Heavy 57Fe in ocean island basalts: A non-unique signature of processes and source lithologies in the mantle
Introduction
Numerous isotopic and trace element studies of ocean island basalts (OIB) suggest that the Earth’s mantle is heterogeneous on scales from hundreds of kilometres, across entire mantle plume systems, to less than a kilometre or metres, as recorded by melt inclusions (e.g., Zindler and Hart, 1986, Weaver, 1991, Hofmann, 1997, Stracke et al., 2005, Maclennan, 2008). The variance in long-lived radiogenic isotopic composition (Sr-Nd-Pb) in most OIB can be explained by mixing between five primary components: depleted mantle, the prevalent mantle (PREMA)/focal zone (FOZO) component, two enriched mantle (EM1/EM2) components and a HIMU component (Stracke, 2012; Fig. 1). Radiogenic isotopes have also been used to argue for enriched (and depleted) components in the mid-ocean ridge basalt source (e.g., Hirschmann and Stolper, 1996, Salters and Dick, 2002, Liu et al., 2008, Byerly and Lassiter, 2014). In addition to these components, osmium isotopes (Walker et al., 1995, Brandon et al., 1998), combined noble gas and short-lived radiogenic isotopic systematics (e.g., 3He/4He and 182W/184W anomalies; Mundl et al., 2017, Mundl-Petermeier et al., 2020) and transition element ratios (e.g., Fe/Mn; Humayun et al., 2004) have been used to support the presence of a small amount of a primordial, lower mantle or core-derived component entrained in some OIB.
Linking the heterogeneity in long-lived radiogenic isotopes to mantle lithology has been achieved through combining radiogenic and stable isotopes (e.g., Day et al., 2009, Day et al., 2010), major element compositions of basalts (Jackson and Dasgupta, 2008, Shorttle and Maclennan, 2011) and trace elements in olivine (e.g., Sobolev et al., 2005, Sobolev et al., 2007, Herzberg, 2011, Neave et al., 2018). These techniques have led to the widely accepted view that recycled oceanic crust generates a significant amount of the variation seen in OIB geochemistry, along with small contributions from sediments and continental crust (e.g., Cohen and O’Nions, 1982, Hofmann and White, 1982, Weaver, 1991, Chauvel et al., 1992; see also Stracke, 2012). The recycled basaltic component is thought to be present as discrete eclogite or, more commonly, pyroxenite (olivine-poor, pyroxene-rich) lithologies, the latter possibly formed by solid-state or melt reaction of eclogite with ambient peridotite (Sobolev et al., 2007, Herzberg, 2011). However, the success of using major elements to identify pyroxenite in OIB sources may depend on the type of enriched component invoked (Lambart et al., 2013); and due to uncertainties over the melting conditions and role of crustal processes (magma recharge events, mixing, diffusional reequilibration; Matzen et al., 2017, Hole, 2018, Gleeson and Gibson, 2019) it is unclear whether source compositional differences unambiguously control the trace element concentrations in olivine phenocrysts.
Non-traditional stable isotope systems, such as Fe, provide an important, alternative method for identifying lithological heterogeneity (of both enriched and depleted components) in the OIB source. Unlike incompatible trace elements and radiogenic isotopes of incompatible elements, both of which are likely to be disproportionately affected by contributions from enriched mantle components (e.g., Burton et al., 2012), the Fe abundances of melts derived from pyroxenitic and peridotitic source mineralogies are similar (e.g., Sobolev et al., 2005). Therefore, neither pyroxenite nor peridotite lithologies should dominate the Fe isotopic composition of erupted melts, and the Fe isotopic composition of the lavas should reflect the relative contributions from these different lithologies (Williams and Bizimis, 2014). Since Fe isotopes show mineral-specific fractionation effects, they provide a unique opportunity to explore the petrological and mineralogical characteristics of mantle heterogeneity, and could help unravel common melting lithologies and processes in the mantle (Williams and Bizimis, 2014, Konter et al., 2016, Nebel et al., 2018, Nebel et al., 2019, Gleeson et al., 2020).
Equilibrium inter-mineral Fe isotope fractionations are driven by the different bonding environments of Fe in mineral structures (Macris et al., 2015, Young et al., 2015). Strong, short Fe–O bonds (due to low coordination numbers and/or the presence of oxidised Fe3+) concentrate heavy Fe, i.e., more 57Fe favoured over 54Fe (Sossi and O’Neill, 2017), which is reported as higher 57Fe, whereThis equilibrium fractionation effect suggests that an olivine-dominated peridotite will be isotopically lighter than a more pyroxene-rich (pyroxenite) recycled component (Macris et al., 2015, Sossi and O’Neill, 2017) due to the presence of a small amount of Fe3+ in pyroxene, and hence stronger Fe–O bonds, relative to olivine. Similarly, pure iron metal (Fe0) is expected to be isotopically lighter than lower mantle silicate (Fe2+, 3+) due to the difference in Fe valence state between the phases (Shahar et al., 2016), although this effect is predicted to be small at pressures and temperatures relevant to core equilibration (Shahar et al., 2016, Liu et al., 2017) and unlikely to be relevant to OIB. Kinetic fractionation effects may also contribute to Fe isotopic variation in OIB melt sources, with thermodiffusion (Soret diffusion) in material diffusing from the outer core into the lowermost mantle recently proposed to generate heavy 57Fe in entrained plume material (Lesher et al., 2020).
Qualitatively consistent with an isotopically heavy recycled source component, OIB show variable 57Fe relative to average N- and T-MORB (the latter two dominated by peridotite melting), generally extending to heavier compositions, such as over 0.25‰ in Samoa and Pitcairn. These heavy Fe isotopic compositions are suggested to relate to pyroxenitic mantle components in the OIB source. There are some N-MORB with 57Fe as heavy as 0.2‰, which could also be consistent with indications of small amounts of enriched pyroxenite or eclogite in the MORB source (c.f., Hirschmann and Stolper, 1996). The radiogenic isotope systematics of hotspots (plumes) that display heavy 57Fe signatures show mixing between a common peridotitic mantle component and recycled crustal endmembers (Konter et al., 2016, Nebel et al., 2019; Fig. 1), and in several cases Fe isotopes correlate positively with indices of recycling (e.g., Sr-Nd-Pb isotopes; Nebel et al., 2019). However, it is unclear whether mixing between different mantle components is represented to the same extent in Fe isotopes as in long-lived radiogenic isotopes, and whether Fe and radiogenic isotopic systems can be linked to identify the mineralogy (e.g., pyroxene enrichment) of different mantle components identified in Fig. 1. In using heavy Fe isotopic compositions to better understand mantle heterogeneity, both the sources and processes generating heavy 57Fe melts in OIB need to be considered.
The Fe isotopic compositions recorded in OIB are heavier (and more variable) than the expected equilibrium isotopic compositions of melts from crustal endmembers contributing to mantle heterogeneity. Isotopic composition estimates for the mantle are 57Fe = 0.03 0.03‰ (DM; Craddock et al., 2013) to 0.05 0.01‰ (BSE; Sossi et al., 2016); for average oceanic crust, represented by MORB, are 57Fe = 0.15‰ (Teng et al., 2013, Sossi et al., 2016); and bulk continental crust is indistinguishable from, or lighter than, oceanic crust (57Fe = 0.08–0.16‰; Johnson et al., 2020). Highly differentiated (SiO2 70 wt%) crust, which could contribute to continentally-derived sediment, records 57Fe 0.9‰ (Du et al., 2017) although the average 57Fe of rocks with SiO2 60 wt% is around 0.3‰ (Johnson et al., 2020). Experimental and theoretical estimates of fractionation during partial mantle melting are known to be small (e.g., Dauphas et al., 2009, Sossi and O’Neill, 2017, Gleeson et al., 2020), likely 57Femelt−mantle (= 57Femelt −57Femantle) 0.1‰ depending on Fe3+ buffering in the mantle (Dauphas et al., 2014). However, data from Pitcairn and Galapagos Spreading Centre lavas require a mantle component with 57Fe = 0.30‰ (Nebel et al., 2019, Gleeson et al., 2020). The heaviest isotopic values in OIB from Samoa have previously been explained by combining source heterogeneity, partial melting, and fractional crystallisation effects (Konter et al., 2016). However, even in this multi-process scenario, equilibrium fractionation factors for these processes are required to be high (crystallisation with 57 = ‰, and mantle melting with 57 0.15‰; Konter et al., 2016), and possibly unrealistically high (Gleeson et al., 2020). It therefore remains unclear if the heaviest Fe isotopic values in OIB (57Fe 0.25‰) can be explained by simple melting processes of recycled crustal components embedded in ambient plume mantle.
Samoan rejuvenated lavas (a later, volumetrically less significant stage than the main shield lavas, erupted far from the plume; Natland, 1980) record the heaviest Fe isotopic compositions in the global OIB dataset (Konter et al., 2016). Samoan shield samples also show correlated 3He/4He-182W, with 3He/4He 33.8 R/Ra (Jackson et al., 2007a) and 182W (Mundl et al., 2017), proposed to relate to core-equilibrated material (Mundl-Petermeier et al., 2020). Thus, Samoan lavas were selected for further Fe isotopic characterisation. We have expanded the shield lava dataset allowing us to study pyroxenite versus core contributions to heavy 57Fe liquids, as the two contributions could be associated with other distinct geochemical signatures. The Azores was chosen as a second OIB locality because it shows radiogenic isotopic mixing between a common Azores mantle component and recycled components, so could allow identification of recycled mantle pyroxenite. There may also be a minor lower mantle or core component associated with the Azores plume, identified by raised 3He/4He relative to MORB (18.4 R/Ra; Moreira et al., 2012) and small negative 182W anomalies (; Mundl-Petermeier et al., 2020). Unlike Samoa, the Azores is also (1) a cooler plume ( 1400C; Beier et al., 2012, compared to 1600C in Samoa; Putirka et al., 2018) meaning melts of enriched (and possibly heavy 57Fe) components will be minimally obscured by contemporaneous melting of ambient, relatively depleted mantle; and (2) shows a well spatially-resolved distribution of melts from different components sampled by volcanoes (Béguelin et al., 2017, Beier et al., 2018; Fig. 1). Therefore, Samoa and the Azores offer different perspectives on the links between distinct mantle components and the source of heavy 57Fe lavas.
Section snippets
Samoa
The Samoan islands show an age-progressive volcanic track (Koppers et al., 2011), with a shield building stage influenced by multiple mantle components (Jackson et al., 2014; Fig. 1). These components include the global EM2 endmember, which is distinguished by high 87Sr/86Sr relative to FOZO (but FOZO-like 206Pb/204Pb) and proposed to relate to recycled continental sediment (White and Hofmann, 1982, Jackson et al., 2007b); and a high 3He/4He-negative 182W common plume component (Jackson et
Results
The measured Fe isotope compositions for Samoa and the Azores are given in Table 1. We discuss the Fe-isotope systematics of the two localities separately below.
What processes could generate heavy 57Fe liquids?
Several processes can modify the whole rock 57Fe from the primary liquid 57Fe, and the primary liquid 57Fe from the isotopic composition of ambient peridotite (0.05‰). In this section, we consider how: (1) post melt emplacement processing (fractional crystallisation, olivine accumulation); (2) partial melting; and (3) pre-melt emplacement considerations of mantle lithological heterogeneity, may contribute to the heavy 57Fe ( 0.20‰) seen in Samoa and the Azores.
How to generate heavy 57Fe mantle components?
The inability of a simple peridotite–eclogitic pyroxenite melting model (Section 4.2.3) to generate the heavy 57Fe of Samoa, Pitcairn and Azores melts means that pre-final melt emplacement processes (i.e., processes affecting the source 57Fe composition) must be considered. After core contributions, we sequentially consider processes operating from a mid-ocean ridge setting, through subduction, to upwelling and melting in a mantle plume (summarised in Fig. 8).
Preservation of heavy 57Fe: Importance of plume variables
Not all OIB record heavy 57Fe (e.g., Hawai’i, Samoan shield, Réunion), which suggests that the processes controlling the generation and preservation of heavy 57Fe in erupted lavas may rely on factors that differ between plumes, e.g., melt fraction and potential temperature, in addition to the 57Fe of source material.
The Azores records heavy 57Fe and is a relatively cool plume (Putirka, 2008, Beier et al., 2012), and the heavy 57Fe rejuvenated Samoan lavas are small-degree melts (Konter and
Summary
Stable Fe isotopes have increasingly been used as a tracer of mineralogical heterogeneity in the mantle, but there are multiple processes that could generate heavy 57Fe mantle melts. We show that the existing dataset of 57Fe, 182W and 3He/4He of OIB are inconsistent with a contribution from heavy 57Fe core liquids. In agreement with previous work, we also calculate that the magnitude of partial melting fractionation of peridotite is small. We show that the partial melting fractionation of
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.
Acknowledgements
We thank Edward Inglis and Geoff Nowell for their help in the labs in Durham, and Marie-Laure Bagard for her help with mass spectrometry in Cambridge. M. H. thanks Luisa Pinto Ribeiro (EMEPC, Lisbon) for the São Jorge samples (SJ48-101). We thank William Miller for valuable discussions and comments throughout this project; Bradley Peters, James Day, and an anonymous reviewer for useful comments that improved the clarity and focus of the manuscript; and Stefan Weyer for editorial handling. This
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Present address: School of Earth and Ocean Sciences, Cardiff University, Cardiff, CF10 3AT, UK.
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Present address: Department of Earth Sciences, University of Cambridge, Cambridge, CB2 3EQ, UK.