Oomurodashi is a newly discovered active, shallow, silicic submarine volcano only 60 km from Tokyo Bay. We reveal its past eruptive activity, and potential future hazards, by examining volatile contents of its subaerial and submarine pumice and lava deposits. These novel data for shallow silicic submarine eruption products were obtained using new Fourier transform infrared spectroscopy (FTIR) analytical techniques for vesicular and hydrated glasses. All matrix glasses have H2O species data consistent with low-temperature hydration following eruption. We therefore used unaltered OH data to investigate past eruptions. Geochemistry confirmed that Oomurodashi was the source of a ca. 13.5 ka subaerial tephra deposit on nearby inhabited islands. We infer from pumice OH contents and tephra characteristics that this deposit was formed by explosive submarine phreatomagmatic activity that produced the shallow crater in the submarine edifice. OH contents of in-place submarine lavas are lower than expected for their current water depth; comparison with past sea level implies that these lavas erupted at ca. 7–10 ka and ca. 14 ka when sea level was lower. Oomurodashi has also erupted submarine pumice with different densities, quench depths, and dispersal histories; however, any pumice sufficiently buoyant to produce floating pumice rafts will have been lost from the local geological record, so pumice rafts remain a potential future hazard.Shallow, silicic submarine volcanoes pose significant, yet poorly constrained, hazards, as shown by the recent eruptions of Fukutoku-Oka-no-Ba (south of Ioto island in the Ogasawara Islands, Japan) in 2021 CE, which produced pumice rafts impacting ships and coastal communities throughout Japan (Geological Survey of Japan, 2021), and Hunga Tonga–Hunga Ha'apai in 2022, which inundated the Kingdom of Tonga with tsunami waves and ash fall, severed communications, and triggered global tsunami activity (Global Volcanism Program, 2022). The difficulties of observing eruptions, sampling submarine deposits, and constraining eruption ages mean much remains unknown about the ways in which submarine eruptions vary with water depth and eruption magnitude, while a lack of detailed eruptive histories prevents assessment of volcanic hazard.A recent study (Mitchell et al., 2018) of pumice volatile contents from the deep, silicic Havre eruption in 2012 (in the Kermadec Islands, southwest Pacific Ocean), obtained using new Fourier transform infrared spectroscopy (FTIR) analytical techniques, demonstrated how these data can yield new insights into submarine eruption processes. Our study used volatile contents of both subaerial and submarine deposits from Oomurodashi, a newly discovered active volcano in the Izu-Bonin arc, to investigate late Quaternary eruptions of a shallow, silicic submarine volcano.Oomurodashi is located in the northern Izu-Bonin arc, 20 km southeast of Izu-Oshima Island (population ~9000) and 60 km southwest of Tokyo Bay (Fig. 1). Its 20-km-wide, flat-topped summit lies ~120 m below sea level (mbsl) and contains a small (1.2 × 0.7 km, ~100 m deep) crater named Oomuro Hole. Oomurodashi was previously thought to be inactive; however, cruise NT07–15 of R/V Natsushima in 2007 (JAMSTEC, 2007), using a remotely operated vehicle (ROV), measured high heat flow in the crater floor and observed fresh rhyolite pumice deposits (Tani et al., 2013). Subsequent ROV surveys (NT12–19 of R/V Natsushima in 2012 [JAMSTEC, 2012]; KS-16–6 of R/V Shinseimaru in 2016 [data available from JAM-STEC]) obtained samples and discovered an active hydrothermal field within the crater. We present volatile and density data for a subset of rhyolite eruption products sampled directly from submarine outcrops and from subaerial tephra deposits on the nearby islands of Izu-Oshima and Toshima (Fig. 1).A distinctive tephra of dark, fine-grained material dotted with white pumice is found among the predominantly basaltic tephra deposits on the islands of Izu-Oshima and Toshima, named layer O58 (14C age of 13.8–13.2 ka; Saito and Miyairi, 2008; recalculated with IntCal20, Reimer et al., 2020) and layer O3T (found between layers dated to 15 and 11 ka; Oikawa and Tani, 2020), respectively (Fig. 1). The pumice clasts are generally <1 cm (longest axis) but range up to 4 cm in size (Fig. 2). We compared their major- and trace-element geochemistry with those of samples from Oomuro Hole as well as published geochemical data for two neighboring rhyolitic volcanic islands (see the Supplemental Material1). These data confirmed that Oomurodashi was the source of both tephra layers.The dark, fine-grained matrix forms the majority of both layers O58 and O3T, with particles <250 μm comprising >70% by mass. Most of these are aggregates, resistant to disaggregation methods, of even finer grains that are dense or only weakly vesicular; some have stepped features indicative of intense brittle fragmentation (Fig. 2C). Such particles may be associated with explosive phreatomagmatic activity (Zimanowski et al., 2015). Other features are particles with manganese coatings likely formed in the submarine environment, and dense angular lithics ≤1 cm in the base of O58 (see the Supplemental Material). We propose that these tephras are the distal deposits resulting from the explosive formation of the Oomuro Hole crater by a shallow submarine, phreatomagmatic eruption.Volatile and porosity data were obtained for both subaerial tephra pumice clasts and submarine samples: a lava (719-R14) and loose, weakly vesicular clast (1409-R10) from the rim of Oomuro Hole, a lava (1970-R01) from a small (100-m-wide, 24-m-high) knoll on the flat summit, and a loose pumice clast (1407-R01) from the northeast flank (Fig. 1). Total water (H2Ot), molecular water (H2Om), and hydroxyl water (OH) contents of matrix glasses were obtained by FTIR using the species-dependent ε3500 (not OH-by-difference) method following the procedures of McIntosh et al. (2017) and Mitchell et al. (2018). CO2 was below detection limits for all samples. All samples had <5% isolated porosity. See the Supplemental Material for full details of the methods, sampling depth, porosity, and FTIR data.The equilibrium concentrations of H2Om and OH (equilibrium speciation) at a given H2Ot concentration and temperature are controlled by a species interconversion reaction and vary in a known way (e.g., Stolper, 1982; Nowak and Behrens, 2001). The interconversion reaction rate decreases dramatically during cooling until reaching the glass transition temperature, Tg, at which H2Om and OH concentrations become fixed (e.g., Dingwell and Webb, 1990; Zhang et al., 1995). Interpreting H2O species data thus requires an assumption of each sample's Tg. Tg depends on H2Ot and cooling rate and is lower for higher H2Ot and/or slower cooling. Sample OH and H2Ot data are therefore plotted (Fig. 3A) against values expected for Tg ranging from 800 °C (instant quench) to 600 °C (equivalent to ~0.8 wt% H2Ot and 10 °C/min cooling; Giordano et al., 2008). Our sample data plot below these curves, so they have excess H2Om; i.e., they have been hydrated by disequilibrium addition of H2Om.Diffusion of water in melts and glasses occurs by movement of H2Om, with some subsequent interconversion of H2Om to OH to maintain local equilibrium (Zhang et al., 1991). Excess H2Om thus indicates H2Om addition at temperatures where the interconversion reaction rate was too slow to maintain equilibrium speciation. This could be due to slow low-temperature (secondary) hydration (e.g., Giachetti and Gonnermann, 2013) or more rapid hydration at intermediate temperatures (e.g., Zhang et al., 1995; McIntosh et al., 2014).FTIR data for pumice from the 2012 submarine Havre eruption revealed they were hydrated, and it could be inferred that hydration occurred during cooling in the water column because their young age excluded slow low-temperature hydration as a possible cause (Mitchell et al., 2018). The older Oomurodashi samples, however, could have experienced either hydration mechanism. H2O species data along hydration profiles can help to distinguish these scenarios; unfortunately, only pumice 1407-R01 was successfully prepared for profile analysis (Figs. 3B and 3C). Unlike H2Ot and H2Om, OH (wt%) does not increase toward its bubble walls. Hydration thus occurred with little to no conversion of excess H2Om to OH. For the measured H2Ot range, even a slow cooling rate of 10 °C/min would give Tg > 650 °C for this pumice (Giordano et al., 2008), which would be reached in ~15 min. This is enough time for the observed H2Om diffusion profile to form, despite the order of magnitude drop in H2Om diffusivity for cooling from 800 °C to 600 °C (Ni and Zhang, 2008); however, at these temperatures and time scales, at least some interconversion of excess H2Om to OH would be expected before OH became fixed at Tg (Zhang et al., 1995). Hydration thus probably occurred at low temperature. Low-temperature H2O diffusivity data span several orders of magnitude (e.g., Giachetti and Gonnermann, 2013), but such a profile could form in ~1–100 k.y., assuming H2Ot diffusivity of ~10-21 to ~10-23 m2/s. Without profiles for the other samples, it is impossible to say conclusively whether all hydration occurred at low temperature. The observed hydration of the ca. 13.5 ka subaerial pumice clasts is, however, also consistent with low-temperature hydration after their eruption, particularly as bulk hydration is enhanced for vesicular samples that experience large diurnal temperature fluctuations (Friedman and Long, 1976).Unlike subaerial tephra layers, it is difficult to date late Quaternary submarine eruption deposits. We estimated eruption ages of the inplace lavas by comparing their OH contents with their sampling water depth and past sea level, in a manner analogous to that used to constrain paleo-ice thickness for subglacial eruptions (e.g., Tuffen et al., 2010). H2O solubility is pressure dependent, so rising magma degasses H2O until final emplacement as a seafloor lava flow. The H2Ot content of (unhydrated) matrix glasses records this emplacement pressure, which can be converted into an equivalent water depth. Hydration has altered the H2Ot record of these lavas, so we instead used their OH contents, fixed at Tg, to estimate their quench pressure, hence emplacement depth. Figure 3D shows sample OH data plotted along curves of OH versus pressure (bottom axis) and equivalent water depth (top axis) calculated for Tg of 600–800 °C, with their ROV sampling depths shown above the top axis. For past sea level, we used a global sea-level curve adjusted for regional tectonic uplift (Fig. 3E; see the Supplemental Material).Cooling rates of submarine rhyolite flows are not constrained, but given the low original H2Ot content of the shallow lavas, we assumed a likely Tg of 700–800 °C. If slow cooling led to Tg < 700 °C, then lavas will be younger than calculated, but this potential error is typically <1 k.y. due to the convergence of Tg curves at low OH. Analytical uncertainties were considered by propagating ±1σ of the FTIR mean (Fig. 3D).Accordingly, the summit knoll lava 1970-R01 has an OH content consistent with quench when sea level was up to 28 m shallower than today, giving an eruption age of ca. 9.8–7.3 ka. Lava 719-R14 also contains less OH than expected for its sampling water depth, but its larger FTIR uncertainty translates to large uncertainty in age, giving an eruption age of ≤14 ka. This lava is exposed at the top of the Oomuro Hole crater with vertical cooling joints, implying it was emplaced on the summit and then cut by the formation of Oomuro Hole, which we infer from tephra O58 to have formed 13.8–13.2 ka. This sequence is also consistent with a preliminary quartz electron spin resonance date of 12 ± 4 ka for this lava (Asagoe et al., 2013). Although FTIR data are consistent within error with an age ≥13.2 ka, we considered additional factors that may have affected the estimated age (see also the Supplemental Material). Ages will be underestimated if lavas have been uplifted since their eruption; however, Oomurodashi's flat summit, formed via wave planation at the last sea-level lowstand, agrees well with our uplift-adjusted sea-level curve and excludes any significant local deformation since that time. Ages will also be underestimated if lava volatiles had not fully equilibrated with the emplacement pressure before quenching. Such incomplete degassing is more likely for lava 719-R14 because the cross-sectioned flow interior was sampled rather than (as for the summit knoll) the exterior, and this may be the cause of the greater variability in its OH data. Generally, flow exteriors are more likely to have fully degassed to their quench pressure, as seen for subaerial lavas at Mount St. Helens (USA) and Santiaguito (Guatemala), although the most vent-proximal samples may retain up to 0.1 wt% excess H2Ot (Anderson et al., 1995). However, even if lava 719-R14 retained as much as 0.15 wt% excess OH from incomplete degassing, its age cannot exceed 15.5 ka.Tephra pumices had higher OH contents than the submarine samples (Figs. 3A and 3D), implying that they quenched at higher pressures despite being deposited subaerially. Assuming Tg of 700–800 °C, their OH contents are equivalent to quench pressures of 3.6–4.0 MPa (O58) and 2.1–2.3 MPa (O3T). At 13.8–13.2 ka, the Oomurodashi summit was ~55 mbsl, equivalent to ~0.7 MPa. These OH contents therefore record fragmentation and quenching within the shallow edifice. Assuming a magma/lithostatic pressure gradient (with density of ρ = 2300 kg/m3), we calculated quench depths within the conduit of ~130–150 m (O58) and ~60–70 m (O3T); these depths would have been shallower if the syneruptive crater had already begun to form (e.g., ~80–100 m and ~20–30 m depth for a 100-m-deep crater like today). Such depths indicate shallow fragmentation, consistent with shallow magma-water interaction (e.g., Dellino et al., 2012), or maar-type explosions within a partly infilled vent that would have been sufficiently shallow to produce surface ejecta (Graettinger et al., 2014). Thus, the volatile data support the interpretation that these tephras were produced by shallow phreatomagmatic explosions that formed the Oomuro Hole crater, and they demonstrate the ability of new FTIR methods to obtain fragmentation depths of juvenile material.Pumice clasts around the rim of Oomuro Hole indicate that the eruption (or subsequent eruption) shifted to a pumice-producing style after crater formation. Clast 1409-R10, collected at 118 mbsl, was dense (33% porosity) and would not have been buoyant in water upon eruption. Its OH content indicates a quench depth of 70–80 mbsl. If it erupted at 13.8–13.2 ka (when the sampling location was 50–56 mbsl), it would thus need ~20 m of nonbuoyant postquench ascent to reach its final deposition location. As momentum-driven ascent in water is limited, it is likely from a more recent eruption, when its deposition location was also 70–80 mbsl (ca. 11.3–10.7 ka). By contrast, pumice 1407-R01 has an OH content equivalent to a quench depth of ~30 mbsl. This pumice (72% porosity) would initially have been buoyant in water, and its final location 5 km from the volcanic center suggests it drifted at shallow depths prior to waterlogging and deposition. Waterlogging of hot pumices that ingest water during cooling can occur rapidly, potentially limiting buoyant ascent and dispersal (Whitham and Sparks, 1986). ROV surveys extending 500 m east of Oomuro Hole (i.e., downstream of the strong Kuroshio current) during Cruise NT12-19 found abundant fresh rhyolitic pumice covering the summit. However, in assessing potential future hazards, it must be noted that if shallow eruptions also produced pumice that cooled within a subaerial plume and thus ingested air rather than water (Whitham and Sparks, 1986), or had a greater percentage of isolated porosity (Manga et al., 2018), such pumice could have formed buoyant rafts that are not preserved in Oomurodashi's local deposits (e.g., Carey et al., 2018).The recently discovered active Oomurodashi volcano has had at least three eruptions since ca. 14 ka, which include submarine lava flows, phreatomagmatic explosions creating subaerial tephras, and submarine eruptions of pumice with varying buoyancies. Oomurodashi's location next to inhabited islands and major shipping lanes entering Tokyo Bay therefore makes it a significant hazard. FTIR volatile data of lavas and pyroclasts integrated with porosity and textural data will provide a new framework for interpreting submarine eruption processes. Acquisition of these data from both local and distal deposits, such as those in marine sediment cores, will enable us to investigate the full spectrum of submarine eruption styles and better understand their potential hazards.We thank the scientists and crew of the R/V Natsushima and the R/V Shinseimaru. This work was supported by Japan Society for the Promotion of Science KAKENHI grants JP00470120 and 16K05584. We thank S. Bryan and five anonymous reviewers for their constructive reviews.

中文翻译：

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日本东京湾附近一座新发现的活跃的浅层硅质海底火山过去的喷发

Oomurodashi 是一座新发现的活跃的浅层硅质海底火山，距离东京湾仅 60 公里。我们通过检查其陆上和海底浮石和熔岩沉积物的挥发性成分来揭示其过去的喷发活动和潜在的未来危害。这些关于浅层硅质海底喷发产物的新数据是使用新的傅里叶变换红外光谱 (FTIR) 分析技术获得的，用于囊泡和水合玻璃。所有基质玻璃都具有与喷发后的低温水合作用一致的 H2O 种类数据。因此，我们使用未改变的 OH 数据来调查过去的喷发。地球化学证实 Oomurodashi 是 ca 的来源。附近有人居住的岛屿上的 13.5 ka 地下火山灰沉积物。我们从浮石 OH 含量和火山灰特征推断该矿床是由爆炸性的海底潜水岩浆活动形成的，该活动在海底大厦中产生了浅火山口。就地海底熔岩的 OH 含量低于当前水深的预期；与过去的海平面比较意味着这些熔岩在 ca 爆发。7–10 ka 和 ca。海平面较低时为 14 ka。Oomurodashi还爆发了具有不同密度、骤冷深度和扩散历史的海底浮石；然而，任何足够浮力以产生漂浮浮石筏的浮石都将从当地地质记录中丢失，因此浮石筏仍然是未来的潜在危险。正如最近在公元 2021 年 Fukutoku-Oka-no-Ba（日本小笠原群岛的 Ioto 岛南部）的喷发所表明的那样，浮石筏影响了日本各地的船舶和沿海社区（日本地质调查局，2021 年），以及 2022 年的洪加汤加-洪加哈帕伊，海啸和火山灰淹没了汤加王国，切断了通讯，并引发了全球海啸活动（全球火山活动计划，2022 年）。观测喷发、海底沉积物取样和限制喷发年龄的困难意味着，关于海底喷发随水深和喷发幅度变化的方式仍有很多未知之处，而由于缺乏详细的喷发历史，无法评估火山危害。最近的一项研究（ Mitchell et al., 2018) 来自深部的浮石挥发性成分，使用新的傅里叶变换红外光谱 (FTIR) 分析技术获得的 2012 年（位于太平洋西南部克马德克群岛）的硅质阿弗尔火山喷发展示了这些数据如何对海底喷发过程产生新的见解。我们的研究使用来自伊豆-Bonin 弧中新发现的活火山 Oomurodashi 的陆上和海底沉积物的挥发性成分来调查浅层硅质海底火山的第四纪晚期喷发。Oomurodashi 位于伊豆-Bonin 弧的北部，伊豆大岛东南 20 公里（人口约 9000 人）和东京湾西南 60 公里（图 1）。其 20 公里宽的平顶山顶位于海平面以下约 120 米（mbsl），并包含一个名为 Oomuro 洞的小型（1.2 × 0.7 公里，约 100 米深）陨石坑。Oomurodashi 以前被认为是不活跃的；然而，2007 年，R/V Natsushima 的 NT07-15 巡航（JAMSTEC，2007），使用遥控飞行器（ROV）测量了火山口底部的高热流，并观察到了新鲜的流纹岩浮石沉积物（Tani 等，2013）。随后的 ROV 调查（2012 年 R/V Natsushima 的 NT12-19 [JAMSTEC，2012 年]；2016 年 R/V Shinseimaru 的 KS-16-6 [数据来自 JAM-STEC]）获取样本并在其中发现了一个活跃的热液场陨石坑。我们提供了直接从海底露头和附近伊豆大岛和丰岛岛上的陆上火山灰沉积物采样的流纹岩喷发产物子集的挥发性和密度数据（图 1）。 一种独特的暗色细粒材料点缀着火山灰在伊豆大岛和丰岛岛的主要玄武质火山灰沉积物中发现了白色浮石，命名为 O58 层（14C 年龄 13.8-13. 2卡；斋藤和宫入，2008；用 IntCal20，Reimer 等人，2020 年）和 O3T 层（在 15 和 11 ka 层之间发现；Oikawa 和 Tani，2020 年）重新计算（图 1）。浮石碎屑一般小于 1 厘米（最长轴），但大小可达 4 厘米（图 2）。我们将它们的主要元素和微量元素地球化学与来自 Oomuro 洞的样品以及已发表的两个相邻流纹岩火山岛的地球化学数据进行了比较（参见补充材料 1）。这些数据证实，Oomurodashi 是两个火山灰层的来源。深色的细粒基质构成了 O58 和 O3T 层的大部分，颗粒 <250 μm 占质量的 >70%。其中大多数是聚集体，对分解方法有抵抗力，甚至是更细的颗粒，致密或只有弱泡状；有些具有表明强烈脆性碎裂的阶梯特征（图2C）。此类颗粒可能与爆炸性的潜水岩浆活动有关（Zimanowski 等人，2015 年）。其他特征是可能在海底环境中形成的具有锰涂层的颗粒，以及 O58 底部的致密角石≤1 cm（参见补充材料）。我们认为这些火山灰是由浅层海底火山爆发形成的 Oomuro Hole 火山口形成的远端沉积物，潜水岩浆喷发。获得了陆地火山灰浮石碎屑和海底样品的挥发性和孔隙度数据：熔岩 (719-R14)以及来自 Oomuro Hole 边缘的松散、弱泡状碎屑（1409-R10），这是来自平坦山顶上一个小（100 米宽，24 米高）小丘的熔岩（1970-R01），和来自东北侧的松散浮石碎屑（1407-R01）（图 1）。基质玻璃的总水 (H2Ot)、分子水 (H2Om) 和羟基水 (OH) 含量是按照 McIntosh 等人的程序使用物种相关的 ε3500（不是 OH-by-difference）方法通过 FTIR 获得的。（2017）和米切尔等人。（2018 年）。所有样品的 CO2 均低于检测限。所有样品均具有<5% 的孤立孔隙率。有关方法、采样深度、孔隙率和 FTIR 数据的详细信息，请参阅补充材料。在给定的 H2Ot 浓度和温度下，H2Om 和 OH（平衡形态）的平衡浓度由物种相互转化反应控制，并且在已知的方式（例如，Stolper，1982；Nowak 和 Behrens，2001）。在冷却过程中，相互转化反应速率急剧下降，直到达到玻璃化转变温度 Tg，此时 H2Om 和 OH 浓度变得固定（例如，Dingwell 和 Webb，1990；Zhang 等人，1995）。因此，解释 H2O 种类数据需要假设每个样品的 Tg。Tg 取决于 H2Ot 和冷却速率，对于较高的 H2Ot 和/或较慢的冷却，Tg 较低。因此，将样品 OH 和 H2Ot 数据绘制（图 3A）与预期的 Tg 值范围为 800 °C（即时淬火）至 600 °C（相当于~0.8 wt% H2Ot 和 10 °C/min 冷却；Giordano 等人., 2008)。我们的样本数据绘制在这些曲线下方，因此它们有过量的 H2Om；即，它们已通过 H2Om 的不平衡添加而水合。水在熔体和玻璃中的扩散通过 H2Om 的运动发生，随后 H2Om 与 OH 相互转化以维持局部平衡（Zhang et al., 1991）。因此，过量的 H2Om 表明在相互转化反应速率太慢而无法维持平衡物种形成的温度下添加 H2Om。这可能是由于低温（二次）水化缓慢（例如，Giachetti 和 Gonnermann，2013 年）或在中等温度下更快速的水化（例如，Zhang 等人，1995；McIntosh 等人，2014 年）。来自 2012 年阿弗尔潜艇喷发的浮石显示它们是水合的，可以推断在水柱冷却过程中发生了水合，因为它们年轻时排除了缓慢的低温水合作为可能的原因（Mitchell 等人，2018 年）。然而，较旧的 Oomurodashi 样本可能经历了任何一种水合机制。沿水合剖面的 H2O 物种数据有助于区分这些情景；不幸的是，只有浮石 1407-R01 成功地准备用于剖面分析（图 3B 和 3C）。与 H2Ot 和 H2Om 不同，OH (wt%) 不会向其气泡壁增加。因此发生水合，过量的 H2Om 几乎没有转化为 OH。对于测量的 H2Ot 范围，即使是 10 °C/min 的缓慢冷却速度也会使这种浮石的 Tg > 650 °C（Giordano 等人，2008 年），这将在约 15 分钟内达到。尽管从 800 °C 冷却到 600 °C 的 H2Om 扩散率下降了一个数量级（Ni 和 Zhang，2008 年），但观察到的 H2Om 扩散曲线形成的时间足够了；然而，在这些温度和时间尺度下，在 OH 固定在 Tg 之前，预计至少会发生一些过量 H2Om 到 OH 的相互转化（Zhang et al., 1995）。因此，水合作用可能发生在低温下。低温 H2O 扩散率数据跨越几个数量级（例如，Giachetti 和 Gonnermann，2013 年），但假设 H2Ot 扩散率为 ~10-21 至 ~10-23 m2/s，这样的剖面可能在 ~1-100 ky 内形成. 如果没有其他样品的概况，就不可能确定所有水合是否发生在低温下。观察到的约水合作用。然而，13.5 ka 的陆上浮石碎屑也与它们喷发后的低温水合作用一致，特别是当经历大的昼夜温度波动的水泡样品的大量水合作用增强时（弗里德曼和朗，1976 年）。与陆上火山灰层不同，它是难以确定晚第四纪海底喷发沉积物的年代。我们通过将其 OH 含量与其采样水深和过去海平面进行比较来估计就地熔岩的喷发年龄，其方式类似于用于限制冰下喷发的古冰厚度（例如，Tuffen 等，2010）。H2O 的溶解度取决于压力，因此上升的岩浆将 H2O 脱气，直到最终以海底熔岩流的形式进入。（未水合的）基质玻璃的 H2Ot 含量记录了这种就位压力，可以将其转换为等效的水深。水合作用改变了这些熔岩的 H2Ot 记录，因此我们改为使用固定在 Tg 的它们的 OH 含量来估计它们的淬火压力，从而估算侵位深度。图 3D 显示了沿 OH 与压力（下轴）和等效水深（上轴）曲线绘制的样品 OH 数据，计算的 Tg 为 600–800 °C，其 ROV 采样深度显示在顶轴上方。对于过去的海平面，我们使用了针对区域构造抬升调整的全球海平面曲线（图 3E；参见补充材料）。海底流纹岩流的冷却速率不受限制，但考虑到浅层熔岩的原始 H2Ot 含量较低，我们假设 Tg 可能为 700–800 °C。如果缓慢冷却导致 Tg < 700 °C，则熔岩将比计算的更年轻，但由于 Tg 曲线在低 OH 下的收敛，这种潜在误差通常小于 1 ky。通过传播 FTIR 平均值的 ±1σ 来考虑分析不确定性（图 3D）。因此，当海平面比今天浅 28 m 时，山顶 knoll 熔岩 1970-R01 的 OH 含量与淬火一致，给出了喷发年龄约 9.8–7.3 卡。Lava 719-R14 的 OH 含量也低于其采样水深的预期值，但其较大的 FTIR 不确定性转化为较大的年龄不确定性，喷发年龄≤14 ka。该熔岩暴露在 Oomuro Hole 火山口的顶部，带有垂直冷却接头，这意味着它被安置在山顶上，然后被 Oomuro Hole 的形成所切割，我们从 tephra O58 推断形成了 13.8-13.2 ka。该序列也与该熔岩的初步石英电子自旋共振日期为 12 ± 4 ka 一致（Asagoe 等，2013）。尽管 FTIR 数据在年龄≥13.2 ka 的误差范围内一致，但我们考虑了可能影响估计年龄的其他因素（另见补充材料）。如果熔岩在喷发后就被抬升，那么年龄将被低估；然而，Oomurodashi的平顶，通过在最后一个海平面低位的波浪平面形成，与我们的抬升调整的海平面曲线非常吻合，并且排除了自那时以来任何显着的局部变形。如果熔岩挥发物在淬火前没有与侵位压力完全平衡，那么年龄也会被低估。熔岩 719-R14 更可能出现这种不完全脱气，因为对横截面流动内部进行采样而不是（就山顶小丘而言）外部采样，这可能是其 OH 数据变异性更大的原因。通常，流动外部更有可能完全脱气至其骤冷压力，如圣海伦斯山（美国）和圣地亚哥（危地马拉）的空中熔岩所见，尽管最靠近排气口的样品可能保留高达 0.1 wt% 的过量H2Ot（安德森等人，1995）。然而，即使熔岩 719-R14 从不完全脱气中保留了多达 0.15 wt% 的过量 OH，其年龄也不能超过 15.5 ka。Tephra 浮石的 OH 含量高于海底样品（图 3A 和 3D），这意味着它们在更高的温度下淬火尽管是在地下沉积的，但仍存在压力。假设 Tg 为 700–800 °C，它们的 OH 含量相当于 3.6–4.0 MPa (O58) 和 2.1–2.3 MPa (O3T) 的淬火压力。在 13.8–13.2 ka，Oomurodashi 峰顶约为 55 mbsl，相当于约 0.7 MPa。因此，这些 OH 含量记录了浅层大厦内的碎裂和淬火。假设岩浆/岩石静压梯度（密度为 ρ = 2300 kg/m3），我们计算出管道内的骤冷深度为 ~130-150 m (O58) 和 ~60-70 m (O3T)；如果协同爆发的陨石坑已经开始形成，这些深度会更浅（例如，对于像今天这样深 100 米的陨石坑，~80-100 m 和~20-30 m 深度）。这样的深度表明浅层碎裂，与浅层岩浆-水相互作用一致（例如，Dellino 等人，2012 年），或部分填充的喷口内的 maar 型爆炸，该喷口足够浅以产生地表喷射物（Graettinger 等人， 2014）。因此，挥发性数据支持解释这些火山灰是由形成 Oomuro Hole 陨石坑的浅层潜水岩浆爆炸产生的，并且它们证明了新的 FTIR 方法能够获得幼体材料的碎裂深度。 Oomuro Hole 边缘周围的浮石碎屑表明火山口形成后，喷发（或随后的喷发）转变为产生浮石的方式。Clast 1409-R10，以 118 mbsl 收集，是致密的（33％的孔隙率）并且在喷发时不会在水中漂浮。它的 OH 含量表明淬火深度为 70-80 mbsl。如果它在 13.8-13.2 ka 喷发（当采样位置为 50-56 mbsl 时），则需要约 20 m 的非浮力淬火后上升才能到达其最终沉积位置。由于水中动量驱动的上升是有限的，它可能来自最近的一次喷发，当时它的沉积位置也是 70-80 mbsl（约 11.3-10.7 ka）。相比之下，浮石 1407-R01 的 OH 含量相当于约 30 mbsl 的淬火深度。这种浮石（72% 的孔隙率）最初在水中是漂浮的，它的最终位置距离火山中心 5 公里，表明它在积水和沉积之前在浅层漂流。在冷却过程中吸水的热浮石会迅速发生积水，可能限制浮力上升和分散（Whitham 和 Sparks，1986 年）。在 NT12-19 巡航期间，在 Oomuro Hole 以东 500 m 处（即强黑潮下游）的 ROV 勘测发现了丰富的新鲜流纹岩浮石覆盖了山顶。然而，在评估潜在的未来危害时，必须注意，如果浅层喷发也产生浮石，这些浮石在空中羽流中冷却，因此吸收空气而不是水（Whitham 和 Sparks，1986），或者具有更大百分比的孤立孔隙（Manga等人，2018 年），这种浮石可能形成了未保存在 Oomurodashi 当地矿床中的浮筏（例如，Carey 等人，2018 年）。14 ka，包括海底熔岩流，潜水岩浆爆炸产生了空中火山灰，以及浮力不同的浮石海底喷发。因此，Oomurodashi 的位置靠近有人居住的岛屿和进入东京湾的主要航道，使其成为重大危险。熔岩和火山碎屑的 FTIR 挥发性数据与孔隙度和纹理数据相结合，将为解释海底喷发过程提供新的框架。从当地和远端沉积物（例如海洋沉积物岩心）中获取这些数据，将使我们能够调查海底喷发类型的全部范围，并更好地了解它们的潜在危害。我们感谢 R/V Natsushima 的科学家和船员和 R/V Shinseimaru。这项工作得到了日本科学促进会 KAKENHI 赠款 JP00470120 和 16K05584 的支持。我们感谢 S。