Rit (Vísindafélag Íslendinga) - 01.06.1984, Blaðsíða 211

MINERAL CHEMISTRY AND RELATIONSHIPS
207
appears in other published feldspar analyses and usually shows comparable
amounts to the ones mentioned here. This might indicate that Ti is
generally present as a trace element in alkali feldspars as well as plagio-
clases. Rahman & MacKenzie (op.cit.) analysed their separates for Mn as
well, but it was not detected. A superficial scan of published feldspar
analyses indicates that Mn and Cr are not generally present in feldspar
minerals, but occur occasionally in low concentrations.
III. Zoning of the feldspars
Zoning in igneous plagioclase is most frequently ascribed to one of three
processes or a combination of these: 1) changes in physical or chemical
parameters (temperature, pressure, volatiles, etc.) controlled by external
changes and in most cases advancing crystallization, 2) the internally
controlled constitutional supercooling (see Sibley et al., 1976) and by 3)
mixing of compositionally different magmas.
The Jan Mayen plagioclases reveal a variety of zoning features, as
previously described, which reflect a complex crystallization history or a
complex interplay of crystals and magma at changing conditions of external
and internal factors. Some information on this crystallization history and its
causes, which can be interpreted from the zoning pattern, will be discussed
here.
The frequently occurring regular oscillatory zoning in cores of plagioclase
crystals of coarse-grained rocks and phenocrysts has been described by
Vance (1962), Bottinga et al. (1968) and Sibley et al. (1976) as the result of
repeated changes in the crystal growth rate and the diffusion rate of the
elements in the liquid. In controlled experiments on plagioclase crystalliza-
tion, Lofgren (1974a) found reversed oscillatory zoning when the liquid was
only slightly undercooled. Lofgren (1974b) further found the crystal
morphology of the plagioclase tp be dependent on the amount of the
undercooling of the liquids and the ratio between the diffusion rate of the
elements in the liquids and the growth rate of the crystals. Under crystalliza-
tion of slight undercooling and D/G close to unity (D=diffusion coefffcient
of the components rejected by plagioclase, G=growth rate of plagioclase),
plagioclase obtained a tabular form during growth, in contrast to skeletal,
dentritic and spherulitic forms when D/G was less than unity and the
undercooling increased. The plagioclase phenocrysts of the Jan Mayen
rocks all have equant to tabular crystal form and have thus probably all
crystallized at minimal undercooling. The contrasting zoning patterns,
oscillatory zoning and non-zoning revealed by these crystal cores, need some
consideration. Both types make up the cores of early formed phenocrysts in
the basalts and are thus most probably formed at high temperatures and
pressures from only slightly undercooled magmas. What then causes these
differences? Pressure changes alone are not likely to be of importance.