Melt supply and magmatic evolution at a large central MOR volcano located in the Lucky Strike segment, 37° N on the Mid-Atlantic Ridge, Azores region

Ferreira, Pedro Lopes (2006) Melt supply and magmatic evolution at a large central MOR volcano located in the Lucky Strike segment, 37° N on the Mid-Atlantic Ridge, Azores region. University of Southampton, Doctoral Thesis.

Record type: Thesis (Doctoral)

Abstract

Lucky Strike basalts have been subdivided into three distinct compositional groups: In Group 1, the basalts have the highest more-to-less incompatible element ratios (e.g. La/Sm, Nb/Zr, Ba/Y) and have the most Sr-Nd-Pb radiogenic values. They are spatially restricted to the central part of the axial volcano, are highly vesicular, and plagioclase phyric. These basalts have a degree of enrichment between those of E-MORB and OIB. Group 3 basalts have the lowest more-to-less incompatible element ratios and the lowest Sr-Nd-Pb radiogenic values. Group 2 basalts have chemical characteristics intermediate between those of Groups 1 and 3.

Taking into account the sample spatial distribution, the basalt textural characteristics, the bathymetric (shallow depths at segment centre), and the gravity data (negative “bull’s eyes” RMBA anomalies at segment midpoint), together with the geochemical data interpretation, a genetic volcanic model is generated. The enriched, more fertile, mantle heterogeneities (Group 1), related to Azores mantle plume material, in the Lucky Strike sub-oceanic mantle, are delivered to the centre of the segment by highly focused, sub-lithospheric processes. Forming wetter domains, they start to melt first, deeper in the mantle (in the garnet stability field), melting more, but generating lower melt fractions, than the ambient mantle (Group 3). Group 1 vesiculating melts, having their density reduced, will be aggregated in a high-level axial magma chamber (AMC). The limit of neutral buoyancy of these magmas is never reached, thus the magmas are delivered vertically to the segment centre. Further magmas, formed from melting of the ambient mantle, will be aggregated in the AMC, which is predicted to be of RTF type. Thus, initial mixing of Group 1 and 3 melts and, lastly, only differentiation of Group 3 melts will produce Group 2 and 3 basalts, respectively.

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