JP3963903B2 - Method for measuring erosion rate of formation - Google Patents

Description

  The present invention relates to a method for measuring an erosion rate of a formation by measuring concentrations of a plurality of long half-life nuclides generated by cosmic ray irradiation.

Measure the concentration of multiple long-lived nuclides such as ¹⁴ C, ¹⁰ Be, and ²⁶ Al generated by cosmic ray irradiation in the vertical direction from the ground surface. Estimating weathering erosion rate or uplift rate of formation is estimated by estimating the time from the start of nuclide generation by the crushing mechanism.

This measurement principle will be described below.
As shown in FIG. 1, when cosmic rays fall on the ground surface, a nuclear reaction occurs between O, Si, which are elements contained in minerals, particularly quartz having high resistance to weathering, and cosmic rays. These nuclear reactions include nuclear disruption (Cosmic-ray Spallation), muon capture (Muon capture), and fast muon induced reactions. As shown in FIG. ¹⁴ C, ¹⁰ Be, ²⁶ Al are produced.

As the exposure time of cosmic rays increases, the amount of nuclides produced by the nuclear reaction increases, and after about 40,000 years for ¹⁴ C, about 5 million years for ²⁶ Al, and about ¹⁰ million years for ¹⁰ Be, It reaches a certain level of radioactivity. FIG. 3 shows the relationship between the exposure time and the amount of radionuclide produced. Based on this relationship, the length of the exposure time at an arbitrary depth near the ground surface (below 10 m.w.e) is estimated. be able to.
Assuming that the cosmic ray intensity does not change with time, the length of cosmic ray exposure time in minerals (here, quartz) at an arbitrary depth under a constant cosmic ray intensity, and ¹⁴ C, ²⁶ Al, ¹⁰ Be The generated radioactivity concentration C is normalized by each saturated radioactivity concentration (radioactivity concentration when exposed to infinite time) Cs, and this figure shows that due to the difference in the length of each half-life. It is generated along the curve shown and will eventually be equal to the saturated radioactivity concentration. (C / Cs = 1). The difference in half-life causes a difference in the time required to reach C / Cs = 1, and this difference is used to estimate the exposure time.

  Cosmic-ray Spallation is prominent in the generation of any nuclide up to 10m.w.e (corresponding to 3-4m in the rock formation) from the ground surface among nuclear reactions of cosmic rays. Yes. This state is shown in FIGS.

Figure 4 shows the relationship between the number of ¹⁴ C atoms produced in quartz (SiO ₂ ) by cosmic ray exposure and the depth in one year, and the mechanism by which nuclides are produced by cosmic ray exposure is nuclear fragmentation, stop muon, It is dominant in the order of the first muon.
Figure 5 shows the relationship between the number of ²⁶ Al atoms generated by cosmic ray exposure in quartz (SiO ₂ ) and the depth in one year, and the mechanism by which nuclides are generated by cosmic ray exposure is nuclear fragmentation, stop muon, It is dominant in the order of the first muon.
Fig. 6 shows the relationship between the number of ¹⁰ Be atoms generated by cosmic ray exposure in quartz (SiO ₂ ) and depth for one year, and the mechanism by which nuclides are generated by cosmic ray exposure is nuclear fragmentation, stop muon, It is dominant in the order of the first muon.

  Especially up to 1m.w.e depth (around 1m near the surface), the nuclear fragmentation is overwhelmingly large compared to the above-mentioned stop muon and first muon mechanisms, and the generation near the surface is large. Since there is no difference, it can be assumed that there is no depth dependence and is almost constant. Therefore, if the exposure time with cosmic rays is sufficiently long compared to the half-life of the produced nuclide, the weathering erosion rate should be slow compared to the half-life of the nuclide and reach radiation equilibrium. Conversely, if the exposure time by cosmic rays is shorter than the half-life of the produced nuclide, the weathering erosion rate is faster than the half-life of the nuclide and the radiation equilibrium is not reached.

Based on these facts, in the present situation, if there is no weathering erosion or uplift, the nuclide concentration in the formation is theoretically calculated based on the elemental composition of quartz from the exposure time by cosmic rays and the concentration (generation rate) of these nuclides. Can be estimated. For example, quartz ¹⁰ Be and ²⁶ Al targeting ratio and the number of ¹⁰ Be atoms 2 dimensionally plotted to estimate the weathering rate of erosion ^{(erosion rate) (10 -6 cm} / y or less), the estimated theoretical curve A technique for obtaining field data by fitting is proposed, and the contents thereof are described in Non-Patent Document 1. An example is shown in FIGS.
In this method, assuming a weathering erosion rates to a suitable size, ¹⁰ Be and ²⁶ analyzes the ratio and the number of ¹⁰ Be atoms Al, 2-dimensionally measured data for these curves keep pulling several theoretical curve And weathering erosion rate is estimated from the nearest curve.

7 (a) is silica (SiO ₂₎ in shows the relationship of cosmic rays number ¹⁰ Be atoms generated per year by exposure and ²⁶ Al / ¹⁰ Be ratio, the upper value is zero erosion The lower value shows the relationship between the exposure time and the erosion rate in a state having a steady erosion rate (10 ⁻¹ to 10 ⁻⁶ cm / y). The further to the right, the lower the erosion rate and the longer the exposure time. FIG. 7 (b) shows the relationship between the number of ²⁶ Al atoms generated in one year by cosmic ray exposure in quartz (SiO ₂ ) and the ²⁶ Al / ¹⁰ Be ratio. Again, the erosion rate decreases and exposure time increases toward the right.

FIG. 8 shows a comparison between the theoretical curve of ¹⁰ Be atoms generated by cosmic ray exposure in quartz (SiO ₂ ) and the ²⁶ Al / ¹⁰ Be ratio and the measured values. It is the exposure time in the case of zero erosion, and the lower value shows the relationship between the exposure time and the erosion rate in a state having a steady erosion rate (10 ⁻¹ to 10 ⁻⁶ cm / y). The actual measurement value is partly out of the theoretical value, and this relationship shows that it is difficult to evaluate the erosion rate.

In this way, when field data is plotted, it is often difficult to judge even when the variation of the data exceeds the theoretical value, and the accuracy is not good. This is due to the long half-life of ¹⁰ Be and ²⁶ Al. Its half-life is relatively long, 1.6 million years at ¹⁰ Be and 716,000 years at ²⁶ Al, and there is only a difference of about twice. For that reason, the difference does not stand out in millions of years. This has the disadvantage that the theoretical curves overlap.

Instead of this method, Phillip et al. Have proposed a method using ³⁶ Cl, but at the same point, Dolomite, another mineral that is not quartz, is used for measurement. When considering the effects of cosmic rays, it includes the elements that must be considered separately in terms of its geometric efficiency, etc., and thus has a drawback that the evaluation becomes complicated. In addition, Dolomite has a drawback that it is weaker to weathering than quartz.

K. Nishiizumi et.al, Earth and Planetary Science Letters, 104 (1991) 440-454

  The present invention has been made in consideration of such circumstances, and focusing on the difference in the half-life of nuclides produced by cosmic ray exposure, the erosion rate of the formation that can accurately measure the exposure time An object is to provide a measurement method.

In order to solve the above problems, the present invention measures the concentration of radionuclides produced in minerals contained in the formation by exposure to cosmic rays, and the exposure time of cosmic rays at any position in the formation from the concentration. wherein estimating the relationship between the growth process of the radionuclide to a method of measuring the exposure time by cosmic rays of the formation of the radionuclide, to determine the concentration of ^{14 C, 10 be, 26 Al} , 14 C To estimate the relationship between the cosmic ray exposure time and the growth process of ¹⁴ C, ²⁶ Al, ¹⁰ Be based on the ratio of C and ²⁶ Al, and the ratio of ¹⁴ C and ¹⁰ Be. It is a characteristic dating method.
^{Since 14} C has a significantly different half-life from ¹⁰ Be and ²⁶ Al, the measurement range of the cosmic ray exposure period extends from a range of thousands of years to a range of millions of years. The accuracy of the elapsed time measurement from is improved.

According to the present invention, the concentration of ¹⁴ C, ¹⁰ Be, ²⁶ Al is measured, and based on the ratio of ¹⁴ C to ²⁶ Al and the ratio of ¹⁴ C to ¹⁰ Be, the cosmic ray exposure time and ¹⁴ C, ²⁶ Al The accuracy of measurement of the elapsed time after the formation of the surface of the formation can be improved by estimating the relationship of the growth process of ¹⁰ Be and measuring the erosion rate of the formation.

Below, this invention is demonstrated based on the embodiment.
FIG. 9 shows a theoretical curve of the saturation concentration of ¹⁰ Be, ²⁶ Al, and ¹⁴ C atoms generated by cosmic ray exposure in quartz (SiO ₂ ) and the exposure time. In quartz, as shown in FIG. 2, but ¹⁴ C in addition to ¹⁰ Be, ²⁶ Al by cosmic rays exposure, are produced by the spallation of oxygen contained in the quartz, ¹⁴ C is ¹⁰ Be and ²⁶ Al The half-life is significantly shorter than that of 5730 years. If there is no change in the intensity of cosmic rays and the formation continues to be exposed to cosmic rays under the same environmental conditions, radiation equilibrium will be reached in about 40000 years under the same conditions.

On the other hand, for ¹⁰ Be and ²⁶ Al, the time required to reach radiation equilibrium is about 5 million years at ²⁶ Al and about 10 million years at ¹⁰ Be. Therefore, in the present invention, the ratio of ¹⁴ C- ²⁶ Al and ¹⁴ C- ¹⁰ Be is measured instead of the conventional ratio of ¹⁰ Be and ²⁶ Al, and the three-dimensional plot is plotted against the exposure time. By drawing, the resolution of the erosion rate measurement of the formation is improved by making the difference in half life stand out.
FIG. 10 shows the relationship between the exposure time and the ¹⁰ Be / ¹⁴ C ratio and ²⁶ Al / ¹⁴ C ratio. With an exposure time of 10 ² to 10 ⁷ years, the ¹⁰ Be / ¹⁴ C ratio changed from 0.4 to 100, and the ²⁶ Al / ¹⁴ C ratio changed from about 2.5 to 300, and the conventional ²⁶ Al / ¹⁰ Be was changed. While the ratio only changed in the range of 2.88 to 6.0, the fluctuation range is increased by about 50 times and the resolution is improved.

By targeting ¹⁴ C, the measurement range of the cosmic ray exposure period, which mainly consists of nuclear spallation, extends from a range of thousands of years to a period of millions of years. The accuracy of the elapsed time measurement is improved.

  INDUSTRIAL APPLICABILITY The present invention can be used as a method for estimating the formation age of a formation and the rate of change associated with formation erosion.

It is a figure which shows a mode that the quartz in the formation raise | generates a nuclear reaction by exposure of a cosmic ray. It is a figure which shows a mode that oxygen and silicon which comprise quartz generate | occur | produce a nuclear reaction, and produce | generate a radionuclide. It is a figure which shows the theoretical curve of the exposure time of a cosmic ray, and the amount of radionuclides produced | generated. By exposure to cosmic rays in quartz (SiO ₂₎ is a diagram showing the relationship between ¹⁴ C atoms and depth generated per year. By exposure to cosmic rays in quartz (SiO ₂₎ is a diagram showing a relationship ²⁶ Al atoms and depth generated per year. Quartz is a diagram showing a (SiO ₂₎ ¹⁰ Be atoms generated per year by exposure to cosmic rays and depth relationship in. (a) is a diagram showing the relationship between quartz (SiO ₂₎ number ¹⁰ Be atoms generated per year by cosmic ray exposure in a ²⁶ Al / ¹⁰ Be ratio, (b) are quartz (SiO ₂ ) is a diagram showing the relationship between ²⁶ Al atoms generated per year by exposure to cosmic rays and ²⁶ Al / ¹⁰ be ratio in. Quartz is a diagram showing a comparison of the (SiO ₂₎ actual measurement value and the theoretical curve of the number of ¹⁰ Be atoms produced by cosmic ray exposure and ²⁶ Al / ¹⁰ Be ratio in. Quartz is a diagram showing a theoretical curve of ^{_{(SiO 2) 10 Be, 26}} Al, 14 C atoms, saturated concentration exposure time generated by cosmic ray exposure in. Exposure time ¹⁰ Be / ¹⁴ C ratio, which is a diagram showing the relationship between the ²⁶ Al / ¹⁴ C ratios.

Claims (1)

Measure the concentration of radionuclides produced in minerals contained in the stratum by exposure to cosmic rays, and estimate the relationship between the cosmic ray exposure time at any position in the strata and the growth process of the radionuclides from the concentration A method for measuring the erosion rate of a formation, wherein the concentration of ¹⁴ C, ¹⁰ Be, ²⁶ Al among the radionuclides is measured, the ratio between ¹⁴ C and ²⁶ Al, and the ratio between ¹⁴ C and ¹⁰ Be. A method for measuring the erosion rate of a formation, characterized in that the cosmic ray exposure time is estimated by estimating the relationship between the cosmic ray exposure time and the growth process of ¹⁴ C, ²⁶ Al, ¹⁰ Be based on the above.

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