Seismicity and crustal structure of the southern main Ethiopian rift: new evidence from Lake Abaya

The Main Ethiopian Rift (MER) has developed during the 18 Ma-Recent separation of the Nubian and Somalian plates. Extension in its central and northern sectors is associated with seismic activity and active magma intrusion, primarily within the rift, where shallow (urn:x-wiley:15252027:media:ggge22586:ggge22586-math-00015 km) seismicity along magmatic centers is commonly caused by fluid flow through open fractures in hydrothermal systems. However, the extent to which similar magmatic rifting persists into the southern MER is unknown. Using data from a temporary network of five seismograph stations, we analyze patterns of seismicity and crustal structure in the Abaya region of the southern MER. Magnitudes range from 0.9 to 4.0; earthquake depths are 0–30 km. urn:x-wiley:15252027:media:ggge22586:ggge22586-math-0002 ratios of urn:x-wiley:15252027:media:ggge22586:ggge22586-math-00031.69, estimated from Wadati diagram analysis, corroborate bulk-crustal urn:x-wiley:15252027:media:ggge22586:ggge22586-math-0004 ratios determined via teleseismic P-to-S receiver function H-urn:x-wiley:15252027:media:ggge22586:ggge22586-math-0005 stacking and reveal a relative lack of mafic intrusion compared to the MER rift sectors to the north. There is a clear association of seismicity with the western border fault system of the MER everywhere in our study area, but earthquake depths are shallow near Duguna volcano, implying a shallowed geothermal gradient associated with rift valley silicic magmatism. This part of the MER is thus interpreted best as a young magmatic system that locally impacts the geothermal gradient but that has not yet significantly modified continental crustal composition via rift-axial magmatic rifting

Rapid solubility and mineral storage of CO2 in basalt

The long-term security of geologic carbon storage is critical to its success and public acceptance. Much of the security risk associated with geological carbon storage stems from its buoyancy. Gaseous and supercritical CO2 are less dense than formation waters, providing a driving force for it to escape back to the surface. This buoyancy can be eliminated by the dissolution of CO2 into water prior to, or during its injection into the subsurface. The dissolution makes it possible to inject into fractured rocks and further enhance mineral storage of CO2 especially if injected into silicate rocks rich in divalent metal cations such as basalts and ultra-mafic rocks. We have demonstrated the dissolution of CO2 into water during its injection into basalt leading to its geologic solubility storage in less than five minutes and potential geologic mineral storage within few years after injection [1–3]. The storage potential of CO2 within basaltic rocks is enormous. All the carbon released from burning of all fossil fuel on Earth, 5000 GtC, can theoretically be stored in basaltic rocks [4]

An experimental study of crystalline basalt dissolution from 2 ≤ pH ≤ 11 and temperatures from 5 to 75 ºC

Steady-state element release rates from crystalline basalt dissolution at far-from-equilibrium were measured at pH from 2 to 11 and temperatures from 5 to 75 °C in mixed-flow reactors. Steady-state Si and Ca release rates exhibit a U-shaped variation with pH where rates decrease with increasing pH at acid condition but increase with increasing pH at alkaline conditions. Silicon release rates from crystalline basalt are comparable to Si release rates from basaltic glass of the same chemical composition at low pH and temperatures ≥ 25 °C but slower at alkaline pH and temperatures ≥ 50 °C. In contrast, Mg and Fe release rates decrease continuously with increasing pH at all temperatures. This behaviour is interpreted to stem from the contrasting dissolution behaviours of the three major minerals comprising the basalt: plagioclase, pyroxene, and olivine. Calcium is primarily present in plagioclase, which exhibits a U-shaped dissolution rate dependence on pH. In contrast, Mg and Fe are contained in pyroxene and olivine, minerals whose dissolution rates decrease monotonically with pH. As a result, crystalline basalt preferentially releases Mg and Fe relative to Ca at acidic conditions. The injection of acidic CO2-charged fluids into crystalline basaltic terrain may, therefore, favour the formation of Mg and Fe carbonates rather than calcite. Element release rates estimated from the sum of the volume fraction normalized dissolution rates of plagioclase, pyroxene, and olivine are within one order of magnitude of those measured in this study

Experimental determination of plagioclase dissolution rates as a function of its composition and pH at 22°C

The steady-state, far-from-equilibrium dissolution rates of nine distinct plagioclases ranging in composition from An2 to An89 were measured in mixed flow reactors at 22 ± 2 °C and pH from 2 to 11. The dissolution rates of all plagioclases based on silica release show a common U-shaped behaviour as a function of pH, where rates decrease with increasing pH at acid condition but rise with increasing pH at alkaline conditions. Consistent with literature findings, constant pH plagioclase dissolution rates increase with increasing anorthite content at acidic conditions; measured anorthite dissolution rates are ∼2.5 orders of magnitude faster than those of albite at pH ∼2. Perhaps more significantly, rates are independent of plagioclase composition at alkaline conditions. Interpretation and data fitting suggests that plagioclase dissolution rates are consistent with their control by the detachment of Si-rich activated complexes formed by the removal of Al from the mineral framework. Taking account of this mechanism and transition state theory yields equations describing plagioclase dissolution rates (r+) as a function of both the mineral and aqueous fluid compositions found in natural Earth surface systems. For pH ⩾ 6 rates are consistent with View the MathML sourceLog(r+/(mol/cm2/s))=0.35Log(aH+3/aAl3+)-11.53 and for pH < 6 rates are consistent with View the MathML sourceLog(r+/(mol/cm2/s))=nacidLog(aH+3/aAl3+)+0.033An%-14.77 where An% represents the percent anorthite in the plagioclase solid solution, ai corresponds to the activity of the ith aqueous species, and nacid is given by nacid=0.004An%+0.05nacid=0.004An%+0.05

Dissolution rates of crystalline basalt at pH 4 and 10 and 25–75°C

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Data from: Differentiation at the MHCIIα and Cath2 loci in sympatric Salvelinus alpinus resource morphs in Lake Thingvallavatn

Northern freshwater fish may be suitable for the genetic dissection of ecological traits because they invaded new habitats after the last ice age (~10.000 years ago). Arctic charr (Salvelinus alpinus) colonizing streams and lakes in Iceland gave rise to multiple populations of small benthic morphotypes, often in sympatry with a pelagic morphotype. Earlier studies have revealed significant, but subtle, genetic differentiation between the three most common morphs in Lake Thingvallavatn. We conducted a population genetic screen on four immunological candidate genes Cathelicidin 2 (Cath2), Hepcidin (Hamp), Liver expressed antimicrobial peptide 2a (Leap-2a), and Major Histocompatibility Complex IIα (MHCIIα) and a mitochondrial marker (D-loop) among the three most common Lake Thingvallavatn charr morphs. Significant differences in allele frequencies were found between morphs at the Cath2 and MHCIIα loci. No such signal was detected in the D-loop nor in the other two immunological genes. In Cath2 the small benthic morph deviated from the other two (FST = 0.13), one of the substitutions detected constituting an amino acid replacement polymorphism in the antimicrobial peptide. A more striking difference was found in the MHCIIα. Two haplotypes were very common in the lake, and their frequency differed greatly between the morphotypes (from 22% to 93.5%, FST = 0.67). We then expanded our study by surveying the variation in Cath2 and MHCIIα in 9 Arctic charr populations from around Iceland. The populations varied greatly in terms of allele frequencies at Cath2, but the variation did not correlate with morphotype. At the MHCIIα locus, the variation was nearly identical to the variation in the two benthic morphs of Lake Thingvallavatn. The results are consistent with a scenario where parts of the immune systems have diverged substantially among Arctic charr populations in Iceland, after colonizing the island ~10.000 years ago

Pre-eruptive storage conditions and magmatic evolution of the Bora-Baricha-Tullu Moye volcanic system, Main Ethiopian Rift

Bora-Baricha-Tullu Moye is a Late Quaternary volcanic system in the Main Ethiopian Rift, characterised by products of both explosive and effusive volcanic eruptions. The petrological and geochemical characteristics of the volcanic products are investigated using a combination of petrography, major and trace element whole rock analyses and in-situ major element analyses of phenocryst phases, matrix glass and melt inclusions. The bulk rock compositions vary from basalt to peralkaline rhyolite (comendite and pantellerite), and the chemical variability can largely be explained by fractional crystallisation processes with minor crustal assimilation and magma mixing. The dominant mineral phases such as clinopyroxenes and feldspars show a tendency for Fe and Na enrichment respectively from the basalts towards the pantellerites. The comendite and pantellerite deposits show systematic variations towards more evolved glass and mineral composition with the stratigraphy. The combination of thermometry (i.e., clinopyroxene-liquid, feldspar-liquid, olivine-liquid and clinopyroxene-only) and barometry (i.e., clinopyroxene-liquid and clinopyroxene-only) modelling suggests that the basaltic magmas are stored at high temperature (1070–1190 °C) at mid-to-deep-crustal levels (~7–29 km). The peralkaline rhyolite melts are stored at lower temperature (i.e., 805–900 °C for comendite; 700–765 °C for pantellerite) at shallow crustal levels (~4 km). The conditions of pre-eruptive storage as recorded in the comendite and pantellerite rocks in combination with stratigraphic constraints, suggests a progressive temporal evolution of the magma reservoirs to cooler storage temperatures

Kapralova2013_Charr2010

Dataset 1 – Arctic Charr from Lake Thingvallavatn, collected in 2010. Captured in the wild, phenotyped and genotyped in the laboratory