Implication on Unknown Radioactivity of Giant and Dwarf Haloes in Scandinavian Rocks
Nature, vol. 274, no. 5670, pp. 457-459, August 3, 1978.
GIANT haloes1-3 attracted little attention until it seemed that
those from Madagascar2 might be associated with
superheavy elements4. Even though this association was not
confirmed5, this renewed interest has generated several
additional suggestions6 for giant-halo origin which will be
evaluated elsewhere (R.V.G. et. al. in preparation). We
report here some new data on the giant haloes found in
certain Swedish biotites1,2 and the implications which these
data furnish for a radioactive origin of the enigmatic dwarf
The majority of U and Th haloes in this Swedish biotite1,2
exhibit darkening which extends to the maximum halo
radius (~38−40 μm for the Th halo). About 1% of haloes,
however, have an inner bleached region which varies from
~2 to 25 μm in radius surrounding a highly radioactive
inclusion. Generally, when the bleached region is small
(≤6−8 μm), no change is evident in the dimensions of the
halo. However, in those haloes in which the bleached region
is more intense and of larger radius (~15 μm), a somewhat
weakly coloured diffuse ring is generally observed outside
the normal U−Th halo boundary.
These are the giant haloes which, because they were
earlier reported2 to surround only dense Th haloes, were
tentatively attributed to the low abundance, high energy αs
from 212Po in the 232Th series. However, we now report that
diffuse, abnormally large rings also surround dense U haloes
in this biotite, and as there are no high energy αs of any
significant abundance in the 238U chain, we therefore
consider this hypothesis untenable. Instead, we are
exploring the possibility that these bleached interior regions
are somehow associated with the formation of the giant
Optical microscopic electron microprobe and ion microprobe
study of a giant halo in Swedish biotite. The giant halo in the optical
photograph is ~47 μm in radius. The X-ray and ion probe maps are
approximately the same magnification as the optical photo. a, K Kα X-ray
map; b, 39K secondary ion map; c, 40Ca
secondary ion map; d, Ca Kα X-ray
map; e, optical photo.|
Even though these giant haloes were found in Swedish
granites obtained from the same location as the specimens
Wiman1 used, the giant haloes described here are different
from those he reported. Our giant haloes surround U and/or
Th rich inclusions and have diffuse boundaries which may
vary from ~42 to ~55 μm in radius. In contrast, Wiman
reported giant haloes in biotite around zircon inclusions
showing normal size inner rings and somewhat weak but
rather sharply defined outer rings of 57 μm and more rarely
of 67 μm.
To show the unusual characteristics of the giant halo we
found in Swedish granitic biotites from Arnö and Rickaby,
we give in Fig. 1 the combined results of applying optical
microscopic, electron microprobe X-ray fluorescence and
ion microprobe mass spectrometer techniques. In particular,
Fig. 1 shows: a transmitted light optical photomicrograph of
a single giant halo of radius ~47 μm; two secondary ion
maps obtained (with the ion microprobe mass spectrometer)
by rastering the halo region with a finely focused 16O− beam
and collecting the sputtered secondary ion signal at mass-to-charge
ratios of 39 (39K) and 40 (40Ca); two X-ray maps
obtained (using electron microprobe X-ray fluorescence) by
rastering the same region with a 30-kV electron beam and
collecting in sequence the K Kα and Ca Kα X rays; and the
complete electron microprobe X-ray fluorescence spectra
obtained by spot focusing the electron microprobe beam
first on the mica completely outside the giant halo and then
on the bright circular area inside it. In Fig. 1, the secondary
ion and X-ray maps, as well as the contrasting X-ray
spectra, show the region corresponding to the bright circular
area in the optical photo (which differs slightly in
magnification from the maps) is considerably diminished in
K (by about a factor of 10) and slightly enhanced in Ca
compared to the surrounding mica. Also, the secondary ion
and X-ray maps in Fig. 1 reflect the composition of the halo
region several micrometres above the central radioactive
inclusion, the rectangular outline of which can be seen in the
optical photo in Fig. 1. Certain mineralogical aspects
relating to this phenomenon have been reported previously
by Rimsaite9. These giant haloes probably did not result
from diffusion of radioactivity into the mica because ion
microprobe mass spectrometer studies showed U, Th and Pb
were confined to the inclusion.
As the radius of the bright circular area in Fig. 1 approximates
the range of the predominant lower energy αs of the
U and Th series, we suggest that extreme radiation damage
effects may have first produced a partial decomposition of
the mica in this region with secondary effects then inducing
the migration of K out of and Ca into this region. Note, for
example, that the highly sensitive ion map of Ca in Fig. 1
shows a Ca depletion in the outer halo region, which is
consistent with the idea that Ca has migrated inwards
toward the central region.
ion microprobe secondary ion maps of two dwarf haloes in
Ytterby mica. a, 39K; b, 40Ca; c,
89Y, and d, 28Si. Dwarf haloes are ~6 μm in
As far as the formation of GH is concerned, the large K
depletion in the central region may have been accompanied
by depletion of other major elements during past epochs. If
this happened, then for a certain period of time the total
mass within this region may have reduced sufficiently to
allow the highest normal energy αs of the Th and U series
(8.73 MeV and 7.68 MeV respectively) to penetrate beyond
the normal halo boundary because of having first passed
through a region of lower density. Because of the large K
depletion and slight Ca enrichment, this region has a slightly
lower density than the adjacent mica. But unless O, which
we have not as yet measured, is also depleted, the K
(comprising only ~5−10 atomic % of the biotite) depletion
alone would not be sufficient to permit normal energy αs to
gain enhanced penetration of ~15 μm.
Although there are unanswered questions about this
phenomenon it seems that the bleached areas are high radiation
damaged regions. This was very important to our
studies of the dwarf haloes10,11, the exact origin of which has
remained a mystery for more than 50 years. Under the
microscope, the dwarf haloes11 exhibit the same type of
bleached appearance as is shown by the giant haloes in Fig.
1. Furthermore, in analyzing the dwarf halo centres for the
presence of some recognizable parent and/or daughter
radionuclides, it became evident that the same type of K−Ca
inversion phenomenon was showing up throughout the
dwarf halo region.
Figure 2a−d shows several secondary ion maps obtained
as an 16O− beam was rastered across two closely spaced
dwarf haloes. The ion microprobe mass spectrometry ion
maps in Fig. 2a−d reveal that these dwarf haloes were highly
depleted in 39K and enriched in 40Ca
and 89Y with no change
in 28Si. The complete mass scans shown in Fig. 3a, b,
contrasting the dwarf halo region (Fig. 3a) with the
surrounding mica (Fig. 3b), reveal a corresponding
depletion of 41K, 85Rb,
and 87Rb. Scanning electron
microscopy X-ray fluorescence maps of many dwarf haloes
also showed the depletion of K and enrichment of Ca
throughout the dwarf halo, showing that the ion microprobe
mass spectrometer results were not an artifact of the
sputtering process. We consider the association of this K−Ca
inversion phenomena with the dwarf haloes as additional
evidence of a radioactive origin of the dwarf haloes.
In this context, it has been reported10 that dwarf haloes are
rapidly etched by hydrofluoric acid. We now report that the
halo periphery is more rapidly etched than the central part,
which suggests the emission of a particle from the halo
centre that caused greater damage (higher specific
ionisation) near the end of its path. While this is a
characteristic of α-particles, except for minute amounts of U
found in some ion microprobe mass spectrometer scans, we
have failed to find evidence that 147Sm (ref. 11) or any other
low energy rare earth α-emitters produced the dwarf haloes.
That is, we searched for but found no significant
concentrations of these nuclides in ion microprobe mass
spectrometer scans of the halo centre. Instead we found that
several rare earths were uniformly enriched (Fig. 3a)
throughout the halo, compared to the surrounding mica.
(Fig. 3b). Even though Fig. 3b does not show any rare
earths in the mica, other ion microprobe mass spectrometry
and spark source mass studies showed that these elements
do exist in the mica in very low abundance. We suggest that
the mechanism for this enrichment is the same as that which
caused the preferential transport of Ca (Fig. 2b) into the halo
region (the ion maps of Ca and the rare earths are equal in
extent). Note also the Y enrichment in the halo (Fig. 3a)
compared to the mica (Fig. 3b).
In conclusion, despite this new evidence for a radioactive
origin of the dwarf haloes, more work is needed before the
radionuclides which produced these haloes can be
This research was supported by the Division of Nuclear
Sciences, US Department of Energy, under contract W-
7405-eng-26 with the Union Carbide Corporation and by
Columbia Union College.
|R. V. GENTRY|
W. H. CHRISTIE
D. H. SMITH
J. W. BOYLE
Oak Ridge National Laboratory,
Oak Ridge, Tennessee 37830
|S. S. CRISTY|
J. F. MCLAUGHLIN
|Laboratory Development Division,|
Y-12 Plant, Oak Ridge, Tennessee 37830
Received 13 March; accepted 17 May 1978.
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