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Abstract On Earth, chemical weathering of basaltic rocks not only modulates the climate change, but also plays a crucial role on the ocean oxygenation and ecosystem evolution. Records of paleo-weathered crusts are a challenge to identify because they are obscured by subsequent alteration processes and buried by the newly formed volcanic lavas. However, late formed volcanic rocks theoretically could record previously weathered crust via their assimilation and fractional crystallization (AFC) processes. Here, we present a systematic study on major, trace elements and Sr-Nd-Mg isotopes in a suite of Changbaishan early formed basalts and late erupted trachytes from Northeast China. Detailed evidence of field observation, geochemical signature and rhyolite-MELTS simulations suggest that trachytes mainly originated from deep fractional crystallization of basaltic melts. The low abundant trachytes from the stratigraphic layer above the basalts display decoupled Mg Sr and Nd isotopes. δ26Mg and 87Sr/86Sr in trachytes range from −0.27 up to +0.94‰ (the heaviest isotopic composition ever reported for Mg in the igneous rocks) and 0.70490 to 0.71065, respectively, which are significantly elevated relative to that of their mantle source. However, they preserved mantle source-like 143Nd/144Nd (0.512584 to 0.512656) ratios. These unusual characteristics cannot be explained by thermal diffusion, fractional crystallization or simple crustal contamination processes. Specifically, δ26Mg correlates positively with SiO 2 and 87Sr/86Sr, and negatively with MgO, CaO, Al 2 O 3 , Sr and Nb/U indicators. The highly elevated Mg and Sr isotopes but limited Nd isotopic changes in trachytes require a component to be the relatively younger weathering residue of basaltic rocks. Taking this component as a wall rock endmember, the well-developed above correlations can be best modelled by AFC processes. This reflects that some trachytes in the aftermath of AFC processes almost fully lost their primary source characteristics, but inherited the intense chemical weathering signature of basalts. This event probably occurred in the middle Miocene as studies of paleosols showed humid and warm climate during that period after the initial eruption of Changbaishan basalts. Therefore, our study as a specific case for the first time provides an efficient way to explore paleo-weathering of basaltic crusts buried in the Earth's surface, through comprehensive investigating Mg, Sr and Nd isotopes of volcanic rocks developed in the late stage of a volcano cone. [ABSTRACT FROM AUTHOR]

Abstract Oceanic subduction zones are important channels for carbon exchange between the Earth’s crust and mantle. However, the nature of carbon cycles at depths from 120 to 410 km in the subduction zone remains unknown. To decipher this issue, high-precision stable Mg isotopes of arc-like volcanic rocks from Tengchong, Southwestern China, have been investigated. The Tengchong volcanic rocks comprise basalts and andesites, with MgO content varying from 2.41 to 8.48 wt.%. Both the basalts and andesites exhibit homogeneous and light Mg isotopic compositions with δ26Mg ranging from −0.51 to −0.45‰ and −0.49 to −0.33‰, respectively. Their δ26Mg values are lower than the average mantle (δ26Mg = −0.25 ± 0.07‰) and island arc lavas (δ26Mg = −0.35 to +0.06‰), but similar to the <110 Ma intra-continental basalts from eastern China (δ26Mg = −0.60 to 0.30‰). This light Mg isotopic composition could not originate from the accumulation of ilmenite in their mantle source because both the Nb/Ta and δ26Mg values of the basalts are invariant with TiO 2 content. The recycling of carbonated eclogites is also unlikely because of the lack of any correlation between δ26Mg and either (Gd/Yb) N or Fe/Mn ratios in the Tengchong basalts. Alternatively, the most probable explanation for the light Mg isotopic composition is the recycling of sedimentary carbonates in the mantle source. This is supported by the high Na 2 O + K 2 O/TiO 2 ratios (3.3–4.1), low Ti/Ti∗ and Hf/Hf∗ values in the basalts, which are consistent with the partial melting trend of carbonated peridotite. Additionally, the high Ba/Th, low Rb/Cs and enriched Sr-Nd isotopes (EMII-like endmember) of the Tengchong basalts indicate the presence of Indian sediments in their mantle source. Furthermore, the extremely high Th/U (6.5–8.3) ratios in basalts suggest the higher mobility of Th than U, which is a unique characteristic of slab-derived supercritical liquids in subduction zones with pressures greater than 6 GPa. Based on the quantitative modeling of Mg-Sr-Nd-Pb isotopes and trace elements, the mantle source of the Tengchong basalts lies along mixing lines of the DMM with 1% supercritical liquids and 15–19% recycled mixture containing 66–72% dolomite, 22–26% calcite and 2–12% Indian sediments. Given that the recycled mixture is mainly dolomite (66–72%), we proposed that the slab-derived supercritical liquids can dissolve dolomite and then metasomatized the overlying mantle to form carbonated peridotite. Partial melting of this mantle source should be located at a depth of ∼120 to 300 km under which supercritical liquids can occur, which is supported by seismic tomographic observations. The Tengchong andesites display many geochemical features similar to the basalts, such as enrichments in LILEs (Large Ion Lithophile Elements), LREEs (Light Rare Earth Elements) and Sr-Nd-Pb isotopes, as well as low δ26Mg values, suggesting a petrogenetic link between these two rock types. Our detailed study suggests that the andesites evolved from the Tengchong basalts via assimilation and fractional crystallization (AFC) processes. This interpretation is also supported by the geophysical tomography, which reveals a low-velocity anomalous zone in the continental crust. This study reveals a new carbon cycle in which Mg-rich carbonate – dolomite – can be dissolved by supercritical liquids and subducted into a deep mantle wedge to depths of 120–300 km in the oceanic subduction zone. This deep metasomatic mantle wedge mixed with the upwelling mantle beneath the Tengchong volcano and partially melted to form the low δ26Mg volcanic rocks. [ABSTRACT FROM AUTHOR]

Whether or not sedimentary carbonates can be recycled into the lower mantle through subduction remains unclear. To further elucidate this issue, we investigate the Mg isotopic composition of Permian basalts from the Emeishan large igneous province (ELIP). Emeishan basalts can be divided into two major groups: the low-Ti and high-Ti basalts, which exhibit distinct major, trace element, and Sr–Nd–Pb isotopic compositions. However, they both possess mantle-like Mg isotopic compositions with δ 26 Mg values of − 0.33 to − 0.19‰ and − 0.35 to − 0.19‰, respectively. Both the low-Ti and high-Ti basalts have experienced compositional evolution, e.g., fractional crystallization or crustal contamination, because their Mg # (39.8–61.1) values are significantly lower than that of primary melt (~ 72.0). However, their δ 26 Mg values do not vary with Mg # , indicating that the Mg isotopic compositions of the basalts were inherited from their mantle source. The low-Ti basalts are enriched in light rare earth elements with low ε Nd (t) values (mostly ranging from − 10.5 to − 8.8) and low Ce/Pb, high Ba/Th ratios, suggesting that they were derived from the lithospheric mantle that was metasomatized by slab-derived fluids. Their mantle-like Mg isotopic composition suggests that the metasomatism of slab-derived fluids did not affect their Mg isotopic composition. This is consistent with most of the reported Arc basalts with mantle-like Mg isotopic compositions. On the other hand, the high-Ti basalts display an OIB-like trace element pattern and positive ε Nd (t) values (+ 0.2 ~ + 2.1), which were possibly derived from a mantle plume that originated from the lower mantle. Their mantle-like Mg isotopic composition indicates that sedimentary carbonates recycled into the lower mantle beneath the ELIP were limited (< 2%). [ABSTRACT FROM AUTHOR]

This study presents stable Mg isotopic data for Cenozoic potassic basalts from Wudalianchi and Erkeshan in northeastern China to determine the interactions between upwelling carbonated asthenosphere and enriched lithospheric mantle. Although the Wudalianchi and Erkeshan basalts have variable MgO contents of 4.45 to 9.47 wt.%, they exhibit a homogeneous Mg isotopic composition with δ 26 Mg values ranging from −0.57‰ to −0.46‰ and averaging −0.51 ± 0.06‰ (2SD, n = 18). This Mg isotopic composition is lighter than that of the average mantle (δ 26 Mg = −0.25 ± 0.07‰) but similar to late Cretaceous (<110 Ma) and Cenozoic basalts from the North China Craton and the South China Block (δ 26 Mg = −0.60 to −0.35‰). The high CaO/Al 2 O 3 and Ba/Rb, and low Hf/Hf ∗ ratios of the Wudalianchi and Erkeshan basalts are typical characteristics of carbonatitic metasomatism, suggesting that the light Mg isotopic composition could derive from involvement of recycled sedimentary carbonates in the mantle source. The high Dy/Er ratios (2.55 to 2.75) and excess of 230 Th ( 230 Th/ 238 U = 1.24 to 1.33) suggest presence of garnet in a relatively deep mantle source. Additionally, the seismic tomographic observations show the existence of the stagnant Pacific slab in the mantle transition zone (410–660 km) under eastern China. This carbonated mantle source should be located in the asthenosphere. However, compared to other low δ 26 Mg basalts from eastern China with MORB-like Sr–Nd–Pb isotopic compositions and OIB-like trace element features, the Wudalianchi and Erkeshan basalts exhibit EM-I Sr–Nd–Pb isotopic compositions combined with high SiO 2 , Ba, K, Pb and LREE contents, high Ba/Th and low Ce/Pb ratios. These geochemical features require a contribution from another mantle source, most likely an EM-I lithospheric mantle. Therefore, an asthenosphere-lithosphere interaction model is proposed for determine the origin of the Wudalianchi and Erkeshan basalts. The original melt was derived from partial melting of the carbonated asthenospheric mantle that was metasomatized by carbonate melt from the stagnant Pacific slab in the mantle transition zone. This ascending melt with a low δ 26 Mg signature subsequently interacted with the EM-I lithospheric mantle. The interaction dissolved pyroxene and crystallized olivine, releasing LILE, REE, Sr and Pb from dissolved minerals into the melt. Simultaneously, Mg was inherited by newly formed olivine. Therefore, this interaction modified the trace elements and the Sr–Nd–Pb isotopic compositions of the melt toward EM-I geochemical features but preserved the light Mg isotope signatures. [ABSTRACT FROM AUTHOR]

The voluminous Eocene postcollisional potassium‐rich magmatism in central Tibet is crucial in understanding the deep geodynamic process and related uplift history of the Tibetan Plateau. Here, we present an integrated study of whole‐rock Sr‐Nd‐Mg‐Li isotopes and zircon U‐Pb ages on two suites of lavas from Duocaima and Nariniya in the northern Qiangtang block. The Duocaima high‐K calc‐alkaline trachyandesites were formed at ~35 Ma and are enriched in light rare earth elements and large ion lithophile elements. They have slightly lower δ26Mg (−0.41‰ to −0.33‰) and δ7Li (+1.0‰ to +2.6‰) relative to the mantle. Together with their high 87Sr/86Sr(i) (0.7066–0.7067) and low ɛNd(t) (−1.96 to −1.61) values, they most likely originated from the lithospheric mantle previously metasomatized by melts from subducted carbonate‐bearing sediments. Model calculation suggests that ~7–11% sediments were added into the mantle via melting. By contrast, the contemporaneous Nariniya adakitic trachytes have higher δ26Mg (−0.13‰ to −0.02‰) and δ7Li (+3.3‰ to +5.4‰) values, as well as higher Pb/Ce and Ba/La ratios, which most likely resulted from the thickened lower crust that was metasomatized by fluids released from downgoing Songpan‐Ganzi oceanic slab. Our combined isotopic study suggests the existence of a remnant oceanic slab beneath central Tibetan Plateau during the late Eocene and implies that the initial uplift of central Tibet was before ~38 Ma. Key Points: Highly variable Mg and Li isotopes in potassium‐rich volcanic rocks from Qiangtang resulted from subducted slab inputsAn ancient remnant oceanic slab still existed beneath central Tibet during the late EoceneThe initial uplift of central Tibet was before ~38 Ma [ABSTRACT FROM AUTHOR]

Carbonate dissolution in subduction zones transfers surface carbon to the mantle and plays an important role in regulating global carbon cycles through geologic time. Petrological observations and thermodynamic models predict substantial dissolution of subducting carbonates induced by slab-derived fluids at arc mantle depths, but this dissolution process has rarely been verified by existing geochemical tracers. In this paper we show that zinc and magnesium isotopes can serve as such tracers through a comparative study on carbonate-bearing and carbonate-free sediments from the South China Sea and the Philippine Sea that are going to be subducted. Leaching experiments are further performed to characterize the isotopic difference between dissolved carbonates and silicate components in sediments. Bulk carbonate-bearing sediments and carbonate-free sediments surprisingly have indistinguishable δ66Zn (0.26 ± 0.04‰ versus 0.25 ± 0.04‰), but the former display systematically lower δ26Mg (−0.85‰ to −0.17‰, n = 10) than the latter (−0.26‰ to +0.13‰, n = 13). Leaching experiments show that dissolved carbonates have much higher δ66Zn (0.82 ± 0.26‰) and lower δ26Mg (−2.70 ± 0.35‰) compared with the silicate residues (δ66Zn = 0.21 ± 0.06‰; δ26Mg = −0.06 ± 0.14‰). These results indicate that the presence of carbonate components has insignificant influence on Zn isotopic compositions of bulk sediments but prominent influence on their Mg isotopic compositions. In this respect, the final fate of carbonates in subducting sediments can be tracked by a combined Mg and Zn isotope analysis on mantle-derived magmas. For instance, bulk carbonate-bearing sediment addition (i.e., no carbonate dissolution) during subduction would produce low δ26Mg but normal δ66Zn values (i.e., Mg-Zn isotopic "decoupling") in mantle-derived magmas. By contrast, if carbonate dissolution occurs during subduction, the high δ66Zn and low δ26Mg (i.e., Mg-Zn isotopic "coupling") signals of dissolved carbonates will be transferred to the sources of mantle-derived magmas. We test this application for a Cenozoic subduction-related, arc-like volcanic exposure from Tengchong, Southwestern China and find significant Mg-Zn isotopic coupling, which demonstrates carbonate dissolution during Indian Oceanic slab subduction. Our results also shed constraints on the amount of dissolved carbonates in the sub-arc mantle. Available data reveal the absence of high δ66Zn and low δ26Mg in global arc lavas, for which we suggest to be the result of a relatively low proportion (<20%) of dissolved calcium carbonates in the sub-arc mantle. [ABSTRACT FROM AUTHOR]