The influence of fractionation of REE-enriched minerals on the zircon partition coefficients

Zircon is widely used to simulate melt generation,migration and evolution within the crust and mantle.The achievable performance of melt modelling generally depends on the availability of reliable trace element partition coefficients(D).However,a large range of D_(REE) values for zircon from natural samples and experimental studies has been reported,with values spanning up to 3 orders of magnitude.Unfortunately,a gap of knowledge on this variability is evident.In this study we model the crystallization processes of common REE-bearing minerals from granitic melts and show that the measured zircon D_(REE) would be elevated if there is crystallization of REE-enriched minerals subsequent to zircon.Nevertheless,compared to zircon D_(REE) values measured from experimental studies,this mechanism appears to have a less significant influence on those from natural granite samples since the quantity of crystallized REE-enriched minerals is very low in natural magmatic systems and/or most of them crystallize prior to zircon.Combined with recently published studies,this work supports that analysis of natural zircon/host groundmass pairs provides more robust D_(REE) values applicable to natural systems than those measured from experimental studies,which can be used to constrain the provenance of detrital zircons.

First-principle study of Ba isotopic fractionation during ion exchange processes

The potential utilization and development of the Ba isotope tool depend on an accurateδ^(137/134)Ba determination of the samples.During the chemical purification,whether the adsorption process on the surface of the ionexchange resin could lead to the Ba isotopic fractionation and the degree of fractionation directly influence the accurateδ^(137/134)Ba determination.In the present work,first-principles calculations based on the density functional theory were used to quantify the Ba isotopic equilibrium fractionation factor between the aqueous solution and the resin in the acid leaching process.By constructing and optimizing the geometric configurations of Ba-containing species,Ba(H_(2)O)_(n)^(2+),Ba(H_(2)O)_(n)Cl_(2),Ba(H_(2)O)_(n)(NO_(3))2,and the adsorbed Ba^(2+)on the surface of the resin,extracting the harmonic vibrational frequencies,we finally at 298 K obtained the fractionations,Δ^(137/134)Ba_(soln-ads)=0.07‰,Δ^(137/134)Ba_(Ba(H_(2)O)_(n)Cl_(2)-ads)=0.05‰,andΔ^(137/134)-Ba^(Ba(H_(2)O)_(n)(NO_(3))2-ads)=0.02‰.Overall,there were almost no Ba isotope fractionations during leaching.Although the Ba isotope fractionation can be magnified by the Rayleigh fractionation process in purification,the difference inδ137/134Ba between the initial and final stages did not exceed0.060‰(or 0.045‰)when leaching the standard sample with HCl or HNO_(3),which is equal to or less than the accuracy of Ba isotopic analysis.At a common yield of89.75%,Ba isotopic fractionation induced by incomplete recovery was 0.015‰for HCl(or 0.011‰for HNO_(3)).Finally,if the influence of an incomplete recovery on theδ137/134Ba determination needs to be ignored,the recovery is suggested to be not less than 67%for HCl(or 46%for HNO_(3)).

Evaluating the soil evaporation loss rate in a gravel-sand mulching environment based on stable isotopes data

In order to cope with drought and water shortages,the working people in the arid areas of Northwest China have developed a drought-resistant planting method,namely,gravel-sand mulching,after long-term agricultural practices.To understand the effects of gravel-sand mulching on soil water evaporation,we selected Baifeng peach(Amygdalus persica L.)orchards in Northwest China as the experimental field in 2021.Based on continuously collected soil water stable isotopes data,we evaluated the soil evaporation loss rate in a gravel-sand mulching environment using the line-conditioned excess(lc-excess)coupled Rayleigh fractionation model and Craig-Gordon model.The results show that the average soil water content in the plots with gravel-sand mulching is 1.86%higher than that without gravel-sand mulching.The monthly variation of the soil water content is smaller in the plots with gravel-sand mulching than that without gravel-sand mulching.Moreover,the average lc-excess value in the plots without gravel-sand mulching is smaller.In addition,the soil evaporation loss rate in the plots with gravel-sand mulching is lower than that in the plots without gravel-sand mulching.The lc-excess value was negative for both the plots with and without gravel-sand mulching,and it has good correlation with relative humidity,average temperature,input water content,and soil water content.The effect of gravel-sand mulching on soil evaporation is most prominent in August.Compared with the evaporation data of similar environments in the literature,the lc-excess coupled Rayleigh fractionation model is better.Stable isotopes evidence shows that gravel-sand mulching can effectively reduce soil water evaporation,which provides a theoretical basis for agricultural water management and optimization of water-saving methods in arid areas.

Estimation of evaporation losses based on stable isotopes of stream water in a mountain watershed

Water stable isotopes(δ^(2) H andδ^(18)O)can record surface water evaporation,which is an important hydrological process for understanding watershed structure and function evolution.However,the isotopic estimation of water evaporation losses in the mountain watersheds remains poorly explored,which hinders understanding spatial variations of hydrological processes and their relationships with the temperature and vegetation.Here we investigatedδ^(2) H,δ^(18)O,and d-excess values of stream water along an altitude gradient of 2130 to 3380 m in Guan’egou mountain watershed at the east edge of the Qinghai-Tibet Plateau in China.The meanδ^(2) H(-69.6‰±2.6‰),δ^(18)O(-10.7‰±0.3‰),and dexcess values(16.0‰±1.4‰)of stream water indicate the inland moisture as the major source of precipitation in study area.Water stable isotopes increase linearly with decreasing altitudes,based on which we estimated the fractions of water evaporation losses along with the altitude and their variations in different vegetations.This study provides an isotopic evaluation method of water evaporation status in mountain watersheds,the results are useful for further understanding the relationship between hydrological processes and ecosystem function under the changing climate surrounding the Qinghai-Tibet Plateau.

The elevation gradient of stable isotopes in precipitation in the eastern margin of Tibetan Plateau

Tibetan Plateau(TP)receives moistures from different directions,so variations of isotope elevation gradient on different sides may exist.In this study,we systematically monitored and modeled the elevation effect of stable isotopes in precipitation of the eastern margin of the Tibetan Plateau,where we collected 265 water samples for stable isotope(δ2H andδ^(18)O)analyses,of which 70 are eventful rainfall samples,and 195 are surface water samples,for comparison.Alpine lakes on the mountain peaks were used to reconstruct the average isotope composition of the local precipitation based on the evaporation effect.An elevation gradient of−4.2‰km^(−1)(R^(2)=0.93)was established for the eastern marginal region of TP.This elevation gradient was further confirmed by fractionation assessment using the Rayleigh fractionation model.This is the highest ever reported for the TP and it is the highest as compared to those of all the other slopes as well as that of the interior part of the TP.Precipitation isotopes in the interior of TP show a lower elevation gradient,which is likely caused by significant local moisture recycling.This paper also discusses the limitations of using river water samples to construct the elevation gradient.