Whole-rock 26Al-26Mg systematics of amoeboid olivine aggregates from the oxidized CV3 carbonaceous chondrite Allende

Mia Bjørg Stolberg Olsen; A.N. Krot; Kirsten Kolbjørn Larsen; Chad Paton; Daniel Kim Peel Wielandt; Martin Schiller; Martin Bizzarro

doi:10.1111/j.1945-5100.2011.01256.x

Whole-rock ²⁶Al-²⁶Mg systematics of amoeboid olivine aggregates from the oxidized CV3 carbonaceous chondrite Allende

Mia Bjørg Stolberg Olsen, A.N. Krot, Kirsten Kolbjørn Larsen, Chad Paton, Daniel Kim Peel Wielandt, Martin Schiller, Martin Bizzarro

4 Citationer (Scopus)

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.

Originalsprog	Engelsk
Tidsskrift	Meteoritics and Planetary Science
Vol/bind	46
Udgave nummer	11
Sider (fra-til)	1688-1702
Antal sider	15
ISSN	1086-9379
DOI	https://doi.org/10.1111/j.1945-5100.2011.01256.x
Status	Udgivet - nov. 2011

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@article{c3966ce29cbc49e5b5c8b5be6a39417d,

title = "Whole-rock 26Al-26Mg systematics of amoeboid olivine aggregates from the oxidized CV3 carbonaceous chondrite Allende",

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.",

author = "Olsen, {Mia Bj{\o}rg Stolberg} and A.N. Krot and Larsen, {Kirsten Kolbj{\o}rn} and Chad Paton and Wielandt, {Daniel Kim Peel} and Martin Schiller and Martin Bizzarro",

year = "2011",

month = nov,

doi = "10.1111/j.1945-5100.2011.01256.x",

language = "English",

volume = "46",

pages = "1688--1702",

journal = "Meteoritics and Planetary Science",

issn = "1086-9379",

publisher = "JohnWiley & Sons, Inc.",

number = "11",

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T1 - Whole-rock 26Al-26Mg systematics of amoeboid olivine aggregates from the oxidized CV3 carbonaceous chondrite Allende

AU - Olsen, Mia Bjørg Stolberg

AU - Krot, A.N.

AU - Larsen, Kirsten Kolbjørn

AU - Paton, Chad

AU - Wielandt, Daniel Kim Peel

AU - Schiller, Martin

AU - Bizzarro, Martin

PY - 2011/11

Y1 - 2011/11

N2 - 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.

AB - 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.

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DO - 10.1111/j.1945-5100.2011.01256.x

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