Abstract
The ascent of hydrous, silica-rich magmas from the lower crust drives volcanic eruptions, builds the upper crust, and concentrates metals such as Cu, Au, and Mo into ore deposits. Owing to the negative slope of the melting curve for granitic materials in the presence of water, it has long been assumed that water-saturated magmas move into the subsolidus field and freeze upon ascent; therefore, for magma to rise it must be water-undersaturated at a temperature well above the solidus. This assumption ignores the considerable energy released by crystallization. Here I show that if magma ascent is treated as an adiabatic, reversible process, then water-saturated magma can rise to the surface, following the solidus to shallow depth and higher temperature as it undergoes modest crystallization and vapor exsolution. Decompression heating is an alternative to magma recharge for explaining pre-eruptive reheating seen in many volcanic systems and accounts for paradoxical growth of quartz during a heating event. The viscosity increase that accompanies vapor exsolution as magma rises to shallow depth explains why silicic magmas tend to stop in the upper crust rather than erupting, producing the observed compositional dichotomy between plutonic and volcanic rocks.
Acknowledgments
This work was inspired by discussions held at a Royal Society meeting on magma reservoirs in November 2017. Careful pre-review by Bill Ussler and Lang Farmer greatly improved the logic and presentation, as did excellent journal reviews by Kelly Russell and Luca Caricchi. This work was supported by the Mary Lily Kenan Flagler Bingham Professorship and National Science Foundation grant 1639724.
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Articles in the same Issue
- Editorial
- Why scientists should study chess
- Highlights and Breakthroughs
- Neither antigorite nor its dehydration is “metastable”
- Geothermometry of the western half of the Central Metasedimentary Belt, Grenville Province, Ontario, and its implications
- Roebling Medal Paper
- An evolutionary system of mineralogy: Proposal for a classification of planetary materials based on natural kind clustering
- The composition and mineralogy of rocky exoplanets: A survey of >4000 stars from the Hypatia Catalog
- New insights into the zircon-reidite phase transition
- The effects of contrasting Ti and Al activities on Mn/Fe systematics in pyroxene from lunar mare basalts
- The condensation temperatures of the elements: A reappraisal
- Estimation of radiation damage in titanites using Raman spectroscopy
- The effect of incorporated carbonate and sodium on the IR spectra of A- and AB-type carbonated apatites
- Re-configuration and interaction of hydrogen sites in olivine at high temperature and high pressure
- The ascent of water-rich magma and decompression heating: A thermodynamic analysis
- High-pressure phase transitions of clinoenstatite
- Compressibility of two Na-rich clinopyroxenes: A synchrotron single-crystal X-ray diffraction study
- Book Review
Articles in the same Issue
- Editorial
- Why scientists should study chess
- Highlights and Breakthroughs
- Neither antigorite nor its dehydration is “metastable”
- Geothermometry of the western half of the Central Metasedimentary Belt, Grenville Province, Ontario, and its implications
- Roebling Medal Paper
- An evolutionary system of mineralogy: Proposal for a classification of planetary materials based on natural kind clustering
- The composition and mineralogy of rocky exoplanets: A survey of >4000 stars from the Hypatia Catalog
- New insights into the zircon-reidite phase transition
- The effects of contrasting Ti and Al activities on Mn/Fe systematics in pyroxene from lunar mare basalts
- The condensation temperatures of the elements: A reappraisal
- Estimation of radiation damage in titanites using Raman spectroscopy
- The effect of incorporated carbonate and sodium on the IR spectra of A- and AB-type carbonated apatites
- Re-configuration and interaction of hydrogen sites in olivine at high temperature and high pressure
- The ascent of water-rich magma and decompression heating: A thermodynamic analysis
- High-pressure phase transitions of clinoenstatite
- Compressibility of two Na-rich clinopyroxenes: A synchrotron single-crystal X-ray diffraction study
- Book Review