seismic evidence for reactivation of a buried hydrated fault in the pacific slab by the 2011 m9 0 tohoku earthquake 1 we employ seismic tomography to estimate detailed 3d seismic velocity structures in the focal area of an intraslab earthquake m7 1 which occurred on april 7 1 month after the 2011 tohoku earthquake m9 0 near its source area the results show a low velocity zone around the focal area of the m7 1 event and that the aftershock activity is limited to the upper 15 km of the oceanic mantle the lateral extent of the low velocity zone is comparable to the distribution of aftershocks suggesting a concentration of fluids in the aftershock area the angle between the aftershock alignment and the dip of the slab surface is estimated to be 60 which is consistent with the dip of an oceanward dipping normal fault observed at the outer trench slope these observations suggest that the m7 1 intraslab event occurred as a result of reactivation of a buried hydrated fault that formed prior to subduction the upper 15 km of the oceanic mantle may be locally hydrated by bending related tensional faulting at the outer trench slope

spinels and mg ilmenites from the noril sk 1 and talnakh intrusions and other mafic rocks of the siberian flood basalt province spinels from the mineralized noril sk 1 and talnakh intrusions show a broad and distinctive compositional spectrum from normal chromite to ti enriched aluminous magnetite in the talnakh intrusion in particular the range of spinel compositions extends across a compositional space where spinels are rare in other layered intrusions spinel compositions vary widely on a scale of individual thin sections and are influenced by the nature of the enclosing silicate phase spinels enclosed in olivines are less ti enriched than those outside olivine grains compositional variance in the spinels is attributed to variable but generally advanced reaction between cumulus chromite grains and solidifying trapped intercumulus liquid the advanced nature of the process is attributed to the open nature of the intrusions continuous prolonged flow through of hot magma enabled cumulates within the intrusions to remain above their solidus temperature for longer than would be normal for sills of comparable thickness the barren chibacheka river intrusion shows some of the same features but differs from talnakh in that spinel olivine pairs record equilibration temperatures higher than those at talnakh temperatures recorded by spinel olivine pairs at talnakh are anomalously low for such a small intrusion again indicating prolonged slow cooling consistent with a flow through model for the sill spinel compositions support a hypothesized comagmatic relationship between the chibacheka river intrusion and the ti cr rich gudchichinsky picritic basalts spinels in the noril sk and talnakh intrusions imply a parent magma with a lower cr al ratio than the gudchichinsky picritic basalts spinels from the siberian intrusions show a similar range in compositions to the feeder intrusions of the karoo flood basalt province jinchuan spinels share the features of high ti contents and anomalously low equilibration temperatures with the noril sk talnakh spinels mg ilmenites are a feature of the siberian and jinchuan intrusions they are attributed to crystallization of ilmenite from evolved trapped intercumulus liquid buffered by surrounding mg rich cumulus phases their existence implies moderately but not unusually ti rich parent magmas consistent with compositions of the siberian traps lavas

compositional variations in the mesoarchean chromites of the nuggihalli schist belt western dharwar craton india potential parental melts and implications for tectonic setting the chromite deposits in the archean nuggihalli schist belt are part of a layered ultramafic mafic sequence within the western dharwar craton of the indian shield the 3 1 ga ultramafic mafic units occur as sill like intrusions within the volcano sedimentary sequences of the nuggihalli greenstone belt that are surrounded by the tonalite trondhjemite granodiorite ttg suite of rocks the entire succession is exposed in the tagdur mining district the succession has been divided into the lower and the upper ultramafic units separated by a middle gabbro unit the ultramafic units comprise of deformed massive chromitite bodies that are hosted within chromite bearing serpentinites the chromitite bodies occur in the form of pods and elongated lenses 60 500 m by 15 m detailed electron microprobe studies reveal intense compositional variability of the chromite grains in silicate rich chromitite 50 modal chromite and serpentinite 2 modal chromite throughout the entire ultramafic sequence however the primary composition of chromite is preserved in the massive chromitites 60 75 modal chromite from the byrapur and the bhaktarhalli mining district of the nuggihalli schist belt these are characterized by high cr ratios cr cr al 0 78 0 86 and moderate mg ratios mg mg fe2 0 38 0 58 the compositional variability occurs due to sub solidus re equilibration in the accessory chromite in the serpentinite mg ratio 0 01 0 38 cr ratio 0 02 0 99 and in silicate rich chromitite mg ratio 0 06 0 48 cr ratio 0 60 0 99 in the massive chromitites the sub solidus re equilibration for chromite is less or absent however the re equilibration is prominent in the co existing interstitial and included olivine fo96 98 and pyroxene grains mg numbers 97 99 compositional variability on the scale of a single chromite grain occurs in the form of zoning and it is common in the accessory chromite grains in serpentinite and in the altered grains in chromitite in the zoned grains the composition of the core is modified and the rim is ferritchromit in general ferritchromit occurs as irregular patches along the grain boundaries and fractures of the zoned grains in this case ferritchromit formation is not very extensive this indicates a secondary low temperature hydrothermal origin of ferritchromit during serpentinization in some occurrences the ferritchromit rim is very well developed and only a small relict core appears to remain in the chromite grain however complete alteration of the chromite grains to ferritchromit without any remnant core is also present the regular well developed and continuous occurrence of ferritchromit rims around the chromite grain boundaries the complete alteration of the chromite grains and the modification of the core composition indicate the alteration in the nuggihalli schist belt to be intense pervasive and affected by later low grade metamorphism the primary composition of chromite has been used to compute the nature of the parental melt the parental melt calculations indicate derivation from a high mg komatiitic basalt that is similar to the composition of the komatiitic rocks reported from the greenstone sequences of the western dharwar craton tectonic discrimination diagrams using the primary composition of chromites indicate a supra subduction zone setting ssz for the archean chromitites of nuggihalli and derivation from a boninitic magma the composition of the komatiitic basalts resembles those of boninites that occur in subduction zones and back arc rift settings formation of the massive chromitites in nuggihalli may be due to magma mixing process involving hydrous high mg magmas or may be related to intrusions of chromite crystal laden magma however there is little scope to test these models because the host rocks are highly altered serpentinized and deformed the present configurations of the chromitite bodies are related to the multistage deformation processes that are common in archean greenstone belts

archean komatiitic sill hosted chromite deposits in the zimbabwe craton lenticular chromite deposits of widely varying age metamorphic grade and strain rate as well as size shape and ore quality are widespread in greenschist to amphibolite grade greenstonegneiss terrain in the south central part of the zimbabwe craton and in granulite grade equivalents in the northern marginal zone of the limpopo belt in the far south in order of age and very approximately decreasing abundance they form three groups 1 early archean 3 5 3 2 ga deposits associated with amphibolite grade greenstone remnants in the 3 6 to 3 35 ga tokwe gneisses e g hornet valley and on the amphibolite grade eastern flank of the shurugwi greenstone belt and granulite grade greenstone remnants in the northern marginal zone e g inyala rhonda 2 mid archean deposits within the ca 3 0 ga shurugwi group in the main greenschist part of the shurugwi belt e g peak railway block and associated with the ca 3 0 ga belingwean supergroup in several other belts and 3 late archean ca 2 74 ga deposits mostly in the western segment of the masvingo belt e g prince with their host and country rocks the deposits together preserve geologic and chromite compositional evidence of formation within poorly differentiated komatiitic sills intruded into predominantly siliciclastic bif platformal sedimentary sequences associated with mafic and or ultramafic lavas unlike the thin laterally extensive chromitite layers associated with macrocyclic units of large layered intrusions the komatiitic sill associated deposits are the remnants of laterally discontinuous up to several tens of meters thick chromitite layers or dunite chromitite zones located at high stratigraphic levels of their host intrusions not far below the upper pyroxenite differentiates in at least some deposits for example at railway block where the chromitites form narrow elongate bodies at least 1 000 m long the lateral discontinuities are most likely primary in many others later deformation obscures the primary geometry chromitite formation may have been delayed by the late attainment of equilibrium crystallization and the reduced nature of the magma the crystallization of thick massive cyclically layered cr rich chromitites was most probably controlled by the stabilization of chromite relative to olivine by absorption of connate water derived from wet host sediments and by episodic flow through processes which at railway block may have been focused along narrow pathways across the magma chamber floor the remarkable abundance of such chromitites and their formation in repeated episodes over 1 000 m y cf other cratons can be attributed to generation of komatiitic magmas in a mantle source that was both cr rich and strongly reduced archean komatiitic sill hosted deposits constitute a third type of chromite deposit in addition to stratiform and podiform deposits in layered intrusions and ophiolites respectively the economically important variability of bulk ore compositions especially cr fe al mg is due to several factors magnetite introduction during serpentinization and talc carbonate alteration at greenschist grade subsolidus fluid related and other metamorphic cation exchange processes dependant on chromite body size and possibly exsolution of cr al and magnetite phase pairs both at midamphibolite to granulite grade as well as fluid related retrogression and partial conversion of chromite to mainly chlorite during amphibolite metamorphism

stability of ilmenite and titanomagnetite in the presence of carbon dioxide a thermodynamic evaluation the stability of ilmenite and titanomagnetite in the presence of co2 gas was evaluated thermodynamically ilmenite with a composition ilm90hm10 breaks down to the assemblage siderite hematite anatase below 270 c at 1 000 bars and below 300 c at 2 000 bars titanomagnetite with a composition usp50mt50 breaks down to the same assemblage below 345 c at 1 000 bars and below 385 c at 2 000 bars during the low grade metamorphism of basaltic rocks a co2 partial pressure of only a few hundreds bars is sufficient to alter ilmenite and titanomagnetite to the assemblage siderite hematite anatase

the dynamic rupture process of the 1993 kushiro oki earthquake we have constructed a dynamic rapture model of the 1993 kushiro oki earthquake mw 7 6 that is consistent with the observed seismograms this earthquake occurred beneath the city of kushiro japan at a depth of about 100 km and the aftershocks are limited to a horizontal plane between a double seismic zone in the subducting slab excellent near field strong motion records are available in our analysis a kinematic inversion is followed by a dynamic crack calculation and this pair of operations is repeated until convergence to the data is obtained two different conditions concerning the dynamic stress drop were considered in the dynamic calculation and a dynamic model was constructed for each condition the difference in these conditions has little effect on the results the final dynamic model shows that this earthquake occurred with a localized high stress drop of about 40 mpa in the subducting slab which indicates that the region between the double seismic zone is strong enough to sustain such a high stress one of the two high strength regions in the final model lies near the upper plane of the double seismic zone and represents a barrier due to a substantial change of stress state in the slab in the region where the initial breaks occurred 5 s before the main rupture both the strength and the stress drop are smaller than in the adjacent region this implies that before the main rupture the accumulated stress had already been released in the area of the initial breaks the aftershocks tend to occur near the high strength area in some of them the p wave first motions are opposite to those of the main shock

seismotectonics beneath the tokyo metropolitan area japan effect of slab slab contact and overlap on seismicity 1 we first determine the configuration of the upper surface of the pacific pac slab beneath kanto japan from the distribution of interplate earthquakes relocated by an appropriate 1 d velocity model then traveltime tomography is carried out to estimate three dimensional seismic velocity structures around kanto using 735 520 p wave and 444 049 s wave arrival times from 6508 local earthquakes the obtained results suggest that the philippine sea phs slab is subducting to depths of 130 140 km without a gap even to the northwest of the izu collision zone we subsequently define the lateral extent of the contact zone between the bottom of the phs slab and the upper surface of the pac slab phs pac interface and reveal that the slab contact zone underlies a wider area beneath kanto in harmony with the kanto plain the downdip limit of interplate thrust type earthquakes on the pac slab is deepened by 30 km locally under the slab contact zone this deepening is probably caused by a lower temperature environment in the pac slab resulting from the overlap with the phs slab subducting above and consequent thermal shielding by the phs slab from the hot mantle wedge we detect an extremely low velocity anomaly in the easternmost portion of the phs slab which is probably attributable to serpentinization of mantle peridotite interplate earthquakes are almost absent along the phs pac interface overlain by the serpentinized mantle in the phs slab suggesting that ductile deformation takes place along the interface because of low viscosity of the serpentine

reaction relationships of chrome spinels in igneous rocks further evidence from the layered intrusions of rhum and mull inner hebrides scotland compositions of chrome spinels in peridotites from the layered igneous rocks of rhum and of the ben buie intrusion mull show evidence of post depositional reaction relationships with the cumulus olivine and or the intercumulus liquid some seam forming spinels in the ben buie intrusion are highly aluminous more so than those in the rhum intrusion occasional zoned spinels in both intrusions show enrichment in al towards their rims the evidence suggests that the spinel reaction trend involving cr al exchange proceeds in the direction of al enrichment equilibrium between chrome spinel and cumulus olivine appears to have been attained rarely thus limiting the application of the olivine spinel geothermometer

composition and textures of chromite and platinum group minerals in chromitites of the western ophiolitic belt from pampean ranges of cordoba argentina abstract chromitite bodies hosted in the neoproterozoic western ophiolitic belt of pampean ranges of cordoba argentina were studied at los congos and los guanacos ultramafic bodies with regard to the composition and textures of the chromite and platinum group minerals primary chromite composition is only preserved in some massive chromitites from the los guanacos ultramafic body and is similar to al rich ophiolitic chromitites suggesting that they crystallized from melts with back arc basin basalts babb affinity in the suprasubduction mantle subsequently these chromitites underwent a prograde metamorphism chromites from chromitites and associated metamorphosed ultramafic rocks show complex replacement and exsolution textures mineral chemistry and texture indicate that the chromite composition records two main metamorphic trends a first trend defined by chromite from massive chromitite in which there is an enrichment in fe3 and fe2 cr remain relatively constant and slightly depleted in al mg a second trend is defined by chromite from disseminated chromitite and metamorphosed dunite and harzburgite in which a fe rich phase is replacing the al rich chromite this alteration trend is characterized by enrichment in the total iron content fe3 fe2 and a strong depletion in al and mg the chemical composition of all analyzed spinels from los guanacos and los congos as plotted on the ternary fe3 cr al diagram correlates well with the cr spinels from the upper amphibolite to granulite facies metamorphism platinum group minerals pgm identified include native osmium laurite erlichmanite irarsite platinum and a number of inadequately identified phases such as an oxide or hydroxide of ru pt and ir ru pt telluride ir ru as se and ir ru ti compounds native osmium was the only pgm which remained unaltered other pgm underwent mineralogical reworking during metamorphism although it is difficult to establish the extent of platinum group element mobilization based on mineralogical observation our results suggest that the ru os ir pgm in the los guanacos and los congos chromitites were modified in situ producing re distribution of these pge on a small scale the presence of rare pt and pge as se minerals was possibly related to remobilization of pt as and se by fluids during the alteration processes

real time magnitude characterization of large earthquakes using the predominant period derived from 1 hz gps data earthquake early warning eew systems performance is driven by the trade off between the need for a rapid alert and the accuracy of each solution a challenge for many eew systems has been the magnitude saturation for large events mw 7 and the resulting underestimation of seismic moment magnitude in this study we test the performance of high rate 1hz gps based on seven seismic events to evaluate whether long period ground motions can be measured well enough to infer reliably earthquake predominant periods we show that high rate gps data allow the computation of a gps based predominant period g to estimate lower bounds for the magnitude of earthquakes and distinguish between large mw 7 and great mw 8 events and thus extend the capability of eew systems for larger events it is also identified the impact of the different value of the smoothing factor on the g results and how the sampling rate and the computation process differentiates g from the commonly used p

trace element fingerprints of chromite magnetite and sulfides from the 3 1 ga ultramafic mafic rocks of the nuggihalli greenstone belt western dharwar craton india the 3 1 ga nuggihalli greenstone belt in the western dharwar craton is comprised of chromitite bearing sill like ultramafic mafic rocks that are surrounded by metavolcanic schists compositionally komatiitic to komatiitic basalts and a suite of tonalite trondhjemite granodiorite gneissic rocks the sill like plutonic unit consists of a succession of serpentinite after dunite peridotite pyroxenite and gabbro with bands of titaniferous magnetite ore the chromitite ore bodies length 30 500 m width 2 15 m are hosted by the serpentinite peridotite unit unaltered chromites from massive chromitites 80 modal chromite of the byrapur and bhaktarhalli chromite mines in the greenstone belt are characterized by high cr 100cr cr al of 78 86 and moderate mg 100 mg mg fe2 of 45 55 in situ trace element analysis la icpms of unaltered chromites indicates that the parental magma of the chromitite ore bodies was a komatiite lacking nickel sulfide mineralization in the ga fe3 versus ti fe3 diagram the byrapur chromites plot in the field of suprasubduction zone ssz chromites while those from bhaktarhalli lie in the mor field the above results corroborate our previous results based on major element characteristics of the chromites where the calculated parental melt of the byrapur chromites was komatiitic to komatiitic basalt and the bhaktarhalli chromite was derived from archean high mg basalt the major element chromite data hinted at the possibility of a ssz environment existing in the archean altered and compositionally zoned chromite grains in our study show a decrease in ga v co zn mn and enrichments of ni and ti in the ferritchromit rims trace element heterogeneity in the altered chromites is attributed to serpentinization the trace element patterns of magnetite from the massive magnetite bands in the greenstone belt are similar to those from magmatic fe ti v rich magnetite bands in layered intrusions and magnetites from andesitic melts suggesting that magnetite crystallized from an evolved gabbroic melt enrichments of ni co te as and bi in disseminated millerite and niccolite occurring within chromitites and in disseminated bravoite within magnetites reflect element mobility during serpentinization monosulfide solid solution inclusions within pyroxenes altered to actinolite in pyroxenite and interstitial pyrites and chalcopyrites in magnetite retain primary characteristics except for fe enrichment in chalcopyrite probably due to sub solidus re equilibration with magnetite disseminated sulfides are depleted in platinum group elements pge due to late sulfide saturation and the pge depleted nature of the mantle source of the sill like ultramafic mafic plutonic rocks in the nuggihalli greenstone belt

complex unmixed spinels in layered intrusions within an obducted ophiolite in the natal namaqua mobile belt spinellids showing unmixed intergrowths of chromite or chromian spinel sensu stricto and magnetite or chromian magnetite are not known in mafic or ultramafic igneous rocks they do occur within metamorphosed rocks that attained temperatures sufficiently high upper amphibolite facies for the formation of homogeneous al cr fe3 ti spinel phases with compositions not matched in slowly cooled igneous rocks in the tugela rand intrusion complex intergrowths of chromian spinel chromian magnetite ulvospinel ilmenite and a transparent aluminous spinel are observed and interpreted in terms of the thermal history of the rocks compositional differences between the separate areas of chromian spinel and chromian magnetite in complex intergrowths exhibited by the metamorphosed tugela rand and mambulu complexes confirm the extension of the magnetite hercynite solvus turnock and eugster 1962 towards magnesium and chromium rich compositions the tugela rand spinellids are compared with those from the carr boyd complex purvis et al 1972 and the ultramafic rocks of the giant nickel mine muir and naldrett 1973 and the red lodge district loferski and lipin 1983 significant differences between the spinels from the red lodge district compared to the other three occurrences may reflect the different metamorphic histories of these areas

accessory chromium spinels their coexistence and alteration in serpentinites abstract accessory chromium spinel occurring in serpentinites is usually altered the extent of this alteration and its relationship to alteration processes such as serpentinization and metamorphism is not defined complicating the use of altered spinel as a petrogenetic indicator here the relationship between different alteration processes and spinel alteration is evaluated in terms of accessory chromium spinel occurring in serpentinites of progressive metamorphism in the eastern central alps two coexisting cr spinels were identified i e an altered fe rich cr spinel i with cr al and an fe poor cr spinel ii with al cr which exsolved a cr fe 3 phase and which resisted alteration alteration of cr spinel i is evident from decreasing al mg and cr values with a trend leading to ferrite chromite trend a an enrichment in cr trend b related to co 2 in the serpentinizing fluid indicated by the presence of carbonate talc or tremolite is superimposed to a alteration is a decomposition of the spinel structure caused by the dissolution of mg al and cr during serpentinization dissolution of cr is favored in a reduced environment whereas reprecipitation occurs under locally oxidizing conditions indicated by the presence of co 2 sio 2 detected in spinel is a relative measure of the degree of alteration metamorphism up to amphibolite facies recrystallized and homogenized preexisting spinel phases to various degrees whether or not and to what extent a spinel alters depends on its fe content fe 2 stabilized in the octahedral site high cfse is responsible for an inverse and less stable spinel structure fe rich spinels cr spinel i are therefore readily subjected to alteration whereas fe poor spinels cr spinel ii are resistant to alteration during the serpentinization processes

diffusion pathways of fe 2 and fe 3 during the formation of ferrian chromite a µxanes study the alteration of chromian spinel is a key process during serpentinization and metamorphism of ultramafic rocks controlled by oxygen fugacity fo2 and fe2 fe3 exchange during fluid rock interaction chromian spinel alteration is better recorded in less permeable chromitite than in peridotites where extensive fluid rock interaction frequently overprints the record of earlier stages of alteration to shed light on that process we have studied the distribution of fe2 and fe3 in variably altered chromian spinel grains from a set of chromitite samples from the same mining district using synchrotron based microscopic chemical imaging and spatially resolved x ray absorption near edge structure spectroscopy our results show that early stages of alteration do not involve changes in cr3 and fe2 contents or in fe speciation but only depletion in al3 and mg2 resulting in the formation of porous chromite with ongoing alteration fe3 migrates into porous chromite mainly along fracture walls and fracture zones as well as along grain boundaries sheared type chromitites record the maximum rates of fluid rock interaction because in these chromitite types the accommodation of deformation on porous chromite allowed higher rates of diffusion of fe3 and fe2 a magnetite component with fe3 fetotal 0 66 into the newly formed neoblasts in porous chromite type texture all the original chromite grains fully transformed to porous chromite the deformation and accompanying diffusion processes result in the formation of homogenous ferrian chromite grains in contrast in partly altered type texture chromite grains with original cores surrounded by porous chromite such processes are only restricted to the porous rims giving rise to zoned chromian spinel ferrian chromite grains

double seismic zone and dehydration embrittlement of the subducting slab 1 dehydration embrittlement of metamorphosed oceanic crust and mantle in the subducting slab may be responsible for the occurrence of intermediate depth earthquakes we explore the possibility that this hypothesis can explain the morphology of the double seismic zones observed in northeast japan southwest japan northeast taiwan northern chile cape mendocino and eastern aleutians we calculate transient temperature structures of slabs based on geologically estimated subduction histories of these regions we then determine dehydration loci of metamorphosed oceanic crust and serpentinized mantle using experimentally derived phase diagrams the depth range of the dehydration loci of metamorphosed oceanic crust and serpentine is dependent on slab age the dehydration loci of serpentine produce a double layered structure because the upper dehydration loci of serpentine are mostly located in the wedge mantle above the slab we regard the upper plane seismicity representing dehydration embrittlement in the oceanic crust and we fix the slab geometry so that the upper plane seismicity is just below the upper surface of the slab we find that the lower plane seismicity is located at the lower dehydration loci of serpentine which indicates that the morphology of the double seismic zones is consistent with the dehydration embrittlement

tomographic imaging of hydrated crust and mantle in the subducting pacific slab beneath hokkaido japan evidence for dehydration embrittlement as a cause of intraslab earthquakes abstract we estimate detailed three dimensional seismic velocity structures in the subducting pacific slab beneath hokkaido japan using a large number of arrival time data from 6902 local earthquakes a remarkable low velocity layer with a thickness of 10 km is imaged at the uppermost part of the slab and is interpreted as hydrated oceanic crust the layer gradually disappears at depths of 70 80 km suggesting the breakdown of hydrous minerals there we find prominent low velocity anomalies along the lower plane of the double seismic zone and above the aftershock area of the 1993 kushiro oki earthquake m7 8 since seismic velocities of unmetamorphosed peridotite are much higher than the observations hydrous minerals are expected to exist in the lower plane as well as the hypocentral area of the 1993 earthquake on the other hand regions between the upper and lower planes where seismic activity is not so high compared to the both planes show relatively high velocities comparable to those of unmetamorphosed peridotite our observations suggest that intermediate depth earthquakes occur mainly in regions with hydrous minerals which support dehydration embrittlement hypothesis as a cause of earthquake in the subducting slab

a great earthquake doublet and seismic stress transfer cycle in the central kuril islands temporal variations of the frictional resistance on subduction zone plate boundary faults associated with the stick slip cycle of large interplate earthquakes are thought to modulate the stress regime and earthquake activity within the subducting oceanic plate here we report on two great earthquakes that occurred near the kuril islands which shed light on this process and demonstrate the enhanced seismic hazard accompanying triggered faulting on 15 november 2006 an event of moment magnitude 8 3 ruptured the shallow dipping plate boundary along which the pacific plate descends beneath the central kuril arc the thrust ruptured a seismic gap that previously had uncertain seismogenic potential although the earlier occurrence of outer rise compressional events had suggested the presence of frictional resistance within minutes of this large underthrusting event intraplate extensional earthquakes commenced in the outer rise region seaward of the kuril trench and on 13 january 2007 an event of moment magnitude 8 1 ruptured a normal fault extending through the upper portion of the pacific plate producing one of the largest recorded shallow extensional earthquakes this energetic earthquake sequence demonstrates the stress transfer process within the subducting lithosphere and the distinct rupture characteristics of these great earthquakes illuminate differences in seismogenic properties and seismic hazard of such interplate and intraplate faults

chromite in komatiites ii modification during greenschist to mid amphibolite facies metamorphism chromite compositions in komatiites are influenced by metamorphic chromite is highly susceptible to modification during processes particularly above 500 c metamorphosed chromite is early hydrous alteration and subsequent prograde metasubstantially more iron rich than igneous precursors as a result of morphism of host rocks this modification is the subject mg fe exchange with silicates and carbonates chromite metaof this paper morphosed to amphibolite facies is enriched in zn and fe and metamorphic modification of chromite has been disdepleted in ni relative to lower metamorphic grades relative cussed extensively in the literature in the context mainly of proportions of the trivalent ions cr al and fe are not ophiolitic or alpine ultramafic complexes onyeagocha greatly modified by metamorphism up to lower amphibolite facies 1974 ulmer 1974 evans frost 1975 hoffman although minor fe depletion occurs during talc carbonate alwalker 1978 loferski lipin 1983 kimball 1990 teration at low temperature significant al is lost from chromite burkhard 1993 and in a few studies of komatiitic rocks cores above 550 c as a result of equilibration with fluids in bliss maclean 1975 donaldson 1983 gole hill equilibrium with chlorite elevated zn content in chromite is 1990 and in a detailed study of the pechenga intrusions restricted to rocks with low metamorphic mg fe ratios and is abzalov 1998 these studies have highlighted two the result of introduction of zn during low temperature alteration important effects first chromites become rimmed and with further concentration and homogenization during prograde progressively replaced by chromian magnetite or fermetamorphism cobalt and mn also behave similarly except where ritchromit second chromite core compositions become carbonate minerals are predominant in the metamorphic assemblage progressively modified during prograde metamorphism chromite at amphibolite facies is typically extensively replaced by as a result of exchange of components with surrounding magnetite this is the result of incomplete metamorphic reaction silicate minerals evans frost 1975 abzalov 1998 between chromite and chlorite bearing silicate assemblages magthis paper examines the nature and magnitude of these netite compositions at the inner chromite magnetite boundary are effects in komatiites from a variety of localities in the indicators of metamorphic grade eastern goldfields province of the archaean yilgarn block in western australia fig 1 brief locality descriptions are given in appendix b

chromite from the swartkop chrome mine an estimate of the effects of subsolidus reequilibration chromites from the lg 3 lg 4 and lg 6 chromitite layers at the zwartkop mine are investigated and subsolidus behavior of chromite in olivine and orthopyroxene hosted environments is compared chromitite layers lg 3 and lg 6 are orthopyroxene hosted but layer lg 4 is olivine hosted with abundant orthopyroxene formed by a reaction relation between olivine and liquid chromite has reequilibrated with host silicate to a degree dependent on the nature of the host silicate and the modal amounts of chromite present in an olivine environment reequilibration between chromite and silicate proceeds to lower temperatures with consequently more iron rich chromite than in an orthopyroxene hosted chromite in a given environment the closure temperature for silicate chromite reequilibration decreases systematically as the modal amount of chromite decreases from this systematic relation the solidus temperature can be estimated and some inferences about the original chromite composition can be made in the lg 6 layer chromite compositions can be projected back to solidus compositions along a trajectory of constant activity of the mga1204 component and at the solidus temperature chromite composition within and adjacent to the chromitite layer is essentially the same the activity of mga1204 remained constant because chromite was in equilibrium with the host orthopyroxene throughout the temperature range in which reequilibration took place in layer lg 4 the activity of mga1204 increased during cooling because the solidus chromite was not in equilibrium with orthopyroxene formed by reaction between olivine and liquid because the liquid which reacted with olivine was more aluminous than the liquid from which the olivine originally crystallized subsolidus reequilibration between orthopyroxene and chromite led to an increase in the concentration of alcoa in chromite consequently the decrease in mg content of chromite during cooling is accompanied by an increase in a1 content in contrast to the more common decrease in a1 content observed in layer lg 6 which results from the higher stability of

elongated prismatic magnetite crystals in alh84001 carbonate globules potential martian magnetofossils using transmission electron microscopy tem we have analyzed magnetite fe 3o 4 crystals acid extracted from carbonate globules in martian meteorite alh84001 we studied 594 magnetites from alh84001 and grouped them into three populations on the basis of morphology 389 were irregularly shaped 164 were elongated prisms and 41 were whisker like as a possible terrestrial analog for the alh84001 elongated prisms we compared these magnetites with those produced by the terrestrial magnetotactic bacteria strain mv 1 by tem again we examined 206 magnetites recovered from strain mv 1 cells natural darwinian selection in terrestrial magnetotactic bacteria appears to have resulted in the formation of intracellular magnetite crystals having the physical and chemical properties that optimize their magnetic moment in this study we describe six properties of magnetite produced by biologically controlled mechanisms e g magnetotactic bacteria properties that collectively are not observed in any known population of inorganic magnetites these criteria can be used to distinguish one of the modes of origin for magnetites from samples with complex or unknown histories of the alh84001 magnetites that we have examined the elongated prismatic magnetite particles 27 of the total are indistinguishable from the mv 1 magnetites in five of these six characteristics observed for biogenically controlled mineralization of magnetite crystals

compositional effects on the solubility of minor and trace elements in oxide spinel minerals insights from crystal crystal partition coefficients in chromite exsolution chromite from los congos and los guanacos in the eastern pampean ranges of cordoba argentinian central andes shows homogenous and exsolution textures the composition of the exsolved phases in chromite approaches the end members of spinel mgal 2 o 4 spl and magnetite fe 2 fe 2 3 o 4 mag that define the corners of the spinel prism at relatively constant cr 3 r 3 ratio where r 3 is cr al fe 3 the exsolution of these phases from the original chromite is estimated to have accounted at 600 c on the basis of the major element compositions of chromite with homogenous and exsolution textures that are in equilibrium with forsterite rich olivine fo 95 the relatively large size of the exsolved phases in chromite up to ca 200 m provided for the first time the ability to conduct in situ analysis with laser ablation inductively coupled plasma mass spectrometry for a suite of minor and trace elements to constrain their crystal crystal partition coefficient between the spinel rich and magnetite rich phases d i spl mag minor and trace elements listed in increasing order of compatibility with the spinel rich phase are ti sc ni v ge mn cu sn co ga and zn d i spl mag values span more than an order of magnitude from d ti spl mag 0 30 0 06 to d zn spl mag 5 48 0 63 our results are in remarkable agreement with data available for exsolutions of spinel rich and magnetite rich phases in other chromite from nature despite their different cr 3 r 3 ratio the estimated crystal crystal partitioning coefficients reflect the effect that crystal chemistry of the exsolved phases from chromite imposes on all investigated elements excepting cu and sc and only slightly for mn the observed preferential partitioning of ti and sc into the magnetite rich phase is consistent with high temperature chromite melt experiments and suggests a significant dependence on fe 3 substitution in the spinel structure a compositional effect of major elements on ga co and zn is observed in the exsolved phases from chromite but not in the experiments this might be due to crystal chemistry differences along the mgfe 1 al 2 fe 2 3 exchange vector which is poorly covered experimentally this inference is supported by the strong covariance of ga co and zn observed only in chromite from layered intrusions where this exchange vector is important a systematic increase of zn and co coupled with a net decrease in ga during hydrous metamorphism of chromitite bodies cannot be explained exclusively by compositional changes of major elements in the chromite which are enriched in the magnetite component the most likely explanation is that the contents of minor and trace elements in chromite from metamorphosed chromitites are controlled by interactions with metamorphic fluids involved in the formation of chlorite

chrome spinels in gabbro wehrlite intrusions of the pechenga area kola peninsula russia emphasis on alteration features abstract chrome spinels are common accessory minerals of the pechenga ultramafic rocks in nw russia varying in metamorphic grade from sub greenschist to amphibolite facies magmatic chrome spinels can be grouped into two main types spinel 1 which crystallized first is represented by al ti chromite it occurs mainly as inclusions in olivine and rather less commonly enclosed within interstitial minerals spinel 2 crystallized after spinel 1 and has composition varying from ti rich chromite to ti rich chrome magnetite spinel 2 occurs in the interstitial spaces between olivine crystals and enclosed within intercumulus silicates zoned crystals with spinel 1 in the core overgrown by spinel 2 are also common cr poor titanomagnetite occurring in pyroxenites and gabbro apparently represents further evolution of spinel 2 phases metamorphic chrome spinels are represented by ferritchromit and cr magnetite which are the dominant chrome spinel phases in the amphibolite facies rocks the continuous trend of the magmatic chrome spinels composition is indicative of their intensive post cumulus alteration by reaction with evolving intercumulus liquid this reaction led to depletion of the earlier crystallized chrome spinels in mg cr and al and their enrichment in ti and fe 3 oxidation exsolution of ilmenite from ti rich chrome spinels is another common process of post crystallization alterations serpentinization of olivine led to formation of magnetite which overgrew and cut chrome spinel grains low temperature hydrothermal processes in particular talc carbonate alterations could cause a local loss of ti and further redistribution of mg mn and zn prograde metamorphic reactions led to formation of ferritchromit and magnetite replacing primary chrome spinels another common process affecting ti rich chrome spinels is a prograde metamorphic recrystallization of exsolved ilmenite which changes from submicron lamellae through a regular grid of coarse ilmenite crystals and further to their irregular blebs and rims metamorphism of epidote amphibolite and amphibolite facies has led to the liberation of most of the elements metamorphically liberated cr was captured by antigorite

earthquakes and plastic deformation of anhydrous slab mantle in double wadati benioff zones double wadati benioff seismic zones dsz with two parallel planes of seismicity separated by 15 30 km are a global feature of subduction zones in the 50 200 km depth range upper plane seismicity is generally attributed to dehydration of the oceanic crust but the origin of the lower seismicity plane is debated serpentine or hydrous phase dehydration embrittlement is a commonly advocated mechanism that implies significant slab mantle hydration high resolution seismic tomography revealed low seismic velocities in the lower seismicity plane that are better explained by seismic anisotropy of anhydrous deformed peridotites than by serpentinization earthquakes correlate with anisotropic planar shear zones and favor a shear instability mechanism as the cause of lower plane seismicity without requiring the presence of water in the center of subducting slabs the contribution of the subducted lithospheric mantle to the water budget of subduction zones is thus likely limited to the first 2 3 kilometers beneath oceanic crust

insight into complex rupturing of the immature bending normal fault in the outer slope of the japan trench from aftershocks of the 2005 sanriku earthquake mw 7 0 located by ocean bottom seismometry 1 the distribution of aftershocks of a large mw 7 0 normal faulting earthquake beneath the outer slope of the japan trench in 2005 measured in 2007 using ocean bottom seismographs indicates that the earthquake was involved with a set of conjugate normal faults although the faults reach to the upper mantle the estimated vp and vp vs show no remarkable changes that can be related to extensive hydration in the crust or uppermost mantle the absence of horst graben topographic structure in the rupture area suggests that immaturity of the bending fault system is responsible for the relatively unhydrated lithosphere several earthquakes below the aftershock zone may belong to the lower plane seismicity of the shallow double seismic zone because no earthquakes were recorded in the area for more than 80 years before the 2005 event shallow extensional and deep compressive earthquakes may be activated concurrently in the focal area of this earthquake

the petrology of some picrites from mauna loa and kilauea volcanoes hawaii the mineralogy and chemistry of picrites from mauna loa and kilauea have been investigated to evaluate for hawaiian tholeiitic picrites the contrasting genetic models which have been proposed for these mg rich volcanics namely products of direct crystallization of high mg melts 20 25 mgo or the result of accumulation of olivine phenocrysts into less mg rich melts genetic interpretations rely heavily on mg fe partitioning relations between olivines and their picrite hosts although the 100 mg mg fe2 ratios m of picrites are wide ranging m 73 6 82 9 for fe2o3 feo 0 15 with mgo as high as 27 8 the average 100 mg mg fe ratios mg of the cores of olivine phenocrysts megacrysts show only restricted compositional variation mg 87 2 89 0 successive olivine generations are progressively more fe rich olivine liquid mg fe partitioning data and the mn and ni abundances in olivine phenocrysts collectively indicate that they were precipitated by mg rich basaltic melts with 12 14 mgo spinel compositions range from liquidus magnesiochromites occurring mainly as inclusions in olivine phenocrysts to groundmass titanomagnetites which crystallized at nearsolidus temperatures the cr2o3 contents and m values of liquidus magnesiochromites suggest that their parent melts were neither mg rich picritic mgo 20 nor relatively mg poor basaltic types