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The cooling kinetics of plagioclase feldspar as revealed by electron-microprobe mapping

  • Gianluca Iezzi EMAIL logo , Silvio Mollo , Edisa Shahini , Andrea Cavallo and Piergiorgio Scarlato
Published/Copyright: May 15, 2014
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American Mineralogist
From the journal American Mineralogist Volume 99 Issue 5-6

Abstract

In this study, we have used electron-microprobe mapping to investigate plagioclase compositional evolution due to cooling kinetics. We re-analyzed five run-products from a prior study (Iezzi et al. 2011), crystallized by cooling a natural andesitic melt from 1300 to 800 °C at 25, 12.5, 3, 0.5, and 0.125 °C/ min under atmospheric pressure and air redox state. As the cooling rate decreases, the texture of large plagioclases changes from skeletal to hollow to nearly equant. In this study, we use X‑ray map data to obtain a database of 12 275 quantitative chemical analyses. The frequency of An-rich plagioclases showing disequilibrium compositions substantially increases with increasing cooling rate. At 25 and 12.5 °C/min the distribution is single-mode and narrow, at 0.5 and 0.125 °C/min is single-mode but very broad, whereas at the intermediate cooling rate of 3 °C/min two distinct plagioclase populations are present. This intermediate cooling rate is fast enough to cause departure from equilibrium for the crystallization of the An-rich population but also sufficiently slow that An-poor plagioclases nucleate from the residual melt. We interpret our findings in the context of time-temperature-transformation (TTT) diagrams, and infer the crystallization kinetics of plagioclase in the experiments. Compositional trends and our inferences regarding TTT systematics are consistent with two discrete nucleation events that produced separate populations of plagioclase (i.e., An-rich and An-poor populations) at 3 °C/min. Using plagioclase-melt pairs as input data for the thermometric reaction between An and Ab components, we find that plagioclase mirrors very high- (near-liquidus) crystallization temperatures with increasing cooling rate. These results have important implications for the estimate of post-eruptive solidification conditions. Lava flows and intrusive bodies from centimeters to a few meters thick are characterized by a short solidification time and a significant thermal diffusion. Under such circumstances, it is possible to crystallize plagioclases with variable and disequilibrium chemical compositions simply by cooling a homogeneous andesitic melt. X‑ray element maps enrich the study of plagioclase compositional variations generated under conditions of rapid cooling.

Keywords : Andesite; plagioclase; EPMA map; cooling; crystallization; thermometer
Published Online: 2014-5-15
Published in Print: 2014-5-1

© 2014 by Walter de Gruyter Berlin/Boston

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https://doi.org/10.2138/am.2014.4626
Keywords for this article
Andesite; plagioclase; EPMA map; cooling; crystallization; thermometer

Articles in the same Issue

  1. Highlights and Breakthroughs
  2. Absence of pressure-induced electron spin-state transition of iron in silicate glasses upon compression
  3. Dolomite discloses a hidden history of subducting slabs
  4. Crossroads in Earth and Planetary Materials
  5. Crystal structures of laihunite and intermediate phases between laihunite-1M and fayalite: Z-contrast imaging and ab initio study
  6. Minerals in the Human Body
  7. A novel technique for fluorapatite synthesis and the thermodynamic mixing behavior of F-OH apatite crystalline solutions
  8. Volcanic Rocks
  9. The cooling kinetics of plagioclase feldspar as revealed by electron-microprobe mapping
  10. Spinels Renaissance–Past, Present, and Future
  11. High-pressure behavior of thiospinel CuCr2S4
  12. Martian Rocks and Soil
  13. Mössbauer parameters of iron in phosphate minerals: Implications for interpretation of martian data
  14. A temperature-controlled sample stage for in situ micro-X-ray diffraction: Application to Mars analog mirabilite-bearing perennial cold spring precipitate mineralogy
  15. Detection of iron substitution in natroalunite-natrojarosite solid solutions and potential implications for Mars
  16. Articles
  17. Harmunite CaFe2O4: A new mineral from the Jabel Harmun, West Bank, Palestinian Autonomy, Israel
  18. An assessment of the reliability of melt inclusions as recorders of the pre-eruptive volatile content of magmas
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  21. Role of silica for the progress of serpentinization reactions: Constraints from successive changes in mineralogical textures of serpentinites from Iwanaidake ultramafic body, Japan
  22. Ophirite, Ca2Mg4[Zn2Mn23+(H2O)2(Fe3+W9O34)2]·46H2O, a new mineral with a heteropolytungstate tri-lacunary Keggin anion
  23. Bubble formation during decompression of andesitic melts
  24. Manganese carbonate formation from amorphous and nanocrystalline precursors: Thermodynamics and geochemical relevance
  25. Insights into the crystal chemistry of Earth materials rendered by electron density distributions: Pauling’s rules revisited
  26. Adsorption of sulfur dioxide on volcanic ashes
  27. Sulfidation of native gold
  28. Electrical conductivity of synthetic mullite single crystals
  29. In situ Raman spectroscopy identification of the S3̅ ion in S-rich hydrothermal fluids from synthetic fluid inclusions
  30. Melting phase equilibria of model carbonated peridotite from 8 to 12 GPa in the system CaO-MgO-Al2O3-SiO2-CO2 and kimberlitic liquids in the Earth’s upper mantle
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  47. Acceptance of the Dana Medal of the Mineralogical Society of America for 2014
  48. Book Review
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