首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 31 毫秒
1.
Clay minerals, although ubiquitous on the ancient terrains of Mars, have not been observed in Martian meteorite Allan Hills (ALH) 84001, which is an orthopyroxenite sample of the early Martian crust with a secondary carbonate assemblage. We used a low‐temperature (20 °C) one‐dimensional (1‐D) transport thermochemical model to investigate the possible aqueous alteration processes that produced the carbonate assemblage of ALH 84001 while avoiding the coprecipitation of clay minerals. We found that the carbonate in ALH 84001 could have been produced in a process, whereby a low‐temperature (~20 °C) fluid, initially equilibrated with the early Martian atmosphere, moved through surficial clay mineral and silica‐rich layers, percolated through the parent rock of the meteorite, and precipitated carbonates (thereby decreasing the partial pressure of CO2) as it evaporated. This finding requires that before encountering the unweathered orthopyroxenite host of ALH 84001, the fluid permeated rock that became weathered during the process. We were able to predict the composition of the clay minerals formed during weathering, which included the dioctahedral smectite nontronite, kaolinite, and chlorite, all of which have been previously detected on Mars. We also calculated host rock replacement in local equilibrium conditions by the hydrated silicate talc, which is typically considered to be a higher temperature hydrothermal phase on Earth, but may have been a common constituent in the formation of Martian soils through pervasive aqueous alteration. Finally, goethite and magnetite were also found to precipitate in the secondary alteration assemblage, the latter associated with the generation of H2. Apparently, despite the limited water–rock interaction that must have led to the formation of the carbonates ~ 3.9 Ga ago, in the vicinity of the ALH 84001 source rocks, clay formation would have been widespread.  相似文献   

2.
Abstract— A number of martian meteorite samples contain secondary alteration minerals such as Ca‐Mg‐Fe carbonates, Fe oxides, and clay minerals. These mineral assemblages hint at hydrothermal processes occurring in the martian crust, but the alteration conditions are poorly constrained. This study presents the results of experiments that examined the alteration of a high‐Fe basalt by CO2‐saturated aqueous fluids at 23 and 75 °C and by mixed H2O‐CO2 vapors at 200 and 400 °C and water‐rock ratios of 1:1 and 1:10. Results indicate that observable alteration of the basalt takes place after runs of only seven days. This alteration includes mobilization of silica into phases such as opal‐CT and quartz, as well as the formation of carbonates, oxides, and at some conditions, zeolites and hydrous silicates. The degree of alteration increases with run temperature and, in high‐temperature vapor experiments, with increasing water content of the vapor. The degree of alteration and the mineralogy observed in the martian meteorites suggests that none of these samples were exposed to aqueous fluids for long periods of time. Nakhla and Lafayette probably interacted with water for relatively brief periods of time; if so, silica may have been leached from the parent rocks by the altering fluids. Allan Hills 84001 shows possible evidence for very limited interaction with an aqueous fluid, but the overall slight degree of alteration described for this meteorite strongly suggests that it never interacted extensively or at high temperature with any water‐bearing fluid. Elephant Moraine A79001 may not have been altered by aqueous fluids at all. The results of this study best support models wherein the meteorite parent rocks were wetted intermittently or for brief periods of time rather than models that invoke long‐term reaction with large volumes of water. Our experiments studied alteration of a high‐Fe basalt by dilute, CO2‐saturated, aqueous solutions at 23 and 75 °C and by mixed H2O‐CO2 vapors at 200 and 400 °C. The results suggest that alteration of the parent rock takes place even after very short reaction times of seven days. All experiments produced carbonate minerals, including calcite, and in some cases, magnesite, siderite, and ankerite. A free silica phase, either opal, quartz, or hydrated silica, formed in most experiments. More altered experiments also contained minerals such as zeolites and hydrous phyllosilicates. Clay minerals were not observed to form in any experiments. In aqueous fluids, higher temperature corresponded with a higher degree of alteration, whereas changing fluid composition had no observable effect. In high‐temperature vapors, the degree of alteration was controlled by temperature and the proportion of H2O to CO2, with water‐rock ratio also playing a role in transport of silica. Application of these results to martian meteorites that contain secondary alteration minerals suggests that none of the martian rocks underwent extensive interaction with aqueous fluids. Nakhla and Lafayette contain clay minerals, which suggests that they interacted with water to some extent, possibly at elevated temperatures. Although ALH84001 shows possible evidence of very limited interaction with aqueous fluids, EETA79001 does not. These results support models for the alteration of these meteorites that do not invoke long‐term interaction with water or reaction with large volumes of water. Except for some models for alteration of ALH84001, this conclusion agrees with most of the literature on alteration of martian meteorites.  相似文献   

3.
Martian meteorites can provide valuable information about past environmental conditions on Mars. Allan Hills 84001 formed more than 4 Gyr ago, and owing to its age and long exposure to the Martian environment, and this meteorite has features that may record early processes. These features include a highly fractured texture, gases trapped during one or more impact events or during formation of the rock, and spherical Fe‐Mg‐Ca carbonates. In this study, we have concentrated on providing new insights into the context of these carbonates using a range of techniques to explore whether they record multiple precipitation and shock events. The petrographic features and compositional properties of these carbonates indicate that at least two pulses of Mg‐ and Fe‐rich solutions saturated the rock. Those two generations of carbonates can be distinguished by a very sharp change in compositions, from being rich in Mg and poor in Fe and Mn, to being poor in Mg and rich in Fe and Mn. Between these two generations of carbonate is evidence for fracturing and local corrosion.  相似文献   

4.
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   

5.
Abstract— The martian meteorite, Allan Hills (ALH) 84001, contains D‐rich hydrogen of plausible martian origin (Leshin et al., 1996). The phase identity of the host(s) of this hydrogen are not well known and could include organic matter (McKay et al., 1996), phlogopite (Brearley, 2000), glass (Mittlefehldt, 1994) and/or other unidentified components of this rock. Previous ion microprobe studies indicate that much of the hydrogen in ALH 84001 as texturally associated with concretions of nominally anhydrous carbonates, glass and oxides (Boctor et al., 1998; Sugiura and Hoshino, 2000). We examined the physical and chemical properties of the host(s) of this hydrogen by stepped pyrolysis of variously pre‐treated subsamples. A continuous‐flow method of water reduction and mass spectrometry (Eiler and Kitchen, 2001) was used to permit detailed study of the small amounts of this hydrogen‐poor sample available for study. We find that the host(s) of D‐rich hydrogen released from ALH 84001 at relatively low temperatures (?500 °C) is soluble in orthophosphoric and dilute hydrochloric acids and undergoes near‐complete isotopic exchange with water within hours at temperatures of 200 to 300 °C. These characteristics are most consistent with the carrier phase(s) being a hydrous salt (e.g., carbonate, sulfate or halide); the thermal stability of this material is inconsistent with many examples of such minerals (e.g., gypsum) and instead suggests one or more relatively refractory hydrous carbonates (e.g., hydromagnesite). Hydrous salts (particularly hydrous carbonates) are common on the Earth only in evaporite, sabkha, and hydrocryogenic‐weathering environments; we suggest that much (if not all) of the “martian” hydrogen in ALH 84001 was introduced in analogous environments on or near the martian surface rather than through biological activity or hydrothermal alteration of silicates in the crust.  相似文献   

6.
Abstract— We report a comprehensive imaging study including confocal microRaman spectroscopy, scanning electron microscopy (SEM), and 3‐D extended focal imaging light microscopy of carbonate globules throughout a depth profile of the Martian meteorite Allan Hills (ALH) 84001 and similar objects in mantle peridotite xenoliths from the Bockfjorden volcanic complex (BVC), Svalbard. Carbonate and iron oxide zoning in ALH 84001 is similar to that seen in BVC globules. Hematite appears to be present in all ALH 84001 carbonate‐bearing assemblages except within a magnesite outer rim found in some globules. Macromolecular carbon (MMC) was found in intimate association with magnetite in both ALH 84001 and BVC carbonates. The MMC synthesis mechanism appears similar to established reactions within the Fe‐C‐O system. By inference to a terrestrial analogue of mantle origin (BVC), these results appear to represent the first measurements of the products of an abiotic MMC synthesis mechanism in Martian samples. Furthermore, the ubiquitous but heterogeneous distribution of hematite throughout carbonate globules in ALH 84001 may be partly responsible for some of the wide range in measured oxygen isotopes reported in previous studies. Using BVC carbonates as a suitable analogue, we postulate that a low temperature hydrothermal model of ALH 84001 globule formation is most likely, although alteration (decarbonation) of a subset of globules possibly occurred during a later impact event.  相似文献   

7.
Abstract— The presence of zoned Fe, Mg, Ca, and Mn in the carbonate phases associated with the cracks and inclusions of the Allan Hills (ALH) 84001 meteorite provides evidence for constraining the thermal history of the meteorite. Using self- and tracer-diffusion coefficients obtained from laboratory experiments on natural calcite, cooling rates are calculated for various temperatures and diffusion distances to assist in the evaluation of the compositional zoning associated with the carbonate phases in ALH 84001. The closure temperature model provides the average temperature below which compositional zoning will be preserved for a given cooling rate, that is, the temperature at which diffusion will be ineffective in homogenizing the phase. The validity of various theories for the formation of the carbonate globules may be examined, therefore, in view of the diffusion-limited kinetic constraints. Experiments using a thin film-mineral diffusion couple and ion microprobe for depth profiling analysis were performed for the temperature range of 550–800 °C to determine self- and tracer-diffusion coefficients for Ca and Mg and in calcite. The resulting activation energies for Ca (Ea(Ca) = 271 ± 80 kJ/mol) and for Mg (Ea(Mg) = 284 ± 74 kJ/mol) were used then to calculate a series of cooling rate, grain size, and closure temperature curves. The data indicate, for example, that by the diffusion of Mg in calcite, a 10 μm compositional zone would be completely homogenized at a temperature of 300 °C for cooling rates <100 K/Ma. These data provide no constraint on formation models that propose a low-temperature fluid precipitation mechanism; however, they indicate that the carbonate globules were not exposed to a high-temperature environment for long time scales following formation.  相似文献   

8.
Abstract— ALH84001 is an igneous meteorite, an orthopyroxenite of martian origin. It contains petrographic evidence of two shock metamorphic events, separated by thermal and chemical events. The evidence for two shock events suggests that ALH84001 is ancient and perhaps a sample of the martian highlands. From petrography and mineral chemistry, the history of ALH84001 must include: crystallization from magma, a first shock (impact) metamorphism, thermal metamorphism, low-temperature chemical alteration, and a second shock (impact) metamorphism. Originally, ALH84001 was igneous, an orthopyroxene-chromite cumulate. In the first shock event, the igneous rock was cut by melt-breccia or cataclastic veinlets, now bands of equigranular fine-grained pyroxene and other minerals (crush zones). Intact fragments of the cumulate were fractured and strained (now converted to polygonized zones). The subsequent thermal metamorphism (possibly related to the first shock) annealed the melt-breccia or cataclastic veinlets to their present granoblastic texture and permitted chemical homogenization of all mineral species present. The temperature of metamorphism was at least 875 °C, based on mineral thermometers. Next, Mg-Fe-Ca carbonates and pyrite replaced plagioclase in both clasts and granular bands, producing ellipsoidal carbonate globules with sub-micron scale compositional stratigraphy, repeated identically in all globules. The second shock event produced microfault offsets of carbonate stratigraphy and other mineral contacts, radial fractures around chromite and maskelynite, and strain birefringence in pyroxene. Maskelynite could not have been preserved from the first shock event, because it would have crystallized back to plagioclase. The martian source area for ALH84001 must permit this complex, multiple impact history. Very few craters on young igneous surfaces are on or near earlier impact features. It is more likely that ALH84001 was ejected from an old igneous unit (Hesperian or Noachian age), pocked by numerous impact craters over its long exposure at the martian surface.  相似文献   

9.
Abstract— Thermodynamic calculations of metastable equilibria were used to evaluate the potential for abiotic synthesis of aliphatic and polycyclic aromatic hydrocarbons (PAHs) in the martian meteorite Allan Hills (ALH) 84001. The calculations show that PAHs and normal alkanes could form metastably from CO, CO2, and H2 below approximately 250–300°C during rapid cooling of trapped magmatic or impact‐generated gases. Depending on temperature, bulk composition, and oxidation‐reduction conditions, PAHs and normal alkanes can form simultaneously or separately. Moreover, PAHs can form at lower H/C ratios, higher CO/CO2 ratios, and higher temperatures than normal alkanes. Dry conditions with H/C ratios less than approximately 0.01–0.001 together with high CO/CO2 ratios also favor the formation of unalkylated PAHs. The observed abundance of PAHs, their low alkylation, and a variable but high aromatic to aliphatic ratio in ALH 84001 all correspond to low H/C and high CO/CO2 ratios in magmatic and impact gases and can be used to deduce spatial variations of these ratios. Some hydrocarbons could have been formed from trapped magmatic gases, especially if the cooling was fast enough to prevent reequilibration. We propose that subsequent impact heating(s) in ALH 84001 could have led to dissociation of ferrous carbonates to yield fine‐grain magnetite, formation of a CO‐rich local gas phase, reduction of water vapor to H2, reequilibration of the trapped magmatic gases, aromatization of hydrocarbons formed previously, and overprinting of the synthesis from magmatic gases, if any. Rapid cooling and high‐temperature quenching of CO‐, H2‐rich impact gases could have led to magnetite‐catalyzed hydrocarbon synthesis  相似文献   

10.
Abstract— In martian orthopyroxenite ALH 84001, pockets of feldspathic glass frequently contain carbonate masses that have been disrupted and dispersed within feldspathic shock melt as a result of impact(s). Transmission electron microscope studies of carbonate fragments embedded within feldspathic glass show that the fragments contain myriad, nanometer‐sized magnetite particles with cuboid, irregular, and teardrop morphologies, frequently associated with voids. The fragments of carbonate must have been incorporated into the melt at temperatures of ?900°C, well above the upper thermal stability of siderite (FeCO3), which decomposes to produce magnetite and CO2 below ?450°C. These observations suggest that most, if not all, of the fine‐grained magnetite associated with Fe‐bearing carbonate in ALH 84001 could have been formed as result of the thermal decomposition of the siderite (FeCO3) component of the carbonate and is not due to biological activity.  相似文献   

11.
Abstract— A single orthopyroxene crystal from the Martian meteorite Allan Hills (ALH) 84001 was studied by X‐ray diffraction (XRD) and electron microprobe analysis (EMPA) to retrieve information about its thermal history. Both sets of data were used to measure the Fe2+‐Mg order degree between the M1 and M2 sites expressed by the distribution coefficient kD. The 529 ± 30°C closure temperature (Tc) of the Fe2+‐Mg ordering process of ALH 84001 orthopyroxene (Fs28) was calculated using Stimpfl (2005a, 2005b) ln kD versus 1/T equation obtained for intermediate iron sample. At this Tc, the orthopyroxene cooling rate, calculated by Ganguly's (1982) numerical method, was 0.1 °C/day. This study puts new constraints on the last high‐temperature thermal episode recorded by orthopyroxene. With reference to the geological history (Treiman 1998), we ascribe this episode to the I3 event, and we interpret the Tc of 529 °C as a lower limit for this impact heating. Our data confirm that experimentally defined physical conditions for the formation of magnetite from decomposition of carbonates took place on the Martian surface during event I3.  相似文献   

12.
Abstract Shock recovery experiments to determine whether magnetite could be produced by the decomposition of iron‐carbonate were initiated. Naturally occurring siderite was first characterized by a variety of techniques to be sure that the starting material did not contain detectable magnetite. Samples were shocked in tungsten‐alloy holders (W = 90%, Ni = 6%, Cu = 4%) to further ensure that any iron phases in the shock products were contributed by the siderite rather than the sample holder. Each sample was shocked to a specific pressure between 30 to 49 GPa. Transformation of siderite to magnetite as characterized by TEM was found in the 49 GPa shock experiment. Compositions of most magnetites are >50% Fe+2 in the octahedral site of the inverse spinel structure. Magnetites produced in shock experiments display the same range of sizes (?50–100 nm), compositions (100% magnetite to 80% magnetite‐20% magnesioferrite), and morphologies (equant, elongated, euhedral to subhedral) as magnetites synthesized by Golden et al. (2001) and as the magnetites in Martian meteorite Allan Hills (ALH) 84001. Fritz et al. (2005) previously concluded that ALH 84001 experienced ?32 GPa pressure and a resultant thermal pulse of ?100–110°C. However, ALH 84001 contains evidence of local temperature excursions high enough to melt feldspar, pyroxene, and a silica‐rich phase. This 49 GPa experiment demonstrates that magnetite can be produced by the shock decomposition of siderite as a result of local heating to > 470°C. Therefore, magnetite in the rims of carbonates in Martian meteorite ALH 84001 could be a product of shock devolatilization of siderite as well.  相似文献   

13.
Abstract— Understanding the origin of carbonate minerals in the Martian meteorite Allan Hills (ALH) 84001 is crucial to evaluating the hypothesis that they contain traces of ancient Martian life. Using arguments based on chemical equilibria among carbonates and fluids, an origin at >650 °C (inimical to life) has been proposed. However, the bulk and stable isotopic compositions of the carbonate minerals are open to multiple interpretations and so lend no particular support to a high-temperature origin. Other methods (possibly less direct) will have to be used to determine the formation temperature of the carbonates in ALH 84001.  相似文献   

14.
Abstract— Phosphates in martian meteorites are important carriers of trace elements, although, they are volumetrically minor minerals. PO4 also has potential as a biomarker for life on Mars. Here, we report measurements of the U‐Th‐Pb systematics of phosphates in the martian meteorite ALH 84001 using the Sensitive High Resolution Ion MicroProbe (SHRIMP) installed at Hiroshima University, Japan. Eleven analyses of whitlockites and 1 analysis of apatite resulted in a total Pb/U isochron age of 4018 ± 81 Ma in the 238U/206Pb‐207Pb/206Pb‐204Pb/206 Pb 3‐D space, and a 232Th‐208Pb age of 3971 ± 860 Ma. These ages are consistent within a 95% confidence limit. This result is in agreement with the previously published Ar‐Ar shock age of 4.0 ± 0.1 Ga from maskelynite and other results of 3.8–4.3 Ga but are significantly different from the Sm‐Nd age of 4.50 ± 0.13 Ga based on the whole rock and pyroxene. Taking into account recent studies on textural and chemical evidence of phosphate, our result suggests that the shock metamorphic event defines the phosphate formation age of 4018 ± 81 Ma, and that since then, ALH 84001 has not experienced a long duration thermal metamorphism, which would reset the U‐Pb system in phosphates.  相似文献   

15.
Abstract— The presence of polycyclic aromatic hydrocarbons (PAHs) in the Martian meteorite Allan Hills 84001 (ALH 84001) was considered to be a major sign of ancient biogenic activity on planet Mars (McKay et al. 1996). An asserted spatial association of PAHs with carbonates, carriers of so‐called nanofossils, was crucial for their suggested connection to early life forms. Although both observations can be explained individually without employing living organisms, a lateral correlation of PAHs and carbonates would suggest a genetic link between PAHs and the microstructures, favoring a biogenic explanation. On the other hand, without such a correlation, a biogenic or even a Martian origin of the PAHs cannot be inferred. Here we show that there is no correlation of PAHs and carbonates in ALH 84001. Furthermore, a general trend of high PAH concentrations at locations where terrestrial lead is present obviously suggests a terrestrial origin for PAHs in ALH 84001.  相似文献   

16.
  相似文献   

17.
Abstract— Compositional and textural relationships of shock‐melted glasses in the Allan Hills (ALH) 84001 meteorite have been examined by optical microscopy, electron microprobe analysis, and compositional mapping. The feldspathic and silica glasses exhibit features which constrain the relative timing of shock events and carbonate deposition in ALH 84001. The feldspathic glasses are stoichiometric and have compositions plausibly described as forming from igneous plagioclase (An27–39Ab58–68Or3–7) or sanidine (Or51Ab46An3), or from a mixture of these phases (mixed‐feldspar glasses). These observations argue against prior interpretations of feldspathic glasses as unflowed maskelynite, hydrothermal precipitates or alteration products, or shock melts that have undergone alkali volatilization. Carbonate was deposited around previously formed mixed‐feldspar glass clasts, suggesting that carbonate deposition occurred after the shock event that formed the granular bands (crushed zones) in this meteorite. SiO2‐rich glasses appear to be silica remobilized during shock, with little addition of other material. A petrogenetic history of ALH 84001 consistent with the observations of feldspathic and silica glasses is (1) igneous crystallization and cumulate formation; (2) a pre‐carbonate shock event that formed the granular bands (crushed zones) and sheared chromites, and melted igneous plagioclase and sanidine to form mixed‐feldspar glasses; (3) carbonate and silica deposition in the granular bands (veining of plagioclase glasses by SiO2 and deposition of carbonate around mixed‐feldspar and plagioclase glass clasts); (4) a post‐carbonate shock event that resulted in invasion of carbonate by feldspathic melts, shock faulting and decarbonation of carbonate, high‐temperature mobilization of silica melts, and minor dissolution of orthopyroxene by silica melts.  相似文献   

18.
Abstract— We report neutron activation analyses, including radiochemical determination of trace siderophile elements (Au, Ge, Ir, Ni, Os and Re), for three SNC/martian meteorites, and Os and Re results for numerous eucrites. Ratios such as Ga/Al in the SNC orthopyroxenite ALH84001 confirm its martian affinity—its many distinctive characteristics, most notably its near-primordial age, notwithstanding. To the list of ALH84001's idiosyncrasies can now be added extraordinarily low concentrations of Au, Ni and, especially, Re (17 pg/g), for a martian meteorite. We consider several possible origins for the anomalously low Re content in ALH84001, including metasomatism or alteration. The pyroxene-cumulate nature of this rock probably does not account for its low Re content. Other SNC meteorites are also cumulates. An examination of Re-Nd variations among terrestrial basalts and komatiites suggests that Re is compatible with mantle minerals in general and only incompatible with olivine (however, olivine dominates the mantle residuum, especially during komatiite genesis). Our preferred model is that the ALH84001 parent melt formed in a mantle source region that was far more Re-depleted, and/or at a substantially lower oxygen fugacity, than the sources of the young SNC meteorites. Such a contrast is consistent with models that replenish siderophile elements in planetary mantles by gradual admixture of late-accreting matter and similarly derive most planetary water (which serves as an oxidant) very late in accretion. According to this model, ALH84001 formed before the siderophile-rich matter and water had been mixed well into the martian interior. Possibly the martian mantle never became generally as Re-rich and/or oxidized as the source region(s) of the younger SNCs.  相似文献   

19.
Abstract— Crystallographic relationships between magnetite, sulfides, and carbonate rosettes in fracture zones of the Allan Hills (ALH) 84001 Martian meteorite have been studied using analytical electron microscopy. We have focused on those magnetite grains whose growth mechanisms can be rigorously established from their crystallographic properties. Individual magnetite nanocrystals on the surfaces of carbonates are epitaxially intergrown with one another in “stacks” of single-domain crystals. Other magnetite nanocrystals are epitaxially intergrown with the surfaces of the carbonate substrates. The observed magnetite/carbonate (hkl) Miller indices orientation relationships are (?, ?, 3)m ‖ (1, ?, 0)c and (1, ?, 1)m ‖ (0,0, 3)c with lattice mismatches of ～13% and ～11%, respectively. Epitaxy is a common mode of vapor-phase growth of refractory oxides like magnetite, as is the spiral growth about axial screw dislocations previously observed in other magnetite nanocrystals in ALH 84001. Epitaxy rules out intracellular precipitation of these magnetites by (Martian) organisms, provides further evidence of the high-temperature (>120 °C) inorganic origins of magnetite in ALH 84001, and indicates that the carbonates also have been exposed to elevated temperatures.  相似文献   

20.
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   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号