Groundmass crystals indicate syneruptive magmatic conditions, and thus their crystal size distributions (CSDs) are used to infer magma ascent histories. Three-dimensional (3D) CSDs are most commonly estimated from two-dimensional (2D) observations and plotted against long-axis length, L (hereafter referred to as “L-plot CSDs”). However, L-plot CSDs have two significant problems: the error because of the conversion from 2D to 3D and a lowered sensitivity to changes in the degree of effective undercooling (ΔTeff), which arises because a crystal’s growth rate varies with ΔTeff most strongly along its long dimension. Although these problems can result in false interpretations of magma ascent dynamics, there has been little discussion of the influence of the size criteria on CSDs.In this study, we investigated which 3D size criterion [i.e., long (L), intermediate (I), or short (S) axis length] is optimum for 2D-estimated CSDs of groundmass crystals from two perspectives: (1) conformity with the actual distributions, and (2) the sensitivity of CSD slopes to the magma ascent conditions in the conduit. We observed groundmass pyroxene crystals in pumice clasts from sub-Plinian and Vulcanian eruptive phases during the 2011 eruption of Shinmoedake (andesitic volcano, Japan) by using synchrotron radiation-based X-ray computed nanotomography (SR-XCT) and field-emission scanning electron microscopy (FE-SEM) and rein-vestigated the crystallization kinetics of pyroxene nanolites ranging in width from a few hundred nanometers to 1 µm. The SR-XCT observations provided the detailed 3D shapes and 3D CSDs (CT-CSDs) of these nanolites directly. The FE-SEM observations allowed us to estimate 3D aspect ratios (S:I:L) and CSDs (SEM-CSDs). L-plot SEM-CSDs, acquired using the program CSDCorrections, were used to calculate S-plot SEM-CSDs and I-plot SEM-CSDs. We compared the data from FE-SEM with those from SR-XCT to evaluate the accuracy of 3D aspect ratios and CSDs estimated from 2D data.The L-plot SEM-CSDs from the sub-Plinian pumice sample showed significant inconsistencies with the CTCSD, a result of the difficulty in estimating representative 3D aspect ratios from 2D observations for elongated groundmass crystals. In contrast, the S- and I-plot SEM-CSDs kept the effect of aspect ratio to a minimum and preserved their actual slopes, except for a vertical discrepancy between the CSDs. Moreover, the slopes of S- and I-plot CSDs of the nanolites differed more markedly between the two eruptive styles (by ~20% more) than those of L-plot CSDs. For estimating magma ascent dynamics, we propose that the optimum method for acquiring SEM-CSDs is to measure the cross-sectional widths of crystals and convert the resulting 2D data set into S-plot CSDs.Our new finding that the 3D shapes and CSDs of pyroxene nanolites differ according to eruptive style means that nanolites indicate distinct differences in ascent histories at the shallow conduit: increasing ΔTeff just before sub-Plinian eruptions and decreasing ΔTeff before Vulcanian eruptions. Given the similarity in CSDs of micro-lites, our results suggest that eruptive style was determined in the shallow conduit. Monitoring the condition of the shallow conduit may contribute to predicting the time evolution of eruptive activity.

中文翻译：

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来自纳米 X 射线计算机断层扫描的辉石纳米岩的 3D 晶体尺寸分布：改进了来自 CSD 对管道中岩浆上升动力学的晶体尺寸分布的校正

地块晶体表明协同爆发的岩浆条件，因此它们的晶体尺寸分布 (CSD) 可用于推断岩浆上升的历史。三维 (3D) CSD 最常从二维 (2D) 观察中估计，并针对长轴长度 L 绘制（以下称为“L-plot CSD”）。然而，L-plot CSD 有两个重大问题：由于从 2D 转换到 3D 导致的误差以及对有效过冷度 (ΔTeff) 变化的敏感性降低，这是因为晶体的生长速率随 ΔTeff 变化最为强烈它的长尺寸。尽管这些问题可能导致对岩浆上升动力学的错误解释，但很少讨论尺寸标准对 CSD 的影响。在这项研究中，我们研究了哪个 3D 尺寸标准 [即长 (L)、中间（I）或短（S）轴长度]从两个角度对地体晶体的二维估计CSD是最佳的：（1）符合实际分布，以及（2）CSD斜坡对岩浆上升条件的敏感性在导管中。我们使用基于同步辐射的 X 射线计算机纳米断层扫描 (SR-XCT) 和场发射扫描电子在 2011 年 Shinmoedake（日本安山火山）喷发期间观察到来自亚普林尼期和火山喷发期的浮石碎屑中的地块辉石晶体显微镜（FE-SEM）并重新研究了宽度从几百纳米到1微米的辉石纳米晶的结晶动力学。SR-XCT 观测结果直接提供了这些纳米颗粒的详细 3D 形状和 3D CSD (CT-CSD)。FE-SEM 观察使我们能够估计 3D 纵横比 (S:I:L) 和 CSD (SEM-CSD)。使用程序 CSDCorrections 获得的 L-plot SEM-CSDs 用于计算 S-plot SEM-CSDs 和 I-plot SEM-CSDs。我们将 FE-SEM 的数据与 SR-XCT 的数据进行了比较，以评估 3D 纵横比和从 2D 数据估计的 CSD 的准确性。来自亚普林尼浮石样品的 L-plot SEM-CSD 与 CTCSD 存在显着不一致，由于难以从细长的地块晶体的 2D 观测中估计有代表性的 3D 纵横比。相比之下，除了 CSD 之间的垂直差异之外，S-和 I-plot SEM-CSD 将纵横比的影响保持在最低限度并保留了它们的实际斜率。而且，与 L-plot CSD 相比，两种喷发方式的 S- 和 I-plot CSD 的斜率差异更明显（高出约 20%）。为了估计岩浆上升动力学，我们提出获取 SEM-CSD 的最佳方法是测量晶体的横截面宽度，并将得到的 2D 数据集转换为 S-plot CSD。我们的新发现是 3D 形状和 CSD辉石纳米岩因喷发方式而异，这意味着纳米岩在浅水道的上升历史上表现出明显的差异：在亚普林尼火山喷发之前增加 ΔTeff，在瓦肯火山喷发之前降低 ΔTeff。鉴于 micro-lites 的 CSD 的相似性，我们的结果表明，喷发风格是在浅水道中确定的。