Petrogenesis of picrites from the Caribbean Plateau and the North Atlanticmagmatic province

We studied the petrography, mineralogy and geochemistry of picrites from th ree different regions: the island of Curacao which forms part of a Cretaceo us oceanic plateau; Iceland, an active hot spot on the mid-Atlantic ridge; and the early Tertiary volcanic margin off the coast of Greenland, which fo rmed during the rifting that created the Atlantic ocean. Using the composit ions of olivine phenocrysts and relations between MgO and FeO, Al2O3 and Ni , we estimated compositions of parental liquids and the proportion of accum ulated olivine in each rock. Picrites from Curacao formed mainly from a liq uid with 12 wt.% MgO and they contain up to 55 wt.% excess olivine in the f orm of phenocrysts. A small proportion of more forsterite-rich olivine grai ns are xenocrysts from a more magnesian source. Picrites from Iceland forme d from a slightly less magnesian liquid but one with also about 12 wt.% MgO and they contain both olivine and plagioclase in the accumulated assemblag e. Picrites from the Greenland volcanic margin formed from a liquid that wa s distinctly more magnesian, with up to 20 wt.% MgO. In some of these rocks the proportion of accumulated olivine was minimal and in these the whole-r ock composition is roughly equivalent to the liquid composition. The picrit es from the three areas formed under different conditions and through contr asting melting processes. The Curacao picrites derive from pooled liquids f ormed through moderate degrees of melting at moderate depths beneath a rela tively old and thick oceanic lithosphere. Iceland picrites, on the other ha nd, formed through advanced fractional melting of mantle that ascended almo st to the base of the crust at the mid-ocean spreading center. An unusual c ombination of relatively high concentrations of incompatible trace elements and high MgO indicates that the Greenland picrites formed by relatively lo w degrees of melting at greater depths in the mantle. (C) 1999 Elsevier Sci ence B.V. All rights reserved.