vertical magnetic structure of ocean crust determined from near bottom magnetic field measurements consensus on the source of marine magnetic anomaly stripes has remained one of the most elusive aspects of the vine and matthews seafloor spreading model which has been of such fundamental importance to global plate tectonics the first attempt to measure the vertical magnetic structure of oceanic crust exposed at a scarp face using near bottom magnetic measurements is reported along with the theoretical basis for the reduction and analysis of these measurements magnetometers mounted to deep tow and submersible platforms were used to determine the horizontal and vertical variation of crustal magnetization of the upper 2 km of young oceanic crust exposed by a steep submarine escarpment at the blanco fracture zone in the northeast pacific ocean results show a large amplitude magnetic anomaly is consistently found at the extrusive to intrusive transition indicating that the extrusives are strongly magnetized compared to the weakly magnetized intrusive crust profiles also show the systematic presence of more than one polarity with depth indicating the possibility of dipping polarity boundaries within the extrusives forward models find that a dipping polarity boundary model correctly predicts the amplitude and shape of the overlying sea surface anomaly while a block model with vertical polarity boundaries overestimates the amplitude these in situ results are used to directly assess the contribution made by the extrusive section to the overlying magnetic anomaly signal in essence to define the magnetic source layer it is concluded that the extrusive lavas constitute the main magnetic source for magnetic anomalies in oceanic crust less than 2 m y in age

complex zoning in apatite from the idaho batholith a record of magma mixing and intracrystalline trace element diffusion apatite crystals in a sample of biotite granodiorite from the idaho batholith display complex chemical zoning characterized by abrupt changes in ree y si content more subtle variations in s na la yb and mn contents and petrographic evidence for multiple episodes of partial resorption the zoning is attributed to changes in melt composition resulting from magma mixing and differentiation although the possibility that some rounded cores may be inherited cannot be disproved high s contents in apatite cores and the presence of an included anhydrite grain indicate crystallization from an oxidized host magma divalent cations that occupy the apatite ca sites sr mn fe show evidence of having been redistributed between zones by intracrystalline diffusion whereas cations that participate in coupled substitutions involving the tetrahedral site si y ree na s were not readily redistributed the main ree substitution in this case is ree super 3 si super 4 ca super 2 p super 5 and ree diffusion is rate limited by slow si diffusion however exchange of lree e g lasm sub 1 on the ca sites does not involve si and proceeds more rapidly resulting in homogenization of la sm between zones within individual crystals relative diffusion rates inferred from zoning profiles in this study are mn sr fe and lasm sub 1 are faster than na s and layb sub 1 which are faster than si these data imply that ree patterns can be decoupled from ree abundances during diffusion and that even apatite zones or cores that appear sharply bounded in backscattered electron images may not retain their original chemical or sr nd isotopic traits

apatite in igneous systems apatite is a minor but ubiquitous mineral in most igneous rocks although the modal proportion of apatite in common rocks is generally low it can reach high concentrations in enclaves cumulates and other rocks of low abundance i e rocks that constitute a small volume of the crust and mantle e g nelsonites the presence of apatite in most rocks is due not only to its low solubility in naturally occurring melts and aqueous solutions but also to the limited ability of common rock forming minerals to accept the amount of phosphorus that occurs in most rocks into their structure in this paper we will discuss some aspects of the occurrence texture composition physical chemistry and petrogenetic significance of apatite in felsic rocks i e andesite to rhyolite and their plutonic analogs in mafic i e basalts and related rocks and plutonic analogs and ultramafic rocks of the earth s crust and mantle fluorapatite is by far the most common member of apatite family found in igneous rocks however most natural fluorapatite contains some chlorine and hydroxyl as well and these constituents can attain high concentrations in some cases the other halogens bromine and iodine also occur in apatite but their concentrations are much lower than chlorine and fluorine many cations commonly substitute for calcium and phosphorus in apatite however they rarely reach concentrations that warrant the definition of a separate mineral species apatite can be described by the general formula a5 xo4 3z following sommerauer and katz lehnert 1985 the a site accommodates large cations e g ca2 sr2 pb2 ba2 mg2 mn2 fe2 ree3 eu2 cd2 na pan and fleet ch 2 in this volume and comprises two sites that exhibit vii fold

upper miocene magnetic stratigraphy at odp site 1092 sub antarctic south atlantic recognition of cryptochrons in c5n 2n summary u channel samples from odp site 1092 in the sub antarctic south atlantic ocean provide an interpretable magnetic stratigraphy from 1 95 to 3 6 ma and from 5 9 to 13 5 ma although the interpretation is unclear within the gilbert chron c3 the record represents one of the more complete upper miocene polarity stratigraphies from the south atlantic eight polarity subchrons that are not included in the standard polarity timescale are identified polarity subchron c4r 2r contains a normal polarity interval that has also been identified in sediments from odp leg 138 three short subchrons are recorded within c4n 2n c5r 3r and c5aar the record of c5n 2n contains four short reverse polarity subchrons of durations estimated to be 3 6 kyr

the central anomaly magnetic high implications for ocean crust construction and evolution the central anomaly magnetic high camh is a long lineated short wavelength 10 km wide at the sea surface magnetic anomaly overlying many of the world s midocean spreading centers this anomaly is particularly well developed over the northern juan de fuca ridge including the northern symmetrical and endeavour segments the ubiquitous presence of this zone of high magnetic contrast directly in the zone of active oceanic plate formation is a key to our understanding of near axis crustal plate evolution recently obtained deep tow magnetometer data from segments of the juan de fuca spreading center have caused us to revise the present models of the source of the camh and to review the implications the new model would have on our understanding of the evolution of the magnetic structure of oceanic plates in this model we attribute the near axis magnetization contrast to tectonically controlled low temperature alteration of the upper vertical section of extrusive basement extensive and systematic fissuring of the upper extrusive rocks allows penetration of sea water and rapid alteration of the magnetic minerals in fissured areas approximately 3 km off axis the correlation between fissured crustal blocks and near bottom magnetic lows and unfissured crustal blocks with magnetic highs which is observed at the axis is also seen in a single off axis tow over an outer flanking ridge

magnetic properties and ferromagnetic mineralogy of oceanic basalts summary rockmagnetic properties of a number of oceanic basalt samples collected by the dsdp odp drillings after leg 45 as well as one dredge haul were investigated in conjunction with palaeomagnetic characteristics of the same samples the electron probe micro analysis epma study indicated that chemical composition of the primary titanomagnetites has a variety particularly in minor constituent elements such as al2o3 mgo mno and v2o3 the variation in chemical composition in the primary phase of titanomagnetites seems to be controlled by changes in temperature oxygen fugacity and volatile pressure during the crystallization as well as chemistry of residual liquid hi magma general crystallization trend is found that is the later crystallized titanomagnetites have higher ulvospinel component higher titanium oxide and lower iron oxide due to decrease in temperature and oxygen fugacity it is established that minor constituent elements are good indicators of crystallization sequence of titanomagnetites in the contrast of variation in major constituent elements in the later stage al2o3 and mgo spinel component and v2o3 decrease whereas mno increases although there is a reversed crystallization trend with a few elements in several cases this reversed trend may suggest that there are discontinuous changes of environmental condition in magma to control titanomagnetite chemistry during a crystallization sequence the primary variation of chemistry shows different trends of the secondary variation due to oxidation oxidation process of titanomagnetites contained in oceanic basalts was examined in detail by means of an improved epma technique being able to directly determine the absolote contents of oxygen in materials the result of the quantitative oxygen measurement shows that the oxidation process of titanomagnetites is not a simple process by changes of ferrous ions to ferric ones with acrretion of oxygen but associated with migration of iron ions of the titanomagnetite lattice this is consistent with the results of recent studies on oxidation processes of titanomagnetites in oceanic basalts

orbital modulation of the earth s magnetic field intensity more than 20 years ago on the basis of data from a pacific sediment core it was suggested that geomagnetic field intensity may vary with the earth s orbital obliquity centred on a period of 41 kyr as a result of the effect of obliquity on precessional forces in the earth s core1 it had also been proposed that precession plays an important role in the energy budget of the earth s geodynamo2 but subsequent analyses indicated that the energy available from precession is at least an order of magnitude less than that required to drive the geodynamo3 here however we report a spectral analysis of sedimentary records of relative geomagnetic palaeointensity from two north atlantic sites which shows significant power both at orbital eccentricity 100 kyr and obliquity 41 kyr the eccentricity power is also present in bulk magnetic properties such as susceptibility and is therefore attributable to lithological variations controlled by eccentricity driven climate change the obliquity power however is not apparent in bulk magnetic properties and seems to be a property of the geomagnetic field itself thus providing evidence for the orbital forcing of geomagnetic field intensity

in situ major and trace element compositions of apatite from the yangla skarn cu deposit southwest china implications for petrogenesis and mineralization abstract the yangla copper deposit is a rare triassic intrusion related cu skarn system in the central part of the jinshajiang tectonic belt central sanjiang orogenic belt southwest china major and trace element compositions of apatites from ores fertile calc alkaline intrusions and its mafic microgranular enclaves and barren calc alkaline intrusions from the yangla skarn copper deposit were measured by electron probe microanalysis and laser ablation inductively coupled plasma mass spectrometry results indicate that apatites grown within ores fertile intrusions and its enclaves show similar features but are different from that grown within barren intrusions apatite from fertile intrusions has higher ca and correspondingly lower total trace element contents which partition onto the ca sites relative to that from barren intrusions apatites from the barren intrusions have high f and low cl contents and correspondingly high f cl ratios which may have resulted from the assimilation of sediment apatites from fertile intrusions show relatively high cl and low f contents which may reflect crystallization from magmas with high water contents redox sensitive elements in apatite can be used as indicators of the oxidation state of magmas apatites from the barren intrusion have lower v and as contents higher mn fe and u contents and more pronounced negative eu anomalies than those from fertile intrusion indicating that the barren magma was more reduced consistent with its lower whole rock fe2o3 feo and zircon ce4 ce3 ratios these findings demonstrate that apatites are effective in distinguishing fertile from barren intrusions and can be used as a robust tool in mineral exploration

lunar apatite with terrestrial volatile abundances the moon is thought to be depleted relative to the earth in volatile elements such as h cl and the alkalis nevertheless evidence for lunar explosive volcanism has been used to infer that some lunar magmas exsolved a co rich and co 2 rich vapour phase before or during eruption although there is also evidence for other volatile species on glass spherules until recently there had been no unambiguous reports of indigenous h in lunar rocks here we report quantitative ion microprobe measurements of late stage apatite from lunar basalt 14053 that document concentrations of h cl and s that are indistinguishable from apatites in common terrestrial igneous rocks these volatile contents could reflect post magmatic metamorphic volatile addition or growth from a late stage interstitial sulphide saturated melt that contained 1 600 parts per million h 2o and 3 500 parts per million cl both metamorphic and igneous models of apatite formation suggest a volatile inventory for at least some lunar materials that is similar to comparable terrestrial materials one possible implication is that portions of the lunar mantle or crust are more volatile rich than previously thought

s rich apatite hosted glass inclusions in xenoliths from la palma constraints on the volatile partitioning in evolved alkaline magmas the composition of s rich apatite of volatile rich glass inclusions in apatite and of interstitial glasses in alkaline xenoliths from the 1949 basanite eruption in la palma has been investigated to constrain the partitioning of volatiles between apatite and alkali rich melts the xenoliths are interpreted as cumulates from alkaline la palma magmas apatite contains up to 0 89 wt so3 3560 ppm s 0 31 wt cl and 0 66 wt ce2o3 sulfur is incorporated in apatite via several independent exchange reactions involving p5 ca2 vs s6 si4 na and ce3 the concentration of halogens in phonolitic to trachytic glasses ranges from 0 15 to 0 44 wt for cl and from 0 2 wt so3 dsapatite glass is only slightly dependent on the melt composition and can be expressed as so3 apatite wt 0 157 ln so3 glass wt 0 9834 the phonolitic compositions of glass inclusions in amphibole and hauyne are very similar to evolved melts erupted on la palma the lower sulfur content and the higher cl content in the phonolitic melt compared to basaltic magmas erupted in la palma suggest that during magma evolution the crystallization of hauyne and pyrrhotite probably buffered the sulfur content of the melt whereas the evolution of cl concentration reflects an incompatible behavior trachytic compositions similar to those of the water rich glass inclusions analyzed in apatite and clinopyroxene are not found as erupted products these compositions are interpreted to be formed by the reaction between water rich phonolitic melt and peridotite wall rock

the distinction between geomagnetic excursions and reversals summary two recent studies of the geomagnetic eld in the last 1 myr have found 14 excursions large changes in direction lasting 5 10 kyr each six of which are established as global phenomena by correlation between diierent sites the older picture of the geomagnetic eld enjoying long periods of stable polarity may not therefore be correct instead the eld appears to suier many dramatic changes in direction and concomitant reduction in intensity for 10 20 per cent of the time during excursions the eld may reverse in the liquid outer core which has timescales of 500 yr or less but not in the solid inner core where the eld must change by diiusion with a timescale of 3 kyr this timescale is consistent with the remarkably uniform duration of well dated excursions the disparity of dynamical timescales between the inner and outer cores a factor of 10 is consistent with the 10 excursions between full reversals

geomagnetic field variations between chrons 33r and 19r 83 41 ma from sea surface magnetic anomaly profiles abstract sea surface magnetic profiles exhibit coherent short wavelength micro anomalies or tiny wiggles superimposed to the main anomalies due to reversals in this study we investigate the nature and distribution of these tiny wiggles on oceanic crust formed during the 42 myr long period following the cretaceous normal superchron to this end we compute stacks of anomaly profiles from different areas in the indian and the pacific oceans using a simple method based on upward continuation we demonstrate that the tiny wiggles are consistent worldwide although their patterns exhibit different resolutions at different spreading rates they are therefore confidently ascribed to past fluctuations of the geomagnetic dipole moment a high resolution record of these fluctuations is obtained by selecting and stacking profiles from areas with the highest spreading rates modeling the micro anomalies as short magnetic polarity intervals yields durations for these intervals generally shorter than 10 kyr likely too short to be indeed true subchrons moreover the number of detected tiny wiggles clearly depends on the spreading rate these results support geomagnetic intensity fluctuations as being the cause of most tiny wiggles as also suggested by recent magnetostratigraphic data the tiny wiggles are uniformly distributed within chrons indicating that paleointensity fluctuations are neither inhibited after nor enhanced before a reversal beyond a blind zone of about 10 km corresponding to 80 to 250 kyr depending on the spreading rate for which the anomalies due to reversals prevent the detection of tiny wiggles most tiny wiggles probably represent a filtered record of a uniform secular variation regime as suggested by their uniform spatial distribution over the whole investigated period

a database for the matuyama brunhes magnetic reversal abstract we present a paleomagnetic database designated mbd97 comprising sedimentary and lava data for the matuyama brunhes reversal transition field using criteria regarding suitability of the rock samples measurement methods and temporal resolution of the data we consider 62 different studies of the matuyama brunhes reversal of these only 11 satisfy our selection criteria four lava sites one loess site three shallow marine sedimentary sites and three deep ocean sedimentary sites we discuss our reasons for accepting or disguarding each candidate data set we investigate the distribution of transitional virtual geomagnetic poles vgps they fall along american and asian australian longitudes the spatial and temporal complexity of the data indicate that the transitional field was almost certainly nonaxisymmetric and nondipolar the directional transition of the matuyama brunhes reversal took 2300 5000 years during which time the surface intensity fell to 10 20 of its pre and post transitional value we do not find any systematic bias due to site distribution or inclination shallowing although the errors in the data do vary over the duration of the reversal mbd97 is a small database but it has a fairly good geographic distribution and is suitable for studies of the matuyama brunhes transition nonetheless it is clear that further analysis of transitions will benefit from a continued program of data collection and analysis the data in mbd97 are listed

seismic structure and anisotropy of the juan de fuca ridge at 45 n a seismic refraction experiment was conducted with air guns and ocean bottom seismometers on the juan de fuca ridge at 45 n at the northern cleft segment and at the overlapping rift zone between the cleft and vance segments these data determine the average velocity structure of the upper crust and map the thickness variability of the shallow low velocity layer which we interpret as the extrusive volcanic layer the experiment is unique because a large number of travel times were measured along ray paths oriented at all azimuths within a small 20 km by 35 km area these travel times provide evidence for compressional velocity anisotropy in the upper several hundred meters of oceanic crust presumed to be caused by ridge parallel fracturing compressional velocities are 3 35 km s in the ridge strike direction and 2 25 km s across strike travel time residuals are simultaneously inverted for anisotropy as well as lateral thickness variations in the low velocity layer extrusive layer thickness ranges from approximately 200 m to 550 m with an average of 350 m the zone of the thinnest low velocity layer is within the northern cleft segment axial valley in a region of significant hydrothermal activity layer thickness variability is greatest near the cleft vance overlapping rift zone where changes of 300 m occur over as little as several kilometers laterally these low velocity layer thickness changes may correspond to fault block rotations in an episodic spreading system where the low side of each fault block accumulates more extrusive volcanics

investigation of a vine matthews magnetic lineation from a submersible the source and character of marine magnetic anomalies we report here the first attempt to map directly the boundary of a vine matthews magnetic stripe on the seafloor our objectives are to study the processes of oceanic crustal accretion as recorded in the reversal transition zone and to investigate the formation of the magnetic source of vine matthews magnetic anomalies our deep tow and alvin based magnetic studies focus on the matuyama brunhes reversal transition on the flanks of the east pacific rise near 21 n while the sea level magnetic anomalies are less than average in clarity for the east pacific rise a three dimensional inversion of deep tow data reveals a sharp strike linear polarity transition less than 1 8 km wide macdonald et al 1980a these measurements have been augmented by mounting a vertical magnetic gradiometer on alvin and making 280 reliable polarity determinations along and across the polarity transition zone even on long traverses across both sides of the boundary we find that nearly every magnetic target has the correct polarity i e the same polarity as the regional magnetic lineation this homogeneity in polarity of the magnetic lineations is surprising the magnetic polarity transition in the outcropping volcanic section is sharp and linear along strike delineated in some cases by a clear geologic contact of opposing flow fronts of different ages several weakly magnetized outcrops mapped within the transition zone may have erupted during the time in which the geomagnetic field was reversing the reversal boundary mapped on the seafloor from alvin is displaced 250 m to 500 m nw away from the spreading axis relative to the position of the average boundary as derived from inversion of the deep tow and sea level magnetic data this offset provides a means for estimating the spillover of lava flows away from the spreading axis during the time the crust was formed the combination of deep tow and alvin measurements suggests that circa 0 7 m y ago the crustal accretion zone magnetized volcanic intrusive and plutonic rocks was 2000 2800 m wide while the zone of recent volcanism alone was only 1000 2000 m wide the determination for the most recent reversal agrees well with submersible observations at the present spreading center where the zone of recent volcanism neovolcanic zone varies between 600 and 2000 m in width this very orderly picture for the formation of magnetic lineations and crustal accretion processes appears to conflict with complex deep sea drilling project dsdp magnetic results from the atlantic we suggest that the crustal generating processes and resulting magnetic structure vary significantly with spreading rate on the slow spreading mid atlantic ridge major episodes of volcanism are likely to be infrequent 104 years the magma chamber may be non steady state and the neovolcanic zone shifts or varies in width considerably this sporadic start and stop spreading process will contribute to a highly heterogeneous and complex crustal and magnetic structure as seen in dsdp holes in addition significant faulting and tilting may disrupt slow spreading crust for intermediate to fast spreading centers more frequent volcanism 50 600 years a nearly steady state magma chamber and a narrow stable neovolcanic zone will create a less complex magnetic and crustal structure both as seen from alvin and as inferred from clear sea level magnetic anomalies in the pacific

co variability of s6 s4 and s2 in apatite as a function of oxidation state implications for a new oxybarometer in this study we use micro x ray absorption near edge structures xanes spectroscopy at the s k edge to investigate the oxidation state of s in natural magmatic hydrothermal apatite durango mexico and mina carmen chile and experimental apatites crystallized from volatile saturated lamproitic melts at 1000 c and 300 mpa over a broad range of oxygen fugacities formula 1 fmq 1 2 fmq 3 fmq fayalite magnetite quartz solid buffer the data are used to test the hypothesis that s oxidation states other than s6 may substitute into the apatite structure peak energies corresponding to sulfate s6 2482 ev sulfite s4 2478 ev and sulfide s2 2470 ev were observed in apatite and the integrated areas of the different sulfur peaks correspond to changes in formula 2 and bulk s content here multiple tests confirmed that the s oxidation state in apatite remains constant when exposed to the synchrotron beam at least for up to 1 h exposure i e no irradiation damages to our knowledge this observation makes apatite the first mineral to incorporate reduced s2 intermediate s4 and oxidized s6 s in variable proportions as a function of the prevailing formula 3 of the system apatites crystallized under oxidizing conditions fmq 1 2 and fmq 3 where the s6 stotal peak area ratio in the coexisting glass i e quenched melt is 1 are dominated by s6 with a small contribution of s4 whereas apatites crystallizing at reduced conditions fmq contain predominantly s2 lesser amounts of s6 and possibly traces of s4 a sulfur oxidation state vs s concentration analytical line transect across hydrothermally altered apatite from the mina carmen iron oxide apatite ioa deposit chile demonstrates that apatite can become enriched in s4 relative to s6 indicating metasomatic overprinting via a so2 bearing fluid or vapor phase this xanes study demonstrates that as the formula 4 increases from fqm to fmq 1 2 to fmq 3 the oxidation state of s in igneous apatite changes from s2 dominant to s6 s4 to s6 s4 furthermore these results suggest that spectroscopic studies of igneous apatite have potential to trace the oxidation state of s in magmas the presence of three s oxidations states in apatite may in part explain the non henrian partitioning of s between apatite and melt our study reveals the potential to use the s signature of apatite to elucidate both oxygen and sulfur fugacity in magmatic and hydrothermal systems 1 embed mml math 1 gif 2 embed mml math 2 gif 3 embed mml math 3 gif 4 embed mml math 4 gif

marine magnetic anomalies evidence that tiny wiggles represent short period geomagnetic polarity intervals since the 1960 s the geomagnetic polarity time scale which is based on marine magnetic anomalies has become fundamentally important in geochronology despite the importance of marine magnetic anomaly records there has been longstanding uncertainty about the meaning of the smallest anomalies observed in these records small amplitude short wavelength anomalies are frequently observed in marine magnetic anomaly records across fast spreading oceanic crust s 50 mm yr the origin of these small scale anomalies referred to as tiny wiggles has remained controversial over the last 30 years tiny wiggles have been interpreted to represent either short period polarity intervals or largescale fluctuations in the ancient field intensity we present palaeomagnetic evidence from a sedimentary record from the north pacific ocean which demonstrates that two short but clearly resolvable polarity zones in addition to a probable geomagnetic excursion occur within chron c5n 2n 9 92 10 95 ma where three tiny wiggles have been reported on marine magnetic anomaly profiles relative palaeointensity data indicate that the field collapsed prior to and during the reversals and during the excursion but that it recovered to higher field intensities within the polarity intervals before collapsing to low values at the succeeding polarity transition this indicates that some tiny wiggles represent real short period geomagnetic polarity intervals while others may represent geomagnetic excursions the existence of such short polarity intervals confirms the predictions of statistical analyses of geomagnetic reversal frequency and indicates that tiny wiggles represent the maximum resolution of geomagnetic polarity intervals in marine magnetic anomaly records fl 2000 elsevier science b v all rights reserved

global changes in intensity of the earth s magnetic field during the past 800kyr recent advances in palaeomagnetic and dating techniques have led to increasingly precise records of the relative intensity of the earth s past magnetic field at numerous field sites the compilation and analysis of these records can provide important constraints on changes in global magnetic field intensity and therefore on the earth s geodynamo itself a previous compilation for the past 200 kyr integrated 17 marine records into a composite curve1 with the geomagnetic origin of the signal supported by an independent analysis of 10be production made on different cores2 the persistence of long term features in the earth s magnetic intensity or the existence of long term periodic changes cannot however be resolved in this relatively short time span here we present the integration of 33 records of relative palaeointensity3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 into a composite curve spanning the past 800 kyr we find that the intensity of the earth s dipole field has experienced large amplitude variations over this time period with pronounced intensity minima coinciding with known excursions in field direction reflecting the emergence of non dipole components no stable periodicity was found in our composite record and therefore our data set does not support the hypothesis that the earth s orbital parameters have a direct and strong influence on the geodynamo

ultrahigh resolution marine magnetic anomaly profiles a record of continuous paleointensity variations a distinctive pattern of small scale marine magnetic anomalies 25 100 nt amplitude 8 25 km wavelength tiny wiggles is superimposed on the more generally recognized seafloor spreading pattern between anomalies 24 and 27 in the indian ocean by normalizing and stacking multiple profiles it is demonstrated that this pattern of tiny wiggles is a high resolution recording of paleodipole field behavior between chrons c24 and c27 the pattern of tiny wiggles between anomalies 26 and 27 is compared to an ultrafast spreading 82 mm yr half rate profile from the southeast pacific where a similar signal is observed confirming the paleodipole field origin of the anomalies two basic models are considered in which the tiny wiggles are attributed either to short polarity intervals or to paleointensity fluctuations we conclude that tiny wiggles are most likely caused by paleointensity fluctuations of the dipole field and are a ubiquitous background signal to most fast spreading magnetic profiles the implications of this study are that 1 tiny wiggles may provide information on the temporal evolution of the geomagnetic dynamo 2 the small scale anomalies observed in the jurassic quiet zones may be due to paleointensity fluctuations 3 tiny wiggles are potential time markers in large regions of uniform crustal polarity such as the cretaceous quiet zones and 4 much of the variance in anomaly profiles normally attributed to crustal eraplacement processes particularly at fast and ultrafast spreading rates is actually due to intensity variations in the paleomagnetic field

the implications of long lived asymmetry of remanent magnetization across the north pacific fracture zones article i nfo large marine magnetic anomalies accompany the pacific fracture zones fzs for thousands of kilometers although the origin of these anomalies is poorly understood their underlying magnetization contrasts should reflect the temporal record of crustal accretion as well as geomagnetic field variations here we present an analysis of archival and newly collected magnetic anomaly profiles measured across three fzs from the north pacific cretaceous quiet zone 120 6 to 83 ma that are characterized by a remarkably uniform shape forward and inverse modeling indicate that these anomalies arise from remanent magnetization with enhanced remanence located on one side of each fz along the entire studied area a comparison of geochemical and magnetic data from active ridge discontinuities and transform faults suggests that elevated iron content near segment ends is likely responsible for the observed anomalies in the cretaceous quiet zone as well a more complex magnetization setting is observed where the fzs contain multiple faults there the simple model of one sided enhancements is only partly valid comparison between 3d forward modeling of the quiet zone magnetization and the calculated magnetization contrasts found across the pioneer and pau fzs suggests that the intensity of the geomagnetic field during the cretaceous superchron had less than 50 percent variability about its average value no major trends in the strength of the geomagnetic field during the superchron are observed the presence of long duration 30 m y zones of enhanced magnetization along the young old sides of the pioneer pau fzs both left stepping requires some long lived asymmetry in crustal construction processes near ridge transform intersections although the underlying mechanism that controls this long lived asymmetry remains unclear absolute plate motions might explain this asymmetry shorter period few m y variations in the amplitudes of the enhancements probably result from oscillations in crustal construction

stability and abundance of the trisulfur radical ion s 3 in hydrothermal fluids the interpretation of sulfur behavior in geological fluids and melts is based on a long standing paradigm that sulfate sulfide and sulfur dioxide are the major sulfur compounds this paradigm was recently challenged by the discovery of the trisulfur ion s 3 in aqueous s bearing fluids from laboratory experiments at elevated temperatures however the stability and abundance of this potentially important sulfur species remain insufficiently quantified at hydrothermal conditions here we used in situ raman spectroscopy on model thiosulfate sulfide and sulfate aqueous solutions across a wide range of sulfur concentration 0 5 10 0 wt acidity ph 3 8 temperature 200 500 degrees c and pressure 15 1500 bar to identify the different sulfur species and determine their concentrations results show that in the low density 0 2 g cm 3 vapor phase h2s is the only detectable sulfur form by contrast in the denser liquid and supercritical fluid phase together with sulfide and sulfate the trisulfur radical ion s 3 is a ubiquitous and thermodynamically stable species from 200 degrees c to at least 500 degrees c in addition the disulfur radical ion s is detected at 450 500 degrees c in most solutions and polymeric molecular sulfur with a maximum abundance around 300 degrees c in s rich solutions these results combined with revised literature data allow the thermodynamic properties of s 3 to be constrained enabling quantitative predictions of its abundance over a wide temperature and pressure range of crustal fluids these predictions suggest that s 3 may account for up to 10 of total dissolved sulfur s tot at 300 500 degrees c in fluids from arc related magmatic hydrothermal systems and more than 50 s tot at 600 700 degrees c in s rich fluids produced via prograde metamorphism of pyrite bearing rocks the trisulfur ion may favor the mobility of sulfur itself and associated metals au cu pt mo in geological fluids over a large range of depth and provide the source of these elements for orogenic au and porphyry epithermal cu au mo deposits furthermore the ubiquity of s 3 in aqueous sulfate sulfide systems offers new interpretations of the kinetics and mechanisms of sulfur redox reactions at elevated temperatures and associated mass dependent and mass independent fractionation of sulfur isotopes

sulfur bearing magmatic accessory minerals almost all magmas contain sulfide or sulfate bearing phases in most natural samples the sulfur bearing phase is a sulfide which typically is pyrrhotite fe1 x s or pyrite fes2 although chalcopyrite cufes2 pentlandite fe ni 9s8 sphalerite zns or molybdenite mos2 may be present as well sulfate minerals are rare at magmatic conditions and anhydrite caso4 is the most common other magmatic so4 bearing minerals include the sodalite group minerals hauyne simplified formula na ca 4 8 al6si6 o s 24 so4 cl 1 2 scapolite minerals silvialite ca na 4al6si6o24 so4 co3 and s bearing apatite ca5 po4 3 f cl oh barite baso4 has been mentioned in rare cases marchev 1991 sulfur bearing minerals usually constitute a negligible fraction of the mineral assemblage in magmatic rocks and thus can be classified as accessory minerals the crystallization of sulfide sulfate and s bearing minerals strongly depends on melt composition temperature and pressure and the s speciation in melt which in turn is strongly dependent on the prevailing oxygen fugacity baker and moretti 2011 this volume wilke et al 2011 this volume irrespectively of the low abundance of s bearing minerals the evolution of sulfur in magmas may be evaluated from the occurrence of sulfides and sulfates these minerals are critical for estimating the activity of various sulfur bearing species in magmas and can be used to constrain the oxygen fugacity and the s concentration in the melt e g pre eruptive sulfur concentration in melts the presence of either sulfide or sulfate in silicate melt indicates that the predominant dissolved sulfur species in the melt are s2 or s6 respectively typically one of these species is dominant but there are conditions where s2

history of the cretaceous osbourn spreading center the osbourn trough is a fossil spreading center that rifted apart the manihiki and hikurangi plateaus during cretaceous time previous models of the osbourn spreading center are based on data collected near the trough axis and therefore only constrain the history of the osbourn spreading center during the last few ma of spreading our data set includes multibeam data collected northward to the manihiki plateau allowing us to examine seafloor morphology created during the entire active period of the osbourn spreading center as well as several additional multibeam data sets that provide the opportunity to examine the relationship between the osbourn paleospreading center and the cretaceous pacific phoenix ridge the axial gravity of the trough is similar to the gravity found at other extinct slow intermediate spreading rate ridges magnetic field measurements indicate that spreading at the trough ceased during chron c34 abyssal hill trends indicate that spreading during the early history of the osbourn spreading center occurred at 15 20 the east west component of this spreading explains the modern east west offset of the manihiki and hikurangi plateaus spreading rotated to 2 5 prior to extinction abyssal hill rms amplitudes show that a decrease in spreading rate from 7 cm yr to 2 6 cm yr full spreading rate occurred 2 6 ma prior to ridge extinction our data analysis is unable to determine the exact spreading rate of the osbourn spreading center prior to the slowing event the temporal constraints provided by our data show that the osbourn spreading center ceased spreading prior to 87 ma or 93 ma depending on whether the manihiki and hikurangi plateaus rifted at 115 ma or 121 ma our model resolves the conflict between regional models of osbourn spreading with models based on trough characteristics by showing that spreading at the osbourn spreading center was decoupled from pacific phoenix spreading

geomagnetic field variability during the cretaceous normal superchron during the cretaceous normal superchron 121 83 million years ago the polarity of the earth s geomagnetic field remained stable for an unusually long time deep tow magnetic data suggest that despite the stability of the polarity the field varied greatly throughout the interval

fluctuations of the paleomagnetic field during chron c5 as recorded in near bottom marine magnetic anomaly data near bottom magnetic data contain information on paleomagnetic field fluctuations during chron c5 as observed in both the north and south pacific the north pacific data include 12 survey lines collected with a spatial separation of up to 120 km and the south pacific data consist of a single long line collected on the west flank of the east pacific rise epr at 19 s the north pacific magnetic profiles reveal a pattern of linear short wavelength 2 to 5 km anomalies tiny wiggles that are highly correlated over the shortest 3 8 km to longest 120 km separations in the survey magnetic inversions incorporating basement topography show that these anomalies are not caused by the small topographic relief the character of the near bottom magnetic profile from anomaly 5 on the west flank of the epr formed at a spreading rate more than twice that of the north pacific displays a remarkable similarity to the individual and stacked lines from the north pacific survey area over distances corresponding to 1 m y 19 lows in the magnetic anomaly profile can be correlated between the north and south pacific lines modeling the lows as due to short polarity events suggests that they may be caused by rapid swings of the magnetic field between normal and reversed polarities with little or no time in the reversed state owing to the implausibly high number of reversals required to account for these anomalies and the lack of any time in the reversed state we conclude that the near bottom signal is primarily a record of paleointensity fluctuations during chron c5 spectral analysis of the north pacific near bottom lines shows that the signal is equivalent to a paleointensity curve with a temporal resolution of 40 to 60 kyr while measurements of the smallest separations of correctable dips in the field suggest a temporal resolution of 36 kyr

astronomical forcing in upper miocene continental sequences implications for the geomagnetic polarity time scale abstract we present an integrated stratigraphic study of cyclically bedded distal alluvial fan to lacustrine deposits in the late miocene continental sections of cascante and canizar teruel basin ne spain the cyclostratigraphic analysis reveals that different scales of sedimentary cyclicity are present with a thickness ratio of about 1 2 5 spectral analysis of colour records in the depth domain indicates the presence of a significant peak at 2 2 m which corresponds to the average thickness of the basic mudstone carbonate cycle other peaks correspond to the large scale cycle which consists of clusters of five basic cycles and to a cycle twice the average thickness of the basic cycle magnetostratigraphic results in combination with small mammal biostratigraphy indicate that the three normal polarity intervals recorded in our sections correspond to c5n 2n c5n 1n and c4ar 2n assigning the ages of ck95 to the polarity reversals implies significant changes in sedimentation rate which is not in agreement with the regularity of the sedimentary cyclicity thus spectral analysis of high resolution colour records in the time domain only produce a spectrum that is consistent with milankovitch climate forcing if several of the assigned age tie points are excluded this indicates that the sedimentary cyclicity in these sections is related to astronomical variations in climate and that the ages of reversal boundaries are in error hence we have calculated the astronomical durations for c4ar 2n 87 ky c4ar 3r 54 ky c5n 1n 141 ky and c5n 1r 33 ky which indeed show significant discrepancies with ck95 the duration pattern of our polarity intervals is confirmed by seafloor anomaly profiles and magnetostratigraphic records of deep sea cores consequently this study demonstrates that astronomically forcing in continental sequences can be a powerful tool to improve the fundamental dating of the geological record

recognition of cryptochron in the polarity subchron c3ar palaeomagnetic results of the late miocene lava sequence from noma peninsula kyushu island japan

the effect of melt composition and oxygen fugacity on manganese partitioning between apatite and silicate melt abstract oxygen fugacity and melt composition are both known to have a strong influence on the partitioning of trace elements between coexisting minerals and melt previous work has suggested that mn partitioning between apatite and silicate melt may be strongly affected by oxygen fugacity and could therefore act as an oxybarometer here we present a new study on the partitioning of mn between apatite and melt at high temperature 1400 1250 c and 1 gpa pressure for various melt compositions and oxygen fugacities nno 4 7 to nno 10 we find that there is no demonstrable variation in the partition coefficient for mn between apatite and silicate melt dmnap m across the range of fo2 conditions studied here instead we find that dmnap m varies significantly with melt composition and that in particular the proportion of non bridging oxygens strongly influences partitioning of mn between apatite and melt we propose that variations in the mn content of natural apatite previously thought to reflect variations in fo2 are instead related to the degree of melt polymerisation these findings are consistent with the results of mn k edge xanes spectroscopy which demonstrate that mn in coexisting apatite and silicate glass is present predominantly as mn2 regardless of fo2 furthermore xanes spectra from a series of silicate glasses synthesised at various oxygen fugacities demonstrate that mn2 is the predominant species and that the average mn oxidation state does not vary over a wide range of fo2 t conditions

central anomaly magnetization high constraints on the volcanic construction and architecture of seismic layer 2a at a fast spreading mid ocean ridge the epr at 9 30 50 n abstract the central anomaly magnetization high camh is a zone of high crustal magnetization centered on the axis of the east pacific rise epr and many other segments of the global mid ocean ridge mor the camh is thought to reflect the presence of recently emplaced and highly magnetic lavas forward models show that the complicated character of the near bottom camh can be successfully reproduced by the convolution of a lava deposition distribution with a lava magnetization function that describes the variation in lava magnetization intensity with age this lava magnetization function is the product of geomagnetic paleofield intensity which has increased by a factor of 2 over the last 40 kyr and low temperature alteration which decreases the remanence of lava with exposure to seawater the success of the forward modeling justifies the inverse approach deconvolution of the magnetic data for lava distribution and integration of that distribution for magnetic layer thickness this approach is tested on two near bottom magnetic profiles al2767 and al2771 collected using alvin across the epr axis at 9 31 n and 9 50 n our analysis of these data produces an estimate of the relative thickness of the magnetic lava layer which is remarkably consistent with existing multichannel estimates of layer 2a thickness from lines cdp31 and cdp27 the similarity between magnetic layer and seismic layer 2a at the 9 10 n segment of the epr crest provides independent support to the notion that seismic layer 2a in young oceanic crust represents the highly magnetic lava layer and that the velocity gradient at the base of layer 2a is related to the increasing number of higher velocity dikes with depth in the lava dike transition zone the near bottom magnetic anomaly character of the camh is a powerful indicator of the emplacement history of upper crust at mors which allows prediction of the relative thickness and architecture of the extrusive lavas independent of other constraints

evolution of subglacial bedforms along a paleo ice stream antarctic peninsula continental shelf geophysical data from the antarctic peninsula continental shelf reveal streamlined subglacial bedforms in a cross shelf trough bedforms exhibit progressive elongation with distance along the trough and record flow of a paleo ice stream from the antarctic peninsula ice sheet during the last glacial maximum downflow evolution of the bedforms indicates increasing flow velocities as the ice stream traversed the shelf this in turn is related to a transition from crystalline bedrock on the inner shelf to a soft sedimentary substrate on the outer shelf although streaming flow operated across both substrates the highest flow velocities occurred over the soft bed spatial variation in the inferred nature of fast flow from sliding to subglacial sediment deformation and or ploughing was also lithologically controlled these data highlight the control of subglacial geology on ice stream dynamics in the geological record and demonstrate a direct relationship between the formation of streamlined subglacial bedforms and paleo ice streams

a new geomagnetic polarity time scale for the late cretaceous and cenozoic we have constructed a magnetic polarity time scale for the late cretaceous and cenozoic based on an analysis of marine magnetic profiles from the world s ocean basins this is the first time since heirtzler et al 1968 published their time scale that the relative widths of the magnetic polarity intervals for the entire late cretaceous and cenozoic have been systematically determined from magnetic profiles a composite geomagnetic polarity sequence was derived based primarily on data from the south atlantic anomaly spacings in the south atlantic were constrained by a combination of finite rotation poles and averages of stacked profiles fine scale information was derived from magnetic profiles on faster spreading ridges in the pacific and indian oceans and inserted into the south ariantic sequence based on the assumption that spreading rates in the south atlantic were smoothly varying but not necessarily constant a time scale was generated by using a spline function to fit a set of nine age calibration points

deep tow magnetic anomaly study of the pacific jurassic quiet zone and implications for the geomagnetic polarity reversal timescale and geomagnetic field behavior 1 the jurassic quiet zone jqz is a region of low amplitude magnetic anomalies whose distinctive character may be related to geomagnetic field behavior we collected deep tow magnetic profiles in pigafetta basin western pacific where previous deep tow data partially covered the jqz sequence our goals were to extend the survey through the jqz examine anomaly correlations and refine a preliminary geomagnetic polarity timescale gpts model we collected a series of closely spaced profiles over anomaly m34 and ocean drilling program hole 801c to examine anomaly correlation in detail one profile in between previous profiles and two long profiles extending the survey deeper into the jqz anomaly features can be readily correlated except in a region of low amplitude short wavelength anomalies in the middle of the survey area low amplitude zone or laz the small multiprofile surveys demonstrate anomaly linearity implying that surrounding anomalies are also linear and likely result from crustal recording of geomagnetic field changes we constructed a gpts model assuming that most anomalies result from polarity reversals the polarity timescale is similar to the polarity sequences from previous studies but its global significance is uncertain because of problems correlating anomalies in the laz and the ambiguous nature of the small jqz anomalies overall anomaly amplitude decreases with age into the laz and then increases again implying low geomagnetic field strength perhaps related to a rapidly reversing field other factors that may contribute to the laz are interference of anomalies over narrow crustal polarity zones and poorly understood local tectonic complexities

sulfur partition coefficient between apatite and rhyolite the role of bulk s content experimental studies have been performed to constrain sulfur behavior during apatite crystallization and to determine sulfur partition coefficient between apatite and melt kd s apatite melt at oxidizing conditions crystallization experiments have been conducted with a hydrous rhyolitic melt and different bulk sulfur contents 0 15 to 2 wt s at 900 and 1 000 c 200 mpa and log f rm o 2 nno 3 6 the sulfur content in the glass increases with increasing amount of added s anhydrite crystallizes for s added 0 75 wt 0 10 and 0 13 wt so3 in glass at 900 and 1 000 c respectively the amount of anhydrite increases and the amount of apatite decreases with increasing amount of added sulfur the sulfur exchange reaction in apatite is influenced by the bulk composition of the melt e g p content however changing melt composition has only little effect on kd s apatite melt for the investigated rhyolitic composition the kd s apatite melt does not depend directly on temperature decreases from 14 2 to 2 7 with increasing s content in glass from so3 0 03 to 0 19 wt respectively and can be predicted from the following equation ln kd 0 0025 s in melt in ppm 2 9178 the combination of experimental data obtained for rhyolitic and andesitic melts reveals that the sulfur partition coefficient tends toward a value of 2 for high sulfur content in the glass 0 2 wt so3 using s in apatite as proxy for determining s content in melt is promising but additional experimental data are needed to clarify the individual effects of t f rm s 2 and p and ca content in the melt on s partitioning

sulfur isotopic zoning in apatite crystals a new record of dynamic sulfur behavior in magmas abstract the mobility and geochemical behavior of sulfur in magmas is complex due to its multi phase solid immiscible liquid gaseous dissolved ions and multi valent from s2 to s6 nature sulfur behavior is closely linked with the evolution of oxygen fugacity fo2 in magmas the record of fo2 evolution is often enigmatic to extract from rock records particularly for intrusive systems we apply a novel method of measuring s isotopic ratios in zoned apatite crystals that we interpret as a record of open system magmatic processes we interrogate the s concentration and isotopic variations preserved in multiple apatite crystals from single hand specimens from the cadiz valley batholith ca via electron microprobe and ion microprobe isotopic variations in single apatite crystals ranged from 0 to 3 8 34s and total variation within a single hand sample was 6 1 34s high s concentration cores yielded high isotopic ratios while low s concentration rims yielded low isotopic ratios we discuss a range of possible natural scenarios and favor an explanation of a combination of magma mixing and open system ascent driven degassing under moderately reduced conditions fo2 at or below nno 1 although the synchronous crystallization of apatite and anhydrite is also a viable scenario our conclusions have implications for the coupled s and fo2 evolution of granitic plutons and suggest that in situ apatite s isotopic measurements could be a powerful new tool for evaluating redox and s systematics in magmatic systems

asymmetry in growth and decay of the geomagnetic dipole revealed in seafloor magnetization abstract geomagnetic intensity fluctuations provide important constraints on time scales associated with dynamical processes in the outer core padm2m is a reconstructed time series of the 0 2 ma axial dipole moment adm after smoothing to reject high frequency variations padm2m s average growth rate is larger than its decay rate the observed asymmetry in rates of change is compatible with longer term diffusive decay of the adm balanced by advective growth on shorter time scales and provides a potentially useful diagnostic for evaluating numerical geodynamo simulations we re analyze the padm2m record using improved low pass filtering to identify asymmetry and quantify its uncertainty via bootstrap methods before applying the new methodology to other kinds of records asymmetry in distribution of axial dipole moment derivatives is quantified using the geomagnetic skewness coefficient s g a positive value indicates the distribution has a longer positive tail and the average growth rate is greater than the average decay rate the original asymmetry noted by ziegler and constable 2011 is significant and does not depend on the specifics of the analysis a long term record of geomagnetic intensity should also be preserved in the thermoremanent magnetization of oceanic crust recovered by inversion of stacked profiles of marine magnetic anomalies these provide an independent means of verifying the asymmetry seen in padm2m we examine three near bottom surveys a 0 to 780 ka record from the east pacific rise at 19 s a 0 to 5 2 ma record from the pacific antarctic ridge at 51 s and a chron c4ar c5r 9 3 11 2 ma record from the ne pacific all three records show an asymmetry similar in sense to padm2m with geomagnetic skewness coefficients s g 0 results from padm2m and c4ar c5r are most robust reflecting the higher quality of these geomagnetic records our results confirm that marine magnetic anomalies can carry a record of the asymmetric geomagnetic field behavior first found for 0 2 ma in padm2m and show that it was also present during the earlier time interval from 9 3 11 2 ma

variations in oceanic crustal magnetization systematic changes in the last 160 million years the amplitudes of marine magnetic anomalies show a clear worldwide pattern of systematic variation when viewed as a function of age globally the amplitudes decrease with age over the first 20 to 30 million years a phenomenon that has been attributed to the low temperature oxidation of the magnetic mineral titanomagnetite in the upper extrusive rocks in oceanic crust older than 40 million years however the amplitudes of the magnetic anomalies increase with increasing age and remain at elevated levels for the entire period between 80 and 160 million years in order to examine the processes responsible for this elevated crustal magnetization in older oceanic crust we compiled all of the existing rock magnetic data from relevant deep sea drilling project and ocean drilling program sites that sampled extrusive rocks from normal ocean crust this compilation shows that the laboratory measurements of the magnetization of the upper basement rocks closely reflect that of the anomaly amplitudes both in the decrease due to oxidation in the first 20 to 30 million years and in the increase in magnetization in older 80 million years crust this positive correlation between anomaly amplitudes and upper crustal magnetization supports the argument that a major source of the marine magnetic anomalies is in the upper extrusive volcanic rocks further the curie temperature data show that oxidation of the magnetic minerals in oceanic basalts occurs largely within the first 30 million years and does not increase significantly beyond that point finally the strong positive correlation between the intensity of the magnetization of the drill core samples and the saturation magnetization argues that the higher magnetic anomaly amplitudes over older crust are related to a change in an intrinsic property of the upper crust specifically we propose that the observed elevated magnetization of older ocean crust is due primarily to an increased abundance of magnetic feti oxides in the older crustal rocks this large increase in abundance of the feti oxides may be related to a systematic increase in bulk feti content in older tholeiitic basalts or more likely is due to a difference in the partitioning of the iron and titanium content between the silicate and oxide phases

the nature of a cryptochron from a paleomagnetic study of chron c4r 2r recorded in sediments off the antarctic peninsula abstract the magnetostratigraphy from ocean drilling program odp site 1095 off the pacific margin of the antarctic peninsula contains an extra normal polarity event that occurs near the base of chron 4r 2r 8 072 8 699 ma which we interpret to be cryptochron c4r 2r 1 owing to the relatively high sedimentation rates about 90 m m y this event is particularly well recorded at the site spanning 4 99 m of the stratigraphic section this allows the characteristics of the cryptochron to be investigated in greater detail than possible from marine magnetic anomalies where it was originally identified or from other sedimentary sections in which it has been recorded at much lower resolution our observations suggest that the cryptochron is a full geomagnetic reversal in which both the direction and paleointensity attain levels similar to that of other normal polarity chrons at the site based on its position within chron 4r 2r the cryptochron started at 8 622 ma and terminated 56 k y later at 8 566 ma at the transition zones bounding the cryptochron the paleointensity collapses to near zero but recovers within a few thousand years our results as well as paleomagnetic observations from other thick sedimentary units indicate that cryptochrons are not always purely paleointensity variations instead they are a record of short term geomagnetic variability that includes short geomagnetic reversals excursions intervals of high paleosecular variation and paleointensity lows all of which are part of a vector field that varies in both strength and direction over time

pre eruptive conditions of the huerto andesite fish canyon system san juan volcanic field colorado influence of volatiles c o h s on phase equilibria and mineral composition crystallization experiments at 400 mpa oxidized condition logfo2 nno 1 where nno is nickel nickel oxide buffer and over a range of temperatures 850 950 c and fluid composition xh2oin 0 3 1 have been carried out to constrain the storage conditions of the sulphur rich magma of the huerto andesite an anhydrite pyrrhotite and s rich apatite bearing post fish canyon tuff mafic lava the results are used to evaluate the role of fluids released from the crystallization of magmas such as the huerto andesite on the remobilization of the largely crystallized dacitic fish canyon magma body experiments were performed using the natural andesitic bulk composition with and without added sulphur the presence of sulphur slightly affects the phase equilibria by changing the phase proportions stability fields of plagioclase pyroxenes and ilmenite and also affects the plagioclase composition phase equilibria and mineral composition data indicate that the magma may have contained 4 5 wt water in the melt and that the pre eruptive temperature was 875 25 c assuming that the magma was in equilibrium with a fluid phase the co2 concentration of the melt is estimated to be in the range 2000 4000 ppm at 400 mpa before eruption the andesite had an oxidation state very close to or slightly within the co stability field of anhydrite pyrrhotite at nno 1 1 at these conditions the sulphur content in the melt is 500 ppm assuming open system degassing resulting from continuing crystallization at depth most of the co2 dissolved in the andesitic melt should be released after the crystallization of 10 vol of the magma corresponding to a cooling from 875 to 825 850 c thus the fluids released owing to crystallization processes should be mainly composed of water at temperatures below 825 c

the geochemistry of apatite from the los colorados iron oxide apatite deposit chile implications for ore genesis apatite grains from the los colorados iron oxide apatite ioa deposit the largest ioa deposit in the chilean iron belt cib exhibit significant intracrystalline spatial variability with respect to the concentrations of f cl and oh and trace elements statistical interrogation of the compositional data indicates that individual apatite grains contain spatially discrete f rich and cl rich domains the chemical composition of the f rich domains is consistent with apatite growth from silicate melts whereas the chemical composition of the cl rich domains is consistent with apatite growth from a magmatic hydrothermal fluid that cooled as it percolated outward from the los colorados fault the structural control for emplacement of the ore body into the surrounding brecciated diorite and andesite host rocks apatite in the deposit is intimately intergrown with magnetite and actinolite for which trace element fe h and o stable isotope data indicate a combined magmatic magmatic hydrothermal genesis for the deposit the compositional data for apatite are consistent with a genetic model wherein f rich apatite cores crystallized with magnetite from silicate melt followed by exsolution of a magmatic hydrothermal fluid during decompression of the parent magma experimental studies demonstrate that magmatic hydrothermal volatile phase bubbles preferentially nucleate and grow on the surfaces of apatite and magnetite microlites during decompression of a magma body continued degassing of the melt results in the volatile phase sweeping up apatite and magnetite microlites and forming a magnetite apatite fluid suspension that is buoyant in the magma chamber and ascends from the source magma along faults during regional extension halite saturated fluid inclusions in magnetite which is paragenetically equivalent to apatite at los colorados indicate that the magmatic hydrothermal fluid was a brine which allows this fluid to efficiently scavenge cl p rare earth elements and other fluid compatible elements from the silicate melt during ascent the xcl xf ratio of apatite increases as it grows from the evolving cl rich magmatic hydrothermal fluid during decompression and cooling

cryptic metasomatism during late stage lunar magmatism implicated by sulfur in apatite the use of volatile bearing h 2 o cl f c s minerals as proxies for constraining the volatile content of lunar magmas remains controversial in this study we apply published f cl s signature trends in apatite from lunar mare basalts new experimentally determined apatitemelt partition coefficients for s and thermodynamic modeling to demonstrate unequivocally that fractional crystallization of lunar silicate melt could not have produced the observed s signatures of lunar apatite instead metasomatic cryptic alteration by a s cl bearing f poor volatile phase plausibly explains the volatile element zoning in lunar apatite consistent with some lunar magmas being more volatile rich than previously suggested

magnetostratigraphy of the vallesian late miocene in the valles penedes basin northeast spain the magnetostratigraphic analysis of the late miocene continental deposits from the valles penedes basin combined with its well documented fossil mammal record has provided a well resolved chronology for the upper basin infill the study is based on the biostratigraphic and magnetostratigraphic cross correlation of 12 sections throughout the alluvial sequences in the western valles area the biostratigraphic framework consists of 21 mammal localities corresponding to the mammal neogene mn9 and mn10 units the composite magnetic polarity sequence is based on 400 paleomagnetic sites correlation with the geomagnetic polarity time scale gpts has led to an absolute dating of the faunal events and a precise chronostratigraphy of the vallesian mammal stage in its type area the hipparion first appearance datum fad at the mn8mn9 boundary is dated at 11 1 ma in the valles penedes basin this age compared to other radiometrically dated localities in europe north africa and turkey is consistent with an isochronous dispersal of this equid through the mediterranean region a possible isochrony at a larger geographical scale old world mesogea must await more reliable ages of the hipparion fad in asia and africa the cricetulodon fad mn9amn9b boundary is dated to 10 4 ma in chron c5n the progonomys fad mn9mn10 boundary corresponding to the intra vallesian faunal crisis is dated at 9 7 ma c4ar 3r the vallesian spans 2 4 myr from 11 1 ma c5r 1n to 8 7 ma c4an and correlates to the early tortonian

preservation of primordial signatures of water in highly shocked ancient lunar rocks abstract spurred by the discovery of water in lunar volcanic glasses about a decade ago the accessory mineral apatite became the primary target to investigate the abundance and source of lunar water this is due to its ability to contain significant amounts of oh in its structure along with the widespread presence of apatite in lunar rocks there is a general understanding that crustal cumulate rocks of the lunar magnesian mg suite are better candidates for recording the original isotopic compositions of volatile elements in their parental melts compared to eruptive rocks such as mare basalts consequently water bearing minerals in mg suite rocks are thought to be ideal candidates for discerning the primary hydrogen isotopic composition of water in the lunar interior mg suite rocks and most other apollo samples that were collected at the lunar surface display variable degrees of shock deformation in this study we have investigated seven apollo 17 mg suite samples that include troctolite gabbro and norite lithologies in order to understand if shock processes affected the water abundances and or h isotopic composition of apatite the measured water contents in apatite grains range from 31 to 964 ppm with associated d values varying between 535 134 and 147 194 2 considering the full dataset there appears to be no correlation between h2o and d of apatite and the level of shock each apatite grain has experienced however the lowest d was recorded by individual water poor 100 ppm h2o regardless of the complexity of the shock induced nanostructures there appears to be no evidence of water loss or alteration in their d the weighted average d value of 24 such water rich apatites is 192 71 and of all 36 analyzed spots is 209 47 indistinguishable from that of other kreepy lunar lithologies or the earth s deep mantle despite experiencing variable degrees of shock deformation at a later stage in lunar history water rich apatite in some of the earliest formed lunar crustal material appears to retain the original isotopic signature of h in the moon

cymrite new occurrence and stability the rare mineral cymrite baal2si2o8 h2o was discovered in nevada in a cambrian bedded barite sequence that exhibits low grade metamorphism the mineral occurs exclusively in thin bedded siliceous rock containing anhedral pyrite crystals up to 1 cm cymrite forms rectangular grains ca 40 m across distributed throughout the chalcedonic quartz matrix an sem image of one such blocky grain shows that it is filled by tiny aggregates instead of a single crystal of cymrite this cymrite may have replaced a pre existing rectangular mineral most likely barite the nevada occurrence of cymrite prompted a restudy of its stability relations conventional hydrothermal techniques were adopted in the experimental work with run durations up to 7 months the univariant curve for the dehydration reaction baa12si2o8 h2o baa12si2o8 h2o passes the following reversed brackets 300 315 c at 3 kbar 290 300 c at a 2 kbar 270 285 c at 1 kbar and 240 270 c at 0 5 kbar these results indicate that cymrite can be stable at much lower pressures than those previously reported the replacement of barite by cymrite was experimentally demonstrated with an alkaline solution as depicted by the reaction baso4 2oh a12o3 2sio2 baa12si2o8 h2o so 4 2

deuterium and 37 chlorine rich fluids on the surface of mars evidence from the enriched basaltic shergottite northwest africa 8657 abstract apatite a major hydrous mineral in martian meteorites has the potential to reveal history of volatiles on mars here we report the h and cl isotopic systematics of apatite from the enriched basaltic shergottite nwa 8657 to better understand the processes influencing volatiles at or near the martian sub surface the apatite in nwa 8657 has the highest reported d value up to 6509 of any phosphates in martian meteorites the positive correlation between the h2o contents 72 2251 ppm and d values 3965 6509 as well as between the cl contents 1 28 6 17 wt and 37cl values 1 7 4 0 in nwa 8657 apatite are consistent with mixing between volatiles derived from the martian mantle and meteoric water fluid the d and 37cl enrichment of apatite in nwa 8657 implies isotopic exchange with sub surficial fluids during post crystallization hydrothermal event s plain language summary mars was probably a wet and warm world during its earliest geological history supported by many lines of evidence present day mars is cold and arid although recent remote sensing data support the existence of subsurface glaciers and seasonal fluid activities at some locations in this study our aim was to use ground truth about the potential water rock interaction on or near surface environments on recent mars via measuring the h and cl isotopic compositions of apatite from a geologically young meteorite from mars the enriched basaltic shergottite nwa 8657 the petrography and mineral chemistry of apatite and other associated phases e g merrillite maskelynite mesostasis and pyroxene were documented prior to in situ h and cl isotopes analysis the water contents of apatite merrillite maskelynite mesostasis and pyroxene are positively correlated with the d values all of the datasets can be reconciled in terms of mixing between two endmembers a d poor 0 mantle reservoir and a d rich 5920 500 near surface underground water reservoir moreover apatite in nwa 8657 displays 37cl enriched 1 1 in average characteristics these signatures are consistent with d and 37cl rich fluid assisted isotopic exchange in recent near surface environment on mars

fidelity and time resolution of the magnetostratigraphic record in siwalik sediments high resolution study of a complete polarity transition and evidence for cryptochrons in a miocene fluviatile section summary the surai khola section in southwest nepal a 5000 m continuously exposed record of fluvial sedimentation since middle miocene was revisited for high resolution magnetostratigraphy in sequences with expected cryptochrons and reversals of the geomagnetic field polarity intervals with durations of a few tens of thousands of years are recorded as zones of stable palaeomagnetic directions polarity transitions are recorded as zones with complex demagnetization behaviour of specimens in the sedimentary column almost antiparallel palaeoremanence directions residing in diverent haematite phases in the same specimens could generally not be separated properly by thermal demagnetization divering demagnetization paths for neighbouring specimens during a reversal suggest that measured transitional directions are not true geomagnetic field directions but rather are generated by the superposition of variable amounts of at least two almost antiparallel components of magnetization accompanying studies of recent river sand deposits demonstrate that these sediments acquire a true depositional remanent magnetization drm with considerable inclination errors and scattered directions for individual specimens

geomagnetic polarity reversal model of deep tow profiles from the pacific jurassic quiet zone the jurassic magnetic quiet zone jqz contains magnetic lineations but their low amplitudes make correlation and interpretation difficult part of the problem is the separation of source and sensor for old deep ocean crust we increased anomaly amplitudes by collecting magnetic data along two deep tow profiles over western pacific jqz lineations a magnetic polarity reversal timescale was constructed by matching deep tow anomalies with a simple rectangular block magnetization model for oceanic crust the polarity sequence covers 11 m y 156 167 5 ma and contains 88 pre m29 polarity chrons extending to chron m41 a limitation of this model is its poor representation of the oldest anomalies upward continued to sea level on deep tow profiles these anomalies have both long and short wavelength components but only the latter are easily modeled on a datum close to the source an alternative polarity model was constructed to match the anomalies upward continued to sea level this model retains only 44 of the deep tow model polarity chrons because of short wavelength attenuation by upward continuation because of the inferred periods and magnetization contrasts we think many of the short wavelength anomalies represent paleofield intensity fluctuations in contrast polarity reversals have been documented by prior magnetostratigraphic work for the younger part of the timescale covered by our model thus our data may show a transition from a geomagnetic field behavior dominated by intensity fluctuations to one dominated by reversals

correlation of paleointensity variation records in the brunhes matuyama polarity transition interval abstract paleomagnetic records of the 0 78 ma brunhes matuyama b m polarity reversal interval described from high resolution 8 11 cm kyr deep sea sediments from western equatorial pacific odp sites 767b and 769a show two successive and marked decreases in paleointensity of about 15 kyr wavelength each and a shift from full reversed to fuli normal polarity directions in only 2 kyr associated with the uppermost paleointensity decrease in contrast transition records from deep sea sediments with lower sedimentation rates tend to show simpler paleointensity variation with more complex patterns of directional change for example the b m transition record described from deep sea sediment core v16 58 from the southern indian ocean we suggest that the twin 10 be peaks reported in v16 58 correspond to increased production of this cosmogenic isotope associated with the double paleointensity dips recorded in odp 767b 769a the low resolution v16 58 paleomagnetic record can be accounted for by modeling of the geomagnetic field history in odp 767b at a nominal recording rate of 1 cm kyr and using an exponential lock in process filter with a half fixing depth of 16 cm the double paleointensity dips become a single broad magnetization low and the change in paleomagnetic directions is offset by about 16 cm below the actual level of the polarity transition similar to what we infer for v16 58 the low resultant magnetizations will be especially prone to overprinting which if in the present field will tend to give an americas vgp path the main features of the 10 be profile will be unaffected by the remanence lock in process and consequently retain their usefulness for correlation as a proxy of geomagnetic dipole intensity variation

age and duration of the matuyama brunhes geomagnetic polarity reversal from 40ar39ar incremental heating analyses of lavas abstract constraints on the timing of geomagnetic polarity reversals have come mainly from k ar or more recently 40 ar 39 ar age determinations of lavas or their k rich phenocrysts that erupted prior or subsequent to particular geomagnetic events we have obtained 40 ar 39 ar isochron ages using incremental heating techniques on groundmass separates phenocryst poor whole rock samples or plagioclase from eight basaltic to andesitic lavas that erupted during the matuyama brunhes m b polarity transition at four geographically dispersed sites these eight lavas range from 784 6 7 1 ka to 770 8 5 2 ka 1 errors the weighted mean 778 7 1 9 ka gives a high precision age that is remarkably consistent with revised astronomical age estimates for the m b polarity transition 6 12 13 despite uncertainties in absolute calibration of 40 ar 39 ar ages relative to mineral standards used as neutron fluence monitors our age determinations are consistent with five other 40 ar 39 ar studies focused on the m b transition these results confirm that the earlier k ar based geomagnetic polarity time scale underestimated the age of the m b reversal by about 6 none of the eight isochron ages are distinguishable from one another at the 95 confidence level however we are tantalizingly close to testing for the duration of this reversal one lava at the base of a sequence of transitionally magnetized flows in chile and the uppermost lava in a similar sequence on maui are only just indistinguishable in age at the 95 confidence level and preserve different magnetic orientations we suggest that the 12 kyr difference in age represents an upper limit for the duration of the reversal and is similar to the period of low magnetic field intensity associated with records of the m b reversal from deep sea sediment cores together with the short duration 2 kyr of the directional reversal observed in several different marine sediment sections our data suggest that reversal of the field s direction could have occurred at slightly different times depending on the position of the recording site

petrogenesis and metallogenic potential of the wulanba granite southern great xing an range ne china constraints from whole rock and apatite geochemistry the wulanba granite consisting of biotite monzogranite and syenogranite is located in the southern part of the great xing an range ne china whole rock major and trace element geochemistry suggests the wulanba granite is a high k shoshonitic slightly peraluminous and highly differentiated i type granite the sr nd hf isotopes indicate that it originated from partial melting of juvenile crust derived from the depleted mantle with a minor input of old crust the relatively young t 2dm and t dm2 ages indicate it was most likely derived from a late neoproterozoic to early palaeozoic source we have demonstrated that the biotite monzogranite is the ore related intrusion of the haobugao zn fe mineralization based on the following geological geochronological and geochemical evidence 1 the chalcopyrite pyrite in the biotite monzogranite and the continuous mineralization of drill core zk2508 2 the consistence of the emplacement age of the biotite monzogranite 141 140 138 ma with the skarn mineralization age 142 ma and 3 the presence of rich ore forming elements fe zn cu in the biotite monzogranite and the similar pb compositions of the sulfides from the haobugao deposit and the biotite monzogranite compared to the barren syenogranite the fertile biotite monzogranite is more oxidized while the edges of the apatite grains in the biotite monzogranite are more oxidized than the centres the average f cl ratio of the fertile biotite monzogranite 123 45 is much higher than that of the barren syenogranite 73 98 we conclude that these differences reflect unique geochemical signatures and the geochemical composition of the apatite can be used to infer the economic potential of granites

kokchetavite a new potassium feldspar polymorph from the kokchetav ultrahigh pressure terrane kokchetavite a new polymorph of k feldspar kalsi3o8 has been identified as micrometer size inclusions in clinopyroxene and garnet in a garnet pyroxene rock from the kokchetav ultrahigh pressure terrane kazakhstan kokchetavite has a hexagonal structure with a 5 27 1 a c 7 82 1 a v 188 09 a 3 z 1 and is found to be associated with phengite a b cristobalite or quartz siliceous glass phlogopite titanite calcite zircon occurring as multi phase inclusions in clinopyroxene and garnet it is concluded that kokchetavite could not be an exsolution phase in host minerals instead it might be metastably precipitated from an infiltrated k rich melt during rock exhumation alternatively although less likely kokche tavite might be derived from dehydration of k cymrite which in turn was formed at high pressures in either case kokchetavite is a metastable polymorph of k feldspar

magnetostratigraphy of the miocene corque basin bolivia implications for the geodynamic evolution of the altiplano during the late tertiary a magnetostratigraphic study of a thick miocene continental red bed sequence totora formation located in the north central altiplano the corque syncline yielded quantitative determinations of the sedimentation rate and better constraints of the timing of the deformation within the altiplano paleomagnetic results obtained in 653 samples from a composite 4 5 km thick red bed sequence across the corque syncline indicate that most of the totora sequence was deposited in the middle miocene from 14 to 9 ma with high sedimentation rates up to 970 m myr from 12 to 9 ma the high rate of infill of the corque basin demonstrates active subsidence of the northern altiplano during the middle miocene deformation ceased in the northern altiplano domain only at the end of the miocene two major tuff beds within the sequence have been previously dated by 39 ar 40 ar of sanidine minerals marshall et al 1992 the excellent agreement between the magnetostratigraphic dating of the tuffs and the radiometric dating independently supports the age calibration of the geomagnetic reference timescale proposed by cande and kent 1995 the numerous paleomagnetic results obtained in middle miocene sediments within the corque syncline a structure which can be traced for more than 100 km along its axis demonstrate that the corque syncline rotated counterclockwise by 10 8 2 9 since 9 ma as a consequence of the internal deformation of the altiplano inclination flattening of 17 is recorded in these red bed sediments this result provides additional evidence that detrital magnetizations especially in red beds should be taken with caution when estimating paleolatitudes or long term variations of the geomagnetic field

factors controlling sulfur concentrations in volcanic apatite apatite crystals from two types of samples were analyzed by electron microprobe for 15 major and trace elements 1 apatite in h2o and s saturated experimental charges of the 1982 el chichon trachyandesite and 2 apatite in volcanic rocks erupted from 20 volcanoes the so3 contents of the experimental apatite increase with increasing oxygen fugacity from 0 04 wt in reduced charges buffered by fayalite magnetite quartz o2 fmq to 1 0 2 6 wt in oxidized charges buffered by manganosite hausmanite mnh or magnetite hematite mth the so3 contents of mnh and mth buffered apatite also generally increase with increasing pressure from 2 to 4 kbar and decreasing temperature from 950 to 800 8c the partition coefficient for so 3 between apatite and oxidized melt increases with decreasing temperature but appears to be independent of pressure apatites in volcanic rocks show a wide range of so3 contents 0 04 to 0 63 wt our sample set includes one group known to contain primary anhydrite and a second group inferred to have been free of primary anhydrite no systematic differences in apatite s contents are observed between these two groups our study was initiated to define the factors controlling s contents in apatite and to evaluate the hypothesis that high s contents in apatite could be characteristic of s rich anhydrite bearing magmas such as those erupted from el chichon in 1982 and pinatubo in 1991 this hypothesis is shown to be invalid probably chiefly a consequence of the slow intra crystalline diffusion that limits re equil ibration between early formed apatite and the evolving silicate melt contributing factors include early crystallization of most apatite over a relatively small temperature interval common late stage magmatic enrichment of s progressive oxidation during magmatic evolution and strong controls on s contents in apatite exerted by temperature and o2 pressure

astronomical ages for miocene polarity chrons c4ar c5r 9 3 11 2 ma and for three excursion chrons within c5n 2n abstract ocean drilling program odp site 1092 from the sub antarctic south atlantic produced a clear magnetic stratigraphy for the late miocene evans h f channell j e t upper miocene magnetic stratigraphy from odp site 1092 sub antarctic south atlantic recognition of cryptochrons in c5n geophys jour int 153 2003 483 496 three short intervals of reverse polarity were identified within the long 1 myr normal polarity subchron c5n 2n these excursion chrons were tentatively correlated to the three cryptochrons identified within subchron c5n 2n from ne pacific marine magnetic anomaly data new oxygen isotope data from odp site 1092 have allowed astronomical calibration of eight polarity chron boundaries in the interval between c4ar 1n and c5r 1n 9 3 11 2 ma as well as calibration of the three excursion chrons identified within c5n 2n the new polarity chron ages in the c4ar 1n c5r 1n interval differ from a current 2004 timescale by up to 48 kyr the astrochronology yields durations in the 3 4 kyr range for the three excursion chrons in c5n 2n the relative paleointensity record from site 1092 and hence the three excursion chrons can be correlated to the deep tow magnetic anomaly record from 19 s on the east pacific rise epr based on published correlation of the epr deep tow record with the sea surface magnetic anomaly stack from the ne pacific the three excursion chrons do not correspond to the cryptochrons recognized in the ne pacific stack

evidence for a new geomagnetic reversal from lava flows in idaho discussion of short polarity reversals in the brunhes and late matuyama polarity chrons k ar ages and paleomagnetic data for basalt samples from a new core hole site e at the idaho national engineering laboratory inel indicate that the age of the reversed polarity event recorded in snake river plain lavas is older than 465 50 ka 1000 years before present reported previously by champion et al 1981 nine basalt flows eight with normal polarity and one with reversed polarity were recognized in the site e core hole the flows above and below the reversed flow have ages of 491 80 ka and 580 93 ka respectively the inclination of the paleomagnetic field direction of the reversed flow at site e agrees with the inclination of reversed flows elsewhere at inel which have an age of 565 14 ka these reversed flows were previously thought to be correlative with the emperor event we suggest that this polarity event is an older event which we name the big lost reversed polarity subchronozone and subchron a review of data documenting short reversal records from volcanic and sedimentary rocks shows that there is evidence for eight polarity subchrons in the brunhes and two besides the jaramillo in the late matuyama these 10 short subchrons begin to indicate the many short events that cox 1968 hypothesized must exist if polarity interval lengths have a poisson distribution these events are true subchrons not excursions and may or may not have low associated paleointensities although low field strengths might explain why the reversal process aborts the mean sustained polarity interval length since late matuyama chron time is 90 000 years the similarity of this number with the 105 year period of the earth s orbital eccentricity suggests anew that linkage between geomagnetic paleoclimatic and possible underlying earth orbital parameters should be evaluated

depositional and diagenetic constraints on the abundance and spatial variability of carbonate associated sulfate abstract marine carbonate rocks are composed in varying abundance of cement micrite abiotic grains and fossils which can provide information about the physical and chemical environments in which they formed geochemical analyses of these carbonates are not always interpreted alongside the wealth of geologic including petrographic information available resulting in potentially faulty reconstructions of biogeochemical and environmental conditions these concerns have prompted closer scrutiny of the effect of depositional lithofacies and diagenesis on carbonate proxies here we have combined x ray absorption near edge structure xanes spectroscopy and x ray fluorescence xrf imaging to map the speciation and abundance of sulfur in carbonate petrographic thin sections in upper ordovician carbonates from anticosti island canada and early silurian carbonates from gotland sweden across multiple depositional facies lithofacies and fossil communities between anticosti island and gotland are similar which allows for comparison of changes in the dominant s species and their abundance in separate basins associated with variations in glacio eustatic sea level sulfide abundance is greatest in mudstone wackestone and packstone facies where interstitial micrite hosts abundant pyrite sulfate abundance as carbonate associated sulfate cas varies within individual fossil fragments as well as within the same fossil phylum and is particularly high in unaltered brachiopods in contrast sulfate abundance is generally very low in micrite near the detection limit and generally arises in situ from sulfide that has been oxidized as opposed to true cas in different cement fabrics sulfate abundance is greatest in drusy pore filling cements organic sulfur compounds are also detected and although low in abundance are mostly found within micrite the detection and characterization of both inorganic sulfur and organic sulfur compounds provides a platform to understand early processes of biomineralization this approach will broaden our understanding of the source of inorganically bound sulfate in ancient carbonates as well as the effect of depositional setting and diagenesis on cas incorporation re mobilization and ultimate abundance in sedimentary carbonates additionally this work has implications for the cas isotopic value of individual carbonate components that may affect interpretations of stratigraphic variability of numerous cas sections throughout earth history

geomagnetic intensity variations over the past 780 kyr obtained from near seafloor magnetic anomalies knowledge of past variations in the intensity of the earth s magnetic field provides an important constraint on models of the geodynamo a record of absolute palaeointensity for the past 50 kyr has been compiled from archaeomagnetic and volcanic materials and relative palaeointensities over the past 800 kyr have been obtained from sedimentary sequences but a long term record of geomagnetic intensity should also be carried by the thermoremanence of the oceanic crust here we show that near seafloor magnetic anomalies recorded over the southern east pacific rise are well correlated with independent estimates of geomagnetic intensity during the past 780 kyr moreover the pattern of absolute palaeointensity of seafloor glass samples from the same area agrees with the well documented dipole intensity pattern for the past 50 kyr a comparison of palaeointensities derived from seafloor glass samples with global intensity variations thus allows us to estimate the ages of surficial lava flows in this region the record of geomagnetic intensity preserved in the oceanic crust should provide a higher time resolution record of crustal accretion processes at mid ocean ridges than has previously been obtainable

analysis of 11 myr of geomagnetic intensity variation we have conducted a detailed exploratory analysis of an 11 million year long almost continuous record of relative geomagnetic paleointensity from a sediment core acquired on deep sea drilling project leg 73 at site 522 in the south atlantic we assess the quality of the paleointensity record using spectral methods and conclude that the relative intensity record is minimally influenced by climate variations isothermal remanence is shown to be the most effective normalizer for these data although both susceptibility and anhysteretic remanence are also adequate statistical analysis shows that the paleointensity variations follow a gamma distribution and are compatible with predictions from modified paleosccular variation models and global absolute paleointensity data when subdivided by polarity interval the variability in paleointensity is proportional to the average and further the average is weakly correlated with interval length spectral estimates for times from 28 77 until 22 74 ma when the reversal rate is about 4 myr 1 are compatible with a poisson model in which the spectrum of intensity variations is dominated by the reversal process in the frequency range 1 50 myr 1 in contrast between 34 7 and 29 4 ma when the reversal rate is about 1 6 myr 1 the spectra indicate a different secular variation regime the magnetic field is stronger and more variable and a strong peak in the spectrum occurs at about 8 myr 1 this peak may be a reflection of the same signal as recorded by the small variations known as tiny wiggles seen in marine magnetic anomaly profiles

structurally bound s2 s1 s4 s6 in terrestrial apatite the redox evolution of hydrothermal fluids at the phillips mine new york usa abstract the oxidation state of sulfur s plays a critical role in the formation of igneous and hydrothermal mineral systems constraining the oxidation state of s during mineralization and alteration is a valuable tool for understanding primary versus secondary processes that affect mineral systems recent experimental studies demonstrate that apatite which is a ubiquitous accessory mineral in igneous and hydrothermal ore forming systems structurally incorporates multiple oxidation states of s i e s6 s4 s2 and that the abundance of reduced versus oxidized s in apatite is systematically related to oxygen fugacity in this study we used micro x ray absorption near edge structure xanes spectroscopy at the s k edge to measure the oxidation states of s in natural apatite from the phillips mine magnetite sulfide mineral deposit putnam county new york micro xanes transects were collected within two apatite grains starting near the edge of 1 a pyrrhotite inclusion and 2 an inclusion assemblage consisting of pyrite ferroan carbonate pyroxene and magnetite significant compositional and textural variations within the apatite were observed by electron probe micro analysis epma wavelength dispersive wds spectroscopy element mapping and cathodoluminescence cl imaging and used in combination with the xanes data to discuss the formation of the phillips mine apatite the xanes analyses reveal that apatite contains variable proportions of s6 s4 s1 and s2 with corresponding peak absorption energies of 2481 7 0 3 ev 2477 9 0 4 ev 2471 8 0 1 ev and 2469 8 0 04 ev respectively notably this marks the first observation of reduced s species s2 s1 in terrestrial apatite peak areas ratios s6 s demonstrate systematic variations in the oxidation state of s within the apatite grains elevated s6 s peak area ratios typically coincide with higher concentrations of s and rare earth elements within the apatite grains several observations including the presence of multiple oxidation states of s in apatite and monazite inclusions that record secondary fluid mediated dissolution reprecipitation of apatite indicate differences in the oxidation of s thus oxygen fugacity during primary mineralization and secondary alteration i e metasomatism we propose a model for the formation of the phillips mine apatite wherein the primary apatite grains crystallized from a reduced s bearing hydrothermal fluid characterized by a low so2 h2s ratio subsequently metasomatism of apatite in the presence of an oxidized fluid which contained an elevated so2 h2s ratio resulted in the exsolution of rare earth elements from apatite and concomitant growth of monazite and the structural incorporation of oxidized s s6 and s4 in apatite this study demonstrates that the oxidation states of s in apatite provide valuable geochemical information regarding the redox evolution of mineralized systems

amphibole and apatite insights into the evolution and mass balance of cl and s in magmas associated with porphyry copper deposits chlorine and sulfur are of paramount importance for supporting the transport and deposition of ore metals at magmatic hydrothermal systems such as the coroccohuayco fe cu au porphyry skarn deposit peru here we used recent partitioning models to determine the cl and s concentration of the melts from the coroccohuayco magmatic suite using apatite and amphibole chemical analyses the pre mineralization gabbrodiorite complex hosts s poor apatite while the syn and post ore dacitic porphyries host s rich apatite our apatite data on the coroccohuayco magmatic suite are consistent with an increasing oxygen fugacity from the gabbrodiorite complex to the porphyries causing the dominant sulfur species to shift from s2 to s6 at upper crustal pressure where the magmas were emplaced we suggest that this change in sulfur speciation could have favored s degassing rather than its sequestration in magmatic sulfides using available partitioning models for apatite from the porphyries pre degassing s melt concentration was 20 200 ppm estimates of absolute magmatic cl concentrations using amphibole and apatite gave highly contrasting results cl melt concentrations obtained from apatite 0 60 wt for the gabbrodiorite complex 0 2 0 3 wt for the porphyries seems much more reasonable than those obtained from amphibole which are very low 0 37 wt for the gabbrodiorite complex 0 10 wt for the porphyries in turn relative variations of the cl melt concentrations obtained from amphibole during magma cooling are compatible with previous petrological constraints on the coroccohuayco magmatic suite this confirms that the gabbrodioritic magma was initially fluid undersaturated upon emplacement and that magmatic fluid exsolution of the gabbrodiorite and the pluton rooting the porphyry stocks and dikes were emplaced and degassed at 100 200 mpa finally mass balance constraints on s cu and cl were used to estimate the minimum volume of magma required to form the coroccohuayco deposit these three estimates are remarkably consistent among each other ca 100 km3 and suggest that the cl melt concentration is at least as critical as that of cu and s to form an economic mineralization

sulfur partitioning between apatite and melt and effect of sulfur on apatite solubility at oxidizing conditions the effect of sulfur on phosphorus solubility in rhyolitic melt and the sulfur distribution between apatite anhydrite melt and fluid have been determined at 200 mpa and 800 1 100 c via apatite crystallization and dissolution experiments the presence of a small amount of sulfur in the system 0 5 wt s under oxidizing conditions increases the solubility of phosphorus in the melt probably due to changing calcium activity in the melt as a result of the formation of ca s complexing cations apatite solubility geothermometers tend to overestimate temperature in ca poor s bearing system at oxidizing conditions in crystallization experiments the sulfur content in apatite decreases with decreasing temperature and also with decreasing sulfur content of the melt the sulfur partition coefficient between apatite and rhyolitic melt increases with decreasing temperature kdsapatite melt 4 5 14 2 at t 1 100 900 c under sulfur undersaturated conditions no anhydrite the sulfur partition coefficient is lower in anhydrite saturated melt 8 at 800 c than in anhydrite undersaturated melt suggesting that kdsapatite melt depends not only on the temperature but also on the sulfur content of the melt these first results indicate that the sulfur content in apatite can be used to track the evolution of sulfur content in a magmatic system at oxidizing conditions

the different sources and petrogenesis of jurassic intrusive rocks in the southern lhasa subterrane tibet evidence from the trace element compositions of zircon apatite and titanite abstract the jurassic magmatism distributed in the southern lhasa subterrane slt has been recognized as a record of the early stage evolution of the neo tethys ocean however the magma compositions and detailed crystallization histories remain an enigma zircon apatite and titanite in igneous rocks can serve as effective petrogenetic indicators in this paper we present the whole rock geochemical zircon u pb data zircon hf isotopic analyses and major and trace elemental compositions of zircon apatite and titanite from baiga granite porphyry egp shanba granodiorite egd pengcuolin granodiorite porphyry mgp and dazhuqu monzonite lm rocks in the slt to constrain their source and petrogenesis the results from this study show that a combination of zircon ti th u hf and rare earth element ree contents apatite halogen ree and sr contents and titanite rees can be used to track the detailed magmatic crystallization histories higher sr contents are found in apatite srap from egp ca 192 ma and egd ca 176 ma than those in their host rocks srwr indicating that their magma genesis may have involved some mafic magma the magma genesis of lm ca 159 ma may have involved more reduced mafic or least evolved magma as evidenced by the higher th u but lower yb gd ratios higher zircon saturation and ti in zircon temperatures in zircon highest srap srwr ratios and large positive eu anomalies in titanite from lm the lower cl contents and higher la sm n values in apatite from mgp ca 172 ma are inherited from the low cl melts the mgp rocks have higher zircon ce4 ce3 and eun eun ratios a greater apatite so3 content and a higher titanite total ree content which indicates that mgp has a higher oxidation state than do egp egd and lm the mgp also exhibits a higher h2o content 11 wt dissolved in the melt greater ba la and nb ta ratios in whole rock and higher nb ta ratios in zircon however mgp demonstrates lower zircon saturation and ti in zircon temperatures which suggest that mgp was generated by fluid fluxed melting below 750 c the low ti in zircon temperatures but high la yb and sr y ratios of mgp indicate that a local thickened crust likely existed in the middle jurassic period in the slt due to mafic magma underplating during this time

assessing the role of bitumen in the formation of stratabound cu ag deposits insights from the lorena deposit las luces district northern chile abstract stratabound cu ag deposits located in north central chile have been grouped into two ns parallel belts according to the age of their volcanic and volcanoclastic host rocks deposits from the cretaceous belt are characterized by an unusual assemblage of cu sulfides with migrated hydrocarbons whereas the presence of organic matter has not yet been recognized in deposits from the jurassic belt here we report the first evidence of pyrobitumen in a jurassic stratabound cu ag deposit and evaluate its role in ore formation we present a micro analytical study of pyrobitumen associated with cu fe sulfides in the lorena deposit located in northern chile our approach involved a combination of surface and drill core sample characterization fe sem observations empa and xrf mapping of pyrobitumen in addition we determined the speciation of sulfur in pyrobitumen by using synchrotron xanes two main events of hypogene mineralization were identified at lorena deposit i e a pre ore and a main cu ore stage the pre ore stage is characterized by globularly shaped pyrobitumen and minor pyrite and a widespread albite alteration of the andesite host rock pre ore pyrite was replaced by chalcopyrite bornite and chalcocite providing evidence for a late cu inception in the main cu ore stage chalcocite is often associated with sharp edged angular pyrobitumen most likely indicating remobilization or a later pulse of hydrocarbons empa data show that pyrobitumen at lorena is enriched in fe 0 02 1 74 wt cu 0 02 0 87 wt s 0 01 0 43 wt and cl 0 01 0 4 wt other elements detected include v co ni cr sb as ag and au both ag and au were only detected in a few spots with au reaching up to 600 ppm desktop and synchrotron xrf mapping shows that s is homogeneously distributed within pyrobitumen with lower s concentrations observed at grain rims on the other hand empa wds x ray maps show that cu precipitated on pyrobitumen grain surfaces filling fractures and forming veinlets in addition textures showing the presence of micro to nano sized pyrobitumen inclusions within both pre ore pyrite and main ore chalcocite grains point unequivocally to the interaction of hydrothermal fluids with hydrocarbons during the formation of this deposit the high s concentrations measured in pyrobitumen and the presence of organosulfur compounds detected by xanes can be attributed to an organic source the pre ore assemblage is interpreted as formed from a petroleum aqueous fluid connate waters mixture that migrated from the sedimentary source rock into the jurassic host rocks where pyrite and pyrobitumen precipitated in the andesite late cu bearing hydrothermal fluids interacted with this pyrobitumen pyrite assemblage to form cu sulfides either by replacement of pyrite or precipitation we conclude that some of the s contained in pyrobitumen could have been released during interaction with the cu bearing hydrothermal fluid contributing at least partially to the s budget required for sulfide mineralization in the main ore stage our results provide evidence supporting pyrobitumen fluid interaction as a key process in the formation of the lorena deposit and possibly other stratabound cu ag deposits in chile

source of tiny wiggles in chron c5 a comparison of sedimentary relative intensity and marine magnetic anomalies 1 in addition to the well established pattern of polarity reversals short wavelength fluctuations are often present in both sea surface data tiny wiggles and near bottom anomaly data while a high degree of correlation between different geographical regions suggests a geomagnetic origin for some of these wiggles anomaly data alone cannot uniquely determine whether they represent short reversals or paleointensity variations independent evidence from another geomagnetic recording medium such as deep sea sediments is required to determine the true nature of the tiny wiggles we present such independent evidence in the form of sedimentary relative paleointensity from chron c5 we make the first comparison between a sedimentary relative paleointensity record odp site 887 at 54 n 148 w and deep tow marine magnetic anomaly data 43 n 131 w bowers et al 2001 for chron c5 the sediment cores are densely sampled at 2 5 kyr resolution the inclination record shows no evidence for reverse intervals within the 1 myr long normal chron c5n 2n rock magnetic measurements suggest that the primary magnetic carrier is pseudo single domain magnetite we choose a partial anhysteretic magnetization parm as our preferred normalizer and the resulting relative paleointensity record is used as input to a forward model of crustal magnetization we then compare the results of this model with the stacked deep tow anomaly records the two records show a significant degree of correlation suggesting that the tiny wiggles in the marine magnetic anomalies are likely produced by paleointensity variations an analysis of our sampling density suggests that if any reverse intervals exist at this site they are likely to be 5 kyr in duration furthermore we suggest that reverse intervals during chron c5n 2n documented in other locations are unlikely to be global

metamorphic conditions and uplift history in central perthshire evidence from mineral equilibria in the foss celsian barite sulphide deposit aberfeldy interlayered lithologies of diverse mineralogy enclosing the middle dalradian syngenetic ba zn pb deposit at foss s of loch tummel in perthshire allow estimates of the p t conditions of metamorphism using several new geothermometers and geobarometers temperatures of 500 560 c are derived from calcite dolomite geothermometry and from data on mg fe partitioning within garnet hornblende and garnet biotite pairs plagioclase biotite garnet muscovite and biotite muscovite chlorite equilibria indicate peak metamorphic pressures of 8 10 kbar which are in agreement with the results of sphalerite geobarometry textural evidence indicates that the rare platy mineral cymrite baa1 2 si 2 o 8 h 2 o was widespread in mineralized lithologies during penetrative deformation and has since largely been replaced by celsian the p t location of the experimentally determined reaction line cymrite ss celsian ss h 2 o places further constraints on the peak metamorphic conditions and suggests that the final episode of penetrative deformation d 3 was followed by a period of near isothermal uplift during which celsian became stable the cymrite field appears to have been re entered during late brittle deformation peak metamorphic conditions recorded by mineral equilibria at foss differ from those of areas 10 km to the n and s

159 ma kjakebeinet lamproites dronning maud land antarctica and their implications for gondwana breakup processes lamproite dykes that cross cut middle jurassic karoo related flood basalts in vestfjella dronning maud land antarctica have been dated at 158 7 1 6 ma by 40ar 39ar incremental heating of phlogopite compared to most lamproites these dykes are unusually low in sio2 40 09 40 18 wt and high in cao 12 35 16 24 wt and p2o5 3 03 3 81 wt they have broad affinities to spatially and temporally related group ii kimberlites of southern africa but the geochemically closest correlatives are cao and p2o5 rich diopside madupitic lamproites from leucite hills usa the kjakebeinet lamproites are notably high in incompatible elements show negative initial end values 6 0 6 7 and positive initial esr values 14 17 with tdm model ages of c 1 1 ga and probably record melting of metasomatized subcontinental lithospheric mantle beneath western dronning maud land their emplacement records the youngest recognized magmatic event in the region coincides with the proposed second and final stage of gondwana breakup between africa and antarctica and may be associated with deep seated coast parallel strike slip faulting in dronning maud land survival of old easily fusible lithospheric mantle material 20 ma after the eruption of voluminous flood basalt magmas suggests that the latter were emplaced within narrow zones of lithospheric thinning and that the source of lamproites remained largely intact between these zones

coherence of signals from magnetometers on parallel paths during a recent marine magnetic survey of the juan de fuca rise two magnetometers were towed near the seafloor one about 300 m above the other to understand how to interpret the records we investigate a simple statistical model two magnetometers moving on parallel paths above a statistically stationary source with known spectrum magnetometers on paths normal to perfectly lineated magnetic anomalies will measure signals that have unit coherence at all wavelengths departure of the system from this ideal state can be diagnosed by a lower coherence and something about the across track structure can be learned from the shape of the coherence spectrum we calculate the power and cross spectra of the profile signals in terms of the two dimensional power spectrum of the field just above the source region hence we obtain the coherence and phase spectra for the special case of a white source spectrum we find surprisingly high coherences a set of inequalities between the spectral estimates is derived and can be used to check the consistency of the measured signals with the model assumptions the theory is applied to a magnetic traverse of the juan de fuca rise when two near bottom magnetometers were deployed the key results are these in the wavelength range above about 1 km the observed coherency is substantially higher than that from the disordered field model consistent with the highly lineated structures observed at the surface over all ocean ridge systems on scales between 500 m and 1 km the coherence falls to levels indistinguishable from those given by an isotropic flat spectrum implying that on these scales there is little or no across track lineation this finding means that the resolution of paleomagnetic field behavior based on seafloor data in this area is no better than 36 000 years

sulfur evolution of oxidized arc magmas as recorded in apatite from a porphyry copper batholith uniformly sulfur rich cores abruptly zoned to sulfur poor rims 1 to 0 2 wt so3 in apatite from the yerington batholith nevada indicate that early magma that is crystal poor oxidizing and sulfate rich evolved to sulfate poor magma via crystallization of anhydrite a mineral observed in magmas from pinatubo and el chichon we predict that the characteristic zonation to sulfur poor rims of apatite in the yerington batholith is common in other oxidized hydrous calc alkaline magmas and can be used to track cryptic anhydrite saturation as well as to monitor sulfur evolution sulfate rich arc magmas such as yerington magmas may crystallize to produce hydrothermal fluids rich in chlorine copper and sulfur and porphyry copper ores

cymrite and celsian in manganese rich metamorphic rocks from andros island greece manganese rich rocks from the island of andros cyclades greece contain cymrite baal2si2o8 h2o and celsian the textural relationships indicate replacement of celsian by cymrite several microprobe analyses of cymrites show a solid solution of 0 7 6 1 mole kalsi3o8 h2o the relic celsians have similar k contents thus the described rocks from andros represent the first natural example for the reaction celsian water cymrite the rocks from andros underwent an eocene high pressure metamorphism lateron the high pressure parageneses were nearly effaced by a late oligocene barrovian type metamorphism it is assumed that both cymrite and celsian were formed at conditions near the reaction curve celsianss water cymritess during the barrovian type event

experimental evidence for the alteration of the fe3 σfe of silicate melt caused by the degassing of chlorine bearing aqueous volatiles the temporal evolution of the redox conditions undergone by degassing magmas has petrologic implications for magmatic phase equilibria we present new experimental data that demonstrate that a cl bearing magmatic volatile phase may affect the concentration and activity of ferrous iron in silicate liquid via the preferential mass transfer of fe 2 into the volatile phase the data are consistent with a model wherein the exsolution of a cl bearing volatile phase from the melt modifies the mgo feo ratio of the residual degassed melt increasing the fe 3 fe ratio of the melt degassing induced alteration of the melt may cryptically affect the phases that constitute the hypersolidus crystallizing assemblage the new results indicate that a low to moderate salinity magmatic volatile phase may contain several weight percent iron present as fecl 2 as such passive degassing over geologically plausible protracted periods has the potential to extract significant quantities of fe 2 resulting in a residual melt depleted in feo relative to fe 2 o 3 notably preferential mass transfer of fe 2 from the melt to the fluid results in self oxidation of the melt driving the oxygen fugacity of the melt across the phase boundary that demarcates the stability fields of sulfide and sulfate phases stabilizing sulfate at the expense of sulfide such a process has significant implications for the remobilization of chalcophile metals during the evolution of magmatic hydrothermal systems

variations in oceanic crustal structure and implications for the fine scale magnetic anomaly signal marine magnetic anomalies are traditionally modelled using a constant thickness source layer usually defined as the extrusive lavas associated with seismic layer 2a recent seismic studies reveal that the thickness of layer 2a can vary by more than a factor of two an observation which has important implications for the magnetic anomaly source layer we model deep tow magnetometer data from the endeavour segment of the northern juan de fuca ridge in combination with nearby seismic data and find a positive correlation between the thickness of the seismically defined layer 2a and the magnetic anomaly source layer this suggests that short wavelength magnetic anomalies observed within individual magnetic polarity units can arise from variations in the thickness of the extrusives rather than geomagnetic field variations or secondary magnetization

geomagnetic field stability reversal events and excursions abstract it has been suggested that there are several reverse subchrons in the brunhes and this suggestion is gaining some acceptance however such a suggestion has fundanmental implications for the reversal process because it implies that the reverse polarity state has been substantially less stable than the normal polarity state during the brunhes chron analysis of the latest polarity reversal timescale confirms that there is no reason to reject the hypothesis of a common stability for the two polarity states the cretaceous superchron appears to be a time when the reversal rate slowed to the point where the reversal process actually halted so does not indicate any difference in the stability of the two polarity states after examining the data we found no convincing evidence for even a single reversal event during the brunhes and we conclude that the only reversal excursions that are sufficiently well documented for the brunhes are the laschamp and the blake further we note that the question of whether of large excursions represent aborted reversals is unresolved so excursion data should not at this stage be grouped with transition polarity data finally we note that most of the time the nearly axially symmetric normal and reverse polarity states appear to be remarkably stable

hydrothermal fluid characteristics and implications of the makou ioa deposit in luzong basin eastern china abstract a large number of iron oxide apatite ioa deposits occur in the middle and lower yangtze river metallogenic belt mlyb eastern china proterozoic phosphorus rich strata are also widely exposed in this belt providing a good opportunity to research the relationship between phosphorus rich layers and ioa deposits the makou ioa deposit is located in the central part of the volcanic luzong basin the ore body is developed in the contact zone between gabbro diorite and cretaceous trachyandesite and it is the only ioa deposit in the basin that spatially associated with a dioritic intrusion the main hydrothermal alteration minerals are albite diopside apatite chlorite and carbonate in this study we employed shrimp si to analyse in situ o isotope ratios of magmatic and hydrothermal apatite and in situ s isotope ratios of pyrite from the makou deposit additionally la icp ms was used for trace element analysis the results show that the sulfur in the pyrite of the makou deposit comes from different sulfur phases e g h2s and so2 in the hydrothermal fluid and s34total is similar to that of magmatic sulfur indicating that the hydrothermal fluid was weakly affected by evaporite which is substantially different from other ioa skarn iron deposits in the mlyb both analysed apatite samples are classified as fluorapatite in which the 18oap of the makou magmatic hydrothermal apatite is similar to that of the initial mantle 3 7 5 5 and much lower than that of the susong sedimentary metasomatic apatite 13 7 14 2 the lree hree ratio increase from magmatic to hydrothermal apatite whereas the sr y decreases indicating that the hydrothermal fluid metasomatised the dioritic intrusive rock resulting in a large amount of albite crystallisation and release fe from pyroxene and biotite to hydrothermal fluid indicates that fe rich fluid in ioa deposit could be formed by albitization of the consolidated intrusive rock by combining this finding with the oxygen isotope data we propose that the mineralization of the makou deposit was mainly controlled by a magmatic hydrothermal process the low co2 and so42 contents in the volcanic rock are a key factor leading to apatite precipitation the addition of a phosphorus rich layer or evaporites is not necessary for the formation of ioa type deposits in the mlyb

direct measurement of magnetic reversal polarity boundaries in a cross section of oceanic crust magnetic field measurements made by submersible define the cross sectional geometry of a magnetic polarity rever sal boundary and the vertical variation of crustal magnetization in upper oceanic crust measured polarity boundaries show a systematic pattern of shallow dip towards the spreading axis within the upper extrusive lavas and steeper dip in the lower extrusive lavas this geometry is a consequence of the em placement of extrusive lava at a midocean ridge reversal boundary geometry and magnetization estimates are used to calculate the magnetic contribution of the extrusive lava se quence to the overlying marine magnetic anomaly signal from the forward modeling the highly magnetized extrusive lavas contribute the majority 50 75 of the observed sea surface magnetic anomaly consistent with the extrusive crust forming the primary source layer for young marine magnetic anomalies