Silicon Isotope Geochemistry of Ocean Island Basalts: Search for Deep Mantle Heterogeneities and Evidence for Recycled Altered Oceanic Crust

Abstract:

Analyses of Ocean Island Basalts (OIB) have shown that the Earth’s mantle contains isotopically distinct components, but debate about the degree and cause of variability persists. The study of silicon (Si) isotopes in OIBs has the potential to elucidate mantle heterogeneities. Relatively large (~several per mil) Si isotopic fractionation occurs in low-temperature environments during precipitation from dissolved Si, where the precipitate is preferentially enriched in the lighter isotopes [1], but only a limited range (~tenths of a per mil) of Si isotope fractionation has been observed due to high-temperature igneous processes [2]. Therefore, Si isotopes may be useful as tracers for the presence of crustal material in OIB source regions in a manner similar to more conventional stable isotope systems, such as oxygen.

Here we present the first comprehensive suite of high-precision Si isotopic data obtained by MC‑ICP‑MS for a diverse set of OIBs representing the EM‑1, EM‑2, and HIMU mantle components. In general, the Si isotopic compositions of OIBs analyzed here are agreement with previous estimates for Bulk Silicate Earth (BSE). However, small systematic variations are present; the HIMU end-member Mangaia and HIMU-type Cape Verde island São Nicolau are enriched in the light isotopes of Si (δ³⁰Si = −0.37 ± 0.06‰ and δ³⁰Si = −0.39 ± 0.04‰, respectively; errors are 2sd), with compositions intermediary between Mid Ocean Ridge Basalts and chondritic values. Additionally, Iceland samples from volcanic complexes in the Northern Rift Zone show similar Si isotope compositions (on average, δ³⁰Si = −0.40 ± 0.06‰). In contrast, the δ³⁰Si averages of the EM-1 end-member Pitcairn (−0.28 ± 0.07‰), the EM-2 end-member Samoa (−0.31 ± 0.07‰) and other OIB localities do not show any significant difference from previous estimates for the δ³⁰Si value of BSE [3].

The Si isotopic variability in some HIMU-type and Icelandic OIBs most likely reflects the incorporation of recycled altered oceanic crust in the plume source. However, the sampling of a primitive reservoir enriched in the light isotopes of Si, as suggested by [4], cannot be ruled out as a potential source of Si isotope variations in OIBs.

References: [1] Ziegler et al., GCA 2005 [2] Savage et al., GCA 2011 [3] Savage et al., EPSL 2010 [4] Huang et al., GCA 2014