Mechanisms of degassing at Nevado del Ruiz volcano, Colombia
Mechanisms of degassing at Nevado del Ruiz volcano, Colombia
复制标题
哥伦比亚内瓦多德尔鲁伊斯火山的排气机制
DOI：
10.1144/0016-764902-028
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发表时间：
2003
影响因子：
2.7
通讯作者：
Stanley N. Williams
中科院分区：
文献类型：
--
作者：
J. Stix;G. Layne;Stanley N. Williams
内华多德鲁伊斯火山是位于哥伦比亚北部安第斯山脉的一座安山岩层状火山。该火山分别于1985年9月11日、1985年11月13日和1989年9月1日喷发。最后两次喷发释放出幼年的固体物质。本文研究了这种年轻物质的熔融包裹体和基质玻璃中挥发性和轻质亲石微量元素的含量，并提出了火山内部脱气的模型。主量元素在玻璃中的分布呈现两种演化趋势，辅助点位于两种趋势之间。这些数据表明，存在两个独立的岩浆，它们在上升和喷发过程中相互作用、混合和混合。由二次电离质谱分析确定的熔体包裹体中的水分含量一般较低，平均在1.6-3.3wt.%之间。玻璃中氟的卤素浓度从400到1200 ppm不等，氯的卤素浓度从1100到1500 ppm不等。硫含量很低，不超过500ppm，大多数玻璃的含硫量为200ppm。锂的浓度在20到40 ppm之间，铍在1.5到2 ppm之间，而硼的浓度在30到100 ppm之间表现出很高的可变性。熔融包裹体和基质玻璃之间唯一显著的区别是水，基质玻璃的浓度(<；0.5wt.%)明显低于熔融包裹体。岩浆中的硼浓度普遍较高，可能是岩浆源区富集的结果，即蚀变洋壳和/或沉积物的俯冲。然而，熔体包裹体和基质玻璃中硼的高度不均一性，使得随后也有必要通过地壳沉积岩的同化或与热液的相互作用，在较浅的深度添加硼。(1)斜长石和辉石的无水矿物组合，(2)玻璃的高二氧化硅含量，(3)熔体包裹体的低水含量，提供了浅层喷发前岩浆侵位的证据。综合起来，这些观察结果表明，在火山顶端下方3公里处的浅深处有一段岩浆驻留的时期。岩浆运移和脱气的多阶段模型包括岩浆上升和岩浆蓄积的交替阶段。最初，在浮力的驱动下，含挥发分的岩浆从9-15公里的深处上升。在减压过程中，岩浆会失去气体，特别是二氧化碳和硫。岩浆最终以其中性浮力水平汇入池塘。此时，饱和气体的岩浆冷却并结晶，从而在等压条件下释放出气体。因此，岩浆中的二氧化碳被亏损，而H2O和SiO_2被浓缩。H2O的富集是由于其在岩浆中的溶解度随着CO2的脱气而增加，而SiO_2是通过分离结晶而富集的。岩浆的密度随着溶解H2O水平的增加而降低，最终导致岩浆再次浮力并继续上升，要么喷发，要么在浅层冻结。
Nevado del Ruiz volcano is an andesite stratovolcano located in the northern Andes of Colombia. The volcano erupted on 11 September 1985, 13 November 1985, and 1 September 1989. The last two eruptions emitted juvenile solid material. This paper examines the volatile and light lithophile trace element contents of melt inclusions and matrix glasses from this juvenile material, and proposes a model for degassing within the volcano. Major element distributions in the glasses show two evolutionary trends, with subsidiary points that lie between the two trends. The data suggest the existence of two separate magmas, which have interacted, mingled, and mixed during their ascent and eruption. Water contents in melt inclusions, as determined by secondary ionization mass spectrometric analysis, are generally low, averaging between 1.6 and 3.3 wt.%. Halogen concentrations in the glasses range from 400 to 1200 ppm for fluorine and from 1100 to 1500 ppm for chlorine. Sulphur contents are low, not exceeding 500 ppm, with most glasses containing <200 ppm. Lithium concentrations range from 20 to 40 ppm, beryllium from 1.5 to 2 ppm, and boron exhibits high variability from 30 to 100 ppm. The only significant difference between melt inclusions and matrix glasses is for water, with matrix glasses having significantly lower concentrations (<0.5 wt.%) than the melt inclusions. The generally elevated concentrations of boron in the magma may be a consequence of enrichment in the source region of the magma, i.e. by subduction of altered oceanic crust and/or sediments. Yet the large degree of boron heterogeneity in both melt inclusions and matrix glasses necessitates subsequent addition of boron at shallower depths as well, by the assimilation of crustal sedimentary rocks or by interaction with hydrothermal fluids. Evidence for pre-eruptive magma emplacement at shallow levels is provided by (1) anhydrous mineral assemblages of plagioclase and pyroxene, (2) high silica contents of glasses, and (3) low water contents in melt inclusions. When combined, these observations suggest a period of magma residence at shallow depths, probably <3 km beneath the summit of the volcano. A multistage model of magma transport and degassing involves alternating periods of magma ascent and magma ponding. Initially, volatile-bearing magma ascends from depths of 9–15 km, driven by buoyancy. During decompression, the magma loses gas, particularly CO2 and sulphur. The magma eventually ponds at its neutral buoyancy level. At this point, the gas-saturated magma cools and crystallizes, thereby liberating gas under isobaric conditions. As a result, CO2 is depleted from the magma whereas H2O and SiO2 are enriched. The H2O enrichment is caused by its increased solubility in the magma as CO2 is degassed, whereas SiO2 is enriched by fractional crystallization. The density of the magma decreases as the level of dissolved H2O increases, eventually causing the magma to become buoyant once more and to continue its ascent, either to erupt or to freeze at shallow depths.