Rit (Vísindafélag Íslendinga) - 01.06.1984, Blaðsíða 213

mineral chemistry and relationships
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the falling pressure, saturation ofvolatiles in the liquid and a separation ofa
volatile phase take place, by either of two processes according to Vance
(1962): 1) gradual increase in volatiles in the residual liquid caused by
crystallization of volatile free phases; 2) rise of the magma to higher levels
causes decrease in the solubility of volatiles in the liquid. The latter process
is favoured by Vance (1962 and 1965) as the more effective and generally
occurring one. In the third step crystallization takes place again, but now at
lower pressures and from a liquid liberating a volatile phase. Once this
separation of volatiles has set in, it continues through the final crystalliza-
tion of the magma and prevents the previous oscillations in the crystalliza-
tion. The migration of the separating volatiles agitates the liquid and thus
tends to keep it more homogeneous, which prevents oscillatory crystalliza-
tion. The resulting crystallization leads to normal zoning of more albitic
plagioclase than at depths, enclosing the resorbed core and occasionally to
the fdling in of the resorption cavities as well (patchy zoning of cores).
According to this theory the resorption cavities in the cores of the Jan
Mayen plagioclase phenocrysts were formed while the magma moved from
the depths of first plagioclase crystallization to higher levels in the crust
where it may have been temporarily delayed before eruption, giving time for
the marginal zone crystallization and gas release, or it was moved directly to
the surface. Differences in thickness of the marginal zone and other zonal
features (of the different samples), indicate that both cases actually occur-
red. Patchy zoning in the Jan Mayen plagioclases is, as noted previously,
scarce and is of two contrasting types. Occasional samples contain plagio-
clases where irregular patches of small compositional difference occur
showing an even or soft transition. In other samples big resorption cavities
in plagioclase cores contain crystalline groundmass but show plagioclase
lining the walls of these cavities which contrasts in composition to that of the
core but is identical to the marginal zones. This is an incomplete patchy
zoning of the type described by Vance (1965). The other type of patchy
zoning mentioned is very scarce but of unknown origin and significance. In
thejan Mayen case the process of resorption and subsequent crystallization
takes place in a basaltic liquid, quite different from the plagioclase composi-
tion. A polymineralic crystal association in the cavities is thus a reasonable
result to expect if the cavities become sealed ofF from the outside liquid and
in fact it occurs in the cavities of a number of cores. Nevertheless, most of the
resorbed plagioclase cores have resorption cavities filled with dark glass.
According to Vance’s (1965) theory this glass should have crystallized
contemporaneously with the marginal zones, but in fact this is not the case.
fhis might indicate that the internal resorption of the cores was in some
cases still going on while or after the crystallization of the marginal zones.
Modelling of this variety of the “resorption-crystallization” process in
plagioclase has not been undertaken.
Resorbed plagioclases, with marginal zones reversely zoned relative to the