Abstract
The North Atlantic V-Shaped Ridges (VSRs) provide a spatially extensive and clear record of unsteady mantle convective circulation over >40 My. VSRs are diachronous ridges of thick crust formed with a periodicity of ∼5 My along the Mid Atlantic Ridge, south of Iceland. We present data from a set of dredged basalt samples that shows chemical variation associated with two complete VSR crustal thickness cycles where they intersect the Mid Atlantic Ridge. The new dataset also records chemical variation associated with a VSR crustal thickness cycle along a plate spreading flow-line. Inverse correlations between crustal thickness and both incompatible trace element concentrations and incompatible element ratios such as Nb/Y and La/Sm are observed. Geochemical and crustal thickness observations can be matched using a time-dependent mid-ocean ridge melting model with a basal boundary condition of sinusoidally varying potential temperature. Our observations and models suggest that VSRs are generated when hot patches are carried up the plume stem beneath SE Iceland and spread radially outward within the asthenosphere. These patches are then drawn upward into the melting region when passing beneath the Mid Atlantic Ridge. The geometry of the VSRs and the size of the dynamically supported swell suggest that the Iceland Plume is the strongest plume in the Earth at present, with a volume flux of 49±14 km3yr−1.
Highlights
Convection within the upper mantle is expected to be timedependent because the Rayleigh number is super-critical by 3 to 5 orders of magnitude (Schubert et al, 2001)
We have developed a model of time-dependent mid-ocean ridge melting to calculate volumes and compositions of erupted melts, in order to test the hypothesis that temperature of the mantle source is the principal control on V-Shaped Ridges (VSRs) generation
Along the Reykjanes (Mid Atlantic) Ridge south of Iceland, spatial variations in incompatible trace element concentrations and ratios correlate with V-Shaped Ridge (VSR) records from bathymetry and gravity
Summary
Convection within the upper mantle is expected to be timedependent because the Rayleigh number is super-critical by 3 to 5 orders of magnitude (Schubert et al, 2001). The North Atlantic V-Shaped Ridges (VSRs) provide a long period, spatially extensive and clear record of the plume pulsing type of unsteady mantle convection over time periods of order 1 to 10 million yr (Vogt, 1971; Jones et al, 2002b). VSRs are diachronous ridges of thick crust formed at the Mid Atlantic Ridge to the north and south of Iceland (Fig. 1) Since their discovery, it has been generally agreed that the diachronous geometry results from melting anomalies that propagate away from Iceland within the asthenosphere (Vogt, 1971; Jones et al, 2002b; Poore et al, 2011). The process that generates VSRs appears to modulate Atlantic oceanic circulation, since the VSR record correlates with stable isotope proxies for meridional overturning circulation (Wright and Miller, 1996; Poore et al, 2006)
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