Mineralogy and39Ar-40Ar age of an old pristine basalt: Thermal history of the HED parent body

doi:10.1016/0012-821X(94)90059-0

Earth and Planetary Science Letters

Volume 122, Issues 1–2, March 1994, Pages 183-194

https://doi.org/10.1016/0012-821X(94)90059-0 Get rights and content

Abstract

Previous investigations of mineral chemistry and Rb single bond Sr and SmNd ages indicated that clast,84 from eucrite Yamato 75011 had preserved the pristine nature of its initial crystallization during an early stage of the HED parent body. Microscale mineralogy and³⁹Ar-⁴⁰Ar ages of this clast, however, revealed local disturbance of micrtextures and partially reset ages. This evidence suggests that, in addition to initial crystallization and rapid cooling, the Y75011,84 clast experienced shock deformation, reheating of short duration at higher temperature, and brecciation. These characteristics suggest two or more impact events. Fe-rich olivine filling fractures in pyroxene may have been introduced during the accompanying shock fracturing. The inferred³⁹Ar-⁴⁰Ar degassing ages for Y75011 matrix and clast,84 are3.94 ± 0.04 Ga and3.98 ± 0.03 Ga, respectively. The suggested degassing age for a clast from Y790020, believed to be paired with Y75011, is ∼ 4.03 Ga, but could be younger. We consider it likely that all three samples experienced a common degassing event3.95 ± 0.05 Ga ago, but we cannot rule out two or more events spaced over a ∼ 0.1 Ga interval. Higher temperature extractions of the two clast samples show significantly older apparent ages up to ∼ 4.5 Ga and suggest that the time/temperature regime of this event was not sufficient to degas Ar totally. Most likely, the K-Ar ages were reset by thermal metamorphism associated with one or more impact events associated with shock fracturing, formation of Fe-rich olivine veins, and/or meteorite brecciation. The pyroxene annealing that commonly occurs in many eucrites is likely to be a much earlier process than the impact-produced textural changes and reset K-Ar ages observed in these meteorites. The existence of mineralogical and chronological evidence for metamorphism in an otherwise pristine eucrite suggests that the HED parent body experienced an extensive degree of early cratering.

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Fluid-assisted metasomatic processes on planetary bodies: Evidence from vestan lithologies
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Citation Excerpt :
These are discussed below in light of our findings. To explain the origin of ferroan olivine veinlets in the eucrite Yamato (Y)-7501, Takeda et al. (1983, 1994) suggested that they formed either during a process of Fe-rich pyroxene decomposition, or during a short impact heating event (at ∼1100 °C) that caused the melting of late-stage, Fe-rich mesostasis. The resulting melts would have been subsequently deposited within pyroxene fractures (see also Warren, 2002).
The mechanisms responsible for the formation of Fe-rich, metasomatic features in eucrites and their debris in howardites are yet controversial. In this study, secondary phases found in polymict eucrite NWA (Northwest Africa) 11911, howardite NWA 1664, monomict eucrite NWA 8675 and unbrecciated basaltic eucrite NWA 13269 were investigated to elucidate their origin and history of formation. Among them, NWA 11911 is the only rock that displays widespread Fe-enrichment near fractures in unequilibrated and slightly equilibrated basaltic clasts and pyroxene fragments. Fayalitic olivine veinlets with various textures, thicknesses and compositions (Fa_64-82) are detected in unequilibrated pyroxenes in NWA 11911, NWA 1664 and NWA 8675. Neighbouring Fe-enriched pyroxene is also Al-depleted compared to magmatic pyroxene (Al₂O₃ = ∼0.3 wt%). NWA 13269 contains Ca-rich plagioclase (An_97-98) veinlets associated with Cr-spinel, but apparently no Fe-enrichment in pyroxene nor ferroan olivine despite a careful search. Olivine veinlets have been also detected in equilibrated pyroxene in NWA 11911. Our observations support the fluid-assisted metasomatism scenario that involves the circulation of hydrothermal fluids throughout the fractures of eucritic material.
Characterization of achondritic cosmic spherules from the Widerøefjellet micrometeorite collection (Sør Rondane Mountains, East Antarctica)
2022, Geochimica et Cosmochimica Acta
Achondritic micrometeorites represent one of the rarest (ca. 0.5–2.1%) particle types among Antarctic micrometeorite collections. Here, we present major, trace element and oxygen isotope compositions on five vitreous, achondritic cosmic spherules (341–526 µm in size) recovered from the Widerøefjellet sedimentary trap in the Sør Rondane Mountains (SRMs) of East Antarctica. We also present the first iron isotope data for four of these achondritic cosmic spherules. The particles were initially identified based on the atomic concentrations of Fe-Mg-Mn and their distribution in Fe/Mg versus Fe/Mn space, spanning a relatively wide range in Fe/Mg ratios (ca. 0.48–1.72). The Fe/Mn ratios cover a more restricted range (22.4–31.7), comparable to or slightly below the values measured for howardite-eucrite-diogenite (HED) and martian meteorites. One particle (WF1801-AC3) displays an elevated Fe/Mn ratio of ∼78, comparable to the values determined for lunar rocks. The negative correlation observed between the CaO + Al₂O₃ contents and the Fe/Si ratios of achondritic spherules reflects both the mineralogy of the precursor materials, as well as the extent of volatilization experienced during atmospheric entry heating. This trend suggests that the primary mineralogy of precursor materials may have been compositionally similar to basaltic achondrites. Based on their distribution in Ca/Si versus Al/Si space, we argue that the majority of achondritic cosmic spherules predominantly sample pyroxene- and/or plagioclase-rich (i.e., basaltic) precursor bodies. Such precursor mineralogy is also inferred from their rare earth element (REE) patterns, which show resemblances to fine-grained basaltic eucrites or Type 1 achondritic spherules (n = 3 – av. REE_N = 11.2–15.5, (La/Yb)_N = 0.93–1.21), pigeonite-rich equilibrated eucrite precursors or Type 2 achondritic spherules (n = 1 – av. REE_N = 27.9, (La/Yb)_N = 0.10), and possibly Ca-phosphates from (primitive) achondritic bodies (n = 1 – av. REE_N = 58.8, (La/Yb)_N = 1.59). This is clearly demonstrated for particle WF1801AC-1, which was likely inherited from a fine-grained eucritic precursor body. The pre-atmospheric oxygen isotope composition was reconstructed through compensation of mass-dependent fractionation processes as well as mixing with atmospheric oxygen, using iron isotope data. Two particles (WF1801AC-2, WF1801-AC4) display corrected oxygen isotope compositions (δ¹⁸O = 3.7–4.4‰) largely consistent with HED meteorites and may thus originate from HED-like parent bodies. The corrected oxygen isotope compositions (δ¹⁸O = 12.6–12.8‰) of the remaining particles (WF1801-AC3, WF1801-AC5) do not correspond to known meteorite fields and may represent two distinct types of unknown achondritic parent bodies or residual atmospheric entry effects. Finally, the abundance (ca. 0.5%) of achondritic cosmic spherules within the Widerøefjellet sedimentary trap is comparable to that observed in the South Pole Water Well (SPWW – ca. 0.5%), Novaya Zemlya glacier (ca. 0.45%) and Transantarctic Mountain (TAM) (ca. 2.1%) collections, confirming their overall rarity in micrometeorite collections. Unambiguous evidence for micrometeorites from the Moon or Mars remains absent from collections to date.
The geologic history of Vesta inferred from combined 207Pb/206Pb and 40Ar/39Ar chronology of basaltic eucrites
2019, Geochimica et Cosmochimica Acta
Eucrites represent samples of mafic crust of the parent body, likely Vesta, and record a complex geologic history involving magmatism, metamorphism, impact processing, and metasomatism. For better understanding of the geologic history, we present combined ²⁰⁷Pb/²⁰⁶Pb and ⁴⁰Ar/³⁹Ar chronology of four basaltic eucrites showing distinct petrologic features: Agoult, an equilibrated eucrite with granulitic textures; Camel Donga, a brecciated equilibrated eucrite enriched in metallic iron; DaG 380, a partially equilibrated monomict eucrite; and NWA 049, a metasomatized polymict eucrite comprising mostly unequilibrated clasts. The ²⁰⁷Pb/²⁰⁶Pb dating in combination with the acid washing technique was applied to pyroxene-rich and plagioclase-rich fractions, while the ⁴⁰Ar/³⁹Ar dating was performed mainly on plagioclase grains using the laser incremental heating method. The Pb isotopic data for acid leaching residues are more radiogenic than those for acid washes, reflecting efficient removal of non-radiogenic Pb components by the acid washing. Among the residues, however, only those of the Agoult and Camel Donga plagioclase-rich fractions yielded isochrons, rather than errorchrons, with ²⁰⁷Pb/²⁰⁶Pb ages of 4532.39 ± 0.87 Ma and 4515.43 ± 0.42 Ma, respectively. The trace element systematics further suggest that these residues essentially represent contaminant oxide minerals such as ilmenite and chromite rather than plagioclase. On the other hand, the Ar isotopic data for Agoult and Camel Donga yielded plateau ages with weighted means of 4494 ± 9 Ma and 3749 ± 25 Ma, in contrast to those for DaG 380 and NWA 049 that did not return any plateau. The ²⁰⁷Pb/²⁰⁶Pb and ⁴⁰Ar/³⁹Ar ages of Agoult are interpreted to reflect the time of the U–Pb and K–Ar system closure during a prolonged thermal metamorphism. By combining the ages reported here and in the literature with the calculated closure temperatures, the cooling rate of Agoult is determined to be 11 ± 2 °C/Ma. This cooling rate corresponds to a burial depth of 10–20 km, indicating that granulitic eucrites were originally crystallized near the surface and were subsequently buried and metamorphosed. The ∼20 Ma younger ²⁰⁷Pb/²⁰⁶Pb ages of Camel Donga than Agoult are considered to reflect delayed burial metamorphism in the source region, whereas the plateau ⁴⁰Ar/³⁹Ar age would record the time of an impact causing brecciation and efficient Ar-degassing of the buried crust. Furthermore, the model ²⁰⁷Pb*/²⁰⁶Pb* dates calculated for acid-washed pyroxenes imply that the burial metamorphism of Agoult and DaG 380 were broadly contemporaneous and also that NWA 049 was metasomatized by fluids within the uppermost crust later than 4.4 Ga.
Achondrites
2013, Treatise on Geochemistry: Second Edition
This chapter covers the major and minor achondrite groups, three newly described unique achondrites, and one recently reclassified achondrite. The silicates of the stony iron mesosiderites show many similarities with the howardites, and the silicate inclusions in IAB irons are closely related to the stony winonaites. Some IIE irons contain nonchondritic silicate inclusions, and they are also considered.
Solar System Time Scales from Long-Lived Radioisotopes in Meteorites and Planetary Materials
2013, Treatise on Geochemistry: Second Edition
Long-lived radioactive isotopes, defined here as those that have half-lives in excess of a few hundred million years, have been utilized for chronology since the early part of the twentieth century. The past several decades have seen significant advances in the precision and accuracy of chronological information based on the decay of long-lived radioisotopes, particularly in the mass spectrometric techniques for measurement of isotope ratios and in the determination of relevant decay constants. Chronometers based on the decay of radioisotopes essentially date the time of isotopic closure following a chemical event that fractionated the parent element from the daughter element. Assuming that parent/daughter isotope ratios can be determined accurately and precisely and that the decay constant is known, meaningful age information based on such chronometers may only be obtained if (1) there was complete equilibration of the isotopic composition of the daughter element prior to fractionation of the parent element from the daughter element and (2) there has been no disturbance of isotope systematics following the isotopic closure event that is to be dated.
Record of S-rich vapors on asteroid 4 Vesta: Sulfurization in the Northwest Africa 2339 eucrite
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In the Northwest Africa (NWA) 2339 eucrite, a few high-Fe pigeonite fragments were partially replaced by fine-grained troilite, Mg-rich augite, silica, with minor ilmenite and Mg-rich ferrosilite. The relict pigeonite has distinctly lower Fe/Mn values (17–18) than those (∼30) of augite and Mg-rich ferrosilite in the fine-grained replacement textures and other eucritic pyroxenes in NWA 2339. The few plagioclase grains associated with these Fe-rich pigeonite fragments contain high albite components (Ab_17–31). Similar replacement textures are also present in a lithic clast consisting of pyroxene and Na-rich plagioclase (Ab_20–25). In this lithic clast, high-Fe subcalcic augite was replaced by troilite, Mg-rich hedenbergite, silica, and minor ilmenite whereas Mg-rich pigeonite remains intact. Troilite also occurs as tiny inclusions in porous regions of plagioclase. Moreover, the porous regions of plagioclase are enriched in Si and depleted in Al, Ca, and Na, compared to relict plagioclase. These textures indicate that the formation of troilite-rich mineral assemblages is related to the reaction between FeSiO₃ component in pyroxene and S-rich vapors; whereas, the formation of tiny troilite in porous regions of plagioclase is related to the reaction between Fe²⁺ in plagioclase and S-rich vapors. X-ray mapping results reveal that during sulfurization of pyroxene and plagioclase, lithophile elements Mn, Al, Ca, Na, and Mg were removed to various extents, very likely by the S-rich vapors. Based on our observations, the S-rich vapors could be dry S–O–P vapors, which most likely have been formed by volatilization of pre-existing S,P-bearing materials due to impact heating on the surface of asteroid 4 Vesta.

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Reference (28)

Samarium-neodymium evolution of meteorites

Geochim. Cosmochim. Acta

Comparisons of Antarctic and non-Antarctic achondrites and possible origin of the differences

Geochim. Cosmochim. Acta

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Absolute age of formation of chondrites studied by the⁸⁷Rb⁸⁷Sr method

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Cited by (39)

Fluid-assisted metasomatic processes on planetary bodies: Evidence from vestan lithologies

Characterization of achondritic cosmic spherules from the Widerøefjellet micrometeorite collection (Sør Rondane Mountains, East Antarctica)

The geologic history of Vesta inferred from combined <sup>207</sup>Pb/<sup>206</sup>Pb and <sup>40</sup>Ar/<sup>39</sup>Ar chronology of basaltic eucrites

Achondrites

Solar System Time Scales from Long-Lived Radioisotopes in Meteorites and Planetary Materials

Record of S-rich vapors on asteroid 4 Vesta: Sulfurization in the Northwest Africa 2339 eucrite

Mineralogy and39Ar-40Ar age of an old pristine basalt: Thermal history of the HED parent body

Abstract

Geochim. Cosmochim. Acta

Geochim. Cosmochim. Acta

Earth Planet. Sci. Lett.

Geochim. Cosmochim. Acta

Earth Planet. Sci. Lett.

Geochim. Cosmochim. Acta

Time differences in the formation of meteorites as determined from the ratio of lead-207 to lead-206

Science

RbSr and SmNd internal isochron ages of a subophitic basalt clast and a matrix sample from the Y75011 eucrite

J. Geophys. Res.

Comparison of Yamato and Victoria Land polymict eucrites: A view from mineralogical and isotopic studies

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

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Chips off of asteroid 4 Vesta: evidence for the parent body of basaltic achondritic meteorites

Science

Forging an asteroid-meteorite link

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Absolute age of formation of chondrites studied by the87Rb87Sr method

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Mineralogy and³⁹Ar-⁴⁰Ar age of an old pristine basalt: Thermal history of the HED parent body

Absolute age of formation of chondrites studied by the⁸⁷Rb⁸⁷Sr method