Kurt Roggensack

Isotopic and elemental partitioning of boron between hydrous fluid and silicate melt

Abstract The fractionation of B and its isotopes between aqueous fluid and silicate melt has been studied from 550 to 1100 °C and 100-500 MPa. Fluid-melt partition coefficients are <1 for basaltic melt and >1 for rhyolite melt. This shows that B is not always strongly extracted from melts into hydrous fluids. Boron isotopic fractionation is large compared with the carbon and oxygen stable isotopic systems (especially at high T) and is most simply explained by differences in coordination (trigonal vs. tetrahedral) among coexisting phases. Combined with earlier measurements on illite-water (300- 350 °C), B isotopic fractionation defines a temperature-dependent trend from 300 to 1100 °C. Because of the large magnitude and apparent low sensitivity to bulk composition, B isotopic fractionation can be readily applied to studies of diagenesis, hydrothermal alteration of planetary bodies, subduction- zone processing and arc magma generation, and magma chamber evolution.

Unraveling the 1974 eruption of Fuego volcano (Guatemala) with small crystals and their young melt inclusions

This study demonstrates that crystal size may be used to temporally constrain melt inclusion data. This approach is applied to the 1974 Vulcanian-type eruption of Volcan Fuego in Guatemala to study magma conditions immediately prior to eruption. The 1974 ash deposit shows significant variations in matrix glass composition, crystal size, crystal content, and volatile (H2O and CO2) content in each of four eruptive phases occurring over a 10-day period. Matrix glass and melt inclusions hosted in small (;0.6 mg) olivine crystals have a particularly wide range in composition (basalt to andesite; 0.52‐1.22 wt% K2O) and a wide range in volatile saturation pressures (;1.0‐4.4 kbar). That small (i.e., young) crystals host diverse melt inclusions demonstrates significant compositional variability in the magma system prior to eruption. The volatile saturation pressures indicate that magma was vertically distributed in the crust. During the eruption, different magmas from varying depths were hybridized, giving rise to short-term (hours to days) variation in the observed products.

Prediction of magmatic water contents via measurement of H2O in clinopyroxene phenocrysts

Water is fundamental to magma genesis, evolution, and eruption. Few direct measurements of magmatic H 2 O exist, however, because rocks found at the surface have extensively degassed upon eruption. Olivine-hosted melt inclusions provide a standard approach to measur ing volatiles in undegassed magma, but many volcanic deposits do not contain melt inclusions large enough for analysis (>30 μm), or olivine at all. Here we use an Al IV -dependent partitioning relationship to calculate magmatic H 2 O from direct measurements of H 2 O in clinopyroxene phenocrysts. We test this approach using phenocrysts from four arc volcanoes (Galunggung, Irazu, Arenal, and Augustine) that span the global range in H 2 O contents as measured in olivine-hosted melt inclusions (from 0.1 to 7 wt% H 2 O). The average and maximum magmatic H 2 O contents calculated from the clinopyroxene measurements agree within 15% of the melt inclusion values for most of the samples. The evolutionary paths recorded in H 2 O-Mg# variations overlap in some clinopyroxene and olivine-hosted melt inclusion populations, and in others, the clinopyroxenes record a larger portion of the liquid line of descent or a different portion of the magma system. Thus, the use of phenocrysts to estimate magmatic H 2 O contents creates a new and powerful tool in igneous petrology and volcanology.

Volatiles from the 1994 Eruptions of Rabaul: Understanding Large Caldera Systems

The 1994 eruption of Rabaul, in Papua New Guinea, involved a small plinian eruption at Vulcan and a vulcanian eruption on the opposite side of the caldera at Tavurvur. Vulcans ash leachates indicate seawater interaction that is consistent with earlier observations of low sulfur dioxide emissions and the presence of ice crystals in the initial plinian eruption cloud. In contrast, Tavurvur ash leachates indicate no seawater interaction, and later sulfur dioxide emissions remained high despite low-level eruptive activity. Silicic melt inclusions indicate that the andesitic melt contained about 2 weight percent water and negligible carbon dioxide. Mafic melt inclusions in Tavurvur ash have water and carbon dioxide contents that vary systematically over the course of the eruption. The mafic melt inclusions suggest that a mafic dike intruded from below the silicic chamber and provide further evidence that mafic intrusions drive caldera unrest.

Sizing up crystals and their melt inclusions: a new approach to crystallization studies

A correlation between melt inclusion composition and host crystal size has been discovered in a basaltic ash deposit from a volcanic arc setting. Least-evolved melt inclusions, featuring high H2O–CO2 saturation pressures, are found in large olivine crystals. Melt inclusion compositions become more evolved and volatile saturation pressures decrease as host crystal size decreases. This trend is interpreted as reflecting crystal nucleation and growth during decompression of a magma saturated with H2O and CO2. In general, melt inclusion compositions show large incompatible element enrichment and modest major element variation consistent with in situ crystallization. Crystal size is used to temporally constrain the formation age of melt inclusions and the decompression path of the magma. An estimated crystal growth rate of 10−8 cm s−1 yields an initial average magma ascent rate of ∼4.8 km yr−1 at mid-crustal depths. At shallower depths the ascent rate was roughly an order of magnitude greater.

Open and almost shut case for explosive eruptions: Vent processes determined by SO2 emission rates at Karymsky volcano, Kamchatka

Vent processes were examined at Karymsky volcano, Kamchatka, by measuring SO2 emissions using a correlation spectrometer (COSPEC). Continuous high-sensitivity COSPEC measurements and physical observations were collected on 11 and 12 September 1999, when Karymsky was producing small ashy eruptions every 5‐20 min. Each eruptive event began with an explosion and audible rumbling (lasting ;30 s) followed in some cases by audible chugging (lasting to 2 min). Gas plumes accompanied each event, and almost without exception the plumes dissipated and became invisible despite significant SO2 emissions. Variations in SO2 output show distinctive patterns that correlate with eruption activity. Maximum SO2 output occurred following each explosion and then declined rapidly to low, but detectable levels. In contrast, a second type of event, often associated with audible chugging, was characterized by high SO2 output long after the initial ash blast. Variations in degassing at Karymsky can be explained by secondary boiling, gas-pressure accumulation, and vent resealing. We developed a new application of the COSPEC technique to study the dynamic vent processes of erupting volcanoes. This application provides insights into the processes that occur at the otherwise inaccessible vents of erupting volcanoes particularly when a volcano changes from passive degassing and small explosions to degassing patterns that may precede a larger and more dangerous eruption.

A low-pressure–high-temperature technique for the piston-cylinder

Abstract A method for conducting successful low pressure (0.3-0.5 GPa) and high temperature (900-1200 °C) experiments in the 19 mm piston-cylinder is presented. The technique is capable of running high fluid/melt experiments with minimum hydrogen loss, attaining precise, reproducible pressures (±10%), and has fast initial quench rates (>150 °C/s). These abilities are invaluable when conducting low pressure, fluid-saturated experiments such as phase equilibria, volatile solubility, and dynamic degassing experiments that are relevant to sub-volcanic magma chamber processes. A double capsule construction is also described that uses a solid oxygen buffer, and minimizes both contamination of the sample by carbon and the loss of iron in the melt to the capsule walls.