Lithium Isotope Geochemistry Of Metasedimentary Rocks From Dabie-Sulu Orogen And Their Implication For Tracing

The development of multi-collector inductively coupled plasma mass spectrometry(MC-ICP-MS)have greatly improve the analytical precision of Lithium(Li)isotopes,making Li isotopes have been widely applied to various kinds of geological processes.Li is the lightest metal element,with two stable isotoples,6Li(7.6%)and 7Li(92.4%).The large relative mass difference(17%)and diffusive rate difference between 6Li and 7Li lead to signigicant Li isotope fractionation in geological processes(up tp 80‰).Over the past few decades,Li isotope compositions of various geological reservoirs have been extensively investigated.Different geological reservoirs have distinguished Li isotope compositions,especially between crust and mantle,making Li isotopes could be potential for tracing crust/mantle materials recycling.However,the Li isotopes data of metamorphic rocks in subduction zones,especially ultra-high pressure(UHP)metamorphic rocks,are incomplete.Except UHP eclogite,the Li isotopes data of other UHP metamorphic rocks are bare,and the Li isotope fractionation scale during subduction dehydration is still controversial.Therefore,in this dissertation,we carried out comprehensive Li isotope geochemical studies by representative samples to shed new constraints for the above scientific issues.The first research subject in this dissertation is to study the Li isotope geochemistry of UHP marbles from the Dabie-Sulu orogenic belt,China,to explore their implication for deep carbon cycling.'Carbon cycle' between Earth's surface and interior is critical for maintaining a habitable climate over geologic time.Subduction zones are main sites for returning carbon to the mantle.Here we report the Li elemental([Li])and isotopic compositions(?7Li)of 36 fresh ultra-high pressure(UHP)marbles from the Dabie-Sulu orogenic belt,aiming to reveal the geochemical behaviour of Li isotopes during deep subduction processes of surficial carbonates,and to test explore the application of Li isotopes for tracing deep carbon cycling.We firstly demonstrate that the Li isotopes compositions of UHP marbles(?7Li=+1.3 to+21.5‰,average=+8.3‰)is significantly heavier than that of the mantle(?7Li=+2.0 to+5.7‰,average=+3.7‰),whereas their Li concentrations([Li]=0.5-6.5 ?g/g,average 1.9 ?g/g)are comparable with the mantle(0.9-2.7 ?g/g,average 1.5 ?g/g).Excluding possible influence from weathering alteration,fluids infiltration,and Li isotopic fractionation during subduction or exhumation,the exhumated UHP marbles can reprensent the Li elemental and isotope composition of the residual carbonates entering into deep mantle.As the UHP marbles contain varying proportion of silicates,the impurity may be the main cause for the large range of ?7Li of UHP marbles.In all,the subducted carbonates could transport the heavy Li signature into the mantle depth to form locally Li isotopic heterogeneity.Such UHP marbles may provide new explanation for mantle-derived samples with elevated Li isotopic composition.Because Li is a trace element in the mantle,and the marked difference of ?7Li between UHP marbles and the mantle,we highlight Li isotopes as a new potential tracer for deep carbon cycling.The second research subject in this dissertation is to study the Li isotope geochemistry of UHP jadeite quartzite from the Dabie-Sulu orogenic belt,China,and to d to provide insights into the action of ultrahigh-pressure(UHP)metamorphic fluids during continental deep subduction.The eight fresh jadeite-quartzite samples show higher Li concentrations(3.6 to 17.0 ?g/g,average 11.2 ?g/g)and lower ?7Li values(-2.6 to-13.5‰,average-6.0‰)compared to the mantle([Li]=1.5 ?g/g,?7Li=+3.7‰).The whole rock ?7Li values positively correlate with the Li concentrations,negatively with Th/U ratios,consisting with fluids metasomatic reaction.For separated minerals,the 17Li(quartz-jadeite)of fresh samples is almost constant at 3‰,indicating equilibrium Li isotopic fractionation during the formation of jadeite-quartzite.Modelling results suggests that the jadeite-quartzite would be precipitated from the UHP metamorphic fluids with low Li concentration and light Li isotopic composition.This is the first case that directly indicates the Li isotopic composition of UHP metamorphic fluids in subduction zones.The seven retrograde samples have lower Li concentrations(2.1 to 13.6 ?g/g,average 5.3 ?g/g)and higher 87Li values(0 to+4.3‰,average+1.9‰)than fresh samples.Plus the ?7Li(quartz-symplectite)(1‰)of retrograde samples is almost constant at 1‰,implying the Li isotopic re-equilibrium between jadeite-quartzite and retrogressive fluids.Fluid-assisted diffusive-driven isotope fractionation over a small scale is likely to explain the Li data of these retrograde samples.Therefore,Li isotopes of jadeite-quartzite could be useful proxy for metamorphic fluids and metasomatic processes.