320,000 years of interaction between a fast-spreading ridge and nearby seamounts monitored using major, trace and isotope composition data from oceanic basalts: Zoom at 15.6°N on the East Pacific Rise

Basaltic samples were collected by the French submersible ``Nautile'' during the “Parisub” cruise (2010, R/V L'Atalante, Ifremer) along a 25 km long sampling profile crossing perpendicularly the current axis of the East Pacific Rise at 15.6°N, as well as the trace of its two former parallel axes located further east. The total length of the profile corresponds to an approximate time interval of ∼ 320,000 years. The corresponding dataset documents the geochemical response of Mid-Ocean Ridge Basalts (MORB) related to the progressive convergence between the ridge segment and a nearby hotspot. It also represents one of the highest-sampling (and in turn geochemical) resolution efforts to date. The major, trace element and isotopic compositions determined through optical and mass spectrometry analysis of 52 samples are presented and compared to other previous data obtained from the same area. The data obtained strictly follow the conventions used in rock geochemistry in terms of data acquisition, reduction, and format, so that they can be compared to similar data from other regions. The different figures present (i) The geological context of study area, (ii) A classification of the samples according to their geochemical composition and geological context for a better legibility of the dataset, (iii) A comparison with data from other oceanic rises, (iv) A detailed method explaining the foundations of the chronology between samples established, and (v) A chronological representation of the geochemical composition of the basalts collected. These data can be useful for anyone interested in marine geosciences and more specifically scientists studying mantle geochemistry, oceanic lithosphere formation, and hotspot-ridge interactions. These data can also be used to model magmatic processes, crust-mantle interactions, and can be integrated in geophysical and geological models of seafloor accretion.

A highly unradiogenic lead isotopic signature revealed by volcanic rocks from the East Pacific Rise

Radiogenic isotopes in oceanic basalts provide a window into the different geochemical components defining the composition of Earth's mantle. Here we report the discovery of a novel geochemical signature in volcanic glasses sampled at a sub-kilometre scale along the East Pacific Rise between 15°37'N and 15°47'N. The most striking aspect of this signature is its unradiogenic lead ((206)Pb/(204)Pb=17.49, (207)Pb/(204)Pb=15.46 and (208)Pb/(204)Pb=36.83). In conjunction with enriched Sr, Nd and Hf signatures, Pb isotopes depict mixing lines that trend away from any known mantle end-members. We suggest that this unradiogenic lead component sampled by magmatic melts corresponds to a novel upper mantle reservoir that should be considered in the Pb isotope budget of the bulk silicate Earth. Major, trace element and isotope compositions are suggestive of an ancient and lower continental origin for this unradiogenic lead component, possibly sulphide-bearing pyroxenites that were preserved even after prolonged stirring within the ambient upper mantle.

The influence of ridge migration on the magmatic segmentation of mid-ocean ridges

The Earth's mid-ocean ridges display systematic changes in depth and shape, which subdivide the ridges into discrete spreading segments bounded by transform faults and smaller non-transform offsets of the axis. These morphological changes have been attributed to spatial variations in the supply of magma from the mantle, although the origin of the variations is poorly understood. Here we show that magmatic segmentation of ridges with fast and intermediate spreading rates is directly related to the migration velocity of the spreading axis over the mantle. For over 9,500 km of mid-ocean ridge examined, leading ridge segments in the 'hotspot' reference frame coincide with the shallow magmatically robust segments across 86 per cent of all transform faults and 73 per cent of all second-order discontinuities. We attribute this relationship to asymmetric mantle upwelling and melt production due to ridge migration, with focusing of melt towards ridge segments across discontinuities. The model is consistent with variations in crustal structure across discontinuities of the East Pacific Rise, and may explain variations in depth of melting and the distribution of enriched lavas.