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1.
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   

2.
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   

3.
Shergottites have provided abundant information on the volcanic and impact history of Mars. Northwest Africa (NWA) 14672 contributes to both of these aspects. It is a vesicular ophitic depleted olivine–phyric shergottite, with average plagioclase An61Ab39Or0.2. It is highly ferroan, with pigeonite compositions En49-25Fs41-61Wo10-14 like those of basaltic shergottites, for example, NWA 12335. Olivine (Fo53-15) has discrete ferroan overgrowths, more ferroan when in contact with plagioclase than when enclosed by pyroxene. The pyroxene (a continuum of augite, subcalcic augite, and pigeonite) is patchy, with ragged “cores” enveloped or invaded by ferroan pyroxene. Magma mixing may be responsible for capture of olivine and formation of pyroxene mantles. The plagioclase is maskelynite-like in appearance, but the original laths were (congruently) melted and the melt partly crystallized as fine dendrites. Most of the 14% vesicles occur within plagioclase. Olivine, pyroxene, and ilmenite occur in part as fine aggregates crystallized after congruent melting with limited subsequent liquid mixing. There are two fine-grained melt components, barred plagioclase with interstitial Fe-bearing phases, and glass with olivine dendrites, derived by melting of mainly plagioclase and mainly pyroxene, respectively. Rare silica particles contain coesite and/or quartz, and silica glass. The rock has experienced >50% melting, compatible with peak pressure >~65 GPa. It is the most highly shocked shergottite so far, at shock stage S6/7. It may belong to the group of depleted shergottites ejected at ~1 Myr from Tooting Crater.  相似文献   

4.
The Loop meteorite was found in 1962 in Gaines County, Texas, at a location very close to that where the Ashmore chondrite was found in 1969. The two specimens were assumed to be fragments of the same meteorite. The Loop meteorite is a type L6 chondrite composed of olivine (Fo75.4Fa24.6), orthopyroxene (En77.6Wo1.5Fs20.9), clinopyroxene (En47.5Wo45.1Fs7.4), plagioclase (Ab84.3Or5.5An10.2), Fe-Ni metal, troilite, and chromite. Fe-Ni metal is represented by kamacite (5.8-6.4 wt % Ni, 0.88-1.00 wt % Co), taenite (30.0–52.9 wt % Ni, 0.16-0.34 wt % Co), and plessite (16.8–28.5 wt % Ni, 0.38-0.54 wt % Co). Native copper occurs as rare inclusions in Fe-Ni metal. Both chondrules and matrix have similar mineral compositions. The mineral chemistry of the Loop meteorite is quite different from that of the Ashmore, which was classified as an H5 chondrite by Bryan and Kullerud (1975). Therefore, the Ashmore and Loop meteorites are two different chondrites, even though they were recovered from the same geographic location.  相似文献   

5.
Abstract— Sayhal Uhaymir (SaU) 094 is a 223.3 g, partially crusted, strongly to very strongly shocked melanocratic olivine-porphyric rock of the shergottite group showing a microgabbroic texture. The rock consists of pyroxene (52.0–58.2 vol%)—dominantly prismatic pigeonite (En60–68Fs20–27Wo7–9) associated with minor augite (En46–49Fs15–16Wo28–31)—brown (shock-oxidized) olivine (Fo65–69; 22.1–31%), completely isotropic interstitial plagioclase glass (maskelynite; An50–64Or0.3-0.9; 8.6–13.0%), chromite and titanian magnesian chromite (0.9-1.0%), traces of ilmenite (Ilm80–86), pyrrhotite (Fe92–100; 0.1-0.2%), merrillite (<<0.1%), and pockets (4.8-6.7%) consisting of green basaltic to basaltic andesitic shock glass that is partially devitrified into a brown to black product along boundaries with the primary minerals. The average maximum dimensions of minerals are: olivine (1.5 mm), pyroxene (0.3 mm) and maskelynite (0.3 mm). Primary melt inclusions in olivine and chromite are common and account for 0.1-0.6% of the rock. X-ray tomography revealed that the specimen contains ˜0.4 vol% of shock-melt associated vesicles, up to 3 mm in size, which show a preferred orientation. Fluidization of the maskelynite, melting and recrystallization of pyroxene, olivine and pyrrhotite indicate shock stage S6. Minor terrestrial weathering resulted in calcite-veining and minor oxidation of sulfides. The meteorite is interpreted as paired with SaU 005/008/051. The modal composition is similar to Dar al Gani 476/489/670/735/876, with the exception that neither mesostasis nor titanomagnetite nor apatite are present and that all phases show little zonation. The restricted mineral composition, predominance of chromite among the oxides, and abundance of olivine indicate affinities to the lherzolitic shergottites.  相似文献   

6.
Grove Mountains (GRV) 020090 is a “lherzolitic” shergottite found in the Grove Mountains, Antarctica. It exhibits two distinct textures: poikilitic and nonpoikilitic. In poikilitic areas, large pyroxene oikocrysts enclose subhedral olivine and chromite chadacrysts. Pyroxene oikocrysts are zoned from pigeonite cores to augite rims. In nonpoikilitic areas, olivine, pyroxene, and interstitial maskelynite occur as major phases, and minor phases include chromite and merrillite. Compared with typical “lherzolitic” shergottites, GRV 020090 contains a distinctly higher abundance of maskelynite (19 vol%). Olivine and pyroxene are more ferroan (Fa28–40, En57–72Fs24–31Wo4–14 and En46–53Fs17–21Wo26–35), and maskelynite is more alkali‐rich (Ab43–65Or2–7). The major phases, whole‐rock (estimated) and fusion crust of GRV 020090, are relatively enriched in light rare earth elements (LREE), similar to those of the geochemically enriched basaltic shergottites, but distinct from those of LREE‐depleted “lherzolitic” shergottites. Combined with a high oxygen fugacity of log fO2 = QFM ? 1.41 ± 0.04 (relative to the quartz‐fayalite‐magnetite buffer), it is clear that GRV 020090 sampled from an oxidized and enriched mantle reservoir similar to those of other enriched shergottites. The calculated REE abundances and patterns of the melts in equilibrium with the cores of major phases are parallel to but higher than that of the whole rock, suggesting that GRV 020090 originated from a single parent magma and experienced progressive fractional crystallization in a closed system. The crystallization age recorded by baddeleyite is 192 ± 10 (2σ) Ma, consistent with the young internal isochron ages of enriched shergottites. Baddeleyite dating results further demonstrated that the young ages, rather than ancient ages (>4 Ga), appear to represent the crystallization of Martian surface lava flow. GRV 020090 shares many similarities with Roberts Massif (RBT) 04261/2, the first enriched “lherzolitic” shergottite. Detailed comparisons suggest that these two rocks are petrologically and geochemically closely related, and probably launch paired.  相似文献   

7.
The only two Nakhlite meteorites, Nakhla and Lafayette, are identical in mineral composition, consisting of augite (Wo39En38Fs23), olivine (Fo32–35), plagioclase (An27), K-feldspar (Or75Ab22An3), titaniferous magnetite with exsolved ilmenite, iddingsite (?), and minor amounts of fluor-chlorapatite, FeS, pyrite, chalcopyrite, and K-rich glass. The texture is suggestive of a cumulative origin.  相似文献   

8.
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   

9.
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   

10.
The unusual achondrite Shergotty resembles terrestrial diabases, and textural and chemical evidence indicates pre-settling and post-settling crystallization of zoned augite (En48Fs19Wo33-En25Fs47Wo28) and pigeonite (En61Fs26Wo13-En21Fs61Wo18) coupled with late crystallization of plagioclase (Ab43An56/Or1-Ab56An41Or3: now shocked to maskelynite), titanomagnetite-ilmenite composite grains, mesostasis (normative Qz34Ab21An5Or38Fs2, assuming Fe as ferrous), whitlockite, pyrrhotite (Fe0.94S), fayalite (Fo10), baddeleyite and chlorapatite. The oxide compositions (Usp62Mt38, Al2O3 2.4, Cr2O3 0.8 wt %; Ilm95Hm5) indicate ～ 850 °C and log oxygen fugacity ? 14, while the occurrence of fayalite rims on mesostasis next to ilmenite indicates 890 °C. Bearing in mind experimental uncertainties, these data are consistent with late-stage crystallization under relatively high oxygen fugacity, as indicated by coexistence of fayalite, Ti-magnetite and a silica glass. The high alkali content of the maskelynite and mesostasis, coupled with the redox state, indicates that the Shergotty meteorite resembles terrestrial basalts more than any other meteorites. Nevertheless the absence of H2O, as shown by the occurrence of phosphorus in whitlockite rather than in hydroxylapatite, distinguish the Shergotty achondrite from typical terrestrial diabases. Whereas the FeO/MnO ratios of pyroxenes from the Moon, Earth and several differentiated meteorites are independent of FeO, the ratio for Shergotty pyroxenes changes from 30 to 40 with increasing FeO, and the linear trend extrapolates to 0.2 MnO for zero iron. Hence caution is needed in using FeO/MnO as a planetary indicator. For pyroxenes, Na is almost independent of Fe/Mg while Ti increases and Cr decreases with increasing Fe/Mg. Maskelynite contains 0.5–0.25 wt % K2O, 0.6 wt % FeO, 0.04 TiO2, 0.04–0.07 MgO, ～ 0.01 BaO and 0.02–0.03 P2O5. A bulk analysis calculated from the mode and compositions of the minerals matches quite well with two bulk chemical analyses but not with a third.  相似文献   

11.
We report on the discovery of a new shergottite from Tunisia, Ksar Ghilane (KG) 002. This single stone, weighing 538 g, is a coarse‐grained basaltic shergottite, mainly composed of maskelynitized plagioclase (approximately 52 vol%) and pyroxene (approximately 37 vol%). It also contains Fe‐rich olivine (approximately 4.5 vol%), large Ca‐phosphates, including both merrillites and Cl‐apatites (approximately 3.4 vol%), minor amounts of silica or SiO2‐normative K‐rich glass, pyrrhotite, Ti‐magnetite, ilmenite, and accessory baddeleyite. The largest crystals of pyroxene and plagioclase reach sizes of approximately 4 to 5 mm. Pyroxenes (Fs26–96En5–50Wo2–41). They typically range from cores of about Fs29En41Wo30 to rims of about Fs68En14Wo17. Maskelynite is Ab41–49An39–58Or1–7 in composition, but some can be as anorthitic as An93. Olivine (Fa91–96) occurs mainly within symplectitic intergrowths, in paragenesis with ilmenite, or at neighboring areas of symplectites. KG 002 is heavily shocked (S5) as indicated by mosaic extinction of pyroxenes, maskelynitized plagioclase, the occurrence of localized shock melt glass pockets, and low radiogenic He concentration. Oxygen isotopes confirm that it is a normal member of the SNC suite. KG 002 is slightly depleted in LREE and shows a positive Eu anomaly, providing evidence for complex magma genesis and mantle processes on Mars. Noble gases with a composition thought to be characteristic for Martian interior is a dominant component. Measurements of 10Be, 26Al, and 53Mn and comparison with Monte Carlo calculations of production rates indicate that KG 002 has been exposed to cosmic rays most likely as a single meteoroid body of 35–65 cm radius. KG 002 strongly resembles Los Angeles and NWA 2800 basaltic shergottites in element composition, petrography, and mineral chemistry, suggesting a possible launch‐pairing. The similar CRE ages of KG 002 and Los Angeles may suggest an ejection event at approximately 3.0 Ma.  相似文献   

12.
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   

13.
Northwest Africa (NWA) 10414 is an unusual shergottite with a cumulate texture. It contains 73% coarse prismatic pigeonite, plus 18% interstitial maskelynite, 2% Si‐rich mesostasis, 2% merrillite, and minor chromite‐ulvöspinel. It contains no olivine, and only ~3% augite. Phase compositions are pigeonite (En68‐43Fs27‐48Wo5‐15) and maskelynite An~54‐36, more sodic than most maskelynite in shergottites. Chromite‐ulvöspinel composition plots between the earliest and most fractionated spinel‐group minerals in olivine‐phyric shergottites. NWA 10414 mineralogically resembles the contact facies between Elephant Moraine 79001 lithologic units A and B, with abundant pigeonite phenocrysts, though it is coarser grained. Its most Mg‐rich pigeonite also has a similar composition to the earliest crystallized pyroxenes in several other shergottites, including Shergotty. The Shergotty intercumulus liquid composition crystallizes pigeonite with a similar composition range to NWA 10414 pigeonite, using PETROLOG. Olivine‐phyric shergottite NWA 6234, with a pure magma composition, produces an even better match to this pigeonite composition range, after olivine crystallization. These observations suggest that after the accumulation of olivine from an olivine‐phyric shergottite magma, the daughter liquid could precipitate pigeonite locally to form this pigeonite cumulate, before the crystallization of overlying liquid as a normal basaltic shergottite.  相似文献   

14.
Abstract— Based on optical microscopy and electron microprobe analysis, Linum is classified as an L6b chondrite that contains olivine (Fa24), orthopyroxene (Fs20), clinopyroxene (Wo45En47Fs8), plagioclase (An10Ab84Or6), nickel-iron, troilite, chromite and accessory amounts of chlorapatite and whitlockite.  相似文献   

15.
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   

16.
Abstract— Dar al Gani 489 (DaG 489) is a meteorite fragment of 2146 g found in the Libyan Sahara by a meteorite finder during one of his search campaigns in 1997–98. It is a porphyritic rock with millimetersized olivine crystals (Fo79–59) set in a fine‐grained groundmass (average grain size 0.1 mm) consisting of pigeonite (En75–57 Wo5–15) crystals and interstitial feldspathic glass (An67–56 Or0–1). Minor phases include enstatite (En82–71 Wo2–4), augite (En48–52 Wo29–32), chromite, Ti‐chromite, ilmenite, pyrrhotite, merrillite, and secondary calcite and iron oxides. On the basis of mineralogical, petrographic, bulk chemical, O‐isotopic, and noble gas data, DaG 489 can be classified as a highly shocked martian meteorite (e.g., Fe/Mn(bulk) = 42.1, Ni/Mg(bulk) = 0.002; δ17O = 2.89, δ18O = 4.98, and Δ17O = 0.305), belonging to the basaltic shergottite subgroup. The texture and modal composition of DaG 489 are indeed those of basalts; nonetheless, the bulk chemistry, the abundance of large olivine and chromite crystals, and enstatitic pyroxene suggest some relationship with lherzolitic shergottites. As such, DaG 489 is similar to the hybrid shergottite Elephant Moraine (EET) A79001 lithology A; however, there are some relevant differences including a higher olivine content (20 vol%), the lack of orthopyroxene megacrysts, a higher molar Mg/(Mg + Fe)(molar) = 0.68, and a lower rare earth element content in the bulk sample. Therefore, DaG 489 has the potential of providing us with a further petrogenetic link between the basaltic and lherzolitic shergottites. Noble gases data show that DaG 489 has an ejection age of ～1.3 Ma. This young age lends support to the requirement of several ejection events to produce the current population of shergottites, nakhlites, and chassignites (SNC) meteorites. In terms of texture, mineral and bulk compositions, shock level, and weathering features, DaG 489 is essentially identical to DaG 476, another basaltic shergottite independently found ～25 km due northnortheast of DaG 489. Because DaG 489 also has the same exposure history as DaG 476, it is very likely that both meteorites are fragments of the same fall. In addition to the existing hypotheses on the petrogenesis of the similar EETA79001 lithology A and the identical DaG 476, we propose that DaG 489 could have formed through high‐degree partial melting of a lherzolite‐like material.  相似文献   

17.
The Beaver-Harrison, Utah chondrite (find July 24, 1979), a single, shock-veined stone of 925 grams, consists of major olivine (Fa25.0), low-Ca pyroxene (En77.3Fs21.1Wo1.6) and metallic nickel-iron; minor troilite and plagioclase (Ab82.6An11.1Or6.3), accessory high-Ca pyroxene (En47.0Fs8.5Wo44.5), chromite (Cm8.7Sp10.6Uv9.4Pc0.6Hc0.7), chlorapatite and whitlockite; and hydrous ferric oxide of terrestrial weathering origin. Mineral compositions indicate L-group classification, and homogeneity of minerals, highly recrystallized texture and presence of clear plagioclase suggest that the meteorite belongs to petrologic type 6.  相似文献   

18.
Abstract– The Grove Mountains (GRV) 021663 meteorite was collected from the Grove Mountains region of Antarctica. The meteorite is composed primarily of olivine (Fa5.4), orthopyroxene (Fs4.7Wo3.0), chromian diopside (En53.6Fs2.4Wo44), troilite, kamacite, and plagioclase (Ab74.5Or4An21.5). Minor phases include schreibersite and K‐feldspar. The meteorite is highly weathered (W3) and weakly shocked (S2). We determine a whole rock oxygen isotopic composition of δ18O = 7.50‰, δ17O = 3.52‰. Comparisons of these data with other primitive achondrites have resulted in the reclassification of this meteorite as a member of the winonaite group. The occurrences of troilite, metal, and schreibersite in GRV 021663 indicate that these minerals were once completely molten. Euhedral inclusions of pyroxene within plagioclase further suggest that these may have crystallized from a silicate melt, while the depletion of plagioclase, metal, and troilite indicates that GRV 021663 could represent a residuum following partial melting on its parent asteroid. Trace element distributions in silicate minerals do not, however, confirm this scenario. As with other winonaite meteorites, the formation of GRV 021663 probably relates to brecciation and mixing of heterogeneous lithologies, followed by varying degrees of thermal metamorphism on the parent body asteroid. Peak metamorphic conditions may have resulted in localized partial melting of metal and silicate mineralogies, but our data are not conclusive.  相似文献   

19.
Abstract— The Yaringie Hill meteorite is a new H5 ordinary chondrite found in the Gawler Ranges, South Australia. The meteorite, which shows only minor signs of terrestrial weathering, is predominantly composed of olivine (Fa17.2), orthopyroxene (Fs15.1Wo1.1), and three distinct phases of nickeliferous iron metal (kamacite, taenite, tetrataenite). Other minerals include troilite, plagioclase (Ab81An16Or3), clinopyroxene (En52Wo42Fs6), chlorapatite, merrillite, ilmenite, and native copper. Three types of spinel with distinctive textures (coarse, skeletal aggregates, rounded aggregates) and with compositions close to the join MgAl2O4‐FeCr2O4 are also present. Chondrules within the Yaringie Hill meteorite, which often have poorly defined boundaries, are placed in a recrystallized matrix. Shock indicators suggest that the meteorite experienced only weak shock metamorphism (S3).  相似文献   

20.
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   

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