Jökull - 01.12.1983, Page 54

(Cao.14NaQ.23Ko.51) (A l2.25Fe2.97Tio ^Mgg 39)
(Alo.43Si7.52) (O.OH)24
Smectite (trioctahedral):
(Ca^ igNa^ i6Ko 34) (Al0 77Fe2 ggTi, g^Mgg 47)
(Ali 33Sig67) (0.0H)24.nH20
Beidellite-nontronite-corrensite:
(OaQ osNaQ 36Kq 66) (Al3 42Fei 80Tio 19MgoJ9)
(Al0.i3Si7 87) (0.0H)24.nH20
The coarse silt and sand fractions are dominated
by pyroxene and plagioclase. Magnetite, limonite,
Na, Ca-zeolites and amphibole occur in minor
amounts.
A similar clay mineralogy has been reported for a
weathered Tertiary basalt in Scotland (fíain et al.
1980, Bain and Russel 1980, 1981).
The mineralogy corresponds to the mordenite-
clinoptilolite zone in the series of progressive mine-
ral changes in pyroclastic rocks suggesting reheat-
ing to 80-90°C after deposition (Iijima and Utada
1972).
CONCLUSION
The interbasalt sediments consist of more or less
weathered tephra that either deposited on vegeta-
tive Iand, in swamps or shallow lakes, or on desert or
savannah like areas. The sediments are mixed with
saprolitic soils and subordinately interbedded with
epiclastic material (sandy tuíf, clayey tuff and hya-
loclastite). The present composition result from sev-
eral processes:
Palagonitization and devitrification
Alteration ofvolcanic glass to allophane, smectite
and opaline silica.
Slight loss of alkali elements
Weathering
Formation of low temperature minerals like gibb-
site, goethite, kaolinite, halloysite and beidellite-
nontronite
Removal ofmobile elements (alkalies)
Relative enrichment of immobile elements (A1
and Fe)
Thermal - metamorphism - metasomatism
The temperature in the sediment-lava contact
might have risen to 500°C.
Formation of maghaemite (y-Fe203) ca. 500°C,
and jacobsite-galaxite (Mn-supply)
Diagenesis and regional burial metamorphism
Formation of a series of zeolites suggesting the
lavas and sediments to have been reheated to temp-
eratures from 70-80°C to 120-130°C.
Primary textures of tephra are frequently seen,
but are largely obscured by argillization and form-
ation ofamorphous material, compaction, and dia-
genetic mineral changes.
Glassy tephra which fell into a swamp is first
altered to smectite, cristobalite and/or zeolite foll-
owed by loss of Na and other alkalies (Peacock 1926,
Jakobsson 1971, Pevear et al. 1980).
By setting the mineralogical and chemical data
for W-Iceland interlava sediments into a generaliz-
ed weathering model (Kuzvart 1978), it is evident
that the oldest Miocene redbeds of NW Iceland
have compositions typical for the late ,,senile“ to
„lateritic" stage of tropical weathering, while the
youngest Pliocene sediments reflect an early „ver-
ile“ stage of weathering.
The result show that lateritic soil could develop
in Middle Miocene at high latitudes even within
geologically short periods of 10, 000-100, 000 years.
ACKNOWLEDGEMENTS
I wish to express my sincere thanks to Sigurður Thor-
arinsson for a most inspiring excursion in summer 1976,
during which the idea for this study was conceived, and for
later discussions during our stay in Iceland.
The sample sites were selected in collaboration with the
Icelandic colleagues Hrefna Kristmannsdóttir, Kristján Ste-
mundsson and Haukur Jóhannesson. Leó Kristjánsson and
Kristján Stemundsson are thankedfor valuable discussions.
The work is financially supported by the Nordic Volcano-
logic Institute (Norrtena Eldfjallastöðin), Reykjavik, and
the study was in part carried out during the author’sfellow-
ship at this institute.
REFERENCES
Aaomine, S. and K. Wada, 1962: Differential weather-
ing ofvolcanic ash and pumice resulting in form-
ation of hydrated halloysite. Am. Min. 47: 1024-
1048.
Ahmétejéf M., 1978: Islenska flóran 3.5 milljón ára
gömul. Tíminn 12. sept.
April, R.H. 1980: Regularly interstratified chlorite
52 JÖKULL 33. ÁR