In-situ garnet 238U-230Th geochronology of Holocene silica-undersaturated volcanic tuffs at millennial-scale precision

Garnet petrochronology reveals the lifetime and dynamics of phonolitic magma chambers at Somma-Vesuvius

“…Garnets extracted from the white pumices of the Pompeii eruption yielded a U-Th isochron date of 3.31 ± 0.99 ka, and garnets from the gray pumices yielded an isochron date of 2.82 ± 0.95 ka. Garnets from the Pollena eruption yielded an isochron date of 2.32 ± 1.42 ka that was previously reported by Wotzlaw et al ( 25 ). While these dates are overlapping within uncertainty, the initial ( 230 Th)/( 232 Th) is distinct between the Pompeii and Pollena, with Pollena garnets having a significantly higher initial ( 230 Th)/( 232 Th) ( Fig.…”

“… ( A ) Location map ( 25 ) showing sampling localities of sample analyses in this study. MER, Mercato; AVE, Avellino; POM, Pompeii; POL, Pollena.…”

“…Instrumental mass bias was determined from the measured 238 U/ 235 U relative to the natural U isotopic ratio of 137.818 ( 48 ). The relative sensitivity factor (RSF) for correcting U-Th elemental fractionation was determined from analyses of the 230 Th/ 238 U ratio of a secular equilibrium garnet reference material (Mali garnet) assuming ( 230 Th)/( 238 U) = 1 [see ( 25 ) for details]. The average U-Th RSF over the course of this study was 1.14 ± 0.08 (1 SD).…”

“…S6 shows results for secular equilibrium garnets analyzed as reference materials. U-Th data for garnets from the Pollena eruption were previously reported by Wotzlaw et al (25). Calculations of isochron ages and initial Th isotopic ratios were performed in IsoplotR (49).…”

“…However, these time scales are still significantly shorter than crystal residence times inferred from U-Th isotope systematics of major phenocrysts in phonotephrites of the 1944 eruption ( 24 ), suggesting that mafic magmas are stored much longer [up to ~40 thousand years (ka)] beneath Vesuvius and/or old crystals are recycled into younger eruptive products. To better constrain the time scales and conditions of phonolite magma storage, we use recently developed techniques that allow in situ U-Th disequilibrium dating of Ca-rich garnet phenocrysts at the subcrystal scale using laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) ( 25 ). Combined with detailed textural and geochemical characterization of a subset of U-Th–dated crystals, this provides a comprehensive petrochronology framework for assessing the time scales and dynamics of magma storage beneath Somma-Vesuvius.…”