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| Titel |
Li isotopic constraints from the Erro-Tobbio serpentinites on Alpine subduction processes |
| VerfasserIn |
M.-F. Chu, M. Scambelluri, W. L. Griffin, S. Y. O'Reilly, N. J. Pearson |
| Konferenz |
EGU General Assembly 2012
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 14 (2012) |
| Datensatznummer |
250069794
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| Zusammenfassung |
| Subduction zones represent a unique feature of the dynamic Earth and provide important
constraints on how plate tectonics works. Subduction of serpentinized oceanic lithosphere,
characterized by releasing water into the mantle wedge via dehydration, i.e. breakdown of
hydrous minerals, plays a critical role in not only the generation of continental crust but also
the Earth’s water cycle. To track the recycling of water or fluid released by subduction, the
stable isotope system of Li, a lithophile and mobile element, shows its high potential because
7Li, relative to 6Li, preferentially moves into the fluid phase when fractionation occurs.
Here we present new Li abundance and isotopic compositions of the Erro-Tobbio
serpentinized peridotite complex, a remnant mantle slice of the Alpine subduction.
Our data indicate that most of the serpentinized ultramafic rocks have uniform Li
concentrations, around 1 ppm. These rocks, however, show variable Li isotopic compositions.
Among them, the high-pressure antigorite-bearing metaperidotites, formed under a
low-strain condition, have a limited range in Li isotopic values, with δ7Li = +1.5 to
+4.0, similar to those of serpentinized peridotites. In contrast, apparently heavier Li
isotopes, up to +10.0, are observed in the high-pressure serpentinite mylonites that also
contain antigorite but formed in high-strain domains. We note that O-H isotope
ratios of the high-pressure ultramafic rocks reported in previous study (Früh-Green
et al., 2001, Contrib. Mineral Petrol. 141: 145-159) show insignificant variations
between the low- and high-strain domains. This demonstrates the superiority of Li
isotope than conventional stable isotope systems in offering critical information about
fluid-releasing processes in subduction zones. Moreover, our new data unsupport the
general assumption that fluid released from subducted slabs is in favor of extracting
7Li than 6Li, thus capable of forming the much lighter δ7Li values observed in
eclogites or the subducted oceanic crust (e.g. Zack et al., 2003, Earth Planet. Sci. Lett.
208: 279-290). A mechanism aside from dehydration is required to account for the
markedly elevated Li isotopic values that we observe from the Alpine subduction zone. |
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