preferred orientation of ferromagnetic phases in rock forming minerals insights from magnetic anisotropy of single crystals in the early days of paleomagnetism david strangway was interested in understanding why igneous rocks are faithful recorders of the earth s magnetic field he recognized that ferromagnetic s l

geomagnetic paleointensity derived from single plagioclase crystals we suggest a new approach for the determination of absolute geomagnetic paleointensity that uses single plagioclase crystals to avoid problems caused by alteration during thellier thellier analyses transmission electron microscope imaging and rock magnetic data indicate that plagioclase crystals from a recent basalt flow in hawaii 1955 kilauea eruption contain pseudo single to single domain titanomagnetite inclusions these feldspars yield paleointensity values that agree within error with magnetic observatory data and paleointensity values reported from whole rock samples these results suggest that single plagioclase crystals from older igneous rocks may provide a viable source of paleointensity data

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

emplacement and temporal constraints of the gondwanan intrusive complexes of northern patagonia la esperanza plutono volcanic case abstract two main lines of evidence disagree whether or not the patagonian blocks collided with gondwana all models invoke the voluminous magmatism of the la esperanza complex as evidence for active subduction magmatic arc or to a postcollisional setting the evolution of this bimodal igneous suite is reassessed with field geochronological shrimp u pb zircon and k ar mica and petrophysical data emplacement of high k calk alkaline granitic magmas occurred at shallow crustal levels 2 8 2 km depth related to the development and collapse of a caldera associated with a regional nw se structural trend magmatism involved intermediate hybrid pulses at 273 2 ma and 255 2 ma prieto granodiorite that shifted like a yo yo to acidic magmas at 260 2 ma and 250 2 ma donosa and calvo granites absence of solid state deformation features and the low anisotropy degrees in the granites indicate that its fabric is magmatic in origin magnetic fabric in granodiorites displays a concentrical pattern with subhorizontal foliations and lineations parallel to the volcanic axis magnetic foliations and moderately plunging lineations indicate a common feeder system for plutonics and volcanics donosa granite shows a discordant pattern with wnw ese ene wsw trending low plunging lineations and foliations the plutono volcanic system construction 273 255 ma followed nw se and ne sw diamond shape faults trends and supracrustal discontinuities magmatic climax is bracketed at 260 ma the collapse of the edifice is evidenced by the intrusion of acid magma plugs and dike swarms between 250 and 246 ma a similar age range was identified in other areas of patagonia related to syn and postcollisional tectonic events no evidence of tectonic activity such as major uplift metamorphism or thrusting was found excepting regional strike slip faulting and extension therefore la esperanza complex is a high crustal level episode and as such may not have structurally recorded an active collision during its crystallization and cooling

recent developments in element concentration and isotope ratio analysis of individual fluid inclusions by laser ablation single and multiple collector icp ms abstract laser ablation inductively coupled plasma mass spectrometry la icp ms has become a most powerful technique for the elemental analysis of individual polyphase inclusions completely enclosed in minerals be they solid or a solid liquid gas mixture at the time of measurement simultaneous accurate quantification of major to ultra trace element concentrations from li to u by well controlled ablation of the entire fluid or melt inclusion content and successful use of largely matrix independent external calibration protocols are unique features of this method this contribution reviews fluid inclusion fundamentals relevant for their bulk analysis by la icp ms and discusses key aspects of the analytical protocol emphasis is on figures of merit precision accuracy obtained from the analysis of individual inclusions and fluid inclusion assemblages and procedures and technical developments to improving data quality are elaborated a new equation for the calculation of detection limits for la icp ms analysis is presented which closely follows iupac conventions applications are reviewed with emphasis on the use of synthetic fluid inclusions in constraining metal solubility and distribution between co existing phases new data for natural bismuth fluid inclusions document the seamless transition to melt inclusion analysis by la icp ms thus highlighting the fact that the procedures presented here are generally applicable to the analysis of inclusions in complex host minerals isotope ratio analysis of individual fluid inclusions by multicollector icp ms mc icp ms is a recent development that requires fast transient signals to be accurately recorded by instrumentation designed for high precision static measurements of long lasting stable ion beams we address the general principles based on pb isotopes and review a first application to the bingham canyon porphyry cu au mo deposit a pilot study using about 50 synthetic fluid inclusions containing srm 987 sr and variable nacl ca and rb concentrations demonstrates that accurate 87 sr 86 sr isotope ratios can be obtained on an individual rb poor fluid inclusion at absolute 2 precisions of 0 0003 to 0 002 a residual trend in 87 sr 86 sr as a function of the rb sr abundance ratio in the fluid inclusions suggests that interference correction of 87 rb on mass 87 assuming identical mass bias coefficients for the two elements may be inaccurate however the offset can be accurately corrected for by regressing the data to zero 87 rb the versatility and detection power of la icp ms makes this technique the method of choice for solute abundance and isotope ratio analysis of individual fluid inclusions significant future progress can be achieved by improvements in ion production transmission and data recording efficiency and by further improving control on inclusion ablation by pulsed laser beams data quantification strategies may also have to be further refined to keep pace with instrumental progress and innovation

crystallographic and shape orientations of magnetite micro inclusions in plagioclase plagioclase hosted oriented magnetite micro inclusions are a frequently observed phenomenon in magmatic and metamorphic rocks understanding the orientation relationships between these inclusions and the plagioclase host is highly relevant for interpreting paleomagnetic measurements the systematics of the shape and crystallographic orientation relationships between needle and lath shaped magnetite micro inclusions and their plagioclase host from oceanic gabbro were investigated using optical microscopy including universal stage measurements scanning electron microscopy and crystal orientation analysis by electron backscatter diffraction the magnetite inclusions show preferred shape orientations following six well defined directions and with specific crystallographic orientation relationships to the plagioclase host these relationships are rationalized based on angular and dimensional similarities between the crystal structures of magnetite and plagioclase which favor the parallel alignment of oxygen layers with similar lattice spacing in both phases the parallel alignment of oxygen layers in plagioclase and magnetite can be traced back to the oriented nucleation of magnetite which occurs by the accommodation of feo6 octahedra in six membered rings of sio4 and alo4 tetrahedra of the plagioclase structure the orientation systematics of the magnetite micro inclusions is related to four orientation variants for placing the feo6 octahedra into the plagioclase structure

preferred orientation and anisotropy of seismic and magnetic properties in gabbronorites from the bushveld layered intrusion abstract the preferred orientation of plagioclase and pyroxene in gabbronorites from the bushveld layered igneous intrusion south africa was investigated using electron backscatter diffraction in two localities 200 km apart belfast and rustenburg strong preferred orientation was observed with the principal fabric features aligned in the subhorizontal foliation plane defined by 010 planes of plagioclase and 100 planes of pyroxene the pattern appears to result from the orientation of tabular crystals during the dynamic intrusion events that formed the layered structure primary magmatic oxides are also influenced by the preferred orientation of plagioclase generating an oblate magnetic susceptibility fabric parallel to the foliation plane plagioclase preferred orientation gives rise to other directionally dependent geophysical properties including a large seismic anisotropy with p waves traveling 4 8 faster in the vertical than in the horizontal direction

tourmalines from appalachian caledonian massive sulfide deposits textural chemical and isotopic relationships tourmaline is a common gangue mineral in several strata bound massive sulfide deposits of the appalachian caledonian orogen most of the tourmaline rich deposits occur in dominantly metasedimentary terranes such as bleikvassli norway black hawk maine ore knob north carolina and elizabeth vermont minor amounts of tourmaline are also known in association with volcanic hosted deposits in the appalachians and in the trondheim district of norway the tourmaline forms isolated euhedral crystals or crystal aggregates in massive sulfide and adjacent wall rocks in a few areas it is found in conformable tourmaline rich layers tourmalinites associated minerals may include quartz plagioclase phlogopite and dolomite with pyrrhotite chalcopyrite galena and sphalerite many of the tourmaline crystals are optically zoned in which brown zones concentrically alternate with pale yellow or green zones a few specimens of tourmaline intergrown with massive sulfide e g ore knob display fine delicate growth lamellae similar to those commonly observed in hydrothermal vein sphalerites skarn garnets and sphalerites from the massive sulfide deposits of the kuroko district japan detailed geochemical studies indicate that the tourmalines have characteristic major and trace element compositions electron microprobe analyses show that with rare exception the tourmalines are mg rich dravites avg feo feo mgo 0 21 abrupt transitions between color zones within some tourmaline are accompanied by relative variations in feo feo mgo of as much as 30 percent yellow brown zones are more mg rich than blue or green zones in contrast the fine growth lamellae are not characterized by such variations in chemistry spectrographic analyses of pure mineral separates indicate that the tourmalines also contain high average contents of cr 78 ppm cu 77 ppm pb 114 ppm sr 98 ppm and v 195 ppm while uniformly low in li 7 ppm and mn 382 ppm in all analyzed samples fluorine is much more abundant than chlorine the average f cl ratio is 33 tourmalines from tourmalinites and massive sulfide deposits are typically characterized by heavier delta 18 o values 9 5 15 5ppm and less variable heavier delta d values generally 45 to 60ppm than tourmalines from unmetamorphosed igneous pegmatites the latter typically have delta 18 o 9 5 per mil and delta d or 60 per mil oxygen isotope fractionations between quartz and tourmaline delta 1 27 4 01 do not correlate directly with inferred grade of subsequent metamorphism although in areas of high metamorphic grade the delta values approach those of igneous quartz tourmaline pairs the delta 18 o values of quartz and tourmaline are believed to reflect variation in the oxygen isotope compositions of the host lithologies the similar narrow range in hydrogen isotope composition for tourmaline in all studied deposits is compatible with a modified seawater origin for the tourmaline and sulfide forming fluids the distinctive textural and chemical characteristics of the tourmalines reported here have both exploration and genetic significance though it may have undergone recrystallization and some accompanying chemical and isotopic exchange during regional metamorphism tourmaline in the massive sulfide deposits is an early formed perhaps diagenetic mineral in which fine lameliar zoning may be a preserved premetamorphic feature in any case the tourmalines from the massive sulfide deposits are mg rich dravites with trace element and stable isotope compositions substantially different from those of the fe rich schorls typical of most granitic rocks these geochemical and isotopic signatures are attributed to the original environment of formation of the associated sulfide deposits and suggest that mg rich tourmaline may be a valuable prospecting guide in the search for massive sulfide deposits

crystallographic preferred orientation magnetic and seismic anisotropy in rocks from the finero peridotite ivrea verbano zone northern italy interplay of anisotropy contributions from different minerals abstract mineral alignment can provide valuable information on a rock s geological history many structural tectonic or geodynamic studies rely on determining the preferred orientation of rock forming minerals either by direct texture determination or using anisotropy measurements as proxies robust interpretations of anisotropy require detailed understanding of its relationship with texture this study investigates the texture magnetic and seismic anisotropies of ultramafic rocks from the ivrea verbano zone ivz northern italy with a special focus on the interplay between anisotropy contributions carried by different minerals texture was obtained from electron backscatter diffraction magnetic anisotropy from low and high field anisotropy of susceptibility measurements and seismic anisotropy from p and s wave velocities along the three principal directions of the macroscopic fabric using 22 mm diameter cores texture based models of magnetic anisotropy agree well with corresponding measurements larger variability is observed for seismic anisotropy possible explanations are 1 velocities were measured in three directions insufficient to determine the full elasticity tensor and 2 seismic anisotropy depends on grain boundaries pores and cracks in addition to crystallographic preferred orientation the contributions to magnetic and seismic anisotropy of each constituent mineral can interfere positively leading to a larger overall anisotropy or negatively resulting in a weaker anisotropy seismic anisotropy is mostly controlled by olivine whereas magnetic anisotropy can be dominated by olivine pyroxene or hornblende this study illustrates that although the relationship between texture and anisotropy can be complex it is possible to quantitatively predict magnetic and to a lesser degree seismic anisotropy from texture data even though both magnetic and seismic anisotropies are common proxies for texture they are not coaxial and no simple relationship between the degree of magnetic and seismic anisotropy was found these results underline the importance of understanding different minerals contributions to anisotropy in upper mantle rocks

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

anisotropy of magnetic susceptibility in alkali feldspar and plagioclase feldspars are the most abundant rock forming minerals in the earth s crust but their magnetic properties have not been rigorously studied this work focuses on the intrinsic magnetic anisotropy of 31 feldspar samples with various chemical compositions because feldspar is often twinned or shows exsolution textures measurements were performed on twinned and exsolved samples as well as single crystals the anisotropy is controlled by the diamagnetic susceptibility and displays a consistent orientation of principal susceptibility axes the most negative or minimum susceptibility is parallel to 010 and the maximum least negative is close to the crystallographic 001 axis however the magnetic anisotropy is weak when compared to other rock forming minerals 1 53 10 9 m3 kg 1 at maximum therefore lower abundance minerals such as augite hornblende or biotite often dominate the bulk paramagnetic anisotropy of a rock ferromagnetic anisotropy is not significant in most samples in the few samples that do show ferromagnetic anisotropy the principal susceptibility directions of the ferromagnetic subfabric do not display a systematic orientation with respect to the feldspar lattice these results suggest that palaeointensity estimates of the geomagnetic field made on single crystals of feldspar will not be affected by a systematic orientation of the ferromagnetic inclusions within the feldspar lattice

the genesis of sediment hosted exhalative zinc lead deposits large sediment hosted lead zinc deposits like mount isa mcarthur river navan rammelsberg and sullivan form a distinctive group characterised by stratiform syngenetic sulphide ores that formed in local basins on the sea floor as a result of protracted hydrothermal activity accompanying continental rifting generally there is a development of a sedimentary pre ore phase mineralization often featuring manganese followed by zinc lead iron and chert lower main phase zinc lead lenses are usually almost devoid of copper but cu tenors increase toward the middle or top of the ore sequences hanging wall trace element haloes are common these characteristics are accounted for by deriving the ore solutions from subsurface convective circulation of modified highly saline seawater the circulation is initiated during rifting and driven by a high geothermal gradient as a result of continued extensional strain and cooling of the rock column the brittle to ductile transition zone is depressed and the circulation penetrates to greater depth with time of the ore metals the downward penetrating convection fluids first leach and transport zinc and lead but with increasing temperature are later able to leach and transport some copper unless convective circulation ceases the metal sequence generally reverses as the cooling phase sets in the minimum distance separating major coeval orebodies of this type is 18 km which is a function of the size of the convective systems

the effect of remanence anisotropy on paleointensity estimates a case study from the archean stillwater complex paleomagnetism of archean rocks potentially provides information about the early development of the earth and of the geodynamo precambrian layered intrusive rocks are good candidates for paleomagnetic studies such complexes are commonly relatively unaltered and may contain some single domain magnetite armored by silicate mineral grains however layered intrusives often have a strong petrofabric that may result in a strong remanence anisotropy magnetic anisotropy can have particularly disastrous consequences for paleointensity experiments if the anisotropy is unrecognized and if its effects remain uncorrected here we examine the magnetic anisotropy of an anorthosite sample with a well developed magmatic foliation the effect of the sample s remanence fabric on paleointensity determinations is significant paleointensities estimated by the method of thellier and thellier range from 17 to 55 t for specimens magnetized in a field of 25 t we describe a technique based on the remanence anisotropy tensor to correct paleointensity estimates for the effects of magnetic fabric and use it to estimate a paleointensity for the stillwater complex mt usa of 32 t adjusted for the effects of slow cooling

exsolved magnetite inclusions in silicates features determining their remanence behavior submicroscopic needle shaped titanomagnetite inclusions exsolved in silicate minerals commonly occur in mafic intrusive rocks and are protected from alteration by their silicate hosts making them excellent candidates for paleomagnetic studies a suite of samples containing clinopyroxene and plagioclase hosted magnetite inclusions from five geologically diverse sites was examined using magnetic force microscopy to image the inclusions magnetic domain state alternating field demagnetization experiments indicate that some inclusions are more stable recorders than others the two factors controlling the remanence behavior of the inclusions are internal microstructures and inclusion dimensions magnetite ulvospinel unmixing within an inclusion subdivides the original titanomagnetite solid solution into a boxwork structure composed of 103 105 magnetite prisms separated by thin ulvospinel lamellae the conversion of multidomain sized needles into assemblages of interacting single domains increases the coercivity and hence relaxation time of the inclusions and results in a thermochemical magnetic remanence in samples without this exsolution microstructure the inclusions diameters determine coercivity and their magnetization is thermoremanent both styles of high coercivity inclusions successfully record paleomagnetic directions in mesozoic rocks and their ubiquity within silicate minerals clinopyroxene and plagioclase of mafic intrusive rocks indicates their value as chemically and magnetically stable tools for elucidating the ancient magnetic field marine magnetic anomalies and crustal kinematics

magnetite rods in plagioclase as the primary carrier of stable nrm in ocean floor gabbros abstract the ubiquitous occurrence of magnetite rods in plagioclase may account for the stable intense magnetization of ocean floor gabbros these single domain grains are formed by the subsolidus exsolution of fe 2 and fe 3 incorporated into the plagioclase at higher temperatures we expect that this process will be particularly important in the upper portions of the oceanic gabbro unit because of the relative paucity of femic minerals at these levels the geometry of accretion prohibits a constructive contribution to marine magnetic anomalies by the lower olivine rich gabbros as a result of the presence of these rods ocean floor gabbros may constitute an important source for marine magnetic anomalies

morphology and microstructure of magnetite and ilmenite inclusions in plagioclase from adirondack anorthositic gneiss it is well known that oriented iron and titanium oxide inclusions occur in pyroxenes and plagioclase of anorthosites and granulites and they are attributed to exsolution at subsolidus conditions the oxides occur as needles or platelets in this study we determine the morphology of oxide needles as well as their orientation in plagioclase an 30 35 in anorthosite gneiss from the adirondack mountains new york the investigation was done with electron backscatter diffraction ebsd in a scanning electron microscope as well as laue diffraction with a microfocus synchrotron x ray beam at the advanced light source in berkeley it was observed that the needle direction is 110 in magnetite and 1010 in ilmenite the needle direction is consistently parallel to 001 of plagioclase furthermore 111 of magnetite and 0001 of ilmenite are sub parallel to 120 and 120 of plagioclase we note that for directions 110 in the magnetite structure and 1010 in ilmenite o atoms are close packed and 111 and 0001 are close packed planes correspondingly in plagioclase 001 is a direction with open channels as well as approximate alignment of si tetrahedral edges thus providing nucleation sites with a coincidence lattice relationship 120 and 120 in this triclinic mineral are planes with approximate tetrahedral sides so that the relationship is structurally plausible from laue diffraction we can determine that the magnetite needle axis is subject to an extensional stress most likely attained during cooling of the inclusions within the plagioclase host

magnetic fabrics in the bjerkreim sokndal layered intrusion rogaland southern norway mineral sources and geological significance abstract magnetic anisotropy can provide important information about mineral fabrics and thus magmatic processes particularly when it is known how multiple mineral species contribute to the anisotropy it may also affect the direction of induced or remanent magnetization with important consequences for paleomagnetic studies or the interpretation of magnetic anomalies here we aim at describing the magnetic fabrics in the bjerkreim sokndal layered intrusion and identifying their carriers anisotropies of magnetic susceptibility and remanence were measured on samples covering different geographic locations and stratigraphic units within the bjerkreim sokndal layered intrusion the intrusion is characterized by magmatic layering and has a synform structure with strong foliation on the limbs detailed comparison between magnetic and mineral fabric shows that they are not necessarily coaxial but the minimum susceptibility and minimum anhysteretic remanence are generally normal to the foliation or the magmatic layering the minimum susceptibility and anhysteretic remanence are associated with pyroxene 100 axes and the maximum susceptibility and anhysteretic remanence are sub parallel to the pyroxene 001 axes in layers mcu ivc and mcu ive for which electron backscatter data are available however the paramagnetic anisotropy of pyroxene is too weak to explain the observed anisotropy we propose that the magnetic anisotropy of magnetite free specimens is carried by hemo ilmenite exsolutions within pyroxene in addition to pyroxene itself when present multi domain magnetite dominates both the anisotropy of magnetic susceptibility and anhysteretic remanence due to shape preferred orientation and distribution anisotropy the orientation of the magnetic fabric appears independent of carrier due to their common deformation history but the degree of anisotropy is stronger for magnetite bearing specimens the results of this study will facilitate future structural interpretations and may be used to correct for magnetization deflection

stratigraphic expression of basement fault zones in northern west virginia stratigraphic relationships of upper devonian lower mississippian subsurface and outcrop strata in northern west virginia indicate that two basement structures influenced the deposition of the catskill and price rockwell delta complexes these structures are herein named the harrison fault zone and the barbour fault zone the harrison fault zone trends north south through north central west virginia and is recognized by the stacking arrangement of shoreline and marine sandstones and by trends of rapid interval thickening of shelf deposits the trend and sense of movement indicated by these features is consistent with that expected for cambrian age basement normal faults along the eastern margin of the rome trough the barbour fault zone is a cross strike feature recognized by the vertical stacking of hampshire group fluvial sandstones facies relationships within the price formation and the configuration of the sub greenbrier lime stone unconformity the barbour fault zone marks the northern edge of the west virginia dome

influence of static alternating field demagnetization on anisotropy of magnetic susceptibility experiments and implications anisotropy of magnetic susceptibility ams indicates the preferred orientation of a rock s constituent minerals however other factors can influence the ams e g domain wall pinning or domain alignment in ferromagnetic minerals therefore it is controversial whether samples should be alternating field af demagnetized prior to ams characterization this may remove the influence of natural remanent magnetization nrm or domain wall pinning on ams however it may also result in field induced anisotropy this study investigates the influence of stepwise af and low temperature demagnetization on mean susceptibility principal susceptibility directions ams degree and shape for sedimentary metamorphic and igneous rocks alternating fields up to 200 mt were applied along the sample x y and z axes rotating the order for each step to characterize the relationship between ams principal directions and the last af orientation the changes in anisotropy defined by the mean deviatoric susceptibility of the difference tensors are between 2 and 270 of the ams in nrm state variations in ams parameters range from small changes in shape to complete reorientation of principal susceptibility axes with the maximum susceptibility becoming parallel to the last af direction this is most prevalent in samples with low degrees of anisotropy in the nrm state no clear correlations were found between field induced anisotropy and hysteresis properties therefore we propose that future studies check any samples whose ams is carried by ferromagnetic minerals and low anisotropy degrees for af induced artifacts these results highlight the need for understanding the ams sources and carriers prior to any structural interpretation

a review of wave dominated clastic shoreline deposits abstract barrier islands beaches wave dominated deltas and chenier plains probably form more than one fifth of present coasts and constitute an even larger proportion of the subsiding lowland coasts likely to be preserved in the stratigraphic record the geometry vertical sequences and composition of their deposits are the result of 1 shoreline behaviour whether transgressive regressive or stable 2 physical processes particularly the relative intensities of waves and tides 3 climate 4 global tectonic setting and 5 topography of underlying deposits published interpretations indicate that equivalent deposits are abundant in the geological record sandstones are mostly quartzose cemented by carbonates and quartz and have sheet or linear geometries ancient deposits can provide evidence of palaeogeography and some are economically significant particularly as hydrocarbon reservoir rocks four end member sedimentation models are proposed for the interpretation of ancient deposits 1 transgressive sheet sands associated with deltas 2 non deltaic transgressive sheet sands 3 regressive sheet sands and 4 linear sand bodies probable depositional settings geometries characteristics of deposits and palaeocurrent patterns are discussed

tourmalinites from the stratiform peraluminous metamorphic suite of the central namaqua mobile belt south africa tourmalinites as proximal fades equivalents of stratiform peraluminous metamorphic rocks occur stratigraphically below base metal deposits and above thick metarhyolite horizons their premetamorphic protoliths are believed to have originated by tourmaline precipitation from exhalative b f and w rich brines also transporting aluminous clay colloids and dissolved silica tourmaline chemistry is used as an effective petrogenetic sensor the tourmalines are al saturated alkali deficient dravite schorl solid solutions which are in the compositional range of tourmalines originated by exhalative processes f substitution in tourmalines is governed by fe f avoidance f is relatively enriched in the tourmalines and can potentially be used as a tracer for the source of primary hydrothermal solutions ti is introduced into the tourmalines by the substitution scheme ti aliv aly si the high ti contents of the tourmalines as well as those of coexisting muscovites represent evidence of high temperature metamorphism many tourmalines exhibit continuous zoning which can partly be attributed to external fluid influx near peak metamorphic conditions

1 66 ga old metamorphosed pbcu deposit in sargipali eastern india manifestations of tidal flat environment and sedex type genesis abstract the metasedimentary rocks and the unusual pb cu mineralization of the sargipali mine area are described and discussed to elucidate the processes involved in their deposition important lines of evidences used for the interpretation of an inter to supratidal flat environment are 1 a general pattern of a shallowing upward sequence from coarse clastics arenaceous through finer clastics argillaceous to carbonate 2 the occurrence of metaconglomerate trough cross bedding and wave ripples within the arenaceous lithostratigraphic unit 3 intercalations of quartzite carbonate skarn dolomite and convolution of chert laminae within the argillaceous lithostratigraphic unit and 4 intercalation of quartz mica chlorite schist laminae within the carbonate lithostratigraphic unit the ores seems to belong to the same environment furthermore in the assessment of the origin of the sargipali ores it is recognized that the orebodies not only represent a sedimentary setting but also show a high pb content and an older age of mineralization than that of deformation metamorphism and granite intrusion along with the lack of stockwork zones wall rock alteration subaerial volcanism and the presence of chert and tourmalinite these features are consistent with a sedex type origin the idea of this origin finds support by yet another observation i e the ore sulphur was mainly derived from the inorganic reduction of the seawater sulphate at more than 300 c while the ore metals were leached from the 238u 204pb enriched singhbhum soda granite of 2200 ma the present study may help in conceiving a new possibility that the sediment hosted pb rich deposits of mvt origin can also be evaluated in terms of the sedex model

anisotropy of magnetic susceptibility in natural olivine single crystals mantle flow dynamics can cause preferential alignment of olivine crystals that results in anisotropy of physical properties to interpret anisotropy in mantle rocks it is necessary to understand the anisotropy of olivine single crystals we determined anisotropy of magnetic susceptibility ams for natural olivine crystals high field ams allows for the isolation of the anisotropy due to olivine alone the orientations of the principal susceptibility axes are related to the olivine s crystallographic structure as soon as it contains 3 wt feo the maximum susceptibility is parallel to the c axis both at room temperature rt and at 77 k the orientation of the minimum axis at rt depends on iron content it is generally parallel to the a axis in crystals with 3 5 wt feo and along b in samples with 6 10 wt feo the ams ellipsoid is prolate and the standard deviatoric susceptibility k0 is on the order of 8 10210 m3 kg for the samples with 1wt feo and ranges from 3 1 1029 m3 kg to 5 7 1029 m3 kg for samples with 3 10 wt feo at 77 k the minimum susceptibility is along b independent of iron content the shape of the ams ellipsoid is prolate for samples with 5 wt feo but can be prolate or oblate for higher iron content the degree of anisotropy increases at 77 k with p0 7757 160 5 the results from this study will allow ams fabrics to be used as a proxy for olivine texture in ultramafic rocks with high olivine content

two sulphur isotope provinces deduced from ores in the mount isa eastern succession australia the sulphur isotopic characteristics of ore deposits in the australian mount isa eastern succession are not well known unlike those of the western succession in this study new detailed analyses are provided for recently discovered eastern succession mineralisation such as the starra and osborne bif hosted cu au ores the dugald river sediment hosted pb zn prospect and four vein hosted cloncurry style cu au deposits hampden mt elliot swan mt cobalt and the answer mine all of the deposits of the eastern succession have 34ssulphide between 8 and 9 regardless of their genesis empirically a moderate 34s range averaging close to 0 characterises starra style cu au and pegmont pb zn bif ores whereas shear and vein style cu mineralisation populations are tighter and do not average close to 0 this is a particularly surprising result for dugald river where a larger isotopic variation more typical of stratiform sediment hosted pb zn ores in the region might have been expected by comparison western succession stratiform pb zn and vein style cu deposits span a huge range of 15 to 51 large sulphur isotope ranges typify sulphate evaporite or organic sulphur rich sedimentary successions the lack of such variation in the eastern succession in turn suggests that primary evaporite sequences there were halite dominated but sulphate poor and or contained only limited volumes of organic sulphur rich sediment eastern succession sequences were therefore less likely hosts for giant stratiform pb zn deposits because of their paucity of sulphur although local sulphur sources permitted small deposits such as dugald river to develop sedimentary conditions were more favourable for the development of sulphur poor synsedimentary hydrothermal systems such as starra osborne and pegmont although sulphur isotope evidence is equivocal about the origin of these epigenetic deposits close to the williams batholith mt dore hampden owe their clustering around 0 to their granitic fluid source

magnetic anisotropy in clinopyroxene and orthopyroxene single crystals pyroxenes constitute an important component in mafic igneous and metamorphic rocks they often possess a prismatic habit and their long axis the crystallographic c axis helps define a lineation in a textured rock anisotropy of magnetic susceptibility ams serves as a fabric indicator in igneous and metamorphic rocks if a rock s ams is carried by pyroxenes it can be related to their crystallographic preferred orientation and degree of alignment this requires knowing the intrinsic ams of pyroxene single crystals this study provides a comprehensive low field and high field ams investigation of chemically diverse orthopyroxene and clinopyroxene crystals in relation to crystal structure chemical composition oxidation state of fe and the possible presence of ferromagnetic inclusions the paramagnetic anisotropy extracted from high field data shows clear relationships to crystallographic directions and fe concentration both in clinopyroxene and orthopyroxene in the diopside augite series the intermediate susceptibility is parallel to b and the maximum is at 45 to the c axis in aegirine the intermediate axis remains parallel to b while the maximum susceptibility is parallel to c the ams of spodumene depends on fe concentration in enstatite the maximum susceptibility aligns with c and the minimum with b and in the case of hypersthene the maximum susceptibility is normal to the exsolution lamellae magnetite inclusions within augite possess a ferromagnetic anisotropy with consistent orientation of the principal susceptibilities which dominates the low field anisotropy these results provide better understanding of magnetic anisotropy in pyroxenes and form a solid basis for interpretation of magnetic fabrics in pyroxene bearing rocks

protracted and polyphased gold mineralisation in the agnew district yilgarn craton western australia abstract the archean orogenic gold deposits exhibits common characteristics including coeval emplacement age often proposed to be associated with a single late tectonic gold mineralisation event although valid for a large majority of deposits in the yilgarn craton this model does not account for certain atypical gold deposits in the world class agnew district yilgarn craton western australia structural and paragenetic relationships combined with a compilation of geochronology and multiple sulfur isotopes indicate that mineralisation developed during a two stage process involving different fluid sources the initial event dated at c 2660 ma is related to the onset of folding during kalgoorlie orogeny and presents characteristics compatible with magmatic intrusion related mineralisation a second mineralisation event dated at c 2630 ma presents a mineral and metal inventory typical to archean orogenic like gold mineralisation and developed at a later stage of the orogenesis measurements of 34s and 33s from spatially constrained samples from the two temporally distinct mineralising events further ascertain contrasting fluid compositions and or reservoirs combined with geochronological data obtained on the poly magmatic lawlers granitic complex we argue for a protracted ore forming process the first stage of mineralisation has a magmatic affinity and is followed by a second stage of mineralisation whereby au bearing hydrothermal solution incorporated an archean sedimentary sulfur reservoir we propose that the change in source reservoir for sulfur in hydrothermal fluids may reflect a regional change in tectonic regime from convergent to transcurrent motion allowing fluids to be first sourced from mantle derived magmas followed later by metamorphic fluids

low temperature magnetic anisotropy in micas and chlorite abstract phyllosilicates such as micas and chlorite are common rock forming minerals and often show preferred orientation in deformed rocks in combination with single crystal anisotropy this leads to anisotropy of physical properties in the rock such as magnetic susceptibility in order to effectively use the magnetic anisotropy to understand a rock fabric it is necessary to identify the minerals responsible for the magnetic anisotropy techniques have been developed to separate contributions of the ferrimagnetic antiferromagnetic paramagnetic and diamagnetic susceptibilities to the anisotropy of magnetic susceptibility because diamagnetic and paramagnetic susceptibility are both linearly dependent on field separation of the anisotropic contributions requires understanding how the degree of anisotropy of the paramagnetic susceptibility changes as a function of temperature note that diamagnetic susceptibility is not dependent on temperature the increase in paramagnetic anisotropy at low temperature is used to separate the paramagnetic and diamagnetic subfabrics and can be expressed by the p77 factor in this study we determined p77 which is the change in the degree of anisotropy k between room temperature 298 k and liquid nitrogen temperature 77 k for a series of micas and chlorite the paramagnetic susceptibility ellipsoid is highly oblate with the minimum principal susceptibility normal to the silicate layers at both 77 k and rt the degree of anisotropy k increases by a factor of approximately 6 3 8 7 for individual samples of muscovite phlogopite and chlorite on cooling from rt to 77 k and between 11 2 and 12 4 for biotite a decrease in temperature enhances the paramagnetic anisotropy in a mineral biotite exhibits a relatively stronger enhancement due to the onset of magnetic ordering below 100 k this can have important implications for interpreting low temperature anisotropy in mudstones mica schists and gneisses

magnetic anisotropy in natural amphibole crystals anisotropy of magnetic susceptibility ams is often used as a proxy for mineral fabric in deformed rocks to do so quantitatively it is necessary to quantify the intrinsic magnetic anisotropy of single crystals of rock forming minerals amphiboles are common in mafic igneous and metamorphic rocks and often define rock texture due to their general prismatic crystal habits amphiboles may dominate the magnetic anisotropy in intermediate to felsic igneous rocks and in some metamorphic rock types because they have a high fe concentration and they can develop a strong crystallographic preferred orientation in this study the ams is characterized in 28 single crystals and 1 crystal aggregate of compositionally diverse clino and ortho amphiboles high field methods were used to isolate the paramagnetic component of the anisotropy which is unaffected by ferromagnetic inclusions that often occur in amphibole crystals laue imaging laser ablation inductively coupled plasma mass spectrometry and mossbauer spectroscopy were performed to relate the magnetic anisotropy to crystal structure and fe concentration the minimum susceptibility is parallel to the crystallographic a axis and the maximum susceptibility is generally parallel to the crystallographic b axis in tremolite actinolite and hornblende gedrite has its minimum susceptibility along the a axis and maximum susceptibility aligned with c in richterite however the intermediate susceptibility is parallel to the b axis and the minimum and maximum susceptibility directions are distributed in the a c plane the degree of anisotropy k increases generally with fe concentration following a linear trend k 1 61 10 9 fe 1 17 10 9 m3 kg additionally it may depend on the fe2 fe3 ratio for most samples the degree of anisotropy increases by a factor of approximately 8 upon cooling from room temperature to 77 k ferroactinolite one pargasite crystal and riebeckite show a larger increase which is related to the onset of local ferromagnetic s l interactions below about 100 k this comprehensive data set increases our understanding of the magnetic structure of amphiboles and it is central to interpreting magnetic fabrics of rocks whose ams is controlled by amphibole minerals

in search of high fidelity geomagnetic paleointensities a comparison of single plagioclase crystal and whole rock thellier thellier analyses the strength of earth s magnetic field on timescales greater than a million years is known primarily from thellier thellier analyses of igneous rocks in the thellier thellier method the natural remanent magnetization nrm is compared with a thermoremanent magnetization trm imparted during heating in a known laboratory field unfortunately the multiple heating steps required by the method often result in alteration of magnetic minerals in clay bearing groundmass of whole rock samples this alteration may result in the creation of new magnetic minerals that could affect trm acquisition analyses of single plagioclase crystals which contain magnetic inclusions that retain paleofield values may offer a way to retrieve paleointensity data that is less affected by alteration we test this hypothesis through a study of whole rocks and single plagioclase crystals from a mid cretaceous 113 115 ma basalt flow of the rajmahal traps northeastern india magnetic hysteresis and thermal demagnetization data from plagioclase crystals and whole rock samples are similar suggesting a common magnetic mineralogy however hysteresis data from heated whole rock samples indicate that a fine grained magnetic phase has formed this effect was either absent or much less pronounced in the single plagioclase crystals tested paleointensity data from these crystals suggest a paleofield value of 65 1 5 3 t n 15 data from whole rock samples overlap these values but are on average lower we attribute the difference in this study to the preferential growth of new magnetic minerals in the whole rock samples resulting in an enhanced acquisition of trm and shallowing of the slope of nrm lost versus trm gained curves these data suggest that subtle alteration effects may bias thellier thellier results from some whole rock samples toward low paleofield values in these cases the single plagioclase crystal approach may yield more reliable estimates of geomagnetic paleointensity

a determination of the three dimensional seismic properties of anorthosite comparison between values calculated from the petrofabric and direct laboratory measurements bernard eront and david mainprice zaboratoire de tectonophysique i h iversit montpe lier ii montpe lier france nikolas i christensen department of earth and atmospheric sciences purdue unh ersity west lafayette btdiana we have undertaken a coinparative study of the seisnfic properties of an anorthosite the seisnfic properties have been calculated froin the lattice preferred orientation lpo of plagioclase 90 by volume and olivine 10 by voluine and the elastic constants of these minerals laboratory measurements of vp 24 directions and vs 6 directions have been made on the same sample at room temperature and pressures up to 1 0 gpa the twe techniques agree with 0 1 km s for all orientations studied when the voigt averaging method is used hill and reuss averaging methods give calculated velocities significantly lower than the experimental ones a study of the difference between the velocity values given by the two methods reveals a systematically larger departure in orientations normal to the foliation the orientation dependent difference between the two tedmiques can be explained if there are small errors in the elastic constants for single crystal plagioclase used in the initial calculations the axial lpo symmetry of the plagioclase with all the crystallographic b axes parallel to the specimen symmetry axis allowed us to calculate c22 norinal to 010 directly from the laboratory measurement of vp some constraints can be put on c22 c44 c66 c12 and c23 on average our data require 10 modification of the single crystal constants reported by ryzhova 1964

magnetic anisotropy as a tracer of crystal accumulation and transport middle banded series stillwater complex montana abstract fabric studies of layered mafic intrusions have led to improved understanding of the mechanical processes operating in large magma chambers including crystal accumulation and crystal mush deformation such studies however are typically limited by a tradeoff between breadth number of sites studied characteristic of field focused work and sensitivity ability to discern subtle fabric elements characteristic of laboratory fabric analyses magnetic anisotropy if analyzed in a systematic way and supported by single crystal and petrofabric measurements permits relatively rapid characterization of magmatic fabrics for large numbers of samples here we present the results of a study of remanence and susceptibility anisotropy from three transects through the middle banded series of the stillwater complex montana all three transects exhibit a magnetic foliation that increases with stratigraphic height up to the top of olivine bearing zone iii consistent with crystal mush compaction perhaps more importantly each transect is characterized by a subtle lineation in the anisotropy of magnetic susceptibility with a consistent direction within that transect the magnetic lineation directions which generally coincide with crystallographic preferred orientations of silicate minerals likely record a pre compaction fabric lineation directions differ from one transect to another implying that the process generating the lineation either slumping of a semiconsolidated crystal mush or magma transport acted on length scales of at most a few km these results demonstrate the sensitivity of magnetic anisotropy to petrofabric in mafic rocks

interpreting magnetic fabrics in amphibole bearing rocks abstract if amphibole is a major constituent in a rock its magnetic fabric can be largely controlled by the crystallographic preferred orientation of amphibole this study describes the para magnetic anisotropy in two amphibolites both containing ca 70 hornblende with rather strong crystallographic preferred orientation both amphibolites display a significant and well defined anisotropy of magnetic susceptibility with the minimum susceptibility approximately normal to foliation however in one amphibolite the maximum susceptibility is parallel to the lineation whereas in the other it is not this seemingly inconsistent observation can be explained by the intrinsic susceptibility anisotropy of single crystals of hornblende and their texture in the rocks numerical models show how the principal susceptibility axes relate to macroscopic foliation and lineation for point and fiber textures this study underlines the potential of using magnetic anisotropy to obtain information about mineral fabrics in mafic rocks at the same time it highlights the necessity for taking into account single crystal properties of the mineral s responsible for the anisotropy and their crystallographic preferred orientation when interpreting magnetic fabrics

a method for the separation of paramagnetic ferrimagnetic and haematite magnetic subfabrics using high field torque magnetometry summary in this study the contribution of the paramagnetic ferrimagnetic and haematite components to the magnetic anisotropy is separated by means of high field torque magnetometry torque measurements at different fields which are high enough to saturate the ferrimagnetic minerals however still low enough that the torque resulting from the haematite is linear with field allow for the separation of the three magnetic anisotropy components the method has been applied to haematite single crystals in which no paramagnetic or ferrimagnetic components have been found contribute to the torque signal the mean direction of the poles to the crystallographic basal plane in the haematite single crystals is subparallel to the minimum susceptibility direction measured in low field the separation analysis has also been applied to highly deformed red beds from the lower glarus nappe complex switzerland no ferrimagnetic phases are present in the rocks and therefore they cannot contribute to the anisotropy of magnetic susceptibility the magnetic fabric arises from a paramagnetic subfabric carried by the phyllosilicate minerals and haematite in which the basal planes of both phases are in the cleavage plane the measured magnetic lineation seen in low field anisotropy of magnetic susceptibility appears to be an apparent lineation that arises from a weak girdling of haematite and the paramagnetic minerals conforming the rock

sargipali galenas unusual lead isotope data from eastern india the sargipali sulphide deposit hosted by a sequence of metasediments of the precambrian gangpur series represents the stratiform and stratabound pb cu mineralization lead isotope analyses obtained from six galena samples of different forms and stratigraphical levels yielded a model pb age of 1665 ma which predates the episodes of deformation and metamorphism 850 ma as well as syn to late tectonic magmatism the model pb age on the contrary shows a relationship to the closing phase of the gangpur sedimentation 1700 ma in a shallow water marine environment the notable paucity of the interlayered volcanic rock in the deposit and its pb rich nature possibly point to its being the sedimentary exhalative sedex type documentary evidence for the pb component from the geological environment of the upper continental crust is revealed by an exceptionally high 238u 204pb and the slightly above average w 232th 238u values these indicate that the source rock was initially rich in u th pb concentration for the sargipali ore pb the parent is assumed to be late archean lower proterozoic soda granite