Advancements in In-Situ Lu-Hf Isotopic Measurements of Zircon Using Tandem LA-MC-ICPMS/LIBS Techniques

Zircon is the primary mineral phase for U-Pb geochronology, providing the precise age of magma crystallization. The Lu-Hf isotopic system in zircon, when paired with U-Pb ages, provides a powerful tool as a geochemical tracer. For terrestrial zircon, this paired measurement can elucidate the temporal evolution of the crust-mantle system. Currently, in-situ studies of the zircon Lu-Hf isotopic composition, which add valuable petrologic context, have increased rapidly with the development and improvement of laser ablation inductively coupled mass spectrometry (LA-ICPMS) [e.g., 1]. Here, we focus on this development with a focus on extraterrestrial zircon, which can advance our understanding of the formation of non-terrestrial bodies (e.g., petrogenesis of lunar [2] and martian [3] zircon).

From the perspective of a precious astromaterial sample, with limited sample mass available and a need for preservation for future studies, there is obligation to continuously document the capabilities and limitations of semi-destructive in-situ geochemical measurement techniques. Building on the initial results of [4], we report the current in-situ Lu-Hf isotopic measurement capabilities of the Applied SpectraTM iX-fs-Tandem LA-LIBS instrument coupled with the Nu InstrumentsTM Sapphire 1700 MC-ICPMS. We also examine the advantages of integrating tandem laser-induced breakdown spectroscopy (LIBS) measurements during ablation.

In-situ Lu-Hf isotopic measurements were made using various laser settings, with spot sizes ranging from 20 µm to 60 µm. However, pit depths were intentionally maintained at approximately 20-25 µm (verified with optical profilometry) to simulate likely limitations regarding sample availability in astromaterials. All ablation settings yield accurate results on natural zircon standards, with a range of 176Lu/177Hf and 176Yb/177Hf from ~0.001 to ~0.1 (R33) and ~0.00002 to ~0.003 (Mud Tank). Spot sizes ≥ 40 µm, with ablation rates of 1 µm s-1, result in uncertainties of ≤ ~2 εHf units (2SE). Using the same laser settings (i.e., power density and ablation rate), smaller laser spots begin to significantly increase the produced uncertainties (e.g., ~5 εHf units for a 30 µm spot). By combining higher energy densities, beam defocusing, and signal smoothing, we can enhance the internal precision of small spot sizes (< 40 um) to be comparable to that of typically large ablation volumes.

An equally important aspect of data reduction and high-precision laser ablation analyses is ensuring that only the target of interest is analyzed, thereby limiting the influence of other mineral phases. In the sample limited scenario described above, the tandem LIBS measurements facilitate simultaneous monitoring of the currently ablated mineral phase. We demonstrate how this technique can be used to clarify both when and additional mineral phase begins to be ablated, as well as adequately identifying what that mineral phase is. We also present insights regarding our early attempts at quantifying select trace elements in zircon via LIBS (e.g., Li, REEs).

References: [1] Wilson et al. (2002) J. Anal. Atom. Spectrom, 17(4), 406-409. [2] Onuk et al. (2017) Geostand Geoanal Res, 41(2), 263-272. [3] Ding et al. (2011) Mineral Mag, 75(2), 279-287. [4] Setera & Simon (2025) Lunar and Planetary Sci Conf, Abstract #1565.