Petrogenetic mixing models and Nd-Sr isotopic patterns

Abstract

The possible Nd and Sr isotopic variations which would result from assimilation of continental crust by mantle-derived magmas can be systematized by considering the chemical and isotopic compositions of the magmas and models for layering within the continental crust. These systematics may provide a valuable framework for studying crustal contamination, the formation of magmas within the crust, and the chemical and age structure of continental segments. The isotopic patterns arise from (1) the overall contrast in Nd and Sr isotopic composition between old continental crust and mantle magma sources, (2) regularities in the variations of ^(143)Nd/^(144)Nd and ^(87)Sr/^(86)Sr in crustal rocks which are related to age and the contrasting geochemical behavior of Sm and Nd relative to Rb and Sr. Consideration of two-component mixing models suggests that the correlation of initial Nd and Sr isotopes in selected young basalts could represent a mixing line, possibly between mantle reservoirs of distinct chemistry and age. However, the possibility that the trend results from mixing of old low - Sm/Nd, high - Rb/Sr crustal material with complementary high - Sm/Nd, low - Rb/Sr mantle reservoirs cannot be eliminated. Such recycling of crust into the mantle could occur in subduction zones. One problem with a recycling model to explain the correlation is that it requires the crustal endmember to have chemical and isotopic properties which do not appear to be common in crustal rocks.

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