Limitations of chemical dating of monazite

Chemical Geology 266 (2009) 218–230

Contents lists available at ScienceDirect

Chemical Geology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / c h e m g e o

Limitations of chemical dating of monazite Frank S. Spear ⁎, Joseph M. Pyle, Daniele Cherniak Department of Earth and Environmental Science, Rensselaer Polytechnic Institute, 110 8th Street, Troy, NY 12180, United States

a r t i c l e

i n f o

Article history: Received 18 February 2009 Received in revised form 10 June 2009 Accepted 10 June 2009 Editor: R.L. Rudnick Keywords: Monazite Chemical dating Geochronology

a b s t r a c t Instrumental and spectral characteristics germane to chemical dating of monazite have been tested using the Cameca SX-100 at Rensselaer Polytechnic Institute. Statistical analysis demonstrates that, for trace element analysis, equal counting time on peak and background is required for optimal statistical precision, thus rendering impractical the procedure of fitting the entire spectrum to obtain background values. Energy shifts require shifting the detector voltage window between peak and background positions, and it is concluded that the differential auto PHA mode works optimally for this. Analyses of Pb-free phosphates, silicates, and oxides are used to measure spectral interferences with the PbMα peak and background positions. Backgrounds were modeled using both linear and exponential fits. It was found that the difference in background counts using the two fits varies with each of the five spectrometers examined, and that the high-pressure (3 bar) detectors show larger differences in exponential vs. linear peak-minusbackground (P-B) values than the low-pressure (1 bar) detectors. In addition, every spectrometer requires a unique correction for every major element in monazite. An analytical protocol is presented that incorporates these results. This protocol was applied to several monazite standards to determine inter-spectrometer variability, and spectrometer reproducibility from session to session. It was found that the difference in composition (and age) between spectrometers on identical spots exceeds the 2 sigma standard error of the mean of composition (or age) on either spectrometer. This means that (a) additional sources of error beyond the counting statistics exist between spectrometers; (b) the precision of microprobe ages cannot be continuously improved by additional counting; and (c) the minimum realistic precision is on the order of ± 2–3% for monazites with around 1500–2000 ppm total Pb, or an additional absolute uncertainty of 20–50 ppm Pb. © 2009 Elsevier B.V. All rights reserved.

1. Introduction The chemical dating of monazite has received considerable attention since the seminal papers of Suzuki and Adachi (1991) Suzuki et al. (1994) and Montel et al. (1996) demonstrated that an electron microprobe could be used to obtain texture-sensitive ages with precisions of a few percent. New spectrometer designs incorporating larger crystals (e.g., the LPET crystals of the Cameca SX-100 and the VLPET crystals of the Cameca Geochron, Williams et al., 2006, 2007) and software to facilitate simultaneous analysis of Pb using multiple spectrometers have enabled analytical precisions based on counting statistics to reach values on the order of 10 ppm Pb or better. This degree of analytical precision holds the analytical promise of age determinations with precisions of 1% or better. However, in practice it has not been possible to achieve this level of precision, and issues of accuracy are still largely unresolved. This paper elucidates some of the significant analytical and statistical considerations in determining the chemical ages of monazite, and

⁎ Corresponding author. E-mail address: [email protected] (F.S. Spear). 0009-2541/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.chemgeo.2009.06.007

provides guidelines for evaluating both the precision and accuracy of monazite ages determined by an electron microprobe. Monazite X-ray spectra are complex owing to the presence of significant REEs, Y, Th, and U, and the spectra are especially complex in the vicinity of the Pb M lines where analyses of Pb are typically made (e.g., Suzuki et al., 1994; Montel et al., 1996; Jercinovic and Williams, 2005; Williams et al., 2006; Pyle et al., 2005). Many attempts have been made to develop strategies to deal with the spectral complexities, but a fundamental problem remains that no secondary monazite standards of known trace Pb concentrations exist that can be used to evaluate the success of a particular strategy. All such previous attempts have used the isotopic age of a monazite as the nominal target of the analysis. Unfortunately, monazite ages determined by TIMS are almost universally U–Pb isotopic ages whereas chemical ages are dominantly Th– Pb ages. In addition, chemical analyses cannot detect common Pb, so all chemical ages (and especially young ones) must carry this uncertainty. Clearly, a suite of well-characterized monazites for which the elemental concentrations (Th, U, Pb, etc.), as well as the U–Pb and Th–Pb ages, are known is desperately needed, and the lack of such a material has hindered the development of this technique. This paper adopts a different approach. Whereas we have no standards with known trace Pb concentrations, we do have numerous

F.S. Spear et al. / Chemical Geology 266 (2009) 218–230

materials that have no measurable Pb (e.g., “blanks”). In particular, we have a superb suite of REE phosphates that has been synthesized by Cherniak et al. (2004), which were made using Pb-free fluxes and are nominally Pb-free. In addition, we have synthetic Th, U, Si, and Ca standards that are also nominally Pb-free, which serve as Pb-blanks for these elemental interferents. Finally, we have identified grains of a monazite sample that are, to within counting statistics, chemically homogeneous. Different grains have slightly different compositions, but each grain is internally homogeneous. Our strategy is to analyze these materials with the goal of developing an analytical protocol with necessary corrections so that we can obtain a value of zero for Pb for each of these materials within the precision of the counting statistics. Taken together, these elements (REE, Th, U, Y, P, Ca, Si) constitute N 98 wt.% of a typical monazite. This same protocol is then applied to our chemically homogeneous monazite grain. Analysis of a chemically homogeneous grain is desirable because it permits statistical treatment of the chemical data. Most monazites are moderately to strongly zoned chemically, even if all at the same age. In samples where chemically similar domains can be identified by either backscattered electron imaging or X-ray mapping, many workers (e.g. Terry et al., 2000; Williams et al., 2006; Pyle et al., 2005; Dahl et al., 2005) have treated such domains as a single age population and treated ages calculated from analyses of such domains statistically. Treating the ages statistically is inherently less satisfactory than treating the compositions statistically but it is the only available option because careful examination of such domains reveals that most are not chemically homogeneous within the statistical precision of the electron microprobe. Unfortunately, treating the ages statistically rather than the compositions raises the possibility of compensating errors obscuring any interference. Moreover, analyzing the same homogeneous monazite grain allows us to test the reproducibility of our protocol for different analytical sessions and thus arrive at a realistic evaluation of the analytical precision in a typical operating environment (i.e., an environment susceptible to changes in ambient external conditions such as P and T). The results of this study indicate that there are non-reducible errors that limit the precision to which monazite ages can be determined on the electron microprobe.

219

width automatically depending on the energy of the chosen X-ray line to be counted. Bias and gain are again set automatically. In “manual differential” mode the user sets the baseline, window, bias, and gain manually for an element. It is important to note that in “auto differential” mode the PHA settings are adjusted independently for the peak and the background measurements, depending on the energies of each. In “manual differential” the same settings are used for the background as were specified for the peak. Materials used in this study were natural quartz, and synthetic diopside-jadeite solid solution (Di85Jd15), ThSiO4, UO2, REEPO4, YPO4, and PbSiO3. The natural monazite is a separate of “Moacyr” that we obtained courtesy of J.-M. Montel (Seydoux-Guillaume et al., 2002). Two sets of experiments were performed. The initial set was conducted on a blank material and was designed to determine the optimal settings for the PHAs, to optimize strategies for measuring background, and to measure correction factors for all possible interferents. These experiments were conducted by analyzing only for Pb on all five spectrometers simultaneously for fifteen replicate analyses of 180 s each. The second set of experiments was designed to test the precision and reproducibility of the analytical protocol on the natural, homogeneous monazite (“Moacyr”). In this phase, the natural monazite was analyzed repeatedly using the full analytical protocol (as described below). In addition, the Pb, U, and Th standards were analyzed with shortened count times before and after each set of monazite analyses (typically every 12 analyses or 2 h) in order to ascertain any instrumental drift.

2.1. Background measurement strategy In the analysis of any trace element, the background determination is equally significant as the peak determination. Recently, Jercinovic and Williams (2005) and Williams et al. (2006, 2007) have argued convincingly that the background in the region of the Pb peak is curved so an exponential fit to the background is more appropriate than a linear fit. The significance of the background curvature in determining the true background value depends, of course, on how far from the peak the background is measured. For most elements, the background is measured sufficiently close to the peak that the linear and exponential fits give very nearly identical background values. However, in the case of PbMα there are interfering peaks that prevent the background from being measured close to the peak, so the curvature becomes important and an exponential fit to background is required. Note that the issue of linear versus exponential background fits is also present when PbMβ is used to measure Pb (although not quantitatively evaluated in the present study). The issue arises because there is no position between the PbMα and PbMβ peaks where the background is interference-free, so similar background positions must be used for either PbMα or PbMβ. However, the magnitude of the difference between the linear and exponential fits is smaller for PbMβ because of the shorter extrapolation from the background measurement position and the PbMβ peak. Analysts attempting to use PbMβ rather than PbMα should evaluate these effects quantitatively for the specific microprobe being used. Note also that the count rate is poorer on PbMβ than on PbMα so the overall statistical precision of the analyses is still better using PbMα.

2. Experimental method The instrument used in this study is the Cameca SX-100 at Rensselaer Polytechnic Institute, which is equipped with five wavelength spectrometers, each of which has a PET crystal. The configuration of each spectrometer is given in Table 1. Four spectrometers house PET crystals of the “large” variety (e.g., LPET: 1320 mm2) and one is “normal” (522 mm2). All detectors use P-10 gas (90% Ar, 10% CH4) with three detectors (spectrometers 1, 2, and 4) at 1 bar pressure and two (spectrometers 3 and 5) at 3 bars pressure. An important part of this study was to determine optimal settings for the pulse-height analyzers (PHAs). The Cameca hardware and software have three different PHA modes. In integral mode, the software automatically specifies a baseline (nominally at 560 mV) and a large “open” window (nominally 10 V) such that essentially all pulses above background noise are counted. The bias and gain are set automatically. In “auto differential” mode the software sets the baseline and window Table 1 Spectrometer configuration for Cameca SX-5100 at Rensselaer Polytechnic Institute. Spectrometer 1 2 3 4 5

Crystal used

Crystal size (mm2)

Bkg −

PET LPET LPET LPET LPET

522 1320 1320 1320 1320

− 3502 − 3502 − 3502 − 3502 − 3502

Bkg + 2239 2239 2239 2239 2239

PHA settings for PbMα peak

Detector gas

Detector gas pressure (bar)

Bias

Gain

Baseline

Window

Mode

P-10 P-10 P-10 P-10 P-10

1 1 3 1 3

1302 1291 1873 1291 1873

891 864 1000 864 1000

Variable Variable Variable Variable Variable

Variable Variable Variable Variable Variable

Differential Differential Differential Differential Differential

auto auto auto auto auto

220

F.S. Spear et al. / Chemical Geology 266 (2009) 218–230

In most microprobe analysis routines, background is measured in two places free of interference on either side of the peak and the background under the peak calculated by fitting the background measurements to either a linear or exponential curve, and interpolating. In this study, background positions of −3502 and +2239 (sin(Θ) 105) were chosen as being free of interferents and both linear and exponential curves were fit to the two measurements. Details of these measurements are discussed below. Jercinovic and Williams (2005) and Williams et al. (2006, 2007), however, advocate collecting a detailed wavelength scan over the region of the Pb peaks, fitting the background parts of the scan to an exponential function, and interpolating the background value under the Pb peak from this function. Although this method does permit the curvature of the background to be evaluated for each monazite chemical domain, we believe that this method does not provide for sufficient statistical precision on the background determination during practical application to warrant its adoption. For trace element analyses, similar statistical precisions are required on the background as on the peak or the error will be greatly weighted towards the error in the less precise value. The following analysis of the statistics of the Jercinovic–Williams method indicates that most of their analytical error resides in their background measurement. Published background scans by Jercinovic and Williams (2005) and Williams et al. (2006, 2007) show scans from around 55,000 to 63,500 (sin(Θ) 105). Although they do not report background locations relative to peak Pb positions for most of their analyses, examination of their published examples leads us to believe that, in practice, they use background counts over three separate intervals of the spectrum, with two sets of counts taken below the peak and one above the peak. Following the general procedure outlined in Table 5 of Jercinovic and Williams (2005) and applying this to the published spectrum for their Elk Mtn. monazite (their Fig. 8), it appears that the entire scan took approximately 30 min to collect and that approximately 6.5 min of this scan was used to model the background (approximately 260 channels total). Based on the spectrometer sensitivity shown in their figure, this equates to around 18,000 total counts collected on background, for which the Poisson statistics are around 0.75% (1 sigma). The error on peak Pb measurement depends of course on the amount of Pb present, but typical analysis times are 10 min/spot and as many as 10 spots are collected to characterize an age domain (Jercinovic and Williams, 2005) for a total peak counting time of 100 min. This approach certainly leads to well-characterized peak counts, but additional spots on the Pb peak only shift a greater percentage of the error onto the background measurement. An illustrative example of this error shifting occurs if the P–B is exactly 0.0 (as it should be in a Pb blank). If the background is measured for around 6 min so that the total background counts are 18,000 (1 sigma = ±134 counts or 0.75%), and the total “peak” counting time is ten times longer (e.g., 60 min), the peak counts will be 180,000 (1 sigma = ±424 counts or 0.23%). The error on the background is over three times larger (i.e., √10) than the error on the peakq and will contribute the largest portion ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 of the total error on the P–B ( σpeak + ð10σÞ2bkg = 1407 counts). As the total time counted on background approaches that counted on peak the relative contributions of errors on peak and background become equal. Whereas this situation improves somewhat as the intensity of the peak measurement increases relative to background (i.e., more of the error is associated with the peak measurement), the fact remains that collecting more spots on the peak only cannot greatly improve the overall statistics of the analysis because most of the error is in the background measurement. The only way to avoid this pitfall is to collect counts on background for the same length of time as on the peak. In the above example, if backgrounds had been collected for 60 minpfor a total of 180,000 counts, then the error on the ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi P–B = 0 would be 180; 000 + 180; 000 = 600 counts, or less than half of the previous value of 1407 counts. In order to collect an

equivalent number of background counts by scanning the spectrum as described by Jercinovic and Williams (2005), one would need to count for approximately 300 min (5 h) for each composition domain, rendering the method somewhat impractical in normal operation. It should be noted that the important values here are the total number of counts used to model the background relative to the total counts collected on the peak and that our analysis can be readily redone if different values are chosen. In summary, although the background is demonstrably curved and doing background scans on every compositional domain is helpful to characterize the chemistry of the sample, it is impractical to count background by fitting parts of the background spectrum. In order to optimize the statistics of trace Pb analyses, our approach is to use two fixed background positions and to measure backgrounds with every spot analysis. Each background (above and below the peak) is measured for half the time of the peak measurement, ensuring similar statistics for peak and background. It is impractical, and probably unwise, to adjust the locations of the background positions for each individual monazite. We have, therefore, adopted the approach of selecting a single set of background positions and then measuring the magnitude of interferences and developing a correction strategy for each. The shape of the background fitting function – linear versus exponential – is still an important consideration, as Jercinovic and Williams (2005) have stressed. We evaluate this following presentation of our results, but some simple theoretical aspects of exponential functions are useful to consider at this point. The curvature of an exponential function in the region of interest depends entirely on the slope of the data being fitted. That is, exponential fits to spectra with larger slopes will have larger curvatures (e.g., Fig. 1). Consequentially, the slope of the spectrum around the Pb region will control the magnitude of the difference between an exponential and a linear background fit. If the slope is zero, the two fits are identical. It turns out, as discussed below, that the slope of the X-ray spectrum around the Pb peaks differs from spectrometer to spectrometer, as well as with the pressure of the detector gas, so it is not possible to generalize for all instruments.

Fig. 1. Illustration of the influence of background slope on the difference between a linear and an exponential fit to background. In both cases the exponential fit yields a smaller estimate of the background but a steeply-sloped background (a) results in a larger difference between the linear and exponential fits than does a shallow-sloped background (b).

F.S. Spear et al. / Chemical Geology 266 (2009) 218–230

2.2. PHA settings Numerous peaks appear in the X-ray spectrum around the Pb M lines (e.g., Jercinovic & Williams, 2005). Some of these are first-order lines (e.g., Y Lγ2,3, Th Mζ1, Th Mζ2, U Mζ1) and others are second-order L lines of the REEs (e.g., Ce and La in particular, Th M2-O4). It is possible to exclude certain parts of second-order lines from the counting by using an appropriate setting of the PHA window, although Pyle et al. (2005) have discussed contributions arising from the escape peaks of these second-order lines where argon is the detector gas rather than xenon. Because the accuracy of trace element analyses depends as much on accurate background measurements as on accurate peak measurement, it is imperative to determine the appropriate PHA settings for both background and peak X-rays. Fig. 2 illustrates a pair of PHA scans for both the 1 bar and 3 bar detectors with the spectrometer set to both the high and low Pb background positions on a sample of Pb-free quartz. The first-order background falls at around 1.5–2 V and a peak corresponding to second-order background between 3 and 4 V. An interfering second-order REE L line would fall in a similar voltage range as the second-order background peak. Setting the PHA window to exclude the higher-order peaks (second-order background and any second-order REE lines) will help eliminate these interferents. However, note that there is also an escape peak for the secondorder line that falls within the voltage range of the first-order background peak, thus making it impossible to completely eliminate second-order peaks using a counter with P-10 (argon) gas. Xenon gas, as discussed by Pyle et al. (2005), has a second-order escape peak that

Fig. 2. PHA spectra (intensity versus voltage) collected at high- and low-background positions for Pb on natural (Pb-free) quartz. (a) Low-pressure detector (1 bar P-10 gas). (b) High-pressure detector (3 bar P-10 gas). Note the shift of the PHA scan from one background position to the other. A scan with the spectrometer at the peak position would lie between these two. The location of the escape peak from the second-order X-rays (dotted peak) is approximate and not drawn to scale. Note that even in the absence of elemental interference, there is a second-order background peak that can contribute to the overall background counts. The “differential auto” window settings are shown for the plus and minus-background scans by the light and heavy bars, respectively.

221

falls to lower voltages and can thus be eliminated by an appropriate PHA setting. It is clear from the plots that there is a significant energy shift in going from the low to the high background settings (the Pb peak scan, which is not shown, would fall between the two spectra). It is thus not possible to use a fixed PHA window for peak and background positions. The “differential auto” setting on the SX-100 solves this problem by shifting the baseline and window voltage as a function of X-ray energy. The “differential auto” settings for the two spectrometer positions shown in Fig. 2 are also shown, and it can be seen that they shift appropriately to encompass the shift in peak energy. In this study, therefore, the “differential auto” mode has been used exclusively.

3. Results In the first set of experiments, we present results of measurements of a range of Pb-free materials, including a suite of Pb-free REE phosphates (Table 2). Peak and background counts were collected on Pb on each of the four spectrometers that contain LPET crystals (spectrometers 2, 3, 4, and 5; see Table 1). Background offsets of −3502 and +2239 relative to the PbMα peak were used for all measurements. The background counts were fit using both a linear and an exponential model, and the background under the peak was calculated and reported in Table 2 as “apparent Pb”. Interferences under the peak result in positive values of “apparent Pb” whereas interferences with one or the other background positions result in negative values of “apparent Pb”. For all materials, the exponential background fits result in larger (more positive) values of apparent Pb, as expected. The difference between exponential and linear fits depends strongly on the material analyzed and the spectrometer used. For example, the difference between the exponential and linear fits on quartz using spectrometer 2 (1 atm) is 11 ppm (±6 = 1 s.e.) whereas on spectrometer 3 (3 atm) it is 40 ppm (± 7 = 1 s.e.). For GdPO4, the difference between exponential and linear fits using spectrometers 2 and 3 is 41 ± 9 (=1 s.e.) ppm and 143 ± 14 (=1 s.e.) ppm, respectively. Furthermore, even between two nominally identical spectrometers the difference between exponential and linear background fits is different, as can seen by comparing spectrometers 3 and 5 for GdPO4 where the differences are 143 ± 14 and 98 ± 11 ppm, respectively. From this we conclude that it is not possible to make a priori inferences about the degree to which background models will affect Pb analyses on any given spectrometer, and that each analysis on each material must be evaluated independently. Also shown in Table 2 is the apparent ppm/weight percent of interfering element (REE, Th, U, etc.), which can be used to correct Pb analyses of natural monazites. As an example, Table 3 shows the magnitude of corrections for two natural monazites. The total amount of correction due to interferences either with the peak or the background positions (positive and negative values, respectively) is significant and exceeds 10% of the total Pb content in each of these analyses. The contributions from individual elements vary with the composition of the monazite. “Moacyr” monazite contains a significant quantity of Th, and over half of the correction is due to Th Mζ1,2 interference. The UCLA 76 monazite contains a somewhat high concentration of Y (3.17 wt.% Y2O3) relative to typical monazite in pelites (Pyle et al., 2005), and most of the correction is due to interference with Y Lγ2,3. The REE contributions (only determined for “Moacyr” in this example) are typically minor but depend on the individual spectrometer, although for monazites with low Pb contents this can be a substantial fraction of the total Pb. The U content of metamorphic monazite is typically low (a few thousand ppm) and the correction can be ignored. However, for monazites with several weight percent U (e.g., monazite UCLA 76), the correction can range up to a few tens of ppm. The U interference is with the background

222

F.S. Spear et al. / Chemical Geology 266 (2009) 218–230

Table 2 Values of apparent Pb in Pb-free standards using linear and exponential background fits. Spec 2

Spec 3

Spec 4

Spec 5

1 atm

3 atm

1 atm

3 atm

Quartz (Si = 46.74 wt.%) Linear ppm Expo ppm Std dev s.e. ppm/wt.% Si

19 30 24 6 0.64

− 39 1 28 7 0.01

7 15 17 4 0.32

− 29 2 27 7 0.04

ThSiO4 (Th = 71.59 wt.%) Linear ppm Expo ppm Std dev s.e. ppm/wt.% Th

1312 1356 37 9 18.94

1436 1568 67 17 21.90

1501 1531 58 15 21.38

1488 1609 59 15 22.47

UO2 (U = 88.15 wt.%) Linear ppm Expo ppm Std dev s.e. ppm/wt.% U

− 856 − 587 77 20 − 6.66

− 1262 − 693 66 17 − 7.86

− 909 − 651 67 17 − 7.39

− 1186 − 666 61 16 − 7.56

YPO4 (Y = 48.35 wt.%) Linear ppm Expo ppm Std dev s.e. ppm/wt.% Y

3580 3598 56 14 74.42

3891 3948 56 14 81.66

3916 3930 70 18 81.28

3725 3779 75 19 78.16

Di85Jd15 (Ca = 15.89 wt.%) Linear ppm Expo ppm Std dev s.e. ppm/wt.% Ca

22 37 28 7 2.30

− 34 9 34 9 0.56

8 17 27 7 1.07

− 13 29 32 8 1.80

LaPO4 (La = 59.39 wt.%) Linear ppm Expo ppm Std dev s.e. ppm/wt.% La

49 79 53 14 1.33

− 79 9 33 9 0.16

25 46 45 12 0.77

− 44 44 59 15 0.75

CePO4 (Ce = 59.60 wt.%) Linear ppm Expo ppm Std dev s.e. ppm/wt.% Ce

− 13 25 39 10 0.41

− 80 22 44 11 0.37

− 17 7 43 11 0.12

− 92 4 40 10 0.07

PrPO4 (Pr = 59.73 wt.%) Linear ppm Expo ppm Std dev s.e. ppm/wt.% Pr

− 42 0 43 11 − 0.01

− 164 − 41 73 19 − 0.69

− 48 − 19 38 10 − 0.31

− 96 7 34 9 0.11

NdPO4 (Nd = 60.29 wt.%) Linear ppm Expo ppm Std dev s.e. ppm/wt% Nd

− 20 16 41 11 0.26

− 84 22 44 11 0.36

− 14 10 51 13 0.17

− 74 28 47 12 0.47

SmPO4 (Sm = 61.29 wt.%) Linear ppm Expo ppm Std dev s.e. ppm/wt.% Sm

− 77 − 30 30 8 − 0.48

− 228 − 84 49 13 − 1.37

− 49 − 16 40 10 − 0.26

− 148 − 35 43 11 − 0.57

EuPO4 (Eu = 61.54 wt.%) Linear ppm Expo ppm Std dev

− 12 25 46

− 117 −4 67

− 22 6 33

− 75 26 38

Table 2 (continued)

EuPO4 (Eu = 61.54 wt.%) s.e. ppm/wt.% Eu GdPO4 (Gd = 62.35 wt.%) Linear ppm Expo ppm Std dev s.e. ppm/wt.% Gd

Spec 2

Spec 3

Spec 4

Spec 5

1 atm

3 atm

1 atm

3 atm

12 0.41

17 − 0.07

9 0.10

10 0.42

− 15 26 36 9 0.41

− 125 − 18 54 14 − 0.29

− 18 11 36 9 0.18

− 100 −2 43 11 − 0.04

Instrument conditions: 15 kV, 100 nA current, 15 spot/analysis, 180 s count on peak, 90 s count on each background. Differential auto PHA mode. Linear background fits calculated by determining intercept using minus (− 3502) and plus (2239) background positions as “x” values and background counts as Y values using EXCEL function LINEST. Exponential background fits calculated as with linear fits following the transformation ln(Y) = ln(b) + ax. Intercept (b) was calculated as exp(ln(b)). Note that the values of ppm/wt.% element are used as correction factors on Pb for natural monazites.

measurement so the U correction results in a decrease in the total Pb content. 4. Monazite chemical dating protocol Based on the above results we have developed a protocol for chemical dating of monazite using our Cameca SX-100 electron probe. A portion of this protocol is executed during analysis and a portion is done as post-processing and each will be discussed in turn. 4.1. Analysis protocol The analysis protocol is set up in the Cameca PeakSight software in the “Quanti-set” file. The values used in our dating protocol are listed in Table 4. Most entries are self-explanatory, but the “sub-counting” option requires some explanation. The “sub-counting” value refers to the number of cycles that the total counting time will be divided into during the analysis. For example, Pb is counted for a total of 180 s on peak, 90 s on minus-background, and 90 s on plus-background. A subcounting value of 6 breaks the total time into 6 separate cycles of 30 s peak, 15 s minus-background, 15 s plus-background. The reason for this approach is to minimize the effects of sample surface damage Table 3 Corrections to Pb analyses (in ppm) due to various interferents in two natural monazites. wt.% element “Moacyr” Si 0.73 Ca 0.31 Y 0.59 Th 6.72 U 0.18 La 12.13 Ce 25.4 Pr 2.9 Nd 9.6 Sm 1.87 Gd 0.85 Total correction REE only correction UCLA 76 Y 3.17 Th 1.62 U 4.05 Total correction

Spec 2

Spec 3

Spec 4

Spec 5

0.5 0.7 43.9 127.3 − 1.2 16.1 10.5 0.0 2.5 − 0.9 0.4 199.6 28.5

0.0 0.2 48.2 147.2 − 1.4 1.9 3.9 − 2.0 3.5 − 2.6 − 0.2 198.6 4.4

0.2 0.3 48.0 143.7 − 1.3 9.3 0.5 − 0.9 1.6 − 0.5 0.2 201.0 10.1

0.0 0.6 46.1 151.0 − 1.4 9.1 0.0 0.3 4.5 − 1.1 0.0 209.2 12.8

235.9 30.7 − 27.0 239.6

258.9 35.5 − 31.8 262.5

257.7 34.6 − 29.9 262.4

247.8 36.4 − 30.6 253.5

F.S. Spear et al. / Chemical Geology 266 (2009) 218–230

223

Table 4 Protocol for monazite chemical analysis on the Cameca SX-100 at RPI. Spectrometer

Element

Standard

Peak count

Bkg count (each)

Sub-counting divisions

PHA

Background fit

s Pass 1 1 2 3 4 5

PET LPET LPET LPET LPET

Y Pb Pb Pb Pb

YPO4 (syn) PbSiO3 (syn) PbSiO3 (syn) PbSiO3 (syn) PbSiO3 (syn)

180 180 180 180 180

90 90 90 90 90

6 6 6 6 6

Diff Diff Diff Diff Diff

auto auto auto auto auto

Doesn't matter Exponential Exponential Exponential Exponential

Pass 2 1 2 3 4 5

PET LPET LPET LPET LPET

Th U Th U

ThSiO4 (syn) UO2 (syn) ThSiO4 (syn) UO2 (syn)

60 60 60 60

30 30 30 30

1 1 1 1

Diff Diff Diff Diff

auto auto auto auto

Doesn't Doesn't Doesn't Doesn't

matter matter matter matter

KV = 15. Current = 100 nA on Faraday Cup. Sub-counting divides total peak and background into n separate sequences of peak, − Bkg, +Bkg. For Pb this means 6 cycles of 30 s (peak) each.

during the analysis. If there is damage or contamination it is highly undesirable that all of the background be collected after the sample is damaged. Thus, sub-counting ensures that if the sample surface is being damaged, it will affect the peak and background counts more or less similarly. Analyses of Th and U are done on two spectrometers each and the results averaged to minimize the total analysis time. Also note that the background fitting routine for Pb is exponential for each spectrometer. The background fitting routines we use for the other elements are also exponential, but the results obtained using linear fits are indistinguishable within counting statistics because the background positions are not far from the peak positions. It should also be noted that standardization is performed with similar settings except that (a) total counting times are shorter and (b) the sample current is set to 10 nA to minimize peak shift in the PHA due to high count rate during standardization. 4.2. Post-processing Analyses are corrected for interferences following the general procedures outlined above and summarized in Table 2. For example, analysis of the Pb-free YPO4 standard yields on the order of 3500– 4000 ppm apparent Pb (Table 2), or 74–82 ppm apparent Pb/weight percent Y. The correction factor is then multiplied by the measured quantity of Y and the concentration of Pb adjusted by that amount. Other interferents are treated in the same way. Note that there is also an interference on the U Mβ peak caused by interference by Th Mγ that must be corrected as well. The REE elements are not routinely analyzed on every spot and the approach taken here is to use an average REE composition for both the ZAF corrections and the REE interference corrections. Although there are numerous excellent reasons to obtain full REE analyses of monazite, variation in the REE content of metamorphic monazite will not affect the calculated REE correction by more than a few ppm at most.

We also examined the effect of REE composition on the ZAF corrections. The ZAF correction for Pb is similar for all REE elements, but the largest concern is that a monazite high in Th and/or U will be low in REE because the two major Th/U substitutions involve REE: Huttonite : Thðor UÞ + Si = REE + P Brabantite : Thðor UÞ + Ca = 2REE Thus, using a fixed REE content might skew the ZAF corrections. To evaluate this, we ran ZAF corrections on three monazite compositions of (a) pure CePO4, (b) CePO4 with 10% huttonite component, and (c) CePO4 with 10% brabantite component (Table 5), We then ran the ZAF corrections using a fixed CePO4 composition and changed the Th content to match that of the 10% huttonite and brabantite compositions, but did not change the REE or P contents (columns b′ and c′). Total calculated Pb ranges from 1852 (huttonite = brabantite = 0) to 1813 ppm (10% huttonite component). This attests to the major influence of REE versus actinide on the absorption of Pb X-rays. What is important in this table, however, is the difference in calculated Pb content using the true composition and the fixed composition (compare b to b′ and c to c′). The largest difference occurs in the 10% brabantite test (compare c to c′) which shows a difference of 5 ppm, or 0.27% of the value (true = 1818 ppm, fixed = 1813 ppm). Whereas this is a small error introduced for most natural monazites, there are monazites where the Th content is on the order of 30%, and the error in these could be as large as nearly 1%. In high-Th monazites, therefore, it would be wise to recalculate the analyses using a more appropriate matrix composition before calculating an age. 4.3. Results on natural monazites Table 6 presents the results of several sets of analyses of a single grain of “Moacyr” monazite collected on different days over several

Table 5 Effect of REE composition on ZAF corrections of Pb.

Ce P Th Si Ca O Pb (ppm)a

(a) Pure CePO4

(b) 10% huttonite true composition

(b′) 10% huttonite fixed REE

(c) 10% brabantite true composition

(c′) 10% brabantite fixed REE

59.6 13.17 0 0 0 27.22 1852

51.68 11.42 9.51 1.15 0 26.23 1813

59.6 13.17 9.51 0 0 27.22 1814

47.85 13.22 9.9 0 1.71 27.32 1818

47.85 13.17 9.9 0 0 27.22 1813

Compare Pb values in two columns (b) and (c). a Pb calculated from an identical K ratio using ZAF corrections for each indicated matrix.

224

F.S. Spear et al. / Chemical Geology 266 (2009) 218–230

Table 6 Chemical and age data for multiple session analyses of the monazite “Moacyr middle”. % error from Poisson statistics

Y Spec 1

Th Spec 2

Th Spec 4

U Spec 3

U Spec 5

Pb Spec 2

Pb Spec 3

Pb Spec 4

Pb Spec 5

Spec 2

Spec 3

Spec 4

Spec 5

0.7

0.4

0.4

5.6

6.3

2.6

3

2.8

3

2.6

3

2.8

3

Composition ppm

Age Ma

Spot

Y Spec 1

Th Spec 2

Th Spec 4

U Spec 3

U Spec 5

Pb Spec 2

Pb Spec 3

Pb Spec 4

Pb Spec 5

Spec 2

Spec 3

Spec 4

Spec 5

May 9, 2008 data 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 Average S.D. measured S.D. Poisson s.e. of the mean (measured)

5613 5612 5566 5637 5551 5501 5559 5557 5603 5556 5563 5582 5627 5499 5465 5575 5526 5531 5443 5556 53 39 12.11

68,485 68,602 68,817 68,432 68,803 68,490 68,355 68,613 68,562 68,990 68,728 68,559 68,513 68,171 68,994 68,147 67,813 68,468 68,255 68,516 293 274 67.26

65,880 66,125 66,617 65,960 66,737 66,276 65,159 66,163 66,069 65,651 65,813 65,388 65,917 65,898 65,847 66,467 65,371 65,917 65,803 65,951 405 264 92.94

1909 2158 1959 1940 1822 1829 1933 2047 1882 1946 1999 2102 2036 1835 1760 1988 1878 2188 2107 1964 119 110 27.41

2144 2035 2054 1913 2362 2028 1975 2030 2004 2086 2039 2173 2137 2079 2174 1829 1990 2219 2193 2077 121 131 27.65

1592 1686 1543 1558 1610 1615 1595 1529 1564 1552 1631 1644 1622 1711 1629 1628 1576 1548 1549 1599 50 42 11.43

1627 1633 1660 1523 1600 1623 1649 1621 1603 1554 1649 1549 1663 1629 1660 1639 1710 1686 1725 1632 51 49 11.80

1486 1445 1630 1573 1572 1545 1500 1581 1466 1665 1452 1515 1536 1601 1622 1577 1584 1457 1531 1544 65 43 14.86

1591 1571 1663 1641 1624 1536 1578 1434 1579 1631 1575 1611 1556 1658 1558 1550 1519 1595 1596 1582 54 47 12.33

497 523 479 489 497 505 502 476 489 484 509 512 505 537 508 510 498 480 482 499 16 13 4

508 507 515 478 494 508 519 504 502 485 514 483 517 511 518 513 540 522 536 509 16 15 4

465 449 506 493 486 483 473 492 459 519 454 473 478 503 506 494 501 452 477 482 20 14 5

497 488 516 515 502 481 497 447 494 508 492 502 485 520 486 486 480 494 497 494 16 15 4

May 19 data 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 Average S.D. measured S.D. Poisson

5801 5800 5777 5777 5703 5794 5683 5736 5789 5751 5839 5747 5744 5811 5733 5792 5781 5692 5698 5760 44 40

68,262 68,204 67,939 67,301 67,834 68,039 67,588 67,694 67,651 67,313 67,322 67,535 67,885 67,959 68,339 67,682 67,474 67,565 67,690 67,751 315 271

67,996 68,102 67,106 67,217 67,738 67,368 67,094 67,528 67,275 67,161 66,995 67,287 67,735 67,533 67,668 67,449 67,195 67,561 67,909 67,469 323 270

2045 1796 1978 1832 1884 1972 1986 2158 2030 1948 1774 1944 1997 1856 1956 1839 1904 1780 1862 1923 101 108

2049 1957 1897 1987 2066 2100 2026 2086 1896 1913 1880 2055 2147 2126 1760 2015 1923 2039 1880 1990 102 125

1687 1620 1609 1551 1577 1649 1599 1512 1642 1679 1549 1652 1508 1651 1644 1615 1570 1730 1637 1615 59 42

1638 1653 1581 1567 1547 1604 1529 1673 1600 1573 1604 1640 1630 1555 1664 1605 1669 1552 1617 1605 44 48

1718 1642 1609 1668 1496 1587 1590 1665 1556 1606 1553 1613 1531 1658 1641 1625 1528 1642 1637 1609 56 45

1596 1641 1527 1498 1628 1490 1634 1585 1624 1637 1679 1660 1713 1602 1538 1559 1643 1557 1588 1600 60 48

522 505 504 489 492 513 501 470 514 528 490 517 469 514 514 506 494 542 513 505 18 13

507 515 496 494 483 499 479 519 501 495 508 513 506 485 520 503 525 487 506 502 13 15

531 511 504 525 467 494 498 517 488 506 492 505 476 517 513 509 481 515 513 503 17 14

494 511 479 472 507 464 512 492 509 515 531 519 531 500 481 489 517 488 498 501 19 15

June 8, 2008 Line 1 1 2 3 4 5 6 7 8 9 10 11 12 Average S.D. measured S.D. Poisson

5833 5739 5693 5720 5743 5780 5710 5796 5738 5768 5733 5724 5748 40 40

69,011 69,349 68,907 68,539 69,419 69,211 69,303 69,052 69,459 68,757 68,803 69,167 69,081 288 276

69,975 69,319 70,094 69,633 69,400 69,328 69,754 69,611 69,724 69,736 69,638 69,691 69,659 235 279

2251 2040 2188 2210 2049 2321 2128 2161 1883 1855 2251 1950 2107 152 118

2067 2101 2156 2042 2189 2001 2163 2062 2079 2127 1977 2134 2092 66 132

1563 1715 1562 1599 1652 1603 1507 1652 1675 1674 1586 1643 1619 59 42

1672 1596 1622 1707 1671 1608 1580 1648 1672 1741 1670 1687 1656 47 50

1643 1595 1573 1611 1640 1647 1646 1546 1603 1564 1662 1549 1607 41 45

1565 1550 1490 1608 1612 1536 1595 1588 1573 1641 1701 1457 1576 65 47

470 518 470 484 498 484 454 499 507 509 480 497 489 19 13

503 483 488 517 504 485 476 498 506 529 505 510 500 15 15

494 483 473 488 495 497 495 467 486 476 502 469 485 12 14

471 469 448 487 486 464 480 480 476 499 514 442 476 20 14

June 8, 2008 Line 2 1 2 3 4

5675 5766 5733 5730

68,990 68,888 68,815 69,263

70,187 69,424 69,241 69,960

1990 1830 2086 1866

2050 1998 2086 2278

1589 1688 1693 1529

1560 1562 1606 1600

1635 1711 1615 1561

1611 1538 1674 1589

480 515 514 461

472 477 488 482

494 522 490 471

487 470 508 479

F.S. Spear et al. / Chemical Geology 266 (2009) 218–230

225

Table 6 (continued) % error from Poisson statistics

Y Spec 1

Th Spec 2

Th Spec 4

U Spec 3

U Spec 5

Pb Spec 2

Pb Spec 3

Pb Spec 4

Pb Spec 5

Spec 2

Spec 3

Spec 4

Spec 5

0.7

0.4

0.4

5.6

6.3

2.6

3

2.8

3

2.6

3

2.8

3

Composition ppm

Age Ma

Spot

Y Spec 1

Th Spec 2

Th Spec 4

U Spec 3

U Spec 5

Pb Spec 2

Pb Spec 3

Pb Spec 4

Pb Spec 5

Spec 2

Spec 3

Spec 4

Spec 5

June 8, 2008 Line 2 5 6 7 8 9 10 11 12 Average S.D. measured S.D. Poisson

5639 5636 5681 5657 5655 5623 5694 5709 5683 44 40

68,798 68,415 68,385 68,301 68,593 68,322 68,712 68,266 68,646 317 275

69,462 69,867 69,714 69,854 69,591 69,338 69,685 69,485 69,651 280 279

2119 2003 1998 2054 1964 2126 2058 2109 2017 95 113

2042 2055 2205 2203 2214 2309 1954 1843 2103 140 132

1692 1553 1608 1657 1648 1471 1619 1619 1614 69 42

1603 1688 1617 1559 1634 1533 1591 1555 1592 42 48

1576 1579 1641 1612 1649 1627 1604 1675 1624 43 45

1532 1577 1577 1602 1635 1652 1695 1576 1605 51 48

513 472 488 502 500 446 492 495 490 22 13

486 513 491 472 495 464 484 476 483 13 15

478 480 498 488 500 492 488 512 493 14 14

465 479 478 485 496 500 515 482 487 15 15

June 8, 2008 Line 3 1 2 3 4 5 6 7 8 9 10 11 12 Average S.D. measured S.D. Poisson

5667 5635 5679 5666 5700 5754 5687 5696 5675 5708 5667 5690 5685 29 40

68,614 69,128 67,575 68,940 68,700 68,475 68,350 67,422 68,021 68,715 67,730 68,302 68,331 543 273

69,603 69,765 69,200 69,757 69,916 68,805 69,107 68,901 69,390 69,362 68,967 69,313 69,340 363 277

2054 1629 1910 1592 1871 2074 1791 1931 1907 2099 2109 1974 1912 172 107

1828 2204 2216 2220 1899 1977 2111 1858 1921 2034 2045 1975 2024 139 128

1736 1583 1673 1553 1691 1629 1553 1561 1567 1719 1609 1591 1622 67 42

1639 1589 1644 1648 1685 1647 1642 1511 1635 1628 1707 1647 1635 48 49

1632 1646 1601 1642 1707 1636 1684 1755 1676 1752 1608 1730 1672 54 47

1565 1712 1551 1648 1650 1684 1572 1611 1700 1667 1652 1513 1627 64 49

529 482 512 474 516 498 476 484 482 522 493 487 496 19 13

500 484 504 502 514 504 503 468 502 495 523 504 500 14 15

498 501 491 500 521 501 516 543 515 532 493 529 511 17 14

478 520 476 502 503 515 482 499 522 506 506 463 498 19 15

June 8, 2008 Line 4 1 2 3 4 5 6 7 8 9 10 11 12 Average S.D. measured S.D. Poisson

5650 5746 5723 5712 5754 5719 5764 5741 5681 5752 5693 5744 5723 34 40

68,670 68,625 68,395 68,598 68,075 68,420 68,306 68,943 67,778 67,736 68,575 68,737 68,405 375 274

69,639 69,981 69,311 69,241 69,914 69,564 68,898 69,698 69,054 70,022 69,322 69,873 69,543 375 278

1954 2135 1830 1799 2106 1866 1746 1797 1965 1839 2061 2062 1930 135 108

2203 1978 2035 2033 2074 2219 2049 2208 2179 2213 2209 2068 2122 90 134

1520 1574 1624 1627 1624 1660 1717 1646 1621 1576 1603 1561 1613 51 42

1557 1612 1596 1545 1588 1715 1616 1691 1556 1614 1654 1634 1615 53 48

1637 1707 1652 1654 1598 1800 1703 1692 1707 1690 1712 1629 1682 52 47

1583 1591 1663 1604 1680 1611 1528 1681 1654 1627 1609 1579 1617 46 49

461 477 497 498 493 505 528 499 497 481 486 473 491 17 13

472 488 489 474 482 522 498 513 477 492 501 495 492 15 15

496 517 506 506 486 547 524 513 522 515 519 493 512 16 14

480 482 509 492 510 490 471 510 506 496 488 478 493 14 15

June 8, 2008 Line 5 1 2 3 4 5 6 7 8 9 10 11 12 Average S.D. measured S.D. Poisson

5759 5757 5634 5719 5640 5743 5732 5692 5724 5763 5745 5748 5721 44 40

68,338 68,208 68,739 67,573 68,590 67,472 67,789 68,447 68,524 68,680 68,853 68,719 68,328 472 273

70,250 69,683 69,887 69,296 69,818 69,521 69,062 69,398 69,368 69,391 69,351 69,725 69,562 323 278

1831 1990 2114 1944 2047 1918 1639 1932 2107 2072 2093 2024 1976 137 111

1906 2046 2070 2091 1947 1947 2055 2014 2135 1887 2038 1988 2010 76 127

1602 1615 1543 1723 1664 1606 1575 1589 1582 1686 1697 1615 1625 55 42

1527 1678 1677 1631 1531 1591 1745 1551 1668 1599 1539 1673 1618 72 49

1638 1673 1569 1628 1707 1576 1593 1710 1642 1583 1695 1703 1643 54 46

1669 1704 1598 1618 1560 1616 1608 1558 1598 1706 1639 1616 1624 48 49

490 492 467 528 506 494 487 486 480 514 515 491 496 17 13

467 511 507 501 466 490 539 474 506 488 467 508 494 23 15

500 510 475 500 519 485 493 522 498 483 514 517 501 15 14

509 519 483 497 475 497 497 476 485 520 498 491 496 15 15

August 15, 2008 1 2 3 4 5 6 7

5739 5784 5743 5820 5832 5953 5968

66,726 67,141 67,102 67,369 67,595 67,822 68,388

67,393 68,017 68,141 67,900 68,627 68,626 68,708

1938 2052 2027 2024 2071 1890 2141

2212 2067 2124 1942 2086 2117 2105

1682 1643 1622 1569 1531 1663 1631

1641 1736 1671 1649 1579 1657 1536

1612 1603 1610 1576 1658 1493 1609

1616 1706 1599 1557 1492 1629 1526

524 509 502 488 471 512 498

511 537 517 512 485 510 469

502 497 498 490 509 460 491

504 528 495 484 459 502 466

(continued on next page)

226

F.S. Spear et al. / Chemical Geology 266 (2009) 218–230

Table 6 (continued) % error from Poisson statistics

Y Spec 1

Th Spec 2

Th Spec 4

U Spec 3

U Spec 5

Pb Spec 2

Pb Spec 3

Pb Spec 4

Pb Spec 5

Spec 2

Spec 3

Spec 4

Spec 5

0.7

0.4

0.4

5.6

6.3

2.6

3

2.8

3

2.6

3

2.8

3

Composition ppm

Age Ma

Spot

Y Spec 1

Th Spec 2

Th Spec 4

U Spec 3

U Spec 5

Pb Spec 2

Pb Spec 3

Pb Spec 4

Pb Spec 5

Spec 2

Spec 3

Spec 4

Spec 5

August 15, 2008 8 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 Average S.D. measured S.D. Poisson

5925 5903 5884 5970 5957 5755 5766 5742 5864 5819 5910 5907 5896 5921 5872 5902 5973 5783 5639 5751 5776 5775 5698 5838 5761 5704 5714 5650 5771 5824 95 16

68,257 68,078 67,529 67,584 67,819 67,763 67,582 68,722 68,289 68,717 68,227 69,038 69,328 69,601 69,622 69,482 70,290 67,452 67,174 67,370 67,626 67,464 66,805 67,152 67,480 67,283 67,561 66,948 68,084 67,958 883 147

68,711 69,352 68,479 68,281 68,496 68,609 68,796 69,079 68,844 69,382 69,606 69,979 69,893 70,217 70,440 70,646 70,419 68,161 67,777 68,692 68,367 67,638 67,244 68,120 68,380 67,511 67,551 67,169 68,416 68,657 933 155

1964 2155 2105 2185 1818 1997 2052 2300 1944 2089 2052 1966 2191 1963 1997 2083 1932 2005 2000 1905 2094 1939 2044 1980 2081 2022 2099 2117 2081 2036 96 16

1907 2058 1979 1925 2084 2193 2253 2269 1985 2147 1912 1992 2065 2036 1956 2220 2187 2107 2155 1955 2013 1933 2248 2215 2217 2187 2271 1927 2206 2090 117 19

1583 1623 1612 1632 1649 1703 1765 1578 1678 1666 1709 1708 1650 1695 1661 1624 1568 1587 1669 1666 1690 1607 1736 1562 1728 1674 1617 1590 1616 1644 54 9

1577 1559 1610 1616 1545 1663 1715 1585 1679 1572 1665 1757 1689 1695 1604 1689 1557 1612 1589 1619 1605 1546 1460 1633 1588 1633 1544 1560 1585 1617 63 11

1580 1599 1464 1522 1555 1613 1652 1579 1598 1605 1668 1643 1635 1756 1714 1563 1602 1669 1663 1601 1663 1590 1615 1641 1605 1580 1596 1488 1527 1604 60 10

1599 1541 1512 1578 1547 1542 1643 1685 1531 1519 1580 1667 1487 1632 1607 1576 1670 1617 1586 1596 1687 1629 1639 1579 1602 1568 1709 1490 1623 1594 61 10

487 494 497 503 509 522 539 476 515 505 522 517 496 510 500 485 469 491 517 516 520 501 539 483 530 519 498 497 494 504 17 3

485 475 496 498 478 510 524 478 515 477 508 532 508 510 483 504 466 498 493 502 495 482 454 505 488 506 476 488 485 496 19 3

486 487 452 470 481 495 506 476 491 487 509 498 492 528 516 467 479 515 515 496 512 496 502 507 493 490 492 466 467 492 17 3

492 470 467 487 478 474 503 508 471 461 483 505 448 492 484 471 499 499 492 495 519 508 509 488 492 486 526 466 496 489 19 3

Grand average S.D. measured S.D. Poisson s.e. of mean

5727 104 40 10.5

68,276 669 273 67.6

68,505 1378 274 139.2

1989 132 111 13.3

2065 117 130 11.8

1622 58 42 5.8

1620 56 49 5.6

1615 67 45 6.8

1600 58 48 5.9

498 18 13 1.6

498 18 15 1.5

496 19 14 1.6

492 18 15 1.6

weeks. Each row presents compositions and ages of a single spot analysis following the protocol presented in Table 4 and the postprocessing of the interference corrections (Table 2). The purpose of Table 6 is to examine the consistency of analyses between spectrometers, within a single session (to ascertain homogeneity), and between sessions. The first point to discuss is the compositional variation among different spots on the same spectrometer. For example, the yttrium data collected on May 9, 2008 show an average content of 5556 ppm with a measured standard deviation of 53 (n = 19). The theoretical standard deviation, calculated from Poisson statistics on the unknown only (so that it is a minimum value, although the contribution from the standard is trivial) is 39. The other elements show similar agreement between measured and theoretical standard deviations. This result suggests that this grain of “Moacyr” is chemically homogeneous, which is a significant finding because it means that the analytical protocol can be evaluated and compared from session to session by comparing the monazite composition, rather than the age. Evaluating chemical dating methods by comparing monazite ages in chemically inhomogeneous monazite is inherently unsatisfactory because the age depends on the concentrations of several different elements (Pb, Th, U, and Y) and may be affected by offsetting errors, as discussed above. Comparison of the chemical data between spectrometers reveals examples where the difference in composition falls outside the 95% (2σ) confidence intervals of the data. For example, the average of the Th data from the May 9, 2008 session differ by 2565, which is far outside the calculated standard error of each respective mean of 67– 90 ppm, and is an order of magnitude larger than the standard

deviation of either spectrometer (ca. 274–264 ppm). Whereas both spectrometers were calibrated identically using the same standard spots, this discrepancy can only be attributed to a faulty standardization and emphasizes the need to monitor the standard during the analysis (see additional discussion in the section on instrument drift below). Note that the Th data from the May 19, 2008 session do not display this discrepancy. 4.4. Analysis reproducibility Data were collected in the same region of a single grain of “Moacyr” over several weeks, and examination of all the data provides perspective on the reproducibility of the entire analytical setup. In order to facilitate comparison of elements with different concentrations, the data were first scaled by dividing the average composition value for each analytical session (Table 6) by the grand average of all the analyses and then plotted (Fig. 3). In addition, during the June 8, 2008 session, the standard material was analyzed before and after each set of “Moacyr” analyses and the K ratios for these analyses are also plotted. The standard K ratio should be 1.0 if the measured value matches the standardization exactly. However, the measurements of the standards bracketing the unknowns were made at the analytical current (100 nA) of the monazite analyses, and not the analytical current of the standardization (10 nA). Some shift of the peak within the PHA is expected owing to significant differences in count rate, so the K ratios are not exactly 1.0. These data on the standard are included in Fig. 3 in their proper time sequence so that variations in the monazite composition can be interpreted as either instrument drift (if both standard and monazite shift in the same

F.S. Spear et al. / Chemical Geology 266 (2009) 218–230

227

June 8 in the first set of “Moacyr” analyses. The former was attributed to a bad standardization, but the latter is the result of unknown causes. If it were sample heterogeneity, then both spectrometers (3 and 5) should show the same spike. However, only one spectrometer records such a high value, so the result must be attributed to unknown analytical uncertainties. The data in Fig. 3 can be used to ascertain a reasonable value for the analytical uncertainty of monazite analyses using the electron microprobe. Most of the data fall within ±2% of the mean, and it is believed that this is a fair representation of the analytical reproducibility of monazite analyses, even with considerable care being taken to optimize the precision of the results. Additional errors also apply such as the uncertainty in the corrections due largely to Y and Th on PbMα, which may add on the order of an additional 0.5–1% uncertainty, and uncertainties in standard composition may contribute further. The minimum uncertainty for chemical ages of monazite can also be inferred from data in Table 6. Compare, for example, the four columns of “Moacyr” ages collected on May 9, which average 499, 509, 482, and 494 Ma, respectively. The similarity of the measured standard deviations (16, 16, 20, and 16) to the expected deviation from Poisson statistics (13, 15, 14, and 15) suggests that each spectrometer is sampling ages from a single population and thus the average is a meaningful value. However, the range of average ages is larger than 4 times the standard error of the mean ages (4, 4, 5, and 4), which must be interpreted to mean that not all spectrometers are sampling the same age population. That is, there are systematic differences in the ages determined on the different spectrometers that exceed statistical expectations and that these represent an irreducible contribution to the age uncertainty. The sources of these uncertainties are not known, but they are undoubtedly present. Therefore, it is not permissible to simply pool ages from the different spectrometers in an attempt to reduce the apparent uncertainty. Similarly, inasmuch as there is no way to determine which of the four spectrometers is giving the correct answer, one cannot reduce the apparent uncertainty from a single spectrometer by collection of more spots, as has been done by, for example, Terry et al. (2000). The true age uncertainty must reflect the vagaries of instrument reproducibility and stability, as well as statistical considerations. To complicate matters further, repeated sets of analyses on different days reveal that the variations between spectrometers are not systematic. As shown in Fig. 4, an individual spectrometer may sometimes record the oldest of the four ages and sometimes the youngest. This observation indicates that this additional uncertainty may not be treated as systematic error (i.e., accuracy) but, rather,

Fig. 3. Plots showing spectrometer performance over time for analyses of “Moacyr middle”. Each line represents a specific element on an individual spectrometer. (a) Y and Th (spectrometers 1, 2, and 4); (b) U (spectrometers 3, and 5); (c) Pb (spectrometers 2, 3, 4, and 5). Symbols aligned vertically represent individual analysis sessions: the first two are May 9, 2008, May 19, 2008. The last 11 are from June 08, 2008 in which groups of 12 analyses of “Moacyr” monazite were bracketed by analyses of the standard material using a modified version of the analysis protocol for the unknown to ascertain drift. Note that the “Moacyr” analyses have been normalized to the grand average of all of the compositions, whereas the standard analyses have been normalized to the standardization value (i.e., the unmodified K ratio is plotted). Thus, the values of the data for “Moacyr” and the standards are not directly comparable, but the trends in the data are. Error bars are 1 sigma from counting statistics.

direction) or sample heterogeneity (if only the monazite composition differs from the mean). As can be seen in Fig. 3, most of the data vary between 0.98 and 1.02. The most notable exception is the Th data on spectrometer 4 collected on May 9, as discussed above, and the U data collected on

Fig. 4. Plot of chemical ages for chemically homogeneous grain of the monazite “Moacyr” calculated for Pb analyses collected on the same spots on each of the four spectrometers for ten separate analytical runs. Error bars on each age are 2 s.e. of the mean (n = 19 for sessions 1 and 2, and n = 12 for the remainder). Note that on some analytical session the four spectrometers give statistically indistinguishable ages, but on others the difference between spectrometers exceeds 4 sigma s.e.

228

F.S. Spear et al. / Chemical Geology 266 (2009) 218–230

should be considered as an additional uncertainty about the mean of the analyses (i.e., precision). It must be concluded that, at this stage of our understanding, an electron microprobe age of the “Moacyr” monazite is not likely to be more precise than ±2–3%, regardless of how many analyses are pooled. Extension of these results to monazites of different compositions and ages is done in the next sections. 4.5. Other monazite analyses Table 7 presents a summary of analyses of other monazite standard grains. The first entry for grain “Moacyr right” is a second grain from the sample originally obtained from J.-M. Montel. The composition of this grain is distinct from those of “Moacyr middle”, especially in Th and Y. However, the ages determined for the two grains are statistically identical. This might suggest that it would be possible to pool all of the ages to obtain an average with a small standard error. In fact, were one to do this the result would be 497±0.7 Ma (n=696), whereas the more realistic 2% uncertainty results in 497±10 Ma. Unfortunately, the isotopic age of this sample does not help constrain the accuracy. Seydoux-Guillaume et al. (2002) cite a concordant U–Pb TIMS age of 474±1 Ma. However, J. Crowley has determined a TIMS age of a grain of our “Moacyr” separate to be 207Pb/206Pb age of 509.3 ±0.5 Ma with the 206Pb/238U age ca. 8 Ma

older (J. Crowley, personal communication, 2007). These discrepancies suggest that the “Moacyr monazite” may be a mixture of more than one monazite age population. The samples labeled “Trebilcock 1” and “Trebilcock 2” are two grains of monazite from the Trebilcock quarry, obtained from Tomascak (1995; Tomascak et al., 1996) and have a 207Pb/235U age of 271–272 Ma and 206Pb /238U age of 279–281 (mild reverse discordance). Individual grains of Trebilcock are zoned, although the set of analyses reported here was taken in a small region, and the statistics indicate that each set is chemically homogeneous, even though the two sets (Trebilcock 1 and Trebilcock 2) are different in composition. The average age for all 12 analyses (240 spot analyses total) is 270 ± 8 Ma, similar to the isotopic age. The monazite labeled UCLA 76 carries a published TIMS age of 336 Ma (T. M. Harrison, personal communication). It is included here as an example of a high-U monazite. The mean chemical age of 339 ± 10 Ma is within the statistical uncertainty of the isotopic age and implies that the correction factor for U provides the proper correction. Monazite labeled UCLA 554 is young (208Pb/232Th age of 45 ± 1 Ma; Harrison et al., 1999) and has only on the order of 130 ppm U. It was included to see whether the correction procedure was capable of obtaining near zero Pb values. Based on the age of 45 Ma and the measured Th content, the predicted radiogenic Pb content should be

Table 7 Compositions and ages of monazite standards and REE phosphates. Y Spec 1

Th Spec 2

Th Spec 4

U Spec 3

U Spec 5

Pb Spec 2

Pb Spec 3

Pb Spec 4

Pb Spec 5

Composition (ppm)

Pb Spec 2

Pb Spec 3

Pb Spec 4

Pb Spec 5

Age (Ma)

Moacyr right May 9 data Average (n = 20) 5731 Measured S.D. 46 Poisson S.D. 40

66,529 349 266

64,491 348 258

1940 116 107

2014 106 113

1575 58 44

1592 41 51

1535 48 45

1524 65 47

504 20 14

509 14 16

492 15 14

488 20 15

Moacyr right May 19 data Average (n = 20) 5816 Measured S.D. 33 Poisson S.D. 41

65,965 236 264

65,746 381 263

1808 130 99

1904 102 107

1566 63 44

1523 52 49

1557 49 45

1573 62 49

504 19 14

491 16 16

501 15 15

506 19 16

Treb 1 May 9 data Average (n = 20) 20,854 Measured S.D. 75 Poisson S.D. 63

116,855 401 351

113,284 396 340

7260 150 123

7505 109 128

1642 57 41

1621 65 45

1626 48 41

1612 63 40

273 9 7

270 11 8

270 8 7

268 10 7

Treb 1 May 19 data Average (n = 20) 21,562 Measured S.D. 63 Poisson S.D. 65

118,047 454 354

116,620 268 350

7247 122 123

7437 136 126

1643 54 41

1624 49 45

1626 43 41

1653 60 41

270 9 7

267 8 7

267 7 7

272 10 7

Treb 2 May 9 data Average (n = 20) 22,401 Measured S.D. 69 Poisson S.D. 67

133,134 744 399

131,243 731 394

7751 143 132

7941 132 127

1854 51 43

1861 67 47

1842 64 41

1810 43 43

272 8 6

273 9 7

270 9 6

266 6 6

UCLA 76 May 9 data Average (n = 20) 28,352 Measured S.D. 1773 Poisson S.D. 57

16,674 1866 133

16,110 1789 145

38,088 2937 190

38,370 2885 192

2151 190 45

2125 178 49

2061 167 43

2087 173 46

346 9 7

342 7 8

332 6 7

336 7 7

UCLA 554 May 9 data Average (n = 20) 12,266 Measured S.D. 421 Poisson S.D. 49

35,868 1723 179

34,614 1512 173

491 112 128

604 89 112

111 42 18

77 54 10

106 37 16

91 51 11

70 26 11

48 35 7

67 24 10

57 32 7

CePO4 August 15 data Average (n = 20) − 283 Measured S.D. 29 s.e. of mean 8

− 13 78 23

−6 75 22

−4 132 38

− 53 124 36

− 13 66 19

− 18 61 18

12 37 11

18 70 20

LaPO4 August 15 data Average (n = 20) − 245 Measured S.D. 40 s.e. of mean 11

− 21 93 27

85 73 21

− 52 138 40

− 24 115 33

33 82 24

−5 47 14

31 54 16

47 61 18

F.S. Spear et al. / Chemical Geology 266 (2009) 218–230

ca. 71 ppm. However, it can be seen that all four Pb analyses are larger (111, 77, 106, and 91 ppm) and at least two of these values lie outside of 2 sigma of the expected deviation from Poisson statistics. One possible explanation is the presence of common Pb (ca. 25 ppm). To verify that the analytical procedure (Table 4) was performing as expected and able to return a value of Pb = 0.0 on Pb-free REE phosphates, we decided to analyze two of the REE phosphates again using the full monazite dating protocol. The results (Table 7) document the success of the analytical procedure, but also emphasize the limitations to chemical dating using the electron microprobe. For CePO4, the average values of Pb (− 13, −18, 12, and 18 ppm) are all within 1 sigma s.e. (19, 18, 11, and 20 ppm) of the means. That is, the values are statistically indistinguishable from zero. For LaPO4, the average Pb values for three of the four spectrometers (Spec 2 = 33 ppm, Spec 3 = −5 ppm, Spec 4 = 31 ppm) are within 2 s.e. (i.e., 24, 14, 16 ppm, respectively) of zero. The Pb value on spectrometer 5 (Spec 5 = 47 ppm) is within 3 s.e. of zero, which, while not as satisfactory as the other values, is still statistically indistinguishable from zero. Unexpectedly, both CePO4 and LaPO4 have apparent Y contents of ca. 250 ppm, indicating some unsuspected interference with the background measurements in the vicinity of the YLα peak. However, the total correction on Pb for this apparent Y content is only 2 ppm, so the effect can be ignored. This result reemphasizes that, although it is possible to correct the Pb values for REE (and Y, Th and U) interferences, it is highly impractical to count for sufficiently long times to reduce the random errors associated with these corrections below around 20 to 30 ppm Pb, which is 1–1.5% of the Pb in a typical monazite containing 2000 ppm total Pb. Furthermore, the range of apparent Pb in the Pbfree REE phosphates for the average of 12 spots on the four spectrometers (Table 7) is −18 to +18 ppm for CePO4 and −5 to +47 ppm on LaPO4. This range is within statistical uncertainty, but must be considered part of the total Pb error on monazite when analyzing on only a single spectrometer. 4.6. Extending the results to monazite of different composition and age The results quoted above are specific to the monazites analyzed in this study which all have Early-to-Mid-Paleozoic ages (ca. 272 Ma to 500 Ma) and total Pb contents that range from ca. 1500 ppm to 2000 ppm (with the exception of UCLA 554). For the quoted error of around 2–3%, this corresponds to an error in the Pb concentration of around 30–60 ppm. The typical theoretical Poisson standard deviation using our analytical protocol is around 50 ppm so it is possible to obtain a standard error of the mean of around 10 ppm by averaging 25 spots. Comparing this with the observed error of around 30–60 ppm suggests that on the order of 20–50 ppm Pb is the magnitude of this irreducible error on our electron microprobe. So for monazites of different Pb concentrations a value of 20–50 ppm should be added to the Poisson uncertainty in evaluating the total age uncertainty.

229

monazite composition. These interferences have been known for some time for Y and Th, but this study also reveals corrections due to REE that range from 4 to 28 ppm, and for U of up to −30 ppm, depending on the spectrometer and the location of the background position. Repeated analyses of the same, chemically homogeneous monazite, yield compositions determined from Pb analyses on four spectrometers that differ by more than the standard error of the means, even after making spectrometer-specific corrections. This implies that there are spectrometer-dependent differences that have yet to be identified. It also implies that there are irreducible errors that cannot be avoided and cannot be reduced by collecting more Pb counts. Comparison of the compositions of monazite collected simultaneously on four spectrometers suggests that this irreducible error is on the order of 2–3% for monazites with 1500–2000 ppm total Pb or an absolute value of around 20–50 ppm Pb uncertainty in addition to the Poisson uncertainty. These errors must be considered when evaluating and interpreting chemical ages of monazite, and especially when comparing chemical ages to other ages. The only way to obtain the optimal age resolution expected from counting statistics is to analyze different age domains on the same spectrometer during the same analytical session, while monitoring the standard counts every dozen analyses or so. Even in this case, it is expected that unknown environmental factors may influence spectrometer response to render the precision larger than the value obtained from Poisson statistics (i.e., the standard error of the mean). Caution is therefore recommended not to over-interpret small differences in apparent age observed within or between crystals of monazite. It should also be emphasized that the analysis presented in this paper addresses only uncertainties arising from counting statistics and spectrometer reproducibility. This study in no way addresses the accuracy of chemical ages of monazite. The similarity in the chemical and isotopic ages of the Trebilcock monazite (270 ± 8 Ma and 272 Ma, respectively) and the UCLA 76 monazite (339 ± 10 Ma and 336 Ma, respectively) validates the chemical dating technique for these monazites. The difference between the chemical and isotopic ages of the “Moacyr” monazite (497 ± 10 Ma and 509.3 ± 0.05 Ma) is larger, but still within analytical uncertainty of each other. The apparent excess of around 25 ppm Pb in the UCLA 554 monazite raises the specter of unknown quantities of common Pb skewing chemical ages to older values, although this possibility clearly requires additional study. Monazite is notoriously zoned, which has rendered previous interlaboratory comparative studies difficult to interpret. An interlaboratory study in which all labs analyzed the exact same grain of “Moacyr”, which this study has shown to be chemically homogenous on a grain scale, would greatly further our confidence in the application of chemical ages to understanding metamorphic histories and tectonic processes and, at the very least, permit meaningful comparisons of chemical ages determined by different laboratories.

5. Conclusions

Acknowledgments

This study demonstrates that even nominally identical spectrometers have individual characteristics that result in different analytical results on analyses collected simultaneously. This study confirms the result of Jercinovic and Williams (2005) and Williams et al. (2006) that an exponential background fit is preferable to a linear background fit for PbMα analyses because the background positions must be taken far from the peak position. For most elemental analysis, linear and exponential background fits give very nearly identical results. However, this study also demonstrates that it is impossible to obtain a 0 ppm Pb value on Pb-free material (silicates and phosphates) on four spectrometers simultaneously using either a linear or an exponential background fit and that, in general, it is necessary to make corrections to the apparent Pb content based on the

This work was supported by grants EAR-0337413 (to Spear) and EAR-0230019 (to Cherniak). Thoughtful reviews by P. Dahl and an anonymous reviewer are gratefully acknowledged.

References Cherniak, D.J., Pyle, J.M., Rakovan, J., 2004. Synthesis of REE and Y phosphates by Pb-free flux methods and their utilization as standards for electron microprobe analysis and in design of monazite chemical U–Th–Pb dating protocol. American Mineralogist 89, 1533–1539. Dahl, P.S., Hamilton, M.A., Jercinovic, M.J., Terry, M.P., Williams, M.L., Frei, R., 2005. Comparative isotopic and chemical geochronometry of monazite, with implications for U–Th–Pb dating by electron microprobe; an example from metamorphic rocks of the eastern Wyoming Craton (U.S.A.). American Mineralogist 90, 619–638.

230

F.S. Spear et al. / Chemical Geology 266 (2009) 218–230

Harrison, T.M., Grove, M., McKeegan, K.D., Coath, C.D., Lovera, O.M., LeFort, P., 1999. Origin and episodic emplacement of the Manaslu intrusive complex, Central Himalaya. Journal of Petrology 40, 3–19. Jercinovic, M.J., Williams, M.L., 2005. Analytical perils (and progress) in electron microprobe trace element analysis applied to geochronology: back-ground acquisition, interferences, and beam irradiation effects. American Mineralogist 90, 526–546. Montel, J.-M., Foret, S., Veschambre, M., Nicollet, C., Provost, A., 1996. Electron microprobe dating of monazite. Chemical Geology 131, 37–53. Pyle, J.M., Spear, F.S., Wark, D.A., Daniel, C.G., Storm, L.C., 2005. Contributions to precision and accuracy of monazite microprobe ages. American Mineralogist 90, 547–577. Seydoux-Guillaume, A.-M., Paquette, J.-L., Wiedenbeck, M., Montel, J.-M., Heinrich, W., 2002. Experimental resetting of the U–Th–Pb systems in monazite. Chemical Geology 191, 165–181. Suzuki, K., Adachi, M., 1991. Precambrian provenance and Silurian metamorphism of the Tsubonosawa paragneiss in the south Kitakami terrane, Northeast Japan, revealed by the chemical Th–U–total Pb isochron ages of monazite, zircon and xenotime. Geochemical Journal 25, 357–376.

Suzuki, K., Adachi, M., Kajizuka, I., 1994. Electron microprobe observations of Pb diffusion in metamorphosed detrital monazites. Earth and Planetary Science Letters 128 (3–4), 391–405. Terry, M.P., Robinson, P., Hamilton, M.A., Jercinovic, M.J., 2000. Monazite geochronology of UHP and HP metamorphism, deformation, and exhumation, Nordøyane, Western Gneiss Region, Norway. American Mineralogist 85, 1651–1664. Tomascak, P.B., 1995. Petrogenesis of the last Paleozoic granites, Maine. Geological Society of America Abstracts with Programs 27, A-468. Tomascak, P.B., Krogstad, E.J., Walker, R.J., 1996. U–Pb monazite geochronology of granitic rocks from Maine: implications for Late Paleozoic tectonics in the northern Appalachians. Journal of Geology 104, 185–195. Williams, M.L., Jercinovic, M.J., Goncalves, P., Mahan, K., 2006. Format and philosophy for collecting, compiling, and reporting microprobe monazite ages. Chemical Geology 225, 1–15. Williams, M.L., Jercinovic, M.J., Hetherington, C.J., 2007. Microprobe monazite geochronology: understanding geologic processes by integrating composition and chronology. Annual Review of Earth and Planetary Sciences 35, 137–175.