T189 Frontiers in Non-Traditional Stable Isotopes: In Honor of Fang-Zhen Teng, Recipient of the 2014 MSA Award

Tuesday, 21 October 2014
8:00 AM - 12:00 PM
213

MAGNESIUM AND OXYGEN ISOTOPIC COMPOSITIONS OF ALKALINE ROCKS IN NORTHWEST XINJIANG, CHINA: IMPLICATIONS FOR ORIGIN OF LOW-D26MG GRANITOIDS

Paper number: T189-11
Program: Technical Sessions
Time: 10:25 AM – 10:40 AM

Authors

  • Shan Ke Presenting Author
    China University of Geosciences, Beijing, N/A
  • Fangzhen Teng
    University of Washington, Seattle, WA

Abstract

Magnesium isotopes of granitoids could be potentially used to trace recycling of supracrustal materials. High-d26Mg granitoids have been previously reported which are attributed to the involvement of sedimentary rocks in the magma sources. Low-d26Mg granitoids, however, have been rarely reported and their origin remains unclear. In this study, we report high-precision (±0.06‰ for d26Mg, 2SD) Mg isotopic analyses for alkaline syenites and granites as well as their magnesian minerals from the Kuzigan region of northwest Xinjiang, China. All syenites have d26Mg values (-0.46 to -0.28‰) which are significantly lighter than the mean of the silicate Earth and igneous rocks (-0.25 ± 0.07‰). The granites representing the products of fractional crystallization of the syenites also have low d26Mg values (-0.41 to -0.19‰) similar to those of the syenites. Furthermore, d26Mg values of syenites and granites show no correlations with SiO2, MgO, FeO and CaO concentrations, indicating limited Mg isotope fractionation during crystal-melt differentiation of alkaline granitic magmas. This is consistent with the behavior of Mg isotopes during differentiation of I-type and A-type granitic magmas, although there is significant inter-mineral Mg isotope fractionation between co-existing aegirine-augite and biotite in the syenites and granites (D26MgAgt-Bt= -0.15 to -0.67‰).

Given the absence of Mg isotope fractionation during alkaline magmatic differentiation, Mg isotopic signatures of the Kuzigan syenites and granites reflect a low-d26Mg source. These syenites and granites also have high radiogenic lead isotopic ratios (206Pb/204Pb = 18.183-18.820) and heavy oxygen isotope ratios (d18O = 8.5-11.6‰), indicating the recycling and involvement of supracrustal materials in their source. There is a negative correlation between d26Mg and d18O in the syenites and granites, which strongly suggests that the recycled materials are most likely carbonate, because carbonate has extremely light Mg and heavy O isotopic compositions relative to the silicate Earth and the crystalline crust. The combined use of Mg and O isotopic ratios is therefore a powerful tool to trace recycling of sedimentary carbonates in magma sources.