Modelización geoquímica de los procesos de mezcla

Abstract

The magmas generated in mantle regions that have been contaminated by the infiltration of subduction-derived fluids, show higher abundances in large-ion lithophile elements relative to the rare-earth elements and the high-strength field elements, when compared to the mid-ocean ridge and oceanic-island basalts. Additionally, their Sr-Nd-Pb-O isotopic ratios show a wider range. On the other hand, the magmas that have been involved in assimilation processes are more enriched in highly incompatible elements as a function of the Cⁱ_A/Cⁱ_O ratio, relative to the magmas generated only by simple fractional crystallization. Both magmas show no significative differences in compatible element abundances. Concerning the isotopic ratios, if assimilation is dominant, the most differentiated liquids can have a signature close to that of the assimilated rocks. However, if this process is not important, even the more evolved liquids have ratios far from those of the assimilated material. When assimilation is not coupled with fractional crystallization, the effects of the first process can be observed in the most primitive magma, and not in the differentiated ones. Therefore, the rocks that are the consequence of thermal erosion, show positive correlations between highly compatible and highly incompatible trace elements and negative correlations when comparing any contamination indicator versus any fractionation index. Since sediments show a wide variation in their Sr/Nd ratio (4-40), Sr and Nd isotopes are not good indicators of the participation of sediments in the generation of lavas. On the contrary, the contrasting concentrations of Pb in be mantle (0,05 ppm) and the sediments (25 ppm) make lead isotopes exceptional indicators, even when sediments participate in low proportions (l %). Nevertheless, the best approach to discriminate between mantle contamination and crustal assimilation, and to estimate the relative roles of these processes, is the combined use of Sr, Nd and O isotopes.