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1.
Abstract— Antarctic meteorite Miller Range (MIL) 03346 is a nakhlite composed of 79% clinopyroxene, ?1% olivine, and 20% vitrophyric intercumulus material. We have performed a petrological and geochemical study of MIL 03346, demonstrating a petrogenetic history similar to previously discovered nakhlites. Quantitative textural study of MIL 03346 indicates long (>1 × 101 yr) residence times for the cumulus augite, whereas the skeletal Fe‐Ti oxide, fayalite, and sulfide in the vitrophyric intercumulus matrix suggest rapid cooling, probably as a lava flow. From the relatively high forsterite contents of olivine (up to Fo43) compared with other nakhlites and compositions of augite cores (Wo38–42En35–40Fs22–28) and their hedenbergite rims, we suggest that MIL 03346 is part of the same or a similar Martian cumulate‐rich lava flow as other nakhlites. However, MIL 03346 has experienced less equilibration and faster cooling than other nakhlites discovered to date. Calculated trace element concentrations based upon modal abundances of MIL 03346 and its constituent minerals are identical to whole rock trace element abundances. Parental melts for augite have REE patterns that are approximately parallel with whole rock and intercumulus melt using experimentally defined partition coefficients. This parallelism reflects closed‐system crystallization for MIL 03346, where the only significant petrogenetic process between formation of augite and eruption and emplacement of the nakhlite flow has been fractional crystallization. A model for the petrogenesis of MIL 03346 and the nakhlites (Nakhla, Governador Valadares, Lafayette, Yamato‐000593, Northwest Africa (NWA) 817, NWA 998) would include: 1) partial melting and ascent of melt generated from a long‐term LREE depleted mantle source, 2) crystallization of cumulus augite (± olivine, ± magnetite) in a shallow‐level Martian magma chamber, 3) eruption of the crystal‐laden nakhlite magma onto the surface of Mars, 4) cooling, crystal settling, overgrowth, and partial equilibration to different extents within the flow, 5) secondary alteration through hydrothermal processes, possibly immediately succeeding or during emplacement of the flow. This model might apply to single—or multiple—flow models for the nakhlites. Ultimately, MIL 03346 and the other nakhlites preserve a record of magmatic processes in volcanic rocks on Mars with analogous petrogenetic histories to pyroxene‐rich terrestrial lava flows and to komatiites.  相似文献   

2.
Abstract— Plagioclase in the Martian lherzolitic shergottite Grove Mountains (GRV) 99027 was shocked, melted, and recrystallized. The recrystallized plagioclase contains lamellae of pyroxene, olivine, and minor ilmenite (<1 μm wide). Both the pyroxene and the olivine inclusions enclosed in plagioclase and grains neighboring the plagioclase were partially melted into plagioclase melt pools. The formation of these lamellar inclusions in plagioclase is attributed to exsolution from recrystallizing melt. Distinct from other Martian meteorites, GRV 99027 contains no maskelynite but does contain recrystallized plagioclase. This shows that the meteorite experienced a slower cooling than maskelynite‐bearing meteorites. We suggest that the parent rock of GRV 99027 could have been embedded in hot rocks, which facilitated a more protracted cooling history.  相似文献   

3.
Abstract— The Yamato nakhlites, Y‐000593, Y‐000749, and Y‐000802, were recovered in 2000 from the bare icefield around the Yamato mountains in Antarctica, consisting of three independent specimens with black fusion crusts. They are paired cumulate clinopyroxenites. We obtained the intercumulus melt composition of the Yamato nakhlites and here call it the Yamato intercumulus melt (YIM). The YIM crystallized to form the augite rims, the olivine rims and the mesostasis phases in the cumulates. The augite rims consist of two layers: inner and outer. The crystallization of the inner rim drove the interstitial melt into the plagioclase liquidus field. Subsequently, the residual melt crystallized pigeonites and plagioclase to form the outer rims and the mesostasis. Three types of inclusions were identified in olivine phenocrysts: rounded vitrophyric, angular vitrophyric, and monomineralic augite inclusions. The monomineralic augite inclusions are common and may have been captured by growing olivine phenocrysts. The rounded vitrophyric inclusions are rare and may represent the composition of middle‐stage melts, whereas the angular vitrophyric inclusions seem to have been derived from fractionated late‐stage melts. Glass inclusions occur in close association with titanomagnetite and ferroan augite halo in phenocryst core augites and the assemblages may be magmatic inclusions in augites. We compared the YIM with compositions of magmatic inclusions in olivine and augite. The composition of magmatic inclusions in augite is similar to the YIM. Phenocrystic olivines contain exsolution lamellae, augite‐magnetite aggregates, and symplectites in the cores. The symplectites often occur at the boundaries between olivine and augite grains. The aggregates, symplectite and lamellae formed by exsolution from the host olivine at magmatic temperatures. We present a formational scenario for nakhlites as follows: (1) accumulation of augite, olivine, and titanomagnetite phenocrysts took place on the floor of a magma chamber; (2) olivine exsolved augite and magnetite as augite‐magnetite aggregates, symplectites and lamellae; (3) the overgrowth on olivine phenocrysts formed their rims, and the inner rims crystallized on augite phenocryst cores; and finally, (4) the outer rim formed surrounding the inner rims of augite phenocrysts, and plagioclase and minor minerals crystallized to form mesostasis under a rapid cooling condition, probably in a lava flow or a sill.  相似文献   

4.
We present a geochemical study of recently discovered Martian meteorite Northwest Africa (NWA) 5790 and use our results to constrain its origin and relationship with the other nakhlites. This nakhlite is a clinopyroxene cumulate composed of phenocrysts of augite, olivine, and rare oxides surrounded by a mesostasis composed of vitrophyric glass, feldspars, oxides, phosphates, and fine‐grained olivines and augite. Petrography, and major and trace element compositions of the phases present are consistent with derivation of NWA 5790 from a parental magma common to all the nakhlites. Olivine cores grew from a distinct, incompatible‐element enriched magma and are surrounded by rims containing augite inclusions that grew from the nakhlite parental liquid, supporting previous arguments for xenocrystic olivine cores in nakhlites. Rare earth element microdistributions suggest derivation of NWA 5790 augites from an evolved, relatively oxidized magma, produced by augite fractionation from the common nakhlite parental liquid. Augite grain shapes and CSD patterns are consistent with rapid cooling and derivation near the top of the nakhlite cumulate pile, but patterns are distinct from other nakhlites thought to have formed near the stratigraphic top. The high mesostasis abundance (~44 vol%) indicates solidification near the top of the nakhlite pile close to locations suggested for nakhlites NWA 817 and Miller Range (MIL) 03346. However, the geochemical and petrographic characteristics of these three samples do not permit their placement in a simple stratigraphic order as would occur in a single lava flow. This lack of simple ordering suggests that the nakhlite lava flow split into multiple sections as would occur during breakouts from a single lava flow. Finally we note that NWA 5790 is unique among currently available nakhlites in having phenocryst abundances low enough to allow it to flow.  相似文献   

5.
Abstract— Quantitative textural data for Northwest Africa (NWA) 032 and the LaPaz (LAP) mare basalt meteorites (LAP 02205, LAP 02224, LAP 02226, and LAP 02436) prvide constraints on their crystallization and mineral growth histories. In conjunction with whole‐rock and mineral chemistry, textural analysis provides powerful evidence for meteorite pairing. Petrographic observations and crystal size distribution (CSD) measurements of NWA 032 indicate a mixed population of slowly cooled phenocrysts and faster cooled matrix. LaPaz basalt crystal populations are consistent with a single phase of nucleation and growth. Spatial distribution patterns (SDP) of minerals in the meteorites highlight the importance of clumping and formation of clustered crystal frameworks in their melts, succeeded by continued nucleation and growth of crystals. This process resulted in increasingly poor sorting, during competition for growth, as the melt crystallized. Based on CSD and SDP data, we suggest a potential lava flow geometry model to explain the different crystal populations for NWA 032 and the LaPaz basalts. This model involves crystallization of early formed phenocrysts at hypabyssal depths in the lunar crust, followed by eruption and flow differentiation on the lunar surface. Lava flow differentiation would allow for formation of a cumulate base and facilitate variable cooling within the stratigraphy, explaining the varied textures and modal mineralogies of mare basalt meteorites. The model may also provide insight into the relative relationships of some Apollo mare basalt suites, shallow‐level crystal fractionation processes, and the nature of mare basalt volcanism over lunar history.  相似文献   

6.
Abstract— We petrologically examined the Miller Range (MIL) 03346 nakhlite. The main‐phase modal abundances are 67.7 vol% augite, 0.8 vol% olivine, and 31.5 vol% mesostasis. Among all known nakhlites, MIL 03346's modal abundance of olivine is the smallest and of mesostasis is the largest. Augite occurs as cumulus phenocrysts having a homogeneous core composition (En36–38Fs24–22Wo40), which is identical with other nakhlites. They accompany thin ferroan rims divided into inner and outer rims with a compositional gap at the boundary between the two rims. Olivine grains have magnesian cores (Fa ≥ 55) and show normal zoning toward ferroan rims (Fa ≤ 84). Mesostasis consists mostly of glass (26.0 vol%) with minor skeletal fayalites, skeletal titanomagnetites, acicular phosphate, massive cristobalite, and sulfides. We conclude that MIL 03346 is the most rapidly cooled nakhlite among all known nakhlites based on the petrography. We obtain the intercumulus melt composition for MIL 03346 from the mass balance calculation using the modal abundances and discuss the crystallization sequence of MIL 03346 in comparison with that of Yamato (Y‐) 000593. Although magnesian olivines of Y‐000593 are phenocrystic, magnesian olivine grains of MIL 03346 seem to have texturally crystallized from the intercumulus melt. After the MIL 03346 magma intruded upward to the Martian surficial zone, the magnesian olivine crystallized, and then the ferroan inner rim formed on phenocrystic core augite. The outer rim of phenocrystic augites formed after the crystallization of skeletal fayalites and skeletal titanomagnetites, resulting in a compositional gap between the inner and outer rims. Finally, glassy mesostasis formed from the residual melt. This crystallization sequence of MIL 03346 is different from those of other nakhlites, including Y‐000593.  相似文献   

7.
Abstract– The oxygen fugacities recorded in the nakhlites Nakhla, Yamato‐000593 (Y‐000593), Lafayette, and NWA998 were studied by applying the Fe,Ti‐oxide oxybarometer. Oxygen fugacities obtained cluster closely around the FMQ (Fayalite–Magnetite–Quartz) buffer (NWA998 = FMQ ? 0.8; Y‐000593 = FMQ ? 0.7; Nakhla = FMQ; Lafayette = FMQ + 0.1). The corresponding equilibration temperatures are 810 °C for Nakhla and Y‐000593, 780 °C for Lafayette and 710 °C for NWA998. All nakhlites record oxygen fugacities significantly higher and with a tighter range than those determined for Martian basalts, i.e., shergottites whose oxygen fugacities vary from FMQ ? 1 to FMQ ? 4. It has been known for some time that nakhlites are different from other Martian meteorites in chemistry, mineralogy, and crystallization age. The present study adds oxygen fugacity to this list of differences. The comparatively large variation in fO2 recorded by shergottites was interpreted by Herd et al. (2002) as reflecting variable degrees of contamination with crustal fluids that would also carry a light rare earth element (REE)‐enriched component. The high oxygen fugacities and the large light REE enrichment of nakhlites fit qualitatively in this model. In detail, however, it is found that the inferred contaminating phase in nakhlites must have been different from those in shergottites. This is supported by unique 182W/184W and 142Nd/144Nd ratios in nakhlites, which are distinct from other Martian meteorites. It is likely that the differences in fO2 between nakhlites and other Martian meteorites were established very early in the history of Mars. Parental trace element rich and trace element poor regions (reservoirs) of Mars mantle ( Brandon et al. 2000 ) must have been kept isolated throughout Martian history. Our results further show significant differences in closure temperature among the different nakhlites. The observed range in equilibration temperatures together with similar fO2 values is attributable to crystallization of nakhlites in the same cumulate pile or lava layer at different burial depths from 0.5 to 30 m below the Martian surface in agreement with Mikouchi et al. (2003) and is further confirmed by similar crystallization ages of about 1.3 Ga ago (e.g., Misawa et al. 2003 ).  相似文献   

8.
Abstract— The petrographic relationships in diogenites between orthopyroxene and minor phases such as chromite, troilite, diopside, plagioclase, and silica are often obscured by the intense brecciation that characterizes these meteorites. Although brecciated, Bilanga preserves numerous clasts displaying primary textural relations between orthopyroxene and these minor phases that are large enough to analyze by electron microprobe. In this study, we focus on the distribution, composition, and origin of the minor phases in Bilanga to provide new insights into the crystallization and metamorphic history of these rocks. The samples examined consist mainly of orthopyroxene grains plus five types of assemblages containing diopside + a Fe‐rich phase (chromite, troilite, and/or Fe‐Ni metal) ± plagioclase ± silica. We interpret type 1 assemblages as being remnants of intercumulus melt trapped in the interstices between orthopyroxene grains after crystal settling in a magma chamber. Type 2 assemblages appear to have formed by heterogeneous exsolution during thermal metamorphism. Type 3 assemblages are believed to be remnants of other assemblages that have been shocked, melted, and rapidly recrystallized by impact events. Type 4 assemblages consist of veins that also appear to have formed from trapped intercumulus melt. Regions of silica‐rich mesostasis (type 5) appear to be larger patches of more evolved intercumulus melt that have been significantly affected by late‐stage impact melting. Finally, large clasts containing plagioclase ± diopside are interpreted to be exotic fragments of a different but possibly related rock type incorporated in the Bilanga breccia.  相似文献   

9.
Abstract– Miller Range (MIL) 03346 is the most oxidized and least equilibrated nakhlite known and displays the highest amount of intercumulus phase. The discovery of three new nakhlites, MIL 090030, MIL 090032, and MIL 090136, in the Miller Range, Antarctica, geographically close to the location of MIL 03346, suggests that they may come from the same parent meteorite. In this study, we investigate the mineralogy and texture of cumulus and intercumulus phases, in situ major and trace element compositions for the cumulus phases, as well as pyroxene crystal size distribution patterns and spatial distribution patterns of MIL 090030, 090032, and 090136. Using these combined results, we conclude that the three nakhlites studied here are paired with MIL 03346. However, modal mineral abundances of MIL 090030, 090032, 090136, and 03346 exhibit variations indicating that a single sample of a Miller Range nakhlite is not modally representative of the parent meteorite and that analyses of multiple samples for a single nakhlite may be necessary to obtain representative modal data for placement in the cumulate pile. Our calculated parental melt composition based on all the paired samples confirms a previous study suggesting that the nakhlite parent melt crystallized as a closed system.  相似文献   

10.
Dynamic crystallization experiments were performed on a liquid having the bulk composition of olivine‐phyric shergottite Yamato 980459, to constrain the igneous thermal history of this meteorite. Key characteristics of the meteorite's mineralogy and texture, including several morphologically distinct olivine and pyroxene crystal populations and a glassy mesostasis devoid of plagioclase, were replicated upon cooling from 1435 to 909 °C at 1 atmosphere under reducing conditions. Three sequential cooling ramps are required to produce synthetic samples with textures and compositions matching Yamato 980459. Olivine phenocrysts formed at <1 °C h?1, presumably at depth in the Martian crust. Pyroxene phenocrysts formed mainly at ~10 °C h?1, consistent with crystallization within a lava flow at depths of 25–45 cm. Increased cooling rate (~100 °C h?1) in a third stage suppressed the formation of plagioclase and produced groundmass crystals, consistent with crystallization at lava flow depths of 5–7 cm. Although Y 980459 is unique among Martian meteorites (i.e., preserving a primary glassy mesostasis), its emplacement did not require unique physical conditions. Rather, the second and third cooling stages may reflect cooling within the core of a pāhoehoe‐like flow and subsequent breakout on the surface of Mars.  相似文献   

11.
Abstract– Six chondritic clasts in the Cumberland Falls polymict breccia were examined: four texturally resemble ordinary chondrites (OCs) and two are impact melt breccias containing shocked OC clasts adjacent to a melt matrix. The six chondritic clasts are probably remnants of a single OC projectile that was heterogeneously shocked when it collided with the Cumberland Falls host. Mayo Belwa is the first known aubrite impact melt breccia. It contains coarse enstatite grains exhibiting mosaic extinction; the enstatite grains are surrounded by a melt matrix composed of 3–16 μm‐size euhedral and subhedral enstatite grains embedded in sodic plagioclase. Numerous vugs, ranging from a few micrometers to a few millimeters in size, constitute ～5 vol% of the meteorite. They occur nearly exclusively within the Mayo Belwa matrix; literature data show that some vugs are lined with bundles of acicular grains of the amphibole fluor‐richterite. This phase has been reported previously in only two other enstatite meteorites (Abee and St. Sauveur), both of which are EH‐chondrite impact melt breccias. It seems likely that in Mayo Belwa, volatiles were vaporized during an impact event and formed bubbles in the melt. As the melt solidified, the bubbles became cavities; plagioclase and fluor‐richterite crystallized at the margins of these cavities via reaction of the melt with the vapor.  相似文献   

12.
Abstract— Quantitative petrographic analysis, using the crystal size distribution (CSD) method, provides a novel approach for examining the crystallization histories of basaltic shergottites. Grain number densities at different sizes are plotted against grain size, and the resulting curve relates to the geologic processes involved with the crystallization of the grain population. Most basaltic shergottites are dominated by pigeonite and augite; and because plagioclase is primarily interstitial, and therefore constrained in its growth by the surrounding pyroxenes, we limited our size measurements to the pyroxene phases. The groundmasses of Elephant Moraine (EET) A79001 lithology A and Dar al Gani (DaG) 476 are fine grained with cumulus pyroxene and interstitial plagioclase glass. Their simple linear CSD plots record a single stage of pyroxene crystallization under steady‐state conditions of continuous nucleation and growth. The textures of Queen Alexandra Range (QUE) 94201 and EETA79001 lithology B are quite different from the other shergottites, with intergrown pyroxene and plagioclase. Likewise, their CSD plots are also distinct, with curved trends that suggest a lack of large grains, most likely because of interference between simultaneously growing silicate phases. However, the CSD plot shapes are smooth, also implying a single stage of growth. Shergotty and Zagami, with coarser cumulus textures, display CSD plots that are generally linear over most grain sizes. This implies that conditions of nucleation and growth were dominant during formation of the pyroxene populations. Both plots, however, also display kinks, implying multiple stages of growth. A similar kink is also visible in a CSD plot of only the Mg‐rich cores of Shergotty pyroxenes, which suggests the feature represents changes in conditions during core crystallization, rather than an event coincident with the change in composition to the Fe‐rich rims. The plot may be interpreted as representing two stages of core growth with an intervening short hiatus of nucleation, with continued crystallization associated with ascent of the magma. Eruption onto the surface probably triggered the compositional change to Fe‐rich rims. The CSD analysis of products from a controlled crystallization study agree with experimental and petrologic estimates that cooling rates for Zagami were on the order of a few tenths of a degree per hour. Growth rates derived from these cooling rates suggest crystallization of Shergotty and Zagami pyroxenes occurred over a period of a few weeks to months.  相似文献   

13.
Amphibole and spinel occur in the Shergotty and Zagami meteorites only in magmatic inclusions in pigeonite. The trapped magma is essentially identical to the parental magmas for Shergotty and Zagami. The amphibole is a kaersutite with minimal halogen content; by inference, it must have been hydrous. If so, the Shergotty and Zagami melts contained at least 0.2 wt % H2O and were probably H2O-undersaturated. Pressures in excess of 1 kilobar seem necessary for the formation of amphibole. Spinel replaces magnetite in the inclusions, and olivine replaces magnetite elsewhere in the meteorites. To stabilize spinel, the melt in the inclusions must have become enriched in Al during fractionation, possibly because the small volume of the inclusions made nucleation of plagioclase unlikely. Pervasive replacement of magnetite through reduction reactions suggests that Shergotty and Zagami interacted with hydrogen-rich fluids during their cooling.  相似文献   

14.
Abstract— Asteroid differentiation was driven by a complex array of magmatic processes. This paper summarizes theoretical and somewhat speculative research on the physics of these processes. Partial melts in asteroids migrate rapidly, taking < 106 years to reach surface regions. On relatively small (<100 km) asteroids with sufficient volatiles in partial melts (<3000 ppm), explosive volcanism accelerated melts to greater than escape velocity, explaining the apparent lack of basaltic components on the parent asteroids of some differentiated meteorites. Partial melting products include the melts (some eucrites, angrites), residues (lodranites, ureilites), and unfractionated residues (acapulcoites). The high liquidus temperatures of magmatic iron meteorites, the existence of pallasites with only olivine, and the fact that enstatite achondrites formed from ultramafic magmas argue for the existence of magma oceans on some asteroids. Asteroidal magma oceans would have been turbulently convective. This would have prevented crystals nucleated at the upper cooling surface (the only place for crystal nucleation in a low-pressure body) from settling until the magma became choked with crystals. After turbulent convection slowed, crystals and magma would have segregated, leaving a body stratified from center to surface as follows: a metallic core, a small pallasite zone, a dunite region, a feldspathic pyroxenite, and basaltic intrusions and lava flows (if the basaltic components had not been lost by explosive volcanism). The pallasite and dunite zones probably formed from coarse (0.5–1 cm) residual olivine left after formation of the magma ocean at >50% partial melting of the silicate assemblage. Iron cores crystallized dendritically from the outside to the inside. The rapid melt migration rate of silicate melts suggests that 26Al could not be responsible for forming asteroidal magma oceans because it would leave the interior before a sufficient amount of melting occurred. Other heat sources are more likely candidates. Our analysis suggests that if Earth-forming planetesimals had differentiated they were either small (<100 km) and poor in volatiles (<1000 ppm) or they were rich in volatiles and large enough (>300 km) to retain the products of pyroclastic eruptions; if these conditions were not met, Earth would not have a basaltic component.  相似文献   

15.
Caleta el Cobre (CeC) 022 is a Martian meteorite of the nakhlite group, showing an unbrecciated cumulate texture, composed mainly of clinopyroxene and olivine. Augite shows irregular core zoning, euhedral rims, and thin overgrowths enriched in Fe relative to the core. Low‐Ca pyroxene is found adjacent to olivine. Phenocrysts of Fe‐Ti oxides are titanomagnetite with exsolutions of ilmenite/ulvöspinel. Intercumulus material consists of both coarse plagioclase and fine‐grained mesostasis, comprising K‐feldspars, pyroxene, apatite, ilmenite, Fe‐Ti oxides, and silica. CeC 022 shows a high proportion of Martian aqueous alteration products (iddingsite) in olivine (45.1 vol% of olivine) and mesostasis. This meteorite is the youngest nakhlite with a distinct Sm/Nd crystallization age of 1.215 ± 0.067 Ga. Its ejection age of 11.8 ± 1.8 Ma is similar to other nakhlites. CeC 022 reveals contrasted cooling rates with similarities with faster cooled nakhlites, such as Northwest Africa (NWA) 817, NWA 5790, or Miller Range 03346 nakhlites: augite irregular cores, Fe‐rich overgrowths, fine‐grained K‐feldspars, quenched oxides, and high rare earth element content. CeC 022 also shares similarities with slower cooled nakhlites, including Nakhla and NWA 10153: pyroxene modal abundance, pyroxenes crystal size distribution, average pyroxene size, phenocryst mineral compositions, unzoned olivine, and abundant coarse plagioclase. Moreover, CeC 022 is the most magnetic nakhlite and represents an analog source lithology for the strong magnetization of the Martian crust. With its particular features, CeC 022 must originate from a previously unsampled sill or flow in the same volcanic system as the other nakhlites, increasing Martian sample diversity and our knowledge of nakhlites.  相似文献   

16.
Abstract— Crystal‐bearing lunar spherules (CLSs) in lunar breccia (14313, 14315, 14318), soil (68001, 24105), and impact‐melt rock (62295) samples can be classified into two types: feldspathic and olivine‐rich. Feldspathic CLSs contain equant, tabular, or acicular plagioclase grains set in glass or a pyroxene‐olivine mesostasis; the less common olivine‐rich CLSs contain euhedral or skeletal olivine set in glass, or possess a barred‐olivine texture. Bulk‐chemical and mineral‐chemical data strongly suggest that feldspathic CLSs formed by impact melting of mixtures of ferroan anorthosite and Mg‐suite rocks that compose the feldspathic crust of the Moon. It is probable that olivine‐rich CLSs also formed by impact melting, but some appear to have been derived from distinctively magnesian lunar materials, atypical of the Moon's crust. Some CLSs contain reversely‐zoned “relic” plagioclase grains that were not entirely melted during CLS formation, thin (≤5 μm thick) rims of troilite or phosphate, and chemical gradients in glassy mesostases attributed to metasomatism in a volatile‐rich (Na‐K‐P‐rich) environment. Crystal‐bearing lunar spherules were rimmed and metasomatized prior to brecciation. Compound CLS objects are also present; these formed by low‐velocity collisions in an environment, probably an ejecta plume, that contained numerous melt droplets. Factors other than composition were responsible for producing the crystallinity of the CLSs. We agree with previous workers that relatively slow cooling rates and long ballistic travel times were critical features that enabled these impact‐melt droplets to partially or completely crystallize in free‐flight. Moreover, incomplete melting of precursor materials formed nucleation sites that aided subsequent crystallization. Clearly, CLSs do not resemble meteoritic chondrules in all ways. The two types of objects had different precursors and did not experience identical rimming processes, and vapor fractionation appears to have played a less important role in establishing the compositions of CLSs than of chondrules. However, the many detailed similarities between CLSs and chondrules indicate that it is more difficult to rule out an origin for some chondrules by impact melting than some have previously argued. Differences between CLSs, chondrules, and their host rocks possibly can be reconciled with an impact‐melt origin for some chondrules when different precursors, the higher gravity of the Moon compared to chondrite parent bodies, and the likely presence of nebular gas during chondrule formation are taken into account.  相似文献   

17.
Abstract— Our studies of the silicate-bearing inclusions in the IIICD iron meteorites Maltahöhe, Carlton and Dayton suggest that their mineralogy and mineral compositions are related to the composition of the metal in the host meteorites. An inclusion in the low-Ni Maltahöhe is similar in mineralogy to those in IAB irons, which contain olivine, pyroxene, plagioclase, graphite and troilite. With increasing Ni concentration of the metal, silicate inclusions become poorer in graphite, richer in phosphates, and the phosphate and silicate assemblages become more complex. Dayton contains pyroxene, plagioclase, SiO2, brianite, panethite and whitlockite, without graphite. In addition, mafic silicates become more FeO-rich with increasing Ni concentration of the hosts. In contrast, silicates in IAB irons show no such correlation with host Ni concentration, nor do they have the complex mineral assemblages of Dayton. These trends in inclusion composition and mineralogy in IIICD iron meteorites have been established by reactions between the S-rich metallic magma and the silicates, but the physical setting is uncertain. Of the two processes invoked by other authors to account for groups IAB and IIICD, fractional crystallization of S-rich cores and impact generation of melt pools, we prefer core crystallization. However, the absence of relationships between silicate inclusion mineralogy and metal compositions among IAB irons analogous to those that we have discovered in IIICD irons suggests that the IAB and IIICD cores/metallic magmas evolved in rather different ways. We suggest that the solidification of the IIICD core may have been very complex, involving fractional crystallization, nucleation effects and, possibly, liquid immiscibility.  相似文献   

18.
Abstract— Dar al Gani 476, the 13th martian meteorite, was recovered from the Sahara in 1998. It is a basaltic shergottitic rock composed of olivine megacrysts reaching 5 mm (24 vol%) set in a finegrained groundmass of pyroxene (59 vol%) and maskelynitized plagioclase (12 vol%) with minor amounts of accessory phases (spinel, merrillite, ilmenite). Dar al Gani 476 is similar to lithology A of Elephant Moraine A79001 (EETA79001) in petrography and mineralogy, but is distinct in several aspects. Low‐Ca pyroxenes in the Dar al Gani 476 groundmass are more magnesian (En76Fs21 Wo3～En58Fs30Wo12) than those in lithology A of EETA79001 (En73Fs22Wo5～En45Fs43Wo12), rather similar to pyroxenes in lherzolitic martian meteorites (En76Fs21 Wo3～En63Fs22Wo15). Dar al Gani 476 olivine is less magnesian and shows a narrower compositional range (Fo76‐58) than EETA79001 olivine (Fo81‐53), and is also similar to olivines in lherzolitic martian meteorites (Fo74‐65). The orthopyroxene‐olivine‐chromite xenolith typical in the lithology A of EETA79001 is absent in Dar al Gani 476. It seems that Dar al Gani 476 crystallized from a slightly more primitive mafic magma than lithology A of EETA79001 and several phases (olivine, pyroxene, chromite, and ilmenite) in Dar al Gani 476 may have petrogenetic similarities to those of lherzolitic martian meteorites. Olivine megacrysts in Dar al Gani 476 are in disequilibrium with the bulk composition. The presence of fractured olivine grains in which the most Mg‐rich parts are in contact with the groundmass suggests that little diffusive modification of original olivine compositions occurred during cooling. This observation enabled us to estimate the cooling rates of Dar al Gani 476 and EETA79001 olivines, giving similar cooling rates of 0.03‐3 °C/h for Dar al Gani 476 and 0.05‐5 °C/h for EETA79001. This suggests that they were cooled near the surface (burial depth shallower than about 3 m at most), probably in lava flows during crystallization of groundmass. As is proposed for lithology A of EETA79001, it may be possible to consider that Dar al Gani 476 has an impact melt origin, a mixture of martian lherzolite and other martian rock (Queen Alexandra Range 94201, nakhlites?).  相似文献   

19.
Lunar mare basalts provide insights into the compositional diversity of the Moon's interior. Basalt fragments from the lunar regolith can potentially sample lava flows from regions of the Moon not previously visited, thus, increasing our understanding of lunar geological evolution. As part of a study of basaltic diversity at the Apollo 12 landing site, detailed petrological and geochemical data are provided here for 13 basaltic chips. In addition to bulk chemistry, we have analyzed the major, minor, and trace element chemistry of mineral phases which highlight differences between basalt groups. Where samples contain olivine, the equilibrium parent melt magnesium number (Mg#; atomic Mg/[Mg + Fe]) can be calculated to estimate parent melt composition. Ilmenite and plagioclase chemistry can also determine differences between basalt groups. We conclude that samples of approximately 1–2 mm in size can be categorized provided that appropriate mineral phases (olivine, plagioclase, and ilmenite) are present. Where samples are fine‐grained (grain size <0.3 mm), a “paired samples t‐test” can provide a statistical comparison between a particular sample and known lunar basalts. Of the fragments analyzed here, three are found to belong to each of the previously identified olivine and ilmenite basalt suites, four to the pigeonite basalt suite, one is an olivine cumulate, and two could not be categorized because of their coarse grain sizes and lack of appropriate mineral phases. Our approach introduces methods that can be used to investigate small sample sizes (i.e., fines) from future sample return missions to investigate lava flow diversity and petrological significance.  相似文献   

20.
Abstract— Experimentally rehomogenized melt inclusions from the nakhlite Miller Range 03346 (MIL 03346) and the lherzolitic shergottite Allan Hills 77005 (ALH 77005) have been analyzed for their rare earth element (REE) concentrations in order to characterize the early melt compositions of these Martian meteorites and to calculate the oxygen fugacity conditions they crystallized under. D(Eu/Sm)pyroxene/melt values were measured at 0.77 and 1.05 for ALH 77005 and MIL 03346, respectively. These melts and their associated whole rock compositions have similar REE patterns, suggesting that whole rock REE values are representative of those of the early melts and can be used as input into the pyroxene Eu‐oxybarometer for the nakhlites and lherzolitic shergottites. Crystallization fO2 values of IW + 1.1 (ALH 77005) and IW + 3.2 (MIL 03346) were calculated. Whole rock data from other nakhlites and lherzolitic shergottites was input into the Eu‐oxybarometer to determine their crystallization fO2 values. The lherzolitic shergottites and nakhlites have fO2 values that range from IW + 0.4 to 1.6 and from IW + 1.1 to 3.2, respectively. These values are consistent with some previously determined fO2 estimates and expand the known range of fO2 values of the Martian interior to four orders of magnitude. The origins of this range are not well constrained. Possible mechanisms for producing this spread in fO2 values include mineral/melt fractionation, assimilation, shock effects, and magma ocean crystallization processes. Mineral/melt partitioning can result in changes in fO2 from the start to the finish of crystallization of 2 orders of magnitude. In addition, crystallization of a Martian magma ocean with reasonable initial water content results in oxidized, water‐rich, late‐stage cumulates. Sampling of these oxidized cumulates or interactions between reduced melts and the oxidized material can potentially account for the range of fO2 values observed in the Martian meteorites.  相似文献   

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