mode of vertical crustal movements during the late quaternary in kyushu japan deduced from heights of ancient shorelines abstract data from the ancient shorelines along the present day coastline of kyushu island western japan show that the island can be divided into several regions on the basis of modes of vertical crustal movements over the past 125 000 years the heights of the ancient shorelines have been determined using paleodepth indicators such as molluscan faunas trace fossils and sedimentary structures the chronology of the sediments and of the tectonic events has been deduced from the well documented history of volcanic ash events tephrostratigraphy of southern japan the difference between the most uplifted and subsided areas the island during the past 125 000 years is about 190 m although it was previously thought that the whole of kyushu island was a region undergoing uplift throughout this time the present work shows that there were marked differential movements north and southwest kyushu were steadily subsiding areas but rapid uplift occurred locally in the southern part of the island a subsiding axis the saeki sendai subsiding axis crosses the island with a ne sw trend and an e w trending structural depression the beppu shimabara graben is present in central kyushu there is a clear correlation between the present day coastal landform of the island and the differential tectonic movements that have occurred during the past 125 000 years

fast and slow spreading ridges structure and hydrothermal activity ultramafic topographic highs and ch4 output different parts of the world ridge system have quite different morphologies which reflect different constructional processes it appears that hydrothermal circulation at all spreading centers is an important exchange process between the ocean and the newly formed oceanic crust this hydrothermal circulation may vary according to morphology and crustal composition and may also affect ridge constructional processes the tag 26 n and snake pit 23 n hydrothermal sites on the mid atlantic ridge mar display mn ch4 ratios in overlying seawater similar to those of nonsedimented east pacific rise sites i e about 0 2 mol l in contrast large methane anomalies with very low mn ch4 ratios of 0 005 mol l are associated with ultramafic topographic highs near 15 n close to the axis and on ultramafic walls of the rift valley the association of ultramafic bodies and ch4 anomalies in seawater indicates active serpentinization processes ch4 is produced during serpentinization according to the fisher tropsch reaction charlou et al 1991 many ultramafic rock assemblages have been sampled on the mar away from fracture zones in particular by the deep sea drilling project changes of mechanical properties and of density of uplifted deep material in the accreting plate boundary zone caused by serpentinization may play an important role in the construction of slow spreading ridges

highly evolved juvenile granites with tetrad ree patterns the woduhe and baerzhe granites from the great xing an mountains in ne china abstract in ne china voluminous granitoids were emplaced in late paleozoic and mesozoic times we report here sr nd o isotopic and elemental abundance data for two highly evolved granitic plutons woduhe and baerzhe from the great xing an mountains they show a rather juvenile sr nd isotopic signature and a spectacular tetrad effect in their ree distribution patterns as well as non charac charge and radius controlled trace element behavior the emplacement ages are constrained at 130 4 ma for the woduhe and 122 5 ma for the baerzhe granites by rb sr and sm nd isotope analyses both granites are also characterized by low but imprecise initial 87sr 86sr ratios of about 0 703 the nd sr isotope data argue for their generation by melting of dominantly juvenile mantle component with subordinate recycled ancient crust this is largely compatible with the general scenario for much of the phanerozoic granitoids emplaced in the central asian orogenic belt the parental magmas for both the woduhe and baerzhe granites have undergone extensive magmatic differentiation during which intense interaction of the residual melts with aqueous hydrothermal fluids probably rich in f and cl resulted in the non charac trace element behavior and the tetrad effect of ree distribution both the woduhe and baerzhe granites show the characteristic trace element patterns of rare metal granites but their absolute abundances differ by as much as two orders of magnitude the oxygen isotope compositions of the two granites have been severely disturbed significant 18o depletion in feldspar but not so much in quartz suggests that the hydrothermal alteration took place in a temperature condition of 300 500 c this subsolidus hydrothermal alteration is decoupled from the late stage magma fluid interaction at higher temperatures despite the two distinct and intense events of water rock interaction the rb sr and sm nd geochronological systems seem to have maintained closed hence suggesting that the two events occurred shortly after the plutonic emplacements

a note on induced stress changes in hydrocarbon and geothermal reservoirs abstract earthquakes have been induced by oil and gas production where pore pressures have decreased in some cases by several tens of mpa it has previously been suggested that such earthquakes are caused by poroelastic stressing of crust surrounding the reservoir induced earthquakes are also common in geothermal fields such as the geysers where strong correlations between both steam production and condensate injection and earthquake activity have been observed over the last several decades stress measurements within hydrocarbon reservoirs show that the least horizontal stress decreases with declining reservoir pressure as predicted by poroelasticity for circular disk shaped reservoirs isothermal reduction in pore pressure induces a relative horizontal tension within the reservoir production induced stressing may promote frictional sliding on pre existing faults within the reservoir itself normal faulting is promoted if the regional stress is extensional and the biot coefficient is sufficiently large 0 85 for reasonable coefficients of friction on the other hand dilatant fracturing and normal faulting are always promoted in extensional environments near the edge of the reservoir or in regions of high pore pressure gradient it is suggested that such fracturing could enhance fracture permeability in tight rocks adjacent to portions of the reservoir that experience large reductions in pore pressure due to production in regional compressional environments production modestly favors reverse faulting above and below the reservoir the ratio of thermal to poroelastic stress can be quite large in geothermal reservoirs such as the geysers reservoir wide energy balance considerations suggest that the average temperature has declined at the geysers by 6 c during the past 20 years reservoir average stress changes are thus on the order of 2 mpa and are certainly much larger near injection wells and steam producing fractures

ultramafic exposures and the gravity signature of the lithosphere near the fifteen twenty fracture zone mid atlantic ridge 14 16 5 n abstract we analyze geological and geophysical data near the fifteen twenty fracture zone to constrain the processes of magmatic accretion and tectonic extension in the area characterized by extensive outcrops of ultramafic rocks at the seafloor close to the fracture zone 60 km the crust is thicker associated with circular or elongated gravity lows bull s eyes axis parallel faults and abyssal hills and no peridotite outcrops consistent with a more homogeneous and magmatic crust simple melting models predict magmatic crustal thicknesses that differ substantially from the gravity estimates and the outcrop of peridotites seems inconsistent with the high degrees of partial melting in the area inferred from basalt and peridotite geochemistry we explain these discrepancies primarily a by the entrapment of melt in the lithosphere at deep levels particularly near the transform b by along axis melt migration away from the transform and c by variations in the degree and depth of serpentinization of a heterogeneous crust composed of gabbro and peridotite the along axis variations in melt supply lithospheric thickness and alteration of the lithosphere can result in a change of rheological structure and therefore in fault pattern as observed along the ridge axis identified detachment faults in the area provide a mechanism for the uplift of ultramafic rocks to the seafloor these detachments are not systematically associated with or formed at the inside corner of discontinuities as described elsewhere the origin and evolution of these structures is yet unconstrained but we suggest that they can be initiated at mechanical heterogeneities in the deep lithosphere and that once formed they localize strain due to the presence of weak serpentinites along the fault planes

hydration and uplift of the oceanic crust on the mid atlantic ridge associated with hydrothermal activity evidence from boron isotopes boron isotope systematics of vent fluids from the mid atlantic ridge mar and the faster spreading east pacific rise epr reveal that at the mar 50 of seawater boron is removed from circulating fluids on the downwelling limb of the hydrothermal convection cell whereas little boron is removed at epr sites geophysical and petrographic studies suggest that downwelling fluids on the mar penetrate deep in the oceanic crust and upper mantle where boron is removed during serpentinization of peridotite the associated hydration of the oceanic crust reduces its density and modelling of the data suggests that this results in uplift leading to significant elevation of the median valley walls in the vicinity of active hydrothermal systems

u pb dating of the madeira suite and structural control of the albite enriched granite at pitinga amazonia brazil evolution of the a type magmatism and implications for the genesis of the madeira sn ta nb ree cryolite world class deposit the madeira sn nb ta deposit corresponds to the albite enriched facies of madeira granite madeira suite that is part of a ne sw alignment of three granitic bodies structural analyses suggest that the albite enriched granite emplacement and development of related structures occurred in a different tectonic setting than earlier facies of the granitic bodies in the ne sw alignment and that the structure was dominated by a n ne trending shortening related to a left lateral motion along a ne sw corridor within a riedel shear three types of structures flat lying near vertical fractures and geodes miaroles concentrated the last magmatic related fluid in the albite enriched granite u pb in situ zircon dating of the albite enriched granite yielded an age of 1 822 22 ma the other granitic plutons of the madeira suite agua boa granite topaz granite facies and europa per alkaline granite formed at 1 825 14 ma 1 831 11 ma and 1 839 10 ma respectively the granite bodies of the madeira suite occur in an area occupied by a cauldron complex composed of volcanic sequences iricoume group and associated a type granites mapuera suite and their formation was controlled by the reactivation of the cauldron structures and the pre existing structures the gap between the cauldron complex and the madeira suite is no greater than 35 ma the geological evolution was reanalyzed from the perspective of an entire extensional environment where three successive episodes of a type magmatism and basin formation urupi formation occurred the data and interpretations of this work imply crucial constraints on the petrologic models proposed for the albite enriched granite and consequently on the genesis of the madeira deposit

oceanic core complex development in modern and ancient oceanic lithosphere gabbro localized versus peridotite localized detachment models abstract drilling and submersible studies of the atlantis bank southwest indian ridge and the atlantis massif mid atlantic ridge oceanic core complexes reveal gabbro localized and peridotite localized end member models of strain localization and deformation during core complex development in which the gabbroic fault rocks exhibit extensive and rare high temperature ductile deformation fabrics respectively both models emphasize a footwall cored by gabbroic intrusions therefore precluding an amagmatic origin of core complex formation we test these models by using relict oceanic core complexes preserved in the western mirdita ophiolite in albania the puka and krabbi massifs display traits of the peridotite localized detachment model whereby amagmatic tectonic extension is not required for the formation of this category of core complexes

incipient hydration of mantle lithosphere at ridges a reactive percolation experiment a reactive percolation experiment was conducted by injecting seawater into a permeable olivine aggregate at 190 c and 19 mpa to explore the relationships between hydration reactions and hydrodynamic properties during the onset of serpentinization in the ultramafic lithosphere exposed at mid ocean ridges the experiment was stopped after 23 days when the sample became impermeable the initial flow rate was 0 2 ml h and then was decreased down to 0 06 ml h after 8 days permeability decreased continuously throughout the experiment the analyses of fluid chemistry time series and of the mineralogy and structure of the reacted sample showed olivine dissolution and precipitation of proto serpentine brucite and fe oxides these reactions are controlled by coupled hydrodynamic and chemical processes interacting at different time and spatial scales their first characteristic is the production of silica rich outlet fluids in disequilibrium with the observed reacted mineral assemblages these compositions are interpreted as resulting from a suite of coupled dissolution precipitation reactions controlled at the pore scale by surface kinetic processes that rapidly reaches steady state constant fluid composition independent of fluid flux hence the effective rate of serpentinization was controlled to the first order by the transport of reactants during the experiment mass balance calculations show that the rate of olivine conversion was fast 0 2 0 5 wt day yet only 8 wt of the olivine sample reacted because the permeability drop limited fluid circulation porosity varied little during the experiment compared to permeability changes the decrease of permeability was explained by the structure of the newly formed serpentine favouring the clogging of fluid paths the rate at which permeability decreased was the fastest at low flux conditions this suggests that permeability changes were not controlled simply by the kinetics of the serpentinization reactions but mainly reflected the development of low peclet pore scale zones forming micro environments that control the serpentinization reaction paths these results outline the need to take into account the coupling of hydrodynamic and chemical processes when modelling the effects of reactive transport within the hydrating oceanic lithosphere

in situ la mc icp ms and id tims u pb geochronology of cassiterite in the giant furong tin deposit hunan province south china new constraints on the timing of tin polymetallic mineralization abstract the furong deposit located in southern hunan province china is a newly discovered giant tin deposit spatially associated with the a type qitianling granite the petrogenetic link between the giant furong tin deposit and the qitianling granite batholith remains controversial because of the lack of precise dating for the tin polymetallic mineralization here we report for the first time in situ u pb data on cassiterite obtained by la mc icp ms and the results are compared to id tims data the in situ la mc icp ms analyses of the cassiterites provide a reliable age constraint for tin polymetallic mineralization in the furong deposit yielding an isochron age of 159 9 1 9 ma at 95 confidence level mswd 18 which is indistinguishable from the id tims 206 pb 238 u weighted mean age of 158 2 0 4 ma the in situ u pb ages corroborate the ar ar dates published for the furong tin deposit which indicate that the mineralization is coeval with the emplacement of the qitianling granite batholith these results provide further evidence of a close temporal link between the pluton s emplacement and tin mineralization in the furong area the data obtained clearly demonstrate that in situ la mc icp ms u pb dating of cassiterite is a robust geochronometer for direct dating of tin polymetallic mineralization

the structure evolution and symmetry of the magma poor rifted margins of the north and central atlantic a synthesis abstract magma poor rifted margins consistently show extreme crustal thinning accompanied by normal faulting the serpentinization of the mantle beneath crust thinned to less than 8 2 km and the unroofing of a broad zone of mantle within the continent ocean transition accompanied by the development of detachment and other large offset faults these observations are the logical result of the progressive extension of cool lithosphere away from thermal anomalies such as plumes although the paucity of magmatism may be explained by depth dependent extension of the lithosphere and by pre depleted sub crustal lithosphere rifting above initially cool sub lithospheric mantle also may explain the subsidence deficit observed at some margins synrift subsidence is buffered by the simultaneous influx of warmer oceanic asthenosphere as it extends thins and cools ductile creeping layers in the mid and deep crust become progressively more brittle resulting in increased coupling between the upper and lower crust and eventually the embrittlement of the entire crust faults cutting from the surface across the moho bringing water into the mantle and causing its serpentinization increased coupling and the development of serpentine detachments predict the development of late stage asymmetry once the entire crust is brittle such detachments are imaged on some margins and inferred on others analysis of the crustal structure across conjugate margins shows that these are approximately symmetric until this late stage when they become markedly asymmetric similar analyses show that depth dependent stretching of the crust is insufficient to explain the discrepancy between the amount of the visible extension along faults and the amount of crustal thinning instead this extension discrepancy may be related to the complex evolution of brittle deformation through multiple phases and styles of faulting related to the changes in the rheological character and strength of the lithosphere as it is thinned complex polyphase faulting continues after complete crustal separation resulting in the exhumation of broad expanses of peridotitic basement the top of which is everywhere marked by an exhumed slip surface similar to the corrugated surface observed at mid ocean ridges the similarity in processes between mantle unroofing and seafloor spreading makes the distinction between the cot and true oceanic crust difficult and possibly moot

serpentinization and heat generation constraints from lost city and rainbow hydrothermal systems abstract the discovery of ultramafic hosted hydrothermal systems at rainbow 36 n mar and lost city a vent site approximately 15 km west of the mar at 30 n provides unique perspectives on chemical and heat generating processes associated with serpentinization at a range of chemical and physical conditions heat balance calculations together with constraints imposed by geochemical modeling indicate that significant changes in temperature are not likely to occur at either vent system as a result of the exothermic nature of olivine hydrolysis at rainbow the relatively high temperatures in subseafloor reaction zones in excess of 400 c which must be linked to magmatic processes inhibit olivine hydrolysis effectively precluding mineralization induced heating effects geochemical modeling of the lost city vent fluids indicates temperatures in excess of those measured 40 75 c the relatively high subseafloor temperatures 200 50 c requires conductive cooling of the fluids on ascent to the seafloor a scenario in keeping with the mineralization of chimney structures actually observed although the intermediate temperatures predicted for subseafloor reaction zones at lost city could be expected to enhance olivine to serpentine conversion dissolved cl k cl and na cl ratios of the lost city vent fluids are virtually unchanged from seawater values and indicate little hydration of olivine which is a necessary condition for exothermic heat generation by serpentinization apparently the fluid rock mass ratio is too high or fluid residence times too low for this to occur to any significant extent thus in spite of the off axis location of the lost city vents and apparent lack of a localized heat source mineralization reactions likely play an insignificant role in accounting for hydrothermal circulation it is more likely that tectonic processes associated with the slow spreading mar permit access of seawater to relatively deep and still hot lithospheric units and or near axis magmatic heat sources before venting additional chemical and physical temperature flow rate data for lost city and similar hydrothermal systems are needed to test key elements of the proposed model

sequence paleogeography and coal accumulation of the early middle jurassic in central qilian mountain belt muli basin qinghai province northwestern china the muli basin in the qinghai province located at the northeastern margin of the tibetan plateau is a jurassic coal bearing basin and is an important area for coal production in china recent explorations have revealed the presence of unconventional types of gases such as coalbed methane and gas hydrates we have investigated the sedimentology sequence stratigraphy and lithofacies paleogeography of this basin based on data from outcrop and borehole sections the coal bearing strata include the lower jurassic reshui and the middle jurassic muli and jiangcang formations lithofacies including conglomerates sandstones mudstones and coal were identified and have been interpreted to be formed in environments ranging from braided fluvial braided fluvial delta to lacustrine the preferred sites of coal accumulation are the interdistributary bays of lower delta plain and upper delta plain environments three types of sequence boundaries are recognized including regional unconformities between the lower jurassic and underlying triassic the basal surface of incised valley fills of thick bedded sandstones in the muli and jiangcang formations and the surfaces that mark abrupt depositional facies shifts four third order sequences were subdivided namely s1 s2 s3 and s4 in ascending order which correspond to the reshui formation the muli formation the lower member of the jiangcang formation and the upper member of the jiangcang formation respectively each sequence consists of a lowstand systems tract a transgressive systems tract tst and a highstand systems tract a series of sequence specific basin wide paleogeographic maps have been constructed based on contour maps of several lithological parameters including stratal thickness sandstone to mudstone ratio and mudstone thickness from s1 to s4 the muli basin experienced an evolution from a braided fluvial system to braided fluvial delta then to a lacustrine system showing a gradual deepening process thick coal seams with thickness greater than 3 5 m 11 5 ft were primarily developed in the delta plain environment where the rate of peat accumulation can be easily balanced by the rate of accommodation generation a five stage coal accumulation model has been summarized and the major coal seams mainly developed in the early and late tst of sequences s1 and s2

deep slab fluids fuel extremophilic archaea on a mariana forearc serpentinite mud volcano ocean drilling program leg 195 1 as the pacific plate subducts beneath the mariana forearc it releases water that hydrates the overlying mantle wedge converting it to serpentinite that protrudes to form mud volcanoes at the seafloor excess h2o ascends through these mud volcanoes and exits as cold springs at their summits the composition of this deep slab derived water has been determined by drilling on two of these seamounts it has a ph of 12 5 and relative to seawater is enriched in sulfate alkalinity na cl k rb b light hydrocarbons ammonia 18o and deuterium and depleted in chloride mg ca sr li si phosphate and 87sr within the upper 20 m below seafloor at south chamorro seamount a microbial community operating at ph 12 5 made up overwhelmingly of archaea is oxidizing methane from the ascending fluid to carbonate ion and organic carbon while reducing sulfate to bisulfide and probably dissolved nitrogen to ammonia

active and relict sea floor hydrothermal mineralization at the tag hydrothermal field mid atlantic ridge the tag hydrothermal field is a site of major active and inactive volcanic hosted hydrothermal mineralization in the rift valley of the slow spreading mid atlantic ridge at 26 degree n the axial high is the principal locus of present magmatic intrusions the tag field contains three main areas of present and past hydrothermal activity 1 an actively venting high temperature sulfide mound 2 two former high temperature vent areas 3 a zone of low temperature venting and precipitation of fe and mn oxide deposits the volcanic centers occur at the intersections between ridge axis parallel normal faults and projected axis transverse transfer faults the intersections of these active fault systems may act as conduits both for magmatic intrusions from sources beneath the axial high that build the volcanic centers and for hydrothermal upwelling that taps the heat sources radiometric dating of sulfide samples and manganese crusts in the hydrothermal zones and dating of sediments intercalated with pillow lava flows in the volcanic center adjacent to the active sulfide mound indicate multiple episodes of hydrothermal activity throughout the field driven by heat supplied by episodic intrusions over a period of at least 140 times 10 sup 3 yr the sulfide deposits are built by juxtaposition and superposition during relatively long residence times near episodic axial heat sources counterbalanced by mass wasting in the tectonically active rift valley of the slow spreading oceanic ridge hydrothermal reworking of a relict hydrothermal zone by high temperature hydrothermal episodes has recrystallized sulfides and concentrated the first visible primary gold reported in a deposit at an oceanic ridge

imaging of vp vs and poisson s ratio anomalies beneath kyushu southwest japan implications for volcanism and forearc mantle wedge serpentinization abstract we determine detailed 3 d vp and vs structures of the crust and uppermost mantle beneath the kyushu island southwest japan using a large number of arrival times from local earthquakes from the obtained vp and vs models we further calculate poisson s ratio images beneath the study area by using this large data set we successfully image the 3 d seismic velocity and poisson s ratio structures beneath kyushu down to a depth of 150 km with a more reliable spatial resolution than previous studies our results show very clear low vp and low vs anomalies in the crust and uppermost mantle beneath the northern volcanoes such as abu kujyu and unzen low velocity anomalies are seen in the mantle beneath most other volcanoes in contrast there are no significant low velocity anomalies in the crust or in the upper mantle between aso and kirishima the subducting philippine sea slab is imaged generally as a high velocity anomaly down to a depth of 150 km with some patches of normal to low seismic wave velocities the poisson s ratio is almost normal beneath most volcanoes the crustal seismicity is distributed in both the high and low velocity zones but most distinctly in the low poisson s ratio zone a high poisson s ratio region is found in the forearc crustal wedge above the slab in the junction area with shikoku and honshu this high poisson s ratio could be caused by fluid filled cracks induced by dehydration from the philippine sea slab the poisson s ratio is normal to low in the forearc mantle in middle south kyushu this is consistent with the absence of low frequency tremors and may indicate that dehydration from the subducting crust is not vigorous in this region

partitioning between seismogenic and aseismic slip as highlighted from slow slip events in hyuga nada japan 1 we present a numerical method to analyze the gps data and examine slow fault slips in hyuga nada a region in western japan where the philippine sea plate subducts under the eurasian plate and earthquakes of magnitude 7 class are frequent we found that after two large earthquakes in 1996 a slow fault slip expanded from the source area to the north and then triggered another slow event with a characteristic source duration of about one year the aseismic slip has increasingly highlighted a particular site where little slow fault slip takes place but where the subducting plate drags the overriding plate it is noteworthy that the highlighted area is just the site of a past large earthquake we propose that this area is a possible site for a future large earthquake

formation of extremely f rich hydrous melt fractions and hydrothermal fluids during differentiation of highly evolved tin granite magmas a melt fluid inclusion study quartz crystals from topaz zinnwaldite albite granites from zinnwald erzgebirge germany contain in addition to primary and secondary fluid inclusions fis abundant crystalline silicate melt inclusions mis with diameters up to 200 m these mis represent various stages of evolution of a highly evolved melt system that finally gave rise to granite related sn w mineralization the combination of special experimental techniques with confocal laser raman microprobe spectroscopy and empa permits precise measurement of elevated contents of h2o f and b in re homogenized mis the contents of h2o and f were observed to increase from 3 to 30 and 1 9 to 6 4 wt respectively during magma differentiation however there is a second mi group very rich in h2o with values up to 55 wt h2o and an f concentration of approximately 3 wt ongoing enrichment of volatiles h2o f b and cl and of cs and rb can be explained in terms of magma differentiation triggered by fractional crystallization and thus is suggested to reflect elemental abundances in natural magmas and not boundary layer melts partitioning between melt and cogenetic fluids has further modified the magmatic concentrations of some elements particularly sn the coexistence of two types of mis with primary fis indicates fluid saturation early in the history of magma crystallization connected with a continuous sequestration of sn f and b the results of this study provide additional evidence for the extraordinary importance of the interplay of h2o f and b in the enrichment of sn during magma differentiation by decreasing the viscosity of and increasing the diffusivity in the melts as well as by the formation of various stable fluoride complexes in the melt and coexisting fluid

solution to the volume problem in serpentinization kernel pattern consists of a rim of completely serpentinized peridotite surrounding a core of partly serpentinized or unserpentinized peridotite it is found adjacent to shear zones in some serpentinites kernels are commonly rectangular with length width ratios of 1 to 3 the boundary between the completely serpentinized rim and the unserpentinized core is commonly cut at a high angle by cross fractures the cross fractures which contain either picrolite or chrysotile asbestos taper out in the core and widen in the rim they are almost always perpendicular to the core rim boundary and point into the core of the kernel in this sense they are radially distributed in the rim the kernel pattern can be observed in thin section in outcrop and on the megascopic scale of tens of metres cross fractures represent expansion fractures in serpentinite in the rim of the kernel caused by an increase in volume acompanying serpentinization of the peridotite in the core of the kernel as the serpentinization front penetrates into the kernel the serpentinized peridotite expands however the serpentinite in the rim cannot expand and must fracture to accommodate the increase in volume as serpentinization proceeds more expansion occurs extending the fractures farther into the core and requiring the fractures in the existing serpentinite to widen these fractures are subsequently filled by serpentine minerals the resulting pattern consists of a rim of serpentinite cut by serpentine veins that surrounds a core of peridotite measured densities of cross fractures in the rims of kernels are consistent with those expected from the magnitude of the volume change accompanying serpentinization of peridotite

calculation of partial melting equilibria in the system na2o cao k2o feo mgo al2o3 sio2 h2o nckfmash a thermodynamic model for haplogranitic melts in the system na2o cao k2o al2o3 sio2 h2o nckash is extended by the addition of feo and mgo with the data for the additional end members of the liquid incorporated in the holland powell 1998 internally consistent thermodynamic dataset the resulting dataset with the software thermocalc is then used to calculate melting relationships for metapelitic rock compositions the main forms for this are p t and t x pseudosections calculated for particular rock compositions and composition ranges the relationships in these full system pseudosections are controlled by the low variance equilibria in subsystems of nckfmash in particular the solidus relationships are controlled by the solidus relationships in nkash and the ferromagnesian mineral relationships are controlled by those in kfmash however calculations in nckfmash allow the relationships between the common metapelitic minerals and silicate melt to be determined in particular the production of silicate melt and melt loss from such rocks allow observations to be made about the processes involved in producing granulite facies rocks particularly relating to open system behaviour of rocks under high grade conditions

geochemistry of a type granites in the huangshaping polymetallic deposit south hunan china implications for granite evolution and associated mineralization abstract the huangshaping granites in hunan province south china were investigated for their geochemical characteristics three types of granites have been petrographically identified quartz porphyry granophyre and granite porphyry whole rock geochemistry suggests that the huangshaping granites especially the granite porphyry exhibit typical a type granite characteristics with their enrichment in si rb u th and nb and significant depletion in ba sr ti eu and p based on the al y and zr contents as well as the ree patterns of the rocks investigated the quartz porphyry and the granophyre are classified as a1 type alkaline granites whereas the granite porphyry is considered as a2 type aluminous granite whole rock and quartz feldspar o isotope data yields a wide range of 18osmow values 11 09 26 32 the granites are characterized by high radiogenic pb isotopic composition the present day whole rock pb isotopic ratios are 206pb 204pb 18 706 19 155 207pb 204pb 15 616 15 711 and 208pb 204pb 38 734 39 296 combining the o pb isotope compositions with major trace and ree geochemistry and regional geology characteristics the huangshaping granites were determined to resemble within plate granites that were mainly derived from a felsic infracrustal source related to continental extension the magma source of the quartz porphyry and the granophyre may have been generated from deeper depths and then ascended rapidly with limited water content and low oxygen fugacity which contributed to cu pb and zn mineralization on the other hand the magma that generated the granite porphyry may have ascended relatively slower and experienced pronounced crystal fractionation upper crustal basement rock contamination assimilation and wall rock interaction producing the sn and w rich granite porphyry this study reveals the crustal extension process and associated magmatic metallogenic activities during 180 150 ma in south hunan

effects of serpentinization on the lithospheric strength and the style of normal faulting at slow spreading ridges rock deformation experiments indicate that serpentinization can strongly influence the strength and tectonics of the oceanic lithosphere strength versus depth profiles calculated for conditions appropriate for slow spreading ridges indicate that the presence of serpentinite can reduce the integrated strength of the lithosphere by up to 30 results from flexural fault models indicate that if serpentinization is isolated to fault zones strain localization should be enhanced providing an explanation for the variations in the style of normal faulting along slow spreading ridge segments at segment centers where serpentinites are scarce deformation is accommodated on closely spaced faults with small throws at the segment ends where serpentinites are most abundant faults are widely spaced and have large throws q 1997 elsevier science b v

late pleistocene crustal uplift and gravity anomaly in the eastern part of kyushu japan and its geophysical implications the miyazaki plain eastern part of kyushu japan is characterized by both significant negative gravity anomalies and aseismic crustal uplifting 1 mm year in the late pleistocene and holocene we examine the relationship between these two phenomena which may provide important constraints on the interaction between the collision and or subduction of the kyushu palau ridge and the forearc we estimate the mass deficiency below 11 km depth by using the gravity anomalies and p wave velocity structure of the upper crust the onset of the load accumulation 0 5 0 4 ma is inferred from the movement of the fluvial terraces considering the tephrochronology the loading history is assumed to be a linear function of time we evaluate the crustal rebound by assuming a viscoelastic plate deformation with an underplating load existing at 20 or 30 km depth the predicted crustal movement for models with a lithospheric crustal viscosity of 1023 1024 pa s can explain the observed altitudes of the shoreline of the marine terraces formed at the last interglacial of about 125 kyr bp and the middle holocene of 5 6 kyr bp although we cannot restrict the origin of the buoyant body the subduction of the kyushu palau ridge remnant arc associated with back arc opening of the shikoku basin may be related to the buoyancy for the uplifting region examined here on the other hand the buoyant body off the miyazaki plain probably plays an important role in the interaction between the subducting oceanic slab and the overriding forearc crust thus the observed lateral variation of the interplate coupling on the convergent boundary along the nankai trough may be attributed to the existence of the buoyant body

stability and dynamics of serpentinite layer in subduction zone abstract the hydrous phyllosilicate serpentines have a strong influence on subduction zone dynamics because of their high water content and low strength at shallow and intermediate depths in the absence of data newtonian rheology of serpentinites has been assumed in numerical models yet experimental data show that serpentine rheology is best described by a power law rheology recently determined in subduction zone conditions hilairet n et al 2007 high pressure creep of serpentine interseismic deformation and initiation of subduction science 318 5858 1910 1913 using a simple 1d model of a serpentinized channel and as opposed to previous models in this power law rheology we examine the influence of channel thickness temperature and subduction angle on serpentine flow driven by density contrast serpentinization degree with the surroundings at temperatures of 200 500 c relevant to intermediate depths a fully serpentinized channel is unlikely to be thicker than 2 3 km for channel thicknesses of 2 km upward velocities are comparable to those using a constant viscosity of 10 18 pa s the velocity profile using power law rheology shows shear zones at the edges of the channel and a low strain rate region at its centre consistent with the frequent observation of weakly deformed hp rocks upward velocities estimated for channels 1 to 3 km thick are comparable to the serpentinization rates for maximum estimates of fluid velocities within shear zones in the literature competition between the upward flow and serpentinization may lead to intermittent behavior with alternating growth periods and thinning by exhumation at shallower levels the thickness allowed for a channel may be up to 8 10 km if the rheology has a higher dependence on stress we therefore propose that the exhumation of hp oceanic units in serpentinite channels is organized in two levels the deepest and fastest motion being driven by density contrast with the surrounding mantle and the shallowest circulation being driven by forced return flow the thicknesses estimated here for serpentinized layers at intermediated depths are similar to the precision of seismic studies the deepest serpentinite channel may thus be difficult to detect by seismic methods but it will have a strong influence on the mechanical coupling between the slab and mantle wedge

deformation and surface uplift associated with serpentinization at mid ocean ridges and subduction zones 1 we employ the classical problem of an inclusion in an elastic half space to model effects of sub surface serpentinization on crustal deformation change of stress state and surface uplift at the tag hydrothermal field on the mid atlantic ridge the model suggests that an anomalous salient 3 km in diameter and 100 m high that projects 3 5 km westward from the east valley wall may have resulted from a relatively deep seated well serpentinized body exhibiting transformational strain the associated large strains would likely result in sub surface fracturing or faulting but surface uplift may be relatively insensitive to the exact depth and shape of the serpentinized region serpentinization of a region beneath the footwall of the tag detachment fault will tend to promote slip along some overlying normal faults which may then enhance fluid pathways to the deeper crust to continue the serpentinization process our solution for the miyazaki plain above the kyushu palau subduction zone in sw japan explains the observed uplift of 120 m the small transformational strains associated with serpentinization in this region may promote thrust type events in the aseismic slip zone near the upper boundary of the subducting philippine sea plate as well as intraplate earthquakes associated with normal faulting the rate of serpentinization needed to produce the observed uplift at the miyazaki plain is significantly greater than that needed at tag though significantly smaller on per unit volume basis thermal effects of serpentinization in both regions appear to be small but uplift data provide an additional constraint on inferring serpentinization from geological and seismological observations

lithium isotopes in global mid ocean ridge basalts abstract the lithium isotope compositions of 30 well characterized samples of glassy lavas from the three major mid ocean ridge segments of the world spanning a wide range in radiogenic isotope and elemental content and sea floor physical parameters have been measured the overall data set shows a significant range in 7li 1 6 to 5 6 with no global correlation between li isotopes and other geochemical or tectonic parameters the samples with the greatest lithophile element depletion n morb k2o tio2

structural development of the lower paleozoic belt of south china genesis of an intracontinental orogen abstract an extensive structural study of wuyishan and surrounding areas south china brings data on the structures formed prior to the devonian unconformity building the lower paleozoic belt an ordovician tectonic event was responsible for i south directed structures in the wuyishan proper and its southern border related to both thin and thick skinned tectonics accompanied by metamorphism and crustal thickening ii north directed structures to the west of ganjiang fault and north of jiangshan shaoxing fault where only thin skinned tectonics is visible the southward deformation accommodating the shortening includes emplacement of thrust sheets involving deep crustal material and some mantle peridotite and likely repetition of continental material both responsible for crustal thickening the orogeny was due to the underthrusting of the southern part of the south china block beneath the northern part of this block closing the pre existing nanhua rift created at around 850 800 ma and involving the rift sedimentary fill in the southward thrusting the syn metamorphic tectonic piling was followed by anatexis and granite emplacement at around 440 390 ma our new u th pb epma monazite dating gives ages of 433 9 ma and 437 5 ma early silurian for the main anatectic event and a younger age of 412 5 ma late silurian early devonian for a late re heating process the lower paleozoic belt is an intracontinental orogen without oceanic suture record it shows some similarities with the cenozoic european pyrenees chain with a strong asymmetry

thermal constraints on the seismogenic portion of the southwestern japan subduction thrust for coastal cities an important factor in earthquake hazard from subduction zone earthquakes is the landward extent of the seismogenic portion of the subduction thrust fault in this study we test the hypothesis that the maximum downdip extent is defined by a critical temperature we have developed a transient thermal model for the nankai subduction zone of southwest japan to allow comparison of the thermally estimated downdip extent of the seismogenic zone with that from 1 seismicity and tsunami data for two great subduction earthquakes 2 the coseismic faulting extent of these events estimated from geodetic deformation data and 3 the interseismic locked zone determined from interseismic geodetic data the nankai margin has extensive heat flow and heat production data to control thermal models and thus crustal temperatures it has earthquake tsunami and geodetic data that constrain the coseismic rupture portion of the subduction thrust fault for past great earthquakes and the portion of the thrust fault that is locked and storing interseismic elastic strain on the nankaido margin off shikoku island the thermal model indicates that a temperature of 350 c taken to be the limit for seismic initiation from laboratory and field data is reached on the subduction thrust fault 150 km from the trench a transition zone into which rupture may extend with decreasing offset taken to be 450 c extends an additional 45 km downdip the thermal model results are in excellent agreement with the maximum downdip extent of coseismic displacement for the 1946 nankaido ms 8 2 earthquake off shikoku island and with the downdip extent of the present locked zone in the region of the 1944 tonankai ms 8 2 earthquake to the northeast the subduction angle is much steeper and the thermal models indicate a narrower downdip seismogenic extent the seismogenic locked zone from earthquake and geodetic data is also narrower thus our analysis of the southwest japan margin indicates that all three constraints on the downdip extent of the seismogenic zone thermal coseismic and interseismic geodetic data are in general agreement the study also supports the hypothesis that the seismogenic portion of subduction thrust faults is limited primarily by temperature the thermal control implies that subduction thrust faults with shallow dip have wider seismogenic zones compared to those with steep dip the subducting plate age and thus heat flow and the thickness of the insulating sediments on the incoming plate are also very important to the thermal regime and thus to the seismogenic width the relation of the maximum seismic rupture area to the interseismic locked zone is particularly important for earthquake hazard estimation on subduction margins such as cascadia where there have been no great historical events

flexural rotation of normal faults a conceptual model is proposed for the generation of low angle normal faults in metamorphic core complexes the model is based on three assumptions 1 the isostatic response to normal fault motion is of regional extent 2 when a fault segment is significantly rotated from the optimum angle of slip relative to the crustal stress field it is replaced by a new planar fault oriented in the optimum direction and 3 the fault cuts the entire upper crust and fault motion always nucleates in the same region at the base of the upper crust the stress field is considered to be uniform through the crust and the regional isostatic response of the crust to loads is computed using the thin plate flexure approximations active low angle normal faults are difficult to reconcile with rock mechanics theories earthquake focal mechanism studies and geochronologic results indicating rapid cooling of core complex rocks the model does not require active fault slip on low angle faults the flexural response to normal faulting is shown to be significantly affected by anelastic behavior of the crust and by loading due to sedimentation the anelastic response to large bending stresses results in significant reduction in the effective elastic rigidity of the upper crust this can explain the observed short wavelength of topographic response to normal fault loads the model results in 1 a nearly flat lying abandoned normal fault or detachment below slices of upper plate rocks and sedimentary infill 2 a strong contrast in metamorphic grade across the abandoned detachment 3 rapid movement of lower plate rocks from midcrustal depths to shallower depths these results are qualitatively in agreement with geologic observations for core complexes

coulomb stress accumulation along the san andreas fault system 1 stress accumulation rates along the primary segments of the san andreas fault system are computed using a three dimensional 3 d elastic half space model with realistic fault geometry the model is developed in the fourier domain by solving for the response of an elastic half space due to a point vector body force and analytically integrating the force from a locking depth to infinite depth this approach is then applied to the san andreas fault system using published slip rates along 18 major fault strands of the fault zone gps derived horizontal velocity measurements spanning the entire 1700 200 km region are then used to solve for apparent locking depth along each primary fault segment this simple model fits remarkably well 2 43 mm yr rms misfit although some discrepancies occur in the eastern california shear zone the model also predicts vertical uplift and subsidence rates that are in agreement with independent geologic and geodetic estimates in addition shear and normal stresses along the major fault strands are used to compute coulomb stress accumulation rate as a result we find earthquake recurrence intervals along the san andreas fault system to be inversely proportional to coulomb stress accumulation rate in agreement with typical coseismic stress drops of 1 10 mpa this 3 d deformation model can ultimately be extended to include both time dependent forcing and viscoelastic response

the changing vision of marine minerals abstract non fuel marine minerals are reviewed from the perspective of resources and their value as active analogs that can advance understanding of types of ancient ore deposits that formed in marine settings the theory of plate tectonics is the largest influence in expanding our vision of marine minerals and in developing our understanding of geologic controls of mineralization in space and time prior to the advent of plate tectonics we viewed the ocean basins as passive sinks that served as containers for particulate and dissolved material eroded from land this view adequately explained marine placer deposits heavy minerals and gems aggregates sand and gravel and precipitates phosphorites and manganese nodules although numerous sites of placer mineral deposits are known on continental shelves worldwide current activity pertains to diamond mining off southwestern africa tin mining off southeastern asia and intermittent gold mining off northwestern north america which are all surpassed economically by worldwide recovery of marine sand and gravel in turn dwarfed by offshore oil and gas with the advent of plate tectonics plate boundaries in ocean basins are recognized as active sources of mineralization in the form of hydrothermal massive sulfide deposits and proximal lower temperature deposits hosted in oceanic crust mafic at ocean ridges and felsic at volcanic island arcs and of magmatic ni cu sulfide chromite and pge deposits inferred to be present in the oceanic upper mantle lower crust based on their occurrence in ophiolites some 300 sites of hydrothermal active and relict mineralization most of them minor are known at this early stage of seafloor exploration on ocean ridges in fore arc volcanoes at back arc spreading axes and in arc rifts deposits formed at spreading axes and transported off axis by spreading are present in oceanic lithosphere but are virtually unknown the tag trans atlantic geotraverse hydrothermal field in the axial valley of the mid atlantic ridge latitude 26 n is considered to exemplify a major volcanogenic massive sulfide vms deposit forming at a spreading axis the most prospective of these occurrences lie within the 200 nautical mile 370 km wide exclusive economic zone eez of the nations of the volcanic island arcs of the western pacific where metal content of massive sulfides ag au ba cu pb sb zn exceeds that at ocean ridges plate tectonics early provided a framework for mineralization on the scale of global plate boundaries and is providing guidance to gradually converge on sites of mineralization through regional scales of plate reorganization with the potential to elucidate the occurrence of individual deposits e g eocene carlin type gold deposits investigation of the spectrum of marine minerals as active analogs of types of ancient mineral deposits is contributing to this convergence consideration of questions posed by brian skinner 1997 of what we do and do not know about ancient hydrothermal mineral deposits demonstrates the ongoing advances in understanding driven by investigation of marine minerals

serpentinized ultramafics and hydrothermal activity at the mid atlantic ridge crest near 15 n the initial occurrence of serpentinized ultramafic rocks at the nontransform intersection of a wall of a rift valley with the wall of a fracture zone is described from a site at the fifteen twenty fracture zone the ultramafics crop out in block faulted terrain on the upper portion of the eastern intersection between the rift valley and fracture zone walls in water depths between 2910 and 3300 m they comprise cumulate harzburgites pyroxenites iherzolites and wehrlites as well as gabbronorites olivine gabbronorites gabbropegmatites and alteration products including serpentinites bastite serpentinites and asbestos the ti zr y relations and relatively constant zr ti ratio in basalts recovered with the ultramafic rocks indicate a cogenetic relation from a common magmatic source that has undergone a late stage differentiation in the lower crust ongoing hydrothermal activity is indicated by chemical anomalies 3he mn in the near bottom water at the ultramafic outcrop the upwelling hydrothermal circulation apparently follows crust penetrating faults that may have controlled the diapiric ascent of the serpentinites and that continue to tap degassing magma and or mantle the observations presented indicate that ultramafic cumulates form beneath the rift valley adjacent to long offset 100 km ridge ridge transform faults where they are serpentinized by hydrothermal processes within the initial 1 106 years of generation of lithosphere at a slow spreading axis the corners formed by the intersections of the walls of a rift valley with both the transform rt corner and the nontransform rn corner portions of these fracture zones are principal loci of diapiric emplacement of serpentinized ultramafics

a new assessment of the abundance of serpentinite in the oceanic crust the gabbro model of the composition of the oceanic crust has prevailed for more than two decades but serpentinites are common along much of the mid atlantic ridge and elsewhere leading to renewed debate over the abundance of serpentinites in crust created at slow spreading ridges based on data from both oceanic and ophiolite samples it has also been suggested that serpentinites are not distinguishable from meta gabbros and diabases by their seismic properties we have made new statistical analyses of the relationship between vs and vp in oceanic gabbro and diabase samples and serpentinized peridotites and compared them to in situ s wave and p wave velocities derived from expanding spread profiles esps acquired in the atlantic our findings demonstrate 1 that there is some overlap of the properties of gabbroic samples taken from ophiolites with the properties of serpentinites but 2 over the range of typical layer 3 seismic velocities the seismic properties of oceanic gabbros and diabases are distinctly different from the properties of serpentinites and 3 that in situ seismic velocities are in excellent agreement with the properties of the gabbros while the shear velocities in serpentinites are significantly lower partially serpentinized peridotite is certainly present but not abundant in normal oceanic crust

lithium isotopes as a probe of weathering processes orinoco river abstract lithium isotopes have the potential to be effective tracers of weathering processes due to their large relative mass difference and therefore fractionation in this study an attempt is made to fill a major gap in the knowledge of li isotope fractionation during continental weathering and of the mechanisms involved finally the relationship between the suspended and dissolved material is made on a basin wide scale the orinoco basin provides a clear contrast in reaction limited and transport limited weathering regimes that has already been documented by a comprehensive study on its fluvial geochemistry edmond et al geochim cosmochim acta 60 1996 2949 2976 edmond et al geochim cosmochim acta 59 1995 3301 3325 conspicuous in our new results is the difference in 6li of the dissolved load between the andean 30 to 22 and shield 22 to 7 tributaries while the 6li of the suspended load is similar between the two to a first approximation during superficial weathering in high relief tectonically active terrains the dissolved load is high in li and isotopically heavy more negative 6li whereas in stable shield regions the concentrations are low and isotopically light in proportion to the increasing degree of weathering

seafloor hydrothermal systems driven by the serpentinization of peridotite 1 heat balance models predict that heat released upon serpentinization of peridotites can result in a wide range of hydrothermal venting temperatures temperatures ranging from a few to a few tens of c are likely temperatures 300 c may be possible in conjunction with significant heat input from below low fluid flow rate 10 kg s a high rate of serpentinization 100 kg s or some combination of these factors the volume of fresh lithosphere accessible to serpentinization may limit the duration of an episode of hydrothermal venting when the serpentinization rate exceeds 10 kg s hydrothermal venting driven by high rates of serpentinization may occur episodically as new reaction surfaces are made available by tectonic activity or crack propagation given the uncertainty of several parameters the current episode of venting at the lost city vent field on the mid atlantic ridge may range from 102 to 104 years

serpentinization faults and their role in the tectonics of slow spreading ridges two major families of faults dominate the tectonics of slow spreading ridges the transform faults paralleling the spreading direction and the gloria faults observed by large scale side scan sonar normal to it the observation of microearthquakes to 8 km depth beneath the mid atlantic ridge axis implies that not only the transform faults but the gloria faults as well penetrate into the mantle since large horizontal temperature gradients are often associated with them both types of fault provide the means of circulating large volumes of sea water through the oceanic mantle under these conditions serpentinization of upper mantle rocks will occur in a predictable way its influence on the tectonic development of slow spreading ridges is discussed

tectonics of the mid atlantic rift valley between the tag and mark areas 26 24 n evidence for vertical tectonism abstract a segment of the mid atlantic ridge between the tag and mark areas between 26 and 24 n was studied during march april of 1988 from the r v akademik mstislav keldysh using mir submersibles the rift valley is depressed here to depths of 3200 4700 m and includes seven independent magmatic cells which are in different phases of their evolution some of them are now in the constructive phase whereas others are in the destructive phase and are strongly fractured slopes of two types flank the rift valley the normal slopes similar to the famous area are block faulted terrains with small elevations above the neovolcanic ridge crest in the center of the rift valley the abnormal slopes are elevated from 1 to 3 km above the neovolcanic ridge crest an almost undisturbed section of the upper oceanic crust from gabbro through sheeted dikes to pillow lavas outcrops on the abnormal slopes as the dikes are dipping nearly vertically and pillow lavas preserve their initial inclinations no block rotation or back tilting occurred during the development of rift mountains pure vertical uplift was responsible for rift mountain growth speculations are made that serpentinization of the uppermost mantle peridodite can be considered as the main process leading to oceanic crust uplift serpentinization takes place during the nonvolcanic destructive phases when seawater can easily penetrate into the deeper crust and uppermost mantle reaching the peridodites as a result serpentinites are added to the oceanic crust of slow spreading ridges

crustal structure of the trans atlantic geotraverse tag segment mid atlantic ridge 26 10 n implications for the nature of hydrothermal circulation and detachment faulting at slow spreading ridges new seismic refraction data reveal that hydrothermal circulation at the trans atlantic geotraverse tag hydrothermal field on the mid atlantic ridge at 26 10 n is not driven by energy extracted from shallow or mid crustal magmatic intrusions our results show that the tag hydrothermal field is underlain by rocks with high seismic velocities typical of lower crustal gabbros and partially serpentinized peridotites at depth as shallow as 1 km and we find no evidence for low seismic velocities associated with mid crustal magma chambers our tomographic images support the hypothesis of tivey et al 2003 that the tag field is located on the hanging wall of a detachment fault and constrain the complex dome shaped subsurface geometry of the fault system modeling of our seismic velocity profiles indicates that the porosity of the detachment footwall increases after rotation during exhumation which may enhance footwall cooling however heat extracted from the footwall is insufficient for sustaining long term high temperature hydrothermal circulation at tag these constraints indicate that the primary heat source for the tag hydrothermal system must be a deep magma reservoir at or below the base of the crust

lithium isotope behaviour during partial melting of metapelites from the jiangnan orogen south china implications for the origin of ree tetrad effect of f rich granite and associated rare metal mineralization abstract we carried out combined nd and li isotopic studies on the neoproterozoic biotite granites late mesozoic evolved two mica muscovite granites and associated the shuangqiaoshan group metapelites from the jiangnan orogen south china which along with petrological and geochemical data were used to constrain the lithium isotopic behavior during partial melting of metapelites the origin of the ree tetrad effect in evolved granites and implications for related rare metal mineralization the neoproterozoic biotite granites show features typical of s types e g high a cnk ratios and have end 6 9 to 10 and 7li 2 2 4 comparable to the shuangqiaoshan group metapelites with end 6 to 11 and 7li 2 3 7 respectively suggesting that the biotite granites were derived from melting of the metapelites and the li isotopic fractionation during the melting process appears negligible the two late mesozoic shimensi and zengjialong granites occur as small stocks showing high silica peraluminous low cao and sr contents variably enrichment in volatiles f b and are accompanied by w cu and sn mineralization the shimensi granites have end t values of 7 0 8 6 and 7li 2 7 3 9 similar to the shuangqiaoshan group metapelites whereas the zengjialong granites have relatively high end t values 5 1 7 3 and heavy li isotopic features mostly 7li 0 18 1 53 and particularly ree tetrad effect is seen in the latter we argue that the late mesozoic evolved granites formed as a result of re melting of the granulitic residues left in the lower crust after extraction of the neoproterozoic biotite granites from the metapelites such re melting of the residues was triggered by underplating of basaltic magma with more mantle input and f b as well in the source of the zengjialong granites the elevated 7li values of the zengjialong granites were probably caused by intense interaction between melt and external fluids during magma emplacement which in turn resulted in the formation of the ree tetrad effect in the granites isotopic modeling suggests that fluid exsolution from evolved granites is minor and is less likely to have been responsible for the extensive rare metal mineralization as traditionally thought

the absence of lithium isotope fractionation during basalt differentiation new measurements by multicollector sector icp ms we report measurements of the isotopic composition of lithium in basalts using a multicollector magnetic sector plasma source mass spectrometer mc icp ms this is the first application of this analytical technique to li isotope determination external precision of multiple replicate and duplicate measurements for a variety of sample types averages 1 1 2 population the method allows for the rapid 8 min sample analysis of small samples 40 ng li relative to commonly used thermal ionization methods the technique has been applied to a suite of samples from kilauea iki lava lake hawaii the samples range from olivine rich cumulitic lava to sio2 and k2o enriched differentiated liquids and have 7li per mil deviation of sample 7li 6li relative to the l svec standard of 3 0 to 4 8 the data indicate a lack of per mil level li isotope fractionation as a result of crystal liquid fractionation at temperatures greater than 1050 c this conclusion has been tacitly assumed but never demonstrated and is important to the interpretation of li isotope results from such geochemically complex environments as island arcs

strength of slightly serpentinized peridotites implications for the tectonics of oceanic lithosphere we deformed cores of peridotite with 10 15 lizardite and chrysotile serpentine to determine the influence of serpentine content on the strength and the style of deformation the strength the pressure dependence of strength and the nominally nondilatant mode of brittle deformation of slightly serpentinized peridotites are comparable to those of pure serpentinites these results indicate that deformation is accommodated primarily by serpentine while olivine despite being the more abundant component remains nominally undeformed on the basis of these data and previous work we determine that the transition from a strong dilatant dunite rheology to a weak nondilatant serpentinite rheology is not a linear function of the degree of serpentinization instead an abrupt transition in strength is observed at low degrees of serpentinization the pressure of the transition from localized to distributed deformation also decreases abruptly from 1000 mpa to 150 350 mpa the change in rheological behavior occurs at a serpentine content of 10 15 or less which corresponds to published compressional seismic velocity of 7 8 7 5 km s at a pressure of 200 mpa the seismic velocity of the oceanic lithosphere particularly of that formed at slow spreading ridges can thus provide constraints on its mechanical properties at depth because slightly serpentinized peridotites have a rheology similar to that of pure serpentinite significant lithospheric weakening may occur after the onset of alteration near or at the ridge axis

inhibition of subduction thrust earthquakes by low temperature plastic flow in serpentine abstract serpentine is considered a candidate hydrous mineral in explaining aseismic zones along faults however recent frictional experiments have shown a marked increase in its shear strength with increasing depth implying its deformation via unstable sliding at a plate interface in subduction zones here we report the results of a series of simple shear experiments designed to determine the dominant deformation style of low temperature serpentine species under p t conditions that correspond to the mantle wedge corner in cool subduction zones ca p 1 gpa and t 200 c we found that with increasing shear strain the dominant flow mechanism of the serpentinite evolves from 1 inhomogeneous semi brittle flow by strain localization into sample scale shear bands to 2 homogeneous ductile flow by intracrystalline deformation within individual serpentine grains with the development of a planar shape fabric and a strong crystallographic preferred orientation tem observations revealed that grain size reduction in lizardite crystals occurs due to 001 interlayer glide represented by the occurrence of tiny sheet like sectors oriented parallel to 001 liz lattice fringes which evolved from large elongated sectors with long axes oriented normal to 001 liz given that a low viscosity serpentinized layer upon a subducting plate interface produces strain localization within the layer and subsequent large bulk shear strains the above results indicate that the presence of low temperature serpentine species at the plate interface in cool subduction zones inhibits the initiation of subduction thrust earthquakes as stress is preferentially accommodated by plastic flow this hypothesis explains the occurrence of an anomalous non seismic region devoid of large thrust earthquakes in locally hydrated forearc mantle within northeast japan

detachment faulting and the evolution of passive continental margins major detachment faults play a key role in the lithospheric extension process in the basin and range province and may also be important in other continental extension terranes such detachment faulting leads to an inherent asymmetry of extensional structure and of uplift subsidence patterns detachment models developed for the formation of metamorphic core complexes can also be applied to the formation of passive continental margins we therefore suggest the existence of upper plate and lower plate passive margins these give rise to a complementary asymmetry of opposing margins after continental breakup transfer faults offset marginal features and allow margins to switch from upper plate to lower plate characteristics along strike

kinematics and geometry of active detachment faulting beneath the trans atlantic geotraverse tag hydrothermal field on the mid atlantic ridge newly acquired seismic refraction and microearthquake data from the trans atlantic geotraverse tag segment of the mid atlantic ridge at 26 n reveal for the first time the geometry and seismic character of an active oceanic detachment fault hypocenters from 19 232 microearthquakes observed during an eight month ocean bottom seismometer deployment form an 15 km long dome shaped fault surface that penetrates to depths 7 km below the seafloor on a steeply dipping 70 interface a tomographic model of compressional wave velocities demonstrates that lower crustal rocks are being exhumed in the detachment footwall which appears to roll over to a shallow dip of 20 5 and become aseismic at a depth of 3 km outboard of the detachment the exhumed lithosphere is deformed by ridge parallel antithetical normal faulting our results suggest that hydrothermal fluids at the tag field exploit the detachment fault to extract heat from a region near the crust mantle interface over long periods of time

reduction of co2 during serpentinization of olivine at 300 c and 500 bar co 2 reduction processes occurring during experimental serpentinization of olivine at 300 c and 500 bar confirm that ultramafic rocks can play an important role in the generation of abiogenic hydrocarbon gas data reveal that conversion of fe ii in olivine to fe iii in magnetite during serpentinization leads to production of h 2 and conversion of dissolved co 2 to reduced c species including methane ethane propane and an amorphous carbonaceous phase hydrocarbon gases generated in the process fit a schulz flory distribution consistent with catalysis by mineral reactants or products magnetite is inferred to be the catalyst for methanization during serpentinization because it has been previously shown to accelerate fischer tropsch synthesis of methane in industrial applications involving mixtures of h 2 and co 2 the carbonaceous phase was predominantly aliphatic but had a significant aromatic component although this phase should ultimately be converted to hydrocarbon gases and graphite if full thermodynamic equilibrium were established its formation in these experiments indicates that the pathway for reduction of co 2 during serpentinization processes is complex and involves a series of metastable intermediates

the lanthanide tetrad effect and its correlation with k rb eu eu sr eu y ho and zr hf of evolving peraluminous granite suites lanthanide tetrad effects are often observed in ree patterns of more highly evolved variscan peraluminous granites of mid eastern germany central erzgebirge western erzgebirge fichtelgebirge and northern oberpfalz the degree of the tetrad effect te1 3 is estimated and plotted vs k rb sr eu eu eu y ho and zr hf the diagrams reveal that the tetrad effect develops parallel to granite evolution and significant tetrad effects are strictly confined to highly differentiated samples mineral fractionation as a cause for the tetrad effect is not supported by a calculated rayleigh fractionation which also could not explain the fractionation trends of sr eu and eu eu the strong decrease of eu concentrations in highly evolved rocks suggests that eu fractionates between the residual melt and a coexisting aqueous high temperature fluid mineral fractionation as a reason for the tetrad effect is even more unlikely as ree patterns of accessory minerals display similar tetrad effects as the respective host rocks the accessory minerals inherit the ree signature of the melt and do not contribute to the bulk rock tetrad effect via mineral fractionation these results point in summary to significant changes of element fractionation behavior in highly evolved granitic melts ionic radius and charge which commonly control the element distribution between mineral and melt are no longer the exclusive control the tetrad effect and the highly fractionated trace element ratios of y ho and zr hf indicate a trace element behavior that is similar to that in aqueous systems in which chemical complexation is of significant influence this distinct trace element behavior and the common features of magmatic hydrothermal alteration suggest the increasing importance of an aqueous like fluid system during the final stages of granite crystallization the positive correlation of te1 3 with bulk rock fluorine contents hints at the importance of ree fluorine complexation in generating the tetrad effect as the evolution of a ree pattern with tetrad effect m type implies the removal of a respective mirroring ree pattern w type the tetrad effect identifies open system conditions during granite crystallization

seismic velocity structure around the hyuganada region southwest japan derived from seismic tomography using land and obs data and its implications for interplate coupling and vertical crustal uplift abstract the hyuganada region a forearc region of southwest japan is characterized by several interesting geological and geophysical features i e significant aseismic crustal uplift of 120 m during the past 120 thousand years at the miyazaki plain negative free air gravity anomalies with the maximum magnitude of 130 mgal and relatively less cohesive interplate coupling compared with that for off the shikoku and kii peninsula in order to examine the causes of these observations we determined a detailed three dimensional seismic velocity structure based on the seismic data observed by ocean bottom seismometers obs and land stations p and s wave tomographic velocity structures clearly indicate the subducting slab and also the zones of high poisson s ratio at 25 35 km depth along the coastline of the northeastern part of the hyuganada the region with high poisson s ratio may correspond to the serpentinized mantle wedge as suggested for other mantle wedges and appears to be coincident with the zone for observed aseismic slips such as the slow slip and after slip events also the detection may be related to a relatively weak interplate coupling in the hyuganada region the tomographic structures also indicate low velocity zones with a horizontal scale comparable to the kyushu palau ridge in and around the subducting slab if we assume that the low velocity zones correspond to the subducted kyushu palau ridge then the predicted gravity anomaly due to the density contrast between the low velocity zones and the surrounding region can explain about 60 of the gravity anomaly in the hyuganada region the buoyancy is probably an important factor for the crustal uplift observed in the miyazaki plain the steep bending of the subducting slab and the normal fault type earthquakes around the hyuganada region

borehole observations of fluid flow from south chamorro seamount an active serpentinite mud volcano in the mariana forearc abstract a sealed borehole observatory cork was deployed on south chamorro seamount an active serpentinite mud volcano in the mariana forearc to explore subduction related processes on a non accretionary convergent plate margin formation fluid was overpressured relative to ambient hydrostatic conditions fluid flowed from the borehole at 0 2 l s when the observatory was opened to recover instruments 2 yr after it was installed the chemical composition of the formation fluid is similar to that extrapolated from trends in pore water data collected during ocean drilling program leg 195 when the observatory was established reduced sulfur is present in this highly alkaline ph 12 4 formation fluid indicative of microbial activity at or below the depth of the screened casing 149 202 m below the seafloor discharge from the open borehole continued for 37 days until the observatory was resealed this discharge requires significant permeability at depth 6 10 14 m2 zones of high permeability may be associated with the formation of headwall scarps consistent with numerous slumps on the southeastern flank of the seamount and likely shape a geochemical environment suitable for an active microbial community

simple approach to predicting ground displacements caused by tunnelling in undrained anisotropic elastic soil an attempt to develop a kinematically based approach to predicting ground settlements due to tunnelling is represented by using the more reliable data of the commonly observed settlement troughs rather than imposing the boundary conditions on the tunnel surface which have a high level of uncertainty the anisotropic elastic model used in this work is highly simplified and the resulting analytical solution is of an approximate nature nevertheless it seems to provide good agreement with both the shape and the magnitude of the observed settlement trough based on this solution a modification is proposed for the widely used empirical gaussian curve by correlating the trough width parameter k with the ratio between the two anisotropic shear moduli ghh gvh

variation of downdip limit of the seismogenic zone near the japanese islands implications for the serpentinization mechanism of the forearc mantle wedge abstract it has been proposed that the downdip limit of interplate thrust type earthquakes in subduction zones dlt is determined either by the brittle ductile transition around 350 450 c of the crustal material at the plate interface or the moho of the forearc of the upper plate the latter is due to possible serpentinization of the forearc mantle wedge above the subducting plate which makes the thrust zone slide in a stable manner i examine this criterion in subduction zones near japan where the pacific and philippine sea plates are subducting i show that in most of n honshu where the old pacific plate is subducting and in kanto and s kyushu where the izu bonin and kyushu palau ridges of the philippine sea plate are subducting the dlt is located at a depth of 60 70 km which is deeper than the moho of the upper plate this shows that the criterion does not hold and the mantle wedge is not likely to be serpentinized in these regions in contrast in the region off iwate in n honshu and in bonin where the old pacific plate is subducting the dlt is located at a depth of 20 30 km around the moho of the upper plate suggesting that the mantle wedge is serpentinized in these regions in sw japan where the young shikoku basin is subducting the dlt is around 30 km in depth which coincides with the moho but also with the 350 c temperature at the plate interface although seismic tomography indicates that the mantle wedge is serpentinized the dlt here does not help to examine the criterion assuming that hydration of the mantle wedge is conducted through hydro fracturing by water released from the dehydrating subducted crust i propose that the stress state of the wedge would determine the extent of serpentinization of the mantle wedge in the reverse fault type stress regime of n honshu kanto and s kyushu hydro fracturing is not favored and fractures in the thrust zone associated with repeated interplate earthquakes provide conduits for water to flow in the updip direction resulting in no serpentinization of the mantle wedge no serpentinization of the mantle wedge in turn causes interplate earthquakes in contrast in the normal fault type stress regime of the region off iwate and bonin and the strike slip fault type stress regime of sw japan hydro fracturing is favored resulting in serpentinization of the mantle wedge the frictional property at the thrust zone is governed by stable sliding resulting in no interplate earthquakes deeper than the moho thus the stress regime of the mantle wedge would determine bifurcation whether it is serpentinized or not and then the depth of dlt

three dimensional p and s wave velocity structures beneath the japan islands obtained by high density seismic stations by seismic tomography abstract we construct fine scale 3d p and s wave velocity structures of the crust and upper mantle beneath the whole japan islands with a unified resolution where the pacific pac and philippine sea phs plates subduct beneath the eurasian eur plate we can detect the low velocity low v oceanic crust of the pac and phs plates at their uppermost part beneath almost all the japan islands the depth limit of the imaged oceanic crust varies with the regions high vp vs zones are widely distributed in the lower crust especially beneath the volcanic front and the high strain rate zones are located at the edge of the extremely high vp vs zone however vp vs at the top of the mantle wedge is not so high beneath northern japan we can image the high v subducting pac plate using the tomographic method without any assumption of velocity discontinuities we also imaged the heterogeneous structure in the pac plate such as the low v zone considered as the old seamount or the highly seismic zone within the double seismic zone where the seismic fault ruptured by the earthquake connects the upper and lower layer of the double seismic zone beneath central japan thrust type small repeating earthquakes occur at the boundary between the eur and phs plates and are located at the upper part of the low v layer that is considered to be the oceanic crust of the phs plate in addition to the low v oceanic crust the subducting high v pac plate is clearly imaged to depths of approximately 250 km and the subducting high v phs zone to depths of approximately 180 km is considered to be the phs plate beneath southwestern japan the iso depth lines of the moho discontinuity in the phs plate derived by the receiver function method divide the upper low v layer and lower high v layer of our model at depths of 30 50 km beneath kyushu the steeply subducting phs plate is clearly imaged to depths of approximately 250 km with high velocities the high vp vs zone is considered as the lower crust of the eur plate or the oceanic crust of the phs plate at depths of 25 35 km and the partially serpentinized mantle wedge of the eur plate at depths of 30 45 km beneath southwestern japan the deep low frequency nonvolcanic tremors occur at all parts of the high vp vs zone within the zone the seaward side and the landward side where the phs plate encounters the mantle wedge of the eur plate we prove that we can objectively obtain the fine scale 3d structure with simple constraints such as only 1d initial velocity model with no velocity discontinuity

ultramafic and gabbroic exposures at the mid atlantic ridge geological mapping in the 15 n region abstract the outcrops of mantle derived ultramafic rocks in the 15 n region are the most extensive yet reported for the mid atlantic ridge north of the fifteen twenty fracture zone these outcrops form a belt at least 20 km long along the west wall of the axial valley and also crop out on the east axial valley wall ultramafic rocks also crop out extensively south of the fifteen twenty fracture zone based on dredging and on a morphological analysis of the bathymetric map we propose that ultramafic outcrops may be common in the crust formed between 14 30 n and 15 50 n during at least the past 2 4 m y moderately dipping fault planes and large expanses of tectonic breccia have been observed during dives on the ultramafic outcrops diving observations also show that the ultramafic rocks are capped in stratigraphic contact by a thin layer of basalt this suggests that these rocks were tectonically emplaced at the axial seafloor or very close to it then uplifted in the footwall of the faults that bound the axial valley the occurrence of ultramafic rocks on both walls of the axial valley may be due to frequent changes of faulting polarity in the axial region instead of one master shear zone there would be a complex array of cross cutting conjugate faults and shear zones that could jump inward in the axial domain as spreading proceeds

melt production during granulite facies anatexis experimental data from primitive metasedimentary protoliths we conducted fluid absent partial melting experiments at 0 5 and 1 0 gpa in the temperature range 750 to 1000 c to investigate the influence of bulk rock mg 100mg mg fe and the effects of additional tio2 on the granulite grade anatectic evolution of relatively magnesian metapelites and metagreywackes in these experiments melting began between 780 and 830 c by the incongruent breakdown of biotite to produce quartz saturated granulite facies residual mineral assemblages in equilibrium with h2o undersaturated granitic melt the glass quenched melt compositions produced in this study vary little generally the glasses have compositions similar to those of many natural strongly peraluminous leucogranites the solidus temperatures in both rock types increase with increasing mg but are unaffected by the presence or absence of a tio2 component at 0 5 gpa the metapelites melted at temperatures up to 50 c lower than the equivalent metagreywackes but at 1 gpa there was no discernible difference this study suggests that the fluid absent solidus has a steep positive dp dt slope in metapelites and steep negative dp dt slope in metagreywackes the pattern of melt production with increasing temperature is strongly controlled by the upper limit of biotite stability in tio2 free compositions this was found to increase by 15 to 20 c in the metapelites and by 30 to 40 c in the metagreywackes as a function of increasing mg from 49 to 81 the presence of a tio2 component increases the upper limit of biotite stability by 50 c in the metapelites and by 80 c in the metagreywackes over that observed in the equivalent tio2 free compositions in consequence in the tio2 free samples large pulses of melt up to 35 wt are produced over narrow temperature ranges as little as 15 c in these experiments between 830 and 875 c in the tio2 bearing samples the major pulse of melt production occurs more gradually between 830 and 900 c

large extensional aftershocks in the continental forearc triggered by the 2010 maule earthquake chile summary the mw 8 8 maule earthquake occurred off the coast of central chile on 2010 february 27 and was the sixth largest earthquake to be recorded instrumentally this subduction zone event was followed by thousands of aftershocks both near the plate interface and in the overriding continental crust here we report on a pair of large shallow crustal earthquakes that occurred on 2010 march 11 within 15 min of each other near the town of pichilemu on the coast of the o higgins region of chile field and aerial reconnaissance following the events revealed no distinct surface rupture we infer from geodetic data spanning both events that the ruptures occurred on synthetic sw dipping normal faults the first larger rupture was followed by buried slip on a steeper fault in the hangingwall the fault locations and geometry of the two events are additionally constrained by locations of aftershock seismicity based on the international maule aftershock data set the maximum slip on the main fault is about 3 m and consistent with field results the onshore slip is close to zero near the surface satellite radar data also reveal that significant aseismic afterslip occurred following the two earthquakes coulomb stress modelling indicates that the faults were positively stressed by up to 40 bars as a result of slip on the subduction interface in the preceding megathrust event in other words the pichilemu earthquakes should be considered aftershocks of the maule earthquake the occurrence of these extensional events suggests that regional interseismic compressive stresses are small several recent large shallow crustal earthquakes in the overriding plate following the 2011 mw 9 0 tohoku oki earthquake in japan may be an analogue for the triggering process at pichilemu

rate of serpentinization in seafloor environments the permeability of partially serpentinized peridotites has been measured near room temperature and found to be in the range of 10 11 to 10 12 darcys or 10 7 to 10 8 cm2 sec 1 comparable to shaley materials serpentinites act as semipermeable membranes to salt ions in ocean crust and deep groundwater environments serpentinization must be a continuous process controlled only by water access to ultramafic rocks through their cover given this it will generate high local strains and stresses with episodic cracking the serpentinization process may also contribute to heat flow and ocean floor topography

low frequency tremors intraslab and interplate earthquakes in southwest japan from a viewpoint of slab dehydration 1 low frequency tremors lft recently found in the southwest japan fore arc likely occur due to hydro fracturing from fluids leaving the dehydrating philippine sea slab however there are some places without such lft they are n izu kanto e shikoku and s kyushu these are the places where island arc type crust is subducting we propose that dehydration of the subducted crust does not occur beneath these regions because it is composed mainly of tonalite lacking the quantity of hydrous minerals seen in normal subducting oceanic crust we show that almost no earthquake occurs within the subducted crust in such regions consistent with dehydration embrittlement hypothesis for intraslab seismicity the lack of dehydration from the crust would also affect the mode of occurrence of interplate earthquakes because pore fluid pressure is more difficult to rise at the thrust zone this might be reflected in the irregular occurrence of great earthquakes in these regions

onto what planes should coulomb stress perturbations be resolved 1 coulomb stress maps are produced by computing the tensorial stress perturbation due to an earthquake rupture and resolving this tensor onto planes of a particular orientation it is often assumed that aftershock fault planes are optimally oriented in other words the regional stress and coseismic stress change are used to compute the orientation of planes most likely to fail and the coseismic stress is resolved onto these orientations this practice assumes that faults capable of sustaining aftershocks exist at all orientations an assumption contradicted by the observation that aftershock focal mechanisms have strong preferred orientations consistent with mapped structural trends here we systematically investigate the best planes onto which stress should be resolved for the landers hector mine loma prieta and northridge earthquakes by quantitatively comparing observed aftershock distributions with stress maps based on optimally oriented planes two and three dimensional main shock orientation and regional structural trend we find that the best model differs between different tectonic regions but that in all cases models that incorporate the regional stress field tend to produce stress maps that best fit the observed aftershock distributions although not all such models do so equally well our results suggest that when the regional stress field is poorly defined or in structurally complex areas the best model may be to fix the strike of the planes upon which the stress is to be resolved to that of the main shock but allow the dip and rake to vary

hydrothermal activity on a 105 year scale at a slow spreading ridge tag hydrothermal field mid atlantic ridge 26 n new radiometric dates are presented for the hydrothermal deposits in the alvin and mir relict hydrothermal zones located on the lower east wall of the rift valley within the tag hydrothermal field and placed in context of the evolution of the tag field as a whole over the past 105 years the field includes an active high temperature sulfide mound situated near the base of the east wall and a low temperature zone at middepth on the east wall all within a 5 by 5 kilometer area the oldest hydrothermal deposits sampled in the mir relict zone the low temperature zone and an inactive sulfide mound in the alvin relict zone are low temperature manganese crusts dated at 140 000 125 000 and 74 000 years respectively high temperature activity began about 100 000 years ago in the mir zone a 50 000 year high temperature event took place at the active sulfide mound in the mir zone and at the inactive sulfide mound in the alvin zone this event is followed by sporadic activity at the mir zone and the active sulfide mound until 10 000 and 5000 years ago respectively a reactivation of the system affected the mir zone from about 2000 to 600 years ago the presently active sulfide mound was reactivated only 60 years ago after a quiescence of about 4000 years the radiochronologic record reveals that the hydrothermal activity has been episodic with a major high temperature event throughout the tag field at 50 000 years and other more localized events

evolution of the mariana convergent plate margin system the mariana convergent plate margin system of the western pacific provides opportunities for studying the tectonic and geochemical processes of intraoceanic plate subduction without the added complexities of continental geology the system s relative geologic simplicity and the well exposed sections of lithosphere in each of its tectonic provinces permit in situ examination of processes critical to understanding subduction tectonics its general history provides analogs to ancient convergent margin terranes exposed on land and helps to explain the chemical mass balance in convergent plate margins the mariana convergent margin s long history of sequential formation of volcanic arcs and extensional back arc basins has created a series of volcanic arcs at the eastern edge of the philippine sea plate the trenchward edge of the overriding plate has a relatively sparse sediment cover rocks outcropping on the trench s inner slope are typical of the early formed suprasubduction zone s lithosphere and have been subjected to various processes related to its tectonic history pervasive forearc faulting has exposed crust and upper mantle lithosphere many large serpentinized peridotite seamounts are within 100 km of the trench axis from these we can learn the history of regional metamorphism and observe and sample active venting of slab fluids ocean drilling recovered suprasubduction zone lava sequences erupted since the eocene that suggest that the forearc region remains volcanologically dynamic seismic studies and seafloor mapping show evidence of deformation throughout forearc evolution large portions of uplifted southern forearc are exposed at the larger islands active volcanoes at the base of the eastern boundary fault of the mariana trough vary in size and composition along strike and record regional differences in source composition their locations along strike of the arc are controlled in part by cross arc structures that also facilitate formation of submarine volcano chains extending from the base of the fault westward into the back arc basin the western boundary is the west mariana ridge the western portion of the volcanic arc active prior to formation of the mariana trough the trough evolved in a two stage extension process of rifting and subsequent seafloor spreading the back arc basin varies along strike from rifted arc lithosphere with scattered volcanoes but no real spreading center in the north to a complex mid ocean type spreading center south of 20 n the change from initial rifting to true seafloor spreading is also evident across the mariana trough from rifted topography near the west mariana ridge to spreading ridges in the central to eastern basin south of 20 n this morphologic change indicates an early stage of extension with basin and range type topography predominant and volcanism restricted to fissure eruptions along fault block boundaries the spreading ridges and abyssal hill morphology evolved later as new lithosphere was generated at elongate volcanic ridges located in the center of rift valleys the center of extension intersects the active volcanic front differently at either end of the mariana trough in the north extension is by rifting of arc lithosphere where it intersects the arc in the south a major strike slip fault extends from the trench axis across the forearc through the volcanic arc and into the back arc basin arc magmas apparently leak along this fault zone into the forearc and the back arc spreading center the complexity of interrelated tectonism and magmatism in this convergent margin is daunting but studies of arc systems such as this provide the best hope of interpreting many of the exposed terranes accreted to continents comparison of subaerial terranes with recent studies of intraoceanic convergent margins will add to our understanding of plate interactions and of the evolution of the volcanic arcs and extensional back arc basins generated within such environments

lithium isotopic systematics of granites and pegmatites from the black hills south dakota to study li isotopic fractionation during granite differentiation and late stage pegmatite evolution li isotopic compositions and concentrations have been measured for the s type harney peak granite the spatially associated tin mountain pegmatite and possible metasedimentary source rocks in the black hills south dakota the harney peak granite is isotopically heterogeneous with 7 li varying from 3 1 to 6 6 the 7 li values of proterozoic metasedimentary rocks that are possible sources of the harney peak granite range from 3 1 to 2 5 and overlap with post archean shales and the harney peak granite for the granite suite there is no correlation between 7 li and elements indicative of degrees of granite differentiation sio2 li rb etc the li isotopic composition of the harney peak granite therefore appears to res ect the source composition minerals from the zoned tin mountain pegmatite have extremely high li contents and heavier li isotopic compositions than the granite or surrounding black hills metasedimentary rocks the heavier compositions may res ect li isotopic fractionation resulting from extensive crystal melt fractionation lithium concentrations decrease in the order spodumene 3 7 wt muscovite 0 2 to 2 0 wt plagioclase 100 1100 ppm quartz 30 140 ppm plagioclase muscovite and spodumene in all zones display a relatively narrow range in 7 li of 7 9 to 11 4 in contrast quartz is isotopically heavier and more variable 14 7 to 21 3 with 7 li showing an inverse correlation with li concentration this correlation res ects the mixing of isotopically heavy li in quartz and lighter li in s uid inclusions as documented by s uid inclusion compositions

lithium isotopic composition of submarine basalts implications for the lithium cycle in the oceans abstract we have measured the li isotope composition of young pristine basalts from active ocean ridge crests of progressively older basalts along a dredging transect and a limited number of hydrothermally altered basalts the data significantly extend our limited knowledge of the isotopic abundance ratio of li in geological material fresh mid ocean ridge basalts have 6li values of between 3 4 and 4 7 relative to isotope standard l svec during low temperature weathering on the seafloor the isotopic composition of the rock becomes increasingly heavier due to addition of seawater li 6li 32 3 the oldest 46 m y and most altered rock studied has an isotopic composition of 14 a linear relationship exists between 6li and the inverse of li concentration suggesting that li in weathered basalts can be regarded as a two component mixture of basaltic li and seawater derived li that has been incorporated in alteration minerals most likely secondary clays such as smectite and phillipsite the inferred li isotopic composition of the alteration endmember indicates an apparent isotopic fractionation factor of 1 019 relative to the seawater source thus li uptake by secondary minerals from the low temperature weathering process and by analogy incorporation in similar authigenic minerals in marine sediments provides a mechanism for preferential removal of the lighter li isotope from ocean water however isotopic fractionation due to authigenic clay formation alone cannot account for the isotopic difference between seawater and its principal sources unless the hydrothermal flux is comparable to the river flux alternatively more important sinks of6li must exist if the steady state isotopic composition of li in ocean water is to be maintained

the lanthanide tetrad effect in apatite from the altay no 3 pegmatite xingjiang china an intrinsic feature of the pegmatite magma abstract apatite and associated minerals such as spessartite tourmaline beryl and alkali feldspar from the altay no 3 granite pegmatite in xinjiang china have highly variable concentrations of rare earth elements rees and possess chondrite normalized ree patterns that show a clear convex tetrad effect the y ho and sr eu ratios of the apatite samples are extremely fractionated relative to the corresponding chondritic values based on the la lu sr eu and y ho ratios of these minerals the crystallization zones of the pegmatite can be divided into two groups that are interpreted to reflect two main stages of magma evolution namely a magmatic and a magmatic hydrothermal transition stage the degree of the lanthanide tetrad effect in ree patterns of the apatite expressed by the quantification factor te 1 3 ranges from 1 3 to 1 9 the size of the tetrads correlates with y ho eu eu and sr eu ratios in addition to the mineral samples from the pegmatite the ree patterns of the altered wall rocks also show a convex tetrad effect a mass balance calculation was carried out to constrain the ree partitioning between the aqueous fluid crystalline phases and the residual melt it was found that the ree patterns of these three phases are dominated by the concentration of the rees in the initial pegmatite forming melt accordingly it is proposed that the ree tetrad effect and the fractionation of the chemically coherent isovalent element pairs in the studied mineral separates represent intrinsic features of the granitic pegmatite forming melt it appears unlikely that these geochemical characteristics developed as a result of melt mineral fluid interactions taking place during magma crystallization after its intrusion interaction of an external fluid with pegmatite magma before emplacement or with parent materials of the highly evolved granitic rocks before melting would be favorable for producing the ree tetrad effect in highly evolved granitic rocks

lithium and boron in late orogenic granites isotopic fingerprints for the source of crustal melts abstract geochemically diverse late and post variscan granites of the erzgebirge vogtland the saxon granulite massif and thuringia germany formed by anatectic melting of palaeozoic sedimentary successions and associated mafic to felsic volcanic rocks the compositional diversity of the least evolved of these granites is largely inherited from the protoliths we present li and b isotopic data of these granites and compare them with the isotopic composition of their protoliths to investigate whether i there exist systematic differences in the li and b isotopic composition among different granite types and ii li and b isotopic compositions provide information on the granite sources complementary to information from the isotopic composition of sr nd and pb and the trace element signatures low f biotite and two mica granite types have flat upper continental crust ucc normalized trace element pattern with variable enrichments in li rb cs sn and w and depletions in sr ba and eu these signatures are least pronounced for the niederbobritzsch biotite granite which has the largest contribution of mafic material and most pronounced for the two mica granites the granites show a relatively narrow range of 7li values 3 0 to 0 5 and a broad range of 11b values 13 4 to 20 1 the 11b values are lower in rocks with distinctly higher contents of li rb cs and sn the high 11b of the niederbobritzsch granite may be explained by the melting of former altered oceanic crust in its source relative to ucc intermediate f to high f low p granites show strong depletions in sr ba eu as well as zr and hf strong enrichments in li rb and cs as well as nb sn ta and w and ree pattern with stronger enrichments for hree than for lree these granites show narrow ranges of 7li 2 0 to 1 6 and 11b values 14 7 to 9 1 reflecting the smaller variability of the li and b isotopic composition in their source lithologies the anomalously high 7li value 14 7 of one granite sample burgberg which is similar to 7li values of its wall rocks up to 14 5 may indicate late magmatic fluid rock interaction with external wall rock derived fluids because of the small compositional range of most source lithologies the li and b isotopic variation in the granites is also small indicating that the isotopic composition of li and b does not represent a particularly sensitive source tracer with the exception of source lithologies characterized by extreme 7li or 11b values

nondilatant brittle deformation of serpentinites implications for mohr coulomb theory and the strength of faults we conducted deformation experiments to investigate the strength deformation processes and nature of the brittle ductile transition of lizardite and antigorite serpentinites a transition from localized to distributed deformation occurs as confining pressure increases from 200 to 400 mpa at room temperature deformation in both brittle localized and ductile distributed regimes is accommodated by shear microcracks which form preferentially parallel to the 001 cleavage axial microcracks mode i are infrequently observed volumetric strain measurements demonstrate that brittle deformation is mostly nondilatant consistent with the shear dominated microcracking three observations indicate that deformation in the ductile regime is accommodated by cataclastic flow 1 a lack of evidence for crystal plastic deformation 2 a positive pressure dependence of the maximum differential stress and 3 abundant evidence for brittle microcracking the weakness of serpentinites relative to other brittle rocks is explained by a low fracture strength along the 001 cleavage combined with the low pressure dependence of strength the transition from brittle to ductile deformation occurs at the crossover between the strength of intact serpentinite and the friction law unique to each type of serpentinite rather than the more general byerlee s law if brittle deformation regimes are defined based on the mode of microcracking and on the occurrence of crystal plasticity serpentinites define an end member style of nondilatant brittle deformation this deformation style may result in extremely weak faults in nature and it may also strongly influence the tectonic evolution of the oceanic lithosphere where serpentinite is present

fissuring near the tag active hydrothermal mound 26 n on the mid atlantic ridge abstract analysis of 12 000 electronic still camera images collected with the argo ii vehicle near the trans atlantic geotraverse tag active hydrothermal mound 26 n on the mid atlantic ridge has made possible the first quantitative in situ assessment of both fissure orientation and width within the median valley of a slow spreading ridge fissures near the tag mound are partially rubble filled extensional fractures that cut lightly sedimented seafloor and in 1 of our observations host pillow lavas fissure widths range from 0 15 to 3 5 m with a mean of 0 7 m and do not vary systematically within the survey area first order estimates of crack depth based on these width measurements and reasonable elastic moduli indicate that fissures are restricted to depths 45 from the strike of the ridge axis the formation of obliquely oriented fissures requires that the local least compressive stress direction varies at least temporarily from that predicted by the regional tectonic stress field associated with plate separation this stress field reorientation may be facilitated by variations in the style of magma emplacement within the rift the close spatial association of long term hydrothermal activity fissure hosted lava flows and faults and fissures trending oblique to the spreading axis suggests a causal relationship between the impact of dike intrusion and the maintenance of localized hydrothermal flow

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

the effect of fluorine on viscosities in the system na2o al2o3 sio2 implications for phonolites trachytes and rhyolites the effect of fluorine on melt viscosities of five compositions in the system na2o al2o3 sio2h as been investigateda t one atmospherea nd 1000 1600 cb y concentric cylinder viscometry the compositions chosen were albite jadeite and nepheline on the join naalolsio2 and two others of the join at 75 mole percent sio2 one peralkaline and one peraluminous all melt viscosities were independent of shear rate over two orders of magnitude indicating newtonian behavior all viscosity temperature relationships were arrhenian within error fluorine reduces the viscosities and activation energies of all melts investigated the viscosity reducing power of fluorine increases with the sio2 content of melts on the join naalo2 sio2 and is a maximum at na al molar i for melts containing 75 mole percent sio2 fluorine and water have similar effects on aluminosilicate melt viscosities probably due to depolymerization of these melts by replacement of si o si al bridges with si oh and si f bonds respectively evidence from slag systems shows that fluorine also reduces the viscosity of depolymerized silicate melts the viscous flow of phonolites trachytes and rhyolites will be strongly afected by fluorine it appears that fluorine contents of igneous rocks may be combined with water in calculation schemes for determining the viscosity of natural melts