136 research outputs found
A sabbatical reboot
No full text“Why have I traded my private office for a desk in a shared room?” I wondered. “Will I get any work done?” It was my first day at my new co-working space on the outskirts of town, 10 kilometers from my university. “I'm here on sabbatical,” I announced to my new officemates. They were directors of tech startups, unaccustomed to mingling with geoscientists like me. I desperately needed some breathing space away from the pressures of university life, but I wasn't totally convinced it would work out. I needn't have worried. The co-working space turned out to be the perfect environment for rebooting my flailing research career
Oklahoma’s induced seismicity strongly linked to wastewater injection depth
No full textThe dramatic rise in Oklahoma seismicity since 2009 is due to wastewater injection. The role of injection depth is an open, complex issue, yet critical for hazard assessment and regulation. We developed an advanced Bayesian Network to model joint conditional dependencies between spatial, operational, and seismicity parameters. We found injection depth relative to crystalline basement most strongly correlates with seismic moment release. The joint effects of depth and volume are critical, as injection rate becomes more influential near the basement interface. Restricting injection depths to 200–500 m above basement could reduce annual seismic moment release by a factor of 1.4–2.8. Our approach enables identification of sub-regions where targeted regulation may mitigate effects of induced earthquakes, aiding operators and regulators in wastewater disposal regions
The role of tephra in enhancing organic carbon preservation in marine sediments
No full textPreservation of organic carbon (Corg) in marine sediments plays a major role in defining ocean-atmosphere CO2 levels, Earth climate, and the generation of hydrocarbons. Important controls over sedimentary Corg preservation include; biological productivity, Corg isolation from oxidants (mainly dissolved O2) in the overlying water column and sediments, and Corg – mineral association in sediments. Deposition of the products of explosive volcanism (tephra) in the oceans directly enhances Corg burial through all these mechanisms, and indirectly through enhanced formation of authigenic carbonate (Cauth) derived from sedimentary Corg. In the modern oceans, it is suggested that tephra deposition may account for 5–10% of the Corg burial flux and 10–40% of the Cauth burial flux. However, during certain periods in Earth's history, extensive explosive volcanism may have led to enhanced Cauth precipitation on a sufficiently large scale to influence the global ocean-atmosphere carbon cycle. Changes in tephra-related Corg preservation may also have played a role in increasing Corg preservation rates in local marine basins, at the oxic-anoxic boundary and enhanced the generation of hydrocarbon deposits in these settings
Did an asteroid impact cause temporary warming during snowball Earth?
No full textThe ca. 717 Ma low-latitude Sturtian “snowball Earth” glaciation lasted ∼56 Myr. However, sedimentological evidence for transient, open ocean conditions during the glaciation appears to contradict the concept of a global deep freeze. We demonstrate multiple lines of geologic evidence from five continents for a temporary, localized sea-ice retreat during the middle of the Sturtian glaciation, which coincides with one, perhaps two, asteroid impacts, and arguably more terrestrial impacts as inferred from the lunar impact record. The well-dated Jänisjärvi impact (ca. 687 Ma) is synchronous with repeated volcanic ash falls whose deposition is most parsimoniously interpreted to indicate a partially ice-free ocean. Temporary greenhouse warming caused by the vaporization of sea ice can explain localized glacial retreat within restricted seaways between these continents, where ice flow would have been constricted and sea ice thinnest before impact
Magmatic evolution during proto-oceanic rifting at Alu, Dalafilla and Borale Volcanoes (Afar) determined by trace element and Sr-Nd-Pb isotope geochemistry
No full textContinental rifting and associated magmatism can eventually result in the formation of new ocean basins. However, the characteristics of magmatism in the latest stages of rifting are poorly understood. The Erta-Ale volcanic segment (EAVS) in the Danakil Depression of Afar, Ethiopia, provides a unique natural laboratory in which to investigate how magma generation evolves during the shift from continental rifting to oceanic spreading. Here we present new trace element data combined with Sr-Nd-Pb isotope ratios for three volcanoes, Alu, Dalafilla and Borale, in the north of the EAVS. These data shed light on the changes in melt production and storage that occur at this late stage in the rifting cycle. Elevated Ce/Pb and ΔNb (33–48, 0.25–0.47 respectively) of the basalts, alongside Sr-Nd-Pb isotope geochemistry indicate the presence of a HIMU component, supplied by the Afar plume, together with contamination by the crust. Melting conditions, estimated using the trace element ratios, Smn/Ybn, Dyn/Ybn and Cen/Smn, indicate that magmas were primarily derived from spinel lherzolite (85–90%) with minor garnet lherzolite (10–15%) with a melt fraction of ~4%. Melt-mantle equilibrium depths are estimated to be on the order of 64 to 83 km, shallower than that previously inferred within Afar. We suggest that this is likely a result of the more plate thinning beneath the EAVS compared to other parts of Afar. Basaltic volcanics are found to have heterogeneous Sr-Nd-Pb isotope compositions whilst those more evolved rocks (i.e., SiO2 ≥52 wt%) exhibit consistent radiogenic compositions. This indicates that homogenisation of all melt compositions occurs prior-to or during melt differentiation, with the latter process occurring rapidly in upper crust with minimal crustal contamination. Overall whilst the Afar plume appears to be the dominant mantle component in the volcanic rocks, the melt characteristics and magmatic storage conditions beneath the EAVS shows variability that is likely controlled by a dynamic interplay between rifting and mantle processes
Depositional processes in a kimberlite crater: the Upper Cretaceous Orapa South Pipe (Botswana)
No full textThe Orapa A/K1 Diamond Mine, Botswana, exposes the crater facies of a bilobate kimberlite pipe of Upper Cretaceous age. The South Crater consists of layered volcaniclastic deposits which unconformably cross-cut massive volcaniclastic kimberlite of diatreme facies in the North Pipe. Based on the depositional structure, grain-size, sorting and composition of kimberlite in the South Crater, six units are distinguished in the similar to 70 m thick stratiform crater-fill sequence and talus slope deposits close to the crater wall, which represents a multistage infill of the volcanic crater. Monolithic basalt breccias (Unit 1) near the base of the crater-fill are interpreted as rock-fall avalanche deposits, generated by the sector collapse of the crater walls. These deposits are overlain by a basal imbricated lithic breccia and upper massive sub-unit (Unit 2), interpreted as the deposits of a pyroclastic flow that entered the South Crater from another source. Vertical degassing structures within the massive sub-unit show evidence for elutriation of fines and probably were formed after emplacement by fluidization due to air entrainment. Units 3 and 5 are thinly stratified deposits, characterized by diffuse bedding, reverse and normal grading, coarse lenticular beds, mudstone beds, small-scale scour channels and load casts. These units are attributed to rapidly emplaced sheet floods on the crater floor. Units 3 and 5 are directly overlain by poorly sorted volcaniclastic kimberlite (Units 4 and 6) rich in basalt boulders, attributed to debris flows formed by the collapse of crater walls. Unit 7 comprises medium sandstones to cobble conglomerates representing talus fans, which were active throughout the deposition of Units 1 to 6. The study demonstrates that much of the material infilling the South Crater is derived externally after eruption, including primary pyroclastic flow deposits probably from another kimberlite pipe. These findings have important implications for predicting diamond grade. Results may also aid the interpretation of crater sequences of ultra-basic, basaltic and intermediate volcanoes, together with the deposits of topographic basins in sub-aerial settings.The Orapa A/K1 Diamond Mine, Botswana, exposes the crater facies of a bilobate kimberlite pipe of Upper Cretaceous age. The South Crater consists of layered volcaniclastic deposits which unconformably cross-cut massive volcaniclastic kimberlite of diatreme facies in the North Pipe. Based on the depositional structure, grain-size, sorting and composition of kimberlite in the South Crater, six units are distinguished in the similar to 70 m thick stratiform crater-fill sequence and talus slope deposits close to the crater wall, which represents a multistage infill of the volcanic crater. Monolithic basalt breccias (Unit 1) near the base of the crater-fill are interpreted as rock-fall avalanche deposits, generated by the sector collapse of the crater walls. These deposits are overlain by a basal imbricated lithic breccia and upper massive sub-unit (Unit 2), interpreted as the deposits of a pyroclastic flow that entered the South Crater from another source. Vertical degassing structures within the massive sub-unit show evidence for elutriation of fines and probably were formed after emplacement by fluidization due to air entrainment. Units 3 and 5 are thinly stratified deposits, characterized by diffuse bedding, reverse and normal grading, coarse lenticular beds, mudstone beds, small-scale scour channels and load casts. These units are attributed to rapidly emplaced sheet floods on the crater floor. Units 3 and 5 are directly overlain by poorly sorted volcaniclastic kimberlite (Units 4 and 6) rich in basalt boulders, attributed to debris flows formed by the collapse of crater walls. Unit 7 comprises medium sandstones to cobble conglomerates representing talus fans, which were active throughout the deposition of Units 1 to 6. The study demonstrates that much of the material infilling the South Crater is derived externally after eruption, including primary pyroclastic flow deposits probably from another kimberlite pipe. These findings have important implications for predicting diamond grade. Results may also aid the interpretation of crater sequences of ultra-basic, basaltic and intermediate volcanoes, together with the deposits of topographic basins in sub-aerial settings
A viable Labrador Sea rifting origin of the Northern Appalachian and related seismic anomalies
No full textThe Northern Appalachian Anomaly (NAA) is a prominent low-seismic-velocity zone, ∼400 km in diameter, in the asthenosphere beneath New England (northeastern USA). Previous studies interpreted this shallow feature, occurring at a depth of ∼200 km, as a thermal anomaly tied to edge-driven convection along the North American continental margins. Those studies recognized, however, that upwelling here is highly unusual given that the passive margin has been tectonically quiescent for ∼180 m.y. We propose an alternative model, based on geologic observations, geotectonic reconstructions, and geodynamic simulations, that the anomaly instead represents a Rayleigh-Taylor instability linked to the breakup of the distant Labrador Sea continental margin. A Labrador Sea origin at breakup, ca. 85−80 Ma, would imply the migration of a chain of Rayleigh-Taylor instabilities at a rate of ∼22 km/m.y., close to expected rates from geodynamic models. A migrating-instability origin for the anomaly can reconcile its spatial characteristics, depth profile, and position near a long-inactive continental margin. A corollary is that the north-central Greenland anomaly, a mirror-image of the NAA, also potentially originated at the time of breakup. Further, The Central Appalachian Anomaly may fit this model if it represents an early-stage instability linked to rifting onset in the Labrador Sea. The NAA and other associated anomalies viably represent a legacy of continental rifting and breakup along the distant Labrador Sea margins
Degassing structures in volcaniclastic kimberlite: Examples from southern African kimberlite pipes
No full textKimberlite pipes are commonly filled with a distinctive structureless facies termed volcaniclastic kimberlite (previously also termed Tuffisitic Kimberlite Breccia, TKB), which constitutes a thorough mix of both juvenile material and lithic clasts derived from all stratigraphic levels. Within this facies at several kimberlite pipes, we have identified steeply-inclined segregation structures, developed on length-scales ranging from several centimetres to decimetres. The structures are pipe-like (in 2D) and characterised by a concentration of coarse crystals and lithic clasts and depletion of the fine components that characterise the host matrix. They are interpreted as degassing structures, generated by the passing of fluids through particulate deposits during the earliest (or latest) stages of fluidisation. In this paper, we identify three circumstances in which degassing structures are generated in volcaniclastic kimberlite emplacement: (1) pyroclastic density current deposits, (2) local segregations caused by degassing through pipe-fill, and (3) fluidisation during channelling of deep-sourced gas-particle dispersions. To our knowledge, these structures have not been described in kimberlites before and provide important evidence for the occurrence of gas-fluidisation during the waning stages of kimberlite eruptions. Using examples from southern African kimberlite pipes, we describe general aspects of their structure, combined with particle size distributions, clast fabric studies and petrographic compositions.Kimberlite pipes are commonly filled with a distinctive structureless facies termed volcaniclastic kimberlite (previously also termed Tuffisitic Kimberlite Breccia, TKB), which constitutes a thorough mix of both juvenile material and lithic clasts derived from all stratigraphic levels. Within this facies at several kimberlite pipes, we have identified steeply-inclined segregation structures, developed on length-scales ranging from several centimetres to decimetres. The structures are pipe-like (in 2D) and characterised by a concentration of coarse crystals and lithic clasts and depletion of the fine components that characterise the host matrix. They are interpreted as degassing structures, generated by the passing of fluids through particulate deposits during the earliest (or latest) stages of fluidisation. In this paper, we identify three circumstances in which degassing structures are generated in volcaniclastic kimberlite emplacement: (1) pyroclastic density current deposits, (2) local segregations caused by degassing through pipe-fill, and (3) fluidisation during channelling of deep-sourced gas-particle dispersions. To our knowledge, these structures have not been described in kimberlites before and provide important evidence for the occurrence of gas-fluidisation during the waning stages of kimberlite eruptions. Using examples from southern African kimberlite pipes, we describe general aspects of their structure, combined with particle size distributions, clast fabric studies and petrographic compositions
Triggering of major eruptions recorded by actively forming cumulates
No full textMajor overturn within a magma chamber can bring together felsic and mafic magmas, prompting de-volatilisation and acting as the driver for Plinian eruptions. Until now identification of mixing has been limited to analysis of lavas or individual crystals ejected during eruptions. We have recovered partially developed cumulate material (‘live’ cumulate mush) from pyroclastic deposits of major eruptions on Tenerife. These samples represent “frozen” clumps of diverse crystalline deposits from all levels in the developing reservoir, which are permeated with the final magma immediately before eruptions. Such events therefore record the complete disintegration of the magma chamber, leading to caldera collapse. Chemical variation across developing cumulus crystals records changes in melt composition. Apart from fluctuations reflecting periodic influxes of mafic melt, crystal edges consistently record the presence of more felsic magmas. The prevalence of this felsic liquid implies it was able to infiltrate the entire cumulate pile immediately before each eruption
Evolving mantle convection from bottom-up to top-down
No full textWhen it comes to convection, what goes up must come down. Or is it, what goes down must come up? The truth is it depends. Although convection must be mass balanced, there is no reason that it must be force balanced: the positive and negative buoyancy forces driving convection up and down, respectively, do not necessarily need to be balanced. The balance, or imbalance, all depends on the top and bottom boundary layers. Thus, convection in Earth's mantle depends on the temperature differences across the core-mantle boundary below and the lithosphere-asthenosphere boundary above. Convective asymmetry predominated by positive buoyancy, or bottom-up convection, would be driven by plume ascent, whereas if it were predominated by negative buoyancy, or top-down convection, it would be driven by plate subduction. Symmetric convection would balance plume ascent and plate subduction. Is mantle convection on Earth balanced, dominantly top down or bottom up, or time dependent?</p
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