[meteorite-list] Dating the Earliest Solids in our Solar System

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Apr 22 10:08:32 2004
Message-ID: <200209260004.RAA21014_at_zagami.jpl.nasa.gov>

http://www.psrd.hawaii.edu/Sept02/isotopicAges.html

Dating the Earliest Solids in our Solar System

--- Lead isotopic analyses give absolute formation ages of Ca-Al-rich
inclusions and chondrules.

Written by Alexander N. Krot
Hawai'i Institute of Geophysics and Planetology
September 25, 2002
 
Chondritic meteorites (chondrites), the oldest rocks in our solar
system, provide a significant record of the processes that
transformed a disk of gas and dust into a collection of planets,
moons, asteroids, and comets. They are considered to be the building
blocks of the inner planets, Mercury, Venus, Earth, and Mars.
Chondrites are aggregates of three major components: refractory Ca-Al-
rich inclusions (CAIs), less refractory ferromagnesian silicate
spherules called chondrules, and a fine-grained matrix. We know that
CAIs and chondrules formed at nearly the same time as the Sun (4.56
billion years ago), but we don't know the details of how or where the
CAIs and chondrules formed. The timing and duration of their
formation remains obscure. My colleagues, Yuri Amelin (Royal Ontario
Museum), Ian Hutcheon (Lawrence Livermore National Laboratory),
Alexander Ulyanov (Moscow State University), and I set out to resolve
these unknowns by determining the absolute formation ages of CAIs and
chondrules using lead isotopic analyses.

Reference:
Amelin, Y., Krot, A. N., Hutcheon, E. D., and Ulyanov, A. A. (2002)
Lead isotopic ages of chondrules and calcium-aluminum-rich
inclusions. Science, vol. 297, p.1678-1683.

             --------------------------------------------------

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.

           [Image]
           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.

      [young star]
      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.

             --------------------------------------------------

Relative Ages

The age relationship between CAIs and chondrules can be established using
the short-lived radioactive isotope 26Al, which has a half-life (t˝) of ~
0.73 million years. Most aluminum is in the form of the isotope 27Al, which
is not radioactive. font size=-1>26Al decays to an isotope of magnesium,
font size=-1>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.

             --------------------------------------------------

Absolute Ages

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.

         [Pb-Pb isochrons]
         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

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. 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
articles 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.

ADDITIONAL RESOURCES

     Amelin, Y., Krot, A. N., Hutcheon, E. D., and Ulyanov, A. A. (2002)
     Lead isotopic ages of chondrules and calcium-aluminum-rich inclusions.
     Science, vol. 297, p.1678-1683.

     Taylor, G. J. "Supernova Debris in the Solar System." PSR Discoveries.
     March 2000. <http://www.psrd.hawaii.edu/Mar00/supernovaDebris.html>.

     Taylor, G. J. "The Oldest Metal in the Solar System." PSR Discoveries.
     September 2000. <http://www.psrd.hawaii.edu/Sept00/primitiveFeNi.html>.

     Taylor, G. J. "Relicts from the Birth of the Solar System." PSR
     Discoveries. March 2001.
     <http://www.psrd.hawaii.edu/Mar01/relicts.html>.
Received on Wed 25 Sep 2002 08:04:46 PM PDT