Crustal isostasy indicates anomalous densities beneath Iceland

Recent seismological determinations of crustal thickness in Iceland, made m ainly through identification of wide-angle PmP and SmS reflections, indicat e that the depth to the Moho deepens from 11 km beneath the low-lying Reykj anes Penninsula to 39 km beneath the central highlands. By compiling all av ailable crustal thickness measurements, we show that this trend holds gener ally for Iceland, with elevation (above m.s.l.) increasing at a rate of 35. 54 +/- 6.4 (1 sigma) meters for each kilometer of Moho depth (below m.s.l.) . If one assumes that the topography and Moho are in local isostatic equili brium with thickness changes in a lower crustal layer, while an upper crust maintains a constant thickness, with the part above sea level having a den sity of of 2500 kg/m(3), then the rate of increase of depth with topographi c elevation implies a density jump of 89 +/- 12 kg/m(3) across the Moho. Pe trological and seismological evidence indicate that the lower-crustal densi ties probably do not exceed about 3060 +/- 50 kg/m(3). Mantle densities mus t therefore be low, 3150 +/- 60 kg/m(3). The combined effects of chemical d epletion, thermal expansion and melt entrainment in the mantle can acount f or some, but not all, of the anomaly.