LITHOSPHERIC CONTROLS ON ALKALINE VOLCANISM: EVIDENCE FROM NYIRAGONGO AND NYAMULAGIRA VOLCANOES, VIRUNGA VOLCANIC PROVINCE, DR CONGO

The active volcanoes Nyiragongo and Nyamulagira, DR Congo have very different physical and geochemical characteristics, despite being situated 15 km apart. Nyiragongo’s foiditic lavas are some of the most silica-undersaturated on earth, whereas Nyamulagira erupts primarily basanites and tephrites. To better understanding the magmatic sources, processes, and eruptive characteristics of these two volcanoes, we have measured major and trace element concentrations, Sr, Nd, Hf, and Pb isotopes, and ²³⁸U-²³⁰Th-²²⁶Ra-²¹⁰Pb-²¹⁰Po and ²³²Th-²²⁸Ra-²²⁸Th nuclides. Nyiragongo lavas are clearly distinct from Nyamulagira lavas in terms of their Sr, Nd, Hf, and Pb isotope compositions, indicating long-lived and small-scale heterogeneity beneath these two volcanoes. Invariant ²⁰⁶Pb/²⁰⁴Pb, ²⁰⁸Pb/²⁰⁴Pb, and (²³⁰Th/²³²Th) ratios for Nyamulagira lavas compared to Nyiragongo imply different controls on the long-term U/Pb and Th/Pb within the sources of the two volcanoes. The marked relationship between eruption age and Sr, Nd, and Hf isotope compositions at Nyamulagira emphasizes the short time scale on which these lavas change geochemically. Nyiragongo lavas are affected by carbonate metasomatism to a greater degree than Nyamulagira lavas. Carbonate metasomatism in Nyiragongo lavas is evidenced by high (²³⁰Th/²³²Th) and variable (²³⁰Th/²³⁸U). The source of both volcanoes likely contains residual garnet evidenced by the relatively high Dy_N/Yb_N ratios and ²³⁰Th excesses. Additionally, there appears to be spatial mixing between these two volcanoes, as samples collected from parasitic cones in between the two have intermediate isotopic compositions. We posit that each volcano’s composition is being controlled by lithospheric metasomes of carbonate for Nyiragongo and phlogopite for Nyamulagira, and that their respective plumbing systems overlap such that there is significant mixing between the two volcanoes.