Degassing-driven crystallisation in basalts

Applegarth, Louisa; Tuffen, Hugh; James, Michael; Pinkerton, Harry

doi:10.1016/j.earscirev.2012.10.007

Cited by 45 publications

(32 citation statements)

References 86 publications

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“…Richet et al (2006) concluded on these grounds that 5 % water exsolution from magmas is associated with temperature changes of only a few degrees at most. Consistent with this, Applegarth et al (2013) found that degassing around 50 % of the volatiles from a basalt causes around 35 % crystallization almost purely due to undercooling. They observed a large net latent heat release; i.e.…”

Section: Crystallization Caused By Degassingsupporting

confidence: 63%

Fred’s Flow (Canada) and Murphy Well (Australia): thick komatiitic lava flows with contrasting compositions, emplacement mechanisms and water contents

Siégel

Arndt

Barnes

et al. 2014

Contrib Mineral Petrol

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pulse was followed by another more voluminous but less magnesian pulse that inflated the flow to its present 120 m thickness. Following the second pulse, the flow crystallized in a closed system and differentiated into cumulates containing 30-38 wt% MgO and a residual gabbroic layer with only 6 wt% MgO. The Murphy Well Flow, in contrast, has a remarkably uniform composition throughout. It comprises a 20-m-thick upper layer of fine-grained dendritic olivine and 2-5 vol% amygdales, a 110-120 m intermediate layer of olivine porphyry and a 20-30 m basal layer of olivine orthocumulate. Throughout the flow, MgO contents vary little, from only 30 to 33 wt%, except for the slightly more magnesian basal layer (38-40 wt%). The uniform composition of the flow and dendritic olivine habits in the upper 20 m point to rapid cooling of a highly magnesian liquid with a composition like that of the bulk of the flow. Under equilibrium conditions, this liquid should have crystallized olivine with the composition Fo94.9, but the most magnesian composition measured by electron microprobe in samples from the flow is Fo92.9. To explain these features, we propose that the parental liquid contained around 32 wt% MgO and 3 wt% H 2 O. This liquid degassed during Abstract Two Archaean komatiitic flows, Fred's Flow in Canada and the Murphy Well Flow in Australia, have similar thicknesses (120 and 160 m) but very different compositions and internal structures. Their contrasting differentiation profiles are keys to determine the cooling and crystallization mechanisms that operated during the eruption of Archaean ultramafic lavas. Fred's Flow is the type example of a thick komatiitic basalt flow. It is strongly differentiated and consists of a succession of layers with contrasting textures and compositions. The layering is readily explained by the accumulation of olivine and pyroxene in a lower cumulate layer and by evolution of the liquid composition during downward growth of spinifex-textured rocks within the upper crust. The magmas that erupted to form Fred's Flow had variable compositions, ranging from 12 to 20 wt% MgO, and phenocryst contents from 0 to 20 vol%. The flow was emplaced by two pulses. A first ~20-m-thick Communicated by J. Hoefs. Electronic supplementary materialThe online version of this article (

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Section: Crystallization Caused By Degassingsupporting

confidence: 63%

Fred’s Flow (Canada) and Murphy Well (Australia): thick komatiitic lava flows with contrasting compositions, emplacement mechanisms and water contents

Siégel

Arndt

Barnes

et al. 2014

Contrib Mineral Petrol

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“…Thermal Gravimetric Analysis (TGA) Multi-species volatile contents and degassing patterns of 16 obsidian samples (representing a wide spatial distribution) were characterised by simultaneous differential scanning calorimetry-thermogravimetric analysis (DCS-TGA) using TA Instruments SDT Q600 and STA 449 devices, following the methods set out in (Denton et al, 2009) and (Applegarth et al, 2013). The DSC-TGA technique subjects the sample to a controlled heating programme (0-1250°C at 5°C/min), during which sample weight and differential heat flow are continually measured.…”

Section: Volatile Content Measurementsmentioning

confidence: 99%

Unravelling textural heterogeneity in obsidian: Shear-induced outgassing in the Rocche Rosse flow

Shields

Mader

Caricchi

et al. 2016

Journal of Volcanology and Geothermal Research

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Obsidian flow emplacement is a complex and understudied aspect of silicic volcanism. Of particular importance is the question of how highly viscous magma can lose sufficient gas in order to erupt effusively as a lava flow. Using an array of methods we study the extreme textural heterogeneity of the Rocche Rosse obsidian flow in Lipari, a 2 km long, 100 m thick,~800 year old lava flow, with respect to outgassing and emplacement mechanisms. 2D and 3D vesicle analyses and density measurements are used to classify the lava into four textural types: 'glassy' obsidian (b15% vesicles), 'pumiceous' lava (N 40% vesicles), high aspect ratio, 'shear banded' lava (20-40% vesicles) and low aspect ratio, 'frothy' obsidian with 30-60% vesicles. Textural heterogeneity is observed on all scales (m to μm) and occurs as the result of strongly localised strain. Magnetic fabric, described by oblate and prolate susceptibility ellipsoids, records high and variable degrees of shearing throughout the flow. Total water contents are derived using both thermogravimetry and infrared spectroscopy to quantify primary (magmatic) and secondary (meteoric) water. Glass water contents are between 0.08-0.25 wt.%. Water analysis also reveals an increase in water content from glassy obsidian bands towards 'frothy' bands of 0.06-0.08 wt.%, reflecting preferential vesiculation of higher water bands and an extreme sensitivity of obsidian degassing to water content. We present an outgassing model that reconciles textural, volatile and magnetic data to indicate that obsidian is generated from multiple shear-induced outgassing cycles, whereby vesicular magma outgasses and densifies through bubble collapse and fracture healing to form obsidian, which then re-vesiculates to produce 'dry' vesicular magma. Repetition of this cycle throughout magma ascent results in the low water contents of the Rocche Rosse lavas and the final stage in the degassing cycle determines final lava porosity. Heterogeneities in lava rheology (vesicularity, water content, microlite content, viscosity) play a vital role in the structural evolution of an obsidian flow and overprint flow-scale morphology. Post-emplacement hydration also depends heavily on local strain, whereby connectivity of vesicles as a result of shear deformation governs sample rehydration by meteoric water, a process previously correlated to lava vesicularity alone.

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“…When corrected for the increase in fragility at decreasing water content, as observed in our measurements of Cp for depolymerized liquids, the viscosity increase of basalt during degassing may be more rapid than previously expected. Moreover, degassing induced crystallization is also known to be more pronounced for basaltic compositions (Applegarth et al, 2013), leading to a further increase in viscosity. The combined effects of degassing-induced crystallization and degassing induced increase in fragility, may be so strong to induce rapid rheological changes with temperature prone to affect the eruption dynamics.…”

Section: Volcanological Implications: Eruptive Thermal Budgetmentioning

confidence: 99%

Heat capacity, configurational heat capacity and fragility of hydrous magmas

Genova

Romano

Giordano

et al. 2014

Geochimica et Cosmochimica Acta

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International audienceThe glassy and liquid heat capacities of four series of dry and hydrous natural glasses and magma as a function of temperature and water content (up to 19.9 mol%) were investigated using differential scanning calorimetry (DSC). The analyzed compositions are basalt, latite, trachyte and pantellerite. The results of this study indicate that the measured heat capacity of glasses (Cpg) is a linear function of composition and is well reproduced by the empirical model of Richet (1987). For the investigated glasses, the partial molar heat capacity of water can be considered as independent of composition, in agreement with Bouhifd et al. (2006). For hydrous liquids, the heat capacity (Cpliq) decreases nonlinearly with increasing water content. Previously published models, combined with the partial molar heat capacity of water from the literature, are not able to reproduce our experimental data in a satisfactory way. We estimated the partial molar heat capacity of water (CpH2O) in hydrous magma over a broad compositional range. The proposed value is 41 ± 3 J mol-1 K-1. Water strongly affects the configurational heat capacity at the glass transition temperature [Cpconf (Tg)]. An increases of Cpconf (Tg) with water content was measured for the polymerized liquids (trachyte and pantellerite), while the opposite behavior was observed for the most depolymerized liquids (basalt and latite). Structural and rheological implications of this behavior are discussed in light of the presented results

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Degassing-driven crystallisation in basalts

Cited by 45 publications

References 86 publications

Fred’s Flow (Canada) and Murphy Well (Australia): thick komatiitic lava flows with contrasting compositions, emplacement mechanisms and water contents

Fred’s Flow (Canada) and Murphy Well (Australia): thick komatiitic lava flows with contrasting compositions, emplacement mechanisms and water contents

Unravelling textural heterogeneity in obsidian: Shear-induced outgassing in the Rocche Rosse flow

Heat capacity, configurational heat capacity and fragility of hydrous magmas

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