| Popis: |
The highest flux of extraterrestrial material entering Earth's atmosphere, approximately 40,000 tons per year, arrives in the form of sub-mm particles of dust, which are found as unmelted micro-meteorites or melted cosmic spherules. Such particles have been collected in areas of low sedimentation, such as deep-sea sediments or by melting large volumes of ice. More recently, large accumula-tions of micrometeorites have been recovered in pits, joints and fractures of glacially eroded granitic nuna-taks (~2600 m a.s.l.) in the northern Victoria Land Transantarctic Mountains. In this abstract, we re-port on a new collection site in a different Antarctic quadrant with excellent preservation of micrometeor-ites and cosmic spherules. Preliminary investigations of 2 locations at the Widerøefjellet site, situated in the Sør Rondane Mountains of Queen Maud Land at S72?8'41" E23?16'41" have revealed a high density of cosmic spherules. Sediment was collected in December 2012 at the top of the Widerøefjellet Mountain after initial study of the region based on satellite images and geological maps of the region. Investigations at two other sites (near Yukidori-toride and Vikinghøgda) did not yield equal amounts of extraterrestrial material. At Wid-erøefjellet, two sediment exposures within the sur-rounding granitic lithologies were taken, with total weights of a few kilograms. Subsequently, the sedi-ment was slowly defrosted at the Antarctic research station. Two samples with initial weights of 850 g and 783 g were split into two sub-samples of approximately equal weight to preserve reference samples for later study. The sediment was then processed by washing and sieving to separate various size fractions. The sediment was divided into 1500 µm splits. Preliminary results reveal the presence of approximately 150 cosmic spherules and micrometeor-ites in 90 g of sediment for the 400-800 µm size frac-tion. One hundred cosmic spherules and micrometeor-ites have been identified in another 50 g of the 150-400 µm size fraction. Thelarger size fractions were of in-terest for preliminary investigations owing to reports from similar Antarctic sites of unusually good preser-vation of micrometeorites of large sizes [4]. Within the set of recovered material, the vast ma-jority are melted cosmic spherules, including eight non-magnetic forsterite-rich spherules. All of the classes of cosmic spherules have been identified amongst the cosmic spherules. The abun-dance of melted cosmic spherules is consistent with the mathematical models, that predict melting of larger dust particles. The recovery of such a high num-ber of cosmic spherules (approximately 2 cosmic spherules and micrometeorites per g) indicates that the Widerøefjellet site is comparable to other reported Antarctic concentration sites. Recovered micrometeorites and cosmic spherules were analyzed with a Scanning Electron Microscope equipped with an electron dispersive spectroscope. The analysis only provides detailed information about the surface of grains, including a demonstrationthat the melted cosmic spherules are composed primarily of olivine to pyroxene composition. Some unu-sual morphologies have been detected that are promis-ing for further study, such as dumbbell-shaped micro-meteorites and a partially melted cosmic spherule that has the ejection and melting of the spher-ule frozen in place. The remainder of the sediment from each of the size fractions is under study to allow for statistics of each category to be determined. Further analysis of the recovered micrometeorites and cosmic spherules is planned, including analysis by laser ablation inductive-ly coupled plasma mass spectrometry, tomographic analysis of some micrometeorites, and sectioning of samples to allow for chemical analysis of the interiors of the samples once all possible non-destructive anal-yses have been completed. The investigation of the Widerøefjellet Antarctic spherules may be placed as a contribution for better understanding the extra-terrestrial flux of dust-sized particles entering Earth's atmosphere and their chemi-cal and mineralogical properties as well as for the im-plementation of worldwide micrometeorite collections. |
