SIMS analysis of U–Pb isotopes in monazite: matrix effects

Chemical Geology 144 Ž1998. 305–312

SIMS analysis of U–Pb isotopes in monazite: matrix effects Xiang-Kun Zhu ) , R. Keith O’Nions, Andrew J. Gibb Department of Earth Sciences, UniÕersity of Oxford, Parks Road, Oxford, OX1 3PR, UK Received 22 July 1997; accepted 7 October 1997

Abstract The calibration of U–Pb and Th–Pb ratios in Th–U-rich minerals measured by SIMS usually assumes that there are no matrix effects arising from the compositional difference between standard and sample. Matrix effects have been observed during attempts to calibrate U–Pb ratios of monazite using a SIMS energy-filtering technique on the ISOLAB-120 ion probe. Thus matrix effects cannot be assumed to be insignificant a priori for SIMS U–Pb or Th–Pb chronometry, and need to be investigated more thoroughly. A general approach to the investigation of matrix effects for SIMS U–Pb and Th–Pb analysis is proposed. q 1998 Elsevier Science B.V. Keywords: Matrix effects; SIMS chronometry; Monazite; U–Pb isotopes; geochronology

1. Introduction Secondary ion mass spectrometry ŽSIMS. has now become a widely used technique for U–Th–Pb dating of U- andror Th-rich minerals Že.g. Hinthorne et al., 1979; Compston et al., 1984, 1992; Williams and Claesson, 1987; DeWolf et al., 1993; Harrison et al., 1995; Vry et al., 1996; Zhu et al., 1997a,b.. A fundamental assumption in the calibration of U–Pb and Th–Pb ratio measurements by SIMS is that no matrix effects exist between standard and sample. The term matrix effects here refers specifically to variations in the secondary ion yields of an isotope of an element with sample composition under a given set of working conditions. Such effects have been a long-standing question in many SIMS applications Že.g. Reed et al., 1979; Shimizu and Hart,

)

Corresponding author. Tel.: q44 1865 272075; Fax: q44 1865 272072; E-mail: [email protected]

1982; Reed, 1986; MacRae et al., 1993; Eiler et al., 1997.. However in the case of SIMS zircon U–Pb ŽCompston et al., 1984, 1992; Williams and Claesson, 1987., monazite U–Pb ŽVry et al., 1996. and monazite Th–Pb ŽHarrison et al., 1995. measurements, it has been generally assumed that matrix effects are not present when natural zircon or monazite is used as a standard. In this contribution we report the presence of matrix effects observed during attempts at a U–Pb calibration of monazite using a SIMS energy-filtering technique on the ISOLAB-120 ion probe.

2. Analytical techniques Energy filtering of secondary ions in SIMS analysis is used widely to suppress the intensities of interfering molecular ions relative to monatomic ions, and is based upon the differences in their kinetic

0009-2541r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII S 0 0 0 9 - 2 5 4 1 Ž 9 7 . 0 0 1 4 0 - X

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X.-K. Zhu et al.r Chemical Geology 144 (1998) 305–312

energy distributions ŽShimizu et al., 1978; Hinton, 1990.. It has also been shown ŽShimizu et al., 1978; MacRae et al., 1993. that the secondary ion yields at energies greater than ; 70 eV show much less matrix dependence than does the lower-energy part of the ion yields. In the energy-filtering mode the secondary ion signal available for measurement is much reduced; however, in the ISOLAB w instrument used here this is to some extent offset by operating at a lower mass resolving power. In the case of the monazites analysed in this study, the energy-filtered 238 q 208 U , Pbq and 206 Pbq secondary ions are about an order of magnitude less intense than unfiltered ions, but are still large enough for precise measurements to be made. All analyses reported in this contribution were performed in situ on polished petrographic thin sections using the ISOLAB-120 in Oxford University. The general procedure of sample preparation for in-situ SIMS analysis of monazite has been detailed elsewhere ŽZhu et al., 1997b.. The analyses were conducted with a medium resolving power of D MrM ( 2000 and an energy acceptance of 50 eV at a q40 eV offset from 238 Uq peak energies. The 238 q U peak was determined with an energy acceptance of 25 eV. The 23 keV and 10 to 20 nA primary ion beams of 16 Oy were focused to ; 20 mm in diameter. Energy-filtered secondary ions of 206 Pbq, 238 q U and 270 UOq 2 were collected alternately by a Daly-photomultiplier ion counting detector which is linked to a SRS counting system. Each analysis

represents 10 to 15 peak switching cycles of 206 Pbq, Uq and 270 UOq 2 . Integration time for each peak was 20 s with an additional 10-s magnet setting time. The statistical internal precision for both 206 238 q Pbqr 238 Uq and 270 UOq U are generally 2r better than 1.5% Ž2 s .. The energy-filtering technique employed here is similar to that described by Shimizu et al. Ž1978. and Hinton Ž1990.. Note that the SHRIMP-technique for zircon U–Pb chronometry ŽCompston et al., 1984, 1992; Williams and Claesson, 1987. and the technique adopted by Harrison et al. Ž1995. for monazite Th–Pb analysis use unfiltered ions Detailed scans of 238 Uq, 254 UOq and 270 UOq 2 from a monazite in a Lewisian granulite have been made at both high mass resolving power of ; 6000 ŽFig. 1. and medium resolving power of ; 2000 Žnot shown.. In both cases it is evident that there are no significant interfering molecules in the vicinities of the peaks for 238 Uq, 254 UOq and 270 UOq 2 species. The observed mass differences between 206 Pb, 207 Pb, 208 Pb and potential interfering molecular peaks are ; 0.09 mass units ŽFig. 2., which is consistent with those noted by Belshaw Ž1994. in the analysis of monazite from the Wind River granulite. These potential molecular interferences could be resolved at 2300 resolving power, if the Pbq and potential interfering peaks were of similar size. However, the intensities of 206 Pbq and 208 Pbq peaks are in fact substantially larger than the adjacent molecular peaks ŽFig. 2.. Thus the resolving power of 2000 combined 238

Fig. 1. The secondary ion mass spectrum in the regions of mass 238, 254 and 270 obtained from the core of monazite a5 in sample 86004 with a mass resolving power of ; 6000. Note that there are no potential interfering molecules in the vicinities of the peaks for 238 Uq, 254 UOq and 270 UOq 2.

X.-K. Zhu et al.r Chemical Geology 144 (1998) 305–312

307

UOq 2 secondary ions, repeated U–Pb isotopic measurements of the core of monazite a4 in sample 86004, which contains ; 6 wt% ThO 2 ŽTable 1., 238 q yield a linear relationship between 270 UOq U 2r 206 q 238 q Ž and Pb r U Fig. 4a. with the following equation: 206

Pbq

238

Uq

270

s a q b)

UOq 2

238

Uq

Ž 1.

where a is intercept and b slope. Here a / 0, which means that the yields of 206 Pbq and 270 UOq 2 vary in different proportions with the

Fig. 2. The secondary ion mass spectrum in the mass range 205.85 to 208.50 obtained from the core of monazite a5 in sample 86004 during 16 Oy sputtering with a mass resolving power of ;6000. Note that the potential interfering molecules are ; 0.09 amu lower than the desired ionic peaks.

with energy filtering, should readily suppress the molecular interferences on 206 Pb and 208 Pb to negligible levels.

3. Results and discussion All the monazites used in this study are from two Lewisian granulite samples, namely 86004 and 86015. These monazites have been dated previously by SIM S 2 0 7 Pbr 2 0 6 Pb chronom etry and 208 Pbr 232 Th chronometry ŽZhu et al., 1997a,b.. Detailed petrography of these samples has been presented elsewhere ŽZhu, 1997; Zhu et al., 1997a,b.. As shown in Fig. 3, the energy distribution of 206 Pbq from monazite is more similar to that of 270 254 UOq UOq and each of these is sub2 than to stantially different from that of 238 Uq, particularly at high energies. Thus, the 206 Pbq, 238 Uq and 270 UOq 2 species were chosen for the 206 Pbr 238 U calibration. Due to the correlated discrimination of Pbq and

Fig. 3. Plots showing the kinetic energy distributions of secondary 206 Pbq, 238 Uq, 254 UOq and 270 UOq 2 ions sputtered from monazite a2L in sample 86015 with a 25 eV energy acceptance. Ža. Core of the monazite which contains ; 2% ThO 2 . Žb. Rim of the monazite which contains 18% ThO 2 . Note that the energy distributions of 206 Pbq and 270 UOq 2 from the high Th rim are parallel at the higher energy part, which implies that they decrease at the same rate with the increase of the kinetic energy.

308

Sample

La 2 O 3

Ce 2 O 3

Pr2 O 3

Nd 2 O 3

Sm 2 O 3

Eu 2 O 3

Gd 2 O 3

Y2 O 3

ThO 2

UO 2

PbO

P2 O5

SiO 2

CaO

Low Th monazite Ma4rcorer86004 a Ma2rcorer86015 Ma3Jr86015

15.81 15.87 15.36

30.91 32.30 32.88

3.27 3.60 3.66

11.10 13.33 14.64

1.16 1.36 1.36

0.08 -d.l. 0.07

0.50 0.24 0.22

0.08 -d.l. -d.l.

5.68 2.40 1.19

0.18 0.54 0.41

0.67 0.46 0.27

28.82 27.72 28.15

0.23 0.27 0.28

1.18 0.44 0.33

99.67 98.47 98.81

High Th monazite Ma5rrimr86004 Ma2Lrrimr86015 Ma1Ar86015

12.26 11.06 9.64

25.81 25.28 25.23

3.08 3.20 3.29

11.01 12.39 12.76

1.24 0.97 1.11

0.08 -d.l. 0.12

0.52 0.18 0.24

0.08 -d.l. -d.l.

14.16 17.51 17.22

0.5 0.80 0.44

1.55 2.05 1.74

26.44 23.20 23.82

1.46 2.11 2.66

1.94 1.65 1.33

100.17 100.39 99.38

Note: a Stands for ‘core of monazite a4 in sample 86004’. d.l.sdetection limit, which is ca. 0.05 wt% for both Eu 2 O 3 and Y2 O 3 . Dy, Er and Er were not detected in any of the monazites analysed.

Total

X.-K. Zhu et al.r Chemical Geology 144 (1998) 305–312

Table 1 Electron probe results of monazites used for SIMS U–Pb calibration

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309

Ž1984, 1992. and to the Th–Pb calibration for monazite reported by Harrison et al. Ž1995.. As for the U–Pb calibration of zircon ŽCompston et al., 1984. and for the Th–Pb calibration of monazite ŽHarrison et al., 1995., it is assumed here that the measured ratio Ž 206 Pbqr 238 Uq . for a monazite sample is related to the actual ratio  206 Pbr 238 U4 by a factor which is the same as for the standard under identical operating conditions. Therefore: 206

ž

/ ½ 5 238

Uq

206

sample

ž

s

U

Pbq

/ ½ 5 238

Uq

206

Pb

238

Fig. 4. Plots showing the linear correlations between 270 238 q UOq U and 206 Pbqr 238 Uq species from monazites. Ža. 2 r Core of monazite a4 in sample 86004, which contains ;6 wt% ThO 2 . In this plot, the intercept as 0.185"0.008 and the slope bs 0.418"0.006. Žb. Rim of monazite a5 in the same sample, which contains ;14 wt% ThO 2 . In this plot, as 0"0.002 and bs 0.496"0.002.

206

Pbq

238

sample

standard

Pb U

Ž 2.

standard

238 q The 270 UOq U ratio is independent of the 2r sample or standard age, but dependent on working conditions. Thus analogous to the U–Pb calibration of zircon ŽCompston et al., 1984. and to the Th–Pb calibration of monazite ŽHarrison et al., 1995., the 270 238 q UOq U ratio may be used in practice as a 2r working conditions reference. That is at identical extraction conditions: 270

ž

270

UOq 2

238

/

Uq

s standard

ž

UOq 2

238

Uq

/

Ž 3. sample

It may be shown from Eqs. Ž1. – Ž3.. that: shift of ion energy. On an energy distribution chart with a linear scale X axis Žion energy. and logarithmic scale Y axis Žion yields., this is reflected by the fact that two lines representing the energy distribuŽ tions of 206 Pbq and 270 UOq 2 are not parallel Fig. 3a.. If a s 0, it is clear from Eq. Ž1. that the ion yields of 206 Pbq and 270 UOq 2 must vary in the same proportion. In that case, the lines representing the energy distributions of 206 Pbq and 270 UOq 2 are parallel on an energy distribution chart with a linear scale X axis Žion energy. and logarithmic scale Y axis Žion yields. ŽFig. 3b.. The relationship expressed by Eq. Ž1. can be repeatedly obtained from monazites with relatively low ThO 2 contents ranging from 1 to 6 wt% ŽTable 1., and this relationship is analogous to the U–Pb calibration for zircon reported by Compston et al.

206

Pb

½ 5 ½ 5 238

U

sample

206

238

s

206

Pb U

P standard 270

aqbP

ž

ž

238

UOq 2 238 q U

Pbq Uq

/

/

sample

Ž 4.

sample

The actual  206 Pbr 238 U4 ratio of the standard is known and a and b are defined by the U–Pb calibration line from the standard. Thus the actual  206 Pbr 238 U4 ratio of the sample can be calculated from Eq. Ž 4 . for any pair of measured 238 q . Ž 206 Pbqr 238 Uq . sample and Ž 270 UOq U sample 2r

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310

ratios, and the sample age can then be readily obtained. However, it should be emphasised that when using Eqs. Ž2. and Ž3. it is assumed that there are no matrix effects on the secondary ion yields of U and Pb arising from the compositional deference between standard and sample. This fundamental assumption usually made in SIMS chronometry has never been fully addressed either for zircon ŽCompston et al., 1984, 1992. or monazite ŽHarrison et al., 1995; Vry et al., 1996., despite the fact that SIMS chronometry has been extensively used for more than one decade. One approach to address this issue is to compare the relationships obtained from the measurements of related secondary ions with those predicted from the calibration obtained assuming no matrix effects. It can be shown from Eqs. Ž1. and Ž2. that: 206

ž

270

Pbq

238

/

Uq

s ka q kb P sample

ž

UOq 2

238

Uq

/

Ž 5. sample

where a and b are the intercept and slope respectively of the standard calibration line and both are not zero, and 206

Pb

½ 5 ½ 5 238

ks

206

U

sample

Pb

238

U

standard

Eq. Ž5. implies that both the slope and intercept in 238 q plots of 270 UOq U versus 206 Pbqr 238 Uq for a 2r monazite sample are functions of age of the monazite analysed. They are expected to differ from those obtained for the standard by the same factor of k, which is a function of the ages of standard and sample. Thus k ) 1 when the sample is older than the standard, and k - 1 when the sample is younger than the standard, and k s 1 when the sample and the standard have the same age. Graphically, the calibration lines for monazite sample and standard originate from the same point OX of which the coordinate is Ž0, yarb.. If there are no matrix effects, the ratios of 270 238 q UOq U and 206 Pbqr 238 Uq obtained from 2r

monazites with different ages should satisfy Eq. Ž5.. 238 q U versus Also in plots of 2 7 0 UO q 2 r 206 q 238 q Pb r U , these data will fall on different lines according to the difference of the sample ages and all the lines originate from the same point OX Ž0, y ar b . . H ow ever, repeated analyses of 270 238 q UOq U and 206 Pbqr 238 Uq from the high 2r Th ŽThO 2 : ; 14 wt%, Table 1. rim of monazite a5 in sample 86004, which coexists in the same thin section with monazite a4, define a linear array which goes through the origin. Thus a s 0 ŽFig. 4b., and: 206

Pbq

238

Uq

238

sb P

270

Uq

UOq 2

Ž 6.

where b is the slope. The relationship expressed by Eq. Ž6. can be obtained repeatedly from different monazites with similar ThO 2 contents ranging from ca. 14 to 18 wt% ŽTable 1.. It is also consistent with expectations from the energy distributions of secondary ions sputtered from monazite with high ThO 2 contents ŽFig. 3b., which are very similar to those of 264 ThOq 2, 232 Thq and 208 Pbq ŽZhu et al., 1997a.. Clearly, Eq. Ž6. defined by measurements of high Th monazites is different from the expected Eq. Ž5. which is based upon the assumption that there are no matrix effects, and the calibration lines for high Th monazite and low Th monazite do not originate from 238 q the same point in plots of 270 UOq U versus 2r 206 q 238 q Ž Pb r U Fig. 4.. The inconsistency between the measured and the expected shows the existence of matrix effects on at least one of the secondary ion species of 206 Pbq, 238 Uq and 270 UOq 2 . More work is required to identify which species is most affected by the matrix. However, it is clear at this stage that the existence of matrix effects impose some limits on the U – Pb calibration of monazite using 270 238 q UOq U and 206 Pbqr 238 Uq ratios. 2r The implications of the matrix effects for dating may be discussed further. If we use the high Th rim of monazite a5 in sample 86004 as a standard to calculate the age of the low Th core of monazite a4 in the same thin section, this implies that, as for the data obtained from the rim of monazite a5, the 238 q ratios of 206 Pbqr 238 Uq and 270 UOq U ob2r tained from the core of monazite a4 are assumed to obey Eq. Ž6.. Accordingly, the data obtained from

X.-K. Zhu et al.r Chemical Geology 144 (1998) 305–312

the low Th core of monazite a4 will define a set of lines starting from the origin ŽFig. 5a.. Rather than giving a unique age Žor a group of ages which are

Fig. 5. Diagrams illustrating the inaccuracy imposed by the matrix 238 q effects on monazite U–Pb calibration using 270 UOq U and 2 r 206 Pbqr 238 Uq ratios. Ža. The high Th rim Ž2532"7 Ma. of monazite a5 in sample 86004 is used to calibrate the low Th core of monazite a4 in the same sample. In that case, it is assumed that the species of 206 Pbq, 238 Uq and 270 UOq 2 from both the low Th core and the high Th rim obey the same rules. In other words, 238 q the ratios of 206 Pbqr 238 Uq and 270 UOq U obtained from 2 r the core of monazite a4 are assumed to obey Eq. Ž6.. Accordingly these data will define straight lines starting from the origin which represent a range of ages bracketed by the lines O–A Ž2669"40 Ma. and O–B Ž2957"32 Ma.. Žb. The low Th core Ž2744"14 Ma. of monazite a4 in sample 86004 is used to calibrate the high Th rim of monazite a5 in the same sample. In that case, the ratios 238 q U obtained from the high of 206 Pbqr 238 Uq and 270 UOq 2 r Th rim are assumed to obey Eq. Ž5.. Accordingly these data X represent a range of ages which are bracketed by the lines O –A X Ž2514"38 Ma. and O –B Ž2669"25 Ma.. The coordinates of X point O are Ž0, y ar b ., where a and b are the intercept and slope of the standard calibration line, respectively Žsee a..

311

statistically indistinguishable. as should be the case, these lines represent a range of ages which are bracketed by the lines O–A and O–B ŽFig. 5a.. When taking the age of 2532 " 7 Ma defined by SIMS 207 Pbr 206 Pb chronometry and 208 Pbr 232 Th chronometry ŽZhu et al., 1997a. as the ‘true’ age of the rim of monazite a5, the ages obtained for the core of monazite a4 using Eq. Ž4. vary widely from 2669 " 40 to 2957 " 32 which are far beyond statistical errors. Alternatively, if the core of monazite a4 is taken as a standard to calibrate the rim of monazite a5, it implies that the ratios of 206 Pbqr 238 Uq 238 q and 270 UOq U obtained from the rim of mon2r azite a5 are assumed to obey Eq. Ž5.. Accordingly these data represent a range of ages which are bracketed by the lines OX –A and OX –B ŽFig. 5b.. When taking 2744 " 14 Ma defined by SIMS 207 Pbr 206 Pb chronometry and 208 Pbr 232 Th chronometry ŽZhu et al., 1997a. as the ‘true’ age of the ‘standard’, a range of ages between 2514 " 38 and 2669 " 25 Ma are obtained for the rim of monazite a5 using Eq. Ž4.. It is worth pointing out that the matrix effects do not necessarily result in systematic error. Both ages older and younger than the true age of a sample may be obtained, as has been shown here. The correct age would be obtained only fortuitously. The physical basis of matrix effects still remains poorly understood Že.g. Plog and Gerhard, 1987; Gerhard and Plog, 1987; Eiler et al., 1997., although the phenomenon is well known. As shown in Table 1, only the contents of ThO 2 and SiO 2 are significantly different between the two groups of monazites. It is of great interest for further investigations to identify whether matrix effects arise from the difference in ThO 2 content or in SiO 2 contents or in both. 4. Concluding remarks The extent to which the matrix effects described in this study can be generalised is not known at the present stage. These results however do demonstrate the existence of matrix effects which limit the accuracy of SIMS U–Pb calibration at least under certain working conditions for monazites. Thus the matrix effects cannot be assumed a priori to be insignificant for SIMS U–Pb or Th–Pb chronometry, and need to be thoroughly investigated. Moreover, this study pro-

312

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posed an approach which is of general significance in the investigation of matrix effects for SIMS U–Pb and Th–Pb analyses.

Acknowledgements We would like to thank Dr. N.S. Belshaw for his useful suggestion and fruitful discussion during the study. Dr. J.M. Eiler and Dr. S.J.B. Reed are thanked for their thorough and insightful reviews. This study was supported by NERC and the Royal Society.

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