2 resultados para amphibole olivine
em Repositório Institucional da Universidade de Aveiro - Portugal
Resumo:
The island of So Jorge (38 45 24 N - 28 20 44W and 38 33 00 N - 27 44 32 W) is one of the nine islands of the Azores Archipelago that is rooted in the Azores Plateau, a wide and complex region which encompasses the triple junction between the American, Eurasia and Nubia plates. So Jorge Island has grown by fissural volcanic activity along fractures with the regional WNW-ESE trend, unveiling the importance of the regional tectonics during volcanic activity. The combination of the volcanostratigraphy (Forjaz & Fernandes, 1975; and Madeira, 1998) with geochronological data evidences that the island developed during two main volcanic phases. The first subaerial phase that occurred between 1.32 and 1.21 Ma ago (Hildenbrand et al. 2008) is recorded on the lava sequence forming the cliff at Faj de So Joo, while the second phase started at 757 ka ago, is still active, and edified the rest of the island. This second phase edified the east side of the island that corresponds to Topo Volcanic Complex, in the period between 757 and 543 ka ago, while the west side named Rosais Volcanic Complex, started at 368 ka ago (Hildenbrand et al. 2008) and was still active at 117 ka ago. After the onset of Rosais, volcanic activity migrates to the center of So Jorge edifying Manadas Volcanic Complex. The volcanism on So Jorge is dominantly alkaline, with a narrow lithological composition ranging between the basanites/tefrites through the basaltic trachyandesites, in spite of this the two volcanic phases show distinct mineralogical, petrographic and geochemical characteristics that should be related with different petrogenetic conditions and growth rates of the island. Abstract viii During the first volcanic phase, growth rates are faster (3.4 m/ka), the lavas are slightly less alkaline and plagioclase-richer, pointing to the existence of a relative shallow and dynamic magma chamber where fractional crystallization associated with gravitational segregation and accumulation processes, produced the lavas of Faj de So Joo sequence. The average growth rates during the second volcanic phase are lower (1.9 m/ka) and the lavas are mainly alkaline sodic, with a mineralogy composed by olivine, pyroxene, plagioclase and oxide phenocrysts, in a crystalline groundmass. The lavas are characterized by enrichment in incompatible trace element and light REE, but show differences for close-spaced lavas that unveil, in some cases, slight different degrees of fertilization of the mantle source along the island. These differences might also result from higher degrees of partial melting, as observed in the early stages of Topo and Rosais volcanic complexes, of a mantle source with residual garnet and amphibole, and/or from changing melting conditions of the mantle source as pressure. The subtle geochemical differences of the lavas contrast with the isotopic signatures, obtained from Sr-Nd-Pb-Hf isotopes, that So Jorge Island volcanism exhibit along its volcanic complexes. The lavas from Topo Volcanic Complex and from the submarine flank, i.e. the lavas located east of Ribeira Seca Fault, sample a mantle source with similar isotopic signature that, in terms of lead, overlaps Terceira Island. The lavas from Rosais and Manadas volcanic complexes, the western lavas, sample a mantle source that becomes progressively more distinct towards the west end of the island and that, in terms of lead isotopes, trends towards the isotopic composition of Faial Island. The two isotopic signatures of So Jorge, observed from the combination of lead isotopes with the other three systems, seem to result from the mixing of three distinct end-members. These end-members are (1) the common component related with the Azores Plateau and the MAR, (2) the eastern component with a FOZO signature and possibly related with the Azores plume located beneath Terceira, and (3) the western component, similar to Faial, where the lithosphere could have been entrained by an ancient magmatic liquid, isolated for a period longer than 2Ga. The two trends observed in the island reinforce the idea of small-scale mantle heterogeneities beneath the Azores region, as it has been proposed to explain the isotopic diversity observed in the Archipelago.
Resumo:
A presente tese tem por objetivo principal contribuir para o conhecimento da geoqumica sedimentar da zona ocenica da crista da Terceira e montanhas submarinas a sul (regio entre 29-39N e 27-32W), integrando tambm a caraterizao dos metais e nutrientes na coluna de gua e propondo concentraes para servirem de referncia nesta regio do Atlntico Central. Para o efeito foram realizadas amostragens na coluna de gua em sete locais e de sedimento em cinco locais, durante a campanha oceanogrfica designada por EMEPC/AORES/G3/2007 a bordo do navio SV Kommandor Jack, no mbito do projeto da Estrutura de Misso para a Extenso da Plataforma Continental (EMEPC). Os perfis de CTD da coluna de gua na regio estudada revelam a presena de massas de gua distintas: a Western North Atlantic Central Water (WNACW), a Eastern North Atlantic Central Water tropical (ENACWt), a Eastern North Atlantic Central Water polar (ENACWp), a Mediterranean Overflow Water (MOW), a Deep Mediterranean Water (DMW) e a North Eastern Atlantic Deep Water (NEADW). Observou-se nos perfis de temperatura e salinidade, referentes aos primeiros 200 m da coluna de gua, um gradiente meridional negativo entre as estaes localizadas na crista da Terceira e as estaes localizadas mais a sul. Observou-se nas guas superficiais valores de oxignio dissolvido de 93% e de pH de 8,1, assim como que as concentraes dos nutrientes NOx, PO4 e SiO2 variam de acordo com a atividade biolgica, tendo-se registado concentraes medianas mais baixas, respetivamente de 6,5, 0,23 e 1,3 mol L-1, que aumentam com a profundidade devido ausncia de produo primria (respetivamente 31, 1,4 e 22 mol L-1). As concentraes de NH4 e de SO4 no variam significativamente nas massas de gua, sendo os valores medianos mnimos e mximos de 0,69 a 0,79 mol L-1 para o NH4 e de 30 a 32 mol L-1 para o SO4. So propostas concentraes de referncia para as massas de gua, para os elementos cobre, cdmio, chumbo e arsnio. Os perfis de sedimento analisados permitem distinguir os sedimentos na crista da Terceira (core A) dos restantes (cores B a E). A grande variabilidade textural encontrada no core A, que contrasta com os outros cores analisados, deve-se a importantes contribuies terrgenas, originadas pela eroso sub-area e pela atividade vulcnica das ilhas prximas. iv resumo (continuao) A anlise mineralgica, efetuada frao areia e frao fina (< 63 m), confirma que os sedimentos do core A derivam de rochas vulcnicas formadas maioritariamente por piroxenas, olivinas, anfbolas, biotite, alterites e ainda calcite, plagiclase e magnetite, tendo-se identificado ao microscpio a glauconite e o vidro vulcnico. De acordo com a composio qumica destes minerais o core A apresenta valores mais elevados de Al, Fe, K, P, Mg, Si, Na, Zn, V, Cr e Mn relativamente aos cores B a E. Os cores B a E apresentam grandes quantidades de calcite (>80%) formada maioritariamente por foraminferos e nanoplncton calcrio (cocolitforos). A frao areia confirma a composio maioritariamente carbonatada com grande abundncia de material biognico formado por oozes de foraminferos (planctnicos e bentnicos) com raras espculas de espongirios e restos de conchas. Os cores B a E apresentam valores muito mais elevados que o core A para os elementos Ca e Sr. Os resultados para o Al, Fe, K, P, Si, Na, As, Cu, Ni, Zn, V, Cr, Li, Pb, Cd e Co presentes nos locais B, C, D e E sugerem que estes cores so comparveis aos sedimentos de fundo carbonatados. Prope-se concentraes de referncia para a regio do Atlntico compreendida entre 29-39N e 27-32W considerando a primeira camada colhida em cada core. Para o core A as concentraes so normalizadas a 5% de Al e CaCO3, enquanto que para os cores B a E so normalizadas a 2% de Al e CaCO3. Assim as concentraes de referncia para o core A so: As 18 mg kg-1, Cr 91 mg kg-1, Cu 127 mg kg-1, Ni 84 mg kg-1, Pb 41 mg kg-1, Hg 41 ng g-1 e Zn 482 mg kg-1. Para os cores B a E as concentraes de referncia so: As 3 mg kg-1, Cr 10 mg kg-1, Cu 36 mg kg-1 Ni 12 mg kg -1, Hg 3 ng g-1 e Zn 20 mg kg-1. Para os restantes metais as concentraes de referncia para o core A so: Al 9%, Si 25%, Fe 6%, Ca 13%, K 2%, Mg 2%, Na 3%, P 0,4%, Sr 900 mg kg-1, Li 10 mg kg-1, Mn 1200 mg kg-1, Ba 700 mg kg-1 e V 140 mg kg-1. Para os cores B a E as concentraes de referncia so: Al 0,9%, Si 2%, Fe 0,2%, Ca 95%, K 0,3%, Mg 0,4%, Na 0,3%, P 0,04%, Sr 2600 mg kg-1, Li 5 mg kg-1, Mn 240 mg kg-1, Ba 345 mg kg-1, Co 2 mg kg-1 e V 6 mg kg-1. Os resultados da presente tese constituem um contributo para a caraterizao geoqumica da regio e podem servir de referncia monitorizao futura do mar dos Aores e montes submarinos a sul.
