The origin of the unique achondrite Northwest Africa 6704

Constraints from petrology, chemistry and Re–Os, O and Ti isotope systematics

Yuki Hibiya, Gregory J. Archer, Ryoji Tanaka, Matthew E. Sanborn, Yuya Sato, Tsuyoshi Iizuka, Kazuhito Ozawa, Richard J. Walker, Akira Yamaguchi, Qing Zhu Yin, Tomoki Nakamura, Anthony J. Irving

Research output: Contribution to journalArticle

6 Citations (Scopus)

Abstract

Northwest Africa (NWA) 6704 is a unique achondrite characterized by a near-chondritic major element composition with a remarkably intact igneous texture. To investigate the origin of this unique achondrite, we have conducted a combined petrologic, chemical, and 187Re–187Os, O, and Ti isotopic study. The meteorite consists of orthopyroxene megacrysts (En55–57Wo3–4Fs40–42; Fe/Mn = 1.4) up to 1.7 cm in length with finer interstices of olivine (Fa50–53; Fe/Mn = 1.1–2.1), chromite (Cr# ∼ 0.94), awaruite, sulfides, plagioclase (Ab92An5Or3) and merrillite. The results of morphology, lattice orientation analysis, and mineral chemistry indicate that orthopyroxene megacrysts were originally hollow dendrites that most likely crystallized under high super-saturation and super-cooling conditions (1–102 °C/h), whereas the other phases crystallized between branches of the dendrites in the order of awaruite, chromite → olivine → merrillite → plagioclase. In spite of the inferred high super-saturation, the remarkably large size of orthopyroxene can be explained as a result of crystallization from a melt containing a limited number of nuclei that are preserved as orthopyroxene megacryst cores having high Mg# or including vermicular olivine. The Re–Os isotope data for bulk and metal fractions yield an isochron age of 4576 ± 250 Ma, consistent with only limited open system behavior of highly siderophile elements (HSE) since formation. The bulk chemical composition is characterized by broadly chondritic absolute abundances and only weakly fractionated chondrite-normalized patterns for HSE and rare earth elements (REE), together with substantial depletion of highly volatile elements relative to chondrites. The HSE and REE characteristics indicate that the parental melt and its protolith had not undergone significant segregation of metals, sulfides, or silicate minerals. These combined results suggest that a chondritic precursor to NWA 6704 was heated well above its liquidus temperature so that highly volatile elements were lost and the generated melt initially contained few nuclei of relict orthopyroxene, but the melting and subsequent crystallization took place on a timescale too short to allow magmatic differentiation. Such rapid melting and crystallization might occur as a result of impact on an undifferentiated asteroid. The O–Ti isotope systematics (Δ17O = −1.052 ± 0.004, 2 SD; ε50Ti = 2.28 ± 0.23, 2 SD) indicate that the NWA 6704 parent body sampled the same isotopic reservoirs in the solar nebula as the carbonaceous chondrite parent bodies. This is consistent with carbonaceous chondrite-like refractory element abundances and oxygen fugacity (FMQ = −2.6) in NWA 6704. Yet, the Si/Mg ratio of NWA 6704 is remarkably higher than those of carbonaceous chondrites, suggesting significant nebular fractionation of forsterite in its provenance.

Original languageEnglish
JournalGeochimica et Cosmochimica Acta
DOIs
Publication statusAccepted/In press - Jan 1 2018

Fingerprint

Petrology
achondrite
Crystallization
orthopyroxene
Isotopes
petrology
Chromite
siderophile element
Supersaturation
isotope
carbonaceous chondrite
Rare earth elements
Melting
Metals
volatile element
olivine
crystallization
Silicate minerals
Meteorites
parent body

Keywords

  • Highly siderophile elements
  • Impact melting
  • Nucleosynthetic anomalies
  • NWA 6704
  • Primitive achondrite

ASJC Scopus subject areas

  • Geochemistry and Petrology

Cite this

The origin of the unique achondrite Northwest Africa 6704 : Constraints from petrology, chemistry and Re–Os, O and Ti isotope systematics. / Hibiya, Yuki; Archer, Gregory J.; Tanaka, Ryoji; Sanborn, Matthew E.; Sato, Yuya; Iizuka, Tsuyoshi; Ozawa, Kazuhito; Walker, Richard J.; Yamaguchi, Akira; Yin, Qing Zhu; Nakamura, Tomoki; Irving, Anthony J.

In: Geochimica et Cosmochimica Acta, 01.01.2018.

Research output: Contribution to journalArticle

Hibiya, Yuki ; Archer, Gregory J. ; Tanaka, Ryoji ; Sanborn, Matthew E. ; Sato, Yuya ; Iizuka, Tsuyoshi ; Ozawa, Kazuhito ; Walker, Richard J. ; Yamaguchi, Akira ; Yin, Qing Zhu ; Nakamura, Tomoki ; Irving, Anthony J. / The origin of the unique achondrite Northwest Africa 6704 : Constraints from petrology, chemistry and Re–Os, O and Ti isotope systematics. In: Geochimica et Cosmochimica Acta. 2018.
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T1 - The origin of the unique achondrite Northwest Africa 6704

T2 - Constraints from petrology, chemistry and Re–Os, O and Ti isotope systematics

AU - Hibiya, Yuki

AU - Archer, Gregory J.

AU - Tanaka, Ryoji

AU - Sanborn, Matthew E.

AU - Sato, Yuya

AU - Iizuka, Tsuyoshi

AU - Ozawa, Kazuhito

AU - Walker, Richard J.

AU - Yamaguchi, Akira

AU - Yin, Qing Zhu

AU - Nakamura, Tomoki

AU - Irving, Anthony J.

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N2 - Northwest Africa (NWA) 6704 is a unique achondrite characterized by a near-chondritic major element composition with a remarkably intact igneous texture. To investigate the origin of this unique achondrite, we have conducted a combined petrologic, chemical, and 187Re–187Os, O, and Ti isotopic study. The meteorite consists of orthopyroxene megacrysts (En55–57Wo3–4Fs40–42; Fe/Mn = 1.4) up to 1.7 cm in length with finer interstices of olivine (Fa50–53; Fe/Mn = 1.1–2.1), chromite (Cr# ∼ 0.94), awaruite, sulfides, plagioclase (Ab92An5Or3) and merrillite. The results of morphology, lattice orientation analysis, and mineral chemistry indicate that orthopyroxene megacrysts were originally hollow dendrites that most likely crystallized under high super-saturation and super-cooling conditions (1–102 °C/h), whereas the other phases crystallized between branches of the dendrites in the order of awaruite, chromite → olivine → merrillite → plagioclase. In spite of the inferred high super-saturation, the remarkably large size of orthopyroxene can be explained as a result of crystallization from a melt containing a limited number of nuclei that are preserved as orthopyroxene megacryst cores having high Mg# or including vermicular olivine. The Re–Os isotope data for bulk and metal fractions yield an isochron age of 4576 ± 250 Ma, consistent with only limited open system behavior of highly siderophile elements (HSE) since formation. The bulk chemical composition is characterized by broadly chondritic absolute abundances and only weakly fractionated chondrite-normalized patterns for HSE and rare earth elements (REE), together with substantial depletion of highly volatile elements relative to chondrites. The HSE and REE characteristics indicate that the parental melt and its protolith had not undergone significant segregation of metals, sulfides, or silicate minerals. These combined results suggest that a chondritic precursor to NWA 6704 was heated well above its liquidus temperature so that highly volatile elements were lost and the generated melt initially contained few nuclei of relict orthopyroxene, but the melting and subsequent crystallization took place on a timescale too short to allow magmatic differentiation. Such rapid melting and crystallization might occur as a result of impact on an undifferentiated asteroid. The O–Ti isotope systematics (Δ17O = −1.052 ± 0.004, 2 SD; ε50Ti = 2.28 ± 0.23, 2 SD) indicate that the NWA 6704 parent body sampled the same isotopic reservoirs in the solar nebula as the carbonaceous chondrite parent bodies. This is consistent with carbonaceous chondrite-like refractory element abundances and oxygen fugacity (FMQ = −2.6) in NWA 6704. Yet, the Si/Mg ratio of NWA 6704 is remarkably higher than those of carbonaceous chondrites, suggesting significant nebular fractionation of forsterite in its provenance.

AB - Northwest Africa (NWA) 6704 is a unique achondrite characterized by a near-chondritic major element composition with a remarkably intact igneous texture. To investigate the origin of this unique achondrite, we have conducted a combined petrologic, chemical, and 187Re–187Os, O, and Ti isotopic study. The meteorite consists of orthopyroxene megacrysts (En55–57Wo3–4Fs40–42; Fe/Mn = 1.4) up to 1.7 cm in length with finer interstices of olivine (Fa50–53; Fe/Mn = 1.1–2.1), chromite (Cr# ∼ 0.94), awaruite, sulfides, plagioclase (Ab92An5Or3) and merrillite. The results of morphology, lattice orientation analysis, and mineral chemistry indicate that orthopyroxene megacrysts were originally hollow dendrites that most likely crystallized under high super-saturation and super-cooling conditions (1–102 °C/h), whereas the other phases crystallized between branches of the dendrites in the order of awaruite, chromite → olivine → merrillite → plagioclase. In spite of the inferred high super-saturation, the remarkably large size of orthopyroxene can be explained as a result of crystallization from a melt containing a limited number of nuclei that are preserved as orthopyroxene megacryst cores having high Mg# or including vermicular olivine. The Re–Os isotope data for bulk and metal fractions yield an isochron age of 4576 ± 250 Ma, consistent with only limited open system behavior of highly siderophile elements (HSE) since formation. The bulk chemical composition is characterized by broadly chondritic absolute abundances and only weakly fractionated chondrite-normalized patterns for HSE and rare earth elements (REE), together with substantial depletion of highly volatile elements relative to chondrites. The HSE and REE characteristics indicate that the parental melt and its protolith had not undergone significant segregation of metals, sulfides, or silicate minerals. These combined results suggest that a chondritic precursor to NWA 6704 was heated well above its liquidus temperature so that highly volatile elements were lost and the generated melt initially contained few nuclei of relict orthopyroxene, but the melting and subsequent crystallization took place on a timescale too short to allow magmatic differentiation. Such rapid melting and crystallization might occur as a result of impact on an undifferentiated asteroid. The O–Ti isotope systematics (Δ17O = −1.052 ± 0.004, 2 SD; ε50Ti = 2.28 ± 0.23, 2 SD) indicate that the NWA 6704 parent body sampled the same isotopic reservoirs in the solar nebula as the carbonaceous chondrite parent bodies. This is consistent with carbonaceous chondrite-like refractory element abundances and oxygen fugacity (FMQ = −2.6) in NWA 6704. Yet, the Si/Mg ratio of NWA 6704 is remarkably higher than those of carbonaceous chondrites, suggesting significant nebular fractionation of forsterite in its provenance.

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KW - Primitive achondrite

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