theoretical models of magnetic anisotropy to strain relationship revisited abstract mathematical modelling of the relationships between the low field magnetic anisotropy and strain are redeveloped and the calculated data are compared with the degree of anisotropy for various rock types it is found that the degree of anisotropy for realistic strain magnitudes is unrealistically high in the passive model the ductile model for which the degree of anisotropy depends on the magnetic grain the matrix viscosity ratio becomes realistic at high viscosity contrasts in the line plane model the theoretical degree of anisotropy corresponds well to the natural degree of anisotropy for rocks in which the carrier of magnetic anisotropy is either magnetite or phyllosilicate minerals by contrast in the case of haematite and pyrrhotite the modelled degree of anisotropy is much higher than the natural values in the viscous model the calculated degree of anisotropy corresponds well to that measured in sedimentary and volcanic rocks in the models imposing pure shear strain the natural logarithm of the degree of anisotropy to natural strain relationship can be approximately represented by a straight line at least for low to intermediate strains the proportionality constant varies according to the specific model and the specific carrier of the anisotropy of magnetic susceptibility

magnetic fabric development in the lower seve thrust from the cosc 1 drilling swedish caledonides abstract the cosc 1 project drilled the several hundred meters thick basal shear zone of the lower seve nappe with mylonites in mica schists amphibole schists and gneisses in zones of high magnetic susceptibility from 1910 to 2450 m we studied magnetic and petrographic fabrics and magnetic mineralogy borehole imaging allowed for geographic reorientation of the samples and offered the opportunity to study anisotropy of magnetic susceptibility ams in relation to tectonic evolution of the seve thrust we measured ams at room temperature added low temperature and field dependent ams for a subset of samples and compared magnetic with petrographic fabrics triaxial and prolate magnetic fabrics with degree of anisotropy p up to 3 2 together with abundant s c fabrics and strain partitioning around porphyroclasts indicate dominant simple shear until 2300 m magnetite and ilmenohematite mimic the rock fabric due to fabric parallel alignment and or magnetic interaction and either contribute to increase or decrease of p depending on the dominating rock fabric elements field dependency of pyrrhotite and magnetite in kmax direction further increases p homogeneous and oblate petrographic and magnetic fabrics in the greenschist grade overprinted rocks below 2300 m with subhorizontal kmax kint girdle distributions indicate dominant flattening ams depicts shear fabrics including magnetite and ilmenohematite and is additionally increased by retrograde magnetite rutile intergrowth in ilmenohematites we interpret that shape and degree of ams are controlled by a tectonic deformation and strain b alteration and magnetic grain interaction and c field dependency of deformed pyrrhotite and or magnetite we observed that all petrographic and magnetic subfabrics are coaxial and lineations are mainly e w to se nw directed confirming the transport direction of the caledonian allochthonous from our microstructural and ams results we suggest that thrusting of the lower seve unit commenced under simple shear conditions at higher metamorphic grades and subsequently switched to more pure shear under greenschist grade conditions

the magnetic stratification of layered mafic intrusions natural examples and numerical models abstract mafic ultramafic intrusions commonly display a prominent petrographic and geochemical layering attributed to differentiation in a magma chamber this layering occurs at scales from several hundreds of meters in some large intrusions down to scales of a few millimeters layering forming processes are numerous and include both internal processes such as crystal settling double diffusive convection flow segregation and compaction and external processes such as magma recharge extraction or mixing mafic magma chambers may operate as a closed system or as an open system several recent investigations have been conducted on borehole cores through the insizwa sill south africa the great dyke zimbabwe and the bushveld complex south africa we present an overview of magnetic results on these intrusions the magnetic properties of mafic rocks are dominated by the presence of magnetite in most layers pyrrhotite the only ferrimagnetic sulfide is generally restricted to sulfide rich reefs merensky j m bohmke or to massive sulfide deposits at the base of the intrusion all intrusions display distinct variations of their magnetic properties anisotropy of magnetic susceptibility magnetic remanence magnetic saturation high field magnetic susceptibility across the layering the significance of these magnetic zones varies depending on the nature of variations from zone to zone for example a drop in magnetic susceptibility does not necessarily correspond to a new magma batch unless it is also associated with a change in magnetic anisotropy parameters in the studied cases all open systems the magnetic susceptibility and the total iron content estimated from high field measurements increase towards the top of these intrusions suggesting a general pattern of iron enrichment a numerical petrologic model for a tholeiitic magma predicts magnetic susceptibility variations across a statically differentiated closed system intrusion no convection the effects of sequential magma recharge and discrete magma extraction are investigated from the magnetic properties point of view conversely these models can be used to interpret the magnetic stratification pattern in natural examples and test for its possible causes

magnetic fabric development in a highly anisotropic magnetite bearing ductile shear zone seve nappe complex scandinavian caledonides magnetite bearing mylonitic garnet micaschists close to the major suture between the baltica and iapetus terranes seve nappe complex scandinavian caledonides show very high anisotropy of magnetic susceptibility ams with corrected degree of anisotropy p up to 4 8 three different magnetic fabric types can be distinguished they correspond to protomylonite type i p 3 respectively the orientation of the ellipsoid axes from all applied magnetic fabric methods in this study is similar with shallow dips of the metamorphic foliation toward wsw and subhorizontal mostly nw se trending mineral lineation differences between subfabrics were minimized under high shear strain as all markers tend to align parallel with the shear plane the very high anisotropies and mostly oblate ellipsoid shapes of type iii correlate with high magnetic susceptibility kmean up to 55 10 3 si units and are related to the concentration of magnetite aggregates with shape preferred orientation they show a distinct field dependence of magnetic susceptibility of up to 10 in the kmax direction we attribute this field dependence to a memory of high strains in the domain walls of the crystals acquired during synkinematic magnetite growth during shear zone fabric development at temperatures of 550 570 c

effect of post magmatic processes on magnetic fabric of teschenite association rocks of the outer western carpathians abstract volcanic and subvolcanic alkaline rocks of the teschenite association of the outer western carpathians are characterized by intensive late magmatic post magmatic and also hydrothermal changes that modified their composition not only in rock forming minerals but also in accessory magnetic minerals magnetite titanomagnetite maghemite as indicated by magnetic susceptibility ranging from 10 5 to 10 1 si units the magnetic fabric of the most sills and dykes investigated is very roughly conformable to the shapes of the bodies corresponding to the most common type of magnetic fabric in dykes and sills which no doubt originates through magma flow however the fabric shows large variation much larger than expected for volcanic magnetic fabric which can be ascribed to late magmatic or post magmatic alterations of magnetic minerals diminishing the magnetic fabric homogeneity even though the teschenite association rocks occur in the sole part of the silesian thrust sheet their magnetic fabric and that of host sedimentary rocks show only very weak indications of ductile deformation consequently the silesian thrust sheet responded to the tectonic movements creating thrust sheet structure by faulting and thrusting of the deformed zones and not by ductile deformation of the bulk of the thrust sheet

distinguishing diapirs from inflated plutons an integrated rock magnetic fabric and structural study on the roundstone pluton western ireland granitoid plutons account for much of the continental crust and are critical in the generation of several economic resources despite over a century of research the fundamental process by which large felsic bodies intrude remains controversial two contrasting models persist 1 mass ascent and subsequent cooling of magma diapirism 2 sheet ascent and subsequent emplacement laccoliths the latter hypothesis is different from the former as distinct ascent and emplacement processes must occur and be identifiable traditional field methods rarely allow this important distinction to be made we present field petrographic and anisotropy of magnetic susceptibility data from the roundstone pluton connemara western ireland which was previously considered a diapiric intrusion the new data reveal anomalous subvertical magmatic lineations in the core of this circular intrusion as well as a suite of magmatic and submagmatic fabrics that parallel nnw sse country rock faults and coeval g2 sheets within the pluton we deduce that these structures reflect a distinct centralized ascent process along a nnw sse conduit lateral emplacement and pluton inflation followed ultimately forming a punched laccolith we conclude that the combined application of the above techniques provides a unique insight into pluton architecture that removes ambiguity between contrasting diapiric and inflated pluton models supplementary material complete rock magnetic data tables and sample site coordinates irish grid are available at http www geolsoc org uk sup18837

quantification of magnetic coercivity components by the analysis of acquisition curves of isothermal remanent magnetisation abstract a new method of analysing isothermal remanent magnetisation irm acquisition curves based on cumulative log gaussian analysis robertson and france phys earth planet inter 82 1994 223 234 is proposed it is based on the curve fitting of the irm acquisition curve versus the logarithm of the applied field with i the acquisition curve on a linear scale ii the acquisition curve expressed as a gradient and iii the acquisition curve on a probability scale even when a sample is not saturated its magnetic properties can be defined although with less certainty the number of magnetic components required for an optimal fit to a measured irm acquisition curve is evaluated statistically the method discriminates on the basis of different mineral coercivity therefore additional rock magnetic tests are still required to separate minerals with similar coercivities

determination of field independent and field dependent components of anisotropy of susceptibility through standard ams measurement in variable low fields ii an example from the ultramafic body and host granulitic rocks at bory in the moldanubian zone of western moravia czech republic abstract a large ultramafic body garnet clinopyroxenite and garnet peridotite embedded in felsic granulite at bory moldanubian zone of western moravia czech republic contains multiple carrier of susceptibility low field variation of anisotropy of magnetic susceptibility ams enables the separation of the field independent and low field dependent ams components to be made in granulite the former is carried by paramagnetic minerals and a subordinate admixture of magnetite while the latter is due to pyrrhotite in the ultramafite the field independent component is carried by pure magnetite formed by serpentinization while the field dependent component is due to pyrrhotite and subordinate paramagnetic minerals despite small differences all ams components are roughly coaxial both within granulite and within ultramafite but very different between granulite and ultramafite the difference indicates that the forces that imposed the later granulite fabric were not strong enough to obliterate the original magnetic fabric of the ultramafites

emplacement of the early miocene pinto peak intrusion southwest utah usa 1 in this contribution we report rock magnetic petrographic and anisotropy of magnetic susceptibility ams data from the pinto peak intrusion all of which bear on volcanic construction rock magnetic data indicate that the dominant magnetic mineral phase is low ti titanomagnetite of multidomain grain size the composition of which varies spatially across the intrusion the intrusion is a porphyritic andesite dominated by ca rich plagioclase 60 as well as biotite amphibole olivine and opaque minerals reflected light petrography reveals mostly euhedral subhedral titano magnetite crystals that often form clustered glomerocrysts and stringers of equant crystals without exhibiting a consistent mineral alignment fabric moderate to shallow plunging prolate magnetic susceptibility ellipsoids dominate the northern part of the intrusion while steeply dipping plunging magnetic susceptibility ellipsoids are generally restricted to the southern part of the intrusion the vent facies rocks yield moderate to steep oblate susceptibility ellipsoids we interpret the flow pattern in the north to reflect subhorizontal flow of magma filling a tabular sheet like body associated with propagation of the intrusion to the north we argue that the southern part of the intrusion represents the ascent site of the magma rising to shallow crustal levels along a steep feeder system the oblate magnetic fabrics in the vent area plausibly represent flattening against the conduit walls as evidenced by a weak planar flow foliation observed in the vent conduit rocks on reaching shallow crustal levels the magma deformed and uplifted the roof rocks leading to gravitational instability as the slide mass released from the roof an explosive eruption ensued resulting in the emplacement of the rocks of paradise tuff and associated effusive lava flows following eruption magma pressure decreased and the magma drained northward forming the northern intrusive phase

a search for seismic reflections from the top of the oceanic crust beneath hawaii three component digital array recordings of a midcrustal microearthquake cluster beneath the southeast flank of kilauea volcano have been analyzed at four nearby portable stations for the presence of s wave reflections from the top of the old oceanic crust and from the m discontinuity beneath hawaii cross spectral correlation was applied to both p and s wave arrivals to obtain very precise relative hypocentral locations for the 29 events in the cluster reciprocal array analysis in which the plane of earthquakes is treated as an observing array and the station as a source spudich and bostwick 1987 was applied to the s wave coda for record sections at each of the four stations used in the joint hypocenter determination results of apparent velocity measurements across the earthquake observing array show that the early s wave coda up to about 1 to 1 5 sec after direct s is dominated by near station reverberations none of the large amplitude arrivals that are seen on the records immediately following direct s including those which were previously interpreted by us to be possible reflections from the upper part of the oceanic crust beneath hawaii are in fact deep reflections array slownesses for two of the stations studied reveal a weak component of backscattered energy at coda arrival times greater than about 1 5 sec that may be due to reflections from lower crustal reflectors and from the moho those signals however originate at reflector depths greater than those postulated for the upper part of the old oceanic crust beneath hawaii the crustal reflections indicate reflector depths around 10 km or 11 km about the predicted depth for the boundary between upper layer 2 and the lower layer 3 sections of the old oceanic crust reflections from the moho give two way travel times indicative of total crustal thickness under that part of hawaii of about 13 to 14 km when the seismograms are source migrated by normalizing the travel times and epicentral distances to remove the effect of varying focal depth both deep crustal and moho reflections are seen as low frequency signals that are roughly aligned on the record sections the apparent absence of strong reflections from the the top of the old oceanic crust beneath hawaii means either that the boundary is not marked by a very strong seismic discontinuity or that the earthquakes studied are so close to the boundary that even large amplitude reflections are effectively masked by the dominating effect of near station reverberations

the magnetic properties of natural and synthetic fex mg1 x 2 sio4 olivines article i nfo olivine is an abundant orthosilicate in the solar system in the upper mantle of rocky planets in meteorites and interstellar dust the magnetic properties of fex m g1 x 2 sio4 olivines result from the silicate matrix and its iron rich inclusions and have not always been separated in previous studies the properties of the matrix are important to understand mantle rocks anisotropy and their deformation both in xenoliths and peridotite massifs while the inclusions are potential paleomagnetic and paleointensity recorders in this study we performed new measurements on 7 natural and 23 synthetic ferromagnesian olivines covering the whole range from forsterite mg2sio4 fo100 to fayalite fe2sio4 fo0 and from 4 to 310 k many of our specimens contain ferromagnetic inclusions magnetite or maghemite with magnetic sizes ranging from super paramagnetic tomultidomain the respectivecontributions of the matrix and the inclusions are systematically isolated using magnetic fields large enough to saturate the ferromagnetic component due to inclusions at room temperature as predicted by molecular field theory while the forsterite end member is diamagnetic xhf 6 8 10 10 m 3 kg and the fayalite end member is paramagnetic xhf 1 10 10 6 m 3 kg their magnetic properties do not vary linearly with iron content these olivines also exhibit one or two magnetic transitions at composition dependent low temperatures at the neel temperature tn olivines exhibit a first magnetic transition from paramagnetic to antiferromagnetic behavior indicated by negative paramagnetic curie temperatures a second transition tt occurs at lower temperatures for specimens with x 0 1 and could be attributed to a change from collinear to canted antiferromagnetic state in the range fo83 fo93 corresponding to the earth s upper mantle the anisotropy of magnetic susceptibility ams is directionally consistent with the ams principal axes k1 k2 and k3 respectively corresponding to the a c and b crystallographic axes at room temperature the degree of anisotropy increases from 1 028 to 1 302 with decreasing iron content the presence of small and submicroscopic iron rich inclusions significantly complicates the investigation of olivine physical properties but also constitutes opportunities to record paleomagnetic field intensities

the magnetic fabric of the wolfcamp shale midland basin west texas understanding petrofabric variability hydrocarbon associations and iron enrichment rock fabrics have long been recognized as a key variable influencing unconventional reservoir properties and yet petrofabric analysis is rarely incorporated in reservoir characterization in a quantitative and scalable manner this work introduces anisotropy of magnetic susceptibility ams coupled with handheld x ray fluorescence to define the origin of petrofabrics in the wolfcamp shale and examine their relationship with total organic carbon toc and oil saturation indices osi standard paleomagnetic plugs were drilled from three vertical cores spanning the entire wolfcamp shale in the midland basin west texas low temperature magnetic experiments suggest paramagnetic minerals contribute between 93 4 and 98 6 to the low field bulk magnetic susceptibility klf across all lithotypes scanning electron microscopy revealed frequent occurrences of paramagnetic minerals such as chlorite illite ferroan dolomite marcasite and pyrite the fe rich clays such as chlorite carry the ams signal in mudrocks with a minor contribution from ferroan dolomite conversely ferroan dolomite carries the ams signal in most carbonate facies with a minor influence from fe rich clays the degree of magnetic anisotropy pj and shape factor t in mudrocks are compositionally dependent and respond to changes in both clay and framework grain concentration for carbonate facies pj and t vary widely because of changes in depositional facies and the concentration of ferroan dolomite the toc correlates well with pj and strongly oblate magnetic fabrics this relationship likely stems from the adsorption of organic matter om to clays during deposition and may indicate that organoclay composites were responsible for delivering om to the midland basin elevated osi are fabric selective favoring low pj and t fabrics suggesting that such fabrics facilitate hydrocarbon migration more so than high pj and t fabrics fuzzy c means clustering provides a framework to describe ams and geochemical transitions at the core scale and could serve as a tool to map barriers and pathways to hydrocarbon migration finally ti nb and klf data point to a mafic provenance lithotype and suggest layered intrusive complexes in the central basin uplift were exposed and eroded during deposition of the wolfcamp shale mafic source rocks probably increased supply of micronutrients such as dissolved iron and may have enhanced productivity in the midland basin

the use of the anisotropy of magnetic remanence in the resolution of the anisotropy of magnetic susceptibility into its ferromagnetic and paramagnetic components abstract the anisotropy of magnetic susceptibility ams is often controlled by both ferromagnetic sensu lato and paramagnetic minerals the anisotropy of magnetic remanence amr is solely controlled by ferromagnetic minerals jelinek trav geophys 37 1993 introduced a tensor derived from the isothermal amr whose normalized form equals the normalized susceptibility tensor provided that the ferromagnetic fraction is represented by multi domain magnetite the present paper shows the close correlation between these tensors for a collection of strongly magnetic specimens containing multi domain magnetite in addition acceptable correlation between the tensors was also found for a collection of specimens containing single domain magnetite a new method is developed for the ams resolution into ferromagnetic and paramagnetic components using the amr some examples are presented of this resolution in mafic microgranular enclaves in granodiorite and in gneisses of the ktb borehole

magnetic anisotropy behaviour of pyrrhotite as determined by low and high field experiments summary here we report on the sources of magnetic anisotropy in pyrrhotite an iron sulphide present in many rocks as an important carrier of the natural remanent magnetization while the magnetic hysteresis parameters of pyrrhotite are well known the existing database concerning its anisotropy behaviour is patchy and ambiguous therefore a collection of 11 seemingly single crystals of natural pyrrhotite was scrutinized before embarking on the anisotropy determinations the set of single crystals was extensively characterized rock magnetically by measuring curie temperatures hysteresis loops irm acquisition curves and forc diagrams the latter three all at room temperature first the variation of the low field susceptibility as function of applied field and grain size was evaluated for fields ranging from 1 to 450 a m 1 existing grain size dependent data and the present larger crystals show a logarithmic grain size dependence this enables estimating the grain size for unimodal pyrrhotite distributions in rocks measured trends are better fitted with an exponential function than with a rayleigh law style function based on the rock magnetic characterization and the behaviour of the anisotropy of magnetic susceptibility six samples of the original 11 were selected for the high field anisotropy determinations within the basal plane those data were acquired with a torque cantilever type magnetometer as expected most single crystals showed a pure 6 curve within their basal plane because of the easy axis configuration in some crystals however lower harmonic terms overlapped the 6 term this may be the dominant source of the observed variation in magnetic anisotropy properties torque data of three of the six samples were of sufficient quality to allow evaluation of k 1 re evaluation of existing torque data and including the present newly derived determinations yields for the anisotropy constant of pyrrhotite within the basal plane k 1 2 7 0 2 10 4 jm 3 this is over an order of magnitude more precise than the sparse existing k 1 data only the value reported by mikami and co authors in 1959 agrees with the new determination with this firmly established k 1 value meaningful anisotropy models are now possible for pyrrhotite bearing rocks

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

magnetic anisotropy of hematite natural crystals increasing low field strength experiments hematite is a very abundant mineral in natural rock samples despite being one of the most important carriers of remanent magnetization its magnetic anisotropy is not well understood partially due to its high coercivity and complex behavior in particular the field intensity beyond which the rayleigh relation no longer holds varies from one crystal to another this field threshold is usually less than the field used in most commercial instruments the nonlinear behavior of low field susceptibility may thus hinder the magnetic fabric analysis we have carried out an intensive study of the low field bulk susceptibility and anisotropy of magnetic susceptibility ams at increasing low fields in the range of 2 450 a m effective value in a collection of hematite natural crystals standard rock magnetic properties x ray diffraction and mass spectrometry have also been determined in order to discover the parameters influencing the low field susceptibility variations with field the ams principal directions the shape of the ams ellipsoid and the degree of anisotropy are the parameters that can vary with different applied fields it has been found that there is no correlation between magnetic properties like coercivity or saturation magnetization and the range in which the rayleigh approximation is valid however there seems to be a correlation with the peak width determined from x ray diffraction suggesting that the rayleigh region in hematite crystals is related to the spatial orientation of the physical domains within the basal plane

variations in magnetic properties of target basalts with the direction of asteroid impact example from lonar crater india abstract the lonar crater in maharashtra state india has been completely excavated on the deccan traps basalt approximately 65 ma at approximately 570 47 ka by an oblique impact of a possible chondritic asteroid that struck the preimpact target from the east at an angle of approximately 30 45o to the horizon where the total duration of the shock event was approximately 1 s it is shown by our early work that the distribution of ejecta and deformation of target rocks around the crater rim are symmetrical to the east west plane of impact misra et al 2010 the present study shows that some of the rock magnetic properties of these shocked target basalts e g low field anisotropy of magnetic susceptibility ams natural remanent magnetization nrm bulk susceptibility and high coercivity and high temperature hc ht magnetization component are also almost symmetrically oriented with reference to the plane of impact studies on the relative displacements of k3 minimum ams axes of shocked basalts from around the crater rim and from the adjacent target rocks to the approximately 2 3 km west of the crater center suggest that the impact stress could have branched out into the major southwestward and northwestward components in the downrange direction immediately after the impact the biaxial distribution of ams axes in stereographic plots for the unshocked basalts transforms mostly into triaxial distribution for the shocked basalts although transitional type distribution also exists the degree of anisotropy p of ams ellipsoids of the shocked basalts decreases by approximately 2 when compared with those of the unshocked target approximately 1 03 the nrm am 1 values of the shocked basalts on the rim of the lonar crater do not show much change in the uprange or downrange direction on and close to the east west plane of impact and the values are only approximately 1 5 times higher on average over the unshocked basalts around the crater however the values become approximately 1 4 16 4 times higher for the shocked basalts on the crater rim which occur obliquely to the plane of impact the target basalts at approximately 2 3 km west of the crater center in the downrange also show a significant increase up to approximately 26 times higher in nrm the majority of the shocked basalt samples approximately 73 from around the crater rim in general show a lowering of rem except those from approximately 2 3 km west of the crater center in the downrange where nearly half of the sample population shows a higher rem of approximately 3 63 in average the shocked target basalts around the lonar crater also acquired an hc ht magnetization component due to impact these hc ht components are mostly oriented in the uprange direction and are symmetrically disposed about the east west plane of impact making an obtuse angle with the direction of impact the low coercivity and low temperature lc lt components of both the unshocked and shocked basalts are statistically identical to the present day field pdf direction this could be chemical and or viscous remanent magnetization acquired by the target basalts during the last 570 47 ka subsequent to the formation of the lonar crater the shocked lonar target basalts appear to have remagnetized under high impact shock pressure and at low temperature of approximately 200 300 c where ti rich titanomagnetite was the main magnetic remanence carrier

magnetic fabric methods and applications fabric is a ubiquitous and significant feature of geological materials the processes involved in the formation and deformation of rocks and sediments leave their mark on the orientations of the constituent mineral grains petrofabrics thus provide essential keys to understanding the history of geological materials magnetic anisotropy is directly related to petrofabric and has become one of the most rapid sensitive and widely used tools for its characterization the relationship between magnetic fabric and petrofabric is complex and depends on various factors including the composition concentration and grain size of mineral grains ongoing research in geological applications is paralleled by studies of the fundamental mineral magnetic phenomena involved the papers in this book represent the current state of investigations in magnetic anisotropy studies as a discipline that integrates geological interpretations mineral fabric development technical advances and rock magnetic properties

field dependence of ac susceptibility in titanomagnetites abstract ac susceptibility measurements as a function of field amplitude hac and of frequency show a strong field dependence for a set of synthetic titanomagnetites fe3 xtixo4 and for certain basalts from the soh 1 hawaiian drill hole and from iceland in phase susceptibility is constant below fields of about 10 100 a m and then increases by as much as a factor of two as hac is increased to 2000 a m both the initial field independent susceptibilities and the field dependence of susceptibility are systematically related to composition initial susceptibility is 3 si for a single crystal sphere of tm0 x 0 and decreases with increasing titanium content field dependence is nearly zero for tm0 and increases systematically to a maximum near tm60 x 0 6 this field dependence can in some cases be mistaken for frequency dependence and lead to incorrect interpretations of magnetic grain size and composition when titanomagnetite is present

an ams study of different silicic units from the southern parana etendeka magmatic province in brazil implications for the identification of flow directions and local sources abstract in the southern portion of the parana etendeka magmatic province in brazil extensive silicic dacite rhyolite deposits occur at the top of a sequence of low ti pahoehoe to rubbly basalts the internal architecture of the silicic deposits and their eruptive style as well as the location of their sources are still unsatisfactorily known in an attempt to provide independent evidence for flow directions in deposits previously characterized as effusive and test the hypothesis of local sources we carried out anisotropy of magnetic susceptibility ams studies on the two main silicic units caxias do sul dacites and santa maria rhyolites with the best exposures in an area previously mapped in detail magnetic anisotropies were determined on oriented cylindrical specimens from a total of 28 sites rock magnetism properties indicate that pseudo single domain magnetite carries the fabrics and the remanence magnetic fabrics were determined by applying anisotropy of low field magnetic susceptibility ams and anisotropy of anhysteretic remanent magnetization aarm both ams and aarm tensors are coaxial indicating that the ams fabric is not affected by the effect of magnetite single domain grains magnetic data from several dacitic coulees caxias do sul unit indicate flows from se to nw the location and spatial distribution of these lavas support the hypothesis of local sources aligned along a ne sw trend these data are in agreement with the alignments of structures dome shaped hills observed in field work and dem images on the other hand magnetic data obtained in santa maria rhyolites indicate that flow directions in two different areas are distinct towards nw ne and w suggesting that they derived from different emission centers so regarding the silicic volcanism in the studied region our data do not support the model which classifies the entire silicic volcanism of the province as extensive rheomorphic pyroclastic deposits released from a central conduit in contrast we propose the occurrence of local volcanic events implying in the existence of different sources possibly characterized by local emission centers

magnetic characterisation of magnetite and hematite from the blotberget apatite iron oxide deposits bergslagen south central sweden rock magnetic measurements were carried out on drill core material and hand specimens from the blotberget apatite iron oxide deposit in the bergslagen ore province south central sweden to chara

primary magnetic fabric in an ultramafic body moldanubian zone european variscides survives exhumation related granulite amphibolite facies metamorphism abstract serpentinized peridotite clinopyroxenite approximately 11 5 m in size enclosed in high pressure felsic granulite occurs in a quarry near the village of bory gfohl unit of the moldanubian zone of western moravia czech republic the anisotropy of magnetic susceptibility ams was used to investigate the fabrics of both the ultramafic body and the host granulite the granulite shows a pt path from above 2 gpa 850 900 c following nearly isothermal decompression to sillimanite stability field with subsequent cooling two foliations s1 preserved only in isoclinal folds and the dominant s2 foliation parallel to leucocratic bands are visible in granulite through investigating low field variation of the ams one could resolve the ams into the field independent and field dependent components in the ultramafic body the former component is due to mafic silicates and pure magnetite while in the host granulite it is due to mafic silicates the latter component is due to pyrrhotite in both the ultramafic body and granulite the field independent and field dependent components are coaxial within ultramafic body and within granulite but show different orientations between the ultramafic body and granulite consequently the magnetic fabric in the ultramafic body is different from that in the host granulite even though they experienced common structural history during granulite facies metamorphism the componental movements forming the granulite fabric were evidently not strong enough to strongly overprint the magnetic fabric of ultramafic rocks the ultramafic rocks have therefore sufficient mechanical strength to maintain their pre granulite facies metamorphic fabric even at such high temperatures and high pressure that are characteristic of granulite facies metamorphism

magnetic fabrics in the basal ice of a surge type glacier 1 anisotropy of magnetic susceptibility ams has been shown to provide specific useful information regarding the kinematics of deformation within subglacially deformed sediments here we present results from debris rich basal glacier ice to examine deformation associated with glacier motion basal ice samples were collected from tunabreen a polythermal surge type glacier in svalbard the magnetic fabrics recorded show strong correlation with structures within the ice such as sheath folds and macroscopic stretching lineations thermomagnetic low temperature susceptibility varying field susceptibility and isothermal remanent magnetism acquisition experiments reveal that the debris rich basal ice samples have a susceptibility and anisotropy dominated by paramagnetic phases within the detrital sediment sediment grains entrained within the basal ice are inferred to have rotated into a preferential alignment during deformation associated with flow of the glacier an up glacier directed plunge of magnetic lineations and subtle deviation from bulk glacier flow at the margins highlight the importance of noncoaxial strain during surge propagation the results suggest that ams can be used as an ice petrofabric indicator for investigations of glacier deformation and interactions with the bed

on the interpretation of normal and inverse magnetic fabric in dikes examples from the eger graben nw bohemian massif abstract recent studies of igneous rocks indicate that the predominant occurrence of normal inverse fabric in dikes may either reflect the presence of multi domain md single domain sd grains or it may result from different orientation mechanisms of magnetic minerals in magmas of different viscosities the ambiguity in physical vs geological cause of normal inverse magnetic fabric must be answered before any successful geological interpretation of magnetic fabric can be made in order to address this problem we studied magnetic fabric of selected dikes associated with the sw ne trending eger graben nw bohemian massif the studied area offered very extensive collection of rock types basanite bostonite camptonite tinguaite and trachybasalt magnetic susceptibility varies according to rock type and reflects the relative contents of magnetic minerals in most cases titanomagnetite with variable ti content was identified as main magnetic carrier the degree of anisotropy is relatively low in most cases less than 10 the shape of anisotropy ellipsoid ranges from slightly prolate to neutral and oblate several different types of magnetic fabric using anisotropy of low field magnetic susceptibility ams were observed in studied dikes so called normal and inverse magnetic fabrics and anomalous magnetic fabric comparing all studied sites it seems that the type of magnetic fabric is lithology dependent normal magnetic fabric with magnetic foliations and subhorizontal magnetic lineations both parallel to the dike margins was found in bostonite and trachybasalt dikes inverse magnetic fabric with magnetic lineations and magnetic foliations perpendicular to the dike margins was found in camptonite dikes anisotropy of anhysteretic remanent magnetization amr indicate that the observed inverse magnetic fabric may be caused by the presence of sd magnetic grains amr fabric being normal with respect to dike margins in contrast to that no single domain particles were revealed using frequency dependence and anhysteretic susceptibility measurements the ams measured in variable weak magnetic fields is field dependent for camptonite dike and field independent for other rock types i e bostonite basanite tinguaite and trachybasalt for further flow direction and tectonic interpretations of magnetic fabric in dikes it is suggested to use preferably the amr fabric at least for dikes which demonstrate significant field dependence of ams as it reflects the true rock fabric more accurately than ams fabric

40ar 39ar chronology and paleomagnetism of quaternary basaltic lavas from the persani mountains east carpathians abstract quaternary volcanism in the persani mountains forms an na alkali basaltic province inside the bend area of the carpathians in the southeastern part of europe previous k ar ages and paleomagnetic data reveal several transitional virtual geomagnetic poles which were tentatively associated with the cobb mountain subchron and a brunhes chron excursion we report a new paleomagnetic and rock magnetic study coupled with 40ar 39ar geochronology to better constrain the age of geomagnetic reversals or excursions that might be recorded and the timing of volcanism of the paleomagnetic directions obtained from sampled lava flows 4 are reversed polarity 19 are normal polarity and 16 have transitional polarity 40ar 39ar plateau ages determined from incremental heating experiments on groundmass indicate that two of the reversely magnetized lavas erupted at 1142 41 and 800 25 ka four of the normally magnetized lavas erupted at 1060 10 1062 24 684 21 and 683 28 ka and two transitionally magnetized lavas formed at 1221 11 and 799 21 ka both the new 40ar 39ar ages and the paleomagnetic data suggest at least five episodes of volcanic activity with the most active periods during the jaramillo and brunhes chrons this results shows that the last phases of alkalic and calc alkaline magmatism in the south east carpathians were contemporaneous the age of the older transitionally magnetized lava flow is within error of recent unspiked k ar and astrochronologic ages for the reversal that defines the onset of the cobb mountain normal polarity subchron the age of the younger transitional lava is similar to that of an excursion that preceded the matuyama brunhes polarity reversal and which has come to be known as the matuyama brunhes precursor omitting the excursion data the dispersion of the virtual geomagnetic poles around 19 is larger than the expected value around 45 n from the global compilation but closer to the value obtained only from the time averaged geomagnetic field initiative studies

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

vertical axis rotations and deformation along the active strike slip el tigre fault precordillera of san juan argentina assessed through palaeomagnetism and anisotropy of magnetic susceptibility palaeomagnetic data from poorly consolidated to non consolidated late cenozoic sediments along the central segment of the active el tigre fault central western precordillera of the san juan province argentina demonstrate broad cumulative deformation up to 450 m from the fault trace and reveal clockwise and anticlockwise vertical axis rotations of variable magnitude this deformation has affected in different amounts miocene to late pleistocene samples and indicates a complex kinematic pattern several inherited linear structures in the shear zone that are oblique to the el tigre fault may have acted as block boundary faults displacement along these faults may have resulted in a complex pattern of rotations the maximum magnitude of rotation is a function of the age of the sediments sampled with largest values corresponding to middle miocene lower pliocene deposits and minimum values obtained from late pleistocene deposits the kinematic study is complemented by low field anisotropy of magnetic susceptibility data to show that the local strain regime suggests a n s stretching direction subparallel to the strike of the main fault

palaeomagnetism of the south harghita volcanic rocks of the east carpathians implications for tectonic rotations and palaeosecular variation in the past 5 ma summary lavasofpliocene quaternaryageweresampledinthesouthharghitamountains whichform the southern end of the cu alimani gurghiu harghita volcanic chain of the east carpathians the analyses of 68 volcanic sites in the south harghita mountains provided 62 sites with well constrained directional data n 5 and k 50 in an age interval ranging from 0 5 to 4 3ma the age and polarity for the 62 sites 38 normal and 24 reversed are consistent with the geomagnetic reversal time scale and support the model of the southward migration of the volcanism the distribution of the vgps is fisherian and the mean pole position both from all contributing sites and from the 53 sites older than 2 ma includes the spin axis this result is consistent with the absence of important vertical axis rotations after the emplacement of the volcanicrocksinagreementwithtectonicmodelsforthepliocene quaternaryevolutionofthe bending area of the east carpathians virtual geomagnetic pole dispersions are consistently high compared with global values obtained between 40 and 55 n but closer to the values obtained only from the time averaged geomagnetic field initiative studies for the same latitudinal band our data are compatible with the prediction of the statistical palaeosecular variationmodeltk03 theinclinationanomalyislessthan1 inaccordwiththetotalaverage field global data these palaeomagnetic data from the south harghita volcanic rocks are the first data from the southeastern europe which can be considered in the databases for time averaged field and palaeosecular variation from lavas analyses in the last 5 ma

palaeomagnetism of the upper cretaceous sânpetru formation haţeg basin south carpathians abstract this paper reports on the palaeomagnetism of 10 localities from upper cretaceous sediments of the s npetru formation exposed in the ha eg basin south carpathians romania the s npetru formation is one of the best exposed latest cretaceous continental successions of eastern europe being famous for its unique assemblage of dinosaurs mammals and other vertebrates a total of 169 samples were collected from 66 strata through an 860 m thick section for most of the samples the remanences were soft and or noisy forcing us to reject 63 of the collection in the rest of the collection both polarities are observed and inclination only analysis indicates that pre tilting remanence is preserved for many samples these data along with published palaeomagnetic results from the ha eg basin pass a regional fold test the palaeomagnetic study has provided an acceptable magnetostratigraphy which shows that the studied part of the s npetru formation was deposited between the beginning of the chron 32n 1n and the end of chron 31n these first palaeomagnetic results obtained from the upper cretaceous sedimentary rocks from the south carpathians confirm the previous palaeomagnetic results obtained from magmatic rocks the subtropical palaeolatitude during late cretaceous and the large clockwise rotation during the tertiary

modification of the magnetic mineralogy in basalts due to fluid rock interactions in a high temperature geothermal system krafla iceland summary active high temperature 150 c geothermal areas like the krafla caldera ne iceland often show distinct magnetic lows in aeromagnetic anomaly maps suggesting a destruction of magnetic minerals by hydrothermal activity the main alteration processes in such an environment are low temperature oxidation 350 c maghemitization and fluid rock interactions we investigated the rock magnetic properties natural remanent magnetization nrm magnetic susceptibility and their temperature and field variation and the mineralogy using x ray diffraction microscopic methods and electron microprobe analyses of two drill cores kh1 and kh3 from the rim of the krafla caldera the drill cores have distinctly lower nrm values average 3 a m 1 compared to younger surface basalts average 20 a m 1 along with a large variation in magnetic susceptibility 1 3 10 7 4 9 10 5 m3 kg 1 the secondary mineral assemblage sulphides sphene rutile and chlorite indicates an alteration within the chlorite smectite zone for both cores without depth zoning optical miscroscopy in combination with the bitter technique and backscatter electron microscopy along with the thermomagnetic analyses allow distinguishing two different magnetomineralogical groups of titanomaghemite 1 titanomaghemite with intermediate titanium concentration and probably high vacancy concentration and 2 titanomaghemite with low titanium concentration and low vacancy concentration the mineral assemblages textures and magnetic properties deduced from the mentioned magnetic measurements indicate two stage transformation mechanism 1 dissolution of titanium at low ph under oxidizing conditions the ulvospinel component of titanomagnetite and ilmenite forms rutile or sphene and fe2 migrates out of the spinel lattice forming titanomaghemite 2 formation of pyrite and dissolution of remaining titanomaghemite under reducing and acidic conditions the latter mechanism produces ghost textures all titanomaghemite is transformed and only their former grain shapes are preserved with only paramagnetic minerals left and ferrimagnetic minerals nearly dissolved this mechanism could explain the significant magnetization loss which is seen in many local magnetic anomaly lows within the oceanic crust and volcanic islands like iceland or hawaii the production of nanoporous textures in titanomaghemites is suggested as a mechanism for the enhancement of the magnetic susceptibility values related to the hydrothermal alteration of krafla

drumlins and flutings in finland their relationships to ice movement and to each other abstract drumlin fields are numerous and widespread in finland and flutings are common in the same areas the impressive kuusamo drumlin field in northeastern finland covers older drumlins whose features are evident from beneath the younger till bed in many cases the orientations of the overlying younger streamlined forms deviate from the orientations of the forms lying beneath them the majority of drumlins and flutings form fan like patterns reflecting the configuration of active ice lobes the orientations of the smallest fluting ridges vary in places more than those of the drumlins in the same area it is suggested that small flutings tend to denote the youngest direction of ice movement bending of some drumlins and flutings indicates a curvilinear motion of the ice and some drumlins with forks pointing in a slightly different direction were perhaps caused by a slight shift in ice movement large groove type flutings in the bedrock took a great deal of time to develop and do not therefore always show the direction of ice movement during deglaciation

late stage oxide growth associated with hydrothermal alteration of the western granite isle of rum nw scotland 1 the paleogene western granite isle of rum nw scotland represents a westward tilted shallow level felsic intrusion associated with the early growth stages of the classic early paleogene rum igneous centre rock magnetic data reveal an east to west variability in magnetic properties across the intrusion that is also reflected in petrographic and geochemical observations it is argued that the subtle intraplutonic variations in oxide grain size and composition observed in the western granite might be a common feature of shallow epizonal igneous intrusions as these mineralogical differences are not easily detectable without detailed rock magnetic study this research underscores the need for comprehensive rock magnetic studies when interpreting the postemplacement history of shallow level plutons paleomagnetic data indicate that the eastern part of the intrusion represents the shallower part of the exposed intrusion as further evidenced by roof pendants of lewisian gneiss and abundant drusy miarolitic cavities indicative of shallow emplacement in contrast the western part represents a deeper 900 m part of the intrusion as evidenced by the lack of roof pendants and existence of only minor drusy cavities rock magnetic data indicate that the dominant magnetic phase is low ti titanomagnetite with some titanomaghemite magnetic grain sizes range from pseudosingle domain in the west and multidomain in the east with a slight increase in ti content of the titanomagnetite phases in the east as revealed by curie point estimates we interpret these data to indicate that late stage subsolidus hydrothermal deuteric fluids associated with emplacement of the stage 2 rum layered suite produced secondary growth of fe ti oxide mineral phases along silicate grain boundaries in the eastern part of the intrusion the secondary oxide growth resulted in a slightly coarser magnetic grain size fraction and an increase in ti content of the titanomagnetite grains the deeper structural levels of the western part of the intrusion and its distance from the contact of the rum layered suite inhibited secondary oxide growth and thus this part of the intrusion preserves a primary magmatic oxide phase of a smaller magnetic grain size and near pure magnetite chemistry

low field variation of magnetic susceptibility as a tool for magnetic mineralogy of rocks abstract low field variation of magnetic susceptibility was investigated on a collection of several hundreds specimens of various minerals and rocks using the kly 4s kappabridge the measurement is fully automated being executed in 21 distinct fields ranging from 2 to 450 a m in one frequency of 875 hz the measurement is rapid 7 min per specimen so that large collections of specimens can be investigated the results can be processed both graphically and mathematically for the latter processing parameters of two kinds were introduced one characterizes the susceptibility change with field the other one characterizes the field in which the susceptibility no longer obeys the rayleigh law and starts becoming more complex the results were evaluated statistically remarkable differences were revealed between individual minerals and between some rock types for example the field variation of susceptibility of pyrrhotite is in general an order of magnitude larger than that of titanomagnetite the susceptibility increase in pyrrhotite starts at the field an order of magnitude lower than that of titanomagnetite low field variation of susceptibility then appears as an interesting phenomeon that helps in the identification of magnetic minerals and in some cases also in assessing the compositional variation of them