Helium and neon isotopes in São Miguel island basalts, Azores Archipelago: New constraints on the “low 3He” hotspot origin

Lavas from the São Miguel Island, Azores Archipelago, have peculiar isotopic compositions, including radiogenic lead and strontium and un-radiogenic neodymium. The peculiar isotopic trend of São Miguel is evident in the lead-lead diagram where both 207Pb/204Pb and 208Pb/204Pb ratios are high for a given 206Pb/204Pb ratio compared to other oceanic island basalts. This signature is unique among OIBs and is particularly evident in the Nordeste area, the oldest part of São Miguel island (≥1 Ma). Only a few olivine samples from the Nordeste volcanic complex have been analyzed for helium. They show radiogenic helium signatures with 4He/3He up to 174,000 (R/Ra ~4) [Moreira et al., Helium and lead isotope geochemistry in the Azores archipelago, Earth Planet. Sci. Lett. 169: 189–205, 1999]. However, because the Nordeste volcano has an age between 1 and 4 Ma and because these samples have low helium concentrations, these radiogenic helium isotopic ratios must be considered with caution as they can also reflect post eruptive radiogenic production. In this paper we present a detailed study of the helium and neon isotopic ratios obtained from 17 Nordeste samples in order to better constrain the helium isotopic signature of the São Miguel mantle source. By coupling helium and the other isotopic systems, we propose that the São Miguel source contains non-degassed material, enriched in U and Th, that was stored in the mantle for the last ~3 Ga. As suggested by Elliot et al. [Elliott et al., The origin of enriched mantle beneath São Miguel, Azores, Geochim. Cosmochim. Acta 71: 219–240, 2007], underplated magma intruded into oceanic lithosphere and subducted ~3 Ga ago is a possible explanation for the peculiar São Miguel source isotopic signatures

Organização das unidades de investigação, estabelecimento de redes e internacionalização

O Congresso de investigação (CI) da UE constituiu um fórum para reflexão e produção de recomendações que serão úteis para um melhor funcionamento e organização interna tendo em vista um melhor desempenho científico, para identificação de fragilidades e de oportunidades, interligação da investigação com o ensino pós-graduado, em suma para definição das políticas universitárias. Foram objectivos do Congresso: 1. Promover a discussão alargada das problemáticas das actividades de investigação; 2. Elaborar um conjunto de recomendações sobre os vários aspectos da actividade de investigação na UE

A ESTRUTURA E COMPOSIÇÃO DO MANTO: CONSTRANGIMENTOS A PARTIR DA ANÁLISE DOS ISÓTOPOS DE He NOS MORB E OIB

As razões isotópicas de He analisadas em basaltos das cristas médias e ilhas oceânicas revelam diferenças compatíveis com a existência de dois reservatórios geoquímicos distintos: um manto superior, caracterizado por razões R/Ra (sendo R a razão 3He/4He no manto e Ra a mesma razão para a atmosfera) pouco variáveis ( 8) e um manto inferior mais primitivo e enriquecido em 3He (e.g. R/Ra = 38 no caso da Islândia). No entanto, os modelos geofísicos recentes sugerem a existência de um único sistema de convecção para o manto, dificilmente compatível com a preservação de heterogeneidades a larga escala ao longo de 4,6 Ga. Serão discutidos diversos argumentos que suportam cada um destes modelos, bem como as falhas inerentes à sua formulação

The Azores mantle plume and its importance to the noble gas composition (He and Ne) of Terceira lavas

The noble gases signature of Terceira lavas was recently described in terms of binary mixing between a MORB magma type and a plume component. The relative proportions estimated for these components show the strong predominance of MORB. This can be interpreted as a consequence from the localization of Terceira Island or, alternatively, can be explained by the last phases of plume activity as imaged by tomographic data

Mantle dynamics and volcanism emplacement in the Azores

The Azores plateau is a triangular shaped topographic feature encompassing the boundary zone where three major tectonic plates (EU, NU and NA) meet. The eastern side of the plateau is delimited by two major tectonic discontinuities: the Mid Atlantic Ridge, and the Terceira Rift, a recently formed ultra-slow-spreading ridge. The origin of the plateau is still under debate. One hypothesis argues that the plateau would have been formed by successive NE jumps of the oblique spreading axis, where the present TR is the latest stage. Other hypotheses invoke the northward jump of the Azores triple junction, during which the Azores region would have been transferred from the Eurasian plate to the Nubian plate. For some authors, the presence of the Azores plume, a low seismic velocity zone in the mantle beneath, is required to explain the observations: the anomalously shallow seafloor depth as well as the geochemistry of the basaltic lavas erupted within the plateau. Here we use a highly resolved tomography model to quantify the influence of this plume and the surrounding mantle.We model the convection pattern, the induced dynamic topography and stresses, and compare them with the surface observations. The dynamic topography shows two maxima: one northwest of St. Miguel, the other encompassing the Terceira, Graciosa, S. Jorge, Faial and Pico islands. Both swells are approximately located on the Terceira Ridge. The convection pattern displays two distinct upwelling towards these two groups of Islands. This may explain the difference in the geochemical signatures, in particular the unique isotopic ratios observed in some lavas from S. Miguel. The stresses induced by the underlying mantle convection are compared with the surface observations (topographic features, seismic and GPS velocities). The modeled and observed tresses fairly correlate west of our study area but their directions depart east

A importância da cartografia geológica para o desenvolvimento de um Território

Neste trabalho apresenta-se uma síntese histórica da evolução da Cartografia Geológica a nível mundial e da evolução do conhecimento geológico no território de Timor-Leste. Salienta-se a importância da Cartografia Geológica como ferramenta para o planeamento territorial e propõe-se uma estratégia para o desenvolvimento do território de Timor-Leste

The Terceira Rift, Azores: a melt inclusion study of submarine lavas

The Azores plateau, a bathymetric high, is seated on an anomalous mantle domain located in the Azores Triple Junction where the American, EUR and AFR lithospheric plates meet. The cause for these anomalies is controversial but many authors consider the presence of an anomalously hot/wet enriched mantle probably supplied by a plume [3]. The origin, size and present location of the plume is under debate but an area near Terceira is the favoured plume centre [2]. The Terceira Rift (TR) defines the EUR/AFR plate boundary of the Azores triple junction. The TR is a 550 km long, generally ESE trending line of volcanic massifs along the axis (e.g. São Miguel, D. João de Castro, Terceira and Graciosa) alternating with deep basins (e.g. Hirondelle basin) interpreted as volcanically unfilled rift valley segments [3]. Vesicular, porphyritic basalts were sampled along the Terceira Rift during Portuguese scientific cruises (EMEPC 2007-2009). Initial studies focused on the adjacent areas Don João de Castro - DJC (submarine volcano) and Hirondelle - Hir (basin). Major and trace element data of phenocrysts (olivine, clinopyroxene, plagioclase), groundmass and exposed melt inclusions found in these vesicular lavas depict only slight differences between the two sites. DJC samples depict higher Fo and Mg# in olivine and clinopyroxene, respectively. Chondrite-normalized REE data indicate that groundmass material is LREE-enriched in both Hir and DJC lavas. Exposed melt inclusions (MI) show similar REE patterns compared to their hosts. Clinopyroxene in both sites display L-MREE enriched, HREE depleted sinusoidal patterns. MI were found in olivine, clinopyroxene and plagioclase phenocrysts. MI are glassy to completely opaque and devitrified, with one or more bubbles, and sulfide globules (SG). SG are spherical, depict two-phase lamellar intergrowths of Fe-Ni and Cu-Fe phases, and are ubiquitous in DJC and Hir lava samples. Some lavas display SG within clinopyroxene-hosted MI and dispersed in the groundmass. Preliminary data suggest that lavas found in these areas of the Terceira Rift were sulfur-saturated in different stages of their evolution; during early fractionation and prior to eruption. The geochemistry of melt inclusion will contribute to the understanding of mantle source, melting and mixing processes in the Terceira Rift, Azores

The Ne isotopic signature of Terceira lavas (Azores): evidence for Ne recycling?

The available noble gas database shows that the presence of an atmospheric component in oceanic basalts is common. Several models have been considered and will be discussed here in order to explain the Ne isotopic data obtained for olivine phenocrysts sampled in Terceira lavas. Analysed samples are porphyritic olivine+clinopyroxene±plagioclase bearing lavas that were erupted from the Fissural and Santa Bárbara eruptive vents. Different incompatible element ratios (e.g. Ba/Nb, La/Nb) were obtained for lavas from these two volcanic systems, which can be related with the presence of crustal recycled components in the Terceira mantle source. Ne isotopic ratios are also distinct for Fissural and Santa Bárbara olivine crystals, the later being invariably similar to air. Indirect barometric data from clinopyroxene-liquid equilibrium point to olivine crystallisation at mantle depths, thus precluding the existence of direct atmospheric contamination during magma ascent and cooling. Alternatively, we argue for a recycled origin of atmospheric Ne in accordance with geochemical data

HELIUM AND NEON ISOTOPIC EVIDENCE FOR LOWER MANTLE CONTRIBUTION TO THE TERCEIRA ISLAND MAGMATISM

The Azores archipelago, located close to the triple junction where the Eurasian, African and American plates meet, is an example of plume related magmatism. Mantle plumes have been the locus of an intense debate. Their ultimate origin has been assigned to the D” layer, to the bottom of transition zone and even to the astenosphere, as the result of passive response to lithosphere cracking (e.g. Courtillot et al., 2003). He isotope geochemistry is a key tool to clear up this question given its ability to identify the incorporation of relatively undegassed lower mantle portions in plumes (with 4He/3He ratios as low as 20.000 or R/Ra=32) and to distinguish them from those generated in a degassed upper mantle generally accepted as the MORB source (with constant 4He/3He ratios around 90.000 or R/Ra8; e.g. Allègre et al., 1995). More recently, it has been demonstrated that the two mantle reservoirs expected from helium isotope systematics can be substantiated by neon data (Moreira et Allègre, 1998). In this work we present the first Ne isotopic data collected from Terceira samples; these data, in addition to new He isotope determinations, are used to better constrain the origin of the Azores mantle plume. The noble gases analyses were performed at IPG in Paris by crushing of olivine phenocrysts (~1g). This method is particularly useful for measuring the mantle gases released by melt inclusions trapped inside the olivine crystals. Terceira island comprises four main strato-volcanoes (Self, 1976): Cinco Picos, Guilherme Moniz, Pico Alto and Santa Bárbara. At least the later two are considered to be active and connected by a fissure zone regarded by some authors as the Terceira rift (e.g. Udías, 1980). One of the oldest features of the island is expressed by the Cinco Picos caldera (7 km in diameter) that should be younger than 300.000 years (K/Ar data from Feraud et al., 1980). Previous studies have shown that Terceira lavas combine relatively high radiogenic lead isotopic signatures typical from HIMU basalts (206Pb/204Pb up to 20.02) and slight primitive He signatures, with 63780 as a minimum value of 4He/3He ratio (R/Ra=11.3; Dupré et al., 1982; Moreira et al., 1999). A lower value of 48821 (R/Ra=14.8) was now obtained, strengthening the contribution of lower mantle to Terceira magmas. Our results also suggest a stratigraphic control on the variation of He isotopic signatures since the most primitive values appear to be confined to the most ancient rocks. In fact, values lower than 68814 (R/Ra ratios higher than 10,5) were found in one lava collected near the village of Säo Sebastiäo (SE of Terceira, in one of the most eroded landscape of the island) and in samples from the lower levels of a 20 m thick lava flow sequence exposed in the actual sea cliff near the village of Santa Bárbara (SW of Terceira). At least for the later samples, we exclude the possibility of a cosmogenic 3He contribution since this sequence is exposed in a relatively recent erosion surface. Our results raise important questions concerning the evolution of the Açores plume; several interpretations seems to be plausible, as the changing with time of the source isotopic signature or the increase of mixing with a MORB. These hypothesis will be better constrained in future works. Some of the analysed samples show higher 20Ne/22Ne and 21Ne/22Ne ratios than air suggesting the presence of a mantle component in the olivine melt inclusions. Moreover, in a three-Ne isotope diagram, the samples define a trend with a higher slope than the MORB line (e.g. Sarda et al., 1988). The Terceira trend is then interpreted as a mixing line between an atmospheric component and a mantle component with a solar like 20Ne/22Ne ratio and with a 21Ne/22Ne ratio of 0.052 (extrapolated to solar). This value is in accord with the mantle mixing model predicted by the 4He/3He-21Ne/22Ne relationship (Moreira et al.,2001) and is also strongly suggestive of a lower mantle contribution to Terceira magmas