precious metals in magmas of east greenland factors important to the mineralization in the skaergaard intrusion a survey of precious metals ni and cu levels and their mutual relationships in several magma suites of the east greenland volcanic passive margin shows that the magma type which was parental to the skaergaard intrusion was markedly enriched in pt pd and especially au and relatively depleted in cu compared to the preceding voluminous plateau basalt volcanism it is likely that elevated levels of platinum group minerals pgm and au were critical to the generation of ore grade precious metals in the intrusion the discovery of a major pcm and au deposit within the intrusion combined with these results suggests a set of criteria which might be critical to the formation of skaergaard type pcm deposits elsewhere in the province and worldwide the skaergaard mineralization appears to be a hitherto undescribed class of pgm and au mineralizations

fluid inclusions in salton sea scientific drilling project core preliminary results fluid inclusions 191 in calcite quartz k feldspar and epidote from l mm veinlets in cores and well cuttings from 604 2560 m homogenize from 217 to 500 c and vary widely in salinity suggesting a complex history of fluids surrounding these samples no daughter minerals were seen and no clathrates were recognized on freezing vapor rich inclusions under pressure presumably containing co2 and or ch4 were found from a wide range of depths suggesting that effervescence has occurred low salinity fluids 1 2 4 0 wt nacl eq were present as deep as 1939 m the data can be explained by a combination of processes such as thermal metamorphism of evaporites and other sediments and mixing of water from metamorphic dehydration reactions with partly evaporated colorado river water

the fluid dynamics of a basaltic magma chamber replenished by influx of hot dense ultrabasic magma this paper describes a fluid dynamical investigation of the influx of hot dense ultrabasic magma into a reservoir containing lighter fractionated basaltic magma this situation is compared with that which develops when hot salty water is introduced under cold fresh water theoretical and empirical models for salt water systems are adapted to develop a model for magmatic systems a feature of the model is that the ultrabasic melt does not immediately mix with the basalt but spreads out over the floor of the chamber forming an independent layer a non turbulent interface forms between this layer and the overlying magma layer across which heat and mass are transferred by the process of molecular diffusion both layers convect vigorously as heat is transferred to the upper layer at a rate which greatly exceeds the heat lost to the surrounding country rock the convection continues until the two layers have almost the same temperature the compositions of the layers remain distinct due to the low diffusivity of mass compared to heat the temperatures of the layers as functions of time and their cooling rate depend on their viscosities their thermal properties the density difference between the layers and their thicknesses for a layer of ultrabasic melt 18 mgo a few tens of metres thick at the base of a basaltic 10 mgo magma chamber a few kilometres thick the temperature of the layers will become nearly identical over a period of between a few months and a few years during this time the turbulent convective velocities in the ultrabasic layer are far larger than the settling velocity of olivines which crystallise within the layer during cooling olivines only settle after the two layers have nearly reached thermal equilibrium at this stage residual basaltic melt segregates as the olivines sediment in the lower layer depending on its density the released basalt can either mix convectively with the overlying basalt layer or can continue as a separate layer the model provides an explanation for large scale cyclic layering in basic and ultrabasic intrusions the model also suggests reasons for the restriction of erupted basaltic liquids to compositions with mgo 10 and the formation of some quench textures in layered igneous rocks

selenium in volcanic environments a review abstract selenium is a chemical element with major environmental implications due to the extremely narrow range between essential and toxic concentrations volcanoes emit significant amounts of se which has an enormous impact on the se cycle and potentially human and animal health this paper presents a broad overview of the current state of knowledge of major aspects of volcanic derived se focussing on the processes in soils and aquifers volcanic soils formed when volcanic products weather rapidly display peculiar characteristics selenium contents can be relatively high 1 2 orders of magnitude higher compared to the worldwide average 0 4 mg kg but often with low mobility the soils can be impacted by local deposition of volcanic emissions which are enriched in se due to volatilization in the magma chamber it seems that the chemical fate of se in volcanic soils is controlled by sorption processes on poorly crystalline phases which presence depends on the parental materials and weathering these sorption processes also determine leaching to groundwaters some volcanic aquifers have se concentrations above the who guideline of 10 g l this might also be related to geothermal activity in spite of the well documented interest on se during the last decades a significant number of uncertainties exist on the behavior of this fascinating element a preliminary model of the se cycle around volcanoes can be constructed but the well known analytical problems associated with se determinations have produced a lack of reliable information on some processes involved in the chemical interaction between soils and water this review therefore also aims to identify the processes where new efforts are needed to produce good quality data to be compared and integrated in global models

magmatic fractionation by compositional convection in a sheet like magma body constraints from the nosappumisaki intrusion northern japan the mechanism of fractionation in a sheet like magma body was investigated based on observations from the nosappumisaki intrusion northern japan this is one of a number of late cretaceous shoshonite sills in the nemuro peninsula and has a compound structure comprising a bottom cumulate zone containing 50 vol cumulus crystals and a overlying middle zone free of such cumulus crystals these zones are sandwiched between porphyritic rocks of upper and lower chilled and marginal zones it is inferred that massive crystal settling or flotation from a magma initially laden with 20 vol of crystals primocrysts in a sheet like magma body formed a cumulus pile and an overlying crystal free melt layer after the formation of the compound structure with partial freezing of the upper and lower marginal zones the internal part of the intrusion slowly solidified during this period the melt in the central part of the intrusive sheet underwent fractional crystallization this is shown by patterns of vertical variation in the composition of the interstitial melt the overall patterns are inverted mirrorimage s shaped and s shaped for incompatible and compatible elements respectively the incompatible element abundances show maxima near the bottom of the middle zone which corresponds to the final solidification horizon they show minima in the cumulate zone which suggests discharge of an evolved melt from the crystal pile fractionation is inferred to have taken place via compositional convection without settling of crystals grown after the intrusion event or collapse of the upper boundary layer the melt in the central melt layer descended into the permeable crystal pile counterbalancing discharge of buoyant evolved melt formed by crystallization of the pore melt in the crystal pile this conclusion is based on the following observations 1 there is no accumulation of crystals grown after the intrusion of the magma above the cumulate zone 2 leucocratic pipe like structures which represent relict pathways for the evolved melt are developed in the cumulate zone these structures increase in diameter decrease in number density and increase in abundance of incompatible elements with height 3 cumulus augite in the cumulate zone has ca rich and al poor dissolved rims the extent of dissolution increases with stratigraphic height suggesting downwelling of an h2o rich melt from the overlying melt layer 4 there is no evidence for extensive compaction accompanying pore melt crystallization in the main part of the cumulate zone 5 the initial melt composition estimated from the groundmass of the chilled margin rocks and the average composition of the material interstitial to the primocrysts for the whole sill are statistically identical a mass balance model for compositional convection was constructed to quantify the melt transportation processes model parameters were optimized by fitting the observed vertical variation of the interstitial melt compositions the optimized results suggest that the evolved melts discharged from the crystal pile and rose through the central melt layer as plumes with minor mingling with the surrounding melt the initially formed compound structure is a highly preferential environment for compositional convection

geological geophysical and thermal characteristics of the salton sea geothermal field california abstract the salton sea geothermal field is the largest water dominated geothermal field in the salton trough in southern california within the trough local zones of extension among active right stepping right lateral strike slip faults allow mantle derived magmas to intrude the sedimentary sequence the intrusions serve as heat sources to drive hydrothermal systems we can characterize the field in detail because we have an extensive geological and geophysical data base the sediments are relatively undeformed and can be divided into three categories as a function of depth 1 low permeability cap rock 2 upper reservoir rocks consisting of sandstones siltstones and shales that were subject to minor alterations and 3 lower reservoir rocks that were extensively altered because of the alteration inter granular porosity and permeability are reduced with depth field permeability is enhanced by renewable fractures i e fractures that can be reactivated by faulting or natural hydraulic fracturing subsequent to being sealed by mineral deposition in the central portion of the field temperature gradients are high near the surface and lower below 700 m surface gradients in this elliptically shaped region are fairly constant and define a thermal cap which does not necessarily correspond to the lithologic cap at the margin of the field a narrow transition region with a low near surface gradient and an increasing gradient at greater depths separates the high temperature resource from areas of normal regional gradient geophysical and geochemical evidence suggest that vertical convective motion in the reservoir beneath the thermal cap is confined to small units and small scale convection is superimposed on large scale lateral flow of pore fluid interpretation of magnetic resistivity and gravity anomalies help to establish the relationship between the inferred heat source the hydrothermal system and the observed alteration patterns a simple hydrothermal model is supported by interpreting the combined geological geophysical and thermal data in the model heat is transferred from an area of intrusion by lateral spreading of hot water in a reservoir beneath an impermeable cap rock

postcumulus processes in layered intrusions during the postcumulus stage of solidification in layered intrusions fluid dynamic phenomena play an important role in developing the textural and chemical characteristics of the cumulate rocks one mechanism of adcumulus growth involves crystallization at the top of the cumulate pile where crystals are in direct contact with the magma reservoir convection in the chamber can enable adcumulus growth to occur to form a completely solid contact between cumulate and magma another important process may involve compositional convection in which light differentiated melt released by intercumulus crystallization is continually replaced by denser melt from the overlying magma reservoir this process favours adcumulus growth and can allow adcumulus growth within the pore space of the cumulate pile calculations indicate that this process could reduce residual porosities to a few percent in large layered intrusions but could not form pure monomineralic rocks intercumulus melt may also be replaced by more primitive melt during episodes of magma chamber replenishment dense magma emplaced over a cumulate pile containing lower density differentiated melt may sink several metres into the underlying pile in the form of fingers reactions between melt and matrix may lead to changes in mineral compositions mineral textures and whole rock isotope compositions another important mechanism for forming adcumulate rocks is compaction in which the imbalance of the hydrostatic and lithostatic pressures in the cumulate pile causes the crystalline matrix to deform and intercumulus melt to be expelled for cumulate layers from 10 to 1000 metres in thickness compaction can reduce porosities to very low values 1 and form monomineralic rocks the characteristic time scale for such compaction is theoretically short compared to the time required to solidify a large layered intrusion during compaction changes of mineral compositions and texture may occur as moving melts interact with the surrounding matrix both compaction and compositional convection can be interrupted by solidification in the pore spaces compositional convection will only occur if the rayleigh number is larger than 40 if the residual melt becomes lower in density and the convective velocity exceeds the solidification velocity measured by the rate of crystal accumulation in the chamber orthocumulates are thus more likely to form in rapidly cooled intrusions where residual melt is frozen into the pore spaces before it can be expelled by compaction or replaced by convection

thermal regime of the state 2 14 well salton sea scientific drilling project temperature logs were made repeatedly during breaks in drilling and both during and after flow tests in the salton sea scientific drilling project well state 2 14 the purpose of these logs was to assist in identifying zones of fluid loss or grain and to characterize reservoir temperatures at the conclusion of the active phase of the project a series of logs was begun in an attempt to establish the equilibrium temperature profile initially we were able to log to depths below 3 km but beginning in late may of 1986 it was impossible to log below about 1 8 km owing to casing failure our best estimates of formation temperature below 1 8 km are 305 5 c at 1890 m and 355 10 c at 3170 m for the upper 1 8 km the latest temperature log october 24 1986 using a digital slickline heat shielded downhole recording device was within a few degrees celsius of equilibrium as confirmed by a more recent log july 31 1987 to a depth of 1 km as in most other wells in the salton sea geothermal field there is an impermeable thermally conductive cap on the hydrothermal system this cap extends to a depth of more than 900 m at the state 2 14 well thermal conductivities of 19 samples of drill cuttings from this interval were measured at room temperature the conductivity values were corrected for in situ porosity as determined from geophysical logs and for the effects of elevated temperature thermal gradients decrease from about 250 mk m 1 same as degrees celsius per kilometer in the upper few hundred meters to just below 200 mk m 1 near the base of the conductive cap using one interpretation thermal conductivities increase with depth mainly because of decreasing porosity resulting in component heat flows that agree reasonably well with the mean of about 450 m w m 2 this value agrees well with heat flow data from shallow wells within the salton sea geothermal field a second interpretation in which measured temperature coefficients of quartz and carbonate rich rocks are used to correct thermal conductivity results in lower mean conductivities that are roughly constant with depth and consequently systematically decreasing heat flux averaging about 350 mw m 2 below 300 m this interpretation is consistent with the inference from fluid inclusion studies that the rocks in this part of the field were once several tens of degrees celsius hotter than they are now the age of this possible disturbance is estimated at a few thousand years

the heating duration and provenance age of rocks in the salton sea geothermal field southern california abstract 40ar 39ar step heating of single k feldspar crystals 500 g in a furnace with a low argon blank and accurate temperature control provide constraints on the thermal history of the salton sea geothermal field southern california estimates of 40ar loss from detrital k feldspars coupled with kinetic information gleaned from laboratory degassing of 39ar allow possible thermal histories to be evaluated samples from core and cuttings of sandstones from the state 2 14 landers 1 dearborn farms 1 magmamax 2 elmore 3 and sinclair 3 wells have been analyzed for this purpose no recent 40ar loss is recognized in any of the 17 crystals measured even for samples currently at temperatures greater than 275 c this observation suggests that the present temperature distributions in these wells have not been maintained for greater than 1000 years however aggregates 100 mg of two k feldspar samples do reveal recent 40ar loss from the state 2 14 well suggesting a time scale of 1000 to 5000 years for near peak thermal conditions these estimates of heating duration are broadly consistent with earlier results from thermal modelling but do not support interpretations that paleotemperatures were once hotter than present provenance ages span the tertiary through middle proterozoic but cluster at ages of 25 70 and 1250 ma these age peaks are very common k feldspar k ar ages in detachment terranes of southeastern california and southwestern arizona both drained by the colorado river this coincidence supports earlier suggestions that the sands of the salton trough were derived mainly from the colorado river unrealistically low apparent activation energies

saline brines and metallogenesis in a modern sediment filled rift the salton sea geothermal system california u s a the salton sea geothermal system ssgs is the site of active hydrothermal metamorphism and metallogenesis in the delta of the colorado river which partially fills the salton trough rift zone at the head of the gulf of california growth of the delta across the rift has isolated the northern part of the salton trough since the pleistocene forming the evaporative salton sea basin whose sediments host the ssgs more than 70 commercial geothermal wells including a 3 2 km deep research borehole drilled as part of the salton sea scientific drilling project sssdp are yielding a wealth of new data from this system within the ssgs active greenschist facies metamorphism is occuring at temperatures 365 c at only 2 3 km depth by reaction of nacakfemncl brines with the deltaic and lacustrine pilo pleistocene sediments two kinds of base metal ore mineralization occur at depth stratabound diagenetic fe sulfides and epigenetic vertical veins containing fezncupb sulfides and fex oxides the vein mineralization occurs as two types an older reduced carbonate sulfide assemblage and a modern oxidized silicate hematite sulfide assemblage the earlier assemblage formed at temperatures up to 100 c higher than the ambient temperatures measured in the wells today implying that cooler oxidized fluids have displaced hot reduced fluids a sharp interface between shallow fluids containing 12 wt tds and deep hypersaline brines containing 15 27 wt tds exists in the ssgs the deeper hypersaline brines are rich in base metals fe 1500 ppm mn 1000 ppm zn 500 ppm whereas the overlying lower salinity fluids contain less than 100 ppm each of fe mn and zn the modern silicate hematite sulfide vein assemblage is precipitating where the two fluids appear to be mixing the two fluid types also have distincty different 18o and d relations the shallow lower salinity fluids have only partially exchanged oxygen with deltaic and lacustrine host rocls the deep hypersaline brines have exchanged oxygen extensively at 250 c with the deltaic sediments d values of the hypersaline brines are typically lighter than either the lower salinity fluids or the modern evaporating groundwaters occupying the salton trough implying a different source for the original fluids the high salinities of the geothermal brines are derived from a combination of evaporation of fossil lakewaters groundwater dissolution of shallow lacustrine evaporites and subsurface hydrothermal metamorphism of buried lacustrine evaporites episodic filling and desiccation of the closed salton sea basin has allowed cold saline brines to form and percolate down into the sedimentary section in some wells anyhydrite meta evaporites and interbedded solution collapse breccias occur at 1 km depth the anyhydrite contains fluid inclusions that are saturated in halite at their homogenization temperatures of 300 c recording the hydrothermal dissolution of bedded salt based on sr and pb isotopic data and whole rock chemical data the source of the metals in the hypersaline brines is from leaching of the host sediments however the origin of reduced s for ore mineralization remains an enigma 34s values for vein sulfides cluster uniformly around zero per mil implying that an isotopically homogeneous source of reduced sulfide exists in the brines it is possible that the vein sulfides receive a constribution from a magmatic s source movement and mixing of brines of different chemistry and oxidation states play a major role in ore genesis our model envisages an early stage in which a deep brine pool accumulated at depth in the sedimentary section by partial section by partial evaporation of basin and dissolution of bedded salts intrusion of rift related basaltic magma into the base of the sedimentary caused heatingand fracturing of the sediments resulting in precipitation of the carbonate sulfide veins during pore fluid expulsion heating also caused a diapir of the hypersaline brine to rise and displace colder less saline shallower pore fluids this brine intrusion was accompanied by pervasive and extensive mineralization as this diapir cooled it began to move downward drawing in shallow more oxidized fluids and causing the formation of the modern silicate hematite sulfide vein ore zone

borehole gravity measurements in the salton sea scientific drilling project well state 2 14 borehole gravity measurements over a depth range from 1737 to 1027 m and the vertical gradient of gravity above ground were measured at the salton sea scientific drilling program well state 2 14 uncorrected borehole gravimetric densities match values from gamma gamma logs indicating that the high densities seen in state 2 14 in the depth range 0 5 to 3 km extend for a few kilometers from the well the above ground gradient was found to be 4 1 micro gal m higher than expected correcting for this value increases the gravimetric density in the borehole combining the borehole gravity and estimated vertical gravity gradients on the surface they find that this densified zone coincides with much of a broad thermal anomaly that has been found to the northeast of the salton sea geothermal field

some effects of viscosity on the dynamics of replenished magma chambers some aspects of the dynamical behavior of magma chambers replenished from below with hotter but denser magma have been modeled in a series of laboratory experiments in previously reported work the fluids used were aqueous solutions of comparable viscosity and thus the results should be applicable to basaltic magma chambers in which the magmas do not vary greatly in viscosity in that case the lower layer cools by convective heat transfer to the fluid above and crystallization causes the density of the residual liquid in the lower layer to decrease when the density becomes equal to that in the upper layer sudden overturning and intimate mixing take place the present paper reports experimental results that allow us to extend the application to systems in which there is a large viscosity ratio between the resident and the injected fluid for example to calcalkaline magmas where magma viscosity can vary by as much as 5 orders of magnitude the largest viscosity ratio in our experiments about 3000 was achieved using cold glycerine for the upper layer above a hot denser kno3 solution the most striking new feature with the very viscous upper layer is that now less dense fluid is released immediately and continuously from the interface and rises as plumes through the upper layer further crystallization occurs in the plumes and the crystals fall out but there is little mixing and a layer of depleted kno3 solution is eventually deposited at the top the transfer process between the layers is dominated by interfacial effects with the high viscosity upper layer acting as a nearly rigid lid that allows buoyant fluid to accumulate just below the interface and then rise in localized plumes across the interface into the viscous layer this physical picture is supported by a series of experiments in which the viscosity ratio is varied systematically the mixing behavior changes gradually between that described above for a large viscosity ratio and the sudden over turning characteristic of layers with comparable viscosity the importance of the viscosity ratio rather than just an increase in viscosity is confirmed by experiments in which both viscosities are increased by the same factor the overturning process is then slower but symmetrical other variations suggested by previous experiments are also described the release of gas by a chemical reaction to model the release of volatiles following an overturning event in a magma chamber the effect of a cold immiscible layer above the cooling crystallizing fluid the influence of two viscous layers with a density step between them and the constraining effects of a density with corresponding viscosity gradient in the upper region the experiments indicate that whatever the stratification whether it be in layers or continuous the form of the initial motion in the upper fluid is determined by the viscosity ratio between the two fluids immediately adjacent to the interface geological applications are not examined in detail in this paper but the experiments suggest that both sudden overturning characteristic of magmas of nearly equal viscosity and continuous release when the upper layer is much more viscous are viable mechanisms for magma mixing in the appropriate circumstances

a laboratory model of a replenished magma chamber abstract it has recently been suggested that periodic influxes of hot but heavy magma into the base of a basaltic magma chamber can remain isolated from the rest of the chamber while the new magma cools and crystallization proceeds when thermal equilibrium is almost complete the suspended crystals settle out and the residual less dense liquid can then mix with the fluid above in the present paper the basic fluid dynamical processes underlying this model have been investigated in laboratory experiments using aqueous solutions the lower layer was hot kno3 solution for which saturated solutions become less dense as the temperature decreases with a cold deeper layer of less dense nano3 or k2co3 above the lower layer there was strong convective transfer of heat through a sharp interface separating the layers at a rate which is predicted here drawing on previous studies carried out with oceanographic applications in mind once crystallization began non equilibrium effects became important and the observed temperatures differ somewhat from those predicted in the experiments crystals grew mainly from the bottom rather than while in suspension but this is not an essential aspect of the model the important fact is that the density of the residual liquid in the lower layer decreased until it became equal to that of the upper layer and then the interface broke down so that the two layers mixed thoroughly together leaving a layer of kno3 crystals at the base no crystallization at all occurred when the hot input liquid was forced to mix initially with the cold solution already in the chamber

floating lids on large mid ocean spreading center magma chambers a discussion rosencrantz and nelson 1982 have concisely presented arguments in favor of the hypothesis that lids overlying ocean ridge magma chambers are supported by flotation of the lid on magma of equal or higher density i concur that given the highly fractured nature of oceanic crust the lids to oceanic magma chambers can only be supported by flotation regardless of the width of the magma chamber in their quantitative analysis rosencrantz and nelson implicitly made two assumptions on the shape of the magma chamber and therefore on the nature and density of the lid and on the nature and therefore the density of the contents of the magma chamber it is important to realize that in a situation in which either one of these assumptions is thought not to hold the acceptance of the flotation hypothesis places severe constraints on other properties of the situation the problem is raised in the light of the archean ocean ridge model of sleep and windley 1982 fig 3 in the same issue of journal of geology but is of general relevance given our ignorance concerning magma chambers there have been several suggestions for the shape of ocean ridge magma chambers in situations where permanent magma chambers are thought to exist sleep 1975 rosendahl 1976 and dewey and kidd 1977 envisaged a magma chamber narrowing upwards with a triangular cross section and a relatively flat base hereafter this shape is referred to as an inverted funnel from their work on the samail ophiolite in oman pallis

rhythmic layering formed by deposition of plagioclase phenocrysts from influxes of porphyritic magma in the cuillin centre isle of skye the outer bytownite gabbros of the cuillin centre on the isle of skye include a 175 m rhythmically layered sequence consisting of alternating layers of coarse grained massive gabbro and finer grained laminated gabbro we infer that the massive layers formed from repeated influxes of plagioclase phyric magmas which flowed into an existing magma chamber and deposited their entrained phenocrysts on the chamber floor the phenocrysts accumulated rapidly trapping large amounts of pore liquid in the cumulus pile during the intervals between influxes laminated gabbro layers formed by crystallization of the resident magma these layers accumulated more slowly by crystal settling or in situ growth allowing sufficient time for their pore liquids to be expelled by compaction or adcumulus crystallization the resulting fractions of trapped pore liquid were510 in laminated gabbros and 20 40 in massive gabbros high porosity in the massive layers is attributable to rapid accumulation of a lowdensity cumulus assemblage with trapping of pore liquids facilitated by the confining effects of impermeable laminated cumulates above and below among layers whole rock sr and nd isotope ratios reveal variable degrees of contamination by the archean crust beneath skye in the lower part of the section influxes of plagioclase phyric magma are associated with a shift to more primitive mineral compositions lower sr sr and higher nd nd indicating that the replenishing magmas were initially more primitive and less contaminated than those already residing in the chamber in accordance with this interpretation massive layers have lower sr sr than laminated layers consistent with derivation of the plagioclase phenocrysts from a less contaminated source in the upper part of the section the shift toward more primitive and less contaminated compositions is reversed suggesting that over time the replenishing magmas underwent increasing degrees of fractionation and contamination within the magmatic plumbing system that fed the intrusion these observations and inferences coupled with published evidence for other isotopic reversals support a model for growth of the cuillin centre by frequent replenishment of a small volume magma chamber with pulses of magma that underwent complex histories of fractionation and contamination during their transit through the archean crust periods dominated by assimilation and fractional crystallization within the feeder system alternated with periods of vigorous recharge by relatively primitive and less contaminated magmas these cycles were superimposed on an overall trend toward less contamination through time consistent with increasing dominance of the system by uncontaminated tholeiitic magmas as the hebridean magmatic province matured

the formation of low volume high tenor magmatic pge au sulfide mineralization in closed systems evidence from precious and base metal geochemistry of the platinova reef skaergaard intrusion east greenland the majority of magmatic platinum group element pge deposits in layered mafic ultramafic intrusions are characterized by visible cu ni sulfide mineralization often located in the lower parts of a magmatic stratigraphy that show evidence of multiple magma injections and crustal contamination the platinova reef hosted by the 55 ma skaergaard intrusion east greenland is an example of a rarer type of pge deposit typified by a pd au cu dominant ni pt poor sulfide assemblage present in the upper parts of the host intrusion such deposits are generally considered to form through prolonged fractional crystallization of the magma with magnetite crystallization playing a role in late stage s saturation in the case of the skargaard intrusion the crystallization of the cumulate rocks and the generation of the mineralization hosted within them occurred within a closed system from a single homogenized batch of magma the platinova reef is characterized by offsets in the stratigraphic position of individual metal peaks an anomalously low sulfide content 1 ppm the intermediate zone pd 0 1 1 ppm the au zone au 1 ppm and the upper cu zone cu 200 ppm with precious metals 3 to 10 4 ppm low cu pd ratios and a remarkable upward decrease in pd au ratio from 40 to 1 whereas the sulfides in the cu zone are lower tenor 10 3 ppm pd higher volume and have high cu pd ratios and pd au to explain these features we present a multistage model of preconcentration dissolution upgrading and further metal enrichment this involves initial s saturation of the skaergaard magma at a late stage in the crystallization history the sulfide liquid produced scavenged metals from the entire volume of the chamber as it sank but was then dissolved as it came into contact with hotter fe rich and s undersaturated magma at the bottom of the chamber producing a highly metal rich magma layer at the bottom of the chamber this layer then became s saturated as a result of continued magnetite crystallization with the tiny sufide droplets that segregated into this precious metal enriched magma layer becoming highly enriched in pge themselves these were then trapped in situ by the crystallizing cumulate pile forming the subzone pd zone and intermediate zone sulfides formed in the rest of the chamber which was pge depleted at this stage would have been of low tenor these sulfide droplets grew and settled concentrating any remaining au when they encounter the floor to form the au zone and formed the low tenor sulfides of the cu zone above this we propose that the platinova reef exhibits an unusual example of a style of magmatic precious metal mineralization formed through sulfide dissolution upgrading in the latter stage of crystallization of a closed system the offsets in individual metal peaks mineralization demonstrate the relative partitioning of pd pt au cu into a sulfide liquid from a silicate magma and demonstrates the importance of partitioning behavior in low volume magmatic sulfide melts this feature can lead to the characteristic stratigraphic offsets in peak metal abundances the role of magnetite crystallization appears to be an important control on s saturation in such systems and thus may be an important indicator of the location of mineralized horizons in addition in such deposits indicators of crustal contamination by external s and multiple injections of magma common features of more ni rich pge reefs are less likely to be present

a study of the trace sulfide mineral assemblages in the stillwater complex montana usa the sulfide assemblages of the stillwater complex away from the well studied ore zones are composed mainly of variable proportions of pyrrhotite chalcopyrite pentlandite and pyrite excluding vein assemblages and those affected by greenschist and lower temperature alteration the majority can be classified into two broad assemblages defined here as pristine multiphase often globular in shape or volatile bearing multiphase high temperature volatile rich minerals such as biotite hornblende or an unmixed calcite dolomite assemblage the volatile bearing assemblages are mainly found within and below the j m reef where native copper and sphalerite are also locally present pristine sulfides are found throughout the stratigraphy both groups can be affected by apparent s loss in the form of pyrite being converted to magnetite and chalcopyrite to a cu fe oxide delafossite with little to no silicate alteration an upward trend from pentlandite rich to pyrrhotite rich to pyrite rich assemblages is observed in the footwall rocks in upper gn i and the same trend repeats from just below the reef and continues into the overlying n ii and gn ii modeling suggests that the sulfide ni in the peridotite zone is largely controlled by silicate ni when taken together observations are most readily explained by the remobilization of selected elements by a high temperature fluid with the apparent loss of s cu ni this could concentrate ore metals by vapor refining eventually producing a platinum group element enriched sulfide ore zone such as the j m reef

fluid density distribution in a high temperature stratified thermohaline system implications for saline hydrothermal circulation density distribution within the salton sea geothermal system of fluids ranging from 20 c to 325 c has been computed using chemical and thermal data from geothermal production well tests and curve fit models of nacak chloride solution properties density corrections can easily be made to 0 01 g cm3 for solute effects of each of the dominant chloride salts as well as pressure above vapor saturation field data on dissolved co2 is too sparse to routinely compute a correction however this moderately but variably abundant component can be shown to produce only minor 0 01 g cm3 errors in density estimates fluid density within the shallow cool 20 wt the density of brine climbs rapidly to near 1 0 g cm3 as the salinity increase across this interface overwhelms the thermal effect on fluid density this steep density gradient precludes all but diffusional conductive or perhaps double diffusive convective mass and heat transfer in this transitional regime measurement uncertainties for reservoir depths temperatures and salinities commonly exceed errors of our density model limiting the accuracy of details in our modeled density distribution gross scale relationships of this salinity stratified geothermal system are however apparent permitting rational discussion of extrapolations to conditions beneath the presently explored reservoir and inference of dynamics during the systems evolution

convective exchange between pore fluid and an overlying reservoir of denser fluid a post cumulus process in layered intrusions abstract we describe experiments on buoyancy driven exchange between fluid in a porous bed of glass balls and an overlying reservoir of denser fluid fingers of the dense fluid penetrated downwards into the porous medium when the two fluids had equal viscosities the rate of convective exchange was proportional to permeability and the density difference between the fluids and was inversely proportional to viscosity when the fluids had different viscosities the rate further depended on the viscosity ratio raised to the power 0 4 if a layer of dense new melt is emplaced above a porous cumulus pile forming the floor of a magma chamber an analogous process may occur as the new melt can percolate down into the pore space and expel the less dense intercumulus liquid convective exchange may proceed at rates of metres per year to tens of metres per year in basic to ultrabasic magmatic systems this conclusion strongly depends on the porosity of the cumulus pile and is less sensitive to the magmatic viscosities chemical and textural effects will result from this process such as re equilibration and resorption of cumulus crystals when intercumulus melt exchange is incomplete lateral geochemical gradients may be preserved in cumulate rocks

geochemical reversals within the lower 100 m of the palisades sill new jersey transects through the lower part of the palisades sill were made at fort lee and alpine new jersey in order to characterize the petrologic signature of previously proposed reversals in the normal tholeiitic differentiation trend petrographic and geochemical data include 1 modal and grain size analyses 2 bulk rock major and trace element concentrations by dcp aes and 3 augite orthopyroxene magnetite and olivine compositions by electron microprobe analysis anomalous horizons defined by increased bulk rock mg cr ni and co concentrations and abrupt modal and grain size changes occur at 10 m the well known olivine zone 27 m 45 m and 95m above the basal contact thermal models coupled with estimates of the emplacement rate and total magma volume indicate that the olivine zone oz is an early stage feature related to the emplacement of initial magma into the palisades chamber stoke s law calculations indicate that the settling velocity of average sized olivine crystals in a high titanium quartz normative htq magma is too slow for significant gravity settling to have occurred prior to the solidification of the basal 20 m of the sill it is suggested that the oz resulted from the emplacement of a heterogeneous initial magma from a compositionally stratified sub palisades storage chamber located within the upper crust however heterogeneity may have been derived directly from the mantle or during rapid ascent geochemical models indicate that the oz contains accumulated olivine that is not in cotectic or constant proportions with the other cumulus phases suggesting a mechanical sorting process magma chamber recharge is proposed to have occurred at the 27 m and 45 m levels when a slightly more primitive htq magma was injected into the palisades sill chamber zones of elevated mg and cr 6 to 10 m thick at these two horizons may indicate the thickness of the hybrid magma formed by the mixing of these two compositions geochemical models indicate that the rocks at these levels have accumulated excess orthopyroxene relative to samples from the rest of the sill normal faulting in the fort lee area at the 95 m level has caused repetition of the stratigraphic section and hence the sharp reversal observed at this level

sulfur and chlorine in late cretaceous deccan magmas and eruptive gas release large volume p hoehoe lava flows erupted 67 to 65 million years ago forming the deccan traps india the impact of these flood basalt eruptions on the global atmosphere and the coeval end cretaceous mass extinction has been uncertain to assess the potential gas release from this volcanism we measured sulfur and chlorine concentrations in rare glass inclusions inside crystals and on glassy selvages preserved within lavas concentrations range from 1400 parts per million of s and 900 parts per million of cl in inclusions down to a few hundred parts per million in the lava these data indicate that eruptions of deccan lavas could have released at most 0 103 weight of s yielding up to 5 4 teragrams of so2 per cubic kilometer of lava a more conservative estimate is 0 07 weight of s and 0 04 weight of cl yielding 3 5 teragrams of so2 and 1 teragram of hcl for every cubic kilometer of lava erupted the flows were very large in volume and these results imply that huge amounts of s and cl gases were released the environmental impact from even individual eruptions during past flood basalt activity was probably severe

shallow drilling in the salton sea region the thermal anomaly during two shallow thermal drilling programs thermal measurements were obtained in 56 shallow 76 2 m and one intermediate 457 3 m depth holes located both onshore and offshore along the southern margin of the salton sea in the imperial valley california these data complete the surficial coverage of the thermal anomaly revealing the shape and lateral extent of the hydrothermal system the thermal data show the region of high thermal gradients to extend only a short distance offshore to the north of the quaternary volcanic domes which are exposed along the southern shore of the salton sea the central thermal anomaly has an arcuate shape about 4 km wide and 12 km long across the center of the anomaly the transition zone between locations exhibiting high thermal gradients and those exhibiting regional thermal gradients is quite narrow thermal gradients rise from near regional 0 09 c m to extreme 0 83 c m in only 2 4 km the heat flow in the central part of the anomaly is greater than 600 mw m2 and in the two local anomalies exceeds 1200 mw m2 the shape of the thermal anomaly is asymmetric with respect to the line of volcanoes previously thought to represent the center of the field with its center line offset south of the volcanic buttes there is no broad thermal anomaly associated with the magnetic high that extends offshore to the northeast from the volcanic domes the salton sea thermal anomaly can be described as three nested thermal features superimposed on the regional salton trough heat flux a broad region of elevated conductive heat flow with constant temperature gradients of about 0 1 c m a central anomaly of high 0 4 c m near surface gradients observed through a conductive thermal cap underlain by a nearly isothermal zone of hydrothermal circulation and two local anomalies with shallow temperature gradients as high as 0 8 c m superimposed on the central geothermal system these observations of the thermal anomaly provide important constraints for models of the circulation of the hydrothermal system thermal budgets based on a simple model for this hydrothermal system indicate that the heat influx rate for local hot spots in the region may be large enough to account for the rate of heat flux from the entire salton trough

cyclic units in the somerset dam layered gabbro intrusion southeastern queensland australia abstract the well preserved somerset dam intrusion probably represents a small relatively shallow subvolcanic magma chamber the 500 m thick exposed sequence consists of 22 macrolayers which are defined by sharp phase modal and textural contacts at least six cyclic units 30 150 m thick are exposed and the sequence from the base to the top of a cyclic unit is inferred to be leucogabbro plagioclase cumulate troctolite plagioclase olivine cumulate olivine gabbro plagioclase augite olivine cumulate and oxide gabbro plagioclase augite olivine magnetite ilmenite cumulate mineral compositions in a typical cyclic unit show a reversed fractionation trend in the sequence leucogabbro troctolite and a normal fractionation trend from troctolite the least fractionated rock type to the oxide gabbro the most fractionated rock type the most sensitive parameters for defining the cryptic trends are an in plagioclase fo and ni in olivine and cr in magnetite and augite whole rock compositions also show marked changes and fe ti v s and cu increase and al mg fe cr and ni decrease from troctolite to oxide gabbro despite the remarkable similarity of successive cyclic units significant differences exist between them in the sequences of layers thicknesses of individual layers and of the cyclic units mineral compositions and cryptic patterns average level of fractionation and the size of the reversals unit 3 is particularly unusual because it is the least fractionated and consists of two incomplete subunits unit 1 the lowest exposed is the most fractionated these differences between the units cannot be explained in terms of a closed system and are strong evidence for an open system involving periodic injections of magma the formation of a cyclic unit appears to reflect the dominant control of the order of crystallisation from a batch of replenished magma which is essentially plagioclase first followed by olivine augite magnetite and ilmenite and brown hornblende the parent magma is inferred to have been a high alumina olivine tholeiite the gradual rather than sharp reversals in the cryptic trends in the lower part of each cyclic unit may reflect some magma mixing though postcumulus processes could have been operative

mixing characteristics and compositional differences in mantle derived melts beneath spreading axes evidence from cyclically layered rocks in the ophiolite of north oman within the ophiolite of north oman interlayered gabbros and peridotites form a unit up to 3 5 km thick between pervasively foliated harzburgites and dunites of mantle origin and isotropic gabbros and plagiogranites spatial and chemical relationships within this sequence require the existence of a large reservoir 20 km wide 3 5 km high of tholeiitic magma that was periodically replenished by mantle derived picritic to tholeiitic melts during the spreading process cyclicity in the appearance of minerals during cooling of the magma is marked by the repetition of stratigraphies in the layered rocks within these cyclic units basal peridotites with magnesian olivines and pyroxenes are abruptly overlain by gabbros with less magnesian mafic phases this abrupt discontinuity is explained by density stratification set up at the base of the chamber whenever a picritic melt influx occurs by virtue of its greater density than tholeiitic magma sparks et al 1979 any picritic magma emplaced into the base of the tholeiitic magma reservoir would spread out along the floor of the chamber with only limited mixing taking place across the liquid liquid interface cooling of this basal layer by contact with the cooler overlying magma would result in the precipitation of olivine chrome spinel pyroxene from the picritic liquid this process would continue until the now more evolved input liquids were incorporated into the tholeiitic magma resevoir the precipitation of a gabbroic layer would then take place until a new magma input occurred to repeat the cycle variable thicknesses of the cyclic units different liquid lines of descent and various entry points within the liquid line of descent represented by the lowermost layered rocks reveal that the input liquids varied in volume primary composition and the extent and nature of prefractionation in the mantle liquids expelled from the chamber as lavas and dykes are shown to be more fractionated than liquids precipitating the layered rocks and are considered to have undergone additional fractionation within a zone of underplated gabbro and plagiogranite beneath the dyke swarm the existence of a large magma reservoir supports earlier interpretations based on age tilton et al 1981 and geochemical data pearce 1980 that the oman ophiolite is an analogue for fast spreading ridges

finger structures in the lille kufjord layered intrusion finnmark northern norway highly irregular contacts are developed between peridotite and troctolite in layers forming the uppermost part of a transition zone between two of the cyclic units of the lille kufjord intrusion the upwardly directed peridotite fingers crosscut both the feldspar lamination and the feldspathic xenoliths in the troctolite and are interpreted as the result of the replacement of troctolite by peridotite similar structures are developed in the rhum ultrabasic pluton replacement may have been caused by the migration of a more hydrous and magnesian magma trapped initially in olivine or olivine clinopyroxene cumulates into plagioclase olivine cumulates precipitated from a basaltic liquid

magma reservoir crystallization processes small scale dikes in cumulate gabbros mauna kea volcano hawaii gabbroic xenoliths that represent cumulate environments within mauna kea volcano are in rare examples penetrated by small scale 7 cm dikes we examined four dike host composite xenoliths to establish how this evidence for magma seemingly injected into cumulate gabbro fits into the evolution of igneous processes in shield volcano magma reservoirs olivine clinopyroxene and plagioclase compositions in both host gabbros and dikes are characteristically tholeiitic and evolved fo71 66 cpx mg 79 77 an72 51 with respect to hawaiian magmatism dikes however when compared with their host gabbros have slightly greater abundances of some incompatible elements and slightly more evolved olivine compositions e g fo68 vs fo71 compared with mauna kea lava compositions both host gabbros and dikes have lower incompatible element abundances positive eu anomalies and notable for dikes major element compositions unlike those of lavas e g sio2 46 wt the small scale dikes therefore also have cumulate characteristics we interpret them as representing late stage liquids e g 5 wt mgo based on fo70 squeezed from solidifying cumulus piles of evolved e g fo70 gabbroic assemblages the compositions of the dikes however do not match those of the most evolved liquids expected in reservoirs because they appear to have lost interstitial liquids e g positive eu anomalies low abundances of some trace elements because minerals in the dikes were in equilibrium with highly evolved liquids conditions for small scale dike formation in cumulate environments apparently occur only at the last stages of reservoir magma differentiation and solidification

diffusion of trace elements during bottom crystallization of double diffusive convection systems the magnetitite layers of the bushveld complex abstract magmas that crystallize to produce layered intrusions may behave as double diffusive convective systems in such an intrusion the magma would be stratified as a stacked series of thick convecting layers separated by thin static diffusive boundary layers crystallization is often inferred to occur at the base of the chamber and chemical changes due to fractional crystallization of the lowermost convecting layer are transmitted by diffusion across the thin diffusive boundary layers separating the thick stacked layers of convecting magma in this work the rayleigh distillation law is extended to describe changes in the concentration of a compatible trace element due to fractional crystallization of the convecting magma layer at the base of the stack as well as diffusion of the trace element into the layer across its top diffusive boundary general equations are presented defining predicted geochemical profiles in terms of partition coefficient rate of accumulation of the crystal pile diffusion rates and thickness of discrete magma layers extraction of a single dense phase from the lowermost convecting layer progressively decreases the density of the residual liquid in the layer ultimately the densities of the basal and immediately overlying layers of magma become equal the diffusive boundary layer breaks down and the two basal layers mix this results in a reversal in the trace element concentration of the crystals being precipitated a sawtooth trace element concentration profile results the bushveld complex provides a good test for the double diffusive convection model envisaged here available data on chromium concentration with respect to height in the main magnetitite layer of the bushveld complex are well described by these equations

formation of hybrid cumulates melatroctolites in intrusion 4 of the honningsvag intrusive suite northern norway new mapping shows that the 1160 m thick layered series of the mafic to ultramafic intrusion 4 of the honningsvag intrusive suite north norwegian caledonides comprises approximately 25 feldspathic peridotite 55 melatroctolite olivine rich troctolite 5 troctolite 2 olivine melagabbro olivine rich olivine gabbro and 13 15 olivine gabbro the cumulate sequence can be divided into eight macrocyclic units the base of each unit resulting from magma replenishment compositions of olivine and plagioclase obtained from ca 1700 electron microprobe point analyses display complex and limited variations fo87 74 and an80 67 relative to stratigraphic height the origin of olivine rich cumulates such as melatroctolites 52 72 olivine and olivine melagabbros 29 37 olivine in large quantities is difficult to explain by either simple fractional crystallisation or conventional layer forming processes a new genetic model is proposed involving suspension of pre existing cumulus crystals during magma recharge in a periodically replenished magma chamber their partial resorption and simultaneous crystallisation of liquidus olivine cumulates formed in this way represent mixtures of crystals that crystallised from compositionally different magmas and are therefore termed hybrid cumulates apart from their distinctly non cotectic bulk modal proportions the hybrid nature of the olivine rich cumulates is evidenced by partially resorbed plagioclases enclosed within cumulus olivines the occurrence of resorbed plagioclase inclusions in olivine gabbros and large variations in modal plagioclase content suggest that these may also be hybrid cumulates d 2002 elsevier science b v all rights reserved

convective hydrothermal c02 emission from high heat flow regions abstract in addition to volatiles released from volcanoes the flux of co2 to the atmosphere from other sources e g metamorphism and subsurface magmatism represents an important aspect of the global carbon cycle we have obtained a direct estimate of the present day atmospheric co2 flux from convective hydrothermal systems within subaerial seismically active high heat flow regions geothermal systems of the salton trough california u s a and the taupo volcanic zone new zealand provide benchmarks for quantifying convective hydrothermal co2 fluxes from such regions co2 fluxes from the salton trough 109 mol yr 1 and the taupo volcanic zone 8 109 mol yr 1 were computed using data on convective heat flow and the temperatures and co2 concentrations of reservoir fluids the similarity in specific co2 flux 106 mol km 2 yr 1 from these two disparate geologic tectonic settings implies that this flux may be used as a baseline to compute convective hydrothermal co2 emission from other areas of high heat flow if this specific flux is integrated over high heat flow areas of the circum pacific and tethyan belts the total global co2 flux could equal or exceed 1012 mol yr 1 adding this flux to a present day volcanic co2 flux of 4 1012 mol yr 1 the total present day earth degassing flux could balance the amount of co2 consumed by chemical weathering 7 1012 mol yr 1

segregation processes in vesiculating crystallizing magmas vesiculation of crystallising magma can produce either a mobile vesicular magma or a rigid network of crystals containing vesicular liquid where partially crystallized rigid mush underlies less crystallized magma such as near the base of a lava flow or in the cumulus pile of a magma chamber evolved interstitial melt and or gas may escape into the main body of magma the consequences of this may include contamination of the overlying liquid with gas and interstitial melt or intrusion of diapirs of vesicular evolved liquids to form vertical vesicle cylinders and other segregation features found in many basaltic lava flows and sills analog experiments were used to investigate some of the phenomena that can arise during vesiculation within a crystal mush which was simulated by pumping air through a porous plate that formed the floor of a container filled with a viscous liquid floored with a layer of glass beads experiments used either a single liquid or two stably stratified liquids with a liquid interface either coincident with the top of the porous layer of beads or slightly above the porous layer for a range of liquid viscosities and air flow rates vesiculation rates individual bubbles emerged from the top of the porous layer of beads and carried a thin trail of interstitial liquid into the overlying liquid the number of bubble trains leaving the surface of the porous bed increased with decreasing liquid viscosity and flow rate and with increasing bead size and hence with increasing permeability analog vesicle cylinders composed of diapirs of bubbly interstitial liquid were produced only when a layer of buoyant bubbly liquid lay above the surface of the porous layer the relative size of the bubbles and constrictions within the porous layer are argued to control whether individual bubbles leading to bubble trains or vesicular liquid leading to vesicle cylinders leaves the porous layer and hence whether vesicle cylinders can form

the skaergaard layered series part iv reaction transport simulations of foundered blocks during the middle stages of crystallization of the skaergaard layered series large numbers of blocks became detached from the upper border series and settled into the mush of crystals on the floor it has been recognized for some time that these blocks now have compositions and textures that differ markedly from those of the units from which they came they tend to be more plagioclase rich and seem to have lost mafic components to the surrounding gabbro numerical simulations coupling crystallization melting and heat and mass transfer for a multicomponent system show how the blocks reacted with the mush in which they were emplaced enhanced cooling and crystallization of a compositionally stratified mush adjacent to the blocks resulted in patterns of melt compositions similar to those of layering around the blocks volume changes during crystallization and melting induced convection of the interstitial melt leading to changes in the bulk compositions of the blocks and the surrounding mush inhomogeneities such as inclusions are likely to facilitate the onset of compositional convection in a chemically stratified solidification zone

macrorhythmically layered gabbronorites in the windimurra gabbroid complex western australia abstract plagioclase orthopyroxene augite cumulates gabbronorites in the 3 5 km thick wagoo hills section show strong rhythmic modal layering over about 200 m of section layers of mafic gabbronorite mgn 1 10 m thick alternate with leucogabbronorite lgn 10 5 m thick with sharp contacts mgn are adcumulates with 65 70 plagioclase cores an68 reverse zoning 20 orthopyroxene mg69 15 augite mg78 and whole rock mg number 65 in contrast lgn are mesocumulates more orthocumulate character with 80 90 plagioclase cores an75 normal zoning 6 augite mg71 4 orthopyroxene mg62 and whole rock compositions have mg numbers ca 54 and higher concentrations of incompatible elements e g k p and lower concentrations of compatible elements and pge than in mgn lgn contain minor interstitial postcumulus quartz hornblende fe ti oxides apatite and 5 25 hydrous alteration products unlike mgn the cause of the presumed cumulus stage differences in pyroxene plagioclase modal ratio noncotectic proportions in lgn and sharp mgn lgn mgn contacts is not clear but could result from rhythmic variations in supercooling and nucleation the postcumulus differences postcumulus mineralogy normal zoned plagioclase more fe rich pyroxenes and higher incompatibles in lgn are interpreted in terms of an in situ crystallisation compositional convection model cumulus mineral assemblages in mgn have a higher fractionation density and their in situ crystallisation released a buoyant residual liquid which was continuously displaced by fresher magma resulting in only 0 5 residual liquid trapped with the mush adcumulate formation in contrast in situ crystallisation of a plagioclase rich cumulus assemblage lower fractionation density in lgn resulted in a denser stagnant residual liquid 10 15 of which was trapped in the crystal mush producing normal zoning in plagioclase 6 8 atom fe shift in pyroxene compositions and higher incompatible element concentrations and h 2 o the latter resulting in selective alteration of lgn as a result mgn had higher solidus t and acted like seals or aquicludes whereas lgn had lower solidus t and were sinks or aquifers for incompatibles and volatiles

effects of h2o on phase relations during crystallization of gabbros in the kap edvard holm complex east greenland in the kap edvard holm complex of east greenland layered monly results in the formation of discordant igneous bodies within crystallizing cumulus sequences in some olivine gabbros host numerous small discordant bodies of ultramafic cases partially molten cumulates appear to have reacted rock and magnetite gabbro that formed by late magmatic metasomatic with liquids or fluids that originated within the cumulus replacement of partially solidified gabbroic cumulates similar repile resulting in resorption of cumulus minerals crysplacive rocks are also present as large semi conformable sheet like tallization of intrusive or replacive bodies and rebodies within the layered gabbro sequence replacive bodies of all distribution of mass within the intrusion e g robins sizes are spatially associated with xenoliths of metabasaltic lava 1982 butcher 1985 butcher et al 1985 helz 1987 the small bodies cluster around xenoliths and the large bodies host irvine 1987 boudreau 1988 mcbirney sonnenthal swarms of xenoliths the replacive bodies are inferred to have formed 1990 larsen brooks 1994 scoon mitchell 1994 by reaction of gabbroic cumulates with pore liquids that were mathez 1995 meurer et al 1997 boudreau 1999 in enriched in h2o by degassing or dehydration melting of the other cases partially molten cumulates are inferred to metabasalts as these liquids migrated through the cumulus pile have reacted with externally derived magmas that inthey resorbed plagioclase and produced ultramafic bodies where the truded the cumulus pile resulting in the development liquids came to rest they reacted with gabbroic cumulates to produce of discordant replacive bodies via the reaction of the bodies of magnetite rich gabbro the most evolved pore liquids cumulates with intruding magmas e g morse et al 1987 segregated to form hornblende rich pegmatitic dikes of roughly dacitic bedard et al 1988 tegner wilson 1993 tegner composition suggesting that hydration and possibly oxidation of the robins 1996 because cumulus rocks preserve records magma shifted the liquid line of descent from a tholeiitic trend toward of magmatic fractionation and replenishment in large a calc alkaline trend these inferences imply that contamination of mafic intrusions recognizing and understanding postmagmas by dehydration of hydrothermally altered crustal rocks may cumulus processes such as these can provide insights significantly influence the evolution of mafic magma systems into the evolution of mafic magma chambers and the differentiation of basaltic magmas within the crust near the western margin of the kap edvard holm

seismic imaging of the metamorphism of young sediment into new crystalline crust in the actively rifting imperial valley california plate boundary rifting between transform faults is opening the imperial valley of southern california and the rift is rapidly filling with sediment from the colorado river three 65 90 km long seismic refraction profiles across and along the valley acquired as part of the 2011 salton seismic imaging project were analyzed to constrain upper crustal structure and the transition from sediment to underlying crystalline rock both first arrival travel time tomography and frequency domain full waveform inversion were applied to provide p wave velocity models down to 7 km depth the valley margins are fault bounded beyond which thinner sediment has been deposited on preexisting crystalline rocks within the central basin seismic velocity increases continuously from 1 8 km s sediment at the surface to 6 km s crystalline rock with no sharp discontinuity borehole data show young sediment is progressively metamorphosed into crystalline rock the seismic velocity gradient with depth decreases approximately at the 4 km s contour which coincides with changes in the porosity and density gradient in borehole core samples this change occurs at 3 km depth in most of the valley but at only 1 5 km depth in the salton sea geothermal field we interpret progressive metamorphism caused by high heat flow to be creating new crystalline crust throughout the valley at a rate comparable to the 2 km myr sedimentation rate the newly formed crystalline crust extends to at least 7 8 km depth and it is shallower and faster where heat flow is higher most of the active seismicity occurs within this new crust

distribution of orthocumulate textures in the jimberlana intrusion the p and u contents and the texture of cumulates have been used to document variations in residual porosity in the jimberlana intrusion the results show that rocks with low residual porosity adcumulate textures occur where heat loss through the floor of the chamber was low or where the viscosity of the melt from which they crystallized was low low residual porosity also correlates with the appearance of plagio clase as a cumulus phase and with zones of well developed rhythmic layering it is suggested that the initial porosity of cumulates is controlled by the balance between nucleation and crystal growth at the crystalliquid interface low initial porosity represents attainment of textural equilibrium and forms when adjacent crystals at the top of the cumulate pile continue to grow expelling the intercumulus magma in the process high initial porosities develop when new crystals nucleate at the top of the crystal pile before a high proportion of the intercumulus magma is expelled low viscosity in

chemistry morphology and origin of magmatic reaction chromite stringers associated with anorthosite in the upper critical zone at winnaarshoek eastern limb of the bushveld complex

the mode of emplacement of the honningsvag intrusive suite mageroya northern norway the honningsvag intrusive suite consists of several layered mafic ultramafic intrusions and a transgressive body of igneous breccia that appears to represent a magma conduit it is emplaced into a silurian flysch type sedimentary sequence that is thermally metamorphosed to spotted slate cordierite andalusite or pyroxene hornfels and agmatitic migmatite folds and flattened reduction spots in the hornfelses suggest that emplacement took place after caledonian deformation and development of a slaty cleavage tectonic rotation subsequent to emplacement has led to exposure of the honningsvag intrusive suite in a natural cross section corresponding to 10 km of crustal depth basaltic magma was initially emplaced as a several kilometre tall pipe that crystallized to form intrusion 1 a second magma chamber was initiated alongside this pipe and subsequently expanded laterally into a sill like magma body as batches of olivine saturated basalt were added a later magma chamber represented by intrusion 4 developed largely within the cumulates forming the upper part of intrusion 2 and appears to have been accompanied by opening of a broad inclined feeder into which blocks and slabs of older cumulates collapsed the resulting igneous breccias of intrusion 3 are chaotic and largely clast dominated in the lower part of the conduit but enclosed slabs are matrix supported and orientated parallel to an originally subhorizontal banding in the feldspathic peridotite matrix in the upper part the core of the breccia body has a troctolite matrix and contains blocks of older breccia suggesting re opening of the conduit either during the crystallization of intrusion 4 or possibly during the development of chambers represented by the younger layered intrusions the cumulates in intrusion 4 subsided sufficiently to invert marginal parts of the layered series before a further magma chamber was initiated in its roof rocks the last major magma chamber opened alongside intrusion 5 and extended upwards as a pipe or broad dyke to the highest structural levels exposed cross cutting relationships show that the honningsvag magma chambers were not active simultaneously but were emplaced sequentially generally at successively higher structural levels olivine tholeiite magma initially pooled in a crustal zone where it had neutral buoyancy subsequent chambers are suggested to have been initiated by emplacement of magma along the density discontinuities that existed above and around crystallized intrusions and their associated hornfelses chambers evolved by fractional crystallization assimilation of country rocks and periodic replenishment the abandonment of magma chambers may have resulted from the expulsion of low density residual melts

infiltration metasomatism of cumulates by intrusive magma replenishment the wavy horizon isle of rum scotland the eastern layered intrusion of the rum layered suite comprises paired peridotite and allivalite troctolite and gabbro layers forming 16 macro rhythmic units whereas the majority of these macro units are believed to have formed by a process of crystal liquid differentiation involving successive accumulation of crystals from multiple picritic replenishments of the chamber the unit 9 peridotite is interpreted as a layer parallel picrite intrusion closely correlated with this discontinuous peridotite body is a distinctive feature generally known as thewavy horizon which divides the overlying allivalite into a lower troctolite and an upper gabbro along a well defined undulating contact we propose that the wavy horizon is a metasomatic feature formed consequent to the removal of clinopyroxene from an original gabbroic mush foundering of the mush into the picritic sill resulted in the replacement of the original interstitial liquid by one saturated only in olivine plagioclase progressive through flow of this liquid resulted in the stripping out of clinopyroxene from the lower parts of the allivalite we interpret thewavy horizon as a reaction front representing the point at which the invading liquid became saturated in clinopyroxene the distinctive pyroxene enriched zone immediately above thewavy horizon could have formed when mixing of the interstitial liquids on either side of the reaction front formed a supercooled liquid oversaturated in pyroxene as a result of the curvature of the olivine plagioclase clinopyroxene cotectic the presence of many such approximately layer parallel features defined by differences in pyroxene content in the eastern layered intrusion of rum suggests that such an infiltration reaction process was not unique to unit 9

textural immaturity of cumulates as an indicator of magma chamber processes infiltration and crystal accumulation in the rum eastern layered intrusion textural maturity describes the extent to which rocks have approached textural equilibrium for igneous rocks this is a function of the time integrated thermal history in the sub solidus which for cumulate rocks generally translates to the cooling history via the rate of solidification of the accumulating crystal mush the most straightforward way to quantify the textural maturity of orthocumulates sensu lato is to measure the angle subtended at the corners of interstitial grains texturally mature cumulates have angle populations with median values of c 120 whereas texturally immature cumulates have lower medians inherited by the interstitial phase which infills either an impingement texture in the accumulating mush or a texturally equilibrated pore topology textural observations of allivalites from the rum layered suite demonstrate that stratigraphically localized excursions towards textural immaturity are the result of either transient relatively rapid solidification rates perhaps a result of the arrival of cold crystal laden bodies of fluid on the chamber floor or late stage infiltration of fully solidified cumulates by evolved liquids expelled upwards by compaction of underlying horizons these two processes have distinct textural and compositional signatures

crystallization and compositional convection in a porous medium with application to layered igneous intrusions growing crystals can provide a local source of buoyancy by depleting melt of certain chemical components and vigorous convective motions in the fluid can result we report a quantitative experimental study in which saturated aqueous solutions of cuso4 and na2so4 in porous media of either glass balls or cuso4 5h2o crystals were crystallized by cooling from below the crystals growing in the pore space released light chemically depleted solution which had the potential to convect out of the porous medium and be replaced by additional saturated solution from above we explore the physical and chemical effects of compositional convection during the complete solidification of the interstitial solution by varying the permeability of the porous medium the cooling rate and the initial composition of the solution this type of convection can form adcumulate rocks by enabling continuous circulation of melt from the main magma reservoir through the pores of a cumulus pile adcumulus growth can occur by this mechanism as long as the convective velocity in the porous medium is greater than the velocity of the solidification front we scale the experimental results to magmatic conditions and infer that this process should occur commonly in layered igneous intrusions when our porous media had a small permeability compositional convection enabled the adcumulate growth of a completely solid contact between melt and cumulate trapping of dynamically unstable melt beneath this impermeable layer of crystals also occurred in these experiments and may be important geologically small shallowly emplaced intrusions and the cumulate zones at the base of komatiites are probably cooled too rapidly for significant intercumulus convection to occur

epidote bearing veins in the state 2 14 drill hole implications for hydrothermal fluid composition epidote bearing veins in state 2 14 drill core from 900 to 2960 m depth were examined using backscattered electron microscopy and electron probe microanalysis to characterize the mineralogy parageneses texture and composition of vein minerals in order of decreasing abundance minerals in epidote bearing veins are pyrite calcite k feldspar quartz anhydrite hematite chlorite fe cu zn sulfides actinolite titanite and allanite the downhole distribution of minerals in epidote bearing veins pyrite and quartz varies as a function of depth and includes 1 calcite above 2000 m 2 k feldspar between 1700 and 2745 m 3 anhydrite between 2195 and 2745 m 4 hematite sulfides above 2773 m and 5 actinolite below 2890 m where present k feldspar was the first mineral to precipitate in veins followed by epidote in all other veins epidote was the earliest vein mineral to form calcite quartz anhydrite hematite and sulfides were paragenetically later compositional zoning common in most vein epidotes is typically symmetric with al rich cores and fe3 rich rims the minimum mole fraction of ca2fe3si3o12 oh xps in vein epidotes decreases systematically with increasing depth from 0 33 at 906 m to 0 21 at 2900 m and the maximum xps at any given depth is greater than 0 33 thermodynamic analyses of phase relations among vein filling minerals and aqueous solutions at depths near 1867 m and 300 c indicate that the modern reservoir fluid in the salton sea geothermal system is in equilibrium with calcite hematite quartz epidote xps 0 33 anhydrite the predicted fugacity of co2 14 bars for the modern salton sea brine is in close agreement with the calculated value of fco2 for the 1867 m production fluid theoretical phase diagrams in the system cao k2o fe2o3 al2o3 sio2 h2o o2 s2 co2 demonstrate that the mineralogies and mineral parageneses recorded hi epidote bearing veins and the observed variations in al fe3 content of vein epidotes may result from only minor changes in the fugacity of co2 o2 and s2 of the geothermal fluid

sulfide oxidation as a process for the formation of copper rich magmatic sulfides typical magmatic sulfides are dominated by pyrrhotite and pentlandite with minor chalcopyrite and the bulk atomic cu fe ratio of these sulfides is typically less than unity however there are rar

portrait of a giant deep seated magmatic conduit system the seiland igneous province abstract the seiland igneous province sip northern norway contains 5000 km2 of mafic and ultramafic intrusions with minor alkaline carbonatite and felsic rocks that were intruded into the lower continental crust at a depth of 25 to as much as 35 km the sip can be geochemically and temporally correlated to numerous dyke swarms throughout scandinavia at 560 610 ma and is linked to magmatic provinces in w greenland and ne america that are collectively known as the central iapetus magmatic province cimp revised mapping show that the sip exposes 85 90 layered tholeiitic alkaline and syeno gabbros 8 10 peridotitic complexes 2 5 carbonatite syenite and diorite that formed within a narrow gravimetric data implies an average thickness of igneous rocks of 4 5 km and also features six deep lithospheric roots of ultramafic rocks extending min 9 km into the crust together the root structures represent the main volcanic conduits conveying thousands of km3 of mafic ultramafic melts from the asthenosphere to the lithosphere the ultramafic complexes were predominantly emplaced into the layered gabbros at four major igneous centres respectively nordre brumandsfjord melkvann kvalfjord and reinfjord all complexes are situated in a right way up position and are steep sided forming large plugs a marginal hybrid zone forms at the contact with country rock and transitions gradually from olivine mela gabbro over pyroxenites that grades in to an olivine clinopyroxenite zone which is followed by a wehrlite zone and finally the centre of the complexes comprises pure dunite from pyroxenite to dunite olivine changes from fo72 to fo85 and clinopyroxene from di80 to di92 i e the complexes observe a reverse fractional crystallisation sequence with time parental melt compositions modelled from early dykes indicate komatiitic to picritic melts with 16 22 wt mgo cr of 1594 ppm and ni of 611 ppm which were emplaced at 1450 1500 c melt compositions calculated from clinopyroxene compositions from reinfjord are oib like with lree enriched over hree the high abundance of carbonatites and lamproites demonstrates the volatile rich nature of the mantle source region and is further corroborated by the unusually high abundance of magmatic sulphides 0 5 1 and carbonated and hydrous assemblages c 1 throughout the region in reinfjord they are also closely associated with pge cu ni reef deposits essentially the ultramafic complexes in the sip comprises deep seated transient magma chambers that facilitated mixing and homogenisation of a rich diversity of fertile asthenospheric melts en route to the upper parts of the continental crust

replenished magma chambers effects of compositional zonation and input rates several recent publications have reported investigations of the dynamical effects and geological consequences of the replenishment of a homogeneous magma chamber by the rapidinjection of hot relatively dense fresh magma the new magmaforms a separate layer at the base of the chamber whereit cools and crystallizes until the density of the residual liquid in the lower layer becomes equal to that of the upperlayer at which time the fluid in the two layers becomes intimately mixed the fluid dynamical aspects of this modelwere confirmed by laboratory experiments using aqueous solutions this paper reports three experimental variations on the basic theme which examine the effects of the rate of injection into homogeneous and compositionally stratified chambers first the rapid injection of a hot dense layer beneath a fluid with a lower temperature and a vertical compositional gradient is considered a series of layers gradually builds up from the base of the upper gradient region while the lower layer cools crystallizes and finally overturns in this situation however the rise of the lower layer fluid is restrained by the density gradient and mixing takes place with only part of the upper fluid second the slow influx of a hot dense liquid beneath a cold less dense homogeneous layer is considered a considerable quantity of the input fluid coolsand crystallizes immediately releasing some lighter fluid to mix with the fluid in the homogeneous layer with time some dense input fluid spreads along the bottom so that locally the situation is the same as the one considered in the previous studies with a local overturning occurring when the density of the lower fluid has decreased sufficiently the deposition of crystals continues and some rather fine chimney like structures can develop which bear a resemblance to the black smokers recently discovered on the sea floor third a combination of the first two variations is studied by the consideration of the slow influx of hot dense liquid beneath a uniformly cooler fluid with a vertical compositional gradient the input fluid is constrained in its rise by the density gradient as the influx continues the additional thermal energy is also confined and causes the crystals to redissolve and build up a lower layer as was observed in the case of a rapid injection beneath a compositional gradient

the volcanology of komatiites as deduced from field relationships in the norseman wiluna greenstone belt western australia abstract komatiites in the western part of the norseman wiluna greenstone belt in the yilgarn block of western australia display a wide variety of volcanic facies ranging from very thin differentiated munro township type flow units through to very thick olivine rich cumulate flow units containing high proportions of adcumulate dunite cumulate flow units have been mapped in detail in the agnew wiluna segment of the norseman wiluna greenstone belt in the yakabindie and mt keith areas thick lenticular bodies of olivine adcumulate are flanked and overlain by thinner sheet like sequences of finer grained olivine orthocumulates they show fine scale internal layering broad scale cryptic layering and upper fractionated sequences containing harrisites pyroxene bearing cumulates and in some cases gabbroic derivatives low grade disseminated sulphide mineralisation occurs within the dunite lenses in the stratigraphically equivalent kathleen east area adcumulate dunite grades laterally into olivine orthocumulates and spinifex textured flows a similar relationship is also seen further south where the perseverance ultramafic complex consists of a thick central dunite lens flanked on one side by thin fine grained orthocumulate sequences and on the other by intercalated orthocumulates and spinifex textured flows these field relationships indicate an extrusive origin for the adcumulate dunite bodies the walter williams formation in the west central part of the norseman wiluna belt is a very large cumulate flow unit 150 km long and 30 km wide in presently exposed extent it consists of a central sheet like body of adcumulate dunite underlain and overlain by olivine orthocumulates the adcumulate sheet is everywhere capped by a distinctive thin layer of olivine harrisite at its northernmost extent the flow unit consists of cyclically layered olivine and pyroxene bearing cumulates capped by gabbros dolerites and pyroxene spinifex textured material interpreted as a periodically replenished lava lake sequence both the lenticular dunite bodies of the agnew wiluna belt and the sheet like dunite body of the walter williams formation are interpreted as crystallisation products of very large submarine komatiite lava flows which erupted and flowed at very high rates the adcumulate dunites are interpreted as the products of in situ crystallisation at low degrees of supercooling at the top of upward and inward accreting crystal piles at the bases of the flows flanking and overlying olivine orthocumulates reflect higher rates of heat loss at the site of crystallisation due to lower lava flow rates field evidence from perseverance and the general geometry of the dunite lenses suggest that the lenses formed within large thermal erosion channels developed by turbulent lava rivers flowing over low melting felsic volcanic substrates the textural range exhibited by komatiites can be integrated into a comprehensive model for the geometry of large komatiite flow fields formed by rapid extrusion dunite sheets form close to the eruption sites and dunite lenses as a result of channellisation further away from the vent or close to the vent in the situation where the substrate is non refractory and thermal erosion can take place readily kambalda type volcanic facies develop in more distal environments where lava emplacement is channellised and episodic and munro type flow units represent small scale lava tubes formed at low flow rates on the distal flanks of major eruptions or close to the vent of very small ones waning eruption rates leads to proximal facies being overridden by distal ones a common observation in komatiite sequences the size of thermal erosion channels requires very rapid eruption rates comparable to those in phanerozoic flood basalt terrains

spatial constraints on magma chamber replenishment events from textural observations of cumulates the rum layered intrusion scotland the eastern layered series of the rum layered suite is formed of 16 macro units each comprising a lower peridotite and an overlying feldspar rich layer the local term is allivalite the origin of the peridotite layers is unresolved with two contrasting models the earlier of the two is based on repeated replenishment of an opensystem magma chamber with deposition of fractionated material on the chamber floor the second is based on the early formation of a troctolitic complex which is then repeatedly intruded by sills of replenishing picritic magma to form the peridotite horizons the lack of resolution of this fundamental problem is a consequence of the reliance of previously published studies on field observations i present evidence to show that the clinopyroxene in the allivalites preserves information about the distribution of the last melt to solidify permitting determination of not only the extent of supersolidus textural equilibration but also the sub solidus history of the allivalite horizons comparison of profiles of clinopyroxene plagioclase plagioclase median dihedral angle across allivalite units demonstrates that it is possible to distinguish between those that were intruded by later picrite sills and those adjacent to peridotite horizons formed by replenishment and subsequent deposition of fractionated crystals above the pre existing pile in the region studied only the main peridotite body of unit 9 was intruded into a pre existing allivalitic mush

fluid dynamic and geochemical evolution of cyclic unit 10 rhum eastern layered series lithological major element trace element and sr isotope data from cyclic unit 10 of the rhum eastern layered series are presented the lower 65 metres of the unit are peridotite subdivided on textural and geochemical grounds into a lower homogeneous portion approximately 50 metres thick and an upper heterogeneous portion approximately 15 metres thick the uppermost 16 5 metres of the unit are allivalite there are steep geochemical gradients across the peridotite allivalite boundary in ni content of olivine and whole rock sr isotope composition calculations are presented on the geochemical evolution of a rhum picritic liquid undergoing olivine precipitation both when the olivines remain suspended in the residual liquid as they precipitate and when they are continuously fractionated quenched groundmass and olivine compositions from the rhum dykes and the unit 10 peridotite olivines show good agreement with the suspension model but are inconsistent with the fractionation model the rhum chamber is thought to have been replenished with a picritic liquid from which olivine crystallized while held in suspension however replenishment by a highly olivine phyric basalt is also possible the peridotite probably accumulated rapidly as olivines were dumped out of suspension onto the chamber floor the lower part of the peridotite is a poikilitic adcumulate it is suggested that this formed by convective circulation of melt in the pores of the pile of cumulus olivines in the latter stages of adcumulus growth more fe rich and isotopically contaminated magma entered the top of the cumulus pile causing cumulus olivines to re equilibrate and giving the intercumulus plagioclase a higher sr isotope ratio than lower down the olivines in the allivalite show steep stratigraphic gradients in major element composition but not in their ni content they also show substantial variation in major element composition laterally within the allivalite it is suggested that these features are a consequence of postcumulus re equilibration of olivine with migrating intercumulus magma

the triple group and the platinova gold and palladium reefs in the skaergaard intrusion stratigraphic and petrographic relations this paper describes the setting structure and textural relations of the platinova reefs in the skaergaard intrusion the reefs comprise a strata bound occurrence of au pge estimated at 91 million tons of rock with 1 8 ppm au and similar amounts of pd the precious metals occur in two major zones a lower major pd reef which is concordant with the modal layering and followed by at least ten minor pd reefs also concordant and an au concentration which fringes the pd reefs outward and upward through the triple group units the major pd concentration and the au concentration are separated in a systematic way across the intrusion near the center of the intrusion the au concentration occurs some 60 m higher in the stratigraphy than the main pd reef but the separation decreases gradually outward being only a few meters near the margins the dominant precious metal minerals are small grains of cu fe au pd pt alloys which typically are included in interstitial sulfides bornite digenite and smaller amounts of chalcopyrite the textural relations suggest a close genetic association between the sulfides and au pge minerals an association which is confirmed by whole rock compositional data our observations demonstrate that the platinova reefs formed after significant concentration of the au pge and s in the residual magma during fractional crystallization before it became saturated with respect to a sulfide fe cu s o liquid this sulfide liquid acted as a collector of au pge and settled into the layered series cumulates we propose that the structure of the deposit evolved through fractional crystallization of the au pge bearing sulfide liquid as it was filter pressed along with the interstitial silicate melt upward into the overlying unconsolidated cumulates the fractional crystallization resulted in an evolution of the au pge bearing phases with progressive fractionation from pd rich alloys at the base of the deposit to au rich alloys at the top we argue that the sulfides evolved to their present extreme cu rich fe poor composition during oxidation desulfurization reactions in the same event which oxidized the iron titanium oxides of the intrusion this oxidation desulfurization probably had no overall effect on the distribution of au and pge

sulphide melt and aqueous fluid saturation in the pge au mineralisation of the skaergaard intrusion evidence from melt inclusions the skaergaard intrusion in east greenland hosts mineralisation of platinum group elements and gold that is characterised by a remarkably low amount of sulphides about 0 02 wt sulphide and a very high metal sulphur ratio here we present the first quantitative analyses of sulphur s copper cu lithium li and other trace elements from melt inclusions in plagioclase to trace their evolution in the magma and to constrain the formation of the mineralisation the concentration of these elements in the melt inclusions varies considerably within each sample the s content in melt inclusions ranges from 165 to 320 ppm in the lower part of the intrusion but increases to 349 512 ppm immediately below the mineralisation interval in the same interval the li content increases from 22 43 to 40 69 ppm and cu ranges from 118 to 1586 ppm with no systematic variations across the mineralisation the li content drops an order of magnitude to 4 7 ppm and s drops to 250 326 ppm we explain these compositional changes as reflecting the magmatic evolution as the result of fractional crystallisation below and coinciding aqueous fluid exsolution and sulphide saturation across the mineralisation normally the s content of basaltic magmas is much higher at sulphide saturation we propose that sulphide saturation is possible nevertheless mostly due to the high cu content and high cu fe in the evolved ferrobasaltic melt resulting in a lowering of the s concentration required for sulphide saturation the relationship between exsolutions of aqueous fluids and sulphides remains unknown

rates of thermal and chemical evolution of magmas in a cooling magma chamber a chronological and theoretical study on basaltic and andesitic lavas from rishiri volcano japan rates of magmatic processes in a cooling magma chamber were investigated for alkali basalt and trachytic andesite lavas erupted sequentially from rishiri volcano northern japan by dating of these lavas using u th radioactive disequilibrium and c dating methods in combination with theoretical analyses we obtained the eruption age of the basaltic lavas to be 29 3 0 6 ka by c dating of charcoals the eruption age of the andesitic lavas was estimated to be 20 2 3 1ka utilizing a whole rock isochron formed by u th fractionation as a result of degassing after lava emplacement because these two lavas represent a series of magmas produced by assimilation and fractional crystallization in the same magma chamber the difference of the ages i e 9 kyr is a timescale of magmatic evolution the thermal and chemical evolution of the rishiri magma chamber was modeled using mass and energy balance constraints as well as quantitative information obtained from petrological and geochemical observations on the lavas using the timescale of 9 kyr the thickness of the magma chamber is estimated to have been about 1 7 km the model calculations show that in the early stage of the evolution the magma cooled at a relatively high rate 40 18c year and the cooling rate decreased with time convective heat flux from the main magma body exceeded 2w m when the magma was basaltic and the intensity diminished exponentially with magmatic evolution volume flux of crustal materials to the magma chamber and rate of convective melt exchange compositional convection between the main magma and mush melt also decreased with time from 0 1m year to 10 3 m year and from 1m year to 10 2 m year respectively as the magmas evolved from basaltic to andesitic compositions although the mechanism of the cooling i e thermal convection and or compositional convection of the main magma could not be constrained uniquely by the model it is suggested that compositional convection was not effective in cooling the main magma and the magma chamber is considered to have been cooled by thermal convection in addition to heat conduction

magma chamber processes in the formation of the low sulphide magmatic au pge mineralization of the platinova reef in the skaergaard intrusion east greenland

dynamics of dikes versus cone sheets in volcanic systems igneous sheet intrusions of various shapes such as dikes and cone sheets coexist as parts of complex volcanic plumbing systems likely fed by common sources how they form is fundamental regarding volcanic hazards yet no dynamic model simulates and predicts satisfactorily their diversity here we present scaled laboratory experiments that reproduced dikes and cone sheets under controlled conditions our models show that their formation is governed by a dimensionless ratio 1 which describes the geometry of the magma source and a dynamic dimensionless ratio 2 which compares the viscous stresses in the flowing magma to the host rock strength plotting our experiments against these two numbers results in a phase diagram evidencing a dike and a cone sheet field separated by a sharp transition that fits a power law this result shows that dikes and cone sheets correspond to distinct physical regimes of magma emplacement in the crust for a given host rock strength cone sheets preferentially form when the source is shallow relative to its lateral extent orwhen the magma influx velocity or viscosity is high conversely dikes form when the source is deep compared to its size or when magma influx rate or viscosity is low both dikes and cone sheets may form fromthe same source the shift fromone regime to the other being then controlled by magma dynamics i e different values of 2 the extrapolated empirical dike to cone sheet transition is in good agreement with the occurrence of dikes and cone sheets in various natural volcanic settings

the relative roles of boundary layer fractionation and homogeneous fractionation in cooling basaltic magma chambers abstract in a cooling magma chamber magmatic differentiation can proceed both by fractionation of crystals from the main molten part of the magma body homogeneous fractionation and by mixing of the main magma with fractionated melt derived from low temperature mush zones boundary layer fractionation in this study the relative roles of boundary layer fractionation and homogeneous fractionation in basaltic magma bodies were examined using a thermodynamics based mass balance model model calculations show that boundary layer fractionation cannot be a dominant fractionation mechanism when magma chambers are located at low pressures 2 wt such as arc basalt and the magma chambers are located at depth 100 mpa because the melt derived from mush zones is enriched in alkalis and h 2 o crystallization from the main magma is suppressed by mixing with the mush melt as a consequence of depression of the liquidus temperature therefore homogeneous fractionation is more effectively suppressed in magma chambers in which boundary layer fractionation is more active if magmatic differentiation proceeds primarily by boundary layer fractionation magmas can remain free of crystals for long periods during magmatic evolution

the salton sea scientific drilling project in march 1986 a research borehole called the state 2 14 reached a depth of 3 22 km in the salton sea geothermal system of southern california this was part of the salton sea scientific drilling project sssdp the first major i e multimillion dollar research drilling project in the u s continental scientific drilling program the principal goals of the project were to investigate the physical and chemical processes of a high temperature high salinity magmatically driven hydrothermal system the borehole encountered temperatures of up to 355 c and produced metal rich alkali chloride brines containing 25 wt of total dissolved solids the rocks penetrated exhibit metamorphism and ore genesis in action they show a progressive transition from unconsolidated lacustrine and deltaic sediments to hornfelses with lower amphibolite facies mineralogy accompanied by pervasive veins containing iron copper lead and zinc ore minerals the sssdp included an intensive program of rock and fluid sampling flow testing and downhole logging and scientific measurement the purpose of this paper is to introduce this special section of the journal of geophysical research on the sssdp to describe briefly the background of the project and the drilling and testing of the borehole to summarize the initial scientific results and to discuss how the lessons learned are applicable to future scientific drilling projects

chemical mass transfer modelling of ore forming hydrothermal systems current practise and problems abstract chemical mass transfer modelling is a numerical approach to predicting the progress of multicomponent fluid rock reactions in natural hydrothermal systems based on thermodynamic data for minerals and fluid species quantitative simulations of hydrothermal wall rock alteration and ore mineral deposition can be used to develop and test geological models for ore formation which are a basis for mineral exploration and resource assessment in practice the application of thermodynamic mass transfer modelling to specific ore forming systems faces two main problems even for the limiting assumption of local equilibration between fluids and minerals the first problem relates to the geometry of fluid rock interaction where the scheme of the model must be adjusted to the geological question of interest one dimensional reactor models defining reaction progress as a function of integrated fluid flux along a path through a chemically reactive rock do not necessarily provide a more realistic description of chemical mass transfer than simple titration models without temporal and spatial dimension because in most ore depositing systems the fluids are partially channelized by structures like veins or faults the efficiency of metal extraction at the ore deposition site is commonly determined by a sensitive balance between fluid focusing on one hand and wall rock reaction promoting ore mineral precipitation on the other an example of gold and scheelite mineralization associated with mesothermal quartz veins illustrates how ore deposition and alteration zoning parallel to the main fluid flow direction in partially channelized systems can be described by a two dimensional multibox model the second problem relates to the extent and quality of thermodynamic data explicit consideration of fe bearing silicate solid solutions is essential for predicting realistic mass balance in complex fluid silicate sulphide reactions associated with hydrothermal ore formation although a high level of internal consistency of multicomponent data sets has been achieved by simultaneous fitting of multiple thermodynamic and equilibrium measurements significant inconsistencies persist among the most comprehensive data compilations published for the three main groups of species relevant to hydrothermal ore deposits aqueous species in high temperature aqueous brines silicate oxide carbonate minerals including fe mg silicates and sulphur bearing minerals including sulphides and sulphates discrepancies between thermodynamic predictions and observed brine composition and mineral assemblages in the salton sea geothermal system suggest that the redox scale for rock forming silicates may be inconsistent with that of sulphides and aqueous s c o h species this analysis is preliminary but illustrates the importance of field testing of thermodynamic data particularly for hydrothermal conditions between 150 and 450 c which lie between the high temperatures of reliable equilibrium experiments among rock forming minerals 500 c and the low temperatures of many experiments defining the thermodynamics of aqueous species mostly

the bushveld complex south africa formation of platinum palladium chrome and vanadium rich layers via hydrodynamic sorting of a mobilized cumulate slurry in a large relatively slowly cooling subsiding magma chamber platinum group element pge deposits in the bushveld complex and other layered intrusions form when large incompletely solidified magma chambers undergo central subsidence in response to crustal loading resulting in slumping of semi consolidated cumulate slurries to the centres of the intrusions and hydrodynamic unmixing of the slurries to form dense layers enriched in sulfides oxides olivine and pyroxene and less dense layers enriched in plagioclase the most economic pge cr and v reefs form in large multiple replenished intrusions because these cool relatively slowly and their central portions subside prior to termination of magmatism and complete cumulate solidification the depth of emplacement has to be relatively shallow as otherwise ductile crust would not be able to flex and collapse in smaller intrusions cooling rates are faster subsidence is less pronounced and where it occurs the cumulate may be largely solidified resulting in insignificant mush mobility and mineral sorting layering is thus less pronounced and less regular and continuous and the grades of the reefs are lower but the reefs can be relatively thicker an additional factor controlling the pge cr and v prospectivity of intrusions is their location within cratons intra cratonic environments offer more stable emplacement conditions that are more amenable to the formation of large layered igneous bodies furthermore intrusions sited within cratons are more readily preserved because cratons are underlain by thick buoyant keels of harzburgite that prevent plate tectonic recycling and destruction of crust

constraints on the formation of cyclic units in ultramafic zones of large basaltic chambers petrological models for the formation of cyclic units in ultramafic zones at the base of large mafic layered intrusions are still few and simple in this study we develop simple physical constraints such as the volume balance and density relationships between the various liquids involved we consider the formation of an entire ultramafic zone made of n cyclic units due to n successive reinjections we tackle the following problems are all the injections of the same chemical composition are the n injections responsible for the formation of the whole magma chamber two end member models are examined in the first one the chamber grows with each injection and does not lose any magma variable volume model vv in the second one the chamber initially forms with a large volume of magma and remains at a constant volume with subsequent reinjections of small volumes balanced by eruptions of equal volume fixed volume model fv several scenarios for the formation of a cyclic unit are envisioned petrological data such as thicknesses of the cumulate layers and their compositions together with the fractionation density of the cumulate phases allow the calculation of the density evolution as a function of the initial density only thus the model yields constraints on the density of the initial liquid and hence on its chemical composition the volumes of the injected magmas are calculated as well as the evolution of the densities of the different involved liquids we apply the calculation to the specific case of the ultramafic series of the stillwater complex the multiple reinjection models require that the initial liquids densities vary between 2 71 and 2 72 g cm3 corresponding to mgo rich liquids for which there is no field evidence either in dikes or in chilled margins they also require that the source produces liquids of different chemical composition thus the evolution of the igneous system requires another larger reservoir where differentiation takes place prior to injection into the shallower one contrary to the multiple reinjection models a closed system model where crystallization proceeds in an isolated bottom layer does not require any specific value of density for the liquids provided of course that it is of basaltic composition on the negative side such a model is not supported by any physical processes which would account for the formation of cyclic units

halogens and trace metal emissions from the ongoing 2008 summit eruption of kīlauea volcano hawai i volcanic plume samples taken in 2008 and 2009 from the halema uma u eruption at k lauea provide new insights into k lauea s degassing behaviour the cl f and s gas systematics are consistent with syn eruptive east rift zone measurements suggesting that the new halema uma u activity is fed by a convecting magma reservoir shallower than the main summit storage area comparison with degassing models suggests that plume halogen and s composition is controlled by very shallow 77 as gaseous elemental mercury at the point of emission sulphate is an important aerosol component modal particle diameter 0 44 m aerosol halide ion concentrations are low compared to other systems consistent with the lower proportion of gaseous hydrogen halides plume concentrations of many metallic elements rb cs be b cr ni cu mo cd w re ge as in sn sb te tl pb mg sr sc ti v mn fe co y zr hf ta al p ga th u la ce pr nd sm eu gd tb dy er tm are elevated above background air there is considerable variability in metal to so2 ratios but our ratios generally at the lower end of the range previously measured at k lauea support assertions that k lauea s emissions are metal poor compared to other volcanic settings our aerosol re and cd measurements are complementary to degassing trends observed in hawaiian rock suites although measured aerosol metal s ratios are about an order of magnitude lower than those calculated from degassing trends determined from glass chemistry plume enrichment factors with respect to hawaiian lavas are in broad agreement with those from previous studies allowing similar element classification schemes to be followed i e lithophile elements having lower volatility and chalcophile elements having higher volatility the proportion of metal associated with the largest particle size mode collected 2 5 m and that bound to silicate is significantly higher for lithophiles than chalcophiles many metals show higher solubility in ph 7 buffer solution than deionised water suggesting that acidity is not the sole driver in terms of solubility nonetheless many metals are largely water soluble when compared with the other sequential leachates suggesting that they are delivered to the environment in a bioavailable form preliminary analyses of environmental samples show that concentrations of metals are elevated in rainwater affected by the volcanic plume and even more so in fog however metal levels in grass samples showed no clear enrichment downwind of the active vents

compositional order disorder and stable isotope characteristics of al fe epidote state 2 14 drill hole salton sea geothermal system epidote ca2fe3si3o12 oh ca2al3si3o12 oh is a common hydrothermal mineral in metasediments and veins at depths 900 m in the state 2 14 drill hole of the salton sea geothermal system the mole fraction of ca2fe3si3o12 oh in epidotes xps from this drill hole ranges from 0 11 to 0 42 and complex compositional zoning of octahedral fe3 and al is typical within single grains with increasing depth there is an overall but irregular decrease in the fe3 content of epidotes in metasandstones in most samples vein epidotes are more fe3 rich and exhibit a wider compositional range than metasandstone epidotes from the same depth octahedral fe3 in the m 1 sites in four epidotes evaluated by 57fe mossbauer spectroscopy ranges from 7 5 1 7 to 11 4 1 5 of the total iron the most ordered epidote formed in veins at 2618 m in the biotite zone this epidote has an ordering parameter 1 2xfe3 m 1 of 0 85 0 03 which corresponds to a calculated state of equilibrium order disorder at 390 60 c in close agreement with the probable downhole temperature of 340 c two epidote samples from the chlorite calcite zone 1420 m 265 c 1867 m 300 c are more disordered corresponding to calculated states of equilibrium order disorder of 450 c these data suggest that the analyzed epidotes from the chlorite calcite zone are in a metastable state of substitutional order disorder the measured value of depidote from 1420 m is 96 which is 5 6 lighter than epidotes from 1867 m depidote 90 2227 m depidote 91 and 2618 m depidote 90 when compared to the reservoir fluid from the december 1985 flow test the derived fractionation of hydrogen isotopes between epidote and the geothermal brine is 19 at 1867 m and 340 c this value is in accord with published experimental hydrogen isotope fractionations between iron rich epidote and aqueous electrolyte solutions therefore considering the complexity of the salton sea brine the isotopic temperature is similar to the inferred downhole temperature

mechanisms of differentiation in the skaergaard intrusion thanks to its magnificent exposures and extraordinary petrologic development the skaergaard intrusion provides an excellent test for three basic mechanisms of crystal liquid fractionation diffusive exchange compaction convective exchange 1 diffusive exchange at the solidification front does not seem to have played a major role for no obvious difference is seen in the behavior of components of widely differing diffusivities the effects of diffusion were probably masked by porous flow of the interstitial liquid 2 compaction and 9filter pressing9 of residual liquids can be tested by comparing concentrations of incompatible elements in parts of the intrusion that crystallized in different orientations with respect to gravity the contrast between the large concentrations of these elements in the upper border series and consistently smaller abundances in the layered series long thought to be due to contamination of the upper levels with crustal rocks is more likely a result of compaction that took place at the floor but not under the roof 3 convective exchange driven by differences in the compositional densities of the main magma and interstitial liquids seems to have been important at the steep walls and to a lesser degree under the roof it may also have been responsible for a large compositional anomaly near the centre of the layered series where an iron rich liquid percolated down through parts of lower and middle zones at later stages of crystallization when density relations were reversed a buoyant liquid rich in volatiles and incompatible elements rose from the layered series and permeated the upper part of the intrusion a liquid of this latter kind could have caused the pervasive metasomatic alteration seen in much of the layered series in both compaction and convective exchange re equilibration and reactions between migrating liquids and early formed crystals augmented permeabilities and brought about extensive compositional and textural changes although inhomogeneities in the initial magma could account for some of the compositional variations the main trend of differentiation can be ascribed to a combination of compaction and convective fractionation the convective flow was of two kinds one of descending iron rich liquids and another of more evolved rising liquids and fluids with low densities and large concentrations of incompatible elements in the early stages the proportions of iron rich liquids were much greater than those of the silica rich variety and until the crystallization fronts converged at the sandwich horizon the former made the greatest contribution to evolution of the main body of magma as crystallization advanced however light silica rich liquids became more important they permeated the upper border series and much of the upper part of the layered series this late stage liquid continued to differentiate after the crystallization fronts met a secondary sandwich horizon was formed where incompatible elements reach maximum concentrations in the lower part of the upper border series

ground based measurements of spatial and temporal variability of snow accumulation in east antarctica the east antarctic ice sheet is the largest highest coldest driest and windiest ice sheet on earth understanding of the surface mass balance smb of antarctica is necessary to determine the present state of the ice sheet to make predictions of its potential contribution to sea level rise and to determine its past history for paleoclimatic reconstructions however smb values are poorly known because of logistic constraints in extreme polar environments and they represent one of the biggest challenges of antarctic science snow accumulation is the most important parameter for the smb of ice sheets smb varies on a number of scales from small scale features sastrugi to ice sheet scale smb patterns determined mainly by temperature elevation distance from the coast and wind driven processes in situ measurements of smb are performed at single points by stakes ultrasonic sounders snow pits and firn and ice cores and laterally by continuous measurements using ground penetrating radar smb for large regions can only be achieved practically by using remote sensing and or numerical climate modeling however these techniques rely on ground truthing to improve the resolution and accuracy the separation of spatial and temporal variations of smb in transient regimes is necessary for accurate interpretation of ice core records in this review we provide an overview of the various measurement techniques related difficulties and limitations of data interpretation describe spatial characteristics of east antarctic smb and issues related to the spatial and temporal representativity of measurements and provide recommendations on how to perform in situ measurements

komatiites of the wildara leonora belt yilgarn craton wa the missing link in the kalgoorlie terrane abstract the wildara leonora belt in the kalgoorlie terrane is located between the world class nickel provinces of the agnew wiluna belt to the north and the kambalda domain to the south it extends from the wildara station to the gold mining center of leonora and consists of a deformed package of mafic komatiitic and sedimentary rocks this package of rocks which hosts a number of small nickel sulfide occurrences has been explored extensively for komatiite hosted nickel deposits most recently within the sullivans unit in the south east of the wildara leonora belt lithostratigraphic similarities together with shared physical geochemical and textural properties support a common affiliation between the wildara leonora and the agnew wiluna belts on this evidence the wildara leonora belt represents the southern extension of the largest komatiite hosted nickel camp in the world the apparently lesser prospectivity for nickel sulfide deposits is interpreted as a consequence of contrasting volcanic facies assemblages komatiites in the wildara leonora belt present geochemical and volcanological characteristics consistent with turbulent lava flow emplacement however compared with the agnew wiluna belt the wildara leonora belt komatiites are not spatially associated with felsic volcanic units but rather overlie a thick sequence of pillowed basalts and sulfidic sediments the lower proportion of adcumulate greater degrees of fractionation and lower degrees of contamination are consistent with komatiites in the wildara leonora belt having been emplaced in a more distal position to the rift environment with lower magma flux and more tranquil emplacement than the agnew wiluna belt komatiites and consequently having lower potential for giant nickel sulfide deposits such as those at perseverance and mt keith

pyroxene oikocrysts in troctolitic cumulates evidence for supercooled crystallisation and postcumulus modification studies of pyroxene oikocrysts in plagioclase olivine cumulates crystallised from high alumina olivine tholeiite magmas in layered intrusions inferred to represent periodically replenished magma chambers show that small plagioclases enclosed within the oikocrysts differ significantly from plagioclases in the host rock plagioclases within oikocrysts are very elongate with a high surface area show extensive mutual attachment at high angles resembling skeletal growth and tend to be randomly orientated some are rounded and embayed are reversely zoned and more sodic by an 5 10 than some of the plagioclase in the host rock plagioclase crystal centres tend to be closer spaced in the oikocrysts than in the host rock in contrast plagioclases in the host rock are discrete coarser grained less elongate strongly aligned and locally polygonised

analysis of geophysical well logs obtained in the state 2 14 borehole salton sea geothermal area california a complete suite of conventional geophysical well logs was obtained in the upper part of a 3220 m deep borehole drilled into geothermally altered alluvial sediments on the southeastern edge of the salton sea in situ temperatures greater than 300 c and an inability to cool parts of the borehole by circulation limited the suite of logs run below 2000 m in depth to deep induction spontaneous potential un calibrated neutron natural gamma and temperature bottom hole temperature trends given by repeat temperature logs were extrapolated to undisturbed temperatures approaching 355 c at a depth of 3220 m geophysical logs obtained in the state 2 14 borehole indicate that neutron porosity gamma gamma and deep induction logs provide useful information on lithologic trends with depth the natural gamma log contains almost continuous high frequency fluctuations that obscure lithologic trends and that may be related to recent radioisotope redistribution and departure from radiometric equilibrium acoustic transit time logs give unrealistically low in situ compressional velocities ranging from 1 8 to 3 0 km s whereas acoustic waveform logs indicate that sediment compressional velocities range from less than 3 0 km s shallower than 1000 m in depth to almost 5 0 km s at depths greater than 2000 m analyses indicate that most log values lie between two lithologic end points an electrically conductive claystone with moderate neutron porosity but no effective porosity and an electrically nonconductive fully cemented siltstone that has small but finite porosity a limited number of clean sandstones depart from this trend geophysical logs from these sandstones indicate an effective porosity ranging from 5 to 14 and saturation with brines having equivalent nacl concentrations greater than 100 000 mg l depth averaged trends in neutron porosity and deep induction logs along with trends in acoustic velocity determined from acoustic waveform logs demonstrate that major changes in the properties of alluvial sediments occur within the depth range from 1200 to 1800 m although caliper logs were not obtained deeper than 2000 m resistivity values less than 10 ohm m at those depths probably correspond to borehole enlargements in production zones rather than local increases in effective porosity the transition in sediment properties indicated by the geophysical logs in the depth interval from 1200 to 2000 m apparently represents a detailed vertical profile of the transition from relatively unaltered clay minerals in alluvial sediments to electrically nonconductive alteration products such as epidote and feldspar

the skaergaard pge and gold deposit the result of in situ fractionation sulphide saturation and magma chamber scale precious metal redistribution by immiscible fe rich melt the skaergaard intrusion greenland is the type locality for skaergaard type mineralizations mineralization levels are perfectly concordant with igneous layering up to 5 m thick internally fractionated and contain crystallized sulphide droplets and precious metal alloys sulphides arsenides and telluride immiscible cu rich sulphide droplets formed in a mush zone below the roof scavenged precious metals they were subsequently dissolved and transported to the floor in late formed immiscible fe rich mush melts mineralized stratigraphic intervals of floor gabbro formed in proto macrolayers owing to local sulphide saturation in melt concentrated between floating plagioclase and sinking clinopyroxene the floor mineralization is divided into four stratigraphic sections formation of the lower platinum group element mineralization lpgem involved 1 crystallization of the bulk liquid liquidus paragenesis and in situ fractionation 2 sulphide saturation and formation of sulphide droplets in melt in the upper part of proto macrolayers after further in situ fractionation the following steps occurred 3 the onset of silicate silicate immiscibility and the consequent loss of buoyant and immiscible si rich melt 4 dissolution of unprotected droplets of sulphide melt present in the fe rich mush melt 5 compaction driven upwards loss of residual mush melt enriched in for example au the lpgem preserves upward increasing bulk pd pt 6 13 owing to a continued supply of pge and au with high pd pt the further development of the lpgem ceased as the supply of precious metals to the floor waned the upper pge mineralization upgem subsequently formed from precious metals recycled in the floor the upgem is characterized by increasing au substitution in pge phases and a decrease in total pge and pd pt owing to upward fractionation in migrating mush melts and exhaustion of pd and pt an upper au rich mineralization level uaum was caused by late remobilization of au and deposition on grain boundaries in fully crystallized gabbro cu concentrations 150 ppm are not correlated with pge and au repeated cu mineralization levels cum attaining 1000 ppm occur above the au levels caused by local mush layer sulphide saturation pge au and cu distributions in the floor mineralization reflect sub liquidus but supra solidus processes and reactions in mushes at the roof wall and floor constraints provided by a new model for the mineralization provide the basis for re evaluation of the solidification processes in the skaergaard intrusion we have identified the importance of extensive in situ fractionation and intrusion wide elemental redistributions in immiscible fe and si rich silicate melts our model characterizes the floor cumulates as bulk liquid orthocumulates containing an upwards increasing proportion crystallized from fe rich immiscible mush melt the roof rocks are complementary to the floor with downwards increasing proportions crystallized from the conjugate si rich melt petrographic observations and the relative timing of crystallization support the hypothesis that crystallization was restricted to marginal mush zones bulk melt remaining in the magma chamber evolved not as generally assumed as a result of loss of crystals grown from the bulk melt but as the consequence of mixing with recycled and evolved melt expelled from the mush by compaction redistribution of fe in immiscible melts may be common to mafic intrusions and puts into question the validity of petrogenetic modelling of bulk liquids in mafic intrusions based only on consideration of floor cumulates

cotectic precipitation of olivine and sulfide liquid from komatiite magma and the origin of komatiite hosted disseminated nickel sulfide mineralization at mount keith and yakabindie western australia an empirical equation for the relationship of sulfur solubility to silicate magma composition and temperature has been combined with numerical simulations of the fractional crystallization of olivine from komatiite magma to predict the relative proportions of olivine and sulfide liquid which should segregate at equilibrium from a fractionating komatiite the predicted cotectic ratio of olivine to sulfide is higher and the s content of the cumulate assemblage lower by a factor of about three compared with the peak observed in type ii mount keith type dunite hosted disseminated ni sulfide deposits for the observed range of olivine compositions in the dunites this implies that a purely cotectic precipitation model for this deposit type is incorrect and that most of the sulfide liquid found in type ii deposits is transported and mechanically deposited

sulfides native metals and associated trace minerals of the skaergaard intrusion greenland evidence for late hydrothermal fluids sulfide assemblages precious metals transition metal alloys and associated accessory phases were characterized throughout the skaergaard intrusion to better constrain the sulfide saturation history of the intrusion and the role of late magmatic volatiles in modifying the skaergaard metal budget and distribution sulfides in and below the middle zone of the layered series of the intrusion are readily replaced by low ti magnetite the ratio low ti magnetite mode sulfide mode indicating oxidation of sulfides reaches maximum values in the lower zone of the layered series sulfide assemblages below the middle zone are typically accompanied by minor biotite apatite and rare calcite as well as trace compositionally distinctive clinopyroxene and orthopyroxene the occurrence of ag au pt cu and metal alloys outside of the middle zone is further evidence of the skaergaard intrusion parental magma being s poor native ag commonly accompanied by trace amounts of cl occurs both in and below the middle zone evidence of coexisting precious metal brine assemblages exists where native metals are accompanied by sylvite halite and ag is accompanied by ag halides ag occurrences in the middle zone are of irregular morphology with trace cl s calcite further evidence supportive of a metal brine assemblage is observed where ag quartz is found in an apparent open clinopyroxene hosted fluid inclusion consisting of na si cl ca k and s ag is used to model the behavior of precious and transition metals in the presence of an exsolving fluid phase numerical modeling suggests that in a sulfide bearing system residual ag concentrations and concentrations in the exsolved fluid are most affected at the point where sulfide is lost to a separating volatile fluid phase it is suggested that owing to the low s nature of the skaergaard system fractional crystallization and early fluid saturation produced enrichment of ag with other precious and transition metals in the interstitial silicate liquid much higher than normal due to delayed sulfide saturation as this interstitial liquid evolved ag was lost to an exsolved volatile phase of high salinity and migrated upward a similar process likely occurred for au and other elements with high affinities for cl

peridotite sills and metasomatic gabbros in the eastern layered series of the rhum complex mapping of the eastern layered series els of the rhum ultrabasic complex on the northern flank of hallival shows that peridotite and allivalite troctolite or gabbro layers are laterally discontinuous and vary both in thickness and lithology peridotite generally has sharp upper and lower contacts against the allivalites which sometimes cut across the layering in the allivalite reaction dissolution and hybridization effects between peridotite and allivalite are developed locally some troctolite layers terminate as isolated fingered blocks in peridotite there are many small peridotite bodies which are clearly intrusive into allivalite and have previously been identified as distinct peridotite sheets and plugs they are petrographically almost identical to the major stratiform peridotites and in some cases are apophyses from them we propose that many of the peridotite layers in the els formed as thick sills of picritic magma emplaced into a partly solidified layered troctolite complex the stratiform gabbros of the els are heterogeneous layered rocks that commonly contain relicts of troctolite and anorthosite wavy metre scale contacts between gabbro and troctolite cut across pre existing grain size modal and rhythmic layering with little disruption these metasomatic gabbros mimic the textures grain size and rhythmic layering of their troctolitic protoliths we propose that many of the els gabbros formed as a result of interaction between porous troctolites and a low temperature basaltic melt residual basaltic melt segregated from solidifying peridotite may have caused this metasomatism

postmagmatic sulphur loss in the skaergaard intrusion implications for the formation of the platinova reef abstract the stratabound platinova au pd reef in the skaergaard intrusion is intimately linked to cu fe sulphide minerals and occurs at a level that shows changes in the sulphur concentration and the cu s ratio these features suggest that the reef is related to silicate sulphide liquid immiscibility in the skaergaard magma however although the evidence is strong present day sulphur concentrations are too low to support such a model the detailed knowledge of the shape volume and compositional relations of the skaergaard lithologies allows for numerical modeling of sulphur and copper concentrations during fractionation of the magma these data can be evaluated with the current models for sulphur saturation in iron rich mafic magmas to assess the original sulphur concentrations the extent of fractionation the composition of sulphide liquids and the extent of postmagmatic sulphur loss mass balance modeling yields initial copper and sulphur concentrations of 250 and 894 ppm for the parental magma to the lower zone a prior to sulphur saturation cu behaves as an incompatible element with a bulk partition coefficient in the cumulus material dcumuluscu of 0 13 indicating some incorporation of cu into mafic silicates probably augite sulphur can be assumed to be perfectly incompatible in the cumulus minerals although it will be included as a component of the interstitial liquid these values yield sulphur saturation at around 635 ppm cu and 2607 ppm s assuming the sulphur to originally be present as a mixture of monosulphide mss and intermediate solid solution sulphide iss the primary sulphides in the cumulates would amount to 0 07 modal through lower and middle zones and 0 53 modal on average through upper zone the expected weight proportion of primary chalcopyrite pyrrhotite evolves from 30 70 at the base of lza through 26 74 at the level of the platinova reef to 18 82 at the sandwich horizon the present day bornite magnetite mineral assemblage is likely to have been derived from a primary pyrrhotite chalcopyrite assemblage through reequilibration and postmagmatic oxidation significant sulphur appears to have been lost from the lower and middle parts of the layered series lower zone a to upper zone b whereas the sulphides of the upper zone c can be explained in the context of a sulphur gain local scale reequilibration of cu from mafic silicates into sulphides may have contributed to the sulphide mineralogy of the lower and middle zones the structure of the platinova reef probably formed from an original single stratabound reef sulphur loss through oxidation in upward migrating deuteric or hydrothermal fluids could have caused dissolution and reprecipitation of pd and au in a cumulate with several stratiform redox barriers this process can thus explain the successive pd peaks of the reef as well as the variable stratigraphic offset of pd and au across the intrusion