【摘 要】
：
Geophysical and geochemical evidence revealed that certain amounts of water are stored in the Earth\'s interior,and its distribution exhibits temporal and spatial differences.Recent studies indicate that the H2O-in-zircon has a potential to reveal magma
【机 构】
：
State Key Laboratory of Isotope Geochemistry,Guangzhou Institute of Geochemistry,Chinese Academy of
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Geophysical and geochemical evidence revealed that certain amounts of water are stored in the Earth\'s interior,and its distribution exhibits temporal and spatial differences.Recent studies indicate that the H2O-in-zircon has a potential to reveal magma water content.Using secondary ion mass spectrometry(SIMS),we analyzed the zircon water content and oxygen isotopes in granitoids(two-mica granite,quartz monzonite,diorite,and granodiorite)from southern Tibet.The results are used to explore the diversity of water content in crust-derived magmas,which is also supported by the LA-ICPMS zircon trace element analysis.There are low zircon water contents for two partial melting products of the ancient crust(peak at 85 and 300 ppm,respectively).Meanwhile,there are high H2O-in-zircon peaks(435 and 475 ppm,respectively)for two partial melting products of the juvenile crust.The calculated radiation damage accumulation in these zircon grains is below 3×1015 α decay events mg-1,suggesting that the zircon grains are well crystalline.There is no correlation between REE+Y and H+P,indicating that the mechanism of charge balance for water uptaking into zircon,in which the excess of REE+Y to P controls H content,is not applicable to the studied samples.The degree of magma fractionation cannot account for the zircon water content variation,because there is no co-variation between zircon Hf and H2O contents.Similarly,the degree of partial melting was unlikely to be responsible for the H2O-in-zircon distinction.We interpret that the H2O-in-zircon variation reflects the influence of melting mode(dehydration vs.water-fluxed)and water content in the crustal source.In the Late Devonian,dehydration melting of the crustal rocks was facilitated at elevated temperatures,which produced magmas with the lowest water contents.Water-fluxed melting of the ancient crust would produce relatively“drier”magma than water-fluxed melting of the juvenile crust.
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