156 research outputs found

    Mantle evolution in the Variscides of SW England: geochemical and isotopic constraints from mafic rocks

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    This is the author accepted manuscript. The final version is available from Elsevier via the DOI in this record.The geology of SW England has long been interpreted to reflect Variscan collisional processes associated with the closure of the Rhenohercynian Ocean and the formation of Pangea. The Cornish peninsula is composed largely of Early Devonian to Late Carboniferous volcanosedimentary successions that were deposited in pre- and syncollisional basins and were subsequently metamorphosed and deformed during the Variscan orogeny. Voluminous Early Permian granitic magmatism (Cornubian Batholith) is broadly coeval with the emplacement of ca. 280-295 Ma lamprophyric dykes and flows. Although these lamprophyres are well mapped and documented, the processes responsible for their genesis and their relationship with regional Variscan tectonic events are less understood. Pre- to syn-collisional basalts have intra-continental alkalic affinities, and have REE profiles consistent with derivation from the spinel-garnet lherzolite boundary. εNd values for the basalts range from +0.37 to +5.2 and TDM ages from 595 Ma to 705 Ma. The lamprophyres are extremely enriched in light rare earth elements, large iron lithophile, and depleted in heavy rare earth elements suggesting a deep, garnet lherzolite source that was previously metasomatised. They display εNd values ranging from -1.4 to +1.4, initial Sr values of ca. 0.706, and TDM ages from 671 Ma to 1031 Ma, suggesting that metasomatism occurred in the Neoproterozoic. Lamprophyres and coeval granite batholiths of similar chemistry to those in Cornwall occur in other regions of the Variscan orogen, including Iberia and Bohemia. By using new geochemical and isotopic data to constrain the evolution of the mantle beneath SW England and the processes associated with the formation of these post-collisional rocks, we may be able to gain a more complete understanding of mantle processes during the waning stages of supercontinent formation

    Geochemistry and Geochronology of the Guajira Eclogites, northern Colombia : evidence of a metamorphosed primitive Cretaceous Caribbean Island-arc

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    The chemical composition of eclogites, found as boulders in a Tertiary conglomerate from the Guajira Peninsula, Colombia suggests that these rocks are mainly metamorphosed basaltic andesites. They are depleted in LILE elements compared to MORB, have a negative Nb-anomaly and flat to enriched REE patterns, suggesting that their protoliths evolved in a subduction related tectonic setting. They show island-arc affinities and are similar to primitive islandarc rocks described in the Caribbean. The geochemical characteristics are comparable to low-grade greenschists from the nearby Etpana Terrane, which are interpreted as part of a Cretaceous intra-oceanic arc. These data support evidence that the eclogites and the Etpana terrane rocks formed from the same volcano-sedimentary sequence. Part of this sequence was accreted onto the margin and another was incorporated into the subduction channel and metamorphosed at eclogite facies conditions. 40Ar-39Ar ages of 79.2±1.1Ma and 82.2±2.5Ma determined on white micas, separated from two eclogite samples, are interpreted to be related to the cooling of the main metamorphic event. The formation of a common volcano-sedimentary protolith and subsequent metamorphism of these units record the ongoing Late Cretaceous continental subduction of the South American margin within the Caribbean intra-oceanic arc subduction zone. This gave way to an arc-continent collision between the Caribbean and the South American plates, where this sequence was exhumed after the Campanian

    The History, Relevance, and Applications of the Periodic System in Geochemistry

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    Geochemistry is a discipline in the earth sciences concerned with understanding the chemistry of the Earth and what that chemistry tells us about the processes that control the formation and evolution of Earth materials and the planet itself. The periodic table and the periodic system, as developed by Mendeleev and others in the nineteenth century, are as important in geochemistry as in other areas of chemistry. In fact, systemisation of the myriad of observations that geochemists make is perhaps even more important in this branch of chemistry, given the huge variability in the nature of Earth materials – from the Fe-rich core, through the silicate-dominated mantle and crust, to the volatile-rich ocean and atmosphere. This systemisation started in the eighteenth century, when geochemistry did not yet exist as a separate pursuit in itself. Mineralogy, one of the disciplines that eventually became geochemistry, was central to the discovery of the elements, and nineteenth-century mineralogists played a key role in this endeavour. Early “geochemists” continued this systemisation effort into the twentieth century, particularly highlighted in the career of V.M. Goldschmidt. The focus of the modern discipline of geochemistry has moved well beyond classification, in order to invert the information held in the properties of elements across the periodic table and their distribution across Earth and planetary materials, to learn about the physicochemical processes that shaped the Earth and other planets, on all scales. We illustrate this approach with key examples, those rooted in the patterns inherent in the periodic law as well as those that exploit concepts that only became familiar after Mendeleev, such as stable and radiogenic isotopes

    The History, Relevance, and Applications of the Periodic System in Geochemistry

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    Geochemistry is a discipline in the earth sciences concerned with understanding the chemistry of the Earth and what that chemistry tells us about the processes that control the formation and evolution of Earth materials and the planet itself. The periodic table and the periodic system, as developed by Mendeleev and others in the nineteenth century, are as important in geochemistry as in other areas of chemistry. In fact, systemisation of the myriad of observations that geochemists make is perhaps even more important in this branch of chemistry, given the huge variability in the nature of Earth materials – from the Fe-rich core, through the silicate-dominated mantle and crust, to the volatile-rich ocean and atmosphere. This systemisation started in the eighteenth century, when geochemistry did not yet exist as a separate pursuit in itself. Mineralogy, one of the disciplines that eventually became geochemistry, was central to the discovery of the elements, and nineteenth-century mineralogists played a key role in this endeavour. Early “geochemists” continued this systemisation effort into the twentieth century, particularly highlighted in the career of V.M. Goldschmidt. The focus of the modern discipline of geochemistry has moved well beyond classification, in order to invert the information held in the properties of elements across the periodic table and their distribution across Earth and planetary materials, to learn about the physicochemical processes that shaped the Earth and other planets, on all scales. We illustrate this approach with key examples, those rooted in the patterns inherent in the periodic law as well as those that exploit concepts that only became familiar after Mendeleev, such as stable and radiogenic isotopes

    Geochemistry of the Neoproterozoic Wadi Ghadir and Fawakhir ophiolites and associated rocks, Central Eastern Desert, Egypt: Implication for geodynamic setting

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    The geochemistry of the ophiolitic complexes and younger late orogenic sedimentary rocks in the Wadi Ghadir and Fawakhir areas has been studied to constrain the tectonic evolution of the Central Eastern Desert. The Wadi Ghadir ophiolite is composed of gabbros and pillow lavas that are intruded by many dike swarms and all of them display subduction zone chemical signatures. The ophiolitic rocks and the surrounding melange were intruded by younger dikes with Andean-type geochemical signature. The Wadi Ghadir ophiolite has chemical similarities to the oceanic crust formed in back-arc basins. The location of the Wadi Ghadir ophiolite to the northeast of the Nugrus volcanic arc and the appearance of forearc amphibolites to the southwest of the Nugrus arc rocks indicates a NE-dipping subduction zone. The accretion of this arcback-arc system onto the passive margin to the west resulted in the reversal in the subduction direction and the establishment of a new Andean-type continental margin. The Fawakhir ophiolite comprises serpentinized ultramafic rocks, gabbros, sheeted dike complexes, and pillow lavas. The geochemical characteristics of the Fawakhir ophiolite are comparable to the forearc oceanic crust developed in the Izu-Bonin-Mariana arc system. However, pillow lavas preserved in the melange to the west have a geochemical signature similar to that of a modern back-arc oceanic crust which suggests formation of an intra-oceanic arc system above an east-dipping subduction zone. As in Wadi Ghadir, the Fawakhir ophiolite is intruded by later calc-alkaline dikes possessing geochemical signatures similar to rocks that occur at modern active continental margins. The initiation of the Andean-type calc-alkaline magmatism may indicate a reversal in subduction polarity from east-dipping during arc accretion to west-dipping and formation of an active continental margin. The Umm Hassa Greywacke member is the top unit of the Hammamat Group which is late orogenic molasse-type sedimentary rocks. Continental arc volcanic rocks and oceanic island arc-ophiolitic sources are the main contributors to the sediments of the Urn Hassa greywackes. Considering the geodynamic evolution of the Central Eastern Desert, the Um Hassa greywackes were deposited in retroarc foreland basin behind the continental arcs developed over a west-dipping subduction zone

    Chondrule formation in the early Solar System:a combined ICP-MS, ICP-OES and petrological study

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    Diese Arbeit untersucht die Formation und Entwicklung von Chondrulen und die nötigen analytischen Werkzeuge und Methoden für diese Art von Untersuchung. Eine Kombination aus Frost-Tauwechsel, „Table-Top“ Aufschlüssen individueller Chondrulen und Analyse mittels ICP-MS und -OES von kleinen Proben (Durchschnitt ~1 mg) ist entwickelt und getestet worden. Die chemische Identifikation von ganzen Chondrulen wird untersucht und die beobachteten Gruppierungen mit klassischen Einzelmineralphasen Klassifikationsschemen verglichen. Die Evolution der Chondrulenarten eines Chondrites, ihre Beziehung zu dessen Zusammensetzung und zu ähnlichen Chondrulen in anderen Chondritarten werden diskutiert. Die Datei werden mit bekannten Daten für das Sonnensystem, CI- und OC-Meteoriten und CAI (Calcium-Aluminium-rich Inclusions) verglichen. Die wahrscheinliche Herkunft von Ausgangsmaterialien und Formationsprozessen werden unter die Lupe genommen. Zum Schluss wird eine Zeitspanne und Formationsumgebung für die untersuchten Objekte vorgeschlagen.  This work looks at the formation and evolution of chondrules and the analytical tools necessary for such an investigation. A combination of freeze-thaw disaggregation, table-top digestion of individual chondrules and ICP-MS and -OES measurements of trace and major elements were tested for applicability to such small samples (average ~1 mg). Chemical fingerprinting of individual whole chondrules is investigated and the observed distinctions compared to classical single mineral phase classification. The data are used to evaluate the evolution of different chondrule types within a chondrite, their relationship to similar chondrules in other meteorites and the ties between chondrules and the bulk meteorite. The data are compared to available values for the Solar System, CI-chondrites, OC-chondrules and CAI. Chondrule source material and the processes operating in the chondrule formation region are evaluated. Finally a nebular vs. planetary formation setting, likely formation mechanisms and timeframes are assigned.<br

    Nature and origin of secondary mineral coatings on volcanic rocks of the Black Mountain, Stonewall Mountain, and Kane Springs Wash volcanic centers, southern, Nevada

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    The following subject areas are covered: (1) genetic, spectral, and LANDSAT Thematic Mapper imagery relationship between desert varnish and tertiary volcanic host rocks, southern Nevada; (2) reconnaissance geologic mapping of the Kane Springs Wash Volcanic Center, Lincoln County, Nevada, using multispectral thermal infrared imagery; (3) interregional comparisons of desert varnish; and (4) airborne scanner (GERIS) imagery of the Kane Springs Wash Volcanic Center, Lincoln County, Nevada

    Isotopic evidence for the origin of Proterozoic massif-type anorthosites and their relation to rapakivi granites in southern Finland and northern Brazil

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    The purpose of this study was to apply novel isotopic methods to shed light on the petrogenesis and magmatic evolution of Proterozoic massif-type anorthosites and related rapakivi granites in southern Finland and northern Brazil. Representative rock types from two AMCG (Anorthosite-Mangerite-Charnockite-Granite) complexes of similar age and composition, the 1640 Ma Ahvenisto complex and the 1530 Ma Mucajaí complex, were studied. Zircon U Pb ages from 1642 Ma to 1636 Ma in the Ahvenisto complex and from 1527 Ma to 1519 Ma in the Mucajaí complex were measured. All major rock types (anorthositic and gabbroic rocks, granites, monzodiorites) have overlapping ages. Only the most evolved granitic rocks in both suites are marginally younger than the rest of the rock types. The whole-rock (ID-TIMS) εNd isotope values obtained for all rock-types in both complexes range from +0.4 to 2.1 (at 1640 Ma) in Ahvenisto and 1.9 to 2.8 (at 1525 Ma) in Mucajaí. Zircon Lu Hf isotopes of the Finnish rapakivi suite rocks revealed a depleted mantle signature (initial εHf values up to +9 at ~1640 Ma) in the most primitive rock types, that was not detected in the Repartimento anorthosite of the Mucajaí complex. Rapakivi granites from both study areas displayed homogeneous Hf isotope compositions (~±2 ε units at 2SD) with average εHf values from +1.0 to 0.1 at ~1640 Ma and +0.1 to 2.2 at 1540 Ma in the Finnish suite and from 2.0 to 3.1 at 1525 Ma in the Mucajaí complex. Zircon oxygen isotope studies in both study areas produced relatively low δ18O value estimates for the primary magmas of the anorthositic rocks (δ18Ozir from 5.5 to 7.5 in Ahvenisto and ~6.5 in Mucajaí) and somewhat higher values for the rapakivi granites (~8.0 in Ahvenisto and from 6.0 to 7.5 in Mucajaí). The lowest δ18Ozir values were observed in the Ahvenisto leucotroctolite (δ18Ozir from 5.4 to 7.0 ), which also had the highest initial εHf values (+0.3 to +5.2 at 1640 Ma) within the Ahvenisto complex. This correlation was taken as evidence of a depleted mantle source component in the Finnish AMCG suite, even though most of them display crustal isotope signatures (δ18Ozir from 6.3 to 7.8 ; εHf from 1.5 to +5.1; 1640 Ma). The somewhat higher δ18Ozir values observed in the rapakivi granites from both complexes combined with the Hf isotope results suggest they were derived from crustal sources.Tässä työssä tutkittiin Etelä-Suomesta tunnettujen n. 1600 miljoonaa vuotta vanhojen rapakivigraniittien ja anortosiittien geokemiallisia koostumuksia ja vertailtiin niitä Pohjois-Brasiliasta, syvältä Amazonin sademetsästä, löydettyjen samanlaisten kivien koostumuksiin. Molemmilla alueilla tutkittujen kivien koostumukset todettiin hyvin samankaltaisiksi ja seurueet miltei samanikäisiksi keskenään. Suomalaiset rapakivigraniitit ja niihin kiinteästi liittyvät tummat, anortosiittiset, kivilajit ovat olleet petrologisten tutkimusten kohteena jo 1800-luvun lopulta lähtien. Nykyään rapakiviä tunnetaan kaikilta mantereilta ja niiden syntymekanismeja tutkitaan aktiivisesti ympäri maailman. Uraani-lyijy -iänmääritysmenetelmän perusteella rapakivigraniitit ja anortosiitit ovat saman ikäisiä keskenään molemmilla tutkimusalueilla, Mäntyharjulla sijaitsevassa Ahveniston rapakivikompleksissa ja Pohjois-Brasiliassa, Mucajaín rapakivikompleksissa. Ahveniston kivien iäksi määritettiin n. 1640 ja Mucajaín n. 1525 miljoonaa vuotta. Nuorimmat Lounais-Suomesta tunnetut rapakiviesiintymät ovat saman ikäisiä kuin Mucajaí-kompleksi. Aiemmin kokokivinäytteistä tehtyjen neodyymi- ja strontium-isotooppimittausten perusteella on saatu viitteitä siitä, että Etelä-Suomen anortosiittiset kivilajit olisivat peräisin maapallon vaipasta ja että samaan aikaan syntyneet rapakivigraniitit edustaisivat maapallon kuorikerroksen osittaisia kivisulia. Tässä tutkimuksessa vertailtujen Ahveniston ja Mucajaín kivilajien neodyymi-isotooppikoostumusten havaittiin paitsi tukevan tätä olettamusta myös olevan keskenään hyvin samankaltaiset. Lisäksi kivilajien tutkimukseen sovellettiin uutta menetelmää, joka hyödyntää yksittäisistä zirkoni-mineraalin kiteistä tehtyjä hafnium- ja happi-isotooppimittauksia. Näiden menetelmien avulla saatiin yksityiskohtaisempaa tietoa graniitit ja anortosiitit synnyttäneiden kivisulien kehityksestä ja voitiin vahvistaa kokokivinäytteistä mitatut tulokset. Seurueiden samankaltaisuus tukee myös SAMBA-supermannerteoriaa, jonka mukaan Fennoskandian ja Pohjois-Amazonian mannerlaatat olisivat keskiproterotsooisella ajalla, noin 1500 miljoonaa vuotta sitten, olleet lähellä toisiaan ja mahdollisesti muodostaneet yhtenäisen supermantereen osan

    A PETROLOGIC, GEOCHEMICAL AND OSMIUM ISOTOPIC STUDY OF SELECTED PRECAMBRIAN KOMATIITES

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    The major and trace elements, and Re-Os isotope systematics of the ca. 2.7-Ga komatiites from the Alexo and Dundonald Beach areas in the Abitibi greenstone belt, Canada, and ca. 2.0-Ga komatiites from Jeesiörova, Finnish Lapland were examined in order to constrain the long-term Os isotopic evolution of their mantle sources and also to evaluate different petrogenetic models for their generation. The Re-Os isotope results for whole-rock komatiites and chromite separates from Alexo, Dundonald Beach and the Jeesiörova areas, all yield precisely chondritic intial Os isotopic compositions of their mantle sources (gamma-Os = -0.1± 1.0, 0.0 ± 0.6 and 0.1± 0.5, respectively), consistent with that for the projected primitive upper mantle. The uniform chondritic initial Os isotopic compositions of these komatiites suggest their derivation from mantle sources that had undergone neither long-term Re-depletion nor enrichments in radiogenic Os-187. The uniform chondritic initial Os isotopic compositions of the Ti-rich Finnish komatiites also suggest that the processes responsible for high Ti concentrations in the Finnish rocks must have either been coeval with the generation of the rocks or Re and Os were not significantly fractionated during the processes. The uniform Os isotopic compositions of the Precambrian mantle source regions for these komatiites are in contrast to the variably radiogenic Os isotopic compositions, commonly reported for the present-day ocean island basalts and modern arc-related rocks. This suggests that the Precambrian mantle was significantly more homogeneous than the present-day mantle. Finally, the high-precision Os isotopic results of this study, combined with those previously published for Precambrian ultramafic rocks suggest that although Os isotopic heterogeneities began to appear in the terrestrial mantle as early as ~2.8 Ga, large portions of the Precambrian mantle were evidently not affected by the processes responsible for long-term Os isotopic heterogeneities

    A geochemical and petrological study of volcanic rocks in the Beardmore-Geraldton Archaean Greenstone Belt, Northwestern Ontario

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    Three repetitive sequences of northward youngIng, east striking, linear, volcano-sedimentary units are found in the late Archaean BeardmoreGeraldton greenstone belt, situated within the Wabigoon subprovince of the Superior Province of northwestern Ontario. The volcanic components are characterised by basaltic flows that are pillowed at the top and underlain by variably deformed massive flows which may In part be intrusive. Petrographic examination of the volcanic units indicates regional metamorphism up to greenschist facies (T=3250 C - 4500 C, P=2kbars) overprinted by a lower amphibolite facies thermal event (T=5750 C, P=2kbars) confined to the south-eastern portion of the belt. Chemical element results suggest olivine, plagioclase and pyroxene are the main fractionating mineral phases. Mobility studies on the varIOUS chemical elements indicate that K, Ca, Na and Sr are relatively mobile, while P, Zr, Ti, Fet (total iron = Fe203) and Mg are relatively immobile. Discriminant diagrams employing immobile element suggests that the majority of the samples are of oceanic affinity with a minor proportion displaying an island arc affinity. Such a transitional tectonic setting IS also refle.cted in REE data where two groups of volcanic samples are recognised. Oceanic tholeiites are LREE depleted with [La/Sm] N = 0.65 and a relatively flat HREE profile with [Sm/Yb] N = 1.2. Island arc type basalts (calc-alkaline) are LREE enriched, with a [La/Sm] N = 1.6, and a relatively higher fractionated HREE profile with [Sm/Yb] N = 1.9. Petrogenetic modelling performed on oceanIC tholeiites suggests derivation from a depleted spinel lherzolite source which undergoes 20% partial melting. Island arc type basalts can be derived by 10% partial melting of a hypothetical amphibolitised oceanic tholeiite source. The majority of the volcanic rocks in the Beardmore-Geraldton Belt are interpreted to represent fragments of oceanic crust trapped at a consuming plate margin. Subsequent post accretionary intrusion of gabbroic rocks (sensu lato) with calc-alkaline affinity is considered to result in the apparent hybrid tectonic setting recognized for the BGB
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