intra oceanic subduction related hydrothermal venting kermadec volcanic arc new zealand intra oceanic volcanic arcs mark the boundaries between converging lithospheric plates where subduction produces volcanic and tectonic activity that ensures a steady supply of magmatic heat and hydrothermal fluids to the seafloor here we report on the first broad and systematic survey of hydrothermal emissions generated along a submarine arc front more than half seven of 13 of the volcanoes surveyed along 260 km of the southern kermadec arc ne of new zealand are hydrothermally active our results indicate that volcanic arcs represent a previously unheeded but potentially extensive source of shallow 2 km water depth vent fields expelling fluids of a unique and heterogeneous composition into the oceans

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

petrophysical properties of the himalayan granitoids implication on composition and source abstract the paper reports the characterization of density magnetic susceptibility magnetic anisotropy seismic wave velocities attenuation as well as mineralogy and major element chemistry of the four generation of granitoids from the indian himalaya based on these petrophysical properties only the cretaceous granitoids of the trans himalayan region by virtue of their mantle affinity and domination of magnetite and or magnetite ilmenite series qualify to be the i type granitoid on the other hand rest of the 3 suites of granitoids have a crustal affinity and can be categorized as s type granitoids enriched with ilmenite and or hemo ilmenite series beside this general classification some anomalous petrophysical properties can be related to distinctive mineralogy stages of magmatic crystallization and intensity of deformation in different class of granitoids for example i presence of heavy minerals like hornblende and magnetite accounts for the significantly high density and seismic wave velocity of the cretaceous granitoids ii fractional crystallization of mantle melts leads to hornblende rich granitoids rich in magnetite in the earlier stage where biotite rich granitoids low magnetite crystallize in the later stage thus explaining bimodal distribution of magnetic susceptibility in cretaceous granitoids iii in s type granitoids high quartz content 45 account for the lowest density recorded in saruna proterozoic granitoids whereas high content of micaceous minerals reduce the seismic wave and are responsible for the lowest s wave velocity in the early palaeozoic mandi granitoids iv further the effect of texture is seen as varying attenuation character of p and s waves on grain size in general the higher the grain size the greater the attenuation once again cretaceous granitoids negate this well established relation incorporation of this anomalous dependence of physical properties on mineralogical tectonic fracturing texture will help the translation of geophysical maps to more a realistic region specific crustal tectonic evolution models

evidence for microbial mediation of subseafloor nitrogen redox processes at loihi seamount hawaii abstract the role of nitrogen cycling in submarine hydrothermal systems is far less studied than that of other biologically reactive elements such as sulfur and iron in order to address this knowledge gap we investigated nitrogen redox processes at loihi seamount hawaii using a combination of biogeochemical and isotopic measurements bioenergetic calculations and analysis of the prokaryotic community composition in venting fluids sampled during four cruises in 2006 2008 2009 and 2013 concentrations of nh4 were positively correlated to dissolved si and negatively correlated to no3 no2 while no2 was not correlated to no3 no2 dissolved si or nh4 this is indicative of hydrothermal input of nh4 and biological mediation influencing no2 concentrations the stable isotope ratios of no3 15n and 18o was elevated with respect to background seawater with 18o values exhibiting larger changes than corresponding 15n values reflecting the occurrence of both production and reduction of no3 by an active microbial community 15n nh4 values ranged from 0 to 16 7 suggesting fractionation during consumption and potentially n fixation as well bioenergetic calculations reveal that several catabolic strategies involving the reduction of no3 and no2 coupled to sulfide and iron oxidation could provide energy to microbes in loihi fluids while 16s rrna gene sequencing of archaea and bacteria in the fluids reveals groups known to participate in denitrification and n fixation taken together our data support the hypothesis that microbes are mediating n based redox processes in venting hydrothermal fluids at loihi seamount

mantle dynamics and seismic tomography three dimensional imaging of the earth s interior called seismic tomography has achieved breakthrough advances in the last two decades revealing fundamental geodynamical processes throughout the earth s mantle and core convective circulation of the entire mantle is taking place with subducted oceanic lithosphere sinking into the lower mantle overcoming the resistance to penetration provided by the phase boundary near 650 km depth that separates the upper and lower mantle the boundary layer at the base of the mantle has been revealed to have complex structure involving local stratification extensive structural anisotropy and massive regions of partial melt the earth s high rayleigh number convective regime now is recognized to be much more interesting and complex than suggested by textbook cartoons and continued advances in seismic tomography geodynamical modeling and high pressure high temperature mineral physics will be needed to fully quantify the complex dynamics of our planet s interior

submarine hydrothermal activity along the mid kermadec arc new zealand large scale effects on venting 1 the 2 500 km kermadec tonga arc is the longest submarine arc on the planet here we report on the second of a series of cruises designed to investigate large scale controls on active hydrothermal venting on this arc the 2002 nzaplume ii cruise surveyed 12 submarine volcanic centers along 580 km of the middle kermadec arc mka extending a 1999 cruise that surveyed 260 km of the southern kermadec arc ska average spacing between volcanic centers increases northward from 30 km on backarc crust along the ska to 45 km on backarc crust along the southern mka to 58 km where the mka joins the kermadec ridge volcanic cones dominate in the backarc and calderas dominate the kermadec ridge the incidence of venting is higher along the mka 83 10 of 12 volcanic centers than the ska 67 8 of 12 but the relative intensity of venting as given by plume thickness areal extent and concentration of dissolved gases and ionic species is generally weaker in the mka this pattern may reflect subduction of the 17 km thick oceanic hikurangi plateau beneath the ska subduction of this basaltic mass should greatly increase fluid loss from the downgoing slab initiating extensive melting in the upper mantle wedge and invigorating the hydrothermal systems of the ska conversely volcanic centers in the southern mka are starved of magma replenishment and so their hydrothermal systems are waning farther north where the mka centers merge with the kermadec ridge fewer but larger magma bodies accumulate in the thicker older crust ensuring more widely separated caldera dominated volcanic centers

natural deformation fabrics of plagioclase implications for slip systems and seismic anisotropy abstract the optical and tem microstructures and fabrics of twelve naturally deformed plagioclase containing rocks of compositions from an0 an80 have been studied with a view to determining the deformation mechanisms the optical indicatrix fabrics have been recorded in all samples with respect to the foliation and lineation in general and form a girdle in the foliation plane with a high concentration of normal to the foliation in some cases the orientation of 010 and 001 cleavages or albite and pericline twins has allowed the construction of pole figures in all cases 010 has been shown to be the dominant slip plane with either 001 or 100 the dominant slip direction tem shows that the dominant slip system is 010 001 with secondary 010 100 the seismic p wave anisotropy has been calculated for the first time from the lpo data of plagioclase the anisotropy coefficient is high at around 15 the minimum p velocity is normal to the lineation in the foliation plane and the maximum is sub parallel to the foliation normal the combined information on deformation kinematics and seismic anisotropy is valuable for interpretations of deep crustal seismic sections

hydrothermal activity on near arc sections of back arc ridges results from the mariana trough and lau basin 1 the spatial density of hydrothermal venting is strongly correlated with spreading rate on mid ocean ridges with the interesting exception of hot spot affected ridges evidently because spreading rate is a reliable proxy for the magma budget this correlation remains untested on spreading ridges in back arc basins where the magma budget may be complicated by subduction induced variations of the melt supply to address this uncertainty we conducted hydrothermal plume surveys along slow spreading 40 60 mm yr and arc proximal 10 60 km distant sections of the southern mariana trough and the valu fa ridge lau basin on both sections we found multiple plumes overlying 15 20 of the total length of each section a coverage comparable to mid ocean ridges spreading at similar rates these conditions contrast with earlier reported results from the two nearest arc segments of a faster spreading 60 70 mm yr back arc ridge the east scotia ridge which approaches no closer than 100 km to its arc there hydrothermal venting is relatively scarce 5 plume coverage and the ridge characteristics are distinctly slow spreading small central volcanic highs bookended by deep median valleys and axial melt lenses restricted to the volcanic highs two factors may contribute to an unexpectedly low hydrothermal budget on these east scotia ridge segments they may lie too far from the adjacent arc to benefit from near arc sources of melt supply and subduction aided migration of mantle from the bouvet hot spot may reduce hydrothermal circulation by local crustal warming and thickening analogous to the reykjanes ridge thus the pattern among these three ridge sections appears to mirror the larger global pattern defined by mid ocean ridges a well defined trend of spreading rate versus hydrothermal activity on most ridge sections plus a subset of ridge sections where unusual melt delivery conditions diminish the expected hydrothermal activity

global anisotropy in the upper mantle inferred from the regionalization of phase velocities a data set of 2600 paths for rayleigh waves and 2170 paths for love waves was selected in order to resolve azimuthal anisotropy of surface waves in the period range 70 250 s the epicenters were chosen so that the paths provide uniform spatial and azimuthal coverage of earth a synthetic seismogram is computed for each path and compared to the observed seismogram in order to infer the phase velocity along each path to process such a large data set a new tomographic technique is designed a good correlation is found with surface tectonics in the whole period range for love waves and at shorter periods for rayleigh waves the directions of maximum velocities are also well correlated with plate tectonics motions for the 2 azimuthal term of rayleigh waves when the power spectrum of heterogeneities is computed it is found that for love waves the even orders are usually larger than the odd orders for rayleigh waves the degrees 2 and 6 dominate only at long periods suggesting a deep origin for these components

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

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

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

microstructures and crystallographic preferred orientation of anorthosites from oman ophiolite and the dynamics of melt lenses 1 microstructures and crystallographic preferred orientation cpo of anorthosite samples interlayered in the upper and lower gabbro sections in the oman ophiolite were analyzed in this paper in the anorthosites registering the dynamics of the melt lenses foliation is flat lying and starts to develop a few meters below the root zone of the sheeted dike complex rzsdc microstructures and cpo of these rocks were developed in response to four different mechanisms 1 density controlled settling of plagioclase on the lens floor 2 deposition of anorthosites related to convection currents 3 melt compaction and 4 uncompacted melt accumulation in these anorthosites the poles to 010 of plagioclase are parallel to the flow plane of convection whereas the 100 axes and poles to 001 express the convection flow direction and the axis of convection rolls respectively the effect of subsidence of melt lens floor is recorded immediately below the rzsdc and is characterized by the rapid but progressive development of dipping foliation and lineation reflecting the increase of deformation downsection the degree of foliation and cpo development in the anorthosites is directly related to the distance of the center of the melt lenses before the subsidence starts despite the uncertain origin of the anorthosites from the lower gabbro section all the samples lost the magmatic microstructural characteristics and presently are reequilibrated aggregates however they still preserve plagioclase cpo where some of these patterns present similarities with the anorthosites from the upper gabbro section but no evidence of intracrystalline deformation under high temperatures

population ecology of the tonguefish symphurus thermophilus pisces pleuronectiformes cynoglossidae at sulphur rich hydrothermal vents on volcanoes of the northern mariana arc abstract flatfish are a major component of the hydrothermal vent community on three seamounts of the northern mariana volcanic arc in the northwest pacific nikko kasuga 2 and daikoku seamounts host vent fields between 375 and 480 m depth where high temperature vents release molten sulphur the small cynoglossid tonguefish symphurus thermophilus munroe and hashimoto is ubiquitous in all vent habitats observed on these seamounts among extensive fields of tubeworms and mussels and on solid sulphur surfaces on nikko on sulphur rich sediments and barnacle covered boulders on kasuga 2 and on recent sulphur flows and on broad areas of loose and semi consolidated sediments on daikoku we recorded repeated forays by individuals onto flows of molten sulphur as these surfaces cooled based on observations using rovs the mean density is 90 fish m 2 with maximum counts over 200 fish m 2 on daikoku sediments compared to collected tonguefish from daikoku and kasuga 2 those from nikko have significantly greater lengths and on average six times the mass otolith data indicate upper ages of 13 years with nikko tonguefish growing significantly faster diets of tonguefish on the three seamounts reflect the different habitats and prey availability in daikoku specimens small crustaceans and polychaetes are most common while on nikko gut contents are predominantly larger shrimp we made the unusual observation of stunned midwater fish falling to the seafloor near the vents where s thermophilus immediately attacked them this tonguefish has a wide diet range and foraging behaviour that likely influence the differing growth rates and sizes of fish inhabiting the different vent sites limited genetic data suggest that larval exchange probably occurs among sites where the common habitat factor is high levels of elemental sulphur forming hard and partly unconsolidated substrata here in the northern range of the mariana trench marine national monument s thermophilus despite having an unusually broad habitat association may be restricted in its overall range to this region of isolated volcanoes with active hydrothermalism

mineralogical characterization and formation of fe si oxyhydroxide deposits from modern seafloor hydrothermal vents we have studied mineralogical characteristics of fe si oxide precipitates from hydrothermal fields of the valu fa ridge lau basin especially the role that the neutrophilic fe oxidizing bacteria played in their formation using various analytical techniques xrd sem epma tg dta and ftir according to this examination the fe si oxide formation can be divided into two stages at the initial stage the fe oxidizing bacteria bound and oxidized fe2 into fe3 to fix co2 triggering precipitation of fe oxyhydroxide ferrihydrite and construction of a loose network of fe rich filaments subsequently the decreased porosity of the network resulting from the gradual growth of the filaments led to a decline in the mixing between seawater and the hydrothermal fluids then the conductive cooling of the network resulted in saturation of the dissolved si with respect to amorphous silica as a result significant precipitation of opal a occurred through inorganic polymerization however part of the silica was immobilized by bonding to fe oh functional groups and yielded unpolymerized silica which is characterized by fe o si bond owing to the incorporation of si into the ferrihydrite structure and its adsorption on the ferrihydrite surface the modern hydrothermal fe si oxides are thermally stable dsc measurements indicate the full segregation of cristobalite from hematite at about 800 c in an o2 atmosphere these observations indicate that primary alternating si and fe rich layers may be absent in the archean ocean and that alternating bands in bifs represent a diagenetic process our work thus provides a potential clue that can be used to unravel the precipitation and diagenetic mechanisms of precambrian banded iron formations bif

low temperature shallow water hydrothermal vent mineralization following the recent submarine eruption of tagoro volcano el hierro canary islands abstract hydrothermal iron fe rich sediments were recovered from the tagoro underwater volcano central atlantic that formed during the 2011 2012 volcanic event cruises in 2012 and 2014 enabled the monitoring and sampling of the early stage establishment of a hydrothermal system degassing vents produced acoustic flares imaged on echo sounders in june 2012 four months after the eruption a novel hydrothermal vent system was discovered and sampled in 2014 during a rov dive the system is characterized by hornito like structures and chimneys showing active co2 degassing and anomalous temperatures at 120 89 m water depth and along the se flank at 215 185 m water depth associated with secondary cones iron and silica rich gelatinous deposits pooled over and between basanite in the hornitos brecciated lavas and lapilli the low temperature shallow water hydrothermal system was discovered by the venting of fe rich fluids that produced a seafloor draped by extensive fe flocculate deposits precipitated from the neutrally buoyant plumes located along the oxic photic zone at 50 70 m water depths the basanite is capped by mm to cm thick hydrothermally derived fe oxyhydroxide sediment and contains micro cracks and degasification vesicles filled by sulfides mostly pyrite mineralogically the fe oxyhydroxide sediment consists of proto ferrihydrite and ferrihydrite with scarce pyrite at their base the fe rich endmember contains low concentrations of most trace elements in comparison with hydrogenetic ferromanganese deposits and the sediments show some dilution of the fe oxyhydroxide by volcanic ash the fe oxyhydroxide phase with a mean particle size of 3 4 nm low average la fe ratios of the mineralized deposits from the various sampling sites and the positive eu anomalies indicate rapid deposition of the fe oxyhydroxide near the hydrothermal vents electron microprobe studies show the presence of various organomineral structures mainly twisted stalks and sheaths covered by iron silica deposits within the mineralized samples reflecting microbial iron oxidation from the hydrothermal fluids sequencing of 16 s rrna genes also reveals the presence of other microorganisms involved in sulfur and methane cycles samples collected from hornito chimneys contain silicified microorganisms coated by fe rich precipitates the rapid silicification may have been indirectly promoted by microorganisms acting as nucleation sites we suggest that this type of hydrothermal deposit might be more frequent than presently reported to occur in submarine volcanoes on a geological scale these volcanic eruptions and low temperature hydrothermal vents might contribute to increased dissolved metals in seawater and generate considerable fe oxyhydroxide deposits as identified in older hot spot seamounts

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

formation of fe mn si oxide and nontronite deposits in hydrothermal fields on the valu fa ridge lau basin abstract hydrothermal fe mn si oxides and nontronite are pervasive in the hine hina vai lili and mariner hydrothermal fields along the central valu fa ridge lau basin morphometric and mineralogical analyses reveal that the iron rich filaments are the most important constituents of these fe mn si oxide deposits both the morphologies and chemical composition of the filaments indicate that neutrophilic fe oxidizing bacteria have played a key role in the formation of these deposits a key process of the formation of these deposits is the creation of a complicated filamentous network in which a series of metabolic activities and passive sorption and nucleation processes occur the precipitation of dissolved si in unsaturated and saturated states leads to a two generation growth model in the hydrothermal vents the precipitation of amorphous opal occurs in a relatively narrow temperature range 41 1 42 9 c based on oxygen isotope analyses indicating a fast precipitation rate of opal a when conductive cooling of the hydrothermal fluid occurrs patchy nontronite in the mariner fields is a product of the direct precipitation from hydrothermal fluids at a temperature of 87 9 c whereas the scattered nontronite at the hine hina field is the product of the replacement of hydrothermal fe si oxides at a temperature of 46 2 c

small scale variations in seismic anisotropy near kimberley south africa summary we have exploited a new data set of broad band seismic waveforms recorded near the region of kimberley south africa to place new constraints on seismic anisotropy in an area of extensive mantle modification within an archaean cratonic setting the kimberley region is significant in that it represents archaean cratonic mantle that has been studied extensively via petrologic geochemical and seismic means and provides a unique opportunity to meld the results of a broad range of data sets to examine processes of crust and mantle deformation shear wave splitting observations using sks phases recorded by a dense broad band seismic array in the kimberley region exhibit consistent fast polarization directions of ne sw and splitting times that range from 0 15 s in the se to nearly 0 75 s in the nw regions of the array multichannel cross correlation of relative arrival times of teleseismic sks phases across the array reveals clear azimuthal variations and a relative arrival time range comparable to the shear wave splitting delay time range a like analysis of teleseismic pkpdf phases does not exhibit significant differences in relative arrival times across the array the combined relative arrival time and splitting analyses indicate that a significant change in the strength of anisotropic structure is required across the region our results are most consistent with a model in which variations in seismic anisotropy near the kimberley region are constrained to the lithosphere with an average anisotropic strength of 1 8 per cent extend to depths no greater than 150 km and are primarily controlled by a significant change in the strength of anisotropy within the lateral bounds of the seismic array this inferred structural boundary is near the location of the ca 2 9 gyr old colesberg magnetic lineament a prominent aeromagnetic feature interpreted as the primary suture zone joining the eastern meso archaean and western neo archaean blocks of the kaapvaal craton the proposed boundary is also located within the southern exposure of the ca 2 7 gyr old ventersdorp volcanic sedimentary sequence a key continental rifting feature in the region we hypothesize that the observed seismic anisotropy is the result of straininduced lattice preferred orientation of olivine caused by the amalgamation and deformation of the eastern and western blocks of the kaapvaal craton this collisional process mechanically weakened the region to generate the observed mantle fabric which was exploited by significant rifting events that may have subsequently enhanced the regional fabric this model is also consistent with sks splitting results for the whole of southern africa which suggest either weak anisotropic or isotropic mantle structure in the eastern kaapvaal and coherent mantle anisotropy across much of the western kaapvaal our results support the notion that seismic anisotropy beneath archaean continental regions is not created or alternatively not preserved during initial continental formation but is instead generated during subsequent significant mantle deforming events

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

albite crystallographic preferred orientation and grain misorientation distribution in a low grade mylonite implications for granular flow abstract crystallographic orientation data from pure albite domains in a low grade sheared metagabbro from the combin zone of the western italian alps were measured by electron backscatter diffraction crystallographic preferred orientations cpos in four high strain domains are non random and have a triclinic symmetry the clusters of 100 010 and 001 show an angular relationship that corresponds to that of the albite crystal lattice however the orientations of axis clusters to the kinematic axes vary from domain to domain cpos from a low strain domain also show clustered axes with triclinic symmetry but with more intense clustering than those in the high strain domains grain misorientation distributions are presented both for the low strain domain and one of the high strain domains in the high strain domain the distribution of misorientation angles between neighbouring grains displays a peak at about 70 the equivalent distribution in the low strain domain has a peak at 30 for both domains the misorientation axis distributions between neighbouring and non neighbouring grains are random except for some of axes with 160 180 misorientation that exhibit a slight concentration around 010 the diversity of cpos among the domains suggests that these cpos could not be produced by dislocation creep they are likely to have been inherited from plagioclase parents as a result of host control on the nucleation of the new albite grains these cpos do not contain any direct information about the deformation kinematics we interpret that deformation of these domains occurred by granular flow crystallographic axis dispersion due to grain boundary sliding gbs caused weakening of cpos modification of misorientation angle distributions and randomisation of misorientation axis distributions the fact that a cpo can survive gbs even after a high strain indicates that cpo is not always a sensitive indicator of deformation mechanisms misorientation distribution may provide a complementary and possibly a more sensitive indicator of deformation mechanisms

molecular evidence for microorganisms participating in fe mn and s biogeochemical cycling in two low temperature hydrothermal fields at the southwest indian ridge we examined two low temperature hydrothermal deposits rich in fe si mn collected from the recently discovered hydrothermal fields at the southwest indian ridge using mineralogical geochemical and molecular biological techniques the mineralogical and geochemical analyses indicated that the low temperature hydrothermal fields would provide a warm and chemical species rich habitat for chemosynthetic based hydrothermal ecosystems analyses of 16s rrna sequences showed that proteobacteria pseudoalteromonas leptothrix and pseudomonas were potential fe and mn oxidizers in the low temperature hydrothermal environments but they were not present in equal abundance among the subniches some potential fe and mn reducers were also recovered they were more commonly found in the exterior black fe mn oxides the difference between the exterior black fe mn oxides and the interior opal a could be related to differences in in situ physicochemical conditions we also identified microbial players that may participate in sulfur s geochemical cycling in these low temperature hydrothermal environments via analyses of 16s rrna sequences and the apra functional gene the results indicated that members of proteobacteria and proteobacteria were involved in the s oxidation process while members of proteobacteria nitrospirae firmicutes and archaea might participate in the s reduction process fe mn and s oxidizers and reducers might actively participate in hydrothermal biogeochemical processes which could influence the transfer of chemical species and the formation of biogenic minerals

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

paleomagnetism anisotropy of magnetic susceptibility and argon argon geochronology of the clearwater anorthosite saskatchewan canada abstract the clearwater anorthosite in saskatchewan is a major gabbro anorthosite layered intrusion lying within the archean rae province along the northwestern hinterland of the paleoproterozoic trans hudson orogen tho the intrusion which could be at least 2 4 ga old carries a well defined remanent magnetization with a direction of declination 112 and inclination 38 k 316 95 3 1 n 8 the corresponding paleomagnetic pole plat 6 5 n plong 48 2 w dp 2 2 dm 3 7 is the first one obtained from the archean western granulite domain of the rae province field observations supplemented by anisotropy of magnetic susceptibility data show that the magnetization post dates ductile deformation of the anorthosite argon argon plateau dates of 1917 7 ma for hornblende and 1839 5 ma for biotite are interpreted as cooling ages with the hornblende age approximating the time of remanence the pole position and others of comparable age from the northwestern margin of the tho form a loose cluster the mean of which is about 90 of arc away from that of a broadly contemporaneous group of poles from the superior province this relation supports previous interpretations of paleomagnetic data that prior to the tho at about 1 9 ga ago a major ocean the manikewan ocean at least 5000 km across separated the superior and rae hearne provinces

mantle and slab contributions in arc magmas destructive plate margins are major sites of terrestrial magmatism that have long had a key role in models for the generation of continental crust and the development of chemical heterogeneities in the upper mantle in these models it is important to evaluate the nature and size of contributions from preexisting crust in the generation of magmas above subduction zones and to constrain the size of the fluxes of different elements from the subducted crust and the overlying mantle wedge in arc magmas kay 1 980 hawkesworth ellam 1 989 hawkesworth et al 1 99 1 a mcculloch gamble 1 99 1 the more mobile components of the subducted oceanic crust are the products of element fractionation processes in the upper continental crust the ocean basins and hydrothermal systems on the ocean floor they therefore have distinctive isotope and even trace element ratios that in principle should be readily distinguished from those in peridotite in the mantle wedge prior to subduction in practice however element ratios are almost certainly fractionated during shallow level dehydration of sub ducted material and at greater depths where material from the slab appears to contribute in the generation of arc magmas moreover the isotope signal of slab derived material may be heavily diluted by exchange with

palaeomagnetism of the upper zone of the bushveld complex abstract palaeomagnetic data were acquired from eight sampling sites situated in the upper zone of the bushveld complex transvaal south africa specimens were subjected to alternating field and thermal demagnetization a mean magnetization direction yielding a palaeomagnetic pole at 16 1 s 148 5 w was found fold tests indicate that the upper zone had acquired its remanent magnetization with the igneous layering in its present position with this new data on the layered sequence it is possible to divide the mafic portion into three magnetic polarity zones namely mpza mpzb and mpzc

induced liquefaction experiment in relatively dense clay rich sand deposits 1 in this paper we report results from a controlled blast induced liquefaction experiment at the field scale the physical and mechanical properties of the materials at the subsurface are characterized by a suite of in situ and laboratory tests including the standard penetration test spt downhole and cross hole seismic velocity tests density porosity and gradation tests and direct shear tests since the blast experiment was performed above groundwater table the subsurface was saturated by a sequence of controlled infiltration tests a 50 kg tnt charge was detonated at a depth of 10 m and seismic ground motions were recorded in a vertical geophone array positioned at a horizontal distance of 30 m from the blast borehole obtained liquefaction features include a water fountain that erupted from the blast borehole prolonged bubbling of the water surface inside the infiltration trench a process equivalent to sand boils typically observed at sites which have experienced liquefaction lateral spreading and surface settlement we argue that in contrast to conventional predictions liquefaction may be induced in relatively dense silty and clayey sands shear wave velocity 300 m s 1 relative density 63 89 relatively rich in clays fines content 30 and that the driving mechanism should not necessarily be restricted to cyclic shear stress loading

plagioclase microfabrics in a ductile shear zone from the jotun nappe norway abstract gabbroic and anorthositic rocks from the jotun nappe are transected by small ductile shear zones in which a high grade paragenesis and a new foliation are formed most plagioclase grains show evidence of plastic deformation and have recrystallized by subgrain rotation and bulge processes to fine grained mylonite during these processes a plagioclase grain shape fabric was destroyed and with increasing strain a pre existing plagioclase crystal fabric was successively modified enhanced and finally obliterated this could be evidence of superplastic flow inverse pole figure analysis of the typical plagioclase crystal fabric in the shear zone indicates that 021 a could be an operative slip system and planes close to 132 could be possible slip planes

high temperature plastic deformation of quartz plagioclase multilayers by layer normal compression it is a common practice in rheological modeling to take the extrapolated compressive flow strengths of quartzite and plagioclase rock as the bulk flow strengths of the upper and lower continental crusts respectively such a practice implies that the bulk flow strength of polyphase rocks is identical to that of the pure weak phase to test this assumption we performed deformation experiments on synthetic layered and particulate quartz anorthite 50 50 composites and aggregates of the end members in a paterson gas medium apparatus at a confining pressure of 300 mpa a constant strain rate of 10 5 s 1 and temperatures from 1273 to 1473 k under these conditions anorthite deforms by recrystallization accommodated dislocation creep while quartz is semibrittle electron backscatter diffraction measurements show a strong lattice preferred orientation lpo of anorthite developed in deformed specimens and the lpo pattern is interpreted as a result of dislocation slip on the 010 100 system our experimental results show that the bulk flow strength of layered composites increases with decreasing thickness of the layers thinlayered composites are significantly stronger than pure anorthite aggregates and a homogeneous quartz anorthite mixture i e particulate composites with the same modal composition but weaker than pure quartz aggregates our experimental results together with the theoretical overview presented previously in the materials science literature demonstrate that 1 compressive flow strength of a polyphase rock cannot be represented by that of the weak phase and 2 thin layered rocks compressed normal to the layering are rheologically stronger than homogeneous isotropic mixtures under the same deformation conditions thus weak phase based rheology will lead to an underestimation of the bulk flow strength of the continental crust in which polyphase rocks dominate

an olivine fabric database an overview of upper mantle fabrics and seismic anisotropy we present a unique database of 110 olivine petrofabrics and their calculated seismic properties the samples come from a variety of the upper mantle geodynamic environments ophiolites subduction zones and kimberlites with a wide range of micro structures a phenomenological relationship is established between p and s wave seismic anisotropy and the degree of crystal alignment fabric strength seismic anisotropy increases rapidly at low fabric strength before reaching a near saturation level of 15 to 20 for p waves and 10 to 15 for s waves despite a large variation in the symmetry of fabric patterns the average seismic properties of the different fabric micro structural and geodynamic settings have similar anisotropies in both magnitude and symmetry hence it would seem possible to determine some measure of fabric strength from seismic anisotropy if the dimensions of the anisotropic region are known but not geodynamic environment or details of the petrofabric pattern a simple pattern of seismic anisotropy characterises the average sample of the database which has the following features the polarisation plane of the fastest s wave is parallel or sub parallel to the foliation plane the maximum shear wave splitting is parallel to the y structural direction in foliation plane and normal to the lineation the maximum of the p wave velocity is parallel to the high concentration of 100 axes which is sub parallel to the lineation the 100 orientation distribution has the greatest influence on the p wave seismic anisotropy the 100 and 001 orientation distributions have the greatest influence on the symmetry of s wave anisotropy although the magnitude of anisotropy is influenced by the distribution of all three principal axes although the database only contains olivine petrofabrics this statistical study clearly shows that seismic anisotropy can be used to deduce the orientation of the structural frame in the upper mantle 1998 elsevier science b v all rights reserved

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

the kaapvaal craton seismic anisotropy petrophysical analyses of upper mantle kimberlite nodules a dense network of seismic stations has been deployed on the kaapvaal craton south africa to investigate the upper mantle seismic structures in order to bring independent petrophysical constraints we analyze a direct sampling of the cratonic upper mantle and determine the seismic properties of 48 mantle nodules brought up to the earth s surface by kimberlite eruptions seismic properties of these nodules are calculated from the olivine and pyroxene crystal preferred orientations and the single crystal elastic constants despite variations in the nodules compositions microstructures and crystallographic preferred orientations seismic anisotropy is rather homogeneous throughout the craton mean s wave anisotropy is weak 2 64 which is compatible with the small measured sks wave splitting mean delay time of 0 62 s

correction for deviations due to magnetic anisotropy of palaeomagnetic directions from the mulcahy gabbro north west ontario summary the mulcahy gabbro is an archean layered intrusion in the wabigoon subprovince of north western ontario alternating field and thermal demagnetization of specimens from it have provided consistent within site characteristic remanence chrm directions in eight of 19 sites antipodal polarities of remanence are recovered and the overall in situ formation mean is d 240 i 51 k 21 this direction is unlike any reported archean direction from the superior province of north america the degree of anisotropy of low field magnetic susceptibility is high with k k3 values up to 1 3 cooling selected specimens in weak fields indicate that trm is deflected towards the magnetic fabric plane accordingly the in situ chrm was corrected for the effect of magnetic anisotropy two methods of correction give similar results however the resultant formation mean remains essentially unchanged as a consequence of sufficient variability in the attitudes of k k planes although uncertainty in the initial dip renders the tilt correction problematical partial tilt correction 36 per cent results in a formational mean with better precision than the in situ formation mean furthermore partial untilting of remanence directions results in a positive reversal test with a better classification than that obtained using in situ directions this partially tilt corrected result d 202 i 49 k 35 yields a pole latitude 62 n longitude 42 e which is archean in aspect

strain softening and microstructural evolution of anorthite aggregates and quartz anorthite layered composites deformed in torsion torsion experiments of anorthite an aggregates and layered composites with equal volume fractions of quartz qtz and an were performed in a gas medium apparatus at a confining pressure of 400 mpa temperatures from 1373 to 1473 k and twist rates from 1 0 x 10 4 to 3 0 x 10 4 rad s dense specimens were fabricated from an glass and qtz crystalline powder using hot isostatic pressing hip techniques both an aggregates and qtz an layered composites show a continuous strain weakening from a peak stress at gamma 0 2 0 3 to gamma 3 2 and steady state flow has not reached under the experimental conditions the weakening is even more pronounced in the layered composites than the monolithic aggregates suggesting channeling or localization of flow into the weak material between strong layers the sheared an specimens developed pervasively c s c structures which are similar to those observed in natural ductile shear zones tem and electron backscattering diffraction ebsd fabric analyses suggest that grain boundary migration recrystallization accommodated dislocation creep with 010 100 as the dominant slip system was operating in the an the strain softening may be due to the development of crystallographic preferred orientation cpo the operation of dynamic recrystallization and the formation of extremely fine grained recrystallized material in the narrow c shear bands c 2004 elsevier b v all rights reserved

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

magmatic accretion and thermal convection at the sheeted dike complex gabbro boundary in superfast spreading crust odp hole 1256d abstract the magmatic processes responsible for accretion of new oceanic plutonic crust at fast spreading centers remain unclear expedition 312 at ocean drilling program hole 1256d on the eastern flank of the east pacific rise barely drilled through the uppermost gabbros but offers unparalleled opportunities to constrain deep magmatic processes and the mode of construction of the gabbroic layer gabbro 1 and gabbro 2 respectively 52 and 24 m thick were intruded in an altered and baked sheeted dike complex both gabbros lack a macroscopic fabric and were until now considered isotropic here we use digital 3 d fabric analysis of plagioclase phenocrysts in 33 non azimuthally oriented samples in contrast with previous observations the two gabbros display a distinct and consistent shape preferred orientation both gabbros exhibit magmatic equilibrated microstructures with minor submagmatic microstructures the calculated rayleigh numbers range between 10 15 and 10 18 proving that the gabbroic magma was initially undergoing turbulent magmatic flow magmatic foliations dip moderately with a mean of 48 in gabbro 1 and 61 in gabbro 2 magmatic lineations plunge in gabbro 1 between 1 and 44 mean 28 while in gabbro 2 plunges range from 6 to 69 mean 41 these inclined magmatic structures combined with the inclined magmatic contacts between the gabbros and their host rock show that these two gabbro bodies instead of being horizontal sills more likely are inclined intrusions that cut obliquely through the sheeted dike complex seismic reflection data around hole 1256d shows inclined reflectors similar to those produced by gabbroic saucer shaped intrusions in passive margins on the basis of the inclined fabrics inclined contacts and the regional seismic data we propose that the uppermost gabbroic intrusions at fast spreading centers form with a saucer shape this new shape of intrusion never reported before in the oceanic environment may constitute a significant component of oceanic accretion

plagioclase preferred orientation by tof neutron diffraction and sem ebsd abstract the lattice preferred orientation lpo of an anorthosite composed of andesine sampled from a highly deformed anorthositic mylonite grenville province quebec was measured by tof neutron diffraction and sem ebsd the quantitative texture analysis of neutron data was accomplished by using the rietveld texture analysis with the wimv algorithm implemented in the program package materials analysis using diffraction maud the texture calculations of the ebsd data were performed by using the program beartex analyses from neutron and electron diffraction data gave similar results if ebsd data are smoothed to account for grain statistics the principal pole figures show 010 roughly parallel to the rock foliation 001 poles exhibiting a low angle 25 to the pole to foliation and 100 poles close to the y direction perpendicular to the lineation and foliation pole the 100 crystallographic direction shows a maximum in the lineation direction 010 directions concentrate near the foliation pole the geological deformation conditions and the constructed pole figure patterns indicate that the preferred orientation could be attributed to intracrystalline slip dominantly on 010 with 100 as slip direction elastic properties calculated by averaging document weak anisotropy that has implications for the seismic structure of the lower crust

postcumulus modification of magnetite grains in the upper zone of the bushveld complex south africa abstract a detailed electron microprobe study has been made of magnetite grains from magnetitite layer 1 of the upper zone of the bushveld complex in order to establish if there is any evidence for postcumulus processes having affected magnetite compositions representative composition profiles obtained from detailed step traverses are presented for touching magnetite grains and where magnetite is in contact with either ilmenite or plagioclase magnetite grains in all textural settings are compositionally heterogeneous touching magnetite grains display systematic compositional zonations the rims of grains show a marked decrease in al and mg towards the grain boundary the cores are characterised by a distinctive peak and trough pattern for these elements these variations can be related to the positions of exsolved phases exsolutions in the cores are either pleonaste or an fe 3 bearing al mg spinel whereas those at the grain boundaries are generally more al rich and include corundum similar exsolution related compositional variations for mg and al were found in magnetites in contact with ilmenite and plagioclase a model has been developed which illustrates the possible sequence of postcumulus events during the interval between accumulation of the magnetite and the cessation of subsolidus reactions the essential feature of the model is the delicate interplay between depletion of the host magnetite by corundum pleonaste and the fe 3 bearing al mg spinel exsolutions and diffusion of al and mg into the depleted areas from the surrounding magnetite data for cr contents in magnetites show that although this particular trace element is apparently resistant to redistribution by exsolution processes it is affected by subsolidus re equilibration between magnetite and ilmenite or magnetite and intercumulus liquid

ab initio calculations of elastic constants of plagioclase feldspars 2014 by walter de gruyter berlin boston plagioclase feldspars comprise a large portion of the earth s crust and are very anisotropic making accurate knowledge of their elastic properties important for understanding the crust s anisotropic seismic signature however except for albite existing elastic constants for plagioclase feldspars are derived from measurements that cannot resolve the triclinic symmetry we calculate elastic constants for plagioclase end members albite naalsi3o8 and anorthite caal2si2o8 and intermediate andesine labradorite nacaal3si5o16 using density functional theory to compare with and improve existing elastic constants and to study trends in elasticity with changing composition we obtain elastic constants similar to measured elastic constants and find that anisotropy decreases with anorthite content

the palaeomagnetism of the main zone in the western bushveld complex abstract sampling was done at thirty four sites in the main zone of the western bushveld complex south africa alternating field af and thermal demagnetization of specimens indicated a multicomponent nrm in the majority of specimens a primary magnetization direction d 357 8 i 70 9 95 2 8 k 80 was recovered from thirty two sampling sites fold tests indicate that the igneous layering was emplaced in a horizontal position and that the nrm predates tilling of the layered sequence two sampling sites situated in the lower part of the main zone yielded magnetization directions approximately antipodal to the above mentioned magnetization direction the palaeomagnetic pole position 9 2 n 27 3 e calculated for the main zone of the western bushveld complex indicates that this part of the main zone had acquired its magnetization later than the main zone in the eastern bushveld complex and later than the underlying critical zone

sense of shear in high temperature movement zones from the fabric asymmetry of plagioclase feldspars abstract a new method for determining the sense of shear in plagioclase bearing tectonites from the 010 orientation of plagioclase feldspar is presented the method is based on the asymmetry of the 010 plane with respect to the structural frame foliation and lineation and the dominant activity of the 010 slip plane in the high temperature plasticity of plagioclase feldspar using examples from the zabargad gneisses red sea the method is applied to plagioclases of an25 an45 and compared with other methods of shear sense determination quartz c axis fabrics and microstructural criteria

quantifying three dimensional silicate fabrics in cumulates using cumulative distribution functions we present a new method for quantifying three dimensional silicate fabrics and the associated uncertainties from grain orientation data on three orthogonal sections our technique is applied to the orientation of crystallographic features and hence yields a fabric related to the lattice preferred orientation although the method could be applied to shape preferred orientations or strain analysis based on passive linear markers the orientation data for each section are represented by their cumulative distribution function and an iterative procedure is used to find the symmetric second rank strain tensor that will simultaneously satisfy the cumulative distribution functions observed on each section for samples with well developed fabrics this technique provides a much closer match to the sectional data than do previous techniques based on eigenparameter analysis of two dimensional orientation data robust uncertainty estimates are derived from a non parametric bootstrap resampling scheme the method is applied to two cumulates one with a well developed fabric and the other with a weak fabric from the stillwater complex montana the silicate petrofabric orientations obtained for these samples compare favorably with independent direct estimates of the volume fabric from electron backscatter diffraction and magnetic techniques

preferred lattice orientations of plagioclase from amphibolite and greenschist facies rocks near the insubric line western alps abstract preferred lattice orientations of plagioclase measured by the universal stage from deformed amphibolite and greenschist facies rocks near the insubric line western alps reflect the type and orientation of the strain system either 001 or 010 are oriented parallel or subparallel to the schistosity and form more or less complete girdles around the stretching direction by the preferred orientation patterns dominant 010 001 slip is indicated and additionally slip on 001 parallel to the a direction possibly at somewhat higher temperatures and or at a higher an content of the plagioclase it is suggested that obliquities of the symmetry planes of the orientation patterns against the principle axes of strain are related to the sense of shearing

plagioclase fabric development in a high grade shear zone jotunheimen norway abstract shear zones of precambrian age transform meta anorthositic rocks to mylonites of high metamorphic grade in jotunheimen norway a plagioclase microfabric study is presented for one of these shear zones the shear zone is heterogeneously and asymmetrically developed regarding the degree of recrystallization and this is inferred as reflecting strain variations pre shear zone plagioclase is preserved in the deformed rocks as porphyroclasts which display large variations in grain elongation in the plane of mylonitic foliation with aspect ratios long dimension average short dimension as large as 25 1 the preferred orientation of lattice and optical directions in the plagioclase porphyroclasts indicates that they are preferentially oriented with the pole to 010 at a moderate angle to the foliation pole in such a way that they are unfavorably oriented for mechanical twinning on the albite and pericline laws in a flattening strain across the plane of the mylonitic foliation this is taken to indicate that the porphyroclasts represent those pre shear zone plagioclase grains which were oriented in order to accommodate such a flattening strain by mechanical twinning while grains otherwise oriented accommodated the strain by kinking with an axis of external rotation at 100 and glide which accumulated lattice strain and eventually led to recrystallization the plagioclase porphyroclasts with large aspect ratios are preferentially oriented with 100 at a large angle to the inferred translation direction of simple shear strain they are the products of preferential recrystallization of zones of large lattice strain in grains which were strongly kinked due to their special orientation this implies that grain elongation is not applicable as a measure of strain the microstructures indicate that the recrystallization was dynamic the produced neoblasts display a preferred orientation which is weak although with a preference for 010 to have a low angle to the foliation plane which mimics the preferred orientation of the porphyroclasts there is a slight indication that the orientation of neoblasts is controlled by the plagioclase host such that 100 of the neoblasts prefer intermediate rather than large angles against 100 of the porphyroclast these features and microstructural observations indicate that rotation recrystallization is a basic recrystallization mechanism under high grade metamorphic conditions

complete texture analysis of a deformed amphibolite comparison between neutron diffraction and u stage data abstract this paper describes a new technique of analysing complex neutron diffraction patterns allowing the quantitative texture analysis of each rock forming mineral up to trielinic lattice symmetry in polymineralic rocks the method is demonstrated using hornblende and plagioclase fabrics in alzenau amphibolite from the spessart mountains mid german crystalline rise hornblende and plagioclase exhibit a pronounced shape preferred orientation with respect to the macroscopic fabric elements for control purposes crystallographic preferred orientations were also determined independently by individual grain measurements using a u stage from the neutron texture analyses we obtain only superposed pole figures hornblende and hornblende hornblende and plagioclase and plagioclase with plagioclase of low indexed lattice planes therefore the critical crystallographic directions were recalculated by the orientation distribution function odf which is given by texture components the optical and neutron derived pole figures are in a good agreement all pole figures display strong preferred orientation although the plagioclase texture is less pronounced the hornblende texture consists of a 001 maximum parallel to the stretching lineation with 010 showing a tendency to develop a girdle around the lineation 100 axes are distributed with a clearly split maximum representing the monoclinic angle between 100 and 001 the plagioclase texture is a rare one with 001 subparallel within the foliation and 100 oriented subparallel to the lineation 010 of plagioclase is concentrated normal to lineation within the foliation the technique outlined here is potentially the most useful one for texture analysis of polyphase rocks independent of crystal symmetry

the tellnes ilmenite deposit rogaland south norway magnetic and petrofabric evidence for emplacement of a ti enriched noritic crystal mush in a fracture zone abstract the tellnes ilmenite deposit a world class titanium deposit occurs in the ana sira anorthosite rogaland anorthosite province south norway it is mainly made up of an ilmenite rich norite that has been previously interpreted as injected in a crystal mush state in a weakness zone of the enclosing anorthosite this emplacement mechanism has produced a faint orientation in the ore due to the flow of the mush the internal flow structure of the orebody is studied here using the low field anisotropy of magnetic susceptibility ams method partial anhysteretic remanent magnetization paarm indicates that coarse magnetite is the main mineral responsible for the magnetic fabric parallelism of the magnetic fabric with the shape preferred orientation of the ore forming minerals is checked using image analysis ia from oriented sections intercept method interpretation of the ams data verified by paarm and ia provides information on the magmatic foliation and lineation emplacement flow of the ilmenite norite crystal mush occurred in the direction of the orebody parallel to its walls and with an average se 18 plunge the feeder zone was likely situated below a network of veins on the se end of the orebody the sickle shaped outcrop of the deposit suggests a transcurrent dextral opening of a wnw ese striking weakness zone across the anorthosite pluton

cryptic variations within chromitites of the upper critical zone northwestern bushveld complex the close association of platinum group elements with chromite in the upper critical zone calls for the appraisal of structures textures and composition of chromitites in the development of genetic models cryptic variations in successive chromitite layers of the ug 1 unit are consistent with fractionation of limited melt volumes individual chromitite bands within the composite ug 2 chromitite layer exhibit distinctive compositions but all display long range compositional trends with height superimposed on short range homogeneity accessory chromites are substantially different from massive chromitites in composition reaction with residual liquids and subsolidus reactions were significant recrystallization of chromitites was ubiquitous but the importance of in situ growth is shown by relict textures long range settling adcumulus growth or deposition from density currents are processes that find little support in the data presented

plagioclase and quartz preferred orientations in a low grade schist the roles of primary growth and plastic deformation albite in a lower greenschist facies haast schist from new zealand has a maximum of poles to 001 parallel to the elongation direction 010 forms a girdle about grain length plastic deformation with slip dominantly on 010 001 is rejected as the orienting mechanism because 1 the relationship of crystal axes to grain length would require multiple slip systems incompatible with the lack of rotation of albite twin planes 2 euhedral terminations and other textural features indicate primary crystallization 3 albites in quartzose layers are less elongate than those in sheet silicate rich layers the albite fabric is ascribed to growth anisotropy and competition in response to stress driven solution and growth during formation of metamorphic segregation layering this explanation is compatible with the common observation that plagioclase is brittle at low metamorphic grades associated weak quartz c axis fabrics are ascribed to plastic deformation and this deformation enhanced a dimensional preferred orientation already established by primary growth

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

heterogeneous nucleation and epitaxial crystal growth of magmatic minerals electron backscatter diffraction analysis of dendritic clinopyroxene cpx forming in rapidly cooled basalt reveals two features that are unexpected for phases growing from a liquid 1 helical growth about 010 cpx the crystallographic b axis with incremental rotation up to 0 4 m 1 within branch segments and large rotational jumps 10 46 between closely spaced branch segments and 2 strong crystallographic preferred orientation cpo between cpx and titanomagnetite timt decorating branch tips such that 010 cpx aligns with one of the six symmetrically equivalent 110 timt face poles more than 80 of timt crystals occur within 5 of the cpo with cpx substrate as do 22 of timt crystals in contact with euhedral cpx in natural etna basalt the probability distribution of distances between an arbitrary unit vector and the nearest 110 timt normal unit vector was found by numerical simulation and indicates that the observed alignment to within 5 occurs with frequency 0 024 thus the cpo occurs 9 33 times more commonly than expected from randomly distributed crystals the cpo matches previous observations of epitaxial relationship between spinel and host pyroxene during subsolidus exsolution but has not previously been reported among magmatic cpx and timt conspicuous contiguity among phenocryst phases is inferred to result from heterogeneous nucleation facilitated by epitaxy associated with cpo an inference supported by high resolution electron microscopy observations of 150 nm timt crystals adhering preferentially to silicate substrates epitaxial relationships among phases nucleating from a melt may contribute to fabrics in magnetic properties dictate the textures of igneous rocks by promoting development of crystal clusters and even influence magma transport and eruption styles through a control over magma rheology

extreme spatial and temporal variability of hydrothermal microbial mat communities along the mariana island arc and southern mariana back arc system 1 twenty five microbial communities were sampled from 18 different hydrothermal systems located at seven different sites along the mariana island arc and at a single site from the southern mariana spreading center over a 3 year period terminal restriction fragment length polymorphism t rflp analysis of the small subunit rrna gene revealed that the microbial community diversity is much greater along the mariana arc back arc than at either hot spot volcanoes or mid ocean ridges along the same spatial scale cluster analysis of t rflp fingerprints reveals the microbial communities formed three distinct clusters designated mariana clusters i ii and iii microbial communities in mariana cluster i are all associated with iron rich microbial mats and are dominated by members of the proteobacteria and by unique phylotypes clustering deeply in the proteobacteria and within the nitrospira division mariana cluster ii communities are all from shallow hydrothermal systems and mostly from colder sediments or microbial mats that are dominated by putative heterotrophic phylotypes usually associated with seawater and sediments not generally associated with hydrothermal fluid inputs mariana cluster iii is generally from much hotter vent sites and is dominated by sulfur oxidizing proteobacteria quantitative polymerase chain reaction q pcr of archaeal abundance reveal that all of the microbial communities are dominated by members of the bacterial domain sampling of microbial mats from iceberg vent at nw rota 1 in 2004 and again in 2006 reveal the community has shown a transition from caminibacter group proteobacteria phylotypes to a mixed population of caminibacter sulfurovum and sulfurimonas group proteobacteria

ultrahigh metamorphic temperatures over 1050 c recorded by fe ti oxides and implications for paleoproterozoic magma induced crustal thermal perturbation in jining area north china craton abstract a pseudobrookite approximately mg0 04fe2 0 85ca0 01al0 05fe3 0 16ti1 89o5 was discovered in fe ti oxide intergrowths in a spinel garnet bearing granulite from the xuwujia paleoproterozoic ultrahigh temperature uht metamorphic region within the khondalite belt of the north china craton the presence of this pseudobrookite indicates a peak temperature possibly over 1120 c when compared with experimental data ferric pseudobrookite grains fe3 up to 0 58 per formula unit from the same granulite outcrop record retrograde temperatures as low as 940 c titanomagnetite and ilmenite intergrowths that resulted from reduction exsolution which was buffered by the decomposition of high ti pseudobrookite grains recorded a post peak temperature up to 1050 c the thermal gradient 100 c kbar required to generate the uht metamorphic bodies in an area over 10 000 km2 around the jining area was modeled by the finite element method the thermal modeling results show that 1 lithospheric thermal erosion induced by a mantle plume with potential temperature of 1550 c could not have provided enough thermal perturbation to allow the observed uht metamorphism 2 underplating at the scale of even the largest known horizontal magma intrusion on earth would not be able to elevate the geotherm to the gradients required for the observed uht metamorphism and 3 intraplating of dykes and intrusions at the depth of this uht metamorphism was able to increase the local temperature field to the required gradients these modeling results suggest that a modern back arc may not be able to provide sufficient heat at the middle crust level for uht metamorphism the uht metamorphism in the study area was probably accompanied by an orogeny plume interaction event which simultaneously supplied heat through thermal erosion magma underplating and magma intraplating

isotope geochemistry tracks the maturation of submarine massive sulfide mounds iberian pyrite belt the massive sulfide deposits of the southern iberian pyrite belt ipb occur as shale hosted exhalative mounds or shallow sub seafloor replacive bodies that formed at the devonian carboniferous boundary in anoxic bottoms the geochemistry of the mineralization and the host shale from some of the major deposits including a detailed 50 m thick section through the migollas orebody shows a marked textural and vertical zonation that is also reflected in the chemical textural and isotopic variations of pyrite and carbonates pyrite evolves from primary framboids and colloform aggregates to late euhedral crystals which accompanies a redistribution of the contained base metals and grain coarsening sims analyses depict an extreme variability of 34s values from 42 3 to 4 4 with a gradual increase that is related to the textural maturation there is also a systematic rise in the 87sr 86sr ratios 0 70846 to 0 71354 13c 12 2 to 5 2 and 18o values 14 1 to 27 8 of the associated carbonates which evolve from early dolomite to late siderite this evolution evidences the maturation of the exhalative mineralization during mixing between two fluids modified seawater enriched in biogenically derived reduced sulfur and upflowing hydrothermal fluids early sulfides precipitated in the seawater mound interface towards the core of the mounds there was an increasing influence of deep fluids that modified the early mineral textures and the isotopic signatures this evolution is similar to that observed in modern submarine hydrothermal systems and confirms the importance of mound refining in the evolution of fossil exhalative mounds and the formation of economic ores

40ar 39ar age constraints on ore deposition and cooling of the bushveld complex south africa the ug 2 chromitite layer in the upper critical zone of the rustenburg layered suite is one of the prominent platinum group element enrichment zones of the c 2 06 ga bushveld complex south africa eleven biotite crystals were selected from a production concentrate from the impala platinum mine and degassed incrementally with a co 2 laser the apparent age spectra display widely varying characteristics ranging from essentially 100 concordant to strongly discordant in some cases undulatory as a result of various degrees of chloritization of the grains the least discordant spectra seven grains 90 100 39 ar released yield integrated and plateau ages ranging from 2037 to 2049 ma relative to the fish canyon sanidine standard at 28 02 ma and share a weighted mean age of 2042 4 3 2 ma 2 excluding systematic errors this age is slightly younger than existing constraints from u pb dating c 2055 2061 ma which seem to indicate a cooling rate of around 20 c ma however the recently inferred c 1 bias between currently used calibrations of the 40 ar 39 ar and u pb systems implies a very rapid cooling rate for the bushveld complex between 700 and

structural and compositional constraints on the emplacement of the bushveld complex south africa abstract despite a plethora of petrological studies the emplacement mechanics of the world s largest layered intrusion the 2 05 ga bushveld complex are still poorly understood early models considered the intrusion to comprise separate lopolithic intrusions or even concentric cone sheets but recently overwhelming support for a sill like intrusional form has emerged examination of the contact aureole reveals three groups of emplacement related structures interfinger deformation zones and bridges formed between intruding and dilating magma fingers preserved in both the western and eastern parts of the complex sub bushveld complex intrusions that reflect finger like emplacement into the underlying contact aureole and diapiric domes that characterize the eastern contact aureole formed by diapiric amplification of initial interfinger deformation zones associated with the earliest mafic ultramafic pulse of the bushveld complex most of these structures exhibit a strong nw se preferred orientation while longitudinal terminations and divergence of the sub bushveld intrusions away from their source horizons indicate magma emplacement towards the se this emplacement direction is supported by petrological data lateral lithological facies variations and mineral chemical variations within specific horizons the thickest and most chemically primitive accumulations of the lower zone of the complex are found adjacent to and thin towards the ne and se away from the thabazimbi murchison lineament tml a crustal scale lineament that has undergone polyphase reactivation since at least 2 7 ga the bushveld complex was most likely fed by a feeder dyke that utilised the tml spreading laterally from the dyke axis to form its current sill like geometry the stress field at 2 05 ga was suitably oriented to allow for dilation of the ene trending tml suggesting the kaapvaal craton was subject to a component of nw se extension during this time nw se directed collision of the kaapvaal and zimbabwe cratons is recorded in the limpopo belt which would suggest emplacement under conditions of far field extensional stress within a back arc setting this is corroborated by petrological data that supports a significant component of subducted crust in the bushveld magmas

preferred orientation of anorthite deformed experimentally in newtonian creep abstract synthetic anorthite aggregates were deformed in a paterson gas deformation apparatus at confining pressures up to 400 mpa in torsion and axial compression at temperatures between 950 c and 1200 c samples deformed in torsion under newtonian creep display development of texture or crystallographic preferred orientation as documented with synchrotron x ray diffraction measurements complex diffraction patterns were deconvoluted with the rietveld method to obtain quantitative texture information torsion samples deformed up to shear strains of 4 and samples deformed in compression at higher stresses to total strains of 0 3 develop clear textures texture and shape preferred orientation spo of torsion samples display a monoclinic pattern with an asymmetry inclined against the sense of shear consistent with polycrystal plasticity simulations that assume the deformation is accomplished by dislocation glide these results show that a material deforming in linear viscous creep can develop a strong texture in striking contrast to the paradigm that the presence of a texture precludes low stress newtonian behavior our observations show that the presence or absence of crystallographic preferred orientation is not sufficient to uniquely infer the dominant rheological mechanical regime as sometimes applied for interpretation of seismic anisotropy in the earth

a chronostratigraphic framework for the north central kaapvaal craton the bushveld complex and the vredefort structure abstract sufficient geochronological data are available pertaining to the north central part of the kaapvaal craton to construct a chronostratigraphic framework for the rock units involved in the vredefort event from the basement rocks at 3170 ma to the waterberg soutpansberg groups deposited between 2025 and 1425 ma the resulting framework forms a convenient chronological guide to the geological evolution of this area and the timing of events such as that at vredefort the basement on which the supracrustal rocks were deposited stabilised from 3170 to 3060 ma but metamorphism and metasomatism of these rocks occurred up to 2800 m y ago in different parts of the craton the first unit of the wits triad the volcaniclastic dominion group was deposited at 3060 ma on the stabilising basement this was followed by the dominantly clastic witwatersrand supergroup and the overlying ventersdorp supergroup the dominantly volcanic ventersdorp rocks are 2700 m y old and form the last unit of the wits triad a considerable time gap of 200 300 ma separates the triad from the lowermost units of the overlying transvaal sequence the age of these volcanic and clastic protobasinal rocks is poorly constrained between 2500 and 2300 ma they may in fact represent eroded remnants of a more extensive depositional cycle the mainly clastic sedimentary part of the transvaal sequence pretoria group has been dated at 2224 ma the upper part of the transvaal sequence rooiberg group is only constrained by the age of the intrusive bushveld complex the bushveld basic layered rocks are well dated at 2060 ma as are the subsequent granites at 2050 ma the vredefort event which post dates the bushveld complex is reasonably well constrained by the age of the bronzite granophyre at 2000 ma deposition of the waterberg and soutpansberg groups which post date the bushveld complex is poorly constrained between the age of the bushveld granites and younger alkaline intrusives at 1425 ma but these sediments and volcanics were probably formed between 2025 and 1752 ma the waterberg group may therefore contain debris from the vredefort structure

the palaeomagnetism of the main zone of the eastern bushveld complex abstract palaeomagnetic data were acquired from eighteen sampling sites situated in the main zone of the eastern bushveld complex transvaal south africa specimens were subjected to alternating field and thermal demagnetization two mean magnetization directions which are approximately antipodal were found one direction represents subzone b of the main zone in the eastern bushveld complex and yields a palaeomagnetic pole at 17 3 35 7 e the second direction represents subzone c of the main zone in the eastern bushveld complex with virtual geomagnetic pole at 28 s 161 7 w the positions of these poles on the apparent polar wander path apw for africa indicate that the critical zone had acquired its remanent magnetization before the main zone fold tests prove that the main zone in the eastern bushveld complex had acquired its remanent magnetization with the igneous layering in a horizontal position

feldspar fabrics in a greenschist facies albite rich mylonite from electron backscatter diffraction abstract the current database of plagioclase cpos in naturally deformed rocks is small and is not supported by cpos from experimentally deformed rocks or by numerical modelling so that the current understanding of plagioclase cpos is limited electron backscatter diffraction ebsd has the potential of measuring the full crystallographic orientation of small domains of any crystalline material including plagioclase we generate and test a file of the relative electron diffraction intensities for lattice planes in the albite structure this enables rapid and accurate individual orientation measurements of albite using ebsd ebsd is used to collect 360 data points from 7 areas across a 2 cm by 1 mm pure albite band in a greenschist facies albite mylonite from a gabbro protolith the cpos are unlike any published the a b and c axes of albite are not symmetrically distributed with respect to the kinematic axes of deformation and do not possess an internal monoclinic or orthorhombic symmetry there is some microstructural evidence for the operation of dislocation creep although this interpretation conflicts with the generally held view that crystal plastic deformation does not occur in plagioclase below about 500 c an alternative mechanism is one of granular flow accommodated by solution transfer however this mechanism would be expected to give a random cpo for either mechanism we need to explain the triclinic nature of the cpo it is possible that the cpo represents the modification by deformation of a pre existing cpo probably defined by a small number of original plagioclase grains

effect of magnetic anisotropy on the natural remanent magnetization in the mcu ive layer of the bjerkreim sokndal layered intrusion rogaland southern norway a strong negative magnetic anomaly caused by an intense natural remanent magnetization nrm approximately opposite today s geomagnetic field is observed above the mcu ive unit of the bjerkreim sokndal layered intrusion the anomaly is strongest in the east close to heskestad and decreases when following the layer toward the north and west this study investigates how the nrm changes along the layer and how its direction and intensity are affected by magnetic fabrics in the intrusion nrm low field anisotropy of magnetic susceptibility and anisotropy of anhysteretic remanence have been measured on 371 specimens from 46 sites the orientation of both the magnetic fabrics and the nrm changes for different locations along the layer and it appears that the nrm is tilted away from the mean paleofield and toward the direction of maximum susceptibility and maximum anhysteretic remanence when nrm directions are corrected for magnetic fabrics the angle between the nrm and mean paleofield direction generally decreases for specimens with a single component nrm no correlation was found between the nrm intensity and the directional relationship between nrm magnetic fabric and mean paleofield

crystallographic preferred orientation in albite samples deformed experimentally by dislocation and solution precipitation creep abstract the crystallographic preferred orientations of a series of experimentally deformed fine grained albite aggregates were measured by synchrotron source x ray diffraction most samples were deformed and extensively recrystallized by low temperature recrystallization accommodated dislocation creep in axial compression as well as simple shear these samples developed weak but distinct crystallographic preferred orientations consistent with intracrystalline slip on 001 the sheared samples have a marked asymmetry of the maxima with respect to the shear zone boundaries one sample was axially compressed by solution precipitation creep it developed a somewhat different but equally strong preferred orientation perhaps reflecting crystallographic anisotropy in rates of dissolution and growth

recrystallization and fabric development in plagioclase the microstructures and fabrics of naturally deformed intermediate composition plagioclase feldspars from the amphibolite granulitic gneisses from the lower crust have been studied special interest in the effects of dynamic recrystallization and deformation fabrics has been taken the plagioclase feldspars an40 50 have undergone extensive ductile deformation by intracrystalline slip and twinning they show strong crystallographic preferred orientation which was produced by dominant 010 001 slip dynamic recrystallization is indicated by fine grained new grains neoblasts interspersed with deformed relicts porphyroclasts the neoblasts have formed preferentially along grain boundaries kink band boundaries kbbs intracrystalline shear zones and around solid inclusions such as garnet and pyroxene optical microstructural evidence suggests that dynamic recrystallization occurred primarily by grain boundary migration with subsidiary subgrain rotation which appears to have occurred only at low st

geodetic evidence of back arc spreading in the mariana trough 1 repeated gps surveys in the mariana islands show that the mariana block is moving apart from the philippine sea plate the velocities of the islands relative to the philippine sea plate range from about 15 mm yr in the northern islands to about 45 mm yr near guam the data also suggest convergence rates for the mariana forearc with respect to the pacific plate of 35 45 mm yr at 19 n increasing to 55 70 mm yr at 13 5 n in addition the velocity vectors show a slight north south expansion of the arc the estimated location of the euler pole of the mariana forearc with respect to the philippine sea plate is well south of the geographical point where the back arc basin narrows to zero width

magnetic fabric interpretation complicated by inclusions in mafic silicates abstract the expected relationships between anisotropy of low field magnetic susceptibility ams and crystal symmetry of mafic silicates are disturbed by the presence of magnetite inclusions measurements of ams anisotropy of anhysteretic remanence aarm and theoretically predicted bulk susceptibilities from chemical composition all favour the exercise of great caution in the interpretation of preferred orientation distributions of silicates from a rock s ams these results pertain mainly to the mafic silicates of lower crustal rocks pyroxene orthopyroxene amphibole and some of their low grade metamorphic alterations serpentine epidote

paleogeographic reconstruction and origin of the philippine sea abstract our reconstruction of the philippine sea suggests that it formed by two distinct episodes of back arc spreading each of which resulted from seaward retreat of the trench in the first episode the protoizu bonin trench retreated northward and the west philippine basin formed behind the northern half of the palau kyushu ridge in the second episode the izu mariana trench retreated eastward and the shikoku and parece vela basins formed behind it during the last 17 ma the philippine sea basin has been moving northwestward with respect to eurasia shifting the ttt triple junction off central japan westward by about 50 km the motion of the philippine sea with respect to eurasia at the triple junction changed from north northwestward to west northwestward 10 5 ma ago for the period before 17 ma ago we construct two models retreating trench model and anchored slab model the izu bonin trench migrated from south to northeast rotating in a clock wise sense since 48 ma ago in the retreating trench model in the anchored slab model the trench has been fixed with respect to eurasian margin since 43 ma ago we prefer the retreating trench model because the deformation of the plate boundary along the eastern margin of eurasia during 30 17 ma ago is much simpler for this model than for the anchored slab model furthermore rotations of the bonin mariana islands are consistent with those predicted from the retreating trench model the 48 ma ages of the northern part of the palau kyushu ridge and of chichi jima of the bonin islands indicate that there was subduction beneath the northern half of the ridge beginning at least 48 ma ago from this and the subparallelism in trend between the northern part of the palau kyushu ridge and the central basin ridge we propose that the major part of the west philippine basin formed by back arc spreading in a n s direction behind the northern part of the palau kyushu ridge the pacific plate was moving northward with respect to hot spots from 48 to 43 ma ago which implies that the pacific plate is not likely to have been subducting beneath the west philippine basin during this time we speculate that another plate existed south of the pacific plate and thai it was subducting beneath both tile west philippine basin and australia the annihilation of this plate might he a cause for the sudden change of the pacific plate motion at 43 ma ago

magnetic susceptibility and ams of the bushveld alkaline granites south africa the bushveld complex in south africa includes one of the world s largest anorogenic alkaline granite intrusions 66 000 km2 the granite forms a composite laccolith of 350 250 km in area and about 2 km in thickness which was emplaced at about 5 km depth into sediments overlying the kaapvaal craton at 2054 ma the bushveld granite and its roof rocks have long been mined for sn w and f the bushveld granites have high magnetic susceptibilities km from 1000 to 4000 si and a quantitative model is presented suggesting that susceptibility fabrics are primarily carried by ferromagnetic minerals the measured ams foliations coincide with observed subhorizontal mineral lineations and compositional layering magnetic lineation trends vary considerably within the horizontal plane the existence of a weak planar fabric and an almost absent linear component may reflect a laccolithic emplacement by roof uplift causing flattening magmatic fabrics or b emplacement of largely crystal free magma crystallizing in situ and developing horizontal compositional layering from thermal chemical diffusion fronts and gravity driven mechanisms weak magnetic fabrics like those identified in the bushveld granites require specific sampling schemes and procedures in addition to rigorous constraint of magnetic mineralogy and crystallization sequence