the paleomagnetism of single silicate crystals recording geomagnetic field strength during mixed polarity intervals superchrons and inner core growth 1 the basic features of the geomagnetic reversal chronology of the last 160 million years are well established the relationship between this history and other features of the field however has been elusive the determination of past field strength paleointensity is especially challenging commonly accepted results have come from analyses of bulk samples of lava historic lavas have been shown to faithfully record the past field strength when analyzed using the thellier double heating method data from older lavas however tend to show effects of in situ and laboratory induced alteration here we review an alternative approach single plagioclase crystals can contain minute magnetic inclusions 50 350 nm in size that are potential high fidelity field recorders thellier experiments using plagioclase feldspars from an historic lava on hawaii provide a benchmark for the method rock magnetic data from older lavas indicate that the feldspars are less susceptible to experimental alteration than bulk samples this resistance is likely related to the lack of clays in addition magnetic minerals are sheltered by the encasing silicate matrix from natural alteration that can otherwise transform the well defined thermoremanent magnetization into an irresolute chemical remanent magnetization if there is a relationship between geomagnetic reversal frequency and paleointensity it should be best expressed during superchrons intervals with few or no reversals thellier data sets based on single plagioclase crystals from lavas erupted during the cretaceous normal polarity superchron 83 120 million years ago suggest a strong 12 1022 am2 stable field consistent with an inverse relationship between reversal frequency and paleointensity superchrons may represent times when the pattern of core mantle boundary heat flux allows the geodynamo to operate at peak efficiency as suggested in some numerical models thellier data from single plagioclase crystals formed during times of moderate 10 reversals million years reversal occurrence suggest a weaker and more variable field these paleointensity data together with a consideration of paleomagnetic directions suggest that geomagnetic reversals field morphology secular variation and intensity are related the linkages over tens of millions of years imply a lower mantle control on the geodynamo on even longer timescales the magnetization held by plagioclase and other silicate crystals can be used to investigate the proterozoic and archean geomagnetic field during the onset of growth of the solid inner core data from plagioclase crystals separated from mafic dikes together with directional data from whole rocks indicate a dipole dominated field similar to that of the modern 2 5 2 7 billion years ago older archean rocks are of great interest for paleomagnetic and paleointensity investigations because they may record a time when the compositionally driven convection of the modern dynamo may not have been operating and a solid inner core did not play its current role in controlling the geometry of outer core flow most rocks of this age have been affected by low grade metamorphism investigations using single silicate grains provide arguably our best hope of seeing through secondary geologic events and reading the early history of the geodynamo absolute paleointensity measurements of the oldest rocks on the planet will require the further development of methods to investigate silicate crystals with exsolved magnetic minerals that address the uncertainties posed by thermocrystallization remanent magnetization anisotropy and slow cooling fortunately prior work in rock magnetism together with advances in analytical equipment and techniques provides a solid foundation from which these frontier issues can be approached

inferring domain state from magnetic hysteresis in high coercivity dolerites bearing magnetite with ilmenite lamellae abstract hysteresis loops were measured for 16 dolerite samples from separate precambrian dykes whose remanence is probably primary remanence is carried by grains of magnetite subdivided into fine particles usually by ilmenite lamellae as seems common in mafic igneous rocks of high coercivity coercive force h c ranges from 9 to 40 10 3 a m 110 500 oe the ratio j rs j s of saturation remanence to saturation magnetization ranges from 0 10 to 0 45 j rs j s varies in approximate proportion to h c with a constant of proportionality appropriate for pseudo single domain magnetite with one or two domain walls per particle a cooling cycle to 77k causes large 40 on average demagnetization of j rs in most samples supporting the dominance of pseudo single domain magnetite most samples also show a large 29 on average decrease in h c on cooling to 140k suggesting magnetostrictive control of h c perhaps through internal stresses opposing domain wall motion only the three dolerites with h c greater than 30 10 3 a m 380 oe show little change in h c on cooling suggesting dominance by single domain particles with shape anisotropy in magnetite grains subdivided into single domain particles by ilmenite lamellae magnetic interaction may lower j rs j s from the expected 0 5 some theories 1 2 suggests that this is due to a self demagnetizing field from magnetic poles on the surface of each grain of magnetite subdivided by ilmenite and should be corrected for another theory 3 suggests that this correction is not necessary because the self demagnetizing field is already approximately cancelled due to magnetic poles inside each subdivided grain at the ends of magnetite particles the latter suggestion is favoured by our three highest h c dolerites whose j rs j s averages 0 42 0 04 before and 0 66 0 06 after correction for the self demagnetizing field from poles on the surface of magnetite ilmenite grains applying this correction to our other dolerites usually raises j rs j s significantly above 0 5 which is unreasonable for pseudo single domain magnetite hence correcting for self demagnetizing fields from poles on the surface of magnetite ilmenite grains in rocks is not recommended whatever the domain state

the architectural variability of small scale cycles in shelf and ramp clastic systems the controlling factors many sedimentary successions are composed of metre to decametre scale cycles exhibiting various stratal architectures that depend on the complex interplay among several syndepositional controlling factors these controlling factors may produce extremely different stratal and facies architectures along relatively brief distances in the same basin but in contrast they also may form similar architectures in basins that experience a radically different tectonic setting the recognition of factors that controlled the architecture of small scale cycles therefore is very important to reconstruct the depositional history of sedimentary successions these factors include 1 the interplay between the rate of accommodation development and the rate of sediment supply a s 2 the amplitude and shape of the relative sea level curve 3 the position in the shoreface shelf system 4 the basin physiography 5 the shoreline trajectory 6 the climate driven environmental energy 7 the siliciclastic input vs carbonate production shoreface to shelf small scale cycles can be grouped into three main categories r cycles showing a dominant regressive interval t r cycles with transgressive and regressive intervals of similar thickness t cycles showing a dominant transgressive interval although some depositional settings may favour the deposition of a particular type of cycle these cycle motifs appear to be mainly controlled by the interaction of local and wider scale controlling factors for example t cycles may be present in both subsiding and uplifting settings depending on the interplay between the a s and the other factors cited above as a result r t r and t cycles may be recognized within the same depositional system these evidences suggest that care must be taken to apply general models that predict the stratal architecture of cycles mainly in function of the long term a s the correct prediction of the cycle architecture is important also for hydrocarbon researches as the thickness of potential reservoir deposits is the result of the interplay among factors discussed here

saturation magnetostriction and its low temperature variation inferred for natural titanomaghemites implications for internal stress control of coercivity in oceanic basalts summary highly oxidized titanomaghemite in oceanic basalts often carries remanent magnetization of high coercivity stability helping preserve the oceanic magnetic anomaly pattern we study the source of this high coercivity in four oceanic basalts from odp sites 238 572d 470a and 556 containing highly oxidized titanomaghemite titanium content parameter x 0 55 and oxidation parameter z 0 9 on average most of the titanomaghemite is likely in singledomain grains with uniaxial anisotropy because the ratio of saturation remanence j rs to saturation magnetization j s approaches 0 50 j rs j s 0 46 on average we show that the uniaxial anisotropy is very likely magnetostrictively controlled through internal stresses i in the titanomaghemite grains this allows us to use a novel indirect method to estimate the saturation magnetostriction s of the titanomaghemite a saturation remanence j rs is given along the axis of a cylindrical sample of each basalt then a small compression is applied

magma flow inferred from preferred orientations of plagioclase of the rio ceara mirim dyke swarm ne brazil and its ams significance

magnetic hysteresis in young mid ocean ridge basalts dominant cubic anisotropy magnetic hysteresis data from young mid ocean ridge basalts include samples with saturation remanence to saturation magnetization mrs ms ratios greater than 0 5 the theoretical limit for an assemblage of single domain grains with uniaxial anisotropy under the usual assumption of dominant uniaxial anisotropy the narrow single domain grain size distribution implied by these high mrs ms values is difficult to reconcile with petrographic and remanence data that suggest the presence of larger multidomain grains dominant cubic anisotropy provides a plausible explanation for the high mrs ms ratios and if generally valid requires reinterpretation of granulometric and domain state inferences made from hysteresis data

reply to the comment by basu on new weathering classifications for granitic rocks based on geomechanical parameters by heidari et al engineering geology 2013 166 65 73

absolute paleointensity at 1 27 ga from the mackenzie dyke swarm canada 1 paleointensity studies have been conducted on 6 mafic dykes from the 1270 ma mackenzie swarm in the slave province canada the mean direction of the characteristic magnetization coincides with results of an earlier study in which the primary origin of the magnetization was established on the basis of a contact test high unblocking temperatures magnetic mineralogy and grain size experiments suggest that the magnetization is dominated by pseudo single domain or single domain grains of magnetite paleointensity experiments were conducted with a specially designed oven using a revised version of the thellier coe method thirteen successful determinations of paleointensity were obtained for 4 dykes the paleofield estimates vary between 4 3 and 22 1 t yielding virtual dipole moments vdms between 1 3 0 2 and 4 5 0 9 1022 am2 these new results increase the number of low field determinations during the precambrian which largely dominate the database with an averaged field of 3 1 2 5 1022 am2 they also emphasize the importance of additional studies to understand the differences with the strong paleointensities obtained using new techniques

thermochemical remanent magnetization in precambrian rocks are we sure the geomagnetic field was weak 1 thellier paleointensity determinations from two dikes of the early proterozoic 2 46 ga matachewan dike swarm canada yield field values of 2 14 0 18 and 9 81 1 65 t corresponding values of virtual axial dipole moment are very low 0 54 0 05 1022 and 2 49 0 42 1022 am2 respectively when compared with the modern field the characteristic remanent magnetization chrm was isolated over a narrow range of high unblocking temperatures 520 580 c detailed rock magnetic analyses indicate that the chrm is carried by nearly stoichiometric pseudo single domain magnetite scanning electron microscopy sem reveals that the magnetite is in the form of fine intergrowths with ilmenite formed by oxyexsolution during cooling the high temperature oxidation defined in the sem images could have continued at temperatures below the curie point of magnetite in this case the chrm would be a thermochemical remanent magnetization tcrm rather than a thermal remanent magnetization trm estimates of the tcrm trm ratio show that the thellier data could underestimate the true field value by a factor of 4 without violating experimental selection criteria this uncertainty in trm fidelity translates into a potential range of field values that spans that defined by the modern field 8 1022 am2 and proposed low precambrian levels 2 1022 am2 therefore understanding further how tcrm is acquired and records the field represents a major challenge if these and many other similar rocks are to be used in thellier studies aimed at defining the strength of the precambrian field

tectonic implications of the 1419 ma nova guarita mafic intrusives paleomagnetic pole amazonian craton on the longevity of nuna abstract the nuna supercontinent was probably assembled in the paleoproterozoic but its paleogeography and the timing for its demise are still a matter of debate a paleomagnetic and geochronological study carried out on the mesoproterozoic nova guarita dyke swarm northern mato grosso state sw amazonian craton provides additional constraints on the duration of this supercontinent paleomagnetic af and thermal treatment revealed south southwest northeast magnetic directions with downward upward inclinations for 19 analyzed sites these directions are carried by psd magnetite with high unblocking temperatures as indicated by additional magnetic tests including thermomagnetic curves hysteresis loops and the progressive acquisition of isothermal remanence in selected samples a positive contact test with the host granite in one of the studied dykes further attests to the primary origin of the characteristic magnetic component a mean site direction was calculated at d m 220 5 i m 45 9 95 6 5 k 27 7 which yielded a paleomagnetic pole located at 245 9 e 47 9 s a 95 7 0 40 ar 39 ar dating carried out on biotites from four analyzed dykes yielded well defined plateau ages with a mean of 1418 5 3 5 ma the nova guarita pole precludes a long lived nuna configuration in which laurentia baltica north china and amazonia formed a long and continuous block as previously proposed for the paleoproterozoic it is nevertheless fully compatible with a samba amazonia baltica link at mesoproterozoic times

magnetic hysteresis of pseudo single domain and multidomain magnetite below the verwey transition abstract hysteresis loops have been measured as a function of temperature between 10 k and room temperature for two size fractions of crushed magnetite 1 5 m pseudo single domain psd and 100 150 m multidomain md the material under study appears to be fairly close to stoichiometric magnetite with a verwey transition temperature of 115 k for the md sample and 118 k for the psd sample for both samples hysteresis properties below the verwey transition depend critically on whether or not the samples were cooled through the transition in the magnetic field for the psd sample an even more complex behavior has been observed temperature dependences of coercive force obtained after zero field cooling differ significantly for the demagnetized versus magnetized starting magnetic state

late cretaceous subsidence in wyoming quantifying the dynamic component the late cretaceous western interior basin of north america is generally considered to have been a retroarc foreland basin flexural backstripping of the stratigraphic record from 97 2 ma to 73 4 ma along a section perpendicular to the wyoming idaho salient of the sevier belt clearly demonstrates that there were components of subsidence in addition to those driven by the thrust and associated sediment loads the simulation demonstrates that the flexural foredeep is only 180 120 km wide and shows a forebulge located near the subsequent laramide rock springs uplift foredeep strata between the rock springs uplift on the east and the thrust front to the west were mostly involved in the eastward overlapping thrust belt or deeply truncated by inferred rebound after 78 5 ma the difference between the observed decompacted cumulative subsidence and the amount of subsidence that can be explained by simple flexural loading the residual subsidence increases from 800 m in eastern wyoming to 1 8 km near the thrust belt the westward thickening upper cretaceous sediment wedge filled the accommodation space generated by downward tilting to the west of the north american cratonic margin by underplating and mantle flow associated with the shallowly subducted farallon plate

the effects of annealing and concentration on the hysteresis properties of magnetite around the psd md transition abstract hysteresis parameters h cr h c j rs j s and their ratios h cr h c j rs j s have been measured for a large number of accurately prepared grain size fractions of magnetite in the range between 5 and 150 m for several grain size fractions three different concentrations of magnetite are used 100 0 1 and 0 002 vol most of the measurements were repeated after annealing the specimens to 600 c for some specimens in the pseudo single psd and multidomain md range h c and h cr have been measured as functions of temperature plots of the results from h c h cr h c and j rs j s versus the grain size reveal curves with a convex and a concave part concentration and annealing affects the values of the hysteresis parameters especially for grains coarser than 25 m but the shape of the curves remains the same the inflection point from convex to concave for all curves occurs at 25 m and it appears to be independent of concentration and annealing it is therefore proposed to define the transition from psd to md as the inflection point of these curves

low temperature demagnetization of saturation remanence in magnetite bearing dolerites of high coercivity we studied 16 magnetite bearing dolerite dyke samples of high coercive force hc ranging from 11 to 50 mt that had been used successfully in precambrian palaeomagnetic studies each dolerite was given a saturation remanent magnetization whose change was measured as the sample was cooled to 77 k in zero field and warmed back to room temperature only the three dolerites of highest hc 40 mt show little change on cooling suggesting that their magnetite is mostly in elongated single domain grains the rest of the dolerites are likely to be dominated by pseudosingle domain magnetite cooling to 135 k causes their remanence to decrease by 37 per cent on average in rough proportion to the decrease in saturation magnetostriction as expected if internal stresses oppose domain wall motion cooling from 135 k to 77 k causes remanence to decrease further by 26 per cent on average probably mostly because of domain reorganization forced by magnetite s verwey crystallographic transition warming back to room temperature causes some of the remanence loss to be recovered perhaps because internal stresses act as a bridge between different easy axes below the verwey temperature 122 k and above the isotropic point 135 k this recoverable low temperature demagnetization averages 23 6 per cent of the initial saturation remanence while the permanent demagnetization averages 40 9 per cent recoverable low temperature demagnetization is even larger for natural remanence averaging 46 9 per cent for the six dolerites measured while the corresponding permanent demagnetization averages 13 6 per cent large recoverable low temperature demagnetization seems to be characteristic of pseudosingle domain magnetite in which high internal stresses block domain wall motion and may be common in mafic igneous rocks like our dolerite samples whose magnetite is intergrown with ilmenite lamellae measuring natural remanence of such rocks before after and while at 77 k should help separate remanence carried by multidomain magnetite mostly permanently demagnetized by single domain magnetite mostly unchanged and by pseudo single domain magnetite mostly responsible for recoverable demagnetization

architecture and formation of transgressive regressive cycles in marginal marine strata of the john henry member straight cliffs formation upper cretaceous of southern utah usa marginal marine deposits of the john henry member upper cretaceous straight cliffs formation were deposited within a moderately high accommodation and high sediment supply setting that facilitated preservation of both transgressive and regressive marginal marine deposits complete transgressive regressive cycles comprising barrier island lagoonal transgressive deposits interfingered with regressive shoreface facies are distinguished based on their internal facies architecture and bounding surfaces two main types of boundaries occur between the transgressive and regressive portions of each cycle i surfaces that record the maximum regression and onset of transgression bounding surface a and ii surfaces that place deeper facies on top of shallower facies bounding surface b the base of a transgressive facies bounding surface a is defined by a process change from wave dominated to tide dominated facies or a coaly shelly interval indicating a shift from a regressive to a transgressive regime the surface recording such a process change can be erosional or non erosive and conformable a shift to deeper facies occurs at the base of regressive shoreface deposits along both flooding surfaces and wave ravinement surfaces bounding surface b these two main bounding surfaces and their subtypes generate three distinct transgressive regressive cycle architectures i tabular shoaling upward marine parasequences that are bounded by flooding surfaces ii transgressive and regressive unit wedges that thin basinward and landward respectively and iii tabular transgressive lagoonal shales with intervening regressive coaly intervals the preservation of transgressive facies under moderately high accommodation and sediment supply conditions greatly affects stratigraphic architecture of transgressive regressive cycles acknowledging variation in transgressive regressive cycles and recognizing transgressive successions that correlate to flooding surfaces basinward are both critical to achieving an accurate sequence stratigraphic interpretation of high frequency cycles

separating geometry from stress induced remanent magnetization in magnetite with ilmenite lamellae from the stardalur basalts iceland

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

sequence stratigraphy of sixth order 41 k y pliocene pleistocene cyclothems wanganui basin new zealand a case for the regressive systems tract this study is based on a late pliocene and early pleistocene approximately 2 6 1 7 ma succession about 1 km thick of 20 sixth order 41 k y duration cyclothems of shelf origin exposed in the rangitikei river valley in the eastern part of wanganui basin the cyclothems correlate with 18 o isotope stages 100 58 and each 41 k y glacialinterglacial stage couplet is represented by an individual depositional sequence comprising transgressive highstand and regressive systems tracts unlike most examples inferred from the stratigraphic record these systems tracts were deposited during phases of known sea level cycles indicated by the contemporary oxygen isotope ice volume curve because of the high rate of subsidence of wanganui basin glacioeustatic sea level falls during most cycles were not of sufficient magnitude to expose the outer shelf thus the rangitikei section provides an exceptional example of regressive strata deposited landward of the contemporary shelf break simple one dimensional modeling shows that moderate to high rates of basin subsidence 1 2 mm yr and low rates of sedimentation 0 2 mm yr during transgressions combined to produce an accommodation surplus at the relative highstand this surplus accommodation was infilled during the late highstand and ensuing fall partly by aggradational highstand systems tract shelf siltstone and chiefly by strongly progradational shoreface sediments of the regressive systems tract rangitikei regressive systems tracts are distinguished from forced regressive systems tracts sensu hunt and tucker 1992 by their different stratal geometry by definition forced regressive systems tracts display an erosional contact with the underlying highstand systems tracts and typically occur as a series of downstepped disjunct shoreline wedges stranded on the shelf and or slope in contrast regressive systems tracts exhibit a gradational lower contact above which parasequences are stacked in a strongly progradational pattern terminated by the superjacent sequence boundary cyclothems display two types of motif termed rangitikei dt depositional transgression and rangitikei nt nondepositional transgression which include the following architectural elements in ascending stratigraphic order 1 a basal sequence boundary that is coincident with either the transgressive surface of erosion which displays small scale up to 50 cm erosional relief and may be penetrated by the ichnofossil ophiomorpha or its deeper water correlative conformity 2 either a thick 5 30 m transgressive systems tract comprising a deepening upward nearshore to inner shelf mixed carbonate siliciclastic lithofacies succession depositional transgression or a thin 2 m transgressive systems tract comprising condensed fossiliferous facies deposited on the sediment starved offshore shelf nondepositional transgression 3 a sharp downlap surface separating condensed fossiliferous facies of the transgressive systems tract from terrigenous siltstone of the superjacent highstand systems tract 4 a highstand systems tract comprising a 10 20 m thick interval of aggradational shelf siltstone and 5 a thick up to 45 m progradational inner shelf to shoreface lithofacies assemblage ascribed to the regressive systems tract condensed shell beds are associated with intrasequence and sequence bounding discontinuities and together with the sedimentological and stratal characteristics of the sequences are important indicators of stratigraphic architecture four types of shell bed are associated with surfaces formed by four different types of stratal termination onlap backlap downlap and flooding surface shell beds cf kidwell 1991 are associated respectively with the transgressive surface of erosion apparent truncation at the top of the transgressive systems tract the downlap surface and local marine flooding surfaces a fifth shell bed type termed a compound shell bed forms in offshore environments where the downlap surface converges with the sequence boundary and elements of both the downlap and the backlap shell beds become mixed or superposed the shell beds mark zones of stratal attenuation and can be used as surrogates for seismic discontinuities when applying sequence stratigraphic concepts at outcrop scale

bedform and stratification characteristics of some modern small scale washover sand bodies washover sand bodies commonly develop along microtidal coastlines in beach barrier island or spit settings wave runup usually in conjunction with an abnormally high water level may overtop the most landward berm of the beach and the foredune crest if one exists to produce overwash and subsequent runoff across the more landward subaerial surface two main elements of the resulting deposit are the washover fan and runoff channel newly formed small scale washover deposits were examined along the outer banks north carolina near pt mugu california and at presque isle lake erie pennsylvania the fans were formed in response to unidirectional landward transport and the runoff channels in response to unidirectional flow usually in a landward direction but sometimes in shore parallel then seaward direction where overwash carried across the fan surface and entered a pond or lagoon a small scale delta microdelta developed in this case the washover fan consisted of two subfacies the wetted but subaerial part of the fan and the subaqueous washover delta flow associated with the development of the fan and runoff channel produced distinctive sets of bedforms and internal stratification high velocity discontinuous surges moving across the fan surface resulted in the development of a plane bed and subhorizontal to low angle landward dipping planar stratification which comprised the major part of the fan similarly rhomboid forms were produced by high velocity sheet flow across the fan surface where flow carried into a standing body of water delta type foreset strata developed for this case the lateral structural sequence was subhorizontal planar stratification merging landward into landward dipping delta tabular foreset strata in the runoff setting where flow became channelized and continuous both upper flow and lower flow regime currents were typical upper flow regime bedforms included antidunes standing waves and plane beds the most commonly observed lower flow regime bedforms included microdelta like bars low amplitude bars linguoid ripples and sinuous crested current ripple trains the sets of sedimentary structures comprising modern washover sand bodies provide criteria for the identification of similar deposits in ancient sediments and for more specific interpretation of the environment

environmental magnetism past present and future environmental magnetism involves the application of rock and mineral magnetic techniques to situations in which the transport deposition or transformation of magnetic grains is influenced by environmental processes in the atmosphere hydrosphere and lithosphere the first explicit description of environmental magnetism as a distinct field was in 1980 since that time environmental magnetism has become a broad field that is finding application in an ever increasing array of scientific disciplines in this review of the present state of environmental magnetic studies we divide the field into three broad but arbitrary categories the first involves the use of mineral magnetic assemblages in the geological record to study physical processes in depositional environments this category includes the correlation of sediment cores using magnetic susceptibility measurements studies of geomagnetic field behavior the analysis of depositional and postdepositional mechanical processes that affect sediments and the examination of magnetic parameters that might represent proxies for paleoclimatic variation the second category encompasses studies of the processes responsible for variations in the magnetic minerals brought into a sedimentary environment these provenance investigations include studies of changes in catchment derived sediment in lakes fluctuations in contributions from terrigenous aeolian and glaciogenic components in deep sea sediments and the origin of atmospheric particulates the final category addresses in situ changes and transformations of magnetic minerals in sedimentary environments including pedogenesis authigenetic diagenetic formation of ferrimagnetic phases dissolution of magnetic minerals due to reductive diagenesis and contributions of biomagnetism to sedimentary magnetism because environmental magnetism can address problems in so many disciplines and because many of these problems may be inaccessible to other techniques it is likely that the scope of environmental magnetism will continue to expand rapidly environmental magnetism is capable of providing important data for studies of global environmental change climatic processes and the impact of humans on the environment all of which are major research initiatives in the international scientific community these factors suggest that environmental magnetism has a bright and diverse future

proterozoic basalt dikes in the nain anorthosite complex labrador two sets of proterozoic between 1290 and 1042 ma alkali to transitional basaltic dikes occur sparsely in an area of at least 5000 km2 within the nain anorthosite complex both are compositionally similar in terms of major elements but the set that trends roughly north south hp dikes contains much higher concentrations of incompatible elements than the more abundant dikes that trend roughly east northeast west southwest lp dikes both types have low normative clinopyroxene relative to olivine a characteristic shared with widespread troctolitic rocks in the nain complex low mg numbers and low ni contents suggest that the magmas producing these dikes have undergone extensive fractional crystallization the low normative di content of these dikes suggests that this fractional crystallization occurred at depths equivalent to at least 10 kbar 1000 mpa high equilibration temperatures for coexisting fe ti oxides in the matrices of these dikes indicate that the dikes cooled rapidly and were emplaced near

low paleointensities recorded in 1 to 2 4 ga proterozoic dykes superior province canada abstract paleointensity studies have been conducted on six mafic dyke swarms from the superior province in canada with u pb ages between 1141 ma and 2473 ma buchan et al tectonophysics 319 2000 167 198 the mean direction of the characteristic magnetization for each dyke swarm coincides with results of earlier studies in which the primary origin of the magnetization was established on the basis of contact and secular variation tests this primary component was isolated over a very narrow range of high unblocking temperatures usually 550 580 c indicating that it is carried by magnetite or low titanium titanomagnetite magnetic mineralogy and grain size experiments suggest that the magnetization is dominated by pseudo single domain or single domain grains taken together the characteristics of this component suggest that it is thermal in origin paleointensity experiments were conducted with a specially designed oven using a revised approach of the thellier coe method eighty five successful determinations of paleointensity were obtained for 20 sites from six dyke swarms doubling the number of existing well constrained precambrian data the paleofield estimates for the dyke swarms vary between 5 and 11 2 t yielding virtual dipole moments between 0 85 0 1 and 2 8 0 87 1022 am2 the overall convergence between these new results and the rest of the database strongly reinforces the existence of a weaker geomagnetic field during the 1000 2400 ma period than has been recorded between 0 3 and 400 ma however additional determinations are required particularly for the 400 2000 ma interval in order to establish whether a long term evolution in the time averaged field intensity can be linked to the onset of the growth of the earth s inner core

oxide mineral formation during the serpentinization of a cr spinel seam insights from rock magnetic experiments 1 serpentinization of ultramafic rocks results in the growth of serpentine and magnetite following hydration of magmatic olivine cr spinel seams in ultramafic intrusions that also contain olivine might produce magnetite during the serpentinization process however hysteresis and temperature dependence of susceptibility experiments on serpentinized chromitite 80 cr spinel from the dawros peridotite western ireland reveal the presence of small quantities of a fe ti oxide phase likely titanomaghemite reflected light petrography reveals that the oxides occur as 0 1 m to 20 m grains that are typically intergrown with serpentine it is suggested that the formation of the minute quantities of low susceptibility high curie point titanomaghemite occurred by low temperature alteration of the magnetite produced during serpentinization the presence of such low quantities of the oxide phase may be either a function of the very high mg content of the magmatic olivine or the decoupling of oxide growth from the process of serpentinization where magnetite formed later than the reaction of olivine with the infiltrating fluids our data highlight the usefulness of rock magnetic techniques to the investigation of oxide mineral phases in serpentinized ultramafic rocks

palaeomagnetism of early triassic limestones from the huanan block south china no evidence for separation between the huanan and yangtze blocks during the early mesozoic summary there has been extensive debate on the timing of the collision between the huanan and yangtze blocks ever since hsu and co workers hsu et al 1987 1988 proposed a mesozoic collision model for south china tectonics we report new rock and palaeomagnetic data from an early triassic limestone formation from the huanan block that help to constrain its tectonic history the samples are from two localities jiupi jt and huangben lt in lianzhou county northern guangdong province only thermal demagnetization is eective in decomposing a multicomponent remanent magnetization the majority of samples show three components a low temperature component a an intermediate temperature component b and a high temperature characteristic remanent magnetization chrm the a component has a steep inclination and is probably drilling induced the b component clusters around the present geomagnetic field direction and is a brunhes age viscous remanence fold test results suggest that the jt chrm is pre tectonic and that the lt chrm is a syn or prefolding remagnetization samples from the two localities show quite similar acquisition and thermal demagnetization of the isothermal remanent magnetization irm and suggest that magnetite carries the chrm component however partial anhysteretic remanent magnetization parm spectra anisotropy of anhysteretic remanence aar rock fabric and morphology of magnetic extracts are quite dierent for the two localities the aar of jt samples shows low anisotropy average 2 per cent and reflects a composite compaction and strain fabric the aar of lt samples shows a high degree of anisotropy average 10 per cent and is clearly consistent with rock deformation fabrics the magnetic extracts of lt samples are dominated by spherical botryoidal ti poor magnetite which is probably precipitated from tectonic fluids during deformation while the jt magnetic extracts are dominated by rod shaped ti poor magnetite probably derived from the erosion of igneous rocks although minor amounts of botryoidal magnetite and rod like magnetite particles can be seen in jt and lt extracts respectively the rock fabric and magnetic fabric data and sem observations of magnetic extracts corroborate the results of the fold test the best clustered lt chrm direction is d 294 i 12 a 95 12 its pole position is 187 e 19 s dp 6 dm 12 the assumption of a reversed polarity for the jt chrm direction d 198 i 20 a 95 9 implies that the huanan block was in the northern hemisphere during the early triassic and its pole position 239 e 68 n dp 5 dm 10 is close to early triassic pole positions from the yangtze block a normal polarity for the jt site chrm would require tectonic movements inconsistent with the local geology therefore the huanan and yangtze blocks were not separated in the early triassic the rotation of the lt locality may have been caused by the collision of the southeast china coastal terranes and or subduction of the pacific plate in the late mesozoic

evidence for magnetostrictive control of intrinsic susceptibility and coercive force of multidomain magnetite in rocks abstract it is shown that intrinsic susceptibility xi susceptibility in the absence of self demagnetizing fields as well as coercive force hc of multidomain magnetite in rocks are likely to be commonly magnetostrictively controlled presumably through internal stresses opposing domain wall motion we use observations of the thermal variation of observed susceptibility x per cm3 of magnetite in the sample and coercive force hc on cooling to 130 k for seven basaltic rocks and for 0 25 0 21 mm magnetite grains dispersed in cement as expected theoretically with magnetostrictively controlled xi a plot of 1 x versus j s 2 for each sample where is the polycrystalline magnetostriction coefficient and js is saturation magnetization gives a reasonably straight line whose 1 x intercept agrees with n average self demagnetizing factor of magnetite grains in the sample given by h c j r n h c j r h c j ri where jr is saturation remanence and ri is jr when n 0 as expected theoretically with magnetostrictively controlled hc a plot of hc versus j s for each sample reveals an approximate proportionality this also implies magnetostrictive control of xi because a plot of 1 x versus h c j s for each sample yields a straight line of 1 x intercept agreeing with n from h c j r n h c j r h c j ri as expected if hc and xi are both magnetorestrictively controlled

possible mechanisms causing failure of thellier palaeointensity experiments in some basalts summary the normally magnetized zone of the jurassic lesotho basalts although providing apparently quite reliable palaeofield directions kosterov perrin 1996 shows anomalous behaviour when studied in vacuum using the thellier palaeointensity method typically the slope of the natural remanent magnetization thermoremanent magnetization nrm trm curves is very steep at intermediate temperatures 200 to 400 460 c in order to elucidate the reasons for such an anomalous behaviour six representative samples from a total of 74 studied using this method were subjected to a variety of analyses these experiments indicate that the magnetic properties are dominated by pseudo single domain psd magnetite grains some 1 mm in size resulting from high temperature oxidation of titanomagnetite laboratory heatings in vacuum up to the curie point do not change significantly the roomtemperature hysteresis characteristics or the initial susceptibility k similarly the k t curves in vacuum are with a single exception rather reproducible since the laboratory trms yield almost ideal nrm trm plots the anomalous nrm trm plot is presumably due to some peculiarity of the natural trm the partial trm ptrm acquisition capacity in the moderate temperature range cooling from 200 to 20 c is generally very strongly reduced after heating to 270 c which indicates that some magnetic alteration has already occurred at these temperatures hysteresis measurements between room temperature and the curie temperature t c show that some small less than 10 per cent but significant irreversible changes in hysteresis characteristics also occur during heating in particular the coercive force h c0 at room temperature is typically reduced after heating at a moderate temperature 175 c but increases after treatments at 475 c and more pronouncedly at 580 c the saturation magnetization j s0 remains unchanged except for a very small decrease less than 5 per cent occurring in some samples after the two latter treatments these changes are most clearly seen on h c t j s t bilogarithmic plots which show that the moderate temperature change in coercivity can extend up to 200 250 c thus hysteresis measurements as a function of temperature over a promising tool for sample pre selection for thellier experiments alternating field demagnetization and cycling of ptrms at liquid nitrogen temperature suggest that the blocking mechanism is largely multidomain like near room temperature but becomes less so as the curie point is approached the main reason for the failure of the thellier experiments is the loss of a fraction of the nrm natural trm at temperatures apparently lower than the blocking temperatures in nature it is suggested that this anomalous behaviour results from the reorganization of the domain structure of the psd grains during heating this transformation which seems to be triggered by the coercivity decrease observed at very moderate temperatures can reduce the nrm intensity without requiring any correlated ptrm acquisition

determination of domain structure in igneous rocks by alternating field and other methods abstract af alternating field demagnetization arm anhysteretic remanent magnetization and strong field hysteresis properties of a large collection of mostly continental igneous rocks are reported here the collection included rocks whose magnetic carriers were believed from previous work to be of one of three types md multidomain sd psd single domain pseudo single domain or a bimodal mixture of md grains e g discrete opaques and sd psd material e g silicate inclusions two series of subaerial basalts with a full range of deuteric oxidation classes included examples of all three classes of behaviour sd psd rocks have relatively hard inflected af decay curves decay rate initially increasing then decreasing md rocks have soft exponential like decay curves and bimodal rocks have a combination of these characteristics relative hardnesses of normalized decay curves of remanences acquired in weak intermediate and strong fields the lowrie fuller test are also distinctively different for the three classes and the results support the theory developed in an accompanying paper 1 that lowrie fuller characteristics are an expression of the shapes of decay curves the lowrie fuller test although its result can be expressed as a numerical parameter is not capable of fine scale classification of domain structure or grain size the shape of the arm induction curve does have a quasi continuous variation with grain size however the parameter ar j rs initial anhysteretic susceptibility normalized to saturation remanence which is easily measured with standard paleomagnetic instrumentation is potentially useful for magnetic granulometry although ar itself was not diagnostic of grain size

determination of self demagnetizing factor n for multidomain magnetite grains in rock abstract the magnetic poles on a multidomain grain with a magnetization j produce a field nj tending to demagnetize the grain strongly affecting many magnetic properties of rocks bearing multidomain magnetite the average self demagnetizing factor n of multidomain grains in a rock is thus an important parameter which is shown to be simply measured from a high field hysteresis loop using h c j r n h c j r h c j r i where h c is the coercive force j r the saturation remanence and j r i an estimate of j r when n 0 measuring magnetic susceptibility x and coercive force h c as a function of low temperature and then plotting 1 x versus h c j s where j s is saturation magnetization gives a straight line whose intercept on the 1 x axis also estimates n these two independent estimates of n are shown to agree for the seven magnetite bearing basaltic rocks studied yielding an average n 1 7 e m u and for the 0 2 mm natural magnetite grains yielding n 2 6 0 2 e m u in contrast estimating n from h c j r 1 x i where x i is x when n 0 is shown to be invalid and even use of n 1 x h c 45 is not recommended measured n is much below the 4 e m u expected of large equidimensional magnetite grains which is understandable for the rocks of higher coercive force but makes further study of rocks of low coercive force desirable

facies architecture and geometry of landward stepping shoreface tongues the upper cretaceous cliff house sandstone mancos canyon south west colorado the campanian cliff house formation represents a series of individually progradational shoreface tongues preserved in an overall landward stepping system in the mancos canyon area the formation consists of four 50 to 55 m thick and 10 to 20 km wide sandstone tongues which pinch out landwards into lower coastal plain and lagoonal deposits of the upper menefee formation and seawards into offshore shales of the lewis shale formation photogrammetric mapping of lithofacies along the steep and well exposed canyon walls was combined with sedimentary facies analysis and mapping of the detailed facies architecture two major facies associations have been identified one comprising the mostly muddy and organic rich facies of lagoonal and lower coastal plain origin and one comprising the sandstone dominated facies of shoreface origin key stratigraphic surfaces were identified by combining the mapped geometry of the lithofacies units with the interpretation of depositional processes the stratigraphic surfaces master ravinement surface shoreface coastal plain contact transgressive surface maximum flooding surface and the sequence boundary allow each major sandstone tongue to be divided into a simple sequence consisting of a basal transgressive system tract tst overlain by a highstand system tract hst within each sandstone tongue a higher frequency cyclicity is evident the high frequency cycles show a complex stacking pattern development and are commonly truncated in the downdip direction by surfaces of regressive marine erosion the complexities of the cliff house sandstone tongues are believed to reflect changes in the rate of sea level rise combined with the responses of the depositional system to these changes synsedimentary compaction causing a thickness increase in the sandstone tongues above intervals of previously uncompacted lagoonal coastal plain sediments also played a role this study of the facies architecture geometry and sequence stratigraphy of the cliff house formation highlights the fact that there may be some problems in applying conventional sequence stratigraphical methods to landward stepping systems in general these difficulties stem from the fact that no single stratigraphic surface can easily be identified and followed from the non marine to the fully marine realm i e from the landward to the basinward pinch out of the sandstone tongues in addition the effects of synsedimentary compaction and changes in the shoreface dynamics are not easily recognized in limited data sets such as from the subsurface

the hysteresis properties of multidomain magnetite and titanomagnetite titanomaghemite in mid ocean ridge basalts abstract hysteresis measurements have been carried out on a suite of ocean floor basalts with ages ranging from quaternary to cretaceous approximately linear yet separate relationships between coercivity bc and the ratio of saturation remanence saturation magnetization mrs ms are observed for massive doleritic basalts with low ti magnetite and for pillow basalts with multi domain titanomagnetites with x 0 6 even when the morb has undergone low temperature oxidation resulting in titanomaghemite the parameters are still distinguishable although offset from the trend for unoxidized multidomain titanomagnetite the parameters for these iron oxides with different titanium content reveal contrasting trends that can be explained by the different saturation magnetizations of the mineral types this plot provides a previously underutilized and non destructive method to detect the presence of low titanium magnetite in igneous rocks notably morb