ALBERT

Description: The Diamante Ignimbrite, which covers an area of 23.000km2 in Chile and Argentina, was emplaced 0.45 Ma ago by a catastophic eruption that caused subsidence of the Diamante Caldera. The ignimbrite is underlain by two fallout deposits, one immediately beneath the ignimbrite, the other separated by sediments. All three deposits have rhyolitic compositions but the older fallout contains higher concentrations of incompatible elements at slightly lower silica contents, reflecting a somewhat different crystal fractionation history. Five juvenile components can be identified in the ignimbrite and its underlying fallout: (1) The dominant white to pink rhyolitic pumice that contains plagioclase, biotite, quartz and ore phenocrysts; alkalifeldspar crystals occur in the matrix. (2) Light-gray rhyolitic pumice, also with plagioclase, biotite, quartz and ore phenocrysts, contains significant amounts of broken crystals and lithic fragments, and mostly occurs mixed with the white component in banded pumice clasts. (3) A highly vesicular, glassy rhyolitic pumice with very few phenocrysts. (4) Foliated, silky shinning white to grey pumice with strongly elongated vesicles and parallel-aligned phenocrysts. (5) A minor component of brown pumice distinct in both its dacitic bulk composition and the presence of amphibole phenocrysts next to plagioclase, biotite and ore. Dark-brown regions in these pumice clasts contain higher amounts of amphibole and plagioclase microlites. There is no systematic vertical compositional zonation through the fallout and ignimbrite but pumice type (5) only occurs in the ignimbrite. Pumice types (1) to (4) share the same mineral and glass compositions. We interpret type (2) as magma that ascended along the conduit walls where high shear stresses caused crystal fragmentation and entrainment of lithic fragments. The dominant type (1) pumice, in contrast, reflects the main, less sheared magma ascending near conduit center. Foliated type (4) pumice may have been magma from in-between these conduit regions. The higher vesicularity of type (3) relative to type (1) indicates that vesiculation in the magma was spatially heterogeneous. The ignimbrite also contains abundant plutonic lithic fragments mostly of andesitic to dacitic bulk compositions. While some of these lie on a fractionation trend leading to the rhyolitic pumice compositions, others have distinct chemical compositions. A fraction of the lithics contains amphibole, and these plutonic rocks have the same dacitic composition as the amphibolebearing pumice (5), which is distinct from the other pumice and lies off the main fractionation trend. We conclude that magma (5) represents a molten crustal component that was added to the rhyolitic magma reservoir.

Description: Rapidly quenched melt inclusions potentially preserve the pre-eruptive volatile concentrations of magmatic melts. We analyze melt inclusions, matrix glasses, phenocrysts and bulk rocks of tephras in order to reveal (1) how magmatic volatile contents are determined by petrogenetic processes, (2) in how far the triggering, intensity and magnitude of explosive volcanic eruptions are controlled by the volatile contents, and (3) how much of the volatiles is actually released into the atmosphere where they may affect atmospheric chemistry and climate. Presently available results show that volatiles (H2O, S, Cl, F) are significantly more enriched relative to incompatible K2O in the tephras from the volcanic front than in those from the back-arc region. Moreover, volatile/K2O ratios vary along the arc such that they converge from both N and S toward a maximum around 38.5°S. This along-arc variation mimicks that of bulk-rock Ba/Th and is a mirror-image of the La/Yb pattern. Thus peak volatile/K2O values coincide with maximum slab-fluid metasomatism (max. Ba/Th) in the mantle source of the magmas and with the highest degree of partial melting (min. La/Yb). Melt inclusions in compositionally zoned tephras show S contents decreasing to higher glass K2O contents, evidence that sulphur exsolution into a fluid phase occurred during progressive crystal fractionation. Melt inclusions in all highly evolved tephras have very low S contents. However, for some of these tephras the H2O and Cl to K2O ratios of melt inclusions decrease to more evolved compositions, suggesting that degassing occurred in the magma chamber and potentially triggered the eruption. Ongoing thermobarometric modelling, however, is needed to better constrain degassing conditions.

Description: We have applied a combination of fluid inclusion and amphibole thermobarometry to felsic tephras from highly explosive volcanic eruptions along the Central American volcanic arc (CAVA) from Guatemala through Nicaragua in order to constrain pre-eruptive magma ascent and storage conditions. We note that this is the first time a combination of pressure estimates from fluid inclusions and amphibole chemistry have been used to quantify multi-stage magma chamber processes and magma ascent velocities of large eruptions. Our data document a stepwise ascent of magmas through the crust, typically involving at least two levels of stagnation. Amphibole and fluid inclusion thermobarometry both indicate a shallow preeruptive magma storage level at 80 to 200 MPa (3-8 km depth) along the entire arc. The deeper levels of magma storage vary along-arc, with a tendency to greater maximum depths of up to 25 km in Guatemala and El Salvador, compared to maximum depths of 15 km in Nicaragua. We assume that the continental crust of about 45 km thickness in Guatemala, compared to the 30km thickness of the largely oceanic crust of Nicaragua, allowed for deeper positions of the magma chambers. Thus the observed along-arc changes in mid-crustal magma storage depths indicate a dependence between magma chamber formation and the composition and probably density of the local crust. The average composition of the pre-eruptive fluid phase for highly explosive eruptions in Central America amounts to 90% water, 5% CO2 and 5% NaCl equivalents, and show no systematic alongarc variations. The pressures obtained from the earliest fluid inclusions were taken as the pressures of fluid oversaturation and thus for the beginning of degassing. They range between 150 and 400 MPa, and do not show systematic along-arc variations. Such fluid oversaturation pressures correspond to water contents between 4-8 wt% in the felsic melts. Our results show that the depths of fluid saturation are mostly independent of crustal properties. Degassing typically started at pressures 150 to 300 MPa higher that those corresponding to the last stagnation level, providing evidence for the pre-eruptive criticality of the systems.

Description: The youngest dacitic Plinian eruption in westcentral Nicaragua, forming the 18 km3 Chiltepe Tephra (CT), occurred about nineteen hundred years ago at Apoyeque stratovolcano, which dominates the Chiltepe volcanic complex 15 km north of the capital Managua, where the CT is 2 m thick. We have traced the CT from its proximal facies at the crater rim, through the medial facies in the lowlands around Apoyeque, and to the distal facies up to 550 km offshore in the Pacific. While medial and distal facies consist of widespread Plinian fall deposits, the proximal facies reveals the complexity of this eruption, which we divide into four phases (I–IV). Interaction of rising magma with a pre-existing crater lake generated the phreatomagmatic opening phase I of the eruption, which produced ash fall with accretionary lapilli. Phase II marked a rapid change to persistent magmatic activity that yielded several large Plinian eruptions, declining through a period of unstable eruption conditions, followed by a short hiatus. Phase III began with unstable conditions, probably as a result of eastward migration and widening of the vent, leading to a second period of Plinian eruptions with three major events reaching magma discharge rates five times larger than those of phase II. Phase III again declined through unstable eruption conditions before magmatic activity terminated. Numerous explosions in the shallow hydrothermal system during the final phase IV resulted in the formation of a phreatic tuff ring on the rim of Apoyeque crater. The white, highly-vesicular, dacitic CT pumice contains plagioclase (An45–68), orthopyroxene, clinopyroxene, and minor hornblende, apatite and titanomagnetite phenocrysts. A very subordinate fraction of gray pumice has the highest crystal content, the least evolved bulk-rock, but the most evolved matrix-glass composition. The CT dacite has two unusual compositional features: (1) all white dacite has the same melt (matrix-glass) composition such that variations in bulk-rock compositions (64– 68 wt% SiO2) simply reflect different phenocryst contents of 10–35%, interpreted as the result of gradual phenocryst settling in the magma chamber. (2) Abundant olivine crystals with a bimodal distribution in Mg# (modes at Mg#=0.75 and Mg#=0.8) are dispersed throughout the erupted dacite. These are clearly out of equilibrium with the dacitic melt and are interpreted as xenocrysts derived from the basaltic Nejapa-Miraflores volcanic lineament that intersects the Chiltepe volcanic complex and was contemporaneously active. Thermobarometric estimates place the dacitic CT magma reservoir in the upper crust (〈250 MPa), with a temperature of about 890°C and about 5 wt% water dissolved in the melt. Comparing water and chlorine contents with respective solubility models suggests that volatile degassing began in the magma reservoir and triggered the CT eruption. From the vertical compositional variation pattern of the CT we deduce that the conduit tapped the magma chamber not at the top but from the side, at some deeper level, and that subsequent magma withdrawal was governed by both variations in discharge rate and possible upward migration and/or widening of the conduit entrance.

Description: Large explosive volcanic eruptions can inject massive amounts of sulfuric gases into the Earth's atmosphere and, in so doing, affect global climate. The January 1835 eruption of Cosigüina volcano, Nicaragua, ranks among the Americas’ largest and most explosive historical eruptions, but whether it had effects on global climate remains ambiguous. New petrologic analyses of the Cosigüina deposits reveal that the eruption released enough sulfur to explain a prominent ca. AD 1835 sulfate anomaly in ice cores from both the Arctic and Antarctic. A compilation of temperature-sensitive tree-ring chronologies indicates appreciable cooling of the Earth's surface in response to the eruption, consistent with instrumental temperature records. We conclude that this eruption represents one of the most important sulfur-producing events of the last few centuries and had a sizable climate impact rivaling that of the 1991 eruption of Mount Pinatubo.

Description: The Lonquimay Volcanic Complex (LVC) in South Central Chile (38.38°S, 71.58°W) is part of the Southern Volcanic Zone of the Andes, which formed in response to the subduction of the Nazca Plate beneath the South American Plate. During the last 10200+-70 years of its magmatic evolution, the LVC produced 23 explosive eruptions documented in the succession of widespread tephra deposits. We investigated this stratigraphic sequence for matrix glass, mineral and bulk rock compositions of the juvenile components. Furthermore, melt inclusions were analyzed for their major element and volatile contents. The tephra succession reflects six mafic replenishments of the LVC magma reservoir followed by progressive magmatic differentiation. Each cycle has been successively tapped by several eruptions. Compositionally zoned tephras were typically deposited early in a cycle, whereas late eruptions discharged more evolved magmas. Intermediate compositions typically contain mixed disequilibrium mineral assemblages. The maximum degree of fractionation reached during a cycle increases with younger ages. Our investigations of melt inclusions, in order to reconstruct the pre-eruptive volatile inventories of the LVC magma chamber, reveal the exsolution of two separate fluid phases. One S-rich fluid phase released from mafic melts in the middle crust and one Cl-rich aqueous phase, released from more ifferentiated melts that resided in the upper part of LVC´s plumbing system. The pre-eruptive saturation state of the LVC melts indicates that felsic eruptions may have been triggered by H2O-supersaturation whereas mafic melts seem to have experienced a complex replenishment history potentially exciting LVC´s mafic eruptions.