A method for determining the uranium-lead age of zircon samples

Abstract

The invention discloses a method for determining the U-Pb age of a zircon sample. The method comprises the steps as follows: the zircon sample and a standard substance are made into a sample target; solid sol is obtained through laser ablation; the solid sol is loaded in a multi-receiving inductive coupled plasma mass spectrograph ion source (plasma) for ionization, and primary ions are obtained; the primary ions pass through an electric field and a magnetic field of a mass spectrograph to realize dual-focusing of energy and direction and arrive at an ion detection system; the original U-Pb ratio of the primary ions of the zircon sample and the zircon standard substance is detected and calculated; the original U-Pb ratio of the zircon sample is corrected by using the standard U-Pb ratio of zircon, and the corrected U-Pb ratio of the zircon sample is obtained; the U-Pb age of the zircon sample is calculated by using the corrected U-Pb ratio. The method has high precision and high sensitivity, has the low sample content requirement, has small ablation depth, and can be used for spatial high-resolution U-Pb age determination of zircon with the ablation diameter smaller than 10 mum.

Description

technical field [0001] The invention relates to a micro-area isotope composition analysis method, in particular to a method for determining the uranium-lead age of a zircon sample. Background technique [0002] Zircon is an important accessory mineral in natural rocks. Due to its high U and Pb content, it has become the most frequently studied object in U-Pb isotope geochronology, and has gradually formed an extremely broad application prospect. Branch discipline - zirconology (zirconology). In particular, the combination of zircon U-Pb age with its trace elements and isotopes such as Hf and O provides important geochemical parameters for exploring the time scale and process of geological processes. [0003] According to the properties of the measured samples, at present, three methods are mainly used in zircon U-Pb isotope geochronology: trace zircon method, single particle zircon method and micro-area analysis. However, in terms of the spatial resolution of the analysis ...

AI Technical Summary

Problems solved by technology

[0007] In the current zircon LA-ICP-MS U-Pb dating, the instruments used are mainly various types of quadrupole (or magnetic) single-reception mass spectrometers, which have the characteristics of fast scanning, but in isotope analysis There are obvious limitations in terms of abundance, such as low abundance sensitivity and non-flat peaks, etc., so the analysis accuracy is usually 3-5%, and the spatial resolution is not high, usually 30-80 microns (diameter of denudation pits). From the low-quality end to the high-quality end or reverse sequential scanning technology, the total measurement time is longer, and the usual ablation depth is about 20 microns

In general, there are problems with laser ablation quadrupole (or magnetic) single-reception plasma mass spectrometry zircon U-Pb dating technology: (1) Due to the use of sequential scanning technology, the reception efficiency is low, such as U-Pb dating. Received in Pb dating 202 Hg, 204 (Hg+Pb), 206 Pb, 207 Pb, 208 Pb, 232 Th and 238 U ion flow, compared with multi-receiver simultaneous reception technology, theoretically needs to consume 7 times the time, and the corresponding erosion depth is increased by 7 times

This results in low vertical resolution, typically around 20 microns

(2) Due to the low sensitivity of the instrument, more ablation materials are required under the same signal intensity conditions, that is, a larger ablation area is required under the same ablation conditions, resulting in poor lateral spatial resolution, usually the ablation diameter 30-80 microns, for individual high-content samples can reach about 20 microns

However, the current zircon U-Pb dating technology using laser ablation multiple receiver plasma mass spectrometry (LA-MC-ICPMS) has the following problems: (1) the use of ion counters to receive 202 Hg, 204 (Hg+Pb), 206 Pb, 207 Pb and 208 Pb, received in a Faraday cup 238 The mixed receiving mode of U, due to the poor sensitivity of the Faraday cup, has extremely poor measurement accuracy for signals below 5mV, making it unsuitable for U-Pb dating analysis with ablation diameters less than 10 microns; in fact, many ring-structured zircon samples The annulus widths are all less than 10 microns

Moreover, the zircon grains of some samples themselves are less than 10 microns, such as zircon grains in ultrabasic rocks and meteorite samples; (2) the U-Pb content in natural zircon samples varies greatly, ranging from high to low. For low U content samples with signal strength less than 5mV, the age accuracy of U-Pb dating is poor

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