[meteorite-list] New Lunar Meteorite Provides its Lunar Address and Some Clues about Early Bombardment of the Moon

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Wed Nov 3 13:25:10 2004
Message-ID: <200411022300.PAA26755_at_zagami.jpl.nasa.gov>

http://www.psrd.hawaii.edu/Oct04/SaU169.html

New Lunar Meteorite Provides its Lunar Address and Some Clues about
Early Bombardment of the Moon

Planetary Science Research Discoveries
October 31, 2004

--- A newly discovered meteorite from the Moon provides a detailed
record of its history, allowing scientists to make a reasonable guess
about where it came from on the Moon and to test ideas for the timing of
early impact bombardment.

Written by G. Jeffrey Taylor
Hawai'i Institute of Geophysics and Planetology

Lunar Meteorite SaU169

Edwin Gnos (University of Bern, Switzerland) and colleagues from
Switzerland, Germany, Sweden, England, and the United States describe an
information-packed meteorite found in Oman, Sayh al Uhaymir 169 (SaU
169). The complicated rock is composed mostly of an impact melt that
contains an exceptionally large amount of thorium, indicative of an
origin in the Imbrium-Procellarum region of the Moon. Gnos and his
colleagues report that the impact melt has an age of 3.909 (?0.013)
billion years, slightly older than estimates of when the huge Imbrium
impact basin formed on the Moon (about 3.850 billion years ago). The
meteorite was involved in a subsequent impact 2.8 billion years ago,
then another 200 million years ago, and a relatively recent one no more
than 340 thousand years ago. It landed on Earth about 10 thousand years
ago. This amazingly detailed record led Gnos to conclude that the rock
was blasted off the Moon from a place not far from Lalande Crater. The
3.9 billion year age of the impact melt adds to the debate about whether
there was an increase in the impact rate 3.9 billion years ago or there
was a continuous decline in the impact rate from 4.5 to 3.8 billon
years. This debate may not be settled until we have samples from the
South Pole-Aitken basin on the farside of the Moon.

Reference:

Gnos E., Hofmann B. A., Al-Kathiri A., Lorenzetti S., Eugster O.,
Whitehouse M. J., Villa I. M., Jull A. J. T., Eikenberg J., Spettle B.,
Krahenbuhl U., Franchi I. A., and Greenwood R. C. (2004) Pinpointing the
source of a lunar meteorite: Implications for the evolution of the Moon.
Science, v. 305, p. 657-659.

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

Lunar Meteorites: Free Samples of the Moon...but from what area? Moon
Meteorites are a great bargain. They come to us for free, from all over
the Solar System. We have them from asteroids, Mars, and the Moon. We
usually do not know exactly where they come from (which asteroid, where
on the Moon or Mars), but you can only expect so much for free. We have
about thirty meteorites from the Moon and these free but valuable
samples supplement the not free but priceless 380 kg of rock and
regolith returned by the Apollo program. Randy Korotev (Washington
University in St. Louis) maintains a list of all known lunar meteorites
<http://epsc.wustl.edu/admin/resources/meteorites/moon_meteorites_list.html>,
a generous contribution to those of us who study lunar meteorites.
Unfortunately, Korotev cannot list where each sample came from on the
Moon. Their chemical and mineral compositions cannot be matched
unambiguously with a specific locale on the Moon. Edwin Gnos and his
colleagues think they might have figured out the location of one of the
lunar meteorites on the basis of its history as revealed by the record
of impact events in it and its chemical composition.

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

A Complicated (But Normal) Lunar Rock

Most of SaU 169 is a typical impact melt rock characterized by numerous
unmelted fragments of minerals and rocks in a matrix of fine-grained
igneous rock. Such rocks are called impact melt breccias. The impact
melt breccia is associated with a regolith breccia, consisting of
fragments of rocks and minerals all smashed together. The regolith
breccia is intruded by yet another regolith breccia, and the whole
intricate mess is crosscut by shock-melted veins. Sounds complicated,
but most rocks from the lunar highlands are this complicated. They
record a long history of igneous and impact events.

photograph of thin section of SaU169

The photograph above shows an entire thin section of SaU 169. The rock
is composed mostly of an impact melt breccia (I) in which fragments
(also called clasts) of plagioclase feldspar (Plg), olivine (Ol), and
orthopyroxene (Opx) are suspended in a matrix of pyroxene and
plagioclase that crystallized from an impact-generated magma. Two
fragmental lithologies (II and III) occur along side the impact melt
breccia. Both are regolith breccias, fragmental material formed at the
very surface of the Moon as determined by the presence of glassy
materials and solar wind gases. Regolith breccia II contains a large
clast of impact melt breccia (II Clast). Regolith breccia III appears to
cut across the boundary between I and II, and Gnos and coworkers infer
that it formed after them. The rock is crisscrossed by dark, irregular
lines (IV). These are shock-melted veins and represent the last impact
event the rock experienced in its tortured history on the Moon.

 
 
 
photograph of slice of SaU169
<http://epsc.wustl.edu/admin/resources/meteorites/sau169.html>

A sawn face of meteorite SaU169 is shown above. Click on image for more
information. Link will open in a new window.

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

Anatomy of a Rock

Gnos and his colleagues analyzed SaU 169 with a vast array of high-tech
analytical gizmos, as summarized in the table below.

Property Measured
Why Measured
Technique
Laboratories

Mineralogy and rock textures

Identify rock type and geologic history

Optical microscopy, X-ray tomography, Raman spectroscopy

Inst. for Geology, Univ. of Bern, Switzerland; Federal Material Testing
Laboratory, Dubendorf, Switzerland

Mineral compositions

Characterized source rocks, determine igneous and impact histories

Electron microprobe analysis

Inst. for Geology, Univ. of Bern, Switzerland

Chemical composition (major and trace elements)

Rock type, igneous history, extent of impact mixing, locale of origin on
moon

Gamma-ray spectroscopy on entire sample, inductively coupled plasma mass
spectrometry, inductively coupled optical emission spectroscopy,
instrumental neutron activation analysis

Depart. of Chemistry, Univ. of Bern, Switzerland; Paul Scherrer Inst.,
Switzerland; Activation Laboratories, Ltd, Ancaster, Canada; Inst. for
Planetology, Max Planck Inst., Mainz, Germany

Oxygen isotopic analysis

Prove lunar origin

Chemical separations followed by mass spectrometry

Open University, England

Lead isotopic analysis of zircons

Determine age of impact melt breccia

Secondary ion mass spectrometry

Swedish Museum of Natural History, Stockholm, Sweden

Argon isotopes in irradiated feldspar grains separated from the impact
melt breccia

Determine ages of impact events in the rock's history

Mass spectrometry

McMasters Reactor, Canada; Inst. for Geology, Univ. of Bern, Switzerland

Noble gas measurements on impact melt and regolith areas

Cosmic ray exposure ages and test for presence of solar wind gases in
regolith breccia

Mass spectrometry

Inst. for Physics, Univ. of Bern, Switzerland

Carbon-14 and Beryllium-10 concentrations in sample of impact melt breccia

Determine how long ago the meteorite arrived on Earth

Accelerator mass spectrometry

Univ. of Arizona, United States

They determined the meteorite's mineralogy and the rock textures (the
geometrical relation of mineral crystals to each other) using optical
microscopy; they measured its chemical composition using neutron
activation analysis and inductively-coupled mass spectrometry; the lead
isotopic composition of the mineral zircon by secondary ion mass
spectrometry to determine the formation age of the impact melt breccia;
the abundance of Ar-40 by neutron-irradiation and mass spectrometry to
determine when subsequent impact events took place; an array other
isotopes by assorted mass spectrometric techniques to determine how long
the rock and its components were exposed to cosmic rays and how long ago
the meteorite arrived on Earth. This was a very thorough study! "Very
thorough" means they produced lots of data, and I can only summarize it
here.

A Highly KREEPy rock: Highly evolved igneous rocks are present on the
Moon. Evolved rocks formed from magmas that fractionally crystallized,
leaving a residual magma rich in elements that do not readily enter the
major rock-forming minerals. The most common class of evolved lunar rock
is KREEP, an acronym referring to enrichments (compared to other lunar
materials) in potassium (K), rare earth elements (REE), and phosphorus
(P). They also contain enrichments in other elements such as zirconium,
thorium, and uranium. The formation of evolved magmas involved extensive
fractional crystallization in the lunar magma ocean, a huge magmatic
system that surrounded the Moon soon after its formation, followed by
partial melting of rocks formed from the residual magma. These secondary
magmas could have fractionally crystallized further, creating rocks much
richer than those observed so far. SaU 169 is without question a KREEP
rock. Its thorium (Th) concentration is 33 parts per million in the
large impact melt breccia that composes most of the rock. This is higher
than all but a few small rock clasts in breccias brought back by the
Apollo missions. An interesting feature of the SaU 169 impact melt
breccia is that it has a much lower K/Th ratio (137) compared to the
average KREEPy rocks (360). This indicates that K and Th, which usually
have similar geochemical behavior in magmas, became decoupled from each
other, signifying a complicated magmatic history of the lithologies that
predated the formation of the impact melt. The rest of the rock is also
KREEPy, but does not contain as much Th or REE as does the impact melt
breccia, and K/Th is the normal KREEP value.

Age of the impact melt breccia: Gnos and his coworkers determined the
lead-lead age of the impact melt breccia by measuring the abundances of
lead isotopes with a secondary ion mass spectrometer. This technique is
based on the decay of uranium isotopes to lead daughter isotopes. To
increase accuracy and precision, they analyzed the mineral zircon
(ZrSiO4), which contained uranium when it formed, but little lead. The
zircon grains crystallized from the impact melt, so determining their
age gives the age of the impact event that produced the impact melt. The
ages of the twelve samples analyzed scatter a bit, but give an excellent
indication of the age of the impact melt: 3909 million years, with an
uncertainty of 13 million years.
lead-lead ages of zircons

Lead-lead ages of 12 zircon crystals from the impact melt portion
(lithology I) in SaU 169. Each bar represents the range of ages defined
by each analysis. The range is really the uncertainty in each
measurement. The horizontal line through the bars is the average of all
the analyses. The age of 3.91 billion years is older than the nominal
age for the Imbrium basin of about 3.85 billion years.

An age-resetting event: About half the impact melt breccia is composed
of plagioclase feldspar. Like zircon, it crystallized from the impact
melt, so its age ought to agree with that given by the zircon analyses.
However, the Moon's impact history complicates that story. The
plagioclase age was determined by determining its 39Ar-40Ar age on
plagioclase separated from a crushed sample of the impact melt. The
Ar-Ar method is really an advanced form of the potassium-argon method.
The difference is that the sample is irradiated by neutrons in a nuclear
reactor, converting some of the potassium-39 into argon-39. Then the
sample is placed in an evacuated extraction line and heated sequentially
to release argon-39 and argon-40, whose abundances are measured with a
mass spectrometer. The ideal sample shows the same age for 60 to 80% of
the gas released. SaU 169, however, gives messier results, a clear
indication that the system has been disturbed by a reheating event after
initial formation of the rock. Gnos and his coworkers show that the gas
released at the highest temperature steps gives an age of about 2800
million years. They also make a good case that the data indicate an even
younger disturbance that affected the potassium feldspar crystals less
than 500 million years ago.

Exposure on the lunar surface: The lead-lead ages indicate formation of
the impact melt from pre-existing rocks at 3909 million years ago,
followed by additional impact events that heated, but did not melt, the
rock at 2800 and 500 million years ago--a complicated history, but the
story does not end there. The rock was sitting around near the lunar
surface before being launched by another meteorite impact. Gnos and his
colleagues determined what we call cosmic ray exposure ages for the
impact melt and the regolith portions of the rock. This type of age
gives the time the material resided in the upper meter of the regolith
(the depth of penetration of cosmic rays). Cosmochemists determine such
ages by melting samples and measuring the concentrations of isotopes of
neon and argon produced by cosmic ray interaction with the rock. The
results show that the impact melt breccia was placed into the upper
meter of regolith about 200 million years ago, where it was mixed with
the regolith portions. Analysis of the beryllium-10 concentration
indicates that it was launched to Earth less than 0.34 million years ago
(340 thousand years ago).

Arrival on Earth: While it was exposed to cosmic rays on the Moon, the
components in SaU 169 built up carbon-14 and beryllium-10 to levels that
were constant--as much decayed as was produced. Once it arrived on
Earth, however, the cosmic ray bombardment effectively ceased, and these
two isotopes began to decay. The saturation levels are known from data
on meteorites that were collected as soon as they fell and from
calculations. Gnos and his team determined that the rock arrived on
Earth 9700 years ago (with an uncertainty of plus or minus 1300 years).

meteorite SaU 169 in the field
<http://epsc.wustl.edu/admin/resources/meteorites/sau169.html>

After falling to earth 9700 years ago, meteorite SaU 169 was found in
the hot desert of Oman in 2002 by Drs. Beda Hofmann of the Natural
History Museum of Berne, Edwin Gnos, and Ali Al-Kathiri both of the
University of Berne, Switzerland. Divisions on the scale bar are in
centimeters. (Photograph courtesy of Beda Hofmann. Click on the image to
link to more information. Link will open in a new window.)

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

Narrowing Down SaU 169's Lunar Home

The chemical characteristics and detailed chronological history of SaU
169 led Gnos and his colleagues to make an intelligent guess about where
on the Moon the rock came from. The high concentrations of KREEP
elements show that it must come from the region of the Moon identified
by Brad Jolliff and his colleagues at Washington University in St. Louis
as the "Procellarum KREEP Terrane." [See PSRD article: A New Moon for
the Twenty-First Century
<http://www.psrd.hawaii.edu/Aug00/newMoon.html>.] This region is
characterized by the highest concentrations of thorium (which associates
with all the other KREEP elements) as measured by the Lunar Prospector
mission.

Th on nearside

The region around the Imbrium and Procellarum areas of the Moon are
characterized by large concentrations of thorium. SaU 169, which has
very high concentrations of thorium, probably comes from the most
thorium-rich areas in the Procellarum-KREEP terrane (outlined in white).

SaU 169 has whopping amounts of thorium and the other elements
associated with KREEP, so it clearly comes from somewhere in the
Procellarum-KREEP terrane. Gnos and colleagues have examined the remote
sensing data in detail and identified places with the highest thorium in
this region. They also used data from the Lunar Prospector and
Clementine missions to find high-thorium areas with concentrations of
iron and titanium like those in SaU 169. Finally, they searched for
areas with these chemical properties that also appeared to have craters
of the right sizes and estimated ages to match the chronology the
deduced from their measurements of SaU 169.

They found some prominent thorium hot spots (see figure below), but only
the area around the prominent craters Lalande and Aristillus have the
right concentrations of iron and titanium. Both areas have a fresh young
crater that could have launched the rock to Earth, but only the Lalande
area has other craters whose estimated ages match the detailed
chronology of SaU 169. They estimate the age of Lalande crater at 2200
to 2800 million years on the basis of crater counting from previous
investigations. Formation of that large crater could have excavated the
SaU impact melt breccia and partially reset the Ar-Ar age of the
plagioclase feldspar. That event would have placed the impact melt
breccia into a pile of ejecta, but buried deeper than a meter to prevent
exposure to cosmic rays at that time. There is another crater, Lalande
A, with an estimated age of 175 to 300 million years, an excellent
choice for the event that moved the impact melt breccia into the upper
meter and mixed it with regolith. Finally, there is a young crater that
is suitable for launching the assembled rock to Earth less than 0.34
million years ago. If the bright young crater is the source, it requires
that the formation of Lalande A tossed the impact melt breccia a few
hundred kilometers to the northeast so it could be excavated by the
youngest crater. The greatest uncertainty, in this complicated but
interesting story, is in the estimated crater ages.

potential source locality

Thorium concentration in part of the Procellarum-KREEP terrane, showing
areas with particularly high thorium concentrations (red). Gnos and his
coworkers argue that the best bet for the lunar home for SaU 169 is the
area around Lalande crater (right). It has a large crater (Lalande) of
approximately the right age (2200 to 2800 million years) to reset the Ar
chronometer in SaU 169, a younger crater (Lalande A) that could toss the
impact melt close to the surface and mix it with regolith about 200
million years ago, and a bright, young crater that might be young enough
to have been the launch site for SaU 169's trip to Earth.

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

Implications for Early Lunar Bombardment

A big debate in lunar science is whether the Moon experienced a sharp
increase in bombardment rate between about 3.95 and 3.85 billion years
ago. This concept, called the "lunar cataclysm," arose because almost
all impact melt breccias have ages in that interval. The alternative is
that the Moon experienced a declining bombardment rate and that the
narrow age range reflects the preservation of only the youngest impact
melts. Still another interpretation is that all the impact melt ages
reflect the age of the event that formed the huge Imbrium impact basin,
which might dominate the nearside chronology. For more information, see
PSRD articles: Uranus, Neptune and the Mountains of the Moon
<http://www.psrd.hawaii.edu/Aug01/bombardment.html> and Lunar Meteorites
and the Lunar Cataclysm
<http://www.psrd.hawaii.edu/Jan01/lunarCataclysm.html>.

The age of the impact melt breccia in SaU 169 adds to this important
debate. It is distinctly older (3.91 billion years) than the nominal age
of the Imbrium event (3.85 billion years), perhaps reflecting formation
before the Imbrium event. Or, it might suggest an older age for the
Imbrium event. If it is older than Imbrium, then some other large impact
occurred and survived resetting during formation of the Imbrium basin.
If Imbrium is actually older than 3.85 billion years, then what does the
3.85 billion-year age represent? Unanswered questions abound.

The whole question of the style of early lunar bombardment is important
as it describes what the environment was like on the early Earth, at the
time when life was originating. It is not straightforward to settle the
issue with our current sample sets (sample returns by the Apollo and
Luna programs and lunar meteorites). We need to return samples from
someplace on the Moon where the effects of Imbrium formation is
minimized. The ideal place is the South Pole-Aitken basin on the lunar
farside. Detailed geologic studies show that this is the oldest basin on
the Moon. Returning samples of impact melt from South Pole-Aitken basin
will allow lunar scientists to test the lunar cataclysm hypothesis. If
the distribution of ages of impact melt breccias from the South
Pole-Aitken region are similar, or perhaps slightly older, than that of
nearside impact melt samples, then the cataclysm hypothesis is favored.
If there is a significant number of samples with much older ages (in the
4.1 to 4.3 billion year range), then the cataclysm hypothesis is less
likely to be correct.




Lunar Topo Map

The huge (2600-km across) South Pole-Aitken impact basin on the lunar
farside is an ideal place to test the lunar cataclysm hypothesis because
it is the oldest impact basin on the Moon and a sample collected from it
will contain samples of impact melts produced by other basin-forming
events. The image shown above is a Clementine topographic map of the
Moon (rotated to be centered on the SPA basin) where red=high,
purple=low. Each color equals 500 meters of elevation.





Ages of Impact Melts

Move your cursor over the buttons below to view the hypothetical histograms.

cataclysm
<#graph> no cataclysm
<#graph>

This figure shows three histograms of the ages of impact melts. Ages of
Apollo and Luna nearside samples are shown in black. The two buttons
allow you to see the cases when the cataclysm hypothesis may be true or
not. Samples from the South Pole-Aitken (SPA) basin will test the idea
of a lunar cataclysm. The hypothetical red distribution for SPA samples
favors the cataclysm. The hypothetical blue distribution suggests that
bombardment occurred over a much longer interval, tending to disfavor
the cataclysm.

The detailed study of SaU 169 done by Edwin Gnos and his large,
interdisciplinary team shows the value of continued studies of lunar
samples, particularly of lunar meteorites. More research will certainly
be done on SaU 169 before its full story is told, and additional finds
of lunar meteorites, remote sensing observations of the Moon, sample
return missions, and eventually field work by humans and teleroperated
robots will lead to a much fuller understanding of the Moon and its
igneous and bombardment history.

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

ADDITIONAL RESOURCES

Cohen, B. A. (2001) Lunar Meteorites and the Lunar Cataclysm. Planetary
Science Research Discoveries.
http://www.psrd.hawaii.edu/Jan01/lunarCataclysm.html

Gnos E., Hofmann B. A., Al-Kathiri A., Lorenzetti S., Eugster O.,
Whitehouse M. J., Villa I. M., Jull A. J. T., Eikenberg J., Spettle B.,
Krahenbuhl U., Franchi I. A., and Greenwood R. C. (2004) Pinpointing the
source of a lunar meteorite: Implications for the evolution of the Moon.
Science, v. 305, p. 657-659.

Jolliff, Bradley L., Gillis, Jeffrey J., Haskin, Larry A., Korotev,
Randy L., and Wieczorek, Mark A. (2000) Major lunar crustal terranes:
Surface expressions and crust-mantle origins. Journal of Geophysical
Research, vol. 105, p. 4197-4216.

Lunar Meteorites
<http://epsc.wustl.edu/admin/resources/moon_meteorites.html>,
comprehensive listing and information compiled by Randy Korotev,
Washington University in St. Louis.

Taylor, G.J. (2000) A New Moon for the Twenty-First Century. Planetary
Science Research Discoveries. http://www.psrd.hawaii.edu/Aug00/newMoon.html

Taylor, G.J. (2001) Uranus, Neptune and the Mountains of the Moon.
Planetary Science Research Discoveries.
http://www.psrd.hawaii.edu/Aug01/bombardment.html
Received on Tue 02 Nov 2004 06:00:25 PM PST