Heterogeneous Water Content Of Crust-Derived Magmas In The Gangdese Belt,Tibet:A H₂O-in-Zircon Perspective

Water is important for the Earth and play a vital role in many geological processes.Due to the incompatibility of H in magma,water preferentially enriches in melt during partial melting and crystallization fractionation.It is crucial to reveal the distribution and migration of water in crust,but few studies have focused on this aim in detail owing to the lack of research method.As a common accessory mineral in felsic rocks,zircon is physically and chemically inert and has high closure temperature,it was widely used in U-Pb dating,Hf-O isotope and trace element analyses.Compared with other rock-forming nominally anhydrous minerals(etc.olivine and pyroxene),only little attention has been paid on the water content of zircon.Method using large geometry secondary ion mass spectrometry(LG-SIMS)to analyze H₂O-in-zircon has been developed recently.In this study,non-metamict zircon grains selected from felsic intrusive rocks in the Gangdese area were used for water content measurement by this new method,which include a suite of co-genetic granitic rocks and four granitoids with different sources.The zircon?¹⁸O simultaneously analyzed by SIMS and trace element concentrations obtained by LA-ICPMS were used to discuss the significance of the water content.The advances of this thesis are listed below:(1)For each sample,their atomic ratios of zircon(REE+Y)/(H+P)are not equal to unity and show a large variation,which are different from the previous research on Mid-Atlantic Ridge crystalline zircons with(REE+Y)/(H+P)equal to unity.These results suggest that the mechanism of charge balance is not the only factor of controlling H₂O-in-zircon.(2)Water content of non-metamict zircon can record the water content of magma and therefore H₂O-in-zircon has a potential to be a new proxy for water content of magma.Zircon water contents of co-genetic Zhengga granites exhibit a broad range.The less evolved biotite granite has a lower H₂O-in-zircon average value(699ppm)and a low peak value(490ppm)in kernel density plot.Meanwhile,the two high evolved garnet-bearing two-mica granite samples have higher H₂O-in-zircon average values(908ppm and 959ppm)and multiple peaks in kernel density plots.There are a positive correlation between Hf and H₂O contents and a negative correlation between Eu/Eu*and H₂O contents,which indicate that the Hf and water contents of zircon are elevating and Eu/Eu*ratios are decreasing during magma evolution with plagioclase fractionation.The elevated trend of H₂O-in-zircon is corresponding to the evolved trend of magma water content.(3)Zircon water contents of the four granitoids derived from different source show distinctive distributions.The two partial melting products of ancient crust exhibit lower water contents of zircon.The H₂O average(223ppm)and kernel density peak(105ppm)of dehydration melting product are slightly lower than water-fluxed melting product(average:307ppm;peak:280ppm).In contrast,the other two water-fluxed melting products of juvenile crust exhibit higher water contents of zircon,which have H₂O average of 432ppm and 472ppm with peak values of 430ppm and500ppm,respectively.Combined with trace element results,the factors of charge balance mechanism,degree of partial melting and magma fractionation cannot be responsible for their different H₂O-in-zircon.We propose that the H₂O-in-zircon records the difference of magma water contents,which relate to the tectonic setting,mode of melting(dehydration vs.water-fluxed)and water content in crustal source.