Abstract The syneruptive decompression rate of basaltic magma in volcanic conduits is thought to be a critical control on eruptive vigor. Recent efforts have constrained decompression rates using models of diffusive water loss from melt embayments, olivine-hosted melt inclusions and clinopyroxene phenocrysts; however, these techniques are difficult to apply because of the rarity of pyroxene crystals or melt embayments suitable for analysis and the complexities associated with modeling water loss from melt inclusions. We have developed a new magma ascent chronometer based on syneruptive diffusive water loss from olivine phenocrysts. We have found water zonation in every olivine phenocryst we have measured, from explosive eruptions of Seguam, Fuego, Kilauea and Cerro Negro volcanoes. The majority of the olivine phenocrysts were polished to expose a central plane normal to the crystallographic ‘b’ axis and volatile concentration profiles were measured along ‘a’ and ‘c’ axes by secondary ion mass spectrometry (SIMS). Profiles are compared to 1D and 3D finite-element models of diffusive water loss from olivine, whose boundaries are in equilibrium with a melt undergoing closed-system degassing. Least-squares fitting of measured water concentration gradients in olivine to a 1D Monte Carlo model produces constraints on magma decompression rates that are in good agreement with independent constraints from melt embayment studies and modeling of water loss from olivine-hosted melt inclusions at Fuego, Seguam, Kilauea, and Cerro Negro. Such agreement confirms the accuracy and sensitivity of the water-in-olivine chronometer over a range of decompression rates (dP/dt) spanning ~2 orders of magnitude (from 0.007 to 0.45 MPa/s). We find that the assumption of a zero-water boundary condition (in which the water concentration at the edges of the olivine phenocrysts is fixed at 0 ppm throughout their ascent) leads to an overestimation of the decompression rate by an order of magnitude compared to the closed-system degassing boundary condition assumed in our model, thereby highlighting the sensitivity of the water-in-olivine chronometer to the host magma degassing path. At Seguam, a wide range of best-fit values of dP/dt is obtained both from the water-in-olivine chronometer (0.04–0.23 MPa/s) and from melt embayments (0.02–0.13 MPa/s). We find systematically higher dP/dt values in the melt embayments that appear to have stalled or crystallized at the shallowest depths. Together, these observations are suggestive of magma acceleration during ascent. A strength of the water-in-olivine chronometer is the prevalence of olivine in mafic to intermediate magmas. This new technique yields many values of dP/dt from a single eruption, providing insight into the diversity of ascent records carried by the crystal cargo and possibly defining changes in dP/dt through time and space. This data density offers a more detailed window into syneruptive conduit processes than has been possible using other techniques for constraining dP/dt. In theory, each crystal is a clock, with the potential to record variable ascent in the conduit, over the course of an eruption, between eruptions, and among volcanic systems.

中文翻译：

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橄榄石岩浆上升计时法：每颗水晶都是一个时钟

摘要 玄武质岩浆在火山通道中的协同减压速率被认为是控制喷发活力的关键。最近的努力使用来自熔体海湾、橄榄石熔体包裹体和单斜辉石斑晶的扩散失水模型来限制减压率；然而，这些技术难以应用，因为适合分析的辉石晶体或熔体的稀有性以及与熔体包裹体失水建模相关的复杂性。我们开发了一种基于橄榄石斑晶协同扩散失水的新型岩浆上升计时器。我们在所测量的每一个橄榄石斑晶中都发现了水分带，它们来自 Seguam、Fuego、Kilauea 和 Cerro Negro 火山的爆发性喷发。大多数橄榄石斑晶被抛光以暴露出垂直于晶体“b”轴的中心平面，并且通过二次离子质谱法 (SIMS) 沿“a”和“c”轴测量挥发性浓度分布。将剖面图与橄榄石扩散失水的 1D 和 3D 有限元模型进行比较，橄榄石的边界与经历封闭系统脱气的熔体处于平衡状态。橄榄石中测得的水浓度梯度与一维蒙特卡罗模型的最小二乘拟合产生对岩浆减压速率的约束，这些约束与来自熔体湾研究的独立约束和 Seguam Fuego 橄榄石熔体包裹体的失水建模非常一致、基拉韦厄和塞罗内格罗。这种一致性证实了橄榄石中水天文钟在约 2 个数量级（从 0.007 到 0.45 MPa/s）的减压率 (dP/dt) 范围内的准确性和灵敏度。我们发现零水边界条件的假设（其中橄榄石斑晶边缘的水浓度在整个上升过程中固定在 0 ppm）导致减压率高估了一个数量级与在我们的模型中假设了封闭系统脱气边界条件，从而突出了橄榄石中的水计时器对宿主岩浆脱气路径的敏感性。在 Seguam，从橄榄石含水量表 (0.04–0.23 MPa/s) 和熔体 (0.02–0.13 MPa/s) 中获得了范围广泛的 dP/dt 最佳拟合值。我们系统地发现熔体海湾中的 dP/dt 值似乎在最浅的深度停滞或结晶。总之，这些观察结果表明上升过程中岩浆加速。橄榄石中的水计时器的一个优势是橄榄石在基性岩浆到中间岩浆中的普遍存在。这种新技术从一次喷发中产生了许多 dP/dt 值，提供了对水晶货物携带的上升记录多样性的洞察，并可能定义 dP/dt 随时间和空间的变化。与使用其他限制 dP/dt 的技术相比，这种数据密度提供了一个更详细的了解协同管道过程的窗口。从理论上讲，每个水晶都是一个时钟，有可能在喷发过程中，在两次喷发之间记录管道中的可变上升，