Basalts Generated by Decompressive Adiabatic Melting of a Mantle Plume: a Case Study from the Isle of Skye, NW Scotland |
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| Authors: | SCARROW JANE H; COX K G |
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| Institution: | DEPARTMENT OF EARTH SCIENCES PARKS ROAD, OXFORD OX1 3PR, UK |
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| Abstract: | The primitive lavas of the Skye Main Lava Series (SMLS) arebasaltic rocks ranging from ne- to hy-normative, and definedas having MgO>7%. They have evolved by olivine(plus minorCr-spinel) fractionation from more picritic parental materialAn artificial data-set has been generated by normalizing allcompositions to 15% MgO by fractional addition of olivine, todefine compositional characteristics of primary magmas. Themost striking feature of the data-set is a very strong negativecorrelation between Si and Fe, as is seen in many oceanic alkalibasalt suites and in localized data-sets from mid-ocean ridgeswhen normalized for fractional crystallization. The SMLS dataare comparable to the compositions of equilibrium melts producedexperimentally by Hirose & Kushiro (Earth and PlanetaryScience Letters, 114, 477–489, 1993) from the relativelySi- and Fe-rich starting composition HK-66. Estimates of depthsand temperatures of last equilibration of the SMLS magmas withtheir mantle source have been made, on the assumption that mantlemelting may have been an equilibrium process. On this basis,it appears that primary magmas, containing  13–15% MgO,were generated by decompressive melting of abnormally hot mantle(estimated minimum mantle potential temperature, TP 1440C),associated with the Iceland plume. Melting was initiated inthe garnet stability field, and segregation is estimated tohave taken place over the pressure range 18–36 kbar (60–112-kmdepth) and a temperature range of 1390–1510 C. The P–Ttrajectory of segregation appears to coincide closely with estimatesof the solid + liquid adiabatfor mantle melting. Alkali basaltswere segregated from the greatest depths and olivine tholeiitesfrom higher levels, though the majority of magmas were derivedfrom near the top of the melting column. After segregation,magmas ascended to the surface, cooling at the rate of 3C/km,and were erupted in a comparatively narrow temperature rangeclose to 1200C. The suitability of HK-66 as a general modelfor mantle composition in ascending plumes is discussed, asmost within-plate primitive basalts in oceanic environmentsshare the same Fe-rich character as the SMLS, in which theycontrast with normal ridge-related magmas. Within-plate plumesmay perhaps tap mantle of abnormally high Fe/Mg, though thepossibility that compositional differences in plume-relatedand normal ridge-related magmas are generated by contrasts inprocess cannot yet be excluded. If, however, a relatively Fe-richreservoir exists deep within the mantle, and acts as the sourcefor plume-related magmatism, then ultimately a global positivecorrelation should exist between estimates ofTp and the Fe/Mgratio of the sources for individual suites.
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