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1.
Abstract— We report here on an ion probe study of rare earth element (REE) geochemistry in the lherzolitic shergottite Grove Mountains (GRV) 99027. This meteorite shows almost identical mineralogy, petrology, and REE geochemistry to those of the lherzolitic shergottites Allan Hills (ALH) A77005, Lewis Cliff (LEW) 88516, and Yamato (Y‐) 793605. REE concentrations in olivine, pyroxenes, maskelynite, merrillite, and melt glass are basically comparable to previous data obtained from ALH A77005, LEW 88516, and Y‐793605. Olivine is the dominant phase in this meteorite. It is commonly enclosed by large (up to several mm) pigeonite oikocrysts. Non‐poikilitic areas consist of larger olivine grains (?mm), pigeonite, augite, and maskelynite. Minor merrillite (up to 150 μm in size) is widespread in non‐poikilitic regions, occurring interstitially between olivine and pyroxene grains. It is the main REE carrier in GRV 99027 and has relatively higher REEs (200–1000 × CI) than that of other lherzolitic shergottites. A REE budget calculation for GRV 99027 yields a whole rock REE pattern very similar to that of other lherzolites. It is characterized by the distinctive light REE depletion and a smooth increase from light REEs to heavy REEs. REE microdistributions in GRV 99027 strongly support the idea that all lherzolitic shergottites formed by identical igneous processes, probably from the same magma chamber on Mars. Despite many similarities in mineralogy, petrography, and trace element geochemistry, subtle differences exist between GRV 99027 and other lherzolitic shergottites. GRV 99027 has relatively uniform mineral compositions (both major elements and REEs), implying that it suffered a higher degree of sub‐solidus equilibration than the other three lherzolites. It is notable that GRV 99027 has experienced terrestrial weathering in the Antarctic environment, as its olivine and pyroxenes commonly display a light REE enrichment and a negative Ce anomaly. Caution needs to be taken in future chronological studies.  相似文献   

2.
Northwest Africa (NWA) 7755 is a newly found enriched lherzolitic shergottite. Here, we report its detailed petrography and mineralogy. NWA 7755 contains both poikilitic and non‐poikilitic lithologies. Olivine has different compositional ranges in the poikilitic and non‐poikilitic lithologies, Fa30–39 and Fa37–40, respectively. Pyroxene in the non‐poikilitic lithology is systematically Fe‐richer than that in the poikilitic lithology. The chromite grains in non‐poikilitic lithology are highly Ti‐richer than those in the poikilitic lithology. The chemical variations of olivine, pyroxene, and chromite between the poikilitic and non‐poikilitic lithologies support a two‐stage formation model of lherzolitic shergottites. Besides planar fractures and strong mosaicism in olivine and pyroxene, shock‐induced melt veins and pockets are observed in NWA 7755. Olivine grains within and adjacent to melt veins and/or pockets have either transformed to ringwoodite, amorphous phase, or dissociated to bridgmanite plus magnesiowüstite. Merrillite in melt veins has completely transformed to tuite; however, apatite only has partially transformed to tuite, indicating a relatively sluggish transformation rate. The partial transformation from apatite to tuite resulted in fractional devolatilization of Cl and F in apatite. The fine‐grained mineral assemblage in melt veins consists mainly of bridgmanite, minor magnesiowüstite, Fe‐sulfide, Fe‐phosphide, and Ca‐phosphate minerals. The coexistence of bridgmanite and magnesiowüstite in these veins indicates a shock pressure of >~24 GPa and a temperature of 1800–2000 °C. Coesite and seifertite are probably present in NWA 7755. The presence of these high‐pressure minerals indicates that NWA 7755 has experienced a more intense shock metamorphism than other enriched lherzolitic shergottites.  相似文献   

3.
Abstract– Rb‐Sr and Sm‐Nd isotopic analyses of the lherzolitic shergottite Grove Mountains (GRV) 99027 are reported. GRV 99027 yields a Rb‐Sr mineral isochron age of 177 ± 5 (2σ) Ma and an initial 87Sr/86Sr ratio (ISr) of 0.710364 ± 11 (2σ). Due to larger uncertainties of the Sm‐Nd isotopic data, no Sm‐Nd isochron age was obtained for GRV 99027. The ε143Nd value is estimated approximately +12.2, assuming an age of 177 Ma. The ISr of GRV 99027 is distinguishable from other lherzolitic shergottites, confirming our previous conclusion that it is not paired with them ( Lin et al. 2005 ). The new data of GRV 99027 support the same age of approximately 180 Ma for most lherzolitic shergottites, and fill the small gap of ISr between Allan Hills A77005 and Lewis Cliff 88516 ( Borg et al. 2002 ). All available data are consistent with a single igneous source for the intermediate subgroup of lherzolitic shergottites.  相似文献   

4.
Northwest Africa (NWA) 6342 is an intermediate, poikilitic shergottite, found in Algeria in 2010. It is comprised of two distinct petrographic areas; poikilitic domains with rounded Mg‐rich olivine chadacrysts enclosed by large low‐Ca pyroxene oikocrysts, and a nonpoikilitic domain mainly comprised of subhedral olivine and vesicular recrystallized plagioclase. Oxygen fugacity conditions become more oxidizing during crystallization from the poikilitic to the nonpoikilitic domain (QFM?3.0 to QFM?2.2). As such, it is likely that NWA 6342 experienced a two‐stage (polybaric) crystallization history similar to that of the enriched poikilitic shergottites. NWA 6342 also experienced relatively high levels of shock metamorphism in comparison to most other poikilitic shergottites as evidenced by the fine‐grained recrystallization texture in olivine, as well as melting and subsequent crystallization of plagioclase. The recrystallization of plagioclase requires an extended period of postshock thermal metamorphism for NWA 6342 and similarly shocked intermediate poikilitic shergottites NWA 4797 and Grove Mountains 99027 most likely due to launch from Mars. The similarities in petrology, chemistry, and shock features between these three meteorites indicate that they have similar crystallization and shock histories; possibly originating from the same source area on Mars.  相似文献   

5.
Grove Mountains (GRV) 020090 is an enriched lherzolitic shergottite, distinct from other lherzolitic shergottites, except RBT 04262/1. Its characteristics include high abundance of plagioclase (24.2 vol% in the nonpoikilitic area), presence of K‐feldspar, common occurrence of baddeleyite, high FeO contents of olivine (bimodal peaks at Fa 33 mol% and Fa 41 mol%) and low‐Ca pyroxenes (bimodal peaks at Fs 23.8–31.7 mol% and Fs 25.7–33.9 mol%), and significant LREE enrichment of phosphates (500–610 × CI). The bulk composition of GRV 020090 suggests derivation from partial melting of an enriched reservoir. However, the REE patterns of the cores of pigeonite oikocrysts and the olivine chadacrysts are indistinguishable from those of GRV 99027 and other moderately depleted lherzolitic shergottites, and reveal a LREE‐depleted pattern of the primordial parent magma. We propose that the primordial parent magma of GRV 020090 was derived from a moderately depleted Martian upper mantle reservoir, and later the residual melt was contaminated by oxidized and enriched Martian crustal materials as it ascended up to the subsurface. GRV 020090 and RBT 04262/1 may have sampled an igneous unit different from other lherzolitic shergottites.  相似文献   

6.
Abstract— We report petrography, mineral chemistry, and microdistribution of rare earth elements (REE) in a new lherzolitic shergottite, Grove Mountains (GRV) 99027. The textural relationship and REE patterns of minerals suggest precipitation of cumulus olivine and chromite, followed by equilibrium crystallization of a closed system with a bulk composition of the inferred intercumulus melt. Subsolidus equilibrium temperatures of pyroxenes and olivine range from 1100 to 1210 °C, based on a two‐pyroxene thermometry and Ca partitioning between augite and olivine. Oxygen fugacity of the parent magma is 1.5–2.5 (av. 2.0 ± 0.4) log units below the quartz‐fayalite‐magnetite (QFM) buffer at 960–1360 °C, according to the olivine‐orthopyroxene‐chromite barometer. The ilmenite‐chromite barometer and thermometer show much wider ranges of oxygen fugacity (1.0–7.0 log unit below QFM) and temperature (1130–480 °C), suggesting subsolidus equilibration of the oxides at low temperatures, probably due to deep burial of GRV 99027 on Mars. The low oxygen fugacity and LREE depletion of the parent magma of GRV 99027 suggest low contamination by martian crust. Characteristics of GRV 99027 demonstrate similarity of lherzolitic shergottites, suggesting a high possibility of launch pairing or a homogeneous upper mantle of Mars if they were ejected by individual impact events. However, GRV 99027 probably experienced severe post‐shock thermal metamorphism in comparison with other lherzolitic shergottites, based on the re‐crystallization of maskelynite, the homogeneity of minerals, and the low subsolidus equilibrium temperatures between chromite and ilmenite.  相似文献   

7.
Abstract— The lherzolitic Martian meteorite Northwest Africa (NWA) 1950 consists of two distinct zones: 1) low‐Ca pyroxene poikilically enclosing cumulate olivine (Fo70–75) and chromite, and 2) areas interstitial to the oikocrysts comprised of maskelynite, low‐ and high‐Ca pyroxene, cumulate olivine (Fo68–71) and chromite. Shock metamorphic effects, most likely associated with ejection from the Martian subsurface by large‐scale impact, include mechanical deformation of host rock olivine and pyroxene, transformation of plagioclase to maskelynite, and localized melting (pockets and veins). These shock effects indicate that NWA 1950 experienced an equilibration shock pressure of 35–45 GPa. Large (millimeter‐size) melt pockets have crystallized magnesian olivine (Fo78–87) and chromite, embedded in an Fe‐rich, Al‐poor basaltic to picro‐basaltic glass. Within the melt pockets strong thermal gradients (minimum 1 °C/μm) existed at the onset of crystallization, giving rise to a heterogeneous distribution of nucleation sites, resulting in gradational textures of olivine and chromite. Dendritic and skeletal olivine, crystallized in the melt pocket center, has a nucleation density (1.0 × 103 crystals/mm2) that is two orders of magnitude lower than olivine euhedra near the melt margin (1.6 × 105 crystals/mm2). Based on petrography and minor element abundances, melt pocket formation occurred by in situ melting of host rock constituents by shock, as opposed to melt injected into the lherzolitic target. Despite a common origin, NWA 1950 is shocked to a lesser extent compared to Allan Hills (ALH) 77005 (45–55 GPa). Assuming ejection in a single shock event by spallation, this places NWA 1950 near to ALH 77005, but at a shallower depth within the Martian subsurface. Extensive shock melt networks, the interconnectivity between melt pockets, and the ubiquitous presence of highly vesiculated plagioclase glass in ALH 77005 suggests that this meteorite may be transitional between discreet shock melting and bulk rock melting.  相似文献   

8.
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   

9.
Abstract— Magnetic properties of 26 (of 32) unpaired Martian meteorites (SNCs) are synthesized to further constrain the lithology carrying Martian magnetic crustal sources. Magnetic properties of ultramafic cumulates (i.e., Chassigny, Allan Hills [ALH] 84001) and lherzolitic shergottites (ALH 77005, Lewis Cliff [LEW] 88516) are one or two orders of magnitude too weak to account for the crustal magnetizations, assuming magnetization in an Earth‐like field. Nakhlites and some basaltic shergottites, which are the most magnetic SNCs, show the right intensity. Titanomagnetite is the magnetic carrier in the nakhlites (7 meteorites), whereas in most basaltic shergottites (11 meteorites) it is pyrrhotite. Dhofar (Dho) 378, Los Angeles, and NWA 480/1460 and 2046 are anomalous basaltic shergottites, as their magnetism is mainly due to titanomagnetite. Pyrrhotite should be among the candidate minerals for the magnetized Noachian crust.  相似文献   

10.
Abstract— We report the concentration of 50 elements, including rare earth elements (REEs) and platinum group elements (PGEs) in bulk samples of the Grove Mountains (GRV) 99027 lherzolitic shergottite. The abundances of REEs are distinctly lower than those of Allan Hills (ALH) A77005 and other lherzolitic shergottites, indicating that GRV 99027 is not paired with them. It may, nevertheless, sample the same igneous unit as the others (Lin et al. 2005b; Wang and Chen 2006). The CI‐normalized elemental pattern of GRV 99027 reveals low (0.004–0.008 × CI) and unfractionated PGEs (except for Pd of 0.018 × CI) without depletion of W. or Ga relative to lithophile element trends. Fractionation between siderophile and lithophile elements become less pronounced with increase of volatility, except for high abundances of Ni and Co. These characteristics are probably representative of the mantle of Mars, which is consistent with previous work that the Martian mantle formed in a deep magma ocean followed by a later accretion of chondritic materials.  相似文献   

11.
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   

12.
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   

13.
Abstract— We examine the occurrences, textures, and compositional patterns of spinels in the olivine‐phyric shergottites Sayh al Uhaymir (SaU) 005, lithology A of Elephant Moraine A79001 (EET‐A), Dhofar 019, and Northwest Africa (NWA) 1110, as well as the Iherzolitic shergottite Allan Hills (ALH) A77005, in order to identify spinel‐olivine‐pyroxene assemblages for the determination of oxygen fugacity (using the oxybarometer of Wood [1991]) at several stages of crystallization. In all of these basaltic martian rocks, chromite was the earliest phase and crystallized along a trend of strict Cr‐Al variation. Spinel (chromite) crystallization was terminated by the appearance of pyroxene but resumed later with the appearance of ulvöspinel. Ulvöspinel formed overgrowths on early chromites (except those shielded as inclusions in olivine or pyroxene), retaining the evidence of the spinel stability gap in the form of a sharp core/rim boundary (except in ALH A77005, where subsolidus reequilibration diffused this boundary). Secondary effects seen in chromites include reaction with melt before ulvöspinel overgrowth, reaction with melt inclusions, reaction with olivine hosts (in ALH A77005), and exsolution of ulvöspinel or ilmenite. All chromites experienced subsolidus Fe/Mg reequilibration. Spinel‐olivine‐pyroxene assemblages representing the earliest stages of crystallization in each rock essentially consist of the highest‐Cr#, lowest‐fe# chromites not showing secondary effects plus the most magnesian olivine and equilibrium low‐Ca pyroxene. Assemblages representing the onset of ulvöspinel crystallization consist of the lowest‐Ti ulvöspinel, the most magnesian olivine in which ulvöspinel occurs as inclusions, and equilibrium low‐Ca pyroxene. The results show that, for early crystallization conditions, oxygen fugacity (fO2) increases from SaU 005 and Dhofar 019 (?QFM ‐3.8), to EET‐A (QFM ‐2.8) and ALH A77005 (QFM ‐2.6), to NWA 1110 (QFM ‐1.7). Estimates for later conditions indicate that in SaU 005 and Dhofar 019 oxidation state did not change during crystallization. In EET‐A, there was an increase in fO2 that may have been due to mixing of reduced material with a more oxidized magma. In NWA 1110, there was a dramatic increase, indicating a non‐buffered system, possibly related to its high oxidation state. Differences in fO2 among shergottites are not primarily due to igneous fractionation but, rather, to derivation from (and possibly mixing of) different reservoirs.  相似文献   

14.
Abstract By mineral and bulk compositions, the Lewis Cliff (LEW) 88516 meteorite is quite similar to the ALHA77005 martian meteorite. These two meteorites are not paired because their mineral compositions are distinct, they were found 500 km apart in ice fields with different sources for meteorites, and their terrestrial residence ages are different. Minerals in LEW88516 include: olivine, pyroxenes (low- and high-Ca), and maskelynite (after plagioclase); and the minor minerals chromite, whitlockite, ilmenite, and pyrrhotite. Mineral grains in LEW88516 range up to a few mm. Texturally, the meteorite is complex, with regions of olivine and chromite poikilitically enclosed in pyroxene, regions of interstitial basaltic texture, and glass-rich (shock) veinlets. Olivine compositions range from Fo64 to Fo70, (avg. Fo67), more ferroan and with more variation than in ALHA77005 (Fo69 to Fo73). Pyroxene compositions fall between En77Wo4 and En65Wo15 and in clusters near En63Wo9 and En53Wo33, on average more magnesian and with more variation than in ALHA77005. Shock features in LEW88516 range from weak deformation through complete melting. Bulk chemical analyses by modal recombination of electron microprobe analyses, instrumental neutron activation, and radiochemical neutron activation confirm that LEW88516 is more closely related to ALHA77005 than to other known martian meteorites. Key element abundance ratios are typical of martian meteorites, as is its non-chondritic rare earth pattern. Differences between the chemical compositions of LEW88516 and ALHA77005 are consistent with slight differences in the proportions of their constituent minerals and not from fundamental petrogenetic differences. Noble gas abundances in LEW88516, like those in ALHA77005, show modest excesses of 40Ar and 129Xe from trapped (shock-implanted) gas. As with other ALHA77005 and the shergottite martian meteorites (except EETA79001), noble gas isotope abundances in LEW88516 are consistent with exposure to cosmic rays for 2.5–3 Ma. The absence of substantial effects of shielding from cosmic rays suggest LEW88516 spent this time as an object no larger than a few cm in diameter.  相似文献   

15.
Northwest Africa (NWA) 7397 is a newly discovered, enriched, lherzolitic shergottite, the third described example of this group. This meteorite consists of two distinct textural lithologies (1) poikilitic—comprised of zoned pyroxene oikocrysts, with chadacrysts of chromite and olivine, and (2) nonpoikilitic—comprised of olivine, low‐Ca and high‐Ca pyroxene, maskelynite, and minor abundances of merrillite, spinel, ilmenite, and pyrrhotite. The constant Ti/Al ratios of pyroxene oikocrysts suggests initial crystallization of the poikilitic lithology at depth (equivalent to pressures of approximately 10 kbar), followed by crystallization of the nonpoikilitic lithology at shallower levels. Oxygen fugacity conditions become more oxidizing during crystallization ranging from fO2 conditions of approximately QFM‐2 to QFM‐0.7. Magma calculated to be in equilibrium with the major rock‐forming minerals is LREE‐enriched relative to depleted or intermediate shergottites and has flat overall profiles. Therefore, we suggest that the parental magma for NWA 7397 had sampled an enriched, oxidized, Martian geochemical source, similar to that of other enriched basaltic and olivine‐phyric shergottites. We present a polybaric formation model for the lherzolitic shergottite NWA 7397, to account for the petrologic constraints. Three successive stages in the development of NWA 7397 are discussed (1) formation of a REE‐enriched parental magma from a distinct Martian mantle reservoir; (2) magma ponding and development of a staging chamber concomitant with initial crystallization of the poikilitic lithology; and (3) magma ascent to the near surface, with entrainment of cumulates from the staging chamber and subsequent crystallization of the nonpoikilitic lithology en route to the surface.  相似文献   

16.
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   

17.
Abstract— Microbeam studies of Martian meteorites Dar al Gani (DaG) 476 and Allan Hills (ALH) 77005 have been conducted to identify potential causes of disequilibrium exhibited in their Sm‐Nd isotopic systematics. Olivine and maskelynite mineral fractions on the DaG 476 isochron are displaced relative to their positions as dictated by measured mineral compositions. The olivine mineral fractions from ALH 77005 not only have a relatively low Sm/Nd ratio, but appear to contain an unradiogenic component that shifts the olivine mineral fraction off the isochron defined by the pyroxene and maskelynite mineral fractions. Trace components such as melt inclusions, impact melt, high‐Si mesostasis, and altered olivine were analyzed using scanning electron microscopy, quantitative electron microscopy, and secondary ion mass spectrometry to determine their potential for disturbing the isotopic systematics of the mineral fractions, assuming that the mineral fractions were not completely pure. Mixing models indicate that the presence of melt inclusions in the DaG 476 olivine mineral fraction lowered its Sm/Nd ratio. The maskelynite mineral fraction contains a related but more evolved mesostasis component that raised the Sm/Nd ratio of the fraction. The position of two olivine mineral fractions below the ALH 77005 isochron is interpreted to reflect small additions of impact melt with a light rare earth element enriched pattern and a non‐indigenous, unradiogenic Nd component. Furthermore, the presence of rare earth elements in olivine and maskelynite from both igneous and non‐igneous components such as melt inclusions, mesostasis, and impact melt is observed on a fine (<30 μm) scale. Despite the addition of this material, the Sm‐Nd ages are not affected. This study demonstrates that detailed mineral separation procedures as employed by modern geochronology laboratories permit reliable ages to be derived from shocked and altered samples.  相似文献   

18.
Northwest Africa (NWA) 1950 is a new member of the lherzolitic shergottite clan of the Martian meteorites recently found in the Atlas Mountains. The petrological, mineralogical, and geochemical data are very close to those of the other known lherzolitic shergottites. The meteorite has a cumulate gabbroic texture and its mineralogy consists of olivine (Fo66 to Fo75), low and high‐Ca pyroxenes (En78Fs19Wo2‐En60Fs26W14; En53Fs16Wo31‐En45Fs14Wo41), and plagioclase (An57Ab41Or1 to An40Ab57Or3; entirely converted into maskelynite during intense shock metamorphism). Accessory minerals include phosphates (merrillite), chromite and spinels, sulfides, and a glass rich in potassium. The oxygen isotopic values lie on the fractional line defined by the other SNC meteorites (Δ17O = 0.312 %o). The composition of NWA 1950 is very similar to the other lherzolitic shergottites and suggests an origin from the same magmatic system, or at least crystallization from a close parental melt. Cosmogenic ages indicate an ejection age similar to those of the other lherzolitic shergottites. The intensity of the shock is similar to that observed in other shergottites, as shown by the occurrence of small melt pockets containing glass interwoven with stishovite.  相似文献   

19.
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   

20.
Abstract— We report the elemental and isotopic composition of the noble gases as well as the chemical abundances in pyroxene, maskelynite/mesostasis glass, and bulk material of Shergotty and of bulk samples from Chassigny and Yamato 793605. The 40K-40Ar isochron for the Shergotty minerals yields a gas retention age of 196 Ma, which is, within errors, in agreement with previously determined Rb-Sr internal isochron ages. Argon that was trapped at this time has a 40Ar/36Ar ratio of 1100. For Chassigny and Y-793605, we obtain trapped 40Ar/36Ar ratios of 1380 and 950, respectively. Using these results and literature data, we show that the three shergottites, Shergotty, Zagami, and QUE 94001; the lherzolites ALH 77005, LEW 88516, and Y-793605; as well as Chassigny and ALH 84001 contain a mixture of Martian mantle and atmospheric Ar; whereas, the trapped 40Ar/36Ar ratio of the nakhlites, Nakhla, Lafayette, and Governador Valadares cannot be determined with the present data. We show that Martian atmospheric trapped Ar in Martian meteorites is correlated with the shock pressure that they experienced. Hence, we conclude that the Martian atmospheric gases were introduced by shock into the meteoritic material. For the Shergotty minerals, we obtain 3He-, 21Ne-, and 38Ar-based cosmic-ray exposure ages of 3.0 Ma, and for the lherzolite Y-793605, 4.0 Ma, which confirms our earlier conclusion that the lherzolites were ejected from Mars ～1 Ma before the shergottites. Chassigny yields the previously known ejection age of 11.6 Ma.  相似文献   

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