(magnesium plus iron) ratio of olivine in the matrix varies less Composition and origin of olivine than 2 percent from a mean composition of 70 percent forsterite xenocrysts in a basanite flow from Hut and 30 percent fayalite, whereas all other olivine crystals are Point Peninsula, Antarctica 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 I. S. LEUNG shadows and kink bands. An olivine nodule associated with a deep red spinel and Department of Geology and Geography minor amounts of phiogopite was traversed by an electron Herbert H. Lehman College, CUNY microprobe which indicated a constant composition of Fo852 for Bronx, New York 10468 all the grains in the central portion of the nodule, but on each side of this portion, olivine crystals become progressively more S. B. TREVES iron-rich until the surrounding host basanite is reached where all the grains have a unique composition of 74.6 percent for- Department of Geology sterite. This seems to indicate that all olivine except the minute University of Nebraska crystals in the matrix are xenocrysts derived from a mechanical Lincoln, Nebraska 68588 disintegration and subsequent continued reaction in the mag- ma of olivine nodules previously equilibrated in the Earths The Dry Valley Drilling Project on Hut Point Peninsula, Ant- mantle. The presence of residual magnesium rich olivine arctica, penetrated and retrieved the core of a 43.77-meter thick xenocrysts implies either a rapid ascent of magma to the sur- basanite flow from the second bore hole at a depth of 101 face, or that they had been prevented from reacting with the meters. Thirty-eight samples chosen at regular intervals of this melt inside large xenoliths which disintegrated at a later stage core were examined. This basalt contains olivine crystals dis- under higher temperature conditions shortly before or during persed abundantly throughout the flow. The 100 magnesium eruption.

Geophysical studies on Mount Erebus (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 seis- T. NAGATA mograms. The arrows at the top of columns show that the exact number of events could not be counted because of high micro- National Institute of Polar Research seismic activity. The mean number of earthquakes per day was Itabashi, Tokyo, Japan 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 Three Japanese scientists visited McMurdo Station for the and the plume activity during the period. International Mount Erebus Seismic Studies (IMESS) during the It seems that the seismic activity of both tectonic and volcanic 1983-1984 field season. The Japanese participants were K. Ka- earthquakes in Ross Island during this period is of a steady state minuma 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 Daily Number of Earthquakes October 02,1983 - January 02, 1984

1983 to 16 January 1984. --no visibility Seismic observations by the telemetry network. This is the main to 42 6 10 15 20 25 301 5 15 20 25 30 5 10 15 20 25 312 program of the IMESS. The telemetry network with five seismic Oct.1983 Nov. Dec. Jan. 1984 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 classi- fied by sight into the following three stages as illustrated in the Figure 1. Daily number of earthquakes recorded at Abbott Peak and right side of the upper part of figure 1; (1) no plume was plume activity observed at Amundsen-Scott Base from 2 October observed, (2) plume was observed only around the summit, and 1983 to 2 January 1984. ("mm" denotes millimeter.)

22 ANTARCTIC JOURNAL as compared with the activities in the last few seasons (Tak- quakes are shown in figure 2. The upper right side of the figure anami etal. 1983-a, 1983-b; Shibuya etal. 1983; Ueki et al. 1984). indicates the projection of hypocenters along the north-south The Japanese members assisted the U.S. members in estab- vertical plane and the lower left side is that along the east-west lishing two new seismic stations of the telemetry network in vertical plane. The earthquakes located below the summit were January. the events associated with eruptions. Observations by temporary seismic network. Eight temporary The lower right side of figure 2 shows the epicenters of seismic stations were established on the flank of Mount Erebus earthquakes from 13 September 1982 to 29 January 1983. An for determining the earthquake locations with higher accuracy extreme earthquake swarm took place in the northwestern flank from 19 November 1983 to 3 January 1984. The earthquake of Mount Erebus on 8-9 October 1982 (Kaminuma, Ueki, and locations were determined for a period from 19 November to 9 Kienle in press; Ueki et al. 1984). December only, because some seismic stations of the telemetry It is remarkable that about 1 year after the October earthquake network had trouble from 9 December 1983 until the seismic swarm in 1982, no earthquake was located in the earthquake station was removed 3 January 1984. Hypocenters of earth- swarm area, as seen in the figure.

W CD E Lfl o DEPTH ( km ) ci z

-5

- 0 19 19 0 0 0 167 l68E 0190 19 19 0 0 0 CD 19 N -o - ABB Ln (i 1p 19 48OM • TEA \H004 I /Th

OO

SEP. la. 1982 - JAN. 29, 1983 0 5 10 km

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. Simple Bouguer Anomaly The purpose of this observation was to investigate local seis- micity in the dry valleys region and to detect a level of p=2.67 microseisms. Gravity measurements. A gravity survey with the LaCoste- Romberg gravimeter model G was carried out at 20 points in- cluding eight temporary seismic stations. The elevation of the 9.3 gravity survey points was measured with a barometric al- timeter. 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. -1.7 Figure 3 shows simple Bouguer anomalies, with assumed •10.8 •-15.8 . . density of 2.67 milligrams per cubic meter. Simple Bouguer _45.1•328 8 6.8 7.3 anomalies range from -92.4 milligals to 15.7 milligals as shown _770••-77.3 in the figure. The Japanese party has also carried out gravity -20.2 • surveys in Ross Island during the austral summer in 1982-1983 -0.5 -92.4 1.3 (Kaminuma, Koyama, and Ueki 1984). Data reductions for the gravity surveys in 1982-1983 and 1983-1984 are under way. 157J1 The IMESS has been supported by the National Science Foun- dation (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 mem- I 0 - lp 20 30km bers in 1982-1983 and 1983-1984, respectively) who maintained Station our recording system. Philip Kyle of New Mexico Institute of McMurdoi-7.9 Mining and Technology has continued to render valuable ad- vice and assistance in the field operation. Special thanks are due to the VXE-6 helicopter crew for their patience while setting up Figure 3. Simple Bouguer anomalies. The density of the mountain of the portable seismic stations and during the gravity surveys. 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. References Shibuya. 1983-a. Seismological observation on Mt. Erebus 1980-1981. In J.B. Jago and R.L. Oliver (Eds.), Antarctic Earth Sciences. Proceed- Kaminuma K., E. Koyama, and S. Ueki. 1984. Preliminary report on ings of the Fourth International Symposium on Antarctic Earth Sci- gravity survey in Ross Island, Antarctica. In T. Nagata (Ed.), Proceed- ences, Adelaide, Australia, August 1982. ings of the fourth symposium on antarctic geosciences 1983. Takanami, T., K. Kaminuma, K. Terai, and N. Osada. 1983-b. Seis- Kaminuma, K., S. Ueki, and J. Kienle. In press. Volcanic earthquake mological observation on Mt. Erebus, Ross Island, Antarctica, swarms at Mt. Erebus, Antarctica. Proceedings of geophysics of the 1980-1981. In T. Nagata (Ed.), Proceedings of the third symposium on Polar Regions, XVIII IUGG, general assembly. Antarctic Geosciences 1982. Shibuya, K., M. Baba, J. Kienle, R.R. Dibble, and P.R. Kyle. 1983. A Ueki, S., M. Baba, K. Kaminuma, E. Koyama, and J . Kienle. 1984. study on the volcanic and seismic activities of Mount Erebus, Ant- Seismic activity of Mount Erebus, Antarctica in 1982-1983. In T. arctica 1981-1982. In T. Nagata (Ed.), Proceedings of the third symposium Nagata (Ed.), Proceedings of the fourth symposium on Antarctic Geosciences on antarctic geosciences 1982. 1983.

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