viscosity of liquid fayalite up to 9 gpa abstract the viscosity of liquid fayalite fe 2 sio 4 was determined up to 9 2 gpa and 1850 c using in situ falling sphere viscometry and x ray radiography imaging the viscosity of liquid fayalite was found to decrease both along the melting curve and an isotherm therefore temperature is thought to have little effect on liquid fayalite viscosity at high pressure the results are in contrast with previous studies on depolymerised silicate melts which found viscosity to increase with pressure in accordance with recent in situ structural measurements on liquid fayalite the viscosity decrease is likely a result of the increase in fe o coordination with pressure the results show that liquid silicate viscosities need to be considered on an individual basis and can be strongly dependent on the melt structure and composition this has important implications for models of planetary differentiation in particular terrestrial bodies with high fe contents and reducing mantle conditions are likely to have had very mobile melts at depth

reply to the comments by marignac and cathelineau on the paper by sanchez espana et al source and evolution of ore forming hydrothermal fluids in the northern iberian pyrite belt massive sulphide deposits sw spain evidence from fluid inclusions and stable isotopes mineralium deposita 38 519 537

self consistent thermodynamic description of silicate liquids with application to shock melting of mgo periclase and mgsio3 perovskite summary we develop a self consistent thermodynamic description of silicate liquids applicable across the entire mantle pressure and temperature regime the description combines the finite strain free energy expansion with an account of the temperature dependence of liquid properties into a single fundamental relation while honouring the expected limiting behaviour at large volume and high temperature we find that the fundamental relation describes well previous experimental and theoretical results for liquid mgo mgsio3 m g2sio4 and sio2 we apply the description to calculate melting curves and hugoniots of solid and liquid mgo and mgsio3 for periclase we find a melting temperature at the core mantle boundary cmb of 7810 160 k with the solid hugoniot crossing the melting curve at 375 gpa 9580 k and the liquid hugoniot crossing at 470 gpa 9870 k for complete shock melting of periclase we predict a density increase of 0 14 g cm 3 and a sound speed decrease of 2 2 km s 1 for perovskite we find a melting temperature at the cmb of 5100 100 k with the perovskite section of the enstatite hugoniot crossing the melting curve at 150 gpa 5190 k and the liquid hugoniot crossing at 220 gpa 5520 k for complete shock melting of perovskite along the enstatite principal hugoniot we predict a density increase of 0 10 g cm 3 with a sound speed decrease of 2 6 km s 1

controls on the siting and style of volcanogenic massive sulphide deposits abstract volcanogenic massive sulphide vms deposits form in subaqueous environments from circulating hydrothermal fluids heated by volcanic activity these deposits form as sulphide mounds stratiform exhalative and or replacive bodies and commonly have stockwork vein mineralization in their immediate footwall these various styles are essentially facies of mineralization each one being the product of a particular set of conditions that control the ore forming processes and the consequent geometry and architecture style of the deposits these controls include the physical and or chemical nature of the host rocks the temperature and composition of the hydrothermal fluids and the redox state of the depositional environment the style of exhalative deposits is controlled by the salinity of the vented fluids and the redox state at the seafloor hydrothermal fluids with salinities less than twice that of seawater that vented into open oxic oceanic environments typically formed small mound and chimney complexes unless they were rapidly covered by sediments or volcanic rocks the massive sulphides were rapidly oxidized and partly dissolved by seawater in contrast stratiform sheet like deposits are typically formed in anoxic bottom waters anoxic marine conditions were periodically of global extent particularly prior to 2 4 ga or of a regional nature local anoxic conditions can also be self induced by the exhalation of saline and reducing hydrothermal fluids that ponded in bathymetric depressions such as second or third order basins to form a brine pool these exhalative systems may have been initiated as chimney vent complexes and subsequently overlain by stratiform sulphides formed under the self induced anoxic conditions deposits formed in anoxic environments can be significantly larger than those in oxic settings and this is attributed to several factors that include longer lived hydrothermal circulation more efficient sulphide precipitation and reduced or inhibited oxidation thereof replacement of volcanic and sedimentary strata by sulphide typically occurs within the feeder zones beneath the exhalative mineralization however successions with abundant porous permeable and or reactive rocks such as glassy and or pumiceous volcaniclastic rocks and in some cases limestone favoured the development of large replacive deposits that may have had little surficial expression on the sea floor vms deposits at spreading centres within oceanic crust formed almost exclusively as mounds most of them have not been preserved likely due to oxidation of the sulphides in the prevailing oxic environment and or destruction of oceanic crust during subsequent subduction intra continental rifts arc rifts and back arc rifts commonly have more complexity in their structure and facies architecture and environments and can host all styles of vms mineralization in these settings early extension favoured the formation of restricted basins with ideal conditions for the onset of hydrothermal activity and development of anoxic bottom waters whereas in mature rifts the conditions were less conducive for the formation of regionally extensive anoxic environments formation of replacive deposits was permissible in all settings with porous or reactive subsea floor strata replacive mineralization is the most likely to be preserved in the geological record due to the sulphides being physically shielded from oxidative weathering and mechanical erosion at the seafloor the various styles of vms mineralization can rarely be distinguished using a single criterion in most cases multiple criteria are required mound style mineralization is distinguished by a mound or lens shaped morphology b presence of chimney fragments c presence of abundant sedimented sulphide breccias d location on a stratigraphic boundary ore horizon and e association with a thin horizon or thicker stratigraphic interval of fine grained clastic rocks e g shale mudstone that accumulated at slow sedimentation rates stratiform exhalative mineralization is distinguished by a sheet like morphology prior to deformation b presence of fine grained clastic host rocks that accumulated at relatively slow rates e g mudstone c presence of local or extensive planar stratification replacive mineralization is characterized by a irregular geometry and distribution of sulphide bodies b gradation from massive sulphides to semi massive sulphides and disseminated mineralization with relict textures of the host rock and c originally pumiceous glassy or reactive host rocks emplaced at high depositional rates mass flow deposits lavas carbonate altered mass flow deposits or limestone one deposit or district may comprise two or more of these main styles of mineralization in many cases the main styles of vms mineralization present in a particular region can be predicted from examination of the facies architecture and depositional environments of the host succession recognition of the style s of mineralization that occur in a particular basin or mineral belt enables exploration models to be improved and should influence the strategy of exploration for vms deposits

textures of the hellyer volcanic hosted massive sulfide deposit tasmania the aging of a sulfide sediment on the sea floor the relatively undeformed polymetallic massive sulfide ore of the hellyer deposit retains many textures developed during the early crystallization history common textures in the least modified ore away from the main feeder zone include pyrite framboids concentric and radial textured spheres dendritic and fibrous forms breccias mineral banding veins and reniform and crustification textures many are typical of early crystallization within a medium at high degrees of supersaturation some result from open space filling and others indicate disturbances of the sulfide mass possibly due to tectonic activity and volume adjustment the banding could be due to current winnowing most of these textures are found in modern midocean ridge and kuroko massive sulfide deposits but at hellyer the framboids are much more common hexagonal outlines in ore sphalerite indicate derivation of much of the sphalerite by inversion of wurtzite the high content of chalcopyrite in the wurtzite derived sphalerite appears in most cases to have developed by exsolution rather than replacement or coprecipitation although there is local evidence of both processes having operated possibly resulting from early asymmetrical crystal growth comparison of hellyer ore and typical kuroko ores reveals other differences than framboid abundance e g metal contents and ratios mineralogy and sulfur isotope compositions the most significant of which are the relatively high salinities of the hellyer ore fluids these together with evidence of a coeval basin imply sulfide deposition in a brine pool as previously proposed the development of a quenched sulfide mud at the bottom of a basin filled with spent ore fluid with little or no involvement of seawater explains the different mineral content e g lack of fe oxides scarcity of barite in the massive sulfide and the pyrite arsenopyrite assemblage the high metal content lack of chimney structures and differences in sulfur isotope compositions the sulfide mud dominated by fe s phases would be an ideal growth medium for framboidal pyrite the weight of experimental evidence concerning the development of framboidal pyrite points to involvement of metastable precursor fe s phases such as mackinawite and greigite in a weakly oxidized environment oxygen may have been added to the reduced acid brine pool as a result of inward diffusion of oxygen from overlying seawater which is shown to have been oxic by the presence of sessile benthic dendroids in the shales overlying the ore deposit however the level of oxidation was insufficient to form marcasite

the stability and structure of primordial reservoirs in the lower mantle insights from models of thermochemical convection in three dimensional spherical geometry large scale chemical lateral heterogeneities are inferred in the earth s lowermost mantle by seismological studies we explore the model space of thermochemical convection that can maintain reservoirs of dense material for a long period of time by using similar analysis in 3 d spherical geometry in this study we focus on the parameters thought to be important in controlling the stability and structure of primordial dense reservoirs in the lower mantle including the chemical density contrast between the primordial dense material and the regular mantle material buoyancy ratio thermal and chemical viscosity contrasts volume fraction of primordial dense material and the clapeyron slope of the phase transition at 660 km depth we find that most of the findings from the 3 d cartesian study still apply to 3 d spherical cases after slight modifications varying buoyancy ratio leads to different flow patterns from rapid upwelling to stable layering and large thermal viscosity contrasts are required to generate long wavelength chemical structures in the lower mantle chemical viscosity contrasts in a reasonable range have a second order role in modifying the stability of the dense anomalies the volume fraction of the initial primordial dense material does not effect the results with large thermal viscosity contrasts but has significant effects on calculations with intermediate and small thermal viscosity contrasts the volume fraction of dense material at which the flow pattern changes from unstable to stable depends on buoyancy ratio and thermal viscosity contrast an endothermic phase transition at 660 km depth acts as a filter allowing cold slabs to penetrate while blocking most of the dense material from penetrating to the upper mantle

shock temperatures of sio2 and their geophysical implications the temperature of sio 2 in high pressure shock states has been measured for samples of single crystal quartz and fused quartz pressures between 60 and 140 gpa have been studied using projectile impact and optical pyrometry techniques at lawrence livermore national laboratory both data sets indicate the occurrence of a shock induced phase transformation at 70 and 50 gpa along the and fused quartz hugoniots respectively the suggested identification of this transformation is the melting of shock synthesized stishovite with the onset of melting delayed by metastable superheating of the crystalline phase some evidence for this transition in conventional shock wave equation of state data is given and when these data are combined with the shock temperature data it is possible to construct the stishovite liquid phase boundaries the melting temperature of stishovite near 70 gpa pressure is found to be 4500 k and melting in this vicinity is accompanied by a relative volume change and latent heat of fusion of 2 7 and 2 4 mj kg respectively the solid stishovite hugoniot centered on quartz is well described by the linear shock velocity particle velocity relation u s 1 822 up 1 370 km s while at pressures above the melting transition the hugoniot centered on quartz has been fit with u s 1 619 u p 2 049 km s up to a pressure of 200 gpa the melting temperature of stishovite near 100 gpa suggests an approximate limit of 3500 k for the melting temperature of sio 2 bearing solid mantle mineral assemblages all of which are believed to contain si 4 in octahedral coordination with o 2 thus 3500 k is proposed as an approximate upper limit to the melting point and the actual temperature in the earth s mantle moreover the increase of the melting point of stishovite with pressure at 70 gpa is inferred to be 11 k gpa using various adiabatic temperature gradients in the earth s mantle and assuming creep is diffusion controlled in the lower mantle the current results could preclude an increase of viscosity by more than a factor of 10 3 with depth across the mantle

the influences of lower mantle viscosity stratification on 3d spherical shell mantle convection we have studied the dynamical effects of a stratified viscosity profile on time dependent mantle circulations by using a 3d boussinesq spherical shell convection model this particular viscosity profile taken from the inversion of geoid data has the distinct feature of having a large viscosity maximum in the middle lower mantle surface rayleigh numbers ranging from 5 104 to 8 106 have been considered there are several distinct differences in both the morphologies of the plumes and the sinking currents from constant viscosity models in a viscously stratified lower mantle plumes are formed invariably only after the cold downwelling has reached the bottom the mean mantle temperature is reduced and this allows for the sinking of very cold material into the deep mantle the number of upwelling plumes is reduced and the overall circulation is stabilized with well organized triple junctions of cold sheets developed the presence of internal heating increases the number of upwelling plumes and breaks up the network of downwelling sheets these plumes become larger in size and have a hotter interior because of the nearly stationary character

a new sulphur isotopic study of some iberian pyrite belt deposits evidence of a textural control on sulphur isotope composition the sulphide deposits of the iberian pyrite belt ipb represent an ore province of global importance our study presents 113 new sulphur isotope analyses from deposits selected to represent the textural spectrum of ores measured 34s values range from 26 to 10 mostly for massive and stockwork ores in agreement with data previously published in situ laser 34s analyses reveals a close correlation of 34s with texture primary diagenetic textures are dominated by relatively low 34s 8 to 2 whereas stockwork feeder textures are dominated by higher 34s 3 to 5 intermediate textures mainly coarse textures in stratiform zones have intermediate 34s although they are mostly dominated by the high 34s component rare barite has a homogeneous 34s around 18 which is consistent with direct derivation from lower carboniferous seawater sulphate a dual source of sulphide sulphur in the ipb deposits has been considered a hydrothermal source derived from reduction of coeval seawater sulphate in the convective systems is represented by sulphide in the feeder zones here variations in 34s are caused by variations in the extent of the sulphate reduction which governs the so4 h2s ratio the second end member was derived from the bacterial reduction of coeval seawater sulphate at or near the surface as reflected in the primary textures a distinct geographical variation in 34s and texture from sw more bacteriogenic and primary textures to ne more hydrothermal textures and 34s which reflects a variation in the relative input of each source was likely controlled by local geological environments given that the sulphur isotope characteristics of the ipb deposits are unlike most vms and kuroko deposits and noting the dominance of a mixed reduced sedimentary and volcanic environment we suggest that the ipb could represent an ore style which is intermediate between volcanic and sedimentary hosted massive sulphide types

the equation of state of a molten komatiite 1 shock wave compression to 36 gpa the equation of state eos of an initially molten 1550 c komatiite 27 wt mgo was determined in the 5 36 gpa pressure range via shock wave compression shock wave velocity u s and particle velocity u p kilometers second follow the linear relationship u s 3 13 0 03 1 47 0 03 u p based on a calculated density at 1550 c 0 bar of 2 745 0 005 g cm 3 this u s u p relationship gives the isentropic bulk modulus k s 27 0 0 6 gpa and its first and second isentropic pressure derivatives k s 4 9 0 1 and k s 0 109 0 003 gpa 1 the calculated liquidus compression curve agrees within error with the static compression results of agee and walker 1988 to 6 gpa but is less dense than their extrapolated values at higher pressures we determine that olivine fo 94 will be neutrally buoyant in komatiitic melt of the composition that we studied near 8 2 gpa clinopyroxene would also be neutrally buoyant near this pressure liquidus garnet majorite may be less dense than this komatiitic liquid in the 20 24 gpa interval however pyropic garnet and perovskite phases are denser than this komatiitic liquid in their respective liquidus pressure intervals to 36 gpa liquidus perovskite may be neutrally buoyant near 70 gpa at 40 gpa the density of shock compressed molten komatiite would be approximately equal to the calculated density of an equivalent mixture of dense solid oxide components this observation supports the model of rigden et al 1989 for compressibilities of liquid oxide components using their theoretical eos for liquid forsterite and fayalite we calculate the densities of a spectrum of melts from basaltic through peridotitic that are related to the experimentally studied komatiitic liquid by addition or subtraction of olivine at low pressure olivine fractionation lowers the density of basic magmas but above 13 14 gpa this trend is reversed all of these basic to ultrabasic liquids are predicted to have similar densities at 13 14 gpa and this density is approximately equal to the density of the bulk preliminary reference earth model mantle in this pressure range this suggests that melts derived from a peridotitic mantle may be inhibited from ascending from depths greater than 400 km

geophysical consequences of planetary scale impacts into a mars like planet all planetary bodies with old surfaces exhibit planetary scale impact craters vast scars caused by the large impacts at the end of solar system accretion or the late heavy bombardment here we investigate the geophysical consequences of planetary scale impacts into a mars like planet by simulating the events using a smoothed particle hydrodynamics sph model our simulations probe impact energies over two orders of magnitude 2 10 27 6 10 29 j impact velocities from the planet s escape velocity to twice mars orbital velocity 6 50 km s and impact angles from head on to highly oblique 0 75 the simulation results confirm that for planetary scale impacts surface curvature radial gravity the large relative size of the impactor to the planet and the greater penetration of the impactor contribute to significant differences in the geophysical expression compared to small craters which can effectively be treated as acting in a half space the results show that the excavated crustal cavity size and the total melt production scale similarly for both small and planetary scale impacts as a function of impact energy however in planetary scale impacts a significant fraction of the melt is sequestered at depth and thus does not contribute to resetting the planetary surface complete surface resetting is likely only in the most energetic 6 10 29 j slow and head on impacts simulated a crater rim is not present for planetary scale impacts with energies 10 29 j and angles 45 but rather the ejecta is more uniformly distributed over the planetary surface antipodal crustal removal and melting is present for energetic 10 29 j fast 6 km s and low angle 45 impacts the most massive impactors with both high impact energy and low velocity contribute sufficient angular momentum to increase the rotation period of the mars sized target to about a day impact velocities of 20 km s result in net mass erosion from the target for all simulated energies and angles the hypothesized impact origin of planetary structures may be tested by the presence and distribution of the geochemically distinct impactor material

role of a low viscosity zone in stabilizing plate tectonics implications for comparative terrestrial planetology 1 earth s near surface layer its lithosphere is broken into quasi rigid plates that form the upper thermal boundary layer for mantle convection since the discovery of plate tectonics it has been widely conjectured but only recently demonstrated that this peculiar style of convection may be facilitated by an upper mantle low viscosity zone lvz over which the plates glide easily the lvz or asthenosphere concept dates from 19th century investigations of isostatic support of mountain belts and is supported by modern evidence for a seismic low velocity zone and by studies of postglacial rebound and dynamic compensation of the earth s gravity field here we show in both two dimensional 2 d cartesian and 3 d spherical earth models that combining a pronounced lvz and a plastic yield stress to allow localized weakening of the cold thermal boundary layer results in a distinctly plate tectonic style of convection with 30 toroidal surface motion for the 3 d case recycling of water into the upper mantle at subduction zones is a plausible cause of earth s lvz whereas venus is dry and lacks both an lvz and plate tectonics

shale basins sulfur deficient ore brines and the formation of exhalative base metal deposits abstract the massive sulfide deposits of the iberian pyrite belt are interbedded with felsic volcanic rocks and shale and underlain by several thousand meters of siliciclastic sedimentary rocks known as the pq group isotope geochemistry and regional geology are both consistent with equilibration of the ore forming fluids with the pq group prior to ore deposition near the former seafloor the average cu zn pb ratio of the pq group rocks ca 26 55 19 is similar to the weighted average of all the massive sulfide orebodies combined ca 25 52 23 the genetic relationship between massive sulfide deposits and a siliciclastic sedimentary metal source is explained here by a thermodynamic model proposing that mildly reducing redox conditions imposed by equilibration with the sedimentary rocks are most critical for the formation of an effective ore forming fluid relatively metal rich but organic poor pyrite bearing shale undergoing dewatering of saline pore fluids is an effective source for the generation of sulfur deficient but relatively iron and base metal rich brines thus we propose that the giant deposits of the iberian pyrite belt owe their existence not to exceptionally metal enriched e g magmatic fluids but to the existence of a fairly ordinary but large metal source in reactive siliciclastic sediments combined with an underlying igneous heat source and a particularly efficient mechanism of sulfide precipitation by mixing with h 2 s rich fluids at or near the seafloor essentially similar mineral equilibria are imposed when saline fluids are buffered by typical continental basement rocks leaching of retrograde minerals and possibly residual salts from their magmatic or metamorphic prehistory is expected to generate similar variably metal rich but relatively sulfide deficient fluids thus the existence of mildly reducing rocks can be the dominant chemical control in the source of fluids generating many volcanogenic irish type or sedex deposits many of which are known to precipitate their metal load in response to biogenic sulfide addition at the ore deposition site

impact megadomes and the origin of the martian crustal dichotomy abstract we show that a sufficiently energetic impact can generate a melt volume which after isostatic adjustment and differentiation forms a spherical cap of crust with underlying depleted mantle depending on impact energy and initial crustal thickness a basin may be retained or impact induced crust may be topographically elevated retention of a martian lowland scale impact basin at impact energies 3 1028 3 1029 j requires an initial crustal thickness greater than 10 km formation of impact induced crust with size comparable to the martian highlands requires a larger impact energy 1 3 1030 j and initial crustal thickness

comparison of rio tinto spain and guaymas basin gulf of california an explanation of a supergiant massive sulfide deposit in an ancient sill sediment complex the volcano sedimentary unit that hosts the stratiform massive sulfide mineralization at rio tinto is reinterpreted as a sill sediment complex that in many respects is analogous to the example that is currently forming in the guaymas basin gulf of california however rio tinto is distinctive because 1 it is a product of bimodal magmatism 2 it has a much higher sill sediment ratio 4 1 lessening the extent of ore forming metal depletion during interaction between hydrothermal fluid and wet sediment and 3 most of the sills were emplaced penecontemporaneously as shown by the ubiquitous development of peperitic sill margins despite the narrowness of the sedimentary screens these data together with evidence for local extrusive behavior imply rapid formation of the complex at exceptionally high levels of emplacement it follows that in the complex variations in thickness of 400 m were directly expressed as basin floor deformation and that troughs in the resultant topography were able to trap exhalative hydrothermal plumes thus preserving the sulfides from dispersion also crucial to the mineralization process was the sheetlike form of the sill complex which by acting as a cap prevented regular convective cell patterns from forming and focused considerable discharge at areas of weakness the intense magmatic pulse the creation of pronounced topography and the capping of the hydrothermal system combined to make a 500 mt supergiant deposit

fluid temperature and salinity characteristics of the matagami volcanogenic massive sulfide district quebec fluid inclusions hosted within various lithologic units of the 40 million metric ton mt matagami district abitibi greenstone belt preserve samples of archean volcanogenic massive sulfide vms and postvolcanogenic massive sulfide hydrothermal fluid microthermometric measurements on ore hosted primary two phase liquid vapor inclusions from matagami s south limb deposits indicate that the vms hydrothermal fluid was highly saline 16 2 4 7 wt nacl cacl2 equiv 1 n 230 and of moderate temperature trapping temperature tt 208 32 c 1 n 230 a fluid with these characteristics is capable of transporting 5 10 4 m 30 ppm zn as zncl3 and zncl4 2 chloride complexes however the low temperature of this fluid precluded efficient cu transport 3 ppm which may in part explain the relatively cu poor nature of the matagami deposits calculated densities of this ore fluid as high as 1 10 g cm3 are consistent with a bottom hugging brine model however a subset of the data indicate a fluid less dense than ambient seawater suggesting that buoyant hydrothermal plumes were also present a microthermometrically determined high cacl2 content xnacl 0 55 molar na ca 2 3 1 for the vms ore hosted primary fluid is consistent with an archean hydrothermal fluid more ca rich than modern day seawater quartz epidote veins located in the hydrothermal cracking zone of the bell river complex host primary liquid vapor halite inclusions these inclusions are interpreted to be samples of the deep seated equivalent to the vms ore hosted hydrothermal fluid described above microthermometry indicates that these inclusions trapped a high temperature brine tt 373 44 c 1 n 92 38 2 1 9 wt nacl equiv 1 n 92 we interpret this brine to be a phase separated product of modified model seawater 3 2 wt nacl an exsolved magmatic fluid or a combination thereof deep within the hydrothermal system at 650 to 670 c and a near lithostatic pressure of 90 mpa phase separation and subsequent convection lowered the temperature of the brine prior to its entrapment within the hydrothermal cracking zone the occurrence of high temperature brine overlain by lower temperature salinity fluid suggests a two cell convection model consistent with metal mass balance calculations for the south limb the high salinity of the ore hosted fluid inclusions indicates two possibilities 1 a significant amount of brine was incorporated into the upper cell and mixed with heated seawater during convection 2 archean seawater itself was very saline and of variable salinity with the cooling of the bell river complex lower temperature fluids dominantly of seawater origin circulated deep within the hydrothermal system modified by water rock interaction yet not phase separated these fluids sealed off the remnant permeability of the fracture network of the hydrothermal cracking zone and were locally trapped as secondary liquid vapor fluid inclusions homogenization temperature th 242 17 c 9 1 1 6 wt nacl equiv 1 n 14 hosted within the bell river complex quartz epidote vein material post and or waning stage vms hydrothermal activity is evident from the presence of quartz epidote veins crosscutting wabassee group hanging wall rocks microthermometry on quartz hosted primary liquid vapor fluid inclusions suggests that this activity occurred at relatively low temperatures th 76 177 c n 212 over a wide range of salinity 6 0 32 4 wt nacl cacl2 equiv n 212 and with a high apparent cacl2 content xnacl 0 06 these fluid inclusion data illustrate the importance of subsea floor chemical and physical processes directly related to metal transport and deposition in vms systems in particular phase separation deep within the hydrothermal system is interpreted as a key process for generating saline brines capable of forming significant ore deposits

hydrothermal alteration of felsic volcanic rocks associated with massive sulphide deposition in the northern iberian pyrite belt sw spain abstract massive sulphide deposits of the northern iberian pyrite belt ipb are mainly hosted by felsic volcanic rocks of rhyolitic to dacitic composition beneath most of the massive ores of this area e g concepcion san miguel aguas tenidas este or san telmo deposits there is usually a wide hydrothermal alteration halo associated with stockwork type mineralization within these alteration envelopes there are two principal rock types 1 chlorite rich rocks linked to the inner and more intensely altered zones and dominantly comprising chlorite pyrite quartz sericite carbonate rutile zircon chalcopyrite and 2 sericite rich rocks more common in the peripheral zones and showing a dominant paragenesis of sericite quartz pyrite chlorite carbonate rutile zircon sphalerite mass balance calculations comparing altered and least altered felsic volcanic rocks suggest that sericitization was accompanied by moderate enrichment in mg fe and h 2 o with depletion in si na and k and a slight net mass loss of about 3 chloritization shows an overall pattern which is similar to that of the sericitic alteration but with large gains in fe mg and h 2 o and minor enrichment in si s and mn and a significant loss of na and k and a minor loss of ca and rb however chloritization has involved a much larger net mass change mass gain of about 28 only a few elements such as nb y zr ti p and lree appear to have remained inert during hydrothermal alteration whilst ti and al have undergone very minor mobilization the results point to the severity of the physico chemical conditions that prevailed during the waxing stage of the ore forming hydrothermal systems further mineralogical and geochemical studies of the altered footwall rocks in the studied deposits indicate that hydrothermal ore bearing fluids reacted with host rocks in a multi stage process which produced a succession of mineralogical and chemical changes as the temperature increased

the ibero armorican arc a collision effect against an irregular continent the ibero armorican arc is the main variscan macrostructure in western europe although it was recognized in the 1920s its genesis is still debatable and the proposed models for its generation questionable however even if some doubts persist it seems that most agree on the general geodynamic evolution of this virgation in both branches in the middle to late devonian iberia was deformed by a sinistral transpressive regime while in the northern branch thrusting events were predominant in the carboniferous a dextral transpression begins to predominate in the armorican branch while the southern branch was deformed by southward thrusting in an attempt to correlate these events we propose that during the late devonian a cantabrian indentor moved northward producing the oblique closure of the southern part of the rheic ocean and an almost orthogonal closure in central europe in the carboniferous the collision with the irregular margin of laurasia induced a rotation of the indentor the intracontinental deformation was then achieved by dextral transpression in the northern branch and thrusting in the southern one

yin yang grid an overset grid in spherical geometry 1 a new kind of overset grid named yin yang grid for spherical geometry is proposed the yin yang grid is composed of two identical component grids that are combined in a complemental way to cover a spherical surface with partial overlap on their boundaries each component grid is a low latitude part of the latitude longitude grid therefore the grid spacing is quasi uniform and the metric tensors are simple and analytically known one can directly apply mathematical and numerical resources that have been written in the spherical polar coordinates or latitude longitude grid the complemental combination of the two identical component grids enables us to make efficient and concise programs simulation codes for geodynamo and mantle convection simulations using finite difference scheme based on the yin yang grid are developed and tested the yin yang grid is suitable for massively parallel computers

dynamic compression of sio2 a new interpretation 1 in light of recent discoveries of post stishovite phases of sio2 with the cacl2 and pbo2 structures we have reassigned the regimes along the hugoniots for initial quartz coesite cristobalite porous coesite and fused silica calculated hugoniots for fused silica cristobalite and porous coesite indicate transition to stishovite and then melt hugoniots for crystal quartz and coesite indicate that transition occurs to stishovite then the cacl2 structure and finally to melt

differentiation and evolution of the iva meteorite parent body clues from pyroxene geochemistry in the steinbach stony iron meteorite we analyzed the steinbach iva stony iron meteorite using scanning electron microscopy sem electron microprobe analysis empa laser ablation inductively coupled plasma mass spectroscopy la icp ms and modeling techniques different and sometimes adjacent low ca pyroxene grains have distinct compositions and evidently crystallized at different stages in a chemically evolving system prior to the solidification of metal and troilite early crystallizing pyroxene shows evidence for disequilibrium and formation under conditions of rapid cooling producing clinobronzite and type 1 pyroxene rich in troilite and other inclusions subsequently type 2 pyroxene crystallized over an extensive fractionation interval steinbach probably formed as a cumulate produced by extensive crystal fractionation 6070 fractional crystallization from a high temperature 1450 1490 c silicate metallic magma the inferred composition of the precursor magma is best modeled as having formed by 30 50 silicate partial melting of a chondritic protolith if this protolith was similar to an ll chondrite as implied by o isotopic data then olivine must have separated from the partial melt and a substantial amount 53 56 of feo must have been reduced in the silicate magma a model of simultaneous endogenic heating and collisional disruption appears best able to explain the data for steinbach and other iva meteorites impact disruption occurred while the parent body was substantially molten causing liquids to separate from solids and oxygen bearing gas to vent to space leading to a molten metal rich body that was smaller than the original parent body and that solidified from the outside in this model can simultaneously explain the characteristics of both stony iron and iron iva meteorites including the apparent correlation between metal composition and metallographic cooling rate observed for metal

fine scale ultralow velocity zone structure from high frequency seismic array data 1 variability of core mantle boundary cmb structure is imaged at the kilometer to tens of kilometers scale by intensive waveform modeling of the scp phase recordings of earthquakes in the tonga fiji subduction zone from the small aperture 10 km high frequency 2 4 hz alice springs array in central australia are used to document anomalous scp waveform variations due to the presence of an ultralow velocity zone ulvz at the cmb these data expand coverage from previous results using the warramunga array which is located slightly to the north and indicate the existence of a geographically confined ulvz at the cmb roughly 100 km in lateral extent south of new caledonia and east of australia synthetic modeling indicates seismic velocity reductions of 8 and 24 for p wave and s waves respectively and a well constrained density increase of 10 in a 8 5 km thick layer above the cmb this ulvz patch is well explained by an elevated iron content of partially molten mantle material likely present as divalent oxidized iron in the low spin state the properties of such an iron enriched assemblage are consistent with both an elevated density and bulk modulus of this region as are dictated from our best fit seismic results furthermore a positive velocity gradient within the ulvz may be present which need not be associated with a decrease in melt content within this region but could rather be associated with a shift in the textural distribution of melt within the ulvz

multi technique equation of state for fe2sio4 melt and the density of fe bearing silicate melts from 0 to 161 gpa we have conducted new equation of state measurements on liquid fe 2 sio 4 in a collaborative multi technique study the liquid density the bulk modulus k and its pressure derivative k were measured from 1 atm to 161 gpa using 1 atm double bob archimedean multi anvil sink float and shock wave techniques shock compression results on initially molten fe 2 sio 4 1573 k fitted with previous work and the ultrasonically measured bulk sound speed c o in shock velocity u s particle velocity u p space yields the hugoniot u s 1 58 0 03 u p 2 438 0 005 km s sink float results are in agreement with shock wave and ultrasonic data consistent with an isothermal k t 19 4 gpa and k 5 33 at 1500 c shock melting of initially solid fe 2 sio 4 300 k confirms that the gruneisen parameter of this liquid increases upon compression where o o q yields a q value of 1 45 constraints on the liquid fayalite eos permit the calculation of isentropes for silicate liquids of general composition in the multicomponent system cao mgo al 2 o 3 sio 2 feo at elevated temperatures and pressures in our model a whole mantle magma ocean would first crystallize in the mid lower mantle or at the base of the mantle were it composed of either peridotite or simplified chondrite liquid respectively in regards to the partial melt hypothesis to explain the occurrence and characteristics of ultra low velocity zones neither of these candidate liquids would be dense enough to remain at the core mantle boundary on geologic timescales but our model defines a compositional range of liquids that would be gravitationally stable

4 transpressional tectonics lower crust decoupling and intrusion of deep mafic sills a model for the unusual metallogenesis of sw iberia abstract sw iberia is interpreted as an accretionary magmatic belt resulting from the collision between the south portuguese zone and the autochthonous iberian terrane in variscan times 350 to 330 ma in the south portuguese zone pull apart basins were filled with a thick sequence of siliciclastic sediments and bimodal volcanic rocks that host the giant massive sulphides of the iberian pyrite belt massive sulphides precipitated in highly efficient geochemical traps where metal rich but sulphur depleted fluids of dominant basinal derivation mixed with sulphide rich modified seawater massive sulphides formed either in porous reactive volcanic rocks by sub seafloor replacement or in dark shale by replacement of mud or by exhalation within confined basins with high biogenic activity crustal thinning and magma intrusion were responsible for thermal maturation and dehydration of sedimentary rocks while magmatic fluids probably had a minor influence on the observed geochemical signatures the ossa morena zone was a coeval calc alkaline magmatic arc it was the site for unusual mineralization particularly magmatic ni cu and hydrothermal fe oxide cu au ores iocg most magmatism and mineralization took place at local extensional zones along first order strike slip faults and thrusts the source of magmas and iocg and ni cu deposits probably lay in a large mafic ultramafic layered complex intruded along a detachment at the boundary between the upper and lower crust here juvenile melts extensively interacted with low grade metamorphic rocks inducing widespread anatexis magma contamination and further exsolution of hydrothermal fluids hypersaline fluids 18 o fluid 5 4 to 12 were focused upward into thrusts and faults leading to early magnetite mineralization associated with a high temperature 500 c albite actinolite salite alteration and subsequent copper gold bearing vein mineralization at somewhat lower temperatures assimilation of sediments by magmas led in turn to the formation of immiscible sulphide and silicate melts that accumulated in the footwall of the layered igneous complex further injection of both basic and sulphide rich magmas into the upper crust led to the formation of ni cu rich breccia pipes younger 330 to 280 ma peraluminous granitoids probably reflect the slow ascent of relatively dry and viscous magmas formed by contact anatexis these granitoids have w sn and pb zn related mineralization that also shows geochemical evidence of major mantle crust interaction late epithermal hg cu sb and pb zn ag mineralization was driven by convective hydrothermal cells resulting from the high geothermal gradients that were set up in the zone by intrusion of the layered igneous complex in all cases most of the sulphur seems to have been derived from leaching of the host sedimentary rocks 34 s 7 to 20 with only limited mixing with sulphur of magmatic derivation the metallogenic characteristics of the two terranes are quite different in the ossa morena zone juvenile magmatism played a major role as the source of metals and controlled the styles of mineralization in the south portuguese zone magmas only acted as heat sources but seem to have had no major influence as sources of metals and fluids which are dominated by crustal signatures most of the magmatic and tectonic features related to the variscan subduction and collision seem to be masked by those resulting from transpressional deformation and deep mafic intrusion which led to the development of a metallogenic belt with little resemblance to other accretionary magmatic arcs

impact origin for the martian crustal dichotomy half emptied or half filled 1 one possible mechanism for martian hemispherical dichotomy formation is excavation of the northern lowland basin in the existing crust by a single giant impact here we explore a related scenario in which the impact melt volume is sufficient to fill the excavated cavity produce a thickened crust and generate a topographic basin centered on the opposite side of the planet we investigate this mechanism by formulating a model for viscous spreading of a partially molten layer over a spherical surface we derive numerical solutions and calculate the final area of the spreading layer as a function of impactor size and shape of the layer boundary due to an initially asymmetric melt region results are compared with observations including lowland basin size shape of the dichotomy boundary age difference between crustal provinces and crustal thickness

global transport on a spherical multiple cell grid abstractsecond and third order upstream nonoscillatory uno advection schemes are applied on a spherical multiple cell smc grid for global transport similar to the reduced grid the smc grid relaxes the courant friedrichs lewy cfl restriction of the eulerian advection time step on the conventional latitude longitude grid by zonally merging cells toward the poles round polar cells are introduced to remove the polar singularity of the spherical coordinate system the unstructured feature of the smc grid allows unused cells to be removed out of memory and transport calculations solid body rotation and deformation flow tests are used for comparison with other transport schemes application on the global ocean surface is used to demonstrate the flexibility of the smc grid by removing all land points and making possible the extension of global ocean surface wave models to cover the arctic in response to the retreating sea ice in recent summers numerical results suggest that uno schemes on the smc grid

distinctive features of late palaeozoic massive sulphide deposits in south china abstract more than 20 sediment hosted massive sulphide deposits occur in late palaeozoic basins in south china these deposits are accompanied by a certain amount of volcanic rocks in the host sequence and are economically important for their cu pb zn au and ag reserves the deposits and their host strata were commonly intruded by mesozoic granitoids remobilisation of sedimentary ores and magmatic hydrothermal overprinting processes resulted in the coexistence of massive sulphides with vein skarn and porphyry type orebodies in the same region or within a single deposit the ore containing basins occur in different tectonic settings the lower yangtze basin occurs on a passive continental margin where the deposits are high in cu and au with minor pb and zn and recoverable ag co and mo the ores have a lower concentration of radiogenic lead and 34s values close to zero fluid inclusions are highly saline and na rich fluids and metals of the lower yangtze region are interpreted to have been derived essentially from deep sources including the precambrian basement by contrast basins of the nanling region formed in an intracontinental setting developed on a folded caledonian basement these deposits are higher in pb zn sn and w as well as cu with recoverable ag sb hg u bi tl and mo the ores are characterised by a higher concentration of radiogenic lead and a wide variation of 34s composition fluid inclusions have lower salinities and higher k na ratios fluids are considered to have been sourced substantially from seawater by convection metals for the nanling deposits were essentially derived from the caledonian basement by leaching the contrast in ore composition between these two regions appears to have been controlled by differences in basement composition of the ore forming basins

time domain parallelization for computational geodynamics 1 i present a time domain parallelization approach for geodynamic modeling this algorithm named parareal is based on the use of coarse sequential and fine parallel propagators to predict and to iteratively correct the solution of the governing equations over a given time interal although the method has been successfully used to solve differential equations in various scientific areas it has not been applied to model solid state convective motions relevant to the earth and other planetary mantles in that case the time dependence of the velocity is only implicit which requires modifications to the original algorithm the performances of this adapted version of the parareal algorithm were investigated using theoretical model predictions in good agreement with numerical experiments i show that under optimum conditions the parallel speedup increases linearly with the number of processors and speedups close to 10 were measured using only few tens of cpus this parareal approach can be used alone or combined with any spatial parallel algorithm allowing significant additional increase in speedup with increasing number of processors

ore deposit types and tectonic evolution of the iberian pyrite belt from transtensional basins and magmatism to transpression and inversion tectonics abstract a new interpretation of the structural evolution of the iberian pyrite belt ipb and volcanogenic massive sulfide mineralization vms is presented in this work based on a review of the ore deposit types the analysis of the hosting volcanic sequences and the tectonic evolution the vms deposits of the ipb are hosted by volcanic and siliciclastic rocks four main volcano sedimentary sequences vsc from vsc0 to vsc3 have been assumed the main deposits being located in the vsc0 and at the top of the vsc2 we have defined three main sectors oriented approximately e w and hosting the vms deposits in the northern sector which is mostly located in spain graben basins and local pull aparts are the main structures in this sector two belts can be distinguished the deposits being located at the top of the vsc2 felsic volcanism rio tinto type ipb deposits in the central sector both in spain and portugal half graben basins are the most common structures and the deposits are mostly located in the vsc0 andesitic volcanic sedimentary sequence tharsis type ipb deposits in the southern sector which is only located in portugal a graben basin with a pull apart is again the main structure and the deposits are located in black slates and at the top of a felsic volcanism strunian in age vsc0 the deposits located in graben basin with a pull apart are essentially felsic volcanic hosted with some siliciclastic material mostly black shales by contrast those located in half graben basins are mainly hosted by black shales with minor amounts of andesitic rocks the tectonic evolution shows that as a result of a counterclockwise rotation of the stress axes the formation of the ipb and the associated ore deposits took place during several episodes from transtension with the development of both graben with pull aparts and half graben basins through left lateral e w shearing to transpression at the beginning of the transtensional process several extensional roughly e w trending faults that developed graben and half graben basins were generated and the first volcanic andesite rhyolite rocks vsc0 formed the tharsis type deposits mainly hosted by black slates with some volcanic rocks were formed in the central sector while the neves corvo type deposit hosted by black slates and felsic volcanism formed in the southern one after a period characterized by barren mafic volcanism vsc1 a sinistral shear affected the previous fractures due to the stress axis rotation and felsic crustal volcanism started vsc2 rhyolites and dacites were particularly abundant in two graben basins which developed rollovers in pull apart zones forming the rio tinto type deposits in the northern sector the thermal increase associated with vsc0 and vsc2 gave rise to the development of crustal scale hydrothermal convective cells which generated both types of deposits after a barren vsc3 felsic volcanism subsequently during the variscan transpressional phase the e w extensional faults were reactivated as reverse faults affecting the volcanic sequence vsc0 to vsc3 as well as the interbedded sedimentary rocks mostly black shales as has been recognized at the rio tinto deposit buttressing must have played a significant role in the geometry of inverted structures and the vms ores were intensely recrystallized it should be emphasized that this new regional geological model for the ipb is an approach to provide a better insight into vms deposits and could be a key point for further studies providing a new tool to improve knowledge of the vms mineralizations and exploration guidelines elsewhere in the ipb

geometry and genesis of feeder zones of massive sulphide deposits constraints from the rio tinto ore deposit spain vein related data have been collected around the giant rio tinto orebody in southern spain within the root zones of the massive sulphide deposits here we report the main results of this study concerning the geometry of the stockwork and the conditions of formation although field and thin section studies have shown that a wide range of vein configurations exist from micro cracks fluid inclusion planes to large paleo flow channels two groups seem to dominate the first corresponds to small constricted micro cracks and capillary flow channels now mainly filled with quartz whereas the veins of the second group have large widths are continuous over several meters and are filled with quartz and sulphides most are tension veins and only very few quartz tends to post date the quartz dominated veins quartz pyrite the vein thickness and spacing distribution is modal rather than logarithmic and their densities are not fractal but are characterized by a poisson distribution from the immediate sub surface zone to more than 100 m below the base of the massive sulphide deposits most hydrothermal quartz sulphide stockwork veins are sub parallel to the base of the massive sulphide deposit the assumption that the base of this deposit corresponds to a paleo horizontal plane implies that most veins were sub horizontal this is particularly evident for small veins but the larger ones can be strongly oblique to the base of the deposit the hydrothermal fluids that generated the massive sulphide deposits and underlying stockworks were very saline and probably underwent sub or super critical phase separation in the root zones of the system this phase separation was the probable mechanism producing the periodic over pressures of at least 20 mpa that were necessary to generate the sub horizontal veins of the stockworks

towards self consistent modeling of the martian dichotomy the influence of one ridge convection on crustal thickness distribution in order to find an explanation for the origin of the martian crustal dichotomy a number of recent papers have examined the effect of layered viscosity on the evolution of a degree 1 mantle convection e g roberts and zhong roberts j h zhong s 2006 j geophys res 111 e06013 and yoshida and kageyama yoshida m kageyama a 2006 j geophys res 111 doi 10 1029 2005jb003905 b03412 it was found that a mid mantle viscosity jump combined with highly temperature and depth dependent rheology are effective in developing a degree 1 convection within a short timescale such a layered viscosity profile could be justified by martian mineralogy however the effect of a degree 1 convective planform on the crustal thickness distribution has not yet been demonstrated it is not obvious whether a thinner crust due to sublithospheric erosion and crustal thinning or a thicker crust due to enhanced crustal production would form above the hemisphere of mantle upwelling also the general shape of the dichotomy which is not strictly hemispherical has not yet been fully investigated here we propose a model of the crustal patterns produced by numerical simulations of martian mantle convection using the finite volume multigrid code stagyy tackley p j 2008 phys earth planet int 107 7 18 doi 10 1016 j pepi 2008 08 005 a self consistent treatment of melting crustal formation and chemical differentiation has been added to models of three dimensional thermal convection this allows us to obtain global maps of the crustal thickness distribution as it evolves with time the obtained results demonstrate that it is indeed possible to form a crustal dichotomy as a consequence of near degree 1 mantle convection early in mars history we find that some of the observed patterns show intriguing first order similarities to the elliptical shape of the martian dichotomy in all models the region of thick crust is located over the region of mantle upwelling which itself is a ridge like structure spread over roughly one half of the planet a planform we describe as one ridge convection

explanation for many of the unusual features of the massive sulfide deposits of the iberian pyrite belt newly published fluid inclusion data from quartz in stockwork veins beneath seven massive sulfide lenses in the iberian pyrite belt suggest that the lenses were formed from fluids that on reaching the sea reversed buoyancy and ponded in basins sulfides quenched in the resulting brine pool would have settled to form a sulfide mud this process provides a relatively efficient trapping mechanism for metal in the fluids and effectively excludes ambient seawater accounting for the deposits tending to have the characteristics of large size sheet like form absence of relict chimney structures and a mineral content characterized by pyrite arsenopyrite and absence or scarcity of barite marcasite and fe oxides if total s was less than total metals in the stockwork fluids some or all of the more soluble zn and pb could have been swept from the basin at the overflow accounting for the variable but generally low zn and pb contents of the ores the lack of sedimentary source for the high salinities implicates magmatic intrusions possibly similar to those related to sn w mineralization

a new 3d geological model and interpretation of structural evolution of the world class rio tinto vms deposit iberian pyrite belt spain abstract a new 3d geological model and interpretation of structural evolution of the rio tinto world class vms deposit are presented in this work the rio tinto volcanogenic massive sulfide vms deposit is located in the spanish segment of the iberian pyrite belt and is hosted by felsic porphyritic volcanic rocks and tuffs computer generated 3d modeling of the different orebodies and host rocks has been carried out using data from around 3000 drill core logs allowing us to build 93 cross sections and 6 plants both 50 m spacing this has enabled us to recognize the geometry and relationships between the mineralization and the earliest carboniferous transtensional tectonics through the development of an extensional pull apart basin with two sub basins separated by the nw se trending eduardo fault the sub basins cerro colorado and san dionisio were limited by two e w strike slip faults the northern and southern faults and bounded in the east and west by the nw se trending nerva and western faults respectively the generated pull apart basin was first filled by a basaltic magmatism of mantle origin and later following the deposition of the intermediate complex sedimentary unit by rhyodacitic volcanic rocks of crustal origin the evolution of the subsiding basins caused the development of an e w oriented rollover anticline that affected these filling rocks as a result of a counterclockwise rotation of the stress axes the primitive pull apart basin evolved into a basin affected by e w transtensional sinistral shearing its northern and southern limits were favorable areas for increased hydrothermal fluid flow which gave way to the huge concentration of vms mineralization located near the limits the northern and to a lesser degree the southern extensional faults thus become channel areas for feeding and discharging of the vms and stockwork ores the main mineralizing period was related to this stage subsequently during the variscan transpressional phase the e w extensional faults were reactivated as inverse faults affecting the volcanic sequence of mafic to felsic composition and the intermediate complex sedimentary unit fault propagation folds developed above these faults affecting the massive sulfides the transition series and the culm flysch sediments with buttressing playing a significant role in the geometry of tectonically inverted structures the vms mineralization and cupriferous stockworks were folded and dismembered from the original conduits in the volcanic series and a dextral reactivation of the nw se trending faults also developed finally it should be emphasized that this new 3d geological model is an approach to provide a better insight into the 3d structure of the world class vms rio tinto deposit and could be a key point for further studies providing a new tool to increase knowledge of the vms mineralizations and exploration guidelines elsewhere in the ipb

brine pool deposition for the zn pb cu massive sulphide deposits of the bathurst mining camp new brunswick canada i comparisons with the iberian pyrite belt abstract the ordovician zn pb cu massive sulphide ore deposits of the bathurst mining camp share many features with those of the devonian carboniferous iberian pyrite belt particularly the tendency to large size tonnage and metal content shape as far as can be determined after allowing for deformation metal content particularly fe cu pb zn and sn mineral assemblages pyrite arsenopyrite pyrrhotite and lack or rarity of sulphates sulphide textures particularly framboidal pyrite lack of chimney structures and rubble mounds irregular metal or mineral zoning and the low degree of zone refining compared to hokuroku ores the major differences between the provinces are the lack of vent complexes and the presence of sn cu ores in the iberian pyrite belt there are also similarities in the geological setting of the two camps both lie within continental terranes undergoing arc continent and continent continent collision and in each case massive sulphide mineralisation followed ophiolite obduction the ore deposits are associated with bimodal volcanic rocks derived from morb and continental crust and marine shales and mineralisation was locally accompanied or followed by deposition of iron formations fluid inclusion data from veins in stockworks from at least six of the iberian massive sulphide deposits point to sulphide deposition having taken place in basins containing mostly spent saline ore forming fluids brine pools and it is suggested that most of the major features of the bathurst deposits can be explained by similar processes the proposed model is largely independent of ocean sulphate and o 2 content whereas low values of each are requisites for the current spreading plume model of sulphide deposition in the bathurst camp

low degree mantle convection with strongly temperature and depth dependent viscosity in a three dimensional spherical shell 1 a series of numerical simulations of thermal convection of boussinesq fluid with infinite prandtl number with rayleigh number 10 7 and with the strongly temperature and depth dependent viscosity in a three dimensional spherical shell is carried out to study the mantle convection of single plate terrestrial planets like venus or mars without an earth like plate tectonics the strongly temperature dependent viscosity the viscosity contrast across the shell is 10 5 makes the convection under stagnant lid short wavelength structures numerous cylindrical upwelling plumes are developed because of the secondary downwelling plumes arising from the bottom of lid this convection pattern is inconsistent with that inferred from the geodesic observation of the venus or mars additional effect of the stratified viscosity at the upper lower mantle the viscosity contrast is varied from 30 to 300 are investigated it is found that the combination of the strongly temperature and depth dependent viscosity causes long wavelength structures of convection in which the spherical harmonic degree l is dominant at 1 4 the geoid anomaly calculated by the simulated convections shows a long wavelength structure which is compared with observations the degree one l 1 convection like the martian mantle is realized in the wide range of viscosity contrast from 30 to 100 when the viscosity is continuously increased with depth at the lower mantle

low velocity zone atop the 410 km seismic discontinuity in the northwestern united states the seismic discontinuity at 410 km depth in the earth s mantle is generally attributed to the phase transition of mg fe 2sio 4 from the olivine to wadsleyite structure variation in the depth of this discontinuity is often taken as a proxy for mantle temperature owing to its response to thermal perturbations for example a cold anomaly would elevate the 410 km discontinuity because of its positive clapeyron slope whereas a warm anomaly would depress the discontinuity but trade offs between seismic wave speed heterogeneity and discontinuity topography often inhibit detailed analysis of these discontinuities and structure often appears very complicated here we simultaneously model seismic refracted waves and scattered waves from the 410 km discontinuity in the western united states to constrain structure in the region we find a low velocity zone with a shear wave velocity drop of 5 on top of the 410 km discontinuity beneath the northwestern united states extending from southwestern oregon to the northern basin and range province this low velocity zone has a thickness that varies from 20 to 90 km with rapid lateral variations its spatial extent coincides with both an anomalous composition of overlying volcanism and seismic receiver function observations observed above the region we interpret the low velocity zone as a compositional anomaly possibly due to a dense partial melt layer which may be linked to prior subduction of the farallon plate and back arc extension the existence of such a layer could be indicative of high water content in the earth s transition zone

the cu stockwork and massive sulfide ore of the feitais volcanic hosted massive sulfide deposit aljustrel iberian pyrite belt portugal a mineralogical fluid inclusion and isotopic investigation the variscan feitais volcanic hosted massive sulfide deposit in the aljustrel district of the iberian pyrite belt consists of 55 million metric tons of zn pb cu massive sulfide overlying a cu rich stockwork the massive ore is overlain by up to 30 m of feldspar phyric rhyolitic volcaniclastic rock and locally by a jasper and or chert layer up to 15 m thick the massive sulfide orebody consists dominantly of pyrite sphalerite galena chalcopyrite tetrahedrite tennantite arsenopyrite and bournonite together with minor quartz chlorite sericite carbonate and barite the orebody is up to 100 m thick and is underlain by a tabular alteration zone of chloritedominated locally silicified felsic volcanic rock the upper 30 to 60 m of which contains chalcopyrite quartzchlorite sericite carbonate bearing stockwork vein let s that prior to deformation were at a shallow angle to the base of the massive orebody chloritized footwall rocks extend up to 20 m below the cu stockwork zone and are underlain by up to 50 m of quartz sericite pyrite altered rhyolitic rock the stockwork veins also contain pyrite tetrahedrite tennantite sphalerite and arsenopyrite pyrite both in stockwork and massive ore locally displays partly recrystallized framboidal reniform and cellular textures two generations of quartz q1 and q2 and carbonate in the stockwork veins contain primary in growth zones and pseudosecondary fluid inclusions with homogenization temperatures of 270 to 315 c and salinities of 2 2 to 8 1 wt percent nacl equiv the 34s cdt values of massive and stockwork ores range from 15 4 to 4 7 mean 2 8 and 11 2 to 11 9 mean 0 4 per mil respectively the lowest values from colloformtextured pyrite with no evidence of oxidation of sulfide sulfur during mineralization the most negative values indicate an origin by biogenic reduction of seawater sulfate the 13c pdb values for carbonates 7 5 to 13 7 and 9 3 to 14 3 per mil in massive and stockwork ore respectively indicate an origin mostly by oxidation of methane derived from organic matter in underlying sedimentary rocks and possibly a contribution of magmatic carbon there are no significant lateral or vertical variations in s isotope values in sulfides or c o isotope values in carbonates either in massive or stockwork ore the 18o smow values for quartz in stockwork and massive sulfide are 11 6 to 13 9 and 16 7 to 17 9 per mil respectively coexisting and texturally contemporaneous carbonate and quartz in stockwork veins are not in isotopic equilibrium indicating that the c o isotope values may have been reset the 18o values of fluid calculated to be in equilibrium with quartz at fluid inclusion homogenization temperatures are 4 2 to 5 2 per mil barite from the hanging wall and massive ore yields 34s values 21 9 27 9 equal to or slightly higher than those of coeval seawater 87sr 86sr ratios 0 708438 0 709063 are slightly more radiogenic than those of coeval seawater 0 7080 0 7085 and much more radiogenic than those of coeval volcanic rocks 0 703304 0 706642 probably representing mixtures between seawater sr and radiogenic sr in fluids sourced in the crustal pile deposition of the massive sulfide on the sea floor is suggested by its stratiform nature the stronger alteration of footwall relative to hanging wall rocks the stockwork system terminating sharply at the base of the massive sulfide the presence of sedimentary like textures in the massive sulfide the absence of replacement fronts and the presence of framboidal and other sea floor depositional textures indicative of fluid quenching the sheetlike form lack of rubble mounds and chimneys scarcity of barite reduced mineral assemblage and metal zoning distinguish feitais from kuroko type deposits it shares most of the characteristics of those iberian pyrite belt deposits for which a brine pool origin has been proposed based on fluid inclusion data

shock wave equation of state of molten and solid fayalite shock wave equations of state eos of initially solid 300 k and molten 1573 k fayalite fe 2sio 4 fa are measured over the pressure ranges of 23 212 and 5 47 gpa respectively the 300 k data indicates that fa undergoes a phase change from the low pressure olivine structure lpp over the 35 55 gpa range in agreement with earlier analyses of rockport fayalite shock data and diamond cell recovery experiments the high pressure phase hpp data are consistent with an oxide 2feo sio 2 stishovite mixture a fit to the hpp hugoniot data in the shock velocity u s particle velocity u p plane yields u s 4 07 0 22 km s 1 43 0 06 u p here the initial density 0 is 4 375 0 027 mg m 3 the 1573 k data yields u s 2 63 0 02 km s 1 59 0 01 u p initial density calculated from temperature data is 3 750 0 018 mg m 3 and k 0s 25 9 0 4 gpa and k 0s 5 36 0 04 gpa the bulk modulus k 0s compares favorably with agee s result geophys res lett 19 1992a 1169 24 4 gpa but the pressure derivative is much less than the k 0t 10 1 gpa previously reported molten fa compression data 40 gpa are closely fit with an ideal mixture of oxides sio 2 stishovite 2feo lpp in support of the hypothesis of rigden et al j geophys res 94 1989 9508 a model molten basalt incorporating previous molten anorthite an diopside di eutectic and the present molten fa eos data implies that a zone of basic silicate liquid could be neutrally buoyant at a depth of 250 400 km upon partial melting of a peridotite mantle as discussed by rigden et al science 226 1984 1071 this conclusion is based on comparison of the density of the model basalt an 0 36 di 0 64 0 85 fa 0 15 with dziewonski and anderson s preliminary reference earth model phys earth planet inter 25 1981 297

melting relations and the equation of state of magmas at high pressure application to geodynamics abstract the melting relations of mantle minerals together with the physical properties of silicate melts at high pressure have been extensively studied during the previous decade the melting relation of minerals and the equation of state of magmas are important properties for deducing the formation and differentiation of the earth and especially for elucidating the nature of the terrestrial magma ocean and the subsequent formation of the core mantle and crust of the earth because these magmas are compressible we expect that the olivine magma density crossover played an important role in controlling the geochemical characteristics of the primitive mantle after the magma ocean stage of the primordial earth the crystal magma density crossover is also expected to occur at the base of the upper mantle in the presence of volatiles and at the base of the lower mantle in the earth today the existence of dense magmas is consistent with seismological observations of low velocity regions existing at the base of the upper mantle beneath japan europe and the usa and the ultralow velocity zone ulvz at the core mantle boundary of the present earth

formation on the sea floor of the hellyer volcanogenic massive sulfide deposit new fluid inclusion data from the stockwork veins beneath the hellyer orebody a relatively undeformed late cambrian polymetallic volcanogenic massive sulfide deposit allow semiquantitative modeling of the behavior of the ore fluids on the sea floor and the manner of growth of the massive sulfide the earliest cool fluids may have been denser than seawater and ponded in a basin identified by sedimentological and structural studies subsequent vent fluids buoyant with respect to the brine pool and subsequently at higher temperature with respect to seawater mixed with the cooler brine during and after filling the basin to the level of its lowest outlet density inhomogeneities within the brine pool were reduced by double diffusive convection across the brine seawater interface and internal boundaries and mixing by currents within the brine pool estimates of likely temperatures and salinities in the brine pool for the first main mineralization stage avg 200 c 11 wt salinity assuming a steady or quasisteady state had been established and using volume fluxes similar to those observed in modern oceans indicate temperatures 100 c sulfides mostly pyrite sphalerite and galena precipitated from the vent fluids after mixing with cooler basin fluid settled from the gravity current to the basin floor a pattern that would have continued whether or not a true steady state was reached if the early fluids were buoyant in seawater they ponded after mixing forming a hybrid brine pool that would have required longer periods to homogenize at the peak temperature 300 c and volume flux the salinity of the vent fluids fell to 7 5 wt percent but the flow paths would have been maintained unless the brine pool had been drained at this time that part of the massive sulfide body over the vent the cu core grew by dilatational veining together with leaching cavity infilling replacement and recrystallization chalcopyrite was deposited both in the cu core and probably at the same time in a layer over the flanking massive sulfide in the final stage as temperature fell and salinity rose avg 250 c 11 wt salinity the fluid oxidation potential increased so that barite precipitation dominated resulting in a formation of a discontinuous barite rich cap over the massive sulfide the brine pool model explains several features of the hellyer ore that differ from those of the ores of the hokuroko basin namely the large metal content high zn cu ratio low aspect ratio absence of chimney fragments presence of mineral banding and high barite sulfur isotope values features seen in many other massive sulfide orebodies

the filon norte orebody tharsis iberian pyrite belt a proximal low temperature shale hosted massive sulphide in a thin skinned tectonic belt the filon norte orebody tharsis iberian pyrite belt is one of the largest pyrite rich massive sulphide deposits of the world the present structure of the mineralization consists of an internally complex low angle north dipping thrust system of variscan age there are three major tectonic units separated by thick fault zones each unit with its own lithologic and hydrothermal features they are internally organized in a hinterland dipping duplex sequence with high angle horses of competent rocks igneous and detritic rocks and massive sulphides bounded by phyllonites the mineralization is within the lower unit and is composed of several stacked sheets of massive sulphides and shales hosting a stockwork zone with no obvious zonation the intermediate unit is made up of pervasively ankeritized shales and basalts spilites here hydrothermal breccias are abundant the upper unit is the less hydrothermally altered one and consists of silicified dacites and a diabase sill the tectonic reconstruction suggests that the sequence is inverted and the altered igneous rocks were originally below the orebody carbon oxygen and sulphur isotopes in the massive sulphides and hydrothermal rocks as well as the mineral assemblage and the paragenetic succession suggest that the sulphide precipitation in the sea floor took place at a low temperature 150 c without indication at least in the exposed section of a high temperature copper rich event sporadic deep subsea floor boiling is probably responsible for the formation of hydrothermal breccias and the wide extension of the stockwork its co au enrichment is interpreted as being related with the superposition of some critical factors such as the relationship with black shales the low temperature of formation and the boiling of hydrothermal fluids the present configuration and thickness of the orebody is due to the tectonic stacking of a thin and extensive blanket 2 4 km2 of massive sulphides with low aspect ratio they were formed by poorly focused venting of hot modified seawater equilibrated with underlying rocks into the seafloor massive sulphide precipitation took place by hydrothermal fluid quenching bacteriogenic activity and particle settling in an unusual restricted euxinic and shallow basin brine pool with a low detritic input but with important hydrothermal activity related to synsedimentary extensional faulting resedimentation of sulphides seems to be of major importance and responsible for the observed well mixed proximal and distal facies the tectonic deformation is largely heterogeneous and has been mostly channelled along the phyllonitic tectonized shales deformation bands thus sedimentary and diagenetic textures are relatively well preserved outside the deformation bands in the massive sulphides superimposed variscan recrystallization is not very important and only some early textures are replaced by metamorphic tectonic ones the stockwork is much more deformed than the massive sulphides the deformation has a critical effect on the present morphology of the orebody and the distribution of the ore minerals this deposit is a typical example of the sheet like shale hosted anoxic low temperature and zn rich massive sulphides developed in a ensialic extensional basin

microthermometry and chemical composition of fluid inclusions from the mt chalmers volcanic hosted massive sulfide deposits central queensland australia implications for ore genesis mt chalmers is a mound style volcanic hosted massive sulfide vhms deposit in central queensland australia the ore lenses are hosted by rhyolitic volcanics and sedimentary rocks of early permian age the two ore lenses main lode and west lode consist of cu zn pb massive sulfide underlain by cu rich stringer mineralization textural and petrographic investigations of fluid inclusions indicate that primary type i inclusions up to 25 microns are found in quartz from the stringer mineralized zone and microthermometric studies of these inclusions yielded homogenization temperatures of 160 285 degrees c and salinities of 5 10 5 nacl equiv wt laser raman spectroscopic lrs analysis indicates the presence of co2 0 1 1 mol in the mt chalmers vhms systems semiquantitative sem wds microprobe analyses of fluid inclusion decrepitates indicate that the mt chalmers ore fluids were enriched in potassium and calcium but depleted in magnesium relative to seawater pixe microanalysis of fluid inclusions in quartz from the stringer zone also indicates a significant base metal concentration in these fluids cation composition and higher salinities relative to seawater suggest that recycled seawater cannot be the sole source of the ore fluids high base metal content and the presence of co2 in the fluid inclusions imply that magmatic input of ore metals copper in particular accompanying seawater leaching of the footwall volcanic pile is a distinct possibility in terms of fluid composition the k ca fe variation of the mt chalmers ore fluid is comparable with those of typical epithermal deposits e g thames new zealand and porphyry copper deposits bingham ut the cu zn fe 10 plot of the mt chalmers ore fluids indicates that there is significant copper in the system comparable to copper enrichment in a porphyry copper system the mt chalmers ore fluids also show similar copper content with the cu rich end member ore fluid composition of the mineralized stage 2b veins of the hellyer vhms deposit whereas the stage 2a veins of hellyer are more enriched in lead and zinc overall the ore fluids have a variable chemistry with a continuum of compositional data from vhms to epithermal porphyry style ore fluids shallow water emplacement 300 m for the vhms mineralization has been postulated for the mt chalmers deposit based on the presence of trace fossils in the footwall and hanging wall sedimentary rocks and volcanic facies studies however fluid inclusion studies do not rule out a moderate to deeper submarine environment as there is no fluid inclusion evidence of boiling at mt chalmers boiling of ore fluids would probably have occurred as in most epithermal systems if the ore fluid exhaled onto the seafloor under a shallow environment

isotope geochemistry tracks the maturation of submarine massive sulfide mounds iberian pyrite belt the massive sulfide deposits of the southern iberian pyrite belt ipb occur as shale hosted exhalative mounds or shallow sub seafloor replacive bodies that formed at the devonian carboniferous boundary in anoxic bottoms the geochemistry of the mineralization and the host shale from some of the major deposits including a detailed 50 m thick section through the migollas orebody shows a marked textural and vertical zonation that is also reflected in the chemical textural and isotopic variations of pyrite and carbonates pyrite evolves from primary framboids and colloform aggregates to late euhedral crystals which accompanies a redistribution of the contained base metals and grain coarsening sims analyses depict an extreme variability of 34s values from 42 3 to 4 4 with a gradual increase that is related to the textural maturation there is also a systematic rise in the 87sr 86sr ratios 0 70846 to 0 71354 13c 12 2 to 5 2 and 18o values 14 1 to 27 8 of the associated carbonates which evolve from early dolomite to late siderite this evolution evidences the maturation of the exhalative mineralization during mixing between two fluids modified seawater enriched in biogenically derived reduced sulfur and upflowing hydrothermal fluids early sulfides precipitated in the seawater mound interface towards the core of the mounds there was an increasing influence of deep fluids that modified the early mineral textures and the isotopic signatures this evolution is similar to that observed in modern submarine hydrothermal systems and confirms the importance of mound refining in the evolution of fossil exhalative mounds and the formation of economic ores

the pulpi gigantic geode almeria spain geology metal pollution microclimatology and conservation the discovery of the giant geode of pulpi almeria spain was considered as an important highlight in the geological heritage of spain projects developed for their conser vation were immediately initiated with legal figures of protection and tourist projects the geode has a tourist interest which must be tem pered by environmental restrictions limiting the public visits first results demonstrate that a continuous visit of two or three people for more than 10 min provokes the appearance of condensation and risks corrosion of the gypsum crystals in addition the electron microprobe analyses con firms 1 the hydrothermal phases of iron manganese in carbonated host rock 2 the presence of sulphides with fe zn pb ag sb cu hg as te se and 3 ba ca and sr sulphates with mercury traces the present proposal to label the geode and the mining environment as geological natural heritage is feasi ble although any tourist adaptation must not permit visits to the geode indoor and hg levels must be con trolled

melt segregation from partially molten source regions the importance of melt density and source region size the compressibility of basic melt at 1 atmosphere is about an order of magnitude higher than that of mantle minerals consequently the density contrast between melt and the principal residual crystals in mantle source regions is expected to decrease with increasing source region depth the increasingly olivine normative character of primary melts produced at greater depths is also expected to result in a decrease in this density contrast with increasing source region depth once vertical permeability is established by melt generated during partial melting buoyancy driven melt percolation can under some circumstances segregate melt from the residual crystals in its source region on a geologically rapid time scale limits to this process are provided by cooling of the source region freezing melt in and rigidity of the crystalline matrix mechanically trapping melt source region size influences these limits strongly consequently small partially molten diapirs km in diameter may be able to trap large melt fractions 30 but larger source regions would be unable to do so the reduction in density contrast with pressure reduces the buoyant force driving melt percolation and provides another limit to melt segregation diapirs at depth may thus stably contain large fractions of melt but may decompress and unload their melt during ascent this effect would be enhanced in small diapirs and may be relevant to the genesis of komatiitic magma melt compression may also be a factor in explaining why the very different maximum depths inferred for typical basic melt segregation from source regions on different planets 500 km on the moon 250 km on mars 100 km on earth correspond to similar pressures 25 35 kbar at greater pressures melt may no longer be capable under ordinary conditions of segregating upwards by buoyancy this may also help to explain why depleted peridotites overlie more fertile peridotites and how deep regions of the mantle are able to remain fertile over geologic time

olivine flotation in mantle melt abstract molten komatiite and peridotite have been compressed in an octahedral multi anvil device up to 10 gpa densities of the melts were measured at pressure intervals in the range 7 to 10 gpa by observing sinking and floating san carlos olivines and synthetic forsterite marker spheres the multi anvil results for komatiite when combined with piston cylinder measurements done at 4 to 6 gpa and a calculated reference density at 105 pa yield a birch murnaghan isothermal bulk modulus of k1900c 26 gpa and pressure derivativek 4 25 the pressure of neutral buoyancy for olivine in komatiite is confirmed to be near 8 gpa as predicted in earlier work olivine flotation in the experimental komatiite commences at a pressure close to where the liquidus phase changes from olivine to denser garnet leading to the possibility of density driven crystal sorting during fractionation molten peridotite klb 1 shows an isothermal compression 2000 c of 0 065 g cm 3 gpa 1 in the interval 105 pa to 8 2 gpa the olivine liquid peridotite density crossover is predicted to lie between 9 and 11 gpa indicating that olivine flotation can operate at depths of 300 500 km in a molten peridotitic mantle

assessing the role of slab rheology in coupled plate mantle convection models reconstructing the 3d structure of the earth s mantle has been a challenge for geodynamicists for about 40 yr although numerical models and computational capabilities have substantially progressed parameterizations used for modeling convection forced by plate motions are far from being earth like among the set of parameters rheology is fundamental because it defines in a non linear way the dynamics of slabs and plumes and the organization of lithosphere deformation in this study we evaluate the role of the temperature dependence of viscosity variations up to 6 orders of magnitude and the importance of pseudo plasticity on reconstructing slab evolution in 3d spherical models of convection driven by plate history models pseudo plasticity which produces plate like behavior in convection models allows a consistent coupling between imposed plate motions and global convection which is not possible with temperature dependent viscosity alone using test case models we show that increasing temperature dependence of viscosity enhances vertical and lateral coherence of slabs but leads to unrealistic slab morphologies for large viscosity contrasts introducing pseudo plasticity partially solves this issue producing thin laterally and vertically more continuous slabs and flat subduction where trench retreat is fast we evaluate the differences between convection reconstructions employing different viscosity laws to be very large and similar to the differences between two models with the same rheology but using two different plate histories or initial conditions

giant pyritic base metal deposits the example of feitais aljustrel portugal detailed study of hanging wall rocks and alteration above the giant feitais orebody of aljustrel iberian pyrite belt portugal shows that cherts that constitute the immediate hanging wall to the sulfide deposit result from hydrothermal alteration of laterally equivalent fe and or mn rich jaspers textures geochemistry and lack of fossil radiolarians indicate that these jaspers are hydrothermal precipitates hydrothermal alteration of the jaspers took place concomitantly with sulfide precipitation through reaction with hot 150 c reduced fluids with replacement of hematite by magnetite pyrite and fe2 phyllosilicates on the basis of the above and considering also the results of previous studies on hydrothermal activity in the aljustrel volcanic rocks and sulfide ores we propose that sulfide precipitation may have taken place under a thin veneer of siliceous sediments present day massive sulfide formation in the guaymas basin and in the salton sea under hundreds of meters of sediments may be approximate analogues in the sense that the presence of a caprock reduces dispersion of black smoke and causes more efficient precipitation

zn pb cu massive sulfide deposits brine pool types occur in collisional orogens black smoker types occur in backarc and or arc basins volcanic hosted massive sulfide deposits of zn pb cu type were derived either from seawater dominated buoyant fluids that built mounds on the seafloor e g the ores of the hokuroku basin japan or from saline fluids that reversed buoyancy on mixing with seawater and filled basins on the seafloor e g several ores of the iberian pyrite belt and the mount read province in tasmania the hokuroku ores formed above subduction zones during protracted periods of regional extension but both the iberian pyrite belt and the mount read province formed under extensional stress during continent continent iberia or arc continent mount read collision and orogenesis in the iberian pyrite belt oblique convergence resulted in transcurrent faulting and this may also be the case for the mount read province transcurrent faulting may have allowed easy vertical access for the ore related magmas some of which were sourced in the asthenosphere as well as the blueschist and eclogitic facies emplaced in adjacent terrains and possibly also saline fluids exsolved from the magmas

zonal flow formation in the earth s core as zonal flows are common in turbulent convection regimes such as in the atmospheres of giant planets and the earth s oceans they might also be present in the earth s outer core which is thought to be in a turbulent state takehiro miyagoshi and colleagues have investigated numerical simulations of the earth s geodynamo with lower viscosities than most previous simulations have been able to use and have identified a new convection regime of core flow that exhibits a dual structure inner sheet like radial plumes surrounded by westward cylindrical zonal flow thus convection in the earth s outer core may therefore resemble that seen in planetary atmospheres and oceans more closely than was expected zonal jets are common in nature and are spontaneously generated in turbulent systems because the earth s outer core is believed to be in a turbulent state it is possible that there is zonal flow in the liquid iron of the outer core by investigating numerical simulations of the geodynamo with lower viscosities than most previous simulations have been able to use a convection regime of the outer core is now found that has a dual structure comprising inner sheet like radial plumes and an outer westward cylindrical zonal flow zonal jets are very common in nature well known examples are those in the atmospheres of giant planets and the alternating jet streams found in the earth s world ocean1 zonal flow formation in nuclear fusion devices is also well studied2 a common feature of these zonal flows is that they are spontaneously generated in turbulent systems because the earth s outer core is believed to be in a turbulent state it is possible that there is zonal flow in the liquid iron of the outer core here we report an investigation at the current low viscosity limit of numerical simulations of the geodynamo we find a previously unknown convection regime of the outer core that has a dual structure comprising inner sheet like radial plumes and an outer westward cylindrical zonal flow we numerically confirm that the dual convection structure with such a zonal flow is stable under a strong self generated dipole magnetic field

late hercynian polymetallic vein type base metal mineralization in the iberian pyrite belt fluid inclusion and stable isotope geochemistry s o h cl late variscan vein type mineralization in the iberian pyrite belt related to the rejuvenation of pre existing fractures during late variscan extensional tectonism comprises pyrite chalcopyrite quartz galena sphalerite quartz stibnite arsenopyrite quartz pyrite quartz cassiterite scheelite fluorite galena sphalerite chalcopyrite and quartz manganese oxide mineral assemblages studies of fluid inclusions in quartz stibnite and barite as well as the sulfur isotopic compositions of stibnite galena and barite from three occurrences in the central part of the iberian pyrite belt reveal compelling evidence for there having been different sources of sulfur and depositional conditions quartz stibnite mineralization formed at temperatures of about 200 c from fluids which had undergone two phase separation during ascent antimony and sulfide are most probably derived by alteration of a deeper lying volcanic hosted massive sulfide mineralization as indicated by 34s signatures from 1 45 to 2 74 sub critical phase separation of the fluid caused extreme fractionation of chlorine isotopes 37cl between 1 8 and 3 2 which correlates with a fractionation of the cl br ratios the source of another high salinity fluid trapped in inclusions in late stage quartz from quartz stibnite veins remains unclear by contrast quartz galena veins derived sulfide and metals by alteration of a sedimentary source most likely shale hosted massive sulfides the 34s values in galena from the two study sites vary between 15 42 and 19 04 barite which is associated with galena has significantly different 34s values 0 2 to 6 44 and is assumed to have formed by mixing of the ascending fluids with meteoric water

a wave prediction system for real time sea state forecasting the considerable increase in requirements for sea state forecasts in recent years has led to development of a numerical wave forecasting system in the meteorological office this is based on a wave prediction model which combines the advantages of the parametric technique in predicting a growing wind sea with those of a discrete spectral model in the swell regime this is done using a discrete model by parametrizing the nonlinear interactions term in the energy balance equation in a way that reproduces the behaviour of the parametric model the main difficulty with the method is in the separation of wind sea and swell required to do this propagation of wave energy is performed using an accurate form of the lax wendroff integration scheme a two term representation of wave growth is used whilst dissipation is modelled by an explicit whitecapping mechanism shallow water effects are included by representations of shoaling refraction and bottom friction the operational numerical atmospheric model at the meteorological office provides the wind input an extensive program of evaluation has shown that the results provide high quality guidance with 24 hour forecasts of wave height having a r m s error ranging from 0 6 m in the southern north sea to 1 0 m east of the shetlands

high pressure equation of state of molten anorthite and diopside new hugoniot equation of state data for molten diopside at 1773 k and molten anorthite at 1923 k are reported to 38 and 35 gpa respectively the diopside data initial density 2 61 mg m 3 are described by a straight line fit to the shock velocity particle velocity results of u s 3 30 1 44 u p km s 1 and our preferred fit to the anorthite data initial density 2 55 mg m 3 is given by u s 2 68 1 42 u p km s 1 reduction of the data to a third order birch murnaghan isentrope assuming the gruneisen ratio times the density is a constant and the mie gruneisen equation of state gives k 0s 22 4 gpa and k s 6 9 for diopside for anorthite we calculate k 0s 17 9 gpa and k s 5 3 the present data for diopside are used to calculate the diopside solidus at high pressures we expect the solidus to be shallow above 10 gpa but the lack of data on the variation of either the gruneisen parameters of the liquid and crystal or the heat capacity and thermal expansion at elevated pressures makes extrapolation of fusion curves uncertain solidus temperatures of 2400 2500 k and 2560 2705 k for diopside are calculated at 10 and 20 gpa respectively the new data are combined with those of rigden et al 1988 for the di 0 64 a 0 36 eutectic composition to examine the degree to which such liquids mix ideally with respect to volume up to 25 gpa for the eutectic composition at 1400 c we calculate the volumes of the an and di mix nearly ideally to 25 gpa we find that the ratio of the partial molar volumes of the oxides in silicate melts to that of the crystal oxides at 1673 k and 1 atm is 1 0 0 1 for a wide range of oxide components for the low pressure tetrahedrally coordinated oxides e g sio 2 al 2o 3 fe 2o 3 the ratio is 1 3 with respect to oxides such as stishovite corundum and hematite in which the cations are octahedrally coordinated by oxygens if changes in coordination of al and si from tetrahedral at low pressures to octahedral at high pressures occur in melts they do so gradually over an interval of 40 gpa although 1 atm bulk moduli for a wide compositional range of silicate melts are similar the differences in integrated compression to mixed oxide like high pressure configurations are reflected mainly by variations in k t k t is found to vary inversely with fraction of network forming initially tetrahedrally coordinated cations e g al 3 si 4 thus k t which may be uncertain by 1 5 is estimated to vary from 7 for molten anorthite enstatite and diopside to 8 for molten ferrosilite to 10 for molten forsterite and to 11 for molten fayalite

effects of low viscosity post perovskite on the stability and structure of primordial reservoirs in the lower mantle we performed numerical experiments of thermochemical convection in 3 d spherical geometry to investigate the effects of a low viscosity of post perovskite ppv on the stability and structure of primordial reservoirs of dense material in the lower mantle of the earth our results show that weak ppv 1000 viscosity reduction in regions containing ppv strongly increases the core mantle boundary cmb heat flux the volume averaged mantle temperature with weak ppv is slightly higher than that with regular ppv except in the lowermost mantle this is because weak ppv weakens the base of the cold downwellings allowing cold slabs to spread more easily and broadly along the cmb the stability and size of the dense reservoirs is not substantially altered by weak ppv in the weak ppv case slabs spreading along the cmb slightly decreases the stability of dense reservoirs i e the amount of dense material entrained upward is slightly larger than in the regular ppv case i e viscosity of ppv identical to that of perovskite furthermore the topography and steepness of these reservoirs slightly increase however as in the regular ppv case the dense reservoirs are maintained over periods of time comparable to the age of the earth

the equation of state of a molten komatiite 2 application to komatiite petrogenesis and the hadean mantle new experimental data for the equation of state of a komatiitic liquid were used to model adiabatic melting in a peridotitic mantle if komatiites are formed by 30 partial melting of a peridotitic mantle then komatiites generated by adiabatic melting come from source regions that began their unmelted ascent in the lower transition zone 500 670 km or the lower mantle 670 km the great depth of incipient melting implied by this model suggests that komatiitic liquids may form in a pressure regime where they are denser than their coexisting crystals possibly the bulk mantle although komatiitic magmas are thought to separate from residual crystals in the mantle at a temperature 200 c greater than modem mid ocean ridge basalts morbs their ultimate sources are predicted to be diapirs that if adiabatically decompressed from initially solid mantle were more than 700 c hotter than the sources of morbs and were derived from great depth we also studied the evolution of an initially molten mantle i e a magma ocean our model considers the thermal structure of the magma ocean density constraints on crystal segregation and approximate phase relationships for a nominally chondritic mantle crystallization will begin at the core mantle boundary perovskite buoyancy at 70 gpa may lead to a compositionally stratified lower mantle with iron enriched magnesiowustite content increasing with depth large convective velocities in the magma ocean would prohibit crystal liquid fractionation by settling or flotation until quiescent boundary layers form such boundary layers could form when the crystal content of the magma reaches a critical value near 44 vol and in the late stages of crystallization around unmelted blocks of foundered protocrust matrix compaction and diapirism could also lead to fractionation effects the upper mantle could be depleted or enriched in perovskite components relative to the bulk mantle olivine neutral buoyancy may lead to the formation of a dunite septum in the upper mantle partitioning the ocean into upper and lower reservoirs but this septum must be permeable

shock compression of liquid silicates to 125 gpa the anorthite diopside join we determined the equation of state eos of three silicate liquid compositions by shock compression of preheated samples up to 127 gpa diopside di ca 2mg 2sio 6 at 1773 k anorthite an caal 2si 2o 8 at 1923 k and the eutectic composition di 64 an 36 at 1673 k were previously studied by shock compression to 38 gpa the new data extend the eos of each composition nearly to the earth s core mantle boundary the previously reported anomaly at 25 gpa for di 64 an 36 eutectic was not reproduced rather all data for this composition fit within error a straight line hugoniot in particle velocity vs shock velocity di also displays a linear hugoniot consistent with ultrasonic data and a third order finite strain eos the full anorthite data set is complex we examine a model with a transition between two structural states and a fourth order finite strain model excluding two points that may not display relaxed behavior we also report an experiment on room temperature solid di 64 an 36 aggregate that clearly demonstrates increase upon compression of the gruneisen parameter of this liquid much as experiment and theory have shown for forsterite and enstatite liquids we construct isentropes and isotherms from our hugoniots using mie gruneisen thermal pressure and evaluate the model of ideal mixing of volumes volume may mix almost linearly at high temperature but deviates strongly when calculated along an isotherm it remains difficult to reach a firm conclusion we compare the densities of liquids to lower mantle solids our results suggest that basaltic liquids rich in cao and al 2o 3 are notably denser than liquids in the mgo sio 2 binary and subject to uncertainties in the behavior of feo and in corrections for thermal pressure such liquids may be the most likely candidates for achieving negative buoyancy in the lowermost mantle

interactions of scales of convection in the earth s mantle abstract the existence of undulations of the geoid gravity and bathymetry in ocean basins as well as anomalies in heat flow point to the existence of small scale convection beneath tectonic plates the instabilities that could develop at the base of the lithosphere are sufficiently small scale 60 ma they form networks that are shaped by the plate evolution slabs plumes and the geometry of continental boundaries plumes impacting the boundary layer from below have a particular influence through rejuvenating the thermal lithosphere they create a wake in which new instabilities form downstream these wakes form channels that are about 1000 km wide and thus are possibly detectable by seismic tomography beneath fast plates cold sinking instabilities are tilted in the direction opposite to plate motion while they sink vertically for slow plates these instabilities are too small to be detected by usual seismic methods since they are about 200 km in lateral scale however this preferred orientation of instabilities below fast plates could produce a pattern of large scale azimuthal anisotropy consistent with both plate motions and the large scale organisation of azimuthal anisotropy obtained from recent surface wave models

static compression and olivine flotation in ultrabasic silicate liquid piston cylinder techniques have been used to observe static compressions of ultrabasic silicate liquids to 6 gpa and 2000 c compressions are measured by bracketing sinking and floating of olivine crystals of known density at run conditions komatiite peridotite and their liquid mixtures with fe2sio4 all show constant density changes with pressure of 0 07 g cm3 gpa along their liquidi these results confirm shock wave studies and ultrasonic studies which show weak compositional effects on compressibility of basic to ultrabasic silicate liquid the techniques converge in supporting the conjecture of stolper et al 1981 that ultrabasic silicate liquids may be denser than their equilibrium coexisting olivines and pyroxenes at high compressions the pressure of this density inversion is 7 8 gpa for the compositions studied olivine flotation provides an important constraint upon earth evolution models that include extensive melting during early differentiation it also provides a mechanism for concentrating high mg si olivine in the upper mantle without unduly perturbing other geochemical parameters a full evaluation of the physical and geochemical consequences is required at an early date