cryptic variation within the cumulate sequence of the oman ophiolite magma chamber depth and petrological implications the cumulate sequence of the oman ophiolite consists of layered mafic and ultramafic cumulates that represent the products of a dynamic long lived crustal magma chamber that underlay a late cretaceous spreading ridge regional variations in cumulate sequence stratigraphy reflect not only contrasting degrees of mantle fractionation but also contrasting primary magma compositions microprobe data are presented from a 600 m section of modally layered olivine gabbros coherent cryptic cyclic variation is shown with both normal and reversed geochemical gradients present these are explained in terms of fractionation mixing and eruption in an open system magma chamber the height of the liquid column required to precipitate the observed cyclic units is calculated to be about 100 m the implications of this result are discussed a contrast between the fo an co variation between olivine and plagioclase in the oman cumulate sequence and that observed for morb votcanics is noted this may reflect compositional stratification in spreading ridge magma chambers or signifcant differences between the petrogenesis of the oman ophiolite and morb

seismicity of the galapagos 95 5 w propagating rift an array of ocean bottom seismometers was deployed for 21 days about the 95 5 w galapagos propagating rift tip to obtain seismic evidence for the propagating rift hypothesis and to investigate the dynamic effects of rift propagation seismicity was fairly constant 10 17 events per day with no indication of swarm activity hypocenters were determined for 304 earthquakes the largest of which is estimated to be mb 2 3 with a seismic moment of 2 9 1020 dyn cm the cumulative seismic moment released by all local earthquakes is estimated to be 6 6 1020 dyn cm a b value of 0 77 0 08 was estimated for the region implying a stress regime dominated by a broad tectonic stress field the distribution of earthquakes is consistent with the propagating rift hypothesis most earthquakes occurred south of the propagating rift within the de steiguer deep seismicity within the transform zone is clustered in such a way as to suggest that two or three discrete fault segments may be active at the same time the seismicity observed along the propagating rift is diffuse earthquakes were observed at the propagating rift tip but not in front of it almost all earthquakes occur south of the northern pseudofault and its extension west of the rift tip that is earthquakes occur almost exclusively in lithosphere that has been or is being overtaken by the propagating rift six composite focal mechanisms are consistent with the propagating rift hypothesis first motions from earthquakes located on the propagating rift indicate extension whereas motions associated with the failing rift area indicate compression a compressional regime is also indicated west of the transform zone twelve earthquakes from the transform zone produce a poorly constrained focal mechanism consistent with right lateral strike slip motion along a fault plane that is roughly parallel to the isochrons of the transferred lithosphere

metamorphism at the base of the samail ophiolite southeastern oman mountains metamorphic rocks showing an inverted metamorphic zonation from upper amphibolite facies to greenschist facies occur beneath the samail ophiolite the metamorphic protolith was a section of marine basalts mn rich cherts and argillites garnet clinopyroxene amphibolites occur within 2 m of the contact hornblende clinopyroxene assemblages occur within 80 m of the contact and lower amphibolite facies rocks persist to 130 m from the contact the higher grade rocks show polyphase deformation cataclastic fabrics and partial retrogression to greenschist facies assemblages however lower grade phyllites show evidence of only one episode of metamorphic crystallization unmetamorphosed sediments in contact with the basal metamorphic sheet do not show evidence of polyphase deformation the distribution coefficient kd feo mgo gar feo mgo cpx ranges from 4 4 to 5 3 suggesting a t range of 755 865 c at 200 mpa low jadeite content in clinopyroxene and low glaucophane content of ca amphiboles are consistent with moderate pressures of crystallization the metamorphic sheet is inferred to be of composite origin the highest grade rocks were tectonically transported the greatest distance and the lower grade rocks were successively incorporated into the sheet as the peridotite was thrust over progressively cooler rocks residual heat from the ophiolite is the dominant heat source for the metamorphism and this could have been augmented by a limited amount of frictional heat generated during thrusting these thermal constraints suggest that the metamorphism occurred early in the tectonic transport history of the samail ophiolite

a new magma chamber model based on structural studies in the oman ophiolite abstract structural mapping in the mafic ultramafic transition zone and in the overlying crustal section of the oman ophiolite has revealed the importance of magmatic flow in the formation of layering in the gabbros and the mechanical coupling between this flow and the asthenospheric flow of the underlying peridotites the crustal section near the spreading axis and in particular the magma chamber are fed by two independent melts a dominant gabbroic melt episodically injected from depth and a subordinate wehrlitic crystal mixture produced by compaction of the transition zone layering in the gabbros can be partly produced by tectonic transposition of the gabbroic and wehrlitic mixtures during magmatic flow whatever its origin the layering orientation in the newly created crust does not necessarily reflect its original attitude the final orientation of the layering is determined by the attitude of the magmatic flow plane at the time it was frozen a new magma chamber model is proposed which solves some of the physical difficulties of previous models and accounts for the seismic imaging of magma chambers beneath oceanic ridges

duality of magmatism in the plutonic sequence of the sumail nappe oman abstract detailed field mapping of the crustal sequence in several areas of the sumail ophiolite nappe mainly in the fizh the salahi and the haylayn blocks and comparisons with other well studied areas have shown that the crustal sequence was constructed by two distinct although nearly contemporaneous magmatic series the first a gabbroic series was emplaced during the accretion stage at an oceanic spreading centre which formed the layered gabbro unit the high level isotropic gabbro unit and their by products the sheeted dyke complex and the lower extrusives fine grained homogeneous planar laminated gabbros commonly occur at the top of the layered gabbro sequence in some specific areas wadi haymiliyah and wadi rajmi these are strongly differentiated fe ti rich gabbronorites and norites which coincide with large scale discontinuities within the plutonic sequence interpreted as axial discontinuities of the spreading magma chambers the second series an ultramafic wehrlitic series is locally rooted in the upper mantle i e the impregnated dunitic transition zone this has intruded the whole crustal sequence up to the pillow lavas countless discordant and concordant wehrlitic bodies intruded the plutonic sequence making up 20 40 of it in volume we describe the various settings and forms of these intrusions which were largely underestimated in previous studies we also describe their mineralogy and textures and show with chemical and mineralogical arguments that these bodies evolved as relatively closed systems independently from each other we infer from the study of the contacts that the gabbros were still hot and consolidated just at the moment of the wehrlitic intrusion at the base of the crust however indications of immiscibility between gabbros and wehrlites are common suggesting that both magmatic series were quite contemporaneous the wehrlites are commonly isotropic and may exhibit an internal layering of the dunite wehrlite clinopyroxenite type sometimes appearing at the upper or lower margins of the intrusions in this case the ultramafic layered cumulates show lobate intrusive contacts with the adjacent layered gabbros these plagioclase free ultramafic layered cumulates therefore are not interbedded within the gabbro cumulates but actually belong to the wehrlitic intrusive series picritic dykes radiating from the uppermost intrusions can be traced up to the upper extrusives which are for the main part cogenetic with the wehrlites the time of this second magmatic event coincides with the cessation of the accretion process and with the beginning of the intraoceanic detachment of the ophiolite at the spreading centre we believe that the compressive regime that initiated the detachment acted as an efficient filter press and helped the extraction and ascent of the wehrlitic magmas towards the uppermost levels of the crust

spreading structure of the troodos ophiolite cyprus orientations of dikes within the sheeted dike complex of the troodos ophiolite reveal primary spreading structure produced at a complex ridge transform intersection three structural grabens are defined by listric and planar normal faults and rotated dikes that dip symmetrically toward graben axes faults flatten at depth into a detachment within the upper parts of the plutonic complex large exhalative massive sulfide deposits occur within the pillow sections of two of the grabens and appear to be associated with underlying altered and mineralized normal fault zones that channeled hydrothermal fluids we suggest that the grabens are fossil axial valleys produced by successive eastward jumps of an approximately north trending present coordinates slow spreading ridge crest a simple model for ridge migration indicates eastward jumps of 8 13 km changes in ridge orientation are suggested by changes in trends of dikes and graben axes comparison of the pattern of dikes near the arakapas fault zone with the structure of active ridge transform intersections suggests that the fault is a right offset sinistral transform in contrast to earlier models in which a ridge to the west of the exposed troodos complex was proposed

petrology and oxygen isotope geochemistry of a fossil seawater hydrothermal system within the solea graben northern troodos ophiolite cyprus hydrothermal mineral zonations and o isotope patterns of the northern troodos complex do not parallel the ophiolite pseudostratigraphy but reflect the convective geometry of an upper cretaceous seawater hydrothermal system large areas of the sheeted intrusive complex sic including the subaxial region of the solea graben are composed of 18o rich subgreenschist mineral assemblages and may represent regions of diffuse seawater recharge other areas of the sic are recrystallized to distinctive epidosite rocks granular high variance assemblages of epidote quartz chlorite that are depleted in 18o al2o3 na2o k2o zr cu and zn and are enriched in cao and sr compared with other mafic volcanic and dike rocks of the solea graben epidosite alteration occurred at temperatures of 310 370 c and involved fluids with 18o values and salinities similar to those of upper cretaceous seawater the epidosite zones are discordant with earlier mineral o isotope zonations and with the axis of spreading in the solea graben suggesting a postspreading off axis origin the seawater hydrothermal system responsible for solea graben massive sulfide deposits was probably driven by hypabyssal intrusions not exposed emplaced in a terminal failed spreading episode the geometries of o isotope surfaces within the solea graben imply that the epidosites formed in fossil upflow and deep recharge conduits depletions in base metals show that epidosite alteration liberated cu and zn to mineralizing fluids within the fossil upflow zone

the volcanic stratigraphy and petrogenesis of the oman ophiolite complex the volcanic stratigraphy and trace element geochemistry of the oman ophiolite complex indicate a multistage magmatic origin comprising 1 magmatism due to sea floor spreading in a marginal basin 2 magmatism associated with discrete submarine volcanic centres or seamounts 3 magmatism associated with crustal uplift and rifting and 4 magmatism associated with continent arc collision trace element petrogenetic modelling is used to investigate the nature of the mantle source region and the partial melting and fractional crystallization history for each magmatic event the petrogenetic pathway for the sea floor spreading lavas requires a high degree of melting of a mantle that was depleted in incompatible elements prior to subduction but subsequently selectively enriched in certain elements mostly lil elements and h2o from an underlying subduction zone it also requires magma mixing in an open system magma chamber prior to eruption the seamount lavas were probably derived by a similar degree of partial melting of a similar source but fractional crystallization was restricted to smaller high level magma chambers the rifting lavas were derived from a mantle source that was more depleted than the seamount lavas prior to subduction but which was later modified by a larger subduction zone component the syn collision lavas were however derived from an enriched mantle source which probably underlay the passive continental margin rather than the marginal basin complex results such as these may provide considerable insight into the petrogenetic changes accompanying the transitions from spreading to arc volcanism in a supra subduction zone setting

flow to a horizontal well in an aquitard aquifer system a horizontal well is sometime installed in an aquifer beneath a water reservoir to get significant amount of groundwater with better quality an aquitard often separates the surface water body from the underneath aquifer previous studies in vertical wells related to this subject treated leakage from the aquitard as a volumetric source term in the governing equation of flow in the main aquifer a hypothesis termed hantush s approximation for partially penetrating wells in this study flow in the aquitard and aquifer is treated as two systems which are linked through the continuity of flux and head at the aquitard aquifer boundary in particular we treat leakage as a boundary at the aquitard aquifer interface not as a volumetric source the leakage induced by the pumping horizontal well depends on several parameters such as the aquitard thickness the vertical hydraulic conductivity of the aquitard the aquitard specific storage the well location and the well length in general we find that the solution based on the hantush s approximation to some extent deviates from that obtained based on the actual scenario of leakage through the interface during the transient flow condition particularly at the early time for steady state flow the hantush s approximation works reasonably well under realistic conditions of aquitard thickness and hydraulic conductivity as long as the aquitard thickness is relatively thick aquitard aquifer ratio greater than 0 001 this approximation also works reasonably well under realistic horizontal well length and well location as long as the well is not too close to the aquitard aquifer boundary q 2005 elsevier b v all rights reserved

petrology of mariana trough and lau basin basalts the crust of actively opening back arc basins has close chemical and mineralogic similarities to normal mid ocean ridge basalts n morb but some subtle but distinctive differences exist these are largely due to differences in volatiles fe oxidation ratio abundances of large ion lithophile elements lil and some high field strength hfs elements isotope data for sr pb nd he and o are variable but broadly are morb like we present data contrasting mariana trough mtb and lau basin basalts lbb showing how they differ in certain trace element and isotopic signatures and compare both to n morb relative to n morb and lbb the mtb are enriched in alkalis alkaline earths light rare earth elements ree h2o and fe fe even for primitive samples while having morb type he 3 4 7 8 to 8 4 ra sr 87 86 mean 0 70293 and nd e 10 2 the lau basin is zoned with a central area having n morb characteristics for h2o lil ree nd e 8 1 to 9 0 and o but a depletion in hfs elements sr is more radiogenic mean 87 86 0 70347 and 3he 4he 3 4 7 2 to 11 0 ra is high relative to n morb the lau basin has a zone bordering the adjacent island arcs which is like mtb in both back arc basins relatively primitive unfractionated basalts with mg 100 mg fe 62 are found which resemble postulated primitive n morb calculated parental magmas obtained by adding xenocrystic olivine plot near or on 3 and 4 phase boundaries at 1 1 5 gpa for the ol di si system low p fractional crystallization and magma mixing explain most of the observed rock types possible explanations for differences between mtb and lbb include differences in the extent of previous melting episodes of their mantle sources and effects of source metasomatism through dehydration and partial melting of subducted lithosphere the long and complex history of mariana trough mantle sources led to a metasomatized mantle which gives rise to the distinctive mtb in contrast the lau basin is dominated by basalts derived from a source depleted in volatiles lil and hfs elements a previous melting history is implied mixing with a mantle component enriched in 87sr and 3he may be due to mantle counterflow driven by the subduction process

east pacific rise from siqueiros to orozco fracture zones along strike continuity of axial neovolcanic zone and structure and evolution of overlapping spreading centers we have used the sea beam multibeam echo sounding system to survey the east pacific rise epr from 8 20 n to 18 30 n obtaining complete coverage of the epr axial neovolcanic zone and all intervening transform faults here we focus on the epr neovolcanic zone and the definition of overlapping spreading centers ocs s between the orozco and siqueiros transform faults the neovolcanic zone is narrow 0 5 2 0 km and continuous along strike and occurs within and near a strike continuous axial summit graben the neovolcanic zone and summit graben reside along the crest of a volcanic axial high which is 2 10 km wide and which continues uninterrupted along strike for 40 140 km within 20 50 km of an intersection with a transform fault the axial neovolcanic zone narrows and deepens and within only 3 6 km of the intersection the neovolcanic zone turns sharply into the transform valley at seven locations between the siqueiros and orozco transform faults the axial neovolcanic zone is discontinuous and is laterally offset a short distance 1 15 km in contrast with a classic ridge transform ridge plate boundary however offset rise terminations overlap each other by a distance approximately 3 times greater than their offset they curve toward each other and often one merges into the other along strike separating the osc s is an elliptical overlap basin up to 600 m deep whose long axis is subparallel to the adjacent spreading centers the overlap basin is characterized by volcanic constructional edifices a key difference between these offsets and small transform faults in the atlantic is that there are no transform fault structures in the overlap basin which link the offset rise axes a continuous axial depth profile reveals a long wavelength 20 60 km undulation of the rise axis which may be associated with variations in the magmatic budget along strike the lowest points in the axial depth profile occur near transform faults and at osc s suggesting that these features are associated with along strike minima in the magmatic budget we suggest that major phases of volcanic activity propagate episodically along the axis away from centers of magmatic inflation and that osc s develop where magmatic pulses fail to meet due to misalignment of fracture systems sinuous bends in the rise axis or other heterogeneities transform faults fail to develop on fast spreading centers where the lateral offsets are small 15 km because the lithosphere is too thin and weak to maintain a classic rigid plate ridge transform fault pattern the osc geometry is unstable and evolves rapidly and the overlapping rises may propagate at rates faster than the local spreading rate one of the two osc s prevails while the other is abandoned while the locations of osc s may not be fixed relative to the rise axis they may tend to recur near the same place creating scars in older lithosphere which resemble fracture zones but whose fine scale structure is quite different there is excellent agreement between the observed structure of osc s and laboratory studies of osc s using wax models photoelastic studies of overlapping cracks and boundary element modeling of overlapping cracks seismic refraction and reflection near two osc s suggests that large offset osc s have separate magma chambers beneath the offset rise axes while small offset osc s may share a single axial magma chamber three dimensional inversion of magnetic anomalies suggests that significant petrologic anomalies may occur near some osc tips waxing and waning of overlapping magma chambers may give rise to the eruption of highly fractionated feti basalts and associated magnetic anomalies

evolution of intra oceanic arc trench systems abstract intra oceanic island arc trench systems represent a locus of crustal thickening through the addition of mantle derived magmas which form the volcanoes and subjacent stratiform plutons the crust of immature island arcs and adjacent back arc basins is oceanic in chemistry in that it is dominated by mafic e g tholeiitic magmas a magma sequence is recognized in many arc systems which progress from boninite to arc tholeiite to calc alkaline and in some arcs to shoshonites the adjacent back arc basins are formed of morb like tholeiite arc systems may be dominated by extension rather than compression if the subducted and overriding plates are decoupled extension of the arc system may be associated with seaward retreat or roll back of the trench axis alternate cycles of extension and compression of the arc system can lead to both tectonic erosion of the fore arc and accretion of fragments from the incoming sea floor the izu bonin mariana tonga yap and palau trench arc systems are examples of different stages in a dynamic process which starts when subduction begins along old fracture zone traces crustal extension causes both fore arc subsidence and opening of back arc basins island arc volcanic chains and back arc basins are new crust formed largely of fractional melts of the mantle the evolution of an arc system may stop when blocking of the subduction zone or changes in relative plate motion causes overthrusting of back arc lithosphere on to former volcanic arcs many ophiolites preserve remnants of arc system crust and upper mantle

petrology and origin of primitive lavas from the troodos ophiolite cyprus parental magmas to the troodos ophiolite are characterised by low tio2 and al2o3 and high sio2 extremely fresh and chemically primitive high mgo rocks are found within the upper pillow lavas and along the arakapas fault belt of cyprus and contain forsteritic olivine enstatite and groundmass clinopyroxene set in glass or plagioclase with accessory magnesiochromite and sometimes hornblende they are quartz normative and may have originally contained up to 3 wt h2o geochemically there are three distinct groups of primitive lavas based on tio2 and zr contents but also reflected by cao na2o and ree abundances these groups cannot be related by crystal fractionation and are considered to have been generated by incremental melting of a variably depleted source region the parental magma to the least depleted group group i was that of the major portion of the troodos plutonic complex and is similar to those postulated for other low ti ophiolites chemically it has close affinities with komatiitic basalts the most depleted lavas group iii all have u shaped ree profiles and variable 143nd 144nd ratios interpreted in terms of metasomatism of the source region by an incompatible element enriched component which was probably derived from a subducted slab these lavas represent an intermediate step in the development of boninite series rocks

geologic section through the samail ophiolite and associated rocks along a muscat ibra transect southeastern oman mountains regional mapping at a 1 60 000 scale of a 30 km strip from the gulf of oman muscat across the oman mountains 130 km to the south provides the geologic setting for the 95 m y ibra section of the samail ophiolite where best preserved the ibra ophiolite section is an 8 km thick section of oceanic consisting of 0 5 km of pillow lavas 1 2 1 6 km of sheeted diabase dike complex 0 2 1 0 km of high level noncumulate gabbro and 3 0 5 0 km of cumulate gabbro that is underlain by tectonite peridotite 9 12 km thick the ibra section is found on the southward dipping limb of the sayah hatat antiform the tectonite peridotite represents uniformly depleted harzburgite and dunite that have been deformed by high temperature low stress asthenospheric flow discordant dunites within the tectonite peridotite appear to represent either flow crystallization products from primary picritic liquids or reaction products of these liquids with the harzburgite the structural base of the tectonite peridotite is overprinted by a high stress low temperature deformation that can be related to its oceanic detachment the layered gabbros are predominantly olivine clinopyroxene plagioclase cumulates and orthopyroxene does not occur as a cumulus phase occurrence of cumulate wehrlites and picrites at high stratigraphic levels within the layered gabbros is evidence that the gabbroic section crystallized predominantly from the bottom upward in a periodically replenished magma chamber high level gabbro represents remnants of crystallization at the roof of the magma chamber and intrudes most overlying diabase dikes both the diabase dike complex and pillow lavas are hydrothermally altered and alteration and metamorphism increase downward zeolite to epidote amphibolite facies in spite of pervasive alteration relict primary mineralogy and bulk chemistry suggest that the diabase dikes and pillow lavas are cogenetic with the underlying gabbros the present day samail thrust surface truncates ophiolite stratigraphy and puts the samail ophiolite on top of unmetamorphosed hawasina melange the last motion on this surface was probably no older than maestrichtian 70 65 m y garnet amphibolites exposed as remnants of earlier thrusting 90 m y record initial ophiolite detachment at a 14 20 km depth within the tethyan oceanic lithosphere the hawasina group which underlays the samail ophiolite is a block melange where exposed near muscat in the north and it grades into an imbricated broken formation at the southern limit of the map area the hawasina group is thrust over permian to late cretaceous shelf carbonates that represent autochthonous arabian continental shelf deposits recent post miocene collapse and dome structures such as the ibra dome have complicated ophiolite stratigraphy south of jabal dimh our geologic studies strongly indicate the samail ophiolite represents a large coherent slab of transported oceanic lithosphere formed at a late cretaceous spreading center in the tethyan sea

k ar ages of metamorphic rocks at the base of the samail ophiolite oman hornblendes from amphiobolities in the sheet of metamorphic rocks beneath the peridotite member of the samail ophiolite and phyllites farther from the peridotite contact have weighted mean sup 40 ar sup 39 ar total fusion ages of 90 0 3 0 m y and 79 5 3 0 m y respectively the amphibolities represent the first tectonic slice welded to the base of the samail ophiolite after it was detached from the tethyan oceanic crust formation of the amphiobolities occurred no more than 3 to 7 m y after crystallization of plagiogranite in the ophiolite the phyllites represent another tectonic slice of ocean floor sediments welded to the ophiolite as it was transported further from the tethyan spreading axis the k ar ages suggest assuming a half spreading rate of 2 to 5 cm yr that detachment of the samail ophiolite and formation of amphibolite facies rocks occurred no more than 60 to 350 km from spreading center using the same spreading rate one can calculate a minimum half width of 300 to 750 km for the tethyan ocean during the late cretaceous

fluid flow patterns in fast spreading east pacific rise crust exposed at hess deep tectonic exposures of a volcanic sequence and sheeted dike complex over a 4 km wide region at hess deep equatorial pacific reveal significant spatial heterogeneity 10 103 m in the extent and nature of hydrothermal alteration in young fast spreading east pacific rise crust the volcanic sequence is fairly uniformly altered with only minor oxidation and alteration to clay minerals sheeted dikes in the eastern part of the field area are highly fractured with narrow intervals of intact dikes that dip up to 60 their alteration characteristics show a simple depth trend such that with increasing depth the dominant secondary mafic mineral changes from chlorite to amphibole clinopyroxene replacement increases 40 whole rock 18o values decreases 4 4 5 5 to 3 5 4 5 and calculated peak metamorphic temperatures increase 250 c to 450 700 c within the deepest dikes localized zones up to 400 m wide are chlorite rich and have low 18o 2 9 4 1 and low peak metamorphic temperatures 345 c these alteration patterns likely formed within broad recharge zones whereby the low 18o zones developed in the regions with the highest fluid flux in the west massive slightly rotated sheeted dikes near the volcanic sheeted dike transition are 18o and cu depleted and display higher peak temperatures 345 c than elsewhere in the shallow dikes these characteristics are consistent with formation within a high temperature hydrothermal discharge zone we propose that the spreading history of a fast spreading ridge segment can create significant spatial heterogeneity in fluid flow and alteration patterns within sheeted dike complexes similar to those preserved in many ophiolites

assessing thermal effects on magnetic fabrics of sedimentary rocks results from synthetic and natural samples 1 in order to examine thermal effects on magnetic fabrics particularly remanence fabrics that test the accuracy of paleomagnetic records the pre and post heating magnetic fabrics of synthetic magnetite caf2 and natural samples were compared the natural samples were collected from a thermally remagnetized baked zone of the triassic passaic fm in southeastern pennsylvania usa and their magnetic fabrics were compared with those from the unremagnetized passaic country rocks the unheated synthetic and natural samples display predominantly oblate magnetic susceptibility and remanence fabrics the heated synthetic samples yield oblate ams and aar fabrics whereas the remagnetized passaic rocks show random ams fabrics but exhibit depositional remanence fabrics apparently natural samples we studied lost their initial ams fabrics but inherited their initial depositional remanence fabrics during thermal remagnetization thus the presence of oblate fabrics may not uniquely indicate a primary remanence the disagreement between ams and aar fabrics could indicate a possible remagnetization

segmentation at a fossil spreading axis the plutonic sequence of the wadi haymiliyah area haylayn block sumail nappe oman abstract wadi haymiliyah in the central part of the haylayn block offers a splendid continuous section across the crustal sequence of the sumail ophiolite the plutonic sequence of this area presents several specific features of great interest for the understanding of the structure and evolution of this ophiolite the petrological moho and the whole plutonic sequence display a subvertical attitude parallel to the mean dyke complex orientation high temperature folding occurred in the mantle sequence and in the lower crust whereas the upper crust remained unfolded and behaved as a rigid slab during this deformation resulting in a tectonic decoupling between lower and upper crust in the gabbro cumulate sequence a remarkable large scale curvature of the magmatic structures layering and lamination planes whose axis is totally discordant with the regional folds axes is interpreted as a primary magmatic feature indicating the southern ending of a magma chamber the lower crust about 2 km thick consists of a primary layered gabbroic sequence consisting of rhythmic mineral graded layers from several metres to several hundred metres thick syndepositional structures such as oblique stratification microslumps eroding mini channels and small normal faults are widespread the mineralogical composition modal variations and cryptic evolution of the whole layered sequence are described both at the megascale and on the simple cyclic scale the data support a model of crystallization of the layered sequence by open system fractionation at the floor and sides of a periodically replenished steady state magma chamber a unique feature of this cumulate sequence is the ubiquitous presence of orthopyroxene throughout the whole cumulate pile this is very unusual in the sumail crust with the exception of the wadi rajmi section in the fizh block two huge plagioclase free dunite wehrlite laccoliths about 300 m thick and numerous ultramafic sills and mushroom like bodies intrude the base of the layered sequence these ultramafic intrusions exhibit in places an internal layering and dunitic differentiation products they have sharp sinuous and lobate contacts with the gabbros which were still hot and probably not entirely consolidated at the moment of the intrusion the mineral compositions of these intrusions are much more primitive than those of the adjacent gabbros higher in the plutonic section the layered gabbros are overlain by 1 km of fine grained non layered planar laminated gabbronorites characterized by the abundance of pleochroic cumulus hypersthene the disappearance of olivine and the development of fe ti oxides and amphibole as post cumulus phases numerous occurrences of concordant coarse grained layered gabbros occur in this unit which is characterized also by a dense net of highly residual dioritic to tonalitic veins resulting in a huge magmatic breccia in the uppermost few hundred metres where the gabbronorites are brecciated by the acidic rocks hydrothermal veins are also extremely abundant in this unit which coincides with the core of the large scale curvature this unit comprising unusually fractionated rocks is interpreted as resulting from the evolution of the haymiliyah magma chamber towards a closed system fractionation due to the cutting off of source supply our interpretation of both structural and petrological data may be summarized by the following major points 1 1 the haymailiyah magma chamber was located at the southern tip of a spreading cell and was cut off from the magma supply at the end of its evolution either vertically when accretion slowed down or horizontally when it became isolated from the central part of the spreading segment we suggest that orthopyroxene bearing cumulates should be looked for all along the sumail nappe and used as markers of segmentation 2 2 early high temperature compressional deformation took place in both the mantle and crust before the intraoceanic detachment of the ophiolite the development of this compression regime activated filter pressing processes and in particular helped with the extraction of wehrlitic magmas from the upper mantle and their ascent to the uppermost levels of the crust

the interrelationship between magmatic and ore forming hydrothermal processes in the oman ophiolite the semail ophiolite of oman contains numerous gossans minor sulfide showings and ferromanganoan deposits as well as several massive sulfide deposits the largest of which 8 x 10 6 metric tons is the deposit of lasail detailed field mapping of the lavas and shallow intrusions around this deposit in conjunction with petrographic studies of their metamorphic assemblages enables the ore forming hydrothermal activity to be related to the location and evolution of an underlying magma chamber it can be demonstrated that the deposit formed in the short time interval between the eruption of a lava unit the geotimes unit at a marginal ocean basin spreading axis and the eruption of a second lava unit the lasail unit at an off axis or arc seamount moreover it was the seamount not the spreading axis magma chamber which drove the ore forming hydrothermal convection providing both a near surface heat source and by virtue of its intrusive pressure a localized fracture system on its flanks subsequent eruptions from this same magma chamber then provided a protective capping for the deposit the orientations and distributions of andesitic cone sheets show that the magma chamber lay some 1 5 km from the deposit itself and was in fact a magma cupola developed on the flanks of a much larger magma chamber similar though less detailed studies of other deposits in oman indicate that only very small mound deposits lie within the spreading axis lavas themselves the majority of deposits lie at the boundary between the spreading axis and seamount lavas and another group of deposits is related to the magma chambers that fed a third submarine lava unit the alley unit also associated with the construction of the seamounts these results may provide useful constraints for future mineral exploration programs in the oman and perhaps also in other ophiolite complexes and on the ocean floor

composite synvolcanic intrusions associated with precambrian vms related hydrothermal systems large subvolcanic intrusions are recognized within most precambrian vms camps of these 80 are quartz diorite tonalite trondhjemite composite intrusions the vms camps spatially associated with composite intrusions account for 90 of the aggregate sulfide tonnage of all the precambrian intrusion related vms camps these low alumina low k and high na composite intrusions contain early phases of quartz diorite and tonalite followed by more voluminous trondhjemite they have a high proportion of high silica 74 sio2 trondhjemite which is compositionally similar to the vms hosting rhyolites within the volcanic host rock successions the quartz diorite and possibly tonalite phases follow tholeiitic fractionation trends whereas the trondhjemites fall within the composition field for primitive crustal melts these transitional m i type primitive intrusive suites are associated with extensional regimes within oceanic arc environments subvolcanic composite intrusions related to the archean sturgeon lake and noranda and paleoproterozoic snow lake vms camps range in volume from 300 to 1 000 km3 three have a sill morphology with strike lengths between 15 and 22 km and an average thickness between 1 500 and 2 000 m the fourth has a gross stock like shape the vms deposits are principally restricted to the volcanic strata above the strike length of the intrusions as are areally extensive thin exhalite units the composite intrusions contain numerous internal phases which are commonly clustered within certain parts of the composite intrusion these clusters underlie eruptive centers surrounded by areas of hydrothermal alteration and which contain most of the vms deposits early quartz diorite and tonalite phases appear to have intruded in rapid succession evidence includes gradational contacts magma mixing and disequilibrium textures they appear to have been emplaced as sill dike swarms these early phases are present as pendants and xenoliths within later trondhjemite phases the trondhjemite phases contain numerous internal contacts indicating emplacement as composite sills common structural features of the composite intrusions include early xenolith phases abundant small comagmatic dikes fractures and veins and in places columnar jointing internal phases may differ greatly in texture from fine to coarse grained aphyric and granophyric through seriate to porphyritic mineralogical and isotopic evidence indicates that early phases of each composite intrusion are affected by pervasive to fracture controlled high temperature 350 450 c alteration reflecting seawater rock interaction trondhjemite phases contain hydrothermal magmatic alteration assemblages within miarolitic cavities hydrothermal breccias and veins this hydrothermal magmatic alteration may in part be inherited from previously altered wall rocks two of the four intrusions are host to cu mo rich intrusive breccias and porphyry type mineralization which formed as much as 14 ma after the main subvolcanic magmatic activity the recognition of these precambrian subvolcanic composite intrusions is important for greenfields vms exploration as they define the location of thermal corridors within extensional oceanic arc regimes which have the greatest potential for significant vms mineralization the vms mineralization may occur for 2 000 m above the intrusions in some cases vms mineralization has been truncated or enveloped by late trondhjemite phases of the composite intrusions evidence that much of the trondhjemitic magmatism postdates the principal vms activity is a critical factor when developing heat and fluid flow models for these subseafloor magmatic hydrothermal systems

boron and oxygen isotope systematics for a complete section of oceanic crustal rocks in the oman ophiolite abstract boron content and the isotopic composition of boron and oxygen were determined for a complete 5 3 km thick oceanic crust in the wadi fizh area of the oman ophiolite a fragment of cretaceous oceanic lithosphere obducted onto the arabian shield the depth profiles of the 18 o values and the secondary mineral assemblages are consistent with successively higher temperature hydrothermal alterations occurring with increasing depth the upper pillow basalts underwent low temperature alteration 18 o 10 the lower pillow basalts and upper sheeted dikes 18 o 6 to 10 the lowermost sheeted dike complex and upper gabbros underwent high temperature alteration at 300 450 c 18 o 450 c 18 o 18 o values compared with the adjacent rocks suggesting the production of 18 o enriched melt the boron content of the rocks in the oceanic crust decreases in both abundance and range with increasing stratigraphic depth 1 4 29 1 g g in pillow lava 7 9 g g on average 1 5 11 6 g g in sheeted dike complex 5 3 g g on average 1 6 5 0 g g in dolerite dike in gabbro 2 9 g g on average 0 25 3 8 g g in gabbro 1 3 g g on average considering an original boron content of 0 72 0 47 g g for basalt and 0 06 0 09 g g for gabbro boron from seawater was incorporated into the rocks through hydrothermal alteration even at temperatures higher than 300 c the 11 b values systematically increase with stratigraphic depth 1 1 to 11 9 in pillow lava 5 5 on average 1 1 to 17 5 in sheeted dike complex 6 3 on average 8 3 to 18 6 in dolerite dike in gabbro 13 9 on average 7 3 to 17 7 in gabbro 12 0 on average the whole rock 11 b values negatively correlate with the 18 o values suggesting that the 11 b values of altered rocks are essentially controlled by isotopic equilibrium with hydrothermal fluids and the increase in the 11 b values is caused by a decrease in the rock fluid boron isotopic fractionation factor with increasing alteration temperatures the 11 b values estimated for hydrothermal fluids from rocks completely altered at 300 450 c range from 28 to 33 values indistinguishable from those of vent fluids observed at modern mid ocean ridges the boron content of the bulk oceanic crust of the oman ophiolite is estimated to be 3 6 g g and the 11 b value is estimated at 7 9 in contrast to previous views the hydrothermally altered gabbro section is a large boron sink accounting for 30 of the total boron in the oceanic crust with a high 11 b value of 13 this boron enriched high 11 b lower oceanic crust may impact estimates of the 11 b value of fluid liberated from the subducted oceanic slab which is believed to largely control the 11 b value of arc magma generated in the mantle wedge

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

volcanic development of small oceanic central volcanoes on the flanks of the east pacific rise inferred from narrow beam echo sounder surveys abstract narrow beam bathymetric surveys of about 30 volcanoes on young crust of the cocos plate were collected in order to help determine the origin and evolution of oceanic central volcanoes the volcanoes are located on ocean crust 1 5 7 5 m y in age and range in size from to 600 km 3 these volcanoes differ greatly in shape development of linear features small scale morphologic characteristics and crater development we conclude that these large differences in morphologic characteristics are due to the complex interaction of many factors in their development such as conduit geometry magma ascent rates eruption rate history tectonic activity crater formation and filling and others elongation directions of individual volcanoes and clear linear bathymetric features near these volcanoes are among the lines of evidence that strongly suggest these volcanoes originate and evolve on and near oceanic fracture zones their origin and morphologic evolution is thus closely tied to the tectonic processes and thermal regimes at fractures we speculate that these volcanoes on fast spreading lithosphere are generated by episodic pulses of melt production beneath ridge crests in the vicinity of fracture zones petrologic evidence shows that these volcanoes cannot be fed from sub axial magma chambers beneath the east pacific rise instead they tap the same mantle source as the ridge crest at mantle depth

oxygen isotope composition of a section of lower oceanic crust odp hole 735b 1 the oxygen and hydrogen isotope compositions of vein minerals and 18o values of 64 whole rocks from the deeper kilometer of the 1 5 km lower crustal section of odp hole 735b in the sw indian ocean were analyzed in order to investigate seawater penetration into and reaction with the lower oceanic crust vein amphiboles have d values of 35 to 65 indicating formation from seawater fluids and hydrothermal fluids containing a component of low d magmatic fluid crystal plastic shear zones reacted with 18o enriched fluids at high temperatures and have 18o values of 6 0 7 0 indistinguishable from surrounding rocks reaction with seawater dominated fluids over a range of temperatures 350 700 c led to local 18o depleted rocks but much of the section exhibits 18o enrichment 18o up to 7 6 as the result of static alteration of plagioclase over a range of temperatures 250 to 600 c and fluid compositions chloritic veins and associated alteration by 18o enriched fluids occurred at temperatures of 150 250 c and late carbonates and fe oxyhydroxides formed from cold 10 seawater associated with open fault related fractures bulk rock 18o values of the lower 1 km range from 3 7 to 10 2 but are mostly 5 6 7 6 and the section has a mean bulk 18o of 6 5 1 0 in contrast rocks from the shallower 500 m exhibit net 18o depletion 5 1 1 0 reflecting high temperature deformation and seawater hydrothermal alteration stakes 1991 the weighted average 18o for the entire gabbro section of hole 735b is 6 0 exhibiting little or no change compared to unaltered crust although tectonism at slow spreading rates may allow exposure of lower crust to greater amounts of low temperature alteration and 18o enrichment there is no evidence for a consistent difference in the bulk 18o of the plutonic sections of oceanic crust formed at fast versus slow spreading rates

oceanic versus emplacement age serpentinization in the josephine ophiolite implications for the nature of the moho at intermediate and slow spreading ridges we present field petrographie and geochemical evidence for oceanic serpentinization in the 162 ma josephine ophiolite of nw california and sw oregon undeformed and unrodingitized dikes that intruded into serpentinized shear zones provide time markers for serpentinization and deformation the dikes intruding serpentinites are of two types 1 fe ti enriched dikes with normal mid ocean ridge basalt nmorb magmatic affinity which are geochemically linked to the uppermost lavas and a late fe ti dike within the crustal sequence of the ophiolite and 2 hornblende bearing calc alkaline dikes intruded during ophiolite emplacement between 150 and 146 ma based on crosscutting relationships between dikes and serpentinites serpentinization of upper mantle peridotites took place at or near the ridge axis during periods of amagmatic structural extension in the absence of a magma chamber lizardite bearing seipentinites in the northern peridotite are constrained to be oceanic and indicate temperatures of 350 c within the oceanic upper mantle oceanic lizardite bearing serpentinized shear zones from the ultramafic cumulate section to the basal sole indicate that the entire josephine peridotite may have been transected by serpentinized shear zones prior to ophiolite emplacement the ultramafic cumulate sequence was completely serpentinized prior to ophiolite emplacement and we suggest that the paleo moho in the josephine ophiolite is a serpentinization boundary the basal sole is interpreted to be a reactivated oceanic fault along which antigorite mylonites formed from a preexisting serpentinite during ophiolite emplacement oceanic serpentinization in the josephine ophiolite took place beneath a 2 3 km crustal sequence and suggests that serpentinization may be an important feature at intermediate spreading rate mid ocean ridges as well as at slower spreading ridges

the death of an accretion zone as evidenced by the magmatic history of the sumail ophiolite oman abstract the ophiolitic crustal sequence of the salahi block in the northern part of oman shows a complex magmatic history the oceanic accretion episode led to the formation of a main plutonic sequence consisting of layered cumulates isotropic gabbros a sheeted dike complex and a first tholeiitic lava series v1 depleted in compatible elements the second magmatic episode immediately following the accretion stage is illustrated by predominantly ultrabasic intrusions crosscutting the main plutonic sequence and by a second volcanic suite v2 directly linked as can be seen from field evidence to more differentiated high level intrusive plutons this volcanic unit includes very primitive members and is in general characterized by depletion in incompatible elements a third volcanic event v3 exhibiting a transitional character enriched in lree s caps the ophiolite these v3 lavas are separated from the v2 unit by a 15 m thick pelagic sedimentary section a geotectonic evolution of the sumail ophiolite is proposed on the basis of field mineralogical and geochemical data after the oceanic accretion phase that gave birth to the main ophiolitic sequence a second stage melting of the upper mantle is suggested to explain the genesis of the second magmatic episode such a model accounts well for the depleted character of the v2 lavas the compression linked to the beginning of the intra oceanic detachment allows the extraction from the mantle of these second stage melts which are then emplaced in the crust and partly extruded the v3 volcanism postdates the intraoceanic detachment of the ophiolite but precedes its obduction onto the arabian continental margin it is interpreted as an intraplate seamount type magmatism

uranium lead isotopic ages of the samail ophiolite oman with applications to tethyan ocean ridge tectonics plagiogranites are a minor but widespread component of the samail ophiolite plutonic member they crystallized from the most fractionated melts generated by magmatic crystallization and differentiation of a steady state magma chamber beneath the tethyan spreading ocean ridge and their ages are thought to mark the time of ocean crust formation isotopic u pb ages of zircons from 13 plagiogranites collected along a 270 km segment of the samail ophiolite subparallel to the regional trend of the sheeted dike complex the former spreading ridge axis direction define a narrow time interval of 93 5 97 9 m y with a pronounced clustering about 95 m y the zircon ages of the plagiogranites agree remarkably well with the early cenomanian to early turonian biostratigraphic ages of sediments that are intercalated within the ophiolite pillow lavas and that lie just above them tippit et al 1981 the agreement of radiometric and biostratigraphic ages provides strong support for the conclusion that the plagiogranite u pb ages closely date the time span of ocean crust formation no step changes in age patterns are observed along the ridge axis sheeted dike direction suggesting that there are no major internal offsets of the ophiolite by transform or other faults along most of the traverse one possible exception occurs at the southeastern end of the sampled interval ibra area where a 3 m y discontinuity might be caused by an unmapped fault assuming that the regional trend of the sheeted dikes n10 25 w marks the direction of the former spreading ridge axis the present array of sample localities spans a distance of 130 to 195 km normal to that axis i e in the spreading direction the data as a whole do not define a clear cut age trend normal to the spreading axis but by eliminating samples that may be aberrant due to faulting the data array suggests a pattern of increasing ages from east to west this would indicate that the samail ophiolite is derived from the western southwestern side of the tethyan spreading system a conclusion reached independently from geological evidence also the gentle age gradient in the spreading direction i e between 130 and 195 km of ocean crust were formed within an age span of no more than 2 m y suggests a very fast spreading rate this conclusion also is supported by independent geologic evidence

deep tow studies of the overlapping spreading centers at 9 03 n on the east pacific rise the deeply towed instrument package of the scripps institution of oceanography was used to study for the first time the fine scale structure of an overlapping spreading center osc system the 9 03 n osc on the east pacific rise epr the eastern and western limbs of this osc system which are 8 km apart and overlap by 27 km show marked differences away from the tips the spreading centers are highly tectonized and differ greatly from the ridge tips the western ridge tip exhibits a few fissures but no sign of recent volcanic activity whereas the eastern ridge tip is highly fissured and has experienced more recent volcanic episodes which are confined to a narrow zone within this highly fissured area in both cases the ridge tip fissures are 10 15 oblique to the strike of the faults that occur on the flanks of the adjacent portion of the opposite spreading axis indicating a recent change in the direction of deviatoric tension and suggesting that the eastern and western spreading centers have been propagating southward and northward respectively the eastern ridge tip fissures are transecting the flank of the western spreading axis suggesting that the eastern spreading center is about to become the through going trace of the east pacific rise in contrast the western spreading center disappears into abyssal hill terrain that was presumably created at the eastern ridge axis some of the very few tectonic features observed in the overlap basin may reflect the existence of a short lived shear couple due to the interaction of the two overlapping spreading centers sediment cover over the whole area and especially in the overlap basin is relatively thick supporting the idea of a time averaged deficit in the magmatic budget at this osc we suggest that the 9 03 n osc developed where two magmatic pulses independent in space and time and propagating along the strike of the east pacific rise away from the loci of melt emplacement failed to meet misalignment of the magmatic conduits resulted in the propagation of the two spreading centers past each other and the development of the ensuing osc geometry independent lines of evidence derived from the three dimensional inversion of the magnetic field at the 9 03 n osc sempere et al 1984 from numerical modelling of the growth of two en echelon elastic cracks in a tensile stress field sempere and macdonald 1986 and from geochemical data langmuir et al 1986 support our interpretation of the deep tow data we suggest that deviations from axial linearity of the spreading centers devals langmuir et al 1986 and small nonoverlapping offsets snoos batiza and margolis 1986 are simply local lows or saddle points along the axial depth profile that arise when two magmatic pulses propagating toward one another meet head on oscs and some saddle points i e devals snoos are all small rapidly evolving ridge axis discontinuities which may represent the surficial expression of the distal ends of small scale longitudinal convection cells beneath the east pacific rise

complexity of the crustal sequence in the northern oman ophiolite fizh and southern aswad blocks the effect of early slicing abstract on the basis of detailed mapping of the fizh and southern aswad blocks in northern oman the complexity of the crustal sequence is shown lateral facies variations are at least as important as vertical ones and no section can be reconstructed as a stratigraphic section sharp and transitional moho boundaries alternate over a distance of 10 km the important residual mantle crust transition zones are root zones for intrusive wehrlites and cause doming in the moho fractures facilitating the penetration of seawater in the upper crustal parts dikes and lavas show intense hydrothermal alteration and can be related to shear zones in the lower parts of the crustal sequence and occasionally be followed into the peridotites intrusive wehrlites reach as high into the crust as the base of the dike complex and occasionally even the lavas they follow fault zones or n s to nw se directions recrystallization of isotropic gabbro occurs in the vicinity of the wehrlites the geometry observed is not compatible with an oceanic transform zone it may indicate the location of an overlapping spreading centre although too little is known of the deeper structures to confirm this there was certainly an important reaction to the early compression at the oceanic spreading ridge prior to final consolidation confirming an earlier model of the setting

a field chemical and stable isotope study of subseafloor metamorphism of the josephine ophiolite california oregon the josephine ophiolite is a large complete ophiolite generated in a late jurassic back arc basin along western north america the ophiolite underwent subseafloor metamorphism under a steep thermal gradient as indicated by downward changes in mineralogy and 18o values characteristic mineral zonation includes mica orthoclase and hematite in the lower extrusives greenschist facies assemblages in the upper sheeted dikes and amphibolite facies assemblages in the lower sheeted dikes and high level gabbro cumulate gabbros are only incipiently altered but are depleted in 18o indicating high temperature alteration with fluids that evolved at low water w rock r ratios a second off axis circulation system is recorded by metalliferous sediments that occur 8 21 m above the ophiolite alteration in the upper sheeted dike complex and especially the extrusive sequence is heterogeneous at outcrop scale as indicated by large variations in mineralogy chemistry and 18o values within individual pillows and dikes the heterogenity is due to 1 variations in w r ratio and starting material during alteration by downwelling seawater e g pillow cores versus glassy rims and 2 localized discharge mineralization mostly epidosites superimposed on the background recharge alteration epidosites occur largely as dike parallel stringers irregular replacement of pillow and massive lava and replacement of interpillow and pillow breccia matrices the chemistry of most extrusives and sheeted dikes is characterized by loss of ca and gain in na and mg consistent with alteration by downwelling fluids in addition the sheeted dikes are depleted in k2o and the lower sheeted dikes and high level gabbros are depleted in zn and cu the 18o and d values for most samples from the extrusive sequence indicate alteration by seawater at 50 rotation of the entire crustal sequence at the spreading axis suggests that the ophiolite formed at a slow spreading center where magma chambers were episodic

chemical and thermal constraints on focussed fluid flow in the lower oceanic crust the mechanism of heat extraction from the lower oceanic crust near the ridge axis is poorly constrained despite its importance for understanding both the process of accretion of the plutonic complex and the mass fluxes associated with ridge hydrothermal systems we have investigated the role of zones of focussed fluid flow in the plutonic complex of the oman ophiolite in the near axis cooling of the oceanic crust lineaments identified on aerial photographs that occur at 1 km spacing show evidence for extensive hydrothermal fluid flow through regions 10 to 50 m wide fluid flow is initiated in these regions at 800 c and continues at least into the lower greenschist facies strontium isotope analyses indicate that the fluid flux through these zones is sufficient to transport a metasomatic front from the base of the sheeted dike complex to close to the moho computed minimum fluid fluxes to transport a metasomatic front through the focussed fluid flow zones are 1x108 kgm2 modeling of diffusive exchange of calcium from olivine to clinopyroxene indicates enhanced cooling rates adjacent to the focussed fluid flow zones heat fluxes estimated from the enhanced cooling rates are broadly consistent with the fluid fluxes determined from modeling the sr isotopic composition of samples from the focussed fluid flow zones the combination of independent estimates of the fluid and heat fluxes such as these can provide more rigorous constraints on the thermal history than either approach used in isolation our results show that focussed fluid flow could play a major role in the cooling in the lower oceanic crust significant focussed fluid flow in the lower oceanic crust has important implications for predicting the total mass flux associated with hydrothermal circulation at mid ocean ridges this is because fluids flowing through channels become chemically rock buffered at smaller fluid fluxes than those flowing pervasively through a rock mass thus if focussed fluid flow is an important mechanism of heat loss from the lower oceanic crust the chemical fluxes from ridge hydrothermal systems into the oceans may be smaller than currently thought

the northern samail ophiolite an oxygen isotope microprobe and field study geological petrological and oxygen isotopic data are presented for 228 whole rock and mineral samples collected from a 100 20 km area of the northern samail ophiolite in oman most of these samples are from three detailed profiles through the pillow lavas sheeted dikes and layered gabbros of this laterally heterogeneous fragment of cretaceous oceanic crust down to and across the petrologic moho the profiles encompass a range of petrologic and tectonic styles and each profile exhibits distinctive 18 16 o variations compared to one another and to mid ocean ridge basalts as a result of pervasive seawater hydrothermal interaction that varied in intensity along strike in the ophiolite in general 18 o depletions are observed in the layered gabbros and 18 o enrichments in most of the sheeted dikes and pillow lavas similar to results previously observed in the southern part of the ophiolite ibra area where 18 o depletions within the gabbroic section are quantitatively balanced by 18 o enrichments in the shallower parts of the oceanic crust the wadi hilti profile selected as an example of relatively intact crust differs from ibra in having more uniform and slightly higher 18 o in the gabbros 5 4 to 6 3 as well as in containing more hydrous alteration minerals amphibole epidote chlorite and prehnite the profiles in the wadi kanut shafan and wadi rajmi sections are much more complex and reveal the impact of off axis intrusions and deep crustal shearing plagiogranite wehrlite intrusions in the shafan kanut area superimposed a local hydrothermal aureole on the ophiolite evident in dikes highly depleted in 18 o quartz sulfide veins abundant epidote thullite and chlorite in shallower rocks and low temperature hydrous alteration of deeper gabbroic rocks the latter produced an overall increase in whole rock 18 o 6 2 to 6 9 such late stage intrusions are found throughout the northern half of the samail ophiolite the wadi rajmi area which is a possible fossil transform or propagating rift represents the most complex of the three profiles it also contains the most abundant highly deformed and hydrothermally altered rocks together with the deepest and largest zone of 18 o depletion yet found in any ophiolite locally 18 o 2 0 conduits for large volumes of high temperature hydrothermal fluids were provided by fractures now occupied by low 18 o gabbro pegmatites and low 18 o dikes material balance estimates for the regional samples and from the various transects through the ophiolite give crustal bulk 18 o averages 5 9 to 6 3 that are within sampling error almost identical to the average morb basalt value of about 5 8 if both vertical and lateral crustal heterogeneities are integrated into a three dimensional model this supports and amplifies the conclusion of earlier workers that the 18 o of seawater is buffered and controlled by hydrothermal interaction with oceanic crust as long as the cumulative effects both spatial and temporal of all seafloor magmatic hydrothermal processes are considered the very slight out of balance enrichment of the integrated crustal average 18 o compared to morb may be explained by the ubiquitous mineralogical and isotopic evidence for a late low temperature alteration event in the basal gabbros these effects are prominent in the vicinity of the petrologic moho and may indicate exchange with low temperature aqueous fluids during or after detachment of the obducted slab

roof assimilation at fast spreading ridges an investigation combining geophysical geochemical and field evidence 1 the role of assimilation at the roof of the axial magma chamber amc at fast spreading mid ocean ridges is investigated using field geophysical and geochemical data field observations from ophiolites indicate that roof assimilation is a common process but provide little constraint on the amount of material assimilated arguments based on geophysical data that include the subsidence of the layer 2a 2b boundary across the width of the amc and the depth of large axial summit troughs require at least 50 100 m of assimilation however significant variations in the depth of the amc on the east pacific rise epr over short along axis distances suggest up to 500 m fluctuations in the depth of the amc may be common on short timescales geochemical modeling of the over enrichment of chlorine in epr basalts suggests that 20 of the oceanic crust may go through a cycle of crystallization alteration then assimilation this is far more than can be accounted for by roof subsidence and suggests that fluctuations in the level of the roof must account for the majority of the assimilation assimilation most likely occurs on a number of different timescales ranging from decadal due to perturbations in the thermal regime related to diking to longer period fluctuations controlled by magma supply from the mantle large scale roof assimilation has implications for the accretion of the lower crust due to heat losses associated with assimilation and for the interpretation of the geochemistry of the oceanic crust

protracted fluid rock interaction in the mesoarchaean and implication for gold mineralization example from the warrawoona syncline pilbara western australia abstract oxygen isotopic and geochemical analyses on whole rock and quartz veins are combined with structural observations in order to constrain the fluid circulation history within the mesoarchaean warrawoona syncline of the north pilbara craton western australia the plumbing system which is the focus of this study is localized in the so called fielding s find shear zone ffsz a km scale shear zone formed during the burial of greenstones and coeval exhumation of granitic complexes this shear zone runs parallel or close to the axial plane of the syncline it involves a prominent quartz vein network and is lined with strongly hydrothermally altered mafic felsic and sedimentary rocks towards the ffsz felsic and mafic volcanic rocks become intensely silicified with an increase in bulk rock 18 o values from 10 8 to 25 1 for altered felsic volcanics and from 7 1 to 18 3 for altered mafic volcanics geochemical modelling ascribes the silicification to a dissolution precipitation process ree and most other trace elements are strongly depleted in the silicified units with the exceptions of elements such as v cr ni and co which are enriched throughout the warrawoona syncline vein quartz 18 o data are within a small range of 13 2 2 significantly lower than their silicified host rocks these data are interpreted as the result of two main paleo fluid circulation stages intense silicification and 18 o enrichment represent alteration driven by low temperature hydrothermal convection probably involving archaean seawater in contrast the quartz veins network is related to the infiltration of metamorphic and or magmatic fluids during a later deformation episode these quartz veins represent the event responsible for the bulk of economic lode gold formation in the area the protracted fluid rock interaction history in the warrawoona syncline may have played a major role in setting the stage for the late mineralizing event the early hydrothermal circulation could have formed an efficient plumbing system characterized by high permeability low reactivity and possibly au enrichment upgrading the au endowment to the late hydrothermal fluids

rare earth element geochemistry of the betts cove ophiolite newfoundland complexities in ophiolite formation the betts cove ophiolite includes the components of typical ocean crust pillow lavas sheeted dikes gabbros and ultramafics however the trace element geochemistry of basaltic rocks is unusual three geochemical units are recognized within the lava and dike members within the pillow lavas the geochemical units correspond to stratigraphic units upper lavas have normal i e typical for ocean floor basalts tio2 contents 0 75 to 2 0 wt heavy rare earth elements hree values in the range 6 20 chondrites and chondrite normalized ree patterns with relative lree depletion intermediate lavas have tio2 contents between 0 30 and 0 50 wt hree contents from 4 7 chondrites and extreme relative lree depletion lower lavas have anomalously low tio2 contents 0 30 wt and unusual convex downwards ree patterns with ree abundances around 2 5 chondrite these geochemical differences can be explained if the three groups were derived from different mantle sources independent mantle sources for the three units are consistent with their different 143nd144nd ratios varying at 480 m y b p from 0 51222 in a lower lava to 0 51238 in an upper lava the upper lavas may be partial melts of a source similar in composition to that of modern morb the intermediate lavas may be from a very depleted oceanic mantle second stage melt and the lower lavas may have formed by melting an extremely depleted mantle that had been invaded by a lree enriched fluid a possible tectonic environment where these different sources could be juxtaposed is a back arc or inter arc basin

oxygen isotope exchange kinetics of mineral pairs in closed and open systems applications to problems of hydrothermal alteration of igneous rocks and precambrian iron formations the systematics of stable isotope exchange between minerals and fluids are examined in the context of modal mineralogical variations and mass balance considerations both in closed and in open systems on mineral pair 18 o plots samples from terranes that have exchanged with large amounts of fluid typically map out steep positively sloped non equilibrium arrays analytical models are derived to explain these effects these models allow for different exchange rates between the various minerals and the external fluids as well as different fluid fluxes the steep arrays are adequately modelled by calculated isochron lines that involve the whole family of possible exchange trajectories these isochrons have initially steep near vertical positive slopes that rotate toward a 45 equilibrium slope as the exchange process proceeds to completion the actual data point array is thus analogous to the hand of an isotopic clock that measures the duration of the hydrothermal episode the dimensionless ratio of the volumetric fluid flux to the kinetic rate parameter u k determines the shape of each individual exchange trajectory in a fluid buffered system u k 1 the solutions to the equations 1 are independent of the mole fractions of the solid phases 2 correspond to taylor s open system water rock equation and 3 yield straight line isochrons that have slopes that approach 1 f where f is the fraction reacted of the more sluggishly exchanging mineral the isochrons for this simple exchange model are closely congruent with the isochrons calculated for all of the more complex models thereby simplifying the application of theory to actual hydrothermal systems in nature in all of the models an order of magnitude of time in units of kt separates steep non equilibrium arrays e g slope 10 from arrays approaching an equilibrium slope of unity on a diagram because we know the approximate lifetimes of many hydrothermal systems from geologic and heat balance constraints we can utilize the 18 o 16 o data on natural mineral assemblages to calculate the kinetic rate constants k s and the effective diffusion constants d s for mineral h 2o exchange these calculated values k qtz 10 14 k feld 10 13 10 12 agree with experimental determinations of such constants in nature once the driving force or energy source for the external infiltrating fluid phase is removed the disequilibrium mineral pair arrays will either 1 remain frozen in their existing state if the temperatures are low enough or 2 re equilibrate along specific closed system exchange vectors determined solely by the temperature path and the mineral modal proportions thus modal mineralogical information is a particularly important parameter in both the open and closed system scenarios and should in general always be reported in stable isotopic studies of mineral assemblages these concepts are applied to an analysis of 18 o 16 o systematics of gabbros plagioclase clinopyroxene and plagioclase amphibole pairs granitic plutons quartz feldspar pairs and precambrian siliceous iron formations quartz magnetite pairs in all these examples striking regularities are observed on and plots but we point out that plots have many advantages over their equivalent diagrams as the latter are more susceptible to misinterpretation using the equations developed in this study these regularities can be interpreted to give semiquantitative information on the exchange histories of these rocks subsequent to their formation in particular we present a new interpretation indicating that precambrian cherty iron formations have in general undergone a complex fluid exchange history in which the iron oxide magnetite precursor has exchanged much faster with low temperature 400 c fluids than has the relatively inert quartz

a survey of hydrous species and concentrations in igneous feldspars the hydrous components in 85 feldspars from various igneous environments spanning the range of naturally occurring compositions were examined with infrared spectroscopy the feldspars contain structural oh 0 512 ppm h 2o h 2o 0 1350 ppm h 2o and nh 4 0 1500 ppm nh 4 groups as well as fluid inclusions and alteration products although composition and structure do influence the type of hydrous species that can be incorporated into a particular feldspar mineral the concentration of these species is not controlled by major element composition coarse perthitic microclines have a heterogeneous distribution of hydrous species and contain h 2o or nh 4 in k rich lamellae and fluid inclusions in na rich areas the structural oh in plagioclase feldspars is not associated with twin boundaries or exsolution lamellae all of the possible structural hydrous species are found in pegmatite feldspars whereas volcanic feldspars contain only structural oh the variation in oh concentration within a given feldspar composition suggests that fluids in the geologic environment play a role in determining the hydrogen concentration of each sample the vast majority of plutonic feldspars have undergone partial or total equilibration with meteoric fluids during low temperature 400 150 c hydrothermal exchange obliterating any structural hydrogen and creating substantial concentrations of fluid inclusions up to 4000 ppm h 2o in the exchanged regions the amount of water stored as fluid inclusions within feldspars in the upper crust 1 10 19 kg is small compared to the 1 35 10 21 kg of water in the oceans but is roughly equivalent to the reservoir of water stored in hydrous minerals in the upper crust

chemical interaction of aqueous solutions with epidote feldspar mineral assemblages in geologic systems ii equilibrium constraints in metamorphic geothermal processes thermodynamic analysis of the system na sub 2 o k sub 2 o cao feo fe sub 2 o sub 3 al sub 2 o sub 3 sio sub 2 h sub 2 o hcl co sub 2 at pressures and temperatures to 5 kb and 600 sup 0 c affords quantitative description and interpretation of phase relations among epidote garnet plagioclase and alkali feldspar solid solutions in hydrothermal systems comparison of computed and observed compositions of these minerals and coexisting fluids suggests that the calculations afford close approximation of equilibrium and mass transfer in metamorphic geothermal processes the standard molal enthalpies of decomposition reactions for plagioclase are of the order of 4 to 50 kcal mole sup 1 which requires the activity of the anorthite component of plagioclase coexisting with epidote solid solutions to increase with increasing temperature at constant pressure in contrast because the standard molal volumes of these reactions are also negative a sub caal sub 2 si sub 2 o sub 8 decreases with increasing pressure at constant temperature either stoichiometric epidote or clinozoisite is compatible with plagioclase quartz calcite and an aqueous phase but only at x sub co sub 2 less than or equal to 0 2 the complex zoning commonly exhibited by both plagioclase and epidote solid solutions in geologic systems can be attributed to minor isothermal isobaric changes in the composition of coexisting aqueous solutions logarithmic activity and fugacitymore diagrams constructed with provision for solid solution permit documentation of such changes and facilitate thermodynamic interpretation of mineral and fluid compositions in metamorphic geothermal systems less

neoarchean seafloor hydrothermal metamorphism of basalts in the zanhuang ophiolitic melange north china craton abstract epidosites are suggested to form in up flow zones located at the base of ore forming oceanic hydrothermal systems and are commonly preserved in ophiolitic sequences however occurrences of epidosites in archean tectonic melanges are rarely documented in this study we report the presence of late neoarchean epidosites in the zanhuang ophiolitic melange in the north china craton using meter scale litho structural and petrologic analysis zircon u pb dating major and trace element and oxygen and strontium isotopic geochemistry field studies show that the epidosites are mainly preserved as the cores of meta pillow lavas and are distributed as blocks in different matrices within the tectonic melange the pillow rims are used as protolith to estimate the element mobility during the alteration process the epidosites display strong depletion in na2o 0 01 0 27 wt mgo 0 43 3 01 wt depletion in large ion lithophile elements lile e g rb ba cs u and transition metals e g co sc cu ni zn and show enrichment of cao 13 35 21 0 wt sr and pb the high field strength elements hfse e g zr hf ta th of epidosites are varied but rare earth elements ree are unchanged the amphibolitic pillow rims are characterized by enrichment of lile flat heavy rare earth elements hree patterns and depletion in hfse with negative ti anomalies and positive pb and ba anomalies oxygen and strontium isotopic ratios of epidosites are 5 0 8 6 and 0 70284 0 70551 respectively both of which are greater than those of fresh basalts the major and trace elements oxygen and strontium isotopic values and extensive element migration indicate that the protolith was altered by a submarine hydrothermal system in the absence of continental influence magmatic zircons from two samples of epidosites yield 207pb 206pb ages of 2512 13 ma mswd 0 51 and 2542 13 ma mswd 1 4 confirming that the epidosites were generated at ca 2 51 2 54 ga based on geochemical data and field observations we suggest that the protolith of the epidosites formed in an arc related tectonic setting during the late neoarchean this study shows that hydrothermal activity operating in the late neoarchean was similar to that of the modern ocean floor

hydrothermal clinopyroxenes of the skaergaard intrusion magmatic augites reacted with high temperature aqueous solutions to form secondary calcic pyroxenes during the subsolidus cooling of the skaergaard intrusion secondary hydrothermal clinopyroxenes replace wall rock igneous augites at the margins of veins filled with calcic amphibole these veins are up to several millimeters wide and tens of meters in length hydrothermal clinopyroxenes are a ubiquitous and characteristic phase in the earliest veins throughout the layered series of the intrusion and occur rarely in late veins that in some places crosscut the early veins associated secondary phases in early veins include amphiboles ranging in composition from actinolite to hornblende together with biotite fe ti oxides and calcic plagioclase hydrothermal clinopyroxenes in late veins may be associated with actinolite hornblende biotite magnetite and albite hydrothermal clinopyroxenes are depleted in fe mg and minor elements and enriched in ca and si relative to igneous augites in the layered series gabbros secondary vein pyroxenes are similar in composition to calcic pyroxenes from amphibolite facies metamorphic rocks clinopyroxene solvus thermometry suggests minimum temperatures of equilibration of between 500 and 750 c these temperatures combined with numerical transport models of the intrusion suggest that vein clinopyroxenes could have formed during 20 000 to 60 000 year time intervals associated with a maximum in the fluid flux through fractures in the layered series

the formation of sio2 rich melts within the deep oceanic crust by hydrous partial melting of gabbros small amounts of felsic evolved plutonic rocks often called oceanic plagiogranites always occur as veins or small stocks within the gabbroic section of the oceanic crust four major models are under debate to explain the formation of these rocks 1 late stage differentiation of a parental morb melt 2 partial melting of gabbroic rocks 3 immiscibility in an evolved tholeiitic liquid and 4 assimilation and partial melting of previously altered dikes recent experimental data in hydrous morb type systems are used to evaluate the petrogenesis of oceanic plagiogranites within the deep oceanic crust experiments show that tio2 is a key parameter for the discrimination between different processes tio2 is relatively low in melts generated by anatexis of gabbros which is a consequence of the low tio2 contents of the protolith due to the depleted nature of typical cumulate gabbros formed in the oceanic crust on the other hand tio2 is relatively high in those melts generated by morb differentiation or liquid immiscibility since the tio2 content of many oceanic plagiogranites is far below that expected in case of a generation by simple morb differentiation or immiscibility these rocks may be regarded as products of anatexis this may indicate that partial melting processes triggered by water rich fluids are more common in the deep oceanic crust than believed up to now at slow spreading ridges seawater may be transported via high temperature shear zones deeply into the crust and thus made available for melting processes

stretching of the deep crust at the slow spreading southwest indian ridge abstract the 500 metres of gabbro drilled in the southwest indian ocean during odp leg 118 allow direct observation of early high temperature extensional deformation in the deep crust of a slow spreading rift valley about 30 of the recovered gabbros have been plastically stretched at progressively decreasing temperatures granulite to lower amphibolite facies metamorphic conditions increasing water rock ratio and probably increasing deviatoric stresses the early solid state strain in granulite facies metamorphic conditions produced low dipping foliations and low strain gradients at this stage mylonitic shear zones only formed in oxide rich gabbros strain localization in the deepest and hottest part of the oceanic crust thus appears to have been influenced by the magmatic stratigraphy in upper to lower amphibolite facies metamorphic conditions the ductile strength of the gabbro was probably higher and deformation concentrated in mylonitic shear bands which may have provided access to seawater derived fluids strain localization at this stage was accompanied by brittle failure and boudinage in the mylonitic foliation of individual porphyroclasts or of thick gabbro intervals the influence of such brittle failure on hydrothermal circulations was critical it allowed for extensive crust seawater interactions at high temperatures upper amphibolite facies conditions in a spreading context where continuous magma supply provides heat and new crustal material to the axial region the mechanical behaviour of the ridge axis lithosphere should be controlled by continuous ductile deformation in granulite facies conditions this may lead to the steady state necking configuration described by tapponnier and francheteau 1978 alternatively a discontinuity of the magma supply allowing the lower crust of the axial region to cool below granulite facies temperatures should cause mylonitic shear zones characteristic of amphibolite facies deformation to extend deeper into the axial domain eventually leading to the detachment fault configuration proposed by karson in press as magma supply to slow spreading oceanic ridges is likely to be discontinuous necking and faulting periods should alternate through time

perspectives on the origin of plagiogranite in ophiolites from oxygen isotopes in zircon abstract the formation of oceanic plagiogranite has been attributed primarily to either 1 extreme fractional crystallization of a mantle melt or 2 partial melting of hydrated mafic crust with support for the latter from field evidence and recent melting experiments remelting of hydrothermally altered ocean crust could yield rocks and minerals with diverse primary magmatic 18 o values forming proximal to the magmatic center where crustal growth is occurring to constrain the magmatic 18 o of a wide range of silicic rocks in oceanic crust and evaluate their petrogenesis we characterized the 18 o of zircons in 22 plagiogranite samples tonalite and trondhjemite from 8 different ophiolites and one dacite sampled along the east pacific rise using secondary ion mass spectrometry sims the 18 o values of 202 magmatic zircons from ophiolites range from 3 9 to 5 6 n 244 spots average 4 9 0 6 2sd extending 1 below typical zircon in equilibrium with mantle and from gabbroic massifs along slow spreading mid ocean ridges 4 7 5 9 east pacific rise dacite zircons range from 4 6 to 5 0 n 12 spots plagiogranite from the dike gabbro transition zone of the northern oman ophiolite yield the lowest 18 o zrn with rock average values of 4 3 5 0 the low 18 o values are best explained by remelting of crust altered by hydrothermal fluids with seawater like isotopic compositions at high temperatures possibly due to vertical migration of the boundary between an active magma chamber and a vigorous high temperature hydrothermal system in the overlying crust if the partial melt was assimilated into a fractionating melt lens with morb like 18 o as envisioned for km scale plagiogranite bodies in oman up to 20 contamination by a protolith with 18 o 2 would be required previous oxygen isotope constraints from quartz in oman plagiogranite suggested melting of both high and low 18 o crust had occurred comparison of quartz zircon pairs indicates that quartz has been modified in most samples and we find no evidence for the involvement of high 18 o rocks during plagiogranite formation

small non overlapping offsets of the east pacific rise high resolution bathymetric surveys1 2 of the east pacific rise epr reveal that in addition to overlapping spreading centres oscs the crest of the epr is offset tiny amounts by smaller features recently langmuir et al have shown that despite their subtle bathymetric expression these small offsets bends and kinks of the epr collectively termed deviations from axial linearity devals represent important petrological and tectonic boundaries some devals are small non overlapping offsets snoos that offset the epr axis by up to a few hundred metres bathymetric contours near snoos are usually disturbed from their axis parallel trend in ways that we interpret as evidence for strike slip faulting and volcanism at high angles to the trend of epr axis we offer a model for the formation of snoos and their associated faults that invokes independent magma supply stochastic active spreading and limited non rigid behaviour of the neovolcanic zone

oxygen isotope evidence for short lived high temperature fluid flow in the lower oceanic crust at fast spreading ridges millimeter scale amphibole veins in the lower oceanic crust record fracture controlled fluid flow at high temperatures but the importance of this fluid flow for the thermal and chemical evolution of the lower oceanic crust is unclear in the section of lower oceanic crust recovered at hess deep from odp hole 894g which formed at the fast spreading east pacific rise these veins are randomly distributed with an average spacing of 1 m we unravel the history of fluid flow through one of these veins by combining in situ o isotope analyses of wall rock plagioclase with major element analyses geothermometry and diffusion modeling thermometry indicates vein sealing by amphibole at 720 c over a narrow temperature interval 20 c in situ oisotope analyses by ion microprobe with a precision of b0 5 reveal zoning of o isotopes in plagioclase adjacent to the vein the zoning profiles can be reproduced using a diffusion model if the duration of o isotope exchange was 100 yr a similar interval of fluid rock exchange is suggested by modeling potassium depletion in plagioclase adjacent to the vein if representative of fracture controlled fluid flow in the lower oceanic crust the limited duration of fluid flow and its occurrence over a narrow temperature interval suggest that high temperature fluid flow in this porosity network does not transport significant heat 2007 elsevier b v all rights reserved

oxygen isotope and chemical studies on the origin of large plagiogranite bodies in northern oman and their relationship to the overlying massive sulphide deposits the extraordinarily well preserved and well exposed semail ophiolite of northern oman hosts several large plagiogranite intrusions in proximity to economic copper sulphide deposits of the lasail mining district a progression of isotopic chemical and mineralogical transformations observed within the plagiogranites and high level gabbros hlg and a comparison of these effects with those in the lowermost dykes of the immediately overlying sheeted dyke complex sdc tracks the evolution of hydrothermal fluids and the alteration of overlying dykes and pillow lavas during discharge of these fluids on the sea floor the largest hydrothermal alteration aureoles and the greatest extent of metamorphic veins and metasomatic replacement features are found adjacent to the largest high level plagiogranite bodies beneath and adjacent to the major ore bodies in northern oman the ubiquitous presence of metamorphic actinolitic hornblende sodic plagioclase epidote and titanite in metabasalts within the high temperature alteration zones points to the most likely mineralogical and structural controls on the development and evolution of the hydrothermal fluids depleted cu contents of the adjacent crustal rocks and cu enrichments above the plagiogranite intrusions demonstrate the redistribution of heavy metals adjacent to the complexes field relationships implicate the formation of both the epidosites and plagiogranites in the genesis of the ore deposits an important process inferred from the field and geochemical data is the assimilation of previously hydrothermally altered basaltic and gabbroic country rocks by stoping into the magma chambers developed near the sdc gabbro horizon in the ophiolite we suggest that this process of combined assimilation fractional crystallization together with replenishment and recharge by injection and quenching of basaltic magma pillows into these plagiogranite magma chambers i e rafc plays a major role in the development of these composite intrusions

oxygen isotope mapping of the archean sturgeon lake caldera complex and vms related hydrothermal system northwestern ontario canada the hydrothermal and magmatic evolution of the sturgeon lake caldera complex is graphically documented by a regional scale 525 km2 analysis of oxygen isotopes spatial variations in whole rock oxygen isotope compositions provide a thermal map of the cumulative effects of multiple stages of hydrothermal metasomatism before during and after volcanogenic massive sulfide vms mineralization there is a progressive upward increase in 18o from less than 2 to greater than 15 through a 5 km thick section above the biedelman bay subvolcanic intrusive complex this isotopic trend makes it clear that at least the earlier phases of this intrusive complex were coeval with the overlying vms hosting cauldron succession and provided thermal energy to drive a convective hydrothermal circulation system the sharp contrast in 18o values between late stage phases of the biedelman bay intrusion and immediate hanging wall strata indicates that the main phase of vms related hydrothermal activity took place before late stage resurgence in the cauldron related magmatic activity mineralogical and isotopic evidence indicates the presence of both syn and postmineralization hydrothermal activity defined by the presence of widespread semiconformable and more restricted discordant alteration zones that affect the pre and syncauldron strata the semiconformable alteration zones formed during early stages of hydrothermal circulation and are defined by widespread silicification and carbonatization in association with relatively high 18o values the discordant alteration assemblages containing al silicate minerals with chloritoid and or fe rich carbonate or chlorite centered on synvolcanic faults represent restricted zones of both seawater inflow and hydrothermal fluid upflow a rapid increase in 18o values 7 9 over a short distance 200 m suggests marked cooling of hydrothermal fluid from 350 c to less than 130 c either just before or during discharge onto the seafloor late emplacement of diorite sills and a dacite dome disrupted the isotopic imprint of cauldron stage hydrothermal activity the abrupt lowering of 18o values at the transition from explosive to passive volcanism andesite flows and dacite domes indicates postcauldron emergence subsequently thrust faulting disrupted the older synvolcanic hydrothermal isotopic patterns

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

stable isotopic evidence for large scale seawater infiltration in a regional metamorphic terrane the trois seigneurs massif pyrenees france oxygen isotopic analyses of 95 metamorphic and igneous rocks and minerals from a hercynian metamorphic sequence in the trois seigneurs massif pyrenees france indicate that all lithologies at higher metamorphic grades than the andalusite in isograd have relatively homogeneous 18o values the extent of homogenization is shown by the similarity of 18o values in metacarbonates metapelites and granitic rocks 11 to 13 and by the narrow range of oxygen isotopic composition shown by quartz from these lithologies these values contrast with the 18o values of metapelites of lower metamorphic grade 18o about 15 homogenization was caused by a pervasive influx of hydrous fluid mass balance calculations imply that the fluid influx was so large that its source was probably high level groundwaters or connate formation water hydrogen isotopic analyses of muscovite from various lithologies are uniform and exceptionally heavy at d 25 to 30 suggesting a seawater origin many lines of petrological evidence from the area independently suggest that metamorphism and anatexis of pelitic metasediment occurred at depths of 6 12 km in the presence of this water rich fluid the composition of which was externally buffered deep penetration of surface waters in such environments has been hitherto unrecognized and may be a key factor in promoting major anatexis of the continental crust at shallow depth three types of granitoid are exposed in the area the leucogranites and the biotite granite quartz diorite are both mainly derived from fusion of local paleozoic pelitic metasediment because all these rocks have similar whole rock 18o values 11 to 13 the post metamorphic biotite granodiorite has a distinctly different 18o 9 5 to 10 0 and was probably derived from a deeper level in the crust rare mafic xenoliths within the deeper parts of the biotite granite quartz diorite also have different 18o 8 0 to 8 5 and possibly represent input of mantle derived magma which may have provided a heat source for the metamorphism