Igneous Petrology: Magma Composition

• Igneous petrology focuses on the origin, composition, distribution, and structure of igneous rocks, which are formed through the cooling and solidification of magma or lava.
• Magma composition is a fundamental aspect of igneous petrology, influencing the physical properties, eruption style, and resulting rock types of igneous systems.

Major Elements in Magmas

• Magmas are complex high-temperature silicate liquids containing dissolved volatiles and suspended crystals.
• The eight most abundant elements in magmas are: Oxygen (O), Silicon (Si), Aluminum (Al), Iron (Fe), Magnesium (Mg), Calcium (Ca), Sodium (Na), and Potassium (K).
• These elements are typically expressed as weight percentages of their oxides: SiO2, Al2O3, FeO, Fe2O3, MgO, CaO, Na2O, and K2O.
• SiO2 content is particularly important, defining magma types:
  • Ultramafic: less than 45 wt% SiO2
  • Mafic: 45-52 wt% SiO2
  • Intermediate: 52-63 wt% SiO2
  • Felsic: greater than 63 wt% SiO2
• The total alkali content (Na2O + K2O) is another key parameter used for magma classification.
• The relative proportions of FeO, MgO, and CaO reflect the abundance of mafic minerals (e.g., olivine, pyroxene) in the magma.
• Al2O3 content is influenced by the abundance of feldspars and other aluminosilicate minerals.

Trace Elements in Magmas

• Trace elements occur in magmas at concentrations of parts per million (ppm) or parts per billion (ppb).
• Although present in small amounts, trace elements provide valuable information about magma sources, differentiation processes, and tectonic settings.
• Trace elements are classified based on their behavior during partial melting and fractional crystallization:
  • Compatible elements: preferentially partition into solid phases (e.g., Ni in olivine).
  • Incompatible elements: preferentially partition into the liquid phase (e.g., Rb, Ba, REE in felsic melts).
• Large ion lithophile elements (LILE: Rb, Ba, K) and high field strength elements (HFSE: Nb, Ta, Zr, Hf) are commonly used in geochemical studies.
• Rare earth elements (REE) are a group of 15 lanthanide elements that exhibit systematic variations in ionic radius and charge, making them useful for tracing magmatic processes.

Volatiles in Magmas

• Volatiles are dissolved gases in magmas, primarily H2O, CO2, SO2, HCl, and HF.
• Water is the most abundant volatile component in most magmas, significantly affecting magma viscosity, melting temperature, and eruption style.
• CO2 is generally less abundant than H2O but plays a crucial role in magma degassing and explosive volcanism.
• Sulfur (SO2) contributes to atmospheric pollution during volcanic eruptions and can influence climate.
• Halogens (HCl and HF) are relatively minor components but can enhance the mobility of metals in hydrothermal systems.
• Volatile content determines the explosivity of volcanic eruptions. High volatile content generally leads to more explosive eruptions.

Silicate Melt Structure

• Magma is primarily a silicate melt, with a structure that influences its physical properties.
• Silicate melts consist of a network of SiO4 tetrahedra, where each silicon atom is bonded to four oxygen atoms.
• The degree of polymerization refers to the extent to which these tetrahedra are linked together.
• High SiO2 magmas have a higher degree of polymerization, resulting in higher viscosity.
• The addition of network-modifying cations (e.g., Na+, K+, Ca2+, Mg2+) breaks up the silicate network, reducing polymerization and viscosity.
• Temperature also affects melt structure, with higher temperatures generally leading to lower viscosity.

Magma Series and Differentiation

• Magma series refer to suites of igneous rocks that are related by a common parental magma and a consistent differentiation process.
• Common magma series include:
  • Tholeiitic: characterized by early iron enrichment and relatively low alkali content.
  • Calc-alkaline: characterized by moderate alkali content and oxidation state, commonly associated with subduction zones.
  • Alkaline: characterized by high alkali content relative to silica.
• Magma differentiation involves processes that change the composition of a magma over time, including:
  • Fractional crystallization: removal of crystals from the melt, altering the composition of the remaining liquid.
  • Partial melting: generation of magma by melting only a portion of the source rock.
  • Assimilation: incorporation of surrounding rocks into the magma.
  • Magma mixing: blending of two or more magmas with different compositions.

Controls on Magma Composition

• The composition of a magma is controlled by several factors:
  • Source rock composition: the starting material that undergoes partial melting.
  • Degree of partial melting: the amount of melt extracted from the source rock.
  • Depth of melting: pressure and temperature conditions influence the minerals that melt and the resulting melt composition.
  • Fractional crystallization: removal of crystals changes the liquid composition.
  • Assimilation: incorporation of crustal material can significantly alter magma composition.
  • Magma mixing: blending of different magma types can create hybrid compositions.
  • Tectonic setting: influences the source rocks, melting processes, and differentiation mechanisms.

Magma Evolution

• Magma evolution involves a complex interplay of physical and chemical processes that modify magma composition and properties over time.
• During ascent, magma undergoes decompression, which can lead to crystallization and degassing.
• Crystallization releases latent heat, which can drive further melting and assimilation.
• Degassing can increase magma viscosity and promote explosive eruptions.
• Magma chambers act as temporary reservoirs where magmas can undergo differentiation, mixing, and assimilation.
• The final composition of an igneous rock reflects the cumulative effects of all these processes.

Importance of Magma Composition

• Magma composition is a critical factor in determining the type of volcanic eruption:
  • High-silica magmas are more viscous and gas-rich, leading to explosive eruptions.
  • Low-silica magmas are less viscous and produce effusive eruptions.
• Magma composition influences the type of igneous rocks that form:
  • Mafic magmas form basalt and gabbro.
  • Felsic magmas form granite and rhyolite.
• Magma composition provides insights into the Earth's mantle and crust:
  • Mantle-derived magmas provide information about the composition and processes occurring in the Earth's interior.
  • Crustal-derived magmas reflect the composition and evolution of the continental crust.
• Understanding magma composition is essential for:
  • Assessing volcanic hazards.
  • Exploring for ore deposits.
  • Interpreting the tectonic history of a region.