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The meteorite flux of the last 2 Myr recorded in the Atacama desert

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The evolution of the meteorite flux to the Earth can be studied by determining the terrestrial ages of meteorite collected in hot deserts. We have measured the terrestrial ages of 54 stony meteorites from the El M\'edano area, in the Atacama Desert, using the cosmogenic nuclide chlorine 36. With an average age of 710 ka, this collection is the oldest collection of non fossil meteorites at the Earth's surface. This allows both determining the average meteorite flux intensity over the last 2 Myr (222 meteorites larger than 10 g per km2 per Myr) and discussing its possible compositional variability over the Quaternary period. A change in the flux composition, with more abundant H chondrites, occurred between 0.5 and 1 Ma, possibly due to the direct delivery to Earth of a meteoroid swarm from the asteroid belt.
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The meteorite flux of the last 2 Myr recorded in the Atacama desert
A. Drouard1,2, J. Gattacceca2, A. Hutzler3, P. Rochette2, R. Braucher2, D. Bourlès2, ASTER
Team2,*, M. Gounelle4, A. Morbidelli5, V. Debaille6, M. Van Ginneken6,7,8, M.
Valenzuela9,10, Y. Quesnel2 , R. Martinez11
1Aix-Marseille Université, CNRS, LAM, Marseille, France
2Aix-Marseille Univ., CNRS, IRD, Coll France, INRA, CEREGE, Aix-en-Provence, France
3LPI, Houston, USA
4IMPMC, Muséum National d'Histoire Naturelle, Sorbonne Universités, CNRS, UPMC & IRD, 57 rue
Cuvier, 75005, Paris, France
5Laboratoire Lagrange, UMR 7293, Université de la Côte d'Azur, CNRS, OCA, Nice, France
6Laboratoire G-Time, Université Libre de Bruxelles, Belgium
7AMGC, Vrije Universiteit Brussel, Belgium
8Geological Survey of Belgium, Royal Belgian Institute of Natural Science, Belgium
9SERNAGEOMIN, Santiago, Chile
10Millennium Institute of Astrophysics MAS, Santiago, Chile
11Museo del Meteorito, San Pedro de Atacama, Chile
*Georges Aumaître, Karim Keddadouche
The evolution of the meteorite flux to the Earth can be studied by determining the
terrestrial ages of meteorite collected in hot deserts. We have measured the terrestrial ages of 54
stony meteorites from the El Médano area, in the Atacama Desert, using the cosmogenic nuclide
36Cl. With an average age of 710 ka, this collection is the oldest collection of non fossil
meteorites at the Earth's surface. This allows both determining the average meteorite flux
intensity over the last 2 Myr (222 meteorites larger than 10 g per km2 per Myr) and discussing its
possible compositional variability over the Quaternary period. A change in the flux composition,
with more abundant H chondrites, occurred between 0.5 and 1 Ma, possibly due to the direct
delivery to Earth of a meteoroid swarm from the asteroid belt.
The delivery of extraterrestrial matter to the Earth is controlled by the complex dynamical
evolution of the Solar System bodies. The past flux of extraterrestrial matter to the Earth's
surface has been studied at different spatial and time scales. Impact craters allow quantifying the
long term flux but only for large impactors (e.g., Mazrouei et al. 2019). At the other end of the
size spectrum, micrometeorites allow studying the extraterrestrial dust reaching the Earth (e.g.,
Genge 2008, Heck et al. 2017). Meteorites allow estimating the flux of intermediate size (cm-m
scale) meteoroids to the Earth’s surface. Fossil meteorites may light on the very ancient flux
(e.g., Schmitz et al. 2001, Schmitz 2013 for a 1.75 Myr time window during the Middle
Ordovician). Meteor observations have allowed estimating the intensity of the current meteorites
flux to the Earth (e.g., Halliday et al. 1989, Zolensky et al. 2006), while observed meteorite falls
(1312 registered meteorites as of July 2018) provide information about its composition over the
last two centuries. On the other hand, meteorite "finds" (whose fall has not been observed, 66165
meteorites as of July 2018) allow constraining the intensity and composition of the meteorite flux
on longer time scales more relevant to geological and astronomical processes (e.g. Bland et al.
1996, Benoit et al. 1996, Graf et al. 2001). Most meteorite finds come from Antarctica (64%)
and hot deserts (~30%), that are suitable areas for both preservation and recovery. The terrestrial
ages of hot desert meteorites are usually in the 0-30 ka range and rarely exceed 50 ka (Jull 2006).
Antarctic meteorites have higher terrestrial ages, variable between icefields, but they rarely
exceed 150 ka (Welten et al. 2006). These timescales are still short with respect to those involved
in dynamical evolution of the Solar System bodies (e.g. Bottke et al. 2005). Moreover, the large
Antarctic meteorite collection, that have older terrestrial ages, cannot be easily used to constrain
the meteorite flux due to biases introduced by meteorite concentration mechanisms (Williams et
al. 1983), and the difficulty in identifying paired fragments (Zolensky et al. 1998), as opposed to
passive in situ accumulation in hot deserts. The Atacama Desert is the oldest and driest of hot
deserts (Clarke 2005, Dunai et al. 2005). It has been shown to be an important meteorite
reservoir, with the highest meteorite density ever determined in hot deserts (Gattacceca et al.
2011, Hutzler et al. 2016). We present in this study the terrestrial ages of 54 meteorites recovered
from the Atacama Desert.
We selected randomly 54 unpaired meteorites within the 388 that were found in the El Médano
and Caleta el Cobre dense collection areas (Figure 1). These two adjacent areas are collectively
called El Médano area in the following. These areas have been shown to bear the highest
meteorite concentration in hot deserts (Hutzler et al. 2016). These 54 meteorites are all ordinary
chondrites. Ordinary chondrites are the most abundant class of meteorites, and are
overrepresented in the El Médano collection (where they represent 96% of the meteorite
population) because of recovery biases (Hutzler et al. 2016). The 54 meteorites span the three
groups of ordinary chondrites: H (25 meteorites), L (26 meteorites), and LL (3 meteorites).
Based on recovery location, petrography, silicate geochemistry and magnetic susceptibility,
special care was taken to discard possibly paired meteorites to avoid statistical
overrepresentation of large falls that can produce multiple meteorites. Terrestrial ages were
determined by measuring the 36Cl (half-life: 301±0.01 kyr) concentration in the FeNi metal
fraction of these meteorites (see appendix: methods and the supplementary material for the
complete data).
Figure 1. Map of the El Médano and Caleta el Cobre areas with the 54 dated samples, with their
class (H, L, or LL) and their age.
The absence of geographical trend in the ages (Figure 1) confirms that pairing does not affect the
age distribution, and was properly assessed by Hutzler et al. (2016). As an extra precaution, the
few groups of meteorites with similar classification and terrestrial age were checked again for
possible pairing using a variety of criteria, in particular petrography. No pairing was evidenced.
Although shielding in large meteoroids may account for overestimation of some ages, measured
cosmogenic radionuclides content of ordinary chondrite falls show that shielding has affected
only 3 out of 31 studied meteorites (Graf et al. 2001, Dalcher et al. 2013). Furthermore, the
meteorite fall population is strongly biased towards large masses (more likely to be observed and
recovered), as evidenced by their mass distribution compared to that of meteorite finds or
theoretical estimates (e.g., Huss 1990). We note that the 3 ordinary chondrites showing a
significant shielding (Richardton, Uberaba and La Criolla meteorites) all have masses above 40
kg. Such large meteorites are exceedingly rare in unbiased meteorites collection like the El
Médano collection. Therefore, only a few percent of the El Médano meteorite collection may
have been affected by shielding. This would correspond to a couple of meteorites with no
consequence on the overall age distribution. With a mean terrestrial age of 710 ka, the El
Médano collection is by far the oldest collection of non-fossil meteorites on Earth's surface
(Figure 2). About 30% of the samples are older than 1 Ma, and two are older than 2 Ma.
For comparison, meteorites from other hot deserts and Antarctica have an average terrestrial age
of only 12 ka and 99 ka respectively (computed on 152 and 398 terrestrial ages from MetBase
database (Koblitz 2005), in agreement with Jull (2006)). The results of the present study are
consistent with the high ages of the Atacama desert surfaces associated to long-standing
hyperaridy (Clarke 2005, Dunai et al. 2005), and offer an explanation for the unusually high
number of meteorites that were found in the El Médano area (~190 unpaired meteorites larger
than 10 g per km2 (Hutzler et al. 2016)). The age distribution can be fitted with an exponential
law decrease with a half-life 590 kyr (Figure 2). The smooth exponential decrease suggests that
no discrete meteorite removal event, such as surface reworking, occurred over the last 2 Myr, but
instead, that meteorites are removed by continuous processes, likely wind abrasion and
fragmentation. This distribution suggests that ordinary chondrites are unlikely to be preserved for
more than 2.5-3 Myr in the Atacama Desert, except perhaps for the large stones (>10 kg) that are
exceedingly rare.
Figure 2. Cumulative terrestrial age distribution measured by 36Cl (blue) and the exponential
best-fit (red). The right panel is a focus on the range 0-65 ka. The shaded area corresponds to the
maximum age uncertainty of 90 ka, derived by propagation. Our results are compared to the
cumulative age distribution of Antarctic (blue) and hot desert (yellow). Both 14C and 36Cl age
measurements were selected for Antarctica meteorites, whereas the data for hot desert meteorites
(Atacama samples excluded) only consist of 14C measurements.
The smooth decrease of the age distribution is consistent with a constant meteorite flux
combined with meteorite removal by weathering. This assumption means that the resulting
meteorite surface density N(t) on the ground satisfies the following evolution equation:
dN(t) = α*dt - λ*N(t)*dN(t)
where α is the meteorite flux and λ is the decay rate (λ= ln(2)/t1/2). This equation can be solved:
N(t) = (1 - e-λt )*α/λ + N(t=0)
Whatever the value of N(t=0), an equilibrium state is reached after a few half-lives (590 kyr),
after which the meteorite surface density remains constant at a saturated value Nsat = α/λ. We
consider here meteorites with mass higher than 10g. Using the meteorite density of 189±14 km-2
according to Hutzler et al. (2016) and the decay rate computed analytically from the terrestrial
age distribution (Figure 2), we determined a fall rate of 222±15 meteorites above 10 g per km2
per Myr. This estimate is similar to the 225±81 meteorites above 10 g per km2 per Myr over the
last 50 kyr, given in Bland et al. (1996) by dating meteorites from Nullarbor (Australia).
However, these values are higher than the estimate of 83 meteorites above 10 g per km2 per Myr
determined by Halliday et al. (1989). This latter estimate is less representative of the average
meteorite flux because of the low number of events (56 meteorite dropping events versus 388
meteorites in this work) and the short time window (11 years versus 2 Myr in this work).
H and L chondrites represent the vast majority (78%) of meteorites and offer the most robust
statistical indicator of possible variation in the composition of the meteorite flux. The H
chondrite fraction with respect to the total (H+L) chondrites is presented in Figure 3. The overall
H/L ratio of our sample selection (25/26) is different from the H/L ratio of the total El Médano
collection (1.74 after pairing, Hutzler et al. 2016). To correct for this bias introduced by our
random selection, we applied scaling factors to the H and L numbers so that the overall H/L ratio
is 1.74 while the total number of H+L meteorites remains 51. The error bars were computed as
the standard deviation of a Poisson distribution, that describes the number of independent events
occuring in a interval of time. The plot shows that H chondrites dominated the flux between 0.4
and 1.2 Ma. A change, leading to nearly equal proportion of H and L, occurs at 0.5 Ma and
broadly fits with the falls and other hot deserts finds proportions (Figure 3). Previous studies
have shown that H chondrites weather more rapidly than L chondrites most likely because of
their higher metallic iron content that is more sensitive to weathering (Bland et al.1998 ;
Munayco et al.2013). This effect may account for the higher proportion of L chondrites before 1
Ma but contradicts the observed higher proportion of H chondrite between 0.5 and 1 Ma.
Therefore, a dynamical explanation must be invoked. The delivery of meteorites to the Earth
from the main-belt starts when the meteoroid orbits are dynamically excited above the Earth-
crossing eccentricity threshold by a resonance in the asteroid belt (e.g., Bottke et al. 2000). A
population of meteoroids on resonant orbits has a typical lifetime of 0.5 Myr before decaying in
number mostly by colliding with the Sun (Gladman et al. 1997). The duration is fully consistent
with the timescale of the bump in Figure 3. Consequently, a potential scenario could be the direct
delivery of a population of meteoroids in a resonance of the main-belt following the break-up of
or cratering event on a close-by asteroid. A population of debris (the meteoroids) entered the
resonance about 1 Myr ago, and the eccentricities of the meteoroids increased rapidly so that
their trajectories started to intersect the Earth's orbit. The flux of meteorites from this population
started to decay after 0.5 Myr. Another hypothesis is that a swarm of meteoroids on very similar
orbits, similar to a cometary trail but generated by the break-up of a Near-Earth asteroid (e.g.,
Wiegert and Brown 2005), had a favorable encounter configuration with the Earth between 0.5
and 1Myr ago. The geometry of intersection of an inclined orbit with the Earth depends on the
value of the argument of perihelion relative to the ecliptic. In that way, the precession rate of the
argument of perihelion sets the duration of the favorable encounter configuration and the
timescale on which the configuration repeats. The typical precession rates of the argument of
perihelion of Near Earth Asteroids are in the range 10-90 arcsec per year. This implies that each
orbital intersection is short-lived and repeats every 40 kyr. This period is 20 times smaller than
the duration of the H-peak evidence by our results, which does not favor this scenario. Both
scenarios remain hypothetic and need to be investigated in details, especially by measuring the
cosmic-ray exposure age of the dataset. Indeed, a short exposure age for the excess of H
chondrites of the bump is required for such a rapid delivery from the asteroid belt to the Earth.
With a mean terrestrial age of 710 ka, the El Médano meteorite collection is by far the oldest
meteorite collection on Earth's surface. This confirms the long-term, multi-Myr stability of the
Atacama desert surfaces and offers a unique opportunity to study the meteorite flux to the Earth
and meteorite weathering over the Myr time scale. Our results suggest that although the bulk flux
has remained constant with about 220 meteorites above 10 g per km2 per Myr, significant
changes in the composition of the flux may have occurred over the last 2 Myr. In particular an
increase in the proportion of H chondrites is suggested between 0.5 and 1 Ma. Our favored
scenario would be the direct delivery of a population of meteoroids from the asteroid main-belt.
Measurement of the cosmic ray exposure ages of these meteorites could confirm this hypothesis.
Figure 3. H chondrite fraction (with respect to the total H+L chondrite) evolution during the last
2.6 Myr. The values derived from our dataset have been corrected by scaling factor so that the
overall H/L ratio matches the H/L ratio of 1.74 estimated for the whole El Médano collection
(see text). Vertical dashed line show the bins, and the numbers in boxes are the number of
samples within the bin. Uncertainties were computed as the standard deviation of a Poisson
distribution. The green dot is the current ratio computed from the meteorite fall population,
whereas the orange dot is the value computed from hot desert collection. Uncertainties are too
small to be shown.
The complete dating protocol, detailed below, was inspired by the works of Vogt and Herpers
1988 and Merchel and Herpers 1999.
Metal extraction
Samples were grinded manually in an agate mortar, and the resulting powder (Ø <200μm) was
completed with ethanol and for a two minutes bath in an ultrasonic cuve to break the silicate
grains. Once dried, a magnetic separation was performed with a hand magnet. The magnetic
extract was then washed in hydrochloric (0.2 mol/L) and hydrofluoric (3.8 mol/L) acids, during
30 and 10 min respectively. The remaining grains were washed, dried, and a final visual
inspection with a binocular microscope ensured the purity of the metallic extract.
Chlorine chemistry
The metal was then dissolved by nitric acid (2 mol/L) and spiked with an enriched 35Cl solution
(6.92 mg/g). Silver chloride precipitate was formed by adding 2 drops of silver nitrate (0.57
mol.L-1) . We stored the tubes in the darkness to prevent photosensibility effects. The solid phase
was centrifuged and washed with milliQ water. To remove all trace of sulfure, we added a
saturated solution of Ba(NO3)2 (0.5 mL) with milliQ water (1mL) and NH3 (2 mL) to form a
mixed precipitate of BaSO4 and BaCO3. The supernatant was filtered and completed with HNO3
to form a new AgCl precipitate. We rinsed it with concentrated HNO3 (4 mL) and twice with
MilliQ water (5 mL) before the final drying.
Terrestrial age measurements
AgCl precipitates were transferred to Nickel cathodes and isotopic ratio 36Cl/35Cl measurements
were performed at ASTER AMS facility. We derived the 36Cl activity Ames for each samples,
what is directly related to the terrestrial age:
Tage = 1/λ ln( Asat / Ames )
where λ is the 36Cl half-life (λ = (301±0.01) kyr, Bartholomew et al. 1955) and Asat the saturation
activity in chondritic meteorites exposed to cosmic rays. This value depends on the elemental
composition of the dissolved metal. We therefore measured the bulk composition of all metallic
fractions (Fe, Ni) by using the Thermoscientific ICap ICP-OES facility at Laboratoire G-Time at
ULB. Some samples samples were analyzed by ICP-MS and ICP-OES at the Service d’Analyse
des Roches et Minéraux (SARM, Nancy, France). The purity of the dissolved metal fraction was
controlled by measuring the mass of dissolved Si for 23 samples: it is only 1‰ of the Fe+Ni
mass on average, and 3‰ maximum. These compositions were then used as inputs in the model
of Leya (2009) to compute the saturated activities assuming exposure in space for longer than 3
Myr, and meteoroid radii < 20 cm (these two conditions ensure that the 36Cl content of the
meteorite had reached saturation and that there was no significant shielding). The first
assumption is validated by the compilation of chondrite exposure ages (e.g. Graf and Marti
1995), typically more than 4 Ma (> 10 λ). The second assumption is validated by the size
distribution of meteoroids hitting the Earth: using a consensual mass distribution for meteorites
hitting the Earth’s surface (Huss 1990), and assuming a mass loss of 99% during atmospheric
entry, meteoroids with radius > 20 cm (about 110 kg) represent less than 2% of meteoroids with
terminal masses above 10 g (the cut-off limit in the El Médano meteorite collection).
We computed a mean saturation activity of 22.9±0.2, 22.6±0.2 and 21.1±0.2 for H, L
and LL chondrites respectively, values that are consistent to those measured in the literature on
meteorite falls (e.g., Nishiizumi et al. 1989, Graf et al. 2001, Dalcher et al. 2013). The
hypothesized absence of high shielding is consistent with the small sizes of the studied
meteorites (diameter < 10 cm). We derived the uncertainties on terrestrial ages by propagation,
which leads to typical range from 60 to 90 ka (Table 1). Finally, we checked that 36Cl terrestrial
production was insignificant to introduce a bias in the results by computing the in situ production
at a typical El Médano location (24°40' S, 70°20' W, 2150 m altitude) following the model
presented in Schimmelpfennig et al. 2009. This terrestrial 36Cl production would lead to a
saturation activity <10-3 that is negligible with respect to the 36Cl activities measured in
this work (average 8.30, median 6.93, minimum 0.060
We are indebted to the many colleagues who contributed to the systematic meteorite search
conducted in the El Médano area: Leonardo Baeza, Lydie Bonal, Cécile Cournède, Bertrand
Devouard, Roger Fu, Albert Jambon, Nejia Lardhidi Ouazaa, Hamed Pourkhorsandi, Neesha
Regmi-Schnepf, Minoru Uehara, Maximilien Verdier. We acknowledge funding from the
Programme National de Planétologie (PNP, INSU/CNES) and the Agence Nationale de la
Recherche (project FRIPON, number ANR13-BS05-0009). This work was also supported by
public funds received in the framework of GEOSUD, a project (ANR-10-EQPX-20) of the
program "Investissements d'Avenir" managed by the French National Research Agency.MV was
supported by the chilean scientific agency CONICYT (FONDECYT projects n°3140562 and
n°11171090). VD thanks Sabrina Cauchies for technical support, and the FRS-FNRS and the
ERC StG "ISoSyC" for funding.
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Most meteorites that fall today are H and L type ordinary chondrites, yet the main belt asteroids best positioned to deliver meteorites are LL chondrites 1,2 . This suggests that the current meteorite flux is dominated by fragments from recent asteroid breakup events 3,4 and therefore is not representative over longer (100-Myr) timescales. Here we present the first reconstruction of the composition of the background meteorite flux to Earth on such timescales. From limestone that formed about one million years before the breakup of the L-chondrite parent body 466 Myr ago, we have recovered relict minerals from coarse micrometeorites. By elemental and oxygen-isotopic analyses, we show that before 466 Myr ago, achondrites from different asteroidal sources had similar or higher abundances than ordinary chondrites. The primitive achondrites, such as lodranites and acapulcoites, together with related ungrouped achondrites, made up ~15–34% of the flux compared with only ~0.45% today. Another group of abundant achondrites may be linked to a 500-km cratering event on (4) Vesta that filled the inner main belt with basaltic fragments a billion years ago 5 . Our data show that the meteorite flux has varied over geological time as asteroid disruptions create new fragment populations that then slowly fade away from collisional and dynamical evolution. The current flux favours disruption events that are larger, younger and/or highly efficient at delivering material to Earth.
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In-situ cosmogenic 36Cl production rates from spallation of Ca and K determined in several previously published calibration studies differ by up to 50%. In this study we compare whole rock 36Cl exposure ages with 36Cl exposure ages evaluated in Ca-rich plagioclase in the same 10 ± 3 ka lava sample taken from Mt. Etna (Sicily, 38° N). The exposure age of the sample was determined by K–Ar and corroborated by cosmogenic 3He measurements on cogenetic pyroxene phenocrysts. Sequential dissolution experiments showed that high Cl concentrations in plagioclase grains could be reduced from 450 ppm to less than 3 ppm after 16% dissolution. 36Cl exposure ages calculated from the successive dissolution steps of this leached plagioclase sample are in good agreement with K–Ar and 3He age. Stepwise dissolution of whole rock grains, on the other hand, is not as effective in reducing high Cl concentrations as it is for the plagioclase. 330 ppm Cl still remains after 85% dissolution. The 36Cl exposure ages derived are systematically about 30% higher than the ages calculated from the plagioclase. We could exclude contamination by atmospheric 36Cl as an explanation for this overestimate. Magmatic 36Cl was estimated by measuring a totally shielded sample, but was found to account for only an insignificant amount of 36Cl in the case of the 10 ka whole rock sample. We suspect that the overestimate of the whole rock exposure age is due to the difficulty in accurately assessing all the factors which control production of 36Cl by low-energy neutron capture on 35Cl, particularly variable water content and variable snow cover. We conclude that some of the published 36Cl spallation production rates might be overestimated due to high Cl concentrations in the calibration samples. The use of rigorously pretreated mineral separates reduces Cl concentrations, allowing better estimates of the spallation production rates.
The terrestrial impact crater record is commonly assumed to be biased, with erosion thought to eliminate older craters, even on stable terrains. Given that the same projectile population strikes Earth and the Moon, terrestrial selection effects can be quantified by using a method to date lunar craters with diameters greater than 10 kilometers and younger than 1 billion years. We found that the impact rate increased by a factor of 2.6 about 290 million years ago. The terrestrial crater record shows similar results, suggesting that the deficit of large terrestrial craters between 300 million and 650 million years ago relative to more recent times stems from a lower impact flux, not preservation bias. The almost complete absence of terrestrial craters older than 650 million years may indicate a massive global-scale erosion event near that time.
We measured the concentrations and isotopic compositions of He, Ne, and Ar in bulk samples and metal separates of 14 ordinary chondrite falls with long exposure ages and high metamorphic grades. In addition, we measured concentrations of the cosmogenic radionuclides 10Be, 26Al, and 36Cl in metal separates and in the nonmagnetic fractions of the selected meteorites. Using cosmogenic 36Cl and 36Ar measured in the metal separates, we determined 36Cl-36Ar cosmic-ray exposure (CRE) ages, which are shielding-independent and therefore particularly reliable. Using the cosmogenic noble gases and radionuclides, we are able to decipher the CRE history for the studied objects. Based on the correlation 3He/21Ne versus 22Ne/21Ne, we demonstrate that, among the meteorites studied, only one suffered significant diffusive losses (about 35%). The data confirm that the linear correlation 3He/21Ne versus 22Ne/21Ne breaks down at high shielding. Using 36Cl-36Ar exposure ages and measured noble gas concentrations, we determine 21Ne and 38Ar production rates as a function of 22Ne/21Ne. The new data agree with recent model calculations for the relationship between 21Ne and 38Ar production rates and the 22Ne/21Ne ratio, which does not always provide unique shielding information. Based on the model calculations, we determine a new correlation line for 21Ne and 38Ar production rates as a function of the shielding indicator 22Ne/21Ne for H, L, and LL chondrites with preatmospheric radii less than about 65 cm. We also calculated the 10Be/21Ne and 26Al/21Ne production rate ratios for the investigated samples, which show good agreement with recent model calculations.
The terrestrial age, or the terrestrial residence time of a meteorite, together with its exposure history provides us with useful insight into the history of the meteorite. It is easy to observe that stony meteorites can weather quickly in humid environments. However, we find that large numbers of meteorites found in semiarid and arid environments can survive for much longer times. Meteorites in desert environments can survive for at least 50,000 yr, and there are some meteorites over 250,000 yr old from these locations. The cold and dry conditions of polar regions such as Antarctica are also good for the storage of meteorites. A considerable number of meteorites survive there for hundreds of thousands of years. Some meteorites have been found in Antarctica with ages of up to 2 m.y. In this paper, we discuss the terrestrial residence times or terrestrial ages of these meteorites. We will show the wide range of terrestrial ages from different environments.
Relict spinel grains (~25–250 μm in diameter) from decomposed extraterrestrial material in Archean to Recent sediments can be used to reconstruct variations in the flux of different types of meteorites to Earth through the ages. Meteorite falls are rare and meteorites weather and decay rapidly on the Earth surface, making it a challenge to reconstruct ancient fluxes. Almost all meteorite types, however, contain a small fraction of spinel minerals that survive weathering and can be recovered by acid-dissolution of large samples (100–1000 kg) of slowly deposited sediments of any age. The spinel grains originate from either micrometeorites, meteorites or asteroids, and can give detailed information on the types of extraterrestrial matter that fell on Earth at specific times in the geological past. Inside the spinels, synchrotron-light X-ray tomography can identify 1–30 μm inclusions of most of the other minerals that made up the original meteorite. With advanced microanalyses of the spinels, such as Ne isotopes (from solar wind, and produced by cosmic rays), oxygen isotopes (meteorite class and group) and cosmic ray tracks, it may be possible to unravel from the geological record fundamental new information about the solar system at specific times through the past ~3.5 Gyr. Variations in flux and types of meteorites may reflect large-scale perturbations of the orbits of planets and other bodies in the solar system, as well as the sequence of disruptions of the parent bodies for the meteorite types known and not yet known. Orbital perturbations may be triggered by near-by passing stars, giant molecular clouds, the galactic gravitational field, supernova shock waves or unusual planetary alignments.
Here I report the first discovery of large numbers of ordinarychondrite-derived micro-meteorites (MMs) recovered fromAntarctic ice. Ordinary chondrite-derived MMs comprise70% coarse-grained igneous particles and are identified on thebasis of their distribution of petrologic types, accessory mineralogy,and minor element compositions, all of which are very similarto those of millimeter-sized igneous objects known as chondrules,from ordinary chondrites, and largely distinct from those ofother meteorite groups. The majority of ordinary chondrite-derivedMMs are unequilibrated materials; however, 15% are equilibrated,indicating parent body metamorphism, and have compositions consistentwith both H (high iron) and L (low iron) chondrites. The totalabundance of ordinary chondrite-derived MMs of ~18% issimilar to predictions by numerical calculations for the abundanceof dust generated by the recent breakup of the Karin group asteroidsof the Koronis asteroid family, suggesting that these asteroidsare the main source of the particles. These MMs represent thefirst to be associated with a known asteroid and imply thatthe Karin group progenitor asteroid was a rubble-pile asteroidthat sampled different depths within the original internallymetamorphosed Koronis parent body.
This paper is part of the special publication Meteorites: flux with time and impact effects (eds M.M. Grady, R. Hutchinson, G.J.H. McCall and D.A. Rothery). It is premature to report firm conclusions regarding the past meteorite flux rate from the Antarctic record, as variables related to ice flow, meteorite catchment area, weathering and removal rate are not well constrained. The relative population of meteorite types appears to be broadly compatible with the modern fall population, although there are minor differences. In particular, there are populations of small unusual irons, carbonaceous chondrites and lunar samples not found among the modern falls. In addition, slight compositional differences between modern and old (Antarctic) H chondrites may signal a time-dependent change in the flux to Earth of more common meteorites.
The age of onset of hyperaridity in the Atacama Desert, Chile, which is needed to validate geological and climatological concepts, has been heretofore uncertain. Measurement of cosmogenic 21Ne in clasts from erosion-sensitive sediment surfaces in northern Chile shows that these surfaces have been barely affected by erosion since 25 Ma. Surface exposure ages of sediment clasts give replicate values at 25, 20, and 14 Ma and individual values at 37 and 9 Ma. Predominantly hyperarid conditions are required to preserve these oldest continuously exposed surfaces on Earth. Our findings are compatible with the hypothesis that the onset of aridity in the Atacama Desert could be the reason for, rather than the consequence of, uplift of the high Andes.