U-series disequilibria in volcanic rocks from the Canary Islands: Plume versus lithospheric melting

U-series disequilibria are presented for Holocene samples from the Canary Islands and interpreted with special emphasis on the separate roles of plume vs. lithospheric melting processes. We report Th and U concentrations and (U-238)/(Th-232), (Th-230)/(Th-232), (Th-230)/(U-238) and (U-234)/(U-238) for 43 samples, most of which are minimally differentiated, along With (Ra-226)/(Th-230) and (Pa-231)/(U-235) for a subset of these samples, measured by thermal ionization mass spectrometry (TIMS). Th and U concentrations range between 2 and 20 ppm. and 0.5 and 6 ppm, respectively. Initial (Th-230)/(U-238) ranges from 1.1 to 1.6. (Ra-226)/(Th-230)(o) ranges between 0.9 and 1.8 while (Pa-231)/(U-235)(o) ranges between 1.0 and 2.0. Our interpretation of results is based on a three-fold division of samples as a function of incompatible element ratio, such as Nb/U. The majority of samples have Nb/U = 47 +/- 10, similar to most MORB and OIB. Higher ratios are found exclusively in alkali basalts and tholeiites from the eastern Canary Islands whereas lower ratios are exclusively found in differentiated rocks from the western Canary Islands. Those with ordinary Nb/U ratios are attributed to melting within the slowly ascending HIMU-dominated Canary plume. Higher Nb/U, generally found in more silica rich basalts from the eastern islands, is attributed to lithospheric contamination. Based on their trace element characteristics, two possible contaminants are amphibole veins (+/- other minerals) crystallized in the mantle from previous plume-derived basanite or re-melted plume-derived intrusive rocks. The high Nb/U signature of these materials is imparted on a melt of the lithosphere created either by the diffusive infiltration of alkalis or by direct reaction between basanites and peridotite. Mixing between plume-derived basanite and lithospheric melt accounts for the U-series systematics of most eastern island magmas including the well-known Timanfaya eruption. Lower Nb/U ratios in differentiated rocks from the western islands are attributed to fractional crystallization of amphibole phlogopite +/- sphene from basanite during its ascent through the lithosphere. Based on changes in disequilibria, phonolites and tephrites are interpreted to result from rapid differentiation of primitive parents within millennia. Copyright (C) 2003 Elsevier Ltd.