Etudes sur les schistes cristallins...

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Volcanic resurfacing and the early terrestrial crust: Zircon U-Pb and REE constraints from the Isua Greenstone Belt, southern West Greenland

The oldest known bona fide succession of elastic metasediments Occurs in the Isua Greenstone Belt. SW Greenland and consists of a variety of mica schists and rare metaconglomerates. The metasediments are in direct contact with a felsic metavolcanic lithology that has previously been dated to 3.71 Ga. Based on trace element geochemical data for 30 metasediments, we selected the six samples with highest Zr concentrations for zircon extraction. These samples all yielded very few or no zircon, Those extracted from mica schists yielded ion probe U/Pb ages between 3.70 and 3,71 Ga. One metaconglomerate sample yielded just a single zircon of 3.74 Ga age. The mica schist hosted zircons have U/Pb ages. Th/U ratios, REE patterns and Eu anomalies indistinguishable from zircon in the adjacent 3.71 Ga felsic metavolcanic unit. Trace element modelling requires the bulk of material in the metasediments to be derived from variably weathered mafic lithologies but some metasediments contain substantial contribution from more evolved source lithologies. The paucity of zircon in the mica schists is thus explained by incorporation of material from largely zircon-free volcanic lithologies. The absence of older zircon in the mica schists and the preponderance of mafic source material imply intense, mainly basaltic resurfacing of the early Earth. The implications of this process are discussed, Thermal considerations suggest that horizontal growth of Hadean crust by addition of mafic ultramafic lavas must have triggered self-reorganisation of the protocrust by remelting. Reworking oft Hadean crust may have been aided by burial of hydrated (weathered) metabasalt due to semi-continuous addition of new voluminous basalt Outpouring,;, This process Causes a bias towards eruption of Zr-saturated partial melts at the surface with O-isotope corn posit ion,, potentially different from the mantle. The oldest zircons hosted in sediments would have been buried to substantial depth or formed in plutons that crystallised at some depth from which it took hundreds of millions of years for them to be exhumed and incorporated into much younger sediments. (C) 2005 Elsevier B.V.All rights reserved.

Chemical characterization of earth's most ancient clastic metasediments from the Isua Greenstone Belt, southern West Greenland

New and published major and trace element abundances of elastic metasediments (mainly garnet-biotite-plagioclase schists) from the similar to 3.8 Ga Isua Greenstone Belt (IGB), southern West Greenland, are used in an attempt to identify the compositional characteristics of the protoliths of these sediments. Compositionally, the metasediments are heterogeneous with enrichment of LREE (La/Sm-chord = 1.1-3.9) and variable enrichment and depletion of HREE (Gd/Yb-chord = 0.8-4.3). Chondrite-normalized Eu is also variable, spanning a range from relative Eu depletion to enrichment (Eu/Eu* = 0.6-1.3). A series of geochemical and geological criteria provides conclusive evidence for a sedimentary origin, in disagreement with some previous studies that questioned the presence of genuine elastic metasediments. In particular, trace element systematics of IGB metasediments show strong resemblance to other well-documented Archaean clastic sediments, and are consistent with a provenance consisting of ultramafic, malic and felsic igneous rocks. Two schists, identified as metasomatized mafic igneous rocks from petrographic and field evidence, show distinct compositional differences to the metasediments. Major element systematics document incipient-to-moderate source weathering in the majority of metasediments, while signs of secondary K-addition are rare. Detailed inspection of Eu/Eu*, Fe2O3 and CIW (chemical index of weathering) relationships reveals that elevated iron contents (when compared to averages for continental crust) and strong relative enrichment in Eu may be due to precipitation of marine Fe-oxyhydroxides during deposition or diagenesis on the seafloor. Some of the IGB metasediments have yielded anomalous Nd-142 and W-182 isotopic compositions that were respectively interpreted in terms of early mantle differentiation processes and the presence of a meteorite component. Alternatively, W and possibly Nd isotopes could have been affected by thermal neutron capture on the Hadean surface. The latter process was tested in this study by analysis of Sm isotope compositions, which serve as an effective monitor for neutron capture effects. As no anomalous variation from terrestrial values was detected, we infer that isotope systematics (including W-182 and Nd-142) of IGB metasediments were not affected by neutron capture, but reflect decay of radioactive parent isotopes. Copyright (c) 2005 Elsevier Ltd.

Chemical and isotopic composition of carbonate veins from ODP Leg 209

Carbonate veins hosted in ultramafic basement drilled at two sites in the Mid Atlantic Ridge 15°N area record two different stages of fluid-basement interaction. A first generation of carbonate veins consists of calcite and dolomite that formed syn- to postkinematically in tremolite-chlorite schists and serpentine schists that represent gently dipping large-offset faults. These veins formed at temperatures between 90 and 170 °C (oxygen isotope thermometry) and from fluids that show intense exchange of Sr and Li with the basement (87Sr/86Sr = 0.70387 to 0.70641, d7Li L-SVEC = + 3.3 to + 8.6 per mil). Carbon isotopic compositions range to high d13C PDB values (+ 8.7 per mil), indicating that methanogenesis took place at depth. The Sr-Li-C isotopic composition suggests temperatures of fluid-rock interaction that are much higher (T > 350-400 °C) than the temperatures of vein mineral precipitation inferred from oxygen isotopes. A possible explanation for this discrepancy is that fluids cooled conductively during upflow within the presumed detachment fault. Aragonite veins were formed during the last 130 kyrs at low-temperatures within the uplifted serpentinized peridotites. Chemical and isotopic data suggest that the aragonites precipitated from cold seawater, which underwent overall little exchange with the basement. Oxygen isotope compositions indicate an increase in formation temperature of the veins by 8-12 °C within the uppermost ~ 80 m of the subseafloor. This increase corresponds to a high regional geothermal gradient of 100-150 °C/km, characteristic of young lithosphere undergoing rapid uplift.

Rare earth element concentrations and geochemistry of carbonate minerals from the Logatchev Hydrothermal Field and the Gakkel Ridge

The ultramafic-hosted Logatchev Hydrothermal Field (LHF) at 15°N on the Mid-Atlantic Ridge and the Arctic Gakkel Ridge (GR) feature carbonate precipitates (aragonite, calcite, and dolomite) in voids and fractures within different types of host rocks. We present chemical and Sr isotopic compositions of these different carbonates to examine the conditions that led to their formation. Our data reveal that different processes have led to the precipitation of carbonates in the various settings. Seawater-like 87Sr/86Sr ratios for aragonite in serpentinites (0.70909 to 0.70917) from the LHF are similar to those of aragonite from the GR (0.70912 to 0.70917) and indicate aragonite precipitation from seawater at ambient conditions at both sites. Aragonite veins in sulfide breccias from LHF also have seawater-like Sr isotope compositions (0.70909 to 0.70915), however, their rare earth element (REE) patterns show a clear positive europium (Eu) anomaly indicative of a small (< 1%) hydrothermal contribution. In contrast to aragonite, dolomite from the LHF has precipitated at much higher temperatures (~100 °C), and yet its 87Sr/86Sr ratios (0.70896 to 0.70907) are only slightly lower than those of aragonite. Even higher temperatures are calculated for the precipitation of deformed calcite veins in serpentine-talc fault schists form north of the LHF. These calcites show unradiogenic 87Sr/86Sr ratios (0.70460 to 0.70499) indicative of precipitation from evolved hydrothermal fluids. A simple mixing model based on Sr mass balance and enthalpy conservation indicates strongly variable conditions of fluid mixing and heat transfers involved in carbonate formation. Dolomite precipitated from a mixture of 97% seawater and 3% hydrothermal fluid that should have had a temperature of approximately 14 °C assuming that no heat was transferred. The much higher apparent precipitation temperatures based on oxygen isotopes (~ 100 °C) may be indicative of conductive heating, probably of seawater prior to mixing. The hydrothermal calcite in the fault schist has precipitated from a mixture of 67% hydrothermal fluid and 33% seawater, which should have had an isenthalpic mixing temperature of ~ 250 °C. The significantly lower temperatures calculated from oxygen isotopes are likely due to conductive cooling of hydrothermal fluid discharging along faults. Rare earth element patterns corroborate the results of the mixing model, since the hydrothermal calcite, which formed from waters with the greatest hydrothermal contribution, has REE patterns that closely resemble those of vent fluids from the LHF. Our results demonstrate, for the first time, that (1) precipitation from pure seawater, (2) conductive heating of seawater, and (3) conductive cooling of hydrothermal fluids in the sub-seafloor all can lead to carbonate precipitation within a single ultramafic-hosted hydrothermal system.

Menires do Alto Algarve oriental : Lavajo I e Lavajo II (Alcoutim)

Neste trabalho apresenta-se o resultado das escavações realizadas respectivamente em 1998 e em 2001 nos núcleos de menires de Lavajo I e de Lavajo II, distanciados cerca de 250 m na direcção NNE e separados pelo pequeno vale do Lavajo. Os locais, actualmente, são intervisíveis, graças à implantação destacada no terreno: Lavajo I situa-se no topo de colina enquanto Lavajo II ocupa a linha de festo de uma encosta, conferindo ao local visibilidade tanto do lado sul como do lado norte. O conjunto de Lavajo I é constituído actualmente por três monólitos, todos de grauvaque: um, quase inteiro, de tendência fálica, é actualmente o maior menir de grauvaque conhecido em território português, atingindo o comprimento máximo de 3,14 m; outro, quase completo, fragmentado em três grandes blocos, possui formato estelar; o restante apresenta-se muito incompleto, dele se conservando apenas uma lasca da sua face frontal. É crível, no entanto, que pudessem existir mais monólitos, tendo em conta os abundantes fragmentos de grauvaque ali observados, quase todos com fracturas frescas. Todos os menires de Lavajo I se apresentam decorados, com destaque para o maior deles, o qual exibe complexa decoração estreitamente relacionada com a morfologia do suporte lítico. Apenas para este foi possível determinar o local de implantação, correspondente a um alvéolo de planta circular e fundo aplanado, parcialmente danificado pelos trabalhos realizados em 1994, que conduziram ao seu reerguimento, infelizmente feito de forma pouco cuidada e incorrecta, visto ter sido colocado no terreno em posição invertida. Seja como for, na zona culminante daquele pequeno cabeço, implantaram-se três menires decorados, os quais não podem ser vistos isoladamente, já que se articulariam directamente com o conjunto de Lavajo II, que se avista ao longe, do outro lado do pequeno vale do Lavajo e na linha de festo da encosta, da qual ocupa a parte média. Neste segundo local, identificaram-se quatro estelas-menir não decoradas, todas de grauvaque, das quais apenas uma, representada por fragmento de pequenas dimensões, se encontrava in situ. Foi, no entanto, possível reconstituir a posição relativa das restantes, através da escavação integral do respectivo alvéolo, correspondente a rasgo alongado, orientado Este-Oeste, aberto no substrato geológico, constituído por xistos do Carbónico Superior finamente folheados. Deste modo, é de concluir que as estelas menir se dispunham em linha, constituindo um painel lítico contínuo. No interior do alvéolo, recolheram-se diversos artefactos ali ritualmente depositados aquando da fundação do monumento, cuja tipologia indica o Neolítico Final, cronologia aliás compatível com a do conjunto megalítico de Lavajo I, tendo presente a iconografia patente nos menires. Muito embora não se conheça ainda suficientemente o padrão de povoamento da região no Neolítico Final, estes dois núcleos megalíticos podem ser interpretados como marcadores de territórios e/ou de espaços sagrados, sendo de destacar a existência, durante todo o ano, de água nas proximidades imediatas, recurso escasso e precioso, que propiciaria a horticultura. Por outro lado, a natureza das matérias-primas utilizadas na confecção dos artefactos encontrados (sílex, anfibolito), para além de outros materiais de circulação transregional muito mais alargada (fibrolite), evidencia a forte interacção destas populações tanto com o interior do Baixo Alentejo (Zona de Ossa/Morena), como com o litoral algarvio ou andaluz, compatível com estádio de desenvolvimento económico do final do Neolítico do sul peninsular. Numa vasta região, correspondente a todo o sotavento algarvio, onde o megalitismo não funerário era até agora totalmente desconhecido, os testemunhos ora estudados constituem, doravante, uma das expressões mais interessantes e significativas do Sudoeste peninsular.