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Ion probe measurements of Os, Ir, Pt, and Au in individual phases of iron meteorites

Hsu, Weibiao and Huss, Gary R. and Wasserburg, G. J. (2000) Ion probe measurements of Os, Ir, Pt, and Au in individual phases of iron meteorites. Geochimica et Cosmochimica Acta, 64 (6). pp. 1133-1147. ISSN 0016-7037. http://resolver.caltech.edu/CaltechAUTHORS:20131119-131332141

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Abstract

We have developed a technique to measure abundances of Os, Ir, Pt, and Au in situ in Fe-Ni metal on a microscale using secondary-ion mass spectrometry. A Cs^+ beam is used to generate negative secondary ions, and interferences from molecular ions are eliminated by a combination of 40 eV of energy filtering and a mass resolution of ∼1900. Ion yields for Fe, Co, Ni, and the PGEs vary considerably from spot to spot in meteoritic metal samples. Ion yield variations for Os and Ir correlate strongly with the Fe ion yield, that for Pt is weakly correlated, and that for Au is essentially uncorrelated. From ion yields for meteorite standards and the correlations with Fe ion yield, it is possible to obtain concentrations of Os, Ir, Pt, and Au in meteorite samples. Using this technique, a Cs^+ beam current of 10 nA gives a spatial resolution of 10–20 μm and detection limits of less than 1 ppm for Os, less than 0.1 ppm for Ir, and 10–20 ppb for Pt and Au. With refinement, it should be possible to measure Rh, Pd, and Ag and to improve the spatial resolution. However, Re, Ru, W, and Hf can only be measured as positive secondary ions. We have measured the abundances of Os, Ir, Pt, and Au in kamacite, taenite, and/or plessite in five iron meteorites: Canyon Diablo (IA), Cape York (Agpalilik) (IIIA), Colomera (IIE), Cruz del Aire (Anom), and Wallapai (IID). Our measurements show that Os, Ir, Pt, and Au partition preferentially into taenite relative to kamacite during slow cooling. Measured abundance ratios (taenite/kamacite) range from ∼1.3 to ∼2.1 for Os, Ir, and Pt, and from ∼2 to ∼6 for Au. These ratios are consistent with those determined recently by laser-ablation ICPMS, but differ significantly from those determined by earlier workers. Low-temperature taenite/kamacite distribution coefficients inferred from our data are ∼2.1 for Os, ∼1.6 for Ir, ∼1.9 for Pt, and ∼6 for Au. PGEs are not enriched in taenite as much as Ni. Partitioning is controlled by the interplay of the size and electron configuration of Os, Ir, Pt, and Au and the crystal structures of kamacite and taenite. The ion probe can be used to further investigate the distribution and subsolidus redistribution of PGEs and Au in iron meteorites and in the metal in chondrites and stony irons.


Item Type:Article
Related URLs:
URLURL TypeDescription
http://dx.doi.org/10.1016/S0016-7037(99)00378-6DOIArticle
http://www.sciencedirect.com/science/article/pii/S0016703799003786PublisherArticle
Additional Information:© 2000 Elsevier Science Ltd. Received April 12, 1999; accepted in revised form October 19, 1999. We thank Herbert Palme for providing a sample of Boguslavska to use as a standard and John Wasson for providing new data for Santa Clara. Reviews by Andy Campbell and two anonymous reviewers led to significant improvements in the paper. Supported by NASA NAG5-4083 to G. J. Wasserburg and NAG5-8158 to G. R. Huss. Caltech Division Contribution Number 8610(1027).
Funders:
Funding AgencyGrant Number
NASANAG5-4083
NASANAG5-8158
Other Numbering System:
Other Numbering System NameOther Numbering System ID
Caltech Division of Geological and Planetary Sciences8610
Lunatic Asylum Lab 1027
Record Number:CaltechAUTHORS:20131119-131332141
Persistent URL:http://resolver.caltech.edu/CaltechAUTHORS:20131119-131332141
Official Citation:Weibiao Hsu, Gary R. Huss, G.J. Wasserburg, Ion probe measurements of Os, Ir, Pt, and Au in individual phases of iron meteorites, Geochimica et Cosmochimica Acta, Volume 64, Issue 6, March 2000, Pages 1133-1147, ISSN 0016-7037, http://dx.doi.org/10.1016/S0016-7037(99)00378-6. (http://www.sciencedirect.com/science/article/pii/S0016703799003786)
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:42572
Collection:CaltechAUTHORS
Deposited By: Ruth Sustaita
Deposited On:19 Nov 2013 22:36
Last Modified:19 Nov 2013 22:36

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