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

effects of hydrothermal alteration on the compositions of chromian spinels chromite spinels in hydrothermally altered rocks from fracture zone ultramafic rocks and from both ultramafic cumulate pods and sheeted dikes in the josephine ophiolite california display a wide variety of compositions alteration of the spinel may not be visible in thin section the primary composition changes accompanying hydrothermal alteration are increase in cr cr al and or fe2 fe2 mg in general altered spinel grains associated with hornblende and chlorite show an increase in cr cr al from core to rim altered spinel grains associated with serpentine show an increase in fe2 fe2 mg from core to rim but may not show an increase in cr cr al the compositional zoning in some altered spinel grains appears to result both from reaction of clinopyroxene plus spinel to form hornblende and from reaction of hornblende to form chlorite these observations suggest that subsolidus hydrothermal metamorphic effects need to be considered when interpreting spinel compositions and the compositions should not be interpreted solely in terms of igneous processes further the presence of highly altered spinels may be indicative of hydrothermal alteration in rocks where other evidence of such alteration is absent

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

initiation of subduction zones along transform and accreting plate boundaries triple junction evolution and forearc spreading centres implications for ophiolitic geology and obduction

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

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

on the correlation of seismic microtremors 1 we present results of the analysis of microtremor measurements using a small array our analysis is based on the computation of cross correlation functions between stations in both frequency and time domains we obtain similar results in both domains and link those results to the application of aki s spatial autocorrelation method of using a single station pair and to recent studies that have shown that the green s function between two stations can be retrieved from the temporal cross correlation of seismic noise we show that the same simple subsoil structure allows interpretation of our correlation results in time and frequency we observe both love and rayleigh waves however love waves dominate the records in our lower frequency range between 3 6 and 6 hz while rayleigh waves are prevalent in the records in our higher frequency band from 6 to 20 hz frequency domain cross correlation yields better results in the lower frequency range while time domain cross correlation shows very clear results in the higher frequency band thus both analyses are complementary our results show that ambient vibration recorded at the free surface includes different types of waves but that the correlation between any two stations is governed by the more stable propagation mode between them surface waves in the case of a layered medium our results shed some light on the nature of microtremors and on the reasons why the spatial autocorrelation method or the horizontal to vertical spectral ratios are useful in geophysical and site response studies

a seafloor and sea surface gravity survey of axial volcano seafloor and sea surface gravity measurements are used to model the internal density structure of axial volcano seafloor measurements made at 53 sites within and adjacent to the axial volcano summit caldera provide constraints on the fine scale density structure shipboard gravity measurements made along 540 km of track line above axial volcano and adjacent portions of the juan de fuca ridge provide constraints on the density over a broader region and on the isostatic compensation the seafloor gravity anomalies give an average density of 2 7 g cm 3 for the uppermost portion of axial volcano the sea surface gravity anomalies yield a local compensation parameter of 23 significantly less than expected for a volcanic edifice built on zero age lithosphere three dimensional ideal body models of the seafloor gravity measurements suggest that low density material with a density contrast of at least 0 15 g cm 3 may be located underneath the summit caldera the data are consistent with low density material at shallow depths near the southern portion of the caldera dipping downward to the north the correlation of shallow low density material and surface expressions of recent volcanic activity fresh lavas and high temperature hydrothermal venting suggests a zone of highly porous crust seminorm minimization modeling of the surface gravity measurements also suggest a low density region under the central portion of axial volcano the presence of low density material beneath axial caldera suggests a partially molten magma chamber at depth

a parallochthonous group of sedimentary rocks unconformable overlying the bay of islands ophiolite complex north arm mountain newfoundland this paper reports and documents the existence on the north arm mountain massif of a parallochthonous group of sedimentary rocks that overlie the bay of islands ophiolite complex with marked angular unconformity some of these rocks are those previously regarded as oceanic sediments lying conformably on top of the igneous rocks of the ophiolite complex and some were mapped as part of the igneous rocks themselves the name crabb brook group is proposed for this sedimentary sequence and three new formation names are also put forward massive sedimentary breccias directly overlie and contain clasts from all units of the ophiolite complex evidence of intense predepositional weathering of the clasts and the underlying bedrock is common bedded shales and their disrupted equivalents in olistostromes containing sedimentary and igneous blocks overlie the breccias two fossil localities within the shales yield acritarchs of llanvirnian age and inarticulate brachiopods probably not older than this a small remnan

terminology for layered intrusions the terminology of igneous cumulates was originally formulated on the concept that the rocks were concentrations of minerals accumulated by crystal settling it is now well established for several major intrusions that plagioclase that had been typed as cumulus should in fact have floated in its parental magmatic liquid nevertheless the terminology has been valuable in the classification of the rocks of these and other intrusions and it has been useful in certain aspects of their genetic interpretation the name cumulate is therefore redefined so that crystal settling is a possible but not essential process in the origins of the rocks to which it is applied the overall nomenclature of layered intrusions is then reviewed and definitions of names are refined and systematized to clarify their applications careful distinction is made between descriptive and genetic names a cumulate is defined as an igneous rock characterized by a framework of touching mineral crystals and grains that evidently were concentrated through fractional crystallization of their parental magmatic liquids a layer is a sheetlike unit of cumulate that is a distinctive entity in its compositional and or textural features a lamina is a thin 2 5 cm sharply defined layer igneous lamination is a general name for planar structure that is pervasive through cumulates on the scale of their grain size layering is the overall structure and fabric manifest through combinations of layers laminae and lamination the difference between layers and layering is essentially the difference between individuals and a population and the terms that describe one are not necessarily appropriate to the description of the other but the fundamental variants of all of layering lamination grading in layers and layer contacts can essentially be described by one or more of the four modifiers grain size modal textural and cryptic where the last refers to cumulus miner al compositions a system for subdividing layered intrusions is proposed in which distinction is made between general divisions ranked as series zones and subzones and prominent units that are called groups members rhythmic units and cyclic units applications to the skaergaard bushveld muskox stillwater and other intrusions are briefly described

the influence of water on the petrogenesis of subductionrelated igneous rocks the presence of dissolved water can significantly change the sequence in which different minerals crystallize from a silicate magma1 3 this will alter the compositional path followed by residual liquids the liquid line of descent and modify the types and compositions of crystals that accumulate at the site of cooling the sharp decrease in the solubility of water in silicate melts with decreasing pressure makes direct measurements of pre eruptive concentrations in magmas difficult because much of the dissolved water boils off before eruption here we report experimental results on the crystallization of basalts that contain substantial dissolved water up to 6 wt and show how the results can be used to infer the amount of dissolved h2o involved in the petrogenesis of lavas that preserve a record of their liquid lines of descent or the accumulated minerals left behind after solidification the presence of abundant magmatic water is a signature of subduction zone volcanism4 and can be used to associate a suite of magmas or igneous cumulates with a convergent margin back arc or interarc tectonic setting

the volcanic tectonic cycle of the famous and amar valleys mid atlantic ridge 36 47 n evidence from basalt glass and phenocryst compositional variations for a steady state magma chamber beneath the valley midsections amar 3 the 1978 amar expedition extended the investigation of the mid atlantic ridge that was begun by the 1973 1974 famous expedition from the northern end of the famous rift the original famous area to the narrow central and southern end of the famous rift narrowgate region and into the broad amar valley south of fracture zone b available field and geochemical data allow us to characterize in detail the volcanic tectonic cycle for these two segments of the mid ocean ridge system a dynamic model of the crust forming processes within these two rift valley segments is deduced from 1 variations in the chemical composition of the erupted basaltic liquids as preserved in chilled glassy margins 2 composition and petrographie relationships of megacrysts and phenocrysts that represent cumulus crystallization onto the floor of a shallow magma chamber of a primitive magma that has reached pyroxene saturation early in its evolution and 3 detailed stratigraphie and regional observations that characterize the periodicity of volcanic construction and superimposed tectonism related to the ongoing extension of the valley floor each volcanic cycle is composed of several eruptive episodes that are initially typified by rapidly extruded sheet flows of primitive composition the decreasing extrustion rates at the end of an eruptive event are associated with more evolved liquid compositions that reflect fractionation within the shallow conduits intermixing of magmas is indicated by the relatively limited variability in glass composition not explicable in terms of simple fractionation processes the common evidence of crystal resorption and the apparent disequilibrium between megacrysts phenocrysts and their enclosing glass this indicates control by a crustal magma chamber liquid and crystal compositions and textural relations can best be explained by a three stage crystallization history 1 crystallization of olivine plagioclase and clinopyroxene onto the floor of a magma chamber 2 partial resorption of mineral phases by a superheated undersaturated liquid above the floor 3 intratelluric crystallization of olivine and plagioclase clinopyroxene during rise of melt to the surface through the conduit system above the magma chamber a magma chamber model is suggested for the amar famous rift valleys that satisfies both seismic and geological constraints we propose that a small steady state magma chamber is maintained beneath the topographically higher midsections of each valley segment amar and narrowgate regions but that these thin and terminate near the intersecting fracture zones expansion and contracton of the central magma body is controlled by the fluctuating imbalance between magma supply and chamber crystallization the famous narrowgate rift is currently in a contraction period so that in the northern famous region primitive liquid has erupted at mount venus and mount pluto without intercepting and mixing into the steady state chamber that supplies the mixed magmas to the narrowgate region

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

a possible explanation of gravity anomalies over mid ocean ridges gravity fields over the crests of slow spreading mid ocean ridges are characterized by small amplitude free air gravity anomaly lows 30 70 mgal with wavelengths of 40 60 km flanked by smaller gravity highs in general the amplitude and wavelength of these gravity lows decrease with increasing spreading rate until at fast spreading ridges 5 cm yr free air gravity highs 10 20 mgal are observed previous explanations of these anomalies which involve uncompensated ridges thermally expanded ridges and elastic plate compensation models are generally not consistent with the geological evidence regarding the nature of the crust and upper mantle beneath the ridge crest in particular the postulated presence of wide zones of partial melt in the uppermost mantle is inconsistent with petrologic data from mid ocean ridge basalts which indicates that primary melts are in equilibrium with and extracted from residual mantle at depths of 30 km or more in addition similar gravity lows observed over extinct spreading ridges suggest that the anomalies are not mainly due to any dynamic aspect of the accretion process taking place at active spreading centers geometrical models of mid ocean ridges derived from detailed structural and petrologic studies of the bay of islands ophiolite complex have been used to generate gravity anomalies over ridges with various spreading rates modeling results show that four main sources serve to define the amplitude and wavelength of the gravity anomaly the ridge topography the crustal level magma chamber the configuration of the isotherms near the ridge crest and a low density partially gabbroic cumulate root extending vertically into the mantle beneath the ridge systematic variations in the contributions from these sources with increasing spreading rate control the changes in character of the gravity anomaly from slow to fast spreading centers major contributions are found to come from the topography and from the lowdensity root which broadens with slower spreading rates resulting in both increasing amplitude and increasing wavelength with decreasing spreading rate at extinct spreading centers contributions from the magma chamber isotherms and topography gradually diminish with time once the ridge ceases to spread whereas the low density root remains a permanent part of the oceanic lithosphere and continues its contribution to the gravity field seismic evidence from slow spreading centers appears to support the existence of the lowdensity gabbroic root zone inferred from the study of the bay of islands ophiolite and the gravity analysis presented here

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

serpentinization of cumulate ultramafic rocks from the north arm mountain massif of the bay of islands ophiolite abstract partially serpentinized dunites and wehrlites comprise the bulk of the cumulate ultramafic unit at the north arm mountain massif of the bay of islands ophiolite complex newfoundland in a suite of 59 dunites and werhlites from the base of the unit the serpentinized portions consist of lizardite chrysotile brucite accessory magnetite the ratio of lizardite chrysotile to brucite 8 2 weight percent petrographic observations show that most serpentinization occurred at the expense of olivine only limited amounts of clinopyroxene were serpentized an estimated volume increase of 32 accompanied serpentinization of the peridotites reconstructions of the primary modal proportions of wehrlites made taking this volume increase into account contain an average of 6 more clinopyroxene and 6 less olivine than do modal reconstructions that ignore the volume increase mass balance calculations provide no clear evidence for appreciable metasomatism of al2o3 cao feo mgo or sio2 during serpentinization the presence of brucite the evidence that most serpentinization occurred at the expense of olivine and the lack of appreciable metasomatism suggest that the primary reaction that controlled serpentinization of the peridotites is 2mg 2 sio 2 3h 2 o mg 3 si 2 o 5 oh 4 mg oh 2 olivine added serpentine brucite

interferometric ground roll removal attenuation of scattered surface waves in single sensor data land seismic data are contaminated by surface waves or ground roll these surface waves are a form of source generated noise and can be strongly scattered by near surface heterogeneities the resulting scattered ground roll can be particularly difficult to separate from the desired reflection data especially when this scattered ground roll propagates in the crossline direction we have used seismic interferometry to estimate scattered surface waves recorded during an exploration seismic survey between pairs of receiver locations where sources and receivers coincide these interreceiver surface wave estimates were adaptively subtracted from the data this predictive subtraction process can successfully attenuate scattered surface waves while preserving the valuable reflected arrivals forming a new method of scattered ground roll attenuation we refer to this as interferometric ground roll removal

evaluation of olivine spinel geothermometry as an indicator of thermal history for peridotites olivine and spinel in peridotites from the miyamori ultramafic complex and the ichinogemata crater of northeast japan show a systematic variation in the mg mg fe ratio which is correlated mainly with the grain size of spinel this correlation can be explained by a diffusion model assuming a semi infinite composite sphere under cooling or heating conditions in order to obtain absolute temperatures of thermal events the olivine spinel geothermometer is applied to pairs of spinel core and olivine core average composition the calculated temperatures range over two hundred degrees and have a systematic relationship with the grain size of spinel in the miyamori complex the calculated temperatures decrease monotonically with decrease in grain size of spinel whereas in the ichinomegata lherzolite nodule those of spinel smaller than 0 2 mm increase as the grain size decreases and those of spinel larger than 0 2 mm remain constant regardless of further increase in grain size these observations in the light of the diffusion model suggest that the miyamori complex may have cooled from higher than 800 c to lower than 600 c and that the lherzolite nodule from the ichinomegata crater may have been in equilibrium at 900 c before it was heated above 1 100 c for less than a few days these two examples indicate that olivine spinel pairs of peridotites do not always indicate an appropriate equilibration temperature we cannot interpret the supposed equilibration temperatures until the existence of isothermal stages in the thermal history of peridotites is established by carefully checking the chemical heterogeneity

seismic and electromagnetic controlled source interferometry in dissipative media abstract seismic interferometry deals with the generation of new seismic responses by crosscorrelating existing ones one of the main assumptions underlying most interferometry methods is that the medium is lossless we develop an interferometry by deconvolution approach which circumvents this assumption the proposed method applies not only to seismic waves but to any type of diffusion and or wave field in a dissipative medium this opens the way to applying interferometry to controlled source electromagnetic csem data interferometry by deconvolution replaces the overburden by a homogeneous half space thereby solving the shallow sea problem for csem applications we demonstrate this at the hand of numerically modeled csem data

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

origin and emplacement of ophiolites ever since their recognition ophiolites have been a source of controversy although all workers have agreed upon their importance a number of conflicting schools of thought have become unified in the interpretation of ophiolite complexes as fragments of oceanic crust and mantle formed at spreading centers an expanded version of the penrose ophiolite definition is proposed herein called the ophiolite association taking into account rocks which bear upon the origin and significance of ophiolite complexes an ideal ophiolite association includes the following units from bottom to top 1 a crystalline basement and shallow water sedimentary sequence 2 a tectonic unit of thrust slices of continental margin and rise and abyssal sediments and or melange 3 a metamorphic unit as much as a few hundred meters thick generally with higher grade rocks over lower grade ones 4 ultramafic tectonite unit composed dominantly of multiply deformed peridotite dunite and minor chromitite 5 a cumulate complex ultramafic at the base grading to mafic or intermediate at the top 6 a noncumulus unit of varitextured gabbro and minor trondhjemite 7 a sheeted dike complex 8 an extrusive section of massive and pillowed flows and intercalated sediments 9 an abyssal or bathyal sediment sequence which may include radiolarian chert red pelagic limestone metalliferous sediments breccias and or pyroclastic deposits and 10 postemplacement deposits of laterite reef limestone or shallow marine or subaerial sediments this expanded ophiolite association allows greater precision in inferring original origin and emplacement some ophiolite complexes particularly in the tethys herein designated tethyan type exhibit units 1 2 3 and 4 whereas others particularly in the cordilleran belt herein termed cordilleran type lack units 1 2 3 and in places 4 the emplacement of the former probably occurred during collision of a passive margin with a subduction zone or incipient subduction zone emplacement of the cordilleran type generally is not clear cut the nature of the spreading center which produced a given ophiolite is indicated more clearly by the presence or absence of units 5 9 than by chemistry or mineralogy some ophiolite complexes lack complete sections and may have formed along transform faults others contain volcanic pyroclastic deposits instead of pelagic sedimentary sequence and probably formed within or near an arc several ophiolite complexes may have originated by conversion of a transform fault into a subduction zone such conversion may result from normal plate evolution or from major plate reorganization major times of ophiolite emplacements appear to correspond with global plate reorganizations

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

a geochemical approach to model periodically replenished magma chambers does oscillatory supply account for the magmatic evolution of epr 17 19 s abstract we propose a new approach to model the geochemical evolution of continuously replenished and tapped steady state magma chambers we use a sinusoidal function to model cyclic magma supply the temporal evolution of a reservoir is described using differential equations in which the amount of refilling magma does not depend on the size of the chamber these equations can be used to calculate incompatible trace element concentrations and magma quantities we examine the geochemical consequences of episodic injections noises and wall rock assimilation we also explore possible variations in crystallization rate to show its potential the theoretical treatment has been applied to the epr 17 19 s a site with a strong magma budget which has been the subject of several geological geophysical studies the practical application requires geological parameters to be constrained as well as the extreme values of the lava concentration range a first step specifies the incompatible trace element composition of the replenishing melt which corresponds in the epr case to a magnesian liquid mgo 9 5 wt it is then possible to determine other parameters such as cycle period 750 years magma residence time 300 years and reservoir size from 4 1 to 8 6 km3 per 20 km segment lastly variations in crystallization rate do not significantly alter the results

mineral relationships in gabbroic rocks from fracture zones of indian ocean ridges evidence for extensive fractionation parental diversity and boundary layer recrystallization

morphotectonic evolution of rifted continental margins inferences from a coupled tectonic surface processes model and fission track thermochronology we use a numerical model to study the topographic evolution and erosional history of rifted continental margins the model combines a kinematic description of lithospheric extension tectonic model with a surface processes model that includes short range hillslope and long range fluvial transport the tectonic model predicts the evolution of the lithospheric temperature and strength elastic thickness distribution as well as the tectonic uplift through time this is input into the surface processes model which calculates the degradation of topography and associated isostatic rebound modeled denudation histories across the margin are used to predict apatite fission track age and length patterns modeling results indicate that depending on the adopted parameters an uplifted rift flank should degrade by erosion within 50 100 m y without significant retreat of the topographic elevation maximum the development of an escarpment system at rifted continental margins is in itself not an indication of tectonic rift flank uplift but results from the existence of a high elevation interior plateau erosional base level lowering as a consequence of rifting and regional isostatic response to erosion of the margin however apatite fission track thermochronology reveals that the areas seaward of the escarpment at a number of rifted margins have been exhumed from several kilometers depth such amounts of denudation cannot be accommodated with isostatic rebound alone and require additional tectonic uplift of the rift flanks modeling of apatite fission track patterns suggests that fission track thermochronology dates the onset of rapid erosion which coincides with the initiation of strong relief i e initiation of rifting fission track ages which are younger than the age of rifting thus cannot be unambiguously interpreted as excluding prerift uplift the timing of margin uplift can be established only by careful track length analysis and integration with regional stratigraphic data the model is applied to the saudi arabian red sea margin and the southeastern australian highlands where it is constrained by present day topography and apatite fission track data as well as seismic and gravity data for the saudi arabian red sea margin synrift regional plateau uplift with a magnitude of approximately 1 km is inferred possibly as a result of asthenospheric upwelling flexurally induced tectonic uplift of the rift flank with a magnitude of 2 km is superimposed on this regional uplift the relatively high elevation of the southeastern australian highlands their steep front and the relatively high amounts of erosion suggest that apart from mesozoic synrift flexural uplift tertiary rejuvenation of topography has occurred possibly as a result of renewed lithospheric thinning and underplating the low elevation of the australian interior would inhibit the evolution of a major escarpment in the absence of such renewed uplift

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

evidence from chromium abundances in mantle rocks for extraction of picrite and komatiite melts one of the more important aspects of understanding the geochemical differentiation of the earth is an accurate knowledge of the compositions of magmas produced by direct melting of the upper mantle which are called primary magmas1 it is clear that a variety of primary magma compositions can develop within the mantle depending on the source composition the presence of volatiles the extent of melting and the pressure of melt segregation1 4 proposed primary magma compositions span a wide composition range from basalts with 8 ll mgo through picrites with 11 20 mgo to komatiites with 20 28 mgo there is no agreement as to which is the dominant type of primary magma we suggest here that the abundances of chromium in samples of the earth s upper mantle indicate that of the wide variety of possible primary liquids produced in experimental studies of mantle melting the dominant type of primary liquid actually extracted from the mantle has been picritic to komatiitic with 15 mgo

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

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

marine tectonics processes at mid ocean ridges the use of multibeam echo sounders and various side looking sonar systems has greatly increased our knowledge and understanding of mid ocean ridge mor processes since the last iugg review on mid ocean ridge research macdonald 1983 the extensive use of swath mapping tools such as sea beam deep tow seamarc i and ii the application of multichannel seismic reflection techniques to the detection of axial magma chambers and the use of satellite data in the study of oceanic lithosphere have rejuvenated marine tectonics and have generated and stimulated a large number of important contributions this review will focus on what we feel were the major developments in marine tectonics with an emphasis on mid ocean ridges in the last four years certain topics such as marine seismicity and satellite studies are only briefly mentionned for reference and will be covered in greater detail in other reviews because of the exponential growth in the number of publications related to mors during 1982 1986 only a few references pertinent to each of the major research topics will be quoted in the text as examples but the reader will find an extensive bibliography at the end of the text references published prior to 1982 were added only where appropriate

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

detrital spinels from alpinotype source rocks in middle devonian sediments of the rhenish massif middle devonian sandy siltstones of the gummersbach area rhenish massif show increased cr contents up to 675 ppm the chromium is mainly associated with detrital cr al spinels the spinels are characterized by a wide spectrum of chemical composition in more than 50 of 134 analyzed spinel grains the cr2o3 content exceeds 50 wt the a12o3 content which is negatively correlated to cr2o3 reaches 40 wt the mgo contents are with few exceptions lower than the feo contents both elements are negatively correlated the fe2o3 contents which were calculated from a quarter of the grains rarely exceed 5 wt besides cr al fe mg tio2 contents were determined the chemical composition of the detrital spinels indicates provenance from peridotites of an ophiolite suite provenance analysis shows that the source area of the middle devonian clastic sediments was situated in the north

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

density changes during the fractional crystallization of basaltic magmas fluid dynamic implications the dynamical behaviour of basaltic magma chambers is fundamentally controlled by the changes that occur in the density of magma as it crystallizes in this paper the term fractionation density is introduced and defined as the ratio of the gram formula weight to molar volume of the chemical components in the liquid phase that are being removed by fractional crystallization removal of olivine and pyroxene whose values of fractionation density are larger than the density of the magma causes the density of residual liquid to decrease removal of plagioclase with fractionation density less than the magma density can cause the density of residual liquid to increase during the progressive differentiation of basaltic magma density decreases during fractionation of olivine olivine pyroxene and pyroxene assemblages when plagioclase joins these mafic phases magma density can sometimes increase leading to a density minimum calculations of melt density changes during fractionation show that compositional effects on density are usually greater than associated thermal effects in the closed system evolution of basaltic magma several stages of distinctive fluid dynamical behaviour can be recognised that depend on the density changes which accompany crystallization as well as on the geometry of the chamber in an early stage of the evolution where olivine and or pyroxenes are the fractionating phases compositional stratification can occur due to side wall crystallization and replenishment by new magma with the most differentiated magma tending to accumulate at the roof of the chamber when plagioclase becomes a fractionating phase a zone of well mixed magma with a composition close to the density minimum of the system can form in the chamber the growth of a zone of constant composition destroys the stratification in the chamber a chamber of well mixed magma is maintained while further differentiation occurs unless the walls of the chamber slope inwards in which case dense boundary layer flows can lead to stable stratification of cool differentiated magma at the floor of the chamber in a basaltic magma chamber replenished by primitive magma the new magma ponds at the base and evolves until it reaches the same density and composition as overlying magma successive cycles of replenishment of primitive magma can also form compositional zonation if successive cycles occur before internal thermal equilibrium is reached in a chamber in a chamber containing well mixed plagioclase saturated magma the primitive magma can be either denser or lighter than the resident magma in the first case the new magma ponds at the base and fractionates until it reaches the same density as the evolved magma mixing then occurs between magmas of different temperatures and compositions in the second case a turbulent plume is generated that causes the new magma to mix immediately with the resident magma