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 racks ranging from ne- to hy-normative, and defined as having MgO > 7%. They have evolved by olivine (plus minor Cr-spinel) fractionation from more picritic parental material. An artificial data-set has been generated by normalizing all compositions to 15% MgO by fractional addition of olivine, to define compositional characteristics of primary magmas. The most striking feature of the data-set is a very strong negative correlation between Si and Fe, as is seen in many oceanic alkali basalt suites and in localized data-sets from mid-ocean ridges when normalized for fractional crystallization. The SMLS data are comparable to the compositions of equilibrium melts produced experimentally by Hirose & Kushiro (Earth and Planetary Science Letters, 114, 477-489, 1993) from the relatively Si- and Fe-rich starting composition HK-66. Estimates of depths and temperatures of last equilibration of the SMLS magmas with their mantle source have been made, on the assumption that mantle melting map have been an equilibrium process. On this basis, it appears that primary magmas, containing similar to 13-15% MgO, were generated by decompressive melting of abnormally hot mantle (estimated minimum mantle potential temperature, T-P similar to 1400 degrees C), associated with the Iceland plume. Melting was initiated in the garnet stability field, and segregation is estimated to have taken place over the 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 coincide closely with estimates of the solid + liquid adiabat for mantle 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 segregation, 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 model for mantle composition in ascending plumes is discussed as most within-plate primitive basalts in oceanic environments share the same Fe-rich character as the SMLS, in which they contrast with normal ridge-related magmas. Within-plate plumes may perhaps tap mantle of abnornally high Fe/Mg, though the possibility that compositional differences in plume-related and normal ridge-related magmas are generated by contrasts in process cannot yet be excluded. If however, a relatively Fe-rich reservoir exists deep within the mantle, and acts as the source for plume-related magmatism, then ultimately a global positive correlation should exist between estimates of T-P and the Fe/Mg ratio of the sources for individual suites.