Jökull - 01.12.1979, Page 72

transparent, but usually yellow to brown alteration
product of the sideromelane. ~—
The palagonitization starts on the surface of the
glass grains and proceeds inward at a speed which
is mainly dependent on moisture and temperature.
In the island of Surtsey, which was created by
volcanic eruptions during 1963—1967, it has been
possible to follow the posteruptional process of
palagonitization and consolidation of basalt tephra
at temperatures between about 35° and 100°C
(Fig. 10). This is a case of mild hydrothermal
activity and at temperatures of about 80° — 100°C,
the tephra was converted to dense palagonitized
tuff in only 1—2 years (Fig. 11).
Palagonitization and consolidation proceeds
much more slowly at subaerial weathering
conditions (up to about 20°C) and according to
present data it may take several thousand years
before any substantial palagonitization occurs.
Palagonitization can be termed a
microsolution:precipitation mechanism. The main
components to be leached out of the sideromelane
are in order on a volume basis: Na20, CaO, A1203,
K20, Si02 and MgO. Instead, HaO enters the glass
and ferrous iron is oxidized to ferric iron to give the
rock the rust brown colour. Table 3 shows typical
sideromelane-palagonite analyses on weight per
cent basis.
The ions which are leached out of the glass form
secondary minerals in the hyaloclastite, which help
to cement the rock together. Most common as
secondary minerals are calcite, chabasite, scolesite,
analcite and opal. Smectites and other clay
minerals form in the palagonite at an advanced
stage of alteration (“fibropalagonite”). It appears
probable that palagonitization in Iceland usually
occurs at subaerial weathering conditions, and
locally by mild hydrothermal activity, where feeder
dykes, other intrusious and possibly pillow lavas act
as heat source.
Burial metamorphism
As the lava pile is becoming thickened by
continuous accumulation of lavas in a volcanic
zone, geoisotherms will rise in the lava pile
resulting in the alteration of the rocks. Geological
mapping in eastern Iceland during the 1950’s
revealed that the secondary minerals formed in the
lava pile, especially zeolites, have a regular
distribution. They occupy near horizontal zones,
which bear no relationship to the stratigraphy of
the lava pile. Several zones, each with distinct
mineral assemblages, have been established (Fig.
12), and these zones are inferred to be approximately
paralled to the top of the lava pile at the time of
zeolitization. Subsequent work has shown that a
similar regional zoning of secondary (amygdale)
minerals exists in the Tertiary basalt lavas of
northern and western Iceland. .This zonal
distribution, along with studies on dyke density,
has made it possible to estimate the height of the
original surface of the lava pile. Up to 1800 m may
have been eroded in southeastern Iceland, some
1000 m in the Eastern Fjords, but only a few
hundred meters may be missing on the Northwest
Peninsula.
The lowest-temperature metamorphism, the
zeolitic facies, is characterized in particular by the
zeolite laumontite. The regional occurrence of this
mineral in the lowest part of the lava pile in eastern
and southeastern Iceland (Fig. 12) therefore
Fig. 12. Diagrammatic section across the Tertiary lava pile in eastern Iceland showing the zonal
distribution of amygdale minerals. The western end of the section corresponds to sections in upper
Jökuldalur and Fljótsdalur; the eastern half corresponds to exposures in the Eastern Fjordlands. After
Walker 1960.
70 JÖKULL 29. ÁR