Barium isotopic composition of the mantle: Constraints from carbonatites

摘要

To investigate the behaviour of Ba isotopes during carbonatite petrogenesis and to explore the possibility of using carbonatites to constrain the Ba isotopic composition of the mantle, we report high-precision Ba isotopic analyses of: (1) carbonatites and associated silicate rocks from the only active carbonatite volcano, Oldoinyo Lengai, Tanzania, and (2) Archean to Cenozoic carbonatites from Canada, East Africa, Germany and Greenland. Carbonatites and associated phonolites and nephelinites from Oldoinyo Lengai have similar delta Ba-137/134 values that range from +0.01 to +0.03 parts per thousand, indicating that Ba isotope fractionation during carbonatite petrogenesis is negligible. The limited variation in delta(137/134 )a values from -0.03 to +0.09 parts per thousand for most carbonatite samples suggests that their mantle sources have a relatively homogeneous Ba isotopic composition. Based on the carbonatites investigated in this work, the average delta Ba-137/134 value of their mantle sources is estimated to be +0.04 +/- 0.06 parts per thousand (2SD, n = 16), which is similar to the average value of +0.05 +/- 0.06 parts per thousand for mid-ocean ridge basalts. The lower delta Ba-137/134 value of -0.08 parts per thousand in a Canadian sample and higher delta Ba-137/134 values of +0.14 parts per thousand and + 0.23 parts per thousand in two Greenland samples suggest local mantle isotopic heterogeneity that may reflect the incorporation of recycled crustal materials in their sources. (C) 2019 Elsevier Ltd. All rights reserved.