Crustal accretion under northern Iceland

Successful models of oceanic crustal accretion should be consistent with bo th geochemical and geophysical observations from active mid-ocean ridges as well as those from ophiolite sections. The major element concentrations of a set of basalt and picrite samples from the Krafla and Theistareykir volc anic systems of the Northern Volcanic Zone of Iceland show two distinct tre nds. The compositional variation in samples with MgO > 9.5 wt% can be expla ined by addition/removal of a wehrlitic cumulate, while the major element v ariability in samples with 5-9.5 wt% MgO is dominated by gabbro removal. Th e results of thermobarometry based on the composition of the samples and th e crystals found within them show that crystallisation took place at a rang e of temperatures (1160-1350 degreesC) and pressures (< 0.3-0.9 GPa) in the crust and uppermost mantle under Krafla and Theistareykir. The geochemical results are consistent with crustal accretion models where crystallisation takes place over a range of depths in the crust and uppermost mantle (<10- 30 km). The geochemical observations allow estimates of the composition, mi neralogy, pressure and temperature of material in the crust and shallow man tle under northern Iceland to be made and these estimates can be used to pr edict the seismic velocity of the material at the ridge axis. These P-wave velocity estimates are in agreement with the results of a seismic survey of the ridge axis at Krafla. The presence of temperatures of over 1000 degree sC and magma chambers at depths greater than 10 km in the Icelandic crust c annot be ruled out using the available geophysical data from the Icelandic rift zones. Therefore both the geochemical and geophysical observations are consistent with models where crustal accretion takes place at a range of d epths under northern Iceland. (C) 2001 Elsevier Science B.V. All rights res erved.