Isotopic Constraints on Magmatic Sources at Nyiragongo and Nyamulagira Volcanoes, Virunga Volcanic Province, DR Congo

Abstract:

The active volcanoes Nyiragongo and Nyamulagira in the DR Congo have very different physical and geochemical characteristics, despite being situated a mere 15 km apart. Nyiragongo’s foiditic lavas are some of the most silica-undersaturated on earth, whereas the highly effusive Nyamulagira erupts primarily basanites and tephrites. To determine the extent and scale of mantle heterogeneities and gain insight into the magmatic sources beneath this portion of the East African Rift, we have measured Hf and Pb isotope compositions for 43 samples from Nyiragongo and Nyamulagira. The Nd and Sr isotope data for the same sample dissolutions are forthcoming.

Nyiragongo lavas are clearly distinct from Nyamulagira lavas in terms of their Hf and Pb isotope compositions, suggesting that a long-lived and small-scale heterogeneous mantle source exists beneath these two volcanoes. Nyiragongo lavas have ε_Hf ranging from +1.8 to +5.5 with an average of +2.9 (n=29) and ²⁰⁶Pb/²⁰⁴Pb ranging from 19.4049 to 19.7252 with an average of 19.6329 (n=29). Nyamulagira lavas have ε_Hf ranging from -0.5 to +1.5 with an average of +0.5 (n=14) and ²⁰⁶Pb/²⁰⁴Pb ranging from 19.2518 to 19.2828 with an average of 19.2663 (n=13). Nyiragongo lavas erupted in 2002 or later have amongst the highest ²⁰⁶Pb/²⁰⁴Pb within this suite of samples. We note that Chakrabarti et al. (2009, Chem Geol 259) measured bulk silicate earth-like Nd and Sr isotope compositions for Nyiragongo lavas and proposed a primitive mantle/bulk-earth plume source for this volcano. Our new Hf isotope compositions for Nyiragongo, however, are higher than bulk silicate earth, suggesting a more depleted source for these highly alkaline lavas. We also note that the He isotope compositions of olivine and clinopyroxene from Nyiragongo lavas (R/Ra = 6.7-8.5; Pik et al., 2006, Chem Geol 226; Tedesco et al., 2010, J Geophys Res 115) are inconsistent with a long-term bulk silicate earth-like source.