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

reconstruction of the geometry of accretion during formation of the bay of islands ophiolite complex two types of highly allochthonous ophiolitic terrains crop out adjacent to one another in the bay of islands region of southwestern newfoundland canada the first called the coastal complex cc is exposed as a long linear belt along the west coast it consists of a highly deformed ophiolitic assemblage interpreted as oceanic crust and upper mantle with a previous transform fault tectonic history the second called the bay of islands complex boic crops out to the east and consists of a series of four relatively undeformed ophiolitic massifs interpreted as oceanic crust and upper mantle with a more normal seafloor spreading tectonic history an autochthonous relationship can be demonstrated between the two types of ophiolitic terrains in the southernmost of these massifs indicating that each evolved along a single ridge transform system the internal structure of both the boic and the cc is consistent with the above interpretations the ne sw structural trends in the cc transform are oriented approximately parallel to the sea floor spreading direction inferred from the average orientation of the sheeted diabase dikes within the boic one way chilling statistics from the sheeted diabase dike unit growth directions of dendritic crystals along inclined comb layers in the plutonic section and the sense of shear along the cc transform all indicate that the boic was accreted on the ne side of a nw se trending spreading center present geographic reference frame juxtaposition of the boic with the ne sw trending cc occurred at a ridge transform intersection the transform exhibited a left lateral offset magma chamber profiles have been identified in the layered gabbroic sections of the boic by using the patterns outlined by the attitudes of fine scale igneous layering these patterns depict arcuate structures which appear to define roughly a cross sectional view of a systematically oriented half cylinder the long axis of which is oriented parallel with the plane of the sheeted dikes and major lithologic unit contacts thus we believe that a large cylindrical shaped crustal level subaxial magma chamber extended in a direction parallel with the ridge axial plane at the time of formation of the bolc the cc marks the abrupt truncation of this long linear magma chamber petrologic constraints demonstrate that the ultramafic and transition zone cumulates which form the base of the plutonic section crystallized at a range of high to intermediate pressures possibly along a long narrow conduit extending to 30 km depth beneath the crustal level magma chamber after formation these cumulates were deformed at high temperatures in the zone of mantle upwelling and divergence then tectonically transported to crustal or near crustal levels to form the basement upon which the remainder of the ophiolite stratigraphy was accreted a geometric and tectonic model is presented for the accretion process responsible for formation of the bay of islands ophiolite

comparison of the geochemistry of ophiolitic pyroxenites with a strongly fractionated dyke of pyroxenite from the sakhakot qila ophiolite pakistan abstract exceptionally strong fractionation has been found amongst the pyroxenite dykes crosscutting the ultramafic rocks of the sakhakot qila ophiolite pakistan one such dyke an fe websterite is particularly fractionated and anomalously feenriched and mg depleted this is supported by the data from whole rock analyses of major trace and rare earth elements as well as by the mineral chemistry of the constituent chromian spinel olivine clinopyroxene orthopyroxene and magmatic amphibole secondary sulphides of the fe websterite include troilite pentlandite instead of the awaruite heazle woodite pentlandite assemblage typical of all the other ultramafic rocks of the host ophiolite the non cumulate parent liquid of the magnesian pyroxenites crystallized in an initial episode when the surrounding cumulates were still hot in a belated second episode the fe websterite may have crystallized in isolated pockets after its differentiation to an extent much greater than that known from most pyroxenite dykes in ophiolites

comment on weathering plants and the long term carbon cycle by robert a berner berner 1992 has asserted that jackson and keller 1970a misinterpreted the conspicuous reddish crust which forms on young lava flows in areas of rock surface colonised by the lichen stereocaulon vulcani but not in adjacent areas of bare rock in regions of high rain fall on the island of hawaii jackson 1968 and jackson and keller 1970a b concluded from the results of a thorough interdisiplinary investigation employing a wide spectrum of techniques and information that his reddish coating is an intensely leached weathering crust formed in situ and that biochemical activities of the lichen or its associated microflora not only accelerate the chemical weathering of the rock by orders of magnitude but also determine the specific mineralogical and chemical properties of the weathering products berner however maintained that the reddish crust is in reality a deposit of open quotes wind blown soil dust close quotes entrapped by a sticky organic substance secreted by the lichen berner fixed his attention on just one aspect of the many sided body of interrelated data on which the conclusions of jackson and keller are founded the observation that the weathering crust is much thicker on lichen covered rock surfaces than on lichen free open quotes control close quotes areas of the same rock more the totality of published evidence overwhelmingly supports the conclusions of jackson and keller an demonstrates that berner s rival hypothesis is untenable less

petrology of the ispendere malatya ophiolite from the southeast anatolia implications for the late mesozoic evolution of the southern neotethyan ocean

mineral chemistry of ultramafic and mafic cumulates as an indicator of the arc related origin of the mersin ophiolite southern turkey the mersin ophiolite represented by approximately 6 km thick oceanic lithospheric section on the southern flank of the taurus calcareous axis formed in the mesozoic neo tethyan ocean some time during late cretaceous in southern turkey the ultramafic and mafic cumulates having over 3 km thickness consist of dunite chromite wehrlite clinopyroxenite at the bottom and pass into gabbroic cumulates in which leucogabbro olivine gabbro and anorthosite are seen crystallization order is olivine fo91 80 chromian spinel cr 60 80 clinopyroxene mg 95 77 plagioclase an95 6 91 6 and orthopyroxene mg 68 77 mineral chemistry of ultramafic and mafic cumulates suggest that highly magnesian olivines clinopyroxenes and absence of plagioclase in the basal ultramafic cumulates are in good agreement with products of high pressure crystal fractionation of primary basaltic melts beneath an island arc environment major trace element geochemistry of the cumulative rocks also indicate that mersin ophiolite was formed in an arc environment coexisting ca rich plagioclase and forich olivine in the gabbroic cumulates show arc cumulate gabbro characteristics field relations as well as the geochemical data support that mersin ophiolite formed in a supra subduction zone tectonic setting in the southern branch of the neo tethys in southern turkey

birth death and resurrection the life cycle of suprasubduction zone ophiolites 1 suprasubduction zone ssz ophiolites display a consistent sequence of events during their formation and evolution that suggests that they form in response to processes that are common to all such ophiolites this sequence includes the following 1 birth which entails the formation of the ophiolite above a nascent or reconfigured subduction zone this stage is typically characterized by the eruption of arc tholeiite lavas and the formation of layered gabbros and sheeted dike complex 2 youth during which is continued melting of refractory asthenosphere depleted during birth occurs in response to fluid flux from the subducting slab with extensional deformation of the older plutonic suite eruption of refractory lavas and the intrusion of wehrlite pyroxenite 3 maturity with the onset of semistable arc volcanism typically calc alkaline as the subduction zone matures and stabilizes and the intrusion of quartz diorite and eruption of silicic lavas and 4 death which is the sudden demise of active spreading and ophiolite related volcanism which in many cases is linked to collision with an active spreading center and the onset of shallow underthrusting of the buoyant spreading axis expressed as dikes and lavas with oceanic basalt compositions that crosscut or overlie rocks of the older suites 5 resurrection with emplacement by obduction onto a passive margin or accretionary uplift with continued subduction the early stages 1 3 may be diachronous and each stage may overlap in both time and space the existence of this consistent progression implies that ophiolite formation is not a stochastic event but is a natural consequence of the ssz tectonic setting

a model of correlated episodicity in magnetic field reversals climate and mass extinctions correlated periodicities of mass extinctions climate magnetic field reversals and other geological phenomena may best be explained by cycles of activity within the mantle and core we propose that these cycles are due to variations in the thickness of the thermal boundary layer at the base of the mantle as the layer alternates between two phases of activity in the quiescent phase little flow occurs and the layer thickens with time by thermal diffusion the active phase begins when the thickening layer becomes dynamically unstable hot material erupts from the layer causing it to become thinner as the layer thins the rate of energy supply to the geodynamo and hence its reversal frequency increases the hot material rises to the surface where it causes widespread basaltic volcanic eruptions these eruptions release large amounts of co 2 and sulfates which have a pronounced effect on the climate and biota

mantle crust transition zone and origin of wehrlitic magmas evidence from the oman ophiolite abstract abundant exposure and well preserved outcrops in the oman ophiolite allow detailed observation of the transition from mantle to crust in an oceanic environment the mantle crust transition zone shows large lateral variations in thickness and composition but consists essentially of residual dunites and a magmatic component present as clinopyroxene and plagioclase impregnations and gabbro lenses compositionally the transition zone marks a gradational passage from mantle to crust the moho may be placed at the base of the continuous gabbroic crustal section structures both within the transition zone and near the base of the crust are nearly parallel to the moho however the dominant flow mechanisms differ from the upper mantle to the lower crust plastic flow dominates in the upper mantle while viscous flow resulting in strong magmatic fabrics dominates at crustal levels wehrlitic bodies within the crustal gabbro section are intruded from the transition zone into which they are rooted the parent magma of the wehrlitic instrusions is in fact a crystal melt mixture composed of olivine and chromite probably including a component of mantle xenocrysts and xenoliths and of reequilibrated basaltic melt

petrology and petrogenesis of cumulate peridotites and gabbros from the marum ophiolite complex northern papua new guinea

textural and compositional evidence for magma mixing and its mechanism abu volcano group southwestern japan quaternary basalts andesites and dacites from the abu monogenetic volcano group sw japan composed of more than 40 monogenetic volcanoes show two distinct chemical trends especially on the feo mgo vs sio2 diagram one trend is characterized by feo mgo enrichment with a slight increase in sio2 content fe type trend whereas the other shows a marked sio2 enrichment with relatively constant feo mgo ratios si type trend the fe type trend is explained by fractional crystallization with subtraction of olivine and augite from a primitive alkali basalt magma rocks of the si type trend are characterized by partially melted or resorbed quartz and sodic plagioclase phenocrysts and or fine grained basaltic inclusions they are most likely products of mixing of a primitive alkali basalt magma containing olivine phenocrysts with a dacite magma containing quartz sodic plagioclase and hornblende phenocrysts petrographic variation as well as chemical variation from basalt to dacite of the si type trend is accounted for by various mixing ratios of basalt and dacite magmas pargasitic hornblende and clinopyroxene phenocrysts in andesite and dacite may have crystallized from basaltic magma during magma mixing olivine and spinel and quartz sodic plagioclase and common hornblende had crystallized in basaltic and dacitic magmas respectively before the mixing within a lava flow the abundance of basaltic inclusions decreases from the area near the eruptive vent towards the perimeter of the flow and the number of resorbed phenocrysts varies inversely suggesting zonation in the magma chamber

petrogenesis of a basalt rhyolite tephra from the west central afar ethiopia the cindery tuff is an unusual tephra fall deposit that contains evidence for the mixing of basaltic and rhyolitic liquids prior to eruption it contains clear rhyolitic glass shards together with brown basaltic glass spheres and a broadly bimodal phenocryst assemblage brown glasses are ferrobasaltic in composition and are similar to the voluminous pliocene tholeiites of the surrounding west central afar volcanic field both are enriched in the light rare earth and incompatible elements and possess higher 87sr 86sr and lower 143nd 144nd than morb rhyolitic glasses are subalkaline and compared to the basaltic glasses are strongly depleted in the compatible elements and enriched in the incompatible elements both glass types have similar incompatible element and isotopic ratios and with the rhyolite glass showing a 2 fold parallel enrichment in rare earth element abundances over the basaltic glass these observations suggest that the two glasses are genetically related

the structure of sheeted dikes and associated rocks in north arm massif bay of islands ophiolite complex and the intrusive process at oceanic spreading centers the structures of north arm mountain ophiolitic basalt and diabase indicate that these formed as a frozen lid to an underlying magma chamber in a system analogous to present day mid ocean spreading systems volcanics consist of pillow lavas flows and breccias volcanics grade downward into sheeted dikes by increase in dike frequency across a transition commonly less than 50 m thick sheeted dikes subparallel throughout dip away from the paleospreading axis located to the west of present ophiolite position as indicated by chilled margin bias implying rotation of the lid down and away from the axis one way chilling statistics dike thickness and dike intrusion sequence analyses all point toward a relatively narrow but complex zone of dike intrusion the mean measured dike thickness is 0 90 m diabase contains zones of fracturing subparallel to dike trends interpreted to reflect oceanic fissuring and faulting these commonly terminate downward in foliated gabbro and or lineated amphibolite diabas

evidence for two stage mixing in magmatic inclusions and rhyolitic lava domes on niijima island japan abstract fine grained inclusions are found in the achiyama and the mukaiyama rhyolitic lava domes on niijima island japan the existence of fine grained rims in the inclusions indicates that these inclusions are a quenched mafic magma incorporated into their rhyolite host the chemical composition and the phenocryst assemblage of the inclusions in the achiyama dome are identical with those of the effusive basaltic lava on niijima island wakago basalt the inclusions of the mukaiyama dome are dacitic and have a disequilibrium phenocryst assemblage olivine clinopyroxene orthopyroxene hornblende biotite plagioclase quartz and opaque minerals whole rock compositions lie on a mixing line between the host rhyolite and wakago basalt the dacitic magma is therefore a product of mixing of the basaltic and rhyolitic magmas the occurrence of two different inclusions basaltic and dacitic in two different domes can be explained by a two stage magma mixing model the dacite is formed in the first stage by mixing in a vertically stratified magma chamber caused by an interfacial instability between rhyolitic and basaltic layers a small amount of rhyolitic magma is entrained into the lower basaltic magma layer and two magmas can mix essentially in a liquid state the mixed dacitic magma is formed in the lower layer the second stage is mixing caused by forced convection during the ascent through a conduit just before eruption the stably stratified silicic and mafic magmas in a magma chamber can turn over and efficiently mix because of the formation of an unstable flow in a conduit under this condition the system reaches thermal equilibrium immediately after the mafic magma is disaggregated and dispersed in the silicic host the mafic magma solidifies when the silicic endmember is predominant in the mixture hence the equilibrium temperature is below the solidus of the mafic magma mafic inclusions are formed at this stage whether the inclusions are basaltic or dacitic depends on whether the final ascent through a conduit eruption occurred before or after the formation of the layer of a mixed dacitic magma in a magma chamber

a seismic refraction study of the oregon cascades a 275 km long reversed refraction profile in the oregon cascades two shallow earthquakes of magnitude 5 in southern washington a shallow earthquake of magnitude 4 6 in northern california and a previously published analysis of the bouguer gravity field are used to develop a crustal p wave velocity model for the oregon cascades travel time analysis of the refraction profile indicates a crustal structure characterized by surface layers with p wave velocities that vary from 2 9 to 5 2 km s and thicknesses that vary from 2 5 to 5 0 km upper crustal velocities of 6 1 to 6 5 km s between the depths of 3 and 29 km lower crustal velocities near 7 0 km s between the depths of 29 and 44 km and a mantle reflector at a depth of 44 km comparison with synthetic seismograms supports this structure and shows that the lower crustal and mono transitions can be better modeled by continuous velocity gradients than by first order discontinuities arrival times from two shallow earthquakes in southern washington in 1981 across 14 telemetry stations in the oregon cascades show an apparent velocity of 7 62 km s on the basis of a previously published gravity analysis the mean north south component of the dip to the crust mantle boundary is estimated at 1 down dip to the south the apparent velocity from the earthquakes and the dip estimate from gravity indicate that the true pn velocity is 7 70 km s arrival times from a shallow earthquake in northern california in 1978 across telemetry stations in the washington cascades are consistent with this model and indicate that the upper mantle velocity reaches 8 20 km s at a depth of approximately 100 km

comments on composition and depth of origin of primary mid ocean ridge basalts by d c presnall and j d hoover presnall and hoover 1984 in their evaluation of the nature of primary mid ocean ridge basalts morbs concluded that earlier data suggesting many of the most primitive morbs are derived from high mgo picritic magmas are not valid in reaching this conclusion presnall and hoover 1984 ignore several important considerations regarding 1 analytical uncertainties 2 tio2abundances in morbs 3 least squares calculations 4 complex models of magmatic evolution and 5 enstatite saturation i briefly discuss these points below

geochemical studies of abyssal lavas recovered by dsrv alvin from eastern galapagos rift inca transform and ecuador rift 2 phase chemistry and crystallization history a diverse suite of lavas recovered by dsrv alvin from the eastern galapagos rift and inca transform includes mid ocean ridge tholeiitic basalts morb iron and titanium enriched basalts feti basalts and abyssal andesites rock types transitional in character ferrobasalts and basaltic andesites were also recovered the most mafic glassy basalts contain plagioclase augite and olivine as near liquidus phases whereas in more fractionated basalts pigeonite replaces olivine and iron titanium oxides crystallize plagioclase crystallizes after pyroxenes and iron titanium oxides in andesites possibly due to increased water contents or cooling rates apatite phenocrysts are present in some andesitic glasses ovoid sulfide globules are also common in many lavas compositional variations of phenocrysts in glassy lavas reflect changes in magma chemistry temperature of crystallization and cooling rate the overall chemical variations parallel the chemical evolution of the lava suite and are similar to those in other fractionated tholeiitic complexes elemental partitioning between plagioclase pyroxene and olivine glass pairs suggests that equilibration occurred at low pressure in a rather restricted temperature range various geothermometers indicate that the most primitive morb began to crystallize between 1150 and 1200 c with fo2 10 7 atm coexisting iron titanium oxides in more evolved lavas yield temperatures 1025 c to as low as 910 c withfo2 from 10 8 to 10 12 atm ph 2 o could have been as high as 1 kbar during andesite crystallization compositions of the lavas from the galapagos rift follow the experimentally determined 1 atm qfm liquid line of descent least squares calculations for the major elements indicate that the entire suite of lavas can be produced by fractional crystallization of successive residual liquids from a morb parent magma feti basalts represent 30 65 cumulative weight percent crystallization of plagioclase augite and olivine an additional 30 50 fractionation of pyroxenes plagioclase titanomagnetite and possible apatite is required to generate andesite from feti basalt liquids the presence of partially resorbed mafic xenocrysts in some andesites feti basalt inclusions in these xenocrysts high silica glass inclusions in basaltic andesites and the transitional chemistry of basaltic andesites are evidence that some magma mixing occurred during crystal fractionation the diversity of lava types recovered at single dive sites suggests that low pressure fractional crystallization is a very efficient process beneath the eastern galapagos rift and that isolated magma bodies must be present at shallow levels beneath the accretionary locus voluminous feti basalts erupted at the rift transform intersection are genetically related to the rift lavas but their restricted chemistry reflects different thermal and tectonic controls on their petrogenesis

reinterpretation of seismic reflection data over the east pacific rise multichannel seismic reflection data over the axial region of the east pacific rise are depth migrated using detailed velocity information from laboratory measurements of ophiolite samples the data are from the lamont doherty geological observatory s line 17 shot across the rise at 9 sup 0 n the migrated data are interpreted to show the structure of the seafloor and what we believe is the magma chamber roof the polarity and apparent root mean squared velocity of the magma chamber roof are asymmetric with respect to the topographic axis of the ridge the asymmetry is probably real and not an artifact of data collection modeling of the magnetic data gathered along the line shows that spreading has not been proceeding normally a reasonable explanation for the asymmetry is the possibility that the line intersects a transform fault or an abandoned ridge segment near the ridge axis the shape of the roof reflection is convex upward with an approximate slope of 10 sup 0 and a width of 4 km if extrapolated symmetrically to the other side of the ridge axis the magma chamber roof event is consistent with the funnel shaped chamber proposed by pallister and hopson 1981 for the samail ophiolite if the chambermore roof steepens rapidly beyond the extent of the reflection it would be consistent with the mush filled model of sleep 1975 1978 and dewey and kidd 1977 less

metabasalts axial hot springs and the structure of hydrothermal systems at mid ocean ridges knowledge of the chemical transfer and mineralogical transformations that occur when sea water reacts with basalt at elevated temperatures and pressures can be used along with geological and geophysical data to deduce the typical structure and evolutionary sequence for hydrothermal systems within the oceanic crust along the axis of a mid ocean ridge studies of metabasalt and metadiabase dredged from fault scarps along the axial valley of the mid atlantic ridge reveal consistent relationships among the bulk chemistry of the altered rocks and their secondary mineralogy mineral abundances and mineral compositions especially for chlorite these relationships can be interpreted in terms of the distribution of alteration with respect to time temperature and water rock ratio in young crust assemblages of chl ab ep act chl ab ep act qtz chl ab qtz and chl qtz are produced at successively higher effective sea water to rock ratios within the temperature range of the greenschist facies 250 to 450 c toward higher ratios chlorites tend to become more mg rich pillow basalts of layer 2 are commonly altered under these conditions by sea water on the descending rather than the ascending limb of a convection system this type of alteration which reflects moderately high water rock ratios may characteristically occur above a still partially molten magma chamber as the magma chamber solidifies and is then penetrated more pervasive alteration of the deeper part of layer 2 and of layer 3 occurs at lower water rock ratios producing solutions such as those recovered from springs on the galapagos rift and the east pacific rise at 21 n localized upwelling limbs of the convection system can produce veins filled with quartz sulfides and fe rich chlorite

authigenesis of amphibole and its relationship to the diagenetic evolution of lower cretaceous sandstones of the potiguar rift basin northeastern brazil abstract authigenic amphibole occurs in the fan delta front arkoses and lithic sandstones of the neocomian pendencia formation in the potiguar rift basin northeastern brazil as overgrowths fracture healing and replacement of detrital amphibole the amphibole which is closely associated with diagenetic albite chlorite and sphene has precipitated from k na cl brines enriched in ti fe mg at temperatures 100 c these ions were derived from the diagenetic alteration of detrital amphibole and pyroxene chloritization of volcaniclastic sediments at the base of the formation and from brines that ascended along faults from the underlying basement

late cretaceous santonian campanian stratigraphy of the northern sacramento valley california the upper cretaceous coniacian lower campanian chico formation of the northeastern sacramento valley california includes three newly defined members at the type locality 1 cobble conglomerate of the basal ponderosa way member 2 coarse grained conglomeratic sandstone of the overlying musty buck member and 3 fine grained silty sandstone of the uppermost ten mile member other outcrops of the chico formation exhibit the same three members plus an additional unit the kingsley cave member composed of mudstone the chico formation resulted from a transgression onto the sierra nevada basement in the late cretaceous the kingsley cave member was deposited locally in quiet water conditions below wave base in an intrashelf basin and may represent a marine connection with regions farther northeast macrofossils allow correlation of the chico formation with marine deposits of the great valley sequence exposed along the western sacramento valley strata of the guinda and forbes formations represent submarine fan channel to outer fan deposits of a shallowing late cretaceous fore arc basin the santonian guinda formation a massive sand unit deposited in fan channel to distal fan turbidite environments is correlative with the musty buck and kingsley cave members of the chico formation the dobbins shale member of the forbes formation a widespread mudstone unit of hemipelagic outer fan to basin plain deposits is correlative with the kingsley cave member of the chico formation and reflects a santonian transgressive event in the northern fore arc basin mudstones and turbidites of the middle of the forbes formation are equivalent to the shallow marine strata of the ten mile member of the chico formation correlatives of younger forbes formation strata have not been positively identified from the chico creek region but may be represented by unfossiliferous cross bedded coarse grained sandstones found above sections of the chico formation

origin and evolution of a peraluminous silicic ignimbrite suite the violet town volcanics the violet town volcanics are a 373 ma old comagmatic s type volcanic sequence mainly comprising crystal rich intracaldera ignimbrites rock types vary from rhyolites to rhyodacites all containing magmatic cordierite and garnet phenocrysts variation in the suite is primarily due to fractionation of early crystallized quartz plagioclase and biotite plus minor accessory phases in a high level magma chamber prior to eruption early magmatic crystallization occurred at around 4 kb and 850 c with melt water contents between 2 8 and 4 wt this high temperature markedly water undersaturated restite poor granitic magma was generated by partial melting reactions involving biotite breakdown in a dominantly quartzofeldspathic source terrain leaving a granulite facies residue

weathering secondary mineral genesis and soil formation caused by lichens and mosses growing on granitic gneiss in a boreal forest environment abstract lichens and mosses growing on granitic gneiss in a boreal forest environment caused intense chemical weathering of the rock producing exclusively biogenic secondary minerals and soils and accumulating solubilised rock forming elements si and metals but weathering in the absence of these organisms was relatively ineffective apart from primary biotite the only phyllosilicates in the rock were sericite and chlorite formed by hydrothermal metamorphism or abiotic weathering or both but the biogenic soils contained expandable clay minerals and in several cases kaolinite and calcite together with illite and chlorite lichens were especially effective in altering feldspars to clay but mosses generally produced a greater abundance of expandable clay minerals and kaolinite was produced only by certain species of moss specifically the ones whose rock derived element concentrations and mg ca ratios were highest rock derived element concentrations were usually highest in mosses and element enrichment increased with the charge radius ratio of the cation of the element suggesting carrier mediated transport across cell membranes furthermore lichens and mosses produced different assemblages of expandable clay minerals and most mosses but none of the lichens produced authigenic amphiboles and pyroxenes biological weathering is attributable to solubilisation of rock forming elements by chelating agents and h2co3 bioaccumulation of the elements and nucleation of secondary silicates by chelate complexes functioning as templates for epitaxial growth of specific crystal structures the different weathering and soil forming processes of lichens and mosses suggest a mutualistic strategy for maximising the release of inorganic nutrients from rocks and retaining them in bioavailable forms in the soil

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

effect of lichen colonization on chemical weathering of hornblende granite as estimated by aqueous elemental flux abstract it has long been suspected that lichens increase the rate of physical and chemical weathering of rock surfaces thus playing a role in biogeochemical cycles however the relative weathering flux of elements from lichen colonized rock versus bare rock has been minimally studied previous attempts to quantify the effect of lichen cover on weathering have focused disproportionately on evidence from altered weathering rinds on basalt here in a field experiment on hornblende granite in new jersey usa we measured the cumulative waterborne net efflux of five elements over 31 days and six rain events from multiple constructed miniature watersheds consisting of either lichen covered or exposed bare rock on average lichen covered watersheds showed approximately double the silicon flux and three times the calcium and magnesium flux compared to bare rock additionally efflux of these elements was higher in lichen covered watersheds across all six rain events this suggests that lichens do indeed promote increased chemical weathering compared to bare rock thus likely increasing sequestration of atmospheric co 2 under equal conditions of atmospheric pco 2 temperature and rainfall it was also observed that lichen covered watersheds showed a 50 reduction in iron flux and had a greater ratio of calcium and magnesium to silicon flux compared to bare rock watersheds the possible causes of these patterns are discussed

magma chamber profiles from the bay of islands ophiolite complex models of the oceanic lithosphere derived from constraints imposed by the geology of ophiolite complexes require a magma chamber to produce the cumulate and other plutonic portions of the oceanic crust and upper mantle evidence from the bay of islands ophiolite complex indicates that layered plutonic rocks have not formed solely due to differential crystal settling to the horizontal floor of a magma chamber but have formed largely by heterogeneous nucleation and in situ crystallization along gently inclined as well as steeply inclined bounding surfaces of a large magma reservoir coherent well defined and systematically oriented large scale arcuate patterns described by mesoscopic igneous layering have been identified in the plutonic sections of two ophiolite massifs in the bay of islands complex these layering patterns seem to outline the shape of a large continuously evolving steady state magma chamber that extended along a ridge axis and ended at a transform fault intersection

crustal structure in the northwestern foothills of the sierra nevada from seismic refraction experiments abstract analysis of travel times from a reversed active seismic refraction experiment in the western foothills of the sierra nevada reveals a localized layer of high velocity material within 10 km of the surface this layer with a velocity of 7 0 km sec has a five fold increase in dip approximately 25 km south of the epicenter of the 1 august 1975 magnitude 5 7 earthquake near oroville california we interpret this material to be a remnant of buried ophiolite these data also suggest a 5 per cent anisotropy in the near surface p wave velocity the compressional velocity is 6 3 km sec parallel to the strike of the sierra range and 6 0 km sec normal to it a series of aftershocks to the magnitude 5 7 oroville earthquake recorded on a linear array of portable seismographs extending 300 km southward along the west edge of the sierra nevada foothills show an apparent p n velocity of 7 8 km sec the mohorovicic discontinuity is located between 30 and 39 km depth this wide range of depth reflects the facts that the profile is unreversed the receivers are widely spaced and the origin times and depths of the sources are not exactly known

soret separation of mid ocean ridge basalt magma chemical differentiation of an initially homogeneous mid ocean ridge basalt morb liquid has been experimentally observed in a temperature gradient above the liquidus the magnitude of this effect in producing differences in composition is comparable to that of crystal liquid fractionation for a given temperature difference however the chemical changes produced by the two processes soret and crystal liquid fractionation are different soret separations resemble those observed in a third process silicate liquid immiscibility this similarity is a reflection of the fact that the soret separable components are the same network former network modifier structural components which segregate during silicate liquid immiscibility

the supra subduction zone pozanti karsanti ophiolite southern turkey evidence for high pressure crystal fractionation of ultramafic cumulates the pozanti karsanti ophiolite pko forming one of the late cretaceous discontinuous oceanic lithosphere remnants in the eastern tauride belt in southern turkey is characterized by mantle tectonites ultramafic and mafic cumulates isotropic gabbros sheeted dikes and volcanics well preserved crustal cumulate rocks are mainly composed of dunitefchromite wehrlite olivine clinopyroxenite clinopyroxenite olivine websterite and low ti gabbro the crystallization order in the plutonic section is olivine fo93 83 fchromian spinel cr cr al 79 clinopyroxene mg mg fe 93 87 orthopyroxene mg mg fe 90 83 the mineral chemistry of the ultramafic cumulates forming the basal portion of the plutonic section of the pozanti karsanti ophiolite pko is not consistent with crystal liquid fractionation of primitive midocean ridge basalts at low pressures the presence of highly magnesian clinopyroxene and orthopyroxene together with the absence of plagioclase as early fractionating phases indicate medium to high pressure up to 10 kbar crystal fractionation of primary basaltic melts mineralogical and geochemical data suggest that the ultramafic cumulates are distinct from rocks in midocean ridge and back arc basin ophiolites the ultramafic rocks instead represent part of the plutonic core of an intraoceanic island arc suprasubduction zone tectonic setting it is believed that the arc crust was thick for the pko during the late cretaceous with the existence of an active magma chamber situated at the base of the crust around pressure of 8 10 kbar for the high pressure products and a related active shallower level magma chamber for the low pressure products d 2002 elsevier science b v all rights reserved

cryptic mineral chemistry variations in a detailed traverse through the cumulate ultramafic rocks of the north arm mountain massif of the bay of islands ophiolite newfoundland

significance of 87 sr 86 sr ratios in the merensky cyclic unit of the bushveld complex in a number of mafic layered intrusions layers with platinum group element pge mineralization coincide with breaks in cumulus mineral composition and isotopic variation trends through the layered sequence1 2 for example in the bushveld complex in south africa abrupt increases in initial 87sr 86sr ratios occur at the ug 2 and merensky pegmatoid or reef horizons both of which are exploited for pges we report here the results of a detailed sr isotope study across the merensky unit and its immediate foot and hangingwall rocks which when considered together with new ideas on processes operating in magma chambers3 5 suggest that magma mixing and post cumulus infiltration of liquids displaced from below were important during crystallization of the merensky pegmatoid

seismic refraction study of upper crustal structure in the lassen peak area northern california in september 1982 the u s geological survey recorded a 125 km long refraction profile through the lassen peak area of the cascade range this profile extended northwestward to the klamath mountains and southeastward to the sierra nevada foothills interpretation of the refraction data determined the upper crustal structure in this volcanic region of northern california near surface velocities 1 5 5 55 km s are consistent with a thickness of 2 4 km for the cascade volcanic rocks between 0 5 km above and 1 8 km below sea level velocities are about 4 7 5 55 km s the highest velocities are modeled below the devils kitchen terminal geyser area possibly indicating hydrothermal alteration of the volcanic rocks the absence of low velocities less than about 3 4 5 km s deeper than 1 km above sea level suggests that the cretaceous sedimentary rocks exposed west of lassen peak do not underlie the volcano southeast of lassen peak at 1 8 km below sea level a 0 3 km thick layer with a velocity of 6 1 km s overlies a lower velocity 5 7 5 8 km s 2 9 km thick layer because these modeled velocities do not change across the boundary between the sierra nevada foothills and the cascade range sierran basement could continue under the cascade volcanic rocks as far northwest as lassen peak the velocity model does not indicate the presence of a magma chamber larger than 3 km in diameter within the top 8 km of the crust

a proposed model for the formation of reversely zoned plutons based on a study of the lacorne complex superior province quebec the lacorne complex is a calc alkaline mass of archean age located in the abitibi region of quebec the complex can be subdivided into three zones the inner zone varies in composition from hornblende biotite gabbro breccia to monzonite with all rocks containing 5 modal quartz the outer zone rocks contain more than 5 modal quartz and range in composition from biotite hornblende quartz diorite to granodiorite the third zone consisting of muscovite biotite garnet leucomonzogranite is exposed only in the northwestern portion of the complex the transition between each zone is relatively abrupt but continuous based on 400 modal estimates of stained slabs and 107 whole rock and trace element analyses it is demonstrated that both the inner zone and the outer zone rocks are reversely zoned mafic cores are surrounded by more felsic borders various methods that could explain the observed geochemical trends are considered the data indicate that crystal liquid equilibria were maintained during cry

evidence for magma mixing processes within the critical and lower zones of the northwestern bushveld complex south africa abstract during deposition of the lower lz lower critical lcz and upper critical ucz zones the course of crystallization was dominated by repeated emplacement of primitive mafic magma evidence for this is provided by textural chemical and isotopic data emplacement of fresh magma into resident residual liquids produced reversal in fractionation trends i e progressive increase in mg mg fe 2 ratios and enrichment of cr in orthopyroxenes through intervals of 350 m in the lz 100 200 m in the lcz and 10 80 m within the lower part of the ucz reversals culminated with the deposition of olivine cumulates upon pyroxenite within the lz and lcz but a change in the style of reversals led ultimately to ultramafic cumulates with or without olivine resting directly upon anorthositic cumulates at the top of the ucz merensky and bastard units these incursions of fresh mafic liquid led to deposition of ultramafic rocks at the base of units constituting the ucz whereas the cumulus feldspar bearing rocks at the tops of such units crystallized from hybrid liquids resulting from the mixing of the new and the resident residual liquids there is a general decoupling of trends of cryptic variation of the feldspar and orthopyroxene populations in the ucz wide variations of initial sr isotope ratios exist throughout the entire section within individual units and within individual members of units isotopic disequilibrium also exists between feldspar and pyroxene fractions of some samples and between individual feldspar grains of different textural habit the first appearance of cumulus feldspar bearing cumulates of the ucz is presaged by the appearance of small partially resorbed plagioclase inclusions within the orthopyroxene grains of pyroxenites at the top of the lcz this texture is conspicuous throughout the lowermost 400 m of the ucz but is still sporadically developed within the bastard unit at the top of the ucz anorthositic rocks of the lower part of the ucz are held to have been derived from residual liquids evolved during crystallization of earlier cumulates but anorthosites with a main zone isotopic signature cap the merensky and bastard units

mineral compositions of plutonic rocks from the lewis hills massif bay of islands ophiolite the lewis hills massif is interpreted as having been formed at an oceanic fracture zone fz prior to its obduction the compositions of minerals in the residual mantle and cumulate rocks of the massif are similar to those in other massifs of the bay of islands ophiolite and those in comparable rocks from the oceanic basins the residual mantle rocks consist of olivine fo89 9 91 7 ni0 0 37 0 46 orthopyroxene 100 mg mg fe ranges from 90 6 to 91 8 spinel 100 cr cr al 43 7 57 4 and 100 mg mg fe2 51 2 67 7 and rare clinopyroxene these minerals have compositions similar to those of the most depleted abyssal peridotites the cumulate units are mixtures of olivine fo88 8 78 0 ni0 0 32 0 08 plagioclase an88 4 68 8 clinopyroxene 100 mg mg fe 93 0 75 6 and cr al spinel 100 cr cr al 36 0 8 8 and 100 mg mg fe2 73 7 50 0 the ultramafic cumulates appear to have crystallized at moderate to high pressures and the gabbroic rocks at low pressures differences between the magmatic processes recorded in the lewis hills massif during its formation adjacent to a fz and those inferred for the north arm mountain massif while it formed more distantly from the fz are 1 the more depleted nature of the mantle near the fz 2 the higher ti02 contents of diabase dikes near the fz 3 the greater abundance of feldspathic ultramafic rocks near the fz and 4 the absence of orthopyroxene as a cumulate phase near the fz

weathering plants and the long term carbon cycle the chemical weathering of ca mg silicate rocks is the principal process whereby co2 is removed from the atmosphere on long or multimillion year time scales at present vascular land plants exert an important influence on weathering by recycling water and by bringing about the secretion of acids and the buildup of high levels of co2 in soils before the spread of vascular plants on the continents during the mid paleozoic chemical weathering must have been achieved by higher levels of atmospheric co2 and or by the intercession of primitive land biota such as lichens and cyanobacteria from examination of several pre vascular weathering scenarios it is concluded that in order to prohibit unreasonably high levels of atmospheric co2 weathering during the precambrian and early paleozoic must have taken place under essentially closed system abiotic conditions linear feedback or via strong regulation by primitive biota preliminary examination of modern weathering by lichens over tens to thousands of years militates against the latter possibility however much more work needs to be done on the quantitative effect of both primitive biota and vascular plants on the rate of weathering and their response to changes in atmospheric co2 before anything more definitive can be said

tectonic significance of the geochemistry and petrology of ophiolites in southeast anatolia turkey abstract the southeast anatolian ophiolites form discontinuous linear belts of oceanic fragments immediately north of the bitlis zagros suture which marks a continental collision zone between the arabian platform to the south and the taurides to the north the late cretaceous ophiolites in the southeast anatolia are represented by the kizildag in the hatay area the goksun and the berit to the north of kahramanmaras ispendere in the malatya area and komurhan guleman in the elazig area the goksun berit ispendere komurhan and guleman ophiolites were attached to the base of the tauride platform i e the malatya keban along its northern active margin and intruded by the i type calc alkaline granitoids of late cretaceous age in contrast the kizildag hatay ophiolite was thrust over the arabian passive margin in the south the cumulate rocks in the ophiolites are represented by dunite wehrlite lherzolite olivine clinopyroxenite olivine gabbronorite olivine gabbro gabbronorite and gabbro highly magnesian olivines fo88 to 74 and pyroxenes mg 95 to 60 as well as highly calcic plagioclases an95 to 68 from the cumulate rocks differ from oceanic equivalents which mainly formed from a mor basaltic melt the order of crystallization in mineral phases whole rock and mineral chemistry data from the cumulates suggest that the primary magma is compositionaly similar to that observed in modern island arc tholeiitic sequences the volcanic and subvolcanic rocks exhibit tholeiitic compositions chondrite normalized ree n morb normalized multi element patterns and tectonic discrimination diagrams suggest the existence of two main types of parental basic magmas in the crustal rocks of the se anatolian ophiolites these are i iat series that characterize the kizildag goksun ispendere komurhan and guleman ophiolites and ii low ti boninitic series known only in the kizildag ophiolite the geochemistry of the crustal rocks suggests that they formed in a suprasubduction zone tectonic setting including arc forearc environments in the southern neotethys these well preserved plutonic oceanic crustal remnants mainly exhibit an intact ophiolite pseudostratigraphy and are overlain by a volcanic sedimentary unit made up of alternations of basic to acidic extrusive rocks debris flows volcaniclastic sandstones and pelagic limestones interpreted as a tholeiitic ensimatic island arc assemblage built on the suprasubduction zone type crust the metamorphism of some of the ophiolites i e berit in the se anatolia might be related to later stages of intraoceanic subduction during the evolution of the southern neotethys

geology and seismic structure of the northern section of the oman ophiolite in the north oman mountains a continuous ophiolite succession is exposed from tectonized harzburgites and dunites at the base through layered gabbros and peridotites high level gabbros and plagiogranite to a dike swarm and pillowed volcanics overlain by pelagic shales the upper part of this sequence possesses a static metamorphic overprint which passes downward from greenschist facies in the lowermost volcanics and most of the dike swarm to amphibolite facies in the lowermost dike swarm and in the high level intrusives these pervasively altered rocks are underlain by layered gabbros and peridotites where hydration is restricted to fractures compressional and shear wave velocities have been measured to confining pressures of 6 kbar for 139 water saturated cores from this sequence the samples were field oriented and collected from known stratigraphic levels which allows the construction of velocity depth profiles and an examination of anisotropy within the ophiolite compressional wave velocities measured for the basalt section are quite variable ranging from 4 5 to 6 0 km s at in situ pressures within the sheeted dike section compressional and shear wave velocities vary from 5 4 to 6 3 km s and from 2 9 to 3 6 km s respectively from the basalt sediment contact through the dike section velocities increase rapidly with depth this gradient is related to increasing metamorphic grade and a decrease in grain boundary porosity the top of seismic layer 3 is located near the lower boundary of the sheeted dike swarm and marks the transition from greenschist facies to amphibolite facies metamorphics the dikes decrease in abundance downward into the high level intrusives coarse grained gabbroic and granitoid rocks which often contain abundant hornblende with isotropic textures and lacking compositional layering mean compressional and shear wave velocities increase uniformly from 6 7 and 3 6 km s respectively in the highlevel intrusives to 7 5 and 3 9 km s respectively at the base of the layered sequence measured velocities of the ultramafic tectonites are highly variable because of the anisotropy and serpentinization petrofabrics of relic olivine in the wadi ragmi region show strong preferred fabrics for the tectonites and calculated compressional wave velocities for the serpentine free rocks range from 7 8 to 8 5 km s with the fast direction parallel to the direction of spreading inferred from dike orientations in excellent agreement with observed upper oceanic mantle seismic anisotropy the crust mantle boundary which is defined by the abrupt increase in seismic velocities coincides with the petrological contact between layered gabbros and peridotites and tectonized harzburgite and dunites

the effect of land plants on weathering rates of silicate minerals abstract land plants and their associated microbiota directly affect silicate mineral weathering in several ways by generation of chelating ligands by modifying ph through production of co2 or organic acids and by altering the physical properties of a soil particularly the exposed surface areas of minerals and the residence time of water in laboratory experiments far from equilibrium 1 mm oxalate a strong chelator of al has a negligible effect on the dissolution rate of alkali feldspars but some effect on calcic feldspars and olivine by analogy to oxalate the overall effect of organic ligands on the weathering rate of silicate minerals in nature is likely to be small except perhaps in microenvironments adjacent to roots and fungal hyphae the effect of ph on silicate mineral dissolution rate depends on ph below ph 4 5 the rate increases with decreasing ph in the circumneutral region the rate is ph independent and at ph values above around 8 the rate increases with increasing ph vegetation should thus cause an increase in weathering rate through the ph effect only where the ph is below 4 5 as an overall generalization the effect of plants on weathering rate through changes in soil solution chemistry is probably small for granitic rocks it may be greater for more mafic rocks it is the release of ca and mg from mafic rocks that has the greatest influence on the global co2 budget the effect of changes in soil physical properties on weathering rate can be major by binding fine particles plants can greatly increase weathering rates in areas of high physical erosion where erosion rates are lower the effect of plants is less clear on long timescales plants may decrease chemical weathering by binding secondary products and isolating unweathered minerals from meteoric water a major unknown in estimating the effect of the advent of land plants on weathering rates is the nature thickness particle size distribution permeability of the regolith on the pre silurian continents the indirect effect of vegetation through changing the regional distribution of precipitation may be as important as the direct effects

plagioclase buoyancy in oceanic basalts chemical effects abstract certain petrological features of oceanic volcanic and plutonic rocks are not completely consistent with previously proposed models of crystal fractionation or magma mixing for example sr is often higher in the differentiated basalts of a suite of aphyric rocks than in the relatively primitive basalts even though the differentiated basalts have apparently been produced by crystallization of large amounts of plagioclase with olivine and clinopyroxene additionally oceanic basalts and gabbroic rocks often contain plagioclase crystals in excess of the appropriate cotectic proportions certain differentiated oceanic basaltic glasses and aphyric rocks crystallize plagioclase as the liquidus mineral which would seem inconsistent with the strongly cotectic nature of the olivine plagioclase liquid surface it is proposed here that plagioclase in mid ocean ridge magma chambers separates from the basaltic liquid that it crystallizes in at a slower rate than does co crystallizing olivine or pyroxene magma mixing in which a portion of the plagioclase remains suspended in the liquid during crystallization results in much more complex liquid lines of descent in mixed magmas and appears to resolve the apparent discrepancies noted above

paleomagnetic results from the central sierra nevada constraints on reconstructions of the western united states paleomagnetic results from three plutons of the central sierra nevada limit the motion of the sierra nevada relative to the north american craton to 5 8 of poleward motion and 7 11 of clockwise rotation since about 90 ma the indicated rotation and translation are not significant at the 95 confidence level a new tectonic model is presented that is consistent with all available paleomagnetic data from the sierra nevada great valley klamaths oregon coast range cascades and western nevada the paleogeographic reconstructions of this model associate the displacement of major crustal blocks with basin and range extension starting at about 30 ma this model accounts for about 15 of clockwise rotation of the klamaths and southern cascades about 2 of clockwise rotation of the sierra nevada and great valley and 26 of clockwise rotation of the oregon coast range since 30 ma the model is consistent with 40 of clockwise rotation of the oregon coast range before 30 ma as proposed by magill and cox 1980 the amount of basin and range extension in this model at latitude 41 n is 225 km directed e w and at latitude 35 n is 245 km directed wsw ene

rare earth element evidence concerning the origin of voluminous mid tertiary rhyolitic ignimbrites and related volcanic rocks sierra madre occidental chihuahua mexico abstract the mid tertiary volcanic sequence of the central sierra madre occidental in chihuahua mexico is about one kilometer thick and is composed predominantly of rhyolitic ignimbrites basaltic andesite to dacitic lavas are interbedded with the rhyolites but they are of minor volumetric importance rare earth element ree data are used to constrain a crustal anatexis model for the origin of the voluminous ignimbrites and to test a fractional crystallization model the ree patterns indicate that if the rhyolites were formed by direct crustal anatexis the residue from partial melting could contain no more than a few percent garnet or about 20 hornblende this eliminates residues with the mineralogy of amphibolite eclogite or garnet granulite but melting of a garnet free granulite source is permitted the crustal anatexis model is difficult to evaluate critically because of a lack of knowledge concerning the mid tertiary geothermal gradient and the composition of the crust beneath the sierra madre occidental in contrast the fractional crystallization model can be tested rigorously rayleigh fractionation calculations are used to closely model ree patterns in the basaltic andesite to rhyolite series the minerals involved are those occurring as phenocryst phases and the mineral proportions were generated by leastsquares major element calculations the results of the calculations are consistent with the hypothesis that the voluminous rhyolites originated by plagioclase dominated crystal fractionation

major element variations in basalts and diabases from the north arm mountain massif bay of islands ophiolite implications for magma chamber processes at mid ocean ridges abstract the major element compositions of aphyric diabases from the north arm mountain massif range from primitive basalts to highly evolved ferrobasalts from modeling of these compositional variations it is clear that the diabases have undergone highly variable 10 to 90 increments of crystallization at low pressures the dominant minerals inferred to have crystallized from the liquids that formed these diabases are olivine plagioclase and clinopyroxene the same minerals that dominate the underlying cumulate gabbroic and ultramafic rocks the projection of these compositions onto pseudo liquidus phase diagrams suggests that the primary magma s from which this north arm suite was produced were generated by melting at 10 25 kbar or greater pressure there is no indication in the major element variations of these diabases of the operation of high pressure crystal factionation processes which is not surprising because low pressure fractionation processes will effectively mask evidence for earlier high pressure processes it is suggested that the tapping of intercumulus liquid from crystal mush areas and the tapping of discrete fractionated magmas within the roof zone of magma chambers is a method of producing a wide variety of magma compositions from a single magma chamber in which the majority of the chamber may be actively convecting and therefore remain relatively uniform in composition

mineralogic variation in a layered ultramafic cumulate sequence at the north arm mountain massif bay of islands ophiolite newfoundland in order to investigate magmatic processes operative within magma chambers underlying oceanic spreading centers a suite of 59 samples was collected over an 8 m stratigraphic interval at the base of the cumulate ultramafic unit exposed on the north arm mountain massif in the bay of islands ophiolite complex newfoundland interlayered dunites and wehrlites are present in the 8 m section low abundances of incompatible elements ti y zr and other evidence suggest that these rocks originally formed as adcumulates olivine compositions are in the range fo84 5 89 9 and clinopyroxene mg mg mg fe2 100 range from 89 7 to 93 3 spinel compositions are highly variable and mg and al2o3 contents of spinels have been modified by subsolidus reequlibration with olivine and clinopyroxene cryptic variation patterns defined by the mg of olivine and clinopyroxene are characterized by successive cryptic regressions that have a repeat interval averaging 1 m the cryptic regressions are believed to record influxes of fresh relatively unfractionated magma into a magma chamber or conduit the gradual decrease in the mg of olivine and clinopyroxene and the decrease in olivine ni contents between successive cryptic regressions indicate that 10 crystallization of each batch of magma occurred before a new batch of magma entered the crystallization site of these cumulate rocks in some cases the cryptic regressions in mineral chemistry correlate with abrupt lithologic changes from wehrlite to dunite suggesting rapid displacement of the fractionated magma by more primitive incoming magma or rapid mixing of fresh incoming magma and resident magma in other cases inverse fractionation trends and fine scale interlayering of the cumulate rocks appear to represent zones of less rapid mixing between fractionated and fresh magmas comparison of cryptic variation patterns at the base and the top of the cumulate ultramafic section on north arm mountain indicates that in some cases the liquids that crystallized to form the upper part of the unit were more primitive higher mgo feo than the liquids that crystallized to form the base of the unit this feature is an indication of the complexity of processes that may operate in magma chambers underlying present day oceanic spreading centers

stratigraphic lithologic and major element geochemical constraints on magmatic evolution at lassen volcanic center california the lassen volcanic center is the most recent of several long lived volcanic centers in the southernmost cascade range these centers have erupted products ranging from basaltic andesite to rhyolite and are superimposed on a background of regional basaltic to andesitic volcanism the evolution of the lassen volcanic center is described in three stages stages i and ii comprise the brokeoff volcano and 80 km3 andesitic stratocone active from 600 to 400 ka brokeoff volcano is compositionally equivalent to the regional basaltic andesite to andesite volcanism in the lassen region and is the result of structurally controlled focusing of the diffuse regional mafic magmatism stage iii comprises a silicic dome field and adjacent area of hybrid andesites and has a total volume of about 100 km3 volcanism during stage iii was episodic and is subdivided into four sequences of lithologically and temporally distinct lavas stage iii began at 400 ka with a rhyolitic caldera forming pyroclastic eruption and chemically related lavas additional sequences of dacite erupted between 250 200 ka and 100 0 ka hybrid andesites erupted adjacent to the silicic dome field between 300 and 0 ka porphyritic andesite and dacite with high al2o3 low tio2 medium k2o and feo mgo ratios of 1 5 2 0 are the most abundant rock types in the lassen volcanic center however the single most voluminous unit is sparsely phyric rhyolite pumice in general the lavas of lassen volcanic center form a single coherent trend on major element variation diagrams and in pseudo quaternary phase space consistant with an origin either by fractional crystallization or magma mixing in detail however the lack of systematic temporal change in silica and subtly crossing trends indicate a complex origin a variety of statistically successful fractional crystallization models can be constructed that derive brokeoff andesites from regional magmas an important conclusion of the modeling is that if fractional crystallization is the process responsible for generation of brokeoff andesite then the parent magma must be low to medium k in geochemical affinity in order to explain the variation in k2o however although major element variation can be modeled by fractional crystallization petrographic and stratigraphic evidence indicates that magma mixing is an important but subtle process in brokeoff lavas and suggests that lavas evolved in small independent batches lavas erupted during stage iii while predominantly silicic range from 53 to 75 sio2 disequilibrium mineral assemblages in the stage iii lavas indicate that they are not directly derived from brokeoff andesite by fractional crystallization mixing of silicic magma with regional mafic magma and disaggregation of andesitic quenched magmatic inclusions play dominant roles in the compositional diversity of stage iii lavas

late cenozoic volcanism subduction and extension in the lassen region of california southern cascade range hundreds of short lived small to moderate volume mostly mafic volcanoes occur throughout the lassen region of ne california and surround five longer lived large volume intermediate to silicic volcanic centers younger than 3 ma volcanic rocks older than 7 ma are scarce in the lassen region we identify 537 volcanic vents younger than 7 ma and we classify these into five age intervals and five compositional categories based on sio2 content maps of vents by age and composition illustrate regionally representative volcanic trends by 2 ma the eastern limit of voicanism had contracted westward toward the late quaternary arc late quaternary volcanism is concentrated around and north of the silicic lassen volcanic center the belt of most recent volcanism 25 0 ka has been active since at least 2 ma most mafic volcanism is cakalkaline basalt and basaltic andesite however lesser volume of low potassium olivine tholeiite lkot a geochemically distinctive basalt type found in the northern basin and range province also has erupted throughout the lassen segment of the cascade arc since the pliocene thus models of the mantle source and tectonic control of lkot magmatism should be applicable both within and behind the subduction related arc normal faults and linear groups of vents are evidence of widespread crustal extension throughout most of the lassen region nnw alignments of these features indicate nnw orientation of maximum horizontal stress ene extension which is similar to the stress regime in the adjacent northwestern basin and range and northern sierra nevada provinces the large long lived volcanic centers developed just west of a zone of closely spaced nnw trending normal faults within that zone of faulting pervasive ene extension has precluded growth of large long lived crustal magma systems we interpret the western limit of the zone of nnw trending normal faults as the western boundary of the basin and range province where it overlaps the lassen segment of the cascade arc in our view the lassen volcanic region occurs above the subducting gorda north plate but also lies within a broad zone of distributed extension that occurs in the north american lithosphere east and southeast of the present cascadia subduction zone an episode of ene extension that began in the late miocene in the northwestern basin and range province appears to have triggered widespread late miocene to quaternary mafic volcanism in the lassen region the scarcity of volcanic rocks older than 7 ma suggests that a more compressive lithospheric stress regime prior to the late miocene extensional episode may have suppressed volcanism even though subduction probably was occurring beneath the lassen region

small scale variations and subtle zoning in granitoid plutons the liberty hill pluton south carolina u s a abstract imperfect flow separation and remixing of crystals and melt produced chemical mineral and textural variability on the scale of several centimeters to meters in coarse grained granitoids of the liberty hill pluton this is believed to develop during emplacement because the resulting layering conforms to the geometry of the pluton and is better developed toward the margins mineral specific equilibrium and fractional crystallization regimes superimposed on the flow separation and remixing produced more complex compositional variations than expected from such granitoid cumulate processes alone the process is chemically comparable to restite unmixing but it is difficult to demonstrate that many minerals involved in the flow are restites local compositional inhomogeneities also formed by deformation and disaggregation of enclaves depending on whether the enclaves are interpreted as xenoliths chilled mafic magma or high melting point restite clots they represent contamination by mechanical assimilation magma mingling and mixing or restite unmixing marginal granitoids distinguished from the central granitoids by pink feldspar biotite muscovite rather than white feldspar amphibole biotite are more evolved but overlap the compositional range of the central granitoids mineralogic and textural similarity and a gradational contact between the two granitoids indicate they are related and that the pluton s subtle reverse zoning was produced by a continuous in situ process flow separation and remixing operating more efficiently and on a larger scale may have produced the more evolved composition of the marginal granitoids however proximity to the contact is more important than rock composition in determining mineralogy there is slight geochemical evidence for contamination of the marginal granitoids by the wall rocks compared to central granitoids they have small lree depletions and hree enrichments which more nearly reflects the lree depletion and hree enrichment of the wall rocks cross cutting contacts and slightly differing compositional trends indicate a later and distinct evolution for the finer grained biotite granitoids they have silica enrichment incompatible element enrichment rb u th compatible element depletion major elements sr ba depleted hree and less frequently lree contents as compared to the central granitoids the fine grained granitoids could be a contemporaneous and unrelated magma or fractionally crystallized from the coarse grained magma compositional mineralogic and textural differences among the later finer grained granitoids indicate either multiple magmas periodic sampling and intrusion of successive residual melt fractions or flow separation and remixing during emplacement

the 1883 eruption of krakatau the 1883 eruption of krakatau was a modest ignimbrite forming event the deposits are primarily coarse grained dacitic non welded ignimbrite large explosions produced pyroclastic flows that entered the sea generating destructive tsunami grain size studies of the ignimbrite suggest that these explosions were not driven by magma seawater interaction the total bulk volume of pyroclastic deposits including co ignimbrite ash is estimated to be 18 21 km3

geochemistry of diabase dikes from the lewis hills massif bay of islands ophiolite evidence for partial melting of oceanic crust in transform faults the lewis hills massif of the bay of islands ophiolite is interpreted to have had a transform fault history in an oceanic fracture zone prior to its abduction diabase dikes from the lewis hills massif are divisible into two groups on the basis of their trace element compositions one group is similar to mid ocean ridge basalts and is termed the lh normal suite the other group termed the lh depleted suite is characterized by high cao ti zr and y zr with relatively low sio 2 zr and p 2 o 5 the low sio 2 abundances are suggestive of silica undersaturated rocks of the alkali basalt nephelinite series of magmas but the very low abundances of incompatible trace elements in these magmas contrast with their high abundances in most silica undersaturated rocks we interpret the geochemical characteristics of thesediabase dikes to indicate that the magmas from which they solidified were produced by partial melting of adcumulate gabbroic and troctolitic rocks at low pressures 2 3 kbar within the ophiolitic fracture zone the effects of partial melting of gabbroic and troctolitic rocks have not been documented in rocks described from present day oceanic fracture zones but samoles have not been obtained from those structural levels of fracture zones where such evidence mightbe found

oceanic ridge crest processes oceanic ridge crest processes constitute a vigorous frontier of multidisciplinary research with global ramifications in the earth atmospheric and biologic sciences this deep ocean frontier is gradually opening in response to investigations which are elucidating the role of these processes in the lithosphere hydrosphere atmosphere and biosphere and the application of advanced instrumental and interpretive techniques which are increasing the quality and quantity of data these techniques include crustal drilling with borehole logging deep sea drilling project reports high resolution bathymetry sea beam etc side scan sonar seamarc i and ii gloria etc imagery with video and photography multichannel seismics determination of water column properties ctd with rosette sampler etc and submersible applications

gravity anomalies over extinct spreading centers a test of gravity models of active centers gravity fields over active mid ocean ridges display long wavelength free air gravity highs with superimposed short wavelength anomalies over the spreading axis these axial anomalies are negative over slow spreading active ridges with amplitudes of typically 30 70 mgal and wavelengths of 40 60 km but decrease in amplitude and wavelength with increasing spreading rate until at fast spreading centers the anomalies are positive 10 20 mgal with wavelengths of about 20 30 km similar free air anomalies are observed over extinct spreading ridges and typically show the same paleo spreading rate dependency suggesting that a large component of the anomaly is not dynamically maintained calculation and removal of gravitational effects from basement relief results in significant residual gravity lows at the extinct ridges analyzed here even though the thermal effects associated with active spreading have diminished and no longer contribute to such short wavelength gravity lows the amplitudes of the residual lows at slow paleo spreading rates are 25 45 mgal with half widths of about 20 45 km and decrease with increasing spreading rate to about 5 mgal amplitude and 10 15 km half width over fast paleo spreading ridges the amplitudes and wavelengths of these residual anomalies are nearly identical to those of the residual anomalies observed over active spreading centers most previously proposed models of mid ocean ridge structure are unable to explain these anomalies and many are inconsistent with a least some aspect of the large body of knowledge currently available for mid ocean ridges this research provides a further constraint on geometric models of accretion as these models predict vastly different gravity fields results indicate that a frozen in low density accretionary structure is probably confined to within about 10 km of either side of the extinct axis this conclusion is consistent with the presence of a low density gabbroic root situated at shallow mantle depths beneath the ridge axis this root tends to narrow with increasing spreading rate producing the required residual gravity field variation

mafic ultramafic sills new insights from m and s shaped mineral and whole rock compositional profiles the origin of mafic and ultramafic sills exhibiting different wholerock compositional profiles e g i c d m and s shaped profiles remains controversial we have addressed this issue by revisiting three 100 m thick siberian dolerite sills vavukansky kuz movsky and vilyuysky that display remarkable internal differentiation the vavukansky sill has an m shaped profile with prominent basal and top reversals showing inward increases in whole rock mgo mg number 100mg mg fe and normative an content 100an an ab followed by the layered and upper border series with inward decreases in these indices the kuz movsky and vilyuysky sills both show s shaped profiles similar to the vavukansky sill but lack a top reversal these whole rock m and s shaped profiles are accompanied by similar profiles in mineral compositions plagioclase and to a lesser extent olivine show systematic inward increases in an content and mg number respectively across basal and top reversals these compositional trends are followed by inward decreases in these ratios in the interiors of the vavukansky and kuz movsky sills currently accepted models attribute whole rock m and s shaped compositional profiles to crystal settling compositional convection or compaction operating in closed systems our observations challenge these traditional interpretations because variations in mineral compositions observed in marginal reversals cannot result from closed system fractionation we suggest instead that initially the sills evolved as open systems that were slowly inflated by magmas that became gradually more primitive with time the inflation was accompanied by in situ crystallization that preserved the preceding fractionation history of the injected magmas by forming basal and top reversals with minerals becoming more primitive inwards this process culminated with rapid inflation of the sills to their current size owing to a major influx of primitive magma subsequently magma flow through the sills ceased and they evolved as closed systems by fractional crystallization this resulted in the layered and upper border series with minerals becoming more evolved inwards this model can be extended to explain other compositional profiles and petrological features in mafic and ultramafic sills

la sio2 and yb sio2 systematics in mid ocean ridge magmas implications for the origin of oceanic plagiogranite analytical expressions for the variation in d la and d yb with increasing liquid sio2 for olivine plagioclase augite hornblende orthopyroxene magnetite and ilmenite brophy in contrib mineral petrol 2008 online first have been combined with numerical models of hydrous partial melting of mid ocean ridge mor cumulate gabbro melting and fractional crystallization of slightly hydrous mid ocean ridge basalt morb magma to assess a melting versus fractionation origin for oceanic plagiogranite for felsic magmas 63 wt sio2 the modeling predicts the following mor cumulate gabbro melting should yield constant or decreasing la and constant yb abundances with increasing liquid sio2 the overall abundances should be similar to those in associated mafic magmas morb fractional crystallization should yield steadily increasing la and yb abundances with increasing sio2 with overall abundances significantly higher than those in associated mafic magmas application to natural occurrences of oceanic plagiogranite indicate that both mor cumulate gabbro melting and morb fractionation are responsible application of the model results to icelandic rhyolites strongly support a fractional crystallization rather than a crustal melting origin

petrology of a leucogabbroic interval within basal layered gabbros at north arm mountain bay of islands ophiolite data from detailed sample traverses in the layered gabbro unit of the north arm mauntain massif bay of islands ophiolite allow meter scale resolution of magmatic processes in spreading ridge magma chambers one suite of 46 samples from a 195 m interval near the base of the layered gabbro unit contains cumulus plagioclase an73 7 87 5 average modal abundance 75 clinopyroxene mg 80 3 86 0 18 and olivine fo76 6 82 1 6 with intergranular orthopyroxene mg 78 0 83 3 1 and accessory cr al spinel cr 32 3 41 4 ilmenite rims spinel in one sample whole rock zr contents range from 6 to 15 ppm plots of stratigraphic height in the traverse versus petrogenetic indicators e g mg s of mafic phases and an in plagioclase reveal both normal and reverse cryptic variation patterns the patterns for all indices are generally correlated the normal portions of the patterns formed during fractional crystallization of basalt batches ranges of mineral compositions in the normal trends suggest that 29 38 crystallization of each batch of basalt occurred before magmatic replenishment the reverse cryptic trends formed by crystallization of hybrid magmas produced during periods of magma mixing other evidence for magma mixing is the systematic association of spinel and reversely zoned plagioclase with the reverse trends experiments and observations of natural assemblages indicate that 55 modal plagioclase crystallizes from basalts at the olivine plagioclase clinopyroxene liquid piercing point the average plagioclase content of this suite of leucogabbros from north arm mountain is too high to have formed from simple crystallization at the piercing point petrologic modeling indicates the leucogabbros may have formed from basalts into which a small amount 10 of plagioclase was resorbed during mixing the initial compositions of these hybrid basalts lie in the plagioclase primary phase volume other suites of layered gabbros from north arm mountain are not so plagioclase rich as the leucogabbros described above crystallization of basalts in the plagioclase primary phase volume and the consequent formation of plagioclaserich gabbros may occur in restricted portions of zoned magma chambers underlying oceanic spreading centers or may occur episodically in the overall lifetimes of the magma chambers

the petrogenesis of an i type volcanic plutonic suite the st marys porphyrite tasmania the devonian i type st marys porphyrite 388 1ma comprises two petrographically similar units an 800 m thick pyroclastic sheet compositionally dacite and rhyolite and a subvolcanic feeder dyke the pyroclastics are crystal rich and contain in order of decreasing abundance plagioclase quartz biotite augite hypersthene and sanidine phenocrysts in an aphanitic groundmass

rollover in volcanic crater lakes a possible cause for lake nyos type disasters abstract seemingly stably stratified fluids that is a heavier layer of fluid underlying an upper layer of lighter fluid can overturn if there is a heat flux through the system such events are termed rollover in the engineering literature occurring for instance in liquid natural gas tanks they take place as well in lakes and ponds in all such cases the stratification starts off with the heavier more dense fluid underlying lighter convection driven mixing at the stratification interface due to the heat flux as well as other processes serves to minimize with time the stabilizing density differences which may eventually cause the stratification to invert if gas has been contained under pressure in the lower layer this gas may explosively vent from the fluid as it suddenly rises to the top such as occurs in liquid natural gas tanks where rollover is a hazard these processes are quantifiable and are applied here to various scenarios that might refer to volcanic crater lakes whose lower layers are charged with volatiles provided herein are some examples of calculated conditions and calculated time of evolution leading to rollover with attendant release of gases from supersaturated ascending fluids these calculations suggest rollover could occur in volcanic lakes the august 1986 gas release at lake nyos is employed as an example these estimates were made taking note that water is commonly supersaturated in co2

mineral chemistry of ultramafic cumulates from the north arm mountain massif of the bay of islands ophiolite evidence for high pressure crystal fractionation of oceanic basalts the mineral chemistry of ultramafic cumulates forming the basal portions of the plutonic section of the north arm mountain massif bay of islands ophiolite complex is not consistent with crystal liquid fractionation of a primitive mid ocean ridge basalt at low pressures the appearance of clinopyroxene and orthopyroxene having very high mg numbers 100 mg mg fe 85 to 92 and the absence of plagioclase as early fractionating phases coprecipitating with forsteritic olivine fo86 92 suggest that moderate to high pressure phase relationships are applicable we propose on the basis of mineral chemistry crystallization sequences and structural relationships that the basal ultramafic cumulates on the north arm massif represent the products of high pressure 10 kbar polybaric crystal fractionation of primary basaltic melts generated within the mantle beneath a mid ocean spreading center these basal ultramafic cumulates were probably precipitated due to in situ crystallization processes along the margin of a vertical conduit extending downward from a crustal level magma chamber to mantle depths that exceed 30 km after their formation the cumulates were transported to the near surface along the axis of upwelling and divergence as the lithosphere was separating beneath a mid ocean ridge

first plants cooled the ordovician the late ordovician period ending 444 million years ago was marked by the onset of glaciations the expansion of non vascular land plants accelerated chemical weathering and may have drawn down enough atmospheric carbon dioxide to trigger the growth of ice sheets

geochemical trends in the ackley granite southeast newfoundland their relevance to magmatic metallogenic processes in high silica granitoid systems the ackley granite occupying an area of approximately 2700 km2 was intruded at about 355 ma across the boundary between the avalon and gander tectonic terranes of southeast newfoundland it is dominated by k feldspar megacristic and equigranular biotite bearing granite with subordinate granodiorite but 10 separate phases have been recognized also including hornblende and muscovite bearing varieties small aplite pegmatite type molybdenite deposits occur in a southwest lobe and cassiterite wolframite bearing quartz topaz greisens are found as steeply dipping veins and pods within a south central lobe of the pluton a geochemical study of 357 rock samples randomly distributed on a grid of 2 km spacing shows systematic geographic variation in the concentrations of a range of elements the concentrations of silica and some other elements show a relatively abrupt change approximately 10 15 km west of the projected boundary between the avalon and gander terranes indicating that the different terranes ha

the geochemistry of the blackstairs unit of the leinster granite ireland the blackstairs unit is the southernmost of the five units which comprise the leinster granite four granite types are recognized which in order of decreasing age are graiguenamanagh granite porphyritic microcline granite non porphyritic granite and type 1 granite using whole rock geochemical data in conjunction with field and petrographic evidence a model is proposed whereby partial melting of a lower palaeozoic immature sedimentary source produced a parental granitic melt which crystallized without further differentiation to form the non porphyritic granites of the northeastern end of the pluton at the sw end this parental melt fractionated to form the porphyritic microcline granite leaving the melt depleted in k 2 o which then crystallized to form a slightly more acidic non porphyritic granite than that at the ne end a mixture of restite minerals plus melt from this partial melting process crystallized forming the graiguenamanagh granite fractionation of thorium into restite allanite in the graiguenamanagh granite accounts for the low th and th u levels which characterize the non porphyritic granite and the leinster granite in general a high degree of partial melting of similar source material produced the low volume type 1 granite magmas which were subsequently intruded along faults a similar petrogenesis is envisaged for the whole of the leinster granite

plagioclase wehrlites and peridotites on the east pacific rise hess deep and the mid atlantic ridge dsdp site 334 evidence for magma percolation in the oceanic upper mantle abstract textural and petrological data are presented that record the formation of plagioclase bearing wehrlites and peridotites at slow spreading and fast spreading ridges that are comparable to the wehrlites and peridotites formed in ophiolitic complexes evidence is provided for locally pervasive magma percolation in the oceanic residual upper mantle the samples studied come from the east pacific rise epr hess deep and the mid atlantic ridge mar dsdp site 334 at both sites parts of the peridotites display poikilitic textures with oikocrysts of clinopyroxene and interstitial plagioclase that include xenocrysts of strained and partly annealed olivine and subidiomorphic spinel crystals petrofabric data for olivine suggest a magmatic origin for the hess deep wehrlite its phase chemistry is comparable to intrusive plagioclase wehrlites and lherzolite from ophiolites that are thought to have crystallized from a crystal mush it differs drastically from the associated hess deep diopside bearing harzburgites and dunites which are quite similar to oceanic and ophiolitic residual peridotites the epr wehrlite is therefore considered as an intrusive rock the leg 37 plagioclase peridotites display a weak lattice fabric of olivive which may have resulted from high temperature plastic flow these rocks have a phase chemistry that is typical of ophiolitic and oceanic residual rocks these rocks are hence interpreted as residual dunite or harzburgite which has been pervasively impregnated by a melt from which the clinopyroxene and plagioclase crystallized these peridotites could have an intrusive origin or may have formed in situ at the crust mantle transition in association with intrusions of large gabbro bodies

shocked minerals at the k t boundary explosive volcanism as a source abstract azimuthal symmetry of p waves first motion up and diminished s wave amplitude indicates an explosion to be the source of the seismicity of the 18 may 1980 eruption at mt st helens explosion coupling efficiencies and scaling from underground mining excavation and nuclear explosions imply that peak pressures could have been as high as 1000 kbar which is more than sufficient for the formation of shocked minerals in addition high load rates as well as contamination may lower phase transition pressures a region of formation of shocked minerals surrounding the magma chamber is indicated to have a thickness on the order of 100 m the outer portions of this region should be sufficiently cool so that shocked features are not annealed hence the source of shocked minerals should be the country rock surrounding the magma chamber it is unlikely any of the magmatic material would be able to retain shocked features due its high temperature the explosive mechanism may be quench supersaturation which theoretically can yield shock producing pressures but chemical processes should not be dismissed as an explosive source tektite distributions show no iridium or mass extinction association industrial and present volcanic atmospheric discharge are known to have worldwide distribution the clays present at the k t boundary may have had a volcanic origin and it is now known that the deccan traps which have been suggested by others as a source of the iridium lie athwart the k t boundary as there is common intimacy between silicic and mafic volcanism both should be anticipated at the k t boundary and their global appearance there may be a manifestation of a surge in whole mantle convection in short there is a definite possibility shocked minerals at the k t boundary have a volcanic source

geochemistry and origin of the regan intrusive suite and other granitoids in the northeastern slave province northwest canadian shield the regan intrusive suite of about 100 plutons of tonalite granodiorite and quartz diorite intruded the yellowknife supergroup and migmatite terrain in the northwest slave structural province 2 59 ga ago rare earth element ree trace element and major element analyses from 39 representative whole rocks from the suite suggest it was derived by batch melting of the crust producing a parental magma of tonalitic or granodioritic composition by analysing ree from different parts of a zoned pluton it was concluded that ree distribution was controlled by early separation of quartz diorite from the parent magma by flow differentiation and that the bulk of the ree were contained in early cumulate accessory apatite and monazite the residual magma was further fractionated in pipelike magma chambers during ascent into more leucocratic rocks chondrite normalized ree patterns of single lithology plutons are similar to lithologies in zoned plutons and it is proposed they initially segregated during ascent

ephemeral magma chambers in the trinity peridotite northern california abstract the trinity massif comprises the major lithologies of an ophiolite as defined at the 1972 penrose conference previous studies have shown however that it differs from the semail oman or table mountain newfoundland ophiolitic massifs particularly because its crustal section is thin and because its mantle section has vertical plastic flow planes these features have led to an interpretation of the trinity massif as a fragment of slow spreading oceanic lithosphere le sueur et al 1984 boudier and nicolas 1985 in this paper we show that the trinity gabbros occur in discontinuous kilometre sized pockets intrusive into the mantle peridotites the internal stratigraphy and the petrological characteristics of these gabbros suggest that they formed in short lived magma chambers these ephemeral magma chambers developed after the end of the plastic deformation in the surrounding mantle when it had cooled down to lithospheric temperatures we discuss the possibility that these small and ephemeral magma chambers formed at a slow spreading oceanic ridge

convective fractionation a mechanism to provide cryptic zoning macrosegregation layering crescumulates banded tuffs and explosive volcanism in igneous processes a large array of igneous and volcanic features has characteristics that are recognized in other disciplines as conclusive and direct evidence of convection in stratified and or solidifying melts e g macroscopic segregation cryptic variation zoning in magma chambers mineral layering in mafic intrusives crescumulates fingering in the vertical and horizontal banding in pyroclastics rollover with attendant flashing of volatiles explosive volcanism etc some quantitative and qualitative aspects of convection in solidifying and or stratified melts e g mineral layer widths such as are observed in the skaergaards are examined to show consistency with field evidence convective fractionation does not possess the physical implausibilities of gravitational segregation crystal settling neither is the field evidence as ambiguous if interpreted in terms of convective fractionation which can explain amongst other things heavier material overlying lighter convective fractionation may operate on larger scales in the interior of planets

weathering rates of bedrock by lichens a mini watershed study the chemical weathering rate of rock substrates by saxicolous lichens was measured quantitatively and compared with the weathering rate of bare rock in the cone pond area at hubbard brook experimental forest usa the experiments used mini watersheds 1 m2 on outcrops of mica schist a laboratory mini watershed experiment and a thin section analysis were also conducted during the course of the study computed cumulative flux ratios of the weathering rate of lichen covered versus bare rock mini watersheds for mg2 16 7 and si 4 0 3 were significant the rate of denudation of the rock was calculated to be higher for the lichen mini watershed than the bare rock mini watershed a laboratory mini watershed experiment demonstrated the same trend of higher elemental fluxes for the lichen mini watershed than the bare rock mini watershed for k mg2 na and si the weathering rind beneath the lichen was measured to be between 0 2 and 0 5 mm thick these results show the significance of biotic enhancement of chemical weathering this research also calculated the rate of denudation of the rock by lichens and supported the mini watershed studies as a reliable method to study the chemical weathering by lichens bryophytes and plants in thin soil and alpine areas

geochemical evidence for formation of the bay of islands ophiolite above a subduction zone the bay of islands boi complex in western newfoundland is one of the world s best known ophiolites and has generally been interpreted as a fragment of ocean floor emplaced on the eastern margin of north america1 2 the similarities of the boi to litho sphere formed at mid ocean ridges has led many investigators to use the boi as an onland laboratory in which to study the geochemical tectonic and geophysical properties of the oceanic lithosphere1 9 there have been nonetheless a few suggestions that the boi ophiolite formed above a subduction zone10 12 as have most other ophiolites13 15 here i report the existence of a substantial tantalum depletion in boi magmas jenner et al 16 demonstrate that boi magmas have a niobium depletion as well these depletions are characteristic of magmas erupted above subduction zones but not at mid ocean ridges indicating that the boi is not after all a good analogue for lithosphere formed at mid ocean ridges

soret convection and rheology viscous dissipation arguments for whole mantle convection abstract excluding phase changes associated with rearrangement of crystal structure with depth i e pressure computations of soret convection for mantle situations indicate that a chemically layered mantle within the earth cannot be maintained for more than several tens of millions of years this result is conservative that is high viscosities etc were employed to determine the longest incubation times lifetime of the layering possible under accepted values of the physical properties of the mantle the layering is removed by roll over there is a possibility that sparging of continental material from the mantle could lead to recurrent stratification and roll over although each layer was assigned various viscosities in determining maximum time scales the short duration of the layering in conjunction with mantle rheology which some workers eliminate by dismissing viscous dissipation supports conjectures that the mantle may assume an effective viscosity of approximately constant value a review of the pathologies of numerical procedures employed in assessing mantle rheologies is provided so that the effects of viscous dissipation may be correctly discerned from numerical artifacts the potential of soret convection in the mantle leads to the possibility of isotope and chemical fractionation across core mantle and mantle crust boundaries the isotope fractionation effect cannot be corrected by the usual corrections that attend mass spectroscopy

geochemical and geological evidence bearing on the origin of the bay of islands and coastal complex ophiolites of western newfoundland abstract suggestions that ophiolitic rocks of the bay of islands complex boic and coastal complex cc of southwestern newfoundland were formed at a seafloor spreading center are supported by 1 1 whole rock trace and major element data from diabases and basalts 2 2 spinel mineral chemistry from ultramafic rocks 3 3 the internal geology of the ophiolite terranes and 4 4 regional geologic and tectonic constraints in the western newfoundland appalachians diabase and basalt samples from the boic and the northern cc massifs are tholeiitic and geochemically indistinguishable from present day mid ocean ridge basalts morbs many diabases from the lewis hills massif the southernmost part of the cc are also identical to morbs but there is also an unusual suite of diabases from the lewis hills that is unlike any other suite yet reported from a variety of tectonic settings including island arcs ocean islands or spreading centers this suite is interpreted to have been produced by unusual magma generation processes in the deep structural levels of an oceanic ridge ridge transform fault approximately 90 of spinels from ultramafic rocks in the boic and cc have cr cr al ratios that are less that 0 60 comparable to those values reported from ultramafic rocks of the present day ocean basins these geochemical constraints in combination with previously described geological relationships within the ophiolite terranes and regional tectonic relationships in the western newfoundland appalachians suggest that the boic and cc ophiolites formed within a main ocean basin i e the early paleozoic iapetus ocean along a seafloor spreading center and ridge ridge transform fault respectively there is little evidence to suggest formation in either an island arc or back arc basin tectonic setting as such the boic and cc ophiolites are probably well suited for detailed petrologic structural and geophysical studies to determine the processes involved in the generation and evolution of contemporary oceanic crust and upper mantle

effects of volatiles on mixing in calc alkaline magma systems the intimate mixing between different magmas of disparate densities characteristic of some calc alkaline magma systems is explained by a new mechanism involving the emplacement of a layer of wet undersaturated mafic magma at the base of a magma chamber containing more differentiated magma turbulent transfer of heat between the mafic magma and the overlying magma leads to crystallization and exsolution of volatiles in the lower layer for an initial water content of a few per cent the bulk density of the mafic magma can become equal to that of the overlying magma causing overturning and intimate mixing of the magmas the mechanism is most probable in the low pressure environment of a high level magma chamber

composition gaps critical crystallinity and fractional crystallization in orogenic calc alkaline magmatic systems the recognition of a three way correlation between magmatic sio2 content critical crystallinity and the size magnitude of crystal fractionation generated composition gaps in calc alkaline magmatic systems suggests an important control of magmatic critical crystallinity on the formation of such composition gaps to explain this correlation it is proposed that fractionation generated composition gaps are caused by 1 simultaneous interior i e non substrate crystallization and vigorous chamberwide convection which leads to progessive crystal suspension 2 cessation of convection when the percentage of suspended crystals reaches the critical crystallinity of the magma and 3 eventual buoyancy driven crystal liquid segregation producing a discrete body of fractionated magma which is separated from the initial magma by a composition gap this mechanism implies that many if not most magma bodies are characterized by interior crystallization and vigorous convection conditions which are not universally agreed upon at present given that such conditions characterize natural magma bodies fractional crystallization through crystal settling in low velocity boundary layers should be an important mechanism of fractional crystallization in a crystallizing and convecting body of magma composition gap formation should represent one endmember of a complete spectrum of possible evolutionary paths governed by the relative rates of crystal settling and crystal retention as a given volcanic plumbing system matures with time average settling retention ratios within individual magma bodies should increase due to higher average wall rock temperatures it follows that within a given volcanic center early stage volcanism should be more likely to display fractionation generated composition gaps than later stage volcanism such a temporal evolution has been documented at at least two aleutian calc alkaline volcanic centers

a model for mixing basaltic and dacitic magmas as deduced from experimental data it has been demonstrated experimentally that basaltic and dacitic magmas can be easily mixed to form both banded dacite and homogeneous andesite in less than a few hours the presence of phenocrysts larger than 0 5 mm increased considerably the efficiency of mixing flow patterns in the experimental system were visualized using pt spheres which indicated that convection occurs in basalt melt but not in dacite melt the reynolds numbers of the basaltic and dacitic melts in the experimental system were calculated to be about 10 3 and 10 6 respectively mixing proceeds initially by mechanical mixing of the two magmas in a large scale but later by coupling interfacial convection and mutual diffusion thus depending on the depth where vesiculation and following disruption of the magma occurs banded pumice homogeneous pumice and homogeneous andesite lava are erupted the observed textures of mixed rocks of plinian type eruption and the limiting occurrence of banded pumice are satisfactorily accounted for on this model

the petrology of tambora volcano indonesia a model for the 1815 eruption abstract tambora is an active volcano in the east sunda arc and is well known for its catastrophic eruption in april 1815 its lavas are of unusual moderately undersaturated k2o rich types ranging from ne trachybasalt to ne trachyandesite they evolved under conditions of high oxygen fugacity about two log units below the m h buffer and crystallised leucite at low pressures the products of the 1815 eruption are black glassy biotite bearing ne trachyandesites together with scoria pumice and tuff of the same composition the parent trachybasalt liquid probably contained about 3 wt h2o and the trachyandesites nearly 6 wt the model presented here suggests that the 1815 eruption which is known to have followed a lengthy period of inactivity was the result of gradual cooling of a hydrous magma in a closed high level magma chamber emplaced at depths between 1 5 and 4 5 km eruption followed the exsolution of a high pressure aqueous fluid phase formed during cooling and crystallisation of the magma second boiling by the time eruption took place overpressures of about 4 5 kbar had been generated and temperatures in the magma chamber ranged from about 850 c to 700 c the catastrophic eruption took place after failure of the roof and the discharge of the vapour phase may have reached velocities of 650 m s 1 calculations indicate that the volume of magma involved in the eruption was of the order of 33 km3 and this may have evolved about 1 2 1027 ergs of energy during the eruption a blast equivalent to 30 000 megatons of tnt