[meteorite-list] The Oldest Moon Rocks

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Apr 22 14:49:16 2004
Message-ID: <200404221815.LAA07301_at_zagami.jpl.nasa.gov>

http://www.psrd.hawaii.edu/April04/lunarAnorthosites.html

The Oldest Moon Rocks
Planetary Science Research Discoveries
April 21, 2004

     --- Rocks from the lunar crust
     provide new clues to the age and
     origin of the Moon and the
     terrestrial planets.

Written by Marc Norman
Lunar and Planetary Institute and Australian National University

Anorthosites, rocks composed almost entirely of plagioclase feldspar, are
the oldest rocks on the Moon. They appear to have formed when feldspar
crystallized and floated to the top of a global magma ocean that surrounded
the Moon soon after it formed. Not all ages determined for anorthosites,
however, are as old as we expected--one appeared to be only 4.29 billion
years old. While 4.29 billion years sounds very ancient, a magma ocean ought
to have solidified well within 100 million years of lunar origin about 4.55
billion years ago. One possibility is that the young ages reflect impact
events, not the original time of igneous crystallization. My colleagues Lars
Borg (University of New Mexico) and Larry Nyquist and Don Bogard (Johnson
Space Center) and I studied an anorthosite (rock 67215) relatively rich in
pyroxene, allowing us to determine a precise crystallization age of 4.40
billion years. But even that age might have been affected by the subsequent
shock heating event that reset the low-temperature components in this rock
about 500 million years after it formed.

By examining data for all of the previously dated lunar anorthosites, we
were able to show that plagioclase feldspar is more prone to shock damage
than are the pyroxenes in these rocks, so we plotted only the pyroxene data
for four different anorthosites on a samarium-neodymium isochron diagram.
These data fall on a well-defined line indicating a crystallization age for
the anorthosites of 4.46 billion years, consistent with very early,
widespread melting of the Moon. Other data for 67215 show that it comes from
a relatively shallow depth in the crust, giving us clues to the structure of
the lunar crust. Studies like this one are filling in the picture of how the
initial crust of the Moon formed, which in turn sheds light on the formation
of the terrestrial planets.

Reference:

     Norman, M. D., Borg, L. E., Nyquist, L. E., and Bogard, D. D. (2003)
     Chronology, geochemistry, and petrology of a ferroan noritic
     anorthosite clast from Descartes breccia 67215: Clues to the age,
     origin, structure, and impact history of the lunar crust. Meteoritics
     and Planetary Science, vol 38, p. 645-661.

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

Keystone for Understanding the Origin of Planetary Systems

Understanding the origins of planetary systems is one of the most central
and challenging questions in planetary science. The idea that the planets in
our Solar System were assembled from a rotating disk of dust and gas known
as the Solar Nebula is reasonably well established, but in detail we know
surprisingly little about the actual events that lead to construction of the
planets.

Chondritic meteorites have revealed an impressive portrait of conditions in
the early nebula (see PSRD articles Dating the Earliest Solids in our Solar
System and The First Rock in the Solar System), whereas igneous meteorites
such as the eucrites provide a glimpse of what the early planets may have
looked like (see PSRD article Asteroidal Lava Flows). The compositions and
textures of eucritic meteorites show that some asteroids were extensively
molten, and it would not be surprising if similar processes occurred on the
early planets. However, asteroids are relatively small bodies and the
existence of now-extinct radioactive isotopes such as 26Al and 182Hf (see
PSRD article Hafnium, Tungsten, and the Differentiation of the Moon and
Mars) in some igneous meteorites show that their parent bodies must have
cooled rapidly and experienced little geological activity since they formed.
Although igneous meteorites provide important information about what was
happening on small bodies in the early Solar System, they provide only a
general guide to the nature of events that built the larger planets.

The internal structure and chemical compositions of the terrestrial planets
provide intriguing clues to their origins, but the record of early events on
Earth, Venus, and Mars has been obscured or erased by billions of years of
geological activity. Processes such as convection, volcanism, weathering,
and erosion have largely obliterated the primary signatures that would
inform us about the mechanisms and timing of planetary formation in the
inner Solar System. Fortunately, nature has provided a keystone that links
the record of early nebular events preserved in meteorites with the
subsequent geological evolution of the terrestrial planets, and that
keystone is the Moon. For example, volcanism on the Earth and Moon
overlapped in time for about a billion years, yet the Moon's crust is
sufficiently old that it preserves direct evidence for planetary-scale
events that occurred before the Earth's surface stabilized. In effect, the
surface of the Moon is a time capsule that carries a record of the physical
processes that created and modified the terrestrial planets.

An essential step in unraveling some of the early planetary history was the
acquisition of samples from the Moon by the Apollo and Luna exploration
missions. While photographs and remote sensing data provide useful
information about distant bodies, having real samples from the Moon
available for detailed laboratory studies has revealed aspects of the
geological evolution of the planets which otherwise could only be imagined.
For example, the first studies of Moon rocks inspired John Wood (Smithsonian
Astrophysical Observatory) to boldly imagine the idea that terrestrial
planets must have been extensively molten soon after they formed.

                         [lunar magma ocean drawing]
               The concept that the Moon melted
               substantially (possibly completely) when it
               formed, nicknamed the "magma ocean concept"
               is a fundamental tenet of lunar science.

This global melting event produced a stratified Moon with a low-density
crust formed by accumulation of the mineral plagioclase overlying a higher
density mantle of olivine and pyroxene. Meteorite impacts have reworked the
lunar crust extensively over the past 4.5 billion years, and most of the
rocks returned from the Moon are breccias. Although these breccias preserve
important clues to lithologic and compositional diversity in the lunar crust
and the impact history of the Earth and Moon, deciphering the primary record
of crustal evolution from these rocks is difficult because they are
mechanical mixtures of unrelated rocks.

                            [lunar breccia 67016]
         67016 is an impact breccia that was collected from the rim
         of North Ray crater, Apollo 16. It consists of fragments
         of plagioclase (white) and glass (dark gray). Rock
         fragments in breccias like these tell us a great deal
         about the early history of the Moon.

Fortunately, the primary record of the early crustal genesis and evolution
on the Moon has not been completely destroyed. Lunar scientists have
developed criteria such as low abundances of siderophile elements (which are
present in high concentrations in most meteorites relative to common igneous
rocks) and other chemical and petrographic data, to identify a suite of
rocks thought to represent primary igneous cumulates from the lunar
highlands. These cumulate rocks are rich in plagioclase, and most are
classified as anorthosites (>90% plagioclase), norites (plagioclase plus
low-Ca pyroxene) and troctolites (plagioclase plus olivine). The
anorthosites are usually referred to as 'ferroan' after the iron-rich
compositions of their olivines and pyroxenes, whereas the norites and
troctolites have more magnesian mineral compositions.

                        [Apollo 15 anorthosite 15415]
  15415 is an anorthosite (more than 90% plagioclase feldspar) collected
  by the Apollo 15 crew in the Hadley-Apennine region.

                        [Apollo 17 troctolite 76535]
         76535 is a troctolite (plagioclase plus olivine) collected
         by the Apollo 17 crew in Taurus-Littrow.

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

Age of the Lunar Crust

Lunar anorthosites in particular have assumed a key role in our
understanding of the early history of the Moon because lunar geochemists
think that these rocks crystallized directly from the global magma ocean.
The ages and chemical compositions of lunar anorthosites therefore provide
ground truth tests for theoretical models of planetary accretion and
differentiation. We have measured the isotopic and trace element
compositions of lunar anorthosites to provide better information about how
and when they formed. Precise crystallization ages of lunar anorthosites are
difficult to determine. The long history of meteorite impacts into the lunar
crust has disturbed or reset their K-Ar and U-Pb isotopic compositions. The
fact that most lunar anorthosites are, by definition, composed almost
totally of plagioclase makes it difficult to obtain enough sample for
mineral isochrons using more robust systems such as 147Sm-143Nd. Our work
has focused on a small group of lunar anorthosites that have enough pyroxene
to enable mineral isochrons to be determined.

Recently we reported the results of a study on a clast of ferroan noritic
anorthosite from Apollo 16 breccia 67215. This clast is especially
interesting as it has one of the best-preserved igneous textures of any
lunar anorthosite (see photo below), and it was found in a type of breccia
collected around North Ray crater in which ancient crustal rocks have been
found by Chantal Alibert, Malcolm McCulloch, and myself in a previous study
back in 1994. Mineral compositions and trace element characteristics of this
clast show that it is genetically related to the main group of lunar ferroan
anorthosites. Our isotopic analyses of plagioclase and pyroxene separated
from 67215c produced a 147Sm-143Nd mineral isochron indicating a
crystallization age of 4.40 ? 0.11 billion years. This very old age supports
the idea that lunar anorthosites formed early in the history of the Moon,
most likely by crystallization from a magma ocean.

                       [clast of lunar breccia 67215]
         Photomicrograph image showing the igneous texture of the
         clast of ferroan noritic anorthosite from Apollo 16
         breccia 67215.

These results also help explain some puzzling features of previous isotopic
studies on other lunar anorthosites. Prior to this study, Sm-Nd isochrons
had been obtained on only three other lunar anorthosites, and these gave an
unexpectedly large range of ages (4.29-4.54 Ga; Carlson and Lugmair, 1988;
Alibert et al, 1994; Borg et al., 1999). This range of ages provoked a
strong challenge to the idea that all of these rocks crystallized from the
magma ocean, and lead to proposals for alternative styles of lunar evolution
perhaps involving formation of the crust through a series of smaller,
unrelated magmatic events. However, we found that the range of ages reported
by the previous studies could be explained by subtle disturbance of the
Sm-Nd isotopic compositions in plagioclase separated from the anorthosites,
and that the pyroxenes and olivines from these rocks defined an age of 4.46
? 0.04 billion years (see graph below). This may represent a robust estimate
for the primary crystallization age of the earliest lunar crust.

     [Sm-Nd data]
     Isochron plot for Nd and Sm isotopes in lunar anorthosites. The
     blue circles are for pyroxene crystals in four lunar anorthosites.
     The red circles are for plagioclase feldspar. The pyroxene data
     define a very precise line, the slope of which defines the age,
     4.456 billion years. Plagioclase data scatter more because of
     chemical exchange of Sm and Nd caused by reheating by a large
     impact event that seems to have affected almost all anorthosites.
     Pyroxene is more resistant to isotopic exchange, and so records
     the original crystallization age of these rocks.

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

Structure of the Lunar Crust

In addition to placing better limits on the age of the Moon, the mineralogy
and textures of 67215c also provide interesting information about the
overall structure of the lunar crust. The fine-scale exsolution lamellae in
the pyroxenes of 67215c (see photo below) are consistent with
crystallization of this rock at relatively shallow depths within the lunar
crust (<0.5 km). This contrasts with the petrographic characteristics of
some other lunar anorthosites, which I. S. McCallum (University of
Washington) and his colleagues show are more consistent with slow cooling at
much greater depths (10-20 km).
          [exsolution of pyroxene in clast of lunar breccia 67215]
     A backscatter photomicrograph image showing the exsolution
     lamellae (light gray stripes) in pyroxene in the ferroan noritic
     anorthosite clast from the Apollo 16 breccia 67215.

The petrographic characteristics of lunar anorthosites can be combined with
recent remote sensing studies of the spatial distribution of lithologic
units exposed in lunar craters and basins (Hawke et al. 2003; Wieczorek and
Zuber 2001) to produce a generalized view of lunar crustal stratigraphy. The
primary upper crust appears to contain a complex mixture of rock types
having affinities with both ferroan anorthosites and the more magnesian
norites and troctolites. This heterogeneous upper crust appears to be
underlain by regionally extensive layers of relatively pure anorthosite at
mid-crustal depths.

In this context, 67215c and the other ferroan noritic anorthosites may
represent samples of relatively shallow anorthositic crust that formed by
accumulation of plagioclase along with some magma. Petrologists call the
magma that occurred between large plagioclase crystals "trapped melt." In
contrast, other lunar anorthosites may have formed at greater depths and
contain very little trapped melt. If all of these samples crystallized from
a common magmatic system, as suggested by their coherent mineralogical,
isotopic, and trace element characteristics, this magma must have been at
least 20 km deep, and probably >45-60 km deep to account for the lack of
complementary mafic and ultramafic cumulates in the lunar crust. Such a deep
magmatic system supports the idea that a global magma ocean was present on
the Moon soon after it formed.
                        [accumulation of plagioclase]
         As the anorthosite crust accumulated by plagioclase
         floatation in the lunar magma ocean, shallower regions
         contained more magma (called "trapped melt") between the
         plagioclase crystals than in deeper zones.

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

Origin of the Earth and Moon

Having a good date for the age of the lunar crust provides an important
constraint on the timing of planetary evolution in the inner Solar System,
and helps us understand the way that planetary systems form. This becomes
especially important as we begin to discover different types of planetary
systems around other stars, and try to predict which types of planets might
have structures and compositions most like our own, and therefore represent
potentially habitable worlds.

The 147Sm-143Nd isotopic compositions of lunar ferroan anorthosites
indicates that the primary lunar crust formed about 100 million years after
the oldest datable materials found in primitive meteorites precipitated from
the solar nebula. As crystallization of a lunar magma ocean is likely to
have been relatively fast, this implies that assembly of the Moon was a
relatively late event during the formation of the Solar System. Such a
scenario is consistent with the planetesimal accretion hypothesis in which
the origin of the Moon was intimately linked to the early evolution of the
Earth through gigantic collisions between proto-planets.

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

ADDITIONAL RESOURCES

     Alibert C., Norman M.D., and McCulloch M.T. (1994) An ancient Sm-Nd age
     for a ferroan noritic anorthosite clast from lunar breccia 67016.
     Geochim. Cosmochim. Acta, v. 58, p. 2921-2926.

     Borg L., Norman M., Nyquist L., Bogard D., Snyder G., Taylor L., and
     Lindstrom M. (1999) Isotopic studies of ferroan anorthosite 62236: a
     young lunar crustal rock from a light-rareearth-element-depleted
     source. Geochim. Cosmochim. Acta, v. 63, p. 2679-2691.

     Carlson R.W. and Lugmair G.W. (1988) The age of ferroan anorthosite
     60025: oldest crust on a young Moon? Earth Planet. Sci. Lett., v. 90,
     p. 119-130.

     Hawke B.R., Peterson C.A., Blewett D.T., Bussey D.B.J., Lucey P.G.,
     Taylor G.J., and Spudis P.D. (2003) The distribution and modes of
     occurrence of lunar anorthosite. J. Geophys. Res., v. 108, No. E6,
     5050, doi:10.1029/2002JE001890.

     Krot, A. N. (2002) Dating the Earliest Solids in our Solar System.
     Planetary Science Research Discoveries.
     http://www.psrd.hawaii.edu/Sept02/isotopicAges.html.

     McCallum I.S. and O'Brien H.E. (1996) Stratigraphy of the lunar
     highland crust: depths of burial of lunar samples from cooling rate
     studies. Am. Mineral., v. 81, p. 1166-1175.

     Norman, M. D., Borg, L. E., Nyquist, L. E., and Bogard, D. D. (2003)
     Chronology, geochemistry, and petrology of a ferroan noritic
     anorthosite clast from Descartes breccia 67215: Clues to the age,
     origin, structure, and impact history of the lunar crust. Meteoritics
     and Planetary Science, vol 38, p. 645-661.

     Norman M.D., Bennett V.C., and Ryder G. (2002) Targeting the impactors:
     highly siderophile element signatures of lunar impact melts from
     Serenitatis. Earth Planet. Sci. Lett., v. 202, p. 217-228.

     Taylor, G. J. (2003) Asteroidal Lava Flows. Planetary Science Research
     Discoveries. http://www.psrd.hawaii.edu/April03/asteroidalLava.html.

     Taylor, G. J. (2002) The First Rock in the Solar System. Planetary
     Science Research Discoveries.
     http://www.psrd.hawaii.edu/Oct02/firstRock.html.

     Wieczorek M.A. and Zuber M.T. (2001) The composition and origin of the
     lunar crust: constraints from central peaks and crustal thickness
     modeling. Geophys. Res. Lett., v. 28, p. 4023-4026.

     Wood, J. A. (1972) Fragments of Terra Rock in the Apollo 12 Soil
     Samples and a Structural Model of the Moon. Icarus, v. 16, p. 462-501.
Received on Thu 22 Apr 2004 02:15:25 PM PDT