Rapid magma ascent and short eruption durations in the Lake Natron-Engaruka monogenetic volcanic field (Tanzania): A case study of the olivine melilititic Pello Hill scoria cone

The Pello Hill scoria cone displays widely different characteristics dependent on which side of the cone is studied. This follows as a direct result of a strong depositional asymmetry (i.e., the crater rim varies between 14 and 111 m in height), preferentially depositing material to the northwestern side of the construct. This is interpreted to reflect sedimentation from a sustained eruption plume (with prevailing winds at the time of the eruption coinciding with the direction of maximum deposition). The scoria deposits on this side of the cone form relatively fine-grained and well-sorted deposits which are laterally continuous over distances of 10's of meters. To all other sides of the vent, deposits are characterized by coarse-grained lenticular deposits rich in mantle xenoliths, occasionally showing inverse grading (consistent with an origin as ballistic ejecta and slight reworking down-slope as grain-flows). The pyroclastic textures are dominated by a moderate vesicularity (similar to 40 vol. %) in combination with smooth, fluidal, outer surfaces. Two other features that stands out in comparison with normal scoria cones, these are defined by the absence of: (i) inward dipping layers into the crater area, and (ii) no agglutination/welding features can be found in any of the exposed outcrops. Calculated magma ascent rates yields values between 8.5 and 36.0 m s(-1), which is similar to that previously reported for kimberlitic magmas. Building on these ascent rates, and the volume of the pyroclastic construct (similar to 5 x 10(6) m(3) DRE), the eruption duration is estimated to less than 6 hours (for any vent area larger than 28 m(2)). Therefore, it may not have taken longer time than 9 hours from the time the magma started to ascend from upper-mantle depth (90 km) and the point at which the eruption halted. Overall, the olivine melilititic Pello Hill scoria cone displays many characteristics that can also be found in the vent-facies deposits of kimberlite eruptions. Therefore, further detailed studies of well-preserved, CO2-rich, olivine melilitite eruptions (such as the provided by the landforms within the LNE-MVF and Pello Hill) could provide a tool to understand the emplacement dynamics of the vent-facies of kimberlitic eruptions. (C) 2012 Elsevier B.V. All rights reserved.