Application of LA-ICP-MS techniques in mineral deposit: elemental mapping and in-situ Rb-Sr dating

Abstract

Laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) is a powerful analytical technique in the research of mineral deposits, which allows in-situ determination of the elemental and isotopic compositions (or ages) of geological and other solid materials. In this thesis, the influence of organic matter impurities on Re-Os dating of pyrite is evaluated by LA-ICP-MS elemental mapping technique, using the giant Jinding sediment-hosted Zn-Pb deposit in China as an example. It is found that pyrite in Jinding trapped abundant organic matter impurities during its growth. Elemental mapping shows that these organic matter impurities are the major host for Re, with Re signals being 1 to 4 orders of magnitude higher than those of the pyrite host. The different Re-Os isochron dates obtained for the organic matter-free pyrite (51 ± 1 Ma) and the organic matter-bearing pyrite (72.9 ± 0.5 Ma) indicate that organic matter impurities can significantly influence the Re-Os dates of pyrite and likely other sulfides. Therefore, to date sulfides formed in organic-rich environments using the Re-Os method, it is necessary to determine the distribution of Re in the samples using detailed petrography and LA-ICP-MS element mapping technique. In addition, in situ Rb-Sr mica dating protocol is established using the triple quadrupole LA-ICP-MS, or LA-ICP-MS/MS technique. This is achieved by conducting 11 sessions of Rb-Sr dating analysis on a well-dated muscovite sample with various laser ablation conditions, data reduction methods, and reference materials. The capacities, advantages, and limitations of this technique are evaluated by dating muscovite (n = 5), biotite (n = 6), and phlogopite (n = 1) from 12 different mineral deposits with a large age range from 306 to 1.4 Ma. The results show that this novel technique can achieve accuracy of 3% or better and precision of ~1-3%, for micas as young as ~15 Ma. However, the dating accuracy (~9% or worse) and precision (~23% or worse) are reduced significantly for minerals younger than 5 Ma due to the short radioactive decay time. Samples with a narrow range in 87Rb/86Sr ratios (< 100) may also have a low precision (e.g., ~21%). Despite these limitations, this technique enables the dating of many types of mineral deposits and rocks, with a relatively high spatial resolution (down to 50 microns), fast turnaround time, and a higher closure temperature for muscovite than the Ar-Ar system. These advantages make this technique an attractive and powerful tool in mineral deposit dating.