Tracing the sources of CO2-rich magmas at Nyiragongo and Nyamulagira using olivine-hosted melt inclusions
Abstract
Nyiragongo, earth's second-highest emitter of volcanic CO2 (1), a significant emitter of SO2 and HF (2), and Nyamulagira, another substantial emitter of CO2, SO2, and HF (3) play a key role in the global cycling of volatile elements. To better understand the origin of such anomalously high volatile fluxes and their implications for parental magmas and their mantle sources beneath both volcanoes, we are investigating olivine-hosted melt inclusions from relatively high-Mg samples. While previous geochemical studies have focused primarily on highly fluid lava flows, tephra from flank scoria cones provide a vital perspective of primitive magma compositions, which may retain signatures of mantle metasomatism and other melting processes. Over 100 young scoria cones and tuff rings occur on the flanks of Nyiragongo and distal flanks of Nyamulagira (many within the rapidly growing city of Goma, Gisenyi and along the populous shores of Lake Kivu), providing rapidly quenched tephra that is ideal for melt inclusion studies. Here, we present Raman spectroscopic measurements of the CO2 density in vapor bubbles within melt inclusions from olivine-melilite (Nyiragongo) and basanite (Nyamulagira) tephra samples. The density of CO2 was calculated using the difference between two Raman peaks (referred to as the Fermi diad), yielding CO2 densities ranging from 0.12-0.58 g/cm3. Bubbles with particularly high (>0.3 g/gm3) CO2 densities suggest coexisting CO2 liquid and vapor phases within the bubble. Additional peaks at 1089 cm-1 indicate the presence of nano-scale carbonate phases along the bubble wall, which must be considered in total CO2 budget calculations. Because the bubbles in the MI likely formed during post-entrapment cooling and crystallization, the measured CO2 densities suggest significant amounts of CO2 were dissolved in the melts at the time of trapping. Future measurements of volatiles, major, and trace elements in the inclusion glasses will provide a more complete record of magmatic volatiles necessary for understanding degassing at these volcanoes and the sources of high CO2 in their mantle sources.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2020
- Bibcode:
- 2020AGUFMV026...02C
- Keywords:
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- 8414 Eruption mechanisms and flow emplacement;
- VOLCANOLOGY;
- 8419 Volcano monitoring;
- VOLCANOLOGY;
- 8434 Magma migration and fragmentation;
- VOLCANOLOGY;
- 8439 Physics and chemistry of magma bodies;
- VOLCANOLOGY