Magma Evolution and Amphibole Dynamics in Volcanic Systems

Magmas generated in Earth’s mantle evolve through a complex interplay of crystallisation, mixing, assimilation and volatile exsolution as they ascend through the crust. Hydrous phases, and amphibole in particular, exert a profound control on the chemical differentiation of arc and continental magmas by sequestering water, trace elements and large-ion lithophile elements deep in the crust. In trans-crustal systems, amphibole can crystallise either as phenocrysts or in cryptic assemblages formed by reaction-replacement of earlier minerals, notably clinopyroxene, thereby creating ‘amphibole sponge’ lithologies that modify melt compositions without leaving abundant crystal records at the surface. Amphibole stability is highly sensitive to pressure, temperature, oxygen fugacity and melt water content, making it both a regulator of magma buoyancy and a recorder of physicochemical processes such as magma mixing, recharge and mush replenishment. Understanding amphibole dynamics thus provides vital insights into crustal growth, magma storage depths, eruption triggers and the evolution of volcanic hazards on a global scale.

Research from Nature Portfolio

Seminal work has demonstrated that amphibole in arc lower crust may form primarily by reaction-replacement of clinopyroxene within mush zones rather than by direct crystallisation from melt. Such studies reveal that evolved hydrous melts overprint existing clinopyroxene crystals to produce granoblastic amphibole assemblages bearing primitive trace-element signatures. These amphibole assemblies act as ‘sponges’, cryptically fractionating water and key trace elements, and imprinting La/Yb and Dy/Yb ratios on residual magmas. This mechanism explains the apparent scarcity of amphibole phenocrysts in erupted lavas, reconciles trace-element geochemistry of arc basalts with petrological observations, and refines models of lower crustal differentiation beneath volcanoes.

Magma Evolution and Amphibole Dynamics in Volcanic Systems publication trend

The graph below shows the total number of articles in magma evolution and amphibole dynamics in volcanic systems across all publications each year (not limited to Nature Index journals).

Technical terms

Amphibole: A group of hydrous double-chain silicate minerals common in intermediate to silicic magmas, sensitive to pressure, temperature and water content.

Cryptic fractionation: Chemical modification of magma by mineral reactions in mush zones that do not produce abundant phenocrysts in erupted lavas.

Peritectic reaction: A mineral-reaction in which an earlier crystallised phase and melt react to form a new mineral, often controlling amphibole appearance.

Thermobarometry: Quantitative estimation of the pressure and temperature conditions of mineral crystallisation from compositional data.

Crystal mush: A partially molten aggregate of crystals and interstitial melt that constitutes a major storage medium in volcanic plumbing systems.

Cumulate: A rock formed by the accumulation of early-crystallised minerals in a magma chamber, recording fractionation processes at depth.

References

1. The Capability of Amphibole in Tracing the Physicochemical Processes of Magma Mixing. Geophysical Research Letters (2024).
2. Clinopyroxene precursors to amphibole sponge in arc crust. Nature Communications (2014).
3. Amp-TB2: An Updated Model for Calcic Amphibole Thermobarometry. Minerals (2021).
4. Polybaric fractional crystallisation of arc magmas: an experimental study simulating trans-crustal magmatic systems. Contributions to Mineralogy and Petrology (2021).