Mathematics – Logic
Scientific paper
Sep 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995metic..30..603y&link_type=abstract
Meteoritics, vol. 30, no. 5, page 603
Mathematics
Logic
1
4 Vesta, Basalts, Crust, Eucrites, Metamorphism, Thermal
Scientific paper
Eucrites are crustal rocks from asteroid 4 Vesta. They cooled rapidly during initial crystallization (0.01-100 degrees C/hr) [1] in lava flows less than about 5 m thick. Most eucrites experienced an extended period of thermal metamorphism after crystallization [2]. We have developed a model that explains the rapid cooling of eucrites during crystallization as well as subsequent thermal metamorphism. We suggest that eucrites formed in lava flows that were metamorphosed through burial by successive lava flows as volcanism built up the crust [3]. The heat for the metamorphism was internally-derived, not from impact events. This model can be examined quantitatively and the results compared to the thermal histories determined from petrologic studies of eucrites [4, 5]. Estimates of the thickness of Vesta's eucritic crust range from 10 [6] to 25 km [7]. For our model, we assume a 20 km thick crust, resulting in a volume of ~1.6x10^7 km^3. If all this basalt erupted during a period of only 1 My, and ignoring the portion of the basaltic magma that would be intrusive, the average mass effusion rate would be ~16 km^3/y, an average accumulation rate of lava of ~0.02 m/yr. The initial temperature gradient in Vesta is problematic. We assume that the temperature of Vesta was uniform throughout the body at the eucrite liquidus about 1200 degrees C, when volcanism began, and that the surface temperature was 0 degrees C. The effect of regolith insulation is important [8], but may be negligible during the short duration of volcanism. Therefore, we use the thermal diffusivity of solid rock of 7x10^-7 m^2/s. Using a diffusion equation, we estimate the thermal histories of basalts that formed at time t (My) after the beginning of volcanism (t=0 My). Fig. 1 shows that the earliest formed basalts, which end up the most deeply buried (20 km), could be heated up almost to their melting temperatures. Even near the end of volcanism (t=0.9 My), the basalts that end up buried only 2 km heat up to 800 degrees C. Thus, the degree of equilibration of most (~70%) of the eucrites could be type 5 and 6 [4], for which peak temperatures exceeded 1000 degrees C for a long time. This is consistent with the time scale of metamorphism for equilibrated eucrites estimated by [2]. Intermediate types (type 3 and 4) [4] and unequilibrated eucrites (type 1 and 2) [4], which extruded near the end of volcanism, are minor. Assuming that the crust was only 10 km thick decreases the proportion of the equilibrated eucrites (~60%), but is still generally consistent with eucrite thermal histories. References: [1] Walker D. et al. (1978) Proc. LPSC 9th, 1369-1391. [2] Miyamoto M. et al. (1985) Proc. LPSC 15th, in JGR, 90, C629-C635. [3] Yamaguchi A. et al. (1995) Proc. Symp. Antarct. Meteorites, 20, in press. [4] Takeda H. and Graham A. L. (1991) Meteoritics, 26, 129-134. [5] Yamaguchi A. et al. (1995) LPS XXVI, 1531-1533. [6] Miyamoto and Takeda (1994) EPSL, 122, 343-359. [7] Delaney (1995) LPS XXVI, 329-330. [8] Warren P. H. et al. (1991) JGR, 96, 5909-5923.
Jeffrey Taylor G.
Keil Klaus
Yamaguchi Akinobu
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