Study On Rubidium Isotope Geochemistry During Chemical Weathering Of Granite

Rubidium(Rb)is one of the alkali metal elements and characterized by moderate volatility,fluid activity and incompatibility,which features make Rb play important roles in tracing the provenance and providing information on various geological processes.What's more,⁸⁷Rb is a radioactive isotope which can decay to ⁸⁷Sr with a long half-life(T_1/2=4.976×10¹⁰ y).Therefore,classical Rb-Sr dating system has been widely applied to determine the age of long-time geological samples.Rb isotopic composition is traditionally considered as a constant value(⁸⁷Rb/⁸⁵Rb=2.593)in geological samples.However,relative mass difference(?m/m)of Rb is about 0.023,mass-dependent fractionation for Rb isotopes may indeed occur during geological processes.Therefore,we can regard Rb isotopes as a stable isotope system.However,the study of Rb isotope geochemistry is in its infancy because of two obstacles including the procedure of Rb purification and Rb isotopic measurement on mass spectrometry,which have prevented high-precision Rb isotopic analysis.Previous studies demonstrated that Rb isotopes could be a useful proxy for constraining the origin of planets in the inner solar system according to its moderate volatility.Questions still remain that whether Rb isotopic fractionated among different reservoirs and geological processes,especially low-temperature processes.Furthermore,whether Rb isotopic fractionation can provide additional clues for Rb tracing requires answers.To answer these questions,it is necessary to develop effective chromatographic procedure and robust instrumental measurement method at the very beginning.Then,determining the Rb composition of various international standard materials,can help us understand the Rb isotope composition of different geological reservoirs.After that,investigating the Rb isotope fractionation during different geological processes and exploring corresponding mechanism,can make a prospective view on of Rb isotope application on different geological processes.Careful chemical pretreatment and cautious instrumental measurement are prerequisites for obtaining high-precision Rb isotopic composition results.The biggest problem is that the distribution coefficients among Rb and Na,Ca,Ti,especially K are similar on conventional cation exchange resins.It is hard to separate Rb from matrix completely during chemical purification by one single column.Considering the relatively obvious distribution coefficients difference between Rb and K in 1?3 mol/L HNO₃ solution on Sr-Spec resin,we raised the height of column to 12.7 cm,and used 3 mol/L HNO₃ as eluting acid to separate most matrix elements,Rb and K in sequence.Then 8 mol/L HNO₃ was used to obtain purified solution of Ba.After that,Sr can be leached out of column by Milli-Q water.A chromatographic procedure for Rb,K,Ba and Sr one-by-one separation from high Rb content geological materials was developed by using a single column with Sr-spec resin.The recovery of four elements are close to100%.However,there are significant differences in chemical composition among different types of geological samples,for example,low Rb content rocks often contain high Na and Ca,seawater contains extremely high Na.Using one Sr-spec resin can not completely remove excessive Na or Ca in these samples.As a result,a small volume(1m L)of AGMP-50 cation exchange resin was used to remove a part of matrix elements such as Na and Ca before Sr-spec resin.Above chemical purification processes can basically meet the separation of Rb isotope from matrix elements of most geological samples.High-precision Rb isotope measurements were carried out by using standard-sample bracketing method(SSB)via multi-collector inductively coupled plasma mass spectrometry(MC-ICP-MS).The short-term precision for?⁸⁷Rb was better than±0.03‰(2SD),and the long-term external reproducibility was better than±0.06‰(2SD)based on repeated analysis of Rb standard solution NIST SRM 984,which can match the best analysis accuracy all over the world.In addition,this study have evaluated the factors that may affect the accuracy and precision of Rb isotope determination,such as digest time of rock samples,recovery of target elements,sample load on resin,acidity of leaching solution,instrumental conditions and matrix effect.To find the optimal conditions involve with chemical purification and instrumental determination are pivotal steps to ensure the quality of Rb isotopic composition we measured.The comprehensive dataset of?⁸⁷Rb value for various international standard materials can provide a reference for quality control and inter-laboratory comparison of high-precision Rb isotopic analyses for future studies,and is helpful to understand the Rb isotope composition of different geological reservoirs as well.Therefore,twenty-one international geological standard materials with different lithologies(including basic rock,intermediate rock,acid rock,sedimentary rock and metamorphic rock)and four water samples have been measured in this study.Among them,the?⁸⁷Rb value of five samples(BHVO-2,BCR-2,W-2a,AGV-2 and GSR-1)obtained by our laboratory are consistent with those of different laboratories within error,implying that our method is credible.The Rb isotope compositions in the rest of reference materials have not been previously analyzed.The?⁸⁷Rb value of the igneous rocks vary from-0.29‰to 0.06‰.The basic rocks hold the heaviest Rb isotope composition,with an average value of-0.07‰;the intermediate rocks define a narrow range of?⁸⁷Rb value,with an average value of-0.11‰;acidic rocks generally have a lighter Rb isotopic composition than basic and intermediate rocks,with an average of-0.20‰.Compared with igneous rocks,samples related to low-temperature geochemical processes have larger isotopic composition differences.The?⁸⁷Rb value of a soil sample(GSS-5)is-0.10‰,while a river sediment(GSD-10)has the lightest Rb isotope composition(-0.43‰).The Rb isotope composition of three river water samples took from the upper,middle and down reaches in Liuxi River range from-0.01‰to-0.38‰,and the seawater standard IAPSO has the heaviest?⁸⁷Rb value of 0.14±0.12‰,suggesting that significant Rb isotopic fractionation exists in surface geological processes.However,whether and how Rb isotopes fractionate during low-temperature geological processes are not known yet.In this study,Rb isotope ratios among the bulk samples and major host minerals from a 40 m-long granite chemical weathering profile in Guangdong Province,South China,and of surrounding stream water samples were investigated,to explore the mechanism triggering Rb isotope fractionation during chemical weathering.The bulk samples show that the?Rb₄2) value(mobility of Rb)fluctuate dramatically from-42.0%to 259%,and?⁸⁷Rb values vary from-0.30±0.13‰in the unaltered bedrock to 0.04±0.08‰in the weathered saprolites.The positive correlations between?Rb₄2),CIA and?⁸⁷Rb suggesting that ⁸⁷Rb trend to enriched in weathering products with the enhancement of chemical weathering.Exogenous input(such as aeolian dust and rainwater)and biological activity have limited influenced on Rb content and?⁸⁷Rb value in Fogang profile.By analyzing the Rb content and Rb isotopic composition of individual minerals from different depths,we suggested that the adsorption and desorption processes of clay minerals is the main reason for triggering the migration of Rb elements and its isotope fractionation.Significant isotopic variations exist among individual mineral species.In parent rock,biotite,chlorite,K-feldspar and plagioclase have the?⁸⁷Rb values of-0.42‰,-0.30‰,-0.23‰and+0.01‰,respectively.With the intensification of chemical weathering,primary minerals such as biotite and K-feldspar decomposed,while the content of clay minerals such as illite and kaolinite increased.In weathered products,major Rb-bearing minerals are K-feldspar,illite and kaolinite.The?⁸⁷Rb values of K-feldspar range from-0.23‰to 0.14‰,which are likely influenced by the secondary clay minerals adhered on the corroded surface of K-feldspar.Illite is the intermediate secondary mineral of decomposition of K-feldspar or biotite to kaolinite and its?⁸⁷Rb values varies from-0.28‰to-0.10‰.The Rb isotope composition of illite may influenced by multiple processes,including adsorbing Rb⁺released from the decomposition of primary minerals,and released Rb⁺during decomposition into kaolinite.The Rb isotope fractionation trend of illite is opposite to that of the bulk samples,suggesting that illite is not the main reason that controls the isotope change of weathering residues during chemical weathering.Kaolinite is the final product of chemical weathering in this profile.Limited by its size and morphology,kaolinite cannot be separated from the bulk samples,nor the Rb concentration and isotopic composition be measured directly.Mass balance calculation results show that kaolinite have the heaviest Rb isotope composition in Fogang profile(-0.07‰to 0.20‰),which play an important role in controlling the?⁸⁷Rb change of weathering saprolites during chemical weathering.Nearby stream water has the average?⁸⁷Rb value of-0.22‰,which is slightly heavier than the value of parent rock but lighter than the saprolites at the upper part of the profile,suggesting that ⁸⁷Rb preferred to be partitioned into weathered saprolites relative to fluids.This phenomenon is opposite to that was preformed by K isotopes.Our results first demonstrate significant Rb isotope fractionation at the Earth's surface and that adsorption and desorption of clay minerals play crucial role in triggering Rb isotope fractionation.In-depth study on Rb isotopic fractionation during chemical weathering may provide unique information of the geochemical behavior of Rb,global Rb cycling,as well as tracing silicate continental weathering.