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Cooling rates of chondrules inferred from porphyritic structures


April 1, 2014, noon - 1 p.m.
3853 Slichter

Presented By:
John Wasson
UCLA

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Chondrules formed by melting in the solar nebula. Their spheroidal shapes indicate high (>50%) degrees of melting at temperatures of 1800-1900 K. Almost all chondrules contain phenocrysts indicating that melting was incomplete. Many chondrules contain relict grains originally formed in an earlier generation of chondrules. The origin of chondrules is still not understood. A key problem is the determination of cooling rates. The ones in use are based on attempts to duplicate the structures of porphyritic chondrules in simple experiments involving a single cooling event in a furnace. These have yielded cooling rates in the range 0.001 to 0.1 K s-1. The cooling rates expected in a relatively transparent nebula (with chondrules radiating into cold space) are far higher, of the order of 100 K s-1. Thus the furnace based experiments require chondrule formation in a well-insulated environment. They require the immersion of chondrules in large dust clouds having temperatures much higher than the 1300 K evaporation temperature of mafic silicates in a nebula with pH2 <10-3 atm. The furnace experiments were carried out in a low-tech environment. They do not simulate the environment of the solar nebula in terms of pressure and composition of the gas nor do they attempt to account for the multiple heatings that chondrules have experienced. They have mainly attempted to duplicate the formation of olivine, the most common mafic phase in chondrules. There are several indicators that cooling rates were far higher than those needed to grow a 200 mm phenocryst from a tiny nucleus. Today I will discuss an orthopyroxene phenocryst in a primitive chondrite that shows 8 overgrowth layers (that increase the linear dimension by a factor of 2) while retaining the overall spheroidal shape of the chondrule. These overgrowths are interpreted to have formed by the melting of glassy sodic mesostasis at about 1450 K while leaving the gross structure of the chondrule unchanged. These layers do not show up on olivine, apparently because diffusion is 40´ more rapid in olivine compared to opx. This is one more indication that cooling rates were orders of magnitude higher than the values used in nearly all chondrule formation models.