TY - JOUR
T1 - Volatile-rich silicate melts from Oldoinyo Lengai volcano (Tanzania)
T2 - Implications for carbonatite genesis and eruptive behavior
AU - de Moor, J. Maarten
AU - Fischer, Tobias P.
AU - King, Penelope L.
AU - Botcharnikov, Roman E.
AU - Hervig, Richard
AU - Hilton, David R.
AU - Barry, Peter H.
AU - Mangasini, Frederick
AU - Ramirez, Carlos
N1 - Funding Information:
Funding was provided by NSF Grant EAR0827352 and the National Geographic Society (TPF) , and a NASA Earth and Space Science student fellowship to JMdM . We acknowledge NSF support for the Arizona State University National SIMS Facility ( EAR-0948878 ). We express deep gratitude to people of Engare Sero village, who were hospitable, allowed access to the volcano, and provided logistical support. We would like to acknowledge COSTECH for the 2009 research permit and support of scientists from the University of Dar Es Salaam. We thank Mike Spilde, Lynda Williams, Adrian Brearley, Stephan Schuth, and Nicole Thomas for analytical support. We would like to express our gratitude to Bruce Kjarsgaard, Tim Elliot (editor) and an anonymous reviewer whose comments and suggestions greatly improved this paper. We also acknowledge two anonymous reviewers who reviewed a previous version of this paper. TF thanks Bernard Marty and Pete Burnard for introducing him to Lengai in 2005.
PY - 2013/1/1
Y1 - 2013/1/1
N2 - 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>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 (<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.
AB - 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>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 (<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.
KW - Explosive eruption
KW - Liquid immiscibility
KW - Mantle source
KW - Oldoinyo Lengai
KW - Volatiles
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U2 - 10.1016/j.epsl.2012.11.006
DO - 10.1016/j.epsl.2012.11.006
M3 - Article
AN - SCOPUS:84872341387
SN - 0012-821X
VL - 361
SP - 379
EP - 390
JO - Earth and Planetary Science Letters
JF - Earth and Planetary Science Letters
ER -