A precise 87Rb87Sr age and initial 87Sr86Sr for the Colomera iron meteorite

doi:10.1016/0016-7037(70)90059-1

Geochimica et Cosmochimica Acta

Volume 34, Issue 11, November 1970, Pages 1227-1228, IN1-IN2, 1229-1239

https://doi.org/10.1016/0016-7037(70)90059-1 Get rights and content

Abstract

Rb-Sr analyses of silicates from the Colomera iron meteorite show a wide range in Rb/Sr and give $^{87} Sr^{86} Sr$ values ranging from 0.699 to 8.45. These analyses define a precise linear array on the Rb-Sr evolution diagram yielding an age of 4.61 ± 0.04 × 10⁹yr. A highprecision analysis of a whitlockite separate having very low Rb/Sr yields an initial ( $^{87} Sr^{86} Sr)_{I} = 0.69940 ± 0.00004$ . An upper limit of 48 ± 7 million years can be set for the time interval between dispersion of the silicate in the metal phase and final Sr isotopic equilibration, if we assume that Colomera silicate formed from a parent material which had the same initial $^{87} Sr^{86} Sr$ as Ca-rich achondrites. The data are consistent with a simple evolutionary history in which Colomera differentiated from a parent material of chondritie $Rb Sr$ ratio at a time 39 million years after the parent material had the initial $^{87} Sr^{86} Sr$ of Ca-rich achondrites and 35 million years before the final Sr equilibration in the Guareña chondrite.

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Cited by (54)

Formation, cooling history and age of impact events on the IIE iron parent body: Evidence from the Miles meteorite
2022, Geochimica et Cosmochimica Acta
Most iron meteorites formed in planetary cores during differentiation, but the IIE iron meteorites have chemical and physical features that are inconsistent with this origin. By combining mineral chemistry, mineral modes and three-dimensional petrography, we reconstruct the bulk chemistry of the felsic silicate-bearing Miles IIE iron meteorite and demonstrate that the silicate inclusion compositions are similar to partial melts produced experimentally from an H chondrite composition. We use the reconstructed bulk composition, mineralogy and thermodynamic modelling to show that melting above ∼1200°C under reducing conditions formed metal (Fe-Ni alloy) and felsic silicate partial melts. Upon cooling, the melts crystallized Mg-rich pyroxenes, Na- and K-rich feldspars, and tridymite. Importantly, this mechanism enriches cosmochemically volatile elements (i.e., those with a 50% condensation temperature of ∼430–830°C, like Na and K) to the level found in the felsic silicate inclusions.
The presence of crystallographically disordered srilankite (only stable above 1160°C) and an absence of Widmanstätten texture require both high peak temperatures and rapid cooling, which cannot be explained by core formation. Instead they point to small melt volumes, a transient heat pulse, and small thermal mass, and imply efficient physical segregation of silicate and metallic melts through buoyancy separation followed by rapid cooling that arrested the separation of metal and silicate liquid phases. In situ ²⁰⁷Pb/²⁰⁶Pb age of 4542.3 ± 4.0 Ma in Zr-oxide and phosphate minerals dates the melting event that formed the silicate inclusions. This age aligns with the earliest ages found in other IIE iron meteorite silicates and requires a heating event ∼25 million years after the solar system formed. We found ³⁹Ar/⁴⁰Ar ages of 3495 ± 52 Ma (low-T) and 4303 ± 7 Ma (high-T) in a K-feldspar grain, with the 3495 Ma age aligning with later thermal events recorded in other IIE iron meteorites. Dating reveals the complex petrogenetic and thermal history of Miles and the IIE iron meteorites. This is the first IIE iron meteorite found to record evidence of heating at 4.5 and 3.5 Ga likely from impact events.
We propose that high-velocity impact(s) into an iron-rich, porous chondritic parent body at ∼4.54 Ga produced immiscible metal and silicate melts that cooled rapidly and trapped low density silicate inclusions within high density metal. Other IIE irons that formed at lower peak temperatures (900–1000°C) contain chondritic silicate inclusions and relict chondrules, supporting this conceptual model. This petrogenesis is consistent with thermodynamic modelling, experimental data and the wide range of peak temperatures and cooling rates observed in the IIE iron meteorites.
Multiple thermal events recorded in IIE silicate inclusions: Evidence from in situ U–Pb dating of phosphates in Weekeroo Station
2021, Geochimica et Cosmochimica Acta
Citation Excerpt :
However, our preliminary Pb–Pb dating results for the phosphates in Miles are much older (>4.5 Ga; Li and Hsu, 2019). This is also the case for Colomera, of which the internal Rb–Sr isochron age (4586 ± 40 Ma; Sanz et al., 1970) overlaps with the time of CAI formation, whereas the Ar–Ar age is ~100 Myr younger (Bogard et al., 2000). The significant discrepancy between the Pb–Pb age and the Ar–Ar age for Miles suggests post-formation impacts as the most plausible mechanism to reset the K–Ar system.
Silicate-bearing iron meteorites record metal–silicate separation and mixing processes of planetesimals. Of particular interest is the IIE group as its silicate inclusions show great petrological diversity, probably resulting from the complex petrogenesis involving early differentiation and impact-induced dynamic mixing, re-melting, and reduction. These geologic records may have been documented by individual minerals to various extents, which blurs the chronological records on the formation of IIEs. Here, by combining X-ray elemental mapping with ion microprobe dating technique, we show that two groups of phosphates are present in the Weekeroo Station IIE iron meteorite. They have distinct petrography, mineralogy, Th/U ratios, and Pb isotopic compositions, which reflects their different formation mechanisms. One group (Group 1) has a fine-grained, euhedral or acicular habit, suggesting rapid crystallization from a melt during the metal–silicate mixing. By contrast, the other group (Group 2) is intimately associated with the decomposition and reduction of pyroxene under subsolidus conditions. The Group 1 phosphates yield a precise Pb–Pb age of 4528 ± 11 Ma and they provide the first robust constraint on the timing of impact-induced metal–silicate mixing on the IIE iron meteorite parent body. The Group 2 phosphates, on the other hand, probably record a Pb–Pb age at least 50 Myr younger, and they give insight into the role of subsequent impacts on the alteration of IIE irons. Combining our results with published chronological data suggests that the formation of IIEs irons is consistent with early partial differentiation followed by multiple impact-driven metal–silicate mixing and that most of them were not shielded from subsequent impacts.
Extreme early solar system chemical fractionation recorded by alkali-rich clasts contained in ordinary chondrite breccias
2017, Earth and Planetary Science Letters
New K–Ca and Rb–Sr isotopic analyses have been performed on alkali-rich igneous rock fragments in the Yamato (Y)-74442 and Bhola LL-chondritic breccias to better understand the extent and timing of alkali enrichments in the early solar system. The Y-74442 fragments yield a K–Ca age of $4.41 \pm 0.28 Ga$ for λ(⁴⁰K) = 0.5543 Ga⁻¹ with an initial ⁴⁰Ca/⁴⁴Ca ratio of $47.1618 \pm 0.0032$ . Studying the same fragments with the Rb–Sr isotope system yields an age of $4.420 \pm 0.031 Ga$ for λ(⁸⁷Rb) = 0.01402 Ga⁻¹ with an initial ratio of ⁸⁷Sr/⁸⁶Sr = 0.7203 ± 0.0044. An igneous rock fragment contained in Bhola shows a similar alkali fractionation pattern to those of Y-74442 fragments but does not plot on the K–Ca or Rb–Sr isochron of the Y-74442 fragments. Calcium isotopic compositions of whole-rock samples of angrite and chondrites are primordial, indistinguishable from mantle-derived terrestrial rocks, and here considered to represent the initial composition of bulk silicate Earth. The initial ε⁴⁰Ca value determined for the source of the alkali clasts in Y-74442 that is ∼0.5 ε-units higher than the solar system value implies an early alkali enrichment.
Multi-isotopic studies on these alkali-rich fragments reveal that the source material of Y-74442 fragments had elemental ratios of K/Ca = 0.43 ± 0.18, Rb/Sr = 3.45 ± 0.66 and K/Rb ∼ 170, that may have formed from mixtures of an alkali-rich component (possibly an alkali-enriched gaseous reservoir produced by fractionation of early nebular condensates) and chondritic components that were flash-heated during an impact event on the LL-chondrite parent body ∼4.42 Ga ago. Further enrichments of potassium and rubidium relative to calcium and strontium as well as a mutual alkali-fractionation (K/Rb ∼ 50 and heavier alkali-enrichment) would have likely occurred during subsequent cooling and differentiation of this melt. Alkali fragments in Bhola might have undergone similar solid–vapor fractionation processes to those of Y-74442 fragments but appear to have formed via a distinct impact melting event.
Oxygen isotope and petrological study of silicate inclusions in IIE iron meteorites and their relationship with H chondrites
2016, Geochimica et Cosmochimica Acta
Citation Excerpt :
A variety of long-lived radioisotopic dating techniques have been applied to the IIE irons. These include Ar–Ar (Niemeyer, 1980; Bogard et al., 2000; Bogard, 2011), K–Ar (Bogard et al., 1968; Olsen et al., 1994; Casanova et al., 1995), Rb–Sr (Sanz et al., 1970; Burnett and Wasserburg, 1967a,b; Evensen et al., 1979), Sm–Nd (Snyder et al., 2001), Pb–Pb (Göpel et al., 1985), Re–Os (Birck and Allègre, 1998) and I–Xe dating (Niemeyer, 1980; Brazzle et al., 1999). These studies have identified two distinct age groups for the IIE silicate inclusions, generally referred to as the old age group and the young age group (Mittlefehldt et al., 1998; Bogard et al., 2000).
The origin of silicate-bearing irons, especially those in groups IAB, IIICD, and IIE, is poorly understood as silicate should have separated rapidly from molten metal. Here we report the results of high precision oxygen isotope analysis of silicate inclusions in eleven group IIE meteorites and a petrological study of silicate inclusions in ten IIE irons including those in Garhi Yasin and Tarahumara, which have not been described in detail before. Oxygen isotopes have also been analysed in 20 H chondrites to investigate their possible relationship with the IIE irons.
Based on petrographic observations and mineral analysis, the silicate-bearing IIE meteorites have been divided into four types according to the nature of their silicate inclusions: (1) primitive chondritic, (2) evolved chondritic, (3) differentiated with >10 vol.% orthopyroxene, and (4) differentiated with <10 vol.% orthopyroxene. Each meteorite contains a single inclusion type. While inclusions in an individual IIE meteorite tend to show relatively limited Δ¹⁷O variation, a wide range of values is seen in the dataset as a whole. Group IIE irons with differentiated silicates, with the exception of Colomera, have a range of mean Δ¹⁷O values that is essentially identical to those of the H4-6 chondrites: 0.60–0.77‰ and 0.61–0.76‰, respectively. Colomera inclusions, which are differentiated with <10 vol.% orthopyroxene, have an anomalously high Δ¹⁷O value and plot ∼2σ away from the next nearest IIE iron. However, in view of the textural similarities to other IIE inclusions, a separate source for Colomera is deemed unlikely. Three IIE irons with primitive chondritic inclusions, Garhi Yasin, Netschaëvo, and Techado, have relatively low mean Δ¹⁷O values of 0.56–0.57‰ as well as relatively reduced silicates with Fa₁₅_–₁₇ olivine, which have been called HH chondrites. Given the significant overlap in their oxygen isotope compositions, a genetic relationship between IIE irons and H chondrites is supported by our new data. However, derivation of both groups from one parent body seems unlikely. Instead, both groups probably sampled similar precursor materials and accreted at a similar nebular location.
Our data suggest that the IIE meteorites formed on an internally heated H/HH chondrite-like body that experienced the initial stages of differentiation in response to radiogenic heating. However, prior to full differentiation the IIE parent body experienced a major hit-and-run style collision that resulted in silicate-metal mixing. The initial stages of this event involved a phase of rapid cooling that prevented unmixing of metal and silicates. Reassembly of the IIE parent body produced a large regolith blanket that facilitated subsequent slow cooling. The IIE parent body has probably experienced numerous subsequent less catastrophic collisions. The development of alkali glass textures in some differentiated inclusions is probably the result of one of these later events.
Silicate-bearing iron meteorites and their implications for the evolution of asteroidal parent bodies
2014, Chemie der Erde
Silicate-bearing iron meteorites differ from other iron meteorites in containing variable amounts of silicates, ranging from minor to stony-iron proportions (∼50%). These irons provide important constraints on the evolution of planetesimals and asteroids, especially with regard to the nature of metal–silicate separation and mixing. I present a review and synthesis of available data, including a compilation and interpretation of host metal trace-element compositions, oxygen-isotope compositions, textures, mineralogy, phase chemistries, and bulk compositions of silicate portions, ages of silicate and metal portions, and thermal histories. Case studies for the petrogeneses of igneous silicate lithologies from different groups are provided. Silicate-bearing irons were formed on multiple parent bodies under different conditions. The IAB/IIICD irons have silicates that are mainly chondritic in composition, but include some igneous lithologies, and were derived from a volatile-rich asteroid that underwent small amounts of silicate partial melting but larger amounts of metallic melting. A large proportion of IIE irons contain fractionated alkali-silica-rich inclusions formed as partial melts of chondrite, although other IIE irons have silicates of chondritic composition. The IIEs were derived from an H-chondrite-like asteroid that experienced more significant melting than the IAB asteroid. The two stony-iron IVAs were derived from an extensively melted and apparently chemically processed L or LL-like asteroid that also produced a metallic core. Ungrouped silicate-bearing irons were derived from seven additional asteroids. Hf–W age data imply that metal–silicate separation occurred within 0–10 Ma of CAI formation for these irons, suggesting internal heating by ²⁶Al. Chronometers were partly re-set at later times, mainly earlier for the IABs and later for the IIEs, including one late (3.60 ± 0.15 Ga) strong impact that affected the “young silicate” IIEs Watson (unfractionated silicate, and probable impact melt), Netschaëvo (unfractionated, and metamorphosed), and Kodaikanal (fractionated). Kodaikanal probably did not undergo differentiation in this late impact, but the similar ages of the “young silicate” IIEs imply that relatively undifferentiated and differentiated materials co-existed on the same asteroid. The thermal histories and petrogeneses of fractionated IIE irons and IVA stony irons are best accommodated by a model of disruption and reassembly of partly molten asteroids.
Rb-Sr isotopic systematics of alkali-rich fragments in the Yamato-74442 LL-chondritic breccia
2013, Earth and Planetary Science Letters
We have undertaken mineralogical, petrographical and Rb–Sr isotopic studies on alkali-rich igneous rock fragments in the Yamato (Y)-74442 LL-chondritic breccia. The fragments are a few mm in size and are composed mainly of porphyritic olivine and dendritic pyroxene set in alkali-rich groundmass glass. Minor phases include chromite, troilite and metallic nickel–iron. Bulk chemical compositions of the fragments are almost identical to the host chondrite except for a depletion of sodium and an enrichment of potassium. Isotopic analyses of nine fragments from Y-74442 yield a Rb–Sr age of 4429±54 Ma (2σ) for λ(⁸⁷Rb)=0.01402 Ga⁻¹ with an initial ratio of ⁸⁷Sr/⁸⁶Sr=0.7144±0.0094 (2σ). Assuming precursors of the fragments formed 4568 Ma with ⁸⁷Sr/⁸⁶Sr=0.69889 when the Solar System formed, a time-averaged Rb/Sr (weight) ratio of the source material for the fragments is calculated to be 2.58+0.91/−0.93.
The extremely high Rb/Sr value of this source is difficult to interpret by any igneous fractionation or liquid immiscibility, but can be explained by mixing of a chondritic component with an alkali-rich component formed in the early solar nebula. In our preferred model, the alkali component with Rb/Sr⪢30 would have condensed from the residual nebular gas after removal of refractory strontium and must have been isolated for a long time in a region where the temperature was sufficiently low to prevent reaction with other silicates/oxides. A mixture of the alkali component (early nebular condensates) and the ferromagnesian component could reflect flash heating induced by impact on an LL-chondritic parent body at least 4429 Ma ago, and further enrichments of rubidium and potassium relative to strontium could have occurred during this event. The resulting impact-melt rocks could have been fragmented by later impact event(s) and finally incorporated into the Y-74442 parent body. Thus, a remarkable signature of alkali enrichments both in the early solar nebula and later on the LL-chondrite parent body is preserved as a minor component of some chondritic breccias such as Y-74442.

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^☆: Contribution No. 1846.

^∗: On leave from Junta de Energia Nuclear, Direccion de Quimica e Isotopes, Ciudad Universitaria, Madrid 3, Spain

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A precise 87Rb87Sr age and initial 87Sr86Sr for the Colomera iron meteorite☆

Abstract

The abundance and distribution of Cl in iron meteorites

Geochim. Cosmochim. Acta

87Rb-87Sr isochron and 40K-40Ar ages of the Norton County achondrite

Earth Planet. Sci. Lett.

40Ar-40K ages of silicate inclusions in iron meteorites

Earth Planet. Sci. Lett.

Meteoritic K-feldspar in the Weekeroo Station, Kodaikanal and Colomera iron meteorites

Science

Evidence for the formation of an iron meteorite at 3.8 × 109 years

Earth Planet. Sci. Lett.

87Rb-87Sr ages of silicate inclusions in iron meteorites

Earth Planet. Sci. Lett.

The rubidium-strontium age of the Bishopville aubrite and its component enstatite and feldspar

Geochim. Cosmochim. Acta

Sobre un hierro meteorico de la provincia de Granada

Anales Soc. Españ Fis. Quim.

The iron meteorites, their thermal history and parent bodies

Geochim. Cosmochim. Acta

Rubidium-strontium age of hypersthene (L) chondrites

J. Geophys. Res.

Rubidium-strontium age of amphoterite (LL) chondrites

J. Geophys. Res.

Implications of shock effects in iron meteorites

Geochim. Cosmochim. Acta

87Rb-87Sr age of bronzite (H group) chondrites

J. Geophys. Res.

A precise $^{87} Rb^{87} Sr$ age and initial $^{87} Sr^{86} Sr$ for the Colomera iron meteorite☆

⁸⁷Rb-⁸⁷Sr isochron and ⁴⁰K-⁴⁰Ar ages of the Norton County achondrite

⁴⁰Ar-⁴⁰K ages of silicate inclusions in iron meteorites

Evidence for the formation of an iron meteorite at 3.8 × 10⁹ years

⁸⁷Rb-⁸⁷Sr ages of silicate inclusions in iron meteorites

⁸⁷Rb-⁸⁷Sr age of bronzite (H group) chondrites