TY - JOUR
T1 - Tracking the lithium isotopic evolution of the mantle using carbonatites
AU - Halama, Ralf
AU - McDonough, William F.
AU - Rudnick, Roberta L.
AU - Bell, Keith
N1 - Funding Information:
Thanks to A. Demény for providing samples from Fuerteventura. A.J. Kaufmann and N. Collins helped with C–O stable isotope analyses and R.D. Ash with ICP-MS measurements. J. Klaudius is thanked for field guidance at the Kaiserstuhl. G. Markl allowed access to his Greenland sample collection. The help of H. Taubald, G. Bartholomä and G. Stoschek in carrying out oxygen whole-rock analyses of silicate rocks and XRF analyses is appreciated. P. Tomascak generously granted access to his unpublished manuscript on Li isotopes in mid-ocean ridge basalts. S. Hier-Majumder is thanked for helpful discussions. Very constructive reviews by L.-H. Chan and A. Simonetti and the careful editing by R. Carlson helped to significantly improve the manuscript. This research was supported by the Alexander von Humboldt foundation (Feodor-Lynen fellowship to R.H.) and the NSF (EAR 0208012 and EAR 0609689).
PY - 2008/1/30
Y1 - 2008/1/30
N2 - 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.
AB - 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.
KW - carbonatites
KW - lithium isotopes
KW - mantle geochemistry
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U2 - 10.1016/j.epsl.2007.11.007
DO - 10.1016/j.epsl.2007.11.007
M3 - Article
AN - SCOPUS:38049092121
SN - 0012-821X
VL - 265
SP - 726
EP - 742
JO - Earth and Planetary Science Letters
JF - Earth and Planetary Science Letters
IS - 3-4
ER -