Tracking the lithium isotopic evolution of the mantle using carbonatites |
| |
Authors: | Ralf Halama William F. McDonough Roberta L. Rudnick Keith Bell |
| |
Affiliation: | 1. Key Laboratory of Mineral Resources, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China;2. University of Chinese Academy of Sciences, Beijing 100049, China;3. Institute of Earth Sciences, Academia Sinica, Taipei 11529, Taiwan;4. Department of Earth Science, School of Physical and Mathematical Sciences, University of Ghana, P.O. Box LG 58, Legon-Accra, Ghana;1. Department of Earth and Planetary Sciences and McDonnell Center for the Space Sciences, Washington University in St. Louis, MO 63130, USA;2. School of Earth and Environmental Sciences, University of St Andrews, St Andrews KY16 9AJ, UK |
| |
Abstract: | Carbonatites are mantle-derived, intraplate magmas that provide a means of documenting isotopic variations of the Earth's mantle through time. To investigate the secular Li isotopic evolution of the mantle and to test whether Li isotopes document systematic recycling of material processed at or near the Earth's surface into the mantle, we analyzed the Li isotopic compositions of carbonatites and spatially associated mafic silicate rocks. The Li isotopic compositions of Archean (2.7 Ga) to Recent carbonatites (δ7Li = 4.1 ± 1.3 (n = 23, 1σ)) overlap the range typical for modern mantle-derived rocks, and do not change with time, despite ongoing crustal recycling. Thus, the average Li isotopic composition of recycled crustal components has not deviated greatly from the mantle value (~ + 4) and/or Li diffusion is sufficiently fast to attenuate significant heterogeneities over timescales of 108 years. Modeling of Li diffusion at mantle temperatures suggests that limited δ7Li variation in the mantle through time reflects the more effective homogenization of Li in the mantle compared to radiogenic isotope systems. The real (but limited) variations in δ7Li that exist in modern mantle-derived magmas as well as carbonatites studied here may reflect isotopic fractionation associated with shallow-level processes, such as crustal assimilation and diffusive isotopic fractionation in magmatic systems, with some of the scatter possibly related to low-temperature alteration. |
| |
Keywords: | |
本文献已被 ScienceDirect 等数据库收录! |
|