Trends in Cretaceous and Tertiary geomagnetic reversals

The statistical theory of modulated renewal processes is used to analyze the polarity reversal scales of Larson & Hilde (1975) and LaBreque, Kent & Cande (1977). The results suggest that the trending effect in these data may be modelled by a rate parameter with an exponential quadratic trend. Short times in one polarity state tend to be followed by short times in the other state. The graphical analysis points to the possibility of an undulating pattern in reversal rates. The empirical distributions of the normal and reversed polarities show slight differences in comparison with each other in most of the statistical tests, but a moving-window analysis indicates possible serial effects for the normal times. As a rough approximation, a statistical two-state model for reversals might be realistic, for example, an alternating renewal process under relaxed assumptions. There has been a gradual stepping-up of the minimum reversal rate from the Oligocene to the present, but little change in the observed range of the reversal rates. With the long quiet (Mercanton) interval removed from the data, the average time spent in the reversed polarity state is slightly greater than for the normal state. A change in statistical properties for the entire set of data considered as a single sample occurs around the Eocene-Oligocene transition (Middle Eocene on earlier time scales). The analyses of the statistical second-order properties of the entire sequence of 271 observations, and the subsets of normal and reversed between-times, reject a renewal hypothesis if theoretical statistical considerations are strictly applied to the results (although this hypothesis is not rejected for the Oligocene to Recent observations); this result is at variance with some geophysical models. A short appendix on the theory of point processes is provided to aid the general reader in following the arguments used in this paper.     

Low δO in the Icelandic mantle and its origins: Evidence from Reykjanes Ridge and Icelandic lavas

Oxygen isotope ratios have been determined using laser fluorination techniques on olivine and plagioclase phenocrysts and bulk glasses from the Reykjanes Ridge and Iceland. δ¹⁸O in Reykjanes Ridge olivines shows hyperbolic correlations with Sr-Nd-Pb isotope ratios that terminate at δ¹⁸O = +4.5‰ at compositions almost identical to those of moderately enriched lavas on the Reykjanes Peninsula, Iceland. Samples with low δ¹⁸O show no indication of contamination by oceanic crust such as elevated Cl/K, and are too deep to have been influenced by meteoric water hydrothermal systems. They cannot represent Icelandic melts contaminated in the crust and transferred laterally along the ridge since fissure systems are strongly oblique to the ridge axis. It follows that Icelandic mantle advected along the ridge has low δ¹⁸O. The hyperbolic ¹⁴³Nd/¹⁴⁴Nd-δ¹⁸O correlation appears to be more strongly curved than magma mixing trajectories and suggests that melt fractions are ∼4.5× greater and source Nd contents ∼9× greater in the mantle at 63°N compared with that at 60°N. Primitive lavas from the Reykjanes Peninsula show linear correlations between olivine δ¹⁸O and ¹⁴³Nd/¹⁴⁴Nd or ²⁰⁶Pb/²⁰⁴Pb, extending to δ¹⁸O of +4.3‰ at ¹⁴³Nd/¹⁴⁴Nd close to the lowest ratios observed in Icelandic magmas. These correlations cannot be produced by melt mixing or crustal contamination because these would yield strongly hyperbolic trajectories. Lower δ¹⁸O seen in more evolved samples from the Eastern Rift Zone may reflect crustal contamination, though there is some evidence of a mantle source with lower δ¹⁸O in eastern Iceland. It is very difficult to explain the low δ¹⁸O of enriched Icelandic mantle sources on current understanding of mantle and crustal oxygen isotopes. There is no obvious reason why such low-δ¹⁸O sources should not contribute to other ocean islands. No oceanic crustal lithologies exist that could produce the low-δ¹⁸O enriched sources by recycling into the mantle, and there is no evidence for changes in δ¹⁸O of ophiolite suites with time, nor of changes during high-P metamorphism. Low δ¹⁸O appears to be associated with high ³He/⁴He, and we speculate that this signature may be characteristic of the host mantle into which ocean crust was recycled.     

Low-latitude and multiple geomagnetic reversals in the Neoproterozoic Puga cap carbonate, Amazon craton

Palaeomagnetic study of the carbonates that ubiquitously cap glacial deposits may constrain the latitudinal extent of Neoproterozoic glaciations and the duration of the greenhouse recovery. We present the first palaeomagnetic data on the Neoproterozoic cap carbonates covering the Amazon craton, which are folded along the Paraguay Belt. Samples collected at deformed beds along the Paraguay Belt present a single‐polarity secondary magnetization acquired by the end of the Brasiliano orogeny (540–520 Ma). In the cratonic area, a dual‐polarity component was isolated in dolostones at the base of the sequence. The presence of a stratabound reversal stratigraphy along with high unblocking temperatures strongly suggest that this magnetization is primary. This result implies a low palaeolatitude (22+6/?5°) for the Amazon block just after deposition of Puga diamictites. In addition, the presence of multiple reversals across the first 20 m of the cap carbonate sequence suggests that their sedimentation must have spanned hundreds of thousands of years at least.     

Petrogenesis of basalt–trachyte lavas from Olmoti Crater, Tanzania

Olmoti Crater is part of the Plio-Pleistocene Ngorongoro Volcanic Highland (NVH) in northern Tanzania to the south of Gregory Rift. The Gregory Rift is part of the eastern branch of the East African Rift System (EARS) that stretches some 4000 km from the Read Sea and Gulf of Aden in the north to the Zambezi River in Mozambique. Here, we (1) characterize the chemistry and mineral compositions of lavas from Olmoti Crater, (2) determine the age and duration of Olmoti volcanic activity through ⁴⁰Ar/³⁹Ar dating of Olmoti Crater wall lavas and (3) determine the genesis of Olmoti lavas and the relationship to other NVH and EARS volcanics and (4) their correlation with volcanics in the Olduvai and Laetoli stratigraphic sequences.Olmoti lavas collected from the lower part of the exposed crater wall section (OLS) range from basalt to trachyandesite whereas the upper part of the section (OUS) is trachytic. Petrography and major and trace element data reflect a very low degree partial melt origin for the Olmoti lavas, presumably of peridotite, followed by extensive fractionation. The ⁸⁷Sr/⁸⁶Sr data overlap whereas Nd and Pb isotope data are distinct between OLS and OUS samples. Interpretation of the isotope data suggests mixing of enriched mantle (EM I) with high-μ-like reservoirs, consistent with the model of Bell and Blenkinsop [Bell, K., Blenkinsop, J., 1987. Nd and Sr isotopic compositions of East African carbonatites: implications for mantle heterogeneity. Geology 5, 99–102] for East African carbonatite lavas. The isotope ratios are within the range of values defined by Oceanic Island Basalt (OIB) globally and moderate normalized Tb/Yb ratios (2.3–1.6) in these lavas suggest melting in the lithospheric mantle consistent with other studies in the region.⁴⁰Ar/³⁹Ar incremental-heating analyses of matrix and anorthoclase separates from Olmoti OLS and OUS lavas indicate that volcanic activity was short in duration, lasting 200 kyr from 2.01 ± 0.03 Ma to 1.80 ± 0.01 Ma. The age of Olmoti activity overlaps with ages reported for Ngorongoro Caldera, implying contemporaneous activity of multiple NVH volcanic centers during part of the eruption interval.Olmoti is considered the source for the bulk of interbedded volcanics and volcaniclastic deposits that comprise much of the upper Bed I section of nearby Olduvai Gorge, and part of the Laetoli sequence, both known for their well preserved fossils and archaeological remains. Age and chemical data reported here are compatible with those derived from tephra and lava interbedded in Bed I at Olduvai Gorge and from the Olpiro Beds at Laetoli.     

Pangea, the geoid, and the paths of virtual geomagnetic poles during polarity reversals

The elongated distribution of virtual geomagnetic poles (VGPs) of South American Jurassic units has been interpreted as reflecting: 1) the behavior of the Earth's magnetic field (EMF), and 2) apparent polar shift of the South American plate before the breakup of Gondwana. New paleomagnetic data do not support the latter. We analyze these Jurassic VGPs together with those of other Pangean continents, considering a model based on the following assumptions: 1) Pangea was assembled over the present African-Atlantic geoid high; 2) the hotspot framework has not been significantly deformed over the last 200 Ma; 3) the present geoid matches the present topography of the core mantle boundary (CMB); 4) the overall topography of the CMB remains unchanged for long periods of time. We plotted Jurassic VGPs of Pangean continents, previously returned to a fixed-hotspot framework, on a map of the present topography of the CMB. The VGPs seem to follow the trend of regions of increased CMB seismic velocities. When a polarity reversal was registered by a Jurassic unit, the path of the VGPs was channeled on zones that have been observed in translational paths for the last 10 Ma.     

Quasi-periodic geomagnetic secular variation (from 1985–2005 world observatory data)

We have discovered a 2–4 year periodicity in geomagnetic secular variation (SV) from data of 110 world magnetic observatories. The periodicity in the horizontal component (H) is most prominent and appears to be globally uniform in different regions, on all continents, and in both hemispheres. The quasi-periodic short-wavelength variations show up in the vertical component (Z) as well but locally superpose on long-wavelength regional anomalies. We presume that the short-period fluctuations may be produced by instability of the eccentric dipole (ED) axis proceeding from the analysis of the SV field and optimization modeling of the dipole field with varied ED parameters.     

New insights into the origin of O–Hf–Os isotope signatures in arc lavas from Tonga–Kermadec

O, Hf and Os isotope data are presented for lavas from the highly depleted Tonga–Kermadec arc. O isotope values overlap with those of MORB limiting the amount of interaction with the arc crust. δ¹⁸O does not increase northwards as would be expected from the ~ 4 fold increase in subduction rate if slab-derived fluids had high ¹⁸O/¹⁶O ratios. Thus, the overall northward decrease in HFSE concentrations likely reflects depletion due to prior melt extraction, not increasing extents of melting. Hf isotopes are strongly negatively correlated with Be isotopes consistent with mixing of subducted pelagic sediment into the mantle wedge and do not require Hf to be fluid mobile. With the exception of a boninite from the north Tongan trench, the northern Tonga lavas do not overlap the Hf isotope composition of either the Samoan plume or the subducting Louisville volcaniclastic sediments. Thus, the Pb isotope signatures in these lavas must have been added by fluids and sediment melts derived from the Louisville volcaniclastics with minimal mobilisation of Hf. This suggests conservative behaviour for this element due to the formation of residual zircon during partial melting of the subducted sediments. ¹⁸⁷Os/¹⁸⁸Os ranges from 0.1275 to 0.4731 and the higher Os isotope ratios reflect the sensitivity of this system to even minor interaction with altered arc crust. Conversely, the lowest Os ratios are subchondritic and indicate that transfer of radiogenic Os from the slab is not all pervasive and provide an important constraint on the composition of the mantle wedge. Remarkably, the least radiogenic sample is a dacite demonstrating that evolved magmas can develop by fractionation from mantle-derived magmas with minimal interaction with the arc crust.     

ULF geomagnetic anomalies of possible seismogenic origin observed at Teoloyucan station, México, in 1999–2001: Intermediate and short-time analysis

The analysis of ULF geomagnetic field measured at Teoloyucan station (Central Mexico, 11′35.735″ W, 19 44′45.100″ N, 2280 m height) is presented in an intermediate (± 15 days) and short time scale (the day of the EQ occurrence) in relation to 7 major earthquakes occurred in Mexico in 1999–2001. Local changes in the fractal dynamics of the magnetic field are found to be important: a pronounced fall of the fractal index is frequently observed prior to the main shock. The study of the ULF resonant structure recently discovered in the frequencies f_R1 = 10.2−11.1 mHz and f_R2 = 13.6−14.5 mHz reveals changes in their character probably related to the processes of the earthquakes preparation. The success of the observation of the mentioned anomalies (specially the fractal index decrease) strongly depends on how close is the station from the epicenter, and what is the magnitude of the earthquake.     

The stability of sapphirine + quartz: calculated phase equilibria in FeO–MgO–Al2O3–SiO2–TiO2–O

The presence in rocks of coexisting sapphirine + quartz has been widely used to diagnose conditions of ultra‐high‐temperature (UHT) metamorphism (>900 °C), an inference based on the restriction of this assemblage to temperatures >980 °C in the conventionally considered FeO–MgO–Al₂O₃–SiO₂ (FMAS) chemical system. With a new thermodynamic model for sapphirine that includes Fe₂O₃, phase equilibra modelling using thermocalc software has been undertaken in the FeO–MgO–Al₂O₃–SiO₂–O (FMASO) and FeO–MgO–Al₂O₃–SiO₂– TiO₂–O (FMASTO) chemical systems. Using a variety of calculated phase diagrams for quartz‐saturated systems, the effects of Fe₂O₃ and TiO₂ on FMAS phase relations are shown to be considerable. Importantly, the stability field of sapphirine + quartz assemblages extends down temperature to 850 °C in oxidized systems and thus out of the UHT range.     

‘Now that the dust has settled…’ the impacts of Icelandic volcanic eruptions

Volcanic eruptions generate hazards, are potent agents of landscape change and have the power to alter global climate. Recent events in Iceland have emphasised the multi‐scale and trans‐boundary nature of hazards from ice‐volcano interactions and have highlighted their local, national and international impacts. Prompted by these recent events, this article reviews a selection of Icelandic volcanic eruptions in order to demonstrate the diversity of hazards and impacts generated by Icelandic volcanic activity. Some of the challenges associated with managing risks from Icelandic volcanic hazards are discussed and future prospects are outlined.     

An enlarged and updated internally consistent thermodynamic dataset with uncertainties and correlations: the system K2O–Na2O–CaO–MgO–MnO–FeO–Fe2O3–Al2O3–TiO2–SiO2–C–H2–O2

We present, as a progress report, a revised and much enlarged version of the thermodynamic dataset given earlier (Holland & Powell, 1985). This new set includes data for 123 mineral and fluid end-members made consistent with over 200 P–T–X_CO2–f_O2 phase equilibrium experiments. Several improvements and advances have been made, in addition to the increased coverage of mineral phases: the data are now presented in three groups ranked according to reliability; a large number of iron-bearing phases has been included through experimental and, in some cases, natural Fe:Mg partitioning data; H₂O and CO₂ contents of cordierites are accounted for with the solution model of Kurepin (1985); simple Landau theory is used to model lambda anomalies in heat capacity and the Al/Si order–disorder behaviour in some silicates, and Tschermak-substituted end-members have been derived for iron and magnesium end-members of chlorite, talc, muscovite, biotite, pyroxene and amphibole. For the subset of data which overlap those of Berman (1988), it is encouraging to find both (1) very substantial agreement between the two sets of thermodynamic data and (2) that the two sets reproduce the phase equilibrium experimental brackets to a very similar degree of accuracy. The main differences in the two datasets involve size (123 as compared to 67 end-members), the methods used in data reduction (least squares as compared to linear programming), and the provision for estimation of uncertainties with this dataset. For calculations on mineral assemblages in rocks, we aim to maximize the information available from the dataset, by combining the equilibria from all the reactions which can be written between the end-members in the minerals. For phase diagram calculations, we calculate the compositions of complex solid solutions (together with P and T) involved in invariant, univariant and divariant assemblages. Moreover we strongly believe in attempting to assess the probable uncertainties in calculated equilibria and hence provide a framework for performing simple error propagation in all calculations in thermocalc, the computer program we offer for an effective use of the dataset and the calculation methods we advocate.     

Revisiting problem of chilled margins associated with marginal reversals in mafic–ultramafic intrusive bodies

Numerous observations on mafic–ultramafic layered intrusions, sills and dykes show that chilled margins always develop as an integral part of their marginal reversals and possess the following features: (a) they are commonly much more evolved or primitive than bulk intrusion compositions, (b) evolved chilled margins are composed of the low temperature cotectic assemblages of relevant magmatic systems and (c) tend to be compositionally similar in intrusions formed from different parental magmas, (d) fine-grained chilled margins are notably absent in many intrusions, with contact rocks being represented by medium- to coarse-grained cumulates. The anomalous features of chilled margins can be partly attributed to contamination, intratelluric inhomogeneity of magma, changes in composition of intruding magma, loss of magma from the chamber, supercooling, etc. A major process still remains, however, illusive, but appears to be universally operating along the cooling margins of magmatic bodies in a liquid state, being gravity-independent and temperature gradient-driven. We recognize this not yet specified process as Soret fractionation and explain the above observations in the following way. Primary chilled margins do not commonly survive because of intensive remelting by heat flux from the interior of the chamber. The subsequently formed “secondary chilled margins” represent cumulates that crystallized from liquids produced by temperature gradient-driven Soret fractionation. At high temperature gradients the process tends to produce similar cotectic liquids crystallizing gabbronorite (or gabbro) from all parental magmas of a given magmatic system, resulting in compositionally similar “secondary chilled margins” that are more evolved than bulk compositions. At low temperature gradients the process produces liquids that are only slightly more fractionated than the parental magma and form “secondary chilled margins” that are more primitive than bulk compositions. This interpretation suggests that, apart from the rare cases of chilled margins that survived remelting, they should not be used as monitors for parental magma compositions of intrusive bodies, even if all conventional complicating factors were not operative.     

Calculated phase equilibria for a morb composition in a P–T range, 450–650 °C and 18–28 kbar: the stability of eclogite

Lower temperature eclogite (with T = 600 °C) represents a significant part of the occurrences of eclogite in orogenic belts. ‘True’ eclogite, with, for example, garnet + omphacite >70%, is well represented in such an occurrence. Calculated phase equilibria in Na₂O–CaO–K₂O–FeO–MgO–Al₂O₃–SiO₂–H₂O–TiO₂–O (NCKFMASHTO), for just one rock composition – that of a representative mid‐ocean ridge basalt, morb – are used to see under what circumstances ‘true’ eclogite is predicted to occur. The variables considered are not only pressure (P) and temperature (T) but also water content and oxidation state. The latter two variables are known to exert a significant control on mineral assemblage but are difficult to establish retrospectively from the observed rocks themselves. It is found that whereas oxidation state does have a strong effect on mineral assemblage, the key control on developing ‘true’ eclogite is shown to be temperature and water content. If temperature is established to be <600 °C, water content has to be low (less or much less than that for H₂O saturation) in order for ‘true’ eclogite to form. Moreover, unless pressure is at the high end in the range considered, lawsonite eclogite and ‘true’ eclogite will tend to be mutually exclusive, with the former requiring high water content at the lower temperature where it occurs, but the latter requiring low water content.     

Radon –in Northamptonshire?

With the new BGS Radon Potential Map of Great Britain published, we focus on Northamptonshire, where reports in 1988 of finding the radioactive gas in houses sent shock waves through the local community – and aroused the interest of Leicester University geologists. With the help of local authorities, research has established the main source as the Northampton Sand Formation, at the base of which is a layer of phosphatic pebbles that are relatively rich in uranium. From more than 3000 field measurements over all rocks, a geologically defined Radon Hazard Map of the county has been compiled.     

A new thermodynamic model for sapphirine: calculated phase equilibria in K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2–Fe2O3

The activity–composition (a–x) relations of sapphirine are re‐evaluated in the light of a recent new internally‐consistent data set of phase end‐members for use in phase equilibria modelling, particularly of ultra‐high‐temperature (UHT) rocks. This is achieved with the aid of relatively oxidized sapphirine+quartz‐bearing granulites from Wilson Lake, Canada. Calculated P–T projections and compatibility diagrams in the K₂O–FeO–MgO–Al₂O₃–SiO₂–H₂O–TiO₂–Fe₂O₃ (KFMASHTO) system are used to illustrate sapphirine+quartz‐bearing phase equilibria in the context of UHT metamorphism. These new a–x relations for sapphirine should allow pseudosection thermobarometry in NCKFMASHTO for estimating peak P–T conditions of sapphirine‐bearing rocks.