bjorn hardarson - Academia.edu (original) (raw)
Papers by bjorn hardarson
… North Iceland and its Geological History. …, 2003
The active volcanic zones in Iceland are characterized by high heat flow and extensive geothermal... more The active volcanic zones in Iceland are characterized by high heat flow and extensive geothermal activity. Iceland is unique in its location astride the diverging Mid-Atlantic Ridge and, furthermore, on top of a mantle plume. These two dynamic systems combine fundamental factors that promote magmatism, tectonics and geothermal activity. The high-temperature geothermal areas are mainly confined to volcanic systems, in particular central volcanoes, and are subject to strong tectonic control. The Hengill central volcano hosts one of the most powerful geothermal fields in Iceland, dominated by NE-SW striking faults. It is located at the western flank of the West Iceland Volcanic Zone (WVZ), which represents the Mid-Atlantic Ridge on land forming a graben-like structure approximately 10-15 km wide and 100 km long. The Hengill low resistivity structure covers about 112 km 2 and presently Reykjavik Energy operates two power plants in the area with installed capacity of about 420 MWe and 4...
… North Iceland and its Geological History. …, 2003
ABSTRACT
The East African Rift System (EARS) is a classic example of continental rifting and provides an e... more The East African Rift System (EARS) is a classic example of continental rifting and provides an excellent framework to study extensional magmatism and the evolution of several central volcanic systems that have formed along the rift from the Tertiary to Recent. Many of the volcanic structures have developed substantial high-temperature geothermal systems where the heat source is magmatic and related to central volcanoes. Detailed studies indicate that the geothermal potential in Eastern Africa is in the excess of 15,000 MWe. However, despite the high geothermal potential of EARS only Kenya has installed significant capacity of about 570 MW. Magmatism along the EARS is generally believed to be associated with mantle plume activities but the number and nature of mantle plumes is still, however, controversial. EARS is divided into two main branches, the Easternand Western rifts, and it is well documented that significantly greater volcanism is observed in the older Eastern rift (i.e. E...
Proceedings of the Ocean Drilling Program, 1998
The southeast Greenland seaward-dipping reflector sequence (SDRS) is composed of Paleocene to Eoc... more The southeast Greenland seaward-dipping reflector sequence (SDRS) is composed of Paleocene to Eocene volcanic rocks erupted during continental breakup. Volcanic rocks recovered from a transect across the SDRS at 63°N, during Ocean Drilling Program Leg 152, range in composition from picrite to dacite and represent all the magmatic phases in the development of the continental margin. The earliest magmas, represented by the pre-breakup succession at Site 917 (Lower and Middle Series), were strongly contaminated with continental crust, but the degree of contamination declined rapidly during the late stages of breakup (Site 917 Upper Series). Very low concentrations of incompatible elements in the uncontaminated primitive magmas made them extremely sensitive to the isotopic effects of crustal contamination. Basaltic rocks from the most seaward part of the transect (Site 918) were erupted after breakup and show no signs of contamination with continental crust. Two distinct crustal contaminants can be recognized: (1) lower crustal basic granulite with unradiogenic Nd, Sr, and Pb; and (2) upper crustal amphibolite-facies gneiss with unradiogenic Nd but highly radiogenic Sr and high 208 Pb/ 204 Pb. The first contaminant affected only the earliest magmas, represented by the lower volcanic units in the Lower Series at Site 917. Later continental magmas were affected by the second contaminant, suggesting storage of magmas at progressively shallower levels in the crust as lithospheric extension proceeded toward continental breakup. The nature and degree of contamination are strikingly similar to those observed in the Hebridean Tertiary igneous province, which would have been adjacent to southeast Greenland during continental breakup.
Proceedings of the Ocean Drilling Program, 1998
Samples were cut into small pieces with a diamond-tipped saw, and any obvious amygdales and veins... more Samples were cut into small pieces with a diamond-tipped saw, and any obvious amygdales and veins were removed. Saw marks were ground away on a diamond wheel, and the fragments were rinsed in clean water and dried before being ground to a fine powder in an agate Tema barrel. Agate was used in preference to tungsten carbide to avoid contamination with Ta and Co. The powders were analyzed in Edinburgh by X-ray fluorescence (XRF) spectrometry, and in Leicester by neutron activation analysis (NAA). A separate suite of volcanic rock samples was prepared and analyzed by XRF at the Geological Survey of Greenland for major elements, and at the University of Copenhagen for trace elements. Agreement between the Edinburgh and Copenhagen XRF data is generally very good
Diachronous V-shaped ridges (VSRs) in basement topography straddling the Reykjanes Ridge south of... more Diachronous V-shaped ridges (VSRs) in basement topography straddling the Reykjanes Ridge south of Iceland, expressed most clearly in the gravity field, reflect ±10% variation in crustal thickness. They appear to be produced by pulses of excess magma productivity propagating from Iceland at ~200 km/Ma with a periodicity of ~5 Ma. Three possible explanations for these crustal thickness variations are: (1) pulses in mantle temperature of ~20°C; (2) pulses of excess fertility; or (3) the regulation of mantle flow by rift relocations in Iceland. If the VSRs are due to pulses in mantle temperature and/or fertility, then this ought to be recorded in the Tertiary lava successions in east and west Iceland. Rift relocation, on the other hand, should leave no such record. Variations in mantle temperature of ~20°C will cause variations in degree of mantle melting and this should affect ratios of trace elements (e.g. Zr/Y) that are sensitive to degree of melting of garnet lherzolite. Icelandic b...
Lithos, 1996
Unequivocal examples of oceanic plateaux older than about 2.1 Ga have yet to he identified from t... more Unequivocal examples of oceanic plateaux older than about 2.1 Ga have yet to he identified from the geological record. The most likely explanation for this is the partial dismemberment of ancient plateaux at convergent plate margins, and their subsequent juxtaposition in accretionary complexes with arc volcanic, sedimentary and plutonic rocks. A review of Cretaceous plateaux in the eastern Indian Ocean suggests that certain geochemical-isotopic criteria (e.g. positive Nb anomalies relative to primitive mantle) employed by petrologists to identify modem oceanic plateau lavas are of little practical use in distinguishing ancient plateau lavas from arc and volcanic rifted margin basal& The extreme chemical and isotopic heterogeneity of Indian Ocean plateaux, together with large variations in area and volume, suggest that these edifices represent one extreme of a spectrum of oceanic plateau compositions. The other extreme, represented by Iceland and Ontong Java, is characterised by relatively homogeneous melt compositions, reflecting steady-state melting conditions and the absence of a continental lithosphere contaminant to plateau magmas. Crustal thickness and compositional estimates for Archaean oceanic plateaux using the McKenzie-Bickle method give a mean thickness of-43 km, and mean MgO content of-19 wt.%. Archaean plateaux were thus compositionally distinct from surrounding ocean floor (basaltic komatiite?), and notably more magnesian than plateaux such as Iceland (mean for Tertiary basalts-6.3 wt.% MgO) and Ontong Java (mean for core samples-6.9 wt.% MgO). By analogy with Cretaceous-Tertiary ultramafic complexes in western Colombia, the deeper portions of these plateaux may have consisted of noritic rocks, underlain by lherzolite, pyroxenite, gabbronorite and dunite. The general absence of such rocks in Archaean terrains suggests that these portions of plateaux are only rarely preserved. The search for remnants of ancient oceanic plateaux has identified examples of Archaean submarine lavas in the Canadian Shield (2 2.7 Ga: southern Abitibi belt, Superior Province) and Kaapvaal craton of southern Africa (3.5-3.3 Ga: Barberton and Pietersberg belts). Chemical and isotopic data suggest that komatiite-tholeiite units from these areas were derived by partial melting of a plume or asthenospheric mantle source, but do not distinguish adequately the tectonic environment (rifted continental margin, ocean floor, oceanic plateau) in which these rocks were erupted. However, the uplift histories of these putative plateau fragments, as recorded in sediments bounding and intercalated with the komatiite-basalt sequences, appear to be broadly comparable to those of the best-documented Phanerozoic oceanic plateaux. Although further stratigraphical investigations of presumed plateaux (ancient and modem) are required to test this hypothesis, we believe that the sedimentary record offers the best evidence yet for the formation of oceanic plateaux as far back as the Mid-Archaean (-3.5 Gal.
Journal of Petrology, 2000
Earth and Planetary Science Letters, 2000
... a NERC Isotope Geosciences Laboratory, Kingsley Dunham Centre, Keyworth NG12 5GG, UK. b Grant... more ... a NERC Isotope Geosciences Laboratory, Kingsley Dunham Centre, Keyworth NG12 5GG, UK. b Grant Institute, University of Edinburgh, Edinburgh EH9 3JW, UK. c Department of Geology, Leicester University, Leicester LE1 7RH, UK. ...
for their enthusiasm, encouragement, numerous valuable discussions, input, critical comments and ... more for their enthusiasm, encouragement, numerous valuable discussions, input, critical comments and guidance during the study period. Special thanks also go to Hjalti Franzson for his helpful suggestions and discussion on various aspects of this work. I would like to thank the entire UNU-GTP and ÍSOR GeoSurvey staff, special thanks go to the Director,
Menengai is a trachytic central caldera volcano in the Kenya rift valley with abundant high-tempe... more Menengai is a trachytic central caldera volcano in the Kenya rift valley with abundant high-temperature geothermal activity. The field is currently in its initial stages of development for geothermal energy in Kenya following Olkaria and Eburru fields. Regional surface geology of Menengai is largely composed of late Quaternary volcanics. The building of a 200,000 year old trachyte shield volcano was followed by piecemeal subsidence through two paroxysmal eruptions 29,000 and 8,000 B.P to produce a caldera of about 84 km in size that has subsequently been largely filled by recent trachyte lavas. Twenty four exploration and production wells, some of them hotter than 390C, have been drilled in Menengai caldera by Geothermal Development Company (GDC). The aim of this study is to reveal the evolutionary history of the Menengai volcano and to describe its igneous lithostratigraphy and secondary mineralization in order to characterize hydrothermal processes within the field. Analytical met...
J Volcanol Geotherm Res, 2000
Nature, 1991
BASALTIC magmatism results when upwelling mantle crosses the peridotite solidus. The greater the ... more BASALTIC magmatism results when upwelling mantle crosses the peridotite solidus. The greater the overstep of the solidus, the greater the degree of mantle melting and the larger the volume of magma produced. In Iceland most of the active volcanism occurs along the central spreading axis, although some occurs in isolated off-axis volcanoes such as Snaefellsjökull. Because the mantle beneath Snaefellsjökull is not upwelling as vigorously as that beneath the spreading axis, it oversteps its solidus by a much smaller amount and consequently the degree of melting is less. The composition of the resulting magma will be much more sensitive to small perturbations in the amount of upwelling than will the mantle beneath the ridge axis. We show here that the reduction of pressure in the mantle caused by the unloading of ice was sufficient to affect magma composition. Unloading was accompanied by a clear shift towards less undersaturated magma compositions, which reflect a transient increase of about 0.5% in the degree of mantle melting, coupled with a decrease in depth of melting.
Geology, 1997
Basalt from the Tertiary lava pile in Iceland is much more homogeneous in composition than that e... more Basalt from the Tertiary lava pile in Iceland is much more homogeneous in composition than that erupted in the axial rift zone. Lavas highly depleted in incompatible elements are occasionally erupted in the latter but have not been reported from the Tertiary succession. This observation leads to a useful test of the Pálmason model of crustal accretion in which axial lava flows ultimately form the lower levels of the lava pile, and only flows emplaced off-axis remain accessible in the upper part of the crust. Axial flows large enough to spill out of the rift axis, and flows erupted on the rift flanks, should therefore have a uniform composition similar to that of the Tertiary flows. Here we test this prediction and thereby the validity of the Pálmason model of crustal accretion in oceanic spreading centers. Our data show that depleted basalt is confined to small-volume lava flows within the rift axis. Furthermore, large flows that originated within the axial zone and spread far enough to become preserved in the accessible part of the future offaxis lava pile are very homogeneous in composition and comparable to those forming the Tertiary lava successions.
Earth and Planetary Science Letters, 1997
A dominant process in the evolution of Iceland is the repeated eastward relocation of the spreadi... more A dominant process in the evolution of Iceland is the repeated eastward relocation of the spreading axis in response to westward migration of the plate boundary relative to the plume centre. Two major former rifts can be identified in western Iceland: the Snaefellsnes rift zone, which last erupted tholeiitic lavas at about 7 Ma, and an older spreading system, lava flows from which can be traced some 100 km along a SW-NE strike in the extreme northwest of Iceland. The extinction of the latter is marked by a 14.9 Ma unconformity with a late&e-lignite horizon representing a maximum 200 k.y. hiatus in the lava succession. Lavas below the unconformity dip northwest towards the older axis from which they were erupted, whereas lavas above the unconformity dip southeast towards their source in the younger Snaefellsnes axis. Thus, two nearly complete rift relocation cycles are preserved in western Iceland, each lasting about 8 m.y. as measured between rift extinction events, and for around 12 m.y. from initial propagation to extinction. In this paper we present major-and trace-element analyses, Sr, Nd and Pb isotope data, and "'Ar/ 3gAr dates on basalt samples from above and below the unconformity in northwest Iceland. The Icelandic Tertiary and Quaternary plateau basalts are remarkably homogeneous in composition, in contrast to the much more diverse compositions found in the presently active rift zone. However, basaltic lava flows beneath the unconformity in northwest Iceland show a wider range of incompatible element and radiogenic isotope ratios than do the younger plateau basalts. At least two mantle components, one depleted and the other less depleted with respect to bulk Earth, are required to explain the composition of post-15 Ma Icelandic basalt. The depleted end-member is chemically and isotopically distinct from the N-MORB source. Basalt from the northwest palaeo-rift, however, contains a significant North Atlantic N-MORB component, suggesting that depleted upper mantle can influence the composition of Icelandic basalt in a dying rift that is too far from the plume centre to be dominated by plume mantle. This may account for the periods of low magma productivity represented by troughs between the V-shaped ridges on the Reykjanes Ridge. We suggest that temporal variation in the composition of Icelandic basalt is better explained by crustal accretion and rift relocation processes than by variations in plume composition and temperature. 0 1997 Elsevier Science B.V.
Earth and Planetary Science Letters, 1997
Basaltic lavas, forming thick offshore seaward-dipping reflector sequences (SDRS) and onshore ign... more Basaltic lavas, forming thick offshore seaward-dipping reflector sequences (SDRS) and onshore igneous provinces around the North Atlantic margins, represent melting of anomalously hot mantle in the head of the ancestral Iceland plume. Some of these lavas are chemically and isotopically indistinguishable from recent Icelandic basalt, but others more closely resemble basalt erupted at normal segments of mid-ocean ridges (N-MORB). In this paper we show that Icelandic basalt and N-MORB define parallel tight arrays on a plot of log(Nb/Y) against log(Zr/Y), with N-MORB relatively deficient in Nb. Deficiency or excess of Nb, relative to the lower bound of the Iceland array, may be expressed as ANb = 1.74 + log(Nb/Y)-1.92 log(Zr/Y) such that Icelandic basalt has ANb > 0 and N-MORB has ANb < 0. ANb is a fundamental source characteristic and is insensitive to the effects of variable degrees of mantle melting, source depletion through melt extraction, crustal contamination of the magmas, or subsequent alteration. We use new and published Nb, Zr and Y data to identify the mantle sources for Palaeocene and Eocene basaltic lavas erupted around the Atlantic margins in order to deduce the thermal and compositional structure of the head of the ancestral Iceland plume. The results show that the head of the plume was zoned, with an axial zone of Icelandic mantle surrounded by a thick outer shell of anomalously hot but compositionally normal N-MORB-source mantle. The zoning is very similar in scale and character to that seen today along the Reykjanes Ridge and is difficult to reconcile with the initiation of rifting and SDRS formation through the impact of a large plume head originating solely from the lower mantle. The thick outer shell of hot, depleted upper mantle, which formed more than half the volume of the plume head, suggests that at least part of the plume originated in the thermal boundary layer at the base of the upper mantle.
Journal of Volcanology and Geothermal Research, 2000
In October 1996 a subglacial fissure to the north of the Grimsvötn caldera in W-Vatnajökull produ... more In October 1996 a subglacial fissure to the north of the Grimsvötn caldera in W-Vatnajökull produced about 0.4 km 3 of Fe-rich basaltic andesite-icelandite-in an area characterized mostly by tholeiitic basalt. In this paper the chemical composition of volcanic systems in the region is discussed with the help of six new analyses and others from the literature, and a tentative model for their evolution is proposed, in which magma produced by the partial melting of a two-component mantle mixes with hydrous, silicic melt in the crust. The Vatnajökull 1996 magma belongs to a separate volcanic system, intermediate between Bardarbunga and Grimsvötn.
… North Iceland and its Geological History. …, 2003
The active volcanic zones in Iceland are characterized by high heat flow and extensive geothermal... more The active volcanic zones in Iceland are characterized by high heat flow and extensive geothermal activity. Iceland is unique in its location astride the diverging Mid-Atlantic Ridge and, furthermore, on top of a mantle plume. These two dynamic systems combine fundamental factors that promote magmatism, tectonics and geothermal activity. The high-temperature geothermal areas are mainly confined to volcanic systems, in particular central volcanoes, and are subject to strong tectonic control. The Hengill central volcano hosts one of the most powerful geothermal fields in Iceland, dominated by NE-SW striking faults. It is located at the western flank of the West Iceland Volcanic Zone (WVZ), which represents the Mid-Atlantic Ridge on land forming a graben-like structure approximately 10-15 km wide and 100 km long. The Hengill low resistivity structure covers about 112 km 2 and presently Reykjavik Energy operates two power plants in the area with installed capacity of about 420 MWe and 4...
… North Iceland and its Geological History. …, 2003
ABSTRACT
The East African Rift System (EARS) is a classic example of continental rifting and provides an e... more The East African Rift System (EARS) is a classic example of continental rifting and provides an excellent framework to study extensional magmatism and the evolution of several central volcanic systems that have formed along the rift from the Tertiary to Recent. Many of the volcanic structures have developed substantial high-temperature geothermal systems where the heat source is magmatic and related to central volcanoes. Detailed studies indicate that the geothermal potential in Eastern Africa is in the excess of 15,000 MWe. However, despite the high geothermal potential of EARS only Kenya has installed significant capacity of about 570 MW. Magmatism along the EARS is generally believed to be associated with mantle plume activities but the number and nature of mantle plumes is still, however, controversial. EARS is divided into two main branches, the Easternand Western rifts, and it is well documented that significantly greater volcanism is observed in the older Eastern rift (i.e. E...
Proceedings of the Ocean Drilling Program, 1998
The southeast Greenland seaward-dipping reflector sequence (SDRS) is composed of Paleocene to Eoc... more The southeast Greenland seaward-dipping reflector sequence (SDRS) is composed of Paleocene to Eocene volcanic rocks erupted during continental breakup. Volcanic rocks recovered from a transect across the SDRS at 63°N, during Ocean Drilling Program Leg 152, range in composition from picrite to dacite and represent all the magmatic phases in the development of the continental margin. The earliest magmas, represented by the pre-breakup succession at Site 917 (Lower and Middle Series), were strongly contaminated with continental crust, but the degree of contamination declined rapidly during the late stages of breakup (Site 917 Upper Series). Very low concentrations of incompatible elements in the uncontaminated primitive magmas made them extremely sensitive to the isotopic effects of crustal contamination. Basaltic rocks from the most seaward part of the transect (Site 918) were erupted after breakup and show no signs of contamination with continental crust. Two distinct crustal contaminants can be recognized: (1) lower crustal basic granulite with unradiogenic Nd, Sr, and Pb; and (2) upper crustal amphibolite-facies gneiss with unradiogenic Nd but highly radiogenic Sr and high 208 Pb/ 204 Pb. The first contaminant affected only the earliest magmas, represented by the lower volcanic units in the Lower Series at Site 917. Later continental magmas were affected by the second contaminant, suggesting storage of magmas at progressively shallower levels in the crust as lithospheric extension proceeded toward continental breakup. The nature and degree of contamination are strikingly similar to those observed in the Hebridean Tertiary igneous province, which would have been adjacent to southeast Greenland during continental breakup.
Proceedings of the Ocean Drilling Program, 1998
Samples were cut into small pieces with a diamond-tipped saw, and any obvious amygdales and veins... more Samples were cut into small pieces with a diamond-tipped saw, and any obvious amygdales and veins were removed. Saw marks were ground away on a diamond wheel, and the fragments were rinsed in clean water and dried before being ground to a fine powder in an agate Tema barrel. Agate was used in preference to tungsten carbide to avoid contamination with Ta and Co. The powders were analyzed in Edinburgh by X-ray fluorescence (XRF) spectrometry, and in Leicester by neutron activation analysis (NAA). A separate suite of volcanic rock samples was prepared and analyzed by XRF at the Geological Survey of Greenland for major elements, and at the University of Copenhagen for trace elements. Agreement between the Edinburgh and Copenhagen XRF data is generally very good
Diachronous V-shaped ridges (VSRs) in basement topography straddling the Reykjanes Ridge south of... more Diachronous V-shaped ridges (VSRs) in basement topography straddling the Reykjanes Ridge south of Iceland, expressed most clearly in the gravity field, reflect ±10% variation in crustal thickness. They appear to be produced by pulses of excess magma productivity propagating from Iceland at ~200 km/Ma with a periodicity of ~5 Ma. Three possible explanations for these crustal thickness variations are: (1) pulses in mantle temperature of ~20°C; (2) pulses of excess fertility; or (3) the regulation of mantle flow by rift relocations in Iceland. If the VSRs are due to pulses in mantle temperature and/or fertility, then this ought to be recorded in the Tertiary lava successions in east and west Iceland. Rift relocation, on the other hand, should leave no such record. Variations in mantle temperature of ~20°C will cause variations in degree of mantle melting and this should affect ratios of trace elements (e.g. Zr/Y) that are sensitive to degree of melting of garnet lherzolite. Icelandic b...
Lithos, 1996
Unequivocal examples of oceanic plateaux older than about 2.1 Ga have yet to he identified from t... more Unequivocal examples of oceanic plateaux older than about 2.1 Ga have yet to he identified from the geological record. The most likely explanation for this is the partial dismemberment of ancient plateaux at convergent plate margins, and their subsequent juxtaposition in accretionary complexes with arc volcanic, sedimentary and plutonic rocks. A review of Cretaceous plateaux in the eastern Indian Ocean suggests that certain geochemical-isotopic criteria (e.g. positive Nb anomalies relative to primitive mantle) employed by petrologists to identify modem oceanic plateau lavas are of little practical use in distinguishing ancient plateau lavas from arc and volcanic rifted margin basal& The extreme chemical and isotopic heterogeneity of Indian Ocean plateaux, together with large variations in area and volume, suggest that these edifices represent one extreme of a spectrum of oceanic plateau compositions. The other extreme, represented by Iceland and Ontong Java, is characterised by relatively homogeneous melt compositions, reflecting steady-state melting conditions and the absence of a continental lithosphere contaminant to plateau magmas. Crustal thickness and compositional estimates for Archaean oceanic plateaux using the McKenzie-Bickle method give a mean thickness of-43 km, and mean MgO content of-19 wt.%. Archaean plateaux were thus compositionally distinct from surrounding ocean floor (basaltic komatiite?), and notably more magnesian than plateaux such as Iceland (mean for Tertiary basalts-6.3 wt.% MgO) and Ontong Java (mean for core samples-6.9 wt.% MgO). By analogy with Cretaceous-Tertiary ultramafic complexes in western Colombia, the deeper portions of these plateaux may have consisted of noritic rocks, underlain by lherzolite, pyroxenite, gabbronorite and dunite. The general absence of such rocks in Archaean terrains suggests that these portions of plateaux are only rarely preserved. The search for remnants of ancient oceanic plateaux has identified examples of Archaean submarine lavas in the Canadian Shield (2 2.7 Ga: southern Abitibi belt, Superior Province) and Kaapvaal craton of southern Africa (3.5-3.3 Ga: Barberton and Pietersberg belts). Chemical and isotopic data suggest that komatiite-tholeiite units from these areas were derived by partial melting of a plume or asthenospheric mantle source, but do not distinguish adequately the tectonic environment (rifted continental margin, ocean floor, oceanic plateau) in which these rocks were erupted. However, the uplift histories of these putative plateau fragments, as recorded in sediments bounding and intercalated with the komatiite-basalt sequences, appear to be broadly comparable to those of the best-documented Phanerozoic oceanic plateaux. Although further stratigraphical investigations of presumed plateaux (ancient and modem) are required to test this hypothesis, we believe that the sedimentary record offers the best evidence yet for the formation of oceanic plateaux as far back as the Mid-Archaean (-3.5 Gal.
Journal of Petrology, 2000
Earth and Planetary Science Letters, 2000
... a NERC Isotope Geosciences Laboratory, Kingsley Dunham Centre, Keyworth NG12 5GG, UK. b Grant... more ... a NERC Isotope Geosciences Laboratory, Kingsley Dunham Centre, Keyworth NG12 5GG, UK. b Grant Institute, University of Edinburgh, Edinburgh EH9 3JW, UK. c Department of Geology, Leicester University, Leicester LE1 7RH, UK. ...
for their enthusiasm, encouragement, numerous valuable discussions, input, critical comments and ... more for their enthusiasm, encouragement, numerous valuable discussions, input, critical comments and guidance during the study period. Special thanks also go to Hjalti Franzson for his helpful suggestions and discussion on various aspects of this work. I would like to thank the entire UNU-GTP and ÍSOR GeoSurvey staff, special thanks go to the Director,
Menengai is a trachytic central caldera volcano in the Kenya rift valley with abundant high-tempe... more Menengai is a trachytic central caldera volcano in the Kenya rift valley with abundant high-temperature geothermal activity. The field is currently in its initial stages of development for geothermal energy in Kenya following Olkaria and Eburru fields. Regional surface geology of Menengai is largely composed of late Quaternary volcanics. The building of a 200,000 year old trachyte shield volcano was followed by piecemeal subsidence through two paroxysmal eruptions 29,000 and 8,000 B.P to produce a caldera of about 84 km in size that has subsequently been largely filled by recent trachyte lavas. Twenty four exploration and production wells, some of them hotter than 390C, have been drilled in Menengai caldera by Geothermal Development Company (GDC). The aim of this study is to reveal the evolutionary history of the Menengai volcano and to describe its igneous lithostratigraphy and secondary mineralization in order to characterize hydrothermal processes within the field. Analytical met...
J Volcanol Geotherm Res, 2000
Nature, 1991
BASALTIC magmatism results when upwelling mantle crosses the peridotite solidus. The greater the ... more BASALTIC magmatism results when upwelling mantle crosses the peridotite solidus. The greater the overstep of the solidus, the greater the degree of mantle melting and the larger the volume of magma produced. In Iceland most of the active volcanism occurs along the central spreading axis, although some occurs in isolated off-axis volcanoes such as Snaefellsjökull. Because the mantle beneath Snaefellsjökull is not upwelling as vigorously as that beneath the spreading axis, it oversteps its solidus by a much smaller amount and consequently the degree of melting is less. The composition of the resulting magma will be much more sensitive to small perturbations in the amount of upwelling than will the mantle beneath the ridge axis. We show here that the reduction of pressure in the mantle caused by the unloading of ice was sufficient to affect magma composition. Unloading was accompanied by a clear shift towards less undersaturated magma compositions, which reflect a transient increase of about 0.5% in the degree of mantle melting, coupled with a decrease in depth of melting.
Geology, 1997
Basalt from the Tertiary lava pile in Iceland is much more homogeneous in composition than that e... more Basalt from the Tertiary lava pile in Iceland is much more homogeneous in composition than that erupted in the axial rift zone. Lavas highly depleted in incompatible elements are occasionally erupted in the latter but have not been reported from the Tertiary succession. This observation leads to a useful test of the Pálmason model of crustal accretion in which axial lava flows ultimately form the lower levels of the lava pile, and only flows emplaced off-axis remain accessible in the upper part of the crust. Axial flows large enough to spill out of the rift axis, and flows erupted on the rift flanks, should therefore have a uniform composition similar to that of the Tertiary flows. Here we test this prediction and thereby the validity of the Pálmason model of crustal accretion in oceanic spreading centers. Our data show that depleted basalt is confined to small-volume lava flows within the rift axis. Furthermore, large flows that originated within the axial zone and spread far enough to become preserved in the accessible part of the future offaxis lava pile are very homogeneous in composition and comparable to those forming the Tertiary lava successions.
Earth and Planetary Science Letters, 1997
A dominant process in the evolution of Iceland is the repeated eastward relocation of the spreadi... more A dominant process in the evolution of Iceland is the repeated eastward relocation of the spreading axis in response to westward migration of the plate boundary relative to the plume centre. Two major former rifts can be identified in western Iceland: the Snaefellsnes rift zone, which last erupted tholeiitic lavas at about 7 Ma, and an older spreading system, lava flows from which can be traced some 100 km along a SW-NE strike in the extreme northwest of Iceland. The extinction of the latter is marked by a 14.9 Ma unconformity with a late&e-lignite horizon representing a maximum 200 k.y. hiatus in the lava succession. Lavas below the unconformity dip northwest towards the older axis from which they were erupted, whereas lavas above the unconformity dip southeast towards their source in the younger Snaefellsnes axis. Thus, two nearly complete rift relocation cycles are preserved in western Iceland, each lasting about 8 m.y. as measured between rift extinction events, and for around 12 m.y. from initial propagation to extinction. In this paper we present major-and trace-element analyses, Sr, Nd and Pb isotope data, and "'Ar/ 3gAr dates on basalt samples from above and below the unconformity in northwest Iceland. The Icelandic Tertiary and Quaternary plateau basalts are remarkably homogeneous in composition, in contrast to the much more diverse compositions found in the presently active rift zone. However, basaltic lava flows beneath the unconformity in northwest Iceland show a wider range of incompatible element and radiogenic isotope ratios than do the younger plateau basalts. At least two mantle components, one depleted and the other less depleted with respect to bulk Earth, are required to explain the composition of post-15 Ma Icelandic basalt. The depleted end-member is chemically and isotopically distinct from the N-MORB source. Basalt from the northwest palaeo-rift, however, contains a significant North Atlantic N-MORB component, suggesting that depleted upper mantle can influence the composition of Icelandic basalt in a dying rift that is too far from the plume centre to be dominated by plume mantle. This may account for the periods of low magma productivity represented by troughs between the V-shaped ridges on the Reykjanes Ridge. We suggest that temporal variation in the composition of Icelandic basalt is better explained by crustal accretion and rift relocation processes than by variations in plume composition and temperature. 0 1997 Elsevier Science B.V.
Earth and Planetary Science Letters, 1997
Basaltic lavas, forming thick offshore seaward-dipping reflector sequences (SDRS) and onshore ign... more Basaltic lavas, forming thick offshore seaward-dipping reflector sequences (SDRS) and onshore igneous provinces around the North Atlantic margins, represent melting of anomalously hot mantle in the head of the ancestral Iceland plume. Some of these lavas are chemically and isotopically indistinguishable from recent Icelandic basalt, but others more closely resemble basalt erupted at normal segments of mid-ocean ridges (N-MORB). In this paper we show that Icelandic basalt and N-MORB define parallel tight arrays on a plot of log(Nb/Y) against log(Zr/Y), with N-MORB relatively deficient in Nb. Deficiency or excess of Nb, relative to the lower bound of the Iceland array, may be expressed as ANb = 1.74 + log(Nb/Y)-1.92 log(Zr/Y) such that Icelandic basalt has ANb > 0 and N-MORB has ANb < 0. ANb is a fundamental source characteristic and is insensitive to the effects of variable degrees of mantle melting, source depletion through melt extraction, crustal contamination of the magmas, or subsequent alteration. We use new and published Nb, Zr and Y data to identify the mantle sources for Palaeocene and Eocene basaltic lavas erupted around the Atlantic margins in order to deduce the thermal and compositional structure of the head of the ancestral Iceland plume. The results show that the head of the plume was zoned, with an axial zone of Icelandic mantle surrounded by a thick outer shell of anomalously hot but compositionally normal N-MORB-source mantle. The zoning is very similar in scale and character to that seen today along the Reykjanes Ridge and is difficult to reconcile with the initiation of rifting and SDRS formation through the impact of a large plume head originating solely from the lower mantle. The thick outer shell of hot, depleted upper mantle, which formed more than half the volume of the plume head, suggests that at least part of the plume originated in the thermal boundary layer at the base of the upper mantle.
Journal of Volcanology and Geothermal Research, 2000
In October 1996 a subglacial fissure to the north of the Grimsvötn caldera in W-Vatnajökull produ... more In October 1996 a subglacial fissure to the north of the Grimsvötn caldera in W-Vatnajökull produced about 0.4 km 3 of Fe-rich basaltic andesite-icelandite-in an area characterized mostly by tholeiitic basalt. In this paper the chemical composition of volcanic systems in the region is discussed with the help of six new analyses and others from the literature, and a tentative model for their evolution is proposed, in which magma produced by the partial melting of a two-component mantle mixes with hydrous, silicic melt in the crust. The Vatnajökull 1996 magma belongs to a separate volcanic system, intermediate between Bardarbunga and Grimsvötn.