Distribution coefficients of major and trace elements; fractional crystallization in the alkali basalt series of Chaîne des Puys (Massif Central,France)

Major and seventeen trace element distribution coefficients between main phenocrysts (olivine, clinopyroxene, amphibole, mica, feldspars and Fe-Ti oxides) and groundmass have been measured in the alkali basalt suite of Chaîne des Puys (Massif Central, France). The suite appears to be a well behaved crystal fractionation series. We pinpoint key elements whose behavior is closely related to the appearance or disappearance of specific crystal phases in the fractionation process. Ta, for instance, clearly indicates the role of hydrous silicates (amphiboles and micas). Distribution coefficients are shown to vary systematically along the differentiation trend. Significantly the hygromagmaphile tendency (Treuilet al., 1979) of U, Th, Ta and La is variable along the series.The mass balance equations,

$\text{D}{}^{\mathrm{i}}\text{=}\text{∑}\text{;}\text{x}{}_{\mathrm{j}}\text{D}{}_{\mathrm{j}}{}^{\mathrm{ii}}$

where Dⁱ and D_jⁱ are the bulk and mineral/liquid distribution coefficients respectively, and x_j the weight fractions of the fractionating phases, are solved by least square resolution of the overdetermined system, taking into account the analytical errors on data. The solution applied to the Chaîne des Puys suite leads to a coherent and quantitative model of the fractional crystallization process. The suite has apparently evolved in three stages. Each stage is characterized by constant bulk distribution coefficients and a specific mineral assemblage. Amphibole fractionation plays an important role in the early stages. Some intensive parameters (T, ? ?_O2, P_H2O) as well as f (weight fraction of residual liquid) are also estimated.