Degree of equilibration of eucritic pyroxenes and thermal metamorphism of the earliest planetary crust

Takeda, Hiroshi; Graham, A. L.

doi:10.1111/j.1945-5100.1991.tb01028.x

Cited by 169 publications

(182 citation statements)

References 7 publications

(8 reference statements)

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“…Unlike most eucrites, which have been metamorphosed and contain compositionally equilibrated pyroxenes, Pasamonte contains pyroxenes with extensive igneous Mg/Fe zoning and it is classed as a rare type 2 eucrite on the Takeda-Graham type 1-6 scale of metamorphism (Takeda and Graham, 1991 The composition of Pasamonte does not appear abnormal for a eucrite (Kitts and Lodders, 1998), although Ni and Ir concentrations are at the high end of the eucrite range. The mean Fe/Mn ratio in Pasamonte's pyroxene (29) lies at the low end of the eucrite range (28-40; Lentz et al, 2007), and its chromite composition appears to be exceptional with very high Al 2 O 3 (18 wt.% vs. 7-9% in other eucrites) and very low TiO 2 (1.4 wt.% vs 3-6% in eucrites) (Bunch and Keil, 1971;Mittlefehldt et al, 1998).…”

Section: Pasamontementioning

confidence: 99%

“…In addition we analyzed four polymict eucrites that contain unequilibrated clasts with subophitic textures and pyroxene compositions like those in Pasamonte: Yamato 75011 (Takeda and Graham, 1991;Warren and Kallemeyn, 2001), the possibly paired samples Y-74159 and Y-74450 , and Y-790260 (Antarctic Meteorite Data Base; see http://metdb.nipr.ac.jp/am_db_public).…”

Section: Samples Analyzed and Terminolgymentioning

confidence: 99%

See 1 more Smart Citation

Oxygen isotopic constraints on the origin and parent bodies of eucrites, diogenites, and howardites

Scott

Greenwood

Franchi

et al. 2009

Geochimica et Cosmochimica Acta

148

177

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A few eucrites have anomalous oxygen isotopic compositions. To help understand their origin and identify additional samples, we have analyzed the oxygen isotopic compositions of 18 eucrites and four diogenites. Except for five eucrites, these meteorites have ∆ 17 O values that lie within 2σ of their mean value viz., -0.242±0.016‰, consistent with igneous isotopic homogenization of Vesta. The five exceptional eucrites-NWA 1240, Pasamonte (both clast and matrix samples), PCA 91007, A-881394, and Ibitira-have ∆ 17 O values that lie respectively 4σ, 5σ, 5σ, 15σ, and 21σ away from this mean value. NWA 1240 has a δ 18 O value that is 5σ below the mean eucrite value. Four of the five outliers are unbrecciated and unshocked basaltic eucrites, like NWA 011, the first eucrite found to have an anomalous oxygen isotopic composition. The fifth outlier, Pasamonte, is composed almost entirely of unequilibrated basaltic clasts. Published chemical data for the six eucrites with anomalous oxygen isotopic compositions (including NWA 011) exclude contamination by chondritic projectiles as a source of the oxygen anomalies. Only NWA 011 has an anomalous Fe/Mn ratio, but several anomalous eucrites have exceptional Na, Ti, or Cr concentrations. We infer that the six anomalous eucrites are probably derived from five distinct Vesta-like parent bodies (Pasamonte and PCA 91007 could come from one body). These anomalous eucrites, like many unbrecciated eucrites from Vesta, are probably deficient in brecciation and shock effects because they were sequestered in small asteroids (~10 km diameter) during the Late Heavy Bombardment following ejection from Vesta-like bodies. The preservation of Vesta's crust and the lack of deeply buried samples from the hypothesized Vestalike bodies are consistent with the removal of these bodies from the asteroid belt by gravitational perturbations from planets and protoplanets, rather than by collisonal grinding.

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Section: Pasamontementioning

confidence: 99%

Section: Samples Analyzed and Terminolgymentioning

confidence: 99%

Oxygen isotopic constraints on the origin and parent bodies of eucrites, diogenites, and howardites

Scott

Greenwood

Franchi

et al. 2009

Geochimica et Cosmochimica Acta

148

177

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“…Previously, we studied a numbcr of noncumulate and cumulate eucrites (Hsu and Crozaz, 1996a,b) and showed that in situ trace element analyses of individual grains can provide reliable information about the formation and evolution of these meteorites despite the fact that they experienced different degrees of metamorphism (Takeda and Graham, 1991). For example, there are plagioclase grains in some equilibrated eucrites that still preserve their original igneous zoning with regard to trace elements.…”

Section: Introductionmentioning

confidence: 99%

Ion microprobe study of the Pomozdino and Peramiho eucrites

Hsu

Crozaz²

1997

Meteorit & Planetary Scien

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Abstract-Electron and ion microprobe measurements of major, minor, and trace element concentrations were made in individual grains of pyroxene, plagioclase, and Ca phosphates in Pomozdino and Peramiho, two eucrites previously classified as anomalous. Although Pomozdino pyroxene is highly magnesian, minor and trace element concentrations in both pyroxene and plagioclase of this meteorite are similar to those in other noncumulate eucrites. High incompatible element concentrations (similar to those in Stannern) coupled with mg# typical of cumulate eucrites confirm the anomalous character of this meteorite but do not allow us to distinguish unequivocally between different possible modes of origin. Peramiho has mg# and trace element concentrations similar to main group eucrites, indicating that this meteorite most probably belongs to this group. A previously reported low incompatible element concentration for Peramiho niay be due to a sampling problem.

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“…Edges of these silica grains extend into surrounding feldspars and pyroxenes indicating that they began to crystallize before the mesostasis. Takeda and Graham (1991) named the type of zoning of the pyroxenes in clast A the "Mg-Fe trend" and pointed out that this pattern commonly displays a "step" or moderate increase in Ca content on a compositional traverse from core to rim ( equilibration as defined by Takeda and Graham (1991). Fine, regularly spaced exsolution lamellae (4 p m in width) are visible throughout the pyroxenes in clast A; pyroxene edges contain moderate numbers of inclusions.…”

Section: Clastsmentioning

confidence: 99%

The South African polymict eucrite Macibini

Buchanan

Lindstrom

Mittlefehldt

et al. 2000

Meteorit & Planetary Scien

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Abstract-The polymict eucrite Macibini is a fragmental breccia, predominantly composed of eucritic materials with minor proportions (maximum 2 ~01%) of diogenitic material. Hence, it is intermediate between the Yamato-74 159-type polymict eucrites, which contain negligible amounts of magnesian orthopyroxene, and the howardites. The present study provides mineralogical and bulk compositional data for the meteorite breccia and for six clasts. These clasts include both volcanic and igneous rocks and a variety of impact-generated rocks. A broad range of degrees of postcrystallization metamorphism affected these materials before the final aggregation of the breccia.Clast A is a fragment of unequilibrated eucrite with subophitic texture. The edges of the zoned pyroxenes in this clast are composed of a host of Fe-rich augite containing vermicules (blebs) and lamellae composed of a mixture of Fe-rich olivine and silica. Similar features occur as fragments in lunar breccias and are attributed by some workers to the breakdown of pyroxferroite, an Fe-rich pyroxenoid. However, textures and compositions of these augite-olivine-silica intergrowths in clast A suggest that, in this case, they are the result of decomposition in a series of steps of Fe-rich subcalcic augite.Among the fragments of impact-generated material in Macibini is clast 2, an earlier-formed clastic breccia that was lithified before being broken apart and included in the meteorite breccia. Clast 3 is an impact-melt breccia that is composed of rock and mineral fragments in a devitrified groundmass. Clast C is also an impact-melt breccia that has a coarser-grained, homfelsic groundmass that resulted from extensive metamorphism after formation.

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Degree of equilibration of eucritic pyroxenes and thermal metamorphism of the earliest planetary crust

Cited by 169 publications

References 7 publications

Oxygen isotopic constraints on the origin and parent bodies of eucrites, diogenites, and howardites

Oxygen isotopic constraints on the origin and parent bodies of eucrites, diogenites, and howardites

Ion microprobe study of the Pomozdino and Peramiho eucrites

The South African polymict eucrite Macibini

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