Titanite (a.k.a Sphene) U-Pb Ages and Trace Element Analyses
Analyses of U-Pb, Th-Pb, and/or Pb-Pb ages of titanite can be performed on SHRIMP-RG, but precision is typically limited by U content. Trace element measurements are routine (e.g., Li, B, F, P, V, Sr, Y, Zr, Nb, Ba, REE, Hf, Ta, Th, U). Analyses can be performed on grains mounted in epoxy, or in-situ from thin section pieces or rock chips
Titanite can be analyzed for U-Pb ages as well as for trace element compositions. Unlike zircon, common-Pb, titanites generally have very low uranium concentrations (<200 ppm) and can have significant proportions of non-radiogenic inherited Pb (i.e., common Pb). This is particularly true for metamorphic mineral which are typically depleted in U, Th, and REE's. In addition, titanite has a closure temperature for Pb between ~550 and 650 deg. C (Cherniak 1993; Corfu 1988; Frost et al. 2000). Therefore, titanite separated from rocks that have had protracted thermal histories can record multiple episodes of titanite growth, or U-Pb ages may have been partially or completely reset. In combination titanite trace element geochemistry and other thermochronometers (e.g., zircon U-Pb, 40Ar/39Ar ages, etc), titantite data can provide useful information about the crystallization or cooling history of rock samples.
Titanite U-Pb Geochronology
U-Pb analyses of titanite are complicated because the uranium concentration is typically low and there can be variable amounts of common-Pb contamination. This variable common-Pb can be an advantage because the data can be used to define a 2D or 3D isochron (see figure below). The low U concentration can be mitigated, in part, by using larger primary beam intensities (5-10 nA) and larger analytical spot sizes ~30 x 30 microns). As a result, it is common for low-U samples to only have low radiogenic yield (1-10%), and therefore have relatively large uncertainties (±10% can be typical). However, for samples that are better behaved, the analytical precision can be ±2% for samples with 50 to 100 ppm U.
The example below shows titanite data from an early Permian granitoid that, when plotted on an inverse isochron (Tera Wasserburg) diagram, define a mixing line between common Pb (207Pb/206Pb upper intercept) and the radiogenic U/Pb age (lower intercept along the concordia line). In this case, the common-Pb composition is 22% different than the calculated Stacey-Kramers (1975) model Pb composition. Therefore, using the data-defined common Pb correction decreasing the age by ~6% relative to a Stacey-Kramers correction. The point here is that the accuracy of the calculated model age for titantie is strongly dependent on the assumptions used to make the calculation.
The run table for titanite typically includes the following: 204Pb, a background measured at 0.045 mass units above the 204Pb peak, TiCa2O2 (guide peak), 206Pb, 207Pb, 208Pb, U, Th, ThO, UO, and UO2. Trace elements (Y, Zr, Nb, REE) can also be added into the U-Pb run table and be analyzed simultaneously (see below).
Titanite Trace Element Analysis
Y, Zr, Nb, REE (La, Ce, Nd, Sm, Eu, Gd, Dy, Yb) Hf, and Ta can be combined with U-Pb analyses (see above) to allow ages and trace element concentrations to be measured from the same analytical volume. Alternatively, a large suite of trace elements can be analyzed without ages, which allows smaller analytical spots (10-12 microns) and shorter analytical time. A full trace element run table could include Li, B, F, Na, Mg, Al, Si,P, K, Ca, Ti, V, Sr, Y, Zr, Nb, Ba, La, Ce, Pr, Ce, Nd, Sm, Eu, Gf, Tb, Dy, Ho, Er, Tm, Yb, Lu, Hf, Ta, Th, U. An example of different compositional domains within a zoned titanite from granodiorite from Joshua Tree National Park, CA, USA (courtesy of Andy Barth).
The Zr in titanite thermobarometer of Hayden et al., (2008) allows model temperatures and/or pressures to be calculated from Zr trace element measurements.