Control of the symmetry of plume‐ridge interaction by spreading ridge geometry

Abstract

[1] The Iceland, Galápagos, and Azores plumes have previously been identified as interacting asymmetrically with adjacent spreading centers. We present evidence that the flow fields in these plume heads are radially symmetric, but the geometry of the mid-ocean ridge systems imparts an asymmetric compositional structure on outflowing plume material. First, we quantify the degree of symmetry in geophysical and geochemical observables as a function of plume center location. For each plume, we find that bathymetry and crustal thickness observations can be explained using a single center of symmetry, with these calculated centers coinciding with independently inferred plume center locations. The existence of these centers of symmetry suggests that the flow fields and temperature structure of the three plume heads are radially symmetric. However, no centers of symmetry can be found for the incompatible trace element and isotopic observations. To explain this, we develop a simple kinematic model to predict the effect of mid-ocean ridge geometry on the chemical composition of outflowing plume material. The model assumes radially symmetric outflow from a compositionally heterogeneous plume source, consisting of a depleted mantle component and enriched blebs. These blebs progressively melt out during flow through the melting regions under spreading centers. Asymmetry in trace element and isotopic profiles develops when ridges on either side of the plume center receive material that has been variably depleted according to the length of flow path under the ridge. This model can successfully explain compositional asymmetry around Iceland and Galápagos in terms of an axisymmetric plume interacting with an asymmetric ridge system.Components: 15,700 words, 13 figures, 1 table.

Constraints on the structure of the crust and lithosphere beneath the Azores Islands from teleseismic receiver functions

“…small-scale convection) that can alter the thickness of the lithosphere over short scale lengths. One such process could be mantle upwelling associated with a mantle plume centred beneath the islands of Faial, Pico, and São Jorge, as has been proposed by Shorttle et al (2010). A mantle plume rising beneath the Central Islands could erode the lithosphere locally, which could explain a steep gradient in lithospheric thickness.…”

The influence of source heterogeneity on the U–Th–Pa–Ra disequilibria in post-glacial tholeiites from Iceland

“…6), with the exception for melting in the spinel stability field (applying the K D values determined by McDade et al (2003a), see also Stracke et al (2006)). Beneath Iceland's rift zones both active and passive mantle upwelling influence the mantle flow regime (Olson et al, 1993;Ribe et al, 1995;Ito et al, 1996;Maclennan et al, 2001;Shorttle et al, 2011). High mantle temperatures near the plume axis result in fast buoyancy-driven mantle upwelling, whereas further away, at lower mantle temperature, the upwelling is slower.…”

A 1.5 Ma record of plume-ridge interaction at the Western Galápagos Spreading Center (91°40′–92°00′W)

“…It is a matter of ongoing discussion, whether the decreasing plume signal reflects radial outflow of plume material along the base of the lithosphere into the spreading center at various locations (Schilling et al, 2003;Shorttle et al, 2010). Alternatively, it could reflect input of plume material into the GSC west and east of the 91°W Transform Fault followed by lateral flow and dilution of plume material as it flows beneath the ridge axis to the west and east, i.e.…”

“…These models are broadly divided into 1) transport to the GSC along channels within the base of the lithosphere (e.g., Morgan, 1978;Schilling et al, 1982;Verma and Schilling, 1982;Braun and Sohn, 2003); 2) deflection of the Galápagos plume head primarily eastwards due to eastward migration of the Nazca Plate but with some of the material in the north reaching the ridge axis (Richards and Griffiths, 1989;Geist, 1992;White et al, 1993;Harpp and White, 2001); 3) gravity driven plume dispersal along the base of the lithosphere (Bercovici and Lin, 1996;Hoernle et al, 2000); 4) radial outflow of plume material away from its stem along the base of the lithosphere to the spreading center (Schilling et al, 2003;Shorttle et al, 2010); 5) subsolidus transport of plume material beneath a viscous residual plug to the ridge (Kokfelt et al, 2005;Ito and Bianco, 2014;Villagomez et al, 2014;Byrnes et al, 2015); and 6) melt transport via veins and channels below the anhydrous peridotite solidus (Gibson et al, 2015).…”