intensity of the geomagnetic field in western europe over the past 2000 years new data from ancient french pottery 1 we studied 14 groups of french pottery fragments dated between the 4th and 16th centuries the potsherds were analyzed using the thellier and thellier 1959 method revised by coe 1967 intensity values were corrected for thermoremanent magnetization trm anisotropy and cooling rate dependence of trm acquisition we first analyzed modern ceramics produced following ancient techniques and fired in a wood burning kiln inside of which field intensity was measured the recovered mean intensity is within 3 of the expected value which proves the reliability of our experimental procedure thermal experiments carried out at rapid and slow cooling rates clearly indicate that the cooling rate correction is critical in archeointensity studies our data indicate that large variations in intensity occurred in france over the last 2000 years two relative maxima in intensity are observed one between the 8th and 10th centuries and the second between the 14th and 15th centuries similarities are observed between the archeointensity data from france and ukraine yielding some evidence for eastward drift of geomagnetic sources between western and eastern europe from a d 800 to a d 1700 we also show that the dipole moment evolution proposed by mcelhinny and senanayake 1982 and yang et al 2000 for the last two millennia is likely biased toward higher values mainly because of the absence of correction for the cooling rate dependence of trm acquisition in most published archeointensity studies we finally underline a possible relationship valid at least in western europe between changes in direction and intensity of the geomagnetic field

cooling rate correction of paleointensity determination for volcanic glasses by relaxation geospeedometry the influence of the cooling rate on paleointensity estimates was investigated for samples from a vertical profile across a 600 a d obsidian lava flow ramp from lipari italy the natural cooling rates at the glass transition which were previously determined for the seven investigated samples by relaxation geospeedometry vary by a factor of more than 4 rock magnetic investigations indicate a magnetic microlite fraction in the single domain grain size range and strong magnetic anisotropy the thermoremanence anisotropy tensor was determined for each specimen to correct the paleointensity results for this anisotropy the cooling rate dependency of the thermoremanence was determined experimentally extrapolation to natural cooling rates indicate an overestimate of the paleointensity by 13 to 20 during experiments with typical laboratory cooling rates correcting for the different cooling rates and the cooling rate dependencies within the vertical profile significantly reduces the standard deviation of the average flow paleointensity the average paleointensity for the 543 19 a d flow ramp results in 52 4 1 1 t corresponding to a virtual axial dipole moment of 9 2 0 2 am2 uncertainties introduced by anisotropy correction and cooling rate extrapolation are considered by error propagation

effet de la vitesse de refroidissement sur l intensite de l aimantation thermoremanente etude experimentale consequences theoriques the experiments reported here compare intensity and thermal resistance of two thermal remanent magnetizations trm successively acquired by the same sample under a magnetic field but at two different cooling speeds the samples one with a base of cro2 powder and the others derived from oceanic basalts were representative of the different types of structure domains characteristic of magnetic grains the experimental procedure established aimed at limiting the various factors of uncertainty mineralogical and structural modifications related to heating inhomogeneity of the sample temperature at crucial stages of each experiment effects of undesirable magnetizations acquired concurrently with the trm the main conclusion of this study is that the trm intensity increases as a function of time of cooling independently of the types of structure domains carried by the magnetic grains the greatest effect is with the cro2 monodomain the trm obtained between 130 and 100 c during a cooling period of 120

paleomagnetism of archean rocks from northwestern ontario iii rock magnetism of the shelley lake granite quetico subprovince the shelley lake granite of northwestern ontario contains five magnetic phases deuteric and post crystallization hematites which are relatively abundant but carry only 1 4 of the natural remanent magnetization nrm primary magnetite in coarse 50 500 m grains both optically homogeneous and subdivided by hematite lamellae micrometre size secondary magnetite in chloritized biotites and submicrometre size magnetite whose presence is inferred from low blocking temperatures in thermal decay curves of the nrm the nrm is a composite of type 1 and type 2 remanences which differ in direction by about 90 see companion paleomagnetic paper both nrm components occur in normal n and reverse r polarities type 1 remanences 1n 1r have the hallmarks of multidomain md behaviour high blocking temperatures but low coercivities exponential alternating field af decay curves generally md results of the lowrie fuller test and md to transitional values 0 3 10 of the koenigsberger qn ratio further

secular variation of the earth s magnetic field and application to paleomagnetic dating of historical lava flows in chile abstract the recent geomagnetic secular variation is mainly characterized by the large growth of the south atlantic magnetic anomaly during the last three centuries first documented by the geomagnetic model gufm1 jackson et al 2000 we report new paleomagnetic results directions and paleointensities from several sites in two well dated lava flows in chile the 1835 ad eruption of the osorno volcano and the 1751 ad eruption of the llaima volcano in addition paleointensities were obtained on 14 samples from bricks of shelters built along the main road across the andes from santiago chile to mendoza argentina in 1770 5 ad the results confirm the high reliability of the global geomagnetic model gufm1 for the last three centuries with a large amplitude of the secular variation in inclination 20 and intensity 25 t results from three 14 c dated volcanic units in the time interval 1400 1750 ad indicate that more paleomagnetic results in well dated lava flows are necessary to improve the robustness of existing global geomagnetic models at this stage precise paleomagnetic or archeomagnetic dating in south america using global models should be restricted to the last 3 centuries to illustrate the potential of paleomagnetic dating in region and time interval with very large geomagnetic secular variation we report paleomagnetic data from several sites in historical lava flows 1700 1900 ad from the antuco llaima and villarrica volcanoes that permit to refine the ages of the major historical effusive volcanic events

petrology geochemistry and geochronology of the meta mafic rocks in the north sulu ultrahigh pressure belt implications for their petrogenetic diversity and complex tectonic evolution abstract the ultrahigh pressure uhp eclogite and country rocks of the north sulu uhp metamorphic belt have traditionally been considered the products of the deep subduction of the yangtze block yb beneath the north china block ncb however widely distributed non uhp meta mafic rocks appearing as irregular lenses and thin layers together with uhp eclogite in the haiyangsuo wuji weihai area along the northwestern margin of the north sulu uhp belt prompted investigation of the petrogenetic diversity and complex tectonic evolution of these rocks the uhp index mineral assemblage of garnet omphacite rutile phengite coesite is common in the matrix of uhp and retrograde eclogites or as inclusions preserved in metamorphic zircons indicating uhp metamorphism in contrast with previous studies herein high pressure and medium pressure hp mp granulite facies index mineral assemblage of garnet clinopyroxene plagioclase quartz orthopyroxene ilmenite was commonly identified in the matrix of non uhp mafic granulites and amphibolites indicating that these rocks underwent hp mp granulite facies metamorphism rather than uhp metamorphism the whole rock geochemical data revealed multiple origins for the uhp and retrograde eclogites with protoliths derived from heterogeneous mantle or lower upper crust sources with weak fractional crystallization in contrast the protolith of the non uhp meta mafic rocks may have derived from a depleted or primitive mantle origin source with varying degrees of crustal contamination during emplacement the uhp and retrograde eclogites exhibited neoproterozoic 803 760 ma protolith ages and underwent uhp eclogite facies metamorphism in the middle triassic 233 225 ma with subsequent late triassic 219 210 ma amphibolite facies retrogression in contrast the non uhp meta mafic rocks exhibited older paleoproterozoic 1944 ma protolith ages the two distinct metamorphic events that occurred in the paleoproterozoic 1944 1825 ma and middle late triassic 226 201 ma are related to the paleoproterozoic jiao liao ji orogeny and the triassic collisional event between the yb and ncb respectively a combined study of petrography whole rock geochemistry and zircon u pb ages revealed that the uhp and retrograde eclogites show a close affinity to the neoproterozoic continental crust on the northern margin of yb whereas the non uhp meta mafic rocks show a close affinity to the paleoproterozoic jiaobei terrane both the uhp and non uhp meta mafic rocks together with their country rocks form an approximately 200 km long melange zone rather than a single uhp belt which extends from the haiyangsuo through the wuji to the weihai area along the northwestern margin of the north sulu uhp belt

neoproterozoic low to negative δ18o volcanic and intrusive rocks in the qinling mountains and their geological significance abstract the yaolinghe group in the central part of the qinling mountains includes neoproterozoic bimodal volcanic and sedimentary sequences and two units with fault contact can be divided the lower unit consists of bimodal volcanic and sedimentary rocks intruded by a coeval plutonic complex granitic dikes and small granitic bodies and the upper unit is made up of greenschist with minor rhyolitic tuff zircon u pb dating yields ages of 719 790 ma for meta rhyolitic tuff in the lower unit 714 721 ma for the intrusive rocks and 635 ma for meta rhyolitic tuff in the upper unit the 18 o values are 4 to 15 for whole rock quartz and zircon from the volcanic rocks and quartz veins in the lower unit and the intrusive rocks and 3 4 12 for whole rock and quartz veins in the upper unit low to negative 18 o values 4 to 4 are widely present in the rocks and quartz veins of the lower unit and the intrusive rocks suggesting that the lower unit and the intrusive rocks suffered meteoric fluids in hydrothermal alteration the 18 o values of whole rock and quartz veins in the upper unit do not exhibit striking 18 o depletion indicating that meteoric fluids in hydrothermal alteration did not occur in this unit low 18 o zircon grains 1 4 are common in some of meta rhyolitic tuffaceous and intrusive rocks and only two analyses have negative 18 o values the low 18 o values of zircon were inherited from crystallization of low 18 o magma which was formed through the remelting of previous magmatic rocks altered by meteoric water in a system of magma chamber caldera and weakly affected by meteoric fluid in hydrothermal alteration thus the presence of low 18 o magmatic zircon is an indicator for meteoric fluid in hydrothermal alteration the low 18 o zircon grains found in different samples yields the ages from 720 to 790 ma implying that meteoric fluid in hydrothermal alterations began earlier than 790 ma whereas the striking 18 o depletion of the intrusive rocks suggest that hydrothermal alteration continued to at least 714 ma and the lack of 18 o depletion in the upper unit constrains that meteoric fluid in hydrothermal alterations stopped before 635 ma the long duration of meteoric fluid in hydrothermal alteration do not support that the glacial meltwater from the neoproterozoic glaciations in the yangtze craton is responsible for the 18 o depletion of the rocks

thermoremanence anhysteretic remanence and susceptibility of submicron magnetites nonlinear field dependence and variation with grain size we have measured initial susceptibility 0 and the dependence of anhysteretic remanent magnetization arm and thermoremanent magnetization trm on applied field for seven samples of magnetite with mean grain sizes from 40 to 540 nm trm acquisition is nonlinear in geomagnetically relevant weak fields contrary to the usual assumption made in paleointensity determination over the 40 540 nm range 0 varies with particle shape but only weakly with particle size d and can be used to correct for varying magnetite concentrations in sediment cores arm is strongly size dependent over the same range and is the best parameter for monitoring grain size variations in natural samples arm and trm have similar variations as d 1 for d 1 m suggesting a common source for this pseudo single domain psd dependence on grain size however trm is 10 20 times more intense than arm in grains around 0 2 m in size the trm microstate seems to be a two domain structure whereas the arm microstate may be a vortex structure which has never before been convincingly demonstrated by magnetic measurements independent evidence comes from theoretical fits to trm and arm field dependence data in moderate and strong fields trm in 215 540 nm magnetites is explained by two domain theory but arm is not in smaller grains d 0 1 m a psd theory predicts that trm is carried by single domain sd moments of entire grains but arm resides in moments 1 per grain 20 40 times weaker these remanence levels match those of metastable sd and vortex states respectively we therefore propose that magnetite grains in the 0 1 0 5 m size range can remain in metastable sd or two domain states following acquisition of trm but revert to a vortex ground state when field cycled for example in arm acquisition or alternating field demagnetization

a new rock magnetic approach to selecting sediments for geomagnetic paleointensity studies application to paleointensity for the last 4000 years the hypothesis that the ratio of detrital remanent magnetization to anhysteretic remanent magnetization drm arm for sediment samples is a measure of relative geomagnetic paleointensity is critically evaluated by two distinct approaches one approach is a detailed rock magnetic examination of the implicit assumptions of the drm arm method and the construction of a selection process by which to identify sediments that conform to requirements satisfying these assumptions sediments are uniform with respect to drm arm ratio if they contain magnetite in the 1 15 ym particle size range as the predominant magnetic mineral and have variations in magnetite content of less than 20 30 times the minimum concentration the drm arm ratios of these sediments should provide estimates of relative geomagnetic paleointensity relative particle size variations in magnetite are detected with a plot of anhysteretic susceptibility xarm versus low field susceptibility x and the size range 1 15 m is approximately identified by high field hysteresis parameters a rock magnetic evaluation of leboeuf lake sediments with these techniques indicates that these sediments are suitable for a relative plaeointensity study the second approach to evaluating the drm arm ratio as a measure of relative paleointensity is direct comparison with absolute paleointensity data a comparison between the leboeuf lake estimates and thellier thellier results from the western united states supports the conclusion that suitable sediments can record geomagnetic paleointensity fluctuations

oxygen isotope aeromagnetic and gravity anomalies associated with hydrothermally altered zones in the yankee fork mining district custer county idaho epithermal ag au vein and disseminated deposits in the yankee fork district are hosted in altered volcanic rocks having low delta 18 o values low magnetic susceptibilities low remanent magnetizations and relatively high densities these isotopic and physical quantities provide sensitive indices of rock alteration that can be contoured over areas that are much larger than zones of strong visible alteration such contour maps reveal a marked spatial association between the ore deposits the steepest delta 18 o gradients and the perimeter of zones where the rocks have extremely low total magnetizations the oxygen isotope results indicate that the ore zones coincide with steep gradients in temperature and water rock ratio in two fossil hydrothermal convective systems that involved fluids derived from tertiary meteoric waters our magnetic measurements suggest that these same high gradient zones also coincide with oxidation reduction boundaries low aeromagnetic intensities and positive bouguer anomalies are also associated with the altered zones in the yankee fork district the intensities of these geophysical features are consistent with the size of the altered zones and with the contrast in physical properties between altered and unaltered rocks contour maps of the magnetic and isotopic properties of host rocks should find application in exploration for a variety of types of hydrothermal ore deposits as well as in the study of ore controls and conditions of mineralization

oriented inclusions of magnetite in clinopyroxene source of stable remanent magnetization in gabbros of the messum complex namibia 1 crystallographically oriented and highly elongate magnetite inclusions in clinopyroxene are the dominant source of highly stable remanent magnetization in gabbros of the early cretaceous messum complex namibia rock magnetic properties determined for individual pyroxene crystals indicate a high proportion of single domain magnetite consistent with the observed sizes and shape anisotropy of the magnetite inclusions as in previous studies of similar inclusions these are inferred to have formed by exsolution two arrays of inclusions are regularly present in the messum clinopyroxenes inclined at about 74 consistent with formation at about 800 c deduced from optimization of phase boundary orientations virtual geomagnetic poles from these rocks are consistent with reference data confirming that the magnetization is of thermoremanent origin bipolar magnetizations are recorded at one site as well in individual clinopyroxene crystals suggesting that remanence acquisition upon initial cooling of the gabbro spanned a geomagnetic polarity reversal

full vector archaeomagnetic records from anatolia between 2400 and 1350 bce implications for geomagnetic field models and the dating of fires in antiquity abstract anatolia as one of the busiest crossroads of ancient civilizations provides an ideal platform for archaeomagnetic studies previous results from the middle east have suggested the occurrence of a strong peak in geomagnetic intensity at 1000 bce associated with dramatic field strength variations that could require a radical rethinking of geodynamo theory the behavior of the field in the centuries preceding this peak remains poorly constrained however here we present the results of full vector archaeomagnetic experiments performed on 18 sets of samples from three archaeological sites belonging to assyrian trade colony and hittite periods associated rock magnetic analyses showed that the major magnetic carrier is magnetite chemically stable up to 700 c and the magnetic mineral assemblage is composed mostly of non interacting psd grains the directional results are compared with existing data and with the most recent global geomagnetic field models pfm9k 1b and sha dif 14k the directions are in remarkably good agreement with sha dif 14k which is based on archaeomagnetic and lava flow data together with our earlier results from anatolia we triple the existing database of directions for the 700 year long period 2200 1500 bce over a large region from greece to azerbaijan and from moldavia ukraine to egypt three archaeointensity methods thermal izzi thellier microwave thellier and the multi specimen protocol msp produced virtual axial dipole moment estimates 9 0 10 9 10 22 am 2 that are somewhat higher than contemporaneous regional and global data and model predictions suggesting that the field was already substantially stronger than today more than 800 years prior to the reported peak in addition to constraining geomagnetic variability our data also allow us to assign relative dates to inferred fire events in the assyrian trade colony period sites this allows us to conclude that the fire events at the largest site kultepe were not all contemporaneous with one another and with the abandonment of the site as has been previously hypothesized

low δ18o zircons u pb dating and the age of the qinglongshan oxygen and hydrogen isotope anomaly near donghai in jiangsu province china zircons from metamorphosed granites exposed near qinglongshan have 18ovsmow values of 7 to 0 in both grain rims and cores the concordant 238u 206pb ages of zircon cores are 684 to 754 ma with rims at 221 ma discordant 238u 206pb ages range from 242 to 632 ma results demonstrate a neoproterozoic age for the origin of the qinglongshan oxygen and hydrogen isotope anomaly the low 18o values were imprinted on the rocks by a hydrothermal system charged with meteoric water from a cold climate groundwater circulation was driven by heat from cooling granitic magma the geologic age of the hydrothermal system correlates with that of the nantuo tillite in the sinian strata of the south china block suggesting that qinglongshan s cold climate may be a manifestation of neoproterozoic snowball earth

long term variations in palaeointensity we compile a dataset of reliable palaeointensity estimates based both on published work and on new data from basaltic glass the basaltic glass data more than double the number of reliable thellie

zircon geochemistry records the action of metamorphic fluid on the formation of ultrahigh pressure jadeite quartzite in the dabie orogen abstract a combined study of mineral inclusions u pb ages trace elements and hf o isotopes was carried out for zircons from a coesite bearing jadeite quartzite in the dabie orogen the results provide insights into the action of ultrahigh pressure uhp metamorphic fluids during continental deep subduction to a mantle depth and thus constraints on the origin of the jadeite quartzite in the continental subduction zone the zircons mostly show core rim structures in cathodoluminescence images the overgrown rims contain rare mineral inclusions and exhibit concordant u pb ages of 225 to 246 ma and flat hree patterns with negligible eu anomalies in contrast the relict cores contain uhp metamorphic mineral inclusions such as coesite jadeite and rutile and show discordant u pb ages ranging from 983 to 2045 ma and steep ree patterns with significant negative eu anomalies the u pb isotope data for the all cores and rims define an apparent discordia line with upper and lower intercept ages of 2000 43 ma and 234 18 ma respectively we interpret the rims with triassic ages as the new growth of metamorphic origin and the cores with precambrian ages as the protolith relics of magmatic origin the relict magmatic zircons underwent two subtypes of metamorphic recrystallization i e solid state transformation and metasomatic alteration the solid state recrystallized zircons exhibit slightly discordant u pb ages close to the protolith age steep mree hree patterns and almost unchanged hf isotope ratios these observations point to the lowest degree of resetting to the geochemical composition of protolith zircons in contrast the metasomatically recrystallized zircons exhibit partial resetting in protolith zircon ree composition and u pb and lu hf isotopic systems all of the zircon domains regardless of the rims and cores show relatively consistent 18 o values of 4 0 0 2 such a consistency indicates not only that the metamorphic fluids are of internal origin from the deeply subducted continental crust but also that the oxygen isotope composition of protolith zircons was reequilibrated with the uhp metamorphic fluids of triassic age the metasomatic recrystallization of protolith zircons is indicated by the occurrence of uhp inclusion minerals such as coesite rutile and jadeite in sealed microcracks in this regard the fluid metasomatism firstly took place along fractures of the relict zircons during prograde subduction of the continental crust and then experienced the metamorphic recrystallization to result in sealing of the fractures under the uhp conditions as such the metasomatic recrystallization has heterogeneously reset the u pb and lu hf isotope systems of protolith zircons the composition of inclusion minerals within the relict zircon cores suggests that the metamorphic fluids were rich in si ti na and al these elements would be acquired by the metamorphic fluids through metasomatic reaction of metagreywackes overlying the granitic orthogneiss therefore the jadeite quartzite would be precipitated from the uhp metamorphic fluids that were derived from dehydration of the underlying basement orthogneiss but reacted with the metagreywackes during the continental subduction zone metamorphism

geomagnetic paleointensity at 2 41 ga as recorded by the widgiemooltha dike swarm western australia abstract absolute geomagnetic paleointensity measurements were conducted on samples from six mafic dikes of the 2 41 ga widgiemooltha swarm western australia rock magnetic analyses indicate that the paleointensity signal is carried by nearly stoichiometric pseudosingle domain magnetite and or low ti titanomagnetite paleointensity values were determined using the thellier double heating method supplemented by low temperature demagnetizations the ltd thellier method in order to reduce the effect of magnetic remanence carried by large pseudosingle domain and multidomain magnetite grains thirty one samples from five dikes yielded successful paleointensity determinations with the mean value of 41 2 3 8 t which corresponds to a virtual dipole moment of 6 65 0 98 a m 2 the mean and range of paleofield strength values are similar to those of the recent earth s magnetic field and are consistent with a compositionally driven geodynamo established by the earliest paleoproterozoic era the existence of a stable dipolar geomagnetic field during the proterozoic indicated by paleointensity and paleodirectional data from this and prior studies seems incompatible with a young age of the inner core suggested by recent models of the earth s thermal evolution acquisition of reliable paleointensity estimates using the novel approaches such as the ltd thellier method is crucial in constraining the development of more realistic earth like models of long term geodynamo behavior

paleointensity determination on the late precambrian tudor gabbro ontario paleointensity determination by the coe modified thellier method was carried out on the 1100 ma tudor gabbro of southern ontario the tudor has a bivectorial natural remanent magnetization nrm the c nrm is a grenvillian uplift thermoviscous remagnetization which overprints the a nrm the surviving primary thermoremanent magnetization trm of the tudor intrusion a sharp junction at 530 c between the a and c vectors in laboratory thermal demagnetization matches the peak metamorphic reheating temperature of 480 500 c in nature when the difference in time scales is taken into account the unblocking temperature distributions of the a nrm and of laboratory trm also match a is carried by single domain or small pseudo single domain grains probably magnetite rods exsolved in plagioclase or pyroxene at temperatures above 580 c arai plots of nrm versus partial trm are linear from 520 c or 530 c to 580 c for 45 specimens of 19 samples from 9 sites the 45 reliable paleointensity values from these specimens give a virtual axial dipole moment vadm of 4 6 0 8 1022 a m2 around 1100 ma this is about one half of the modern field intensity but is similar to the mean cretaceous cenozoic intensity and to most archean and proterozoic vadms keweenawan rocks whose ages are within 20 myr of the tudor gabbro have a vadm of 11 4 1022 a m2 the difference between the tudor and keweenawan vadms is similar to differences recorded over similar time intervals in the cretaceous and seems to represent normal variation in geomagnetic field strength throughout earth s history

formation of low δ18o rhyolites after caldera collapse at yellowstone wyoming usa we present a new model for the genesis of low 18o rhyolites of the yellowstone caldera based on analyses of zircons and individual quartz phenocrysts low 18o rhyolites were erupted soon after the massive caldera forming lava creek tuff eruption 602 ka 1000 km3 and contain xenocrysts of quartz and zircon inherited from precaldera rhyolites these zircons are isotopically zoned and out of equilibrium with their host low 18o melts and quartz diffusion modeling predicts that magmatic disequilibria of oxygen isotopes persists for as much as tens of thousands of years following nearly total remelting of the hydrothermally altered igneous roots of the depressed cauldron in which the alteration resistant quartz and zircon initially retained their 18o values these results link melting to caldera collapse rule out rapid or catastrophic magma meteoric water interaction and indicate wholesale melting rather than assimilation or partial melting

refining the timing and mechanism of the triassic partial melting in the sulu uhp orogen china zircon and garnet evidence from a felsic vein and its host granitic gneiss abstract partial melting of high pressure hp to ultrahigh pressure uhp metamorphic rocks in subduction zones has great influence on both the chemical and physical properties of deeply subducted continental crust felsic veins in hp uhp metamorphic rocks are products of partial melting and thus would be a key target to trace partial melting in hp uhp terranes this contribution is an integrated study of petrology zircon and titanite u pb ages as well as zircon hf o and major minerals o isotope compositions for a felsic vein and its host granitic gneiss in the sulu uhp terrane to decipher the timing and mechanism of partial melting zircon u pb dating for the inherited magmatic zircon cores of the granitic gneiss constrains the protolith age at ca 750 ma the inherited zircons had an average 18o value of 0 03 0 15 and yielded a weighted mean ehf t value of 1 16 0 54 and two stage hf model ages of 1707 65 ma these results suggest that the protolith was derived from reworking of paleoproterozoic crust in a neoproterozoic rift in the two samples broad garnet core mantle domains are of metamorphic origin and narrow garnet rims of peritectic origin there are large differences in 18o values between the protolith magmatic zircon 18o 0 03 0 15 and metamorphic garnet 4 14 0 21 for the felsic vein and 4 13 0 11 for the host granitic gneiss suggesting that the negative 18o fingerprints might result from hydrothermal alteration or multi episodes of rift magmatism after the formation of the protolith zircon from the felsic vein yielded a u pb age of 212 3 ma titanites from the two studied samples show undistinguishable u pb ages of 227 14 ma and 212 6 ma within the analytical uncertainties these ages register the timings of the partial melting the approximately consistent 18o values of the metamorphic zircon 4 78 0 15 and metamorphic garnet indicate that the melt was sourced from the surrounded gneiss in a closed system which might be triggered by breakdown of phengite according to a compilation of partial melting in the sulu uhp terrane there exists a clear spatio temporal variation the ca 237 224 ma partial melting in the northern uhp zone might be due to its relatively high temperature metamorphism the most extensive partial melting occurred at ca 220 215 ma in both the southern and northern uhp zones which might be triggered by the detachment of the preceding subducted slab and partial melting of the northern uhp zone continued to ca 210 ma due to heat supply from the underneath mantle such a variation might be dictated by the tectonic evolution of the different uhp slices accordingly the whole partial melting process of uhp rocks in the sulu uhp terrane might have lasted for about 27 myr with two major episodes at ca 230 224 and 220 215 ma

surface vitrification caused by natural fires in late pleistocene wetlands of the atacama desert we describe extended occurrences of unusual silicate glass surface layers from the atacama desert chile these glasses found near the town of pica at four localities separated by up to 70 km are neither fulgurites nor volcanic glasses nor metallurgical slags related to anthropic activity but show close similarities to other glasses that have been previously attributed to large airbursts created by meteoroids entering the earth s atmosphere the glasses are restricted to specific late pleistocene terrains paleo wetlands and soils rich in organic matter with sio2 rich plant remains salts and carbonates 14c dating and paleomagnetic data indicate that the glasses were formed during at least two distinct periods this rules out the hypothesis of a single large airburst as the cause of surface melting instead burning of organic rich soils in dried out grassy wetlands during climate oscillations between wet and dry periods can account for the formation of the pica glasses large oases did indeed form in the hyperarid atacama desert due to elevated groundwater tables and increased surface discharge during the central andean pluvial event roughly coeval with the mystery interval and younger dryas finally we discuss the implications of our results for the other surface glasses previously attributed to extraterrestrial events

fluid regime in continental subduction zones petrological insights from ultrahigh pressure metamorphic rocks abstract intensive studies of ultrahigh pressure uhp metamorphic rocks in continental collision zones have provided petrological insights into the fluid regime during subduction and exhumation recent studies in the dabie sulu orogen of china have made important contributions to understanding fluid action in such systems hence these rocks are the main focus of this review sources of metamorphic fluid can be deduced from study of hydrous minerals structural hydroxyl and molecular water in nominally anhydrous minerals fluid and solid inclusions of uhp metamorphic minerals are also a record of fluid action in deep subduction zones the presence or absence of metamorphic fluid during subduction and exhumation dictates zircon growth and recrystallization making available materials for u pb dating isotopic dating demonstrates that after peak uhp metamorphism retrograde fluid with deuteric origin became active the decomposition of hydrous minerals and the exsolution of molecular water and structural hydroxyl during exhumation can provide sufficient amounts of aqueous fluid for quartz veining and amphibolite facies retrogression within uhp slabs aqueous fluid released by the exsolution of molecular water and structural hydroxyl from uhp nominally anhydrous minerals is characterized by low salinity and light hydrogen and oxygen isotope compositions therefore one part of the uhp slab is a source of aqueous fluid whereas another part is a sink in both cases the fluid action during subduction and exhumation is episodic rather than continuous supplementary material a description of the dabie sulu uhp metamorphic rocks analytical methods la icpms zircon u pb isotope data and rare earth element compositions are available at http www geolsoc org uk sup18355

paleointensity of the geomagnetic field recovered on archaeomagnetic sites from france abstract twenty new paleointensity determinations have been obtained from archaeomagnetic sites from france time intervals investigated range from 65 to 380 ad and from 1370 to 1700 ad two hundred forty two samples of tiles and bricks were treated with the classical thellier method for paleointensity determination rock magnetic studies identify magnetite substituted maghemite and hematite as the main magnetic minerals as is usual with archaeomagnetic samples especially tiles magnetic anisotropy was observed in all samples the effect of thermoremanent magnetization trm anisotropy upon the paleointensity values was corrected after the determination of the tensor of trm anisotropy in each sample these determinations allow corrections of the field strength obtained which are usually lower than 20 the effect of the cooling rate on the trm intensity acquired was also determined and seems relatively low on our samples most frequently eight to 10 samples per site were used to calculate the mean intensity standard deviations are typically lower than 7 of the average value during the roman period small variations of the field intensity in paris are observed between 62 and 70 t while during the more recent period the intensity of the field decreases from 62 to 52 t an analysis of the paleointensity data available for the last 2000 years obtained from sites in western europe was performed using a weighing factor which takes into account the number and type of the samples studied as well as the technique used for the paleointensity determination this weighing factor clearly shows that most of the existing data might not be reliable the analysis performed indicates that more data is needed especially from the time interval 500 to 1400 ad before it would be possible to determine a reference curve of the time variations of the archaeomagnetic intensity of the field in western europe

toward an optimal geomagnetic field intensity determination technique 1 paleointensity determinations based on double heating techniques in field zero field cooling zero field in field cooling and two in field steps with opposite laboratory fields are generally considered to be functionally interchangeable producing equally reliable paleointensity estimates to investigate this premise we have developed a simple mathematical model we find that both the zero field first and in field first methods have a strong angular dependence on the laboratory field parallel orthogonal and anti parallel while the two in field steps method is independent of the direction of the laboratory produced field contrary to common practice each method yields quite different outcomes if the condition of reciprocity of blocking and unblocking temperatures is not met even with marginal 10 tails of partial thermoremanence our calculations suggest that the zero field first method with the laboratory produced field anti parallel to the natural remanence nrm is the most robust paleointensity determination technique when the intensity of the lab induced field is smaller than ancient field however the zero field first method with the laboratory field parallel to the nrm is the optimum approach when the intensity of the lab induced field is larger than the ancient field by far the best approach however is to alternatethe infield zerofield iz steps with zerofield infield zi steps

introduction to tectonics of china abstract the continental crust of china is a mosaic of cratonic blocks and orogenic belts containing small cratons and terranes with various tectonic settings they have diverse origins and complex histories of amalgamation and often suffered repeated reworking after multiple episodes of amalgamation in the last three decades extensive geological geochemical and geophysical investigations have been carried out on these cratonic blocks and intervening orogenic belts producing an abundant amount of new data and competing interpretations this provides important insights into understanding the formation and evolution of the chinese continents the papers assembled in this volume present a timely and comprehensive overview on major advancements and controversial issues related to the formation and evolution of continental crust in china complex tectonic histories were experienced not only by the large scale cratonic blocks and orogenic belts but also by small scale terranes and orogens between and inside these blocks nevertheless our understanding of lithotectonic units and geological processes has been greatly advanced by recent studies of zirconology and geochemistry for various rock types from major petrotectonic units in china it has been further advanced from integrated interpretations of geochemical and petrological data for petrogenesis of magmatic rocks an overview of these observations and interpretations provides new insights into understanding the continental plate tectonics and the chemical geodynamics of subduction zones

paleointensity determination magnetic domain structure and selection criteria this special section is focused on the impact of kittel s theory of magnetic domains on rock magnetism and this paper is focused on the implications for paleointensity determinations absolute paleointensity determination is based on specific properties of thermoremanent magnetizations trms mainly the linear dependence of a trm s intensity on the inducing field and the additivity and independence of partial trms these properties are exhibited by populations of noninteracting single domain sd grains but not by populations of multidomain grains the situation is more complex in the pseudo single domain population the most common one in natural rocks usable for paleointensity studies which sometimes exhibit sd like behavior but not always if the experiments were restricted to sd rocks it would not be possible to find enough outcrops to have the spatial and temporal resolution needed for interpretation of the data in terms of geodynamo however as paleointensity determination is very time consuming a preselection of suitable samples is necessary a new procedure will be proposed using a direct test of partial trm s properties with a fully automated experiment fast to implement

low δ18o rhyolites from yellowstone magmatic evolution based on analyses of zircons and individual phenocrysts the yellowstone plateau volcanic field is one of the largest centers exchange time to form zoned zircons is between a few hundred and a few thousand years which reflects the residence time of lowo of rhyolitic magmatism on earth major caldera forming eruptions are followed by unusual lowo rhyolites new oxygen isotope magmas after formation and before eruption petrologic and geochemical data from rhyolites belonging to the 2 0 my eruptive history of yellowstone are presented with emphasis on the genesis of lowo magmas erupted after the huckleberry ridge tuff 2 0 ma 2500 km and lava creek tuff 0 6 ma

a cooling rate bias in paleointensity determination from volcanic glass an experimental demonstration 1 the suitability of volcanic glass for paleointensity determinations is the basis of many studies the dominant single domain sd magnetic remanence carriers the pristine character of volcanic glass and the possibility to correct paleointensity data for cooling rate dependence using relaxation geospeedometry are all arguments that have been made in favor of this technique in the present study the validity of cooling rate correction is tested using remelted volcanic glass to obtain a stable multicomponent glass with ideal magnetic properties a natural phonolitic glass from tenerife was remelted in air to avoid heterogeneity and degassing in later experiments further it was tempered for altogether 10 hours at 900 c to yield a sufficient concentration of magnetic remanence carriers to exclude nucleation or crystallization six samples were then heated to about 60 c above the calorimetric glass transition temperature 660 c and quenched at different rates from 0 1 to 15 k min rock magnetic measurements show that low titanium titanomagnetite in the sd range is the main remanence carrier after performing paleointensity experiments using a modified thellier method the dependence of the thermoremanence on cooling rate was investigated using the synthesis cooling rates and the experimentally determined magnetic cooling rate dependencies we were able to correct the data and obtained a mean paleointensity of 46 9 1 3 t which reflects the ambient field of 48 t within error the uncorrected mean paleointensity corresponds to a 18 larger value of 56 5 0 9 t therefore application of a cooling rate correction is essential to obtain the correct ancient magnetic field intensity from sd assemblages in volcanic glass

precise determination of phanerozoic zircon pb pb age by multicollector sims without external standardization 1 zircon has long been recognized as the best geochronometer and the most important timekeeper in geosciences modern microbeam techniques such as sims and la icpms have been successfully applied to in situ u pb zircon age determinations at spatial resolutions of 20 30 m or less matrix matched calibration by external standardization of well characterized natural zircon references is a principal requirement for precise microbeam u pb zircon age determination due to fractionation effects between pb and u which usually result in an external age error exceeding 1 alternatively zircons with a closed u pb system can be directly dated by measurement of 207pb 206pb isotopic ratio without external standardization which has been a common practice for zircons older than 1 0 ga but not for relatively young 1 0 ga and particularly phanerozoic ones because of limitations of analytical precision we describe in this paper a method of 207pb 206pb measurement on phanerozoic zircons using a new generation of large radius magnetic sector multicollector cameca ims 1280 sims in combination with multicollector mode a nuclear magnetic resonance nmr magnet controller and oxygen flooding techniques we achieve precisions of 207pb 206pb ratio of 0 1 and 0 1 0 2 propagating to pb pb age errors 0 4 and 1 3 excluding u decay constant uncertainties for zircons of latest neoproterozoic and late paleozoic to mesozoic age respectively therefore the multicollector sims is capable of direct determination of zircon pb pb ages as young as mesozoic age with uncertainties of geological significance this technique is useful for direct dating of zircons in thin sections moreover it has significance for dating of some other u rich minerals i e baddeleyite and zirconolite that are not suitable for sims u pb dating by external standardization

a low δ 18 o intrusive breccia from koegel fontein south africa remobilisation of basement that was hydrothermally altered during global glaciation abstract the cretaceous koegel fontein igneous complex is situated on the west coast of south africa and has a high proportion of rocks with abnormally low 18o values the rocks with the lowest 18o values 5 2 belong to intrusive matrix supported breccia pipes and dykes containing a variety of clast types the breccia rocks range in sio2 from 44 to 68 wt and their whole rock 18o values vary between 5 2 and 1 8 the major and trace element composition of the breccia rocks is consistent with them containing variable proportions of clasts of cretaceous intrusive rocks and basement gneiss and the matrix being fluidized material derived from the same source as the clasts based on the nature of the clasts contained in the breccia it was emplaced just prior to intrusion of the main rietpoort granite at 134 ma all components of the breccia have low 18o value and at least in the case of the gneiss clasts this predates incorporation in the fluidized material although the early cretaceous appears to have been a period of cold climate it is unlikely that the 18o values of ambient precipitation 10 would have been low enough to have generated the required 18o depletion the basement gneiss was probably 2 3 km below the cretaceous surface minimizing the possibility of interaction with isotopically unmodified meteoric water and there is no evidence for foundered blocks of cover rocks in the breccia there is therefore no evidence for downwards movement of material we favour a model where basement gneiss interacted with extremely 18o depleted fluid during crustal reworking at 547 ma a time of global glaciation low 18o metamorphic fluids produced by dehydration melting of 18o depleted gneiss became trapped and as the fluid pressure increased failure of the seal resulted in explosive upwards movement of fluidized breccia migration was along pre existing dykes incorporating fragments of these dykes as well as the country rock gneiss

juxtaposition of allochthonous terranes in the central korean peninsula evidence from zircon u pb ages and o hf isotopes in jurassic granitoids abstract the eventual docking of an exotic continental mass over the course of a wilson cycle is marked by intracontinental sites of prograde metamorphism contractional deformation and syn or post collisional magmatism this study shows that inherited premagmatic zircon in subsequently emplaced granitoid rocks provides another line of evidence for the collisional orogeny following the continental collision between the north and south china cratons jurassic granitoids intruded widely throughout the central korean peninsula in association with the subduction of the paleo pacific plate the latest early to middle jurassic 177 167 ma granitoids occur throughout the cratonic gyeonggi massif whereas early jurassic 194 184 ma granitoids occur locally along the okcheon metamorphic belt to the southeast of the massif these two spatiotemporal groups differ profoundly in their age patterns and o hf isotopic signatures of inherited zircon the younger group yields a typically north china like archean paleoproterozoic mostly 2 5 1 8 ga zircon inheritance in contrast inherited zircons from the older group exhibit exclusively neoproterozoic ca 0 9 0 7 ga and paleozoic ca 0 4 ga cluster along with archean paleoproterozoic populations the exotic feature of the protolith of the older group is further indicated by significantly lower 18o 0 9 to 4 9 and positive ehf 3 to 15 values of the cryogenian zircon cores the results of zircon analysis demonstrate that south and north china like terranes were juxtaposed infracrustally along the southeastern margin of the gyeonggi massif during the pre jurassic collisional orogeny and then selectively provided major source material to the older and younger jurassic granitoids respectively

are arm and trm analogs thellier analysis of arm and pseudo thellier analysis of trm abstract we test the possibility of using the pseudo thellier method as a means of determining absolute paleointensity thellier analysis of anhysteretic remanent magnetization arm and pseudo thellier analysis of thermoremanent magnetization trm have been carried out on a large collection of sized synthetic magnetites and natural rocks in all samples the intensity of trm is larger than that of arm and the ratio r trm arm is strongly grain size dependent the best fit slope b ta from pseudo thellier analysis of trm shows a linear correlation with r the ratio b ta r yielded approximately correct paleointensities although uncertainties are larger than in typical thellier type determinations for single domain and multidomain magnetites alternating field and thermal stabilities of arm and trm are fairly similar however for 0 24 m magnetite arm is both much less intense and less resistant to thermal demagnetization than trm reflecting different domain states for the two remanences and resulting in severely non linear arai plots for thellier analysis of arm

holocene paleointensity of the island of hawai i from glassy volcanics this study presents new high quality paleointensity records and 14c radiocarbon age determinations from the island of hawai i during the holocene previous studies on hawai i use experimental methods and statistical selection criteria that may produce inaccurate geomagnetic field strength estimates additional high quality paleointensity results can be used to evaluate the existing hawaiian dataset and investigate holocene geomagnetic field behavior new paleointensity sites from 22 lava flows were calculated using the izzi thellier laboratory technique and a strict set of selection criteria rapidly cooled glassy volcanic material was collected for all sites isotopic age determinations range from 270 10 000 years before present nine new 14c ages are also presented as part of this study the median intensity for the 22 flows is 47 5 t with a median absolute deviation uncertainty of 5 6 t substantially greater than the present day field strength at hawai i 36 t these new results are comparable to previously published data from this location and are consistent with global paleointensity models there is no evidence of an intensity spike at 3 000 years before present as seen in the levant and elsewhere previously published data vary in intensity by experimental technique relative to data using glassy material and strict selection criteria non thellier type data are biased low a result of these techniques estimating intensity from possibly non single domain magnetic carriers thellier thellier data are biased high the reasons for which remain unclear as no cooling rate effect was demonstrated and we were unable to reproduce the high bias with different selection criteria

unusually low δ18o ultra high pressure metamorphic rocks from the sulu terrain eastern china eclogite and quartzite interlayers containing ultra high pressure uhp metamorphic mineral assemblages occur at qinglongshan in the sulu terrain eastern china these rocks exhibit some of the most negative oxygen isotope compositions that have ever been reported for metamorphic rocks the 18 value of minerals in eclogite ranges from 10 4 to 9 0 o and in quartzite ranges from 10 2 to 7 3 near isotopic equilibrium between minerals and rocks suggests that these rocks must have acquired such low oxygen isotope compositions before the uhp metamorphism through near surface meteoric water rock interactions the rocks were then subducted to a great depth during continent continent collision forming uhp minerals with concomitant local oxygen isotope redistribution it is significant that the rocks have retained such negative oxygen isotope compositions despite subduction into the upper mantle

oxygen and hydrogen isotope geochemistry of gneisses associated with ultrahigh pressure eclogites at shuanghe in the dabie mountains the oxygen and hydrogen isotope compositions of minerals and whole rock were determined for two types of gneiss biotite gneiss and granitic gneiss associated with ultrahigh pressure uhp eclogites in the shuanghe district of the eastern dabie mountains there are significant differences in 18o between the two gneisses the uhp biotite gneiss varying from 4 3 to 10 6 similar to the associated eclogites whereas the non uhp granitic gneiss ranges only from 3 8 to 1 2 the d values are similar in the two gneisses with 37 to 64 for epidote zoisite 92 to 83 for amphibole and 63 to 109 for biotite phengite hydrogen isotope disequilibrium among the coexisting hydroxyl bearing minerals is ascribed to retrograde exchange subsequent to amphibolite facies metamorphism oxygen isotopic equilibrium has been preserved among various minerals in both gneisses regardless of the large variation in rock 18o oxygen isotopic geothermometers yield different but regular temperatures corresponding to the closure temperatures of oxygen diffusion in the minerals the metamorphic temperatures of both eclogite facies and amphibolite facies have been recovered in mineral pairs from the biotite gneiss the isotopic temperatures for the granitic gneiss are mostly in accordance with amphibolite facies metamorphism however high temperatures of 550 to 650 c are obtained from those minerals resistant to retrograde oxygen isotope exchange implying that the granitic gneiss may have experienced higher temperature metamorphism than expected from petrologic thermometers the 18o depletion of both gneisses is interpreted to result from meteoric hydrothermal exchange before during plate subduction therefore the measured 18o values of the gneisses reflect the oxygen isotope compositions of their protoliths prior to the uhp metamorphism it is inferred that the uhp unit is in foreign contact with the non uhp unit like a tectonic melange but both of them experienced the two common stages of geodynamic evolution 1 18o depletion prior to the uhp metamorphism 2 uplifting since the amphibolite facies metamorphism

metamorphic growth and recrystallization of zircon distinction by simultaneous in situ analyses of trace elements u th pb and lu hf isotopes in zircons from eclogite facies rocks in the sulu orogen simultaneous in situ analyses of trace elements u th pb and lu hf isotopes were carried out on distinct domains of zircons in ultrahigh pressure uhp eclogite facies metamorphic rocks from the main hole of the chinese continental scientific drilling ccsd in the sulu orogen for the first time trace elements are directly linked to lu hf isotopes in metamorphic zircons with reference to their u pb dates this enables methodological integration to distinguish four types of metamorphic zircon solid state replacement and dissolution recrystallizations of protolith zircons and new growth from the aqueous fluid metamorphically grown zircons are characterized by concordant u pb ages for the metamorphism flat hree patterns typical of the garnet effect low contents of ree especially hree y nb ta and th u high contents of hf low lu gd n lu hf and th u 0 1 ratios and elevated 176hf 177hf ratios relative to solid state recrystallized zircons this suggests the effects of both garnet and fluid on the growth of metamorphic zircons in contrast metamorphic recrystallization has reset the u th pb isotope system of protolith zircons to different extents depending on the extents of fluid action during metamorphism solid state recrystallized zircons exhibit the lowest degrees of resetting and thus almost inherit all geochemical features from the protolith zircons which are characterized by discordant u pb ages close to or below the protolith age steep mree hree patterns typical of magmatic origin high contents of trace elements and their ratios and low 176hf 177hf ratios on the other hand dissolution recrystallized zircons show the highest degrees of reworking and thus have concordant or nearly concordant u pb ages for the metamorphism steep mree hree patterns lowered contents of trace elements such as ree th u y nb ta and ti relative to the protolith zircons and almost unchanged hf isotope ratios replacement recrystallized zircons display intermediate degrees of reworking and thus have their many features of elements and isotopes in between while the metamorphic growth in the presence of both garnet and fluid is characterized by both depletion of hree with flat pattern and the low contents of trace elements the metamorphic recrystallization in the presence of aqueous fluid is indicated by gradual decreases of mree to hree without the flat hree pattern therefore the simultaneous in situ analyses of metamorphic zircons have the advantage over single term analyses in making distinction between the new growth and the different types of recrystallization

tracking a non dipole geomagnetic anomaly using new archaeointensity results from north east china summary a collection of ceramics and samples collected from north east china with ages ranging from 1000 to 7000 years have been investigated using a modified version of the shaw palaeointensity techniques shaw 1974 rolph shaw 1985 in which only partial nrms and trms pnrms and ptrms with blocking temperatures t above 300 c are used after pre selection of samples by mineral magnetic analysis a secular variation curve obtained from this study is quite consistent with previous results from other areas of china wei et af 1987 tang et af 1991 as well as with the global model of mcelhinny senanayake 1982 comparison of the chinese results with contemporaneous results from greece aitken el af 1989 has allowed us to track the movement of a large non dipole anomaly as it drifts westwards

solving the mystery of the 1960 hawaiian lava flow implications for estimating earth s magnetic field studying historic lava flows provides a rare comparison between direct measurements of the earth s magnetic field and the field information recorded in the rock record connecting direct and indirect measurements provides a way to bridge the gap between historic data in the last 50 100 yr to geologic data over kyr to gyr the field strength in hawaii in 1960 was directly measured at the honolulu observatory to be 36 47 t so our palaeointensity analysis of the 1960 flow is expected to give the same value two vertical sections of the 1960 flow section 1 and section 2 were the focus of a previous microwave palaeointensity study the microwave experiments were run using the non standard perpendicular modified thellier type protocol and produced either apparently good quality data that were biased to low values section 1 or more scattered results averaging close to the expected value section 2 the cause of the non ideal behaviour observed in the data from the 1960 flow is a long standing mystery that it is important to resolve to confirm the reliability of palaeointensity measurements in general and the microwave demagnetization mechanism in particular here we test the hypothesis that higher quality unbiased only random noise measurements are possible using an improved thellier type protocol coupled to an updated microwave system new palaeointensity experiments were performed primarily using the izzi protocol which allows alteration checks during the experiment adapted for the microwave system the specimens from section 1 produced more linear arai plots and gave an estimate of 36 8 3 4 t whereas those from section 2 gave an estimate of 39 1 4 6 t our new experiments demonstrate the microwave system s ability to produce accurate results and efficiently run any thellier style experiment we investigate correcting perpendicular data for undetected alteration and find that using too strict selection criteria can be counterproductive to obtaining accurate and precise microwave palaeointensity results

experiments on trm intensity dependence on cooling rate palaeointensity studies have an important role to play in understanding the origin and time variation of the geomagnetic field however theoretical considerations indicate that the intensity of magnetization recorded by a rock or archaeological sample will be dependent on the rate at which it cooled as well as field magnitude experiments on synthetic magnetite and titanomagnetites of varying grain sizes indicate that for acicular magnetite at low concentration slower cooled trm is stronger than faster cooled trm but for all other samples slower cooled trm is weaker this is the predicted behaviour however the magnitude of the effect is much greater than the predicted kinetic effects and indicates that the magnetization of sub microscopic magnetite rich inclusions in the titanomagnetites strongly controls the trm intensity of the matrix

partial anhysteretic remanent magnetization in magnetite 1 additivity 1 we have tested the additivity of partial anhysteretic remanent magnetization parm for suites of eight synthetic magnetites with mean grain sizes from 65 nm to 18 m and 18 natural samples including lake sediments oceanic and continental volcanic rocks gabbros and granites in both synthetic and natural sample suites domain states inferred from hysteresis and other magnetic properties vary from single domain sd through pseudosingle domain psd to multidomain md for each sample total arm intensity was compared with sums of partial arms of three different types four conjugate pairs of parallel parms four pairs of perpendicular parms and one set of five neighboring parallel parms in each case the intervals of alternating field af over which a steady field h was applied to produce the partial arms are nonoverlapping and cover the entire af range 0 100 mt used to produce the total arm additivity of partial arms was verified to better than 3 for all the samples whatever the domain state sd psd and md or composition ranging from pure magnetite to x 0 6 titanomagnetite the universality of parm additivity is unexpected because its analog partial thermoremanent magnetization ptrm deviates from ideal behavior as the grain size increases and the domain structure becomes md the different behaviors probably result from the fact that parm is produced at ordinary temperatures over short times whereas the most intense ptrm is produced at temperatures approaching the curie point with significant dwell times promoting such processes as isothermal remanence acquisition domain nucleation and domain wall reequilibration verification of the law of additivity of parms is an encouraging first step toward validating pseudo thellier and other methods of paleointensity determination that use arm in place of or in addition to trm

nonlinear thermoremanence acquisition and implications for paleointensity data abstract in paleointensity studies thermoremanence is generally regarded as a linear function of ambient magnetic field at low fields comparable to that of the present day earth we find pronounced nonlinearity at low fields for a class of materials with silicate hosted magnetite that otherwise perform well in paleointensity experiments we model this nonlinearity with narrow size ranges of large acicular single domain grains which are most likely in a vortex state i e nonuniformly magnetized sometimes labeled pseudosingle domain simple trm theory predicts that even certain single domain particles will also exhibit a nonlinear response saturating in fields as low as the earth s such behavior although likely to be rare may bias some paleointensity estimates the bias is especially pronounced when the laboratory field is higher than the ancient field fortunately the fundamental assumption that thermoremanence is proportional to applied field can and should be routinely checked at the end of successful paleointensity experiments by adding two extra heating steps

geochronological and geochemical constraints on the subduction modified lithospheric origin of the early cretaceous volcanic rocks in the western north huaiyang belt of dabie orogen china geochronological elemental and sr nd hf isotopic data for intermediate acid volcanic sequences from the western north huaiyang belt of the dabie orogen china provide new constraints for the nature of the lithosphere and the potential involvement of subducted yangtze crust these rocks are calc alkaline andesite trachyandesite and dacite they yield ages of 129 127 ma and show a geochemical affinity to arc volcanic rocks 87sr 86sr i ratios and end t values range from 0 7065 to 0 7080 and 16 9 to 13 2 respectively zircon in situ ehf t values range from 29 4 to 20 2 and hf model ages from 3 03 to 2 46 ga these volcanic rocks were likely derived from a hybridized source involving enriched lithosphere of the north china craton ncc and deep recycled continental crust of yangtze block our data along with geological observations from early cretaceous igneous rocks suggest an east west extensional magmatic pulse at 130 120 ma along the entire north huaiyang belt the extent of involvement of the yangtze continental crust with the lithosphere of the ncc decreases from east to west this reflects along strike changes in triassic northward deep subduction and subsequent exhumation of the yangtze crust further lithospheric extension in the early cretaceous led to unrooting of the thickened lithosphere and generation of cretaceous volcanic rocks along the north huaiyang belt supplementary material microprobe data for all samples are available at https doi org 10 6084 m9 figshare c 4611911

high temperature glacial meltwater rock reaction in the neoproterozoic evidence from zircon in situ oxygen isotopes in granitic gneiss from the sulu orogen abstract to seek the occurrence of negative 18o magmas in the neoproterozoic we conducted in situ zircon o isotope analysis and u pb dating for granitic gneiss from the northeastern sulu orogen east central china zircon u pb dating yields protolith ages of 753 15 ma to 780 13 ma and metamorphic ages of 209 3 to 244 7 ma the neoproterozoic cores with concordant u pb ages exhibit a wide 18o range from 11 0 to 5 8 which is nearly the same as those for cores with discordant u pb ages the triassic rims of some samples have homogeneous 18o values of around 10 whereas the rims of the other samples show a wider range from 9 8 to 5 0 the 18o values as negative as 11 0 for zircons with concordant neoproterozoic u pb ages are reported for the first time representing the primary record of negative 18o magma in the neoproterozoic the continental subduction zone metamorphism in the triassic did not erase the abnormal 18o record in the protolith zircon cores despite metamorphic dehydration and partial melting under high pressure to ultrahigh pressure conditions a conservative estimate suggests that the hydrothermal fluid reacted with the rocks should have 18o values lower than 9 2 corresponding to the meteoric water in cold paleoclimate or the meltwater of local continental glaciation the spatial variation in the o isotope compositions of neoproterozoic zircons is a manifestation of the o isotope heterogeneity in the extinct hydrothermal magmatic system the hydrothermal alteration during the neoproterozoic was incongruent which was lately recorded by the wide range of 18o values in the metamorphic zircons of triassic age the extensive o isotope exchange between the surface water and the deep rock requires high temperature and high water rock ratios in continental rifting zones this is ascribed to neoproterozoic splitting of the south china block from the rodinia supercontinent

fluid generation and evolution during exhumation of deeply subducted uhp continental crust petrogenesis of composite granite quartz veins in the sulu belt china

cautionary note on magnetic grain size estimation using the ratio of arm to magnetic susceptibility the ratio of anhysteretic remanent magnetization arm to magnetic susceptibility is widely used for rapid estimation of magnetic grain size variations on interpreting arm variations no significant influence of interactions among magnetic grains on arm acquisition is usually assumed implicitly although some previous experiments using artificial sediments and biogenic magnetites opposed this assumption our rock magnetic study of deep sea surface sediments from the pacific ocean showed that arm depends strongly on the concentration of magnetic minerals and on the r value of cisowski 1981 an index of the strength of magnetic interactions this result demonstrates that arm can be influenced significantly by magnetic interactions in natural sediments we corrected for the influence of the magnetic interactions using the observed concentration dependence of arm regional variations of arm values before the correction were geologically unnatural in terms of magnetic grain size variations previous interpretations of arm variations on magnetic grain size without consideration of magnetic interactions may not be valid

zircon u pb age and o isotope evidence for neoproterozoic low 18o magmatism during supercontinental rifting in south china implications for the snowball earth event identification of neoproterozoic low 18 o igneous rocks is a key to demonstrate that high t water rock interaction and low 18 o magmatism in rift tectonic zones can serve as an efficient interface to transport heat and material from the earth s interior to exterior at the onset of the snowball earth event low 18 o values of 10 9 to 4 4 permil characteristic of meteoric origin were observed on an areal extent over 30 000 km2 for mid neoproterozoic metaigneous zircons from the ultrahigh pressure eclogite facies metamorphic belt of triassic ages along the dabie sulu orogenic belt china although the zircon o isotope system could be reset to acquire its low 18 o values by exchange with matrix minerals during wet eclogite facies metamorphism the abnormal 18 o depletion is confirmed by occurrence of neoproterozoic low 18 o zircons in low grade metaigneous rocks in the same belt cathodoluminscence imaging of zircons from low grade metagranitoid and metavolcanic rocks shows preservation of the internal structures of oscillatory zoning typical of magmatic origin hornblende from two granites of low 18 o zircons gave ar ar plateau ages of 747 6 and 776 12 ma ruling out that phanerozoic metamorphism effected mineral o isotope values two groups of u pb ages were measured at 756 5 ma and 782 5 ma respectively for the low 18 o zircons this indicates that low 18 o zircons crystallized during two episodes of low 18 o magmatism in response to tectonic evolution from supercontinental rifting at 780 ma to breakup at 750 ma the second episode of low 18 o magmatism is contemporaneous with the continental kaigas iceage a plate rift model is advanced to account for the tectonic connection between the low 18 o magmatism the continental glaciation and the supercontinental rifting during the cryogenian period high t meteoric water rock interaction occurred prior to low 18 o magmatism in rift tectonic zones and caldera collapse was responsible for melting of hydrothermally altered low 18 o rocks during rifting of the south china block from the rodinia supercontinent

permo triassic collision subduction zone metamorphism and tectonic exhumation along the east asian continental margin abstractconvergent plate motion at 320 210 ma generated the tongbai dabie sulu east central china imjingang gyeonggi central korea renge suo southwestern japan sikhote alin orogen along the paleo pacific edge of cratonal asia this amalgamated belt reflects collision between the sino korean and yangtze cratons on the sw portion and accretion of outboard oceanic arcs sialic fragments against the ne margin subducted proterozoic paleozoic continental and oceanic crustal complexes underwent high and ultrahigh pressure metamorphism at low to moderate temperatures tectonic slices of sialic crust episodically disengaged from the downgoing plate and driven by buoyancy ascended rapidly to midcrustal levels from depths exceeding 90 200 km after continental collision in east central china plus or minus korea and from 30 50 km after arrival of far traveled oceanic terranes in sw japan and the russian far east on achieving neutral buoyancy and stalling out at 10 20 km depth later doming gravitation

strong ground motion simulation of the 6 april 2009 l aquila italy earthquake on 6 april 2009 an earthquake of mw 6 13 herrmann et al 2011 occurred in central italy close to the town of l aquila although the earthquake is considered to be a moderate size event it caused extensive damage to the surround ing area the earthquake is identified with significant directivity effects high amplitude short duration motions are observed at the stations that are oriented along the rupture direction whereas low amplitude long duration motions are observed at the stations oriented in the direction opposite to the rupture the complex nature of the earthquake combined with its damage potential brings the need for studies that assess the seismological characteristics of the 2009 l aquila mainshock in this study we present the strong ground motion simulation of this particular earthquake using a stochastic finite fault model with a dynamic corner frequency ap proach for modeling the resulting ground motions we choose two finite fault source models that take into account the source complexity of the l aquila mainshock in order to test the sensitivity of ground motion parameters to the seismic wave attenua tion parameters we use two different attenuation models obtained in the study region using weak motion and strong motion databases comparisons are made between the attenuation of synthetics and ground motion prediction equations gmpes synthetic ground motions are further compared with the observed ones in terms of fourier amplitude and response spectra at 21 strong ground motion stations that recorded the mainshock within an epicentral distance of 100 km the spatial distribution of shaking intensity obtained from the did you feel it project and site survey results are compared with the spatial distributions of simulated peak ground motion intensity parameters our results show that despite the limitations of the method in simulating the directivity effects the stochastic finite fault model seems an effective and fast tool to simulate the high frequency portion of ground motions

paleointensity record from the 2 7 ga stillwater complex montana 1 the record of geomagnetic intensity captured in the 2 7 ga stillwater complex montana usa provides a statistical description of the archean geodynamo we present results of modified thellier paleointensity experiments on 441 core specimens 114 of which pass strict reliability criteria the specimens are from 53 sites spanning most of the banded series rocks in the stillwater complex on the basis of thermochronologic and petrologic evidence we interpret the highest temperature component of remanence to be a late archean thermoremanence though the possibility remains that it is a thermochemical remanence thermal models indicate that the highest temperature magnetization component at each of the sites averages 20 200 ka of geomagnetic secular variation the suite of sites as distributed through the banded series samples a roughly a 1 ma time interval the average of the most reliable paleointensity measurements uncorrected for the effects of anisotropy or cooling rate is 38 2 11 3 t 1 remanence anisotropy cooling rate and the nonlinear relationship between applied field and thermoremanence have a significant effect on paleointensity results a corrected average of 30 6 8 8 t is likely a more appropriate value earth s average dipole moment during the late archean 5 05 1 46 1022 am2 pmag 44 5 was well within the range of estimates from phanerozoic rocks the distribution of site mean paleointensities around the mean is consistent with that expected from slow cooling over timescales expected from thermal models and with secular variation comparable to that of the phanerozoic field

protolith control on fluid availability for zircon growth during continental subduction zone metamorphism in the dabie orogen abstract different episodes of zircon growth are recognized by a combined study of cl images mineral inclusions u pb ages trace elements and lu hf isotopes for ultrahigh pressure uhp eclogite facies metamorphic rocks in the dabie orogen the results provide insights into the effect of protolith property on fluid liberation during continental collision fluid availability from premetamorphic protoliths of different origins is recognized as a key to the zircon growth zircon u pb dating for uhp metabasalt and metasediment eclogite and its host paragneiss yields two groups of ages at 244 3 and 225 2 ma respectively mineral inclusion trace element and lu hf isotope analyses also suggest that these two groups of zircon grew from hydrous melt during the final subduction and supercritical fluid during the initial exhumation respectively in contrast zircon u pb dating for uhp metaintrusive rocks granitic orthogneiss and its hosted eclogite gave only one group of age at 222 2 ma mineral inclusion trace element and lu hf isotope analyses suggest that the metamorphic zircon grew from aqueous fluid during the initial exhumation the difference between the two groups of zircon u pb dates is attributed to the difference in their protolith origin volcanic and sedimentary rocks contain large amounts of water primarily in the form of molecular water pore fluid so that considerable amounts of aqueous fluid can be released from them during subduction this episode of fluid action was recorded by the growth of anatectic zircon at 244 ma in contrast intrusive rock only contains small amounts of water primarily in the form of structural hydroxyl in crystalline minerals so that little fluid can be released from them during subduction nevertheless large amounts of retrograde fluid were released from uhp metamorphic rocks regardless of their protolith origin during decompression exhumation this episode of fluid action was recorded by the growth of metamorphic and anatectic zircons at 220 225 ma therefore the protolith property is a key to the liberation of aqueous fluid from metamorphosing rocks during subduction which has great bearing on partial melting element transport and mineral growth during continental subduction zone metamorphism

an evaluation of the influence of the experimental cooling rate along with other thermomagnetic effects to explain anomalously low palaeointensities obtained for historic lavas of mt etna italy methodological aspects in obtaining reliable absolute palaeointensity estimates have attracted renewed attention in recent years obtaining a reliable palaeointensity from lavas however still is notoriously difficult in many cases lavas have been shown to be a non ideal recorder of the palaeofield here we assess the influence of various thermomagnetic effects on the outcome of palaeointensity experiments on a suite of recent lavas from mt etna several palaeointensity studies have targeted recent flows of mt etna before often palaeointensity underestimates were found we apply both thellier and multispecimen style palaeointensity experiments and obtain estimates generally 20 35 per cent below the expected reference value in line with previous studies we explored subtle chemical alteration viscous magnetic behaviour and differences between the natural and laboratory cooling rates used during the palaeointensity experiments as possible causes for the observed underestimates of the palaeofield for the latter a furnace with controlled programmable cooling rates was constructed it can cool samples very slowly with rates that are in the same order of magnitude as natural cooling rates typical of mt etna lava flows 10 k day both viscous magnetic behaviour and differences in cooling rate appear to influence the outcome of the palaeointensity experiments however the magnitude of the changes observed is not sufficient to explain the observed underestimates we therefore hypothesize that the observed underestimates may be primarily explained by transdomain processes occurring during the heating steps of the palaeointensity experiments

paleointensity record in zero age submarine basalt glasses testing a new dating technique for recent morbs abstract thellier thellier paleointensity experiments were conducted on a collection of glasses from three very recent submarine axial flows two were erupted along the juan de fuca ridge at around 46 n and one along the east pacific rise south at around 18 s the within sample dispersion of paleointensity results from the animal farm flow epr south is very low and leads to a well defined mean value of 35 6 1 t 95 error on the mean based on 11 glass chips from four independent samples today s geomagnetic field intensity in the area is 31 2 t comparing animal farm results with published field model reference curves developed for the past 400 yr suggests an eruptive date estimated between 1880 a d and 1950 a d taking into account different sources of errors this is consistent with qualitative evidence for the age of this flow and constitutes the first precise demonstration of using paleointensity as a dating tool for very recent mid ocean ridge basalts morbs however in the juan de fuca area results show a more erratic pattern with samples varying by up to 30 higher and lower from the expected value of about 55 t the dispersion is attributed to the large local crustal magnetic anomalies in this area that can lead to inconsistent intensity values over the same unit local magnetic anomalies should thus always be checked when doing paleointensity on morb samples which should also be distributed as widely as possible in a flow unit when no significant magnetic anomalies are detected the paleointensity dating tool is anticipated to be especially efficient to investigate the volcanic cyclicity along the epr axis during the last several hundred years

importance of cooling rate dependence of thermoremanence in paleointensity determination 1 the practical effect of cooling rate on the magnitude of thermal remanent magnetization trm is experimentally tested using both natural and synthetic magnetites for synthetic and natural single domain sd magnetites trm intensity increases as the cooling rate decreases because a longer exposure time for slower cooling offers more chances to achieve an equilibrium magnetization multidomain md magnetites have the opposite response trm decreases as the cooling rate increases because slower cooling allows more time to achieve lower magnetization because of self demagnetization for pseudo single domain psd magnetite the effect of cooling rate on the remanence intensity appears to be insignificant intermediate in behavior between sd and md for a sd trm differences are restricted to lower temperatures but diminish near the curie point strongly indicating a serious problem in a conventional treatment of cooling rate correction on the practical side merely applying trm differences as a correction can overcorrect the true paleointensity another important significance of the present study is that a cooling rate difference alone can cause a slight nonlinearity in an arai plot for a sd

geology petrology and o and h isotope geochemistry of remarkably 18o depleted paleoproterozoic rocks of the belomorian belt karelia russia attributed to global glaciation 2 4 ga abstract this paper deals with strongly 18o depleted down to 27 3 vsmow 1 9ga paleoproterozoic mid grade metamorphic rocks found in the belomorian belt of karelia e baltic shield the protolith of these rocks is attributed to have been altered by glacial meltwaters during the world s first 2 4 2 3 ga paleoproterozoic slushball glaciation when karelia was located near equatorial latitudes we describe in detail three and report seven new localities with unusually depleted 18o signatures that now span 220 km across the belomorian belt hydrogen isotope ratios measured in amphibole biotite and staurolite also display remarkably low values of 212 to 235 isotope mapping in the three best exposed localities has allowed us to identify the world s most 18o depleted rock located at khitostrov with a 18o value 27 in khitostrov samples zircons have normal 18o detrital cores and low 18o metamorphic rims mapping demonstrates that zones of 18o depletion occur in a concentric pattern 100 400 m in dimension and each locality displays significant 18o and d heterogeneity on a meter to centimeter scale characteristic of meteoric hydrothermal systems worldwide the zone of maximum 18o depletions usually has the highest concentration of metamorphic corundum rutile and zircon and also display doubled concentrations of insoluble trace elements zr ti cr hree these results are explained by elemental enrichment upon mass loss during hydrothermal dissolution in ph neutral meteoric fluid remarkably low 18o and d values suggest that alteration could have only happened by glacial meltwaters in a subglacial rift zone many localities with 18o depletions occur inside metamorphozed 2 4 ga gabbro noritic intrusions or near their contact with belomorian gneisses implying that the intrusions were driving meteoric hydrothermal systems during the known 2 4 ga episode of belomorian rifting given that the isotopically depleted localities now spread over 200 km the extent of the karelian ice cap is estimated to be at least that large svecofennian 1 9 ga metamorphism is seen to cause metamorphic recrystallization of hydrothermally altered rocks into coarse grained assemblages and causing local metasomatism through devolatilization of the underlying hydrous low 18o protolith further depleting d via volatilization this process led to gem quality rubies and kyanites that preserve these remarkable 18o values in the geologic record

dissolution reprecipitation of zircon at low temperature high pressure conditions lanzo massif italy abst r a c t an eclogite facies meta plagiogranite from the lanzo massif western alps italy contains crystals of zircon intimately associated with allanite zircon displays different microtextures ranging from pristine euhedral and magmatic to fractured porous varieties with mosaic zoning and pervasive recrystallization into euhedral microcrystals fractures and voids in the recrystallized zircon microcrystals are mainly filled by high pressure na rich pyroxene electron backscattered diffraction analysis revealed a similar crystallographic orientation for primary magmatic zircon crystals and microcrystals with less than 2 misorientation among neighboring microdomains the textural change is coupled with chemical and isotopic modifications recrystallized zircon domains contain significantly less th and light to mid ree but are richer in sr than magmatic zircon crystals magmatic zircon preserves the protolith u pb age of 163 5 1 7 ma whereas zircon microcrystals have a mean age of 55 1 ma the coexisting allanite also contains inclusions of na rich pyroxene and has chemical features elevated sr and ni contents and lack of eu anomaly indicating formation at high pressure despite being associated texturally with zircon allanite yields a younger th pb age of 46 5 3 0 ma suggesting that the lanzo unit remained at relatively high pressure conditions for 8 m y zircon recrystallization proceeded with volume reduction and loss of material to an alkaline metamorphic fluid that acted as the agent for a coupled dissolution reprecipitation process recrystallization occurred with minimum transport in a low strain environment and was not significantly enhanced by metamictization the source of the fluid for zircon recrystallization is most probably related to prograde devolatilization reactions in the surrounding serpentinite

strength of the geomagnetic field in the cretaceous normal superchron new data from submarine basaltic glass of the troodos ophiolite 1 we present here new paleointensity data from 39 sampling sites collected from the quenched margins of pillow lavas and dikes exposed within the troodos ophiolite similar to 92 ma formed during the cretaceous normal superchron cns a period of approximately 40 million years when the geomagnetic field reversed extremely infrequently if at all monte carlo simulations suggest that a minimum of 25 estimates are necessary for a reasonably robust estimate for the average field strength our data suggest a dipole strength equivalent to the present field or nearly twice the post cns average the mean and standard deviation of the dipole moment 81 43 zam 2 z 10 21 from the 57 data points compiled here agree remarkably well with those predicted from the long paleointensity record derived from dsdp site 522 the new data set for the cns suggests a picture of a strong and stable field during the period of time when it stopped reversing moreover the similarity of the cns data with the present geomagnetic field suggests that it is presently in a state of unusual polarity stability

origin and evolution of silicic magmatism at yellowstone based on ion microprobe analysis of isotopically zoned zircons the origin of large volume yellowstone ignimbrites and smallervolume intra caldera lavas requires shallow remelting of enormous volumes of variably o depleted volcanic and sub volcanic rocks altered by hydrothermal activity zircons provide probes of these processes as they preserve older ages and inherited do values this study presents a high resolution oxygen isotope examination of volcanism at yellowstone using ion microprobe analysis with an average precision of 0 2o and a 10 mm spot size we report 357 analyses of cores and rims of zircons and isotope profiles of 142 single zircons in 11 units that represent majoryellowstone ignimbrites and post caldera lavas many zircons from these samples were previously dated in the same spots by sensitive high resolution ion microprobe shrimp and all zircons were analyzed for oxygen isotope ratios in bulk as a function of grain size by laser fluorination we additionally report oxygen isotope analyses of quartz crystals in three units the results of this work provide the following new observations 1 most zircons from post caldera low do lavas are zoned with higher do values and highly variable u pb ages in the cores that suggest inheritance from pre caldera rocks exposed on the surface 2 many of the higher do zircon cores in these lavas have u pb zircon crystallization ages that postdate caldera formation but pre date the eruption age by 10 20 kyr and represent inheritance of unexposed post caldera sub volcanic units that have do similar to the lava creek tuff 3 young and voluminous 0 25 0 1ma intra caldera lavas which represent the latest volcanic activity atyellowstone contain zircons with both high do and lowdo cores surrounded by an intermediate do rim this implies inheritance of a variety of rocks from high do pre caldera and low do post caldera units followed by residence in a common intermediate do melt prior to eruption 4 major ignimbrites of huckleberry ridge and to a lesser extent the lava creek and mesa falls tuffs contain zoned zircons with lower do zircon cores suggesting that melting and zircon inheritance from the lowdo hydrothermally altered carapace was an important process during formation of these large magma bodies prior to caldera collapse 5 the do zoning in the majority of zircon core rim interfaces is step like rather than smoothly inflected suggesting that processes of solution reprecipitation were more important than intracrystalline oxygen diffusion concave downward zircon crystal size distributions support dissolution of the smaller crystals and growth of rims on larger crystals this study suggests that silicic magmatism atyellowstone proceeded via rapid shallow level remelting of earlier erupted and hydrothermally alteredyellowstone source rocks and that pulses of basaltic magma provided the heat for melting each postcaldera yellowstone lava represents an independent homogenized magma batch that was generated rapidly by remelting of source rocks of various ages and do values the commonly held model of a single large volume super solidus mushy state magma chamber that is periodically reactivated and produces rhyolitic offspring is not supported by our data rather the source rocks for theyellowstone volcanism were cooled below the solidus hydrothermally altered by heated meteoric waters that caused low do values and then remelted in distinct pockets by intrusion of basic magmas each packet of new melt inherited zircons that retained older age and do values this interpretation may have significance for interpreting seismic data for crustal low velocity zones in which magma mush and solidified areas experiencing hydrothermal circulation occur side by side new basalt intrusions into this solidifying batholith are required to form the youngest volcanic rocks that erupted as independent rhyolitic magmas we also suggest that the lava creek tuff

low δ 18 o mantle derived magma in panjal traps overprinted by hydrothermal alteration and himalayan uhp metamorphism revealed by sims zircon analysis abstract we report two generations of low 18o zircons from the himalayan eclogites and their host gneisses in situ sims 18o analyses on single zircon crystals with known age and hf isotope ratios from two populations of chemically distinct zircons demonstrate a complex history 1 an early low 18o mantle derived magma 2 followed by post emplacement high temperature meteoric water alteration and finally 3 crystallization of new low 18o minerals during the ultrahigh pressure metamorphism magmatic zircon 269 ma in group i eclogites yielded 18o values from 1 9 to 4 6 vsmow with an average value of 4 0 0 2 n 35 the error is 2sd analytical precision and n represents number of analyzed spots which is lower than the typical mantle values 5 3 0 6 2sd in contrast metamorphic zircons 45 ma in group ii eclogites preserve unusually low negative 18o values from 3 9 to 2 7 average 3 4 0 4 n 35 2sd zircons in felsic gneiss that surround group ii eclogites have inherited magmatic cores ca 260 ma with 18o values of ca 2 9 which decrease to 0 1 in metamorphic ca 45 ma rims these zircons preserve lower 18o values than would be equilibrated with typical mantle the low 18o values in magmatic zircons suggest that the mafic protolith to these eclogites formed from a hydrothermally altered subducted oceanic crust and the negative 18o values in metamorphic zircons indicate hydrothermal alteration after crystallization of the mafic magmas but before growth of metamorphic zircons this study reports evidence for melting of subducted low 18o ocean crust to form low 18o mantle derived mafic magmas as previously proposed by cartwright and valley 1991 geology 19 578 581 for proterozoic scourie dikes

hydrogen and oxygen isotope evidence for fluid rock interactions in the stages of pre and post uhp metamorphism in the dabie mountains abstract hydrogen and oxygen isotope studies were carried out on high and ultrahigh pressure metamorphic rocks in the eastern dabie mountains china the 18 o values of eclogites cover a wide range of 4 2 to 8 8 but the d values of micas from the eclogites fall within a narrow range of 87 to 71 both equilibrium and disequilibrium oxygen isotope fractionations were observed between quartz and the other minerals with reversed fractionations between omphacite and garnet in some eclogite samples the 18 o values of 4 to 1 for some of the eclogites represent the oxygen isotope compositions of their protoliths which underwent meteoric water rock interaction before the high to ultrahigh pressure metamorphism heterogeneous 18 o values for the eclogite protoliths implies not only the varying degrees of the water rock interaction before the metamorphism at different localities but also the channelized flow of fluids during progressive metamorphism due to rapid plate subduction retrograde metamorphism caused oxygen and hydrogen isotope disequilibria between some of the minerals but the fluid for retrograde reactions was internally buffered in the stable isotope compositions and could be derived from structural hydroxyls dissolved in nominally anhydrous minerals

eight thousand years of geomagnetic field intensity variations in the eastern mediterranean 1 twenty new intensity determinations of the ancient geomagnetic field have been obtained from groups of potsherds and brick fragments from syria these artifacts archeologically well dated from 6000 b c to approximately a d 1200 have been analyzed using the thellier and thellier 1959 method as modified by coe 1967 intensity values have been corrected for the effects of anisotropy of thermal remanent magnetization and cooling rate our results indicate that field intensities were moderate in syria from 6000 b c to 3500 b c with values of 30 40 t there was a significant increase in intensity by a factor of 2 from 3500 b c to 700 b c which was interrupted by a moderate decrease between 2550 b c and 1750 b c during more recent periods our results show an intensity minimum approximately a d 200 and a maximum around the tenth century comparison with different data sets from the eastern mediterranean and central asia shows that geomagnetic field intensity variations were consistent at this large regional scale at least over the last 5 millennia

tectonic driving of neoproterozoic glaciations evidence from extreme oxygen isotope signature of meteoric water in granite abstract the global context of glaciation in the neoproterozoic has been hypothesized to result from the massive reorganization of the earth s land and ocean systems due to breakup and dispersal of the supercontinent rodinia at about 750 ma this hypothesis gains support from unusually light o isotope records of hydrothermally altered rocks in a rift tectonic zone at that time which is indicative of interaction with meteoric water of low mean annual temperature and thus tectonic driving of cold paleoclimate at the time of hydrothermal alteration very negative 18 o values of 14 4 to 10 0 are found for garnet from neoproterozoic granite in south china which are the lightest o isotope record so far reported for minerals from igneous rocks negative d values of 129 to 109 are obtained for garnet magnetite and zircon thus high t meteoric hydrothermal alteration occurred during magma emplacement shrimp u pb dating for magmatic and hydrothermal zircons yields two groups of ages at 782 3 ma and 748 3 ma respectively responsible for granite crystallization and hydrothermal alteration the garnet is in o isotope disequilibrium with zircon indicating differential effects of subsolidus hydrothermal alteration on the minerals of different o diffusion rates occurrence of the unusually negative 18 o granite at mid low paleolatitudes provides a geochemical proxy for a cold paleoclimate at 748 3 ma possibly a continental glaciation corresponding to the kaigas iceage it suggests a tectonic link to the climatic effect of uplifted rift flanks due to the rodinia breakup at about 750 ma and thus the ice fire interaction by syn rift magmatism of low 18 o imprints in association with the mantle event of asthenospheric upwelling hence the tectonic driving is evident for regional glaciations in supercontinental rift basins

decay rate dependence of anhysteretic remanence fundamental origin and paleomagnetic applications 1 we have measured the intensity of anhysteretic remanent magnetization arm as a function of alternating field af decay rate for synthetic and natural single domain sd and pseudo single domain psd magnetites arm intensity increases as decay rate decreases multidomain md magnetites have the opposite response arm increasing as the decay rate increases these are identical to the sd psd and md dependences of thermoremanent magnetization on cooling rate for all grain sizes and domain structures arm intensity increases as the af decay rate used to achieve an initial demagnetized state decreases decay rate differences in arm intensity are a property of low and medium coercivity grains as shown by annealing and by stepwise af demagnetizing samples we interpret the sd results to mean that increased af exposure time permits a closer approach to equilibrium magnetization an approximate thermal activation theory based on neel 1949 and an exact theory by egli and lowrie 2002 predict 6 11 increases in arm for an order of magnitude decrease in decay rate in reasonable accord with the observed 12 increase for 0 065 m sd grains for md grains we hypothesize that increased exposure time slower decay permits more efficient self demagnetization reducing arm low coercivity grains experience the largest self demagnetizing fields and therefore have the largest decay rate response initial state decay rate response is attributed to longer exposure times leaving domain walls more strongly pinned in deeper potential wells the net self demagnetizing field is zero in the demagnetized state acquisition decay rate annealing and initial state responses of psd grains are a blend of sd and md responses because arm is the most frequently used normalizer in relative paleointensity determination it is important either to use a standard decay rate or else to remove the decay rate dependence by demagnetizing the arm to 30 of its initial value arm0 3 a standard demagnetization level for the normalizing arm is particularly important when comparing paleointensity records from different laboratories

cooling rate effects on paleointensity estimates in submarine basaltic glass and implications for dating young flows 1 cooling rate effects on the intensity of thermoremanent magnetization trm have been well documented in ceramics in that case laboratory cooling is generally more rapid than the initial cooling leading to an overestimate of the paleofield by 5 10 in thellier type paleointensity experiments the reverse scenario however has never been tested we examine the effects of cooling rate on paleointensity estimates from rapidly quenched submarine basaltic glass sbg samples from 13 sites at 17 30 18 30 s on the east pacific rise absolute cooling rates determined by relaxation geospeedometry at five of these sites range from 10 to 330 c min 1 at the glass transition 650 c over the dominant range of remanence blocking temperatures 200 400 c the natural cooling rates are approximately equal to or slightly slower than the laboratory cooling rates during the thellier experiment these results suggest that while the cooling rate effect might introduce some within site scatter it should not result in a systematic bias in paleointensity from sbg paleointensity estimates from the 15 sites range from 29 to 59 t with an average standard error of 1 t comparison with models of geomagnetic field intensity variations at the site indicate the youngest group of samples is very recent indistinguishable from present day and the oldest is at least 500 and probably several thousand years old these age estimates are consistent with available radiometric ages and geologic observations

metamorphic growth and recrystallization of zircons in extremely 18o depleted rocks during eclogite facies metamorphism evidence from u pb ages trace elements and o hf isotopes abstract a combined in situ sims and la mc icpms study of u pb ages trace elements o and lu hf isotopes was conducted for zircon from eclogite facies metamorphic rocks in the sulu orogen the two microbeam techniques sampled various depths of zircon domains revealing different element and isotope relationships between residual magmatic cores and new metamorphic rims and thus the geochemical architecture of metamorphic zircons which otherwise cannot be recognized by the single microbeam technique this enables discrimination of metamorphic growth from different subtypes of metamorphic recrystallization magmatic cores with u pb ages of 769 9 ma have positive 18o values of 0 1 10 1 high th u and 176lu 177hf ratios high ree contents and steep mree hree patterns with negative eu anomalies they are interpreted as crystallizing from positive 18o magmas during protolith emplacement in contrast newly grown domains have concordant u pb ages of 204 4 to 252 7 ma and show negative 18o values of 10 0 to 2 2 low th u and 176lu 177hf ratios low ree contents and flat hree patterns with weak to no eu anomalies they are interpreted as growing from negative 18o fluids that were produced by metamorphic dehydration of high t glacial hydrothermally altered rocks during continental subduction zone metamorphism differences in 18o between different domains within single grains vary from 0 8 to 12 5 suggesting different degrees of o isotope exchange between the positive 18o magmatic core and the negative 18o metamorphic fluid during the metamorphism the magmatic zircons underwent three subtypes of metamorphic recrystallization depending on their accessibility to negative 18o fluids the zircons recrystallized in solid state maintained positive 18o values and ree and lu hf isotopes of protolith zircon but their u pb ages are lowered the zircons recrystallized through dissolution exhibit negative 18o values similar to the metamorphic growths almost completely reset u pb ages and partially reset ree systems the zircons recrystallized through replacement show variably negative 18o values and partially reset ree and u pb and lu hf isotopic systems therefore this study places robust constraints on the origin of metamorphic zircons in eclogite facies rocks and provides a methodological framework for linking the different types of metamorphic zircons to petrological processes during continental collision

metamorphic zirconology of continental subduction zones abstract zircon is widely used to date geological events and trace geochemical sources in high pressure hp to ultrahigh pressure uhp metamorphic rocks of continental subduction zones however protolith zircons may be modified by three different types of metamorphic recrystallization via mechanisms of solid state transformation metasomatic alteration and dissolution reprecipitation new zircon growth may be induced by dehydration reactions below the wet solidus of crustal rocks metamorphic zircon or peritectic reactions above the wet solidus peritectic zircon as a consequence there are different origins of zircon domains in high grade metamorphic rocks from collisional orogens thus determining the nature of individual zircon domains is substantial to correct interpretation of their origin in studies of isotopic geochronology and geochemical tracing we advocate an integrated study of zircon mineragraphy internal structure and external morphology u pb ages mineral inclusions trace elements and lu hf and o isotope compositions only in this way we are in a position to advance the simple zircon applications to metamorphic zirconology enabling discrimination between the different origins of zircon and providing constraints on the property of fluid activity at subduction zone conditions the metamorphic recrystallization of protolith zircons and the new growth of metamorphic and peritectic zircons are prominent in hp to uhp metamorphic rocks of collisional orogens these different types of recrystallized and grown zircons can be distinguished by their differences in element and isotope compositions while the protolith nature of metamorphosed rocks dictates water availability the p t conditions of subduction zones dictate the property of subduction zone fluids the fluids of different properties may be produced at different positions of subducting and exhuming crustal slices and they may physically and chemically mix with each other in continental subduction channels such fluids can act as an important agent not only for the physical transport of protolith zircons but also for the chemical transport of element zr and other fluid mobile incompatible trace elements from the subducted crust to the mantle wedge therefore the discrimination between the different types of zircons provides a powerful means to decipher the role of fluids in subduction zone processes