constraints from alkaline basalts in Payenia, Argentina
The processes of magma generation in upwelling eclogite bodies of recycled lithospheric material are not fully understood but are important for our understanding and modelling of major and trace element variations in many ocean island basalts (OIB). The primitive alkaline intraplate basalts from the Payenia volcanic province (34–38 °S) in Argentina, for which Sr, Nd and double-spike Pb isotope ratios are presented, and from other north Patagonian volcanic fields may provide details of the eclogite melt–peridotite reactions taking place in the melting column of an upwelling OIB-type mantle. The isotopic composition of the uncontaminated lavas is highly restricted but the Payenia basalts fall in two distinct trace element groups termed the high and low Nb/U groups, which both have EM1-type trace element patterns but with subtle differences that cannot be explained by contamination, fractionation or simple variations in degrees of mantle melting. The difference is also clear in major elements where the low Nb/U basalts have markedly higher alkali contents but lower FeO and Ni than the high Nb/U basalts. Four melt components have been identified based on olivine fractionation corrected compositions: a low Nb/U pyroxenite melt component which is interpreted as an eclogite melt that experienced low degrees of melt–peridotite interaction, a high Nb/U pyroxenite melt component which is interpreted as an eclogite melt that experienced high degrees of melt–peridotite interaction, a low pressure peridotite melt component which contributes to most high Nb/U, Payún Matrú, Matancilla and Auca Mahuida basalts and finally, a melt component with similarities to peridotite melts contributing to many low Nb/U lavas. The high Nb/U type basalts are interpreted to have been formed from mantle with higher temperature than ambient mantle while the low Nb/U basalts may have been formed from normal temperature mantle.