Tracing The Deep Carbon Cycle Using Zn-Mg Isotopes

The deep carbon cycle refers to transportation of sedimentary carbonates into the Earth's deep interior via slab subduction and subsequent return of CO₂ to the atmosphere via volcanic degassing.This cycle is an essential part of the global carbon cycle and has profound impact on the carbon budget of the atmosphere and climate change over geological timescale.However,valid approaches that can effectively identify the recycled crustal carbon in the Earth's deep interior are still lacking.This dissertation aimed to trace the deep carbon cycle by using Mg and Zn isotopes,which have great potential in tracing the recycled carbonates.The magnitudes and mechanisms of Zn isotope fractionation during magmatic processes have been first explored.The Mg and Zn isotopic compositions of igneous and metamorphic rocks formed in various geological environments were then investigated.The results demonstrate the presence of recycled crustal carbonates in the deep lithosphere and asthenosphere.Major findings in this dissertation are summarized below.?1?The Zn isotopic difference between peridotites and basalts indicates the occurrence of Zn isotope fractionation by approximately 0.1‰during partial melting of the upper mantle.The Zn isotopic composition of the Bulk Silicate Earth is estimated to be 0.17±0.08‰based on the result of peridotites.?2?The<110 Ma intraplate basalts from Eastern China display anomalously light Mg and heavy Zn isotopic compositions relative to the mantle,providing robust evidence for the presence of recycled marine carbonates in the asthenospheric mantle beneath Eastern China.The systematic Mg-Zn-Sr-Nd isotopic difference between strongly alkaline basalts and weakly alkaline basalts suggests the important role of interaction of carbonated silicate melt with lithospheric mantle in the compositional transition in intraplate basalts from Eastern China.?3?Two types of post-collisional mantle-derived lavas,showing distinct petrological,elemental and Mg-Zn-Sr-Nd isotopic features,have been recognized in the southeastern margin of the Himalayan-Tibetan orogen.The earlier potassic-ultrapotassic lavas were likely formed by partial melting of metasomatized lithosphere,while the later alkaline basaltic lavas were likely formed by partial melting of carbonated asthenosphere associated with the subduction of Neo-Tethyan oceanic crust.?4?A series of deep-seated xenoliths have been investigated for their Mg and Zn isotopic compositions.The Mg and Zn isotope anomalies observed in some granulite,pyroxenite and peridotite xenoliths from North China Craton document multiple metasomatic events that the deep lithosphere has been modified by carbonate-dissolved fluids or carbonated melts.The Zn isotopic signatures of eclogite xenoliths from West African Craton support the protolith origin of mantle eclogite as carbonated oceanic crust.Recyling of carbonated eclogite into the mantle may significantly modify the Mg and Zn isotopic compositions of the mantle and mantle-derived melts.?5?The Himalayan leucogranites display large Mg and Zn isotopic variations,primarily as a result of strong differentiation of high-silica granitic magmas.The isotope fractionation caused by magmatic differentiation must be considered when applying these isotopes to trace intracrustal carbon cycle.