Mechanisms of degassing at Nevado del Ruiz volcano, Colombia

Abstract

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 CO 2 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, CO 2 is depleted from the magma whereas H 2 O and SiO 2 are enriched. The H 2 O enrichment is caused by its increased solubility in the magma as CO 2 is degassed, whereas SiO 2 is enriched by fractional crystallization. The density of the magma decreases as the level of dissolved H 2 O increases, eventually causing the magma to become buoyant once more and to continue its ascent, either to erupt or to freeze a...

Breathing of the Nevado del Ruiz volcano reservoir, Colombia, inferred from repeated seismic tomography

“…The lowest glass water content of 1.6 wt% provides an estimate of the shallowest storage depth, which must be at least 0.8 km23. Similarly, the largest water content of 3.3 wt% indicates a minimum depth of 3.1 km23. These two different estimates may be interpreted as indicative of the thickness of the shallow NRV reservoir and are are consistent with the seismological evidence.…”

“…The water content of volatile-saturated melt is dictated by solubility and varies as a function of pressure and CO 2 content. The lowest glass water content of 1.6 wt% provides an estimate of the shallowest storage depth, which must be at least 0.8 km23. Similarly, the largest water content of 3.3 wt% indicates a minimum depth of 3.1 km23.…”

Multiple levels of magma storage during the 1980 summer eruptions of Mount St. Helens, WA

“…This classification ambiguity points to limitations in traditional descriptive classification schemes, especially for pulsatory behavior where there is no direct relationship between eruption intensity and magnitude (e.g., Pyle 2000). Furthermore, it is interesting to note that magma feeding another subplinian eruptionthat of Nevado del Ruiz, Columbia, in 1985 -appears to have experienced similar multiple levels of storage and decompression-driven crystallization en route to surface (Stix et al 2003).…”

Chlorine variations in the magma of Soufrière Hills Volcano, Montserrat: Insights from Cl in hornblende and melt inclusions

“…Values from other silicic systems are shown for comparison. MSH, Mount St. Helens ; Aug, augustine (Roman et al, 2006); Maz, Mount Mazama (Bacon et al, 1992); SVL, Shiveluch ; NdR, Nevado del Ruiz (Stix et al, 2003); DOM, Dominica (Gurenko et al, 2005). .…”