Composition and origin of olivine xenocrysts in a
Composition and origin of olivine xenocrysts in a basanite flow from Hut Point Peninsula, Antarctica I. S. LEUNG
Department of Geology and Geography Herbert H. Lehman College, CUNY Bronx, New York 10468
S. B. TREVES Department of Geology University of Nebraska Lincoln, Nebraska 68588
The Dry Valley Drilling Project on Hut Point Peninsula, Antarctica, penetrated and retrieved the core of a 43.77-meter thick basanite flow from the second bore hole at a depth of 101 meters. Thirty-eight samples chosen at regular intervals of this core were examined. This basalt contains olivine crystals dispersed abundantly throughout the flow. The 100 magnesium
Geophysical studies on Mount Erebus T. NAGATA
National Institute of Polar Research Itabashi, Tokyo, Japan
Three Japanese scientists visited McMurdo Station for the International Mount Erebus Seismic Studies (IMESS) during the 1983-1984 field season. The Japanese participants were K. Kaminuma of National Institute of Polar Research, S. Ueki of Tohoku University, and E. Koyama of Earthquake Research Institute, University of Tokyo. They conducted three research programs while staying at McMurdo Station from 11 November 1983 to 16 January 1984. Seismic observations by the telemetry network. This is the main program of the IMESS. The telemetry network with five seismic stations has been established by the U.S. party on the summit and the flank of Mount Erebus since 1980, and the Japanese party has installed the recording system for the seismic network at Scott Base (Takanami et al. 1983-a, 1983-b; Shibuya et al. 1983). The Japanese party played back seismic magnetic tapes to make seismograms to be distributed to the U.S. and New Zealand participants during their stay at McMurdo. Figure 1 shows the daily number of volcanic earthquakes occurring in and around Mount Erebus, counted at Abbott Peak station, and the plume activity observed at Scott Base from 2 October 1983 to 2 January 1984. The plume activity was classified by sight into the following three stages as illustrated in the right side of the upper part of figure 1; (1) no plume was observed, (2) plume was observed only around the summit, and 22
(magnesium plus iron) ratio of olivine in the matrix varies less than 2 percent from a mean composition of 70 percent forsterite and 30 percent fayalite, whereas all other olivine crystals are variably zoned, showing a whole range of composition from Fo89 to 75 percent forsterite near the core, and from 70 to 80 percent forsterite at the rim. Most crystals also display strain shadows and kink bands. An olivine nodule associated with a deep red spinel and minor amounts of phiogopite was traversed by an electron microprobe which indicated a constant composition of Fo852 for all the grains in the central portion of the nodule, but on each side of this portion, olivine crystals become progressively more iron-rich until the surrounding host basanite is reached where all the grains have a unique composition of 74.6 percent forsterite. This seems to indicate that all olivine except the minute crystals in the matrix are xenocrysts derived from a mechanical disintegration and subsequent continued reaction in the magma of olivine nodules previously equilibrated in the Earth's mantle. The presence of residual magnesium rich olivine xenocrysts implies either a rapid ascent of magma to the surface, or that they had been prevented from reacting with the melt inside large xenoliths which disintegrated at a later stage under higher temperature conditions shortly before or during eruption.
(3) huge plume was observed. The columns and the hatched columns in the figure indicate the daily number of earthquakes of which maximum amplitudes are larger than 2 millimeters and 20 millimeters, respectively, on the played-back seismograms. The arrows at the top of columns show that the exact number of events could not be counted because of high microseismic activity. The mean number of earthquakes per day was 67, and the mean number of earthquakes with the maximum amplitude larger than 20 millimeters was seven events. We also looked for any new relationship between the seismic activity and the plume activity during the period. It seems that the seismic activity of both tectonic and volcanic earthquakes in Ross Island during this period is of a steady state
Daily Number of Earthquakes October 02,1983 - January 02, 1984 --no visibility
4 to 2 6 10 15 20 25 301 5 Oct.1983
15 20 25 30 5 10 15 20 25 312 Nov. Jan. 1984 Dec.
Figure 1. Daily number of earthquakes recorded at Abbott Peak and plume activity observed at Amundsen-Scott Base from 2 October 1983 to 2 January 1984. ("mm" denotes millimeter.) ANTARCTIC JOURNAL
as compared with the activities in the last few seasons (Takanami etal. 1983-a, 1983-b; Shibuya etal. 1983; Ueki et al. 1984). The Japanese members assisted the U.S. members in establishing two new seismic stations of the telemetry network in January. Observations by temporary seismic network. Eight temporary seismic stations were established on the flank of Mount Erebus for determining the earthquake locations with higher accuracy from 19 November 1983 to 3 January 1984. The earthquake locations were determined for a period from 19 November to 9 December only, because some seismic stations of the telemetry network had trouble from 9 December 1983 until the seismic station was removed 3 January 1984. Hypocenters of earth-
W
quakes are shown in figure 2. The upper right side of the figure indicates the projection of hypocenters along the north-south vertical plane and the lower left side is that along the east-west vertical plane. The earthquakes located below the summit were the events associated with eruptions. The lower right side of figure 2 shows the epicenters of earthquakes from 13 September 1982 to 29 January 1983. An extreme earthquake swarm took place in the northwestern flank of Mount Erebus on 8-9 October 1982 (Kaminuma, Ueki, and Kienle in press; Ueki et al. 1984). It is remarkable that about 1 year after the October earthquake swarm in 1982, no earthquake was located in the earthquake swarm area, as seen in the figure.
E Lfl o DEPTH ( km )
CD
ci
z
-5 -
0
19
19
0
0 0190 00
0 19
19 19
0
167
CD
N
-o Ln 19
l68E
ABB (i \H004
1p
I
48OM
• TEA
/Th
OO
0 5 10 km
SEP. la. 1982 - JAN. 29, 1983
Figure 2. Epicenter locations of earthquakes and their vertical projection along the north-south and east-west profiles from 19 November to 9 December 1983. The right side of the lower part presents epicenter locations of earthquakes from 13 September 1982 to 29 January 1983. ("km" denotes kilometer.)
1984 REVIEW
23
A seismograph was installed near Vanda Station in the Wright Valley for a period from 19 December 1983 to 7 January 1984. The purpose of this observation was to investigate local seismicity in the dry valleys region and to detect a level of microseisms. Gravity measurements. A gravity survey with the LaCosteRomberg gravimeter model G was carried out at 20 points including eight temporary seismic stations. The elevation of the gravity survey points was measured with a barometric altimeter. The elevation corrections for the air pressure change during the survey were made using the barogram at Scott Base. However, the accuracy of elevation at each point was estimated to be approximately 10 meters. Figure 3 shows simple Bouguer anomalies, with assumed density of 2.67 milligrams per cubic meter. Simple Bouguer anomalies range from -92.4 milligals to 15.7 milligals as shown in the figure. The Japanese party has also carried out gravity surveys in Ross Island during the austral summer in 1982-1983 (Kaminuma, Koyama, and Ueki 1984). Data reductions for the gravity surveys in 1982-1983 and 1983-1984 are under way. The IMESS has been supported by the National Science Foundation (United States), Antarctic Division of Department of Scientific and Industrial Research (New Zealand), and National Institute of Polar Research (Japan). We are extremely grateful to Andrew Harrall and Jermey Ireland (Scott Base wintering members in 1982-1983 and 1983-1984, respectively) who maintained our recording system. Philip Kyle of New Mexico Institute of Mining and Technology has continued to render valuable advice and assistance in the field operation. Special thanks are due to the VXE-6 helicopter crew for their patience while setting up of the portable seismic stations and during the gravity surveys.
References Kaminuma K., E. Koyama, and S. Ueki. 1984. Preliminary report on gravity survey in Ross Island, Antarctica. In T. Nagata (Ed.), Proceedings of the fourth symposium on antarctic geosciences 1983. Kaminuma, K., S. Ueki, and J . Kienle. In press. Volcanic earthquake swarms at Mt. Erebus, Antarctica. Proceedings of geophysics of the Polar Regions, XVIII IUGG, general assembly. Shibuya, K., M. Baba, J. Kienle, R.R. Dibble, and P.R. Kyle. 1983. A study on the volcanic and seismic activities of Mount Erebus, Antarctica 1981-1982. In T. Nagata (Ed.), Proceedings of the third symposium on antarctic geosciences 1982.
24
Simple Bouguer Anomaly p=2.67 9.3
-1.7
•10.8 •-15.8 _45.1•328 8 _770••-77.3 -20.2 • -0.5 -92.4
. . 6.8 7.3 1.3
157J1'
0 I
Station McMurdo'i' -7.9
lp 20 30
'km
Figure 3. Simple Bouguer anomalies. The density of the mountain was assumed to be 2.67 milligrams per cubic meter. ("km" denotes kilometers; "p" denotes density.)
Takanami, T., J . Kienle, P.R. Kyle, R.R. Dibble, K. Kaminuma, and K. Shibuya. 1983-a. Seismological observation on Mt. Erebus 1980-1981. In J.B. Jago and R.L. Oliver (Eds.), Antarctic Earth Sciences. Proceedings of the Fourth International Symposium on Antarctic Earth Sciences, Adelaide, Australia, August 1982. Takanami, T., K. Kaminuma, K. Terai, and N. Osada. 1983-b. Seismological observation on Mt. Erebus, Ross Island, Antarctica, 1980-1981. In T. Nagata (Ed.), Proceedings of the third symposium on Antarctic Geosciences 1982. Ueki, S., M. Baba, K. Kaminuma, E. Koyama, and J . Kienle. 1984. Seismic activity of Mount Erebus, Antarctica in 1982-1983. In T. Nagata (Ed.), Proceedings of the fourth symposium on Antarctic Geosciences 1983.
ANTARCTIC JOURNAL
Department of Geology and Geography Herbert H. Lehman College, CUNY Bronx, New York 10468
S. B. TREVES Department of Geology University of Nebraska Lincoln, Nebraska 68588
The Dry Valley Drilling Project on Hut Point Peninsula, Antarctica, penetrated and retrieved the core of a 43.77-meter thick basanite flow from the second bore hole at a depth of 101 meters. Thirty-eight samples chosen at regular intervals of this core were examined. This basalt contains olivine crystals dispersed abundantly throughout the flow. The 100 magnesium
Geophysical studies on Mount Erebus T. NAGATA
National Institute of Polar Research Itabashi, Tokyo, Japan
Three Japanese scientists visited McMurdo Station for the International Mount Erebus Seismic Studies (IMESS) during the 1983-1984 field season. The Japanese participants were K. Kaminuma of National Institute of Polar Research, S. Ueki of Tohoku University, and E. Koyama of Earthquake Research Institute, University of Tokyo. They conducted three research programs while staying at McMurdo Station from 11 November 1983 to 16 January 1984. Seismic observations by the telemetry network. This is the main program of the IMESS. The telemetry network with five seismic stations has been established by the U.S. party on the summit and the flank of Mount Erebus since 1980, and the Japanese party has installed the recording system for the seismic network at Scott Base (Takanami et al. 1983-a, 1983-b; Shibuya et al. 1983). The Japanese party played back seismic magnetic tapes to make seismograms to be distributed to the U.S. and New Zealand participants during their stay at McMurdo. Figure 1 shows the daily number of volcanic earthquakes occurring in and around Mount Erebus, counted at Abbott Peak station, and the plume activity observed at Scott Base from 2 October 1983 to 2 January 1984. The plume activity was classified by sight into the following three stages as illustrated in the right side of the upper part of figure 1; (1) no plume was observed, (2) plume was observed only around the summit, and 22
(magnesium plus iron) ratio of olivine in the matrix varies less than 2 percent from a mean composition of 70 percent forsterite and 30 percent fayalite, whereas all other olivine crystals are variably zoned, showing a whole range of composition from Fo89 to 75 percent forsterite near the core, and from 70 to 80 percent forsterite at the rim. Most crystals also display strain shadows and kink bands. An olivine nodule associated with a deep red spinel and minor amounts of phiogopite was traversed by an electron microprobe which indicated a constant composition of Fo852 for all the grains in the central portion of the nodule, but on each side of this portion, olivine crystals become progressively more iron-rich until the surrounding host basanite is reached where all the grains have a unique composition of 74.6 percent forsterite. This seems to indicate that all olivine except the minute crystals in the matrix are xenocrysts derived from a mechanical disintegration and subsequent continued reaction in the magma of olivine nodules previously equilibrated in the Earth's mantle. The presence of residual magnesium rich olivine xenocrysts implies either a rapid ascent of magma to the surface, or that they had been prevented from reacting with the melt inside large xenoliths which disintegrated at a later stage under higher temperature conditions shortly before or during eruption.
(3) huge plume was observed. The columns and the hatched columns in the figure indicate the daily number of earthquakes of which maximum amplitudes are larger than 2 millimeters and 20 millimeters, respectively, on the played-back seismograms. The arrows at the top of columns show that the exact number of events could not be counted because of high microseismic activity. The mean number of earthquakes per day was 67, and the mean number of earthquakes with the maximum amplitude larger than 20 millimeters was seven events. We also looked for any new relationship between the seismic activity and the plume activity during the period. It seems that the seismic activity of both tectonic and volcanic earthquakes in Ross Island during this period is of a steady state
Daily Number of Earthquakes October 02,1983 - January 02, 1984 --no visibility
4 to 2 6 10 15 20 25 301 5 Oct.1983
15 20 25 30 5 10 15 20 25 312 Nov. Jan. 1984 Dec.
Figure 1. Daily number of earthquakes recorded at Abbott Peak and plume activity observed at Amundsen-Scott Base from 2 October 1983 to 2 January 1984. ("mm" denotes millimeter.) ANTARCTIC JOURNAL
as compared with the activities in the last few seasons (Takanami etal. 1983-a, 1983-b; Shibuya etal. 1983; Ueki et al. 1984). The Japanese members assisted the U.S. members in establishing two new seismic stations of the telemetry network in January. Observations by temporary seismic network. Eight temporary seismic stations were established on the flank of Mount Erebus for determining the earthquake locations with higher accuracy from 19 November 1983 to 3 January 1984. The earthquake locations were determined for a period from 19 November to 9 December only, because some seismic stations of the telemetry network had trouble from 9 December 1983 until the seismic station was removed 3 January 1984. Hypocenters of earth-
W
quakes are shown in figure 2. The upper right side of the figure indicates the projection of hypocenters along the north-south vertical plane and the lower left side is that along the east-west vertical plane. The earthquakes located below the summit were the events associated with eruptions. The lower right side of figure 2 shows the epicenters of earthquakes from 13 September 1982 to 29 January 1983. An extreme earthquake swarm took place in the northwestern flank of Mount Erebus on 8-9 October 1982 (Kaminuma, Ueki, and Kienle in press; Ueki et al. 1984). It is remarkable that about 1 year after the October earthquake swarm in 1982, no earthquake was located in the earthquake swarm area, as seen in the figure.
E Lfl o DEPTH ( km )
CD
ci
z
-5 -
0
19
19
0
0 0190 00
0 19
19 19
0
167
CD
N
-o Ln 19
l68E
ABB (i \H004
1p
I
48OM
• TEA
/Th
OO
0 5 10 km
SEP. la. 1982 - JAN. 29, 1983
Figure 2. Epicenter locations of earthquakes and their vertical projection along the north-south and east-west profiles from 19 November to 9 December 1983. The right side of the lower part presents epicenter locations of earthquakes from 13 September 1982 to 29 January 1983. ("km" denotes kilometer.)
1984 REVIEW
23
A seismograph was installed near Vanda Station in the Wright Valley for a period from 19 December 1983 to 7 January 1984. The purpose of this observation was to investigate local seismicity in the dry valleys region and to detect a level of microseisms. Gravity measurements. A gravity survey with the LaCosteRomberg gravimeter model G was carried out at 20 points including eight temporary seismic stations. The elevation of the gravity survey points was measured with a barometric altimeter. The elevation corrections for the air pressure change during the survey were made using the barogram at Scott Base. However, the accuracy of elevation at each point was estimated to be approximately 10 meters. Figure 3 shows simple Bouguer anomalies, with assumed density of 2.67 milligrams per cubic meter. Simple Bouguer anomalies range from -92.4 milligals to 15.7 milligals as shown in the figure. The Japanese party has also carried out gravity surveys in Ross Island during the austral summer in 1982-1983 (Kaminuma, Koyama, and Ueki 1984). Data reductions for the gravity surveys in 1982-1983 and 1983-1984 are under way. The IMESS has been supported by the National Science Foundation (United States), Antarctic Division of Department of Scientific and Industrial Research (New Zealand), and National Institute of Polar Research (Japan). We are extremely grateful to Andrew Harrall and Jermey Ireland (Scott Base wintering members in 1982-1983 and 1983-1984, respectively) who maintained our recording system. Philip Kyle of New Mexico Institute of Mining and Technology has continued to render valuable advice and assistance in the field operation. Special thanks are due to the VXE-6 helicopter crew for their patience while setting up of the portable seismic stations and during the gravity surveys.
References Kaminuma K., E. Koyama, and S. Ueki. 1984. Preliminary report on gravity survey in Ross Island, Antarctica. In T. Nagata (Ed.), Proceedings of the fourth symposium on antarctic geosciences 1983. Kaminuma, K., S. Ueki, and J . Kienle. In press. Volcanic earthquake swarms at Mt. Erebus, Antarctica. Proceedings of geophysics of the Polar Regions, XVIII IUGG, general assembly. Shibuya, K., M. Baba, J. Kienle, R.R. Dibble, and P.R. Kyle. 1983. A study on the volcanic and seismic activities of Mount Erebus, Antarctica 1981-1982. In T. Nagata (Ed.), Proceedings of the third symposium on antarctic geosciences 1982.
24
Simple Bouguer Anomaly p=2.67 9.3
-1.7
•10.8 •-15.8 _45.1•328 8 _770••-77.3 -20.2 • -0.5 -92.4
. . 6.8 7.3 1.3
157J1'
0 I
Station McMurdo'i' -7.9
lp 20 30
'km
Figure 3. Simple Bouguer anomalies. The density of the mountain was assumed to be 2.67 milligrams per cubic meter. ("km" denotes kilometers; "p" denotes density.)
Takanami, T., J . Kienle, P.R. Kyle, R.R. Dibble, K. Kaminuma, and K. Shibuya. 1983-a. Seismological observation on Mt. Erebus 1980-1981. In J.B. Jago and R.L. Oliver (Eds.), Antarctic Earth Sciences. Proceedings of the Fourth International Symposium on Antarctic Earth Sciences, Adelaide, Australia, August 1982. Takanami, T., K. Kaminuma, K. Terai, and N. Osada. 1983-b. Seismological observation on Mt. Erebus, Ross Island, Antarctica, 1980-1981. In T. Nagata (Ed.), Proceedings of the third symposium on Antarctic Geosciences 1982. Ueki, S., M. Baba, K. Kaminuma, E. Koyama, and J . Kienle. 1984. Seismic activity of Mount Erebus, Antarctica in 1982-1983. In T. Nagata (Ed.), Proceedings of the fourth symposium on Antarctic Geosciences 1983.
ANTARCTIC JOURNAL
