| Halogens?F,Cl,Br and I?are highly incompatible,volatile and mobile,which have been applied for many studies such as evolution of planetary,curst-mantle differentiation,climate change and the formation of ore deposit.Due to the high incompatibility of volatile halogens,the abundance of halogens in mantle-derived rocks?e.g.peridotite,komatiite?or general igneous rocks?e.g.basalts,andesite?are low,especially for Br and I(<1?g g-1).It is difficult to accurate determination of halogens with low abundances in geological materials.The main challenge is to overcome the loss of halogens in digestion process due to their strong volatility.Although pyrohydrolysis,alkali fusion and neutron activation analysis have been used for the determination of halogens in geological materials,some drawbacks limit their applications such as time-consuming,energy-consuming,lots of cost and variable recoveries of halogens.The analysis technique is the major obstacle for the development of halogen geochemistry.To develop halogen geochemistry,the simple,rapid,economical high-performance methods have been developed to analysis halogens in geological materials in this study.Furthermore,these methods have been applied for the determination of halogen in arc basalts and dacites to investigate effects of subducted fluids on halogens in arc system.This study was carried out from these four aspects.?1?A closed rapid acid digestion for determination of halogens was developed which overcome the loss of halogens and memory effect.It is rapid for the analysis of halogens in geological materials using this method which break through the bottleneck of acid digestion.The closed acid digestion has been mostly used to decompose geological materials.However,this method is rarely used for the determination of halogen due to the loss of halogens and serious memory effect in acidic environment.To obtain a method with amounts of applications,it is a good chance to develop a acid digestion method which is suitable for analysis of halogens.In this study,a rapid acid digestion?1 ml HNO3+1 ml HF?for the simultaneous determination of bromine and iodine in soils and sediments using ICP-MS was developed.The recoveries of Br and I were controlled by the synergic effect of temperature and time.It only takes15 minutes at 140°C for the complete recoveries of Br and I in sediment and soil,which is a process that needs 2-6 hours at 90°C.A serious loss of Br and I is found at a high digestion temperature of 190°C.A 5%v/v NH4OH dilution effectively eliminates the memory effects and stabilizes the signals of Br and I.Moreover,the ammonia dilution also avoids the corrosiveness of HF on the sample introduction system?nebulizer,spray chamber and torch?of ICP-MS.Te is a more suitable internal standard element than In in the ammonia medium.To avoid the adsorption of residues of dissolution on Te,addition of Te should be carried out after centrifuging the solution.The developed method was successfully applied to determine Br and I in fifty-three Chinese soil and sediment reference materials.The simple and effective method shows great potential for the rapid determination of Br and I in large batches of geological and environmental samples,which is common for mineral exploration and environmental geochemistry studies.?2?The method based on NH4HF2/NH4F digestion in open vessels is proposed for the simultaneous analysis of Cl,Br and I in geological materials.It is the most effective analysis method for halogen,which had been used for the analysis of low abundance halogens in rocks.Due to the short digestion time of rapid acid digestion in closed system,it can not completely decompose the silicate minerals?e.g.refractory mineral zircon?,which underestimate the abundances of halogens in rocks.Compared to closed acid digestion,newly NH4HF2/NH4F digestion in open vessels effectively decompose silicate minerals in short digestion time,which have been used for determination of trace elements in a range of rocks.In this study,a simple and effective method for the simultaneous analysis of Cl,Br and I in geological materials based on NH4HF2/NH4F digestion in open vessels?Savillex Teflon vials?is proposed.Effects of digestion parameters?temperature,time,the ratio of fusion and sample,open vessel?on recoveries of halogens are discussed in details.It is very interesting to note that Cl,Br and I are not lost during NH4HF2/NH4Fdigestion at temperature of 200-240°C for 0.5-12h in open vessels.This should be related to the alkaline atmosphere environment caused by the NH3 produced during NH4HF2/NH4F digestion,which suppresses the volatilization loss of Cl,Br and I.A digestion time of 2 h for 400 mg of NH4HF2/NH4F in Savillex Teflon vials at 220°C is sufficient to digest 100 mg of geological materials.The use of new Savillex Teflon vials significantly reduced the procedural blanks of halogens compared with the old Savillex Teflon vials.The developed method was successfully applied to the simultaneous determination of Cl,Br and I in a series of international geological reference materials.Most of the results were found to be in reasonable agreement with reported values in the literature.This simple,rapid,effective,and economical method shows great potential for the analysis of Cl,Br and I in a large number of samples for mineral exploration and geochemistry studies,which is most effective analytical method for halogen in geological materials.?3?The mechanism of sensitivity enhancement by N2 and interface in LA-ICP-MS is investigated,which provides the theories foundation for the in-situ analysis of halogen by LA-ICP-MS at high spatial resolution.To investigate the micro-geological reactions between minerals and fluids,it needs to obtain the composition of halogens in situ.However,the sensitivity of in-situ analysis techniques directly influences its applications in halogen geochemistry.High sensitivity is required to analysis of halogen in situ.The effects of assorted cone combinations on the signal enhancement of nitrogen in LA-ICP-MS are presented.In Ar plasma,the standard sample cone+X skimmer cone?S+X cone?increases the sensitivity for most investigative elements by a factor of 1.5-9.7 compared to the standard sample cone+H skimmer cone?S+H cone?.The effect of the addition of nitrogen on sensitivities of elements obtained by LA-ICP-MS was minimal for the S+H cone but significant for the S+X cone.The degree of the signal enhancement by the addition of nitrogen is positively correlated with the oxide bond dissociation energy of the element.Compared to the Ar plasma,the oxide ratio in Ar-N2 mixed plasma(N2=4 ml min-1)is reduced from 31.5%to 1.8%in the S+H cone and from 88.7%to 12.6%in S+X cone.The hydride yield is also reduced by a factor of 10 and 46 in the S+H cone and S+X cone by the addition of 4 ml min-1 nitrogen,respectively.Our results indicate that the geometry of the skimmer cone is an important factor for signal enhancement and the reduction of oxide and hydride yields by the addition of nitrogen.In addition,the increased Zr/Rb ratio by the addition of nitrogen indicated the reduction of ICP-related elemental fractionation.We successfully apply the S+X cone with Ar-N2 mixed plasma to analysis of low content REEs in four geological reference materials at a 16?m single-hole ablation mode by LA-ICP-MS.?4?Halogens in Niuatahi-Motutahi lavas provide useful information about effects of subducted-fluids on arc magma.The cycle of fluids is the key role in mass transfer processes and the cycle of elements in the Earth.And subduction is the main channel for the exchange of materials between surface and interior of the Earth.During the process of subduction,there are metasomatisms between fluids derived subducted slab and arc magma,which change the compositions and properties of magmas and influence the evolution of arc magma.The composition of halogens in arc lavas have good constrains on the contributions of subducted fluids to arc magma by metasomatism.In this study,the compositions of halogens in Niuatahi-Matutahi submarine lavas?basalts,high-K dacites and low-K dacites?were analyzed by NH4HF2 digestion.The combination of our results and other data obtained in different arc and back arc basalts is used to investigate the cycle of subducted fluids in Tonga arc.The concentration ranges of Cl,Br and I in Niuatahi-Matutahi submarine lavas were 1222-4510?g g-1 3.64-13.55?g g-1 and0.085-0.349?g g-1,respectively.The similar concentration ranges of Cl between bulk rock and melt inclusions suggested that Niuatahi-Matutahi submarine lavas including basalts,high-K dacites and low-K dacites were not affect by degassing.The Br/Cl(2.88-3.37×10-3)and I/Cl(47-126×10-6)ratios in Niuatahi-Matutahi submarine lavas are similar to values in MORB/OIB,suggesting the effect of seawater assimilation is negligible.During the evolution of arc magma,Br/Cl ratio is constant,implying there is no fractionation between halogens and lavas retain the halogens ratios derived magma source.However,there a large variation of I/Cl ratio in arc magma due to inputs of iodine from sediment and alter oceanic crust in subduction.There is a good correlation between Cl/Nb,Br/Nb,I/Nb and Ba/Nb in Tonga arc basalts and BABBs,which implies the composition of halogens in the arc magma are influence by the subducted-slab fluids.It should be note that the spatial distribution of halogens concentrations and fluid-mobile elements ratios are distinct,which may be related with the specific tectonic background. |
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