• Ramirez, Carlos
• Mangasini, Frederick
• Barry, Peter H.
• Fischer, Tobias P.
• Moor, J. Maarten De
• King, Penelope
• Botcharnikov, Roman E.
• Hilton, David R.
• Hervig, Richard L.

Abstract

<p>This study presents volatile, trace, and major element compositions of silicate glasses (nepheline-hosted melt inclusions and matrix glass) from the 2007-2008 explosive eruption at Oldoinyo Lengai volcano, Tanzania. The bulk compositions of the heterogeneous ash erupted in 2007-2008 are consistent with physical mixing between juvenile nephelinite magma and natrocarbonatite emplaced during the preceding ~25 years of effusive carbonatite eruption. The melt inclusions and matrix glasses span a wide range of silica-undersaturated compositions, from ~46wt% SiO₂ and (Na+K)/Al~3 in the least evolved melt inclusions to 38wt% SiO₂ and (Na+K)/Al up to 12 in the matrix glass. The depletion in SiO₂ between melt inclusions and matrix glass is accompanied by strong enrichment in all of the incompatible trace elements measured (Ba, Nb, La, Ce, Sr, Zr, Y), which is consistent with fractional crystallization of a bulk mineral assemblage with SiO₂ higher than that of the melt inclusions but inconsistent with silicate melt evolution by assimilation of carbonatite. The melt inclusions are volatile-rich with 2.7wt% to 8.7wt% CO₂ and 0.7wt% to 10.1wt% H₂O, indicating that Oldoinyo Lengai is a hydrous system. This is contrary to the long-held assumption that Oldoinyo Lengai is relatively anhydrous, which is based on the observation that natrocarbonatite lavas are water-poor. We argue that natrocarbonatites are derived from hydrous carbonate liquid that degas H₂O at low pressure. The silicate glass data show that H₂O concentration is negatively correlated with incompatible element enrichment, which we attribute to crystallization of the melt in response to decompression degassing of H₂O. The eruptive cycle at Oldoinyo Lengai reflects changes in bulk silicate magma viscosity due to extensive H₂O-driven crystallization and explosive eruptions occur when volatiles (i.e. H₂O&gt;CO₂ gas, and carbonate liquid) cannot separate from the crystal-rich nephelinite magma. Melt H₂O content decreases as a function of pressure; however CO₂ concentration in the melt inclusions is buffered by the presence of immiscible carbonate liquid. CO₂ content increases with melt evolution parameters (e.g. increasing (Na+K)/Al) due to enhanced solubility with alkali enrichment and SiO₂ depletion in the melt. The matrix glasses and evolved melt inclusions, on the other hand, experienced low pressure (&lt;50MPa) CO₂ degassing and were not buffered by a coexisting carbonate liquid. Whereas the melt inclusions are the most CO₂-rich yet identified, their CO₂/Nb ratios are without exception lower than that in MORB, indicating that a volatile-rich mantle source is not required for Oldoinyo Lengai.</p>

Topics

• impedance spectroscopy
• mineral
• inclusion
• melt
• glass
• glass
• viscosity
• crystallization
• degassing
• trace element