This is the peer-reviewed, final accepted version for American Mineralogist, published by the Mineralogical Society of America. The published version is subject to change. Cite as Authors (Year) Title. American Mineralogist, in press. DOI: https://doi.org/10.2138/am-2020-7449. http://www.minsocam.org/ 1 1 Revision N°1 231 2 Quantification of excess Pa in late Quaternary igneous baddeleyite 3 Word count: 7324. 4 Yi Sun1,*,a, Axel K. Schmitt1, Lucia Pappalardo2, Massimo Russo2 5 1Institute of Earth Sciences, Heidelberg University, Im Neuenheimer Feld 236, D-69120 6 Heidelberg, Germany. 7 2Osservatorio Vesuviano, Istituto Nazionale di Geofisica e Vulcanologia, via Diocleziano 328, 8 80124 Naples, Italy. 9 *E-mail:
[email protected] 10 Abstract 11 Initial excess protactinium (231Pa) is a frequently suspected source of discordance in baddeleyite 12 (ZrO2) geochronology, which limits accurate U/Pb dating, but such excesses have never been 13 directly demonstrated. In this study, Pa incorporation in late Holocene baddeleyite from Somma- 14 Vesuvius (Campanian Volcanic Province, central Italy) and Laacher See (East Eifel Volcanic 15 Field, western Germany) was quantified by U-Th-Pa measurements using a large-geometry ion 16 microprobe. Baddeleyite crystals isolated from subvolcanic syenites have average U 17 concentrations of ~200 ppm and are largely stoichiometric with minor abundances of Nb, Hf, Ti, 18 and Fe up to a few weight percent. Measured (231Pa)/(235U) activity ratios are significantly above 19 the secular equilibrium value of unity and range from 3.4(8) to 14.9(2.6) in Vesuvius baddeleyite 20 and from 3.6(9) to 8.9(1.4) in Laacher See baddeleyite (values within parentheses represent 21 uncertainties in the last significant figures reported as 1σ throughout the text).