An eclogitic component in the Pitcairn mantle plume: Evidence from olivine compositions and Fe isotopes of basalts. (1st February 2022)

• Record Type: Journal Article
• Authors: Shi, Jin-Hua; Zeng, Gang; Chen, Li-Hui; Hanyu, Takeshi; Wang, Xiao-Jun; Zhong, Yuan; Xie, Lie-Wen; Xie, Wen-Li
• Is Part Of: Geochimica et cosmochimica acta. Volume 318(2022)
• Page Start: 415
• Page End: 427
• Publication Date: 2022-02-01
• Subjects: Eclogite -- Pitcairn -- EM1 -- Crustal recycling -- Fe isotopes -- Olivines; Geochemistry -- Periodicals; Meteorites -- Periodicals; Géochimie -- Périodiques; Météorites -- Périodiques; Geochemie; Astrochemie; Electronic journals; 551.905
• DOI: 10.1016/j.gca.2021.12.017
• Languages: English
• ISSNs: 0016-7037

Abstract

Abstract: Subducted oceanic crust can transform into eclogite in the upper mantle, and generate chemical heterogeneity of mantle plumes, as recorded by elemental and radiogenic isotopic variations in oceanic island basalts (OIBs). The secondary pyroxenite produced by the reaction between eclogite-derived melt and peridotite is increasingly regarded as a major source component of OIBs, as well as peridotite. However, it remains unclear whether eclogite can be a direct source component of OIBs. To test this possibility, we present high-precision whole-rock Fe isotopes and the chemical compositions of olivine phenocrysts from well-characterized EM1-type basalts from Pitcairn Island. The Pitcairn basalts are characterized by moderate 87 Sr/ 86 Sr, low 143 Nd/ 144 Nd and 206 Pb/ 204 Pb isotopic ratios, and lowest δ 26 Mg values among OIBs, suggesting a contribution from recycled ancient crustal components (oceanic crust plus sediment). For comparison, we also report the Fe isotope compositions of FOZO-type basalts (with low 87 Sr/ 86 Sr, moderately high 143 Nd/ 144 Nd and moderate 206 Pb/ 204 Pb ratios) from the Louisville hot spot track, which were suggested to be a typical peridotite-derived OIB. The Louisville basalts have MORB-like δ 57 Fe values (0.06‰–0.15‰), whereas the Pitcairn lavas have substantially heavier Fe isotopic compositions (δ 57 Fe = 0.17‰–0.31‰) than MORBs. Quantitative evaluations suggest that magmatic differentiation, partial melting, and elevated oxygen fugacity in the source are insufficient to generate the heavy Fe isotopic compositions of the Pitcairn basalts. A good correlation between δ 57 Fe and εNd(i) (or δ 26 Mg) values in the Pitcairn basalts indicates binary mixing, and the melts derived from the EM1 endmember have unusually heavy Fe and light Mg isotopic compositions. In order to explain the origin of the Fe and Mg isotopic compositions, we calculated the Fe, Mg, and Nd isotopic compositions of partial melts of eclogite, secondary pyroxenite and peridotite, respectively. The results indicate that eclogite is the only suitable candidate to generate melts with both heavy Fe and light Mg isotopes. This inference is strengthened by the major and minor elemental compositions of olivine phenocrysts from the Pitcairn basalts, which show low Fo (73.2–82.5) and Ni contents (620–1949 ppm), and low Mn/Fe (0.011–0.015) and Ni/(Mg/Fe) (524–1041) ratios. These characteristics are markedly different from those of olivines from MORBs and the Koolau lavas from Hawaii, which represent phenocrysts that equilibrated with peridotite-derived and secondary pyroxenite-derived melts, respectively. We therefore argue that eclogite is the source lithology of the EM1 endmember of the Pitcairn basalts. Binary mixing between our modelled eclogite- and peridotite-derived melts produced magmas with relatively low Mg# value, Mg/Fe ratios and moderate Ni contents characteristics, which are preserved in the low-Fo Pitcairn olivines. Our results highlight that, in addition to peridotite and secondary pyroxenite, eclogite may survive in mantle plumes at shallow depths and make a substantial contribution to the source of OIBs.