期刊名称：Proceedings of the National Academy of Sciences
印刷版ISSN：0027-8424
电子版ISSN：1091-6490
出版年度：2019
卷号：116
期号：4
页码：1132-1135
DOI：10.1073/pnas.1809164116
语种：English
出版社：The National Academy of Sciences of the United States of America
摘要：Indirect evidence for the presence of a felsic continental crust, such as the elevated 49Ti/47Ti ratios in Archean shales, has been used to argue for ongoing subduction at that time and therefore plate tectonics. However, rocks of intermediate to felsic compositions can be produced in both plume and island arc settings. The fact that Ti behaves differently during magma differentiation in these two geological settings might result in contrasting isotopic signatures. Here, we demonstrate that, at a given SiO2 content, evolved plume rocks (tholeiitic) are more isotopically fractionated in Ti than differentiated island arc rocks (mainly calc-alkaline). We also show that the erosion of crustal rocks from whether plumes (mafic in average) or island arcs (intermediate in average) can all produce sediments having quite constant 49Ti/47Ti ratios being 0.1–0.3 per mille heavier than that of the mantle. This suggests that Ti isotopes are not a direct tracer for the SiO2 contents of crustal rocks. Ti isotopes in crustal sediments are still a potential proxy to identify the geodynamical settings for the formation of the crust but only if combined with additional SiO2 information.
关键词：titanium isotopes ; plume ; island arc ; magma differentiation ; plate tectonics
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