Geochemical heterogeneity and element mobility in deeply subducted oceanic crust; insights from high-pressure mafic rocks from New Caledonia

Bulk-rock major and trace element geochemistry of a range of eclogite, garnet blueschist and garnet amphibolite rocks from northern New Caledonia has been determined in order to geochemically characterise subducted oceanic crust. The rocks experienced peak metamorphic conditions of 1.9 GPa and 600 °C and represent excellent samples of oceanic crust that was subducted to depths of approximately 60 km. The rocks can be divided into seven rock types that respectively have geochemical characteristics of enriched and normal mid-ocean ridge basalt, back-arc basin basalt, alkaline basalt, plagioclase-rich cumulate, seafloor-altered basalt and Fe-Ti basalt. All of the samples studied represent a single slice of oceanic crust interpreted to have formed in a back-arc or marginal basin setting. Examination of modern oceanic crust suggests that most subducting crust also contains a diverse range of mafic rock-types. The presence of minor amounts of alkaline and seafloor-altered basalts in the slab can greatly influence the recycling of incompatible elements and the depth of fluid release during subduction. Comparison of the high-grade metamorphic rocks with equivalent igneous rocks from western New Caledonia demonstrates that the main chemical variations of the rocks are related to differences in their magmatic history and to different degrees of seafloor alteration, whereas high-pressure metamorphism produced only minor changes. There is evidence for some depletion of LILE and B during subduction in a few of the analysed samples. However, most of the blueschists and eclogites with protoliths showing high LILE contents such as the back-arc basin and alkaline basalts still contain high LILE contents of 10-100 times the amount found in normal mid ocean ridge basalts. Therefore, even fluid mobile elements (B, LILE) may be efficiently subducted to sub-arc depths. Trace elements are most likely to be removed from the slab in regions of elevated temperature or in zones of intense fluid-rock interaction or partial melting. In contrast to the trace elements, large volumes of fluid are liberated from mafic rocks prior to eclogite-facies metamorphism, providing evidence for a decoupling of fluid and trace element release in subducted oceanic crust