Temporal And Spatial Variations In Chalcophile Element Signature Of Subduction-related Magmas Of Izu-bonin-mariana Arc/back-arc Basin System

Metal transfer and enrichment driven by subduction recycling have been considered to exert profound influences on earth’s supergene environment and global metallogeny.Because most economic-valued chalcophile metals（e.g.,Cu,Au,Pd）are hosted by sulfide phases,understanding the behavior of sulfides and related chalcophile metals during slab subduction and arc magmatism is fundamental if we are to identify the subduction-related processes that are responsible for the formation of subduction-related ore deposits.Izu-Bonin-Mariana（IBM）arc-basin system provides a classic example of convergent plate margin developed above cold subduction zone.On the perspective of arc development from initiation to mature stages of oceanic subduction,this thesis present comprehensive analyses of chalcophile element contents and Cu isotopes of volcanic rocks with spatiotemporal relationships from IBM system,with the aims of studying influences of subduction-related processes（e.g.,subduction recycling,prior mantle depletion）on the composition and behavior of chalcophile metals and Cu isotopes in IBM magmas and placed constraints on redox conditions of slab materials at various stages of oceanic subduction.The oxidized and hydrous characteristics of arc and back-arc magmas,which is induced by subduction input,not only regulate the sulfide saturation history of the magma but also have profound influence on metal transfer and enrichment for ore mineralization.How slab subduction controls spatial difference in composition and behavior of chalcophile element in arc and back-arc magmas is still unclear.Based on the detailed petrological and geochemical studies,we present high-precision analyses of whole-rock chalcophile element（S,Cu,Ag,Re,Pd,Pt,Ru,Ir）contents for a suit of basaltic lavas（n=15）from mature spreading center of the Mariana Trough.The Mariana Trough of this study were influenced by subducted components,however,the extent was limited so that these basalts remain overall reducing features similar to MORB magmas,as indicated by low V/Ti（<0.05）and low Fe³⁺/∑Fe（<0.2）ratios in basaltic glasses.Consequently,sulfide saturation is achieved at the early stages of magmatic evolution,as reflected by changes in whole-rock contents and ratios of strongly chalcophile elements,as well as occurrence of sulfide inclusions in high-Mg olivine crystals(Fo₈₀).Combined with compiled data from other island arc and back-arc lavas,we propose that the extent of adding slab-derived components,regardless of specific tectonic settings,are determinants of sulfide saturation states and behavior of chalcophile element in magmatic systems developed above cold subduction zones.Early magmatic sulfide saturation at central Mariana Trough would deplete the magmas in most precious metals such as Cu,Ag and Au,resulting in barren magmatic systems that are not suitable for large-scale Cu-Au mineralization at the seafloor.The behavior of chalcophile elements in subduction metamorphism and arc magmatism do not necessarily identical at all stages of oceanic subduction.Based on the petrogenesis of early IBM lavas,the chalcophile element contents of forearc basalts（FABs）,boninites and adakaites were analyzed to constraint the effects of adding slab-derived fluids and melts on the composition and behavior of chalcophile metals in subduction-related magmas of various stages of oceanic subduction.The MORB-like FABs display smooth,inclined primitive mantle-normalized Re-Cu-Ag-Au-PGE patterns similar to those of MORBs,suggesting retention of sulfide in their mantle source.The boninites,which have formed during fluid-fluxed melting of the previously depleted mantle,are uniformly enriched in Pd（>10-40 ng/g）and Pt（>5-13ng/g）with variable low Cu（35-84μg/g）,Ag（14-47 ng/g）and Au（0.63-2.76 ng/g）contents and low Cu/Pd（～1690-8320）ratios,comparable to previously published data for IBM boninites and other magnesian low-Ti lavas worldwide.These results suggest that the fluid-induced melting and oxidation promote the complete sulfide dissolution and effective release of chalcophile metals from mantle wedge to primitive boninites,reconciling with sulfide-undersaturated early-stage evolution of boninites.The subsequent adakites were also derived from highly-depleted mantle source,but incorporate higher proportions of slab melt in their composition（e.g.,Sr/Y>50）than those in boninites.Adakites display lower strongly chalcophile element contents（e.g.,Cu<50μg/g,Pd<5ng/g and Pt<3ng/g）than boninites and show MORB-like Cu/Pd（～16800-41500）as well as slightly lower Cu/Ag（844-4802）ratios,implying retention of sulfide phases in their source（s）.These features probably reflect that,compared with fluid-induced melting,the addition of Si-rich and Fe-poor slab melts lowered the sulfur content at sulfide saturation（SCSS）of adakitic magmas,inhibiting the complete sulfide dissolution and extraction of chalcophile elements.It’s remains controversial if slab-driven metal addition is effective and whether it occurs at certain stages of subduction.We explore this issue on a perspective of arc development from initiation of subduction to mature arc volcanism,by studying the ore-forming and redox-sensitive metal Cu and its isotopes(δ⁶⁵Cu)in FABs,boninites,adakites as well as MORBs from the East Pacific rise.The oxidized,hydrous boninites and subsequent adakites display low Cu contents,and their Cu/Sc ratios andδ⁶⁵Cu are similar to those of FABs,MORBs and unmetasomatized mantle rocks,suggesting that subducted Pacific slabs from initial to mature subduction contribute little Cu to the mantle wedge.This likely further reflects the overall reducing nature of slab materials added to the mantle wedge,whereas Cu and other chalcophile metals remains trapped by reduced sulfides in the subducting slabs.However,we propose that the subsequent reaction of such reducing,hydrous slab melts with peridotites and flux melting produce oxidized primitive arc magmas deep in the mantle wedge.This model reconciles the oxidized nature but no significant Cu enrichment in the metasomatized mantle sources of magmatic arcs.These results also highlight the importance of the magmatic-hydrothermal differentiation in the crust for the formation of subduction-related ore deposits.In summary,this study presented comprehensive datasets of high-precision chalcophile element contents of arc-related lavas from IBM system.The results revealed how subduction recycling controls the spatial and temporal variations in composition and behavior of chalcophile elements in magmatic systems above cold subduction zones.The Cu contents and Cu isotopes of IBM lavas reveals ineffective slab-driven Cu addition and overall reducing nature of slab-derived materials during all stages of oceanic subduction.These results lays a foundation for better understanding the influence of subduction recycling on mobilization,transfer and enrichment of chalcophile metals within ore-bearing arc systems.