Mantle Petrology and Melting Quiz
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Questions and Answers

What are ophiolites primarily composed of?

  • Igneous rocks exclusively
  • Mafic and ultramafic rocks (correct)
  • Metamorphic rocks
  • Solely sedimentary rocks

    • Which of the following best describes alpine-type units?

  • Unaltered volcanic rock samples from the upper mantle
  • Small slivers of oceanic lithospheric mantle that are highly metamorphosed (correct)
  • Large sections of unaltered oceanic lithosphere
  • Unmetamorphosed fragments of continental crust

    • What type of samples provide direct access to rocks from the mantle that are not heavily deformed?

  • Ophiolites
  • Alpine-type units
  • Dredge samples
  • Nodules/xenoliths (correct)

    • Which mineral is NOT typically found in the dominant lithologies of the Earth's upper mantle?

    <p>Feldspar</p> Signup and view all the answers

    What is the primary composition of dredge samples taken from oceanic lithospheric mantle?

    <p>Ultramafic and mafic rocks</p> Signup and view all the answers

    What is necessary for partial melting of the mantle to occur under normal conditions?

    <p>Raising temperature</p> Signup and view all the answers

    Which of the following describes hotspots in the context of mantle melting?

    <p>Stationary source of hotter material</p> Signup and view all the answers

    What happens to the residue when extraction melt occurs?

    <p>It depletes U, Th, and K concentration</p> Signup and view all the answers

    What is the main heat source attributed to the process of mantle melting?

    <p>Decay of radioactive elements</p> Signup and view all the answers

    What is the consequence of the long timescale required for temperature increases in mantle peridotite?

    <p>Heat generated dissipates before significant melting</p> Signup and view all the answers

    Which of the following methods can raise the temperature in the mantle to allow for partial melting?

    <p>Increasing local heat flow</p> Signup and view all the answers

    How long does it approximately take to raise the temperature of peridotite by 1°C through radioactive decay?

    <p>107 years</p> Signup and view all the answers

    Which process is associated with volcanic tracks as a result of mantle melting?

    <p>Mantle plumes creating hotspots</p> Signup and view all the answers

    What do hotspots typically originate from?

    <p>Stationary source in the mantle</p> Signup and view all the answers

    What process leads to decompression melting in the mantle?

    <p>Constant temperature with lowering pressure</p> Signup and view all the answers

    What conditions are necessary for the rising mantle material to avoid heat loss during decompression?

    <p>Rapid rise of material</p> Signup and view all the answers

    What is the significance of the 10° C/GPa geotherm in mantle processes?

    <p>It helps to determine the depth at which melting begins</p> Signup and view all the answers

    Which composition change can primarily influence mantle melting?

    <p>Varied volatile composition</p> Signup and view all the answers

    How do scientists know that volatiles are present in the mantle?

    <p>Discovery of hydrous minerals</p> Signup and view all the answers

    What does the latent heat of fusion do during mantle melting?

    <p>Absorbs heat from the rising mass</p> Signup and view all the answers

    What estimation is made regarding mantle material at mid-ocean ridges?

    <p>Needs to rise ~ 150 km and suffer 20-30% decompression</p> Signup and view all the answers

    What is the effect of adding H2O to the solidus temperature in the mantle?

    <p>It drastically lowers the solidus temperature.</p> Signup and view all the answers

    What are the two criterions that must be satisfied for mantle melting to occur?

    <p>Free H2O must be present and P-T must be sufficient for melting.</p> Signup and view all the answers

    What is the normal weight percentage of H2O in the mantle?

    <p>0.1-0.2 wt.% H2O</p> Signup and view all the answers

    What is a consequence of the breakdown of amphibole/phlogopite in the mantle?

    <p>It releases low amounts of H2O.</p> Signup and view all the answers

    What is the primary reason for the presence of volatiles in the mantle?

    <p>Presence of hydrous minerals like Amphibole and Phlogopite</p> Signup and view all the answers

    Which condition is NOT necessary for mantle melting according to the content?

    <p>Traditional mantle composition must change</p> Signup and view all the answers

    What does a Low-Velocity Layer (LVL) in the mantle indicate?

    <p>Interconnected water film reducing seismic wave velocity.</p> Signup and view all the answers

    How much partial melting occurs in the mantle from H2O presence?

    <p>Around 1%</p> Signup and view all the answers

    How does the addition of H2O affect the solidus temperature in the mantle?

    <p>It lowers the solidus temperature</p> Signup and view all the answers

    What could prevent the extraction of water in the mantle?

    <p>The formation of interconnected films on grain surfaces.</p> Signup and view all the answers

    What percentage of H2O is typically found in 'normal' mantle composition?

    <p>0.1 - 0.2 wt.% H2O</p> Signup and view all the answers

    What depth range corresponds to the breakdown experiments and seismic refraction data?

    <p>60-200 km</p> Signup and view all the answers

    Which of the following minerals is associated with volatiles in the mantle?

    <p>Phlogopite</p> Signup and view all the answers

    What are fluid inclusions in mantle minerals primarily composed of?

    <p>H2O and CO2</p> Signup and view all the answers

    What is the likely significance of carbonate inclusions in mantle minerals?

    <p>They provide evidence of volatile recycling processes</p> Signup and view all the answers

    Why is it stated that the elemental composition of the mantle is mostly fixed?

    <p>It rarely undergoes significant changes</p> Signup and view all the answers

    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.

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