No ¹⁸²W excess in the Ontong Java Plateau source

Journal Article · Sun Mar 18 00:00:00 EDT 2018 · Chemical Geology

DOI:https://doi.org/10.1016/j.chemgeo.2018.03.024· OSTI ID:1466143

Kruijer, Thomas S. ^[1]; Kleine, Thorsten ^[1]

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1. University of Münster (Germany). Institut für Planetologie

We report that small-scale W isotope variations in ancient and modern terrestrial rocks provide insights into Earth's accretion and early differentiation history as well as the long-term evolution of the Earth's mantle. Tungsten isotope studies on such rocks have exploited advances in mass spectrometry, both NTIMS and MC-ICPMS, which now permit the determination of W isotope ratios at unprecedented precision. While W isotope studies performed in different labs by MC-ICPMS and NTIMS generally exhibit excellent agreement, obtaining accurate W isotope data at this level of precision remains analytically challenging. For example, a recent NTIMS study reported a relatively large, +24 ppm excess in ¹⁸²W/¹⁸⁴W for a Phanerozoic sample from the Ontong Java Plateau (OJP), but no such ¹⁸²W/¹⁸⁴W anomaly was found in another study by MC-ICPMS. The present study aims to resolve the discrepancy between these two previous studies, and more generally to evaluate the agreement between different recent W isotope studies by NTIMS and MC-ICPMS. To this end, we report new W isotope data for OJP drill core samples obtained by MC-ICPMS. Additionally, the OJP samples analyzed here exhibit no resolvable ¹⁸²W/¹⁸⁴W excess relative to the standards and most terrestrial rocks. Moreover, the OJP samples as well as the terrestrial rock standards analyzed here exhibit small but variable W isotope variations for ratios involving ¹⁸³W, producing coupled variations in both ‘radiogenic’ (i.e., ¹⁸²W/¹⁸³W) and ‘non-radiogenic’ (i.e., ¹⁸³W/¹⁸⁴W) ratios. These W isotope variations are analytical in origin, induced during sample preparation, and very likely caused by a nuclear field shift isotope fractionation affecting primarily the odd isotope (¹⁸³W). The recently reported ¹⁸²W excess for an OJP sample may result from this nuclear field shift effect, as the NTIMS analyses had to rely on a double normalization involving the ¹⁸³W/¹⁸⁴W ratio. More generally, these results demonstrate that using ¹⁸³W data from any MC-ICPMS or NTIMS study requires a careful quantification of any potential analytical ¹⁸³W effect. Nevertheless, once such effects are taken into account, then both ¹⁸²W/¹⁸⁴W and ¹⁸³W/¹⁸⁴W can accurately be determined to a very high level of precision.

• Research Organization: Lawrence Livermore National Lab. (LLNL), Livermore, CA (United States)
• Sponsoring Organization: USDOE National Nuclear Security Administration (NNSA)
• Grant/Contract Number: AC52-07NA27344; SFB TRR 170 subproject C3-1; LLNL-JRNL-744639
• OSTI ID: 1466143
• Alternate ID(s): OSTI ID: 1775612
• Report Number(s): LLNL-JRNL-744639; 899688
• Journal Information: Chemical Geology, Vol. 485, Issue C; ISSN 0009-2541
• Publisher: Elsevier
• Country of Publication: United States
• Language: English

Related Subjects

58 GEOSCIENCES
07 ISOTOPE AND RADIATION SOURCES
Archean rocks
Hf-W system
MC-ICPMS
NTIMS
Mantle differentiation
Late accretion