USING TRACE ELEMENT AND NOBLE GAS GEOCHEMISTRY TO EVALUATE VOLCANIC HAZARDS AT THE NYIRAGONGO AND NYAMURAGIRA VOLCANOES, D.R. OF CONGO

Goma, a city located in the Virunga Volcanic Province along the border between the Dem. Rep. of Congo and Rwanda, has experienced explosive population growth following the civil war started in 1996 and persisting today related to politcal unrest in the area. The population growth occurs despite the presence of two very active volcanoes (Mt. Nyiragongo and Mt. Nyamuragira) within ~18 km of the city center. In January 2002 Mt. Nyiragongo erupted pouring lava into the city of Goma killing ~180 people and leaving 130000 homeless. Because of the poor knowledge of this volcano, its silica-undersaturated lava composition and fissural flank eruptive style traditional volcanic hazard prediction techniques are difficult to apply.

Presently both volcanoes are extremely active emitting lava and gas with unique and paradoxical composition despite their close geospatial proximity (~12km). Mt. Nyiragongo has an actively rising lava lake characterized by on of the most alkaline and foiditic compositions on earth. Radiogenic isotope, trace element, and noble gas compositions are paradoxical with bulk-earth radiogenic isotopic, carbonatitic HFSE, ‘upper mantle’ noble gas (e.g. 3He/4He ~8Ra) composition, and highly enriched ‘crustal’ LREE patterns. Conversely, Nyamuragira, which last erupted on Jan 2010, is characterized by trachybasaltic/tephritic major element composition and crustal-trending (EM II) radiogenic isotopic (Sr, Nd, Pb) and elemental composition (REEs and HFSE’s). We present trace element data for lava samples taken from Nyamuragira (2006 and 2010 eruptions) and Nyiragongo (2002 eruption and 2004 and 2010 lava lake samples), focusing on REEs, HFSEs, and actinides (Th/U) in the context of volatile emissions (δ13C-(CO2), CO2, noble gases (e.g. 3He/4He, 40Ar/36Ar) to explore mantle processes leading to the unique lava compositions and trace element compositional changes throughout this period. This geochemical approach, then combined with geophysical measurements, will be used to generate a volcanic hazard model to make predictions about the state of Nyiragongo feeding system, and its possible future activity, trying to understand the geologic behaviour of the region on both a short and a long-term timescale.