Nitrogen and noble gases reveal a complex history of metasomatism in the Siberian lithospheric mantle

https://doi.org/10.1016/j.epsl.2020.116707Get rights and content

Highlights

  • We present N, Ne and Ar data from two suites of peridotitic xenoliths from Siberia.

  • Siberian xenoliths show evidence of ancient radiogenic metasomatic and/or recycled signatures.

  • Siberian xenoliths show evidence of mantle re-fertilization, attributed to the SFB event.

  • The geochemical nature of the SCLM is highly variable and poorly constrained.

  • Ancient radiogenic metasomatic fluids have profoundly modified the Siberian CLM.

Abstract

The Siberian flood basalts (SFB) erupted at the end of the Permian period (∼250 Ma) in response to a deep-rooted mantle plume beneath the Siberian Sub-Continental Lithospheric Mantle (SCLM). Plume-lithosphere interaction can lead to significant changes in the structure and chemistry of the SCLM and trigger the release of metasomatic material that was previously stored within the stable craton. Here, we investigate the nature of the Siberian-SCLM (S-SCLM) by measuring nitrogen abundances and isotopes (δ15N) in 11 samples of two petrologically-distinct suites of peridotitic xenoliths recovered from kimberlites which bracket the eruption of the SFB: the 360 Myr old Udachnaya and 160 Myr old Obnazhennaya pipes. Nitrogen isotope (δ15N) values range from -5.85 ± 1.29‰ to +3.94 ± 0.63‰, which encompasses the entire range between depleted Mid-Ocean Ridge Basalt (MORB) mantle (DMM; -5 ± 2‰) and plume-derived (+3 ± 2‰) endmembers. In addition, we present neon (n=7) and argon (n=8) abundance and isotope results for the same two suites of samples. The 20Ne/22Ne and 21Ne/22Ne range from atmospheric-like values of 9.88 up to 11.35 and from 0.0303 to 0.0385, respectively, suggesting an admixture of DMM and plume-derived components. Argon isotopes (40Ar/36Ar) range from 336.7 to 1122 and correlate positively with 40Ar contents. We show that volatile systematics of Siberian xenoliths: (1) exhibit evidence of ancient metasomatic and/or recycled signatures, and (2) show evidence of subsequent plume-like re-fertilization, which we attribute to the emplacement of the SFB. Metasomatic fluids are highly enriched in radiogenic gases and have elevated Br/Cl and I/Cl values, consistent with an ancient subducted crustal component. The metasomatic component is marked by light N isotope signatures, suggesting it may be derived from an anoxic Archean subducted source. Taken together, these N2-Ne-Ar isotope results suggest that mantle plume impingement has profoundly modified the S-SCLM, and that N, Ne and Ar isotopes are sensitive tracers of metasomatism in the S-SCLM. Metasomatic fluids that permeate the S-SCLM act to archive a “subduction-fingerprint” that can be used to probe relative volatile-element recycling efficiencies and thus provide insight into volatile transport between the surface and mantle reservoirs over Earth history.

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 δ15N of ∼ -5 ± 2‰ (Javoy et al., 1986; Marty and Zimmermann, 1999) (normalized to air, δ15Nair = 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 δ15N value of +3 ± 2‰. Modern ocean floor sediments are also enriched in 15N, with δ15N values ranging from +5 to +7‰ (e.g., Peters et al., 1978), indicating that high δ15N 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 δ15N 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 δ15N (where δ15N = [(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 δ15N 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 δ15N (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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