δ11B COMPOSITIONS OF CARBONATITES FROM WORLDWIDE SOURCES: POSSIBLE IMPLICATIONS FOR MANTLE SOURCES

The origin of carbonatite melt and associated alkaline silicate counterparts has been linked to metasomatized upper mantle sources and related to mantle plume involvement. However, the exact origin of the carbon within mantle-derived carbonatite, whether derived from lithosphere or asthenosphere, is still under debate. In order to investigate this, carbonatite samples from worldwide locations of varying ages have been analyzed for their major and trace element contents, stable and radiogenic isotope (Sr, Nd, Pb, C and O) and for the first time, boron (B) isotope compositions. It is well documented that the global boron geochemical cycle is closely tied to the recycling of geological material via subduction processes. Therefore, we hypothesize that boron isotope compositions will aid in testing for a recycled component present in carbonatite magmas, and provide a unique way of assessing their mantle sources. As part of a preliminary investigation, 8 carbonatite samples from the Oka Complex (Québec, Canada) were chosen because of their unique Pb isotope compositions; the latter exhibit well-defined linear trends on Pb-Pb isotope diagrams indicative of mixing between at least three distinct mantle components (HIMU, EMI, and an ancient, depleted mantle). Boron abundances for 8 carbonate separates (one from each sample) yield values between 0.1 and 0.5 ppm, which are consistent with other mantle-derived rocks, such as MORBS and OIBs. δ¹¹B values for the samples from Oka indicate a large variation between ~-25 and ~0‰. The B isotopic data define a bimodal distribution when compared to their corresponding δ¹³C values, some of which are consistent with the involvement of recycled crustal material (i.e., heavier δ¹¹B values coupled with isotopically lighter δ¹³C). The δ¹¹B values also indicate bimodal trends when compared to their corresponding Pb isotopic data, and may be attributed to mixing between EM1 and HIMU components, or possibly a late-stage ‘autometasomatic’ event that affected the B isotope compositions.