Other
Scientific paper
Sep 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995metic..30q.517h&link_type=abstract
Meteoritics, vol. 30, no. 5, page 517
Other
1
Ablation, Antarctic Meteorites, Chondrites, H, Dust, Cosmic
Scientific paper
Where the East Antarctic ice sheet meets the Transantarctic Mountains, old, deep glacial ice is tilted upward and exposed.Within this visible cross-section of the ice sheet, layers of dark volcanic tephra serve as stratigraphic markers and datable age horizons [1,2]. Systematic sampling of these layers at a well-known meteorite collection site (the Allan Hills Main icefield) has revealed a band consisting of unusually dark and rounded particles, many of which are spheroidal. This debris layer (BIT- 58) extends parallel to the stratigraphy of the ice established from the tephra bands, and thus apparently marks a single depositional event. Several kg of ice from two sites along this band were subsequently collected and melted, yielding a few grams of sediment for further study. Microscopic examination of sieved samples reveals that roughly 95% of the particles consist of a singular olivine-rich hyaloclastic litholo gy; more that 40% of these are spheroidal. The remaining 5% of the sediment consists of grains derived from local bedrock exposures. Particles range in size from sub-micrometer to over 100 micrometers in diameter, with a strong mode around 85 micrometers suggesting sorting by aeolian processes. However, preservation of delicate particle morphologies such as small parasitic spheres suggests that saltation and/or abrasion was limited. A representative group of particles was mounted in epoxy and sectioned for subsequent electron microprobe analysis. All particles show a mixture of three dominant phases; euhedral and/or skeletal olivine, an Fe-rich glass mesostasis, and abundant Fe-Ni opaques (mostly metals and sulfides). There is a strong correlation between particle shape and the size of olivine grains: angular particles contain larger, more distinct (presumably relict) grains, while the most spheroidal particles are so fine-grained they appear homogenous at this scale. Defocused beam major-element analyses of spheroidal particles show good agreement with bulk H chondrite composition (Table 1). Euhedral olivine grains also correspond to typical H-chondrite composition with Mg-rich cores around 17% Fa zoned to rims of 24% Fa near contact with Fe-rich glass. Opaques include some relatively exotic Ni-rich phases, such as a Ni3S2 / gamma Ni,Fe assemblage. Although direct evidence of an extraterrestrial origin for this debris layer (such as the presence of cosmogenic 10Be and 26Al ) has not yet been obtained, the available data strongly suggest that this sediment originated as meteoritic spallation debris. This debris is distinct from other Antarctic "cosmic dust" collections by virtue of its uniform, recognizable ordinary chondrite composition and the consistent relation shown between grain- size and texture. The BIT-58 layer probably originated from a single transient event, the passage and/or impact of a single large meteorite over the East Antarctic ice sheet. Ar-Ar dating of the tephra layers that bracket the BIT-58 layer should yield a well-constrained age for this event. References: [1] Dunbar N. W. et al. (1995) Abstracts for IUGG XXI General Assembly, in press. [2] Dunbar N. W. et al. (1995) Intl. Symp. Antarc. Earth Sciences VII, in press. [3] Jarosewich E. (1990) Meteoritics, 25, 323-338. Table 1 shows a comparison between average bulk major element composition of debris layer spherules and H chondrite falls. +/- values represent sample standard deviation.
Dunbar Nelia W.
Esser Richard P.
Harvey Ralph P.
McIntosh William C.
Taylor Scott
No associations
LandOfFree
A Meteoritic Event Layer in Antarctic Ice does not yet have a rating. At this time, there are no reviews or comments for this scientific paper.
If you have personal experience with A Meteoritic Event Layer in Antarctic Ice, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and A Meteoritic Event Layer in Antarctic Ice will most certainly appreciate the feedback.
Profile ID: LFWR-SCP-O-830015