Abstract

COSTA, I. Ribeiro da  and  BARRIGA, F.J.A.S.. The upper mantle beneath the Mid-Atlantic Ridge in the Azores sector. Comunicações Geológicas [online]. 2010, n.97, pp.23-34. ISSN 1647-581X.

Low 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.

Keywords : Refractory harzburgitic oceanic mantle; thermo-barometry; Mid-Atlantic Ridge (MAR).

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