First in situ Re-Os dating of molybdenite by LA-ICP-MS/MS

Högmalm, Johan; Dahlgren, I.Lena; Fridolfsson, Irma; Zack, Thomas

doi:10.1007/s00126-019-00889-1

Cited by 24 publications

(53 citation statements)

References 20 publications

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“…Increased resolution and sensitivity for micro-scale quantification of elemental and isotopic contents has sparked renewed interest in molybdenite Re-Os geochronology. Surprisingly, conclusions derived from NanoSIMS [26] and LA-ICP-MS/MS [27] molybdenite spot analysis proclaimed there was no decoupling of 187 Re from 187 Os, contrary to prior observations. By visually contrasting 185 Re and mass 187 (presumed to be 187 Re + 187 Os) NanoSIMS isotopic 2-D maps, it was concluded that sufficient similarity precluded decoupling [26] .…”

Section: Recent Micro-scale Observationscontrasting

confidence: 94%

“…NanoSIMS analysis additionally re-confirmed Re oscillatory zoning, Os nanoparticles, and Re overgrowths. Based on collision cell LA-ICP-MS/MS data yielding concordant Re-Os ages, it was concluded to be "limited to essentially nonexistent decoupling" [27] . Given the surprising conclusion that decoupling is rare or absent, we decided to investigate their methodology, observations, and conclusions in comparison to our known expectations and procedures.…”

Section: Recent Micro-scale Observationsmentioning

confidence: 99%

“…Limitations to conventional micro-scale Re-Os in-situ dating using LA-MC-ICP-MS were overcome by implementing a methane-filled reaction chamber imparting a mass shift between 187 Re and 187 Os [27] . The reaction product is predominantly 187 OsCH2 (up to 70% conversion) with minor 187 ReCH2 (~2% conversion).…”

Section: Nanosims Determination Of 192mentioning

confidence: 99%

“…The reaction product is predominantly 187 OsCH2 (up to 70% conversion) with minor 187 ReCH2 (~2% conversion). By tracking the non-reacted 185 and 187 masses along with 199 and 201 methane adjusted masses, they were able to date molybdenite within 1% of the conventionally determined age [27] . Yet, determining Re-Os ages still required an externally-determined factor of F = ( 187 OsCH2/ 185 ReCH2)/age derived from a molybdenite "standard".…”

Section: Nanosims Determination Of 192mentioning

confidence: 99%

“…Precision varied between 1-5% 2 RSD. Due to the small spot size, usually 70 um, and matching ages, it was concluded the molybdenite analyzed did not show significant parent-daughter decoupling [27] .…”

Section: Nanosims Determination Of 192mentioning

confidence: 99%

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Addressing Molybdenite187 Re Parent- 187 Os Daughter Intra-Crystalline Decoupling In Light of Recent In-Situ Micro-Scale Observations

Zimmerman

Yang

Stein

et al. 2021

Preprint

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Every so often, an analytical advancement or challenging geological occurrence necessitates re-evaluation of well-established geochronological methods. Rhenium-osmium (Re-Os) dating of sulfide minerals, especially molybdenite, by isotope dilution-negative thermal ionization mass spectrometry (ID-NTIMS) yields demonstrably accurate, robust ages. Difficulty in determining an age is caused by either 1) geological complexity including intra-crystalline heterogeneity or 2) inadequate analytical capabilities. Established, systematic methods overcome most of these difficulties, with exceptions. With respect to molybdenite, geological complexity is encountered as macro-scale polyphase overgrowths. Micro-scale complexity is observed as primary Re oscillatory zoning and secondary intra-granular 187Re parent-187Os daughter decoupling. Macro- and micro-scale geologic heterogeneities are overcome by systematic, targeted sampling protocols built upon careful hand sample and microscopic observations linking molybdenite crystallization to the host rock's geologic history.Advances in instrumentation permit more accurate and precise determination of elemental and isotopic compositions at the micro-scale, which ultimately reveals geological complexity and heterogeneity. Yet, newly produced high-resolution data must be carefully scrutinized and interpreted correctly. Two new micro-scale analytical techniques have been proposed: (1) to use NanoSIMS imaging to reveal heterogeneities that could preclude micro-scale geochronology (spot dating) methods, and (2) enable micro-scale geochronology using LA-ICP-MS/MS to distinguish the interfering masses of parent Re and daughter Os isotopes. We turn first to NIST Reference Material 8599, Henderson molybdenite, to compare its Re-Os characteristics and ID-NTIMS results with the newly published Re-Os molybdenite data. We identify the "best possible results" by quantifying the homogeneity of the NIST molybdenite, including model age variability, precision of ID-NTIMS analyses, and statistical treatment. Subsequently, we examine the limitations of the two newly proposed methods. We demonstrate that parent-daughter decoupling cannot be precluded by visual inspection of NanoSIMS isotopic maps. In addition, we prove mathematically that the quantifying the contribution of 187Os to the total measured mass 187 by LA-ICP-MS/MS is far too imprecise to achieve high-precision geochronology. Statistical data treatment and reporting can also result in misguided conclusions and applications.

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Section: Recent Micro-scale Observationscontrasting

confidence: 94%

Section: Recent Micro-scale Observationsmentioning

confidence: 99%

Section: Nanosims Determination Of 192mentioning

confidence: 99%

Section: Nanosims Determination Of 192mentioning

confidence: 99%

Section: Nanosims Determination Of 192mentioning

confidence: 99%

See 3 more Smart Citations

Addressing Molybdenite187 Re Parent- 187 Os Daughter Intra-Crystalline Decoupling In Light of Recent In-Situ Micro-Scale Observations

Zimmerman

Yang

Stein

et al. 2021

Preprint

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The 187Re-187Os and 190Pt-186Os Radiogenic Isotope Systems: Techniques and Applications to Metallogenic Systems

Norman

2023

Mineral Resource Reviews

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Rhenium, Os, and Pt are redox sensitive elements that are concentrated in highly reducing environments such as those associated with black shales but mobile under more oxidizing conditions such as those associated with arc volcanism. They are chalcophile in many terrestrial ore-forming environments, and their isotopic systematics provide unique opportunities to date the formation of sulfide ore deposits and understand their petrogenesis. Fractional crystallization of magmatic sulfide ores generates primary variations in Re/Os and Pt/Os that allow mineral and whole rock isochron ages to be determined and discrimination of crustal and mantle sources based on initial Os isotopic compositions. Molybdenite is especially well suited for geochronology due to its high Re/Os and resistance to resetting. Rhenium concentrations in molybdenite tend to reflect the composition or provenance of the ore-forming fluids, with higher concentrations associated with more primitive sources or more oxidized fluids and lower concentrations with more evolved and/or reduced conditions, although local and regional factors also have a significant influence. Many studies have used pyrite for dating but its typically low Re concentration, variable initial Os isotopic composition (reflecting fluid mxing), and susceptibility to re-equilibration makes its use as a geochronometer problematic in many cases. Other sulfide minerals such as bornite and arsenopyrite have shown promise for Re–Os isotope geochronology but additional studies are needed to evaluate their broader applicability for dating of ore deposits. The isobaric beta decay of parent isotope 187Re to 187Os has restricted investigation of this system by microbeam techniques such as ion microprobe or laser ablation mass spectrometry, especially for geochronology. This requires either chemically processing the sample to separate the elements or novel techniques such as collision-cells that preferentially ionize the Re and Os during the analysis. Thermal ionization mass spectrometry (TIMS) and inductively-coupled plasma mass spectrometry (ICPMS) are the most widely applied techniques for Re-Pt-Os isotopic analyses. Specialized techniques for sample digestion to ensure redox equilibrium between Os in the sample and the isotopically enriched spikes used for isotope dilution measurements are typically required. This chapter briefly reviews development of the 187Re-187Os and 190Pt-186Os isotopic systems for earth science, physico-chemical controls on their behavior in ore-forming environments, and applications to metallogenic systems.

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