Other
Scientific paper
Jul 1993
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993metic..28r.447t&link_type=abstract
Meteoritics, vol. 28, no. 3, volume 28, page 447-448
Other
Differentiated Meteorites, Fractures, Magma, Magma Migration, Permeability
Scientific paper
Complicated processes produced differentiated meteorites inside asteroids. As recently reviewed [1], some asteroids may have had magma oceans (e.g., those producing magmatic iron meteorites and pallasites), in which numerous processes could have operated, including convection too vigorous to allow crystals smaller than 1 cm to settle, sinking and partial dissolution of quenched crust and crystal suspensions, and accumulation of coarse, residual crystals. Other asteroids partially melted. We have residues from such melting (e.g., lodranites, ureilites [2]), and samples of some of the partial melts (eucrites, angrites). In some cases, volatile abundances were high enough to drive explosive volcanism and loss of initial basaltic melts and eutectic Fe,Ni-FeS [3,4]. I focus here on the efficient way in which melts migrate, even in bodies as small as asteroids. The grain sizes of lodranites (0.5 mm) allow us to place limits on how long melts were in contact with unmelted silicates. Grain size increases by Ostwald ripening during partial melting [1]. Growth from 0.1 to 0.5 mm in the presence of silicate melt requires <10^3 yr. This implies that basaltic melts leave their source regions in <10^3 yr, otherwise lodranites would be coarser grained. This is much faster than migration by porous flow, which seems to take 10^5-10^6 yr [1]. However, calculating melt percolation by porous flow becomes complicated by the constant increase in grain size, hence in permeability of the solid matrix, during partial melting. For instance [1], a grain size before melting of 0.1 mm would grow to almost 2 mm in 10^3 yr, almost 1 cm in 10^5 yr, and almost 2 cm in 10^6 yr. Furthermore, because cracks readily open during partial melting [4,5], the actual porosity may be much higher than indicated by porous flow models. For example, for porous flow, McKenzie [6] suggests calculating permeability (K) from K = a^2f^3/1000, where a is the mean grain radius and f is the porosity (equivalent to the melt fraction). For a grain size of 1 mm and melt fraction of 0.1, the permeability is 10^-12 m^2. A standard approach in hydrology [7] uses K = Nb^3/12, in which N is the number of fractures of width b per meter of rock face (two-dimensional model). (We need to determine the crack density and size distribution during asteroid melting; L. Wilson and K. Keil are working on this problem.) Assuming the density of cracks is 10/m and the cracks are 1 mm wide, the permeability increases by three orders of magnitude to about 10^-9 m^2, and magmas would migrate from inside asteroids in 10^3-10^4 yr, about the time suggested by lodranite grain sizes. Similarly, if grains could coarsen to 1 cm during melting, the permeability increases to 10^-10 m^2, yielding almost the same migration rates as for flow in fractures. It appears that partial melts flow out of asteroidal interiors so rapidly that there is barely enough time to equilibrate minerals in the sources (it takes >10^4 yr to equilibrate a pyroxene 1 mm in radius), suggesting that fractional partial melting may have been common. Thus, calculations and experiments that assume equilibrium partial melting may be only approximations to the conditions that existed in asteroids. The rapid migration time also suggests, along with the lack of strong density traps, that melts will not readily form intrusions; thus, it may be difficult to explain the formation of cumulates such as diogenites by conventional intrusive processes. Perhaps they formed in a magma ocean. References: [1] Taylor G. J. et al. (1993) Meteoritics, 28, 34-52. [2] Scott E. R. D. et al. (1993) GRL, 20, 415-418. [3] Wilson L. and Keil K. (1991) EPSL, 104, 505-512. [4] Keil K. and Wilson L. (1993) EPSL, in press. [5] Muenow D. W. et al. (1992) GCA, 56, 4267-4280. [6] McKenzie D. (1984) J. Petrol., 25, 713-765. [7] Snow D. T. (1969) Water Resour. Res., 5, 1273-1289.
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