# An oceanic subduction origin for Archaean granitoids
revealed by silicon isotopes

Zhengbin Deng, Marc Chaussidon, Martin Guitreau, Igor Puchtel, Nicolas Dauphas, Frederic Moynier

► To cite this version:

Zhengbin Deng, Marc Chaussidon, Martin Guitreau, Igor Puchtel, Nicolas Dauphas, et al. An oceanic subduction origin for Archaean granitoids revealed by silicon isotopes. Nature Geoscience, 2019, 12 (9), pp.774-778. 10.1038/s41561-019-0407-6. hal-02278676

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# An oceanic subduction origin for Archaean granitoids revealed by silicon isotopes

Zhengbin Deng<sup>1*</sup>, Marc Chaussidon<sup>1</sup>, Martin Guitreau<sup>2</sup>, Igor S. Puchtel<sup>3</sup>, Nicolas Dauphas<sup>3,4</sup> and Frédéric Moynier<sup>1,5</sup>

Modern oceanic crust is constantly produced at oceanic ridges and recycled back into the mantle at subduction zones via plate tectonics. An outstanding question in geology is whether the Earth started in a non-plate tectonic regime, and if it did, when the transition to the modern regime occurred. This is a complicated question to address because Archaean rocks lack modern equivalents to anchor interpretations. Here, we present a silicon isotopic study of 4.0–2.8-Gyr-old tonalite–trondhjemite–granodiorites, as well as Palaeozoic granites and modern adakites. We show that Archaean granitoids have heavier silicon isotopic compositions than granites and adakites, regardless of melting pressure. This is best explained if Archaean granitoids were formed by melting of subducted basaltic crust enriched in sedimentary silica through interaction with seawater. Before the appearance of silica-forming organisms 0.5–0.6 billion years ago, the oceans were close to silicon saturation, which led to extensive precipitation of cherts on the seafloor. This is in contrast to modern oceans, where silica biomineralization maintains dissolved silicon at low concentration. The unique heavy silicon isotope signature of cherts has been transferred to Archaean granitoids during an oceanic subduction process, which was probably responsible for the formation of felsic rocks on Archaean emerged lands.

Modern subduction-zone lavas are derived primarily from melting of the mantle wedge that is fluxed with fluids from the subducting plate. Except for rare rock occurrences known as adakites that are found at some subduction zones with a high thermal gradient of ~40 °C kbar⁻<fnref n="1" /> (ref. <fnref n="1" />; Fig. 1), modern subducting oceanic crust experiences only dehydration rather than melting because thermal gradients along the Benioff zone are too low on average² (~25 °C kbar⁻<fnref n="1" />). A remarkable feature of Archaean cratons is the prevalence of the tonalite–trondhjemite–granodiorite (TTG) suite (granitoids with high Na₂O/K₂O ratios), which are thought to have been formed by partial melting of hydrated mafic crust at great depths³ (>50 km). However, the exact tectonic setting of this melting regime and whether it is indicative of subduction-driven magmatism in the Archaean are hotly debated⁴⁻¹⁰. Several authors have proposed a subduction-like setting, with thermal gradients between ~40 and 100 °C kbar⁻<fnref n="1" /> (refs. <fnref n="1" />,³,8,9) (Fig. 1), whereas others have suggested a stagnant-lid environment, such as partial melting at the base of thickened oceanic plateaus⁶,⁷,¹⁰. A telltale signature of Archaean oceanic crust would be its extensive interaction with seawater, which, if TTGs have been produced by melting of subducted oceanic crust, could have left an imprint on their chemical and Si and/or O isotopic compositions. A recent study¹¹ suggested that combined analyses of O and Si isotopes could help unravel the contributions of different sedimentary components, such as hydrothermal silica, pelites and cherts, to the magmas from which Hadean and Archaean zircons crystallized. In this study, we show that Si isotope data can be used to directly trace the origin of Archaean TTGs.

## Elevated Si isotopic ratios in TTGs

We report the Si isotopic compositions as δ³⁰Si values (that is, the per mil deviation of the ³⁰Si/²⁸Si ratio in a sample from that of

the NBS28 standard) of 3.50–2.70-Gyr-old komatiites (bulk rock samples), 3.98–2.79-Gyr-old TTGs (quartz separates and bulk-rock samples) and a set of quartz separate samples from Palaeozoic granites previously studied for their bulk-rock Si isotopic compositions¹² (Supplementary Dataset 1). The komatiites (that are, ultra-mafic volcanic rocks interpreted as mantle-derived high-degree melts¹³) have an average δ³⁰Si value of −0.29 ± 0.02‰ (95% confidence interval (95% CI hereafter unless stated otherwise); n=11), which is identical to the modern bulk silicate Earth value estimated from modern mantle-derived rocks (δ³⁰Si = −0.29 ± 0.01‰; n=64) (Fig. 1 and Supplementary Dataset 2). Bulk-rock TTG samples have an average δ³⁰Si value of −0.01 ± 0.02‰, n=14, which is significantly higher than an average for bulk rock I- and A-type granites¹² (δ³⁰Si = −0.18 ± 0.02‰, n=19), but only slightly higher than that for modern adakites (δ³⁰Si = −0.06 ± 0.02‰, n=4; Fig. 1 and Supplementary Dataset 2). The δ³⁰Si values of bulk-rock TTGs are consistent with other reported data for TTGs from the Kaapvaal Craton that have δ³⁰Si = −0.02 ± 0.02‰ (n=37; ref. ¹⁴). The difference in δ³⁰Si value between the quartz and bulk-rock samples of TTGs is similar to that measured between the quartz and bulk rock samples in I- and A-type granites (Supplementary Fig. 1 and Supplementary Text). This observation provides evidence that quartz from TTGs is at Si isotopic equilibrium with other coexisting minerals, and that the elevated δ³⁰Si values in bulk TTGs reflect the composition of their parental magmas, rather than late contamination of the silicate melts by sedimentary components isotopically heavy in Si. In the following section, we show that some of the variations in the δ³⁰Si values of granites, TTGs and adakites can be accounted for by isotopic fractionation during partial melting, but that for the same pressure-temperature conditions of partial melting, TTGs have systematically higher δ³⁰Si values than Phanerozoic granites and adakites (Fig. 2).

<footnote n="1">¹Université de Paris, Institut de Physique du Globe de Paris, CNRS, Paris, France. ²Université Clermont Auvergne, Laboratoire Magmas et Volcans, IRD, CNRS, UMR 6524, Aubière, France. ³Department of Geology, University of Maryland, College Park, MD, USA. ⁴Origins Laboratory, Department of the Geophysical Sciences and Enrico Fermi Institute, The University of Chicago, Chicago, IL, USA. ⁵Institut Universitaire de France, Paris, France.
*e-mail: deng@ipgp.fr</footnote>
<figure><img src="image_2.png" /></figure>

**Fig. 1 | Pressure-temperature diagram and Si isotopic results.** Phase equilibria relations for partial melting of a hydrous tholeiitic basalt under the thermal gradients of various geological settings<sup>1,2,7,10</sup>, taken from ref. ¹. The stability curve for plagioclase (black dashed curve) was not quantitatively constrained in ref. ¹, so the one calculated from the H₂O-saturated solidus at 12 kbar from ref. ¹⁰ is shown instead. Si isotopic data for Archaean TTGs and komatiites, Phanerozoic I- and A-type granites, and modern basalts, peridotites and adakites are shown as insets (from this study (Supplementary Dataset 1) and the literature (as compiled in Supplementary Dataset 2)). In the bottom-right inset, Si isotopic data for mantle-derived rocks, including the modern (black dashed curve) and Archaean (black solid curve) ones, are compared with those of Phanerozoic granites. Note that the decomposition of hornblende would release water and leave behind a dry refractory residue. Plg, plagioclase; Hbl, hornblende; Grt, garnet.

## Quantifying the effects of melt-crystal segregation

Silicon isotopic variations in igneous rocks are controlled by crys-
tal-melt equilibrium fractionation during either partial melting or
fractional crystallization processes¹² (see details in Supplementary
Text). First-principles calculations show that, at equilibrium, tecto-
silicates (for example, quartz and feldspar) have higher δ³⁰Si values
compared with nesosilicates (for example, olivine) and inosilicates
(for example, pyroxene)¹⁵,¹⁶. The situation is not as clear for melts,
but one can reasonably estimate the Si isotopic fractionation fac-
tors between minerals and silicate melts by considering the melt
normative mineral compositions¹⁶,¹⁷ (for example, predominance
of neso- and inosilicates in basaltic melts, and predominance
of tectosilicates in felsic melts; Supplementary Dataset 5 and
Supplementary Information), which has been shown to be a good
approximation for O isotopes¹⁸. Future experimental and theoreti-
cal work will be needed to assess the validity of this approach for Si
isotopic fractionation involving silicate melts, which is beyond the
scope of the present study. Under the temperatures relevant to man-
tle partial melting (≥1,400 °C), Si isotopic fractionation between
olivine/pyroxene and basaltic melt is limited, and, at equilibrium,
basaltic melts should have δ³⁰Si values shifted by less than 0.04‰
relative to their mantle sources, as corroborated by the isotopic sim-
ilarity between peridotites and mid-ocean ridge basalts¹⁹⁻²¹, as well
as most oceanic island basalts²². During crustal magma differen-
tiation, however, Si isotopic fractionation between melt and clino-
pyroxene/orthopyroxene/hornblende/garnet can be significantly
enhanced due to the lower temperatures involved and stronger

contrast in Si polymerization between the melt and the residual
minerals<sup>17,23</sup>. At 850–900 °C (typical liquidus temperatures for
TTGs, I- and A-type granites, and SiO<sub>2</sub>-rich adakites), Si isotopic
fractionation between felsic melt on the one hand, and pyroxene,
garnet, and hornblende on the other hand, is Δ³⁰Si<sub>melt-cpx/opx/hbl/grt</sub> ≈
+0.30‰, while the isotopic fractionation between granodio-
ritic melt and plagioclase is only Δ³⁰Si<sub>melt-plg</sub> ≈ +0.03‰ (ref. ¹⁶;
Supplementary Dataset 5). During crustal anatexis, the stabil-
ity of plagioclase in the source is sensitive to the partial melting
pressure¹,¹⁰,²⁴ (Fig. 1). Therefore, melting of the crust under vari-
ous thermal gradients should produce melts of varying Si isotopic
compositions (Fig. 2).

Mineral assemblages in magma sources, including major (plagioclase, clinopyroxene, orthopyroxene, hornblende and garnet) and accessory (for example, ilmenite, titanite and rutile) phases, during partial melting of a basaltic protolith are intimately related to the pressure-temperature-H₂O conditions of melting⁷,²⁴ (see details in Supplementary Information). In the discussion below, we assume water saturation, but our conclusions do not rely heavily on this assumption, as Si isotopic fractionation during melting of the crust is mostly sensitive to the presence of plagioclase in the source, which we trace directly using the Sr/Sr⁺ anomaly, that is, Sr/Sr⁺ = Sr<sub>N</sub>/√Ce<sub>N</sub> × Nd<sub>N</sub> (subscript ‘N’ represents a normalization to the bulk-silicate Earth values²⁵; Fig. 2).

(1) Under a thermal gradient of 40°C kbar⁻¹ (~13°C km⁻¹), partial melting of tholeiitic basalt takes place at pressures >20 kbar in
<figure><img src="image_3.png" /><figcaption>**Fig. 2** | Trace-element and Si isotopic compositions. a, Nb concentrations and Sr/Sr⁺ (Sr/Sr⁺ = Sr<sub>N</sub> / √C<sub>N</sub> × Nd<sub>N</sub>) values of Archaean TTGs, Phanerozoic granites and modern adakites (subscript 'N' represents normalization to the primitive mantle values<sup>25</sup>). Archaean granitoids from ref. ⁸ are shown as potassic, low-pressure (LP) TTG, medium-pressure (MP) TTG and high-pressure (HP) TTG groups. b, δ³⁰Si and Sr/Sr⁺ values of studied samples. Partial melts of 5–30% (5% increments by batch melting models) under thermal gradients of 40, 70 and 100 °C kbar⁻¹ are represented by circles, which trend towards lower δ³⁰Si and Sr/Sr⁺ values with increasing melting degrees. The dark grey area delineates the δ³⁰Si and Sr/Sr⁺ range by partial melting, with the light grey field indicating the effects from plagioclase xenocrysts, which increase Sr/Sr⁺ values in bulk without significantly changing δ³⁰Si values. Uncertainties on δ³⁰Si are shown at the 95% CI.</figcaption></figure>

the presence of residual garnet, clinopyroxene, hornblende and rutile<sup>24</sup>. The absence of plagioclase means that Sr and Eu are incompatible, while garnet preferentially incorporates Y and Yb, and rutile sequesters Nb and Ta. The felsic melts generated under such conditions have low concentrations of Y, Yb, Nb and Ta, and positive Sr anomalies, as typically observed in modern adakites that are considered to be partial melts from young and hot subducted oceanic crust<sup>26,27</sup> (Fig. 2, Supplementary Fig. 2 and Supplementary Dataset 3). The plagioclase-free residue has a Δ³⁰Si<sub>melt-residue</sub> ≈ +0.30‰ at 850–900 °C, and, therefore, a basaltic source with SiO₂ = 50 wt% and δ³⁰Si = −0.30‰ would produce 5–30% partial melts with δ³⁰Si = −0.02 to −0.12‰ (Fig. 2, Supplementary Dataset 6 and Supplementary Text), consistent with the δ³⁰Si values of modern adakites (δ³⁰Si = −0.06 ± 0.02‰).

(2) Under a thermal gradient of 100 °C kbar⁻¹ (~33 °C km⁻¹), partial melting occurs at pressures <8–10 kbar, in the presence of residual clinopyroxene, plagioclase and hornblende<sup>7,24</sup> (Fig. 1). In contrast to adakites, felsic melts produced at these, relatively low, pressures would have higher concentrations of Y, Yb, Nb and Ta, and negative Sr anomalies, as documented for Phanerozoic granites (Fig. 2, Supplementary Fig. 2 and Supplementary Dataset 3). The plagioclase-rich nature of the melting residues

under this thermal gradient decreases Δ³⁰Si<sub>melt-residue</sub> to ~+0.17–0.21‰. Using the same basaltic source with SiO₂ = 50 wt% and δ³⁰Si = −0.30‰, felsic melts derived from 5–30% partial melting at 850–900 °C would have δ³⁰Si = −0.20 to −0.10‰ (Fig. 2, Supplementary Dataset 6 and Supplementary Text), which is consistent with the δ³⁰Si values of Palaeozoic I- and A-type granites (δ³⁰Si = −0.18 ± 0.02‰; Fig. 1).

## Presence of sedimentary cherts in the sources of TTGs

Depending on the modal fractions of plagioclase in the source, felsic rocks produced by melting under thermal gradients between 40 and 100 °C kbar⁻¹ (that is, between 13 and 33 °C km⁻¹) should have δ³⁰Si and Sr/Sr⁺ values between those of adakites and granites (Fig. 2). Although Archaean TTGs have a range of chemical compositions encompassing those of adakites and granites, at identical Sr/Sr⁺ values, TTGs have Si isotopic compositions shifted towards higher δ³⁰Si values by +0.1 to +0.2‰ (except for one TTG sample, KV2.1/08; Fig. 2). The similarity of δ³⁰Si values between Archaean komatiites and modern peridotites demonstrates the absence of a secular change in the Si isotopic composition of the Earth's mantle since ~3.5 billion years ago (Fig. 1). Therefore, the higher δ³⁰Si values of Archaean TTGs relative to the Phanerozoic rocks generated at equivalent pressures require the presence of a component with a heavy Si in the source of TTGs. TTGs also have δ¹⁸O values that are ~1‰ higher than those of eclogite-derived felsic melts<sup>28–30</sup>, and the most likely explanation for these heavy-isotope enrichments for O and Si is the presence of a marine sedimentary chert component in the source of TTGs (Fig. 3). A similar interpretation has been previously proposed for the increased δ³⁰Si values in some Phanerozoic andesites<sup>17</sup>, which, based on this study, can also be related to the addition of adakite-like slab melts.

## Significance for Si cycle and geodynamics of the Archaean

Archaean marine sedimentary cherts have variable, but high δ³⁰Si values between 0 and +4‰ (refs. ³¹,³²). They are derived from (1) precipitation of amorphous silica in oceans saturated with silica due to the lack of marine organisms that fix Si in their skeleton³³, and (2) subsequent diagenetic reactions that form microquartz³⁴. During the Archaean, silicic acid in the oceans acquired a high δ³⁰Si value³⁵ due to the isotopic fractionations at depth in the oceanic crust between hydrothermal fluids (heavy Si isotope enrichment) and silicified basalts and gabbros (light Si isotope enrichment) (Δ³⁰Si<sub>precipitated silica-dissolved silicon</sub> ≈ −1‰ (ref. ³⁵)). Marine cherts are also enriched in ¹⁸O, due to the temperature-dependent oxygen isotopic fractionation between quartz and seawater (for example, Δ¹⁸O<sub>quartz-seawater</sub> = +6‰ at 300 °C to = +31‰ at 25 °C (ref. ³⁶; Fig. 3). Addition of 2.5–5.0% of sedimentary chert with δ³⁰Si = +2‰ and δ¹⁸O = +25‰ to a basaltic source with δ³⁰Si = −0.30‰ and δ¹⁸O = +5.5‰ can account for the O and Si isotopic compositions of TTGs (Fig. 3 and Supplementary Fig. 4). Sedimentary chert is the most probable contaminant that can account for both the O and Si heavy-isotope enrichment of Archaean TTGs.

The evidence for contamination by sedimentary cherts implies that the basaltic source of TTGs had ample time for interacting with seawater, which precludes TTG formation by melting at the base of an oceanic plateau in a stagnant-lid regime. Oceanic subduction under moderate to high thermal gradients of 40–100 °C kbar⁻¹, therefore, appears to be the most likely mechanism to bring this basaltic source in the appropriate pressure–temperature conditions for melting. Our data further suggest that only the top part (for example, the first few kilometres) of the subducted oceanic crust is likely to be melted. This is because the deeper parts of the oceanic crust are probably enriched in silica with low δ³⁰Si values, since it precipitated from hydrothermal fluids with a δ³⁰Si value around −0.3‰ (ref. ³¹), which is opposite to what is observed in TTGs
<figure><img src="image_4.png" /></figure>

**Fig. 3** | Combining O and Si isotopes to trace hydrothermal or sedimentary addition. A indicates a sedimentary chert component selected from the chert dataset in ref. ³¹ (δ¹⁸O = +25‰ and δ³⁰Si = +2‰), B indicates an uncontaminated basaltic source (δ¹⁸O = +5.5‰ and δ³⁰Si = -0.30‰ (refs. ¹⁸,¹⁹)), C indicates low-temperature hydrothermal silica⁴² (δ¹⁸O = +25‰ and δ³⁰Si = -2‰) in silificated sheeted dykes, D indicates high-temperature silica⁴² (δ¹⁸O = +10‰ and δ³⁰Si = -2‰ (ref. ⁴²)) in silificated gabbros and E indicates greywackes, sandstones and turbidites in average (δ¹⁸O = +25‰ and δ³⁰Si = -0.30‰ (ref. ¹²)). The δ¹⁸O and δ³⁰Si values for partial melts from uncontaminated basalts at pressures of 4-8 kbar (F = 25%; δ¹⁸O = +6.3‰ and δ³⁰Si = -0.18‰) and >20 kbar (F = 15%; δ¹⁸O = +6.3‰ and δ³⁰Si = -0.07‰)²⁵, as well as those of felsic rocks in this study, and refs. ¹²,⁴¹ are shown for comparison. Here F indicates the degree of partial melting. The δ³⁰Si values for the green and blue fields, respectively for adakites and TTGs, are from this study. The δ¹⁸O range for the adakite field includes the whole-rock δ¹⁸O values from ref. ²⁷ and those derived from olivines in ref. ²⁸, while the δ¹⁸O range of the TTG field is limited by the bulk δ¹⁸O values calculated from zircons in refs. ²⁹,³⁰ (Supplementary Dataset 7). Note that the whole-rock δ¹⁸O values of Archaean TTGs can be altered by metamorphism, thus deviating from the TTG field defined by zircon values. Uncertainties on δ³⁰Si are shown at the 95% CI.

(Fig. 3). In the modern oceanic crust, the deeper plutonic rocks are indeed slightly lower in δ³⁰Si compared with shallow lava flows³⁷, and Archaean sedimentary cherts are 0 to +4‰ higher in δ³⁰Si than other Archaean uncontaminated ultramafic and mafic rocks¹²,¹⁷,³¹,³². The same conclusion holds for oxygen isotopes, since the bulk δ¹⁸O value of altered oceanic crust is close to that of fresh oceanic crust³⁸ (+5.5‰), as observed in the Oman ophiolite³⁹. For oxygen, this is due to the fact that the isotopic fractionation factor between water and carbonate (or silica) changes from being positive to being negative below the ~300°C isotherm³⁹. Partial melting of the top basaltic and chert layers of the subducted crust seems to be in line with the expectations from petrological considerations, as evidenced by experimentally determined phase relations⁴⁰ in the nepheline–diopside–silica ternary diagram. Although silica itself is refractory, adding silica to basalts (5–20 wt%, depending on basalt type) can reduce the solidus temperatures by 65–120°C (ref. ⁴¹; Supplementary Fig. 5, Supplementary Dataset 4 and Supplementary Information). Thus, in addition to the presence of high thermal gradients in the Archaean, the fact that the subducted basaltic crust could contain significant amounts of cherts would also have promoted its partial melting. It is noteworthy that a higher contribution of the upper high-δ¹⁸O layers of the subducted slab compared with the deeper low-δ¹⁸O layers has also been proposed for the origin of modern adakites²⁸. The exact mechanism to melt refractory cherts and underlying mafic crust remains to be elucidated, but we speculate that melting could start at the interface between chert and basalt. Future experimental studies are necessary to further investigate this aspect. Overall, the evidence for incorporation of sedimentary cherts within magma sources of Archaean TTGs indicates that lateral motion of lithosphere (horizontal tectonics) and downward transport of sediments must have been operating, at least locally, since as early as ~4.0 billion

years ago. We argue that oceanic subduction settings involving high thermal gradients were most likely responsible for the build-up of felsic continents in the Archaean.

## Online content

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Published online: 26 August 2019

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## Acknowledgements

We thank P. Louvat, T. Sontag and P. Burckel for help with the (multi-collector) inductively coupled plasma mass spectrometry. We thank G. Libourel for suggestions. I. Bindeman, M. Krawczynski, F. Poitrasson, E. J. Chin and M. Harrison are appreciated for their comments on an earlier version of this manuscript. I.S.P.’s komatiite sample collection benefited from contributions of E. Nisbet, G. Byerly and C. Anhaeusser. We thank O. Sigmarsson for providing adakite samples. F.M. acknowledges funding from the ERC under the H2020 framework programme/ERC grant agreement no. 637503 (Pristine). F.M. and M.C. thank the financial support of the UnivEarthS Labex programme at Sorbonne Paris Cité (ANR-10-LABX-0023 and ANR-11-IDEX-0005-02). Parts of this work were supported by IPGP platform PARI, and by Région Île-de-France Sesame grant no. 12015908. M.G. acknowledges financial support from Région Auvergne (Auvergne Fellowship programme), LabEx ClerVolc (ANR-10-LABX-0006) and Agence National de la Recherche (ANR-17-CE31-0021 Zircontinents). This is ClerVolc contribution 353.

## Author contribution

Z.D., M.C., M.G. and F.M. designed the research project. M.G., I.S.P. and N.D. selected the TTGs/adakites, komatiites and granites, respectively, for study. Z.D. performed the research and analysed the data. Z.D., M.C., M.G., I.S.P., N.D. and F.M. contributed to interpreting the data and writing the paper.

## Competing interests

The authors declare no competing interests.

## Additional information

Supplementary information is available for this paper at https://doi.org/10.1038/s41561-019-0407-6.

Reprints and permissions information is available at www.nature.com/reprints.

Correspondence and requests for materials should be addressed to Z.D.

Publisher's note: Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
## Methods

The TTG and granite samples were crushed and the quartz grains were handpicked under a microscope. Aliquots of the coarse-grained powders of the TTG samples (~1 g for each sample) were sieved in alcohol and crushed into fine powders using agate mortar and pestle. The selected quartz grains, as well as fine powders of TTGs, komatiites, adakites, NBS28 quartz and USGS GRM BHVO-2, were processed following the protocols described previously¹³. The sample powders (11–22 mg for quartz grains of granites and TTGs, 12–37 mg for bulk komatities and adakites, and 44–86 mg for bulk TTGs) were fused using NaOH as flux (10–15 times of the mass of the samples) in Ag crucibles at 720 °C for 12 min. The fusion cake was dissolved in MQ-e H₂O (18.2Ω) and acidified with 1% HNO₃. Small aliquots of sample solutions were further diluted and measured on an Agilent quadrupole inductively coupled plasma mass spectrometer for Si concentrations and Al/Si ratios, which were used to quantify the purity of the quartz grains from the TTG and granite samples. Solution aliquots containing ~20 µg Si from the samples were processed through BioRad Poly-Prep chromatography columns filled with 1.8 ml AG50X-12 (200–400 mesh) cation exchange resin to separate Si from the matrix. At low to neutral pHs, dissolved Si is present as neutral or anionic species, which can be efficiently eluted by H₂O, while the cationic species are retained on the resin⁴¹. Sample solutions of 2 ppm Si in 0.2 M HNO₃ were measured for their Si isotopic compositions on a Thermo-Fisher Neptune Plus multi-collector inductively coupled plasma mass spectrometer housed at the Institut de Physique de Globe de Paris using a quartz-made spray chamber. A medium resolution (M/ΔM ≈ 5,000, with M the mass of the peak, and ΔM the mass difference between 5% and 95% relative peak heights) mode was used to avoid an isobaric interference from ¹⁴N¹⁶O⁺ on mass ³⁰Si⁺. The alkali fusion solutions of NBS28 were processed through the same chemistry, and the purified solutions were run between samples to correct for instrumental mass bias. In every analytical session, the basalt reference material BHVO-2 was processed following the same procedure

to monitor the accuracy of the analysis. Each measurement consisted of 25 cycles with an integration time of 8.389 s. A two-step wash-out was performed after each measurement by rinsing with 0.5 M HNO₃ and 0.2 M HNO₃. The Si isotopic compositions of the samples are reported in Supplementary Dataset 1 using a per mil delta notation for ³⁰Si/²⁸Si or ²⁹Si/²⁸Si ratio relative to the NBS28 standard, that is, δ³⁰Si or δ²⁹Si.

Two granites, four TTGs and four adakites measured for Si isotopes were also measured for whole-rock O isotopic composition by Activation Laboratories (Actlabs) following the procedures described in ref. ⁴⁵. The O isotopic results are reported in Supplementary Dataset 8 using a per mil notation for the ¹⁸O/¹⁶O ratio relative to that of the Vienna standard mean ocean water (V-SMOW).

## Data availability

The authors declare that the data supporting the findings of this study are available within the article and its supplementary information files (that is, the Supplementary Information and additional datasets).

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