PETROGENESIS OF ALKALINE BASALTS FROM SOCORRO ISLAND, MEXICO - TRACE-ELEMENT EVIDENCE FOR CONTAMINATION OF OCEAN ISLAND BASALT IN THE SHALLOW OCEAN CRUST

Postcaldera mafic volcanism on Socorro Island, Mexico, an island locat ed in the eastern Pacific Ocean on a segment of a mid-ocean ridge spre ading center abandoned at similar to 3.5 Ma, dominantly comprises alka lic basalt, hawaiite, and mugearite with subordinate mildly alkalic-tr ansitional basalt. Most major element data are consistent with differe ntiation of alkalic basalt to hawaiite and mugearite by up to 50% frac tionation of plagioclase+clinopyroxene+olivine+/-Fe-Ti oxides. High-fi eld strength element abundances are consistent with this interpretatio n, and the narrow range in Zr/Nb suggests that parental magmas formed by similar degrees of partial melting of a relatively homogeneous mant le source. Sr and Nd isotopic ratios exhibit relatively narrow ranges (0.7031-0.7032, 0.5128-0.5130, respectively). Despite this, a subset o f the alkalic basalts has negative Ce anomalies and abundances of P2O5 , Ba, Y, and some rare earth elements in excess of those predicted by fractional crystallization models; these chemical features require ope n-system processes. The observed heterogeneities are perhaps best expl ained by assimilation of ocean crustal components by basalt in a shall ow magma reservoir. Regional mantle heterogeneity is evident from elem ental and isotopic data for mildly alkalic-transitional basalt from So corro Island, mugearite from San Benedicto Island, and submarine basal ts from near Socorro Island. Evidence presented here indicates that co mpositional variations in ocean island basalts can be introduced by oc ean crustal contamination and underscores the importance of considerin g this source of chemical heterogeneity in basalts before characterizi ng associated mantle.