Magma chambers are the heart of volcanic systems, where molten rock accumulates and evolves. These dynamic environments shape the composition and behavior of magmas, influencing volcanic eruptions. Understanding magma chamber processes is crucial for predicting volcanic activity and interpreting the rock record.

From formation to differentiation, magma chambers undergo complex changes. Fractional crystallization, assimilation, and mixing alter magma composition, while internal structures develop through cooling and convection. These processes create diverse magma types and volcanic products, shaping Earth's crust and surface landscapes.

Magma chamber formation and structure

Magma chamber development

• Magma chambers form when magma rising from depth stalls and accumulates in the Earth's crust
  • Typical depths range from a few kilometers to a few tens of kilometers
• The formation of magma chambers is influenced by various factors
  • Magma supply rate determines the amount of magma available for chamber growth
  • Magma composition affects its density and viscosity, which influence its ability to rise and accumulate
  • Properties of the surrounding crustal rocks (density, strength, thermal conductivity) control the ease of magma ascent and storage
• Magma chambers can have various shapes and sizes
  • Small, isolated pockets of magma (individual chambers beneath volcanoes)
  • Large, interconnected reservoirs (extensive magmatic systems)

Internal structure and complexity

• The structure of magma chambers often includes distinct zones
  • Main magma body represents the bulk of the stored magma
  • Solidified outer shell forms as the magma cools and crystallizes along the chamber margins
  • Partially molten or mushy zone exists between the main magma body and the solidified shell
• Magma chambers can develop complex internal structures due to various processes
  • Magma differentiation leads to the formation of compositionally distinct layers or regions within the chamber
  • Crystallization results in the growth of minerals and the development of cumulate layers
  • Convection currents within the magma chamber can redistribute heat and crystals, creating heterogeneities

Magma differentiation and crystallization

Fractional crystallization and Bowen's reaction series

• Magma differentiation is the process by which a single parent magma evolves into different magma compositions
  • Driven by various physical and chemical processes
• Fractional crystallization is a key process in magma differentiation
  • Early-forming crystals are removed from the melt, changing the composition of the remaining magma
  • Occurs through crystal settling, compaction, or filter pressing
• The order of mineral crystallization in a magma is determined by the Bowen's reaction series
  • Describes the sequence of mineral formation based on their melting temperatures and compositions
  • Continuous series (plagioclase feldspars) and discontinuous series (olivine, pyroxene, amphibole, biotite)
• Crystal settling and compaction can lead to the formation of cumulate rocks
  • Accumulation of early-formed crystals at the base of magma chambers
  • Enriches the remaining magma in incompatible elements (potassium, sodium, titanium)

Alternative differentiation processes

• Magmatic differentiation can also occur through liquid immiscibility
  • Magma separates into two or more distinct liquid phases with different compositions
  • Example: separation of a silicate melt and a sulfide melt in mafic magmas
• Magmatic differentiation can be influenced by volatile exsolution
  • Release of dissolved gases (water, carbon dioxide, sulfur dioxide) from the magma
  • Affects the stability of minerals and the evolution of the remaining melt
• Thermal diffusion and Soret fractionation can contribute to magma differentiation
  • Elements migrate in response to thermal gradients within the magma chamber
  • Results in the concentration of certain elements in hotter or cooler regions of the chamber

Magmatic assimilation and composition

Assimilation process and controls

• Magmatic assimilation is the process by which magma incorporates and melts surrounding crustal rocks
  • Changes magma composition and potentially its physical properties
• The extent of assimilation depends on various factors
  • Temperature and composition of the magma
  • Composition and melting temperature of the crustal rocks
  • Duration of contact between the magma and the country rocks
• Assimilation is more likely to occur when the magma is hot and the crustal rocks are relatively fusible
  • Mafic magmas have higher temperatures and are more likely to assimilate crustal material than felsic magmas
  • Sedimentary rocks (shales, limestones) are more easily assimilated than crystalline basement rocks (granites, gneisses)

Compositional and isotopic effects

• Assimilation can lead to the enrichment of magma in certain elements
  • Silica, alkalis (sodium, potassium), and volatiles are commonly enriched
  • Depletion of other elements depends on the composition of the assimilated material
• Assimilation can affect the isotopic composition of the magma
  • Incorporated crustal material may have distinct isotopic signatures compared to the original magma
  • Radiogenic isotopes (strontium, neodymium, lead) are commonly used to trace assimilation processes
• The energy required for assimilation can lead to cooling and crystallization of the magma
  • Triggers eruptions or the formation of intrusive bodies (sills, dikes)
  • Assimilation-fractional crystallization (AFC) processes can significantly modify magma compositions

Magma mixing and mingling

Mixing processes and efficiency

• Magma mixing involves the physical and chemical interaction between two or more magmas
  • Magmas have different compositions and temperatures
  • Mixing occurs within a magma chamber or conduit
• Magma mingling refers to the incomplete mixing of magmas
  • Results in the formation of distinct domains or enclaves of one magma within another
  • Preserved evidence of the original magma compositions and textures
• Mixing and mingling can occur in various scenarios
  • Injection of new magma pulses into an existing magma chamber
  • Convective currents within the chamber bring different magma compositions into contact
• The efficiency of magma mixing depends on several factors
  • Viscosity contrast between the magmas (lower contrast promotes mixing)
  • Volume ratio of the interacting magmas (similar volumes enhance mixing)
  • Duration of the mixing process (longer time allows for more complete homogenization)

Textural and compositional indicators

• Magma mixing can lead to the formation of hybrid magmas
  • Intermediate compositions between the end-member magmas
  • Development of disequilibrium textures in the resulting volcanic rocks
• Disequilibrium textures provide evidence for magma mixing
  • Reverse zoning in crystals (core composition differs from rim composition)
  • Resorption textures (partial dissolution of crystals)
  • Sieve textures (fine-grained inclusions within crystals)
• Magma mingling can result in distinctive textural features
  • Mafic enclaves (inclusions of one magma within another)
  • Banded pumices (alternating layers of different magma compositions)
  • Disaggregated enclaves and crystal transfer between magmas
• Geochemical and isotopic analyses can reveal the extent and timing of magma mixing
  • Linear trends in element variation diagrams indicate mixing between end-member compositions
  • Isotopic disequilibrium between minerals and host magma suggests mixing shortly before eruption