Geological survey method for recovering protolith type of metamorphic rock by utilizing boron isotope fractionation characteristics

Abstract

The invention provides a geological survey method for recovering metamorphic rock protolith types by utilizing boron isotope fractionation characteristics. The method comprises the following steps: establishing a benchmark database containing a plurality of non-metamorphic protorock boron isotopes and key inactive element contents; a double-control correction model of boron isotope fractionation is established, quantitative function relationships between a distribution coefficient and pressure and between a fractionation coefficient and temperature are respectively established through high-temperature and high-pressure experimental data, and a fluid activity factor is introduced to form an inversion calculation equation. The model is used for inversion to obtain original boron isotope composition, then the original boron isotope composition and key element indexes are subjected to collaborative analysis, a two-dimensional discrimination diagram is constructed, and the protolith type is finally determined by comparing discrimination regions in a reference database. According to the method, the metamorphic fractionation effect is effectively corrected, and the accuracy and reliability of high-grade metamorphic rock protolith recovery are remarkably improved in combination with double geochemical index constraints. The method can be matched with portable on-site analysis equipment, and field rapid discrimination is realized.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of geological survey, and particularly relates to a geological survey method for recovering protolith types of metamorphic rocks by using boron isotope fractionation characteristics. BACKGROUND

[0002] The recovery of protoliths of high-grade metamorphic rocks such as granulite facies rocks has long been a difficult problem for the geological community. Traditionally, geochemical discriminant diagrams are widely used, but under high-grade metamorphic conditions, the reliability is greatly reduced due to the significant migration of large elements during metamorphism. Therefore, finding stable geochemical indicators that can still effectively trace the protolith genesis under strong metamorphic modification has become the key in this field.

[0003] Boron element and its stable isotope δ 11 Boron is a sensitive geochemical tracer with significant differences in protoliths of different genesis. However, boron also has high fluid activity during metamorphism and is easily migrated and isotopically fractionated in open-system tectonic-thermal events. This makes it difficult to directly use the measured boron isotope composition of metamorphic rocks for protolith discrimination, as the signal is the product of the superposition of the protolith signal and the multi-stage metamorphic fractionation effect.

[0004] Currently, there is a lack of systematic and effective technical means to quantitatively strip the metamorphic process of boron isotope composition modification and extract the original boron isotope information that can represent the protolith type. Existing research has neither established a sufficient protolith boron isotope benchmark database, nor lacks a correction model that can simulate the effects of pressure conditions and fluid activity on fractionation, and has not formed a complete method system integrating correction and multi-element discrimination. This technical gap has seriously limited the practical application of boron isotope tracing technology in the protolith recovery of high-grade metamorphic terrains.

[0005] Therefore, it is urgent to develop a new geological survey method that can systematically correct the modification of boron isotope composition by metamorphism and cooperatively use stable geochemical indicators to reliably recover the protolith types of high-grade metamorphic rocks. SUMMARY

[0006] To solve the above problems in the prior art, the present application provides a geological survey method for recovering protolith types of metamorphic rocks by using boron isotope fractionation characteristics.

[0007] The present application specifically provides the following technical solutions: A geological survey method for recovering protolith types of metamorphic rocks by using boron isotope fractionation characteristics, the method comprising the following steps: S1. Collect and measure the boron content, boron isotope composition and key non- active element content of a plurality of known unmetamorphosed protoliths, and establish a benchmark database including marine sedimentary rocks, oceanic crust basalts, island arc magmatic rocks and terrigenous clastic rocks; S2. Establish a double-control correction model of boron isotope fractionation, which is used to simulate the distribution behavior of boron element between mineral phases and fluid phases and the boron isotope fractionation effect under metamorphic conditions, and the double-control correction model takes the pressure condition and fluid activity during metamorphism as the core control variable; S3. Obtain measured whole rock boron isotope composition data and key non- active element content data of a sample to be measured metamorphic rock, and determine the temperature and pressure conditions of the main metamorphic stage experienced by the sample; S4. Based on the determined temperature and pressure conditions, the double-control correction model is applied to correct the measured whole rock boron isotope composition data, and the original boron isotope composition of the sample before metamorphism is calculated by inversion; S5. The original boron isotope composition calculated by inversion is analyzed in conjunction with the key non-active element content data to form a comprehensive discrimination index; S6. The comprehensive discrimination index is compared with the benchmark database, and the original rock type of the sample to be measured metamorphic rock is determined according to the preset double-index discrimination rule.

[0008] Optionally, in the S1, the establishment of the benchmark database specifically includes: Select the marine sedimentary rocks, oceanic crust basalts, island arc magmatic rocks and terrigenous clastic rocks with clear geological background and without significant metamorphic modification as standard samples; Determine the whole rock boron content and whole rock boron isotope δ 11 B composition of the standard samples, and additionally measure the single mineral boron isotope δ 11 B value of the representative boron-bearing minerals in the sample; Correlate the measured boron content data, whole rock boron isotope δ 11 B value, single mineral boron isotope δ 11 B value with the key non-active element content data and geological background information corresponding to the standard samples, and construct a standardized benchmark database.

[0009] Optionally, in the S2, the establishment of the double-control correction model includes establishing a quantitative function relationship between the distribution coefficient and the pressure, and the quantitative function relationship is determined by the following method: Based on the data of boron distribution coefficient between the target metamorphic mineral and the coexisting fluid phase obtained under different pressure conditions in high temperature and high pressure experiments simulating the temperature and pressure conditions of the geological metamorphic process, a function relationship between the distribution coefficient D and the pressure P is obtained by nonlinear regression fitting, and the function relationship is specifically: wherein, , , is a fitting parameter, is the pressure.

[0010] Optionally, the establishing a double-control correction model further comprises establishing a quantitative function relationship between the boron isotope fractionation coefficient and the temperature, and constructing an inversion calculation equation, wherein: the quantitative function relationship between the boron isotope fractionation coefficient and the temperature is determined by the following way: Based on the data of boron isotope equilibrium fractionation value between the target metamorphic mineral and the fluid phase obtained under different temperature conditions in high temperature and high pressure experiments simulating the temperature and pressure conditions of the geological metamorphic process, a function relationship between the fractionation value Δ and the temperature T is obtained by nonlinear regression fitting, and the function relationship is specifically: wherein, A, B, C are fitting parameters, and T is the absolute temperature; The inversion calculation equation is: wherein, is the original boron isotope composition, is the measured whole rock boron isotope composition, is the fluid activity factor, is the distribution coefficient, is the fractionation value.

[0011] Optionally, in the S3, the measured whole rock boron isotope composition data is obtained by using a multi-receiving inductively coupled plasma mass spectrometer to determine the powder pressed sheet or the molten glass target prepared from the metamorphic rock sample to be measured; The temperature and pressure conditions of the main metamorphic stage experienced by the sample are determined according to the petrographic observation results of the metamorphic rock sample to be measured, and are obtained by quantitative calculation using a paragenetic mineral geobarometer; wherein, the paragenetic mineral includes a typical paragenetic combination of the target boron-rich mineral, and the temperature and pressure conditions are calculated according to the chemical composition of the characteristic mineral in the paragenetic combination.

[0012] Optionally, the determination of the temperature and pressure conditions of the main metamorphic stage experienced by the sample specifically comprises: Based on the petrographic observation of the metamorphic rock sample to be tested, a specific mineral pair associated with the target boron-rich mineral is identified, wherein the specific mineral pair includes at least one of a garnet-biotite mineral pair for temperature calculation or a garnet-orthopyroxene-plagioclase-quartz mineral combination for pressure calculation; and chemical compositions of the specific mineral pair are determined by an electron probe; The chemical composition data are input into corresponding geothermobarometer calculation formulas to obtain temperature and pressure values, respectively.

[0013] Optionally, the double-control correction model is called for fractionation correction and inversion calculation, specifically including: The values of the metamorphic temperature and pressure determined in S3 are input into the double-control correction model to obtain corresponding boron isotope fractionation coefficients Δ and mineral-fluid distribution coefficients D, respectively; Based on the petrographic analysis result of the metamorphic rock sample to be tested, the value of the fluid activity factor in the double-control correction model is set; The measured whole-rock boron isotope composition obtained in S3, the calculated Δ and D, and the set are input into the inversion calculation equation for solving, and the original boron isotope composition is output.

[0014] Optionally, in S5, the collaborative analysis and formation of the comprehensive discrimination index are implemented by constructing a “boron isotope-key element” collaborative discrimination system, including: The original boron isotope composition obtained by inversion calculation is taken as a core isotope index; At least one element ratio having discrimination significance for protolith types is calculated from the obtained key inactive element content data as a key element index, and the element ratio is composed of elements that are chemically stable and have strong migration inertia in high-grade metamorphic processes; the key inactive elements are at least two of Ti, Nb, Ta, Zr, Hf, and Th; A collaborative correspondence relationship is established between the core isotope index and the key element index, and a two-dimensional discrimination diagram is constructed with the core isotope index as the horizontal axis and the key element index as the vertical axis, and the comprehensive discrimination index corresponds to a coordinate point falling in the two-dimensional discrimination diagram.

[0015] Optionally, in S6, the double-index discrimination rule is preset, specifically: In the two-dimensional discrimination diagram, an exclusive discrimination area is demarcated for the standard sample data of each type of protolith in the reference database, and the boundary of the discrimination area is determined by the statistical distribution range of the core isotope index and the key element index of the corresponding type of protolith. If the coordinate point of the metamorphic rock sample to be tested falls within the discrimination region of a certain type of protolith, it is determined that the protolith of the sample belongs to the corresponding category.

[0016] Optionally, the determination of the protolith type of the metamorphic rock sample to be tested specifically comprises: calculating the Euclidean distance from the coordinate point of the metamorphic rock sample to be tested to the center point of the discrimination region of each type of protolith in the reference database; discriminating the metamorphic rock sample to be tested as the protolith type corresponding to the discrimination region with the minimum Euclidean distance.

[0017] The present application has the following beneficial technical effects: the present application provides a geological survey method for restoring the protolith type of metamorphic rock by using the fractionation characteristics of boron isotopes. The present application establishes a boron isotope reference database, constructs a double-control correction model taking pressure conditions and fluid activity as core control variables, and constructs a "boron isotope-key element" collaborative discrimination system. The accuracy and reliability of the protolith restoration of high-grade metamorphic rocks are significantly improved, which has important value for regional geological survey and resource evaluation. BRIEF DESCRIPTION OF DRAWINGS

[0018] In order to more clearly illustrate the technical solutions of the embodiments of the present application, the drawings needed in the embodiments will be briefly introduced as follows. Obviously, the drawings in the following description are only some embodiments of the present application, and other drawings can be obtained by those skilled in the art without creative labor.

[0019] Figure 1 A geological survey method for restoring the protolith type of metamorphic rock by using the fractionation characteristics of boron isotopes is provided for the embodiments of the present application.

[0020] Figure 2 A metamorphic system conceptual model diagram is provided for the embodiments of the present application.

[0021] Figure 3 A boron isotope fractionation double-control correction model core calculation relationship diagram is provided for the embodiments of the present application.

[0022] Figure 4 A boron isotope-key element collaborative discrimination diagram is provided for the embodiments of the present application. DETAILED DESCRIPTION

[0023] The embodiments of the present application will be described in detail below with reference to the drawings.

[0024] The following detailed description illustrates embodiments of the application by way of example and not by way of limitation. The description will clearly enable one of ordinary skill in the art to make and use the application. Numerous alternatives to the embodiments described herein will be readily apparent to those skilled in the art and the general principles defined herein can be applied to other alternatives without departing from the scope of the application. Accordingly, the description is not intended to limit the scope of the application. The description is intended to cover all alternatives consistent with the principles of the application and the appended claims.

[0025] It is to be understood that the embodiments described hereinbelow within the scope of the appended claims. It will be apparent to one of ordinary skill in the art that aspects described herein can be practiced with less than all of the aspects, and alternatives to the aspects described herein can be practiced within the scope of the claims. Indeed, variations and / or modifications can be made to the described embodiments, and any aspect of the application can include one or more of the features described herein.

[0026] It is also to be understood that the following description is only illustrative of the aspects of the embodiments within the scope of the appended claims.

[0027] Furthermore, in the following description, numerous specific details are set forth in order to provide a thorough understanding of the examples. However, it will be apparent to one of ordinary skill in the art that the aspects can be practiced without these specific details.

[0028] The purpose of the present application is to provide a geological survey method for restoring protolith types of metamorphic rocks by using the boron isotope fractionation characteristics, to establish a quantitative correlation model between boron isotopes and protolith types, to propose a pressure-fluid double control model of boron isotope fractionation during metamorphism, and to realize real-time protolith discrimination in the field.

[0029] In order to make the above-mentioned purposes, characteristics and advantages of the present application more apparent and easy to understand, the present application will be further described in detail below with reference to the drawings and specific embodiments.

[0030] All formula operations involved in the present application are presented by parameter normalization and dimension removal.

[0031] The present embodiment takes the protolith restoration of high-grade metamorphic boron-rich rock series in the Larsemann Hills area of East Antarctica as an example. Referring to Figure 1This paper illustrates a geological survey method for reconstructing the protolith type of metamorphic rocks using boron isotope fractionation characteristics, according to an embodiment of this application. The method includes the following steps: S1. Collecting and measuring the boron content, boron isotope composition, and key inactive element content of various known unmetamorphosed protoliths, and establishing a benchmark database including marine sedimentary rocks, oceanic basalt, island arc magmatic rocks, and terrigenous clastic rocks.

[0032] In S1, establishing the benchmark database specifically includes: selecting marine sedimentary rocks, oceanic basalts, island arc igneous rocks, and terrigenous clastic rocks with clear geological backgrounds and no significant metamorphic alteration as standard samples; and determining the whole-rock boron content and whole-rock boron isotope δ¹⁸O of the standard samples. 11 Composition B, and for the aforementioned marine sedimentary rock standard samples, the single-mineral boron isotope δ¹⁸ of representative boron-bearing minerals in the samples were additionally measured. 11 B value; the measured boron content data and whole-rock boron isotope δ¹⁸O values. 11 B value, boron isotope δ of single mineral 11 The B-value is correlated with the content data of key inactive elements corresponding to the standard sample and geological background information to construct a standardized benchmark database.

[0033] Specifically, typical protoliths with clear geological backgrounds and no significant metamorphic alteration were selected as standard samples. These included marine sedimentary rocks such as organic-rich shale and mudstone known to have formed in stable continental margin environments, or boron-bearing strata within evaporite systems; oceanic basalts collected from modern mid-ocean ridges or well-preserved fragments of ancient oceanic crust, such as basalt within ophiolite suites; island arc igneous rocks including typical island arc basalts and andesites; and terrigenous clastic rocks such as quartz sandstone with high compositional maturity. Detailed geological background information, including field occurrence, geographical location, and stratigraphic unit, was recorded for each type of sample. Systematic testing was performed on the collected standard samples. All standard samples were pulverized to below 200 mesh, prepared using alkali fusion or acid dissolution methods, and their whole-rock boron content and the content of key inactive elements, including but not limited to Ti, Nb, Ta, Zr, Hf, and T, were determined using inductively coupled plasma mass spectrometry (ICP-MS). Simultaneously, the whole-rock boron isotopic composition δ¹⁸O was determined using a multi-receiver inductively coupled plasma mass spectrometer (MC-ICP-MS). 11 B. Single-mineral supplementation analysis was performed, especially for standard samples of marine sedimentary rocks. Since boron may be enriched in specific minerals such as tourmaline, representative boron-bearing mineral grains within these samples needed to be selected under a microscope. In-situ micro-area boron isotope determination of these single minerals was performed using laser ablation multi-collector inductively coupled plasma mass spectrometry (LA-MC-ICP-MS) to obtain more refined isotopic information that may represent the initial sedimentary fluid. All the above test data, including whole-rock boron content and whole-rock δ¹⁸O₂, were then analyzed. 11 B value, δ of boron-loaded minerals 11The B-value, the content of key inactive elements, and the corresponding geological background information of the sample, including but not limited to rock type, origin, and tectonic environment, are correlated and entered into a computer database management system to construct a structured and standardized benchmark database. This database forms the basis for subsequent quantitative comparisons in discrimination.

[0034] S2. Establish a dual-control calibration model for boron isotope fractionation. This model simulates the distribution behavior of boron between the mineral and fluid phases and the boron isotope fractionation effect under metamorphic conditions. The model uses pressure conditions and fluid activity during the metamorphic process as the core control variables. Figure 2 As shown, the conceptual model of the metamorphic system illustrates the coexistence of mineral particles and intergranular fluids at the microscale. Pressure P and fluid activity X are two key external factors controlling boron isotope fractionation.

[0035] First, for the target metamorphic minerals in the study area, namely tourmaline and columnar quartz, commonly found in the boron-rich rocks of the Larsmann Hills, a dataset of boron partition coefficients between these minerals and coexisting fluid phases, such as chloride-rich fluids, was collected or obtained through high-temperature and high-pressure experiments at a specific temperature (800°C) and under different pressure conditions, including 0.5 GPa, 1.0 GPa, 1.5 GPa, and 2.0 GPa. The partition coefficient D is defined as D=C 矿物 / C 流体 Where C is the boron concentration. Then, a nonlinear regression fitting is performed on this dataset, with pressure P as the independent variable, to establish a predictive equation for the partition coefficient. This equation describes how pressure changes systematically alter the partitioning behavior of boron between the solid and liquid phases. The fitted relationship is: in, , , For the fitting parameters, The pressure is the factor. This fitted relationship function can reflect the rapid adjustment of the distribution coefficient due to changes in fluid density in the initial stage of pressure increase, as well as the linear effect of the compression effect of mineral crystal structure under high pressure.

[0036] Secondly, for the same mineral-fluid system, a dataset of fractionation values ​​Δ (in units of ‰) of boron isotopes at equilibrium between the two phases was collected or obtained experimentally under different temperature conditions, including 600℃, 700℃, 800℃, and 900℃. This dataset was then fitted to establish a quantitative relationship between the fractionation value Δ and the absolute temperature T. The fitting equation describing the complex fractionation behavior is as follows: Where A, B, and C are the fitting parameters.

[0037] Finally, combining the distribution coefficient D and the fractionation value Δ, and introducing a key variable, namely the fluid activity factor,... Its value ranges from 0 to 1. =0 represents a completely closed system. =1 represents an extremely open system with complete fluid exchange, and the core inversion calculation equation is constructed as follows: This equation is the core of the calculation of the dual-control correction model, and it can convert measured values... Correct back to original value Among them, pressure P and temperature T determine the values ​​of D and Δ, while the factor This quantifies the impact of fluid activity on the fractionation path, thus achieving a dual-control simulation of the altered fractionation effect. For example... Figure 3 The figure shows two core calculation relationships of the dual-control correction model for boron isotope fractionation. The left figure shows the relationship between the distribution coefficient D and the pressure P. D changes non-linearly with increasing pressure, reflecting the response of boron's distribution behavior between minerals and fluids to pressure. The right figure shows the relationship between the fractionation value Δ and the temperature T. It shows that Δ decreases with increasing temperature, reflecting the response of the degree of boron isotope fractionation to temperature.

[0038] S3. Obtain the measured whole-rock boron isotope composition data and key inactive element content data of the metamorphic rock sample to be tested, and determine the temperature and pressure conditions of the main metamorphic stages experienced by the sample.

[0039] The boron-rich metamorphic rock sample to be tested, corresponding to tourmaline-bearing quartz gneiss, was pulverized and ground to a particle size of less than 200 mesh to prepare a uniform powder. A portion of the powder was then used in an alkaline fusion method, including co-melting with sodium hydroxide, to prepare a uniform glass target. The boron isotope composition δ¹⁸O₅ of the prepared target was analyzed using a multi-receiver inductively coupled plasma mass spectrometer (MC-ICP-MS). 11 Accurate determination of the B value. Simultaneously, the content of key inactive elements in the same sample powder, including but not limited to Ti, Nb, Ta, Zr, Hf, and Th, was determined using inductively coupled plasma mass spectrometry (ICP-MS). These data will be used for subsequent correction and discrimination.

[0040] Probe slides were prepared for the test samples, and detailed petrographic observations were conducted under a microscope. The focus was on identifying specific mineral assemblages that were in equilibrium with the target boron-rich mineral, tourmaline, and suitable for geothermobaric calculations. For example, if tourmaline was observed to be in stable contact with garnet (Grt) and biotite (Bt) without twinning, reaction edges, or other imbalance phenomena, the Grt-Bt mineral pair could be identified as the thermometer's calculation object. If further coexistence with garnet (Grt), orthopyroxene (Opx), plagioclase (Pl), and quartz (Qtz) was found, this could be used as the pressure gauge's calculation combination. Using an electron probe microanalyzer (EPMA), precise in-situ major elemental chemical composition analysis was performed on the identified coexisting mineral pairs: the Gr core and adjacent Bt, and the Gr core and adjacent Opx and Pl, ensuring that the measurement points were in a homogeneous equilibrium region.

[0041] Substituting the chemical composition of Grt and Bt, including the contents of elements such as Fe, Mg, and Mn, into the specific calculation formula of the garnet-biotite Grt-Bt, Fe-Mg exchange thermometer, the primary metamorphic temperature experienced by the sample was calculated, yielding T = 800 ± 30℃. Substituting the chemical composition of Grt, Opx, Pl, and Qtz—the Ca, Mg, and Fe contents in Grt, the Al content in Opx, and the An molecular percentage in Pl—into the formula of the garnet-orthopyroxene-plagioclase-quartz Grt-Opx-Pl-Qtz geobaric pressure gauge, the corresponding pressure value was calculated, yielding P = 1.2 ± 0.1 GPa.

[0042] Finally, the calculated temperature T≈800℃ and pressure P≈1.2GPa were used as representative temperature and pressure conditions for the main metamorphic phase of the sample and input into the subsequent correction model.

[0043] S4. Based on the determined temperature and pressure conditions, the measured whole-rock boron isotope composition data is fractionated and corrected using the dual-control calibration model, and the original boron isotope composition of the sample before metamorphism is obtained through inversion calculation. The fractionation correction and inversion calculation using the dual-control calibration model specifically includes: inputting the metamorphic temperature and pressure values ​​determined in S3 into the dual-control calibration model to calculate the corresponding boron isotope fractionation coefficient Δ and mineral-fluid partition coefficient D; and setting the fluid activity factor in the dual-control calibration model based on the petrographic analysis results of the metamorphic rock sample to be tested. The numerical value; the measured whole-rock boron isotope composition obtained in S3. The calculated Δ and D, and the set The inversion calculation equation is input and solved to output the original boron isotope composition. .

[0044] Specifically, the representative metamorphic temperature and pressure conditions determined above are used as inputs: temperature T≈800℃, i.e. 1073 K; pressure P≈1.2GPa.

[0045] Calculate the distribution coefficient D: Substitute the pressure value P = 1.2 GPa into the established distribution coefficient-pressure function relationship. In the middle. Assuming that the parameters for the tourmaline-chlorinated fluid system have been obtained through previous experimental fitting as D0=5.0, α=0.8, β=0.1, then we can calculate: D=5.0×e 0.8×1.2 +0.1×1.2≈5.0×0.383+0.12≈1.915+0.12≈2.035 Calculate the fractionation coefficient Δ: Substitute the temperature value T=1073K into the established fractionation coefficient-temperature function relationship. In the middle. Assume the parameters fitted for the same system are A=5000, B= 2.5, C=15, then we can calculate: Δ=5000 / 1073+( 2.5)×ln(1073)+15≈4.66+( 2.5)×6.978+15≈4.66 17.445 + 15 ≈ 2.215‰ Setting fluid activity factor The setting of this factor should be based on a comprehensive evaluation of geological evidence obtained from petrographic analysis of the samples.

[0046] Observations revealed that the contact boundaries between tourmaline and minerals such as garnet and biotite in the tested samples were straight and clear, without obvious dissolution or reaction rim structures, indicating that the minerals were basically in equilibrium during the main metamorphic stage. However, a small number of late-stage quartz-carbonate veinlets were found in the rock, cutting through the early foliation, suggesting the existence of localized and limited fluid activity after the metamorphic peak.

[0047] Based on the above evidence, it can be concluded that the system was close to equilibrium during the main metamorphic stage, but not absolutely closed, with limited external fluid intervention or local reequilibrium of internal fluids. Therefore, the fluid activity factor... Set to a small value between 0 and 1. Based on empirical evaluation, set... =0.2, used to characterize the system state of "mainly in near-closed equilibrium, accompanied by slight fluid activity".

[0048] Perform inversion calculations to solve for the original boron isotope composition: The obtained measured whole-rock boron isotope values ​​are then used to determine the composition. The set values ​​are +5.0‰, and the calculated Δ = 2.215‰ and D = 2.035. =0.2 and substitute it into the core inversion equation: Perform numerical calculations: =5.0+2.215×(1 0.2)×2.035(1 2.035) = 5.0 + 2.215 × 0.8 × 2.035( 1.035) = 5.0 + 1.772 × ( 0.5086) = 5.0 0.901≈4.099‰ The original whole-rock boron isotope composition before metamorphism of the boron-rich metamorphic rock sample from the Larsmann Hills was obtained by calculation using a dual-control calibration model. It is approximately +4.1‰. This value has largely eliminated the influence of boron isotope fractionation caused by metamorphism at approximately 800℃ and 1.2 GPa, providing a geochemical basis closer to its initial origin for subsequent protolith identification.

[0049] S5. The original boron isotope composition obtained from the inversion calculation is synergistically analyzed with the content data of the key inactive elements to form a comprehensive discrimination index. In S5, the synergistic analysis and formation of the comprehensive discrimination index is specifically achieved by constructing a "boron isotope-key element" synergistic discrimination system, including: using the original boron isotope composition obtained from the inversion calculation as the core isotope index; calculating at least one element ratio that is significant for discriminating the protolith type from the acquired key inactive element content data as the key element index, wherein the element ratio consists of elements with stable chemical properties and strong migration inertness during advanced metamorphism; the key inactive elements are at least two of Ti, Nb, Ta, Zr, Hf, and Th; establishing a synergistic correspondence between the core isotope index and the key element index, constructing a two-dimensional discrimination diagram with the core isotope index as the horizontal axis and the key element index as the vertical axis, and the comprehensive discrimination index corresponding to the coordinate point in the two-dimensional discrimination diagram.

[0050] Specifically, the original boron isotope composition output directly from the inversion calculation is used. =+4.1‰ was used as the core isotope index. This index carries the boron isotope signal, after correction, that best reflects the characteristics of the original source region. From the obtained data on the content of key inactive elements in the samples to be tested, elements that are chemically extremely stable during advanced metamorphism and hardly migrate with fluid activity were selected, and their ratios were calculated.

[0051] For the regional tectonic setting identification studied in this embodiment, Th and Nb elements were selected. Th is strongly enriched in felsic crust, while Nb is relatively enriched in mantle-derived rocks, and both exhibit high inactivity under granulite facies metamorphic conditions. Therefore, the ratio Th / Nb is an effective indicator for determining whether the protolith was added with ancient continental crustal material. Assuming that ICP-MS analysis shows that the sample has a Th content of 12.5 ppm and an Nb content of 8.0 ppm, the key element index is calculated as: Th / Nb = 12.5 / 8.0 = 1.56. This ratio of 1.56 is the quantified key element index.

[0052] Constructing a two-dimensional discriminant diagram and determining the comprehensive discriminant coordinate points: based on the original boron isotope composition. A two-dimensional Cartesian coordinate system is established with the x-axis as the horizontal axis and the key element ratio Th / Nb as the y-axis. This system provides a discriminant diagram. The two index values ​​X= in this embodiment are used to represent the sample under test. =+4.1‰, Y=Th / Nb=1.56 is calibrated in this diagram. This unique coordinate point (+4.1, 1.56) is the comprehensive discrimination index corresponding to this sample. This point integrates the sample's corrected boron isotope characteristics and elemental ratio characteristics unaffected by metamorphism, forming the core data pair for subsequent comparison with the protolith database. For example... Figure 4 The figure illustrates the specific process of determining the type of protolith using the "boron isotope-key element" synergistic discrimination system. The figure also shows the details of Euclidean distance discrimination: calculating the distance from the sample point to the center point of each type of protolith region.

[0053] S6. Compare the comprehensive discrimination index with the benchmark database, and determine the protolith type of the metamorphic rock sample to be tested according to the preset dual-index discrimination rule. In S6, the preset dual-index discrimination rule specifically means: in the two-dimensional discrimination diagram, a dedicated discrimination region is defined for the standard sample data of each type of protolith in the benchmark database. The boundary of the discrimination region is determined by the statistical distribution range of the core isotope index and the key element index of the corresponding type of protolith. If the coordinate point of the metamorphic rock sample to be tested falls within the discrimination region of a certain type of protolith, then the protolith of the sample is determined to belong to the corresponding category.

[0054] The determination of the protolith type of the metamorphic rock sample to be tested specifically includes: calculating the Euclidean distance from the coordinate point of the metamorphic rock sample to be tested to the center point of the various protolith discrimination regions in the benchmark database; and classifying the metamorphic rock sample to be tested as the protolith type corresponding to the discrimination region with the smallest Euclidean distance.

[0055] Specifically, before performing the discrimination, a dedicated discrimination region needs to be pre-defined for each type of protolith in the benchmark database on the constructed two-dimensional discrimination diagram. From the benchmark database, the original boron isotope composition δ¹⁸, corresponding to two key indicators of all standard samples of each type of protolith, including marine sedimentary rocks and oceanic basalt, should be used after method calibration. 11 The ratio of B to the selected key element Th / Nb.

[0056] Statistical analysis was performed on the two sets of data for each type of protolith, including but not limited to calculating the mean and standard deviation. Taking "marine sedimentary rocks" as an example, assuming that the δ0.05 of all its standard samples... 11 The B value is mainly distributed between -10‰ and +10‰, and the Th / Nb ratio is mainly distributed in the range >5. Based on this, a polygonal or elliptical region roughly covering this densely distributed data area can be drawn on the discriminant diagram as the discriminant region for "marine sedimentary rocks". Similarly, for "oceanic basalt", its δ... 11 B is approximately +5‰ to +10‰, Th / Nb < 0.2, and other types such as "terrigenous clastic rocks" are delineated into their respective regions. These regions serve as the direct basis for subsequent comparisons.

[0057] Plot the coordinates (+4.1‰, 1.56) of the sample to be tested onto the diagram with the pre-defined discrimination area, and make a discrimination based on the following rules: observe which preset discrimination area the coordinate point falls into. For example, if the point happens to fall into the preset discrimination area for "island arc igneous rock", then it is preliminarily determined that its protolith may be an island arc igneous rock.

[0058] To improve the objectivity and accuracy of the discrimination, especially when the coordinate point is close to the regional boundary, a secondary confirmation is performed by calculating the Euclidean distance. The Euclidean distance from the coordinate point to the mean point of each data index corresponding to the statistical center point of each type of protolith discrimination region in the database is calculated. The calculated center point coordinates for the "Island Arc Igneous Rock" category in the benchmark database are (+5.0‰, 1.2), and the center point coordinates for the "Oceanic Crust Basalt" category are (+7.0‰, 0.1). The distances from the measured point (+4.1, 1.56) to these two centers are calculated: Distance to the center of "Island Arc Igneous Rock": Distance to the center of the "oceanic basalt": Based on the rule of "identifying the sample as the protolith type corresponding to the region with the smallest Euclidean distance", since 0.969 < 3.247, the protolith type of the boron-rich metamorphic rock sample to be tested in the Larsmann Hills was finally determined to be island arc magmatic rock.

[0059] The solution of this invention is not limited to a laboratory environment. To meet the needs of rapid, real-time field operations in geological surveys, a dedicated portable boron isotope rapid on-site analyzer has also been developed. This integrated on-site analysis equipment typically includes at least the following modules: Portable boron isotope analysis module: The core is a miniaturized, interference-resistant multi-receiver inductively coupled plasma mass spectrometer (MC-ICP-MS). This module is used to directly perform boron isotope delta analysis on rock powder pellets or simply prepared target samples on-site. 11 Rapid, semi-quantitative, or quantitative determination of boron (B). Key element rapid analysis module: Integrates a portable X-ray fluorescence spectrometer. This module is used for rapid, non-destructive, or minimally destructive on-site determination of the content of key inactive elements in rock samples, including but not limited to Ti, Nb, Zr, and Th, to calculate key element ratios such as Th / Nb. Data processing and model calculation unit: Built-in high-performance embedded computer system, pre-stored with: a benchmark database and dual-control calibration models including the functional relationship between distribution coefficient and pressure, the functional relationship between fractionation coefficient and temperature, and inversion calculation equations. The established thermo-barotropic calculation program integrates common geological thermo-barotropic calculation methods. An established "boron isotope-key element" co-discrimination program and discrimination diagrams. Human-computer interaction and display module: Includes a touch screen, data interface, and result report printing output unit.

[0060] This integrated field analysis equipment eliminates the reliance on large, fixed laboratories, compressing the process of sample delivery, batch testing, offline calculations, and manual interpretation—which previously took weeks or even months—into hours in the field. This significantly improves the efficiency and responsiveness of geological surveys, especially in remote areas and extreme environments.

[0061] Based on the same inventive concept, according to another aspect of the present invention, embodiments of the present invention also provide a computer device, comprising: At least one processor; and The memory stores a computer program that can run on a processor. When the processor executes the program, it performs the steps of a geological survey method described above for reconstructing the protolith type of metamorphic rocks using boron isotope fractionation characteristics.

[0062] Based on the same inventive concept, according to another aspect of the present invention, embodiments of the present invention also provide a computer-readable storage medium storing computer program instructions, which, when executed by a processor, perform the steps of the geological survey method described above for reconstructing the protolith type of metamorphic rocks using boron isotope fractionation characteristics.

[0063] Finally, it should be noted that those skilled in the art will understand that all or part of the processes in the above embodiments can be implemented by a computer program instructing related hardware. The program can be stored in a computer-readable storage medium, and when executed, it can include the processes of the embodiments of the above methods. The storage medium can be a magnetic disk, optical disk, read-only memory (ROM), or random access memory (RAM), etc. The above computer program embodiments can achieve the same or similar effects as any of the corresponding foregoing method embodiments.

[0064] Furthermore, typically, the devices and equipment disclosed in the embodiments of this invention can be various electronic terminal devices, such as mobile phones, personal digital assistants (PDAs), tablet computers (PADs), smart TVs, etc., or they can be large terminal devices, such as servers. Therefore, the scope of protection disclosed in the embodiments of this invention should not be limited to a specific type of device or equipment. The client disclosed in the embodiments of this invention can be applied to any of the above-mentioned electronic terminal devices in the form of electronic hardware, computer software, or a combination of both.

[0065] Furthermore, the method disclosed in the embodiments of the present invention can also be implemented as a computer program executed by a CPU, which can be stored in a computer-readable storage medium. When the computer program is executed by the CPU, it performs the functions defined in the method disclosed in the embodiments of the present invention.

[0066] Furthermore, the above-described method steps and system units can also be implemented using a controller and a computer-readable storage medium for storing a computer program that enables the controller to perform the functions of the above-described steps or units.

[0067] Furthermore, it should be understood that the computer-readable storage medium (e.g., memory) described herein can be volatile memory or non-volatile memory, or may include both volatile and non-volatile memory. By way of example, and not limitation, non-volatile memory may include read-only memory (ROM), programmable ROM (PROM), electrically programmable ROM (EPROM), electrically erasable programmable ROM (EEPROM), or flash memory. Volatile memory may include random access memory (RAM), which can act as external cache memory. By way of example, and not limitation, RAM may be available in various forms, such as synchronous RAM (DRAM), dynamic RAM (DRAM), synchronous DRAM (SDRAM), dual data rate SDRAM (DDRSDRAM), enhanced SDRAM (ESDRAM), synchronous link DRAM (SLDRAM), and direct Rambus RAM (DRRAM). The storage devices disclosed herein are intended to include, but are not limited to, these and other suitable types of memory.

[0068] The above are exemplary embodiments disclosed in this invention. However, it should be noted that various changes and modifications can be made without departing from the scope of the embodiments of this invention as defined by the claims. The functions, steps, and / or actions of the methods according to the disclosed embodiments described herein do not need to be performed in any marked order. Furthermore, although the elements disclosed in the embodiments of this invention may be described or claimed individually, they may be understood as multiple unless explicitly limited to a singular.

[0069] In this specification, the same or similar parts between the various embodiments can be referred to mutually. Each embodiment focuses on describing the differences from other embodiments. In particular, the descriptions of the embodiments described later are relatively simple, and relevant parts can be referred to the descriptions of the foregoing embodiments.

[0070] The above description is merely a specific embodiment of this application, but the scope of protection of this application is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in this application should be included within the scope of protection of this application. Therefore, the scope of protection of this application should be determined by the scope of the claims.

Claims

1. A geological survey method for reconstructing the protolith type of metamorphic rocks using boron isotope fractionation characteristics, characterized in that, The method includes the following steps: S1. Collect and determine the boron content, boron isotope composition and key inactive element content of various known unmetamorphosed protoliths, and establish a benchmark database including marine sedimentary rocks, oceanic crust basalt, island arc magmatic rocks and terrigenous clastic rocks; S2. Establish a dual-control calibration model for boron isotope fractionation. The dual-control calibration model is used to simulate the distribution behavior of boron between the mineral phase and the fluid phase and the boron isotope fractionation effect under metamorphic conditions. The dual-control calibration model uses the pressure conditions and fluid activity during the metamorphic process as the core control variables. S3. Obtain the measured whole-rock boron isotope composition data and key inactive element content data of the metamorphic rock sample to be tested, and determine the temperature and pressure conditions of the main metamorphic stages experienced by the sample. S4. Based on the determined temperature and pressure conditions, the measured whole-rock boron isotope composition data are fractionated and corrected using the dual-control correction model, and the original boron isotope composition of the sample before metamorphism is obtained by inversion calculation. S5. The original boron isotope composition obtained from the inversion calculation is analyzed in conjunction with the content data of the key inactive elements to form a comprehensive discrimination index; S6. Compare the comprehensive discrimination index with the benchmark database, and determine the protolith type of the metamorphic rock sample to be tested according to the preset dual-index discrimination rule.

2. The geological survey method for reconstructing the protolith type of metamorphic rocks using boron isotope fractionation characteristics according to claim 1, characterized in that, In S1, establishing the benchmark database specifically includes: Marine sedimentary rocks, oceanic basalts, island arc igneous rocks, and terrigenous clastic rocks with clear geological backgrounds and no significant metamorphic alteration were selected as standard samples. The whole-rock boron content and whole-rock boron isotope δ¹⁸O of the standard sample were determined. 11 Composition B, and for the aforementioned marine sedimentary rock standard samples, the single-mineral boron isotope δ¹⁸ of representative boron-bearing minerals in the samples were additionally measured. 11 B value; The measured boron content data and whole-rock boron isotope δ¹⁸O will be used to... 11 B value, boron isotope δ of single mineral 11 The B-value is correlated with the content data of key inactive elements corresponding to the standard sample and geological background information to construct a standardized benchmark database.

3. The geological survey method for reconstructing the protolith type of metamorphic rocks using boron isotope fractionation characteristics according to claim 1, characterized in that, In S2, establishing the dual-control correction model includes establishing a quantitative functional relationship between the distribution coefficient and the pressure, which is determined in the following way: Based on high-temperature and high-pressure experiments simulating geological metamorphic processes, data on the distribution coefficients of boron between the target metamorphic minerals and coexisting fluid phases under different pressure conditions were obtained. A functional relationship between the distribution coefficient D and the pressure P was derived through nonlinear regression fitting. Specifically, the functional relationship is as follows: in, , , For the fitting parameters, For pressure.

4. A geological survey method for reconstructing the protolith type of metamorphic rocks using boron isotope fractionation characteristics, as described in claim 3, is characterized in that... The establishment of the dual-control correction model also includes establishing a quantitative functional relationship between the boron isotope fractionation coefficient and temperature, and constructing an inversion calculation equation, wherein: the quantitative functional relationship between the fractionation coefficient and temperature is determined in the following way: Based on high-temperature and high-pressure experiments simulating geological metamorphic processes, boron isotope equilibrium fractionation values ​​between target metamorphic minerals and fluid phases under different temperature conditions were obtained. A nonlinear regression fitting was used to derive a functional relationship between the fractionation value Δ and temperature T. Specifically, the functional relationship is as follows: Where A, B, and C are fitting parameters, and T is the absolute temperature; The inversion calculation equation is: in, It has the original boron isotopic composition. To measure the whole-rock boron isotope composition, As a fluid activity factor, For allocation coefficients, This is the fractionation value.

5. A geological survey method for reconstructing the protolith type of metamorphic rocks using boron isotope fractionation characteristics, as described in claim 1, is characterized in that... In S3, the measured whole-rock boron isotope composition data are obtained by using a multi-receiver inductively coupled plasma mass spectrometer to measure the powder pellets or fused glass targets prepared from the metamorphic rock sample to be tested. The determination of the temperature and pressure conditions of the main metamorphic stages experienced by the sample is specifically based on the petrographic observation results of the metamorphic rock sample to be tested, and quantitative calculation is performed using a symbiotic mineral geothermobarometer; wherein, the symbiotic minerals include typical symbiotic assemblages of target boron-rich minerals, and the temperature and pressure conditions are calculated based on the chemical composition of the characteristic minerals in the symbiotic assemblage.

6. A geological survey method for reconstructing the protolith type of metamorphic rocks using boron isotope fractionation characteristics, as described in claim 5, is characterized in that... The temperature and pressure conditions for determining the main metamorphic stages experienced by the sample specifically include: Based on petrographic observations of the metamorphic rock samples to be tested, specific mineral pairs associated with the target boron-rich minerals are identified; wherein, the specific mineral pairs include at least one of the following: a garnet-biotite mineral pair for temperature calculation, or a garnet-orthopyroxene-plagioclase-quartz mineral assemblage for pressure calculation; the chemical composition of the specific mineral pairs is determined by electron probe microanalysis. Input the chemical composition data into the corresponding geothermal barometer calculation formula to calculate the temperature and pressure values ​​respectively.

7. A geological survey method for reconstructing the protolith type of metamorphic rocks using boron isotope fractionation characteristics, as described in claim 4, is characterized in that... The dual-control correction model is invoked to perform fractionation correction and inversion calculations, specifically including: The values ​​of metamorphic temperature and metamorphic pressure determined in S3 are input into the dual-control correction model to calculate the corresponding boron isotope fractionation coefficient Δ and mineral-fluid partition coefficient D, respectively. Based on the petrographic analysis results of the metamorphic rock sample to be tested, the fluid activity factor in the dual-control calibration model was set. The value; The measured whole-rock boron isotope composition obtained in S3 The calculated Δ and D, and the set The inversion calculation equation is input and solved to output the original boron isotope composition. .

8. A geological survey method for reconstructing the protolith type of metamorphic rocks using boron isotope fractionation characteristics, as described in claim 1, is characterized in that... In S5, the collaborative analysis and formation of comprehensive discrimination indicators are specifically achieved by constructing a "boron isotope-key element" collaborative discrimination system, including: The original boron isotope composition obtained from the inversion calculation is used as the core isotope index; From the obtained key inactive element content data, at least one element ratio that is significant in distinguishing the original rock type is calculated as a key element index. The element ratio is composed of elements that are chemically stable and have strong migration inertness during advanced metamorphism. The key inactive element is at least two of Ti, Nb, Ta, Zr, Hf, and Th. A synergistic correspondence is established between the core isotope index and the key element index. A two-dimensional discriminant diagram is constructed with the core isotope index as the horizontal axis and the key element index as the vertical axis. The comprehensive discriminant index corresponds to the coordinate point in the two-dimensional discriminant diagram.

9. A geological survey method for reconstructing the protolith type of metamorphic rocks using boron isotope fractionation characteristics, as described in claim 8, is characterized in that... In step S6, the step of judging according to the preset dual-index rule specifically includes: In the two-dimensional discrimination diagram, a specific discrimination region is defined for the standard sample data of each type of protolith in the benchmark database. The boundary of the discrimination region is determined by the statistical distribution range of the core isotope index and the key element index of the corresponding type of protolith. If the coordinates of the metamorphic rock sample to be tested fall within the discrimination area of ​​a certain type of protolith, then the protolith of the sample is determined to belong to the corresponding category.

10. A geological survey method for reconstructing the protolith type of metamorphic rocks using boron isotope fractionation characteristics, as described in claim 9, is characterized in that... Determining the protolith type of the metamorphic rock sample to be tested specifically includes: Calculate the Euclidean distance from the coordinates of the metamorphic rock sample to be tested to the center point of each protolith discrimination region in the benchmark database; The metamorphic rock sample to be tested is identified as the protolith type corresponding to the discrimination region with the smallest Euclidean distance.

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