CN102141539A - Method for uranium lead dating of baddeleyite by using secondary ion mass spectroscopy - Google Patents

Abstract

The invention discloses a method for determining uranium lead age of baddeleyite by using secondary ion mass spectroscopy, which belongs to the field of secondary ion mass spectroscopy dating method in geochronology and particularly relates to a method applied to a large secondary ion mass spectrometer having oxygen blowing conditions. In the measurement method described by the invention, the degree of vacuum in a sample cavity of the spectrometer is enabled to reach 1E-8Torr at first, and an oxygen leak valve is then opened to blow the oxygen with the purity of 99.99% to the surface of the sample so that the oxygen partial pressure of the sample cavity reaches 1E-5Torr. The method of the invention can reduce the optical axis effect, which is exhibited by secondary ion mass spectroscopy during the uranium-lead dating of baddeleyite, to be below 2%, thus the accuracy of uranium-lead age when the baddeleyite is subjected to secondary ion mass chromatographic analysis is significantly improved. After uranium-lead measurement ratio is obtained, uranium-lead fractionation correction is performed by adopting a linear relation between ln(Pb+/U+) and ln(UO2+/U+) in order to obtain accurate age.

Description

A kind of secondary ion mass spectrum of utilizing is carried out the plumbous fixed year method of baddeleyite uranium

Technical field

The present invention is a kind of method of utilizing ion microprobe (SIMS) to measure the baddeleyite uranium lead age, belongs to the category of the fixed year method of secondary ion mass spectrum mineral in the geologic chronology.

Background technology

The secondary ion mass spectrum analytic approach is a kind of advanced method that micro-zone in situ is analyzed, and has high sensitivity, high spatial resolution and high-precision advantage.Use through the primary ions bundle that quickens and focus on bombardment sample surface, back, the sample surfaces composition is sputtered, and the atom, molecule and the atomic group that have part to be sputtered be ionized, and promptly forms secondary ion.Secondary ion enters mass spectrometer and carries out quality analysis after the sample surfaces high pressure quickens, obtain element, the isotope information of sample in the microcell scope.When using sims analysis, the productive rate difference between different elements or the isotope, promptly the different elements, isotopic signal intensity and its true abundance in sample that receive of the ion acceptor of instrument is disproportionate.The phenomenon of this outwardness is called the instrument fractionation.And for the sample of different substrates, fractionation factor has difference, and this effect is called matrix effect.General sims analysis need one or more known elements, isotopic ratio and with the sample same matrix, the standard model of promptly identical principal ingredient and same substance structure carries out element, isotope fractionation is proofreaied and correct.

There are two kinds of isotopes in the occurring in nature uranium element ²³⁸U and ²³⁵U, decay becomes respectively ²⁰⁶Pb and ²⁰⁷Pb.The plumbous fixed year technology of uranium is further to calculate the method that is closed to current elapsed-time standards from sample by this two group elements relative abundance in the measuring samples.The decay computing formula is as follows.

${}^{206}\mathrm{<figure-callout\; id="206"\; label="Pb"\; filenames="HSA00000010285800021.png"\; state="\{\{state\}\}">Pb</figure-callout>}={}^{238}U(e^{{\mathrm{\&lambda;}}_{238}t}-1)$ Formula (1)

${}^{207}\mathrm{<figure-callout\; id="207"\; label="Pb"\; filenames="HSA00000010285800021.png"\; state="\{\{state\}\}">Pb</figure-callout>}={}^{235}U(e^{{\mathrm{\&lambda;}}_{235}t}-1)$ Formula (2)

In formula (1) and the formula (2) ²⁰⁶Pb, ²³⁸Present this abundance ratio of isotopes in sign such as the U representative sample, wherein lead isotope is the decay daughter of corresponding uranium isotope, λ ₂₃₅And λ ₂₃₈Be respectively two isotopic disintegration constants of uranium, t is the fall time of experience, i.e. age.Through the mathematics replacement, can obtain the formula of two corresponding compute age, be shown below.

$t=\frac{\ln (\frac{{}^{206}\mathrm{Pb}}{{}^{238}U}-1)}{{\mathrm{\&lambda;}}_{238}}$ Formula (3)

$t=\frac{\ln (\frac{{}^{207}\mathrm{Pb}}{{}^{235}U}-1)}{{\mathrm{\&lambda;}}_{235}}$ Formula (4)

Two kinds of lead elements of indication all are that uranium is radiogenic in the formula, and two kinds of isotopes of occurring in nature uranium ²³⁵U and ²³⁸The content ratio of U is constant, so formula (1) compares respectively with formula (2) both members, can obtain formula (5).This formula is a lead isotope ²⁰⁷Pb and ²⁰⁶The ratio R of Pb ₇₆About the function of age t, be transcendental equation to t, 1/137.88 is the fixed ratio of two uranium isotopes.But by the t that this transcendental equation obtains is the age of an independence test, is called plumbous lead age.

$R_{76}=\frac{1}{137.88}\&times;\frac{e^{{\mathrm{\&lambda;}}_{235}t}-1}{e^{{\mathrm{\&lambda;}}_{238}t}-1}$ Formula (5)

More than age of going out of three Equation for Calculating in error range when consistent, this age is called concordant age.Show that the disturbance in later stage has not taken place since self-enclosed for the U-Pb system of mineral.

Plumbous lead age is that two isotopic ratios by lead calculate, the isotope of identity element be it is generally acknowledged the influence that is not subjected to the instrument fractionation or is influenced very I and ignores, but when age value hour, age value is too responsive to lead isotope ratio, it is too high that the correlative value measuring accuracy requires, so plumbous lead age method generally only is used to measure age (＞800Ma) baddeleyite always.

Abundance by the lead element that measures in the sample is the summation of radiogenic lead element and non-radioactive origin cause of formation lead element (common lead).Common lead may be that sample has existed when forming or the lead of laboratory pollution during specimen preparation.Formula (1) abundance of all lead elements to the formula (4) all is meant the radiogenic lead that the isotope emission decay of uranium element forms, and does not comprise common lead.The common lead that also will deduct after the abundance of lead element is wherein measured on the historical facts or anecdotes border, just can obtain the correct sample age.In the isotope of lead element ²⁰⁴Pb is non-radioactivity genesis fully, measures ²⁰⁴The abundance of Pb is according to known ²⁰⁴Pb and ²⁰⁶The ratio of Pb reaches ²⁰⁴Pb and ²⁰⁷The ratio of Pb is deducted in the sample ²⁰⁶Pb and ²⁰⁷Common lead among the Pb obtains the radiogenic lead isotopic abundance.Hereinafter, all lead elements that relates to all refer in particular to radiogenic lead when not having specified otherwise.

Use SIMS to carry out uranium in the time of plumbous fixed year, the relative productive rate of lead element and uranium element differs bigger, the Pb that promptly measures ⁺/ U ⁺Not bigger with the Pb/U value difference in the actual sample.So needing to carry out the plumbous fractionation of uranium with known standard model of age proofreaies and correct.In actual measurement, for common zircon sample (molecular formula ZrSiO ₄), the Ln that directly measures ( ²⁰⁶Pb ⁺/ ²³⁸U ⁺) and Ln (UO ₂ ⁺/ U ⁺) the better linearity correlativity is arranged, so carrying out the fractionation of uranium lead, this relation of employing proofreaies and correct.The zircon sample of same age Ln ( ²⁰⁶Pb ⁺/ ²³⁸U ⁺) and Ln (UO ₂ ⁺/ U ⁺) linear distribution on the correlogram, with Ln (UO ₂ ⁺/ U ⁺) be horizontal ordinate x, Ln ( ²⁰⁶Pb ⁺/ ²³⁸U ⁺) be ordinate y, y=ax+b is arranged.Once measure for the same of same matrix, all samples should have identical a value.The intercept b difference that the sample of all ages and classes obtains.As shown in Figure 1.

Among Fig. 1, four samples all are international Zircon age standard models.Be respectively for 1 to No. 4: 91500 zircons, BR266 zircon, TEMORA zircon and Plesovice zircon, the age is respectively 1,062 hundred ten thousand (Ma), 559Ma, 417Ma and 337Ma.Article four, the parallel lines of match be respectively the corresponding Ln of four standard models ( ²⁰⁶Pb ⁺/ ²³⁸U ⁺) and Ln (UO ₂ ⁺/ U ⁺) relation line.Identical and the intercept difference of straight slope.The difference of intercept has been represented the gap size at age separately.In the actual measurement, known by standard model ²⁰⁶Pb/ ²³⁸U value and actual measurement ²⁰⁶Pb ⁺/ ²³⁸U ⁺Calculate the fractionation factor of uranium element and lead element, this coefficient is applied to the age that can calculate unknown sample in the unknown sample.

The plumbous fixed year method of SIMS uranium is widely used in fixed year at zircon, is the dating method of development comparative maturity in the geologic chronology.Be suitable for forming the above zircon sample of about 1Ma of age, almost comprised the earth all time ranges since about 4567Ma forms.Being mainly used in the rock that the saturated environment of silicon is formed down carried out fixed year.

The baddeleyite that relates among the present invention is considered to the first-selected mineral in fixed year of the pyrogene rock of silicon insatiable hunger, molecular formula ZrO ₂Often crystalline particle is very little owing to baddeleyite, usually less than 30 microns, so the fixed year technology of the micro-zone in situ of secondary ion mass spectrum (SIMS) ought to be the first-selected means in fixed year of baddeleyite.Plumbous fractionation but has serious " optical axis effect " yet SIMS is for baddeleyite uranium, has influenced the SIMS technology in the application of baddeleyite in fixed year.Here " optical axis effect " is meant, when the primary ions bundle and the baddeleyite optical axis of crystal of SIMS became different angles to incide the baddeleyite surface, the uranium element that is produced was different with the fractionation of lead element.For baddeleyite, because its particle is extremely small, the optical axis that can not guarantee all particles when carrying out specimen preparation all have identical towards, it is the direction optical axis direction difference relatively separately that SIMS primary ions bundle incides each sample surfaces, too discrete by measuring observed uranium lead ratio value distribution, be not suitable for practical application.Influence that but lead isotope ratio is not subjected to substantially in the baddeleyite " optical axis effect ", measured value is comparatively accurate.So far, worldwide does not all use SIMS to carry out the plumbous fixed year data announces of baddeleyite uranium, generally adopts the plumbous fixed year method of SIMS.Particular content is asked for an interview document " Crystal orientation effectsduring ion microprobe U-Pb analysis of baddeleyite " Chemical Geology, 2000,168 volumes, 75-97 page or leaf.

Summary of the invention

The objective of the invention is to by the technical measures in the optimization experiment process, " the optical axis effect " that baddeleyite was existed in the technology at SIMS uranium in plumbous fixed year is decreased in the geologic chronology in the acceptable range of indeterminacy.Make the application of technology in baddeleyite in plumbous fixed year of SIMS uranium become possibility, thereby be applied to the extensive undersaturated pyrogene rock of silicon that exists fixed year.

Core of the present invention is near the continual and steady pure oxygen that is blown into certain flow in the space baddeleyite sample surfaces, to improve the lead element productive rate of baddeleyite sample when carrying out sims analysis, reduce its " optical axis effect " simultaneously, improve the plumbous fixed year precision of baddeleyite SIMS uranium, and can be applied in the measurement of actual sample and go.

Technical scheme of the present invention is:

1) the baddeleyite sample is selected about 100 of individual particle mineral and make epoxy resin sample target, 1 inch of sample target diameter, the about 5mm of thickness measures to adapt to SIMS.

2) the sample target is placed in the SIMS sample chamber, and vacuumizes, and the chamber vacuum degree should be less than 1E-8Torr.The manual adjustment sample makes the primary ions bundle focus on best condition at sample surfaces in the position of instrument secondary ion optical axis direction.

3) measure before, use vacuum pump to take out oxygen pipeline 5 minutes, and with high purity oxygen gas flushing line inside, assurance is blown into the purity of sample chamber oxygen.Ventilation channel through cleaning after, being blown into purity at sample surfaces is 99.99% high purity oxygen gas.By the leakage valve on instrument sample chamber control oxygen flow, make the vacuum degree control instrument sample chamber in positive and negative 10% scope of 1E-5Torr, and in whole standard model and unknown sample measuring process, keep stablizing.Oxygen is by the two-stage voltage stabilizing, and adjusting back pressure is 0.5Bar, and supplies with instrument internal through the mechanical valve that leaks.Variation of temperature can cause the variation of oxygen pressure in the pipeline in the laboratory, so need accurately control indoor temperature, per hour changes being no more than ± 1 ℃.

4) adopt SIMS to jump the peak mode, single receiving system electron-multiplier (EM) carries out the reception of all mass peaks.

5) in the measuring process, standard model and unknown sample are alternately measured, and guarantee that standard model is identical with the unknown sample measuring condition.

6) obtain each element ratios after, according to ²⁰⁴Pb measured value and present common lead are formed and are carried out common lead and proofread and correct, and with Ln ( ²⁰⁶Pb ⁺/ ²³⁸U ⁺) and Ln (UO ₂ ⁺/ U ⁺) the age correspondence of linear relationship according to standard sample ²⁰⁶Pb ⁺/ ²³⁸U ⁺The plumbous fractionation factor of uranium that standard value obtains instrument carries out the plumbous fractionation correction of uranium to unknown sample, and by the last ratio calculation age.

The present invention is by being blown into pure oxygen at the baddeleyite sample surfaces, increase on the one hand about 6 times of the ionization productive rates of the lead element in the baddeleyite, never the 3cps/nA/ppm under the oxygen blast gas high vacuum brings up to 21cps/nA/ppm, promptly, if contain the lead element of 1ppm after the oxygen blast in the baddeleyite sample, use the primary ions bundle bombardment sample of 1nA so, will under 5000 resolving powers, use the electron-multiplier (EM) of 95% above efficient to obtain the signal intensity of 21 countings p.s.; " the optical axis effect " that suppressed on the other hand when baddeleyite uses sims analysis to about 2%, significantly improved analysis precision, reaches the accuracy rating that geologic chronology has application value, makes plumbous fixed year of baddeleyite SIMS uranium become possibility.

Description of drawings:

Fig. 1. synoptic diagram is proofreaied and correct in the plumbous fractionation of the SIMS uranium of general zircon sample

Fig. 2. the oxygen blast of baddeleyite sample surfaces is to its plumbous influence of productive rate relatively in this method

Fig. 3. the oxygen blast of baddeleyite sample surfaces is to the inhibiting effect of its " optical axis effect " in this method

Embodiment:

Below in conjunction with concrete sample the technical scheme that the present invention relates to is described further, but not as the restriction to content of the present invention.

With 6 baddeleyite sample: Phalaborwa, FC-4b, 05-JX02, Kovdor, LGXL-01 and SK10-2 is example, introduces the concrete enforcement of the technology of the present invention method.Wherein Phalaborwa is an international standard baddeleyite age sample, and the age is 2059.6 ± 3.5Ma.All the other each sample ages all record by additive method, or can accurately define according to the known age of isogenetic other mineral.Except that standard specimen Phalaborwa, remaining five sample age or the mineral age same period are respectively 1098.2 ± 1.5Ma, 810 ± 6Ma, 377.52 ± 0.94Ma, 195 ± 1Ma and 32.1 ± 0.49Ma.These known reference ages are checked the levels of precision of this method as the standard of verifying in this method.

Above-mentioned five samples as sample with international baddeleyite age criterion Phalaborwa direction putting at random, become the sample target with poured with epoxy resin.Through exposing the baddeleyite tangent plane after the slight polishing, again through polishing, clean, dry, gold-plated back is stand-by.

Put into instrument after the sample target is ready to and vacuumize, at first the vacuum with the sample chamber drops to below the 1E-8Torr, uses backing vacuum pump to take out 5 minutes on the oxygen deliverng pipe road then, puts into the pure oxygen flushing again more than five times, guarantees to send into the oxygen purity of sample surfaces.Open the leakage valve of instrument sample chamber then, make vacuum degree control in the instrument sample chamber, and in whole standard model and unknown sample measuring process, keep stable in positive and negative 10% scope of 1E-5Torr.Oxygen is by the two-stage voltage stabilizing, and adjusting back pressure is 0.5Bar, and supplies with instrument internal through the mechanical valve that leaks.The control indoor temperature is in ± 1 ℃/h variation range.

Adjust SIMS and once restraint spot, be shaped as 20 * 30 microns ellipse, line is 10nA, and a beam accelerating voltage-13KV uses the magnetic-type mass filter on the beam optical path to select once to restraint material and is O ₂ ^-Add on the sample+the 10KV accelerating potential, use 200 times of transmission magnifications.Energy filter slits is made as and is equivalent to the 60eV energy width.Mass resolution is made as 5400 (definition of 10% peak height) and disturbs 55 microns of secondary ion entrance slit width, 150 microns of exit slit width with the isobary of removing on the mass peak that needs to measure in most baddeleyites.

Adopt single collector magnetic field to jump the peak scan mode, use EM to accept all secondary ion signals.EM efficient is more than 95%, dead time 26nS.

In the present invention, need ion, sign mass number, measurement integral time and the role thereof of measurement to see also table 1.

Among table 1. the present invention by measured ion

Ion	Mass number (amu)	Integral time (S)	Effect
				90Zr 2 16O +	196	1.04	Stroma ground substance, the quality and the content that are mainly used in each measurement are proofreaied and correct
92Zr 2 16O +	200	0.56	200.5 the reference mass peak of mass number
					200.5	4.16	The receiver background noise is measured quality, and this locates no secondary ion signal
94Zr 2 16O +	204	0.56	204Pb +The quality references peak
				204Pb +	204	6.24	Common lead content measurement peak
206Pb +	206	4.16	Lead isotope

207 Pb +	207	6.24	Lead isotope
				238U +	238	2.08	Uranium isotope
238U 16O 2 +	270	2.08	Uranium oxide is used to proofread and correct the plumbous quality fractionation of uranium

Before the measurement, the software that uses instrument to carry carries out mass calibration, wherein 200.5 mass numbers and ²⁰⁴Pb ⁺Its magnetic field position can not directly be determined because signal is very weak in the position, so its calibrating as the reference quality than the large-signal peak of closing on of use.Determine earlier the center of its reference peak, the magnetic field biasing is set, determine the magnetic field intensity of two signals according to the Theoretical Mass difference at reference peak and two small-signal peaks separately.

Behind the mass calibration, make magnetic field in tested scan round qualitatively, the magnetic field temperature tends towards stability about 45 minutes, carries out repeatedly mass calibration again, check magnetic field stability, the central value that each calibration obtains and last time the mass peak center differ smaller or equal to beginning measurement behind the 3ppm.

Standard model and other samples are alternately measured, guaranteed that the measuring condition of other samples and standard model is consistent in whole measuring process.Each measurement point respectively scans 7 circulations to measured mass number.Can reject exceptional value wherein during statistics.

The isotopic ratio that records and as shown in table 2 through the result after the standard model correction, listed lead isotope ratio is through the common lead deduction in the table.

Table 2. measurement result

By tables of data as can be seen, by the baddeleyite uranium lead age that this method obtains, promptly go up the t in the table _207/235And t _207/235Consistent in error range with the reference age of above introducing separately.By statistics, take all factors into consideration t _207/235And t _206/238Age carries out obtaining concordant age after error is synthesized and is respectively 1105.8 ± 9.1Ma, 805 ± 10Ma, 381.4 ± 4.5Ma, 194.8 ± 1.4Ma and 32.9 ± 0.5Ma, and above error is 2 σ standard errors (standard error).The correction age error of sample is all less than 2% level, and being that geologic chronology is general uses acceptable error range.

Validity for demonstration baddeleyite sample surfaces oxygen blast technology, added contrast test in this example, measured according to above-mentioned normal process for first group, second group under the prerequisite of experiment condition unanimity, the oxygen of closing the sample chamber leaks valve, keeps the high vacuum state (being better than 1E-8Torr) of sample chamber.Many are arbitrarily put, and promptly optical axis direction baddeleyite age criterion sample Phalaborwa at random tests.The result projects to ²⁰⁷Pb/ ²⁰⁶Pb and ²³⁸U/ ²⁰⁶On the Pb coordinate system, as shown in Figure 3.Among the figure, the measurement point of the circular first group of experiment of some expression of loosing, square hollow diffusing point is represented second group of measurement point.As can be seen, two groups of data are in the ordinate direction, and promptly plumbous lead age direction is all more concentrated, and at horizontal ordinate, it is more discrete promptly to represent the uranium lead age direction to distribute.Illustrate that " optical axis effect " truly has certain influence to the measurement of baddeleyite uranium lead age.But first group of result is more concentrated at both direction than second group of result, and the influence that can reduce " optical axis effect " by this method is described, and the productive rate of lead element also can improve measuring accuracy in the oxygen blast raising baddeleyite.

Data in the above example are all finished on CAMECA IMS1280 type SIMS.Illustrative example only supplies explanation the present invention's usefulness, and is not limitation of the present invention.Those skilled in the art can obtain similar result according to this method on the SIMS with oxygen blast condition.

About those skilled in the art can make various variations and modification without departing from the scope of the invention, so all technical schemes that are equal to also should belong to category of the present invention, and scope of patent protection of the present invention should be defined by the claims.

Claims (6)

1. one kind is used secondary ion mass spectrum to carry out the method in fixed year of baddeleyite uranium-lead, it is characterized in that: at the baddeleyite sample surfaces amount of blowing over oxygen.

2. the method for claim 1, it is characterized in that: the initial depression of instrument sample chamber should reach 1E-8Torr.

3. method as claimed in claim 1 or 2 is characterized in that: after the sample surfaces oxygen blast, sample vacuum chamber should be controlled at 1E-5Torr.

4. as method as described in above-mentioned arbitrary claim, it is characterized in that: indoor temperature be controlled at per hour change be no more than ± constant to guarantee oxygen pressure in 1 ℃ the scope.

5. as method as described in above-mentioned arbitrary claim, it is characterized in that: the purity of the oxygen that uses should be more than or equal to 99.99%.

6. as the method for above-mentioned arbitrary claim, be characterised in that: the present invention is only at the situation of utilizing the plumbous fixed year method of secondary ion mass spectrum uranium that the baddeleyite sample is measured.

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