Jökull - 01.12.1980, Page 38

lamellae of pure ilmenite appear at this stage
restricted to directions parallel to octahedral
planes in the titanomagnetite host.
Hydrothermal alteration is defined by granu-
lation and titanohematite flecking but may
now also include alteration of ilmenite lamel-
lae to blue-black iron-bearing sphene
replacements at temperatures higher than
250°C.
Class 3: This only differs from class 2 in the
extent of the ilmenite lamellae which now
cover half or more of the grain so that the
residual magnetite approaches Fe304 in com-
position. Hydrothermal alteration is similar to
class 2 although grains are less susceptible to
granulation.
Class 4: With further oxidation the ilmenite
lamellae break down into fine titanohematite
and ferrirutile aggregates causing a whitening
of the lamellae and volume increase.
Hydrothermal alteration may be evidenced
by pod-like titanohematite and ferrirutile
lamellae.
Class 5: In this class islands of magnetite
contain black spinel rods and are separated by
broad oxidised ilmenite lamellae.
Class 6: The original titanomagnetite grain
is now entirely pseudomorphed by tit-
anohematite, ferri-rutile and pseudobrookite.
Like titanomagnetite, ilmenite also responds
to hydrothermal alteration with formation of
a dark blue-black iron-bearing sphene at
temperatures of about 250° and goes to form a
clothwork texture of two phases at temper-
atures in excess of 300°C.
Maghemitisation of titanomagnetite grains
is quite extensively observed in this collection
(Table 1). It is identified here exclusively in
grains which have undergone conchoidal
fracture due to possible lattice compression,
and have taken on a pale blue-grey
colouration (often milky or cloudy in texture).
These grains have developed in them a
lattice-like network of alteration which nor-
mally starts from the grain edges and proceeds
inwards. All of these characteristics are dis-
tinct frorn the effects of deuteric oxidation or
granulation.
METHOD
Polished sections were analysed with a
Zeto-pan Pol microscope at x 1200 magnifi-
cation by making continuous traverses across-
the sample, and classifying the deuteric
oxidation state of each magnetite and ilmenite
grain with more than 50% of its area falling
within the field of view; any doubt concerning
the correct class of a grain was resolved by
placing it in the lower class. Since there are
often considerable and rapid spatial vari-
ations within a rock unit with individual
grains only a few microns apart differing in
their oxidation state, it is essential to adopt a
statistical approach, and in this study a
minimum of 50—60 grains were examined in
each section to establish an arithmetic mean
oxidation state. Grain dimensions were also
measured and a granulation (G) and magh-
emitisation (Mh) index running from 0 to 3
assigned to the titanomagnetite grains
according to the area of grain (0%,< 33%,
33—66% and 66%) affected. Maghemite is '1
Fe203, a low temperature oxidised version of
magnetite with an inverse spinel structure in-
cluding a few missing iron atoms; its extent
proved to be a difficult parameter to quantify
and it was subjectively considered that a 0—3
subdivision by area was the most realistic
assessment.
RESULTS
Seventy-eight dykes showing no visible
weathering in hand specimen were selected for
study and salient results are listed in Table 1
(copy of full results obtainable from authors).
Thirty-four dykes exhibit no deuteric
oxidation and the remainder show oxidation
classes up to but not exceeding 3; only in one
site (64) is deuterically-oxidised ilmenite ob-
served. In a less rigorous survey by Fowler
(1975) of 40 Reydarfjördur lavas, the majority
of which were massive tholeiities (sampling
localities given by Piper et al. 1977, Fig. 4), no
significant difference in average oxidation
state was found between normal and reversed
36 JÖKULL 30. ÁR