Jökull - 01.12.1983, Page 39

productive fissure swarms, positioned more centr-
ally within the rift zone. These more efficient fissure
swarms are the ones that are most productive in
primitive rocks, and these rocks do generally not
drift out of the rift zones on the surface; they subside
within the rift zone to depths below general later
exposure. In the Tertiary lava piles of Iceland, the
most primitive tholeiites are thus underrepresent-
ed, when compared to the entire rift zones.
This causes a bias in the chemical spectrumofthe
Tertiary rocks, when they are compared to those of
the present rift zones ofboth Iceland and theocean
fioor. On the other hand, a similarity emerges bet-
ween the Eastern Iceland lava pile and theoff-ridge
oceanic floor, as the primitive MORB-type basalts
are practically confined to the spreading centers in
case of the ocean floor, as stated by Bryan et al.
(1976). This parallelism is in harmony with the
kinematic evolution of the ocean floor in general,
and is not surprising, when it is kept in mind that
Iceland is a natural part of the ocean floor, regard-
ing both materials and crustal structures, as well as
processes of formation.
The “Iceland geochemical anomaly”, when it
refers to the Tertiary lava piles of Iceland, is the
result of compositionally selective kinematic pro-
cesses at the spreading axis.
CONCLUSION
The “Iceland geochemical anomaly” is a mis-
coined phrase which does not stand for differences
in rock types, magmatic evolutionary processes, or
magma generating sources. As found in the litera-
ture, the idea stands for skewness in volume relat-
ions and increased total productivity. Ifused at all,
the phrase should be the „Iceland productivity ano-
maly”, which is the reason for the ocean floor above
sea level, called Iceland. The idea of a chemical
anomaly in Iceland originates through different
sampling methods and compositionally selective
kinematic evolution within the volcanic rift zones.
ACKNOVVLEDGEMENTS
K. Grönmld, M. Olajsson, N. Oskarsson and G.E.
Sigvaldason are thanked jor discussions and constructive
criticism oj manuscripts. The Nordic Volcanological Instit-
ute is thankedJor providingjinancial support andJacilities.
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Manuscript accepted 18 June 1982.
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