Project

Discipline

carbon dioxide; lithofacies analyses; Fragmentation; explosive volcanic eruptions; nephelinite; carbonatite; Oldoinyo Lengai; magma mixing; pyroclastic deposits

Lead
Lay summary
Volcanic eruptions involving mixing of two magmas of widely different characteristics have been occurring on numerous occasions at the Oldoinyo Lengai volcano in northern Tanzania, including a recent eruptive episode in 2007-2008. Eruptions of these mixed magmas have resulted in surprisingly vigorous explosions that predominantly produce fine ash. One goal of this project is to investigate the processes involved in the fragmentation of the magmas, i.e., is the fragmentation process driven by exsolution of volatiles from the magmas themselves, due decreased solubility of CO2 in the hybrid magma, or by explosive interactions with an external water source? This project is based on extensive fieldwork, sampling and mapping of volcanic deposits to detect spatial and temporal variations in magma fragmentation and deposition occurring during the eruptions. The collected material will be subject to laboratory-based analyses in order to characterize their chemistry and mineralogy. The samples will be subject to mechanical sieving to retrieve grain-size distributions, and a variety of imaging techniques will be applied to quantify the shapes of the resulting pyroclasts as well as their vesicle content. In combination, these techniques will allow to constrain the physical mixing, eruption, and depositional processes involving two magmas of distinctively different chemistry and subsequently also different physical properties. Special emphasis is placed on the deposits generated in the most recent eruption of Oldoinyo Lengai, as this volcano is currently active and understanding the past behavior is an integral part of building a realistic hazard assessment for future eruptions.

Direct link to Lay Summary

Last update: 21.02.2013

Number	Title	Start	Funding scheme

In order to understand carbonatite volcanism, and the genesis of carbonatitic magmas, a wide spectrum of complementary studies needs to be conducted. By combining information retrieved from mineralogy, petrology, isotope geochemistry, with extensive field-based studies a more genuine understanding of how carbonatitic volcanoes work, and how the carbonatitic magmas are linked to the silicate rocks, will emerge. This continuation of SNF project 200021_129985 focuses on pyroclastic deposits generated in recent explosive mixed carbonatite-silicate eruptions of Oldoinyo Lengai volcano in northern Tanzania. This specific volcano is chosen for the present project because there exist abundant petrological, mineralogical and isotopic data in the published literature. However, detailed and systematic studies of its pyroclastic deposits and their field relations are lacking. Therefore, this project will close this informational gap by providing detailed field data that can be easily integrated with the existing studies to better understand carbonatite volcanism and to add constraints on the petrogenesis of the carbonatitic magmas. The project will give insights into the mixing, eruption, and depositional process involving two magmas of widely different chemical compositions and subsequently also very different physical properties. The project will also provide data on how natrocarbonatites and silicate magmas occur within the natural deposits at Oldoinyo Lengai. The pyroclastic deposits generated during the recent mixed eruption of Oldoinyo Lengai in 2007-08 is studied in detail, and the eruption dynamics will be established based on the measured spatial and temporal variations within these deposits. This proposal seeks funding for a 2-year continuation for a currently ongoing PhD-project to complete the field-based stratigraphic logging of the pyroclastic deposits, and the laboratory-based granulometric, petrographic, textural and geochemical analyzes. By combining the results from the field with the laboratory analyzes interpretations of eruption dynamics and fragmentation modes, as well as different emplacement mechanisms can be made. Understanding the past eruptive behavior is an integral part in building a realistic hazard assessment for any volcano.