The effects of superheating and cooling rate on olivine growth in chondritic liquid
Résumé
Chondrules, the major constituent of chondrites, are millimeter-sized igneous objects resulting from the crystallization of silicate liquids produced by the partial or complete melting of chondritic precursors, whose exact nature remains disputed. Various chondrule textures are observed as a function of the extent of the initial melting event. Here, we report dynamic crystallization experiments performed with a broad range of cooling rates (2-750 °C h−1) from superliquidus or subliquidus initial conditions to demonstrate the control of nucleation on the final chondrule texture. Classical crypto-porphyritic, micro-porphyritic, and porphyritic olivine textures were reproduced in subliquidus experiments in which heterogeneous nucleation dominates. In contrast, we were unable to reproduce barred olivine textures, regardless of the cooling rates investigated from superliquidus conditions; instead, macro-porphyritic textures were systematically obtained at low cooling rates (<10 °C h−1). The small number and large size of crystals in the macro-porphyritic texture are consistent with the initial step of superheating and the presence of long embayments that indicate an initial episode of rapid growth due to delayed nucleation. Crystals then acquired polyhedral shapes during a subsequent episode of slow growth. When the growth rate is too slow to decrease the degree of supersaturation in the liquid, a new episode of rapid growth produces a new generation of melt inclusions.