Genesis of the Iceland melt anomaly by plate tectonic processes

Abstract

Iceland is the best studied, large-volume, active volcanic region in the world. It features the largest subaerial exposure of any hotspot at a spreading ridge, and it is conventionally attributed to a thermal plume in the mantle. However, whereas the apparently large melt productivity and low-wavespeed mantle seismic anomaly are consistent with this attribution, at any more detailed level, the observations are poorly predicted by the plume hypothesis. There is no time-progressive volcanic track, the melt anomaly having been persistently centered on the Mid-Atlantic Ridge. Spatial variations in crustal structure are inconsistent with the southeastward migration that is required of a plume fixed with respect to other Indo-Atlantic hotspots. The mantle seismic anomaly weakens with depth and does not extend into the lower mantle. Estimates of excess temperature using a broad range of methods are inconsistent with a mantle potential temperature anomaly greater than a few tens of K. Much of the lava erupted in Iceland has geochemistry little different from normal mid-ocean ridge basalt, and the detailed spatial geochemical pattern bears little resemblance to what is predicted for a plume beneath central Iceland. We propose an alternative model in an attempt to explain the observations at Iceland with fewer difficulties. Our model involves only shallow plate tectonic processes and attributes the large melt volume to the remelting of subducted oceanic crust trapped in the Caledonian suture in the form of eclogite or mantle peridotite fertilized by resorbed eclogite. Delaminated continental mantle lithosphere may also be involved. Such a source can produce several times more melt than pure peridotite without the need for high temperatures. The longevity of anomalous volcanism at the Mid-Atlantic Ridge at the latitude of Iceland is attributed to its location on a Caledonian structure that runs transversely across the north Atlantic. Many aspects of the geochemistry of Icelandic lavas fit this model, which also provides an explanation for the high maximum helium isotope ratios observed there. The "depleted plume component" may be derived from abyssal olivine gabbro cumulates and the "enriched plume component" from recycled enriched material that forms part of the crustal section of subducted slabs. Such a model for the Iceland melting anomaly raises new questions concerning how much thermal energy can be generated by isentropic upwelling of eclogite at a ridge, the location of the homogenizing 595 *on June 18, 2015 specialpapers.gsapubs.org Downloaded from reservoirs required, and the mechanism by which fertile material is incorporated into the asthenosphere beneath new oceans. Most fundamentally, if validated, such a model can explain the generic observations associated with hotspots as shallow processes associated with plate tectonics, and thus raises the question of whether thermal plumes are required in general in the Earth.

Ambient and excess mantle temperatures, olivine thermometry, and active vs. passive upwelling

“…Opponents of the mantle plume hypothesis thus contend that T ex (= T p hot spot − T p ambient ) is small, on the order of tens of degrees C (Anderson, 1998;Foulger et al, 2005;Green and Falloon, 2005), or in the case of shear heating, possibly as high as 120°C (Doglioni et al, 2005). But even when mantle compositions are well constrained, seismic estimates of temperature are uncertain.…”

“…Anti-plume arguments lead to two significant implications: either (1) thermally buoyant upwelling currents do not exist or (2) such thermal currents have no impact on terrestrial volcanism. Alternative models for intra-plate volcanism include: lithospheric delamination (Foulger et al, 2005), small-scale passive mantle upwelling and lithosphere rifting (Christiansen et al, 2002), focusing of magma along lithosphere-scale cracks (Shaw and Jackson, 1973;Jackson et al, 1975), or compositional anomalies that depress the mantle solidus and promote partial melting (Bonatti, 1990;Green et al, 2001;Presnall and Gudfinnsson, 2005). In contrast to the thermal plume model, these models do not require a source of excess heat.…”

Are ‘hot spots’ hot spots?

“…Temperature-anomaly estimates for the Iceland region. Bars correspond to temperature ranges and stars to individual estimates [from Foulger et al, 2005a].…”

Formation and evolution of Shatsky Rise oceanic plateau: Insights from IODP Expedition 324 and recent geophysical cruises

“…An example of anomalous melt production in a similar setting is ridge-centered Iceland, which is proposed to originate from the re-melting of relatively young subducted oceanic crust that was trapped in the shallow upper mantle and eventually tapped by the melt extraction zone of the westward-migrating mid Atlantic ridge (Foulger et al, 2005b(Foulger et al, , 2005c. Oceanic crust, transformed to eclogite during subduction, can produce several times more melt during adiabatic decompression melting than regular upper mantle peridotite without requiring higher melting temperatures (e.g.…”