# Rubidium isotopic compositions of the mantle sources of ocean island basalts

Baoliang Wang, Frederic Moynier, Matthew G. Jackson, James M. D. Day

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Baoliang Wang, Frederic Moynier, Matthew G. Jackson, James M. D. Day. Rubidium isotopic compositions of the mantle sources of ocean island basalts. Geochimica et Cosmochimica Acta, 2025, 394, pp.194-204. 10.1016/j.gca.2025.02.011. insu-05080910

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# Rubidium isotopic compositions of the mantle sources of ocean island basalts

Baoliang Wang<sup>a,*</sup> <sup>ID</sup>, Frederic Moynier<sup>a</sup>, Matthew G. Jackson<sup>b</sup>, James M.D. Day<sup>c</sup> <sup>◎</sup>

<figure><img src="image_3.png" /></figure>

## ARTICLE INFO

Associate Editor: Sheng-Ao Liu

**Keywords:**

Ocean island basalts

Rubidium isotope

Mantle

Crustal recycling

## ABSTRACT

Ocean island basalts (OIB) offer valuable insights into the chemical composition of Earth's mantle. Distinct radiogenic isotope compositions recorded in OIB have been used to characterize diverse mantle sources, indicating the presence of recycled crustal components within plume sources. Rubidium isotopes have the potential to trace crustal recycling, given the significant enrichment of Rb in the crust relative to the mantle, and the inherent variability of Rb isotopes among crustal materials. Here we present Rb isotope data of twenty-eight OIB and two peridotite xenoliths representing EM1, EM2, HIMU, and FOZO mantle, with locations spanning the Pacific, Atlantic, and Indian Oceans. Except for one sample from Mangaia, which was affected by low-temperature alteration, the Rb isotopic compositions of remaining samples reflect the characteristics of their mantle source, exhibiting a δ<sup>87</sup>Rb range of variations from -0.19 to +0.01 ‰. To a first order, the Rb isotopic similarity between OIB, peridotite xenoliths, and previously reported mid-ocean ridge basalts (MORB) suggests a relatively homogeneous Rb isotope composition of the mantle, displaying an average δ<sup>87</sup>Rb value of -0.12 ± 0.09 ‰ (2SD, n = 52). Nevertheless, a discernible difference exists in primary lava compositions from Mangaia, with their Rb isotopic compositions (-0.02 ± 0.06 ‰, 2SD, n = 3) being 0.1 ‰ heavier than other OIB samples and the Bulk Silicate Earth (BSE) estimate. The heavy Rb isotope enrichment in Mangaia, the most extreme (highest 206Pb/204Pb) HIMU-type OIB examined in this study, is attributed to incorporation of high δ<sup>87</sup>Rb signal within the mantle source, potentially due to the deep recycling of ancient altered oceanic crust.

## 1. Introduction

Mid-ocean ridge basalts (MORB) and ocean island basalts (OIB) are both produced through partial melting of mantle materials, providing essential information for understanding the geochemical complexity of the mantle. In contrast to MORB, which are likely to originate from a relatively uniform depleted upper mantle, OIB provide a window into the otherwise inaccessible deeper mantle. Significant heterogeneity in terms of various radiogenic isotope systems, including Sr, Nd, Hf, Os, and Pb, has been observed in oceanic basalts (e.g., White and Hofmann, 1982; Zindler and Hart, 1986; Hart et al., 1992; Hauri and Hart, 1993, 1997; Hanan and Graham, 1996; Hofmann, 1997; Blichert-Toft et al., 1999, 2003; Eisele et al., 2002; Salters and Stracke, 2004; Workman et al., 2004; Workman and Hart, 2005; Day et al., 2010; Hanyu et al., 2014; Harðardóttir and Jackson, 2024). Accordingly, different mantle 'endmembers' or 'components' have been defined

based on the radiogenic isotopic signatures, including depleted MORB mantle (DMM, which is the source of MORB and displays low 87Sr/86Sr, high 143Nd/144Nd as well as low 206Pb/204Pb; Zindler and Hart, 1986; Workman and Hart, 2005), Enriched Mantle 1 (EM1, characterized by intermediate 87Sr/86Sr, low 143Nd/144Nd and low 206Pb/204Pb; Hofmann, 1997; Eisele et al., 2002), Enriched Mantle 2 (EM2, which exhibits high 87Sr/86Sr, low 143Nd/144Nd and intermediate 206Pb/204Pb; White and Hofmann, 1982; Workman et al., 2004), High-μ (where μ = 238U/204Pb) mantle (HIMU, characterized by low 87Sr/86Sr, intermediate to high 143Nd/144Nd and high 206Pb/204Pb; Hauri and Hart, 1993, 1997), and Focus zone (FOZO, geochemically depleted—including high 143Nd/144Nd, low 87Sr/86Sr, and low 206Pb/204Pb—and may be characterized by high 3He/4He ratios; Hart et al., 1992; Hanan and Graham, 1996).

'EM1' and 'EM2'—with variability in radiogenic isotope compositions have been considered as endmembers of a continuous mixing array

<footnote><sup>a</sup> Université Paris Cité, Institut de Physique du Globe de Paris, CNRS, 1 Rue Jussieu, 75005 Paris, France</footnote>

<footnote><sup>b</sup> Department of Earth Science, University of California, Santa Barbara 93106, USA</footnote>

<footnote><sup>c</sup> Scripps Institution of Oceanography, University of California San Diego, La Jolla, CA 92093-0244, USA</footnote>

<footnote>* Corresponding author.</footnote>

<footnote>E-mail address: bwang@ipgp.fr (B. Wang).</footnote>
of enriched mantle components—as well as ‘HIMU’ reflect inputs of recycled continental and oceanic crust to the mantle (e.g., Soderman et al., 2024). For example, EM2 is generally believed to involve continental crust-derived material (e.g., White and Hofmann, 1982; Jackson et al., 2007; Moynier et al., 2021), and the concept of the contribution of recycled oceanic crust to HIMU has been well-developed (Hofmann and White, 1982; Hauri and Hart, 1993; Day et al., 2010; Hanyu et al., 2011). On the other hand, the origin of EM1 remains a matter of debate, possibly incorporating continental crust, delaminated sub-continental lithosphere, or pelagic sediments (McKenzie and O’Nions, 1983; Woodhead and McCulloch, 1989; Eisele et al., 2002; Garapić et al., 2015; Delevault et al., 2016). In this paper, ‘EM1’, ‘EM2’, and ‘HIMU’ are not taken to represent “endmembers” but reflect distinct mantle sources with extreme radiogenic isotopic compositions. In addition to radiogenic isotope systems, analytical advancements in recent years have enabled various stable isotope systems (e.g., O, Eiler, 2001; Cl, John et al., 2010; Ca, Huang et al., 2011; Eriksen and Jacobsen, 2022; Huang and Jacobsen, 2022; Si, Pringle et al., 2016; Mg, Zhong et al., 2017; Fe, Soderman et al., 2021; Hg, Moynier et al., 2021; Ba, Bai et al., 2022; Mo, Ahmad et al., 2023; Ti, Deng et al., 2023; S, Cabral et al., 2013) to serve as tracers of recycled signals in the mantle, which can offer additional insights into the nature and origin of mantle composition.

Rubidium isotopes have the potential to be an effective tracer for the recycling of crustal materials in the mantle. Rubidium possesses two naturally occurring isotopes: the stable isotope $^{85}$Rb (72.17 ‰) and the radioactive isotope $^{87}$Rb (27.83 ‰) (Berglund and Wieser, 2011). Since Rb is a highly incompatible element, it is significantly enriched in Earth's crust compared to the mantle. The Rb abundances of the upper, middle, and lower continental crust are estimated to be 84 µg/g, 65 µg/g, and 11 µg/g, respectively (Rudnick and Gao, 2003), which are much higher than that in the primitive mantle (0.6 µg/g; McDonough and Sun, 1995) and the depleted upper mantle (0.05 µg/g; Workman and Hart, 2005). Due to seafloor alteration, altered oceanic crust (AOC) exhibits Rb enrichment (13.7 µg/g; Kelley et al., 2003) compared to fresh MORB average (2.88 µg/g; Gale et al., 2013). Measurable Rb

isotopic variability has been observed among crustal materials (Fig. 1). For example, Rb isotopic analyses of materials associated with low-temperature processes (e.g., soils, stream sediments, and sedimentary rocks) show a wide range of isotopic compositions ($\delta^{87}$Rb = -0.45 to +0.07 ‰; Hu et al., 2021, 2022; Zhang et al., 2023). Modern seawater displays a heavier Rb isotopic signature ($\delta^{87}$Rb = +0.13 ± 0.04 ‰, 2SD, n = 13; Zhang et al., 2024) compared to the average composition of the upper continental crust (UCC, $\delta^{87}$Rb = -0.14 ± 0.09 ‰, 2SD, n = 73; Hu et al., 2022). Additionally, heavy Rb isotopic enrichments, with $\delta^{87}$Rb values even as high as +1.2 ‰, have been observed in leucogranites from Kampa, in the Himalayan orogen, which also significantly exceeds the average UCC value. Given the wide range of Rb isotopic variations in surface materials, as well as the remarkable discrepancy in Rb abundance between the mantle and crust, incorporation of recycled crustal materials may produce Rb isotope heterogeneity in the mantle.

Samples of MORB from different mid-ocean ridges exhibit a limited range of Rb isotopic compositions ($\delta^{87}$Rb = -0.19 ± 0.01 ‰ to -0.02 ‰ ± 0.02 ‰; 2SD), reflecting a relatively uniform Rb isotopic signature of the DMM ($\delta^{87}$Rb = -0.12 ± 0.09 ‰, 2SD, n = 15; Wang et al., 2023). In contrast, estimate for the Rb isotopic composition of the deeper OIB-source mantle remains relatively unconstrained. To date, Rb isotopic data have been reported for only one sample from the Galapagos Island and six basaltic samples from Hekla volcano (Pringle and Moynier, 2017; Wang et al., 2023). Given the potential of Rb isotopic variability in the deep mantle, a systematic study of Rb isotopes in OIB samples is warranted.

Here, we present Rb isotopic compositions for a set of OIB samples as well as two peridotite xenoliths representative of different mantle sources including EM1, EM2, HIMU, and FOZO. Based on the Rb isotopic data, we identify the Rb isotope characteristics of the OIB-source mantle and evaluate whether Rb isotopes can be used to trace recycled crustal materials in the mantle.

<figure><img src="image_4.png" /><figcaption>**Fig. 1.** Rubidium isotopic variability observed among crustal materials. Rubidium isotopic compositions of granites, loess, and sedimentary rocks are from Hu et al. (2022). Data of river sediments and soil are from Hu et al. (2021) and Zhang et al. (2023). The composition of modern seawater has been reported in Zhang et al. (2024). The shaded box represents the average Rb isotopic composition of the mantle (-0.12 ± 0.08 ‰; 2SD, n = 25) estimated by Wang et al. (2023).</figcaption></figure>
## 2. Samples and methods

### 2.1. Samples

Twenty-eight OIB samples and two peridotite xenoliths representing different mantle sources were selected for Rb isotopic analyses, with locations spanning the Pacific, Atlantic, and Indian Oceans. The detailed lithology of samples is presented in the Supplementary material. Among them, four samples are from Pitcairn (which has EM1 characteristics; Garapić et al., 2015), and thirteen samples are from Samoa (which host EM2 signatures, except samples from Ta'u and Ofu islands displaying FOZO signatures; Workman et al., 2004; Jackson et al., 2007; Hart and Jackson, 2014; Bai et al., 2022). Four samples from Mangaia exhibit HIMU compositions (Hanyu et al., 2011; Herzberg et al., 2014). While the two Tubuai samples are peridotite xenoliths rather than OIB, their Sr-Nd-Pb isotopic signatures resemble those of basalts from Tubuai, reflecting a HIMU mantle composition (Hauri et al., 1993). Four samples were measured from La Palma (Canary Islands), where the mantle source has also been considered to involve recycled oceanic crust and lithosphere, as indicated by He-O-Os-Sr-Nd-Pb isotope analyses (Day et al., 2010; Day and Hilton, 2011; Day et al., 2022). The other samples correspond to hotspots characterized by less extreme radiogenic isotopic compositions. For example, samples from Tristan da Cunha and Gough are EM1-type lavas (Gibson et al., 2005; Willbold and Stracke, 2006), and the isotopic homogeneity recorded in La Réunion lavas suggests little influence from crustal recycling (e.g., Vlastélic et al., 2006; Peters et al., 2018).

### 2.2. Analytical methods

Sample dissolution and chemical procedures were performed in a clean laboratory at the Institut de Physique du Globe de Paris (IPGP). All reagents used throughout the processes were prepared with high-purity concentrated acids after sub-boiling distillation. Approximately 50 mg of whole rock basalt and whole rock xenolith powders were weighed into Savillex Teflon beakers, followed by sequential digestion using concentrated HF-HNO₃ (3:1, v/v), HCl-HNO₃ (3:1, v/v), and 6 mol L⁻¹ HCl. Subsequently, samples were re-dissolved in 1 mL of 1 mol L⁻¹ HNO₃ for column chemistry. Rubidium purification was achieved through four steps with Eichrom DGA resin (50–100 µm, 1.8 mL), Bio-Rad AG50W-X12 resin (200–400 mesh, 20 mL and 10 mL, respectively) and Eichrom Sr-spec resin (20–50 µm) successively, following the method as described in Wang et al. (2023). The Rb recovery of the whole procedure for both unknowns and standard reference materials was > 95 %, and the blank was less than 0.05 ng, which was negligible (< 0.1 %) compared to the amount of Rb in sample aliquots (>50 ng).

Rubidium isotopic measurements were performed on a Nu Sapphire collision-cell multi-collector inductively-coupled-plasma mass spectrometer (CC-MC-ICP-MS) at IPGP. Sample solution was introduced through an Apex Omega desolvation nebulizer system. The isotopes ⁸⁵Rb and ⁸⁷Rb were collected on L3 and H1 Faraday cups, respectively. Simultaneously, ⁸⁸Sr was collected on the H3 Faraday cup for the purpose of correcting the ⁸⁷Sr interference on ⁸⁷Rb by assuming a constant ⁸⁷Sr/⁸⁸Sr ratio (i.e., ⁸⁷Sr/⁸⁸Sr = 0.085, corresponding to ⁸⁷Sr/⁸⁶Sr = 0.71; Nebel et al., 2005; Pringle and Moynier, 2017; Wang et al., 2023). Our previous Sr doping test demonstrated that Sr correction using a fixed ⁸⁷Sr/⁸⁸Sr value of 0.085 is effective when Sr/Rb ≤ 0.005 (i.e., ⁸⁸Sr/⁸⁵Rb intensity ≤ 0.006) (Wang et al., 2023). In this study, all samples have ⁸⁸Sr/⁸⁵Rb intensity ≤ 0.005 after chemical purification, ensuring the quality of the ⁶⁸Rb data. The high sensitivity achieved here (up to ~ 500 V per µg/g for ⁸⁵Rb) allowed the sample and standard solutions to be diluted to 2 ng/g for measurements. The sample introduction system was cleaned twice using 0.5 mol·L⁻¹ HNO₃ after each analysis in order to avoid potential cross-contamination. The instrumental mass bias was corrected by standard-sample bracketing method. Rubidium isotopic data is reported in ⁶⁸Rb notation, relative to the

bracketing standard NIST SRM 984: δ⁸⁷Rb (‰) = [(⁸⁷Rb/⁸⁵Rb)ₘₚₐₙ/ (⁸⁷Rb/⁸⁵Rb)ₘₚₐₙ − 1] × 1000. The long-term reproducibility of our Rb isotopic analyses is 0.04 ‰ (2SD), based on repeated measurements of BHVO-2 (δ⁸⁷Rb = −0.12 ± 0.04 ‰, 2SD, n = 39) across different analytical sessions over one year (where, in each session, BHVO-2 was measured at least three times) (Fig. S1; Table S1).

## 3. Results

Twenty-eight OIB samples and two peridotite xenoliths from different ocean island localities (including Pitcairn, Samoa, Mangaia, La Palma, Tristan, Gough, La Réunion, and Tubuai) were analyzed for Rb isotopic compositions. Two USGS reference materials (i.e., BHVO-2 and BCR-2) were processed using the same chemical methods, and the Rb isotope values acquired for them are consistent with the previously published data (Pringle and Moynier, 2017; Zhang et al., 2018; Hu et al., 2021; Nie et al., 2021). The Rb isotopic data of these samples are presented in Table 1. Major- and trace-element compositions, as well as radiogenic isotopic ratios, have been reported previously (LeRoex, 1985; Albarede and Tamagnan, 1988; Hauri et al., 1993; Workman et al., 2004; Jackson et al., 2007; Day et al., 2022; Herzberg et al., 2014; Hart and Jackson, 2014; Garapić et al., 2015; Bai et al., 2022), and are provided in the Supplementary file. It is worth noting that the radiogenic isotopic values of our specific Mangaia samples are not reported here. Nevertheless, the reported data on Mangaia lavas exhibit extremely uniform radiogenic isotopic compositions (Hanyu et al., 2011), and the average values are therefore directly adopted to represent the characteristics of the Mangaia samples in this study.

Rubidium isotopic compositions of the studied samples span a range from −0.19 to +0.16 ‰ (Table 1). Nonetheless, the vast majority of samples display consistent Rb isotopic characteristics regardless of different localities, with an average δ⁸⁷Rb value of −0.13 ± 0.07 ‰ (2SD, n = 26), which is similar to the average value of MORB (δ⁸⁷Rb = −0.12 ± 0.09 ‰, 2SD, n = 15) and the estimate of the Bulk Silicate Earth (BSE, δ⁸⁷Rb = −0.13 ± 0.06 ‰, 2SD) (Wang et al., 2023). One exception is Mangaia, where the samples have heavier Rb isotopic signatures (Fig. 2). Specifically, three Mangaia samples have Rb isotopic compositions ranging from −0.05 to +0.01 ‰, slightly heavier than that of BSE; one Mangaia sample exhibits a particularly anomalous enrichment in heavy Rb isotope (MG-1008; +0.16 ‰), likely influenced by low-temperature alteration, which will be discussed in detail in section 4.1.

While Mangaia OIB exhibit heavier Rb isotope characteristics relative to other samples, the difference is small. To exclude the possibility of the discrepancies arising from analytical uncertainties, three Mangaia samples underwent repeated analyses (including dissolution, chemistry, and isotopic measurement), and consistent results were obtained over two different analytical sessions (Table 1). Additionally, we used the basalt geostandard BHVO-2 as the bracketing standard in a separate analytical session to directly discern Rb isotopic variations between Mangaia OIB and BHVO-2. BHVO-2 was selected as a bracketing standard because it is an OIB from Kilauea volcano (Hawaii), it demonstrates an excellent long-term reproducibility (δ⁸⁷Rb = −0.12 ± 0.04 ‰, 2SD, n = 39), and its δ⁸⁷Rb is consistent with the average composition of MORB (δ⁸⁷Rb = −0.12 ± 0.09 ‰, 2SD, n = 15) and the majority of OIB (δ⁸⁷Rb = −0.12 ± 0.07 ‰, 2SD, n = 24). Considering that the propagation of error may compound the uncertainty when comparing data bracketed using the international standard NIST SRM 984, the direct comparison between OIB facilitates the resolution of non-zero isotopic offsets by removing a potential source of analytical error. The data of the three Mangaia samples, measured against BHVO-2 and denoted as δ⁸⁷Rb<sub>BHVO-2</sub>, are presented in supplementary Table 1 and Fig. S2, which further confirm the Rb isotopic variations within the Mangaia OIB. Therefore, despite the overlap in Rb isotope composition when errors are considered, the repeated analyses of the three Mangaia samples and the direct measurement against BHVO-2 collectively demonstrate the systematically heavier (~0.1 ‰) Rb isotopic signature of OIB from
<table><caption>**Table 1**

Rubidium isotopic compositions of OIB samples and two peridotite xenoliths.</caption><thead><tr><th>Sample</th><th>Lithology</th><th>Island</th><th>Ocean</th><th>Rb (µg/g)<sup>a</sup></th><th>δ<sup>87</sup>Rb (‰)</th><th>2SD<sup>b</sup></th><th>n</th></tr></thead><tbody><tr><td>PIT3</td><td>basalt</td><td>Pitcairn</td><td>Pacific</td><td>31.4</td><td>-0.09</td><td>0.03</td><td>4</td></tr><tr><td>PIT6</td><td>basalt</td><td>Pitcairn</td><td>Pacific</td><td>42.5</td><td>-0.18</td><td>0.06</td><td>4</td></tr><tr><td>PIT12</td><td>basalt</td><td>Pitcairn</td><td>Pacific</td><td>34.5</td><td>-0.11</td><td>0.05</td><td>4</td></tr><tr><td>PIT16</td><td>basalt</td><td>Pitcairn</td><td>Pacific</td><td>37.5</td><td>-0.18</td><td>0.01</td><td>4</td></tr><tr><td>ALIA115-03</td><td>hawaiite</td><td>Samoa-Savai'i</td><td>Pacific</td><td>83.1</td><td>-0.19</td><td>0.05</td><td>4</td></tr><tr><td>ALIA115-18</td><td>trachybasalt</td><td>Samoa-Savai'i</td><td>Pacific</td><td>97.4</td><td>-0.12</td><td>0.06</td><td>4</td></tr><tr><td>OFU04-14</td><td>ankaramite</td><td>Samoa-Ofu</td><td>Pacific</td><td>11.4</td><td>-0.13</td><td>0.03</td><td>4</td></tr><tr><td>OFU04-06</td><td>ankaramite</td><td>Samoa-Ofu</td><td>Pacific</td><td>25.6</td><td>-0.13</td><td>0.02</td><td>4</td></tr><tr><td>T10</td><td>basalt</td><td>Samoa-Ta'u</td><td>Pacific</td><td>19.9</td><td>-0.17</td><td>0.04</td><td>4</td></tr><tr><td>T16</td><td>basalt</td><td>Samoa-Ta'u</td><td>Pacific</td><td>19.8</td><td>-0.13</td><td>0.04</td><td>4</td></tr><tr><td>T21</td><td>hawaiite</td><td>Samoa-Ta'u</td><td>Pacific</td><td>49.1</td><td>-0.12</td><td>0.07</td><td>4</td></tr><tr><td>T32</td><td>basalt</td><td>Samoa-Ta'u</td><td>Pacific</td><td>19.0</td><td>-0.17</td><td>0.07</td><td>4</td></tr><tr><td>T38</td><td>basalt</td><td>Samoa-Ta'u</td><td>Pacific</td><td>16.7</td><td>-0.09</td><td>0.01</td><td>4</td></tr><tr><td>128-21</td><td>basalt</td><td>Samoa-Taumatau Seamount</td><td>Pacific</td><td>28.4</td><td>-0.09</td><td>0.03</td><td>4</td></tr><tr><td>AVON3-63-2</td><td>porphyritic basalt</td><td>Samoa-Vailulu'u Seamount</td><td>Pacific</td><td>16.5</td><td>-0.08</td><td>0.03</td><td>4</td></tr><tr><td>AVON3-78-1</td><td>porphyritic basalt</td><td>Samoa-Malumalu Seamount</td><td>Pacific</td><td>41.8</td><td>-0.10</td><td>0.05</td><td>4</td></tr><tr><td>AVON3-74-4</td><td>aphyric basalt</td><td>Samoa-Ta'u</td><td>Pacific</td><td>22.3</td><td>-0.09</td><td>0.06</td><td>4</td></tr><tr><td>MG-1001</td><td>porphyritic basalt</td><td>Mangaia</td><td>Pacific</td><td>10.8</td><td>-0.04</td><td>0.06</td><td>4</td></tr><tr><td>MG-1001 (Replicate)</td><td>porphyritic basalt</td><td>Mangaia</td><td>Pacific</td><td>10.8</td><td>-0.05</td><td>0.01</td><td>4</td></tr><tr><td colspan="8"><strong>average</strong></td></tr><tr><td>MG-1002</td><td>porphyritic basalt</td><td>Mangaia</td><td>Pacific</td><td>16.0</td><td>-0.01</td><td>0.03</td><td>4</td></tr><tr><td>MG-1002 (Replicate)</td><td>porphyritic basalt</td><td>Mangaia</td><td>Pacific</td><td>16.0</td><td>0.03</td><td>0.03</td><td>4</td></tr><tr><td colspan="8"><strong>average</strong></td></tr><tr><td>MG-1006</td><td>porphyritic basalt</td><td>Mangaia</td><td>Pacific</td><td>18.4</td><td>-0.04</td><td>0.06</td><td>4</td></tr><tr><td>MG-1006 (Replicate)</td><td>porphyritic basalt</td><td>Mangaia</td><td>Pacific</td><td>18.4</td><td>-0.02</td><td>0.04</td><td>4</td></tr><tr><td>MG-1006 (Replicate 2)</td><td>porphyritic basalt</td><td>Mangaia</td><td>Pacific</td><td>18.4</td><td>0.02</td><td>0.04</td><td>4</td></tr><tr><td colspan="8"><strong>average</strong></td></tr><tr><td>MG-1008</td><td>porphyritic basalt</td><td>Mangaia</td><td>Pacific</td><td>10.1</td><td>0.13</td><td>0.04</td><td>4</td></tr><tr><td>MG-1008 (Replicate)</td><td>porphyritic basalt</td><td>Mangaia</td><td>Pacific</td><td>10.1</td><td>0.19</td><td>0.02</td><td>4</td></tr><tr><td colspan="8"><strong>average</strong></td></tr><tr><td>TBA4-11</td><td>wehritic dunite</td><td>Tubuai</td><td>Pacific</td><td>15.6</td><td>-0.12</td><td>0.03</td><td>4</td></tr><tr><td>TBA1-9</td><td>wehritic dunite</td><td>Tubuai</td><td>Pacific</td><td>21.4</td><td>-0.17</td><td>0.06</td><td>4</td></tr><tr><td>LP21-4</td><td>basalt</td><td>Canary Islands-La Palma</td><td>Atlantic</td><td>42.8</td><td>-0.11</td><td>0.05</td><td>4</td></tr><tr><td>LP21-77</td><td>basalt</td><td>Canary Islands-La Palma</td><td>Atlantic</td><td>80.4</td><td>-0.09</td><td>0.04</td><td>4</td></tr><tr><td>LP21-79</td><td>basalt</td><td>Canary Islands-La Palma</td><td>Atlantic</td><td>36.5</td><td>-0.10</td><td>0.03</td><td>4</td></tr><tr><td>LP21-81</td><td>basalt</td><td>Canary Islands-La Palma</td><td>Atlantic</td><td>30.0</td><td>-0.13</td><td>0.06</td><td>4</td></tr><tr><td>BM.1962.128 (114)</td><td>basalt</td><td>Tristan da Cunha</td><td>Atlantic</td><td>24.7</td><td>-0.11</td><td>0.07</td><td>4</td></tr><tr><td>ALR 40G Gough</td><td>basalt</td><td>Gough</td><td>Atlantic</td><td>30.0</td><td>-0.14</td><td>0.06</td><td>4</td></tr><tr><td>Eruption 1931 La Réunion</td><td>basalt</td><td>La Réunion</td><td>Indian</td><td>7.0</td><td>-0.13</td><td>0.06</td><td>4</td></tr><tr><td colspan="8"><strong>Geostandards</strong></td></tr><tr><td>BHVO-2</td><td></td><td></td><td></td><td></td><td>-0.10</td><td>0.04</td><td>4</td></tr><tr><td>BCR-2</td><td></td><td></td><td></td><td></td><td>-0.13</td><td>0.07</td><td>4</td></tr></tbody></table>

Mangaia (δ<sup>87</sup>Rb = -0.02 ± 0.06 ‰, 2SD, n = 3), in contrast to other OIB (δ<sup>87</sup>Rb = -0.12 ± 0.07 ‰, 2SD, n = 24) and the BSE (-0.13 ± 0.06 ‰, 2SD; Wang et al., 2023).

# 4. Discussion

## 4.1. Post-eruptive alteration

Low-temperature alteration can lead to significant Rb isotopic fractionation (Zhang et al., 2021). Hence, it is imperative to evaluate whether the Rb isotopic compositions have been modified by low-temperature alteration in the studied samples prior to utilizing them for obtaining the characteristics of the mantle sources.

Elemental ratios like Rb/Cs, Ba/Rb, and Th/U have been employed as indicators of low-temperature alteration in OIB samples, as these elements exhibit different responses to chemical weathering or seawater alteration (e.g., Hart et al., 2004; Workman et al., 2004). Specifically, a typical impact of submarine alteration on OIB is reflected in the addition of alkali elements from seawater (Hart, 1969; Philpotts et al., 1969; Hart et al., 2004). In contrast to submarine alteration, subaerial chemical weathering can result in Rb, Cs, and Ba being preferentially removed from the rock (Hart et al., 2004; Workman et al., 2004; Bai et al., 2022). The higher mobility of Rb compared to Ba often leads to an increase in

the Ba/Rb ratio during subaerial chemical weathering. Uranium behaves in a manner akin to alkali elements during low-temperature alteration, being introduced from seawater in submarine settings and removed from the rock during subaerial weathering, whereas Th is generally resistant to alteration, making the Th/U ratio a useful indicator of the presence of low-temperature alteration (Hart and Staudigel, 1989; Hart et al., 2004).

The samples analyzed in this study display a considerable dispersion in their Rb/Cs ratios, with only a few samples falling within the range of canonical value (84.5 ± 35.8, 2SD) established by Hofmann and White (1983) based on a large quantity of fresh oceanic basalts (Fig. 3a). Most of our samples have a higher Rb/Cs ratio compared to the canonical oceanic basalts value, which suggests mobilization of these elements by low temperature processes. In addition, Th/U > 4.5 (due to U loss) and Ba/Rb > 30 (typically due to Rb loss) can be used as thresholds to distinguish samples that have been altered after eruption (Workman et al., 2004; Pietruszka et al., 2013; Bai et al., 2022). The Th/U ratios of the analyzed samples span a wide range, with values exceeding 4.5 for all Pitcairn samples and some Samoa samples (Fig. 3b). Conversely, most of the samples exhibit a Ba/Rb ratio below 30, apart from one sample from Mangaia (i.e., MG-1008) that has a ratio of 32.2 (Fig. 3a). The elevated Th/U ratio observed in Pitcairn samples may not result from seawater interaction or subaerial weathering; and may rather reflect a

<footnote><sup>a</sup> Rubidium concentrations are from previous study (LeRoex, 1985; Albarede and Tamagnan, 1988; Hauri et al., 1993; Workman et al., 2004; Jackson et al., 2007; Day et al., 2022; Herzberg et al., 2014; Hart and Jackson, 2014; Garapić et al., 2015; Bai et al., 2022).</footnote>

<footnote><sup>b</sup> 2SD of each sample represents 2 times the standard deviation of the population of three repeat measurements of a sample solution.</footnote>
<figure><img src="image_5.png" /><figcaption>**Fig. 2.** Rubidium isotopic compositions of oceanic basalts and two Tubuai peridotite xenoliths. The dashed line and shaded box represent the estimate for the Rb isotopic composition of the Bulk Silicate Earth (BSE, $\delta^{87}\text{Rb} = -0.13 \pm 0.06\,\text{‰}$, 2SD; Wang et al., 2023). Most samples fall in the range of the estimate for BSE, except for Mangaia OIB and one E-MORB which exhibit heavier Rb isotopic characteristics. Rubidium isotopic data of Hekla basalts and MORB are from Wang et al. (2023).</figcaption></figure>

high Th/U mantle source (Eisele et al., 2002). However, the increase of the Th/U ratio in some Samoa samples could be attributed to the preferential loss of fluid mobile U compared to relatively immobile Th during subaerial chemical weathering. The Rb/Cs and Ba/Rb ratios of the La Palma samples (e.g., Rb/Cs: 49.3 to 90.9; Ba/Rb: 7.2 to 17.2) align with the canonical values (Rb/Cs = 84.5 ± 35.8, 2SD and Ba/Rb = 11.6 ± 4.5, 2SD; Hofmann and White, 1983) of fresh oceanic basalts (Fig. 3a), showing no apparent signs of influence on Rb with low-temperature alteration. In contrast, the variability in Rb/Cs and Th/U ratios indicates a certain extent of low-temperature alteration in some samples from Samoa, Pitcairn, and Tubuai. However, samples from these hotspots show homogeneous Rb isotopic compositions (–0.08 to –0.19 ‰) within the range of the BSE (–0.13 ± 0.06 ‰, 2SD; Wang et al., 2023) (Fig. 3b and c). One possible reason is that the chemical weathering resulted in significant mobilization of Cs and U, while having a comparatively minimal effect on Rb. Consequently, limited Rb isotopic variations are observed regardless of the changes in Rb/Cs and Th/U ratios.

In contrast, Mangaia OIB exhibit Rb isotopic characteristics heavier than the BSE (Fig. 2). Among them, MG-1008 stands out with the highest Ba/Rb ratio (32.2), and heaviest Rb isotopic composition (+0.16 ‰) (Fig. 3d). Both the significant increase in $\delta^{87}\text{Rb}$ value and Ba/Rb ratio in MG-1008 may be attributed to low-temperature alteration. This would be consistent with the notion of Rb loss during chemical weathering, given that Rb is more fluid mobile compared with Ba (Hart et al., 2004), and the lighter Rb isotope preferentially migrates with fluids (Zhang et al., 2021). Loss of Rb by chemical weathering is further supported by the lowest Rb/Zr and Rb/Th ratios (among the OIB samples studied here) observed in MG-1008 (Fig. 3d and f). Considering the potential for

low-temperature alteration, MG-1008 is precluded from further discussion. Unlike MG-1008, the other three samples from Mangaia have Ba/Rb (11.5 to 15.2) ratios within the canonical range (Ba/Rb = 11.6 ± 4.5, 2SD; Hofmann and White, 1983) of fresh oceanic basalts (Fig. 3a), suggesting they have undergone limited low-temperature alteration. Alternatively, if alteration has occurred, its impact on the distribution of Rb and Ba appears to be insignificant. Consequently, the heavier Rb isotopic compositions observed in these three Mangaia samples, relative to the BSE, are probably not the result of low-temperature alteration. This contrasts MG-1008, which shows clear loss of Rb and has a correspondingly heavy Rb isotopic composition.

## 4.2. Magmatic evolution

While low-temperature alteration is not considered as a contributing factor to the Rb isotopic compositions in most of the samples in this study, it remains essential to test whether these Rb isotope data have potentially undergone modification due to partial melting and fractional crystallization. Rubidium is a highly incompatible element, e.g., the bulk distribution coefficient $D_{\text{solid/melt}}$ of Rb is estimated to be 0.00001 during mantle partial melting (Workman and Hart, 2005). This implies that over 99 % of Rb preferentially incorporates into the basaltic melt phase even at just 0.1 % aggregated fractional melting, except when Rb-bearing phases (e.g., phlogopite) are present. The presence of abundant Rb-rich phases in the mantle sources corresponding to the samples studied here is unlikely, given the consistent incompatibility of Rb with K and La during mantle partial melting, as evidenced by the positive correlations observed between Rb and these two elements (Fig. 4a and b): residual Rb-rich phases in the mantle source would be expected to
<figure><img src="image_6.png" /><figcaption>Fig. 3. Rubidium isotopic compositions of OIB and two peridotite xenoliths as a function of low-temperature alteration. (a) Rb/Cs–Ba/Rb relationships of the studied samples. The shaded box represents the canonical range of fresh oceanic basalts (Rb/Cs = 84.5 ± 35.8, 2SD and Ba/Rb = 11.6 ± 4.5, 2SD; Hofmann and White, 1983). The unfilled circles symbolize the reported data for fresh oceanic basalts, and the square cross represents the average value. (b) to (f) The δ$^{87}$Rb values are plotted against Th/U, Rb/Cs, Ba/Rb, Rb/Zr, and Rb/Th ratios, respectively.</figcaption></figure>

fractionate these three elements from each other such that such clear
correlations would not be present. Therefore, it is reasonable to infer
that Rb isotopic fractionation is negligible during mantle partial
melting. In addition, the low solid/melt partition coefficients of Rb (for
example, D<sub>ol-melt</sub> = 1.8 × 10<sup>-4</sup>, D<sub>opx, pig-melt</sub> = 6 × 10<sup>-4</sup>, D<sub>cpx-melt</sub> =
0.001; McKenzie and O'Nions, 1995) indicate that limited amount of Rb
would enter solid phases during fractional crystallization of basaltic
magma. Consequently, basaltic differentiation may exert an insignifi-
cant influence on the Rb isotopic signature of the source melt, which
aligns with the homogeneous Rb isotopic compositions observed in a
series of Hekla volcanic rocks derived from the evolution of an OIB

magma (Wang et al., 2023).

The OIB in this study, as well as the previously reported MORB and
Hekla basalts, exhibit a wide range of Rb concentrations (0.4 to 97.4 µg/
g) and MgO contents (varying from 3.2 to 24.2 wt%). However, no
simple binary mixing relationship is evident as revealed by the plot of
δ<sup>87</sup>Rb values against Th/Rb ratios (Fig. 4c). Furthermore, there is no
resolvable trend between their δ<sup>87</sup>Rb values and La/Sm ratios, Na<sub>8</sub> (an
indicator of the extent of partial melting, which is defined as Na<sub>8</sub> = Na<sub>2</sub>O
+ 0.373 × MgO – 2.98; Klein and Langmuir, 1987), as well as MgO
concentrations (Fig. 4d, e, and f). The absence of such correlations in
these oceanic basalts further reinforces the inference that mantle partial
<figure><img src="image_7.png" /><figcaption>**Fig. 4.** (a) and (b) Plots of K and La concentrations versus Rb contents. (c) The $\delta^{87}$Rb values are plotted against Th/Rb. (d) to (f) Rubidium isotopic compositions of oceanic basalts as a function of magmatic evolution. $Na_8$ ($Na_{8.0} = Na_2O + 0.373 \times MgO - 2.98$) are calculated for samples with MgO between 5 and 8.5 wt% only. Data of Hekla basalts and MORB samples are from She et al. (2023) and Wang et al. (2023). The average Rb isotopic composition of the mantle is estimated to be –0.12 ± 0.09 ‰ (2SD, n = 52) based on OIB, Tubuai peridotite xenoliths and previously reported MORB (Wang et al., 2023).</figcaption></figure>

melting and basaltic magma differentiation do not lead to significant Rb isotope fractionation.

### 4.3. Rubidium isotope characteristics of OIB mantle sources

After excluding the possible effects of low-temperature alteration and magmatic differentiation, we deem that the Rb isotopic compositions of these samples reflect the characteristics of their mantle sources. Based on radiogenic isotope systematics (such as Sr-Nd-Pb), the studied samples selected from different hotspots are representative of EM1 (Pitcairn), EM2 (Samoa-Savai'i, Vailulu'u, and Malumalu), HIMU-type

(Tubuai, La Palma, and Mangaia), and FOZO (Samoa Ta'u and Ofu) mantle, respectively (Fig. 5a; Hauri et al., 1993; Workman et al., 2004; Jackson et al., 2007; Day et al., 2010; Hanyu et al., 2011; Hart and Jackson, 2014; Garapić et al., 2015; Bai et al., 2022). While these samples originate from diverse localities spanning the Pacific, Atlantic, and Indian Oceans, and represent different mantle domains, their Rb isotopic compositions show a relatively limited range of variations (–0.19 to +0.01 ‰). The average $\delta^{87}$Rb value of these samples is –0.11 ± 0.09 ‰ (2SD, n = 29), which is identical to the composition of the upper mantle (–0.12 ± 0.09 ‰, 2SD, n = 15; Wang et al., 2023) estimated using a MORB sample suite.
<figure><img src="image_8.png" /><figcaption>Fig. 5. Rubidium isotopic characteristics of different mantle sources. (a) Distinct mantle sources defined based on the radiogenic isotopic signatures. (b) to (d) The Rb isotopic compositions are plotted versus Pb isotopes, ¹⁴³Nd/¹⁴⁴Nd, and ⁸⁷Sr/⁸⁶Sr. The shaded box represents the average of other OIB (excluding Mangaia) and the two peridotite xenoliths analyzed in this study (δ⁸⁷Rb = -0.13 ± 0.07 ‰; 2SD, n = 26).</figcaption></figure>

The similarity of the average Rb isotopic composition of the OIB mantle sources and the DMM, as represented by MORB, implies a relatively uniform Rb isotopic composition of the mantle. Our newly acquired dataset from the studied samples (including Mangaia OIB), when combined with the previously reported MORB and Hekla basalts data, yields an average δ⁸⁷Rb value of -0.12 ± 0.09 ‰ (2SD, n = 52), thereby serving as the current best estimate of Rb isotopic composition of the mantle (Fig. 4).

### 4.4. Contribution of crustal recycling to Rb isotopic variability in OIB mantle sources

A wide variety of crustal materials, encompassing continental crust, oceanic crust and lithospheric mantle, as well as sediments, can be recycled into the mantle through subduction (Plank and Langmuir, 1998). Incorporation of these geochemically diverse materials into hotspot mantle sources contributed to the chemical and isotopic heterogeneities in the products of OIB magmas.

The variability in radiogenic isotopes, such as Sr, Nd, Hf, Os and Pb, as well as stable isotopes, like O, Cl, Ca, Si, Mg, Fe, Hg, Ba and S, has been observed in OIB (White and Hofmann, 1982; Eiler, 2001; Workman et al., 2004; Jackson et al., 2007; Day, 2013; John et al., 2010; Huang et al., 2011; Cabral et al., 2013; Garapić et al., 2015; Pringle et al., 2016; Zhong et al., 2017; Moynier et al., 2021; Soderman et al., 2021; Bai et al., 2022; Huang and Jacobsen, 2022), which are commonly considered as compelling evidence for the incorporation of crustal components

into specific mantle sources of OIB. For example, the mantle source of some Samoan lavas has been recognized as containing continental crust components (White and Hofmann, 1982; Jackson et al., 2007). While ongoing debate persists regarding the specific components that recycled into the EM1 domains (e.g., continental crust, delaminated sub-continental lithosphere, or pelagic sediments, etc.), the presence of crustal materials in the Pitcairn mantle plume is widely acknowledged (McKenzie and O'Nions, 1983; Woodhead and McCulloch, 1989; Eisele et al., 2002; Garapić et al., 2015). In addition, three oceanic islands targeted in this study are identified as HIMU-type—Mangaia, Tubuai, and La Palma—and they have been proposed to have variably-aged recycled oceanic crust and lithosphere in their mantle sources (Hauri et al., 1993; Day et al., 2010; Day and Hilton, 2011; Hanyu et al., 2011; Pringle et al., 2016; Moynier et al., 2021).

Although an array of isotope systems investigated in lavas from Samoan, Pitcairn, Tubuai, and La Palma suggest the involvement of crustal components in their mantle sources, Rb isotopes offer no additional constraint on the role of crustal recycling in these hotspots. On average, the δ⁸⁷Rb values of samples from Samoan, Pitcairn, Tubuai, and La Palma are -0.12 ± 0.07 ‰ (2SD, n = 13), -0.14 ± 0.09 ‰ (2SD, n = 4), -0.15 ± 0.07 ‰ (2SD, n = 2) and -0.11 ± 0.03 ‰ (2SD, n = 4), respectively, which generally aligns with the previous estimate for the BSE (δ⁸⁷Rb = -0.13 ± 0.06 ‰, 2SD; Wang et al., 2023) (Figs. 2 and 5). The absence of Rb isotope heterogeneity within Samoan and Pitcairn lavas can be explained by a limited contribution of abnormal δ⁸⁷Rb signals to their mantle sources, as the average Rb isotopic composition of
the UCC (−0.14 ± 0.09 ‰, 2SD, n = 73; Hu et al., 2022) closely resembles that of the mantle (−0.12 ± 0.09 ‰, 2SD, n = 52). Although currently reported data reveals a wide range of Rb isotopic variations in some crustal materials (e.g., −0.45 ‰ in some soils to +1.2 ‰ in the Kampa leucogranites, Fig. 1; Zhang et al., 2023; Hu et al., 2021, 2022), it may remain insufficient to generate observable Rb isotope variability in the mantle if the input of these subducted materials is small. For example, when the proportion of abnormal Rb (even with δ⁸⁷Rb as high as +1.2 ‰) in the total recycled Rb is less than 5 % (the remaining 95 % represents average UCC), a 5 % recycled continental protolith (as suggested for the Samoan source by Jackson et al. (2007)) would produce no more than 0.05 ‰ variation in δ⁸⁷Rb of the mantle source, which is not resolvable with current methods. Similarly, the Rb isotopic composition of the recycled oceanic crust likely remained largely unchanged (i.e., exhibiting a characteristic δ⁸⁷Rb close to fresh MORB: −0.12 ± 0.09 ‰, 2SD, n = 15; Wang et al., 2023) before entering the mantle sources of Tubuai and La Palma, thereby not contributing to Rb isotopic heterogeneity.

In contrast, Mangaia OIB samples have heavier Rb isotopic compositions (average δ⁸⁷Rb = −0.02 ± 0.06 ‰, 2SD, n = 3) relative to other samples (−0.13 ± 0.07 ‰, 2SD, n = 26) and the BSE (−0.13 ± 0.06 ‰, 2SD; Wang et al., 2023) (Figs. 2 and 5). Since the elevated δ⁸⁷Rb values observed in the Mangaia lavas cannot be attributed to low-temperature alteration or magmatic evolution, the most plausible explanation is that their mantle source is enriched with a high δ⁸⁷Rb signature, potentially due to the incorporation of recycled crustal materials with heavy δ⁸⁷Rb. The low $^{87}$Sr/$^{86}$Sr, high $^{143}$Nd/$^{144}$Nd, and high $^{206}$Pb/$^{204}$Pb ratios in Mangaia samples, together with major and trace element compositions, cannot be explained by the recycling of continental crust or sediments, but rather indicate a contribution of oceanic lithosphere (Hofmann and White, 1982; Hanyu et al., 2011; Herzberg et al., 2014). Considering the potential similarity in the Rb isotopic composition between the oceanic crust (as inferred from the fresh MORB: −0.12 ± 0.09 ‰, 2SD, n = 15; Wang et al., 2023) and mantle (−0.12 ± 0.09 ‰, 2SD, n = 52), the recycled oceanic crust must experience Rb isotopic fractionation by seafloor alteration or Rb loss during subduction, to explain the isotopically heavy Rb measured in Mangaia lavas (if the high δ⁸⁷Rb signal comes from subducted oceanic crust).

Numerous processes can influence the distribution of Rb in the oceanic crust prior to their subduction into the mantle. For example, due to seafloor alteration, AOC exhibits a significant enrichment in Rb (13.7 µg/g, Kelley et al., 2003), which is ~ 4.7 times higher than fresh MORB (2.88 µg/g, Gale et al., 2013). In addition, high partition coefficients between fluid and solid suggest a high mobility of Rb during metamorphic dehydration (Kessel et al., 2005; Zheng, 2019), potentially reaching up to 81 % Rb loss to explain the low $^{87}$Sr/$^{86}$Sr signature of HIMU source (Stracke et al., 2003). Thus, Rb isotopic composition of the oceanic crust can potentially be modified with the redistribution of Rb during seawater and hydrothermal alteration, as well as during metamorphic dehydration.

Based on Pb–Os isotope systematics, it has been estimated that 20 % to 40 % oceanic crust mixing with ambient mantle contributed to the formation of the HIMU Mangaia source (Hauri and Hart, 1993; Hanyu et al., 2011; Day, 2013). The Rb isotopic composition of recycled oceanic crust required to account for the elevated δ⁸⁷Rb values observed in Mangaia OIB therefore can be back-calculated through a mass balance calculation: $δ^{87}\text{Rb}_{\text{Mangaia}} = f \times δ^{87}\text{Rb}_{\text{recycled}} + (1-f) \times δ^{87}\text{Rb}_{\text{BSE}}$, where f is the proportion of Rb hosted by recycled crustal materials, which can be calculated through an equation: $f = \frac{[\text{Rb}]_{\text{recycled}} \times F_{\text{recycled}}}{[\text{Rb}]_{\text{recycled}} \times F_{\text{recycled}} + [\text{Rb}]_{\text{primitive}} \times (1-F_{\text{recycled}})}$. The $F_{\text{recycled}}$ is the proportion of recycled materials within the mantle source, and $[\text{Rb}]_{\text{recycled}}$ represents Rb concentration of crustal material entering the mantle. The initial Rb concentration of subducted crustal material can be represented by its abundance in altered oceanic crust (i.e., 13.7 µg/g; Kelley et al., 2003). Assuming a maximum of 81 % Rb (Stracke et al., 2003) was removed

from the subducted oceanic crust due to metamorphic dehydration, the final Rb concentration ($[\text{Rb}]_{\text{recycled}}$) is calculated to be 2.6 µg/g when the crustal material entering the mantle source. The Rb content of the primitive mantle ($[\text{Rb}]_{\text{primitive}}$) is 0.6 µg/g (McDonough and Sun., 1995). If we further assume that the oceanic crust makes up 20 % to 40 % of the mixture with ambient mantle (Hauri and Hart, 1993; Hanyu et al., 2011; Day, 2013), we calculate that the relatively heavier Rb isotopic compositions observed in Mangaia OIB can be explained by the incorporation recycled altered oceanic crust with a $δ^{87}\text{Rb}_{\text{recycled}}$ value of +0.02 to +0.08 ‰.

The elevated value calculated for $δ^{87}\text{Rb}_{\text{recycled}}$ is consistent with the Rb isotope data recently reported for the AOC from IODP Site U1365, where the $δ^{87}\text{Rb}$ values range from −0.16 ± 0.03 ‰ to +0.07 ± 0.02 ‰, with a weighted average of −0.02 ± 0.06 ‰ (Jiang et al., 2024). However, the studied AOC samples represent only 53 m of the oceanic crust, likely corresponding to the uppermost ~ 10 % of the entire U1365 AOC, while the lower 90 ‰ may have a lower average $δ^{87}\text{Rb}$ value (Jiang et al., 2024). Additionally, high-temperature hydrothermal alteration significantly leaches Rb from the lower oceanic crust (Staudigel, 2014), yet the Rb isotopic composition of the hydrothermally altered oceanic crust has not been determined. Therefore, further Rb isotope studies on the compositions of altered oceanic crust will be important to evaluate the effects of crustal recycling on mantle heterogeneity.

On the other hand, the Rb isotopic composition of the residual slab entering the deep mantle may be modified due to slab dehydration and/or partial melting during subduction. A recent study on Mariana arc lavas revealed across-arc variation in Rb isotopes, with $δ^{87}\text{Rb}$ values decreasing from the fore-arc (−0.03 ± 0.04 ‰ to +0.09 ± 0.02 ‰) to the frontal arc (−0.12 ± 0.04 ‰ to −0.01 ± 0.03 ‰) (Jiang et al., 2024). This variation is attributed to the differing contributions from AOC-derived fluids and sediment melts at different subduction depths, with AOC-derived fluids being preferentially enriched in $^{87}\text{Rb}$, while sediment melts are preferentially depleted in $^{87}\text{Rb}$ (Jiang et al., 2024). In this case, HIMU OIB, with mantle sources having the involvement of dehydrated AOC, are speculated to exhibit low $δ^{87}\text{Rb}$ because AOC-derived fluids have removed $^{87}\text{Rb}$ from the slab. This is not consistent with the observation that HIMU-type Mangaia lavas exhibit slightly heavier Rb isotope compositions relative to other OIB and the BSE. In contrast, EM-type OIB, whose mantle sources contain residual sediments, may be characterized by high $δ^{87}\text{Rb}$ (Jiang et al., 2024). However, this hypothesis is also not consistent with the homogeneous Rb isotopic compositions observed in most OIB in this study, unless the proportion of sediment in EM OIB is small as suggested above.

Alternatively, the elevated $δ^{87}\text{Rb}$ values in Mangaia OIB may arise from recycled surficial carbonates. Multiple lines of evidence support the notion that the HIMU source is genetically linked to a carbonate component (Hauri et al., 1993; Saal et al., 1998; Weiss et al., 2016; Castillo et al., 2018). Notably, carbonate globules in melt inclusions from Mangaia lavas suggest a CO₂-rich primary magma (Saal et al., 1998). Mangaia lavas exhibit heavier Zn isotopic compositions ($δ^{66}\text{Zn} = +0.38 \pm 0.03 \text{ ‰}$) compared to the average MORB ($δ^{66}\text{Zn} = +0.27 \pm 0.05 \text{ ‰}$; Wang et al., 2017; Huang et al., 2018), a characteristic attributed to contributions from isotopically heavy recycled carbonates (Zhang et al., 2022). Similarly, recycled carbonates may also account for the relatively heavier Rb isotopic compositions in Mangaia OIB, in contrast to other OIB and the BSE. However, the present lack of Rb isotope data for carbonates (and carbonatites) prevents a test of this hypothesis.

Nevertheless, the enrichment of heavy Rb isotope in Mangaia OIB relative to other samples is well-established, which cannot be explained by low-temperature alteration or magmatic evolution, but likely reflects the addition of high $δ^{87}\text{Rb}$ signal in their HIMU mantle source, though the origin of this high $δ^{87}\text{Rb}$ signal remains to be investigated. Therefore, further studies on the compositions of AOC, especially hydrothermally altered oceanic crust (to test for the effects of submarine alteration on Rb isotopes), as well as the behavior of Rb isotopes during subduction
(including exhumed ultra-high pressure metamorphic rocks of MORB protolith to test for the effects of slab dehydration and partial melting), will be essential for assessing the effectiveness of Rb isotope as a tool for tracing crustal recycling.

While the Rb isotopic signature observed in Mangaia samples reflect the involvement of altered oceanic crust within their mantle source, the other HIMU reservoir studied here (i.e., Tubuai and La Palma) do not exhibit comparable recycled Rb isotope signals. Negative $\Delta^{33}\text{S}$ values, which were exclusively generated in the Archaean atmosphere, have been reported in olivine-hosted sulphides from Mangaia lavas, indicating the incorporation of hydrothermally altered Archaean oceanic crust into their HIMU domain (Cabral et al., 2013). The distinct Rb isotopic compositions of the Mangaia OIB compared to other HIMU-type samples may stem from the Archaean age of their recycled oceanic crust, or possibly from the fact that the thermally regimes of subduction zones may have been hotter in the Archean. In other words, due to seafloor hydrothermal alteration, metamorphic dehydration during Archean subduction, or their combined effects, the Archean altered oceanic crust exhibited a distinctive heavy Rb isotopic signature upon entering the mantle, thereby leading to the Rb isotopic heterogeneity within the Mangaia HIMU source, but not for 'younger' HIMU-type sources, such as the Canary Islands (La Palma).

## 5. Conclusions

In this study, Rb isotopic compositions were analyzed in a variety of lavas representing different mantle sources, including EM1, EM2, HIMU, and FOZO. In general, the studied samples exhibit relatively homogeneous Rb isotopic compositions, with an average $\delta^{87}\text{Rb}$ value of -0.11 ± 0.09 ‰ (2SD, n = 29), which is consistent with the estimate of the BSE and DMM. The Rb isotopic similarity between OIB mantle sources and the DMM suggests a relatively homogeneous Rb isotopic composition of the mantle (-0.12 ± 0.09 ‰; 2SD, n = 52). Nonetheless, Mangaia lavas unaffected by low-temperature alteration exhibit systematic and slightly heavier Rb isotope characteristics (-0.05 to +0.01 ‰) compared to other OIB. This heavy Rb isotope signature is attributed to the incorporation of a high $\delta^{87}\text{Rb}$ signal in Mangaia mantle source, potentially due to the recycling of altered and likely ancient oceanic crust.

## CRediT authorship contribution statement

Baoliang Wang: Writing – review & editing, Writing – original draft, Methodology, Investigation, Formal analysis, Data curation. Frederic Moynier: Writing – review & editing, Supervision, Project administration, Funding acquisition. Matthew G. Jackson: Writing – review & editing. James M.D. Day: Writing – review & editing.

## Data availability

Data are available through the Zenodo database at https://doi.org/10.5281/zenodo.14215531.

## Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

## Acknowledgements

We thank Prof. Caroline R. Soderman and four anonymous reviewers for their thorough comments that have greatly improved the quality of this manuscript. We are grateful to the editorial handling of Prof. Jeffrey G. Catalano and Prof. Sheng-Ao Liu. We thank Tu-Han Luu and Dimitri Rigoussen for their assistance with mass spectrometer maintenance. This work was partly supported by the IPGP analytical platform PARI,

Region Ile-de-France SESAME Grants no. 12015908, and DIM ACAV +, the ERC grant agreement No. 101001282 (METAL) (F.M.), the -UnivEarthS Labex program (numbers: ANR-10-LABX-0023 and ANR-11-IDEX-0005-02) (F.M.). B.W thanks the support of CSC scholarship (No. 202106340025).

## Appendix A. Supplementary material

Major and trace elements data, as well as radiogenic isotope data of OIB samples, are presented in the supplementary file. Supplementary material to this article can be found online at https://doi.org/10.1016/j.gca.2025.02.011.

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