Unravelling disequilibrium in basaltic crystal cargoes: an experimental and petrological study of mixing in primitive Icelandic magmas

• Applicant: David Neave, Ph.D.
• Subject Area: Mineralogy, Petrology and Geochemistry
• Term: from 2017 to 2021
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 345810429

Oceanic basalts, which form the bulk of material erupted at the surface Earth, are complex hybrids of melts and crystals assembled during mixing and crystallisation of melts during ascent from the mantle and processing in magma chambers. Records of disequilibrium, i.e., the juxtaposition of two or more phase assemblages that are out of equilibrium with each other, are widely preserved in the textures and compositions of basaltic crystal cargoes. Realistic models of magma assembly and evolution must therefore incorporate the effects of disequilibrium into their descriptions of magmatic processes. Although phase equilibria studies have provided innumerable insights into the nature of magmatic processes, the validity of using equilibrium experiments to understand disequilibrium features in natural samples remains unclear. Carrying out well-constrained disequilibrium experiments to understand reaction kinetics is thus a crucial next step in improving the interpretation of natural basalts.The primary aim of this project is to document quantitatively how the crystal contents of basaltic magmas respond to chemical and thermal disequilibrium as a result of magma mixing. This will be achieved by performing high-pressure (3 kbar), high-temperature (1160-1280 °C) experiments designed to the mimic mixing processes that have been identified in well-characterised natural basalts from Iceland. In order to separate the effects of chemical and thermal disequilibrium as much as possible, one series of experiments will investigate the isothermal and isobaric mixing of crystal-bearing and compositionally heterogeneous primitive basalts (open system chemical disequilibrium). A second series will investigate the effect of cooling and heating on a single crystal-bearing primitive magma composition, simulating entrainment and recharge respectively (closed system thermal disequilibrium). Disequilibrium experiments will only be performed on starting compositions for which equilibrium phase relationships have already been determined in a previous project. Run products will be imaged (BSE) and analysed to quantify the textural (e.g. crystal size distribution) and compositional response of crystals to both chemical and thermal disequilibrium. Rates of crystal growth, dissolution and re-equilibration will be evaluated by performing experiments over a range of timescales that overlap with those of pre-eruptive unrest in Icelandic volcanic systems. The new experimental data will then be used to re-assess disequilibrium features in natural crystals from a selection of Icelandic eruptions including the 1784-1784 Laki and 2014-2015 Holuhraun eruptions. For example, the ability of mixing to bias the crystalline record of magmatic processes will be investigated. While this project will focus on Icelandic systems, because of their excellent geological context, its findings will be widely relevant to any basalts that carry records of disequilibrium in their crystal cargoes.

• DFG Programme: Research Grants
• International Connection: Iceland
• Co-Investigators: Professor Dr. Francois Holtz; Professor Dr. Jürgen Koepke; Professor Dr. Olivier Namur
• Cooperation Partners: Professorin Dr. Enikö Bali; Professor Dr. Harald Behrens; Dr. Sæmundur Halldórsson; Professor Dr. Thorvaldur Thordarson