Magma Evolution Processes
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Questions and Answers

What is the primary characteristic of crustal-derived magmas such as rhyolites and granites?

  • They are rich in magnesium and iron.
  • They are formed by partial melting of the mantle.
  • They show low isotopic ratios of Sr-87/Sr-86.
  • They have high silica and incompatible elements. (correct)

    • Which process is linked to systematic depletion of magnesium and iron in magma?

  • Partial melting
  • Fractional crystallization (correct)
  • Magma mixing
  • Crustal contamination

    • What isotopic ratios are important for identifying the source of magma?

  • Rb, Sr, and La
  • Sr, Nd, and Pb (correct)
  • Sr, U, and Pb
  • Sr, La, and Nd

    • Which factor is primarily introduced during crustal contamination in magma formation?

    <p>Silica, potassium, and incompatible elements.</p> Signup and view all the answers

    What is the result of magma mixing in geological processes?

    <p>It generates magmas with hybrid geochemical signatures.</p> Signup and view all the answers

    What is the main effect of fractional crystallization on magma composition as it cools?

    <p>Magma becomes richer in silica.</p> Signup and view all the answers

    What is a characteristic result of magma mixing?

    <p>Formation of intermediate compositions with mixed textures.</p> Signup and view all the answers

    How does the assimilation of crustal material affect magma composition?

    <p>It adds incompatible elements and silica from the crust.</p> Signup and view all the answers

    What happens during partial melting of rocks?

    <p>The melt is enriched in incompatible elements.</p> Signup and view all the answers

    Which scenario is likely to produce a felsic magma?

    <p>Incorporation of surrounding crustal material.</p> Signup and view all the answers

    What does a high value of Sr-87/Sr-86 indicate in magmas?

    <p>Incorporation of crustal materials.</p> Signup and view all the answers

    What type of magma is produced from a depleted mantle source?

    <p>Basaltic magma with low incompatible elements.</p> Signup and view all the answers

    What is a result of early crystallizing minerals during the cooling of magma?

    <p>Magma becomes increasingly enriched in incompatible elements.</p> Signup and view all the answers

    What effect does fractional crystallization have on the concentrations of incompatible elements in magma?

    <p>It leads to their enrichment.</p> Signup and view all the answers

    Which of the following best describes the primary result of crustal contamination during magma formation?

    <p>Alters isotopic ratios to reflect crustal material.</p> Signup and view all the answers

    During which process would you expect to see a hybrid geochemical signature in magma?

    <p>Magma mixing.</p> Signup and view all the answers

    What do trace element concentrations like Rb/Sr and La/Yb indicate in relation to magma formation?

    <p>The extent of fractional crystallization or contamination.</p> Signup and view all the answers

    Which tectonic environment is associated with an enriched mantle source for magma production?

    <p>Ocean island settings.</p> Signup and view all the answers

    Which of the following processes leads to the enrichment of magma in incompatible elements?

    <p>Partial Melting</p> Signup and view all the answers

    What is a characteristic feature of intermediate magmas formed through magma mixing?

    <p>Resorbed crystals</p> Signup and view all the answers

    Which phase of magma evolution is primarily responsible for the transition from mafic to felsic compositions?

    <p>Fractional Crystallization</p> Signup and view all the answers

    What geochemical signature is associated with high values of Sr-87/Sr-86 in magmas?

    <p>Crustal contamination</p> Signup and view all the answers

    Which of the following statements correctly describes the effect of fractional crystallization on magma?

    <p>It increases the silica concentration in the remaining liquid.</p> Signup and view all the answers

    What results from the assimilation of crustal material during magma evolution?

    <p>Enriched isotopic signatures</p> Signup and view all the answers

    Which condition is most likely to lead to the production of basaltic magmas?

    <p>Partial melting of mantle rocks</p> Signup and view all the answers

    What is the overall effect of magma mixing on magma evolution?

    <p>Leads to complex compositions and textures</p> Signup and view all the answers

    Study Notes

    Magma Evolution Processes

    • Magma's composition and texture change as it moves, interacts with rocks, and cools.
    • These changes are reflected in the resulting rock textures and geochemical signatures.

    Fractional Crystallization

    • As magma cools, minerals crystallize in a specific order (Bowen's Reaction Series).
    • Remaining magma becomes richer in elements not incorporated into early crystals (incompatible elements).
    • Early-forming crystals (like olivine, pyroxene) remove Mg and Fe, increasing silica (Si) in the magma.
    • This process creates a gradation from mafic (like basalt) to intermediate (andesite) to felsic (rhyolite) compositions.
    • Early crystals (e.g., olivine, pyroxene), crystallize out according to Bowen's reaction series. Remaining liquid becomes enriched in incompatible elements.
    • Resulting magmas show a progression from mafic to intermediate to felsic compositions.

    Magma Mixing

    • Two magmas with different compositions combine.
    • This merging occurs when a more mafic magma intrudes into a felsic magma chamber.
    • Mixed magmas exhibit disequilibrium textures (resorbed crystals) and mineral zoning, highlighting the merging process.

    Assimilation of Crustal Material

    • Magma incorporates surrounding crustal material as it rises.
    • This process is common in thick continental crust where magma stays longer.
    • The magma gains silica and incompatible elements from the incorporated crust.
    • Added crustal material leads to felsic magmas, reflected in enriched isotopic signatures (e.g., higher Sr-87/Sr-86).

    Partial Melting

    • Rocks partially melt, and the separated melt from the solid residue is enriched in incompatible elements.
    • Mantle melts generally yield basaltic magmas.
    • High Sr-87/Sr-86 values indicate crustal contamination.
    • High Nd-143/Nd-144 values point to a depleted mantle source, while low values suggest an enriched source.

    Geochemical Manifestations and Magma Formation

    • Geochemical data act as a "fingerprint" of magma formation and evolution.

    Source of Magma

    • Mantle-Derived (Basalts):
      • Depleted mantle sources (MORB) produce magmas with low incompatible elements and simple isotopic signatures.
      • Enriched mantle sources (OIB, LIPs) generate magmas with high incompatible elements (e.g., Nb, U).
    • Crust-Derived (Rhyolites, Granites):
      • Formed by crustal partial melting; high in silica and incompatible elements.
      • Isotopic ratios (e.g., high Sr-87/Sr-86) indicate crustal contribution.

    Processes of Evolution

    • Fractional Crystallization:
      • Leads to Mg and Fe depletion and incompatible element enrichment.
      • REE patterns reveal a progressive enrichment in light rare earth elements (REEs).
    • Crustal Contamination:
      • Adds silica, potassium, and incompatible elements, resulting in felsic magmas.
      • Changes isotopic ratios to reflect incorporated crustal material.
    • Magma Mixing:
      • Creates intermediate compositions with mixed geochemical signatures, exhibiting disequilibrium textures.

    Identifying and Understanding Magma's Formation

    • Identifying Sources:
      • Isotopic ratios (Sr, Nd, Pb) determine if the magma originated from the mantle, crust, or a mix.
    • Understanding Processes:
      • Trace element concentrations (e.g., Rb/Sr, La/Yb) expose the extent of fractional crystallization, partial melting, or crustal contamination.
    • Tectonic Settings:
      • Geochemical signatures confirm tectonic settings (e.g., depleted mantle for MORB, enriched mantle for OIB).

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    Description

    Explore the intricate processes of magma evolution including fractional crystallization, magma mixing, and assimilation of crustal material. Understand how these mechanisms affect rock formation and composition through various geological scenarios. Test your knowledge of geochemical signatures and rock textures in relation to magma processes.

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