Calcium isotope compositions of mantle pyroxenites

Journal Pre-proofs Calcium isotope compositions of mantle pyroxenites Wei Dai, Zaicong Wang, Yongsheng Liu, Chunfei Chen, Keqing Zong, Lian Zhou, Ganglan Zhang, Ming Li, Frederic Moynier, Zhaochu Hu PII: DOI: Reference:

S0016-7037(19)30732-X https://doi.org/10.1016/j.gca.2019.11.024 GCA 11536

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Geochimica et Cosmochimica Acta

Received Date: Revised Date: Accepted Date:

10 June 2019 19 November 2019 19 November 2019

Please cite this article as: Dai, W., Wang, Z., Liu, Y., Chen, C., Zong, K., Zhou, L., Zhang, G., Li, M., Moynier, F., Hu, Z., Calcium isotope compositions of mantle pyroxenites, Geochimica et Cosmochimica Acta (2019), doi: https://doi.org/10.1016/j.gca.2019.11.024

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Calcium isotope compositions of mantle pyroxenites Wei Daia, Zaicong Wanga,*, Yongsheng Liua, Chunfei Chena, Keqing Zonga, Lian Zhoua, Ganglan Zhanga, Ming Lia, Frederic Moyniera,b, Zhaochu Hua a State

Key Laboratory of Geological Processes and Mineral Resources, School of Earth Sciences,

China University of Geosciences, Wuhan 430074, China b Institut

de Physique du Globe de Paris, Université de Paris, 1 rue Jussieu, 75005 Paris, France

*Corresponding author: Zaicong Wang, E-mail: [email protected]

Abstract Variations of the stable Ca isotopic compositions (noted as δ44/40Ca relative to the SRM915a standard) of basalts are interpreted as effects of mantle sources. Mantle pyroxenites are a minor but integral part of the mantle and, as fusible components, they are important source rocks to understand chemical and isotopic heterogeneity in mantle-derived magmas. However, the effect of pyroxenites on the Ca isotopic composition of the mantle has been poorly constrained. To which extent mantle pyroxenites and their formation processes such as melt-peridotite reaction, particularly involving recycled crustal materials, lead to heterogeneity in δ44/40Ca of the mantle is unknown. Here, we report δ44/40Ca of different types of pyroxenites (spinel pyroxenites, garnet pyroxenites, and phlogopite-bearing spinel clinopyroxenites) and minerals separates, along with surrounding peridotites from Hannuoba xenoliths, North China Craton to address the issue. Initial 87Sr/86Sr ratio indicates that recycled crustal materials were incorporated into parental magmas of garnet pyroxenites (0.70391-0.70715) and phlogopite-bearing clinopyroxenites (0.7142-0.7149), consistent with previous conclusions from Sr-Nd isotopes. Overall, the δ44/40Ca of garnet pyroxenites ranges from 0.86‰ to 0.98‰ (average 0.90±0.05‰, n=10) and the host peridotites affected by the infiltrating melts from 0.87‰ to 0.93‰ (0.89±0.04‰, n=8). Each pair (n=8) of garnet pyroxenite and host peridotite displays no measurable difference in δ44/40Ca. The spinel pyroxenites and phlogopite-bearing spinel clinopyroxenites also show similar δ44/40Ca (0.94±0.06‰ and 0.98±0.05‰, respectively). The indistinguishable δ44/40Ca among these different types of 1 / 41

pyroxenites and surrounding peridotites suggest no obvious Ca isotope variations during silicate melt-peridotite interaction and fractional crystallization, even if recycled silicate materials were involved. These results indicate that the mantle source with variable proportions of pyroxenites in equilibrium conditions overall would show uniform δ44/40Ca. It implies that the basic magmas derived from such peridotite-pyroxenite source would display limited variations in δ44/40Ca, even if their radiogenic isotopes show strong heterogeneity. The conclusion is consistent with no systematic variations in the available δ44/40Ca of different basalt types such as DMM, EM1 and HIMU. However, garnets in mantle rocks of recent work and this study generally display heavier δ44/40Ca than co-existing clinopyroxenes, implying that melts generated by partial melting of mantle sources with abundant residual garnets may show lighter δ44/40Ca values than MORBs.

Key words: mantle heterogeneity, pyroxenite, Ca stable isotope, melt-peridotite reaction, basalts, recycling

1. Introduction Chemical and isotopic heterogeneity recorded in mantle rocks and mantle-derived melts is a natural result of differentiation of the dynamic Earth and is intimately related to magma migration, redistribution or recycling (e.g., Hofmann, 1997; Rampone and Hofmann, 2012). Mantle pyroxenites, mainly as layers or dykes, form a small (generally <10 vol.%) but integral part of the mantle. They display diverse lithologies and represents the mineralogical manifestation of elemental and isotopic heterogeneity (e.g., Allegre and Turcotte, 1986; Becker, 1996; Bodinier and Godard, 2014; Downes, 2007; Garrido and Bodinier, 1999; Pearson et al., 1991; Suen and Frey, 1987; van Acken et al., 2008). The formation of pyroxenites in the mantle mainly results from crystal accumulation from migrating melts and melt-rock reaction between existing pyroxenite, host peridotite and percolating melts (e.g., Bodinier and Godard, 2014; Downes, 2007; Garrido and Bodinier, 1999). Their parental magmas often display diverse origins such as asthenospheric mantle (Mukasa and Shervais, 1999; Sinigoi et al., 1983), subducted crustal materials or hybrid melts derived from reaction products of subducted crustal materials with peridotites (e.g., Borghini et al.,

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2016; Marchesi et al., 2013; Montanini et al., 2012; Pearson et al., 1991; Sobolev et al., 2005; Yaxley and Green, 1998). Importantly, because of their lower solidus temperatures compared to peridotites, a number of pyroxenites preferentially form melts during mantle melting and contribute to considerable fractions of pyroxenite-derived melts (e.g., Kogiso and Hirschmann, 2001; Lambart et al., 2009; Pertermann and Hirschmann, 2003). Therefore, mantle pyroxenites have been proposed as important source rocks for the genesis of basaltic magmas and are critical components to understand chemical and isotopic heterogeneity in both mantle sources and mantle-derived melts (e.g., Hirschmann and Stolper, 1996; Sobolev et al., 2005, 2007). In recent years, non-traditional stable isotopes such as calcium isotopes were regarded as good tracers for crust-mantle recycling especially for carbonate-related circulation and metasomatism based on their large variations in surficial reservoirs compared to mantle processes (e.g., Banerjee and Chakrabarti, 2019; Blättler and Higgins, 2017; Fantle and Tipper, 2014; Huang et al., 2011). Up to now, the majority of mantle peridotites and mantle-derived magmas displays consistent Ca isotope compositions while part of them shows a large variation (e.g. Amini et al., 2009; Chen et al., 2018; Huang et al., 2010, 2011; John et al., 2012; Kang et al., 2017; Liu et al., 2017a; Simon and DePaolo, 2010; Valdes et al., 2014; Zhao et al., 2017a). Compared to the δ44/40Ca value of the Bulk Silicate Earth (BSE, δ44/40Ca=0.94±0.10‰ relative to SRM915a standard) estimated from fertile mantle peridotites (Chen et al., 2019b; Kang et al., 2017) and the mineral separates (Huang et al., 2010; Simon and DePaolo, 2010) and komatiites (Amsellem et al., 2019), the peridotites and basalts from different tectonic settings often show large δ44/40Ca variations from -0.08‰ to 1.85‰. The huge Ca isotopic variation could not simply be attributed to partial melting or mineral differentiation as these processes result in negligible or limited (< 0.1‰) Ca isotopic fractionation (e.g., Chen et al., 2019a, 2019b; Kang et al., 2017; Zhang et al., 2018). Instead, they were attributed to the incorporation of recycled materials such as carbonates (Huang et al., 2011; Liu et al., 2017a), melt-rock interaction/metasomatism (Ionov et al., 2019; Kang et al., 2017, 2019; Zhao et al., 2017a) or the existence of garnet in the mantle source (e.g. He et al., 2017; Kang et al., 2019; Wang et al., 2019). In this case, the variable δ44/40Ca of mantle-derived basalts would reflect petrologic and isotopic heterogeneity in their mantle sources. However, the mantle sources seem to show generally consistent δ44/40Ca for different types of basalts such as DMM, EM1 and HIMU (Valdes et al., 2014). 3 / 41

It is also argued that carbonate metasomatism does not necessarily affect Ca isotope composition of the mantle peridotites (Ionov et al., 2019). All together, these studies raise the question of possible mantle source heterogeneities in δ44/40Ca and the link to mantle-derived rocks. Although mantle peridotites affected by metasomatism show large variations in Ca isotope ratios (e.g. Chen et al., 2018; Ionov et al., 2019; Kang et al., 2017; Zhao et al., 2017a), to which extent silicate melt-rock reaction and involvement of recycled component which are often related to the formation of pyroxenites would lead to variations in the δ44/40Ca of the mantle remains poorly understood. Compared to surrounding peridotites, the pyroxenites are more fusible than peridotites and have potential to contribute more to basic magmas (e.g. Kelemen et al., 1998; Lambart et al., 2013; Matzen et al., 2017). If any isotopic variations are present among pyroxenites, it would thus affect the δ44/40Ca of their melts. However, very limited Ca isotopic data have been available for mantle pyroxenites (Chen et al., 2019b; Kang et al., 2019). These pyroxenites are mainly crystal accumulates from asthenosphere-derived magmas and those from Balmuccia massif peridotite body have no evidence for the involvement of recycled crustal materials (Mukasa and Shervais, 1999; Sinigoi et al., 1983). Given the importance of pyroxenites for mantle chemical heterogeneity and the origin of basalts, it is necessary to investigate the Ca isotopic composition of mantle pyroxenites, particularly those which exhibit the effects of recycled crustal materials and melt-peridotite reaction. In this study, we selected a set of well-characterized pyroxenite xenoliths from Hannuoba, northeastern North China Craton (NCC, Liu et al., 2005; Zong et al., 2005) to constrain the Ca isotope variation among different types of mantle pyroxenites. These pyroxenites are comprised of three different types: spinel pyroxenites, garnet pyroxenites and phlogopite (Phl)-bearing clinopyroxenites. Eight composite xenoliths of garnet pyroxenites and peridotites show clear petrological evidence of melt-peridotite reaction (Liu et al., 2005). Previous studies have suggested that the parental melts of the pyroxenites in Hannuoba could have variably incorporated recycled crustal materials due to crustal-derived Sr-Nd isotopic compositions (e.g., initial 87Sr/86Sr of 0.71064 and εNd of -25) and we present Sr isotopic composition to test this in our samples. These samples are well-suited to understand the Ca isotope variation during the melt-peridotite reaction,

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particularly recycled crustal materials are involved.

2. Geologic settings and samples The North China Craton is one of the world’s oldest craton which experienced replacement of old, cold, thick and depleted Archean lithosphere by young, hot, thin and fertile lithospheric mantle (e.g., Gao et al., 2002; Rudnick et al., 2004; Wu et al., 2003; Wu et al., 2019; Zheng et al., 2004). Such event has been attributed to subduction of oceanic slabs in margins of the NCC at different stages and related delamination of lower crustal rocks into the mantle (e.g., Gao et al., 2004; Griffin et al., 1998; Windley et al., 2010). The intense modification with multi-stage melt-rock interactions were recorded by different types of pyroxenites and metasomatized peridotites carried out by eruption of later host basalts (Liu et al., 2005; Tang et al., 2008; Xu, 2002). The Tertiary Hannuoba basaltic plateau occurs along the north margin of the NCC (e.g., Song et al., 1990; Zhi et al., 1990; Zhu, 1998). Multiple types of mantle xenoliths have been found in the Hannuoba basalts and these peridotite and pyroxenite xenoliths have been considered as good samples to trace recycling of crust materials and melt-rock interaction event beneath NNC (e.g., Chen et al., 2016; Hu et al., 2016; Liu et al., 2005; Xu, 2002). A number of mantle xenoliths, which have been previously well characterized (Liu et al., 2005; Zong et al., 2005), were selected for Ca and Sr isotope measurements. These xenoliths can be classified into three types based on petrographic differences and formation processes: clinopyroxene-rich spinel pyroxenites (type I, cumulates), garnet pyroxenites (type II, products of silicate melt-peridotite reaction) and phlogopite-bearing spinel clinopyroxenites (type III, cumulates with later hydrous metasomatism by silicate melts). Type I spinel pyroxenites include garnet-free pyroxenite layers hosted in lherzolite with a sharp boundary (DMP314) and a single pyroxenite layer xenolith (DMP407-PV). The minerals are characterized by assemblage of clinopyroxene (Cpx)+olivine (Ol)+orthopyroxene (Opx)+spinel (Sp), with medium- to coarse-grained protogranular and equilibration texture (see detailed petrological features in Figure 4 in Liu et al., 2005). These pyroxenites have the similar mineral assemblages and equilibrium temperatures (839-1072°C) with their host peridotites, and have been interpreted as accumulation products of asthenospheric mafic magmas at spinel-peridotite facies

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conditions (Xu, 2002; Zhao et al., 2017b). Type II garnet pyroxenites are characterized by garnet pyroxenite layers/veins infiltrating to the spinel lherzolite with clear reaction boundary (see Fig. 1a and Fig. 4 in Liu et al., 2005). Previous studies have suggested that the Type II garnet pyroxenites and host lherzolites represent melt-rock reaction between evolved melts and peridotites (Liu et al., 2005). The mineral phases in pyroxenites are mostly coarse-grained clinopyroxenes, orthopyroxenes and garnet with rare anhedral spinel and olivine. This petrographic feature suggests that garnet pyroxenites may be formed at the expense of olivine and spinel during reaction between melt and peridotites. The host reactive lherzolites show the same mineral assemblage with other peridotite xenoliths found in Hannuoba. However, their enrichments in CaO and Al2O3 content suggest that these lherzolites could be influenced by the addition of silicate melt during reaction (Fig. 2a). Hydrous phases such as phlogopite in mantle rocks are indicative of fluid metasomatism (O’Reilly and Griffin, 2013; Zanetti et al., 1999). Type III Phl-bearing spinel clinopyroxenites were original accumulation products interacted with hydrous melts, as reflected by presence of abundant phlogopites (Zong et al., 2005). This type of pyroxenites is characterized by grey-green mediumcoarse sized clinopyroxene, intergranular phlogopite and spinel (Fig. 1b, d). The three types of mantle pyroxenites and, if present, their associated peridotites were selected for the present study. Among these samples, the type II garnet pyroxenites and the host peridotites were dominant. Furthermore, in order to understand inter-fractionation of coexisting phases, four garnet pyroxenites were picked for mineral separates of garnet, clinopyroxene and orthopyroxene. During the melt-rock reaction, element exchange between minerals and melt, caused composition zoning and isotopic kinetic fractionation such as observed for Ca (Zhao et al., 2017a). The in-situ analysis of major and trace element composition of minerals were performed to check whether the reaction reached equilibrium for type II samples and to calculate the distribution of Ca and its isotopes among different minerals of the whole rock.

3. Analytical Methods

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3.1. Calcium isotope analyses Sample dissolution, chemical purification and mass spectrometric analyses of Ca isotope were performed at the State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Wuhan (GPMR-CUG). The detailed analytical procedure has been reported before (Chen et al., 2019b; Feng et al., 2018; Li et al., 2018) and only a brief description is provided here. Sample powder of both mineral separates and bulk rocks was thoroughly digested by the mixture acid of HF-HNO3 and subsequent HCl-HNO3. Sample solutions were converted to nitride form and dissolved in 4 mol l-1 HNO3 for chemical purification. An aliquot sample solution of 40 μg Ca was loaded into the pre-cleaned column filled with 250 µl DGA resin (Ca-selective resin). Other elements present as matrix were removed by addition of 6.8 ml 4 mol l-1 HNO3 after loading the sample, and the Ca fraction was collected in 3 ml H2O. High recovery (>99%), efficient separation of Ca and a low total procedural blank of <10 ng were achieved. Calcium isotopes were analysed using a Nu Plasma 1700 MC-ICP-MS (Li et al., 2018). Isotope measurements were performed using standard-sample bracketing to correct instrumental drift. The Ca isotopic compositions were reported by using δ-notation: δn/42Ca relative to standard NIST SRM915a = [(nCa/42Ca)sample/(nCa/42Ca)SRM915a-1]×1000, where, n=44 or 43. Considering that 44Ca/40Ca

is commonly used in literature, all Ca isotopic ratios in this study are reported as δ44/40Ca

where δ44/40Ca=δ44/42Ca*2.048 (Heuser et al., 2016). Four USGS reference materials (BCR-2, BHVO-2, DTS-2, BIR-1a) were digested, purified and analysed during the same sequences as the other samples (Table 1). All results of these geological reference materials are consistent with literature data within the external analytical uncertainty of 0.14 ‰ (2sd, for 44Ca/40Ca) (Amsellem et al., 2017; Feng et al., 2017; He et al., 2017; Li et al., 2018; Liu et al., 2017b; Schiller et al., 2012; Valdes et al., 2014). Sample replicates digested from different fractions of sample powder show identical results within error. All measured δ44/42Ca and δ43/42Ca of the peridotites, pyroxenites, seawater, NIST SRM 915b and other reference materials were plotted on a mass-dependent fractionation line (Fig. S1).

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3.2. Strontium isotope analyses To identify the origin of melts for melt-peridotite reactions, seven pairs of type II composite xenoliths that include pyroxenites and reacted host lherzolites were chosen for bulk rock Sr isotope analysis. Because Rb-rich phlogopite is present in type III pyroxenites, 87Rb decay may strongly affect the bulk rock initial

87Sr/86Sr

ratio. Thus, the Sr isotope ratios of the Phl-bearing type III

clinopyroxenites were determined by laser ablation on clinopyroxene grains. The bulk rock Sr isotopic ratios of type II samples were analysed by a Fisher Scientific Triton TIMS. The sample powders were thoroughly digested and then purified by using cation exchange resin (Bio-Rad AG-50 W-X8) and Eichrom Ln-Spec resin. Full details of the procedure and data correction have been reported in Gao et al. (2004). Reference material NBS-987 was analysed along with samples. The result (87Sr/86Sr= 0.71024±0.00008, 2SD, n=8) agreed well with the reference value (87Sr/86Sr = 0.710236 ± 0.000016, 2SD) (Gao et al., 2004). Based on bulk rock trace element composition, the initial 87Sr/86Sr values of these samples were corrected back to 135 Ma, which is the approximate formation age of most garnet pyroxenites (Xu, 2002). The in-situ Sr isotope composition of Cpx were analysed by a Fisher Scientific Neptune Plus MC-ICP-MS coupled with a 193 nm laser ablation system. The strategy of data reduction and interference correction applied was reported by Tong et al. (2016). All spots analysed in this study had 87Rb/87Sr ratio < 0.002, indicating the negligible effect of Rb decay on 87Sr/86Sr (Table S1). The uncertainty of

87Sr/86Sr

were mostly < 0.0003 (2σ). In-house standards HNB-8 (natural Cpx

megacryst) and CPX05 (glass with Cpx composition) were analysed as the monitors. As the Table S1 shows, the results agree well with reference values within analytical uncertainty (Tong et al., 2016). Off-line selection and integration of signals and mass-dependent calibrations were performed using ICPMSDataCal (Lin et al., 2016).

3.3 In situ analyses of major and trace elements Major element compositions of minerals were analysed on polished thick sections by LA-ICPMS (GeoLas 2005 + Agilent 7900) at the GPMR-CUG. Detailed operation conditions for the laser system and the ICP-MS instrument were described in Liu et al. (2008). Laser sampling was

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performed using a spot size of 44 μm. Each mineral was analysed with 2-5 dots from core to rim. Off-line selection, integration of background, sample signals with time-drift correction and quantitative calibration were performed by ICPMSDataCal (Lin et al., 2016; Liu et al., 2008). Analyses of the USGS glasses suggest that the analytical accuracies were generally 5–10% for the trace elements and better than 5% for the major elements (Table S2).

4. Results Calcium isotopic compositions of the bulk rock pyroxenites and peridotites are reported in Table 1 and illustrated in Fig. 3 and S2. Two samples from the type I spinel pyroxenites have δ44/40Ca 0.88 and 0.92‰ (n=2) and the type III phlogopite-bearing spinel clinopyroxenites show similar δ44/40Ca (0.94‰ and 0.98‰, n=2). The δ44/40Ca of the type II garnet pyroxenites ranges from 0.86‰ to 0.98‰ with the average of 0.90‰ ± 0.08‰ (2sd, n=10). The harzburgite DMP314-PE which hosts the type I pyroxenite contains the lowest CaO content among measured peridotites (0.6 wt.%) with δ44/40Ca (1.01±0.08‰). The δ44/40Ca of host lherzolites of type II pyroxenites range from 0.86 to 0.93‰ with an average of 0.89‰ ± 0.04‰ (2sd, n=8). Importantly, each pair (n=8) of garnet pyroxenites and the host reactive lherzolites show no resolvable Ca isotope variation. The δ44/40Ca of these different types of pyroxenites and lherzolites overall display indistinguishable δ44/40Ca (Fig. 3), which overlaps the BSE value of 0.94 ± 0.10‰. Four garnet pyroxenites were selected for mineral separation, and garnets of two samples were successfully picked out. The data of mineral separates of four garnet pyroxenites are listed in Table 2 and shown in Fig. S3. The Cpx and Opx separates from garnet pyroxenites have similar δ44/40Ca (0.85-0.94‰, 0.83-0.98‰, respectively) and show identical δ44/40Ca with the bulk rock (0.86-0.91‰, Fig. 4 and S3). However, the co-existing garnets show heavier δ44/40Ca (1.14-1.29‰) than the Cpx by 0.24-0.44 ‰ (Fig. 4a). Bulk rock initial 87Sr/86Sr of type II samples are shown in Table 1. The initial 87Sr/86Sr ratios of type II garnet pyroxenites and host lherzolites show a broad range from 0.70391 to 0.70715 and 0.70338 to 0.70600, respectively. The garnet pyroxenites often show higher

87Sr/86Sr

than host

reactive lherzolites (Fig. 5). The in-situ 87Sr/86Sr ratios of Cpx grains in the same type III Phl-bearing

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spinel clinopyroxenites show limited change from rim, mantle and core within the analytical uncertainty (see Table S1). As a result, the average of 87Sr/86Sr ratios in Cpx from the same sample were calculated to represent its bulk rock ratio without the effect of phlogopite (Table 1). The Phlbearing pyroxenites display similar but noticeably high 87Sr/86Sr of 0.7142 and 0.7149. Major and trace element compositions of garnet, Opx, Cpx, olivine and spinel in the representative samples are summarized in Table S2. No significant chemical zonation has been found in pyroxenes (Table S3). The mass proportion of these minerals in the garnet pyroxenites was calculated from major-element mass balance between the bulk rock and the constituent minerals (Table 2). In-situ LA-ICP-MS results indicate that the CaO contents of the Cpx are 18.2-21.3 wt.%, which are far higher than for Opx (0.54-0.84 wt.%), olivine (<0.03 wt.%) and spinel (< 0.03 wt.%) (Table S2). Garnet is the other Ca-rich mineral in pyroxenites (4.10-8.29 wt.% CaO). Because of different proportions of garnet and pyroxene in garnet pyroxenites, the Ca contents hosted by Cpx vary from 80 wt.% to 86 wt.% and the remaining Ca is mainly hosted in garnet. Although Opx (together with olivine) could have a mineral proportion up to 30%, they account for < 1.5 wt.% of the bulk rock Ca budget (Table 2). Cpx and garnet host most of REE elements of the rock, and clinopyroxenes show a strong depletion of heavy REE relative to co-existing garnets which are enriched in heavy REE (Fig. S4).

5. Discussion 5.1 Inter-mineral Ca isotope variations in garnet pyroxenites Constraining inter-mineral isotope fractionation is helpful to explain the isotope fractionation of bulk rock and its variation during magmatic processes (Doucet et al., 2016; Millet et al., 2016; Williams and Bizimis, 2014; Zhong et al., 2017). The Ca isotope fractionation between co-existing Cpx and Opx has been well studied (e.g. Feng et al., 2014; Huang et al., 2010; Kang et al., 2016; Wang et al., 2017). However, the inter-mineral fractionation of Ca isotope in garnet-bearing mantle rocks remains limited (Kang et al., 2019; Wang et al., 2019). For the garnet pyroxenites from Hannuoba, the δ44/40Ca of garnets (1.16-1.29‰) are higher

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than the co-existing Cpx (0.85-0.93‰) with Δ44/40CaGrt-Cpx (δ44/40CaGrt-δ44/40CaCpx) = 0.24-0.44‰ (Fig. 4a). We suggest that such δ44/40Ca difference could record inter-mineral equilibrium isotope fractionation between garnet and co-existing Cpx. While the garnets in this study were decomposed into kelyphite assemblages, their bulk compositions still preserve the major element composition of pyrope-rich garnet (Liu et al., 2003), implying that the δ44/40Ca of garnet grains should not be influenced by decomposing process. Garnets show typical enriched patterns of heavy REE whereas the Cpxs show relative depletion in heavy REE (Fig. S4). The complementary feature would suggest that garnet and clinopyroxene formed in equilibrium condition, as shown by experimental data on co-existing garnet and Cpx (e.g., Hauri et al., 1994). The higher value of δ44/40Ca in garnets than in co-existing Cpx is consistent with the theoretical prediction that the stronger bonding in garnet favors the incorporation of heavier Ca isotope (Antonelli et al., 2019; Huang et al., 2019; Magna et al., 2015; Urey, 1947; Zhou et al., 2016). Recent studies on garnet peridotites and eclogites also show similar results (Kang et al., 2019; Wang et al., 2019). According to natural samples from these previous work and this study, the Δ44/40CaGrt-Cpx very likely depends on the equilibrium temperature (Fig. 4a, the empiric function: Δ44/40CaGrt-Cpx=0.47*106/T2), which generally follows the prediction of theoretical calculation (Huang et al., 2019). The Cpx and co-existing Opx show similar δ44/40Ca, leading to a very limited fractionation (Δ44/40CaOpx-Cpx (δ44/40CaOpx-δ44/40CaCpx) = -0.11~0.02‰, n=3). This result is consistent with the negative correlation between Δ44/40CaOpx-Cpx and Ca/Mg ratio in Opx observed in natural samples (Fig. 4b). Such Ca isotope variation between Opx and Cpx could be attributed to compositional effect (Kang et al., 2016; Zhao et al., 2017a), equilibrium temperature (Wang et al., 2017), or disequilibrium caused by chemical diffusion (Antonelli et al., 2019). In this study, the core and rim of the Cpx do not show chemical zonation (Table S3). The Ca/(Ca+Mg+Fe) ratio in Opx and Cpx display a negative trend (Fig. S5) which has been considered as the indicator of chemical equilibrium (Chen et al., 2019b; Zhao et al., 2017a). Furthermore, the Fe–Mg exchange factor between Opx and Cpx mostly ranges from 0.92 to 0.96 (Table 2) which are close to the calibrated equilibrium value of 1.09 ± 0.14 in Putirka (2008). These results indicate that the Δ44/40CaOpx-Cpx values of this study should record the inter-mineral equilibrium Ca isotope fractionation. Although the theoretical Δ44/40CaOpx-Cpx is expected to be within a range of 0.26-0.50‰ for mantle rocks 11 / 41

(Wang et al., 2017), the theoretical prediction cannot explain the large inter-mineral equilibrium Ca isotope fractionation recorded in the mantle rocks (e.g., Kang et al., 2016). Negative correlation between Δ44/40CaOpx-Cpx values and Ca/Mg ratio indicates the control of chemical compositions of Opx (Kang et al., 2016; Zhao et al., 2017a). Here, we suggest the low Δ44/40CaOpx-Cpx values of the Hannuoba garnet pyroxenites could result from high Ca/Mg ratio in Opx. Irrespective of the details, such internal fractionation hardly affects the bulk rock δ44/40Ca due to the very low Ca contents and low Ca budget in Opx (Table 2) except for Cpx-poor mantle rocks (Chen et al., 2019b). For spinel peridotites or pyroxenites, bulk rock Ca contents is dominated by Cpx (90-96 wt.%) and δ44/40Ca of Cpx are generally reflecting that of the bulk rock (e.g., Chen et al., 2019b; Kang et al., 2016; Zhao et al., 2017a). However, the garnet pyroxenites have two main Ca-host minerals: Cpx (~ 40-54 vol.%) and garnet (~ 20-42 vol.%). The garnet in this study accounts for 14-19 wt.% of the Ca in bulk rock (Table 2). Based on the mass balance calculation, the existence of large volume fraction of garnet only slightly elevates bulk rock δ44/40Ca such as by 0.04-0.06‰ at ~ 40 vol.% garnet compared to garnet-free pyroxenites (see Table 2). Such change is minor and implies that adding garnet mode in lherzolites and pyroxenites hardly changes bulk rock δ44/40Ca due to the existence of Cpx.

5.2 Limited Ca isotope variations among different types of pyroxenites Mantle pyroxenites measured in this study include different types such as spinel pyroxenites, garnet pyroxenites and Phl-bearing clinopyroxenites. These pyroxenites formed through mineral accumulation or melt-rock reaction by different kinds of melts and display large variation on mineral proportions and chemical compositions (e.g. CaO content ranges from 6.1 wt.% to 23.5 wt.%, see Fig. 2). The combined high La/Yb and low Ti/Eu ratios are often considered to be the typical feature of carbonate metasomatism (Coltorti et al., 1999; Rudnick et al., 1993). As shown in Fig. 2c, the different pyroxenites and peridotites have variable (La/Yb)N ratio but relatively high Ti/Eu ratio. Such feature indicates that all types of pyroxenites in this study are related to silicate melt/fluid addition without noticeable influence of carbonates. Their variable (La/Yb)N ratio might reflect the addition of silicate melts with different compositions. Due to the complex origins and formation

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processes, these three types of pyroxenites will be discussed below, respectively.

5.2.1 Type I spinel pyroxenites The type I spinel pyroxenites are accumulation products of migrating asthenospheric basic melts in the mantle (Choi et al., 2008; Liu et al., 2005; Tang et al., 2008; Xu, 2002), similar to the pyroxenites in Balmuccia peridotite massif (Mukasa and Shervais, 1999; Wang and Becker, 2015). They show convex REE abundance patterns with slight enrichment of light LREE (Fig. 2b, Liu et al., 2005). Such REE pattern is typical for cumulates derived from basaltic magma (Frey and Prinz, 1978). The type I spinel pyroxenites and their host lherzolites show similar δ44/40Ca. These δ44/40Ca values are also consistent with those in Balmuccia peridotite massif, supporting the previous conclusion that negligible fractionation of δ44/40Ca during accumulation of asthenospheric-derived melts to form pyroxenites (Chen et al., 2019b).

5.2.2 Type II garnet pyroxenites and host reactive lherzolites The type II garnet pyroxenites are reaction products between silicate melts and spinel peridotite host rocks (Fig. 1, and Liu et al., 2005). According to the calculation functions (Brey and Kohler, 1990; Putirka, 2008), the equilibrium temperature and pressure between Cpx and Opx were 9831067℃ and 7.4-17.2Kbar (KD(Fe-Mg)d = 0.92-0.96 for most samples, Table 2), consistent with previous estimates (Liu et al., 2003; Zhao et al., 2017b). The P-T range reflects the last equilibrium condition of these samples and should be lower than that for melt-peridotite reaction. The initial 87Sr/86Sr ratios of type II garnet pyroxenites and host lherzolite range from 0.70391 to 0.70715 and 0.70338 to 0.70600, respectively (Table 1 and Fig. 5). Such variable 87Sr/86Sr is noticeably higher than that of asthenospheric-derived melts (e.g. mostly ~0.7026 for N-MORBs, Workman and Hart, 2005). Xu (2002) analysed the Sr isotope composition of pyroxenites from Hannuoba and showed highly variable initial 87Sr/86Sr (e.g., 0.7049 to 0.7113). Combined with Nd isotope ratios (εNd = -1.1 to +7.4), the feature has been interpreted to be the results of mixing of asthenospheric melts with a variable amount of recycled altered oceanic crust and/or continental crustal components (Xu, 2002). A number of zircons were also separated from garnet pyroxenites

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from Hannuoba and they displayed variable ages which agree with detrital zircon age of North China Craton (Liu et al., 2010). These results clearly indicate that the recycled crustal materials have contributed to the melts and led to the silicate melt-peridotite reaction to form garnet pyroxenites. As shown in Fig. 5a, the garnet pyroxenites overall show higher CaO contents and higher initial 87Sr/86Sr

ratio than host reactive lherzolites. It suggests that during melt-peridotite reaction, the

silicate melts adding Ca to peridotites corresponded to a component with radiogenic Sr. Although, garnet pyroxenites overall show more radiogenic Sr relative to their host reactive peridotites, the δ44/40Ca values of each pair remain identical (Fig. 5b). Such remarkable constancy in δ44/40Ca values in different pyroxenites and affected peridotites suggests that addition of recycled crustal components and silicate melt-peridotite reaction have limited influence on the bulk-rock Ca isotope compositions (Fig. 5). Mantle-like δ44/40Ca values of peridotites metasomatized by silicate melts were also reported in previous studies (Ionov et al., 2019; Kang et al., 2017). The results from Hannuoba also imply that the parental melts of garnet pyroxenites could have similar δ44/40Ca values with host peridotites. The affected peridotites with variable extents of meltperidotite reaction remain the δ44/40Ca similar to the BSE (Fig. 5b). In turn, it suggests that the bulk rock δ44/40Ca value of garnet pyroxenites should be unchanged during silicate melt-peridotite reaction and largely reflect their hybrid parental melts. Previous studies have suggested that the fractionation between Cpx and their forming melts is close to 0 (Δ44/40CaCpx-melt=0) and it could be used to estimate the δ44/40Ca of silicate melt (e.g., Zhang et al., 2018). As shown in Table 2, the δ44/40Ca of parental melts show an undetectable variation with that of bulk rock (< 0.12‰), even taking effect of temperature during differentiation of Cpxs into account (Zhang et al., 2018). The garnet pyroxenites display δ44/40Ca identical with mantle peridotites collected from Hannuoba (this study and Kang et al., 2016), also suggesting that the δ44/40Ca of hybrid melts from recycled crustal materials would be indistinguishable with the mantle peridotites. The newly-estimated δ44/40Ca values of continental silicate sediments (0.83‰ ± 0.1 ‰, 2SE, Wang et al., 2019) is close to that of BSE and the eclogites would retain δ44/40Ca of the primary basaltic rocks during subduction (Lu et al., 2019). Such limited change in δ44/40Ca values of recycling silicate materials is consistent with the δ44/40Ca values of the parental melt of pyroxenites from Hannuoba.

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5.2.3 Type III phlogopite-bearing spinel clinopyroxenites Previous studies have suggested that early Al-pyroxenites underwent later interaction with fluid-bearing melts to form type-III Phl-bearing clinopyroxenites (Zhao et al., 2017b; Zong et al., 2005). These samples show the strongest enrichment in light REE among all the pyroxenites studied here with (La/Yb)N of 5.6 and 6.4 (Fig. 2b). The in-situ

87Sr/86Sr

ratios of Phl-bearing

clinopyroxenites show a very radiogenic feature (0.7142 to 0.7149). Given the rather low Rb/Sr ratio in Cpx (<0.002), 87Sr/86Sr ratios measured in-situ should reflect the initial 87Sr/86Sr ratios of parental magmas of type III pyroxenites. Although Sr isotopic diffusion from phlogopite potentially affects the Cpx grains, Xu (2002) also reported a series of Al-pyroxenites from Hannuoba with high bulk rock initial

87Sr/86Sr

ratio (e.g., up to 0.7113). Therefore, these radiogenic Sr enrichments

would be attributed to the addition of crust-derived components such as continental sediments (Xu, 2002). Although these Phl-bearing clinopyroxenites inherited a strong crust-derived signature and formed from hydrous silicate melts, their δ44/40Ca of 0.94-0.98‰ show no difference from the value of BSE and other pyroxenites in Hannuoba. The result further strengthens that reaction of peridotite with recycled crustal silicate materials does not result in obvious variations in δ44/40Ca.

5.3 Calcium isotopes of the pyroxenite-veined mantle and the implications for mantle-derived basalts Basic melts migrating in the mantle often interact with peridotites and form different veins/layers of pyroxenites along the transport channels (e.g., Becker, 1996; Bodinier and Godard, 2014; Downes, 2007; Garrido and Bodinier, 1999; Pearson et al., 1991; Suen and Frey, 1987; van Acken et al., 2008). The ubiquitous presence of pyroxenite veins in the mantle peridotites leads to a highly heterogenous, veined mantle, also called a “marble-cake” mantle (Allegre and Turcotte, 1986). Up to now, the large Ca isotope variation has been observed among mantle rocks but is mainly displayed for depleted peridotites with low Ca contents; whereas the fertile lherzolites and pyroxenites in equilibrium conditions show consistent δ44/40Ca value as the BSE (Fig. 3). The pyroxenites from Hannuoba have diverse origins and formed from a very different history. However, 15 / 41

notably, they still show consistent δ44/40Ca. This result indicates that no resolvable Ca isotope fractionation occurs during silicate melt-peridotite reaction or mineral differentiation in an equilibrium system. The pyroxenites from Alps and Tariat also show similar δ44/40Ca to surrounding peridotites (Chen et al., 2019b; Kang et al., 2019). Together, these results suggest the pyroxeniteveined mantle should have relatively homogeneous δ44/40Ca values, even if they are affected by recycled crustal silicate materials and show heterogenous radiogenic isotopes (Fig. 6). Due to limited fractionation (< 0.1‰) during melting of upper mantle, partial melts of such heterogenous, pyroxenite-veined mantle would inherit the radiogenic isotope anomalies but not necessarily lead to the change of δ44/40Ca. The conclusion is supported by the remarkably indistinguishable δ44/40Ca of currently available basalts from a wide range of mantle end-members such as DMM, EM1, HIMU (Fig. 6, Chen et al., 2019a; Valdes et al., 2014; Zhang et al., 2018), suggesting the possible decoupling of Ca stable isotope and other radiogenic tracers. Many peridotites indeed show very different δ44/40Ca values from the BSE, but the kinetic fractionation or disequilibrium could be one of the main reasons (e.g., Ionov et al., 2019; Zhao et al., 2017a). The basaltic magmas are generally derived from a large scale of mantle source and such variations in δ44/40Ca of dis-equilibrated peridotites could be diluted or homogenized during mantle melting and magmas mixing. However, some basalts located in different tectonic regions have been reported with variable δ44/40Ca values from 0.65‰ to 1.07‰ (e.g. John et al., 2012; Liu et al., 2017a; Zhu et al., 2018). The recycled carbonates have been thought to play an important role (Banerjee and Chakrabarti, 2019; Huang et al., 2011; Liu et al., 2017a). Alternatively, the existence of garnet in the mantle source could be another explanation. Some basalts are suggested to originate from a deep, garnetbearing mantle source, e.g. Hawaii OIBs (Davis et al., 2011; Hirschmann et al., 2003; Hofmann and White, 1982; Salters and Sachi-Kocher, 2010). As garnets are enriched in heavy Ca isotopes relative to co-existing Cpx (this study, Kang et al., 2019; Wang et al., 2019), melts generated from mantle sources with garnet and jadeite-rich Cpx should display lower δ44/40Ca. The detailed modelling has indicated that the partial melts from garnet-abundant mantle sources like eclogites can be up to 0.3‰ lower than the mantle residues, which would explain the low δ44/40Ca values in some OIB-like basalts (Kang et al., 2019; Wang et al., 2019). 16 / 41

6 Conclusions Different types of mantle pyroxenites and associated peridotites from Hannuoba mantle xenoliths display large variations in mineralogy, chemistry and Sr isotopic compositions, which were mainly influenced by recycled crustal materials in different extents. However, their δ44/40Ca values are indistinguishable from each other. The associated host peridotites, even strongly affected by infiltrating parental magmas of pyroxenites (Type II garnet pyroxenite-lherzolite composite xenoliths), also show no difference in δ44/40Ca values from pyroxenites, suggesting negligible Ca isotope fractionation during silicate melt-peridotite reaction. The fertile peridotites and these different types of pyroxenites also show δ44/40Ca values consistent with that of the BSE. These results indicate that magmatic processes dominantly by silicate melt, either derived from asthenosphere or recycled silicate rocks or a combination of both origins, would not lead to significant Ca isotope variations in equilibrium conditions. Therefore, the veined, heterogeneous mantle consisting of peridotites and pyroxenites should show similar δ44/40Ca values of around 0.94 ± 0.10‰, even if they show highly radiogenic isotopic compositions such as 87Sr/86Sr ratios. The observed lower or higher δ44/40Ca values in some mantle rocks would be attributed to other factors such as kinetic or disequilibrium fractionation or incorporation of other surficial components like carbonates. Melting of garnet-poor, pyroxenite-veined mantle would not lead to noticeable variations in δ44/40Ca values of derivative magmas. The mantle-derived magmas may show variable Sr isotopic compositions, but their δ44/40Ca values remain similar as reflected in the homogeneous Ca isotopic compositions of basalts derived from DMM, EM1 and HIMU mantle components. However, available data of mantle rocks indicate that garnets generally display heavier δ44/40Ca than coexisting clinopyroxenes. It implies that abundant garnets residual in the source of melting would lead to δ44/40Ca values of the derivative magmas noticeably lighter than the MORBs.

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Acknowledgements We thank Haihong Chen, Lanping Feng and Yu Lei for support in the lab. This study was supported by National Science Foundation of China (No. 41673027, 41722302), Chinese Fundamental Research Funds for the Central Universities (No. CUG170602) and the MOST Special Fund from GPMR-CUG (No. MSFGPMR10) to Z. Wang. F. Moynier thanks ERC under the European Community’s H2020 framework program/ERC grant agreement (No. 637503, Pristine) and the UnivEarthS Labex program (No. ANR-10-LABX-0023 and ANR-11-IDEX-0005-02). We appreciate Vincent Salters, Anupam Banerjee and an anonymous reviewer for their constructive comments and the editors Jeffrey G. Catalano and Shichun Huang for patient and kind editorial help.

7. Figure captions Figure 1. Representative microphotographs of type II and III Hannuoba pyroxenite xenoliths. (a) Garnet pyroxenite shows irregular rim with its host reactive peridotite. (b) Type III pyroxenite contains large grains of phlogopite. The rectangular areas of a and b are displayed in detail in c and d, respectively (plane-polarized light). Type I mantle xenoliths have been shown in Liu et al. (2005).

Figure 2. (a) Bulk rock major and trace elements of mantle pyroxenites and peridotites of this study. Different types of pyroxenites show a large range of MgO and CaO contents. Overall, they contain higher CaO contents than the host peridotites. Data of DMP peridotite xenoliths from Rudnick et al. (2004) and Xia et al. (2004) are shown for comparison. (b) Three types of pyroxenites display different REE contents and patterns, suggesting different origins or formation processes. (c) Plots of Ti/Eu vs. (La/Yb)N indicate the predominant control of silicate melt metasomatism for Hannuoba samples. The black square represent the value of primitive mantle from Palme and O'Neill (2014). The data for chondrite (CI) normalization is from McDonough and Sun (1995).

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Figure 3. Plots of bulk rock CaO content versus δ44/40Ca values for pyroxenites and their host lherzolites. Although fertile peridotites and pyroxenites show a significant range of CaO contents, they display similar δ44/40Ca. Literature data is also shown in the figure (Chen et al., 2018, 2019b; Ionov et al., 2019; Kang et al., 2016, 2017, 2019; Zhao et al., 2017a). The grey range represents the δ44/40Ca of the BSE (0.94±0.10‰, see Introduction).

Figure 4. (a) Plots of Δ44/40CaGrt-Cpx (‰) vs. equilibrium temperature of the rocks. The published data from Dabie and Siberia are also shown (Kang et al., 2019; Wang et al., 2019). The Δ44/40CaGrt-Cpx from these natural samples overall show a negative correlation with the equilibrium temperature, with an empiric fractionation of Δ44/40CaGrt-Cpx = 0.47*106/T2 (dashed line ①). Solid line ② is the theoretical equation of equilibrium fractionation between garnet and jadeite-free Cpx (Δ44/40CaGrt-Cpx ≈ 0.56*106/T2, Huang et al., 2019). (b) Plots of Δ44/40CaOpx-Cpx (‰) vs. Ca/Mg of Opx in type II garnet pyroxenites. The theorical calculation lines of Δ44/40CaOpx-Cpx come from Wang et al. (2017). The literature data between Cpx-Opx in natural samples are compared (Chen et al., 2019b; Huang et al., 2010; Kang et al., 2016; Zhao et al., 2017a). The results indicate the strong effect of Ca/Mg in Opx on Δ44/40CaOpx-Cpx.

Figure 5. Plots of initial

87Sr/86Sr

ratios vs. bulk rock CaO contents (a) and δ44/40Ca (b) for

seven pairs of type II garnet pyroxenites and host reactive lherzolites. Garnet pyroxenites mostly show higher 87Sr/86Sr than host reactive lherzolites (dashed lines in a and b) and two pairs show similar 87Sr/86Sr.

Figure 6. Plot of δ44/40Ca values and initial

87Sr/86Sr

ratio for Hannuoba pyroxenites and

peridotites. The δ44/40Ca values of different types of basalts such as MORBs, HIMU and EM1 are similar (Chen et al., 2019a; Valdes et al., 2014; Zhu et al., 2018). Their 87Sr/86Sr ratios were not reported and thus is estimated from literature (Jackson and Dasgupta, 2008). The data of Hawaii basalts (Huang et al., 2011), Tengchong basalts (Liu et al., 2017a) and Fanshi peridotites (Chen et al., 2018) which were all interpreted with the contribution of recycled carbonates are also shown for comparison. The yellow rectangle area represents the hydrothermal carbonates in altered oceanic 19 / 41

crust from ODP Site 801 (see Blättler and Higgins, 2017 and references therein). The marine carbonates show huge Ca isotope variation from -1.09‰ to 1.81‰ which are not plotted in the figure. The average δ44/40Ca of continental silicate sediments is estimated at 0.83‰ ± 0.1 ‰ (2SE) (see Wang et al., 2019 and references therein). Due to the variable 87Sr/86Sr ratio, these values are not plotted in the figure.

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Table 1. Ca isotopic compositions of the Hannuoba xenoliths and reference materials. Sample s

MgOa(wt.

CaOa(wt.

Al2O3a(wt.

Ti/E

%)

%)

%)

u

44.2

0.63

1.22

(La/Yb)

δ44/40Ca(‰

N

)b

2.04

1.03

0.17

1.01

0.08

2sd

δ44/42Ca( ‰)

2sd

δ43/42Ca( ‰)

2sd

n

87Rb/86

87Sr/86S

Sr

rc

0.3354

0.70664

0.2404

0.70444

2σ

87Sr/86Sr

(135M

d

a)

Type I lherzolite DMP31

WR

4-PE

1124 1

Replica te

0.50

0.08

0.23

0.08

3

0.49

0.04

0.25

0.03

3

0.45

0.05

0.26

0.12

4

0.43

0.06

0.23

0.22

3

0.45

0.01

0.20

0.04

Type I spinel pyroxenites DMP31 4-PV DMP40 7-PV

WR

26.8

9.97

4.00

WR

24.6

6.12

4.28

4692

1.66

0.92

0.10

6967

1.42

0.88

0.11

8260

3.36

0.93

0.01

Type II metasomatic lherzolites DMP12 3-PE DMP42 5-PE

WR

32.7

4.10

5.24

WR

38.8

2.62

3.02

8101

0.62

2

0.88

0.12

0.43

0.06

0.19

0.06

3

0.87

0.10

0.43

0.05

0.25

0.16

3

0.0000 9

0.70600

Replica te DMP44 1-PE

WR

35.3

4.64

4.25

6370

0.39

0.87

0.08

0.43

34 / 41

0.04

0.23

0.09

3

0.0000 3

0.70397

DMP44 8-PE DMP45 8-PE

WR

36.9

3.55

4.61

WR

36.3

3.93

4.53

7327

1.19

0.89

0.06

0.43

0.03

0.23

0.01

3

6456

0.46

0.88

0.05

0.43

0.03

0.20

0.05

3

0.90

0.08

0.44

0.04

0.17

0.09

Replica te DMP46 0-PE DMP46 4-PE DMP46 6-PE

WR

38.4

4.52

2.77

WR

35.9

3.19

5.16

WR

28.0

6495 8493

5.81 3.09

0.86 0.91

0.01 0.08

5.25

7.38

8564

0.79

0.87

0.02

24.8

6.81

8.35

6275

0.62

0.88

0.09

19.4

15.0

6.76

0.42 0.44

0.01 0.04

0.24 0.21

0.00 0.04

0.43

0.01

0.17

0.13

0.43

0.04

0.16

0.02

0.2213

0.70381

0.2212

0.70488

0.2725

0.70458

0.3296

0.70401

0.0993

0.70444

0.1000

0.70734

0.2313

0.70435

0.1297

0.70603

0.0000 8 0.0001 1

0.70339 0.70446

3

2 3 3

0.0000 1 0.0000 2 0.0000 2

0.70405 0.70338 0.70425

Type II garnet pyroxenites DMP12 3-PV DMP42 5-PV

WR WR

5396

1.07

0.86

0.12

0.83

0.11

Replica te DMP44 1-PV DMP44 8-PV

WR WR

21.5

26.6

9.34

7.50

11.7

8.33

5096 6566

0.33 0.82

0.90

0.01

0.80

0.18

0.86

0.06

Replica te

0.42

0.06

0.15

0.08

0.41

0.05

0.17

0.09

0.44

0.01

0.22

0.07

0.39

0.09

0.16

0.10

0.42

0.03

0.24

0.05

35 / 41

3 3

0.0000 7

0.70715

5 3 3 4

0.0000 2 0.0000 7

0.70391 0.70578

DMP45 8-PV DMP46 0-PV DMP46 4-PV

WR WR WR

19.8

11.2

11.3

21.1

11.4

9.32

17.6

12.4

13.8

4847

0.45

0.95

0.06

5577

1.04

0.91

0.14

5509

0.70

0.46

0.03

0.26

0.08

0.44

0.07

0.24

0.06

3 3

0.90

0.05

0.44

0.03

0.24

0.10

2

0.91

0.12

0.45

0.06

0.27

0.06

3

0.2376

0.70583

0.1659

0.70478

0.0738

0.70454

0.0509

0.70518

0.0000 9 0.0000 3 0.0000 2

0.70537 0.70446 0.70440

Replica te DMP46 6-PV

WR

19.8

8.92

12.5

WR

21.1

11.7

10.3

WR

23.2

10.8

10.2

14.0

23.5

14.1

22.7

6744

0.16

0.86

0.05

0.42

0.03

0.18

0.01

3

4952

1.38

0.91

0.09

0.44

0.04

0.19

0.07

3

5016

1.61

0.98

0.10

0.48

0.05

0.17

0.08

3

9.38

3197

5.60

0.94

0.09

0.46

0.04

0.17

0.11

10.8

3752

6.39

0.98

0.07

0.48

0.03

0.27

0.08

NIST SRM 915b

0.67

0.10

0.33

0.05

0.17

0.06

6

Alfa Ca

1.85

0.09

0.90

0.05

0.43

0.07

5

DTS-2b

1.21

0.13

0.59

0.06

0.29

0.09

8

DMP13 4 DMP25 4

0.0000 2

Type III Phl-bearing clinopyroxenites DMP10 7 DMP13 8

WR WR

3 3

Reference materials

36 / 41

0.7149

0.0001

0.7142

0.0002

0.70508

SEA

1.92

0.16

BHVO2

0.77

BCR-2 BIR-1a

0.09

0.74

0.11

0.85

0.10

0.94

0.08

0.47

0.08

0.37

0.04

0.22

0.10

0.36

0.06

0.18

0.16

0.42

0.05

0.22

0.08

6 1 1 1 3 4

Note: WR-whole rock; Replicate-replicate digestion of the different fraction of sample powder. a: Major and trace element compositions of the whole rock are from Liu et al., (2005). b: δ44/40Ca=δ44/42Ca*2.048 (Heuser et al., 2016) c: 87Sr/86Sr of type III clinopyroxenites are averages of in-situ 87Sr/86Sr of Cpx grains in each sample. All spots analysed in this study had 87Rb/87Sr ratio < 0.001. See Table S1 for the specific values. d: The initial 87Sr/86Sr values of these samples were corrected back to 135 Ma, which is the approximate formation age of most garnet pyroxenites (Xu, 2002).

37 / 41

Table 2. Ca distribution and δ44/40Ca in constituent minerals of the Hannuoba garnet pyroxenites.

CaOa

Samples DMP441PY

(wt.%)

The Mode(%)

proportion of Cab

WR

9.34

Cpx

19.6

39

Opx

0.54

13

4.21

41

DMP464PV

DMP460PV

WR

<0.03

4

δ43/42Ca(‰)

2sd n

Δ44/40Cax-

Δ44/40Cax-

KD(Fe-

Cpx

c melt(T)

Mg)d

0.01

0.22

0.07

3

0.04

0.10

81%

0.93

0.03

0.45

0.02

0.17

0.04

3

0.00

0.06

1%

0.98

0.02

0.48

0.01

0.21

0.12

2

0.05

0.11

0.91

0.10

0.45

0.05

0.24

0.10

3

1.14

0.13

0.55

0.06

0.27

0.03

2

1.16

0.08

0.57

0.04

0.27

0.13

3

0.24

0.29

0.90

0.05

0.44

0.03

0.24

0.10

2

0.06

0.12

0.85

0.15

0.42

0.07

0.24

0.13

3

0.00

0.05

1.29

0.13

0.63

0.06

0.32

0.12

3

0.44

0.49

0.91

0.14

0.44

0.07

0.24

0.06

3

0.91

0.11

0.44

0.05

0.21

0.03

3

0.88

0.01

0.43

0.01

0.17

0.07

2

18%

T(℃)

P(kbar)

0.92

1004

15.1

0.95

1025

12.6

0.93

1051

7.4

0%

Cpx

19.9

54

86%

Opx

0.73

1

0%

Grt

4.47

40

14%

Ol&Sp

<0.03

1

0%

WR

11.4

Cpx

21.3

Replicate

2sd

0.44

12.4

44

δ44/42Ca(‰)

0.01

Replicate Ol&Sp

(‰)

2sd

0.90

Replicate Grt

δ44/40Ca

82%

38 / 41

0.00

0.05

DMP448PV

DMP458PV

DMP466PV

Grt

5.77

33

17%

Opx

0.84

20

1%

Ol&Sp

<0.03

0

0%

0.84

0.11

7.50

0.86

0.06

Cpx

18.8

0.94

Opx

0.71

0.83

WR

11.20

Cpx

20.2

48

Opx

0.62

19

1%

Grt

8.29

20

14%

Ol&Sp

<0.03

10

0%

WR

8.90

Cpx

18.5

39

80%

Opx

0.56

15

1%

Grt

4.10

42

19%

Ol&Sp

<0.03

2

0%

WR

0.41

0.05

0.16

0.06

3

0.42

0.03

0.24

0.05

4

0.05

0.46

0.03

0.20

0.08

3

0.08

0.41

0.04

0.18

0.07

3

-0.07

-0.02

0.96

1040

14.5

0.96

983

12.7

1.41

1067

17.2

85%

39 / 41

Note: Cpx-clinopyroxene; Opx-orthopyroxene; Ol&Sp-olivine and spinel; Replicate-replicate digestion of a different fraction of sample powder. a. The major element contents of the minerals are averages from several grains (Table S2). b. The proportion of Ca hosted by minerals is obtained by calculation bsaed on CaO content and mineral mode (specific method could be found in Chen et al., 2019b). The CaO content of Ol and Spl were considered to be <0.03%. c: Temperature effect of Δ44/40CaCpx-melt(T) is from Zhang et al., (2018). Δ44/40CaGrt-melt = Δ44/40CaCpx-melt + Δ44/40CaGrt-Cpx. Δ44/40Cawhole rock-melt could be obtained through calculation based on the proportion of Ca hosted by Cpx and Grt and intermineral Ca isotope fractionation. d. The Fe-Mg exchange factor of two pyroxenes, temperature and pressure of garnet pyroxenites are calculated by two-pyroxene thermobarometry (Brey and Kohler, 1990; Putirka, 2008)

40 / 41

Declaration of interests ☒ 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.

☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

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