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1.
Abstract— We report the elemental and isotopic composition of the noble gases as well as the chemical abundances in pyroxene, maskelynite/mesostasis glass, and bulk material of Shergotty and of bulk samples from Chassigny and Yamato 793605. The 40K-40Ar isochron for the Shergotty minerals yields a gas retention age of 196 Ma, which is, within errors, in agreement with previously determined Rb-Sr internal isochron ages. Argon that was trapped at this time has a 40Ar/36Ar ratio of 1100. For Chassigny and Y-793605, we obtain trapped 40Ar/36Ar ratios of 1380 and 950, respectively. Using these results and literature data, we show that the three shergottites, Shergotty, Zagami, and QUE 94001; the lherzolites ALH 77005, LEW 88516, and Y-793605; as well as Chassigny and ALH 84001 contain a mixture of Martian mantle and atmospheric Ar; whereas, the trapped 40Ar/36Ar ratio of the nakhlites, Nakhla, Lafayette, and Governador Valadares cannot be determined with the present data. We show that Martian atmospheric trapped Ar in Martian meteorites is correlated with the shock pressure that they experienced. Hence, we conclude that the Martian atmospheric gases were introduced by shock into the meteoritic material. For the Shergotty minerals, we obtain 3He-, 21Ne-, and 38Ar-based cosmic-ray exposure ages of 3.0 Ma, and for the lherzolite Y-793605, 4.0 Ma, which confirms our earlier conclusion that the lherzolites were ejected from Mars ～1 Ma before the shergottites. Chassigny yields the previously known ejection age of 11.6 Ma.  相似文献   

2.
Abstract— In a study of the isotopic signatures of trapped Xe in shock-produced glass of shergottites and in ALH 84001, we observe three components: (1) modern Martian atmospheric Xe that is isotopically mass fractionated relative to solar Xe, favoring the heavy isotopes, (2) solar-like Xe, as previously observed in Chassigny, and (3) an isotopically fractionated (possibly ancient) component with little or no radiogenic 129Xerad. In situ-produced fission and spallation components are observed predominantly in the high-temperature steps. Heavy N signatures in ALH 84001, EET 79001 and Zagami reveal Martian atmospheric components. The low-temperature release of ALH 84001 shows evidence for the presence of a light N component (δ15N ≤ -21%), which is consistent with the component observed in the other Shergotty, Nakhla and Chassigny (SNC) group meteorites. The highest observed 129Xe/130Xe ratio of 15.60 in Zagami and EET 79001 is used here to represent the present Martian atmospheric component, and the isotopic composition of this component is compared with other solar system Xe signatures. The 129Xe/130Xe ratios in ALH 84001 are lower but appear to reflect varying mixing ratios with other components. The consistently high 129Xe/130Xe ratios in rocks of different radiometric ages suggest that Martian atmospheric Xe evolved early on. As already concluded in earlier work, only a small fission component is observed in the Martian atmospheric component. Assuming that a chondritic 244Pu/129I initial ratio applies to Mars, this implies that either Pu-derived fission Xe is retained in the solid planet (in fact, in situ-produced fission Xe is observed in ALH 84001) or may reflect a very particular degassing history of the planet.  相似文献   

3.
Abstract— Argon-isotopic abundances were measured in neutron-irradiated samples of Martian meteorites Chassigny, Allan Hills (ALH) 84001, ALH 77005, Elephant Moraine (EET) 79001, Yamato (Y) 793605, Shergotty, Zagami, and Queen Alexandra Range (QUE) 94201, and in unirradiated samples of ALH 77005. Chassigny gives a 39Ar-40Ar age of 1.32 ± 0.07 Ga, which is similar to radiometric ages of the nakhlites. Argon-39-Argon-40 data for ALH 84001 indicate ages between 3.9 and 4.3 Ga. A more precise definition of this age requires detailed characterization of the multiple trapped Ar components in ALH 84001 and of 39Ar recoil distribution. All six shergottite samples show apparent 39Ar-40Ar ages substantially older than the ～165–200 Ma range in ages given by other isotope dating techniques. Shergottites appear to contain ubiquitous Ar components acquired from the Martian atmosphere, the Martian mantle, and commonly terrestrial atmospheric contamination. Zagami feldspar also suggests inherited radiogenic 40Ar. These data analyses indicate that the recent Martian atmospheric component trapped in shergottites has a 40Ar/36Ar ratio possibly as low as ～1750 and no greater than ～1900. These ratios are less than the value of 3000 ± 500 reported by Viking. The 40Ar/36Ar ratio for the Martian mantle component is probably <500 but is poorly constrained. The correlation between trapped 40Ar/36Ar and 129Xe/132Xe ratios in shergottite impact glasses and unirradiated samples of ALH 77005 shows considerable scatter and suggests that the 36Ar/132Xe ratio in the Martian components may vary. Resolution of Martian atmospheric 40Ar/36Ar ratio at different time periods (i.e., at ～4.0 and 0.2 Ga) is also difficult without an understanding of the composition of various trapped components.  相似文献   

4.
Abstract— We report a high‐resolution 40Ar‐39Ar study of mineral separates and whole‐rock samples of olivine‐phyric (Dhofar 019, Sayh al Uhaymir [SaU] 005) and basaltic (Shergotty, Zagami) shergottites. Excess argon is present in all samples. The highest (40Ar/36Ar)trapped ratios are found for argon in pyroxene melt inclusions (?1500), maskelynite (?1200), impact glass (?1800) of Shergotty and impact glass of SaU 005 (?1200). A high (40Ar/36Ar)trapped component‐usually uniquely ascribed to Martian atmosphere‐can also originate from the Martian interior, indicating a heterogeneous Martian mantle composition. As additional explanation of variable high (40Ar/36Ar)trapped ratios in shocked shergottites, we suggest argon implantation from a “transient atmosphere” during impact induced degassing. The best 40Ar‐39Ar age estimate for Dhofar 019 is 642 ± 72 Ma (maskelynite). SaU 005 samples are between 700–900 Ma old. Relatively high 40Ar‐39Ar ages of melt inclusions within Dhofar 019 (1086 ± 252 Ma) and SaU 005 olivine (885 ± 66 Ma) could date entrapment of a magmatic liquid during early olivine crystallization, or reflect unrecognized excess 40Ar components. The youngest 40Ar‐39Ar age of Shergotty separates (maskelynite) is ?370 Ma, that of Zagami is ?200 Ma. The 40Ar‐39Ar chronology of Dhofar 019 and SaU 005 indicate >1 Ga ages. Apparent ages uncorrected for trapped (e.g., Martian atmosphere, mantle) argon components approach 4.5 Ga, but are not caused by inherited 40Ar, because excess 40Ar is supported by 36Artrapped. Young ages obtained by 40Ar‐39Ar and other chronometers argue for primary rather than secondary events. The cosmic ray exposure ages calculated from cosmogenic argon are 15.7 ± 0.7 Ma (Dhofar 019), 1.0–1.6 Ma (SaU 005), 2.1–2.5 Ma (Shergotty) and 2.2–3.0 Ma (Zagami).  相似文献   

5.
Abstract— Isotopic abundances of the noble gases were measured in the following Martian meteorites: two shock glass inclusions from Elephant Moraine (EET) 79001, shock vein glass from Shergotty and Yamato (Y) 793605, and whole-rock samples of Allan Hills (ALH) 84001 and Queen Alexandra Range (QUE) 94201. These glass samples, when combined with literature data on a separate single glass inclusion from EET 79001 and a glass vein from Zagami, permit examination in greater detail of the isotopic composition of Ne, Ar, Kr, and Xe trapped from the Martian atmosphere. The isotopic composition of Martian Ne, if actually present in these glasses, remains poorly defined. The 40Ar/36Ar ratio of trapped Martian atmospheric Ar is probably considerably lower than the nominal ratio of 3000 measured by Viking, and data on impact glasses suggest a value of ～1900. The atmospheric 36Ar/38Ar ratio is ≤4.0. Martian atmospheric Kr may be enriched in lighter isotopes by ～0.5%/amu compared to both solar-wind Kr and to the Martian composition previously reported. The isotopic composition of Xe in these glasses agrees with that previously reported in the literature. The Martian atmospheric 36Ar/132Xe and 84Kr/132Xe elemental ratios are higher than those reported by Viking by factors of ～2.5–1.6 (depending on the 40Ar/36Ar ratio adopted) and ～1.8, respectively, and are discussed in a separate paper. Cosmogenic gases indicate space exposure ages of 2.7 ± 0.6 Ma for QUE 94201 and Shergotty and 14 ± 1 Ma for ALH 84001. Small amounts of 21Ne produced by energetic solar protons may be present in QUE 94201 but are not present in ALH 84001 or Y-793605. The space exposure age for Y-793605 is 4.9 ± 0.6 Ma and appears to be distinctly older than the ages for basaltic shergottites. However, uncertainties in cosmogenic production rates still makes somewhat uncertain the number of Martian impact events required to produce the exposure ages of Martian meteorites.  相似文献   

6.
Abstract— ALH84001, originally classified as a diogenite, is a coarse-grained, cataclastic, orthopyroxenite meteorite related to the martian (SNC) meteorites. The orthopyroxene is relatively uniform in composition, with a mean composition of Wo3.3En69.4Fs27.3. Minor phases are euhedral to subhedral chromite and interstitial maskelynite, An31.1Ab63.2Or5.7, with accessory augite, Wo42.2En45.1Fs12.7, apatite, pyrite and carbonates, Cc11.5Mg58.0Sd29.4Rd1.1. The pyroxenes and chromites in ALH84001 are similar in composition to these phases in EETA79001 lithology A megacrysts but are more homogeneous. Maskelynite is similar in composition to feldspars in the nakhlites and Chassigny. Two generations of carbonates are present, early (pre-shock) strongly zoned carbonates and late (post-shock) carbonates. The high Ca content of both types of carbonates indicates that they were formed at moderately high temperature, possibly ～700 °C. ALH84001 has a slightly LREE-depleted pattern with La 0.67x and Lu 1.85x CI abundances and with a negative Eu anomaly (Eu/Sm 0.56x CI). The uniform pyroxene composition is unusual for martian meteorites, and suggests that ALH84001 cooled more slowly than did the shergottites, nakhlites or Chassigny. The nearly monomineralic composition, coarse-grain size, homogenous orthopyroxene and chromite compositions, the interstitial maskelynite and apatite, and the REE pattern suggest that ALH84001 is a cumulate orthopyroxenite containing minor trapped, intercumulus material.  相似文献   

7.
Abstract— The magnetic properties of samples of seven Martian meteorites (EET 79001, Zagami, Nakhla, Lafayette, Governador Valadares, Chassigny and ALH 84001) have been investigated. All possess a weak, very stable primary natural remanent magnetization (NRM), and some have less stable secondary components. In some cases, the latter are associated with magnetic contamination of the samples, imparted since their recovery, and with viscous magnetization, acquired during exposure of the meteorites to the geomagnetic field since they fell. The magnetic properties are carried by a small content (<1%) of titanomagnetite and, in ALH 84001, possibly by magnetite as well. The most likely source of the primary NRM is a thermoremanent magnetization acquired when the meteorite material last cooled from a high temperature in the presence of a magnetic field. Current evidence is that this was 1.3 Ga ago for the nakhlites and Chassigny and 180 Ma for shergottites: the time of the last relevant cooling of ALH 84001 is not presently known. Preliminary estimates of the strength of the magnetizing field are in the range 0.5–5 üT, which is at least an order of magnitude greater than the present field. It is tentatively concluded that the magnetic field was generated by a dynamo process in a Martian core with appropriate structure and properties.  相似文献   

8.
Abstract— The objective of this study was to identify and map possible source regions for all 5 known martian meteorite lithologies (basalt, lherzolite, clinopyroxenite, orthopyroxenite, and dunite) using data from the Mars Global Surveyor Thermal Emission Spectrometer (MGS TES). We deconvolved the TES data set using laboratory spectra of 6 martian meteorites (Los Angeles, Zagami, ALH A77005, Nakhla, ALH 84001, and Chassigny) as end members, along with atmospheric and surface spectra previously derived from TES data. Global maps (16 pixels/degree) of the distribution of each meteorite end member show that meteorite‐like compositions are not present at or above TES detectability limits over most of the planet's dust‐free regions. However, we have confidently identified local‐scale (100s‐1000s km2) concentrations of olivine‐ and orthopyroxene‐bearing materials similar to ALH A77005, Chassigny, and ALH 84001 in Nili Fossae, in and near Ganges Chasma, in the Argyre and Hellas basin rims, and in Eos Chasma. Nakhla‐like materials are identified near the detection limit throughout the eastern Valles Marineris region and portions of Syrtis Major. Basaltic shergottites were not detected in any spatially coherent areas at the scale of this study. Martian meteorite‐like lithologies represent only a minor portion of the dust‐free surface and, thus, are not representative of the bulk composition of the ancient crust. Meteorite‐like spectral signatures identified above TES detectability limits in more spatially restricted areas (<tens of km) are targets of ongoing analysis.  相似文献   

9.
Abstract– Xenon‐isotopic ratios, step‐heating release patterns, and gas concentrations of mineral separates from Martian shergottites Roberts Massif (RBT) 04262, Dar al Gani (DaG) 489, Shergotty, and Elephant Moraine (EET) 79001 lithology B are reported. Concentrations of Martian atmospheric xenon are similar in mineral separates from all meteorites, but more weathered samples contain more terrestrial atmospheric xenon. The distributions of xenon from the Martian and terrestrial atmospheres among minerals in any one sample are similar, suggesting similarities in the processes by which they were acquired. However, in opaque and maskelynite fractions, Martian atmospheric xenon is released at higher temperatures than terrestrial atmospheric xenon. It is suggested that both Martian and terrestrial atmospheric xenon were initially introduced by weathering (low temperature alteration processes). However, the Martian component was redistributed by shock, accounting for its current residence in more retentive sites. The presence or absence of detectable 129Xe from the Martian atmosphere in mafic minerals may correspond to the extent of crustal contamination of the rock’s parent melt. Variable contents of excess 129Xe contrast with previously reported consistent concentrations of excess 40Ar, suggesting distinct sources contributed these gases to the parent magma.  相似文献   

10.
Abstract— Cosmic-ray produced nuclear tracks and noble gases have been studied in the martian orthopyroxenite Allan Hills 84001 to delineate its cosmic-ray exposure history, preatmospheric size, and fall characteristics. A K-Ar age of 3.9 Ga, cosmic-ray exposure duration of 16.7 Ma, and a preatmospheric radius of 10 cm have been deduced from the noble gas and track data. The track data suggest ALH 84001 to be a single fall that has suffered atmospheric mass ablation in excess of 85%, higher than the value deduced for the shergottites, ALHA 77005, EETA 79001, and Shergotty. The formation age, as well as the cosmic-ray exposure duration, determined in this work are in good agreement with values reported earlier and are distinctly different from other shergottite, nakhlite, and chassignite (SNC) meteorites analysed so far. The high cosmogenic 22Ne/21Ne ratio of 1.22 most probably reflects an effect due to non-chondritic composition of ALH 84001 as the track data suggest high shielding (<5cm) for the analysed samples. There are signatures in the noble gas data that indicate the possible presence of trapped Ar and Ne of martian atmospheric origin in ALH 84001.  相似文献   

11.
Abstract— The Zagami shergottite experienced a complex, petrogenetic formation history (McCoy et al. 1992, 1999). Like several shergottites, Zagami contains excess 40Ar relative to its formation age. To understand the origin of this excess 40Ar, we made 39Ar‐40Ar analyses on plagioclase and pyroxene minerals from two phases representing different stages in the magma evolution. Surprisingly, all these separates show similar concentrations of excess 40Ar, ?1 × 10?6 cm3/g. We present arguments against this excess 40Ar having been introduced from the Martian atmosphere as impact glass. We also present evidence against excess 40Ar being a partially degassed residue from a basalt that actually formed ?4 Gyr ago. We utilize our experimental data on Ar diffusion in Zagami and evidence that it was shock‐heated to only ?70 °C, and we assume this heating occurred during an ejection from Mars ?3 Myr ago. With these constraints, thermal considerations necessitates either that its ejected mass was impossibly large, or that its shock‐heating temperature was an order of magnitude higher than that measured. We suggest that this excess 40Ar was inherited from the Zagami magma, and that it was introduced into the magma either by degassing of a larger volume of material or by early assimilation of old, K‐rich crustal material. Similar concentrations of excess 40Ar in the analyzed separates imply that this magma maintained a relatively constant 40Ar concentration throughout its crystallization. This likely occurred through volatile degassing as the magma rose toward the surface and lithostatic pressure was released. These concepts have implications for excess 40Ar in other shergottites.  相似文献   

12.
The Martian meteorites record a wide diversity of environments, processes, and ages. Much work has been done to decipher potential mantle sources for Martian magmas and their interactions with crustal and surface environments. Chlorine isotopes provide a unique opportunity to assess interactions between Martian mantle‐derived magmas and the crust. We have measured the Cl‐isotopic composition of 17 samples that span the range of known ages, Martian environments, and mantle reservoirs. The 37Cl of the Martian mantle, as represented by the olivine‐phyric shergottites, NWA 2737 (chassignite), and Shergotty (basaltic shergottite), has a low value of approximately ?3.8‰. This value is lower than that of all other planetary bodies measured thus far. The Martian crust, as represented by regolith breccia NWA 7034, is variably enriched in the heavy isotope of Cl. This enrichment is reflective of preferential loss of 35Cl to space. Most basaltic shergottites (less Shergotty), nakhlites, Chassigny, and Allan Hills 84001 lie on a continuum between the Martian mantle and crust. This intermediate range is explained by mechanical mixing through impact, fluid interaction, and assimilation‐fractional crystallization.  相似文献   

13.
Abstract— We report noble gas data for the second chassignite, Northwest Africa (NWA) 2737, which was recently found in the Moroccan desert. The cosmic ray exposure (CRE) age based on cosmogenic 3He, 21Ne, and 38Ar around 10–11 Ma is comparable to the CRE ages of Chassigny and the nakhlites and indicates ejection of meteorites belonging to these two families during a discrete event, or a suite of discrete events having occurred in a restricted interval of time. In contrast, U‐Th/He and K/Ar ages <0.5 Ga are in the range of radiometric ages of shergottites, despite a Sm‐Nd signature comparable to that of Chassigny and the nakhlites (Misawa et al. 2005). Overall, the noble gas signature of NWA 2737 resembles that of shergottites rather than that of Chassigny and the nakhlites: NWA 2737 does not contain, in detectable amount, the solar‐like xenon found in Chassigny and thought to characterize the Martian mantle nor apparently fission xenon from 244Pu, which is abundant in Chassigny and some of the nakhlites. In contrast, NWA 2737 contains Martian atmospheric noble gases trapped in amounts comparable to those found in shergottite impact glasses. The loss of Martian mantle noble gases, together with the trapping of Martian atmospheric gases, could have occurred during assimilation of Martian surface components, or more likely during shock metamorphism, which is recorded in the petrology of this meteorite.  相似文献   

14.
Abstract— Isotopic signatures and concentrations of xenon have been measured in Shergotty mineral separates by laser step heating. Martian atmosphere and ‘martian interior’ xenon are present, as is a spallation component. Martian atmospheric xenon is 5–10 times more concentrated in opaque minerals (magnetite, ilmenite, and pyrrhotite) and maskelynite than in pyroxenes, perhaps reflecting grain size variation. This is shown to be consistent with shock incorporation. A component consisting of solar xenon with a fission contribution, similar to components previously identified in martian meteorites and associated with the martian interior, is best defined in the pyroxene‐dominated separates. This component exhibits a consistent 129Xe (129Xe/132Xe ?1.2) excess over solar/planetary (129Xe/132Xe ?1.04). We suggest that gas present in the melt, perhaps a mixture of interior xenon and martian atmosphere, was incorporated into the pyroxenes in Shergotty as the minerals crystallized.  相似文献   

15.
Abstract— North West Africa (NWA) 480 is a new martian meteorite of 28 g found in the Moroccan Sahara in November 2000. It consists mainly of large gray pyroxene crystals (the largest grains are up to 5 mm in length) and plagioclase converted to maskelynite. Excluding the melt pocket areas, modal analyses indicate the following mineral proportions: 72 vol% pyroxenes extensively zoned, 25% maskelynite, 1% phosphates (merrillite and chlorapatite), 1% opaque oxides (ilmenite, ulvöspinel and chromite) and sulfides, and 1% others such as silica and fayalite. The compositional trend of NWA 480 pyroxenes is similar to that of Queen Alexandra Range (QUE) 94201 but in NWA 480 the pyroxene cores are more Mg‐rich (En77‐En65). Maskelynites display a limited zoning (An42–50Ab54‐48Or2–4). Our observations suggest that NWA 480 formed from a melt with a low nuclei density at a slow cooling rate. The texture was achieved via a single‐stage cooling where pyroxenes grew continuously. A similar model was previously proposed for QUE 94201 by McSween et al. (1996). NWA 480 is an Al‐poor ferroan basaltic rock and resembles Zagami or Shergotty for major elements and compatible trace element abundances. The bulk rock analysis for oxygen isotopes yields Δ17O = +0.42%, a value in agreement at the high margin, with those measured on other shergottites (Clayton and Mayeda, 1996; Romanek et al., 1998; Franchi et al., 1999). Its CI‐normalized rare earth element pattern is similar to those of peridotitic shergottites such as Allan Hills (ALH)A77005, suggesting that these shergottites shared a similar parent liquid, or at least the same mantle source.  相似文献   

16.
Abstract— We report new 39Ar‐40Ar measurements on 15 plagioclase, pyroxene, and/or whole rock samples of 8 Martian shergottites. All age spectra suggest ages older than the meteorite formation ages, as defined by Sm‐Nd and Rb‐Sr isochrons. Employing isochron plots, only Los Angeles plagioclase and possibly Northwest Africa (NWA) 3171 plagioclase give ages in agreement with their formation ages. Isochrons for all shergottite samples reveal the presence of trapped Martian 40Ar (40Arxs), which exists in variable amounts in different lattice locations. Some 40Arxs is uniformly distributed throughout the lattice, resulting in a positive isochron intercept, and other 40Arxs occurs in association with K‐bearing minerals and increases the isochron slope. These samples demonstrate situations where linear Ar isochrons give false ages that are too old. After subtracting 40Ar*that would accumulate by 40K decay since meteorite formation and small amounts of terrestrial 40Ar, all young age samples give similar 40Arxs concentrations of ?1–2 × 10?6cm3/g, but a variation in K content by a factor of ?80. Previously reported NASA Johnson Space Center data for Zagami, Shergotty, Yamato (Y‐) 000097, Y‐793605, and Queen Alexandra Range (QUE) 94201 shergottites show similar concentrations of 40Arxs to the new meteorite data reported here. Similar 40Arxs in different minerals and meteorites cannot be explained as arising from Martian atmosphere carried in strongly shocked phases such as melt veins. We invoke the explanation given by Bogard and Park (2008) for Zagami, that this 40Arxs in shergottites was acquired from the magma. Similarity in 40Arxs among shergottites may reveal common magma sources and/or similar magma generation and emplacement processes.  相似文献   

17.
Abstract— Quantitative petrographic analysis, using the crystal size distribution (CSD) method, provides a novel approach for examining the crystallization histories of basaltic shergottites. Grain number densities at different sizes are plotted against grain size, and the resulting curve relates to the geologic processes involved with the crystallization of the grain population. Most basaltic shergottites are dominated by pigeonite and augite; and because plagioclase is primarily interstitial, and therefore constrained in its growth by the surrounding pyroxenes, we limited our size measurements to the pyroxene phases. The groundmasses of Elephant Moraine (EET) A79001 lithology A and Dar al Gani (DaG) 476 are fine grained with cumulus pyroxene and interstitial plagioclase glass. Their simple linear CSD plots record a single stage of pyroxene crystallization under steady‐state conditions of continuous nucleation and growth. The textures of Queen Alexandra Range (QUE) 94201 and EETA79001 lithology B are quite different from the other shergottites, with intergrown pyroxene and plagioclase. Likewise, their CSD plots are also distinct, with curved trends that suggest a lack of large grains, most likely because of interference between simultaneously growing silicate phases. However, the CSD plot shapes are smooth, also implying a single stage of growth. Shergotty and Zagami, with coarser cumulus textures, display CSD plots that are generally linear over most grain sizes. This implies that conditions of nucleation and growth were dominant during formation of the pyroxene populations. Both plots, however, also display kinks, implying multiple stages of growth. A similar kink is also visible in a CSD plot of only the Mg‐rich cores of Shergotty pyroxenes, which suggests the feature represents changes in conditions during core crystallization, rather than an event coincident with the change in composition to the Fe‐rich rims. The plot may be interpreted as representing two stages of core growth with an intervening short hiatus of nucleation, with continued crystallization associated with ascent of the magma. Eruption onto the surface probably triggered the compositional change to Fe‐rich rims. The CSD analysis of products from a controlled crystallization study agree with experimental and petrologic estimates that cooling rates for Zagami were on the order of a few tenths of a degree per hour. Growth rates derived from these cooling rates suggest crystallization of Shergotty and Zagami pyroxenes occurred over a period of a few weeks to months.  相似文献   

18.
Abstract— Isotopic and trace element compositions of Martian meteorites show that early differentiation of Mars produced complementary crustal and mantle reservoirs that were sampled by later magmatic events. This paper describes a mass balance model that estimates the rare earth element (REE) content and thickness of the crust of Mars from the compositions of shergottites. The diverse REE and Nd isotopic compositions of shergottites are most easily explained by variable addition of light rare earth element (LREE)–enriched crust to basaltic magmas derived from LREE-depleted mantle source regions. Antarctic shergottites EET 79001, ALH 77005, LEW 88516, and QUE 94201 all have strongly LREE-depleted patterns and positive initial 143Nd isotopic compositions, which is consistent with the generation of these magmas from depleted mantle sources and little or no interaction with enriched crust. In contrast, Shergotty and Zagami have negative initial 143Nd isotopic compositions and less pronounced depletions of the LREE, which have been explained by incorporation of enriched crustal components into mantle-derived magmas (Jones, 1989; Longhi, 1991; Borg et al., 1997). The mass balance model presented here derives the REE composition of the crustal component in Shergotty by assuming it represents a mixture between a mantle-derived magma similar in composition to EET 79001A and a LREE-enriched crustal component. The amount of crust in Shergotty is constrained by mixing relations based on Nd-isotopic compositions, which allows the REE pattern of the crustal component to be calculated by mass balance. The effectiveness of this model is demonstrated by the successful recovery of important characteristics of the Earth's continental crust from terrestrial Columbia River basalts. Self-consistent results for Nd-isotopic compositions and REE abundances are obtained if Shergotty contains ～10–30% of LREE-enriched crust with >10 ppm Nd. This crustal component would have moderately enriched LREE (Sm/Nd = 0.25–0.27; 147Sm/144Nd = 0.15–0.17; La/Yb = 2.7–3.8), relatively unfractionated heavy rare earth elements (HREE), and no Eu anomaly. Crust with these characteristics can be produced from a primitive lherzolitic Martian mantle by modest amounts (2–8%) of partial melting, and it would have a globally averaged thickness of <45 km, which is consistent with geophysical estimates. Mars may serve as a laboratory to investigate planetary differentiation by extraction of a primary basaltic crust.  相似文献   

19.
Abstract— Magnetic properties of 26 (of 32) unpaired Martian meteorites (SNCs) are synthesized to further constrain the lithology carrying Martian magnetic crustal sources. Magnetic properties of ultramafic cumulates (i.e., Chassigny, Allan Hills [ALH] 84001) and lherzolitic shergottites (ALH 77005, Lewis Cliff [LEW] 88516) are one or two orders of magnitude too weak to account for the crustal magnetizations, assuming magnetization in an Earth‐like field. Nakhlites and some basaltic shergottites, which are the most magnetic SNCs, show the right intensity. Titanomagnetite is the magnetic carrier in the nakhlites (7 meteorites), whereas in most basaltic shergottites (11 meteorites) it is pyrrhotite. Dhofar (Dho) 378, Los Angeles, and NWA 480/1460 and 2046 are anomalous basaltic shergottites, as their magnetism is mainly due to titanomagnetite. Pyrrhotite should be among the candidate minerals for the magnetized Noachian crust.  相似文献   

20.
Abstract– We present 40Ar‐39Ar dating results of handpicked mineral separates and whole‐rock samples of Nakhla, Lafayette, and Chassigny. Our data on Nakhla and Lafayette and recently reported ages for some nakhlites and Chassigny ( Misawa et al. 2006 ; Park et al. 2009 ) point to formation ages of approximately 1.4 Ga rather than 1.3 Ga that is consistent with previous suggestions of close‐in‐time formation of nakhlites and Chassigny. In Lafayette mesostasis, we detected a secondary degassing event at approximately 1.1 Ga, which is not related to iddingsite formation. It may have been caused by a medium‐grade thermal event resetting the mesostasis age but not influencing the K‐Ar system of magmatic inclusions and the original igneous texture of this rock. Cosmic‐ray exposure ages for these meteorites and for Governador Valadares were calculated from bulk rock concentrations of cosmogenic nuclides 3He, 21Ne, and 38Ar. Individual results are similar to literature data. The considerable scatter of T3, T21, and T38 ages is due to systematic uncertainties related to bulk rock and target element chemistry, production rates, and shielding effects. This hampers efforts to better constrain the hypothesis of a single ejection event for all nakhlites and Chassigny from a confined Martian surface terrain ( Eugster 2003 ; Garrison and Bogard 2005 ). Cosmic‐ray exposure ages from stepwise release age spectra using 38Ar and neutron induced 37Ar from Ca in irradiated samples can eliminate errors induced by bulk chemistry on production rates, although not from shielding conditions.  相似文献   

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