|Event:||Earth and Planetary Science Seminar|
|Date:||13:00, Tue 08 May, 2012|
|Speaker:||Dr Hannes Mattsson|
|Title:||Magma ascent, fragmentation and depositional characteristics of CO2-rich silicate magmas: Examples from the Gregory Rift (northern Tanzania)|
|Venue:||Main Lecture Theatre, Grant Institute|
|Abstract:||Many eruptions in the Gregory Rift of northern Tanzania involve extrusion of silicate-undersaturated, CO2-rich magmas. Nearly all of these exhibit characteristics of being moderately- to highly-explosive, especially when considering the low viscosities of the magmas involved (i.e., melilititic to nephelinitic in composition). Here, two different eruptions are considered in detail: (i) the scoria-cone forming eruption of Pello Hill, and (ii) the most recent explosive episode of the much larger Oldoinyo Lengai volcano in 2007-2008. |
Both of these eruptions show striking similarities in pyroclast textures as well as depositional characteristics, indicating similar magma ascent/fragmentation histories. Modeling of magma ascent rate, magma supply rate and eruption duration for the monogenetic melilititic cones in northern Tanzania indicate very fast ascent rates (and short-lived eruptions) driven by exsolution of CO2 as a result of decompression and cooling. For the Oldoinyo Lengai volcano, experimental data suggests that magma mixing between nephelinitic and natrocarbonatitic magmas produced a hybrid silicate melt, which lowered the overall solubility of CO2 in the system. The forced exsolution of CO2 at shallow levels (from the hybrid melt) may explain the vigorous activity, and high eruption columns, observed at Oldoinyo Lengai during the 2007-2008 eruption(s).
These types of CO2-rich silicate eruptions also show strong similarities to vent-facies kimberlite deposits as described previously in the literature. Therefore, more detailed studies of the well-preserved melilititic cones may offer important insights into the shallow-level emplacement processes of kimberlitic magmas.
This article was published on Aug 16, 2010