Quantifying Crustal Assimilation in Historical to Recent (1329-2005) Lavas at Mt. Etna, Italy: Insights from Thermodynamic Modeling
Date of Degree Completion
Master of Science (MS)
Wendy A. Bohrson
Second Committee Member
Third Committee Member
The nearly continuous volcanic eruption record at Mt. Etna dating back approximately 700 years provides an excellent opportunity to investigate the geochemical evolution of a highly active volcano. Of particular interest is elucidating the cause of a selective enrichment in alkali elements (K, Rb, Cs) and 87Sr/86Sr. This alkali enrichment trend, which began in the 17th century and accelerated after 1971, was accompanied by an increase in the volume, frequency, and explosivity of eruptions. To explain this enrichment, two major arguments are invoked: (1) crustal contributions (e.g., assimilation of the sedimentary basement), and (2) changes in the mantle source, possibly due to increased interaction between the mantle source and subduction related fluids, and/or a mantle source that was melted to different degrees. This study quantitatively examines the role of crustal contributions to post-1971 Etnean magmas via the Magma Chamber Simulator, on the basis of published and unpublished whole rock major oxides, trace elements, and 87Sr/86Sr, and mineral compositional data. Over 200 models were run with varied pressures, initial fO2 buffers, parental magma compositions, and initial magma H2O contents, and wallrock compositions, masses, and initial temperatures. Best-fit results for whole rock trends and mineral compositions for pre-1971 lavas are satisfactorily reproduced by fractional crystallization of a parental magma that is a high-Mg (~17 wt.% MgO) basalt to picrite composition. Such a magma is documented in Etna’s ~4 ka pyroclastic fallout deposit. The observed post-1971 whole rock trends and mineral compositions are reproduced via stoping and assimilation of skarn and flysch, which compose the uppermost 10-15 km of sedimentary substrata beneath the volcanic pile. Specifically, K2O and Rb behave incompatibly in skarn/flysch wallrock melts, and elevated 87Sr/86Sr in the post-1971 samples is consistent with the addition of radiogenic Sr from these wallrock components. In the best-fit model, which yields the post-1971 K2O, Rb, and 87Sr/86Sr trends, 5% of wallrock was stoped and 12% of anatectic melt was assimilated; percentages are relative to the starting mass of the magma body. Based on these modeling outcomes, I propose that the post-1971 alkali enrichment signature is due to both crustal contamination and mantle heterogeneity; up to ~20% crustal input is coupled with mantle heterogeneity introduced by magma recharge and mixing. The influence crustal contamination has on post-1971 lavas is, in part, the result of frequent recharge of hot magma that thermally primed the shallow crust for melting. Furthermore, the significant liberation of CO2 from the wallrock via magma-carbonate interaction has the potential to increase the volatile budget and thus the explosivity at the volcano, which would coincide with the observed increase in the explosivity of Mt. Etna after 1971.
Takach, Marie, "Quantifying Crustal Assimilation in Historical to Recent (1329-2005) Lavas at Mt. Etna, Italy: Insights from Thermodynamic Modeling" (2018). All Master's Theses. 1006.