A H_(2)O-in-zircon perspective on the heterogeneous water content of crust-derived magmas in southern Tibet

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 H_(2)O-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 and300 ppm,respectively).Meanwhile,there are high H_(2)O-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 3×10^(15)αdecay 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 Hcontent,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 H_(2)O contents.Similarly,the degree of partial melting was unlikely to be responsible for the H_(2)O-in-zircon distinction.We interpret that the H_(2)O-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.