Lithium Isotope Geochemistry In Subduction Zone Metamorphism And Magmatic Process

In recent years,with the development of multi-collector inductively coupled plasma mass spectrometry(MC-ICP-MS)and the advance of stable isotope measurement technology,non-traditional isotope geochemistry has obtained a rapid progress and has been widely applied to various kinds of geological processes.Lithium(Li),the lightest metal element,has two stable isotoples,6Li(7.6%)and 7Li(92.4%),with large relative difference in mass(?17%).Hence,Li isotope fractionation is signigicant in geological processes(up tp 800‰).As a trace element,Li is widely existed from ultrabasic rocks to acidic rocks and ocean,lake,river and various geological hydrothermal systems.Meanwhile,Li is a incompatibility and highly fluid mobile element.Therefore,it can trace the crust-mantle and fluid-rock interactions in the subduction zones.In addition,over the past few decades,Li isotope compositions of different geological reservoir has been extensively investigated,the results show that different geological reservoirs have distinguished Li isotope compositions.Therefore,Li isotope system can be widely used to trace various geological processes and sources.However,the genetic mechanism of the Li isotopic composition in the high pressure metamorphic rocks in subduction zones,the Li isotope fractionation scale during subduction dehydration,the genetic mechanism of the Li isotopic composition in the island are magma and the Li isotopic fractionation in mineral scale are hotly debated.Therefore,in this dissertation,conprehensive Li isotope geochemical studies have been carried out on the representative samples to constrain the above scientific controversy.In this dissertation,firstly,we carried out the studies of the metamorphism and fluid evolution in Sumdo eclogite.The Sumdo metamorphic belt,located in the Lhasa terrane of Tibet,is a key area for unraveling the Paleo-Tethys evolution.In order to identify the fluid evolution in oceanic subduction zones,the eclogite derived from altered oceanic crust has been investigated in a combined study of petrography,mineral geochemistry,fluid inclusion and oxygen isotopes.On the basis of petrography and mineral geochemistry,two kinds of garnet were distinguished.The core part of Grt-I shows increasing grossular and contains prograde mineral inclusions,suggesting that it grew at a prograde stage,while the rim of Grt-I shows varying grossular and pyrope contents,representing subsequent overgrowth.Grt-II displays the same trend as the outermost of Grt-I,indicating that Grt-II grew at the peak or initial retrograde stage.Garnet trace element zoning patterns and P-T estimate suggest that the Sumdo eclogite experienced a heating stage at an initial exhumation after the peak.Based on textural criteria and host minerals,three types of fluid inclusions were distinguished:Type-I are primary NaCl dominated intermediate to high-salinity(10 to 22 wt%)fluid inclusions hosted by omphacite;Type-II are primary low-salinity(1 to 6 wt%)fluid inclusions hosted by epidote/clinozoisite;Type-III are secondary low-salinity(0 to 7 wt%)fluid inclusions hosted by matrix(i.e.,minerals other than the mineral inclusions)quartz and rarely by omphacite.Type-I inclusions hosted in omphacite reflect the composition of eclogite-facies fluids,which were most likely released from the dehydration of the subducting oceanic crust.During uplift the fluids evolved towards low-salinity solutions as represented by type-? fluid inclusions in epidote/clinozoisite.Type-III inclusions are secondary and may be formed at a very late stage.In addition,the eclogite displays similar 8180 values(8180 = +5.0 to +8.9‰)compared to its protolith,a low temperature altered oceanic crust(+4.9 to +12.7‰).The equilibrium oxygen isotope fractionations(?18Qtz-Min)between quartz and garnet/omphacite suggest isotope equilibrium at the peak metamorphic stage,indicating closed fluid system conditions during eclogite facies metamorphism.Combined with the present data,we conclude that the Sumdo eclogite experienced "hot" exhumation,during which hydrous minerals decompressed and released large amounts of fluids.The fluids in Sumdo eclogite were dominated by aqueous fluids with various salinities and preserved by fluid inclusions in HP metamorphic rocks.In comparison with continental subduction zones,the fluids regime in oceanic subduction zones are relatively simple and may reflect that the sources are dominated by the seawater altered slabs.Aiming to constrain the Li isotope characteristics of eclogite rocks in subduction zone,this dissertation have investigated whole-rock major and trace elements,Li isotopes as well as oxygen and radiogenic(Sr,Nd,and Pb)isotopes on 20 Sumdo eclogite samples.Samples display REE patterns similar to N-MORB,relatively homogeneous bulk sNd(t=260Ma)values ranging from +4.0 to +7.8,but heterogeneous initial 87Sr/86Sr ratios from 0.703703 to 0.706645 and ?180 values from +4.9 to +8.9‰.These data suggest that protolith of the Sumdo eclogite is N-MORB.Meanwhile,the correlation between Sr-Nd isotopic compositions and oxygen isotope characteristics suggest that the MORB protoliths had experienced low temperature(T)seafloor alteration prior to subduction.The Li isotope study shows that Sumdo eclogite has high contents(3.64 to 29.7 ppm)and lighter Li isotopic composition(-1.1 to +4.0‰)compared to N-MORBs.There is no correlation between 87Li values and 818O values,indicating that the eclogite did not inherit the ?7Li values from the protolith.According to Rayleigh fractionation model,this dissertation simulated the Li isotope fractionation of altered oceanic crust during the subduction with varying parameters.The results show that the residual of the slab display low Li content and higher ?7Li values and are inconsistent with the observed Li isotopic compositions in Sumdo eclogite.Considering the eclogite underwent the amphibole facies retrograde metamorphism,which may be enough to hydrate the eclogite.Therefore,the dehydration of sediments are modelled by Rayleigh fractionation.The fluid metasomatism between sediments derived fluids(<5%)and residul slab could simulate the Li isotopic compositions in Sumdo eclogite.Therefore,the dissertation suggest that the fluid metasomatism in the later stage of eclogite may be an important reason for the formation of its high Li contents and low?7Li values.In order to explore the origin of the Li isotopes signatures of the island arc magma,this dissertation systematically studied the geochemical compositions of the Kamchatka arc.The Kamchatka peninsula,located in the far east of Russia,is a key area for the convergence of the northeast of Eurasia and the Pacific plate.The volcanism in Kamchatka is extensively developed.According to the depth of benioff,it can be divided into three areas:(1)the Eastern Volcanic Front(EVF),(2)the Central Kamchatka Depression(CKD)and Northern Central volcano(NCKD),(3)Sredinny Ridge(SR)back arcs.Li isotopic research of Kamchatka arc shows that the arc lavas contain Li concentration of 1.45-16.7 ppm(average 8.8 ppm),?7Li values of + 1.8?+ 8.6 ‰(average + 4.2 ‰),similar to N-MORB values(+ 3.4 ± 1.4 ‰),falling in the range of published island arc data(-8.6?+ 10.0 ‰).There is no significant correlation between ?7Li and subduction depth,SiO2,MgO and K2O,indicating that there is no Li isotope fractionation in the high temperature magmatism.NCKD lavas,with adakite signature,show no correlation between ?7Li and Sr/Y,indicating that there is no significant fractionation during the melting of subducting plate.Kluchevskoy lavas in CKD area display significantly higher ?7Li values(+4.5?+8.6 ‰)than Tolbachik volcanoe(40 km away form KLU,+3.6?+4.7 ‰),suggesting that the subduction of Emperor Seamounts derived more fluid.The EVF lavas show lower ?7Li and higher Pb isotopic composition,indicating that there may be the sediment relased fluids.While,the CKD lavas display high 87Li(+ 3.6-+8.6 ‰)and the lower Pb isotopic composition,which indicates that there may be the fluids derived from the altered oceanic crust.Different fluid source in different depth may be one of the important causes for irregular change of Li isotopic compositions in most of the subducted island arc in the world.The Jinshan granite intrusion,located at the southeast edge of north China craton,was derived from partial melting of the subducted crustal materials of the Yangtze craton at 162 Ma.Pegmatite dikes in Jingshan granite intrusion,mainly garnet granites,contain large garnet grains,showing high Li content,which are suitable for the study of Li isotope geochemistry in mineral scale.Major and trace elements compostions of garnet in pegmatite display strong zonations.From the core to the rim,grossular components decrease gradually,while,spessartine,almandine increase,which represents that the pressure and the temperature is increasing.Trace element study shows the depeletion of light rare earth element and the enrichment of heavy rare earth element.Garnet displays REE zonation,that the core contains higher concentration than the rim and the mantle show the minimum and less in between.Especially,there is a positive correlation between Li elements and heavy rare earth element(e.g.,Dy).The zonations in garnet and the positive correlation between Li and heave rare earth element demonstrated that the growth information during magmatism was preserved and diffusion process has no obvious effect.Li isotope study of micro drill samples shows that the 87Li values range from-1.2 to + 0.6 ‰.From core to rim,garnet shows zonation in ?7Li values,namely the ?7Li values decrease to the mantle,and then increasing until the rim.The above evidences suggest that Li isotope information in magmatism can be preserved in the garnet and the study of mineral scale Li isotope geochemistry is an important means to trace the geological processes.