Relating vesicle shapes in pyroclasts to eruption styles

Vesicles in pyroclasts provide a direct record of conduit conditions during explosive volcanic eruptions. Although their numbers and sizes are used routinely to infer aspects of eruption dynamics, vesicle shape remains an underutilized parameter. We have quantified vesicle shapes in pyroclasts from fall deposits of seven explosive eruptions of different styles, using the dimensionless shape factor , a measure of the degree of complexity of the bounding surface of an object. For each of the seven eruptions, we have also estimated the capillary number, Ca, from the magma expansion velocity through coupled diffusive bubble growth and conduit flow modeling. We find that Ω is smaller for eruptions with Ca 1 than for eruptions with Ca 1. Consistent with previous studies, we interpret these results as an expression of the relative importance of structural changes during magma decompression and bubble growth, such as coalescence and shape relaxation of bubbles by capillary stresses. Among the samples analyzed, Strombolian and Hawaiian fire-fountain eruptions have Ca 1, in contrast to Vulcanian, Plinian, and ultraplinian eruptions. Interestingly, the basaltic Plinian eruptions of Tarawera volcano, New Zealand in 1886 and Mt. Etna, Italy in 122 BC, for which the cause of intense explosive activity has been controversial, are also characterized by Ca 1 and larger values of Ω than Strombolian and Hawaiian style (fire fountain) eruptions. We interpret this to be the consequence of syn-eruptive magma crystallization, resulting in high magma viscosity and reduced rates of bubble growth. Our model results indicate that during these basaltic Plinian eruptions, buildup of bubble overpressure resulted in brittle magma fragmentation.National Science Foundation EAR-1019872National Science Foundation EAR-081033

The rheology of particle-liquid suspensions, the shape and connectivity of vesicles in pyroclasts and implications for the Plinian eruption of basaltic magma

This thesis consists of three projects based on magma ascent dynamics during volcanic eruptions. In the first project, I quantified vesicle shapes in pyroclasts, from different styles of volcanic eruptions, using a dimensionless shape factor. I found that this shape factor can be related to a dimensionless Capillary number, estimated from coupled bubble growth and magma ascent modeling and thus, to the eruption styles. My second project dealt with understanding the effect of crystals on the rheological properties of magma from dynamically similar analog laboratory experiments. I found that the rheological properties of particulate suspensions depend on the applied shear rate and maximum packing fraction of a particulate system, which is a function of particle size- and shape-modality. Using empirical formulations, I showed that non-Newtonian rheology of crystalline magma may cause large changes in magma discharge rates for small changes in driving pressure gradient and/or crystal shape- and size-modality. In the third project, I measured permeability of pyroclasts from the Plinian style eruptions of basaltic magma at Mt. Etna (122 BCE) and Mt. Tarawera (1886) and found that the permeability of these pyroclasts are 1-2 orders of magnitude larger than that of the pyroclasts from Plinian style eruptions of silicic magmas. Using numerical modeling I found that the permeability thresholds are approximately at 35% of magma porosity and formulated the porosity-permeability relationships for pyroclasts from both the studied eruptions

Controls of Hydrogen Partitioning on the Formation of Wet Reservoirs During Lunar Magma Ocean Crystallization

Recent studies indicate the presence of hydrogen (H) in lunar samples. H inherited from the proto-Earth and impactors, as well as added during the phase of accretion (contemporaneous to lunar magma ocean (LMO) crystallization) have been retained in spite of losses during the Moon-forming impact and magma ocean degassing [1]. The bulk H in the LMO or the Bulk Silicate Moon (BSM) is an important constraint to understand the dynamics of the Moon-forming impact as well as determine the origin of volatiles in the Earth-Moon system [2]. Recent analysis of Apollo samples indicates that the Moon has heterogeneous H reservoirs [3], which may be explained by the partitioning of H between nominally anhydrous cumulates and liquid as well as the entrapment of residual liquid during progressive crystallization of the LMO. The recent estimates of H2O in the BSM (5 to 1650 μg/g; [4]) rely heavily on the partition coefficients of H (DH) between minerals and melt used in the models (where DH = H concentration in mineral/ H concentration in the melt). Here we demonstrate the effect of DH between nominally anhydrous minerals (NAMs) and melt on mantle and crustal H contents by modeling the fractional crystallization of a 600 km deep LMO. We follow published crystallization sequences as well as use a combination of the codes SPICES and alphaMELTS. We use lower and upper limits of DH for each mineral-melt pair as published in the literature and vary the initial bulk H assuming 1% residual melt in the crystal mush after compaction. Using joint H2-H2O solubility, we further evaluate the extent of degassing in the LMO. We demonstrate that H in plagioclase may be explained by either DHmin or DHmax, if the initial H content of the LMO was 100 μg/g. However, with higher initial H content, i.e. 1000 μg/g, only DHmin would explain plagioclase chemistry. This demonstrates that the current range of published values of DH are not sufficient to fully capture the dynamics of LMO crystallization, and highlight the necessity in future studies to experimentally constrain the DH values between the NAMs and melt compositions, specific to LMO crystallization conditions. [1] Barnes et al., 2016. Nat. Comm. [2] Desch and Robinson, 2019. Geochem [3] Robinson et al., 2016. GCA [4] McCubbin et al., 2015. Am Min

Controls on determining the bulk water content of the Moon

The Moon is much wetter than previously thought. The estimated bulk H2O concentrations based on the analyses of H2O in lunar materials show a wide range from 5 to 1650 ppm. To better constrain bulk H2O in the lunar magma ocean (LMO), we model LMO crystallization and vary DH (concentration of H2O in LMO mineral/concentration of H2O in melt), interstitial melt fraction, and initial LMO depth. We take the highest and lowest values of DH reported in the literature for the LMO minerals. We assess the bulk H2O content required in the initial magma ocean to satisfy two observational constraints: (1) H2O measured in plagioclase grains from ferroan anorthosites and (2) crustal mass from GRAIL. We find that the initial bulk LMO H2O that best explains the H2O content in crustal plagioclase is strongly dependent on DH rather than interstitial melt fractions or initial LMO depths, with a drier magma ocean (10 ppm H2O) being favored with higher DH and a wetter magma ocean (100-1000 ppm H2O) with lower DH. This underscores the importance of constraining DH specific to lunar conditions in future studies. We also demonstrate that crustal mass is not an effective hygrometer

Replication Data for: Controls on determining the bulk water content of the Moon.

The Moon is much wetter than previously thought. The estimated bulk H2O concentrations based on the analyses of H2O in lunar materials show a wide range from 5 to 1650 ppm. To better constrain bulk H2O in the lunar magma ocean (LMO), we model LMO crystallization and vary DH (concentration of H2O in LMO mineral/concentration of H2O in melt), interstitial melt fraction, and initial LMO depth. We take the highest and lowest values of DH reported in the literature for the LMO minerals. We assess the bulk H2O content required in the initial magma ocean to satisfy two observational constraints: (1) H2O measured in plagioclase grains from ferroan anorthosites and (2) crustal mass from GRAIL. We find that the initial bulk LMO H2O that best explains the H2O content in crustal plagioclase is strongly dependent on DH rather than interstitial melt fractions or initial LMO depths, with a drier magma ocean (10 ppm H2O) being favored with higher DH and a wetter magma ocean (100–1000 ppm H2O) with lower DH. This underscores the importance of constraining DH specific to lunar conditions in future studies. We also demonstrate that crustal mass is not an effective hygrometer

Fragmentation and Plinian eruption of crystallizing basaltic magma

Basalt is the most ubiquitous magma on Earth, erupting typically at intensities ranging from quiescently effusive to mildly explosive. The discovery of highly explosive Plinian eruptions of basaltic magma has therefore spurred debate about their cause. Silicic eruptions of similar style are a consequence of brittle fragmentation, as magma deformation becomes progressively more viscoelastic. Magma eventually crosses the glass transition and fragments due to a positive feedback between water exsolution, viscosity and decompression rate. In contrast to silicic eruptions, the viscosity of basaltic magmas is thought to be too low to reach conditions for brittle fragmentation. Pyroclasts from several basaltic Plinian eruptions, however, contain abundant micron-size crystals that can increase magma viscosity substantially. We therefore hypothesize that magma crystallization led to brittle fragmentation during these eruptions. Using combined oscillatory and extensional rheometry of concentrated particle-liquid suspensions that are dynamically similar to microcrystalline basaltic magma, we show that high volume fractions of particles and extension rates of about 1 s?1or greater result in viscoelastic deformation and brittle fracture. We further show that for experimentally observed crystallization rate, basaltic magma can reach the empirical failure conditions when erupting at high discharge rates.by Pranabendu Moitra, Helge M. Gonnermann, Bruce F. Houghton and Chandra S. Tiwar