Nitrogen and noble gases reveal a complex history of metasomatism in the Siberian lithospheric mantle
Introduction
The SCLM, which formed through the repeated under-thrusting of oceanic slabs beneath stable continental crust (Griffin and O'Reilly, 2007), represents a relatively minor component of the Earth's depleted mantle (∼ 1.5 vol%). It has remained isolated from the convecting asthenosphere over billion-year time-scales (Pernet-Fisher et al., 2015) and now forms the interface between the mantle and the stable continental crust. Due to its isolation from the convecting mantle, the SCLM has retained local geochemical heterogeneities introduced through interactions with mantle-plumes, crustal and/or subduction-related sources since its formation. The SCLM therefore represents a potentially important geological reservoir where energy and mass fluxes are greatly-enhanced and focused, acting as magnifying lenses into metal and volatile transport fractionation, ultimately leading to ore deposition (Holwell et al., 2019). The ability of the SCLM to retain metasomatic components over geologically-significant periods of time indicates that it could potentially constitute a significant long-term reservoir of volatile elements (Broadley et al., 2018a). Furthermore, the release of volatiles stored in the SCLM to the surface during thermal events associated with plume impingement and rifting could have globally-significant effects on the environment, which further highlights the importance of understanding the contribution of the SCLM to the global volatile budget (Broadley et al., 2018a). This is particularly the case for S-SCLM, where the thick stable cratonic lithosphere may have accumulated significant quantities of metasomatic volatiles over billions of years (Broadley et al., 2018a).
Isotopic and elemental signatures of volatiles such as nitrogen, noble gas and halogens can be used to quantify the extent of metasomatic modification and potentially reveal important information about the source(s) of volatiles within the SCLM. Coupled noble gas and halogen studies have been used in the past to suggest that a significant proportion of volatiles in the mantle may have been introduced by the subduction of marine pore fluids, serpentinites and altered oceanic crust (AOC) (Sumino et al., 2010; Kendrick et al., 2011; Chavrit et al., 2016; Broadley et al., 2016). Mantle xenoliths sourced from the SCLM contain volatiles hosted in self-contained fluid inclusions that are unlikely to be contaminated by surface components. They can therefore provide a direct window into the volatile composition of the SCLM. From the analysis of SCLM xenoliths, it has been suggested that the volatiles trapped in the SCLM may also originate from surface-derived metasomatic fluids (e.g., Broadley et al., 2016) that were introduced during periods of subduction and went on to pervasively modify the composition of the SCLM.
Geochemical investigations of N isotopes in various terrestrial reservoirs have revealed a discernible N isotopic contrast between surface reservoirs and the mantle. For example, the DMM is estimated to have a N of ∼ -5 ± 2‰ (Javoy et al., 1986; Marty and Zimmermann, 1999) (normalized to air, Nair = 0‰). In contrast, the deep mantle, as sampled by the Kola magmatic province, Iceland, Yellowstone, Loihi Seamount, Hawaii and the Society Islands, is enriched in 15N relative to air by up to +12‰ (Dauphas and Marty, 1999; Halldórsson et al., 2016; Labidi et al., 2020), with a mean N value of +3 ± 2‰. Modern ocean floor sediments are also enriched in 15N, with N values ranging from +5 to +7‰ (e.g., Peters et al., 1978), indicating that high N material may be recycled into the deep mantle by modern subduction processes (Barry and Hilton, 2016; Bekaert et al., 2021). Nitrogen isotopes may therefore be able to provide a new insight into the origin of volatiles in the SCLM.
Despite a number of studies over the past two decades (e.g., Matsumoto et al., 2002; Yokochi et al., 2009; Yamamoto et al., 2020), the nitrogen isotope composition of SCLM remains poorly constrained. Several investigations of N and Ar in peridotitic fluid inclusions showed N values similar to those of oceanic basalts (Yamamoto et al., 2020), with slightly higher N2/Ar values, attributed to recycled crustal material (Matsumoto et al., 2002). The S-SCLM beneath Udachnaya is one of the most geochemically well-characterized sections of the S-SCLM (e.g., Pokhilenko et al., 1999; Sumino et al., 2006). However, to date, relatively few studies have investigated the origin of the metasomatic processes responsible for the compositional variations reported in associated peridotites (e.g., Pokhilenko et al., 1999; Howarth et al., 2014; Barry et al., 2015; Pernet-Fisher et al., 2015, 2019; Broadley et al., 2018a). Here we use Ne-N-Ar isotopes and halogen elemental ratios in order to identify the different volatile components present in the S-SCLM. This multi-tracer approach provides unique insights into the geochemical composition of the S-SCLM and enables the history of volatile interaction, potentially dating back to the Archean, to be determined.
Section snippets
Geologic settings
Cratonic areas represent ideal settings for studying the temporal evolution of the SCLM. The Siberian craton (see Fig. 1 in Barry et al., 2015) encompasses approximately 4.4 × 106 km2 of north-central Asia. It is composed of several island-arc terrains, which amalgamated during the Archean and Proterozoic (Pearson et al., 1995), and has subsequently experienced a complex history of Phanerozoic metasomatism and kimberlite emplacement (e.g., Pearson et al., 1995; Griffin et al., 1999). Between
Nitrogen abundances and isotopes
The N (where N = [(15N14N/14N14N)sample/(15N14N/14N14N)air - 1] x 1000) determined for the Siberian peridotite xenolith samples range between -5.85 and +3.94‰ relative to air (i.e., 0‰) (Table 1). Obnazhennaya samples (n=6) are 15N-depleted, with N values ranging from -0.71 to -3.12‰ (Fig. 1; Table 1), which notably fall above the DMM range (Marty and Zimmermann, 1999; Cartigny and Marty, 2013). In contrast, Udachnaya samples (n=5) span the range from DDM to plume-like N (Fig. 1),
Discussion
This suite of Siberian mantle peridotites was previously analyzed for a wide array of geochemical parameters, including major and trace elements (Howarth et al., 2014), Re-Os (Pernet-Fisher et al., 2015), He isotopes (Barry et al., 2015) and halogens (Broadley et al., 2018a). In brief, Howarth et al., 2014 showed that garnet compositions have two distinct trends in CaO–Cr2O3 space: increasing CaO at constant Cr2O3 within the harzburgite field, and decreasing CaO and Cr2O3 within the lherzolite
Summary
In summary, data presented here – together with previous studies of mantle xenoliths (Kim et al., 2005; Yamamoto et al., 2004, 2020) – suggests that metasomatism of the SCLM may be a globally significant process. Critically, the metasomatic material that infiltrates the S-SCLM records an important “subduction-fingerprint” that can be used to gain insight into relative volatile element recycling efficiencies and shed light on volatile movements between Earth's surface and interior over our
CRediT authorship contribution statement
Peter H. Barry: Conceptualization, Methodology, Data acquisition, Data processing, Data curation, Writing-Original draft preparation.
Michael W. Broadley: Conceptualization, Visualization, Writing-Original draft preparation.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
We acknowledge The National Science Foundation (NSF) awards (EAR-1144559; MGG-2015789) to PHB. We thank the late Dave Hilton and the late Larry Taylor for strong mentorship, friendship and access to their laboratories. We also thank David Bekaert, David Byrne, John Pernet-Fisher, Geoff Howarth, Ray Burgess, James Day, Sæmi Halldórsson and Sami Mikhail for fruitful discussions about these samples. We'd also like to thank the editor (Raj Dasgupta) and the two anonymous reviewers.
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2021, Chemical GeologyCitation Excerpt :The cold Mazuku seeps have slightly lower 3He/4He values (up to 5.2RA) than those measured by Barry et al., 2013 (7.2RA) and higher 4He concentrations, suggesting that radiogenic helium production in the crust may have diluted the mantle endmember, as has been observed in other geothermal systems globally (Füri et al., 2010). Such helium isotope variations are not likely related to mantle metasomatism, as metasomatic fluids are characterized by high 4He/40Ar*, due to the fact that K is more efficiently subducted relative to U and Th (Barry and Broadley, 2021), yet measured samples have 4He/40Ar*, with values close to or below expected mantle production (Fig. 5). Neon isotopes are nearly indistinguishable from air, likely due to dilution of mantle-derived neon by air-derived neon, circulating in the shallow hydrothermal system beneath the RVP (Table 2).
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2021, Chemical GeologyCitation Excerpt :Such high N2/4He values have previously been reported in volcanic arc settings (1870 ± 15; Giggenbach, 1996; Fischer et al., 1998; Sano et al., 2001) and are attributed to the preferential recycling of N2 relative to He, however it can explain the values in the EARS as it is far from any modern subduction zone. Nevertheless, if ancient subduction has metasomatized the mantle, such high N2/4He signatures could be retained in the mantle source, as N2 is preferentially recycled relative to He, resulting in a metasomatically N2 enriched mantle source (Barry and Hilton, 2016; Foley and Fischer, 2017; Aiuppa et al., 2021; Barry and Broadley, 2021; Bekaert et al., 2021). Alternatively, samples could be N2 rich due to incorporation of N2 during gas emplacement - there are abundant nitrogen-rich sediments in the subsurface beneath the EARS and N2 rich cratonic material has formed in the region from the amalgamation of multiple paleo-arc environments (Smirnov et al., 1973; Sklyarov et al., 1998; Boniface and Schenk, 2012; Boniface et al., 2012; Boniface and Tsujimori, 2019).