Page 32 - Volcanic Textures A Guide To The Interpretation of Textures In Volcanic Rocks

process that accompanies cooling of hot, coherent glass maintenance of higher temperatures, or the presence, of
and, hence, affects lavas, shallow intrusions and densely aqueous solutions, especially alkali-rich solutions, favor
welded pyroclastic deposits. At least in principle, the development of spherulitic-stage textures. Lofgren
primary, high-temperature devitrification is distinct (1971b) predicted that glass subject to prolonged heat,
from crystallization in response to metamorphism, pressure and solutions would be granophyric, consisting
hydrothermal alteration or weathering. Spherulites, of fine, equigranular quartz and feldspar, and lacking in
lithophysae, orb texture and micropoikilitic texture textural evidence of the former presence of glass.
composed of fine-grained quartz and feldspar are Central parts of very thick (several tens to hundreds of
characteristic products of high-temperature metres), densely welded ignimbrites commonly display
devitrification of silicic glass (Lofgren, 197la, b). granophyric texture due to slow cooling and
Subsequent recrystallization to a mosaic of quartz and crystallization of the formerly glassy components
feldspar can destroy or modify the original (welded shards and pumice) (24.3-4, 28.5).
devitrification textures.
Spherulites
Lofgren (1971a, b; 1974) artificially generated many of
the devitrification textures found in natural rhyolitic Spherulites consist of radiating arrays of crystal fibres
glasses and identified important controls on the rate and (3.1-6, 8.2, 25.3, 25.6, 44.3-7). Each fibre is a single
products of devitrification. The rate of devitrification is crystal that has only slightly different crystallographic
dependent on temperature and on the presence and orientation from adjacent crystals. Spherulites are a
composition of aqueous solutions (Marshall, 1961; characteristic product of the high-temperature
Lofgren, 1970). In particular, below about 300°C or devitrification of natural glass. In formerly glassy silicic
under dry conditions, or if only pure water is present, igneous rocks, crystal fibres consist of alkali feldspar
rates of devitrification are very slow. The presence of and/or quartz (or cristobalite). In mafic rocks, spherulite
alkali-rich solutions increases devitrification rates by fibres consist of plagioclase and/or pyroxene.
four to five orders of magnitude (Lofgren, 1970). OH in
these solutions helps to transform polymeric chains of Spherulites are not spherical throughout their growth
SiO, into separate SiO 4 tetrahedra and allows more history (Lofgren, 1971a; 1974). Lofgren (1971a)
rapid diffusion of Na and K; both changes promote demonstrated that the morphology of spherulites in
crystallization of quartz and feldspar. In addition to the rhyolitic glasses varied according to the temperature of
textural changes noted above, devitrification results in formation (Fig. 14). Spherulites formed at high
significant changes in the bulk rock chemistry, temperatures (700°C) consist of open clusters of widely
particularly affecting SiO 2, H 2O, Na 2O, K 2O and Al 2O 3 spaced crystal fibres. At low temperatures (<400°C),
contents (Lipman, 1965; Lofgren, 1970) and, in some spherulites comprise bundles of radiating fibres. Bow-
cases, trace and rare earth element abundances (Weaver tie-shaped sheafs of fibres characterize intermediate
et al., 1990). temperatures of formation (400°-650°C). In addition,
crystal fibre width increases with increasing temperature
Lofgren (1971b) distinguished two textural associations of formation. The internal crystal fibre structure can be
among the devitrification products of silicic glasses. recrystallized to a quartz-feldspar mosaic as a result of
Glassy-stage texture consists of glass that contains later alteration, metamorphism or deformation.
isolated spherulites (spherulitic obsidian) (44.3). Perlitic
fractures and a variety of quench crystallites may be Spherulites typically have diameters of 0.1-2.0 cm but
present in the glassy portions. This textural stage may be much larger (e.g. 10-20 cm in welded ignimbrite
reflects rapid cooling and low-temperature (200°C) — Steven and Lipman, 1976). Isolated spherulites are
hydration of relatively dry magma. Devitrification is commonly spherical. Adjacent spherulites may impinge
complete in spherulitic-stage texture, in which former on each other and produce elongate trains of spherulites,
glass is crystallized to spherulites and/or micropoikilitic often aligned along flow layering.
texture (3.2, 4). Relatively slow cooling and

Fig. 14 Spherulite morphologies. Outlines of spherical spherulites are often irregular due to impingement on
adjacent spherulites. Bow-tie spherulites consist of two conical bundles of fibres joined at their apices. Plumose
spherulites are open, coarse and commonly fan shaped. Fibres in axiolitic spherulites radiate from a line. Modified
from Lofgren (1974).

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