Insight into hydrothermal and subduction processes from copper and nitrogen isotopes in oceanic metagabbros
Introduction
The geochemical evolution of Earth's mantle and external reservoirs are highly dependent on the fluxes of material that are output from mid-ocean ridges, volcanic arcs and hot spots, and input by lithospheric plates consumed in subduction zones. Chemical exchanges are mediated by magma degassing for volatile species (CO2, H2O, SO2, rare gases, and N2; e.g. Marty, 1995, Cartigny et al., 2008), or fluid–rock interaction during hydrothermal alteration on the seafloor and/or metamorphism in subduction zones (e.g. for elements such as Sr, K, Na, Fe, Mn, Cu, Zn, as well as volatile species; e.g. Staudigel, 2003, Bebout, 2013). The behavior and fluxes of elements transferred to the mantle can be explored by studying metamorphic rocks buried in paleo-subduction zones, and exhumed back to the surface by tectonic processes (Bebout and Fogel, 1992, Spandler et al., 2004, Halama et al., 2010). These rocks include various lithologies, which can be grouped as metasediments, metabasalts, metagabbros and metaperidotites. For a single lithology, the fate of volatiles in subduction zones is mainly controlled by the stability of the main water-bearing phases, which are strongly dependent on the geothermal gradient within the subduction zone (Bebout et al., 1999, Kerrick and Connolly, 2001). For instance, in subducted metagabbros, the dominant water-bearing phase is amphibole and rock dehydration corresponds to eclogitization (e.g. Nadeau et al., 1993). Although the behavior of any element in metagabbros could in theory be simply derived by comparing several samples subducted to different depths, a complication arises from the fact that fluid–rock interactions often occur during both hydrothermal alteration and subduction processes. This potential limit can be overcome by coupling multiple isotope tracers, with variable geochemical behaviors during hydrothermal alteration and subduction. In the present contribution, we illustrate that Cu and N isotopes can be used together as a powerful tracer for distinguishing subduction zone metamorphic effects from hydrothermal chemical modifications, and for understanding the fate of Cu and N in the subducting oceanic crust.
Nitrogen geochemical data on mafic and ultramafic rocks buried in subduction zones are scarce. Nitrogen concentrations and N values (, where the standard is atmospheric N2) of serpentinized metaperidotite analyzed to date range from 1.4 to 15 ppm, and 4 to 15‰, respectively (Halama et al., 2014, Philippot et al., 2007). In contrast, the N concentration in unaltered lithospheric peridotites is always lower than 1 ppm (Yokochi et al., 2009), supporting N enrichment during interaction with hydrothermal fluids on the seafloor. Although serpentinized metaperidotites may transfer a significant amount of N to the deep mantle, this N flux has never been assessed because of the poorly constrained mass flux of total subducting serpentinites. Interaction of seawater with the basaltic part of the oceanic crust produces secondary mineral phases that are able to store N. The concentrations of N in altered oceanic basalts vary from ∼1.3 to 18.2 ppm, with N values from −11.6 to 8.3‰ (Busigny et al., 2005a, Li et al., 2007). Nitrogen concentrations and N values of eclogitic metabasalts (2–20 ppm and −1 to 8‰, respectively) have similar ranges as altered oceanic basalts, suggesting that N that is added during hydrothermal processes on the seafloor is largely preserved during metamorphism in subduction zones (Halama et al., 2010). To our knowledge, N concentrations and isotope compositions of fresh/unaltered gabbros have never been measured. In a companion study, we analyzed ophiolitic metagabbros that were subducted to different depths in the Alps and found N contents and N values of 2.6 to 55 ppm and 0.8 to 8.1‰, respectively (Busigny et al., 2011). N values of low-strain metagabbros showed a negative correlation with Cu concentrations, interpreted as resulting from a release of a Cu–N chemical compound during hydrothermal alteration on the seafloor. No specific relation with metamorphic grade in subduction was observed, suggesting that minor modification affected Cu and N during burial metamorphism.
In order to further understand the origin of the relations between N and Cu, and to determine their fate during hydrothermal alteration and subduction, we measured Cu isotope compositions in the same sample set as reported in Busigny et al. (2011). We also conducted petrological analyses by optical and scanning electron microscopy (SEM) to determine the Cu-hosting minerals in Alpine metagabbros, which is crucial for interpreting the isotopic data. Primary igneous rocks generally display a restricted range of Cu isotope composition, with values averaging (Larson et al., 2003, Dekov et al., 2013, Liu et al., 2015, Savage et al., 2015; see compilation in Moynier et al., 2017). In contrast, significant Cu isotope variability is inherited from redox reactions during low-temperature supergene and hydrothermal alteration of primary Cu minerals (Larson et al., 2003, Rouxel et al., 2004, Mathur et al., 2005, Markl et al., 2006, Mathur and Fantle, 2015), with a total range of values from −16.5 to 10‰ (Moynier et al., 2017). Experimental determination of Cu(II)aq–Cu(I)sulfide isotope fractionation shows enrichment of aqueous Cu(II) in the heavy isotope by ∼2.5 to 3.5‰ for the temperature range between 0 and 100 °C (Ehrlich et al., 2004, Mathur et al., 2005, Asael et al., 2007). Slightly lower Cu isotope fractionation of ∼1.4‰ was obtained for chalcopyrite leaching under abiotic oxidative conditions at low pH (∼2) and a temperature of 25 °C, which possibly resulted from a kinetic isotope effect (Mathur et al., 2005, Kimball et al., 2009). Laboratory experiments of Cu partitioning between chalcopyrite and fluid at high temperature (250 and 300 °C) showed that the Cu isotope fractionation is controlled by several parameters such as pH, salinity, and partitioning between liquid and vapor phases (Maher et al., 2011). Additionally, Cu isotope fractionation between minerals and fluids depends heavily on the Cu speciation in the fluid, and therefore on the fluid composition. This is also illustrated by thermodynamic calculations of the equilibrium isotope fractionation for Cu complexes relevant to hydrothermal ore-forming fluids (Seo et al., 2007), Cu-bearing minerals and various aqueous Cu complexes and organic compounds (Sherman, 2013, Fujii et al., 2014). Overall, the mobility of Cu during fluid–rock interactions and associated isotope fractionation provide a strong potential for tracing hydrothermal alteration of the oceanic crust and subduction processes.
Section snippets
Sample description
The samples have been described in detail previously (Busigny et al., 2011) and are only briefly presented here. They correspond to ophiolitic metagabbros embedded in pelagic metasediments from the Schistes Lustrés nappe of the Piemonte-Ligurian domain, western Alps. The metagabbros experienced hydrothermal alteration on the seafloor and some of them were subsequently subducted in a cold slab environment (∼8 °C/km; Le Pichon et al., 1988). Samples have been collected in three different zones:
Analytical methods
Optical microscopy and scanning electron microscopy (SEM) in back-scattered electron and secondary electron imaging modes were used to characterize polished thin sections of Alpine metagabbros. The morphology and composition of the mineral phases were determined by SEM, with special emphasis on Cu-bearing phases and the potential association with nitrogen. Analyses were carried out in the electron microscopy platform at Institut de Physique du Globe de Paris using a Carl Zeiss EVO MA10 SEM.
Petrographic observations
Thin section observation of Alpine metagabbros by optical microscopy can be summarized as follows. The two metagabbros used as reference materials predating subduction zone processes (CH-129 and CH-80-02) are mainly composed of large crystals (mm to cm scale) of magmatic pyroxene (augite), plagioclase, apatite and ilmenite. These rocks experienced ocean crust hydrothermal alteration under greenschist- to amphibolite-facies conditions. Primary pyroxenes are partially replaced by actinolite,
Mantle-derived Cu and N signatures
All low-strain metagabbro samples show trends suggesting Cu and N release during either hydrothermal and/or subduction processes. The primary mantle-derived composition of the gabbros is difficult to establish but one can reasonably assume that it was close to the least depleted sample (i.e. sample CH80-02; Table 1), in terms of Cu concentration and isotope composition. This sample presents a Cu concentration of 73 ppm, typically in the range of Mid-Ocean Ridge Basalts (Fellows and Canil,
Conclusions
The present study demonstrates the coupling between Cu and N isotope systematics in metagabbros, and shows that they can be used successfully to decipher their geochemical behaviors during hydrothermal alteration and subduction zone processes. In the oceanic crust, Cu in primary magmatic gabbros was present as Cu+ in chalcopyrite while N likely occurred as NH in Ca–Na minerals. Although initially carried by different mineral phases, N and Cu were concomitantly released into hydrothermal
Acknowledgments
Colleagues from the Laboratory of Stable Isotope Geochemistry in IPGP are thanked for fruitful discussions. Jean-Louis Birck is thanked for long discussions about the data. Pascale Louvat and Julien Moureau are acknowledged for their technical assistance for MC-ICP-MS analyses. Peter Michael is thanked for careful reading and advise on the manuscript. Marc Quintin is thanked for making thin sections of all samples. Marguerite Godard and Catherine Mével greatly helped in evaluating Cu speciation
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