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1.
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   

2.
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   

3.
Abstract— The geologic history of Martian meteorite Allan Hills (ALH) 84001 is more complex than previously recognized, with evidence for four or five crater-forming impacts onto Mars. This history of repeated deformation and shock metamorphism appears to weaken some arguments that have been offered for and against the hypothesis of ancient Martian life in ALH 84001. Allan Hills 84001 formed originally from basaltic magma. Its first impact event (I1) is inferred from the deformation (D1) that produced the granular-textured bands (“crush zones”) that transect the original igneous fabric. Deformation D1 is characterized by intense shear and may represent excavation or rebound flow of rock beneath a large impact crater. An intense thermal metamorphism followed D1 and may be related to it. The next impact (I2) produced fractures, (Fr2) in which carbonate “pancakes” were deposited and produced feldspathic glass from some of the igneous feldspars and silica. After I2, carbonate pancakes and globules were deposited in Fr2 fractures and replaced feldspathic glass and possibly crystalline silicates. Next, feldspars, feldspathic glass, and possibly some carbonates were mobilized and melted in the third impact (I3). Microfaulting, intense fracturing, and shear are also associated with I3. In the fourth impact (I4), the rock was fractured and deformed without significant heating, which permitted remnant magnetization directions to vary across fracture surfaces. Finally, ALH 84001 was ejected from Mars in event I5, which could be identical to I4. This history of multiple impacts is consistent with the photogeology of the Martian highlands and may help resolve some apparent contradictions among recent results on ALH 84001. For example, the submicron rounded magnetite grains in the carbonate globules could be contemporaneous with carbonate deposition, whereas the elongate magnetite grains, epitaxial on carbonates, could be ascribed to vapor-phase deposition during I3.  相似文献   

4.
Abstract— Rare earth element (REE) and other selected trace and minor element concentrations were measured in individual grains of orthopyroxene, feldspathic glass (of plagioclase composition) and merrillite of the ALH 84001 Martian meteorite. Unlike in other Martian meteorites, phosphate is not the main REE carrier in ALH 84001. The REE pattern of ALH 84001 bulk rock is dependent on the modal abundances of three REE-bearing phases, namely, orthopyroxene, which contains most of the heavy rare earth elements (HREEs); feldspathic glass, which dominates the Eu abundances; and merrillite, which contains the majority of the light rare earth elements (LREEs). Variations in the REE abundances previously observed in different splits of ALH 84001 can easily be explained in terms of small variations in the modal abundances of these three minerals without the need to invoke extensive redistribution of LREEs. At least some orthopyroxenes (i.e., those away from contacts with feldspathic glass) in ALH 84001 appear to have preserved their original REE zonation from igneous fractionation. An estimate of the ALH 84001 parent magma composition from that of the unaltered orthopyroxene “core” (i.e., zoned orthopyroxene with the lowest REE abundances) indicates that it is LREE depleted. This implies that the Martian mantle was already partly depleted within ～100 Ma of solar system formation, which is consistent with rapid accretion and differentiation of Mars. Although equilibration and exchange of REEs between phases (in particular, transport of LREEs into the interstitial phases, feldspathic glass and merrillite) cannot be ruled out, our data suggest that the LREE enrichment in melts “in equilibrium” with these interstitial phases is most likely the result of late-stage infiltration of the cumulate pile by a LREE-enriched melt.  相似文献   

5.
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   

6.
Abstract— We have studied carbonate and associated oxides and glasses in a demountable section of Allan Hills 84001 (ALH 84001) using optical, scanning, and transmission electron microscopy (TEM) to elucidate their origins and the shock history of the rock. Massive, fracture‐zone, and fracture‐filling carbonates in typical locations were characterized by TEM, X‐ray microanalysis, and electron diffraction in a comprehensive study that preserved textural and spatial relationships. Orthopyroxene is highly deformed, fractured, partially comminuted, and essentially unrecovered. Lamellae of diaplectic glass and other features indicate shock pressures >30 GPa. Bridging acicular crystals and foamy glass at contacts of orthopyroxene fragments indicate localized melting and vaporization of orthopyroxene. Carbonate crystals are >5 mm in size, untwinned, and very largely exhibit the R3c calcite structure. Evidence of plastic deformation is generally found mildly only in fracture‐zone and fracture‐filling carbonates, even adjacent to highly deformed orthopyroxene, and appears to have been caused by low‐stress effects including differential shrinkage. High dislocation densities like those observed in moderately shocked calcite are absent. Carbonate contains impactderived glasses of plagioclase, silica, and orthopyroxene composition indicating brief localized impact heating. Stringers and lenses of orthopyroxene glass in fracture‐filling carbonate imply flow of carbonates and crystallization during an impact. Periclase (MgO) occurs in magnesite as 30–50 nm crystals adjacent to voids and negative crystals and as ?1 μm patches of 3 nm crystals showing weak preferred orientation consistent with (111)MgO//(0001)carb, as observed in the thermal decomposition of CaCO3 to CaO. Magnetite crystals that are epitaxially oriented at voids, negative crystals, and microfractures clearly formed in situ. Fully embedded, faceted magnetites are topotactically oriented, in general with (111)mag//(0001)carb, so that their oxygen layers are aligned. In optically opaque rims, magnetites are more irregularly shaped and, except for the smallest crystals, poorly aligned. All magnetite and periclase crystals probably formed by exsolution from slightly non‐stoichiometric, CO2‐poor carbonate following impact‐induced thermal decomposition. Any magnetites that existed in the rock before shock heating could not have preserved evidence for biogenic activity.  相似文献   

7.
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   

8.
Abstract— The carbonate mineralogy of several complex carbonate‐rich regions in Allan Hills (ALH) 84001 has been examined. These regions contain familiar forms of carbonate, as well as textural forms previously unreported including carbonate rosettes, planiform “slab” carbonates, distinct “post‐slab” magnesites, and carbonates interstitial to feldspathic glass and orthopyroxene. Slab carbonates reveal portions of the carbonate growth sequence not seen in the rosettes and suggest that initial nucleating compositions were calcite‐rich. The kinetically controlled growth of rosettes and slab carbonates was followed by an alteration event that formed the magnesite‐siderite layers on the exterior surfaces of the carbonate. Post‐slab magnesite, intimately associated with silica glass, is compositionally similar to the magnesite in these exterior layers but represents a later generation of carbonate growth. Feldspathic glasses had little or no thermal effect on carbonates, as indicated by the lack of thermal decomposition or any compositional changes associated with glass/carbonate contacts.  相似文献   

9.
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   

10.
Abstract— Microstructures in the Allan Hills 84001 meteorite were studied using optical and electron microscopy, putting emphasis on shock effects, which are widespread. Some orthopyroxene exhibits only (100) slip, but more typical grains suffered extensive slip, microfracturing, and frequently contain (100) clino‐inversion lamellae. In fracture zones, shock deformation of orthopyroxene has produced all three effects in profusion, together with intergranular pockets of orthopyroxene glass and intragranular glass lamellae, which were apparently created by shearing on low index planes, usually (100) or {110}. Both types of plane are loci that pseudo‐planar fractures tend to follow. Thus, the glass lamellae, which have not been observed in other meteorites, probably formed by frictional heating during the sliding of microscale corrugated surfaces, one over another, leading to local melting. We infer that the orthopyroxene glass and the fracture zones both formed from shear stresses created by strong shock. Ubiquitous undeformed micrometer and submicrometer euhedral chromites in orthopyroxene and plagioclase glasses and carbonate probably crystallized after shock heating and fracture zone formation. Nanocrystals of eskolaite (Cr2O3) coating silica glass grains are probably also a result of shock‐induced thermal decomposition of chromite. Iron sulfides (pyrite and pyrrhotite were identified) tended to be associated with plagioclase glass. A carbonate disk showing no evidence for shock deformation had a substructure of elongated, slightly misoriented subcells in the exterior; interior regions had more eqiaxed subcells. Both microstructures probably formed during growth, but the conditions are undetermined. Chemical composition varied on a micron scale, but the rim of the disk was more ferroan; oxide precipitates and voids were widely distributed as in fracture‐filling carbonates. If the fracture zones and opx glass are the result of strong shock, as we deduce, it is very unlikely that pores could have filled by carbonate long after the fracture zones formed. We infer that the carbonate, like the phosphate, olivine, pyrrhotite, eskolaite, and many euhedral, submicrometer chromites, crystallized during the final stages of the impact that created the fracture zones and glasses with compositions of plagioclase, silica, and orthopyroxene.  相似文献   

11.
Abstract— A crusted stone weighing 3.10 kg was found in 1983 near Tabbita in south central New South Wales (ca. 34°03′S, 145°50′E), Australia. Compositions of the ferro-magnesian silicates (olivine Fa24.6; orthopyroxene Fs20.9) show that the meteorite belongs to the L-group of chondrites. Uniformity of silicate compositions and the presence of abundant crystalline plagioclase feldspar (An10.8Ab81.7Or7.5) show that the meteorite belongs to petrologic type 6. Silicates that display undulose extinction, and the absence of any thermal effects induced by shock indicate that Tabbita is shock facies c. Tabbita is distinct from several other L6 chondrites found in the same general area.  相似文献   

12.
Abstract— Spherical carbonate globules of similar composition, size, and radial Ca‐, Mg‐, and Fe‐zonation to those in martian meteorite Allan Hills (ALH) 84001 were precipitated from Mg‐rich, supersaturated solutions of Ca‐Mg‐Fe‐CO2‐H2O at 150 °C. The supersaturated solutions (pH ? 6–7) were prepared at room temperature and contained in TeflonTM‐lined stainless steel vessels, which were sealed and heated to 150 °C for 24 h. Experiments were also conducted at 25 °C and no globules comparable to those of ALH 84001 were precipitated. Instead, amorphous Fe‐rich carbonates were formed after 24 h and Mg‐Fe calcites formed after 96 h. These experiments suggest a possible low‐temperature inorganic origin for the carbonates in martian meteorite ALH 84001.  相似文献   

13.
Feldspar in ordinary chondrites (OCs) is often associated with thermal metamorphism, as a secondary mineral that forms from the crystallization of matrix and chondrule mesostasis. However, studies of feldspar in equilibrated OCs show that there is a range of plagioclase compositions within chondrules, some of which may be primary products of chondrule crystallization. It is important to recognize primary feldspar within chondrules because it can be used to help understand the secondary effects of thermal metamorphism and aqueous alteration. The presence of primary feldspar also provides important petrologic constraints on chondrule formation time scales. We undertook a careful study of Semarkona (LL3.00) and observed feldspar in 18% of chondrules. The feldspar is plagioclase covering a wide range of compositions (An2–An99) with little K‐feldspar component (<Or3). We show that plagioclase is a primary igneous phase, based on grain morphology and compositions consistent with growth from a melt having the bulk compositions of the host chondrules. Based on experimental studies, the presence of plagioclase suggests chondrules cooled slowly at temperatures close to the solidus. We also observed several secondary features consistent with the aqueous alteration. These features include zoning of Na and Ca in plagioclase, heterogeneity in plagioclase compositions in altered chondrules, development of porosity from the dissolution of chondrule glass, and alteration of glass to phyllosilicates. Alteration of major Al‐bearing phases, like plagioclase and glass, has important implications for interpretations of ages derived from Al‐Mg dating of chondrules, if they have been affected by secondary processes.  相似文献   

14.
Abstract— NWA 1950 is a new lherzolitic shergottite recently recovered from Morocco and is the first sample of this group found outside Antarctica. Major constituent phases of NWA 1950 are olivine, pyroxenes, and plagioclase glass (“maskelynite”) and the rock shows a two distinct textures: poikilitic and non‐poikilitic typical of lherzolitic shergottites. In poikilitic areas, several‐millimeter‐sized pyroxene oikocrysts enclose cumulus olivine and chromite. In contrast, pyroxenes are much smaller in non‐poikilitic areas, and olivine and plagioclase glass are more abundant. Olivine in non‐poikilitic areas is more Fe‐rich (Fa29–31) and shows a narrower distribution than that in poikilitic areas (Fa23–29). Pyroxenes in non‐poikilitic areas are also more Fe‐rich than those in poikilitic areas that show continuous chemical zoning suggesting fractional crystallization under a closed system. These observations indicate that pyroxene in non‐poikilitic areas crystallized from evolved interstitial melts and olivine was re‐equilibrated with such melts. NWA 1950 shows similar mineralogy and petrology to previously known lherzolitic shergottites (ALH 77005, LEW 88516, Y‐793605 and GRV 99027) that are considered to have originated from the same igneous body on Mars. Olivine composition of NWA 1950 is intermediate between those of ALH 77005‐GRV 99027 and those of LEW 88516‐Y‐793605, but is rather similar to ALH 77005 and GRV 99027. The subtle difference of mineral chemistry (especially, olivine composition) can be explained by different degrees of re‐equilibration compared to other lherzolitic shergottites, perhaps due to different location in the same igneous body. Thus, NWA 1950 experienced a high degree of re‐equilibration, similar to ALH 77005 and GRV 99027.  相似文献   

15.
Abstract— Miller Range (MIL) 05035 is a low‐Ti mare basalt that consists predominantly of pyroxene (62.3 vol%) and plagioclase (26.4 vol%). Pyroxenes are strongly shocked and complexly zoned from augite (Wo33) and pigeonite (Wo17) cores with Mg# = 50–54 to hedenbergite rims. Coexisting pyroxene core compositions reflect crystallization temperatures of 1000 to 1100 °C. Plagioclase has been completely converted to maskelynite with signs of recrystallization. Maskelynite is relatively uniform in composition (An94Ab6–An91Ab9), except at contacts with late‐stage mesostasis areas (elevated K contents, An82Ab15Or3). Symplectites (intergrowth of Fe‐augite, fayalite, and silica) of different textures and bulk compositions in MIL 05035 suggest formation by decomposition of ferro‐pyroxene during shock‐induced heating, which is supported by the total maskelynitization of plagioclase, melt pockets, and the presence of a relict pyroxferroite grain. Petrography and mineral chemistry imply that crystallization of MIL 05035 occurred in the sequence of Fe‐poor pyroxenes (Mg# = 50–54), followed by plagioclase and Fe‐rich pyroxenes (Mg# = 20–50), and finally hedenbergite, Fe‐Ti oxides, and minor late‐stage phases. Petrography, bulk chemistry, mineral compositions, and the age of MIL 05035 suggest it is possibly source crater‐paired with Asuka (A‐) 881757 and Yamato (Y‐) 793169, and may also be launch‐paired with Meteorite Hills (MET) 01210. MIL 05035 represents an old (?3.8–3.9 Ga), incompatible element‐depleted low‐Ti basalt that was not sampled during the Apollo or Luna missions. The light‐REE depleted nature and lack of Eu anomalies for this meteorite are consistent with an origin distant from the Procellarum KREEP Terrane, and genesis from an early cumulate mantle‐source region generated by extensive differentiation of the Moon.  相似文献   

16.
Abstract— Hydrogen‐isotopic compositions of carbonate and maskelynite in Allan Hills (ALH) 84001 were measured by secondary ion mass spectrometry (SIMS). the δd values of both minerals show considerable deviation. The deviation seems to be caused by addition of varying amounts of terrestrial water in the case of carbonate. In the case of maskelynite, H is heterogeneously distributed and the deviation in δD values seems to be due to mixing of this indigenous heavy H with isotopically normal H present in the SIMS chamber. The indigenous δD value in ALH 84001 seems to be ～2000%‰. Carbonate rather than maskelynite seems to be the main carrier of H in ALH 84001. Because ALH 84001 is ～4 Ga old, the H‐isotopic composition suggests that a large fraction of the initial martian atmosphere had already escaped by 4 Ga.  相似文献   

17.
Abstract— The enstatite chondrite reckling peak (rkp) a80259 contains feldspathic glass, kamacite, troilite, and unusual sets of parallel fine‐grained enstatite prisms that formed by rapid cooling of shock melts. Metallic Fe,Ni and troilite occur as spherical inclusions in feldspathic glass, reflecting the immiscible Fe‐Ni‐S and feldspathic melts generated during the impact. The Fe‐Ni‐S and feldspathic liquids were injected into fractures in coarse‐grained enstatite and cooled rapidly, resulting in thin (≤ 10 μm) semicontinuous to discontinuous veins and inclusion trails in host enstatite. Whole‐rock melt veins characteristic of heavily shocked ordinary chondrites are conspicuously absent. Raman spectroscopy shows that the feldspathic material is a glass. Elevated MgO and SiO2 contents of the glass indicate that some enstatite and silica were incorporated in the feldspathic melt. Metallic Fe,Ni globules are enclosed by sulfide and exhibit Nienrichment along their margins characteristic of rapid crystallization from a Fe‐Ni‐S liquid. Metal enclosed by sulfide is higher in Si and P than metal in feldspathic glass and enstatite, possibly indicating lower O fugacities in metal/sulfide than in silicate domains. Fine‐grained, elongate enstatite prisms in troilite or feldspathic glass crystallized from local pyroxene melts that formed along precursor grain boundaries, but most of the enstatite in the target rock remained solid during the impact and occurs as deformed, coarsegrained crystals with lower CaO, Al2O3, and FeO than the fine‐grained enstatite. Reckling Peak A80259 represents an intermediate stage of shock melting between unmelted E chondrites and whole‐rock shock melts and melt breccias documented by previous workers. The shock petrogenesis of RKPA80259 reflects the extensive impact processing of the enstatite chondrite parent bodies relative to those of other chondrite types.  相似文献   

18.
Abstract— Olivine from Martian meteorite Allan Hills (ALH) 84001 occurs as clusters within orthopyroxene adjacent to fractures containing disrupted carbonate globules and feldspathic shock glass. The inclusions are irregular in shape and range in size from ～40 μm to submicrometer. Some of the inclusions are elongate and boudinage-like. The olivine grains are in sharp contact with the enclosing orthopyroxene and often contain small inclusions of chromite. The olivine exhibits a very limited range of composition from Fo65 to Fo66 (n = 25). The δ18O values of the olivine and orthopyroxene analyzed by ion microprobe range from +4.3 to +5.3‰ and are indistinguishable from each other within analytical uncertainty. The mineral chemistries, O-isotopic data, and textural relationships indicate that the olivine inclusions were produced at a temperature >800 °C. It is unlikely that the olivines formed during the same event that gave rise to the carbonates in ALH 84001, which have more elevated and variable δ18O values, and were probably formed from fluids that were not in isotopic equilibrium with the orthopyroxene or olivine. The reactions most likely instrumental in the formation of olivine could be either the dehydration of hydrous silicates that formed during carbonate precipitation or the reduction of orthopyroxene and spinel. If the olivine was formed by either reaction during a postcarbonate heating event, the implications are profound with regards to the interpretations of McKay et al. (1996). Due to the low diffusion rates in carbonates, this rapid, high-temperature event would have resulted in the preservation of the fine-scale carbonate zoning, while partially devolatilizing select carbonate compositions on a submicrometer scale (Brearley, 1998a). This may have resulted in the formation of the minute magnetite grains that McKay et al. (1996) attributed to biogenic activity.  相似文献   

19.
Abstract— Sayh al Uhaymir (SaU) 300 comprises a microcrystalline igneous matrix (grain size <10 μm), dominated by plagioclase, pyroxene, and olivine. Pyroxene geothermometry indicates that the matrix crystallized at ?1100 °C. The matrix encloses mineral and lithic clasts that record the effects of variable levels of shock. Mineral clasts include plagioclase, low‐ and high‐Ca pyroxene, pigeonite, and olivine. Minor amounts of ilmenite, FeNi metal, chromite, and a silica phase are also present. A variety of lithic clast types are observed, including glassy impact melts, impact‐melt breccias, and metamorphosed impact melts. One clast of granulitic breccia was also noted. A lunar origin for SaU 300 is supported by the composition of the plagioclase (average An95), the high Cr content in olivine, the lack of hydrous phases, and the Fe/Mn ratio of mafic minerals. Both matrix and clasts have been locally overprinted by shock veins and melt pockets. SaU 300 has previously been described as an anorthositic regolith breccia with basaltic components and a granulitic matrix, but we here interpret it to be a polymict crystalline impact‐melt breccia with an olivine‐rich anorthositic norite bulk composition. The varying shock states of the mineral and lithic clasts suggest that they were shocked to between 5–28 GPa (shock stages S1–S2) by impact events in target rocks prior to their inclusion in the matrix. Formation of the igneous matrix requires a minimum shock pressure of 60 GPa (shock stage >S4). The association of maskelynite with melt pockets and shock veins indicates a subsequent, local 28–45 GPa (shock stage S2–S3) excursion, which was probably responsible for lofting the sample from the lunar surface. Subsequent fracturing is attributed to atmospheric entry and probable breakup of the parent meteor.  相似文献   

20.
Abstract— We have performed petrologic and geochemical studies of Patuxent Range (PAT) 91501 and Lewis Cliff (LEW) 88663. PAT 91501, originally classified as an L7 chondrite, is rather a unique, near total impact melt from the L‐chondrite parent body. Lewis Cliff 88663 was originally classified as an “achondrite (?)”, but we find that it is a very weakly shocked L7 chondrite. PAT 91501 is an unshocked, homogeneous, igneous‐textured ultramafic rock composed of euhedral to subhedral olivine, low‐Ca pyroxene, augite and chrome‐rich spinels with interstitial albitic plagioclase and minor silica‐alumina‐alkali‐rich glass. Only ～10% relic chondritic material is present. Olivine grains are homogeneous (Fa25.2–26.8). Low‐Ca pyroxene (Wo1.9–7.2En71.9–78.2Fs19.9–20.9) and augite (Wo29.8–39.0En49.2–55.3Fs11.8–14.9) display a strong linear TiO2‐Al2O3 correlation resulting from igneous fractionation. Plagioclase is variable in composition; Or3.0–7.7Ab79.8–84.1An8.2–17.2.‐Chrome‐rich spinels are variable in composition and zoned from Cr‐rich cores to Ti‐Al‐rich rims. Some have evolved compositions with up to 7.9 wt% TiO2. PAT 91501 bulk silicate has an L‐chondrite lithophile element composition except for depletions in Zn and Br. Siderophile and chalcophile elements are highly depleted due to sequestration in centimeter‐size metal‐troilite nodules. The minerals in LEW 88663 are more uniform in composition than those in PAT 91501. Olivine grains have low CaO and Cr2O3 contents similar to those in L5–6 chondrites. Pyroxenes have high TiO2 contents with only a diffuse TiO2‐Al2O3 correlation. Low‐Ca pyroxenes are less calcic (Wo1.6–3.1En76.5–77.0Fs20.4–21.4), while augites (Wo39.5–45.6En46.8–51.1Fs7.6–9.4) and plagioclases (Or2.6–5.7Ab74.1–83.1An11.2–23.3) are more calcic. Spinels are homogeneous and compositionally similar to those in L6 chondrites. LEW 88663 has an L‐chondrite bulk composition for lithophile elements, and only slight depletions in siderophile and chalcophile elements that are plausibly due to weathering and/or sample heterogeneity.  相似文献   

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