Systematics of U-series nuclides in primitive lavas from the 1730-36 eruption on Lanzarote, Canary Islands, and implications for the role of garnet pyroxenites during oceanic basalt formations

Abstract

Inferences on mantle melting and related parameters are often derived from U-series results on oceanic basalts. However, most of these basalts come from magma chambers in which magma mixing amongst other processes is likely to have reduced the true compositional spread created during partial melting. Here are presented U–Th–Ra disequilibrium results on historical lavas of near-primary composition from Lanzarote, Canary Islands, where magma chambers have not been detected. During the 1730–36 fissure eruption, the magma composition varied with time from basanites through alkali basalts to tholeiites. Approximately 3–5 km3 of primitive and often ultramafic xenolith-bearing lavas were produced. Abundances of Ba, Th and U generally decreased towards the end of the eruption, and show more than four-fold variations (e.g. Th = 5.75–1.55 ppm). Ba/Th and Ba/U vary by more than 75%, whereas Th/U vary by less than 10%. Excesses of 230Th over 238U range from 18% to 33% and generally increase with alkalinity. (230Th/232Th) decreased significantly from 1.02 to 0.94 with time and is positively correlated with the alkalinity of the lavas. Isotope ratios of Sr and (226Ra/230Th) also correlate with SiO2. (226Ra/230Th) increased with time from 1.10 to 1.56. The correlations between (230Th/232Th), 87Sr/86Sr and SiO2 suggest that the lava compositions represent nearly unmodified primary melts from a lithologically and compositionally heterogeneous mantle source. Decreasing (230Th/238U) and increasing (226Ra/230Th) in basanites to tholeiites can be explained by partial melting of a mixed garnet pyroxenite–lherzolite source, during which the proportion of pyroxenite and extent of melting diminished. Alternatively, the large variations of excess 226Ra and Ba/Th may reflect fluid addition to such a heterogeneous mantle source. In order to conserve the systematic variations of (226Ra/230Th) with other geochemical variables, transfer time through the lithosphere is likely to have been short relative to the half-life of 226Ra. A magma velocity through the lithosphere on the order of 10−5 m/s (0.5 km/year) is compatible with the data. Both the Th–U disequilibria and the Th-isotope ratios of the Lanzarote lavas appear to be related to the mantle source lithology and composition, where garnet pyroxenite melting produced the early alkaline basalts and increasing melting of garnet lherzolite produced the tholeiites. Furthermore, correlations between (230Th/238U) and (232Th/238U) for different volcanic provinces worldwide, strongly suggest that Th–U disequilibria in oceanic basalts are principally caused by partial melting of garnet pyroxenites.