The Melt Anomaly at the Iceland Volcanic Province can be Explained by Mantle Fertility in the Caledonian Suture

1G.R. Foulger, 2D.L. Anderson& 3J.H. Natland

1University of Durham, Science Laboratories, South Rd., Durham, DH1 3LE U.K.,

2California Institute of Technology, Seismological Laboratory MC 252-21, Pasadena, CA 91125 USA,

3RSMAS/MGG University of Miami, 4600 Rickenbacker Causeway, Miami, FL 33149,

The Iceland volcanic province may be explained by a mantle fertility anomaly associated with oceanic crust in the Caledonian suture and shallow mantle, in the midst of the broad, warm mantle region that occupies the entire north Atlantic. Contrasting styles of spreading to the north and south of the Caledonian suture, over which Iceland presently lies, have furthermore given rise to complex, unstable, leaky-microplate tectonics which encourage magmatism by providing pathways along which melt may rise. The moderate, broad, regional temperature high of the north Atlantic may be a result of earlier continental insulation. The melting of a source containing a few tens of percent of old oceanic crust in the Caledonian suture, mixed with normal mantle, can explain both the quantity of melt at Iceland and the geochemistry, including REE patterns, major elements and isotope ratios. The Caledonian suture is orientated easterly at this latitude, parallel to the direction of plate motion, and this is consistent with the constant, high rate of magmatism at this part of the MAR for the last ~ 54 Myr. The chronic local tectonic disequilibrium in this region has featured the persistent, southward-migration of a pair of parallel, spreading axes, local variations in spreading direction and extension across easterly orientated shear zones. Such complex, transient tectonics may both encourage, and be a consequence of, high magmatism, thus comprising a positive feedback system. This hypothesis requires fewer forced explanations and contradictions than a plume model, which cannot explain many first-order observations from the Iceland region without special pleading. These include the absence of evidence for high, plume-like temperatures, the presence of a prominent volcanic zone oriented parallel to the spreading direction in Iceland, the fixity of the melt extraction anomaly over the mid-Atlantic ridge, the absence of the predicted hotspot track, the pattern of crustal thickness and the confinement of seismic velocity anomalies to the upper mantle.