high temperature alteration and porosity generation in upper ordovician microbial reefs hudson bay intracratonic basin arctic canada abstract upper ordovician biohermal mounds of the red head rapids river formation in the hudson bay basin consist of microbial and sponge boundstone and cementstone the cementstone is made up of isopachous layers and botryoids of former aragonite now calcite secondary dissolution porosity and small fractures are cutting through the bioherm secondary pore fillings consist of drusy calcite cement and subsequent bitumen the 18ovpdb and 13cvpdb values of late cements are invariably more negative than those of the marine cements fluid inclusion microthermometry documents the presence of two distinct populations of homogenization temperatures one in secondary pore filling burial calcite cement th average 93 10 c and a second one in recrystallized marine calcite cements th average 118 25 c the combined 18ovpdb and th data suggest that burial cements precipitated from a fluid having 18osmow values between 0 5 and 2 whereas the marine cement data indicate resetting of the fluid inclusions in the presence of a high temperature 18osmow heavy brine 2 5 to 12 the lower temperature fluid inclusions of the late pore filling cement agree with recent apatite fission track data suggesting early oil window temperature at maximum burial the higher th values recorded in the marine cement represent resetting of initial or entrapment of new fluid inclusions from fracture controlled circulation of basement derived fluids the petrographic and geochemical data suggest that fracture controlled high temperature brine circulation occurred after the inception of burial and recrystallization of original marine aragonite to calcite which resulted in the generation of significant secondary porosity that was later filled by lower temperature burial cements and hydrocarbons

the use of palaeomagnetism and rock magnetism to understand volcanic processes introduction this special publication provides a snapshot of our understanding of volcanic processes through the use of palaeomagnetic and rock magnetic techniques here we provide a context for the book placing individual chapters within the milieu of previous work including some magnetic techniques that were not used in the particular studies described herein thermoremanent magnetization is a powerful tool to understand processes related to heating and cooling of rocks including estimating the temperature of emplacement of pyroclastic deposits which may allow us to better understand the rates of cooling during eruption and transport anisotropy of magnetic susceptibility and anisotropy of remanence are used primarily to investigate rock fabrics and allow the interpretation of flow dynamics in dykes lava flows and pyroclastic deposits as well as the location of the eruptive vents rock magnetic characteristics can help in the correlation of volcanic deposits but also provide means to date volcanic deposits and to better understand the processes of cooling of the deposits as the magnetic minerals can change with temperature in addition volcanic rocks may be key recorders of past magnetic fields allowing a better understanding of changes in field intensity and perhaps providing clues of how the magnetic field is formed over the past several decades palaeomagnetic and rock magnetic techniques have been applied to numerous problems in volcanology since aramaki akimoto s pioneering study in 1957 which used natural remanent magnetization nrm data to qualitatively estimate the temperature of emplacement of selected pyroclastic deposits in japan aramaki akimoto 1957 many studies have utilized and refined the approach of using progressive thermal demagnetization data to estimate emplacement temperatures of pyroclastic deposits and have demonstrated the utility of this approach in many settings another extensively utilized technique is that of anisotropy of magnetic susceptibility ams which was first applied to pyroclastic volcanic rocks by ellwood 1982 on ignimbrites in the san juan mountains sw colorado usa to estimate flow fabrics and in that study to infer source locations through triangulation it is staggering to think of the number and diversity of ams studies on volcanic rocks that have been undertaken since that time in a bit more than three decades remanent magnetization characteristics have long been used to correlate volcanic rocks and such data together with rock magnetic properties have been employed to study crystallization and cooling processes e g schlinger et al 1988 wells hillhouse 1989 as techniques have been refined and the sensitivity precision reliability and or speed of measurements for an array of modern instrumentation all have improved workers have been able to study volcanic problems in more detail pulling out details of the processes that have not previously been considered in addition the types of problems addressed have diversified considerably we intend this special publication to provide a representative suite of contributions that help to summarize the current array of studies that apply palaeomagnetism and rock magnetism to volcanic processes and to stimulate further research in this exciting science this book is a result of two sessions on volcanic processes and palaeomagnetism held at the international union of geodesy and geophysics congress in melbourne australia in july 2011 and at the american geophysical union annual meeting in san francisco usa in december 2011 from ort m h porreca m geissman j w eds 2015 the use of palaeomagnetism and rock magnetism to understand volcanic processes geological society london special publications 396 1 11 first published online january 20 2015 http dx doi org 10 1144 sp396 17 2015 the geological society of london for permissions http www geolsoc org uk permissions publishing disclaimer www geolsoc org uk pub ethics angharad hills at the geological society of london graciously and tactfully guided this book and the editors through the entire process from chapter solicitation to publication thermoremanent magnetization in volcanic rocks and emplacement temperature

an oscillating trend in phanerozoic non skeletal carbonate mineralogy textural and compositional data on ancient ooids and cements suggest the existence of an oscillating phanerozoic temporal trend in non skeletal carbonate mineralogy the trend shows good correlation with eustatic climatic oscillations proposed by others the most likely mechanism appears to be plate tectonically influenced oscillations in pco2 the vapour pressure of co2 rather than change in oceanic mg ca ratio

anatomy of an extinct magmatic system along a divergent plate boundary alftafjordur iceland recent rifting episodes highlight the role of magmatic systems with propagating dikes on crustal spreading however our knowledge of magmatic systems is usually limited to surface observations and geophysical data eastern iceland allows direct access to extinct and eroded deeper magmatic systems here we collected field structural and ams anisotropy of magnetic susceptibility data on 187 and 19 dikes respectively in the 10 12 ma old alftafjordur magmatic system at a paleodepth of 1 5 km the extension due to diking is at least 1 2 orders of magnitude larger than that induced by regional tectonics confirming magmatism as the key mechanism for crustal spreading this magma induced extension inferred from the aspect ratio of the magmatic system was of 8 mm yr lower than the present one ams data suggest that most of dikes have geometrically normal fabric at least at the margins consistent with prevalent subvertical magma flow and propagation

aragonite calcite seas quantifying the gray area oscillations between the dominance of aragonite and calcite in abiotic marine caco3 precipitates throughout earth history are closely coupled with the evolution of earth s seawater composition and represent the environmental context in which organisms evolved their ability to biomineralize the most important factor controlling these phanerozoic oscillations in caco3 polymorph composition is the ratio of mg ca in seawater which is thought to separate aragonite and calcite precipitation along a distinct temperature controlled threshold a sharp threshold however is contradicted by overlapping aragonite and calcite precipitation fields at a range of experimental conditions here we present experimental data that enable us to quantify the proportions of caco3 polymorphs as a function of mg ca ratio and temperature this allows us to convert published mg ca ratio proxy data and models of the phanerozoic mg ca ratio into proportions of abiotic caco3 polymorphs at a given temperature and thus provides a temperature corrected view of aragonite calcite sea conditions in this revised view abiotic calcite precipitation was inhibited during aragonite sea intervals at temperatures above 20 c whereas calcite sea intervals were characterized by the co precipitation of aragonite and calcite in environments above 20 c this continuous prominence of aragonite precipitation in phanerozoic warm water environments explains the phanerozoic increase of aragonite over calcite skeletal composition in calcifying organisms

anisotropy of full and partial anhysteretic remanence across different rock types 1 are partial anhysteretic remanence anisotropy tensors additive

magma flow directions in dikes of the koolau complex oahu determined from magnetic fabric studies we compared the magnetic fabric with the macroscopic surface lineation direction previously assumed to be parallel with magma flow direction for 35 dikes of the koolau complex oahu anisotropy of magnetic susceptibility ams was determined for 530 samples from 59 dikes including a sill and a pluglike intrusive body and two filled lava tubes and statistically significant ams clusters were found in all but five dikes for the 35 lineated dikes the mean maximum ams direction 1 coincides with the macroscopic lineations to within 25 in 18 and to within 15 in eight the ams ellipsoid shape in about half of the samples is prolate cigar shaped and in about half is oblate pancake shaped we interpret the ams to be determined by shape anisotropy of ferromagnetic grains the mean total anisotropy is 2 2 and total anisotropy h ranges from 0 24 to as high as 14 12 the principal ams axes for samples from a dike are tightly grouped on equal area plots and for most of the dikes the minimum ams axes plot near to the pole of the dike our ams measurements enable us to infer absolute magma flow directions for half the dikes studied the maximum ams axes of samples from one side of the dike form a tight cluster on an equal area plot and those from the other side of the dike form another tight cluster the two clusters are close together separated by 10 30 for narrow dikes 100 cm wide and are farther apart 30 60 for wide dikes and are symmetrically disposed on either side of the plane of the dike we infer that these paired clusters are caused by imbrication of nonequant ferromagnetic crystals against each margin of the dike during deposition in a velocity gradient and hence indicate the flow azimuth i e the absolute magma flow direction nearly half of the dikes measured 25 gave paired ams plots in 12 the magma flowed up at low oblique angles 5 30 from the horizontal in five it flowed down at 5 20 in four it flowed horizontally 5 in three it flowed up at a steep angle 50 75 and in one it flowed down at a steep angle the magma flow in general was rarely vertical or horizontal

magnetic study of a late alpine dike crosscutting the regional foliation abstract dikes crosscutting existing folds or foliation are one of the classic time markers used in establishing relative geological chronology we examine the paleomagnetic directions and magnetic fabric in a deformed gneiss from the southern steep belt which lies immediately north of the insubric fault and in an aplite dike that transects this foliation no clear foliation or other evidence of internal deformation is discernible in the aplite dike in the field and it was originally interpreted as post tectonic with its intrusion age placing a young limit on the timing of deformation in the adjacent gneiss the aplite dike does not carry a stable remanent magnetization and cannot be used to resolve the age of intrusion the characteristic paleomagnetic remanence in the gneiss is compatible with eurasian paleomagnetic directions during the last 20 ma these results suggest that the timing of magnetization postdates dextral motion along the insubric and rhine rhone faults the low field anisotropy of magnetic susceptibility ams in the gneiss is controlled by the regional schistosity high field ams suggests that both paramagnetic and ferromagnetic minerals contribute to the magnetic fabric whereby the ferromagnetic minerals show a stronger lineation this is further confirmed by the anisotropy of anhysteretic remanent magnetization aarm rock magnetic experiments show that multi domain magnetite is the main ferromagnetic mineral in the gneiss unexpectedly the ams results from the aplite dike show that it also has a magnetic fabric which is shown by high field ams aarm and rock magnetic experiments to be carried by superparamagnetic magnetite the presence of magnetic fabric in the dike demonstrates that regional deformation was still on going in the southern steep belt of the central alps at the time of its intrusion

magma flow within dykes in submarine hyaloclastite environments an ams study of the miocene cabo de gata volcanic units this research has been funded by projects cgl2005 03511 bte hi2006 0073 prin2009 prin 2009h37m59 and prin 2010 11 2010tt22sc and 2010tt22sc 003

rock eval tmax suppression documentation and amelioration the rock eval thermal maturation parameter tmax may occasionally be suppressed relative to surrounding rock units this is commonly assumed to result from either a bitumen contribution to the s2 peak or the presence of sulfur rich and hence thermally labile kerogen the phenomenon is frequently coincident with a high natural gamma ray signature and elevated rock eval hydrogen index hi and total organic carbon toc contents determination of thermal maturity of such units relies either on the interpolation of tmax from surrounding units or the direct estimation of the extent of suppression on a given sample the latter may be required if the available samples are restricted to conventional or sidewall core material representative of imited stratigraphic intervals the second white speckled shale part of the cretaceous colorado group in the alberta basin displays tmax suppression over part of its thickness a consistent relationship emerges from calculating a linear regression of tmax as a function of either toc or hi for several wells at different levels of thermal maturity below values of about 453 c late oil window tmax is suppressed by about 1 c for every increase in hi of 50 mg hydrocarbon g toc thus tmax suppression in the second white speckled shale can be corrected by extrapolating all tmax values to a constant hi of 100 to 150 mg hydrocarbon g toc values typical of type iii kerogen

did the hudson strait in arctic canada record the opening of the labrador sea abstract the hudson strait evans strait foxe channel hef is a major composite topographic feature that connects hudson bay and foxe basin with the labrador sea hudson strait is underlain by several fault controlled sub basins with a half graben geometry the sedimentary succession in the sub basins is thicker than the one preserved onshore and reaches a maximum of 2 6 km in the eastern part of the hudson strait the lower part of the offshore succession correlates with paleozoic rocks exposed in nearby islands but the nature and age of the upper part remains poorly constrained faults have a clear morphological expression strike wnw and ene dip predominantly toward the north and record an extensional tectonic event that predates an episode of shortening a model is proposed linking half graben development to the initial stages of extension in the future labrador sea and subsequent inversion to the kinematic decoupling of the labrador sea from the canadian landmass by the ungava fault zone

characterization of the magnetic fabric of rocks abstract an inextensive system of magnetic susceptibility anisotropy factors is suggested which is sufficient in the majority of practical applications for characterizing the magnetic fabric of rocks the system involves three newly devised factors the corrected anisotropy degree the shape factor and the difference shape factor the first of these characterizes the quantity of anisotropy somewhat better than the commonly used anisotropy degree the shape factor characterizes the quality of anisotropy i e the shape of the susceptibility ellipsoid in a convenient way its definition is based on a certain analogy between the anisotropy ellipsoid and the strain ellipsoid the difference shape factor can replace the shape factor when only the differences between the principal susceptibilities are given while the mean susceptibility is unknown

unravelling textural heterogeneity in obsidian shear induced outgassing in the rocche rosse flow abstract obsidian flow emplacement is a complex and understudied aspect of silicic volcanism of particular importance is the question of how highly viscous magma can lose sufficient gas in order to erupt effusively as a lava flow using an array of methods we study the extreme textural heterogeneity of the rocche rosse obsidian flow in lipari a 2 km long 100 m thick 800 year old lava flow with respect to outgassing and emplacement mechanisms 2d and 3d vesicle analyses and density measurements are used to classify the lava into four textural types glassy obsidian 40 vesicles high aspect ratio shear banded lava 20 40 vesicles and low aspect ratio frothy obsidian with 30 60 vesicles textural heterogeneity is observed on all scales m to m and occurs as the result of strongly localised strain magnetic fabric described by oblate and prolate susceptibility ellipsoids records high and variable degrees of shearing throughout the flow total water contents are derived using both thermogravimetry and infrared spectroscopy to quantify primary magmatic and secondary meteoric water glass water contents are between 0 08 0 25 wt water analysis also reveals an increase in water content from glassy obsidian bands towards frothy bands of 0 06 0 08 wt reflecting preferential vesiculation of higher water bands and an extreme sensitivity of obsidian degassing to water content we present an outgassing model that reconciles textural volatile and magnetic data to indicate that obsidian is generated from multiple shear induced outgassing cycles whereby vesicular magma outgasses and densifies through bubble collapse and fracture healing to form obsidian which then re vesiculates to produce dry vesicular magma repetition of this cycle throughout magma ascent results in the low water contents of the rocche rosse lavas and the final stage in the degassing cycle determines final lava porosity heterogeneities in lava rheology vesicularity water content microlite content viscosity play a vital role in the structural evolution of an obsidian flow and overprint flow scale morphology post emplacement hydration also depends heavily on local strain whereby connectivity of vesicles as a result of shear deformation governs sample rehydration by meteoric water a process previously correlated to lava vesicularity alone

glaciation of north america in the james bay lowland canada 3 5 ma glaciers expanded in the northern hemisphere during the middle pliocene however whether they extended into the midlatitude lowlands remains unknown a pliocene sequence from a buried deep bedrock trench in the midlatitude james bay lowland canada 52 49 5 n 83 52 5 w contains a till and an overlying lacustrine deposit rich in fossil pollen magneto stratigraphy together with pollen derived biostratigraphy constrains it to a time span from 3 6 to 3 0 ma based on multiple lines of evidence we are able to prove the deposition of the till by an early ice sheet and hence glaciation of the lowland at ca 3 5 ma 3 6 3 4 ma after glaciation rapid warming permitted thermophilic trees now exotic to this area to grow which include oak sweetgum and cypress furthermore pollen analysis indicates alternating carolinian deciduous and boreal evergreen forests under a climate that oscillated and cooled gradually during a prolonged postglacial period from 3 5 to 3 0 ma

bathymetric and isotopic evidence for a short lived late ordovician glaciation in a greenhouse period the end ordovician glaciation is distinct among phanerozoic glaciations in that co 2 levels were generally high yet major continental ice sheets accumulated on the gondwana supercontinent new oxygen isotopic data indicate substantial changes in sea water temperatures and ice volume coinciding with glacio eustatic changes in sea level reflecting the growth and decay of the gondwana ice cap major glaciation was apparently confined to the hirnantian and was 0 5 1 m y long rather than the 35 m y of earlier estimates carbon isotope values indicate significant changes in carbon cycling as the oceans changed from a state with warm saline bottom waters to a state with cold deep water circulation and then back again we believe that the changes in the carbon cycle effected a reduction in p co 2 levels in the oceans and atmosphere and thus promoted glaciation but were unable to sustain icehouse conditions in a greenhouse world

solidification and turbulence non laminar during magma ascent insights from 2d and 3d analyses of bubbles and minerals in an etnean dyke

rare earth element geochemistry of the molybdenum bearing granitoids in the jinduicheng huanglongpu district shaanxi province northwest china located in the luonan county shaanxi province northwest china jinduicheng shijiawan and huanglongpu molybdenum deposits constitute the most important molybdenum mineralized district in china among these three deposits the jinduicheng and shijiawan molybdenum deposits are connected spatially and genetically with granitoid porphyry 124 6 ma k ar biotite and consist of disseminated veinlet ores geochemical studies of rare earth elements ree furnish further evidence for understanding the rock and ore forming processes of these two porphyry molybdenum deposits and their related granitoid rocks the ree distribution in molybdenum ore granitoids and their middle proterozoic meta volcanic wall rocks is discussed the similarities between the ree signatures of the shijiawan molybdenum bearing monzogranite porphyry and the neighbouring laoneushan monzogranite 130 5 ma k ar biotite show that they were produced at the same evolutional stage of granitoid magma derived mainly from crustal anatexis the shijiawan biotite monzogranite porphyry may be an apophysis of the laoneushan granitoid batholith compared to the shijiawan monzogranite porphyry the jinduicheng molybdenum bearing granite porphyry is characterized by a high content of hree and depletion in lree the unique ree patterns indicates that the molybdenum bearing granite porphyry was formed by thermogravitation diffusion of a granitoid magma the slight depletion of ree abundance in the altered granitoid porphyry and meta volcanic wall rocks shows that leaching of ree occurred during breakdown of the primary mineral assemblage and crystallization of secondary minerals the high ree content of molybdenum ore represented re deposition of the mobilized molybdenum and ree

an unusual sponge microbe synsedimentary cement framework in a late ordovician reef southampton island nunavut canada a large resistant buildup at the top of the upper ordovician hirnantian red head rapids formation on southampton island nunavut arctic canada is dominated by massive boundstone and cementstone facies these massive facies have more in common with the sponge microbial reefs that dominated worldwide in the early ordovician including the following primary components early calcified sponge material microbial elements and synsedimentary cement a close spatial relationship between sponge and microbial framework elements suggests that a poorly preserved decaying sponge framework provided substrates for the attachment and development of microbes and that the microbes played essential roles as reef consolidators centimetre scale colonial metazoans are present and locally intergrown with the sponge and microbial components other mound dwelling invertebrates or calcareous algae are rare although altered now to calcite cement fabrics suggest that aragonite was ubiquitous as seafloor precipitate prior to its subaerial exposure in the latest ordovician the red head rapids formation buildup developed on the margin of a shallow marine evaporative epicratonic basin where a diverse community of reef building metazoans was unable to flourish

the system na 2 si 2 o 5 k 2 si 2 o 5 mos 2 h 2 o with implications for molybdenum transport in silicate melts the behavior of molybdenite in the system na 2 si 2 o 5 k 2 si 2 o 5 mos 2 h 2 o has been investigated from 340 bars to 1 36 kb in the temperature range 450 degrees to 850 degrees c phase relations in the system show enhanced molybdenite solubility in the alkali silicate liquids and coexisting vapors mos 2 solubility is up to 10 wt percent in vapor saturated na 2 si 2 o 5 melt and 12 5 wt percent in vapor saturated k 2 si 2 o 5 melt at 680 bars and 650 degrees c there is a positive pressure dependence of solubility but retrograde solubility is indicated as a function of temperature above about 650 degrees c at the pressures investigated the experimental data combined with published data on structure of silicate melts mysen and virgo 1980 suggest that mo silicate and or mo hydroxylated silicate complexes such as mosi 2 o super 2 5 mosi 2 o 6 and mosi 2 o 4 oh super 3 may account for the observed molybdenite solubility if these results can be extrapolated to nature it seems likely that geologically significant quantities of molybdenum may be transported in hydrous alkalic silicic melts as mo silicate complexes for eventual fractionation into the vapor phase during vapor generation such a transport mechanism would be consistent with the commonly observed association of mo with silica rich rocks

when an inverse fabric is not inverse an integrated ams spo study in morb like dykes terra nova 23 49 55 2011 abstract magma flow direction is recorded in the macroscopic mineral petrofabric and magnetic fabric of the morb like rooi rand dyke swarm the bulk magnetic fabric is sub parallel to the dyke plane resulting from preferred orientation of grains during magma flow this is referred to as type a fabric a study of mineral shape preferred orientation reveals an additional fabric which is orthogonal to the average dyke orientation in 30 of the data this fabric is carried by both plagioclase and opaque grains and would typically be interpreted as inverse it is referred to here as type b fabric however because it is carried by the shape preferred orientation of macroscopic grains and is in most cases not coaxial to the anisotropy of magnetic susceptibility fabric it is interpreted as having been acquired during increased grain interaction during late stage magma flow associated with decreasing magma pressure

influences of temperature and mg ca ratio on caco3 precipitates from seawater the mg ca ratio at which the calcium carbonate mineral that has pseudohomogeneously precipitated from seawater changes from calcite to aragonite was experimentally determined as a function of temperature results indicate a dramatic change in the critical mg ca ratio over a relatively small temperature range for example in normal seawater with a mg ca of 5 1 the critical temperature is 6 3 c at a temperature of 25 c however mg ca must be less than 1 4 0 1 for calcite to form these data indicate that paleoclimatic conditions at the time of carbonate deposition must be considered when attempting to make inferences about variation in other factors such as paleo p co 2 p pressure and paleo mg ca ratios that may control carbonate mineral formation by abiotic processes however the inverse relation between inferred global paleoclimatic conditions and the dominant type of caco 3 polymorph being abiotically deposited i e warm conditions and low mg calcite versus cool conditions and aragonite suggest that substantial changes must have taken place in ocean chemistry over phanerozoic time

anisotropy of out of phase magnetic susceptibility as a potential tool for distinguishing geologically and physically controlled inverse magnetic fabrics in volcanic dykes abstract among the mechanisms of the mass transport within the earth s crust and the upper mantle the magma movement within the dykes is very important in some dykes the magma movement is represented by almost free flow infilling more or less open fractures in the host rocks while it has character of forceful injection of the host rocks in the other dykes the magnetic fabric originated by the free flow is characterized by the magnetic foliation and magnetic lineation approximately parallel to the dyke plane normal fabric whereas the forceful injection gives rise to the magnetic foliation perpendicular to the dyke plane defining the intermediate or inverse magnetic fabric it is known however that the inverse magnetic fabric can also originate through the free flow of magma in the case when the minerals carrying the ams are represented by single domain sd grains in order to distinguish between the two mechanisms controlling the intermediate inverse fabric the anisotropy of magnetic remanence amr is often used because it indicates the shape preferred orientation of ferromagnetic minerals regardless of their domain state we show that not only amr but also the anisotropy of out of phase magnetic susceptibility opams is conformable with particle shape regardless of the magnetic state of the particle consequently the opams which is measured automatically with the standard ams using the kly 5 kappabridge may substitute the amr at least in some cases of volcanic dykes

is this magnetic fabric normal a review and case studies in volcanic formations abstract when studying weakly anisotropic rocks non standard relationships between magnetic and expected petrofabric axes are sometimes observed in particular the so called inverse or intermediate fabrics a review of possible explanations for these effects is presented and leads to the conclusion that a majority of abnormal magnetic fabrics often encountered in magnetite rich volcanic rocks are due to abnormal preferred orientations or distributions of magnetite grains through case studies in dikes from skye island yemen and the iberian peninsula and magma and ash flows from the ethiopian traps and southern corsica the various possible origins of such fabrics are examined it is possible to retrieve reliable magma flow directions from these sites provided proper data selection is performed leading to non unique interpretations

flow lobes in granite the determination of magma flow direction in the trawenagh bay granite northwestern ireland using anisotropy of magnetic susceptibility anisotropy of magnetic susceptibility ams has been determined from 152 sites over the 64 km2 trawenagh bay granite part of the 400 ma caledonian donegal batholith microscopic observations thermomagnetic analyses high field torque magnetometry and heating experiments demonstrate that the ams fabric is carried by biotite with varying amounts of mimetic magnetite in the biotite cleavage planes the ams fabric is interpreted as a flow fabric because microstructures indicate magmatic state deformation it defines large scale macroscopic flow features flow lobes in this weakly deformed granite at pluton scale the data set shows a gentle westward plunging lineation at a smaller scale many lobate magnetic foliation patterns can be discerned in which the magnetic lineation is usually symmetrically disposed about the long axes of the lobes in three dimensions the lobes are tongue like 1500 m wide subunit scale and elongated east west parallel with the general lineation they close consistently westward and younger lobes in the east appear to deflect older lobes in the west the trawenagh bay granite is west of a major synplutonic shear zone that hosts the main donegal granite pluton these structures are interpreted as frozen flow lobes and indicate that the magma flowed in a westward direction from within the active shear zone of the main donegal granite into the weakly deformed wall rock trawenagh bay granite intra unit pulses or surges have long been suspected in granite plutons however this study reports the most detailed description of such structures to date

grain size dependence of initial susceptibility and saturation magnetization related parameters of four natural magnetites in the psd md range summary initial susceptibility coercive force and several types of remanent coercive forces were measured on a set of artificial rock specimens containing grain size fractions of a natural magnetite and titanomagnetite both of which are optically homogeneous and a natural titanomagnetite and titanomaghemite both of which show exsolution phenomena it was found subsequently that the optically homogeneous titanomagnetite contains some submicroscopic fe rich inclusions these inclusions are magnetically screened in coarse grains only for the lower grain sizes studied they have an appreciable influence on the data the size of the fractions varied from 5 to 250 m the parameters and several parameter ratios are plotted as a function of grain size several of these parameters can be used as mineralogical and grain size indicators the varying degree of exsolution of the minerals is reflected in the gradients of the plots from plots of initial susceptibility versus grain size it is concluded that the shape of the grains does not play a dominant role in determining the change of this parameter with grain size variation of the various parameters with grain size is explained in terms of a gradual transition from large psd grains to md grains without there being any sharp transition for titanomagnetite x 0 55 the transition takes place at grain sizes that are about 15 m larger than for magnetite

theory and application of the day plot mrs ms versus hcr hc 2 application to data for rocks sediments and soils 1 new theoretical curves relating the hysteresis parameters mrs ms and hcr hc for single domain sd superparamagnetic sp pseudo single domain psd and multidomain md grains and their mixtures are applied to published data for natural materials the day plot of mrs ms versus hcr hc has been used to crudely classify samples into box like sd psd and md or sometimes incorrectly md sp regions with arbitrary boundaries new type curves for md psd sd md and sd sp grains and mixtures permit more subtle and precise modeling the predicted md trend and its junction with the psd trend are observed in two data sets for magnetite spherules from carbonate rocks and for temperature varying hysteresis results spanning the verwey transition the latter data are the basis of a suggested new method for pinpointing the psd md threshold size selected data for pottery clays soils and paleosols generally follow sd md type curves and indicate intermediate size psd magnetite with narrow to broad size distributions a lake sediment section with known grain size progression tracks in the predicted sense along the sd md trend selected data for glaciomarine and pelagic sediments are also generally compatible with sd md trends examples of remagnetized carbonate rocks submarine basaltic glasses and glassy rims of pillow basalts all follow predicted sp sd or sp psd mixing curves with a large range in volume fraction of sp grains 0 75 but a narrow range of sp particle sizes 10 2 nm larger sp grains spanning the range to sd size 25 30 nm are absent for unknown reasons oceanic dolerites gabbros and serpentinized peridotites in some cases fall in a novel region of the day plot parallel to but below magnetite sd md mixing curves

lithosphere structure beneath the phanerozoic intracratonic basins of north america four intracratonic basins of north america the hudson bay michigan illinois and williston basins have similar ages and are close to one another yet they exhibit different subsidence histories characterised by different time scales and sediment thicknesses they can be explained by local lithosphere thinning and by the cooling of the induced thermal anomaly within the framework of 1d thermal models for vertical heat transport each basin requires a different lithosphere thickness or a different boundary condition at the base of the lithosphere heat flow and seismic studies show that beneath the north american craton the lithosphere is too thick for the assumption of purely vertical heat transfer to be valid thermal models are developed to account for finite thermal anomaly width and for two types of basal boundary conditions fixed temperature or fixed heat flux different subsidence histories are explained by deep lithospheric anomalies of different sizes the stability of thick continental roots requires the mantle part of the lithosphere to be compositionally buoyant with respect to normal convecting mantle localised lithospheric thinning due for example to plume penetration results in the emplacement of compositionally denser mantle into the lithosphere this represents a load which drives permanent flexure the cooling time and the characteristics of flexure allow constraints on the dimensions of these deep lithospheric anomalies there are no solutions for lithosphere thicknesses less than 170 km the williston and illinois basins are associated with wide v200 km and thin anomalies v100 km whereas the michigan and hudson bay are located on top of narrow v100 km and tall v200 km anomalies fl 2000 elsevier science b v all rights reserved

secular oscillations in the carbonate mineralogy of reef building and sediment producing organisms driven by tectonically forced shifts in seawater chemistry the primary mineralogy of oolites and early marine carbonate cements led sandberg nature 305 1983 19 22 to divide the phanerozoic eon into three intervals of aragonite seas and two intervals of calcite seas hardie geology 24 1996 279 283 has shown that these oscillations together with synchronous oscillations in the mineralogy of marine potash evaporites can be explained by secular shifts in the mg ca ratio of seawater driven by changes in spreading rates along mid ocean ridges the hardie model also predicts that high mg calcite should precipitate along with aragonite as it does in today s aragonite sea we have uncovered oscillations in the carbonate mineralogy of hypercalcifying organisms ones that have produced massive skeletons large reefs or voluminous bodies of sediment that correspond to sandberg s aragonite seas and calcite seas and that are predicted by the hardie model particular groups of corals sponges and algae appear to have been dominant reef builders only when favored by an appropriate mg ca ratio in seawater in early and middle paleozoic calcite seas calcite i reefs were dominated by calcitic tabulate heliolitid and rugose corals and calcitic stromatoporoids in contrast during the period of late paleozoic early mesozoic aragonite seas aragonite ii aragonitic groups of sponges scleractinian corals and phylloid algae as well as high mg calcitic red algae were principal reef builders during late cretaceous time at the acme of calcite ii massive rudists displaced aragonitic hermatypic corals in today s aragonite sea aragonite iii scleractinian corals are again dominant reef builders along with high mg calcitic coralline algae major sediment producing algae exhibit temporal patterns similar to those of reef builders calcitic receptaculitids flourished during calcite i whereas aragonitic dasycladaceans did not become dominant rock formers until aragonite ii during calcite ii calcitic nannoplankton formed massive coccolith chalks in warm shallow seas of the late cretaceous after the mg ca ratio of seawater had reached a very low value and calcium concentration a very high value as the mg ca ratio of seawater rose and calcium concentration fell during the cenozoic era individual coccoliths on average became less massive and encrusted cells less thickly by pliocene time during aragonite iii the prominent genus discoaster secreted only narrow rayed coccoliths that covered less than 25 of the cell surface also during aragonite iii the aragonitic green alga halimeda emerged as the dominant skeletal sediment producer in reef tracts the influence of seawater chemistry on skeletal secretion appears to have been especially strong for morphologically simple taxa that exert relatively weak control over their own calcification such groups include algae sponges corals and bryozoans morphological simplicity also permits these groups to adopt vegetative or colonial modes of growth that confer success in competition for space on reefs this linkage in addition to the basic chemical demands of hypercalcification has given the mg ca ratio of seawater strong control over the success of individual reef building taxa more generally this ratio appears to have strongly influenced evolutionary changes in the skeletal mineralogy of sponges and cheilostome bryozoans throughout their history we conclude that throughout phanerozoic time a chain of causation has extended from mid ocean ridge processes via seawater chemistry to the mineralogical and biological composition of reef communities and bioclastic carbonate deposits

magnetic anisotropy study of a columnar basalt from san anton morelos mexico anisotrophy of magnetic susceptibility ams results from 27 specimens drilled from the top and two sides of a single columnar basalt segment are presented the magnetic foliation plane is nearly horizontal for all parts of the column which is consistent with a primary magma flow pattern without evidence of local convection or differentiative processes the shape of ams ellipsoids is however predominantly prolate which may be indicative of increased magnetic grain elongation due to crystal growth or grain realignment normal to a vertical stress field due to thermal contraction apparent systematic variations related to column shape are found in bulk susceptibility anisotropy degree and degree of lineation and foliation some of the variation may also be related to weathering effects the results are consistent with a primary ams pattern resulting from thermal contractive stresses during column formation comparison of results from previous studies of columnar basalts reveals that there is a relatively large variation in ams properties there appears to exist a number of factors which may locally control the magnetic anisotropy of columns and very likely some of their other characteristics

magnetic fabric and petrology of miocene sub volcanic sills and dykes emplaced into the sw flysch belt of the west carpathians s moravia czech republic and their volcanological and tectonic implications abstract magnetic fabrics were investigated in the sub volcanic bodies occurring within the sw flysch belt of the western carpathians magnetite or low ti titanomagnetite is the principal carrier of the anisotropy of magnetic susceptibility ams in these rocks though various fe ti oxides were also identified in minor amounts the magnetic fabric in almost all sills and dykes is conformable to the shapes of the bodies and no doubt originated through more or less free magma flow that requires relatively open voids to be injected that likely originated in an extensive tectonic regime in an ankaramite sill where the rock creates typical hexagonal columns the magnetic fabric is related rather to the column axes than to the overall shape of the sill the magnetic fabric formation in this sill is controlled by the post emplacement process of the creation of the columns at some studied localities signs of post magmatic middle miocene tectonic deformation are observed in the field the youngest known in the flysch belt as the magnetic fabric is unrelated to those signs we conclude that these young tectonic movements did not penetrate ductilely in the volume of the volcanic body it seems that the strain was focused to thin zones along the faults since it did not change the preferred orientation of magnetic minerals in the rock volume substantially

cathodoluminescence in quaternary carbonate deposits abstract the cathodoluminescent oscillatory and sectoral growth zones common in crystals formed in ancient limestone successions in a variety of putative environments appear to be rare or absent from recent and pleistocene marine carbonate sequences the factors controlling cathodoluminescence and reasons for this disparity are examined the cathodoluminescent zones in the cements of ancient rocks have been interpreted as responses to variations in the redox potential of formative pore waters during crystal growth although similar cathodoluminescent behaviour is recorded from some deposits including travertines and quaternary speleothems formed in what are thought to have been strongly oxidizing environments the apparent absence of cathodoluminescence in the most recent and pleistocene marine deposits that presumably reflect deposition and diagenesis in environments that are also characteristically oxidized therefore seems anomalous the controlling influences on cathodoluminescence are reviewed together with evidence relating to observations of pleistocene marine deposits and likely conditions of formation but where it is present the mechanism s for its development remain elusive

sr c and o isotope geochemistry of ordovician brachiopods a major isotopic event around the middle late ordovician transition here we present sr c and o isotope curves for ordovician marine calcite based on analyses of 206 calcitic brachiopods from 10 localities worldwide these are the first ordovician wide isotope curves that can be placed within the newly emerging global biostratigraphic framework a total of 182 brachiopods were selected for c and o isotope analysis and 122 were selected for sr isotope analysis seawater 87sr 86sr decreased from 0 7090 to 0 7078 during the ordovician with a major quite rapid fall around the middle late ordovician transition most probably caused by a combination of low continental erosion rates and increased submarine hydrothermal exchange rates mean 18o values increase from 10 to 3 through the ordovician with an additional short lived increase of 2 to 3 during the latest ordovician due to glaciation although diagenetic alteration may have lowered 18o in some samples particularly those from the lower ordovician maximum 18o values which are less likely to be altered increase by more than 3 through the ordovician in both our data and literature data we consider that this long term rise in calcite 18o records the effect of decreasing tropical seawater temperatures across the middle late ordovician transition superimposed on seawater 18o that was steadily increasing from 3 standard mean ocean water smow by contrast 13c variation seems to have been relatively modest during most of the ordovician with the exception of the globally documented but short lived latest ordovician 13c excursion up to 7 nevertheless an underlying trend in mean 13c can be discerned changing from moderately negative values in the early ordovician to moderately positive values by the latest ordovician these new isotopic data confirm a major reorganization of ocean chemistry and the surface environment around 465 to 455 ma the juxtaposition of the greatest recorded swings in phanerozoic seawater 87sr 86sr and 18o at the same time as one of the largest marine transgressions in phanerozoic earth history suggests a causal link between tectonic and climatic change and emphasizes an endogenic control on the o isotope budget during the early paleozoic better isotopic and biostratigraphic constraints are still required if we are to understand the true significance of these changes we recommend that future work on ordovician isotope stratigraphy focus on this outstanding middle late ordovician event

internal flow structures in columnar jointed basalt from hreppholar iceland ii magnetic anisotropy and rock magnetic properties internal flow structures in columnar jointed basalt from hreppholar iceland ii magnetic anisotropy and rock magnetic properties

current challenges and future developments in magnetic fabric research abstract magnetic fabrics are used extensively as proxies for mineral fabrics and to correct paleomagnetic data and have been proposed to quantify pore fabrics today s understanding and interpretation of magnetic anisotropy benefits from decades of carefully observing anisotropic mineral and rock properties establishing and testing empirical relationships between magnetic and mineral fabrics and developing models to quantify aspects of anisotropy each advance was preceded by some puzzling observations i e data that could not be explained based on the models available at the time e g oblique or inverse fabrics or lithology dependent anisotropy strain relationships these observations led to numerous experimental and numerical techniques designed to characterize magnetic fabrics and determine their origin despite the successful application of magnetic fabrics in many structural and tectonic problems there are still phenomena and measurements that cannot be explained based on today s magnetic fabric theory with the purpose of fostering future development i will touch on three main areas where i see challenges in magnetic fabric research 1 experimental characterization of magnetic anisotropy including the non uniqueness of reported tensors and non linearity 2 anisotropy modelling and defining the carrier minerals and origin of the magnetic fabric 3 selection of adequate sets of anisotropy tensors to correct paleomagnetic data the description of the challenges presented here will hopefully help define directions for future research inform those that are new to anisotropy and advance our field

the use of field dependence of ac susceptibility for the interpretation of magnetic mineralogy and magnetic fabrics in the hsdp 2 basalts hawaii abstract we applied the field dependence parameter hd k 300a m k 30a m k 300a m 100 given by de wall for the subaerial and submarine basalts drilled by the 3109 m deep hsdp 2 borehole on hawaii in order to verify the hypothesis that mainly composition controls the field dependence of ac susceptibility in titanomagnetite of natural occurrences when we used this parameter our data showed a significant scattering compared to data presented in earlier studies in addition to composition the effect of measurement temperature grain size and anisotropy on the field dependent susceptibility were examined and found to be critical the impact of grain size is weaker than the other effects it cannot be totally excluded that the observed effects arise indirectly through an overlap of the other effects for the investigated basalts the most important factor for the variation of field dependence is the degree of oxidation causing a modification of the titanomagnetite composition or formation of titanomaghemite and the mixing of ti rich with ti poor titanomagnetites which strongly reduces the hd parameter field dependence is not only related to titanomagnetite composition especially for intermediate titanomagnetites with t c s between 100 and 300 c temperature dependent susceptibility measurements at different field amplitudes for these intermediate types showed at constant geometry of the k t curve great differences in susceptibility resulting in significant changes of the field dependence parameter over the temperature interval from 100 to 260 c therefore variations of the ambient measurement temperatures are able to influence the field dependence the second important effect is the degree of particle shape and alignment which controls the field dependence in different orientations especially for the intermediate titanomagnetite which is intensively intergrown with elongated hemoilmenite grains as a consequence samples with higher degrees of anisotropy exhibit differences of the field dependence parameter if measured parallel to k max or k min axis therefore in addition to compositional effects and the temperature dependence the magnetic fabric has to be considered for the interpretation of field dependent susceptibility measurements the influence of intrinsic ti content magnetocrystalline anisotropy and extrinsic shape and alignment of grains factors for the interpretation of the degree of anisotropy has to be kept in mind when interpreting ams data in terms of strain rates experienced by moving lava during emplacement

single domain particles in rocks and magnetic fabric analysis magnetic susceptibility anisotropy is commonly used as a guide for determining stress and flow axes in rocks and for assessing the suitability of anisotropic rocks for palaeomagnetic purposes we have recently demonstrated however in laboratory produced samples that as theoretically expected susceptibility anisotropy is dependent on particle size multidomain particles of magnetite or similar strongly magnetic particles with shape anisotropy exhibit a maximum susceptibility parallel to their easy remanent magnetization axes whereas uniaxial single domain particles exhibit a maximum susceptibility perpendicular to their easy magnetization axes hence susceptibility anisotropy does not yield an unambiguous determination of particle alignment in rocks independent of domain state by demonstrating uniaxial single domain anisotropy characteristics for the first time in a rock specimen we show here that susceptibility measurements by themselves may not correctly identify the degree or type of alignment of ferro or ferrimagnetic particles in rocks in particular apparently foliated rocks can actually be lineated and vice versa moreover rocks with low anisotropy of susceptibility can have a much higher anisotropy of remanence remanence anisotropy methods are shown to give unambiguous determinations of the degree and type of alignment

degradative calcification of a modern siliceous sponge from the great bahama bank the bahamas a guide for interpretation of ancient sponge bearing limestones abstract organic colloids are involved in the early calcification of the modern siliceous sponge spheciospongia vesparium lamarck 1815 from the great bahama bank electron microscopy and in situ fluorescence microspectometry studies indicate that colloids attached within or onto a collagen network promote the precipitation of aragonite crystals in these sponges calcification occurs within those portions of the sponge that are buried in the sediment preferentially in regions of agglutinated sediment particles with sponge connective tissue being subjected to necrosis and significant degradation of the extracellular collagen matrix the dismantling of collagen bundles leads to collagen scaffolds which act as a sorbent and have significant adhesive effects for ions and or organic colloids bacteria and other microorganisms support tissue degradation but neither act as a substrate for aragonite precipitation or are present in significant numbers at calcification sites this process of early calcification may explain the origin of fossil calcified siliceous sponges sponge mummies and tuberoids as well as the occurrence of patchy calcified sponge materials thereby calling into question the commonly accepted idea that pelletoidal texture associated with these fossil sponges indicates that bacteria are directly responsible for the calcification

a review of the geological characteristics and geodynamic setting of the late early cretaceous molybdenum deposits in the east qinling dabie molybdenum belt east china abstract the east qinling dabie molybdenum belt which is located in the nearly east west trending qinling dabie orogen east china is the largest mo belt in the world and is characterized by extensive mesozoic mineralization although the late early cretaceous ca 120 100 ma is the latest stage of the mo mineralization in the east qinling dabie mo belt this period shows relatively intensive mo mineralization such as the world class shapinggou mo deposit all of the late early cretaceous mo deposits are closely related to granitic magmatism some of which are typical porphyry type deposits e g donggou tangjiaping shapinggou homogenization temperatures of fluid inclusions in the late early cretaceous mo deposits range from 115 to 550 c with salinities ranging from 0 02 to 62 1 wt nacl equivalent different amount of co2 have been recognized in the hydrothermal systems of these mo deposits indicating h2o nacl co2 systems the ore forming fluids in these mo deposits were magmatic water mixed with different amount of meteoric water except the donggou deposit the 34s values of sulfides from the late early cretaceous mo deposits have a narrow range of 0 4 6 3 suggesting a deep magmatic source for the sulfur the donggou mo deposit has slightly higher 34s values 7 5 9 4 reflecting that the ore sulfur was mainly derived from magma probably with some upper crust materials available isotopic data show that the ore associated magmas of the late early cretaceous mo deposits were mainly derived from crustal materials probably with some contribution of mantle or newly added crust the donggou tangjiaping and shapinggou mo deposits are genetically related to a type magmatism suggesting that they formed in an extensional setting the late early cretaceous mo deposits in the east qinling dabie mo belt formed in an intraplate extensional setting accompanied by the lithospheric thinning and asthenospheric upwelling

emplacement of magma in eastern iceland dikes insights from magnetic fabric and rock magnetic analyses abstract the anisotropy of magnetic susceptibility ams coupled with mineral magnetic identification has been investigated in about 640 samples from 27 basaltic dikes of tertiary age in eastern iceland the dikes are a few meters thick and they have been sampled all across three types of magnetic fabrics with well defined mean principal axes of anisotropy are recognized in the first one the maximum axis is vertical and the minimum axis coincides with the pole of the dike plan geometrically normal fabric the second fabric shows opposite orientation geometrically inverse fabric the third type is when these two fabrics coexist with the normal one at the margins and the inverse one in the center of the dike mixed fabric no relationship is observed between the type of fabric and the width of the dikes each fabric is carried by clear magnetic assemblages made of low x 0 15 ti content fine grained magnetites for the normal one and high ti content around tm60 coarse magnetite grains for the inverse fabric upon heating the magnetic properties and the orientation of the anisotropy ellipsoid characterizing the inverse fabric in the mixed dikes change and evolve toward the normal type these two geometrically normal and inverse fabrics are interpreted as primary ones representing different physico chemical conditions during the emplacement of the magma the ams results indicate that during the volcanic events occurring about 10 myr ago the extrusion of the magma was vertical at the depth of the studied sites in eastern iceland

evidence for an albian hudson arm connection between the cretaceous western interior seaway of north america and the labrador sea numerous researchers have alluded to the existence of a cretaceous hudson arm connection between the labrador sea and the western interior seaway of north america however the evidence for this marine connection has been circumstantial in this paper we present sedimentary geochemical data that indicate a marine influence in the albian mattagami formation of the moose river basin james bay lowlands ontario the facies associations between dinoflagellate bearing laminated mudstones fluvial sandstones and early pyrite mineralization are interpreted to indicate deposition in the central basin of an estuary we use the facies association between the estuarine fill and coeval kaolinitic paleosols in the moose river basin and in similar deposits in quebec and labrador to reconstruct a southern shoreline of the albian hudson arm to the cretaceous western interior seaway we suggest that development of the hudson arm connection between the labrador sea and the cretaceous western interior seaway may be related to a regional extensional regime associated with rifting between labrador and greenland and the passage of eastern north america over cretaceous hotspots

ordovician conodont biostratigraphy and redefinition of the age of lithostratigraphic units on northeastern melville peninsula nunavut northeastern melville peninsula nunavut canada preserves the stratigraphic record of the northwestern margin of the foxe basin the ordovician sequence on the peninsula includes the lower ordovician ship point formation and upper ordovician frobisher bay amadjuak akpatok and foster bay formations their biostratigraphic ages and correlations are poorly understood in particular it is unclear whether the organic rich boas river formation exists on the peninsula following extensive sampling of these stratigraphic units studies of numerous conodont elements from both outcrops and rubble at about 60 localities have established five conodont assemblages through the five lithostratigraphic units on the peninsula oepikodus communis jumudontus gananda assemblage in the ship point formation is correlated to the reutterodus andinus zone in the uppermost ibexian lower ordovician the other four assemblages from the upper ordovician are as follows appalachignathus delicatulus polyplacognathus ramosus

relic aragonite from ordovician silurian brachiopods implications for the evolution of calcification understanding the influence of aragonite calcite sea conditions on the evolution of biocalcification relies strongly on the correct interpretation of the original composition of calcareous taxa aragonite dissolves or inverts into calcite over geological time and its preservation is currently unknown to predate the pennsylvanian here we present direct evidence for the common occurrence of relic aragonite in ordovician and silurian trimerellid brachiopods thereby extending the known range of aragonite preservation by more than 130 million years together with associated hypercalcifying taxa of putatively original aragonite or high magnesium calcite composition and considerations of the temperature dependence of aragonite and calcite precipitation our results suggest that the evolution of aragonite biomineralization might have presented an adaptive advantage in shallow marine tropical waters of calcite seas a targeted search for paleozoic aragonite should both resolve the original composition of consistently recrystallized taxa and enable the reassessment of the aragonite calcite sea paradigm in a paleoenvironmental context

anisotropy of magnetic susceptibility in the ponta grossa dyke swarm brazil and its relationship with magma flow direction abstract measurements of anisotropy of magnetic susceptibility ams in 95 mafic dykes mainly tholeiites 10 200 m in width from the mesozoic ponta grossa swarm southern brazil revealed two main types of magnetic fabric type i fabric plane k 1 k 2 parallel to the dyke plane represents magma flow within the dykes whereas type ii plane k 1 k 3 parallel to the dyke plane is compatible with a fabric pattern reflecting vertical compaction of the magma column fabric type i dominates 51 of the dykes within the swarm whereas type ii 38 of the dykes concentrates mainly in the western region where the dykes intrude sediments considering the dykes with type i fabrics it is concluded that 58 of the dykes were fed by horizontal or sub horizontal k 1 inclinations less than 30 magmatic flow and 42 were fed by inclined to vertical k 1 inclinations more than 30 magma flow the latter are more frequent in the southeastern part of the swarm suggesting a magma source close to this area although there may have been other sources in other regions where dykes with inclined flow and distinct chemical characteristics are also found

origin of internal flow structures in columnar jointed basalt from hreppholar iceland i textural and geochemical characterization basalt columns from hreppholar iceland show distinct internal structures produced by alternating brighter and darker bands through the column locally exhibiting viscous fingering features here we present geochemical and petrographic data retrieved from analyses of major and trace elements and mineral chemistry from a cross section of a single basaltic column this is combined with petrographic descriptions and data on crystal size distributions of plagioclase we use our data from hreppholar to test four existing models that have been proposed to explain banded structures inside columns 1 deuteric alteration 2 double diffusive convection 3 constitutional supercooling and 4 crystallization induced melt migration we find that the internal structures at hreppholar represent primary magmatic features because approximately 20 of the observed structures crosscut the column bounding fracture for each meter along the main axis of the column these features must thus have been formed before the column delimiting crack advanced major and trace element analyses show small but significant variations across the column and strong correlation between oxides like feo and tio2 as well as k2o and p2o5 the geochemical variations correlate with the presence of darker brighter bands visible on a polished surface and can be explained by a variation in the modal proportions of the main phenocryst phases specifically variable plagioclase and titanomagnetite content this banding enhances the internal structures apparent in the polished cross section from columnar joints at hreppholar the measured variations in major and trace element geochemistry as well as mineral chemistry are too small to distinguish between the proposed band forming models plagioclase crystal size distributions however display a systematic change across the column that is consistent with late stage migration of melt inside the column i e the crystallization induced melt migration hypothesis the central part of the columns have plagioclases indicative of slow cooling and these are also more steeply oriented i e subparallel to the column axis compared with plagioclases present in the more rapidly cooled edges this redistribution of melt within individual columns may significantly affect the cooling rate of columnar jointed lava flows and intrusions