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1.
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   

2.
We present a study of the petrology and geochemistry of basaltic shergottite Northwest Africa 2975 (NWA 2975). NWA 2975 is a medium‐grained basalt with subophitic to granular texture. Electron microprobe (EMP) analyses show two distinct pyroxene compositional trends and patchy compositional zoning patterns distinct from those observed in other meteorites such as Shergotty or QUE 94201. As no bulk sample was available to us for whole rock measurements, we characterized the fusion crust and its variability by secondary ion mass spectrometer (SIMS) measurements and laser ablation inductively coupled plasma spectroscopy (LA‐ICP‐MS) analyses as a best‐available proxy for the bulk rock composition. The fusion crust major element composition is comparable to the bulk composition of other enriched basaltic shergottites, placing NWA 2975 within that sample group. The CI‐normalized REE (rare earth element) patterns are flat and also parallel to those of other enriched basaltic shergottites. Merrillite is the major REE carrier and has a flat REE pattern with slight depletion of Eu, parallel to REE patterns of merrillites from other basaltic shergottites. The oxidation state of NWA 2975 calculated from Fe‐Ti oxide pairs is NNO‐1.86, close to the QFM buffer. NWA 2975 represents a sample from the oxidized and enriched shergottite group, and our measurements and constraints on its origin are consistent with the hypothesis of two distinct Martian mantle reservoirs: a reduced, LREE‐depleted reservoir and an oxidized, LREE‐enriched reservoir. Stishovite, possibly seifertite, and dense SiO2 glass were also identified in the meteorite, allowing us to infer that NWA 2975 experienced a realistic shock pressure of ~30 GPa.  相似文献   

3.
Abstract– Northwest Africa (NWA) 2977 is an olivine‐gabbro lunar meteorite that has a distinctly different petrographic texture from other lunar basalts. We studied this rock with a series of in situ analytical methods. NWA 2977 consists mainly of olivine and pyroxene with minor plagioclase. It shows evidence of intense shock metamorphism, locally as high as shock‐stage S6. Olivine adjacent to a melt vein has been partially transformed into ringwoodite and Al,Ti‐rich chromite grains have partially transformed into their high‐pressure polymorph (possibly CaTi2O4‐structure). Olivine in NWA 2977 contains two types of lithic inclusions. One type is present as Si,Al‐rich melt inclusions that are composed of glass and, in most cases, dendritic pyroxene. The other type is mafic and composed of relatively coarse‐grained augite with accessory chromite, RE‐merrillite, and baddeleyite. Two Si,Al‐rich melt inclusions are heavy rare earth elements (REE) enriched, whereas the mafic inclusion has high REE concentrations and a KREEP‐like pattern. The mafic inclusion could be a relict fragment captured during the ascent of the parent magma of NWA 2977, whereas the Si,Al‐rich inclusions may represent the original NWA 2977 melt. The calculated whole‐rock composition has a KREEP‐like REE pattern, suggesting that NWA 2977 has an affinity to KREEP rocks. Baddeleyite has recorded a young crystallization age of 3123 ± 7 Ma (2σ), which is consistent with results from previous whole‐rock and mineral Sm‐Nd and Rb‐Sr studies. The petrography, mineralogy, trace element geochemistry, and young crystallization age of NWA 2977 support the possibility of pairing between NWA 2977 and the olivine‐gabbro portion of NWA 773.  相似文献   

4.
Knowledge of Martian igneous basaltic compositions is crucial for constraining mantle evolution, including early differentiation and mantle convection. Primitive magmas provide direct information about their mantle source regions, but most Martian meteorites either contain cumulate olivine or crystallized from fractionated melts. The recently discovered Martian meteorite Northwest Africa (NWA) 5789 is an olivine‐phyric shergottite. NWA 5789 has special significance among the Martian meteorites because it appears to represent one of the most magnesian Martian magmas known, other than Yamato (Y) 980459. Its most magnesian olivine cores (Fo85) are in Mg‐Fe equilibrium with a magma of the bulk rock composition, suggesting that the bulk represents a magma composition. Based on the Al/Ti ratio of its pyroxenes, we infer that the rock began to crystallize at a high pressure consistent with conditions in Mars’ lower crust/upper mantle. It continued and completed its crystallization closer to the surface, where cooling was rapid and produced a mesostasis of radiating sprays of plagioclase and pyroxene. The mineralogy, petrology, mineral chemistry, and bulk rock composition of NWA 5789 are very similar to those of Y‐980459. The similarities between the two meteorites suggest that NWA 5789 (like Y‐980459) represents a primitive, mantle‐derived magma composition. They also suggest the possibility that NWA 5789 and Y‐980459 formed in the same lava flow. However, based on the mineralogy and texture of its mesostasis, NWA 5789 must have cooled more slowly than Y‐980459. NWA 5789 will help elucidate the igneous geology and geochemistry of Mars.  相似文献   

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

6.
Petrology of Martian meteorite Northwest Africa 998   总被引：1，自引：0，他引：1  
Abstract— Nakhlite Northwest Africa (NWA) 998 is an augite-rich cumulate igneous rock with mineral compositions and oxygen isotopic composition consistent with an origin on Mars. This 456-gram, partially fusion-crusted meteorite consists of (by volume) ∼75% augite (core composition Wo39En39Fs22), ∼9% olivine (Fo35), ∼7% plagioclase (Ab61An35) as anhedra among augite and olivine, ∼3.5% low-calcium pyroxenes (pigeonite and orthopyroxene) replacing or forming overgrowths on olivine and augite, ∼1% titanomagnetite, and other phases including potassium feldspar, apatite, pyrrhotite, chalcopyrite, ilmenite, and fine-grained mesostasis material. Minor secondary alteration materials include “iddingsite” associated with olivine (probably Martian), calcite crack fillings, and iron oxide/hydroxide staining (both probably terrestrial). Shock effects are limited to minor cataclasis and twinning in augite. In comparison to other nakhlites, NWA 998 contains more low-calcium pyroxenes and its plagioclase crystals are blockier. The large size of the intercumulus feldspars and the chemical homogeneity of the olivine imply relatively slow cooling and chemical equilibration in the late- and post-igneous history of this specimen, and mineral thermometers give subsolidus temperatures near 730 °C. Oxidation state was near that of the QFM buffer, from about QFM-2 in earliest crystallization to near QFM in late crystallization, and to about QFM + 1.5 in some magmatic inclusions. The replacement or overgrowth of olivine by pigeonite and orthopyroxene (with or without titanomagnetite), and the marginal replacement of augite by pigeonite, are interpreted to result from late-stage reactions with residual melts (consistent with experimental phase equilibrium relationships). Apatite is concentrated in planar zones separating apatite-free domains, which suggests that residual magma (rich in P and REE) was concentrated in planar (fracture?) zones and possibly migrated through them. Loss of late magma through these zones is consistent with the low bulk REE content of NWA 998 compared with the calculated REE content of its parent magma.  相似文献   

7.
Abstract– Northwest Africa (NWA) 4797 is an ultramafic Martian meteorite composed of olivine (40.3 vol%), pigeonite (22.2%), augite (11.9%), plagioclase (9.1%), vesicles (1.6%), and a shock vein (10.3%). Minor phases include chromite (3.4%), merrillite (0.8%), and magmatic inclusions (0.4%). Olivine and pyroxene compositions range from Fo66–72,En58–74Fs19–28Wo6–15, and En46–60Fs14–22Wo34–40, respectively. The rock is texturally similar to “lherzolitic” shergottites. The oxygen fugacity was QFM?2.9 near the liquidus, increasing to QFM?1.7 as crystallization proceeded. Shock effects in olivine and pyroxene include strong mosaicism, grain boundary melting, local recrystallization, and pervasive fracturing. Shock heating has completely melted and vesiculated igneous plagioclase, which upon cooling has quench‐crystallized plagioclase microlites in glass. A mm‐size shock melt vein transects the rock, containing phosphoran olivine (Fo69–79), pyroxene (En44–51Fs14–18Wo30–42), and chromite in a groundmass of alkali‐rich glass containing iron sulfide spheres. Trace element analysis reveals that (1) REE in plagioclase and the shock melt vein mimics the whole rock pattern; and (2) the reconstructed NWA 4797 whole rock is slightly enriched in LREE relative to other intermediate ultramafic shergottites, attributable to local mobilization of melt by shock. The shock melt vein represents bulk melting of NWA 4797 injected during pressure release. Calculated oxygen fugacity for NWA 4797 indicates that oxygen fugacity is decoupled from incompatible element concentrations. This is attributed to subsolidus re‐equilibration. We propose an alternative nomenclature for “lherzolitic” shergottites that removes genetic connotations. NWA 4797 is classified as an ultramafic poikilitic shergottite with intermediate trace element characteristics.  相似文献   

8.
Abstract— The newly found meteorite Northwest Africa 6234 (NWA 6234) is an olivine (ol)‐phyric shergottite that is thought, based on texture and mineralogy, to be paired with Martian shergottite meteorites NWA 2990, 5960, and 6710. We report bulk‐rock major‐ and trace‐element abundances (including Li), abundances of highly siderophile elements, Re‐Os isotope systematics, oxygen isotope ratios, and the lithium isotope ratio for NWA 6234. NWA 6234 is classified as a Martian shergottite, based on its oxygen isotope ratios, bulk composition, and bulk element abundance ratios, Fe/Mn, Al/Ti, and Na/Al. The Li concentration and δ7Li value of NWA 6234 are similar to that of basaltic shergottites Zagami and Shergotty. The rare earth element (REE) pattern for NWA 6234 shows a depletion in the light REE (La‐Nd) compared with the heavy REE (Sm‐Lu), but not as extreme as the known “depleted” shergottites. Thus, NWA 6234 is suggested to belong to a new category of shergottite that is geochemically “intermediate” in incompatible elements. The only other basaltic or ol‐phyric shergottite with a similar “intermediate” character is the basaltic shergottite NWA 480. Rhenium‐osmium isotope systematics are consistent with this intermediate character, assuming a crystallization age of 180 Ma. We conclude that NWA 6234 represents an intermediate compositional group between enriched and depleted shergottites and offers new insights into the nature of mantle differentiation and mixing among mantle reservoirs in Mars.  相似文献   

9.
Northwest Africa (NWA) 4898 is the only low‐Ti, high‐Al basaltic lunar meteorite yet recognized. It predominantly consists of pyroxene (53.8 vol%) and plagioclase (38.6 vol%). Pyroxene has a wide range of compositions (En12–62Fs25–62Wo11–36), which display a continuous trend from Mg‐rich cores toward Ca‐rich mantles and then to Fe‐rich rims. Plagioclase has relatively restricted compositions (An87–96Or0–1Ab4–13), and was transformed to maskelynite. The REE zoning of all silicate minerals was not significantly modified by shock metamorphism and weathering. Relatively large (up to 1 mm) olivine phenocrysts have homogenous inner parts with Fo ~74 and sharply decrease to 64 within the thin out rims (~30 μm in width). Four types of inclusions with a variety of textures and modal mineralogy were identified in olivine phenocrysts. The contrasting morphologies of these inclusions and the chemical zoning of olivine phenocrysts suggest NWA 4898 underwent at least two stages of crystallization. The aluminous chromite in NWA 4898 reveals that its high alumina character was inherited from the parental magma, rather than by fractional crystallization. The mineral chemistry and major element compositions of NWA 4898 are different from those of 12038 and Luna 16 basalts, but resemble those of Apollo 14 high‐Al basalts. However, the trace element compositions demonstrate that NWA 4898 and Apollo 14 high‐Al basalts could not have been derived from the same mantle source. REE compositions of its parental magma indicate that NWA 4898 probably originated from a unique depleted mantle source that has not been sampled yet. Unlike Apollo 14 high‐Al basalts, which assimilated KREEPy materials during their formation, NWA 4898 could have formed by closed‐system fractional crystallization.  相似文献   

10.
The meteorite Mount DeWitt (DEW) 12007 is a polymict regolith breccia mainly consisting of glassy impact‐melt breccia particles, gabbroic clasts, feldspathic clasts, impact and volcanic glass beads, basaltic clasts, and mingled breccia clasts embedded in a matrix dominated by fine‐grained crystals; vesicular glassy veins and rare agglutinates are also present. Main minerals are plagioclase (typically An>85) and clinopyroxene (pigeonites and augites, sometimes interspersed). The presence of tranquillityite, coupled with the petrophysical data, the O‐isotope data (Δ17O = ?0.075), and the FeOtot/MnO ratios in olivine (91), pyroxene (65), and bulk rock (77) indicate a lunar origin for DEW 12007. Impactites consist of Al‐rich impact‐melt splashes and plagioclase‐rich meta‐melt clasts. The volcanic products belong to the very low titanium (VLT) or low titanium (LT) suites; an unusual subophitic fragment could be cryptomare‐related. Gabbroic clasts could represent part of a shallow intrusion within a volcanic complex with prevailing VLT affinity. DEW 12007 has a mingled bulk composition with relatively high incompatible element abundances and shows a high crustal diversity comprising clasts from the Moon's major terranes and rare lithologies. First‐order petrographic and chemical features suggest that DEW 12007 could be launch‐paired with other meteorites including Y 793274/981031, QUE 94281, EET 87521/96008, and NWA 4884.  相似文献   

11.
Abstract— The meteorite Northwest Africa 773 (NWA 773) is a lunar sample with implications for the evolution of mafic magmas on the moon. A combination of key parameters including whole‐rock oxygen isotopic composition, Fe/Mn ratios in mafic silicates, noble gas concentrations, a KREEP‐like rare earth element pattern, and the presence of regolith agglutinate fragments indicate a lunar origin for NWA 773. Partial maskelynitization of feldspar and occasional twinning of pyroxene are attributed to shock deformation. Terrestrial weathering has caused fracturing and precipitation of Carich carbonates and sulfates in the fractures, but lunar minerals appear fresh and unoxidized. The meteorite is composed of two distinct lithologies: a two‐pyroxene olivine gabbro with cumulate texture, and a polymict, fragmental regolith breccia. The olivine gabbro is dominated by cumulate olivine with pigeonite, augite, and interstitial plagioclase feldspar. The breccia consists of several types of clasts but is dominated by clasts from the gabbro and more FeO‐rich derivatives. Variations in clast mineral assemblage and pyroxene Mg/(Mg + Fe) and Ti/(Ti + Cr) record an igneous Fe‐enrichment trend that culminated in crystallization of fayalite + silica + hedenbergite‐bearing symplectites. The Fe‐enrichment trend and cumulate textures observed in NWA 773 are similar to features of terrestrial ponded lava flows and shallow‐level mafic intrusives, indicating that NWA 773 may be from a layered mafic intrusion or a thick, differentiated lava flow. NWA 773 and several other mafic lunar meteorites have LREE‐enriched patters distinct from Apollo and Luna mare basalts, which tend to be LREE‐depleted. This is somewhat surprising in light of remote sensing data that indicates that the Apollo and Luna missions sampled a portion of the moon that was enriched in incompatible heatproducing elements.  相似文献   

12.
Abstract— We report on the bulk composition and petrography of four new basaltic meteorites found in Antarctica—LAP (LaPaz Icefield) 02205, LAP 02224, LAP 02226, and LAP 02436—and compare the LAP meteorites to other lunar mare basalts. The LAP meteorites are coarse‐grained (up to 1.5 mm), subophitic low‐Ti basalts composed predominantly of pyroxene and plagioclase, with minor amounts of olivine, ilmenite, and a groundmass dominated by fayalite and cristobalite. All of our observations and results support the hypothesis that the LAP stones are mutually paired with each other. In detail, the geochemistry of LAP is unlike those of any previously studied lunar basalt except lunar meteorite NWA (Northwest Africa) 032. The similarities between LAP and NWA 032 are so strong that the two meteorites are almost certainly source crater paired and could be two different samples of a single basalt flow. Petrogenetic modeling suggests that the parent melt of LAP (and NWA 032) is generally similar to Apollo 15 low‐Ti, yellow picritic glass beads, and that the source region for LAP comes from a similar region of the lunar mantle as previously analyzed lunar basalts.  相似文献   

13.
Abstract— We report on a new angrite, Northwest Africa (NWA) 1296, a fine‐grained rock with a magmatic texture of rapid cooling. Dendritic olivine (?Fo50) crystallized first in association with anorthite microcrysts (An98–100) forming composite chains separated from one another by intergrown Al‐Fe diopside‐hedenbergite pyroxenes. In addition, some olivines with lower Mg# and increased CaO (up to 12%) are found between the chains as equant microphenocrysts. Pyroxenes and olivines are both normally zoned from Mg# = 0.52 to less than 0.01 in the rims. Ca‐rich olivines are surrounded by, intergrown with, or replaced by subcalcic kirschsteinite. They appear after plagioclase crystallization stopped, at the end of the crystallization sequence. Minor phases are pyrrhotite, F‐apatite, and titanomagnetite. Pyroxene is the last silicate phase to grow, interstitial to idiomorphic olivine‐kirschsteinite. Numerous small vesicles and some channels are filled with microcristalline carbonate. The mode (vol%) is about 28% olivine, 3% kirschsteinite, 32% anorthite, 34% pyroxene, and 3% of the minor phases—close to that reported previously for D'Orbigny and Sahara (SAH) 99555. The bulk chemical composition of NWA 1296 is similar to D'Orbigny and SAH 99555; NWA 1296 differs by its texture and mineralogy, which are interpreted as resulting from rapid crystallization—an evidence of impact melting. Angrites cannot be produced by partial melting of a CV source because segregation of a “planetary” core is necessary to explain the low FeO/MgO ratio of magnesian olivines. Neither the odd Ca/Al ratio nor the very low SiO2 content can be explained by conventional partial melting scenarios. We suggest that carbonate is the key to angrite genesis. This is supported by the striking similarities with terrestrial melilitites (low SiO2, superchondritic Ca/Al ratio, presence of carbonate). The lack of alkalies could be the result of either loss after impact melting or absence of alkalies in the source.  相似文献   

14.
Major, minor and trace element abundances have been determined by instrumental neutron activation analysis (INAA) in whole rock and plagioclase separates of Serra de Magé (SdM). The whole rock contains 52% normative plagioclase and its chondritic normalized REE abundance pattern shows a large Eu anomaly, dominated by the plagioclase REE distribution, and nearly unfractionated La-Sm and Sm-Lu abundances. The plagioclase separates contained ～ 6% pyroxenes and exhibited a typical plagioclase REE distribution. The REE abundances in the derivative equilibrium magmas from which SdM and Moore County (MC) plagioclases crystallized have been estimated from the plagioclase data and the plagioclase mineral/liquid partition coefficients. The REE distributions in possibly earlier parental magmas were calculated by assuming that various degrees of plagioclase and pigeonite (plagioclase/pigeonite = 1) fractional crystallization had been operative prior to the crystallization of SdM and MC. The calculated La/Sm and Sm/Yb ratios for the earlier magmas are essentially the same as the equilibrium magmas over a wide range (10–95%) of the assumed fractional crystallization. Considering the REE distributions and the Fe/Fe+Mg ratios, calculation shows that there is no simple genetic relationship between MC and SdM via fractional crystallization processes. A hypothesis for the derivation of these cumulate eucrites in the plutonic environment from residual diogenitic liquid, which was produced by the extensive partial melting of an eucritic source material followed by the crystallization of diogenite, also fails to account for the fractionated REE patterns calculated for the equilibrium and the possible parental magmas for either SdM or MC. Equilibrium non-modal partial melting calculations indicate that SdM and MC could be genetically related by a factor ～ 6 difference in the degrees of partial melting from a similar source material. However, this common source material which should contain > 30% high-Ca clinopyroxene and has a chondritic normalized La/Yb ～ 3, is different than that proposed for the non-cumulate eucrites.  相似文献   

15.
Abstract— Northwest Africa (NWA) 428 is an L chondrite that was successively thermally metamorphosed to petrologic type‐6, shocked to stage S4–S5, brecciated, and annealed to approximately petrologic type‐4. Its thermal and shock history resembles that of the previously studied LL6 chondrite, Miller Range (MIL) 99301, which formed on a different asteroid. The petrologic type‐6 classification of NWA 428 is based on its highly recrystallized texture, coarse metal (150 ± 150 μm), troilite (100 ± 170 μm), and plagioclase (20–60 μm) grains, and relatively homogeneous olivine (Fa24.4 ± 0.6), low‐Ca pyroxene (Fs20.5 ± 0.4), and plagioclase (Ab84.2 ± 0.4) compositions. The petrographic criteria that indicate shock stage S4–S5 include the presence of chromite veinlets, chromite‐plagioclase assemblages, numerous occurrences of metallic Cu, irregular troilite grains within metallic Fe‐Ni, polycrystalline troilite, duplex plessite, metal and troilite veins, large troilite nodules, and low‐Ca clinopyroxene with polysynthetic twins. If the rock had been shocked before thermal metamorphism, low‐Ca clinopyroxene produced by the shock event would have transformed into orthopyroxene. Post‐shock brecciation is indicated by the presence of recrystallized clasts and highly shocked clasts that form sharp boundaries with the host. Post‐shock annealing is indicated by the sharp optical extinction of the olivine grains; during annealing, the damaged olivine crystal lattices healed. If temperatures exceeded those approximating petrologic type‐4 (?600–700°C) during annealing, the low‐Ca clinopyroxene would have transformed into orthopyroxene. The other shock indicators, likewise, survived the mild annealing. An impact event is the most plausible source of post‐metamorphic, post‐shock annealing because any 26Al that may have been present when the asteroid accreted would have decayed away by the time NWA 428 was annealed. The similar inferred histories of NWA 428 (L6) and MIL 99301 (LL6) indicate that impact heating affected more than 1 ordinary chondrite parent body.  相似文献   

16.
Knowledge of Martian igneous and mantle compositions is crucial for understanding Mars' mantle evolution, including early differentiation, mantle convection, and the chemical alteration at the surface. Primitive magmas provide the most direct information about their mantle source regions, but most Martian meteorites either contain cumulate olivine or crystallized from fractionated melts. The new Martian meteorite Northwest Africa (NWA) 6234 is an olivine‐phyric shergottite. Its most magnesian olivine cores (Fo78) are in Mg‐Fe equilibrium with a magma of the bulk rock composition, suggesting that it represents a melt composition. Thermochemical calculations show that NWA 6234 not only represents a melt composition but is a primitive melt derived from an approximately Fo80 mantle. Thus, NWA 6234 is similar to NWA 5789 and Y 980459 in the sense that all three are olivine‐phyric shergottites and represent primitive magma compositions. However, NWA 6234 is of special significance because it represents the first olivine‐phyric shergottite from a primitive ferroan magma. On the basis of Al/Ti ratio of pyroxenes in NWA 6234, the minor components in olivine and merrillite, and phosphorus zoning of olivine, we infer that the rock crystallized completely at pressures consistent with conditions in Mars' upper crust. The textural intergrowths of the two phosphates (merrillite and apatite) indicate that at a very last stage of crystallization, merrillite reacted with an OH‐Cl‐F‐rich melt to form apatite. As this meteorite crystallized completely at depth and never erupted, it is likely that its apatite compositions represent snapshots of the volatile ratios of the source region without being affected by degassing processes, which contain high OH‐F content.  相似文献   

17.
Abstract— Mineralogy, major element compositions of minerals, and elemental and oxygen isotopic compositions of the whole rock attest to a lunar origin of the meteorite Northwest Africa (NWA) 032, an unbrecciated basalt found in October 1999. The rock consists predominantly of olivine, pyroxene and chromite phenocrysts, set in a crystalline groundmass of feldspar, pyroxene, ilmenite, troilite and trace metal. Whole‐rock shock veins comprise a minor, but ubiquitous portion of the rock. Undulatory to mosaic extinction in olivine and pyroxene phenocrysts and micro‐faults in groundmass and phenocrysts also are attributed to shock. Several geochemical signatures taken together indicate unambiguously that NWA 032 originated from the Moon. The most diagnostic criteria include whole‐rock oxygen isotopic composition and ratios of Fe/Mn in the whole rock, olivine, and pyroxene. A lunar origin is documented further by the presence of Fe‐metal, troilite, and ilmenite; zoning to extremely Fe‐rich compositions in pyroxene; the ferrous oxidation state of all Fe in pyroxene; and the rare earth element (REE) pattern with a well‐defined negative europium anomaly. This rock is similar in major element chemistry to basalts from Apollo 12 and 15, but is enriched in light REE and has an unusually high Th/Sm ratio. Some Apollo 14 basalts yield a closer match to NWA 032 in REE patterns, but have higher concentrations of Al2O3. Ar‐Ar step release results are complex, but yield a whole‐rock age of ?2.8 Ga, suggesting that NWA 032 was extruded at 2.8 Ga or earlier. This rock may be the youngest sample of mare basalt collected to date. Noble gas concentrations combined with previously collected radionuclide data indicate that the meteorite exposure history is distinct from currently recognized lunar meteorites. In short, the geochemical and petrographic features of NWA 032 are not matched by Apollo or Luna samples, nor by previously identified lunar meteorites, indicating that it originates from a previously unsampled mare deposit. Detailed assessment of petrographic features, olivine zoning, and thermodynamic modelling indicate a relatively simple cooling and crystallization history for NWA 032. Chromite‐spinel, olivine, and pyroxene crystallized as phenocrysts while the magma cooled no faster than 2 °C/h based on the polyhedral morphology of olivine. Comparison of olivine size with crystal growth rates and preserved Fe‐Mg diffusion profiles in olivine phenocrysts suggest that olivine was immersed in the melt for no more than 40 days. Plumose textures in groundmass pyroxene, feldspar, and ilmenite, and Fe‐rich rims on the phenocrysts formed during rapid crystallization (cooling rates ?20 to 60 °C/h) after eruption.  相似文献   

18.
Abstract— We report on the discovery of a new shergottite from South Morocco. This single stone weighing 320 g is referenced as Northwest Africa (NWA) 856 with Djel Ibone as a synonymous name. It is a fresh, fine‐grained basaltic rock consisting mainly of two pyroxenes (total ?68 vol%: 45% pigeonite, En61‐16Wo9–22Fs26–68; 23% augite, En46‐26Wo34‐29Fs21–43) and plagioclase converted to maskelynite (?23 vol%, Ab43–57Or1–5An54‐36). Accessory minerals include merrillite, Cl‐apatite, pyrrhotite, ilmenite, ulvöspinel, silica (stishovite and glass), amorphous K‐feldspar and baddeleyite. Amorphous mixtures of maskelynite and silica occur most commonly as median layers inside maskelynite laths. In addition, melt pockets (?2 vol%) were recognized with relics of maskelynite, pyroxene and both dense silica glass and stishovite occurring as both grains and submicrometer needles. The compositions of the melt pockets are consistent with mixtures of maskelynite and pyroxenes with an average of ?50 vol% maskelynite. The meteorite is highly fractured at all scales. The bulk composition of NWA 856 has been measured for 44 elements. It is an Al‐poor ferroan basaltic rock which strongly resembles Shergotty and Zagami in its major and trace element composition. The nearly flat rare earth element (REE) pattern (La/Lu)n = 0.9, is similar to that of Shergotty or Zagami and differs significantly from NWA 480, another Moroccan shergottite recently described. According to the U, Ba and Sr abundances, NWA 856 is not significantly weathered. The oxygen isotopes (δ18O = +5.03%, δ17O = +3.09%, and Δ17O = +0.47%) are in agreement with the martian origin of this meteorite. On the basis of grain size, pyroxene zoning and composition, abundance of silica inclusions associated with maskelynite, trace element abundances, REE pattern and oxygen isotopes, pairing with NWA 480 is excluded. The similarity with Shergotty and Zagami is striking. The only significant differences are a larger grain size, a greater abundance of silica and melt pockets, a slightly more restricted range of pyroxene compositions and the absence of significant mesostasis.  相似文献   

19.
We report the results of a detailed study of the basaltic eucrite Northwest Africa (NWA) 7188, including its mineralogical and bulk geochemical characteristics, oxygen isotopic composition, and 147,146Sm‐143,142Nd mineral isochron ages. The texture and chemical composition of pyroxene and plagioclase demonstrate that NWA 7188 is a monomict eucrite with a metamorphic grade of type 4. The oxygen isotopic composition and the Fe/Mn ratios of pyroxene confirmed that NWA 7188 belongs to the howardite–eucrite–diogenite meteorite suite, generally considered to originate from asteroid 4 Vesta. Whole‐rock TiO2, La, and Hf concentrations and a CI chondrite‐normalized rare earth element pattern are in good agreement with those of representative Stannern‐group eucrites. The 147,146Sm‐143,142Nd isochrons for NWA 7188 yielded ages of 4582 ± 190 and 4554 +17/?19 Ma, respectively. The closure temperature of the Sm‐Nd system for different fractions of NWA 7188 was estimated to be >865 °C, suggesting that the Sm‐Nd decay system has either been resistant to reheating at ~800 °C during the global metamorphism or only partially reset. Therefore, the 146Sm‐142Nd age of NWA 7188 corresponds to the period of initial crystallization of basaltic magmas and/or global metamorphism on the parent body, and is unlikely to reflect Sm‐Nd disturbance by late reheating and impact events. In either case, NWA 7188 is a rare Stannern‐group eucrite that preserves the chronological information regarding the initial crustal evolution of Vesta.  相似文献   

20.
Northwest Africa (NWA) 8657 is an incompatible trace element-enriched, low-Al basaltic shergottite, similar in texture and chemistry to Shergotty, Zagami, and NWA 5298. It is composed of zoned pyroxene, maskelynite, merrillite, and Ti-oxide minerals with minor apatite, silica, and pyrrhotite. Pyroxene grains are characterized by patchy zoning, with pigeonite or augite cores zoned to Fe-rich pigeonite mantles. The cores have rounded morphologies and irregular margins. Combined with the low Ti/Al of the cores, the morphology and chemistry of the pyroxene grains are consistent with initial crystallization at depth (30–70 km) followed by partial resorption en route to the surface. Enriched rare earth element (REE) equilibrium melt compositions and calculated oxygen fugacities (fO2) conditions for pigeonite cores indicate that the original parent melts were enriched shergottite magmas that staged in chambers at depth within the Martian crust. NWA 8657 does not represent a liquid but rather entrained a proportion of pyroxene crystals from magma chambers where fractional crystallization was occurring at depth. Variation between fO2 and bulk-rock (La/Yb)N of the enriched and intermediate shergottites suggests that oxidation conditions and degree of incompatible element enrichment in the source may not be correlated, as thought previously. Shock melt pockets are characterized by an absence of phosphates and oxide minerals. It is likely that these phases were melted during shock. REEs were redistributed during this process into maskelynite and to a lesser extent the shock melt; however, the overall normalized REE profile of the shock melt is like that of the bulk-rock, but at lower absolute concentrations. Overall, shock melting has had a significant effect on the mineralogy of NWA 8657, especially the distribution of phosphates, which may be significant for geochronological applications of this meteorite and other Martian meteorites with extensive shock melt.  相似文献   

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