Computer Science
Scientific paper
Jul 1993
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993metic..28q.378k&link_type=abstract
Meteoritics, vol. 28, no. 3, volume 28, page 378
Computer Science
Agglutinates, Aggregates, Fine-Grained, Interplanetary Dust Particles
Scientific paper
Anhydrous interplanetary dust particles (IDPs) comprise a major fraction of collected IDPs, and are believed to be derived largely from cometary sources because of their porous morphology, unusal mineralogy, and high carbon contents. Among the major components of anhydrous IDPs are fine-grained aggregates (FGAs) consisting of nanometer-sized grains of FeNi metal and FeNi sulfides dispersed in a glassy matrix. During our transmission electron microscope (TEM) studies, we noted that these FGAs in IDPs are texturally very similar to agglutinate glasses in lunar soils and to shock melts in meteorites. In this abstract, we explore the possibility that FGAs in IDPs are a product of micrometeorite impacts into the regolith of asteroidal bodies. Fine-grained aggregates (the unequilibrated aggregates of [1]) are typically ~0.2 micrometers in diameter, and contain a high density of tiny (1 to 50 nm) FeNi metal and sulfide inclusions embedded in a highly enstatite-normative glass. We find that the distribution of Ca and Al in the glassy matrix varies on the submicrometer scale, and [1] has suggested compositional heterogeneity at the nanometer scale. The bulk compositions of these FGAs are chondritic (within a factor of 2 for most major elements). Lunar agglutinitic glass, by comparison, consists of fine-grained (1-50 nm) Fe metal (and occasional sulfides and phosphides) embedded in feldspathic glass. Although most lunar agglutinates are relatively coarse grained (typically 250 micrometers) relative to typical IDPs, comminuted agglutinitic glass is a common constituent of the <10-micrometer fraction of most mature lunar soils. Agglutinitic glass is compositionally similar to the bulk <10-micrometer fraction of lunar soils [2]. The formation of fine-grained aggregates in IDPs involved a high-temperature process operating under reducing conditions. The glassy matrix is highly pyroxene normative, which is a difficult composition to quench without devitrification. Thus, these FGAs were rapidly quenched after their formation. Further evidence that a rapid quench was necessary is the compositional heterogeneity of the glass at the submicrometer scale. It is well known that shock melting results in heterogeneous glass compositions at the micrometer scale [e.g., 3]. Could fine-grained aggregates in IDPs form in asteroid regoliths? Asteroids have regoliths, and even modest impact velocities will induce local melting (the average collisional velocities in the main belt are on the order of 6 km/s [4]). The presence of metal and sulfide in the aggregates indicates reducing conditions. In lunar agglutinates, much of the fine-grained Fe metal is believed to form by reduction of Fe^2+ in silicates, with solar-wind-implanted hydrogen as the reducing agent. The reducing agent in the formation of FGAs in IDPs is probably carbon (typical IDPs contain, on average, ~10 wt% carbon [5,6]). Local impact melting of this carbon-rich, fine-grained chondritic material would reproduce the textures and mineralogy of fine-grained aggregates, a hypothesis we have tested by flash-heating mixtures of powdered Allende with carbon black in a vacuum. The material produced in this experiment consists of fine-grained FeNi metal in a heterogeneous silicate and carbonaceous matrix similar to the FGAs in IDPs. We believe that the argument for a regolith origin for FGAs in IDPs is compelling, and has important implications for sources of IDPs and for remote sensing studies of the asteroids. Acknowledgments: This work was supported by NASA RTOPs 152-17-40-23 and 199- 52-11-02. References: [1] Bradley J. P. (1993) LPS XXIV, 171-172. [2] Walker R. J. and Papike J. J. (1981) Proc. LPS 12B, 421-431. [3] Schaal R. B. et al. (1979) Proc. LPSC 10th, 2547-2571. [4] Farinella P. and Davis D. R. (1992) Icarus, 97, 111-123. [5] Thomas K. L. et al. (1993) GCA, 57, 1551-1566. [6] Keller L. P. et al. (1993) LPS XXIV, 785-786.
Keller Lindsay P.
McKay David S.
Thomas Kathie L.
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