Abstract Initial excess protactinium (231Pa) is a frequently suspected source of discordance in baddeleyite (ZrO2) geochronology, which limits accurate U/Pb dating, but such excesses have never been directly demonstrated. In this study, Pa incorporation in late Holocene baddeleyite from Somma-Vesuvius (Campanian Volcanic Province, central Italy) and Laacher See (East Eifel Volcanic Field, western Germany) was quantified by U-Th-Pa measurements using a large-geometry ion microprobe. Bad-deleyite crystals isolated from subvolcanic syenites have average U concentrations of ~200 ppm and are largely stoichiometric with minor abundances of Nb, Hf, Ti, and Fe up to a few weight percent. Measured (231Pa)/(235U) activity ratios are significantly above the secular equilibrium value of unity and range from 3.4(8) to 14.9(2.6) in Vesuvius baddeleyite and from 3.6(9) to 8.9(1.4) in Laacher See baddeleyite (values within parentheses represent uncertainties in the last significant figures reported as 1σ throughout the text). Crystallization ages of 5.12(56) ka (Vesuvius; MSWD = 0.96, n = 12) and 15.6(2.0) ka (Laacher See; MSWD = 0.91, n = 10) were obtained from (230Th)/(238U) disequilibria for the same crystals, which are close to the respective eruption ages. Applying a corresponding age correction indicates average initial (231Pa)/(235U)0 of 8.8(1.0) (Vesuvius) and 7.9(5) (Laacher See). For reasonable melt activities, model baddeleyite-melt distribution coefficients of DPa/DU = 5.8(2) and 4.1(2) are obtained for Vesuvius and Laacher See, respectively. Speciation-dependent (Pa4+ vs. Pa5+) partitioning coefficients (D values) from crystal lattice strain models for tetra- and pentavalent proxy ions significantly exceed DPa/DU inferred from direct analysis of 231Pa for Pa5+. This is consistent with predominantly reduced Pa4+ in the melt, for which D values similar to U4+ are expected. Contrary to common assumptions, baddeleyite-crystallizing melts from Vesuvius and Laacher See appear to be dominated by Pa4+ rather than Pa5+. An initial disequilibrium correction for baddeleyite geochronology using DPa/DU = 5 ± 1 is recommended for oxidized phonolitic melt compositions.

中文翻译：

---

晚第四纪火成斜斜方岩中过量 231 Pa 的量化

摘要 初始过量的镥 (231Pa) 是斜锆石 (ZrO2) 年代学中经常被怀疑的不一致性来源，这限制了准确的 U/Pb 测年，但这种过量从未被直接证明过。在这项研究中，使用大几何离子微探针通过 U-Th-Pa 测量量化了来自 Somma-Vesuvius（Campanian 火山省，意大利中部）和 Laacher See（东埃菲尔火山场，德国西部）的全新世晚期斜斜岩中的 Pa 掺入. 从亚火山正长岩中分离出的坏三长石晶体具有~200 ppm 的平均 U 浓度，并且主要是化学计量的，少量的 Nb、Hf、Ti 和 Fe 丰度高达几个重量百分比。测得的 (231Pa)/(235U) 活性比显着高于 1 的长期平衡值，范围从 3.4(8) 到 14.9(2. 6) 在 Vesuvius 斜斜岩中和从 3.6(9) 到 8.9(1.4) 在 Laacher 中见斜斜岩（括号内的值代表最后一个有效数字的不确定性，在全文中报告为 1σ）。5.12(56) ka (Vesuvius; MSWD = 0.96, n = 12) 和 15.6(2.0) ka (Laacher See; MSWD = 0.91, n = 10) 的结晶年龄是从 (230Th)/(238U) 不平衡中获得的相同的晶体，接近各自的喷发年龄。应用相应的年龄校正表明平均初始 (231Pa)/(235U)0 为 8.8(1.0) (Vesuvius) 和 7.9(5) (Laacher See)。对于合理的熔体活动，Vesuvius 和 Laacher See 的模型斜斜岩-熔体分布系数分别为 DPa/DU = 5.8(2) 和 4.1(2)。物种依赖（Pa4+ vs. Pa5+) 分配系数（D 值）来自四价和五价替代离子的晶格应变模型，显着超过从直接分析 231Pa Pa5+ 推断出的 DPa/DU。这与熔体中显着减少的 Pa4+ 一致，预期 D 值与 U4+ 相似。与通常的假设相反，来自维苏威火山和 Laacher See 的斜斜辉石结晶熔体似乎以 Pa4+ 而不是 Pa5+ 为主。对于氧化音质熔体成分，建议使用 DPa/DU = 5 ± 1 对斜锆石地质年代学进行初始不平衡校正。来自维苏威火山和 Laacher See 的斜锆石结晶熔体似乎以 Pa4+ 而不是 Pa5+ 为主。对于氧化音质熔体成分，建议使用 DPa/DU = 5 ± 1 对斜锆石地质年代学进行初始不平衡校正。来自维苏威火山和 Laacher See 的斜锆石结晶熔体似乎以 Pa4+ 而不是 Pa5+ 为主。对于氧化音质熔体成分，建议使用 DPa/DU = 5 ± 1 对斜锆石地质年代学进行初始不平衡校正。