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

作　　者：Juntong MENG Xiaoping XIA Lin MA Ziqi JIANG Jian XU Zexian CUI Qing YANG Wanfeng ZHANG Le ZHANG 

机构地区：[1]State Key Laboratory of Isotope Geochemistry,Guangzhou Institute of Geochemistry,Chinese Academy of Sciences,Guangzhou 510640,China [2]CAS Center for Excellence in Deep Earth Science,Guangzhou 510640,China [3]College of Earth and Planetary Science,University of Chinese Academy of Sciences,Beijing 100049,China [4]School of Earth Science,Guilin University of Technology,Guilin 541004,China

出　　处：《Science China Earth Sciences》2021年第7期1184-1194,共11页中国科学（地球科学英文版）

基　　金：supported by the National Natural Science Foundation of China(Grant No.41673010);the Second Tibetan Plateau Scientific Expedition and Research(STEP)(Grant No.2019QZKK0702)。

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 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.

关 键 词：H_2O-in-zircon SIMS dehydration melting water-fluxed melting magma water content 

分 类 号：P588.11[天文地球—岩石学]