Experimental Study On The Dehydration Process Of Minerals In Subduction Zones

Volatile elements(hydrogen,nitrogen,carbon,halogens)are closely associated with the origin of life,the habitability of planets,environmental change,natural disaster,and mineralization.Among them,hydrogen is the most abundant element in the solar system.Water formed by combination of hydrogen and oxygen not only occurs in the surface spheres,but also resides in the Earth’s interior.Water distribution and migration in the Earth’s interior significantly affect the physical and chemical properties of minerals and rocks,and therefore exert effects on the geodynamics of the Earth’s interior dynamics and the environment of the Earth system.The subduction zone plays an important role in connecting the surface and interior of the Earth,which consists of marine sediments,altered oceanic crust,and mantle wedge.Subduction zone minerals are important water carriers to the deep Earth.Although a large amount of research has been conducted on water migration in subduction zones,it is still unclear about the dehydration process and mechanism of the minerals.This prevents further understanding of the water cycle in the Earth’s interior.Based on the above,this thesis selects its research objects on phlogopite,apatite,and rutile,as representative minerals of silicate,phosphate,and oxide,respectively.The samples were analyzed by analytical instruments such as electron microprobe analysis,in situ high-temperature infrared spectroscopy,in situ Raman spectroscopy,and in situ high-temperature X-ray diffraction.The thesis provides systematic research on the dehydration process and its influence factors,including the effects of coexisting volatile element fluorine on the stability of OH and crystal lattice,the effects of fluorine on the hydrogen diffusion kinetices,and the effects of iron on hydrogen diffusion kinetics.The research provides new constraints on the water cycle in the Earth’s interior.A summary of research results see below:(1)The effects of fluorine on phlogopite dehydration processPhlogopite is an important hydrous mineral in the mantle wedge,not only transporting a significant amount of water to the mantle but also acting as an important carrier of halogens.Fluorine significantly improves the thermal stability of phlogopite;natural fluorine-rich phlogopite can stabilize under ultrahigh temperature conditions in the Earth’s interior.However,it is still unclear how fluorine affects phlogopite stability and its physiochemical mechanism.To solve the problem,this thesis selects fluorine-rich and fluorine-poor natural phlogopite,analyzing them with in situ Raman spectroscopy,and X-ray diffraction under conditions from room temperature to 1000–1200 ℃.The experimental results show an increase in fluorine can prevent the weakening of hydroxyl and lattice at high temperatures.Therefore,this postpones the dehydration and decomposition of phlogopite,with 4 wt.% of fluorine can increase the decomposition temperature by 100 ℃.The results reveal the physicochemical mechanism of fluorine impact on the phlogopite stability,providing new information to understand phlogopite dehydration process.(2)The effects of fluorine on apatite dehydration processApatite is a prevalent mineral in the Earth,Moon,and Martian meteorite,enriching in volatile elements(F,Cl,OH,etc.).The contents of these volatile element and diffusion behavior can inverse the volatile concentration of different sources and estimate magma ascent velocity.Furthermore,apatite is an important carrier of volatile elements in the mantle wedge.To understand the dehydration process of apatite,this thesis selects apatite with various fluorine contents(Mole fraction of fluorine,XF is 0.92 and 0.67,respectively).The apatite is analyzed by high-temperature infrared spectroscopy for the OH stability and water diffusion kinetics.The results suggest that fluorine-related OH(OHF)has a stronger thermal stability than the other two types of OH(OHOH and OHCl),due to the strong electronegativity of fluorine.Diffusion experimental results at 900,1000,and 1000 ℃ suggest that fluorine controls the dehydration mechanism of apatite.Hydrogen diffusion happens in apatite with high fluorine content,with the activation energy of about 77 k J/mol,while hydroxyl diffusion occurs in apatite with low fluorine content whose activation energy is 247 k J/mol.At 900–1100 ℃,the hydrogen diffusion rate in apatite with high fluorine content is 2–3 orders of magnitude faster than hydroxyl diffusion rate in apatite with low fluorient content.The study reveals the impacts of fluorine on OH stability and hydrogen diffusion mechanism.The study proposes that fluorine and hydrogen are interconnected with each other in the crystal lattice.Therefore,it requires considering links between multicomponents in the evaluation of apatite dehydration and inversion of magma ascending velocity using water diffusion.(3)The effects of iron on the rutile dehydration processRutile is a prevalent accessory mineral in the subduction zone.The enrichment of trace elements in it can track various metamorphic processes.Numerous natural samples from ultrahigh-pressure metamorphic melt suggest that rutile is an important water carrier.Although the previous study conducts research on water solubility in rutile,little is known of its dehydration process.This thesis uses high-temperature infrared spectroscopy to conduct analyses of water in natural rutile with broad compositional ranges.The experimental results suggest rutile dehydration at 500–600 ℃,with a negative correlation between dehydration rate and its iron contents.Furthermore,water with different defect sites in rutile has different thermal stabilities.The dehydration rate of iron-related OH(Fe and H couple to replace Ti)is lower than the others.Based on the hydrogen diffusion experiments at 500,550,and 600 ℃,rutile with iron has an activation energy of 95 ± 12 to 100 ± 11 k J/mol,less than that of the iron-free rutile activation energy reported in previous studies(107–123 k J/mol).Therefore,the presence of iron provides a possibility of storing more water in rutile and its high-pressure phase Ti O2(II)and playing an important role in transporting water into the deep Earth.Due to the structural similarity between rutile and stishovite,the results shed light on the stishovite dehydration process as well.In summary,the thesis works on the dehydration process of subduction zone minerals,selecting different types of silicate,phosphate,and oxide as research objectives,to reveal the dehydration process and their impact factors on phlogopite,apatite,and rutile.The study provides a new understanding of the water cycle in the Earth’s interior and its geodynamic consequences.This study still leaves much to be desired due to technical limits and samples.In the subsequent work,it will be expanded to minerals in different areas of subduction zones,and explore the impact factors on dehydration process under high-temperature and high-pressure conditions.