Volatiles Flashcards
At Mid-Ocean ridges, what are the characteristics of partial melting?
Hot, dry diverging plates.
Mantle ascent and decompression causes melting.
Geotherm crosses solidus at shallow depths, melt generated.
At subduction zones, what are the characteristics of partial melting?
Addition of water causes melting not heat.
get magma at greater depths.
get a wet solidus; lower temps, due to fluids.
How does water effect melt properties?
Density - decreases with increasing water content.
Diffusion - increases with water content, as simpler melt structure, diffusion easier.
Solubility - increases with water content due to increased pressure.
- if saturated, decompression occurs.
Whats a melt inclusion?
pocket of melt trapped inside a crystal as it grows.
What is the excess sulphur problem?
SO2 emissions don’t correspond (greater than) to SO2 expected to be lost from magma.
sulphur volatiles are lost from magma when magma ascends and decompresses.
What are the causes of excess sulphur?
1) The crystals have to be growing to trap melt inclusions. Means that sulphur may have exsolved before crystals grew, meaning there is an underestimation in the melt.
2) More magma is involved than whats erupted - Transcrustal magmatic systems.
How do bubbles containing volatiles grow by decompression? (Two ways)
1) By Diffusion - at depth bubbles initially grow by diffusion of volatiles into bubbles. Due to pressure drop, bubbles form.
2) By Expansion - magma rises, decompresses and bubbles expand. Expansion is limited by viscosity of melt and neighbouring bubbles, creates an overpressure.
How do bubbles drive explosive eruptions?
High volatiles content.
Rapid nucleation and bubble growth.
Gas cant escape at high melt viscosity and low permeability. Overpressure. At high magma flow rates.
How does gas escape work?
When bubbles rise due to their low density, gas can escape through connected bubble networks.
Open system degassing - gas is removed from the system (escapes) as it is exsolved from the melt. Normally low magma flow rates (E.g. lava flow or dome)
Closed system volatile exsolution - gas remains in contact with the melt after its exsolved. (Explosive eruptions)
What are the two types of bubble nucleation and characteristics?
Heterogenous - Bubbles form on a crystal. Vesiculation from magma is easier if bubbles nucleate on crystals
Homogeneous - Bubble form from melt alone. Harder to nucleate. But at high enough change in pressurw, lots of bubbles form.
Characteristics of Limnic Eruptions?
Form when lake overturning occurs. Get the release of CO2.
CO2 is magmatic in origin, enters the lakes by percolating through rocks into the lake.
CO2 dissolved in water is denser than pure water so tends to remain at depths.
This forms a density-stratified lake.
More CO2 is dissolved at depth with high pressure, it is anoxic.
Amelioration of Limnic Eruptions?
Vent Gas Lift Systems:
2 pipes - 1 with a pump and one without.
pump draws CO2 rich water up, pipe draws water up by pressure differences.
Causes the slow release of CO2 to prevent rapid release by overturning.
Limnic Eruptions Example?
Lake Nyos
- CO2 release triggered by landslides
- CO2 gas cloud was denser than the atmosphere so didnt disperse, hugged ground and caused suffocation.
Types of basaltic lava fountain foams?
Scoria (bubbles) and reticulite (polygonal glass struts).
Three eruption types and their flow regimes?
Lava flows - dispersed bubble flow.
Low gas speed, high liquid speed. Liquid carrying gas.
Lava Fountains - annular flow.
High gas speed, gas carrying liquid blobs.
Strombolian - slug flow.
Large bubbles, bubbly liquid between.
