162,097 research outputs found
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
Geological constraints on the eruption of the Jwaneng Centre kimberlite pipe, Botswana
No full textGeological mapping has allowed constraints to be placed on the eruption mechanisms involved in the formation of the Late Permian–Early Triassic Jwaneng Centre kimberlite pipe, Botswana. Twelve lithofacies and three lithofacies associations (LFA 1–3) are recognised. LFA 1 comprises massive to bedded volcaniclastic kimberlite and marginal shale breccias and outcrops over 65% of the surface area of the pipe. It is characterised by a lithic population dominated by Transvaal shale clasts. LFA 1 grades into LFA 2, which comprises massive and bedded volcaniclastic kimberlite and volcaniclastic breccias and outcrops over 19% of the surface area of the pipe. The lithic population of LFA 2 is dominated by contorted and fluidal-outlined Karoo-age mudstones and siltstones. LFA 3 comprises a wedge-shaped unit in the north of the pipe and consists of a series of allochthonous megablocks of sedimentary and volcaniclastic strata. The juvenile clast type and matrix mineral assemblages of the volcaniclastic deposits in the Centre Pipe differ from those in many other southern African kimberlite pipes. Various emplacement models for kimberlite pipes are discussed and evaluated in the light of the new geological data. Both a maar–diatreme model and an explosive volatile-driven eruption model could account for much of the geology of the Centre Pipe and distinguishing between the two models based on deposits alone is difficult. There is strong circumstantial evidence for ambient conditions favourable to phreatomagmatism at the time of the eruption, and the influence of external water may explain the differences between the Jwaneng Centre Pipe and the Class 1 kimberlites common across Southern Africa in terms of both the juvenile clasts and of the inter-clast cement. However, low abundances of some types of lithic inclusions derived from major country rock units pose an unresolved problem for a classic maar–diatreme model of pipe formation.Geological mapping has allowed constraints to be placed on the eruption mechanisms involved in the formation of the Late Permian–Early Triassic Jwaneng Centre kimberlite pipe, Botswana. Twelve lithofacies and three lithofacies associations (LFA 1–3) are recognised. LFA 1 comprises massive to bedded volcaniclastic kimberlite and marginal shale breccias and outcrops over 65% of the surface area of the pipe. It is characterised by a lithic population dominated by Transvaal shale clasts. LFA 1 grades into LFA 2, which comprises massive and bedded volcaniclastic kimberlite and volcaniclastic breccias and outcrops over 19% of the surface area of the pipe. The lithic population of LFA 2 is dominated by contorted and fluidal-outlined Karoo-age mudstones and siltstones. LFA 3 comprises a wedge-shaped unit in the north of the pipe and consists of a series of allochthonous megablocks of sedimentary and volcaniclastic strata. The juvenile clast type and matrix mineral assemblages of the volcaniclastic deposits in the Centre Pipe differ from those in many other southern African kimberlite pipes. Various emplacement models for kimberlite pipes are discussed and evaluated in the light of the new geological data. Both a maar–diatreme model and an explosive volatile-driven eruption model could account for much of the geology of the Centre Pipe and distinguishing between the two models based on deposits alone is difficult. There is strong circumstantial evidence for ambient conditions favourable to phreatomagmatism at the time of the eruption, and the influence of external water may explain the differences between the Jwaneng Centre Pipe and the Class 1 kimberlites common across Southern Africa in terms of both the juvenile clasts and of the inter-clast cement. However, low abundances of some types of lithic inclusions derived from major country rock units pose an unresolved problem for a classic maar–diatreme model of pipe formation
[Report to Chief J. E. Curry, by an unknown author #1]
No full textReport to Chief J. E. Curry, by an unknown author. The report contains a list of officers who gave depositions to the United States Attorney
[Report to Chief J. E. Curry, by an unknown author #2]
No full textReport to Chief J. E. Curry, by an unknown author. The report contains a list of officers who gave depositions to the United States Attorney
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
Greenlandic debris in Iceland likely tied to Bond 1 ice-rafting in the Dark Ages
No full textWe report the discovery of exotic igneous, metamorphic, and sedimentary cobbles in raised beach deposits near Breiðavík, northern Iceland. These deposits consist of alternating cobble-, sand-, and silt-dominated facies. A nearby package of sands and silts, dated to the Late Antique Little Ice Age (LALIA; ca. 536−660 CE), provides age constraints for the raised terraces. While the upper terraces are composed exclusively of local basaltic material, the lowermost terraces (∼2 m above high tide) contain a mix of basaltic and nonbasaltic cobbles, including quartzofeldspathic gneiss, granitoid, rhyolite, sandstone, and serpentinite. U-Pb geochronologic analysis of zircon revealed dominant age modes of ca. 2800, 1150, 500, and 240 Ma with Lu-Hf isotopic compositions suggesting derivation from Greenland’s North Atlantic craton and Caledonian fold belt. The colder conditions of the LALIA, coupled with increased iceberg calving from the Greenland ice sheet, would have led to enhanced ice-rafted debris (IRD) transport to disparate areas south and east of Greenland. The East Greenland and East Iceland currents transported this IRD from Greenland, with deposition occurring along the Icelandic coast as the icebergs melted. This IRD was likely transported across the North Atlantic during Bond event 1. This process, along with those during other transient cooling events, may explain the age discrepancies between local bedrock and detrital zircons in the Arctic
Eruptive history of an alkali basaltic diatreme from Elie Ness, Fife, Scotland
No full textThe Elie Ness diatreme (Fife, Scotland) is an ideal place to study the internal architecture and emplacement processes of diatremes. Elie Ness is one of approximately 100 alkali basaltic diatremes and intrusions in the East Fife area, emplaced during Upper Carboniferous to Early Permian times into an extensive rift system in the northern Variscan foreland. Within the diatreme, seven lithofacies and three lithofacies associations (LFAs 1-3) are recognised. Field, petrographic and geochemical studies demonstrate that the diatreme experienced a protracted history of eruption and infill, initially driven by volatile expansion and later by magma-water interaction. Massive lapilli tuffs of LFA 1 contain abundant highly vesicular juvenile scoria and magma-coated clasts, which are best explained by a magmatic origin for the early explosive eruptions. On a large-scale, the tuffs are well mixed and locally exhibit small-scale degassing structures attributed to fluidisation processes occurring within the diatreme fill. The occurrence of abundant volcaniclastic autoliths and megablocks within LFA 1 can be explained by subsidence of volcaniclastic strata from the maar crater and upper diatreme during emplacement. Pyroclastic density current deposits of LFA 2 form a series of continuous sheets across the diatreme, some of which may have originated from phreatomagmatic explosions in a neighbouring vent. We attribute the overall bedding pattern to a combination of primary volcanic processes and post-depositional folding related to movement along an adjacent fault. Minor steeply inclined breccias and tuffs of LFA 3 cross-cut the LFA 2 succession and are interpreted as late-stage volcaniclastic dykes and conduits, signalling the final phase of eruptive activity at Elie Ness. The study offers new insights into the volcanic evolution of diatremes fed by low viscosity, alkali-rich magmas
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
Geology of the Snap Lake kimberlite intrusion, Northwest Territories, Canada: field observations and their interpretation
No full textThe Cambrian (523 Ma) Snap Lake hypabyssal kimberlite intrusion, Northwest Territories, Canada, is a complex segmented diamond-bearing ore-body. Detailed geological investigations suggest that the kimberlite is a multi-phase intrusion with at least four magmatic lithofacies. In particular, olivine-rich (ORK) and olivine-poor (OPK) varieties of hypabyssal kimberlite have been identified. Key observations are that the olivine-rich lithofacieshas a strong tendency to be located where the intrusion is thickest and that there is a good correlation between intrusion thickness, olivine crystal size and crystal content. Heterogeneities in the lithofacies are attributed to variations in intrusion thickness and structural complexities. The geometry and distribution of lithofacies points to magmaticco-intrusion, and flow segregation driven by fundamental rheological differences between the two phases. We envisage that the low-viscosity OPK magma acted as a lubricant for the highly viscous ORK magma. The presenceof such low-viscosity, crystal-poor magmas may explain how crystal-laden kimberlite magmas (>60 vol.%) are able to reach the surface during kimberlite eruptions. We also document the absence of crystal settling and the development of an unusual subvertical fabric of elongate olivine crystals, which are explained by rapid degassing-induced quench crystallization of the magmas during and after intrusio
- …
