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Mourao C.,University of Lisbon | Moreira M.,CNRS Paris Institute of Global Physics | Mata J.,University of Lisbon | Raquin A.,CNRS Paris Institute of Global Physics | And 3 more authors.
Contributions to Mineralogy and Petrology | Year: 2012

We present and discuss noble gas compositions of minerals from silicate rocks (olivines) and carbonatites (apatites and calcites) from Brava Island. The presence of an almost ubiquitous atmosphere-derived fingerprint is explained as reflecting contamination by seawater. Because of the high U and Th content in apatites, which are responsible for 4He production by α-decay, the high measured 4He/ 3He ratios do not represent magmatic signatures. In contrast, low values of 4He/ 3He in calcites (≥61,223; R/R a ≤ 11.80) and olivines (≥56,240; R/R a ≤ 12. 85) are considered to be representative of signatures trapped at the time of crystallization, given that there are no evidences for significant cosmogenic additions. These relatively low 4He/ 3He ratios depicted by silicate and carbonatite rocks imply the contribution of a reservoir that evolved under low (U + Th)/ 3He; this is considered a strong evidence for the genesis of Brava by a mantle plume deeply anchored in the lower mantle. The inferred low 4He/ 40Ar* ratio (≈0.3), before degassing, is thought to reflect the contribution to the carbonatites source of a mantle domain evolving under high K/U, which cannot be explained by recycling of crustal components. The possible link between the low 4He/ 40Ar* source and the "missing Ar reservoir" is discussed. The usually referred geochemical dichotomy between Northern and Southern Cape Verde islands, which is markedly evident from Sr, Nd, and Pb isotope signatures, is not apparent from Brava Island (Southern Cape Verde), where some samples present relatively unradiogenic 4He/ 3He signatures, similar to those reported for the Northern islands of the archipelago. © 2011 Springer-Verlag.

Mourao C.,University of Lisbon | Mata J.,University of Lisbon | Doucelance R.,CNRS Magmas and Volcanoes Laboratory | Madeira J.,University of Lisbon | And 8 more authors.
Journal of African Earth Sciences | Year: 2010

The Cape Verde volcanic archipelago, located in the oceanic portion of the African plate some 500 km west of the Senegal coast, is renowned for the occurrence of carbonatites on at least 5 of its 10 islands. In this study we report the occurrence of about twenty new small outcrops of extrusive carbonatites on Brava Island (64 km2), the south-westernmost island of the archipelago. These new occurrences are studied from geological, petrographic, mineral chemistry and whole rock (elemental and isotopic) geochemical points of view, allowing for a discussion of their petrogenesis and emphasising their geological and geochemical peculiarities in the context of the Cape Verde carbonatites. Most of the extrusive carbonatitic formations correspond to pyroclastic rocks, comprising magmatic and/or phreatomagmatic ash and lapilli fall deposits and one probable pyroclastic flow. Lava flows occur at one locality. The predominance of pyroclastic facies demonstrates the significant explosivity of these magmas characterised by very low viscosity. Independent of the modes of emplacement, all samples are calciocarbonatites and exhibit a remarkable compositional uniformity, considering that they represent several different eruptions and present a wide geographical dispersion. Brava extrusive carbonatites belong to the younger (probable Holocene - Pleistocene) volcanic sequence of the island. This feature is unique in the context of Cape Verde geology, because in the other islands (including Brava) of the archipelago carbonatites are commonly assigned to the basal complexes, having formed during a fairly early stage of the emerged evolution of volcanic construction. Compared with the older intrusive sövitic rocks occurring at Middle Unit of Brava Island, extrusive facies are more iron and manganese rich and yield higher contents of trace elements like Ba, Th, U, Nb, Pb and REE, but somewhat lower Sr abundances. New initial Sr and Nd isotope data (0.703557-0.703595 and 0.512792-0.512816, respectively) determined in extrusive calciocarbonatitic rocks are clearly different from those obtained in intrusive rocks (0.703340-0.703356 and 0.512910-0.512912, respectively), which demonstrates that the studied rocks were ultimately the product of a source distinct from those that produced the older intrusive carbonatites. Brava extrusive carbonatites yield isotope signatures that are clearly distinct from all other Cape Verde carbonatites, but akin to the Southern Cape Verde silicate rocks. We propose that the extrusive carbonatites resulted from an immiscibility process that also produced conjugate melts of nephelinitic composition. © 2009 Elsevier Ltd. All rights reserved.

Mourao C.,University of Lisbon | Mata J.,University of Lisbon | Doucelance R.,CNRS Magmas and Volcanoes Laboratory | Madeira J.,University of Lisbon | And 5 more authors.
Chemical Geology | Year: 2013

Elemental and isotopic (Sr, Nd and Pb) analyses of silicate and carbonatite rocks from Brava, the southernmost island of the Cape Verde archipelago, are reported here to assess the nature, origin and location of the main contributors to magma composition and to evaluate the chemical evolution of magma through time. In contrast to other islands of the archipelago where a continuous range is usually observed, Brava is clearly characterized by two groups of distinct isotopic compositions. Rocks from the Upper Unit (<. 0.5. Ma) are less Nd- and Pb-radiogenic and more Sr-radiogenic than rocks belonging to the Basal Complex (~. 3 to 1.4. Ma), which are also characterized by more unradiogenic He signatures. Here, the chemistry of the Basal Complex is mainly explained by the mixing of a high-μ (HIMU)-type local end-member, which represents an ancient (~. 1.3. Ga) recycled oceanic crust, with the lower mantle. For the Upper Unit, elemental and isotopic signatures suggest the involvement of an additional local end-member akin to the EM1-type. Such differences in the contributing end-members to the Upper Unit and Basal Complex sources mirror those usually assigned to the southern and northern Cape Verde islands, respectively. This temporal evolution is discussed in light of plume interaction with a shallow EM1-type domain, which is considered to represent the subcontinental lithosphere floating in the asthenosphere. Brava carbonatites define two distinct groups with similar isotopic ranges as those observed for the coeval alkaline silicate rocks. This observation suggests that, in each of the volcano-stratigraphic units, carbonatite and silicate magmas are ultimately derived from the same sources. We propose that calciocarbonatites from either the Basal Complex or the Upper Unit resulted from nephelinite-carbonatite liquid immiscibility, while the subordinate Basal Complex magnesiocarbonatites represent residual liquids after calcite fractionation from carbonatite magma. © 2012 Elsevier B.V.

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