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Earth and Space Sciences · 术语表

什么是 Mantle end-member？

Definition 4.8 University Earth and Space Sciences — Year 2 · 第 4 章 — Isotope Geochemistry

A mantle end-member is one of the extreme isotopic compositions from which the Sr, Nd and Pb isotope ratios of oceanic basalts can be built by mixing. Four are commonly named: DMM, the depleted MORB mantle (low $^{87}\mathrm{Sr}/^{86}\mathrm{Sr}$ , high $\varepsilon_{\mathrm{Nd}}$ ); HIMU, for a high $\mu = {}^{238}\mathrm{U}/^{204}\mathrm{Pb}$ in its source (very radiogenic lead, $^{206}\mathrm{Pb}/^{204}\mathrm{Pb}$ above 20, with low $^{87}\mathrm{Sr}/^{86}\mathrm{Sr}$ ); EM1, enriched mantle 1 (moderately high $^{87}\mathrm{Sr}/^{86}\mathrm{Sr}$ , negative $\varepsilon_{\mathrm{Nd}}$ , unradiogenic lead); and EM2, enriched mantle 2 (high $^{87}\mathrm{Sr}/^{86}\mathrm{Sr}$ ).

Strontium and neodymium isotopes of 89 basalts from five ocean islands and of the mean mid-ocean ridge basalt. The HIMU-type islands plot below the main array, with the low strontium ratio of MORB but a lower _ Nd; the EM1-type islands lie on the enriched side of CHUR. Samoan lavas of the EM2 type reach 87 Sr/86 Sr = 0.7205, far off this scale to the right. Data: Stracke and others (2022), CC BY 4.0; Gale and others (2013); Edwards and others (2019). — Strontium and neodymium isotopes of 89 basalts from five ocean islands and of the mean mid-ocean ridge basalt. The HIMU-type islands plot below the main array, with the low strontium ratio of MORB but a lower $\varepsilon_{\mathrm{Nd}}$ ; the EM1-type islands lie on the enriched side of CHUR. Samoan lavas of the EM2 type reach $^{87}\mathrm{Sr}/^{86}\mathrm{Sr} = 0.7205$ , far off this scale to the right. Data: Stracke and others (2022), CC BY 4.0; Gale and others (2013); Edwards and others (2019).

One widely used interpretation of the mantle reservoirs, not to scale: the ridges sample the depleted upper mantle; oceanic crust and sediment return to the mantle at subduction zones; plumes rising from the deep mantle bring up material in which old recycled components are mixed, and give the islands their HIMU, EM1 or EM2 character.

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