os isotopic variation in basalts from haleakala volcano maui hawaii a record of magmatic processes in oceanic mantle and crust os isotopic data are presented for a suite of tholeiitic and alkalic basaltic lavas from the honomanu gulch section of haleakala volcano maui spanning the shield building post shield and post erosional stages of the typical hawaiian volcanic cycle os is shown to behave compatibly during fractional crystallization of the basalts with a bulk crystal melt partition coefficient of 10 5 os isotopic compositions are inversely related to os concentrations and exhibit no simple temporal trend in contrast to sr nd pb and he isotopic data from the suite 1 the os sr and he isotopic systematics in the haleakala basalts are explained by the presence of three components the dominant component has 187os188os 0 132 0 002 187os186os 1 10 0 02 87sr86sr 0 704 and elevated 3he4he and is interpreted to reflect melts derived from the hawaiian mantle plume a second component with low 187os186os 87sr86sr and 3he4he is considered representative of depleted upper mantle morb source temporal and petrologic trends in the isotopic data support a model for progressive mixing between plume derived melts and up to 30 of small degree partial melts of the morb source during the later stages of volcanic activity direct exchange of plume derived melts with morb source peridotite is precluded a third component with 187os188os 0 135 187os186os 1 12 is diagnostic of old crustal materials with high time integrated reos it is present only in some of the most differentiated basalts and is probably derived through contamination of melts with aged oceanic crust beneath the volcano this study demonstrates that os isotopes in ocean island basaltic magmas are sensitive to interactions with oceanic crust and mantle as a consequence the os isotopic compositions of such basalts can record a variety of magmatic processes both within and outside their mantle plume source regions

an alternative interpretation of the cayman trough evolution from a reidentification of magnetic anomalies magnetic data were collected during the wilkes 1973 and seacarib ii 1987 cruises to the cayman trough a new interpretation of magnetic data is carried out an isochron pattern is drawn up from our anomaly identifications an early eocene age 49 ma ypresian for cayman trough opening is proposed instead of the late oligocene or middle eocene ages suggested by previous studies our plate tectonic reconstruction is simpler and fits the on land geology jamaica and cuba and the tectonics our reconstruction shows a southward propagation of the spreading centre between magnetic anomalies 8 and 6 26 and 20 ma the trough width increases by 30 km in this period the southward propagation of the cayman spreading centre from the middle oligocene to the early miocene induced the development of the restraining bend of the swan islands the formation of a 1 km high scarp on the eastern trace of the cayman trough transform fault walton fault and the formation of a pull apart basin hendrix pull apart magnetic anomalies and magnetization maps give information about the deformation and the rocks the proposed evolutionary model of the cayman trough from the inception of seafloor spreading to the present configuration is presented in relation to the tectonic escape of the northern boundary of the caribbean plate from the maastrichtian to the present

chalcophile element geochemistry and petrogenesis of high ti and low ti magmas in the permian emeishan large igneous province sw china sulfide poor mafic layered intrusions sills dykes and lava flows in the funing region sw china are part of the 260 ma emeishan large igneous province they belong to either a high ti group tio2 1 6 4 4 wt with elevated ti y ratios 351 1 018 or a low ti group tio2 1 low y pd pm ratios 0 2 7 1 and nearly constant cu pd pm ratios 1 5 3 8 the even and parallel chalcophile element patterns of the high pge low ti subgroup rocks are likely a result of olivine dominated fractionation under s undersaturated condition the low pge low ti group rocks have low mgo 4 5 8 9 wt and very poor pge pt pd 0 5 1 6 ppb ir 0 004 0 02 ppb with low cu zr ratios 0 1 0 5 high y pd pm 26 70 and variable cu pd pm ratios 2 8 14 the trough like chalcophile element patterns of the low pge low ti subgroup rocks indicate that the magmas were sulfide saturation and sulfide melts were extracted from the magmas the extracted sulfide melts might be potential ni cu sulfide ores at depth in the funing region

anomalous sulfur poor platinum group element mineralization in komatiitic cumulates mount clifford western australia an unusual occurrence of sulfur poor platinum group element pge mineralization has been discovered at mount clifford in the agnew wiluna belt western australia within the lower part of a 1 000 m thick komatiitic dunite body the host body consists primarily of strongly serpentinized olivine adcumulates with accessory chromite high precision whole rock icp ms data show that the pge mineralization extends over tens of meters whereas the lateral extent is unknown palladium and pt abundances are closely associated with base metal sulfides but show offset distribution patterns with maximum concentrations associated with the relatively sulfide poor base of a disseminated sulfide bearing layer whole rock concentrations range up to 4 100 ppb pd and 920 ppb pt iridium rh and ru follow the trends in pd and pt but are strongly depleted ir 0 4 ppb ru 0 5 ppb rh 0 1 ppb platinum group minerals pgm were located in essentially sulfide free serpentine 2 m below the peak in sulfide abundance and can be categorized into three groups pd antimonides pt dominated pgm and pt bearing ni antimonides the pge mineralization zone is underlain by an interval of strong ni depletion within olivine mesocumulates that is characterized by an unusual negative correlation between ni and the mg number of the rock this relationship is interpreted to be the result of an equilibration of olivine of variable forsterite content with a relatively large mass of magmatic sulfide however there is no direct evidence of this sulfide mass anywhere in the entire mount clifford dunite body the ni depletion is accompanied by strong depletion in ipge ir ru and rh the association of the pge zone with a reversal in the mg fe ratio of the host cumulates and a distinctive offset relationship of multiple pt pd and cu s peaks is suggestive of a primary magmatic origin however this is inconsistent with current models for offset reef pge concentrations which involve the onset of sulfide liquid saturation from a previously sulfur undersaturated pge undepleted magma the association of enriched pt and pd with highly depleted ni and ipge in immediately underlying cumulates conflicts with this model we suggest a combination of processes early sulfide segregation at low r values producing ni and ipge depleted olivine partial redissolution and removal of most of this sulfide component during a major replenishment event and subsequent overprint by a migrating pt pd cu rich fluid

pge in fresh basalt hydrothermal alteration products and volcanic incrustations of kilauea volcano hawaii abstract the concentrations of os ir pd and au in fresh unaltered kilauean tholeiite were determined by radiochemical neutron activation analysis for a suite of 18 samples averages were os 0 38 0 23 ir 0 38 0 14 pd 2 40 1 04 and au 1 78 0 57 in ppb with a 1 sd correlations of these metals with co cr cu ni and mgo in fresh basalts and petrographic observations indicate that os and ir are carried mainly in chromite much of which occurs as inclusions in olivine phenocrysts palladium correlations suggest its occurrence partly in olivine and partly in the matrix whereas au seems to be predominantly a matrix constituent altered basalts were analyzed for ir pd and au in a suite of 19 samples from five different locations minor changes only in either concentrations or element ratios were found for ir and pd when fresh and altered rock data were compared however au was consistently enriched in altered relative to fresh rocks these results imply that pd and ir in contrast to au will likely retain their eruptive signatures upon burial in a subaerial eruptive setting high temperature sulfate dominated condensates generate incrustations enriched in ir os au and pd by approximately 50 20 10 and 3 respectively relative to fresh rocks in contrast low temperature native sulfur deposits are the most depleted material found in the study with ir pd and au lower by factors of 10 4 and 5 compared with fresh rock averages the strong enrichments of os and ir in the high temperature suite are attributed mainly to enhanced volatility in highly oxygenated magmatic hydrothermal fluids contaminated by meteoric water near the structural top of volcanic conduits the relatively smaller pd enrichment which is dependent on the chloride content of fluids implies that pge partitioning into volcanic fume may fractionate these metals e g pd versus ir relative to host basalt in the eruptive process

the krafla geothermal field iceland 2 the natural state of the system a model of the natural state of the krafla reservoir system in iceland has been developed the model consists of a vertical cross section which includes reservoirs in both old and new well fields the physical processes considered include mass transport conductive and convective heat transfer and boiling and condensation natural heat losses to surface manifestations springs are also included the model matches very well all relevant data from the krafla field the natural flow of hot fluids through the reservoirs is estimated as approximately 20 kg s steam escaping to surface springs constitutes the bulk of the heat losses in the area modeled conductive heat losses through the caprock are approximately 1 w m2 and heat recharge from below is about 2 w m2

rotation of the chortis block causes dextral slip on the guayape fault the chortis block is undergoing deformation as a direct result of large magnitude sinistral strike slip faulting on the north america caribbean plate boundary we have studied the chortis block using geologic mapping remote imagery and earthquake seismology to evaluate the effect that large magnitude strike slip faulting has on an adjacent microcontinental block in the eastern part of the chortis block recent geologic mapping has shown that the northeast striking guayape fault is dextral instead of sinistral as previously assumed because this is a major fault that cuts most of the central america isthmus the new evidence for dextral slip necessitates a reinterpretation of the active tectonics of the chortis block the presence of several north trending rifts in the western part of the chortis block has led previous workers to propose that the chortis block is divided into smaller independently rotating blocks we use earthquake seismology and remote imagery data to extend this interpretation to the central portion of the chortis block however we have modified their model to account for dextral slip on the guayape fault we believe that rotation of the chortis block causes dextral slip on the guayape fault even though central america is undergoing large sinistral displacement with respect to north america

re os and s isotope evidence for the origin of platreef mineralization bushveld complex abstract the bushveld complex contains the largest platinum group element pge deposits of the world that are represented by persistent stratiform reefs highly enriched with pge with respect to underlying and overlying rocks new re os isotope and elemental la mc icpms data on platinum group minerals pgm from the mineralized reefs are presented with implications to correlation between the different segments of the bushveld complex and a role of superimposed processes at the reef formation we analyzed laurite rus 2 hollingworthite rhass sperrylite ptas 2 and pt fe alloys from the merensky reef pseudoreef and the pge reef of the platreef the measured 187 os 188 os value for platreef laurite is 0 1751 0 0004 whereas the ratios for sperrylite and hollingworthite range to slightly higher values 0 1713 0 1818 and 0 1744 0 1835 respectively the observed textures of the analyzed pgm such as pt fe symplectites in base metal sulfides bms laurite inclusions in chromite and sperrylite rims around sulfide silicate aggregates are interpreted as features of primary magmatic crystallization whereas hollingworthite overgrowths and exsolutions in sperrylite are likely to have originated from later solid state transformation or metasomatic processes the platreef is a composite sill like body in the northern limb correlative to the critical zone in terms of stratigraphic position whole rock geochemistry and isotope characteristics the pristine magmatic character of sulfides and pgm in the stratiform reefs at the top of the platreef strongly resembles the style of merensky reef mineralization however the basal part of the platreef pyroxenitic sequence is variably contaminated and mineralized with a significant hydrothermal overprint sulfides from underlying lower zone peridotite yield 34 s values varying from 9 to 14 2 that are much higher than the values for the overlying platreef and are a consequence of sulfur assimilation from sedimentary sulfates the same homogeneous mantle like s isotope compositions in the high grade pge reef of the platreef and the merensky reef can be explained by enhanced s isotope exchange through the orthomagmatic process of sulfide upgrading the similar values and the limited variations of the initial os isotopic ratios of all pgms in both reefs also support their magmatic crystallization and origin from a common source

fractionation of the platinum group elments and re during crystallization of basalt in kilauea iki lava lake hawaii article i nfo kilauea iki lava lake formed during the 1959 summit eruption of kilauea volcano then crystallized and differentiated over a period of 35 years it offers an opportunity to evaluate the fractionation behavior of trace elements in a uniquely well documented basaltic system a suite of 14 core samples recovered from 1967 to 1981 has been analyzed for 5 platinum group elements pge ir os ru pt pd plus re these samples have mgo ranging from 2 4 to 26 9 wt with temperatures prior to quench ranging from 1140 c to ambient 110 c five eruption samples were also analyzed osmium and ru concentrations vary by nearly four orders of magnitude 0 0006 1 40 ppb for os and 0 0006 2 01 ppb for ru and are positively correlated with mgo content these elements behaved compatibly during crystallization mostlylikelybeingconcentratedintracephases alloyorsulfide presentinolivinephenocrysts or included chromite iridium also correlates positively with mgo although less strongly than os and ru the somewhat poorer correlation for ir compared with os and ru may reflect variable loss of ir as volatile irf6 in some of the most magnesian samples rheniumisnegativelycorrelated withmgo behavingas anincompatibletraceelement itsbehavior inthe lava lake is complicated by apparent volatile loss of re as suggested by a decrease in re concentration with time of quenching forlakesamplesvs eruptionsamples platinum andpdconcentrationsarenegatively albeit weakly correlatedwithmgo sotheseelementsweremodestlyincompatible duringcrystallizationof themajorsilicate phases palladium contents peaked before precipitation of immiscible sulfide liquid however and decline sharply in the most differentiated samples in contrast pt appears to have been unaffected by sulfide precipitation microprobe data confirm that pd entered the sulfide liquid before re and that pt is not strongly chalcophile in this system occasional high pt values in both eruption and lava lake samples suggest the presence of unevenly distributed unidentified pt rich trace phases in some kilauea iki materials estimated mineral olivine chromite melt d values for os ir ru and pt for equilibrium crystallization for samples from 7 to 27 wt mgo are 26 8 2 19 and 0 55 respectively these os ir and ru estimates are somewhat higher than previous estimates for similar systems if fractional crystallization is instead assumed d values are much more similar results confirm many prior observations in other mafic systems that olivine together with included phases has a major effect on absolute and relative abundances of re and the pge the relatively linear correlations between these elements and mgo potentially permit accurate estimation of the concentrations of these elements in the primary melts of comparable systems especially in instances where the mgo content of the primary melt is well constrained

the heber geothermal field california natural state and exploitation modeling studies using numerical simulation techniques and an axisymmetric model of the heber geothermal field the natural preexploitation state of the system and its response to fluid production are analyzed the results of the study indicate that the heber geothermal anomaly is sustained by the upflow of hot water through a central zone of relatively high permeability the best model suggests that in its natural state the system is recharged at depth by a 15 mwt megawatts thermal reference temperature 0 c convective heat source the existence of an axisymmetric convection pattern whose axis coincides with the center of the heber anomaly is also suggested at the lower part of the ascending hot water plume the deep recharge water mixes with colder water moving laterally toward the axis of the system in the upper part the rising plume spreads radially outward after reaching the bottom of the cap rock at about 550 m depth the model results suggest that the so called cap rock is quite permeable 5 10 15 m2 with convection controlling its temperature distribution the results also show reduced permeability 10 10 15 m2 of the upper zones in the outer region of the system that may be due to mineral precipitation in modeling the exploitation of the field the generation rate is allowed to build up over a period of 10 years after that 30 years of constant power production is assumed full 100 injection of the spent brines is considered the fluids being injected 2250 m near injection or 4250 m far injection from the center of the system the study shows that a maximum of 6000 kg s equivalent to approximately 300 mwe megawatts electric of fluids may be produced for the near injection case but only 3000 kg s equivalent to approximately 150 mwe for the far injection case the results indicate that the possible extraction rates generating capacity are generally limited by the pressure drop in the reservoir the average temperature of the produced fluids will decline 10 18 c over the 40 year period

replacement of base metal sulfides by actinolite epidote calcite and magnetite in the ug2 and merensky reef of the bushveld complex south africa interaction of base metal sulfides pyrrhotite pentlandite and chalcopyrite with aqueous fluids in the ug2 chromitite layer and the merensky reef in the western bushveld complex has produced four common replacements 1 replacement of sulfides and orthopyroxene by actinolite or tremolite 2 replacement of sulfides and plagioclase by epidote 3 replacement of sulfides by calcite and 4 replacement of sulfides by magnetite the first three replacements are directly related to the presence of sulfides in the rocks and occur at the grain scale silicate minerals that are not in direct contact with sulfides are usually not affected the replacement of sulfides by magnetite accompanies regional serpentinization superimposed on the union section platinum group minerals including ru pt and pd sulfides pt and pd tellurides and pd arsenides occur in replacement aureoles around sulfide aggregates as well as within the aggregates suggesting that the platinum group minerals are more resistant to hydrothermal alteration than the associated base metal sulfides redistribution of chalcopyrite at least at the millimeter scale variable losses of s and ni and perhaps decoupling of pt and pd at the centimeter scale took place during secondary hydrothermal alteration these effects should be considered before models of primary mineralization processes can be meaningfully applied

cumulative deformation and original geometry of the bushveld complex abstract a structural review and re interpretation of the bushveld complex based on 254 georeferenced maps from research technical reports and geophysical surveys suggests that its current geometry differs markedly from its original geometry broadly n s flattening left lateral transpressional shearing along an interconnected fault network steepening of cumulate layering and 10 km of uplift to the north of the northern margin of the complex s main chamber were accompanied by an eastward verging escape structure burgersfort bulge and syn cooling gap areas on its western limb southward verging deformation was transmitted from the villa nora fragment via thrusting in the rooiberg area generating 50 70 km of southward displacement uplift and erosional loss of the northern limb s western portion a string of anomalous occurrences of bushveld complex units and dense areas outlined by unconstrained gravity isoshells suggest a further 170 km southward displacement through the center of the main chamber the southern limb underwent predominantly right lateral ductile offset from the western limb along the crocodile river and rustenburg faults expressed by an in line series of nnw trending magnetic highs to the south of their mapped terminations left lateral shearing and progressive splaying of a nne trending dyke swarm to the west of an inflection point provides strain markers for this offset and 35 anticlockwise rotation of the burgersfort bulge and central parts of the main chamber although recent work suggests that the complex cooled within 1 myr late intrusion syn cooling sse wards movement of its deeper ductile central parts within at least 5 10 myr after its initial intrusion the transvaalide fold and thrust event and the expulsion of fluids to form the high ti igneous suite hitis and related bodies appear to have closely post dated the complex s intrusion removal of the effects of cumulative strain produces a more equant complex shortly after its intrusion occupying an orthogonal network of normal faults and perpendicular strike slip transverse faults

a plausible two dimensional vertical model of the east mesa geothermal field california a two dimensional conceptual model of the east mesa geothermal system is developed on the basis of existing geological geophysical geochemical heat flux and borehole logging data hot water rising in a set of faults is assumed to charge the reservoir which is overlaid by a clay rich cap the temperature depth distribution observed at the site implies that the liquid is convecting at a high rayleigh number in this approximation liquid rises up the fault and spreads isothermally into the nearby sections of the reservoir the cooling effect of the surface on the flow in the reservoir is confined to a thin layer adjacent to the cap reservoir interface near the fault this layer grows with the distance from the fault eventually the entire depth of the reservoir is cooled by the surface the mathematical model is based on the flow of liquid water in a saturated porous medium results are obtained for the velocities pressures and temperatures in the entire system consisting of fault zone aquifer and clay cap finally we compare the predicted surface heat flux to that measured at the site in shallow wells it is concluded that the model represents a plausible description of fault zone controlled systems likemore that at east mesa less

actively evolving microplate formation by oblique collision and sideways motion along strike slip faults an example from the northeastern caribbean plate margin abstract the pattern of folding faulting and late quaternary coral reef uplift rates in western and central hispaniola haiti and dominican republic suggest that the elongate gonave microplate a 190 000 km2 area of the northeastern caribbean plate is in the process of shearing off the caribbean plate and accreting to the north american plate late cenozoic transpression between the southeastern bahama platform and the caribbean plate in hispaniola has inhibited the eastward motion of the northeastern corner of the plate transpression is manifested in western and central hispaniola by the formation of regional scale folds that correspond to present day anticlinal topographic mountain chains continuous with offshore anticlinal ridges areas of most rapid quaternary uplift determined from onland coral reefs 125 ka and younger coincide with the axial traces of these folds offshore data suggest recent folding and faulting of the seafloor onshore reef data do not conclusively require late quaternary folding but demonstrate that tectonic uplift rates of the axial areas of the anticlines decrease from the northwest peninsula of haiti 0 37 mm yr to to the central part of the coast of western haiti 0 19 mm yr to the south central part of western haiti 0 mm yr formation of the 1200 km long enriquillo plantain garden walton fault zone as a bypass strike slip fault has isolated the southern edge of the gonave microplate and is allowing continued unimpeded eastward motion of a smaller caribbean plate past the zone of late neogene convergence and quaternary uplift of coral reefs in hispaniola offshore seismic reflection data from the jamaica passage the marine strait separating jamaica and haiti show that the enriquillo plantain garden fault zone forms a narrow but deep active fault bounded trough beneath the passage the active fault is continuous with active faults mapped onshore in western haiti and eastern jamaica the bathymetric deep is present because the jamaica passage fault segment represents a 50 km wide transtensional left step of the fault trace between haiti and jamaica onshore satellite imagery and field observations suggest that the enriquillo plantain garden fault forms a continuous trace extending from central hispaniola east of lake enriquillo dominican republic to the westernmost end of the southern peninsula of haiti the regional lineament corresponds to a recent fault scarp in quaternary alluvium of the clonard pull apart basin in the central part of the southern peninsula of haiti and suggests that at least this part of the lineament has undergone recent slip calmus 1983 has suggested a total offset of 30 50 km of the enriquillo plantain garden fault zone in the southern peninsula of haiti using the apparent offset of lithologic units seismic reflection data from lake enriquillo document recent deformation of quaternary lake sediments where the lineament crosses the lake leveling of the crest of a late holocene coral reef and associated algal tufa around lake enriquillo demonstrate late holocene vertical movement and tilting in a 1500 m wide zone parallel to the fault trend lateral offset is difficult to show in the enriquillo valley area because of rapid recent sedimentation into the valley the pattern of inactive strike slip faults and fold belts of cenozoic age in cuba and the yucatan basin suggest that two elongate microplates were sheared off the proto caribbean plate and accreted to the north american plate by a similar process in paleocene and eocene times age of terminal deformation in western central and eastern cuba is consistent with southeastward younging and migration of arc collision the similarity of the size and sequence of events in cuba suggests that the process of oblique collision and sideways motion of the plate along a new strike slip fault towards a free face may be an important process of microplate formation and interplate transfer in other areas

a seismic refraction survey of the imperial valley region california the u s geological survey conducted an extensive seismic refraction survey in the imperial valley region of california in 1979 the imperial valley is located in the salton trough an active rift between the pacific and north american plates forty shots fired at seven shot points were recorded by 100 portable seismic instruments at typical spacing of 0 5 1 km more than 1300 recording locations were occupied and more than 3000 usable seismograms were obtained we analyzed five profiles using a standard ray tracing program constructed a contour map of reduced travel times from our most widely recorded shot point and modeled an existing gravity profile across the salton trough results are itemized 1 all models have in common a sedimentary layer vp 1 8 5 0 km s a transition zone vp 5 0 5 65 km s a basement vp 5 65 km s in the imperial valley 5 9 km s on the bordering mesas and subbasement vp 7 2 km s 2 the sedimentary layer ranges in thickness along the axis of the salton trough from 3 7 km salton sea to 4 8 km u s mexican border on the bordering mesas it is quite variable in thickness 3 the transition zone is about 1 km thick in most places in the imperial valley there are no marked velocity discontinuities in this zone between the sedimentary layer and basement on the bordering mesas however there is a discontinuity at the top of this zone 4 there are apparently two types of basement on the bordering mesas basement is crystalline igneous and metamorphic rocks in the imperial valley basement is mostly lower greenshist facies sedimentary rocks based primarily on the smooth transition in character from sediment to basement arrivals the low value of basement velocity and the fact that deep 4 km wells in the valley penetrate only the upper part of the known cenozoic stratigraphic column for the salton trough 5 the subbasement or intermediate crustal layer ranges in depth along the axis of the salton trough from 16 km salton sea to 10 km u s mexican border gravity modeling requires that this layer deepen and or pinch out beneath the bordering mesas and mountain ranges based on its high velocity and the presence of intrusive basaltic rocks in the sedimentary section in the imperial valley the subbasement is thought to be a mafic intrusive complex similar to oceanic middle crust 6 several structures are seen that affect basement transition zone and deeper parts of the sedimentary layer they include a scarp along the imperial fault as much as 1 km down to the northeast and a scarp passing roughly along the topographic boundary between the imperial valley and the bordering mesa to the west as much as 3 km down to the east we interpret the latter scarp to be the suture or rift boundary between the older crystalline basement on the mesa and the younger metasedimentary basement in the imperial valley 7 on a contour map of reduced travel time from our most widely recorded shot point subtle patches of early arrivals among otherwise late arrivals in the central imperial valley correlate well with known geothermal resource areas having reservoir temperatures of more than 150 c apparently the salton trough is a location where new crust is being generated as the rift opens mafic intrusive rocks fill it from below as sedimentary rocks fill it from above rifting and intrusion produce high heat flow that metamorphoses the sedimentary rocks to shallow depth metasedimentary basement in the imperial valley and thus consolidates the new crust

reservoir scale heterogeneity in depositional packages and diagenetic patterns on a reef rimmed platform upper miocene mallorca spain predictive models for interwell scale variations in heterogeneous carbonate rock are best made from outcrop studies of well exposed limestone and dolomite such as the upper miocene reef complex that crops out in the sea cliffs of mallorca spain the sea cliff sections reveal highly complex lithofacies stacking patterns that could lead to ambiguous lateral correlations of coeval units the stratigraphic complexity and distribution of primary and secondary porosity are the result of a sea level driven hierarchical stacking of different magnitudes of accretional units thickest sections of porous and permeable rocks are in the aggradational portions of the reef upper slope and outer lagoon units the relative volume of the various accretional units and heterogeneity in the lithofacies architecture were dependent on the amount of carbonate production which was related to 1 accommodation changes controlled by sea level fluctuations and 2 depositional profile moldic porosity mostly from the dissolution of aragonitic constituents is the predominant porosity type and its heterogeneous distribution is related to the lateral and vertical distribution of lithofacies the secondary porosity in much of this reef complex was produced mainly during early dolomitization dolomite patterns are complicated apparently mainly related to shallow flooding of the platform during third or fourth order sea level highs and to geographic location of permeable pathways for brine reflux probably primarily through fourth order aggradational reef units many of these stratigraphic complexities and diagenetic patterns are below the resolution of seismic and well analyses thus models based on outcrop data such as this can enhance reservoir development in certain shallow water carbonate rocks

the origin of chromitites and related pge mineralization in the bushveld complex new mineralogical and petrological constraints this article reports a study of chromitites from the lg 1 to the ug 2 3 from the bushveld complex chromite from massive chromitite follows two compositional trends on the basis of cation ratios trend a decreasing mg mg fe with increasing cr cr al trend b decreasing mg mg fe with decreasing cr cr al the chromitites are divided into five stages on the basis of which trend they follow and the data of eales et al chemical geology 88 261 278 1990 on the behaviour of the mg fe ratio of the pyroxene and whole rock sr isotope composition of the environment in which they occur following eales et al chemical geology 88 261 278 1990 the different characteristics of the stages are attributed to the rate at which new magma entered the chamber and the effect of this on aal2o3 and in the case of stage 5 the appearance of cumulus plagioclase buffering the aal2o3 the similarity of pge profiles across the mg 3 and mg 4 chromitites that are separated laterally by up to 300 km and the variation in v in the ug 2 argue that the chromitites have largely developed in situ modelling using the programme melts shows that increase in pressure mixing of primitive and fractionated magma felsic contamination of primitive magma or addition of h2o do not promote crystallization of spinel before orthopyroxene in general they hinder it and that the cr2o3 content of the magma was of the order of 0 25 wt less than 20 of the chromite in the magma is removed before orthopyroxene joins chromite which implies a 13 km thickness of magma for the critical zone it is suggested that the large excess of magma has escaped up marginal structures such as the platreef the pge profile of chromitites depends on whether sulphide accumulated or not along with chromite modelling shows that contamination of critical zone magma with a felsic melt will induce sulphide immiscibility although not chromite precipitation the lg 1 to lg 4 chromitites developed without sulphide whilst those from the lg 5 upwards had associated liquid sulphide much of the sulphide originally in the lg 5 and above has been destroyed as a result of reaction with the chromite

basin modeling of late cretaceous mio pliocene petroleum system of the deep water eastern colombian basin and south caribbean deformed belt abstract previous interpretation of 2d and 3d seismic reflection data have shown widespread direct hydrocarbon indicators from deep water successions of the eastern colombian basin ecb and adjacent south caribbean deformed belt scdb that suggest the presence of an extensive but poorly defined petroleum system we integrate rock eval analysis seismic interpretation and 1d 2d thermal and basin modeling to better understand the origin of gas chimneys and bright spots identified in the ecb our dataset consists of 36 rock eval samples of coniacian to santonian source rocks from the dsdp well sites 153 151 and 146 located in the beata ridge tied to a 270 km long composite 2d seismic line covering the ecb rock eval analyses reveal intervals of immature source rocks containing 1 kerogen types ii and iii 2 an average toc of 3 17 with a maximum toc of 11 4 3 an average hi of 241 and 4 an average oi of 124 seismic interpretation reveals 1 wedging of post coniacian organic rich successions from the beata ridge 35 73 m towards the ecb 500 1700 m 2 late cretaceous to eocene slow sedimentation rate of 38 m my followed by faster oligocene to recent sedimentation rate of 149 m my from the magdalena river and 3 late oligocene to recent thrusting and propagation of the scdb thermal and basin modeling show 1 mature source rocks in the scdb and ecb 2 miocene to pliocene reservoir and seal rocks related to deltaic and turbiditic sandstone and mass transport deposits of the magdalena fan 3 trap formation since the late oligocene related to thrust front structures compaction folds and stratigraphic traps 4 hydrocarbon expulsion since pliocene times 5 vertical and lateral migration in the scdb and vertical up dip migration in the ecb and 6 hydrocarbon accumulation and preservation may be affected by remobilization due to biodegradation and trap breaching

numerical studies of the heat and mass transport in the cerro prieto geothermal field mexico numerical simulation techniques are employed in studies of the natural flow of heat and mass through the cerro prieto reservoir mexico and of the effects of exploitation on the field s behavior the reservoir model is a two dimensional vertical east to west southwest cross section which is based on a recent hydrogeologic model of this geothermal system the numerical code mulkom is used in the simulation studies the steady state pressure and temperature distributions are computed and compared against observed preproduction pressures and temperatures a reasonable match is obtained a natural hot water recharge rate of about 1 x 10 sup 2 kg s per meter of field length measured in a north south direction is obtained the model is then used to simulate the behavior of the field during the 1973 1978 production period the response of the model to fluid extraction agrees to what has been observed in the field or postulated by other authors there is a decrease in temperatures and pressures in the produced region no extensive two phase zone develops in the reservoir because of the strong fluid recharge most of the fluid recharging the system comes from colder regions located above and west of the produced reservoir 34 references 22 figures more 5 tables less

iridium bearing sublimates at a hot spot volcano piton de la fournaise indian ocean sublimates and incrustations derived upon the cooling of volcanic gases have been collected on various sites piton de la fournaise poas momotombo etna ardoukoba and erta ale they have been analysed for ir and other volatile elements se as cu au ag pb tl by means of instrumental neutron activation analysis inaa and proton induced x ray emission pixe among the investigated volcanoes only piton de la fournaise is found to release detectable amounts of iridium ir in piton de la fournaise sublimates is associated with f minerals this confirms its gaseous transport as a volatile fluoride compound iridium seems to be preferentialy released by hot spot type volcanoes and its detection in piton de la fournaise sublimates provides a positive argument in favour of a volcanic hypothesis to explain the ktb events

cenozoic rift formation in the northern caribbean rifts form in many different tectonic environments where the lithosphere is put into extension we outline the distribution orientation and relative ages of 16 cenozoic rifts along the northern edge of the caribbean plate and suggest that these structures formed successively by localized extension as the caribbean plate moved eastward past a continental promontory of north america evidence leading to this conclusion includes 1 recognition that the rifts become progressively younger westward 2 a two phase subsidence history in a rift exposed by upthrusting in jamaica 3 the absence of rifts east of jamaica and 4 the observation that removal of 1400 km of strike slip displacement on the cayman trough fault system places the paleogene rifts of jamaica in an active area of extension south of yucatan where the rifts of honduras and guatemala are forming today

cooperite and braggite from the bushveld complex implications for the miscibility gap and identification analyses of the pt pd ni sulphides cooperite braggite and vysotskite reported from worldwide occurrences seem to imply a continuum of compositions between vysotskite and cooperite with no obvious miscibility gap this is contrary to the experimentally confirmed miscibility gap between cooperite and braggite and the established compositional gap between co existing cooperite and braggite from the merensky reef although the only unambiguous way of distinguishing between cooperite and braggite is to obtain structural information through x ray diffraction or equivalent techniques most identification of pt pd ni sulphides is based on microanalytical techniques due to the small grain size of most platinum group minerals ni contents also have to be considered because a classification based on the pt pd ratio alone can be very misleading naming of pt pd ni sulphide compositions with high pt contents based on qualitative or semi quantitative analyses should be avoided natural pt pd ni sulphides which project into the compositional gap established by experimental work cannot be named without supporting structural information compositions of grains which plot inside the gap are considered to be metastable and to result from the loss of pd through interaction with hydrothermal fluids

the merensky reef at winnaarshoek eastern bushveld complex a primary magmatic hypothesis based on a wide reef facies the merensky reef on the winnaarshoek property in the central sector of the eastern limb of the bushveld complex belongs to what may be described as a wide reef facies in which platinum group element pge mineralization is largely contained within a relatively thick succession of medium grained nonpegmatoidal feldspathic orthopyroxenite bounded top and bottom by thin chromite stringers the wide reef merensky reef facies at winnaarshoek presents the opportunity to study subtle processes which could potentially be obscured or obliterated in the thinner reef facies found in many areas of the western limb especially in places where the reef zone contains a component of pegmatoidal orthopyroxenite at winnaarshoek a pegmatoidal feldspathic orthopyroxenite is developed below the lower chromite stringer and a second pegmatoidal orthopyroxenite is intermittently developed several centimeters above the upper chromite stringer in contrast to most occurrences in the western limb however both these pegmatoidal orthopyroxenites are only sporadically mineralized if at all in addition rather than being at the base of a feldspathic orthopyroxenite unit as is the case throughout most of the western limb the merensky reef at winnaarshoek occurs toward the top of a sequence of feldspathic orthopyroxenite despite the gross relative uniformity of the merensky reef on a property wide scale at winnaarshoek we demonstrate using underground channel samples that individual intersections spaced a matter of meters apart commonly display remarkable variations in grade distribution our study is based primarily on whole rock and mineral chemical profiles from drill cores through three separate and distinctive subfacies of essentially normal i e nonpothole reef one of the drill core intersections represents typical reef in which pge mineralization forms an upper reef extending from the upper chromitite stringer approximately 1 m into the underlying feldspathic pyroxenite and a much thinner lower reef centered on the lower chromitite stringer the other two drill core intersections represent extremes in terms of the vertical distribution of the pge one having mineralization confined to the immediate vicinity of the two chromitite stringers while the other is richly mineralized throughout the reef zone peaks in pge concentrations in all three intersections are coincident with the most primitive compositions of orthopyroxene plagioclase and chromite the dominant rock forming minerals whereas the cumulate layers below major pge peaks are the most evolved as indicated by mineral compositions and elevated whole rock incompatible element concentrations furthermore the lowest grade merensky reef intersection has the most chemically evolved footwall whereas the richest intersection has a relatively primitive footwall we consider the origin of the merensky reef in the context of its position as the culmination of a sequence of several pge mineralized chromite bearing layers in the upper critical zone and suggest that the regular upward variations in the chemistry of the chromitite from one layer to the next and the overall cyclicity are indicative of complex magma replenishment and mixing processes in the chamber we apply a model of lateral streaming of magma with chromite crystallization and consequent sulfide and platinum group mineral pgm precipitation resulting from magma mixing specifically the merensky reef cannot be properly interpreted outside the context of the sequence of successive events that evolved throughout the development of the upper critical zone genetic models also need to account for lateral facies variations in the reef

re and os concentrations in arc basalts the roles of volatility and source region fo2 variations olivine and spinel compositions major elements including ferric and ferrous iron s re and os contents have been measured for a suite of primitive most 6 wt mgo basalts from the trans mexican volcanic belt tmvb including the western mexican volcanic belt the michoacan guanajuato volcanic field sierra chichinautzin pico de orizaba region palma sola san martin tuxtlas and the eastern alkaline province eap sulfur contents at sulfide saturation were calculated to determine whether the measured s contents are representative of sulfide saturated liquids most of the samples have s contents much lower than expected for sulfide saturation a few have higher contents than calculated perhaps due to the presence of sulfate in the measured total sulfur i e more oxidized samples comparison of the tmvb samples along with previously analyzed morb oib babb and arc samples reveals a continuum of re and os contents that is best explained by variation in oxygen fugacity and thus sulfide stability in the source region high re and os magmatic suites are best explained by derivation by melting of oxidized mantle where sulfide is no longer stable and re and os behave incompatibly whereas low re and os magmatic suites are derived from melting of relatively reduced mantle where sulfide is stable and re and os behave compatibly intermediate examples abound and arc magmas span a wide range of re and os concentrations due to variation of fo2 in the source during genesis of arc magmas low re magmatic suites are furthermore potentially affected by volatility which can lower re by a factor of 3 5

lowstand carbonates highstand sandstones the sedimentary facies sediment dynamics and sequence architecture of modern high energy shelves in the mid and high latitudes are largely governed by wave abrasion processes cool water carbonates may form there if the influx and or net accretion of siliciclastics is kept at a minimum little dilution of the carbonate produced in situ is generally promoted by a wide epicontinental shelf subdued topography of the adjacent mainland the predominance of limestone outcrops and an arid climate the aforementioned requirements are rarely met and thus will automatically lead to the formation of mixed siliciclastic cool water carbonates such an example is found in the early to mid miocene lagos portimao formation algarve s portugal which formed on a narrow high energy shelf of the atlantic ocean that was bounded by a mountain range the sediments of the formation consist of fossiliferous sandstone fs shell beds and rhodolith blankets along strike the stratification of the formation is monotonous for tens of kilometres and well exposed in coastal cliffs whereas no outcrops of dip sections exist the bulk skeletal composition of the sediments is typical for the warm temperate climatic zone various endo and epibenthic bivalves bryozoans coralline algae echinoderms gastropods and large foraminifers heterostegina in some very rare beds a few isolated not framework forming specimens of zooxanthellate corals porites tarbellastrea indicate temporally elevated surface water temperatures close to the lower threshold of the coral reef ecosystem in sandstones the fauna is well preserved and burrowing bivalves are commonly found in life position in limestone beds the state of preservation of the grains ranges from intact to disintegrated and abraded specimens we infer an accumulation of the shell beds through winnowing of fine materials siliciclastic sand and carbonate mud at wave abrasion depth and concentration of calcareous skeletons associated with the subsequent attraction of new epibiota in a complex shell bed the vertical alternation of fossiliferous sandstone and shell beds and in phase variations of the photo index photic biota vs bryozoans and bryozoan index bivalves vs bryozoans is envisaged to document variations of water depth and sea level sandstone units built up when wave abrasion depth wad rose above the sea floor during tst and early hst whereas the shell beds formed during lst when the wad for sand intersected with the sea floor clastic sediments were probably brought on the outer shelf during early transgression and by longshore currents sea level signatures inferred in the mixed siliciclastic cool water carbonate shelf setting of s portugal therefore significantly deviate from conventional concepts of carbonate sequence stratigraphy which were developed for flat topped platforms successful interpretations of ancient mixed sequences must therefore take into consideration the processes of production concentration and accretion of the carbonate sediments

evolution of mauna kea volcano inferences from lava compositions recovered in the hawaii scientific drilling project the lower 776 m of core recovered during the initial phase of the hawaii scientific drilling project hsdp contains lavas erupted from mauna kea volcano tholeiitic and alkalic basalts including an fe ti rich flow are intercalated in the upper 58 m of mauna kea lavas similar basaltic sections are subaerially exposed on the lower east flank of mauna kea the fe ti rich lavas reflect large amounts of clinopyroxene plagioclase and olivine fractionation within the crust and upper mantle but the range from tholeiitic to alkalic compositions reflects variable extents of melting of a garnet bearing source based on abundances of incompatible elements the extent of melting for a basanitoid was a factor of 2 less than that for nearly coeval tholeiitic lavas all flow units in the lower 718 m of the hsdp core are tholeiitic lavas their variability in major element compositions reflect variable accumulation of olivine incompatible element abundance ratios in these lavas reflect a complex temporal variation in extent of melting within the tholeiitic part of the core lavas from 800 m to 950 m formed by the largest extent of melting whereas tholeiitic lavas from the bottom of the core and from just below the tholeiitic to alkalic transition formed by lower degrees of melting inferred melt compositions at 16 mgo show that the 200 to 400 ka mauna kea lavas from the hsdp core and the 250 ka subaerial exposures define an inverse correlation between sio2 and feo contents based on experimental studies this correlation is caused by differing pressures of melt segregation furthermore abundances of nb and sio2 are also inversely correlated in these calculated melts in general the younger lavas are relatively enriched in feo and incompatible elements but are depleted in sio2 these trends are interpreted to reflect an overall trend of increasing pressure of melt segregation and decreasing extent of melting with decreasing eruption age there are however geochemical variations which indicate short term reversals in this long term trend previously the geochemical trends accompanying the transition from tholeiitic to alkalic volcanism at hawaiian volcanoes have been interpreted as reflecting the effects of increasing distance from the plume axis the long term geochemical trends of tholeiitic lavas in the hsdp core also reflect migration of mauna kea away from the hawaiian plume

trace elements in the gas emissions from the erta ale volcano afar ethiopia high temperature 1084 c fumarolic emissions from a spatter cone at the erta ale north pit crater granted a rare opportunity to collect giggenbach bottles condensates and sublimates from the fumarolic gases of a rift plume volcano in the immediate proximity of degassing lava these results represent the highest temperature gases sampled directly for trace elements and the first high t fumarolic samples for a rift plume volcano sixty seven metals and metalloids were measured quantitatively by icp aes and icp ms in liquid gas condensates and solid sublimates the total amount of trace elements in the gas without si exceeded 170 ppm the ten most abundant elements transported in nonsilicate modes were na 61 ppm fe 41 ppm k 32 ppm br 12 4 ppm cd 3 0 ppm se 2 3 ppm cu 2 0 ppm zn 2 0 ppm te 1 7 ppm and as 1 2 ppm the gases contained 27 ppb re 1 5 ppb au 0 5 ppb pt and 0 27 ppb os rh and pd containing phases were observed in solid sublimates the only metal not detected by all analytical methods was ruthenium chalcophile elements cd cu zn se te as were transported primarily in the gaseous form alkali metals were emitted at least partially as liquid micron size sulfate droplets whereas alkali earths aluminum and ree had a high affinity to silicate aerosol in the form of basaltic microspheres the silicate aerosol content was evaluated at 59 ppm on average but varied within a 7 5 fold range during sampling because of the extremely high temperature refractory elements ca ba ta u and lree showed evidence of transport in the form of gaseous species the so2 flux in january 2011 was evaluated at 91 24 t d for the entire volcano including 31 t d from the north crater and 60 t d from the lava lake the daily output of trace elements from both erta ale craters without si was estimated at 53 kg d including 20 kg of na 13 5 kg of fe 10 5 kg of k 4 5 kg of chalcophile elements 9 g of re 0 5 g of au and 0 17 g of pt the trace element pattern of erta ale fumarolic gases do not possess any distinct geochemical specialization and resemble te patterns from fumarolic emissions of studied arc and plume volcanoes

nicaragua petroleum geology of the caribbean margin located on the caribbean margin of nicaragua the miskito basin covers an area of 45000km2 this region has undergone hydrocarbon exploration by different oil companies mostly over the continental shelf with scattered efforts onshore over 20 000 km of seismic reflection data have been shot and 26 exploratory wells were drilled between 1943 and 1977 figure 1 the first well punta gorda 1 was drilled onshore in 1943 followed by sporadic activities the touche 1 well drilled offshore in 1957 was abandoned after a blowout at a total depth of 4420 m

gas analyses from the pu u o o eruption in 1985 kilauea volcano hawaii volcanic gas samples were collected from july to november 1985 from a lava pond in the main eruptive conduit of pu u o o from a 2 week long fissure eruption and from a minor flank eruption of pu u o o the molecular composition of these gases is consistent with thermodynamic equilibrium at a temperature slightly less than measured lava temperatures comparison of these samples with previous gas samples shows that the composition of volatiles in the magma has remained constant over the 3 year course of this episodic east rift eruption of kilauea volcano the uniformly carbon depleted nature of these gases is consistent with previous suggestions that all east rift eruptive magmas degas during prior storage in the shallow summit reservoir of kilauea minor compositional variations within these gas collections are attributed to the kinetics of the magma degassing process

transverse intra arc rifting palaeogene wagwater belt jamaica abstract the wagwater belt in eastern jamaica is an exceptionally well exposed example of a transverse intra arc rift formed during an abrupt transition from convergent to strike slip tectonics the belt consists of the uplifted wagwater half graben containing at least 5 6 km of coarse clastic terrestrial sedimentary rocks which is overlain and flanked by a much wider basin filled with about 1 2 km of upward fining marine sedimentary rocks neogene strike slip movements structurally inverted the half graben and exposed thick sections of both the half graben and overlying basin early palaeocene early eocene rifting preceded the middle eocene formation of oceanic crust in the cayman trough pull apart basin and affected a 50000 km 2 area near the eastern distal end of the pull apart fifteen half grabens identified in jamaica and in offshore areas using seismic profiling range in strike from north east to north west rifts in jamaica are presently oblique to the generally east west strike of plutons and sedimentary facies belts within a maestrichtian palaeocene island arc based on their apparent relationship to the strike of the enclosing palaeo arc we interpret the jamaican rifts as transverse intra arc rifts following the deposition of palaeocene redbeds and evaporites wagwater formation in the half graben subsidence was widespread in a marine sedimentary basin this marine basin was filled in early eocene time by marine conglomerates port maria and albany members of the richmond formation sandstone and siltstones roadside member and finally in the latest early eocene to earliest middle eocene time by a calcareous mudstone langley member a bimodal suite of volcanic rocks newcastle and nutfield volcanics was erupted during the deposition of the wagwater formation and the roadside member the structure and tectonic setting of transverse intra arc basins in jamaica is somewhat similar to rifts of the neogene pannonian basin in hungary in both areas an abrupt transition between convergent and strike slip tectonics resulted in transverse intra arc rifts striking almost perpendicular to a dying volcanic arc transverse intra arc rifts may be common features of small arc fringed plates such as the caribbean and pannonian plates that respond quickly to collision by strike slip motion towards an unopposed oceanic free face

age and spreading history of the cayman trough as determined from depth heat flow and magnetic anomalies the opening of the cayman trough records aspects of relative motion between the north american and caribbean plates essential to accurate reconstruction of caribbean evolution assessment of opening rates based upon basement subsidence analysis as well as magnetic anomaly interpretation shows that the trough may have opened at one third to one half previously proposed rates depths to basement show that the crust has subsided away from the present axis in two stages a younger stage characterized by substantial and asymmetric greater to the east subsidence preceded by an older stage of slight increases of depth with distance depth to age relations calculated from subsidence curves which accommodate lateral as well as vertical cooling of the accreting slab indicate that the trough has opened at an overall rate of less than 15 mm yr since 25 30 ma and at 20 30 mm yr prior to 30 ma marine magnetic anomalies show lineations perpendicular to the trough including a distinct central anomaly but the pattern is neither well defined nor symmetrical about the present axis however partial anomaly sequences can be identified on both sides of the axis and the distribution of these sequences indicates that spreading may have involved frequent short ridge jumps mostly eastward the overall rate of opening as indicated by this interpretation of the magnetics is about 15 and 30 mm yr since and prior to 26 ma both subsidence and magnetics indicate that the trough opened by at least 45 50 ma

platinum group minerals in chromite seams of the stillwater complex montana platinum group minerals are present as inclusions in both disseminated and massive chromite but are modally more abundant in massive chromite in the ultramafic zone of the stillwater complex they are small generally less than 20 mu in diameter and characterized by euhedral and subhedral habits their distribution is random not crystallographically controlled within the chromite host the platinum group mineral inclusions may occur with silicate and base metal sulfide inclusions in an unfractured chromite host all identified inclusions in unfractured chromite are laurite ru os ir s 2 the ru ru os ir ruranges from 0 92 to 0 83 with ru os substitution more extensive than ru ir both pd and rh abundances in laurite are less than 2 wt percent no pt was detected at present the chemical data do not indicate a correlation between stratigraphic height and the ru ratio several pt pd and rh bearing interstitial minerals and a single unusual inclusion phase have been identified in the a chromite seam at west fork and the b seam at mountain view the interstitial platinum group minerals are sperrylite and isoferroplatinum whereas the unusual inclusion is a polyphase intergrowth of pt pd ni arsenides with chalcopyrite and pent landite whole rock platinum group element analyses of the chromite seams range from a few ppb to nearly 16 ppm with pd pt rh ru ir os was not analyzed this is in opposition to the data for the inclusions which show high ru ir and os low rh and pd and no detectable pt thus in the stillwater magma chamber ru ir and os bearing minerals appear to have precipitated early and have been included in chromite in contrast pd pt and rh apparently remained in solution in the magma and are in phases that precipitated late and are interstitial to chromite

evolution of the northern nicaragua rise during the oligocene miocene drowning by environmental factors abstract possible causes to explain platform drowning have been hotly debated by carbonate sedimentologists for more than a decade now in this paper we present multiple evidence to explain the drowning of a carbonate megabank that covered most of the modern northern nicaragua rise nnr during an interval spanning from late oligocene to early miocene by the interaction of several environmental factors the recovery during odp leg 165 of late oligocene to middle miocene sedimentary sequences in the sub seafloor of the modern channels and basin pedro channel and walton basin respectively that dissect the nnr site 1000 and south of the rise in the colombian basin site 999 combined with information from dredged rock samples allows us to explore in more detail the timing and possible mechanisms responsible for the drowning of the megabank and its relationship to miocene climate change the modern system of isolated banks and shelves dissected by a series of intervening seaways and basins on the nnr has evolved from a continuous shallow water carbonate megabank that extended from the honduras nicaraguan mainland to the modern island of jamaica available information suggests that this megabank broke apart and partially drowned in the late part of the late oligocene at around 27 ma and finally foundered during the late early miocene around 20 ma resulting in limited neritic coral growth in the areas where the modern isolated carbonate banks and shelves are occurring today available information also suggests that the southern and central parts of pedro channel were already a deep water area before the major episode of platform drowning and its formation predates the initiation of the caribbean current however after the partial drowning of the megabank the channel has become a major pathway for the caribbean current stratigraphic units identified in deep water carbonates sampled at odp sites 999 and 1000 help to constrain the environmental setting leading to the drowning of the banks changes in lithology and mass accumulation rates of both the carbonate and non carbonate fraction parallel stable isotope shifts and likely indicate regional changes in climate and circulation during the late oligocene middle miocene interval carbonate mass accumulation rates mars at site 999 suggest increased regional productivity during the early miocene terrigenous mars at both sites 999 and 1000 show a general increase from the burdigalian through the serravallian the temporal association among episodes of neritic platform deposition followed by increased productivity as identified by higher carbonate mars and positive excursion in carbon isotopes suggests that oceanographic changes such as local upwelling and nutrification have led to the partial drowning of the nnr megabank

the distribution of pge and the role of arsenic as a collector of pge in the spotted quoll nickel ore deposit in the forrestania greenstone belt western australia the spotted quoll pge bearing ni deposit in the forrestania greenstone belt in the archean yilgarn block in western australia is a komatiite associated massive sulfide orebody tectonically displaced from its original host the brecciated ore contains clasts of quartz and garnet schist and is located along a shear zone overlain by banded iron formation bif and underlain by bif and quartz biotite metasediments the deformation of the ore has destroyed its magmatic textures and it has been sheared and recrystallized at amphibolite facies then the deformed ore has been subjected to a hydrothermal event that concentrated the pge with au and as often at the edge of the ni ore the pge are distributed between pgm and in solid solution in ni sulfarsenides and pd also occurs in pentlandite the pgm include sudburyite pdsb sperrylite ptas2 and irarsite irass all six pge and minor au are hosted in gersdorffite niass two generations of gersdorffite have been recognized a higher temperature magmatic euhedral co rich gersdorffite encloses ir pt and rh bearing pgm surrounded by halos of rh ir and os rich gersdorffite a lower temperature ni rich gersdorffite forms anhedral grains and rims on grains of nickeline nias in this low temperature gersdorffite pge are concentrated toward the mineral edges sudburyite and gold occur associated predominantly with nickeline the pge and gold are now predominantly associated with sulfarsenides that are the controlling factor for their distribution

depositional history of the western colombian basin caribbean sea revealed by seismic stratigraphy seismic stratigraphic analysis of multichannel seismic reflection profiles indicates that the sedimentary section in the western colombian basin comprises five principal units the deposition of these units was strongly influenced by the basement relief of an oceanic plateau the five units define three depositional episodes the middle three units defining a single episode the depositional history of the western colombian basin supports plate tectonic models of this area based on the insertion of pacific lithosphere the first depositional episode is interpreted to be an extended period of biogenic pelagic sedimentation before and during the insertion of pacific lithosphere it is characterized by accumulation of a thick 0 5 km layer of pelagic deposits on the crest of the plateau above the carbonate compensation depth these deposits probably correlate with similar rocks of late cretaceous to eocene age bounded by reflection horizons a and b in the venezuelan basin the second depositional episode represents a major change in sedimentation with the influx of a large volume of clastic sediment transported mainly by unconfined turbidity flows this episode is inferred to span eocene through miocene time sediments were derived from sources north and west of the basin resuspension of some of these sediments in nepheloid layers probably led to hemipelagic deposition on topographically elevated areas pervasive internal deformation and physical properties suggest that this hemipelagic layer is undercompacted the beginning of this episode apparently corresponds to initiation of predominantly eastward motion of the caribbean plate relative to the americas the final depositional episode began in the late miocene epoch and extends to the present it is characterized by deposition of turbiditefan complexes including a large fan dominated by leveed channel systems offshore of costa rica development of this fan was in response to uplift and increased volcanism in southern central america caused by the collision between the panama arc and south america

methods for identification of isolated carbonate buildups from seismic reflection data isolated carbonate buildups icbs are commonly attractive exploration targets however identifying icbs based only on seismic data can be difficult for a variety of reasons these include poor quality two dimensional data and a basic similarity between icbs and other features such as volcanoes erosional remnants and tilted fault blocks to address these difficulties and develop reliable methods to identify icbs 234 seismic images were analyzed the images included proven icbs and other features such as folds volcanoes and basement highs which may appear similar to icbs when imaged in seismic data from this analysis 18 identification criteria were derived to distinguish icbs from non icb features these criteria can be grouped into four categories regional constraints analysis of basic seismic geometries analysis of geophysical details and finer scale seismic geometries systematically assessing the criteria is useful because it requires critical evaluation of the evidence present in the available data working from the large scale regional geology to the fine details of seismic response it is also useful to summarize the criteria as a numerical score to facilitate comparison between different examples and different classes of icbs and non icbs our analysis of scores of different classes of features suggests that the criteria do have some discriminatory power but significant challenges remain

correlation between chromite composition and pge mineralization in the critical zone of the western bushveld complex detailed mineralogical investigations of chromite in the lower and critical zones in the northwestern sector of the bushveld complex have revealed significant compositional variations with regard to modal proportions host rock lithology and stratigraphic height superimposed on these variations are long range systematic trends in the composition of chromite in the massive layers these long range trends are closely linked with the evolution of the silicate cumulates the massive chromitite layers are divided into two types type 1 comprises the chromitites hosted entirely within ultramafic cumulates while type 2 chromitites are within cyclic units in which plagioclase cumulates occur the types are also distinguishable by their respective contents of platinum group elements pges and distribution patterns thereof viz the ratios between ru os ir and pt pd rh or relative element proportions both of which display a systematic change with height in accordance with chromite composition the relation between silicate geochemistry chromite composition and pge tenor leads to the development of a model explaining the formation of pge mineralized sulphide poor chromitite layers in the critical zone of the bushveld complex

layering and precious metals mineralization in the rincon del tigre complex eastern bolivia geologic investigations supporting recent technically successful platinum exploration of the rincon del tigre complex a ca 4 6 km thick layered sill have provided significant new understandings of the layering development and precious metals mineralization of one of south america s largest mafic ultramafic intrusions three macrocyclic units are recognized from the base up basal unit unit 1 and unit 2 each most likely the product of a major replenishment with mafic magma of one compositional type the ultramafic rocks of all three macrocyclic units largely comprise repeated minor cyclic units of poikilitic harzburgite granular harzburgite olivine bronzitite each a few meters thick with occasional development of bronzitite at the top mafic rocks absent from unit 1 and present only locally in the basal unit form a well developed norite gabbro magnetite gabbro sequence in the upper half of unit 2 associated with the base of the magnetite gabbro is the precious metals zone a persistent zone of low grade subeconomic sulfide and precious metals mineralization 80 to 185 m thick the precious metals zone comprises an upper cu sulfide rich portion its base the sulfide phase boundary about 23 to 70 m above the magnetite phase boundary and a lower precious metals rich portion the lower part straddling the magnetite phase boundary the individual precious metals are concentrated in separate sulfide poor subzones each up to many tens of meters thick in the stratigraphic order rh pt pd au with peak offsets at the same scale modal layering of silicates and magnetite is well developed in the mafic rocks as is modal and cryptic precious metals layering of the sulfides in the precious metals zone all at three different superimposed scales from about 3 m to several tens of meters a possible interpretive framework for the cryptic layering and perhaps for much of the other layering including the meter scale layering of the ultramafic rocks is provided by the likely compositional stratification of the resident magma initiated by the fluid dynamics of the replenishment and mixing processes and in the magnetite gabbro by repeated convective overturns caused by the onset of magnetite precipitation the characteristics of the precious metals zone fit an orthomagmatic model the composition stratigraphic relations and modal correlation with magnetite of the sulfides all imply that s saturation was induced by loss of feo to magnetite precipitation this took place after prolonged closed system fractionation following the last major magma injection at the base of unit 2 during which crystallization of olivine pyroxene plagioclase and finally magnetite precipitation led to overall s cu and precious metals enrichment and ni and finally fe depletion of the magma the development of the mineralization and its complex layering was controlled by sulfide segregation rayleigh fractionation the different but generally very high partition coefficients dsilicate sulfide for each precious metal and repeated convective overturns at three different superimposed scales the decoupled metals distribution is analogous and similar in origin to that of the main sulfide zone of the great dyke zimbabwe and of the main sulfide layer offset portion of the munni munni complex western australia but the stratigraphic order of metal enrichment peaks is different pt pd au vs pd pt au this suggests a reversal in the relative magnitudes of the partition coefficients dpt and dpd that was possibly brought about by changes in the temperature and o s and fe activities of the magmas between the middle and late stages of crystallization constancy of d values during fractionation of mafic magma should not be assumed the precious metals zone is one of several skaergaard type strata bound precious metals zones associated with magnetite gabbros in the upper levels of many layered intrusions and the origin of the skaergaard mineralized zone itself including the potentially economic platinova reefs can be explained in terms of the precious metals zone model presented here the very low grade great thickness and wide separation of individual metal subzones in the precious metals zone are similar to those of the subeconomic axial facies of the main sulfide zone in the wider parts of the great dyke cf the economic more compact marginal facies on the basis of the lateral and vertical thermal model of the great dyke magma chamber the subeconomic status of the precious metals zone may be in large part a function of the specific fluid dynamics of the rincon del tigre magma chamber

ruthenium in komatiitic chromite abstract the distinction between ru in solid solution and ru bearing inclusions is essential for the predictive modeling of platinum group element pge geochemistry in applications such as the lithogeochemical exploration for magmatic sulfide deposits in komatiites this study investigates the role of chromite in the fractionation of ru in ultramafic melts by analyzing chromite grains from sulfide undersaturated komatiites and a komatiitic basalt from the yilgarn craton in western australia in situ analysis using laser ablation icp ms yields uniform ru concentrations in chromites both within grain and on a sample scale with concentrations between 220 and 540 ppb all other platinum group elements are below the detection limit of the laser ablation icp ms analysis carius tube digestion isotope dilution icp ms analysis of chromite concentrates confirms the accuracy of the in situ method time resolved laser ablation icp ms analyses have identified the presence of sub micron ir bearing inclusions in some chromite grains from the komatiitic basalt however ru bearing inclusions have not been recognized in the analyzed chromites and this combined with the in situ data suggests that ru exists in solid solution in the crystal lattice of chromite these results show that chromite can control the fractionation and concentration of ru in ultramafic systems

regional transect across the western caribbean sea based on integration of geologic seismic reflection gravity and magnetic data we analyze western caribbean structural styles and depositional controls associated with late cretaceous cenozoic deformational events using a 1600 km 994 mi long regional northwest southeast transect extending from the cayman trough in honduras to northern colombia different structural provinces defined along the transect include 1 the cayman trough and adjacent honduran borderlands marking the north american caribbean transtensional plate boundary characterized by late eocene holocene fault controlled depocenters 2 the nicaraguan rise that includes continental paleocene eocene rocks deposited in sag basins which are overlain by relatively undeformed miocene holocene carbonate and clastic shelf deposits of the northern nicaraguan rise following a late cretaceous convergent phase 3 the colombian basin that includes thick miocene clastic depocenters and the localized presence of upper cretaceous rocks overlying the basement and where much of the subsidence is likely isostatic and flexurally driven given its proximity to the subduction zone of northern colombia 4 the south caribbean deformed belt an active accretionary prism produced by the subduction of the caribbean large igneous province beneath the south american plate which has deformed the cenozoic prism and fore arc section and produced thrust fault controlled accommodation space for upper miocene holocene piggyback deposits and 5 the onshore cesar rancheria basin in northern colombia which has recorded the uplift of its bounding mountain ranges the sierra de santa marta massif to the west and perija range to the east plate reconstructions place the various crustal provinces along the transect into the context of the late cretaceous cenozoic deformation events that can be partitioned into strike slip convergent and extensional components

platinum group minerals in the middle group of chromitite layers at marikana western bushveld complex indications for collection mechanisms and postmagmatic modification the platinum group minerals in a drill core taken through the middle group of chromitite layers in the critical zone at marikana in the western bushveld complex were found to consist mainly of laurite as inclusions in chromite grains the platinum group minerals containing pt pd and rh are concentrated in the intercumulus silicates and frequently associated with base metal sulphides up to about 20 of all platinum group minerals in the investigated chromitite layers contain sub stantial amounts of as the base metal sulphides are strongly modified in the postmagmatic stage which led to a significant loss of fe and s in this way concentrating cu ni and the platinum group elements by factors of up to 10 interaction between chromite and base metal sulphides cannot account for all the fe lost in chromite poor samples and the importance of additional processes is indicated inclusions in chromite and orthopyroxene indicate the formation of discrete platinum group minerals and as rich phases before the

iridium ruthenium and rhodium in komatiites evidence for iridium alloy saturation abstract a large body of new data over 400 samples on platinum group element pge concentrations in yilgarn craton komatiites is combined with all available high quality literature data to reveal trends in a spectrum of komatiite lithologies ranging from spinifex textured rocks to pure olivine and olivine chromite adcumulates the data set is restricted to carius tube isotope dilution analyses and high precision nickel sulfide fire assay icp ms analyses and we demonstrate good agreement between the two methods only s poor samples are considered and the data set is filtered to eliminate the effects of addition of cumulus chromite and addition or segregation of magmatic sulfide liquid the residual trends indicate that ir ru and rh are decoupled from one another and from pt and pd rhodium behaves more like pt and pd whereas ru is intermediate between ir and rh rhodium and ru both show indications of olivine control with partition coefficients less than unity the maximum ir whole rock content is a roughly linear positive function of mgo but the minimum is constant sub 1 ppb and independent of mgo ruthenium shows some evidence for limited control by chromite but anomalous ru enrichment is also found in some olivine cumulate samples with no evidence for accumulated chromite or sulfide neither rh nor ir shows evidence for any detectable control by chromite the distribution of ir and to a lesser degree ru and rh cannot be adequately explained by olivine control and a major influence of an ir rich phase is strongly indicated the pattern of ir variance in relation to mgo bears some important similarities to that of cr this similarity arises from the fact that both elements are controlled by a temperature dependent solubility limit the chromite saturation surface in the case of cr and saturation in ir or ir os alloy in the case of ir combining this effect with moderate compatibility in olivine allows us to successfully model the entire range of ir data the upper limit of ir concentration in olivine cumulates in an exact analogy to that for cr is interpreted as the cotectic ratio of olivine to alloy as determined by the average slope of the tangent to the alloy solubility curve sulfide undersaturated mantle melts of a wide range of compositions are likely to be ir alloy saturated experimental predictions of very low ir solubilities in magmas are borne out with the further implication that true thermodynamic sulfide silicate liquid partition coefficients at least for ir and by inference for the other pges are of the order of 107 or even higher

nickel copper sulfides from the 1959 eruption of kilauea volcano hawaii contrasting compositions and phase relations in eruption pumice and kilauea iki lava lake iron nickel copper sulfides in picritic eruption pumice from the 1959 eruption of kilauea volcano hawaii and associated kilauea iki lava lake have been studied in detail the sulfides occur as micrometer sized subspherical blebs in glass inclusions within olivine phenocrysts and in glass within quenched groundmass the proportion of blebs present within the volume of a thin section and within individual olivine grains is widely variable sulfide minerals present in the eruption pumice include monosulfide solid solution mss isocubanite pentlandite bornite and an unidentified metal excess iron rich sulfide bulk sulfides in the eruption pumice are enriched in ni relative to cu these minerals are also present in kilauea iki lava lake along with chalcopyrite but the bulk sulfides are markedly enriched in cu relative to ni in the eruption pumice the high temperature history of the sulfide liquid precursor to the blebs involved crystallization entirely to mss and subsequent exsolution of isocubanite on cooling or crystallization of mss and formation of residual cu rich sulfide liquid in kilauea iki lava lake the high temperature history of the sulfide liquid precursor to the olivine hosted blebs involved crystallization and subsequent unmixing of isocubanite whereas that of the sulfide liquid precursor to groundmass glass hosted blebs involved exsolution of bornite solid solution from cubic chalcopyrite isocubanite and chalcopyrite isocubanite from bornite solid solution the presence of pentlandite chalcopyrite and pentlandite bornite solid solution suggests that the blebs have equilibrated internally below approximately 600 degrees c the ni rich bulk sulfide composition in the pumice reflects the higher temperature and rate of quenching the cu rich bulk sulfide composition in kilauea iki reflects a lower temperature of quenching and more extensive annealing and crystallization history of the lava the wide variability in bulk sulfide composition in the samples and lack of equilibration with coexisting olivine are inconsistent with an origin solely by early magmatic sulfide liquid immiscibility instead bulk sulfide compositions reflect late magmatic processes blebs in the eruption pumice and kilauea iki lava largely represent cooling and quench induced segregation of sulfides with metals scavenged from the silicate liquid

the behaviour of platinum group elements in basalts from the east greenland rifted margin the continental flood basalts of the east greenland volcanic rifted margin were extruded during continental breakup above the ancestral iceland mantle plume at 55 ma three distinct magma types the low ti high ti and very high ti series lts hts and vhts respectively are found intercalated in the 6 km thick plateau lava sequence incompatible trace elements indicate that the lts are derived from a more depleted mantle source compared to hts and vhts the lts is characterised by increasing cu 105 to 248 ppm and pd 7 to 24 ppb constant cu pd ratio 10 000 and decreasing ir 1 1 to 0 05 ppb and ru 1 8 to 0 3 ppb concentrations during magmatic differentiation 16 to 7 wt mgo the constant cu pd ratio reflects silicate and chromite dominated fractionation without concurrent segregation of sulphide s undersaturated differentiation is also indicated in the hts which also displays increasing pd 6 16 ppb and decreasing ir concentrations 1 to 0 05 ppb during differentiation and the cu pd ratios for the entire series average 21 000 however some hts samples have elevated cu pd ratios up to 33 000 cu pd ratios in the hts do not correlate with mgo and this is interpreted to reflect varying cu pd ratios of hts parental magmas rather than s saturated differentiation during s undersaturated differentiation of the lts and hts pt pd ratios decrease from 1 3 to 0 11 and 1 1 to 0 2 respectively which indicates that pd is much more incompatible than pt during s undersaturated differentiation the vhts consists exclusively of highly evolved samples with low mgo 6 6 6 1 wt and pd ir ratios 98 228 here cu pd ratios increase from 17 500 to 35 000 with decreasing cr concentrations which indicate that these magmas experienced silicate fractionation with concurrent segregation of sulphide the lts represent melting of a depleted source and show high pge concentrations and constant cu pd ratios during s undersaturated differentiation melting of a normal depleted upper mantle source generates s saturated melts morb and the depleted lts source is therefore considered an extraordinary s poor component within the ancestral iceland plume of the three series the vhts contain the largest contribution from enriched mantle portions the vhts have similar pge but much higher nb concentrations for instance compared to the most evolved lts and hts samples indicating that the enriched source contributes nb but not pge

some theoretical and field aspects of reinjection in geothermal reservoirs theoretical and field aspects of reinjection into geothermal reservoirs is discussed using examples from various geothermal fields reinjection effects can be observed in terms of pressure maintenance changes in chemical concentrations and temperature decline pressure support due to injection is observed in many liquid dominated fields and in two phase field pressure effects are often felt through mobility changes in most geothermal fields where injection is employed changes in chemical concentrations of the produced fluids have been observed due to the higher salinity of the injected fluids in some cases the returns of the injected fluids observed through chemical changes have resulted in temperature declines at the producing wells in those cases the fluid flow is probably concentrated through a few fractures or faults with insufficient surface area for heat exchange with the host rock

stabilities of laurite rus2 and monosulfide liquid solution at magmatic temperature we have reversed the equilibria ru s2 rus2 and 2feo s2 2fes o2 under controlled temperature oxygen fugacity fo2 and sulfur fugacity fs2 at 1 atm to test if the platinum group mineral laurite rus2 has a stability field at magmatic temperature outside that of magmatic monosulfide liquid the fs2 in equilibrium with ru and rus2 is given by log fs2 6 9 12190 t 1173 to 1673 k whereas the fs2 at sulfide saturation of a basaltic melt may be approximated from log fs2 6 7 12800 t 2 log xfeo log fo2 1473 to 1673 k our experiments support the notion that the sulfide mineral laurite can crystallize directly out of a sulfur bearing sulfide undersaturated silicate melt as implied by laurite chromite intergrowth textures in natural chromite ore

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

escape of unradiogenic osmium during sub aerial lava degassing evidence from fumarolic deposits piton de la fournaise reunion island abstract this study presents new re os isotope and elemental data in gas condensates and corresponding lavas in order to examine the geochemical behavior of these two elements during magma degassing at piton de la fournaise reunion island gas sublimates formed between 2007 and 2011 at temperature ranging from 400 to ca 100 c include na k sulfate aphthitalite na sulfate thenardite ca cu sulfate e g gypsum ca mg al fe fluoride e g ralstonite and native sulfur the high temperature deposits show trace element typical of volcanic gas with high enrichment in re 24 to 79 ppb almost two order of magnitude higher than the corresponding lavas but with os abundances similar to those of the lavas 14 132 ppt in contrast the os contents of the low temperature fluoride deposits 13 77 ppb are higher than any of the other condensates the fluorides are also enriched in re albeit to lesser extent than os 2 9 15 3 ppb based on high temperature samples the fluid melt partition coefficients estimated for re and os are 100 80 and 1 2 respectively considering 1 of fluid loss these partition coefficients translate into emanation coefficients of 0 50 0 17 0 65 for re and 0 01 0 0 03 for os these results indicate that re unlike os is highly volatile at piton de la fournaise osmium isotopic compositions of samples collected at medium and low temperature 187 os 188 os between 0 130 0 135 and plot within the range of the april 2007 lava flow and the historical lavas of piton de la fournaise i e 0 130 0 137 however the highest temperature condensates na k sulfates with t of 384 400 c yield lower 187 os 188 os ratios i e 0 124 0 129 within the field of mantle signal such unradiogenic compositions are best explained if old mantle sulfides occur in lavas and contribute to volcanic gases within the general frame of osmium mantle geochemistry loss of unradiogenic os during magmas degassing could help to explain osmium isotope disequilibrium between lavas and melting residues

platinum group element distribution in the main zone and upper zone of the bushveld complex south africa abstract the platinum group element pge contents of the upper portions of the bushveld complex were investigated with three questions in mind a in natural systems does magnetite concentrate os ir ru and rh ipge as has been observed in experimental systems b is there a au pd enriched layer present such as observed in the upper parts of the skaergaard intrusion c can changes in metal ratios be used for prospecting for pge deposits in the sulfide poor main magnetite layer of the eastern bushveld ir and rh are enriched relative to pt pd and au and this could be because magnetite preferentially concentrated ir and rh over pt pd and au however in most other magnetite layers no enrichment was observed this could be because most magnetite layers contain approximately 1 sulfides and the pge budget is dominated by the sulfides these sulfides obscured the effects of magnetite collecting ipge because sulfides collect all the pge and the partition coefficients for the pge into a sulfide liquid are much greater than the partition coefficient for ipge into magnetite the weighted average of the platinum group elements pge and au over the 2000 m of sampled stratigraphy is au 2 1 ppb pd 1 7 ppb pt 1 7 ppb rh 0 18 ppb ru comparison of the composition of the marginal rocks of the bushveld with a weighted average for the complete 6 km of the bushveld cumulates shows that the cumulate pile is much richer in compatible elements ir rh cr and poorer in incompatible elements sm and hf than the marginal rocks two possible solutions to this are a the magma emplaced into the chamber was a crystal mush and thus more mafic than marginal rocks to the intrusion or b fractionated magma has been removed from the cumulate sequence and either irrupted or intruded the country rocks as granites and granophyres

late cretaceous cenozoic sequence stratigraphic and paleogeographic controls on petroleum system elements of the nicaraguan platform western caribbean sea late cretaceous to present day mixed carbonate clastic deposition along the nicaraguan platform western caribbean sea has evolved from a tectonically controlled rifted upper eocene shallow to deep marine carbonate siliciclastic shelf to an upper miocene to present day tectonically stable shallow marine carbonate platform and passive margin by integrating subsurface data of 287 two dimensional seismic lines and 27 wells we interpret the cenozoic stratigraphic sequence as 3 cycles of transgression and regression beginning with an upper eocene rhodolitic algal carbonate shelf that interfingered with marginal siliciclastic sediments derived from exposed areas of central america bordering the margin to the west during the middle eocene a carbonate platform was established with both rimmed reefs and isolated patch reefs a late eocene forced regression produced widespread erosion and subaerial exposure across much of the platform and was recorded by a regional unconformity the oligocene upper miocene sedimentary record includes a southeastward prograding delta of the proto coco river which drained the emergent area of what is now northern nicaragua the late miocene to present day period marks a period of strong subsidence with the development of small pinnacle reefs we describe favorable petroleum system elements of the nicaraguan platform that include 1 eocene fossiliferous limestone source rocks documented as thermally mature in vintage exploration wells and seen as active gas chimneys emanating from inferred carbonate reservoirs 2 upper to middle eocene reservoirs in patch and pinnacle reefs middle eocene calcareous slumps and oligocene fluvial deltaic facies documented in wells and 3 regional seal intervals that consist of both regional unconformities and eocene oligocene intraformational shale

western caribbean intraplate deformation defining a continuous and active microplate boundary along the san andres rift and hess escarpment fault zone colombian caribbean sea the san andres rift sar located on the lower nicaraguan rise is a previously poorly studied active 015 trending bathymetric and structural rift basin that is 11 27 km 7 17 mi wide and extends for 346 km 215 mi across the western flank of the caribbean plate in this study we integrate bathymetric maps potential field data and high resolution two dimensional 2 d seismic lines to understand the crustal structure tectonic history and tectonic origin of the sar which is one of the active areas within the otherwise stable caribbean plate we compiled regional gravity and magnetic data that revealed a negative gravity anomaly and positive magnetic anomaly that we interpret as a result of crustal thinning and an elevated moho along the main rift axis of the sar forward models of gravity data show four possible interpretations for the origin of the crust underlying and surrounding the sar interpretations of 2 d seismic reflection data show structural features within the upper crust and sedimentary sections typical of other active rift systems including a sar parallel north south alignment of earthquakes with the larger events showing normal and strike slip focal mechanisms sequential kinematic restorations based on 2 d seismic profiles reveal three major phases of sar opening 1 the initial early eocene rifting stage 2 middle eocene extension and 3 a rapid middle miocene to early pliocene extension accompanied by emergence of the san andres island as a rift shoulder we propose slab rollback and intraplate extension as main tectonic mechanisms to explain all rift phases and neogene volcanism found in the western caribbean region

trace element geochemistry of volcanic gases and particles from 1983 1984 eruptive episodes of kilauea volcano compositional data have been obtained for volcanic gases and particles collected from fume emitted at the pu u o o vent on the east rift zone of kilauea volcano the samples were collected by pumping fume through a filter pack system consisting of a front stage particulate filter followed by four base treated filters 7lioh particles and condensed phases are trapped on the particulate filter and acidic gases are collected on the treated filters the filters are analyzed for 30 elements by instrumental neutron activation analysis fume samples were collected from the pu u o o vent for two eruptive episodes 1 7 days after episode 11 cooling vent samples and 2 the waning stage of episode 13 active vent samples additional samples were collected by aircraft from the gas plume released during the lava fountaining phase of episode 17 aircraft samples element concentrations in the vent gases were 104 g m 3 for s cl and f enrichment factors efs for the volcanic fume versus the source magma were calculated using the volatile element br as the reference element for normalization and the u s geological survey standard bhvo 1 as the magma standard this removes the ash dilution effect obtained by using an ash constituent al sc or mg as the reference element bromine normalized efs 105 range from 101 to 102 for na k and cu 102 to 105 for zn w sb in ir ag f and as and 105 for au cd re cl se and s the highest enrichment factors are for aircraft samples collected during the most gas rich phase of an eruption cycle metal and volatile element data form two groups 1 elements showing little or no variation in abundance ratios with sample type group 1 cl br and re and 2 elements that show significant variation in abundance ratios by sample type group 2 zn w sb in ir au and cd bivariate plots of elements of the first group versus elements of the second group separate by sample type the separation corresponds to samples collected during eruptive activity versus samples collected during repose periods monitoring trace metal ratios in volcanic fume could provide an additional tool for predicting volcanic eruptions the f cl ratio of cooling vent samples is higher than those of active vent or aircraft samples and the ratio is inversely correlated with efs for most volatile metals

late cenozoic alkaline volcanism in the northwestern caribbean tectonic setting and sr isotopic characteristics a province of alkaline volcanism has developed over the last 10 m y in the northwestern part of the caribbean plate most of the volcanism is quaternary in age and follows an apparent halving of the spreading rate at the cayman rise spreading center 2 4 m y ago intraplate deformation in central america and the nicaraguan rise has produced a series of north south orientated grabens this extensional tectonism is associated temporally and spatially with some of the alkaline magmatism strontium isotopic ratios of rocks from sixteen of these centers of volcanism enable three separate areas with different isotopic characteristics to be identified the largest area corresponds to the nicaraguan rise and is characterized by low87sr 86sr ratios 0 7026 0 7031 a more concentrated area of alkaline magmatism in northeastern costa rica has intermediate87sr 86sr ratios 0 7036 0 7038 which are within the range shown by the adjacent calc alkaline volcanoes in central hispaniola high87sr 86sr ratios 0 7047 0 7063 are found in strongly alkalic rocks and in rocks that are transitional to calc alkaline in nature in both costa rica and hispaniola the increased radiogenic strontium may have come from volatile rich fluids escaping from adjacent subducting slabs of oceanic crust the isotopic differences between the two areas may be related to the relative longevity and high rate of subduction in costa rica compared to hispaniola the costa rican alkaline rocks overlie a segment of the cocos plate which is being subducted at a smaller angle 35 than at the rest of the central american arc

native gold in a hawaiian alkalic magma native gold found in fresh basanite glass from the early submarine phase of kilauea volcano hawaii may be the first documented case of the transport of gold as a distinct precious metal phase in a mantle derived magma the gold bearing glass is a grain in bedded volcanic glass sandstone japan marine science and technology center jamstec sample s508 r3 collected by the submersible shinkai 6500 at 3879 m depth off kilauea south flank extensive outcrops there expose debris flow breccias and sandstones containing submarine erupted alkalic rock fragments and glasses from early kilauea precipitation of an immiscible gold liquid resulted from resorption of magmatic sulfides during crystallization differentiation with consequent liberation of sulfide hosted gold elevated whole rock gold concentrations to 36 ppb for fresh lavas and clasts from early kilauea further show that some magmas erupted at the beginning stages of hawaiian shield volcanoes were distinctly gold rich most likely owing to limited residual sulfide in their mantle source alkalic magmas at other ocean islands may also be gold rich and oceanic hot spot provinces may contain underappreciated gold resources

character of the ug2 chromitite and host rocks and petrogenesis of its pegmatoidal footwall northeastern bushveld complex the ug2 chromitite and its immediate footwall have been mapped at the centimeter scale in a characteristic section exposed in a wall in the middelpunt mine northeast bushveld complex in this region the ug2 chromitite and its coarse grained footwall rocks are underlain and overlain by plagioclase pyroxenite the ug2 chromitite itself is a massive 70 cm thick layer composed of 75 to 90 percent chromite with plagioclase as the dominant interstitial mineral immediately beneath the chromitite is the so called mixed layer a previously undescribed but distinct mappable layer 50 to 70 cm thick consisting of chromite bearing pegmatoidal pyroxenite and chromitite intimately mixed at scales of centimeters to decimeters the mixture is chaotic with no megascopic fabric and contains in bulk about 15 percent chromite the contact between the mixed layer and overlying chromitite is well defined but sinuous at scales of centimeters to decimeters based on observations of drill core and outcrop the mixed layer appears to exist throughout the central sector of the eastern bushveld and may be ubiquitous throughout the entire complex the mixed layer usually rests directly on the ug2 pegmatoid which is a well known and widespread pegmatoidal plagioclase pyroxenite the pegmatoid is much coarser grained than the underlying pyroxenite but the two lithologic units are similar in terms of mineral modes and compositions the contact between them is irregular at scales of decimeters to meters with vertical fingers of pegmatoid extending into the pyroxenite the mixed layer and ug2 pegmatoid contain rare lenses of massive anorthosite encased by continuous centimeter thick rims of chromitite a model is proposed to account for the footwall sequence of the ug2 chromitite in this model the ug2 pegmatoid formed when hydrous interstitial melt percolated up through the partially molten crystal pile accumulated beneath the preexisting compacted massive chromitite and metasomatized and partially melted the pyroxenite protolith which upon cooling recrystallized to a coarse grained rock on the basis of published experiments illustrating the effect of water on melting in mafic systems it is shown how the hydrous melt was able to penetrate the pyroxene bearing assemblages below the chromitite but unable to partially melt the chromitite itself which in consequence remained relatively impermeable the anorthosite lenses in the pegmatoid mark local areas of relatively high degrees of hydration partial melting with the anorthosite representing the crystalline residue the mixed layer formed by gradual collapse of the base of the ug2 chromitite into the less dense relatively fluid footwall the mixed layer and ug2 pegmatoid are diagnostic of the ug2 chromitite and the three lithologic units should be considered a single petrologic unit