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Internal and External Fluid Sources for Eclogite-Facies Veins In JOURNAL OF PETROLOGY VOLUME 52 NUMBER 6 PAGES 1207 ^ 1236 2011 doi:10.1093/petrology/egr025 Internal and External Fluid Sources for Eclogite-facies Veins in the Monviso Meta-ophiolite, Western Alps: Implications for Fluid Flow in Subduction Zones CARL SPANDLER1*, THOMAS PETTKE1 AND DANIELA RUBATTO2 1INSTITUTE OF GEOLOGICAL SCIENCES, UNIVERSITY OF BERN, BERN, CH-3012, SWITZERLAND 2RESEARCH SCHOOL OF EARTH SCIENCES, AUSTRALIAN NATIONAL UNIVERSITY, CANBERRA, ACT, 0200, AUSTRALIA RECEIVED DECEMBER 17, 2009; ACCEPTED APRIL 28, 2011 ADVANCE ACCESS PUBLICATION JUNE 3, 2011 To contribute to our understanding of the mechanisms and path- at least two separate growth mechanisms for the Monviso veins. ways of fluid movement through deeply subducted crust, we Most vein infillings were formed during progressive prograde meta- investigate high-pressure veins cutting eclogite-facies (2·0GPa morphism from locally derived fluid. Influx of the serpentinite- and 6008C) metagabbros of the Monviso Ophiolite, Italian derived or other external fluid was transient and episodic and was Western Alps. The veins consist mainly of omphacite with minor probably achieved via brittle fractures, which preferentially formed garnet, rutile, talc and accessory zircon. Most of the vein minerals along the pre-existing vein structures.The dehydration of serpentinite have major and trace element compositions that are comparable with at high pressures in subduction zones may provide crucial volatiles the host-rock minerals, and vein and host-rock zircons have similar and trace elements for arc magmas. Our results indicate that Hf isotopic compositions.These observations support the conclusions the movement of these fluids through subducted oceanic crust is of previous studies that these veins largely formed from a locally likely to be highly channeled and transient so the progressive develop- sourced hydrous fluid during prograde or peak metamorphism. ment of vein systems in mafic rocks may also be crucial for forming However, the bulk-rock Cr and Ni contents of the veins are signifi- channelways for long-distance fluid flow at depth in subduction cantly higher than those of the surrounding host eclogites. We also zones. document distinct Cr-rich (up to weight per cent levels) zones in | downloaded: 30.9.2021 omphacite, garnet and rutile in some vein samples.Vein garnet and talc also have relatively high MgO and Ni contents. X-ray maps of KEY WORDS: eclogite; fluid; Monviso; serpentinite; subduction; veins vein garnet and rutile grains reveal complex internal zoning features, which are largely defined by micrometre-scale variations in Cr con- tent. Some grains have concentric and oscillatory zoning in Cr, INTRODUCTION whereas others feature a chaotic fracture-like pattern.These Cr-rich Fluid flow through crustal rocks represents one of the most zones are associated with high concentrations of Ni, B, As, Sb, Nb, important processes governing mass and heat transfer in Zr and high ratios of light rare earth elements (LREE) to middle the Earth. In particular, the fluids produced by mineral REE (MREE) compared with low-Cr vein and host-rock min- dehydration in subducting plates are thought to be crucial erals. Petrological and mass-balance constraints verify that the for instigating mantle melting and the production of Cr-rich zones in the veins were not derived from internally sourced arc magmas (e.g. Peacock, 1993; Schmidt & Poli, 1998; fluids, but represent precipitates from an external fluid.The external Stern, 2002). Fluid movement in the slab at depth is also source that is consistent with the distinctive trace element character- invoked as the principal source of intermediate-depth istics of the vein components is antigorite serpentinite, which forms intra-slab earthquakes (Peacock, 2001; Hacker et al., 2003). the structural basement of the high-pressure metagabbros.We propose Quantification of the composition, evolution and flow of https://doi.org/10.7892/boris.10624 *Corresponding author. Present address: School of Earth and ß The Author 2011. Published by Oxford University Press. All Environmental Sciences, James Cook University, Townsville, QLD, rights reserved. For Permissions, please e-mail: journals.permissions@ source: 4811, Australia. E-mail: [email protected] oup.com JOURNAL OF PETROLOGY VOLUME 52 NUMBER 6 JUNE 2011 fluids in subduction zones concerns many aspects of the on anomalous Cr-rich zones within vein minerals in an Earth Sciences. attempt to better constrain the sources of the fluid respon- The most important sources of fluids in subduction sible for vein formation. Our new results indicate that zones are expected to be devolatilizing mafic oceanic vein formation is significantly more complex than indi- crust and serpentinite (Ulmer & Trommsdorff, 1999; cated by previous models and, hence, provide new insights Poli & Schmidt, 2002). However, direct evidence of into the mechanisms of fluid flow through subducting high-pressure (HP) serpentinite dehydration is rarely pre- crust. This work may also ultimately contribute to a bet- served in the rock record. Recent experimental efforts ter understanding of mass transfer and seismicity in sub- have made progress in characterizing the chemistry of duction zones. fluids at high pressure and temperature (e.g. Manning, 2004; Kessel et al., 2005; Spandler et al., 2007; Hermann & Spandler, 2008; Klimm et al., 2008), but relatively little is GEOLOGICAL SETTING known of how fluids migrate through, or react with, sub- The Monviso Ophiolite is a 35 km long, 8 km wide north^ ducting slabs at depth (see Scambelluri & Philippot, 2001; south-trending complex that is tectonically sandwiched Hermann et al., 2006; Zack & John, 2007). between the Queyras sedimentary Unit (Schistes Lustre¤s) Exhumed HP and ultrahigh-pressure (UHP) meta- to the west and the Dora Maira continental unit to the morphic rocks represent fragments of previously subducted east. Detailed geological descriptions and maps of the lithosphere and, hence, may be studied to gain insights ophiolite have been published elsewhere (e.g. Lombardo et al., 1978; MONVISO, 1980; Schwartz et al., 2000), hence, into deep subduction-zone processes. Evidence that fluids here we present only a geological overview of the region. flowed through these rocks at high pressure is preserved The Monviso complex consists of a series of serpentinite, as cross-cutting veins that comprise eclogite-facies mineral metagabbro and metabasalt units with subordinate mica- assemblages (e.g. Philippot, 1993; Scambelluri et al., 1998; and calc-schist (Lombardo et al., 1978, 2002). Serpentinite Becker et al., 1999; Rubatto & Hermann, 2003; Spandler & is the dominant rock type of the complex (Schwartz et al., Hermann, 2006; Gao et al., 2007; John et al., 2008). A com- 2001). The entire sequence originally formed part of the prehensive understanding of how such veins form is yet to Jurassic westernTethys oceanic lithosphere, which was sub- be reached, as these veins often have complex growth his- sequently subducted and metamorphosed under eclogite- tories and many properties of fluids under high pressures facies conditions during the Tertiary (Lombardo et al., are poorly known. Nevertheless, careful study can reveal 1978, 2002; Cliff et al., 1998; Rubatto & Hermann, 2003). important information on the source, composition and Conditions of metamorphism vary from unit to unit, evolution of the fluids that formed the veins and, hence, which indicates that the current arrangement of some can provide insights into the fluid flow regimes in subduc- units may have occurred only during the rapid exhumation tion zones. (1cm aÀ1) of the massif (Schwartz et al., 2000, 2001). Perhaps the most extensively studied examples of The samples examined in this study derive from the HP veining in eclogitic rocks occur within the Monviso Lago Superiore Unit in the central part of the complex Ophiolite of the Western Alps, Italy. The Monviso (see Lombardo et al., 1978; Schwartz et al., 2000). The Ophiolite represents a relatively coherent section of ocean- Lago Superiore Unit consists of highly deformed Mg^Al ic crust, which was subducted during the Tertiary and Fe^Ti metagabbros and talc schists, which are struc- (Lombardo et al., 1978, 2002; Rubatto & Hermann, 2003). turally underlain by antigorite serpentinites. The presence Within metagabbroic rocks are omphacite-rich veins, of rodingite dykes in the serpentinite body and oxygen iso- which have been the subject of detailed structural and tope compositions of the metagabbros indicate that these petrological (Philippot, 1987; Philippot & Kienast, 1989; rocks underwent hydration and alteration at, or near, Philippot & van Roermund, 1992), fluid inclusion the seafloor, prior to subduction (Nadeau et al., 1993; (Philippot & Selverstone, 1991; Nadeau et al., 1993), stable Philippot et al., 1998; Lombardo et al., 2002). Continuity of isotope (Nadeau et al., 1993), geochronological (Rubatto & structural fabrics (Schwartz et al., 2000), serpentine min- Hermann, 2003) and geochemical (Philippot & eralogy (Auzende et al., 2006), the presence of eclogitic Selverstone, 1991; Rubatto & Hermann, 2003) studies. The blocks within the serpentinite unit (Blake et al., 1995) and general consensus reached by these studies is that the widespread retrograde overprinting features (Blake et al. veins formed as precipitates of locally derived hydrous 1995; Schwartz et al., 2000) indicate that both metagabbro fluid during ductile deformation at high pressure. and serpentinite
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