INSITU MICRO-BADDELEYITE U-PB DATING METHOD: ‘AN AGE FROM ANY DIKE'

Baddeleyite (ZrO₂) is a favorite target for U-Pb dating of mafic and ultramafic rocks. It is challenging to identify suitable samples, however, which have cooled slowly enough to produce 100-200 micron long grains, the minimum size for traditional separation and dating methods. The ages of many paleomagnetic poles are inferred from dated locations within a suite, rather than the actual paleomagnetic sample sites. It has recently become recognized that micro-baddeleyite crystals (<30 microns long) are ubiquitous in mafic dikes and sills (U. Söderlund and L. Heaman, pers. comm.). With refined Wilfley Table techniques (Söderlund and Johansson, 2002) it is possible to isolate micro-baddeleyite grains for IDTIMS analysis, greatly increasing the number of dateable mafic samples. We are currently developing an in-situ technique to date micro-baddeleyite crystals using the CAMECA ims 1270 ion microprobe, avoiding the separation and dissolution steps. Due to the ion microscope optics of the ims 1270, secondary ions emitted from host minerals can be blocked by an aperture located in the transfer section of the instrument, resulting in an effective sampling diameter of a few to 10 microns from a 30 micron primary ion beam. The method involves locating suitable crystals on a polished thin section and mounting portions of the thin section alongside baddeleyite standards for U-Pb isotopic analysis. Analysis times average 30 minutes per spot. Average 2 sigma precision on Phanerozoic ²⁰⁶Pb/²³⁸U dates are ~6%; Precambrian ²⁰⁷Pb/²⁰⁶Pb dates are 1% or less. Our preliminary results show that using oxygen-flooding of the sample chamber enhances Pb secondary ion yields for baddeleyite approximately 10-fold. Under these conditions, crystal orientation-related bias in U-Pb baddeleyite data is less than has been documented on SHRIMP instruments.

A preliminary date from an Ordovician-aged mafic sill agrees with IDTIMS date on isolated micro-baddeleyite crystals from the same suite (Suordakh, Siberia: 451 +/- 24 vs. 450 +/- 24 Ma, respectively, 2 sigma ²⁰⁶Pb/²³⁸U dates). The technique is potentially applicable to most unmetamorphosed to low-grade mafic dikes and sills. Sample size can be very small for either approach. Direct dates on paleomagnetic sites can address fundamental questions concerning the timing and duration of LIPs, supercontinent reconstructions and the geodynamo.