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1.
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   

2.
Abstract— Textures, mineral assemblages, and Al‐Mg isotope systematics indicate a protracted, episodic secondary mineralization history for Allende Ca‐Al‐rich inclusions (CAIs). Detailed observations from one type B1 CAI, one B2, one compact type A (CTA), and one fluffy type A (FTA) indicate that these diverse types of CAIs are characterized by two distinct textural and mineralogic types of secondary mineralization: (1) grossular‐rich domains, concentrated along melilite grain boundaries in CAI interiors, and (2) feldspathoid‐bearing domains, confined mostly to CAI margins just interior to the Wark‐Lovering rim sequence. The Al‐Mg isotopic compositions of most secondary minerals in the type B1 CAI, and some secondary minerals in the other CAIs, show no resolvable excesses of 26Mg, whereas the primary CAI phases mostly yield correlated excesses of 26Mg with increasing Al/Mg corresponding to “canonical” initial 26Al/27Al ～ 4.5–5 × 10?5. These secondary minerals formed at least 3 Ma after the primary CAI minerals. All but two analyses of secondary minerals from the fluffy type‐A CAI define a correlated increase in 26Mg/24Mg with increasing Al/Mg, yielding (26Al/27Al)0 = (4.9 ± 2.8) × 10?6. The secondary minerals in this CAI formed 1.8–3.2 Ma after the primary CAI minerals. In both cases, the timing of secondary alteration is consistent with, but does not necessarily require, alteration in an asteroidal setting. One grossular from the type B2 CAI, and several grossular and secondary feldspar analyses from the compact type A CAI, have excesses of 26Mg consistent with initial 26Al/27Al ～ 4.5 × 10?5. Especially in the compact type A CAI, where 26Mg/24Mg in grossular correlates with increasing Al/Mg, these 26Mg excesses are almost certainly due to in situ decay of 26Al. They indicate a nebular setting for formation of the grossular. The preservation of these diverse isotopic patterns indicates that heating on the Allende parent body was not pervasive enough to reset isotopic systematics of fine‐grained secondary minerals. Secondary mineralization clearly was not restricted to a short time interval, and at least some alteration occurred coincident with CAI formation and melting events (chondrule formation) in the nebula. This observation supports the possibility that alteration followed by melting affected the compositional evolution of CAIs.  相似文献   

3.
Abstract— We report in situ magnesium isotope measurements of 7 porphyritic magnesium‐rich (type I) chondrules, 1 aluminum‐rich chondrule, and 16 refractory inclusions (14 Ca‐Al‐rich inclusions [CAIs] and 2 amoeboid olivine aggregates [AOAs]) from the ungrouped carbonaceous chondrite Acfer 094 using a Cameca IMS 6f ion microprobe. Both AOAs and 9 CAIs show radiogenic 26Mg excesses corresponding to initial 26Al/27Al ratios between ～5 × 10?5 ～7 × 10?5 suggesting that formation of the Acfer 094 CAIs may have lasted for ～300,000 years. Four CAIs show no evidence for radiogenic 26Mg; three of these inclusions (a corundum‐rich, a grossite‐rich, and a pyroxene‐hibonite spherule CAI) are very refractory objects and show deficits in 26Mg, suggesting that they probably never contained 26Al. The fourth object without evidence for radiogenic 26Mg is an anorthite‐rich, igneous (type C) CAI that could have experienced late‐stage melting that reset its Al‐Mg systematics. Significant excesses in 26Mg were observed in two chondrules. The inferred 26Al/27Al ratios in these two chondrules are (10.3 ± 7.4) × 10?6 (6.0 ± 3.8) × 10?6 (errors are 2σ), suggesting formation 1.6+1.2‐0.6 and 2.2+0.4‐0.3 Myr after CAIs with the canonical 26Al/27Al ratio of 5 × 10?5. These age differences are consistent with the inferred age differences between CAIs and chondrules in primitive ordinary (LL3.0–LL3.1) and carbonaceous (CO3.0) chondrites.  相似文献   

4.
Abstract– Mg isotope data were collected by NanoSIMS with high‐precision and high‐spatial resolution from a coarse‐grained type B Ca‐, Al‐rich inclusion (CAI), EK1‐6‐3, in the Allende CV3 chondrite to evaluate the time scale of parent body thermal metamorphism. The CAI melilite and fassaite contain excesses of 26Mg (26Mg*) from the in‐situ decay of 26Al; the inferred initial ratio, (26Al/27Al)0 = (5.8 ± 2.4) × 10?5, is consistent with many previously reported coarse‐grained CAIs from CV chondrites (e.g., MacPherson et al. 1995 ). However, the anorthite has heterogeneous (26Al/27Al)0, ranging from 1.8 × 10?5 to 3.3 × 10?6. The 26Al‐26Mg systematics within the anorthite is consistent with thermal diffusion of Mg isotopes during metamorphism. We also show that the heterogeneous distribution of 26Mg* in anorthite could have resulted from thermal diffusion of 26Mg* over a 0.6–0.8 Ma time span. Mg diffusion thus may be responsible for the (26Al/27Al)0 heterogeneity within anorthite in CAIs.  相似文献   

5.
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   

6.
We report on the primary and secondary mineralogies of three coarse-grained igneous calcium-aluminum-rich inclusions (CAIs) (Compact Type A [CTA], Type B [B], and forsterite-bearing type B [FoB]) from the Northwest Africa (NWA) 5343 (CK3.7) and NWA 4964 (CK3.8) carbonaceous chondrites, compare them with the mineralogy of igneous CAIs from the Allende (CV3.6) chondrite, and discuss the nature of the alteration processes that affected the CK and CV CAIs. The primary mineralogy and mineral chemistry of the CK3 CAIs studied are similar to those from Allende; however, primary melilite and anorthite are nearly completely absent. Although the secondary minerals identified in CK CAIs (Al-diopside, andradite, Cl-apatite, clintonite, forsterite, ferroan olivine, Fe,Ni-sulfides, grossular, ilmenite, magnetite, plagioclase, spinel, titanite, and wadalite) occur also in the Allende CAIs, there are several important differences: (i) In addition to melilite and anorthite, which are nearly completely replaced by secondary minerals, the alteration of CK CAIs also affected high-Ti pyroxenes (fassaite and grossmanite) characterized by high Ti3+/Ti4+ ratio and spinel. These pyroxenes are corroded and crosscut by veins of Fe- and Ti-bearing grossular, Fe-bearing Al,Ti-diopside, titanite, and ilmenite. Spinel is corroded by Fe-bearing Al-diopside and grossular. (ii) The secondary mineral assemblages of grossular + monticellite and grossular + wollastonite, commonly observed in the Allende CAIs, are absent; the Fe-bearing grossular + Fe-bearing Al-diopside ± Fe,Mg-spinel, Fe-bearing grossular + Fe,Mg-olivine ± Fe,Mg-spinel, and Ca,Na-plagioclase + Fe-bearing Al-diopside + Fe-bearing grossular assemblages are present instead. These mineral assemblages are often crosscut by veins of Fe-bearing Al-diopside, Fe,Mg-olivine, Fe,Mg-spinel, and Ca,Na-plagioclase. The coarse-grained secondary grossular and Al-diopside often show multilayered chemical zoning with distinct compositional boundaries between the layers; the abundances of Fe and Ti typically increase toward the grain edges. (iv) Sodium-rich secondary minerals, nepheline and sodalite, commonly observed in the peripheral portions of the Allende CAIs, are absent; Ca,Na-plagioclase is present instead. We conclude that coarse-grained igneous CAIs from CK3.7–3.8 s and Allende experienced an open-system multistage metasomatic alteration in the presence of an aqueous solution–infiltration metasomatism. This process resulted in localized mobilization of all major rock-forming elements: Si, Ca, Al, Ti, Mg, Fe, Mn, Na, K, and Cl. The metasomatic alteration of CK CAIs is more advanced and occurred under higher temperature and higher oxygen fugacity than that of the Allende CAIs.  相似文献   

7.
Abstract— We describe the mineralogy, petrology, oxygen, and magnesium isotope compositions of three coarse‐grained, igneous, anorthite‐rich (type C) Ca‐Al‐rich inclusions (CAIs) (ABC, TS26, and 93) that are associated with ferromagnesian chondrule‐like silicate materials from the CV carbonaceous chondrite Allende. The CAIs consist of lath‐shaped anorthite (An99), Cr‐bearing Al‐Ti‐diopside (Al and Ti contents are highly variable), spinel, and highly åkermanitic and Na‐rich melilite (Åk63–74, 0.4–0.6 wt% Na2O). TS26 and 93 lack Wark‐Lovering rim layers; ABC is a CAI fragment missing the outermost part. The peripheral portions of TS26 and ABC are enriched in SiO2 and depleted in TiO2 and Al2O3 compared to their cores and contain relict ferromagnesian chondrule fragments composed of forsteritic olivine (Fa6–8) and low‐Ca pyroxene/pigeonite (Fs1Wo1–9). The relict grains are corroded by Al‐Ti‐diopside of the host CAIs and surrounded by haloes of augite (Fs0.5Wo30–42). The outer portion of CAI 93 enriched in spinel is overgrown by coarse‐grained pigeonite (Fs0.5–2Wo5–17), augite (Fs0.5Wo38–42), and anorthitic plagioclase (An84). Relict olivine and low‐Ca pyroxene/pigeonite in ABC and TS26, and the pigeonite‐augite rim around 93 are 16O‐poor (Δ17O ～ ?1‰ to ?8‰). Spinel and Al‐Ti‐diopside in cores of CAIs ABC, TS26, and 93 are 16O‐enriched (Δ17O down to ?20‰), whereas Al‐Ti‐diopside in the outer zones, as well as melilite and anorthite, are 16O‐depleted to various degrees (Δ17O = ?11‰ to 2‰). In contrast to typical Allende CAIs that have the canonical initial 26Al/27Al ratio of ～5 × 10?5 ABC, 93, and TS26 are 26Al‐poor with (26Al/27Al)0 ratios of (4.7 ± 1.4) × 10?6 (1.5 ± 1.8) × 10?6 <1.2 × 10?6 respectively. We conclude that ABC, TS26, and 93 experienced remelting with addition of ferromagnesian chondrule silicates and incomplete oxygen isotopic exchange in an 16O‐poor gaseous reservoir, probably in the chondrule‐forming region. This melting episode could have reset the 26Al‐26Mg systematics of the host CAIs, suggesting it occurred ～2 Myr after formation of most CAIs. These observations and the common presence of relict CAIs inside chondrules suggest that CAIs predated formation of chondrules.  相似文献   

8.
Abstract— We have made Be‐B measurements in six calcium‐aluminum‐rich inclusions (CAIs) (mostly type B inclusions) from CV chondrites and compared them to Al‐Mg measurements. All CAIs show 10B excesses in melilite that are correlated with Be/B ratios. The initial 10Be/9Be ratio inferred from the correlation line is 6.2 times 10?4. In contrast to the Be‐B system in melilite, the Al‐Mg system in anorthite is disturbed. This is probably due to B diffusion in melilite being slow compared with Mg diffusion in anorthite. This suggests that Be‐B chronology may be useful for measuring time differences of high‐temperature (melting, condensation, etc.) events in the early solar system.  相似文献   

9.
Abstract— MacAlpine Hills (MAC) 87300 and 88107 are two unusual carbonaceous chondrites that are intermediate in chemical composition between the CO3 and CM2 meteorite groups. Calcium‐aluminum‐rich inclusions (CAIs) from these two meteorites are mostly spinel‐pyroxene and melilite‐rich (Type A) varieties. Spinel‐pyroxene inclusions have either a banded or nodular texture, with aluminous diopside rimming Fe‐poor spinel. Melilite‐rich inclusions (Åk4–42) are irregular in shape and contain minor spinel (FeO <1 wt%), perovskite and, more rarely, hibonite. The CAIs in MAC 88107 and 87300 are similar in primary mineralogy to CAIs from low petrologic grade CO3 meteorites but differ in that they commonly contain phyllosilicates. The two meteorites also differ somewhat from each other: melilite is more abundant and slightly more Al‐rich in inclusions from MAC 88107 than in those from MAC 87300, and phyllosilicate is more abundant and Mg‐poor in MAC 87300 CAIs relative to that in MAC 88107. These differences suggest that the two meteorites are not paired. The CAI sizes and the abundance of melilite‐rich CAIs in MAC 88107 and 87300 suggests a genetic relationship to CO3 meteorites, but the CAIs in both have suffered a greater degree of aqueous alteration than is observed in CO meteorites. Aluminum‐rich melilite in CAIs from both meteorites generally contains excess 26Mg, presumably from the in situ decay of 26Al. Although well‐defined isochrons are not observed, the 26Mg excesses are consistent with initial 26Al/27Al ratios of approximately 3–5 times 10?5. An unusual hibonite‐bearing inclusion is isotopically heterogeneous, with two large and abutting hibonite crystals showing significant differences in their degrees of mass‐dependent fractionation of 25Mg/24Mg. The two crystals also show differences in their inferred initial 26Al/27Al ratios, 1 × 10?5 vs. ≤3 × 10?6.  相似文献   

10.
CK chondrites are the only group of carbonaceous chondrites with petrologic types ranging from 3 to 6. Although CKs are described as calcium‐aluminum‐rich inclusion (CAI)‐poor objects, the abundance of CAIs in the 18 CK3–6 we analyzed ranges from zero to approximately 16.4%. During thermal metamorphism, some of the fine‐grained CAIs recrystallized as irregular assemblages of plagioclase + Ca‐rich pyroxene ± olivine ± Ca‐poor pyroxene ± magnetite. Coarse‐grained CAIs display zoned spinel, fassaite destabilization, and secondary grossular and spinel. Secondary anorthite, grossular, Ca‐rich pyroxene, and spinel derive from the destabilization of melilite, which is lacking in all CAIs investigated. The Al‐Mg isotopic systematics measured in fine‐ and coarse‐grained CAIs from Tanezrouft (Tnz) 057 was affected by Mg redistribution. The partial equilibration of Al‐Mg isotopic signatures obtained in the core of a coarse‐grained CAI (CG1‐CAI) in Tnz 057 may indicate a lower peak temperature for Mg diffusion of approximately 540–580 °C, while grossular present in the core of this CAI indicates a higher temperature of around 800 °C for the metamorphic event on the parent body of Tnz 057. Excluding metamorphic features, the similarity in nature and abundance of CAIs in CK and CV chondrites confirms that CVs and CKs form a continuous metamorphic series from type 3 to 6.  相似文献   

11.
Abstract— Fine‐grained, spinel‐rich inclusions in the reduced CV chondrites Efremovka and Leoville consist of spinel, melilite, anorthite, Al‐diopside, and minor hibonite and perovskite; forsterite is very rare. Several CAIs are surrounded by forsterite‐rich accretionary rims. In contrast to heavily altered fine‐grained CAIs in the oxidized CV chondrite Allende, those in the reduced CVs experienced very little alteration (secondary nepheline and sodalite are rare). The Efremovka and Leoville fine‐grained CAIs are 16O‐enriched and, like their Allende counterparts, generally have volatility fractionated group II rare earth element patterns. Three out of 13 fine‐grained CAIs we studied are structurally uniform and consist of small concentrically zoned nodules having spinel ± hibonite ± perovskite cores surrounded by layers of melilite and Al‐diopside. Other fine‐grained CAIs show an overall structural zonation defined by modal mineralogy differences between the inclusion cores and mantles. The cores are melilite‐free and consist of tiny spinel ± hibonite ± perovskite grains surrounded by layers of anorthite and Al‐diopside. The mantles are calcium‐enriched, magnesium‐depleted and coarsergrained relative to the cores; they generally contain abundant melilite but have less spinel and anorthite than the cores. The bulk compositions of fine‐grained CAIs generally show significant fractionation of Al from Ca and Ti, with Ca and Ti being depleted relative to Al; they are similar to those of coarsegrained, type C igneous CAIs, and thus are reasonable candidate precursors for the latter. The finegrained CAIs originally formed as aggregates of spinel‐perovskite‐melilite ± hibonite gas‐solid condensates from a reservoir that was 16O‐enriched but depleted in the most refractory REEs. These aggregates later experienced low‐temperature gas‐solid nebular reactions with gaseous SiO and Mg to form Al‐diopside and ±anorthite. The zoned structures of many of the fine‐grained inclusions may be the result of subsequent reheating that resulted in the evaporative loss of SiO and Mg and the formation of melilite. The inferred multi‐stage formation history of fine‐grained inclusions in Efremovka and Leoville is consistent with a complex formation history of coarse‐grained CAIs in CV chondrites.  相似文献   

12.
Abstract— Petrographic, compositional, and isotopic characteristics were studied for three calcium‐aluminum‐rich inclusions (CAIs) and four plagioclase‐bearing chondrules (three of them Al‐rich) from the Axtell (CV3) chondrite. All seven objects have analogues in Allende (CV3) and other primitive chondrites, yet Axtell, like most other chondrites, contains a distinctive suite of CAIs and chondrules. In common with Allende CAIs, CAIs in Axtell exhibit initial 26Al/27Al ratios ((26Al/27Al)0) ranging from ～5 × 10?5 to <1.1 × 10?5, and plagioclase‐bearing chondrules have (26Al/27Al)0 ratios of ～3 × 10?6 and lower. One type‐A CAI has the characteristics of a FUN inclusion. The Al‐Mg data imply that the plagioclase‐bearing chondrules began to form >2 Ma after the first CAIs. As in other CV3 chondrites, some objects in Axtell show evidence of isotopic disturbance. Axtell has experienced only mild thermal metamorphism (<600 °C), probably not enough to disturb the Al‐Mg systematics. Its CAIs and chondrules have suffered extensive metasomatism, probably prior to final accretion. These data indicate that CAIs and chondrules in Axtell (and other meteorites) had an extended history of several million years before their incorporation into the Axtell parent body. These long time periods appear to require a mechanism in the early solar system to prevent CAIs and chondrules from falling into the Sun via gas drag for several million years before final accretion. We also examined the compositional relationships among the four plagioclase‐bearing chondrules (two with large anorthite laths and two barred‐olivine chondrules) and between the chondrules and CAIs. Three processes were examined: (1) igneous differentiation, (2) assimilation of a CAI by average nebular material, and (3) evaporation of volatile elements from average nebular material. We find no evidence that igneous differentiation played a role in producing the chondrule compositions, although the barred olivine compositions can be related by addition or subtraction of olivine. Methods (2) and (3) could have produced the composition of one chondrule, AXCH‐1471, but neither process explains the other compositions. Our study indicates that plagioclase‐bearing objects originated through a variety of processes.  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— 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   

15.
Abstract— Rumuruti chondrites (R chondrites) constitute a well‐characterized chondrite group different from carbonaceous, ordinary, and enstatite chondrites. Many of these meteorites are breccias containing primitive type 3 fragments as well as fragments of higher petrologic type. Ca,Al‐rich inclusions (CAIs) occur within all lithologies. Here, we present the results of our search for and analysis of Al‐rich objects in Rumuruti chondrites. We studied 20 R chondrites and found 126 Ca,Al‐rich objects (101 CAIs, 19 Al‐rich chondrules, and 6 spinel‐rich fragments). Based on mineralogical characterization and analysis by SEM and electron microprobe, the inclusions can be grouped into six different types: (1) simple concentric spinel‐rich inclusions (42), (2) fassaite‐rich spherules, (3) complex spinel‐rich CAIs (53), (4) complex diopside‐rich inclusions, (5) Al‐rich chondrules, and (6) Al‐rich (spinel‐rich) fragments. The simple concentric and complex spinel‐rich CAIs have abundant spinel and, based on the presence or absence of different major phases (fassaite, hibonite, Na,Al‐(Cl)‐rich alteration products), can be subdivided into several subgroups. Although there are some similarities between CAIs from R chondrites and inclusions from other chondrite groups with respect to their mineral assemblages, abundance, and size, the overall assemblage of CAIs is distinct to the R‐chondrite group. Some Ca,Al‐rich inclusions appear to be primitive (e.g., low FeO‐contents in spinel, low abundances of Na,Al‐(Cl)‐rich alteration products; abundant perovskite), whereas others were highly altered by nebular and/or parent body processes (e.g., high concentrations of FeO and ZnO in spinel, ilmenite instead of perovskite, abundant Na,Al‐(Cl)‐rich alteration products). There is complete absence of grossite and melilite, which are common in CAIs from most other groups. CAIs from equilibrated R‐chondrite lithologies have abundant secondary Ab‐rich plagioclase (oligoclase) and differ from those in unequilibrated type 3 lithologies which have nepheline and sodalite instead.  相似文献   

16.
We report an occurrence of hexagonal CaAl2Si2O8 (dmisteinbergite) in a compact type A calcium‐aluminum‐rich inclusion (CAI) from the CV3 (Vigarano‐like) carbonaceous chondrite Northwest Africa 2086. Dmisteinbergite occurs as approximately 10 μm long and few micrometer‐thick lath‐shaped crystal aggregates in altered parts of the CAI, and is associated with secondary nepheline, sodalite, Ti‐poor Al‐diopside, grossular, and Fe‐rich spinel. Spinel is the only primary CAI mineral that retained its original O‐isotope composition (Δ17O ~ ?24‰); Δ17O values of melilite, perovskite, and Al,Ti‐diopside range from ?3 to ?11‰, suggesting postcrystallization isotope exchange. Dmisteinbergite, anorthite, Ti‐poor Al‐diopside, and ferroan olivine have 16O‐poor compositions (Δ17O ~ ?3‰). We infer that dmisteinbergite, together with the other secondary minerals, formed by replacement of melilite as a result of fluid‐assisted thermal metamorphism experienced by the CV chondrite parent asteroid. Based on the textural appearance of dmisteinbergite in NWA 2086 and petrographic observations of altered CAIs from the Allende meteorite, we suggest that dmisteinbergite is a common secondary mineral in CAIs from the oxidized Allende‐like CV3 chondrites that has been previously misidentified as a secondary anorthite.  相似文献   

17.
Abstract— High‐precision Mg isotopic compositions of Ca‐Al‐rich inclusions (CAIs) from both Ningqiang (ungrouped) and Allende (CV3) carbonaceous chondrites and amoeboid olivine aggregations (AOAs) from Allende were analyzed by multicollector inductively coupled plasma mass spectrometry (MC‐ICP‐MS). The CAIs from Allende plot on a line, with an inferred initial 26Al/27Al ratio of (4.77 ± 0.39) × 10?5 close to the canonical value. This indicates a relatively closed Al‐Mg system in the CAIs and no significant Mg isotope exchange with ambient materials, although two of the CAIs are severely altered. The AOAs contain excess 26Mg and plot close to the CAI regression line, which is suggestive of their contemporary formation. The CAIs from Ningqiang define a different line with a lower inferred (26Al/27Al)0 ratio of (3.56 ± 0.08) × 10?5. None of the CAIs and AOAs studied in this work shows significant mass fractionation with enrichment of the heavier Mg isotopes, arguing against an evaporation origin.  相似文献   

18.
The distribution of the short‐lived radionuclide 26Al in the early solar system remains a major topic of investigation in planetary science. Thousands of analyses are now available but grossite‐bearing Ca‐, Al‐rich inclusions (CAIs) are underrepresented in the database. Recently found grossite‐bearing inclusions in CO3 chondrites provide an opportunity to address this matter. We determined the oxygen and magnesium isotopic compositions of individual phases of 10 grossite‐bearing CAIs in the Dominion Range (DOM) 08006 (CO3.0) and DOM 08004 (CO3.1) chondrites. All minerals in DOM 08006 CAIs as well as hibonite, spinel, and pyroxene in DOM 08004 are uniformly 16O‐rich (Δ17O = ?25 to ?20‰) but grossite and melilite in DOM 08004 CAIs are not; Δ17O of grossite and melilite range from ~ ?11 to ~0‰ and from ~ ?23 up to ~0‰, respectively. Even within this small suite, in the two chondrites a bimodal distribution of the inferred initial 26Al/27Al ratios (26Al/27Al)0 is seen, with four having (26Al/27Al)0 ≤1.1 × 10?5 and six having (26Al/27Al)0 ≥3.7 × 10?5. Five of the 26Al‐rich CAIs have (26Al/27Al)0 within error of 4.5 × 10?5; these values can probably be considered indistinguishable from the “canonical” value of 5.2 × 10?5 given the uncertainty in the relative sensitivity factor for grossite measured by secondary ion mass spectrometry. We infer that the 26Al‐poor CAIs probably formed before the radionuclide was fully mixed into the solar nebula. All minerals in the DOM 08006 CAIs, as well as spinel, hibonite, and Al‐diopside in the DOM 08004 CAIs retained their initial oxygen isotopic compositions, indicating homogeneity of oxygen isotopic compositions in the nebular region where the CO grossite‐bearing CAIs originated. Oxygen isotopic heterogeneity in CAIs from DOM 08004 resulted from exchange between the initially 16O‐rich (Δ17O ~?24‰) melilite and grossite and 16O‐poor (Δ17O ~0‰) fluid during hydrothermal alteration on the CO chondrite parent body; hibonite, spinel, and Al‐diopside avoided oxygen isotopic exchange during the alteration. Grossite and melilite that underwent oxygen isotopic exchange avoided redistribution of radiogenic 26Mg and preserved undisturbed internal Al‐Mg isochrons. The Δ17O of the fluid can be inferred from O‐isotopic compositions of aqueously formed fayalite and magnetite that precipitated from the fluid on the CO parent asteroid. This and previous studies suggest that O‐isotope exchange during fluid–rock interaction affected most CAIs in CO ≥3.1 chondrites.  相似文献   

19.
Abstract— Anorthite‐rich chondrules in CR and CH carbonaceous chondrites consist of magnesian low‐Ca pyroxene and forsterite phenocrysts, FeNi‐metal nodules, interstitial anorthite, Al‐Ti‐Cr‐rich low‐Ca and high‐Ca pyroxenes, and crystalline mesostasis composed of silica, anorthite and high‐Ca pyroxene. Three anorthite‐rich chondrules contain relic calcium‐aluminum‐rich inclusions (CAIs) composed of anorthite, spinel, ±Al‐diopside, and ± forsterite. A few chondrules contain regions which are texturally and mineralogically similar to magnesian (type I) chondrules and consist of forsterite, low‐Ca pyroxene and abundant FeNi‐metal nodules. Anorthite‐rich chondrules in CR and CH chondrites are mineralogically similar to those in CV and CO carbonaceous chondrites, but contain no secondary nepheline, sodalite or ferrosilite. Relatively high abundances of moderately‐volatile elements such as Cr, Mn and Si in the anorthite‐rich chondrules suggest that these chondrules could not have been produced by volatilization of the ferromagnesian chondrule precursors or by melting of the refractory materials only. We infer instead that anorthite‐rich chondrules in carbonaceous chondrites formed by melting of the reduced chondrule precursors (olivine, pyroxenes, FeNi‐metal) mixed with the refractory materials, including relic CAIs, composed of anorthite, spinel, high‐Ca pyroxene and forsterite. The observed mineralogical and textural similarities of the anorthite‐rich chondrules in several carbonaceous chondrite groups (CV, CO, CH, CR) may indicate that these chondrules formed in the region(s) intermediate between the regions where CAIs and ferromagnesian chondrules originated. This may explain the relative enrichment of anorthite‐rich chondrules in 16O compared to typical ferromagnesian chondrules (Russell et al., 2000).  相似文献   

20.
Abstract— We report detailed chemical, petrological, and mineralogical studies on the Ningqiang carbonaceous chondrite. Ningqiang is a unique ungrouped type 3 carbonaceous chondrite. Its bulk composition is similar to that of CV and CK chondrites, but refractory lithophile elements (1.01 × CI) are distinctly depleted relative to CV (1.29 × CI) and CK (1.20 × CI) chondrites. Ningqiang consists of 47.5 vol% chondrules, 2.0 vol% Ca,Al‐rich inclusions (CAIs), 4.5 vol% amoeboid olivine aggregates (AOAs), and 46.0 vol% matrix. Most chondrules (95%) in Ningqiang are Mg‐rich. The abundances of Fe‐rich and Al‐rich chondrules are very low. Al‐rich chondrules (ARCs) in Ningqiang are composed mainly of olivine, plagioclase, spinel, and pyroxenes. In ARCs, spinel and plagioclase are enriched in moderately volatile elements (Cr, Mn, and Na), and low‐Ca pyroxenes are enriched in refractory elements (Al and Ti). The petrology and mineralogy of ARCs in Ningqiang indicate that they were formed from hybrid precursors of ferromagnesian chondrules mixed with refractory materials during chondrule formation processes. We found 294 CAIs (55.0% type A, 39.5% spinel‐pyroxene‐rich, 4.4% hibonite‐rich, and several type C and anorthite‐spinel‐rich inclusions) and 73 AOAs in 15 Ningqiang sections (equivalent to 20 cm2surface area). This is the first report of hibonite‐rich inclusions in Ningqiang. They are texturally similar to those in CM, CH, and CB chondrites, and exhibit three textural forms: aggregates of euhedral hibonite single crystals, fine‐grained aggregates of subhedral hibonite with minor spinel, and hibonite ± Al,Ti‐diopside ± spinel spherules. Evidence of secondary alteration is ubiquitous in Ningqiang. Opaque assemblages, formed by secondary alteration of pre‐existing alloys on the parent body, are widespread in chondrules and matrix. On the other hand, nepheline and sodalite, existing in all chondritic components, formed by alkali‐halogen metasomatism in the solar nebula.  相似文献   

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