PETROGENESIS OF AXIAL LAVAS FROM THE SOUTHERN CHILE RIDGE - MAJOR-ELEMENT CONSTRAINTS

We present major element glass data for 163 rock samples collected fro m four ridge segments of the southern Chile Ridge between the Chiloe F racture Zone and the Chile Margin Triple Junction, including the segme nt currently being subducted at the Chile Trench (segment 1). The subr idge 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 be en 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 o cean island basalts, display a large range of major element variations at constant and high MgO, and are spatially associated with N-MORB. T ype 2 E-MORB have trace element affinities with suprasubduction zone s ettings. They are found at two segment 1 sites and along most of segme nt 3. in order to minimize fractionation and source heterogeneity effe cts and assess melting conditions, E-MORB compositions were double-bac ktracked to 8 wt % MgO and a K/Ti ratio of 0.1. Although the magnitude s 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 obse rvations indicate that spreading rate and mantle temperature exert pri mary control on the southern Chile Ridge thermal regime. We see no inf luence of ridge subduction on the major element systematics and meltin g conditions of segments closest to the trench.