Nebular Conditions in the Early Solar System from the 3D Tomography of Chondrules.

English Abstract:

Chondrules are millimetre-sized igneous melt droplets formed in the early Solar System over 4.5 Gyr ago, and preserved in chondritic meteorites. Molten silicate droplets form spheroidal shapes due to a minimization of surface energy. In microgravity, a droplet may become perfectly spherical if no internal or external forces act during cooling and crystallization, but non-spherical shapes can arise if a droplet is perturbed by any number of forces during or after solidification. Many processes and diverse conditions in a range of nebular settings may be involved in chondrule formation. One key feature is the geometric shapes of chondrules themselves; the departure from perfect sphericity may provide clues for possible formation mechanisms. Here, X-ray micro-computed tomography (CT) was used to obtain the 3D shapes of >100 distinct chondrules in a 1 in² slab of the CR2 chondrite NWA801 at 50 μm/voxel resolution. For each chondrule, orientations of the three orthogonal symmetry axes and the lengths of the principal semi-diameters, A, B and C were determined by defining a minimum of 54 independently distributed points on the chondrule surface. A best-fitting ellipsoid was then calculated for each chondrule, characterized by a centre X_i, and a symmetric 3×3 matrix B_i. Axial ratios between 1.1< A/C<1.9 and 1.0<B/C<1.3 were observed with a clear preferred orientation of the short axes and the girdle pattern for the long axes. Thus some minor compaction (~10%) has acted on the chondrules. However, a range of original “nebular” shapes are present, from spheres, to oblate and prolate spheroids. Chondrule rotation (oblate sphereoids) or exposure to a ram-pressure headwind (prolate) could account for departures. Other forms may have been produced by ductile mergers. An alternate explanation may be that the chondrule shapes were produced by droplet vibration caused by melting of precursor dustballs in a nebular shock event.