An Experimental Study on COH-bearing Peridotite up to 3·2 GPa and Implications for Crust–Mantle Recycling

JOURNAL OF PETROLOGY VOLUME 54 NUMBER 3 PAGES 453^479 2013 doi:10.1093/petrology/egs074 An Experimental Study on COH-bearing Peridotite up to 3·2 GPa and Implications for Crust^Mantle Recycling DIPARTIMENTO DI SCIENZE DELLA TERRA, UNIVERSITA' DI MILANO, VIA MANGIAGALLI 34, I-20133 MILANO, ITALY RECEIVED SEPTEMBER 14, 2011; ACCEPTED SEPTEMBER 14, 2012 ADVANCE ACCESS PUBLICATION NOVEMBER 12, 2012 We experimentally investigated phlogopite- and C^O^H-bearing lherzolite to model the mantle wedge fluxed by volatiles released from a subducting crustal slab. Experiments have been carried out at 900^10508C and 1·6^3·2 GPa, at fluid- and carbon-saturated conditions. We used an end-loaded piston cylinder apparatus and a conventional double-capsule technique to constrain the redox state of the experiments, using the nickel^nickel oxide oxygen buffer (NNO). Following thermodynamic calculations, we expect inner fO2 values to be systematically below NNO, with fluids that are mixtures of CO2 and H2O. Estimated fO2 in the runs are between FMQ  ^0·7 at 3 GPa and FMQ  ^1·1 at 1·8 GPa, values that have been reported for natural mantle-wedge xenoliths. At the conditions investigated, the hydrous phases are phlogopite and pargasitic amphibole. Whereas phlogopite is ubiquitous, amphibole disappears at 3·1 GPa at 9008C and 2·7 GPa at 10508C, where the solidus is encountered. The amphibole-out reaction also consumes orthopyroxene and liberates water. From low to high P, we observed first carbonate-free, amphibole-bearing assemblages, then carbonate þ amphibole-bearing assemblages, and finally amphibole-free, carbonate-bearing assemblages. Carbonate-free assemblages melt to produce trachyandesite at T410508C, whereas dolomitic carbonatites have been found beyond the solidus of carbonate-bearing assemblages. Carbonates occur as dolomite at 51·9 GPa, 9008C and at 52·1 GPa, 10508C; magnesite at 42·4 GPa, 9008C and 42·7, 10508C; between these limits, a magnesite þ dolomite-bearing assemblage constitutes a two-carbonate field. P^T pseudosections fail to reproduce the experimental results concerning amphibole breakdown and reaction positions involving carbonates. The amount of COH fluid is thought to have a major role, even in fluid-saturated peridotites. Clinopyroxene and olivine are not expected at *Corresponding author. Telephone: þ39-02-5031-5625. Fax: þ39-02-5031-5597. E-mail: fluid-oversaturated conditions, for which dolomite or magnesite are stable respectively. The presented results are useful for unravelling the exhumation history of orogenic lherzolites bearing COH phases and to suggest a way to transfer carbon species to the mantle wedge. We suggest that once carbon-bearing fluids react with mantle-wedge peridotites, a sort of buoyant ‘cold plume’ will form containing low-density phases such as amphibole, carbonates and carbonatitic melt. This plume could represent an important source of CO2 and H2O, and it is one of a series of processes that ultimately lead to arc magmatism. KEY WORDS: peridotite; experimental petrology; subduction; arc mag- matism; redox I N T RO D U C T I O N There is a general consensus that the mantle wedge overlying a subducting slab is fluxed by aqueous fluids originating from the dehydration of the down-going oceanic lithosphere and its sedimentary cover (see review by Schmidt & Poli, 2003). These fluids are enriched in large ion lithophile elements (LILE) and light rare earth elements (LREE) (e.g. Morris & Ryan, 2003). Therefore, they can promote the crystallization of hydrous minerals in mantle-wedge peridotite, notably amphibole and phlogopite, and even epidote-group minerals (e.g. allanitic epidote; Yang & Enami, 2003; Tumiati et al., 2005). Hydration of peridotite may favour asthenospheric flow in the mantle wedge by decreasing density, viscosity and ß The Author 2012. Published by Oxford University Press. All rights reserved. For Permissions, please e-mail: journals.permissions@ oup.com SIMONE TUMIATI*, PATRIZIA FUMAGALLI, CARLA TIRABOSCHI AND STEFANO POLI JOURNAL OF PETROLOGY VOLUME 54 MARCH 2013 influence its carbon dissolution potential (e.g. chlorine; Newton & Manning, 2010). The geological record from orogenic peridotites High-pressure (HP) peridotite samples containing both hydrous and carbonate phases are relatively scarce, mostly because carbonates tend to decompose upon decompression (Canil, 1990). Recently, dolomite- and magnesite-bearing peridotites have been described from Su-Lu in China (Zhang et al., 2007), Bardane in Norway (van Roermund et al., 2002; Scambelluri et al., 2008), Ulten and Finero in the Italian Alps (Zanetti et al., 1999; Sapienza et al., 2009) and the Moldanubian Massif (Naemura et al., 2009). Carbonate occurrence is in most cases recorded as preserved inclusions in garnet and pyroxenes. The hydrous phases invariably associated with the carbonates are phlogopite in ultra-HP terranes (e.g. Su-Lu, Bardane), and phlogopite þ amphibole in HP terranes (e.g. Ulten, Moldanubian Massif). Carbonates are magnesite  dolomite in ultra-HP terranes, whereas dolomite alone occurs at HP conditions. At Bardane, the dolomite exhibits a corona of magnesite and clinopyroxene, interpreted as a prograde reaction of the type dolomite þ orthopyroxene ¼ magnesite þ clinopyroxene (Scambelluri et al., 2008). Together with carbonates and hydrous phases, elemental carbon has also been found, for example in multiphase inclusions, occurring as diamond in ultra-HP peridotites and as graphite in lower-P peridotites (e.g. van Roermund et al., 2002; Naemura et al., 2009). However, graphite and diamond may occur as accessory phases more frequently than previously believed. Microdiamonds and nanoscale graphite interlayered in phlogopite (e.g. Finero peridotite, Ferraris et al., 2004) are difficult to detect and can easily be overlooked. Nevertheless, their presence is of primary interest because the variance of the system is lowered. Precipitation of a carbon polymorph implies that mixed fluids metasomatizing peridotites should be adequately described in the COH system. The boundary that defines the carbon-saturated system is often referred to as the graphite-boundary (Holloway & Reese, 1974) or graphite saturation surface (Connolly, 1995). Along the graphite saturation surface, fluid speciation is fixed if a redox condition, fO2, is imposed externally. This implies that the fluid composition cannot be arbitrarily chosen independently from the iron oxidation state of the rock and from this observation stems the importance of conducting reference experimental studies at controlled fO2 conditions. Although the dataset on carbonate-bearing orogenic peridotites is relatively limited, carbonates, whenever they occur, are invariably associated with hydrous phases. This actually often applies to subcratonic peridotites as well (e.g. Logvinova et al., 2008), suggesting that COH-bearing 454 rock strength (Gerya et al., 2002; Arcay et al. (...truncated)