BASALTS GENERATED BY DECOMPRESSIVE ADIABATIC MELTING OF A MANTLE PLUME - A CASE-STUDY FROM THE ISLE-OF-SKYE, NW SCOTLAND

The primitive lavas of the Skye Main Lava Series (SMLS) are basaltic r acks ranging from ne- to hy-normative, and defined as having MgO > 7%. They have evolved by olivine (plus minor Cr-spinel) fractionation fro m more picritic parental material. An artificial data-set has been gen erated by normalizing all compositions to 15% MgO by fractional additi on of olivine, to define compositional characteristics of primary magm as. The most striking feature of the data-set is a very strong negativ e correlation between Si and Fe, as is seen in many oceanic alkali bas alt suites and in localized data-sets from mid-ocean ridges when norma lized for fractional crystallization. The SMLS data are comparable to the compositions of equilibrium melts produced experimentally by Hiros e & Kushiro (Earth and Planetary Science Letters, 114, 477-489, 1993) from the relatively Si- and Fe-rich starting composition HK-66. Estima tes of depths and temperatures of last equilibration of the SMLS magma s with their mantle source have been made, on the assumption that mant le melting map have been an equilibrium process. On this basis, it app ears that primary magmas, containing similar to 13-15% MgO, were gener ated by decompressive melting of abnormally hot mantle (estimated mini mum mantle potential temperature, T-P similar to 1400 degrees C), asso ciated with the Iceland plume. Melting was initiated in the garnet sta bility field, and segregation is estimated to have taken place over th e pressure range 18-36 kbar (60-112-km depth) and a temperature range of 1390-1510 degrees C. The P-T trajectory of segregation appears to c oincide closely with estimates of the solid + liquid adiabat for mantl e melting. Alkali basalts were segregated from the greatest depths and olivine tholeiites from higher levels, though the majority of magmas were derived from near the top of the melting column. After segregatio n, magmas ascended to the surface, cooling at the rate of similar to 3 degrees C/km, and were erupted in a comparatively narrow temperature range close to 1200 degrees C. The suitability of HK-66 as a general m odel for mantle composition in ascending plumes is discussed as most w ithin-plate primitive basalts in oceanic environments share the same F e-rich character as the SMLS, in which they contrast with normal ridge -related magmas. Within-plate plumes may perhaps tap mantle of abnorna lly high Fe/Mg, though the possibility that compositional differences in plume-related and normal ridge-related magmas are generated by cont rasts in process cannot yet be excluded. If however, a relatively Fe-r ich reservoir exists deep within the mantle, and acts as the source fo r plume-related magmatism, then ultimately a global positive correlati on should exist between estimates of T-P and the Fe/Mg ratio of the so urces for individual suites.