Osmium isotope heterogeneity in Earth's mantle: insights from peridotite xenoliths of petit-spot volcano

Abstract

The Re-Os isotopic composition of oceanic peridotites can be used to detect mantle heterogeneity created by ancient partial melting events in the Earth's interior. In this study, we analyzed the Re-Os isotope and highly siderophile element (HSE) compositions of mantle xenoliths from petit-spot volcanoes off the coast of Tohoku, Japan. Four of the analyzed harzburgite samples exhibited low Os isotope ratios (＜ 0.12), with the lowest value reaching 0.1133. This is the most unradiogenic Os isotope ratio ever reported in Pacific peridotites, corresponding to a Re-depletion model age of ca. 2 Ga. Most notably, there is a clear negative correlation between the Os isotope ratios and the Fo content in olivine. This suggests that a depleted mantle domain that underwent partial melting during the Proterozoic exists beneath the petit-spot volcanoes. Considering that basalts from the petit-spot volcanoes show EM-1 type isotopic signatures originating from ancient recycled materials, the unradiogenic Os isotope ratios detected in this study may also reflect the incorporation of ancient recycled components within the convective mantle.