A high concentration of meteorites were discovered on a blue ice field northeast of the Frontier Mountains. As a result of a systematic search, a total of 42 meteorites were recovered. The current glacial situation has evolved through various stages, which are discussed in relationship to the concentration of meteorites. Ice flow patterns are summarized. The chemical composition and terrestrial ages of the meteorites are discussed

Delisle, G.

Hoefle, H. C.

Schultz, L.

Thierbach, R.

English

NASA Technical Reports Server

^|^ N86-28815
A. 30
" Discovery of Meteorites on a Blue-Ice Field near thei/Frontier
Mountains, North Victoria Land, Antarctica
G. Delisle, H.-C. Hbfle, R. Thierbach, Federal1 Institute for
Geosciences and Natural Resources, 3000 Hannover, FRG
L. Schultz, Max Planck Institute for Chemistry, 6500 Mainz, FRG
Members of the West German GANOVEX IV expedition 1984/85 (German
Antarctic North Victoria Land Expedition) discovered a high con-
centration of meteorites on a blue-ice field northeast of the
Frontier Mountains. As a result of a systematic search, a total
of 42 meteorites were discovered.
The Frontier Mountains pierce the polar ice sheet over a length
of approximately 8 km with a NW-SE trend. Glacial flow from the
polar regions approaches from WSW, flowing around the mountain
range at both ends (Figure 1). Due to the barrier function of
the range, the ice surface to the southwest is higher than in
the northeast by an estimated 200 m. Large areas of crevasses
caused by the stepwise descent of the ice sheet occur at both
ends of the Frontier Mountains. A blue-ice field has formed on
the lee side of the range, characterized by high rates of subli-
mation and abrasion due to frequent strong catabatic and foehn
winds, as well as compressive ice flow, as evident from the oc-
currence of numerous dust bands.
The crevasse pattern outside the valley is obviously controlled
by the f low pattern of the ice masses around the Frontier Mts.
Meter-wide crevasses run parallel to the trend of the Frontier
Mts. in zones in which the elevation rapidly decreases. Past
the range to the E, the crevasse orientation swings gradually
from NW-SE to SW-NE, whi le crevasse widths decrease to a few cm.
Noteworthy are the surprisingly deep depressions in the blue
ice, reflecting changes in elevation by more than 100 m (esti-
mated).
The current glacial situation has evolved through various stages.
By the orientation of grooves and striae on bedrock at various
levels of elevation, at least three different stages of glacia-
tion characterized by different ice-f low directions can be di-
stinguished:
- Grooves only at the highest points of the range were formed by
an SSE-NNW trending ice-flow (long arrows in Fig. 1). In ana-
https://ntrs.nasa.gov/search.jsp?R=19860019343 2020-03-20T14:44:14+00:00Z
31
logy to glaciological observations in the Dry Valleys, we as-
sume an Upper Miocene age for this episode.
- A lowering of the ice surface of the polar ice sheet (Pliocene?)
resulted in a change of the flow direction toward SW-NE
(medium size arrows in Fig. 1).
- A further reduction of surface elevation of the polar ice
sheet exposed the Frontier Mountains to a large extent. Now,
glacial f low controlled by topography prevails (short arrows
in Fig. 1).
Today, the following situation has evolved:
The greatest depression in the blue-ice field to the E of the
range occurs in a valley an the southeastern end of the Frontier
Mountains. The valley serves as a preferred channel for cataba-
tic and foehn winds that are the cause of the locally high de-
gree of sublimation and abrasion. The mass losses are counterac-
ted by blue-ice moving into the valley from the. east (nearly op-
posite of the regional f low direction).
The thickness of the blue ice was measured-with a radio echo
sounding instrument (RES) along the profile A-B (see Fig. 1) pa-,
rallel to the long axis of the valley. From the 0 to 300 m mark,
a complicated reflection pattern was observed for depths of more
than 150 m indicative of an irregular subsurface pattern at the
interface ice-bedrock. Reflections from this interface are indi-
cated in Figure 2a as bars. Dots represent reflections from a
non-horizontal surface.
The interpretation of the available RES-data is depicted in Figu-
re 2b. A local glacier entering the western end of the valley
apparently sustains at its bottom a ground moraine judging from
the extensive reflection pattern for depths below the first major
reflection. The side reflections (dots in Fig 2a) are apparently
caused by a buried lateral moraine at the glacier front. No evi-
dence, however, is seen at the ice surface. The pressure on the
lateral moraine is balanced by the inflowing blue-ice from the
E. RES-measurements on the blue-ice in general yield a less com-
plicated reflection pattern past the first major reflection indi-
cative of a smoother ice-bedrock interface.
An obvious connection of the reflectors with steeply dipping dust
bands, trending almost perpendicularly to the long axis of the
32
valley, was not detected. The lateral separation of these dust
bands, slightly parabolic, ranged from a few dm to about 30 m.
The highest concentration of meteorites were found within an area
of 300 m by 1 500 m within the valley. Wi th reference to profile
A-B, most meteorites were found between the 600 m to 900 m marks
(see Fig. 2).
The largest meteorite, about 14 cm long, was discovered about
3 km ESE of the entrance to the valley. The blue ice there is
very sparcely covered by wind-blown rock fragments of local
origin.
The main discovery site, is covered by an almost unbroken" veneer
of rock fragments. Trending from W to E, the size of the rock
fragments increases steadily from pebble size to boulders with
diameters in excess of 3 m. The latter are on top of an ice
ridge at the E-entrance to the valley (identical with point B of
Fig. 2), surrounded by a rock veneer several cm thick.
The total weight of the recovered meteorites is 1.2 kg, however,
only 7 specimens have masses larger than 10 g. The specimens
from the main discovery site are moderately weathered; cuts
through the specimens show brown limonitic staining throughout,
but most of the metal has been preserved. Preliminary minera-
logical investigations of the larger specimens show that these
meteorites belong to different petrological and chemical types
of H- and L-group chondrites. Noble gas and ^Be analyses of 7
meteorites yield different records of irridation which exclude
pairing of these investigated samples. Two H-group chondrites -
FRO 8405 (H3) and FRO 8409 (H4) - contain large amounts of so-
lar trapped gases (4He: 1.8xlO~3ccSTP/g). A brecciation, howe-
ver, is not visible in hand specimens because the brown staining
obscures any primary structure.
has been measured in 4 of these meteorites. FRO 8403, an H6
chondrite with a ^Ne-exposure age of about 5 million years, has
a 2fy\l-activity
 Of 29.2 dpm/kg. This number corresponds to a
terrestrial age of about 6.6x10$ years and is one of the longest
terrestrial ages for Antarctic meteorites observed so far.
ORIGINAL PAGE IS
OF POOR QUALITY
Figure 1: Ice flow-directions around Frontier Mountains. The
orientation of grooves and striae on bedrock, caused
by three differnt stages of glaciation, are shown:
long arrows: Upper Miocene glaciation
medium size arrows: Pliocene (?) glaciation
short arrows: third recognized stage of glaciation
33
The meteorite field is shown as hatched area. Most meteo-
rites were found along the eastern margin of the field.
RES-measurements were carried out along line A - B.
Figure 2a: Results of RES-measurements along A-B. Bars mark first
major reflections indicative of the ice-bedrock interfa-
ce. Dots mark reflections from inclined surfaces.
Figure 2b: Interpretation: The ice masses on the left end of the
profile are part of a local glacier approaching from the
west. At its base we suspect a ground moraine of un-
known thickness. The glacier is apparently bounded at
its front by an ice-buried lateral moraine. From the
east, blue-ice enters the valley. The internal structure
of the blue-ice, as shown, is inferred from the occur-
rence of dust bands at the blue-ice surface.
A
0 300 600
2100
B
900m


Discovery of meteorites on a blue-ice field near the Frontier Mountains, North Victoria Land, Antarctica

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server