Low δ26Mg volcanic rocks of Tengchong in Southwestern China: A deep carbon cycle induced by supercritical liquids
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 TiO2 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 Na2O + K2O/TiO2 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.