The Formation of Chondrules
Conel M. O'D. Alexander
At 4.55 billion years in age, chondritic meteorites are the oldest known rocks in the solar system. When looked at in detail, they appear to be essentially sedimentary rocks and are made up of primitive materials that formed in the first 10 million years or so of the solar system. This was when the solar nebula -- the disk of gas and dust that surrounded the young Sun -- was still present and dust was beginning to accumulate into planets. If we can only decipher them, chondrites will provide us with a window into processes that occurred in the nebula during this important but distant period.
The most abundant of the materials in chondrites (50-80% by volume) are chondrules. These are millimeter-size spherical objects that appear to have formed as free-floating molten droplets in the nebula. Melting material with their compositions requires temperatures of 1500-1800°C. Clearly the process that formed them was very energetic. If their abundance in chondrites is any guide, chondrule formation was one of the most energetic processes in the nebula, at least in the region of the asteroid belt between Mars and Jupiter where the chondrites come from.
Despite having been studied for over 100 years, there remains no consensus on what the energy source was for making chondrules. More progress has been made in trying to pin down the conditions under which chondrules formed, however. Simulation experiments of chondrules suggest that they were rapidly heated to near their total melting temperatures (1500-1800°C) and then cooled at 100-1000°C/hr. The cooling rate of a millimeter-size droplet radiating freely into space at such high temperatures would be at least 100 times faster than these estimates. To slow the cooling rates probably requires that the chondrule-forming regions were fairly large and dusty. How large and how dusty remains very uncertain.
At present, we have few independent means of determining conditions in the nebula, such as dust densities or gas pressure, and we must rely on astrophysical models for estimating them. Pressures in the nebula were probably low, about 10-3-10-6 atmospheres. At these pressures and at the high temperatures of chondrule formation, many elements become volatile. The degree of volatility of some elements can be very sensitive to the conditions, such as gas pressure. Therefore, it might be possible to constrain nebular conditions if we can determine the degree of volatile loss of one or more elements. During evaporation, the heavy isotopes of an element become increasingly enriched in the residue, in this case the chondrule. The isotopic composition of an element can therefore be a sensitive indicator of the degree of evaporative loss. I have been developing techniques for making precise isotopic measurements of four elements, potassium, iron, magnesium, and oxygen. These elements will have had quite different volatilities during chondrule formation.
The potassium and iron isotope measurements were made with the DTM's ion microprobe in collaboration with Jianhua Wang. The magnesium isotopes were measured with the DTM's new Plasma 54 multicollector inductively coupled plasma mass spectrometer (MC-ICP-MS) in collaboration with Richard Carlson, Timothy Mock, and summer intern Lan-Anh Nguyen. Joint DTM-Geophysical Laboratory-Smithsonian Institution fellow Richard Ash made the oxygen isotope measurements with the laser fluorination system developed by Douglas Rumble.
The picture that emerges from these four elements is still hazy. The two most volatile elements, potassium and iron, show no evidence of evaporative loss; the more refractory magnesium exhibits isotopic fractions consistent with small degrees of loss; and oxygen seems to have undergone isotopic mixing between two reservoirs with quite different isotopic compositions but not the isotopic fractionation associated with evaporation. There are conditions under which the fractionation associated with evaporation can be suppressed in some elements. The challenge is to explain the results for all four elements, a task that will require detailed numerical modeling.