The geochemical and petrological evolution of subduction zones : insights from blueschist to eclogite-facies rocks from New Caledonia and high-pressure hydrothermal experiments

The return of oceanic crust, sediments, and lithosphere into the mantle at subduction zones is arguably the most important process affecting the chemical differentiation and evolution of Earth. In this thesis the geochemical and petrological processes governing subduction are investigated using blueschist and eclogite-facies rocks from New Caledonia and experiments designed to model subduction-zone conditions. The high-pressure metamorphic belt of northern New Caledonia is an excellent analogue of subducted oceanic crust. Bulk-rock geochemistry coupled with U-Pb SHRIMP dating of zircon from the belt reveal that the protoliths to the belt were formed in a back-arc basin between 85-55 Ma. The basin was subsequently subducted and metamorphosed up to eclogite-facies at 44 Ma. Subduction jamming at this time may also have led to a tectonic reconfiguration of the southwest Pacific region. Remarkably, evidence of lowtemperature (<100 °C) seafloor alteration of the oceanic crust prior to subduction is also preserved as a mineral assemblage in zircon from a mafic rock. This mineral assemblage includes the rare minerals thortveitite (SciSbCb) and yttrialite (Y2 Si2 C>7 ) that exsolved from zircon during recrystallisation and provide information on the mechanisms of trace element incorporation in zircon. The geochemistry of the eclogite-facies mafic rocks indicates that a variety of mafic rocktypes may comprise subducting oceanic crust. In particular, alkaline basalts and altered MORB may be important sources of many key trace elements (e.g. LILE) for arc magmas. Comparison between the high-grade rocks and unmetamorphosed equivalent rocks demonstrates that mafic and sedimentary rocks do not undergo significant geochemical modification during prograde dehydration up to eclogite facies. Trace elements are efficiently redistributed from lawsonite (REE, Sr, Pb) and titanite (REE, Nb, Ta, U) into garnet (HREE), epidote (Sr, Pb, U, Th, REE), and rutile (Nb, Ta) during conversion from blueschist to eclogite facies. Other important trace-element hosts such as phengite (LILE), allanite (LREE, Th, U, Sr, Pb), and zircon (Zr, Hf) are stable throughout the blueschist and eclogite facies. As significant volumes of H2 O are released across the blueschist to eclogite transition, there must be a decoupling of fluid and trace element release in subducting oceanic crust. High-pressure melange zones of northern New Caledonia contain talc and chlorite schists that formed prior to or during subduction by metasomatic mixing between mafic, ultramafic, and sedimentary rocks in the melange zones. These hybrid rocks dehydrate at higher pressures and temperatures than other subducting rock-types and hence may recycle volatiles to deep levels of subduction zones. Veins that cut eclogite from New Caledonia also provide insights into fluid recycling in subduction zones. These veins were formed over a prolonged period by locally cycling fluids trapped within the host rock and indicate that mineral dehydration may be decoupled from major fluid flow and seismic activity in slabs. Hydrothermal piston-cylinder experiments conducted on a synthetic pelite establish the wet solidus for representative subducted sediments at 675 °C at 2.2 GPa. Subsolidus fluid compositions determined from synthetic fluid inclusions have low element concentrations, implying that hydrous fluids in subduction zones do not flux significant amounts of trace elements from subducting slabs. By contrast, sediment-derived melts can completely account for the slab component of arc magmas, which requires that subducting slabs are warmer than many thermal models predict. The results of this work have crucial implications for mass transfer and element recycling in subduction zones. Subducting slab are now recognised to be petrologically and geochemically much more complex than previously acknowledged. Fluids released during prograde metamorphism are unlikely to alter the trace-element composition of the slab or contribute significantly to arc magmatism, but may produce new hybrid rock-types during localised metasomatism. At sub-arc depths dehydration of ultramafic and hybrid rocks or fluid escape from mafic rocks may induce partial melting of metasedimentary rocks to provide a flux of volatiles and trace elements for arc magma-genesis.