Greinar (Vísindafélag Íslendinga) - 01.01.1977, Blaðsíða 86

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to the truth. But Frechen then jumps to the unacceptable hypo-
thesis that the substratum is an eclogitic low velocity layer
of seismology, from which plumes rise and melt into basaltic
magma. Along with our above remarks, already the low radio-
activity of eclogite casts doubt on this explanation of primary
basaltic magma.
Many petrologists and geochemists may consider, that cer-
tain systematic differences in the “primary basalts” have been
overlooked in the above general picture of the basalts. In parti-
cular, differences in the alkalis and the trace elements have
not been taken into account directly. But differences in the
smallest components must, in the present context, rather be
considered as qualitative than as quantitative differences. The
similarity in the main components, say 99% of the bulk com-
position, seems more fundamental.
We may now add that in 1950 (59) the present author con-
cluded that there must be a general potentiál basaltic magma
layer under Iceland. This was based on studies of the physical
and chemical properties of the magma that was erupted from
Hekla during 13 months in 1947—48, and described in several
papers in the Monograph on the eruption. (60, 61, 62). Further,
the general features of the postglacial volcanism in the coun-
try, and our studies of fractures and their systematic mean-
ing, were used as a basis.
One point is that, close to the Hekla fracture and „reservoir”
in which the Hekla magma is very markedly differentiated bet-
Ween the main eruptions (the final lava of 1845 had 54%
SiOo, while the first material to appear in 1947 had 63% Si02;
the silica content in 1947 first dropped to 58% after the initial
phase, to decline gradually towards 54% at the end of the
eruption), the apparent mother material of Hekla magmas,
stays at depth without any differentiation during times of at
least thousands’ of years. We consider this to be a volcano-
logically very important fact, and since I realized this point
in 1947, it has always remained in my mind as one of the keys
to the solution of the formation of basaltic magma, but only
now I have found more of the necessary keys. It seems clear