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CN-121983161-A - For U-Pb isotope systems238U/235U ratio correction method and application thereof

CN121983161ACN 121983161 ACN121983161 ACN 121983161ACN-121983161-A

Abstract

The invention belongs to the application field of isotope geology theory, in particular to a correction method of 238 U/ 235 U ratio for a U-Pb isotope system and application thereof, wherein the method comprises the steps of arranging 238 U、 235 U atomic number calculation formula according to radioactive decay law; according to a 238 U、 235 U atomic number calculation formula, calculating the ratio of initial abundance N i and 238 U/ 235 U of parent isotope 238 U、 235 U at different moments, analyzing the relative deviation of the ratio of 238 U/ 235 U to 137.88, establishing 238 U/ 235 U calculation formula for the middle stage of the three stages of U-Pb and evaluating, comparing the 238 U/ 235 U calculation value with 207 Pb age error calculated by the original parameter 137.88, and determining qualitative and quantitative application range and conditions. The method can effectively improve the reliability of age calculation and is applied to U-Pb isotope age calculation and isotope evolution research.

Inventors

  • HAN JUN
  • GUO CHUNYING
  • NIE JIANGTAO

Assignees

  • 核工业北京地质研究院

Dates

Publication Date
20260505
Application Date
20251230

Claims (8)

  1. 1. A method for correcting 238 U/ 235 U ratio for a U-Pb isotope system, the method comprising: step (1), sorting 238 U、 235 U atomic number calculation formulas according to radioactive decay law; Calculating the ratio of initial abundance N i and 238 U/ 235 U of the parent isotope 238 U、 235 U at different moments according to a 238 U、 235 U atomic number calculation formula, and analyzing the relative deviation of the calculated 238 U/ 235 U ratio relative to the common ratio 137.88; establishing 238 U/ 235 U arithmetic expression for the middle stage of the U-Pb three stages and evaluating; And (4) comparing 238 U/ 235 U calculated values with 207 Pb age errors calculated by the original parameters 137.88, and determining qualitative and quantitative application ranges and conditions.
  2. 2. The method for correcting the 238 U/ 235 U ratio for a U-Pb isotope system according to claim 1, wherein the 238 U、 235 U atomic number calculation formula in step (1) is: N=N 0 ×e -λt0 ..................(9) N i ＝N×e λti ..................(10) N i-i+1 ＝N×e λ(ti-ti+1) .................(11) Where N 0 is the initial atomic number of the radioactive parent at t0 (at earth formation), N is the remaining atomic number of the parent at t, N i is the initial atomic number of the radioactive parent at ti, λ is the decay constant, and e -λt0 、e λti is the decay factor from t0 to ti, respectively.
  3. 3. The method for correcting the 238 U/ 235 U ratio for a U-Pb isotope system of claim 1 wherein step (2) includes: the number of element atoms multiplied by the Avgalileo constant corresponds to a relative abundance value, and the uranium isotopes have the same atomic mass due to the same multiplier, and the relative abundance is adopted to represent the number of atoms when the proportional relationship is discussed; The abundance of 238 U、 235 U nowadays is 99.275%, 0.72%, decay constant λ5 of 235 U, decay constant λ8 of 0.98485 ×10 - 9 a -1 , 238 U, and decay constant of 0.155125 ×10 -9 a -1 respectively; The 238 U、 235 U initial abundance and 238 U/ 235 U ratio at different times are calculated by using the formula (10), and the relative deviation of the 238 U/ 235 U ratio relative to the common ratio 137.88 is calculated.
  4. 4. The method for correcting 238 U/ 235 U ratio for U-Pb isotope system according to claim 1, wherein the step (3) comprises the steps of adopting 238 U/ 235 U ratio k 1 、k 2 of the middle stage to replace the common ratio 137.88, correcting the three-stage middle stage definite year evolution formula to obtain a corrected three-stage middle stage definite year evolution formula; In the U-Pb intermediate stage mode, k 1 、k 2 is calculated according to a calculation formula of 238 U、 235 U initial abundance, and 238 U/ 235 U ratio of the intermediate stage is obtained.
  5. 5. The method for correcting 238 U/ 235 U ratio in U-Pb isotope system according to claim 4, wherein in the step (3), the corrected three-stage intermediate-stage definite year evolution formula is: β 1 ＝β 0 +μ 0 /k 1 (e λ5t0 -e λ5t1 ).....................(12) β 2 ＝β 1 +μ 1 /k 2 (e λ5t1 -e λ5t2 ).....................(13) Where k 1 represents the evolution of uranium parent from the earth formed t 0 to the time period 238 U/ 235 U,k 2 of t 1 and from t 1 to the time period 238 U/ 235 U of t 2 ; 238 U、 235 The calculation formula of the initial abundance of U is as follows: N(8) t1 0 ＝N×e λ8(t0-t1) .....................(14) N(5) t1 0 ＝N×e λ5(t0-t1) .....................(15) k 1 ＝N(8) t1 0 /N(5) t1 0 .....................(16) N(8) t2 1 ＝N×e λ8(t1-t2) .....................(17) N(5) t2 1 ＝N×e λ5(t1-t2) .....................(18) k 2 ＝N(8) t2 1 /N(5) t2 1 .....................(19) Wherein N (8) t1 0 、N(5) t1 0 represents the 238 U、 235 U initial abundance of t 0 evolving to t 1 period, respectively, N (8) t2 1 、N(5) t2 1 represents the 238 U、 235 U initial abundance of t 1 evolving to t 2 period, respectively; 238 U/ 235 U of the first stage (t 0 ～t 1 ) and the second stage (t 1 ～t 2 ) represented by k 1 、k 2 is calculated by the formulas (16) and (19).
  6. 6. The method of claim 1, wherein the step (4) comprises analyzing 207 Pb age error caused by 238 U/ 235 U ratio reduction in the middle of the three-stage system by comparing 207 Pb age values obtained by the original parameters 137.88 for 207 Pb ages at different ages, quantitatively giving out the age range applicable to the 238 U/ 235 U formula established in the step (3), and qualitatively giving out the applicable U-Pb isotope pattern.
  7. 7. The method for correcting the 238 U/ 235 U ratio for a U-Pb isotope system of claim 6 wherein the qualitative, quantitative use range and conditions of the 238 U/ 235 U formula determined in step (4) are: ① 238 U/ 235 The U ratio has an influence on 207 Pb ages, and the 238 U/ 235 U formula established in the step (3) is suitable for the middle stage of a U-Pb multi-stage system and is not suitable for single-stage age calculation; ② When 238 U/ 235 U deviates by more than 20% from 137.88, no less than experimental and data processing errors are generated for the calculated 207 Pb age; ③ When 238 U/ 235 U deviates more than 10% from 137.88, only a non-negligible error is introduced for the 207 Pb age of less than 30 hundred million years; ④ When 238 U/ 235 U deviates more than 5% from 137.88, only a non-negligible error is introduced for the 207 Pb age of less than 10 hundred million years; ⑤②～④ Under the condition, after 238 U/ 235 U ratio is calculated by adopting 238 U/ 235 U formula established in the step (3), 207 Pb age is calculated, so that extra errors caused by 238 U/ 235 U unreasonable are avoided; ⑥ The 238 U/ 235 U formula established in the step (3) is suitable for calculating the age in the U-Pb isotope H-H mode.
  8. 8. The application of the 238 U/ 235 U ratio for the U-Pb isotope system is characterized in that the application comprises the steps of obtaining a 238 U、 235 U value in a geological history period by utilizing a radioactive decay rule, comparing initial 238 U、 235 U values of geologic bodies with known diagenetic and mineous ages, providing evidence of geologic body evolution by traceable object source uranium isotope composition change degree and mechanism, and simultaneously judging whether an object source is a supply source of uranium or not by comparing uranium isotope abundance calculated according to multi-stage U-Pb isotopes, and providing isotope evidence for prospecting.

Description

Method for correcting 238U/235 U ratio of U-Pb isotope system and application thereof Technical Field The invention belongs to the field of application of isotope geology theory, and particularly relates to a method for correcting 238U/235 U ratio of a U-Pb isotope system and application thereof. Background In the evolution and definite year of uranium-lead isotope system, the generally adopted 238U/235 U ratio is 137.88, and the ratio is 238 U and 235 U which serve as radioactive precursors, and the homogenization is achieved from the earth formation, and the uranium isotope ratio which reaches isotope balance nowadays is evolved. Because uranium is an incompatible element and active in chemical property, the uranium mainly exists in the nature in the forms of crystalline uranium ore, asphaltic uranium ore and uranyl complex, isotope system opening is easy to occur under the low-temperature condition, and uranium and lead isotopes are in a multi-stage or continuous open system. The 238U/235 U ratio of the radioactive parent U isotope at different evolution stages is obviously different when age calculation is performed, and it is impossible to consider all in evolving to the present mode. This ratio is also an important clue for judging the relationship between uranium isotope evolution and time. In general, when calculating the age in U-Pb multi-stage, a classical U-Pb three-stage age-determining formula is adopted, and the formula is also a basic formula for calculating the age by uranium lead isochrone, lead isochrone, coincidence line and other methods, and the formula is as follows: 206Pb/204Pb＝206Pb/204Pb0+μ0(eλ8t0-eλ8t1)+μ1(eλ8t1-eλ8t2)+μ2(eλ8t2-1)…(1) 207Pb/204Pb＝207Pb/204Pb0+(μ0/137.88)×(eλ5t0-eλ5t1)+(μ1/137.88)×(eλ5t1-eλ5t2)+(μ2/137.88)×(eλ5t2-1)…(2) 206Pb/204Pb0、207Pb/204Pb0 - 206Pb/204Pb、207Pb/204 Pb atomic ratio at earth formation (t 0), mu 0、μ1、μ2 - 238U/204 Pb atomic ratio corresponding to t 0、t1、t2 and following evolution under closed conditions, mu 2 being 238U/204 Pb present measurement, lambda 8、λ5 - 238U、235 U decay constant, e λ8t0、eλ8t1、eλ8t2 being 238 U decay factor from t 0、t1、t2, e λ5t0、eλ5t1、eλ5t2 being 235 U decay factor from t 0、t1、t2, mu 0/137.88、μ1/137.88、μ2/137.88 representing 235U/204 Pb values from t 0、t1、t2 to evolution under closed conditions. The above-described U-Pb three-stage formula is obviously equivalently applicable to three evolutions, and each stage can be represented by the following formula: α1＝α0+μ0(eλ8t0-eλ8t1).....................(3) α2＝α1+μ1(eλ8t1-eλ8t2).....................(4) α3＝α＝α2+μ2(eλ8t2-1).....................(5) β1＝β0+μ0/137.88(eλ5t0-eλ5t1).....................(6) β2＝β1+μ1/137.88(eλ5t1-eλ5t2)...............(7) β3＝β＝β2+μ2/137.88(eλ5t2-1)..............(8) (3) In the formula (5), alpha=alpha 3、α0、α1、α2 corresponds to the current (current measured value) and 206Pb/204 Pb value at the moment t 0、t1、t2 respectively, wherein t 0 is the earth formation time, and other symbols have the same meaning as those in the formulas (1) and (2). (6) In the formulas (8), beta=beta 3、β0、β1、β2 corresponds to the current value (current measured value) and the 207Pb/204 Pb value at the moment t 0、t1、t2 respectively, and mu 0/137.88、μ1/137.88、μ2/137.88 respectively represents the 235U/204 Pb value which is evolved to the present under the closed condition from the moment t 0、t1、t2. According to decay law, mu 0/137.88 of formula (6) should be from t 0 to t 1 formed by the earth, at 235U/204 Pb value formed by the evolution of an isotope closed system. Similarly, mu 1/137.88 represents 235U/204 Pb values from t 1 to t 2 evolved in an isotope closed system, and mu 2/137.88 represents 235U/204 Pb values from t 2 to present day evolved in an isotope closed system, inherited from the original uranium isotope composition. The general 137.88 multiple relationship of 235 U to 238 U in the above formula has the meaning that isotope homogenization is achieved in a short time of earth formation, and the uranium isotopes consist of 238U、235U、234 U in original abundance, and when the uranium isotopes respectively evolve until the isotope equilibrium is achieved, 235U/238 U value is 1/137.88. Since 235 U decays often about 6.35 times the decay constant of a few 238 U (the parent decay speed of a 5/λ8≈6.35),238 U is significantly less than 235 U, the original uranium isotope composition must be gradually increased from 238U/235 U <137.88 up to the present value. For the reasons described above, only the effects due to decay of the parent radioactive element are considered. The 235U/238 U value of the multi-stage evolution system cannot be simply scaled with a constant 137.88, and the ratio of the different evolution stages under an open system is not the same. Disclosure of Invention The invention aims to provide a method for correcting 238U/235 U ratio of a U-Pb isotope system and application thereof, wherein the method can effectively improve the reliability of age calculation and is a