Geochemical Composition and Stratigraphy of Tephra Layers in Antarctic Blue Ice : Insights into Glacial Tephrochronology

Nelia W. Dunbar, Philip R. Kyle, William C. McIntosh, & Richard P. Esser

Geoscience Department and New Mexico Bureau of Geology and Mineral Resources, New Mexico Institute of Mining and Technology, Socorro, NM, 87801; (575) 835- 5783; nelia@nmt.edu

Abundant "dust bands" are interbedded with blue ice at Allan Hills, and at a number of other locations along the Transantarctic Mountains, Antarctica (Table 1). Approximately 50 of these layers were sampled during the 1994/1995 field season, and are revealed to be tephra, terrestrial windblown (?) dust, or mixed tephra and dust. The layers range from thin, faint laminae to distinct bands as thick as 50 cm. They dip from near-horizontal to near-vertical, depending on the geometry of local ice flow.
Detailed GPS mapping reveals that individual ash and dust bands can be traced for up to 10 km and further tracing was only inhibited by snow cover, not by disappearance of the layer within the ice (Fig. 1). The same sequence of tephra and dust bands can be recognized throughout a single geographic area, irrespective of local ice flow conditions. The tephra and dust layers appear to have been deposited as stratigraphic layers rather having been emplaced by shear at the base of the ice sheet, based on their consistent orientation, coherent stratigraphy and the nature of their contacts with adjacent ice. The lower contact with the ice is invariably sharp, and is interpreted to be the depositional surface, whereas the upper contact is diffuse, apparently due to mixing with later snow. No significant shearing or brittle deformation of the dust and tephra section was observed.
Petrographic observations show that many of the layers appear to be composed of volcanic glass shards and crystals. In many layers, the glass and crystals appear to be fresh and unabraded, and very delicate volcanic glass textures, such as finely vesicularity and bubble wall fragments are preserved. The average size of tephra within layers ranges between <2 to ~100 m, and tephra within individual layers is well-sorted. Glass color and bubble morphology is consistent within individual tephra layers. The glass color ranges from brown to light green, and the glass contains variable amounts of microlites. The browner mafic glass tends to be more microlite- rich than the greener silicic glass. Bubbles in the brown glass tend to be small (~10- 20 microns) and round, whereas stretched vesicles are more typical in the greenish glass. In addition to the tephra layers, a number of layers are composed of terrestrial rock fragments, and/or rock fragments mixed with tephra. Petrographically, these layers are distinctive from the purer tephra layers because the glass, crystal and rock fragments are abraded and the range of grain sizes within an individual layer is greater.

Microbeam analyses of major and trace element composition of individual tephra shards have been accomplished using the electron and ion microprobe techniques. These analyses allow the chemical homogeneity of tephra layers to be assessed, and allow detailed chemical fingerprinting in order to tie tephra layers to source volcanoes. The composition of tephras include basanite, trachyte, phonolite and rhyolite (Fig. 2a). Individual glass shards appear to be chemically homogeneous with respect to major and trace elements, although detailed chemical maps to assess leaching along the margins of individual shards have not yet been made. Within individual tephra layers, the glass compositions are extremely homogeneous, with the exception of one tephra unit, which ranges from 57-70 wt.% SiO2 (Fig. 2a). However, examination of the major and trace elements trends exhibited by 10 analysed fragments suggests that this unit was erupted from a zoned magma chamber, rather than representing shards from a number of unrelated eruptions. The trace element compositions of the tephra units, as determined by ion microprobe

Chemical correlations of tephra layers with their source volcanoes, some of which, for the coarser tephras, may be up to 500 km away, are underway. A phonolitic tephra layer at Allan Hills has been correlated with the large, currently active volcano, Mt. Erebus, on Ross Island. Many of the basanitic layers may have been erupted from vents in the McMurdo Sound area. Because of their ease of access and sampling, blue ice areas offer an alternative to deep ice cores for the reconstruction of regional and possibly global volcanic records. 40Ar/39Ar dating of large tephra samples, which can be readily collected from blue ice areas, offers a means of establishing a chronology which may extend back to 300 ka or more. Furthermore, dated layers found in blue ice areas may be geochemically correlated with tephra in deep ice cores, enabling a reliable chronology to be established.

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