Possible Clues to the Physical Nature of Chondrule Precursors: an Experimental Study Using Flash Melting Conditions

Details Possible Clues to the Physical Nature of Chondrule Precursors: an Experimental Study Using Flash Melting Conditions Possible Clues to the Physical Nature of Chondrule Precursors: an Experimental Study Using Flash Melting Conditions

Jul 1993

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993metic..28r.338c&link_type=abstract

Meteoritics, vol. 28, no. 3, volume 28, page 338

Computer Science

2

Chondrites, Chondrules, Meteorites, Nebula, Olivine

Scientific paper

In a continuing series of experiments to assess how flash melting conditions can reproduce chondrule textures we have investigated how the initial grain size of the starting material can affect chondrule textures. A previous series of experiments [1] has been carried out using an Fa analog composition (T(sub)L 1211 degrees C) with initial grain size ranges from 23-45 micrometers, 45-63 micrometers, 63-125 micrometers, and 125-250 micrometers. New experiments ([2], this research) use an average Type IIA chondrule composition (TL 1550 degrees C) with initial grain size ranges of 23-45 micrometers and 125-250 micrometers. All experiments utilized the flash melting techniques of [1,2]. Experiments performed with the finer grain fractions (<125 micrometers) of these composition for flash melting conditions less than 50 degrees C above their respective TL yield microporphyritic textures typical of Type IA chondrules [3]. Higher initial flash melting temperatures generate textures typical of Type IIA chondrules (i.e., PO,BO). However, the same experimental conditions that produced Type IA textures produced porphyritic textures typical of Type IIA chondrules [4] with starting grain size of 125-250 micrometers. Experiments with 125- 250-micrometer grain size produced only the typical Type IIA chondrule textures from initial flash melting temperatures ranging to 125 degrees C above TL. These charges also had a high number of large (100-200 micrometers) relict grains indicating that melting was not very extensive. Fewer nucleation sites survived the melting process allowing larger crystals to grow in the Type IIA textured charges than charges that produced Type LA textures. From our experiments it is clear that for the flash melting conditions used we cannot produce the microporphyritic textures typical of Type IA chondrules from precursors with an initial grain size of 125-250 micrometers with our compositions. If chondrules were produced by flash melting conditions similar to our experimental conditions it is clear that Type IA chondrules had precursors with a relatively homogeneous grain size that is less than 125 micrometers. Based on our experiments, in order to obtain the high nucleation density needed after melting to produce textures truly analogous to Type IA chondrules a grain size of 45 micrometers or less is favored. Therefore, Type IA chondrules could not have experienced high degrees of melting, either due to a relatively short, high-temperature melting event (i.e. flash melting) or a longer, lower-temperature (subTL) melting event(s). The presence of BO textures and large phenocrysts within PO textures of Type IIA group indicates that melting was more extensive, thus fewer nucleation sites survived, for Type IIA than for Type IA chondrules. Relict grains produced from flash melting with 125250-micrometer precursor material in the lab may be analogous to relict grains found in Type IIA chondrules. However, these experiments cannot rule out the possibility that Type IIA chondrules were totally melted and their texture is a function of the number of nucleation sites caused by collisions with dust and the temperatures at which those collisions occur. Type IA chondrules were made by partial melting of fine grained precursor dust. Type IIA chondrules could either have been made by partial melting of coarse grained precursor dust or they could have been totally melted precursor dust (with any size characteristics) provided dust of any size range collided with molten chondrules during their formation to act as nucleation sites. References: [1] Connolly H. C. Jr. et al. (1991) Meteoritics, 26, 329. [2] Connolly H. C. Jr. et al. (1993) LPSC XXIV, 329-330. [3] Jones R. H. and Scott E. R. D. (1989) Proc. LPSC 14th, 559-566. [4] Jones R. H. (1990) GCA, 54, 1784-1802.

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