Other
Scientific paper
Jul 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992metic..27r.241k&link_type=abstract
Meteoritics, vol. 27, no. 3, volume 27, page 241
Other
3
Scientific paper
Wilson and Keil (1991) showed that a few hundred ppm of expanding volatiles present in early partial (basaltic) melts on the aubrite parent body (or other differentiated asteroids <100 km in radius), upon ascent of the magma to the surface of the body, would cause disruption into a spray of droplets moving with velocities in excess of the escape velocities of small asteroidal-sized bodies and, thus, escape and be lost into space 4.55 Ga ago. Hence, no such basaltic rocks exist as individual meteorites nor as clasts in brecciated aubrites. This concept has also been applied to explain the lack of complementary basaltic rocks to the ureilites (Warren and Kallemeyn, 1991; Scott et al., 1992). Here we summarize the theory of pyroclastic volcanism on small Solar System bodies and briefly describe how pressure rises of at least tens of MPa due to melting cause the growth of pathways (dikes) that can deliver melt to the surface (Wilson and Keil, 1991; Muenow et al., 1992). We then introduce the concept that this process has had major effects on the compositions of the cores of some of the differentiated parent bodies of the magmatic iron meteorite groups (Keil and Wilson, in prep.). Some magmas from which the magmatic iron meteorite groups crystallized are depleted in initial S content relative to reasonable precursor materials such as H group chondrites and, with decreasing S content, show an increase in their cooling rates and hence, decrease in their parent body radii (Table 1). We suggest that S depletion is the result of removal from the parent body of cotectic Fe,Ni-FeS melts by explosive pyroclastic volcanism, which is more effective the smaller the bodies. The first partial melt in an H group chondritic precursor body of a differentiated asteroid forms at about 980 degrees C and consists of about 85 wt% FeS and 15 wt% Fe,Ni. The negative buoyancy of this dense melt can be compensated by the presence of a few 100 to 1000 ppm volatiles, the subsequent expansion of which then drives pyroclastic volcanism of the type envisaged by Wilson and Keil (1991). Removal of about 88 wt% of the Fe,Ni-S cotectic liquid would deplete the residue in S content to approximately that of IVB iron meteorites, yet leave about 96 wt% of the original Fe,Ni to form a core. Thus, it appears that this process can account for the puzzling depletions of certain magmatic iron meteorite magmas in S. References: Keil and Wilson Explosive volcanism and the compositions of cores of differentiated asteroids, EPSL (in prep.); Muenow, Keil, and Wilson (1992) GCA (in press); Wilson and Keil (1991) EPSL 104, 505-512; Warren and Kallemeyn (1991) EOS 72, 281; Scott, Keil, and Taylor (1992) LPS XXIII, 1253-1254. Table 1. Estimated initial S contents of magmas of magmatic iron meteorite groups, their metallographic cooling rates (MCR), and parent body radii (R). Group S(%) MCR (Kmy^-1) R (km) II AB 10-17 6-12 100-73; 87 IIIAB 4-5 3-75 138-31; 85 IV A 1.0-1.8 11-500 75-13; 44 IV B 0.6-1.2 30-260 47-17; 32 H chondrites* 8.6 This is the S content of the metallic Fe,Ni-FeS portion of H chondrites.
Keil Klaus
Wilson Leslie
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