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Zircon U-Th-Pb and U-Th Ages and Trace Element Analyses

The reverse geometry and a highly stable laminated magnet allows large run tables (i.e., large number of isotopes) to be performed in a single analysis on the SHRIMP-RG. Thus, we routinely combine trace elements with U-Pb and U-Th isotope measurements. This allows compositional data (e.g., Ti, Y, REE's, Hf, etc) to be measured from the exact same analytically volume as the radiometric age. The biggest drawback is that individual analyses will take longer than measurements of U-Pb, Th-Pb, Pb-Pb, or U-Th ages alone.

Approximately 70% of the analyses performed on the SHRIMP-RG are focused on zircon geochronology and trace element analyses. For U-Pb ages, the analytical time depends on the U concentration and the age of the sample. In general, if the sample is young (e.g., <25 Ma), the analyses will generally require longer count times to obtain the same statistical uncertainty as, say, a 250 Ma sample. Below is an example of the approximate time for an individual zircon analysis with 3, 13, or 15 trace elements measured simultaneously with U-Pb ages. 

Analytical Time (per spot)      Zircon Age Trace Elements Measured with U-Pb ages
          12 to 15 (min.)          > 250 Ma U-Pb age + Hf, U, Th
          14 to 18 (min.)             ~ 5 Ma U-Pb age  + Hf, U, Th
          16 to 18 (min.)          > 250 Ma U-Pb + 9-REE (La, Ce, Nd, Sm, Eu, Gd, Dy, Er, Yb), Y, Hf, U, Th
          18 to 20 (min.)             ~ 5 Ma U-Pb + 9-REE (La, Ce, Nd, Sm, Eu, Gd, Dy, Er, Yb), Y, Hf, U, Th
          20 to 22 (min.)          > 250 Ma U-Pb + Ti, Fe, 9-REE (La, Ce, Nd, Sm, Eu, Gd, Dy, Er, Yb), Y, Hf, U, Th
          23 to 25 (min.)             ~ 5 Ma U-Pb + Ti, Fe, 9-REE (La, Ce, Nd, Sm, Eu, Gd, Dy, Er, Yb), Y, Hf, U, Th

This table only represents an example of what is available on SHRIMP-RG. The increased time necessary for young samples reflects the increased counting time necessary to measure the radiogenic 206Pb*. If you are interested in a specific subset of these elements, the run table can easily be tailored to suit your needs. Trace elements can also be measured separately (see below).

Zircon U-Pb ages and uncertainties

In general, the analytical uncertainty for an individual zircon U-Pb spot ranges from 2 to 5%, depending primarily on U concentration and the age of the samples. Therefore, the interpreted zircon crystallization age for a particular sample is often interpreted to be the weighted mean of 8 to 16 individual zircon analyses, and the uncertainty (reported as the 2 sigma standards error) typically ranges from 1 to 2% for a homogeneous population of zircon. Zircon populations that have protracted crystallization histories, or contain older anticrystic or xenocrystic age populations, can be more difficult to interpret and often require >20 analyses per sample.

Zircon age standards are analyzed every 5th analysis to bracket unknowns throughout the analytical session.

Below, is an example of weighted mean age and inverse isochron age for a Miocene zircon sample. Also highlighted is the reproducibility of the results from session to session, as well as the better than 1% analytical uncertainty of this calculated age. The inverse isochron diagram (akam Tera-Wasserburg) shows mixing between 100% radiogenic grains and common Pb.

Zircon 238U-230Th disequilibrium ages and uncertainties

Ages can be obtained from zircon which are <350 ky by exploiting the disequilibrium between 238U and 230Th that is produced during crystallization. These analyses are challenging because 230Th is present at the ppb level and thus requires a large primary ion spot size (~30 x 35 microns) and zircons which have an average U concentration >500 ppm. These analyses typically take 30 to 35 minutes each, and will require knowledge of the U and Th isotope composition of associated whole-rocks or glasses to constrain the initial 230Th disequilibrium and calculate dates. Allanite and monazite are also good candidate minerals for in-situ U-Th analyses. Mineral grains can either be mounted in epoxy or thin section for analyses of polished grain interiors, or mounted in indium to analyze the surfaces (see depth profiling at the bottom of the page)

Below is an example of U-Th zircon crystallization ages for a ignimbrite sample from Valles Caldera. Data is presented on a probability density ranked plot and on a isochron diagram  Analyses were performed on zircon surfaces, mounted in indium. The 40Ar/39Ar eruption age for this unit is 74.4 ± 1.3 ka, which is within uncertainty of the U-Th zircon age measured on SHRIMP-RG. See the full paper here: http://dx.doi.org/10.1016/j.jvolgeores.2015.11.021

Zircon Trace Elements

Combining zircon U-Pb ages with trace elements has its limits. The analytical limitations include: [1] the fact that it is difficult to maintain magnet stability going from high mass (238U) to low mass (48Ti), and [2] the high mass resolving power is necessary to resolve some trace elements from overlapping interferences (e.g., Nb and Sc in zircon). Also, in terms of sample throughput, having excessive numbers of trace elements can make the duration of each analysis very long, which can influence the analytical precision of individual U-Pb analyses. Thus, we also offer zircon analyses of trace elements, without ages.  

Below is an example of U-Th zircon ages and trace elements from the Central Plateau Member (CPM) lavas in Yellowstone Caldera, measured on SHRIMP-RG. Each point represents the weighted average and 2-sigma error for each sample. The horizontal gray fields represent the times when Yellowstone was actively erupting. Glass data is also shown for comparison. The ability to combine both age and compositional information on the same a samples can provide more insight into complex petrogenetic problems in earth science. For more information about these data, see the whole paper by Stelton et al (2015) here: http://dx.doi.org/10.1093/petrology/egv047.

The following is a list of our routinely measured zircon trace element suite.

Li, Be, B, F, Na, Al, Si, P, K, Ca, Sc, Ti, Fe, Y, Nb, Zr, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Th, Yb, Lu, Hf, Pb, Th, U.

The Ti in zircon thermometer of (Watson et al., 2006; Ferry et al., 2007) allows model temperatures to be calculated from Ti trace element measurements. This is routinely measured at the same time as U-Pb or U-Th age measurements.

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. 

Zircon U-Pb depth profiling

The greatest advantage of secondary ion mass spectrometry (SIMS) is the high spatial resolution. Unlike other techniques, ion-probe analyses have the ability to perform measurements from analytical volumes of 1 to 5 nanograms of material. As a results, the sputtered pit geometry typically has a diameter of ~15 5o 25 microns, and a depth of ~2 microns.  To take advantage of the relatively slow sputter rate, we will mount mineral grains (e.g., zircons) in soft indium metal to expose the euhedral mineral face parallel to the sample surface (see Sample Prep. page for more details about mounting). Using this approach, analyses performed on the sample surface will provide information about the change in composition and/or age with depth below the initial surface.

We commonly use this approach for measuring the age of thin (<1 micron) metamorphic rims on zircons (see example). For obtaining zircon U-Pb or U-Th ages that from young volcanic rocks, we analyze the zircon surfaces by depth profiling because the zircon surface records the last crystallization event and has the highest probability to correspond to the eruption age sample. There are many examples where this approach yields ages that are within error of Ar-Ar or other measure of eruption age for the same sample, and many examples where the zircon surfaces are record only older crystallization ages.

The example below is from a high-U (2000 ppm) metamorphic zircon rim, showing a well-resolved 22.5 Ma rim, that is approximately 450 nanometer thick. After 450 nanometers, the age begins to steadily increase because the pit is sputtering into an older interior of the grain. However, because the pit geometry is bowl-shaped, the increase in age after 450 nanometers defines a mixing trend between the 22.5 Ma rim and ~500 Ma core. If the sputtered spot was better focused, with steep sides and a flat bottom, this transition could be better resolved. However, SIMS analyses will always implant material over the duration of the analysis, so there will always be some mixing between depth-resolved zones. The 3D image of the pit geometry and pit-depths was acquired by white-light interferometry at UC Santa Cruz.