Revisiting the exhumed mantle at the Iberia margin to get new insight about break-up processes

Previous studies from the Western Iberia magma-poor rifted margin enabled to describe the evolution of the mantle lithosphere during rifting and breakup based on the study of dredged and drilled magmatic and mantle samples. These data together with those from the present-day Australia-Antarctica and the fossil Alpine Tethys rifted margins and Pyrenean hyperextended basins provide insights about the role of the mantle processes and inheritance on the tectono-magmatic evolution of rift systems during rifting and breakup. However, key questions remain in understanding lithospheric breakup such as when, where, and how much magma is produced during breakup; how first magma interacts with the percolated subcontinental mantle and how these mantle-melt processes interrelate with the extensional processes operating during breakup.

This study focuses on samples drilled during ODP Legs 103, 149, 173 and 210 from the conjugate Iberia-Newfoundland margins and included also previously little studied dive-recovered samples from the Galicia Bank (Galinaute I and II). Bulk-rock, in-situ chemical and isotopic analysis of ultramafic rocks are used to constrain mantle dynamics during final rifting and breakup along the southern North Atlantic margins. Major and trace-element concentrations of primary minerals like olivine, pyroxenes and spinel are used to distinguish between different mantle domains, i.e., depleted oceanic or refertilized and/or inhertited subcontinental mantle. Thermo-barometry calculations are applied to define rates and thermal conditions during mantle exhumation.

Preliminary results from textural observations and geochemical data from Galinaute ultramafic rocks show two mantle types: subcontinental and refertilized mantle (T1/T2 mantle types). Indeed, plagioclase texture in corona around spinel together with spinel compositions are consistent with lherzolite formation by sub-solidus re-equilibration, similar to those of subcontinental mantle exposed in the Alps (Tasna and Malenco). However, some clinopyroxene compositions show evidence of low pressure mantle-melt interaction, which may indicate a refertilization process by ascending MORB-type melts. Diffusion modeling of sub-solidus major element and REE re-equilibration between OPX and CPX from Galinaute peridotites show that the exhumed mantle along the Galicia Bank cooled at rates between 10^-6 and 10^-4°C/yr across the sp-pl peridotite facies transition, slower than cooling rates determined for samples from the Alpine Tethys and the present-day Australia-Antarctica magma-poor rifted margins.