Apatite high-precision uranium-lead ratio testing system and method

Abstract

The present invention discloses a high-precision uranium-lead ratio testing system and method for apatite, which relates to the technical field of uranium-lead dating. The testing system includes a Resolution LR-S155 193nm ArF excimer laser ablation system and a Thermo Scientific iCap TQ inductively coupled plasma mass spectrometer; the testing system follows the standard methods commonly used in laboratories. The testing method includes the following steps: S1. Pretreatment: cutting, cleaning, drying, and target preparation of the apatite sample; S2. Instrument analysis: including instrument debugging, pre-ablation of the sample, and data processing; S3. Data processing: obtaining the U-Pb age calculation result of the apatite sample according to the data results obtained in the instrument analysis in step S2 and the corresponding age calculation method. The testing system and method of the present invention can meet the limitation that the dating method has no age time range, and solve problems such as the lack of suitable absolute dating techniques for the early Cenozoic strata and the rarity of absolute dating of late Miocene sedimentary rocks and fossils.

Description

Technical Field

[0001] The present invention relates to the technical field of uranium-lead dating, and particularly relates to a high-precision uranium-lead ratio testing system and method for apatite. Background Art

[0002] U-Pb dating based on the enamel of tooth fossils has higher precision and stronger anti-interference ability, and has broad application prospects in the field of stratigraphy. This relatively independent absolute dating method is helpful for stratigraphic research. Especially in the case where the late Cenozoic strata lack obvious sedimentary sequences, it is possible to establish a chronology based on fossil materials, or to be applied to the determination of the age of meteorite impacts, geological landslides or collapse events (provided that there is burial of biological fossils during the event, that is, the lithology of the sedimentary strata and the buried strata changes). The feasibility of U-Pb dating of fossils has been confirmed, but it is still in its infancy. The current small amount of results mainly focus on marine fossil types, and the research on terrestrial fossils is still blank.

[0003] Uranium-lead (U-Pb) dating is based on the accumulation of radioactive Pb and has no age time range limit. It has been applied to the dating of accessory minerals in igneous and metamorphic rocks for decades. For samples of several million years, the calculation of the Tera-Wasserburg age first assumes that there is no disequilibrium effect in the U decay chain ([234U / 238U]i = [231Pa / 235U]i = [230Th / 238U]i = 1), and then corrects the estimated disequilibrium value. The U-Pb dating method has also been gradually applied to the dating of carbonates and apatites in the past decade or so. It is theoretically feasible to apply it to bones and tooth enamel mainly composed of carbonates and apatites, and continuous attempts have been made, but it has not been popularized because the accuracy of this method is poor (the mean square weighted deviation MSWD is usually greater than dozens). The reason for the poor accuracy is that tooth enamel, dentin and bones have significant chemical and structural differences and different resistance degrees to diagenetic changes.

[0004] In view of this, the testing and analysis of the U-Pb ratio of high-precision, high-accuracy and low-signal Cenozoic fossil samples have become one of the technical problems urgently to be solved in the field of geochemical analysis technology.

[0005] Therefore, the present invention aims to provide a high-precision uranium-lead ratio testing system and method for apatite to solve the above problems. Summary of the Invention

[0006] The purpose of the present invention is to solve the above problems and provide a high-precision uranium-lead ratio testing system and method for apatite.

[0007] In order to achieve the above purpose, the technical solution of the present invention is as follows:

[0008] The present invention provides a test system for accurately measuring the uranium-lead ratio of apatite. The test system includes a Resolution LR-S155 193nm ArF excimer laser ablation system (LA) and a Thermo Scientific iCap TQ inductively coupled plasma mass spectrometer (ICP-MS); the test system follows the standard methods commonly used in laboratories.

[0009] The present invention also provides a test method for a test system for accurately measuring the uranium-lead ratio of apatite, comprising the following steps:

[0010] S1. Pretreatment: cutting, cleaning, drying, and target preparation (or slide preparation) of the apatite sample;

[0011] S2. Instrument analysis: including instrument debugging, pre-ablation of the sample, and data processing;

[0012] S3. Data processing: obtaining the U-Pb age calculation result of the apatite sample according to the data results obtained in the instrument analysis in step S2 and the corresponding age calculation method. In order to evaluate the influence of initial value differences and diagenesis on the U-Pb age of fossils, two-dimensional and three-dimensional linear regression was used to compare and analyze the U-Pb age data using Isoplot (Pieter V., 2018; Ludwig, K.R., 1998) to evaluate the influence of subsequent diagenetic transformation and initial value inhomogeneity on the accuracy of the U-Pb age.

[0013] The reported results are: MSWD = u,p(χ 2 ) = v.

[0014] Where: u: mean square weighted deviation (MSWD) of isochron fitting.

[0015] v: chi-square p-value of isochron fitting.

[0016] Age = x ± y(|z).

[0017] Where: x: maximum likelihood estimate of age or initial Pb ratio using the algorithm of Ludwig (1998). y: analytical uncertainty of x(2σ). z: analytical uncertainty of x with overdispersion. The calculation formula is: z = y√MSWD (reported only when the p value of the chi-square test < α).

[0018] Further, the cutting in step S1 is to cut the apatite sample into a suitable size.

[0019] Further, the cleaning in step S1 is to clean the apatite sample with MilliQ water in an ultrasonic cleaner to remove surface impurities.

[0020] Further, the drying in step S1 is to dry the washed apatite sample overnight on a hot plate or in an oven at 45°C - 60°C.

[0021] Further, the target preparation is to fix the apatite sample with epoxy resin and polish it to expose a flat surface of the apatite sample for testing.

[0022] Further, the instrumental analysis specifically is as follows:

[0023] Set the laser time as follows: 5 seconds for surface cleaning, 7 seconds for rinsing, 10 seconds for background, 20 seconds for ablation, 5 seconds for rinsing, with a repetition frequency of 8 Hz and a fluence of 3 J / cm 2 , and the apatite sample spot is 30 μm;

[0024] Gas flow settings: Use helium as the carrier gas, set the flow rate to 280 mL / min, the nebulizer gas of iCap TQ is argon, and the flow rate is adjusted according to the oxide yield and sensitivity; Introduce a small amount of nitrogen into the mass spectrometer and introduce it into the receiving cup at a flow rate of 5 mL / min to mix with the He carrier gas;

[0025] Optimize the instrument status parameters. Mainly, iCap TQ adjusted the mass spectrometry parameters such as gas flow according to NIST 612 to optimize the sensitivity and acceptable oxidation rate (232Th / 16O 232Th, less than 0.2% in this experiment).

[0026] Optimize the ICP parameters; The ICP conditions are as follows: cooling gas, 14 L / min; RF power, 1550 W; auxiliary gas, 0.8 L / min; Since Hg in He and Ar gases 204 has a great interference on 204 Pb, so masses 202 (Hg) and 204 (Pb + Hg) were not measured, and only the masses of 206 Pb, 207 Pb, 208 Pb, 232 Th, 238 U data were collected.

[0027] Conduct standard instrument tests; The sensitivity of U is greater than 500 k cps, equivalent to greater than 13 kcps / ppm, and the Th / U ratio of NIST612 is between 0.9 - 1.0.

[0028] The difficulty and significance of the present invention in solving technical problems are as follows:

[0029] From the perspective of the development of biogenic apatite U-Pb dating technology: Uranium-lead (U-Pb) dating is based on the accumulation of stable radiogenic lead and has no age range limitation. It has been widely used in the dating of igneous rocks and metamorphic accessory minerals for decades. This technology has also been gradually applied to the dating of carbonates and apatites in the past decade or so. It is theoretically feasible to apply it to bones and tooth enamel mainly composed of biogenic apatite, and continuous attempts have been made. However, it has not been popularized because of its poor accuracy (the mean square weighted deviation MSWD is usually greater than dozens), and the reasons are as follows: Tooth enamel, dentin and bones have significant chemical and structural differences and different resistance degrees to diagenetic changes; the proportion of common lead is high, resulting in a low μ value; low-μ value samples have high requirements for testing accuracy; the initial values are multi-source and there are differences, etc. From the perspective of the needs of stratigraphic research: Aiming at the problem that it is difficult to accurately date by ordinary dating methods in the case of the lack of obvious sedimentary sequences in the late Cenozoic strata. The absolute dating of sedimentary rocks and fossils in the late Miocene is still a rare problem. Based on the uncertainty of U-Pb ages generated by previous studies, the inventors of this application selected sample characteristics and data processing methods to jointly solve the above existing problems. Through the solution of the present invention, the significance in geological stratigraphy lies in: U-Pb dating based on tooth fossil enamel has broad application prospects in the field of stratigraphy. Especially in the case of the lack of obvious sedimentary sequences in the late Cenozoic strata, a chronology based on fossil materials can be established.

[0030] Compared with the prior art, the beneficial effects of this solution are as follows:

[0031] 1. Through the application of the test system of the present invention, the analysis and test requirements of low-content and high-precision apatite samples can be met. In particular, the LA-ICP-MS method can help obtain high-precision U / Pb ages with spatial resolution in different types of fossil materials;

[0032] 2. Since the paleomagnetic age is judged according to the magnetic pole cycle, the essence of the paleomagnetic absolute age is a time interval (or two-pole interval), while the attribute of the U-Pb absolute age is a time point, which is theoretically more accurate and greatly reduces the age error obtained by testing. Therefore, in the test method of the present invention, U-Pb fossil dating can not only be carried out in an independent absolute dating system, but also be combined with traditional methods and conclusions. The absolute dating of fossils helps to determine the stratigraphic research and the geological age of sedimentary sequences;

[0033] 3. Through the application of the test system of the present invention, the limitation of the dating method without an age time range can be met, and problems such as the lack of suitable absolute dating techniques in the early Cenozoic strata and the still rare absolute dating of sedimentary rocks and fossils in the late Miocene can be solved. Description of the Drawings

[0034] Figure 1 They are the sampled horse, deer, and elephant fossil samples in the present invention;

[0035] Figure 2 They are the experimental results of the horse fossil samples in the present invention;

[0036] Figure 3 They are the selected sampling points of the horse fossil samples in the present invention;

[0037] Figure 4 They are the preliminary results of the horse fossil samples in the present invention;

[0038] Figure 5 They are the results of sampling giraffe fossils of the same horizon in Fugu with different genera and species for anchoring the initial value in the present invention;

[0039] Figure 6 They are the results of one of the treatment methods for deer fossils in the present invention;

[0040] Figure 7 They are the results of another treatment method for deer fossils in the present invention;

[0041] Figure 8 They are the results of one of the treatment methods for rhino fossils in the present invention;

[0042] Figure 9 They are the results of another treatment method for rhino fossils in the present invention. Detailed implementation manners

[0043] To enable those skilled in the art to better understand the solution of the present invention, the technical solution of the present invention will be further described in detail below in conjunction with the embodiments and drawings of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without making creative efforts shall fall within the protection scope of the present invention.

[0044] It should be noted that, without conflict, the embodiments in the present invention and the features in the embodiments can be combined with each other. The present invention will be described in detail below in conjunction with the embodiments.

[0045] The solution provided by the present invention is: a high-precision uranium-lead ratio test system and method for apatite. The test system includes a Resolution LR-S155 193nm ArF excimer laser ablation system (LA) and a Thermo Scientific iCap TQ inductively coupled plasma mass spectrometer (ICP-MS); the test system follows the standard methods commonly used in the laboratory. The test method includes three steps: pretreatment, instrumental analysis, and data processing.

[0046] Wherein:

[0047] The pretreatment mainly includes cutting, cleaning, drying, and target preparation (or specimen preparation).

[0048] Cutting mainly cuts the apatite sample required for the experiment into appropriate sizes for testing on the machine.

[0049] Cleaning mainly removes surface impurities by cleaning the sample with MilliQ water in an ultrasonic device.

[0050] Drying mainly dries the cleaned sample overnight on a hot plate or in an oven at 45°C - 60°C.

[0051] Target preparation mainly fixes the sample with epoxy resin and polishes it to expose a flat surface for testing.

[0052] The instrument analysis steps mainly include instrument debugging, pre - ablation of the sample, and data reception, etc.

[0053] The laser time in the experiment is set as follows: 5 seconds for surface cleaning, 7 seconds for rinsing, 10 seconds for background, 20 seconds for ablation, 5 seconds for rinsing, the repetition frequency is 8 Hz, and the fluence is 3 J / cm 2 , and the sample spot size is 30 μm.

[0054] First, the gas flow is set. Helium (He) is used as the carrier gas, and the flow rate is set to 280 mL / min. The nebulizing gas of iCapTQ is argon (Ar), and the flow rate is adjusted according to the oxide yield and sensitivity. To improve the sensitivity of the instrument, a small amount of nitrogen (N2) is introduced into the mass spectrometer and introduced into the receiving cup at a flow rate of 5 mL / min to be mixed with the He carrier gas.

[0055] Secondly, the instrument state parameters are optimized. Mainly, iCap TQ adjusts the mass spectrometry parameters such as gas flow according to NIST 612 to optimize the sensitivity and acceptable oxidation rate (232Th / 16O 232Th, less than 0.2% in this experiment).

[0056] Then, the ICP parameters are optimized. The ICP conditions are mainly as follows: cooling gas, 14 liters / minute; RF power, 1550 watts; auxiliary gas, 0.8 liters / minute. Since Hg in He and Ar gases 204 has a great interference on 204 Pb, mass 202 (Hg) and 204 (Pb + Hg) are not measured, and only the data of masses 206 Pb,[[]] 207 Pb,[[]] 208 Pb,[[]] 232 Th,[[]] 238 U are collected.

[0057] Finally, standard instrument testing. Specifically, the sensitivity of U is greater than 500 kcps, which is equivalent to greater than 13 kcps / ppm, and the Th / U ratio of NIST612 is between 0.9 and 1.0.

[0058] The data analysis steps mainly involve calculating the U-Pb isotope ratio from the raw data obtained in the instrument analysis steps to obtain the U-Pb age results of the samples. To evaluate the influence of initial value differences and diagenesis on the U-Pb ages of fossils, Isoplot (Pieter V., 2018; Ludwig, K.R., 1998) was used to conduct a comparative analysis of the U-Pb age data using two-dimensional and three-dimensional linear regression to evaluate the influence of subsequent diagenetic transformation and initial value heterogeneity on the accuracy of U-Pb ages.

[0059] The reported results are: MSWD = u,p(χ 2 ) = v.

[0060] Where: u: Mean square weighted deviation (MSWD) of isochron fitting.

[0061] v: Chi-square p-value of isochron fitting.

[0062] Age = x ± y(|z).

[0063] Where: x: Maximum likelihood estimate of age or initial Pb ratio using the algorithm of Ludwig (1998). y: Analytical uncertainty of x(2σ). z: Analytical uncertainty of x with overdispersion. The calculation formula is: z = y√MSWD (reported only when the p of the chi-square test < α).

[0064] Example 1

[0065] Six fossilized three-toed horse teeth collected from Section 2 of Wang Dafu Liang, Fugu County, Shaanxi Province (as shown) were tested using a Resolution LR-S155 193 nm ArF excimer laser ablation system (LA) and a Thermo Scientific iCap TQ inductively coupled plasma mass spectrometer (ICP-MS) to obtain higher accuracy and reproducibility. The results prove (as shown) Figure 3 as shown Figure 1 to obtain higher accuracy and reproducibility. The results prove (as shown) Figure 2As shown, the U-Pb ages obtained from 6 tooth fossils of Hipparion vary, ranging from 6.87±0.13 Ma to 7.71±0.39 Ma (2s), with an average of 7.26 Ma. This result is consistent with the paleomagnetic and stratigraphic cycle results. Additionally, the youngest U-Pb chronostratigraphy determined from 1 collected Hipparion tooth U-Pb fossil can reach 0.9 million years, and this result is also consistent with the paleomagnetic result. The above two U-Pb age results of horse fossils from different times provide new evidence for determining the age of the Hipparion chiai-Dinocrocuta gigantean-Hezhengia-Shaanxispira standard fossil zone (index fossil assemblage) in the late Miocene of northern China. The age of this zone may be 7.26 Ma, rather than the previously estimated 10 - 9 Ma. And it provides a connection between global environmental perturbations, carbon cycle disruptions, mass extinctions, and the recovery on the sub-millennial time scale.

[0066] Example 2

[0067] The laboratory used a Resolution LR-S155 193 nm ArF excimer laser ablation system (LA) and a Thermo Scientific iCap TQ inductively coupled plasma mass spectrometer (ICP-MS) to determine the geological age of horse fossils at certain positions based on the initial value of 207Pb / 206Pb obtained from calcareous nodules. This method made contributions in sample selection, data processing, and initial value determination respectively. Generally, the common lead in apatite is very high, while for special samples, the proportion of radiogenic apatite selection reaches about 75%. The initial value is determined through the results of co-stratigraphic calcareous nodules, and the data processing adopts the method of anchoring the initial value. For samples with high common lead, more points are better, at least 50 points, and for samples with high common lead and no 232Th, at least 70 points. The samples taken in this example have high U content, low proportion of common lead, and no 232Th. The initial value of 232Th entering the sample system is zero, greatly reducing the uncertainty of sample error. The preliminary results show (as Figure 4 shown) that for samples with a relatively low proportion of common lead (30%), the MSWD value of the results can be controlled at about 2.

[0068] Example 3

[0069] The laboratory used a Resolution LR-S155 193nm ArF excimer laser ablation system (LA) and a Thermo Scientific iCap TQ inductively coupled plasma mass spectrometer (ICP-MS) to determine the geological ages of other species (cattle, giraffe, deer, rhinoceros) at the same stratigraphic position. The results of different methods for deer are not the same. For samples with a high content of common lead, there may be a range of variations. For samples with a low proportion of common lead (such as the horse fossil in the above example), this is not the case. However, due to the influence of the proportion of common lead, there will be differences in age between the results of different species, but they are credible within a certain range. In addition, sample modification also affects dating and is related to the degree of sample modification. The greater the sample modification, the greater the difference in age between the two treatment methods. The results of deer (as shown in Figure 6 and Figure 7 ) are relatively larger than those of rhinoceros (as shown in Figure 8 and Figure 9 ). The modification of rhinoceros is slightly weaker, but there are still variations. Therefore, for samples with large modification, it may be more reliable to preferentially select those with a large MSWD. For unmodified samples of different species at the same point, it is more reliable to preferentially select the results of samples with a low MSWD, and secondly, to select the results of samples with a low MSWD. In addition, although there are significant differences in age between different species in the same stratum, the samples of giraffe, rhinoceros, and horse fossils (as shown in Figure 1 ) all match the corresponding geological ages, proving that the difference between the two-dimensional and three-dimensional linear regression results of this method can be used as an evaluation index for whether the initial Pb isotope composition and diagenesis affect the U-Pb age of fossils. When the age results are consistent within the error range of 2D and 3D, we usually choose the age with a smaller MSWD. When there are limited differences within the error range in the age results, if it is due to different initial values and usually there is a large difference in MSWD between 2D and 3D, then we choose the age with a smaller MSWD between 2D and 3D as the final possible age of the fossil. When there are significant differences between the 2D and 3D ages, if it is due to late diagenesis and there are large MSWD differences in both 2D and 3D, then when setting the initial Pb values of 2D and 3D to the measured Pb isotope ratio of carbonate nodules in the same layer as the fossil, we choose the age with a lower MSWD as the possible fossil age. At the same time, this method can also be used to judge whether there is diagenetic modification and whether the original sample is homogeneous.

[0070] Comparative Example 1

[0071] Six Hipparion teeth fossils were tested in the laboratory using a Resolution LR-S155 193 nm ArF excimer laser ablation system (LA) and a Thermo Scientific iCap TQ inductively coupled plasma mass spectrometer (ICP-MS). At the same time, traditional sedimentary stratigraphy and paleomagnetic methods were compared. The data of the proposed solution of the present invention are closer to the existing paleomagnetic absolute age of 7.8 Ma. At the same time, the difference between the U-Pb age and the paleomagnetic age is also worthy of attention. The paleomagnetic age is judged according to the magnetic pole cycle, so the essence of the paleomagnetic absolute age is a time interval (or the two poles of the interval). The attribute of the U-Pb absolute age is a time point, which is theoretically more accurate.

[0072] Finally, it should also be noted that the term "comprising", "including" or any other variant thereof in the present invention is intended to cover non-exclusive inclusion, so that a process, method, article or device comprising a series of elements not only includes those elements, but also includes other elements not expressly listed, or further includes elements inherent to such process, method, article or device.

[0073] The above specific embodiments are only explanations of the present invention, and they are not limitations of the present invention. Those skilled in the art can make modifications without creative contributions to the embodiments according to needs after reading this specification, but as long as they are within the scope of the claims of the present invention, they are protected by the patent law.

Claims

1. A test method for apatite high-precision uranium-lead ratio test system, characterized in that: The test system includes a Resolution LR-S155 193nm ArF excimer laser ablation system and a Thermo Scientific iCap TQ inductively coupled plasma mass spectrometer; the test system follows the standard method commonly used in laboratories; The testing method of the apatite high-precision uranium-lead ratio testing system comprises the following steps: S1. Pre-treatment: cutting, cleaning, drying and target preparation of apatite samples; S2, instrument analysis: including instrument debugging, pre-stripping samples and data processing; S3, data processing: according to the data results obtained in the instrument analysis in step S2 and the corresponding age calculation method, the U-Pb age calculation results of the apatite sample are obtained, and the U-Pb age data are compared and analyzed using two-dimensional and three-dimensional linear regression using IsoplotR and Isoplot to evaluate the impact of subsequent diagenetic modification and initial value heterogeneity on the accuracy of U-Pb age; The instrumental analysis is specifically as follows: The laser time was set to: 5 seconds surface cleaning, 7 seconds rinse, 10 seconds background, 20 seconds ablation, 5 seconds rinse, repetition rate of 8 Hz, and flux of 3 J / cm 2 , the spot of apatite sample is 30um; Gas flow settings: Helium was used as the carrier gas, and the flow rate was set to 280 mL / min. The nebulizer gas of iCap TQ was argon, and the flow rate was adjusted according to the oxide yield and sensitivity. A small amount of nitrogen was introduced into the mass spectrometer and introduced into the receiving cup at a flow rate of 5 mL / min to mix with the He carrier gas. Optimize instrument status parameters; The ICP parameters were optimized; the ICP conditions were: cooling gas, 14 L / min; RF power, 1550 W; auxiliary gas, 0.8 L / min; Standard instrument tests were performed; the sensitivity of U was greater than 500k cps, equivalent to greater than 13kcps / ppm, and the Th / U ratio of NIST 612 was between 0.9-1.

0.

2. The method for testing a high-precision uranium-lead ratio test system of apatite according to claim 1, characterized in that: The cutting in step S1 is to cut the apatite sample into suitable sizes.

3. The method for testing a high-precision uranium-lead ratio test system of apatite according to claim 1, characterized in that: The cleaning in step S1 is to clean the apatite sample with MilliQ water in an ultrasonicator to remove impurities on its surface.

4. The method for testing a high-precision uranium-lead ratio test system of apatite according to claim 1, characterized in that: The drying in step S1 is to dry the cleaned apatite sample on a hot plate or in an oven at 45° C.-60° C. overnight.

5. The method for testing a high-precision uranium-lead ratio test system of apatite according to claim 1, characterized in that: The target making method is to fix the apatite sample with epoxy resin and polish it to expose a flat surface of the apatite sample for testing.

Images