| Titanium(Ti)is a lithophile and dispersive element widely distributed in the earth's crust,where the content of TiO2 could reach 0.64 wt%.Ti has 5 isotopes,46Ti,47Ti,48Ti,49Ti,and 50Ti.There are up to 2 variations of Ti isotope compositions(?49Ti)measured in terrestrial and extraterrestrial materials.Furthermore,Ti isotopes are supposed to be a potential proxy to identify geodynamic settings of magma generation even after alteration for its conservative characteristics in fuilds.However,the high-precision Ti isotope composition measurement is still a challenge.The accuracy and precision of Ti isotope on MC-ICP-MS measurement are affected by many isobaric interferences.There are some common problems in present Ti purification methods,such as time-consuming purification processes,low recovery rates and artificial Ti isotope fractionation introduced in purification.Moreover,it is vital to understand the Ti isotope behaviors before its wide application as a burgeoning isotope proxy.Previous studies were mainly focused on high-temperature magma processes.There still lack of systematic studies on Ti isotope behaviors in the highly fractionated granites and in weathering processes.In this paper,we present two convenient and efficient procedures to separate Ti from matrix elements on geological materials for MC-ICP-MS analysis:alkaline fusion method coupled with AG50W-Χ12 two-column purification procedure;and acid solution method coupled with Ln-Spec+AG50W-Χ12 two-column purification procedure.Alkaline fusion method takes advantage of the high-temperature that favors fusing refractory minerals.Meanwhile,it is convenient to use the fused glass for major element measurement on XRF.Moreover,this procedure is free of HF aicds that could avoid precipitation of insoluble fluorides and achieve high Ti recovery.However,the alkali fusion method introduces a large amount of Li and B,which results in a long AG50W-Χ12 two-column purification procedure.The acid solution method requires less sample weight,which is in favor of valuable and rare samples.Our work showed that adding an appropriate amount of boric acid to the sample could effectively diminish the interference of HF introduced by the acid solution method.Both of these two chemical separation methods could remove almost all matrix elements in the sample and obtain a pure Ti solution,with the recovery rate of close to 100%.Ti isotope measurement was carried out on a Neptune Plus multi-collector inductively coupled plasma mass spectrometry(MC-ICP-MS)using standard-sample-bracketing(SSB)mode.The long-term(9 months)?49Ti precision of the NIST SRM 3162a solution was better than ±0.047(2SD,N=130).Our method will provide a powerful tool for investigating Ti isotope variations during variable geological processes.In the surface geological processes,Ti is a fluid immobile and chemically conservative element,and its isotopic variations have seldom been investigated.The previous pioneering study formulated a hypothesis that Ti isotopic composition is immune to processes involving water-rock interactions like weathering and diagenesis.Several studies,however,indicated that weathering and fluid-transport processes can fractionate Ti and Ti isotopes.Bulk saprolites,parent-rock minerals,extracted crystalline Fe(hydr)-oxide phase and residue phase from the Neogene tholeiitic basalt weathering profile from Hainan Island,China,were measured for their titanium(Ti)isotope composition,to test the hypothesis if Ti isotope composition is immune to the water-rock interactions and to elucidate the potential Ti isotope fractionation mechanisms during the extreme weathering process.Relative to the OL-Ti isotope standard,the ?49Ti values in the saprolites range from-0.066 to+0.078,with an average of 0.028,slightly lighter than the unaltered basalt(0.054 ± 0.032).Significant Ti isotope fractionations exist between extracted crystalline Fe(hydr)-oxide phase and residue phase(consisting of clay minerals and Fe-Ti oxides),which is up to 0.6.The residue phase(account for 72%to 94%of the total Ti)and has light Ti isotope compositions(-0.163 to 0.045),inherited from the primary weather-resistant ilmenite(-0.198 ± 0.038).Whereas the crystalline Fe(hydr)-oxides phase(account for 5%to 26%of the total Ti)has heavy ?49Ti values,ranging from 0.110to 0.542,transferred from the primary pyroxene phenocryst(0.178 ± 0.004)and groundmass(0.155 ± 0.041).Therefore,the Ti isotope composition of this weathering profile was controlled by formation and dissolution of titaniferous minerals,resulting different Ti isotope compositions in different minerals in the weathering product.Considering the physical sorting,induced by long-distance transport and deposition processes,will change the mineral assemblages of the sediments and hence the ?49Ti gaps among different minerals,we therefore highlight that the application of Ti isotopic compositions in sediment provenance studies needs to be cautious.Since large variations of Ti isotope composition were measured in the magmatic processes,the Ti isotope fractionation mechanism have been widely concerned.As noted by the previous studies,Ti isotope compositions of silicic melts(>65 wt%SiO2)from different magmatic series were different considerably.However,most Ti fractionation model were built on a mafic melts.We here present a comprehensive set of Ti isotopic compositions of I-type,S-type and A-type highly fractionated granites from South China and their minerals to further explore the mechanism of Ti isotope fractionation in magmas.The results showed that the Ti isotope fractionation of South China highly fractionated granites could reach 3,and the ?49Ti value ranged from 0.133 to 3.288.whilst all granites displayed an increase in ?49Ti relative to increasing SiO2 during the magma differentiation.The A-type granites displayed the most substantial range of variation(?49Ti=0.648 to 3.288),followed by I-type granites(?49Ti=0.133 to 0.964)and S-type granites(?49Ti=0.224 to 0.303).From the lightest ilmenite(?49Ti=-0.050±0.040,2SD)to the heaviest magnetite(?49Ti=7.112 ± 0.029,2SD),the isotopic fractionation between minerals could reach 7.Rock-forming minerals(hornblende,biotite,potash feldspar,plagioclase)generally had heavy Ti isotopic composition,while the Ti isotopic composition of accessory minerals(ilmenite,sphene and pyrite)had lighter ?49Ti values.The Ti isotopic fractionation of granite is controlled by the material source,mineral compositions,temperature,oxygen fugacity and water content of magma.The Ti isotope composition of S-type granite was mainly controlled by melted metasediment than the magmatic differentiation process,which indicated that it has the potential to limit the Ti isotopic composition of magma provenance(less than 0.224).Compared to the I-type granites,the A-type granites,originated from mantle-derived mafic alkaline magma,had the characteristics of anhydrous,low oxygen fugacity,and high initial melt TiO2 content,which lead to the ilmenite saturation at orthomagmatic stage.Thus,the A-type granites are extremely enriched in heavy Ti isotopes.The results of this study confirmed that Ti isotopes have the potential to identify geodynamic settings of magma generation. |
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