How to find rare zircon in mafic and ultramafic rocks: an integrated in-situ detection workflow

Zircon is a key accessory mineral because it can retain geochronological and geochemical information. In mafic and ultramafic rocks, zircon is scarce and restricted to localised microstructural domains, making its detection difficult and largely dependent on sampling strategy. We present a reproducible workflow for locating zircon directly within mafic and ultramafic rocks, designed to improve detection efficiency while preserving textural context. The approach is based on preparing multiple small rock slabs with carefully controlled polishing quality, allowing systematic inspection of large surface areas. This slab-scale screening strategy departs from conventional thin-section-based searches and is essential for accessing zircon in zircon-poor systems. Large-area elemental mapping is performed using SEM, EPMA, and micro-XRF, all of which can identify Zr-enriched domains. Comparative testing shows that micro-XRF provides the most favourable balance between acquisition time, analysed surface area and sensitivity to zirconium, making it suitable for first-pass screening of large sample sets. Across the different analytical methodologies used, the Zr signal can be affected by spectral interferences from other elements. For this reason, zircon detection in this study relies on the combined behaviour of multiple elements and on their statistical consistency across the mapped area. Zircon candidates are retained only when they meet several independent criteria, thereby reducing misidentification due to background noise or overlapping mineral phases. This approach enables the recovery of zircon grains down to ~50 µm and can be readily adapted for the in-situ detection of other scarce accessory minerals. More generally, it provides a practical framework for accessing mineral-scale records in systems where key phases are sparse, heterogeneous and difficult to locate using conventional approaches.

Work supported by the Spanish Ministerio de Ciencia e Innovación, Fondos Feder, PID2023-149105NA-I00. M.S.F. benefits from the FPI-PRE2023-002262.