17:15 〜 18:30
[SGC32-P08] Helium isotope heterogeneity recorded in different phenocrysts in Samoan lavas
キーワード：Helium、Ocean island basalt、Samoa
Ocean island basalts (OIBs) are generally considered to be fed by upwelling mantle plumes sourced from deep mantle domain. Geochemical variation observed in OIBs should reflect heterogeneous deep mantle materials including subducted slabs. Isotopically enriched signature of Samoan OIBs have been recognized to be the manifestation of subducted sedimentary materials. Samoan OIBs also show depleted and primordial isotopic signature, suggesting that variety of source materials are involved in the magma sources as demonstrated by geochemical heterogeneity in the inter-island scale to phenocryst scale. For example, lavas on Ofu Island show large 3He/4He range (19-34 Ra) despite limited Sr and Nd isotopic variation. Lavas with very high 87Sr/86Sr show Sr isotopic difference between clinopyroxene and whole rock. Moreover, variation in 87Sr/86Sr has been found in melt inclusions in olivine. In this study, we show 3He/4He isotopic difference between olivine and clinopyroxene in a single lava from Tutuila Island.
Radiogenic isotope data from the literatures and this study indicate the presence of at least four mixing components; EM2 (enriched mantle-2), HIMU (high-mu), FOZO (focal zone mantle), and depleted mantle. Olivine and clinopyroxene in lavas with FOZO-like compositions show similar 3He/4He. In contrast, olivine show higher 3He/4He than clinopyroxene in lavas with radiogenic 87Sr/86Sr. Some lavas with radiogenic 87Sr/86Sr also have 87Sr/86Sr that is different from clinopyroxene hosted in them. The 3-D isotopic plot (e.g., Sr-Pb-He) of Samoan OIBs displays a cone-shape distribution with the FOZO component at the top (high 3He/4He) and EM2, HIMU and depleted component at the bottom (low 3He/4He). Tutuila lavas plot on a steep slope of the cone. I suggest that small difference in mixing proportion of the components is responsible for relatively large isotopic difference. Such isotopic difference may not be caused by the coupled effect of temporal change in magma composition and mineral crystallization, as Mg# of olivine and clinopyroxene overlaps with each other. I infer that compositional variation existed in a magma chamber where olivine and clinopyroxene crystallized, and thereby geochemical heterogeneity in a deep-rooted mantle plume was preserved to the shallow depth.
Radiogenic isotope data from the literatures and this study indicate the presence of at least four mixing components; EM2 (enriched mantle-2), HIMU (high-mu), FOZO (focal zone mantle), and depleted mantle. Olivine and clinopyroxene in lavas with FOZO-like compositions show similar 3He/4He. In contrast, olivine show higher 3He/4He than clinopyroxene in lavas with radiogenic 87Sr/86Sr. Some lavas with radiogenic 87Sr/86Sr also have 87Sr/86Sr that is different from clinopyroxene hosted in them. The 3-D isotopic plot (e.g., Sr-Pb-He) of Samoan OIBs displays a cone-shape distribution with the FOZO component at the top (high 3He/4He) and EM2, HIMU and depleted component at the bottom (low 3He/4He). Tutuila lavas plot on a steep slope of the cone. I suggest that small difference in mixing proportion of the components is responsible for relatively large isotopic difference. Such isotopic difference may not be caused by the coupled effect of temporal change in magma composition and mineral crystallization, as Mg# of olivine and clinopyroxene overlaps with each other. I infer that compositional variation existed in a magma chamber where olivine and clinopyroxene crystallized, and thereby geochemical heterogeneity in a deep-rooted mantle plume was preserved to the shallow depth.