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Abstract Radioisotopic dates for Late Pleistocene to Holocene silica-undersaturated volcanic rocks are often imprecise, limiting our ability to assess the frequency of eruptions in alkaline volcanic provinces, evaluate related volcanic risk and date associated archaeological sites. Here, we present a new approach to dating alkaline volcanic rocks employing 238U-230Th disequilibrium dating of Ca-rich garnet phenocrysts by laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS). We document analytical protocols and apply this technique to date magmatic garnets from the 472 AD Pollena eruption of Vesuvius (Southern Italy) and from the Engare Sero Footprint Tuff, a volcaniclastic unit attributed to Oldoinyo Lengai (Northern Tanzania). Garnet phenocrysts from the Pollena phonolite yield a well-defined U-Th isochron with a date of 2.28 ± 0.71 ka (1σ) that is indistinguishable from the historical date and suggests that garnet crystallized close to eruption with a pre-eruption residence time of less than 1.6 kyr. Garnets from the Engare Sero Footprint Tuff yield a U-Th isochron date of 4.91 ± 0.58 ka (1σ). This date is compatible with 14C and 40Ar/39Ar dates that bracket tuff deposition to between ∼5 and ∼19 ka but confidently constrains the age of the Engare Sero Footprint Tuff to the mid-Holocene. These examples demonstrate the potential of this new approach for dating Late Pleistocene to Holocene silica-undersaturated volcanic rocks at millennial-scale precision and for investigating magma chamber processes beneath active alkaline volcanoes. [ABSTRACT FROM AUTHOR]

Volcanic ash leachate studies have been conducted on various volcanoes on Earth, but few have been done on African volcanoes until now. Tephra emissions may affect the environment and the health of people living in this area, and therefore we conducted a first tephra (ash and lapilli sized) leachate study on the Oldoinyo Lengai volcano, situated in northern Tanzania. The recent explosive eruption in 2007-2008 provided us with fresh samples from the first three weeks of the eruption which were used for this study. In addition, we also used a natrocarbonatitic sample from the activity prior to the explosive eruption, as the major activity at Oldoinyo Lengai is natrocarbonatitic. To compare the leaching process affecting the natrocarbonatitic lavas and the tephras from Oldoinyo Lengai, the 2006 natrocarbonatitic lava flow was resampled 5 years after the emplacement and compared to the initial, unaltered composition. Special interest was given to the element fluorine (F), since it is potentially toxic to both humans and animals. A daily intake of fluoride (F − ) in drinking water of > 1.5 mg/l can lead to dental fluorosis, and higher concentrations lead to skeletal fluorosis. For this reason, a guideline value for fluoride in drinking water was set by the WHO (2011) to 1.5 mg/l. However, surface waters and groundwaters in the Gregory Rift have elevated fluoride levels of up to 9.12 mg/l, and as a consequence, an interim guideline value for Tanzania has been set at 8 mg/l. The total concentration of fluorine in the samples from the natrocarbonatitic lava flow is high (3.2 wt%), whereas we observed a significant decrease of the fluorine concentration (between 1.7 and 0.5 wt%) in the samples collected three days and three weeks after the onset of the explosive 2007–08 eruption. However, the total amount of water-extractable fluoride is lower in the natrocarbonatitic lavas (319 mg/l) than in the nephelinitic tephra (573–895 mg/l). This is due to the solubility of the different F-bearing minerals. In the natrocarbonatites, fluorine exists predominantly in fluorite (CaF 2 ), and in the early tephra as Na-Mg bearing salts such as neighborite (NaMgF 3 ) and sellaite (MgF 2 ). All these three minerals have very low solubility in water (16–130 mg/l). The later nephelinitic tephras contain surface coating of villiaumite (NaF), which is highly soluble (42,200 mg/l) in water and can thus release the fluoride more readily upon contact with water. Although there is still the need for further data and a more precise study on this topic in Tanzania, we can already draw a first conclusion that the intake of water during or directly following the deposition of the tephra is not advisable and should be avoided, whereas the release of fluoride from the lava flow has less influence on the river waters. [ABSTRACT FROM AUTHOR]

Abstract: 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 111m 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 (~40vol. %) 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 36.0m s–1, which is similar to that previously reported for kimberlitic magmas. Building on these ascent rates, and the volume of the pyroclastic construct (~5×106 m3 DRE), the eruption duration is estimated to less than 6hours (for any vent area larger than 28m2). Therefore, it may not have taken longer time than 9hours from the time the magma started to ascend from upper-mantle depth (90km) 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. [Copyright &y& Elsevier]

Abstract: The 2500 BP Hverfjall eruption in northern Iceland produced pyroclastic and effusive deposits of widely different characteristics along the length of the eruptive fissure. The southern half of the fissure was located in a shallow lake, whereas the northern part of the fissure extended onto dry land. This specific setting, with overlapping periods of activity at the different vents, resulted in various mingling features between fine-grained phreatomagmatic deposits and lava flows. Here we reconstruct the course of events during this eruption based on field observations and granulometric analyses of the resulting deposits. The eruption can be divided into three main phases depending on the main depositional characteristics and vent locations. The initial phase is dominated by phreatomagmatic fall deposits which are attributed to an overall high eruption rate in a shallow lacustrine setting. The second phases involved opening of two new vents on dry ground, and deposition of scoria and lava flows. The third and final phase of the eruption is associated with a lowering of the eruption rate in the southernmost vent, with a shift in the activity from continuous uprush and fall deposits to discrete explosions and emplacement of base surges. These surges display features consistent with drying up with increasing distance from the vent, suggesting that their dynamics changed during emplacement. Most wet surges were channelized within a preexisting graben structure close to the vent, but some more dilute (i.e., dry) surges were able to flow over this obstacle and continued to flow for more than 5km away from the vent and 100m uphill before stopping. [Copyright &y& Elsevier]

Abstract: Here I present textural data (i.e., vesicularity, vesicle size distributions (VSD), plagioclase crystallinity, crystal size distributions (CSD), combined with fractal analyses of particle outlines) from a natural succession of alternating fall and surge deposits in the emergent Capelas tuff cone (Azores). The textural variation in the Capelas succession is surprisingly small considering the wide variety of fragmentation processes, vent activity and emplacement mechanisms that are characteristic of emergent eruptions. The plagioclase crystal content varies between 24 and 33vol.%. CSD analyses of plagioclase show near-linear trends with a slight increase in time for the smallest crystal sizes (with surge deposits having more groundmass plagioclase when compared with fall deposits). This is consistent with crystallization induced by degassing and decompression at lower eruption rates. The vesicularities of the Capelas pyroclasts are more variable (18 to 59vol.%), with VSDs displaying kinked trends characteristic of coalescence. This is especially evident in the fall deposits, and consistent with being formed in continuous uprush (jetting) with an overall shallow fragmentation level within the conduit. Bubble coalescence can also be identified in the surge deposits, although to a much lesser extent. The amount of bubble coalescence is negatively correlated with the amount of groundmass crystallization (i.e., plagioclase) in the Capelas deposits. A relatively broad range of fractal dimensions (with average D box =1.744 and σ =0.032) for the outlines of pyroclastic fragments emplaced by fall or as surges indicate that there is little difference in the fragmentation process itself at Capelas. In addition to this, the fact that the fractal dimensions for both the fall and surge end-members completely overlap suggests that shape modification due to abrasion and chipping of grain edges was minor during emplacement of base surges. These results are consistent with emergent eruptions, building tuff cones, to be a relatively low-energy phreatomagmatic landform (e.g., at least when compared with more energetic phreatomagmatic eruptions producing tuff rings and maar volcanoes). [Copyright &y& Elsevier]

Abstract: The Capelas tuff cone is an emergent Surtseyan-type tuff cone that erupted in shallow seawater off the coast of São Miguel, Azores. In this paper, we present a detailed stratigraphic study which is used to infer depositional processes and modes of fragmentation for the Capelas tuff cone deposits. The growth of the tuff cone can be divided into three stages based on variations in depositional processes that are probably related to differences in water/magma (W/M) ratios. The first stage corresponds well to wet Surtseyan-type activity where wet fallout is the dominant depositional process, with only minor representation of pyroclastic surge deposits. The second stage of the eruption is suggested to be the result of alternating wet and slightly drier periods of Surtseyan activity, with an overall lower W/M-ratio compared to the first stage. The drier Surtseyan periods are characterized by the presence of minor grain-flow deposits and undulating pyroclastic surge deposits that occasionally display relatively dry structures such as strongly grain-segregated layers and brittle behavior when impacted by ballistic ejecta. The first deposits of the second stage show an intense activity of pyroclastic surges but fallout, commonly modified by surges, is still the dominant depositional process during the second stage. The third stage represents a final effusive period, with the build-up of a scoria cone and ponded lava flows inside the tuff cone crater. Phreatomagmatic fragmentation, as seen by studies of the fine ash fraction (<64 μm), is dominant in the Capelas tuff cone. However, particles with shapes and vesicularities characteristic of magmatic fragmentation are abundant in proximal deposits and present in all investigated beds (in various amounts). Emergent Surtseyan-type tuff cones are characterized by a domination of fallout deposits, both wet and dry, where dry periods are characterized by the deposition of relatively dry falling tephra transforming into grain-flow deposits. However, this study of the Capelas tuff cone shows that drier Surtseyan periods may also be represented by an increased amount of thin surge deposits that occasionally display dry features. [Copyright &y& Elsevier]