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Method for determining U-Pb age of zircon sample

A zircon and sample technology, applied in the field of micro-area isotope composition analysis, can solve the problems of increased denudation depth, low abundance sensitivity, and poor lateral spatial resolution

Inactive Publication Date: 2017-06-30
INST OF GEOLOGY & GEOPHYSICS CHINESE ACAD OF SCI
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  • Abstract
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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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  • Method for determining U-Pb age of zircon sample
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  • Method for determining U-Pb age of zircon sample

Examples

Experimental program
Comparison scheme
Effect test

Embodiment 1

[0054] Example 1 Comparative experiment on the ability of ion counters and Faraday cups to measure small signals (the instrument used in this experiment is Neptune Plus MC-ICP-MS, manufacturer: Thermo Fisher Scientific, Germany)

[0055] In this experiment, the conditions of laser ablation zircon shown in Table 1 were used to contain different concentrations of zircon (see figure 1 ) lead solution for experiment, first use ion counter IC3 and IC2 to measure simultaneously 206 Pb and 207 See figure 1 , indicated by open circles, measured at IC3 206 The intensity of the Pb ion current is marked next to the circle, and the unit is cps; then it is measured simultaneously by Faraday cup L1 and ion counter IC1 respectively 206 Pb and 207 Pb (mixed reception mode), measurement results see figure 1 , represented by a solid circle, measured at L1 206Pb ion current intensity is marked next to the circle in mV. The experimental results prove that when measuring small signals (less...

Embodiment 2

[0056] Example 2 Comparative experiment of multi-ion counter receiving mode and ion counter-Faraday cup mixed receiving mode

[0057] In this experiment, under the same laser ablation zircon conditions as in Example 1, the ion counter IC3 and the Faraday cup L4 were used to simultaneously receive 206 Pb and 238 U ion current, the resulting signal changes in figure 2 shown in a. It can be seen from this figure that the Faraday cup L4 measured 238 When the U signal reaches its peak value, it lags behind that measured by the ion counter IC3. 206 Pb signal, this hysteresis effect makes the measured 206 Pb and 238 The variation of U isotope ratio increases, which seriously affects the accuracy of U-Pb dating results. Then use ion counters IC3 and IC7 to receive simultaneously 206 Pb and 238 U ion current, the resulting signal changes in figure 2 b shows that it can be seen from the figure that the measured 206 Pb and 238 U signal changes are fully synchronized, meaning...

Embodiment 3

[0059] Based on the results of the above experiments 1 and 2, the following technical solutions are designed to solve the main problems in the current laser ablation in situ micro-area U-Pb dating technology.

[0060] Sample preparation: In this experiment, seven standard zircons were used: Phalaborwa, 91500, GJ-1, Temora, Plěsovice, Qinghu and SK10-2. These 7 zircon particles were embedded in a 2.54 cm (1 inch) epoxy resin sample target. The unknown sample was placed in the same or another epoxy resin sample target. After polishing, a cathodoluminescence image was taken with a scanning electron microscope to identify Core, mantle and edge structure relationships of zircon mineral grains.

[0061] Instrument composition: Mass spectrometer used for determination: Neptune Plus multi-receiver inductively coupled plasma mass spectrometer (Neptune Plus MC-ICP-MS, manufacturer: Thermo Fisher Scientific, Germany), equipped with 7 ion counters IC4, IC5, IC3A, IC2, IC1B1, IC6 and IC7,...

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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 ...

Claims

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Application Information

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Patent Type & Authority Applications(China)
IPC IPC(8): G01N27/62
CPCG01N27/62
Inventor 谢烈文杨进辉杨岳衡刘景波黄超
Owner INST OF GEOLOGY & GEOPHYSICS CHINESE ACAD OF SCI
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