posted September 25, 2002
Dating the Earliest Solids in our Solar System
Written by Alexander N. Krot
High Temperature Processing
Most meteorite experts believe that CAIs and chondrules formed in the solar nebula by high temperature processes. These processes included condensation, evaporation, and, for all chondrules and many CAIs, subsequent melting during multiple brief heating episodes.
This combined X-ray elemental map shows Mg (red), Ca (green) and Al (blue) of the CR carbonaceous chondrite PCA 91082. CAIs and chondrules are labeled. Rocks like these preserve a record of the processes and timing of events in the solar nebula.
There are two mechanisms proposed for CAI and chondrule formation: shock waves and jet flows. According to the shock models, for example by S. J. Desch (Carnegie Institution of Washington) and H. C.Connolly, Jr. (Kingsborough College-CUNY), chondrules and CAIs were heated by shock waves that originated in the asteroid belt region. These shock waves moved through the dusty cloud at supersonic speeds, produced frictional heating, and melted the dust particles. According to the jet flow model developed by Frank Shu (University of California, Berkeley), chondrules and CAIs formed near the Sun (at ~0.04-0.08 AU) by sunlight and radiation associated with solar flares and were transported later to the asteroid belt region by a bipolar outflow [see PSRD articles Relicts from the Birth of the Solar System and The Oldest Metal in the Solar System for more information.] Ages (relative or absolute) of CAIs and chondrules can provide important constraints on their origin, but past calculations are either controversial or insufficiently precise.
This drawing depicts some of the processes that might have operated in the nebular disk surrounding the young Sun. It shows the jet flow model of CAI and chondrule formation. The yellow region near the Sun is very hot, which vaporizes all the dust falling into the nebula. The young Sun emits vast quantities of energetic particles, which create winds in the nebula. Rising plumes above the dashed lines are blown out to cooler parts of the disk. According to Shu, powerful jets accelerate CAIs and chondrules to hundreds of kilometers per second, allowing them to reach the asteroid belt in only a day or two.
The age relationship between CAIs and chondrules can be established using the short-lived radioactive isotope 26Al, which has a half-life (t½) of ~ 0.73 million years. Most aluminum is in the form of the isotope 27Al, which is not radioactive. 26Al decays to an isotope of magnesium, 26Mg. The tricky thing about determining age differences is that some 26Mg was already present in CAIs and chondrules, so not all the 26Mg originated from the decay of radioactive 26Al. We look for an excess of 26Mg (designated 26Mg* ) by comparing the 26Mg/27Mg ratio to that of other solar system materials.
CAIs and chondrules formed with different initial contents of 26Al. Most CAIs show large excesses of 26Mg*, corresponding to an initial 26Al /27Al ratio of ~4-5x10-5. Chondrules, in contrast, show only small or undetectable 26Mg*, implying an initial 26Al /27Al of less than or equal to 1.5x10-5.
Glenn MacPherson (Smithsonian Institution) and colleagues suggest that the difference in the initial 26Al /27Al indicates that CAIs formed at least 1-2 million years (My) earlier than chondrules. This chronological interpretation is based on the assumption that 26Al had an external stellar origin and was injected and homogenized in the solar nebula over a time scale shorter than its half-life. A second school of thought proposed by Gounelle (CSNSM-Université, Paris) and colleagues involves a local origin of 26Al by energetic particle irradiation near the forming Sun. This would result in a radial heterogeneity of 26Al distribution and limit the utility of using 26Al as a chronometer.
In contrast to the relative age dating achieved with 26Al-26Mg radioactive decay, absolute formation ages of CAIs and chondrules may be measured with the 207Pb/206Pb chronometer. This lead-lead age is based on radioactive decay of two long-lived radioactive isotopes 235U and 238U. The uncertainties of 207Pb/206Pb dates can be as low as 0.5-1.5 My, thus the 207Pb/206Pb chronometer may be suitable for resolving a potential 2-3 My age difference between CAIs and chondrules. C. Allégre, G. Manhes, and C. Göpel (Paris Geophysical Institute) report an impressively precise Pb-Pb age of 4566±2 Ma for CAIs from the Allende CV chondrite. Unfortunately, this is not quite precise enough.
My colleague, Yuri Amelin, has developed an even more precise technique for determining Pb-Pb ages. The key to improving the precision of the age determinations comes from Amelin's ability to analyze many small samples. This allows him to find the samples that have the least amount of common (or initial) lead, the lead that was in a chondrule or CAI during its formation and subsequent modification in an asteroid. For example, a chondrule sample could be contaminated with the fine-grained, relatively lead-rich matrix of a chondrite. Or lead might have migrated into the chondrule when the rock was heated in an asteroid. A smaller amount of common lead means that there is a smaller correction required to determine how much lead is due to radioactive decay of uranium. By analyzing many tiny samples, researchers can choose those with the least amount of common lead, hence the largest amount of lead formed by radioactive decay. This decreases the scatter on the Pb-Pb diagrams, leading to much higher precision.
In our recently published paper in Science magazine, we report Pb-Pb isochron ages for two CAIs (E60 and E49) from the CV carbonaceous chondrite Efremovka and a similarly precise Pb isotopic age for chondrules from the CR carbonaceous chondrite Acfer 059. We also report the 26Al-26Mg systematics for the CAI E60.
The Pb-Pb isochron age for the Acfer 059 chondrules is 4564.7±0.6 Ma. The weighted average Pb-Pb isochron age for the Efremovka CAIs is 4567.2±0.6 Ma.
On lead isotope plots, materials with the same age will fall along a single line. The data for chondrules from Acfer 059 (solid line) and for CAIs from Efremovka (dashed lines) define precise lines and indicate ancient ages. Error ellipses are 2-sigma; isochron age errors are 95% confidence intervals; MSWD is mean square weighted deviation.
Combining the age of the Acfer 059 chondrules with the age of the Efremovka CAIs gives an interval of 2.5±1.2 My between formation of the CV CAIs and CR chondrules. This indicates that CAI- and chondrule-forming events in the solar nebula continued for at least 1.6 My. This estimate of the interval during which CAIs and chondrules formed is within the range of the jet flow and shock-wave models of chondrule formation.
Implications of the Age Dates
It appears that 26Al can be used as a clock to measure small time differences, allowing us to understand more about the formation of the first solids in the solar system. Preliminary Al-Mg results for chondrules from the CR and CV chondrites by K. K. Marhas (Physical Research Lab, India) and colleagues suggest a range of initial 26Al /27Al from ~1x10-5 to less than 3x10-6. Our analyses and those of Marhas and J. N. Goswami show initial 26Al/27Al ratios of ~4-5x10-5 for the CR CAIs and Efremovka CAI E60. This suggests a 2-3 My age difference between CAIs and chondrules in these chondrite groups. Ernst Zinner and Christa Göpel [see PSRD article Using Aluminum-26 as a Clock for Early Solar System Events] have also shown a correspondence between Pb-Pb ages and 26Al.
Together, the Pb-Pb and Al-Mg isotopic studies support the chronological significance of 26Al-26Mg systematics. These isotopic age results are inconsistent with a local origin of 26Al by energetic particle irradiation. This implies uniform mixing of 26Al and perhaps other isotopes with short half lives [see PSRD article Supernova Debris in the Solar System]. Using the Pb-Pb dating technique, we are planning to date chondrules from other chondrite groups. This will help to define the total duration of chondrule formation in the early Solar System, and possibly discriminate between the jet flow and shock-wave formation models.
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