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1.
Spatially resolved argon isotope measurements have been performed on neutron-irradiated samples of two Martian basalts (Los Angeles and Zagami) and two Martian olivine-phyric basalts (Dar al Gani (DaG) 476 and North West Africa (NWA) 1068). With a ∼50 μm diameter focused infrared laser beam, it has been possible to distinguish between argon isotopic signatures from host rock (matrix) minerals and localized shock melt products (pockets and veins). The concentrations of argon in analyzed phases from all four meteorites have been quantified using the measured J values, 40Ar/39Ar ratios and K2O wt% in each phase. Melt pockets contain, on average, 10 times more gas (7-24 ppb 40Ar) than shock veins and matrix minerals (0.3-3 ppb 40Ar). The 40Ar/36Ar ratio of the Martian atmosphere, estimated from melt pocket argon extractions corrected for cosmogenic 36Ar, is: Los Angeles (∼1852), Zagami (∼1744) and NWA 1068 (∼1403). In addition, Los Angeles shows evidence for variable mixing of two distinct trapped noble gas reservoirs: (1) Martian atmosphere in melt pockets, and (2) a trapped component, possibly Martian interior (40Ar/36Ar: 480-490) in matrix minerals. Average apparent 40Ar/39Ar ages determined for matrix minerals in the four analyzed meteorites are 1290 Ma (Los Angeles), 692 Ma (Zagami), 515 Ma (NWA 1068) and 1427 Ma (DaG 476). These 40Ar/39Ar apparent ages are substantially older than the ∼170-474 Ma radiometric ages given by other isotope dating techniques and reveal the presence of trapped 40Ar. Cosmic ray exposure (CRE) ages were measured using spallogenic 36Ar and 38Ar production. Los Angeles (3.1 ± 0.2 Ma), Zagami (2.9 ± 0.4 Ma) and NWA 1068 (2.0 ± 0.5 Ma) yielded ages within the range of previous determinations. DaG 476, however, yielded a young CRE age (0.7 ± 0.25 Ma), attributed to terrestrial alteration. The high spatial variation of argon indicates that the incorporation of Martian atmospheric argon into near-surface rocks is controlled by localized glass-bearing melts produced by shock processes. In particular, the larger (mm-size) melt pockets contain near end-member Martian atmospheric argon. Based on petrography, composition and argon isotopic data we conclude that the investigated melt pockets formed by localized in situ shock melting associated with ejection. Three processes may have led to atmosphere incorporation: (1) argon implantation due to atmospheric shock front collision with the Martian surface, (2) transformation of an atmosphere-filled cavity into a localized melt zone, and (3) shock implantation of atmosphere trapped in cracks, pores and fissures.  相似文献   

2.
A series of crystallization experiments have been performed on synthetic glasses matching the composition of a melt pocket found in Allan Hills (ALH) 77005 in order to evaluate the heterogeneous nucleation potential of the melt and the effect of oxygen fugacity on crystallization. The starting temperature of the experiments varied from superliquidus, liquidus and subliquidus temperatures. Each run was then cooled at rates of 10, 500 and 1000 °C/h at FMQ. The results of this study constrain the heating and cooling regime for a microporphyritic melt pocket. Within the melt pocket, strong thermal gradients existed at the onset of crystallization, giving rise to a heterogeneous distribution of nucleation sites resulting in gradational textures of olivine and chromite. Skeletal olivine in the melt pocket center crystallized from a melt containing few nuclei cooled at a fast rate. Nearer to the melt pocket margin elongate skeletal shapes progress to hopper and equant euhedral, reflecting an increase in nuclei in the melt at the initiation of crystallization and growth at low degrees of supercooling. Cooling from post-shock temperatures took place on the order of minutes.An additional series of experiments were conducted for a melt temperature of 1510 °C and a cooling rate of 500 °C/h at the FMQ buffer, as well as 1 and 2 log units above and below FMQ. Variation in chromite stability in these experiments is reflected in crystal shapes and composition, and place constraints on the oxygen fugacity of crystallization of the melt pocket. We conclude that the oxygen fugacity of the melt pocket was set by the Fe3+/Fe2+ ratio imparted by melting of the host rock, rather than external factors such as incorporation of CO2-rich Martian atmosphere, or melting and injection of oxidized surface (e.g., regolith) material.Comparison with previous crystallization experiments on melt pockets in Martian basalts indicate that the predominance of dendritic crystal shapes reflects the likelihood that those melt pockets with lower liquidus temperatures will be more completely melted, destroying most or all nuclei in the melt. In this case, crystal growth takes place at high degrees of supercooling, yielding dendritic shapes. It appears as though the melting process is just as important as cooling rate in determining the final texture of the melt pocket, as this process controls elimination or preservation of nuclei at the onset of cooling and crystallization.  相似文献   

3.
Samarium-neodymium isotopic analysis of the martian meteorite Dar al Gani 476 yields a crystallization age of 474 ± 11 Ma and an initial εNd143 value of +36.6 ± 0.8. Although the Rb-Sr isotopic system has been disturbed by terrestrial weathering, and therefore yields no age information, an initial 87Sr/86Sr ratio of 0.701249 ± 33 has been estimated using the Rb-Sr isotopic composition of the maskelynite mineral fraction and the Sm-Nd age. The Sr and Nd isotopic systematics of Dar al Gani 476, like those of the basaltic shergottite QUE94201, are consistent with derivation from a source region that was strongly depleted in incompatible elements early in the history of the solar system. Nevertheless, Dar al Gani 476 is derived from a source region that has a slightly greater incompatible enrichment than the QUE94201 source region. This is not consistent with the fact that the parental magma of Dar al Gani 476 is significantly more mafic than the parental magma of QUE94201, and underscores a decoupling between the major element and trace element-isotopic systematics observed in the martian meteorite suite.Combining the εNd142-εNd143 isotopic systematics of the martian meteorites yields a model age for planetary differentiation of 4.513+0.033−0.027 Ga. Using this age, the parent/daughter ratios of martian mantle sources are calculated assuming a two-stage evolutionary history. The calculated sources have very large ranges of parent/daughter ratios (87Rb/86Sr = 0.037-0.374; 147Sm/144Nd = 0.182-0.285; 176Lu/177Hf = 0.028-0.048). These ranges exceed the ranges estimated for terrestrial basalt source regions, but are very similar to those estimated for the sources of lunar mare basalts. In fact, the range of parent/daughter ratios calculated for the martian meteorite sources can be produced by mixing between end-members with compositions similar to lunar mare basalt sources. Two of the sources have compositions that are similar to olivine and pyroxene-rich mafic cumulates with variable proportions of a Rb-enriched phase, such as amphibole, whereas the third source has the composition of liquid trapped in the cumulate pile (i.e. similar to KREEP) after ∼99% crystallization. Correlation between the proportion of trapped liquid in the meteorite source regions and estimates of fO2, suggest that the KREEP-like component may be hydrous. The success of these models in reproducing the martian meteorite source compositions suggests that the variations in trace element and isotopic compositions observed in the martian meteorites primarily reflect melting of the crystallization products of an ancient magma ocean, and that assimilation of evolved crust by mantle derived magmas is not required. Furthermore, the decoupling of major element and trace element-isotopic systematics in the martian meteorite suite may reflect the fact that trace element and isotopic systematics are inherited from the magma source regions, whereas the major element abundances are limited by eutectic melting processes at the time of magma formation. Differences in major element abundances of parental magma, therefore, result primarily from fractional crystallization after leaving their source regions.  相似文献   

4.
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 347 ± 13 Ma and an value of +40.1 ± 0.9. Maskelynite fractions do not lie on the Sm-Nd isochron and appear to contain a martian surface component with low 147Sm/144Nd and 143Nd/144Nd 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 87Sr/86Sr ratio of 0.7016 is estimated by passing a 347 Ma reference line through the maskelynite fraction that is least affected by contamination. The high initial value and the low initial 87Sr/86Sr 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 similar 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 and 575 Ma and trace element and/or isotopic systematics that are indicative of derivation from incompatible-element-depleted sources. This suggests that the oldest group of meteorites is more closely related to one another than they are to the younger meteorites that are derived from less incompatible-element-depleted sources. Closed-system fractional crystallization of this suite of meteorites is modeled with the MELTS algorithm using the bulk composition of Yamato 980459 as a parent. These models reproduce many of the major element and mineralogical variations observed in the suite. In addition, the rare earth element systematics of these meteorites are reproduced by fractional crystallization using the proportions of phases and extents of crystallization that are calculated by MELTS. Other shergottites that demonstrate enrichments in incompatible-elements and have evolved Sr and Nd isotopic systematics have some geochemical systematics that are similar to those observed in the depleted group. Most notably, although they exhibit a very limited range of incompatible trace element and isotopic compositions, they have highly variable major element compositions. This is also consistent with evolution from a common mantle source region by variable amounts of fractional crystallization. If this scenario is correct, it suggests that the combined effects of source composition and fractional crystallization are likely to account for the major element, trace element, and isotopic diversity of all shergottites.  相似文献   

5.
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 non-glassy 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 (20Ne/36Ar > 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 20Ne/22Ne, 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.  相似文献   

6.
Low-temperature aqueous processes have been implicated in the generation of jarosite and hematite on the martian surface, but little is known regarding the role that high-temperature magmatic fluids may have played in producing similar assemblages on Mars. We have identified jarosite and hematite in a clinopyroxene-hosted melt inclusion in martian meteorite MIL 03346 that shows evidence of having been hydrothermally precipitated. In addition to jarosite and hematite, the melt inclusion contains titanomagnetite, pyrrhotite, potassic-chlorohastingsite, an iron-rich silicate glass and possibly goethite. These phases were identified and characterized using scanning electron microscopy (SEM), con-focal Raman-spectroscopy and electron probe microanalysis (EPMA).Based on observed textural relationships and the compositions of the hosted phases, we report that the jarosite-bearing melt inclusion in MIL 03346 has recorded a fluid-rich history that began in the magmatic stage and continued to low-temperatures. This history begins at entrapment of a volatile-rich silicate melt that likely reached fluid-saturation after only minor crystallization within the melt inclusion. This fluid, rich in chlorine, reacted with surrounding silicate material to produce the potassic-chlorohastingsite. As cooling proceeded, the liquid phase eventually became more oxidized and reacted with the pyrrhotite. Sulfide oxidation resulted in SO42− formation and concomitant acid production, setting the stage for jarosite formation once the fluid cooled beyond the upper thermal stability of jarosite (∼200 °C). As the fluid cooled below 200 °C, jarosite continued to precipitate with hematite and/or goethite until equilibrium was established or reactions became kinetically unfavorable.This work suggests an additional jarosite-hematite formation pathway on Mars; one that may be important wherever magmatic-hydrothermal fluids come into contact with primary sulfide grains at the martian surface or subsurface. Moreover, hydrothermal fluids rich in chlorine, sulfur, and iron are important for ore-forming processes on Earth, and their indirect identification on Mars may have important implications for ore-formation on Mars.  相似文献   

7.
We have determined metallographic cooling rates of 9 IVB irons by measuring Ni gradients 3 μm or less in length at kamacite-taenite boundaries with the analytical transmission electron microscope and by comparing these Ni gradients with those derived by modeling kamacite growth. Cooling rates at 600-400 °C vary from 475 K/Myr at the low-Ni end of group IVB to 5000 K/Myr at the high-Ni end. Sizes of high-Ni particles in the cloudy zone microstructure in taenite and the widths of the tetrataenite rims, which both increase with decreasing cooling rate, are inversely correlated with the bulk Ni concentrations of the IVB irons confirming the correlation between cooling rate and bulk Ni. Since samples of a core that cooled inside a thermally insulating silicate mantle should have uniform cooling rates, the IVB core must have cooled through 500 °C without a silicate mantle. The correlation between cooling rate and bulk Ni suggests that the core crystallized concentrically outwards. Our thermal and fractional crystallization models suggest that in this case the radius of the core was 65 ± 15 km when it cooled without a mantle. The mantle was probably removed when the IVB body was torn apart in a glancing impact with a larger body. Clean separation of the mantle from the solid core during this impact could have been aided by a thin layer of residual metallic melt at the core-mantle boundary. Thus the IVB irons may have crystallized in a well-mantled core that was 70 ± 15 km in radius while it was inside a body of radius 140 ± 30 km.  相似文献   

8.
The oxygen fugacity of the Dar al Gani 476 martian basalt is determined to be quartz-fayalite-magnetite (QFM) −2.3 ± 0.4 through analysis of olivine, low-Ca pyroxene, and Cr-spinel and is in good agreement with revised results from Fe-Ti oxides that yield QFM −2.5 ± 0.7. This estimate falls within the range of oxygen fugacity for the other martian basalts, QFM −3 to QFM −1. Oxygen fugacity in martian basalts correlates with 87Sr/86Sr, 143Nd/144Nd, and La/Yb ratios, indicating that the mantle source of the basalts is reduced and that assimilation of crust-like material controls the oxygen fugacity. This allows constraints to be placed on the oxidation state of the martian mantle and on the nature of assimilated crustal material. The assimilated material may be the product of early and extensive hydrothermal alteration of the martian crust, or it may be amphibole- or phlogopite-bearing basaltic rock within the crust. In either case, water may play a significant role in the oxidation of basaltic magmas on Mars, although it may be secondary to assimilation of ferric iron-rich material.  相似文献   

9.
We have studied the formation conditions of Al-rich chondrules by doing isothermal and dynamic crystallization experiments at one atmosphere on four different chondrule analogue compositions within the pure CaO-MgO-Al2O3-SiO2 system. For the dynamic crystallization experiments, we cooled from both liquidus and subliquidus peak temperatures (Tmax), at cooling rates from 5-1000 °C/h. The starting compositions include two with anorthite and two with forsterite as the dominant liquidus phases, all at or near spinel-saturation. One of each pair evolves towards diopside crystallization, and the others cordierite or enstatite crystallization, giving a total of four completely different crystallization sequences analogous to the four basic varieties of Al-rich chondrule recently proposed. Bulk composition is the main controlling factor, both in terms of mineralogy and texture. The textures of the anorthite-rich compositions are more sensitive to Tmax than they are to cooling rate, whereas the textures of the forsterite-rich compositions are more sensitive to cooling rate. Comparisons of natural Al-rich chondrules having similar compositions to our synthetic analogues indicate that the natural objects reflect a range of peak heating temperatures, ∼1400-1500 °C, and cooling rates of 10-500 °C/h for porphyritic chondrules and possibly higher (1000 °C/h) for barred chondrules. These conditions are consistent with the conditions inferred for ferromagnesian chondrules but differ from those inferred for some calcium-aluminum-rich inclusions.  相似文献   

10.
We have determined the metallographic cooling rates for 13 IVA irons using the most recent and most accurate metallographic cooling rate model. Group IVA irons have cooling rates that vary from 6600 °C/Myr at the low-Ni end of the group to 100 °C/Myr at the high-Ni end of the group. This large cooling rate range is totally incompatible with cooling in a mantled core which should have a uniform cooling rate. Thermal and fractional crystallization models have been used to describe the cooling and solidification of the IVA asteroid. The thermal model indicates that a metallic body of 150 ± 50 km in radius with less than 1 km of silicate on the outside of the body has a range of cooling rates that match the metallographic cooling rates in IVA irons in the temperature range 700-400 °C where the Widmanstätten pattern formed. The fractional crystallization model for Ni with initial S contents between 3 and 9 wt% is consistent with the measured variation of cooling rate with bulk Ni and the thermal model. New models for impacts in the early solar system and the evolution of the primordial asteroid belt allow us to propose that the IVA irons crystallized and cooled in a metallic body that was derived from a differentiated protoplanet during a grazing impact. Other large magmatic iron groups, IIAB, IIIAB, and IVB, also show significant cooling rate ranges and are very likely to share a similar history.  相似文献   

11.
Shock veins and melt pockets in Lithology A of Martian meteorite Elephant Moraine (EETA) 79001 have been investigated using electron microprobe (EM) analysis, petrography and X-ray Absorption Near Edge Structure (XANES) spectroscopy to determine elemental abundances and sulfur speciation (S2− versus S6+). The results constrain the materials that melted to form the shock glasses and identify the source of their high sulfur abundances. The XANES spectra for EETA79001 glasses show a sharp peak at 2.471 keV characteristic of crystalline sulfides and a broad peak centered at 2.477 keV similar to that obtained for sulfide-saturated glass standards analyzed in this study. Sulfate peaks at 2.482 keV were not observed. Bulk compositions of EETA79001 shock melts were estimated by averaging defocused EM analyses. Vein and melt pocket glasses are enriched in Al, Ca, Na and S, and depleted in Fe, Mg and Cr compared to the whole rock. Petrographic observations show preferential melting and mobilization of plagioclase and pyrrhotite associated with melt pocket and vein margins, contributing to the enrichments. Estimates of shock melt bulk compositions obtained from glass analyses are biased towards Fe- and Mg- depletions because, in general, basaltic melts produced from groundmass minerals (plagioclase and clinopyroxene) will quench to a glass, whereas ultramafic melts produced from olivine and low-Ca pyroxene megacrysts crystallize during the quench. We also note that the bulk composition of the shock melt pocket cannot be determined from the average composition of the glass but must also include the crystals that grew from the melt - pyroxene (En72-75Fs20-21Wo5-7) and olivine (Fo75-80). Reconstruction of glass + crystal analyses gives a bulk composition for the melt pocket that approaches that of lithology A of the meteorite, reflecting bulk melting of everything except xenolith chromite.Our results show that EETA79001 shock veins and melt pockets represent local mineral melts formed by shock impedance contrasts, which can account for the observed compositional anomalies compared to the whole rock sample. The observation that melts produced during shock commonly deviate from the bulk composition of the host rock has been well documented from chondrites, rocks from terrestrial impact structures and other Martian meteorites. The bulk composition of shock melts reflects the proportions of minerals melted; large melt pockets encompass more minerals and approach the whole rock whereas small melt pockets and thin veins reflect local mineralogy. In the latter, the modal abundance of sulfide globules may reach up to 15 vol%. We conclude the shock melt pockets in EETA79001 lithology A contain no significant proportion of Martian regolith.  相似文献   

12.
Plagioclase is not only the most abundant mineral in the Earth’s crust, but is present in almost all terrestrial tectonic settings and is widespread in most extraterrestrial material. Applying the K-Ar system to this common mineral would provide a powerful tool for quantifying thermal histories in a wide variety of settings. Nonetheless, plagioclase has rarely been used for thermochronometry, largely due to difficulties in simultaneously acquiring precise geochronologic data and quantifying argon diffusion kinetics from a mineral with low-K concentration. Here we describe an analytical technique that generates high-precision 40Ar/39Ar data and quantifies Ar diffusion kinetics of low-K minerals. We present results of five diffusion experiments conducted on single crystals of plagioclase from the Bushveld Complex, South Africa. The observed diffusion kinetics yield internally consistent thermochronological constraints, indicating that plagioclase is a reliable thermochronometer. Individual grains have activation energies of 155-178 kJ/mol and ln(D0/a2) varies between 3.5 and 6.5. These diffusion parameters correspond to closure temperatures of 225-300 °C, for a 10 °C/Ma cooling rate. Age spectra generally conform to single-domain diffusive loss profiles, suggesting that grain-scale diffusion dominates argon transport in this fairly simple plagioclase. Conjointly examining several single-grain analyses enables us to distinguish episodic reheating from slow cooling and indicates that the Bushveld Complex cooled rapidly and monotonically from magmatic temperature to <300 °C over 3 Ma, followed by protracted cooling to ambient crustal temperatures of 150-200 °C over ∼600 Ma.  相似文献   

13.
We report on the petrography and geochemistry of the newly discovered olivine-phyric shergottite Larkman Nunatak (LAR) 06319. The meteorite is porphyritic, consisting of megacrysts of olivine (?2.5 mm in length, Fo77-52) and prismatic zoned pyroxene crystals with Wo3En71 in the cores to Wo8-30En23-45 at the rims. The groundmass is composed of finer grained olivine (<0.25 mm, Fo62-46), Fe-rich augite and pigeonite, maskelynite and minor quantities of chromite, ulvöspinel, magnetite, ilmenite, phosphates, sulfides and glass. Oxygen fugacity estimates, derived from the olivine-pyroxene-spinel geo-barometer, indicate that LAR 06319 formed under more oxidizing conditions (QFM -1.7) than for depleted shergottites. The whole-rock composition of LAR 06319 is also enriched in incompatible trace elements relative to depleted shergottites, with a trace-element pattern that is nearly identical to that of olivine-phyric shergottite NWA 1068. The oxygen isotope composition of LAR 06319 (Δ17O = 0.29 ±0.03) confirms its martian origin.Olivine megacrysts in LAR 06319 are phenocrystic, with the most Mg-rich megacryst olivine being close to equilibrium with the bulk rock. A notable feature of LAR 06319 is that its olivine megacryst grains contain abundant melt inclusions hosted within the forsterite cores. These early-trapped melt inclusions have similar trace element abundances and patterns to that of the whole-rock, providing powerful evidence for closed-system magmatic behavior for LAR 06319. Calculation of the parental melt trace element composition indicates a whole-rock composition for LAR 06319 that was controlled by pigeonite and augite during the earliest stages of crystallization and by apatite in the latest stages. Crystal size distribution and spatial distribution pattern analyses of olivine indicate at least two different crystal populations. This is most simply interpreted as crystallization of megacryst olivine in magma conduits, followed by eruption and subsequent crystallization of groundmass olivine.LAR 06319 shows close affinity in mineral and whole-rock chemistry to olivine-phyric shergottite, NWA 1068 and the basaltic shergottite NWA 4468. The remarkable features of these meteorites are that they have relatively similar quantities of mafic minerals compared with olivine-phyric shergottites (e.g., Y-980459, Dho 019), but flat and elevated rare earth element patterns more consistent with the LREE-enriched basaltic shergottites (e.g., Shergotty, Los Angeles). This relationship can be interpreted as arising from partial melting of an enriched mantle source and subsequent crystal-liquid fractionation to form the enriched olivine-phyric and basaltic shergottites, or by assimilation of incompatible-element enriched martian crust. The similarity in the composition of early-trapped melt inclusions and the whole-rock for LAR 06319 indicates that any crustal assimilation must have occurred prior to crystallization of megacryst olivine, restricting such processes to the deeper portions of the crust. Thus, we favor LAR06319 forming from partial melting of an “enriched” and oxidized mantle reservoir, with fractional crystallization of the parent melt upon leaving the mantle.  相似文献   

14.
Two glassy refractory Al-rich chondrules in Semarkona (LL3.0), the most primitive unequilibrated ordinary chondrite, provide direct evidence for condensation of Si and Mg on melt droplets during cooling. The chondrules are completely rounded, rich in Ca and Al, and poor in Fe and alkalis. They have extraordinarily abundant glass (70-80 vol%) with a subordinate amount of forsterite as the only crystalline phase that occurs mostly rimming the chondrule edge. The groundmass glass is concentrically zoned in terms of Si with an outward increase, which is overlapped with local heterogeneity of Mg and Al induced by crystallization of forsterite. The outward increase of Si, mostly compensated by Al, cannot be formed solely by crystallization of forsterite from a homogeneous melt in a closed system. Combined with skeletal or dendritic morphology and sector zoning of forsterite, it is suggested that Si condensed onto totally molten droplets (“initial melts”) accompanied by nucleation and rapid growth of forsterite with lowering temperature. The “initial melts”, the compositions of which were estimated from the Ca contents of the first crystallized forsterite, are very similar to Type C CAI but are notably poorer in Mg and Si than the bulk chondrules, indicating condensation of Mg in addition to Si with an atomic ratio of Mg:Si ∼ 3:2. The condensation after the nucleation of forsterite took place below ∼1300 °C under cooling at ∼70 °C/h and amounted to 30 wt% of the current chondrule. This study suggests a model that a short-time and local shock heating event induced melting of Type C CAI and concomitant evaporation of dusts, ferromagnesian chondrules of earlier generation, and their fragments to generate Mg and Si-rich gas, which condensed onto the melt droplets upon cooling accompanying condensation of Type I chondrules.  相似文献   

15.
The texture of Los Angeles (stone 1) is dominated by relatively large (0.5−2.0 mm) anhedral to subhedral grains of pyroxene, and generally subhedral to euhedral shocked plagioclase feldspar (maskelynite). Minor phases include subhedral titanomagnetite and ilmenite, Fe-rich olivine, olivine+augite-dominated symplectites [some of which include a Si-rich phase and some which do not], pyrrhotite, phosphate(s), and an impact shock-related alkali- and silica-rich glass closely associated with anhedral to euhedral silica grains. Observations and model calculations indicate that the initial crystallization of Mg-rich pigeonitic pyroxenes at ≤1150 °C, probably concomitantly with plagioclase, was followed by pigeonitic and augitic compositions between 1100 and 1050 °C whereas between 1050 and 920 to 905 °C pyroxene of single composition crystallized. Below 920 to 905 °C, single composition Fe-rich clinopyroxene exsolved to augite and pigeonite. Initial appearance of titanomagnetite probably occurred near 990 °C and FMQ-1.5 whereas at and below 990 °C and ≥FMQ-1.5 titanomagnetite and single composition Fe-rich clinopyroxene may have started to react, producing ilmenite and olivine. However, judging from the most common titanomagnetite compositions, we infer that most of this reaction likely occurred between 950 and 900 °C at FMQ-1.0±0.2 and nearly simultaneously with pyroxene exsolution, thus producing assemblages of pigeonite, titanomagnetite, olivine, ilmenite, and augite. We deem this reaction as the most plausible explanation for the formation of the olivine+augite-dominated symplectites in Los Angeles. But we cannot preclude possible contributions to the symplectites from the shock-related alkali- and silica-rich glass or shocked plagioclase, and the breakdown of Fe-rich pigeonite compositions to olivine+augite+silica below 900 °C. Reactions between Fe-Ti oxides and silicate minerals in Los Angeles and other similar basaltic Martian meteorites can control the T-fO2 equilibration path during cooling, which may better explain the relative differences in fO2 among the basaltic Martian meteorites.  相似文献   

16.
Assessing the ferric-ferrous ratio in magmas prior to eruption remains a challenging task. X-ray absorption near-edge structure (μXANES) spectra were collected at the iron K-edge in water-rich peralkaline silicic melt/glass inclusions trapped in quartz. These experiments were carried out between 800 and 20 °C. The chemical environment of iron was also determined in the naturally quenched samples (glass inclusions and matrix glass) and in the peralkaline rhyolitic reference glasses, with variable [Fe3+ / ∑Fe] ratios.In the reference glasses, both the intensity of the pre-peaks (Fe2+, Fe3+) and site geometry of iron change as the oxidation state increases. Fourfold-coordinated Fe3+ prevails in highly oxidised peralkaline silicic glasses, using alkalis for charge balance. The position of the pre-edge centroid of the 1s-3d transition correlates with the Fe3+ / ΣFe ratios that allowed calibration of the redox state of iron of our natural samples.At high temperatures, Fe2+ dominates in the pre-edge structure of melt inclusions. Upon cooling down to 20 °C, the intensity of the Fe3+ peak increases, the centroid position of the pre-edge features shifts by nearly 0.5 eV and the main edge moves slightly towards higher energies. The slower the cooling rate, the higher the ferric iron contribution. Iterative μXANES experiments performed on the same samples show that the process is reversible. However, this apparent oxidation of iron upon cooling is an artefact of changes in Fe coordination. It implies that the [Fe3+ / ΣFe] ratio of glassy samples, measured at 20 °C, may be overestimated by a factor > 1.7, and that this ratio cannot be reliably retrieved by probing naturally cooled glass inclusions, and most silicate glasses. High temperature μXANES experiments led first to an assessment of the ferric-ferrous ratio in the water-rich peralkaline melt in pre-eruptive magmatic conditions and second to the determination of the corresponding oxygen fugacity at 740 °C.  相似文献   

17.
Rb and Sr partitioning between haplogranitic melts and aqueous solutions   总被引：2，自引：0，他引：2  
Rubidium and strontium partitioning experiments between haplogranitic melts and aqueous fluids (water or 1.16-3.56 m (NaCl + KCl) ± HCl) were conducted at 750-950 °C and 0.2-1.4 GPa to investigate the effects of melt and fluid composition, pressure, and temperature. In addition, we studied if the applied technique (rapid and slow quench, and in-situ determination of trace element concentration in the fluid) has a bearing on the obtained data. There is good agreement of the data from different techniques for chloridic solutions, whereas back reactions between fluid and melt upon cooling have a significant effect on results from the experiments with water.The Rb fluid-melt partition coefficient shows no recognizable dependence on melt composition and temperature.For chloridic solutions, it is ∼0.4, independent of pressure. In experiments with water, it is one to two orders of magnitude lower and increases with pressure. The strontium fluid-melt partition coefficient does not depend on temperature. It increases slightly with pressure in Cl free experiments. In chloridic fluids, there is a sharp increase in the Sr partition coefficient with the alumina saturation index (ASI) from 0.003 at an ASI of 0.8 to a maximum of 0.3 at an ASI of 1.05. At higher ASI, it decreases slightly to 0.2 at an ASI of 1.6. It is one to two orders of magnitude higher in chloridic fluids compared to those found in H2O experiments. The Rb/Sr ratio in non-chloridic solutions in equilibrium with metaluminous melts increases with pressure, whereas the Rb/Sr ratio in chloridic fluids is independent of pressure and decreases with fluid salinity.The obtained fluid-melt partition coefficients are in good agreement with data from natural cogenetic fluid and melt inclusions. Numerical modeling shows that although the Rb/Sr ratio in the residual melt is particularly sensitive to the degree of fractional crystallization, exsolution of a fluid phase, and associated fluid-melt partitioning is not a significant factor controlling Rb and Sr concentrations in the residual melt during crystallization of most granitoids.  相似文献   

18.
Uranium-lead ratios (commonly represented as 238U/204Pb = μ) calculated for the sources of martian basalts preserve a record of petrogenetic processes that were active during early planetary differentiation and formation of martian geochemical reservoirs. To better define the range of μ values represented by the source regions of martian basalts, we completed U-Pb elemental and isotopic analyses on whole rock, mineral and leachate fractions from the martian meteorite Queen Alexandra Range 94201 (QUE 94201). The whole rock and silicate mineral fractions have unradiogenic Pb isotopic compositions that define a narrow range (206Pb/204Pb = 11.16-11.61). In contrast, the Pb isotopic compositions of weak HCl leachates are more variable and radiogenic. The intersection of the QUE 94201 data array with terrestrial Pb in 206Pb/204Pb-207Pb/204Pb-208Pb/204Pb compositional space is consistent with varying amounts of terrestrial contamination in these fractions. We calculate that only 1-7% contamination is present in the purified silicate mineral and whole rock fractions, whereas the HCl leachates contain up to 86% terrestrial Pb. This terrestrial Pb contamination generated a 206Pb-207Pb array in the QUE fractions that appears to represent an ancient age, which contrasts with a much younger crystallization age of 327 ± 10 Ma derived from Rb-Sr and Sm-Nd isochrons (Borg L. E., Nyquist L. E., Taylor L. A., Wiesmann H. and Shih C. -Y. (1997) Constraints on Martian differentiation processes from Rb-Sr and Sm-Nd isotopic analyses of the basaltic shergottite QUE 94201. Geochim. Cosmochim. Acta61, 4915-4931). Despite the contamination, and accepting 327 ± 10 Ma as the crystallization age, we use the U-Pb data to determine the initial 206Pb/204Pb of QUE 94201 to be 11.086 ± 0.008 and to calculate the μ value of its mantle source to be 1.82 ± 0.01. The μ value calculated for the QUE 94201 source is the lowest determined for any martian basalt source, and, when compared to the highest values determined for martian basalt sources, indicates that μ values in martian source reservoirs vary by at least a factor of two. Additionally, the range of source μ values indicates that the μ value of bulk silicate Mars is approximately three. The amount of variation in the μ values of the mantle sources (μ ∼ 2-4) is greater than can be explained by igneous processes involving silicate phases alone. We suggest the possibility that a small amount of sulfide crystallization may generate greater extents of U-Pb fractionation during formation of the mantle sources of martian basalts.  相似文献   

19.
A synthetic composition representing the Yamato 980459 martian basalt (shergottite) has been used to carry out phase relation, and rare earth element (REE) olivine and pyroxene partitioning experiments. Yamato 980459 is a sample of primitive basalt derived from a reduced end-member among martian mantle sources. Experiments carried out between 1-2 GPa and 1350-1650 °C simulate the estimated pressure-temperature conditions of basaltic melt generation in the martian mantle. Olivine-melt and orthopyroxene-melt partition coefficients for La, Nd, Sm, Eu, Gd and Yb (DREE values) were determined by LA-ICPMS, and are similar to the published values for terrestrial basaltic systems. We have not detected significant variation in D-values with pressure over the range investigated, and by comparison with previous studies carried out at lower pressure.We apply the experimentally obtained olivine-melt and orthopyroxene-melt DREE values to fractional crystallization and partial melting models to develop a three-stage geochemical model for the evolution of martian meteorites. In our model we propose two ancient (∼4.535 Ga) sources: the Nakhlite Source, located in the shallow mantle, and the Deep Mantle Source, located close to the martian core-mantle boundary. These two sources evolved distinctly on the ε143Nd evolution curve due to their different Sm/Nd ratios. By partially melting the Nakhlite Source at ∼1.3 Ga, we are able to produce a slightly depleted residue (Nakhlite Residue). The Nakhlite Residue is left undisturbed until ∼500 Ma, at which point the depleted Deep Mantle Source is brought up by a plume mechanism carrying with it high heat flow, melts and isotopic signatures of the deep mantle (e.g., ε182W, ε142Nd, etc.). The plume-derived Deep Mantle Source combines with the Nakhlite Residue producing a mixture that becomes a mantle source (herein referred to as “the Y98 source”) for Yamato 980459 and the other depleted shergottites with the characteristic range of Sm/Nd ratios of these meteorites. The same hot plume provides a heat source for the formation of enriched and intermediate shergottites. Our model reproduces the REE patterns of nakhlites and depleted shergottites and can explain high ε143Nd in depleted shergottites. Furthermore, the model results can be used to interpret whole rock Rb-Sr and Sm-Nd ages of shergottites.  相似文献   

20.
We present compositional data for 358 lithic fragments (2-4-mm size range) and 15 soils (<1-mm fines) from regolith samples collected at the Apollo 12 site. The regolith is dominated by mare basalt, KREEP impact-melt breccias (crystalline and glassy), and regolith breccias. Minor components include alkali anorthosite, alkali norite, granite, quartz monzogabbro, and anorthositic rocks from the feldspathic highlands. The typical KREEP impact-melt breccia of Apollo 12 (mean Th: 16 μg/g) is similar to that of the Apollo 14 site (16 μg/g), 180 km away. Both contain a minor component (0.3% at Apollo 12, 0.6% at Apollo 14) of FeNi metal that is dissimilar to metal in ordinary chondrites but is similar to metal found in Apollo 16 impact-melt breccias. The Apollo 12 regolith contains another variety of KREEP impact-melt breccia that differs from any type of breccia described from the Apollo sites in being substantially richer in Th (30 μg/g) but with only moderate concentrations of K. It is, however, similar in composition to the melt breccia lithology in lunar meteorite Sayh al Uhaymir 169. The average composition of typical mature soil corresponds to a mixture of 65% mare basalt, 20% typical KREEP impact-melt breccia, 7% high-Th impact-melt breccia, 6% feldspathic material, 2.6% alkali noritic anorthosite, and 0.9% CM chondrite. Thus, although the site was resurfaced by basaltic volcanism 3.1-3.3 Ga ago, a third of the material in the present regolith is of nonmare origin, mainly in the form of KREEP impact-melt breccias and glass. These materials occur in the Apollo 12 regolith mainly as a result of moderate-sized impacts into surrounding Fra Mauro and Alpes Formations that formed craters Copernicus (93 km diameter, 406 km distance), Reinhold (48 km diameter, 196 km distance), and possibly Lansberg (39 km diameter, 108 km distance), aided by excavation of basalt interlayers and mixing of regolith by small, local impacts. Anomalous immature soil samples 12024, 12032, and 12033 contain a lesser proportion of mare basalt and a correspondingly greater proportion of KREEP lithologies. These samples consist mainly of fossil or paleoregolith, likely ejecta from Copernicus, that was buried beneath the mixing zone of micrometeorite gardening, and then brought to the near surface by local craters such as Head, Bench, and Sharp Craters.  相似文献   

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