Noble Gases In the Martian Meteorite Northwest Africa 2737: A New Chassignite Signature (original) (raw)
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… et Cosmochimica Acta, 2009
Meteorite ''finds" from the terrestrial hot deserts have become a major contributor to the inventory of Martian meteorites. In order to understand their nitrogen and noble gas components, we have carried out stepped heating experiments on samples from two Martian meteorites collected from hot deserts. We measured interior and surface bulk samples, glassy and nonglassy portions of Dar al Gani 476 and Sayh al Uhaymir 005. We have also analyzed noble gases released from the Antarctic shergottite Lewis Cliff 88516 by crushing and stepped heating. For the hot desert meteorites significant terrestrial Ar, Kr, Xe contamination is observed, with an elementally fractionated air (EFA) component dominating the low temperature releases. The extremely low Ar/Kr/Xe ratios of EFA may be the result of multiple episodes of trapping/loss during terrestrial alteration involving aqueous fluids. We suggest fractionation processes similar to those in hot deserts to have acted on Mars, with acidic weathering on the latter possibly even more effective in producing elementally fractionated components. Addition from fission xenon is apparent in DaG 476 and SaU 005. The Ar-Kr-Xe patterns for LEW 88516 show trends as typically observed in shergottites -including evidence for a crush-released component similar to that observed in EETA 79001. A trapped Ne component most prominent in the surface sample of DaG 476 may represent air contamination. It is accompanied by little trapped Ar ( 20 Ne/ 36 Ar > 50) and literature data suggest its presence also in some Antarctic finds. Data for LEW 88516 and literature data, on the other hand, suggest the presence of two trapped Ne components of Martian origin characterized by different 20 Ne/ 22 Ne, possibly related to the atmosphere and the interior. Caution is recommended in interpreting nitrogen and noble gas isotopic signatures of Martian meteorites from hot deserts in terms of extraterrestrial sources and processes. Nevertheless our results provide hope that vice-versa, via noble gases and nitrogen in meteorites and other relevant samples from terrestrial deserts, Martian secondary processes can be studied.
The age of SNC meteorites and the antiquity of the Martian surface
Earth and Planetary Science Letters, 2005
We report new Sm-Nd, Lu-Hf, and Pb-Pb mineral and whole-rock isotope data for the basaltic shergottite Zagami, as well as Pb-Pb whole-rock isotope data for the basaltic shergottite Los Angeles, the lherzolitic shergottite Dar-al-Gani 476 (DaG 476), and the clinopyroxenite Nakhla. In agreement with previous findings, our new Sm-Nd and Lu-Hf mineral ages on the Martian meteorite Zagami are young (155 and 185 Ma, respectively). The 207 Pb/ 206 Pb-204 Pb/ 206 Pb compositions of the insoluble fractions of shergottites (Zagami, Los Angeles, and literature data for Shergotty and EETA79001) form an excellent alignment indicative of a 4.0 Ga crystallization age. The range of Pb isotope compositions observed in the leachates of these samples attests to negligible contamination of the shergottites by terrestrial Pb and argues against mixing relationships. The age of 4.048 F 0.017 Ga (MSWD = 1.5) provided by the Pb isotope compositions of the Zagami whole-rock and residues is therefore taken to date the crystallization of this rock, which, so far, was believed to be only~180 Ma old. Based on this result, we argue that the lithosphere of Mars is extremely old and that most mineral ages were reset recently by acidic aqueous solutions percolating through the Martian surface. This interpretation is consistent with photographic interpretations of erosional features on Mars. It also relieves the constraint imposed by the presence of anomalies of 142 Nd and 182 W (both products of extinct radioactive nuclides) that the Martian mantle should have preserved primordial isotopic heterogeneities, thus allowing for the planet interior to be actively convecting.
Geochimica et Cosmochimica Acta, 2008
Samarium-neodymium isotopic analyses of unleached and acid-leached mineral fractions from the recently identified olivine-bearing shergottite Northwest Africa 1195 yield a crystallization age of 348 ± 19 Ma and an 143 Nd ε value of +40.1 ± 1.3. Maskelynite fractions do not lie on the Sm-Nd isochron and appear to contain a martian surface component with low 147 Sm/ 144 Nd and 143 Nd/ 144 Nd ratios that was added during shock. The Rb-Sr system is disturbed and does not yield an isochron. Terrestrial Sr appears to have affected all of the mineral fractions, although a maximum initial 87 Sr/ 86 Sr ratio of 0.701614 ± 16 is estimated by passing a 348 Ma reference isochron through the maskelynite fraction that is least affected by contamination. The high initial 143 Nd ε value and the low initial 87 Sr/ 86 Sr ratio, combined with the geologically young crystallization age, indicate that Northwest Africa 1195 is derived from a source region characterized by a long-term incompatible element depletion. The age and initial Sr and Nd isotopic compositions of Northwest Africa 1195 are very similar to those of Queen Alexandra Range 94201, indicating these samples were derived from source regions with nearly identical Sr-Nd isotopic systematics. These similarities suggest that these two meteorites share a close petrogenetic relationship and might have been erupted from a common volcano. The meteorites Yamato 980459, Dar al Gani 476, Sayh al Uhaymir 005/008, and Dhofar 019 also have relatively old ages between 474-575 Ma and trace element and/or isotopic systematics that are indicative of derivation from incompatible-element-depleted
Nitrogen and heavy noble gases in ALH 84001: Signatures of ancient Martian atmosphere
Geochimica Et Cosmochimica Acta, 1997
Nitrogen and noble gases have been studied in a bulk sample and three density separates of the Martian orthopyroxenite ALH 84001. The δ 15N values which lie between 85%. and −18%. (and after correcting for cosmogenic contribution, between 46%. and −23%.), define a two component mixing trend in a plot of δ15N vs. 1/N, with Chassigny as one endmember and another component with ° 15N ≥ 46%.. This trend is different from the one defined by the data from EET 79001,C and glass from Zagami. Most of the krypton and xenon are of trapped origin; the ratios 129Xe/132Xe and 136Xe/132Xe being similar to the Martian atmospheric values as found in EET 79001,C. In addition, small contributions from in situ 238U fission and live 129I decay are evident in some high temperature steps, the later observation attesting to the antiquity of this Martian meteorite. Excesses at 80, 82Kr and 128Xe due to neutron capture effects on bromine and iodine, respectively, are observed in all the samples. These neutron effects are not consistent with in situ production in the meteoroid during cosmic ray exposure and hence should be produced in the Martian atmosphere or surface and enterd the meteorite as a trapped component. The lower δ 15N (≥46%.) and 40Ar/36Ar ≤ 1400 in the trapped component of ALH 84001, as compared to the values from EET 79001,C, together with the fact that radiogenic 40Ar and trapped 36Ar, 84Kr, and 132Xe have similar release pattern, are strongly suggestive that the trapped component in ALH 84001 represents Martian atmosphere of ∼4Ga ago.The noble gas elemental ratios 36Ar/132Xe and 84Kr/132Xe show an elemental fractionation trend, enriching the heavy noble gases, similar to what has been observed in Nakhla (Drake et al., 1994). Comparing the nitrogen and xenon isotopic records and the radiogenic and stable isotope ratios (40Ar/129Xe and 39Ar/14N) from ALH 84001 representing Martian atmospheric component of ∼4 Ga ago, with those from EET 79001,C representing Martian atmospheric component of recent past, we infer the following on the evolution of the Martian atmosphere: (a) Xenon isotopic composition, as well as the amounts of xenon have been completely evolved at 4 Ga in Martian atmosphere and almost remained unchanged to the present; (b) The radiogenic 40Ar has not been completely degassed into the atmosphere at 4 Ga; (c) Nitrogen has been lost in a continuous process, leading to an increase in the ratio of 36Ar/14N as well as the δ 15N in the present Martian atmosphere as compared to 4 Ga ago. These inferences are consistent with the model predictions (Pepin, 1994).
Understanding Mars from meteoritesthe nitrogen and noble gas perspective
Current Science, 2004
Mars has inspired our imagination and scientific research. While space missions in the 1960 and 1970s set up a milestone in the study of Mars, discovery of meteorites apparently from the Red Planet led scientific research into a new dimension. Easily identified and linked by their nitrogen and noble gas isotopic signatures, martian meteorites have provided a unique means of acquiring geochemical data about Mars. These in turn, complemented by results from various geophysical studies and cosmochemical modelling, provide important constraints on the planet's origin and evolution, which constitute the subject matter for the present article.
Planetary and Space Science, 2020
Two Martian meteorites, Chassigny and Nakhla, were analyzed by non-destructive gamma-ray spectrometry to determine concentrations of cosmogenic (26Al) and primordial (40K, 238U, 232Th) radionuclides, and to estimate their pre-atmospheric sizes and cosmic-ray exposure ages. The pre-atmospheric radii of the Chassigny and Nakhla meteorites were estimated using the cosmogenic 26Al to be 17 ± 4 and 30 ± 5 cm, which would result in their total masses of 30–130 and 210–580 kg, respectively. This is comparable to the size and mass estimated for other chassignites and nakhlites. The cosmic-ray exposure ages of the Chassigny and Nakhla meteorites estimated using the 26Al – 21Ne isotope pair were 12.8 ± 2.5 and 11.6 ± 1.8 Myr, respectively, in agreement with published ages of chassignites and nakhlites averaged from several isotope methods, i.e. 11.6 ± 0.8 and 12.2 ± 0.8 Myr, respectively.
Noble gases in mineral separates from three shergottites: Shergotty, Zagami, and EETA79001
… & Planetary Science, 2007
This study provides a complete data set of all five noble gases for bulk samples and mineral separates from three Martian shergottites: Shergotty (bulk, pyroxene, maskelynite), Zagami (bulk, pyroxene, maskelynite), and Elephant Moraine (EET) A79001, lithology A (bulk, pyroxene). We also give a compilation of all noble gas and nitrogen studies performed on these meteorites. Our mean values for cosmic-ray exposure ages from 3 He, 21 Ne, and 38 Ar are 2.48 Myr for Shergotty, 2.73 Myr for Zagami, and 0.65 Myr for EETA79001 lith. A. Serious loss of radiogenic 4 He due to shock is observed. Cosmogenic neon results for bulk samples from 13 Martian meteorites (new data and literature data) are used in addition to the mineral separates of this study in a new approach to explore evidence of solar cosmic-ray effects. While a contribution of this low-energy irradiation is strongly indicated for all of the shergottites, spallation Ne in Chassigny, Allan Hills (ALH) 84001, and the nakhlites is fully explained by galactic cosmic-ray spallation. Implanted Martian atmospheric gases are present in all mineral separates and the thermal release indicates a near-surface siting. We derive an estimate for the 40 Ar/ 36 Ar ratio of the Martian interior component by subtracting from measured Ar in the (K-poor) pyroxenes the (small) radiogenic component as well as the implanted atmospheric component as indicated from 129 Xe * excesses. Unless compromised by the presence of additional components, a high ratio of ~2000 is indicated for Martian interior argon, similar to that in the Martian atmosphere. Since much lower ratios have been inferred for Chassigny and ALH 84001, the result may indicate spatial and/or temporal variations of 40 Ar/ 36 Ar in the Martian mantle.
Rb–Sr, Sm–Nd and Ar–Ar isotopic systematics of Martian dunite Chassigny
Earth and Planetary Science Letters, 2006
Isotopic analysis of the Martian meteorite Chassigny yields a Rb-Sr age of 1406 ± 14 Ma with an initial 87 Sr/ 86 Sr ratio of 0.702251 ± 0.000034, a Sm-Nd age of 1386 ± 28 Ma with an initial ε 143Nd -value of + 16.9 ± 0.3 and an 39 Ar-40 Ar age of 1360 + 40 − 20 Ma. The concordance of these ages and the Rb-Sr and Sm-Nd initial isotopic signatures suggest that Chassigny crystallized from low Rb/Sr, light rare earth element depleted source materials ∼1390 Ma ago. The ages and ε 143Nd -values of Chassigny and the nakhlites Governador Valadares and Lafayette overlap, suggesting that they could have come from very similar mantle sources. Nakhla, Northwest Africa 998 and Yamato 000593 appear to be from similar but distinct sources. Chassigny and all nakhlites so far studied have undergone similar evolution histories. That is, chassignites/nakhlites were derived from a region where volcanism lasted at least 50 Ma and crystallized from different lava flows or subsurface sills. They probably were launched from Mars by a single impact event. The trapped Martian atmospheric 40 Ar/ 36 Ar ratios in Chassigny, nakhlites and shergottite impact glass are similar and possibly indicate minimal change in this ratio over the past ≥ 600 Ma.
The Ksar Ghilane 002 shergottite-The 100th registered Martian meteorite fragment
Meteoritics & Planetary Science, 2013
We report on the discovery of a new shergottite from Tunisia, Ksar Ghilane (KG) 002. This single stone, weighing 538 g, is a coarse-grained basaltic shergottite, mainly composed of maskelynitized plagioclase (approximately 52 vol%) and pyroxene (approximately 37 vol%). It also contains Fe-rich olivine (approximately 4.5 vol%), large Ca-phosphates, including both merrillites and Cl-apatites (approximately 3.4 vol%), minor amounts of silica or SiO 2 -normative K-rich glass, pyrrhotite, Ti-magnetite, ilmenite, and accessory baddeleyite. The largest crystals of pyroxene and plagioclase reach sizes of approximately 4 to 5 mm. Pyroxenes (Fs 26-96 En 5-50 Wo 2-41 ). They typically range from cores of about Fs 29 En 41 Wo 30 to rims of about Fs 68 En 14 Wo 17 . Maskelynite is Ab 41-49 An 39-58 Or 1-7 in composition, but some can be as anorthitic as An 93 . Olivine (Fa 91-96 ) occurs mainly within symplectitic intergrowths, in paragenesis with ilmenite, or at neighboring areas of symplectites. KG 002 is heavily shocked (S5) as indicated by mosaic extinction of pyroxenes, maskelynitized plagioclase, the occurrence of localized shock melt glass pockets, and low radiogenic He concentration. Oxygen isotopes confirm that it is a normal member of the SNC suite. KG 002 is slightly depleted in LREE and shows a positive Eu anomaly, providing evidence for complex magma genesis and mantle processes on Mars. Noble gases with a composition thought to be characteristic for Martian interior is a dominant component. Measurements of 10 Be, 26 Al, and 53 Mn and comparison with Monte Carlo calculations of production rates indicate that KG 002 has been exposed to cosmic rays most likely as a single meteoroid body of 35-65 cm radius. KG 002 strongly resembles Los Angeles and NWA 2800 basaltic shergottites in element composition, petrography, and mineral chemistry, suggesting a possible launch-pairing. The similar CRE ages of KG 002 and Los Angeles may suggest an ejection event at approximately 3.0 Ma.