Petrogenesis of axial lavas from the southern Chile ridge: Major element constraints

We present major element glass data for 163 rock samples collected from four ridge segments of the southern Chile Ridge between the Chiloe Fracture Zone and the Chile Margin Triple Junction, including the segment currently being subducted at the Chile Trench (segment 1). The subridge mantle is heterogeneous at small spatial scales. Normal mid-ocean ridge basalts (N-MORB), recovered from all four ridge segments, have experienced variable extents of low pressure fractionation but have been generated by relatively uniform extents (F) and initial pressures (P-o) of melting of a slightly heterogeneous depleted source. Type 1 E-MORB, found only on segment 4, have trace element affinities to some ocean island basalts, display a large range of major element variations at constant and high MgO, and are spatially associated with N-MORB. Type 2 E-MORB have trace element affinities with suprasubduction zone settings. They are found at two segment 1 sites and along most of segment 3. in order to minimize fractionation and source heterogeneity effects and assess melting conditions, E-MORB compositions were double-backtracked to 8 wt % MgO and a K/Ti ratio of 0.1. Although the magnitudes of F and P-o are model-dependent, we find that N-MORB and both types of E-MORB were generated under similar melting conditions. These observations indicate that spreading rate and mantle temperature exert primary control on the southern Chile Ridge thermal regime. We see no influence of ridge subduction on the major element systematics and melting conditions of segments closest to the trench.