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Monazite U-Th-Pb Ages and Trace Element Analyses

Monazite is a REE-rich phosphate mineral that can be challenging to analyze because it has very high concentrations of Th as well as La, Ce, Pr, and Nd. Molecules of these elements can combine with H, O, and P during the sputtering and ionization process and create molecular and/or doubly-charged isobaric interferences at the Pb masses positions. To eliminate these interferences, we operate the SHRIMP-RG using the energy selection window to only accept high-energy ions into the collector. Because metal ions (e.g., Pb+) have high higher energy than molecules with the same mass, this procedure dramatically reduced potential isobaric interferences. We monitored the 205 AMU mass spectra where the 205Pb+ peak would occur, and adjust the energy selection window until the interference at mass 205 is removed or minimized.  Although moving the energy window in this manner reduced the total secondary ion transmission by a factor of 5 to 15, this reduction is necessary to optain accurate U-Pb, Th-Pb, and Pb-Pb isotopic measurements.

Because monazites are enriched in Th relative to U (typical Th/U >10), Th-Pb ages have similar analytical precision to U-Pb ages. However, the major limitation to applying Th-Pb ages to in-situ analyses of monazite is the general lack of monazite standards that have Th-Pb TIMS ages. The majority of monzaite standards are dated by U-Pb, and although one can assume concordance between the U-Pb and Th-Pb systems, this might not always be an accurate assumption. Each monazite analysis on SHRIMP-RG measures U, Th and Pb isotopes so that both U-Pb and Th-Pb ages can be calculated, but we currently have the highest confidence in the U-Pb ages. New Th-Pb TIMS analyses on existing and new monazite standards is ongoing.

The example below shows U-Pb results of three well-behaved igneous monazite samples analyzed on SHRIMP-RG. Samples are well-resolved at 2 sigma uncertainty, are reproducible, and concordant (with the exception of one grain in sample 2 that was reversely discordant and omitted - grey).

Monazite Analyses on SHRIMP-RG

1) The shortest option is to collect monazite U-Pb ages without trace elements (this likely offers *slightly* higher precision than option 2, depending on instrument conditions). The run table for ages includes the following: 204Pb, a background measured at 0.045 mass units above the 204Pb peak, 206Pb, 207Pb, 208Pb, Th, and U. REE molecules (LaPO2, CePO2, and NdPO2) are also measured to monitor matrix issues between the standard and unknown monazite. This run table allows U-Pb, Pb-Pb and/or Th-Pb ages, to be calcuated - depending on the sample's age and/or composition. Individual analyses usually take 15-17 min. per spot.

2)  The second option is combining Age and REE analysis of monazite.  The run table is similar to option one, but also includes Y, Sm, Eu, Gd, Dy, Er, and Yb. These trace element measurements (Y, REE) are measured briefly (typically 1 to 4 sec/mass) immediately before the geochronology peaks, and in mass order. REE molecules (LaPO2, CePO2, PrPO2, and NdPO2) are also measured to monitor matrix issues between the standard and unknown monazite. These analyses take approximately 20 min. per analysis.

3) The third option is monazite trace elements, measured without ages. The following is a list of our routinely measured zircon trace element suite:

Li, F, Na, Al, Si, P, K, Ca, Sc, V, Mn, Fe, Ge, As, Sr, Zr, Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Th, Yb, Lu, Hf, Pb, Th, U.  If you are interested in a specific subset of these elements, the run table can easily be tailored to suit your needs. Other elements my also be possible.


The example below from Aleinikoff et al. (2012) shows reflected light and backscattered electron (BSE) images of monazites, showing patchy textures over older igneous primary zoning. Individual SHRIMP-RG spots and ages are shown in the BSE image, and the complete data set for this sample is shown on the concordia diagram and as a probability density plot, revealing two populations of monazite ages in this sample. On the right are chondrite normalized REE plots for the older and younger poputions of monazites. The younger, metamorphic monazites are depleted in HREE relative to the older zones. For a full discussion of the data, see Aleinikoff et al. (2012). These data were collected with option #2 above; combined U-Th-Pb ages and REE from the same analytical volume, which are routine on SHRIMP-RG.