Bromine is a key halogen element in the quantification of volcanic volatiles, but analytical difficulties in measuring its very low abundances have prevented progress in understanding its behavior and its role in volcanic emissions. We present a new data set of bromine, chlorine, and fluorine concentrations in melt inclusions and matrix glasses for two rhyolitic super-eruptions from the Toledo and Valles calderas, New Mexico, USA. We show that before eruption, Br and Cl were efficiently partitioned from the gas-saturated magma into a separate fluid phase, and we calculate the mass of halogens in the fluid phase. We further demonstrate that syn-eruptive magma degassing was negligible during the super-eruptions, so that the main source of halogen emissions must have been the fluid phase. If the fluid phase were erupted, the large mass of Br and Cl could have severely impacted the atmospheric chemistry upon eruption.Halogens are an essential component of magmatic volatile systems and play a crucial role in magma degassing (Aiuppa et al., 2009), atmospheric ozone destruction (Aiuppa et al., 2009; von Glasow et al., 2009), and metal transport (Williams-Jones and Heinrich, 2005). Bromine concentrations in magmas are typically two orders of magnitude lower than those of chlorine, yet its higher volatility (Bureau and Métrich, 2003) and potential for ozone destruction (Daniel et al., 2007) make bromine a particularly useful element to study when evaluating the potential impact of an explosive eruption. We analyzed the enrichment of halogens in a super-volcanic system and quantified the masses of Br and Cl that were fractionated from the melt and potentially released into the atmosphere.We studied the Bandelier Tuff eruptions, which were vented from the Toledo and Valles calderas in northwestern New Mexico, United States. The first super-eruption from Toledo caldera took place at 1.61 Ma (Spell et al., 1990), erupted an estimated 400–500 km3 dense-rock equivalent (DRE) of magma (Cook et al., 2016), and deposited the high-silica rhyolitic lower Bandelier Tuff (LBT). The second super-eruption, similar in both composition and volume, deposited the upper Bandelier Tuff (UBT) at 1.26 Ma (Phillips et al., 2007). Both eruptions experienced an initial Plinian phase followed by ignimbrites. Between the two super-eruptions, the top of the magma chamber was periodically tapped by small rhyolitic eruptions that formed the tephras, flows, and domes of the Valle Toledo Member, which span ∼350 k.y. (Spell et al., 1996; Gardner et al., 2010). The initial phase of the Valle Toledo eruptions is geochemically related to the LBT and interpreted as the last pulse from the emptying magma chamber, after which new silicic magma entered the chamber and started to differentiate (Stix and Gorton, 1993). Similarly, the Deer Canyon Member of the Valles Rhyolite (Deer Canyon Rhyolite herein) erupted immediately after the UBT as resurgence was taking place in Valles caldera (Phillips et al., 2007). This unit is geochemically related to the UBT and represents the final pulse from the UBT magma chamber before fresh magma replenished the system anew (Spell et al., 1993; Wilcock et al., 2013).We collected pumice samples from (1) the base and the top of the LBT and UBT Plinian phases, (2) the two key units of the Valle Toledo representing the initial phase related to the LBT and the first phase immediately after magma recharge, and (3) the Deer Canyon Rhyolite. Phenocrysts in the pumices dominantly consist of melt inclusion–rich quartz and sanidine. Bromine (Br), chlorine (Cl), and fluorine (F) were measured by ion microprobe in melt inclusions, with ∼10 melt inclusions analyzed per unit as well as one to two matrix glasses per unit. Analytical details are provided in the Supplemental Material1.In the LBT sequence, F measured in melt inclusions shows a steady decrease upsection, hence deeper into the magma chamber (Fig. 1). Flourine decreases from 2700 to 3340 ppm in the base of the Plinian, then to 1880–2830 ppm in the top, and finally to 260–550 ppm in the first-erupted Valle Toledo unit. Chlorine follows a similar, although less pronounced, trend, decreasing from 1920 to 2440 to 1860–2660 ppm through the Plinian, and further to 1030–2190 ppm in the first-erupted Valle Toledo. By contrast, Br exhibits no variation in the LBT sequence; Br values of 1.3–1.7 ppm are found at the base of the Plinian, 1.2–1.8 ppm at the top of the Plinian, and 1.1–1.9 ppm at the end of the LBT. For the entire LBT sequence, F exhibits a 6.6× decline from base of the LBT to the earliest Valle Toledo, while the decrease for Cl is 1.5×, and Br remains constant.Trends in the UBT are similar to those in the LBT, with several key differences: (1) the ranges in concentrations are substantially larger in the UBT; (2) the halogen concentrations themselves are higher; (3) there is a slight increase in Cl and F from the base to top of the Plinian, rather than a decrease; and (4) Br also decreases, rather than remaining constant. Fluorine evolves from 2800 to 3570 ppm to 2740–4100 ppm through the Plinian, then plummets to 340–930 ppm in the Deer Canyon Rhyolite, which is the deepest erupted magma, with one high outlier at 2230 ppm. Chlorine starts at 2560–3440 ppm in the base of the Plinian, increases to 2710–4080 ppm, then strongly declines to 1310–2050 ppm in the Deer Canyon. In the same sequence, Br varies from 2.7 to 3.7 to 2.2–3.8 ppm, and finally to 1.5–3.2 ppm. The decrease over the entire UBT sequence is 3.6× for F (4.4× if the high Deer Canyon outlier is not considered), 1.7× for Cl, and 1.2× for Br.The Valle Toledo samples that we analyzed comprise three units: the first-erupted material, which marks the end of the LBT sequence (ca. 1.61 Ma; Spell et al., 1996); the first post-replenishment eruption (1.54 Ma; Spell et al., 1996); and the base of the UBT Plinian (1.26 Ma; Phillips et al., 2007), which represents the end of Valle Toledo differentiation. The three halogens show variable enrichment during this 0.35 m.y. interval. Fluorine starts at 260–550 ppm in the earliest Valle Toledo, then goes to 190–830 ppm after replenishment, culminating in strong enrichment of 2800–3570 ppm at the base of the UBT. Chlorine concentrations follow a similar trend, from 1030 to 2190 ppm to 800–1720 ppm after replenishment, then enrichment to 2560–3440 ppm. The Br trend is similar yet less pronounced than for F and Cl, ranging from 1.1 to 1.9 ppm to 1.2–2.7 ppm and ending at 2.7–3.7 ppm at the base of the UBT. Enrichment factors for F, Cl, and Br from immediately after replenishment to the UBT eruption are 5.6, 2.3, and 1.6, respectively.The systematic difference in enrichment factors for the three halogens indicates the presence of a fluid-saturated magma, whereby Br and Cl partitioned principally into the fluid while F accumulated almost entirely in the magma. Fluorine has been shown in previous studies to be nearly perfectly incompatible in these rocks, similarly to Cs (Stix and Layne, 1996), indicating that mineral phases such as apatite play a negligible role in sequestering halogens. Bromine has higher fluid-melt partition coefficients than Cl (Bureau et al., 2000; Bureau and Métrich, 2003; Cadoux et al., 2018), which results in a lesser enrichment in the melt as proportionally more Br fractionates to the fluid phase. If the magma were not fluid saturated, Br and Cl would exhibit the same enrichment as F (Balcone-Boissard et al., 2010). Hence, the difference indicates the amounts of Br and Cl that were lost to the fluid phase. The observed enrichment of F, Cl, and Br throughout the Valle Toledo eruptions reflects re-establishment of concentration gradients, with F accumulating in the melt while Cl and Br were partially lost to a fluid phase. Taken together, our results demonstrate that the Bandelier magmatic system was always fluid saturated during its evolution.For each sample we analyzed, we also measured F, Cl, and Br concentrations in one to two matrix glasses (Fig. 1). For all samples and all three halogens, the matrix glass analyses plot in the same range as the melt inclusions. Only at the base of the LBT do both matrix glasses plot at the lower end of the range for melt-inclusion concentrations. Given there is no systematic difference between the matrix glasses and the melt inclusions, syn-eruptive degassing cannot have played any significant role in the Bandelier eruptions, which is explained by the slow diffusion of halogens in rhyolitic magma compared to other volatiles such as H2O (Baker and Balcone-Boissard, 2009). The similar Br and Cl contents also indicate that the magma erupted shortly after crystallization of the host crystals, given that little further loss of Br and Cl to the fluid phase took place between melt inclusion capture and eruption.Continental crust is estimated to have a Cl/Br ratio of 273, while Cl/Br ratios measured in magmatic rocks cluster at ∼300 (Bureau et al., 2000). Given the higher fluid-melt partition coefficient of Br versus Cl, Cl/Br ratios in the melt phase would progressively increase in the presence of a fluid phase. The melt inclusions we analyzed have Cl/Br ratios from 600 to 1600 (Fig. 2), indicating that pre-eruptive halogen fractionation to a fluid phase played a dominant role in the magma chamber. Despite some spread for a few samples, plotting the Cl/Br ratios reveals three distinct groups: (1) the LBT Plinian has the highest Cl/Br ratios, with most melt inclusions between 1300 and 1600, and therefore experienced the most Br and Cl loss to a fluid phase; (2) the UBT Plinian exhibits less spread in values, with all melt inclusions having Cl/Br ratios ∼900–1000, indicating that the higher Br and Cl values in the UBT can be explained by less-efficient partitioning to a fluid phase than in the LBT; and (3) the two Valle Toledo samples and the Deer Canyon samples exhibit the lowest Cl/Br values, which are tightly clustered between 600 and 700 (except for a few outliers with high Cl/Br), reflecting the lesser degree of fractionation to a fluid phase in the more primitive rhyolitic magma.Previous studies of Br in melt inclusions have focused on arc magmas (Kutterolf et al., 2013, 2015; Cadoux et al., 2015, 2017, 2018; Balcone-Boissard et al., 2018). In all these studies, Br contents are substantially higher (5–15 ppm) than those we measured for the Bandelier system, with high Br levels linked to input from subducted sediments and serpentinized subducted mantle (Straub and Layne, 2003; Kutterolf et al., 2015). We ascribe the lower Br values observed in the Bandelier melt inclusions to a combination of the non-arc setting of Toledo and Valles calderas and the major role of pre-eruptive halogen loss.The lack of syn-eruptive degassing during the Bandelier eruptions, indicated by the similar halogen concentrations in melt inclusions and matrix glasses, means that the fluid phase is the only major source of Cl and Br released to the atmosphere. To calculate the mass of Cl and Br lost to the fluid phase, we assume they would have exhibited the same concentration gradient as F if no fluid phase had been present. We integrate this theoretical concentration gradient for Cl and Br over the depth of the LBT magma chamber and assume a quasi-cylindrical chamber with the same cross-sectional area as the caldera and a magma volume of 550 km3 (maximum estimate; Cook et al., 2016) to calculate the mass of Cl and Br in the magma if no fluid phase were present. Doing the same exercise for the observed concentration gradient and then calculating the difference yields the mass of Cl and Br released from the melt into the fluid phase. For the LBT, this results in 4800 Tg Cl and 5.7 Tg Br, while the same calculation for the UBT, assuming a magma volume of 400 km3 (best estimate; Goff, 2010), yields 2100 Tg Cl and 3.6 Tg Br.Although part of the fractionated fluid phase may be lost through passive degassing or via fractures into a hydrothermal system, and the eruption efficiency depends upon the physical state of the fluid phase, the release of buoyant gases directly from a vapor-rich fluid phase located at the top of the magma chamber (Stix and Layne, 1996; Wallace et al., 2003) can propel them efficiently into the stratosphere, as was observed for the 1982 El Chichón (Mexico) and 2000 Hekla (Iceland) eruptions (Schneider et al., 1999; Rose et al., 2006). The calculated masses in the fluid phase are thus maximum amounts of Br and Cl released from the system upon eruption. For comparison, maximum estimates for the Minoan eruption of Santorini (Greece), with an erupted volume an order of magnitude less than that of the Bandelier eruptions, are 675 Tg Cl and 1.5 Tg Br emitted (Cadoux et al., 2015); a study of Plinian eruptions along the Central American volcanic arc yielded maxima of 800 Tg Cl and 1.1 Tg Br (Kutterolf et al., 2015); and an estimated 227 Tg Cl and 1.3 Tg Br were emitted by the 1257 Samalas (Indonesia) eruption, which erupted 40 km3 DRE (Vidal et al., 2016).The calculation of Cl and Br masses exsolved into the fluid phase allows us to establish the Cl/Br ratio in the fluid. We calculate ratios of 843 for the LBT and 580 for the UBT. Our analysis of halogen partitioning to a fluid phase also permits an independent estimate of partition coefficients for Cl and Br in the Bandelier system. H2O concentrations in the LBT and UBT indicate respective fluid phases of 6.7 and 4.2 wt% of the total magma mass (details in the Supplemental Material). This fluid-mass estimate allows us to calculate an average Br and Cl concentration in the fluid (68 ppm Br and 57 × 103 ppm Cl for the LBT, 93 ppm Br and 54 × 103 ppm Cl for the UBT), which we then divide by average Br and Cl concentrations in the melt to yield associated fluid-melt partition coefficients (Df-m). DBrf-m is 41 for the LBT and 33 for the UBT, while DClf-m is 30 for the LBT and 23 for the UBT. These coefficients are higher than experimentally determined for rhyodacites (DBrf-m = 20.2: Cadoux et al., 2018; DClf-m = 16: Webster et al., 2009; both at 900 °C and 200 MPa), probably because the Bandelier magmas were both more silicic and colder.Halogens emitted from volcanoes can impact atmospheric chemistry and lead to ozone depletion (Textor et al., 2003; von Glasow et al., 2009). Although typically present in lower concentrations than Cl, Br is ∼60× more efficient than Cl at destroying ozone molecules, hence is of major significance in evaluating ozone destruction potential from volcanoes (Daniel et al., 2007). An influx of 5.7 Tg Br to the atmosphere results in an increase of the bromine mixing ratio (moles of Br per moles of air in the whole atmosphere) of 397 pptv (parts per trillion per volume of air), while 3.6 Tg Br during the UBT yields an additional increase of the bromine mixing ratio of 248 pptv. As a comparison, the anthropogenic, chlorofluorocarbon (CFC)–fueled input from pre-industrial times to 1995 has been only ∼15 pptv (Daniel et al., 2007). A similar calculation for Cl results in an increase of the atmospheric chlorine mixing ratio of 753 ppbv for the LBT and 327 ppbv for the UBT eruption. The scale of this Cl input dwarfs anthropogenic, CFC-generated increases of ∼3 ppbv for Cl (Daniel et al., 2007). The exact impact of the eruptions depends on how efficiently the fluid phase was erupted and on scavenging of halogens in the eruption column. Nevertheless, the large masses of Br and Cl fractionated from the Bandelier magma could have severely impacted the paleo-atmosphere.We present a new data set of halogen concentrations in eruptive products and calculate the maximum amount of halogens released during eruption. We interpret similar halogen concentrations in melt inclusions and matrix glasses as indicative of a negligible role for syn-eruptive halogen diffusion and degassing, while large masses of Cl and Br were stored in a fluid phase. The halogen-rich fluid phases of the Bandelier magma chamber could have been released upon eruption to cause severe ozone depletion in the troposphere and stratosphere. Measured Br concentrations for the Bandelier system are lower than in arc magmas (5–15 ppm), which suggests similar-sized eruptions in arc settings such as the 26.5 ka Oruanui eruption in Taupo (New Zealand) could have emitted even larger masses of Br. Our study confirms that large explosive volcanic eruptions can play an important role in atmospheric chemistry and that the contribution of volatiles from erupted fluid phases should not be underestimated.We thank Anita Cadoux and Kim Berlo for reference glasses, Lang Shi for assistance on the electron microprobe, and Nicole Bobrowski and two anonymous reviewers for constructive comments. This research was supported by a Geological Society of America grant (C. Waelkens), Natural Sciences and Engineering Research Council of Canada (NSERC) Discovery and CREATE grants (J. Stix), and Institut de Physique du Globe de Paris multidisciplinary program PARI and Paris–IdF region SESAME grant 12015908 (P. Burckel). The Northeast National Ion Microprobe Facility is subsidized by U.S. National Science Foundation (NSF) facility support grant NSF-EAR-1664308.

中文翻译：

---

通过超级喷发有效释放溴

溴是量化火山挥发物的关键卤素元素，但测量其极低丰度的分析困难阻碍了对其行为及其在火山排放中作用的理解。我们展示了美国新墨西哥州托莱多和瓦莱斯火山口的两次流纹岩超级喷发的熔体包裹体和基质玻璃中溴、氯和氟浓度的新数据集。我们表明，在喷发之前，Br 和 Cl 从气体饱和的岩浆中有效地分配到单独的流体相中，并且我们计算了流体相中卤素的质量。我们进一步证明，在超级喷发期间，同步喷发的岩浆脱气可以忽略不计，因此卤素排放的主要来源一定是流体相。如果流体相喷发，大量的 Br 和 Cl 可能在喷发时严重影响大气化学。卤素是岩浆挥发系统的重要组成部分，在岩浆脱气（Aiuppa 等人，2009 年）、大气臭氧破坏（Aiuppa 等人）中起着至关重要的作用., 2009; von Glasow et al., 2009) 和金属运输 (Williams-Jones and Heinrich, 2005)。岩浆中的溴浓度通常比氯低两个数量级，但其较高的挥发性（Bureau 和 Métrich，2003 年）和破坏臭氧的潜力（Daniel 等人，2007 年）使溴成为评估时特别有用的研究元素爆炸性喷发的潜在影响。我们分析了超级火山系统中卤素的富集，并量化了从熔体中分离出来并可能释放到大气中的 Br 和 Cl 的质量。美国新墨西哥州西北部。托莱多火山口的第一次超级喷发发生在 1.61 Ma（Spell 等人，1990 年），喷发了约 400-500 平方公里的岩浆致密岩当量（DRE）（Cook 等人，2016 年），并沉积了高硅流纹质下班德利尔凝灰岩 (LBT)。第二次超级喷发，在成分和体积上都相似，在 1.26 Ma 沉积了上班德利尔凝灰岩 (UBT)（Phillips 等，2007）。两次喷发都经历了最初的普林尼阶段，然后是结冰凝灰岩。在两次超级喷发之间，岩浆房的顶部周期性地被小的流纹岩喷发所挖掘，这些喷发形成了 Valle Toledo 段的火山灰、流和穹顶，其跨度约为 350 ky（Spell 等人，1996 年；Gardner 等人，2010 年）。Valle Toledo 喷发的初始阶段在地球化学上与 LBT 相关，并被解释为来自排空岩浆房的最后一次脉冲，之后新的硅质岩浆进入岩浆房并开始分化（Stix 和 Gorton，1993）。类似地，随着巴勒斯火山口的回潮，在 UBT 之后立即喷发了山谷流纹岩的鹿峡谷成员（此处为鹿峡谷流纹岩）（Phillips 等，2007）。该单元在地球化学上与 UBT 相关，代表了在新鲜岩浆重新补充系统之前来自 UBT 岩浆房的最后脉冲（Spell 等人，1993 年；Wilcock 等人，2013). 我们从 (1) LBT 和 UBT Plinian 相的底部和顶部收集了浮石样品，(2) 代表与 LBT 相关的初始阶段和岩浆之后的第一阶段的 Valle Toledo 的两个关键单元充电，和 (3) 鹿峡谷流纹岩。浮石中的斑晶主要由富含熔体包裹体的石英和三色胺组成。溴 (Br)、氯 (Cl) 和氟 (F) 通过离子微探针测量熔体夹杂物，每单位分析约 10 个熔体夹杂物以及每单位 1 到 2 个基质玻璃。补充材料 1 中提供了分析细节。在 LBT 序列中，在熔体包裹体中测得的 F 显示出稳定下降的上部，因此更深入岩浆房（图 1）。氟在普林尼阶底部从 2700 ppm 减少到 3340 ppm，然后在顶部减少到 1880–2830 ppm，最后在首次喷发的 Valle Toledo 单元中达到 260–550 ppm。氯遵循类似但不那么明显的趋势，从 1920 年到 2440 年，通过 Plinian 减少到 1860-2660 ppm，并在首次喷发的 Valle Toledo 中进一步减少到 1030-2190 ppm。相比之下，Br 在 LBT 序列中没有表现出变化；在 Plinian 的底部发现 1.3-1.7 ppm 的 Br 值，在 Plinian 的顶部发现 1.2-1.8 ppm，在 LBT 的末端发现 1.1-1.9 ppm。对于整个 LBT 序列，F 从 LBT 底部到最早的 Valle Toledo 下降 6.6 倍，而 Cl 下降 1.5 倍，Br 保持不变。 UBT 中的趋势与 LBT 中的趋势相似，其中几个主要区别： (1) UBT 中的浓度范围要大得多；(2)卤素浓度本身较高；(3) Cl和F从Plinian的底部到顶部略有增加，而不是减少；(4) Br 也减少，而不是保持不变。氟从 2800 到 3570 ppm 再到 2740-4100 ppm，然后在鹿峡谷流纹岩中骤降到 340-930 ppm，这是最深的喷发岩浆，有一个高异常值在 2230 ppm。氯在 Plinian 底部从 2560-3440 ppm 开始，增加到 2710-4080 ppm，然后在鹿峡谷中急剧下降至 1310-2050 ppm。按照相同的顺序，Br 从 2.7 到 3.7 再到 2.2–3.8 ppm，最后到 1.5–3.2 ppm。F 的整个 UBT 序列的减少为 3.6 倍（如果不考虑高鹿峡谷异常值，则为 4.4 倍），Cl 为 1.7 倍，Br 为 1.2 倍。我们分析的 Valle Toledo 样本包括三个单元：第一个- 喷发的材料，这标志着 LBT 序列的结束（约 1.61 Ma；Spell 等，1996）；补给后的第一次喷发（1.54 Ma；Spell 等，1996）；和 UBT Plinian (1.26 Ma; Phillips et al., 2007) 的基础，它代表了 Valle Toledo 分化的结束。在这 0.35 米的间隔期间，三个卤素显示出不同的富集。氟在最早的 Valle Toledo 开始为 260-550 ppm，然后在补充后达到 190-830 ppm，最终在 UBT 底部达到 2800-3570 ppm 的强富集。氯浓度遵循类似的趋势，在补充后从 1030 到 2190 ppm 再到 800-1720 ppm，然后富集到 2560-3440 ppm。Br 趋势与 F 和 Cl 相似，但不如 F 和 Cl 明显，范围从 1.1 到 1.9 ppm 到 1.2-2.7 ppm，并在 UBT 底部以 2.7-3.7 ppm 结束。F、Cl 的富集因子，补充后到 UBT 喷发的 Br 和 Br 分别为 5.6、2.3 和 1.6。三种卤素富集因子的系统差异表明存在流体饱和的岩浆，其中 Br 和 Cl 主要分配到流体中，而F几乎完全在岩浆中积累。以前的研究表明，氟在这些岩石中几乎完全不相容，与 Cs 类似（Stix 和 Layne，1996 年），表明磷灰石等矿物相在螯合卤素方面的作用可以忽略不计。溴具有比 Cl 更高的流体-熔体分配系数（Bureau 等人，2000 年；Bureau 和 Métrich，2003 年；Cadoux 等人，2018 年），这导致熔体中的富集较少，因为更多的 Br 分馏到流体相中. 如果岩浆不是流体饱和的，Br 和 Cl 将表现出与 F 相同的富集（Balcone-Boissard 等，2010）。因此，差异表明损失到流体相中的 Br 和 Cl 的量。在整个 Valle Toledo 火山喷发中观察到的 F、Cl 和 Br 富集反映了浓度梯度的重新建立，F 在熔体中积累，而 Cl 和 Br 部分损失到流体相中。总之，我们的结果表明班德利尔岩浆系统在其演化过程中始终处于流体饱和状态。对于我们分析的每个样品，我们还测量了一到两种基质玻璃中的 F、Cl 和 Br 浓度（图 1）。对于所有样品和所有三种卤素，基质玻璃分析在与熔体夹杂物相同的范围内绘制。只有在 LBT 的底部，两种基质玻璃才会在熔体夹杂物浓度范围的下限处绘图。鉴于基质玻璃和熔体夹杂物之间没有系统差异，协同喷发在班德利尔喷发中不能发挥任何重要作用，这是由于卤素在流纹质岩浆中的缓慢扩散与其他挥发物如 H2O 相比（ Baker 和 Balcone-Boissard，2009 年）。相似的 Br 和 Cl 含量也表明岩浆在主体晶体结晶后不久喷发，因为在熔体包裹体捕获和喷发之间几乎没有 Br 和 Cl 进一步损失到流体相中。 估计大陆地壳有一个 Cl /Br 比率为 273，而在岩浆岩中测得的 Cl/Br 比率约为 300（Bureau 等，2000）。鉴于 Br 相对于 Cl 的较高的流体-熔体分配系数，熔体相中的 Cl/Br 比率将在流体相的存在下逐渐增加。我们分析的熔体包裹体的 Cl/Br 比为 600 到 1600（图 2），表明喷发前卤素分馏成流体相在岩浆房中起主导作用。尽管一些样品有一些分布，但绘制 Cl/Br 比率显示三个不同的组：(1) LBT Plinian 具有最高的 Cl/Br 比率，大多数熔体夹杂物在 1300 和 1600 之间，因此经历了最多的 Br 和 Cl流体相损失；(2) UBT Plinian 值的分布较小，所有熔体夹杂物的 Cl/Br 比约为 900-1000，表明 UBT 中较高的 Br 和 Cl 值可以解释为分配到流体相的效率低于在 LBT 中；(3) 两个 Valle Toledo 样品和 Deer Canyon 样品表现出最低的 Cl/Br 值，它们紧密聚集在 600 和 700 之间（除了少数具有高 Cl/Br 的异常值），反映了较小的分馏程度更原始的流纹质岩浆中的流体相。以前对熔体包裹体中 Br 的研究主要集中在弧形岩浆上（Kutterolf 等，2013，2015；Cadoux 等，2015，2017，2018；Balcone-Boissard 等， 2018）。在所有这些研究中，溴含量比我们在 Bandelier 系统中测量的含量高得多（5-15 ppm），高溴含量与俯冲沉积物和蛇纹石化俯冲地幔的输入有关（Straub 和 Layne，2003 年；Kutterolf 等人。 , 2015)。我们将 Bandelier 熔体包裹体中观察到的较低 Br 值归因于 Toledo 和 Valles 火山口的非弧形环境以及喷发前卤素损失的主要作用。 Bandelier 喷发期间缺乏同步喷发脱气，表明由于熔体夹杂物和基质玻璃中的卤素浓度相似，这意味着流体相是释放到大气中的 Cl 和 Br 的唯一主要来源。为了计算在流体相中损失的 Cl 和 Br 的质量，我们假设如果不存在流体相，它们将表现出与 F 相同的浓度梯度。我们在 LBT 岩浆房的深度上对 Cl 和 Br 的这一理论浓度梯度进行积分，并假设一个准圆柱形的房间与火山口具有相同的横截面积和 550 km3 的岩浆体积（最大估计值；Cook 等人，2016 年）计算岩浆中 Cl 和 Br 的质量（如果不存在流体相）。对观察到的浓度梯度进行相同的练习，然后计算差异，得出从熔体释放到流体相中的 Cl 和 Br 的质量。对于 LBT，这导致 4800 Tg Cl 和 5.7 Tg Br，而对于 UBT 的相同计算，假设岩浆体积为 400 km3（最佳估计；Goff，2010），产生 2100 Tg Cl 和 3.6 Tg Br。尽管部分分馏的流体相的一部分可能通过被动脱气或通过裂缝进入热液系统而损失，喷发效率取决于流体相的物理状态，浮力气体直接从位于顶部的富含蒸汽的流体相中释放出来岩浆房（Stix 和 Layne，1996 年；Wallace 等人，2003 年）可以有效地将它们推入平流层，正如在 1982 年 El Chichón（墨西哥）和 2000 年 Hekla（冰岛）喷发中观察到的那样（Schneider 等人，1999 年；Rose 等人，2006 年）。因此，流体相中计算出的质量是喷发时从系统中释放出的最大 Br 和 Cl 量。相比之下，圣托里尼（希腊）米诺斯火山喷发的最大估计值是 675 Tg Cl 和 1.5 Tg Br（Cadoux 等人，2015 年），其喷发量比班德利尔火山喷发小一个数量级；对中美洲火山弧沿线普林尼亚火山喷发的研究得出最大值为 800 Tg Cl 和 1.1 Tg Br（Kutterolf 等，2015）；1257 年的 Samalas（印度尼西亚）喷发估计排放了 227 Tg Cl 和 1.3 Tg Br，喷发了 40 平方公里的 DRE（Vidal 等人，2016 年）。计算溶出到流体相中的 Cl 和 Br 质量使我们能够确定流体中的 Cl/Br 比率。我们计算了 LBT 的 843 和 UBT 的 580 的比率。我们对卤素分配到流体相的分析还允许对 Bandelier 系统中 Cl 和 Br 的分配系数进行独立估计。LBT 和 UBT 中的 H2O 浓度表明各自的流体相分别占总岩浆质量的 6.7 和 4.2 重量％（补充材料中的详细信息）。这种流体质量估计使我们能够计算流体中的平均 Br 和 Cl 浓度（LBT 为 68 ppm Br 和 57 × 103 ppm Cl，UBT 为 93 ppm Br 和 54 × 103 ppm Cl），然后我们将其除以通过熔体中 Br 和 Cl 的平均浓度来产生相关的流体-熔体分配系数 (Df-m)。DBrf-m 对于 LBT 是 41，对于 UBT 是 33，而 LBT 的 DClf-m 为 30，UBT 的 DClf-m 为 23。这些系数高于流纹岩的实验确定（DBrf-m = 20.2：Cadoux 等人，2018 年；DClf-m = 16：Webster 等人，2009 年；均在 900 °C 和 200 MPa），可能是因为 Bandelier岩浆的硅质含量更高，温度更低。火山释放的卤素会影响大气化学并导致臭氧消耗（Textor 等人，2003 年；von Glasow 等人，2009 年）。尽管通常以低于 Cl 的浓度存在，但 Br 在破坏臭氧分子方面的效率比 Cl 高约 60 倍，因此在评估火山破坏臭氧的潜力方面具有重要意义（Daniel 等人，2007 年）。涌入5。向大气中添加 7 Tg Br 导致溴混合比（整个大气中每摩尔空气中的 Br 摩尔数）增加 397 pptv（每体积空气中的万亿分之一），而在 UBT 期间添加 3.6 Tg Br 会产生额外增加了 248 pptv 的溴混合比例。相比之下，从前工业化时代到 1995 年，人为的、以氯氟烃 (CFC) 为燃料的输入只有约 15 pptv（Daniel 等人，2007 年）。Cl 的类似计算导致 LBT 喷发的大气氯混合比增加 753 ppbv，UBT 喷发增加 327 ppbv。这种 Cl 输入的规模使人为的、CFC 产生的 Cl 增加了约 3 ppbv（Daniel 等人，2007 年）。喷发的确切影响取决于流体相喷发的效率和喷发柱中卤素的清除。然而，从班德利尔岩浆中分离出的大量溴和氯可能严重影响了古大气。我们提供了喷发产物中卤素浓度的新数据集，并计算了喷发期间释放的卤素的最大量。我们将熔体夹杂物和基质玻璃中类似的卤素浓度解释为对同步爆发卤素扩散和脱气的作用可以忽略不计，而大量的 Cl 和 Br 储存在流体相中。Bandelier 岩浆房中富含卤素的流体相可能在喷发时被释放出来，导致对流层和平流层臭氧严重消耗。Bandelier 系统测得的 Br 浓度低于弧形岩浆（5-15 ppm），这表明在弧形环境（如 26. 陶波（新西兰）的 5 ka Oruanui 喷发可能会释放出更大质量的 Br。我们的研究证实，大型爆炸性火山喷发可以在大气化学中发挥重要作用，并且不应低估喷发流体相挥发物的贡献。感谢 Anita Cadoux 和 Kim Berlo 提供参考玻璃，感谢 Lang Shi 在电子微探针方面的帮助，以及 Nicole Bobrowski 和两位匿名审稿人的建设性意见。这项研究得到了美国地质学会资助 (C. Waelkens)、加拿大自然科学和工程研究委员会 (NSERC) 发现和 CREATE 资助 (J. Stix) 以及巴黎地球物理研究所多学科项目 PARI 和巴黎-IdF 地区 SESAME 拨款 12015908 (P. Burckel)。