Crustal contamination versus subduction zone enrichment: Examples from the Lesser Antilles and implications for mantle source compositions of island arc volcanic rocks

Details Crustal contamination versus subduction zone enrichment: Examples from the Lesser Antilles and implications for mantle source compositions of island arc volcanic rocks Crustal contamination versus subduction zone enrichment: Examples from the Lesser Antilles and implications for mantle source compositions of island arc volcanic rocks

Aug 1987

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1987gecoa..51.2185d&link_type=abstract

Geochimica et Cosmochimica Acta, vol. 51, Issue 8, pp.2185-2198

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36

Scientific paper

Isotopic and geochemical data are presented from the northern and central Lesser Antilles arc. Island arc tholeiites from the northern islands are restricted in 143 Nd / 144 Nd and 87 Sr / 86 Sr and show large enrichments in LILE relative to other trace elements, as exemplified particularly in high Ba/La ratios. These volcanic rocks represent typical island arc compositions, similar to those described from the Aleutians, Marianas and South Sandwich Islands. In contrast, samples from the central islands (Martinique and St. Lucia) are highly variable in isotopic and trace element ratios, and reflect overprinting of primitive island arc magma chemistry by subsequent crustal contamination. Such magmas therefore do not represent island arc mantle source characteristics. Northern Lesser Antilles arc mantle sources are displaced from MORB Pb isotopic compositions towards the higher 207 Pb / 204 Pb ratios characteristic of sediments. Also, Sr and Nd isotopic compositions are highly restricted and distinct from those of MORB. As a result, mixing models between MORB source and subducted sediment are tightly constrained in terms of the allowable proportion of sediment. The characteristically high LILE / REE and LILE / HFSE ratios of island arc volcanics cannot be easily reconciled with sediment + MORB source mixing. Buffering of isotopic compositions together with considerable relative LILE enrichment might be achieved by mixing between mantle and a hydrous fluid which has equilibrated with not only sediment but also a large proportion of variably altered oceanic crust. The fluid has isotopic compositions closer to those of MORB, thereby permitting a greater range of mixing proportions. At the same time, the LILE characteristics of the modified mantle source are dominated by the very high concentrations of these elements in the fluid. High degrees of partial melting of mantle modified by a hydrous fluid reproduce the trace element pattern of the source in the derivative volcanic rocks and account for the depletion of HFSE and HREE relative to MORB.

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