Investigation of 238U–230Th–226Ra and 232Th–228Ra–228Th radioactive disequilibria in volcanic rocks from Trindade and Martin Vaz Islands (Brazil; Southern Atlantic Ocean)

This paper presents the first results of 238U–230Th–226Ra and 232Th–228Ra–228Th radioactive disequilibria of alkaline volcanic rocks from Trindade and Martin Vaz islands (Southern Atlantic Ocean — Brazil). Th and U concentrations reinforce the role of fractional crystallization in the genesis of Trindade volcanics and allow dividing the rocks in three groups: Group I is characterized by phonolites depleted in U in relation to Th, whose genesis can be explained by mineral assemblage fractionation with high uranium partition coefficients (magnetite, apatite and/or titanite); Group II is composed of ultrabasic, basic and intermediate rocks, with relatively high U contents in respect to Th, whose enrichment is probably caused by the presence of small comagmatic xenoliths containing accumulation of those fractionated mineral phases; Group III is composed of remaining rocks, which tend to present despite of some variation homogeneous Th/U ratios (average = 4.6 ± 0.8). Very low degrees (1–2%) of partial melting generated ultrabasic rocks of the last group, which are highly SiO2 undersaturated, and together with Ascension volcanics have the highest Th/U ratios of all OIB from Southern Atlantic Ocean. Activity ratios of (234U/238U), (230Th/238U), (226Ra/238U), (226Ra/230Th), (228Ra/232Th), and (228Th/232Th) are equal to unity in all phonolites and some basic and ultrabasic rocks belonging to Trindade Complex and Desejado Sequence, agreeing with ages more than 300 ky. Martin Vaz peralkaline phonolite also presents activity ratios indicating radioactive equilibrium, except for (226Ra/238U), which is significantly smaller than unity as the result of 226Ra steady mobilization. Rocks from the three youngest volcanic episodes of Trindade (Morro Vermelho Formation, Valado Formation, and Vulcão do Paredão) present (230Th/238U) and (226Ra/238U) ratios distinct from unity, showing also a significant 230Th enrichment, and all pairs of activity ratios indicate system closure between 8 and 300 ky ago. (230Th/232Th) and (238U/232Th) data of Morro Vermelho rocks allowed determining an age of 111 ± 20 ky. Radioactive disequilibrium between 238U and 226Ra of Valado Formation rocks, associated with literature information constrain the system was closed between 8 and 11 ky. For ultrabasic rocks, calculated 230Th0 excesses, along with Th/U ratios do not show significant correlations with K2O contents, therefore if phlogopite remained as a residual phase during K-poor magma generation, this mineral did not fractionate Th because of its very small partition coefficient. The relatively low (230Th/232Th)0 in comparison with measured 87Sr/86Sr, plotting slightly below the mantle array, may be explained either by a relatively long time (maximum of about 110 ky for Morro Vermelho Formation) of magma transfer towards surface or, more probably, by melting of heterogeneous mantle due to a metasomatic event occurred between 111 and 300 ky, followed by a very short magma transfer time (hundreds of years).