Lithium and delta7Li behavior during metamorphic dehydration processes and crustal evolution

Abstract

Li isotopes have been used as a tracer of a wide range of geological processes, because the two stable isotopes of Li have significant relative mass difference and large elemental dispersion in Earth. In order to understand better Li behavior during the processes of sub-greenschist to granulite facies metamorphism and partial melting, fluid transport in subduction zones, weathering of continental crust and the evolution of juvenile continental crust, a large variety of samples have been studied in this thesis. These samples include mudrocks from British Caledonides, schists from Otago, New Zealand, high-grade metapelites and metabasites from Ivrea-Verbano Zone, Italy, Archean shales from the Kaapvaal Craton, South Africa, and Archean granitoids from Scotland, South Africa and Canada.

Investigations of metapelites from several localities show that: 1) sedimentary provenance exerts the greatest control on Li in fine grained sediments, and Li concentrations generally increase while delta7Li decreases with CIA (Chemical Index of Alteration) in post Archean shales, 2) sub-greenschist facies metamorphism has negligible effect on Li concentrations and isotopic compositions, 3) metamorphic dehydration from greenschist to granulite facies may cause significant Li depletion, but has had little influence on delta7Li, and 4) a key factor controlling [Li] in metapelites during metamorphism is the stability of Mg-bearing phyllosilicates. Furthermore, based on the understanding gained of the Li behavior above, the Li signature in Archean shales and granitoids may indicate that: 1) more severe weathering conditions prevailed during the Archean, 2) the Archean upper continental crust had a heavier Li isotopic composition than post Archean upper continental crust, and 3) Archean juvenile granitoids have heavier Li isotopic compositions than post-Archean equivalents. Collectively, the results for Archean samples may indicate that Li isotopes can be used as tracers of paleoclimate change, and also can be used to understand the nature of the source of Archean juvenile magmas.