Compositional zonation and thermodynamic conditions of formation of basalts of the Bouguer anomaly (Central sector of the Mid-Atlantic Ridge)

In the Central sector of the Mid-Atlantic Ridge (MAR), the earlier distingu ished area of E-MORB occurrence (13-15 degrees N, "Bouguer anomaly") is cha racterized by a zonal distribution of basalts of no less than eight statist ically different petrochemical groups: 1) Nonvolcanic basalts of the axial rift are richer in Mg and Fe but poorer in Ti than basalts of the eastern a nd western slopes of the MAR crest; 2) basalts of the eastern and western s lopes of the MAR differ only in Fe content; 3) basalts of the southern and northern parts of the "Bouguer anomaly" differ in average composition; 4) t he composition of nonvolcanic basalts changes from south to north along the axial rift. A petrogenetic analysis of the composition of these basalts suggests two tr ends of their formation: a) outmelting of basic magmas (normal and water-en riched) from two sources and b) fractionation of primary melts in intermedi ate chambers. Among the lavas of the MAR crest, N-MORB amount to only a few percent. It is estimated that within the studied area the mantle-derived m elts intruded in separate places at nearly the same rate (0.05-0.5 m/s), wh ich suggests their similar geneses.