Dr. Stuart N. Thomson
Research Scientist
Department of Geosciences
University of Arizona

Apatite LA-MC-ICPMS U-Pb Dating

Over the last 2 years I have been involved in developing routine apatite U-Pb analysis using the Nu Plasma HR Multicollector ICPMS at the Arizona LaserChron Center.

The methods have now been published in the AGU Journal G-Cubed (February 2012).

Thomson, S.N., Gehrels, G.E., Ruiz, J. & Buchwaldt, R. (2012). Routine low-damage U-Pb dating of apatite using laser ablation-multicollector-ICPMS. Geochemistry, Geophysics, Geosystems, 13, Q0AA21, doi:10.1029/2011GC003928

Nu Plasma HR Multicollector Inductively Coupled Plasma Mass Spectrometer

Summary of Methods

1) Challenges to reliable U-Pb dating of apatite

U-Pb dating of apatite is made difficult by its generally low U concentrations and hence limited production of measurable concentrations of radiogenic Pb), as well its tendency to incorporate high amounts of common lead during formation and/or recrystallization. This has previously limited accurate apatite U-Pb dating to destructive isotope dilution methods. Furthermore, attempts to apply in situ SHRIMP and laser ablation (LA) ICPMS U-Pb techniques on apatite have been hindered by the lack of well-characterized matrix-matched standards to correct for elemental fractionation, as well as by the difficulty in accurately and precisely measuring 204Pb to provide a robust common lead correction that does not rely on an assumption of concordance.

2) New analytical methods developed at the Arizona LaserChron Center

To overcome these limitations we have identified two new well-characterized matrix-matched apatite standards to correct for elemental fractionation and directly measure 204Pb isobarically corrected for 202Hg to allow for common Pb correction without the assumption of concordance.

3) Data Acquisition

Machine tuning and data acquisition is performed in the same way as zircon, although sample bracketing is done with a primary apatite standard (usually Madagascar, see below) with monitoring of machine performance done by ablating either a secondary apatite standard, NIST SRM 614 glass, or our in-house SL zircon standard.

Data are first corrected for background and any excess 204Hg. If necessary, measured 206Pb/204Pb values from ablating NIST SRM 614 glass are used to monitor and correct for ion counter gain, with an average value obtained from analyses at the beginning and end of each run compared to the known NIST SRM 614 206Pb/204Pb isotope composition of 17.842 [Baker et al., 2004]. Data are also corrected for down-hole laser fractionation, elemental fractionation, and common Pb correction.

4) Down-hole laser fractionation

Down-hole fractionation in apatite during lasing can be treated similarly to zircon.

Figure 1: Apatite down-hole laser fractionation. Note that the relatively low abundance of 207Pb in many apatite samples results in the typically noisy 206Pb/207Pb signal when using a smaller spot szie. In contrast to zircon, most apatite shows either a slightly decreasing (30 micron spot) or flat (65 micron spot) 206Pb/238U ratio during down-hole lasing. The higher 207Pb signal acquired when using the larger 65 micron spot results in significantly reduced run-time scatter in 206Pb/207Pb ratio.

5) New apatite matrix-matched elemental fractionation standards

A previous limitation to using in situ LA-ICMPS for U-Pb dating of apatite has been the lack of a well characterized matrix-matched standard to correct for the differential fractionation of U, Th, and Pb during laser ablation. Finding a suitable standard is made difficult by the tendency of apatite to have low and variable U (and hence radiogenic Pb) concentration, variable common Pb, and to lose Pb by thermally induced diffusion at temperatures > ca. 500°C. Despite this, we have identified two seemingly reliable natural standards: apatite from the McClure Mountain syenite, Colorado and our now preferred gem rough apatite from Madagascar. During analysis we employ a standard bracketing approach (five standards at the start, two standards between every four or five unknowns, and three standards at the end).

5.1) McClure Mountain apatite fractionation standard (MMap)

The Cambrian McClure Mountain syenite of Colorado is well known as the source of the widely used 40Ar/39Ar hornblende standard MMhb and apatite from this rock has a published U-Pb TIMS age of 523.5 Ma. Typical self-normalized concordia age uncertainty for 10-12 spots during a typical sample run is between 1.2-2.0% at the 2 sigma level. Over the course of 14 machine runs and 160 spot analyses the 2 sigma concordia age uncertainty is 0.6% (Figure 2a). The variable U concentrations and hence µ (238U/204Pb) between grains allows production of a well-defined isochron plot that does not rely on a common Pb correction (Figure 2b).

Figure 2: (a) Self-normalized MMap reference apatite data from 160 spots over 14 separate machine runs showing excellent concordia age precision of 1.2% (2 sigma). (b) Isochron plot from 169 individual spots from 14 machine runs (not corrected for common Pb).

5.2) Madagascar apatite fractionation standard (MAD)

In our exploration of other potential apatite fractionation standards, we had notable success with blue/green gem roughs of Madagascan apatite from the “1st Mine Discovery”. We have this apatite available in relatively large quantities in small gem quality chips about 1cm in size.

Figure 3: Typical selection of “1st Mine Discovery” blue/green gem rough apatite chips from Madagascar. These grains typically come from carbonatite pegmatite of late Cambrian (Pan-African) age.

We have obtained ID-TIMS U-Pb data from randomly chosen mm-sized shards taken from two larger fragments of the MAD apatite (labeled MAD1 and MAD2) as part of the NSF EARTHTIME initiative (www.earth-time.org) through Robert Buchwaldt at MIT. The smaller shards from each larger MAD crystal show excellent internal consistency in ID-TIMS U-Pb age and 206Pb/204Pb ratio. The data show some slight (ca. 1%) discordance most likely reflecting the common Pb correction being dependent of the Stacey and Kramers [1975] common lead evolution model rather than the true value. However the quoted 206Pb/238U weighted mean ages are insensitive to the correction used. The weighted mean 206Pb/238U age for each crystal differ by about 11 Ma. One explanation for this age difference is that variable Pb diffusion occurred during cooling through ca. 500°C owing to the dated crystal fragments being derived from different sized whole crystals.

Figure 4: ID-TIMS U-Pb ages from two randomly selected chips of Madagascar apatite showing weighted average 206Pb/238U ages and concordia plots. Data represented by the grey ellipses in the concordia plots were not used in the weighted mean age calculations.

During a typical apatite U-Pb laser ablation run using MAD1 apatite as the primary fractionation reference, very consistent self-normalized data is obtained (~1% uncertainty at the 2 sigma level from 132 spot analyses - Figure 5). The self-normalized concordia age MSWD value for the combined MAD1 LA-ICPMS data is low (0.033) indicative of uncertainty overestimation. The source of this excess uncertainty is most likely attributable to noise on the Faraday collector affecting the low 207Pb signal of ~15 kcps for this apatite using a 30 µm spot size.

Figure 5: Self-normalized MAD1 reference apatite data from 132 spots over 9 separate machine runs over course of 8 months showing excellent concordia age precision of 1.0% (2 sigma). The low MSWD value is indicative of ~4% overestimation of uncertainties.

6) Standard-Sample Fractionation bracketing

Figure 6: Examples of standard-unknown 206Pb/238U and 206Pb/207Pb fractionation correction bracketing using both the MAD-1 and MMhb apatite standards for both 30 micron and 65 micron spot szie. The blue points represent the standard fractionation value (measured ICPMS ratio compared to ratio expected given known ID-TIMS age). The thick red line represents the 30 point running average of these values with 1 sigma standard error shown by the thin red lines. The grey bars represent a ±2% error about the average.

7) Iterative Common Pb Correction

To accurately measure 204Pb, we assign its measurement to an independent ion-counter. The 206Pb, 207Pb, and 208Pb peaks are corrected for common Pb using measured and corrected 204Pb according to the Stacey and Kramers common lead model. However, owing to the high common to radiogenic Pb ratio (Pbc/Pb*) in most apatite, the initial age estimate from the raw measured data considerably overestimates the true age. To address this we apply a five-step iterative process to determine the 206Pb/238U age used to estimate initial 206Pb/204Pb according to the Stacey and Kramers model. We use the initial measured age to revise the Stacey and Kramers model value to calculate a new corrected 206Pb/238U ratio and age. This corrected age is then used to calculate a new Stacey and Kramersmodel value, and calculate a new 206Pb/238U ratio and age. We repeat this process five times. We find that for almost all apatite, ages converge after 2 or 3 iterations.

Some Example Data

For older apatite with high proportion of radiogenic Pb, we have found that using a 30 micron spot size, and pooling ages from 10+ individual grains provides good data without the need to unnecessarily damage the grain, and hence allow further analysis using other techniques. However, for younger grains ( ~75 Ma) with low proportion of radiogenic Pb, or older grains with very low uranium concentrations (5-10 ppm) data quality can be significantly improved by upping the spot size to as large as is reasonable given the size of the grain. some example data using different spot sizes are shown below.

1) Using a 30µm spot size

A) Bancroft Terrane Apatite: Large centimetre-sized apatite crystals from various Grenville aged pegmatites in Ontario, Canada are widely available. U-Pb sphene ages from the Bancroft terrane range from 1024 to 1074 Ma and hornblende Ar/Ar total fusion and plateau ages with closure temperature of ca. 500°C similar to that of U-Pb in apatite range between 956 and 996 Ma.

Figure 7: Apatite U-Pb data plots from 2 crystals of the Grenville Bancroft terrane, Canada. The weighted mean U-Pb ages obtained from both of these Bancroft terrane apatite crystals (958±13 Ma and 938±31 Ma, respectively) match well with similar closure temperature hornblende Ar/Ar cooling ages from the same region supporting the accuracy of our U-Pb apatite ages.

B) Forest Center Anorthosite, Duluth Complex, Minnesota: Zircon from anorthosite of the Duluth Complex, near Forest Center, Minnesota is a widely used standard for both LA-ICPMS and SIMS U-Pb dating. High precision ID-TIMS analysis of these zircons yield an age of 1099.0±0.6.

Figure 8: Apatite U-Pb data from anorthosite from Forest Center, Duluth Complex, Minnesota. These ages are within the uncertainty of the zircon age and imply fast cooling of these rocks to below 500°C following their intrusion. Common Pb uncorrected data plotted as an 238U-206Pb isochron give an age of 1070±140 Ma (95% confidence) with low MSWD of 0.056 implying some uncertainty overestimation.

2) Using 65µm and 110µm spot sizes

A) Mud Tank Apatite: Large zircon crystals from the Neoproterozoic Mud Tank carbonatite, Strangways Range, Northern Territory, Australia, are used widely as reference zircon. Large centimetre to decimetre-sized apatite is also found in the same deposit, and has been used as a calibration reference in (U-Th)/He and fission track dating. The Mud Tank apatite, with its low uranium concentrations (less than 3 ppm) and high proportion of common Pb (206Pb/204Pb values less than 30; Pb*/Pbc ratios less than 1) yields poor data when using the 30 µm spot size with a 2 sigma age uncertainty of ~11%. With larger spot size, a similar age is obtained. However, the data quality is accurate and much improved. With a ten spot ten 65 µm spots the uncertainly is 2.1%, while using a very large 110µm spot, a 2 sigma uncertainty of 1.7% was obtained from only 5 spots.

Figure 9: Large laser spot size apatite U-Pb data on a low uranium apatite, Mud Tank.

B) Durango Apatite: Gem apatite from the early Oligocene Cerro de Mercado iron ore deposit, Durango, Mexico (31.4 Ma) is a widely used reference material for both fission track and (U-Th)/He dating. Data quality using larger spot size with the young Durango apatite is markedly improved. While the uranium concentration and 206Pb/204Pb values of this apatite are unremarkable (ca. 10-15ppm, and between 90 and 160, respectively), the amount of total Pb (radiogenic + common Pb) is very low with typical 207 signal often close to 1000 cps using a 30 µm spot making the acquisition of good quality data challenging. However, when using a 65 µm spot a concordia age of 32.2±5.3 Ma (2 sigma, MSWD = 0.21) was obtained from only 10 spots (uncertainty of 16%), albeit with two spots showing large uncertainties. With the 110µm spot size, 5 spots yielded a concordia age of 32.0±3.1 (2 sigma, MSWD = 0.32) - a 2 sigma uncertainty of 9.7%. These ages compare well with the precise sanidine-anorthoclase Ar/Ar reference age of 31.44±0.18 Ma.

Figure 10: Large laser spot size apatite U-Pb data on the young 31 Ma Durango apatite.

Last Modified: January 12th, 2012