Browsing by Author "Gerdes, Axel"

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    An assessment of monazite from the Itambé pegmatite district for use as U–Pb isotope reference material for microanalysis and implications for the origin of the “Moacyr” monazite.
    (2016) Gonçalves, Guilherme de Oliveira; Lana, Cristiano de Carvalho; Cipriano, Ricardo Augusto Scholz; Buick, Ian S.; Gerdes, Axel; Kamo, Sandra L.; Corfu, Fernando; Marinho, Moacyr Moura; Chaves, Alexandre de Oliveira; Valeriano, Claudio de Morisson; Nalini Júnior, Hermínio Arias
    Large quantities of monazite from different pegmatite bodies of the Itambe pegmatite district were investigated to assess their suitability as U–Pb and Sm–Nd isotope reference materials for LA-ICP-MS and to track the origin of a piece of theMoacyrmonazite (termed here Itambé), awidely used reference material for LA-ICP-MS U–Pb geochronology. Monazite fromthe largest pegmatite bodies in the district (the Bananeira, Coqueiro and Paraíso pegmatites) are Ce-monazite, with negligible amounts of the huttonite and brabantite components. They are homogeneous in major and trace elements, which makes them potential candidates as compositional reference materials. U–Pb LA-ICP-MS and ID-TIMS analyses yielded identical ages within error. Although the ID-TIMS ages (507.7 ± 1.3 (207Pb⁎/235U) and 513.6 ± 1.2 Ma (206Pb⁎/238U)) were reversely discordant, spot ages determined by LA-ICP-MS geochronology were concordant at ca 508 Ma. The Bananeiro monazite was assessed as a LA-ICPMS U–Pb primary reference material against other known reference materials (treated as unknowns). This approach successfully reproduced the previously published ages of the reference materials.MREE/HREE fractionation (ie, (La/Gd)N and (Gd/Lu)N values), Eu/Eu⁎ and the chondrite-normalized REE patterns suggest that the “Itambé” monazite aliquot is very similar to that fromthe Coqueiro pegmatite. This similarity is likewise apparent in their Sm–Nd isotope compositions. Moreover, the εNdi values of the “Itambé” monazite fragment (εNdi=−4.2) and those fromall the major pegmatites in the district, are distinct fromother reference materials (eg, Managountry; εNdi = −22.3) as well as gem-quality monazite from c. 490–520 Ma pegmatites from the Araçuaí Orogen, further to the south. The εNdi can provide a further distinction for tracing Brazillian gemquality monazite reference materials.
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    Archean evolution of the southern São Francisco craton (SE Brazil).
    (2017) Albert, Capucine; Lana, Cristiano de Carvalho; Farina, Federico; Lana, Cristiano de Carvalho; Gerdes, Axel; Storey, Craig Darryl; Cipriano, Ricardo Augusto Scholz; Pimentel, Márcio Martins
    Os continentes modernos são fundamentalmente diferentes de suas contrapartes Arqueanas, devido à influência de um manto substancialmente mais quente sobre a produção e as propriedades reológicas da crosta no início da Terra. Particularmente, a natureza e a composição do registro de rochas ígneas sofreram modificações significativas no final do eon Arqueano, atestando mudanças importantes nos processos geodinâmicos. Este trabalho concentra-se nas rochas granitóides posicionadas na porção sul do craton São Francisco no sudeste do Brasil. Uma combinação de observações de campo e petrográficas, geoquímica de rocha total e análises de isotópicas (U-Pb, Lu- Hf, O e B) foram realizadas para um melhor entendimento nos mecanismos que levaram à formação e estabilização deste bloco cratônico, além de compreender as modificações geodinâmicas globais ocorridas durante esse período. O embasamento do Quadrilátero Ferrífero, no sul do cráton São Francisco, consiste em ortognaisses, intrudidos por abundantes plútons granitóides e associados produtos magmáticos (veios pegmatíticos/aplíticos, por exemplo). O embasamento do Quadrilátero Ferrífero registra três períodos principais de magmatismo, denominados de Rio das Velhas I (RVI) e II (RVII) de 2.92-2.85 e 2.80-2.76 Ga, respectivamente, e o último evento Mamona (2.75-2.68 Ga). Granitóides e gnaissses dos três complexos do embasamento estudados (Bação, Belo Horizonte e Bonfim) foram subdivididos em dois grupos, refletindo diferentes processos petrogenéticos e/ou fontes: rochas de médio- e alto-K. Os gnaisses e granitóides de médio-K foram formados durante os eventos Rio das Velhas I e II. Essas rochas apresentam algumas semelhanças com os TTG Arqueanos, apesar de possuírem um pequeno enriquecimento em SiO2 e K2O e depleção em Al2O3, e foram interpretadas como o resultado da mistura entre um membro final derivado da fusão parcial de uma rocha metabasáltica e um fundido resultante da reciclagem de uma crosta mais antiga de composição TTG. Por outro lado, os granitos de alto-K colocados na crosta durante subsequente evento Mamona, assemelham-se aos típicos granitóides à biotita do Arqueano tardio, com origem interpretada da fusão parcial de metassedimentos imaturos. O envolvimento de metassedimentos na petrogênese destes magmas de alto-K Neoarqueanos é ainda suportado por: (i) uma tendência para valores pesados de δ18O(Zrc), indicando equilíbrio isótopico de O em condições de subsuperfície, e (ii) a presença fundidos magmáticos ricos em boro e fluídos intrudidos em 2.70-2.60 Ga, provavelmente derivados de um protólito metassedimentar rico em boro. Em geral, propomos que a evolução magmática e geodinâmica da porção sul do craton São Francisco ocorreu da seguinte maneira. De ~ 3.50 a 2.90 Ga, fragmentos de crosta juvenil (possivelmente TTGs) foram produzidos, formando o protólito do atual sul do cráton São Francisco. Este é um período de crescimento crustal, como evidenciado pela modelagem isotópica de hafnium à partir de zircões detríticos. Em ~ 2.90 Ga, a crosta sofreu significativas modificações, devido ao início de uma tectônica colisional. Diversos fragmentos crustais com histórias distintas foram acrescidos progressivamente, construindo, por fim, um núcleo continental rígido e soerguido. Isto é suportado por: (i) a transição para um regime dominado por reciclagem crustal, com diminuição de contribuição mantélica em magmas recém-gerados, (ii) o envolvimento de componentes TTG mais antigos na geração de magmas de médio-K durante os eventos RVI e RVII e a simultânea ausência de materiais crustais preservados mais velhos que ~ 2.90 Ga na porção sul do craton São Francisco, (iii) a abundância de sistemas hidrotermais na porção superior da crosta evidenciados por valores leves de δ18O(Zrc), indicando o soerguimento de grandes porções da crosta continental durante o Neoarqueano, (iv) a progressiva maturação crustal, registrada por uma tendência para o magmatismo mais potássico e rico em HFSE (High Field Strength Element). O soerguimento e rápido soterramento de grandes bacias sedimentares dominadas por componentes clásticos no Neoarqueano (~ 2.75 Ga) levaram à produção de granitóides de alto-K durante o evento Mamona. Fluidos magmáticos diferenciados portadores de turmalina percolaram a crosta e interagiram de forma generalizada com o greenstone belt adjacente. A presença de turmalina hidrotermal de origem magmática (e Neoarqueana) em associação espacial próxima aos complexos do embasamento sugere que a formação das estruturas domo e quilhas ocorreu antes do último evento magmático (ou seja, antes de 2.70 Ga), o que está em desacordo com a idéia de que a região adquiriu sua arquitetura atual durante o Paleoproterozóico.
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    Archean magmatic-hydrothermal fluid evolution in the Quadrilátero Ferrífero (SE Brazil) documented by B isotopes (LA MC-ICPMS) in tourmaline.
    (2018) Albert, Capucine; Lana, Cristiano de Carvalho; Gerdes, Axel; Schannor, Mathias; Narduzzi, Francesco; Queiroga, Gláucia Nascimento
    In the Archean Quadrilátero Ferrífero district (SE Brazil), tourmaline occurs as a major constituent in a leucogranitic intrusion and numerous pegmatitic/aplitic veins within magmatic basement complexes, as well as in quartzo-feldspathic veins, in quartz-tourmaline rocks (tourmalinites) and as disseminated grains in the surrounding greenstone belt metasediments. The chemical and boron isotope composition of these tourmalines was analysed by electron microprobe and LA MC-ICP-MS to determine the origin of the fluids involved and to shed light on the hydrothermal evolution of the region. The tourmalines exhibit an overall decrease in Fe/(Fe + Mg) ratio and a net increase in Cr (up to 0.75 wt%) from tourmaline hosted in the leucogranite, the quartzo-feldspathic veins, the tourmalinites to the disseminated grains in the schists. These variations mirror the bulk composition of the host schists, and illustrate a strong protolith control on tourmaline major element composition. The full range of tourmaline δ11B is from −27.1 to −9.2‰, with a major cluster between −12 and −19‰, which includes the magmatic tourmaline in the leucogranite (−15.2 to −12.5‰). Most of these isotope compositions can be reconciled with a model involving tourmaline growth from late-stage exsolved magmatic fluids percolating through the magmatic basement and into the nearby metasediments. This model agrees well with the trends of major element compositions, as well as with the critical observation that tourmaline occurrence is restricted to the vicinity of the basement complexes. δ11B values lower than ~−19‰ in our dataset are beyond the reach of isotope fractionation during magmatic fluid exsolution and temperature decrease, and suggest the presence of a distinct fluid component. We propose that an isotopically light fluid was released from mica breakdown in the country rocks during local upper amphibolite facies metamorphism in the dome border shear zone.
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    Assessing the isotopic evolution of S-type granites of the Carlos Chagas Batholith, SE Brazil : clues from U–Pb, Hf isotopes, Ti geothermometry and trace element composition of zircon.
    (2016) Melo, Marilane Gonzaga de; Lana, Cristiano de Carvalho; Stevens, Gary; Soares, Antônio Carlos Pedrosa; Gerdes, Axel; Alkmin, Leonardo Azevedo Sá; Nalini Júnior, Hermínio Arias; Alkmim, Fernando Flecha de
    The Carlos Chagas batholith (CCB) is a very large (~14,000 km2) S-type granitic body formed during the syn-collisional stage of the Araçuaí orogen (southeastern Brazil). Zircons extracted from the CCB record a wide range of U–Pb ages (from 825 to 490 Ma), indicating a complex history of inheritance, magmatic crystallization and partial melting during the evolution of the orogeny.Magmatic zircons (ca. 578–588Ma) aremarked by similar Hf isotope compositions and REE patterns to those of inherited cores (ca. 825–600Ma), indicating that these aspects of the chemical signature of the magmatic zircons have likely been inherited from the source. The U–Pb ages and initial 176Hf/177Hf ratios from anatectic and metamorphic zircon domains are consistent with a twostage metamorphic evolution marked by contrasting mechanisms of zircon growth and recrystallization during the orogeny. Ti-in-zircon thermometry is consistent with the findings of previous metamorphic work and indicates that the twometamorphic events in the batholith reached granulite facies conditions (N800 °C) producing two generations of garnet via fluid-absent partial melting reactions. The oldest metamorphic episode (ca. 570– 550Ma) is recorded by development of thin anatectic overgrowths on older cores and by growth of newanatectic zircon crystals. Both domains have higher initial 176Hf/177Hf values compared to relict cores and display REE patterns typical of zircon that grewcontemporaneouslywith peritectic garnet through biotite-absent fluid partial melting reactions. Hf isotopic and chemical evidences indicate that a second anatectic episode (ca. 535–500Ma) is only recorded in parts from the CCB. In these rocks, the growth of new anatectic zircon and/or overgrowths is marked by high initial 176Hf/177Hf values and also by formation of second generation of garnet, as indicated by petrographic observations and REE patterns. In addition, some rocks contain zircon crystals formed by solid-state recrystallization of pre-existing zircon, which exhibit similar Hf isotope composition to those of inherited/magmatic core domains. The first anatectic event is interpreted as result of crustal thickening after the intrusion of the batholith. This introduced the batholith to a depth in excess of 30 km and produced widespread anatexis throughout the batholith. The second event was associated with asthenospheric upwelling during extensional thinning and gravitational collapse of the orogen, this produced anatexis in parts fromthe CCB that had been re-fertilized for anatexis by retrogression along shear zones following the first granulite facies event.
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    Implications for sedimentary transport processes in southwestern Africa : a combined zircon morphology and age study including extensive geochronology databases.
    (2022) Gärtner, Andreas; Hofmann, Mandy; Zieger, Johannes; Sagawe, Anja; Krause, Rita; Stutzriemer, Marika; Gesang, Subani; Gerdes, Axel; Marko, Linda; Lana, Cristiano de Carvalho; Linnemann, Ulf
    Extensive morphological and age studies on more than 4600 detrital zircon grains recovered from modern sands of Namibia reveal complex mechanisms of sediment transport. These data are further supplemented by a zircon age database containing more than 100,000 single grain analyses from the entire southern Africa and allow for hypothesising of a large Southern Namibian Sediment Vortex located between the Damara Orogen and the Orange River in southern Namibia. The results of this study also allow assuming a modifed model of the Orange River sand highway, whose origin is likely located further south than previously expected. Moreover, studied samples from other parts of Namibia give frst insights into sediment movements towards the interior of the continent and highlight the potential impact of very little spatial variations of erosion rates. Finally, this study points out the huge potential of detrital zircon morphology and large geo-databases as an easy-touse additional tool for provenance analysis.
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    A new appraisal of sri lankan bb zircon as a reference material for LA-ICP-MS U-Pb geochronology and Lu-Hf isotope tracing.
    (2017) Santos, Maristella Moreira; Lana, Cristiano de Carvalho; Buick, Ian S.; Schmitz, Mark D.; Kamo, Sandra L.; Gerdes, Axel; Corfu, Fernando; Tapster, Simon; Lancaster, Penelope; Storey, Craig Darryl; Basei, Miguel Ângelo Stipp; Tohver, Eric; Alkmim, Ana Ramalho; Nalini Júnior, Hermínio Arias; Krambrock, Klaus Wilhelm Heinrich; Leite, Cristiano Fantini; Wiedenbeck, Michael
    A potential zircon reference material (BB zircon) for laser abla tion-in ductively coupled plasma-mass spectrometry (LA-ICP-M S)U-Pb geochronology and Hf isotope geochemistry is described. A batch of twenty zircon megacrysts (0.5–1.5 cm3) from SriLanka was studied. Within-grain rare earth element (REE) compositions are largely homogeneous, albeit with somevariation seen between fractured and homogeneous domains. Excluding fractured cathodoluminescence bright domains,the variation in U content for all analysed crystals ranged from 227 to 368 lgg-1and the average Th/U ratios werebetween 0.20 and 0.47. The Hf isotope composition (0.56–0.84 g/100 g Hf) is homogeneous within and between thegrains – mean176Hf/177Hf of 0.281674 ± 0.000018 (2s). The calculated alpha dose of 0.59 3 1018g-1for a numberof BB grains falls within the trend of previously studied, untreated zircon samples from Sri Lanka. Aliquots of the samecrystal (analysed by ID-TIMS in four different laboratories) gave consistent U-Pb ages with excellent measurementreproducibility (0.1–0.4% RSD). Interlaboratory assessment (by LA-ICP-MS) from individual crystals returned results that arewithin uncertainty equivalent to the TIMS ages. Finally, we report on within- and between-grain homogeneity of theoxygen isotope systematic of four BB crystals (13.16‰ VSMOW).
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    The Diamantina monazite : a new Low‐Th reference material for microanalysis.
    (2018) Gonçalves, Guilherme de Oliveira; Lana, Cristiano de Carvalho; Cipriano, Ricardo Augusto Scholz; Buick, Ian S.; Gerdes, Axel; Kamo, Sandra L.; Corfu, Fernando; Rubatto, Daniela; Wiedenbeck, Michael; Nalini Júnior, Hermínio Arias; Oliveira, Luiz Carlos Alves de
    Most monazite reference materials (RMs) for in situ U-Pb geochronology are rich in Th; however, many hydrothermal ore deposits contain monazite that is low in trace element contents, including Th, U and Pb. Because of potential problems with matrix effects and the lack of appropriate matrix-matched RMs, such variations can bias dating of hydrothermal deposits. In this paper, we describe a polycrystalline low-U and low-Th Diamantina monazite from the Espinhac o Range, SE Brazil. It has a U-Pb ID-TIMS weighted mean 207Pb*/235U ratio of 0.62913 ± 0.00079, 206Pb*/238U of 0.079861 ± 0.000088 and 207Pb*/206Pb* of 0.057130 ± 0.000031, yielding a weighted mean 206Pb*/238U date of 495.26 ± 0.54 Ma (95% c.l.). In situ dates acquired with different methods (LA-(Q, SF, MC)-ICP-MS and SIMS) are within uncertainty of the ID-TIMS data. U-Pb LA-(Q, MC)-ICP-MS runs, using Diamantina as a primary RM, reproduced the ages of other established RMs within < 1% deviation. The LA-MC-ICP-MS analyses yielded homogeneous Sm-Nd isotopic compositions (143Nd/144Nd = 0.511427 ± 23, 2s; 147Sm/144Nd = 0.1177 ± 13, 2s) and eNd(495 Ma) of -18.7 ± 0.5 (2s). SIMS oxygen isotope determinations showed measurement reproducibility better than ± 0.3‰ (2s), confirming Diamantina’s relative homogeneity at test portion masses below 1 ng.