Polybaric fractional crystallisation of arc magmas - an experimental study
Abstract
Differentiation of mantle-derived, hydrous calc-alkaline magmas by fractional crystallisation is now generally accepted as one of the main driving mechanism controlling the compositional evolution of arc magmas. However, previous experimental studies failed to reproduce the predominantly metaluminous natural rock record by lower crustal crystal fractionation. At high pressures, experimental liquids rather evolve towards peraluminous differentiates. The absence of abundant cumulates in the upper crust does not support the alternative scenario of major differentiation at low pressure conditions. Therefore, we propose an alternative process, namely polybaric fractional crystallisation, inferring that arc magmas differentiate progressively by crystal fractionation and interaction with crystal mushes/cumulates upon ascent through the crust.
This hypothesis is tested through a series of experiments along several pressure - temperature ascent trajectories, simulating the buoyant rise of differentiating magmas in the crust. Running concomitant equilibrium (closed-system) crystallisation experiments allows us to directly elaborate the impact of crystal fractionation on magma differentiation. Phase equilibria data, chemical compositions of mineral phases, liquid lines of descent as well as the evolution of crystal/melt ratios provide crucial information to improve our understanding of the evolution of the calc-alkaline magmatic series and clarify if fractional crystallisation can be considered as a major process in the differentiation of arc magmas. Experimental results support theoretical considerations on the effect of decreasing pressure on mineral phase equilibria: the olivine-clinopyroxene cotectic curve is shifted towards more cpx-rich compositions (equivalent to a destabilisation of clinopyroxene) rendering residual melts more metaluminous and, therefore, circumventing a rapid evolution of liquid lines of descent towards and into the peraluminous compositional field. Enhanced plagioclase precipitation with decreasing pressure further affects the liquid line of descent. The polybaric evolution of experimental liquids more closely approaches the prevalent compositions of calc-alkaline intermediate to SiO2-rich plutonic and volcanic rocks observed in nature.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFM.V13A..04M
- Keywords:
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- 1036 Magma chamber processes;
- GEOCHEMISTRY;
- 1199 General or miscellaneous;
- GEOCHRONOLOGY;
- 7299 General or miscellaneous;
- SEISMOLOGY;
- 8439 Physics and chemistry of magma bodies;
- VOLCANOLOGY