Distribution of Mg in the Earth is extremely uneven with more than 99% of the total Mg in the mantle. Therefore, the Mg isotopic composition of the mantle can represent that of the Earth, which may provide important constrains on the early evolution of the Earth. However, the Mg isotopic composition of the mantle has not yet been well estimated due to the lack of systematic studies.

Recently, Dr. Wei Yang and his co-advisor Pf. Hong-Fu Zhang of State Key Laboratory of Lithospheric Evolution, IGGCAS, and Pf. Fang-Zhen Teng of University of Arkansas, investigated Mg isotopic compositions of mantle peridotite xenoliths from Sanyitang and Beiyan, North China craton. Their study was published in Earth and Planetary Science Letters (Yang W, Teng F Z, Zhang H F. Chondritic magnesium isotopic composition of the terrestrial mantle: A case study of peridotite xenoliths from the North China craton. Earth and Planetary Science Letters, 2009, 288: 475-482).

Their study indicates that the Sanyitang and Beiyan peridotites, which have diverse origins with different mineralogy, chemical composition and degrees of partial melting and metasomatism, display a small variation in Mg isotopic composition, with δ²⁶Mg varying from -0.48 to -0.12 and an average value of -0.29 ± 0.19 (2SD, n = 21) in olivines, varying from -0.27 to -0.10 and an average value of -0.21 ± 0.09 (2SD, n = 12) in orthopyroxenes and varying from -0.35 to -0.08 and an average value of -0.22 ± 0.14 (2SD, n = 15) in clinopyroxenes. The Mg isotopic compositions of the coexisting olivine, orthopyroxene and clinopyroxene in all peridotites are identical within our external precision (~ ± 0.1‰, 2SD), suggesting that Mg isotope fractionation between olivine and pyroxenes at the temperatures of > 900 ^oC is insignificant. These results are thus consistent with the absence of Mg isotopic fractionation during basalt differentiation. The ~0.4‰ Mg isotopic variations in these peridotites samples, which are slightly larger than our precision, might result from thermal diffusion isotope fractionation or melt-rock interactions. Overall, the δ²⁶Mg of the mantle, based on peridotite minerals analyzed here, is estimated to be -0.26 ± 0.16 (2SD). This value is in excellent agreement with recent studies based on peridotites and oceanic basalts. The Mg isotopic composition of the silicate Earth is concluded to have a chondritic Mg isotopic composition.

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