METHOD FOR IN-SITU U-Pb DATING OF HETEROGENEOUS MINERALS

Abstract

In a method for in-situ U-Pb dating of heterogeneous minerals, a target mineral is separated and purified by carrying out data analysis, extraction and enhancement on original mapping data. The method highlights distribution features of dating indexes (U content, Pb content, Pb.sup.206/U.sup.238 ratio, Pb.sup.207/U.sup.235 ratio and Th/U ratio) in a target mineral phase with a noise data processing technology, and more intuitively displays a favorable dating portion and a crystal growth change trend of the dating mineral. Further, a fine spot design is provided for high-precision dating analysis in a later stage. A plurality of geological events experienced by a heterogeneous mineral crystal in a growth process is accurately limited, so as to invert the mineral formation history which provides the basis for better study of mineralogy, mineral geochemistry and mineral geochronology. It is hence a novel indispensable auxiliary means of mineral geochronology and an auxiliary method for mineral exploration.

Claims

1. A method for in-situ U-Pb dating of heterogeneous minerals, comprising: S1, collecting a bedrock sample, wherein the bedrock sample is rich in any one of zircon, rutile or garnet; S2, carrying out petrographic analysis of the sample comprising the following sub-steps: S21, grinding the collected bedrock sample into a probe piece having a width of 25 mm, a length of 30 mm-35 mm and a thickness of 0.05 mm and a laser sheet having a width of 25 mm, a length of 30 mm-35 mm and a thickness of 0.08 mm, microscopically observing mineral features corresponding to the probe piece and the laser sheet, and recording petrography and paragenetic association features of the probe piece and the laser sheet; S22, selecting a single mineral crystal having a desirable crystal form, complete particles and no impurity inclusion as a sample; and S23, designing a rectangular area greater than a particle size of the single mineral crystal as an experimental test location with the single mineral crystal as a target on the bedrock sample, marking the rectangular area with a marking pen, and carrying out in-situ element mapping analysis on the selected experimental test location by a laser ablation inductively coupled plasma mass spectrometer; S3, carrying out mineral phase separation and purification based on mapping data comprising the following sub-steps: classifying matrix data of Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) mapping obtained in step S23 by K-means semi-automatic supervised multi-channel classification, and hence achieving mineral phase separation to purify the single mineral crystal; S4, carrying out data cleaning and filtering analysis comprising the following sub-steps: for high-U minerals, carrying out data cleaning with purified and separated high-U mineral data, and replacing negative numbers and null values to carry out median filtering on processed data, so as to highlight an element distribution law; and for low-U minerals, carrying out data cleaning with purified and separated low-U mineral data, replacing negative values and null values to carry out median filtering on processed data, and determining a compositional zonation having high U.sup.238 and low Pb.sup.204; S5, carrying out stage determination and spot design comprising the following sub-steps: for the high-U minerals, drafting a U-Pb concordia diagram with filtered data, determining whether multiple stages and Pb loss exist, computing a relation between Th/U and the Pb loss, excluding an area in which U.sup.238 elements are suddenly changed or gradually changed with an area in which the U.sup.238 elements are stably distributed as an area designing LA-ICP-MS SPOT analysis spots, and designing an LA-ICP-MS spot analysis spot distribution diagram according to the above principle; and for the low-U minerals, searching for the compositional zonation having high U and low Pb, determining whether multiple stages exist, and designing an LA-ICP-MS spot analysis spot distribution diagram for an area in which U.sup.238 elements are stably distributed and have a content greater than 10 ppm; and S6, accurately limiting a mineral formation age comprising the following sub-steps: for the high-U minerals, processing and interpreting dating data by using ICPMSDataCal software, and drafting a U-Pb concordia diagram and a U-Pb weighted mean age diagram in combination with ISOPLOTR software; and for the low-U minerals, processing and interpreting data by using the ICPMSDataCal software, and under the condition that single-spot Pb.sup.206/U.sup.238 age concordances are greater than 80%, drafting a U-Pb concordia diagram and a U-Pb weighted mean age diagram by using the ISOPLOTR software; and under the condition that most of single-spot Pb.sup.206/U.sup.238 age concordances are less than 80%, drafting a Tera-Wasserburg U-Pb concordia diagram with the ISOPLOTR software.

2. The method according to claim 1, wherein the single mineral crystal in step S22 has a particle size greater than 0.1 mm.

3. The method according to claim 1, wherein the laser ablation inductively coupled plasma mass spectrometer comprises an Agilent 8900 quadrupole plasma mass spectrometer, a COMPexPro 102 ArF 193 nm excimer laser and a MicroLas optical system, a U-Pb isotope is dated and contents of trace elements are processed with a mineral established standard sample and an international standard material glass standard sample NIST610/NIST612 as calibration standards, and data are restored with iolite4 software and matrix data for mapping elements are exported.

4. The method according to claim 3, wherein test parameters of the laser ablation inductively coupled plasma mass spectrometer comprise laser working parameters and ICP-MS working parameters, wherein the laser working parameters are as follows: in a laser ablation process, high-purity helium is used as carrier gas and high-purity argon is used as compensation gas to adjust sensitivity, the high-purity helium and the high-purity argon are mixed by means of a T-shaped joint before entering plasma, mapping is used in an early stage, a sampling mode is rapid spot ablation, spots are connected to form a line, so as to form a plane, each analysis spot lasts for 3 s to 5 s, and comprises a 1 s-2 s of blank signal and 2 s-3 s of sample ablation and cleaning time, a helium flow rate is 0.6 L/min-0.9 L/min, a laser energy density is 1.5 J/cm.sup.2, a laser beam spot diameter is 5 ?m-10 ?m, a frequency is 10 Hz, and a scanning speed is 3 ?m/s-6 ?m/s; and the ICP-MS working parameters are as follows: radio frequency (RF) power is 1550 W, a plasma gas flow speed is 15 L/min, a sampling depth is 2 ?m-5 ?m, integration time is 2 s-5 s, and an auxiliary argon flow rate is 1.0 L/min.

5. The method according to claim 4, wherein spot analysis is used in a later stage, a sampling mode is spot ablation, each analysis spot lasts for 70 s-90 s, and comprises 15 s-20 s of blank signal, 40 s of sample ablation signal and 15 s-20 s of cleaning time, a helium flow rate is 0.8 L/min, laser energy is 80 mJ, a laser beam spot diameter is 32 ?m-60 ?m, a frequency is 2 Hz-8 Hz, and a pulse number is 90 times-200 times; and the ICP-MS working parameters are as follows: RF power is 1550 W, a plasma gas flow speed is 15 L/min, a sampling depth is 5 ?m-5.5 ?m, integration time is 40 s, and an auxiliary argon flow rate is 1.0 L/min.

6. The method according to claim 5, wherein the K-means semi-automatic supervised multi-channel classification in step S3 is carried out by using MATLAB software.

7. The method according to claim 6, wherein the mineral phase separation in S3 comprises separating different minerals by means of contents of major elements of measured minerals, wherein the major elements of the measured minerals are elements having contents greater than 5 wt. %, and only dating minerals are left while other minerals are removed, thereby obtaining a two-dimensional matrix image.

8. The method according to claim 7, wherein in a specific implementation process of data cleaning and filtering analysis in step S4, data cleaning is carried out on the high-U minerals with the purified and separated high-U mineral data, the negative numbers and the null values are replaced with 0.1 time of a minimum value, median filtering is carried out on the processed data with MineralMAPPING software, and then the matrix data are converted into a two-dimensional element image with XMapTools software to highlight the element distribution law.

9. The method according to claim 8, wherein data cleaning in step S4 is carried out on the low-U minerals with the purified and separated low-U mineral data, the negative values and the null values are replaced with a value 0, median filtering is carried out on the processed data with the MineralMAPPING software, then the matrix data are converted into a two-dimensional element image with the XMapTools software, and the compositional zonation having high U.sup.238 and low Pb.sup.204 is determined.

10. Use of the method according to claim 1 in mineral exploration.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0042] FIG. 1 is a map showing a favorable sampling section delineated in a north margin of a Qaidam basin;

[0043] FIG. 2 is an image showing a rutile probe piece and microscopic petrographic features in a Lvliangshan area;

[0044] FIG. 3 shows (a-c) a mapping data set configured to perform classification and (d) a mineral phase separation effect according to Embodiment 1 of the present disclosure;

[0045] FIG. 4 shows a compositional zonation having high U238 and low Pb204 after median filtering according to the Embodiment 1 of the present disclosure;

[0046] FIG. 5 is an image showing a rutile isotope ratio mapping multi-stage formation zonation and spot design according to the Embodiment 1 of the present disclosure;

[0047] FIG. 6 is a graph showing results of LA-ICP-MS SPOT analysis data according to the Embodiment 1 of the present disclosure;

[0048] FIG. 7 shows (a) a rutile multi-stage SPOT high-precision U-Pb concordia diagram and (b) a U-Pb weighted mean age diagram according to the Embodiment 1 of the present disclosure;

[0049] FIG. 8 is an image showing a zircon probe piece in a Lvliangshan area;

[0050] FIG. 9 shows (a-c) a mapping data set configured to perform classification and (d) a mineral phase separation effect according to Embodiment 2 of the present disclosure;

[0051] FIG. 10 is an image showing resampling and data preprocessing of purified mapping data according to the Embodiment 2 of the present disclosure;

[0052] FIG. 11 is an image showing a compositional zonation having high U238 and low Pb204 after median filtering according to the Embodiment 2 of the present disclosure;

[0053] FIG. 12 is a graph showing a zircon isotope ratio mapping multi-stage formation zonation and spot design;

[0054] FIG. 13 is an image showing a zircon isotope ratio mapping multi-stage formation zonation, Pb loss and spot design;

[0055] FIG. 14 is a graph showing results of LA-ICP-MS SPOT analysis data according to the Embodiment 2 of the present disclosure; and

[0056] FIG. 15 is a zircon multi-stage SPOT high-precision U-Pb concordia diagram.

DETAILED DESCRIPTION OF THE EMBODIMENTS

[0057] If no specific technology or conditions are specified in the present disclosure, the present disclosure shall be carried out according to a technology or conditions described in the literature in the art or according to the product description. All of used reagents or instruments that are not specified by manufacturers are conventional commercially-available products.

Professional Terms and Technical Parameters Involved in the Present Disclosure

[0058] (1) K-means semi-automatic supervised multi-channel classification: it is an iterative clustering analysis algorithm, and includes: pre-divide data into K groups, randomly select K objects as an initial clustering center, then compute a distance between each object and each seed clustering center, and assign each object to the clustering center closest to the object. The clustering centers and the objects assigned to the clustering centers represent a cluster. Each time a sample is assigned, the clustering center of the cluster is recomputed according to existing objects in the cluster. The process will continue being repeated until a certain termination condition is satisfied. The termination condition may be that no (or a minimum number) objects are reassigned to different clusters, no (or a minimum number) clustering centers are changed, and the sum of squared errors is locally minimized. A K-means algorithm defines a distance metric, which is represented by d.sub.ij. A distance between a sample i and a sample j is

[00001]$d_{\mathrm{ij}}=\underset{k=1}{\overset{p}{.Math.}}{\left(x_{\mathrm{ik}}-x_{\mathrm{jk}}\right)}^2={.Math.x_i-x_j.Math.}^2$

to represent a square Euclidean distance. Specifically, p is a dimension of data (which is usually two dimensions). A K value is selected according to actual situations. In order to distinguish minerals studied from other minerals herein, the K value is generally selected as 2. A defined loss function is utilized to iterate and converge continuously to recompute a center of each cluster. The loss function is defined as the sum of a distance between the sample and a center of the cluster to which the sample belongs:

[00002]$W\left(C\right)=\underset{l=1}{\overset{k}{.Math.}}\underset{C\left(i\right)=l}{.Math.}{.Math.x_i-{\stackrel{\_}{x}}_l.Math.}^2.$

x.sub.l=(x.sub.1l, x.sub.2l, . . . x.sub.pl) is a center of an l-th cluster.

[0059] (2) High-U minerals and low-U minerals: a threshold of a U content is set as 200 ppm herein. That is, minerals having the U content greater than or equal to 200 ppm are defined as the high U minerals, such as zircon, monazite and xenotime; and minerals having the U content between 1 ppm and 200 ppm are defined as low U minerals, such as rutile, apatite and garnet.

[0060] (3) Multiple stages: the multiple stages herein refer to significant differences in a U content, a Pb content, a Pb.sup.206/U.sup.238 ratio, a Pb.sup.207/U.sup.235 ratio and a Th/U ratio in heterogeneous minerals. By comparing a relation between an element content and a ratio of the minerals, the number of times of stages of formation events that the minerals have experienced is determined, and a mineral formation history is inverted.

[0061] (4) Median filtering: it is a sorting statistical theory based nonlinear signal processing technology that may effectively suppress noise. A principle thereof is to replace a value of a spot in a digital image or digital sequence with a median of a value of each spot in a neighborhood of the spot, so as to make surrounding pixel values approach to the value, thereby eliminating isolated noise spots. The median filtering is to remove a two-dimensional sliding template of a certain structure, sort pixels in the template according pixel values, and compute the median for replacement. An output of two-dimensional median filtering is g(x,y)-med({f(x?k,y?1), (k,1?W)}). Specifically, f(x,y) and g(x,y) are an original signal and a processed signal respectively. Specifically, W is the two-dimensional template, and is usually a 3*3 area. Parameters of W may be adjusted according to specific situations.

[0062] (5) U-Pb concordia diagram: a Pb.sup.206/U.sup.238-Pb.sup.207/U.sup.235 coordinate map. According to a known age, a ratio is inversely computed, and spots are projected to obtain a curve representing a trajectory of a U-Pb system having consistent ages, i.e. a concordance line. A formed map is referred to as a concordance map. A specific style is shown in the examples.

[0063] (6) U-Pb weighted mean age diagram: a weighted mean age is computed according to t.sub.average=?(t.sub.i/s[t.sub.i].sup.2)/?(1/s[t.sub.i].sup.2) with a Pb.sup.206/U.sup.238 age (t.sub.i) and error (s[t.sub.i]) data of the age, and a diagram presented by the Pb.sup.206/U.sup.238 age (t.sub.i) and the error (s[t.sub.i]) data of the age in a form of a box graph is the U-Pb weighted mean age map. A size of the weighted mean age not only depends on variation between ages of single zircon, but also depends on a number of times of occurrences of each zircon age. Generally, data having a concordance less than 80% are not used, and a specific style is shown in examples.

[0064] (7) Tera-Wasserburg U-Pb concordia diagrams: Pb.sup.206/U.sup.238-Pb.sup.207/Pb.sup.206 coordinate map. A trajectory formed by different time concordance age spots is referred to as a Tera-Wasserburg concordance curve. On the curve, a degree of concordance of relatively young minerals may be fully displayed. Generally, an age of a lower intersection spot is taken as an age of mineral formation.

[0065] MineralMAPPING software used in the present disclosure is independently developed by the applicant, and a computer software copyright registration certificate is 2020SR0341873.

[0066] Mapping analysis in the present disclosure is completed by a laser ablation inductively coupled plasma mass spectrometer. The laser ablation inductively coupled plasma mass spectrometer consists of an Agilent 7900 quadrupole plasma mass spectrometer, a COMPexPro 102 ArF 193 nm excimer laser and a MicroLas optical system. A U-Pb isotope is dated and contents of trace elements are processed with a mineral established standard sample and an international standard material glass standard sample NIST610 as calibration standards. Data are restored with iolite4 software and matrix data for mapping elements are exported.

[0067] Test parameters of the laser ablation inductively coupled plasma mass spectrometer include laser working parameters and ICP-MS working parameters. The laser working parameters are as follows: in a laser ablation process, high-purity helium is used as carrier gas and high-purity argon is used as compensation gas to adjust sensitivity, the high-purity helium and the high-purity argon are mixed by means of a T-shaped joint before entering plasma, mapping is used in an early stage, a sampling mode is spot ablation, each analysis spot lasts for 70 s, and includes a 15 s-20 s of blank signal and 40 s of sample ablation signal and 15 s-20 s of cleaning time, a helium flow rate is 0.8 L/min, laser energy is 80 mJ, a laser beam spot diameter is 44 ?m, a frequency is 5 Hz, and a pulse number is 300 times. The ICP-MS working parameters are as follows: radio frequency (RF) power is 1550 W, a plasma gas flow speed is 15 L/min, a sampling depth is 5 mm-5.5 mm, integration time is 40 s, and an auxiliary argon flow rate is 1.0 L/min.

[0068] A laser ablation system is provided with a signal smoothing device. A laser beam spot and a step size of this analysis are 5 ?m?5 ?m, and ablation duration of each spot is 3 s. In U-Pb isotope dating and trace element content processing, a mineral established standard sample (for example, Zircon is calibrated by a 91500 standard sample) and a glass standard material NIST610 are used as external standards for fractionation correction of isotopes and trace elements respectively. In a later stage, data are restored with the iolite4 software and the matrix data of mapping elements are exported.

Embodiment 1

Inversion of In-Situ Chronological History of Low-U Heterogeneous Minerals

[0069] (1) Systematically collect existing data of a stratum, a structure and magmatic rock in a North Qaidam, comprehensively analyze study potential of the North Qaidam, and systematically study and delineate a favorable sampling section, i.e. Lvliangshan study area, as shown in FIG. 1.

[0070] (2) Select a Lvliangshan area, and collect surface eclogite, i.e. felsic vein body sample; and in a sampling process, faithfully record the following information in detail, as shown in Table 1:

TABLE-US-00001 TABLE 1 Sampling Borehole Symbolic No. No. X Y Lithology Occurrence mineral Photograph Location 1 365417 3275439 Retrograde Block Rutile ZP03 Surface eclogite shape 2 TC201 365450 3275441 Felsic vein Block Rutile ZP04 Trial shape trench 3 TC101 365465 3275413 Felsic vein Veined, Rutile ZP07 Trial 172?65 trench

[0071] (3) Grind the collected bedrock sample into a probe piece and a laser sheet, microscopically observe microscopic petrographic features of rutile, and detailedly describe and record optical features, paragenetic association and special phenomena of the rutile; and select the rutile having representative features, and mark a location and cataloging a number (as shown in FIG. 2) on a probe piece or a laser sheet for accurate rutile minerals with a marking pen, and carry out in-situ element mapping analysis on a selected experimental test location by LA-ICP-MS.

[0072] (4) Select Fe, Nb and Sn elements in LA-ICP MS mapping data as a classification basis (as shown in FIGS. 3a-3c), set a classification quantity into two categories by K-means semi-automatic supervised multi-channel classification, achieve phase separation of rutile from other minerals, and purify a rutile crystal (as shown in FIG. 3d).

[0073] (5) Re-sample purified and separated rutile data, replace negative values and null values, carry out median filtering on processed data, and determine a compositional zonation having high U.sup.238 and low Pb.sup.204 (as shown in FIG. 4), so as to make preparations for genetic determination and LA-ICP-MS spot analysis spot design of single minerals.

[0074] (6) Search for the compositional zonation having high U and low Pb, a Pb.sup.206/U.sup.238 ratio and a Pb.sup.207/U.sup.235 ratio, determine whether multiple stages exist (FIGS. 5a-5c), design an LA-ICP-MS spot analysis spot distribution diagram (FIG. 5d), and carry out spot experiment test. LA-ICP-MS SPOT analysis data results are shown in FIG. 6.

[0075] (7) Process and interpret data by using ICPMDataCal software. It is found that single-spot Pb.sup.206/U.sup.238 age concordances are all greater than 80%, and therefore a U-Pb concordia diagram (as shown in FIG. 7a) and a U-Pb weighted mean age diagram (as shown in FIG. 7b) are manufactured with ISOPLOTR software.

[0076] Therefore, it is found that there are two stages of U element enrichment in a growth process of a rutile crystal, which is highly enriched at 428 Ma in an early stage. With growth of the crystal, an enrichment degree is reduced at 426 Ma, such that an in-situ chronological history of heterogeneous minerals (Rutile) is inversed finely.

Embodiment 2

Inversion of In-Situ Chronological History of High-U Heterogeneous Minerals

[0077] (1) Systematically collect existing data of a stratum, a structure and magmatic rock in a North Qaidam, comprehensively analyze study potential of the North Qaidam, and systematically study and delineate a favorable sampling section, i.e. Lvliangshan study area (as shown in FIG. 1).

[0078] (2) Select a Lvliangshan area, and collect surface eclogite, i.e. felsic vein body sample; and in a sampling process, faithfully record the following information in detail, as shown in Table 2:

TABLE-US-00002 TABLE 2 Sampling Borehole Symbolic No. No. X Y Lithology Occurrence mineral Photograph Location 1 365477 3275489 Retrograde Block Zircon ZP01 Surface eclogite shape 2 TC301 365410 3275491 Felsic vein Nervation Zircon ZP06 Trial trench 3 TC201 365865 3275998 Felsic vein Nervation Zircon ZP08 Trial trench

[0079] (3) Grind the collected bedrock sample into a probe piece and a laser sheet (as shown in FIG. 8), microscopically observe microscopic petrographic features of zircon, and detailedly describe and record optical features, paragenetic association and special phenomena of the zircon; and select the zircon having representative features, and mark a location and cataloging a number on a probe piece or a laser sheet for accurate zircon minerals with a marking pen, and carry out in-situ element mapping analysis on a selected experimental test location by LA-ICP-MS.

[0080] (4) Select Hf, Ti and Zr elements in LA-ICP MS mapping data as a classification basis (FIGS. 9a-9c), set a classification quantity into two categories by K-means semi-automatic supervised multi-channel classification, achieve phase separation of zircon from other minerals, and purify a zircon crystal (FIG. 10d).

[0081] (5) Re-sample purified and separated zircon data, replace negative values and null values (as shown in FIG. 10), carry out median filtering on processed data, and determine a compositional zonation having high U.sup.238 and low Pb.sup.204 (as shown in FIG. 11) to highlight element zonation information, so as to make preparations for genetic determination and LA-ICP-MS spot analysis spot design of single minerals.

[0082] (6) Draft a U-Pb concordia diagram (as shown in FIG. 12) with filtered data, search for the compositional zonation (as shown in FIGS. 13a-13b) having high U and low Pb, a Pb.sup.206/U.sup.238 ratio and a Pb.sup.207/U.sup.235 ratio, determine whether multiple stages and Pb loss exist (as shown in FIGS. 13a-13b), compute a relation (as shown in FIG. 13c) between Th/U and the Pb loss, design an LA-ICP-MS spot analysis spot distribution diagram (as shown in FIGS. 13d-13e), and carry out SPOT experiment test. LA-ICP-MS SPOT analysis data results are shown in FIG. 14.

[0083] (7) Process and interpret data by using ICPMDataCal software, and draft a U-Pb concordia diagram (as shown in FIG. 15) and a U-Pb weighted mean age diagram with ISOPLOTR software. Therefore, it is found that there are multiple stages of crystal growth phenomena in zircon. A core of zircon is formed at around 439 Ma and formed in an environment having a high U/Th ratio. With long growth of a zircon crystal, an edge of the zircon is formed at around 426 Ma and formed in an environment with low U/Th ratio. In such an environment, a phenomenon of Pb loss exists due to geological events in a later stage, which is not conducive to determination of an age of the zircon. In the method, the in-situ chronological history and formation conditions of heterogeneous minerals (zircon) are finely inverted.

[0084] The above embodiments and the features in the embodiments herein can be combined with each other without conflict.

[0085] What are described above are merely preferred embodiments of the present disclosure, and are not intended to limit the present disclosure. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present disclosure shall fall within the protection scope of the present disclosure.