COSTA, I. Ribeiro da    BARRIGA, F.J.A.S.. O manto superior subjacente à Crista Média Atlântica no sector dos Açores. []. , 97, pp.23-34. ISSN 1647-581X.

^lpt^aOs serpentinitos aflorantes em zonas de falhas não-transformantes no sector Açoreano da Crista Média Atlântica apresentam aspectos texturais, mineralógicos e cristaloquímicos que permitiram caracterizar o manto superior subjacente a este sector e confirmar a influência do hotspot dos Açores nessa região. O estudo de manto superior serpentinizado aflorante nas áreas Rainbow e Saldanha, neste sector da Crista Média Atlântica, pôs em evidência esses aspectos: a abundância de texturas pseudomórficas e a mineralogia relíquia (Fo_olivina = 87-92, En_piroxena = 89-92, Cr#_espinela = 46-52) indicam um protólito refractário correspondente a um harzburgito com espinela, pontualmente associado a dunitos e harzburgitos tectonizados com anfíbola. O carácter refractário destas rochas ultramáficas e o metassomatismo anfibolítico, coadjuvados pelos dados termobarométricos obtidos por RIBEIRO DA COSTA et al. (2006), são consistentes com fusão parcial significativa do Manto Superior e com actividade metassomática intensificada nas proximidades do hotspot dos Açores, tal como sugerem estudos anteriores neste sector da Crista Média Atlântica.^len^aLow magma flow and significant crustal-scale extensional tectonics characterize the Mid-Atlantic Ridge (MAR), favouring seawater circulation down to the upper-mantle and fault-related exposures of serpentinized upper-mantle peridotite (e.g., FRANCIS, 1981; CANNAT, 1993; TUCHOLKE & LIN, 1994; ESCARTÍN & CANNAT, 1999). Samples of such serpentinized ultramafic rocks were collected on two locations of the Azores sector of the MAR: the Rainbow hydrothermal field (36°14'N), and the Saldanha massif (36°34'N). Textural, mineralogical and crystalchemical features of such serpentinite outcrops from non-transform offsets in the Azores area of the Mid-Atlantic Ridge (MAR) have provided enough evidence to identify their protolith and to characterize the upper mantle beneath this oceanic sector, confirming the relevance of the Azores hotspot on the nature of this mantle sector. Serpentinized upper mantle exposures on the Rainbow and Saldanha hydrothermal fields, located in the Azores sector of the Mid-Atlantic Ridge (MAR), exhibit pseudomorphic textures and relict mineralogy (Fo_olivine = 87-92, En_pyroxene = 89-92, Cr#_spinel = 46-52) indicative of a dominant refractory spinel-harzburgite protolith, occasionally accompanied by minor dunites and strongly tectonized amphibole-bearing harzburgites. The refractory nature of these ultramafics and modal amphibole metasomatism are consistent with a significant extent of upper mantle partial melting and metasomatic activity on approaching the Azores hotspot, as suggested by previous studies in the area (e.g., MICHAEL & BONATTI, 1985; BONATTI & MICHAEL, 1989; JUTEAU et al., 1990; MÉVEL et al., 1991). Moreover, mantle oxygen fugacities estimated by oxygen barometry in the relict mineralogy of the Rainbow and Saldanha rocks, considering likely harzburgite depths for mantlemelt segregation temperatures of 1200-1330°C (RIBEIRO DA COSTA et al., 2006), also match the data and interpretations of those authors for the upper-mantle beneath the Azores sector of the MAR.

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