A laser in situ uranium-lead isotope dating method for ancient marine carbonate rocks

Abstract

The invention discloses a laser in-situ uranium-lead isotope dating method for ancient marine carbonate rocks, comprising the following steps: 1) Sample target production: carbonate rock samples and carbonate rock dating standard samples Make sample targets respectively; 2) select the area to be measured; 3) single-point ablation analysis: use a laser ablation system to perform single-point laser ablation on the area to be measured selected in step 2) to obtain multiple sampling points, and multiple sampling points Load the point into the multi-receiver inductively coupled plasma mass spectrometer for isotope analysis and export data; 4) Data processing and age mapping: The data obtained in step 3) is processed and age mapped to obtain the age of the carbonate rock sample. The laser in-situ uranium-lead isotope dating method of the invention has the advantages of simple sample preparation process, small sample consumption, high spatial resolution, fast analysis speed and high success rate.

Description

technical field [0001] The invention relates to the technical field of isotope geological dating. More specifically, it relates to a laser in situ uranium-lead isotope dating method for ancient marine carbonate rocks. Background technique [0002] Dating is an eternal theme of geology, and the uranium-lead isotope dating techniques of carbonate rock uranium series method and solution method are also mature technologies. [0003] Among the existing methods, the carbonate rock U-Th solution method can be used to determine the absolute age of carbonate rock samples from 0 to 500,000 years old. Among them, Quaternary corals and Quaternary carbonate sediments without diagenetic alteration Objects are the most suitable research objects. For example, Wang Zhaorong et al. have determined the age of stalagmites in Guizhou Rhinoceros Cave to be 85±5ka (Wang Zhaorong et al., Research on the age of cave sediments (stalagmites) by high-precision thermal ionization mass spectrometer (TIM...

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Problems solved by technology

However, generally ancient marine carbonate minerals have obvious low U characteristics, generally between 0.05-0.5ppm (usually 2-4 orders of magnitude lower than zircon, that is, its measurement signal is only 1 / 100-1 / 10 of zircon. 10000), so the detection is very difficult, mainly in the following aspects: first, carbonate rock minerals have obvious low U characteristics, and it is easy to bring in external pollution during the raking process of conventional samples, which will affect the age results; second , how to reduce the transmission loss in the process of loading the laser ablation products into the mass spectrometer, and input the ablation products into the mass spectrometer as much as possible; Third, compared with high U content minerals such as zircon, calcite or dolomite usually has ordinary Pb interference, so it is necessary to select the area to be measured with a change in the U / Pb ratio for measurement and analysis, so that the final data can be fitted to an isochrone to calculate the age; fourth, the predecessors have used sector field single reception ICP-MS (such as Elements 2, Elements XR, or Attom), or multi-receiving MC-ICPMS (such as Nu Plasma or Neptune) for carbonate mineral U-Pb isotope determination, there are a few successful examples, but the success rate is low, mainly due to the sector field The sensitivity of single-reception ICP-MS is not as high as MC-ICPMS, and the measurement efficiency is low because of the peak-hopping method

In addition most MC-ICPMS although realize 238 U- 208 Pb- 207 Pb- 206 Static measurement of Pb, but only 208 Pb- 207 Pb- 206 Pb was measured on an ion counter, 238 U tends to be at 10 11 Ω is measured on a general-purpose Faraday cup, usually when U238 U is difficult to measure precisely

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