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Partial Melting and Counterclockwise P T Path of Subducted Oceanic Crust (Sierra del Convento Melange, Cuba) 总被引：1，自引：0，他引：1
Garcia-Casco Antonio; Lazaro Concepcion; Rojas-Agramonte Yamirka; Kroner Alfred; Torres-Roldan Rafael Luis; Nunez Kenya; Neubauer Franz; Millan Guillermo; Blanco-Quintero Idael 《Journal of Petrology》2008,49(1):129-161
Partial melting of subducted oceanic crust has been identifiedin the Sierra del Convento mélange (Cuba). This serpentinite-matrixmélange contains blocks of mid-ocean ridge basalt (MORB)-derivedplagioclase-lacking epidote ± garnet amphibolite intimatelyassociated with peraluminous trondhjemitic–tonalitic rocks.Field relations, major element bulk-rock compositions, mineralassemblages, peak metamorphic conditions (c. 750°C, 14–16kbar), experimental evidence, and theoretical phase relationssupport formation of the trondhjemitic–tonalitic rocksby wet melting of subducted amphibolites. Phase relations andmass-balance calculations indicate eutectic- and peritectic-likemelting reactions characterized by large stoichiometric coefficientsof reactant plagioclase and suggest that this phase was completelyconsumed upon melting. The magmatic assemblages of the trondhjemitic–tonaliticmelts, consisting of plagioclase, quartz, epidote, ±paragonite, ± pargasite, and ± kyanite, crystallizedat depth (14–15 kbar). The peraluminous composition ofthe melts is consistent with experimental evidence, explainsthe presence of magmatic paragonite and (relict) kyanite, andplaces important constraints on the interpretation of slab-derivedmagmatic rocks. Calculated P–T conditions indicate counterclockwiseP–T paths during exhumation, when retrograde blueschist-faciesoverprints, composed of combinations of omphacite, glaucophane,actinolite, tremolite, paragonite, lawsonite, albite, (clino)zoisite,chlorite, pumpellyite and phengite, were formed in the amphibolitesand trondhjemites. Partial melting of subducted oceanic crustin eastern Cuba is unique in the Caribbean realm and has importantconsequences for the plate-tectonic interpretation of the region,as it supports a scenario of onset of subduction of a youngoceanic lithosphere during the early Cretaceous (c. 120 Ma).The counterclockwise P–T paths were caused by ensuingexhumation during continued subduction. KEY WORDS: amphibolite; Cuba; exhumation; partial melting; trondhjemite; subduction 相似文献
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The aim of this study is to investigate the nature of the solutions and the temperature during the bentonite formation by using δ18O and δ2H values and the relationship between chemical and isotopic composition of smectites from Cabo de Gata region. Therefore, 36 bentonite samples had been selected. For analyses the <2 μm fraction were separated. Most of the samples contain smectite above 90 wt%. The stable oxygen and hydrogen isotopic composition of the analysed smectites is close to the “montmorillonite line”. However, from isotopic composition two types of bentonite can be distinguished. The Sierra de Gata (SG) and Serrata de Nijar (SN) types show δ18O and δ2H values above and below this line, respectively. Temperatures obtained from geothermometer equation are similar to the temperatures values obtained from the equation of Savin and Lee (1988). Estimated formation temperatures of SG type are 25 and 49 °C higher compared to those of the SN type (6 and 25 °C).Correlations between structural cations versus isotopic composition show no statistical significance concerning all samples. However, separating the samples of SG (Sierra de Gata) from SN (Serrata of Nijar) a statistical significance for the individual areas is obtained. In both cases the effect is similar for 18O and for 2H. This could be due to the different composition of the solutions forming these bentonites. 相似文献
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Concepcion MB 《Singapore journal of tropical geography》1982,3(2):109-118
This study is concerned with the dynamics of labor force composition in selected countries bordering the Pacific. The estimates presented are based on national definitions of urban and rural places and of the labor force and are derived primarily from published U.N. data. Variations in labor force composition and in urbanization levels by country are discussed. Countries considered include Canada, Costa Rica, Ecuador, Japan, the Republic of Korea, Nicaragua, Peru, the Philippines, and the United States. 相似文献
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The aim of this study is to determine the isotopic composition (δ18O and δ2H) of interstitial water in bentonites. The study had been carried out from adsorbed vapour in homoionic bentonites. We have worked with the fraction <20 μm of a bentonite from Serrata de Níjar (Almería, Spain). Homoionic sodium and calcium bentonites were prepared as well as variable quantities of exchangeable Na/Ca (75Ca/25Na; 50Ca/50Na; 25Ca/75Na), by mixing pure sodium and calcium suspensions in the appropriate quantities. To carry out the hydration of the samples, every one of them was previously dried at 300 °C overnight, they were subjected to controlled saturation conditions in an atmosphere of different relative humidity and at a constant temperature of 20 °C, until equilibrium was achieved. The different vapour pressures were: P/Po = 0.05; P/Po = 0.078; P/Po = 0.15; P/Po = 0.32; P/Po = 0.45; P/Po = 0.63; P/Po = 0.8; P/Po = 1, obtained from the saturated solutions of different salts or sulphuric acid.The saturation water was extracted for isotopic analysis from an aliquot of a saturated sample. The technique used was vacuum extraction based on a modification of the one described by Araguás-Araguás et al. (1995).The differentiation between the isotopes of 18O and 2H from interstitial water in the bentonite samples, depending on the different relative humidity, may affect the ions diffusion in the interstitial solutions, since in those from low relative humidities, there could be a higher diffusion velocity as they are formed by molecules of light isotopes, versus interstitial solutions from higher relative humidities or those near saturation. This fact should be taken into account in studies on cation and solute transport from these solutions inside bentonite, even though it is important to continue studying and corroborating it with a larger number of solutions isotopically marked. 相似文献
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Bacterially mediated mineralization of vaterite 总被引：1，自引：0，他引：1
Carlos Rodriguez-Navarro Concepcion Jimenez-Lopez Maria Teresa Gonzalez-Muñoz 《Geochimica et cosmochimica acta》2007,71(5):1197-1213
Myxococcus xanthus, a common soil bacterium, plays an active role in the formation of spheroidal vaterite. Bacterial production of CO2 and NH3 and the transformation of the NH3 to and OH−, thus increasing solution pH and carbonate alkalinity, set the physicochemical conditions (high supersaturation) leading to vaterite precipitation in the microenvironment around cells, and directly onto the surface of bacterial cells. In the latter case, fossilization of bacteria occurs. Vaterite crystals formed by aggregation of oriented nanocrystals with c-axis normal to the bacterial cell-wall, or to the core of the spherulite when bacteria were not encapsulated. While preferred orientation of vaterite c-axis appears to be determined by electrostatic affinity (ionotropic effect) between vaterite crystal (0001) planes and the negatively charged functional groups of organic molecules on the bacterium cell-wall or on extracellular polymeric substances (EPS), analysis of the changes in the culture medium chemistry as well as high resolution transmission electron microscopy (HRTEM) observations point to polymorph selection by physicochemical (kinetic) factors (high supersaturation) and stabilization by organics, both connected with bacterial activity. The latter is in agreement with inorganic precipitation of vaterite induced by NH3 and CO2 addition in the protein-rich sterile culture medium. Our results as well as recent studies on vaterite precipitation in the presence of different types of bacteria suggest that bacterially mediated vaterite precipitation is not strain-specific, and could be more common than previously thought. 相似文献