evaluation of paleomagnetism as a correlation criterion for mogollon datil ignimbrites southwestern new mexico paleomagnetic and rock magnetic data show that 36 24 ma ignimbrite outflow sheets in the mogollon datil volcanic field carry thermoremanent magnetizations trms that provide useful reliable correlation criteria over 90 of 404 ignimbrite sites exhibit well grouped 95 500 m intracaldera ignimbrite sequences agree in polarity with associated outflow sheets but show less consistent trm directions probably reflecting syncooling secular variation and discrepancies between eutaxitic foliations and the syncooling paleohorizontal mean virtual geomagnetic pole positions vgps of normal and reverse polarity ignimbrites are nearly antipodal 175 6 apart showing no evidence of long term differences in the zonal harmonics of the normal and reverse polarity states less antipodal mean vgps 161 2 apart from normal and reverse polarity lavas are interpreted as defects in the data set probably including incompletely removed normal polarity viscous overprints the mean paleomagnetic pole position for mogollon datil ignimbrites n 68 independent unit mean vgps is located at 78 8 n 166 6 e k 19 8 a95 4 0 18 2

cooling rate correction of paleointensity determination for volcanic glasses by relaxation geospeedometry the influence of the cooling rate on paleointensity estimates was investigated for samples from a vertical profile across a 600 a d obsidian lava flow ramp from lipari italy the natural cooling rates at the glass transition which were previously determined for the seven investigated samples by relaxation geospeedometry vary by a factor of more than 4 rock magnetic investigations indicate a magnetic microlite fraction in the single domain grain size range and strong magnetic anisotropy the thermoremanence anisotropy tensor was determined for each specimen to correct the paleointensity results for this anisotropy the cooling rate dependency of the thermoremanence was determined experimentally extrapolation to natural cooling rates indicate an overestimate of the paleointensity by 13 to 20 during experiments with typical laboratory cooling rates correcting for the different cooling rates and the cooling rate dependencies within the vertical profile significantly reduces the standard deviation of the average flow paleointensity the average paleointensity for the 543 19 a d flow ramp results in 52 4 1 1 t corresponding to a virtual axial dipole moment of 9 2 0 2 am2 uncertainties introduced by anisotropy correction and cooling rate extrapolation are considered by error propagation

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

palaeoenvironmental change recorded in the palaeodunefields of the western murray basin south australia new data from single grain osl dating this study is concerned with the reconstruction of the palaeoenvironmental history of southeastern australia for the last 300 ka by establishing a luminescence chronology of dune sand deposition in the western murray basin south australia in the study area vast fields of palaeodunes stabilised by vegetation provide evidence of past environmental change in total 98 samples were collected from dune sand layers at 13 different dune sections the time of their deposition was determined using optically stimulated luminescence dating of single quartz grains accounting for the impact of post depositional mixing by the use of a finite mixture model the oldest depositional phase demonstrates that dune sand layers of great antiquity are preserved in the western murray basin ranging up to at least 380 ka phases of substantial dune sand deposition were identified for the periods 18 38 ka and 63 72 ka older depositional phases also exist but are poorly resolved due to relatively large errors of the luminescence ages aeolian deposition during the last termination and the holocene is relatively limited with a slight clustering of ages at the time of the antarctic cold reversal and from 5 8 ka two modern ages give evidence of very recent dune sand deposition comparison with other palaeoclimate records from the region suggests that phases with high aeolian sedimentation coincide with more arid conditions and breaks in the dune record with more humid phases thus although dune records are often discontinuous and their interpretation in palaeoclimatic terms is not always straightforward the palaeodunes of the western murray basin show a good preservation of phases of aeolian activity and provide useful information for palaeoenvironmental reconstruction

age of desert dunes near birdsville southwest queensland abstract a mild controversy has arisen concerning the age of sand ridges in the eastern simpson desert around birdsville stratigraphic evidence indicates a holocene age but thermoluminescence tl dating of two sand samples from the area gave ages of about 34 ka and almost 78 ka wollongong laboratory in an attempt to resolve the difficulty sand samples were collected and dated by optically stimulated luminescence osl from a dune east of birdsville though low the dune is fully exposed in section so that basal sand as well as sediment from the substrate below the gibber were sampled the substrate gave dates of 36 ka the basal sand about 10 ka and sand from higher levels about 9 ka no younger sand was present and the dune is construed as the clay cemented core of a dune from which loose sand has been stripped by the wind samples of uncemented sand from the crests and flanks of higher dunes and from a clayey lee side mound in the same area gave dates around 1 ka or less

late quaternary climates of the australian arid zone a review abstract aridity in australia has generally been associated with glacial intervals of the last few glacial cycles the causes and nature of aridity varied from north to south over the continent in northern and central australia reduced monsoon rain in global cold stages caused lakes and rivers to dry vegetation to become more sparse sand dunes to become active and dust advection to increase while the monsoon is correlated with warmer global temperatures and high sea levels overall the exact timing of the greatest development of the monsoon in stage 5 remains uncertain in southeastern australia the proxy records give a less clear picture while vegetation seems to have been more sparse in cold stages and sand dune activity and dust flux consequently greater records of runoff in rivers and lakes have mixed or opposite behaviour to northern or central australia the nature of aridity in southeastern australia is not analogous to conditions anywhere on the continent today greatly reduced temperatures over the central and southern interior greater influence of snow and periglaciation on runoff high groundwater tables lower rainfall and reduced atmospheric carbon dioxide levels may all have contributed to and interacted to produce the reconstructed picture of a cold sparsely vegetated arid landscape with large rivers and perennial lakes the largest changes in circulation patterns over the glacial cycle probably occurred in the location and or intensity of summer tropical convergence in northern australia over southern australia changes to the temperature and humidity of the westerly circulation have been more significant than the small fluctuations in latitude of the sub tropical high pressure ridge

is there evidence for global scale forcing of southern hemisphere quaternary desert dune accumulation a quantitative method for testing hypotheses of dune system development luminescence dating of desert dune sediments has generated many hundreds of ages many used in reconstructions of quaternary environmental changes others in attempts to elucidate dune processes environmental and climatic interpretations of these records have proved problematic and it remains challenging to test hypotheses of the systematic response of dunefields to changes in external forcing in the past and to make predictions of the future we use a method that quantifies dune sediment accumulation to interpret dune luminescence age datasets rather than simply using the ages themselves as proxies of change the accumulation intensity method allows periods of dune sediment accumulation here over the timescale 102 105 years to be identified from compilations of dated sand sea stratigraphic sequences we apply this approach to two of the largest dune age datasets from southern africa and australia testing whether or not dunefield accumulation has co varied in the late quaternary and whether systematic relationships to external drivers at global hemispheric regional and local scales can be identified

luminescence dating of fluvial and coastal red sediments in the se coast india and implications for paleoenvironmental changes and dune reddening abstract the holocene and late pleistocene environmental history of the teri sandy waste in local parlance red sands in the southeast coastal tamil nadu was examined using remote sensing stratigraphy and optically stimulated luminescence osl dating geomorphological surveys enabled the classification of the teri red sands as 1 inland fluvial teri 2 coastal teri and 3 near coastal teri dunes the inland teri sediments have higher clay and silty sand component than the coastal and near coastal teri suggesting that these sediments were deposited by the fluvial process during a stronger winter monsoon around 15 ka the coastal teri dunes were deposited prior to 11 4 0 9 ka and the near coastal dunes aggraded at around 5 6 0 4 ka we interpret that the coastal dunes were formed during a period of lower relative sea level and the near coastal dunes formed during a period of higher sea level dune reddening is post deposition occurred after 11 4 0 9 ka for the coastal teri dunes and after 5 6 0 4 ka for the near coastal teri dunes presence of microlithic sites associated with the coastal dunes suggest that the cultures existed in the region during 11 4 0 9 ka and 5 6 0 4 ka

evolution of the lower saxony basin abstract the lower saxony basin is a 300 km long and 65 km wide late jurassic and early cretaceous trough which parallels the northern margin of the rhenish massif this basin became inverted during the senonian and paleocene the lower saxony basin is the most important oil province of germany past production amounts to 200 10 6 t of oil 1 5 10 9 bbl the lower saxony basin is superimposed on the late carboniferous variscan foredeep basin and the frontal structures of the variscan fold belt during the permian to middle jurassic the area formed part of the northwest european basin differential subsidence of the lower saxony basin was initiated during the late jurassic and persisted till albian time this involved reactivation of permo carboniferous fracture systems by divergent wrenching a major aptian deep seated laccolith is evident in the western part of the lower saxony basin during the senonian and paleocene sinistral convergent wrenching induced uplift of the basin floor and folding of its sedimentary fill the margins of the inverted lower saxony basin are thrust over the adjacent stable platform areas the differential subsidence of the lower saxony basin can be related to a period of major crustal extension in the north sea rift basin inversion coincides with important orogenic events in the alpine system and is thought to have been induced by intra plate compressional stresses

quartz veins record vertical flow at a graben edge fulmar oil field central north sea the fulmar oil field lies at the faulted western margin of the central graben in the central north sea chalcedony and quartz and later dolomite occur as diagenetic cements in veins within the upper jurassic fulmar formation quartz in one particular vein contains primary aqueous fluid inclusions with homogenization temperatures ranging from 85 degrees to 140 degrees c with a mean of 117 degrees c the measured d18o of the authigenic quartz ranges from 25 1 to 27 1o oo smow the d18o of the water from which most of the quartz precipitated is calculated to have been around 8o oo which is significantly more positive than the measured present day formation water d18o of 4o oo quartz and chalcedony cements in the vein are postdated texturally by dolomite that has a d18o of 23 5o oo smow high salinities are recorded in the fluid inclusions in the quartz the majority ranging from 15 to 19 wt nacl equivalent also the quartz contains inclusions of anhydrite high salinity aqueous inclusions combined with anhydrite inclusions in the quartz and the 18o enriched calculated water composition suggest that the quartz precipitated from an evaporite influenced fluid the same fluid contributed mg for dolomite precipitation more deeply buried sections including the permian zechstein evaporites underlying the fulmar field are the most obvious source of an 18o enriched saline fluid mineralization of the vein resulted from cross formational flow of warm fluids from depth through the zechstein and into the jurassic succession over vertical distances of 600 2000 m fluids most probably migrated up faults and fractures during episodes of salt movement or overpressure release

discrete single domain and pseudo single domain titanomagnetite particles in silicic glass of an ash flow tuff single domain and pseudo single domain titanium poor magnetite grains are present in silicic vitrophyre glass of an ash flow tuff from western nevada conventional transmission electron microscopy and analytical electron microscopy corroborate inferences made solely from rock magnetic and remanence data that the thermoremanent magnetization in the vitrophyre is carried by such grains the magnetite grains vary in size shape and distribution throughout the glass they probably crystal lized during violent eruption and welding of the ash flow at temperatures well in excess of 600 c

magnetic grain size variations through an ash flow sheet influence on magnetic properties and implications for cooling history rock magnetic studies of tuffs are essential to the interpretation of paleomagnetic data derived from such rocks provide a basis for interpretation of aeromagnetic data over volcanic terranes and yield insights into the depositional and cooling histories of ash flow sheets a rhyolitic ash flow sheet the miocene aged tiva canyon member of the paintbrush tuff contains both titanomagnetite phenocrysts present in the magma prior to eruption and cubic fe oxide microcrystals that grew after emplacement systematic variations in the quantity and magnetic grain size of the microcrystals produce large variations in magnetic properties through a section of the ash flow sheet penetrated in a borehole on the nevada test site natural remanent magnetization varies from less than 1 10 4 to more than 8 10 4 a m3 kg 1 and in phase magnetic susceptibility varies from less than 1 10 6 to more than 10 10 6 m3 kg 1 the microcrystals which include both magnetite and maghemite have curie points and maximum unblocking temperatures between 580 c and 640 c rock magnetic data including in phase and quadrature magnetic susceptibilities as well as hysteresis parameters demonstrate that these microcrystals are of superparamagnetic and single domain sizes titanomagnetite phenocrysts are the dominant remanence carriers in the central 50 m of the section whereas microcrystals are important contributors to remanent magnetization and magnetic susceptibility in two 15 m thick zones at the top and bottom within these zones the size of microcrystals decreases both toward the quenched margins and toward the interior of the sheet the decrease in microcrystal size toward the interior of the sheet is interpreted to indicate the presence of a cooling break possibly represented by a concentration of pumice laboratory heating produces changes in magnetic properties which vary complexly from sample to sample such changes include 1 decrease in saturation magnetization 2 both increase and decrease in in phase and quadrature magnetic susceptibilities and 3 shifts in coercivity spectra the complex sample to sample variation in response to heating is attributed to differences in initial grain size distributions

lateral migration thermoluminescence chronology and colour variation of longitudinal dunes near birdsville in the simpson desert central australia the longitudinal dunes of the simpson desert in the vicinity of birdsville have been reworked largely during the holocene from dunes deposited up to 80000 years ago or earlier the widespread asymmetry of these roughly northward trending dunes with steeper eastern faces and more gentle western faces supports wind rose data showing sand transporting winds from the southwest obliquely intersecting the dunes while this suggests a change in the wind pattern since the dune field was oriented it does not indicate that the dunes are necessarily shifting leeward eastward as a consequence it is hypothesized that the direction of migration is controlled by the extent to which the dunes are vegetated relatively well vegetated dunes can accrete sand on their gentle stoss slopes and erode on their lee slopes causing them to shift westward and hence obliquely into the wind a condition that probably prevails in wetter regions and during episodes of relatively humid climate in contrast in very dry areas or during arid phases sand can move unimpeded up a sparsely vegetated stoss face and over the crest to form an avalanche or slip face on the lee side thereby causing the dunes to shift eastward despite evidence that longitudinal dune crests can shift laterally to some extent the dunes in the western part of the simpson desert have not migrated either westward or eastward more than 100m or so from their pleistocene cores aeolian transport and partial or complete removal of iron cutans from around quartz grains results in dunes of widely varying colour yet of similar age

paleomagnetic evidence for high temperature emplacement of the 1883 subaqueous pyroclastic flows from krakatau volcano indonesia the paroxysmal 1883 eruption of krakatau volcano in indonesia discharged at least 6 5 km3 dense rock equivalent of pyroclastic material into the shallow waters of the sunda straits within a 15 km radius of the volcano progressive thermal demagnetization studies of individually oriented pumice clasts from a core sample of the submarine pyroclastic deposits show that 41 out of 47 clasts exhibit single component remanence with mean inclination of 2 4 the partial thermoremanent magnetization components of both pumice and lithic clasts are well grouped in orientation indicating that substantial cooling of clasts must have occurred following deposition estimated subaqueous emplacement temperature for such clasts is greater than 500 c rare two component lithic fragments exhibit inflection points on vector endpoint diagrams that mark the temperature below which the fragments acquired magnetization of similar orientation these inflection points range from 350 to 550 c indicating a minimum subaqueous emplacement temperature of 350 c paleomagnetic evidence for high emplacement temperature supports the hypothesis that proximal 1883 submarine pyroclastic deposits resulted from entrance of hot subaerially generated pyroclastic flows into the sea similar deposits have been interpreted from the geologic record but this is the first documented example of submarine pyroclastic flows from a historic eruption the krakatau deposits thus serve as an important modern analog for the study of pyroclastic flow seawater interactions

optical dating of an aeolian deposit on the murrumbidgee floodplain abstract optical dating of quartz grains from an aeolian deposit using multiple aliquot additive dose and regenerative dose growth curves combined using the australian slide method produced ages slightly exceeding the 14 c 2 age range of 3470 4230 yr from charcoal fragments from the same horizon a slightly greater degree of scatter was observed about the additive dose growth curve than about the regenerative dose growth curve raising the possibility that the sample might have been composed of grains containing different palaeodose p as a result of incomplete predepositional bleaching this possibility was first investigated using small aliquots 10 grains and a single aliquot regenerative dose sar protocol asymmetric p distributions were observed initially interpreted as supporting the presence of a partially bleached population single grains of quartz were then examined to determine if several discrete p populations could be resolved thereby providing information on reworking events and larger aliquots were then measured in an attempt to reconcile the australian slide findings with the small aliquot and single grain results

formation and age of desert dunes in the lake eyre depocentres in central australia the sand dunes of the deserts surrounding lake eyre record the latest episode of climatic development from the warm humid conditions of the early tertiary to the hot and arid climate prevailing today the dunes are typical seif dunes which rest on a substrate of late pleistocene fluviatile and lacustrine deposits the sand ridges are thus essentially of holocene age although earlier cycles of aridity are evidenced by remnants of older dune fields the dunes are still advancing across the desert plains blocking river channels and causing tracks to be diverted the dune sands are white in the source areas but they become increasingly red downwind due to the development of a hematite coating on the sand grains the dunes derive from sand accumulations bordering the lee side of sand sources such as playas and alluvial plains large numbers of short sand ridges develop from transverse barchan like forms the incipient dunes coalesce downwind from the generating area and progressively fewer but larger dunes evolve from the many small ones variations in dune spacing both along and normal to the dune trend are related to sand supply and reinforcement mechanisms dune height is directly related to the transverse spacing internal structure and observed short term changes in dune asymmetry caused by different wind directions suggest that the dunes are depositional and that once beyond the generating zone the dunes are maintained by a bidirectional wind regime the resultant of the dominant wind directions determines the long axis of the sand ridges and the trend of migration direction several man years of field observations let us reject helicoidal flow regimes as a significant dune forming mechanism

the evolution of the deep flow regime at soultz sous forets rhine graben eastern france evidence from a composite quartz vein drilling at soultz sous for cflx e ts france conducted as part of the european hot dry rock project intersected a fracture lined with vein quartz and actively producing hot 150h degree c water at 2174 m depth in the granite basement to the rhine graben at least seven generations of quartz are present within the vein but fluid inclusion homogenization temperatures show that temperatures remained within 15h degree c of the present field temperature throughout its development apart from a few rare pulses of hotter water in contrast freezing measurements indicate two distinct fluids the more saline one ranging from 10 to 15 wth percent nacl eq dominated the early stages of vein fill while the second ranging from 0 to 8 wth percent nacl eq become more prominent in later stages oxygen isotope analyses of different quartz generations also show two populations in addition to magmatic relicts in the early stages a population with delta sup 18 o from 14 per thousand to 18 per thousand predominates and is consistent with growth from a sedimentary basin brine later quartz shows lighter values 12 per thousand endash 13 per thousand resulting from precipitation from a fluid isotopically identical to the modern fluid which is in equilibrium with granite feldspar at a temperature approximately 25h degree c higher than the present vein temperature the changemore in delta sup 18 o does not however correlate with that in fluid salinity reflecting the different effects of fluid rock interaction on the two parameters changes in fluid salinity possibly resulting from meteoric inputs into the deep formation waters of the rhine graben occurred early in the flow path of the soultz fluids because there is no decrease in fluid temperature associated with even the most dilute inclusion fluids with time the path by which these fluids from deep in the mesozoic sediments of the graben have made their way to the soultz site has changed resulting in greater interaction with granite as recorded in the oxygen signature copyright 1998 american geophysical union less

late cretaceous intraplate thrusting in central europe effect of africa iberia europe convergence not alpine collision a well established event of intraplate basin inversion and basement thrusting affected central europe in late cretaceous time it is widely accepted to have resulted from the collision of the alpine orogen with europe s margin at that time an early alpine orogen located on the leading edge of the adria microplate still lay far southeast of its present day position and had entered a phase of extension after a first orogenic event characterized by w to nw directed thrusting this configuration is not likely to have induced ssw nne directed thrusting and folding in the future european foreland that was still separated from the alpine wedge by a strip of oceanic lithosphere by contrast the onset of intraplate contraction coincides with an important change in relative motion between the european and african plates at ca 90 ma africa s sse directed sinistral transform motion relative to europe changed to ne directed convergence this agrees well with the timing and kinematics of intraplate thrusting in central europe structures of similar age and kinematics occurring in southern france spain and north africa suggest that the late cretaceous pulse of contraction was caused by pinching west central europe s thin lithosphere between baltica and africa only since the onset of n directed thrusting in the alps in paleocene or eocene time are the kinematics of the alps and their european foreland compatible indicating that mechanical coupling between africa europe and the adria microplate had been achieved

the onset of dune formation in the strzelecki desert south australia abstract this study is concerned with the late quaternary climatic chronology of the strzelecki desert dunefields in central australia the sand ridges comprise layers of quartz sand some of which include palaeosol horizons with carbonated rootlets providing excellent opportunity for dating of alternations of dune building and stability by using optically stimulated luminescence osl deduced from the osl age of the oldest aeolian layer dated we conclude that the onset of aridity dates back to at least 65 ka older phases of aeolian activity though following a fluvial depositional phase 160 ka ago cannot be excluded although no aeolian layers giving evidence for this have been found in the two dunes dated here unconsolidated dune sands in the upper part of one section with late holocene 4 ka to modern depositional ages indicate a reactivation of the dunefield in recent times from the crosscheck of 14 c ages of the carbonated rootlets with osl results it is concluded that under the given environmental conditions radiocarbon dating of the calcareous rootlets is not able to provide reliable ages for the phase of soil development

intraformational fluid flow in the thuringian syncline germany evidence from stable isotope data in vein mineralization of upper permian and mesozoic sediments abstract vein mineralization observed in boreholes quarries and outcrops of upper permian and mesozoic sediments in the thuringian syncline germany was investigated for the first time by petrography electron microprobe analysis and stable isotope analysis the objective was a characterization of paleo fluid systems within this basin veins are mostly restricted to nw se trending fault systems and comprise carbonates calcite dolomite minor sulfates gypsum anhydrite celestite barite and rarely sulfides they are variably distributed within the late paleozoic to mesozoic succession calcite vein mineralization occurs in almost all triassic strata though dominating in the lower muschelkalk subgroup mu gypsum anhydrite veins occur in the middle keuper subgroup km middle muschelkalk subgroup mm middle and upper buntsandstein subgroup sm so as well as in the upper permian zechstein sulfate and carbonate isotope analyses indicate that most vein mineralization dominantly crystallized from intraformational fluids within the upper permian and triassic sediments crystallization from descending meteoric waters was observed especially for calcite veins in the lower muschelkalk but also for gypsum anhydrite veins in the upper buntsandstein mineralization from ascending carbonate ph buffered warm waters was restricted to individual parts of the southern deep reaching main border fault zone of the thuringian syncline mixing and fault crossing fluid movements are subordinate within the thuringian syncline we conclude that no large scale cross formational fluid flow occurred in thuringian syncline over the last millions of years

the age of the palaeodunefield of the northern murray basin in south australia preliminary results abstract a vast field of old desert dunes extends from northwestern eyre peninsula across yorke peninsula and the northern adelaide plains into the murray basin and northwestern victoria there are patches of parabolic forms but the sand ridges under review are of seif linear or longitudinal type they trend nw se in the west and west east in the east here we record luminescence dates for three dune sites in the waikerie district of the northwestern murray basin they range from respectively 151 25 3 ka 157 33 3 ka and a basal age of 59 6 ka with sand movement also indicated around 1906 and 1933 ce apart from the last named no unconformities are discernible in the sampled sections

palaeomagnetic stratigraphy of lake bungunnia plio pleistocene precursor of aridity in the murray basin southeastern australia abstract the murray darling river system in southeastern australia formerly flowed into a large lake lake bungunnia before reaching the sea fine grained clastics of this system the blanchetown clay formation extend over an area of about 68 000 km2 underlying the mallee region with its surface array of dunes and salt lakes palaeomagnetic analyses are used to date the major hydrologic transition from the expansive freshwater system to the saline deflation playas and dunes that characterize this landscape today two sites were selected for sampling one a cliff section on the left bank of the murray river at chowilla the other from within sediments of lake tyrrell a saline playa cut into sediments of the older freshwater system two sequences were sampled at tyrrell one by coring to 30 m in the basin centre the other a composite cliff and 5 m core on the western margin of the playa post bungunnia sediments representing modern landforms were sampled in both surficial dune palaeosol units of the woorinen formation and in saline sediments the tyrrell beds within the modern saline playa the sequence of multiple aeolian units of the woorinen formation and gypseous sediments of the tyrrell beds are normally magnetized indicating deposition within the past 0 7 million years by contrast the uppermost horizons of bedded blanchetown clay below a surface zone of pedogenic alteration are reversed the brunhes matuyama b m boundary key to the age of draining of lake bungunnia by deepening the overflow channel to the sea is obscured within the zone of pedogenesis separating unaltered blanchetown clays from overlying aeolian deposits the onset of aeolian and saline gypseous deposits characteristic of the arid zone facies is placed soon after the b m boundary about 500 000 years ago sequences through the blanchetown clay pass down through the matuyama reversed chron into the gauss normal chron with the lowermost section on the tyrrell west bank recording a basal reversed zone of possible gilbert age thus the origins of lake bungunnia extend back beyond basal pleistocene into the late pliocene the lake s duration covers more than 2 million years spanning the plio pleistocene boundary in this important region of southeastern australia

electron optical characterization of bacterial magnetite abstract magnetic grains isolated from magnetococcoid bacterial cells were studied by means of transmission electron microscopy electron diffraction and electron microprobe analysis observed in situ the magnetic grains are each surrounded by an organic membrane and are usually found in a random array although chains are also seen electron diffraction confirms the magnetite mineralogy and provides additional evidence in favor of vacancies in the structure electron microprobe analysis shows the magnetite to be slightly titaniferous electron microscopy indicates that the grains rather than being flake shaped are parallelepiped crystals with a mean length of 99 3 8 7nm a mean width of 62 3 6 1nm yielding a width to length ratio of 0 63 these data support the contention that the magnetic bacterial grains are single domain crystals capable of producing a natural remanent magnetization in sediments

silicon in magnetite high resolution microanalysis of magnetite ilmenite intergrowths magnetite ilmenite oxidation exsolution intergrowths from an original titanomagnetite microphenocryst from an ash flow tuff unit have been studied using conventional transmission electron microscopy and analytical electron microscopy silicon has been found to be in solid solution in all of the magnetite studied and in some of the coexisting ilmenite the average value in magnetite is 1 2 wt si equivalent to solid solution of 9 mole fe2sio4 silicon is also present in very small silicate inclusions and as unusual si rich domains of uncertain origin in magnetite the inclusions and domains may be irregularly distributed through the magnetite in sizes well below those resolvable with the electron microprobe microprobe analyses for si in magnetite generally reflect these heterogeneities in addition to a component presumably in solid solution the petrologic implications of the data can be assessed only when relevant thermochemical data become available and the distribution of si in magnetite is better understood

geomagnetic paleointensity in historical pyroclastic density currents testing the effects of emplacement temperature and postemplacement alteration thellier type paleointensity experiments were conducted on welded ash matrix or pumice from the 1912 novarupta nv and 1980 mt st helens msh pyroclastic density currents pdcs with the intention of evaluating their suitability for geomagnetic paleointensity studies pdcs are common worldwide but can have complicated thermal and alteration histories we attempt to address the role that emplacement temperature and postemplacement hydrothermal alteration may play in nonideal paleointensity behavior of pdcs results demonstrate two types of nonideal behavior unstable remanence in multidomain md titanomagnetite and nonideal behavior linked to fumarolic and vapor phase alteration emplacement temperature indirectly influences msh results by controlling the fraction of homogenous md versus oxyexsolved pseudo single domain titanomagnetite nv samples are more directly influenced by vapor phase alteration the majority of nv samples show distinct two slope behavior in the natural remanent magnetization partial thermal remanent magnetization plots we interpret this to arise from a thermo chemical remanent magnetization associated with vapor phase alteration and samples with high water content 0 75 loss on ignition generate paleointensities that deviate most strongly from the true value we find that pdcs can be productively used for paleointensity but that as with all paleointensity studies care should be taken in identifying potential postemplacement alteration below the curie temperature and that large welded flows may be more alteration prone one advantage in using pdcs is that they typically have greater areal spatial exposure than a basalt flow allowing for more extensive sampling and better assessment of errors and uncertainty

luminescence dating of late holocene dunes showing remnants of early settlement in cuddalore and evidence of monsoon activity in south east india abstract in this study dunes from south east india were dated using optically stimulated luminescence osl to reconstruct the depositional history a belt of dunes has developed parallel to the coast between pondicherry and karikal in tamil nadu south east india in the area between cuddalore and porto novo the dune belt is 5 km wide a transect from the coast to the most western dune inland was investigated changes in the environmental conditions are recorded in the dunes they show features including unconformities changes in the direction of bedding erosional features water escape structures and remnants of human settlements the osl results show that strong changes in the environmental conditions occurred about 100 and 300 years ago the latter event marks also the termination of settlement in this place the settlement period started about 1500 years previously the periods of sand mobility and stabilisation of the land surface by soil formation correlate with changes in the precipitation record of india the investigated dunes very likely reflect fluctuations in the monsoon activity during the last 3500 years in south east india

magnetic anisotropy in rhyolitic ignimbrite snake river plain implications for using remanent magnetism of volcanic rocks for correlation paleomagnetic studies and geological reconstructions some of the field and laboratory expenses for this work were covered by the natural environment research council nerc grant ne g005372 1 the salary and school fees for the first author were covered in part by a national science foundation nsf grant ear 443549 81179 awarded to x zhao

magnetism and magnetic mineralogy of ash flow tuffs from yucca mountain nevada the magnetic susceptibility and remanent magnetization of an ash flow sheet are profoundly influenced by cooling history after emplacement spatial variations in these properties therefore provide a potentially useful tool for geological studies maxima and minima in measured along profiles at yucca mountain nevada identify persistent magnetic marker horizons within vitric portions of the tiva canyon and underlying topopah spring members of the paintbrush tuff the correlatable variations of with vertical position in these units exist over the 8 km of strike examined the observed stratigraphic changes in magnetic properties reflect variations in amounts and mineralogy of fe ti oxide phenocrysts and the presence shape size and mineralogy of magnetic fe oxide microcrystals that precipitated at high temperature after emplacement of each sheet vitrophyre near the top of the topopah spring member has a distinctive maximum in due to titanomagnetite phenocrysts susceptibility in the lower portion of the overlying tiva canyon member reaches maximum values of about 1 3 10 2 si near the base of the member beneath the columnarly jointed vitrophyre due to the presence of abundant superparamagnetic cubic fe oxide these elongate microcrystals are several tens of nm long and are typically deformed and tapered up section in the tiva canyon member the susceptibility decreases going to a minimum within the columnar vitrophyre at this stratigraphic level the precipitated fe oxides are larger and carry intense nrm of up to 30 a m is only 3 10 3 the whisker like microcrystals of fe oxide which typically nucleated on what appears to be chain silicate grew as needles up to 25 m in length the size variations of the precipitated fe oxides which were established using transmission electron microscopy tem and petrographic observation are consistent both with variations in magnetic susceptibility measured at the outcrop and with variations in the intensity of remanent magnetization several interpretations of the shape anisotropy of the precipitated feoxide are possible including growth by a dislocation mechanism additionally the observed elongation of precipitated microcrystals is consistent with theoretical predictions for growth in a uniaxial stress field susceptibility variations as established at the outcrop as well as in the borehole offer a potentially useful tool for stratigraphic correlation of ash flow sheets

late quaternary palaeoenvironmental change in the australian drylands in this paper we synthesise existing palaeoenvironmental data from the arid and semi arid interior of the australian continent for the period 40 0 ka moisture is the predominant variable controlli

carbonate diagenesis and feldspar alteration in fracture related bleaching zones buntsandstein central germany possible link to co 2 influenced fluid mineral reactions fracture related bleaching of lower triassic buntsandstein red beds of central germany was related to significant carbonate diagenesis and feldspar alteration caused by co2 rich fluids using cathodoluminescence microscopy and spectroscopy combined with electron microprobe analysis and stable carbon isotope study two major fluid mineral interactions were detected 1 zoned joint filling calcites and zoned pore filling calcite cements the latter replacing an earlier dolomite were formed during bleaching during the calcite formation and dolomite calcite transformation iron was incorporated into the calcite cement crystal cores due to fe availability from the coeval bleaching the dedolomitisation was ultimately associated with a volume increase the related permeability decrease implies a certain degree of sealing and increasing retention of co2 and the volume increase offers a minor co2 sink carbonate rich sandstone therefore can provide advantages for underground co2 storage especially when situated in the fringes of the reservoir 2 alkali feldspar alteration due to the bleaching fluids is reflected in cathodoluminescence spectra predominantly by the modulation of a brown luminescence emission peak 620 nm this peak represents a newly discovered effect related to alkali feldspar alteration not solely associated with bleaching its modulation by the bleaching is interpreted to be due to na depletion or a lattice defect in the si o bonds of the sio4 tetrahedron alteration reflected by this luminescence feature has a destructive effect on the feldspars implying the possibility of diminished rock integrity due to bleaching and hence co2 rich fluids two further cl spectral changes related to bleaching occur a decreased intensity between around 570 nm assigned to mn depletion and b increased amplitude and wavelength shift of the red 680 nm band converging evidence from carbonate and feldspar diagenesis stable carbon isotope data and analysis of fracture directions suggests that co2 fluids contributed to a significant extent to the bleaching phenomena and alteration in the studied buntsandstein strata

distinguishing and correlating deposits from large ignimbrite eruptions using paleomagnetism the cougar point tuffs mid miocene southern snake river plain idaho usa in this paper we present paleomagnetic geochemical mineralogical and geochronologic evidence for correlation of the mid miocene cougar point tuff cpt in southwest snake river plain srp of idaho the new stratigraphy presented here significantly reduces the frequency and increases the scale of known srp ignimbrite eruptions the cpt section exposed at the black rock escarpment along the bruneau river has been correlated eastward to the brown s bench escarpment six common eruption units and cassia mountains three common eruption units regions of southern idaho the cpt records an unusual pattern of geomagnetic field directions that provides the basis for robust stratigraphic correlations paleomagnetic characterization of eruption units based on geomagnetic field variation has a resolution on the order of a few centuries providing a strong test of whether two deposits could have been emplaced from the same eruption or from temporally separate events to obtain reliable paleomagnetic directions the anisotropy of anhysteretic remanence was measured to correct for magnetic anisotropy and an efficient new method was used to remove gyroremanence acquired during alternating field demagnetization

lake bungunnia a plio pleistocene megalake in southern australia abstract between 2 5 ma and 0 7 ma a plio pleistocene megalake lake bungunnia existed in the western murray basin australia study of hydrologic and geologic constraints on this lake has enabled the climate of southeastern australia during the late pliocene and early pleistocene to be partly quantified modelling shows that lake bungunnia could not exist under current climatic conditions and that an average rainfall of at least 500 mm yr 1 over the catchment of the murray darling river system was likely considerably more than at present minor changes in climate caused large fluctuations in the shoreline of the shallow lake which are reflected in the geologic record possible histories for lake bungunnia are suggested in the light of this evidence demise of the megalake heralded the onset of aridity in southern australia and modern landforms date from that time salt lakes in part of the western murray basin may have directly descended from lake bungunnia

how do longitudinal dunes respond to climate forcing insights from 25 years of luminescence dating of the australian desert dunefields abstract this paper presents a meta analysis of 689 luminescence age estimates of australian desert sand dunes this analysis had two aims to examine the hypothesis that quaternary climate changes have forced dune accumulation and to understand longitudinal dune behaviour from the age architecture of sand dunes for which stratigraphic information is often absent or poorly known a novel approach to the analysis of probability density curves of dune age frequency shows that in the mallee dunefield of southeastern australia only the period leading up to and including the last glacial maximum lgm 18 34 ka experienced marked dune growth which cannot be explained by random stochastic activity during the lgm most but not all mallee longitudinal dunes were accumulating more rapidly than before and after in the strzelecki dunefield source bordering sbd longitudinal dune tl n 61 and osl n 40 ages are more frequent at the pleistocene holocene transition but nearby non sbd longitudinal dunes n 21 ages were only active above background levels during the lgm however the strzelecki dune age data set suffers from being drawn from a small number of individual dunes 10 sbd dated using tl 12 sbd dated by osl 7 non sbd dated by osl with a marked bias away from surficial young and basal old depths even during the lgm many dunes do not record active growth and the dunefield was most likely a mosaic of stable dune surfaces and bare mobile patches supported by the scant pollen data from the arid zone hyper aridity has not been experienced in these dunefields these vegetated dunes exhibit conservative behaviour with limited response to the range of climate forcing experienced in the late pleistocene a more complete understanding of dunefield response to quaternary climates would require an intensive and structured dating program to overcome the noise from stochastic processes leading to small scale activity in these dunes and the simultaneous removal of parts of the sedimentary record

elongated prismatic magnetite crystals in alh84001 carbonate globules potential martian magnetofossils using transmission electron microscopy tem we have analyzed magnetite fe 3o 4 crystals acid extracted from carbonate globules in martian meteorite alh84001 we studied 594 magnetites from alh84001 and grouped them into three populations on the basis of morphology 389 were irregularly shaped 164 were elongated prisms and 41 were whisker like as a possible terrestrial analog for the alh84001 elongated prisms we compared these magnetites with those produced by the terrestrial magnetotactic bacteria strain mv 1 by tem again we examined 206 magnetites recovered from strain mv 1 cells natural darwinian selection in terrestrial magnetotactic bacteria appears to have resulted in the formation of intracellular magnetite crystals having the physical and chemical properties that optimize their magnetic moment in this study we describe six properties of magnetite produced by biologically controlled mechanisms e g magnetotactic bacteria properties that collectively are not observed in any known population of inorganic magnetites these criteria can be used to distinguish one of the modes of origin for magnetites from samples with complex or unknown histories of the alh84001 magnetites that we have examined the elongated prismatic magnetite particles 27 of the total are indistinguishable from the mv 1 magnetites in five of these six characteristics observed for biogenically controlled mineralization of magnetite crystals

punctuated aridity in southern africa during the last glacial cycle the chronology of linear dune construction in the northeastern kalahari abstract the mega kalahari of central southern africa is one of the most extensive quaternary desert basins on a regional scale present day aeolian activity is restricted to episodic dune crest reactivation in the most arid southwestern desert core there is however abundant evidence of former periods of both more arid and more humid conditions many of which have little or no chronological control we have employed optical dating of quartz sand grains to develop a chronology of arid intervals as recorded by phases of linear dune construction in the northeastern sector of the mega kalahari we identify repeated phases of aeolian deposition during the last interglacial glacial cycle at ca 95 115 41 46 20 26 and post 20 ka which are separated by depositional hiatuses that we infer to correspond to more humid periods these aeolian depositional events correlate with and are inferred to relate to millennial scale cold sea surface temperature events in the southeast atlantic which have been linked to sub milankovitch climate changes recognised in northern hemisphere oceanic and cryospheric environmental archives covering the same time period while the present landscape is the product of either post 20 ka hwange national park dune field or 20 30 ka victoria falls dune field aeolian activity and subsequent erosion and reworking much of the vertical expression of the larger dune forms corresponds to the earlier periods of activity the linear ridges of the area are rich archives of late quaternary terrestrial climate change

synchrony of southern hemisphere late pleistocene arid episodes a review of luminescence chronologies from arid aeolian landscapes south of the equator abstract luminescence dating chronologies that have been reported in various studies of late pleistocene arid land aeolian sequences from australia southern africa and south america are reviewed with the objective of determining whether any pan hemispheric synchroneity existed in the timing of landscape responses to late quaternary climate changes south of the equator the results broadly show that contemporaneous regional arid land aeolian activity occurred in australia and southern africa during the periods ca 65 41 and 36 9 ka the relatively limited luminescence data from the south american subcontinent also point to recurrent arid land aeolian deposition between 63 and 8 ka with increasing frequency in the period ca 32 8 ka records from all three continents suggest a period of reduced aeolian activity between 41 and 36 ka overall the data are indicative of extensive landscape instability of the low to mid latitudes in the southern hemisphere during the last glacial period particularly at the last glacial maximum which accords with mainstream opinions aeolian activity during the holocene has generally been localised the rigour with which comparisons can be made between the different studies however is constrained by variations in practices between laboratories and changes that have occurred to luminescence dating procedures over the years

the finne fault zone central germany structural analysis of a partially inverted extensional fault zone by balanced cross sections the finne fault zone located in central germany to the southwest of the harz mountains was studied by means of detailed map analysis investigations of fault displacement and balanced cross sections for the most strongly deformed area in the center of the fault zone ca 50 of total fault zone length the system of the finne fault zone shows a nearly 100 km long straight flexure that symbolizes the morphological and geological northeastern border of the thuringian basin in the central part which should be surveyed here the fault zone corresponds to a distinctive narrow band of highly deformed triassic sedimentary rocks the northwestern and especially the southeastern parts of the research area are developed as several parallel branch faults in the southeastern elongation of the fault zone which is not part of our survey the sedimentary cover is missing here it is possible to gain insight to the fact that the basement is also involved to the kinematics of the fault zone based on our results we propose a subdivision of the fault zone into four sectors from the northwest to the southeast we interpret the structure of these sectors to reflect 1 a compressional flexure 2 an overthrust graben and 3 a partially inverted and folded half graben in the extreme southeast 4 the fault zone is characterized by an anticline with some strike slip movement parallel to the fold axis this segmentation is caused by a thrust fault system whose strike direction deviates slightly from that of the earlier formed graben system the structural configuration can be explained by a two phase deformation in which the contractional strain exceeded the preceding extensional deformation in the investigated area the horizontal shortening attains a maximum of ca 1 km the present study confirms many earlier hypotheses presents new results on the deformation history of the fault zone and attempts to evaluate the deformation in a regional geological context the results of earlier studies were refined and revised using modern methods and a unified structural and kinematic model of the finne fault zone was created

fabrics facies and flow through a large volume ignimbrite pampa de oxaya chile the miocene oxaya formation exposed along the western andean slope in northern chile represents one of the largest ignimbrite provinces on earth in this study magnetic fabric data were acquired from a 1 km long core drilled vertically through a single cooling unit of the oxaya formation the ca 22 ma cardones ignimbrite samples for magnetic analysis were obtained every 20 m from the fine grained matrix of the core detailed measurements of the variation in bulk magnetic properties including natural remanent magnetization nrm susceptibility and anisotropy of magnetic susceptibility ams were used to monitor changes in magnetic mineralogy as well as changes in the strength and orientation of the magnetic fabric throughout the flow ams ellipsoid orientation and shape reflect rapid deposition from a concentrated granular fluidized flow and constrain both the location of the source caldera for this catastrophic eruption and processes of transport and deposition in this large volume ignimbrite after utilizing the magnetic remanence to correct for rotation about the core axis well grouped and imbricated petrofabric orientations reveal a well defined sw 247 transport direction down a proto western andean slope indicating syn or post welding flow and confirming the deformed lauca caldera as the likely source of the eruption systematic variations in fabric shape t and intensity p with depth reveal a predominately oblate fabric toward the top and base of the flow and predominately prolate fabrics in the center these vertical changes in fabric reveal that this massive apparently homogeneous deposit has a systematic layering this layering reflects depth dependent temperature variations temporal changes in the flow boundary zone during deposition and post depositional processes

paleomagnetic evidence for the timing of collapse and resurgence of the lake city caldera san juan mountains colorado rocks of the 23 1 m y old lake city caldera consist of the compositionally zoned sunshine peak tuff postcollapse intracaldera lava flows and resurgent quartz syenite intrusions declinations of reversely magnetized i 45 to 75 sunshine peak tuff change from easterly d 93 130 throughout most of the tuff to southerly d 195 207 within the late eruptive phases the postcollapse lava flows are also reversely magnetized and their emplacement appears to have spanned resurgence of the caldera in contrast the uppermost levels of the resurgent intrusion record two distinct components of magnetization a reversed component isolated by thermal demagnetization typically at temperatures greater than 400 c and a normal component defined by thermal demagnetization at lower temperatures a baked contact test implies that emplacement and initial cooling of the intrusion occurred during a time of reversed polarity and that continued cooling at lower blocking temperatures took place during normal polarity assuming that the reversed directions of the extrusive and intrusive rocks record a single period of reversed polarity as suggested by the paleomagnetic and radiometric age data the time for caldera development from eruption of the ash flow tuffs to emplacement of the resurgent intrusion was less than about 300 000 years on the basis of best estimates of durations of reversed polarities in the early miocene the estimated 300 000 year time span for the development of the lake city caldera is very close in duration to the radiometrically determined time spans of much younger pleistocene calderas having similar dimensions and similar volumes of erupted material

diagenesis of celestite in quartz geodes from the basque cantabric basin northern spain evidence from sulphur and strontium isotopes

carbon isotope stratigraphy using carbonate cements in the triassic sherwood sandstone group corrib field west of ireland abstract carbon stable isotopes from carbonate minerals mainly dolomite from six wells from the lower triassic sherwood sandstones of the corrib gas field slyne basin west of ireland allow stratigraphic correlation the results also provide information on palaeoenvironmental change during the deposition of these continental redbed sedimentary rocks the triassic reservoir rocks have been buried to 4000 m and heated to 165 c and now contain methane rich gas although the oxygen isotopic signal has been at least partially reset during burial and heating a primary carbon isotopic signal appears to have survived diagenesis the carbon isotope ratio varies from 3 2 to 2 1 all six wells show similar stratigraphic changes when all the carbon isotope data are plotted relative to a major playa horizon 13 c increases from about 3 at the base of the sherwood to about 2 170 m above the base 13 c then decreases to about 2 for the next 70 m and remains steady for the following 50 m the top 20 m of the sherwood contains carbonate with a 13 c values decreasing to about 3 the occurrence of a stratigraphically correlatable carbon isotope pattern implies that the primary evolution signal has been preserved the change in 13 c correlates with indicators of aridity and biological stress such that the highest 13 c values are in sedimentary rocks deposited in a playa lake arid times these rocks contain the greatest quantity of dolomite cement conversely the lowest 13 c values correspond to sedimentary rocks deposited from well developed rivers relatively humid times from the lowest quantity of dolomite cement the same carbon isotope evolution has been found in another well in the slyne basin and in belgium suggesting that the palaeoenvironmental isotope signal in the triassic sedimentary rocks of the corrib field may have a regional significance

paleointensity estimates from ignimbrites an evaluation of the bishop tuff 1 ash flow tuffs or ignimbrites typically contain fine grained magnetite spanning the superparamagnetic to single domain size range that should be suitable for estimating geomagnetic field intensity however ignimbrites may have a remanence of thermal and chemical origin as a result of the complex magnetic mineralogy and variations in the thermal and alteration history we examined three stratigraphic sections through the 0 76 ma bishop tuff where independent information on postemplacement cooling and alteration is available as a test of the suitability of ignimbrites for paleointensity studies thermomagnetic curves suggest that low ti titanomagnetite tc 560 c 580 c is the dominant phase with a minor contribution from a higher tc phase s significant remanence unblocking above 580 c suggests that maghemite and or titano maghemite is an important contributor to the remanence in most samples we obtained successful paleofield estimates from remanence unblocked between 440 c and 580 c for 46 of 89 specimens 15 sites at two of three total localities these specimens represent a range of degrees of welding and have variable alteration histories and yet provide a consistent paleofield estimate of 43 0 t 3 2 equivalent to a vadm of 7 8 1022 am2 the most densely welded sections of the tuff have emplacement temperatures inferred to be as high as 660 c suggesting that the remanence may be primarily thermal in origin though a contribution from thermochemical remanence cannot be excluded these results suggest that ignimbrites may constitute a viable material for reliable paleointensity determinations

paleointensities of phonolitic obsidian influence of emplacement rotations and devitrification 1 a paleomagnetic study on phonolitic obsidian from six sites in tenerife spain is presented two sites are located at the 750 ka el pasajiron at the southern wall of las canadas caldera four sites correspond to the 115 17 bce montana blanca complex paleointensity determinations are performed with a modified thellier technique using checks for alteration and domain state additionally the anisotropy of the thermoremanence and the magnetic cooling rate dependency of each specimen are measured it was not possible to obtain good quality paleointensity data for el pasajiron thermal alteration was observed in most measurements systematic changes in rock magnetic properties and paleointensities within the profile of one site also suggest the presence of a chemical remanence crm which was probably acquired during devitrifiction of the upper part of the flow this crm cannot be seen in the arai plots themselves but only by comparison of data from different samples this underlines the importance of sampling only fresh glassy looking obsidians and not devitrified ones paleointensity determinations at montana blanca are of very good quality leading to an high success rate of 82 samples at one site show acquisition of trm during emplacement rotations however it could be shown that paleointensity data of rotated samples are reliable if the remanence carriers are in the single domain range an atrm anisotropy tensor of thermoremanent magnetization and cooling rate corrected paleointensity of 48 4 2 1 t and a vdm of 9 64 0 42 1022am2 were determined which are in very good agreement with other data from spain and morocco and to various geomagnetic field models