Earth and Planetary Science Letters, 2009, 288: 475-482

Chondritic magnesium isotopic composition of the terrestrial mantle: A case study of peridotite xenoliths from the North China craton

Yang W , Teng FZ, Zhang HF

Abstract

In order to further investigate inter-mineral Mg isotope fractionation at mantle temperatures and to better constrain the Mg isotopic composition of the terrestrial mantle, we have studied a set of well-characterized mantle peridotite xenoliths from Sanyitang and Beiyan, North China craton. The Sanyitang and Beiyan peridotites, which have diverse origins with different mineralogy, chemical composition and degree of partial melting and metasomatism, display a small variation in Mg isotopic composition, with delta Mg-26 varying from -0.48 to -0.12 and an average value of -0.29 +/- 0.19 (2SD, n =21) in olivines, from -0.27 to -0.10 and an average value of -0.21 +/- 0.09 (2SD, n = 12) in orthopyroxenes and 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 (similar to +/- 0.1 parts per thousand, 2SD), suggesting that Mg isotope fractionation between olivine and pyroxenes at the temperatures of >900 degrees C is insignificant. These results are thus consistent with the absence of Mg isotope fractionation during basalt differentiation. The similar to 0.4 parts per thousand Mg isotopic variations in these peridotite samples, which are larger than our external precision, might result from thermal diffusion-cl riven isotope fractionation or melt-rock interactions.