Characterization of Mali Andradite Garnets as U-Pb Reference Materials

Andradite-grossular series garnets that form within contact metamorphic skarns around igneous intrusions can incorporate trace levels of uranium (U), enabling robust U-Pb geochronology for the direct dating of metasomatic processes. However, in situ U-Pb geochronology by laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) requires homogenous matrix matched standards of high U content and U/Pb ratios. Alluvial andradite and melanite (Ti-rich) garnets sourced from skarns associated with ca 200 Ma Central Atlantic Magmatic Province intrusions in Mali (West Africa) are readily available in the gem and mineral market and have been described as possible reference materials in recent literature. We developed a screening procedure for three lots of Mali garnets to determine their suitability as reference materials using scanning electron microscopy, energy dispersive x-ray spectroscopy, and LA-ICPMS transects across crystals.

Crystals of Mali garnet were sliced into 1mm-thick sections and polished for analysis. A first lot of tumbled and polished, reddish, inclusion-rich garnet crystals with a composition of Alm ₆₆ Pyr _15S pes ₁₁ Gros ₅ were heterogeneous at the spot to crystal scale, with generally low and highly variable U and high common Pb making them unsuitable as a standard material. A single large (5 g) red-orange crystal from Bafoulabé Cercle, in the Kayes Region of Mali, is uniform in major element composition (Gros ₇₆ And ₁₆ ), but symmetrically zoned in TiO ₂ (0.1 to 0.6 wt%) and U (0.1 to 0.3 ppm) from rim to core. Its U-Pb isotope ratios form a correlated array in Tera-Wasserburg space toward a ca 200 Ma concordia intercept. A crystal from a third lot of black ‘melanite’ garnets are also uniform in major element composition (Gros ₅₅ And ₃₆ ) with 1 wt% TiO ₂ and consistently high, homogenous U contents (3.4 ± 0.8 ppm). This garnet has high and tightly clustered ²³⁸ U/ ²⁰⁶ Pb ratios (33.23 ± 3%), producing nearly concordant ca 200 Ma dates across the crystal. The correlation of color, andradite component, and Ti and U concentrations hold promise for creating a series of Mali garnets for standardization of in situ U–Pb dating.