the time averaged paleomagnetic field we review indications of persistent deviations from the geocentric axial dipole model of the time averaged geomagnetic field and present a zonal harmonic model derived from 185 deep sea sediment piston cores taken from low to middle latitudes to approximately 45 analysis of the paleomagnetic inclination recorded in these cores for the brunhes normal polarity 0 73 ma and matuyama reverse polarity 0 73 2 47 ma chrons after plate motion correction gives well constrained estimates of the dominant long term nondipole contributions the axial quadrupole and axial octupole and shows no significant deviation from axial symmetry the amplitude of the axial quadrupole is found to vary with polarity 2 6 of the geocentric axial dipole for normal 4 6 for reverse while the axial octupole does not show appreciable change 2 9 for normal 2 1 for reverse these estimates of the quadrupole contribution agree well with prior determinations for the plio pleistocene 0 5 ma however the octupole contribution we find is opposite in sign to previous estimates we suggest that a negative octupole is representative of the actual time averaged paleomagnetic field while prior positive octupole estimates probably reflect spurious inclination shallowing the lack of polarity asymmetry in the octupole suggests that this nondipole component may be more closely linked to the main dipole field than is the quadrupole and so supports models of the geodynamo in which dipole and quadrupole families do not interact

thermal core mantle interactions and time averaged paleomagnetic field recent studies of historical geomagnetic secular variation and lower mantle seismic velocity suggest the earth s magnetic field is controlled by lateral variations in mantle temperature in this paper the average expected magnetic field pattern is compared with the time averaged field obtained from paleomagnetic pole positions from the last 5 m y the historical record suggests that the time averaged inclination anomalies are negative far sided in the atlantic region north of 30 s and the west pacific and positive in the south atlantic extreme north and south pacific and east pacific the pole positions give positive inclination anomalies in the south atlantic and extreme north pacific and negative anomalies elsewhere in agreement with the theory except for the east pacific this suggests that the historical record is not long enough to give an accurate picture of long term secular change in the eastern pacific but that elsewhere the time averaged field of paleomagnetism is predicted satisfactorily the preponderance of far sidedness in the paleomagnetic record is a consequence of the present distribution of temperature at the base of the mantle rather than a fundamental property of the geodynamo and it may therefore change on the long time scale of mantle convection in the historical record declinations average to negative values in the atlantic hemisphere and positive values in the pacific which if typical of longer term behavior could explain the preponderance of positive declinations for reversed polarity which are mainly in the atlantic hemisphere

two magma series and associated ore deposit types in the permian emeishan large igneous province sw china the late middle permian 260 ma emeishan large igneous province in sw china contains two magmatic series one comprising high ti basalts and fe rich gabbroic and syenitic intrusions the other low ti basalts and mafic ultramafic intrusions the fe rich gabbros are spatially and temporally associated with syenites each series is associated with a distinctive type of mineralization the first with giant fe ti v oxide ore deposits such as panzhihua and baima the second with ni cu pge sulfide deposits such as jinbaoshan limahe and zhubu new shrimp zircon u pb isotopic data yielded 263 3 ma for the limahe intrusion 261 2 ma for the zhubu intrusion and 262 2 ma for a syenitic intrusion these new age dates together with previously reported shrimp zircon u pb ages suggest that all these intrusions are contemporaneous with the emeishan flood basalts and formed during a major igneous event at ca 260 ma the oxide bearing intrusions have higher al2o3 feo as total iron and total alkalis na2o k2o but lower mgo than the sulfide bearing intrusions all intrusions are variably enriched in lree relative to hree the oxide bearing intrusions display positive nb and ti anomalies and in certain cases negative zr hf anomalies whereas the sulfide bearing intrusions have obvious negative nb and ti anomalies a feature of crustal contamination individual intrusions have relatively small ranges of nd t values all the intrusions however have nd t values ranging from 3 9 to 4 6 and initial 87sr 86sr ratios from 0 7039 to 0 7105 the syenites have very low mgo 2 wt but highly variable fe2o3 2 5 to 13 wt with initial 87sr 86sr ratios ranging from 0 7039 to 0 7089 magmas from both series could have derived by melting of a heterogeneous mantle plume the high ti series from a fe rich more fertile source and the low ti series from a fe poor more refractory source in addition the low ti series underwent significant crustal contamination the two magma series evolved along different paths that led to distinct mineralization styles

origin of the ug2 chromitite layer bushveld complex chromitite layers are common in large mafic layered intrusions a widely accepted hypothesis holds that the chromitites formed as a consequence of injection and mixing of a chemically relatively primitive magma into a chamber occupied by more evolved magma this forces supersaturation of the mixture in chromite which upon crystallization accumulates on the magma chamber floor to form a nearly monomineralic layer to evaluate this and other genetic hypotheses to explain the chromitite layers of the bushveld complex we have conducted a detailed study of the silicate rich layers immediately above and below the ug2 chromitite and another chromitite layer lower in the stratigraphic section at the top of the lower critical zone the ug2 chromitite is well known because it is enriched in the platinum group elements and extends for nearly the entire 400 km strike length of the eastern and western limbs of the bushveld complex where we have studied the sequence in the central sector of the eastern bushveld the ug2 chromitite is embedded in a massive 25 m thick plagioclase pyroxenite consisting of 60 70 vol granular cumulus orthopyroxene with interstitial plagioclase clinopyroxene and accessory phases throughout the entire pyroxenite layer orthopyroxene exhibits no stratigraphic variations in major or minor elements mg number1 4 79 3 81 1 however the 6 m of pyroxenite below the chromitite footwall pyroxenite is petrographically distinct from the 17 m of hanging wall pyroxenite among the differences are 1 phlogopite k feldspar and quartz are ubiquitous and locally abundant in the footwall pyroxenite but generally absent in the hanging wall pyroxenite and 2 plagioclase in the footwall pyroxenite is distinctly more sodic and potassic than that in the hanging wall pyroxenite an45 60 vs an70 75 the lower critical zone chromitite is also hosted by orthopyroxenite but in this case the rocks above and below the chromitite are texturally and compositionally identical for the ug2 we interpret the interstitial assemblage of the footwall pyroxenite to represent either interstitial melt that formed in situ by fractional crystallization or chemically evolved melt that infiltrated from below in either case the melt was trapped in the footwall pyroxenite because the overlying ug2 chromitite was less permeable if this interpretation is correct the footwall and hanging wall pyroxenites were essentially identical when they initially formed however all the models of chromitite formation that call on mixing of magmas of different compositions or on other processes that result in changes in the chemical or physical conditions attendant on the magma predict that the rocks immediately above and below the chromitite layers should be different this leads us to propose that the bushveld chromitites formed by injection of new batches of magma with a composition similar to the resident magma but carrying a suspended load of chromite crystals the model is supported by the common observation of phenocrysts including those of chromite in lavas and hypabyssal rocks and by chromite abundances in lavas and peridotite sills associated with the bushveld complex indicating that geologically reasonable amounts of magma can account for even the massive 70 cm thick ug2 chromitite the model requires some crystallization to have occurred in a deeper chamber for which there is ample geochemical evidence

weak palaeointensity results over a pliocene volcanic sequence from lesser caucasus georgia transitional record or time averaged field author s sanchez moreno elisa m calvo rathert manuel goguitchaichvili avto tauxe lisa vashakidze george t lebedev vladimir a abstract summary a palaeointensity study has been carried out on a pliocene sequence of 20 consecutive lava flows where previous directional results seem to reflect anomalous behaviour of the earth s magnetic field emf which can be explained by a polarity transition record or non averaged palaeosecular variation or both here we perform a total of 55 palaeointensity determinations using the original thellier thellier tt method and 100 with the izzi method we assess the performance of our selection criteria using a set of strict threshold values applied to a set of test data whose trms were acquired in known fields absolute palaeointensity determinations that passed our selection criteria were obtained on four specimens with the tt method and on 41 specimens with the izzi method application of reliability criteria at a site level yielded palaeointensity results in 8 of 20 studied lava flows we obtained median values of vadm between 28 9 and 45 6 zam2 for the reverse polarity lower apnia section while the normal polarity upper section displayed a single value of 54 6 zam2 the low palaeointensity values before a transitional direction lava flow and the higher value after it suggest the common behaviour at the start of a polarity reversal and the recovery after it however an isolated record of a stable emf where the intensity is lower than the current for the same location 83 7 zam2 cannot be discarded consequently this interpretation would support a weak time averaged field

paleomagnetic field variation with strong negative inclination during the brunhes chron at the banda sea equatorial southwestern pacific abstract we reconstruct the earth magnetic field in the brunhes epoch at the banda sea by studying the paleomagnetic data from core md012380 collected during the international marine global change study images vii cruise in 2001 magnetic analysis is carried out for whole core with a sampling spacing of 1 cm by using u channel magnetic susceptibility nature remanent magnetization nrm anhysteretic remanent magnetization arm and isothermal remanent magnetization irm are measured in our paleomagnetic experiment results show the low latitude geomagnetic field variation at the banda sea during the last 820 kyr except for the brunhes matuyama boundary bmb there is no clear signal of reverse events in paleo inclination and paleo declination patterns however the synthetic paleointensity curve displays the asymmetrical saw tooth pattern that can be used for determining reverse events and shows a maximum intensity drop at the bmb the characteristics of paleointensity provide a useful tool to identify reverse signals and improve the difficulties from only using inclination and declination patterns especially at low latitude with the help of paleointensity inclination and declination we have identified five reverse events furthermore if we consider the secular variation effect we think that the strong negative inclination observed in our study may be the zonal time averaged field with paleo secular variation rather than non dipole effect within the brunhes epoch

paleomagnetic secular variation of the honolulu volcanic series 33 700 ka o ahu hawaii abstract thirty years after the precursory study j geophys res 13 1972 2129 19 dated sites with ages between 33 and 700 ka from the honolulu volcanic series hvs southeastern part of the island of o ahu in hawaii have been revisited a minimum of 16 samples per site have been studied using alternating field and thermal demagnetization both techniques yielded similar results coeval flows could be identified by relying on their k ar ages similar rock magnetic parameters and identical directions reliable paleomagnetic directions have been obtained for all sites two site mean directions which deviate significantly from the geocentric axial dipole gad correspond to periods of low dipole intensity and may thus reflect a larger proportion of non dipole components the average value of the site mean declinations is 358 6 while the mean inclination of 34 5 is 3 5 lower than the value of the geocentric dipole at the site the north geographic pole is largely contained within the uncertainties surrounding the position of the mean virtual geomagnetic poles vgp latitude the deviation of 2 3 in latitude is further reduced by 1 if we consider the existence of a standing small quadrupole component that would represent 5 of the dipole the angular dispersion of the virtual geomagnetic poles 12 is only 1 7 smaller than expected at the site latitude and thus not a firm indication of low secular variation thus there is no significant difference with the global time averaged field for the same period other recent results from the same island for an older time period display similar characteristics but with reverse and normal polarities that are not perfectly antipodal it is suggested that this could result from changes in the mean dipole field intensity during these time intervals this points out to the importance of scrutinizing the entire field vector in any analysis of paleosecular changes these results do not bring any evidence for a persistent field anomaly under hawaii

mineral lamination development in layered gabbros of the british palaeogene igneous province a combined anisotropy of magnetic susceptibility quantitative textural and mineral chemistry study mineral lamination development in layered gabbros of the british palaeogene igneous province a combined anistotropy of magnetic susceptibility textural and mineral chemistry study

timing of ti magnetite crystallisation and silicate disequilibrium in the panzhihua mafic layered intrusion implications for ore forming processes abstract the 260 ma panzhihua mafic layered intrusion is one of a suite of intrusions related to the emeishan large igneous province elip sw china the panzhihua intrusion hosts a large 60 m thick fe ti oxide ore body at the base of the intrusion this study provides new constraints on the genesis of fe ti oxide ore layers at the panzhihua intrusion using geochemistry petrography and modeling of parent magma crystallisation with variation in f o 2 and h 2 o content whole rock major element geochemical trends are controlled by the modal abundance of fe ti oxides ti magnetite and ilmenite the lower 270 m of the intrusion is clearly dominated by a ti magnetite accumulation trend whereas above this level the geochemical variation is consistent with a ti magnetite and ilmenite accumulation trend this suggests that the lower 270 m of the intrusion crystallised at higher f o 2 conditions relative to that above 270 m detailed petrographic analysis of exsolution microtextures of the ti magnetite from within ore layers and gabbroic host rocks also indicates a relative increase in f o 2 within the ore rocks modeling of panzhihua parent magma shows that ti magnetite crystallises late for dry 1 5 wt h 2 o compositions this corresponds to a distinct variation in plagioclase crystallisation temperature which decreases with increasing h 2 o content of the parent magma the initial plagioclase composition varies from an 55 for a dry magma up to an 73 for a magma containing 3 wt h 2 o the average plagioclase composition for the panzhihua intrusion is an 58 indicating an initial magma with low h 2 o content textures within the fe ti oxide ore rocks clearly indicate that ti magnetite crystallised after the silicate phases plag cpx and disequilibrium textures indicate that the fe ti oxide ores were not in equilibrium with the enclosed silicates we present a model for fe ti oxide ore formation in an open system by multiple replenishments of magma with variable h 2 o contents ti magnetite crystal load and volume intruding h 2 o and crystal rich magmas effectively thermo chemically erode previously formed gabbroic cumulates forming the footwall and incorporates previously crystallised silicate grains these grains are consumed by the magma due to the high h 2 o content and higher temperature resulting in the association of consumed silicate primocrysts enclosed in fe ti oxide ore layers

long term geometry of the geomagnetic field for the last five million years an updated secular variation database lee 1983 assembled more than two thousand paleomagnetic directional data from lava flows in 65 sampling sites with ages spanning the last 5 million years constable 1992 recently suggested that the virtual geomagnetic poles vgp derived from this data base have been preferentially located within two antipodal bands of longitude we have reanalyzed and updated the lee data base yielding a total of 3179 data from 86 distinct sites neither the total data set nor various subsets of the data unambiguously show any large and clear maxima in the longitude distribution of vgps the shape of the common site longitude distribution is far more striking with a pronounced and robust minimum close to the common site longitude and secondary maxima about 120 o away from it the former first order feature is equivalent to the far sided effect discovered by wilson 1970 and corresponds to a persistent axial quadrupole amounting to about 5 of the axial dipole as previously found by several authors the latter second order feature could correspond to the vgp biasing effect described by egbert 1992 although its observed amplitude appears larger than predicted part of this effect an asymmetry in the maxima may also correspond to wilson s 1971 right handed effect given only a small axial quadrupolar component and the very uneven actual site distribution the expected longitude distribution of vgps can be calculated this is found to be in good agreement with the paleomagnetic observations i e rather flat with a moderate maximum near 120oe this study emphasizes problems related to the unsatisfactory distribution of sites the amplitude and significance of other terms beyond the dominant axial dipole and significant axial quadrupole contribution remain to be assessed

computer synthesis of geomagnetic palaeosecular variations computer models designed to synthesize palaeosecular variations of the geomagnetic field cast doubt on some widely accepted palaeomagnetic dogmas westward or eastward drift cannot be uniquely deduced from the sense of rotation of the geomagnetic vector the use of unit vectors in statistical analyses leads to systematic errors which render investigation of the global symmetry of the palaeofield more difficult than previously supposed at least some geomagnetic excursions may be regarded as large amplitude secular variations

the marcona magnetite deposit ica south central peru a product of hydrous iron oxide rich melts marcona the preeminent andean magnetite deposit 1 9 gt 55 4 fe and 0 12 cu is located in the iron oxide copper gold iocg subprovince of littoral south central peru fe oxide and cu zn pb sulfide mineralization was controlled by northeast striking faults transecting a middle jurassic aalenian to oxfordian andesitic shallow marine arc and a succession of contiguous plate boundary parallel late jurassic to mid cretaceous volcanosedimentary basins at marcona hydrothermal activity was initiated in the earliest middle jurassic 161 177 ma by high temperature mg fe metasomatism represented by cummingtonite and phlogopite magnetite assemblages subsequently during the terminal eruptions 156 162 ma of the arc widespread albite marialite alteration na cl metasomatism was followed by the emplacement of an en echelon swarm of massive magnetite ore bodies with subordinate overprinted magnetite sulfide assemblages hosted largely by paleozoic metasilici clastics the magnetite orebodies exhibit abrupt smoothly curving contacts dike like to tubular apophyses and intricate amoeboid interfingering with dacite porphyry intrusions there is no convincing megascopic or microscopic evidence for large scale fe metasomatism associated with the main sulfide poor mineralization the largest 400 mt minas 2 3 4 orebody is interpreted as a bimodal magnetite dacite intrusion comprising commingled immiscible melts generated through the dissolution of metasedimentary quartz in parental andesitic magma oxygen and sulfur stable isotope geothermometry indicates that the evolution at ca 159 ma from magnetite biotite calcic amphibole phlogopite fluorapatite to magnetite phlogopite calcic amphi bole pyrrhotite pyrite assemblages coincided with quenching from above 800 c to below 450 c and the concomitant exsolution of dilute aqueous brines subsequently chalcopyrite pyrite calcite pyrrhotite sphalerite galena assemblages in part metasomatic were deposited from lower temperature 360 c brines the cu poor marcona kiruna type magnetite and cu rich iocg deposits in the district therefore although spatially contiguous represent contrasting ore deposit types the former are interpreted as the product of fe oxide melt coexisting with dacite magma within an andesitic arc which failed during the closure of a back arc basin the weak associated magmatic hydrothermal cu sulfide mineralization at marcona was generated through melt vesiculation and contrasts with the considerably higher grade cu and ag rich orebodies of the major cu rich iocg deposits in the central andes e g la candelaria punta del cobre mantoverde raul condestable and mina justa which were the products of cool oxidized hydrothermal fluids plausibly expelled from the adjacent basins during tectonic inversion

a fluid dynamic model for the potholes of the merensky reef the cyclic unit that hosts the merensky reef is attributed to the entry of a new pulse of bronzite saturated magma into a chamber containing a melt that was crystallizing plagioclase mixing of these magmas produced a hybrid magma that was superheated with respect to both bronzite and plagioclase this magma melted the plagioclase cumulates that formed the floor of the chamber and because the products of erosion were light they were swept away by turbulent convection and replaced by hot uncontaminated magma allowing erosion to continue thermochemical erosion leading to the release of a light fluid has been modeled in a series of experiments in which a block of ice was melted by an overlying salt solution two features of the experiments are noteworthy first melting produced a pitted texture on the surface of the ice similar to the dimpled contact between the merensky reef and the underlying plagioclase cumulates and second small holes in the ice rapidly expanded into larger holes which were circular in plan and rounded in section the similarity between these structures and the potholes suggests that they formed by similar processes it is proposed that the erosion in the larger potholes was arrested at olivine rich layers because the products of thermochemical erosion were dense and could not convect away from the melting surface

the implications of non suppressed geomagnetic secular variation during the permo carboniferous reversed superchron abstract we have investigated variations in the natural remanent magnetisation nrm in red beds which were deposited during the permo carboniferous reversed superchron pcrs near the paleo equator these red beds are reliable recorders of the geomagnetic field because the nrm is carried by detrital hematite we analysed the angular standard deviation asd and spectral content of the directional records to quantify the amount of paleosecular variation psv during the time of deposition the asd of a high resolution directional record of the dome de barrot which spans approximately 27 kyr is 10 this is comparable to recent 0 5 ma low latitude lavas from the psvrl database mcelhinny and mcfadden 1997 and to recent lake sediments from the secvr database mcelhinny and lock 1996 the frequency spectra of declination and inclination are also similar to those from the recent lake sediments spectral power significant at the 95 level is concentrated in periods of 1 5 kyr higher frequencies are not significant from the asd and spectral analyses it appears that the amount of psv during at least part of the pcrs was comparable to that of recent times several heuristic and numerical geodynamo models are discussed in the context of superchrons and psv there is a discrepancy between the amount of sv shown by our data and that suggested by numerical models which use lateral heat flux variations at the core mantle boundary cmb however computational limitations still force some model parameters to have values that are unrealistic for the earth for the phenomenological geodynamos low energy state models which suggest that psv is suppressed during a superchron are not supported by the dome de barrot data rather a high energy state of the geodynamo in which outer core convection is vigorous is favoured as a prerequisite for a superchron to occur

exsolution features in titanomagnetites from massive magnetite layers and their host rocks of the upper zone eastern bushveld complex a systematic investigation of the exsolution textures in titanomagnetite grains of massive magnetite layers and their associated host rocks from the entire upper zone sequence has revealed noticeable differences that can be related to changes in the oxidation state of the magma during crystallization and cooling the observed oxidation exsolution intergrowths indicate a higher f sub o 2 during formation of the magnetite layers than during crystallization of the disseminated titanomagnetite as well as a decrease in the f sub o 2 required to precipitate successive titanomagnetite rich layers upward in the sequence peculiar composite lamellar intergrowths of magnetite and ilmenite in the uppermost magnetite layers and in some magnetite plugs are ascribed to a subsolvus oxidation of ulvoespinel in ulvoespinel rich magnetite the configuration of the magnetite ulvoespinel solvus dictates that ulvoespinel in ti rich magnetite ulvoespinel solid solution high in the succession exsolves at considerably higher temperature than in ti poor solid solution lower in the sequence subsolvus oxidation of ulvoespinel to ilmenite at higher temperatures near the top of the intrusion facilitated diffusion of ilmenite to produce the variety of different composite exsolution textures the textures are not developed where abundant oxidation exsolution of ilmenite has taken place at temperatures above the solvus

deflection of paleomagnetic directions due to magnetization of the underlying terrain in the summers of 1989 and 1991 we made 344 near ground level measurements of the ambient geomagnetic field above recent basalts on the island of hawaii using a three component fluxgate magnetometer we studied 12 surface features including a lava pond lava channels long tilted blocks smooth sloping surfaces two fissures and a deep u shaped road cut we observed substantial differences up to 20 between the observed and expected international geomagnetic reference field igrf magnetic field directions over these features except those composed of shelly pahoehoe and a flat horizontally thin lava pond we also observed inclinations that were systematically shallower than the igrf field by up to 5 we show that these shallower inclinations can be explained by the magnetization of the regionally sloping surface of the southern side of the island we found that all of the observed inclination deflections can be explained by simple two dimensional models which assume uniform induced and remanent magnetization parameters in the local terrain our observations imply that the inclination deflections cannot be corrected without a complete knowledge of the preexisting terrain and the remanence in the underlying flows upon which the lavas cooled since this information is rarely available it is difficult or impossible to discriminate between dispersion of paleomagnetic directions caused by the magnetic terrain effect and dispersion due to other factors such as paleosecular variation psv we therefore conclude that psv dispersion parameters cannot be accurately determined from paleomagnetic measurements on highly magnetic volcanic flows we also suggest that some of the geomagnetic excursions inferred from measurements on volcanic rocks may be at least in part due to the magnetic terrain effect it is unnecessary to invoke ad hoc mechanisms such as clastic block or crustal rotations distortion of the top crust or flow deformation to explain the large between site dispersions or inclination anomalies observed in many of the paleomagnetic data from volcanic rocks our observations also bring into question the general reliability of paleomagnetic pole positions inferred from volcanic rocks as a systematic inclination deflection due to local and regional slopes and irregular terrain such as those we observed would lead to a corresponding error in the inferred paleolatitude the magnetic terrain effects also offer alternative explanations for anomalous paleomagnetically inferred plate motions

petrogenesis and metallogenesis of the wajilitag and puchang fe ti oxide rich intrusive complexes northwestern tarim large igneous province abstract the wajilitag and puchang intrusive complexes of the tarim large igneous province tlip are associated with significant resources of fe ti oxide ores these two mafic ultramafic intrusions show differences in lithology and mineral chemistry clinopyroxenite and melagabbro are the dominant rock types in the wajilitag complex whereas the puchang complex is generally gabbroic and anorthositic in composition with minor plagioclase bearing clinopyroxenites in the marginal zone disseminated fe ti oxide ores are found in the wajilitag complex and closely associated with clinopyroxenites whereas the puchang complex hosts massive to disseminated fe ti oxide ores mainly within its gabbroic rocks the wajilitag intrusive rocks are characterized by a restricted range of initial 87sr 86sr ratios 0 7038 0 7048 and positive end t 0 04 3 01 indicating insignificant involvement of continental crustal contamination the slightly higher initial 87sr 86sr ratios 0 7039 0 7059 and lower end t values 1 05 2 35 of the puchang intrusive rocks also suggest that the isotopic characteristics was controlled primarily by their mantle source rather than by crustal contamination both complexes have sr nd isotopic compositions close the neighboring kimberlitic rocks and their hosted mantle xenoliths in the tlip this indicates that the ferrobasaltic parental magmas were most probably originated from partial melting of a metasomatized subcontinental lithospheric mantle modified recently with subduction related materials by the impingement of the ascending mantle plume the recycled subduction related materials preserved in the lithospheric mantle could play a key role in the formation of the parental fe rich magmas in the context of an overall lip system the distinct variations in mineral assemblage for each complex and modeled results indicated that the wajilitag and puchang complexes experienced different crystallization path fe ti oxides in wajilitag joined the liquidus earlier in the crystallization sequence especially relative to the crystallization of plagioclase this is attributed to the relatively high tfeo tio2 and initial h2o contents of the parental magma in combination with definitive field and petrological evidence the enrichment of highly incompatible elements e g zr hf nb and ta and the depletion of rare earth elements in the fe ti oxide ores is consistent with the plausible interpretation that the ores could be formed by fractional crystallization and crystal accumulation of the fe ti oxide crystals from the ferrobasaltic parental magmas a considerable amount of the fe ti oxides in the puchang has transported and sunk from higher up in the chamber to the underlying unconsolidated silicate crystal pile the highly dense fe ti oxide crystal slurries further tended to effective accumulate fe ti oxides to form high grade fe ti oxide ore bodies and subsequent rapid collapse and intrusive into lower lithologies within the complex under a h2o rich environment during the late stage of magmatic differentiation the development of massive fe ti oxide ores in the case of the puchang could plausibly result from a combination of the protracted differentiation history of a fe highly enriched parental magma and the later addition of external h2o from the country rocks e g carbonates to the slowly cooling magma chamber

abundant fe ti oxide inclusions in olivine from the panzhihua and hongge layered intrusions sw china evidence for early saturation of fe ti oxides in ferrobasaltic magma abundant fe ti oxide inclusions in cumulus olivine fo77 81 from the panzhihua and hongge intrusions emeishan large igneous province sw china document the first evidence for early crystallization of fe ti oxides in ferrobasaltic systems in nature the intrusions also contain significant stratiform fe ti v oxide ores the oxide inclusions are sub rounded or irregular range from 5 to 50 m in diameter and are dominated by either titanomagnetite or ilmenite the fact that the inclusions are either titanomagnetite or ilmenite dominant suggests that they are trapped crystals instead of immiscible oxide melt formed during growth of the host olivine the absence of other silicate phases in the inclusion bearing olivine is difficult to reconcile with a possible xenocrystic origin of the oxide inclusions these oxide inclusions are thus interpreted to be cumulus minerals crystallized together and trapped in olivine from the same parental magma in addition to fe ti oxides some inclusions contain amphibole biotite fluorapatite that might have formed by reaction of trapped hydrous liquid with the host olivine numerical modeling of high ti emeishan basalts using the melts program successfully simulates early crystallization of olivine fo81 and fe ti spinel in the presence of a moderate amount of h2o 1 5 wt under pressure and fo2 conditions generally pertinent to the panzhihua and hongge intrusions the modal mineralogy of the oxide inclusions is in good agreement with the bulk compositions of the ore as inferred from whole rock data in a given intrusion this is consistent with the interpretation that the stratiform oxide ores in the intrusions formed by accumulation of fe ti oxide crystals that appeared on the liquidus with olivine and clinopyroxene

crustally contaminated komatiite primary source of the chromitites and marginal lower and critical zone magmas in a staging chamber beneath the bushveld complex in the lowermost rocks of the rustenburg layered suite bushveld complex levels of total cr 2 o 3 are up to ten times higher than its putative solubility 0 16 cr 2 o 3 in the magmas that many investigators have accepted as parental to the dunites harzburgites pyroxenites and norites of the lower lower critical and upper critical zones various existing models fail to explain this chromium paradox in so far as they may explain why chromite is precipitated but do not account for its abundance in many cumulates simple mixing calculations and quantitative adiabat 1ph runs support our model in which a heterogeneous liquid crystals system was evolved in a staging chamber beneath the complex by mixing of komatiitic melts with 20 to 30 of crustal melts at depths appropriate to pressures between 10 and 4 5 kbars we contend that crystal settling and the effects of elutriation during magma ascent had generated a high sio 2 cap with modal olivine orthopyroxene and chromite increasing with depth in the staging chamber the magma from the high sio 2 cap of the staging chamber was first injected into the largely sedimentary transvaal supergroup generating what is now termed the marginal zone slurries of crystals plus liquid from the spasmodically rejuvenated main body of the staging chamber were then injected into the shallower bushveld chamber to form the lower and critical zones chromite within the slurries introduced in this way could account for their high gross cr content frequent reversals of normal evolutionary trends evident in the cumulates are attributable to the episodic nature of the influxes from the staging chamber the lithologically diverse nature of the floors upon which the chromitite layers accumulated peridotites pyroxenites norites and anorthosites was therefore more a function of the timing of emplacement of the cr rich slurries generated in the deeper reservoir than of specific evolutionary processes within the bushveld chamber itself

is 600 myr long enough for the random palaeogeographic test of the geomagnetic axial dipole assumption summary a simple analysis suggests that a time interval of more than about 5000 myr is needed for the random palaeogeographic test to be effective in assessing the geomagnetic axial dipole assumption this time is so long as to render the test ineffective consequently intervals of 600 myr should be recognized as providing only a small sample that is likely to be biased towards low inclination values

an experimental study of the influence of oxygen fugacity on fe ti oxide stability phase relations and mineral melt equilibria in ferro basaltic systems

bookshelf faulting and horizontal block rotations between overlapping rifts in southern afar lateral slip on initially rift parallel normal faults may be a particularly efficient mechanism to accomodate strain between overlapping oceanic rifts it occurs in southern afar where clockwise block rotations result from distributed dextral shear between the overlapping ghoubbet asal manda inakir and manda hararo abhe bad rifts faulting observed during the 1969 serdo earthquakes and on spot images is consistent with the shear being taken up by left lateral slip on steep nw se striking faults which formed as normal faults before extensional strain became localized in the two rifts this bookshelf faulting accounts quantitatively for the 14 5 7 5 rotation documented by paleomagnetism in the 1 8 0 4 ma old afar stratoid basalts given the 17 5 5 mm yr rate of separation between arabia and somalia

the time averaged geomagnetic field as recorded by lava flows over the past 5 myr summary a recently compiled lava flow data base spanning the last 5 million years is used to investigate properties of the time averaged geomagnetic field more than 90 per cent of the power in the palaeofield can be accounted for by a geocentric axial dipole however there are significant second order structures in the field declination and inclination anomalies for the new data base indicate that the main second order signal is the far sided effect and there is also evidence for non zonal structure vgp virtual geomagnetic pole latitude distributions indicate that over the last 5 million years normal and reverse polarity morphologies are different and that any changes in the normal polarity field morphology are undetectable given the present data distribution regularized non linear inversions of the palaeomagnetic directions support all these observations we test the hypothesis that zonal models for the time averaged field are adequate to describe the data and find that they are not non zonal models are needed to fit the data to within the required tolerance level normal and reverse polarity field models obtained are significantly different field models obtained for the brunhes epoch data alone are much smoother than those obtained from combining all the normal polarity data simulations indicate that these differences can be explained by the less extensive data distribution for the brunhes epoch the field model for all of the normal polarity data ln1 contains features observed in the historical field maps although the details differ ln1 suggests that although the two northern hemisphere flux lobes observed in the historical field are stationary to a first order approximation they do show changes in position and amplitude a third less pronounced flux lobe is observed in ln1 over central europe the lack of structure in the southern hemisphere is due in part to the paucity of data jackknife estimates of the field models for different subsets of the data suggest that a few sites contribute significant structure to the final field models more conservative estimates of the time averaged field morphology are obtained by removing these sites

plate motions and the geomagnetic field i quaternary and late tertiary summary we have tested the hypothesis that the apparent increase in the northward offset of the axial dipole i e the ratio g gy with age for the last 25 ma wilson mcelhinny is due to the failure to correct for plate motions spherical harmonic analyses were performed on two types of data palaeomagnetic poles from continents islands and seamounts and magnetic inclinations from deep sea cores after returning the sampling sites to their predrift locations in fixed hotspots coordinates the results show that the quadrupole term g maintained a value of about 0 05 gy throughout the last 35 ma and that the axial octupole had a small value of about 0 02 gy for the last 5 ma sea core inclinations analysed separately gave essentially the same results as continental palaeopoles for the last few million years independent data sets for the past 2 ma and for the period 2 6 ma gave nearly identical solutions showing that the persistence of the axial terms is not a phenomenon of the more densely sampled quaternary field alone nor is it an artifice of predominantly northward motion of the plates returning all 0 5 ma data to a 5 ma reconstruction failed to eliminate the quadrupole the non zonal coefficients for the 0 5 ma field are typically an order of magnitude less than the zonal terms with the exception of h and are probably insignificant suggesting that longitudinal drift is generally effective in averaging out these components the present distribution of data is inadequate to determine coefficients of third and higher degree prior to 5 ma but there is evidence that g may have changed little during the last 35 ma in addition there has been very little relative motion between the palaeomagnetic dipole axis as defined by the first degree terms and the axis of the hotspots reference frame

the myth of the pacific dipole window the angular dispersion of virtual geomagnetic poles vgp from lava flows is often cited as having been anomalously low in the pacific during the brunhes epoch because the dispersion from hawaiian lavas is said to be much lower than measured elsewhere this led to the concept of the pacific dipole window or pacific non dipole low because lavas tend to be erupted in bursts of activity many of the hawaiian data are serially correlated and thus cannot necessarily be treated as independent observations geochronological controls as are available for hawaii have been used in a previous analysis to thin the data to try to avoid repeated sampling of the same geomagnetic field unfortunately in that analysis the angular dispersion of vgps was calculated about the mean vgp instead of about the spin axis and thus underestimated the dispersion we have therefore reanalyzed the relevant data using the following criteria 1 only those lavas with 95 10 have been considered 2 a latitude dependent cut off angle is used to eliminate those vectors that are not part of the normal secular variation 3 we have developed a statistical method to rationalise those flows that have repeatedly sampled the same geomagnetic field vector because suitable geochronological controls are rarely available application of this new method to the hawaiian data shows excellent agreement with the results from using geochronological controls where possible we have therefore included this method in our overall analysis as expected the resulting global data are compatible with a fisher distribution about the spin axis brunhes age data from hawaii n 96 independent measurements give a vgp angular dispersion about the spin axis of sf 12 4 and for the pacific region as a whole sf 12 5 n 190 between latitudes 15 and 30 north or south these values are the same as sf 12 4 n 160 calculated for the rest of the world in the same latitude range this clearly demonstrates that the hypothesis of the pacific dipole window may confidently be rejected

paleomagnetic directions and k ar dating of 0 to 1 ma lava flows from la guadeloupe island french west indies implications for time averaged field models twenty six lava flows spanning the last million years were sampled in la guadeloupe french west indies because of the lack of continuous volcano stratigraphic sections in la guadeloupe dating is necessary in order to describe the temporal evolution of the geomagnetic field in this time interval new k ar ages ranging from 50 ka to 1 ma have been obtained on andesites using the cassignol gillot technique at the universite paris sud institut de physique du globe de paris orsay laboratory additional flows with k ar ages obtained using the same dating technique were also sampled for paleomagnetic investigations more than 200 samples were analyzed using both alternating field af and thermal stepwise demagnetization techniques duplicate samplings of two flows at three different sites demonstrate that within flow dispersion is negligible for the andesitic lava flows sampled in this study direct comparison with an earlier paleomagnetic study performed on the island indicates that for the three investigated flows modern demagnetization techniques yield much better defined paleomagnetic directions the matuyama brunhes transition was recorded in a three flow section and is dated at 781 18 ka in good agreement with other recent radiometric age determinations the mean paleomagnetic pole calculated from the 23 normal polarity flows is indistinguishable from geographic north which implies that no significant persistent axial quadrupole term can be identified at this site for the last million years this result contradicts earlier results and has important implications for models of the time averaged field taf an occam algorithm was used to construct a taf model from the global volcanic database from the last 5 ma substitution of the mean direction calculated from the earlier study for the lesser antilles by our new mean value reduces the quadrupole term by more than 30 this effect which was produced by changing data from a single site demonstrates that older paleomagnetic sites may need to be reinvestigated furthermore it also highlights the limitations of taf models that can be inferred from paleomagnetic databases in their present state

mineral compositional constraints on petrogenesis and oxide ore genesis of the late permian panzhihua layered gabbroic intrusion sw china a systematic study of mineral compositions has been carried out on stratigraphic sections through the panzhihua intrusion a layered gabbroic sill of late permian age that hosts significant magmatic fe ti v oxide deposits in the emeishan large igneous province sw china the intrusion which is 19 km long and 2 km thick is divided into a marginal zone mgz lower zone lz middle zone sub zones mza and mzb and upper zone uz from the base upwards compositions of the most primitive cumulus olivine fo81 and plagioclase an73 found in the intrusion suggest that the parental magma was similar to the mg rich end member of the coeval high ti basalts in the region two reversals in terms of mineral compositions are found in the intrusion one in the lower mza and the other at the transition between the mzb and the uz the first reversal is marked by a gradual increase in the an content of plagioclase an60 to an70 over a 100 m vertical interval the other is characterized by abrupt changes in the compositions of olivine fo28 to fo70 clinopyroxene mg 51 to 75 and plagioclase an24 to an51 over a 20 m vertical interval these suggest that the panzhihua intrusion opened at least twice for recharge of more primitive magmas strong depletion of incompatible trace elements such as zr in the cumulate sequence without corresponding enrichment in any border series is suggested to be evidence for significant loss of residual liquid at a late stage during solidification of the intrusion compared to other well studied layered intrusions the panzhihua intrusion contains fe ti oxides crystallized at a high temperature consistent with evidence obtained in earlier studies early appearance of liquidus fe ti oxides in the panzhihua magma is most likely a combined effect of a rather high degree of initial oxidation and elevated volatile fugacity

geochemistry petrogenesis and metallogenesis of the panzhihua gabbroic layered intrusion and associated fe ti v oxide deposits sichuan province sw china the panzhihua gabbroic layered intrusion is associated with the 260ma emeishan large igneous province in sw china this sill like body hosts a giant fe ti v oxide deposit with 1333 million ton ore reserves which makes china a major producer of these metals the intrusion has a marginal zone of fine grained hornblende bearing gabbro and olivine gabbro followed upward by lower middle and upper zones the lower and middle zones consist of layered melanogabbro and gabbro composed of cumulate clinopyroxene plagioclase and olivine these zones also contain magnetite layers the upper zone consists chiefly of leucogabbro composed of plagioclase and clinopyroxene with minor olivine most rocks in the body show variable scale rhythmic modal layering in which dark minerals primarily clinopyroxene dominate in the lower parts of each layer and lighter minerals primarily plagioclase dominate in the upper parts the oxide ores occur as layers and lenses within the gabbros and are concentrated in the lower parts of the intrusion ore textures and associated mineral assemblages indicate that the ore bodies formed by very late stage crystallization of v rich titanomagnetite from an immiscible oxide liquid in a fluidrich environment the rocks of the panzhihua intrusion become more evolved in chemistry upward and follow a tholeiitic differentiation trend with enrichment in fe ti and v they are enriched in light rare earth elements relative to heavy rare earth elements and exhibit positive nb ta and ti anomalies and negative zr and hf anomalies the silicate rocks and oxide ores of the panzhihua intrusion formed from highly evolved fe ti v rich ferrobasaltic or ferropicritic magmas the textures of the ores and the abundance of minor hydrous phases indicate that addition of fluids from upper crustal wall rocks induced the separation of the immiscible oxide melts from which the fe ti v oxide ore bodies in the lower part of the intrusion crystallized

variations of trace element concentration of magnetite and ilmenite from the taihe layered intrusion emeishan large igneous province sw china implications for magmatic fractionation and origin of fe ti v oxide ore deposits abstract in situ la icp ms trace elemental analysis has been applied to magnetite and ilmenite of the taihe layered intrusion emeishan large igneous province sw china in order to understand better fractionation processes of magma and origin of fe ti v oxide ore deposits the periodic reversals in mg ti mn in magnetite and mg sc in ilmenite are found in the middle zone of the intrusion and agree with fractionation trends as recorded by olivine fo plagioclase an and clinopyroxene mg compositions these suggest the taihe intrusion formed from open magma chamber processes in a magma conduit with multiple replenishments of more primitive magmas the v and cr of magnetite are well correlated with v and cr of clinopyroxene indicating that they became liquidus phases almost simultaneously at an early stage of magma evolution ilmenite from the middle and upper zones shows variable cr ni v mg nb ta and sc contents indicating that ilmenite at some stratigraphic levels crystallized slightly earlier than magnetite and clinopyroxene the early crystallization of magnetite and ilmenite is the result of the high feot and tio2 contents in the parental magma the ilmenite crystallization before magnetite in the middle and upper zones can be attributed to higher tio2 content of the magma due to the remelting of pre existing ilmenite in a middle level magma chamber compared to the coeval high ti basalts the relatively low zr hf nb and ta contents in both magnetite and ilmenite throughout the taihe intrusion indicate that they crystallized from fe ti p rich silicate magmas positive correlations of ti with mg mn sc and zr of magnetite and zr with sc hf and nb of ilmenite also suggest that magnetite and ilmenite crystallized continuously from the homogeneous silicate magma rather than an immiscible fe rich melt therefore frequent replenishments of fe ti p rich silicate magma and gravitational sorting and settling are crucial for the formation the massive and apatite rich disseminated ores in the lower and middle zones of the taihe intrusion

paleomagnetic tomography of the core mantle boundary abstract analysis of updated global paleomagnetic data indicates that over the last 5 million years the normal and reverse polarity states are discernibly different this difference which can be characterized by a higher ratio of the axial quadrupole to dipole field for the reverse polarity relative to the normal polarity is attributed to lateral variations in temperature or composition at or near the core mantle boundary although several mechanisms are considered thermal electric effects associated with core mantle topography and or with chemical variations in d the seismic layer approximately 150 km thick at the base of the mantle are judged to be the most likely sources of the observed polarity asymmetry this asymmetry should not necessarily be expected to give rise to systematic trends in polarity transitions

apparent polar wander paths for the major continents 200 ma to the present day a palaeomagnetic reference frame for global plate tectonic reconstructions summary synthetic apparent polar wander apw paths for north america south america eurasia india central africa australia and antarctica for the last 200 myr are proposed computation of these apw paths is based upon the latest version 4 5a of the global paleomagnetic database gpmdb a revised global plate tectonic model since the early jurassic and a new technique for generating smoothed apw paths the smoothing technique includes the following steps 1 pre selection of palaeopoles including pre filtering parameters number of sites number of samples per site 95 per cent confidence circle about mean direction cleaning procedure and time uncertainty 2 generation of palaeolatitude and declination plots for a reference site on each continent that combines palaeopoles via a global plate tectonic circuit 3 independent spline regression analyses of the palaeolatitude and declination plots 4 removal of palaeolatitude or declination data that deviate by more than 10 from the regression curves post filtering process 5 generation of synthetic apw paths from the resulting palaeolatitude and declination plots these synthetic apw paths are then rotated into african coordinates to determine the best fit apw path and a global palaeomagnetic reference frame four representative plate tectonic reconstructions and global plate velocity fields are presented for the three time intervals that correspond to globally synchronous changes in plate motion

sampling bias in vgp longitudes i derive probability densities for virtual geomag netic pole vgp longitudes for a general statistical model of local magnetic field variations i show that even for very simple statistically homogeneous models of secular varia tion the distribution of vgp longitudes is peaked 90 o away from the sampling longitude thus when sites are distri buted unevenly a non uniform overall distribution of vgp longitudes is to be expected analysis of the recent geomag netic field indicates that this bias can be significant particu laxly when random errors in paleomagnetic data are allowed for it is possible that some of the deviations from unifor mity seen in recent compilations of paleomagnetic reversal and secular variation data are a statistical artifact resulting from the distortion of the vgp transformation and the non uniformity of paleomagnetic sampling sites

incompatible ediacaran paleomagnetic directions suggest an equatorial geomagnetic dipole hypothesis abstract paleomagnetic results obtained from rocks of ediacaran age in several localities in laurentia and baltica persistently display co existence of two magnetization components one shallowly and the other steeply inclined both components pass criteria for a primary magnetization while geological considerations and radiometric age dating indicate that these magnetizations are surprisingly close in age the conventional interpretation of these results translating the inclination into paleolatitudes using the geocentric axial dipole hypothesis would imply that rocks acquired magnetizations in positions switching back and forth between equatorial and near polar latitudes in a geographic reference frame such large scale and fast 45 cm yr migrations of a continent have been rejected as dynamically implausible neither plate tectonics nor true polar wander are thought to be able to attain the required velocities a highly irregular behavior of the geomagnetic field during the ediacaran possibly an alternation of the geomagnetic dipole axis between a co axial and an equatorial alignment remains the only viable explanation for the paleomagnetic data such a behavior entails specific outer core conditions which in turn impose strong constraints on the possible models for the thermal evolution of the earth s interior

iron isotope fractionation during crystallization and sub solidus re equilibration constraints from the baima mafic layered intrusion sw china abstract to better understand fe isotope fractionation between mafic minerals and oxides during crystallization of mafic magma and sub solidus re equilibration the fe isotopes of whole rocks and separated minerals olivine clinopyroxene magnetite and ilmenite of the baima mafic layered intrusion sw china have been investigated the separated minerals show a systematical decrease in 57 fe values from magnetite 0 15 to 0 51 to olivine 0 11 to 0 15 and clinopyroxene 0 35 to 0 05 and then to ilmenite 0 82 to 0 10 demonstrating regular fractionation between these minerals except for a few of samples most of the olivine and clinopyroxene are similar to those of mantle xenoliths in fe isotopes indicating that fe isotope equilibrium reached during magma crystallization was well preserved by contrast the fe isotopes of the magnetite and ilmenite may be evidently modified by sub solidus re equilibration via the fe 3 versus ti 4 and fe 2 exchange between the oxides furthermore the sub solidus re equilibration in fe isotope is strongly controlled by the proportions of magnetite and ilmenite in rocks therefore although the 57 fe ilm of the lower zone rocks with magnetite ilmenite ratios as high as 6 10 was reduced evidently by the sub solidus re equilibration the magnetite preserved their original fe isotope compositions by contrast the fe isotopes of both most magnetite and ilmenite in the middle zone had been markedly modified by sub solidus re equilibration owing to the moderate magnetite ilmenite ratios 4 7 the decreases of both 57 fe mt and 57 fe ol upwards in the cyclic units of the lower zone reveal that extensive early fractional crystallization of the magnetite resulted in depletion of heavier fe isotopes in the magma on the other hand early crystallization of olivine and clinopyroxene gave rise to the slight elevation of 57 fe ol values upwards in the cyclic units of the middle and upper zones the stratigraphic reversals in the 57 fe mt and 57 fe ol values suggest multiple magma recharges additionally the 57 fe ol values in the base of the lower zone 0 10 to 0 15 indicate that the parental magma were heavy in fe isotope due to extensive silicate mineral fractionation at depth this study indicates fractionation in fe isotope between silicates and oxides during magma crystallization and sub solidus re equilibration

origin of fe ti oxide ores in mafic intrusions evidence from the panzhihua intrusion sw china economicconcentrationsof fe ti oxidesoccuras massive conform able lenses or layers in the lower part of the panzhihua intrusion emeishan large igneous province sw china mineral chemistry textures and quilfequilibria indicate that oxides in rocks of theintrusion were subjected to extensive subsolidus re equilibration andexsolution the primary oxide reconstructed from compositions oftitanomagnetite in the oresand associated intergrowths isan alumi nous titanomagnetite usp

disequilibrium iron isotopic fractionation during the high temperature magmatic differentiation of the baima fe ti oxide bearing mafic intrusion sw china abstract iron isotopic compositions of olivine clinopyroxene and titanomagnetite of the baima fe ti oxide bearing layered mafic intrusion sw china are used to investigate fe isotopic fractionation during the formation of the fe ti oxide ore bodies and to constrain the origin of silicate minerals and fe ti oxides the baima intrusion comprises the lower zone of interlayered troctolite clinopyroxenite and oxide ores and the upper zone of gabbros significant differences of fe isotope values between olivine fe 56 0 01 to 0 11 clinopyroxene fe 56 0 11 to 0 22 and titanomagnetite fe 56 0 20 to 0 31 were observed in rocks and ores of the lower zone iron isotopic fractionation between olivine and clinopyroxene olivine and titanomagnetite and clinopyroxene and titanomagnetite exhibits large variations of 0 06 to 0 22 0 12 to 0 27 and 0 00 to 0 20 respectively suggesting disequilibrium fractionation this disequilibrium cannot be explained by thermal or chemical gradient induced kinetic fractionation or by possible subsolidus exsolution of granular ilmenite from titanomagnetite instead it could be attributed to crystallization of silicates and titanomagnetite from two immiscible si rich and fe rich liquids continuous segregation of the fe rich liquid changed the fe isotopic composition of the si rich liquid and thus the fe isotopic compositions of olivine and clinopyroxene crystallized from it resulting in disequilibrium fe isotopic fractionation between them the effect of oxygen fugacity on the crystallization order of titanomagnetite and ilmenite from the fe rich melts on the other hand gave rise to the disequilibrium fe isotope fractionation between titanomagnetite and olivine clinopyroxene

key factors controlling the accumulation of the fe ti oxides in the hongge layered intrusion in the emeishan large igneous province sw china abstract the hongge layered intrusion hosts the largest fe ti v oxide ore deposit in the central part of the emeishan large igneous province sw china it is divided into lower zone lz middle zone mz and upper zone uz from the bottom to the top for the lz relatively higher cr 250 3000 ppm and ni 50 200 ppm contents of clinopyroxene lower end259ma and higher 87sr 86sr 259ma values 2 82 to 0 07 and 0 7057 0 7076 respectively and plenty of hornblende demonstrate a more primitive parental hydrous magma in contrast relatively low cr

apatite geochemistry of the taihe layered intrusion sw china implications for the magmatic differentiation and the origin of apatite rich fe ti oxide ores abstract the taihe intrusion is one of the layered intrusions situated in the central zone of the emeishan large igneous province elip sw china the cyclic units in the middle zone of the intrusion are composed of apatite magnetite clinopyroxenite at the base and gabbro at the top the apatite rich oxide ores contain 6 12 modal apatite and 20 50 modal fe ti oxides evidently distinguished from the coeval intrusions in which apatite rich rocks are poor in fe ti oxides most of apatites of the taihe middle and upper zones are fluorapatite although four samples show slightly high cl content in apatite suggesting that they crystallize from a hydrous parental magma compared to the apatite from the gabbro of the panzhihua intrusion situated 100 km to the south of the taihe intrusion the apatite of the taihe rocks is richer in sr and depleted in hree relative to lree the calculated magma in equilibrium with apatite of the taihe middle and upper zones also shows weakly negative sr anomalies in primitive mantle normalized trace element diagrams these features indicate that the apatite of the taihe middle and upper zones crystallizes after clinopyroxene and before plagioclase the apatite of the taihe middle and upper zones shows weakly negative eu anomalies suggesting a high oxygen fugacity condition the high iron and titanium contents in the oxidizing magma result in crystallization of fe ti oxides crystallization of abundant fe ti oxides and clinopyroxenes lowers the solubility of phosphorus and elevates sio2 concentration in the magma triggering the saturation of apatite the positive correlations of sr v total ree contents and ce yb ratio in apatite with cumulus clinopyroxene demonstrate approximately compositional equilibrium between these phases suggesting they crystallized from the same ferrobasaltic magma early crystallization and accumulation of fe ti oxide together with apatite produced the apatite rich oxide ores at the base of the cyclic units of the taihe middle zone

the time averaged paleomagnetic field 0 5 ma persistent departures from the geocentric axial dipole field model of the time averaged paleomagnetic field over the past 5 myr have been analyzed using oceanic data from deep sea cores and continental data from igneous rocks and sediments the data set comprises the equivalent of 9490 spot readings of the field 5831 normal and 3659 reverse from 930 groups of data variations in declination anomalies between groups of data strongly suggest the existence of second order tectonic effects small rotations that make it difficult if not impossible to identify any nonzonal terms that might exist inclination anomalies have therefore been modeled in terms of low degree zonal harmonics the calculated inclination anomalies within 10 latitude bands are widely scattered for both oceanic and continental data and do not appear to have been drawn from distributions with common means we attribute this large scatter to previously undetected or uncorrected second order tectonic effects in the continental data or in the oceanic data to small departures from the vertical in deep sea cores if it is assumed that these effects are random between groups of data and without a systematic bias they can be averaged out to a large extent if there are sufficient numbers of groups within each latitude band this method also has the major advantage that it maximizes time averaging of the field when this is done an acceptable fit to the inclination anomaly data is found using a zonal harmonic model although the point estimates for the reverse polarity zonal quadrupole term are consistently larger than those for the normal polarity zonal quadrupole term the difference is not significant and is likely due to contamination effects this finding is true for a combination of all the data or for separate combinations of continental igneous rocks and ocean sediments this conclusion differs from all previous analyses the major differences occur in model estimates of the zonal octupole term the estimates varying widely depending on the particular data combination we show that false octupole terms can be introduced by several factors including inclination errors in sediments and lavas the use of unit vectors in the analyses and incomplete magnetic cleaning particularly of reversely magnetized rocks thus we cannot impute a geomagnetic significance to the estimates of a zonal octupole term we suggest that the best estimates for the zonal quadrupole g2 g20 g10 are obtained from the combined brunhes age igneous rocks and oceanic normal data 0 033 0 019 and for the combined matuyama age igneous rocks and oceanic reverse data 0 042 0 022 when these two sets are combined the overall best estimate of g2 is 0 038 0 012

holocene paleomagnetic secular variation records from the western equatorial pacific ocean abstract we have carried out detailed rock magnetic and paleomagnetic studies on a suite of four deep sea sediment cores from indonesia and the philippine islands which form a transect from 6 3 n to 10 6 s the cores have average sediment accumulation rates of 35 65 cm kyr that all yield high resolution paleomagnetic secular variation psv records the psv records contain 44 correlatable inclination and declination features that corroborate the individual psv records and indicate that they are all continuous with no significant stratigraphic gaps thirty four ams radiocarbon dates provide a high resolution time stratigraphic framework for the psv records which extend from 0 18 000 yrs bp these are the first high resolution holocene psv records ever recovered from anywhere on earth within 17 of the equator almost 1 3 of the earth s surface area statistical analysis of the holocene psv records indicates that they have i anomalies of 5 to 9 consistent with 5 ma averaged results the strong similarity in i anomaly pattern of holocene and longer term 5 ma averaged psv data suggest that the dynamo process causing the anomaly is largely described by holocene psv by contrast the holocene psv records have vgp angular dispersions that are significantly lower than 5 ma averaged psv data yet significantly higher than psv data averaged over only the last few thousand years this difference indicates that whatever dynamo process causes the overall directional variability of the earth s magnetic field is not totally described by recent last few thousand year psv or even by late quaternary last 20 000 yrs psv

phase relations and liquid lines of descent in hydrous ferrobasalt implications for the skaergaard intrusion and columbia river flood basalts crystallization experiments using a hydrous ferrobasalt as starting material conducted at 200mpa 940 12008c at a wide range of water activities 0 1 1 and redox conditions qfm 3 to qfm 4 where qfm is the quartz fayalite magnetite oxygen buffer show that h2o influences significantly the differentiation history of ferrobasaltic magmas a combination of our data with published experiments on dry ferrobasalt at 1atm provides an extensive experimental database for modeling and quantifying crystallization and differentiation processes within a typical fe rich tholeiitic system under both dry and hydrous conditions the addition of h2o decreases liquidus temperatures and changes significantly the proportions temperature range and sequence of crystallizing mineral phases the dry liquidus is at about 11708c whereas the liquidus for h2o saturated melts is at 10608c the main phases crystallizing from h2o bearing ferrobasalt at the investigated conditions are olivine ol clinopyroxene cpx plagioclase pl magnetite mt hematite hm ilmenite ilm and amphibole am the phase assemblage is similar to that of the dry system except for the presence of hm at extremely oxidizing conditions and am at low temperatures 59508c and h2o saturated conditions the important observation made in this study is that the stability of fe ti oxides and in particular mt as well as the simultaneous coprecipitation of mtand ilm are almost independent of the activity of h2o ah2o in the system whereas the liquidus temperatures of the silicate minerals are dramatically depressed by increasing ah2o the stabilities of oxides are controlled mainly by the redox conditions prevailing in the system the most pronounced effect of ah2o on the liquidus temperatures of silicates is observed for pl which shows a considerable delay in crystallization with progressive magma differentiation early crystallization of fe ti oxides in h2o bearing ferrobasaltic compositions precludes any significant fe enrichment during differentiation as fe enrichment is a characteristic feature of the skaergaard intrusion it implies that the skaergaard parental magma did not contain considerable amounts of water on the other hand our experiments indicate that the differentiation of some ferrobasaltic series from the columbia river flood basalt province might have occurred in magmatic systems containing significant amounts of volatiles 0 5 3 wt h2o

linking the late cretaceous to paleogene pacific plate and the atlantic bordering continents using plate circuits and paleomagnetic data 1 late cretaceous to paleogene paleomagnetic data from the pacific plate the emperor seamounts can be compared with data from the atlantic bordering continents through the use of plate circuit reconstructions here we summarize the uncertainties in all data sets and present formal tests we report agreement between late cretaceous pacific paleomagnetic data and predictions based on the estimates of non pacific pole positions from synthetic apparent polar wander paths this congruency points to the veracity of the plate circuits and the accuracy of the paleomagnetic estimates in contrast to the agreement seen for the late cretaceous small discrepancies are observed in the comparisons of the pacific paleogene data and predictions from synthetic apparent polar wander paths such a disparity in a younger time interval is unexpected given the agreement of the late cretaceous data the possibility that minor temporally variable nondipole field components contribute to the discrepancy cannot be completely discounted however an alternative and more straightforward explanation is suggested by further comparisons of the mean non pacific paleomagnetic data and the highest quality poles that contribute to the means in particular we note that 1 the pacific paleogene data are in full agreement with coeval poles from north america meeting strict reliability criteria and 2 the non pacific paleogene poles of synthetic apparent polar wander paths are dominated by results from the north atlantic igneous province naip but taken as a whole the naip data fail a paleomagnetic reversal test hence minor discrepancies between paleocene paleomagnetic data from the pacific and atlantic hemispheres may point to limitations of the latter which incorporate a relatively large number of older lower quality data these findings call for renewed data collections utilizing comprehensive rock magnetic and paleomagnetic demagnetization procedures to improve resolution of paleocene non pacific data the uncertainties of the pacific paleolatitude data and the non pacific reference poles are larger than the differences related to the use of the two alternative plate circuits through east to west antarctica and through australia to the lord howe rise that link the atlantic and pacific hemispheres while no preference can thus be given to either plate circuit their overall consistency with paleomagnetic data suggests that they can be used to investigate long term motion of the pacific plate since late cretaceous times

paleosecular variation and the average geomagnetic field at 20 latitude we assembled a new paleomagnetic directional data set from lava flows and thin dikes for four regions centered on 20 latitude hawaii mexico the south pacific and reunion we investigate geomagnetic field behavior over the past 5 myr and address whether geographical differences are recorded by our data set we include inclination data from other globally distributed sites with the 20 data to determine the best fitting time averaged field taf for a two parameter longitudinally symmetric zonal model values for our model parameters the axial quadrupole and octupole terms are 4 and 6 of the axial dipole respectively our estimate of the quadrupole term is compatible with most previous studies of deviations from a geocentric axial dipole gad field our estimated octupole term is larger than that from normal polarity continental and igneous rocks and oceanic sediments but consistent with that from reversed polarity continental and igneous rocks the variance reduction compared with a gad field is 12 and the remaining signal is attributed to paleosecular variation psv we examine psv at 20 using virtual geomagnetic pole vgp dispersion and comparisons of directional distributions with simulations from two statistical models regionally the hawaii and reunion data sets lack transitional magnetic directions and have similar inclination anomalies and vgp dispersion in the pacific hemisphere hawaii has a large inclination anomaly and the south pacific exhibits high psv the deviation of the taf from a gad contradicts earlier ideas of a pacific dipole window and the strong regional psv in the south pacific contrasts with the generally low secular variation found on short timescales the taf and psv at hawaii and reunion are distinct from values for the south pacific and mexico demonstrating the need for time averaged and paleosecular variation models that can describe nonzonal field structures investigations of zonal statistical psv models reveal that recent models are incompatible with the empirical 20 directional distributions and cannot fit the data by simply adjusting relative variance contributions to the psv the 20 latitude data set also suggests less psv and smaller persistent deviations from a geocentric axial dipole field during the brunhes

statistical properties of paleomagnetic directions in kerguelen lava flows implications for the late oligocene paleomagnetic field 40 ar 39 ar ages are reported to aid in the calibration of the paleomagnetic results the primary contribution of this report however is a compilation of these new data with those already published in order to describe statistically the characteristics of the paleomagnetic field as recorded by the kerguelen flood basalts in total 258 paleomagnetic directions sampled at 13 stratigraphic sections through the lava pile are available and span an approximately 5 ma window from 25 to 30 ma the composite section represents at least 11 polarity zones that are correlated to the reference geomagnetic polarity timescale our approach is to investigate the average normal and reversed polarity field directions over this 5 ma window we calculated a paleomagnetic pole found to be located at l 85 5n f 189 3 e a95 2 3 k 16 5 n 233 this pole is in close agreement with the coeval paleomagnetic poles obtained from different worldwide places when analyzed in the indo atlantic hot spot reference frame in the statistical analysis we tackle the specific question is the secular variation isotropic this question is directly related to the occurrence of a longitudinal confinement of the virtual geomagnetic poles which is still a matter of debate among paleomagnetists by means of statistical tests we show that the paleomagnetic data from kerguelen agree with an isotropic model for paleosecular

oligo miocene magnetostratigraphy and rock magnetism of the xishuigou section subei gansu province western china and implications for shallow inclinations in central asia magnetostratigraphy of 222 remanent directions together with late oligocene to early miocene mammal and charophyte paleontology suggest that 2179 m of the xishuigou section subei gansu province china were deposited from 26 to 19 ma stratigraphic patterns of bulk susceptibility anisotropy of magnetic susceptibility parameters and natural and anhysteretic remanent magnetization intensities demonstrate that 1 faulting does not significantly affect the record 2 sediment deposition was relatively continuous 3 sediment source changed around 23 ma and 4 rapid uplift near subei occurred at 21 ma subei rotated 27 5 counterclockwise with respect to the 20 ma pole from the eurasian synthetic apparent polar wander path folding and rotation of the section took place after 19 ma the paleolatitude of subei is 14 less than at present and 19 3 less than predicted from the reference pole both rock magnetic and paleomagnetic data sets suggest that the unusually low paleolatitude is the result of synsedimentary inclination shallowing a phenomenon which has likely affected other paleomagnetic data from central asia

multiple magma evolution and ore forming processes of the hongge layered intrusion sw china insights from sr nd isotopes trace elements and platinum group elements abstract the hongge layered intrusion 259 ma which is located in the inner zone of the emeishan large igneous province elip is one of the most typical fe ti v ore deposits in the pan xi area mafic ultramafic layered intrusions of the elip have attracted a lot of attention lately because these intrusions host world class fe ti v oxide deposits plus interesting cu ni pge mineralization which may have economic potential this paper reports new whole rock major and trace element compositions pge abundances and sr nd isotopic data for selected cumulate rocks and basalts we use these data to investigate the nature of parental magmas and the controls on its evolution from the source mantle en route to the surface involving the hongge ore bearing intrusion two abrupt changes in mt ilm and trace element ratios such as ba th with depths in the hongge layered intrusion indicate that this intrusion formed by at least two pluses of relatively primitive magma the whole rock sr nd isotopic data of basaltic and intrusive rocks plot in the region of emeishan low ti basalts and the compositions of residual liquid at 1260 c and 1155 c calculated by melts are similar to our actual high ti bfq 2 and low ti bc 1 basltic samples indicate they are co magmatic rather than derivation from a distinct source total pge abundances in the hongge samples are extremely low ranging from 0 5 to 10 ppb sulfide bearing rocks in the hongge intrusion and the nearby coeval banfangqing and baicao basalts have similar mantle like pd pt ratios 2 6 and extremely high cu pd ratios 3 104 to 4 105 indicating that sulfide segregation took place at depth prior to emplacement at hongge and eruption in this region sulfide saturation in the hongge magma may have resulted from such crustal contamination event crystallization of silicate minerals under the anhydrous magma magma hydration plus fe ti enrichments in the parental magma are three critical factors for the formation of fe ti oxide ore layers in the hongge intrusion

persistent patterns in the geomagnetic field over the past 2 5 myr historical geomagnetic measurements covering the past 400 years reveal a symmetrical pattern of four relatively stationary flux concentrations lobes at the surface of the liquid core and regions of rapid change extending from the atlantic to the indian ocean1 palaeomagnetic data define a time average over several thousand years2 which might reflect the stationary parts of the present field but unfortunately the historical record is too short to provide a satisfactory average3 here we model palaeomagnetic directions from the past 2 5 myr using the same methods as for modern data4 and find the two northern lobes in the same position as today over arctic canada and siberia in southern regions by contrast the field appears to have been smoothed out as might be expected from the current rapid secular variation1 we propose that the present geomagnetic field morphology and pattern of secular variation have persisted for several million years as would occur if the solid mantle controls flow at the top of the core

palaeomagnetism and chronology of the central taupo volcanic zone new zealand summary the central taupo volcanic zone tvz of new zealand is a region of intense quaternary silicic volcanism active since 1 6 ma we report palaeomagnetic measurements from 59 distinct volcanic units sampled at 98 sites in the tvz these are mainly rhyolitic ignimbrites and lava domes with a few basaltic andesitic and dacitic lavas most have new k ar or 4oar 39ar ages the remanent magnetizations are generally stable to both thermal and alternating field demagnetization and well determined mean palaeodirections were obtained for all sites our findings suggest that the taupo whakamaru maroa reporoa rotorua and okataina volcanic centres were magnetized during the brunhes normal chron kapenga is an older volcanic centre where activity commenced around 0 89 ma and extended into the brunhes mangakino volcanic centre is significantly older and was active from 1 6 to 0 95 ma transitional or intermediate palaeodirections were obtained from ahuroa ignimbrite 1 18 ko 02 ma and mamaku ignimbrite 0 22k0 01 ma the former almost certainly corresponds to the cobb mountain event the latter is significantly older than the blake event and probably corresponds to the recently reported pringle falls summer lake magnetic episode multiple sites from the whakamaru ignimbrite have indistinguishable 4oar 39ar ages 0 33 f 0 01 ma and glass composition but divergent palaeomagnetic directions this contrast suggests that either 1 the different sites were formed during a phase of extremely violent activity lasting up to a few hundred years during which geomagnetic secular variation was recorded or 2 that they were formed in a single eruption and rotation during subsequent extensional tectonism has caused divergence of the palaeodirections 40ar 39ar ages of 0 77 0 03 ma for the reversely magnetized rahopeka ignimbrite and 0 71 k 0 06 ma for the overlying normally magnetized waiotapu ignimbrite bracket and constrain the age of the matuyama brunhes transition

the nature and origin of titaniferous magnetite rich layers in the upper zone of the bushveld complex a review and synthesis the occurrence and geologic relationships of ti magnetite layers indicate that their genesis is intimately related to fractional crystallization processes that were responsible for the formation of their silicate host rocks the exact mechanism by which these oxide rich layers form is however still debated most recent models recognize that precipitation of copious quantities of ti magnetite is triggered by episodic increases in f sub o 2 but the process whereby this occurs is not well understood the nature and occurrence of ti magnetite rich layers in the bushveld complex is reviewed and the requirements of any satisfactory genetic model are outlined the formation and subsequent textural evolution of ti magnetite layers is believed to have taken place as follows 1 favorable conditions for the precipitation of large quantities of ti magnetite were created by a lengthy period of fractional crystallization that resulted in the concentration of large amounts of fe ti and v in the residual magma 2 large scale in situ bottom crystallization of plagioclase resulted in a marked increase in the total fe content and density of the immediately surrounding melt which collected as a dense layer on the bottom of the magma chamber this dense fe ti v enriched liquid did not mix with the overlying magma and formed a stagnant layer from which copious amounts of ti magnetite may have crystallized 3 crystallization of ti magnetite is controlled by the fe 2 o 3 feo ratio of the liquid and is a function of f sub o 2 temperature and the f sub h 2 o f sub h 2 ratio the fe 2 o 3 content of the liquid will also increase during the crystallization of ilmenite and fe super 2 bearing silicates clinopy roxene pigeonitc and fayalitic olivine the h 2 o content of the residual liquid will also be increased by the fractionation of anhydrous silicate phases 4 a complex interplay of these factors resulted in the precipitation of relatively large amounts of ti magnetite required for the development of ore rich layers the bulk of the ti magnetite formed by in situ bottom crystallization the nucleation and growth of magnetite within the stagnant layer and gravitative settling of these crystals over a short distance on the order of meters may have augmented the growth of the layers 5 variations in the relative proportions of coexisting oxide and silicate phases may be related to differences in concentrations and diffusion rates of ions in the liquid immediately above the crystallizing layer this resulted in differences in the rates of nucleation and growth of individual phases 6 precipitation of abundant magnetite lowered the density of the stagnant layer until it reached that of the overlying magma the two liquids then mixed thus terminating the cycle of magnetite layer formation and marking a return to silicate dominated fractionation 7 the ore rich layers originally consisted of ti magnetite crystals and variable amounts of silicate crystals which contained a certain amount of interstitial residual liquid densification of these layers and expulsion of significant amounts of this liquid were accomplished by annealing at high subsolidus temperatures possibly augmented by the addition of suitable material from interprecipitate liquids 8 variations in f sub o 2 during subsolidus cooling resulted in the development of a wide range of ulvoespinel and ilmenite microintergrowths

fluvial discharge and sea level changes controlling black shale deposition during the paleocene eocene thermal maximum in the dababiya quarry section egypt abstract the formation of black shales along the tethyan margins at the paleocene eocene thermal maximum petm 55 ma may have been an important feedback mechanism to reduce the warming by excess carbon burial however the detailed evolution of sediment redox conditions during this event is still poorly constrained we address this issue by a high resolution mineralogical and geochemical investigation of the outer neritic dababiya quarry petm section in central egypt which serves as the global boundary stratotype section and point for the base of the eocene there the base of the petm beds corresponds to the onset of sediment lamination indicative of oxygen deficiency at the seafloor the absence of calcium carbonate and a major increase in phyllosilicate abundance and change in detritus sensitive trace elements is indicative of severe carbonate dissolution in addition to enhanced fluvial input erosion of coastal low lands and deposition during low or slightly rising sea level subsequently fully anoxic conditions were established for a short period during the peak phase of the petm as recorded by the strong relative enrichment of redox sensitive trace elements and organic carbon compared to background sediments the decoupling of the fe and mo contents from the detrital fraction may even indicate that euxinic conditions existed with free hydrogen sulfide in the water column concurrent to the onset of anoxia high carbonate and quartz contents as well as high elemental ratios of si al suggest a major drop of siliciclastic sediment supply sediment starvation most likely caused by a rapid sea level rise the final recovery phase of the petm is associated with a progressive restoration of the pre petm siliciclastic sedimentation temporary seasonal anoxic to dysoxic conditions and high phosphorus enrichment followed by carbonate dominated sedimentation and the return to well oxygenated conditions we conclude that after a period of strong fluvial discharge several parameters inherent to the petm e g warming water column stratification and enhanced fluvial discharge may have contributed to the development of anoxia and increased the burial efficiency of organic carbon by reducing water column oxygenation the temporal coincidence of sediment starvation and onset of black shale deposition during the petm suggests that a rapid sea level rise was an additional important triggering factor therefore our results support not only current views that tethyan continental shelf areas may have acted as large carbon sinks during the petm but also indicate that transgressions may have provided an important feedback mechanism for organic carbon drawdown during hyperthermal events

the geochemistry of titanomagnetite in magnetite layers and their host rocks of the eastern bushveld complex more than 430 analyses of magnetite separates revealed pronounced differences in composition between magnetite from magnetite layers and disseminated magnetite from the host rocks magnetite from the massive layers is among others enriched in elements such as ti mg al and si which suggests that this magnetite crystallized under conditions of disequilibrium with the magma i e by spontaneous nucleation and rapid crystallization lower vanadium values in the magnetites from the massive layers suggest crystallization at higher f sub o 2 than from the rocks containing the disseminated magnetite variation diagrams reveal a cyclicity in compositional trends upward in the sequence which is related to nucleation of the magma in the roof zone of the intrusion the periodic collapse of this crystal laden magma and its incorporation in the zone of crystallization at the base of the magma chamber deviations of this normal trend are considered to reflect a stratification of the roof zone and a gradual incorporation of the stratified roof zone liquid into the convecting magma a pronounced upward increase in the cr content of magnetite is tentatively ascribed to a combination of olivine crystallization at the expense of pyroxene in the upper subzones and a possible increase in the partition coefficient of chromium between spinel and liquid with a decrease in the crystallization temperature

the genesis of the newly discovered giant wuben magmatic fe ti oxide deposit in the emeishan large igneous province a product of the late stage redistribution and sorting of crystal slurries a giant fe ti oxide deposit hosted by the wuben mafic intrusion has recently been discovered in the pan xi area of the emeishan large igneous province elip the evolved compositions of the gangue minerals within the fe ti oxide ores indicate that they formed during later stages of magma differentiation than those within the neighboring panzhihua intrusion or other ore bearing intrusions in this area the rocks from the wuben intrusion and mzb of the panzhihua intrusion contain compositionally similar silicate minerals and have similar titanomagnetite ilmenite ratios suggesting that the former is related to and probably connected to the latter by subsurface magmatic conduits this indicates that unconsolidated minerals that formed in the mzb flowed as crystal slurries into the wuben magma chamber during the later stages of evolution of the parental magma the later secondary enrichment of fe ti oxides by mechanical redistribution and the sorting of crystals as a result of density and size differences generated the wuben massive fe ti oxide bodies the ilmenite was commonly saturated in the magma at late stage of differentiation in the elip thereby the associated deposit contains much higher contents of ilmenite this indicates that future exploration for fe ti oxide mineralization in the elip should not merely focus on the lower parts of large layered intrusions but should also include nearby relatively small intrusions or even the upper parts of large intrusions especially as ilmenite enriched fe ti oxide deposits may have greater economic value than ilmenite poor deposits

open magma chamber processes in the formation of the permian baima mafic ultramafic layered intrusion sw china abstract the baima mafic ultramafic layered intrusion of the 260 ma emeishan large igneous province elip hosts the second largest fe ti v oxide deposit in the panxi region sw china it is a 1600 m thick layered body intruded by slightly younger syenitic and granitic plutons the intrusion includes the lower and upper zones troctolite and olivine pyroxenite of the lower zone contains conformable oxide ore layers whereas the upper zone consists of olivine gabbro and gabbro with abundant apatite in the higher level the crystallization order of the silicates in the baima intrusion is olivine plagioclase clinopyroxene fe ti oxides titanomagnetite and ilmenite crystallized after olivine and possibly plagioclase the oxide ores in the lower zone show slightly lree enriched patterns with la yb n values between 2 0 and 6 4 and positive eu anomalies eu eu of 1 0 to 2 7 in contrast olivine gabbros in the lower zone display stronger lree enrichments la ybn 7 7 14 0 and positive eu anomalies eu eu 2 8 3 3 gabbros in the upper zone have ree profiles characterized by intermediate lree enrichments with la yb n values of 3 2 to 11 2 and positive eu anomalies of 2 1 to 3 0 primitive mantle normalized trace element patterns are characterized by negative la ce th sm and positive nb ta ba and ti anomalies in oxide ores and negative th u zr hf and positive ba sr and ti anomalies in olivine gabbro and gabbro fo of olivine and an of plagioclase remain roughly constant from 0 to 90 m in the lower zone indicating that the magma chamber was continuously filled by compositionally similar magmas during the initial stage three magma replenishments occurred afterwards in the upper part of the lower zone and the upper zone based on compositional reversals of plagioclase olivine and sr isotope mass balance calculations show that the baima parental magma can produce all oxide ores under closed system conditions a wide range of an values of plagioclase within thin sections and disequilibrium sr isotopic compositions of plagioclase both along the stratigraphic profile 87sr 86sri 0 70312 0 70510 and within thin sections indicate convection and co accumulation of cumulus plagioclase that had crystallized from different magmas we propose that the 1600m thick baima intrusion formed in an open and isotopically heterogeneous magma chamber that was periodically recharged by compositionally similar or more primitive magmas in each replenishment cycle the magma underwent progressive crustal contamination and fractional crystallization convection and sorting of crystals based on density differences result in igneous layering characterized by intervals of oxide ores troctolite and olivine gabbro

petrogenesis of the merensky reef in the rustenburg section of the bushveld complex a petrogenetic model for the merensky reef in the rustenburg section of the bushveld complex has been developed based on detailed field and petrographic observations and electron microprobe data the model maintains that the reef formed by reaction of hydrous melt and a partially molten cumulate assemblage the model is devised to account for several key observations 1 although the dominant rock type in the rusterburg sections is pegmatoidal feldspathic pyroxenite there is a continous range of reef lithology from pyroxenite to pegmatoidal harzburgite and dunite and small amounts of olivine are present in nearly all pegmatoids 2 the pegmatoid is usually bounded above and below by chromitite seams and the basal chromitite separated from underlying norite by a centimeter thick layer of anorthosite the thicknesses of the two layers exhibit a well defined positive correlation 3 inclusion of pyroxenite identical to the hanging wall and of leuconorite identical to the footwall are present in the pegmatoid the leuconorite inclusions are surrounded by thin anorthosite and chromitite layers in the same sequence as that at the base of the reef 4 chromite in seams adjacent to plagioclase rich rocks is characterized by higher mg mg fe and al r3 and lower cr r3 than that in seams adjacent to pyroxene rich rocks similar variations in mineral compositions are observed across individual chromitite seams where the underlying and overlying rock types differ the chromite compositional variations cannot be rationalized in terms of either fractional crystallization or reequilibration with surrounding silicates it is proposed that the present reef was originally a melt rich horizon in norite immediately overlain by relatively crystallized pyroxenite magmatic vapor generated by crystallization of intercumulus melt migrated upward through fractures in the cumulate pile below the protoreef the melt rich protoreef became hydrated because fractures were unable to propagate through it and because the melt itself was water undersaturated hydration of the intercumulus melt was accompanied by melting and the hydration melting front migrated downward into the footwall and upward into the hanging wall in the footwall melting resulted first in the dissolution of orthopyroxene and then of plagioclase with continued hydration chromite was stabilized as melt alumina content increased the regular variations in chromite compositions reflect the original gradients in melt composition at the hydration front the stratigraphic sequence downward through the base of the reef or pegmatoid melt chromitite anorthosite norite represents the sequence of stable mineral assemblages across the hydration melting front the sequence is shown to be consistent with knowledge gained from experiments on melting of hydrous mafic systems at crustal pressures with cooling the hydrated mixture from partial melting of norite footwall and more mafic hanging wall crystallized in the sequence chromite olivine pyroxene plagioclase with peritectic loss of some olivine calculations of mass balance indicate that a significant proportion of the melt was lost from the melt rich horizon variations in the development of the pegmatoid and associated lithologies and amount of modal olivine in the pegmatoids along the strike of the merensky reef resulted because the processes of hydration melting and melt loss operated to varying extents

character of holocene paleomagnetic secular variation in the tangent cylinder evidence from the chukchi sea abstract we have carried out a paleomagnetic study on three deep sea cores from the chukchi sea 72 n in order to characterize the holocene paleomagnetic secular variation psv in this high latitude region the chukchi sea lies within the geomagnetic field tangent cylinder and psv variability in this region might be expected to have a different pattern than psv at sites located outside the tangent cylinder at lower latitudes we have recovered correlatable directional psv records and relative paleointensity records from all three cores 15 radiocarbon dates were used to develop a chronostratigraphy for the psv records these records constitute the highest resolution full vector psv records ever recovered from such high latitudes we have compared our results with other previous studies from the region and find that our overall psv is consistent with these other studies although there are sometimes age differences up to 1000 years between correlatable psv features our statistical psv characteristics indicate that field variability vgp angular dispersion is lower than in regions just south of the chukchi sea and outside the tangent cylinder but our records are probably not long enough to completely characterize psv however our results are consistent with the only other published vgp angular dispersion results from inside the tangent cylinder antarctica 79 s

paleomagnetic and geochronological identification of the reunion subchron in ethiopian afar this paper reports the paleomagnetic and geochronological analysis of a suite of samples from a basaltic fault related scarp of pliocene age in the central part of the afar depression ethiopia paleomagnetic work uncovers a clear characteristic remanent magnetization carried by titano magnetites with high unblocking temperatures close to the curie temperature of pure magnetite the characteristic direction based on 27 non transitional flows is d 8 1 o i 10 1 o 5 4 1 o this is consistent with earlier determinations of the clockwise tectonic rotation due to rift propagation and overlap of the block to which the section belongs magnetic stratigraphy consists of a succession from bottom to top of 16 reversed 7 normal and 4 reversed flows one flow at the upper normal to reverse transition demonstrates complex behavior upon thermal and alternating field af demagnetization which is due to remagnetization by the overlying flow k ar dating of five samples provides consistent determinations averaging 2 07 0 05 ma all ages agree with this average given their rather large individual uncertainties from 0 04 to 0 08 ma at the lc level comparison with recent reference geomagnetic polarity timescales gpts indicates that the normal subchron recorded in the gamarri section must be linked with one of the r6union event s the normal subchron and an earlier episode of large secular variation could provide the best volcanic record of a double r6union event although uncertainties in ages are too large to allow unequivocal inferences our age determination for the r6union event is in better agreement with earlier determinations of mcdougall and watkins 1973 than with a more recent value proposed by baksi et al 1993 and used in the gpts