Geochemistry of Basalt from Escanaba Trough: Evidence for Sediment Contamination

Mid-ocean ridge basalts (MORBs) erupted in Escanaba Trough, INTRODUCTION the southernmost segment of the Gorda Ridge, have greater isotopic Crustal contamination of basaltic magmas is generally and geochemical variability than those from the remainder of the recognized for continental volcanoes (e.g. Taylor, 1980; ridge. Samples from the sediment-free northern sites in the trough Leeman & Hawkesworth, 1986; Glazner et al., 1991). The are depleted normal-MORB similar to those from the northern composition, thickness, and low melting temperatures of Gorda Ridge. However, samples from the sediment-covered portion continental crustal rocks facilitate assimilation that can of the trough are significantly enriched in incompatible elements, be easily identified. However, the extent to which basaltic and Sr, Nd and Pb isotopic ratios are shifted toward the composition magmas are contaminated by oceanic crust is a topic of of local sediments. Pb isotopic ratios, in particular, lie on a mixing intense debate (e.g. Bohrson & Reid, 1995; Clague et al., line with local sediments. Assimilation–fractional crystallization 1995; Eiler et al., 1996; Bohrson et al., 1996, and references (AFC) calculations suggest that 25–40% fractional crystallization therein; Thirlwall et al., 1997). The debate has focused coupled with assimilation of 1–2% of local sediments can produce on distinguishing the chemical characteristics magmas the enrichment observed for some Escanaba basalts. Variations in acquire during ascent, crustal storage, and emplacement the amount of enrichment of elements with similar incompatibility from those that have been used to identify heterogeneous for comparably differentiated basalts, and uncorrelated isotopic mantle sources. For most ocean island volcanoes, the ratios suggest variable amounts of assimilation of compositionally crustal components available to serve as contaminants heterogeneous assimilants by parental magmas. Assimilation of are poorly characterized, but include the underlying sediment by MORB magmas probably resulted when laccolithic altered oceanic crust, pelagic to hemipelagic sediment, intrusions ponded at the sediment–basement interface beneath the and the hydrothermally altered volcanic edifice. thick sediment cover in the southern part of Escanaba Trough. Mid-ocean ridge basalts (MORBs) are relatively uniGeochemical and isotopic evidence for contamination can be detected form geochemically with depleted incompatible element in these lavas because of the strong compositional contrast between and radiogenic isotope compositions compared with lavas the parental magma and assimilant. The depleted MORB magma from ocean island or continental volcanoes. However, and highly enriched sediment assimilant are end-member comexcess chlorine content and Cl/K ratios in fresh MORB positions that provide a sensitive indicator for the smallest amount glasses support the idea that MORB magmas may also of contamination of oceanic magma by crustal processes. be affected by assimilation of hydrothermally altered oceanic crust (e.g. Michael & Schilling, 1988; Michael & Cornell, 1996). Chlorine, like other incompatible elements, shows the greatest enrichment in strongly frac