Determination of high-precision Zr/Hf ratios in bulk meteorites using LA-ICP-MS

Introduction: Zirconium (Zr) and hafnium (Hf) are incompatible, lithophile, and refractory trace elements. In general, trace element distribution and relative abundances may provide very valuable information of geological processes. With respect to meteorites, Zr /Hf ratios may help decipher condensation conditions and sequences during the early stages of solar system formation. With analytical technology being continuously improved, the relative chondritic abundance of Zr and Hf was recently corrected to Zr/Hf = 34.2 ± 0.3 [1] and 34.3 ± 0.3 (2σ) [2]. The former accepted value was 36.3 with an uncertainty of approx. 10% (for the CI1 chondrite Orgueil, [3]). As opposed to [1] and [2] who used an isotope dilution method coupled with a multiple collector ICPMS, our first objective was to verify the accuracy and precision of a relatively fast and inexpensive sample preparation method combined with expeditious laser ablation ICP-MS techniques. Secondly, we intend to identify possible systematic fine-scale Zr/Hf variations within bulk meteoritic matter of different classes. Sample preparation: As a first step, we analyzed 8 different standard reference materials (SRMs) including 7 terrestrial rock samples and the synthetic soda-lime glass NIST614 (Tab. 1). The natural SRMs were processed as follows: Individual aliquots of approximately 10 mg powder were transferred into a graphite crucible and melted using a CO2 laser. The resulting, approx. 1 mm-sized spherules were placed on top of the gas stream of an aerodynamic levitation device for complete homogenization. Subsequently, they were quenched and embedded in resin. Four aliquots were prepared of each SRM and fused between one and four times. For further details on sample preparation see [4]. Almost all samples exhibited a glassy to finely crystallized texture after quenching. Only RGM-1 aliquots that were fused multiple times displayed more progressive recrystallization. Instrumentation: Zr-Hf analyses were carried out by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) at Frankfurt University using a Thermo-Finnigan Element II sector field ICPMS system coupled to a New Wave Research UP213 (213 nm) ultraviolet laser system [5]. Each analysis consisted of 25 s background acquisition followed by approx. 60 s data acquisition. The laser was set to a beam diameter of 120 μm and fired in line-scan mode. Isotopes measured include Zr, Zr, Hf, Hf, and Ca as internal standard.