Physics
Scientific paper
Jul 1993
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993metic..28q.422r&link_type=abstract
Meteoritics, vol. 28, no. 3, volume 28, page 422
Physics
1
Dust, Interplanetary, Interplanetary Dust Particles, Micrometeorites, Petrology, Protoplanets
Scientific paper
Chondritic porous (CP) and chondritic smooth (CS) interplanetary dust particle (IDP) texture and petrology alone cannot distinguish asteroidal from cometary debris and give no evidence that the IDPs are debris of different protoplanets, e.g., active, dormant, and extinct comet nuclei and carbon-rich asteroids [1]. There is probably a gradual transition from active to extinct comet nuclei with asteroidlike behaviour, and cometary orbits might evolve into asteroidlike orbits [2], which will hamper efforts to match IDPs to a particular protoplanet type. Recently, chondritic IDP morphology, mineralogy, and He release temperatures are used to link individual IDPs to asteroids or comets. Yet the observation that CP IDPs occur with both cometary and asteroidal orbits [3] suggests we should explore the notion that chondritic IDP protoplanets are texturally heterogeneous bodies containing both CS IDP and CP IDP-like domains. Particle L2005B22, 7.5 mm x 5.1 mm in size, has a CI bulk composition in FeO- MgO-SiO2 space. Its highly vesicular texture indicates stage C of the type S sphere ablation sequence [4] and supports degassing of a volatile-rich precursor. A relic forsterite crystal, Mg/(Mg + Fe^2+) = 0.93-1.0 is the only large (1.8 mm x 0.8 mm) grain in a poorly crystalline matrix. The olivine interior contains a minor laihunite component. The grain has a ~45-nm-wide vesicular rim where it abuts the particle outer surface. The matrix contains densely packed round crystals up to 10 nm in size and larger, up to 12 nm x 37 nm, euhedral olivine, orthopyroxene, and iron oxide crystals. Vesicles in the frotty matrix increase in size toward the particle exterior and along vugs. The matrix shows local flow textures. Single maghemite nanocrystals and partial maghemite rims (<180 nm wide) occur along the exterior. Thin (~30 nm) crescents decorate the vugs. This maghemite is evidence that the IDP was heated during atmospheric entry. The vesicular rim on olivine suggests evaporation at ~1200 degrees-1350 degrees C along the steep portion of the solidus in the olivine solid-gas phase diagram [5]. The compositions support melting and fractionation of the chondritic matrix at >1350 degrees C, quenching into the subsolidus ol-opx- SiO2 field at ~1200 degrees C, and low fO2 [6]. A model of atmospheric entry heating predicts this type of cooling behavior for cometary dust [7]. In general, these models [7,8] support that IDP L2005B22 entered the atmosphere as a smaller volatile-rich particle traveling at cometary velocity. While CP IDPs are considered typical cometary dust, the icy-glue model to explain active comet behavior [9] also predicts texturally heterogenous comet nuclei with carbon- and layer-silicate-rich boulders. Comparisons of infrared spectra for CS and CP IDPs with those dust tails in active comets showed that mixtures of these IDP types best explain the comet data [10]. I submit that highly vesicular particle L2005B22 is a molten pebble of these refractory boulders. This observation is the first confirmation of texturally heterogeneous comet nuclei. I conclude that the proposition that CS and CP IDPs represent different protoplanet types is untenable. This work is supported by NASA. References: [1] Rietmeijer F. J. M. (1992) Trends Mineral., 1, 23-41. [2]McFadd (1993) LPS XXIV, 205-206. [4] Brownlee D. E. et al. (1983) In Chondrules and their Origins (E. A. King, ed.), 10-25, LPI, Houston. [5] Nagahara H. et al. (1991) Annu. Rept. Dir. Carnegie Inst. Wash. 1990-1991, 88-92. [6] Kitayama K. and Katsura T. (1968) Bull. Chem. Soc. Japan, 41, 1146-1151. [7] Love S. G. and Brownlee D. E. (1991) Icarus, 89, 26-43. [8] Flynn G. J. (1989) Proc. LPSC 19th, 673-682. [9] Gombosi T. I. and Houpis H. L. F. (1986) Nature, 324, 43- 44. [10] Sanford S. A. (1987) Fund. Phys., 1-73.
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