A Correlation Between Remanent Magnetism and
Petrological and Chemical Properties of Tertiary
Basalt Lavas from Mull, Scotland.
J. M. Ade-Hall
(Received 1963 October
I)*
Summary
A description is given of a petrological examination of a set of
normally and reversely magnetized samples from tertiary basalt lava
flows on the Isle of Mull.
The collected evidence suggests that the normally magnetized
samples are most likely to retain part of their original thermoremanence. The reversely magnetized samples probably acquired their
magnetization during a diffusion process in the opaque minerals which
took place at an undetermined time after the consolidation of the lavas.
The distribution of four features is found to correlate with the two
senses of natural remanent magnetization. These features are the
alteration products of decomposed olivine, the grain size and crystal
form and the internal nature of the primary opaque minerals-especially
exsolution of ilmenite from titanomagnetite.
Normally magnetized samples are found to contain olivine replaced
only by serpentinous minerals together with relatively small wellcrystallized, optically-homogeneous (unexsolved) primary magnetite
grains.
On the other hand, reversely magnetized samples are found to
contain olivine replaced by serpentine, secondary iron oxides and a
red hydrated iron oxide (iddingsite). I n addition, the primary magnetite
grains in this group are relatively large, show poor crystal form and are
typified by varying amounts of ilmenite exsolved as lamellae in the
octahedral planes of the magnetite.
Introduction
A detailed study of the natural remanence of a sequence of 36 Tertiary plateau
and central-type basalt lava flows from Mull, Scotland was carried out by S. K.
Dutt of the Geophysics Department, Imperial College, from 1952 to 1955. This
present paper describes the results of a re-examination of the material collected
and examined by Dr Dutt, to whom I am indebted for permission to make use of
I.
his results and specimens. All the magnetic measurements quoted in this paper
are derived from Dutt’s original measurements of declination and inclination of
natural remanence, and are published here with his kind permission.
* Received in original form 1963 May 9.
403
404
J. M. Ade-Hall
Other palaeomagnetic investigations on Mull have been carried out by Bruckshaw & Vincenz (1954) and Vincenz (1954).
Dutt's collecting area, Maol Buidhe, was a hill slope 3 miles ESE of Salen, in
the north eastern part of Mull (Map I). I n his main sequence, Dutt found that
samples from the lower flows with a few exceptions, were reversely magnetized,
while the upper flow samples were mainly normally magnetized. Samples from
several flows from different parts of the sequence showed random, or consistently
oblique or intermediate magnetization. Eleven flows contained both normal and
reversed specimens. Dutt applied a statistical test to his results and concluded
that, of the 36 flows examined, 1 1 were reversely magnetized, 13 were normally
magnetized, 4 had consistent oblique directions and 8 were randomly magnetized.
But this classification is not of great significance because of the great variation
from sample to sample.
SO,,.
f
.FLOWS SAMPLED
( S o m r numbered)
---ROADS
ri DIPS
MAP I.-The Northern part of the Isle of Mull, off Scotland. Collection sites
for Mull lavas, relative to the coast and to the major intrusive bodies and
recent sediments. Some flows are numbered as in Dutt's thesis.
Dutt examined 12 thin sections representing 9 flows and did not find a significant petrological difference between the normal and reversed ones: all were
serpentinized olivine basalts and the ranges of grain sizes were rather similar.
T o avoid the necessarily arbitrary classification used by Dutt into normal,
reverse, oblique and random specimens, it is convenient to introduce the quantity
cos 4,where 4 is the angle of divergence between the direction of the magnetic
remanence of a particular sample and the mean axis of magnetization of the whole
sequence (Blackett, Clegg & Stubbs 1960). It is convenient to designate a sample
with cos 4positive as normally magnetized and one with cos 4 negative as reversely
magnetized. For a tightly grouped set of normally magnetized specimens, cos 4
will have values near + I .o and for a reversely magnetized group a value near
- I '0.Obliquely or randomly magnetized flows will have intermediate values of
cos 4.
The mean direction of Dutt's specimens is 351' east from true north and
67-5"downward. In Figure I and Table I are shown the values of cos 4 for 43
selected samples from Dutt's 36 flows, arranged serially with the lowest flow (No.
58) at the bottom of the figure and the highest one (No. 126) at the top. This
representation of the magnetic directions shows at once the result obtained by
Dutt that the lower flows are mainly reversed and the upper ones are mainly
normal. Each sample used in this analysis was, whenever possible, representative
of the direction of magnetization of the flow to which it belonged. Mention will
Chemical properties of tertiary basalt lavas from Mull, Scotland
405
be made later of flow 100,part of which was normal and part reversed. A detailed
analysis of the magnetic properties of this flow has been made by R. L. Wilson
(following paper).
The main content of this paper describes a detailed re-examination of the
petrology of this series of lavas, and makes a comparison of the results with the
t
I
I
I
I
-
I
I
I
1
I
126-1
125-8
120-7
lf4-I
f08-I
107-5
106-I
f#-/
fO4-4
JCX-1
100-9
I00 -I
99-1
98- 6
98-1
97-6 .
96-1
94-1
93-2
-
9/-2
9 0-4
89-8
89-1
86-2
a
84-1
83-2
el-3
-
794
77-3
76-8
762
75- 4
744
74-1
73-1
70-7
706
67-9
67-f
65 4
6l-f
604
58f
sense of their magnetization. For this purpose thin and polished sections respectively were made of the samples of Table I and Table 3.
This sequence of plateau basalts lies just within the zone of pneumatolysis
associated with the central volcanic complex of Mull, and within this zone all the
lavas show evidence of alteration by steam under pressure. I n the field this zone
may be recognized by the absence of the step-like trap featuring, which results
from the weathering away of the softer tops of lava flows in areas of un-pneumatolysed plateau basalts: and microscopically by the serpentinization of olivine
406
J. M. Ade-Hall
Table I
I
2
3
4
5
12.3
3.4
13'4
12.7
7.1
2.8
3'4
1.5
2.3
14'5
140
9.6
9.1
-
6
7
8
9
I0
I1
al
a
5
m
4
3
cr,
.+0
I 26-1
-f I 25-8
?I207
14-1
108-1
107-5
106-1
I
105-1
104-4
t103-I
I 00-9
100-1
"99-1
"98-6
"98-1
97-6
96-1
94-1
93-2
9 1-2
90-4
89-8
89-1
86-2
84-1
83-2
81-3
79-2
"77-3
76-8
76-2
79-4
74-5
74-1
74-1
73-1
70-7
70-6
67-9
67-1
65-4
61-1
60-1
58-1
18.0
16.4
14.2
12.4
22.7
10.6
5'8
22'0
10' I
5'6
242
8.9
8.5
3
10.9
30.2
4 9
20'
5'2
8.2
38. I
6.4
6-9
5' 3
7'9
'3'3
12.6
5'5
3-8
9'3
10.4
0.6
-
1'7
6.8
5'3
49
3.1
2-6
9'7
8.1
2'1
-
-
1.7
-
-
2.1
9.5
1.2
4' 5
0' I5
10.6
2.4
3'5
0.62
1'4
7'9
10.0
3.9
2.5
7'7
5-4
4.8
15.8
6.0
2.4
4'0
11.4
3.6
18.0
19'7
19'7
8.5
9'3
16.1
6.0
12.6
13'3
9-7
17.0
6.1
-
6,9
49
49
2.5
15'7
7.1
11.7
20.8
9'2
08
16.4
5.9
-
I' I
-
-
-
-
-
14.8
9.1
8.5
0.94
4'2
0'93
> o0
1'5
3'6
-
-
3.9
I' I
8.1
0'01
44
3.5
6.1
6.1
46
1'5
1.7
6.3
1'5
2.8
12.2
0'2
3.0
10.4
6.6
48
4.5
4.8
1.2
2' 3
0.80
0.80
0'89
2' I
43
1'2
1'3
0'5
0'5
0'2
3' 3
3'6
-
0'44
0 02
-
1.6
0.43
0.75
26
26
+ 0.90
+0.90
I9
I4
I9
16
163
+ 0'97
21
I5
40
148
17
130
162
I45
I73
6
IOI
53
176
I35
158
I35
I57
>I0
-
49
49
4' 1
3' 3
7' 4
10
28
23
14
24
6
7
30
47
76
30
32
53
36
27
36
53
68
29
34
0.63
0.08
5'0
45
too fine to estimate
7.8
3.9
3'5
8.8
0.8
11.8
2.1
o 24
2' 7
-
58
52
28
36
63
63
33
36
38
55
37
54
61
38
169
169
+095
+0.96
-0.96
+0.93
+097
+0.77
-0.85
+0.96
-0.64
-0.95
-0.82
-0.99
+0.99
- 0 19
+0.60
-1.0
-0.71
-0.93
-0.71
-0.92
-0.57
+0.87
+097
+0.98
-0.99
-0.96
-0.96
-0.96
-0.93
-0.89
+0.75
+0.93
+0.42
- 0 98
-0.98
-0.98
N.B.-Over-estimation of opaque material is likely in the finer samples.
All the specimens are of the olivine plateau basalt type except those marked with *,
which are central porphyritic or tholeiitic type, and those marked 7, which are mugearites.
When cos 4 is positive, the magnetization is normal: when negative it is reversed.
Chemical properties of tertiary basalt lavas from Mull, Scotland
407
(magnesium iron silicate), partial decomposition of the plagioclase felspar (calcium
sodium alumino-silicate), and by vesicle infilling with particular secondary minerals
such as epidote (hydrated calcium iron aluminium silicate).
The plateau basalts from the slopes of Maol Buidhe lie very near to the margin
of this zone of alteration. Trap featuring is retained but the characteristic features
of pneumatolytic alteration can be seen microscopically. Dutt thought it unlikely
that the lavas in this peripheral region were reheated by steam to much above
200' C, and he found that heating samples in air to this temperature and cooling
in the Earth's magnetic field produced negligible effect on the natural remanence.
2. Quantitative study of the petrology of the lava sequence using
transmitted light
2. I General petrology
Thin sections were made from at least one sample from each flow. Where only
one sample was sectioned, I tried to chose one with a typical direction of magnetization. More than one sample was sectioned from five flows and all the samples from
three flows. Examination of these thin sections showed the sequence to consist of
the lava types shown in Table 2.
Table 2
Serial No.
of $ow
No. of
Sections
measured
total I
petrological type
olivine (plateau) basalt
mugearite
6
olivine (plateau) basalt
I03
I
mugearite
I00
2
olivine (plateau) basalt
( + 10sections examined qualitatively)
98-99
2
porphyritic central basalt
79-97
IZ
olivine (plateau) basalt
77
I
tholeiitic basalt (possibly intrusive)
58-76
10
olivine (plateau) basalt
126
120-125
104-114
2
Briefly, the lava types shown in Table 2 have the following petrological
properties.
OZivim plateau basalt-Olivine rich basalt o f relatively coarse grain size.
Typical Maol Buidhe plateau basalt consists of titaniferous augite, calcic plagioclase felspar, serpentinized olivine, and iron oxides.
Porphyritic Central type basalt-A fine grained olivine-poor basalt with
titanium-poor augite, and porphyritic and ground mass plagioclase.
Mugearite-A fine grained basalt, with aligned plagioclase felspar and iron
oxides as the main minerals.
Tholeiitic Basalt-A fine grained basalt consisting of plagioclase felspar, augite,
iron oxides, devitrified glass and a little serpentinized olivine.
Pneumatolytic alteration
The basalts of this sequence show the effects of pneumatolytic alteration, both
in the serpentinization of the olivine and the turgidity of the plagioclase felspar.
Not only have the primary olivines, in all but a few cases, been completely replaced
2.2
408
J. M. Ade-Hall
in situ by secondary minerals, the resulting aggregate retaining the olivine crystal
outline, but serpentinous minerals now infill what were originally small holes in
the rock. The titaniferous augite usually appears to be fresh but in some of the
more highly altered lavas peripheral chloritization has occurred.
The products of the serpentinization of olivine show considerable variety.
O n e sample, from flow 75a, shows unaltered olivine. A few samples show shreds
of unaltered olivine in serpentine replacements or pseudomorphs. I n all the other
samples the primary olivine has been wholly replaced by serpentine and other
secondary minerals. The principal secondary mineral is a blue-green or brownishgreen serpentine but minerals of the bowlingite (green fibrous hydrated magnesium
silicate) and sepiolite (colourless fibrous hydrated magnesium silicate) type also
occur. Other non-serpentinous minerals also contribute to the pseudomorphs, in
particular a red transluscent mineral of the iddingsite type and iron oxides of
obvious secondary origin. I n some samples iddingsite and secondary oxides
together constitute a greater bulk of the pseudomorphs of olivine than the green
secondary minerals which occur mainly as a vesicle and hole infilling.
When both red iddingsite and black secondary oxide occur, the latter always
appears amorphous, even at a magnification of 400, and frequently shows a transition into adjacent iddingsite. When iddingsite is absent, either amorphous
secondary iron oxide or another morphological type of secondary iron oxide may
occur. I n this latter form the oxide appears as small discrete imperfectly cubic
grains, often forming rims to pseudomorphs after olivine. This type of secondary
oxide can always be resolved into discrete grains at a magnification of 400 and may
represent recrystallized amorphous secondary type oxide.
2.3 Quantitative estimation of minerals in thin sections
A normal point counting method was found to be useless as the fineness of
secondary mineral areas would require inordinately long traverses to measure their
quantities satisfactorily. Instead, a calibrated eye piece scale with a medial line
was used. The section was moved slowly under the objective and the intercept
made by the medial line on different minerals measured.
Note was kept of these intercepts made on different grains of the same mineral
along a certain traverse, allowing the aggregate intercept for each mineral present
to be computed for the traverse length. This method gives an immediate measure
of the proportions of each mineral in the sample.
Data obtained by this method were used to calculate a number of parameters
for which correlation with natural magnetization seemed possible. The results
are given in various columns of Table I .
Column
Amount of original olivine (calculated as equivalent to volume of
pseudomorphs).
Column 3. Amount of displaced green secondary minerals.
Column 4. Amount of primary iron oxide.
Column 5 . Amount of secondary iron oxide-amorphous type.
Column 6. Amount of secondary iron oxide-discrete type.
Column 7. Ratio of secondary oxide to primary oxide.
Column 8. Amount of iddingsite.
Column 9. Grain size index (aggregate primary oxide divided by number
of grains).
2.
409
The term primary oxide is used to describe opaque grains which are likely to
have been formed at the time when the originally molten lava solidified. Secondary
oxide describes opaque material most likely formed during the process of serpentinization which took place at an undetermined later time.
The distinction between primary and secondary oxide is usually clear. Primary
oxide grains are relatively large with imperfect square or sharp outlines, and are
not preferentially associated with any particular non-opaque mineral. On the other
hand, secondary oxide in these rocks is always closely associated with serpentine
pseudomorphs after olivine. Typically it occurs in the serpentine as crack infillings,
cores or rims. I n outline the material may be imperfectly cubic but very fine
compared with primary grains, or even so fine as to be optically amorphous. Where
primary grains occur near to serpentine pseudomorphs, amorphous secondary
oxide may form a bridge from the pseudomorph to the primary grain. I n those
cases where two different opaque forms are in contact, differentiation is arbitrary.
Chemical properties of tertiary basalt lavas from Mull, Scotland
2.4 Comparison of the mineral content of normally and reversely magnetixed samples
In Figures za to 2g are shown frequency histograms of the occurrence of the
various parameters given above. The frequency for normal specimens, that is
cos 4 between + I and o are shown upwards on the figures and for reversed specimens, cos 4 between o and - I, are shown downwards. A clear difference between
the normal and reversed samples exists only for the amount of secondary oxide
'"1
'"1
5REVERSE
SAMPLES
f0-
5-
-
REVERSE
SAMPLES
f0A
(Figure zd) the ratio of secondary to primary oxide (Figure ze) the amount of
iddingsite (Figure zg) and the size of the opaque grains (Figure zf).
Secondary oxide of the discrete cubic type (column 6 of Table I) is restricted to
six normal samples representing all the flows in the range 104 to 114. Amorphous
secondary oxide and iddingsite are found in 15 out of zz reverse samples. I n no
case is iddingsite found in normal samples.
Within the sequence of these 36 flows examined there can be no doubt that the
distribution of secondary minerals correlates with the sense of the magnetic remanences, reversely magnetized specimens having more amorphous secondary
oxide and iddingsite than normal ones.
Figure 3a shows this relationship in another way-by plotting the amount of
amorphous secondary oxide against cos (6, both for individual samples (black dots)
5
J. M. Ade-Hall
410
and also for the lavas collected together into Dutt's six groups of magnetically
stable flows by large open circles, (Group 111: flows 58, 60, 61, Group V: flows
67, Group VI: flows 70, 73, 74, Group VIII: flows 75, 76, Group X: flows 79,
83, 91, 98, Group XI: flows 103, 105, 108, 114))so reducing the scatter of the
1
10-
I0
NORMAL
SAMPLES
5-
NORMAL
SAMPLES
5
OhVOLUME
L
I
I
L
Y O
I
3
2
E
410%
5
REVERSE
SAMPLES
10
15J
1.5.
FIG.ac.-Quantity
FIG.zd.-Quantity of amorphous
secondary oxide.
of primary oxide.
20-
*"1
ISNORMAL
SAMPLES
10-
L 5-
18
o--
5-
REERE
SAMPLES
I0J
FIG.ae.-Ratio
of secondary amorphous
oxidelprimary oxide.
lo-
FIG.zg.-Quantity
of iddingsite.
results. T h e greater amount of alteration products in the reversed flows is very
evident. Figure 3b shows similar results for iddingsite.
A few figures may be usefully given of the amount of these alteration products
as measured along a traverse across a thin section. The average amount of iddingsite present in the reversed rocks is I -2 per cent and is zero in the normal ones.
T h e amorphous oxide present is 4.9 per cent in the reversed ones and I . I in the
normal ones.
411
If one compares a typical thin section from the basalt of the upper and more
normally magnetized flows (e.g. nos. 98-125) with similar types from the lower
and generally reversely magnetized flows (e.g. 58-96) the optical difference is very
striking and can be recognized without expert petrological training. The normal
specimen has little colour and mainly isolated sharp-edged opaques : the reverse
specimens are highly coloured with green, yellow and brown areas, around which
are found quantities of associated amorphous black oxide material. Thin sections
from all the samples from two flows with closely grouped sample directions 83,
Chemical properties of tertiary basalt lavas from Mull, Scotland
NORMAL
SAMPLES ’Ol
REVERS€
SAMPLES
FIG.zf.-Grain
size index.
-transmitted light,
- - - reflected light.
(R) and 105 (N) were examined. In every sample the typical features for the relevant direction of magnetization were found.
From the results shown in Table I and in Figure 2f, it can be calculated that
the mean size of the opaque grains, as measured by transmitted light in thin
sections, in 17 normally magnetized samples is 31 3pm while for 19 reversely
magnetized samples it is 45 2 3pm. In a later section measurements of opaque grains
by reflected light are described and the results shown by the dotted histogram of
Figure 2f. The normal and reverse samples of Flow IOO show a similar grain
size difference.
The limits of precision quoted are the arithmetic mean error of the arithmetic
mean, that is, the sum of the residuals divided by n z / n where n is the number
of samples included.
2.5 A single $ow with both normal and reversed magnetization
Flow IOO of Dutt’s series is an example of the above. Of the available 10
samples, six were normal and four reversed. Their magnetic “hardness” is
investigated in the following paper. Thin sections were made of all 10samples and
all the reversed were found to have abundant secondary amorphous oxide, while
the normal had only a trace-the optical difference between the two is very striking.
I t is very probable that all the samples were similar petrologically before serpentinization took place.
J. M.Ade-Hall
-10%
-8
0
-6
Reverse
-4
0
0
' 0
0
.
h
i
-0
0-
-0-0
-2
O1
FIG.ga.-Percentage of secondary amorphous oxide in samples versus cos 4.
0 individual samples.
0 means for Dutt's 6 groups of statistically stable flows.
0
0
c
L
.-i-.-,a@g
Em-"
I
-)O
-0.6
I
I
I
-03
+02
+O*6
FIG.3b.-Volume
of iddingsite versus cos 4.
I
+/.O
Chemical properties of tertiary basalt lavas from Mull, Scotland
4’3
2.6 Summary of alteration type
This study of the Mull lavas suggests that there are three chief types of alteration which are of magnetic importance. These are listed below in order of apparently increasing alteration. The first two occur in normal samples and the third
in reverse ones.
Type A: Olivine is usually completed pseudomorphed by pale green or blue
green serpentine. Fresh olivine is seen in two samples. No secondary oxide or
iddingsite occurs in I I normal samples. (Plate fa).
Type B: Olivine is usually pseudomorphed by green serpentine, accompanied
by secondary oxides of discrete imperfect cubic type, occurring as rims or crack
infillings. (Plates I b and IC) Iddingsite is always absent. Shreds of fresh olivine also
occur sometimes. Flows 104 to 114 showing alteration of type B also show some
microscopic evidence of contact metamorphism. In particular, amphiboles belonging to the tremolite-actinolite series have partially replaced both serpentinous
minerals and the margin of primary augite grains. Temperatures might have been
as high as 500-600° C during this metamorphic process.
A map in Dutt’s thesis (fpllowing page 49) shows the collecting sites of these
flows to be within 400 m of the outcrop of a large later granophyre intrusion. This
intrusion could well be responsible for these observed metamorphic effects. (Map I.)
Type C: Olivine completely serpentinized to dark green or brownish-green
serpentine accompanied by amorphous secondary oxide. Of 26 samples all
showing amorphous secondary oxide, 21 are reversely magnetized. 15 contain
iddingsite, and all of these 15 are reversely magnetized (Plate Id).
These three types of alteration characterize this particular sequence of lava
flows but it is not claimed that they will characterize all other sequences.
3. Quantitative study of opaque minerals in reflected light
A search for further correlation between magnetic and petrological properties
was made by the examination by reflected light of the minerals seen to be opaque
in transmitted light. Such an examination is made possible by the use of polished
sections of the samples. Where possible a polished section was made from the
same 2 cm cube from which the thin section had been made. Where no material
remained a cube with a similar polarity from the same or an adjacent block was
used.
Thirty eight polished sections were examined, using a Reichert “Zetopan-Pol”
microscope. I n each section the first 50 primary opaque grains to pass under the
objective in a traverse were described and measured. The conventional term used
here to describe such grains is “opaque”; in fact in a polished surface these grains
show a high metallic reflectivity. A transparent graticule marked with concentric
circles centred on the origin of rectangular axes was used for measuring grain
dimensions.
The term “primary opaque” will be used here to describe an opaque grain
formed when the lava crystallized from a molten state, which may or may not have
been considerably changed at a later time by the above mentioned processes. It
was also hoped to identify the mineralogical nature of the secondary opaque
material seen in transmitted light. However, in the single plane through the
sample presented by a polished surface this material was seen to occur as fine,
highly-reflecting particles with diameters of < Ipm in a non-opaque matrix. Grains
of this size could not be resolved at the maximum magnification available of
414
J. M. Ade-Hall
1250. T h e continuous opaque appearance of this material in transmitted light must
be due to the distribution of many of these fine particles throughout the 30pm
section thickness, resulting in a continuous opacity over large areas due to complete
over-lapping.
Eventually the following features were noted, allowing the calculation of related
parameters :
(a) The mineralogical constitution of the primary opaques was noted, and also
the quantity of different mineralogical phases present as a percentage of
the total volume of primary opaque material. I n practice it was found that
two typical associations of different mineralogical phases occurred in the
samples. These associations are magnetite with ilmenite lamellae exsolved
in octahedral planes, and magnetite without such lamellae.
(b) T h e volume of an exsolved phase in exsolved grains (e.g. ilmenite or its
alteration products).
(c) Magnetite grain size.
(d) Perfection of crystal form.
(e) T h e spatial relationship between different types of primary opaque grains
and areas of serpentinous minerals.
3 . I . Types of opaque grains present and their relative volumes
In the majority of samples the primary opaques show secondary oxidation
effects obviously due to the same agency responsible for the serpentinization of
olivine and kaolinization of felspar. Peripheral hematite occurs which is mostly
highly developed when the primary opaque grains are near to serpentinized areas.
Similar alteration is found along cracks and the margins of exsolved lamellae of
ilmenite in magnetite grains. Such lamellae are usually more decomposed than the
surrounding magnetite and are sometimes replaced near to the edges of grains by a
dark blue-brown non-opaque material, later shown to be rutile (TiO2). Rutile is
produced from the oxidation of ilmenite. However, these secondary products,
together with the fine particles of secondary opaque material found in serpentine
pseudomorphs after olivine form only a very small fraction of the volume of the
apparently fresh phases within the primary opaque grains and will not be considered
further.
Magnetite forms the main opaque phase in all 38 sections but associations in
the same grains with minor quantities of other opaque and non-opaque phases
characterize different samples. These two typical associations are as follows :
Magnetite without exsolved lamellae-Plate 2(a)
Cracked greyish magnetite showing good crystal form. No lamellar exsolution
of the ilmenite occurs in I I I planes. Patches of fine felt-like ilmenite intergrowth
and patches of fine non-opaque granules are typical features in some samples.
These granules do not appear to be a peripheral oxidation product since they do
not occur preferentially near to grain margins. The nature of the granules could
not be determined owing to their small size. Separate elongated grains of ilmenite
frequently occur in samples containing magnetite of this type.
Magnetite with exsolved lamellae-Plate 2( b)
Brownish magnetite grains of irregular outline with various amounts of ilmenite
exsolved in octahedral (Miller index I I I ) planes in the magnetite. These ilmenite
PLATEIa.-(
PLATEIb.-(
x 6 5 ) Normal samples. Olivine alteration type A-Serpentinous
minerals : dotted, primary opaques : black.
x
6 5 ) Normal samples. Olivine alteration type B-Serpentinous
minerals : dotted Cubic secondary opaque : black.
[Facing page 414
PLATEIC.-(
PLATE id.-(
x400) Enlargement of area in ib.
x 6 5 ) Reverse samples. Olivine alteration type C-Iddingsite:
hatched; serpentinous : dotted.
(iii)
PLATEpa.-Normal samples-(i) 67-9 : small homogeneous magnetites, (ii)
120-7: as for (i), (iii) 76-8: magnetite with granule patches and conchoidal
cracking, (iv) 89-1: magnetite with more extensive granule patches than in
76-8 (All x 1250).
(i i)
(Vi)
samples-(i) 60-1 : magnetite (grey) with ilmenite (white)
exsolved in I I I planes in the magnetite, (ii) 126-1 : magnetite with fine exsolution lamellae of ilmenite mainly replaced by rutile, (iji) 74-1 magnetite (grey)
with ilmenite (white) exsolved in 111 planes, (iv) 100-9: magnetite with
rutile replaced ilmenite lamellae and enclosed separate ilmenite. (All x 1250).
(iii)
PLATE
zb.-Reverse
PLATE3 .-Opaques
: double outline. Serpentinous minerals : dotted. Augite-A.
Plagioclase felspar--F.
Chemical properties of tertiary basalt lavas from Mull, Scotland
4'5
Table 3
Amounts of diflerent opaque mineralogical phases expressed as a percentage of the
volume of total measured primary opaque material
I
2
126-1
125-8
12-7
I 14-1
108-1
107-5
106-1
37'2
105-1
104-1 I
103-1
I 00-9
100-5
99-1
98-1
97-6
96-1
94-1
93-2
91-2
90-5
89-8
89-1
86-1
84-1
83-1
81-3
79-2
77-3
76-8
75-6
74-1
73-1
7 07
67.9
65-3
61-1
60-1
58-1
3
62.5
4
3
100' 0
0
0
100'0
0
0
0' I
0
0
0
100'0
0
0
93-6
70' 6
0
0
29.4
97'4
3'3
64.8
6.3
NM
NM
NM
I5
98.3
99'4
I' I
95'9
3'9
93' 0
9 49
48.8
63.2
0 2
22' 9
0'9
49' 1
95.0
57' 5
79'4
0.6
7' 5
98. I
99.6
0.4
0
5
-
9
R
N
N
N
N
N
6.3
N
0'3
-
1.6
0.6
-
1'5
0.8
7' 9
0'7
5' f
N
N
N
R
N
N
0
0
40' o
26.9
99' 8
52'3
97' 2
35.8
5'0
NM
NM
16
11'2
R
9' 6
N
28
21.8
33
33
13.8
0
-
0
0
42' 5
0
0
20.6
98.9
92.0
3 '4
NM
-
0
0
1'2
0
0
0' I
16
-
99.6
-
R
R
R
R
N
R
N
R
R
R
R
R
R
I 0 0 09
0
0
-
99' 9
99' 7
0'5
0
0
0' I
0
0
0' 2
N
N
24
0
99' 2
9 02
99'4
NM
NM
0'3
4' 0
0'5
100'0
0
0
-
R
R
R
0'5
0
0' I
N
N
0
99' 1
0.7
8.2
98.8
21'0
79'0
85.0
50
NM
I4
-
-
R
R
R
* Skeletal unexsolved magnetite in this tholeiitic basalt differs from the unexsolved
magnetite of other samples by its occurrence in devitrified glass at a late period in the crystallization sequence.
Column I-Flow and Sample.
2-Magnetite without lamellar exsolution of ilmenite (Type I).
3-Magnetite with lamellar exsolution of ilmenite (Type 2).
4-Volume of exsolved ilmenite as percentage of volume of magnetite with
exsolved ilmenite.
5-Separate ilmenite.
6-Pyrite.
7-Possible secondary opaque.
8-Too small or corroded to determine.
9-Normal or reversed magnetization.
NM = not measured.
416
J. M. Ade-Hall
bodies have a characteristic lamellar form and frequently many parallel lamellae
occur.
Table 3 (columns 2 and 3) shows the measurements of these two types of
grains, together with other minor phases in columns 5,6 and 7. Mean grain size and
grain volume were found from measurements of two normal axes of regular
sections, and of estimated equivalents for irregular sections of grains. The volumes
of the different mineralogical types are expressed as a percentage of the total
volume of measured opaque material.
The measurements given in Table 3 reveal a high correlation between dominant
type of the main opaque grains in a sample and the sense of the natural remanent
magnetization. Reversely magnetized samples are characterized by magnetite with
ilmenite lamellae exsolved in I I I planes, while normally magnetized samples are
characterized by magnetite with no lamellar exsolution of ilmenite. Only three
o u t of 38 samples do not satisfy this classification. Numerically the reversely
magnetized samples contain a mean value of 65 _+ 8 per cent total volume of magnetite with some ilmenite lamellae, while the normal samples contain a mean value of
9 ? 3 per cent.
The results are shown graphically in Figures 4a and 4b. The former is a histogram of the frequency N of opaque grains, of which a fraction F contain some
exsolved ilmenite. The frequency is plotted upwards for normal and downwards
for reverse magnetization. Figure 4b shows the actual value of F and cos 4 for each
individual sample. Both methods of plotting show clearly the greater prevalence
in reverse rocks of magnetite grains with ilmenite lamellae. The disadvantages of
the histogram method is that it gives equal weight to all values of cos (6, only noting
whether it is positive or negative and so giving too much weight to samples with
widely scattered magnetic directions.
3.2. Volume of exsolved lamellae as a fraction of the whole grain
Only measurements of ilmenite exsolved in the I I I planes of magnetite were
feasible. T h e fine felt-like ilmenite and granule patches were too fine and irregular
t o measure.
Measurements of grain width and net lamellae width were made normal to the
parallel I I I exsolution lamellae of ilmenite lamellae in magnetite grains. From the
ratio of these figures an approximate measure can be made of the volume of the
exsolved ilmenite as a fraction of the magnetite grain. (The results are given in
Column 4 of Table 3). In Figure 5 is shown the ratio plotted against cos 4. It will
be seen that all the normal samples (cos 4 > 0)have no ilmenite lamellae, whereas
most of the reverse ones (cos 4 < 0)have some. Though the data are scanty, it does
seem that the average amount of ilmenite increases as cos 4 falls from o to minus
unity.
Numerically one notices that the 15 reversed samples have an average of 18 per
cent lamellae in the magnetite, whereas all 14 normal ones have no lamellae.
A comment is perhaps required about the difference between the data displayed
in Figure 4b and those in Figure 5 . The former show the percentage of all magnetite grains which have some exsolution lamellae, whereas the latter show the average
amount of exsolution lamellae in all the magnetite grains.
3.3. Size of opaque grains
The mean grain size, as measured using reflected light for the different flows, is
shown in Table 4, together with the nature of the grains. The mean size of both
Chemical properties of tertiary basalt lavas from Mull, Scotland
417
12-
/ONormof
8-
64-
,rJo
2-
/o
p"
3 ,-8
MAGNETITE
4-
-
6ReVQrSed 8-
l01
I2
FIG.4a.-Volume
percentage of total magnetite which contains exsolved
ilmenite .
-100
WO
a
80
0
0
60
Normal
Reverse
40
4
.20
-0-o-
FIG. &.-Volume
am2
-0
percentage of total magnetite which contains exsolved
ilmenite.
J. M. Ade-Hall
4'8
normal and reversed groups is 34pm which is close to the value of 3 Ipm found for
the grains of the normal lavas as measured by transmission. The greater size (45pm)
of the opaques as compared with the reverse samples seen in transmitted light is
probably due to the amorphous oxides in the reverse flows, which appear opaque
in transmission but only to a very small extent give metallic-like reflections. The
scatter of the grain size in reflection of the normal flows is larger than that of the
FIG. 5.-Exsolved
ilmenite as percentage of volume of magnetite containing
exsolved ilmenite.
reverse flows, but the statistical weight of this result is not high. The grain size
measurements are shown by the dotted histogram in Figure 2f.
In the lower part of the sequence of flows (100 to 58) a significant difference
occurs between the grain size of the primary opaques of normally and reversely
magnetized samples. The normal samples have a mean cube edge of 23 rt: 2pm
and the reverse samples have a mean cube edge of 37 rt: zpm.
It is not possible to examine the upper group of flows (103 to 126) for such a
correlation as the group contains no definitely reversely magnetized sample.
3.4. Perfection of crystal form
Perfection of crystal form can be used to determine the crystallization sequence
in an igneous rock. The first minerals to crystallize as the molten material cools
will generally show a good crystal form while the last will show little sign of crystal
form as they will have to fit into any remaining spaces in the near solid rock. This
results in the later crystals growing around and between earlier crystals, so giving
them an irregular outline.
Chemical properties of tertiary basalt lavas from Mull, Scotland
4'9
Table 4
Size of opaque grains and crystalline perfection
Main Phase
Sample
126-1
Size
Pm
Type of
Grain
Size
Pm
Type of
Grain
33
2
53
I
-
-
I20-7
I14-1
I4
I
45
I
108-1
42
I
107-5
68
r
106-1
105-1I
104-1
I
103-1
100-9
100-5
22
I
I
98-1
97-6
96-1
94-1
93-2
91-2
.
39
67
58
40
32
25
8
29
42
24
33
34
2
I
2
I
I
2
2
2
I
I
2
2
2
I
I
2
2
2
65-3
61-1
60-1
58-1
37
69
I9
I0
26
39
I0
22
I:
2
I
I
I
37
47
I7
28
I
76-8
75-45
74-1
67-9
B
I
79-2
I7
40
30
I
36
I2
I
27
22
22
I9
25
39
A
A
2
2
I3
20
2
2
2
76
I
2
2
B
45
78
89-8
89-1
86-1
84-1
83-1
81-3
Main
Phase
A
A
A
I
I
2
90-5
73-1
70-7
Degree of Crystalline perfection
w-,
&r
125-8
99-1
Minor Phase
'I3
I5
r
B
A
A
41
9
I
I
Type of Grain
Type I-Magnetite without exsolution lamellae.
Type 2-Magnetite with exsolution larnellae.
Degree of Crystallization Perfection
Type A-well formed crystals.
Type B-irregular crystals.
R
N
N
N
N
N
N
N
N
N
R
N
N
A
R
A
B
B
B
B
N
A
A
A
B
B
A
A
B
A
A
B
I
Minor
Phase
Sense of
Magnetization
B
B
A
B
B
B
B
R
R
R
R
N
R
N
R
R
R
R
R
N
N
R
R
R
N
N
R
R
R
420
J. M. Ade-Hall
With practice the non-opaque constituents of an igneous rock with only a few
constituent minerals can be distinguished by their form and texture in a polished
section.
It was found that two degrees of crystalline perfection of the opaques characterized the majority of the samples.
Degree A :Relatively small equi-dimensional grains showing various regular
sections of cubic crystal forms and irregular conchoidal cracking due to later stress
in the solid crystals. These features show the relatively early termination of
crystallization of the primary opaques, before much felspar, or any augite, has
crystallized. Grains with these crystallization features characteristically consist
of pale brown or greyish magnetite sometimes containing felt-like ilmenite or
patches of granules.
Degree B: Relatively large grains with irregular angular outlines. These
features show the grains to have finished crystallizing after most of the plagioclase
felspar has crystallized and the augite has partly crystallized. No cracking is
seen in the grains suggesting that little further crystallization can have taken place
in the lava after that of the primary opaque. Grains with these features characteristically consist of medium brown magnetite with ilmenite exsolved within the 111
planes.
The distribution of these degrees of crystalline perfection in the samples is
shown in Table 4. This table shows that the grouping of samples on the basis
of the two different degrees of crystalline perfection of the primary opaque grains
is essentially the same as the grouping on the basis of whether the magnetite does
or does not contain exsolved ilmenite lamellae.
We find that the smaller, well-crystallized primary opaques usually consist of
magnetite without ilmenite lamellae while the larger irregular primary opaques
usually contain ilmenite lamellae.
3.5. The spatial relationships between the occurrence of opaque phases and regions
of serpentinous materials
Some samples containing opaque material both with and without ilmenite
exsolved in 111 planes, show a spatial relationship between exsolved grains and
areas of serpentinous secondary minerals. This is particularly well shown by sample
84-1(Plate 3). The spatial relationship strongly suggests that the agency responsible for the patches of secondary serpentinous minerals also caused the exsolution
of ilmenite in I I I planes from the originally homogeneous primary opaque grains.
Grains of opaque material protected from such areas of alteration by unaltered
augite and felspar remain unexsolved.
This exsolution of ilmenite in I I I planes in magnetite may well be due to the
influence of temperature and of water vapour on homogeneous titaniferous
magnetites. This takes place near to areas of serpentinous minerals which represent
the passages by which hot water vapour percolates through lava flows in the lower
part of a lava sequence.
This spatial relationship does not, however, hold throughout, and the statistical
validity of the correlation is still under study. Some samples, such as 67-9, 100-5
show unexsolved grains contiguous with serpentinous areas. Some preliminary
measurements with an electron probe instrument suggest the possibility that such
unexsolved grains may contain enough bivalent impurities (calcium, manganese
and zinc) as to inhibit exsolution. This will be further investigated.
Chemical properties of tertiary basalt lavas from Mull, Scotland
421
4. Summary of results
Four definite statistical petrological differences between normally and reversely
magnetized samples have been found in a collection of lava samples. These are
as follows:
Alteration products of olivine
I n normally magnetized samples the pseudomorphs after olivine almost always
consist essentially of pale green secondary minerals without secondary opaques.
The pseudomorphs in reversely magnetized samples consist of large quantities
of secondary iron oxides and iddingsite in addition to green secondary minerals.
(I)
Mineral associations now found in primary opaque grains
Normal samples almost always contain pale brown or grey cracked magnetite
with some transparent granules. Reverse samples almost always contain medium
brown magnetite and are typified by varying amounts of ilmenite exsolved in
octahedral planes in the magnetite.
(2)
(3) Grain size of opaques
Measurement of primary opaque grain size in transmitted light shows that
the reverse samples have a significantly higher average grain size, probably due to
secondary amorphous oxides attached to primary grains, than the normal samples.
The measurements made in reflected light give the same average size for both N
and R specimens since the amorphous oxide, which appears opaque in transmission
does not, in general, appear metallic in reflection.
(4)Degree of crystalline perfection of primary opaques
The primary opaques of normal samples frequently show fairly well developed
cubic form, while those in reverse samples almost always show no regular crystal
form indicating that these have finished crystallizing after, and so around and
between, plagioclase felspar.
The observed distinctions between normally and reversely magnetized lava
samples can be tentatively explained by postulating the existence of two different
crystallization sequences or compositional types of lava followed by subsequent
petrological alteration affecting all the flows in the sequence.
These are as follows:
( a ) Normal flows
These are typified by pseudomorphs after olivine consisting only of green
secondary minerals and by small well-crystallized primary opaques which, although
containing appreciable titanium (shown by electron-probe measurement), have
failed to exsolve this element in the form of ilmenite lamellae.
(6) Reverse flows
These are typified by pseudomorphs after olivine consisting of secondary iron
oxides, iddingsite and green secondary minerals, together with large primary
opaque grains with exsolution lamellae. These opaque grains rarely show any
simple crystal form, but are often moulded onto the well-formed felspars, indicating a later termination of crystallization than the felspars.
Several flows contain both normal and reverse directions of magnetization. The
422
J. M. Ade-Hall
normal and reverse samples of flow IOO show the same differences as samples from
wholly normal or reverse flows. This fact suggests that different parts of the same
flow can have different crystallization histories.
From the standpoint of the natural remanent magnetization of the samples, it is
most likely that the primary opaques of the normally magnetized flows show their
original thermo-remanence since there is no sign of appreciable ionic diffusion
within these minerals. However, the reversely magnetized flows contain primary
opaques showing abundant evidence of ionic diffusion, i.e. exsolution of ilmenite.
I n general, both normal and reverse samples have subsequently undergone
serpentinization, presumably due to the effect of heat and water during pneumatolysis. Different effects produced by rather similar pneumatolytic conditions in
the normal and reverse samples are very probably due to the (assumed) slight
differences in the original samples.
5. Conclusions
Many workers have emphasized that if a reversal of the Earth’s magnetic field
is to be deduced from a set of normal and reversed lavas, then there must be no
statistically significant difference of magnetic or petrological nature between the
two sets of lavas: unless one were to make the highly improbable assumption that
the Earth’s field reversed at just the epoch when the petrological character of the
lava changed. So when one finds, as with the Mull plateau basalts, a petrological
difference between N and R specimens, then one is led to suppose either that
the N or the R specimens are self-reversed, or that one set gained its natural
remanence at a later date in a field having the reverse direction from that acting
during the extrusion of the lavas.
T h e petrological evidence collected in this paper makes it more probable that it
is the normally magnetized lavas which have retained the direction of the contemporary field, and the reversely magnetized flows acquired their N R M at a later date.
For the former are petrologically slightly less altered than the latter, as indicated by
both the transparent and opaque minerals. T h e evidence for this statement is
both that two normal samples contain fresh olivine and also that the primary
magnetite of all the normal samples does not show ilmenite exsolved in octahedral
planes. Moreover, the high correlation of reversed magnetization with the presence
of exsolution 1amelIae i n the magnetite grains suggests a correlation of reverse
magnetization with ionic diffusion in general.
Now theoretical studies by Nkel (1951)and Verhoogen (1956,1962)have
shown that ionic diffusion can in some circumstances lead to magnetic selfreversal. So one may tentatively assume that the Earth‘s field was normally
directed throughout the period when the Mull lavas were laid down, and that
the reversely magnetized flows have become self-reversed due to some petrological
process. It is not possible at present to decide for certain whether this physicalchemical self-reversal occurred during the latter stages of cooling of the original
magma, or at some geologically later epoch, when perhaps the lavas were buried
and under the influence of heat and steam. T h e terms “sample” and “flow” have
been used synonymously in this paper; in fact, only samples showing distinctly
normal and reverse directions were examined. A large number of discordant
samples (for definition see the following paper) also occur. No petrological
examination has been made of these but the magnetic investigations suggest magnetic stability of the natural magnetization.
423
Recently Van Zijl, Graham & Hales (1962 a, b) have made detailed magnetic
and petrological studies of the Triassic-Jurassic Stormberg lavas of South Africa.
As with the Tertiary Mull lavas studied here, the lower flows are reversed and the
upper ones normal. These authors, however, find no significant magnetic, petrological or x-ray differences between the normal and reverse flows, and so conclude
that the transition from the lower reverse to the higher normal lavas represents a
reversal of the Earth’s field. In addition, several cases of lava and sediment baked
by a dyke provide additional evidence for field reversal. Balsley & Buddington
(1954 and 1957) have investigated and discussed differences which they did find
between normal and reversed rocks in the Adirondack Mountains, although they
offered no final explanation of these differences. Nor can we for Dutt’s rocks.
There is no direct evidence that the observed exsolution structure in Dutt’s reversed
samples has directly resulted in self-reversal. The mineral phase exsolved (near
pure ilmenite) is probably not ferromagnetic at ordinary temperatures, so cannot
directly have had anything directly to do with self-reversal. One may postulate
that both exsolution and self-reversal have a common cause, that is, re-heating
and hydro-thermal changes.
It is also possible that self-reversal, if it has occurred, may be the result of Fe
rich and T i rich solid solutions unmixing, but on such a fine scale as to be difficult
to resolve.
Chemical properties of tertiary basalt lavas from Mull, Scotland
Acknowledgments
I am greatly indebted to Professor P. M. S. Blackett for suggesting this study
and for the free provision of facilities. Professor J. McG. Bruckshaw of the
Department of Geophysics, kindly allowed me much free time to work in the
Physics Department of the Imperial College. Miss J. Langbein was very helpful
with much of the preparation of specimens.
References
Balsley, J. R. & Buddington, A. F., 1954, J . Geomag. @ Geoelec. 6 , 176.
Balsley, J. R. & Buddington, A. F., 1957. Records Geol. Surv. of India, 86, 553.
Blackett, P. M. S., Clegg, J. A. & Stubbs, P. H. S., 1960. Proc. Roy. SOC.256,291.
Bruckshaw, J. Mc G. & Vincenz, S. A., 1954. M.N.R.A.S.,Geophys. Suppl. 6,
579.
Dutt, S. K., 1955. Thesis, Imperial College, London.
NCel, L., 1951. Ann. Geophys. 7 , 90.
Van Zijl, J. S. V., Graham, K. W. T. & Hales, A. L., 196za. Geophys. J., 7 , 2 3 .
Van Zijl, J. S. V., Graham, K. W. T. &Hales, A. L., 1962b. Geophys.J., 7 , 169.
Verhoogen, J., 1956. J. Geophys. Res., 61, 201.
Verhoogen, J., 1962. J . Geol., 70, 168.
Vincenz, S. A., 1954. M.N.R.A.S., Geophys. Suppl., 6, 590.