883 research outputs found
The iodine-plutonium-xenon age of the Moon-Earth system revisited
From iodine-plutonium-xenon isotope systematics, we re-evaluate time
constraints on the early evolution of the Earth-atmosphere system and, by
inference, on the Moon-forming event. Two extinct radioactivites (129I, T1/2 =
15.6 Ma, and 244Pu, T1/2 = 80 Ma) have produced radiogenic 129Xe and
fissiogenic 131-136Xe, respectively, within the Earth, which related isotope
fingerprints are seen in the compositions of mantle and atmospheric Xe. Recent
studies of Archean rocks suggest that xenon atoms have been lost from the
Earth's atmosphere and isotopically fractionated during long periods of
geological time, until at least the end of the Archean eon. Here we build a
model that takes into account these results. Correction for Xe loss permits to
compute new closure ages for the Earth's atmosphere that are in agreement with
those computed for mantle Xe. The minimum Xe formation interval for the Earth-
atmosphere is 40 (-10+20) Ma after start of solar system formation, which may
also date the Moon-forming impact.Comment: 27 pages, 3 figures, 2 table
Perspectives on atmospheric evolution from noble gas and nitrogen isotopes on Earth, Mars & Venus
The composition of an atmosphere has integrated the geological history of the
entire planetary body. However, the long-term evolutions of the atmospheres of
the terrestrial planets are not well documented. For Earth, there were until
recently only few direct records of atmosphere's composition in the distant
past, and insights came mainly from geochemical or physical proxies and/or from
atmospheric models pushed back in time. Here we review innovative approaches on
new terrestrial samples that led to the determination of the elemental and
isotopic compositions of key geochemical tracers, namely noble gases and
nitrogen. Such approaches allowed one to investigate the atmosphere's evolution
through geological period of time, and to set stringent constraints on the past
atmospheric pressure and on the salinity of the Archean oceans. For Mars, we
review the current state of knowledge obtained from analyses of Martian
meteorites, and from the direct measurements of the composition of the
present-day atmosphere by rovers and spacecrafts. Based on these measurements,
we explore divergent models of the Martian and Terrestrial atmospheric
evolutions. For Venus, only little is known, evidencing the critical need for
dedicated missions
Nitrogen isotopic fractionation during abiotic synthesis of organic solid particles
The formation of organic compounds is generally assumed to result from
abiotic processes in the Solar System, with the exception of biogenic organics
on Earth. Nitrogen-bearing organics are of particular interest, notably for
prebiotic perspectives but also for overall comprehension of organic formation
in the young solar system and in planetary atmospheres. We have investigated
abiotic synthesis of organics upon plasma discharge, with special attention to
N isotope fractionation. Organic aerosols were synthesized from N2-CH4 and
N2-CO gaseous mixtures using low-pressure plasma discharge experiments, aimed
at simulating chemistry occurring in Titan s atmosphere and in the protosolar
nebula, respectively. Nitrogen is efficiently incorporated into the synthesized
solids, independently of the oxidation degree, of the N2 content of the
starting gas mixture, and of the nitrogen speciation in the aerosols. The
aerosols are depleted in 15N by 15-25 permil relative to the initial N2 gas,
whatever the experimental setup is. Such an isotopic fractionation is
attributed to mass-dependent kinetic effect(s). Nitrogen isotope fractionation
upon electric discharge cannot account for the large N isotope variations
observed among solar system objects and reservoirs. Extreme N isotope
signatures in the solar system are more likely the result of self-shielding
during N2 photodissociation, exotic effect during photodissociation of N2
and/or low temperature ion-molecule isotope exchange. Kinetic N isotope
fractionation may play a significant role in the Titan s atmosphere. We also
suggest that the low delta15N values of Archaean organic matter are partly the
result of abiotic synthesis of organics that occurred at that time
Origin and significance of cosmogenic signatures in vesicles of lunar basalt 15016
Lunar basalt 15016 (~3.3 Ga) is among the most vesicular (50% by volume) basalts recovered by the Apollo missions. We investigated the possible occurrence of indigenous lunar nitrogen and noble gases trapped in vesicles within basalt 15016, by crushing several cm‐sized chips. Matrix/mineral gases were also extracted from crush residues by fusion with a CO_2 laser. No magmatic/primordial component could be identified; all isotope compositions, including those of vesicles, pointed to a cosmogenic origin. We found that vesicles contained ~0.2%, ~0.02%, ~0.002%, and ~0.02% of the total amount of cosmogenic ^(21)Ne, ^(38)Ar, ^(83)Kr, and ^(126)Xe, respectively, produced over the basalt's 300 Myr of exposure. Diffusion/recoil of cosmogenic isotopes from the basaltic matrix/minerals to intergrain joints and vesicles is discussed. The enhanced proportion of cosmogenic Xe isotopes relative to Kr detected in vesicles could be the result of kinetic fractionation, through which preferential retention of Xe isotopes over Kr within vesicles might have occurred during diffusion from the vesicle volume to the outer space through microleaks. This study suggests that cosmogenic loss, known to be significant for ^3He and ^(21)Ne, and to a lesser extent for ^(36)Ar (Signer et al. 1977), also occurs to a negligible extent for the heaviest noble gases Kr and Xe
Primordial Origins of Earth's Carbon
International audienceIt is commonly assumed that the building blocks of the terrestrial planets were derived froma cosmochemical reservoir that is best represented by chondrites, the so-called chondritic Earthmodel. This view is possibly a good approximation for refractory elements (although it hasbeen recently questioned; e.g., Caro et al. 2008), but for volatile elements, other cosmochemicalreservoirs might have contributed to Earth, such as the solar nebula gas and/or cometary matter(Owen et al. 1992; Dauphas 2003; Pepin 2006). Hence, in order to get insights into the originof the carbon in Earth, it is necessary to compare: (i) the elemental abundances and isotopiccompositions of not only carbon, but also other volatile elements in potential cosmochemical“ancestors,” and (ii) the ancestral compositions with those of terrestrial volatiles. This approachis the only one that has the potential for understanding the origin of the carbon in Earth butit has several intrinsic limitations. First, the terrestrial carbon budget is not well known, and,for the deep reservoir(s) such as the core and the lower mantle, is highly model-dependent(Dasgupta 2013; Wood et al. 2013). Second, the cosmochemical reservoir(s) that contributedvolatile elements to proto-Earth may not exist anymore because planet formation might havecompletely exhausted them (most of the mass present in the inner solar system is now in Venusand Earth). Third, planetary formation processes (accretion, differentiation, early evolutionof the atmospheres) might have drastically modified the original elemental and isotopiccompositions of the volatile elements in Earth. Despite these limitations, robust constraints onthe origin(s) of the carbon in Earth can be deduced from comparative planetology of volatileelements, which is the focus of this chapter
Volatiles in protoplanetary disks
Volatiles are compounds with low sublimation temperatures, and they make up
most of the condensible mass in typical planet-forming environments. They
consist of relatively small, often hydrogenated, molecules based on the
abundant elements carbon, nitrogen and oxygen. Volatiles are central to the
process of planet formation, forming the backbone of a rich chemistry that sets
the initial conditions for the formation of planetary atmospheres, and act as a
solid mass reservoir catalyzing the formation of planets and planetesimals.
This growth has been driven by rapid advances in observations and models of
protoplanetary disks, and by a deepening understanding of the cosmochemistry of
the solar system. Indeed, it is only in the past few years that representative
samples of molecules have been discovered in great abundance throughout
protoplanetary disks - enough to begin building a complete budget for the most
abundant elements after hydrogen and helium. The spatial distributions of key
volatiles are being mapped, snow lines are directly seen and quantified, and
distinct chemical regions within protoplanetary disks are being identified,
characterized and modeled. Theoretical processes invoked to explain the solar
system record are now being observationally constrained in protoplanetary
disks, including transport of icy bodies and concentration of bulk
condensibles. The balance between chemical reset - processing of inner disk
material strong enough to destroy its memory of past chemistry, and inheritance
- the chemically gentle accretion of pristine material from the interstellar
medium in the outer disk, ultimately determines the final composition of
pre-planetary matter. This chapter focuses on making the first steps toward
understanding whether the planet formation processes that led to our solar
system are universal.Comment: Accepted for publication as a chapter in Protostars and Planets VI,
University of Arizona Press (2014), eds. H. Beuther, R. Klessen, C.
Dullemond, Th. Hennin
Mercury (Hg) in meteorites: variations in abundance, thermal release profile, mass-dependent and mass-independent isotopic fractionation
We have measured the concentration, isotopic composition and thermal release
profiles of Mercury (Hg) in a suite of meteorites, including both chondrites
and achondrites. We find large variations in Hg concentration between different
meteorites (ca. 10 ppb to 14'000 ppb), with the highest concentration orders of
magnitude above the expected bulk solar system silicates value. From the
presence of several different Hg carrier phases in thermal release profiles
(150 to 650 {\deg}C), we argue that these variations are unlikely to be mainly
due to terrestrial contamination. The Hg abundance of meteorites shows no
correlation with petrographic type, or mass-dependent fractionation of Hg
isotopes. Most carbonaceous chondrites show mass-independent enrichments in the
odd-numbered isotopes 199Hg and 201Hg. We show that the enrichments are not
nucleosynthetic, as we do not find corresponding nucleosynthetic deficits of
196Hg. Instead, they can partially be explained by Hg evaporation and
redeposition during heating of asteroids from primordial radionuclides and
late-stage impact heating. Non-carbonaceous chondrites, most achondrites and
the Earth do not show these enrichments in vapor-phase Hg. All meteorites
studied here have however isotopically light Hg ({\delta}202Hg = ~-7 to -1)
relative to the Earth's average crustal values, which could suggest that the
Earth has lost a significant fraction of its primordial Hg. However, the late
accretion of carbonaceous chondritic material on the order of ~2%, which has
been suggested to account for the water, carbon, nitrogen and noble gas
inventories of the Earth, can also contribute most or all of the Earth's
current Hg budget. In this case, the isotopically heavy Hg of the Earth's crust
would have to be the result of isotopic fractionation between surface and
deep-Earth reservoirs.Comment: 43 Pages, 9 Figures. Accepted for publication in Geochimica et
Cosmochimica Act
Salinity of the Archaean oceans from analysis of fluid inclusions in quartz
Fluids trapped in inclusions in well-characterized Archaean hydrothermal quartz crystals were analyzed by the extended argon–argon method, which permits the simultaneous measurement of chlorine and potassium concentrations. Argon and nitrogen isotopic compositions of the trapped fluids were also determined by static mass spectrometry. Fluids were extracted by stepwise crushing of quartz samples from North Pole (NW Australia) and Barberton (South Africa) 3.5–3.0-Ga-old greenstone belts. The data indicate that fluids are a mixture of a low salinity end-member, regarded as the Archaean oceanic water, and several hydrothermal end-members rich in Cl, K, N, and radiogenic parentless ^(40)Ar. The low Cl–K end-member suggests that the salinity of the Archaean oceans was comparable to the modern one, and that the potassium content of the Archaean oceans was lower than at present by about 40%. A constant salinity of the oceans through time has important implications for the stabilization of the continental crust and for the habitability of the ancient Earth
Nitrogen Isotopic Composition and Density of the Archean Atmosphere
Understanding the atmosphere's composition during the Archean eon is a
fundamental issue to unravel ancient environmental conditions. We show from the
analysis of nitrogen and argon isotopes in fluid inclusions trapped in 3.0-3.5
Ga hydrothermal quartz that the PN2 of the Archean atmosphere was lower than
1.1 bar, possibly as low as 0.5 bar, and had a nitrogen isotopic composition
comparable to the present-day one. These results imply that dinitrogen did not
play a significant role in the thermal budget of the ancient Earth and that the
Archean PCO2 was probably lower than 0.7 bar
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