Probable relations between plagioclase zoning and magma dynamics, Fuego Volcano, Guatemala

Millimeter-sized plagioclase phenocrysts were explosively erupted in high-alumina basaltic ash from Fuego volcano, Guatemala on October 14, 1974. The crystals have unzoned, patchy-zoned and oscillatory-zoned parts. Unzoned, inclusion-poor cores, up to 2 mm thick have round edges and compositions between An₉₇ and An₈₈. Patchy-zoned cores and regions are rich in inclusions of glass and gas, about 50 μm thick and An₉₀ to An₈₃. Oscillatory-zoned regions surround the cores and occur between patchy-zoned regions; compositions are An₉₀ to An₈₃ except for thin rims (An₇₅ to An₇₀).

Patchy-zoned, inclusion-rich regions probably crystallized in gas-saturated environments during episodes of relatively large supersaturation because: (1) such regions are either cores (homogeneous nucleation?) or have flat inner terminations suggestive of a constructive rather than resorptive origin, (2) most such regions are relatively sodic (relatively large concentration gradients in the nearby melt?), (3) such regions terminate outward with irregular, convolute oscillatory zones suggestive of morphological instability, and (4) such regions are rich in inclusions of gas many of which are large and flat against the inner boundary suggesting onset of patchy-zoned growth together with effervescent decompression. Crystallization with effervescence is supported by decreasing Cl/K₂O with increasing K₂O for inclusions of glass. Patchy-zoned regions alternating with oscillatory-zoned regions suggest episodes of accelerated decompression.

Oscillatory zones are 0.5 to 8 μm thick; average internal compositional variation is about 1% An, with the most calcic portion being earliest. Although individual zones are thinnest on {010}, many thin zones on some forms correspond to non-oscillatory-zoned growth on {010} and other slow-growing forms, not cessation of growth. Consequently, zones are generally thinnest and most numerous on the fastest growing forms: {001}, {201}, {110}, and {110}. Discontinuous zones suggest dominant sideways rather than radial growth for individual zones. Thickening of zones on irrational or curved surfaces and in reentrants suggest dominant surface-attachment control of growth rate. Most of the zones correlate between opposite faces of the same crystal form on individual crystals. Some zones correlate between different crystals. The number of zones from the outside of the crystals to patchy-zoned regions or other disturbances cluster around 25 and 50. The various textural and compositional features of the zones are consistent with modulation of growth by hypothetical pulses of upward motion of a gas-saturated magma which sheared the boundary layer of melt near the growing crystals, thereby renewing the level of supersaturation at the crystal face. Tidal triggering of the motion seems likely. The oscillatory zones on the fast growing faces of the Fuego plagioclases probably are twice daily “ticks” of a volcanological “clock”.

Probably the Fuego magma body inherited initial unzoned crystals (cores). Subsequent growth probably accompanied ascent and was alternately patchy or oscillatory, possibly associated with various distances from the walls of the body: a crystal temporarily near a wall might ascend relatively slowly, experience only moderate decompression and supersaturation, and consequently grow only oscillatory zones; a crystal near the center of the body might ascend rapidly, experience greater decompression and supersaturation, and consequently grow some patchy-zoned regions. Explosive eruption could coextrude crystals from the various environments.