Siderophile element constraints on the thermal history of the H chondrite parent body

Archer, Gregory J.; Walker, Richard J.; Tino, Jonathan; Blackburn, Terrence; Kruijer, Thomas S.; Hellmann, Jan L.

doi:10.1016/j.gca.2018.11.012

Title: Siderophile element constraints on the thermal history of the H chondrite parent body

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Abstract

The abundances of highly siderophile elements (HSE: Re, Os, Ir, Ru, Pt, Pd), as well as ¹⁸⁷Re-¹⁸⁷Os and ¹⁸²Hf-¹⁸²W isotopic systematics were determined for separated metal, slightly magnetic, and nonmagnetic fractions from seven H4 to H6 ordinary chondrites. The HSE are too abundant in nonmagnetic fractions to reflect metal-silicate equilibration. The disequilibrium was likely a primary feature, as ¹⁸⁷Re-¹⁸⁷Os data indicate only minor open-system behavior of the HSE in the slightly and non-magnetic fractions. ¹⁸²Hf-¹⁸²W data for slightly magnetic and nonmagnetic fractions define precise isochrons for most meteorites that range from 5.2 ± 1.6 Ma to 15.2 ± 1.0 Ma after calcium aluminum inclusion (CAI) formation. By contrast, 182W model ages for the metal fractions are typically 2–5 Ma older than the slope-derived isochron ages for their respective, slightly magnetic and nonmagnetic fractions, with model ages ranging from 1.4 ± 0.8 Ma to 12.6 ± 0.9 Ma after CAI formation. This indicates that the W present in the silicates and oxides was not fully equilibrated with the metal when diffusive transport among components ceased, consistent with the HSE data. Further, the W isotopic compositions of size-sorted metal fractions from some of the H chondrites also differ, indicating disequilibrium among somemore »« less

Authors:

Archer, Gregory J. ^[1]; Walker, Richard J. ^[2]; Tino, Jonathan ^[2];

^[3];

^[4]; Hellmann, Jan L. ^[5]

Univ. of Maryland, College Park, MD (United States). Dept. of Geology; Univ. of Münster, Münster (Germany). Inst. für Planetologie
Univ. of Maryland, College Park, MD (United States). Dept. of Geology
Univ. of California, Santa Cruz, CA (United States). Earth and Planetary Sciences
Univ. of Münster, Münster (Germany). Inst. für Planetologie; Lawrence Livermore National Lab. (LLNL), Livermore, CA (United States). Nuclear and Chemical Sciences Division
Univ. of Münster, Münster (Germany). Inst. für Planetologie

Publication Date:: Tue Nov 20 00:00:00 EST 2018

Research Org.:: Lawrence Livermore National Lab. (LLNL), Livermore, CA (United States)

Sponsoring Org.:: USDOE National Nuclear Security Administration (NNSA)

OSTI Identifier:: 1488788

Report Number(s):: LLNL-JRNL-748443
Journal ID: ISSN 0016-7037; 933552

Grant/Contract Number:: AC52-07NA27344

Resource Type:: Accepted Manuscript

Journal Name:: Geochimica et Cosmochimica Acta

Additional Journal Information:: Journal Volume: 245; Journal Issue: C; Journal ID: ISSN 0016-7037

Publisher:: The Geochemical Society; The Meteoritical Society

Country of Publication:: United States

Language:: English

Subject:: 58 GEOSCIENCES; H chondrites; Thermochronology; Onion shell; Highly siderophile elements; Os isotopes; W isotopes; CR chondrites; Nucleosynthetic

Citation Formats


                    Archer, Gregory J., Walker, Richard J., Tino, Jonathan, Blackburn, Terrence, Kruijer, Thomas S., and Hellmann, Jan L. Siderophile element constraints on the thermal history of the H chondrite parent body.  United States: N. p., 2018. 
Web.  doi:10.1016/j.gca.2018.11.012.

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                    Archer, Gregory J., Walker, Richard J., Tino, Jonathan, Blackburn, Terrence, Kruijer, Thomas S., & Hellmann, Jan L. Siderophile element constraints on the thermal history of the H chondrite parent body.  United States.  https://doi.org/10.1016/j.gca.2018.11.012

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                    Archer, Gregory J., Walker, Richard J., Tino, Jonathan, Blackburn, Terrence, Kruijer, Thomas S., and Hellmann, Jan L. Tue .  
"Siderophile element constraints on the thermal history of the H chondrite parent body".  United States.  https://doi.org/10.1016/j.gca.2018.11.012.  https://www.osti.gov/servlets/purl/1488788.

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@article{osti_1488788,

  title        = {Siderophile element constraints on the thermal history of the H chondrite parent body},

  author       = {Archer, Gregory J. and Walker, Richard J. and Tino, Jonathan and Blackburn, Terrence and Kruijer, Thomas S. and Hellmann, Jan L.},

  abstractNote = {The abundances of highly siderophile elements (HSE: Re, Os, Ir, Ru, Pt, Pd), as well as 187Re-187Os and 182Hf-182W isotopic systematics were determined for separated metal, slightly magnetic, and nonmagnetic fractions from seven H4 to H6 ordinary chondrites. The HSE are too abundant in nonmagnetic fractions to reflect metal-silicate equilibration. The disequilibrium was likely a primary feature, as 187Re-187Os data indicate only minor open-system behavior of the HSE in the slightly and non-magnetic fractions. 182Hf-182W data for slightly magnetic and nonmagnetic fractions define precise isochrons for most meteorites that range from 5.2 ± 1.6 Ma to 15.2 ± 1.0 Ma after calcium aluminum inclusion (CAI) formation. By contrast, 182W model ages for the metal fractions are typically 2–5 Ma older than the slope-derived isochron ages for their respective, slightly magnetic and nonmagnetic fractions, with model ages ranging from 1.4 ± 0.8 Ma to 12.6 ± 0.9 Ma after CAI formation. This indicates that the W present in the silicates and oxides was not fully equilibrated with the metal when diffusive transport among components ceased, consistent with the HSE data. Further, the W isotopic compositions of size-sorted metal fractions from some of the H chondrites also differ, indicating disequilibrium among some metal grains. The chemical/isotopic disequilibrium of siderophile elements among H chondrite components is likely the result of inefficient diffusion of siderophile elements from silicates and oxides to some metal and/or localized equilibration as H chondrites cooled towards their respective Hf-W closure temperatures. The tendency of 182Hf-182W isochron ages to young from H5 to H6 chondrites may indicate derivation of these meteorites from a slowly cooled, undisturbed, concentrically-zoned parent body, consistent with models that have been commonly invoked for H chondrites. Overlap of isochron ages for H4 and H5 chondrites, by contrast, appear to be more consistent with shallow impact disruption models. The W isotopic composition of metal from one CR chondrite was examined to compare with H chondrite metals. In contrast to the H chondrites, the CR chondrite metal is characterized by an enrichment in 183W that is consistent with nucleosynthetic s-process depletion. Once corrected for the correlative nucleosynthetic effect on 182W, the 182W model age for this meteorite of 7.0 ± 3.6 Ma is within the range of model ages of most metal fractions from H chondrites. Finally, the metal is therefore too young to be a direct nebular condensate, as proposed by some prior studies.},

  doi          = {10.1016/j.gca.2018.11.012},

  journal      = {Geochimica et Cosmochimica Acta},

  number       = C,

  volume       = 245,

  place        = {United States},

  year         = {Tue Nov 20 00:00:00 EST 2018},

  month        = {Tue Nov 20 00:00:00 EST 2018}

}

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