# Strontium isotope and trace element compositions of olivine-hosted melt inclusions from the Song Da ultramafic volcanic suite, northern Vietnam: Implications for chemical heterogeneity in mantle plumes

Charbel Kazzy, Alexander Sobolev, Adrien Vezinet, Valentina Batanova, Igor Puchtel, Evgeny Asafov, Andrey Izokh, Leonid Danyushevsky, Vũ Hoàng Ly, Can Pham-Ngoc, et al.

► To cite this version:

Charbel Kazzy, Alexander Sobolev, Adrien Vezinet, Valentina Batanova, Igor Puchtel, et al. Strontium isotope and trace element compositions of olivine-hosted melt inclusions from the Song Da ultramafic volcanic suite, northern Vietnam: Implications for chemical heterogeneity in mantle plumes. Chemical Geology, 2025, 674, pp.122564. 10.1016/j.chemgeo.2024.122564. hal-04963997

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# Strontium isotope and trace element compositions of olivine-hosted melt inclusions from the Song Da ultramafic volcanic suite, northern Vietnam: Implications for chemical heterogeneity in mantle plumes

Charbel Kazzy<sup>a</sup>, Alexander V. Sobolev<sup>a,*</sup>, Adrien Vezinet<sup>a</sup>, Valentina G. Batanova<sup>a</sup>, Igor S. Puchtel<sup>b</sup>, Evgeny V. Asafov<sup>c</sup>, Eero Hanski<sup>d</sup>, Andrey E. Izokh<sup>e</sup>, Leonid V. Danyushevsky<sup>f</sup>, Vũ Hoàng Ly<sup>g</sup>, Can Pham-Ngoc<sup>g</sup>, Tran Tuan Anh<sup>g</sup>

## ARTICLE INFO

Editor: S Aulbach

**Keywords:**

Emeishan LIP

Olivine

Melt inclusions

Strontium isotopes

Mantle heterogeneity

Mantle plume

## ABSTRACT

Ultramafic magmas are the ideal tracers of mantle composition, as they are derived from high degrees of partial melting and their compositions approach that of mantle peridotite. The Permian Song Da ultramafic volcanic rocks in northern Vietnam represent a rare example of well-preserved Paleozoic picrites of the Emeishan Large Igneous Province (ELIP), with the most Mg-rich olivine phenocrysts of up to Fo93.5. As such, they are invaluable for constraining temperature, pressure, and composition of the ELIP mantle source. Here, we report the results of a study of olivine-hosted melt inclusions from ultramafic lavas of both low-Ti and high-Ti types in the Song-Da zone of the ELIP, providing new information on the concentrations of mobile trace elements, volatile components, and Sr isotope compositions of parental melts. Our data suggest a significant difference between the mantle source compositions of low-Ti and high-Ti primary melts of the Song Da zone. The latter melt likely originally contained 2.7 ± 1.2 wt% CO2 and 0.6 ± 0.1 wt% H2O, but lost most of its volatile inventory during degassing. Its enrichment in trace elements is attributed to a low degree of partial melting (less than 9 %) of a PREMA-type peridotitic source, as evidenced by the bulk rocks Nd and melt inclusions Sr isotopic data, contaminated by metasomatic fluids or recycled components bringing Pb, Sr, and Na. The low-Ti komatiite-type Song Da melts contain an excess of H2O (H2O/Ce up to 5500), similarly to some Archean komatiites, show a decoupling of Nd and Sr isotopes, and reveal mixing of at least two unusual components: (1) one with low initial 87Sr/86Sr (down to 0.7040), low Nb/U and Ce/Pb ratios and relatively enriched in La, Th, U, Pb, and Sr, and (2) the other with high initial 87Sr/86Sr (up to 0.7080), high Nb/U and Ce/Pb ratios, and strongly depleted in highly incompatible lithophile elements such as Ba, Rb, Nb, K, U, Th, La, and Sr. We suggest that component (1) reflects contamination with lower continental crust or other subduction-related constituents in the mantle source, while component (2) represents partial melt derived from dehydrated seawater-altered recycled harzburgite that was part of a subducted oceanic slab 300 to 500 Ma in age. The latter component can be recognized only in the melts that are exceptionally depleted in highly incompatible elements. The data from this study and Kazzy et al. (2024) suggest that the ELIP was produced by a mantle plume with a maximum potential temperature (T<sub>p</sub>) of ~1600 °C at ~260 Ma. Because of its high temperature, this plume passed through the hydrated mantle transition zone (MTZ) in a partially molten state, which allowed it to entrain in its hottest part H2O and components of the subducted 300–500 Ma Paleo-Tethyan oceanic plate stagnated in the MTZ for ~200 Ma. An alternative source of dehydrated serpentinites of recycled Paleo-Tethyan slab in the Emeishan mantle plume could be a core-mantle boundary if this slab had been subducted shortly (less than 240 Ma) before the initiation of this plume. This option requires a fast Tethyan crust recycling rate of >2.3 cm/year. Low-Ti komatiite-type melts of the Song Da

<footnote><sup>a</sup> Univ. Grenoble Alpes, Univ. Savoie Mont Blanc, CNRS, IRD, Univ. Gustave Eiffel, ISTERRE, 38000 Grenoble, France</footnote>

<footnote><sup>b</sup> Department of Geology, University of Maryland, College Park, MD 20742, USA</footnote>

<footnote><sup>c</sup> Vernadsky Institute of Geochemistry and Analytical Chemistry RAS, Moscow, Russia</footnote>

<footnote><sup>d</sup> Oulu Mining School, University of Oulu, 90014 Oulu, Finland</footnote>

<footnote><sup>e</sup> V.S. Sobolev Institute of Geology and Mineralogy, Novosibirsk, Russia</footnote>

<footnote><sup>f</sup> Friendly Solutions, Australia</footnote>

<footnote><sup>g</sup> Institute of Geological Sciences, Vietnam Academy of Science and Technology, Ha Noi, Viet Nam</footnote>

<footnote>* Corresponding author.</footnote>

<footnote>E-mail address: alexander.sobolev@univ-grenoble-alpes.fr (A.V. Sobolev).</footnote>
area came from the hottest part of the Emeishan mantle plume and contain traces of these components, whereas the high-Ti Song Da melts came from the coldest part of the Emeishan mantle plume (Tp ~1450 °C) and reflect a PREMA-type mantle source metasomatized by addition of a component enriched in Na, Sr, and Pb.

# 1. Introduction

Mantle plumes, originating from the thermal boundary layers in the mantle (Loper and Lay, 1995; Labrosse, 2002; Rost et al., 2005; Burke et al., 2008) traverse the mantle before reaching the surface. Some of the largest plumes undergo partial melting resulting in vast magma eruptions and intrusions that form large igneous provinces (LIP) at the initial stage of plume impingement. Studies of samples from LIPs have played a major role in understanding the compositional variation of the Earth's mantle through time (Campbell et al., 1989; McDonough and Ireland, 1993; Arndt et al., 2008), the history of Earth's crustal evolution, and geological processes leading to past mass extinctions. The Permian (~260 Ma) Emeishan LIP (ELIP), located in SW China, is one of the largest Phanerozoic continental flood basalt provinces and has been extensively studied (Chung and Jahn, 1995; Xu et al., 2001; Ali et al., 2005; Zhang et al., 2006; Li et al., 2010; Anh et al., 2011; Hanski et al., 2010; Kamenetsky et al., 2012; Tran et al., 2015; Zhang et al., 2019; Yu et al., 2020; Zhang et al., 2021; Yu et al., 2024). It is characterized by the exposure of its plumbing system at the surface, therefore, offering a unique research target for studying the activity of a mantle plume (Shellnutt, 2014). Previous studies of the ELIP have left several key questions unsolved: 1) How much did the continental crust and the subcontinental lithospheric mantle (SCLM) contribute to the chemical diversity that is observed in the lavas? 2) What are the isotopic and geochemical signatures of the Emeishan mantle plume? 3) Did subducted slabs affect the composition of the ELIP?

Crustal contamination has long been identified in magmas from the ELIP, a process that obscures the primary chemical and isotopic composition of the source (Xu et al., 2001; Hanski et al., 2004; He et al., 2010). Ultramafic lavas are the most primitive rocks of the ELIP. In the first part of our study (Kazzy et al., 2024), we divided the primitive volcanic rocks in the Song Da zone into the low-Ti and high-Ti ultramafic suites. Both have positive $\epsilon_{Nd}$ (Hanski et al., 2004; Anh et al., 2011; this study) and contain olivine grains with the highest forsterite content (93.5 mol%) of the ELIP lavas (Hanski et al., 2004; Kazzy et al., 2024). These are the best samples for characterizing the mantle source of the Emeishan LIP.

Bulk-rock compositions have long been used as a proxy to estimate the source compositions of magma suites by assuming that the bulk rock, when corrected for olivine fractionation, represents the primary melt and is in equilibrium with the mantle source. Commonly, when modern mass-spectrometry is used, the amount of material available for analysis of bulk rocks is sufficient to determine the isotopic signatures of even those elements that are present in low abundances. However, rocks have frequently experienced post-magmatic alteration and have always lost their magmatic volatiles. This leads to the loss of some of the most informative elemental and isotope tracers of mantle processes, such as Rb, K, Cl, U, Pb, Sr, H₂O, and CO₂, which are mobile during metamorphism, alteration and degasing. In addition, many bulk rocks are crystallization products of melts affected by magma mixing and crustal contamination, which make it difficult to reconstruct the composition of their primary melt (e.g. Sobolev, 1996). In contrast, this information can be recovered via a study of melt inclusions. When hosted by minerals such as olivine crystals in komatiites, the inclusions represent, at least partially, closed systems shielded by the host mineral (Sobolev, 1996).

Olivine-hosted melt inclusions from ELIP picrites in the Chinese part of the ELIP have been studied for major and trace elements, H₂O contents, Pb and Sr isotopes (Hanski et al., 2010; Kamenetsky et al., 2012; Ren et al., 2017; Zhang et al., 2019; Zhang et al., 2021). Recently, Kazzy et al. (2024) used the major and trace element compositions of olivine

phenocrysts (antecrysts) and olivine- and Cr-spinel-hosted melt inclusions in ultramafic volcanic rocks of the Song Da suites, to reconstruct the compositions of melts, as well as their crystallization temperatures and oxygen fugacity, and estimated the PT conditions of their formation. It was shown that the parental melt for low-Ti Song Da suite was komatiitic in composition with 21–23 wt% MgO and 0.7 wt% H₂O formed by 26% partial melting of a depleted mantle source at ~1610 °C and 3.7 GPa. It crystallized olivine and Cr-spinel at a low pressure in the temperature range of 1450–1150 °C and the oxygen fugacity that evolved in a closed system from ΔQFM –0.5 to +1.5 compared to Quartz-Fayalite-Magnetite oxygen buffer (QFM). The parental melt of the high-Ti Song Da suite was alkaline picritic in composition with an MgO content of 17–18 wt%, formed by less than 9% melting of mantle peridotite at around 6 GPa and 1600 °C. It crystallized olivine and Cr-spinel at a lower pressure and in the temperature range of 1320–1200 °C, under buffered oxygen fugacity conditions at ΔQFM +0.5 to +1.0. Kazzy et al. (2024) also showed that the studied low-Ti and high-Ti picrites of the Song Da area and their olivine-hosted melt inclusions differ from picrites in the main Chinese part of the ELIP and represent the most depleted and most enriched end-members of the ELIP magmas (see Figs. 4 and 7 of Kazzy et al., 2024). This justifies the goal of the present study, which is to use trace elements and Sr and Nd isotopes in olivine-hosted melt inclusions of the Song Da high-Ti and low-Ti picrites to reconstruct the composition of their parental melts and help understand the chemical heterogeneity of the mantle plume that produced the ELIP.

# 2. Geological context

The ~260 Ma ELIP is located in SW China on the western margin of the Yangtze craton. This continental flood basalt province covers an area of more than 250,000 km² and has an erupted magma volume that exceeds $0.3 \times 10^6$ km³ (Chung and Jahn, 1995; Xu et al., 2001; Hanski et al., 2010). The geotectonic architecture of SE Asia constitutes an assemblage of five small continental blocks, the Yangtze Block, Indochina Block, Simao terrane, Sibumasu terrane, and West Burma terrane, which are separated by suture zones or fault lines (Metcalfe, 2002). The Song Da zone, a southern part of the Emeishan flood basalt province, is located in northern Vietnam. It is bordered by the Ailao Shan-Red River Fault to the northeast and the Song Ma structure to the southwest. The Song Da suite, located between the Yangtze Block and the Indochina Block, originally erupted in the center of the ELIP, but due to the collision between the Indian and Eurasian plates, it has moved approximately 600 km from its original position along the Ailao Shan-Red River Fault (Chung et al., 1997, 1998; Hanski et al., 2004; Ali et al., 2010).

# 3. Methodology

## 3.1. Sample preparation

Sample locations are presented in Fig. 1. We studied 5 low-Ti samples: VS6-1 (from Nicklas et al., 2019), SD22-2-1, SD22-2-2, SD22-13-3, and SD22-4-2, and a single high-Ti sample, SD22-13-4. They all represent high-MgO, olivine-phyric picrites and are described in Kazzy et al., 2024, who also report major and trace element compositions of bulk-rock samples. For the convenience of readers, we present these data in the main text table, along with new bulk-rock Nd isotope compositions.

To prepare samples with melt inclusions, fresh olivine grains in platinum ampules were placed in a high temperature Nabertherm
furnace at Vernadsky Institute of Geochemistry in Moscow, Russia, heated at 1350 °C for 3 mins and then quenched following the method described by Sobolev et al. (2019). Olivine grains that contain melt inclusions larger than 38 µm in diameter were selected for Sr isotope and trace element analysis, whereas decrepitated (cracked) inclusions were filtered out. Host olivine grains were polished until the melt inclusions were partly exposed at the surface.

Major and some trace element concentrations of prepared melt in-
clusions and host olivine, as well as the H₂O content of melt inclusions,

were determined by non-destructive analytical methods: electron
microprobe and RAMAN spectroscopy and are considered in detail in
Kazzy et al. (2024). Here, we use some of these data, together with
additional data obtained by LA-ICP-MS in this study.

3.2. Laser-ablation MC-ICP-MS and QQQ-ICP-MS

In-situ strontium isotope ratios and trace element abundances of
olivine-hosted melt inclusions were obtained in ISTerre, Grenoble,

<figure><img src="image_3.png" /><figcaption>**Fig. 1.** Geological map of the Emeishan LIP with the focus on the Song Da zone, north-eastern Vietnam (modified after Kamenetsky et al., 2012). The locations of the studied samples are represented by stars.</figcaption></figure>
France. Depending on the size of melt inclusions, the single stream or split stream mode was applied. A RESOlution-SE instrument equipped with ATL 193 nm excimer laser of 5 ns width, S-155 Laurin Technic two-volume ablation cell and Squid was coupled with Agilent 8900 QQQ-ICP-MS or Neptune XT MC-ICP-MS or both. Melt inclusions were ablated with a repetition rate of 10–15 Hz and a laser fluence of 4 J.cm⁻². Usually, 15 pulses of pre-ablation were used to eliminate surface contamination. The carrier gas was He (0.35–0.90 l/min) mixed with Ar (1.0–2.0 l/min) and N₂ (2.5–4 ml/min) before introduction into the mass spectrometer. Time of each single analysis included 10–15 pre-ablation shots followed by 30s of washout, 30s of background measurement, and 30s to 40s of ablation. Spot size for single-stream trace elements was 20 µm, and 38 µm for all other analyses.

An Agilent 8900 ICP-QQQ-MS was tuned for ThO/Th ratio less than 0.1 and U/Th ratio of 1–1.02 using a line ablation on the NIST SRM 612. The following masses were analyzed: ⁴³Ca, ⁴⁵Sc, ⁴⁹Ti, ⁵¹V, ⁵³Cr, ⁸⁵Rb, ⁸⁸Sr, ⁸⁹Y, ⁹⁰Zr, ⁹³Nb, ¹³⁷Ba, ¹³⁹La, ¹⁴⁰Ce, ¹⁴¹Pr, ¹⁴⁶Nd, ¹⁴⁷Sm, ¹⁵³Eu, ¹⁵⁷Gd, ¹⁵⁹Tb, ¹⁶³Dy, ¹⁶⁶Er, ¹⁷²Yb, ¹⁷⁵Lu, ¹⁷⁸Hf, ¹⁸¹Ta, ²⁰⁸Pb, ²³²Th, and ²³⁸U. Dwell times ranged from 10 to 50 ms depending on the expected abundances of the isotope. Data were reduced using LADR (https://norsci.com/?p=ladr; Norris and Danyushevsky, 2018). NIST SRM612 was used as the calibration standard and GSD -1G and KL2-G as the secondary standards (GeoREM Data base http://georem.mpch-mainz.gwdg.de). Mass ⁴³Ca was used as the internal standard. For quantification, conventional approaches (Longerich et al., 1996) were applied. Reference materials GOR128-G and ML3-G were run as unknown with each 5–10 samples. Reproducibility of GOR128-G (close to measured low-Ti melts) and uncertainty calculated with LADR are shown in Supplementary Table 1. Most values reproduced within the errors except small (ca 2–4 ppb, or 23 % relative) underestimation of Th and Ta values for GOR128-G.

Measurements of Sr isotopic compositions were conducted on the ThermoFisher Neptune XT ICP-MS following a modified protocol of Zhu et al. (2020): ⁸⁷Rb was estimated through the measurement of ⁸⁵Rb. All masses were measured in static mode on the Faraday cups. Masses 84, 85, 86 and 87 were obtained using 10¹³ Ω amplifiers, while other masses were measured using 10¹¹ Ω amplifiers. To compensate for the different response and decay times of the 10¹³ Ω amplifiers a tau correction was applied, following the principles outlined in Craig et al. (2017). Reference glasses NIST-614 and GOR-128G (Jochum et al., 2006, 2011), which cover the entire range of Sr contents and Rb/Sr ratios of measured low-Ti inclusions were used to evaluate the accuracy of the analyses. Reference glasses KL2-G and StHs6/80-G were used to evaluate the accuracy of analyses of the high-Ti melt inclusions because of their higher Rb/Sr ratios. No REE²⁺ correction has been applied. The absence of REE²⁺ correction is not generating any bias in any of the 3 references glasses used as validation material here. All reference materials, which represent the full compositional range of studied melt inclusions in Rb/Sr ratios and the contents of Sr and major elements, were measured as unknowns and matched their accepted ⁸⁷Sr/⁸⁶Sr ratios within reported errors (Fig. 2 and Supplementary Fig. 1 and Supplementary Table 2).

Uncertainties in individual Sr isotope analyses were calculated using the in-built Iolite code (Sr-isotope DRS provided on the Iolite Github; see https://github.com/iolite-LA-ICP-MS for the full view of the python code). Exports from Iolite include uncertainties of all background, interference corrections and standardization if conducted. Note that including REE²⁺ correction changes the final uncertainty to the sixth decimal only, i.e., 2 orders of magnitude lower than the typical precision obtained with our LASS protocol for such Sr-depleted melt inclusions.

Since several analytical runs were conducted for analyzing melt inclusions, an evaluation of the inter-sequence (i.e. long-term) excess variance (i.e. long-term variance) was conducted. This approach follows the widely accepted workflow used in U-Pb isotope analyses (Horstwood et al., 2016) and consists of calculating the degree of homogeneity of the reference glasses over all sequences: if the MSWD value or corresponding p(χ²) value is within the acceptable range of 5–95 %, then no inter-

sequence excess variance is propagated into individual analyses. If not, then uncertainties are expanded up to the level where all validation materials (here glasses) return a MSWD value or p(χ²) value indicating statistical homogeneity. No inter-sequence excess variance was required for the ⁸⁷Sr/⁸⁶Sr values: MSWD and p(χ²) values for the two reference glasses GOR128-G and NIST-614-G are (i) 0.53 and 99 %, (ii) 1.3 and 14 %, respectively (see data table in Supplementary Table 2 and Supplementary Fig. 1), while an inter-sequence excess variance of 5 %, was propagated into ⁸⁷Rb/⁸⁶Sr (reference glasses and melt inclusions). Similar to the standardization mentioned above, the need of inter-sequence excess variance for these 3 ratios likely reflects the contribution of elemental fractionation, which is unaccounted for in our data reduction protocol; an effect absents in the ⁸⁷Sr/⁸⁶Sr calculation, hence explaining the better accuracy over several analytical runs for this latter ratio. Consequently, we consider the uncertainty reported in individual ⁸⁷Sr/⁸⁶Sr, ⁸⁷Rb/⁸⁶Sr, Rb/Zr and Sr/Zr values as external reproducibility. The precision we obtained in this study is similar to that of the most recent apatite Sr-isotope analyses (e.g. Buzenchi et al., 2023).

Lastly, all melt inclusions of low-Ti lavas discussed in this work have ⁸⁷Rb/⁸⁶Sr values lower than the ⁸⁷Rb/⁸⁶Sr of KL2-G, the reference glass with the most elevated ⁸⁷Rb/⁸⁶Sr ratio. Melt inclusions displaying ⁸⁷Rb/⁸⁶Sr ratio higher than that of KL2-G were disregarded since the accuracy of the ⁸⁷Rb correction could not be assessed. However, for the high-Ti suite, as its ⁸⁷Rb/⁸⁶Sr ratio is higher than that of KL2-G, an additional standard StHs6 was analyzed along with the sequence to ensure the accuracy of the ⁸⁷Rb correction. In addition, those inclusions were also excluded that showed 2 standard errors of the measured ⁸⁷Sr/⁸⁶Sr ratio over ±0.0015 (thin inclusions where signal was collected for less than 8 s).

### 3.3. Nd isotopes

The Sm-Nd isotopic data were obtained at the Isotope Geochemistry Laboratory and Plasma Laboratory (IGL/PL), University of Maryland, College Park, following the protocol of Puchtel et al. (2020, 2022). Between 100 and 200 mg of whole-rock sample powder were sealed in 15 ml screw-cap Savillex Teflon vials with 7 ml of double-distilled concentrated HF, 1.5 ml HNO₃, and appropriate amounts of mixed ¹⁴⁹Sm-¹⁵⁰Nd spike and digested on a hotplate at 180 °C for 96 h. The REE were separated from the silicate matrix using the standard cation-exchange chromatography. Neodymium and Sm were further separated from the other REE and each other using HDEHP column extraction, or reversed-phase partition, chromatography. The resultant Nd and Sm cuts were used for the measurements of the Nd and Sm isotopic compositions.

Measurements of Nd isotopic compositions were performed on Faraday cups of the ThermoFisher Neptune Plus ICP-MS in static mode using 10¹¹ Ω amplifiers on all masses except for ¹⁴⁷Sm, on which a 10¹³ Ω amplifier was used. The measurements were followed by off-line correction for spike contributions. For each sample, 150–200 ratios were collected with 8 s integration times in blocks of 50 ratios each. For every block of data collected, the peaks were centered, and a 100 s baseline measurement was performed for each Faraday cup/amplifier pair without beam deflection, and this baseline was then automatically subtracted from the sample beam. The effects of instrumental mass fractionation were corrected for relative to ¹⁴⁶Nd/¹⁴⁴Nd = 0.7219 using the exponential law. A total of six 50 ppb AMES Nd standard solutions were run during the analytical session, with 100–200 ratios collected during each measurement. The in-run precision of the measured ¹⁴³Nd/¹⁴⁴Nd ratio for both the samples and standards was between 5 and 7 ppm (2SE). During the course of the present analytical campaign, the external precision of the 50 ppb AMES Nd standard solution measurements for ¹⁴³Nd/¹⁴⁴Nd was ±10 ppm (2SD). The average ¹⁴³Nd/¹⁴⁴Nd measured for the AMES Nd standard during the analytical session was used to calculate the instrumental mass-bias coefficient for correction of the measured ¹⁴³Nd/¹⁴⁴Nd in the samples using the long-term average ¹⁴³Nd/¹⁴⁴Nd value of 0.512152 ± 2 measured precisely
<figure><img src="image_4.png" /><figcaption>**Fig. 2.** $^{87}$Sr/$^{86}$Sr ratios of melt inclusions and reference glasses and their weighted means. Melt inclusions are from olivines from the low-Ti and high-Ti suites, recalculated for 260 Ma age. Colour coded are inclusions from different samples (a): red- sample VS6–1; blue- sample SD22–2–1; magenta-sample SD22–4–2; green-sample SD22–13–3 and orange -sample SD22–2–2. (b) inclusions in olivine from high-Ti picrite, sample SD22–13–4. (c)-(f) Weighted mean of $^{87}$Sr/$^{86}$Sr ratios for standards Gor128, ML3B, NIST614 and STHs6 used for the analysis of Sr isotopes in melt inclusions to ensure accuracy and reproducibility. Different colors represent different sequences. StHs6 and ML3B were only analyzed for high-Ti sequence. Uncertainty bars here and elsewhere correspond to 95 % confidence interval. Plots were made using IsoplotR toolbox (Vermeesch, 2018). Note that most samples of low-Ti picrites contain olivine-hosted melt inclusions with variable initial $^{87}$Sr/$^{86}$Sr ratios. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)</figcaption></figure>
for the AMES Nd standard on the IGL ThermoFisher Triton using a multi-
dynamic routine (Puchtel et al., 2018).

In order to assess the accuracy of the analyses, several separate
powder aliquots of USGS GRM BCR-1 and BIR-1 were processed and
analyzed using the same analytical protocol. The measured ¹⁴³Nd/¹⁴⁴Nd
ratios for both GRM are identical, within their respective uncertainties,
to the GeoRem preferred values (Puchtel et al., 2022), as well as to the
average high-precision value for BCR-1 (0.512645 ± 1) obtained on the
IGL ThermoFisher Triton using a multi-dynamic routine (Puchtel et al.,
2018).

Measurements of the spiked Sm isotopic compositions were also
performed on the Faraday cups of the *ThermoFisher Neptune Plus* ICP-MS
in static mode using 10¹¹ Ω amplifiers on all masses except for ¹⁵⁵Gd, on
which a 10¹³ Ω amplifier was used. At the start of the measurements for
each sample, the peaks were centered, and a 100 s baseline measure-
ment was performed for each Faraday cup/amplifier pair without beam
deflection, and this baseline was then automatically subtracted from the
sample beam. For each sample, 20 ratios were collected with 8 s inte-
gration times in blocks of 10 ratios. The effects of instrumental mass
fractionation were corrected for relative to ¹⁴⁷Sm/¹⁵²Sm = 0.56081
using the exponential law. The uncertainty on the Sm/Nd ratio in the
samples analyzed was conservatively estimated on the basis of replicate
analyses of the USGS GRM BCR-1 and BIR-1 to be 0.20 % relative (2SD).

The eruption age-corrected $\epsilon^{143}\text{Nd}$ values were calculated based on the present-day parameters of the Chondritic Uniform Reservoir (CHUR): $^{147}\text{Sm}/^{144}\text{Nd} = 0.1967$ (Jacobsen and Wasserburg, 1980) and $^{143}\text{Nd}/^{144}\text{Nd} = 0.512638$ (Hamilton et al., 1983), and the $^{147}\text{Sm}$ decay constant $\lambda = 6.54 \times 10^{-12}$ year$^{-1}$ (Lugmair and Marti, 1978) for the age of 260 Ma.

3.4. Data processing

The loss of volatile elements such as Cl, S, and Na under high tem-
perature and 1 atm pressure is a characteristic feature of decrepitated
melt inclusions (Sobolev et al., 2016). In Song Da low-Ti melt inclusions,
concentrations of Cl and S are close to the detection limits at low probe
current microprobe analysis. Therefore, the filtering was only done
based on their Na content and visible signs of decrepitation, such as the
presence of unusually large shrinkage bubbles or cracks filled with melt.
Regarding Song Da high-Ti melt inclusions, as they are volatile-rich, we
were able to filter out decrepitated inclusions based on their Cl, S, and
Na contents. The average value of Na₂O in the low-Ti suite is 1.5 wt%.
Any inclusions that showed a significant drop in Na were filtered out
(Fig. 3). Most of them also show visible signs of decrepitation. Post-
entrapment correction was applied using Petrolog4 software (https://p
etrologsoftware.com Danyushevsky and Plechov, 2011). Total iron in
the trapped melt was set for low-Ti and high-Ti suites at 10.6 and 11.8
wt%, respectively (Kazzy et al., 2024). The models of Danyushevsky
(2001) and Weaver and Langmuir (1990) were used to correct the data
for the low-Ti and high-Ti melt inclusions, respectively. Strontium
isotope and trace element data for melt inclusions were filtered based on
3 criteria: 1) Any analyses of Sr isotopes with 2 standard errors that
exceeded ±0.0015 were rejected. 2) Analyses with an integration time
of signal less of than 10s was considered too short to be statistically
meaningful. 3) As Ti was analyzed by both EPMA and laser-ablation ICP-
MS, it was used to control potential ablation of host olivine or spinel
during glass analysis. All outliers were filtered out. The results of the
analyses before and after filtering are presented in Fig. 3. Similarly,
$^{87}$Rb/$^{86}$Sr measured by Neptune XT ICP-MS was cross-correlated with
$^{87}$Rb/$^{86}$Sr calculated from trace elements analyzed by *Agilent 8900 ICP-*

<figure><img src="image_5.png" /><figcaption>Fig. 3. a), b) Ti concentration in ppm from LA-ICP-MS and EPMA before and after filtering. c), d) Na₂O concentrations in melt inclusions before and after filtering showing decrepitated inclusions. Error bars correspond to ±2SE.</figcaption></figure>
MS-MS to ensure strong correlation (R² = 1) reflecting reproducibility of the analysis (Supplementary Fig. 2).

# 4. Results

## 4.1. Bulk rocks vs melt inclusions: Trace elements

Fig. 4 compares the mantle-normalized trace element compositions of whole-rock samples and melt inclusions from this study with the bulk-rock compositions of primitive volcanic rocks from the Emeishan flood basalt province as compiled by Kamenetsky et al. (2012) and Yu et al. (2024). Melt inclusion and host bulk-rock compositions of the samples studied here are normalized to the average TiO₂ content of their parental melts, 1.92 wt% for the high-Ti suite and 0.464 wt% for the low-Ti suite; Kazzy et al., (2024) to correct for olivine fractionation or accumulation. Fig. 4a displays compositions of individual rocks and inclusions, whereas Fig. 4b-d show the average compositions of melt inclusions for each sample compared with those normalized to corresponding parental melts host rocks and ELIP picrites out of Song Da region. For Song Da low-Ti picrites the abundances of immobile trace elements are similar in host rocks and melt inclusions, while mobile trace elements such as K, Rb, Ba, U, Pb and sometimes Th and Nb (sample SD22-13-3 on Fig. 4c) differ considerably and are generally much lower in melt inclusions than in host rocks (Fig. 4b-d). Figs. 4b-d also show a dramatic difference between Song Da and other ELIP low-Ti picrites. Song Da is the only area of ELIP where the ultradepleted by most incompatible elements ultra-mafic lavas occur (e.g. Hanski et al., 2004). The Song Da high-Ti picrite sample and its melt inclusions show enrichment in the highly incompatible trace elements relative to the Bulk Silicate Earth (BSE, similar to primitive mantle), along with a depletion in heavy REE. They overlap significantly with compositions of high-Ti picrites from China's regions of ELIP. The average compositions of melt inclusions from two low-Ti

samples (SD22-4-2 and SD22-13-3) are distinguished from the others by having the most depleted compositions, and hence we will refer to them as ultra-depleted melts (UDMs). All studied Song-Da low-Ti picrites are significantly more depleted in the most incompatible trace elements than any other low-Ti picrites from Chinese part of the ELIP.

## 4.2. Elemental and Sr isotope compositions of melt inclusions

### 4.2.1. H₂O contents

The original mobile and volatile element compositions in bulk-rock samples are commonly modified during alteration. A common way to estimate the volatile content in a magma is to analyze melt inclusions trapped in olivine crystals during their crystallization, as they represent partly closed systems (Sobolev, 1996). Kazzy et al. (2024) reported and discussed H₂O contents of Song Da olivine-hosted melt inclusions, as measured by Raman spectroscopy. In this paper, we use these data to compare with the trace element compositions of these melt inclusions. Because the bulk partition coefficient of H₂O between peridotite minerals and melt is similar to that of Ce, the ratio of H₂O/Ce is independent of olivine fractionation or the degree of melting of the source (Michael, 1995). Considering the H₂O contents, the Song Da low-Ti melt inclusions fall into two separate groups with low and high water concentrations, as illustrated in Fig. 5. The H₂O/Ce ratio of the low-Ti, high-H₂O suite ranges between 1500 and 5500, with Ba/Nb falling between 5 and 10.5, i.e., typical values observed in komatiites (Sobolev et al., 2019). On the other hand, H₂O/Ce of the low-Ti, low-H₂O suite ranges between 200 and 800, with Ba/Nb varying between 5 and 9. However, as was shown in Kazzy et al. (2024), the low water contents in these melt inclusions are likely an artifact of hydrogen loss by volume diffusion through the host olivine crystal. Thus, in later discussions, we only consider these inclusions for other trace elements and isotopes. All published data on H₂O/Ce ratios of olivine-hosted melt inclusions or estimates from Ca-

<figure><img src="image_6.png" /></figure>

Fig. 4. a) Compositions of melt inclusions from high-Ti and low-Ti samples from this study compared to the Emeishan picrites from Kamenetsky et al. (2012) and Yu et al. (2024). Melt inclusion and host rocks compositions are normalized to the Ti content of the parental melt. b-d) Averaged melt inclusion compositions compared with host rock compositions for each sample and ELIP picrites out of Song Da region, all normalized to the Ti content of the corresponding parental melts. Primitive mantle composition after McDonough and Sun (1995), the normalization value for H₂O (shown as H on spider diagram) was set to 320 p.p.m. (Sobolev et al., 2019). Uncertainty bars in this figure and all other figures correspond to 95 % confidence interval.
<figure><img src="image_7.png" /><figcaption>**Fig. 5.** a) H₂O/Ce versus Ba/Nb ratios for Song Da melt inclusions. The colored fields are from Sobolev et al. (2019) and correspond to back-arc basin basalts (BAB), mid-ocean ridge basalts, ocean island basalts (MORB + OIB) and island-arc basalts and continental margin basalts and andesites (IAB + CMB). b) H₂O/Ce versus forsterite content of the host olivine. Song Da rocks are divided into three different suites: Song Da low-Ti with high water (Song Da LTi HW), Song Da low-Ti with low water (Song Da LTi LW) and Song Da high-Ti (Song Da HTi). Small diamonds represent olivine-hosted melt inclusions from Dali picrites (Chen et al., 2023). Large colored circles (b) are calculated H₂O/Ce ratios of mantle sources versus highest Fo content of picrites, based on H₂O content of Ca-pyroxene phenocrysts (Liu et al., 2022). The primitive mantle composition constrained from the data on glasses from OIB and MORB is from Kendrick et al. (2017). Forsterite, Fo = Mg/Mg + Fe in at. %. Error bars are ±2SE of at least 3 analytical points for H₂O within melt inclusion and for Fo in olivine around melt inclusion.</figcaption></figure>

pyroxene H₂O contents from other localities of ELIP are also shown in Fig. 5. Being slightly elevated compared to typical mantle values, these ratios are much lower than those of the Song Da low-Ti melts.

The melt inclusions of the high-Ti suite show H₂O/Ce between 30 and 50, which are significantly lower than the values in typical ocean island basalts and mid-ocean ridge basalts (Dixon et al., 2017) due to near-surface degassing of H₂O and/or H loss by diffusion through the host olivine (Kazzy et al., 2024). Assuming a typical mantle H₂O/Ce ratio of 200, (Dixon et al., 2017), the reconstructed original H₂O content in the parental melt of the high-Ti suite is 0.6 ± 0.1 wt%.

### 4.2.2. Trace elements and Sr and Nd isotopes

Sr isotope ratios of Song Da olivine-hosted melt inclusions are presented in Fig. 2 along with the data on the reproducibility of reference materials, which cover the compositional range of analyzed inclusions. Most samples of the low-Ti picrites contain olivine-hosted melt inclusions with variable age- corrected ⁸⁷Sr/⁸⁶Sr ratios, while high-Ti melt inclusions comprise a homogeneous group of age corrected ⁸⁷Sr/⁸⁶Sr ratios. Trace-element ratios and Sr isotope compositions are plotted against forsterite contents in Fig. 6. To help determine the sources of the magmas, we use Nb/U and Ce/Pb ratios. In the continental crust, Nb is depleted relative to U and the Nb/U ratio is lower (6) than that of the BSE (33), whereas Pb is relatively enriched, yielding the Ce/Pb ratios of 3.9 and 11.2, respectively (McDonough and Sun, 1995; Rudnick and Gao, 2003). In modern MORB and OIB, Nb/U is 47 ± 10 and Ce/Pb is 25 ± 5 (Hofmann et al., 1986; Weis et al., 2023).

The Ce/Pb ratios of melt inclusions range between 6 and 25 in the Song Da low-Ti suite and between 7 and 13 in the high-Ti suite. The range of Ce/Pb in the low-Ti suite indicates the presence of components of both present-day mantle and continental crust. The same components can be identified on the basis of the Nb/U ratio, which varies between 14 and 55. However, the Ce/Pb ratio of the high-Ti melt inclusions plots well below mantle-derived melts and closer to the continental crust value, in contrast to their high Nb/U ratio, which ranges between 44 and 74, being within and above the range of the present-day mantle composition.

Ratios of La/Sm and ⁸⁷Sr/⁸⁶Sr are not affected by olivine fractionation and any variation in these ratios can therefore be related to crustal contamination, mantle source heterogeneity or, for La/Sm, variable degrees of mantle melting. In low-Ti melt inclusions, the La/Sm ratio vary in a large range of between 0.5 and 1.5, with a maximum in olivine with the composition of 89 mol% Fo (Fig. 6e). Their age-corrected ⁸⁷Sr/⁸⁶Sr (260 Ma) ratios show a large range from 0.7043 to 0.7083,

indicating an isotopically heterogeneous population (Fig. 2a). The maximum range of the ⁸⁷Sr/⁸⁶Sr ratios is observed for inclusions trapped in the most Fo-rich olivine (over 92 mol% Fo, Fig. 6d). In contrast, in the high-Ti melt inclusions, the ⁸⁷Sr/⁸⁶Sr ratios form a homogenous population (p(χ²) >0.05), with an average value of 0.70405 ± 0.00028 (2SE) (Fig. 6b). Their La/Sm ratios also show a narrow range from 3.3 to 3.9 (Fig. 6e) in the inclusions trapped in olivine with various Fo contents indicating that the composition of high-Ti melts was mostly controlled by fractional crystallization of olivine.

The Sr isotopic compositions and trace element ratios of the Song Da melt inclusions are compared in Fig. 7 to those of present-day ocean island basalts (OIB), mid-ocean ridge basalts (MORB) (Huang and Zheng, 2017) and olivine-hosted melt inclusions from Dali picrites, ELIP (L. Zhang et al., 2019). In terms of the Nb/U and Nb/Th ratios, the compositions of most of the low-Ti and high-Ti melts match the present-day mantle composition. However, the low-Ti group shows clearly lower Rb/Sr and higher ⁸⁷Sr/⁸⁶Sr than typical mantle-melt compositions. The highest ⁸⁷Sr/⁸⁶Sr ratio in the low-Ti volcanics is similar to that of Mauna Loa ultra-depleted melts (UDMs) which are trapped by high-Fo olivine crystals in the Puu Wahi scoria cone, Hawaii (Sobolev et al., 2011). The Sr isotope ratio of melt inclusions from the Dali picrites overlaps with those of the Song Da high-Ti picrites and the less radiogenic low-Ti picrites. But Dali picritic melts are generally more enriched in Rb than any other analyzed Song Da melts.

The ⁸⁷Sr/⁸⁶Sr ratios of low-Ti melt inclusions show significant negative correlations (p-value <0.001 corresponding to >99.9 % confidence) with the contents of most incompatible trace elements (Rb, Ba, K, Th, U, Nb, La, Pb) and Ce and Sr (p-value <0.003). They also display significant negative correlations (p-value <0.001) with Th/U and La/Sm and positive correlations with Nb/Th (p-value = 0.001) and Nb/U (p-value = 0.03). Statistical analyses of linear correlations are reported in Supplementary Table 3.

All studied samples from the low-Ti suite have radiogenic Nd isotopic compositions, with initial εNd values ranging from +5.5 ± 0.1 to +8.4 ± 0.1, being similar to the values that Hanski et al. (2004) reported for the same suite. The initial Nd isotopic ratio of the high-Ti picrite is significantly less radiogenic, with εNd = +3.7 ± 0.1. The low contents of Nd in the melt inclusions (typically a few ppm) and their small sizes (typically 40–50 µm) currently preclude direct analysis of their Nd isotopic compositions. Thus, we apply two indirect approaches to estimate the initial εNd value for the Song Da melt inclusions. In the first approach, we assume that the initial εNd values are similar in the bulk-rock samples and melt inclusions of the Song Da suites (Fig. 9a). This assumption is
<figure><img src="image_8.png" /><figcaption>**Fig. 6.** Trace element concentrations and age corrected Sr isotope ratios of melt inclusions hosted by olivine from Song Da ultramafic lavas. Fields for MORB + OIB and Subducting Sediments + Arc Front Volcanoes are from Hofmann et al. (1986, 2022). The continental crust composition is from Rudnick and Gao (2003) and the BSE (primitive mantle) composition is from McDonough and Sun (1995). FC- fractional crystallization trend of olivine; AFC-assimilation fractional crystallization trend. Olivine was measured in 3 to 4 points around each inclusion. All errors are 2 standard errors. Some significant uncertainties of host olivine Fo come from its zoning. Song Da LoTi = Song Da low-Ti melt inclusions. Song Da HiTi = Song Da high-Ti melt inclusions.</figcaption></figure>

supported by a close match of the average composition of melt in-
clusions and host rocks for most immobile incompatible elements
including Nd and Sm (Fig. 4). In the second approach, we use the strong
correlation between La/Sm and ¹⁴³Nd/¹⁴⁴Nd and ¹⁴⁷Sm/¹⁴⁴Nd ratios of
the Song Da low-Ti ultramafic and mafic rocks
(y = -0.0003 × La/Sm + 0.5133; R² = 0.94 and y = -0.04 × La/Sm + 0.30;

R² = 0.95 for ¹⁴³Nd/¹⁴⁴Nd and ¹⁴⁷Sm/¹⁴⁴Nd, respectively, Supplemen-
tary Fig. 3). This correlation yields a linear relationship between La/Sm
ratio of the melt and its εNd value which, when applied to the low-Ti melt
inclusions, yields an εNd value similar to the bulk rocks for inclusions
with low La/Sm (majority of population), but predicts much lower
values to those with elevated La/Sm ratios. The high-Ti melt matches the
<figure><img src="image_9.png" /><figcaption>Fig. 7. Trace element and $^{87}$Sr/$^{86}$Sr ratios (calculated for the age of 260 Ma) in melt inclusions from Song Da low-Ti and high-Ti lavas compared to the data for olivine-hosted melt inclusions in Dali picrites and OIB compositions. Data on melt inclusions from Dali picrites are from Zhang et al. (2019); because the same inclusions were not analyzed for Sr isotopes and trace elements in this study, we used average values for Sr isotopes (14 inclusions) and trace elements (75) inclusions. OIB data are from Huang and Zheng (2017) and Mauna Loa UDM after Sobolev et al. (2011).</figcaption></figure>

mantle array for both $^{87}$Sr/$^{86}$Sr and $\varepsilon_{Nd}$. In contrast, most of the low-Ti melt inclusions indicate major decoupling of Nd and Sr isotopic ratios for both approaches of estimation of melt inclusions $\varepsilon_{Nd}$ (Fig. 9). The subset of the low-Ti melts matches the Nd-Sr isotope mantle array if their $\varepsilon_{Nd}$ is estimated as a function of La/Sm.

# 5. Discussion

## 5.1. Melt inclusions versus bulk-rock compositions

Fig. 4 compares the compositions of bulk rocks (Fig. 4a) and melt inclusions normalized to the TiO$_2$ of the primary melt (Fig. 4b-d). Fluid-mobile trace elements (such as Rb, Ba, K, U, Pb, and Sr) are important tracers of mantle evolution and formation of continental crust (Hofmann, 1988). As it is clear from Fig. 4, the melt inclusions provide more reliable concentrations of these elements than whole rocks: the contents of these elements are more consistent with concentrations of immobile elements. Also, melt inclusions provide insights into the volatile content of the melt, such as H$_2$O. Therefore, melt inclusions have an advantage over bulk rocks in tracking mantle processes for certain elements. Unfortunately, even when isolated and unaltered, melt inclusions can still be affected by crustal contamination if it occurred prior to crystallization of the host mineral and entrapment of the inclusion. In the case of the ELIP, the crystallization occurred within the continental crust leading to potential contamination by wall-rock assimilation. However, crustal contamination can be identified and corrected by using certain canonical trace element and isotope ratios informative for crustal contamination, such as Nb/U and Sr isotopes.

## 5.2. CO$_2$ contents

One way to estimate the CO$_2$ content in the parental melt of the Song Da high-Ti suite is based on the Ba concentrations of melt inclusions and the CO$_2$/Ba ratio in the mantle. Assuming a moderately depleted mantle source (initial $\varepsilon_{Nd}$ = +3.7 ± 0.1 of bulk rock, Table 2) with an average CO$_2$/Ba ratio of 133 ± 44 (Rosenthal et al., 2015), and the average Ba content in the parental melt of the Song high-Ti suite of 233 ± 37 ppm (Table 1), the CO$_2$ content of the parental melt of the Song Da high-Ti melt is estimated to be 3.1 ± 1.4 wt%. A similar approach using 2–8 ppm of Ba for the parental melts of low-Ti suite yields 0.02–0.14 wt% of CO$_2$.

## 5.3. Olivine as an indicator of source lithology

Sobolev et al. (2007) showed that the amount of recycled material in deep mantle melts can be estimated using the major and minor element contents of liquidus olivine in these melts. Fig. 10 plots the Song Da olivine compositions from this study and Hanski et al. (2004) and shows that the sources of both high-Ti and low-Ti melt samples are dominated by olivine-bearing peridotite similarly to most Archean komatiite melts (Sobolev et al., 2019). These data, however, do not preclude the presence in the source of recycled silica-undersaturated mafic rocks produced at a high pressure from olivine-bearing pyroxenites (Sobolev et al., 2007).
<caption>**Table 1**</caption>

Major oxydes (in wt% on dry basis), trace element (in ppm), and Nd isotope analyses of Song Da samples.

<table><thead><tr><th>Sample</th><th>SD22-2-1</th><th>SD22-2-2</th><th>SD22-13-3</th><th>SD22-4-2</th><th>VS-6-1</th><th>SD22-13-4</th></tr></thead><tbody><tr><td>Type</td><td>low-Ti</td><td>low-Ti</td><td>low-Ti</td><td>low-Ti</td><td>low-Ti</td><td>high-Ti</td></tr><tr><td>SiO<sub>2</sub></td><td>43.7</td><td>43.4</td><td>45.6</td><td>44.3</td><td>44.7</td><td>44.4</td></tr><tr><td>TiO<sub>2</sub></td><td>0.316</td><td>0.354</td><td>0.538</td><td>0.392</td><td>0.532</td><td>1.521</td></tr><tr><td>Al<sub>2</sub>O<sub>3</sub></td><td>8.8</td><td>6.3</td><td>10.2</td><td>7.5</td><td>10.4</td><td>9.3</td></tr><tr><td>Fe<sub>2</sub>O<sub>3</sub></td><td>11.3</td><td>10.7</td><td>11.4</td><td>11.4</td><td>11.6</td><td>13.1</td></tr><tr><td>MnO</td><td>0.166</td><td>0.157</td><td>0.173</td><td>0.167</td><td>0.172</td><td>0.201</td></tr><tr><td>MgO</td><td>28.0</td><td>32.5</td><td>21.4</td><td>28.8</td><td>22.5</td><td>19.9</td></tr><tr><td>CaO</td><td>6.9</td><td>5.7</td><td>9.4</td><td>6.6</td><td>8.7</td><td>8.7</td></tr><tr><td>Na<sub>2</sub>O</td><td>0.324</td><td>0.315</td><td>0.806</td><td>0.377</td><td>0.994</td><td>1.967</td></tr><tr><td>K<sub>2</sub>O</td><td>bdl</td><td>0.036</td><td>0.051</td><td>bdl</td><td>0.016</td><td>0.307</td></tr><tr><td>P<sub>2</sub>O<sub>5</sub></td><td>bdl</td><td>bdl</td><td>bdl</td><td>bdl</td><td>0.048</td><td>0.282</td></tr><tr><td>Cr<sub>2</sub>O<sub>3</sub></td><td>0.459</td><td>0.462</td><td>0.329</td><td>0.45</td><td>0.267</td><td>0.3</td></tr><tr><td>LOI</td><td>9.6</td><td>11.6</td><td>8.7</td><td>9.6</td><td>2.2</td><td>6.9</td></tr><tr><td>Cr (ppm)</td><td>3142</td><td>3162</td><td>2251</td><td>3078</td><td>1827</td><td>985</td></tr><tr><td>Ni</td><td>1359</td><td>1697</td><td>949</td><td>1408</td><td>1206</td><td>730</td></tr><tr><td>Co</td><td>99.8</td><td>103.9</td><td>82.5</td><td>102.1</td><td>95.4</td><td>82.9</td></tr><tr><td>Sc</td><td>22.6</td><td>22.6</td><td>35.4</td><td>27.5</td><td>n.d.</td><td>21.6</td></tr><tr><td>V</td><td>124.0</td><td>123.5</td><td>195.5</td><td>141.4</td><td>212.9</td><td>207.1</td></tr><tr><td>Zr</td><td>13.5</td><td>16.7</td><td>22.1</td><td>16.0</td><td>n.d.</td><td>69.2</td></tr><tr><td>Hf</td><td>0.418</td><td>0.503</td><td>0.693</td><td>0.525</td><td>0.698</td><td>1.823</td></tr><tr><td>Ta</td><td>0.031</td><td>0.053</td><td>0.06</td><td>0.026</td><td>n.d.</td><td>1.219</td></tr><tr><td>Nb</td><td>0.376</td><td>0.629</td><td>0.838</td><td>0.333</td><td>0.706</td><td>17.60</td></tr><tr><td>Th</td><td>0.088</td><td>0.167</td><td>0.071</td><td>0.036</td><td>0.124</td><td>1.102</td></tr><tr><td>U</td><td>0.029</td><td>0.05</td><td>0.023</td><td>0.015</td><td>0.032</td><td>0.256</td></tr><tr><td>Rb</td><td>2.63</td><td>3.27</td><td>2.63</td><td>0.86</td><td>n.d.</td><td>8.48</td></tr><tr><td>Sr</td><td>38.3</td><td>34.3</td><td>38.2</td><td>26.2</td><td>n.d.</td><td>355.4</td></tr><tr><td>Cs</td><td>0.246</td><td>0.102</td><td>0.496</td><td>0.071</td><td>n.d.</td><td>1.731</td></tr><tr><td>Ba</td><td>n.d.</td><td>10.5</td><td>8.3</td><td>n.d.</td><td>n.d.</td><td>360.7</td></tr><tr><td>Y</td><td>7.4</td><td>8.1</td><td>14.3</td><td>9.9</td><td>n.d.</td><td>12.2</td></tr><tr><td>Cu</td><td>62.1</td><td>64.1</td><td>100.8</td><td>69.1</td><td>n.d.</td><td>122.6</td></tr><tr><td>Ga</td><td>8.5</td><td>7.0</td><td>10.9</td><td>8.5</td><td>n.d.</td><td>13.3</td></tr><tr><td>Pb</td><td>n.d.</td><td>n.d.</td><td>n.d.</td><td>n.d.</td><td>n.d.</td><td>2.8</td></tr><tr><td>Zn</td><td>76.7</td><td>69.7</td><td>73.7</td><td>74.2</td><td>n.d.</td><td>107.4</td></tr><tr><td>La</td><td>0.47</td><td>0.76</td><td>0.73</td><td>0.45</td><td>1.33</td><td>10.66</td></tr><tr><td>Ce</td><td>1.34</td><td>1.92</td><td>2.06</td><td>1.42</td><td>3.31</td><td>22.10</td></tr><tr><td>Pr</td><td>0.23</td><td>0.31</td><td>0.38</td><td>0.28</td><td>0.51</td><td>2.81</td></tr><tr><td>Nd</td><td>1.38</td><td>1.68</td><td>2.27</td><td>1.71</td><td>2.80</td><td>12.08</td></tr><tr><td>Sm</td><td>0.61</td><td>0.73</td><td>1.08</td><td>0.79</td><td>1.14</td><td>2.85</td></tr><tr><td>Eu</td><td>0.28</td><td>0.30</td><td>0.47</td><td>0.34</td><td>0.48</td><td>0.97</td></tr><tr><td>Gd</td><td>0.95</td><td>1.07</td><td>1.68</td><td>1.20</td><td>1.87</td><td>2.65</td></tr><tr><td>Tb</td><td>0.19</td><td>0.20</td><td>0.34</td><td>0.24</td><td>0.34</td><td>0.41</td></tr><tr><td>Dy</td><td>1.34</td><td>1.47</td><td>2.44</td><td>1.71</td><td>2.43</td><td>2.48</td></tr><tr><td>Ho</td><td>0.29</td><td>0.32</td><td>0.55</td><td>0.40</td><td>0.52</td><td>0.49</td></tr><tr><td>Er</td><td>0.81</td><td>0.91</td><td>1.55</td><td>1.07</td><td>1.54</td><td>1.24</td></tr><tr><td>Tm</td><td>0.13</td><td>0.13</td><td>0.23</td><td>0.16</td><td>0.22</td><td>0.18</td></tr><tr><td>Yb</td><td>0.81</td><td>0.90</td><td>1.51</td><td>1.06</td><td>1.45</td><td>1.09</td></tr><tr><td>Lu</td><td>0.12</td><td>0.14</td><td>0.23</td><td>0.16</td><td>0.21</td><td>0.16</td></tr><tr><td><sup>147</sup>Sm/<sup>144</sup>Nd</td><td>0.26397</td><td>0.24791</td><td>0.28091</td><td>0.27454</td><td>0.24979</td><td>0.13941</td></tr><tr><td>±2SD</td><td>0.00053</td><td>0.00050</td><td>0.00056</td><td>0.00055</td><td>0.00050</td><td>0.00028</td></tr><tr><td><sup>143</sup>Nd/<sup>144</sup>Nd</td><td>0.513067</td><td>0.513008</td><td>0.513188</td><td>0.513202</td><td>0.513116</td><td>0.512730</td></tr><tr><td>±2SD</td><td>0.000005</td><td>0.000005</td><td>0.000005</td><td>0.000005</td><td>0.000005</td><td>0.000005</td></tr><tr><td>±2SE</td><td>0.000005</td><td>0.000004</td><td>0.000003</td><td>0.000004</td><td>0.000003</td><td>0.000004</td></tr><tr><td><sup>143</sup>Nd/<sup>144</sup>Ndi</td><td>0.512618</td><td>0.512586</td><td>0.512710</td><td>0.512734</td><td>0.512691</td><td>0.512493</td></tr><tr><td><sup>143</sup>Nd/<sup>144</sup>NdCH</td><td>0.512303</td><td>0.512303</td><td>0.512303</td><td>0.512303</td><td>0.512303</td><td>0.512303</td></tr><tr><td>ε<sup>143</sup>Nd(T)</td><td>6.14</td><td>5.51</td><td>7.95</td><td>8.41</td><td>7.57</td><td>3.69</td></tr><tr><td>±2SD</td><td>0.10</td><td>0.10</td><td>0.10</td><td>0.10</td><td>0.10</td><td>0.10</td></tr></tbody></table>

n.d.- not determined; bdl- below detection limit; i- age-corrected; CH- CHUR; (T)- age-corrected VS-6-1 major elements are from Nicklas et al. (2019)

## 5.4. Crustal contamination

Most of the studies conducted on the ELIP argue for an interaction between a mantle plume and the subcontinental lithospheric mantle (SCLM) (Chung et al., 1998; Xu et al., 2001; Xiao et al., 2004; Zhou et al., 2008). Others argue for the presence of crustal contamination during the formation of Emeishan basalts (Xu et al., 2003; Shellnutt, 2014; Liu et al., 2022; Yi et al., 2022).

The La/Sm and $^{87}$Sr/$^{86}$Sr ratios in the melt are independent of fractional crystallization, and, hence, any variations in these ratios can be explained by crustal contamination, source heterogeneity or mixing between magmas of significantly different degrees of melting (for La/Sm only). The negative correlation between La/Sm and $^{87}$Sr/$^{86}$Sr, along with the positive correlation between Nb/U and $^{87}$Sr/$^{86}$Sr (Fig. 8), may

indicate contamination of the parental melt by a component with Nb/U, Ce/Pb and La/Sm ratios similar to those of continental crust. We propose that the best candidate for the uncontaminated parental melt is one that is most depleted in most incompatible elements (Ba, Rb, Th, U, Nb, K, La) trapped in the highest-Fo olivine (average between melt inclusions SD22-4-2-ol15-i1 and SD22-4-2-ol22-i1, Table 2). The crustal contaminant, however, must have less radiogenic Sr ($^{87}$Sr/$^{86}$Sr < 0.7040) than required for mature continental crust of Precambrian age in the region (Hanski et al., 2004). However, such a component could be mafic material in the lower continental crust (Rudnick and Gao, 2003). With its Rb/Sr ratio of 0.03, similar to Bulk Silicate Earth or PM (McDonough and Sun, 1995), its $^{87}$Sr/$^{86}$Sr ratio could be as low as 0.7030 or even lower at the time of ELIP formation (Lee et al., 1993). Considering the lower crust composition from Rudnick and Gao (2003) with the
<figure><img src="image_10.png" /><figcaption>**Fig. 8.** Trace element ratios versus initial $^{87}$Sr/$^{86}$Sr ratios for Song Da low-Ti melt inclusions. The uncertainty bars correspond to 95 % confidence interval. The large open circle is the Mauna Loa ultra-depleted melt composition from Sobolev et al. (2011). R is the correlation coefficient, and the dashed line represents the linear correlation trend. Yellow circles represent mixing between Song Da UDM (average between melt inclusions SD22–4-2-ol15-i1 and SD22–4-2-ol22-i1 (Table 2) and result of 50 % of melting of lower crust, with garnet and pyroxene in the residue (see section 5.2 for details). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)</figcaption></figure>

$^{87}$Sr/$^{86}$Sr ratio of 0.7030, and assuming its 50 % melting with pyroxene and garnet with negligible contents of Th, U, Nb, Sr, La and Sm in the residue, we estimated the effects of contamination of the parental melt of the low-Ti ultramafic suite by this material. As is shown in Fig. 8, up to 10 % of contamination with such a material can reasonably explain the observed trend of melt inclusion compositions. However, inclusions with low Nb/U and relatively unradiogenic Sr were also trapped by the most magnesian olivine (Fig. 6). This can be explained by contamination starting early during or before parental melt crystallization. This is also supported by the fact that the amount of radiogenic-Sr depleted inclusions gradually decreases with decreasing Fo content of host olivine.

The drawback of the hypothesis of lower crust contamination is the likely rather shallow crystallization depth (less than 10 km) of the studied lavas. The fact that most olivine-hosted melt inclusions are not decrepitated during annealing at high temperatures and one-atmosphere pressure suggests that the internal pressure of inclusions does not exceed 200–300 MPa (MacLennan, 2017). A possible explanation for the shallow emplacement of the lower continental crust could be the local crustal uplifts caused by ELIP. However, this may also suggest an alternative explanation for the origin of this component, which is mantle source contamination by subduction-related fluids or melts or even by recycled continental crust. These materials must have relatively unradiogenic Sr (Fig. 8), which may restrict their age, considering their commonly high Rb/Sr ratios. Modelling mantle source contamination requires defining too many unknowns and more data, and thus, it is beyond the scope of this paper.

The constant La/Sm and $^{87}$Sr/$^{86}$Sr values in the high-Ti melts indicate a system controlled by fractional crystallization only. However, these melts show a range of Ce/Pb between 7 and 13, which is much lower than what is expected for present-day mantle-derived melts and which is also inconsistent with their high Nb/U ratios. The average Na$_2$O/TiO$_2$ ratio of high-Ti melts is 2.2 ± 0.2 (2SE), which is higher than 1.8 in the Bulk Silicate Earth (McDonough and Sun, 1995) and 1.0 in the

depleted mantle (Workman and Hart, 2005). This requires contamination of their mantle source by Na together with Sr (Fig. 4). These elements are highly mobile in carbonatite melt or CO$_2$ fluid (Dasgupta et al., 2009), which thus may contribute these elements to the source of these silica-undersaturated melts. This, however, cannot explain the low Ce/Pb (BSE-type) ratio Fig. 4), which requires a source unusually enriched or at least undepleted in Pb compared to BSE.

### 5.5. The source composition of the Song Da rocks

In terms of the Nb/Th, Nb/U, and Rb/Sr ratios and Sr and Nd isotopes, the Song Da high-Ti melts plot within the mantle array, indicating that their source is similar to the PREMA mantle component (Fig. 7 and Fig. 9). The trace element enrichment of these melts is attributed to the low degrees of partial melting (less than 9 %, Kazzy et al., 2024), with some possible contamination by metasomatic fluids resulting in the elevated Pb, Sr and Na concentrations (Fig. 4b).

The Song Da low-Ti melt inclusions reveal the presence of at least two components. The first component had high La/Sm and Th/U and high contents of the most incompatible elements associated with low initial $^{87}$Sr/$^{86}$Sr (0.7040), Nb/U, Nb/Th, and Ce/Pb. The second component – ultra-depleted melt (UDM) – is strongly depleted in the most incompatible elements (Rb, Ba, K, Th, U, Nb, La, Pb), with low La/Sm and Ba/Nb associated with high initial $^{87}$Sr/$^{86}$Sr (up to 0.7080) and high Nb/U (>34) (Fig. 8). The first component can be related either to lower crust contamination by wall rock assimilation during crystallization, or by the subduction-related component in the mantle source as discussed in section 5.4.

The second component (UDM) is an exotic one with inverse relations between Sr isotopes and contents of most incompatible elements. The enrichment in $^{87}$Sr/$^{86}$Sr associated with depletion of most incompatible trace elements in this component cannot be explained either by direct continental crust contamination or by its recycling. The very low
<caption>Table 2</caption>

Compositions of representative melt inclusions from Song Da picrites corrected for post-entrapment crystallization of olivine.

<table><thead><tr><th>Name</th><th>M5-VS6-1-ol-A5</th><th>MSIMS-VS6-1-olA1</th><th>SD22-2-2-ol27</th><th>SD22-2-2-ol38</th><th>SD22-13-3-ol7</th><th>SD22-13-3-ol16</th><th>SD22-2-1-ol5</th><th>SD22-2-1-ol13</th><th>SD22-4-2-ol1</th><th>SD22-4-2-ol11</th><th>SD22-4-2-ol15-i1</th><th>SD22-4-2-ol22-i1</th><th>SD22-4-2-ol25</th><th>SD22-13-4-ol1</th><th>SD22-13-4-ol11</th><th>SD22-13-4-ol14</th></tr></thead><tbody><tr><td colspan="18" style="text-align: left; font-weight: bold;">Host olivine</td></tr><tr><td>Fo (mol %)</td><td>86.63</td><td>88.29</td><td>87.08</td><td>92.38</td><td>87.97</td><td>92.19</td><td>92.13</td><td>92.69</td><td>92.21</td><td>93.16</td><td>92.52</td><td>91.91</td><td>92.93</td><td>82.04</td><td>92.12</td><td>87.28</td></tr><tr><td>oxydes (wt %)</td><td>Low-Ti</td><td>Low-Ti</td><td>Low-Ti</td><td>Low-Ti</td><td>Low-Ti</td><td>Low-Ti</td><td>Low-Ti</td><td>Low-Ti</td><td>Low-Ti</td><td>Low-Ti</td><td>Low-Ti</td><td>Low-Ti</td><td>Low-Ti</td><td>High-Ti</td><td>High-Ti</td><td>High-Ti</td></tr><tr><td>SiO<sub>2</sub></td><td>48.78</td><td>48.79</td><td>48.31</td><td>46.80</td><td>51.68</td><td>48.16</td><td>48.45</td><td>46.76</td><td>46.73</td><td>45.99</td><td>47.54</td><td>46.61</td><td>46.78</td><td>43.51</td><td>42.85</td><td>43.28</td></tr><tr><td>TiO<sub>2</sub></td><td>0.70</td><td>0.69</td><td>0.81</td><td>0.61</td><td>0.47</td><td>0.62</td><td>0.56</td><td>0.55</td><td>0.60</td><td>0.59</td><td>0.52</td><td>0.63</td><td>0.51</td><td>2.89</td><td>1.95</td><td>2.20</td></tr><tr><td>Al<sub>2</sub>O<sub>3</sub></td><td>14.93</td><td>13.08</td><td>14.00</td><td>10.81</td><td>11.23</td><td>9.98</td><td>9.76</td><td>10.62</td><td>11.06</td><td>10.05</td><td>10.01</td><td>11.41</td><td>9.74</td><td>15.27</td><td>11.10</td><td>12.71</td></tr><tr><td>Fe<sub>2</sub>O<sub>3</sub></td><td>1.65</td><td>1.42</td><td>1.58</td><td>1.06</td><td>1.46</td><td>1.07</td><td>1.08</td><td>1.04</td><td>1.07</td><td>1.01</td><td>1.05</td><td>1.09</td><td>1.02</td><td>2.81</td><td>2.64</td><td>2.73</td></tr><tr><td>FeO</td><td>9.08</td><td>9.29</td><td>9.14</td><td>9.62</td><td>9.26</td><td>9.61</td><td>9.60</td><td>9.64</td><td>9.61</td><td>9.67</td><td>9.63</td><td>9.59</td><td>9.65</td><td>9.26</td><td>9.42</td><td>9.39</td></tr><tr><td>MnO</td><td>0.11</td><td>0.08</td><td>0.12</td><td>0.10</td><td>0.11</td><td>0.10</td><td>0.10</td><td>0.10</td><td>0.11</td><td>0.10</td><td>0.11</td><td>0.11</td><td>0.10</td><td>0.21</td><td>0.09</td><td>0.11</td></tr><tr><td>MgO</td><td>10.33</td><td>12.45</td><td>10.81</td><td>19.39</td><td>12.33</td><td>19.23</td><td>19.21</td><td>20.25</td><td>18.99</td><td>21.64</td><td>20.23</td><td>18.19</td><td>21.30</td><td>8.34</td><td>16.02</td><td>11.13</td></tr><tr><td>CaO</td><td>12.17</td><td>11.02</td><td>12.57</td><td>9.58</td><td>11.51</td><td>9.59</td><td>9.50</td><td>9.48</td><td>10.00</td><td>9.11</td><td>9.12</td><td>10.23</td><td>8.89</td><td>10.41</td><td>10.13</td><td>11.86</td></tr><tr><td>Na<sub>2</sub>O</td><td>1.99</td><td>1.55</td><td>1.64</td><td>1.35</td><td>1.71</td><td>1.37</td><td>1.36</td><td>1.23</td><td>1.13</td><td>1.03</td><td>1.11</td><td>1.19</td><td>1.02</td><td>5.80</td><td>4.49</td><td>5.08</td></tr><tr><td>Cr<sub>2</sub>O<sub>3</sub></td><td>0.04</td><td>0.23</td><td>0.03</td><td>0.10</td><td>0.14</td><td>0.13</td><td>0.14</td><td>0.10</td><td>0.13</td><td>0.10</td><td>0.16</td><td>0.10</td><td>0.15</td><td>0.01</td><td>0.05</td><td>0.05</td></tr><tr><td>K<sub>2</sub>O</td><td>0.067</td><td>0.037</td><td>0.038</td><td>0.023</td><td>0.021</td><td>0.020</td><td>0.037</td><td>0.049</td><td>0.020</td><td>0.016</td><td>0.019</td><td>0.018</td><td>0.023</td><td>0.928</td><td>0.816</td><td>0.950</td></tr><tr><td>Cl</td><td>bdl</td><td>bdl</td><td>bdl</td><td>bdl</td><td>bdl</td><td>bdl</td><td>0.0033</td><td>bdl</td><td>bdl</td><td>bdl</td><td>bdl</td><td>bdl</td><td>bdl</td><td>bdl</td><td>0.073</td><td>0.049</td><td>0.057</td></tr><tr><td>P<sub>2</sub>O<sub>5</sub></td><td>0.07</td><td>0.05</td><td>0.06</td><td>0.04</td><td>0.04</td><td>0.04</td><td>0.08</td><td>0.05</td><td>0.05</td><td>0.03</td><td>0.03</td><td>0.04</td><td>0.05</td><td>0.43</td><td>0.32</td><td>0.38</td></tr><tr><td>H<sub>2</sub>O</td><td>0.10</td><td>1.33</td><td>0.90</td><td>0.51</td><td>0.065</td><td>0.089</td><td>0.137</td><td>0.146</td><td>0.51</td><td>0.68</td><td>0.47</td><td>0.78</td><td>0.76</td><td>0.15</td><td>0.14</td><td>0.14</td></tr><tr><td>Total</td><td>100.0</td><td>100.0</td><td>100.0</td><td>100.1</td><td>100.0</td><td>100.0</td><td>100.0</td><td>100.1</td><td>100.0</td><td>100.0</td><td>100.0</td><td>100.1</td><td>100.0</td><td>99.9</td><td>99.9</td><td>99.9</td></tr><tr><td colspan="18" style="text-align: left; font-weight: bold;">Elements (ppm)</td></tr><tr><td>Ca</td><td>86,666</td><td>79,289</td><td>89,683</td><td>67,615</td><td>82,101</td><td>67,726</td><td>67,072</td><td>66,872</td><td>70,645</td><td>64,197</td><td>64,270</td><td>72,294</td><td>62,697</td><td>74,383</td><td>72,372</td><td>84,798</td></tr><tr><td>Ti</td><td>4199</td><td>4193</td><td>4686</td><td>3622</td><td>2822</td><td>3739</td><td>3244</td><td>3295</td><td>3526</td><td>3460</td><td>3196</td><td>3629</td><td>3005</td><td>16,634</td><td>11,336</td><td>12,437</td></tr><tr><td>Rb</td><td>1.204</td><td>0.635</td><td>0.902</td><td>0.375</td><td>0.281</td><td>0.261</td><td>0.615</td><td>0.869</td><td>0.293</td><td>0.211</td><td>0.226</td><td>0.228</td><td>0.369</td><td>24,297</td><td>19,654</td><td>22,166</td></tr><tr><td>Sr</td><td>79.0</td><td>72.1</td><td>71.6</td><td>51.9</td><td>59.8</td><td>53.4</td><td>51.6</td><td>51.9</td><td>34.9</td><td>35.9</td><td>42.1</td><td>43.0</td><td>39.4</td><td>653.7</td><td>459.2</td><td>526.1</td></tr><tr><td>Y</td><td>20.0</td><td>17.1</td><td>19.0</td><td>15.8</td><td>17.9</td><td>16.9</td><td>15.0</td><td>15.1</td><td>16.3</td><td>14.8</td><td>15.8</td><td>16.7</td><td>14.9</td><td>23.6</td><td>14.9</td><td>17.5</td></tr><tr><td>Zr</td><td>29.1</td><td>30.8</td><td>33.8</td><td>26.0</td><td>32.3</td><td>28.2</td><td>24.1</td><td>23.5</td><td>21.0</td><td>19.1</td><td>21.5</td><td>23.3</td><td>19.8</td><td>172.5</td><td>84.8</td><td>98.1</td></tr><tr><td>Nb</td><td>0.989</td><td>0.910</td><td>0.912</td><td>0.672</td><td>0.463</td><td>0.454</td><td>0.781</td><td>0.679</td><td>0.522</td><td>0.390</td><td>0.537</td><td>0.473</td><td>0.554</td><td>30.553</td><td>24.534</td><td>27.692</td></tr><tr><td>Ba</td><td>11.46</td><td>6.94</td><td>7.21</td><td>4.43</td><td>2.75</td><td>2.51</td><td>8.00</td><td>5.80</td><td>3.73</td><td>2.18</td><td>2.86</td><td>2.25</td><td>4.72</td><td>332.7</td><td>251.4</td><td>295.0</td></tr><tr><td>La</td><td>1.885</td><td>1.081</td><td>0.938</td><td>0.669</td><td>0.682</td><td>0.560</td><td>0.620</td><td>0.644</td><td>0.491</td><td>0.430</td><td>0.569</td><td>0.522</td><td>0.532</td><td>21.55</td><td>13.29</td><td>15.32</td></tr><tr><td>Ce</td><td>4.452</td><td>3.327</td><td>2.987</td><td>2.222</td><td>2.532</td><td>2.061</td><td>2.011</td><td>1.940</td><td>1.628</td><td>1.410</td><td>1.792</td><td>1.862</td><td>1.718</td><td>47.32</td><td>27.16</td><td>30.52</td></tr><tr><td>Nd</td><td>3.756</td><td>3.709</td><td>3.529</td><td>2.841</td><td>3.185</td><td>3.062</td><td>2.631</td><td>2.401</td><td>2.532</td><td>2.104</td><td>2.333</td><td>2.507</td><td>2.367</td><td>28.47</td><td>15.07</td><td>16.86</td></tr><tr><td>Sm</td><td>1.324</td><td>1.524</td><td>1.594</td><td>1.201</td><td>1.383</td><td>1.434</td><td>1.271</td><td>1.124</td><td>1.305</td><td>1.059</td><td>1.272</td><td>1.347</td><td>1.184</td><td>6.48</td><td>3.47</td><td>3.93</td></tr><tr><td>Dy</td><td>3.463</td><td>2.893</td><td>3.267</td><td>2.691</td><td>3.059</td><td>2.974</td><td>2.603</td><td>2.509</td><td>2.925</td><td>2.534</td><td>2.698</td><td>2.885</td><td>2.589</td><td>4.58</td><td>3.11</td><td>3.55</td></tr><tr><td>Yb</td><td>2.079</td><td>2.028</td><td>2.280</td><td>1.458</td><td>1.751</td><td>1.577</td><td>1.494</td><td>1.429</td><td>1.769</td><td>1.601</td><td>1.551</td><td>1.779</td><td>1.499</td><td>2.27</td><td>1.36</td><td>1.44</td></tr><tr><td>Pb</td><td>0.250</td><td>0.178</td><td>0.233</td><td>0.137</td><td>0.101</td><td>0.111</td><td>0.159</td><td>0.162</td><td>0.070</td><td>0.067</td><td>0.087</td><td>0.084</td><td>0.108</td><td>3.59</td><td>3.81</td><td>3.89</td></tr><tr><td>Th</td><td>0.184</td><td>0.092</td><td>0.110</td><td>0.047</td><td>0.042</td><td>0.039</td><td>0.061</td><td>0.100</td><td>0.043</td><td>0.035</td><td>0.041</td><td>0.039</td><td>0.048</td><td>2.43</td><td>1.36</td><td>1.61</td></tr><tr><td>U</td><td>0.040</td><td>0.022</td><td>0.028</td><td>0.017</td><td>0.012</td><td>0.011</td><td>0.014</td><td>0.025</td><td>0.012</td><td>0.009</td><td>0.012</td><td>0.014</td><td>0.013</td><td>0.601</td><td>0.331</td><td>0.383</td></tr><tr><td>Er</td><td>2.438</td><td>1.810</td><td>n.d.</td><td>n.d.</td><td>n.d.</td><td>n.d.</td><td>n.d.</td><td>n.d.</td><td>n.d.</td><td>n.d.</td><td>n.d.</td><td>n.d.</td><td>n.d.</td><td>2.40</td><td>1.51</td><td>1.75</td></tr><tr><td>Sc</td><td>33.7</td><td>32.6</td><td>n.d.</td><td>n.d.</td><td>n.d.</td><td>n.d.</td><td>n.d.</td><td>n.d.</td><td>n.d.</td><td>n.d.</td><td>n.d.</td><td>n.d.</td><td>n.d.</td><td>14.0</td><td>19.5</td><td>22.6</td></tr><tr><td>V</td><td>261</td><td>184</td><td>n.d.</td><td>n.d.</td><td>n.d.</td><td>n.d.</td><td>n.d.</td><td>n.d.</td><td>n.d.</td><td>n.d.</td><td>n.d.</td><td>n.d.</td><td>n.d.</td><td>279</td><td>234</td><td>231.3</td></tr><tr><td>Pr</td><td>0.654</td><td>0.600</td><td>n.d.</td><td>n.d.</td><td>n.d.</td><td>n.d.</td><td>n.d.</td><td>n.d.</td><td>n.d.</td><td>n.d.</td><td>n.d.</td><td>n.d.</td><td>n.d.</td><td>6.26</td><td>3.51</td><td>4.00</td></tr><tr><td>Eu</td><td>0.609</td><td>0.592</td><td>n.d.</td><td>n.d.</td><td>n.d.</td><td>n.d.</td><td>n.d.</td><td>n.d.</td><td>n.d.</td><td>n.d.</td><td>n.d.</td><td>n.d.</td><td>n.d.</td><td>2.14</td><td>1.16</td><td>1.33</td></tr><tr><td>Gd</td><td>2.623</td><td>2.293</td><td>n.d.</td><td>n.d.</td><td>n.d.</td><td>n.d.</td><td>n.d.</td><td>n.d.</td><td>n.d.</td><td>n.d.</td><td>n.d.</td><td>n.d.</td><td>n.d.</td><td>5.91</td><td>3.66</td><td>4.30</td></tr><tr><td>Tb</td><td>0.441</td><td>0.421</td><td>n.d.</td><td>n.d.</td><td>n.d.</td><td>n.d.</td><td>n.d.</td><td>n.d.</td><td>n.d.</td><td>n.d.</td><td>n.d.</td><td>n.d.</td><td>n.d.</td><td>n.d.</td><td>n.d.</td><td>n.d.</td></tr><tr><td>Cr</td><td>193</td><td>1444</td><td>n.d.</td><td>n.d.</td><td>n.d.</td><td>n.d.</td><td>n.d.</td><td>n.d.</td><td>n.d.</td><td>n.d.</td><td>n.d.</td><td>n.d.</td><td>n.d.</td><td>n.d.</td><td>n.d.</td><td>n.d.</td></tr><tr><td>Lu</td><td>0.302</td><td>0.237</td><td>n.d.</td><td>n.d.</td><td>n.d.</td><td>n.d.</td><td>n.d.</td><td>n.d.</td><td>n.d.</td><td>n.d.</td><td>n.d.</td><td>n.d.</td><td>n.d.</td><td>n.d.</td><td>n.d.</td><td>n.d.</td></tr><tr><td>Hf</td><td>0.913</td><td>0.918</td><td>n.d.</td><td>n.d.</td><td>n.d.</td><td>n.d.</td><td>n.d.</td><td>n.d.</td><td>n.d.</td><td>n.d.</td><td>n.d.</td><td>n.d.</td><td>n.d.</td><td>n.d.</td><td>n.d.</td><td>n.d.</td></tr><tr><td>Ta</td><td>0.047</td><td>0.052</td><td>n.d.</td><td>n.d.</td><td>n.d.</td><td>n.d.</td><td>n.d.</td><td>n.d.</td><td>n.d.</td><td>n.d.</td><td>n.d.</td><td>n.d.</td><td>n.d.</td><td>n.d.</td><td>n.d.</td><td>n.d.</td></tr><tr><td><sup>87</sup>Sr/<sup>86</sup>Sr</td><td>0.70413</td><td>0.70445</td><td>0.70614</td><td>0.70726</td><td>0.70632</td><td>0.70698</td><td>0.70601</td><td>0.70452</td><td>0.70688</td><td>0.70813</td><td>0.70523</td><td>0.70835</td><td>0.70573</td><td>0.70390</td><td>0.70467</td><td>0.70499</td></tr><tr><td>±2SE</td><td>0.00060</td><td>0.00069</td><td>0.00055</td><td>0.00137</td><td>0.00071</td><td>0.00067</td><td>0.00075</td><td>0.00076</td><td>0.00079</td><td>0.00061</td><td>0.00107</td><td>0.00059</td><td>0.00134</td><td>0.00062</td><td>0.00106</td><td>0.00055</td></tr><tr><td><sup>87</sup>Rb/<sup>86</sup>Sr</td><td>0.0346</td><td>0.0235</td><td>0.0365</td><td>0.0221</td><td>0.0139</td><td>0.0137</td><td>0.0340</td><td>0.0486</td><td>0.0256</td><td>0.0168</td><td>0.0169</td><td>0.0152</td><td>0.0287</td><td>0.1047</td><td>0.1226</td><td>0.1202</td></tr><tr><td>±2SE</td><td>0.0020</td><td>0.0014</td><td>0.0019</td><td>0.0012</td><td>0.0007</td><td>0.0007</td><td>0.0019</td><td>0.0025</td><td>0.0013</td><td>0.0009</td><td>0.0009</td><td>0.0008</td><td>0.0017</td><td>0.0057</td><td>0.0068</td><td>0.0068</td></tr><tr><td><sup>87</sup>Sr/<sup>86</sup>Sr(i)</td><td>0.70400</td><td>0.70436</td><td>0.70601</td><td>0.70718</td><td>0.70627</td><td>0.70693</td><td>0.70589</td><td>0.70435</td><td>0.70678</td><td>0.70807</td><td>0.70517</td><td>0.70829</td><td>0.70563</td><td>0.70352</td><td>0.70423</td><td>0.70456</td></tr><tr><td>±2SE</td><td>0.00060</td><td>0.00069</td><td>0.00055</td><td>0.00137</td><td>0.00071</td><td>0.00067</td><td>0.00075</td><td>0.00076</td><td>0.00079</td><td>0.00061</td><td>0.00107</td><td>0.00059</td><td>0.00134</td><td>0.00062</td><td>0.00106</td><td>0.00055</td></tr></tbody></table>

Major elements and H₂O contents of melt inclusions and Fo of host olivine are from Kazzy et al. (2024). Fo = Mg/Mg + Fe in mol%, n.d.-not determined, bdl- below detection limit, i- age-corrected.
<figure><img src="image_11.png" /><figcaption>**Fig. 9.** Initial ε<sub>Nd</sub> values of whole-rock samples versus initial Sr isotopic compositions of melt inclusions compared to modern mantle-derived magmas. a) ε<sub>Nd</sub> for melt inclusions is assumed to be the same as in the host-rock samples; b) ε<sub>Nd</sub> for low-Ti melt inclusions calculated from their La/Sm ratios and correlation from the host rocks (see text for details). ε<sub>Nd</sub> data for different mantle reservoirs and for the present-day magmas are from Stracke et al. (2022). The Nd isotopic compositions of the studied Song Da picrites are reported in Table 2.</figcaption></figure>

concentration of trace elements, such as Rb, Pb, and U, and the high values of Nb/U, Nb/Th, Ce/Pb and low Rb/Sr ratios cannot reasonably be explained by metasomatic fluids of any continental origin. Another potential explanation of the highly radiogenic Sr isotopic composition is direct contamination of the melt by seawater-altered oceanic crustal material. However, the ELIP was emplaced on the continental crust and even if some oceanic crust was present locally at the time of the emplacement of the Song Da lavas, the seawater ⁸⁷Sr/⁸⁶Sr ratio at that time was below 0.7080 (Veizer et al., 1999) (Fig. 12). Another argument against contamination of the melt by seawater-altered oceanic crust is the absence of excess of Cl and K in these melt inclusions (Cl below 0.01 wt%, K₂O less than 0.06 wt%).

A similar exotic component has been reported from UDM inclusions in olivine of the Mauna Loa volcano, Hawaii (Sobolev et al., 2011). According to Sobolev et al. (2011), the most likely explanation for the origin of the enrichment in ⁸⁷Sr/⁸⁶Sr of the Mauna Loa ultra-depleted melts involves incorporation in the mantle source of recycled ultra-

depleted oceanic crust that was heavily altered by seawater. Because of low Sr contents in such rocks (from few ppm to a few tens of ppm (Sobolev and Shimizu, 1993; Benoit et al., 1996, 1999; Boschi et al., 2008; Sobolev et al., 2011), their original Sr isotope ratio was completely overprinted by that of seawater without any effects on the Nd isotope ratios because of negligible Nd contents in the seawater (Van De Flierdt et al., 2016). In such a model, the mobile components of seawater alteration (e.g. Cl, K, Rb, Ba, and Pb) have likely been removed during subduction (Tatsumi and Kogiso, 2003). However, the Sr isotopic signature of seawater, as well as the most incompatible elements of recycled oceanic crust, were retained in the pyroxene of SiO₂-saturated olivine-free eclogite or/and of dehydrated serpentinite in the mantle source. This explanation can be adapted to the similarly exotic component of Song Da low-Ti melts, specifying that the primary source of exotic Sr was dehydrated serpentinite. This clarification is required because the olivine composition of these rocks does not support a significant contribution of recycled eclogite in their source (Fig. 10). To test
<figure><img src="image_12.png" /><figcaption>**Fig. 10.** Plots of olivine compositions from Song Da zone (data from Kazzy et al., 2024). Fields of pyroxenite and peridotite are from Sobolev et al. (2007) and data for Archean komatiites from Sobolev et al. (2016, 2019) and from Asafov et al. (2018). The field of eclogite is from Le Roux et al. (2011). Song Da LTi = Song Da low-Ti. Song Da HTi = Song Da high-Ti.</figcaption></figure>

the feasibility of this hypothesis, we estimated the composition of the mantle source of the Song Da low-Ti parental melt for different degrees of melting of depleted harzburgite assemblage (see supplementary Tables 6 and 7) and compared the results (Fig. 11) with the most refractory serpentinites of the Vema fractures zone, Central Atlantic (Boschi et al., 2013). Seawater almost entirely altered these serpentinites, yielding an ⁸⁷Sr/⁸⁶Sr ratio of >0.7083. As is seen in Fig. 11a, the estimated composition of the mantle source of the Song Da low-Ti melts at the degree of melting of 20 ± 5 % matches reasonably well the fluid immobile element compositions of serpentinites. Fluid-mobile elements Rb, Th, U, (and Ba, K, Pb, not shown) of these serpentinites should be extracted due to dehydration reaction during subduction. Strontium is not mobile in these rocks as it is stored in the relicts of clinopyroxene and/or in amphibole (Boschi et al., 2013), and its contents thus significantly correlate with Pr (p-value = 0.008, Fig. 11b) and does not look anomalous compared to adjacent immobile elements (Fig. 11a). Therefore, we propose that radiogenic Sr in such rocks will not be lost during dehydration in a "subduction factory" and could finally be stored in high-temperature Ca-poor pyroxene in dehydrated harzburgite after their processing in a subduction zone.

If the proposed scenario for the origin of the Song Da low-Ti parental melt is valid, its Sr isotope composition reflects that of seawater-altered source serpentinite. In the Earth's history, the only possible period before the eruption of the Emeishan flood basalt, when the seawater ⁸⁷Sr/⁸⁶Sr ratio was within the error of the mean UDM or higher, is between 300 and 550 Ma (Fig. 12). This estimate provides a direct time stamp for the origin of the source of the Emeishan mantle plume.

The proposed source of low-Ti Song Da melts is highly refractory harzburgite, which requires significantly higher temperatures to melt than fertile peridotite. The partial melts of such a source must be ultra-depleted in all incompatible trace elements, including Sr. This means

that such harzburgites could be common components of mantle plumes but utterly invisible in the presence of more fusible fertile peridotite or recycled pyroxenites. The conditions making this component visible in the Song Da region are the high potential temperature (1590 °C) of the source, elevated water content (0.70 wt%) of the parental melt (Kazzy et al., 2024) and the absence of other fertile components in the mantle source.

### 5.6. The origin of the chemical and isotope heterogeneity of the ELIP mantle source

Significant differences in the Nd and Sr isotope compositions of the studied Low-Ti and High-Ti Song Da picrites suggests their origin from a heterogenous mantle source. Three different hypotheses have been proposed to explain the origin of the heterogeneity of the mantle source of the Emeishan lavas: 1) the presence of >2 Ga old recycled oceanic crust and sediments in the source (Ren et al., 2017; Zhang et al., 2021; Liu et al., 2022); 2) interaction between a mantle plume and a subducted oceanic slab during the Neoproterozoic (~860–760 Ma) on the western margin of the Yangtze craton that entrained oceanic crust and marine sediments into the mantle (Wang et al., 2022; Yu et al., 2024); 3) interaction between a mantle plume and the 300–500 Ma old Paleozoethan oceanic slab subducted eastward during the late Carboniferous–Early Triassic entraining relatively young marine sediments (Bai et al., 2022; Lin et al., 2023).

The isotope and trace element data obtained in this study directly support the third hypothesis, as it explains the relatively young ages (between 300 and 550 Ma, Fig. 12) of the recycled “ghost seawater” component of low-Ti melts. We propose that the recycled seawater-altered material (partly dehydrated former serpentinite) of the Paleozoethan oceanic slab (300–500 Ma old) can be entrained together
<figure><img src="image_13.png" /><figcaption>**Fig. 11.** Comparison of predicted source composition of the primary melt of the Song Da low-Ti suite with the composition of most refractory serpentinites of the Vema fracture zone, Central Atlantic (Boschi et al., 2013). a -The blue field indicates the composition of serpentinite (Supplementary Table 6); the colored solid lines are calculated source compositions of the primary melt of the Song Da low-Ti suite for melt fractions 0.15–0.25 during modal batch melting (Supplementary Table 7). b-c – composition of serpentinites (Supplementary Table 6).</figcaption></figure>

with the rest of water from the mantle transition zone (MTZ) if this slab stagnated there or the plume picked up a completely dehydrated former serpentinite at the core-mantle boundary, and excess water was entrained from the MTZ (Kazzy et al., 2024). In the latter case, this requires a rather fast Tethyan crust recycling rate of more than 2.3 cm/ year because of the relatively small difference in the ages of ELIP (260 Ma) and Paleo-Tethyan slab (300–500 Ma) and the considerable distance to travel: rate > 2*2800 km/(500–260) Ma >2.3 cm/a. Also, we cannot exclude the presence of other components in the plume source or entrainment by the plume in the subduction-modified lithosphere, which yields continental crust-like flavors in low-Ti melts and mantle sources of the high-Ti melts rich in a Pb-Sr-Na component. However, due to their Sr and Nd isotopic compositions, these components could be restricted in the age.

**6. Concluding remarks**

1. The present study and the results of Kazzy et al. (2024) on olivine-hosted melt inclusions from the Song Da ultramafic lavas provide new information on the concentrations of mobile trace elements and volatile components and Sr isotope compositions of parental melts, which are not attainable from bulk-rock analysis. The results suggest a significant difference between the source compositions of the low-Ti and high-Ti primary melts of the Song Da zone.

2. The Song Da high-Ti parental melt was silica-undersaturated and likely originally contained 2.7 ± 1.2 wt% CO₂ and 0.6 ± 0.1 wt% H₂O, but lost most of its volatiles by degassing. Its trace element enrichment is attributed to a low degree of melting (less than 9 %) of a peridotite source contaminated by fluids or silica-undersaturated

recycled components enriched in Pb, Sr, and Na. The trace element concentrations and Sr and Nd isotope compositions of the high-Ti melts suggest a predominantly PREMA-type mantle source.

3. The low-Ti komatiite-type melts contained excess of H₂O (H₂O/Ce of up to 5500), similarly to some Archean komatiites, and reveal mixing of at least two components: (1) one with low ⁸⁷Sr/⁸⁶Sr (down to 0.7040), Nb/U, and Ce/Pb and relatively enriched in La, Th, U, Pb and Sr, and (2) the other with high ⁸⁷Sr/⁸⁶Sr (up to 0.7080), Nb/U and Ce/Pb and strongly depleted in the most incompatible elements Ba, Rb, Nb, K, U, Th, La and Sr. We suggest that component 1 reflects contamination of component 2 with lower continental crust or other subduction-related constituents, while component 2 corresponds to a 15-25 % partial melt of seawater-altered recycled peridotite from oceanic slab 300 to 500 Ma in age. The latter component can be recognized only in the melts exceptionally depleted in highly incompatible elements.

4. The data from this study and that of Kazzy et al. (2024) suggest that a mantle plume with a maximum potential temperature (Tp) of ~1600 °C produced the Emeishan Large Igneous Province at ~260 Ma. Because of its high temperature, this plume passed through the hydrated MTZ in a partially molten state which allowed it to entrain in its hottest part refractory harzburgites (former seawater-altered serpentinites) with elevated H₂O coming from subducted 300–500 Ma Paleo-Tethyan oceanic crust stagnated in the MTZ. An alternative source of dehydrated serpentinites of the recycled Paleo-Tethyan slab in the Emeishan mantle plume could be a core-mantle boundary if this slab had been subducted there less than 240 Ma ago (the difference between the maximum source age of 500 Ma and emplacement age of 260 Ma) before the initiation of this plume. The
<figure><img src="image_14.png" /><figcaption>**Fig. 12.** $^{87}$Sr/$^{86}$Sr in the most radiogenic melt inclusions from Song Da compared to the seawater $^{87}$Sr/$^{86}$Sr. Plot modified after Sobolev et al. (2011) with seawater $^{87}$Sr/$^{86}$Sr ratios from Veizer et al. (1999). The dashed line represents the average value of the 2 most radiogenic melt inclusions and the black line corresponds to the upper and lower limits of two standard errors of the average. The green star represents the seawater $^{87}$Sr/$^{86}$Sr ratio at the age of emplacement of the Emeishan large igneous province (~260 Ma), and the blue field represents the possible age of seawater contamination that could cause the enrichment in radiogenic Sr. The red arrows represent the ages of subducted oceanic plates proposed by several authors (see text), which may have contributed to the source of the ELIP. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)</figcaption></figure>

low-Ti komatiite-type melts of the Song Da province came from the hottest part of the Emeishan mantle plume and inherited the most refractory components.

5. The Song Da low-Ti picrites are unique in their extremely refractory and depleted composition of the ELIP and worldwide, with the only close Phanerozoic analogue being the Gorgona komatiites and picrites (Hanski et al., 2004). The mantle source of such melts has to be a refractory harzburgite. To melt this type of material, an elevated temperature and/or elevated H₂O content are needed. Without these conditions, such a material can stay unmelted and thus invisible in the resulting magmas. Moreover, even if these conditions are fulfilled, the melts from this component will be significantly or entirely overprinted by melts from more fertile mantle components. We thus suggest that refractory harzburgite originated from recycled dehydrated serpentinite could be a significant component of the ELIP mantle plume, but the results of its melting are visible only in a particular region of the plume where there are no other mantle components and temperature and H₂O contents are high enough for its melting.

6. The high-Ti Song Da melts came from the coldest part of the Emeishan mantle plume (Tp ~1450 °C) and reflect a PREMA-type mantle source enriched in Na, Pb and Sr.

### CRediT authorship contribution statement

**Charbel Kazzy:** Writing – review & editing, Writing – original draft, Visualization, Validation, Investigation, Data curation. **Alexander V. Sobolev:** Writing – review & editing, Validation, Supervision, Resources, Project administration, Investigation, Funding acquisition, Formal analysis, Data curation, Conceptualization. **Adrien Vezinet:**

Writing – review & editing, Validation, Methodology, Investigation, Data curation. **Valentina G. Batanova:** Writing – review & editing, Validation, Methodology, Investigation, Data curation. **Igor S. Puchtel:** Writing – review & editing, Validation, Resources, Methodology, Investigation, Data curation. **Evgeny V. Asafov:** Resources, Investigation. **Eero Hanski:** Writing – review & editing, Resources. **Andrey E. Izokh:** Resources. **Leonid V. Danyushevsky:** Software, Methodology. **Vũ Hoàng Ly:** Resources. **Can Pham-Ngoc:** Resources. **Tran Tuan Anh:** Resources.

### 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.

### Data availability

Data will be made available on request.

### Acknowledgments

This project received funding from the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation program (grant agreement No 856555). ISP acknowledges support from the United States National Science Foundation Petrology and Geochemistry grant EAR-2220936. We thank GeoThermoChronology Platform, ISTerre, for grinding, cleaning, sieving, and separating olivines. We thank N. Arndt for very useful comments on the manuscript, Martine Lanson and Sylvain Campillo for technical support in powder preparation, Julien Leger for the assistance in olivine and melt inclusions laser ablation ICP-MS analysis and Alexander Chugunov for the help with olivine quenching. We thank Dr. S. Aulbach for efficient editorial handling and valuable advices, Dr. J. Koornneef and anonymous reviewer for their feedback and helpful comments on the first version of the manuscript.

### Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.chemgeo.2024.122564.

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