Upper Mantle Amphiboles and Micas and TiO2, K2O, and P2O5 Abundances and 100 Mg/(Mg+Fe2+ Ratios of Common Basalts and Andesites: Implications for Modal Mantle Metasomatism and Undepleted Mantle Compositions

Modal mantle metasomatism, involving the re-enrichment of depleted mantle by the introduction or production of new hydrous phases, apatite and other minerals, has been proposed as a critical precursor to alkaline volcanism. The merits of the modal metasomatism model are evaluated by examining whole-rock 100 Mg/(Mg+Fe2+) ratios and the abundances of TiO2, K2O and P2O5 in mafic volcanics spanning the mafic alkaline-subalkaline compositional spectrum. Upper mantle amphiboles and micas are also discussed because they would be major donors of Ti, Fe, and K to melts during anatexis of either modally metasomatized depleted mantle or undepleted mantle. Compared with tholeiitic and calc-alkaline basalts and andesites, basanites and alkali basalts and alkali andesites are neither distinctive nor unique by virtue of persistant or well-defined higher abundances of TiO2, K2O, and P2O5 or lower 100 Mg/(Mg+Fe2+ )ratios, features which might reflect precursor modal metasomatism of the alkaline sources. Some basanites and alkali basalts do have higher abundances of TiO2, K2O, and P2O5 than some tholeiites but these abundances may be the result of lower degrees of melting of similar undepleted mantle sources for both magma types. The most widespread mantle phases of inferred metasomatic origin are interstitial amphiboles and micas in Group I spinel peridotite xenoliths. These have high 100 Mg/(Mg+Fe) ratios (˜ 90) and high Cr2O3 and low TiO2 abundances, and the K2O/Na2O ratios of the amphiboles (chromian pargasites) are low, generally less than 0·3. Interstitial amphiboles and micas developed as a result of near-isochemical hydration reactions which largely involved Cr-spinel and Cr-diopside. Their formation was probably induced in many instances by fluids derived from crystallizing mafic magmas. Metasomatized Group I xenoliths with interstitial hydrous phases remain depleted in TiO2, K2O, and P2O5, and they retain the high 100 Mg/(Mg+Fe) ratios characterizing depleted Group I xenoliths. Together with the low K2O/Na2O ratios, these features preclude such peridotites as suitable sources of most alkaline (and subalkaline) volcanics. It is suggested that modal metasomatism plays an insignificant role in the genesis of most mantle-derived mafic volcanics. Compared with the interstitial phases, kaersutitic amphiboles and titaniferous micas from vein, Group II inclusion and megacryst upper mantle parageneses have lower 100 Mg/(Mg+Fe) ratios and Cr2O3 contents, and much higher TiO