Depositional characteristics and volcanic landforms in the Lake Natron–Engaruka monogenetic field, northern Tanzania

Abstract

The Lake Natron–Engaruka monogenetic volcanic field (LNE-MVF) is situated in the East African Rift of northern Tanzania, where it comprises approximately 200 vents scattered over an area of 2500 km 2. Similar to most other monogenetic volcanic fields in the world, the landforms of the LNE-MVF are characterized by a wide array of morphologies, such as maar-diatreme volcanoes, tuff cones and tuff rings, scoria cones and spatter cones. However, in contrast to most other MVFs (which are basaltic in composition) the magmas erupted within the LNE-MVF are predominantly of olivine-melilititic to nephelinitic compositions. Here we show by field observations, granulometric analyses and morphological studies of particle shapes from a large selection of different landforms from the LNE-MVF that there are some crucial differences compared to the more common basaltic equivalents. These differences are reflected in both eruption dynamics and fragmentation mechanisms as well as the overall characteristics of the deposits. Landforms within the LNE-MVF that resemble those produced by phreatomagmatic eruptions in morphology, display strong evidence for dry fragmentation and also dry deposition. Therefore, to reconcile the observed depositional characteristics with eruptive processes within the LNE-MVF we propose a hypothetical eruption scenario. Partial melting of a carbonate-bearing mantle source produces small volume volatile-rich melilititic melts (rich in phlogopite and amphibole). These volatile-rich, and mantle xenolith-bearing, magmas ascend rapidly from the mantle to the surface without being subject to significant degassing. The volatile-rich nature of these melilitite magmas, in combination with a significant exsolution of CO 2 during decompression (ascent) can explain the dry characteristics of these apparent "phreatomagmatic" landforms.