Extending Laser-Ablation (U-Th)/He to Ultra-Low-eU Magnetite and Chromite: A New Tool for Mafic and Ultramafic Rocks

Mafic and ultramafic rocks provide critical insights into ophiolite formation and obduction, orogenic suturing, and rock magnetism, yet they are difficult to date due to the absence of conventional radioisotopic chronometers. Magnetite and chromite are common primary or secondary mineral phases that often preserve growth zoning and complex overprinting, but extremely low eU concentrations and intragrain heterogeneity have limited the application of (U–Th)/He dating. Here we report recent progress at the University of Texas at Austin (UTChron) toward establishing protocols of routine laser ablation (LA) (U–Th)/He in-situ dating of magnetite and chromium spinel (chromite) with eU <10 ppb and ultra-low He concentrations. Our analytical procedures couple in-vacuo laser ablation and ultra-low-blank magnetic-sector noble gas mass spectrometry (Thermo Fisher SFT) for He with U–Th quantification by laser-ablation ICP-MS (Thermo Element2) and ablation pit volume measurements using white-light interferometry. This integrated workflow supports targeted analyses of discrete textural growth zones and complex intragrain textures at ~100 μm spatial resolution, with typical analytical precision of ~5–10%.

We present three applications for in-situ He dating of magnetite and chromite from mafic and ultramafic rocks: (1) magnetite and chromite from kimberlitic diatremes that yield LA-(U–Th)/He ages consistent with independent emplacement constraints, providing a practical alternative where conventional datable minerals are absent; (2) He ages from primary chromite from obducted orogenic ophiolitic units that record thermal resetting and subsequent cooling, offering new leverage on the tectonic histories of ultramafic sequences that are traditionally challenging to date; and (3) He dates from secondary magnetite growth in sheared serpentinites that provide direct constraints on fluid-rock interaction during deformation in ultramafic shear zones. Collectively, these results establish magnetite and chromite as viable targets for in situ (U–Th)/He thermochronometry and broaden the range of geological problems accessible to in-situ dating.