Type B), and externally derived H2O-CO2 fluids (Types A and C), interpreted in terms of mixing with a magmatic component, probably derived from local granitic rocks
Sr-Nd-Pb-O isotopic evidence for decreasing crustal contamination with ongoing magma evolution at Alicudi volcano (Aeolian arc, Italy): implications for style of magma-crust interaction and for mantle source compositions
No full textNew isotopic (Sr-Nd-Pb-O) data are reported for a suite of basalts (MgO up to 10 wt.%) to andesites and associated mineral separates from the Island of Alicudi, Aeolian arc, Southern Tyrrhenian Sea, with the aim of investigating the effects of magma-wall rock interaction in continental magmatic systems. Major and trace elements at Alicudi exhibit smooth variations, suggesting derivation of the entire suite from a single type of parental magma that underwent fractional crystallisation. However, (87)Sr/(86)Sr and delta(18)O values decrease, while (143)Nd/(144)Nd increases with increasing SiO(2) and decreasing MgO. Moreover, phenocryst phases show isotopic disequilibrium relative to whole rocks and groundmass. Such an unusual isotope vs. major and trace element variation and isotopic disequilibria suggest interaction between magmas and crustal wall rocks, but the decrease of (87)Sr/(86)Sr with increasing differentiation excludes assimilation-fractional crystallisation processes. We suggest that mafic calcalkaline magmas with primitive Sr-Nd-O isotopic signatures were intruded into a deep reservoir, where they underwent fractional crystallisation. Contamination occurred shortly before eruption, as the magmas ascended to the surface. Interactions between magmas and wall rocks were more extensive for the basalts than basaltic andesites and andesites, generating a positive correlation between MgO and (87)Sr/(86)Sr ratios. The variable degrees of magma-wall rock interaction are suggested to depend on the differing capabilities of magmas to dissolve crustal rocks due to variable temperature and viscosity. The present study, therefore, provides Sr-Nd-Pb-O isotopic evidence on an uncommon modality of magma contamination by upper crust. The Alicudi andesites have the most primitive isotope signatures over the entire Aeolian arc. In Plots of (206)Pb/(204)Pb vs. (87)Sr/(86)Sr or (143)Nd/(144)Nd isotopes, the bulk of the Aeolian volcanoes fall along a curved trend that connects HIMU mantlereservoir and the upper crust. In contrast, the Alicudi andesites fall outside this trend and plot on a binary trend connecting HIMU and EM1 mantle reservoirs. It is suggested that the Alicudi source consisted of a hybrid HIMU-EM1 mantle that was modified by fluids or melts coming from an oceanic stab. These fluids did not change significantly the isotopic signatures of the wedge but generated high LILE/HFSE ratios, which were inherited by the Alicudi magma. In contrast, the compositions of other Aeolian volcanoes and of central Italian magmatism reflect addition of upper crust to a HIMU or FOZO mantle. Collectively, the data record the complex interplay between different mantle reservoirs and the addition of various amounts and types of subduction-related components in the southern Tyrrhenian Sea. (C) 2004 Elsevier B.V. All rights reserved
Fluid-rock interaction in UHP phengite-kyanite-epidote eclogite from the Sulu orogen, eastern China
No full textCombined petrographic, minerochemical, fluid inclusion, and stable isotope studies have been carried out on phengite-kyanite-epidote (Phe-Ky-Ep) eclogites from Hushan and Qinglongshan (Donghai area, UHP Sulu orogen, China) to unravel their metamorphic evolution and fluid-rock interaction. A complex metamorphic evolution, from coesite-eclogite (P = 3.5-4.0 GPa and T = 840 +/- 50 degrees C) to greenschist facies conditions (P congruent to 0.2 GPa and T congruent to 350 degrees C), through quartz-eclogite and HT amphibolite-facies conditions, was recognized. The associated fluids are aqueous and show a progressive change in salinity and composition, reflecting different metamorphic stages. Stable isotope data confirm that, prior to subduction, the Sulu eclogite protolith experienced a meteoric-hydrothermal alteration, and indicate that during metamorphic evolution the rocks recrystallized without pervasive fluid infiltration
Multiphase solid inclusions in UHP rocks (Su-Lu, China): Remnants of supercritical silicate-rich aqueous fluids released during continental subduction
No full textPrimary multiphase solid (MS) inclusions without preserved fluid are found within peak minerals in kyanite quartzite topaz and kyanite-phengite-epidote eclogite from Donghai area (Su-Lu terrane). Typical mineral association in inclusions is: paragonite + muscovite + anhydrite +/- corundum +/- "alunite-type" sulphate +/- zircon +/- calcite +/- chlorite +/- SiO2 +/- barite +/- pyrite +/- apatite in quartzites, and paragonite + rutile + apatite +/- amphibole +/- Zn-staurolite +/- magnetite +/- plagioclase +/- zircon +/- pyrite +/- "alunite-type" sulphate +/- Zn-Mg-Fe-Al-Ti spinel in eclogites. On the basis of the fluid inclusion textures and of the daughter-phase assemblage, calculated fluid composition is as follows: in quartzites 24 wt.% SiO2, 30 wt.% Al2O3, 9 Wt.% CaO, 5 wt.% K2O, 3 wt.% Na2O, 11 Wt-% SO3, 18 wt.% H2O, with traces of TiO2, Fe2O3, FeO, MgO, BaO, P2O5, Cl-, F, and (CO3)(2-), and in eclogites 26 wt.% SiO2, 21 wt.% TiO2, 20 wt.% Al2O3, 2 wt.% MgO, 4 wt.% FeO, 6 wt.% Fe2O3, 7 wt.% CaO, 3 wt.% NaO, 3 Wt-% P2O5, 7 wt.% H2O, 1 wt.% Cl, and traces of ZnO, MnO, K2O, SO3, and F. Originally trapped chloride-poor aqueous fluids contained very high amounts - in the order of tens of wt.% - of Si4+, Al3+, and Ti4+. Solute species reflect the chemical composition of the host rocks: Mg2+, Fe2+, Ti4+, P5+, and Na+, are abundant in the fluids present in eclogites, while fluids are enriched in Al3+, K+, Na+, (SO4)(2-),(CO3)(2-) in quartzites. We propose that multiphase solid inclusions represent remnants of high-density supercritical silicate-rich aqueous fluids that were in equilibrium with peak minerals at UHP conditions. These fluids show characters which are transitional between aqueous fluids and silicate melts, and were probably produced by dehydration reactions of the host rocks during the latest stages of subduction. (c) 2005 Published by Elsevier B.V
GEOGRAPHIC VARIATIONS IN PETROGRAPHY AND COMPOSITION IN AUSTRALASIAN MICROTEKTITES: IMPLICATIONS FOR THEIR FORMATION AND PARENT CRATER LOCATION
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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
Coexisting calc-alkaline and ultrapotassic magmatism at Monte Ernici, Mid Latina Valley (Latium, central Italy)
No full textNew major and trace element data, and Sr-Nd-Pb-O isotopic ratios for volcanic mafic rocks outcropping at Monti Ernici in the Mid Latina Valley (southern Latium) are reported, with the aim of investigating the nature and evolution of Plio-Quaternary K-rich volcanism in Central Italy. Petrographical and geochemical studies allow us to identify mafic rocks ranging from ultrapotassic (HKS) to shoshonitic (SHO), and calc-alkaline (CA), these last ones being identified for the first time. The CA rocks exhibit the most primitive signatures for Sr, Nd, and Pb isotopes (87Sr/ 86Sr = 0.706326-0.706654; 143Nd/I44Nd = 0.512388-0.512361; 206Pb/204Pb = 18.944-18.940). The δ180 values are variable (δ18Ocpx from +5.75 to +7.08 ‰ and , δ180ol, from +5.50 to +6.23‰), suggesting interaction with carbonate wall rocks. Radiogenic isotope ratios and incompatible elements distribution have several characteristic in common with equivalent rocks from Pontine Islands (Ventotene), Campania and Aeolian arc volcanoes. Conversely, the HKS rocks closely resemble the ultrapotassic rocks from the Roman Province (87Sr/86Sr = 0.709679-0.711102; δ 18Ocpx from +6.27 to +7.08 ‰). The high ratios of LILE (Large Ion Lithophile Elements: Rb, Cs, Th, U, K, LREE) and HFSE (High Field Strength Elements: Ta, Nb, Zr, Hf, Ti), and radiogenic isotope compositions of CA to HKS rocks indicate that all suites contain subduction-related components, and suggest a N-MORB-type mantle source variably contaminated by hydrous fluids and/or melts released by undergoing slabs, possibly during two distinct stages of metasomatism. The coexistence of ultra-alkaline and sub-alkaline orogenic magmatism, combined with tectonic, geophysical and geological evidence, support the possibility that the mantle beneath central-southern Italy (Ernici-Roccamonfina Province) was vertically zoned and produced different magma suites during time. © 2007 E. Schweizerbart'sche Verlagsbuchhandlung
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