Studies of MORB and of MORP (Mid Ocean Ridge Peridotite) show that the oceanic upper mantle is heterogeneous. Long and short wavelength chemical variability has been reported from the Mid Atlantic Ridge. The subridge mantle degree of melting, estimated from MORP mineral chemistry of mantle equilibrated spinel, opx and cpx, as well as from MORB glasses, decreases from the Azores swell at ~40ο N to the equator, in parallel to the decrease of crustal thickness inferred from the near zero-age Mantle Bouguer anomaly. The temporal evolution of this portion of a slow-spreading mid ocean ridge can be studied along seafloor spreading flowlines normal to ridge segments. A lithospheric section exposed between 10ο and 11ο N, south of the Vema Fracture Zone is giving us the opportunity to study how generation of lithosphere at a 80-km long ridge segment evolved since 25 Ma. Gravity data, and MORP-MORB chemistry correlations suggest a steady increase in crustal thickness and in mantle degree of melting from 20 Ma to Present, with superimposed 3-4 Ma oscillations. Variations of degree of melting, peridotite temperature of equilibration and crustal thickness are not proportional to spreading rate, suggesting variations in mantle source composition and a non-purely passive lithosphere formation at ridge axis. The combination of the zero-age axial ridge trend with the 20 Ma to Present trend could be explained by a subaxial hot/fertile mantle flow from the Azores swell toward an equatorial “cold” belt. The equatorial belt, characterized by long offset transforms, is magma starved and nearly free of basaltic crust. Small quantities of basalt, generated mostly in the garnet stability region by low degrees of partial melting, could not be expelled from the mantle, and froze to form a “constipated”, crust free lithosphere. Numerical calculations show a strong decrease of crustal production as a ridge approaches a transform, proportional to slip rate and offset length. When the amount of melt produced is small, the capability of melt to aggregate and to mix on the way to the surface is low; therefore, the chemical and isotopic signatures of preaggregated melts can be detected. Therefore, long-offset RTI's are potential areas to detect short-scale mantle heterogeneities through the study of MORB. Accordingly, basalts sampled in the equatorial Atlantic close to the Romanche transform, show an increase in trace element variability within a single dredge haul approaching the RTI, with wavelengths comparable to those observed in suites of melt inclusions. The correlation between MORB trace elem

Present day versus Temporal Heterogeneity of the Subridge Mantle in the Central Atlantic / M., Ligi; E., Bonatti; Brunelli, Daniele; Cipriani, Anna. - In: EOS. - ISSN 0096-3941. - STAMPA. - 90:(2009), pp. 1-1. ((Intervento presentato al convegno AGU Fall Meeting tenutosi a S. Francisco - CA nel 14/12/2009.

Present day versus Temporal Heterogeneity of the Subridge Mantle in the Central Atlantic

BRUNELLI, Daniele;CIPRIANI, Anna
2009

Abstract

Studies of MORB and of MORP (Mid Ocean Ridge Peridotite) show that the oceanic upper mantle is heterogeneous. Long and short wavelength chemical variability has been reported from the Mid Atlantic Ridge. The subridge mantle degree of melting, estimated from MORP mineral chemistry of mantle equilibrated spinel, opx and cpx, as well as from MORB glasses, decreases from the Azores swell at ~40ο N to the equator, in parallel to the decrease of crustal thickness inferred from the near zero-age Mantle Bouguer anomaly. The temporal evolution of this portion of a slow-spreading mid ocean ridge can be studied along seafloor spreading flowlines normal to ridge segments. A lithospheric section exposed between 10ο and 11ο N, south of the Vema Fracture Zone is giving us the opportunity to study how generation of lithosphere at a 80-km long ridge segment evolved since 25 Ma. Gravity data, and MORP-MORB chemistry correlations suggest a steady increase in crustal thickness and in mantle degree of melting from 20 Ma to Present, with superimposed 3-4 Ma oscillations. Variations of degree of melting, peridotite temperature of equilibration and crustal thickness are not proportional to spreading rate, suggesting variations in mantle source composition and a non-purely passive lithosphere formation at ridge axis. The combination of the zero-age axial ridge trend with the 20 Ma to Present trend could be explained by a subaxial hot/fertile mantle flow from the Azores swell toward an equatorial “cold” belt. The equatorial belt, characterized by long offset transforms, is magma starved and nearly free of basaltic crust. Small quantities of basalt, generated mostly in the garnet stability region by low degrees of partial melting, could not be expelled from the mantle, and froze to form a “constipated”, crust free lithosphere. Numerical calculations show a strong decrease of crustal production as a ridge approaches a transform, proportional to slip rate and offset length. When the amount of melt produced is small, the capability of melt to aggregate and to mix on the way to the surface is low; therefore, the chemical and isotopic signatures of preaggregated melts can be detected. Therefore, long-offset RTI's are potential areas to detect short-scale mantle heterogeneities through the study of MORB. Accordingly, basalts sampled in the equatorial Atlantic close to the Romanche transform, show an increase in trace element variability within a single dredge haul approaching the RTI, with wavelengths comparable to those observed in suites of melt inclusions. The correlation between MORB trace elem
EOS
90
1
1
M., Ligi; E., Bonatti; Brunelli, Daniele; Cipriani, Anna
Present day versus Temporal Heterogeneity of the Subridge Mantle in the Central Atlantic / M., Ligi; E., Bonatti; Brunelli, Daniele; Cipriani, Anna. - In: EOS. - ISSN 0096-3941. - STAMPA. - 90:(2009), pp. 1-1. ((Intervento presentato al convegno AGU Fall Meeting tenutosi a S. Francisco - CA nel 14/12/2009.
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Utilizza questo identificativo per citare o creare un link a questo documento: http://hdl.handle.net/11380/644644
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