Pleistocene alkaline rocks of Martin Vaz volcano, South Atlantic: low-degree partial melts of a CO2-metasomatized mantle plume

Several alkaline massifs on inland southeastern Brazil extend offshore, roughly parallel to similar to 20 degrees S, through a seamount chain of the Vitoria-Trindade ridge. This paper presents the first extensive work on the Martin Vaz volcano through whole-rock and Sr and Nd isotopic composition of volcanic and subvolcanic lithotypes from the Martin Vaz Island, located at the easternmost of this volcanic chain. These alkaline rocks were generated during the Plio-Pleistocene (similar to 0.47 My, Ar-40/Ar-39 dating in whole-rock) and represent the crystallization of sodic magmas of nephelinitic composition that evolved through fractional crystallization towards phonolites. Calculations from P-T-Liquidus using PELE software show temperatures of 1045 degrees C and 818 degrees C, viscosity of 2.47 log Poise and 5.02 log Poise, and densities of 2.57 g/cm(3) and 2.26 g/cm(3) for nephelinite and phonolite, respectively. Like in Trindade Island, the nephelinitic volcanism in Martin Vaz may represent a Strombolian and/or Hawaii-type eruption due to low viscosity magma according to its physical properties whereas phonolitic intrusions present higher viscosity characteristics forming lava domes. The Sr-87/Sr-86 (similar to 0.703800) and Nd-143/Nd-144 (similar to 0.512750) ratios of lavas from the seamounts and Martin Vaz do not vary significantly, pointing to partial melting process from a homogeneous mantle source showing isotope signature close to HIMU. Beside the restrict variation on these isotopic ratios, a conspicuous enrichment in incompatible trace elements, mainly LREE, indicates that metasomatism is a recent process and not a long-term source characteristic. Non-modal partial melting models (fractional melting and batch melting) suggest that the source of the Martin Vaz magmatism is consistent with the garnet-lherzolite mantle stability field (>90 km depth; Tb/Yb >0.7), generated about 3.0 GPa by very small degree of partial melting of an enriched wet mantle source (F = 0.03-0.04) with 2.5 wt. % of CO2.