Computer Science
Scientific paper
Jul 1993
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993metic..28q.356g&link_type=abstract
Meteoritics, vol. 28, no. 3, volume 28, page 356
Computer Science
Cold Bokkeveld, Fusion Crust, Micrometeorites, Antarctic
Scientific paper
Deceleration in the Earth's atmosphere causes frictional heating in micrometeorites. The duration and maximum temperature attained are a function of the entry angle, mass, and initial velocity of the particle [1]. Frictional heating may deplete volatile elements, in particular S and Zn [2], cause partial or total melting, and lead to the formation of magnetite rims [3]. To improve our understanding of the processes involved in atmospheric heating we are undertaking a comparative investigation using both micrometeorites and larger samples. Compared to micrometeorites, which decelerate above 70km, larger meteorites experience significant atmospheric heating at much lower altitudes [1]. As a consequence there are some significant differences between their respective fusion products [4]. However, one advantage of using fusion crust as an analogue for melted micrometeorites is that melted and unmelted material can be directly compared. In view of estimates that meteorite ablation losses may be as much as 90% [5] we are also interested in assessing whether such material has been sampled by recent micrometeorite surface collections [6]. Method: 45 particles hand picked from the 50-100 micrometer fraction collected during the 1991 EUROMET Antarctic expedition [6] were mounted and polished in Orsay, France. Back-scattered electron images of each particle were taken and the major and minor element composition determined using a Cameca SX-50 WDS microprobe. Analytical work on a second batch of these particles is in progress. Raster-beam analysis (integrated area 4.5 cm^2) of the central unheated portion of a crusted stone from the Cold Bokkeveld (CM2) shower and major element characterization of its fusion crust were carried out using an Hitachi S2500 analytical SEM. Results: Fusion crust enclosing Cold Bokkeveld varies in thickness from 500-1200 micrometers and is composed of two distinct zones, an outer highly-vesicular glassy layer 100-600 micrometers thick, and an inner, 400-600 micrometer-thick layer, in which meteorite matrix displays a highly porous texture. The contact between the two zones is extremely sharp. The outer glassy layer consists of abundant 10-30 micrometer diameter strongly-zoned olivines (Fo(sub)80-98.7) and 2-5 micrometer diameter magnetites set in an aphanitic mesostasis. The crystal content of this layer is 40-60%, falling to zero near the contact with the inner porous layer. The unheated interior and the outer glassy layer have similar major element abundances (100% anhydrous basis), the only significant difference being their SO3 contents, which are 8 wt% and 2.9 wt% respectively. Of the 45 50-100 micrometer Antarctic particles analyzed thus far, 25 had distinct magnetite rims and are considered to be of extraterrestrial origin. On a textural basis these may be separated into melted (n=16) (cosmic spherules n=5, scoria n=11) and unmelted types (n=9)(phyllosilicate-bearing n=3,coarse- grained n=6). Melted micrometeorites show highly variable compositions (100% anhydrous basis) SiO2 22.8-45.0 wt%, MgO 11.8- 29.6 wt%, FeO 18.7-61.1 wt%, CaO 0.3-1.6 wt%, Al2O3 1.8-4.7 wt%. However, when compared to Cl or CM2 compositions the most striking feature is their uniformly low SO3 content 0.1-1.5 wt%. NiO contents tend to be low in scoria particles, but close to Cl values in spherules. Conclusions: The uniformly low SO3 content in both glassy fusion crust and melted Antarctic micrometeorites lends support to the argument that S depletions in these particles were produced during atmospheric heating [7]. NiO is not uniformly low in these micrometeorites, nor is it depleted in glassy fusion crust, so that where Ni depletions do occur these would not have been produced by ejection of a NiS phase [7]. References: [1] Brownlee D. E. (1985) Ann. Rev. EPS, 13, 147-173. [2] Flynn G. J. et al. (1993) LPS XXIV, 497-498. [3] Rietmeijer F. J. M. (1993) LPSC XXIV, 1201-1202. [4] Robin E. et al. (1992) EPSL, 108, 181-190. [5] Wasson J. T. (1974) Meteorites. [6] Maurette M. et al. (1992) Meteoritics, 27, 473-475. [7] Maurette M. et al. (1992) LPSC XXIII, 861-862.
Greenwood Richard C.
Hutchison Robert
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