GEOCHEMISTRY OF BASALT FROM ESCANABA TROUGH - EVIDENCE FOR SEDIMENT CONTAMINATION

Mid-ocean ridge basalts (MORBs) erupted in Escanaba Trough the souther nmost segment of the Gorda Ridge, have greater isotopic and geochemica l variability than those from the remainder of the ridge. Samples from the sediment-free northern sites in the trough are depleted normal-MO RB similar to those from the northern Gorda Ridge. However, samples fr om the sediment-covered portion of the trough are significantly enrich ed in incompatible elements, and Sr, Nd and Pb isotopic ratios are shi fted toward the composition of local local sediments. Pb isotopic rati os, in particular, lie on a mixing line with local sediments. Assimila tion-fractional crystallization (AFC) calculations suggest that 25-40% fractional crystallization coupled with assimilation of 1-2% of local sediments can produce the enrichment observed for some Escanaba basal ts. Variations in the amount of enrichment of elements with similar in compatibility for comparably differentiated basalts, and uncorrelated isotopic ratios suggest variable amounts of assimilation of compositio nally heterogeneous assimilants by Parental magmas. Assimilation of se diment by MORE magmas probably resulted when laccolithic intrusions po nded at the sediment-basement interface beneath the thick sediment cov er in the southern part of Escanaba Trough. Geochemical and isotopic e vidence for contamination can be detected zn these lavas because of th e strong compositional contrast between the parental magma and assimil ant. The depleted MORE magma and highly enriched sediment assimilant a re end-member compositions that Provide a sensitive indicator for the smallest amount of contamination of oceanic magma by crustal processes .