An uplifted, relatively undeformed sliver of oceanic lithospheric (Vema Lithospheric Section or VLS), exposing a >26 Myr record of lithosphere generation at a segment of the Mid Atlantic Ridge (EMAR segment, 10°-11°N, central Atlantic), provides the ideal setting to tackle the problem of temporal variations in the processes of creation of oceanic lithosphere at a slow-spreading ridge. A first study, that combined the temporal variations of the mineral chemistry of mantle peridotites sampled along the VLS and gravity measured along a flow line starting from the center of the EMAR segment, detected a steady increase of crustal thickness and of mantle degree of melting from ~20 Ma to 4 Ma before present. Additional sampling was carried out in 2005 along the VLS, extending the coverage of mantle peridotites from a lithospheric age of 26 Ma to 2 Ma. The 26 Ma to 18.5 Ma interval shows short wavelength (~4 Myr) variations of the mantle degree of melting, superimposed on a weak trend of decreasing degree of melting. In the 18.5 to 2 Ma interval a steady increase of degree of melting was observed, with superimposed 3-4 Myr oscillations. Temporal variations of RMBA and crustal thickness inferred from gravity data reveal similar trends. The older (26 to 18.5 Ma) and the younger (18.5 to 2Ma) peridotite suites differ in clinopyroxene CaO/Al2O3 ratio, suggesting that not only the thermal regime, but also the composition of the mantle source might be different in the two suites. Peridotite two-pyroxene geothermometry shows a decreasing trend of the calculated equilibration temperature from 26 Ma to 18.5 Ma ago, followed by a steady increase from 16 Ma to 2 Ma ago, suggesting that the solid mantle upwelling speed varied through time. A ~2 Myr long interval with strongly deformed ultramafic mylonites as the dominant rock type lies between the two opposite trends, as if marking a change in mantle thermal regime. It may reflect a thermal minimum in the subridge upwelling mantle, with nearly a-magmatic emplacement of the lithosphere resulting in strong deformation of the mantle rocks. When upwelling of hot and/or fertile mantle resumes a new cycle starts with injection of igneous crust.

26 Million Years of mantle upwelling below a segment of the Mid Atlantic Ridge: the Vema Lithosperic Section revisited / Cipriani, Anna; Bonatti, E.; Brunelli, Daniele; Ligi, M.. - In: EOS. - ISSN 0096-3941. - ELETTRONICO. - 87 (52):(2006), pp. Abstract V23E-0684-Abstract V23E-0684. ((Intervento presentato al convegno AGU Fall Meeting tenutosi a S. Francisco CA, USA nel Dec. 2006.

26 Million Years of mantle upwelling below a segment of the Mid Atlantic Ridge: the Vema Lithosperic Section revisited

CIPRIANI, Anna;BRUNELLI, Daniele;
2006

Abstract

An uplifted, relatively undeformed sliver of oceanic lithospheric (Vema Lithospheric Section or VLS), exposing a >26 Myr record of lithosphere generation at a segment of the Mid Atlantic Ridge (EMAR segment, 10°-11°N, central Atlantic), provides the ideal setting to tackle the problem of temporal variations in the processes of creation of oceanic lithosphere at a slow-spreading ridge. A first study, that combined the temporal variations of the mineral chemistry of mantle peridotites sampled along the VLS and gravity measured along a flow line starting from the center of the EMAR segment, detected a steady increase of crustal thickness and of mantle degree of melting from ~20 Ma to 4 Ma before present. Additional sampling was carried out in 2005 along the VLS, extending the coverage of mantle peridotites from a lithospheric age of 26 Ma to 2 Ma. The 26 Ma to 18.5 Ma interval shows short wavelength (~4 Myr) variations of the mantle degree of melting, superimposed on a weak trend of decreasing degree of melting. In the 18.5 to 2 Ma interval a steady increase of degree of melting was observed, with superimposed 3-4 Myr oscillations. Temporal variations of RMBA and crustal thickness inferred from gravity data reveal similar trends. The older (26 to 18.5 Ma) and the younger (18.5 to 2Ma) peridotite suites differ in clinopyroxene CaO/Al2O3 ratio, suggesting that not only the thermal regime, but also the composition of the mantle source might be different in the two suites. Peridotite two-pyroxene geothermometry shows a decreasing trend of the calculated equilibration temperature from 26 Ma to 18.5 Ma ago, followed by a steady increase from 16 Ma to 2 Ma ago, suggesting that the solid mantle upwelling speed varied through time. A ~2 Myr long interval with strongly deformed ultramafic mylonites as the dominant rock type lies between the two opposite trends, as if marking a change in mantle thermal regime. It may reflect a thermal minimum in the subridge upwelling mantle, with nearly a-magmatic emplacement of the lithosphere resulting in strong deformation of the mantle rocks. When upwelling of hot and/or fertile mantle resumes a new cycle starts with injection of igneous crust.
EOS
87 (52)
Abstract V23E-0684
Abstract V23E-0684
Cipriani, Anna; Bonatti, E.; Brunelli, Daniele; Ligi, M.
26 Million Years of mantle upwelling below a segment of the Mid Atlantic Ridge: the Vema Lithosperic Section revisited / Cipriani, Anna; Bonatti, E.; Brunelli, Daniele; Ligi, M.. - In: EOS. - ISSN 0096-3941. - ELETTRONICO. - 87 (52):(2006), pp. Abstract V23E-0684-Abstract V23E-0684. ((Intervento presentato al convegno AGU Fall Meeting tenutosi a S. Francisco CA, USA nel Dec. 2006.
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Utilizza questo identificativo per citare o creare un link a questo documento: http://hdl.handle.net/11380/855903
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