Properties Of Subduction Zone Fluids As Constrained By Metal Stable Isotopes

Subduction zone fluids released from hydrated minerals are crucial to crust-mantle material exchange,arc volcanism,redox state of mantle and arc lavas,earthquake,and mineralization.Although numerous studies related to subduction zone fluids have been done,many questions still remain unclear,such as the influence of subducted carbonate and siliceous rocks on fluid compositions,the redox state of subduction zone fluids,the alteration of fluid during migrating in the subduction channel,and the origin of the fluids.Metal stable isotopes have been developed rapidly in the past two decades.With the improvement of analytical methods,the exploration of different reservoirs,and the study of isotope fractionation mechanisms,metal stable isotopes have been widely applied to cosmochemistry,crust-mantle formation and evolution,magmatism,mineralization,and palaeoenvironment research.Meanwhile,with the advantage of tracing sources and geological processes,metal stable isotopes are also a novel tool to study the properties of subduction zone fluids.For example,silicon is enriched in subduction zone fluids,and silicon isotopes can be highly fractionated in the high-temperature hydrothermal process.Iron is a multivalence element and its isotopes can reflect the redox state.Barium is highly enriched in crustal materials,and barium isotope is sensitive to water-rock interactions.Combining these metal stable isotope systematics with fluid-related metamorphic rocks,some problems that often be ignored by traditional geological methods may be solved.In this thesis,we study the Si-Fe isotopes of Myanmar jadeitite related to oceanic subduction zone fluids,and the Ba isotopes of whiteschists in Western Alps and jadeite quartzites in Dabie area that associated with the continental subduction zone fluids,respectively.We wish to provide new constraints on the source of fluid solutes and on the variation of fluid and rock compositions during fluid-rock interactions.Myanmar jadeitites resulted from fluid crystallization or fluid-ultramafic rock interaction in the forearc area.They have provided a unique record for the chemical compositions and properties of subduction zone fluids.The white jadeitites,directly precipitated from fluids,have higher ?30Si(-0.04‰ to 0.23‰)than any igneous rocks reported in the literature.The green jadeitites(-0.35‰ to 0.03‰)and amphibole-rich blackwalls(-0.33‰ to 0.05‰),formed by fluid metasomatism,have ?30Si values generally lower than those of white jadeitites.These data indicate that the jadeitite-forming fluids have heavier Si isotope compositions than the bulk silicate Earth.Based on the equilibrium Si isotope fractionation factors among jadeite-quartz-fluid,it is inferred that the subduction zone fluids have high ?30Si of about 0.7‰ to 1.2‰.The?30Si of jadeitites and amphibole-rich blackwalls show a positive correlation with SiO2 content with a slope of 0.0639,which is larger than that of the igneous rock array(0.0056).Such a correlation cannot be produced by magmatic differentiation or mineral fractional crystallization from fluids.Instead,it can be explained by a binary mixing model between fluids and mafic-ultramafic rocks.The model shows that about 0 to 25%of Si in green jadeitites and amphibole-rich blackwalls are from the subduction zone fluids.Since the fluids derived from abyssal clay sediment,altered oceanic crust,and mantle serpentinite have relatively low ?30Si,it is proposed that the deep-sea siliceous rocks with high ?30Si are likely to be the main source of Si in subduction zone fluids.In summary,this study shows that Si isotopes have useful application in tracing the source of solute in subduction zone fluids.In contrast to the Si isotope features,two types of jadeitites in Myanmar display much lower ?56Fe values than normal igneous rocks.White jadeitites have variable?56Fe from-0.60‰ to-0.04‰,while green jadeitites(?56Fe=-0.44‰ to-0.19‰)the associated amphibole-rich blackwalls(?56Fe=-0.47 to-0.23‰)have similar variation.The analysis shows that the light Fe isotope compositions of the jadeitites are not induced by weathering,kinetic fractionation,or dissolution-reprecipitation processes.The correlations of ?56Fe with Mg/(Mg+Fe),V/Sc,Zn contents,and Eu/Eu*in white jadeitites suggest the mixing between low-?56Fe fluids and oceanic crust-derived fluids.The correlations of ?56Fe with Mg/(Mg+Fe),Ba/Th,and Cr contents in green jadeitites record the interaction of low-?56Fe fluids with chromites.Such low-?56Fe fluids contain iron released from Fe-bearing carbonates.The low ?56Fe signature and CH4-bearing fluid inclusions in the jadeitites suggest the locally reducing property of subduction zone fluids.Subducted Fe-bearing carbonates can effectively influence the Fe isotope compositions of ambient mantle and arc lavas,which can also facilitate abiotic hydrocarbon generation.The whiteschists in the western Alps are ultrahigh-pressure metamorphic rocks that resulted from strong infiltration of the metagranites by fluids released from serpentinite.The Ba isotope signature of whiteschists can record whether fluid compositions have been modified during transport through the channel,because Ba isotopes can be significantly fractionated in the fluid-related processes.Here we measured Ba isotope compositions of the whiteschists and metagranite protoliths.The metagranites have a relatively small variation of ?138Ba(-0.25‰ to 0.26‰),while the?138Ba of whiteschists dramatically increase from-0.99‰ to 0.48‰ with the progressive loss of Ba.Such fractionation is not induced by weathering,retrograde metamorphism,or kinetic effects,but reflects fluid-rock interactions in the subduction channel.We applied a conceptual model to simulate the Ba isotope fractionation process during the intensive fluid-rock interactions.Our results demonstrate that the subduction channel is not only a mass transporting pathway but also the critical fluid-rock interaction zones where the chemical compositions of the slab-derived fluids are strongly altered.The Ba isotope compositions of the fluids may be heavier than the rocks during fluid-rock interaction processes.The modification of fluid composition in the subduction channel will significantly affect the recycling of subducted materials and Ba isotope compositions of arc lavas.Different from whiteschists in the Western Alps,the jadeite quartzites in Dabie area have variable but lower ?138Ba values(-0.50‰?0.11‰)than the average upper continental crust.The ?138 Ba is positively correlated with the logarithm of Ba contents.The Ba isotope compositions become lighter with the decrease of fluid-mobile elements(K,Cs,and Rb)and the increase of Al,Mg,and Ca contents.These observations reflect the loss of heavy Ba isotopes and fluid mobile elements with the influence of biotite-released fluids.Likely suffered from more intense fluid metasomatism,the jadeite quartzites at the east of the studied area have lighter Ba isotopes than the western ones.The continental subduction zone fluids are of great significance for the formation of post-collisional arc-like magmatisms.In summary,it is shown that subducted materials,especially some special sedimentary endmembers,have a great influence on fluid compositions.The subduction channel is a strong fluid-rock interaction zone,where the fluid and rock compositions are changed greatly.Therefore,metal stable isotopes are powerful tools to study the source of fluid solutes and the variation of fluid compositions in the subduction zones.