Deep carbon cycles constrained by a large-scale mantle Mg isotope anomaly in eastern China

Abstract

Although deep carbon recycling plays an important role in the atmospheric CO2 budget and climate changes through geological time, the precise mechanisms remain poorly understood. Since recycled sedimentary carbonate through plate subduction is the main light-δ26Mg reservoir within deep-Earth, Mg isotope variation in mantle-derived melts provides a novel perspective when investigating deep carbon cycling. Here, we show that the Late Cretaceous and Cenozoic continental basalts from 13 regions covering the whole of eastern China have low δ26Mg isotopic compositions, while the Early Cretaceous basalts from the same area and the island arc basalts from circum-Pacific subduction zones have mantle-like or heavy Mg isotopic characteristics. Thus, a large-scale mantle low δ26Mg anomaly in eastern China has been delineated, suggesting the contribution of sedimentary carbonates recycled into the upper mantle, but limited into the lower mantle. This large-scale spatial and temporal variation of Mg isotopes in the mantle places severe constraints on deep carbon recycling via oceanic subduction.

Marine Carbonate Component in the Mantle Beneath the Southeastern Tibetan Plateau: Evidence From Magnesium and Calcium Isotopes

“…According to modern GPS observations, the India Oceanic plate is currently moving northward with a velocity of 30 mm per year (Figure ; e.g., Lei et al, ), which may account for the ~410 km discontinuity area beneath Tengchong region (Zhou et al, ). As shown in Figure , the stagnant Neo‐Tethyan slab might feed the Tengchong volcanoes, similar to the model of the stagnant Pacific slab feeding the Cenozoic basalts in eastern China (e.g., S. G. Li et al, ). However, the active Tengchong volcanism commenced at about 5 Ma (e.g., B. Q. Zhu et al, ), delayed more than 50 Ma after India–Asia collision, implying that the trigger for mantle melting cannot be simply attributed to volatile additions related to the subducted Neo‐Tethyan Oceanic slab.…”

“…Mg–Sr–O isotopes were employed to study the origin of syenites from northwest Xinjiang, China, showing that syenites with low δ 26 Mg were caused by the incorporation of dolostone, limestone, and Indian sediment (Ke et al, ). In the modeling of Mg and Sr isotopes, S. G. Li et al () suggested that 1–10% recycled carbonates were responsible for the low δ 26 Mg anomaly of the continental basalts (<110 Ma) in eastern China. However, to date, there is no work simultaneously using Mg and Ca isotopes to study the recycling carbonate components in the mantle source.…”

Light Mg Isotopic Composition in the Mantle Beyond the Big Mantle Wedge Beneath eastern Asia

“…Total alkalis versus SiO 2 diagram (after Le Bas et al, ) for the Huihe nephelinites. Data for Shandong nephelinites (gray diamonds; Zeng et al, ) and Chaihe basalts (gray circles; Ho et al, ; Li et al, ; Zhao & Fan, ; Xue et al, ) are shown for comparison.…”

“…Heavy δ 66 Zn signatures (δ 66 Zn = 0.30‰ to 0.63‰; Liu et al, ) of the basalts, compared to peridotite mantle values (δ 66 Zn = 0.16 ± 0.06‰; Sossi et al, ), reflect contributions from isotopically heavy subducted carbonated material (average δ 66 Zn = 0.91 ± 0.47‰; Pichat et al, ). Highly variable but consistently light Mg isotopic compositions of these rocks (δ 26 Mg = −0.60‰ to −0.25‰; Huang, Li, et al, ; Tian et al, ; Wang et al, ; Yang et al, ; Li et al, ; Yu et al, ) are also usually attributed to recycled sedimentary carbonates (with extremely light Mg isotopic compositions; δ 26 Mg = −5.57‰ to −0.38‰; Huang, Ke, et al, ; Wombacher et al, ; Young & Galy, ) in the mantle sources.…”

“…The region above this stagnant slab has been termed the “Big Mantle Wedge” (BMW; Zhao et al, ), which extends to over 1,000 km from the trench, where the BMW and the crustal section above are considered as an intraplate setting (Xu et al, ). Cenozoic basalts occurring to the east of the DTGL in eastern China have consistently light Mg isotopic compositions and define a large low δ 26 Mg anomaly within the BMW (δ 26 Mg = −0.60‰ to −0.34‰; Huang, Li, et al, ; Tian et al, ; Wang et al, ; Yang et al, ; Sun, Teng, et al, ; Su et al, ; Li et al, ). Basalts erupted at Chaihe in the nearby DTGL area (Figure a), in contrast, show terrestrial, mantle‐like Mg isotopic composition (δ 26 Mg = −0.34‰ to −0.19‰; Li et al, ; Sun, Teng, et al, ), which may suggest a spatial relationship between the large‐scale Mg isotopic anomaly and the stagnant Pacific slab or the BMW beneath eastern Asia (Li et al, ).…”