# Deciphering the origin of a basanite-alkali basalt-tholeiite suite using Zn isotopes

Yangtao Zhu, Yongsheng Liu, Rong Xu, Frédéric Moynier, Ming Li, Haihong

Chen

► To cite this version:

Yangtao Zhu, Yongsheng Liu, Rong Xu, Frédéric Moynier, Ming Li, et al., Deciphering the origin of a basanite-alkali basalt-tholeiite suite using Zn isotopes. *Chemical Geology*, 2021, 585, pp.120585. 10.1016/j.chemgeo.2021.120585. hal-03840848

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Deciphering the origin of a basanite-alkali basalt-tholeiite
suite using Zn isotopes

Yangtao Zhu¹, Yongsheng Liu¹*, Rong Xu¹, Frédéric Moynier¹,², Ming Li¹, Haihong

Chen¹

5

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

7 China University of Geosciences, Wuhan 430074, China

8 ² Institut de Physique du Globe de Paris, Université de Paris, CNRS UMR 7154, 75238 Paris

9 Cedex 05, France

10

11 *Corresponding author: yshliu@cug.edu.cn or yshliu@hotmail.com (Yongsheng Liu).

12

13 Abstract

14
Basanite-alkali basalt-tholeiite suites have unique chemical and isotopic
15
signatures associated with recycled crustal materials. However, the relative
16
contribution of recycled carbonates and silicates is unclear owing to the inability of
17
traditional isotopic approaches (e.g., Sr, Nd, Pb, and O isotopes) to fully identify trace
18
amounts of recycled crustal materials. On the other hand, the identification of the
19
types of recycled components would help decipher origin of these magmatic suites.
20
Since the zinc isotopic ratios (reported as the per mil deviation of the ⁶⁶Zn/⁶⁴Zn ratio
21
from the JMC-Lyon standard, δ⁶⁶Zn) are distinct between marine carbonates and
22
silicate reservoirs (e.g., mantle, oceanic mafic crust, and siliciclastic sediments), they
could be used to identify the different source endmembers in magmatic rocks. In this
work, we used the Zn and Sr-Nd isotopic ratios of Cenozoic continental basalts from
the Central North China Craton (also known as Trans-North China Orogen, TNCO) to
identify the nature of the recycled materials present in the mantle source of these
basanite-alkali basalt-tholeiite suite. We found that, basanites and alkali basalts have
high δ⁶⁶Zn values (ranging from 0.32‰ to 0.46‰), while tholeiites have low δ⁶⁶Zn
values (0.28 ± 0.04‰, 2sd) similar to mid-oceanic ridge basalts (MORB) (δ⁶⁶Zn =
0.27 ± 0.05‰, 2sd). Given the limited extent of Zn isotopic fractionation (< 0.1‰)
during crystallization and <20% partial melting, the elevated δ⁶⁶Zn values observed in
the basanites and alkali basalts reflect the involvement of recycled marine carbonates
(which have an average δ⁶⁶Zn of ~0.91‰) in the mantle sources. Moreover, the δ⁶⁶Zn
values of these basalts show clear correlations with carbonate metasomatism indexes
(such as, Ti/Eu and Zr/Hf ratios). Mixing models for δ⁶⁶Zn values vs. ⁸⁷Sr/⁸⁶Sr ratios
and εNd(t) values demonstrate that the mantle sources of basanites, alkali basalts, and
tholeiites were metasomatized by different proportions of carbonates and siliceous
sediments. Furthermore, both isotopic and geochemical compositions exhibit
symmetrical variations from the central zone towards the south and the north along
the TNCO, that is, δ⁶⁶Zn values and ¹⁴³Nd/¹⁴⁴Nd and Ce/Pb ratios decrease, while
SiO₂ content and ⁸⁷Sr/⁸⁶Sr ratios increase. We suggest that more carbonates sediments
and fewer siliceous sediments are released from the subducted slab, with an increase
in the depth of subduction. We further propose that this process is one of the most
important causes of compositional variations in the basanite-alkali basalt-tholeiite
# 1. Introduction

Basanite-alkali basalt-tholeiite suites are widely distributed in oceans and continents and exhibit a transitional chemical compositional variation, from basanite through alkali basalt and finally to tholeiite, with a gradual increase in silica and decrease in alkali content (Herzberg, 2011; Hofmann, 1997). There has been debate over the origin of this compositional variation as a foundation to study the composition and evolution of the Earth's mantle. For instance, the trace element compositional results from previous studies indicate that an increase in the degree of melting of garnet peridotite or fractional crystallization promotes the transition from basanite through alkali olivine basalt to tholeiite (Caroff et al., 1997; Frey et al., 1978; O'Hara and Yoder, 1967). However, experiments have shown that melting of volatile-free peridotites cannot produce the complete range of primary compositions observed in natural alkali lavas (Kogiso et al., 2003), whereas, melting of carbonated peridotites/pyroxenes/eclogites can form highly silica-deficient melts similar to natural nephelinite, basanite, and alkali basalts (Dasgupta et al., 2007; Gerbode and Dasgupta, 2010; Gudfinnsson and Presnall, 2005; Gupta and Green, 1988). Isotopic studies (e.g., Sr, Nd, Pb, and Hf isotopes) of basanite-alkali basalt-tholeiite suites usually suggest heterogeneous and hybrid mantle sources (Chauvel et al., 2007; Hofmann, 1997; Jackson and Dasgupta, 2008); recycled oceanic crust, with or without sediments, is often regarded as a source component for alkaline lavas to explain their
trace element and isotopic signatures (Hofmann, 1997). Previous radiogenic isotopic
systems (e.g., the Sr-Nd-Pb-Hf isotopes) fail to clearly distinguish between the
carbonate and silicious endmembers (e.g., the Sr-Nd isotopes of carbonates and
siliciclastic sediments are similar; Ben othman et al. (1989)), thus creating an
ambiguity regarding the relative contributions of carbonates versus silicates (mantle,
siliceous sediments, oceanic crust). On the other hand, Zn isotopic ratios (expressed
as δ⁶⁶Zn, the per mil deviation of the ⁶⁶Zn/⁶⁴Zn ratio from the JMC-Lyon standard) are
significantly different between silicates (δ⁶⁶Zn = ~0.16–0.3‰) (Bentahila et al., 2008;
Chen et al., 2013; Huang et al., 2018; Inglis et al., 2017; Moynier et al., 2017; Pons et
al., 2016; Pons et al., 2011; Sossi et al., 2018; Wang et al., 2017) and marine
carbonates (up to 1.61 ‰; Pichat et al. (2003), Kunzmann et al. (2013), Liu et al.
(2017), Sweere et al. (2018), Wang et al. (2018a) and Lv et al. (2018)) and therefore
have great potential for tracing the nature of recycled crustal materials (in particular,
carbonates) in the mantle. A combination of radiogenic isotopes with stable Zn
isotopes can thus be used to decipher the origin of compositional variation in the
basanite-alkali basalt-tholeiite suite.

Cenozoic basalts from the Trans-North China Orogen (TNCO) encompass a
broad range of compositions, from basanite through alkali basalt to tholeiite (Tang et
al., 2006; Xu et al., 2017), and are distributed vertically along a subduction zone (the
subduction of the Paleo-Asian oceanic slab) (Windley et al., 2010) (Fig. 1b, c).
Previous studies found that the geochemical compositions of the TNCO basalts vary
symmetrically from the central zone of TNCO towards the south and north, that is, the
¹⁴³Nd/¹⁴⁴Nd and Ce/Pb ratios decreased, while the SiO₂ contents and ⁸⁷Sr/⁸⁶Sr ratios increased (Xu et al., 2017). Xu et al. (2017) suggested that the formation of the TNCO basanite-alkali basalt-tholeiite suite is likely related to recycled oceanic crustal components being transported by southward subduction of the Paleo-Asian oceanic slab and/or the northward subduction of the Tethyan oceanic slab. In particular, recycled carbonates have been found to exist extensively beneath the northern North China Craton, closely related to the subduction of the Paleo-Asian oceanic slab (Chen et al., 2017; Chen et al., 2016a; Liu et al., 2015b; Wang et al., 2016). Some of the recycled carbonates possibly transferred from the subduction zone into the mantle wedge (Chen et al., 2016a; Liu et al., 2015b), and the residual carbonates got carried into the deep mantle. As for the latter, carbonate metasomatism is widely recorded in the xenoliths from the lithospheric mantle of the Trans-North China Craton (Chen et al., 2017; Wang et al., 2016). The Cenozoic basalts from the TNCO thus provided a window for studying the origin of the basanite-alkali basalt-tholeiite suite.

In recent studies, zinc isotopic compositions have been used to trace recycled carbonates (Jin et al., 2020; Liu et al., 2016; Wang and Liu, 2021; Wang et al., 2018b). Igneous processes, including fractional crystallization (Chen et al., 2013; Huang et al., 2018) and <30% partial melting (Doucet et al., 2020; Doucet et al., 2016; Sossi et al., 2018; Wang et al., 2017) produce very limited isotopic fractionation of Zn (< 0.1‰). Therefore, the Zn isotopic compositions in basalts generally reflect the composition of the mantle source within 0.1‰. Among various geochemical reservoirs, only marine carbonates are distinct and have higher δ⁶⁶Zn values (up to 1.61‰) (Kunzmann et al.,
111 2013; Liu et al., 2017; Lv et al., 2018; Pichat et al., 2003; Sweere et al., 2018; Wang
112 et al., 2018a) compared to mid-oceanic ridge basalts (MORB) (0.27 ± 0.05‰, 2sd)
113 (Huang et al., 2018; Wang et al., 2017). Zinc isotopes are well suited to trace the
114 presence of marine carbonates recycled in the mantle source of erupted lavas (Jin et
115 al., 2020; Liu et al., 2016; Wang and Liu, 2021; Wang et al., 2018b). Furthermore, a
116 recent study has used zinc isotopes to show that carbonate can be transported to
117 mantle depths in the form of carbon-bearing eclogite (Beunon et al., 2020).

118 In this research, we present the Zn isotopic compositions of a series of Cenozoic
119 basalts distributed along the N-S direction of the TNCO (Fig. 1c), to identify whether
120 carbonates are present in the source of these basalts. We aim to establish the origin of
121 the basanite-alkali basalt-tholeiite suite by combining the Zn isotopic data with
122 whole-rock chemical and Sr-Nd isotopic compositions.

## 2. Geological settings and samples

123 The North China Craton is one of the oldest Archean cratons in the world, with
124 crustal remnants ≥ 3.8 Ga (Liu et al., 1992a). Based on the lithology, geochemistry,
125 geochronology, and tectonic evolution, the North China Craton can be divided into
126 Western and Eastern Blocks, and the intervening TNCO (Fig. 1b) formed by the
127 assembly of the former during the Paleoproterozoic (~1.85 Ga) (Zhao et al., 2001).
128 The TNCO underwent multiple subduction events including (1) the northward
129 subduction of the Paleozoic Tethyan oceanic slab, (2) the southward subduction of the
130 Late Permian Paleo-Asian oceanic slab, and (3) the westward subduction of the
131 Pacific oceanic slab, since the Cretaceous (Windley et al., 2010).
132
The North China Craton experienced widespread tectono-thermal reactivation during the Mesozoic and the Cenozoic, as indicated by extensive Mesozoic granitic and mafic magmatism, and Cenozoic alkali basalt volcanism (Wu et al., 2005). These events were accompanied by replacement of the old, cold, thick and depleted “cratonic” lithospheric mantle by young, hot, thin and fertile “oceanic” lithospheric mantle (Gao et al., 2002). According to seismic tomographic imaging, the Eastern Block has a thin crust (~30 km), and the underlying lithosphere is inferred to be ~80 km thick (Chen, 2009). In contrast, the seismic waveform imaging for TNCO shows a thick crust (> 40 km), with the underlying lithosphere being 90–120 km thick (Chen, 2009).

The samples in this study were collected from seven basaltic fields along the TNCO: Fengzhen (7–14 Ma), Yangyuan (30–35 Ma), Datong (< 1 Ma), Fanshi (40–38 Ma), Pingding (7.9–7.3 Ma), Xiyang (7.9–7.3 Ma), and Zuoquan (5.6 Ma) (Liu et al., 1992b; Tang et al., 2006) (Fig. 1c). These samples have been well characterized for petrology, major-trace element, and Sr-Nd isotope geochemistry (Xu et al., 2017). The results from these studies show that, these samples span a compositional range from basanite through alkali basalt to tholeiite, with SiO₂ contents ranging from 45.7 to 53.2 wt% and the total alkali contents (K₂O and Na₂O) ranging from 3.55 to 7.55 wt%. The dominant phenocrysts in these basalts are olivine, along with minor clinopyroxenes and plagioclase. The groundmass is variable and mainly consists of olivine, clinopyroxene, plagioclase, and glass. All TNCO Cenozoic basalts are characterized by ocean island basalt (OIB)-like trace element patterns,
enriched in light rare earth elements (LREE) and large ion lithophile elements (LILE)
(Xu et al., 2017). Similar to the Hawaiian OIBs (~66–68) (Humayun et al., 2004), the
TNCO Cenozoic basalts have high Fe/Mn ratios (62.9 to 78.9) (Xu et al., 2017).
Fractionation of clinopyroxene could not produce such high Fe/Mn ratios, because
this would require fraction unrealistically high clinopyroxene crystallization
(>30–70%) to match the observed Fe/Mn ratios of the TNCO Cenozoic basalts (Liu et
al., 2008; Xu et al., 2017). This suggests that the high Fe/Mn ratios of TNCO
Cenozoic basalts are similar to partial melts of pyroxenite but significantly higher
than partial melts of peridotite at a given MgO or MnO content (Liu et al., 2008; Xu
et al., 2017). In addition, since olivine and orthopyroxene have D<sub>Zn/Fe</sub> ≈ 1, whereas
clinopyroxene and garnet have D<sub>Zn/Fe</sub> << 1 (Le Roux et al., 2010), the high Zn/Fe*10<sup>4</sup>
(11.9 to 16.5) ratios indicate an eclogite/pyroxenite-rich mantle source as well (Le
Roux et al., 2010; Xu et al., 2017). Their <sup>87</sup>Sr/<sup>86</sup>Sr ratios varying from 0.70332 to
0.70481 and ε<sub>Nd</sub>(t) values varying from 0.35 to 6.51, implying a hybrid source (Xu et
al., 2017).

3. Analytical methods

We conducted Zn isotopic measurements at the State Key Laboratory of
Geological Processes and Mineral Resources, China University of Geosciences
(Wuhan). Approximately 10–30 mg (containing ~1 µg Zn) of the sample powder was
mixed with 1 mL concentrated HF and 1 mL concentrated HNO₃ and was dissolved
on a hotplate at 120 °C for 24 h. Later, the solutions were dried and an additional 1
mL of concentrated HNO₃ was added and evaporated to ensure the complete removal
177 of HF. This was followed by the addition of aqua regia (1.5 mL concentrated HCl + 178 0.5 mL concentrated HNO₃) to the sample and the beaker was kept on a hotplate at 179 120 °C for 48 h, to digest the residues, including fluorides. After complete digestion, 180 an additional 1 mL of concentrated HCl was added and evaporated to dryness. Finally, 181 the residual sample was dissolved in 1 mL of 8 mol/L HCl + 0.001% H₂O₂ and 182 prepared for chemical purification.

183 Chemical purification, as described by Zhu et al. (2019) was conducted as per 184 protocols adapted from Maréchal et al. (1999). The sample solution was loaded onto 2 185 mL pre-cleaned AG-MP-1M resin. After the elution of matrix elements (e.g., K, Na, 186 Mg, Cu, Fe, etc.) in 8 mol/L HCl + 0.001% H₂O₂ and 0.5 mol/L HCl, Zn was rinsed 187 by 10 mL 0.5 mol/L HNO₃. The final Zn cut elution was dried and dissolved in 2% 188 (m/m) HNO₃ for analysis.

189 Zinc isotopic ratios were measured using a Nu Plasma 1700 MC-ICP-MS 190 instrument at the State Key Laboratory of Geological Processes and Mineral 191 Resources, China University of Geosciences (Wuhan). Instrumental isotopic 192 fractionation was corrected by the sample-standard bracketing method and by adding 193 Cu as an external standard (Maréchal et al., 1999). The bracketing standard is NIST 194 SRM 683, which is fractionated by 0.12 ± 0.04‰ compared to JMC Lyon (Chen et al., 195 2016b). The Zn concentrations of the samples were checked and adjusted to match the 196 NIST SRM 683 solution within 10% error. Zinc solutions were introduced to the Ar 197 plasma using a Glass Expansion Nebulizer with an uptake rate of ~100 µL min⁻¹. 198 Intensities of a total of seven signals (⁶⁰Ni, ⁶³Cu, ⁶⁴Zn, ⁶⁵Cu, ⁶⁶Zn, ⁶⁷Zn, and ⁶⁸Zn)
were collected using Faraday cups at positions L6, L4, L3, L2, Ax(C), H2, and H4, respectively; all Faraday amplifiers used 10¹¹ Ω resistors. Among these, ⁶⁰Ni was used to monitor the interference of ⁶⁴Ni on ⁶⁴Zn. ⁶⁴Ni interference was negligible for all samples (⁶⁰Ni/⁶⁴Zn < 5×10⁻⁴). The samples were analyzed in low-resolution mode and a 300 ppb Zn solution returned ~4 V for ⁶⁴Zn. Each analysis consisted of an integration of 25 cycles, 8 s each. Finally, zinc isotope ratios were reported in standard δ-notation in per mil relative to JMC Lyon 3-0749L: δ¹⁸Zn(‰) = [((¹⁸Zn/⁶⁴Zn)sample/((¹⁸Zn/⁶⁴Zn)JMC Lyon 3-0749L - 1)] × 1000, calculated using X = 66 and 68.

International standards were used and the samples underwent repeated analysis to test the accuracy of the method. Nine replicates of these samples (different batches of dissolution from aliquots of a homogeneous powder, chemical purification, and mass spectrometry measurements) showed identical δ⁶⁶Zn values within an uncertainty of 0.05‰ (2sd) (Table 1). Details of the analytical procedures and standard samples have been reported by Zhu et al. (2019). The results of standard samples (BHVO-2: δ⁶⁶Zn = 0.31 ± 0.04‰ (2sd, n = 47); BCR: δ⁶⁶Zn = 0.26 ± 0.03‰ (2sd, n = 24); BIR-1: δ⁶⁶Zn = 0.24 ± 0.04‰ (2sd, n = 17); AGV-2: δ⁶⁶Zn = 0.28 ± 0.04‰ (2sd, n = 19); W-2a: δ⁶⁶Zn = 0.22 ± 0.03‰ (2sd, n = 7); Table S1) are consistent with recommended values within the margin of error (0.05‰) (Moynier et al., 2017).

## 4. Results

Zinc isotopic compositions of the studied basalts, along with their major and trace elements and Sr-Nd isotopes reported by Xu et al. (2017), have been listed in Table 1. All samples plot on a mass fractionation line (δ⁶⁶Zn vs. δ⁶⁸Zn) (Fig. S1) with
a slope of 1.9923 ($R^2$ = 0.9916; $n$ = 42), indicating that the data is unaffected by isobaric interference. Since the Zn isotopic data follow mass-dependency, all isotopic data will be discussed in terms of $\delta^{66}$Zn. The tholeiites from Fengzhen and Datong have homogeneous $\delta^{66}$Zn values (0.28 ± 0.04‰, 2sd), covering the range of the MORB ($\delta^{66}$Zn = 0.27 ± 0.05‰, 2sd) (Huang et al., 2018; Wang et al., 2017). Alkali basalts and basanites have higher $\delta^{66}$Zn values (from 0.32 to 0.46‰) than MORB, similar to the previously analyzed alkali basalts from eastern China (Jin et al., 2020; Liu et al., 2016; Wang et al., 2018b). The $\delta^{66}$Zn values of all the basalts correlate negatively with Ti/Eu ratios and $SiO_2$ contents (Fig. 4a and c), and positively with the Zr/Hf ratio (Fig. 4b)

These basalts are characterized by geochemical compositions varying with latitude (Xu et al., 2017). The compositions show symmetrical variations from the central zone towards the south and north along the TNCO, that is, $SiO_2$ contents and $^{87}$Sr/$^{86}$Sr ratios (Fig. 7b, c) increase, whereas the $\delta^{66}$Zn values, and $^{143}$Nd/$^{144}$Nd, Sm/Yb, and Ce/Pb ratios decrease (Fig. 7a, d, e, and f).

# 5. Discussion

## 5.1 Origin of the high $\delta^{66}$Zn values of basanites and alkali basalts

Studies on non-metasomatized peridotites, komatiites, and picrites give an estimate for the zinc isotopic compositions of the Bulk Silicate Earth (BSE) between 0.16‰ and 0.20‰ (Doucet et al., 2020; Doucet et al., 2016; McCoy-West et al., 2018; Sossi et al., 2018; Wang et al., 2017). Compared to BSE and MORB (0.27 ± 0.05‰, 2sd) (Huang et al., 2018; Wang et al., 2017), the Cenozoic TNCO basanites and alkali
basalts have abnormally high δ⁶⁶Zn values (0.32‰ to 0.46‰). Magmatic processes and hybridization in the mantle source could lead to the fractionation of Zn isotopes.

Three magmatic processes could possibly affect the Zn isotopic compositions in basalts: crustal contamination, fractional crystallization, and partial melting. Tang et al. (2006) and Xu et al. (2017) showed that the TNCO basalts had no obvious imprints of continental crust in the primitive mantle-normalized trace element patterns (e.g., depletion of Nb and Ta) (Rudnick and Gao, 2003), and thus experienced negligible crustal contamination during eruption. Xu et al. (2017) suggested that the CaO/Al₂O₃ ratios and CaO and Sc contents of basanites and alkali basalts from the TNCO did not vary with decreasing MgO at MgO > 7.5 wt%, indicating that the basanites and alkali basalts could have only experienced olivine fractionation at MgO > 7.5 wt%.

However, the CaO/Al₂O₃ ratios and CaO and Sc contents clearly decreased with MgO < 7.5 wt%, suggesting that clinopyroxenes were involved in the crystallizing phase (Xu et al., 2017). The absence of significant Eu anomalies indicates very little plagioclase fractionation (Xu et al., 2017). The δ⁶⁶Zn values show no correlation to MgO, Zn, Ni, and Sc (Fig. 2a, b, c, d). This suggests that the zinc isotopic compositions of the TNCO basalts are not modified by olivine and pyroxene fractionation. In addition, the chromite fractionation has also been proposed to lead to heavy Zn isotopes enrichment in basalts (Yang et al., 2021), but previous studies have shown that there is no fractionation of chromite in TNCO Cenozoic basalts (Tang et al., 2006; Xu et al., 2017). Zinc has a different coordination in basaltic melts compared to typical mantle minerals, leading to potential zinc isotopic fractionation
during partial melting (Huang et al., 2018; Sossi et al., 2018). A recent study proposed that even if they are generated at a very low melting degree (e.g. 0.5%), partial melts from a normal peridotitic mantle (δ⁶⁶Zn = 0.18‰), still have a MORB-like δ⁶⁶Zn (0.25‰) (Wang and Liu, 2021). Furthermore, the fractionation of Zn isotopes induced by 0.5% to 20% partial melting of a single peridotitic source is lower than 0.02‰ (Wang and Liu, 2021). The study of Doucet et al. (2016) suggested that high degrees of melt extraction (>30%) may significantly fractionate Zn isotopes (up to 0.16‰).

However, such high-degree melt extraction, is only occurring during the first half of the Earth's history (Herzberg and Rudnick, 2012), and the degree of partial melting of TNCO Cenozoic basalts is less than 10% (Tang et al., 2006; Xu et al., 2017).

Therefore, Zn isotopic fractionation induced by low degrees of melt extraction (< 20%) is ~0.1‰ at most (Huang et al., 2018; Sossi et al., 2018; Wang et al., 2017), which is supported by the difference between the isotopic composition of the BSE (ranging from 0.16‰ to 0.20‰) (Doucet et al., 2020; McCoy-West et al., 2018; Sossi et al., 2018; Wang et al., 2017) and of MORB (0.27 ± 0.05‰, 2sd) (Huang et al., 2018; Wang et al., 2017). The fractionation at equilibrium of Zn isotopes between carbonate and silicate phases at mantle temperatures (e.g. 1300 °C) is below 0.04‰ (Ducher et al., 2016; Macris et al., 2013), also indicating that Zn isotopes experience limited fractionation during partial melting of carbonated mantle. Therefore, partial melting fails to explain the heavy Zn isotopic composition (up to 0.46‰) of the TNCO basanites and alkali basalts. Since light isotopes diffuse faster than heavy isotopes (Richter et al., 2008), and the basalts have a higher Zn concentration (commonly > 80
287 µg/g⁻¹) than mantle peridotites (~55 µg/g⁻¹) (Le Roux et al., 2010), diffusion-driven
288 kinetic fractionation may also result in elevated δ⁶⁶Zn of basalts (Huang et al., 2010).

289 However, the diffusion of zinc from basaltic melts to ambient peridotites is strongly
290 unsupported by the positive relationship between Zn concentration and δ⁶⁶Zn ratios in
291 TNCO alkali basalts and basanites (Fig. 2b). Thus, the elevated δ⁶⁶Zn values must
292 reflect the hybridization of δ⁶⁶Zn-enriched agents in the mantle source.

293 Subduction can transport large amounts of crustal materials into the depths of the
294 mantle, including various siliceous-sediments, oceanic crust and carbonates. Previous
295 studies have suggested that both various siliceous-sediments and oceanic crust, have
296 δ⁶⁶Zn = ~0.2–0.3‰ (Bentahila et al., 2008; Chen et al., 2013; Huang et al., 2018;
297 Inglis et al., 2017; Moynier et al., 2017; Pons et al., 2016; Pons et al., 2011; Sossi et
298 al., 2018; Wang et al., 2017). Fluids released from the subducted slab could have high
299 δ⁶⁶Zn values (up to 0.75 ± 0.25‰, 2sd) (Pons et al., 2016). However, the δ⁶⁶Zn values
300 of fluids would decrease with the depth of subduction (Pons et al., 2016). As reported
301 by Pons et al. (2016), at the melting depth relevant to the TNCO basalts (~90–120 km)
302 (Xu et al., 2017), fluids released from the subducting slab should have δ⁶⁶Zn values
303 between 0.16 ± 0.06‰ (2sd) and 0.30 ± 0.06‰ (2sd). Since Ba is more mobile in the
304 fluid than La and Th, the absence of positive correlation of Ba/La and Ba/Th ratios
305 with δ⁶⁶Zn values (Fig. 3a, b) indicates that the addition of a relatively high
306 proportion of fluid components does not result in high δ⁶⁶Zn values. Therefore, under
307 the conditions relevant to the source of the TNCO magmas, various siliceous
308 sediments, oceanic crust, and fluids released by the subducted slab are not isotopically
309 heavy.

310 Marine carbonates are thus the most likely candidate for high δ<sup>66</sup>Zn agents (up to
311 1.61 ‰) (Kunzmann et al., 2013; Liu et al., 2017; Lv et al., 2018; Pichat et al., 2003;
312 Sweere et al., 2018; Wang et al., 2018a). Previous works failed to identify carbonate
313 components in the TNCO basanites and alkali basalts sources based on traditional
314 Sr-Nd isotopes and trace elements (Xu et al., 2017). Our Zn isotopic data further
315 constrains the nature of the subducted components by identifying previously
316 unrecognized carbonate components. Combined with elevated partition coefficients of
317 Ti and Zr in carbonate systems relative to silicate systems (Rudnick et al., 1993),
318 metasomatism by carbonatite melts is thought to be distinguishable from silicate melt
319 metasomatism by highly enriched incompatible trace element contents, notably low
320 Ti/Eu ratios and high and Zr/Hf ratios. The carbonate metasomatism can be confirmed
321 by the negative correlation between the Ti/Eu ratio and δ<sup>66</sup>Zn values (Fig. 4a), and the
322 positive correlation between the Zr/Hf ratio and δ<sup>66</sup>Zn values (Fig. 4b). The negative
323 correlation between SiO<sub>2</sub> contents and δ<sup>66</sup>Zn values (Fig. 4c) also provides evidence
324 for the presence of carbonates, since the SiO<sub>2</sub> contents of carbonated silicate melts
325 from carbonated mantle decrease with an increase in dissolved CO<sub>2</sub> in the melt
326 (Dasgupta et al., 2007). All these observations imply that the high δ<sup>66</sup>Zn values
327 (0.32‰ to 0.46‰) of the TNCO alkali basalts and basanites can only be explained by
328 the presence of marine carbonates in their mantle source.

329 **5.2 Recycled sediments-related compositional diversity of mantle-derived melts**

330 The previously proposed model suggested that the subducted altered oceanic
crust (mostly in the form of eclogites) mixed with subducted sediments in the asthenosphere, followed by the reaction of the mixtures with the depleted mantle source, explaining the Sr-Nd isotopic compositions of TNCO Cenozoic basalts (Xu et al., 2017). Several previous studies suggested that lavas ascending through the overlying lithosphere cause the silica-undersaturated basanitic melts to react with the lithospheric mantle, thus resulting in the transition of the Sr-Nd isotopic compositions from basanites into alkali basalts and even tholeiites (Tang et al., 2006; Wang and Liu, 2021; Wang et al., 2018b; Xu et al., 2004). Sm-Nd model ages (Song and Frey, 1989) and Re-Os isotopes (Gao et al., 2002) suggested that the lithospheric mantle beneath the north of TNCO suffered from melting during the Proterozoic, but the basalts in this work is the product of Cenozoic melting. In addition, many lithospheric mantle xenoliths are depleted in LREE, and some enriched lithospheric mantle xenoliths are even depleted in Nb-Ta (Chen et al., 2017). Therefore, this lithospheric mantle source is unlikely to be the mantle source of OIB type enriched basalts (Xu et al., 2017). The reactions between an asthenospheric magma and the lithospheric mantle would lead to a low Nb/La ratio, while the Nb/La ratio of the TNCO Cenozoic basalts (> 0.98; Table.1) is generally higher than the N-MORB (~ 0.9; Hofmann (1988)), therefore, making the mixing of the ancient lithosphere mantle an unlikely source (Liu et al., 2015a). The study of Kuritani et al. (2011) indicated that the EMI-like isotopic signature should be related to an ancient event (>1.0 Ga), but the Sr-Nd isotopes of modern global subducted sediments (GLOSS) are similar to EMII (Plank and Langmuir, 1998). Following the mixing models of Kuritani et al. (2011), Li et al.
(2016) and Xu et al. (2017), we propose that the altered oceanic crust mixed with either recycled ancient subducted sediments (>1.0 Ga) or modern Ca-rich subducted sediments as the enrich agent (Fig. 5). The mixing model of $^{87}$Sr/$^{86}$Sr ratios vs. $^{143}$Nd/$^{144}$Nd ratios suggests that the Sr-Nd isotopic compositions of the TNCO Cenozoic basalts can be explained by recycling of subducted sediments plus altered oceanic crust into the depleted mantle sources (Fig. 5). This mixing model also suggests that altered oceanic crust may be prevalent in the mantle source of the TNCO Cenozoic basalts and that the type and proportion of the subducted sediments control the compositional variation in these basalts. Subducted sediments consist of subordinate carbonates and dominant siliceous sediments (Plank and Langmuir, 1998). However, Sr-Nd isotopes fail to clearly distinguish between the carbonates and silicious endmembers (the Sr-Nd isotopic compositions of basanites, alkali basalts and tholeiites overlap; Fig. 5), thus creating an ambiguity regarding the relative contributions of carbonates versus siliceous sediments.

In section 5.1, we show that the high $\delta^{66}$Zn values (0.32‰ to 0.46‰) of the TNCO alkali basalts and basanites point to the presence of marine carbonates in their mantle source. In order to identify the relative contributions of carbonates and siliceous sediments affecting the mantle source of TNCO basanites, alkali basalts and tholeiites, we chose a mixing model for $\delta^{66}$Zn values versus $^{87}$Sr/$^{86}$Sr ratios and $\varepsilon_{Nd}$ values (Fig. 6a, b proposed by Jin et al. (2020)). This mixing model is particularly well suited to estimate the relative contributions of carbonates, siliceous sediments and MORB-eclogites in the mantle source of erupted lavas, and also reflect that the
375 hybridization of carbonates and siliceous sediments into the depleted asthenospheric
376 mantle can well explain the Zn-Sr-Nd isotopic variations of TNCO Cenozoic basalts
377 (Jin et al., 2020). This Zn-Sr-Nd isotope modeling calculations (Fig. 6a, b) show that
378 the mantle sources of the basanites contained a relatively high proportion of
379 carbonates (carbonates sediments / siliceous sediments = 4:1), while that of the
380 tholeiites contained a relatively high proportion of siliceous sediments (carbonates
381 sediments / siliceous sediments = 1:4). This indicates that the difference between the
382 TNCO basanites and alkali basalts lies in the different proportion of carbonates mixed
383 in the mantle source. The mantle sources of tholeiites are chiefly affected by the melts
384 derived from siliceous sediments. The results obtained by this model are very similar
385 with the Cenozoic continental basalts from eastern China (Jin et al., 2020). Thus, we
386 summarize that the compositional variations from basanites through alkali basalts to
387 tholeiites reflect the decrease of the proportions of carbonates and the increase of the
388 proportions of siliceous sediments in their mantle source.

389
5.3 Origin of the compositional variation in basanite-alkali basalt-tholeiite suite

390 The compositional variation from basanites through alkali basalts to tholeiites
391 widely observed in oceans and continents (e.g., Hawaii, southeastern Australia, and
392 Tubuai [French Polynesia]), has been conventionally attributed to increasing degrees
393 of mantle melting (Caroff et al., 1997; Clague and Frey, 1982; Frey et al., 1978).
394 However, systematic isotopic variations (e.g., 87Sr/86Sr and 143Nd/144Nd ratios) (Fig.
395 7c, d) found in Cenozoic TNCO basalts from a spatially and temporally limited
396 volcanic province indicate multiple sources (Xu et al., 2017). Wang et al. (2018b)
suggests that lavas ascending through the overlying lithosphere cause the silica-undersaturated basanitic melts to react with the lithospheric mantle, resulting in the compositional transition from basanites into alkali basalts and even tholeiites.

Previous studies have indicated that Cenozoic TNCO basalts primarily originated from the asthenospheric mantle, with negligible contributions from the lithospheric mantle (Xu et al., 2017). Based on the analyses of multiple subduction events experienced by TNCO, Xu et al. (2017) suggested that the N-S spatio-temporal variation of these basalts was likely related to the southward subduction of the Paleo-Asian oceanic slab and/or the northward subduction of the Tethyan oceanic slab.

These subduction events not only provided geodynamic driving forces that triggered the instability of the convective mantle but also ultimately led to widespread modification in mantle geochemistry through peridotite-melt interactions. On the one hand, the depth of the subducted slab gradually reduces from the central zone to the north and south of the study area (Fig. 8). Owing to the deep mantle sources of basalts, the mantle sources predominantly contain garnet peridotites, while the basalts are characterized by higher Sm/Yb ratios. The Sm/Yb ratios symmetrically decrease from the central zone towards the south and the north (Fig. 7e), which proves that the depth of the mantle source of the TNCO basalts gradually decreases from the central zone towards the north and south. On the other hand, Xu et al. (2017) suggested that the mantle sources of the TNCO tholeiites and alkali basalts are silica-excess and silica-deficient pyroxenites, respectively, which could have been formed by the reaction between subducted oceanic crust-derived melts and ambient mantle
peridotites, with a high proportion of melts producing silica-excess pyroxenites and a low proportion of melts generating silica-poor pyroxenites. The δ<sup>66</sup>Zn values here show that the compositions of the mantle sources of TNCO basanites and alkali basalts are controlled by the variations in proportion of recycled carbonates. The δ<sup>66</sup>Zn values also vary symmetrically from the central zone towards the south and north (Fig. 7a) and decrease gradually along the basanite-alkali basalt-tholeiite suite (Fig. 4). Therefore, based on the results of Sr-Nd isotopes and stable isotopes of Zn, we focused on making some improvements to the previous model and further explored the origin of the compositional variation in the basanite-alkali basalt-tholeiite suite.

Tholeiites are distributed only on the northern side (e.g., Fengzhen, closer to the subduction zone of the Paleo-Asian oceanic slab) (Fig. 8) and originate from silica-excess pyroxenites (Xu et al., 2017). Since the depth of subducted slab along the mantle of the northern side is shallow (Tang et al., 2006) and the subducted carbonates cannot be melted owing to their high solidus temperatures (Dasgupta and Hirschmann, 2006; Spandler et al., 2010), the mantle on the northern side was mainly affected by melts/fluids derived from siliceous sediments and MORB-eclogite (Fig. 6a and b); thus, a silica-excess pyroxenite mantle source may have formed along the edge of the northern side of the TNCO (Xu et al., 2017). This second-stage pyroxenite featured high SiO₂ contents, ⁸⁷Sr/⁸⁶Sr ratios (Fig. 7b and c), and low ¹⁴³Nd/¹⁴⁴Nd, Sm/Yb, and Ce/Pb ratios (Fig. 7d, e, and f), and MORB-like δ⁶⁶Zn values (0.28 ± 0.04‰, 2sd).

Alkali basalts are widely distributed on the central (Fanshi, Datong, Xiyang, and
Pingding) and southern sides (Zuoquan); however, basanites only exist in the central zone (Fig. 8). As the carbonates cannot melt at the top of the subducted slab at shallow depth, most of them possibly got carried into the deep mantle during subduction (Beunon et al., 2020), and formed a second-stage carbonated pyroxenite with high δ<sup>66</sup>Zn values and Zr/Hf ratios, and low SiO<sub>2</sub> content and Ti/Eu ratios. This model is supported by previous studies which show that the mantle of the central zone (e.g., Fanshi and Datong) experienced carbonate metasomatism (Chen et al., 2017; Wang et al., 2016) and is also consistent with the results of our hybrid model (Fig. 6a and b). The mantle source of the basanites was chiefly influenced by high proportions of carbonates (Fig. 6a and b), as the solidus temperature of carbonates is higher than that of siliceous sediments (Dasgupta and Hirschmann, 2006; Spandler et al., 2010), and basanites are only distributed in the central area of the TNCO, where the depth of the subducted slab is maximum (Fanshi, Datong, Xiyang, and Pingding) (Fig. 8).

Compared to the basanites and alkali basalts from the central zone, the Zn isotopic compositions of the alkali basalts from the southern side (e.g., Zuoquan) are slightly lighter, but still higher than the tholeiites of the northern side which have MORB-like Zn isotopic compositions (Fig. 7a). These Zn isotopic compositions reflect the presence of carbonates in the mantle source of the alkali basalts in the south. The SiO₂ contents of carbonated silicate melts from carbonated mantle decrease with increasing dissolved CO₂ in the melt (Dasgupta et al., 2007). The SiO₂ contents of the alkali basalts in the south are intermediate between those in the central zone and those in the north (Fig. 7b), indicating that the mantle source of the southern
side is an intermediate form of the carbonates and siliceous sediments. Alkali basalts
are the dominant rock type on the south side; the mixing models show that the
proportion of carbonates in the mantle source of alkali basalts is lower than basanites
(Fig. 6a and b). Based on the fact that the solidus temperature of siliceous sediments
is lower than that of carbonates (Dasgupta and Hirschmann, 2006; Spandler et al.,
2010), we suggest that the depth of the mantle source for alkali basalts in the south is
between that of mantle source in the central zone and that in the north.

Since the depth of the subducted slab of the central zone is deeper and the
solidus of the carbonate mantle is lower (Dasgupta et al., 2007), the mantle of the
central zone melts preferentially at deeper depth during asthenosphere upwelling
compared to both the northern and southern sides (Fig. 8). This explains the temporal
distribution of Cenozoic basalts in the TNCO; the eruption ages of these basalts, from
north to south, are roughly divided into a variation tendency, with the intermediate
zone being older than the north and south sides: Fengzhen (7–14 Ma), Yangyuan
(30–35 Ma), Datong (< 1 Ma), Fanshi (40–38 Ma), Xiyang-Pingding (7.9–7.3 Ma),
and Zuoquan (5.6 Ma) (Liu et al., 1992b; Tang et al., 2006). Although our model can
explain the geochemical composition of all TNCO Cenozoic basalts, we do not rule
out the reaction of asthenospheric melts with the lithospheric mantle in a localized
area, such as Datong. Tholeiites also occur in Datong. In addition, there is a
prominent low-velocity anomaly under the Datong (Huang and Zhao, 2006; Lei,
2012), suggesting that these tholeiites may be the products for the reaction of
basanitic melts with the lithospheric mantle (Wang and Liu, 2021). However, the
basalts from Datong are very young (< 1 Ma), and there is no evidence of
low-velocity anomaly under the other areas of TNCO.

## 6. Conclusion

The aim of this study was to systematically investigate the roles of various
recycled crustal materials in the formation of a basanite-alkali basalt-tholeiite suite,
and to further decipher the origin of the compositional variation in the suite.

We reported a comprehensive Zn isotopic dataset of the Cenozoic basanite-alkali basalt-tholeiite suite from TNCO. Owing to the limited Zn isotopic fractionation during magmatic processes (crustal contamination, fractional crystallization, and partial melting) and hybridization of silicates (siliceous sediments, oceanic crust), the high $\delta^{66}\text{Zn}$ values (0.32‰ to 0.46‰) of basanites and alkali basalts reflect the involvement of recycled marine carbonates in their mantle source. Although both basanites and alkali basalts have high $\delta^{66}\text{Zn}$ values, the different evolutionary trends in their $^{87}\text{Sr}/^{86}\text{Sr}$ ratios and $\varepsilon_{\text{Nd(t)}}$ values indicate that the proportion of recycled marine carbonates in the mantle sources of basanites is higher than alkali basalts. The mantle sources of tholeiites are chiefly affected by the melts derived from siliceous sediments. The $\delta^{66}\text{Zn}$ values of the tholeiites match the expected $\delta^{66}\text{Zn}$ values of MORB.

Furthermore, geochemical compositions (e.g., $\delta^{66}\text{Zn}$ values, $^{143}\text{Nd}/^{144}\text{Nd}$ and $^{87}\text{Sr}/^{86}\text{Sr}$ ratios, and $\text{SiO}_2$ contents) of the basanite-alkali basalt-tholeiite suite vary symmetrically with latitude and are vertically distributed along the subduction zone (the northward subduction of the Tethyan oceanic slab and the southward subduction of the Paleo-Asian oceanic slab). We suggest that owing to the different solidus lines
of recycled crustal materials, more carbonates sediments and fewer siliceous sediments are released from the subducted slab, with an increase in the depth of subduction. We conclude that this is one of the most important causes of compositional variations in the basanite-alkali basalt-tholeiite suite.

This study thus provides a fresh perspective to explore the origin of a natural basanite-alkali basalt-tholeiite suite formed under subducted settings.

## 513 Acknowledgments

We thank Dr. Luc Doucet, Dr. Sheng'Ao Liu and two anonymous reviewers for thorough reviews that have greatly improved the quality of the paper and Editor Don Porcelli for efficient editorial handling.

We thank Dr. Sheng'Ao Liu for generous support during the development of the analytical methods of Zn isotopes, and Dr. Fang Huang for providing the NIST SRM 683 solution. This research was co-supported by NSFC (41530211), Key R&D Program of China (2019YFA0708400), and MOST Special Funds of the State Key Laboratory of Geological Processes and Mineral Resources (MSFGPMR01).

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743

Figure captions

744

745

746 **Figure. 1.** (a) Simplified tectonic map of the major units of China, after Zhao et al.

747 (2001). KL=Kunlun orogen; QL=Qilian orogen; QDSL = Qinling-Dabie-Sulu fold

748 belt. (b) Subdivisions of the North China Craton (NCC). Two dashed lines outline the
749 Central Zone (Trans-North China Orogen) that separates the West and East NCC. The
750 blue arrow indicates the direction of the subduction. (c) Distribution of Cenozoic
751 basalts in the Trans-North China Orogen, modified after Tang et al. (2006).

752
753 **Figure. 2.** Variations in δ<sup>66</sup>Zn values, with (a) MgO (wt%), (b) Zn (ppm), (c) Ni (ppm), and (d) Sc (ppm). The data for Kilauea Iki and Hekla are from Chen et al. (2013). The major and trace elements of our sample are from Xu et al. (2017). There is no correlation between δ<sup>66</sup>Zn values and these geochemical indexes. The δ<sup>66</sup>Zn values of MORB is 0.27 ± 0.05‰; 2sd (Huang et al., 2018; Wang et al., 2017).
755
756
757
758

758
759 **Figure. 3.** Variations in δ<sup>66</sup>Zn values, with (a) Ba/La and (b) Ba/Th ratios. The trace element data for our sample are from Xu et al. (2017).
760
761

761
762 **Figure. 4.** Correlations of δ<sup>66</sup>Zn values vs. (a) Ti/Eu ratios, (b) Zr/Hf ratios, and (c) SiO<sub>2</sub> (wt%), for the basalts from TNCO. The major and trace element data are from Xu et al. (2017). The δ<sup>66</sup>Zn values of the basalts of Eastern China are from Wang et al. (2018b).
763
764
765
766

766
767 **Figure. 5.** ¹⁴³Nd/¹⁴⁴Nd ratios versus ⁸⁷Sr/⁸⁶Sr ratios diagrams showing the TNCO basalts together with the modeled isotopic arrays. The blue line corresponds to the recycled oceanic crust mixing array between subducted ancient sediment (~1.5 Ga) and altered oceanic crust (altered N-MORB), with tick marks shown for 2%, 4% and
771 6% sediment in the mixture, parameters for altered oceanic crust (AOC) and sediment
772 used in the mixing calculations are from Rehka¨mper and Hofmann (1997);
773 parameters for the green line are similar, but the sediment end-member is the modern
774 average Ca-rich subducted sediment from Plank and Langmuir (1998). The red and
775 orange line shows the array for the source region composed of 6% previously mixed
776 subducted oceanic crust and depleted mantle peridotite (DM) from Salters and Stracke
777 (2004). The data for Eastern China are from Jin et al. (2020), Wang et al. (2018b) and
778 Li et al. (2017). See Table S2 (Appendix) for parameters and modeling results.
779

780 **Figure. 6.** Zn, Sr, and Nd isotopic compositions of the TNCO Cenozoic basalts, along
781 with those of three end-member components. Zn, Sr and Nd elemental and isotopic
782 compositions for N-MORB are from Sun and McDonough (1989), Workman and Hart
783 (2005), Hofmann (1988), Liu et al. (2016), Wang et al. (2017), Huang et al. (2018)
784 (Zn = 80 ppm, δ<sup>66</sup>Zn = 0.27‰, Sr = 113 ppm, <sup>87</sup>Sr/<sup>86</sup>Sr = 0.7026, Nd = 7.3 ppm, ε<sub>Nd</sub> =
785 9.6). For carbonate end-member, the Zn, Sr, and Nd elemental and isotopic
786 compositions of magnesites were used (Zn = 280 ppm, δ<sup>66</sup>Zn = 0.91‰, Sr = 1.84 ppm,
787 <sup>87</sup>Sr/<sup>86</sup>Sr = 0.7164, Nd = 0.10 ppm, ε<sub>Nd</sub> = -9.7, (Demény et al., 2004; Huang and Xiao,
788 2016; Pichat et al., 2003). The siliceous sediments are represented by GLOSS, and
789 their Zn-Sr-Nd contents and Sr-Nd isotopic compositions are from Plank and
790 Langmuir (1998) (Zn = 86.4 ppm, Sr = 327 ppm, <sup>87</sup>Sr/<sup>86</sup>Sr = 0.7173, Nd = 27 ppm,
791 ε<sub>Nd</sub> = -8.9). Average δ<sup>66</sup>Zn (0.28‰) of siliceous sediments are taken from Moynier et
792 al. (2017). C : S = Carbonates : Siliceous sediments. The data of eclogite component
793 is from Hauff et al. (2003) and Inglis et al. (2017). The data for Eastern China are
794 from Jin et al. (2020), Wang et al. (2018b) and Li et al. (2017). See Table S3
795 (Appendix) for parameters and modeling results.

796

797 **Figure. 7.** Isotopic ratios ($^{87}$Sr/$^{86}$Sr and $^{143}$Nd/$^{144}$Nd ratios and $\delta^{66}$Zn values), major
798 element compositions (SiO$_2$ contents), and trace element ratios (Sm/Yb and Ce/Pb
799 ratios) of the Cenozoic basalts in the TNCO plotted against latitude. A systematic
800 geochemical zonation exists that symmetrically varies from the center towards the
801 north and south. Major and trace-element concentrations and Sr-Nd isotopic ratios of
802 whole-rock have been measured by Xu et al. (2017). The data for TNCO Cenozoic
803 basalts of previous work are from Tang et al. (2006), Liu et al. (2016), Wang and Liu
804 (2021), and Xu et al. (2004).

805

806 **Figure. 8.** Illustration of mantle heterogeneity caused by subduction of the
807 Paleo-Asian oceanic slab and Tethyan oceanic slab. Figure modified after Chen et al.
808 (2016a) and Tang et al. (2006).

809
<figure><img src="image_2.png" /><figcaption>Figure 1</figcaption></figure>
<figure><img src="image_3.png" /><figcaption>Figure 2</figcaption></figure>
<figure><img src="image_4.png" /><figcaption>Figure 3</figcaption></figure>
<figure><img src="image_5.png" /><figcaption>Figure 4</figcaption></figure>
<figure><img src="image_6.png" /><figcaption>Figure 5</figcaption></figure>
<figure><img src="image_7.png" /><figcaption>Figure 6</figcaption></figure>
<figure><img src="image_8.png" /><figcaption>Figure 7</figcaption></figure>
<figure><img src="image_9.png" /><figcaption>Figure 8</figcaption></figure>