We report both oxygen-and magnesium-isotope compositions measured in situ using a Cameca ims-1280... more We report both oxygen-and magnesium-isotope compositions measured in situ using a Cameca ims-1280 ion microprobe in 20 of 166 CAIs identified in 47 polished sections of 15 CR2 (Renazzo-type) carbonaceous chondrites. Two additional CAIs were measured for oxygen isotopes only. Most CR2 CAIs are mineralogically pristine; only few contain secondary phyllosilicates, sodalite, and carbonates -most likely products of aqueous alteration on the CR2 chondrite parent asteroid. Spinel, hibonite, grossite, anorthite, and melilite in 18 CAIs have 16 O-rich (D 17 O = À23.3 ± 1.9&, 2r error) compositions and show no evidence for postcrystallization isotopic exchange commonly observed in CAIs from metamorphosed CV carbonaceous chondrites. The inferred initial 26 Al/ 27 Al ratios, ( 26 Al/ 27 Al) 0 , in 15 of 16 16 O-rich CAIs measured are consistent with the canonical value of (4.5-5) Â 10 À5 and a short duration (<0.5 My) of CAI formation. These data do not support the ''supra-canonical" values of ( 26 Al/ 27 Al) 0 [(5.85-7) Â 10 À5 ] inferred from whole-rock and mineral isochrons of the CV CAIs. A hibonite-grossite-rich CAI El Djouf 001 MK #5 has uniformly 16 O-rich (D 17 O = À23.0 ± 1.7&) composition, but shows a deficit of 26 Mg and no evidence for 26 Al. Because this inclusion is 16 O-rich, like CAIs with the canonical ( 26 Al/ 27 Al) 0 , we infer that it probably formed early, like typical CAIs, but from precursors with slightly nonsolar magnesium and lower-than-canonical 26 Al abundance. Another 16 O-enriched (D 17 O = À20.3 ± 1.2&) inclusion, a spinel-melilite CAI fragment Gao-Guenie (b) #3, has highly-fractionated oxygen-and magnesium-isotope compositions ($11 and 23&/amu, respectively), a deficit of 26 Mg, and a relatively low ( 26 Al/ 27 Al) 0 = (2.0 ± 1.7) Â 10 À5 . This could be the first FUN (Fractionation and Unidentified Nuclear effects) CAI found in CR2 chondrites. Because this inclusion is slightly 16 O-depleted compared to most CR2 CAIs and has lower than the canonical ( 26 Al/ 27 Al) 0 , it may have experienced multistage formation from precursors with nonsolar magnesium-isotope composition and recorded evolution of oxygen-isotope composition in the early solar nebula over 0:9 À0:7 þ2:2 My. Eight of the 166 CR2 CAIs identified are associated with chondrule materials, indicating that they experienced late-stage, incomplete melting during chondrule formation. Three of these CAIs show large variations in oxygen-isotope compositions (D 17 O ranges from À23.5& to À1.7&), suggesting dilution by 16 O-depleted chondrule material and possibly exchange with an 16 O-poor (D 17 O > À5&) nebular gas. The low inferred ( 26 Al/ 27 Al) 0 ratios of these CAIs (<0.7 Â 10 À5 ) indicate melting >2 My after crystallization of CAIs with the canonical ( 26 Al/ 27 Al) 0 and suggest evolution of the oxygen-isotope composition of the inner solar nebula on a similar or a shorter timescale. Because CAIs in CR2 and CV chondrites appear to have originated in a similarly 16 O-rich reservoir and only a small number of CR2 and CV CAIs were affected by chondrule melting events in an 16 O-poor gaseous reservoir, the commonly observed oxygen-isotope heterogeneity in CAIs from metamorphosed CV chondrites is most likely due to fluid-solid isotope exchange on the CV asteroidal body rather than gas-melt exchange. This conclusion does not preclude that some CV CAIs experienced oxygen-isotope exchange during remelting, 0016-7037/$ -see front matter Ó Geochimica et Cosmochimica Acta 73 instead it implies that such remelting is unlikely to be the dominant process responsible for oxygen-isotope heterogeneity in CV CAIs. The mineralogy, oxygen and magnesium-isotope compositions of CAIs in CR2 chondrites are different from those in the metal-rich, CH and CB carbonaceous chondrites, providing no justification for grouping CR, CH and CB chondrites into the CR clan.
a b s t r a c t R chondrites are a distinct group of chondritic meteorites with unique mineralogi... more a b s t r a c t R chondrites are a distinct group of chondritic meteorites with unique mineralogical and chemical compositions. They contain various types of Al-rich objects [Ca,Al-rich inclusions (CAIs), Al-rich chondrules and fragments], whose mineralogical compositions and classifications were previously determined by . Here, we report on the bulk compositions of 126 such Al-rich objects determined by broad-beam electron probe microanalysis.
Abstract— Rumuruti chondrites (R chondrites) constitute a well-characterized chondrite group diff... more 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.
We report both oxygen-and magnesium-isotope compositions measured in situ using a Cameca ims-1280... more We report both oxygen-and magnesium-isotope compositions measured in situ using a Cameca ims-1280 ion microprobe in 20 of 166 CAIs identified in 47 polished sections of 15 CR2 (Renazzo-type) carbonaceous chondrites. Two additional CAIs were measured for oxygen isotopes only. Most CR2 CAIs are mineralogically pristine; only few contain secondary phyllosilicates, sodalite, and carbonates -most likely products of aqueous alteration on the CR2 chondrite parent asteroid. Spinel, hibonite, grossite, anorthite, and melilite in 18 CAIs have 16 O-rich (D 17 O = À23.3 ± 1.9&, 2r error) compositions and show no evidence for postcrystallization isotopic exchange commonly observed in CAIs from metamorphosed CV carbonaceous chondrites. The inferred initial 26 Al/ 27 Al ratios, ( 26 Al/ 27 Al) 0 , in 15 of 16 16 O-rich CAIs measured are consistent with the canonical value of (4.5-5) Â 10 À5 and a short duration (<0.5 My) of CAI formation. These data do not support the ''supra-canonical" values of ( 26 Al/ 27 Al) 0 [(5.85-7) Â 10 À5 ] inferred from whole-rock and mineral isochrons of the CV CAIs. A hibonite-grossite-rich CAI El Djouf 001 MK #5 has uniformly 16 O-rich (D 17 O = À23.0 ± 1.7&) composition, but shows a deficit of 26 Mg and no evidence for 26 Al. Because this inclusion is 16 O-rich, like CAIs with the canonical ( 26 Al/ 27 Al) 0 , we infer that it probably formed early, like typical CAIs, but from precursors with slightly nonsolar magnesium and lower-than-canonical 26 Al abundance. Another 16 O-enriched (D 17 O = À20.3 ± 1.2&) inclusion, a spinel-melilite CAI fragment Gao-Guenie (b) #3, has highly-fractionated oxygen-and magnesium-isotope compositions ($11 and 23&/amu, respectively), a deficit of 26 Mg, and a relatively low ( 26 Al/ 27 Al) 0 = (2.0 ± 1.7) Â 10 À5 . This could be the first FUN (Fractionation and Unidentified Nuclear effects) CAI found in CR2 chondrites. Because this inclusion is slightly 16 O-depleted compared to most CR2 CAIs and has lower than the canonical ( 26 Al/ 27 Al) 0 , it may have experienced multistage formation from precursors with nonsolar magnesium-isotope composition and recorded evolution of oxygen-isotope composition in the early solar nebula over 0:9 À0:7 þ2:2 My. Eight of the 166 CR2 CAIs identified are associated with chondrule materials, indicating that they experienced late-stage, incomplete melting during chondrule formation. Three of these CAIs show large variations in oxygen-isotope compositions (D 17 O ranges from À23.5& to À1.7&), suggesting dilution by 16 O-depleted chondrule material and possibly exchange with an 16 O-poor (D 17 O > À5&) nebular gas. The low inferred ( 26 Al/ 27 Al) 0 ratios of these CAIs (<0.7 Â 10 À5 ) indicate melting >2 My after crystallization of CAIs with the canonical ( 26 Al/ 27 Al) 0 and suggest evolution of the oxygen-isotope composition of the inner solar nebula on a similar or a shorter timescale. Because CAIs in CR2 and CV chondrites appear to have originated in a similarly 16 O-rich reservoir and only a small number of CR2 and CV CAIs were affected by chondrule melting events in an 16 O-poor gaseous reservoir, the commonly observed oxygen-isotope heterogeneity in CAIs from metamorphosed CV chondrites is most likely due to fluid-solid isotope exchange on the CV asteroidal body rather than gas-melt exchange. This conclusion does not preclude that some CV CAIs experienced oxygen-isotope exchange during remelting, 0016-7037/$ -see front matter Ó Geochimica et Cosmochimica Acta 73 instead it implies that such remelting is unlikely to be the dominant process responsible for oxygen-isotope heterogeneity in CV CAIs. The mineralogy, oxygen and magnesium-isotope compositions of CAIs in CR2 chondrites are different from those in the metal-rich, CH and CB carbonaceous chondrites, providing no justification for grouping CR, CH and CB chondrites into the CR clan.
a b s t r a c t R chondrites are a distinct group of chondritic meteorites with unique mineralogi... more a b s t r a c t R chondrites are a distinct group of chondritic meteorites with unique mineralogical and chemical compositions. They contain various types of Al-rich objects [Ca,Al-rich inclusions (CAIs), Al-rich chondrules and fragments], whose mineralogical compositions and classifications were previously determined by . Here, we report on the bulk compositions of 126 such Al-rich objects determined by broad-beam electron probe microanalysis.
Abstract— Rumuruti chondrites (R chondrites) constitute a well-characterized chondrite group diff... more 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.
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