238U–230Th–226Ra fractionation in historical lavas from the Azores: long-lived source heterogeneity vs. metasomatism fingerprints

This study reports 238U–230Th–226Ra nuclides measured by mass spectrometry, trace elements and 87Sr/86Sr isotopes on historical alkali basalts from the Azores region. The (230Th/232Th) activity in the alkali basalts spans a range (from 0.897 in Sao Miguel to 1.382 in Terceira island) that is almost as large as the global variation of mid ocean ridge basalts (MORB) and oceanic island basalts (OIB). The (230Th/238U) activity (10–40%), Th/U (2.92–3.74) and the 87Sr/86Sr (0.70354–0.70467) also display large variations. The (226Ra/230Th) activity varies from 0.84 in Sao Miguel to 1.60 in Pico island. Samples from Pico, Faial and Terceira islands are found to plot on the negative (230Th/238U) vs. (226Ra/230Th) correlation defined by the 1730–1736 eruption of Lanzarote, Canary islands [Sigmarsson et al., Earth Planet. Sci. Lett. 162 (1998) 137], except for the sample from Sao Miguel island. We propose that the negative correlation reflects a mixing between two end-member melts, one displaying a low (230Th/238U) and a high (226Ra/230Th) and the other end-member displaying a high (230Th/238U) and a low (226Ra/230Th). Numerical model suggests that the high 226Ra, low 230Th excesses end-member represented by samples from Pico island can be explained by partial melting of a garnet lherzolite. The other end-member represented by the Terceira lava is shown to result from the interaction of low-degree melts with the metasomatized mantle containing phlogopite. Phlogopite crystals in contact with a melt enriched in 226Ra and Ba have a (226Ra/230Th)≫1 and high Ba/Th because of crystal affinities for Ra and Ba relative to Th and U. After 8000 years, however, the unsupported 226Ra excess will have decreased down to 0 as phlogopite displays little amount of U and Th. So, during the process of interaction with phlogopite crystals, low-degree melts formed in the garnet stability field that are characterized by high (230Th/238U), (226Ra/230Th) and Ba/Th will loose the 226Ra excess, scavenged by phlogopite while the (230Th/238U) and Ba/Th will remain unaffected. One finds that this process may be valid over 250 years maximum so that only the first interacting melts may preserve a high (230Th/238U), low (226Ra/230Th) activity. On Sao Miguel island, it is shown that the low (230Th/238U), while the source is clearly enriched in Th, cannot be explained by a lack of garnet in the mantle neither by a higher degree of melting. The 226Ra deficit (15%) and low Ba/Th and Sr/Th are explained by partial melting of an enriched mantle in the presence of phlogopite.