Magma Generation and Composition Notes

Magma Generation and Composition Notes

Introduction to Magma

  • Magma: Molten rock beneath Earth’s surface; consists of:

    • Liquid portion (melt)

    • Solids (silicate minerals)

    • Volatiles (dissolved gases such as H2O, CO2, SO2)

Mantle Petrology

  • Earth's composition: Crust, mantle, and core made primarily of solid rock.

  • Magma generation location: Upper mantle, from melting solid rock.

  • Key materials in the mantle:

    • Peridotite: Main type, composed of olivine, pyroxene, and aluminium phases.

    • Oceanic crust: Basalt/gabbro mixed with Plagioclase and Clinopyroxene.

    • Continental crust: Andesite or granite.

Composition of the Mantle

  • Upper Mantle: Primarily peridotite (combination of olivine, orthopyroxene (Opx), and clinopyroxene (Cpx)).

  • Types:

    • Lherzolite: Fertile, unaltered mantle rock.

    • Dunite and Harzburgite: Residuary rocks post partial melting.

Mantle Sampling Techniques

  • Xenoliths: Fragments of mantle rock, different from surrounding rocks, found in basalt.

  • Ophiolites: Ancient oceanic crust and upper mantle formed into mountain ranges during subduction.

Partial Melting Process

  • Partial melting occurs when:

    • Not all of the rock melts at the same temperature.

    • Only a fraction melts (crossing the solidus).

    • Produces basaltic magmas.

  • Solidus: Temperature at which melting starts; varies with pressure (+ depth).

Factors Influencing Melting

  1. Temperature: Increasing heat leads to melting.

    • Example: Hawaiian mantle plumes increase mantle temperatures.

  2. Pressure: Decompression melting occurs as confining pressure drops.

    • At mid-ocean ridges, the lithosphere thins, allowing melting to occur.

  3. Volatiles: Water lowers melting points in mantle rocks.

    • Particularly at subduction zones, leading to wet melting.

  4. Composition: Varies depending on the source material and minerals present.

Magma Generation Scenarios

  • Intraplate Mantle Plumes: Hotspot volcanoes (e.g., Hawaii).

  • Divergent Margins: Ocean ridges produce the greatest amounts of magma.

  • Convergent Margins: Subduction zones produce lesser amounts.

Types of Magmas

  • Basaltic Magma:

    • Composition: SiO2 45-52 wt%.

    • Erupted by ~80% of volcanoes (e.g. Kilauea, Mauna Loa).

  • Andesitic Magma:

    • Intermediate composition (SiO2 52-66 wt%).

    • Forms from basaltic magmas assimilating crustal rocks (e.g. Mount St. Helens).

  • Rhyolitic Magma:

    • Composition: SiO2 >65 wt%, higher in K and Na, lower in Fe and Mg.

    • Example: Volcanoes at Yellowstone.

Viscosity of Magmas

  • Defined as substance's resistance to flow; depends on:

    • Temperature: Higher temperature = Lower viscosity.

    • Composition: More silica = Higher viscosity.

      • Rhyolitic > Andesitic > Basaltic in viscosity levels.

Bowen's Reaction Series

  • Crystallization sequence of minerals from cooling magma:

    • High-Temperature Minerals: (e.g., Olivine).

    • Low-Temperature Minerals: (e.g., Quartz).

Magma Evolution Processes

  1. Magmatic Differentiation: Changes magma composition towards felsic through crystallization or settling.

  2. Assimilation: Incorporating surrounding rock materials into magma, altering its composition.

  3. Magma Mixing: Interaction of two different magma bodies, leading to hybrid compositions.

Important Definitions

  • Primary Magma: Original magma formed directly from mantle melting.

  • Parental Magma: Most primitive form in a sequence; high Mg, low silica, high eruption temperature.

  • Derived Magma: Evolves from primary due to differentiation.

End of Notes.