| Titanite is a common accessory mineral in a wide variety of igneous and metamorphic rocks.Titanite usually incorporates considerable amounts of U(10->100?g g-1)and Nd(500~5000 ?g g-1)in its structure,making it a versatile mineral for U-Pb dating and Sm-Nd isotopic measurement.In recent years,the rapid development of laser ablation technique has become an important tool for in situ microanalysis of trace elements,U-Pb dating and Sm-Nd isotope analysis because of its simple sample preparation and rapid analysis.The lack of matrix matching reference material is one of the important factors limiting the wide use of this technique.Bearing this consideration in mind,we have undertaken a comprehensive study of the U-Pb age and Sm-Nd isotopic composition of twelve natural titanite crystals,including six well-characterised reference materials(Khan,BLR-1,OLT1,Ontario,MKED1 and YQ82)and six candidate reference materials(T3,T4,TLS-36,NW-IOA,Pakistan and C253).The aim is to find suitable titanite reference materials for in situ U-Pb dating and Sm-Nd measurement so that a suite of well-calibrated titanite materials can be made available to the geochemical research community.The results of LA-ICP-MS trace element analysis and U-Pb dating show that titanite BLR-1,OLT1,Ontario,MKED1,T3 and Pakistan are homogeneous in trace elements and U-Pb ages,thus are ideal reference materials for in situ U-Pb dating.In this paper,ID-TIMS U-Pb isotope data show that Ontario has 207Pb/206Pb,207Pb/235U and 206Pb/38U age of 1048.4 ± 3.3 Ma(2?,n=9),1044.8 ± 2.0 Ma(2?,n=9)and 1043.1 ± 2.8 Ma(2?,n=9),respectively.Titanite T3 has 207Pb/06Pb,207Pb/235U and 206Pb/238U age(ID-TIMS)of 1096.6 ±2.1 Ma(2?,n=9),1097.1± 3.6 Ma(2?,n=9)and 1096.7± 5.5 Ma(2?,n=9),respectively.Titanite Pakistan has a concordia intercept age of 20.77 ± 0.05 Ma(2?,n=10)and 206Pb/238U age(ID-TIMS)of 20.79±0.08 Ma(2?,n=10)obtained by ID-TIMS.The results of LA-MC-ICP-MS analysis show that the Sm-Nd isotopic compositions of titanite BLR-1,OLT1,Ontario,MKED1 and T3 are very homogeneous in space,and thus are ideal in situ Sm-Nd isotopic analysis reference materials.In this paper,the results obtained by ID-MC-ICP-MS indicate that the 147Sm/144Nd and 143Nd/144Nd ratios of BLR-1 were 0.1921 and 0.512815 ± 0.000004(2?,n=1),respectively;the 147Sm/144Nd and 143Nd/144Nd ratios of Ontario were 0.1928 ± 0.0011(2?,n=4)and 0.512833 ± 0.000008(2?,n=4),respectively,and the 147Sm/144Nd and 143Nd/144Nd ratios of T3 were 0.1958± 0.0009(2?,n=4)and 0.512611±0.000012(2?,n=4),respectively.Titanite OLT1 has 147Sm/144Nd and 143Nd/144Nd ratios of 0.1237 ± 0.0020(2SD,n=184)and 143Nd/144Nd=0.512214 ± 0.000041(2SD,n=184),respectively,obtained by LA-MC-ICP-MS.The trace elements,U-Pb ages,and Sm-Nd isotopes of titanites in the alkaline igneous rocks(Taohua,Ninglang-Yongsheng,Habo,Tongchang and Shilicun pluton)in Sanjiang region were determined in detail by the established method.The trace elements of titanite in this study have typical characteristics of magmatic titanite.Titanites from Shigu,Ninglang-Yongsheng,Habo,Tongchang and Shilicun pluton yielded U-Pb ages of 37.9±2.4 Ma(2?,n=21),32.5±0.6 Ma(2?,n=40),35.5 ±0.4 Ma(2?,n=33),33.7±0.7 Ma(2?,n=19)and 35.6 ± 1.5 Ma(2?,n=42).The ?Nd(t)values of titanite in the alkaline igneous rocks in this study are ranging from-6.8 to-2.1,which are consistent with the Nd isotopic composition of the whole rock.The Nd isotopes in combination with previous whole rock Sr-Pb isotopes and zircon Hf isotopic data indicate that these five intrusions were likely derived from the fractional crystallization of magmas originating from an enriched lithospheric mantle,which was resulted from metasomatism related to ancient oceanic subduction.The eNd(t)of Tongchang porphyry is significantly lower than that of other alkaline igneous rocks in this study,indicating that the magma source of Tongchang syenite porphyry is more enriched,or more crustal materials are added in the process of magma assimilation and mixing.In terms of the Cenozoic alkaline igneous rocks in this study,the U-Pb age of the titanite represents the age of magma emplacement,and the Sm-Nd isotopic compositions are basically the same as that of the whole rock,which represent the Nd isotopic composition of the magma during crystallization.Thus,through the laser ablation analysis of titanites,geological information such as geochronology,chemical composition and isotopic composition at the same time on the mineral scale are obtained,and important parameters for the genesis and evolution of the minerals and rocks are provided. |
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