Constraining the Cooling Rates of Chondrules

The evolution of the protoplanetary disk is inferred in large part from chondritic meteorites which contain the oldest Solar System material. A major component of these meteorites are chondrules, small, ~mm, silicate melt droplets. These formed as a result of transient, widespread heating of dust in the protoplanetary disk. The processes that created chondrules remain poorly understood, with many different competing mechanisms proposed. These range from shockwaves in the solar nebula, impacts between planetesimals and interactions with the early Sun. The cooling rates chondrules experienced during their formation are key constraints on chondrule formation mechanisms and can distinguish between models that are consistent with these constraints and those which are not. Many type II, FeO-rich porphyritic chondrules contain features called forsteritic-olivine relict grains which originated in the chondrule precursor assemblage and survived the melting event. These grains were able to exchange with the chondrule melt during cooling creating a diffuse boundary. Twelve type II porphyritic chondrules containing forsteritic-olivine grains have been identified and characterised in samples of ALHA 77307 (CO3.00), NWA 8276 (L3.00) and NWA 4910 (L3.1). Fe-Mg compositional profiles have been measured across the grain boundary using calibrated BSE greyscales. These profiles are compared to model diffusion profiles generated using a 1-D explicit finite difference forward modelling program to determine chondrule cooling rates. Determined chondrule cooling rates for chondrules range from 5 to 8000 Kh-1, with one chondrule in ALHA 77307 showing evidence for extremely rapid cooling at 50000 Kh-1. Chondrules also often show complex, non-linear cooling histories which are most consistent with formation in planetary embryo bow shocks with shock speeds of 6 to 7 kms-1. This places chondrule formation after the formation of large planetary embryos, suggesting that chondrules are a by-product of planet formation rather than representing an important stage in the planet-building process