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- Title
Linking Chemical Heterogeneity to Lithological Heterogeneity of the Samoan Mantle Plume With Fe‐Sr‐Nd‐Pb Isotopes.
- Authors
Wang, Xiao‐Jun; Chen, Li‐Hui; Hanyu, Takeshi; Shi, Jin‐Hua; Zhong, Yuan; Kawabata, Hiroshi; Miyazaki, Takashi; Hirahara, Yuka; Takahashi, Toshiro; Senda, Ryoko; Chang, Qing; Vaglarov, Bogdan S.; Kimura, Jun‐Ichi
- Abstract
The Samoan mantle plume is thought to host three isotopically (radiogenic) distinct low‐3He/4He components including EM2 (enriched mantle 2), dilute HIMU (high μ = 238U/204Pb), and a depleted mantle (DM) component, which were sampled by shield‐stage lavas from the Malu, Vai, and Upo trend volcanoes, respectively. However, it is unclear whether the isotopically distinct components are present as different lithologies. Using new Fe–Sr–Nd–Pb isotope data for Tutuila basalts (Samoa), combined with literature data for other Samoan basalts, we attempt to infer the lithological structure of the Samoan plume. The results show that "Malu trend" basalts have heavier Fe isotopic compositions (δ57Fe = 0.15–0.24‰) than "Vai trend" and "Upo trend" basalts. The latter two groups have average δ57Fe of 0.14 ± 0.07‰ (2SD) and 0.11 ± 0.03‰ (2SD), respectively, similar to normal midocean ridge basalts (N‐MORBs, δ57Fe = 0.15 ± 0.05‰, 2SD). The fractional‐crystallization‐corrected δ57Fe values of all shield lavas are positively correlated with (Gd/Yb)N, Pb/Nd and 87Sr/86Sr ratios whereas negatively correlated with Nb/Th and εNd ratios, which cannot be explained by partial melting of a single garnet peridotite but point to heterogeneous source lithologies. The EM2 lavas are characterized with high δ57Fe and (Gd/Yb)N, low Nb/Th, and enriched Sr–Nd isotopic ratios, requiring a pyroxenitic source component with imprints of both recycled terrigenous sediments and oceanic crust. The Vai‐ and Upo‐trend lavas with MORB‐like δ57Fe can be explained by partial melting of peridotitic sources, although different extents of refertilization by recycled crust are essential for generating their distinct radiogenic isotope signatures. These observations highlight the lithological heterogeneity of the Samoan plume and relates the EM2 component with a pyroxenitic lithology. Plain Language Summary: Lavas erupted on Samoan islands/seamounts construct three parallel volcanic lineaments: Upo‐, Malu‐, and Vai‐trend. Basalt samples from each of the lineaments have distinctive chemical and isotopic compositions. They are thought to be the surface expressions of three isotopically distinct components in the Samoan mantle plume beneath the volcanic lineaments. The physical nature (lithological identity) of the three distinct mantle components is still unclear. Recent efforts have found that iron isotope ratios (e.g., the ratio of iron‐57 to iron‐54) are a new tracer of mantle source mineralogy of basaltic lavas. In this study, we use iron isotopic compositions of basalts from Tutuila Island, American Samoa, together with literature data for other Samoan basalts, to infer the lithological identity of the three distinct mantle components. We find that these lavas have variable iron isotopic compositions, which cannot be ascribed to fractional crystallization and partial melting process. The correlations between iron isotopes and radiogenic isotopes of these lavas indicate that their difference in iron isotopic compositions is most likely caused by difference in mantle source mineralogy. Specifically, a pyroxene‐dominated lithology (e.g., pyroxenite) is required to explain the heavy iron isotopic compositions (high ratio of iron‐57 to iron‐54) of lavas from the Malu trend, while the partial melting of peridotitic mantle sources can reproduce the iron isotopic signatures of lavas from the Vai‐ and Upo‐trend volcanoes. These observations highlight the heterogeneous lithological structure of the Samoan mantle plume. Key Points: Samoan shield‐stage lavas show heterogeneous Fe isotopic compositionsThe EM2 (enriched mantle 2) component in the Samoan mantle plume is pyroxenitic with heavy Fe isotopic compositionsThe other components in this mantle plume are most likely to be peridotitic with primitive‐mantle‐like Fe isotopic compositions
- Subjects
SAMOAN Islands; PETROLOGY; MANTLE plumes; ISOTOPES; CRYSTALLIZATION; PYROXENITE
- Publication
Journal of Geophysical Research. Solid Earth, 2021, Vol 126, Issue 12, p1
- ISSN
2169-9313
- Publication type
Article
- DOI
10.1029/2021JB022887