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Experimental constraints on the crystallization of natrocarbonatitic lava flows
Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Mineralogy Petrology and Tectonics.ORCID iD: 0000-0002-5433-0908
2009 (English)In: Bulletin of Volcanology, ISSN 0258-8900, E-ISSN 1432-0819, Vol. 71, no 10, p. 1179-1193Article in journal (Refereed) Published
Abstract [en]

Natrocarbonatitic magmas are characterized by their extremely low viscosities and fast elemental diffusion, and as a consequence of this, their chemistry and crystallinity can change significantly during residence in shallow reservoirs or even due to cooling during lava flow emplacement. Here, we present the results of a series of crystallization experiments conducted at 1-atm confining pressure and in a temperature range between 630A degrees C and 300A degrees C. The experiments were set up to characterize the chemistry and growth processes of the phenocryst phases present in natrocarbonatites. The results are applicable to (1) processes occurring during residence in shallow magma reservoirs and/or (2) during lava flow emplacement. We show that during crystallization of natrocarbonatites at atmospheric pressure, gregoryite is the first mineral to crystallize at 630A degrees C, followed by nyerereite at 595A degrees C. Crystal size distributions of the gregoryites show that the crystals grow rapidly by textural coarsening (i.e., Ostwald ripening). As the crystallization is a continuous process at this pressure, the composition of the residual melt changes in response to the crystallization. However, the experiments also show that individual crystals completely reequilibrate with the changes in melt composition in as little time as < 11 min. We therefore conclude that crystallization and diffusion are extremely fast processes in the natrocarbonatitic system and that the measured chemical variations in phenocrysts from Oldoinyo Lengai can be explained by different cooling histories. Finally, we model the rheological control on the emplacement of highly crystallized natrocarbonatitic lavas at Oldoinyo Lengai.

Place, publisher, year, edition, pages
2009. Vol. 71, no 10, p. 1179-1193
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Geology
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URN: urn:nbn:se:uu:diva-375177DOI: 10.1007/s00445-009-0288-4OAI: oai:DiVA.org:uu-375177DiVA, id: diva2:1283062
Available from: 2019-01-28 Created: 2019-01-28 Last updated: 2019-08-08Bibliographically approved

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Mattsson, Hannes B.

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