Tracing The Physicochemical Influence Of The Recycled Carbonate On The Mantle Using Zn-Fe-Cu Isotopes

Deep carbon cycle is a process of exchange between carbon in the Earth’s surface and carbon in the deep Earth,which directly affect Earth’s atmospheric climate,the origin and evolution of life,the physical and chemical state of the mantle and the migration of metallogenic elements.Especially,plate subduction process is the main way to transport carbon from the Earth’s surface to the deep Earth.In the oceanic subduction zone,the carbon mainly exists in the form of sedimentary carbonate rocks,carbonated sediments,carbonated oceanic crust and organic matter.However,due to the limited samples and trace tools,the research about the physical and chemical influence of recycled carbonate on the mantle is restricted.Basalts,as the melt formed by the partial melting of mantle,can help us to infer the physical and chemical state of its mantle source,and is an ideal sample to study the deep carbon cycle.In addition,with the rapid development of analytical techniques,stable isotope of metallic element has gradually become a"sharp tool"to reveal the geological processes such as crust-mantle material cycle and the migration metallic elements.Based on the analysis of Zn-Fe-Cu isotopes of the Cenozoic basalts in the Trans-North China Orogen（TNCO,the central zone of the North China Craton）,combined with petrographic observation,the effects of recycled carbonates on the geochemical composition of the mantle,redox state and potential migration of metallic elements are discussed in this dissertation.The basalts in this study come from 8 regions,including Zuoquan,Xiyang,Pingding,Fanshi,Datong,Yangyuan,Fengzhen and Zhouba,which were distributed latitudinally along the TNCO and the eruption age gradually became younger from the central area（Fanshi）to both the north and south sides.In addition,these basalts are divided into basanites,alkali basalts and tholeiites,and their geochemical compositions also vary systematically with latitude,showing the characteristics of OIB（ocean island basalt）type trace elements.Furthermore,the Sr-Nd isotopic compositions,high Fe/Mn ratios and high Zn/Fe ratios of these basalts indicated that the mantle sources are asthenosphere pyroxenites,and there are recycled crust materials in the mantle sources.In conclusion,the Cenozoic basalts from the TNCO thus provided a window for studying the deep carbon cycle.The premise of exploring the deep carbon cycle is to trace the recycled carbonate in the mantle.Since the zinc isotopic ratios are distinct between sedimentary carbonate rocks and silicate reservoirs（e.g.,mantle,oceanic mafic crust,and siliciclastic sediments）,they could be used to identify the different source endmembers in magmatic rocks,especially recycled carbonates.We found that,basanites and alkali basalts have highδ⁶⁶Zn values（ranging from 0.32‰to 0.46‰）,while tholeiites have lowδ⁶⁶Zn values（0.28±0.04‰,2SD）similar to mid-oceanic ridge basalts（MORB）(δ⁶⁶Zn=0.27±0.05‰,2SD).Given the limited extent of Zn isotopic fractionation（<0.1‰）during crystallization and partial melting,the elevatedδ⁶⁶Zn values observed in the basanites and alkali basalts reflect the involvement of recycled marine carbonates(which have an averageδ⁶⁶Zn of～0.91‰)in the mantle sources.Moreover,theδ⁶⁶Zn values of these basalts show clear correlations with carbonate metasomatism indexes（such as,Ti/Eu ratios and Zr/Hf ratios）.The basanites and alkali basalts with highδ⁶⁶Zn values have both low Ti/Eu ratios and high Zr/Hf ratios.In addition,during the partial melting of the carbonated mantle,the Si O₂ contents of the melt will decrease with the increase of the dissolved CO₂ contents.The negative correlation between theδ⁶⁶Zn values and Si O₂ contents of these basalts also indicates the presence of carbonates in the mantle sources of the basanites and alkali basalts.Mixing models forδ⁶⁶Zn values vs.⁸⁷Sr/⁸⁶Sr ratios andε_Nd（t）values demonstrate that the mantle sources of basanites,alkali basalts,and tholeiites were metasomatized by different proportions of recycled carbonates and siliceous sediments.Furthermore,both isotopic and geochemical compositions exhibit symmetrical variations from the central zone towards the south and the north along the TNCO,that is,δ⁶⁶Zn values and ¹⁴³Nd/¹⁴⁴Nd ratios and Ce/Pb ratios decrease,while Si O₂ contents and ⁸⁷Sr/⁸⁶Sr ratios increase.We suggest that more recycled carbonates and fewer siliceous sediments are released from the subducted slab,with an increase in the depth of subduction.We further propose that this process is one of the most important causes of compositional variations in the basanite-alkali basalt-tholeiite suite.The recycled carbonate not only affects the petrology,element content and isotopic characteristics of the mantle,but also affects the redox state of the mantle.Fe,as the most abundant polyvalent element in the mantle,is the most important redox buffer in the mantle.However,Fe³⁺is more incompatible than Fe²⁺.Fe³⁺is more inclined to enter the melt during the partial melting of mantle rocks,and Fe³⁺usually forms stronger chemical bonds,so it is relatively enriched with heavier Fe isotopes.This allows Fe isotopes capable of measuring Fe³⁺/∑Fe ratios,and indirectly reflect the redox state of the mantle.We find that the Fe isotopic compositions of tholeiites are relatively uniform(δ⁵⁶Fe=0.10±0.04‰,2SD),similar to that of mid-ocean ridge basalts(δ⁵⁶Fe=0.10±0.06‰,2SD).The higher Fe isotopic compositions（0.14‰to 0.22‰）of the basanites and alkali basalts can not be explained by fractional crystallization,partial melting,silicate sediments/fluids metasomatism,and the presence of pyroxenites in the mantle source.At the same time,theδ⁵⁶Fe values has a good positive correlation with the Fe³⁺/∑Fe ratios.Theδ⁵⁶Fe values and the Fe³⁺/∑Fe ratios also have a good correlation with theδ⁶⁶Zn values and the carbonate metasomatism indexes（such as,Ti/Eu ratios and Zr/Hf ratios）,respectively.Therefore,we believe that the recycled carbonates were gradually reduced with the increase of the subduction depth,and at the same time,Fe²⁺in mantle rocks are oxidized to Fe³⁺,and the Fe³⁺/∑Fe ratios increases,which make the mantle rocks become more oxidized.In the partial melting of mantle,Fe³⁺are more incompatible than Fe²⁺,and relatively enriched with heavier Fe isotopic compositions,which leaded to the higher Fe³⁺/∑Fe ratios andδ⁵⁶Fe values in the melt phase.The high Fe³⁺/∑Fe ratios of the original melts widened the stable domain of magnetite,resulting in the early separation crystallization of magnetite at Mg^#=60 and the removal of heavy Fe isotopes,leading to a slight decrease of Fe isotopes in the residual melt.The redox state of the mantle controls the dissolution and precipitation of sulfides,which makes the recycled carbonate possibly indirectly affect the geochemical behavior of sulfides in the mantle.We found that the tholeiites have relatively uniform Cu isotopic compositions(δ⁶⁵Cu=0.16±0.09‰,2SD),which is slightly higher than that of the MORB(δ⁶⁵Cu=0.07±0.06‰,2SD).However,the Cu isotopes of basanites and alkali basalts vary greatly（from-0.13‰to 0.27‰）and decrease with the decrease of Cu contents.The relatively high Fe³⁺/∑Fe ratios of primitive basanites and alkali basalts broadened the stable domain of magnetite,resulting in the separation crystallization of magnetite at Mg^#=60,and the Cu contents andδ⁶⁵Cu values of these samples also plummeted at Mg^#=60.Combined with petrographic observations,we believe that the separation crystallization of magnetite causes"SO₄^2-+8Fe²⁺O=S^2-+8Fe³⁺O_1.5"to proceed to the right and promotes the early sulfide saturation.In addition,the separation crystallization of sulfide in the relative oxidizing condition is relatively enriched with heavy Cu isotope,which will lead to the decrease of Cu isotopes and Cu contents in the residual melts.In addition,combined with the distribution coefficients of Cu and Ag in different sulfides,the Cu/Ag ratios of basanites and alkali basalts decreases with the decrease of MgO contents,indicating that the separation crystallization of sulfide is monosulfide solid solution（MSS）.In addition,petrographic observations have demonstrated that CO₂-rich volatile phases form complex droplets with sulfides,and low-density CO₂-rich volatile phases provide additional buoyancy and may be the physical agent that assists and facilitates the migration of sulfides from the mantle to the crust.This may be a common but mysterious mechanism that promotes the cycle of volatile materials and metals from the mantle to the shallow crust,but leaves little footprint when magma reaches the Earth’s surface due to degassing.