A natural example of the disequilibrium breakdown of biotite at high temperature: TEM observations and comparison with experimental kinetic data

Abstract

Transmission electron microscopy has been used to investigate the mechanism of natural biotite breakdown under pyrometamorphic disequilibrium conditions. Biotite in a xenolith of pelitic gneiss collected from a Tertiary dolerite sill, Isle of Mull, Scotland, shows evidence of an incipient reaction, characterised by a darkening in colour and the appearance of areas of fine-grained reaction products. TEM and analytical electron microscope data show that the reaction can be described as:Fe-A1 biotite --* Mg-A1 biotite + magnetite + hercynitic spinel + K-feldspar/melt + vapour. The orientations of the product phase are controlled by the crystallography of the reacting biotite, demonstrating that the transformation proceeds by a topotactic mechanism. An empirical method, based on the Mg/(Fe 2+ + Fe 3 +) ratios of coexisting spinel and biotite from experimental data, is used to deduce that the reaction occurred above ~ 770 ~ A comparison of the natural reaction microstructures with those produced experimentally suggest that the xenolith was probably above 800 ~ for less than 48 hours and cooled to temperatures of 770 ~ after ~ 150-200 hours. K E Y W O R D S :transmission electron microscopy, biotite, pyrometamorphism.

Origin of high-silica liquids at Stromboli volcano (Aeolian Islands, Italy) inferred from crustal xenoliths

“…3f) but only partially reacted to form Ti-magnetites (TiO 2 7.9±8.9%; Al 2 O 3 3.8±5.3%; Table 11). These textural and chemical transformations of the biotite, with or without the presence of the melt, are consistent with the results of Day and Allen (1925) and Knesel and Davidson (1999) and with the biotite breakdown reaction proposed by Brearley (1987 Restitic plagioclase showing spongy texture is An 38±49 Ab 48±58 Or 3±4 (Table 9). Quenched microlites of plagioclase (swallow-tailed and hollow-shaped) are An 35±43 Ab 53±58 Or 3±8 (Table 9).…”

Transmission electron microscopy (TEM) applied to ancient pottery

“…Detailed textural, compositional and crystal structural data for mineral assemblages define the extent of reactions between coexisting minerals, and define the specific reaction processes. Such relations depend on both the temperature and the duration of reaction, as demonstrated for naturally 'fired' rocks that have been subjected to high temperatures for relatively short periods of times, a process called pyrometamorphism (Brearley 1987;Worden et al 1987;Clark and Peacor 1992). Those relations are therefore especially significant for determining characteristics of source materials, temperatures and durations of firing, or the kind and degree of post-burial alteration of ceramic materials.…”

Pyrometamorphism and partial melting of shales during combustion metamorphism: mineralogical, textural, and chemical effects

“…Scanning Cosca et al (1989). Sample locality is marked as shown electron microscopy (SEM) and scanning transmission electron microscopy (STEM) resolve the textures of these rocks and have been used to examine reactions in pelitic xenoliths (Grapes 1986;Brearley 1986Brearley , 1987 and alteration of phyllosilicates in contact aureoles (Smith 1969;Wirth 1985;Worden et al 1987). However, those studies involved mineralogical changes that occurred at depth at lower temperatures, higher fluid pressures, and over a longer time period.…”