Zn-Ca Isotopic Compositions Of Oceanic Crust And Arc Igneous Rocks And Their Significance For The Cycling Of Subduction Components

The exchange of materials and energy between the Earth’s surface and interior has widely affected the habitability,physical and chemical composition of crust and mantle,and the migration of ore-forming elements.Specifically,the migration and cycling of carbonates in subduction zones are closely related to climate change and are also one of the most important scientific issues in recent years.The subducted oceanic crust,as the main source of subduction components,makes an important contribution to the cycling of carbonates,while the arc magmas are the most direct manifestation of subduction components returning to the surface,and both connect the Earth’s surface and interior.The slab-derived fluid is the most important medium for the migration of materials and elements,including carbonates.However,how to trace the cycle of initial subduction materials and its impact on the mantle wedge and arc magma is the most critical link in the process of subduction.Zinc and Ca stable isotopes show great potential in tracing the migration and cycling of subducted materials,especially carbonates.Accurately constraining the Zn-Ca isotopic compositions of the initial subduction material（oceanic crust）and subduction products（island arcs）,as well as understanding the behavior of Zn-Ca isotopes during oceanic crust formation and subduction-related magmatic processes,are prerequisites for using Zn-Ca isotopes to trace carbonate-bearing fluid cycling.In this thesis,a systematic study of major and trace elements,Zn and Ca stable isotopes,was conducted on a series of oceanic gabbros,mid-ocean ridge basalts（MORBs）,and arc lavas,combined with petrology and other conventional geochemical studies,to constrain the Zn and Ca isotopic compositions of the oceanic crust and arcs,and to explore the migration and cycling process of subducted materials,especially carbonate-bearing fluids.This study presents Zn isotopes for gabbors from ultraslow Southwest Indian Ridge（SWIR）and MORBs from East Pacific Rise（EPR）and South Mid-Atlantic Ridge（SMAR）to understand the behavior of Zn isotopes during magmatic differentiation and Zn isotopic composition of oceanic crust.The gabbros from the SWIR underwent multiple magmatic events and strong differentiation,exhibiting a wide range ofδ⁶⁶Zn values（0.11‰to 0.34‰）with a negative correlation with MgO content,indicating that magmatic differentiation has a significant impact on Zn isotopes.In contrast,EPR MORB（0.27±0.04‰）and SMAR MORB（0.26±0.06‰）have relatively homogeneousδ⁶⁶Zn values,which are higher than the average value of gabbros（0.22±0.03‰）.These results suggest that Zn isotopes get heavier during the formation of the oceanic crust,resulting in variable Zn isotopes at different depths of oceanic crust.This variation likely a result of the continuously crystallization of olivine and clinopyroxene will preferentially remove light Zn isotopes and increaseδ⁶⁶Zn in the residual melt.The differentiation model shows that theδ⁶⁶Zn increase caused by the primary MORB melt differentiation does not exceed 0.08‰,and theδ⁶⁶Zn of gabbros with great changes is the result of superposition of melt-rock reaction.Using the weighted averageδ⁶⁶Zn values of gabbros and MORBs,theδ⁶⁶Zn of oceanic crust was estimated to be 0.23±0.04‰,which is about0.05‰lower than the MORB-based values（0.28±0.05‰）.This value enhances the Zn isotopic discrepancy between oceanic crust and OIBs（up to 0.40‰）,further emphasizing the importance of recycled surficial carbonates in improving Zn isotopes in their mantle sources.The study of the Zn isotopic composition of oceanic crust provides a basis for using Zn isotopes to trace the cycling of carbonates in the oceanic crust.This study analyzes the Zn isotopic compositions of a series of arc lavas with spatiotemporal relationships from the Izu-Bonin-Mariana（IBM）arc system,including fore-arc basalts（FAB）,boninites,and adakites,Mariana arc lavas,and back-arc basin lavas.These rocks represent magma products from the subduction initiation to mature stages and are from fore-arc to arc to back-arc basin,aiming to trace the influences of slab melts/fluids on the mantle wedge in different subduction stages and depths from same arc system.Each rock type has a relatively constantδ⁶⁶Zn value with small variations,including FAB（0.23±0.04‰）,boninites（0.22±0.04‰）,adakites（0.29±0.04‰）,Mariana arc lavas（0.26±0.04‰）,and back-arc basin lavas（0.26±0.04‰）.The lack of correlations betweenδ⁶⁶Zn and indicators such as MgO and Ba/La suggests limited influences of hydrous arc magma differentiation and slab fluid on the mantle wedge.However,the higherδ⁶⁶Zn in adakites from a highly depleted mantle source compared to boninites may represent a significant contribution from slab melts.The Zn isotopic features of all arc rocks can be effectively explained by partial melting of mantle sources ranging from refractory harzburgite to relatively fertile lherzolite mantle.The relatively lowδ⁶⁶Zn of FAB and boninites compared to MORBs and other rock types are primarily formed by partial melting of depleted-highly depleted refractory harzburgite mantle,while Mariana arc lavas and back-arc basin lavas withδ⁶⁶Zn consistent with MORBs are formed by partial melting of source regions similar to DMM.Compared with other mantle-derived magmas,significantly higherδ⁶⁶Zn in some OIBs and alkaline rocks require contributions from low-degree partial melting and recycled materials,such as oceanic crust or carbonates.These results suggest that the migration of Zn in slab fluid is limited,and Zn and carbonate may be decoupled,while slab melts have a stronger migration capacity for carbonates and Zn.Given that Ca is a major component of carbonates,it may reflect the migration of carbonate in subduction zones more directly.We further investigated the Ca isotopes of arc lavas from Tonga and Mariana arcs to explore the role of fluid-mediated carbonate transfer in subduction zones.Fresh basalts（0.84±0.01‰）and dacites（0.84±0.10‰）from the Tonga arc show indistinguishableδ^44/40Ca,reflecting negligible Ca isotopic fractionation during differentiation of hydrous arc magmas.More importantly,arc lavas from both the Tonga arc（0.84±0.09‰）and Mariana arc（0.79±0.12‰）display MORB-likeδ^44/40Ca values.The MORB-likeδ^44/40Ca of arc lavas indicates that the carbonates released from altered oceanic lithosphere（AOL）do not significantly modify the Ca isotopic composition of the mantle wedge,although extensive volcanic CO₂ degassing at both arcs suggests that slab fluids might introduce abundant carbonate into the depleted mantle wedge.These results could be attributed to a limited Ca budget in the slab fluids added to the mantle wedge and/or homogenization effect of variableδ^44/40Ca for the slab fluids.At cold subduction zones,a fraction of carbonates from the AOL may survive during slab dehydration and recycle into the deep mantle.This study first constrained the Zn isotopic compositions of the most important initial subduction material（oceanic crust）,providing data support for evaluating the proportion and scale of oceanic crust and carbonate cycling using Zn isotopes.And first discussed the contribution of subduction components from different subduction stages and deep by Zn isotopes of volcanic rocks from the same island arc system with different subduction time and space.Finally,combined with Ca isotopes in different arc systems,this study explored the migration and cycling of carbonate fluid in subduction zones.These research results provide necessary theoretical support for a better understanding of the behavior and fate of carbonate fluids in slabs and also lay a foundation for utilizing Zn and Ca isotopes to trace deep carbon cycling.