Sigurdur Jakobsson

Gradual caldera collapse at Bárdarbunga volcano, Iceland, regulated by lateral magma outflow

Driven to collapse Volcanic eruptions occur frequently, but only rarely are they large enough to cause the top of the mountain to collapse and form a caldera. Gudmundsson et al. used a variety of geophysical tools to monitor the caldera formation that accompanied the 2014 Bárdarbunga volcanic eruption in Iceland. The volcanic edifice became unstable as magma from beneath Bárdarbunga spilled out into the nearby Holuhraun lava field. The timing of the gradual collapse revealed that it is the eruption that drives caldera formation and not the other way around. Science, this issue p. 262 Magma flow from under the Bárdarbunga volcano drove caldera collapse during the 2014 eruption. INTRODUCTION The Bárdarbunga caldera volcano in central Iceland collapsed from August 2014 to February 2015 during the largest eruption in Europe since 1784. An ice-filled subsidence bowl, 110 square kilometers (km2) in area and up to 65 meters (m) deep developed, while magma drained laterally for 48 km along a subterranean path and erupted as a major lava flow northeast of the volcano. Our data provide unprecedented insight into the workings of a collapsing caldera. RATIONALE Collapses of caldera volcanoes are, fortunately, not very frequent, because they are often associated with very large volcanic eruptions. On the other hand, the rarity of caldera collapses limits insight into this major geological hazard. Since the formation of Katmai caldera in 1912, during the 20th century’s largest eruption, only five caldera collapses are known to have occurred before that at Bárdarbunga. We used aircraft-based altimetry, satellite photogrammetry, radar interferometry, ground-based GPS, evolution of seismicity, radio-echo soundings of ice thickness, ice flow modeling, and geobarometry to describe and analyze the evolving subsidence geometry, its underlying cause, the amount of magma erupted, the geometry of the subsurface caldera ring faults, and the moment tensor solutions of the collapse-related earthquakes. RESULTS After initial lateral withdrawal of magma for some days though a magma-filled fracture propagating through Earth’s upper crust, preexisting ring faults under the volcano were reactivated over the period 20 to 24 August, marking the onset of collapse. On 31 August, the eruption started, and it terminated when the collapse stopped, having produced 1.5 km of basaltic lava. The subsidence of the caldera declined with time in a near-exponential manner, in phase with the lava flow rate. The volume of the subsidence bowl was about 1.8 km3. Using radio-echo soundings, we find that the subglacial bedrock surface after the collapse is down-sagged, with no indications of steep fault escarpments. Using geobarometry, we determined the depth of magma reservoir to be ~12 km, and modeling of geodetic observations gives a similar result. High-precision earthquake locations and moment tensor analysis of the remarkable magnitude M5 earthquake series are consistent with steeply dipping ring faults. Statistical analysis of seismicity reveals communication over tens of kilometers between the caldera and the dike. CONCLUSION We conclude that interaction between the pressure exerted by the subsiding reservoir roof and the physical properties of the subsurface flow path explain the gradual near-exponential decline of both the collapse rate and the intensity of the 180-day-long eruption. By combining our various data sets, we show that the onset of collapse was caused by outflow of magma from underneath the caldera when 12 to 20% of the total magma intruded and erupted had flowed from the magma reservoir. However, the continued subsidence was driven by a feedback between the pressure of the piston-like block overlying the reservoir and the 48-km-long magma outflow path. Our data provide better constraints on caldera mechanisms than previously available, demonstrating what caused the onset and how both the roof overburden and the flow path properties regulate the collapse. The Bárdarbunga caldera and the lateral magma flow path to the Holuhraun eruption site. (A) Aerial view of the ice-filled Bárdarbunga caldera on 24 October 2014, view from the north. (B) The effusive eruption in Holuhraun, about 40 km to the northeast of the caldera

THE SYSTEM C-O IN EQUILIBRIUM WITH GRAPHITE AT HIGH PRESSURE AND TEMPERATURE : AN EXPERIMENTAL STUDY

Abstract The fluid system C-O has been studied experimentally in equilibrium with graphite at 7.5 to 20 kbars and 1100 to 1500°C. The results are compared to calculated results from the Redlich-Kwong equation of state and the Saxena and Fei corresponding states equation and are also fitted to a virial equation of state to obtain interaction parameters and a new equation to calculate the oxygen fugacity of the graphite-oxygen (CCO) buffer. Experimental results were found to be lower in CO than calculated values, especially at high pressures and temperatures. Several experiments performed at different quench rates showed decreasing CO with increased quench time suggesting that CO reacts during the quench to form CO 2 . The data obtained from the quench experiments allow determination of the rate constant, k , for the quench reaction at different P and T and calculation of zero-time concentrations.

Crystal — Liquid experiments in the presence of a C-O-H fluid buffered by graphite + iron + wustite: Experimental method and near-liquidus relations in basanite

Abstract A capsule assembly for use at high pressures and temperatures in the piston-cylinder apparatus has been developed which prevents iron loss to capsules and fixes fluid fugacities. The oxygen fugacity of the assembly is fixed by the iron-wustite buffer, and fugacities of other fluid species present are fixed by the buffer and the presence of graphite. The conditions of buffering and no iron loss to capsules allow a very high degree of precision from run to run. This allows determination of subtle melting reactions. The solubility of the fluid phase in albite, basanite and tholeiite liquids was determined using infrared, near-infrared and Raman spectral analysis along with quadrupole mass spectrometry, thermogravimetric analysis and total C-H analysis. Only H2O and CO show significant solubility compared to their activities. CH4, CO2 and H2 are rejected by the melt. Liquids produced by different degrees of melting of a low-K basanite, which fractionates hornblende, show an alkali trend similar to fractionation trends observed in nature.

A note on the Raman spectra of water-bearing albite glasses

The Raman spectra of albite glasses with 4.5 and 6.6 weight percent water have been obtained, and are compared with that of a dry sample. The hydrous glasses show bands near 3600 cm^(−1) due to O-H stretching, and a previously unreported weak band near 1600 cm^(−1) due to bending of molecular H_2O. Other weak spectral features are discussed, and the effect of dissolved water on the aluminosilicate framework vibrations is considered.

Experimental determination of fluid compositions in the system C-O-H at high P and T and low fo2

Abstract In order to verify theoretical calculations on speciation in the system C-O-H at iron-wustite oxygen fugacity we have performed piston-cylinder experiments on fluids in the system at pressures ranging from 5 to 15 kb and temperatures from 900 to 1200°C. The samples were quenched isobarically and analyzed with a quadrupole mass analyzer. Our results show that CH 4 , H 2 O, and H 2 are major components of the fluid with minor amounts of C 2 H 6 , CO, and CO 2 being present. The theoretical values for H 2 O fit our experimental values quite well whereas the fit for other species ranges from moderate to poor.

Determination of Si/Al Ratios in Semicrystalline Aluminosilicates by FT-IR Spectroscopy

Infrared spectra of synthetic, semicrystalline aluminosilicates in the region 500–1200 cm−1 show a dominant band due to Si–O stretching at 980–1030 cm−1 and two combined bands due to Si–O–Al bending at 600–700 cm−1. These bands can be used to determine quantitatively the Si/Al ratio of the aluminosilicates. A calibration curve has been obtained by integrating the intensities of the bands and relating them to Si/Al ratios of 10 aluminosilicate samples as measured with inductively coupled plasma (ICP) with Si/Al ratios ranging from 0.19 to 1.46.

Oxygen fugacity control in piston-cylinder experiments: A re-evaluation

Jakobsson (Contrib Miner Petrol 164(3):397–407, 2012) investigated a double capsule assembly for use in piston-cylinder experiments that would allow hydrous, high-temperature, and high-pressure experiments to be conducted under controlled oxygen fugacity conditions. Using a platinum outer capsule containing a metal oxide oxygen buffer (Ni–NiO or Co–CoO) and H2O, with an inner gold–palladium capsule containing hydrous melt, this study was able to compare the oxygen fugacity imposed by the outer capsule oxygen buffer with an oxygen fugacity estimated by the AuPdFe ternary system calibrated by Barr and Grove (Contrib Miner Petrol 160(5):631–643, 2010). H2O loss or gain, as well as iron loss to the capsule walls and carbon contamination, is often observed in piston-cylinder experiments and often go unexplained. Only a few have attempted to actually quantify various aspects of these changes (Brooker et al. in Am Miner 83(9–10):985–994, 1998; Truckenbrodt and Johannes in Am Miner 84:1333–1335, 1999). It was one of the goals of Jakobsson (Contrib Miner Petrol 164(3):397–407, 2012) to address these issues by using and testing the AuPdFe solution model of Barr and Grove (Contrib Miner Petrol 160(5):631–643, 2010), as well as to constrain the oxygen fugacity of the inner capsule. The oxygen fugacities of the analyzed melts were assumed to be equal to those of the solid Ni–NiO and Co–CoO buffers, which is incorrect since the melts are all undersaturated in H2O and the oxygen fugacities should therefore be lower than that of the buffer by 2 log

Volatile solubilities in magmas: Transport of volatiles from mantles to planet surfaces

a_{{{\text{H}}_{ 2} {\text{O}}}}