magnetite exsolution in ilmenite from the fe ti oxide gabbro in the xinjie intrusion sw china and sources of unusually strong remnant magnetization this study investigates magnetite exsolution in ilmenite from fe ti oxide gabbro in the xinjie intrusion sw china exsolved magnetite lamellae in ilmenite contain nearly pure fe3o4 with 1 wt tio2 ebsd based analyses indicate that the magnetite lamellae have close packed oxygen planes and directions parallel to those in the host ilmenite with 111 mag 0001 ilm and formula 1 the fe2 in the magnetite lamellae is probably derived from adjacent titanomagnetite by sub solidus inter oxide cation repartitioning of fe2 ti4 2fe3 during cooling it is thus suggested that only fe3 cations in the magnetite lamellae should be included into the composition of the ilm hemss precursor for the fe ti oxide oxy thermometer the existence of magnetite exsolution in ilmenite also provides an alternative explanation for unusually strong natural remnant magnetization in natural ilmenite 1 embed mml math 1 gif

fluid inclusion evidence for minimum 11 km vertical offset on the wasatch fault utah the footwall of the wasatch fault in the corner creek area near salt lake city is hydrothermally altered and deformed quartz monzonite of the oligocene little cottonwood stock secondary fluid inclusions are associated with hydrothermal alteration minerals and structural deformation thermometric measurements of fluid inclusion characteristics indicate entrapment of a co2 h2o nacl fluid at minimum temperatures of 223 353 c and minimum fluid pressures of 900 2800 bar the 2800 bar fluid pressure is near lithostatic pressure at a depth of 11 km the minimum displacement of the fault that is required for exhumation of the observed alteration and fluid inclusions and more than three times greater than most previous estimates the large displacement estimate is supported by the occurrence of pyrophyllite in shales in the footwall by the petrology of metamorphosed shale surrounding an unroofed pluton in the footwall and by geologic reconstruction of the eroded footwall

oxide and sulphide minerals in highly oxidized rb depleted archaean granulites of the shevaroy hills massif south india oxidation states and the role of metamorphic fluids the shevaroy hills of northern tamil nadu southern india expose the highest grade granulites of a prograde amphibolite facies to granulite facies deep crustal section of late archaean age these highly oxidized quartzofeldspathic garnet charnockites generally show minor high tio2 biotite and amphibole as the only hydrous minerals and are greatly depleted in the incompatible elements rb and th peak metamorphic temperatures garnet orthopyroxene and pressures garnet orthopyroxene plagioclase quartz are near 750 degrees c and 8 kbar respectively pervasive veinlets of k feldspar exist throughout dominant plagioclase in each sample and show clean contact with orthopyroxene they are suggested to have been produced by a low h2o activity migrating fluid phase under granulite facies conditions most likely a concentrated chloride carbonate brine with high alkali mobility accompanied by an immiscible co2 rich fluid silicate oxide and sulphide mineral assemblages record high oxygen fugacity pyroxenes in the felsic rocks have high mg mg fe 0 5 0 7 the major oxide mineral is ilmenite with up to 60 mole per cent exsolved hematite utilizing three independent oxygen barometers ferrosilite magnetite quartz ferrosilite hematite quartz and magnetite hematite in conjunction with garnet orthopyroxene exchange temperatures samples with x hm ilm 0 1 yield a consistent oxygen fugacity about two log units above fayalite stability less oxidized samples x hm ilm 0 4 pyrite is the dominant sulphide and pyrrhotite is absent pyrite grains in these samples have marginal alteration to magnetite along the rims signifying a high temperature oxidation event moderately oxidized samples 0 1 x hm ilm 0 4 have abundant co existing pyrrhotite pyrite and magnetite the most reduced granulite samples have pyrrhotite as the dominant sulphide with little or no pyrite and no coexisting magnetite chalcopyrite is a common accessory mineral of pyrite and pyrrhotite in all the samples textures in some samples suggest that it formed as an exsolution product from pyrrhotite extensive vein networks of magnetite and pyrite associated principally with the pyroxene and amphibole give evidence for a pervasive highly oxidizing fluid phase thermodynamic analysis of the assemblage pyrrhotite pyrite and magnetite yields consistent high oxidation states at 700 800 degrees c and 8 kbar the oxygen fugacity in our most oxidized pyrrhotite bearing sample is 10 12 65 bar at 770 degrees c there are strong indications that the shevaroy hills granulites recrystallized in the presence of an alkali rich low h2o activity fluid probably a concentrated brine it cannot be demonstrated at present whether the high oxidation states were set by initially oxidized protoliths or effected by the postulated fluids the high correspondence of maximally rb depleted samples with the highest recorded oxidation states suggests that the rb depletion event coincided with the oxidation event probably during breakdown of biotite to orthopyroxene k feldspar we speculate that these alterations were effected by exhalations from deep seated alkali basalts which provided both heat and high oxygen fugacity low a h2o fluids it will be of interest to determine whether greatly rb depleted granulites in other precambrian terranes show similar highly oxidizing signatures

interplate quasi static slip off sanriku ne japan estimated from repeating earthquakes 1 space time distribution of quasi static slip on the plate boundary off sanriku northeastern ne japan was estimated by using small repeating earthquakes we found 2509 repeating earthquakes occurring on the plate boundary for the last 17 years based on the interpretation that these events are caused by repeating slips of small asperities surrounded by stable sliding area we regarded these repeating events as creepmeters embedded on the plate boundary the results show that deeper portions of the plate boundary are sliding almost stably and earthquakes with magnitudes more than 6 are accompanied by quasi static slips in the surrounding areas

trace elements in magnetite from massive iron oxide apatite deposits indicate a combined formation by igneous and magmatic hydrothermal processes iron oxide apatite ioa deposits are an important source of iron and other elements e g ree p u ag and co vital to modern society however their formation including the namesake kiruna type ioa deposit sweden remains controversial working hypotheses include a purely magmatic origin involving separation of an fe p rich volatile rich oxide melt from a si rich silicate melt and precipitation of magnetite from an aqueous ore fluid which is either of magmatic hydrothermal or non magmatic surface or metamorphic origin in this study we focus on the geochemistry of magnetite from the cretaceous kiruna type los colorados ioa deposit 350 mt fe located in the northern chilean iron belt los colorados has experienced minimal hydrothermal alteration that commonly obscures primary features in ioa deposits laser ablation inductively coupled plasma mass spectroscopy la icp ms transects and electron probe micro analyzer epma wavelength dispersive x ray wdx spectrometry mapping demonstrate distinct chemical zoning in magnetite grains wherein cores are enriched in ti al mn and mg the concentrations of these trace elements in magnetite cores are consistent with igneous magnetite crystallized from a silicate melt whereas magnetite rims show a pronounced depletion in these elements consistent with magnetite grown from an fe rich magmatic hydrothermal aqueous fluid further magnetite grains contain polycrystalline inclusions that re homogenize at magmatic temperatures 850 c smaller inclusions 500 ppm concentrations

interplate coupling in northeast japan deduced from inversion analysis of gps data abstract the spatial distribution of the strength of interplate coupling between the subducting pacific plate and the overlying continental plate in northeast japan was investigated through an inversion analysis of gps data employing akaike s bayesian information criterion the gps data used for the inversion analysis are rates of baseline length changes and vertical displacement rates of crustal movements during the period from april 6 1996 to march 20 1998 which were obtained at 161 continuous gps observation stations operated by the geographical survey institute of japan the result shows the average drag rates of the overriding plate on the model source regions off the tohoku and hokkaido districts are 6 7 cm yr and 6 8 cm yr respectively which are 0 85 and 0 87 as interplate coupling ratio these values are much higher than those in previous studies the average directions of drag rates are oriented n73 w 15 and n78 w 13 on the model source regions off the tohoku and hokkaido districts respectively these directions agree well with the direction of coseismic slip of the foot wall side n75 w of the 1978 miyagiken oki earthquake m 7 4 but differ slightly from the direction estimated from the plate motion model n65 w seno et al j geophys res 101 1996 11305 11315 we also found afterslip of 5 5 cm yr associated with the 1994 sanriku haruka oki earthquake m 7 5 off aomori prefecture interestingly enough large afterslip occurred on the plate boundary even when more than a year had passed after the sanriku haruka oki earthquake the amount of drag rate becomes dramatically small at depths greater than about 50 km on the model source regions indicating weak interplate coupling there

oxide and sulphide isograds along a late archean deep crustal profile in tamil nadu south india oxide sulphide fe mg silicate and titanite ilmenite textures as well as their mineral compositions have been studied in felsic and intermediate orthogneisses across an amphibolite north to granulite facies south traverse of lower archean crust tamil nadu south india titanite is limited to the amphibolite facies terrane where it rims ilmenite or occurs as independent grains pyrite is widespread throughout the traverse increasing in abundance with increasing metamorphic grade pyrrhotite is confined to the high grade granulites ilmenite is widespread throughout the traverse increasing in abundance with increasing metamorphic grade and occurring primarily as hemo ilmenite in the high grade granulite facies rocks magnetite is widespread throughout the traverse and is commonly associated with ilmenite it decreases in abundance with increasing metamorphic grade in the granulite facies zone reaction rims of magnetite quartz occur along fe mg silicate grain boundaries magnetite also commonly rims or is associated with pyrite both types of reaction rims represent an oxidation effect resulting from the partial subsolidus reduction of the hematite component in ilmenite to magnetite this is confirmed by the presence of composite three oxide grains consisting of hematite magnetite and ilmenite magnetite and magnetite pyrite micro veins along silicate grain boundaries formed over a wide range of post peak metamorphic temperatures and pressures ranging from high grade so2 to low grade h2s dominated conditions oxygen fugacities estimated from the orthopyroxene magnetite quartz orthopyroxene hematite quartz and magnetite hematite buffers average 2 5 log units above qfm it is proposed that the trends in mineral assemblages textures and composition are the result of an external infiltrating concentrated brine containing an oxidizing component such as caso4 during high grade metamorphism later acted upon by prograde and retrograde mineral reactions that do not involve an externally derived fluid phase

full interseismic locking of the nankai and japan west kurile subduction zones an analysis of uniform elastic strain accumulation in japan constrained by permanent gps we analyze permanent global positioning system gps data obtained over japan between 1995 and 1997 to estimate the instantaneous interseismic coupling ratio of the seismogenic zones due to the subduction of the pacific and philippine sea plates below the japanese islands we first derive the gps strain rate fields that characterize the crustal deformation of southern and northern japan and invert them to determine the effective subduction velocity along the central nankai trough on one side and the japan west kurile trench on the other these reference free velocities are close to those predicted by plate motion models with respect to eurasia we conclude that the eurasian reference frame gives a good approximation to the subduction motion and that to first order both subduction zones were fully locked during the period of measurements we then test whether the coupling ratio shows local variations within the seismogenic zones to do this we divide the subduction interface into 35 km 30 km elements that we model by point source groups and we invert the gps velocity field referenced to eurasia to derive the coupling ratio between 0 and 1 on each fault element the results are coherent over the 3 years and confirm that both the central nankai and the japan west kurile seismogenic zones are homogeneously fully locked most of the coupling ratios are close to 1 and a few are close to 0 intermediate values are rare the zones of decoupling correspond either to strong postseismic afterslip associated with the 1994 sanriku oki interplate earthquake japan trench or to a small overestimation of the actual lower limit of the locked zone we conclude that within the resolution of the gps data and the model 1 partial coupling did not exist during these 3 years along the nankai and japan west kurile trenches 2 the small seismic coupling ratio previously derived from earthquakes analysis for the japan and kurile trenches may indicate that a significant part of the elastic energy is dissipated silently through slow earthquakes and postseismic afterslip and 3 the heterogeneous coseismic slip pattern observed for the large and great earthquakes that rupture both subduction zones is in great contrast to the homogeneous loading finally we discuss the nonelastic residual deformation within the frame of the long term deformation of the japanese islands

newly imaged shape of the deep seismic zone within the subducting pacific plate beneath the hokkaido corner japan kurile arc arc junction 1 the geometry of the deep seismic zone associated with the subducting pacific plate beneath the north american plate in the hokkaido corner japan kurile arc arc junction has been investigated based on the hypocenters accurately relocated by a dense local seismographic network with three dimensional p and s wave velocity structures the model suggests that the lateral changes in the dip of the subduction zone are more rapid than previously believed we found that the deep seismic zone in the pacific plate dips at 20 to 30 on the side of the japan arc the dip angle increased in a narrow region around 143 144 e by 10 to 20 and the dip was 40 to 50 on the side of the kurile arc in the transitional zone around 143 144 e an unusual distribution of microearthquakes has been discovered within the pacific plate which was located on a near vertical plane with an area of 50 100 km2 with strike that was perpendicular to the kurile trench axis this feature was named the tokachi oki slab cracking zone we suggest that this is a slab cracking zone torn by the extensional stress due to the rapid lateral changes in the dip of the subduction zone the slab cracking zone may possibly grow into the large disruption or tear of the slab in the future

in situ la icp ms trace elemental analyses of magnetite the mesozoic tengtie skarn fe deposit in the nanling range south china abstract the nanling range in south china hosts numerous world class w sn deposits and some fe deposits the mesozoic tengtie fe skarn deposit in the southern nanling range is contemporaneous with the regional sn mineralization the deposit is composed of numerous ore bodies along the contacts between the late paleozoic or mesozoic carbonate rocks and the yanshanian lianyang granitic complex interaction of the magma with hosting dolomitic limestone and limestone formed calcic ca rich and magnesian mg rich skarns respectively the tengtie deposit has a paragenetic sequence of the prograde stage of anhydrous skarn minerals followed by the retrograde stage of hydrous skarn minerals and the final sulfide stage magnetite in the prograde and retrograde skarn stages is associated with diopside garnet chlorite epidote and phlogopite whereas magnetite of the final stage is associated with chalcopyrite and pyrite massive magnetite ores crosscut by quartz and calcite veins are present mainly in the retrograde skarn stage laser ablation icp ms was used to determine trace elements of magnetite from different stages some magnetite grains have unusually high ca na k and si possibly due to the presence of silicate mineral inclusions magnetite of the prograde stage has the highest co contents but that of the sulfide stage is extremely poor in co which partitions in sulfides magnetite of magnesian skarns contains more mg mn and al than that of calcic skarns attributed to the interaction of the magma with compositionally different host rocks magnetite from calcic and magnesian skarns contains 6 185 ppm sn and 61 1246 ppm sn respectively the high sn contents are not due to the presence of cassiterite inclusions which are not identified in magnetite instead we believe that sn resides in the magnetite structure regionally intensive mesozoic sn mineralization in south china indicates that concurrent magmatic hydrothermal fluids may be rich in sn and contribute to the formation of high sn magnetite our study demonstrates that trace elements of magnetite can be a sensitive indicator for the skarn stages and wall rock compositions and as such trace elemental chemistry of magnetite can be a potentially powerful fingerprint for sediment provenance and regional mineralization

spatial variation in the frequency magnitude distribution of earthquakes under the tectonic framework in the middle east abstract spatial variations of seismic energy released and b value over the middle east region are investigated based on a seismicity catalog from 1995 to 2007 the goal is to use these seismic parameters and based on other geodetic and geophysical observations such as gps measurements strain rate model fault distribution focal mechanism crustal model q model and gravity measurements etc to uncover spatial patterns that seem anomalous areas of high energy released cumulative seem to correspond to the areas of relatively high b values areas of high energy released and high b values seem to correspond very well with the location of continental collision where earthquake activities are high the divergent boundary between arabia and african plates and subduction zone at makran seem to correspond to low to moderate energy release northern pamir azerbaijan caucasus the lower part of zagros mountains eastern turkey owen fracture zone strait of bob el mandeb and south of the sulaiman shear zone seem to correspond to high cumulative energy released high strain rate high b values and high fault density while the central and eastern iran southern zagros northern pakistan gulf of aden alborz southwest of the caspian sea western caucasus and kopeh dagh seem to correspond with lower b values the cross section map for hindu kush shows general decreasing of the b values with depth however a region of high b value is observed underneath pamir at depths from 170 to 230 km this anomaly region can be due to dehydration of pamir crustal slab at depth

slip distribution of the 1952 tokachi oki earthquake m 8 1 along the kuril trench deduced from tsunami waveform inversion 1 we inverted 13 tsunami waveforms recorded in japan to estimate the slip distribution of the 1952 tokachi oki earthquake m 8 1 which occurred southeast off hokkaido along the southern kuril subduction zone the previously estimated source area determined from tsunami travel times hatori 1973 did not coincide with the observed aftershock distribution our results show that a large amount of slip occurred in the aftershock area east of hatori s tsunami source area suggesting that a portion of the interplate thrust near the trench was ruptured by the main shock we also found more than 5 m of slip along the deeper part of the seismogenic interface just below the central part of hatori s tsunami source area this region which also has the largest stress drop during the main shock had few aftershocks large tsunami heights on the eastern hokkaido coast are better explained by the heterogeneous slip model than previous uniform slip fault models the total seismic moment is estimated to be 1 87 1021 n m giving a moment magnitude of mw 8 1 the revised tsunami source area is estimated to be 25 2 103 km2 3 times larger than the previous tsunami source area out of four large earthquakes with m 7 that subsequently occurred in and around the rupture area of the 1952 event three were at the edges of regions with relatively small amount of slip we also found that a subducted seamount near the edge of the rupture area possibly impeded slip along the plate interface

randomness of megathrust earthquakes implied by rapid stress recovery after the japan earthquake the 2011 tohoku oki earthquake released stress within a subduction zone analysis of seismic data shows that stresses returned to pre quake levels within a few years implying that large quakes could occur more often than previously thought

exsolution features in titanomagnetites from massive magnetite layers and their host rocks of the upper zone eastern bushveld complex a systematic investigation of the exsolution textures in titanomagnetite grains of massive magnetite layers and their associated host rocks from the entire upper zone sequence has revealed noticeable differences that can be related to changes in the oxidation state of the magma during crystallization and cooling the observed oxidation exsolution intergrowths indicate a higher f sub o 2 during formation of the magnetite layers than during crystallization of the disseminated titanomagnetite as well as a decrease in the f sub o 2 required to precipitate successive titanomagnetite rich layers upward in the sequence peculiar composite lamellar intergrowths of magnetite and ilmenite in the uppermost magnetite layers and in some magnetite plugs are ascribed to a subsolvus oxidation of ulvoespinel in ulvoespinel rich magnetite the configuration of the magnetite ulvoespinel solvus dictates that ulvoespinel in ti rich magnetite ulvoespinel solid solution high in the succession exsolves at considerably higher temperature than in ti poor solid solution lower in the sequence subsolvus oxidation of ulvoespinel to ilmenite at higher temperatures near the top of the intrusion facilitated diffusion of ilmenite to produce the variety of different composite exsolution textures the textures are not developed where abundant oxidation exsolution of ilmenite has taken place at temperatures above the solvus

sulfide saturation and magma emplacement in the formation of the permian huangshandong ni cu sulfide deposit xinjiang northwestern china the eastern tianshan orogenic belt northwestern china hosts 120 mafic ultramafic intrusions and at least six of them host significant ni cu sulfide mineralization of which the permian huangshandong intrusion is the largest the mafic ultramafic rocks of the huangshandong intrusion comprise lherzolite gabbronorite gabbro and diorite individual orebodies are composed of pyrrhotite pentlandite and chalcopyrite as disseminated to net textured or massive to semimassive sulfide mineralization oxide bearing sulfide mineralization contains 1 to 5 magmatic magnetite that has ilmenite and spinel exsolution lamellae silicate rocks and sulfide mineralization have variable whole rock platinum group elements pge 0 38 362 ppb cu 42 97 000 ppm and ni 26 33 000 ppm concentrations the mineralization is broken out into two types termed oxide rich and oxide poor the oxide rich sulfide mineralization has high pge concentrations with low 187re 188os ratios 38 9 151 whereas oxide poor sulfide mineralization has low pge concentrations with high 187re 188os 159 781 both oxide rich and poor sulfide mineralization have highly variable os values 49 189 and 30 278 respectively oxide rich sulfide mineralization contains magnetite with variable amounts of mgo tio2 al2o3 feototal cr v zn and sn indicating that they crystallized from different stages during magma differentiation we propose that the host intrusion and associated sulfide mineralization were derived from high mg basaltic magmas from a mantle source that was previously modified by subducted oceanic slab oxide poor sulfide mineralization of the huangshandong intrusion formed from a magma that underwent 0 03 sulfide removal before emplacement and was thus pge depleted oxide rich sulfide mineralization formed from magma that had much lower amounts of sulfide 0 003 removed in the early stage before entering the huangshandong magma chamber an early sulfide saturation event was likely associated with fractionation of silicate minerals in a deep staging magma chamber the second sulfide saturation event that formed the sulfide mineralization was probably triggered by selective crustal contamination and fractionation in the high level chamber where mixing of magmas and accumulation of immiscible sulfide occurred

mapping the b value anomaly at 100 km depth in the alaska and new zealand subduction zones a positive anomaly in the frequency magnitude distribution b value is detected at approximately 90 100 km depth in two subduction zones the b value in this anomalous zone is 40 higher than in adjacent volumes we use regional catalogs with a magnitude of completeness of 2 6 central alaska and cook inlet 2 0 shumagin islands and 3 5 new zealand to resolve the b value as a function of space in more detail we project all earthquakes onto planes perpendicular to the strike of the slab these cross section views of the b value distribution locate the high b value anomalies at a depth of 90 100 km on the upper surface of the wadati benioff zone at these depths slab dehydration may increase pore pressure thus lowering the effective stress and increasing the b value increased pore pressure would also lower the liquidus in the overlying asthenosphere giving rise to the volcanism that occurs directly above this zone

analysis of the b values before and after the 23 october 2011 mw 7 2 van ercis turkey earthquake abstract the b values from the gutenberg richter frequency magnitude relation are investigated in the lake van region in eastern turkey where accommodated the 23 october 2011 m w 7 2 van ercis earthquake earthquake catalog is provided by the kandilli observatory and earthquake research institute koeri the catalog consists of 6709 well located events with local magnitudes m l from 2 6 to 5 8 the time span of the catalog covers the period 2002 and 2012 the b values were significantly lower at the van ercis mainshock area prior to 23 october 2011 the mainshock occurred near to the high stress low b value asperity in the east of lake van the decrease in b values supports low b value high stress hypothesis prior to the van ercis earthquake the temporal change of b value before the mainshock exhibits a strong trend of decreasing which could be used as an indicator of the approaching mainshock this b value analysis provides an evidence for significant difference that exist between pre seismic and post seismic periods of the mainshock area distribution of low b values for van ercis aftershock sequence relates the termination of the mainshock rupture to the east may be the precursory phenomena for the future moderate or large magnitude earthquakes in the lake van region

analysis of the 2005 2016 earthquake sequence in northern iran using the visibility graph method we present an analysis of the seismicity of northern iran in the period between 2005 and 2016 using a recently introduced method based on concepts of graph theory the method relies on the inter event visibility defined in terms of a connectivity degree parameter k which is correlated with the earthquake magnitude m previous studies show that the slope m of the line fitting the k m plot by the least squares method also observes a relationship with the b value from the gutenberg richter law thus rendering the graph analysis useful to examine the seismicity of a region these correlations seem to hold for the analysis of relatively small sequences of earthquakes offering the possibility of studying seismicity parameters in time we apply this approach to the case of the seismicity of northern iran using an earthquake catalog for the tectonic seismic regions of azerbaijan alborz and kopeh dagh we use results drawn for this region with the visibility graph approach in combination with results from other similar studies to further improve the universal relationship between m and b and show that the visibility graph approach can be considered as a valid alternative for analyzing regional seismicity properties and earthquake sequences

modeling the rupture process of the 2003 september 25 tokachi oki hokkaido earthquake using 1 hz gps data received 10 september 2004 accepted 3 october 2004 published 3 november 2004 1 high rategpshasthepotentialtorecoverbothdynamic and static displacements accurately we analyze 1 hz gps data recorded during the 2003 tokachi oki earthquake the 1 hz gps displacement waveforms show good agreement with integrated accelerometer records except for low frequency noise that are inherently present in integrated seismic records the gps waveforms were inverted to model the spatio temporal evolution of the fault slip during the rupture the slip is found to propagate downdip in the subduction zone with largest moment release 50 km northwest of the hypocenter the region of largest slip agrees in general with traditional seismic studies indicating that 1 hz gps can be used for finite fault studies the 1 hz gps slip model shows clearer contrast with afterslip distributions than those inferred from strong motion data possibly because 1 hz gps is more sensitive to cumulative slip distribution index terms 1294 geodesy and gravity instruments and techniques 7209 seismology earthquake dynamics and mechanics 7212 seismology earthquake ground motions and engineering citation miyazaki s k m larson k choi k hikima k koketsu p bodin j haase g emore and a yamagiwa 2004 modeling the rupture process of the 2003 september 25 tokachi oki hokkaido earthquake using 1 hz gps data geophys res lett 31 l21603 doi 10 1029 2004gl021457

new textural and mineralogical constraints on the origin of the hongge fe ti v oxide deposit sw china the hongge magmatic fe ti v oxide deposit in the panxi region sw china is hosted in a layered mafic ultramafic intrusion this 2 7 km thick lopolith like intrusion consists of the lower middle and upper zones which are composed of olivine clinopyroxenite clinopyroxenite and gabbro respectively abundant fe ti oxide layers mainly occur in the middle zone and the lower part of the upper zone fe ti oxides include cr rich and cr poor titanomagnetite and granular ilmenite cr rich titanomagnetite is commonly disseminated in the olivine clinopyroxenite of the lower parts of the lower and middle zones and contains 1 89 to 14 9 wt cr2o3 and 3 20 to 16 2 wt tio2 whereas cr poor titanomagnetite typically occurs as net textured and massive ores in the upper middle and upper zones and contains much lower cr2o3 0 4 wt but more variable tio2 0 11 to 18 2 wt disseminated cr rich titanomagnetite in the ultramafic rocks is commonly enclosed in either olivine or clinopyroxene whereas cr poor titanomangetite of the net textured and massive ores is mainly interstitial to clinopyroxene and plagioclase the lithology of the hongge intrusion is consistent with multiple injections of magmas the lower zone being derived from a single pulse of less differentiated ferrobasaltic magma and the middle and upper zones from multiple pulses of more differentiated magmas cr rich titanomagnetite in the disseminated ores of the lower and middle zones is interpreted to represent an early crystallization phase whereas clusters of cr poor titanomagnetite granular ilmenite and apatite in the net textured ores of the middle and upper zones are thought to have formed from an fe ti p rich melt segregated from a differentiated ferrobasaltic magma as a result of liquid immiscibility the dense fe ti p rich melt percolated downward through the underlying silicate crystal mush to form net textured and massive fe ti oxide ores whereas the coexisting si rich melt formed the overlying plagioclase rich rocks in the intrusion

spatiotemporal variation in b value within the subducting slab prior to the 2003 tokachi oki earthquake m 8 0 japan 1 the b value of an earthquake catalogue defined as the slope of the gutenberg richter frequency magnitude relationship logn a bm is typically close to unity in many tectonic settings however analysis of seismicity data for the subducting slab along the kuril trench reveals a zone of anomalously low b values near the hypocenter of the 2003 tokachi oki earthquake m 8 0 the zone of low b value 0 50 0 65 at depths of less than 80 km in the subducting slab changes in location with time in 1990 2003 and intensifies leading up to the 2003 earthquake this transient low b value zone corresponds roughly with the location of an asperity inferred from waveform inversion in recent research in the shallow zone corresponding to the immediate region 125 km along the subducting slab of the 2003 event the b value is shown to have decreased in the year prior to the main shock from initial values of 0 65 to values as low as 0 5 the occurrence of the 2003 earthquake a large thrust event on the subducting plate interface in this zone of anomalously low b values suggests that low b values in the subducting slab prior to the main shock and during the aftershock series are concordant with high stress concentrations associated with interseismic strain accumulation and redistribution in the vicinity of the rupture

on the stress dependence of the earthquake b value laboratory experiments have shown that the b value in the size distribution of acoustic emission events decreases linearly with differential stress there have been a number of observations that indicate that this relation may also hold for earthquakes here using a simple frictional strength model for stresses in the continental lithosphere combined with earthquake b values measured as a function of depth in a wide variety of tectonic regions we verify and calibrate that relation finding b 1 23 0 06 0 0012 0 0003 1 3 where the stress difference 1 3 is in megapascal for subduction zones we find that b value correlates linearly with the slab pull force and with the net reduction of plate interface normal force both of which also indicate a negative linear relation between b value and differential stress

whole rock and mineral composition constraints on the genesis of the giant hongge fe ti v oxide deposit in the emeishan large igneous province southwest china the hongge giant fe ti v oxide ore deposit is hosted by a layered intrusion located in the central part of the emeishan large igneous province sw china the intrusion is relatively small in comparison with other typical oxide bearing intrusions worldwide it consists of a lower olivine clinopyroxenite zone a middle clinopyroxenite zone and an upper gabbro zone herein referred to as the lower middle and upper zones most of the economic fe ti v oxide ore layers occur within the middle zone the hongge oxide ores are depleted in ree and enriched in zr hf nb and ta as compared to the associated clinopyroxenites this enrichment of elements that are compatible in titanomagnetite is consistent with the interpretation that the ores formed by accumulation of magnetite and ilmenite as in the nearby coeval panzhihua fe ti v oxide deposit described previously by others mafic silicates in the hongge deposit have much higher mgo contents than those in other oxide deposits associated with large layered intrusions in the world this highlights the importance of relatively primitive parental magma becoming saturated in titanomagnetite at an early stage in the genesis of the giant fe ti v oxide deposits in the emeishan large igneous province phase equilibrium constraints suggest that the parental magma of the hongge deposit is similar to that of some of the most primitive high ti basalts in the emeishan large igneous province the ferrobasaltic parental magma and the ferropicritic primary magma of the hongge intrusion are similar in major and trace element composition to the ferropicritic ferrobasaltic magma in the pechenga belt kola peninsula russia depletion of incompatible trace elements in the oxide ores and associated rocks in the hongge intrusion as compared to the coeval high ti basalts suggest that not all the magma involved in the development of the hongge intrusion has been retained in the intrusion the occurrence of multiple fe ti oxide layers alternating with fe ti oxide bearing silicate layers within a single zone and the repetitive appearance of sulfides olivine and cr rich layers suggest that multiple pulses of magma were involved in the formation of the hongge intrusion and related fe ti v oxide deposit we propose that the middle zone of the hongge intrusion was a magma stepwise flow through system and that some of the liquid was lost to the peripheral lavas to form the basalt

in situ la icp ms trace elemental analyses of magnetite cu au fe deposits in the khetri copper belt in rajasthan province nw india abstract magnetite is common in many ore deposits and their host rocks and is useful for petrogenetic studies in the khetri copper belt in rajasthan province nw india there are several cu au fe deposits associated with extensive cu fe au ag co ree u mineralization hosted in phyllites schists and quartzites of the paleoproterozoic delhi supergroup ore bodies of these deposits comprise dominantly disseminated and vein type cu sulfide ores composed of chalcopyrite pyrite and pyrrhotite intergrown with minor magnetite there are also fe oxide ores with minor or no cu sulfides which are locally overprinted by the mineral assemblage of the cu sulfide ores in addition to the fe oxide and cu sulfide ores the protolith of the delhi supergroup includes banded iron formations bifs with original magnetite preserved i e magnetite quartzites and their sheared counterparts in the sheared magnetite quartzites their magnetite and quartz are mobilized and redistributed to magnetite and quartz bands trace elemental compositions of magnetite from these types of ores rocks were obtained by la icp ms the dataset indicates that different types of magnetite have distinct concentrations of ti al mg mn v cr co ni zn cu p ge and ga which are correlated to their forming environments magnetite grains in magnetite quartzites have relatively high al 800 8000 ppm ti 150 900 ppm and v 300 600 ppm contents compared to those of bifs in other regions such as the yilgarn craton western australia and labrador canada the high al ti and v contents can be explained by precipitation of the magnetite from relatively reduced al ti rich water possibly involving hotter seafloor hydrothermal fluids derived from submarine mafic volcanic rocks magnetite in sheared magnetite quartzites is generally irregular and re crystallized and has ni mn al cu and p contents lower than the magnetite from the unsheared counterparts suggesting that the shearing related mobilization is able to extract these elements from original magnetite however elevated contents of ti v co cr ge and mg of the magnetite in the sheared magnetite quartzites can be ascribed to involvement of external hydrothermal fluids during the shearing consistent with occurrence of some hydrothermal minerals in the samples compositions of magnetite from the fe oxide and cu sulfide ores are interpreted to be controlled mainly by fluid compositions and or oxygen fugacity f o 2 other potential controlling factors such as temperature fluid rock interaction and co precipitating minerals have very limited impacts magnetite in the cu sulfide ores has higher v but lower ni contents than that of the fe oxide ores likely indicating its precipitation from relatively reduced evolved fluids however it is also indicated that the two types of magnetite do not show large distinctions in terms of concentrations of most elements suggesting that they may have precipitated from a common evolving fluid we propose a combination of ge versus ti al and cr versus co ni co variation plots to discriminate different types of magnetite from the khetri copper belt our work agrees well with previous studies that compositions of magnetite can be potentially useful for provenance studies but also highlights that discrimination schemes would be more meaningful for deposits in a certain region if fluid water chemistry and specific formation conditions reflected in compositions of magnetite are clearly understood

variations of trace element concentration of magnetite and ilmenite from the taihe layered intrusion emeishan large igneous province sw china implications for magmatic fractionation and origin of fe ti v oxide ore deposits abstract in situ la icp ms trace elemental analysis has been applied to magnetite and ilmenite of the taihe layered intrusion emeishan large igneous province sw china in order to understand better fractionation processes of magma and origin of fe ti v oxide ore deposits the periodic reversals in mg ti mn in magnetite and mg sc in ilmenite are found in the middle zone of the intrusion and agree with fractionation trends as recorded by olivine fo plagioclase an and clinopyroxene mg compositions these suggest the taihe intrusion formed from open magma chamber processes in a magma conduit with multiple replenishments of more primitive magmas the v and cr of magnetite are well correlated with v and cr of clinopyroxene indicating that they became liquidus phases almost simultaneously at an early stage of magma evolution ilmenite from the middle and upper zones shows variable cr ni v mg nb ta and sc contents indicating that ilmenite at some stratigraphic levels crystallized slightly earlier than magnetite and clinopyroxene the early crystallization of magnetite and ilmenite is the result of the high feot and tio2 contents in the parental magma the ilmenite crystallization before magnetite in the middle and upper zones can be attributed to higher tio2 content of the magma due to the remelting of pre existing ilmenite in a middle level magma chamber compared to the coeval high ti basalts the relatively low zr hf nb and ta contents in both magnetite and ilmenite throughout the taihe intrusion indicate that they crystallized from fe ti p rich silicate magmas positive correlations of ti with mg mn sc and zr of magnetite and zr with sc hf and nb of ilmenite also suggest that magnetite and ilmenite crystallized continuously from the homogeneous silicate magma rather than an immiscible fe rich melt therefore frequent replenishments of fe ti p rich silicate magma and gravitational sorting and settling are crucial for the formation the massive and apatite rich disseminated ores in the lower and middle zones of the taihe intrusion

dissolution reprecipitation process of magnetite from the chengchao iron deposit insights into ore genesis and implication for in situ chemical analysis of magnetite abstract magnetite formed in different environments commonly has distinct assemblages and concentrations of trace elements that can potentially be used as a genetic indicator of this mineral and associated ore deposits in this paper we present textural and compositional data of magnetite from the chengchao iron deposit daye district china to provide a better understanding in the formation mechanism and genesis of the deposit and shed light on analytical protocols for in situ chemical analysis of hydrothermal magnetite magnetite grains from the ore related granitoid pluton mineralized endoskarn magnetite dominated exoskarn and vein type iron ores hosted in marine carbonate intruded by the pluton were examined using scanning electron microscopy and analyzed for major and trace elements using electron microprobe back scattered electron images reveal that primary magnetite from the mineralized skarns and vein type ores were all partly reequilibrated with late stage hydrothermal fluids forming secondary magnetite domains that are featured by abundant porosity and have sharp contact with the primary magnetite these textures are interpreted as resulting from a dissolution reprecipitation process of magnetite which however are mostly obscure under optically primary magnetite grains from the mineralized endoskarn and vein type ores contain high sio 2 0 92 3 21 wt al 2 o 3 0 51 2 83 wt and low mgo 0 15 0 67 wt whereas varieties from the exoskarn ores have high mgo 2 76 3 07 wt and low sio 2 0 03 0 23 wt and al 2 o 3 0 54 1 05 wt this compositional contrast indicates that trace element geochemical composition of magnetite is largely controlled by the compositions of magmatic fluids and host rocks of the ores that have reacted with the fluids compared to its precursor mineral secondary magnetite is significantly depleted in most trace elements with sio 2 deceasing from 1 87 to 0 47 wt on average and al 2 o 3 from 0 89 to 0 08 wt in mineralized endoskarn and vein type ores and mgo from 2 87 to 0 60 wt in exoskarn ores on the contrary average content of iron is notably increased from 69 2 wt to 71 9 wt in the secondary magnetite grains the results suggest that the dissolution reprecipitation process has been important in significantly removing trace elements from early stage magnetite to form high grade high quality iron ores in hydrothermal environments the textural and compositional data confirm that the chengchao iron deposit is of hydrothermal origin rather than being crystallized from immiscible iron oxide melts as previously suggested this study also highlights the importance of textural characterization using various imaging techniques before in situ chemical analysis of magnetite as is the case for texturally complicated u th bearing accessory minerals that have been widely used for u pb geochronology study

some premonitory phenomena of the 1995 hyogo ken nanbu kobe earthquake seismicity b value and fractal dimension abstract this paper presents a detailed and complex analyses of the spatio temporal evolution of seismic activity between 1976 and 1998 in a broad area surrounding the epicentre of the 1995 hyogo ken nanbu kobe earthquake as well as in the source area various precursory changes in seismicity such as quiescence followed by increased seismic activity b value and fractal dimension correlation dimension d2 anomalies appear approximately two years before the occurrence of the major event an increased level of background seismicity is clearly observed after the occurrence of the kobe earthquake particularly in the northeast extension of the source area all these anomalous changes in seismic activity before and after the occurrence of the 1995 hyogo ken nanbu earthquake appear in a relatively large region surrounding the epicentre of the main shock as well as in the source area

geochemical evolution of the banded iron formation hosted high grade iron ore system in the koolyanobbing greenstone belt western australia the banded iron formation bif hosted iron ore deposits in the lower greenstone succession of the koolyanobbing greenstone belt 50 km north of southern cross in western australia are a 200 mt high grade fe 58 pre mining resource and represents one of the most important iron ore districts in the yilgarn craton four hypogene alteration ore forming stages and one supergene upgrading event took place 1 during ore stage 1 lree depleted transition metal enriched mg fe ca carbonates replaced quartz in bifs the deposit scale alteration was most likely induced by devolatilization of sea floor altered ca si depleted mafic rocks in the vicinity of the bif during early regional syn d 1 very low to low grade metamorphism and was most strongly developed on reactivated bif basalt contacts 2 ore stage 2 involved the formation of patchy magnetite ore by a syn d 2 to d 4 dissolution of early carbonate enrichment of fe 2o 3total in magnetite iron ore was by a factor of 2 to 2 4 and compatible trace elements in magnetite such as ga v and al were immobile a subdeposit scale ferroan talc footprint proximal to magnetite iron ore in the largest deposit k deposit was associated with ore stage 2 and resulted from dissolution of magnesite due to reaction with silica in the bif under greenschist facies conditions and potentially high fluid rock ratio 3 magnetite growth during ore stage 3 forming granular magnetite martite ore is related to a subsequent hydrothermal event occurring locally throughout the belt especially in d 2b faults 4 ore stage 4 was associated with fe ca p l ree y enriched hydrothermal fluids possibly from a magmatic source such as the postmetamorphic lake seabrook granite that crops out about 10 km west of the koolyanobbing deposits and at the southern margin of the greenstone belt these ca enriched fluids interacted with distal metamorphosed mafic rock and influenced the bif ore system in a small number of deposits they were channelled through regional d 4 faults and caused specularitedolomite quartz alteration resulting in fe grades of up to 68 5 supergene upgrade ore stage 5 by further gangue leaching in the weathering zone was most effective in carbonate altered bifs and magnetite ore this process together with supergene martitization and goethite replacement of magnetite led to the formation of high grade locally at k deposit high p goethite martite ore at koolyanobbing the two geochemically distinct stages of archean carbonate alteration clearly controlled the formation of hypogene magnetite specularite martite rich ore and recent supergene modification including the further upgrade of fe ore 2012 by economic geology

geochemistry petrogenesis and metallogenesis of the panzhihua gabbroic layered intrusion and associated fe ti v oxide deposits sichuan province sw china the panzhihua gabbroic layered intrusion is associated with the 260ma emeishan large igneous province in sw china this sill like body hosts a giant fe ti v oxide deposit with 1333 million ton ore reserves which makes china a major producer of these metals the intrusion has a marginal zone of fine grained hornblende bearing gabbro and olivine gabbro followed upward by lower middle and upper zones the lower and middle zones consist of layered melanogabbro and gabbro composed of cumulate clinopyroxene plagioclase and olivine these zones also contain magnetite layers the upper zone consists chiefly of leucogabbro composed of plagioclase and clinopyroxene with minor olivine most rocks in the body show variable scale rhythmic modal layering in which dark minerals primarily clinopyroxene dominate in the lower parts of each layer and lighter minerals primarily plagioclase dominate in the upper parts the oxide ores occur as layers and lenses within the gabbros and are concentrated in the lower parts of the intrusion ore textures and associated mineral assemblages indicate that the ore bodies formed by very late stage crystallization of v rich titanomagnetite from an immiscible oxide liquid in a fluidrich environment the rocks of the panzhihua intrusion become more evolved in chemistry upward and follow a tholeiitic differentiation trend with enrichment in fe ti and v they are enriched in light rare earth elements relative to heavy rare earth elements and exhibit positive nb ta and ti anomalies and negative zr and hf anomalies the silicate rocks and oxide ores of the panzhihua intrusion formed from highly evolved fe ti v rich ferrobasaltic or ferropicritic magmas the textures of the ores and the abundance of minor hydrous phases indicate that addition of fluids from upper crustal wall rocks induced the separation of the immiscible oxide melts from which the fe ti v oxide ore bodies in the lower part of the intrusion crystallized

minimum magnitude of completeness in earthquake catalogs examples from alaska the western united states and japan we mapped the minimum magnitude of complete reporting m c for alaska the western united states and for the junec earthquake catalog of japan mc was estimated based on its departure from the linear frequency magnitude relation of the 250 closest earthquakes to grid nodes spaced 10 km apart in all catalogs studied mc was strongly heterogeneous in offshore areas the mc was typically one unit of magnitude higher than onshore on land also mc can vary by one order of magnitude over distance less than 50 km we recommend that seismicity studies that depend on complete sets of small earthquakes should be limited to areas with similar mc or the minimum magnitude for the analysis has to be raised to the highest com mon value of mc we believe that the data quality as reflected by the mc level should be used to define the spatial extent of seismicity studies where mc plays a role the method we use calculates the goodness of fit between a power law fit to the data and the observed frequency magnitude distribution as a function of a lower cutoff of the magnitude data mc is defined as the magnitude at which 90 of the data can be modeled by a power law fit mc in the 1990s is approximately 1 2 0 4 in most parts of california 1 8 0 4 in most of alaska aleutians and panhandle excluded and at a higher level in the junec catalog for japan various sources such as ex plosions and earthquake families beneath volcanoes can lead to distributions that cannot be fit well by power laws for the hokkaido region we demonstrate how neglecting the spatial variability of m c can lead to erroneous assumptions about deviations from self similarity of earthquake scaling

crystallization sequence and magma chamber processes in the ferrobasaltic sept iles layered intrusion canada the sept iles layered intrusion quebec canada 564 ma is a large plutonic body with a diameter of 80 km and a thickness of 6 km made up from its base to top of a layered series with troctolite and gabbro and an upper border series with anorthosite capped by cupolas of a type granite chilled margin compositions suggest a ferrobasaltic parental magma close to that of the skaergaard intrusion but much richer in iron and titanium samples from drill cores and surface sampling of the 4 7 km thick layered series reveal a succession of massive troctolites and layered gabbros that contain 24 fe ti oxide layers cm to m thick and many anorthositic autolithic blocks the sequence of crystallization in the layered series is plagioclase an72 34 and olivine fo75 21 followed by magnetite and ilmenite then ca rich pyroxene and finally apatite an olivine gap is observed between fo66 and fo59 the saturation of fe ti oxides before ca rich pyroxene is interpreted to be the result of the high feot and tio2 contents and the low cao content of the parental magma contamination by old continental crust has occurred during crystallization of the layered series as indicated by sr isotopic compositions 87sr 86sr564 0 70360 0 70497 the differentiation trend of the intrusion is interrupted by two large and many small reversals to more primitive compositions of cumulus phases cr content of magnetite and lower sr isotope ratios these reversals and the intermittent disappearance of some phases are interpreted as resulting from magma chamber replenishments by undifferentiated primitive basaltic magma and mixing with the resident magma the author 2010 published by oxford university press all rights reserved for permissions please e mail journals permissions oxfordjournals org

the partitioning of trace elements between ilmenite ulvospinel armalcolite and silicate melts with implications for the early differentiation of the moon mineral melt trace element partition coefficients for a large number of trace elements zr hf ta nb v co cu zn sr ree sb sn mo cr w u th were determined experimentally for ilmenite fetio3 ulvospinel fe2tio4 minerals of the armalcolite solid solution fe mg ti2o5 the measured partition coefficients indicate that the high field strength elements zr hf nb and ta and some transition metals v cr co sn are moderately compatible in iron titanium oxides whereas other elements ree y sr u th mo sb and w are strongly incompatible the partition coefficients for u are always significantly higher than the th partition coefficients which makes fe ti oxides efficient in fractionating u from th our data also indicate that ilmenite and other iron titanium oxides are able to fractionate w from hf the latter of which is much more compatible this makes the iron titanium oxides very effective agents to influence hf w isotopic ratios during the evolution of the moon in addition we also present some trace element partition coefficients for iron and titanium bearing orthopyroxene

precursory seismic quiescence before the mw 8 3 tokachi oki japan earthquake on 26 september 2003 revealed by a re examined earthquake catalog 1 the 2003 tokachi oki earthquake mw 8 3 occurred on 26 september 2003 off the pacific coast of hokkaido japan in the present study an earthquake catalog is used that lists 2 000 earthquakes with m 3 3 all of the earthquake waveforms were recorded by the institute of seismology and volcanology hokkaido university in the present study these waveforms are manually re examined and hypocenters and magnitudes are re calculated a detailed analysis of the re determined earthquake catalog between 1994 and 2003 using a gridding technique zmap shows that the 2003 tokachi oki earthquake is preceded by two neighboring seismic quiescence anomalies that start around the beginning of 1999 and last about 5 years until the main shock occurs these quiescence anomalies are located around the asperity ruptured by the main shock and the z values are 3 9 and 4 0 for a time window of tw 4 years using a sample size of n 100 earthquakes the detected seismic quiescences can be interpreted as being caused by a decrease of 50 in the stressing rate based on dieterich s theory it is proposed that a quasi static pre slip occurs at the northeastern edge of the asperity ruptured by the main shock and lasts for five years until the main shock occurs by calculating the change in the coulomb failure stress cfs it is found that negative cfs areas are consistent with the two quiescence anomalies and a positive cfs area corresponds to the hypocenter of the main shock indicating that the quasi static pre slip model is a plausible one

geodetic and seismological investigation of crustal deformation near izmir western anatolia abstract the aegean region including western turkey mainland greece and the hellenic arc is the most seismological and geodynamical active domain in the alpine himalayan belt in this study we processed 3 years of survey mode gps data and present the analysis of a combination of geodetic and seismological data around izmir which is the third most populated city in turkey the velocities obtained from 15 sites vary between 25 mm yr and 28 mm yr relative to the eurasian plate the power law exponent of earthquake size distribution b value ranges from 0 8 to 2 8 in the izmir region between 26 2 e and 27 2 e the lowest b value zones are found along karaburun fault b 0 8 and between seferihisar and tuzla faults b 0 8 a localized stress concentration is expected from numerical models of seismicity along geometrical locked fault patches therefore areas with lowest b values are considered to be the most likely location for a strong earthquake a prediction that is confirmed by the 2005 mw 5 9 seferihisar earthquake sequences with epicentres located to the south of the karaburun fault the north south extension of the izmir area is corroborated by extension rates up to 140 nanostrain yr as obtained from our gps data we combined the 3 year gps velocity field with the published velocity field to determine the strain rate pattern in the area the spatial distribution of b value reflects the normal background due to the tectonic framework and is corroborated by the geodetic data b values correlate with strain pattern this relationship suggests that decrease of b values signifies accumulating strain

seismicity anomalies of the 2003 bam iran earthquake abstract the bam earthquake occurred in an area without any historically recorded major earthquake based on active tectonics and cmt solutions the area is under the influence of shortening and right lateral strike slip faulting seismicity is of a shallow crustal type which is mainly distributed within the koohbanan golbaf lakarkooh shahdad gowk fault systems and the terminal sections of zagros in order to detect temporal seismicity anomalies before this earthquake we compiled a catalog from the isc events and then analyzed it for the magnitude of completeness after declustering this catalog changes in seismicity were assessed by the z value test the temporal variations indicate a relative decrease in z value before the bam earthquake additional to seismic quiescence and decrease in b value before the main shock there are pieces of evidence corresponding to increase in triggered background seismicity after this earthquake

signature of fault healing in an aftershock sequence the 2008 wenchuan earthquake we analyzed the aftershock sequence of the 2008 wenchuan earthquake from may 12 2008 to may 12 2013 using the earthquake catalog of the china earthquake networks center cenc in the analysis performed we took under consideration the temporary variation in the completeness of the earthquake catalog just after the wenchuan mainshock the cutoff completeness magnitude from may 12 to june 27 2008 was above 3 0 due to the impact of the earthquake sequence on the seismological observatory practice it was observed that the b value has an increasing trend from june 27 2008 to late april 2009 while since may 2009 the b value has remained stable if these characteristics were associated with the possible signature of fault healing the apparent healing time could be pinpointed by this measure as around 1 year due to two strong asperities present on the rupture of the wenchuan mainshock the aftershock zone can be divided into two segments namely the north and the south segment the b values of the two segments seem to show different trends of temporal variation the main contribution of the increasing trend comes from the south segment or the initiation segment of the main rupture

fe ti oxides in metamorphic basites from the eastern alps austria a contribution to the formation of solid solutions of natural fe ti oxide assemblages the development of fe ti oxide assemblages in basic rocks from the penninic series of the southern venediger rea austria during polyphase alpine metamorphism has been studied textural and compositional relations indicate thorough reequilibration of the opaque mineral assemblages during late barrovian metamorphism at essentially static conditions of lower amphibolite to greenschist facies in contrast silicate mineralogy of the preceeding blueschist to eclogite facies metamorphism might still be preserved to a large extent

source and path of magma for volcanoes in the subduction zone of northeastern japan the assumption that the source of magma for volcanoes in subduction zones is located in the mantle immediately above the top of the subducting slab at approximately 100 km depth and straight beneath the volcanoes is incorrect in northeastern japan the combination of evidence from velocity tomography in the mantle wedge above the slab and mapping of earthquake size distribution within it strongly points to a source of fluids at the top of the slab at 140 to 150 km depth from where material rises along an inclined path to the volcanoes

minerals exsolution features and geochemistry of fe ti ores of the suwalki district north east poland the fe ti deposits of the suwalki district north east poland are magmatic in origin and were formed as a consequence of differentiation of magma melt that served as a parent for mineralization in the host and ore rocks ore mineralization in the host rocks is of accessory and segregational type and was formed in conditions characterized by low content of interprecipitable liquid relatively high oxygen fugacity and slow continuous cooling nearly all ti was removed from titanomagnetite above the solvus hence the true exsolution of ulvospinel was of minor importance in the host ore mineralization is characterized there by low content of impurities and low fe ti ratio high content of admixtures extreme mineralogical and compositional variations lack of geochemical cyclicity patterns in particular sections are typical of ore rocks both an increased amount of interprecipitable liquid and higher temperatures stimulated by intermediate fo2 controlled the formation of ore rocks the fo2 here remained credibly intermediate from early to late stages of mineral evolution the formation of magma and minerals enriched in fe ti settling in a bottom part of the magma chamber governed the formation of large ore bodies some other mechanisms such as filterpressing or immiscible segregation may be responsible for the formation of some discordant and concordant smaller ore bodies in the host the disclosed partitioning pattern of major and minor elements in ore minerals of host and ore rocks suggests that titanum free magnetite concentrate should be not practicable

fractal properties of seismicity in regions affected by large shallow earthquakes in western japan implications for fault formation processes based on a binary fractal fracture network model 1 the temporal variation in the b value which is defined as the slope of the gutenberg richter frequency n magnitude m relationship log n a bm and the multifractal dimension of the spatial distribution dq of seismic activity are analyzed using earthquake catalog data for regions affected by two recent large earthquakes in japan an anomalous decrease in dq and an increase in b value appear as a common seismic rupture response during the period of decreasing seismic energy release in the lead up to these large shallow earthquakes the change in fractal dimensions of faults inferred from sequential seismic events reflects the transition of fracture from a dispersive rupture process to an anomalous rupture process and shows that the growth of the fault network through fault linkage is abruptly restrained in the fault formation process leading up to the main shock the anomalies in the fractal and multifractal dimensions prior to large earthquakes indicate that the anomalous activation and clustering of smaller earthquakes represents criticality of the state around asperities along a fault

the geochemistry of titanomagnetite in magnetite layers and their host rocks of the eastern bushveld complex more than 430 analyses of magnetite separates revealed pronounced differences in composition between magnetite from magnetite layers and disseminated magnetite from the host rocks magnetite from the massive layers is among others enriched in elements such as ti mg al and si which suggests that this magnetite crystallized under conditions of disequilibrium with the magma i e by spontaneous nucleation and rapid crystallization lower vanadium values in the magnetites from the massive layers suggest crystallization at higher f sub o 2 than from the rocks containing the disseminated magnetite variation diagrams reveal a cyclicity in compositional trends upward in the sequence which is related to nucleation of the magma in the roof zone of the intrusion the periodic collapse of this crystal laden magma and its incorporation in the zone of crystallization at the base of the magma chamber deviations of this normal trend are considered to reflect a stratification of the roof zone and a gradual incorporation of the stratified roof zone liquid into the convecting magma a pronounced upward increase in the cr content of magnetite is tentatively ascribed to a combination of olivine crystallization at the expense of pyroxene in the upper subzones and a possible increase in the partition coefficient of chromium between spinel and liquid with a decrease in the crystallization temperature

mapping the frequency magnitude distribution in asperities an improved technique to calculate recurrence times we hypothesize that highly stressed asperities may be defined by mapping anomalously low b values along the san andreas fault near parkfield the asperity under middle mountain with its b 0 46 can be distinguished from all other parts of the fault surface likewise along the calaveras fault the northern asperity of the morgan hill 1984 m6 2 rupture can be identified by its low b of 0 5 as a high stress patch along the fault we add further evidence to the observations that the b value of the frequency magnitude relationship of earthquakes is inversely proportional to stress by showing that it decreases with depth in the parkfield segment of the san andreas and along the calaveras fault in both of these areas b values above and below 5 km depth are 1 2 and 0 8 respectively we propose that probabilistic recurrence times tr based on the seismicity parameters a and b should be calculated from their values within asperities only instead of from the values of the entire rupture area of the maximum expected earthquake the strong patches on faults control the time of rupture because they are capable of accumulating larger stresses than the rest of the fault zone which slips along passively when an asperity breaks therefore no information on tr is contained in the passive fault segments only in the asperities at parkfield the probabilistic estimates of tr derived from the data in the whole rupture and in the asperity only are 72 18 24 and 23 12 18 years respectively compared to the historically observed repeat time of 22 years at morgan hill the tr estimates are 122 46 76 and 78 47 110 years respectively compared to the observed repeat time of 72 years

variation in trace element content of magnetite crystallized from a fractionating sulfide liquid sudbury canada implications for provenance discrimination abstract laser ablation icp ms analysis has been applied to many accessory minerals in order to understand better the process by which the rock formed and for provenance discrimination we have determined trace element concentrations of fe oxides in massive sulfides that form ni cu pge deposits at the base of the sudbury igneous complex in canada the samples represent the crystallization products of fractionating sulfide liquids and consist of early forming fe rich monosulfide solution mss cumulates and residual cu rich intermediate solid solution iss this study shows that fe oxide geochemistry is a sensitive petrogenetic indicator for the degree of fractionation of the sulfide liquid and provides an insight into the partitioning of elements between sulfide and fe oxide phases in addition it is useful in determining the provenance of detrital fe oxide in a sulfide melt all lithophile elements cr ti v al mn sc nb ga ge ta hf w and zr are compatible into fe oxide the concentrations of these elements are highest in the early forming fe oxide titanomagnetite which crystallized with fe rich mss upon the continual crystallization of fe oxide from the sulfide liquid the lithophile elements gradually decrease so that late forming fe oxide magnetite which crystallized from the residual cu rich liquid is depleted in these elements this behavior is in contrast with fe oxides that crystallized from a fractionating silicate melt whereby the concentration of incompatible elements such as ti increases rather than decreases the behavior of the chalcophile elements in magnetite is largely controlled by the crystallization of the sulfide minerals with only ni co zn mo sn and pb present above detection limit in magnetite nickel mo and co are compatible in fe rich mss and thus the co crystallizing fe oxide is depleted in these elements in contrast magnetite that crystallized later from the fractionated liquid with cu rich iss is enriched in ni mo and co because fe rich mss is absent the concentrations of sn and pb which are incompatible with fe rich mss are highest in magnetite that formed from the fractionated cu rich liquid at subsolidus temperatures ilmenite exsolved from titanomagnetite whereas al spinel exsolved from the cores of some magnetite locally redistributing the trace elements however during laser ablation icp ms analysis of these fe oxides both the magnetite and its exsolution products are ablated so that the analysis represents the original magmatic composition of the fe oxide that crystallized from the sulfide melt

imaging b value depth variations within the cocos and rivera plates at the mexican subduction zone abstract by a systematic mapping of the b value along profiles perpendicular to the mexican wadati benioff zone we obtained important characteristics pertaining the stress state and faulting style related to the subduction process to this purpose we used data from the earthquake catalog reported by the servicio sismologico nacional 1988 2016 we investigate depth variations of the b value for the cocos and rivera under north american plates interface by a detailed analysis of 15 cross sections the obtained b value profiles vary from 0 50 to 2 50 which nevertheless appear related to the faulting style and stress state by comparing the locations and focal mechanism of the largest events with the b values of the surrounding regions our analysis corroborates the dependence of the b value on the faulting style thrust events occur in regions of low and high b value at depths 30 km in agreement with global studies these results support the hypothesis that differential stress processes may be behind the occurrence of the different faulting style on the contrary by analyzing the mean b values for both types of faulting mechanism at each of the cross sections we found a significantly lower mean b value related to normal faulting for those regions where the 8 mw 8 2 and 19 mw 7 1 september 2017 earthquakes occur these results lead us to conclude that those regions experienced an increased stress state prone to the occurrence of normal intraplate events we also compare the b value distribution with vp and q tomography studies obtaining a good correlation between them we found evidence to relate b value variations with subduction processes such as stress state due to tectonic and flexural conditions and to a lesser extent to material heterogeneity and fluid dehydration

open magma chamber processes in the formation of the permian baima mafic ultramafic layered intrusion sw china abstract the baima mafic ultramafic layered intrusion of the 260 ma emeishan large igneous province elip hosts the second largest fe ti v oxide deposit in the panxi region sw china it is a 1600 m thick layered body intruded by slightly younger syenitic and granitic plutons the intrusion includes the lower and upper zones troctolite and olivine pyroxenite of the lower zone contains conformable oxide ore layers whereas the upper zone consists of olivine gabbro and gabbro with abundant apatite in the higher level the crystallization order of the silicates in the baima intrusion is olivine plagioclase clinopyroxene fe ti oxides titanomagnetite and ilmenite crystallized after olivine and possibly plagioclase the oxide ores in the lower zone show slightly lree enriched patterns with la yb n values between 2 0 and 6 4 and positive eu anomalies eu eu of 1 0 to 2 7 in contrast olivine gabbros in the lower zone display stronger lree enrichments la ybn 7 7 14 0 and positive eu anomalies eu eu 2 8 3 3 gabbros in the upper zone have ree profiles characterized by intermediate lree enrichments with la yb n values of 3 2 to 11 2 and positive eu anomalies of 2 1 to 3 0 primitive mantle normalized trace element patterns are characterized by negative la ce th sm and positive nb ta ba and ti anomalies in oxide ores and negative th u zr hf and positive ba sr and ti anomalies in olivine gabbro and gabbro fo of olivine and an of plagioclase remain roughly constant from 0 to 90 m in the lower zone indicating that the magma chamber was continuously filled by compositionally similar magmas during the initial stage three magma replenishments occurred afterwards in the upper part of the lower zone and the upper zone based on compositional reversals of plagioclase olivine and sr isotope mass balance calculations show that the baima parental magma can produce all oxide ores under closed system conditions a wide range of an values of plagioclase within thin sections and disequilibrium sr isotopic compositions of plagioclase both along the stratigraphic profile 87sr 86sri 0 70312 0 70510 and within thin sections indicate convection and co accumulation of cumulus plagioclase that had crystallized from different magmas we propose that the 1600m thick baima intrusion formed in an open and isotopically heterogeneous magma chamber that was periodically recharged by compositionally similar or more primitive magmas in each replenishment cycle the magma underwent progressive crustal contamination and fractional crystallization convection and sorting of crystals based on density differences result in igneous layering characterized by intervals of oxide ores troctolite and olivine gabbro

detrital magnetite as a provenance indicator abstract detrital magnetite grains carry unique petrographic and chemical fingerprints that can be used in provenance research petrographic analysis of 2 941 detrital magnetite grains in holocene sands from felsic plutonic and volcanic intermediate volcanic mafic plutonic and volcanic and metamorphosed mafic ultramafic parent rocks has found that 58 are polymineralic polymineralic detrital magnetite grains are most common in sands derived from felsic volcanic mafic plutonic and metamorphosed mafic ultramafic parent rocks sands from felsic volcanic parent rocks are characterized by grains with composite or trellis type magnetite ilmenite intergrowths c c t 0 5 whereas sands from marie plutonic and metamorphosed mafic ultramafic parent rocks are characterized by grains with exsolved pleonaste or ulvospinel monomineralic homogeneous grains are characteristic of sands derived from felsic plutonic and intermediate volcanic sources sands from mafic volcanic sources are characterized by a 1 1 relationship between polymineralic grains with trellis or composite type magnetite ilmenite intergrowths c c t 0 5 and monomineralic grains a ternary plot with homogeneous grains grains with trellis or composite type magnetite ilmenite intergrowths and grains with exsolved pleonaste or ulvospinel as corners successfully grouped grains from these parent rocks into fields with minimum overlap electron probe microanalysis of 433 homogeneous grains identified the presence of a chemical fingerprint step wise discriminant function analysis found that tio2 mgo v2o3 and al2o3 could best discriminate between detrital magnetite grains from felsic plutonic and volcanic intermediate volcanic and mafic plutonic parent rocks within this control set detrital magnetite grains were correctly classified 98 of the time however grains from mafic volcanic and metamorphosed mafic ultramafic sources were not chemically distinct emphasizing the importance of integrating petrographic and chemical analyses in provenance research

acoustic emission microstructure and damage model of dry and wet sandstone stressed to failure twenty three uniaxial compression tests were performed on dry and wet flechtingen sandstone from germany compressive strength of wet core is 60 of the strength of dry core before fracture the transverse p wave speed drops by 13 and the pulse amplitude by 22 for wet and 37 for dry cores accumulated strain energy doubles for dry core acoustic emissions ae are detected with 10 sensors for 19 cores ae activity starts at 84 of the fracture strength of wet cores 55 mpa and at 91 of the strength of dry cores 87 mpa the ratio of located to recorded ae is 0 37 for dry and 0 13 for fully wet cores ae hypocenter patterns document the development of two opposite fracture cones the negative slope of cumulative ae amplitude frequency distribution drops by 50 before failure in dry cores the slope of the wet core drops and recovers energy discrimination of ae detected by a broadband sensor resolves different stages of damage and captures the onset of the dilatant throughgoing macrofracture using the analogy to visible light microfracturing events are separated into high energy short pulses blue ae and low energy pulses with long duration times red ae blue ae are explained by intragranular grain breakage red ae by multiple stick slip on crack planes or grain boundaries deformed cores show highly fractured calcite cement and mostly intact quartz grains the stochastic damage model for brittle composites developed highlights that microfracturing of the sandstone is controlled by the amount and distribution of the weak mineral calcite

reequilibration processes in magnetite from iron skarn deposits textural and compositional data for magnetite from nine iron skarn deposits in canada romania and china show that most samples have reequilibrated by dissolution and reprecipitation oxy exsolution and or recrystallization the dissolution and reprecipitation processes are most extensive and are present in most magnetite samples examined whereas the oxy exsolution occurs only in high ti magnetite forming exsolution lamellae of fe ti al oxides electron microprobe analysis indicates that the reequilibration processes have significantly modified the minor and trace element compositions of magnetite notably si mg ca al mn and ti whereas oxy exsolution is effective in decreasing the ti content of high ti magnetite many analyses of magnetite grains from the skarn deposits plot variably in the banded iron formations bif iron oxide copper gold iocg or porphyry cu fields using the ti v versus ca al mn discrimination diagram this pattern suggests that trace element data for magnetite that has unusual composition and or reequilibrated cannot be reliably used as a petrogenetic indicator mixing of externally derived saline fluids with fe rich magmatic hydrothermal solutions an increase in temperature and local decreasing pressure and f o 2 are considered the most important causes for the dissolution and reprecipitation or recrystallization of the magnetite increasing f o 2 and decreasing temperature may facilitate oxy exsolution of fe ti al oxides in high ti magnetite results presented here highlight the importance of detailed textural characterization prior to in situ chemical analysis of magnetite grains so that mineral compositions can be properly evaluated in terms of the genesis and evolution of iron skarn deposits

pressure temperature time paths of regional metamorphism i heat transfer during the evolution of regions of thickened continental crust the development of regional metamorphism in areas of thickened continental crust is investigated in terms of the major controls on regional scale thermal regimes these are the total radiogenic heat supply within the thickened crust the supply of heat from the mantle the thermal conductivity of the medium and the length and time scales of erosion of the continental crust the orogenic episode is regarded as consisting of a relatively rapid phase of crustal thickening during which little temperature change occurs in individual rocks followed by a lengthier phase of erosion at the end of which the crust is at its original thickness the principal features of pressure temperature time ptt paths followed by rocks in this environment are a period of thermal relaxation during which the temperature rises towards the higher geotherm that would be supported by the thickened crust followed by a period of cooling as the rock approaches the cold land surface the temperature increase that occurs is governed by the degree of thickening of the crust its conductivity and the time that elapses before the rock is exhumed sufficiently to be affected by the proximity of the cold upper boundary for much of the parameter range considered the heating phase encompasses a considerable portion of the exhumation decompression part of the ptt path in addition to the detailed calculation of ptt paths we present an idealized model of the thickening and exhumation process which may be used to make simple calculations of the amount of heating to be expected during a given thickening and exhumation episode and of the depth at which a rock will start to cool on its ascent path an important feature of these ptt paths is that most of them lie within 50 c of the maximum temperature attained for one third or more of the total duration of their burial and uplift and for a geologically plausible range of erosion rates the rocks do not begin to cool until they have completed 20 to 40 per cent of the total uplift they experience considerable melting of the continental crust is a likely consequence of thickening of crust with an average continental geotherm a companion paper discusses these results in the context of attempts to use metamorphic petrology data to give information on tectonic processes 1984 oxford university press

in situ u pb zircon dating using laser ablation multi ion counting icp ms high resolution in situ u pb zircon geochronology on zoned grains can obtain isotope signatures from multiple growth or thermal events we present a method using laser ablation multicollector inductively coupled plasma mass spectrometry la mc icp ms to overcome complications associated with intricately zoned zircon crystals through in situ sampling of zircon volumes as small as 12 m 25 m and 40 m in diameter by about 10 m in depth high precision u pb age of a series of zircon standard covering a wide age range of 30 to 1 065 ma was acquired using la mc icp ms the precision of measured pb u ratios in homogeneous zircon is about 2 2 resulting in routinely achieved precision of u pb ages obtained by external calibration of 1 2 or better all masses of interest can be simultaneously recorded with a multi ion counting system mic operating in static mode and the short ablation required to achieve such precision results in spatial resolution that is superior to comparable u pb zircon analyses by single collector icp ms the resulting present u pb age for five zircon reference samples and two geological samples show an excellent agreement with the previously reported id tims or shrimp data

micro textures and chemical compositions of metamorphic magnetite and ilmenite insights from the mianhuadi mafic complex in sw china abstract the compositions of titano magnetite in magmatic and hydrothermal ore deposits have shown the potential to discriminate mineral deposit types however primary fe ti oxides were sometimes overprinted by metamorphic event s over the long geologic history to what extent the fe ti oxide aggregates in mafic rocks can be modified either by re equilibration or interaction with the silicates during high grade metamorphism has not been systematically studied so far the mianhuadi mafic complex intruded at 260 ma as a layered intrusion in the emeishan large igneous province sw china has undergone granulite facies metamorphism in eocene during the development of the ailao shan red river shear zone the primary igneous titanomagnetite underwent oxyexsolution and reacted with the silicates to form magnetite ilmenite and pleonaste resulting in an increased volume proportion of ilmenite and pleonaste a retrograde chlorite carbonate assemblage was found to replace the magnetite whereas alo oh sometimes replaced pleonaste redistribution of elements associated with the oxyexsolution occurred that trace elements with larger partition coefficients in magnetite e g al ga v and cr than in ilmenite are more enriched in magnetite after metamorphism and vice versa iron ti oxide equilibrium calculations indicate that the increased oxygen fugacity is mainly responsible for the oxyexsolution of ilmenite and therefore the geochemical variation of magnetite metamorphic magnetite from the mianhuadi mafic complex can be clearly distinguished from its magmatic precursor by high ga zr and v ti ratios we further propose that v versus fe ti diagram can be used to discriminate between magmatic metamorphic and metasomatised titano magnetite

corundum cr muscovite rocks at o briens zimbabwe the conjunction of hydrothermal desilicification and lil element enrichment geochemical and isotopic evidence monomineralic domains of chlorite corundum and cr muscovite coexist over a kilometer scale within ultramafic schists of the harare greenstone belt 2 73 ga this exotic lithological association includes the conjunction of some of the most aluminous al2o3 88 wt and potassic k2o 10 wt rocks known the paragenetic sequence developed from chlorite corundum corundum diaspore cr muscovite variably overprinted both the corundum and chloritite domains terminal stages were marked by sporadic production of andalusite quartz and finally margarite

percolation conditions in fractured hard rocks a numerical approach using the three dimensional binary fractal fracture network 3d bffn model 1 we numerically investigate fracture connectivity and percolation conditions in fractured hard rocks using a three dimensional binary fractal fracture network 3d bffn model based on three fractal geometric parameters the fractal dimensions d2 of the spatial distribution of fractures the exponent of the power law cumulative fracture length distribution a and the maximum fracture length lmax normalized by the domain length l lmax l numerical results clarify that the percolation threshold in 3d bffn models is strongly controlled by fractal geometric parameters and is independent of any anisotropy in the orientations in addition when a 0 55 are percolated domains the zone of percolation within seismogenic fracture networks between mi and ki reveals that the networks formed from seismic swarm related seismogenic fractures over a 7 week period related to the intrusion of a dyke inferred previously from seismicity and deformation data

the sanriku oki japan earthquake of december 28 1994 mw 7 7 rupture of a different asperity from a previous earthquake fault geometry depth and slip distribution of the sanriku oki earthquake of december 28 1994 ms 7 5 are estimated from seismic waveforms geodetic measurements and tsunami waveforms and compared with those of the 1968 tokachi oki earthquake mw 8 2 the most recent large earthquake in the epicentral region seismic wave inversions indicate a shallowly dipping thrust type mechanism and the focal depth of 22 28 km representing an underthrust event at the subduction interface the source time function has an initial stage of 20 s followed by a main pulse of 30 s duration a shape similar to that of the 1968 tokachi oki earthquake joint inversion of geodetic and tsunami data shows that the region of the largest slip 1 7 m corresponds to a region of relatively small slip for the 1968 tokachi oki earthquake furthermore the joint inversion shows that the region of the largest asperity of the 1968 earthquake had essentially zero slip during the 1994 sanriku oki earthquake these results indicate that the dominant asperities ruptured by the 1994 sanriku and 1968 tokachi oki earthquakes are different the result of the joint inversion also shows that the aseismic zone near the japan trench was not ruptured by the 1994 earthquake