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 C i A /C i 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.
Highlights
On the other hand, the magmas that have been more enriched in highly incompati)le elements as involved in a function oafsstihmeilCatii/oqn processes are ratio, relative to the magmas generated only by,imple fractional crystallization
When assimilation is not coupled with fractional cl'ystallization, the effects of the first process can be observed in the most primitive magma, and not in the differentiated ones
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 indic ator versus any fractionation indexo
Summary
La erosión térmica produce unos efectos geoquímicos en el magma resultante, que contrastan con los que se producen en la asimilación. Ej., Sr87, KzO...) y cualquier parámetro que refleje grado de fraccionación (por ejemplo, contenido en SizO), puesto que en este caso la contaminación es mayor en los magmas más primitivos que en los más evolucionados. Como la temperatura provoca que los magmas más primitivos sean menos selectivos que los más evolucionados, ya que los primeros pueden asimilar rocas de composición intermedia y ácida mientras que los segundos sólo pueden asimilar rocas ácidas, la consecuencia es que las rocas de una región que han sufrido erosión térmica se proyectan sobre diferentes hipérbolas de mezcla. Por lo que respecta a las relaciones isotópicas, la asimilación preferencial de los magmas más primitivos, produce que aquellos con altos valores Mg/(Mg + Fe) y bajos contenidos en Sr son los que tienen las más altas relaciones Sr87/Sr86, lo que se traduce en una correlación inversa entre Sr y Sr87/Sr86 en las rocas generadas. Si la contaminación tiene lugar después de que han cristalizado algunos fenocristales, existirá desequilibrio isotópico entre éstos y la matriz contaminada
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
