Abstract
Water occurs in Earth’s interior mostly as trace hydroxyl in nominally anhydrous minerals. Clinopyroxene is known to be an important water carrier in the uppermost mantle, and eclogite, which forms a subordinate part of the cratonic lithosphere, contains some 50% of jadeite-rich clinopyroxene, making this potentially a significant H2O reservoir in the bulk lithospheric mantle. Mantle metasomatism, in particular by small-volume melts like kimberlite, is known to enrich the lithosphere in highly incompatible components, but its effect on H2O contents in cratonic eclogite remains unclear. We report H2O concentrations for clinopyroxene and garnet in eclogite and pyroxenite xenoliths from several African kimberlites, obtained by Fourier-Transform Infrared Spectroscopy (FTIR). Except one sample showing evidence for minor within-grain variability of H2O concentrations (< 15%), FTIR images demonstrate that H2O is homogeneously distributed in optically clear areas of clinopyroxene fragments mounted for this study. The samples were variably metasomatised by a kimberlite-like melt, as evidenced by elevated MgO contents and abundances of highly incompatible elements (e.g., Sr, Ce, Th). Although metasomatised eclogites and pyroxenites on average show higher H2O abundances than pristine ones, mantle metasomatism decreases the Al2O3 content in clinopyroxene, which is known to enhance hydrogen incorporation in this mineral. As a consequence, hydrogen incorporation is inhibited, and c(H2O) becomes increasingly decoupled from other highly incompatible components, such as LREE. Thus, eclogite – metasomatised or not - does not significantly contribute to the H2O inventory in the bulk cratonic mantle.
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Introduction
Water in Earth’s interior is held dominantly as trace hydroxyl in nominally anhydrous minerals (NAMs). Relative to its minor abundance, this “deep water” has a disproportionate effect on mantle physical properties, such as its solidus temperature and viscosity, thereby strongly influencing its phase relations and rheological behaviour as well as its geophysical expression (e.g., Kohlstedt 2006; Garber et al. 2018). During metasomatism, depending also on H2O fugacity, water may be added via crystallisation of hydrous minerals (e.g., amphibole, phlogopite), but can also be incorporated in NAMs as trace amounts of OH associated with point defects following diffusion into NAMs from fluids or melts (Demouchy and Bolfan-Casanova 2016). Water, sensu lato, in NAMs is reported as equivalent H2O concentration by weight, and may include intrinsic or structural protons or hydroxyl from lamellae of hydrous minerals (e.g., Stalder 2004; Chen et al. 2011; Moine et al. 2020). Subsequently, water in NAMs from magma-borne xenoliths may be lost upon transport to the surface depending on H diffusivities, which are mechanism-specific (Demouchy and Bolfan-Casanova 2016; Yang et al. 2019). However, arguments for the preservation of pre-entrainment signatures include co-variation between mineral compositions and H2O contents or the lack of zonation, with clinopyroxene considered more robust than olivine or orthopyroxene (Sundvall and Stalder 2011; Yang et al. 2019).
Water contents in ultramafic xenoliths from hitherto investigated cratons show strong variability (e.g., Peslier et al. 2010; Baptiste et al. 2012; Xia et al. 2013; Doucet et al. 2014; Jean et al. 2016). Archaean and Palaeoproterozoic eclogite reservoirs in the cratonic lithospheric mantle, representing km-sized pods or lenses ultimately emplaced during subduction of ancient oceanic crust (e.g., Aulbach and Smart 2023), represent a volumetrically subordinate lithology in the cratonic lithosphere (< 20%; Garber et al. 2018). However, in a discussion of the overall water content of the cratonic lithosphere mantle, these pods may have a disproportionate significance. This is because eclogite is a rock with subequal proportions of clinopyroxene and garnet (e.g., Huang et al. 2014) and because jadeite-rich clinopyroxene (omphacite) typical of eclogite can accommodate 100s of wt.ppm H2O (e.g., Skogby et al. 1990; Smyth et al. 1991; Katayama et al. 2006).
In a prior study (Aulbach et al. 2023), we focussed specifically on relatively pristine eclogite xenoliths from the Siberian and Slave cratons, which show little evidence for mantle metasomatism. This is also suggested by clinopyroxene H2O abundances that almost all fall below those expected for equilibrium with kimberlite, which is a typical small-volume volatile-rich agent of metasomatism with ∼ 3 to 7 wt% H2O (Bell et al. 2004; Becker and Le Roex 2006). Here, we investigate geochemically well-characterised eclogite and pyroxenite xenolith suites from several African kimberlite localities that show a spectrum of compositions indicative of metasomatic overprinting. In this study, we do not aim to constrain the nature of OH defects or the coupled substitutions by which it may be incorporated. Instead, using the combined isogenous datasets of oriented clinopyroxene crystals measured by Fourier-Transform Infrared Spectroscopy (FTIR) in polarised light (this study and Aulbach et al. 2023), we assess the effects of metasomatism on H2O contents and distribution between eclogite minerals, by comparison to the dataset from pristine eclogite xenoliths. We complement these data with a preliminary set of fabric analysis by Electron Backscatter Diffraction (EBSD) to test the extent to which the minerals have recovered from metasomatic effects.
Samples and prior work
Eclogite and pyroxenite xenoliths in this study are medium- to coarse-grained, with subequal abundances of garnet and clinopyroxene (Fig. 1) and derive from four African kimberlite localities. All have been subject to detailed prior geochemical investigation. Samples from the 146 Ma (Skinner et al. 2004) Koidu kimberlite cluster in the West African craton comprise bimineralic low-Mg eclogites that were possibly emplaced during Neoarchaean subduction, during which they lost a partial melt, followed by mantle metasomatism leading to their transformation to high-Mg eclogites and pyroxenites (Barth et al. 2001, 2002; Aulbach et al. 2019). This metasomatism not only increased the samples’ MgO content, but also caused enrichment in Cr2O3 and highly incompatible elements (LREE, Pb, Th, U), as well as dilution or loss of Li±Zn (Aulbach et al. 2019). Along with reduced jadeite component in clinopyroxene and grossular component in garnet, these geochemical characteristics have been identified as hallmarks of metasomatism by kimberlite-like melts that are observed in eclogite and pyroxenite xenolith suites globally (Aulbach et al. 2020) (Fig. 2a-c). Similar signatures are recognised in eclogite and pyroxenite xenoliths from the late Cretaceous (Davis 1977) Orapa kimberlite cluster in the Zimbabwe craton, where pre-entrainment melt metasomatism, fed by earlier-emplaced phlogopite-rich metasomes, imparted enriched Sr and mantle-like O isotopic compositions, which is particularly prevalent at temperatures < 800 °C (Aulbach et al. 2017). For a preliminary comparison, we also investigated four previously-described (Dludla et al. 2006) eclogite and pyroxenite xenoliths from the Cullinan (formerly known as Premier) kimberlite, which was emplaced in the Mesoproterozoic (e.g., Tappe et al. 2018).
All samples were classified according to Aulbach and Jacob (2016), whereby gabbroic eclogites have protoliths that experienced plagioclase accumulation based on Eu/Eu*, while complementary MORB-like eclogites are further subdivided: high-Mg and high-Ca eclogites having less- and more-differentiated protoliths, respectively, whereas low-Mg eclogites have intermediate compositions. Pyroxenites and some high-Mg eclogites acquired their compositions from a kimberlite-like metasomatic melt (Aulbach et al. 2020). The metasomatic signature is dominantly seen at low temperatures (< 800 °C) for Orapa, whereas it is more broadly distributed for Koidu (Fig. 2d).
Sample preparation and analytical methods
FTIR spectroscopy
The sample preparation and analytical routines for acquisition of FTIR spectra are described in detail in Aulbach et al. (2023). Briefly, fragments of clinopyroxene (n ∼ 50) and of garnet (n ∼ 10) typically 0.5–1.0 mm in the longest dimension were picked from lightly crushed xenoliths, placed in epoxy in 1-inch mounts and doubly polished to thicknesses of mostly 100–350 μm (Fig. S1 in Online Resource 1). Conoscopic interference figures were used to identify clinopyroxene fragments oriented perpendicular to the obtuse or acute bisectrix, or parallel to the optical axis plane. Optically clear parts, free of visible alteration or cracks, of clinopyroxene and garnet fragments were analysed at Institut für Mineralogie und Petrographie, Universität Innsbruck (Austria) using a Bruker Vertex 70 FTIR spectrometer equipped with a SiC globar MIRE light source and a KBr beamsplitter. The spectrometer was connected to a Hyperion 3000 microscope equipped with an MCTD316-025 (mercury cadmium telluride) detector and a wire ZnSe grid polariser. Samples were placed on a BaF2 plate, and spectra were acquired by acquisition of 32 scans with a spectral resolution of 4 cm− 1 in transmittance mode, using non-polarised and polarised IR radiation for garnet and oriented clinopyroxene sections, respectively. The preferred aperture size was 50 μm × 50 μm, which was adjusted to avoid impurities if necessary.
Clean clinopyroxene spectra were background-corrected with a linear baseline between 3700 and 3100 cm− 1 and integrated in this wavenumber interval. A linear baseline was subtracted for garnet in the range 3700 and 3300 cm− 1, the low value of which was adjusted upward when a strong baseline curvature, typical of Fe-bearing minerals, such as eclogitic garnet, was observed. For both clinopyroxene and garnet, wavy spectra with variably strong amplitudes and variable frequency caused by reverberations were also mostly rejected.
Recognising impurities in FTIR spectra
Clinopyroxene in many of the samples studied here and in prior work contain impurities that are manifest as poorly resolved bands at wavenumbers ∼ 3630 − 3600 and 3470 –3430 cm− 1 and as slowly declining absorbance from the OH-related bands towards lower wavenumbers. This affects some 40–50% of the spectra per sample on average, which were rejected accordingly (the reader is referred to Figure S1 in Aulbach et al. 2023; which shows the entirety of accepted and rejected spectra for two samples, which are fully representative of the results and data reduction approach adopted here). Extracted spectra for different domains of a fracture-bearing crystal - from an optically clear core to visible micro-inclusions and cracks contaminated by small amounts of alteration ± epoxy - illustrate gradual differences within the fragment, with varying degree of H2O band absorbance addition to the intrinsic H2O signal in clinopyroxene (Fig. S2 in Online Resource 1). As in the spectra we rejected, this results in poorly resolved OH-related bands and slowly rather than sharply declining absorbance from the OH-related bands towards lower wavenumbers and shows the importance of detecting these impurities and excluding them from H2O concentration estimates.
As demonstrated by transmission electron microscopy (Koch-Müller et al. 2004), impurities in eclogite minerals are related to the presence of submicroscopic sheet silicates, mostly clinochlor and amesite. On the other hand, in eclogite xenoliths from Obnazhennaya (Siberian craton), crystallographically oriented zoisite lamellae were originally present in the clinopyroxene lattice and exsolved along with garnet from a H2O- and Al-rich precursor (Radu et al. 2022), thus being internally-derived. Therefore, the question arises to which extent the inferred sheet silicates may relate to H2O that was originally incorporated in clinopyroxene and exsolved due to changes in solubility, for example related to changes in pressure, temperature or composition (metasomatism). We note that the impurities are not ubiquitously present in different clinopyroxene fragments of a given sample, and that they typically, though not always, appear in both spectra polarised along the main refractive indices of an individual crystal section, thus do not appear to have a systematic orientation. Therefore, we conclude that these impurities are randomly oriented rather than representing topotaxial exsolution lamellae. Furthermore, spectra with broad absorption features are only observed in some but not all fragments of a given sample (Table S4) and total absorbance profiles (method described below) show little variability along optically clear paths (Fig. 3; Online Resource 2). In contrast, if spectra we consider contaminated were included in the totals, the variability (1σ) would increase substantially. In sum, we consider the broad absorbance over the OH bands a consequence of impurities and such spectra are not considered for H2O estimates.
Quantification of H2O concentrations
The modified Beer-Lambert law (c(H2O) = Atot/ε × ρ × t - integrated absorbance Atot, specific absorption coefficient ε, density ρ and plate thickness t) allows to convert integrated absorbances into water contents (in wt.ppm). Total c(H2O) for clinopyroxene were calculated by adding the average of the three contributions (α, β, γ; Libowitzky and Rossman 1997) for polarised measurements, whereas for unpolarised measurements (garnet and clinopyroxene in selected samples), the absorbance was multiplied by three (Kovács et al. 2008). The contributions along the refractive indices α, β and γ in omphacite and the final total H2O concentrations are reported in Table 1. Tables S1 and S2 in Online Resource 3 report the thickness (average of 1–3 measurements, with an average 1σ of 1.5 μm per 100 μm thickness), density (calculated for clinopyroxene and garnet endmembers) and the number of spectra taken in each fragment of clinopyroxene or garnet. Table S3 in Online Resource 3 reports these data for clinopyroxene fragments measured in unpolarised mode (comprising both oriented and unoriented fragments). Table S1 in Online Resource 3 additionally reports the quality of the orientation, as mismatches between perfect conoscopic interference figures (i.e. a symmetrical section through the indicatrix or a single isogyre rotating about the centre, depending on orientation) and the observed figures, which may be tilted by up to 15o, and rarely up to 25o. Table S4 in Online Resource 3 lists the results for all spectra taken along the three refractive indices for clinopyroxene, the number and percentage of accepted spectra per refractive index, statistics for multiple accepted spectra per sample to assess inter-grain variability, and for multiple spectra per grain to assess intra-grain variability (shown as an example for samples from Koidu). Results for spectra taken in unpolarised light for three samples are listed for comparison. Finally, Table S5 in Online Resource 3 reports results for garnet, where the thicknesses and absorbances of multiple analyses were added in order to achieve better signal-to-background ratio.
Both the mineral-specific absorption coefficients of Bell et al. (1995; hereafter B + 95) for diopside and the wavenumber-dependent calibration of Libowitzky and Rossman (1997; hereafter L + R97) are applied in this study. Here, we assess in more detail how the two calibrations compare for xenolithic eclogites, combining data obtained in this and a previous study (Aulbach et al. 2023). The derived H2O concentrations are higher using B + 95 for both clinopyroxene and garnet, and, consequently, also reconstructed whole rocks (Fig. S3 in Online Resource 1). Figure S3 shows that the difference cannot be clearly attributed to changes in jadeite component in clinopyroxene, but may be slightly higher when diopside component in clinopyroxene is low. The average difference in estimated bulk-rock H2O concentrations between B + 95 and L + R97 is 33%, which is taken as a measure of uncertainty related to the choice of absorption coefficient in typical xenolithic eclogites. In earlier work (Aulbach et al. 2023), we had opted to use B + 95 for illustration and interpretation because their calibration is more widely used in the literature, allowing for direct comparison with published data. As in this work we focus on an isogenous dataset to directly compare metasomatised against relatively pristine eclogites, hereafter we will use H2O concentrations derived with L + R97, in keeping with prior work advocating a wavenumber-dependent calibration (e.g., Stalder and Ludwig 2007; Sundvall and Stalder 2011).
Based on the water contents of the grains with the least detectable IR signal for the typical sample thicknesses reported in the Online Resource, the detection limit is conservatively estimated to be ∼ 2 wt.ppm. Random errors reflecting those related to thickness measurement, baseline correction, possibly undetected contamination by optically invisible sheet silicates and contribution from non-ideal orientation (clinopyroxene), combined with uncertainties in experimentally-determined extinction coefficients are estimated to result in a total uncertainty in mineral c(H2O) of ∼ 40% (50% when uncertainties in bulk rock reconstruction are added). For comparison, the average relative uncertainty for c(H2O) based on one standard deviation for multiple measurements per sample is 11–19%, while the intra-grain heterogeneity, gauged by multiple measurements per mineral fragment is 5–7% (1σ) (Table S4 in Online Resource 3), both well within the estimated total uncertainty.
FTIR imaging
In order to further test sample homogeneity, transmission FTIR imaging was carried out at the Institute of Geological Sciences of the University of Bern using a Bruker Tensor II spectrometer with a globar MIRE light source and a KBr beam-splitter, coupled to a Bruker Hyperion 3000 microscope with a dry air-purged sample chamber. The instrument is equipped with a focal plane array (FPA) detector with 64 × 64 = 4096 liquid-nitrogen-cooled MCT elements on a square array with a pixel size of 2.7 μm × 2.7 μm. A 2 × 2 binning was used resulting in a 5.4 × 5.4 μm pixel resolution, higher signal quality and an improved signal to noise ratio. Spectra were acquired with 8 cm− 1 wavenumber resolution and 64 scans between 900 and 3850 cm− 1. For all acquired FTIR-FPA images, the atmospheric correction and concave rubber band correction with 64 points and four iterations was performed in OPUS® version 8.5. The spectra were then exported and further processed with the in-house developed MATLAB software SpecXY (Gies et al. 2024). The spectra were normalized to 1 cm thickness and integrated absorbance for different images were generated by extracting the chosen part of the signal, performing a linear baseline correction between the integration endpoints, and integrating the corrected spectra. In total, 23 clinopyroxene fragments from K1-2, K1-4, K1-6, K1-10, K1-13, K1-14, K1-16, K5-5 covering clear areas, as well as domains with cracks, inclusions and/or alteration features, were investigated. Grains were selected for imaging based on two criteria: firstly, the largest possible grains with clear areas and, secondly, grains with identifiable grain boundaries, for example, by direct contact with garnet. Two representative examples are shown in Fig. 4, and all imaging in Online Resource 2.
EBSD
In order to estimate whether a crystallographic-preferred orientation (CPO) formed and to visualise internal misorientation indicative of grain boundaries and subgrain formation, EBSD data were acquired – as a preliminary investigation – for two metasomatised samples from Orapa (pyroxenite OE72 and eclogite OE83). The samples were polished to 0.25 micron with diamond and then for 10–15 min with colloidal silica. We employed a JEOL JSM-6490 scanning electron microscope, using an acceleration voltage of 15 kV, beam current of ∼ 8 nA, and a working distance of 20 mm. A Nordlys camera (Oxford Instruments) and the Channel 5 software (Flamenco, Oxford Instruments) were used to acquire the data. Maps were made of some garnet and omphacite grains with a step size of 5–7 μm. Due to the large grain size, the orientations were measured manually in Flamenco, with one measurement per grain. The orientations were plotted in equal area, upper hemisphere pole figures. Because of the typical medium to coarse grain size of xenolithic eclogite (Fig. S4 in Online Resource 1), only < 50 grains could be analysed per mineral. Consequently, the pole figures were not contoured, as more than 100–150 grains need to be measured to get a good representation of the orientations and to confirm the omphacite CPO, but in practice a limited number of grains are exposed in a given section of medium- to coarse-grained eclogite. The orientation is illustrated by three pole figures of [100], (010) and [001] for omphacite, and of {100}, {110} and {111} for garnet.
Results
For measurements along the refractive indices α and β in clinopyroxene in this study, bands with varying absorbance ratios are always recognised at wavenumbers ∼ 3630 − 3600 (Type I) and 3470 –3430 cm− 1 (Type III), whereas bands of Type II (3540 –3510 cm− 1) are weak or not observed. Three types of bands have been commonly described for clinopyroxene (Koch-Müller et al. 2004; Libowitzky and Beran 2006; Huang et al. 2014). The dominant absorption band ||γ is typically linked to the Type III region (Fig. 4). Averaging the results per sample as listed in Table S4 in Online Resource 3, OH absorption contributions in clinopyroxene (recalculated as water contents) from eclogite xenoliths range from 22 to 160 wt.ppm (average 81 wt.ppm) for the α-polarised component, from 24 to 160 wt.ppm (average 79 wt.ppm) for the β-polarised component, and from 34 to 130 wt.ppm for the γ-polarised component (average 67 wt.ppm), while the calculated total clinopyroxene c(H2O) ranges from 92 to 420 wt.ppm (average 230 wt.ppm; Table 1). Contributions for measurement along the refractive indices α and β are identical to within 20%, consistent with previous findings (Koch-Müller et al. 2004; Kolesnichenko et al. 2018; Agasheva et al. 2021; Radu et al. 2022; Aulbach et al. 2023). The OH band positions for garnet are between ∼ 3700 and 3300 cm− 1 (Fig. 4) (see also Schmädicke and Gose 2017; Gose and Schmädicke 2018). As in prior work, c(H2O) in garnet is on average lower, ranging from 2 to 48 wt.ppm (average 14 wt.ppm) when quantifiable, and H2O was entirely undetectable in garnet in eclogite xenoliths from Orapa.
Bulk rock c(H2O) was reconstructed assuming 55 wt% garnet and 45 wt% clinopyroxene in the bulk (see Aulbach et al. 2020 for rationale) and neglecting the presence of accessory rutile. Due to the low abundance of rutile in eclogite (median 0.6 wt; see Appendix 1 in Aulbach 2020), its contribution is neglected here. While rutile is indeed a very hydrogen-rich NAM, and can contain up to 380 wt.ppm H2O (median reported for rutile from mafic to felsic samples; see Appendix 3 in Lueder et al. 2024), rutile would only contribute 2.3 wt.ppm to the bulk eclogite. Bulk rock c(H2O) reconstructed from only garnet and clinopyroxene ranges from 43 to 190 wt.ppm, with an estimated total uncertainty of 50%, which reflects the uncertainties on mineral c(H2O) and on the mineral modes used in the bulk rock reconstruction. When no absorption related to OH in garnet was detected, its c(H2O) was calculated 2 wt.ppm (i.e. the detection limit) × sqrt(2)/2 = 1.4 wt.ppm, following Antweiler (2015) who suggests this procedure to safeguard against biasing datasets toward high values.
FTIR images (Fig. 3; Online Resource 2) show the total integrated absorbance of H2O (3050–3720 cm− 1), the C-H bonds (2750–3000 cm− 1) as a measure of epoxy contamination, which often also shows bands in the OH range, and the Si-O overtones of as a measure of clinopyroxene thickness changes (1450–2350 cm− 1). Of the 23 fragments analysed, 21 show flat hydrogen profiles with no clear diffusive loss or gain (Fig. 3a, b). In these grains the total H2O absorption is only affected by the contamination of epoxy or hydrous inclusions, which can be detected by changes in the absorption of the C-H bonds or a change in the absorption of the Si-O overtones (Fig. S2 in Online Resource 1). Only two grain fragments of sample K1-6, which exhibit constant Si-O overtones and no contamination by epoxy, have profiles with lower total integrated H2O absorbance towards the edge, but still show a plateau in the core of the fragment overtones (Online Resource 2). However, the differences in absorption only decrease for grain fragment K1-6_1 and K1-6_2 from 670 cm− 2 to 600 cm− 2 and from 750 cm− 2 to 650 cm− 2, respectively. The within-grain H2O concentration variability is therefore estimated < 15%, which is well within the total estimated uncertainty of 40% relative. The images also validate the measurement strategy for the water quantification and clearly show that only measurements of optically clear areas with no sign of epoxy contamination in the spectrum are representative for the sample.
Sample OE72 is a medium-grained pyroxenite, with correspondingly jadeite-poor, apple-green clinopyroxene (Jadeite mole fraction 0.07) and grossular-poor garnet (grossular mole fraction 0.12), which shows a mild foliation defined by some grains of both garnet and omphacite (Fig. S4 in Online Resource 1). The preliminary data suggest the formation of a CPO where [100] and (010) form girdles in the YZ plane and [001] forms a point maximum parallel to X (Fig. S5 in Online Resource 1). Orientation was determined for 46 omphacite grains. The garnet grains show some minor internal misorientation (Fig. S5 in Online Resource 1). Sample OE83 is an eclogite with jadeite mole fraction of 0.28 and grossular mole fraction of 0.20, and also shows a foliation (Fig. S4 in Online Resource 1). Again, subgrain boundaries (i.e. lines with a misorientation < 10°) or variability in misorientation are observed in garnet, but not in omphacite. CPO is detected in both omphacite and garnet (based on only 42 grains each). The omphacite CPO is similar to the CPO of sample OE72; the [001] axes are parallel to X, and [100] and poles to (010) form a girdle in the YZ plane (Fig. S5 in Online Resource 1). The foliation and lineation of the samples are not known, and the reference frame for the pole figures was based on Fig. S4 in Online Resource 1, where the horizontal of the image is X, and vertical is Y. Therefore, X was parallel to the long axes of the elongated grains in sample OE72, but perpendicular to the long grain axes in sample OE83 (Fig. S4 in Online Resource 1). Modelling and previous measurement of omphacite CPO shows that the [001] axes are expected to align with the lineation, and that [100] and (010) form girdles (e.g., Bascou et al. 2001; Ulrich and Mainprice 2005).
Discussion
Nature of mantle metasomatism in cratonic eclogite and its timing
Making use of observational data, thermodynamic models and experimental constraints, mantle eclogite was shown to have an origin as spreading ridge-derived and variably differentiated oceanic crust (comprising melts and cumulates) that was subsequently exposed to varying degrees of seawater alteration and metamorphic processes (dehydration, partial melting) during recycling, probably in subduction zones, followed by mantle metasomatism (e.g., Aulbach and Smart 2023). The geochemical effects of mantle metasomatism in eclogites include – patently – addition of hydrous minerals or carbonates and – cryptically – enrichment in incompatible elements as well as enriched radiogenic isotope compositions, with melts from the kimberlite-carbonatite spectrum frequently invoked as metasomatic agents (e.g., Ireland et al. 1994; Barth et al. 2002; Heaman et al. 2006; Jacob et al. 2009; Smart et al. 2009; Czas et al. 2018). The geochemical hallmarks of this metasomatism are recognised in eclogite and pyroxenite suites globally, and include a decrease of jadeite component in clinopyroxene and of grossular component in garnet, a decrease in FeO and Li, Cu ± Zn abundances, and an increase in MgO, Cr2O3 and moderately to highly incompatible elements (Sr, Pb, Th, U ± Zr, Nb) (Aulbach et al. 2020), as illustrated in Fig. 2b and c. These signatures, which are often prevalent at mid-lithospheric depths (60–150 km; see Fig. 2d), occur in some 20–40% of individual xenolith populations. As a result of the varied protoliths and multi-stage evolution of xenolithic eclogite, bivariate plots typically exhibit significant scatter and weak correlations among various parameters.
Mantle metasomatism in xenolithic eclogite was suggested to involve addition of a diopside-rich pyroxene from a kimberlite-like melt, followed by diffusive homogenisation and/or recrystallisation to account for the general compositional homogeneity of garnet and clinopyroxene in metasomatised eclogite and pyroxenite xenoliths (Aulbach et al. 2020), although exceptions exist (e.g., Korolev et al. 2021). In peridotite xenoliths, secondary clinopyroxene introduction was inferred for samples having a coherent olivine ± orthopyroxene fabric, but clinopyroxene of different orientation based on EBSD (Puziewicz et al. 2023). Similar observations could not be made for the two metasomatised eclogite xenoliths studied here. Several samples amongst the Orapa suite do show weak shape-preferred orientation of some but not all grains of both clinopyroxene and garnet (Fig. S4 in Online Resource 1), and clinopyroxene orientation data additionally suggest a CPO (Fig. S5 in Online Resource 1). Based on profiles for mineral pairs in metasomatised vs. pristine Koidu eclogite xenoliths (Aulbach et al. 2019), trace element abundances show no consistent zoning across garnet-clinopyroxene interfaces of strongly metasomatised eclogites, except possibly at the outermost 50–100 μm. This includes Li, which is the fastest-diffusing element amongst those quantifiable by laser ablation inductively-coupled plasma mass spectrometry (Fig. S6 in Online Resource 1). This homogeneity suggests that any chemical gradients associated with metasomatism were removed by diffusion by the time of entrainment, perhaps aided by melt-advected heat, or obliterated by melt-assisted recrystallisation.
Subgrain boundaries (i.e. lines with a misorientation < 10°) in garnet from eclogite OE83 (Fig. S3 in Online Resource 1) may reflect partial recovery after deformation. Conversely, no subgrain boundaries are observed in clinopyroxene. This is taken to indicate that, in contrast to garnet, subgrain boundary migration in clinopyroxene led to formation of new high-angle grain boundaries (e.g., Buatier et al. 1991). Combined with the compositional equilibration, this qualitatively indicates that some time had elapsed between metasomatism and exhumation in the host kimberlite. More data are needed to assess whether this interpretation is robust.
Effects of mantle metasomatism on H2O distribution
Values of clinopyroxene−garnetDH2O for Orapa vary widely because none of the garnets contained measurable H2O, as also reported for other localities on the Kaapvaal craton (Huang et al. 2014) (a value of 1.4 wt.ppm was assumed for calculation, see Results). Prior work showed that clinopyroxene−garnetDH2O decreases with increasing grossular content of garnet and with increasing temperature, both of which favour partitioning of incompatible components, including H2O, in eclogitic garnet (Aulbach et al. 2023). However, such systematics are not (or only weakly) observed in the present dataset (Fig. 5a-b), nor is there a relationship with jadeite component in clinopyroxene (Fig. 5c). There is a weak relationship of clinopyroxene−garnetDH2O with indicators of metasomatism, such as elevated MgO, Ce or Th abundances (Ce shown as an example in Fig. 5d). While some scatter in clinopyroxene−garnetDH2O could reflect diffusive loss or gain of hydrogen related to entrainment, we note that that pristine eclogite xenoliths, which should likewise be affected by hydrogen diffusion, do show systematic variations. This is confirmed by comprehensive FTIR imaging of clinopyroxene grain fragments from some of the samples analysed in this study, which, with one exception (K1-6) show homogeneous H2O absorption (Fig. 3; Online Resource 2). We therefore conclude that the scattered clinopyroxene−garnetDH2O in metasomatised eclogites reflects changes in the crystal chemistry (mostly lowering the jadeite and grossular components in clinopyroxene and garnet, respectively; Fig. 2b-c). As these changes are also prevalent over specific temperature intervals (Fig. 2d), there are superposed effects on how much hydrogen is incorporated in these two minerals in equilibrium with a hydrous melt.
Effects of mantle metasomatism on mean OH dipole orientation in omphacite: further evidence for H2O retention in xenolithic eclogite minerals
Hydroxyl in clinopyroxene occurs in different local environments with correspondingly different orientations. The mean orientation of the OH dipole in clinopyroxene may be gauged as the ratio of the absorption contribution of the γ and the α component (hereafter γ/α), given that contributions from α and β are indistinguishable. Prior work on pristine eclogite xenoliths finds that mean dipole orientation shows weak anticorrelations with temperature, diopside component and with some incompatible elements, such as Nb (Aulbach et al. 2023). In this study comprising also strongly metasomatised eclogite and pyroxenite xenoliths, the weak anticorrelation with temperature and diopside persists (Fig. 6a-b), and a marked relationship with a wider range of incompatible elements is highlighted (Sr, Nb, Ce, Th) that was not evident from the pristine eclogite dataset. That is, γ/α for samples with strong enrichment in these elements is restricted to values < ∼ 1.2, whereas nearly the full range of γ/α (0.4–3.1) is observed for unenriched samples (shown for MgO, Ce, Th and Li in Fig. 6c-f). The relationship with REE disappears for Gd and heavier elements (not shown). This is suggested to reflect that mantle metasomatism affects both the concentrations of incompatible elements and mean OH band polarisation via its effect on crystal chemistry. More importantly, we take this regularity – where hydrogen and trace element concentration estimates are obtained by completely different methods (FTIR and laser ablation microprobe inductively-coupled plasma mass spectrometry, respectively) – as circumstantial evidence that our estimates of the contributions from the three refractive indices are largely accurate, with insignificant effects of diffusive H2O loss or contributions from contamination.
Effects of mantle metasomatism on H2O in cratonic eclogite – expectations and observations
Omphacite, with its high vacancy contents related to jadeite or Ca-Tschermaks component, has the highest H2O contents among rock-forming minerals in the uppermost mantle (Demouchy and Bolfan-Casanova 2016), and it additionally constitutes some 40–60% of xenolithic eclogite (e.g., Aulbach et al. 2020). Combined with the knowledge that H2O is incompatible, therefore enriched in small-volume kimberlite-like mantle melts, metasomatised eclogite might be expected to be a particularly H2O-rich reservoir in cratonic lithospheric mantle. Aubaud et al. (2008) derived the dependence of clinopyroxene-melt and garnet-melt H partition coefficients (clinopyroxene−garnetDH2O) in the system metabasalt + 6 wt% H2O on Al2O3 and TiO2 content, respectively. This allows estimating the hypothetical bulk c(H2O) in eclogite minerals for equilibrium with a melt containing, as an example, 3 wt% H2O. The uncertainties on the estimates so obtained are large, due to possible pressure-temperature effects, differences in the crystal chemistry of experiments and natural samples (Fig. 2a), additional crystal-chemical effects not considered by Aubaud et al. (2008) and the possibility that CO2 in the metasomatic kimberlite melt decreases the activity of H2O (Novella et al. 2015). Bearing this in mind, bulk rock c(H2O) reconstructed based on these hypothetical values are then compared to bulk c(H2O) reconstructed from measured mineral c(H2O), hereafter referred to as “hypothetical” and “observed” for brevity (Fig. 7a).
For samples from all suites investigated in this and a prior study (Aulbach et al. 2023), therefore representing an isogenous dataset, the difference between hypothetical and observed bulk c(H2O) increases with increasing clinopyroxene jadeite content (Fig. 7b). At the same time, the difference between hypothetical and observed c(H2O) is smaller for samples with high MgO, Cr2O3, Sr, Ce and Th concentrations (shown for MgO and Ce in Fig. 7c-d), which along with lower jadeite and grossular contents are identified as geochemical signatures of kimberlite melt metasomatism. If CO2 in kimberlite (∼ 5–10 wt%; e.g., Becker and Le Roex 2006) significantly reduces H2O activity, then the actual H2O content in the kimberlite would have had to be correspondingly higher, which is possible given the range of reported concentrations (3–7 wt%; Bell et al. 2004; Becker and Le Roex 2006). Despite the uncertainties, the observations suggest that metasomatised eclogites with jadeite-poor clinopyroxene did interact with an agent containing H2O, whereas samples with high-jadeite clinopyroxene, therefore little affected by metasomatism (including carbonatite), indeed contain less H2O than expected for equilibrium with a hydrous agent.
Inhibited hydrogen uptake in metasomatised cratonic eclogite: the role of crystal chemistry
As discussed in the previous section, most eclogite and pyroxenite xenoliths identified as metasomatised have reconstructed bulk rock H2O abundances commensurate with equilibrium with hydrous melt, whereas those identified as pristine have lower abundances than expected for such equilibrium. This finding translates into overall higher H2O abundances for metasomatised than for pristine samples, with broad correlations with various indicators of metasomatism (Fig. 8a-e). Although there is no correlation of bulk c(H2O) with Zr/Hf, high ratios of which have been linked to carbonatite metasomatism (Rudnick et al. 1993), we note that samples with the highest Zr/Hf values are restricted to bulk c(H2O) < 150 wt.ppm (Fig. 8f), which may reflect low H2O activity in carbonatite (Novella et al. 2015).
Increasing bulk-rock Ce concentrations, as a gauge for mantle metasomatism in eclogite, are accompanied by a decrease in Al2O3 content (Fig. 9a), and this is consequently similarly true for Ce and jadeite component (hence Al2O3 content) in clinopyroxene (Fig. 2b). Due to interaction with an H2O-bearing melt, metasomatised clinopyroxene with correspondingly low Al2O3 content does show elevated H2O concentrations (Fig. 9b). However, the positive effect of Al2O3 concentration in clinopyroxene clinopyroxene−meltDH is well established (Aubaud et al. 2008 and references therein), and the lowering of Al2O3 during metasomatism is thus expected to counteract the uptake of hydrogen from the metasomatic melt. As a consequence, the decrease in Al2O3 content and jadeite component in clinopyroxene leads to a decoupling of H2O and Ce, which have similar compatibilities during mantle melting (Aubaud et al. 2004), such that the lowest H2O/Ce are observed at low jadeite component in clinopyroxene and vice versa (Fig. 9c). Because the H2O inventory in eclogite is dominated by clinopyroxene, reconstructed bulk rocks similarly show low H2O/Ce at low Al2O3 contents (Fig. 9d).
In sum, in contrast to other incompatible components, the uptake in particular by clinopyroxene of hydrogen from kimberlite-like metasomatic agents seems to be impeded by attendant compositional and crystal-chemical changes that diminish hydrogen partitioning into clinopyroxene, and possibly additionally by lower H2O activity in carbonated melts. With a median H2O abundance of 94±47 wt.ppm (this study and Aulbach et al. 2023; assuming a 50% total uncertainty), eclogite and pyroxenite do not disproportionately (relative to their subordinate volume of < 20%; Garber et al. 2018) contribute to the bulk cratonic H2O inventory, compared to median abundances of 80 wt.ppm in peridotites from the Slave, Siberian and Kaapvaal cratons (Peslier et al. 2010, 2012; Doucet et al. 2014; Kilgore et al. 2020; see also Jackson and Gibson 2023).
Summary
We estimated, by FTIR, hydroxyl contents in clinopyroxene and garnet from eclogite and pyroxenite xenoliths that show evidence for metasomatism by kimberlite-like melt evident in elevated contents of MgO and of highly incompatible elements (e.g., Sr, Ce, Th), and compare them to published data for xenoliths that do not show strong evidence for metasomatic overprint. The main findings are as follows:
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In this study, the difference between H2O abundances derived from mineral-specific (Bell et al. 1995) vs. wavenumber-dependent calibrations (Libowitzky and Rossman 1997) is on average 33%.
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FTIR imaging reveals that the rim of only one of 8 samples shows variability (< 15%) of H2O, but well within the estimated overall uncertainty on mineral H2O of 40%, mostly related to choice of absorption coefficient.
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The calculated total clinopyroxene c(H2O), estimated for both pristine and metasomatised eclogite and pyroxenite xenoliths using the calibration of Libowitzky and Rossman (1997), ranges from 90 to 420 wt.ppm, while that in garnet is on average lower (< 2–50 wt.ppm).
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Bulk rock c(H2O) ranges from 40 to 210 wt.ppm, and – to the extent that experimental hydrogen partition coefficients derived in the eclogite-H2O system can be applied to kimberlite-metasomatised eclogites and pyroxenites - for many metasomatised samples corresponds within ~100 wt.ppm with hypothetical contents calculated for equilibrium with a melt having a typical H2O content of 3 wt%. In contrast, most pristine samples have lower bulk H2O abundances than expected for equilibrium with such melt.
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Although metasomatised eclogite and pyroxenite xenoliths have on average higher H2O abundances than pristine ones, metasomatism by kimberlite-like melt is shown to lower clinopyroxene Al2O3 contents, which in turn inhibits hydrogen incorporation, such that the abundances of highly incompatible elements like Ce become increasingly decoupled from those of hydrogen.
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Eclogite does not disproportionately contribute to the H2O inventory of the bulk cratonic mantle.
Data availability
New data acquired in this study are available in the article, and ancillary information is provided in its Online Resource.
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Acknowledgements
We gratefully acknowledge three reviewers for incisive comments that significantly improved the paper and the editor, Hans Keppler, for handling the manuscript and for additional comments. We thank Christel Tinguely and Anton Le Roex for providing samples from the Cullinan kimberlites, and Koidu Limited for those from Koidu. We gratefully acknowledge the De Beers Group for permission to use the Orapa samples.
Funding
This work was supported by the Deutsche Forschungsgemeinschaft (German Research Foundation) - Project number 432249855 (AU356/11) to SA. Nils B. Gies acknowledges SNF grant 200020 − 196927 for support of this research. Fanus Viljoen acknowledges funding from the South African Department of Science and Innovation under their Research Chairs Initiative (geometallurgy), as administered by the National Research Foundation (grant number 64779).
Open Access funding enabled and organized by Projekt DEAL.
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Aulbach, S., Gies, N.B., Linckens, J. et al. Inhibited hydrogen uptake in metasomatised cratonic eclogite. Contrib Mineral Petrol 179, 77 (2024). https://doi.org/10.1007/s00410-024-02157-6
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DOI: https://doi.org/10.1007/s00410-024-02157-6