Timescales of magma ascent during explosive eruptions: Insights from the re-equilibration of magmatic volatiles
Creators: Lloyd, Alexander
Publisher: name: Columbia University
Year: 2014
Subjects: subject: Geochemistry
subject: FOS: Earth and related environmental sciences
subject: Petrology

description: The explosivity of volcanic eruptions is governed in part by the rate at which magma ascends and degasses. Because the timescales of eruptive processes can be exceedingly fast relative to standard geochronometers, magma ascent rate remains difficult to quantify. As an exception to this principle, magmatic volatiles can re-equilibrate on timescales relevant to explosive eruptions, producing evidence for diffusion that can be assessed by various micro-beam techniques.
           Because the solubility of water and other magmatic volatiles decreases substantially at lower pressures, magmas erupt with a minute fraction of that which was initially dissolved. Melt inclusions, melt embayments, and trace amounts of H2O incorporated into the structure of nominally anhydrous minerals have the potential to retain information about the initial concentrations of magmatic volatiles prior to degassing. In this thesis, I present an assessment of the viability of these hydrous inclusions and mineral phases in preserving initial magmatic conditions in light of post-eruptive cooling effects. In addition, I also present an investigation of the potential of utilizing this volatile loss to estimate time scales of magma ascent during the 1974 sub-plinian eruption of Volcán de Fuego in Guatemala.
           To test the possibility of systematic H2O re-equilibration in olivine-hosted melt inclusions, I designed a natural experiment using ash, lapilli, and bomb samples that cooled at different rates owing to their different sizes. Ion microprobe, laser ablation-ICPMS, and electron probe analyses show that melt inclusions from ash and lapilli record the highest H2O contents, up to 4.4 wt%. On the other hand, MIs from bombs indicate up to 30% lower H2O contents (loss of ~ 1 wt% H2O) and 10% post-entrapment crystallization of olivine. This evidence is consistent with the longer cooling time available for a bomb-sized clast, up to 10 minutes for a 3-4 cm radius bomb, assuming conductive cooling and the fastest H+ diffusivities measured in olivine (D ~ 10-9 to 10-10 m2/s). On the other hand, several lines of evidence point to some water loss prior to eruption, possibly during magma ascent and degassing in the conduit. The duration of magma ascent that could account for the measured H2O loss was calculated to range from 10 to 30 minutes for the fast mechanism of H+ diffusion and 3.7 to 12.3 hours for the slow mechanism of H+ diffusion. Thus, results point to both slower post-eruptive cooling and slower magma ascent affecting MIs from bombs, leading to H2O loss over the timescale of minutes to hours. 
           Utilizing an established method for assessing magma ascent rates, concentration gradients of volatile species along open melt embayments within olivine crystals were measured for use as a chronometer. Continuous degassing of the external melt during magma ascent results in diffusion of volatile species from embayment interiors to the bubble located at their outlets. The wide range in diffusivity and solubility of these different volatiles provides multiple constraints on ascent timescales over a range of depths. We focused on four 100-200 micron, olivine-hosted embayments which exhibit decreases in H2O, CO2, and S towards the embayment outlet bubble. Compared to the extensive melt inclusion suite also presented in this thesis, the embayments have lost both H2O and CO2 throughout the entire length of the embayment. We fit the profiles with a 1-D numerical diffusion model that allows varying diffusivities and external melt concentration as a function of pressure. Assuming a constant decompression rate from the magma storage region at approximately 220 MPa to the surface, H2O, CO2 and S profiles for all embayments can be fit with a relatively narrow range in decompression rates of 0.3-0.5 MPa/s, equivalent to 11-17 m/s ascent velocity and an 8 to 12 minute duration of magma ascent from ~10 km depth. A two-stage decompression model takes advantage of the different depth ranges over which CO2 and H2O degas, and produces good fits given an initial stage of slow decompression (0.05 - 0.3 MPa/s) at high pressure ( &gt; 145 MPa), with similar decompression rates to the single-stage model for the shallower stage. The magma ascent rates reported here are among the first for explosive basaltic eruptions and demonstrate the potential of the embayment method for quantifying magmatic timescales associated with eruptions of different vigor.
           I investigated the utility of clinopyroxene as a recorder of the initial water and magma ascent rate using natural phenocrysts erupted during the 1974 eruption of Volcán de Fuego and the 1977 eruption on Seguam Island. The partitioning of water between clinopyroxene and melt was determined by analyzing melt inclusions and the adjacent clinopyroxene host by ion microprobe. For 10 Cpx-hosted MIs from Seguam, the partition coefficient is best predicted by the temperature-dependent parameterization by O'Leary et al. (2010). The diffusivity of H2O in clinopyroxene exhibits a four order of magnitude range in previous experimental studies that prevents a direct interpretation of concentration profiles as a chronometer. To constrain the diffusivity in magmatic phenocrysts, H2O concentration profiles were measured in Cpx from Fuego by ion microprobe and exhibit characteristics that are consistent with diffusive re-equilibration during magma ascent. Using the duration of ascent calculated from the melt inclusions and embayments (10 to 30 minutes), a range of H+ diffusivity was determined that would satisfy these timescales (10-9.20 to 10-10.45 m2/s). The calculated DH+ values are on the same order as the highest diffusivities for H+ in Cpx measured in the laboratory. A comparison of H2O concentrations measured in Cpx from lava and tephra samples from the Seguam eruption demonstrated that Cpx from lava retains less H2O in comparison to the H2O measured in the tephra. Using the DH+ values obtained from the Fuego Cpx, I showed that the difference in H2O between the lava and tephra Cpx can be attributed to post-eruption H2O loss during the estimated ~ 13 minute emplacement of the lava flow. The results from this work indicate that iron-rich clinopyroxene from slowly-cooled basaltic lavas should not be used to reconstruct initial magmatic water contents.
           The novel findings reported in this thesis are two-fold. Based on evidence from olivine-hosted melt inclusions in volcanic bombs and clinopyroxene in a pahoehoe lava flow, it is unlikely that the initial concentration of water can be preserved if a volcanic product undergoes slow post-eruptive cooling. This fact implies that a portion of the published data on H2O concentrations in olivine-hosted melt inclusions and clinopyroxene may reflect unrecognized H2O loss via diffusion and highlights the importance of reporting the type of volcanic deposit or the clast size from which a sample is extracted. The second novel finding of this thesis concerns the convergence in magma ascent rate estimates from three independent chronometers. In one of the first studies of this magma type, I report relatively fast time scales for magma ascent (~10 minutes from mid-crustal depths) for a basaltic, sub-plinian eruption. Furthermore, the similarity of the estimated timescales from melt inclusions, embayments, and clinopyroxene indicate the validity of any of these chronometers in tracking magma ascent rate. This further expansion of the methods for assessing time scales of volcanic eruptions enables researchers to pursue the complicated relationship between magmatic volatiles, ascent rate, and volcanic explosivity.