The lunar regolith provides an archive of data that can potentially
be used to date the chronology of ancient impact events on the Moon. The ApolloMoonlander missions of the 1970s returned numerous samples of impact melt
breccias (rocks made up of heat-fused pieces of older rocks) which can
potentially be used to date the timings of the formations of the ancient impact
basins which cover the surface of the Moon. However recent observations by NASA’s
Lunar Reconnaissance Orbiter Camera suggest that even quite small impacts,
creating craters as little as 100 m across, can result in some melting,
suggesting that the impact chronology recorded in lunar rocks is likely to be
extremely complicated.
The impact melt breccias recovered by the Apollo Moonlander missions
were dated using 40Argon/39Argon dating techniques, which rely on determining the ratio of radioactive Argon⁴⁰ to
non-radioactive Argon³⁹ within minerals from igneous or metamorphic rock
to determine how long ago the mineral
cooled sufficiently to crystaliz. If the ratio of Argon⁴⁰ to Argon³⁹ is
constant in the environment, then this ratio will be preserved in a
mineral at the time of crystallization; no further Argon³⁹ will enter
the mineral from this point, but Argon⁴⁰ is produced by the decay of
radioactive Potassium⁴⁰, and increases in the mineral at a steady rate,
providing a clock which can be used to date the mineral.
However even the most advanced methods available at the time, such as
infrared laser techniques (which focus an infra-red laser on a small sample of
rock in order to vaporize some of the rock for spectrographic analysis), were
prone to causing collateral heating (heating of the rock outside the area being
focussed on), which made it hard to quantify how much of the outgassed sample
analysed actually came from the area of the rock under investigation. Even at
their very best these methods could not be focussed on areas of rock smaller
than a few hundred micrometres across.
In a paper published in the journal Science Advances on 12 February
2014, Cameron Mercer of the School of Earth and Space Exploration at ArizonaState University, Kelsey Young of the School of Earth and Space Exploration at
Arizona State University and the Planetary Geodynamics Laboratory at NASA’s
Goddard Space Flight Center, John Weirich of the School of Earth and Space Exploration
at Arizona State University and the Centre for Planetary and Space Exploration
at the Department of Earth Sciences at Western University, Kip Hodges of the School
of Earth and Space Exploration at Arizona State University, Bradley Jolliff of
the Department of Earth and Planetary Sciences and McDonnell Center for theSpace Sciences at Washington University, Jo-Anne Wartho of the School of Earth
and Space Exploration at Arizona State University and the GEOMAR HelmholtzCentre for Ocean Research in Kiel, and Matthijs van Soest, also of the School
of Earth and Space Exploration at Arizona State University, present a new study
of two rock samples recovered by the Apollo 17 mission using ultraviolet laser
ablation microprobe techniques, which use pulsed ultraviolet lasers to ablate,
rather than melt, small volumes of a polished sample, thereby eliminating the
problem of collateral heating.
Mercer et al. selected two
100-μm-thick sections from the Apollo 17 material for analysis.
The first sample, 77115,121 (that is to say the 121st
section cut from sample 77115) has a fine grained crystalline matrix consisting
largely of plagioclase and pyroxenewith scattered olivine and ilmenite grains and
xenoclasts of anorthosite, norite and dunite.
Backscattered electron mosaic of 77115,121 with the
locations and dimensions of UVLAMP melt analyses shown as red circles. Mercer et al. (2015).
The second sample, 73217,83,
is more complex, showing several distinct zones with different
mineralogy. The first zone is a medium-grained noriticanorthosite assemblage
with plagioclaseand orthopyroxenegrains; this is split into two subsections,
(1a), which has orthopyroxene gains up to 750 μm across and plagioclase grains
in excess of a millimetre, and (1b) where all the grains are much smaller. The
second zone, (2) gabbronoritic melt breccia with a potassic, glassy matrix and
large quartz crystals, and the third zone, (3), is a very fine-grained
crystalline,noritic melt breccia with abundant plagioclase and low-Ca pyroxene
but few larger inclusions.
Backscattered electron mosaic of 73217,83 with the locations
and dimensions of UVLAMP melt analyses in domains 1 (yellow), 2 (blue), and 3
(red). The coarse-grained region in the upper right is a gabbroic granulite. Mercer
et al. (2015).
Mercer et al. took fifteen
samples from the matrix of 77115,121, and a further eleven samples from larger
crystals and clasts (fragments of older rock incorporated into the matrix). The
matrix from this sample yielded an approximate age of 3.834 billion years,
while the included clasts were considerably older, ranging from 3.892 to 4.23
billion years old.
For sample 73217,83 thirteen samples were taken from the matrix of
zone (1), ten from the melt volume of zone (2) and forty from the melt volume
of zone (3), as well as eight from clasts in zones (2) and (3). The zone (1)
matrix yielded an age of 3.808 billion years, zone (2) yielded an age of 3.644
years and zone (3) yielded a range of dates averaging 3.609 billion years old, but
with a distinct subset averaging 3.27 billion years old.
Sample 77115,121 is thought to have been the result of a single
impact-related melt event, approximately 3.8 billion years ago, but sample
73217,83 appears to have a more complex history. Mercer et al. suggest that it was initially formed by an impact related
melt event about 3.8 billion years ago, then partially melted about 3.644
billion years ago when zone (2) was formed. Zone three could represent two
further impact melting events, at 3.609 and 3.27 billion years ago, or a single
event at about 3.27 billion years ago which altered some of the zone (2)
material without fully melting it, producing an intermediate age close to that
of zone (2).
Data from the Lunar Reconnaissance Orbiter Camera suggests that the
area from which the Apollo 17 samples were recovered is likely to be dominated
by ejecta from the formation of the Imbrium Basin, or (less likely) ejecta from
the formation of the Serenitatis Basin. Estimates for the ages of these two events
vary, but most of the estimates of the age of the Imbrium formation event
cluster around 3.8 billion years ago, which coincides with the age of sample
77115,121 and with zone (1) of sample 73217,83, while all estimates suggest the
Serentis Basin formation event was considerably earlier, ruling it out as a
source of the melts, though leaving open the possibility it may have
contributed some of the clasts incorporated in the samples.
See also…
A recent volcanic lava flow in the Lowell Crater? The Lowell Crater is a 66 km diameter crater located in the northwest
Oriental Basin, just beyond the Moon's western limb (i.e. not directly
observable from Earth). It is thought to be early Copernican in age
(about 1.1...
How the Moon got its strange magnetic field. The Moon has a much weaker magnetic field than the Earth, but much
stronger than can easily accounted for, and not arranged in a bipolar
bar-magnet style like that of the Earth, but made up of a patchwork of
magnetic fields, with different orientations on different parts of the
surface. This patchwork is seem on Earth too, where it is useful for
dating and placing rocks. When liquid magma cools to form rocks,
magnetic particles...
Recent tectonic activity on the moon? The moon is thought to have formed about 4.5 billion years ago, thrown
off from the Earth by a collision with another early planetesimal. After
this collision it cooled rapidly, with the last of the flood basalts
that cover much of the surface of the moon erupting between 3 and 3.5
billion years ago (the most recent major flood...
Follow Sciency Thoughts on Facebook.
