Oceanic mantle beneath the southern Rio Grande rift

Major element and modal trends in spinel lherzolites from Salt Lake Crater, Hawaii, and Kilbourne Hole, New Mexico, are essentially identical. These trends have developed through 5–30% partial melting of the same pyrolite-like parent composition. The melting events probably took place at mid-ocean ridges.Using Cr/(Cr + Al + Fe³⁺) in spinel as an index of the degree of depletion in fusible components in each of a large number of lherzolite inclusions, we have subdivided both suites into relatively parental, intermediate, and refractory groups, and computed an average bulk composition for each group. Compositions of hypothetical melts generated from these rocks are characterized by high MgO (about 18%), low Ti and K, and CaO/Al₂O₃ around 1.0.The strongly similarity between the two xenolith suites suggests that south central New Mexico is underlain by oceanic mantle. We propose that the Kilbourne Hole lherzolites represent oceanic lithosphere which was buried beneath offshore sediments during a Precambrian ocean basin formation, later to be preserved when subduction further offshore created new additions to the continent above that near-shore oceanic lithosphere. In addition, we propose that the Mg-rich, Ti- and K-poor calculated melts reported here are the primitive precursors to mid-ocean ridge basalts. Early dunite and wehrlite cumulates from some ophiolite sequences may record the fractionation histories these liquids follow prior to eruption at the surface.