HR: 09:30h
AN: V51B-05    [Abstracts]
TI: Talc and Chlorite 'Hybrid' Rocks in Subduction Melanges; Their Role in Fluid and Element Recycling Through Subduction Zones
AU: * Arculus, R J
EM: Richard.Arculus@anu.edu.au
AF: Department of Earth and Marine Sciences, Australian National University, Canberra, ACT 0200 Australia
AU: Spandler, C
EM: Carl.Spandler@anu.edu.au
AF: Department of Earth and Marine Sciences, Australian National University, Canberra, ACT 0200 Australia
AU: Hermann, J
EM: Joerg.Hermann@anu.edu.au
AF: Research School of Earth Sciences, Australian National University, Canberra, ACT 0200 Australia
AU: Mavrogenes, J
EM: John.Mavrogenes@anu.edu.au
AF: Department of Earth and Marine Sciences, Australian National University, Canberra, ACT 0200 Australia
AU: Mavrogenes, J
EM: John.Mavrogenes@anu.edu.au
AF: Research School of Earth Sciences, Australian National University, Canberra, ACT 0200 Australia
AB: The fluxing of H2O, CO2 and trace elements through subduction zones is one of the most important geochemical cycles on Earth. Volatile components released during progressive metamorphism of subducting oceanic crust are thought to cause melting of the overlying mantle wedge, producing arc magmas. The petrological evolution and devolatilisation of the principle components of subducting slabs (MORB, sediments, serpentinite) are now well constrained. However, volatile and trace element flux through subduction zones remains highly contentious. The eclogite-facies rocks of northern New Caledonia represents oceanic crust that was previously subducted to depths of ~60 km. Within the terrane are high-pressure melange zones consisting of a range of mafic, metasedimentary and ultramafic lithologies that have been strongly sheared and intermixed. These melange zones have been observed on sub-metre to kilometre scales and comprise a significant proportion of the terrane. The melanges contain hybrid rock-types, such as carbonate-rich talc and chlorite schists, that are not equivalents of any typical protolith found at the Earth's surface. Using field relations, bulk-rock geochemistry and oxygen isotopes, we suggest that these hybrid rocks formed during subduction by metasomatism and mixing of serpentinites, mafic rocks and metasediments. Subducted mafic rocks and serpentinites may deliver H2O to sub-arc depths, but fluid release occurs at temperatures too low (>650 ›¦C) to dissolve significant amounts of the slab-sourced elements that are found in arc magmas. Hybrid rock-types may comprise a significant proportion of subducting slabs and have the potential to transport large amounts of volatiles to sub-arc depths. Talc and chlorite-rich hybrid rocks will undergo almost complete dehydration at relatively high temperatures (~ 800 ›¦C) allowing for elevated element solubility in fluids or partial melting of adjacent pelitic and mafic rocks. By contrast, carbonates in these rocks are stable to very great depths and may be important for recycling C into the deep mantle. Therefore, hybrid rock-types in subducting slabs may be critical for element and fluid recycling through subduction zones and the evolution of arc magmas.
DE: 3660 Metamorphic petrology
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2005 Joint Assembly