Madeira Island alkaline lava spinels: petrogenetic implications

Summary

Four groups of spinels have been identified in Madeira Island alkaline basalts: 1) Al-rich chromian-spinels (Cr#<0.15; 51<Mg#<62) characteristic of olivine xenocrysts from dismembered, high-pressure, cognate, ultramafic xenoliths; 2) Al-poor chromian-spinel/magnesiochromite/chromite (Cr#>31; 30<Mg#<61) included in olivine phenocryst cores; 3) chromian-titanomagnetites included in olivine phenocryst rims (11<Mg#<34) and chromian-titanomagnetites to titanomagnetites in clinopyroxene phenocrysts (6<Mg#<25) and in the groundmass (3<Mg#<36); 4) rare, TiO₂-poor, MnO-rich titanomagnetite inclusions in green-core clinopyroxene xenocrysts, derived from metasomatized upper mantle xenoliths. Chemical variations among spinel groups 1) to 3) largely reflect physical conditions and the extent of fractionation of crystallising magmas. Extensive solid solution among chromian-spinel – ulvospinel – magnetite and the ulvospinel enrichment exhibited by titanomagnetite evolutionary trends are attributed to the combined effects of low-a _SiO2 and oxidizing conditions (0.2≤Δlog(fO₂)_NNO≤1.8) during crystallisation from Madeira alkaline magmas. Pressure does not seem to have a direct influence on the stability of high-Al spinels; however, Cr/Al contrasts between spinels in high-pressure olivine xenocrysts and those in olivine phenocryst cores are envisaged as reflecting compositional effects of distinct crystallisation sequences during polibaric fractionation of Madeira magmas.