Mineralogy, geochemistry, and Nd isotope composition of the Rainbow hydrothermal field, Mid-Atlantic Ridge

Petrological, geochemical, and Nd isotopic analyses have been carried out on rock samples from the Rainbow vent field to assess the evolution of the hydrothermal system. The Rainbow vent field is an ultramafic-hosted hydrothermal system located on the Mid-Atlantic Ridge characterized by vigorous high-temperature venting (∼365°C) and unique chemical composition of fluids: high chlorinity, low pH and very high Fe, and rare earth element (REE) contents (Douville et al., Chemical Geology 184:37–48, 2002). Serpentinization has occurred under a low-temperature (<270°C) retrograde regime, later overprinted by a higher temperature sulfide mineralization event. Retrograde serpentinization reactions alone cannot reproduce the reported heat and specific chemical features of Rainbow hydrothermal fluids. The following units were identified within the deposit: (1) nonmineralized serpentinite, (2) mineralized serpentinite—stockwork, (3) steatite, (4) semimassive sulfides, and (5) massive sulfides, which include Cu-rich massive sulfides (up to 28wt% Cu) and Zn-rich massive sulfide chimneys (up to 5wt% Zn). Sulfide mineralization has produced significant changes in the sulfide-bearing rocks including enrichment in transition metals (Cu, Zn, Fe, and Co) and light REE, increase in the Co/Ni ratios comparable to those of mafic Cu-rich volcanic-hosted massive sulfide deposits and different ¹⁴³Nd/¹⁴⁴Nd isotope ratios. Vent fluid chemistry data are indicative of acidic, reducing, and high temperature conditions at the subseafloor reaction zone where fluids undergo phase separation most likely under subcritical conditions (boiling). An explanation for the high chlorinity is not straightforward unless mixing with high salinity brine or direct contribution from a magmatic Cl-rich aqueous fluid is considered. This study adds new data, which, combined with the current knowledge of the Rainbow vent field, brings compelling evidence for the presence, at depth, of a magmatic body, most likely gabbroic, which provides heat and metals to the system. Co/Ni ratios proved to be good tools used to discriminate between rock units, degree of sulfide mineralization, and positioning within the hydrothermal system. Deeper units have Co/Ni <1 and subsurface and surface units have Co/Ni >1.     

Multi-phase water-rhyolite interaction and ore fluid generation at Aljustrel, Portugal

Detailed study of the Aljustrel volcanic rocks, through petrography, mineral chemistry, whole-rock geochemistry, and previously published stable isotope data, show that the rocks experienced extreme metasomatism. The metasomatic phenomena presented are explained accurately by a sea-water convective model. The evolution of the sea-water-derived fluid as it progresses downwards through the rocks is evident, from cold, oxidising, Mg-bearing, to hot, reducing, Mg-free, and enriched in Si, Fe, Cu, Zn. Several lines of evidence (feldspar mineralogy, paragenetic relations between regional and ore-zone alteration, and stable isotope data) show that the temperature of the hydrothermal circulation increased through time, and that the water-to-rock ratio evolved from very high to low values. According to our data, about 30 ppm Cu and some 100 ppm Zn were removed from the Aljustrel Volcanics. This implies, for both metals, the same amount of leached rock to produce the 2 Mt Cu and 6 Mt Zn contained in the massive sulphide deposits of the Aljustrel area. With regard to Fe, about 1% was on average removed from the rocks, producing enough metal to account for all the Fe in the MS deposits and overlying chemical sediments. This study documents semi-quantitatively the provenance of the main metals in major massive sulphide deposits via sea water leaching of the immediate host rocks.

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Resumen (translated by E. Pascual) Un estudio detallado de las rocas volcánicas de Aljustrel, mediante petrografia, química mineral, geoquímica de roca total y datos de isótopos estables previamente publicados muestra que estas rocas han sufrido metasomatismo en grado extremo. Los fenómenos metasomáticos presentados se explican con precisión mediante un modelo convectivo con agua de mar. Es evidente la evolución del fluido derivado del agua de mar conforme progresa en profundidad a través de las rocas, desde un fluido frío, oxidante y con magnesio a otro reductor, sin magnesio y enriquecido en Si, Fe, Cu y Zn. Varias líneas de evidencia (mineralogía de los feldespatos, relaciones paragenéticas entre la alteración regional y de las zonas mineralizadas, isótopos estables) muestran que la temperatura y la circulación hidrotermal se incrementaron con el tiempo y que la relación agua-roca evolucionó de valores muy altos a bajos. De acuerdo con nuestros datos, de las rocas volcánicas de Aljustrel se han extraído unas 30 ppm de Cu y unas 100 ppm de Zn. Esto implica, para ambos metales, que la misma roca lixiviada ha producido los 2 Mt de Cu y los 6 Mt de Zn contenidos en los depósitos de sulfuros masivos del área de Aljustrel. En cuanto al Fe, una media de alrededor de 1% fue extraída de las rocas, produciendo metal suficiente para explicar todo el Fe de los depósitos de sulfuros masivos y de los sedimentos químicos suprayacentes. Este estudio documenta de forma semicuantitativa la procedencia de los principales metales en depósitos mayores de sulfuros masivos, mediante lixiviado por agua de mar de sus rocas huéspedes inmediatas.

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Received: 1 February 1997 / Accepted: 30 July 1997     

Geochemistry of a sediment push-core from the Lucky Strike hydrothermal field,Mid-Atlantic Ridge

Hydrothermal sediment mineralogy and geochemistry can provide insights into seafloor mineralization processes and changes through time. We report a geochemical investigation of a short (22 cm) near-vent hydrothermal metalliferous sediment core from the Lucky Strike site (LS), on the Mid-Atlantic Ridge (MAR). The sediment was collected from the base of an active white smoker vent and comprises pure hydrothermal precipitates, mainly chalcopyrite, sphalerite, pyrite and barite, with negligible detrital and biogenic inputs. Geochemically, the core is enriched in elements derived from high-temperature hydrothermalism (Fe, Cu, Zn and Ba) and depleted in elements derived from low-temperature hydrothermalism (Mn), and metasomatism (Mg). The U/Fe content ratio is elevated, particularly in the deeper parts of the core, consistent with uptake from seawater associated with sulphide alteration. Rare earth elements (REE) concentrations are low and chondrite-normalized patterns are characteristic of high-temperature vent fluids with an enrichment in light REE and a pronounced positive Eu anomaly. A stronger positive Eu anomaly associated with higher La_n/Sm_n at the core top is controlled by barite precipitation. The hydrothermal influence on the REE decreases downcore with some evidence for a stronger seawater influence at depth. Nd isotopes also exhibit an increased detrital/seawater influence downcore. Pb isotope ratios are uniform and plot on the Northern Hemisphere Reference Line in a small domain defined by LS basalts and exhibit no detrital or seawater influence. Lucky Strike sediments are derived from high-temperature mineralization and are overprinted by a weak seawater–sediment interaction when compared with other Atlantic hydrothermal sites such as TAG. The larger seawater input and/or a larger detrital contribution in deeper layers can be explained by variable hydrothermal activity during sediment formation, suggesting different pulses in activity of the LS hydrothermal system.     

GPS-derived Deformation of the Central Andes Including the 1995 Antofagasta Mw = 8.0 Earthquake

—In order to study both the interplate seismic loading cycle and the distribution of intraplate deformation of the Andes, a 215 site GPS network covering Chile and the western part of Argentina was selected, monumented and observed in 1993 and 1994. A dense part of the network in northern Chile and northwest Argentina, comprising some 70 sites, was re-observed after two years in October/November, 1995. The M _w = 8.0 Antofagasta (North Chile) earthquake of 30th July, 1995 took place between the two observations. The city of Antofagasta shifted 80 cm westwards by this event and the displacement still reached 10 cm at locations 300 km from the trench. Three different deformation processes have been considered for modeling the measured displacements (1) interseismic accumulation of elastic strain due to subduction coupling, (2) coseismic strain release during the Antofagasta earthquake and (3) crustal shortening in the Sub-Andes.¶Eastward displacement of the sites to the north and to the south of the area affected by the earthquake is due to the interseismic accumulation of elastic deformation. Assuming a uniform slip model of interseismic coupling, the observed displacements at the coast require a fully locked subduction interface and a depth of seismic coupling of 50 km. The geodetically derived fault plane parameters of the Antofagasta earthquake are consistent with results derived from wave-form modeling of seismolog ical data. The coseismic slip predicted by the variable slip model reaches values of 3.2 m in the dip-slip and 1.4 m in the strike-slip directions. The derived rake is 66°. Our geodetic results suggest that the oblique Nazca–South American plate convergence is accommodated by oblique earthquake slip with no slip partitioning. The observed displacements in the back-arc indicate a present-day crustal shortening rate of 3–4 mm/year which is significantly slower than the average of 10 mm/year experienced during the evolution of the Andean plateau.     

The three-point function in large-scale structure – I. The weakly non-linear regime in N-body simulations

This paper presents a comparison of the predictions for the two- and three-point correlation functions of density fluctuations, ξ and ζ , in gravitational perturbation theory (PT) against large cold dark matter (CDM) simulations. This comparison is made possible for the first time on large weakly non-linear scales (>10  h ⁻¹ Mpc) thanks to the development of a new algorithm for estimating correlation functions for millions of points in only a few minutes. Previous studies in the literature comparing the PT predictions of the three-point statistics with simulations have focused mostly on Fourier space, angular space or smoothed fields. Results in configuration space, such as those presented here, were limited to small scales where leading-order PT gives a poor approximation. Here we also propose and apply a method for separating the first-order and subsequent contributions to PT by combining different output times from the evolved simulations. We find that in all cases there is a regime where simulations do reproduce the leading-order (tree-level) predictions of PT for the reduced three-point function   Q ₃∼ ζ / ξ ²  . For steeply decreasing correlations (such as the standard CDM model) deviations from the tree-level results are important even at relatively large scales, ≃20 Mpc  h ⁻¹. On larger scales ξ goes to zero and the results are dominated by sampling errors. In more realistic models (such as the ΛCDM cosmology) deviations from the leading-order PT become important at smaller scales   r ≃10 Mpc  h ^-1  , although this depends on the particular three-point configuration. We characterize the range of validity of this agreement and show the behaviour of the next-order (one-loop) corrections.     

Geochemistry and stable isotope constraints on high-temperature activity from sediment cores of the Saldanha hydrothermal field

The Saldanha hydrothermal field is hosted atop a mafic–ultramafic seamount, located at a non-transform offset on the Mid-Atlantic Ridge. Previous observations revealed a field where transparent low-temperature fluids discharge through centimeter-sized vents without the formation of chimney structures. We present geochemical and stable isotope (O and C) analyses from sediment samples collected at this field, both at and far from the vent area. Most sediments, including some directly adjacent to orifice vents, are pelagic oozes with only a weak hydrothermal overprinting. Hydrothermal precipitates are characterized by Fe–Mn oxyhydroxides and a minor amount of Cu–Zn sulphide minerals. However, one of the cores (SCD7) collected at the vent area shows a much stronger hydrothermal signature. This core is composed of a matrix of serpentine + talc ± chlorite with high porosity, where calcite + chalcopyrite + sphalerite/wurtzite ± pyrite–pyrrhotite were precipitated. In this core, metal enrichments, REE patterns, and the oxygen and carbon isotope composition of calcites indicate that mineralization must have occurred in the subsurface by high-temperature fluids, with minor mixing with seawater and with a significant magmatic contribution. Thus, while most samples confirm previous findings indicating that Saldanha hydrothermal fluid discharge is mainly diffuse and of low temperature, data from core SCD7 suggest that areas of high-temperature hydrothermal activity also occur, where temperatures of the fluids could reach > 260 °C and maximum temperatures of 330 °C. We suggest that fluids can flow through faults at the top of the mount and discharge in a more focused way through vent orifices, producing intense hydrothermal alteration of the sediments. At these locations complex hydrothermal processes occur, including reactions of the hydrothermal fluids with mafic and ultramafic rocks and magma degassing, as suggested by the carbon isotope composition of hydrothermal calcites. The high temperature of the fluid inferred from the geochemistry of the hydrothermal minerals requires a significant heat input to the system, suggesting an additional magmatic heat source to the already proposed exothermic serpentinization reactions.     

Processing and compression of noise-dominated data: application to the cosmic microwave background data on board the Planck satellite

We address the problem of encoding and compressing data dominated by noise. Information is decomposed into 'reference' sequences plus arrays containing noisy differences susceptible to being described by a known probability distribution. One can then give reliable estimates of the optimal compression rates by estimating the corresponding Shannon entropy. As a working example, this idea is applied to an idealized model of the cosmic microwave background (CMB) data on board the Planck satellite. Data reduction is a critical issue in space missions because the total information that can be downloaded to Earth is sometimes limited by telemetry allocation. Similar limitations might arise in remotely operated ground based telescopes. This download-rate limitation could reduce the amount of diagnostics sent on the stability of the instruments and, as a consequence, curb the final sensitivity of the scientific signal. Our proposal for Planck consists of taking differences of consecutive circles at a given sky pointing. To a good approximation, these differences could be made independent of the external signal, so that they are dominated by thermal (white) instrumental noise, which is simpler to model than the sky signal. Similar approaches can be found in other individual applications. Generic simulations and analytical predictions show that high compression rates,     can be obtained with minor or zero loss of sensitivity. Possible effects of digital distortion are also analysed. The proposed scheme is flexible and reliable enough to be optimized in relation to other critical aspects of the corresponding application. For Planck , this study constitutes an important step towards a more realistic modelling of the final sensitivity of the CMB temperature anisotropy maps.