Náttúrufræðingurinn - 1989, Blaðsíða 21

thermal evolution and the age of the as-
phalt.
Retreat of the glacier since 1945 (fig. 3)
may be one of the reasons that the as-
phalt-bearing amygdales were found. Oth-
erwise, weathering would most likely have
masked the outcrop and definitely spoiled
the low resistant asphaltic petroleum. The
petrological study of the asphalt-bearing
lava shows the lava matrix to be quite
dense, while large vesicles and primary
fractures are quite common. Chemical
analyses of this lava as well as the under-
lying and overlying lavas are presented in
Table 1. The petroleum seems to have
penetrated the lava via fractures, occa-
sionally to become trapped in half-empty
amygdales.
Study of the secondary mineralization
indicates that three hydrothermal episodes
may be distinguished. The first one began
during burial, and involved low tempera-
ture hydrothermal alteration products
(siderite, opaline silica, zeolite, smectite,
calcite, celadonite and limonite). The
temperature at formation is estimated to
have been 40-80°C (Table 2). Small ves-
icles and primary fractures are more or
less filled by these early-forming minerals,
while the larger amygdales and some sec-
ondary veins, are lined by some of them.
The larger amygdales were precipitated in
a second hydrothermal episode, when
high-temperature effluent, saturated with
respect to calcite and quartz, flowed into
cooler fissures between the volcanoes.
The cooling resulted in widespread
precipitation of calcite, silica minerals
(agate, quartz and jasper), pyrite and pos-
sibly tridymite. Unsuccessful attempts
were made to nreasure the homogeniza-
tion temperature in fluid inclusions in
quartz and calcite. The lack of inclusions
may imply slow crystallization during
growth of both the calcite and the quartz.
The water temperature is estimated to
have been between 80-120°C. The palaeo-
surface during the second hydrothermal
episode is believed to liave been only
about 300-500 m above the asphalt-bear-
ing lava. This is deduced from the absence
of calcite above 600 m altitude as well as a
5 degree tilting unconformity as mapped
by Torfason (1979).
Later both the central volcanoes dis-
cussed were buried by an approximately 1
km thick lava succession. During burial,
regional zeolitization was superimposed
on the earlier hydrothermal alteration.
The maximum depth to the asphalt-bear-
ing lava during this time may have been
nearly 1.5 km. The asphaltic petroleum
may have formed during this time, from
lignite beds. The asphalt was thus, along
with quartz, deposited during a third
hydrothermal episode.
The asphaltic petroleum occurs in two
ways in the amygdales. Either it fills the
space between quartz crystals (fig. 5) or it
occurs as spherical bodies on the floor of
the amygdales, usually covered with fine
grained rock crystal (front page and fig.
6). The asphalt spheres range between 2
and 8 mm in diameter. The result of liq-
uid-solid chromatography of two asphalt
samples are shown in Table 3. Non-hydro-
carbons and aromatic hydrocarbons are
the major constituents. Figs. 7A and B
show the results of the gas chromato-
graphic analysis of one asphalt sample.
The dominance of aliphatic hydrocarbons
with odd numbers over even indicates that
the source is of terrestrial origin. The iso-
prenoid hydrocarbons, pristine and phy-
tane, which are derived from biological
precursors, are also present. The aromatic
hydrocarbons form a complex mixture, as
illustrated in Fig. 7B. The sulfur content
of one asphalt sample was determined as
being 0.9 wt. %.
As mentioned previously, the asphalt
was deposited during the third hydrother-
mal episode, which may have been gener-
ated by intrusive activity. The emplace-
ment of the igneous sheet in the lignite
may be part of that igneous phase, the age
of which is uncertain. Calculation of the
cooling time of the sheet is presented in
table 4, assuming thernral conduction. It
appears to have taken about 3 years for
the sheet to cool down to 115°C, which
ntay be considered the oil generation
187