A method for the quantitative measurement of rare earth elements in the ion microprobe

International Journal of Mass Spectrometry and Ion Processes, 69 (1986) 17-38 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

A METHOD FOR THE QUANTITATIVE MEASUREMENT EARTH ELEMENTS IN THE ION MICROPROBE

ERNST

17

OF RARE

ZINNER

McDonnell Center for the Space Sciences and Physics Department, St. Louis, MO 63130 (U.S.A.) GHISLAINE

Washington University,

CROZAZ

McDonnell Center for the Space Sciences and Earth and Planetary Washington University, St. Louis. MO 63130 (U.S.A.) (First received

1 July 1985; in final form 13 September

Sciences

Department,

1985)

ABSTRACT An ion probe method for the quantitative measurement of rare earth elements (REE) in phosphates is presented. The measurements are made with a Cameca IMS 3F ion microprobe. All complex molecular interferences are removed effectively with moderate energy filtering and the remaining mass spectrum is deconvoluted into contributions from REE and their monoxides only. Corrections for fluorides, which are present in fluorapatites, can be made. Sensitivity factors relative to Ca are determined from a terrestrial apatite standard. Comparison with other standards of known REE concentrations shows that working curves are linear over a wide range of concentrations. The method allows the measurements of all REE in spots of 5-20 pm in diameter. Detection limits depend on the overall REE pattern but are generally < 50 p,p.b. for the light REE and - 200 p.p.b. for the heavy REE. Preliminary measurements on oxides show that this technique can be extended successfully to other phases.

INTRODUCTION

Since it was hailed as the “ ultimate weapon” of the chemical geologist [ 11, the ion probe has led to considerable progress in the study of geological samples. Specifically, isotopic measurements of elements such as H, Mg, Si, Ti and Pb have been very successful (see, for example, refs. 2-8). Several investigators have applied the ion probe to the quantitative elemental analysis of minerals [9-131, but progress in this area has been limited. The presence of molecular and multiply charged ions in the secondary ion mass spectrum and the existence of matrix effects are major complications inherent in secondary ion mass spectrometry (SIMS). For these reasons, 016%1176/86/$03.50

0 1986 Elsevier Science Publishers

B.V.

18

SIMS applications have generally been restricted to the analysis of “pure” systems such as semiconductor materials implanted with various ions [14-181. Molecular interferences can be eliminated by high mass resolution or energy filtering. High mass resolution has been used for isotopic measurements [3,6,7] as well as for trace element analysis [lO,ll] and has been most successful for elements with an atomic mass of less than 60 [3,7,20] or for heavy elements like Pb [6]. Energy filtering, based on the difference in the energy distribution of atomic and molecular ions [21,22], has been successfully applied to the analysis of trace elements in geological samples [9,12,22-241. A modified form of energy filtering, the “isolated specimen technique” [25-291, suffers from a smaller loss of signal than conventional energy filtering [26] but requires a primary beam spot of more than - 50 pm diameter. Considerable efforts have been spent on the so-called local thermal equilibrium (LTE) model and derivatives thereof [1,30,31] to predict ion yields but none of these models has had any practical impact on the analysis of geological samples. The use of ion implantation to obtain an internal calibration for quantitative analysis [32-341 has great promise but its application to geological samples has been limited. The most successful method so far involves the use of standards to obtain relative sensitivity factors of the measured elements 19-121. This approach works well for trace elements in a uniform matrix (see, for example, ref. 13) but not for major elements whose sensitivities can be a strong function of the matrix composition [ll]. The major problems with this technique involve the difficulty of finding suitable standards and the large number of standards needed.

MEASUREMENTS

OF RARE EARTH

ELEMENTS

Because of the diagnostic importance of the rare earth elements (REE) in geochemistry, their study occupies a special place in trace element analysis of geological samples [35]. The two best established methods to measure REE abundances are neutron activation and mass spectrometric isotope dilution, which generally require a few mg of sample but have occasionally been used to analyze microgram aliquots. After neutron activation, the analysis is done either on a whole rock, without chemical separation (instrumental neutron activation analysis or INAA) or on the chemically separated REE fraction (radiochemical neutron activation analysis or RNAA). For measurement of the elements La, Ce, Nd, Sm, Eu, Tb, Dy, Yb and Lu, an accuracy of 2% or better can be achieved; many of these elements can be measured at the p.p.m. level and some at the p.p.b. level.

19

For the mass spectrometric isotope dilution method, light and heavy REE have to be separated chemically in order to reduce oxide interferences. This method is also very sensitive and particularly accurate. All the REE, with the exception of Pr, Tb, Ho, and Tm, which each have only one stable isotope, can be analyzed. The precision is l-2% for most REE but is somewhat less for La, Gd and Lu [36]. So far, electron and proton probe methods, which measure compositional variations between grains and crystal zoning, have been of limited usefulness for REE measurements. The electron microprobe sensitivity is of the order of 500 p.p.m., adequate only to measure a few REE in REE-rich phases such as terrestrial apatites or REE minerals. The principal limitations of this technique are low peak-to-background ratios and considerable overlap between X-ray peaks. The proton-induced X-ray emission method (PIXE) has already been shown to be more sensitive for the REE than the electron probe method [37]. This is due to the fact that the X-ray signal to beam-induced background ratio is much higher for heavy charged particles than for electrons. However, only Ce and Nd concentrations at the level of 100 p.p.m. in meteoritic merrillite were reported by Benjamin et al. [37], although they expect, with technical improvements, to lower the detection limit to 50 p_p.m. An X-ray fluorescence technique using synchrotron radiation is presently under development and offers great promise. It is based on the analysis of the K spectra of the REE. Rivers et al. [38] have recently indicated that all K lines from the REE can easily be resolved with a Si(Li) detector and detection limits of the order of 1 p.p.m. are expected. The potential capabilities of the ion probe for REE analysis were recognized some 10 years ago [l]. Both approaches, high mass resolution and energy filtering, have been used in REE ion probe studies. Reed and co-workers [39-441 employed an MRP of = 6000 to evaluate molecular interferences. Actual analyses were made at low mass resolution. The authors measured either interference-free peaks or corrected signals for interferences. A disadvantage of this approach is the fact that not all REE can be measured [43] and that the sensitivity is limited [41]. Energy filtering has been applied in the form of the specimen isolation (SI) technique by Metson and his team [25,26,29]. These investigators obtained the relative concentrations of all REE [26] from spectra which were virtually free of molecular interferences. However, primary current density and specimen conductivity cannot be well controlled and, in order to achieve the energy selection effect, the primary ion beam spot diameter has to be larger than 50 pm. We have developed an ion probe method for the analysis of REE based on energy filtering which avoids some of the above problems. The method was

20

first employed for the analysis of phosphates [45-481 but has also been applied successfully to oxides [49,50]. Measurements are performed with moderate energy filtering which removes complex molecular ions very effectively. The remaining mass spectrum contains, in addition to the REE ions, only simple oxides and in some cases fluorides of the REE, but can be deconvoluted into element and oxide components. A preliminary description of the method was given by Crozaz and Zinner [45]. The present paper provides the instrumental and computational details for the REE analysis in phosphates.

EXPERIMENTAL

The ion probe used for the measurements is a Cameca IMS 3F [19]. Samples are analyzed as polished sections. Commercial diamond paste, used in sample preparation, was found to be sufficiently clean for the measurements of REE [4]. Samples are coated with 1000-2000 A of Au. This relatively thick metal coating has the advantage that the beam spot sputtered through the Au film stays relatively small, resulting in fairly stable charging conditions of the analyzed area. For a thinner metal coat, the beam halo enlarges the sputter-cleaned area during the course of a measurement and this leads to increased sample charging. Samples are bombarded with an O- primary beam of either 14.5 or 17 keV energy focussed into a spot 5-25 pm in diameter. Typical beam currents range from 3 to 20 nA. Positive secondary ions are accelerated through a voltage of 4500 V and focussed by an immersion lens and a set of transfer lenses on to the entrance slit of the mass spectrometer [19,51,52]. The plane of the entrance slit is an image plane of the crossover or illumination pupil [52] and contains a series of beam-confining apertures (“contrast apertures”) in addition to the entrance slit. The instrument is equipped with three transfer lenses which, if used individually, give three different magnifications of the sample surface. The corresponding imaged fields are 25, 150, and 400 pm. This set-up has been modified so that any combination of the three transfer lenses can be energized simultaneously, allowing us to vary the magnification continuously. REE measurements are performed with an imaged field of - 75 pm. Under this condition, the image of the crossover in the entrance slit plane has a diameter of - 180 pm, smaller than the largest contrast aperture diameter of 400 pm [19]. In this plane, neither the aperture nor the entrance slit are used to reduce the beam. As a consequence, the angular energy distribution of the secondary ion beam is defined entirely by the acceleration space and extraction hole of the secondary ion collection optics [52].

21

(b)

Offset

of Sample

Voltage

IV

I

Fig. 1. Energy distributions of atomic and molecular secondary ions sputtered from terrestrial apatite and extracted into the mass spectrometer of the Cameca IMS 3F ion probe. (a) “Ca+ ion signal vs. the sample voltage offset with the energy slit opened to 1 V (curve A) and 32.5 V (curve B). The selected energy window under energy filtering condition is indicated in curve A. (b) Energy distribution of the ion signal at atomic mass 159 (solid line). The ion signal consists of contributions from a Ca,P’60; and Tb+ (broken lines). At zero voltage offset, the molecular ion signal dominates but falls off steeply with increasing energy filtering. From a voltage offset of - 50 V on, the signal is almost entirely from Tb+ (the Tb concentration in this sample is 30.4 p.p.m.). The additional contribution seen between -35 and -55 V is probably from CeF+.

In certain cases, when small phosphate grains imbedded in a foreign matrix are analyzed, a field aperture close to the image plane of the sample surface is inserted into the ion beam to accept ions only from a restricted area (diameters 10 or 15 pm) of the sample surface. This procedure proved advantageous over the use of a small primary beam spot which would have resulted in unacceptably low ion signals in samples with low REE concentrations. Energy filtering is obtained by offsetting the sample high voltage from the nominal 4.5 kV while maintaining the voltages of the electrostatic analyzer and the position of the energy slit at values for the transmission of ions of a total energy of 4.5 keV. The width of the energy slit is set to accept a 32.5 V window. Figure l(a) shows the energy distribution of 44Ca+ secondary ions if the energy slit is almost closed (curve A). Widening of the slit to 32.5 V results in a wider distribution (curve B), which is the convolution of the energy distribution with the energy window. For the REE measurements, a voltage offset of - 100 V is chosen relative to the voltage at which this latter distribution (B) drops to 10% of the maximum. Since the distribution drops off sharply at low energy, the edge voltage can be determined accurately and does not depend on the exact voltage offset at which the maximum is measured. Figure l(a) shows the energy window of the original energy distribution (curve A) selected under the above conditions. The restriction to

22

an energy window of 32.5 V causes a loss of signal of 40% for the REE when compared with a maximum window of 120 V. However, for such a wider window, it would not have been possible to cover the voltage range required for determining the low energy edge and for providing the required offset in the present instrument. The exit slit is set to its maximum opening, resulting in an MRP of - 500. Secondary ions are detected by an electron multiplier in a pulse counting mode. Counts are corrected for dead time determined from measurements of isotopic ratios of a Ti metal standard. The deadtime was typically 20 ns, leading to a correction of 2% for a count rate of lo6 counts s-‘. Measurements are performed by cycling through a series of mass peaks in an automatic peak jumping mode. The Cameca IMS 3F ion probe is interfaced with a Hewlett Packard HP 9845 computer [65] which controls various instrument functions, notably the sample high voltage offset and the magnetic field of the mass spectrometer. The measurement program was written for automatic operation but retains the possibility of monitoring the progress of the run. The operator’s role.is restricted to the selection of the spot to be analyzed and the tuning of the secondary ion beam sputtered from the selected area. During automatic peak jumping, the positive ion signals of 0, F, P, Cl, Ca and all the REE and their oxides are measured. Special attention has to be paid to possible hysteresis effects of the magnetic field control, to the proper calibration of the magnetic field settings as a function of atomic mass, to possible sample charging during the course of a run, and to the calibration of the ion conversion efficiency in the electron multiplier. MEASUREMENT PROGRAM

Before the start of an analysis, the operator checks whether the mass calibration is accurate. If not, a new mass calibration array is obtained by measuring an appropriate standard which yields ion signals at the desired masses. It is important to cycle through the same set of masses for calibration and analysis in order to minimize hysteresis effects. In either case, the magnet is conditioned through a series of hysteresis cycles before measurement. Following the hysteresis loops in the analysis program, the operator obtains a local mass calibration after which the apparent energy distribution of 48Ca+ ion is measured with a 32.5 V window. The program determines the maximum of the distribution and the lower energy edge (10% of maximum). This edge is the reference for the sample voltage offset and is tracked during each cycle of the run in order to compensate for possible sample charging. Once the sample voltage is offset, the operator has another opportunity to tune the primary and secondary beam. This is necessary since a change of

23

the sample voltage causes a slight shift of the obliquely incident primary beam and since the crossover of high energy secondary ions differs from that of low energy ions [52]. After another series of 5 hysteresis loops, the program finds the center of the 160, 48Ca and lNCe peaks for a last mass calibration correction. The edge of the energy distribution is measured once more before preliminary count rates are obtained for all masses. On the basis of these values, the program decides on how long it will count on each mass and how many cycles (between 10 and 30) it will use to obtain the desired counting statistics. Runs last between 0.5 and 3 h depending on concentration levels and measurement conditions (primary beam current and field aperture used) which, in turn, depend on the size of the sample to be analyzed. At the end of a run, a preliminary REE pattern is calculated for monitoring purposes. The data are stored for subsequent, mo’re extensive, analysis.

AUXILIARY

PROGRAMS

(1) The mass calibration array is obtained in a program which automatically determines the magnetic field, Bi, for the centers of 20 selected mass peaks, m,. The relationship m = aB 2 is not exactly satisfied in the instrument and a correction is obtained by choosing parameters rni for each mi so that rni = aB2 for a constant a. The accuracy of the calibration is improved if a linear interpolation in the mi/rn; vs. fi curve is made as compared with a linear interpolation in the rnj vs. m, curve. (2) The registration efficiency of ions in the electron multiplier (EM) is a function of the element detected, but also varies with parameters like the EM voltage, pre-amplifier gain and discriminator threshold. Since, even under constant conditions, the EM gain changes with time, the registration efficiency has to be measured periodically. This is accomplished by comparing

TABLE

1

Conversion Element

efficiency

of REE+

Conversion efficiency

ions in the electron Element

87.7 88.4 88.4 87.2 84.4

Conversion efficiency

relative to 40Ca+ Element

@,>

(%) La Ce Pr Nd Sm

multiplier

+ + + * f

1.1 0.9 0.8 0.7 0.6

EU Gd Tb DY

80.6 83.3 82.4 83.5

Conversion efficiency ($1

+ f f +

0.8 0.9 0.8 1.2

Ho Er Yb LU

83.7 + 0.9 82.4kO.7 78.8 k 0.6 78.2kO.6

24

the count rates measured in the EM with the current measured in a Faraday cup (FC). Measurements are performed at a count rate of lo6 counts s-l. Secondary ions of the elements 0, Al, Si, Ca, Y and all the REE are obtained from a series of standard glasses [53] which have a sufficiently high REE concentration that a signal of lo6 counts s-l can be obtained. Although absolute ion registration efficiencies vary considerably with time, efficiencies of the REE relative to Ca were found to be fairly constant. Table 1 gives the REE+ registration efficiencies relative to 40Ca+ in the electron multiplier. During the course of our measurements, these relative efficiencies changed by less than 3% and no corrections for these changes were applied. DECONVOLUTION

The analysis method described here is based on the fact that the energy filtering effectively removes all complex molecular ions interfering with the REE. This is demonstrated in Fig. 2 which shows two mass spectra in the atomic mass range 140-180 measured in an apatite grain of the meteorite St. Severin with and without energy filtering. The spectra are normalized to the number of counts at mass 140 (lNCe). It is evident that, without energy filtering, the ion signals at most masses are dominated by molecular interferences. Most pronounced are the signals of 40Ca,‘60, at mass 152, 40Ca,P’60, at mass 159 interfering with ‘59Tb (100% of Tb is “‘Tb) and 40Ca,P’60, at mass 175 interfering with 175Lu (97% of the Lu). However, energy filtering reduces these molecular ion signals by more than five orders

lS0

160

170

180

Mass Number

Fig. 2. Positive ion mass spectra of St. Severin apatite measured without (open bars) and with (solid bars) energy filtering. For comparison, the two spectra are normalized to 1000 counts at mass 140 (lace+ ).

25

of magnitude compared with the REE ion signals. In this St. Severin apatite grain, the Tb and Lu concentrations are respectively 0.8 and 0.4 p.p.m. From the overall REE pattern of this sample, one finds that the molecular interferences at masses 159 and 175 are less than 15% of the elemental signals. Without energy filtering, the interferences give up to 10000 times as large a signal as the elements Tb and Lu themselves. How sharply the energy distribution of ““CaZP’603 falls off can be seen in Fig. l(b). Extrapolation of the curve indicates that, under the standard energy filtering conditions, the contribution of this molecular ion is less than the equivalent of 50 p.p.b. Tb. In phosphates, the only molecular ions in the mass range of the REE which are not suppressed by energy filtering below equivalent concentration levels of 50 p.p.b. are monoxides and fluorides. Fluorides make a noticeable contribution only in apatite samples with high fluorine concentrations and pronounced light rare earth (LREE) enrichments, but corrections can be made (see below). Hydride, hydroxide and chloride interferences were found to be so low in all cases that they could be neglected. As a result, after correction for fluoride, the mass spectrum from 133 to 191 can be deconvoluted into contributions from the elements Cs to Hf and the monoxides CsO to LuO. The measured count rate, C,,,, at a given mass 111can be written as a linear superposition of these components

i=l

where Ii is the intensity (signal) of the component i (element or oxide) and ai is the isotopic abundance of this component at mass m. The 59 equations for C,,, form an overdetermined system for the 33 unknown intensities Ii. A least squares fit with the requirement

cm-&(Ji X*=X

i

mi

%I

1 2

+min

(1)

yields 33 linear equations

for the Ijs. The a,,, are the measurement Ijs are found from Ii = c b;;B, k

errors on C,,,. The solutions for the

(3)

26

with

(4) and

The uncertainties

SIj =

on the components

I, determined

in this way are

J

(6)

where bijl is the inverse of the matrix bkj and a, are the counting statistical errors. The chi square (x2) of Eq. (1) is a measure of the goodness of the fit; a large x2 should alert the experimenter that additional components (interferences) exist that have not been taken into account or that the actual errors are larger than assumed. For most of our measurements, x2 values were consistent with the assumption of no additional interferences and of counting statistical errors. In a few cases (analysis of lunar phosphates with REE/chondritic values as high as 104), x2 values were much larger. Since interferences can be excluded in these cases, it was probably the fact that isotopic mass fractionation inherent in SIMS analysis [54-571 was neglected which caused large x2 for samples with high count rates. It should also be

TABLE 2 Oxide-to-elemental ion ratios (in 4%) of the REE measured in whitlockite 14321 with different values for the sample voltage offset V& MO+/M+

Voff -

Lao/La CeO/Ce PrO/Pr NdO/Nd SmO/Sm DYO/DY HoO/Ho ErO/Er TmO/Tm YbO/Yb LuO/Lu

of lunar

125 v

13.6kO.5 17.2kO.4 13.9 + 1.2 11.3+1.2 5.5k1.6 5.8kO.4 5.3kO.4 6.0 k 0.4 5.3 + 0.9 4.4kO.8 6.5k1.5

-loov

-75

v

23.1 f 0.2 32.4 k 0.2 23.3 + 0.4 18.3+0.4 9.3+0.5 9.7 +0.1 9.3 & 0.2 9.5 f 0.2 7.7kO.3 6.2 + 0.3 9.8+2.1

42.8 * 0.4 58.6 + 0.3 42.6 + 0.6 33.3 + 0.6 17.2 f 0.8 19.4+0.3 18.8fO.3 18.7kO.4 14.9 +0.8 12.3 & 1.0 15.5 +4.5

sample

27 TABLE

3

Ion ratios of different REE Durango) as function of the The ratios are normalized contributions from Tb+ and

-160 -140 -120 -100 -80 -60

0.94 0.94 0.97 =l 1.03 1.12

ion signals relative to Ce+ measured in terrestrial apatite (from sample voltage offset, Vorr to the values at Vorr = - 100 V. The “Tb” signal consists of CeF+.

Pr+

Nd+

Sm+

ce+

Ce+

Ce+

0.98 0.96 0.98 cl

0.98 0.96 0.98 =l

1.02 1.06

1.03 1.10

0.95 0.94 0.97 ZG1 1.04 1.08

Dy+ Ce+

,Tb+, Ce+

1.09 1.00 1.04 cl

0.55 0.60 0.81 cl

1.06 1.25

1.61 2.80

noticed that, at high count rates, fluctuations of the secondary ion signal are greater than those expected from counting statistics alone. With the exception of the elements Yb and Lu, and to a lesser degree Gd, for which the isotopic abundances make the deconvolution difficult if not impossible and which will be discussed in more detail, the above algorithm yields all the elemental and monoxide components of the REE. Measurements were performed to determine the relative variation of these components with the degree of energy filtering. Table 2 shows the oxide-to-element ion ratios measured in a lunar phosphate (whitlockite) as function of the sample voltage offset, V,,,. As expected, the ratios vary strongly with energy filtering. In contrast, the contributions for ions of the different REE elements depend only slightly on the voltage offset (Table 3). This means that the inferred REE pattern does not depend sensitively on the degree of energy filtering, especially for larger absolute values of J&_ SPECIAL

PROBLEMS

Fluorides In apatites with high fluorine concentration, the fluoride contributions cannot be neglected. Most pronounced is the interference of 14CeF with ‘59Tb. In the Durango apatite, which has a Ce/Tb ratio of 280, the ““‘CeF+ signal amounts to 60% of the signal at mass 159. Whereas most elemental signals relative to Ce+ stay constant as a function of V,,, (Table 3), the signal at mass 159, which would normally be attributed to 159Tb+, varies strongly with energy filtering relative to Ce+. Figure 3 shows the mass 159 signal normalized to l”Ce+ as a function of CeO+/Ce+. The advantage of

vottC~’

O&95-

/

/ z-100

Vott

t

/

O.OW-

“‘AX /

‘%eF*/ce’

-1 I

0005-

/

Oo

I

I

I

a1

a2

0.2

/I, a4

1 b*Ge’ , a5

oa

,

,

,

47

0.5

a9

0

CeO+/Ce+

Fig. 3. Ion signal at mass 159 relative to the Ce+ signal versus the CeO+/Ce+ ratio measured in Durango apatite by applying different degrees of energy filtering (V,, ranges from - 60 to -200 V). Whereas the CeO+/Ce+ ratio goes to zero with increasing energy filtering, the normalized mass 159 ion signal approaches a finite value (Tb+/Ce+ ) as the CeF+ component is removed.

this plot is that it can be extrapolated to CeO+/Ce+= 0, corresponding to infinite energy filtering, at which point the lNCeF+ contribution will also be approaching zero. This allows one to determine the 14CeF+ interference as a function of I&,. A correction was made for all measured samples by assuming the rNCeF+ signal relative to the Ca+ signal to be proportional to the Ce+/Ca+ and F+/Ca+ ratios for a given l&r value and determining the correction constant from measurements on the Durango apatite. Interferences from other fluorides are smaller (in Durango apatite, 139LaF is 50% of i’*Gd and PrF is 6% of 16’Gd), but were also corrected for by assuming that MF+/Ce+ is proportional to MO+/Ce+ for any element M. Of all the samples analyzed by us, the Durango apatite, with an F concentration of 3.5% and a steeply decreasing REE pattern, was by far the worst case of fluoride interferences. In the apatite used as standard, the CeF+ interference

29

is - 20% of the apparent 159Tb+ signal; in all other samples, it was less. Meteoritic apatites measured so far have REE patterns which do not decrease like those in the terrestrial apatites and much lower F concentrations and, as a consequence, the correction for the CeF+ contribution was less than 5% of the Tb+ signal. The elements Yb, Lu and Gd

Problems are encountered during the mass spectral deconvolution of the isotopic abundance patterns of these elements.

because

Ytterbium

This element has seven isotopes with masses and abundances given in Table 4. The 160 monoxides of Gd fall at the same masses and have very similar abundances. A way to visualize the problem of deconvolution is to consider the Yb and the GdO components as two vectors in a 7-dimensional space. The deconvolution of the measured mass spectrum then amounts to the decomposition of the “measurement vector” (the count rates at the seven masses in Table 4) into two vectors parallel to the “basis vectors” Yb and GdO. However, the angle between the Yb and GdO vectors is only 14.2”, which means that even small errors in the count rates at the masses of the Yb isotopes result in a relatively large error in the Yb component. To reduce this error in samples with low REE concentrations, we subtracted the GdO+ contribution by using the GdO+/Gd+ ratio measured in the terrestrial apatite standard (15.8 f 0.7%).

TABLE 4 Isotopic abundances of Yb and Gd”‘O in % The last two columns give the abundances normalized as unit vectors in the 7-dimensional space spanned by the masses in the first column. The dot product of these two vectors is the cosinus of the angle between them. Mass 168 170 171 172 173 174 176

Yb 0.14 3.0 14.3 21.8 16.1 31.8 12.7 Dot product

Gd16 0

Yb

Gd160

0.2 2.2 14.7 20.5 15.7 24.9 21.9

0.003 0.065 0.311 0.473 0.350 0.691 0.278 0.9634

0.004 0.049 0.330 0.460 0.352 0.559 0.491

30

0.040.03

3

I

I

1

I

I

I

4

5

6

7

8

9

10

EIMo IeVI

Fig. 4. Oxide-to-element ion ratios of the REE plotted as a function of the modified linear relationship seen dissociation energy EL, of the oxide molecule MO. The approximate between log(MO+/M+) and EL, for EL, > 6 eV is used to obtain the TbO+/Tb+ ratio by interpolation.

Lutetium

The situation is even worse for Lu: 97.4% of this element is 175Lu, but ‘59Tb0 falls at the same mass. Since Tb has only one isotope, it is impossible to decompose the signal at mass 175 into Lu and TbO. On the other hand, at mass 176, contributions from other components are dominating (in the terrestrial apatite standard, the signals of 176Hf+, 176Yb+, r6’GdO+ and 16’DyO+ are 1.3, 39, 90 and 3.5 times that of ‘76Lu+). For this reason, the signal at mass 175 must be used and ‘75Lu obtained by subtracting the TbO+ contribution which is determined from the Tb+ signal and the TbO+/Tb+ ratio. Since the TbO+/Tb+ ratio cannot be measured, it was obtained by interpolation from a relationship between the oxide to elemental ion ratios and the modified oxide dissociation energy E’ MO

=

EMO

+

IM

-

IMO

Here, EM0 is the oxide dissociation energy, IM the first ionization potential Of the element and IMo that of the oxide. Reed [40] observed an approximate exponential relationship between MO+/M+ and EL0 for REE in a silicate matrix. Figure 4 shows a plot of the logarithm of MO+/M+, measured in the terrestrial apatite standard under our normal instrumental conditions (V,, = -lOO), vs. E&,. With the exception of La, a good linear relationship is exhibited for EL0 values above 6 eV. At 6 eV, there is a break in the slope of the line. By interpolation, the TbO+/Tb+ ratio was determined to be 13.7 + 2.0% as indicated in the graph.

31 TABLE

5

Isotopic

abundances

in % of Gd, CeO and NdO at the atomic masses 156, 158 and 160

Mass

Gd

CeO

NdO

156 158 160

20.5 24.9 21.9

88.5 11.1 0

0 27.1 23.6

Gadolinium

Of the seven Gd isotopes, iS2Gd and is4Gd have abundances of only 0.2 and 2.2% and are therefore not useful for the determination of the concentration of this element. The isotopes “‘Gd and 15’Gd cannot be used either since masses 155 and 157, respectively, see the contributions of 99.9% ‘39La0 and 100% 14’Pr0. The abundances of the remaining Gd isotopes at the masses 156, 158 and 160, as well as those of CeO and NdO, are shown in Table 5. The count rates at these masses have to be decomposed into the components Gd, CeO and NdO. The problem is that the three vectors representing these components are almost coplanar. The volume of the parallelepiped spanned by unit vectors in the direction of the Gd, CeO and NdO components at the masses 156,158 and 160 is only 0.047. How well the Gd can be determined depends on the NdO determination from masses 161, 162, 164 and 166. To improve the precision of the Gd concentration measurement in samples with low REE concentrations, we made a correction by assuming a fixed pattern of the MO+/M+ ratios for the elements La, Ce, Pr, Nd and Sm. This pattern was determined with high precision by measurements on the terrestrial apatite standard. The least squares fit was then obtained by adjusting an overall multiplication factor for the oxide/element pattern.

QUANTIFICATION

The deconvolution method yields the relative ion signals of all the REE. In order to convert these relative intensities to elemental concentrations, we determined the ion yields of the REE relative to a major element in a terrestrial phosphate standard of known REE concentrations. Calcium was chosen for the reference element since around V,,, = - 100 V, the Ca+ signal relative to Ce+ changes less as a function of the sample voltage offset than do the O+ and P+ signals. Our standard was a terrestrial apatite (from a gabbronorite) graciously provided by R. Dymek. REE concentrations were measured by isotopic dilution [66]. Neutron activation (INAA) analysis [67] confirmed some of

32 TABLE 6 Element

La Ce Pr Nd Sm Eu Gd Tb DY Ho Er Tm Yb LU

Cont. (p.p.m. wt.)

Source a

610 1610 216 1130 250 46.8 202 30.4 154 29 66.5 8.1 42.9 5.9

2 192 3 1,2 192 172 1 2 1 3 1 3 1,2 2

REE+/ Ca+ (X103)

Std. dev. (W)

Sensitivity factor 4

(45)

Ion yield b REE+/ Ca+

Ion yield ’

33.0 78.9 12.9 68.2 17.8 3.9 13.3 1.32 7.70 1.28 2.80 0.33 1.51 0.19

12.0 8.8 7.6 5.9 4.3 4.3 4.0 7.7 3.9 4.6 4.4 6.1 6.0 8.1

3.37 3.73 3.05 3.02 2.56 2.20 2.80 4.18 3.64 4.12 4.31 4.37 5.21 5.81

3.3 2.8 2.7 2.5 2.4 2.4 2.4 5.7 2.4 2.4 2.4 2.6 6.1 15.3

0.74 0.67 0.82 0.85 1.05 1.23 1.00 0.68 0.79 0.71 0.69 0.69 0.59 0.53

0.38 0.36 0.44 0.50 0.63 0.81 0.30 0.30 0.41 0.30 0.52 0.38 0.33 0.12

Error

a 1 = isotope dilution; 2 = INAA; 3 = interpolation. b Measured in apatite standard. ’ Measured in silicate glass [40].

these values and provided additional REE determinations (Table 6). The abundances of Pr, Ho and Tm were obtained by interpolation. From the REE concentrations and ion signals relative to Ca+, we obtained sensitivity factors I;1 which were used to calculate the REE concentrations [REE,] of the unknown samples. [REE,] = qs

[CaO]

The REE concentrations [REE,] and the Ca oxide concentration [CaO] are measured in weight fraction (or p.p.m. weight). In Table 6, column 4 contains the REE,?/Ca+ ratios measured in the apatite standard and column 6 the sensitivity factors determined from these measurements. Column 8 gives the secondary ion yields of the REE relative to Ca in the phosphate matrix. These values differ in their absolute magnitude from the ion yields determined by Reed [39,40] in Ca-Al silicate glasses which are given in the last column of Table 6. While a difference in magnitude can be expected, since the ion yields in silicate were measured without energy filtering, the patterns of the ion yields also differ significantly, especially for Eu and Gd, and the elements Er and Lu (Fig. 5).

33

18 -

i

0.20

““““““’

La Co R Nd SmEu Gd Tb Dy Ho Er TmYb

_ ’

Lu

Fig. 5. Ion yields of the REE relative to Ca measured in the terrestrial apatite standard w. Also plotted are measurements by Reed (39,401 in Ca-AI silicate glasses (0). The latter are normalized so that the Ce+/Ca+ ratio is the same as that of the phosphate.

The relatively high absolute ion yields of all the REE (Ca is an element with a very high ion yield [58]) is the reason that detection limits are low even with the loss of signal due to energy filtering. With V,,, = - 100 V, the La signal is 0.25 counts (s X p.p.m. La X 1 nA)-’ primary O- beam current. This means that detection limits are < 50 p.p.b. if only the LREE are measured and the crystal to be analyzed is large enough ( - 50 pm) so that a 50 nA O- beam can be used. The REE+/Ca+ ratios in column 4 of Table 6, which are the basis for quantification, are the averages of 26 measurements on different phosphate grains. The standard deviations of these measurements in % are given in column 5. Note that the standard deviations of the lightest REEs, especially La, are much larger than those of elements like Gd, Dy and Er. Since the intrinsic precision of the measurement of La, Ce and Pr is much higher than that for these heavier elements, we conclude that a variation of the concentration of the light REE between different grains and not measurement uncertainties is probably the reason for the larger scatter of the measurements. Grains were selected at random and, if we assume that they are a representative sample of the phosphate, the errors on the averages are less than 2-3%. The errors from counting statistics are automatically calculated in the deconvolution program from Eq. (6) and range from < 1 to - lo%, depending on the REE concentrations and the element analyzed. Additional errors result from uncertainties of the CaO concentration, which is used as a

34

reference (Eq. 7) in the standard and the analyzed sample. The largest possible uncertainties are systematic errors stemming from the correction procedures. The fluoride correction introduces an error in the Tb determination. In our terrestrial apatite standard, the lNCeF+ correction is 25% of the Tb, with an estimated error of < 5%. The error on the Yb concentration depends on that of the GdO+/Gd+ ratio and on the Gd/Yb ratio in the sample analyzed. In the apatite standard, a la error of 0.7% on the GdO+/Gd+ ratio of 15.8% gives rise to a 5.6% error in the Yb determination. In a sample whose REE abundances differ from chondritic abundances by a constant factor (“flat REE pattern”), this error would be only 1.5%. Finally, our estimated error of 2.0% in the estimated TbO+/Tb+ ratio of 13.7% results in an error of 14.2% on the Lu concentration. With an estimated error of 5% on the Tb concentration itself, that amounts to 15.1% on the Lu determination. Column 7 in Table 6 gives the combined la errors on the determination of the different REE in the apatite standard. These are therefore the errors on the sensitivity factors Fi and the systematic errors made in the analysis of an unknown sample. It is important to obtain calibration data from other samples to check those from the terrestrial apatite standard and to find out over what range of concentrations the sensitivity factors are valid, or, in other words, to determine whether or not there is a linear relationship between the REE concentrations in the sample and the concentrations measured in the ion probe. For this purpose, we have measured REE in phosphates of three additional samples for which REE abundance data obtained by other methods are available: terrestrial apatite from Durango [59], merrillite from

TABLE 7 Comparison of ion probe measurements REE concentrations are in p.p.m. wt. Element

La Ce Nd Sm Eu Tb DY Yb Lu

Durango

in phosphates

with measurements

Angra dos Reis

67975

by other methods

INAA

Ion probe

INAA

Ion probe

Electron probe

3800 4350 1180 153 15.5 15.7

4230 4700 1095 143 15.2 17.6

370 750 370 72 20.5 11.5 66 20 2.4

377 660 318 68 17.6 10.6 63 15.7 1.8

9600 22800 15000 3800

10600 28100 16900 4500

5350

5390

36.4 4.26

44.4 4.8

Ion probe

35

the meteorite (achondrite) Angra dos Reis, and whitlockite from the Apollo 16 lunar sample 67975 [60]. The REE concentrations of Durango apatite had been determined before [42] but we used more recent INAA results [68] obtained from an aliquot of the crystal measured in the ion probe. In Angra dos Reis merrillite, Ma et al. [61] determined the abundance of nine REE by INAA and their Nd and Sm data were confirmed by isotope dilution measurements [62,63]. In the 67975 whitlockite, which has extremely high REE abundances [47], the concentrations of the LREE and of Dy could be measured by electron microprobe [69]. The comparison with the ion probe determination is shown in Table 7. It can be seen that the linear relationship extends over a large range of concentrations and that deviations are usually less than 15%. CONCLWION

AND OUTLOOK

An ion probe method for measuring the REE concentrations in ng samples has been developed. This method uses energy filtering and is based on the deconvolution of the mass spectrum in the REE mass region into contributions from elements and simple oxides. The precision of the method is generally better than 10%. Down to - 10 X chondritic abundances, all the REE can be determined; measurements of the LREE can easily be done down to - 1 X chondritic. The method was developed for phosphates, but has already been applied to other phases. Fahey et al. [49] used it for the measurement of REE in hibonites from the meteorites Murchison and Murray. For lack of a hibonite calibration standard with known REE concentrations, these authors used the sensitivity factors Fi determined from the apatite standard (Table 6). Figure 6 shows the REE pattern measured in a Murchison hibonite. The fact that this pattern matches one of the classes defined by Martin and Mason [64] can be taken as evidence that the relative ion yields between REE are the same as those in phosphates. Similar REE determinations were made in perovskites of the meteorite Efremovka [50]. And, again, while the determination of the absolute values of the sensitivity factors has to await measurements on a terrestrial perovskite calibration standard, the uniformity of REE pattern suggests that the relative ion yields of the REE are the same as for phosphates also in this oxide phase. The method is presently being extended to other minerals which contain REE. Another natural extension of the REE analysis method is the measurement of other trace elements of geochemical interest. Strictly speaking, this is not an extension of the REE method since the unique abundance pattern of REE makes the deconvolution probably necessary only for these elements. For other trace elements, it has to be decided in each case (i.e. for each

36

I:)/ ----v--v 100

; ; : ; LaCePrNd

1 1 I I I I

I I I I

SmEuGdTbDyHoErTmYbLu

Fig. 6. Rare earth concentrations relative to chondritic measured by ion probe in a hibonite grain from the CM chondrite Murchison. For the concentration determination, relative sensitivity factors measured in the phosphate standards were used.

particular matrix) which interferences are present and which method should be used to eliminate them. And again, as for the REE, measurements on well-characterized calibration standards will have to be performed to determine the sensitivity factors of the desired elements in the analyzed matrix. ACKNOWLEDGEMENTS

Robert F. Dymek generously provided the apatite standard and the following individuals shared some of their unpublished results: L. Peter Gromet, Odette James, Randy Korotev and Marilyn Lindstrom. The efforts of Albert Fahey and Kevin McKeegan in keeping the ion probe at its high level of performance are gratefully acknowledged. Esther Koenig helped in the manuscript preparation and NASA (NAG9-55) and NSF (EAR8415168) provided support. REFERENCES 1 J.F. Lovering, Natl. Bur. Stand. (U.S.) Spec. Publ., 427 (1975) 135. 2 S.R. Hart, N. Shimizu and D.A. Svejensky, Econ. Geol., 76 (1981) 1873. 3 J.C. Huneke, J.T. Armstrong and G.J. Wasserburg, Geochim. Cosmochim. Acta, 47 (1983) 1635. 4 I.D. Hutcheon, in L.A. Curie (Ed.), Nuclear and Chemical Dating Techniques: Interpreting the Environmental Record, American Chemical Society, Washington, DC, 1982, p. 95. 5 R.N. Clayton, G.J. MacPherson, I.D. Hutcheon, A.M. Davis, L. Grossman, T.K. Mayeda, C. Molini-Velsko and J.M. Allen, Geochim. Cosmochim. Acta, 48 (1984) 535. 6 W. Compston, I.S. Williams and C. Meyer, J. Geophys. Res., 89 (1984) B525. 7 A. Fahey, J.N. Goswami, K.D. McKeegan and E. Zinner, Astrophys. J., 296 (1985) L17. 8 K.D. McKeegan, R.M. Walker and E. Zinner, Geochim. Cosmochim. Acta, 49 (1985) 1971.

37 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47

N. Shimizu, M.P. Semet and J.C. All&e, Geochim. Cosmochim. Acta, 42 (1978) 1321. S.J.B. Reed, Scanning, 3 (1980) 119. I.M. Steele, R.L. Hervig, I.D. Hutcheon and J.V. Smith, Am. Mineral., 66 (1981) 526. G. Ray and S.R. Hart, Int. J. Mass Spectrom. Ion Phys., 44 (1982) 231. N. Shimizu and S.R. Hart, Annu. Rev. Earth Planet. Sci., 10 (1982) 483. P. Williams, Appl. At. Collision Phys., 4 (1983) 327. E. Zinner, Scanning, 3 (1980) 57. C.W. Magee, R.E. Honig and C.A. Evans, Jr., Secondary Ion Mass Spectrometry, SIMS III, Springer-Verlag, Heidelberg, Berlin, New York, 1982, p. 172. W. Katz and G.A. Smith, Scanning Electron Microscopy, IV (1984) 1557. N.H. Turner, B.I. Dunlap and R.J. Colton, Anal. Chem., 56 (1984) 373R. M. Lepareur, Rev. Tech. Thomson-CSF, 1271980) 225. I.D. Hutcheon, J.T. Armstrong and G.J. Wasserburg, Lunar Planet. Sci., XVI (1985) 384. Z. Jurela, Int. J. Mass Spectrom. Ion Phys., 18 (1975) 101. N. Shimizu, Earth Planet. Sci. Lett., 39 (1978) 398. N. Shimizu, Secondary Ion Mass Spectrometry, SIMS II, Springer-Verlag, Berlin, Heidelberg, New York, 1980, p. 62. A. Havette and G. Slodzian, Secondary Ion Mass Spectrometry, SIMS III, Springer-Verlag, Berlin, Heidelberg, New York, 1982, p. 288. J.B. Metson, G.M. Bencroft, N.S. McIntyre and W.J. Chauvin, Surf. Interface Anal., 5 (1983) 181. J.B. Metson, G.M. Bancroft, H.W. Nesbitt and R.G. Jonassen, Nature (London), 307 (1984) 347. N.S. McIntyre, D. Fichter, J.B. Metson, W.H. Robinson and W.J. Chauvin, Surf. Interface Anal., 7 (1984) 69. N.S. McIntyre, W.J. Chauvin, J.B. Metson and G.M. Bancroft, J. Phys. (Paris) Colloq. C2, 45 (1984) 143. J.B. Metson, G.M. Bancroft and H.W. Nesbitt, Scanning Electron Microsc., in press. C.A. Andersen and J.R. Hinthorne, Anal. Chem., 45 (1973) 1421. D.E. Newbury, Scanning, 3 (1980) 110. E. Zinner and R.M. Walker, Proc. 6th Lunar Sci. Conf., Geochim. Cosmochim. Acta, Suppl. 6 (1975) 3601. V.R. Deline, W. Katz, C.A. Evans, Jr. and P. Willi’lms, Appl. Phys. Lett., 33 (1978) 832. D.P. Leta and G.H. Morrison, Anal. Chem., 52 (1980) 277. P. Henderson (Ed.), Rare Earth Element Geochemistry, Elsevier, Amsterdam, 1984. G.N. Hanson, Annu. Rev. Earth Planet. Sci., 8 (1980) 371. T. Benjamin, W.R. Heuser and D.S. Burnett, Proc. 9th Lunar Planet. Sci. Conf., Geochim. Cosmochim. Acta, Suppl. 10 (1978) 1393. M.L. Rivers, I.M. Steele and J.V. Smith, Eos, 66 (1985) 401. S.J.B. Reed, in R.H. Geiss (Ed.), Microbeam Analysis 1981, 1981, p. 87. S.J.B. Reed, Int. J. Mass Spectrom. Ion Processes, 54 (1983) 31. S.J.B. Reed, D.G.W. Smith and J.V.P. Long, Nature (London), 306 (1983) 172. S.J.B. Reed, Scanning Electron Microscopy, II (1984) 529. S.J.B. Reed and D.G.W. Smith, Earth Planet. Sci. Lett., 72 (1985) 238. C.A. Goodrich, G.J. Taylor, K. Keil, S.J.B. Reed, W.V. Boynton and D.H. Hill, Lunar Planet. Sci., 16 (1985) 282. G. Crozaz and E. Zinner, Earth Planet. Sci. Lett., 73 (1985) 41. S. deChaza1, G. Crozaz and E. Zinner, Lunar Planet. Sci., XVI (1985) 169. M. Lindstrom, G. Crozaz and E. Zinner, Lunar Planet. Sci., XVI (1985) 493.

38 48 G. Crozaz, J.S. Delaney and E. Zinner, Meteoritics, 20 (1985) in press. 49 A.J. Fahey, J.N. Goswami, K.D. McKeegan and E. Zinner, Lunar Planet Sci., XVI (1985) 221. 50 A. Fahey, E. Zinner, G. Crozaz, A.S. Kornacki and A.A. Ulyanov, Meteoritics, 20 (1985) in press. 51 G. Slodzian, Natl. Bur. Stand. (U.S.) Spec. Pub]., 427 (1975) 33. 52 G. Slodzian, Adv. Electron. Electron Phys. Suppl., 13B (1980) 1. 53 M.J. Drake and D.F. Weill, Chem. Geol., 10 (1972) 179. 54 G. Slodzian, J.C. Lorin and A. Havette, J. Phys. (Paris), 23 (1980) 555. 55 N. Shimizu and S.R. Hart, J. Appl. Phys., 53 (1982) 1303. 56 T. Jull, Int. J. Mass Spectrom. Ion Phys., 41 (1981) 135. 57 J.C. Lorin, A. Havette and G. Slodzian, Secondary Ion Mass Spectrometry, SIMS III, Springer-Verlag, Berlin, Heidelberg, New York, 1982, p. 140. 58 C. Meyer, Jr., Secondary Ion Mass Spectrometry, SIMS II, Springer-Verlag, Berlin, Heidelberg, New York, 1980, p. 67. 59 E.J. Young, A.T. Myers, E.L. Munson, and N.M. Conklin, U.S. Geol. Surv. Prof. Pap., 650-D (1969) D84. 60 M.M. Lindstrom, J. Geophys. Res., 89 (1984) C50. 61 M.S. Ma, A.V. Murali and R.A. Schmitt, Earth Planet. Sci. Lett., 35 (1977) 331. 62 G.W. Lugmair and K. Marti, Earth Planet. Sci. Lett., 35 (1977) 273. 63 G.J. Wasserburg, F. Tera, D.A. Papanastassiou and J.C. Huneke, Earth Planet. Sci. Lett., 35 (1977) 294. 64 P.M. Martin and B. Mason, Nature (London), 249 (1974) 333. 65 Hewlett Packard, Fort Collins, CO 80525, U.S.A., Mode1 9845B. 66 L.P. Gromet, private communication, 1984. 67 R. Korotev, private communication, 1984. 68 R. Korotev and M. Lindstrom, private communication, 1985. 69 0. James, private communication, 1985.