Calc-Alkaline Magma Ascent Rates: An Experimental Investigation

建议对角闪石分解反应速率进行实验研究，以估计许多安山岩、英安岩和流纹岩的岩浆上升速率。上升速率估计与实验确定的喷发前岩浆室条件(P、T、fH2O)结合使用，以约束喷发动力学模型。要研究的反应是(1)角闪石与熔体反应生成辉石、斜长石、铁钛氧化物和更具演化的熔体，以及(2)伴生的斜长石结晶。对天然火山灰和穹顶岩石样品的初步研究表明，这些反应在圣海伦斯火山和长谷火山口喷发产物中进行了不同程度的反应。反应的程度是通过测量角闪石分解边的宽度，以及斜长石和熔体成分的变化来表示的。从最近圣海伦斯英安岩喷发产物(1980-1986)的氧化物地质测温来看，反应的程度主要是在大致恒定的温度下减压(降低fH2O)的函数。反应程度，如角闪石分解边缘宽度和斜长石结晶，将被实验校准为减压速度的函数。这项研究的主要目标是(1)估计研究充分的圣海伦斯火山和长谷破火山口的盖弗拉和穹顶喷发的岩浆上升速度，以及(2)将估计的上升速度与(A)晶体尺寸分布分析和(B)震源迁移分析得出的结果进行比较。

An experimental investigation of amphibole decomposition reaction rates is proposed which should yield estimates of magma ascent rates for many andesite, dacite and rhyolite magmas. Ascent rate estimates are used in conjunction with experimentally determined pre-eruption magma chamber conditions (P,T, fH2O)) to constrain models of eruption dynamics. Reactions to be studied are (1) decomposition of amphibole by reaction with melt to form pyroxenes, plagioclase, Fe-Ti oxides and a more evolved melt, and (2) associated plagioclase crystallization. Preliminary studies of natural tephra and dome rock samples indicate that these reactions have proceeded to different degrees in Mount St. Helens and long Valley caldera eruption products. The extent of reaction is indicated by measurements of the widths of amphibole breakdown rims, and by changes in plagioclase and melt composition. The extent of reaction is primarily a function of decompression (decreasing fH2O) at roughly constant temperature, judging from oxide geothermometry of recent Mount St. Helens dacite eruption products (1980-1986). Extent of reaction, as measured by amphibole breakdown rim width and plagioclase crystallization, will be experimentally calibrated as a function of decompression rate. Primary objectives of the research are (1) to estimate magma ascent rates for well-studied tephra and dome eruptions of Mount St. Helens and Long Valley caldera, and (2) to compare the ascent rate estimates with those derived from (a) crystal size distribution analysis and (b) earthquake focus migration analysis.