ABSTRACT

Lithospheric xenoliths were collected from basanitic scoria cones of the Mc Murdo Volcanic Province at a number of localities on and adjacent to Ross Island (Hut Point Peninsula, TurtleRock), Black Island and in the foothills of the Transantarctic Mountains.

The xenoliths vary from texturally variable, spinel lherzolites and dunites representative of upper mantle assemblages to ultramafic Al-augite ± kaersutite bearing ultramafic rocks and plagioclase bearing ultramafic to mafic granulites thought to represent the transition zone between upper mantle and lower crust. In some localities (e.g. Foster Crater) supracrustal rocks(sintered . granitoids and basement metasediments) occur in significant proportions. Many of the lower crustal mafic granulites are strongly layered mineralogically and, moreover, are notably denser than accepted upper mantle peridotites sampled from the same vent. This observation has an important bearing on the transition zone from lower crust to uppermost mantle and on model parameters for future geophysical modelling of sedimentary basin formation in the region.

The suites of xenoliths collected this season complement and in some cases (Black Island, Cape Bird and Foster Crater) greatly extend our existing xenolith sample sets. They will be used to expand previous models of ¹ lithospheric structure for the Ross Sea Embayment region⁴ and to make comparisons with similar work being undertaken in West Antarctica as a part of the WAVE research programme, field work for which was undertaken in the 1989/90/91 seasons.

Proposed Programme

Lithospheric xenoliths are a convenient (and relatively cost efficient) means of gaining an insight into the petrology of the deep earth. As such, they provide important information on lithospheric structure and processes and can be used to gauge thermal regime and, possibly, the timing of events. In addition, they provide vital first order information from which to constrain geophysically based models.

Gamble and coworkers^{1, 2} have undertaken detailed petrological studies of mantle and lower crustal xenoliths from Foster Crater in the foothills of the Trans Antarctic Mountains. This work has documented a variety of physico-chemical processes influencing the lithosphere and isotopic studies on a subset of these samples (McGibbon, 1991³) have yielded ages ranging from Proterozoic to Phanerozoic.

Further, and more rigorous sampling of the localities visited in the 1992/93 season was necessary to complete xenolith collections from the Ross Embayment area so that comparison could be attempted with the comprehensive suites collected in West Antarctica as part of the WAVE research programme.

Scientific Endeavours and Achievements

The Antarctic Ross Sea Lithosphere Study (ARSLS) was planned to visit a number of key volcanic cones in the vicinity of Ross Island and the foothills of the Transantarctic Mountains to collect xenoliths derived from the lower continental crust and lithospheric mantle. Most of the localities visited had been sampled by Gamble on previous occasions, but many of these were helicopter supported visits with little opportunity for detailed sampling. The aim for the 1992/93 season was to establish a field camp and spend several days undertaking exhaustive sampling. At this stage we are pleased to report that our sampling was both rigorous and highly successful as may be judged by the 10001b + of rocks being returned to New Zealand.

The ARSLS team consisted of John Gamble (Event Leader), Richard Wysoczanski (PhD student) and Jonathan Brooker (BSc Hons student).

The party departed for Antarctica on Monday 10th December 1992 after a delay of 1 day due to bad weather in McMurdo. Between the 11th - 13th November Wysoczanski and Brooker attended Field Training (Gamble day only) and assembled field gear. During this exercise, Wysoczanski and Brooker took the opportunity to sample xenoliths from Half-Moon Crater, adjacent to Castle Rock on Hut Point Peninsula. On Saturday 14th November the party travelled north along the length of Hut Point Peninsula and collected xenoliths from Sulphur Cones (166° 45′E, 77° 48′S) and Turtle Rock (166° 47′E, 77° 44.5′S). A wide range of xenoliths was collected from both localities. On 16th November the party moved by helicopter to Black Island, to a locality immediately south of Mt Aurora (163° 23′E,78° 14′S) to visit a cone initially sampled by Gamble in the 1984/85 season. Careful collecting and more thorough exploration of the locality resulted in a complete crust-mantle suite being collected, considerably extending that made on the previous visit. The party returned to Scott Base on 18th November 1992.

The party left Scott Base for Foster Crater (162° 57.00′E, 78° 23.73′S) on the 20th November, arriving at the crater amidst gathering cloud conditions which actually obscured the previous camp site used by Gamble in 1982/83 and 1984/85. As a result we camped on hard snow on the south of the crater, several hundred feet above the Koettlitz Glacier. The party encountered page break extreme winds (>75 - 80 knots) at this locality which eventuated in an early rescue (see logistic report) on 23rd November. However, before the high wind situation had developed, a comprehensive set of xenoliths was recovered. In particular, we added to the number of upper mantle spinel lherzolite samples and the granulite lower crustal suite.

The final destination in our search for xenoliths was Cape Bird and the basalt scoria cone called Cinder Hill (166° 25′E, 77° 16.53′S). On this trip the K-048 party had an extra field assistant in Vivienne Taylor, from the Scott Base staff. The field party departed Scott Base on 27th November, returning 1st December. Fresh granular spinel lherzolite xenoliths were recovered from a new locality to the south of Cinder Hill. These xenoliths were contained in dense black lava, in contrast to most other xenolith occurrences, which are in poorly welded, frequently oxidised, scoria deposits.

In summary, the 1992/93 season proved very successful, a new xenolith locality was located at Cape Bird and previous collections from Turtle Rock, Sulphur Cones, Black Island and Foster Crater were greatly enlarged and expanded. Work on these samples will commence as soon as they are returned from the ice.

Publications

Publication of our results will be by way of scientific journals and meetings. R.J. Wysoczanski will complete his PhD thesis (on West Antarctic xenoliths) in the next few months. J. Brooker will embark on a MSc thesis on rocks collected this season. Gamble and Wysoczanski will present results of this season at the International Association of Volcanology and Chemistry of the Earth's Interior congress in Canberra (September 1993) and the Geological society of New Zealand Annual meeting (November 1993). They will draw comparisons between West Antarctica suites and those of the Ross Sea Embayment and Transantarctic Mountains.

Environmental Impact

A completed environmental impact form is enclosed with this report. All waste products, including solid human wastes, were returned to Scott Base for disposal.

Future Research

Future research will involve detailed geochemical studies of the suites of samples collected this season. This will involve electron microprobe study and careful mineral separation followed by stable and radiogenic isotope analysis. The results will then be integrated into our extensive data-base of xenoliths from West Antarctica.

To some extent, this season marks a conclusion to the field work associated with xenolith studies in the Ross Sea area and West Antarctica. Time will be needed to work through the data and publish our results. A major aim is that this work can be integrated with available and planned geophysical studies of the lithosphere.

Management of Science in the Ross Dependency

An original intention of this year's programme was to have a visiting foreign scientist (Dr M.A. Menzies of the University of London) collaborate on the field aspect of the programme. Much of the radiogenic isotope work in the WAVE programme has been carried out in his laboratories at Royal Holloway and Bedford New College, University of London and it was fitting that we extend this to the field aspect. Unfortunately, due to other committments, Dr Menzies was forced to withdraw, but is still involved as a collaborating scientist.

Concerning Scott Base, Field equipment and logistics we have no complaints, in fact, to the contrary, I found the Base staff entirely accommodating and helpful during my preparations for the field and on return from the field.