Calc-Alkaline Magma Ascent Rates: An Experimental Investigation

钙碱性岩浆上升速率：实验研究

• 批准号: 9004842
• 负责人: Malcolm Rutherford
• 金额: $ 9.5万
• 依托单位: Brown University
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 1990
• 资助国家: 美国
• 起止时间: 1990-07-01 至 1992-12-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=9004842&HistoricalAwards=false
• 关键词: Calc; Alkaline; Magma; Ascent; Rates

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.

本文提出了一种角闪洞分解反应速率的实验研究方法，可用于估算安山岩、英安岩和流纹岩岩浆的岩浆上升速率。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。研究的反应有：(1)角闪洞与熔体反应分解形成辉石、斜长石、Fe-Ti氧化物和更进化的熔体；(2)伴随的斜长石结晶。对天然火山和圆顶岩石样本的初步研究表明，这些反应在圣海伦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孔破裂边缘的宽度，以及斜长石和熔体成分的变化来表示的。从1980-1986年圣海伦斯火山英安岩喷发产物的氧化物地温测量结果来看，反应程度主要是在大致恒定的温度下减压（减少fH2O）的函数。反应程度，由角闪孔破裂边缘宽度和斜长石结晶测量，将作为减压率的函数进行实验校准。本研究的主要目的是：(1)估算得到充分研究的St. Helens火山和Long Valley火山口的火山穹窿喷发的岩浆上升速率；(2)与(a)晶体尺寸分布分析和(b)地震震源迁移分析得出的上升速率估算值进行比较。