CN106124606B - A kind of 10 μm of monazite small beam spot LA-Q-ICP-MS U-Th-Pb age dating method - Google Patents

Abstract

The invention discloses a kind of 10 μm of monazite small beam spot LA-Q-ICP-MS U-Th-Pb age dating methods, being first introduced directly into the standard solution sample that concentration is 1ppb is made to level four bars icp ms by optimizing instrument parameter²⁰⁵The response intensity of Tl is greater than 200000cps/ppb, and relative standard deviation is 1%~2%, and double charge is less than 1.5%, and oxide yield is less than 1%；It recycles the mode of laser ablation to introduce 610 solid etalon sample of man-made glass NIST to level four bars icp ms, matches the carrier gas of laser and the carrier gas of level four bars icp ms, make²³⁸The counting of U reaches highest；The mode of laser ablation is recycled to introduce monazite standard sample 44069 to level four bars icp ms, the laser energy density used is 3J/cm², pulse frequency 4Hz, with the 10 μm small laser beam spot measurement monazite U-Th-Pb age.Present invention greatly enhances the spatial resolutions of monazite LA-Q-ICP-MS U-Th-Pb age dating, meet monazite testing requirement.

Description

A kind of 10 μm of monazite small beam spot LA-Q-ICP-MS U-Th-Pb age dating method

Technical field

The invention belongs to geological test technical field more particularly to beam spot LA-Q-ICP-MS U- 10 μm of a kind of monazite small Th-Pb age dating method.

Background technique

Monazite ((LREE, Th) PO₄) be lree enrichment mineral, be widely present in sedimentary rock, metamorphic rock and middle acid Property igneous rock in, while being also relatively conventional hydrothermal solution accessory mineral.Monazite Th, U content usually with higher, low is common Pb content and higher Pb closure temperature (~750 DEG C) are a kind of common geological epoch meters.Especially interestingly, solely It is more sensitive to the change of environment in geologic process to occupy stone, easily record hydrothermal activity information, ingredient domain not only records The generation epoch of different geologic events, and can clearly reflect geologic process and hydrothermal activity history, U-Th- Pb the study of isotope chronology has important geological Significance.

But just because of to the movable sensibility of fluid, monazite is subject to hydrothermal alteration, forms complicated ingredient point Area.In carrying out micro-zone in situ analysis test process, it is desirable that analysis beam spot is the smaller the better.Electron probe chemical method is common one Kind monazite micro-zone in situ U-Th-Pb dating methods, analysis beam spot diameter, are 1 μm or so, have very high spatial resolution, But the detection limit for height of electron probe, it is generally difficult to measure young or in the lower monazite sample of U, Th content Pb and contains Amount, can not provide the U-Th-Pb age.Laser ablation level four bars inductively coupled plasma mass spectrometry also belongs to micro-zone in situ point Analysis, and detection limit is low, for monazite LA-Q-ICP-MS U-Th-Pb determine the common laser ablation beam spot diameter, in year for 32 μm or 24 μm, guarantee the high sensitivity of Q-ICP-MS with enough sample volumes, and then obtains stationary signal to measure the U- of monazite The Th-Pb age.But monazite particle is usually smaller (~50 μm), and has complicated ingredient subregion, 32 μm or 24 μm of larger beam Spot can only generally obtain the U-Th-Pb age of mixing, and not only data are difficult to explain, but also have no geological Significance, or even cause mistake Geologic interpretation accidentally.When laser ablation beam spot diameter, is reduced into 10 μm, it is able to satisfy in test process to laser beam spot size Demand, but it is inadequate to will lead to measurement sensitivity, and detection signal stabilization is deteriorated, meanwhile, under the conditions of small beam spot, with erosion pit Intensification, detection signal can sharply decline, cause element fractionating degree increase severely, it is difficult to the Accurate Determining monazite U-Th-Pb age.

Summary of the invention

The purpose of the present invention is to provide a kind of 10 μm of the monazite small age dating sides beam spot LA-Q-ICP-MS U-Th-Pb Method, it is intended to it is usually smaller and have complicated ingredient subregion to solve monazite particle, current 32 μm can only be obtained compared with large beam spot it is mixed At the U-Th-Pb age of conjunction, data are inaccurate and have no geological Significance, or even cause mistake geologic interpretation, and 10 μm of small beam spots It is inadequate to will lead to measurement sensitivity, detection signal stabilization is deteriorated, meanwhile, with the intensification of erosion pit, detecting signal can be sharply Decline causes element fractionating degree to increase severely, it is difficult to the problem of the Accurate Determining monazite U-Th-Pb age.

The invention is realized in this way a kind of 10 μm of monazite small age dating side beam spot LA-Q-ICP-MS U-Th-Pb Method, the 10 μm small beam spot LA-Q-ICP-MS U-Th-Pb age dating method of the monazite first introduce 1ppb standard solution sample To level four bars icp ms, optimizes mass spectrometer parameters, make²⁰⁵The response intensity of Tl is greater than 200000cps/ Ppm, relative standard deviation are 1%~2%, and double charge is less than 1.5%, and oxide yield is less than 1%；It is re-introduced into man-made glass 610 solid etalon sample of NIST matches level four bars inductive coupling to laser ablation level four bars icp ms The carrier gas Ar gas of plasma mass spectrograph and the carrier gas He gas of laser；It is to degrade object with monazite standard sample 44069, uses Laser ablation energy density 3J/cm², laser pulse frequency 4Hz, with 10 μm small beam spot measurement monazite the U-Th-Pb age.

Further, the 10 μm small beam spot LA-Q-ICP-MS U-Th-Pb age dating method of the monazite includes following step It is rapid:

Step 1, optimization level four bars icp ms parameter.Due to temperature in instrument local environment, wet The variation of degree, under causing identical tuner parameters to be arranged, the indexs such as element response intensity, double charge and oxide yield difference is very Greatly, it needs by adjusting quarter bend position, sampling depth, extraction lens voltage, Omega lens voltage, RF power, carrier gas flux etc. Parameter makes in 1ppb concentration standard solution²⁰⁵The response intensity (i.e. sensitivity) of Tl is greater than 200000cps, relative standard deviation It (RSD) is 1%~2%, double charge (70/140) is less than 1.5%, and oxide yield (156/140) is less than 1%.Specific tuning is each The step of item parameter, is as follows: preparing 1ppb standard solution, and is introduced into level four bars icp ms, monitors²⁰⁵The counting of Tl and RSD, and¹⁵⁶CeO⁺/¹⁴⁰Ce and⁷⁰Ce⁺⁺/¹⁴⁰Ce⁺Ratio tunes each instrument parameter item by item, repeatedly；Usually Sampling depth is smaller,²⁰⁵The counting of Tl is higher, and RSD is smaller, but double charge and oxide yield can also increase；With carrier gas flux Raising,²⁰⁵The counting of Tl can be increased first to be declined again；Collision cell exit potential on oxide yield substantially without influence, but to double electricity Lotus yield impact is very big；The influence of Omega lens voltage centering mass number is very big, right²⁰⁵The counting of Tl slightly influences；Omega is inclined The inverse change of light weight mass number can be caused by turning voltage；Plate Bias voltage is affected to sensitivity；There are also some parameters Variation can cause the irregular variation of sensitivity, double charge and oxide yield, can only adjust repeatedly to reach level four bars electricity Feel the optimal detection state of couple plasma mass spectrometer.

Step 2, the carrier gas (Ar gas) for matching laser carrier gas (He gas) and level four bars icp ms, make 610 standard sample of man-made glass NIST degrades parameter in fixed laser and (degrades 24 μm of beam spot, energy density 6J/cm², pulse frequency Rate 7Hz) under,²³⁸U, which is counted, is greater than 450000cps, and relative standard deviation (RSD) is less than 3%.Specific matching process is as follows: with people Making 610 standard sample of glass NIST is test object, utilizes laser ablation sample introduction to level four bars inductivity coupled plasma mass spectrometry Instrument, fixed laser degrade parameter (24 μm, 6J/cm², 7Hz), guarantee that sample volume is the same, level four bars inductively coupled plasma constitution He flow is separately fixed at 700,750,800 and 850mL/ referring to the parameter after optimization in step 1 by the parameter setting of spectrometer Under the magnitude of min, change Ar flow (variation range: 0.65~0.95L/min), record²³⁸U is counted and RSD is obtained by comparing It is flux matched to optimal He gas and Ar air-flow.

Step 3, the parameter for optimizing laser.Degraded beam spot (10 μm) by fixed laser, transformation laser energy density and The mode of pulse frequency explores the laser ablation for being suitble to the 10 μm small beam spot LA-Q-ICP-MS U-Th-Pb age dating of monazite Parameter makes monazite²³⁸The sensitivity of U instantaneous signal is greater than 300000cps, and²⁰⁶Pb-²³⁸The element fractionation index of U is less than 3%.Specific Optimization Steps are as follows: being test object with monazite standard sample 44069, utilize laser ablation sample introduction to level four bars The parameter setting of icp ms, level four bars icp ms optimizes referring in step 1 Carrier gas (Ar gas) flow set reference of parameter afterwards, laser carrier gas (He gas) and level four bars icp ms Optimum Matching in step 2 first fixes beam spot diameter, (10 μm) and laser ablation energy density (2J/cm²), conversion pulse frequency (1 ~7Hz), in such a way that single-point degrades, the signal of monazite standard sample 44069 under the conditions of recording every group；Then it fixes Beam spot diameter, (10 μm) and laser ablation energy (3J/cm²), conversion pulse frequency (1~7Hz), in such a way that single-point degrades, The signal of monazite standard sample 44069 under the conditions of recording every group；Finally the signal recorded is handled, is sifted out solitary Stone²³⁸The maximum sensitivity of U instantaneous signal calculates monazite under the conditions of each group²⁰⁶Pb-²³⁸U element fractionation index and²⁰⁶Pb /²³⁸The RSD of U value compares to obtain the optimal laser stripping for determining year suitable for beam spot LA-Q-ICP-MS U-Th-Pb 10 μm of monazite small Lose parameter.

Under step 4, level four bars icp ms and laser ablation parameter after optimization, 10 μm are utilized Small beam spot measures the monazite U-Th-Pb age, makes²⁰⁸Pb-²³²Th and²⁰⁶Pb-²³⁸U element fractionation index less than 3%,²⁰⁸Pb /²³²Th and²⁰⁶Pb/²³⁸The single-spot testing precision of U isotopic ratio is respectively less than 3%,²⁰⁶Pb-²³⁸U and²⁰⁸Pb-²³²Th weighted average Age is consistent in error range, and within the scope of recommendation.Specific step is as follows: being to survey with monazite standard sample 44069 Object is tried, laser ablation sample introduction to level four bars icp ms, level four bars inductively coupled plasma body are utilized Mass spectrometric parameter setting is referring to the parameter after optimization in step 1, laser carrier gas (He gas) and level four bars inductively coupled plasma The mass spectrometric carrier gas of body (Ar gas) flow set is referring to Optimum Matching in step 2, and the parameter setting of laser is referring to excellent in step 3 Laser ablation parameter after change, test process are all made of single-point and degrade mode, and each measuring point bulk analysis time is 60s, wherein carrying on the back Scape signal 10s, sample signal 40s purge signal 10s.Data acquisition, which uses, jumps peak mode,²⁰⁴Pb、²⁰⁶Pb、²⁰⁷Pb、²⁰⁸Pb、²³⁸The detection residence time of U is 20ms,²³²The residence time of Th is 10ms.5 unknown sample points of every measurement, alternately measurement 2 Secondary standard sample.The school of U-Th-Pb isotopic fractionation and instrumental shift is carried out as external standard using monazite standard specimen 44069 Positive calculating, correction, which calculates, uses software Glitter 4.0, does not carry out any common lead correction to data, and data throw use of desiring to make money or profit Isoplot 3.7 is completed, the final monazite that obtains²⁰⁸Pb-²³²Th and²⁰⁶Pb-²³⁸U weighted average age.

Further comprise: preparing 1ppb⁷Li,⁵9Co,⁸⁹Y,¹⁴⁰Ce,²⁰⁵Tl standard solution is optimized using the solution example Level four bars icp ms parameter.

Further, the summation of He flow and Ar flow is between 1.55~1.65L/Min.

Another object of the present invention is to provide one kind to be applied to 10 μm of monazite small beam spot U-Th-Pb age dating method Laser ablation level four bars icp ms.

Further, the laser ablation level four bars icp ms are to degrade object with monazite, are adopted With laser ablation energy density 3J/cm², laser pulse frequency 4Hz, with 10 μm small beam spot measurement monazite the U-Th-Pb age.

Technical effect of the invention is as follows:

1, under conditions of solution sample introduction, the element response intensity of raising level four bars icp ms, Calibrating quality axis, guarantee mass resolution, the balanced consistent element response of acquisition, reduction are double in entirely monitoring mass range Charge and the interference of oxide etc. guarantee the optimal detection state of level four bars icp ms

2, carrier gas (He) and level four bars inductively coupled plasma body during laser ablation sample introduction, by laser ablation system Mass spectrometric carrier gas (Ar), is transferred to mass spectrometric detection system for solid sample particle jointly.The matching of the two flow is direct Efficiency of transmission is influenced, sensitivity is influenced very big.Only optimal Ar gas and He air-flow is flux matched, could utilize LA-Q- When ICP-MS measures the monazite U-Th-Pb age, there is high sensitivity.

3, excessively high energy density and pulse frequency, may puncture mineral, and too low energy density and pulse frequency, It may cause sample volume deficiency, it is difficult to detect, therefore need for monazite, it is fixed that acquisition is appropriate for beam spot U-Th-Pb 10 μm small The laser ablation parameter in year, guarantees sufficiently high sensitivity, and low element fractionating degree.

4, solitary suitable for small beam spot in the Q-ICP-MS parameter of optimization, matching two-way carrier gas flux (Ar and He), acquisition On the basis of stone U-Th-Pb determines the laser ablation parameter in year, the U-Th-Pb age of monazite standard sample 44069 is measured, is established A kind of 10 μm of monazite small beam spot LA-Q-ICP-MS U-Th-Pb age dating method.

In short, 10 μm of monazite small beam spot LA-Q-ICP-MS U-Th-Pb age dating method provided by the invention, greatly Ground improves the spatial resolution of monazite U-Th-Pb age dating, is allowed to meet the testing requirement of most of monazite；It utilizes Laser ablation level four bars icp ms instrument, under the conditions of instrument parameter after optimization, with 10 μm of tuftlets Spot measures the U-Th-Pb age of monazite standard specimen 44069, the applicability of inspection institute's method for building up.

Detailed description of the invention

Fig. 1 is 10 μm of monazite provided in an embodiment of the present invention small beam spot U-Th-Pb age dating method flow diagrams.

Specific embodiment

In order to make the objectives, technical solutions, and advantages of the present invention clearer, with reference to embodiments, to the present invention It is further elaborated.It should be appreciated that the specific embodiments described herein are merely illustrative of the present invention, it is not used to Limit the present invention.

Application principle of the invention is explained in detail with reference to the accompanying drawing.

As shown in Figure 1, the age dating side the monazite of embodiment of the present invention beam spot LA-Q-ICP-MS U-Th-Pb 10 μm small Method the following steps are included:

S101: solution sample introduction optimizes Q-ICP-MS parameter, makes it have high sensitivity, and RSD is small；

S102: solid sampling (man-made glass standard specimen NIST610) matches the load of carrier gas (the Ar gas) and laser of Q-ICP-MS Gas (He gas), make it is online after LA-Q-ICP-MS there is high sensitivity, and RSD is small；

S103: first group of laser ablation condition experiment of design (being to degrade object with monazite standard specimen 44069), it is preliminary to obtain The laser ablation parameter area in year is determined suitable for the small beam spot of monazite；

S104: second group of laser ablation condition experiment of design (being to degrade object with monazite standard specimen 44069) is applicable in In the laser ablation parameter for carrying out monazite U-Th-Pb age dating under the conditions of 10 μm of beam spots；

S105: utilizing LA-Q-ICP-MS, under the conditions of instrument parameter after optimization, measures monazite with 10 μm small beam spot The U-Th-Pb age of standard specimen 44069, the applicability of inspection institute's method for building up.

Application principle of the invention is further described combined with specific embodiments below.

Embodiment 1

10 μm of monazite small beam spot U-Th-Pb age dating methods of the embodiment of the present invention the following steps are included:

Step 1, preparation 1ppb tuning liquid (⁷Li,⁵9Co,⁸⁹Y,¹⁴⁰Ce,²⁰⁵Tl), the solution example is then introduced into four Grade bar icp ms are aided with manual tuning on the basis of automatic tuning, the level four bars electricity optimized Sense couple plasma mass spectrometer parameter specifically includes that quarter bend position, sampling depth, extracts lens voltage, Omega lens electricity Pressure, RF power, carrier gas flux etc., target be improve the response intensity of element to be measured, calibrating quality axis, guarantee mass resolution, Balanced consistent element response is obtained in entirely monitoring mass range, reduces double charge and the interference of oxide etc., wherein ,²⁰⁵The response intensity (i.e. sensitivity) of Tl is greater than 200000cps/ppm, and relative standard deviation (RSD) is 1%~2%, double charge (70/140) less than 1.5%, oxide yield (156/140) is less than 1%.Introducing solution example is excellent to plasma mass spectrograph Change plasma mass spectrograph parameter, makes plasma mass spectrograph that there is high sensitivity, and relative standard deviation is small；

Step 2 carries out the online of laser and level four bars icp ms, it may be assumed that by laser beam focus in 610 surface man-made glass standard specimen NIST is allowed to corrode gasification, is sent sample particle (aerosol) to level four bars by carrier gas (He gas) In icp ms, quantitative analysis is carried out.During laser ablation sample introduction, by the carrier gas of laser ablation system (He) and the carrier gas of plasma mass spectrograph (Ar) solid sample particle, is transferred to mass spectrometric detection system jointly.The two The matching of flow directly influences efficiency of transmission, influences on sensitivity very big.Parameter, which is degraded, in fixed laser (degrades 24 μ of beam spot M, energy density 6J/cm², pulse frequency 7Hz), that is, in the case where guaranteeing that sample volume is the same, when the summation of He flow and Ar flow When between 1.55~1.65L/Min,²³⁸U counting is higher, and wherein He flow and Ar flow are respectively 800ml/Min, 0.8L/ When Min,²³⁸U is counted as 460000cps, and relative standard deviation (RSD) is 3% or so.Man-made glass standard specimen NIST610 is introduced to arrive Laser plasma Mass Spectrometry instrument, the carrier gas He gas of the mass spectrometric carrier gas Ar gas of match plasma and laser, make it is online after swash Light plasma mass spectrograph has high sensitivity, and relative standard deviation is small；

Step 3, according to common zircon laser ablation parameter, under larger beam spot diameter, (32,24 μm), desk study It is suitble to the laser ablation energy density and pulse frequency of monazite.Specific operation process is as follows: fixed laser beam spot diameter, first (32 μm) are respectively 2,4,6,8J/cm in energy density²Under conditions of, conversion pulse frequency (5,7,10Hz)；Then fixed to swash Light beam spot diameter (24 μm) is respectively 2,4,6,8J/cm in energy density²Under conditions of, conversion pulse frequency (5,7,10Hz), Comprehensive Correlation²⁰⁶Pb、²⁰⁸Pb、²³⁸U、²³²The element fractionating degree of the sensitivity of Th instantaneous signal and RSD and Pb, Th, U (with²⁰⁸Pb-²³²Th and²⁰⁶Pb-²³⁸U is calculated).

The result shows that monazite²⁰⁶Pb、²⁰⁸Pb、²³⁸U、²³²The sensitivity of Th instantaneous signal is higher, and RSD is small.Phase Than in zircon laser parameter (24 μm, 6J/cm², 7Hz) under²⁰⁶Pb-²³⁸The element of U is fractionated index (12%), monazite 's²⁰⁸Pb-²³²Th and²⁰⁶Pb-²³⁸The element fractionation index of U is lower, respectively 0.2%~6.1%, 0.1%~17.0%.Separately Outside, the (>=6J/cm when energy density is excessively high²), even if being also easy using small pulse frequency (5Hz) by sample breakdown.Explanation Monazite is easier to degrade compared with zircon, and the laser ablation condition (energy density, pulse frequency) of initial setting is all higher, needs again One group of experiment is set.

First group of laser ablation condition experiment is designed, is to degrade object with monazite standard specimen 44069, is tentatively suitable for The small beam spot of monazite determines the laser ablation parameter area in year；

Step 4, according to monazite elementary laser condition experiment as a result, the present invention has carried out one group of orthogonal test, beam spot again Diameter is 10 μm；Energy density is 2,3J/cm²；Pulse frequency is 1~7Hz.Since laser energy density is 1J/cm²When, it is insufficient To degrade monazite, even if pulse frequency is raised to 7Hz, after degrading lasting 10s, sample signal just can be detected obviously, therefore real Testing process not being related to laser energy density is 1J/cm²Correlated condition.Experimentation is as follows: fixed beam spot diameter, (10 μm), Laser ablation energy density is respectively 2,3J/cm²Under conditions of, conversion pulse frequency (1~7Hz), Comprehensive Correlation²⁰⁶Pb、²⁰⁸Pb、²³⁸U、²³²The sensitivity of Th instantaneous signal and RSD, Pb, Th, U element fractionating degree (with²⁰⁸Pb-²³²Th and²⁰⁶Pb-²³⁸U Calculate) and²⁰⁸Pb/²³²Th value and²⁰⁶Pb/²³⁸The RSD of U value explores under the conditions of small beam spot the measurement monazite U-Th-Pb age most Excellent laser ablation parameter.

Experiment shows under the conditions of small beam spot, monazite²⁰⁶Pb、²⁰⁸Pb、²³⁸U、²³²The sensitivity of Th instantaneous signal according to It is so higher, and RSD is small.When beam spot diameter, is 10 μm, in 3J/cm², under the conditions of 4Hz,²⁰⁸Pb-²³²Th and²⁰⁶Pb-²³⁸The element of U Being fractionated index is respectively 1.5%, 2.7%,²⁰⁸Pb/²³²Th and²⁰⁶Pb/²³⁸The RSD difference 9.6%, 11.4% of U value, it is smaller, Optimum Matching in this group of condition experiment.

Second group of laser ablation condition experiment is designed, is to degrade object with monazite standard specimen 44069, obtains and be suitable for 10 The laser ablation parameter of monazite U-Th-Pb age dating is carried out under the conditions of μm beam spot；

Step 5 is measured under 10 μm of beam spot diameter,s solely using laser ablation level four bars inductively coupled plasma mass spectrometry Occupy the U-Th-Pb age of stone standard specimen 44069.Under 10 μm of beam spot diameter,s, the laser energy density used is 3J/cm², pulse frequency Rate is 4Hz, and monazite standard specimen 44069 can achieve the sensitivity of enough measurements,²⁰⁸Pb-²³²Th and²⁰⁶Pb-²³⁸The element of U is fractionated Index less than 3%,²⁰⁸Pb/²³²Th and²⁰⁶Pb/²³⁸The RSD of U isotopic ratio less than 12%,²⁰⁸Pb/²³²Th and²⁰⁶Pb/²³⁸U is same The single-spot testing precision of the plain ratio in position is respectively less than 3%.It is surveyed²⁰⁷Pb/²³⁵U-²⁰⁶Pb/²³⁸The U age is harmonious,²⁰⁶Pb-²³⁸U weighting is flat The equal age is 425.7 ± 7.2Ma,²⁰⁸Pb-²³²Th weighted average age is 426.6 ± 6.1Ma.

Utilize SHRIMP method measurement monazite 44069²⁰⁶Pb-²³⁸U weighted average age is 426 ± 3Ma, utilizes TIMS Method measures monazite 44069²⁰⁷Pb/²³⁵U-²⁰⁶Pb/²³⁸U concordant age is 424.9 ± 0.4Ma, with the surveyed monazite of the present invention 44069 LA-ICP-MS method²⁰⁶Pb-²³⁸U and²⁰⁸Pb-²³²Th weighted average age is consistent.Show to utilize laser ablation level four bars It is feasible that inductively coupled plasma mass spectrometry measures the method at monazite U-Th-Pb age under 10 μm of beam spot diameter,s.

Using laser ablation level four bars icp ms instrument, instrument parameter condition after optimization Under, with the U-Th-Pb age of 10 μm small beam spot measurement monazite standard specimen 44069, the applicability of inspection institute's method for building up.

The foregoing is merely illustrative of the preferred embodiments of the present invention, is not intended to limit the invention, all in essence of the invention Made any modifications, equivalent replacements, and improvements etc., should all be included in the protection scope of the present invention within mind and principle.

Claims (3)

1. a kind of 10 μm of monazite small beam spot LA-Q-ICP-MS U-Th-Pb age dating method, which is characterized in that described solitary 10 μm of stone small beam spot LA-Q-ICP-MS U-Th-Pb age dating method is first introduced directly into the standard solution sample that concentration is 1ppb Made to level four bars icp ms by optimizing instrument parameter²⁰⁵The response intensity of Tl is greater than 200000cps/ppb, relative standard deviation are 1%~2%, and double charge is less than 1.5%, and oxygenation efficiency is less than 1%；Recycle laser The mode degraded introduces man-made glass NIST610 solid etalon sample to level four bars icp ms, matching The carrier gas He gas of laser and the carrier gas Ar gas of level four bars icp ms, make²³⁸The counting of U reaches highest； It recycles the mode of laser ablation to introduce monazite standard sample 44069 to level four bars icp ms, adopts Laser energy density is 3J/cm², pulse frequency 4Hz, with the 10 μm small beam spot measurement monazite U-Th-Pb age；

The small beam spot LA-Q-ICP-MS U-Th-Pb age dating method of described 10 μm of monazite the following steps are included:

Step 1, optimize level four bars icp ms parameter；By adjusting quarter bend position, sampling depth, extraction Lens voltage, Omega lens voltage, RF power, carrier gas flux make in 1ppb concentration standard solution²⁰⁵The response intensity of Tl is greater than 200000cps, relative standard deviation are 1%~2%, and double charge is less than 1.5%, and oxide yield is less than 1%；

The step of specific tuning parameters, is as follows: preparing 1ppb standard solution, and is introduced into level four bars inductively coupled plasma Constitution spectrometer, monitoring²⁰⁵The counting of Tl and RSD, and¹⁵⁶CeO+/¹⁴⁰Ce and⁷⁰Ce++/¹⁴⁰Ce+ ratio tunes each item by item, repeatedly Instrument parameter；

Step 2, the carrier gas Ar gas for matching laser carrier gas He gas and level four bars icp ms, make man-made glass NIST610 standard sample in the case where fixed laser degrades parameter,²³⁸U, which is counted, is greater than 450000cps, and relative standard deviation is less than 3%；

Specific matching process is as follows: using 610 standard sample of man-made glass NIST as test object, being arrived using laser ablation sample introduction Level four bars icp ms, fixed laser degrade parameter, guarantee that sample volume is the same, level four bars inductive coupling etc. He flow is separately fixed at 700,750,800 and referring to the parameter after optimization in step 1 by the mass spectrometric parameter setting of gas ions Under the magnitude of 850mL/min, change Ar flow, record²³⁸U is counted and RSD obtains optimal He gas and Ar air-flow by comparing It is flux matched；

Step 3, the parameter for optimizing laser；Beam spot is degraded by fixed laser, converts laser energy density and pulse frequency Mode is explored the laser ablation parameter for being suitble to the 10 μm small beam spot LA-Q-ICP-MS U-Th-Pb age dating of monazite, is made solitary Stone²³⁸The sensitivity of U instantaneous signal is greater than 300000cps, and²⁰⁶Pb-²³⁸The element fractionation index of U is less than 3%；

Specific Optimization Steps are as follows: being test object with monazite standard sample 44069, utilize laser ablation sample introduction to level four bars The parameter setting of icp ms, level four bars icp ms optimizes referring in step 1 The carrier gas Ar throughput setting of parameter afterwards, laser carrier gas He gas and level four bars icp ms is referring to step Laser beam spot diameter is first fixed as 24 μm by Optimum Matching in 2, is respectively 2J/cm in energy density², 4J/cm², 6J/cm², 8J/cm²Under conditions of, conversion pulse frequency, preliminary obtain is suitable for the laser ablation parameter model that monazite U-Th-Pb determines year It encloses；Beam spot diameter, and laser ablation energy density are separately fixed at 10 μm and 2J/cm again², conversion pulse frequency from 1Hz to 7Hz, in such a way that single-point degrades, the signal of monazite standard sample 44069 under the conditions of recording every group；Then beam spot is straight Diameter and laser ablation energy are separately fixed at 10 μm and 3J/cm², conversion pulse frequency degraded from 1Hz to 7Hz using single-point Mode, the signal of monazite standard sample 44069 under the conditions of recording every group；Finally the signal recorded is handled, is sieved Monazite under the conditions of each group out²³⁸The maximum sensitivity of U instantaneous signal calculates monazite under the conditions of each group²⁰⁶Pb-²³⁸The member of U Element fractionation index and²⁰⁶Pb/²³⁸The RSD of U value compares to obtain suitable for beam spot LA-Q-ICP-MS U-Th-Pb 10 μm of monazite small Determine the optimal laser ablation parameter in year；

Under step 4, level four bars icp ms and laser ablation parameter after optimization, 10 μm of tuftlets are utilized Spot measures the monazite U-Th-Pb age, makes²⁰⁸Pb-²³²Th and²⁰6Pb-²³⁸U element fractionation index less than 3%,²⁰⁸Pb/²³²Th With²⁰⁶Pb/²³⁸The single-spot testing precision of U isotopic ratio is respectively less than 3%,²⁰⁶Pb-²³⁸U and²⁰⁸Pb-²³²Th weighted average age exists It is consistent in error range, and within the scope of recommendation；

Specific step is as follows: being test object with monazite standard sample 44069, utilizes laser ablation sample introduction to level four bars inductance Couple plasma mass spectrometer, after the parameter setting of level four bars icp ms in step 1 referring to optimizing The carrier gas Ar throughput setting of parameter, laser carrier gas He gas and level four bars icp ms is referring in step 2 Optimum Matching, for the parameter setting of laser referring to the laser ablation parameter after optimization in step 3, test process is all made of single-point stripping Erosion mode, each measuring point bulk analysis time are 60s, and wherein background signal 10s, sample signal 40s, purge signal 10s；Data are adopted Collection, which uses, jumps peak mode,²⁰⁴Pb、²⁰⁶Pb、²⁰⁷Pb、²⁰⁸Pb、²³⁸The detection residence time of U is 20ms,²³²The residence time of Th is 10ms；5 unknown sample points of every measurement alternately measure 2 standard samples；It is carried out using monazite standard specimen 44069 as external standard The correction of U-Th-Pb isotopic fractionation and instrumental shift calculates, and correction, which calculates, uses software Glitter 4.0, non-logarithm According to any common lead correction is carried out, data throwing, which is desired to make money or profit with Isoplot3.7, to be completed, final acquisition monazite²⁰⁸Pb-²³²Th With²⁰⁶Pb-²³8U weighted average age.

2. 10 μm of monazite small beam spot LA-Q-ICP-MS U-Th-Pb age dating method as described in claim 1, feature It is, further comprises: prepares 1ppb⁷Li,⁵⁹Co,⁸⁹Y,¹⁴⁰Ce,²⁰⁵Tl standard solution optimizes level Four using the solution example Bar icp ms parameter.

3. 10 μm of monazite small beam spot LA-Q-ICP-MS U-Th-Pb age dating method as described in claim 1, feature Be, the summation of laser carrier gas He flow and level four bars inductivity coupled plasma mass spectrometry carrier gas Ar flow between 1.55~ 1.65L/Min。