Rit (Vísindafélag Íslendinga) - 01.06.1984, Síða 219

mineral chemistry and relationships
215
increases with increasing pressure (Moore, 1965). Alkaline magmas, rich in
water, moving upwards from depths, could thus be supposed to release
volatiles at greater depths than subalkaline magmas. Vesicles are known in
ptllows extruded on the ocean floor down to ~4.5 km (Moore, 1965) which
equals ~0.5 kb pressure. Vesicles occur in dikes down to considerable
depth. In the current episode of rifting and magmatism in the Krafla fissure
swarm in Northern Iceland (Björnsson et al., 1977), changes in composition
of the gases, of the hydrothermal system of the area, accompany magmatic
events. These changes are rooted in gas release from the magma (Óskars-
son, 1978). The magma is collected in a stationary magma reservoir lying
between 7 and 3 km depth in the rift zone (Einarsson, 1978). Here the
tholeiitic magma is intruded and collected over a period of several months
and then moves over a period of several hours out of the magma chamber
•nto the fissure system towards the north and south, and occasionally erupts
m small amounts. This shows that magma relatively poor in voladles can
release these at depths corresponding to at least 1 kb pressure. The alkaline
Jan Mayen magmas may thus be expected to be able to release gases at even
more than 1 kb pressure. It is thus not unreasonable to estimate the depth of
microphenocryst crystallization in the Jan Mayen basalts as starting
between 1 and 2 kb pressure or at ~5 km depth.
Optical pyrometer determinations of the temperature of the glowing lava,
flowing from the vents of the 1970 eruption on NE Jan Mayen, made by
Sigvaldason (personal communication and Siggerud, 1972) gave 1010—
1030°C. These determinations indicate that the basaltic lava was at least
1030°C at the time of eruption, when the groundmass was still uncrystal-
bzed. These temperatures are in good agreement with the oxide tempera-
tures obtained for microphenocrysts. The crystallization of the groundmass
of the Jan Mayen basalts seems thus to have taken place at temperatures
below ~1040°C.
In summary a generalized crystallization history of the plagioclases of the
basalts may have been as follows: Phenocryst cores started to crystallize at
depths probably approaching 10 kb pressure at temperatures between 1160
und 1200°C. These cores occasionally became resorbed en route to a near
surface magma reservoir at ~5 km depth, where marginal zones became
deposited on the resorbed cores and microphenocrysts started to crystallize.
I his crystallization continued during the final ascent of the magma and took
place between 1040 and 1090°C. The groundmass plagioclase crystallized at
temperatures below 1040°C after extrusion.
Trachytes are generally believed to crystallize at higher temperatures
than both rhyolites and phonolites where the liquidus temperatures are
lowered by silica and nepheline respectively (Carmichael, 1963 and 1965
and Rahman & MacKenzie, 1969).
The feldspars of the more evolved rocks of the Jan Mayen rock suite, the
tristanites and trachytes, range from plagioclase via anorthoclase to more