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1.
Abstract– Bunburra Rockhole is the first meteorite fall photographed and recovered by the Desert Fireball Network in Australia. It is classified as an ungrouped achondrite similar in mineralogical and chemical composition to eucrites, but it has a distinct oxygen isotope composition. The question is if achondrites like Bunburra Rockhole originate from the same parent body as the howardite‐eucrite‐diogenite (HED) meteorites or from several separate, differentiated parent bodies. To address this question, we measured cosmogenic radionuclides and noble gases in the Bunburra Rockhole achondrite. The short‐lived radionuclides 22Na and 54Mn confirm that Bunburra Rockhole is a recent fall. The concentrations of 10Be, 26Al and 36Cl as well as the 22Ne/21Ne ratio indicate that Bunburra Rockhole was a relatively small object (R approximately 15 cm) in space, consistent with the photographic fireball observations. The cosmogenic 38Ar concentration yields a cosmic‐ray exposure (CRE) age of 22 ± 3 Myr, whereas 21Ne and 3He yield approximately 30% and approximately 60% lower ages, respectively, due to loss of cosmogenic He and Ne, mainly from plagioclase. With a CRE age of 22 Myr, Bunburra Rockhole is the first anomalous eucrite that overlaps with the main CRE peak of the HED meteorites. The radiogenic K‐Ar age of 4.1 Gyr is consistent with the U‐Pb age, while the young U,Th‐He age of approximately 1.4 Gyr indicates that Bunburra Rockhole lost radiogenic 4He more recently.  相似文献   

2.
The Emmaville eucrite is a relatively poorly studied basaltic achondrite with an anomalous oxygen isotope signature. In this study, we report comprehensive mineralogical, petrographic, and geochemical data from Emmaville in order to understand its petrogenesis and relationship with the basaltic eucrites. Emmaville is an unusually fine‐grained, hornfelsic‐textured metabasalt with pervasive impact melt veins and mineral compositions similar to those of typical basaltic eucrites. The major and trace element bulk composition of Emmaville is also typical of a basaltic eucrite. Three separated individual lithologies were also analyzed for O isotopes; a dark gray fraction (E1), a shocked lithology (E2), and a lighter gray portion (E3). Fractions E1 and E2 shared similar O isotope compositions to the bulk sample (E‐B), whereas the lighter gray portion (E3) is slightly elevated in Δ17O and significantly elevated in δ18O compared to bulk. No evidence for any exogenous material is observed in the thin sections, coupled with the striking compositional similarity to typical basaltic eucrites, appears to preclude a simple impact‐mixing hypothesis. The O‐isotopes of Emmaville are similar to those of Bunburra Rockhole, A‐881394, and EET 92023, and thus distinct from the majority of the HEDs, despite having similarities in petrology, mineral, and bulk compositions. It would, therefore, seem plausible that all four of these samples are derived from a single HED‐like parent body that is isotopically distinct from that of the HEDs (Vesta) but similar in composition.  相似文献   

3.
Comparative planetary geochemistry provides insight into the origin and evolutionary paths of planetary bodies in the inner solar system. The eucrite and angrite achondrite groups are particularly interesting because they show evidence of early planetary differentiation. We present 147Sm‐143Nd and 176Lu‐176Hf analyses of eight noncumulate (basaltic) eucrites, two cumulate eucrites, and three angrites, which together place new constraints on the evolution and differentiation histories of the crusts of the eucrite and angrite parent bodies and their mantle mineralogies. The chemical compositions of both eucrites and angrites indicate similar evolutionary paths and petrogenetic models with formation and isolation of differentiated crustal reservoirs associated with segregation of ilmenite. We report a 147Sm‐143Nd mineral isochron age for the Moama cumulate eucrite of 4519 ± 34 Ma (MSWD = 1.3). This age indicates protracted magmatism within deep crustal layers of the eucrite parent body lasting up to about 50 Ma after the formation of the solar system. We further demonstrate that the isotopic compositions of constituent minerals are compromised by secondary processes hindering precise determination of mineral isochron ages of basaltic eucrites and angrites. We interpret the changes in geochemistry and, consequently, the erroneous 147Sm‐143Nd and 176Lu‐176Hf internal mineral isochron ages of basaltic eucrites and angrites as the result of metamorphic events such as impacts (effects from pressure, temperature, and peak shock duration) on the surfaces of the eucrite and angrite parent bodies.  相似文献   

4.
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5.
Olivine‐dominated (70–80 modal %) achondrite meteorite Lewis Cliff (LEW) 88763 originated from metamorphism and limited partial melting of a FeO‐rich parent body. The meteorite experienced some alteration on Earth, evident from subchondritic Re/Os, and redistribution of rhenium within the sample. LEW 88763 is texturally similar to winonaites, has a Δ17O value of ?1.19 ± 0.10‰, and low bulk‐rock Mg/(Mg+Fe) (0.39), similar to the FeO‐rich cumulate achondrite Northwest Africa (NWA) 6693. The similar bulk‐rock major‐, minor‐, and trace‐element abundances of LEW 88763, relative to some carbonaceous chondrites, including ratios of Pd/Os, Pt/Os, Ir/Os, and 187Os/188Os (0.1262), implies a FeO‐ and volatile‐rich precursor composition. Lack of fractionation of the rare earth elements, but a factor of approximately two lower highly siderophile element abundances in LEW 88763, compared with chondrites, implies limited loss of Fe‐Ni‐S melts during metamorphism and anatexis. These results support the generation of high Fe/Mg, sulfide, and/or metal‐rich partial melts from FeO‐rich parent bodies during partial melting. In detail, however, LEW 88763 cannot be a parent composition to any other meteorite sample, due to highly limited silicate melt loss (0 to <<5%). As such, LEW 88763 represents the least‐modified FeO‐rich achondrite source composition recognized to date and is distinct from all other meteorites. LEW 88763 should be reclassified as an anomalous achondrite that experienced limited Fe,Ni‐FeS melt loss. Lewis Cliff 88763, combined with a growing collection of FeO‐rich meteorites, such as brachinites, brachinite‐like achondrites, the Graves Nunataks (GRA) 06128/9 meteorites, NWA 6693, and Tafassasset, has important implications for understanding the initiation of planetary differentiation. Specifically, regardless of precursor compositions, partial melting and differentiation processes appear to be similar on asteroidal bodies spanning a range of initial oxidation states and volatile contents.  相似文献   

6.
We look at the relationship between the value of ε54Cr in bulk meteorites and the time (after calcium‐aluminum‐rich inclusion, CAI) when their parent bodies accreted. To obtain accretion ages of chondrite parent bodies, we estimated the maximum temperature reached in the insulated interior of each parent body, and estimated the initial 26Al/27Al for this temperature to be achieved. This initial 26Al/27Al corresponds to the time (after CAI formation) when cold accretion of the parent body would have occurred, assuming 26Al/27Al throughout the solar system began with the canonical value of 5.2 × 10?5. In cases of iron meteorite parent bodies, achondrite parent bodies, and carbonaceous chondrite parent bodies, we use published isotopic ages of events (such as core formation, magma crystallization, and growth of secondary minerals) in each body's history to obtain the probable time of accretion. We find that ε54Cr correlates with accretion age: the oldest accretion ages (1 ± 0.5 Ma) are for iron and certain other differentiated meteorites with ε54Cr of ?0.75 ± 0.5, and the youngest ages (3.5 ± 0.5 Ma) are for hydrated carbonaceous chondrites with ε54Cr values of 1.5 ± 0.5. Despite some outliers (notably Northwest Africa [NWA] 011 and Tafassasset), we feel that the correlation is significant and we suggest that it resulted from late, localized injection of dust with extremely high ε54Cr.  相似文献   

7.
Abstract– We report an analysis of the first instrumentally observed meteorite fall in Australia, which was recorded photographically and photoelectrically by two eastern stations of the Desert Fireball Network (DFN) on July 20, 2007. The meteoroid with an initial mass of 22 kg entered the atmosphere with a low speed of 13.36 km s?1 and began a luminous trajectory at an altitude of 62.83 km. In maximum, it reached ?9.6 absolute magnitude and terminated after a 5.7 s and 64.7 km long flight at an altitude of 29.59 km with a speed of 5.8 km s?1. The angle of the atmospheric trajectory to the Earth’s surface was 30.9°. The first organized search took place in October 2008 and the first meteorite (150 g) was found 97 m southward from the predicted central line at the end of the first day of searching (October 3, 2008). The second stone (174 g) was recovered 39 m northward from the central line, both exactly in the predicted mass limits. During the second expedition in February 2009, a third fragment of 14.9 g was found again very close (～100 m) from the predicted position. Total recovered mass is 339 g. The meteorite was designated Bunburra Rockhole (BR) after a nearby landscape structure. This first DFN sample is an igneous achondrite. Initial petrography indicated that BR was a brecciated eucrite but detailed analyses proved that BR is not a typical eucrite, but an anomalous basaltic meteorite ( Bland et al. 2009 ). BR was delivered from an unusual, Aten type orbit (a < 1 AU) where virtually the entire orbit was contained within Earth’s orbit. BR is the first achondrite fall with a known orbit and it is one of the most precise orbits ever calculated for a meteorite dropping fireball.  相似文献   

8.
Abstract— A compilation of over 1500 Mg-isotopic analyses of Al-rich material from primitive solar system matter (meteorites) shows clearly that 26Al existed live in the early Solar System. Excesses of 26Mg observed in refractory inclusions are not the result of mixing of “fossil” interstellar 26Mg with normal solar system Mg. Some material was present that contained little or no 26Al, but it was a minor component of solar system matter in the region where CV3 and CO3 carbonaceous chondrites accreted and probably was a minor component in the accretion regions of CM chondrites as well. Data for other chondrite groups are too scanty to make similar statements. The implied long individual nebular histories of CAIs and the apparent gap of one or more million years between the start of CAI formation and the start of chondrule formation require the action of some nebular mechanism that prevented the CAIs from drifting into the Sun. Deciding whether 26Al was or was not the agent of heating that caused melting in the achondrite parent bodies hinges less on its widespread abundance in the nebula than it does on the timing of planetesimal accretion relative to the formation of the CAIs.  相似文献   

9.
Abstract— In order to investigate whether or not 26Al can be used as a fine‐scale chronometer for early solar system events we measured, with an ion microprobe, Mg isotopes and Al/Mg ratios in separated plagioclase, olivine, and pyroxene crystals from the H4 chondrites Ste Marguerite (SM), Forest Vale (FV), Beaver Creek and Quenggouk and compared the results with the canonical 26Al/27Al ratio for calcium‐aluminum‐rich inclusions (CAIs). For SM and FV, Pb/Pb and Mn‐Cr ages have previously been determined (Göpel et al., 1994; Polnau et al., 2000; Polnau and Lugmair, 2001). Plagioclase grains from these two meteorites show clear excesses of 26Mg. The 26Al/27Al ratios inferred from these excesses and from isotopically normal Mg in pyroxene and olivine are (2.87 ± 0.64) × 10?7 for SM and (1.52 ± 0.52) × 10?7 for FV. The differences between these ratios and the ratio of 5 times 10?5 in CAIs indicate time differences of 5.4 ± 0.1 Ma and 6.1 ± 0.2 Ma for SM and FV, respectively. These differences are in agreement with the absolute Pb/Pb ages for CAIs and SM and FV phosphates but there are large discrepancies between the U‐Pb and Mn‐Cr system for the relative ages for CAIs, SM and FV. For example, Mn‐Cr ages of carbonates from Kaidun are older than the Pb/Pb age of CAIs. However, even if we require that CAIs are older than these carbonates, the time difference between this “adjusted” CAI age and the Mn‐Cr ages of SM and FV require that 26 Al was widely distributed in the early solar system at the time of CAI formation and was not mostly present in CAIs, a feature of the X‐wind model proposed by Shu and collaborators (Gounelle et al., 2001; Shu et al., 2001). From this we conclude that there was enough 26Al to melt small planetary bodies as long as they formed within 2 Ma of CAIs, and that 26Al can serve as a fine‐scale chronometer for early solar system events.  相似文献   

10.
There is variability in the Mg isotopic composition that is a reflection of the widespread heterogeneity in the isotopic composition of the elements in the solar system at approximately 100 ppm. Measurements on a single calcium‐aluminum‐rich inclusion (CAI) gave a good correlation of 26Mg/24Mg with 27Al/24Mg, yielding an isochron corresponding to an initial (26Al/27Al)o = (5.27 ± 0.18) × 10?5 and an initial (26Mg/24Mg)o = ?0.127 ± 0.032‰ relative to the standard. This isochron is parallel to that obtained by Jacobsen et al. (2008) , but is distinctively offset. This demonstrates that there are different initial Mg isotopic compositions in different samples with the same 26Al/27Al. No inference about uniformity/heterogeneity of 26Al/27Al on a macro scale can be based on the initial (26Mg/24Mg)o values. Different values of 26Al/27Al for samples representing the same point in time would prove heterogeneity of 26Al/27Al. The important issue is whether the bulk solar inventory of 26Al/27Al was approximately 5 × 10?5 at some point in the early solar system. We discuss ultra refractory phases of solar type oxygen isotope composition with 26Al/27Al from approximately 5 × 10?5 to below 0.2 × 10?5. We argue that the real issues are: intrinsic heterogeneity in the parent cloud; mechanism and timing for the later production of 16O‐poor material; and the relationship to earlier formed 16O‐rich material in the disk. 26Al‐free refractories can be produced at a later time by late infall, if there is an adequate heat source, or from original heterogeneities in the placental molecular cloud from which the solar system formed.  相似文献   

11.
Abstract– Hibonite‐bearing Ca,Al‐rich inclusions (CAIs) usually occur in CM and CH chondrites and possess petrographic and isotopic characteristics distinctive from other typical CAIs. Despite their highly refractory nature, most hibonite‐bearing CAIs have little or no 26Mg excess (the decay product of 26Al), but do show wide variations of Ca and Ti isotopic anomalies. A few spinel‐hibonite spherules preserve evidence of live 26Al with an inferred 26Al/27Al close to the canonical value. The bimodal distribution of 26Al abundances in hibonite‐bearing CAIs has inspired several interpretations regarding the origin of short‐lived nuclides and the evolution of the solar nebula. Herein we show that hibonite‐bearing CAIs from Ningqiang, an ungrouped carbonaceous chondrite, also provide evidence for a bimodal distribution of 26Al. Two hibonite aggregates and two hibonite‐pyroxene spherules show no 26Mg excesses, corresponding to inferred 26Al/27Al < 8 × 10?6. Two hibonite‐melilite spherules are indistinguishable from each other in terms of chemistry and mineralogy but have different Mg isotopic compositions. Hibonite and melilite in one of them display positive 26Mg excesses (up to 25‰) that are correlated with Al/Mg with an inferred 26Al/27Al of (5.5 ± 0.6) × 10?5. The other one contains normal Mg isotopes with an inferred 26Al/27Al < 3.4 × 10?6. Hibonite in a hibonite‐spinel fragment displays large 26Mg excesses (up to 38‰) that correlate with Al/Mg, with an inferred 26Al/27Al of (4.5 ± 0.8) × 10?5. Prolonged formation duration and thermal alteration of hibonite‐bearing CAIs seem to be inconsistent with petrological and isotopic observations of Ningqiang. Our results support the theory of formation of 26Al‐free/poor hibonite‐bearing CAIs prior to the injection of 26Al into the solar nebula from a nearby stellar source.  相似文献   

12.
Abstract— According to a currently popular model for petrogenesis on the howardite, eucrite, and diogenite (HED) parent asteroid, the diogenites are not comagmatic with most eucrites but instead formed in separate orthopyroxenite-dominated plutons. This model can be tested for consistency with mass balance for MgO and FeO, assuming the overall diogenite/(diogenite + eucrite) ratio, d, of the parent asteroid is at least comparable to the average d for the eucrite + diogenite dominated howardite regolith breccias. Average mg# (=MgO/[MgO + FeO]) is much lower for eucrites, especially noncumulate eucrites, than for diogenites. Unless the diogenite parent magmas eventually produced a large proportion of low-mg# residual basalt and gabbro (RBG), the implied initial magma's mg# is vastly higher than that of any noncumulate eucrite. Starting from a source previously depleted by putative primary eucrite genesis, melt mg# can be estimated as a function of the exchange reaction KD and degree of melting. Using several very conservative assumptions (e.g., assuming that the total [MgO + FeO] concentration is nearly the same in the nascent melt as in the residual solids), the degree of melting required to yield a melt with mg# high enough to satisfy mass balance, without implying an RBG component that accounts for >50% of all eucrites, is an implausibly high 60–80 wt%. The separate orthopyroxenitic plutons (SOP) model also seems inconsistent with the uniform density of melts across the diogenite-eucrite compositional spectrum (2.77–2.82 g/cm3), which implies that diogenitic magmas should have been as capable as eucrites of extruding to form lavas. This difficulty cannot be reduced by simply assuming that later-formed magmas were systematically both more plutonic and more MgO-rich than earlier ones, because the plutonic cumulate eucrites equilibrated with melts systematically lower in mg# than noncumulate eucrites. Conceivably, the bulk mg# of the asteroid's silicate system was increased between primary-melt eucrite genesis and SOP diogenite genesis by graphite-fueled reduction of FeO. However, the graphite oxidation process generates a huge proportion of gas, which would have enhanced the buoyancy of the nascent diogenite-parent magmas, thus exacerbating the difficulty of achieving the implied high degrees of partial melting. To avoid these difficulties but still form most eucrites as rapidly cooled extrusives, I propose the NERD (noncumulate eucrites as extruded residua of diogenites) model. In this model, the diogenites form as early cumulates from a large magma system (probably a global “magma ocean”) that yields a large proportion of eucritic melt as residuum. This residual melt zone undergoes relatively little crystallization during a period when it is episodically tapped to produce extrusions, dikes and sills of rapidly cooled noncumulate eucrites. Slight (～5–10%) porosity in the nascent eucritic crust keeps it marginally buoyant over the residual melt zone. The common thermal metamorphism of noncumulate eucrites results from baking by superjacent flows during the episodic venting of the melt zone. The NERD model's greatest advantage is that it does not require implausibly high degrees of localized melting in the mature stages of igneous evolution of the HED asteroid.  相似文献   

13.
Abstract— In order to investigate the distribution of 26A1 in chondrites, we measured aluminum‐magnesium systematics in four calcium‐aluminum‐rich inclusions (CAIs) and eleven aluminum‐rich chondrules from unequilibrated ordinary chondrites (UOCs). All four CAIs were found to contain radiogenic 26Mg (26Mg*) from the decay of 26A1. The inferred initial 26Al/27Al ratios for these objects ((26Al/27Al)0 ? 5 × 10?5) are indistinguishable from the (26Al/27Al)0 ratios found in most CAIs from carbonaceous chondrites. These observations, together with the similarities in mineralogy and oxygen isotopic compositions of the two sets of CAIs, imply that CAIs in UOCs and carbonaceous chondrites formed by similar processes from similar (or the same) isotopic reservoirs, or perhaps in a single location in the solar system. We also found 26Mg* in two of eleven aluminum‐rich chondrules. The (26Al/27Al)0 ratio inferred for both of these chondrules is ～1 × 10?5, clearly distinct from most CAIs but consistent with the values found in chondrules from type 3.0–3.1 UOCs and for aluminum‐rich chondrules from lightly metamorphosed carbonaceous chondrites (～0.5 × 10?5 to ～2 × 10?5). The consistency of the (26Al/27Al)0 ratios for CAIs and chondrules in primitive chondrites, independent of meteorite class, implies broad‐scale nebular homogeneity with respect to 26Al and indicates that the differences in initial ratios can be interpreted in terms of formation time. A timeline based on 26Al indicates that chondrules began to form 1 to 2 Ma after most CAIs formed, that accretion of meteorite parent bodies was essentially complete by 4 Ma after CAIs, and that metamorphism was essentially over in type 4 chondrite parent bodies by 5 to 6 Ma after CAIs formed. Type 6 chondrites apparently did not cool until more than 7 Ma after CAIs formed. This timeline is consistent with 26Al as a principal heat source for melting and metamorphism.  相似文献   

14.
Abstract— Eucrite meteorites are igneous rocks that derived from a large asteroid, probably 4 Vesta. Past studies have shown that after most eucrites formed, they underwent metamorphism in temperatures up to ≥800°C. Much later, many were brecciated and heated by large impacts into the parent body surface. The less common basaltic, unbrecciated eucrites also formed near the surface but, presumably, escaped later brecciation, while the cumulate eucrites formed at depths where metamorphism may have persisted for a considerable period. To further understand the complex HED parent body thermal history, we determined new 39Ar‐40Ar ages for 9 eucrites classified as basaltic but unbrecciated, 6 eucrites classified as cumulate, and several basaltic‐brecciated eucrites. Precise Ar‐Ar ages of 2 cumulate eucrites (Moama and EET 87520) and 4 unbrecciated eucrites give a tight cluster at 4.48 ± 0.02 Gyr (not including any uncertainties in the flux monitor age). Ar‐Ar ages of 6 additional unbrecciated eucrites are consistent with this age within their relatively larger age uncertainties. By contrast, available literature data on Pb‐Pb isochron ages of 4 cumulate eucrites and 1 unbrecciated eucrite vary over 4.4–4.515 Gyr, and 147Sm‐143Nd isochron ages of 4 cumulate and 3 unbrecciated eucrites vary over 4.41–4.55 Gyr. Similar Ar‐Ar ages for cumulate and unbrecciated eucrites imply that cumulate eucrites do not have a younger formation age than basaltic eucrites, as was previously proposed. We suggest that these cumulate and unbrecciated eucrites resided at a depth where parent body temperatures were sufficiently high to cause the K‐Ar and some other chronometers to remain as open diffusion systems. From the strong clustering of Ar‐Ar ages at ?4.48 Gyr, we propose that these meteorites were excavated from depth in a single large impact event ?4.48 Gyr ago, which quickly cooled the samples and started the K‐Ar chronometer. A large (?460 km) crater postulated to exist on Vesta may be the source of these eucrites and of many smaller asteroids thought to be spectrally or physically associated with Vesta. Some Pb‐Pb and Sm‐Nd ages of cumulate and unbrecciated eucrites are consistent with the Ar‐Ar age of 4.48 Gyr, and the few older Pb‐Pb and Sm‐Nd ages may reflect an isotopic closure before the large cratering event. One cumulate eucrite gives an Ar‐Ar age of 4.25 Gyr; 3 additional cumulate eucrites give Ar‐Ar ages of 3.4–3.7 Gyr; and 2 unbrecciated eucrites give Ar‐Ar ages of ?3.55 Gyr. We attribute these younger ages to a later impact heating. Furthermore, the Ar‐Ar impact‐reset ages of several brecciated eucrites and eucritic clasts in howardites fall within the range of 3.5–4.1 Gyr. Among these, Piplia Kalan, the first eucrite to show evidence for extinct 26Al, was strongly impact heated ?3.5 Gyr ago. When these data are combined with eucrite Ar‐Ar ages in the literature, they confirm that several large impact heating events occurred on Vesta between ?4.1–3.4 Gyr ago. The onset of major impact heating may have occurred at similar times for both Vesta and the moon, but impact heating appears to have persisted for a somewhat later time on Vesta.  相似文献   

15.
Abstract– The Northwest Africa (NWA) 1500 meteorite is an olivine‐rich achondrite containing approximately 2–3 vol% augite, 1–2 vol% plagioclase, 1 vol% chromite, and minor orthopyroxene, Cl‐apatite, metal and sulfide. It was originally classified as a ureilite, but is currently ungrouped. We re‐examined the oxygen three‐isotope composition of NWA 1500. Results of ultra‐high precision (～0.03‰ for Δ17O) laser fluorination analyses of two bulk chips, and high precision (～0.3‰) secondary ion mass spectrometry (SIMS) analyses of olivine and plagioclase in a thin section, show that the oxygen isotope composition of NWA 1500 (Δ17O = ?0.22‰ from bulk samples and ?0.18 ± 0.06‰ from 16 mineral analyses) is within the range of brachinites. We compare petrologic and geochemical characteristics of NWA 1500 with those of brachinites and other olivine‐rich primitive achondrites, including new petrographic, mineral compositional and bulk compositional data for brachinites Hughes 026, Reid 013, NWA 5191, NWA 595, and Brachina. Modal mineral abundances, texture, olivine and pyroxene major and minor element compositions, plagioclase major element compositions, rare earth element abundances, and siderophile element abundances of NWA 1500 are within the range of those in brachinites and, in most cases, well distinguished from those of winonaites/IAB silicates, acapulcoites/lodranites, ureilites, and Divnoe. NWA 1500 shows evidence of internal reduction, in the form of reversely zoned olivine (Fo ～65–73 core to rim) and fine‐grained intergrowths of orthopyroxene + metal along olivine grain margins. The latter also occur in Reid 013, Hughes 026, NWA 5191, and NWA 595. We argue that reduction (olivine→enstatite + Fe0 + O2) is the best hypothesis for their origin in these samples as well. We suggest that NWA 1500 should be classified as a brachinite, which has implications for the petrogenesis of brachinites. Fe‐Mn‐Mg compositions of brachinite olivine provide evidence of redox processes among bulk samples. NWA 1500 provides evidence for redox processes on a smaller scale as well, which supports the interpretation that these processes occurred in a parent body setting. SIMS data for 26Al‐26Mg isotopes in plagioclase in NWA 1500 show no 26Mg excesses beyond analytical uncertainties (1–2‰). The calculated upper limit for the initial 26Al/27Al ratio of the plagioclase corresponds to an age younger than 7 Ma after CAI. Compared to 53Mn‐53Cr data for Brachina ( Wadhwa et al. 1998b ), this implies either a much younger formation age or a more protracted cooling history. However, Brachina is atypical and this comparison may not extend to other brachinites.  相似文献   

16.
Abstract– We report on mineralogy, petrography, and whole‐rock 26Al‐26Mg systematics of eight amoeboid olivine aggregates (AOAs) from the oxidized CV chondrite Allende. The AOAs consist of forsteritic olivine, opaque nodules, and variable amounts of Ca,Al‐rich inclusions (CAIs) of different types, and show evidence for alteration to varying degrees. Melilite and anorthite are replaced by nepheline, sodalite, and grossular; spinel is enriched in FeO; opaque nodules are replaced by Fe,Ni‐sulfides, ferroan olivine and Ca,Fe‐rich pyroxenes; forsteritic olivine is enriched in FeO and often overgrown by ferroan olivine. The AOAs are surrounded by fine‐grained, matrix‐like rims composed mainly of ferroan olivine and by a discontinuous layer of Ca,Fe‐rich silicates. These observations indicate that AOAs experienced in situ elemental open‐system iron‐alkali‐halogen metasomatic alteration during which Fe, Na, Cl, and Si were introduced, whereas Ca was removed from AOAs and used to form the Ca,Fe‐rich silicate rims around AOAs. The whole‐rock 26Al‐26Mg systematics of the Allende AOAs plot above the isochron of the whole‐rock Allende CAIs with a slope of (5.23 ± 0.13) × 10?5 reported by Jacobsen et al. (2008) . In contrast, whole‐rock 26Al‐26Mg isotope systematics of CAIs and AOAs from the reduced CV chondrite Efremovka define a single isochron with a slope of (5.25± 0.01) × 10?5 ( Larsen et al. 2011 ). We infer that the excesses in 26Mg* present in Allende AOAs are due to their late‐stage open‐system metasomatic alteration. Thus, the 26Al‐26Mg isotope systematics of Allende CAIs and AOAs are disturbed by parent body alteration processes, and may not be suitable for high‐precision chronology of the early solar system events and processes.  相似文献   

17.
Abstract— An acapulcoite, Northwest Africa (NWA) 725, a transitional acapulcoite, Graves Nunataks (GRA) 95209, and a lodranite, NWA 2235, have been studied with the short‐lived chronometer 182Hf‐182W system in order to better constrain the early evolution history in the acapulcoite‐lodranite parent body. Unlike the more evolved achondrites originating from differentiated asteroids—e.g., eucrites and angrites—bulk rock acapulcoites and lodranite are characterized by distinct 182W deficits relative to the terrestrial W, as well as to the undifferentiated chondrites, εw varies from ?2.7 to ?2.4. This suggests that live‐182Hf was present during the formation of acapulcoites and lodranites, and their parent body probably had never experienced a global melting event. Due to the large uncertainties associated with the isochron for each sample, the bulk isochron that regressed through the mineral separates from all 3 samples has provided the best estimate to date for the timing of metamorphism in the acapulcoite‐lodranite parent body, 5 (+6/‐5) Myr after the onset of the solar system. It is thus inconclusive whether acapulcoites and lodranites have shared the same petrogenetic origin, based on the Hf‐W data of this study. Nevertheless, the formation of acapulcoite‐lodranite clan appears to have post‐dated the metal‐silicate segregation in differentiated asteroids. This can be explained by a slower accretion rate for the acapulcoite‐lodranite parent body, or that it had never accreted to a critical mass that could allow the metal‐silicate segregation to occur naturally.  相似文献   

18.
Abstract— Antarctic meteorite Queen Alexandra Range (QUE) 94201 is a 12 g basaltic achondrite dominated by plagioclase (now maskelynite) and zoned low‐ and high‐Ca pyroxene. Petrologic, geochemical, and isotopic analyses indicate that it is related to previously described basaltic and Iherzolitic shergottites, which are a group of igneous meteorites that are believed to be from Mars. Unlike previous shergottites, however, QUE 94201 represents a bulk melt rather than a cumulate fraction, meaning it can be used to infer magmatic source regions and the compositions of other melts on Mars. This melt has much more Fe and P than basaltic melts produced on Earth and formed at a much lower oxygen fugacity. This has altered the crystallization sequence of the melt, removing olivine from the liquidus to produce a plagioclase and 2‐pyroxene assemblage. If the high‐phosphorus and low‐oxygen fugacity conditions represented by QUE 94201 are common in magmatic regions of Mars, then olivine may be rare in marrian basalts. No solar cosmic ray effects were seen in the concentrations of 10Be, 26A1, and 36C1 with depth in the meteorite, implying at least 3 cm of ablation during entry to Earth. Significant excesses of neutron capture noble gas isotopes (80,82Kr and 128,131Xe) suggest that the QUE 94201 sample came from a depth >22 cm in a meteoroid of at least that radius. The meteorite also has very low 21Ne/22Ne, which would often be interpreted to mean little ablation (contradicting above evidence) but, in this case, appears to reflect a very low abundance of Mg (the principal target element for Ne) in the meteorite, consistent with our bulk chemical analyses. The meteorite has a terrestrial 36C1 age of 0.29 ± 0.05 Myr and a 10Be exposure age of 2.6 ± 0.5 Myr in a 47π geometry, implying an ejection age of 2.9 ± 0.5 Myr.  相似文献   

19.
Abstract— We studied the texture, mineralogy, and bulk chemical composition of Dhofar 007, a basaltic achondrite. Dhofar 007 is a polymict breccia that is mostly composed of coarse‐grained granular (CG) clasts with a minor amount of xenolithic components, such as a fragment of Mg‐rich pyroxene. The coarse‐grained, relict gabbroic texture, mineral chemistry, and bulk chemical data of the coarse‐grained clast indicate that the CG clasts were originally a cumulate rock crystallized in a crust of the parent body. However, in contrast to monomict eucrites, the siderophile elements are highly enriched and could have been introduced by impact events. Dhofar 007 appears to have experienced a two‐stage postcrystallization thermal history: rapid cooling at high temperatures and slow cooling at lower temperatures. The presence of pigeonite with closely spaced, fine augite lamellae suggests that this rock was cooled rapidly from higher temperatures (>0.5 °C/yr at ˜1000 °C) than typical cumulate eucrites. However, the presence of the cloudy zone in taenite and the Ni profile across the kamacite‐taenite boundaries indicates that the cooling rate was very slow at lower temperatures (˜1–10 °C/Myr at <600–700 °C). The slow cooling rate is comparable to those in mesosiderites and pallasites. The two‐stage thermal history and the relative abundance of siderophile elements similar to those for metallic portions in mesosiderites suggest that Dhofar 007 is a large inclusion of mesosiderite. However, we cannot rule out a possibility that Dhofar 007 is an anomalous eucrite.  相似文献   

20.
We report petrology and geochemistry of an achondrite EET 92023 and compare it with normal and anomalous eucrites. EET 92023 is an unbrecciated achondrite and shows a granular texture mainly composed of low‐Ca pyroxene and plagioclase, petrologically similar to normal cumulate eucrites such as Moore County. However, this rock contains a significant amount of kamacite and taenite not common in unbrecciated, crystalline eucrites. EET 92023 contains a significant amount of platinum group elements (PGEs) (ca. 10% of CI), several orders of magnitude higher than those of monomict eucrites. We suggest that the metallic phases carrying PGEs were incorporated by a projectile during or before igneous crystallization and thermal metamorphism. The projectile was likely to be an iron meteorite rather than chondritic materials, as indicated by the lack of olivine and the presence of free silica. Therefore, the oxygen isotopic signature is indigenous, rather than due to contamination of the projectile material with different oxygen isotopic compositions. A significant thermal event involving partial melting and metamorphism after the impact event indicates that EET 92023 records early impact events which took place shortly after the crust formation on a differentiated protoplanet when the crust was still hot.  相似文献   

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