Petrology and Trace Element Budgets of High-pressure Peridotites Indicate Subduction Dehydration of Serpentinized Mantle (Cima di Gagnone, Central Alps, Switzerland)

JOURNAL OF PETROLOGY VOLUME 55 NUMBER 3 PAGES 459^498 2014 doi:10.1093/petrology/egt068 Petrology and Trace Element Budgets of High-pressure Peridotites Indicate Subduction Dehydration of Serpentinized Mantle (Cima di Gagnone, Central Alps, Switzerland) MARCO SCAMBELLURI1*, THOMAS PETTKE2, ELISABETTA RAMPONE1, MARGUERITE GODARD3 AND ERIC REUSSER4 1 DIPARTIMENTO DI SCIENZE DELLA TERRA, AMBIENTE E VITA, UNIVERSITY OF GENOVA, CORSO EUROPA 26, 16132 GENOVA, ITALY 2 INSTITUTE OF GEOLOGICAL SCIENCES, UNIVERSITY OF BERN, BALTZERSTRASSE 1, CH-3012, BERN, SWITZERLAND 3 GEOSCIENCES MONTPELLIER, UNIVERSITE MONTPELLIER 2, PLACE EUGENE BATAILLON 34095, MONTPELLIER, FRANCE 4 ETH ZU«RICH, INSTITUT FU«R GEOCHEMIE UND PETROLOGIE, CLAUSIUSSTRASSE 25, 8092 ZU«RICH, SWITZERLAND RECEIVED JANUARY 19, 2013; ACCEPTED OCTOBER 24, 2013 ADVANCE ACCESS PUBLICATION JANUARY 7, 2014 At Cima di Gagnone, garnet peridotite and chlorite harzburgite lenses within pelitic schists and gneisses correspond to eclogite-facies breakdown products of hydrated peridotites and are suitable for studying dehydration of serpentinized mantle.Thermobarometry and pseudosection modelling yield peak temperatures of 750^8508C and pressures53 GPa. The minimum temperature recorded by the garnet peridotite corresponds to the maximum conditions experienced by the chlorite harzburgite, suggesting that these rocks recrystallized cofacially at 8008C. Alternatively, they might have decoupled during subduction, as achieved in tectonically active plate interface boundaries. The major and rare earth element (REE) variability of the peridotites was mostly acquired during pre-subduction mantle evolution as a result of partial melting and reactive melt flow. The ultramafic suite is also characterized by fluid-mobile element enrichments (B, Pb, As, Sb, Cs, Li, U, Be), which confirm derivation from variably serpentinized protoliths. Similarity in the U, Pb, B, Li and Sr contents of the Gagnone peridotites to present-day oceanic serpentinites suggests that these elements were partly taken up during initial serpentinization by seawater-derived fluids. Positive Be, As and Sb anomalies suggest involvement of fluids equilibrated with crustal (metasedimentary) reservoirs during *Corresponding author. E-mail: subsequent subduction metamorphism and peridotite entrainment in (meta)sediments. Fluid-mobile element enrichment characterizes all peak eclogitic minerals, implying that multiple hydration events and element influx pre-dated the eclogite-facies dehydration. Peak anhydrous minerals retain B, Li, As and Sb concentrations exceeding primitive mantle values and may introduce geochemical anomalies into the Earth’s mantle.The relatively low contents of large ion lithophile elements and light REE in the Gagnone peridotites with respect to much higher enrichments shown by metasomatized garnet peridotite pods hosted in migmatites (Ulten Zone, Eastern Alps) suggest that the crustal rocks at Gagnone did not experience partial melting. The Gagnone garnet peridotite, despite showing evidence for chlorite dehydration, retains significant amounts of fluid-mobile elements documenting that no partial melting occurred upon chlorite breakdown. We propose that the Gagnone ultramafic rocks represent a prime example of multi-stage peridotite hydration and subsequent dehydration in a plate interface setting. KEY WORDS: garnet peridotite; chlorite harzburgite; subduction; serpentinized mantle; trace elements; subduction fluid; subduction me¤lange ß The Author 2014. Published by Oxford University Press. All rights reserved. For Permissions, please e-mail: journals.permissions@ oup.com JOURNAL OF PETROLOGY VOLUME 55 I N T RO D U C T I O N In recent years there has been an explosion of interest in serpentinites. This is due to their role as (1) volatile and fluid-mobile element repositories in the ocean basins and in subduction zones, (2) triggers of arc magmatism and of seismic activity, (3) the main constituents of low-viscosity channels located at plate interfaces, which affect both subduction and exhumation dynamics, (4) producers of hydrogen and methane with implications for the potential origin of life in Earth, (5) reservoirs for CO2 sequestration (Ulmer & Trommsdorff, 1995; Scambelluri et al., 1997, 2004a,b; Schmidt & Poli, 1998; Hermann et al., 2000; Guillot et al., 2001; Schwartz et al., 2001; Dobson et al., 2002; Gerya et al., 2002; Straub & Layne, 2003; Jung et al., 2004; Sharp & Barnes, 2004; Federico et al., 2007; Hilairet et al., 2007; Kelemen & Matter, 2008; Barnes & Straub, 2010; Deschamps et al., 2010; Evans, 2010). Interest in serpentinites has progressively increased since the second half of the 1990s, when ocean drilling programs documented their wide exposure in the Atlantic seafloor and combined experimental and field studies demonstrated antigorite stability to eclogite-facies conditions and to subarc depths (Cannat et al., 1995; Scambelluri et al., 1995; Ulmer & Trommsdorff, 1995). These rocks have also been identified along mega-fractures in bending subducting plates (Ranero et al., 2003; Alt & Shanks, 2006) and in mantle-wedge domains flushed by ascending slab fluids (Peacock, 1993; Bostock et al; 2002; Savov et al., 2005). Since these discoveries, the contribution of hydrated (serpentinized) mantle to volatile and light element recycling in arc magmas, and its involvement in subduction and exhumation tectonics, has been investigated and modelled (Gerya et al., 2002; Scambelluri et al., 2004b; Marschall et al., 2006; Bonifacie et al., 2008; Faccenda et al., 2008; Healy et al., 2009; Barnes & Straub, 2010; John et al., 2011; Kendrick et al., 2011; Scambelluri & Tonarini, 2012). Understanding the role of serpentinites in subduction zones is thus highly relevant and timely for constraining geochemical recycling and tectonics at convergent plate margins. A major event in the subduction cycle of hydrated mantle concerns the relevant fluid loss (5^13 wt % water) related to final breakdown of antigorite serpentine to olivine plus orthopyroxene at sub-arc depths (Ulmer & Trommsdorff, 1995; Schmidt & Poli, 1998; Scambelluri et al., 2001; Alt et al., 2012). De-serpentinized peridotites are made of anhydrous minerals with subordinate modal amounts of chlorite and Ti-clinohumite (Trommsdorff et al., 1998; Ulmer & Trommsdorff, 1999; Fumagalli & Poli, 2005), which control the water budgets and partial melting of ultramafic rocks beyond antigorite stability. Several experimental studies have recently discussed the relevance of chlorite breakdown to garnet-bearing assemblages for fluid release at sub-arc depths (Fumagalli & NUMBER 3 MARCH 2014 Poli, 2005; Grove et al., 2006). Besides providing additional water loss from subducting ultramafic systems, the chlorite dehydration curve may intersect the H2O-saturated peridotite solidus from 2 to 3·6 GPa, potentially triggering peridotite dehydration-melting and arc magmatism (Grov (...truncated)