Lu-Hf data in zircons by LA-MC-ICP-MS: Application in gabbro of the Aburrá Ophiolite, Colombia

Abstract

The Lu-Hf methodology in zircon by LA-MC-ICP-MS established at the Laboratory of Isotope Geology, Rio Grande do Sul Federal University, and applied in igneous zircons from a gabbroic body of the Aburrá Ophiolite, Colombia, is described. The interest in the application of this methodology when it is available a system of in situ zircon analysis by LA-ICP-MS, should be consigned to relative simplicity, sensitivity and speed of analysis. The high concentration of Hf in zircon, which can reach thousands of ppm, provides highly accurate isotopic ratios and model ages that can support not only important inferences about the age of mantle extraction, but also as to its possible evolutionary history, and provenance related studies. The accuracy in order of 20 ppm in standard error of the ¹⁷⁶Hf/¹⁷⁷Hf ratios and about 10% in ¹⁷⁶Lu/¹⁷⁷Hf ratios provided by the Neptune (Thermo Finnigan) should be consigned to the modern technology of simultaneous measurements of the isotopes involved in the methodology. The Nd: YAG (213 nm) laser from New Wave used proved to be very appropriate using the following parameters: energy density of 5-6 J/cm² , frequency of 10 Hz, 50 s of ablation and diameter of 55 µm. A flow of approximately 4 mL/min of nitrogen added to the carrier gas to inhibit the oxide formation in plasma proved to be effective in stabilizing the sign and brought a remarkable increase in ionization efficiency estimated at up to 100% in the signs, seen in GJ-1 and Mud Tank standards. These standards are slightly different in isotopic compositions for the176Hf/177Hf ratio and provided values of 0.282004 ± 0.000004 and 0.282466 ± 0.000022, respectively. The analytical procedure adopted followed a protocol in which three zircons were interspersed with the international standard GJ-1. In situ analyses of Hf in 12 grains separated from the gabbroic body, previously dated by U-Pb, revealed ε_Hf(t) between +2.01 and +5.35, while the T_DM model age values resulted in a range between 1.15 and 1.44 Ga.