Rit (Vísindafélag Íslendinga) - 01.06.1984, Blaðsíða 212

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PÁLL IMSLAND
Lofgren (1974a) found homogeneous centres in crystals showing zoning at
the margins when the crystals grew at a constant temperature and Bottinga
et al. (op.cit.) consider the crystallization of groups of oscillatory zones in
plagioclases to be essentially an isothermal process. The temperature does
thus not seem to be important in these processes. Lofgren (1974a) found in
his experiments features closely resembling the convolute zoning described
from natural plagioclases by Blackerby (1968) and ascribed by the same
author to diíferences in the viscosity of the magma in contact with the
growing crystal. The viscosity of the magma affects the diffusion rate of the
elements in that magma, and since oscillatory zoning results from changes
affected by this diffusion rate, the zoning must depend on the viscosity. The
more viscous the magma, the slower the diffusion rate, and hence crystal
growth, will be. It is thus probable that the oscillatory zoned and unzoned
cores of the plagioclase phenocrysts of the Jan Mayen rocks only reflect
relative changes in the viscosity of the magmas.
These cores of the Jan Mayen plagioclase phenocrysts are enclosed by
marginal zones of relatively contrasting compositions. Mostly these margin-
al zones are Ab richer than the cores, but An richer margins occur in some
samples. The process of this marginal zoning is frequently accompanied or
preceded by resorption. The resorption features are of two types: 1) resorp-
tion cavities, occurring irregularly distributed throughout the core, and 2) a
distinct zone of small resorption cavities between the core and the marginal
zones. It seems to be a general rule that the distinct resorption zones are
enclosed by more An rich margins than are the cores (reverse zoning), while
the marginal zones on the cores containing irregularly distributed resorption
cavities are primarily Ab richer than the cores (normal zoning). We are thus
most probably dealing with two processes of origin of accompanied zoning
and resorption.
Vance (1962 and 1965) i.a. has discussed the appearance and origin of
normal zoning in igneous plagioclase and the frequently accompanying
patchy zoning of the crystal cores. Most of his studies centre around granitic
rocks, but the features of zoning and resorption described are the same as in
the Jan Mayen basalts, and his discussions are oriented towards general
magmatic environments. He simplifles the evolution of plagioclase showing
these features to three steps: 1) crystallization of plagioclase at depths; 2)
partial resorption of the plagioclase related to a fall in pressure; 3) renewed
crystallization at lower pressure, necessitating development of more sodic
plagioclase. His first steps would correspond to the cores of the Jan Mayen
phenocrysts. During this step, crystallization from magma, undersaturated
in water or other volatiles, takes place. In the second step the falling
pressure affects both the liquidus and the solidus of plagioclase such that
they are moved towards lower temperatures (Vance 1965; Yoder, 1968).
The moving of the magma upwards thus takes the plagioclase into an
environment of lower melting point and its resorption takes place. Following