Journal of Petrology, Volume 39, Issue 5: May 1 1998.

Geochemistry of Basalt from Escanaba Trough: Evidence for Sediment Contamination

ALICÉ S. DAVIS¹, DAVID A. CLAGUE¹ AND WILLIAM M. WHITE²

¹MONTEREY BAY AQUARIUM RESEARCH INSTITUTE, P.O. BOX 628, 7700 SANDHOLDT ROAD, MOSS LANDING,CA 95039-0628, USA ²CORNELL UNIVERSITY, DEPARTMENT OF GEOLOGICAL SCIENCES, SNEE HALL, ITHACA, NY 17853-1504, USA

Mid-ocean ridge basalts (MORBs) erupted in Escanaba Trough, the southernmost segment of the Gorda Ridge, have greater isotopic and geochemical variability than those from the remainder of the ridge. Samples from the sediment-free northern sites in the trough are depleted normal-MORB similar to those from the northern Gorda Ridge. However, samples from the sediment-covered portion of the trough are significantly enriched in incompatible elements, and Sr, Nd and Pb isotopic ratios are shifted toward the composition of local sediments. Pb isotopic ratios, in particular, lie on a mixing line with local sediments. Assimilation-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 basalts. Variations in the amount of enrichment of elements with similar incompatibility for comparably differentiated basalts, and uncorrelated isotopic ratios suggest variable amounts of assimilation of compositionally heterogeneous assimilants by parental magmas. Assimilation of sediment by MORB magmas probably resulted when laccolithic intrusions ponded at the sediment-basement interface beneath the thick sediment cover in the southern part of Escanaba Trough. Geochemical and isotopic evidence for contamination can be detected in these lavas because of the strong compositional contrast between the parental magma and assimilant. The depleted MORB magma and highly enriched sediment assimilant are end-member compositions that provide a sensitive indicator for the smallest amount of contamination of oceanic magma by crustal processes.

Keywords: assimilation; basalt; Escanaba Trough; lead isotopes; mid-ocean ridge

Pages 841-858

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