Pt-Os isotope systematics do not prove an ultra-deep origin for intraplate volcanism

Alan D. Smith

Dept. Earth Science, University of Durham, Durham DH1 3LE, U.K.

muicmohr@ecosse.net

Abstract

Suprachondritic ¹⁸⁶Os/¹⁸⁸Os ratios in intraplate volcanic rocks have been interpreted as indicating incorporation of outer core material into plume sources. Such signatures may, however, also be generated in pyroxenites precipitated from MgO-rich melts by the preferential incorporation of Pt relative to Os in ortho- and clinopyroxenes. The ¹⁸⁶Os/¹⁸⁸Os signatures do not therefore give any indication of depth of origin, and are compatible with shallow-source models for intraplate volcanism.

Variations in osmium isotopic composition result from the alpha decay of ¹⁹⁰Pt to ¹⁸⁶Os and the beta decay of ¹⁸⁷Re to ¹⁸⁷Os. Suprachondritic ¹⁸⁷Os/¹⁸⁸Os ratios in intraplate volcanic rocks have generally been used to support plume models for generation of this type of volcanism from recycled oceanic crust. The Pt-Os system has been applied to comparatively few rock types on account of the long half life of ¹⁹⁰Pt and corresponding small variation in ¹⁸⁶Os abundances. Analyses of Hawaiian picrites and Gorgona Island komatiites have also revealed suprachondritic ¹⁸⁶Os/¹⁸⁸Os ratios (¹⁸⁶Os/¹⁸⁸Os = 0.119832 to 0.119854) in intraplate volcanic rocks [Brandon et al., 1998; 2000; 2003].

A problem for the crustal recycling model is, however, that the coupled ¹⁸⁶Os/¹⁸⁸Os -¹⁸⁷Os/¹⁸⁸Os variation in such samples lies above mixing lines between recycled crust and peridotite compositions. The intraplate signatures have therefore been interpreted as resulting from entrainment of small amounts (1%) of outer core material [¹⁸⁶Os/¹⁸⁸Os estimated between 0.118952 and 0.118973, Brandon et al., 1998; 2000; 2003] into plume sources. This maintains the concept of an ultra-deep origin for this category of volcanism.

This interpretation is, however, non-unique. High Pt/Os and Re/Os ratios, which should lead to generation of suprachondritic ¹⁸⁶Os/¹⁸⁸Os -¹⁸⁷Os/¹⁸⁸Os, are also features of pyroxenites precipitated from MgO-rich melts such as bonninites [Smith, 2003]. Using average Pt/Os and Re/Os ratios of pyroxenite dykes from the Bay of Islands ophiolite complex [orthopyroxenite: Pt/Os = 199, Re/Os = 3.9; clinopyroxenite: Pt/Os = 264, Re/Os = 7.1 Edwards, 1990], it can be calculated that the ¹⁸⁶Os/¹⁸⁸Os and ¹⁸⁷Os/¹⁸⁸Os signatures found in intraplate picrites and komatiites could be produced from peridotite:pyroxenite mixtures containing 30 – 60% pyroxenite in 150 million years. This corresponds to the lifetime of oceanic lithosphere [Smith, 2003]. If isotopic evolution took place over the longer time period of 2 billion years, as might be appropriate for evolution in a continental mantle reservoir, the required percentage of pyroxenite would decrease to 5 – 10%.

Figure: The ¹⁸⁶Os/¹⁸⁸Os and ¹⁸⁷Os/¹⁸⁸Os isotopic variation in the depleted mantle and intraplate volcanic rocks (after Brandon et al., 2003). ¹⁸⁷Os/¹⁸⁸Os ratios are expressed in terms of gamma-Os which represents the percent deviation from the chondritic ¹⁸⁷Os/¹⁸⁸Os ratio at a given time. The low Pt content of basaltic crust causes mixing lines between recycled basalt:sediment mixtures and peridotite to have a shallow trajectory irrespective of the type of sediment involved. Plume models must therefore invoke contamination with outer core material to explain the isotopic signatures observed in Gorgona komatiites and Hawaiian picrites. However, the plume model is not proven as the intraplate arrays may also be generated from pyroxenite:peridotite mixtures (mixing lines from Smith, 2003) which could be formed at shallow levels in the mantle. Tick marks on mixing lines indicate 10% increments in all models.

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