[meteorite-list] Using Aluminum-26 as a Clock for Early Solar System Events

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Apr 22 10:06:56 2004
Message-ID: <200210062217.PAA17591_at_zagami.jpl.nasa.gov>

http://www.psrd.hawaii.edu/Sept02/Al26clock.html

Using Aluminum-26 as a Clock for Early Solar System Events
Planetary Science Research Discoveries
September 30, 2002

     --- Correspondence between 26Al and Pb-Pb ages shows
     that 26Al records a detailed record of events in the
     early solar system.

Written by Ernst Zinner
Washington University, St. Louis, MO

Our solar system formed 4.6 billion years ago. Primitive meteorites provide
samples that were formed in its earliest days and thus can give us
information about this period. To establish the sequence of events during
solar system formation on a time scale of a million years radioactive
isotopes that decay with half-lives comparable to this time scale can
potentially serve as clocks for dating these events. 26Al, which has a
half-life of 0.73 million years appeared to be an ideal chronometer.
However, for this to be the case, 26Al had to be uniformly distributed in
the early solar system and this fact had not been clearly established.
Comparison measurements with two different clocks, 26Al and the decay of
uranium isotopes, in refractory Ca-Al-rich inclusions (CAIs) and in feldspar
crystals from ordinary chondrites indicate that both techniques give the
same ages. It appears that 26Al can indeed be used as a fine-scale
chronometer for early solar system events.

     References:

     Zinner E. and Göpel C. (2002) Aluminum-26 in H4 chondrites:
     implications for its production and its usefulness as a fine-scale
     chronometer for early-solar-system events. Meteoritics and Planetary
     Science, v. 37, p. 1001-1013.

     Zinner E., Hoppe P. and Lugmair G. (2002) Radiogenic 26Mg in Ste.
     Marguerite and Forest Vale plagioclase: can 26Al be used as
     chronometer? Lunar Planet. Sci. XXXIII, Abstract #1204.

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

A Clock and a Heat Source

26Al is a radioactive isotope that decays into 26Mg, a stable isotope, with
a half-life of 0.73 million years. Although this is so short that all of it
has decayed billions of years ago, its presence at the beginning of the
solar system has been conclusively established by the discovery of excesses
of its daughter isotope 26Mg in the most primitive solar system objects. If
these objects containing 26Al at the time of their formation remained
relatively undisturbed (i.e., did not experience high temperatures), the
decay product 26Mg was frozen in and today provides a record of the original
26Al. The ratio of 26Mg excess measured now relative to the amount of the
stable isotope 27Al yields the original 26Al/27Al ratio.

         [graph of Mg isotopic ratios]
         Magnesium isotopic ratios measured in different minerals
         with different ratios of aluminum to magnesium from a
         refractory inclusion in the meteorite Allende. Magnesium
         shows excesses in the isotope 26 that are correlated with
         the aluminum/magnesium ratio, indicating that the 26 Mg
         excesses originated from the decay of the radioactive
         isotope 26Al. This finding is evidence for the initial
         presence of 26 Al in early solar system objects.

The discovery of evidence for 26Al in the 1970s offered two very exciting
prospects. The first was that this isotope could be used as a clock. The
reason is that because of its radioactive decay, the 26Al/27Al ratio varies
in objects that formed at different times. By measuring the
aluminum-magnesium system today, the relative ages of these objects can be
established. The second was that the radioactive decay of 26Al produces heat
and this heat could have melted small asteroidal bodies. We have evidence
for the melting of such bodies from certain types of meteorites that were
produced from magmas. However, for 26Al to serve as a clock and as a heat
source, two conditions had to be satisfied. The 26Al had to be distributed
uniformly in the solar system (otherwise different 26Al/27Al ratios cannot
be uniquely interpreted as being due to a time difference) and enough of it
had to be present to provide the heat necessary for melting.

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

Was 26Al Uniformly Distributed?

Measurements in refractory Ca-Al-rich inclusions (CAIs) from primitive
meteorites established an initial 26Al/27Al ratio of 5x10-5. This would have
been enough for asteroidal melting as long as 26Al was uniformly distributed
throughout the solar system and not concentrated only in CAIs and as long as
small asteroids formed within a couple of million years after CAIs. It was
assumed that 26Al, together with other short-lived radioisotopes, had been
produced by nuclear processes (nucleosynthesis) in stars prior to the
collapse of the nebular cloud giving birth to our solar system. Other
primitive objects from meteorites such as chondrules show initial 26Al/27Al
ratios of approximately 10-5 and smaller. This has generally been
interpreted as indicating that chondrules formed approximately 2 million
years after CAIs. However, it could also have meant that chondrules formed
at the same time as CAIs but were endowed with less 26Al. Thus, nagging
doubts remained whether 26Al was uniformly distributed. These nagging doubts
were amplified by the recent discovery by Kevin McKeegan (University of
California, Los Angeles) and colleagues that another short-lived isotope,
beryllium-10 (half-life 1.5 million years) was also originally present in
CAIs. This radioisotope is not produced by stellar nucleosynthesis but most
likely formed as energetic particles from the early sun bombarded material
in the accretion disk. This bombardment could, in principle, also have
produced other short-lived isotopes including 26Al. If this happened mostly
in CAIs, as was proposed by Frank Shu (University of California, Berkeley)
and collaborators, a uniform distribution of 26Al was not assured.

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

Feldpars from H4 Chondrites to the Rescue

One way to establish whether 26Al can be used as a clock was to compare it
to a different radioactive clock where a uniform distribution in the solar
system is not in doubt. Such a clock is uranium whose isotopes 235U
(half-life 0.7 billion years) and 238U (half-life 4.5 billion years) decay
into lead isotopes. One fundamental difference with respect to 26Al is that
the uranium half-lives are long enough that these isotopes are still around
today. As a consequence, absolute ages can be measured by the uranium clock,
while only relative ages can be determined with the 26Al clock. Furthermore,
uranium is the only clock based on long-lived isotopes that has a precision
(less than a million years) that allows the resolution of different events
in the early solar system. Because lead isotopes are the daughter products
of uranium decay, uranium ages are usually called Pb-Pb ages.

With my collaborators Christa Göpel
(Laboratoire Géochimie et Cosmochimie,
Paris, France) and Peter Hoppe
(Max-Planck-Institut für Chemie, Mainz,
Germany) I selected feldspar grains from
two ordinary chondrites of type H4, Ste.
Marguerite and Forest Vale, for such a
comparative study. There were several
reasons for the selection of H4
chondrites. H chondrites are believed to
come from a parent body that was heated
(presumably by the decay of 26Al). This
heating was the cause of metamorphic
changes in the rocks making up this
asteroid. Rocks from different depths
experienced different peak temperatures
and duration of heating. Correspondingly,
H chondrites exhibiting different
metamorphic grades are assumed to come
from different depths in this parent
body.

Another reason was that Christa Göpel had
previously used the uranium clock on
phosphate crystals from Ste. Marguerite
and Forest Vale and had obtained absolute
ages of 4562.7±0.6 and 4560.9±0.7 million
years. These ages are so-called
metamorphic ages because phosphates are
metamorphic minerals that formed during
heating of the H4 region on the parent
body. The uranium clock thus measures a
time when the temperature became low
enough that the uranium and lead isotopes
did not equilibrate any more with their
surroundings. Compared to a uranium age
of 4567.2±0.6 million years for CAIs, the
time differences given by these ages are
such that we could expect to find
evidence for initial 26Al in H4
chondrites provided that they contain
phases with a very high aluminum to
magnesium ratio. This is because the 26Mg
excess from 26Al decay is proportional to
this ratio. Fortunately, the two H4
chondrites of our study contain fairly
large (up to 0.2 millimeter) feldspar
crystals with aluminum/magnesium ratios
exceeding 10,000.

       [temperature profiles of H chondrites]
       Models of the temperature profiles experienced by different
       metamorphic grades of the H ordinary chondrites. The
       temperature at which the uranium-lead system stops being
       equilibrated during decreasing temperature is believed to be
       approximately 730 degrees Kelvin. This temperature is reached
       at different times for different H chondrites.

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

Ion Microprobe Measurements of Initial 26Al/27Al Ratios

This picture shows the recently
installed NanoSIMS at Washington
University. The NanoSIMS is a new type
of ion microprobe that allows elemental
and isotopic analysis with very high
spatial resolution and with high
sensitivity. Peter Hoppe and I measured
the magnesium isotopic ratios and the
aluminum/magnesium ratios in many
different spots on a single feldspar
crystal with such an instrument at the
Max-Planck-Institute for Chemistry in
Mainz, Germany.

The determination of 26Mg excesses as a function of aluminum/magnesium
ratios was made with a special type of mass spectrometer, the ion
microprobe. In this instrument a finely focused ion beam (in our case
oxygen) bombards the surface of the sample to be analyzed (in our case
polished thin sections of the meteorites). This ion bombardment results in
the emission of atoms and ions from the sample. The ions are accelerated and
analyzed for their mass in a mass spectrometer. This analysis technique is
therefore called secondary ion mass spectrometry (SIMS). The ion probe
allows the elemental and isotopic analysis of small samples and even
measurements of many different spots on a given crystal.

                                Ion microprobe measurements of the
                                aluminum-magnesium system in feldspar
                                crystals from the H4 chondrites Ste.
                                Marguerite and Forest Vale show 26Mg
 [ion microprobe measurements]
                                excesses that are correlated with the
                                aluminum-magnesium ratio in both
                                meteorites. The slopes of the correlation
                                lines yield initial 26Al/27Al ratios.

We measured the ratios of all three stable magnesium isotopes (24Mg, 25Mg,
and 26Mg) and 27Al (the only stable isotope of aluminum) in several crystals
from the two meteorites. On a large crystal from Forest Vale we could make
these measurements in many different areas. Measurements are made by
changing the magnetic field of the mass spectrometer to different values so
that only ions of a given isotope are transmitted and counted. This is done
through many cycles. Because of the very low magnesium concentrations,
measurements take up to 10 hours for a single spot. Comparison with the
magnesium isotopic ratios in terrestrial rocks revealed clear 26Mg excesses
in the feldspar grains from both meteorites. The inferred initial 26Al/27Al
ratios obtained from these measurements are (2.87±0.64)x10-7 for Ste.
Marguerite and (1.55±0.32)x10-7 for Forest Vale.

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

26Al and Uranium Age Differences Between CAIs and H4 Chondrites Agree

If we interpret the differences between the widespread ("canonical") initial
26Al/27Al ratio of 5x10-5for CAIs and the ratios determined for the H4
chondrites of the present study as being due to a time difference, then we
obtain for the 26Al ages of these meteorites relative to CAIs 5.4±0.1
million years for Ste Marguerite and 6.1±0.1 million years for Forest Vale.
This compares to differences of 4.5±0.9 and 6.3±0.9 million years,
respectively, obtained with the uranium clock. The ages obtained by the two
methods are in excellent agreement.

    [isotopic ages]
    26Al/ 27Al ratios measured in CAIs and in Ste. Marguerite and Forest
    Vale as well as the ages of these objects determined with the
    uranium clock (Pb-Pb ages). The lower scale indicates the absolute
    ages, the upper scale ages relative to CAIs. The line with the
    arrow indicates the decrease of the 26Al/27 Al ratio because of the
    decay of 26 Al, the blue area around this line is due to the
    uncertainty in the absolute age of CAIs. The ellipses around the
    data points for the two H4 chondrites express the uncertainties of
    their uranium ages and 26Al/26Al ratios. Within these uncertainties
    the difference in the ages between CAIs and the two H4 chondrites
    measured by the uranium and inferred from the 26Al clock agree.

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

Remaining Questions

>From our analysis we have obtained an affirmative answer to our original
question whether or not 26Al can be used as a fine-scale clock for early
solar system events. However there are several remaining questions.

  1. Are the feldspar crystals of our study of metamorphic or igneous
     origin? We have already mentioned that there is little doubt that the
     phosphate used for uranium dating of the H4 chondrites is of
     metamorphic origin. The question is whether also feldspar in these
     meteorites formed from preexisting other phases during metamorphic
     heating of the parent body and the 26Al age measures the ceasing of
     equilibration of the aluminum-magnesium system during parent body
     cooling. The relatively high concentrations of sodium and the extremely
     low concentrations of magnesium, much lower than any observed in
     feldspar from CAIs and chondrules, indicate a metamorphic origin.

  2. Do the 26Al and uranium chronometers measure the same event? Not
     necessarily. The start of the clock, namely the time when the
     parent-daughter system becomes frozen in (this is called "closure" of
     the system by scientists working on geo- and cosmochronology) depends
     on the temperature when the respective isotopic systems stop
     equilibration. Unfortunately, the diffusive behavior of aluminum and
     magnesium in feldspar has not been determined. Thus the start of the
     two clocks could be different and, in principle, one cannot compare
     radiometric ages based on different chronometers. What helps in our
     case is that previous measurements indicated a high cooling rate of
     more than 1000 degrees Kelvin per million years for the H4 chondrites.
     If this is correct, then the difference in the start of the 26Al and
     uranium clocks must have been much less than a million years and the
     general agreement still holds within the experimental errors involved.

  3. Do the relative ages obtained from the 26Al and uranium clocks agree
     with those obtained from other short-lived isotopes? Besides 26Al,
     manganese-53 (53Mn, half-life 3.7 million years) and iodine-129 (129I,
     half-life 16 million years) have also been used for radiometric dating
     of early solar system events. However, while there is some agreement
     between the 53Mn and 129I chronometers, inconsistencies remain between
     them and the 26Al and uranium systems.

     [ages of rocks from different clocks]
     Ages of different objects from the early solar system determined
     with different clocks. Only the uranium (Pb-Pb) clock gives
     absolute ages. The other chronometers have to be anchored to the
     uranium clock by measuring both systems in the same object or a
     set of objects. For the manganese-chromium (Mn-Cr) clock that has
     been done on a type of meteorite called angrites, for the
     iodine-xenon (I-Xe) clock the age calibration has been made on
     the meteorite Acapulco. For the 26Al clock we assigned absolute
     ages by anchoring the relative 26Al ages to the uranium age of
     CAIs. As can be seen, while the 26 Al ages of chondrules, Ste.
     Marguerite (SM) and Forest Vale (FV) agree well with their
     uranium ages, this is not the case for the other clocks based on
     short-lived isotopes. For example, the 53 Mn ages of Ste.
     Marguerite, chondrules, and especially CAIs are much older than
     their uranium (Pb-Pb) ages. These inconsistencies are still not
     understood.
             --------------------------------------------------

Supporting Evidence

Two recent experimental findings support our tentative conclusion that 26Al
can indeed be used as a chronometer. First, Amelin (Royal Ontario Museum)
and coworkers used the uranium clock to determine the absolute ages of CAIs
and chondrules. [See PSRD article Dating the Earliest Solids in our Solar
System.] According to their measurements, CAIs are 2.5 years older than
chondrules. This is in good agreement with the relative age difference
inferred from the 26Al chronometer. Second, Marhas (Physical Research Lab,
India) and colleagues reported ion microprobe measurements in unusual
refractory inclusions that show the initial presence of beryllium-10 (10Be)
but lack any evidence of 26Al. This indicates that 26Al was not produced
together with 10Be by irradiation with energetic particles in the early
solar system and removes a constraint on its uniform distribution.

While the detailed chronology of early solar system events is still far from
being consistently established, our and other recent experimental studies
indicate that 26Al is after all an important clock. We hope that its further
usefulness can be shown in future studies.

[ADDITIONAL RESOURCES]

     Krot, A. N. "Dating the Earliest Solids in our Solar System." PSR
     Discoveries. September 2002.
     <http://www.psrd.hawaii.edu/Sept02/isotopicAges.html>.

     MacPherson G. J., Davis A. M., and Zinner E. K. (1995) The distribution
     of aluminum-26 in the early Solar System-A reappraisal. Meteoritics, v.
     30, p. 365-386.

     McKeegan K. D., Chaussidon M., and Robert F. (2000) Incorporation of
     short-lived 10Be in a calcium-aluminum-rich inclusion from the Allende
     meteorite. Science, v. 289, p. 1334-1337.

     Shu F. H., Shang H., Gounelle M., Glassgold A. E., and Lee T. (2001)
     The Origin of Chondrules and Refractory Inclusions in Chondritic
     Meteorites. Astrophys. J., v. 548, p. 1029-1050.

     Zinner E. and Göpel C. (2002) Aluminum-26 in H4 chondrites:
     implications for its production and its usefulness as a fine-scale
     chronometer for early-solar-system events. Met. & Planet. Sci., v. 37,
     p. 1001-1013.

     Zinner E., Hoppe P. and Lugmair G. (2002) Radiogenic 26Mg in Ste.
     Marguerite and Forest Vale plagioclase: can 26Al be used as
     chronometer? Lunar Planet. Sci. XXXIII, Abstract #1204.
Received on Sun 06 Oct 2002 06:17:04 PM PDT