13c and 18o compositions of carbonates from a cyclic carbonate evaporite rock sequence evidences for meteoric water input abstract twenty five carbonate rock samples from a core section composed of seven thick rhythmic carbonate evaporite sequences lower mcknight formation early cretaceous south texas u s a were analyzed for carbon and oxygen isotopic compositions results show the carbonate rocks to have 13 c 0 38 to 1 68 and 18 o 4 60 to 2 48 vs pdb consistent with the reported world wide values for carbonate rocks of cretaceous age despite this low 2 overall variation 18o shows more variability than 13c throughout the core section studied furthermore 13c and 18o are covariant becoming progressively lighter toward the overlying evaporites within each carbonate evaporite sequence the tendency toward lighter isotopic compositions together with the covariance between 13c and 18o in these carbonates is thus not reconcilable with a simple model of carbonate deposition in a marginal evaporitic basin upon continuous evaporation of seawater instead the data imply contributions from isotopically light meteoric waters

submarine metamorphism of gabbros from the mid cayman rise petrographic and mineralogic constraints on hydrothermal processes at slow spreading ridges the coarse grained submersible collected rocks from the mid cayman rise in our study span a vertical section of about 700 m and range from amphibolite to gabbro petrographic and mineralogic studies showed that 1 some deformation is present in all rocks 2 the distribution of deformation is not uniform within a specimen and 3 alteration has taken place preferentially in the deformed zones the abundance of amphibole decreases with sample depth and argues that seawater flux into the oceanic crust decreased with depth the compositional changes in rocks indicate that partially exchanged seawater was the metamorphic fluid that supplied na k and h2o to and removed ca from the rock in all about 15 of the original rock has been transformed to amphibolite in the 700 m vertical section our study indicates that deformation of oceanic crust is necessary for providing pathways for seawater penetration which in turn is necessary for the submarine metamorphism to occur if deformation continues to take place during the spreading of seafloor alteration may also continue to occur along newly formed fractures and cracks

oxygen 18 and carbon 13 in the carbonates of the salina formation of southwestern ontario the oxygen and carbon isotopic composition of the carbonates of the upper silurian salina formation of the michigan basin was investigated to aid in interpretation of depositional environments 13c results indicate that a change from generally anoxic bottom conditions to oxic conditions occurred during deposition of the b evaporite unit the organic rich a carbonate units were deposited in a shallow water evaporitic setting most likely adjacent to a sabkha type environment a positive water balance maintained the anoxic conditions and buffered the carbon isotopes above the b evaporite the isotopic composition suggests that the development of a similar depositional environment a subaerial prograding sabkha occurred over wide areas of the basin 18o results support the conclusion that silurian oceans were depleted in 18o with respect to modem oceans by 5 6

natural deformation and recrystallization of some intermediate plagioclase feldspars abstract intermediate an25 an48 plagioclase feldspars from the nordre stromfjord shear zone central west greenland have undergone extensive ductile deformation by intracrystalline slip and by twinning the plagioclases were deformed under low granulite facies conditions optical substructures include undulatory extinction deformation bands subgrains and deformational twins mainly on the albite law dynamic recrystallization occurred primarily by subgrain rotation with bulge nucleation migration of a high angle boundary as a secondary mechanism in some cases recrystallization is accompanied by a small decrease in the an content which is thought to indicate prograde metamorphic conditions the plagioclases exhibit a good crystallographic preferred orientation which is compatible with 010 001 being the principal slip system quartz has in the same rocks undergone more plastic deformation and more extensive recovery than plagioclase this is at least in part due to the fact that most dislocations in plagioclase are dissociated

novel type of crust produced during continental rifting a normal oceanic crust cannot be generated after continental crust splitting of rifts filled by a thick pile of young sediments the magma released by the rising mantle is trapped mainly in these sediments as basalts sills and plugs the sediments are themselves metamorphosed by the heat flux from the underlying mantle a metasedimentary crust is formed in direct contact with an abnormal mantle vp 7 5 km s l produced by partial serpentinization

zabargad st john s island an uplifted fragment of sub red sea lithosphere zabargad is a small island located in the red sea about 50 km w of its axis between 23o and 24on the island is not volcanic but probably represents an uplifted block of upper mantle and crustal rocks and yields information on the nature of the underlying lithosphere peridotites are the main rock type consisting of exceptionally fresh spinel lherzolites which equilibrated last at a depth 30 km in the mantle other ultramafic facies some representing rocks which mixed with a basaltic melt fraction and some which underwent metasomatic exchange at depth are also present the peridotite bodies are in tectonic contact with a metamorphic formation consisting of gneissic and amphibolitic rocks similar to those outcropping in the pan african metamorphic basement of eastern egypt peridotites and metamorphic rocks are both crossed by several generations of basaltic dykes the sedimentary zabargad formation of probable cretaceous or palaeocene age consists of alternations of silicified limestones sandstones black shales and phosphoritic lenses and is overlain by evaporites probably of miocene age and by reef limestone units one probably of early pleistocene age the other probably deposited during the last interglacial various formations are affected by localized metasomatic mineralizations recrystallized large gem quality olivine cancrinite scapolite garnierite the uplift of upper mantle ultramafic bodies with fragments of crust which created the island might have occurred in connection with the early stages of development of the rift which preceded the formation of the red sea an alternative explanation that the zabargad peridotites are remnants of a pan african palaeozoic ophiolitic complex is possible but less likely

fluid circulation related to deformation in the zabargad gneisses red sea rift abstract a fluid inclusion study in relation to textures has been performed on pan african granulite facies gneisses from zabargad island red sea rift egypt the gneiss samples were collected at a maximum distance of 60 m above the contact with mantle peridotites with which they are co structured by a high t plastic deformation during an isothermal decompression starting from conditions estimated at 35 km depth and 850 c base of the pan african crust a first metasomatism of the crustal section during decompression is represented by co2 fluid inclusions trapped at minimum conditions of 15 km depth and 800 c the co2 fluid inclusion planes are controlled in orientation by the high t plastic flow structures but have been subsequently deformed during medium t plastic deformation in quartz the second metasomatism is represented by solid rich aqueous fluid inclusions which were trapped at minimum p t conditions of 3 7 km depth and 450 c corresponding to a very high thermal gradient they trace the introduction either of seawater or of fluids resulting from leaching of the middle miocene evaporites within the gneisses during their residence at a shallow depth below the seafloor two models are proposed in the literature in order to reconstruct the geodynamic history of zabargad island one brueckner h k elhaddad m a hamelin b hemming s kroner a reisberg l seyler m 1995 a pan african origin and uplift for the gneisses and peridotites of zabargad island red sea a nd sr pb and os isotope study j geophys res 100 22283 22297 suggests that most of the decompression p t path is pan african in age and only the aqueous metasomatism is related to the rifting history of the red sea the second nicolas a boudier f montigny r 1987 structure of zabargad island and early rifting of the red sea j geophys res 92 461 474 boudier f nicolas a ji s kienast j r mevel c 1988 the gneiss of zabargad island deep crust of a rift tectonophysics 150 209 227 supposes the existence of an asthenospheric diapir intrusive through the deep pan african crust leading to a continuous deformation event during early rifting of the red sea although it is impossible to date trapping of fluid inclusions and to know with certainty the origin of the fluids our preference is for the second model but slightly modified the high t decompression path traced by co2 fluid inclusions relayed by solid rich aqueous inclusions traces a continuous process of crustal thinning such a continuous path would assign to a single event the thinning and uplift of the deep crust and uppermost lithosphere to shallow basin bottom during the red sea early rifting in such an interpretative model the northern and central peridotite bodies are representative of the uppermost continental lithosphere and only the southern body could belong to an ascending asthenospheric diapir centred on the southeast of zabargad island

st john s island red sea a new geophysical model and its implications for the emplacement of ultramafic rocks in fracture zones and at continental margins abstract new gravity and magnetic data from the northern red sea reveal the extent of the large gravity anomaly 164 mgal and the presence of significant magnetic anomalies over st john s island spectral transformation and three dimensional potential field modelling delineate the surface configuration and vertical extent of the causative body and the enormous density contrast required 1 2 g cm 3 suggests that it is composed of unserpentinised peridotite density 3 4 g cm 3 to a depth of at least 8 km st john s island is uniquely located not only at a passive continental margin but also within a fracture zone at the transition from plate separation by seafloor spreading to extension by lithospheric attenuation this precludes several suggested mechanisms for the emplacement of ultramafic bodies in fracture zones thermal contraction serpentinite diapirism and changes in the poles of rotation do not seem possible mechanisms in this tectonic environment and the emplacement is most probably related to the spreading readjustment necessary to create a continent to continent fracture zone a post mesozoic age of emplacement associated with the onset of continental rifting and the rejuvenation of a pre existing continental fracture seems most probable

granulite xenoliths from western saudi arabia the lower crust of the late precambrian arbian nubian shield mafic and intermediate granulite xenoliths collected from cenozoic alkali basalts provide samples of the lower crust in western saudi arabia the xenoliths are metaigneous two pyroxene and garnet granulites mineral and whole rock compositions are inconsistent with origin from red sea rift related basalts and are compatible with origin from island arc calc alkaline and low potassium tholeiitic basalts most of the samples are either cumulates from mafic magmas or are restites remaining after partial melting of intermediate rocks and extraction of a felsic liquid initial87sr 86sr ratios are less than 0 7032 except for two samples at 0 7049 the sm nd data yield tdm model ages of 0 64 to 1 02 ga similar to typical arabian nubian shield upper continental crust the isotopic data indicate that the granulites formed from mantle derived magmas with little or no contamination by older continent crust calculated temperatures and pressures of last reequilibration of the xenoliths show that they are derived from the lower crust calculated depths of origin and calculated seismic velocities for the xenoliths are in excellent agreement with the crustal structure model of gettings et al 1986 based on geophysical data from western saudi arabia estimation of mean lower crustal composition using the granulite xenoliths and the gettings et al 1986 crustal model suggests a remarkably homogeneous mafic lower crust and an andesite or basaltic andesite bulk composition for pan african juvenile continental crust

the emplacement of peridotites and associated oceanic rocks from the lizard complex southwest england upper mantle peridotites and associated oceanic rocks from the lizard complex south west england preserve evidence for a multistage geological history steeply dipping pre emplacement fabrics record high temperature 900 1100 c shearing and exhumation of the mantle peridotites apparently formed during localized ne sw rifting in a pull apart basin setting c 400 390 ma associated oceanic rocks landewednack amphibolites preserve a pre emplacement prograde brown amphibole bearing metamorphic assemblage and steeply dipping fabric thought to have formed as the newly formed oceanic crust was juxtaposed with newly exhumed hot mantle peridotite during ne sw rifting in both the peridotites and landewednack amphibolites steep pre emplacement structures are cross cut by low angle mylonitic fabrics thought to have formed during the initial phases of emplacement of mantle over crustal rocks in a partially intra oceanic setting c 390 375 ma the fabrics in peridotites and amphibolites exhibit retrograde mineral assemblages c 500 800 c with the amphibolites preserving two superimposed assemblages green amphibole titanite and colourless magnesio hornblende respectively that are thought to record progressive down temperature deforma tion during thrusting emplacement related structures in both the basal peridotites and amphibolites consistently dip at low to moderate angles nw with down dip lineations and kinematic indicators showing consistent top to the nw senses of shear syn emplacement magmatism is recorded by intrusions of foliated kennack gneiss anastomosing serpentine filled faults mark many existing low angle contacts between the peridotites and landewednack amphibolites and appear to represent the final lowest temperature 250 c stages of emplacement c 370 ma this study shows that dynamothermal aureoles in ophiolites may preserve evidence for tectonothermal events that pre date thrust emplacement

formation of 18o depleted dolomite within a marine evaporitic sequence triassic reichenhall formation austria dark grey bituminous dolostones interbedded with marine derived anhydrite horizons occur in the triassic reichenhall formation of western austria fossils are rare and indicate a hostile hypersaline depositional environment the dolomites are finely crystalline fairly stoichiometric well ordered and non ferroan closely spaced samples 94 in total of individual dolomite units have been analysed for their carbon and oxygen isotopic composition the data indicate surprisingly low 18o values 5 7 to 2 1 0 pdb whereas the 13c values are comparable to the contemporary triassic seawater 0 2 to 2 6 0 pdb sedimentological evidence including i lack of any evidence for extensive dissolution ii distinct oxygen and carbon isotope ratios of individual dolomite units iii covariance of carbon and oxygen isotopes within some dolomite layers and iv inclusions of celestite in dolomite indicates a nearly closed system after early diagenesis combining this information with water rock interaction calculations suggests that the lightest oxygen isotope compositions are the result of freshwater influx into the basin during very early dolomite formation a secondary factor may be dolomite recrystallization at elevated temperatures during burial

a pan african age for the hp ht granulite gneisses of zabargad island implications for the early stages of the red sea rifting abstract up to now the age of granulite gneisses intruded by the zabargad mantle diapir has been an unsolved problem these gneisses may represent either a part of the adjacent continental crust primarily differentiated during the pan african orogeny or new crust composed of miocene clastic sediments deposited in a developing rift crosscut by a diabase dike swarm and gabbroic intrusions and finally metamorphosed and deformed by the mantle diapir previous geochronological results obtained on zabargad island and al lith and tihama asir complexes saudi arabia suggest an early miocene age of emplacement for the zabargad mantle diapir during the early opening of the red sea rift in contrast sm nd and rb sr internal isochrons yield pan african dates for felsic and basic granulites collected 500 600 m from the contact zone with the peridotites devoid of evidence for retrograde metamorphic minerals from a felsic granulite provide well defined rb sr and sm nd dates of 655 8 and 699 34 ma for the hp ht metamorphic event 10 kbar 850 c the thermal event related to the diapir emplacement is not recorded in the sm nd and rb sr systems of the studied gneisses in contrast the development of a retrograde amphibolite metamorphic paragenesis strongly disturbed the rb sr isotopic system of the mafic granulite the initial 143 nd 144 nd ratio of the felsic granulite is higher than the contemporaneous value for chur and is in agreement with other nd isotopic data for samples of upper crust from the arabian shield this result suggests that source rocks of the felsic granulite were derived at 1 0 to 1 2 ga from either an average morb type mantle or a local 2 2 ga lree depleted mantle zabargad gneisses represent a part of the disrupted lower continental crust of the pan african afro arabian shield during early stages of the red sea rifting in the miocene these precambrian granulites were intruded and dragged upwards by a rising peridotite diapir

fracture zone tectonics at zabargad island red sea egypt abstract zabargad island which lies along the western margin of the red sea rift is a remarkable place because it provides fresh exposures of undepleted mantle peridotite how this peridotite came to be exposed on zabargad remains unclear our field mapping indicates that most of the contacts between peridotite and the adjacent bodies of pan african gneiss and cretaceous zabargad formation on the island are now high angle brittle faults zabargad formation strata have been complexly folded partly in response to this faulting overall the array of high angle faults and associated folds on the island resembles those found in cross rift transfer zones we suggest therefore that the zabargad fracture zone a band of submarine escarpments on the floor of the red sea north of the island crosses zabargad island and has actively resolved differential movement between the central red sea rift and the northern red sea rift the final stage of uplift that brought the unusual peridotite to the earth s surface is related to shallow crustal transpression which may have inverted an earlier transtensional regime

sense of shear in high temperature movement zones from the fabric asymmetry of plagioclase feldspars abstract a new method for determining the sense of shear in plagioclase bearing tectonites from the 010 orientation of plagioclase feldspar is presented the method is based on the asymmetry of the 010 plane with respect to the structural frame foliation and lineation and the dominant activity of the 010 slip plane in the high temperature plasticity of plagioclase feldspar using examples from the zabargad gneisses red sea the method is applied to plagioclases of an25 an45 and compared with other methods of shear sense determination quartz c axis fabrics and microstructural criteria

isotope shifts in the late permian of the delaware basin texas precisely timed by varved sediments closely spaced samples 285 in number of varved sediments from the upper permian in delaware basin texas have been analyzed for 13ccarb 13corg 18ocarb corg ccarb and calcite dolomite 13c records a dramatic rise from 2 8 to 5 7 in only 4400 years detected in three sections across the basin extrapolating smoothly through a 600 year interruption by a local west side of the basin fresh water inflow evidenced by low 18o this continuity and low corg within the basin both indicate that the excess net deposition of corg necessary to generate the rise in 13c took place in the ocean external to the delaware basin correlation with similar records from the zechstein basin suggest that the event was world wide although this poses obvious difficulties for the carbon cycle the rate of rise of 13c and its sustained high level must imply conversions of oxidized carbon to reduced carbon that are very large depending on which reservoirs were involved

upper mantle beneath a young oceanic rift peridotites from the island of zabargad red sea information on the nature of the upper mantle in a young rift zone with lithosphere transitional from continental to oceanic was obtained from the island of zabargad probably an uplifted fragment of red sea lithosphere the island exposes exceptionally fresh mantle derived peridotite spinel iherzolite bodies the mineralogy and major element rare earth element and mineral chemistry of the spinel iherzolites suggest that they equilibrated last at depth 30 km in the mantle and that they have oceanic rather than continental affinity it is unlikely that the zabargad ultramafics are part of a late precambrian early paleozoic ophiolite as found elsewhere in circum red sea regions rather they were uplifted probably in connection with the development of the red sea rift the zabargad peridotites are in tectonic contact with continental metamorphic basement rocks it appears therefore that regions of an embryonic rift such as the red sea can be covered with continental type crust but be underlain by an upper mantle having already oceanic affinities

crustal underplating and evolution in the red sea rift uplifted gabbro gneiss crustal complexes on zabargad and brothers islands the nature of the crust of the red sea outside the oceanic axial trough segments can be inferred from uplifted crustal sections exposed on the islands of zabargad and the brothers in the central and northern red sea zabargad exposes in addition to mantle derived peridotites a crustal section consisting of metagabbros and metapyroxenites associated with silicic gneisses and cut by doleritic dikes seismic velocities of samples from this section are compatible with crustal velocities obtained by seismic refraction experiments in the central and northern red sea the zabargad peridotitesilicic gneiss gabbroic association provides a sample of upper mantle lower crust in a continent before the inception of a major rifting event the petrology and mineral chemistry of the zabargad metagabbros suggest that they were originally part of a basic layered complex which crystallized at relatively high pressure 900 1000 mpa or 9 10 kbar i e over 30 km deep the gneisses have tonalitic trondhjemitic affinity and were originally equilibrated in the garnet granulite facies implying also high pressure and derivation from the lower continental crust this suggests that after the subcrustal continental lithosphere along the proto red sea rift had been consumed possibly by convective thinning but before any significant thinning of the crust basaltic melts intruded the base of the continental crust underplating it with gabbroic complexes thus crustal rifting was probably preceded by a thermal anomaly in the mantle which supports an active rifting hypothesis the gabbro pyroxenite complex and the gneisses underwent subsequently retrometamorphic events under decreasing pt conditions from granulite to amphibolite to greenschist facies implying decompression and uplift probably related to the gradual thinning of the continental crust during rifting during this stage additional gabbroic intrusions and basaltic dikes exemplified by the gabbros and dolerites from the brothers islands were emplaced at decreasing depths beneath the thinned continental crust the northern red sea crust is presently in this stage it is neither oceanic nor continental it lacks a continuous thick basaltic upper crustal layer and is made prevalently of gabbroic and doleritic intrusives with relicts of sialic continental crust subsequently basaltic injections became progressively focused in a narrow axial zone and seafloor spreading initiated at discrete equidistant hot points as observed presently in the central red sea which by axial propagation evolved into nearly continuous zones of oceanic crust accretion as observed presently in the southern red sea processes similar to those proposed here for the evolution of the red sea rift can be documented in passive margins of the atlantic and in some alpine ophiolite complexes

geochemistry of brachiopods oxygen and carbon isotopic records of paleozoic oceans abstract combined trace element and isotope studies of 319 brachiopods covering the ordovician to permian time span show that 13c and 18o in well preserved specimens varied during the paleozoic the overall 13c secular trend is in accord with the previously published observations but its details are obscured by vital isotopic fractionation effects at generic level nonetheless the results suggest that the negative correlation between marine 13ccarbonate and 34ssulphate deteriorates at time scales of 106 years due to the long residence time and thus slow response of so42 in the ocean for oxygen isotopes all devonian and older specimens have 18o of 4 while the well preserved permian samples have near present day 18o of about 1 pdb this isotopic dichotomy is probably not due to post depositional phenomena salinity or biogenic fractionation effects this leaves open the perennial arguments for a change in 18 o 16 o of sea water versus warmer ancient oceans the present data are difficult to explain solely by the temperature alternative the coincidence of the proposed shift in 18o with the large late paleozoic changes in marine 87 sr 86 sr 13 c 12 c 34 s 32 s and sea level stands argues for a tectonic cause and for a change in 18 o 16 o of sea water although such explanation is difficult to reconcile with global balance considerations and with isotopic patterns observed in alteration products of ancient basalts and ophiolites whatever the precise cause or combination of causes the implications for tectonism and or paleoclimatology are of first order significance

the gneiss of zabargad island deep crust of a rift abstract the gneisses of the zabargad island are in contact with lherzolites that were emplaced by mantle diapirism during the early rifting of the red sea the gneisses and the peridotites at their contact develop similar structures kinematics and common granulite metamorphism away from the contact the shear strain and associated recrystallizations decrease rapidly thus the zabargad gneisses are metamorphosed and deformed by the mantle diapiric intrusion this is confirmed by preliminary isotopic dates the gneisses have a tripartite polygenic composition of felsic granulites metagabbros and amphibolites the metagabbros and amphibolites are metamorphosed only by the mantle diapir intrusion which also induces a partial anatexis of the gneisses the felsic granulites record a proceeding high p granulite facies metamorphism they are assumed to represent screens of ancient continental crust

carbon cycle changes of the zechstein sea isotopic transition zone in the marl slate the carbon isotope composition of carbonate rocks is one parameter used to study the carbon sulphur and oxygen cycles through geological time1 2 the understanding of these interrelated cycles is important in quantitative flux modelling of sedimentary rocks the carbon isotope composition in the large reservoir of dissolved carbonate in ocean water is a result of mass balance between the amount of carbon in the reduced organic reservoir and the oxidized carbonate reservoir at any given stage the reduced carbon reservoir preferentially sequesters 12c owing to biological fractionation processes any change in the removal ratio of oxidized reduced carbon will be reflected by the change of the 13c value of the oceanic carbonate system here we present a detailed study of the change in 13c which took place at the base of the upper permian zechstein sequence

charnockite geobarometers based on coexisting garnet pyroxene plagioclase quartz numerous geothermometers based on compositions of coexisting minerals have been calibrated experimentally or theoretically during the past 10 yr in contrast few continuous reaction geobarometers exist which have had wide application one based on the reaction of cordierite to garnet sillimanite and quartz is limited by a relatively narrow pressure range of application and also by conflicting calibrations1 2 because the mixing relations of mg and fe and the role of h2o in cordierite3 are not understood another successful barometer uses the assemblage garnet plagioclase al2sio5 and quartz4 5 continuous geobarometers based on the reaction of orthofer rosilite to fayalite plus quartz6 and on the aluminium content of enstatite in equilibrium with garnet7 have been used to a lesser extent the former barometer is limited by rarity of the requisite assemblage and the latter by uncertain experimental calibration for crustal rocks we calibrate here two mineralogical geobarometers from measured thermodynamic data the participating minerals pyroxenes garnet plagioclase and quartz are frequently associated in high grade metamorphic rocks calculation of pressures for several terrains show that the geobarometers yield reasonable and consistent results for the entire range of crustal pressures

evolution of oceanic gabbros from dsdp leg 82 influence of the fluid phase on metamorphic crystallizations abstract gabbro breccias were recovered from an anomalously shallow level of the ocean crust during dsdp leg 82 the rocks display evidence of metamorphic crystallization related either to localized deformation or to hydrothermal circulation of a seawater derived fluid under static conditions secondary phases consist of plagioclase amphibole and minor clinopyroxene ilmenite sphene and chlorite petrological study indicates that deformation took place at high temperature under anhydrous conditions and was followed by hydrothermal circulation the compositions of secondary minerals i e strong zonations presence of chlorine in amphiboles varying compositions of secondary plagioclase indicate that reactions of the gabbros with the fluids occurred at a low water rock ratio relations between cl na and k in amphiboles suggest penetration of at least two distinct fluids of different compositions metamorphic crystallization stopped when greenschist facies conditions were reached 350 c probably because hydrothermal circulation faded out

a pan african origin and uplift for the gneisses and peridotites of zabargad island red sea a nd sr pb and os isotope study a sr nd pb and os isotopic study of peridotites and granulite facies gneisses from zabargad island in the red sea suggests that the tectonothermal petrogenetic and geochemical evolution of these rocks occurred largely during the pan african orogeny rather than the recent opening of the red sea sm nd model ages and whole rock errorchrons indicate that spinel iherzolites and gneisses differentiated from a common depleted mantle source about 700 ma the iherzolites were mylonitized metasomatized and amphibolitized during a structural event that juxtaposed the peridotites with the gneiss complex and uplifted the gneiss peridotite complex to relatively shallow crustal levels most radiometric dating schemes suggest a pan african age for this event the gneisses generally have lower 143nd 144nd 87sr 86sr 208pb 204pb 207pb 204pb and 206pb 204pb ratios than the peridotites they extend linear trends defined by the spinel and amphibole peridotites on sr nd sm nd and pb pb diagrams suggesting the gneisses were either the source or buffering medium for the pan african metasomatism only one post pan african event had a significant effect on the geochemistry of the gneiss peridotite complex shallow level metasomatism by ultrahot 750 900 c hypersaline solutions with high 87sr 86sr 0 710 ratio led to the development of gem quality olivine crystals as well as low pressure mineral assemblages in the peridotites gneisses and younger rocks plagioclase rich assemblages with apparent igneous textures troctolites that are most common in the southern peridotite body may have formed by interaction of these fluids with peridotite i e are pseudo troctolites metasomatism changed the 87sr 86sr sm nd and re os ratios of the plagioclase peridotites making them unsuitable representatives of the pan african mantle

zabargad and the isotopic evolution of the sub red sea mantle and crust abstract zabargad island in the red sea exposes ultramafic bodies in contact with partially retrograded granulite facies lower crustal rocks the contact could represent the sub red sea moho prior to the opening of the red sea the ultramafic rocks are largely protogranular spinel lherzolites that grade locally to porphyroclastic to mylonitic amphibole peridotites some of the peridotites are infiltrated by high mg gabbros troctolites the peridotites and the lower crustal rocks are cut by diabase dikes the zabargad peridotites are isotopically heterogeneous on a scale of meters or less sr and nd isotopic ratios of clinopyroxenes cpx from spinel lherzolites lie in the depleted portion of the 143 nd 144 nd 87 sr 86 sr mantle array whereas cpx and amphiboles from amphibole peridotites fall off the array towards the low 143 nd 144 nd and sol 87 sr 86 sr field that may characterize portions of the sub continental mantle cpx from troctolites plot in the middle of the 143 nd 144 nd 87 sr 86 sr mantle array and have higher 87 sr 86 sr ratios and more depleted lree patterns than the intruded spinel lherzolites and amphibole peridotites all whole rock lherzolites and peridotites and most diabase dikes from zabargad as well as a variety of rocks from other parts of the red sea area morb glasses a diabase dike from the brothers island basanites and clinopyroxenes from associated nodules from the ataq diatreme in yemen define a linear array on a sm nd isochron diagram with a best fit age of ca 675 ma and an initial nd of 6 7 this array is probably a mixing line between depleted morb type mantle and a low 143 nd 144 nd 87 sr 86 sr metasomatic component the age defined by the linear array may date either an episode of pan african mantle homogenization or in our preferred model the pan african segregation of a previously homogeneous mantle into two or more reservoirs of identical 143 nd 144 nd but different sm nd ratios the subsequent partial remixing of these isotopic reservoirs during metasomatism associated with the opening of the red sea did not perturbed the slope of the array the lree enriched reservoir may have been the pan african lower crust since there is field and textural evidence that the metasomatic fluids were derived from or equilibrated with the adjacent crustal complex alternatively the lree enriched component may have been a mantle reservoir similar to that which generated basalts and nodules from the nearby ataq diatreme the troctolites are isotopically distinct from the host peridotites and retain igneous textures suggesting they intruded the peridotites during the opening of the red sea their isotopic patterns are similar to those of recent volcanics from afar portions of the ataq diatreme and some zabargad dikes indicating derivation from a similar mantle source their very lree depleted patterns suggest they are either re melted fractions of previously very depleted spinel lherzolites or early formed mineral aggregates that became enmeshed in the peridotites as fractionating melt ascended to higher levels our results indicate that at least two major processes metasomatism and intrusion have modified the isotopic pat terns of the spinel lherzolites of zabargad had the resultant chemically and isotopically heterogeneous bodies stayed in the mantle instead of being uplifted to their present positions they would have proven to be enigmatic sources for future basalts and xenoliths

mixed evaporative and meteoric water dolomitization isotope study of the zechstein limestone upper permian southwestern poland abstract dolomites of the zechstein limestone in western poland have high 13c values characteristic of the zechstein carbonates and there is some variation in 18o profiles dolomites are mostly stoichiometric and well ordered and are characterized by variable crystal textures the area studied underwent constant burial since the late permian and the isotopic curves despite their complexity show distinct regularities and signals of meteoric water input that in most cases could occur only during the permian moldic porosity interpreted as originated shortly after dolomitization commonly occurs stratigraphic patterns and facies analyses suggest that dolomitization phases occurred at the top of each eustatically controlled subcycle of the zechstein limestone in association to subaerial exposures most of the dolomites therefore originated during the permian a significant proportion of the dolomites originated via meteoric marine mixing during deposition of the zechstein limestone the mixing mechanism was closely related to sea level falls conventionally evaporite related dolomites are believed to have originated in a sabkha environment and or by the reflux mechanism the zechstein limestone case suggests that the mixing of evaporative and meteoric water may be responsible for dolomitization of other evaporate related carbonates

a new explanation for cyclic deposition in marine evaporite basins meteoric water input abstract one of the least discussed processes affecting the deposition of marine evaporites is the role of meteoric water sand and fine grained clastic sediments deposited from river system in a marginal evaporite basin facies the oxygen isotope record in the carbonate minerals mainly dolomites and the br cl ratios of second cycle salts are some of the evidence for influxes of meteoric water important meteoric water influxes are not limited to sabkha and salina evaporites but may also affect deep water evaporite basins one can see in the oxygen isotope record of carbonate minerals an imprint of meteoric water just as the sulfur isotope record reflects the imprint of the marine water sedimentological characteristics of evaporite sequences could be interpreted as variations in the amount of precipitation in the evaporite basin catchment area rather than as a result of changes in sea level for example the varved structures may reflect seasonal flooding of the basin by meteoric water evaporite facies cycles may be the result of longer term weather variation for example the paradox member cycles pennsylvanian utah could reflect periods of increased precipitation which are associated with glacial interglacial cycles