textures and chemical compositions of magnetite from iron oxide copper gold iocg and kiruna type iron oxide apatite ioa deposits and their implications for ore genesis and magnetite classification schemes

time resolved seismic tomography detects magma intrusions at mount etna the continuous volcanic and seismic activity at mount etna makes this volcano an important laboratory for seismological and geophysical studies we used repeated three dimensional tomography to detect variations in elastic parameters during different volcanic cycles before and during the october 2002 january 2003 flank eruption well defined anomalous low p to s wave velocity ratio volumes were revealed absent during the pre eruptive period the anomalies trace the intrusion of volatile rich 4 weight percent basaltic magma most of which rose up only a few months before the onset of eruption the observed time changes of velocity anomalies suggest that four dimensional tomography provides a basis for more efficient volcano monitoring and short and midterm eruption forecasting of explosive activity

shallow intrusive processes during 2002 2004 and current volcanic activity on mt etna 1 the understanding of shallow intrusive processes during 2002 2004 as well as the causes of the volcano tectonic seismicity has been improved at mt etna by comparing the inversion results from gps data with accurate 3d hypocentral locations our findings indicate that short periods of deflation about six months were followed by recharging phases after the end of both the 2001 and 2002 2003 flank eruptions during the last recharging phase june 2003 august 2004 modeling results and seismic observations suggest a composite mechanism of re injection of magma into the rift zones s and ne similar to that leading to the 2002 2003 flank eruption which could have triggered the summit eruption started on september 7 2004

constant holocene southern ocean 14c reservoir ages and ice shelf flow rates article i nfo southern ocean radiocarbon reservoir ages i e non zero radiocarbon ages in seawater are the highest in the world s surface ocean constraining these reservoir ages at present and in the past is important not only because unknown reservoir ages limit the interpretation of antarctic radiocarbon chronologies but also because reservoir ages provide information about ocean circulation as a recorder of past circulation and as an end member for major deep water masses in today s ocean in this study we use paired u th and 14 c ages of an unusual set of solitary coral samples trapped by fringing ice shelves in the ross sea to provide the first detailed study of holocene reservoir ages for the southern ocean our results indicate a relatively constant marine radiocarbon reservoir age of 1144 120 years for the past 6000 years these results are consistent with extrapolation of the relationship between 14 c and alkalinity seen elsewhere supporting the use of this empirical relationship in high latitudes the results also suggest constant deep ocean circulation and air sea exchange during the holocene and provide a good target for tuning ocean models of modern circulation combining the new ages for corals with their distance from the modern day ice shelf grounding line provides some of the first long term records of ice shelf velocities for any region and indicates constant flow of the mcmurdo ice shelf during the holocene at a rate similar to that observed today

holocene elephant seal distribution implies warmer than present climate in the ross sea we show that southern elephant seal mirounga leonina colonies existed proximate to the ross ice shelf during the holocene well south of their core sub antarctic breeding and molting grounds we propose that this was due to warming including a previously unrecognized period from 1 100 to 2 300 14c yr b p that decreased coastal sea ice and allowed penetration of warmer than present climate conditions into the ross embayment if as proposed in the literature the ice shelf survived this period it would have been exposed to environments substantially warmer than present

viscoelastic modeling of deformation and gravity changes induced by pressurized magmatic sources abstract gravity and height changes which reflect magma accumulation in subsurface chambers are evaluated using analytical and numerical models in order to investigate their relationships and temporal evolutions the analysis focuses mainly on the exploration of the time dependent response of gravity and height changes to the pressurization of ellipsoidal magmatic chambers in viscoelastic media firstly the validation of the numerical finite element results is performed by comparison with analytical solutions which are devised for a simple spherical source embedded in a homogeneous viscoelastic half space medium then the effect of several model parameters on time dependent height and gravity changes is investigated thanks to the flexibility of the numerical method in handling complex configurations both homogeneous and viscoelastic shell models reveal significantly different amplitudes in the ratio between gravity and height changes depending on geometry factors and medium rheology the results show that these factors also influence the relaxation characteristic times of the investigated geophysical changes overall these temporal patterns are compatible with time dependent height and gravity changes observed on etna volcano during the 1994 1997 inflation period by modeling the viscoelastic response of a pressurized prolate magmatic source a general agreement between computed and observed geophysical variations is achieved

heavy mineral compositions of the changjiang yangtze river sediments and their provenance tracing implication abstract a total of 71 floodplain sediments were collected from the mainstream and major tributaries of the changjiang for heavy mineral examinations the upper changjiang samples contain more heavy minerals in the very fine sand fraction than the middle lower samples which are relatively enriched in polygenetic heavy minerals the heavy mineral assemblages are characterized by fe ti oxide minerals calcic amphibole epidote garnet biotite and zircon which exhibit large variations in percent between different tributaries and the mainstream the widely distributed granitoids and permian emeishan basalt in the drainage basins account for the high contents of zircon and hypersthene in the xiangjiang and daduhe samples respectively most of the detrital magnetite grains are homogeneous in typomorphic feature and almost stoichiometric in chemical composition the discrimination plot of tio2 v2o3 versus mgo mgo al2o3 suggests that most of the detrital magnetite grains are sourced from felsic plutonic volcanic and metamorphic parent rocks and those from different tributaries have distinct chemical compositions our study suggests that the combination of transparent heavy mineral assemblages and varietal study of individual heavy minerals sheds new light on the provenance discrimination of the changjiang sediments and contributes to the mineralogical determination of ancient source rocks within a large drainage basin despite the sediment recycling and complex source rock types

226ra in barite absolute dating of holocene southern ocean sediments and reconstruction of sea surface reservoir ages absolute dating of marine sediments is necessary to achieve reliable correlations of paleoclimate records the 2 2 6 ra decay in barite separated from a deep sea core of the southern ocean was used to determine absolute ages for the holocene epoch comparison with 1 4 c ages measured on planktic foraminifers provides the first past sea surface reservoir ages in the antarctic zone of the southern ocean throughout the middle to late holocene our results indicate a reservoir age of ca 1100 yr comparable to modern estimates a significantly higher reservoir age ca 1900 yr is obtained for the early holocene which suggests a major change in the southern ocean circulation at that time

subsolidus evolution and alteration of titanomagnetite in ocean ridge basalts from deep sea drilling project ocean drilling program hole 504b9 leg 83 implications for the timing of magnetization magnetic minerals in six samples of oceanic basalts of the transition zone and upper sheeted dikes from deep sea drilling project ocean drilling program dsdp odp hole 504b leg 83 were studied by methods of rock magnetism and transmission electron microscopy tem tem observations showed that the magnetic mineral in these basalts is end member magnetite tmo of extremely fine grain size 30 100 nm primarily in the range of pseudosingle domain magnetite consistent with the rock magnetic properties including hysteresis parameters curie temperature and low temperature measurements verwey transition magnetite formed by two different processes 1 oxidation exsolution true exsolution and hydrothermal alteration and 2 oxidation exsolution a second stage of oxidation exsolution and hydrothermal alteration the primary titanomagnetite tm60 70 that crystallized from the melt thus evolved to end member magnetite coexisting with titanite sphene kassite ulvospinel tm 87 and ilmenite on a submicroscopic scale on the basis of the formation mechanisms of the magnetic carrier the primary titanomagnetite tm 60 with curie temperature of 180 c did not acquire thermoremanent magnetization trm in these basalts instead the ti bearing magnetite tm 10 20 that formed as oxidized or exsolved lamellae acquired its first thermal chemical remanent magnetization crm at 500 400 c during subsolidus cooling upon the onset of hydrothermal alteration the recrystallized end member magnetite acquired a second crm the natural remanent magnetization of the basalts from the transition zone and upper sheeted dikes is therefore characteristic of crms that were acquired when titanomagnetite altered in part to magnetite during subsolidus cooling and hydrothermal alteration close to the ridge axis

textural and chemical variations of magnetite from porphyry cu au and cu skarn deposits in the zhongdian region northwestern yunnan sw china abstract the zhongdian region in northwestern yunnan sw china hosts a number of important porphyry and skarn deposits in this study we integrate petrographic observations sem eds elemental mapping epma and la icp ms geochemical analyses on magnetite from several key deposits in the region magnetite grains from the pulang porphyry cu au lannitang porphyry au cu langdu cu skarn disuga porphyry and andesite of tumugou formation were analyzed to unravel the magnetite formation mechanism and the relative oxygen fugacity of the porphyry skarn ore related hydrothermal fluids in this region two types of magnetites magnetite i and ii have been identified magnetite i is commonly distributed in the chlorite and or sericite alteration zone s magnetite i grains are characterized by being textually inhomogeneous the magnetite i core with local ilmenite exsolution lamellae is ti cr rich indicative of a dominantly igneous origin the magnetite i core and its surrounding primary igneous minerals biotite amphibole and plagioclase underwent late alteration magnetite i grains are characterized by core rim texture sharp core rim contact and irregular grain boundaries these textural features indicate that magnetite i may have undergone dissolution reprecipitation processes during which the magnetite i core was chemically purified via losing ti and gaining fe magnetite ii in potassic and sericite alteration zones is textually homogeneous with relatively low contents of ti mg al v ni and mn we suggest that magnetite ii was formed by hydrothermal alteration compared to magnetite ii grains from the pulang and langdu deposits those from the lannitang deposit have lower mn and v but higher sn contents indicative of higher oxygen fugacity of the hydrothermal fluids we propose that binary plots such as ti vs sc v vs sn and ga vs sn can distinguish the different origins of magnetites from an exploration perspective magnetite ii from the altered disuga porphyries has similar texture and composition to its counterpart from the potassic altered pulang porphyries this together with the presence of cu sulfide inclusions in the disuga magnetite i suggests significant mineralization potential in these porphyries

geothermometry using minor and trace elements in igneous and hydrothermal magnetite abstract the temperature t and oxygen fugacity fo2 dependence of minor and trace elements mg al mn cr ni in magnetite mag in equilibrium with water bearing silicate liquids varying in composition from basalt to rhyolite 49 77 wt sio2 is examined in a compilation of experimental data n 299 from 700 to 1050 c 50 900 mpa and five order of magnitude range in fo2 the xmg mg mg fetot of mag is strongly dependent on t and can be formulated into an empirical geothermometer tmg mag c 8344 320 lnxmg 4 1 0 28 273 r2 0 83 that reproduces 72 of the experimentas to 50 c and conventional fe ti oxide thermometry in tuffs and vitrophyres to better than 60 c for 85 of the samples the tmg mag thermometer may have potential application in igneous or hydrothermally altered rocks where conventional thermometry or oxybarometry is not possible application to natural mag n 476 compiled from several settings shows igneous rocks mostly record 700 c but with some lower temperatures in plutonic rocks due to slow cooling hydrothermal samples skarns porphyry deposits show consistently subsolidus conditions tmg mag of 700 350 c consistent with other mineral and o isotopic equilibria the element correlations in natural mag from various settings are explored using multivariate statistical analysis to identify which minor or trace elements most critically depend on t fo2 or fluid composition

composite ground deformation pattern forerunning the 2004 2005 mount etna eruption 1 after the end of the 2002 2003 eruption mount etna activity was characterized only by gentle degassing at the summit craters and some earthquake swarms suddenly an eruption started on 7 september 2004 in complete absence of summit crater volcanic activity seismicity or seismic tremor and ground deformation this is the first time that magma poured out passively without preeruptive and coeruptive volcanic and or geophysical phenomena the primary key to understanding this event is represented by the ground deformation pattern recorded through gps measurements during the year before the eruption the ground deformation shows inflation superimposed by a predominant eastward movement of the eastern sector at a rate never observed before in a noneruptive period the images from satellite radar interferometry confirmed this pattern the deformation field clearly shows that the maximum tension in the eastern sector of the volcano caused the opening of the eruptive fracture which favored the silent pouring out of already resident magma

trace elements in magnetite as petrogenetic indicators we have characterized the distribution of 25 trace elements in magnetite mg al si p ca sc ti v cr mn co ni cu zn ga ge y zr nb mo sn hf ta w and pb using laser ablation icp ms and electron microprobe from a variety of magmatic and hydrothermal ore forming environments and compared them with data from the literature we propose a new multielement diagram normalized to bulk continental crust designed to emphasize the partitioning behavior of trace elements between magnetite the melt fluid and co crystallizing phases the normalized pattern of magnetite reflects the composition of the melt fluid which in both magmatic and hydrothermal systems varies with temperature thus it is possible to distinguish magnetite formed at different degrees of crystal fractionation in both silicate and sulfide melts the crystallization of ilmenite or sulfide before magnetite is recorded as a marked depletion in ti or cu respectively the chemical signature of hydrothermal magnetite is distinct being depleted in elements that are relatively immobile during alteration and commonly enriched in elements that are highly incompatible into magnetite e g si and ca magnetite formed from low temperature fluids has the lowest overall abundance of trace elements due to their lower solubility chemical zonation of magnetite is rare but occurs in some hydrothermal deposits where laser mapping reveals oscillatory zoning which records the changing conditions and composition of the fluid during magnetite growth this new way of plotting all 25 trace elements on 1 diagram normalized to bulk continental crust and elements in order of compatibility into magnetite provides a tool to help understand the processes that control partitioning of a full suit of trace elements in magnetite and aid discrimination of magnetite formed in different environments it has applications in both petrogenetic and provenance studies such as in the exploration of ore deposits and in sedimentology

the application of heavy indicator mineralogy in mineral exploration with emphasis on base metal indicators in glaciated metamorphic and plutonic terrains abstract indicator mineralogy is used to explore for a wide variety of mineral commodities the method utilizes minerals which are sufficiently heavy to be readily concentrated in the laboratory often colourful and possess other useful physical and chemical properties the minerals also must be source specific some indicator minerals are true resistate minerals the others although less resistant are stable in oxidized glacial drift and many non glacial sediments a few such as gold grains are silt sized but most are coarse grained grain size has a major impact on indicator mineral dispersal patterns in glacial drift the coarse grained indicator minerals are of two main types 1 kimberlite indicator minerals kims and 2 metamorphosed or magmatic massive sulphide indicator minerals mmsims kims are enriched in mg and cr and most mmsims are enriched in mg mn al or cr these indicator elements cannot be diagnosed geochemically in anomalous heavy mineral concentrates because the concentrates contain other more plentiful non indicator minerals containing the same elements in the same chemical form chalcopyrite is also a very useful mmsim but the number of surviving grains in a dispersal train is too low for detection by selective geochemical analysis mmsims are derived from three main types of base metal deposits and their associated alteration or reaction zones 1 volcanosedimentary massive sulphides encompassing volcanogenic sedex and mississippi valley subtypes in medium to high grade regional metamorphic terrains 2 skarn and greisen deposits and 3 magmatic ni cu sulphides the variety of mmsims associated with ni cu deposits is astonishing apparently reflecting mineral hybridization related to assimilation of sulphurous sedimentary rocks by ultramafic magmas cr diopside is one of the best indicators of fertile ni cu environments although not necessarily of the actual ni cu deposits heavy indicator mineralogy is much more sensitive than heavy mineral geochemical analysis and offers many exploration benefits in regional exploration programs including 1 sampling efficiencies 2 enlargement of both the bedrock target and dispersal train 3 coverage of a wider range of mineral commodities 4 undiminished sensitivity in areas of overabundant non indicator heavy minerals 5 visual evidence of points of origin of dispersal trains and 6 indications of the economic potential of the source mineralization it is most effective as a reconnaissance exploration tool and is particularly well suited for testing gneissic volcanosedimentary and plutonic terranes where base and precious metal deposits are highly modified and difficult to recognize by other prospecting methods

a new holocene relative sea level curve for terra nova bay victoria land antarctica more than 100 radiocarbon dates of penguin guano and remains shells and seal skin afford ages for raised beaches adjacent to terra nova bay antarctica these dates permit construction of a new relative sea level curve that bears on the timing of deglaciation recession of the ross sea ice sheet grounding line from terra nova bay occurred no earlier than 7200 14c yr 8000 cal yr bp retreat along the victoria land coast may have been rapid possibly contributing to eustatic sea level rise centred at ca 7600 cal yr bp the presence of a significant amount of ice remaining in the ross sea embayment in holocene time lessens the chance that antarctica contributed significantly to meltwater pulse 1a several thousand years earlier copyright 2004 john wiley sons ltd

trace elements in magnetite from porphyry cu mo au deposits in british columbia canada abstract this study examines trace elements in hydrothermal magnetite from five porphyry cu mo au deposits and two skarns in british columbia canada trace element concentrations vary several orders of magnitude both within and between magnetite from skarn and porphyry deposit settings the heterogeneous composition of hydrothermal magnetite may in part be due to the short duration low temperature and multiple fluid events that attend the formation of porphyry ore deposits principal component analysis shows two dominant patterns of trace element abundances in hydrothermal magnetite firstly positive correlations of ti al and v which account for nearly 40 of the total variation in magnetite are inferred to depend on temperature and oxygen fugacity secondly antithetic abundances of lower valence cations co mn with higher valence cations sn and mo may reflect variations in the redox potential acidity and metal speciation of hydrothermal fluids the cu fe and mn fe ratios calculated for fluids in equilibrium with the hydrothermal magnetite using experimental partitioning data are similar to those measured directly in brines trapped in quartz hosted fluid inclusions from porphyry cu mo au deposits

in situ la icp ms trace element analysis of magnetite from mesozoic iron oxide apatite ioa deposits in the luzong volcanic basin eastern china abstract the iron copper gold belt in the middle and lower yangtze river region is among the most important metallogenic provinces in eastern china the luzong lujiang zongyang ore district hosts many significant iron deposits in the luzong basin and is an important part of this belt the yangshan deposit is hosted in the cretaceous volcanic rocks of the middle luzong basin the makou deposit in the southern central luzong basin has orebodies at the contact between gabbro diorite and volcanic rocks of the zhuanqiao formation there are three types of magnetite in these two deposits 1 magmatic magnetite from the makou gabbro diorite 2 hydrothermal magnetite from the makou deposit and 3 hydrothermal magnetite in the volcanic rocks from the yangshan deposit magmatic magnetite from gabbro diorite in the makou deposit has concentrations of elements mg al k sc ti v cr mn co ni and zn higher than hydrothermal magnetite magmatic magnetite from the makou deposit are the highest concentrations of cr sc ni al and zn but hydrothermal magnetite from this deposit has the lowest concentrations of these elements whereas hydrothermal magnetite from the yangshan deposit contains intermediate concentrations of these elements overall there are positive correlations between mg and al and ti and mn the makou and yangshan hydrothermal magnetite show highly variable mg values apparently influenced by clinopyroxene diopside crystallisation from hydrothermal fluids the ti ga and sn contents of the makou hydrothermal magnetite are generally higher than those from the yangshan deposit suggesting the makou magnetite formed under higher temperatures the formation of ival rich clinopyroxene in the yangshan deposit may have resulted in the low v content of magnetite whereas the high cr content may have been derived from the associated trachyandesite leaching of thick volcanic sequences via magmatic fluids may have led to the incorporation of certain elements into the ore forming fluids resulting in higher al mn and zn concentrations in the yangshan magnetite the makou hydrothermal magnetite has na and k contents similar but ca content higher than that of the magmatic magnetite indicating that na and k were retained in the melt but ca was enriched in the fluid both the makou and yangshan hydrothermal magnetites contain high v and low cr concentrations and resemble magnetite in ioa deposits elsewhere combined with the wall rocks and mineral assemblages the makou and yangshan deposits are best classified as magnetite apatite deposits intensive diopside alteration in these two deposits may be related to the marine sedimentary strata at the depth

delayed triggering of the 1999 hector mine earthquake by viscoelastic stress transfer stress changes in the crust due to an earthquake can hasten the failure of neighbouring faults and induce earthquake sequences in some cases1 2 3 4 5 the 1999 hector mine earthquake in southern california magnitude 7 1 occurred only 20 km from and 7 years after the 1992 landers earthquake magnitude 7 3 this suggests that the hector mine earthquake was triggered in some fashion by the earlier event but uncertainties in the slip distribution and rock friction properties associated with the landers earthquake have led to widely varying estimates of both the magnitude and sign of the resulting stress change that would be induced at the location of the hector mine hypocentre with estimates varying from 1 4 bar ref 6 to 0 5 bar ref 7 more importantly coseismic stress changes alone cannot satisfactorily explain the delay of 7 years between the two events here we present the results of a three dimensional viscoelastic model that simulates stress transfer from the ductile lower crust and upper mantle to the brittle upper crust in the 7 years following the landers earthquake using viscoelastic parameters that can reproduce the observed horizontal surface deformation following the landers earthquake our calculations suggest that lower crustal or upper mantle flow can lead to postseismic stress increases of up to 1 2 bar at the location of the hector mine hypocentre during this time period contributing to the eventual occurrence of the 1999 hector mine earthquake these results attest to the importance of considering viscoelastic processes in the assessment of seismic hazard8 9 10 11

u and th concentrations and isotope ratios in modern carbonates and waters from the bahamas the short residence times of th and pa in seawater make them very responsive to changes in the ocean environment we use a new multi ion counting technique to make th and pa isotope measurements in seawaters from a near shore environment in which oceanic chemical tracers are not overwhelmed by terrestrial inputs the bahamas an unusual feature of the bahamas setting is the shallow depth of water residing on the bank tops these waters have significantly lower 232th 230th 10 000 than those immediately adjacent to the banks 24 000 31 000 and a 231pa 230th near the production ratio 0 1 the change in 232th 230th and 231pa 230th on the bank tops is explained by almost quantitative removal of th and pa by scavenging and their replacement with a mixture of 230th and 231pa alpha recoiled from the underlying carbonates together with th from dust dissolution analysis of a water profile in the tongue of the ocean which separates the great and little bahama banks allows us to trace the movement of bank top water to depth a distinct minimum in both 232th 230th 13 000 and 231pa 230th 0 5 is observed at 430 m and is interpreted to reflect density cascading of bank top water with entrained carbonate sediment these results suggest that th and pa can be used as water mass tracers in near shore environments uranium concentration measurements on the same waters demonstrate that u is conservative across a range in salinity of 2 psu with a concentration of 3 33 ppb at a salinity of 35 the incorporation of u and th isotopes into marine carbonates has also been assessed by analyzing carbonate samples from the same location as these bahamas waters such incorporation is critical for u th geochronology u isotope analyses demonstrate that seawater 234u averages 146 6 and does not vary by more than 2 5 o and that carbonates capture this value additional high precision measurements 1 o on modern carbonates confirm that all oceans have identical 234u modern marine carbonates are shown to have 232th 230th ratios that reflect the local seawater in which they formed

etna 2002 eruption imaged from continuous tilt and gps data 1 on the night of october 26 2002 intense explosive activity and lava effusion began suddenly on the southern flank of mt etna at an altitude of 2750 m during the 27 and 28 october a long field of eruptive fractures propagated radially along the north eastern flank of the volcano ground deformation changes recorded between 26 and 27 october from gps and tilt data collected at the permanent geodetic network of mt etna were modeled to infer the positions and dimensions of the two dikes the observed deformation pattern was consistent with a response of the edifice to a composite mechanism consisting of a vertical uprising dike in the upper southern flank and a lateral intrusion propagating along the north eastern sector the first dike which triggered the eruption crossed the volcano edifice in a few hours and was located in the same area as the 2001 eruption while the second lateral dike which crossed the ne flank was the primary cause of the recorded deformation pattern

changes in north atlantic radiocarbon reservoir ages during the allerod and younger dryas estimates of the radiocarbon age of seawater are required in correlations between marine and terrestrial records of the late quaternary climate we radiocarbon dated marine shells and terrestrial plant remains deposited in two bays on norway s west coast between 11 000 and 14 000 years ago a time of large and abrupt climatic changes that included the younger dryas yd cold episode the radiocarbon age difference between the shells and the plants showed that sea surface reservoir ages increased from 400 to 600 years in the early yd stabilized for 900 years and dropped by 300 years within a century across the yd holocene transition

precise th u dating of small and heavily coated samples of deep sea corals marine carbonate skeletons like deep sea corals are frequently coated with iron and manganese oxides hydroxides which adsorb additional thorium and uranium out of the sea water a new cleaning procedure has been developed to reduce this contamination in this further cleaning step a solution of na2edta na2h2tb and ascorbic acid is used which composition is optimised especially for samples of 20 mg of weight it was first tested on aliquots of a reef building coral which had been artificially contaminated with powdered ferromanganese nodule applied on heavily contaminated deep sea corals scleractinia it reduced excess 230th by another order of magnitude in addition to usual cleaning procedures the measurement of at least three fractions of different contamination together with an additional standard correction for contaminated carbonates results in th u ages corrected for the authigenic component a good agreement between th u and 14c ages can be achieved even for extremely coated corals

apatite geochronology and chemistry of luanchuan granitoids in the east qinling orogen china implications for petrogenesis metallogenesis and exploration abstract the luanchuan granitoids in the east qinling orogen central china host multiple genetic types of mo w pb zn ag polymetallic deposits here we present results from systematic geochronological and geochemical investigations on apatite with a view to gain insights into the petrogenesis metallogenesis and potential for mineral exploration we analyzed 388 apatite grains from eight late mesozoic granitic plutons in the luanchuan ore district apatite u pb geochronology yields 206pb 238u mean ages in the range of 151 5 102 3 ma representing the crystallization ages through integrating our results with those from previous studies on the multi stage magmatism 160 130 ma 111 108 ma and metallogeny 150 139 ma 138 135 ma 114 ma the newly obtained apatite ages can be correlated with coeval magmatic and metallogenetic events of this ore district in comparison except for the nannihu pluton which is ca 19 ma younger than the corresponding zircon u pb ages the other studied plutons show almost consistent apatite and zircon u pb ages within error ranges suggesting rapid post magmatic cooling below 450 c in response to shallow emplacement apatite chemistry displays ree concentrations in the range of 2379 15 925 ppm characterized by rather high lree concentrations depleted to flat hree profiles and distinctive negative eu anomalies suggesting crust mantle mixing in the magma source prior to apatite crystallization which incorporated different degrees of sediment melting and slab melting components triggered by the subducted paleo pacific slab derived fluids in conjunction with element concentrations and ratios e g sr mn cl y la y eu eu sm nd n la sm n eu eu n ce ce n la yb n in apatite the luanchuan granitoids are classified as moderately oxidized and mineralized i type granites we suggest that the huangbeiling pluton and other igneous rocks in its vicinity likely have relatively higher potential for future w mo mo w exploration and that different degrees of magmatic differentiation might have generated the porphyry skarn mo w deposit in the yuku ore field moreover our study also demonstrates that the apatite ree concentrations and ratios can be used as potential indicators for mineral exploration

oceanic radiocarbon separation of the natural and bomb components an improved method has been developed for the separation of the natural and bomb components of the radiocarbon in the ocean the improvement involves the use of a very strong correlation between natural radiocarbon and dissolved silica this method is applied to radiocarbon measurements made on samples collected during the geochemical ocean sections study geosecs transient tracers in the ocean tto and south atlantic ventilation experiment save expeditions on the basis of this new separation we provide not only an estimate of the global inventory of bomb 14c at the time of the geosecs survey but also the distribution of bomb radiocarbon along four thermocline isopycnals in each ocean we also document the evolution of the bomb 14c inventory and penetration along thermocline isopycnals in the north atlantic ocean between the times of the geosecs 1972 1973 and tto 1980 1982 surveys and in the south atlantic ocean between the times of the geosecs 1973 and save 1987 1989 surveys in addition we show that the bomb tritium to bomb 14c ratio expressed in the tritium unit tu 81 units 100 for waters entering the thermocline of the northern hemisphere is about 9 times higher than for those entering the southern hemisphere thermocline this contrast offers long term potential as an indicator of inter hemispheric transport of upper ocean waters

trace element composition of detrital magnetite from coastal sediments of northwestern japan sea for provenance study abstract typomorphic varieties of magnetite in coastal sediments of the nw japan sea were distinguished by their contents of 11 trace elements ti mn cr v ni co zr sn zn pb cu magnetite from principal magmatic complexes differing in composition and crystallization conditions were used as standard samples the trace elements show that magnetite in sediments of the inlets near river mouths corresponds to that in rocks occurring in the basin of the rivers flowing into these inlets magnetite corresponding to that in rocks exposed on the coast and forming active cliffs indicates that these rocks supply abrasion material in the coastal zone different magnetite composition in modern nearshore sediments from that of relict shelf deposits is due to the more extensive provenance at sea leve low stands during cold phases of the late pleistocene

modeling the effect of abrupt ocean circulation change on marine reservoir age abstract radiocarbon surface reservoir age is required for precise dating of marine organisms although often assumed constant changes in atmospheric radiocarbon content ocean circulation and ocean mixing imprint changes on this quantity the spatial and temporal variations of marine surface and bottom reservoir ages in response to a shutdown and subsequent recovery of atlantic meridional overturning circulation moc are analyzed using a cost efficient three dimensional ocean circulation model generally surface reservoir age changes are limited to the atlantic ocean with a reduction of about 100 yr after the moc shutdown followed by a slow increase and a peak at the time of moc resumption parameter sensitivity studies with respect to the roles of gas exchange diffusivity and north atlantic ice cover show that ice cover has the largest effect on the transient evolution of surface reservoir age during the shutdown our model results agree well with a recent reconstruction of surface reservoir age changes during the younger dryas when we reduce the rate of gas exchange in the model and include a parametrization of seasonal ice cover in the north atlantic

genesis of chating cu au deposit in the middle lower yangtze river metallogenic belt eastern china implications from magnetite and biotite geochemistry abstract porphyry deposits developed in intracontinental settings have some geological characteristics that differ from those developed in arc related continental margins the chating deposit is a newly discovered cu au deposit in the middle lower yangtze river metallogenic belt mlyb eastern china we carried out detailed petrographic observations and major and trace element analyses of different types of magnetite and biotite from the host quartz diorite porphyry and different alteration zones in chating to elucidate the genesis of chating deposit alteration types in chating include skarn sodic calcic potassic and phyllic and copper mineralization is closely associated with potassic alteration of quartz diorite porphyry the different types of magnetite and biotite at chating show distinctive geochemical characteristics higher ti and al in magmatic magnetites and biotites compared to the hydrothermal minerals indicate higher temperature and pressure in the quartz diorite porphyry higher v in magmatic magnetites and lower xmg in magmatic biotites indicate higher fo2 similar mineral geochemistry from typical worldwide porphyry deposits for example al2o3 tio2 feot mgo na2o iv f and iv cl contents in biotite and ni cr ti al sn and ga in magnetite indicate a predominately magmatic fluid source differences in si ca co ni mg and mn in different types of magnetite and mno cao sio2 differences in two types of biotite from typical porphyry copper deposits show the influence of external fluids moreover high iv f values and low iv cl values in biotites demonstrate that the magmatic hydrothermal system in chating was enriched in cl and lacked f consistent with the importance of cl in the transport of cu based on different discrimination diagrams for magnetite such as ni cr versus ti ni cr mn versus ti v and al mn versus ti v and the geochemical similarities of biotite from chating and other porphyry deposits combined with the similarity of alteration assemblages we conclude that chating has porphyry cu au affinities but the hydrothermal system was affected by external fluids in contrast to more typical subduction related porphyry deposits

ice sheet grounding line dynamics steady states stability and hysteresis 1 the ice sheet ice shelf transition zone plays an important role in controlling marine ice sheet dynamics as it determines the rate at which ice flows out of the grounded part of the ice sheet together with accumulation this outflow is the main control on the mass balance of the grounded sheet in this paper we verify the results of a boundary layer theory for ice flux in the transition zone against numerical solutions that are able to resolve the transition zone very close agreement is obtained and grid refinement in the transition zone is identified as a critical component in obtaining reliable numerical results the boundary layer theory confirms that ice flux through the grounding line in a two dimensional sheet shelf system increases sharply with ice thickness at the grounding line this result is then applied to the large scale dynamics of a marine ice sheet our principal results are that 1 marine ice sheets do not exhibit neutral equilibrium but have well defined discrete equilibrium profiles 2 steady grounding lines cannot be stable on reverse bed slopes and 3 marine ice sheets with overdeepened beds can undergo hysteresis under variations in sea level accumulation rate basal slipperiness and ice viscosity this hysteretic behavior can in principle explain the retreat of the west antarctic ice sheet following the last glacial maximum and may play a role in the dynamics of heinrich events

magnetic and electric field observations during the 2000 activity of miyake jima volcano central japan magnetic and electric field variations associated with the 2000 eruption of miyake jima volcano are summarized for about 1 week prior to the july 8 phreatic explosion significant changes in the total intensity were observed at a few stations which indicated uprising of a demagnetized area from a depth of 2 km towards the summit this nonmagnetic source can be regarded as a vacant space itself electric and magnetic field variations were observed simultaneously associated with the tilt step event which was the abrupt v50 s inflation at a few km depth within the volcano followed by gradual recovery vseveral hours the electric field is ascribed to the electrokinetic effect most probably due to forced injection of fluids from the source while the magnetic field to the piezomagnetic effect due to increased pressure large magnetic variations amounting to a few tens of nt were observed at several stations since july 8 and they turned almost flat after the august 18 largest eruption magnetic changes are explained mostly by the vanishing of magnetic mass in the summit and additionally by the thermal demagnetization at a rather shallow depth a large increase in the self potential by 130 mv was also observed near the summit caldera associated with the august 18 eruption which suggests that the hydrothermal circulation system sustained within the volcano for the past more than 10 years was destroyed by this eruption 9 2002 published by elsevier science b v

the 1992 m 7 cape mendocino california earthquake coseismic deformation at the south end of the cascadia megathrust we invert geodetic measurements of coseismic surface displacements to determine a dislocation model for the april 25 1992 m 7 cape mendocino california earthquake the orientation of the model slip vector which nearly parallels north america juan de fuca relative plate convergence and the location and orientation of the model fault relative to the offshore cascadia megathrust suggest that the 1992 cape mendocino earthquake is the first well recorded event to relieve strain associated with the cascadia subduction zone we use data from three geodetic techniques 1 the horizontal and vertical displacements of 13 monuments surveyed with the global positioning system corrected for observed horizontal interseismic strain accumulation 2 88 section elevation differences between leveling monuments and 3 the uplift of 12 coastal sites observed from the die off of intertidal marine organisms maximum observed displacements are 0 4 m of horizontal movement and 1 5 m of uplift along the coast we use monte carlo techniques to estimate an optimal uniform slip rectangular fault geometry and its uncertainties the optimal model using all the data resolves 4 9 m of slip on a 14 by 15 km fault that dips 28 se the fault extends from 1 5 to 8 7 km in depth and the main shock hypocenter is close to the downdip projection of the fault the shallowly dipping fault plane is consistent with the observed aftershock locations and the estimated geodetic moment is 3 1 1019 n m 70 of the seismic moment other models that exclude leveling data collected in 1935 and 1942 are more consistent with seismological estimates of the fault geometry if the earthquake is characteristic for this segment the estimated horizontal slip vector compared with plate convergence rates suggests a recurrence interval of 140 years with a 95 confidence range of 100 670 years the coseismic uplift occurred in a region that also has high quaternary uplift rates determined from marine terrace studies if repeated ruptures of this southernmost segment of the cascadia megathrust are responsible for the quaternary uplift a comparison of the coseismic uplift with coastal uplift rates suggests a recurrence interval of 200 400 years thus comparing horizontal and vertical coseismic to long term deformation suggests a recurrence interval of about 100 300 years for m 7 events at the south end of the cascadia megathrust

u th dating of marine isotope stage 7 in bahamas slope sediments in order to understand the driving forces for pleistocene climate change more fully we need to compare the timing of climate events with their possible forcing in contrast to the last interglacial marine isotope stage mis 5 the timing of the penultimate interglacial mis 7 is poorly constrained this study constrains its timing and structure by precise u th dating of high resolution n 18 o records from aragonite rich bahamian slope sediments of odp leg 166 sites 1008 and 1009 the major glacial interglacial cycles in n 18 o are distinct within these cores and some mis 7 substages can be identified these sediments are well suited for u th dating because they have uranium concentrations of up to 12 ppm and very low initial 230 th contributions with most samples showing 230 th 232 th activity ratio of s 75 u and th concentrations and isotope ratios were measured by thermal ionisation mass spectrometry and multiple collector inductively coupled plasma mass spectrometry with the latter providing dramatically better precision twenty nine of the 41 samples measured have a n 234 u value close to modern seawater suggesting that they have experienced little diagenesis ages from 27 of the 41 samples were deemed reliable on the basis of both their u and their th isotope ratios ages generally increase with depth although we see a repeated section of stratigraphy in one core extrapolation of constant sedimentation rate through each substage suggests that the peak of mis 7e lasted from v237 to 228 ka and that 7c began at 215 ka this timing is consistent with existing low precision radiometric dates from speleothem deposits the beginning of both these substages appears to be slightly later than in orbitally tuned timescales the end of mis 7 is complex but also appears to be somewhat later than is suggested by orbitally tuned timescales although this event is not particularly well defined in these cores fl 2002 elsevier science b v all rights reserved

temporal relationships of carbon cycling and ocean circulation at glacial boundaries evidence from high sedimentation rate south atlantic deep sea cores indicates that global and southern ocean carbon budget shifts preceded thermohaline circulation changes during the last ice age initiation and termination and that these were preceded by ice sheet growth and retreat respectively no consistent lead lag relationships are observed during abrupt millennial warming events during the last ice age allowing for the possibility that ocean circulation triggered some millenial climate changes at the major glacial interglacial transitions the global carbon budget and thermohaline ocean circulation responded sequentially to the climate changes that forced the growth and decline of continental ice sheets

volcanomagnetic signals associated with the 2001 flank eruption of mt etna italy 1 geomagnetic changes were observed during the 2001 flank eruption at mt etna after differential magnetic fields were filtered from the seasonal thermic noise by using temperature data we recognized three stages in the total intensity changes related to different volcanic events a rapid variation of 4 nt associated with july 12 seismic swarm recorded beneath the summit craters b step like variation of 7 nt occurring during the quick propagation of fractures in the south flank which was followed by the off set of 2 nt before july 17 when the lava outpoured from fissures and c quick variation of 3 nt coincident with july 19 eruptive fissures opening up in the valle del leone these observations are consistent with those calculated from volcanomagnetic models in which the magnetic changes are generated by stress redistribution due to magmatic intrusions at different depth and by the thermal demagnetization at a rather shallow depth

volcanomagnetic changes accompanying the onset of the 2002 2003 eruption of mt etna italy abstract remarkable changes in the local magnetic field were associated with the onset of the 2002 2003 flank eruption at mt etna after differential magnetic field measurements were filtered from the external noise by using adaptive filters we recognized two stages in the total intensity changes which are closely related to different volcanic events a rapid variations of about 4 5 nt associated with october 26 seismic swarm recorded beneath the summit craters b step like variations of 9 10 nt coincident with october 27 eruptive fissures opening up in the north flank these observations are generally consistent with those calculated from simple magnetic models of these volcanic processes in which the magnetic changes are generated by stress redistribution due to magmatic intrusions at different depth the magnetic data not only allow the timing of the intrusive event to be described in greater detail but also together with other volcanological and geophysical evidences permit some constraints to be set on the characteristics of propagation of a shallow dike firstly at around midnight on 26 october magma was rapidly injected to a depth of 3 4 km just below the summit craters secondly after 1 00 on 27 october continued intrusion magma occurred upward and culminated a few hundred meters below the free surface fractured along a n e direction thirdly at about 2 28 magma gave rise to an explosive fissural vent at the northern base of the ne crater near 3000 m a s l finally at about 5 00 the first eruptive fissure became active along the eastern border of the ne rift at 2500 m a s l the rate of growth of the magnetic anomalies moreover leads to the interpretation that the magmatic intrusion travelled northward from base of the ne crater to the ne rift at approximately 14 m min

silicon in magnetite high resolution microanalysis of magnetite ilmenite intergrowths magnetite ilmenite oxidation exsolution intergrowths from an original titanomagnetite microphenocryst from an ash flow tuff unit have been studied using conventional transmission electron microscopy and analytical electron microscopy silicon has been found to be in solid solution in all of the magnetite studied and in some of the coexisting ilmenite the average value in magnetite is 1 2 wt si equivalent to solid solution of 9 mole fe2sio4 silicon is also present in very small silicate inclusions and as unusual si rich domains of uncertain origin in magnetite the inclusions and domains may be irregularly distributed through the magnetite in sizes well below those resolvable with the electron microprobe microprobe analyses for si in magnetite generally reflect these heterogeneities in addition to a component presumably in solid solution the petrologic implications of the data can be assessed only when relevant thermochemical data become available and the distribution of si in magnetite is better understood

causes of pre collapse changes of the larsen b ice shelf numerical modelling and assimilation of satellite observations satellite observations revealed that beside a rapid thinning the larsen b ice shelf lbis was undergoing a significant acceleration before its collapse in 2002 this paper investigates the ice shelf acceleration between 1995 and 1999 using a combination of data assimilation and numerical modelling based on a flow model adjusted to the 1995 insar velocities perturbation experiments are performed such as ice front retreat thinning increase in tributary flow and rheological weakening furthermore an inversion for ice shelf rheology and tributary flow velocity is performed for both the 1995 and the 1999 insar velocities the perturbation experiments together with the inversion strongly suggest that the acceleration cannot solely be explained by the retreat of the ice shelf front but relies on a further significant rheological weakening of the already weak shear zones within the lbis minor tributary acceleration is found to be an effect rather than a cause of the ice shelf acceleration furthermore the observed acceleration cannot be explained by the observed recent thinning we conclude that for smaller ice shelves such as the lbis such weak shear margins play a crucial role in controlling their dynamics and are the key to understand changes in the future finally we compare the dynamic thinning likely to be associated with the observed acceleration with the observed thinning for the ice shelf as a whole this thinning accounts for 20 of the observed value which implies that factors such as enhanced basal melt were the primary cause of the observed thinning

magnetite as an indicator mineral in the exploration of porphyry deposits a case study in till near the mount polley cu au deposit british columbia canada the investigation of this paper focused on whether the composition of hydrothermal ore related magnetite in till could be used to locate porphyry deposits in terrane where glacial overburden overlies rocks that host porphyry cu au mineralization this hypothesis was tested using 20 till samples collected in a 900 km2 area surrounding the mount polley porphyry cu au deposit in south central british columbia canada at least 100 magnetite grains were randomly selected from the magnetic fraction of each till sample nineteen trace elements in 50 magnetite grains in each sample were measured by laser ablation inductively coupled plasma mass spectrometry la icp ms the large beam or raster size used for laser ablation to 100 m homogenizes any heterogeneous trace element distributions in magnetite that result from oxy exsolution and or and dissolution reprecipitation avoiding this issue with the few micron size of an electron beam linear discriminant analysis lda performed on a compilation of magnetite compositions measured by la icp ms from worldwide porphyry deposits and intrusive igneous rocks define the chemical signature mg al ti v mn co ni of hydrothermal magnetite exclusive to porphyry systems application of our two lda models to the 985 magnetite compositions we measured in the till samples surrounding mount polley showed anomalous amounts of hydrothermal magnetite grains in till up to 2 5 km west southwest and 4 km northwest of the deposit a pattern that is consistent with the ice flow history of the region our lda models for magnetite trace element compositions have strong potential to be an effective tool in exploration for buried porphyry systems

principal component analysis of magnetite composition from volcanogenic massive sulfide deposits case studies from the izok lake nunavut canada and halfmile lake new brunswick canada deposits abstract magnetite grains from the izok lake nunavut canada and the halfmile lake new brunswick canada volcanogenic massive sulfide deposits and from till covering the nearby areas were investigated using the scanning electron microscopy sem electron probe micro analyzer epma laser ablation inductively coupled plasma mass spectrometry la icp ms and optical microscopy the method of robust estimation for compositional data rob composition was applied to censored geochemical data and the results were analyzed by principal component analysis pca textural relationships and mineral association of magnetite reveal the history of formation and contribute to the explanation of characteristic compositional differences of magnetite from different geological settings the integration of petrography and mineral chemistry allows discriminating magmatic metamorphic and hydrothermal magnetite grains in the vms deposits bedrock samples magmatic magnetite is found in izok lake gabbro and halfmile lake syenite felsic ash tuff and gossan samples whereas magnetite in izok lake massive sulfides gahnite rich dacite and iron formations formed during the amphibolite facies metamorphism in halfmile lake andesite magnetite recrystallized during greenschist facies metamorphism in the magnetite alteration zone associated to the halfmile lake deposit hydrothermal magnetite has been overprinted by metamorphic magnetite halfmile lake massive sulfides in chloritic argillite contain hydrothermal magnetite pca identifies discriminator elements and their contributions to magnetite composition from different izok lake lake and halfmile lake bedrock samples the results suggest that si ca zr al ga mn mg ti zn co ni and cr are discriminator elements for vms deposits and their host bedrocks the distinct chemical signatures for magnetite from various bedrock lithologies demonstrate that magnetite grains of the same origin share more similarities in chemistry as high ti indicates magmatic sources for magnetite whereas high si ca and mg are indicative of hydrothermal settings variable compositions of metamorphic magnetite suggest that the chemistry of this type of magnetite is controlled by the composition of host rocks the grade of metamorphism and oxygen fugacity pca of epma and la icp ms data of magnetite from the izok lake and halfmile lake bedrock samples yield discrimination models for classification of magnetite grains from till decreases in the proportion of magnetite grains with the chemical signature of the izok lake massive sulfides and gahnite rich dacite down ice from the izok lake deposit show the use of magnetite chemistry in geochemical exploration in the halfmile lake area till magnetite grains with the signature of vms mineralization make a glacial dispersal train more than 2 km down ice from the deposit

magnetic features of the magmatic intrusion that occurred in the 2007 eruption at stromboli island italy significant changes in the local magnetic field marked the resumption of eruptive activity at stromboli volcano on february 27 2007 after differential magnetic fields were obtained by filtering out external noise using adaptive filters and seasonal thermal noise using temperature data we identified step like changes of 1 4 nt coincident with the opening of eruptive fissures in the upper part of the sciara del fuoco the magnetic variations detected at two stations are closely related to the propagation of a shallow ne sw magmatic intrusion extending beyond the summit craters area these observations are consistent with those calculated using piezomagnetic models in which stress induced changes in rock magnetization are produced by the magmatic intrusion no significant magnetic changes were observed when the first fractures opened along the ne crater rim indeed the stress induced magnetization caused by this magmatic activity is expected to be too low because of the structural weakness and or thermal state of the summit area the continuous long term decay characterizing the post eruptive magnetic pattern can be related to a time dependent relaxation process a maxwell rheology was assumed and the temporal evolution of the piezomagnetic field was evaluated this allowed us to estimate the rheological properties of the medium in particular an average viscosity ranging between 1016 and 1017 pa s was a relaxation time of about 38 days

post 1906 stress recovery of the san andreas fault system calculated from three dimensional finite element analysis 1 the m 7 8 1906 san francisco earthquake cast a stress shadow across the san andreas fault system inhibiting other large earthquakes for at least 75 years the duration of the stress shadow is a key question in san francisco bay area seismic hazard assessment this study presents a three dimensional 3 d finite element simulation of post 1906 stress recovery the model reproduces observed geologic slip rates on major strike slip faults and produces surface velocity vectors comparable to geodetic measurements fault stressing rates calculated with the finite element model are evaluated against numbers calculated using deep dislocation slip in the finite element model tectonic stressing is distributed throughout the crust and upper mantle whereas tectonic stressing calculated with dislocations is focused mostly on faults in addition the finite element model incorporates postseismic effects such as deep afterslip and viscoelastic relaxation in the upper mantle more distributed stressing and postseismic effects in the finite element model lead to lower calculated tectonic stressing rates and longer stress shadow durations 17 74 years compared with 7 54 years all models considered indicate that the 1906 stress shadow was completely erased by tectonic loading no later than 1980 however the stress shadow still affects present day earthquake probability use of stressing rate parameters calculated with the finite element model yields a 7 12 reduction in 30 year probability caused by the 1906 stress shadow as compared with calculations not incorporating interactions the aggregate interaction based probability on selected segments not including the ruptured san andreas fault is 53 70 versus the noninteraction range of 65 77

radiocarbon simulations for the glacial ocean the effects of wind stress southern ocean sea ice and heinrich events simulations of oceanic radiocarbon for the last glacial maximum are presented using a three dimensional global ocean circulation model forced with glacial background states according to various reconstructions we investigate the influence of sea surface temperatures sea ice margins wind stress and antarctic sea ice formation on the glacial tracer distribution and meridional overturning circulation the aim of these sensitivity studies is to reconcile available radiocarbon data from marine sediments with reconstructed sea surface temperatures and estimated sea ice production rates model runs with a modified freshwater balance in the southern ocean mimicking increased brine release due to enhanced divergence of antarctic sea ice arrive at radiocarbon values close to observations these experiments also yield abyssal temperatures and salinities which are consistent with recent inferences in the simulation with the best agreement with radiocarbon observations north atlantic deep water export is reduced by 40 compared to present day while antarctic bottom water flow is intensified to similar strength in the south atlantic transient simulations show that glacial freshwater discharge into the north atlantic can cause abrupt increases of atmospheric radiocarbon as observed during heinrich event 1 however the effect is only significant in scenarios with a massive short time discharge at the beginning which is followed by low level freshwater input for the rest of the event or if it is assumed that the meridional overturning circulation was already in a modern operational mode

debris characteristics and ice shelf dynamics in the ablation region of the mcmurdo ice shelf antarctica this paper presents observations and measurements of debris characteristics and ice shelf dynamics in the ablation region of the mcmurdo ice shelf in the ross sea sector of antarctica ice shelf surface processes and dynamics are inferred from a combination of sedimentological descriptions ground penetrating radar investigations and through ablation velocity and ice thickness measurements field data show that in the study area the ice shelf moves relatively slowly 1 5 18 3 m a 1 has high ablation rates 43 441 mm during 2003 04 summer and is thin 6 22 m the majority of debris on the ice shelf was originally transported into the area by a large and dynamic ice sheet ice shelf system at the last glacial maximum this debris is concentrated on the ice shelf surface and is continually redistributed by surface ablation creating an ice cored landscape of large debris rich mounds ice shelf flow forming medial moraines and meltwater streams locally reworking material and redistributing it across the ice shelf surface a conceptual model for supraglacial debris transport by contemporary antarctic ice shelves is presented which emphasizes these links between debris supply surface ablation and ice shelf motion low velocity ice shelves such as the mcmurdo ice shelf can maintain and sequester a debris load for thousands of years providing a mechanism by which ice shelves can accumulate sufficient debris to contribute to sediment deposition in the oceans

using apatite to discriminate synchronous ore associated and barren granitoid rocks a case study from the edong metallogenic district south china abstract in order to find criteria to discriminate the synchronous ore associated and barren granitoid rocks we have determined apatite chemistry associated with ore associated cu au and barren granitoid rocks in the edong district of the middle and lower yangtze river metallogenic belt south china both rock types give zircon u pb ages between 135 0 and 138 7 ma apatite has a higher volatile and li content cl 0 19 0 57 wt average 0 35 wt so3 0 08 0 71 wt average 0 32 wt li 0 49 7 99 ppm average 3 23 ppm in ore associated rocks than those in barren rocks cl 0 09 0 31 wt average 0 16 wt so3 0 06 0 28 wt average 0 16 wt li 0 15 0 89 ppm average 0 36 ppm apatite la yb n ratios and eu eu values are relatively high and show wider variation in ore associated rocks than those in barren rocks apatite la sm n and yb sm n show positive correlation in ore associated rocks but negative in barren rocks the higher volatile content occurs in ore associated magma favoring cu au transportation and deposition furthermore amphibole fractional crystallization in ore associated magma further enriched the ore elements in the residual melt barren rocks may have undergone fluid exsolution before emplacement which makes it barren in cl s and ore elements cu s these signatures emphases the significance of volatile and magma evolution in mineralization and indicate that analyses of magmatic apatite can serve to distinguish ore associated from barren intrusions

chronology of late wisconsin ice retreat from the western ross sea antarctica lithologic data from marine sedimentary cores and accelerator mass spectrometer ams radiocarbon dates indicate that grounded ice did not advance to the western ross sea continental shelf edge during the last glacial maximum lgm a chronology of the timing of ice retreat was provided by 26 ams dates obtained from 12 cores dates ranging from 20 to 29 ka suggest that the outer continental shelf beyond ca lat 74 s was not covered by grounded glacial ice prior to and during the early stages of the lgm 14c dates just above transitions from subglacial diamictons to marine muds indicate that the area around the drygalski ice tongue was deglaciated by at least 11 5 ka the ross ice shelf reached its present day position near ross island by about 7 ka a hiatus of about 15 ka is apparent in radiocarbon dates in cores from the outer continental shelf the hiatus is interpreted to represent an absence of sedimentation caused by the presence of an ice shelf

magnetic monitoring at merapi volcano indonesia abstract merapi volcano located 30 km north of the heavily populated city of yogjakarta java is one of the most active of the 129 volcanoes in indonesia about every 2 years a new phase of activity is observed depending on the past activity the unrest gives rise either to an endogenous dome which partly collapses in the southwest direction or to pyroclastic flows which travel as far as 15 km the 1990 1997 period has involved a plume emission on 30 august 1990 an extrusion on 20 january 1992 and a pyroclastic eruption on 22 november 1994 the intensity of the earth magnetic field has been measured simultaneously and digitally recorded at four stations since 1990 two overhauser magnetometers with resolution of 0 01 nt have been installed in the summit area to strengthen the volcano monitoring outstanding magnetic changes appear to correlate with volcanic activity three types of volcanomagnetic signals can be identified long term trends up to 15 nt with period 10 years medium term cyclic variations at most 3 nt in amplitude and with 1 2 years period and small events reaching 1 5 nt lasting a few months and associated with any remarkable volcanic activity merapi volcano began a new cycle of activity in 1995 leading to a dome growth in july 1996 and accompanied by 27 nuees ardentes in august the comparison between magnetic data seismicity and surface phenomena suggests that some long term trends of decade periods could be of thermomagnetic origin while mid term volcanomagnetic variations associated with the cycles of merapi activity could be of piezomagnetic origin short term variations of a few weeks duration less than 1 5 nt are well correlated with the 1995 1996 seismic activity

hydrothermally reset magmatic spinels in retrograde serpentinites formation of ferritchromit rims and chlorite aureoles serpentinized harzburgites from southern tuscany italy host different kinds of spinels a relic magmatic al spinels b hydrothermally altered spinels occurring as ferritchromit rims c syn and post serpentinization magnetites the composition of relic al spinels suggests 5 15 partial melting of a fertile spinel lherzolite after the main serpentinization al spinels are progressively replaced by ferritchromit rims by a dissolution recrystallization process transmission electron microscopic investigation shows that ferritchromit actually consists of a complex nanometric association of cr magnetite mg0 03 fe0 972 al0 11 cr0 89 fe3 1 00 o4 chlorite and lizardite with 001 chl liz always parallel to 111 cr mag mg and al released during the al spinel ferritchromit replacement interact with mesh textured serpentine giving rise to chloritic aureoles i e randomly intergrown chlorite lizardite and septechlorite that overgrow and postdate mesh textures

separating natural and bomb produced radiocarbon in the ocean the potential alkalinity method alkalinity method provides an estimate of surface ocean prebomb d 14 c concentrations predictions with the technique appear to be unbiased and have uncertainties which are less than previous techniques index terms 4860 oceanography biological and chemical radioactivity and radioisotopes 4894 oceanography biological and chemical instruments and techniques

paleomagnetism and magnetic mineralogy of the nahant gabbro and tonalite eastern massachusetts paleomagnetic analyses of the nahant gabbro and an associated tonalite of eastern massachusetts reveal five separate populations of paleomagnetic directions the gabbro and tonalite exposed in the northernmost portion of the boston basin were selected for this study because of the lack of significant macroscopic evidence of penetrative metamorphism or deformation the tonalite has not been radiometrically dated however an ordovician date has been reported for the gabbro detailed petrographic data indicate that the five populations of paleomagnetic directions obtained for the gabbro and tonalite correspond to five styles of magnetic mineral alteration a mean alteration index was calculated for each site in the nahant gabbro and tonalite there is an apparent relationship between magnetite alteration and normalized intensity at a given demagnetization level with normalized intensity decreasing as magnetite alteration increases this relationship illustrates the interdependence between paleomagnetic pr

pre bomb radiocarbon and the reservoir correction for calcareous marine species in the southern ocean four antarctic marine mollusc shells which were collected alive between 1917 and 1940 were analyzed by accelerator mass spectrometry to provide the first pre bomb radiocarbon measurements of biogenic carbonates from the southern ocean after correcting for the impact of fossil fuel combustion suess effect radiocarbon activities of the pre bomb shells averaged 149 8 10 4 in contrast the 14c values for post bomb molluscs echinoderms brachiopods and foraminifera averaged 96 1 25 2 these biogenic carbonate 14c values are nearly identical to pre bomb estimates 148 to 152 and post bomb measurements 98 4 22 0 of the surface waters in the southern ocean average radiocarbon ages of the biogenic carbonates before and after 1950 1303 84 years and 811 205 years respectively along with those from seals and penguins indicate that the antarctic marine radiocarbon reservoir has decreased in age by nearly 500 years during the second half of the 20th century marine species and seawater measurements firmly place the radiocarbon reservoir correction at 1300 100 years for calcareous marine fossils which are widespread abundant and well preserved organic materials for interpreting ice sheet climate and sea level impacts on the antarctic marine ecosystem during the holocene

volcanomagnetic signals associated with the quasi continuous activity of the andesitic merapi volcano indonesia 1990 1995 abstract merapi volcano in java island indonesia is an andesitic stratovolcano which presents long periods of effusive activity during which an endogeneous dome is continuously growing the viscous lava dome gives rise to unstable blocks which collapse or turn into pyroclastic flows when the volcano does not exhibit any surface activity the overpressure within the volcano slowly increases depending on the quietness duration the unrest of the volcano can start with an explosive phase during which the former dome is partly destroyed magnetic variations of different time constant are observed during the 1990 1995 period which includes one gas plume emission on august 26 1990 and two eruptions on january 20 1992 and on november 22 1994 compared with other types of active volcanoes the observed volcanomagnetic variations are very small at the most a few nanoteslas nt to discriminate the variations associated with the global activity from the signals correlated with each unrest phase one has to dissociate the different time constant variations over the six year time span when long term trends are removed from the magnetic field in each station of the network an outstanding correlation between all the magnetic differences is emphasised the midterm variations point out 2 cycles of activity which fit the stress field evolution within the edifice leading to the 1992 and 1994 eruptions a new cycle has started in may 1995 in every identified cycle rapid volcanomagnetic signals are well associated with stress field changes may 1991 september 1991 february 1993 december 1993 some of the volcanomagnetic variations are short term precursory signals as the three months decrease up to 1 3 nt preceding the 1992 eruption the comparison between magnetic data seismicity and surface phenomena implies that the midterm volcanomagnetic variations associated with the cycles of merapi activity are of piezomagnetic origin

review of magnetic field monitoring near active faults and volcanic calderas in california 1974 1995 abstract differential magnetic fields have been monitored along the san andreas fault and the long valley caldera since 1974 at each monitoring location proton precession magnetometers sample total magnetic field intensity at a resolution of 0 1 nt or 0 25 nt every 10 min data samples are transmitted via satellite telemetry to menlo park ca for processing and analysis the number of active magnetometer sites has varied during the past 21 years from 6 to 25 with 12 sites currently operational we use this network to identify magnetic field changes generated by earthquake and volcanic processes during the two decades of monitoring five moderate earthquakes m5 9 to m7 3 have occurred within 20 km of magnetometer sites located along the san andreas fault and only one preseismic signal of 1 5 nt has been observed during moderate earthquakes coseismic magnetic signals with amplitudes from 0 7 nt to 1 3 nt have been identified for 3 of the 5 events these observations are generally consistent with those calculated from simple seismomagnetic models of these earthquakes and near fault coseismic magnetic field disturbances rarely exceed one nanotesla these data are consistent with the concept of low shear stress and relatively uniform displacement of the san andreas fault system as expected due to high pore fluid pressure on the fault a systematic decrease of 0 8 1 nt year in magnetic field has occurred in the long valley caldera since 1989 these magnetic field data are similar in form to observed geodetically measured displacements from inflation of the resurgent dome a simple volcanomagnetic model involving pressure increase of 50 mpa a at a depth of 7 km under the resurgent dome can replicate these magnetic field observations this model is derived from the intrusion model that best fits the surface deformation data

piezomagnetic fields due to an inclined rectangular fault in a viscoelastic half space an application to the 2002 2003 etna eruption summary time dependent piezomagnetic fields due to inclined rectangular faults embedded in a viscoelastic homogeneous half space were investigated a viscoelastic rheology of the surrounding medium was assumed to relate piezomagnetic changes at the surface to the stress field at depth the viscosity of the medium strongly influences time dependent stress changes especially in volcanic areas rocks near magmatic sources are considerably heated the presence of higher temperatures produces a lower effective viscosity in the crust making it necessary to consider its inelastic properties rocks no longer behave in a purely elastic manner but permanently deform because the viscosity is significantly lowered to determine the time dependent piezomagnetic fields in a viscoelastic medium we applied the correspondence principle to the analytical elastic solutions for dislocation sources among all the possible rheological models we investigated three cases in which the bulk modulus is purely elastic and the shear modulus relaxes as for i a maxwell solid ii a standard linear solid sls and iii a kelvin solid the piezomagnetic field completely vanishes after the relaxation process for a maxwell rheology whereas it is found to decrease over time and reach some finite offset values for sls and kelvin rheologies a real case study concerning the magnetic anomalies observed during the 2002 2003 mt etna eruption is also investigated post eruptive magnetic variations were in general agreement with a viscoelastic relaxation process of a sls rheology undergoing in the volcano edifice

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

close temporal correspondence between geomagnetic anomalies and earthquakes during the 2002 2003 eruption of etna volcano 1 the early stages of the 2002 2003 lateral eruption at mount etna were accompanied by slow changes over some hours and some rapid step offsets in the local magnetic field at five monitoring locations the total magnetic field intensity has been measured using continuously operating overhauser magnetometers at a sampling rate of 10 s the very unique aspect of these observations is the close temporal correspondence between magnetic field offsets and earthquakes that occurred in the upper northern flank of the volcano on 27 october 2002 prior to a primary eruption rapid coseismic changes of the magnetic field were clearly identified for three of the most energetic earthquakes which were concentrated along the northeast rift at a depth of about 1 km below sea level coseismic magnetic signals with amplitudes from 0 5 to 2 5 nt have been detected for three of the largest seismic events located roughly midway between the magnetic stations we quantitatively examine possible geophysical mechanisms which could cause the magnetic anomalies the comparison between magnetic data seismicity and surface phenomena implies that piezomagnetic effects are the primary physical mechanism responsible for the observed magnetic anomalies although the detailed cause of the rapid high stress change required is not clear the modeling of the observed coseismic magnetic changes in terms of piezomagnetic mechanism provides further evidence of the complex interaction between volcanic and tectonic processes during dike propagation along the northeast rift

new relative sea level curves for the southern scott coast antarctica evidence for holocene deglaciation of the western ross sea here we present new relative sea level rsl curves developed from holocene aged raised beaches along the southern scott coast of the western ross sea antarctica fifty four dates of marine shells seal skin and elephant seal remains incorporated within raised beaches during storms afford a chronology for these curves all of the curves show the same pattern and timing of rsl change within a small range of error the best dated curve suggests that final unloading of grounded ross sea ice from the southern scott coast and mcmurdo sound region occurred shortly before 6500 14c yr bp this age is consistent with glacial geological evidence that places deglaciation between 5730 and 8340 14c yr bp our data strongly suggest that grounding line retreat of the ross sea ice sheet southward through the mcmurdo sound region occurred in mid and late holocene time if this is correct then rising sea level could not have driven ice recession to the present day grounding line on the siple coast because global deglacial sea level rise was essentially accomplished by mid holocene time copyright 1999 john wiley sons ltd

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

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

radiocarbon variability in the western north atlantic during the last deglaciation we present a detailed history of glacial to holocene radiocarbon in the deep western north atlantic from deep sea corals and paired benthic planktonic foraminifera the deglaciation is marked by switches between radiocarbon enriched and depleted waters leading to large radiocarbon gradients in the water column these changes played an important role in modulating atmospheric radiocarbon the deep ocean record supports the notion of a bipolar seesaw with increased northern source deep water formation linked to northern hemisphere warming and the reverse in contrast the more frequent radiocarbon variations in the intermediate deep ocean are associated with roughly synchronous changes at the poles

ice composition evidence of marine ice transfer along the bottom of a small antarctic ice shelf the existence of marine ice transfer along the underside of the hell s gate ice shelf victoria land is indicated by an isotopic and chemical study of ice cores because of top surface ablation the marine ice formed at the ice shelf ocean interface ultimately appears at shelf surface a succession of congelation platelet and frazil ice is shown to occur the combined study of stable isotope composition and of the sodium content of these different ice types proves to be a valuable tool for specifying the ice shelf ocean interactions in this area two different freezing zones separated by a melting rone exist the parent water for the frazil ice is meltwater from congelation ice which appears in the upstream zone

geochemistry of magnetite from porphyry cu and skarn deposits in the southwestern united states a combination of petrographic observations laser ablation inductively coupled plasma mass spectrometry la icp ms and statistical data exploration was used in this study to determine compositional variations in hydrothermal and igneous magnetite from five porphyry cu mo and skarn deposits in the southwestern united states and igneous magnetite from the unmineralized granodioritic inner zone batholith japan the most important overall discriminators for the minor and trace element chemistry of magnetite from the investigated porphyry and skarn deposits are mg al ti v mn co zn and ga of these the elements with the highest variance for i igneous magnetite are mg al ti v mn zn for ii hydrothermal porphyry magnetite are mg ti v mn co zn and for iii hydrothermal skarn magnetite are mg ti mn zn and ga nickel could only be detected at levels above the limit of reporting lor in two igneous magnetites equally cr could only be detected in one igneous occurrence copper as mo ag au and pb have been reported in magnetite by other authors but could not be detected at levels greater than their respective lors in our samples comparison with the chemical signature of igneous magnetite from the barren inner zone batholith japan suggests that v mn co and ga concentrations are relatively depleted in magnetite from the porphyry and skarn deposits higher formation conditions in combination with distinct differences between melt and hydrothermal fluid compositions are reflected in al ti v and ga concentrations that are on average higher in igneous magnetite than in hydrothermal magnetite including porphyry and skarn magnetite low ti and v concentrations in combination with high mn concentrations are characteristic features of magnetite from skarn deposits high mg concentrations 1 000 ppm are characteristic for magnetite from magnesian skarn and likely reflect extensive fluid rock interaction in porphyry deposits hydrothermal magnetite from different vein types can be distinguished by varying ti v mn and zn contents titanium and v concentrations are highly variable among hydrothermal and igneous magnetites but ti concentrations above 3 560 ppm could only be detected in igneous magnetite and v concentrations are on average lower in hydrothermal magnetite the highest ti concentrations are present in igneous magnetite from gabbro and monzonite the lowest ti concentrations were recorded in igneous magnetite from granodiorite and granodiorite breccia and largely overlap with ti concentrations found in hydrothermal porphyry magnetite magnesium and mn concentrations vary between magnetite from different skarn deposits but are generally greater than in hydrothermal magnetite from the porphyry deposits high mg and low ti and v concentrations characterize hydrothermal magnetite from magnesian skarn deposits and follow a trend that indicates that magnetite from skarn calcic and magnesian commonly has low ti and v concentrations

automated system for magnetic monitoring of active volcanoes in order to provide a basis for short term decision making in the forecasting and monitoring of volcanic activity we developed an entirely automated system of data acquisition and reduction for magnetic data the system mag net is designed to provide monitoring and analysis of magnetic data on etna volcano at large distances from the central observatory the mag net system uses data from an array of continuously recording remote stations spread over the volcanic area and linked by mobile phone to the control center at the local observatory at this location a computer receives the data and performs data sorting and reduction as well as limited evaluation to detect abnormal behavior or breakdown of remote sensors communication software called magtalk is also designed to provide data to distant users with a view to using continuous magnetic observations in advanced analysis techniques for volcano monitoring the mag net system also delivers two graphical user interface based applications to provide an interpretation capability the former called madap speeds up all the data reduction processes in order to evaluate the reliability of magnetic signals the latter called vmm is a procedure for modeling magnetic fields associated with tectonic and volcanic activity to facilitate the identification and interpretation of the sources of a wide spectrum of magnetic signals

the link between climate warming and break up of ice shelves in the antarctic peninsula a review of in situ and remote sensing data covering the ice shelves of the antarctic peninsula provides a series of characteristics closely associated with rapid shelf retreat deeply embayed ice fronts calving of myriad small elongate bergs in punctuated events increasing flow speed and the presence of melt ponds on the ice shelf surface in the vicinity of the break ups as climate has warmed in the antarctic peninsula region melt season duration and the extent of ponding have increased most break up events have occurred during longer melt seasons suggesting that meltwater itself not just warming is responsible regions that show melting without pond formation are relatively unchanged melt ponds thus appear to be a robust harbinger of ice shelf retreat we use these observations to guide a model of ice shelf flow and the effects of meltwater crevasses present in a region of surface ponding will likely fill to the brim with water we hypothesize building on weertman 1973 hughes 1983 and van der veen 1998 that crevasse propagation by meltwater is the main mechanism by which ice shelves weaken and retreat a thermodynamic finite element model is used to evaluate ice flow and the strain field and simple extensions of this model are used to investigate crack propagation by meltwater the model results support the hypothesis