Zircon ID-TIMS Pb isotope determination method using multiple ion counters with dynamic multi-collection protocol

Abstract

A zircon ID-TIMDS Pb isotope determination method by multiple ion counters with a dynamic multi-collection protocol is provided. Compared with a commonly used multi-ion counter static determination method, the method provided by the present invention completely eliminates influences of gain differences of the different ion counters on determination results of Pb isotopes. Compared with a conventional single-ion counter determination method with five times of peak-jumps, the method provided by the present invention can obtain all of Pb isotope ratios with two times of peak-jumps, which increases the collection efficiency of Pb isotope ion beams and decreases influences of ion beam stability on Pb isotope analysis results. Consequently, compared with a multi-ion counter static method and a single-ion counter peak-jumping method, the method provided by the present invention improves the Pb isotope analysis precision for the single-grain zircon ID-TIMS U—Pb dating method (with a .sup.205Pb tracer), having application potentials.

Claims

1. A zircon ID-TIMS (Isotope Dilution-Thermal Ionization Mass Spectrometry) Pb Isotope determination method using multiple ion counters with a dynamic multi-collection protocol, comprising steps of: (S1) for a Pb sample in which a .sup.205Pb tracer is added, adopting at least four ion counters of a thermal ionization mass spectrometer, respectively denoted as IC1, IC2, IC3 and IC4; (S2) designing two times of peak-jumps for isotope determination, respectively denoted as J1 and J2; (S3) determining all of five isotopes of .sup.204Pb, .sup.205Pb, .sup.206Pb, .sup.207Pb and .sup.208Pb, particularly comprising steps of: at a first jump (J1), determining .sup.205Pb, .sup.206Pb, .sup.207Pb and .sup.208Pb respectively by the four ion counters IC1, IC2, IC3 and IC4; at a second jump (J2), determining .sup.204Pb, .sup.205Pb, .sup.206Pb and .sup.207Pb respectively by the four ion counters IC1, IC2, IC3 and IC4; and (S4) through appropriately combining Pb isotope signal intensities obtained by the first and second jumps, calculating and obtaining Pb isotope ratios of .sup.204Pb/.sup.206Pb, .sup.205Pb/.sup.206Pb, .sup.207Pb/.sup.206Pb and .sup.208Pb/.sup.206Pb, particularly comprising steps of: (S41) with .sup.207Pb determined by the IC3 from the first jump and .sup.206Pb determined by the IC3 from the second jump, obtaining .sup.207Pb/.sup.206Pb, denoted as .sup.207Pb.sub.IC3-J1/.sup.206Pb.sub.IC3-J2; (S42) with .sup.208Pb determined by the IC4 from the first jump and .sup.207Pb determined by the IC4 from the second jump, obtaining .sup.208Pb/.sup.207Pb, denoted as .sup.208Pb.sub.IC4-J1/.sup.207Pb.sub.IC4-J2; through calculating with a formula of .sup.208Pb/.sup.206Pb=.sup.207Pb.sub.IC3-J1/.sup.206Pb.sub.IC3-J2×.sup.208Pb.sub.IC4-J1/.sup.207Pb.sub.IC4-J2, obtaining .sup.208Pb/.sup.206Pb; (S43) with .sup.205Pb determined by the IC2 from the second jump and .sup.206Pb determined by the IC2 from the first jump, obtaining .sup.205Pb/.sup.206Pb, denoted as .sup.205Pb.sub.IC2-J2/.sup.206Pb.sub.IC2-J1; and (S44) with .sup.204Pb determined by the IC1 from the second jump and .sup.205Pb determined by the IC1 from the first jump, obtaining .sup.204Pb/.sup.205Pb, denoted as .sup.204Pb.sub.IC1-J2/.sup.205Pb.sub.IC1-J1; through calculating with a formula of .sup.204Pb/.sup.206Pb=.sup.201Pb.sub.IC2-J2/.sup.206Pb.sub.IC2-J1×.sup.204Pb.sub.IC1-J2/.sup.205Pb.sub.IC1-J1, obtaining .sup.204Pb/.sup.206Pb.

2. The determination method, as recited in claim 1, wherein: the obtained ratios of .sup.204Pb/.sup.206Pb, .sup.205Pb/.sup.206Pb, .sup.207Pb/.sup.206Pb and .sup.208Pb/.sup.206Pb are all determined by peak-jumps with a same ion counter, which completely eliminates influences of gain differences of the ion counters on determination results of the Pb isotopes.

3. The determination method, as recited in claim 1, wherein: in the steps of S41-S44, a linear interpolation method is adopted to correct influences of ion beam stability on determination results of each isotope ratio during a peak-jump determination process by the multiple ion counters.

4. The determination method, as recited in claim 1, wherein: when determining Pb isotope ratios of a sample by the determination method, because .sup.204Pb/.sup.206Pb is obtained through a calculation of .sup.204Pb/.sup.206Pb=.sup.205Pb.sub.IC2-J2/.sup.206Pb.sub.IC2-J1×.sup.204Pb.sub.IC1-J2/.sup.205Pb.sub.IC1-J1, the method is only applicable to Pb isotope analysis of a .sup.205Pb—Pb mixture.

5. The determination method, as recited in claim 1, wherein: the method completes determination of all Pb isotope ratios with only two times of peak-jumps; compared with a conventional single-ion counter (SEM (Secondary Electronic Multiplier) or Daly detector) method which completes determination of all Pb isotopes with five times of peak-jumps, the determination method increases a collection efficiency of Pb ion beams by 2.5 times and meanwhile decreases influences of ion beam stability during a Pb isotope determination process on determination results of each isotope ratio, so that time-normalized precision for Pb isotope determination of a .sup.205Pb—Pb mixture is improved; compared with a commonly used Pb isotope determination method with a static multi-collection protocol by the multiple ion counters, the determination method completely eliminates influences of gain differences of the different ion counters on the determination results of the Pb isotopes, so that an analytical precision is greatly improved.

6. The determination method, as recited in claim 1, wherein: the determination method is especially applicable to Pb isotope analysis for a single-grain zircon ID-TIMS U—Pb dating method.

7. The determination method, as recited in claim 1, wherein: the protocol of the established determination method has universality and is suitable for various types of mass spectrometers equipped with a multi-ion counter system which can be applied for Pb isotope analysis of a .sup.205Pb—Pb mixture.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a sketch view to illustrate a dynamic multi-collection method for Pb isotopic analysis of a .sup.205Pb—Pb mixture using multiple ion counters according to the present invention.

(2) FIG. 2 is a configuration diagram of a multi-ion counter system of a TRITON PLUS thermal ionization mass spectrometer according to the present invention.

(3) FIG. 3 shows comparison of determination results of .sup.207Pb/.sup.206Pb of NIST981 with a method provided by the present invention, a multi-ion counter static method, and a single-ion counter peak-jumping method. In FIG. 3, a full line represents a mean, and dotted lines represent a range of mean±2SD.

(4) FIG. 4A and FIG. 4B show determination results of Qinghu zircon U—Pb age, wherein: FIG. 4A is a U—Pb Concordia age diagram; and FIG. 4B is a diagram of weighted average .sup.206Pb/.sup.238U age.

(5) FIG. 5A and FIG. 5B show determination results of TEMORA zircon U—Pb age, wherein: FIG. 5A is a U—Pb Concordia age diagram; and FIG. 5B is a diagram of weighted average .sup.206Pb/.sup.238U age.

(6) In figures: J1 and J2 respectively represent a first jump and a second jump; IC1, IC2, IC3 and IC4 respectively represent the first, second, third and fourth ion counters; SEM represents a secondary electronic multiplier; and CDD represents a compact discrete dynode multiplier.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

(7) The present invention is further described in detail with examples as follows.

(8) Referring to FIG. 1, the present invention provides a determination method for Pb isotopes of a .sup.205Pb—Pb mixture with a dynamic multi-collection protocol by multiple ion counters of a thermal ionization mass spectrometer. The method is suitable for various types of mass spectrometers equipped with a multi-ion counter system which can be applied for Pb isotope analysis of a .sup.205Pb—Pb mixture. The established method comprises steps of: adopting four ion counters; with two times of peak-jumps, determining all of the Pb isotopes including .sup.204Pb, .sup.205Pb, .sup.206Pb, .sup.207Pb and .sup.208Pb, wherein .sup.205Pb is from a tracer; then, through designing algorithms, eliminating the influences of the gain differences of the multiple ion counters and decreasing the influences of ion beam stabilities on the Pb isotope determination results. The established method is especially applicable to high-precision Pb isotope determination for a single-grain zircon ID-TIMS (Isotope Dilution-Thermal Ionization Mass Spectrometry) U—Pb method (using a .sup.205Pb tracer). With the TRITON PLUS thermal ionization mass spectrometer produced by the Thermo Fisher Scientific Company as an example, the present invention is described as follows.

(9) 1. Mass Spectrometric Determination Method

(10) The TRITON PLUS thermal ionization mass spectrometer of Thermo Fisher Scientific Company is equipped with a multi-ion counter (MIC) system specifically for Pb isotope analysis for zircon ID-TIMS U—Pb dating (as shown in FIG. 2). The MIC system comprises three SEM (Secondary Electronic Multiplier) and two CDD (Compact Discrete Dynode) electronic multipliers, wherein the ion counter IC5 (CDD) is bundled on the L4 Faraday cup (a position of the IC5 can be slightly adjusted through adjusting the position of the L4 Faraday cup); the ion counters IC2 (SEM) and IC4 (CDD) are located on the L5 Faraday cup position. According to the present invention, three SEM ion counters and one CDD ion counter therein are used with a dynamic multi-collection peak-jumping method to determine the Pb isotopes of a .sup.205Pb—Pb mixture. For the multiple ion counters of TRITON PLUS, the detailed settings of the multiple ion counters and the peak-jumping method thereof are listed in Table 1. Through designing two times of peak-jumps, the ion beams of different Pb isotopes are collected, wherein: at the first jump, the virtual mass referring to the center of the multi-collection system of the instrument is set to be 223.04, the ion counter IC5-L4 (CDD) collects .sup.208Pb, IC1 B (SEM) collects .sup.207Pb, IC2-L5 (SEM) collects .sup.206Pb, and IC3A (SEM) collects .sup.205Pb; at the second jump, the virtual mass referring to the center of the multi-collection system of the instrument is set to be 221.95, the ion counter IC5-L4 (CDD) collects .sup.207Pb, IC1 B (SEM) collects .sup.206Pb, IC2-L5 (SEM) collects .sup.205Pb, and IC3A (SEM) collects .sup.204Pb. For each jump, the integration time is 4.194 seconds, and the idle time before integrations is set to 1 second. Because only the position of IC5-L4 can be slightly adjusted through adjusting the position of the L4 Faraday cup and the positions of IC2-L5, IC3A and IC1-B cannot be adjusted, the present invention ensures perfect peak alignments of different ion counters for the two jumps through setting Zoom dispersion parameters and adjusting the position of the L4 Faraday cup.

(11) When using the multi-ion counter system, it is required to determine the dead time and the yield of each ion counter. The present invention firstly determines the dead time of each ion counter by measuring the .sup.208Pb/.sup.206Pb of NIST981 Pb standard using a peak-jumping method with an ion counter; that is to say, for each ion counter, the .sup.208Pb signal intensity is increased from 1 mV to 10 mV stepwise, and under different signal intensities, the ratios of .sup.208Pb/.sup.206Pb are respectively determined by a peak-jumping method; through monitoring the correlation of the determination results of the ratio of .sup.208Pb/.sup.206Pb with the .sup.208Pb signal intensity, the dead time of each ion counter is determined. Moreover, the yield of the ion counters was determined by switching a stable 5-10 mV .sup.208Pb ion beam sequentially into the center Faraday cup, IC5, IC1B, IC2 and IC3A as shown in Table 2; that is to say, the .sup.208Pb signal is adjusted to be stable at 5-10 mV, and the .sup.208Pb signal is determined successively with the center Faraday cup and the different ion counters one by one; through the ratios of the .sup.208Pb measured by each ion counter to that determined by the center Faraday cup, the yield of each ion counter relative to the center Faraday cup is obtained. It is required to ensure that the yield of each ion counter is greater than 90%; if the yield for an ion counter is smaller than 90%, the high voltage applied for the ion counter should be appropriately increased.

(12) TABLE-US-00001 TABLE 1 Dynamic multi-collection method for Pb isotopic determination of a .sup.205Pb—Pb mixture by multiple ion counters of TRITON PLUS thermal ionization mass spectrometer Virtual mass of center Integration Idle Faraday IC4-L5 IC3-A IC2-L5 RPQ/IC1-B IC5-L4 time time cup (CDD) (SEM) (SEM) (SEM) (CDD) (s) (s) J1 223.04 .sup.204Pb .sup.205Pb .sup.206Pb .sup.207Pb .sup.208Pb 4.194 1 J2 221.95 .sup.203Tl .sup.204Pb .sup.205Pb .sup.206Pb .sup.207Pb 4.194 1

(13) TABLE-US-00002 TABLE 2 Method for Yield determination of the multiple ion counters Center Integration Idle RPQ/ Faraday time time Line IC3 A IC2-L5 IC1 B IC5-L4 cup (s) (s) 1 .sup.208Pb 4.194 3 2 .sup.205Pb .sup.206Pb .sup.207Pb .sup.208Pb 222.93 4.194 3 3 .sup.207Pb .sup.208Pb 224.01 4.194 3 4 .sup.208Pb 225.08 4.194 3 5 .sup.208Pb 226.13 4.194 3

(14) 2. Sample Determination Process

(15) When determining the Pb isotopes, firstly slowly increasing a temperature of the filament to about 1000° C.; tuning and focusing the ion beam with the .sup.208Pb signal detected by the IC5-L4 or with the .sup.206Pb signal detected by the IC2-L5; subsequently, slowly ramping the temperature of the filament to increase the ion beam intensity; after the ion beam intensity reaches the expected value, starting to collect the data. For each data block, 25 cycles of data are collected; and there are totally 20 blocks of data to be acquired. Prior to each bock, peak centering is run for the first and the second jumps with the .sup.206Pb signal detected by IC2-L5 and IC1-B, respectively, to re-locate the ion beams into the corresponding ion counters; every four blocks, the ion beam is re-focused with the .sup.206Pb signal detected by IC2-L5 once.

(16) For the zircon sample, after completing the determination of the Pb isotopes, further increasing the temperature of the filament to about 1200° C.-1300° C.; and determining the U isotope composition (determining UO.sub.2.sup.+). The U isotopes are determined by peak-jumping using the center channel SEM (IC1 C). With .sup.17O/.sup.16O=0.00039 and .sup.18O/.sup.16O=0.00205, the interferences of .sup.235U.sup.17O.sup.18O on .sup.238U.sup.16O.sub.2 are corrected. The fractionation effects of the U isotopes are corrected through an external calibration method with the U 500 determination results. The detailed U mass spectrometric determination method refers to document 1. Reference document 1: Chu et al., Ultra-low blank analytical procedure for high precision CA-ID-TIMS U—Pb dating of single grain zircons, Chinese Science Bulletin, 2016, Volume 61, pages 1121-1129.

(17) 3. Method for Processing Pb Isotope Data

(18) The method comprises steps of:

(19) (1) with .sup.207Pb determined from the first jump and .sup.206Pb determined from the second jump by the IC1-B (SEM), obtaining the ratio of .sup.207Pb/.sup.206Pb; with a linear interpolation method, correcting the influences of ion beam stability on the determination results of .sup.207Pb/.sup.206Pb;

(20) (2) with .sup.208Pb determined from the first jump and .sup.207Pb determined from the second jump by the IC5 (CDD), obtaining the ratio of .sup.208Pb/.sup.207Pb; with the linear interpolation method, correcting the influences of the ion beam stability on the determination results of .sup.208Pb/.sup.207Pb;

(21) (3) calculating with a formula of .sup.208Pb/.sup.206Pb=.sup.208Pb/.sup.207Pb×.sup.207Pb/.sup.206Pb, obtaining .sup.208Pb/.sup.206Pb;

(22) (4) with .sup.205Pb determined from the second jump and .sup.206Pb determined from the first jump by the IC2-L5 (SEM), obtaining the ratio of .sup.205Pb/.sup.206Pb; with the linear interpolation method, correcting the influences of the ion beam stability on the determination results of .sup.205Pb/.sup.206Pb;

(23) (5) with .sup.204Pb determined from the second jump and .sup.205Pb determined from the first jump by the IC3 (SEM), obtaining the ratio of .sup.204Pb/.sup.205Pb; with the linear interpolation method, correcting the influences of the ion beam stability on the determination results of .sup.204Pb/.sup.205Pb; and

(24) (6) calculating with a formula of .sup.204Pb/.sup.206Pb=.sup.205Pb/.sup.206Pb×.sup.204Pb/.sup.205Pb, obtaining .sup.204Pb/.sup.206Pb.

(25) Through the above method, all of the Pb isotope ratios after correcting the influences of the ion beam stabilities through the linear interpolation method are obtained, including .sup.204Pb/.sup.206Pb, .sup.205Pb/.sup.206Pb, .sup.207Pb/.sup.206Pb and .sup.208Pb/.sup.206Pb.

(26) The ratios of .sup.204Pb/.sup.206Pb, .sup.205Pb/.sup.206Pb, .sup.207Pb/.sup.206Pb and .sup.208Pb/.sup.206Pb obtained through the above method are all equivalently determined by a same ion counter using a peak-jumping method, thus completely eliminating influences of gain differences of different ion counters on determination results of the Pb isotopes.

(27) According to the present invention, the isotope signal data collected by the IC4-L5 ion counter is not used.

(28) During the isotope determination process of ultra-small amount of Pb (pg level), the trace amount of impurities in the sample, such as the organics, may influence the determination result of the Pb isotopes. Thus, the impurities require to be gradually burned off; and because the influences of the interfering substances may exist, the data at the beginning stage of a determination are generally required to be discarded. During the data processing process, these abnormal data are deleted with the Tripoli software (can be downloaded from http://www.earth-time.org/), and then the final Pb isotope determination results can be obtained.

(29) The fractionation effects of the Pb isotopes for the zircon sample are required to be corrected through an external calibration method with the NIST981 Pb isotope determination results.

Example 1: Pb Isotope Analysis of .SUP.205.Pb-NIST981 Mixed Solution

(30) The present invention firstly conducts the Pb isotope determination on a mixed solution of a NIST981 standard solution and a .sup.205Pb tracer, so as to evaluate the precision and accuracy of the determination method. The Pb isotope determination results obtained through the multi-ion counter dynamic multi-collection method provided by the present invention are compared with the Pb isotope determination results obtained through other methods, including the multi-ion counter static multi-collection method and the single-ion counter peak-jumping method.

(31) The .sup.205Pb-NIST981 mixed solution is prepared through following steps of: taking 500 μL of .sup.205Pb-.sup.235U tracer (the concentration of .sup.205Pb is 9.223 pmol/g; the abundances of .sup.204Pb, .sup.205Pb, .sup.206Pb, Pb and .sup.208Pb are respectively 0.0046%, 99.85%, 0.0384%, 0.0308% and 0.0731%) and 50 μL of 290 ng/g NBS981 standard solution; adding into a 3 mL Teflon PFA beaker, and capping the beaker tightly; placing the beaker onto a hot-plate, and fluxing at 80° C. for at least one week, so as to ensure the sample-tracer Pb isotopic equilibration. 2 μL of the .sup.205Pb-NIST981 mixed solution (corresponding to 50 pg of Pb) are loaded on zone-fined high purity Re filaments for mass spectrometric determination, and the detailed sample loading method refers to document 1. Preparation of the .sup.205Pb-NIST981 mixed solution and the sample loading are conducted in a class 100 fume hood and/or class 100 clean bench in a class 1000 clean room.

(32) After subtraction of the contribution of the tracer from the Pb isotope determination results of the mixed solution, the determination results of .sup.204Pb/.sup.206Pb, .sup.207Pb/.sup.206Pb and .sup.208Pb/.sup.206Pb of the NIST981 are obtained, listed in Table 3.

(33) Correspondingly, the mean (n=20) of the determination results of .sup.204Pb/.sup.206Pb, .sup.207Pb/.sup.206Pb and .sup.208Pb/.sup.206Pb of the NIST981 Pb standard (also with a sample load amount of 50 pg) by the multiple ion counters of the TRITON PLUS thermal ionization mass spectrometer with a static collection method (the IC4-L5 CDD collects .sup.204Pb, the IC2-L5 SEM collects .sup.206Pb, the RPQ/IC1B SEM collects .sup.207Pb, and the IC5-L4 CDD collects .sup.208Pb) are also listed in Table 3 (determining 30 blocks for each sample, and collecting 20 cycles of data for each block). The mean (n=20) of the determination results of .sup.204Pb/.sup.206Pb, .sup.207Pb/.sup.206Pb and .sup.208Pb/.sup.206Pb of the NIST981 Pb standard (again with a sample load amount of 50 pg) through a peak-jumping method (totally four jumps, for respectively determining .sup.204Pb, .sup.206Pb, .sup.207Pb and .sup.208Pb) with the center channel SEM ion counter of the TRITON PLUS thermal ionization mass spectrometer are also listed in Table 3 (determining 15 blocks for each sample, and collecting 20 cycles of data for each block).

(34) FIG. 3 shows comparison of .sup.207Pb/.sup.206Pb determination results of NIST981 with above three determination methods.

(35) It can be seen from Table 3 and FIG. 3 that: the determination internal precision and external precision of .sup.204Pb/.sup.206Pb, .sup.207Pb/.sup.206Pb and .sup.208Pb/.sup.206Pb of the NIST981 Pb standard with the method provided by the present invention are obviously better than that with the single-ion counter peak-jumping method and the multi-ion counter static method.

(36) TABLE-US-00003 TABLE 3 Pb isotope determination results of NIST981 with multi-ion counter dynamic multi-collection method 2RSE 2RSE 2RSE Determination times .sup.204Pb/.sup.206Pb (%) .sup.207Pb/.sup.206Pb (%) .sup.208Pb/.sup.206Pb (%) 1 0.05895 0.080 0.91367 0.045 2.1635 0.036 2 0.05898 0.043 0.91393 0.028 2.1592 0.024 3 0.05898 0.037 0.91243 0.023 2.1597 0.019 4 0.05896 0.038 0.91287 0.020 2.1573 0.022 5 0.05884 0.050 0.91246 0.021 2.1609 0.026 6 0.05918 0.084 0.91376 0.027 2.1603 0.040 7 0.05892 0.062 0.91387 0.043 2.1624 0.035 8 0.05901 0.091 0.91339 0.032 2.1599 0.038 9 0.05908 0.081 0.91409 0.051 2.1609 0.034 10 0.05912 0.079 0.91390 0.025 2.1615 0.030 11 0.05913 0.12 0.91320 0.025 2.1608 0.046 12 0.05912 0.059 0.91284 0.026 2.1603 0.027 13 0.05920 0.062 0.91320 0.032 2.1617 0.025 14 0.05915 0.070 0.91362 0.033 2.1586 0.032 15 0.05917 0.067 0.91391 0.025 2.1598 0.034 16 0.05912 0.066 0.91298 0.026 2.1629 0.028 17 0.05924 0.063 0.91411 0.031 2.1579 0.031 18 0.05908 0.072 0.91349 0.027 2.1618 0.033 19 0.05910 0.068 0.91413 0.029 2.1592 0.026 20 0.05915 0.048 0.91375 0.034 2.1588 0.029 Mean ± SD 0.05907 ± 0.00011 0.91348 ± 0.00054 2.1604 ± 0.0016 IC1-C peak-jumping 0.05909 ± 0.00021 0.91323 ± 0.00071 2.1610 ± 0.0023 method: Mean ± SD (n = 20) MIC static method: 0.05830 ± 0.00028 0.9128 ± 0.0014 2.1680 ± 0.0032 Mean ± SD (n = 20) *RSE: relative standard error; SD: standard deviation

Example 2: ID-TIMS U—Pb Age Determination for Qinghu Standard Zircon

(37) The Qinghu zircon is a calibration standard used for SIMS (Secondary Ion Mass Spectrometry) and LA-ICP-MS (Laser Ablation-Inductively Coupled Plasma-Mass Spectrometry) zircon U—Pb age determination. Researchers have previously conducted U—Pb age determination for the Qinghu standard zircon with the ID-TIMS method, and the obtained .sup.206Pb/.sup.238U weighted average age is 159.45±0.16Ma (±2SE) (referring to the document 2). Recently, the ID-TIMS U—Pb laboratory of Massachusetts Institute of Technology has also conducted ID-TIMS age determination for the Qinghu zircon, and the .sup.206Pb/.sup.238U weighted average age is 159.36±0.06 Ma (±2SE).

(38) The present invention conducts ID-TIMS U—Pb age determination on the Qinghu standard zircon with a .sup.205Pb-.sup.235U tracer, and the sample digestion and chemical separation method refers to the document 1. During the mass spectrometric determination, the Pb isotopes are determined with the multi-ion counter dynamic method provided by the present invention, and U is determined with the center channel SEM using a peak-jumping method. The .sup.206Pb/.sup.204Pb determination results are between 360-2560, and the age determination results are illustrated in FIG. 4A and FIG. 4B. The .sup.206Pb/.sup.238U weighted average age is 159.51±0.13 Ma (±2SE) (2RSE=0.08%, n=8; RSE represents relative standard error, the same hereafter), which is consistent with the values reported in the document 2 and the determination results by the ID-TIMS U—Pb laboratory of Massachusetts Institute of Technology within analytical errors. It is indicated that: with the multi-ion counter dynamic Pb isotope analysis method provided by the present invention, accurate U—Pb age determination results of a zircon sample can be obtained. Reference document 2: Li X H, Liu Y, Li Q L, et al., Precise determination of Phanerozoic zircon Pb/Pb age by multi-collector SIMS without external standardization, Geochemistry Geophysics Geosystems, 2009, 10: Q04010.

Example 3: ID-TIMS U—Pb Age Determination for TEMORA Standard Zircon

(39) The TEMORA zircon is an international reference standard zircon for U—Pb geochronology. Researchers have previously conducted U—Pb age determination on the TEMORA zircon with the ID-TIMS method, and the obtained .sup.206Pb/.sup.238U weighted average age is 416.78±0.33 Ma (±2SE) (referring to the document 3). Recently, the inventors have also conducted ID-TIMS age determination on the TEMORA zircon at the ID-TIMS U—Pb laboratory of Massachusetts Institute of Technology, and the .sup.206Pb/.sup.238U weighted average age is 417.71±0.12 Ma (±2SE).

(40) The present invention conducts ID-TIMS U—Pb age determination on the TEMORA standard zircon with a .sup.205Pb-.sup.235U tracer, similarly as the example 2, and the sample digestion and chemical separation method refers to the document 1. During the mass spectrometric determination, the Pb isotopes are determined with the multi-ion counter dynamic method provided by the present invention, and U is determined with the center channel SEM using a peak-jumping method. The .sup.206Pb/.sup.204Pb determination results are between 300-2630, and the age determination results are illustrated in FIG. 5A and FIG. 5B. The .sup.206Pb/.sup.238U weighted average age is 418.49±0.31Ma (±2SE) (2RSE=0.07%, n=7); although it is relatively higher than the values (.sup.206Pb/.sup.238U age=416.78±0.33 Ma) reported in the document 3, it is consistent with the determination results (.sup.206Pb/.sup.238U age=417.71±0.12 Ma) of the same batch of TEMORA standard zircon by the inventors at the ID-TIMS U—Pb laboratory of Massachusetts Institute of Technology within analytical errors. The example 3 further indicates that: with the dynamic multi-ion counter Pb isotope analysis method provided by the present invention, accurate zircon U—Pb age determination results can be obtained. Reference document 3: Black L P, Kamo S L, Allen C M, et al., TEMORA 1: A new zircon standard for Phanerozoic U—Pb geochronology, Chem. Geol., 2003, 200: 155-170.

(41) Compared with the conventional single-ion counter peak-jumping method, the multi-ion counter dynamic collection method provided by the present invention increases the collection efficiency of the Pb isotope ion beams by 2.5 times and meanwhile decreases the influences of the ion beam stability on the analysis results of the Pb isotopes. Thus, time-normalized precision for Pb isotope determination for the single-grain zircon ID-TIMS U—Pb analysis is improved, and thus the mass spectrometric determination time of the Pb isotopes can be shortened. In order to obtain a high-precision single-grain zircon ID-TIMS U—Pb age (better than 0.1%), the conventional single-ion counter peak-jumping method generally requires 3.5 hours to complete the Pb isotope analysis of one zircon U—Pb dating sample (including the impurity burn-off time), while the method provided by the present invention generally requires only 2 hours (also including the impurity burn-off time).

(42) It should be understood that: for one of ordinary technique in the field, improvements and variations can be made based on the above description, which should be all encompassed in the protection scope of the claims of the present invention.