View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by Bern Open Repository and Information System (BORIS) Per. Mineral. (2007), 76, 2-3, 253-265 doi:10.2451/2007PM0028 SPECIAL ISSUE: In honour of Ezio Callegari on his retirement http://go.to/permin An International Journal of l PERIODICO di MINERALOGIA MINERALOGY, CRYSTALLOGRAPHY, GEOCHEMISTRY, established in 1930 ORE DEPOSITS, PETROLOGY, VOLCANOLOGY and applied topics on Environment, Archaeometry and Cultural Heritage Subduction fluids and their interaction with the mantle wedge: a perspective from the study of high-pressure ultramafic rocks MARCO SCAMBELLURI 1 *, NADIA MALASPINA 2 and JOERG HERMANN 3 1 Dipartimento per lo Studio del Territorio e delle sue Risorse, Università di Genova, Italy 2 Dipartimento di Scienze della Terra, Università di Milano, Italy 3 Research School of Earth Sciences, The Australian National University, Canberra, Australia ABSTRACT. — We review three case studies bodies. Another aspect regards the debate whether emphasizing the role of ultramafic rocks in the supercritical fluids or hydrous melts are effective recycling of volatiles and trace elements at convergent media for trace element transport. Since both agents plate margins. Serpentinites are major water carriers are saturated in silica, they will react with the silica- in subduction zones and their breakdown liberates undersaturated mantle wedge peridotites to produce large quantities of water at sub-arc depths. The aqueous, incompatible trace element-rich residual incompatible elements incorporated during oceanic fluids. Hence, while hydrous melt and/or supercritical serpentinization are released into the fluid phase fluids are important for scavenging incompatible produced once antigorite dehydrates to olivine + elements from the slab, they may not be the agents orthopyroxene. Importantly, the antigorite breakdown that transfer the metasomatic subduction signature to | downloaded: 13.3.2017 can trigger either wet melting or production of the inner parts of the mantle wedges. supercritical fluids in altered basalts and sediments. The produced fluid phases incorporate substantial RIASSUNTO. — Questo contributo riassume tre casi di amounts of incompatible element, initially residing in studio che evidenziano il ruolo delle rocce ultrafemiche the crustal reservoirs. The fluid phase which exits the nei processi di riciclo delle sostanze volatili e degli slab is highly reactive with respect to the overlying, elementi in traccia ai margini di placca convergenti. silica undersaturated, mantle rocks. This leads to Le serpentiniti sono i sistemi maggiormente formation of reactive (ortho)pyroxenite layers, which responsabili per il trasporto dell’acqua nelle zone di filter the uprising hydrous melt/supercritical fluid to subduzione, dove liberano grandi quantità di acqua a produce aqueous, solute-rich solutions. This fluid profondità di sub-arco a causa della disidratazione del has equilibrated with peridotites and is mobile in the serpentino. Gli elementi incompatibili incorporati da mantle. queste rocce durante l’alterazione oceanica, vengono A consequence of these subduction fluid/mantle rilasciati nel fluido prodotto dalla disidratazione del reactions is that the mantle wedge domains overlying serpentino. L’acqua rilasciata dall’antigorite può the slabs can be heterogeneous in composition and innescare la fusione parziale o la formazione di fluidi http://boris.unibe.ch/85742/ layered, due to the presence of reactive pyroxenite supercritici nei livelli di rocce basaltiche e meta- sedimentarie costituenti la placca subdotta. I fusi o i * Corresponding author, E-mail: fluidi così prodotti incorporano quantità significative [email protected] di elementi maggiori (oltre il 50 % in peso) e in traccia source: 254 M. SCAMBELLURI, N. MALASPINA and J. HERMANN originariamente presenti nelle rocce crostali. La fase distinction between aqueous fluids and hydrous fluida rilasciata dallo slab subdotto è altamente silicate melts, which characterizes all rock systems reattiva rispetto alle soprastanti rocce di mantello e at relatively low pressures and temperatures, causa la formazione di livelli reattivi a ortopirosseno. vanishes at ultrahigh-pressure conditions, where Questi livelli ‘filtrano’ i fluidi supercritici e/o i complete miscibility between water and silicate fusi idrati uscenti dallo slab per produrre un fluido melts has been experimentally attained in a range acquoso residuale ricco in soluto: quest’ultimo si of P-T conditions and of bulk rock compositions è equilibrato con le peridotititi di mantello ed è in (�ureau and Keppler, 1999; Stalder et al., 2001; grado di migrare all’interno del wedge di mantello. Schmidt et al., 2004; Hermann et al., 2006; Kessel Una conseguenza di queste reazioni fluido/mantello è et al., 2005). The existence of a second critical che i dominii del cuneo di mantello soprastanti lo slab end point, where the wet solidus terminates and a sono composizionalmente eterogenei e ‘stratificati’ a causa della presenza dei livelli di pirosseniti supercritical liquid forms, opened the debate on the reattive. Un altro aspetto di queste ricerche riguarda role of supercritical fluid phases as metasomatic l’efficienza dei fluidi supercritici o dei fusi idrati agents in deep subduction environments. come agenti di trasporto degli elementi in traccia nel Studies of natural eclogite-facies rocks provide mantello. Entrambi gli agenti sono ricchi in silice e important constraints to the understanding of deep la loro reazione con il mantello libera fluidi acquosi subduction fluids and their interaction with slab and mobili arricchiti in elementi incompatibili. Di mantle wedge rocks. The high (HP) and ultrahigh- conseguenza, mentre i fusi idrati e i fluidi supercritici pressure (UHP) rocks exposed in orogenic sono importanti per incorporare elementi dai serbatoi terrains provide independent constraints on deep crostali nello slab, essi non sono gli agenti che metamorphism in slabs, and represent exceptional trasferiscono alle parti interne del cuneo di mantello natural laboratories on subduction-zone processes l’impronta metasomatica subduttiva. in a depth window between 50 and 200 kilometers. Some ultradeep coesite-, diamond- and majorite- bearing rocks preserve primary solid multiphase INTRODUCTION inclusions (Van Roermund et al., 2002; Stoeckert et al., 2001; Ferrando et al., 2005; Malaspina et Subduction zone fluids play a fundamental al., 2006; Scambelluri et al., 2007), which have role in large-scale mass transfer at convergent been interpreted in some case as remnants of a plate margins, as they transfer volatiles and supercritical fluid phase. incompatible elements from crustal reservoirs in Ultramafic rocks play a fundamental role in the subducting plates to the overlying mantle. The volatile and element recycling at convergent plate fluid transport leads to metasomatism of the mantle margins. Field studies have shown that serpentinite wedge peridotites and triggers partial melting in is stable at eclogite-facies conditions and hence regions where peridotites are above the wet solidus can transport water into the mantle (Scambelluri et temperatures. �����������������������������ased on detailed geochemical al., 1995). Experiments demonstrate the prolonged studies of arc lavas, it has been inferred that stability of antigorite serpentine to 200 km depth and subduction fluids are enriched in large ion litophile identify hydrous ultramafic systems as exceptional (LILE) and light rare earths (LREE) relative to the water carriers into the Earth’s mantle (Ulmer and high field strength elements (HFSE; McCulloch Trommsdorff, 1995; Wunder and Schreyer, 1997; and Gamble, 1991; �renan et al., 1994). ����The �romiley and Pawley, 2003). These findings have crust-to-mantle exchange at subduction zones thus important consequences on subduction dynamics impacts on mantle re-fertilization and is a major because serpentinites provide a particularly driving force for the chemical differentiation of fertile water reservoir for arc magmatism (Ulmer the Earth. The role of fluids in such a cycle has and Trommsdorff, 1995), and because their been increasingly emphasized in the last decade dehydration can generate intermediate-depth (50- and an ongoing debate concerns their nature, 200 Km) earthquakes (Peacock, 2001; Dobson et composition and effective mobility (Scambelluri al., 2002). Serpentinites also act as low density and and Philippot, 2001; Manning, 2004; Hermann low viscosity media enabling the exhumation of et al., 2006; Zack and John, 2007). The clear high and ultrahigh pressure rocks (Hermann et al., Subduction fluids and their interaction with the mantle wedge: a perspective from the study ... 255 2000; Guillot et al., 2001; Rupke et al., 2004). The inherited from a previous oceanic lithosphere, as mantle domains overlying the subducting plates are documented in present-day fast spreading ridges. other environments where ultramafic rocks play a In such settings, serpentinization of the oceanic key role, as the fluid/peridotite interactions at the mantle occurs at outer rises, where fractures in the slab/mantle interface can control the composition bending plates enhance seawater infiltration at and of fluids, which are transferred to the inner parts deep mantle serpentinization (Ranero et al., 2003; of the mantle wedges. However,�������������������������� the understanding Peacock, 2001). Alternatively, part of the layered of mechanisms