Silicon Isotope Fractionations During Magmatism,Metamorphic Fluid Activities,and Chemical Weathering

Silicon(Si)is one of the most important elements in the crust and the mantle.As the main component of Si-O tetrahedron,Si is involved in almost all geochemical processes in the earth(BSE).Previous studies have shown that many geological processes can lead to fractionation of Si isotopes.Therefore,as a useful tool,Si isotope has been widely used in the studies on different geological fields including high-and low-temperature processes.In this dissertation,I have investigated Si isotope fractionations during magmatic processes,subduction-zone fluid activities,and basalt weathering.Establishment of a high-precision Si isotope analytical method is the precondition of geological applications of Si isotopes.Based on previous works,I have established Si isotope analytical method,including chemical purification of Si,and measurement of Si isotopes by MC-ICP-MS at the USTC.The chemical procedure includes alkaline fusion followed by ion-exchange chromatography.The overall recovery of Si and matrix tests show the robustness of our method.Silicon isotope ratios were determined by MC-ICP-MS at the USTC,and sample-standard bracketing method was utilized,with the international standard NBS-28 as the bracketing standard.I have analyzed four different kinds of USGS standards,and the results are consistent with data reported in the literature.Measurement of the basalt standard BHVO-2 yielded a long-term average ?30Si of-0.29±0.06‰(2SD,n=249)in over two years,showing the high accuracy and precisionof our data.At present,only a few studies have been published on the Si isotope fractionations during high-temperature geological processes.There are many problems remaining unresolved.For example,in the subduction zone,large amounts of crustal materials are subducted into the mantle,and it is still unclear whether this will lead to Si isotopic heterogeneity of the upper mantle.Beside this,igneous rocks may record their formation and evolution processes through Si isotope characteristics.To understand the mechanism of Si isotope fractionation during highly differentiated igneous processes,I have presented Si isotope data for a suite of bimodal volcanic rocks from Hailar Basin,northeast China.The results show that Si isotopes are resolvably fractionated between the mafic and felsic rocks.Among all samples,basaltic trachyandesites have the lowest ?30Si values,from-0.34±0.03‰ to-0.20±0.01‰,trachytes-rhyodacites have 30Si values from-0.25±0.01‰to-0.05±0.06‰,and rhyolites have the highest 30Si of-0.16±0.05‰ to-0.07±0.03‰.The 30Si values of these samples are linearly correlated to their SiO2 contents,and the trend matches with the magma differentiation trend.Furthermore,the 30Si values also correlate with other indicators of magma differentiation such as MgO contents.These evidences indicate that magma differentiation processes are the main causes of Si isotope fractionation in the Hailar volcanic rocks.To understand Si isotope fractionation during metamorphic fluid activities in the subduction zone,I present the Si isotopic compositions of two high-to ultrahigh-pressure(HP-UHP)eclogite-vein systems from the Ganghe and Hualiangting areas in the Dabie orogen,eastern China.The results show that Si isotopes are significantly fractionated between the veins and their host eclogites in both areas.The ?30Si values of Ganghe eclogites range from-0.50‰ to-0.39‰,higher than that of the studied omphacite--epidote vein(-0.63±0.04‰).The Hualiangting eclogites have 830Si values of-0.36‰ to-0.29‰,whereas the ?30Si values of the Hualiangting multi-stage veins show greater variation,from-0.45‰ to 0.05‰,revealing significant Si isotope fractionation during metamorphic fluid evolution and vein formation.For the Hualiangting and Ganghe samples the enrichment of heavy Si isotopes in minerals follows the order of ?30Simuscovite(-0.01 to 0.13‰)?830Siquartz(-0.14 to 0.10‰)>?30Siomphacite(-0.63 to-0.33‰)??30Siepidpte(-0.60 to-0.30‰)??30Sikyanite(-0.42 to-0.28‰)830Sigarnet(-0.92 to-0.44‰).The equilibrium Si isotope fractionation factors between metamorphic minerals were calculated from first-principles methods based on density-functional theory.The reduced partition function ratio(103ln?)of 30Si/28Si decreases in the order of quartz?muscovite>epidote"kyanite,which is generally consistent with the observed sequence of metamorphic mineral isotopic compositions.These results suggest that the vein minerals are likely in Si isotopic equilibrium with each other.The ?30Si of the Hualiangting veins are linearly correlated to SiO2 contents with a steeper slope than that of the magma differentiation trend.By also considering the mineralogy of the veins,I conclude that this linear relationship reflects the sequential variation of mineral composition and the Si isotope signature of the fluids.The SiO2 content and ?30Si of veins increased with continuing crystallization from the fluid,and the S30Si of the fluid also increased.Assuming ?30Siquartz-fluid=0.15±0.03‰,the initial ?30Si of the Hualiangting fluid is calculated as-0.22±0.08‰.The ?30Si of the fluid subsequently evolved to-0.17±0.09‰ during the second stage of vein formation and to-0.07±0.10‰ during the third stage.The results suggest that Si isotope data can be used to constrain element recycling and metamorphic fluid activities in the subduction zones.Si isotopes are easily fractionated by low-temperature aqueous processes,making them a useful weathering proxy.Weathering of mother rock and forming of latosol are important in migration and Si cycle in the crust.To investigate the mechanism of Si migration and Si isotope fractionation during intense weathering and latosol formation,I present the Si isotopic compositions of a basalt weathering profile from Zhanjiang,south China.The latosol has lower ?30Si than the unweathered rock(?30Si=-0.29‰),with ?30Si values of-2.45‰to-0.63‰.This is because light Si isotopes are preferentially retained in secondary phases(such as kaolinite)compared with soil solution.The Si isotope compositions are also heterogeneous in the profile,and the factors that influence the Si isotopic signatures in latosol are the intensity of weathering,the Si migration rate,and precipitation of Fe-and Al-hydroxides.The latosol near the bedrock has lower intensity of weathering,and have higher ?30Si.As the intensity of weathering increases,the proportion of secondary minerals and the lost of Si increases,and the ?30Si value decreases.For the latosol that is highly weathered,the ?30Si values show a negative correlation with ?Th,si.Furthermore,the precipitation of Fe-and Al-hydroxides can lead to enrichment of Si and decrease in?30Si,and this may be due to the adsorption of Si from solution onto these precipitated hydroxides.The results above show promising future of Si isotopes in the studies of magmatic processes,subduction-zone fluid activities,and chemical weathering.These work also present useful information for further research of these fields,especially in metamorphic fluid activities and chemical weathering.