Synchronisation of sedimentary records using tephra : a postglacial tephrochronological model for the Chilean Lake District

Well-characterised tephra horizons deposited in various sedimentary environments provide a means of synchronising sedimentary archives. The use of tephra as a chronological tool is however still widely underutilised in southern Chile and Argentina. In this study we develop a postglacial tephrochronological model for the Chilean Lake District (ca. 38 to 42 degrees S) by integrating terrestrial and lacustrine records. Tephra deposits preserved in lake sediments record discrete events even if they do not correspond to primary fallout. By combining terrestrial with lacustrine records we obtain the most complete tephrostratigraphic record for the area to date. We present glass geochemical and chronological data for key marker horizons that may be used to synchronise sedimentary archives used for palaeoenvironmental, palaeoclimatological and palaeoseismological purposes. Most volcanoes in the studied segment of the Southern Volcanic Zone, between Llaima and Calbuco, have produced at least one regional marker deposit resulting from a large explosive eruption (magnitude >= 4), some of which now have a significantly improved age estimate (e.g., the 10.5 ka Llaima Pumice eruption from Llaima volcano). Others, including several units from Puyehue-Cordon Caulle, are newly described here. We also find tephra related to the Cha1 eruption from Chaiten volcano in lake sediments up to 400 km north from source. Several clear marker horizons are now identified that should help refine age model reconstructions for various sedimentary archives. Our chronological model suggests three distinct phases of eruptive activity impacting the area, with an early-to-mid-Holocene period of relative quiescence. Extending our tephrochronological framework further south into Patagonia will allow a more detailed evaluation of the controls on the occurrence and magnitude of explosive eruptions throughout the postglacial

Geochemical processes controlling the groundwater chemistry and fluoride contamination in the aquifer systems on the eastern, western and northern flanks of Mount Meru, Tanzania

In the Arusha volcanic region in northern Tanzania, within the eastern branch of the East African Rift, water shortage is common and much of the surface water carries unacceptable levels of dissolved fluoride; hence groundwater is the main source of drinking water. Unfortunately, the quality of groundwater in this region is also very poor due to a high fluoride (F⁻) content because of natural contamination from the surrounding geological environment. Consumption of this water leads to dental and skeletal fluorosis among the local population. Existing borehole completion reports and field stratigraphic description of sub-surface sediments during the digging of new hand dug wells were used to characterise the geology of aquifers. On the western flank, the main aquifer is a fractured and weathered basalt while on the south-western and north-eastern flanks the aquifers are composed of volcanic deposits (ash, tuffs, pumice, lapilli) and fractured and weathered basalt. A total of 158 groundwater samples from 113 water points (61 hand-dug wells and 52 springs) were collected for chemical analysis. Laboratory results show Sodium (Na+) and bicarbonate (HCO3⁻) are the dominant ions hence the main water type in this area is Sodium bicarbonate (NaHCO3). Preliminary results show strong contrasts in pH, EC, TDS and F⁻ concentration across the study area, with some spatial patterns for water samples from springs (on the north-eastern flank) where pH, EC, TDS and F⁻ concentration increase with a decrease in elevation. In all 158 groundwater samples, 91% (143 samples) have F⁻ concentration higher than the WHO limit (1.5 mg/l) recommended for drinking water. The 9% (15 samples) with F⁻ concentration lower than the WHO limit are water from the springs found in the recharge area at higher elevation on the eastern flanks of Mount Meru inside the Arusha National Park. The chemistry of the groundwater shows that weathering and dissolution of silicate minerals (especially chemical weathering of Na-K-rich volcanic rocks), dissolution of fluoride-rich minerals and calcite precipitation play great roles on the chemical evolution of groundwater in the study area while long residence time and nature of the geological formations (including degree of weathering) play great roles in the groundwater fluoride contamination in the area

Compositional variability in mafic arc magmas over short spatial and temporal scales: evidence for the signature of mantle reactive melt channels

Understanding arc magma genesis is critical to deciphering the construction of continental crust, understanding the relationship between plutonic and volcanic rocks, and for assessing volcanic hazards. Arc magma genesis is complex. Interpreting the underlying causes of major and trace element diversity in erupted magmas is challenging and often non-unique. To navigate this complexity mafic magma diversity is investigated using sample suites that span short temporal and spatial scales. These constraints allow us to evaluate models of arc magma genesis and their geochemical implications based on physical arguments and recent model results. Young volcanic deposits (≲18 kyr) are analysed from the Southern Volcanic Zone (SVZ), Chile, in particular suites of scoria cones on the flanks of arc stratovolcanoes that have erupted relatively primitive magmas of diverse compositions. Our study is centred on the high-resolution post-glacial tephrochronological record for Mocho-Choshuenco volcano where tight age constraints and a high density of scoria cones provide a spatially well-resolved mafic magma dataset. Two compositional trends emerge from the data. Firstly, magmas from cones on the flanks of the main edifice become more mafic with distance from the central vent. This is attributed to fractional crystallisation processes within the crust, with distal cones sampling less differentiated magmas. Secondly, there is a set of cones with distinct major and trace element compositions that are more primitive but enriched in incompatible elements relative to the central system and other ‘normal SVZ’ magmas. This distinct signature – termed the ‘Kangechi’ signature – is observed at three further clusters of cones within the SVZ. This is attributed to greater preservation of the enriched melt signature arising from reactive melt transport within the mantle wedge. Our model has important implications for arc magma genesis in general, and in particular for the spatial and temporal scales over which compositional variations are preserved in erupted magmas

Mixing and crystal scavenging in the Main Ethiopian Rift revealed by trace element systematics in feldspars and glasses

For many magmatic systems, crystal compositions preserve a complex and protracted history which may be largely decoupled from their carrier melts. The crystal cargo may hold clues to the physical distribution of melt and crystals in a magma reservoir and how magmas are assembled prior to eruptions. Here we present a geochemical study of a suite of samples from three peralkaline volcanoes in the Main Ethiopian Rift. Whilst whole-rock data shows strong fractional crystallisation signatures, the trace element systematics of feldspars, and their relationship to their host glasses, reveals complexity. Alkali feldspars, particularly those erupted during caldera forming episodes, have variable Ba concentrations, extending to high values that are not in equilibrium with the carrier liquids. Some of the feldspars are antecrysts, which we suggest are scavenged from a crystal-rich mush. The antecrysts crystallised from a Ba-enriched (more primitive) melt, before later entrainment into a Ba-depleted residual liquid. Crystal-melt segregation can occur on fast timescales in these magma reservoirs, owing to the low viscosity nature of peralkaline liquids. The separation of enough residual melt to feed a crystal-poor post caldera rhyolitic eruption may take as little as months to tens of years (much shorter than typical repose periods of 300-400 years). Our observations are consistent with these magmatic systems spending significant portions of their life cycle dominated by crystalline mushes containing ephemeral, small (< 1 km3) segregations of melt. This interpretation helps to reconcile observations of high crustal electrical resistivity beneath Aluto, despite seismicity and ground deformation consistent with a magma body.This project is funded by the Natural Environment Research Council grant NE/L013932/1 (RiftVolc)

Structural controls on magma pathways in Bora-Baricha-Tullu Moye (BBTM) volcanic system, Main Ethiopian Rift

The Bora-Baricha-Tullu Moye (BBTM) volcanic complex is located at a transitional zone in the Main Ethiopian Rift where tectonic and volcanic features show complex interplays. We mapped and characterised volcanic and tectonic features using high-resolution digital elevation models and performed morphometric and vent spatial distribution analyses. Structural analysis reveals NNE–SSW, NE–SW, and NW–SE trending faults in the region. The dominant post-caldera volcanic landforms are lava domes, pumice cones, scoria cones, maars, obsidian coulees and lava flows, which have distinct morphological characteristics. Vent elongation and alignment highlight close association between these landforms and the caldera(s) as well as with tectonic structures, suggesting these structures acted as the main magma pathways during the BBTM recent eruptions. We estimate that during the entire BBTM post-caldera phase a total bulk volume of 10.9 km3 of material was erupted. This would represent a time-averaged magma flux of 0.05 km3 ky-1 in the BBTM

Contrasting styles of post-caldera volcanism along the Main Ethiopian Rift : implications for contemporary volcanic hazards

This work was funded by the Natural Environment Research Council grant NE/L013932/1 (RiftVolc) and a Boise Fund grant from the Department of Zoology, University of Oxford.The Main Ethiopian Rift (MER, ~7–9°N) is the type example of a magma-assisted continental rift. The rift axis is populated with regularly spaced silicic caldera complexes and central stratovolcanoes, interspersed with large fields of small mafic scoria cones. The recent (latest Pleistocene to Holocene) history of volcanism in the MER is poorly known, and no eruptions have occurred in the living memory of the local population. Assessment of contemporary volcanic hazards and associated risk is primarily based on the study of the most recent eruptive products, typically those emplaced within the last 10–20 ky. We integrate new and published field observations and geochemical data on tephra deposits from the main Late Quaternary volcanic centres in the central MER to assess contemporary volcanic hazards. Most central volcanoes in the MER host large mid-Pleistocene calderas, with typical diameters of 5–15 km, and associated ignimbrites of trachyte and peralkaline rhyolite composition. In contrast, post-caldera activity at most centres comprises eruptions of peralkaline rhyolitic magmas as obsidian flows, domes and pumice cones. The frequency and magnitude of events varies between individual volcanoes. Some volcanoes have predominantly erupted obsidian lava flows in their most recent post-caldera stage (Fentale), whereas other have had up to 3 moderate-scale (VEI 3–4) explosive eruptions per millennium (Aluto). At some volcanoes we find evidence for multiple large explosive eruptions (Corbetti, Bora-Baricha, Boset-Bericha) which have deposited several centimeters to meters of pumice and ash in currently densely populated regions. This new overview has important implications when assessing the present-day volcanic hazard in this rapidly developing region.PostprintPeer reviewe

Mixing and crystal scavenging in the Main Ethiopian Rift revealed by trace element systematics in feldspars and glasses

For many magmatic systems, crystal compositions preserve a complex and protracted history which may be largely decoupled from their carrier melts. The crystal cargo may hold clues to the physical distribution of melt and crystals in a magma reservoir and how magmas are assembled prior to eruptions. Here we present a geochemical study of a suite of samples from three peralkaline volcanoes in the Main Ethiopian Rift. Whilst whole‐rock data shows strong fractional crystallisation signatures, the trace element systematics of feldspars, and their relationship to their host glasses, reveals complexity. Alkali feldspars, particularly those erupted during caldera‐forming episodes, have variable Ba concentrations, extending to high values that are not in equilibrium with the carrier liquids. Some of the feldspars are antecrysts, which we suggest are scavenged from a crystal‐rich mush. The antecrysts crystallised from a Ba‐enriched (more primitive) melt, before later entrainment into a Ba‐depleted residual liquid. Crystal‐melt segregation can occur on fast timescales in these magma reservoirs, owing to the low viscosity nature of peralkaline liquids. The separation of enough residual melt to feed a crystal‐poor post‐caldera rhyolitic eruption may take as little as months to tens of years (much shorter than typical repose periods of 300‐400 years). Our observations are consistent with these magmatic systems spending significant portions of their life cycle dominated by crystalline mushes containing ephemeral, small (< 1 km3) segregations of melt. This interpretation helps to reconcile observations of high crustal electrical resistivity beneath Aluto, despite seismicity and ground deformation consistent with a magma body