Questions and Answers
What are ophiolites primarily composed of?
Which of the following best describes alpine-type units?
What type of samples provide direct access to rocks from the mantle that are not heavily deformed?
Which mineral is NOT typically found in the dominant lithologies of the Earth's upper mantle?
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What is the primary composition of dredge samples taken from oceanic lithospheric mantle?
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What is necessary for partial melting of the mantle to occur under normal conditions?
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Which of the following describes hotspots in the context of mantle melting?
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What happens to the residue when extraction melt occurs?
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What is the main heat source attributed to the process of mantle melting?
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What is the consequence of the long timescale required for temperature increases in mantle peridotite?
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Which of the following methods can raise the temperature in the mantle to allow for partial melting?
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How long does it approximately take to raise the temperature of peridotite by 1°C through radioactive decay?
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Which process is associated with volcanic tracks as a result of mantle melting?
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What do hotspots typically originate from?
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What process leads to decompression melting in the mantle?
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What conditions are necessary for the rising mantle material to avoid heat loss during decompression?
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What is the significance of the 10° C/GPa geotherm in mantle processes?
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Which composition change can primarily influence mantle melting?
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How do scientists know that volatiles are present in the mantle?
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What does the latent heat of fusion do during mantle melting?
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What estimation is made regarding mantle material at mid-ocean ridges?
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What is the effect of adding H2O to the solidus temperature in the mantle?
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What are the two criterions that must be satisfied for mantle melting to occur?
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What is the normal weight percentage of H2O in the mantle?
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What is a consequence of the breakdown of amphibole/phlogopite in the mantle?
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What is the primary reason for the presence of volatiles in the mantle?
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Which condition is NOT necessary for mantle melting according to the content?
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What does a Low-Velocity Layer (LVL) in the mantle indicate?
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How much partial melting occurs in the mantle from H2O presence?
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How does the addition of H2O affect the solidus temperature in the mantle?
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What could prevent the extraction of water in the mantle?
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What percentage of H2O is typically found in 'normal' mantle composition?
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What depth range corresponds to the breakdown experiments and seismic refraction data?
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Which of the following minerals is associated with volatiles in the mantle?
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What are fluid inclusions in mantle minerals primarily composed of?
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What is the likely significance of carbonate inclusions in mantle minerals?
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Why is it stated that the elemental composition of the mantle is mostly fixed?
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Study Notes
Mantle Petrology
- Ophiolites are fragments of oceanic lithosphere that end up on continental edges or in mountain ranges during tectonic collisions.
- Alpine-type units consist of smaller slivers of oceanic mantle incorporated into deformed mountain ranges, typically showing high metamorphism.
- Dredge samples collect oceanic lithospheric mantle exposed in transform faults, facilitating study of deeper mantle.
- Nodules or xenoliths are mantle fragments brought to the surface by basalt eruptions, providing access to deeper mantle materials.
- The Earth's upper mantle is primarily composed of peridotitic lithologies including olivine, clinopyroxene, and orthopyroxene.
Mantle Melting
- Under "normal" conditions, partial melting is not expected in the mantle; changes in temperature, pressure, or composition are required.
- Raising the temperature through local heat flow perturbations, such as hotspots, can lead to melting.
- Hotspots are stationary magma conduits, often creating volcanic tracks due to plate movements.
- Radioactive decay of elements (U, Th, K) contributes to heat accumulation, but the process is slow and limited.
Decompression Melting
- Decompression melting occurs when hot mantle rocks rise, causing a reduction in pressure and potential melting.
- Localized pressure drops facilitate ductile flow, restoring lithostatic equilibrium.
- A rapid rate of ascent (an adiabatic process) is crucial to retain heat and encourage melting.
- Melting is particularly notable at divergent plate boundaries like mid-ocean ridges.
Composition Changes in Mantle Melting
- The elemental composition of the mantle is generally fixed, yet volatile components (H2O and CO2) can vary.
- Presence of hydrous minerals (e.g., amphibole, phlogopite) indicates that the mantle contains volatiles.
- Free H2O significantly lowers the solidus temperature of the mantle, leading to partial melting when conditions allow.
- A threshold of 0.1-0.2 wt.% H2O is typical in the mantle; more free H2O is necessary to generate substantial melt.
- The breakdown of hydrous minerals releases small amounts of H2O, promoting a low-velocity layer that slows seismic waves in the mantle.
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Description
Test your knowledge on mantle petrology and the processes of mantle melting. Explore concepts such as ophiolites, alpine-type units, xenoliths, and the conditions required for partial melting in the Earth's mantle. This quiz covers essential characteristics and components of the upper mantle.
