977 resultados para 986.103


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Fil: Saguí, Teresita.

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Fil: Pró, Diego F.. Universidad Nacional de Cuyo. Facultad de Filosofía y Letras. Instituto de Filosofía Argentina y Americana

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Interstitial waters were squeezed from strata recovered at Sites 637-641 of ODP Leg 103 on the Galicia margin, along the northwestern Iberian continental margin in the northeast Atlantic. Chemical profiles of Site 638 show the most complexity, which appears to be related to an unconformity in the strata between Cretaceous and Neogene sediments and to rapid deposition of Cretaceous syn-rift sediments upon pre-rift strata. Analyses of waters from all of the Leg 103 sites show generally antithetical trends for calcium and magnesium; calcium increases with depth as magnesium decreases. No calcium-magnesium 'crossover' profiles are observed in these data. Data from Site 637 show an unusual pattern; calcium increases with increasing depth, but magnesium remains relatively constant. Sulfate is either stable or shows an overall decrease with depth, and boron profiles show some structure. At all but one site (Site 638), strontium profiles do not show marked depth structure. The structure of alkalinity and silica profiles is highly site dependent. Bromide profiles are, in general, constant. In nearly every case, observed bromide concentrations are near average seawater values. Relatively low concentrations of iron and manganese are common within the upper 10 m of the sediment sequence and typically are near detection limits at deeper depths

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Mineral compositions of the plagioclase-bearing ultramafic tectonites dredged and cored seaward of the continental slope of the Galicia margin (Leg 103, Site 637) were compared to mineral compositions from onshore low-pressure ultramafic bodies (southeastern Ronda, western Pyrenees, and Lizard Point), on the basis of standardized (30-s counting time) probe analyses. The comparison was extended to some plagioclase-free harzburgites related to ophiolites (Santa Elena in Costa Rica, north Oman, and the Humboldt body in New Caledonia) on the basis of new analytical data and data from the literature. The behavior of Cr, Na, Al, Mg, Fe, Ni, and Ti in olivine, pyroxenes, and spinel was examined in order to distinguish between the effects of partial melting and mineral facies change, from the spinel to plagioclase stability fields. The peridotite from the Galicia margin appears slightly depleted in major incompatible elements and experienced a minor partial melting. However, it experienced large scale but heterogeneous recrystallization in the plagioclase field. These features are very similar to those observed in Ronda, whereas in the western Pyrenees the minerals exemplify a very minor partial-melting event (or none at all) and have retained compositions corresponding to those of the relatively high-pressure Seiland sub facies. The minerals from the Lizard Point peridotite have characteristics (low Mg/(Mg + Fe) ratio; high Cr/(Cr + Al) ratio in spinel) more related to cumulate from a differentiated tholeiitic melt than related to ophiolitic tectonite. Diffusion profiles of Al and Cr across pyroxenes and spinel show that recrystallization features occurred at different speeds or temperatures in the different bodies. The pyroxenes from Ronda would have experienced recrystallization about 14 times faster than the peridotite from the Galicia margin. The western Pyrenean lherzolites also experienced rapid recrystallization; nevertheless, because they are of a different mineral facies, the data are not directly comparable to that from Ronda and Galicia. The harzburgite at Santa Elena as well as a xenolith from alkali basalt exemplify rapid cooling characterized by very weak re-equilibration. Recrystallization speed is related to emplacement speed in the present geological environment. The slow-rising Galicia margin peridotite was emplaced by thinning of the lithospheric subcontinental mantle near an incipient mid-oceanic ridge. The fast-rising peridotites from Ronda and the western Pyrenees were hot diapirs emplaced from the asthenosphere along transcurrent faults, possibly related to the opening of the Atlantic Ocean.