highly fractionated i type granites in ne china ii isotopic geochemistry and implications for crustal growth in the phanerozoic ne china is the easternmost part of the central asian orogenic belt caob the area is distinguished by widespread occurrence of phanerozoic granitic rocks in the companion paper part i we established the jurassic ages 184 137 ma for three granitic plutons xinhuatun lamashan and yiershi we also used geochemical data to argue that these rocks are highly fractionated i type granites in this paper we present sr nd o isotope data of the three plutons and 32 additional samples to delineate the nature of their source to determine the proportion of mantle to crustal components in the generation of the voluminous granitoids and to discuss crustal growth in the phanerozoic despite their difference in emplacement age sr nd isotopic analyses reveal that these jurassic granites have common isotopic characteristics they all have low initial 87 sr 86 sr ratios 0 7045f0 0015 positive end t values 1 3 to 2 8 and young sm nd model ages 720 840 ma these characteristics are indicative of juvenile nature for these granites other late paleozoic to mesozoic granites in this region also show the same features sr nd and oxygen isotopic data suggest that the magmatic evolution of the granites can be explained in terms of two stage processes 1 formation of parental magmas by melting of a relatively juvenile crust which is probably a mixed lithology formed by pre existing lower crust intruded or underplated by mantle derived basaltic magma and 2 extensive magmatic differentiation of the parental magmas in a slow cooling environment the widespread distribution of juvenile granitoids in ne china indicates a massive transfer of mantle material to the crust in a post orogenic tectonic setting several recent studies have documented that juvenile granitoids of paleozoic to mesozoic ages are ubiquitous in the central asian orogenic belt hence suggesting a significant growth of the continental crust in the phanerozoic d 2003 elsevier science b v all rights reserved

oxygen diffusion in quartz diffusional oxygen exchange between quartz and water was studied by determining 18o profiles in the quartz using an ion microprobe inverse error function plots of the data are linear as required for a diffusion mechanism one natural and two synthetic samples were used and although they had different impurity concentrations oh na and al per 106 si atoms are 7 4000 35 835 and 67 127 respectively the d values were the same within experimental uncertainty diffusion parallel to c is about 2 orders of magnitude faster than that normal to c at 700 c and diffusion normal to the rhombohedron is intermediate the data indicate a change in slope on an arrhenius plot at the boundary at 100 mpa 1 kbar water pressure the preexponential factors do in square centimeters per second and activation energies q in kilojoules per mole for quartz 500 550 c are as follows do 190 and q 284 parallel to c and do 8 10 2 and q 238 normal to the rhombohedron for quartz 600 800 c do 4 10 7 and q 142 parallel to c do 9 10 7 and q 155 normal to the rhombohedron and do 1 10 4 and q 234 normal to c d increases with ph 2 o and between 25 and 350 mpa log d versus log h 2 o has a slope of approximately 1 1 these d values are all several orders of magnitude larger than previously reported values for oxygen diffusion in water free systems

petrogenesis of the hercynian tichka plutonic complex western high atlas morocco trace element and rbzsr and smznd isotopic constraints the late tectonic hercynian tichka plutonic complex in morocco was emplaced in the lower cambrian volcanosedimentary sequence of the western high atlas and displays an exceptional acid basic association which consists of five well defined plutonic intrusions gabbros diorites granodiorites tonalites monzogranites and two types of leucogranites biotite amphibole leucogranites and two mica leucogranites with the exception of the late two mica leucogranites the plutonic suites present field relationships clearly indicating an almost synchronous magmatic emplacement the monzogranites and biotite amphibole leucogranites plot along the same rbzsr isochron and most of them also plot along the same smznd isochron yielding respectively ages of 291 5 ma and 283 24 ma with initial87sr 86sr and143nd 144nd ratios of 0 70348 4 and 0 51243 4 major and trace element data together with the rbzsr and smznd isotopic results rule out any genetic process involving a unique homogeneous source the smznd isotopic heterogeneity even at the scale of one unit implies the existence of contrasting source rocks and the combination of different mechanisms the gabbro and diorite magmas are assumed to be derived from the melting to different degrees of a depleted upper mantle source in a within plate geotectonic environment whereas a combined wallrock assimilation fractional crystallisation model is proposed for the evolution of the diorite suite in contrast the granitoids are mainly the result of the anatexis of a dominant basic to intermediate igneous heterogeneous continental crust

anatomy of a pressure induced ferromagnetic to paramagnetic transition in pyrrhotite implications for the formation pressure of diamonds 1 meteorites and diamonds encounter high pressures during their formation or subsequent evolution these materials commonly contain magnetic inclusions of pyrrhotite because magnetic properties are sensitive to strain pyrrhotite can potentially record the shock or formation pressures of its host moreover pyrrhotite undergoes a pressure induced phase transition between 1 6 and 6 2 gpa but the magnetic signature of this transition is poorly known here we report room temperature magnetic measurements on multidomain and single domain pyrrhotite under nonhydrostatic pressure magnetic remanence in single domain pyrrhotite is largely insensitive to pressure until 2 gpa whereas the remanence of multidomain pyrrhotite increases 50 over that of initial conditions by 2 gpa and then decreases until only 33 of the original remanence remains by 4 5 gpa in contrast magnetic coercivity increases with increasing pressure to 4 5 gpa below 1 5 gpa multidomain pyrrhotite obeys neel theory with a positive correlation between coercivity and remanence above 1 5 gpa it behaves single domain like yet distinctly different from uncompressed single domain pyrrhotite the ratio of magnetic coercivity and remanence follows a logarithmic law with respect to pressure which can potentially be used as a geobarometer owing to the greater thermal expansion of pyrrhotite with respect to diamond pyrrhotite inclusions in diamonds experience a confining pressure at earth s surface applying our experimentally derived magnetic geobarometer to pyrrhotite bearing diamonds from botswana and the central african republic suggests the pressures of the pyrrhotite inclusions in the diamonds range from 1 3 to 2 1 gpa these overpressures constrain the mantle source pressures from 5 4 to 9 5 gpa depending on which bulk modulus and thermal expansion coefficients of the two phases are used

igneous banding schlieren and mafic enclaves in calc alkaline granites the budduso pluton sardinia abstract this study deals with the origin of igneous layering in plutons and especially the extent layering is related to mafic silicic magma interactions the budduso pluton sardinia shows three main scales of organization i large scale lithological variations correspond to three main magmatic units with differentiation increasing from the outer hornblende bearing biotite granodiorite monzogranite to the middle biotite monzogranite and the inner leucomonzogranite units the striking homogeneity of 87 sr 86 sr initial ratios 0 7090 4 and e nd t values 5 6 0 1 strongly suggests that magma isotopic equilibration was achieved prior to emplacement whereas mixing mingling structures observed within the pluton reflect second stage processes involving broadly cogenetic components ii metre to decametre scale igneous layering may be isomodal or modally graded locally with cross layering biotite and plagioclase compositions are similar in both biotite rich and quartzofeldspathic layers as are the trace element patterns which differ only by relative abundances this precludes an origin by fractional crystallization a penetrative submagmatic fabric superimposed on the layering and corresponding mainly to flattening can be ascribed to interference between pluton growth and regional deformation iii composite layering and schlieren are commonly associated to mafic microgranular enclaves locally within synmagmatic shear zones or disrupted synplutonic dykes in that case there is a progressive shift in biotite x fe values from the core of enclave 0 65 to the host monzogranite 0 72 schlieren in the monzogranite show biotite x fe values similar to that of the host rock whereas schlieren close to mafic enclaves show lower x fe values 0 69 towards those of enclave rims these features can be ascribed to three main processes i assembly of differentiated mixed mingled magmatic pulses ii local hydrodynamic sorting related to density currents in a mush and segregation of residual melt iii mechanical disruption and chemical hybridization of mafic magmas during ascent or within the pluton related to magma dynamics none of these processes affect the whole pluton but they are limited to specific magmatic units therefore pluton growth by incremental assembly of magma batches is not incompatible with magma chamber processes

an interpretation of magnetic signatures of subduction zones detected by magsat abstract the magnetic anomalies detected by magsat over subduction zones are related to the thermal state and the location with respect to the geomagnetic poles of the subducting oceanic lithosphere it is shown that an old about 100 m y subducting lithosphere produces about twice as strong a magnetic anomaly as that associated with a young about 40 m y one also a given subducting oceanic lithosphere produces a stronger magnetic anomaly at high geomagnetic latitudes than near the geomagnetic equator based on the thermal evolution models of the oceanic lithosphere and assuming a mean magnetic susceptibility of 13 10 3 si the pacific plate subducting along the kuril trench produces a magnetic anomaly similar to the observed one the nazca plate subducting along the peru chile trench produces a magnetic anomaly of less than 0 5 nt no distinct anomaly observed by magsat over this region

depositional features and stratigraphic sections in granitic plutons implications for the emplacement and crystallization of granitic magma abstract many granitic plutons contain sheet like masses of dioritic to gabbroic rocks or swarms of mafic to intermediate enclaves which represent the input of higher temperature more mafic magma during crystallization of the granitic plutons small scale structures associated with these bodies e g load cast and compaction features silicic pipes extending from granitic layers into adjacent gabbroic sheets indicate that the sheets and enclave swarms were deposited on a floor of the magma chamber on granitic crystal mush and beneath crystal poor magma while the mafic magma was incompletely crystallized these structures indicate way up typically toward the interior of the intrusions and appear to indicate that packages of mafic sheets and enclave concentrations in these plutons are a record of sequential deposition hence these plutons preserve a stratigraphic history of events involved in the construction filling replenishment and crystallization of the magma chamber the distinctive features of these depositional portions of plutons allow them to be distinguished from sheeted intrusions which usually preserve mutual intrusive contacts and dike sill relations of different magma types the considerable thickness of material that can be interpreted as depositional and the evidence for replenishment suggest that magma chamber volumes at any one time were probably much less than the final size of the pluton thus magma chambers may be constructed much more slowly than presently envisaged the present steep attitudes of these structures in many plutons may have developed gradually as the floor of the chamber along with the underlying solidified granite and country rock sank during continuing episodes of magma chamber replenishment these internal magmatic structures support recent suggestions that the room problem for granites could be largely accommodated by downward movement of country rock beneath the magma chamber

magnetic mineralogy of the lower continental crust rocks that were once present in the deep continental crust can be found in surface exposure as xenoliths as parts of high grade metamorphic and plutonic terrains and as obducted lower crustal cross sections a review of magnetic and mineralogic studies of samples representing all such occurrences shows that magnetite is the only magnetic phase present in significant amounts in these rocks thermodynamic considerations indicate that tipoor magnetite is the expected magnetic oxide under equilibrium conditions that exist in the lower crust regardless of the primary magmatic composition of the spinel phase furthermore it seems unlikely that serpentinites with metal alloys contribute significantly to the magnetization of the lower crust or mantle thus magnetite in the deep crust is the most likely candidate for the source of the long wavelength magnetic anomalies observed in regional aeromagnetic and satellite magnetometer surveys if so the entire crust should be magnetic in most continental areas including shields in areas of high heat flow the bottom of the magnetic crust may lie considerably above the moho

deciphering the impact of diagenesis overprint on negative δ13c excursions using rock magnetism case study of ediacaran carbonates yangjiaping section south china rock magnetism is used here to investigate the genesis of one of the puzzling negative carbon isotopic excursions of the neoproterozoic in the yangtze platform south china a detailed characterization of the magnetic mineralogy which includes low temperature and high field magnetometry and classical magnetic measurement arm irm susceptibility was therefore performed along upper doushantuo and lower dengying formations outcropping in the yangjiaping section the derived magnetic parameters show variations that can be interpreted as variations in magnetic grains size and in oxide contents they show that the magnetic content is significantly reduced in samples presenting negative 13ccalcite values we interpret this as a result of magnetite dissolution and secondary carbonate precipitation during early diagenesis bacterial sulfate reduction combined with c and o isotopic data paleomagnetic techniques thus show that the upper doushantuo lower dengying negative excursion of the yangjiaping section is largely due to diagenesis although the preservation of a genuine 13c excursion of lower magnitude from 7 down to 0 instead of down to 9 as usually considered cannot be ruled out a corrected 13ccarbonate chemostratigraphic curve is therefore proposed the unambiguous identification of a strong diagenetic component for this excursion casts doubts on the primary nature of other potentially time equivalent negative excursions of the yangtze platform and thus to its correlation to negative excursions in other cratons i e shuram excursion more generally this study illustrates the potential of magnetic mineralogy characterization a low cost time efficient and non destructive technique as screening tool for diagenetic overprints of 13c and 18o

dynamics of the shallow meridional circulation around intertropical convergence zones the generally accepted view of the meridional circulation in the tropical east pacific is that of a single deep overturning cell driven by deep convective heating in the intertropical convergence zone itcz similar to the zonal mean hadley circulation however recent observations of the atmosphere from the tropical eastern pacific have called this view into question in several independent datasets significant meridional return flows out of the itcz region were observed not only at high altitudes but also at low altitudes just above the atmospheric boundary layer this paper presents a theory and idealized simulations to understand the causes and dynamics of this shallow meridional circulation smc fundamentally the smc can be seen as a large scale sea breeze circulation driven by sea surface temperature gradients when deep convection is absent in the itcz region a simple model of this circulation is presented using observed values the sea breeze model shows that the pressure gradient above the boundary can indeed reverse leading to the pressure force that drives the shallow return flow out of the itcz the weather research and forecast model wrf is used to simulate an idealized hadley circulation driven by moist convection in a tropical channel the smc is reproduced with reasonable similarity to the circulation observed in the east pacific the simulations confirm that the smc is driven by a reversal of the pressure gradient above the boundary layer and that the return flow is strongest when deep convection is absent in the itcz and weakest when deep convection is active the model also shows that moisture transport out of the itcz region is far greater in the low level shallow return flow than in the high altitude return flow associated with the deep overturning and that a budget for water transport in and out of the itcz region is grossly incomplete without it much of the moisture carried in the shallow return flow is recycled into the boundary layer but does not appear to contribute to enhanced cloudiness in the subtropical stratocumulus poleward of the itcz

high temperature alteration of oceanic gabbros by seawater hess deep ocean drilling program leg 147 evidence from oxygen isotopes and elemental fluxes two major stages of high temperature water rock interactions have been identified in gabbros of oceanic layer 3 exposed in the hess deep rift valley at the east pacific rise 2 n 101 w the 154 m of plutonic rocks cored at site 894g show mineralogies and textures that suggest that they represent the roof of an approximate 1 myr magma chamber a late stage of hydrothermal alteration produced mineral assemblages typical of lower amphibolite upper greenschist facies conditions the hydration of the gabbros occurred locally through the development of well crystallized green amphiboles significant chemical fluxes were associated with the fluid flow and precipitation dissolution mechanisms oxygen isotope data suggest an earlier cryptic stage of hydrothermal alteration of the hess deep gabbros at temperatures above 500 c hydration was limited with only minute amphibole lamellae in pyroxenes that trapped cl na and minor k from seawater ca plagioclases remained stable and preserved their magmatic cationic compositions and zoning patterns we propose that this high temperature event lowered the 18o of the gabbros by isotopic exchange with a hydrothermal fluid through solid state oxygen diffusion although it is unclear whether the 18o depletion of hole 894g gabbros is representative of the entire lower oceanic crust it is nevertheless thought to contribute to the oxygen isotope buffering of the oceans during this stage water rock ratios calculated using sr isotopes are underestimated 0 1 0 5 because of insignificant diffusion of this cation in plagioclases better estimates of water rock ratios are provided by oxygen isotopes 0 2 1 which are important in unraveling the cooling history of the oceanic crust

remagnetization of carbonate rocks in southern tibet perspectives from rock magnetic and petrographic investigations netherlands organization for scientific research nwo with a rubicon grant 825 15 016 institute for rock magnetism irm at the university of minnesota instruments and facilities program of nsf erc starting grant 306810 nwo vidi 864 11 004

magnetic properties of single and multi domain magnetite under pressures from 0 to 6 gpa 1 using novel experimental methods we measured the acquisition of isothermal remanent magnetization direct field demagnetization and alternating field demagnetization of multi domain md and single domain sd magnetite under hydrostatic pressures to 6 gpa we find that the saturation remanence of md magnetite increases 2 8 times over initial non compressed values by 6 gpa while its remanent coercivity remains relatively constant for sd magnetite remanent coercivity and saturation remanence vary little from 0 to 1 gpa increase markedly from 1 to 3 gpa then plateau above 3 gpa these new findings suggest that by 3 gpa sd magnetite either undergoes a magnetic phase transition or that it reaches an optimal magnetic state where magnetostriction and or magnetocrystalline energy constants attain some threshold state without reorganization of the pre existing magnetic lattices similar behavior is not observed in md magnetite likely due to domain wall effects

petrology and geochemistry of late stage intrusions of the a type mid proterozoic pikes peak batholith central colorado usa implications for petrogenetic models abstract the 1 08 ga anorogenic a type pikes peak batholith front range central colorado is dominated by coarse grained biotite amphibole syenogranites and minor monzogranites collectively referred to as pikes peak granite ppg the batholith is also host to numerous small late stage plutons that have been subdivided into two groups e g wobus 1976 studies in colorado field geology colorado school of mines professional contributions colorado 1 a sodic series sio 2 44 78 wt k na 0 32 1 36 composed of gabbro diabase syenite quartz syenite and fayalite and sodic amphibole granite and 2 a potassic series sio 2 70 77 wt k na 0 95 2 05 composed of biotite granite and minor quartz monzonite differences in major and trace element and nd isotopic characteristics for the two series indicate different petrogenetic histories potassic granites of the late stage intrusions appear to represent crustal anatectic melts derived from tonalite sources based on comparison of their major element compositions with experimental melt products in addition nd isotopic characteristics of the potassic granites e nd 1 08 ga 0 2 to 2 7 overlap with those for tonalites granodiorites ca 1 7 ga boulder creek intrusions e nd 1 08 ga 2 4 to 3 6 exposed in the region some of the partial melts evolved by fractionation dominated by feldspar the late stage potassic granites share geochemical characteristics with most of the ppg which is also interpreted to have an anatectic origin involving tonalitic crust the origin of monzogranites associated with the ppg remains unclear but mixing between granitic and mafic or intermediate magmas is a possibility syenites and granites of the sodic series cannot be explained as crustal melts but are interpreted as fractionation products of mantle derived mafic magmas with minor crustal input high temperature and low oxygen fugacity estimates e g frost et al 1988 american mineralogist 73 727 740 support a basalt fractionation origin as do e nd values for sodic granitoids e nd 1 08 ga 2 2 to 0 7 which are higher than e nd values for colorado crust at 1 08 ga ca 1 0 to 4 0 enrichments in incompatible elements e g rare earth elements rb y and depletions in compatible elements e g cr sr ba in the sodic granitoids compared to coeval mafic rocks are also consistent with fractionation accessory mineral fractionation release of fluorine rich volatiles and or removal of pegmatitic fluids could have modified abundances of ce nb zr and y in some sodic granitoid magmas gabbros and mafic dikes associated with the sodic granitoids have e nd 1 08 ga of 3 0 to 3 5 which are lower than depleted mantle at 1 08 ga and their trace element characteristics suggest derivation from mantle sources that were previously affected by subduction related processes however it is difficult to characterize the mantle component in these magmas because assimilation of crust during magma ascent could also result in their observed geochemical features the pikes peak batholith is composed of at least two petrogenetically different granite types both of which exhibit geochemical characteristics typical of a type granites models proposed for the petrogenesis of the granitoids imply the existence of mafic rocks at depth and addition of juvenile material to the crust in central colorado at 1 1 ga

petrography and uplift history of the quaternary takidani granodiorite could it have hosted a supercritical hdr geothermal reservoir abstract the quaternary takidani granodiorite japan alps is analogous to the type of deep seated 3 5 km deep intrusive hosted fracture network system that might support supercritical hot dry wet rock hdr hwr energy extraction the i type takidani granodiorite comprises porphyritic granodiorite porphyritic granite biotite hornblende granodiorite hornblende biotite granodiorite biotite hornblende granite and biotite granite facies the intrusion has a reverse chemical zonation characterized by 70 wt sio2 at its inferred margin and 600 c hypersaline 35 wt nacleq fluids of magmatic origin with inferred minimum pressures of formation being 600 750 bar which corresponds to fluid entrapment at 2 4 3 0 km depth al in hornblende geobarometry indicates that hornblende crystallization occurred at about 1 45 ma 7 7 9 4 km depth in the marginal eastern takidani granodiorite but later at 1 25 ma and shallower 6 5 7 0 km near the core of the intrusion the average rate of uplift across the takidani granodiorite from the time of hornblende crystallization has been 5 1 5 9 mm yr although uplift was about 7 5 mm yr prior to 1 2 ma which is faster than average uplift rates in the japan alps 3 mm yr during the last 2 million years a temperature depth time window when the takidani granodiorite had potential to host an hdr system would have been when the internal temperature of the intrusive was cooling from 500 c to 400 c taking into account the initial 7 5 mm yr rate of uplift and effects of erosion an optimal temperature time depth window is proposed for 500 c at 1 54 1 57 ma and 5 2 0 9 km drilling depth and 400 c at 1 36 1 38 ma and 3 3 0 8 km drilling depth which is within the capabilities of modern drilling technologies and similar to measured temperature depth profiles in other active hydrothermal systems e g at kakkonda japan

cooling history of nested plutons from the variscan tichka plutonic complex morocco four imbricated mafic to felsic plutons of variscan age from morocco have been investigated for their cooling history and geochemical interactions with surrounding continental rocks oxygen isotope compositions of whole rocks and minerals have been used to model the cooling rates of these kilometer sized intrusions by combining both the knowledge of oxygen self diffusion data of rock forming minerals and the determination by ir spectroscopy of the water content of quartz the cooling times are estimated ranging from 10 to 5 10 years in agreement with the shallow emplacement 4 6 km depth of these intrusions into the continental crust such fast cooling rates could explain why after assimilation of the various country rocks heterogeneities of both neodymium and strontium isotope ratios were still preserved a progressive o increase from the mafic to felsic terms of the plutonic suite which does not excess 1 to 1 5 could be explained by the assimilation of metamorphosed pelitic and volcanic rocks that constitute the basement of the tichka plutonic complex

rates of magma transfer in the crust insights into magma reservoir recharge and pluton growth plutons have long been viewed as crystallized remnants of large magma reservoirs a concept now challenged by high precision geochronological data coupled with thermal models similarly the classical view of silicic eruptions fed by long lived magma reservoirs that slowly differentiate between mafic recharges is being questioned by petrological and geophysical studies in both cases a key and yet unresolved issue is the rate of magma transfer in the crust here we use thermal analysis of magma transport to calculate the minimum rate of magma transfer through dikes we find that unless the crust is exceptionally hot the recharge of magma reservoirs requires a magma supply rate of at least 0 01 km 3 yr much higher than the long term growth rate of plutons which demonstrates unequivocally that igneous bodies must grow incrementally this analysis argues also that magma reservoirs are short lived and erupt rapidly after being recharged by already differentiated magma these findings have significant implications for the monitoring of dormant volcanic systems and our ability to interpret geodetic surface signals related to incipient eruptions

the effect of hydrostatic pressure up to 1 61 gpa on the morin transition of hematite bearing rocks implications for planetary crustal magnetization we present new experimental data on the dependence of the morin transition temperature tm on hydrostatic pressure up to 1 61 gpa obtained on a well characterized multidomain hematite bearing sample from a banded iron formation we used a nonmagnetic high pressure cell for pressure application and a superconducting quantum interference device magnetometer to measure the isothermal remanent magnetization irm under pressure on warming from 243 k to room temperature t0 irm imparted at t0 under pressure in 270 mt magnetic field irm270mt is not recovered after a cooling warming cycle memory effect under pressure was quantified as irm recovery decrease of 10 gpa tm determined on warming reaches t0 under hydrostatic pressure 1 38 1 61 gpa the pressure dependence of tm up to 1 61 gpa is positive and essentially linear with a slope dtm dp 25 2 k gpa this estimate is more precise than previous ones and allows quantifying the effect of a pressure wave on the upper crust magnetization with special emphasis on mars

shallow meridional circulations in the tropical atmosphere abstract a shallow meridional circulation smc in the tropical atmosphere features a low level e g 700 hpa flow that is in the opposite direction to the boundary layer monsoon or trade wind flow and is distinct from the meridional flow above representations of the smc in three global reanalyses show both similarities and astonishing discrepancies while the smc over west africa appears to be the strongest it also exists over the eastern atlantic and eastern pacific oceans and over the indian subcontinent with different strength and structure all smcs undergo marked seasonal cycles the smcs are summarized into two types one associated with the marine itcz and the other with the summer monsoon the large scale conditions for these two types of smcs are similar a strong meridional gradient in surface pressure linked to surface temperature distributions and an absence of deep moist convection the processes responsible for these conditions are different for the two types of smcs as are their str

robust elements of snowball earth atmospheric circulation and oases for life 1 atmospheric circulation in a snowball earth is critical for determining cloud behavior heat export from the tropics regions of bare ice and sea glacier flow these processes strongly affect snowball earth deglaciation and the ability of oases to support photosynthetic marine life throughout a snowball earth here we establish robust aspects of the snowball earth atmospheric circulation by running six general circulation models with consistent snowball earth boundary conditions the models produce qualitatively similar patterns of atmospheric circulation and precipitation minus evaporation the strength of the snowball hadley circulation is roughly double modern at low co2 and greatly increases as co2 is increased we force a 1 d axisymmetric sea glacier model with general circulation model gcm output and show that neglecting zonal asymmetry sea glaciers would limit ice thickness variations to o 10 global mean ice thickness in the 1 d sea glacier model is well approximated by a 0 d ice thickness model with global mean surface temperature as the upper boundary condition we then show that a thin ice snowball solution is possible in the axysymmetric sea glacier model when forced by output from all the gcms if we use ice optical properties that favor the thin ice solution finally we examine snowball oases for life using analytical models forced by the gcm output and find that conditions become more favorable for oases as the snowball warms so that the most critical time for the survival of life would be near the beginning of a snowball earth episode

lower crustal melting and the role of open system processes in the genesis of syn orogenic quartz diorite granite leucogranite associations constraints from sr nd o isotopes from the bandombaai complex namibia abstract the bandombaai complex southern kaoko belt namibia consists of three main intrusive rock types including metaluminous hornblende and sphene bearing quartz diorites allanite bearing granodiorites and granites and peraluminous garnet and muscovite bearing leucogranites intrusion of the quartz diorites is constrained by a u pb zircon age of 540 3 ma quartz diorites granodiorites and granites display heterogeneous initial nd and o isotope compositions end 540 ma 6 3 to 19 8 18o 9 0 11 6 but rather low and uniform initial sr isotope compositions 87sr 86srinitial 0 70794 0 70982 two leucogranites and one aplite have higher initial 87sr 86sr ratios 0 70828 0 71559 but similar initial end 11 9 to 15 8 and oxygen isotope values 10 5 12 9 the geochemical and isotopic characteristics of the bandombaai complex are distinct from other granitoids of the kaoko belt and the central zone of the damara orogen our study suggests that the quartz diorites of the bandombaai complex are generated by melting of heterogeneous mafic lower crust based on a comparison with results from amphibolite dehydration melting experiments a lower crustal garnet and amphibole bearing metabasalt probably enriched in k2o is a likely source rock for the quartz diorites the granodiorites granites show low rb sr 1 and are most likely generated by biotite dehydration melting of heterogeneous felsic lower crust all segments of the lower crust underwent partial melting during the pan african orogeny at a time 540 ma when the middle crust of the central damara orogen also underwent high t medium p regional metamorphism and melting geochemical and isotope data from the bandombaai complex suggest that the pan african orogeny in this part of the orogen was not a major crust forming episode instead even the most primitive rock types of the region the quartz diorites represent recycled lower crustal material

paleomagnetic and rock magnetic study of the mistastin lake impact structure labrador canada implications for geomagnetic perturbation and shock effects abstract we carried out an integrated rock magnetic and paleomagnetic study of the 36 ma mistastin lake labrador canada meteorite impact structure in order to investigate whether energy from the collision influenced the geodynamo and to assess the effects of shock on the magnetic properties of the target basement rocks stepwise demagnetization of 114 specimens isolates a well defined magnetization component throughout the crater whose overall mean deviates slightly from the expected direction for north america at the time of impact paleointensity results from seven samples meeting stringent selection criteria show no significant difference with a global compilation from 40 to 30 ma the combined results including those from a 80 m thick profile of an impact melt unit discovery hill lend no support that the impact caused an aberration of the geodynamo within a few centuries of a bolide collision that created the 28 km diameter crater both titanium rich and titanium poor titanomagnetite carry the magnetic remanence in the impact melt rocks their relative proportions compositions and domain states are cooling rate dependent magnetic hysteresis parameters of the magnetite bearing anorthositic basement rocks reveal systematic changes as a function of distance from the crater s center with an increasing prevalence of single domain like grains toward the center changes with radial distance are also found in the character of the verwey transition in magnetite basement rocks were thermally overprinted when lying less than a meter from the impact melt rocks mesoproterozoic basement rocks more than a meter below the impact melt rocks hold similar magnetization directions to those expected from a 1500 ma result for laurentia no evidence exists that shock heating of the basement rocks exceeded 200 c at distances of 6 7 km from the crater s center

relative rheological evolution of chemically contrasted coeval magmas example of the tichka plutonic complex morocco the tichka plutonic complex presents many unusual structures related to physical interaction between mafic and felsic coeval magmas in this paper a rheological approach is used to attempt to understand the various observed structures after a general presentation of rheological laws and a discussion about the present state of our understanding of the rheology of suspensions we analyse the main steps of rheological evolution of magmas during crystallization taking into account the effect of cooling increase of silica content and increase of crystallinity on viscosity during thermal evolution of magma a general rheological diagram is presented two main rheological thresholds at about 35 crystallinity th1 and about 65 70 th2 allow us to distinguish three major steps in the rheological behaviour of magmas during cooling comparative study of evolution curves for viscosity of coeval mafic m and felsic f magmas shows that an inversion temperature ti exists at which viscosities of both magmas are the same this point has major petrologic significance because the viscosity ratio m f inverses during cooling when the temperature of magma falls below the ti point a model for the evolution of the tichka magmas during cooling has been derived from the knowledge of the chemical and mineralogical composition of the different rocks representing the original m and f magmas as well as some hybrid and differentiated samples viscosities and crystal fractions of magmas as functions of temperature have been calculated from chemical and mineralogical data curves representing the evolution of apparent viscosity of the three initial magmas of the complex are presented from these vicosity temperature curves and from the calculted temperatures corresponding to rheological thresholds th1 and th2 for the different coeval magmas five main evolution stages are defined the different structures observed at the contracts between m and f magmas are developed throughout the entire cooling history of the pluton each structure being related to one of the main evolution stages

seasonal simulations of the planetary boundary layer and boundary layer stratocumulus clouds with a general circulation model abstract the ucla general circulation model gcm has been used to simulate the seasonally varying planetary boundary layer pbl as well as boundary layer stratus and stratocumulus clouds the pbl depth is a prognostic variable of the gcm incorporated through the use of a vertical coordinate system in which the pbl is identified with the lowest model layer stratocumulus clouds are assumed to occur whenever the upper portion of the pbl becomes saturated provided that the cloud top entrainment instability does not occur as indicated by arakawa and schubert cumulus clouds are assumed to originate at the pbl top and tend to make the pbl shallow by drawing on its mass results are presented from a three year simulation starting from a 31 december initial condition obtained from an earlier run with a different version of the model the simulated seasonally varying climates of the boundary layer and free troposphere are realistic the observed geographical and seasonal variations of stratocumulus cloudi

tectonic record magmatic history and hydrothermal alteration in the hercynian guerande leucogranite armorican massif france abstract the guerande peraluminous leucogranite was emplaced at the end of the carboniferous in the southern part of the armorican massif at the scale of the intrusion this granite displays structural heterogeneities with a weak deformation in the southwestern part whereas the northwestern part is marked by the occurrence of s c and mylonitic extensional fabrics quartz veins and pegmatite dykes orientations as well as lineations directions in the granite and its country rocks demonstrate both e w and n s stretching therefore during its emplacement in an extensional tectonic regime the syntectonic guerande granite has probably experienced some partitioning of the deformation the southwestern part is characterized by a muscovite biotite assemblage the presence of restites and migmatitic enclaves and a low abundance of quartz veins compared to pegmatite dykes in contrast the northwestern part is characterized by a muscovite tourmaline assemblage evidence of albitization and gresenization and a larger amount of quartz veins the southwestern part is thus interpreted as the feeding zone of the intrusion whereas the northwestern part corresponds to its apical zone the granite samples display continuous compositional evolutions in the range of 69 8 75 3 wt sio 2 high initial 87 sr 86 sr ratios and low e nd t values suggest that the peraluminous guerande granite a cnk 1 1 was formed by partial melting of metasedimentary formations magmatic evolution was controlled primarily by fractional crystallization of k feldspar biotite and plagioclase an20 the samples from the apical zone show evidence of secondary muscovitization they are also characterized by a high content in incompatible elements such as cs and sn as well as low nb ta and k rb ratios the apical zone of the guerande granite underwent a pervasive hydrothermal alteration during or soon after its emplacement u th pb dating on zircon and monazite revealed that the guerande granite was emplaced 309 7 1 3 ma ago and that a late magmatic activity synchronous with hydrothermal circulation occurred at ca 303 ma these new structural petrological and geochronological data presented for the guerande leucogranite highlight the interplay between the emplacement in an extensional tectonic regime magmatic differentiation and hydrothermal alteration and provide a general background for the understanding of the processes controlling some mineralization in the western european hercynian belt

the influence of stress path on thermoremanent magnetization the influence of stress path on thermoremanent magnetization trm was studied experimentally specimens with a trm were cyclically loaded under three different conditions hydrostatic pressure uniaxial compression or uniaxial strain initial loading resulted in a large decrease in magnetization at stress and permanent demagnetization at the termination of the stress cycle the observed magnetic changes were much larger for non hydrostatic loading than for hydrostatic compression with subsequent loading to the same peak stress over the same path stress induced variations in magnetization were smaller and tended to become reproducible shear stress produced greater magnetic perturbations

what is magnetic in the lower crust long wavelength and satellite magnetic anomalies require that some regions of the earth s crust contain minerals that are magnetic at lower crustal conditions studies of high grade metamorphic rocks emphased multi domain magnetite as a likely source members of the hematite ilmenite series were not considered because hematite commonly has a relatively weak magnetization and though ilmenite compositions in the range ilm 50 70 can be strong ferrimagnets their low curie t 260 c makes them an unlikely source however recent investigations have described hematite ilmenite magnetism and coercivity in terms of a new ferrimagnetic substructure related to interfaces between ilmenite and hematite in exsolution lamellae down to unit cell thickness in this lamellar magnetism ferrimagnetism is coupled to principal af moments of hematite and retains many properties of titanohematite including high coercivity and thermal stability if exsolution lamellae are the source of the magnetism a temperature above that predicted for lamellar resorption should destroy it to look at behaviour of lamellae at mid to lower crustal conditions we selected a hemo ilmenite sample ilm 84 on which detailed studies of microstructures mineral chemistry and magnetic properties had been made we designed experiments at 10 kbar as an analog for lower crustal conditions after initial 5 day runs at 540 685 and 950 c a temperature of 580 c was chosen for 8 and 21 day runs at one atmosphere this t for ilm 84 is above that predicted for complete resorption of all lamellae to produce a homogeneous paramagnetic r3 ilmenite in these experiments exsolution lamellae coarsened if these features were reproduced in the lower crust hemo ilmenite or ilmeno hematite would retain a magnetic moment and higher than expected coercivity due to the microstructures emp and tem analyses were used to understand the compositional differences between the natural sample products of the experiments and one atmosphere phase diagrams if the experimental changes are interpreted as moving the compositions closer to equilibrium at 580 c 10 kbar then a considerable widening of miscibility gaps as a function of pressure and the 20 geikielite component in the ilmenite as well as an increase in neel temperature of hematite is predicted an alternate is to consider the response as ostwald steps short of equilibrium experimental results indicate that exsolution microstructures play a key role in mineral magnetism and that such microstructures as well as coarse hematite may be preserved at pressures and temperatures of the lower crust

shallow meridional circulation in the tropical eastern pacific abstract based on four independent datasets this study presents observational evidence of a shallow meridional circulation cell in the eastern tropical pacific in this shallow meridional circulation cell the northerly cross equatorial return flow from the itcz into the southern hemisphere is found immediately above the atmospheric boundary layer in contrast to the classic concept of the hadley type deep meridional circulation whose northerly return flow resides in the upper troposphere the strength and depth of this shallow meridional circulation undergo a distinct annual cycle possible causes and climatic implications of this shallow meridional circulation are discussed

remanent magnetization and coercivity of rocks under hydrostatic pressure up to 1 4 gpa 1 we designed an isothermal remanent magnetization irm acquisition system based on permanent magnets and sized to accommodate an amagnetic hydrostatic pressure cell this pressure cell fits in a superconducting rock magnetometer allowing for the measurement of remanent magnetization of pressurized samples with this system we determined the coercivity of remanence bcr at different hydrostatic pressures up to 1 4 gpa for rock and dispersed mineral samples with various magnetic mineralogy and domain state irm and bcr are nearly identical before compression and after decompression indicating no permanent changes in the magnetic properties during pressure cycling hydrostatic pressure up to 1 4 gpa does not significantly increases irm under pressure except for multidomain pyrrhotite and magnetite which show an increase of about 40 relative increase of bcr under pressure is mild except for a near single domain titanomagnetite where bcr doubles

extent of hadley circulations in dry atmospheres the subtropical terminus of the hadley circulation is interpreted as the latitude poleward of which vertical wave activity fluxes meridional eddy entropy fluxes become sufficiently deep to reach the upper troposphere this leads to a sign change of the upper tropospheric divergence of meridional wave activity fluxes convergence of meridional eddy angular momentum fluxes and marks the transition from the tropical hadley cell to the extratropical ferrel cell a quantitative formulation for determining the depth of vertical wave activity fluxes and thus the terminus of the hadley circulation is proposed based on the supercriticality a measure of the slope of isentropes the supercriticality assumes an approximately constant value at the terminus of the hadley circulation in a series of simulations with an idealized dry general circulation model however it is unclear how to generalize this supercriticality based formulation to moist atmospheres

diffusion effects on oxygen isotope temperatures of slowly cooled igneous and metamorphic rocks recently obtained data on oxygen diffusion in feldspars quartz and hornblende permit the prediction of the apparent18o16o temperatures that would be measured in a rock that consisted only of those three minerals and cooled slowly from high temperature the computed temperatures would be based on the differences in the18o16o ratios between coexisting pairs of minerals the present calculation takes into account the diffusion rates for oxygen as a function of temperature the cooling rate of the rock the mineral grain sizes and the mode of the rock for mineral grains 1 mm in radius and a cooling rate of 10 c m y the minimum difference in apparent temperature between quartz feldspar and feldspar hornblende pairs will be 115 c despite the assumption of a normal uneventful slow cooling history to room temperature further the apparent quartz hornblende temperature will range over 30 c 590 620 c depending on the mode of the rock for a cooling rate of 1000 c m y the apparent difference in temperature can be as much as 400 c consequently consistency in temperatures obtained by oxygen isotope analysis should not be expected in most high grade metamorphic rocks or igneous rocks which are cooled slowly departures from the pattern of temperatures obtained in this model would imply a very rapid quench from high temperature or a complex history for the rock for some minerals including hornblende the relation between temperature and the equilibrium fractionation of oxygen isotopes between coexisting phases has been derived from observed relations in natural specimens the choice of the specimens used for such calibrations needs to be re evaluated in light of these findings this may result in a change in the equilibrium equation constants an example from the literature the san jose tonalite baja california mexico was modelled and yields 18o concentrations in the minerals that correspond closely with the measured values this suggests that the model used is appropriate that the rock has had a simple thermal history and that it cooled at 100 200 c m y over the temperature range 800 500 c the set of paleotemperatures obtained for a rock will in general yield neither the mineral closure temperatures nor the formation or crystallization temperatures on the other hand the cooling rate of the rock may be derived from the data this in turn may have important tectonic implications with regard to denudation and uplift rates

magnetic characterization of non ideal single domain monoclinic pyrrhotite and its demagnetization under hydrostatic pressure up to 2 gpa with implications for impact demagnetization here we present a comprehensive magnetic characterization of synthesized non ideal single domain sd monoclinic pyrrhotite fe 7 s 8 the samples were in the form of a powder and a powder dispersed in epoxy non ideal refers to a powder fraction of predominantly sd size with a minor contribution of small pseudo single domain grains such non ideal sd pyrrhotite was found to be a remanence carrier in several types of meteorites carbonaceous chondrites snc which justifies the usage of synthetic compositions as analogous to natural samples data were collected from 5 to 633 k and include low field magnetic susceptibility 0 thermomagnetic curves major hysteresis loops back field remanence demagnetization curves first order reversal curves forcs alternating field and pressure demagnetization of saturation isothermal remanent magnetization sirm low temperature data such as zero field cooled and field cooled remanence datasets together with room temperature sirm cooling warming cycles as well as xrd and mossbauer spectra the characteristic besnus transition is observed at 33 k forc diagrams indicate interacting sd grains the application of hydrostatic pressure up to 2 gpa using nonmagnetic high pressure cells resulted in the demagnetization of the sample by 32 38 repeated cycling from 1 8 gpa to atmospheric pressure and back resulted in a total remanence decrease of 44 after 3 cycles pressure demagnetization experiments have important implications for meteorite paleomagnetism and suggest that some published paleointensities of meteorites with non ideal sd monoclinic pyrrhotite as remanence carrier may be lower limits because shock demagnetization was not accounted for

the cooling history of the late pliocene eldzhurtinskiy granite caucasus russia and the thermochronological potential of grain size age relationships abstract isotopic age investigations have been carried out on samples from the late pliocene eldzhurtinskiy granite great caucasus russia in order to elucidate the cooling history of this kilometere sized plutonic body the samples were taken from a horizontal profile from the edge to the centre of the granite and from a vertical profile from the top to a depth of 4570 m the 40ar 39ar results on biotites ranging from 2 5 to 1 2 ma indicate a vertical age zoning of the granite caused by regional uplift an uplift rate of 4 mm a is calculated from the age differences together with the results of fission track dating on apatite and zircon and the present day rock temperatures the biotite ages can be used to evaluate cooling rates below 400 c the cooling rates of about 180 c ma are relatively high and spatially and temporarily fairly uniform throughout the granite a geothermal gradient of 45 c km is derived from uplift and cooling rates this is identical to the present day borehole gradient the 40ar 39ar ages of distinct biotite grain fractions from one and the same sample specimen are discordant generally they show clear age grain size correlation as expected from ar diffusion theory for cooling systems thus the biotite results support the additional detailed thermochronological information the cooling rates in the temperature range of biotite closure based on the grain size age correlations of the biotites are close to 180 c ma they provide evidence that within a certain temperature interval the time temperature path of a rock may be determined exclusively by isotopic dating of different grain size fractions of only one type of mineral

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

thermoremanence acquisition and demagnetization for titanomagnetite under lithospheric pressures the geological sources of large scale lithospheric magnetic field anomalies are poorly constrained understanding the magnetic behavior of rocks and minerals under the pressures and temperatures encountered at large crustal depths is particularly important in that task the impact of lithospheric pressure is not well known and most of the time neglected in numerical models of the geological sources of magnetic anomalies we present thermal remanent magnetization trm acquisition and stepwise thermal demagnetization on synthetic titanomagnetite dispersed powder within an amagnetic cell under hydrostatic pressure up to 1 gpa trm is measured after thermal cycling within a cryogenic magnetometer pressure dependent increase in the curie temperature initially in the 50 70 c range is observed mostly between 0 3 and 0 6 gpa on the order of 20 k gpa trm intensity also increases with pressure up to 200 at 675 mpa although the pressure variation with temperature inside the cell complicates the interpretation

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

magnetic discrimination between al substituted hematites synthesized by hydrothermal and thermal dehydration methods and its geological significance received 22 june 2011 revised 20 december 2011 accepted 24 december 2011 published 23 february 2012 1 hematite a ubiquitous mineral in aerobic sediments and soils of temperate and warm areas is weakly magnetic however it carries a stable natural remanent magnetization and thus can reflect paleoenvironment changes to quantify the influence of al content in hematite on its magnetic properties two series of hematite particles were prepared by hydrothermal transformation of ferrihydrite in aqueous suspension hfh series and by thermal dehydration of goethite hg series crystal morphological and mineral magnetic properties of these two types of hematites differ distinctively more specifically the hfh series samples display oblate plate like morphologies while the hg series samples are prolate highly acicular hfh series samples display higher saturation magnetization but lower magnetic coercivity than that of the hg series it is tenable that a better lattice ordering of al substitution occurs during the process of dehydration of goethite than after transformation from ferrihydrite resulting in weaker saturation magnetization for hg series samples the origin of single domain sd hematite in nature can be diagnosed by the correlation of unblocking temperature and magnetic coercivity a positive correlation indicates the presence of pure al free sd hematite while a negative correlation indicates a chemical origin of sd al substituted hematite these results bear new information on decoding the complex magnetic properties of sd al hematite in nature environments and thus deepen our understanding of the mechanism of variations in both paleomagnetic and paleoenvironmental signals carried by al hematite

physical controls and depth of emplacement of igneous bodies a review the formation and growth of magma bodies are now recognised as involving the amalgamation of successive discrete pulses such as sills sills would thus represent the building blocks of larger plutons sensu lato mechanical and thermal considerations on the incremental development of these plutons raise the issue of the crustal levels at which magma can stall and accumulate as sills reviewing the mechanisms that could a priori explain sill formation it is shown that principal physical controls include rigidity contrast where sills form at the interface between soft strata overlaid by comparatively stiffer strata rheology anisotropy where sills form within the weakest ductile zones and rotation of deviatoric stress where sills form when the minimum compressive stress becomes vertical comparatively the concept of neutral buoyancy is unlikely to play a leading control in the emplacement of sills although it could assist their formation these different controls on sill formation however do not necessarily operate on the same length scale the length scale associated with the presence of interfaces separating upper stiffer layers from lower softer ones determines the depth at which rigiditycontrolled sills will form on another hand the emplacement depths for rheology controlled sills is likely to be determined by the distribution of the weakest ductile zones whereas the emplacement depth of stress controlled sills is determined by a balance between the horizontal maximum compressive stress which favours sill formation and the buoyancy of their feeder dykes which drives magma vertically ultimately the depth at which a sill forms depends on whether crustal anisotropy or stress rotation is the dominant control i e which of these processes operates at the smallest length scale using dimensional analysis it is shown that sill formation controlled by remote stress rotation would occur on length scales of hundreds of meters or greater this therefore suggests that crustal heterogeneities and their associated anisotropy are likely to play a larger role than remote stress rotation in controlling sill emplacement unless these heterogeneities are several hundred meters or more apart this also reinforces the role of local stress barriers owing to interactions between deviatoric stress and crustal heterogeneities in the formation of sills

magnetic characterisation of magnetite and hematite from the blotberget apatite iron oxide deposits bergslagen south central sweden rock magnetic measurements were carried out on drill core material and hand specimens from the blotberget apatite iron oxide deposit in the bergslagen ore province south central sweden to chara

a new look at stable isotope thermometry interdiffusion between coexisting minerals affects all rocks and causes resetting and discordance of stable isotope geothermometers that is commonly observed in slowly cooled igneous and metamorphic rocks the fast grain boundary fgb model describes the stable isotope fractionations and intracrystalline zonation which result from closed system interdiffusion eiler et al 1991 1992 this model assumes that grain boundary diffusion is much faster than volume diffusion and it accounts for exchange among all minerals in a rock previous models of closure temperature violate mass balance restrictions and will be inaccurate in most rocks modeling results are described for amphibolites and hornblende granites and gneisses biotite granites schists and gneisses pelitic and semi pelitic rocks garnet peridotites anorthosites gabbros pyroxenites and related rocks and calc silicate rocks examples of mineral pairs and specific rock types that allow accurate stable isotope thermometry include plagioclase pyroxene in pyroxene bearing anorthosites and garnet quartz in garnetiferous quartzites in contrast the same mineral pairs in related rocks such as pyroxenites and pelitic schists will exhibit reset apparent temperatures closed system processes are capable of producing a variety of patterns of stable isotope resetting discordance mineral zonation and fractionation reversals examples include large reversals of quartz feldspar fractionations in micaceous rocks and oscillatory zonation in feldspar from some quartz rich rocks these results permit reinterpretation of many studies of stable isotope thermometry speedometry and retrograde alteration history fgb modeling of mineral zonation provides an important new guide to applying recently developed micro analytical tools to slowly cooled rocks application of the fgb model to quartzo feldspathic gneisses from the adirondack mountains new york demonstrates the usefulness of diffusion modeling in discriminating closed system diffusion controlled retrogression from open system retrogression and illustrates the possible importance of incorporating the effect of water activity on mineral diffusivity

mantle and crustal sources in the genesis of late hercynian granitoids nw portugal geochemical and sr nd isotopic constraints abstract large volumes of granitoids were emplaced in the hercynian central iberian zone during the last ductile deformation phase d3 300 320 ma the biotite rich granitoids are the most abundant 1 syn d3 granodiorites monzogranites 313 319 ma with calc alkaline and aluminopotassic affinities 2 late d3 granodiorites monzogranites 306 311 ma related to subalkaline and aluminopotassic series these granitoids are associated with coeval gabbro norite to granodiorite bodies and or mafic microgranular enclaves both granitoids and basic intermediate rocks show petrological geochemical and isotopic evidence of interaction between felsic and mafic magmas the mantle derived melts represented by shoshonitic gabbro norites were probably derived from an enriched and isotopically homogeneous source sr i 0 7049 to 0 7053 e nd 2 1 to 2 5 in some syn and late d3 plutons there are evidences of essentially crustal granites represented by moderately peraluminous monzogranites of aluminopotassic affinity they have similar nd model ages 1 4 ga but different isotopic compositions sr i 0 7089 to 0 7106 e nd 5 6 to 6 8 revealing a heterogeneous crust potential protoliths are metasedimentary immature sediments and or felsic meta igneous lower crust materials large amounts of hybrid magmas were generated by the interaction of these coeval mantle and crust derived liquids giving rise to slightly peraluminous monzogranites granodiorites of calc alkaline and subalkaline affinities which display more depleted isotopic compositions than the crustal end members sr i 0 7064 to 0 7085 e nd 4 4 to 6 2 petrogenetic processes involving mingling and or mixing and fractional crystallization at variable degrees in multiple reservoirs are suggested a major crustal growth event occurred in late hercynian times 305 320 ma related to the input of juvenile mantle magmas and leading to the genesis of composite calc alkaline and subalkaline plutons largely represented in the central iberian zone

crust and mantle contributions to granite genesis an example from the variscan batholith of corsica france studied by trace element and ndsro isotope systematics the corsican batholith formed during the variscan orogeny was studied with particular emphasis on the respective roles played by crustal and mantle derived material in granite genesis major and trace element data identify the various types of magma and their genetic relationships the magmatic processes that gave rise to the observed rocks and the primitive liquids o sr nd isotope systematics constrain the origin of primitive liquids trace element models provide data on the various source materials and their possible geodynamic setting the evolution of two main calc alkaline plutonic associations is compared an older high mg k association u1 with both ultrapotassic mafic and silicic rocks the latter ranging from monzonite to leucosyenogranite and a younger calc alkaline composite association u2 involving a mafic cumulate sequence and granodiorite to leucomonzo granite both types of granite are accompanied by mafic units that were non cogenetic with surrounding granitic melts trace element calculations indicate that a primitive liquid of monzodioritic composition gave rise to the u1 mg k granite by fractional crystallization amphibole and titanite playing a major role which with increasing crystallization gave decreasing ree contents without a strong negative eu anomaly the u2 calc alkaline suite was formed by fractional crystallization involving feldspar and lree rich minerals i e monazite from a liquid of monzogranitic composition field and petrographical data identify magma mingling in both u1 and u2 between mafic and silicic rocks but geochemical data only indicate how magma mixing processes led to u2 granodiorite geochemical modelling shows that a single protolith of calculated graywacke composition yielded the two associations under different melting conditions the high mg k monzodiorite melt was formed after partial melting of a 30 non modal batch of granulite facies metamorphic protolith low ph2o but the calc alkaline monzogranite was formed by the same process in an amphibolite facies source of similar composition higher ph2o the primitive granite magmas of both associations show the same crustal characteristics i e sri 0 706 0 707 nd t 4 3 to 2 2 and 18o 7 to 8 this provides evidence that composition of both u1 and u2 granite melts probably was controlled by physico chemical fusion rather than protolith composition parameters the u1 ultrapotassic mafic rock is thought to be of mantle origin i e a deep source containing phlogopite garnet zone refining led to a significant increase of incompatible trace element contents during ascent of the magma mantle crust interaction lowered the la yb and 143nd144nd ratios but the 87sr86sr ratio increased nevertheless interaction with associated u1 mg k granite was not important and the crustal component is thought to be different from the associated granitic magma in u2 the mafic cumulate sequence clearly of mantle origin has e morb characteristics and seems the result of 10 non modal partial melting of a heterogeneous mantle of spinel or amphibole peridotite without garnet some interaction occurred with the associated granitic melt the e morb character of the mafic rock indicates that a large part of the batholith and at least the u2 magmatic association was generated under post collisional extensional conditions

interplay between geochemistry and magma dynamics during magma interaction an example from the sithonia plutonic complex ne greece orogenic granitoids often display mineralogical and geochemical features suggesting that open system magmatic processes played a key role in their evolution this is testified by the presence of enclaves of more mafic magmas dispersed into the granitoid mass the occurrence of strong disequilibrium textures in mineralogical phases and or extreme geochemical and isotopic variability in this contribution intrusive rocks constituting the sithonia plutonic complex northern greece are studied on the basis of mineral chemistry whole rock major trace element geochemistry and sr and nd isotopic composition sithonia rocks can be divided into a basic group bearing macroscopic mafic enclaves microscopic disequilibrium textures geochemical and isotopic evidence of magma interaction and an acid group in which most geochemical and isotopic features are consistent with a magma mixing process but macroscopic and microscopic features are lacking a two step mixing plus fractional crystallization mfc process is considered responsible for the evolution of the basic group the firststepexplainsthechemicalvariationinthemaficenclavegroup abasicmagma representedbytheleastevolvedenclaves interacted

intercrystalline stable isotope diffusion a fast grain boundary model we formulated a numerical model for stable isotope interdiffusion which predicts the temperatures recorded between two or more minerals and the intragranular distribution of stable isotopes in each mineral as functions of mineral grain sizes and shapes diffusivities modes equilibrium isotopic fractionations and the cooling rate of a rock one of the principal assumptions of the model is that grain boundaries are regions of rapid transport of stable isotopes this fast grain boundary fgb model describes interdiffusion between any number of mineral grains assuming that local equilibrium and mass balance restrictions apply on the grain boundaries throughout the volume modeled the model can be used for a rock containing any number of minerals and number of grain sizes of each mineral several grain shapes and any thermal history or domain size desired previous models describing stable isotope interdiffusion upon cooling have been based on dodson s equation or an equivalent numerical analogue the closure temperature of dodson is the average bulk temperature recorded between a mineral and an infinite reservoir by using dodson s equation these models have treated the closure temperature as an innate characteristic of a given mineral independent of the amounts and diffusion rates of other minerals such models do not accurately describe the mass balance of many stable isotope interdiffusion problems existing models for cation interdiffusion could be applied to stable isotopes with some modifications but only describe exchange between two minerals under specific conditions the results of fgb calculations differ considerably from the predictions of dodson s equation in many rock types of interest actual calculations using the fgb model indicate that closure temperature and diffusion profiles are as strongly functions of modal abundance and relative differences in diffusion coefficient as they are functions of grain size and cooling rate closure temperatures recorded between two minerals which exchanged stable isotopes by diffusion are a function of modal abundance and differences in diffusion coefficient and may differ from that predicted by dodson s equation by hundreds of degrees c either or both of two minerals may preserve detectable zonation which may in some instances be larger in the faster diffusing mineral rocks containing three or more minerals can record a large span of fractionations resulting from closed system processes alone the results of fgb diffusion modeling indicate that the effects of diffusive exchange must be evaluated before interpreting mineral fractionations concordant or discordant recorded within any rock in which diffusion could have acted over observable scales the predictions of this model are applicable to thermometry evaluation of open or closed system retrogression and determination of cooling rates or diffusion coefficients

mineral magnetic characterization of the upper pleniglacial nussloch loess sequence germany an insight into local environmental processes 1 first comprehensive environmental magnetism study of the nussloch rhine river valley germany loess palaeosol deposit 2 bulk ferrimagnetic concentration parameters are dominantly controlled by variations in coarse grained md ferrimagnetic particles of detrital aeolian origin 3 the imprint of waterlogging induced redoxomorphic processes on the magnetic record is observed by dissolution of fine grained magnetic minerals 4 the interpretation of magnetic susceptibility variations alone within loess and palaeosol deposits following the wind vigour or pedogenic enhancement models will be hindered by waterlogging induced redoxomorphic processes if present s u m m a r y presently most loess palaeosol magnetic susceptibility records are interpreted as following either the wind vigour model or the pedogenic enhancement model however redoxomorphic processes induced by waterlogging often referred to gleying in the loess literature are also known to alter loess deposits but their impact on loess palaeosol magnetic susceptibility records has received little attention the reported rock magnetic study aims to characterize the mineral magnetic response of loess to waterlogging induced redoxomorphic processes thus improving our understanding of mineral magnetic changes within loess deposits with respect to environmental and climate conditions the nussloch loess palaeosol deposit rhine valley germany was targeted because it is one of the best studied pleniglacial deposits for western europe in which numerous tundra gley intervals have been identified moreover a comprehensive high resolution environmental magnetism study has never been undertaken for this site various rock magnetism experiments were conducted at both room and low temperatures to characterise the composition concentration and relative magnetic grain size of the mineral magnetic assemblage the relative changes in magnetic parameters within the investigated loess interval are primarily controlled by 1 varying concentrations of coarse grained ferri magnetic particles of detrital aeolian origin and 2 dissolution of fine grained ferrimagnetic particles related to in situ post depositional alteration promoted by waterlogging induced re doxomorphic processes goethite is found to be ubiquitous throughout the studied interval and is argued to have both a primary aeolian and secondary in situ origin we conclude that redoxomorphic processes induced by waterlogging if present will hinder the interpretation of magnetic susceptibility variations within loess and palaeosol deposits following the expected relationships dictated by the wind vigour and the pedogenic enhancement magnetism models

can remanent magnetization in the deep crust contribute to long wavelength magnetic anomalies long wavelength aeromagnetic anomalies require sources in the deep crust that are far more magnetic than the measured induced magnetization of lower crustal rocks several scientists have suggested that remanence in the deep crust may provide the missing magnetization this is not likely either through stable and or viscous remanence bodies large enough to contribute to surface anomalies and carrying stable remanence probably cooled slowly enough that they are subdivided into zones of alternating polarity this subdivision sharply reduces the power in the long wavelength portion of the anomaly spectrum on the other hand if such a body carries a viscous remanence the viscous component will be acquired in such a way as to bring the total magnetization of the body into equilibrium with the earth s field this equilibrium total magnetization is only about 25 greater than the induced magnetization not enough to explain the discrepancy

a fluid inclusion and stable isotope study of the moonta copper gold deposits south australia evidence for fluid immiscibility in a magmatic hydrothermal system abstract the copper gold ores of the moonta mineral field in the eastern part of the archaean mesoproterozoic gawler craton south australia are vein deposits occupying fractures and shear zones within the palaeoproterozoic moonta porphyry the veins show a spatial relationship with mesoproterozoic granites equivalents of the hiltaba suite granites that are widespread in the gawler craton the ores were deposited in four hydrothermal stages separated by episodes of fracturing the first two stages were dominated by fe oxides the third by fe sulfides and the final stage by a cu fe co au zn pb s assemblage three types of fluid inclusions are present in quartz associated with the oxide and sulfide minerals type 1 fluid inclusions are two phase vapour rich with homogenization temperatures up to 473 c and low to moderate salinity 2 to 22 wt nacl equivalent type 2 are two phase liquid rich with lower homogenization temperatures up to 269 c and low to moderate salinities 0 7 to 22 wt nacl equivalent type 3 are liquid rich solid s bearing with homogenization temperatures up to 467 c and high salinities 28 to 55 wt nacl equivalent the coexistence of types 1 and 3 suggests that these fluid inclusions resulted either from trapping of boiling fluids or represent two immiscible fluids most likely derived from a magma the fluid history reflected in fluid inclusion characteristics was complex involving variable amounts of boiling cooling and mixing in the vein system salinity homogenization temperature relationships indicate mixing of the hypersaline fluid and the vapour rich fluid with a surface derived fluid of low temperature and low to moderate salinity d h 2 o values 22 to 31 and 18 o h 2 o values 2 9 to 5 1 calculated from analyses of chlorite and 18 o h 2 o values calculated from analyses of quartz 0 6 to 8 8 on the basis of mean to maximum fluid inclusion homogenization temperatures are intermediate between magmatic water and surface derived fluids i e seawater or meteoric water the narrow range of 18 o values of quartz 10 8 to 12 7 may be explained by mixing of a magmatic fluid and either seawater or a low latitude moderately evolved meteoric water with a coupled decrease in fluid temperature and 18 o content similar ranges for 34 s values of ore sulfides 2 3 to 6 4 and disseminated sulfides in the moonta porphyry and interdigitated doora schist 1 5 to 4 6 suggest assimilation of crustal sulfur by the granitic magma or ore fluids the combined fluid inclusion and stable isotope data support previous proposals based on field and mineralogical evidence for a genetic relationship between the moonta ores and the mesoproterozoic granitoids

rock magnetic record of the triassic jurassic transition in pelagic bedded chert of the inuyama section japan the end triassic mass extinction event is regarded as one of the fi ve largest extinction events of the phanerozoic the emerging consensus points to volcanic activity at the central atlantic magmatic province camp as the ultimate cause of the extinction yet the underlying mecha nisms and the nature of global environmental changes that accompanied the biotic turnover remain elusive we present a rock magnetic study of the extinction interval found within a continuous chert sequence that provides an uninterrupted record of pelagic sedimentation in the panthalassa ocean the variations in the relative abundances and characteristics of authi genic magnetic phases indicate that the triassic jurassic transition progressed in two stages the initial stage characterized by a disappearance of the previously ubiquitous magnetofos sils started a few tens of thousands of years to 100 k y prior to the formal triassic jurassic boundary as identifi ed by the diagnostic radiolarian species the second stage defi ned by signifi cant changes in optical and magnetic properties of hematite pigment lasted a few tens of thousands of years the stepwise change in magnetic properties is suggestive of the protracted environmental deterioration likely prompted by the early episodes of the camp volcanism which was followed by a sudden ocean acidifi cation event perhaps triggered by a catastrophic release of gas hydrates

novel high pressure magnetic measurements with application to magnetite 1 we report a novel system designed to measure reversible magnetic susceptibility of micron sized samples under high pressures in a diamond anvil cell we find that magnetite reversible hysteresis parameters vary 15 below 0 6 to 1 0 gpa while at higher pressures significant increases occur in 1 bulk coercivity hc and 2 the ratio of saturation remanent magnetization mrs to saturation magnetization ms the net effect of pressure is to displace magnetite toward a truer single domain state with both higher mrs ms and hc our data together with the fact that magnetite curie temperature increases with pressure suggest that magnetite can account for geomagnetic anomalies related to some subduction zones and potentially to meteorite impact sites on earth as well as magnetic signatures observed on some planetary bodies like mars

thermal buffering effects at the solidus implications for the equilibration of partially melted metamorphic rocks abstract a simple thermal model is used to investigate to what extent buffering effects of latent heat of fusion may prolong the thermal evolution of a one dimensional statically cooling partially or completely melted heat source for a melting model appropriate for crustal rocks the time of cooling of rocks through the solidus may be about tripled by this process depending on the melting model cooling may halt completely near the solidus for time spans comparable to the thermal time constant of the heat source in order to test the influence of this buffering effect on the equilibration of partially melted metamorphic rocks these thermal model results are coupled with a simple diffusion model that relates the degree of equilibration of a simple assemblage to temperature and cooling rate it is shown that for a broad range of initial heat source temperatures freezing of selected mineral equilibria may occur in a comparably narrow temperature range at and around the solidus the calculations may have some relevance to low p high t metamorphic terranes in as much as they may provide an explanation for the widely observed equilibration and partial re equilibration of rocks near the crystallisation temperatures of partial melts

morin transition in hematite size dependence and thermal hysteresis 1 hematite is a frequently used mineral in paleomagnetic and environmental magnetic studies just below room temperature it undergoes a magnetic phase transition the morin transition whose nature is an important part of our basic understanding of hematite s magnetism and magnetic memory we have determined the temperature tm of the morin transition from saturation remanence warming curves to be 250 261 k for 0 5 6 mm hematite natural single crystals 257 260 k for 45 600 m sieved crystal fractions and 241 256 k for submicron synthetic hematites with grain sizes between 120 and 520 nm the variation must be due to differences in crystal morphology lattice strain and crystal defects common in both synthetic and natural crystals our results are compatible with published data for 100 nm to 10 mm hematites and show that tm is nearly size independent decreasing very gradually as particle size decreases over this broad range which includes both multidomain md and single domain sd structures however tm decreases sharply between 90 and 30 nm below 20 nm the transition disappears entirely as near surface spins deviate strongly from the antiferromagnetic easy axis our sd and md hematites exhibit a thermal hysteresis in the morin transition the values of tm in cooling and in heating are different for the same cooling warming rate tm is much greater for submicron hematites than for larger crystals we attribute the lag in the transition in both cooling and heating to crystal imperfections and resulting internal stresses and speculate that defects may serve to pin and stabilize surface spins preventing spin rotation in a region large enough to trigger the phase transition would inhibit destabilization of the weakly ferromagnetic phase in cooling and the antiferromagnetic phase in heating the wide distribution of particle sizes in our submicron samples may also play a role

casto ring zone a 4 500 km2 fossil hydrothermal system in the challis volcanic field central idaho 18 o analyses of granitic and volcanic rocks reveal the largest hydrothermal system ever documented around a single granite pluton occupying a 4 500 km 2 area in central idaho the remains of this meteoric hydrothermal system are principally preserved within a sharply bounded 15 km wide 70 km diameter annulus of low 18 o rock 2 0 to 8 8 termed the casto ring zone the zone centered on a less depleted 4 5 core zone consisting of granitic rocks of the casto pluton this 700 km 2 eocene subvolcanic batholith has intruded domed and hydrothermally metamorphosed a thick sequence of challis volcanics the stratigraphically low rocks in the 2 000 km 2 van horn peak and the 1 000 km 2 thunder mountain cauldron complexes being most strongly altered less extreme 18 o depletions occur in the youngest major ash flow sheets of these complexes indicating a vertical 18 o gradient water rock ratios of geothermal systems are surprisingly insensitive to the circulation scale

eddy mediated regime transitions in the seasonal cycle of a hadley circulation and implications for monsoon dynamics in a simulation of seasonal cycles with an idealized general circulation model without a hydrologic cycle and with zonally symmetric boundary conditions the hadley cells undergo transitions between two regimes distinguishable according to whether large scale eddy momentum fluxes strongly or weakly influence the strength of a cell the center of the summer and equinox hadley cell lies in a latitude zone of upper level westerlies and significant eddy momentum flux divergence the influence of eddy momentum fluxes on the strength of the cell is strong the center of the cross equatorial winter hadley cell lies in a latitude zone of upper level easterlies and is shielded from the energy containing midlatitude eddies the influence of eddy momentum fluxes on the strength of the cell is weak mediated by feedbacks between eddy fluxes mean zonal winds at upper levels and the mean meridional circulation the dominant balance in the zonal momentum equation at the center of a hadley cell shifts at the transitions between the regimes from eddies dominating the momentum flux divergence in the summer and equinox cell to the mean meridional circulation dominating in the winter cell at the transitions a feedback involving changes in the strength of the lower level temperature advection and in the latitude of the boundary between the winter and summer cell is responsible for changes in the strength of the cross equatorial winter cell the transitions resemble the onset and end of monsoons for example in the shift in the dominant zonal momentum balance rapid shifts in the latitudes of maximum meridional mass flux and of maximum convergence at lower levels rapid changes in strength of the upward mass flux and changes in direction and strength of the zonal wind at upper and lower levels in the monsoonal regime the maximum upward mass flux occurs in an off equatorial convergence zone located where the balance of the meridional geopotential gradient in the planetary boundary layer shifts from nonlinear frictional to geostrophic similar dynamic mechanisms as at the regime transitions in the simulation mechanisms that can act irrespective of land sea contrasts and other inhomogeneities in lower boundary conditions may be implicated in large scale monsoon dynamics in earth s atmosphere

rock magnetic investigation of possible sources of the bangui magnetic anomaly the bangui magnetic anomaly bma is the largest lithospheric magnetic field anomaly on earth at low latitudes previous studies investigated its geological source using constraints from satellite and ground magnetic field measurements as well as from surface magnetic susceptibility measurements on rocks from the panafrican mobile belt zone pmbz here we combine magnetic field data modelling and rock magnetic property measurements susceptibility and natural remanent magnetization nrm on many samples from this pmbz and the surrounding formations it reveals that nrm is a significant component of the total magnetization mt of the bma source which reaches 4 3 a m with maximum thicknesses of 38 and 54 km beneath the western and eastern parts of the bma only the isolated and relatively thin banded iron formations and some migmatites show such mt values thus we suggest that the thick bma source may be composed either by overlapped slices of such metamorphic rocks or by an iron rich mafic source or by a combination of these two geological structures

petrogenesis of a mesoproterozoic quartz latite granitoid suite from the roxby downs area south australia abstract mesoproterozoic granitoids from the roxby downs area south australia show a range in composition from mafic quartz monzodiorites 56 sio 2 to felsic granites 71 sio 2 and are interpreted as consanguineous on the basis of similar age isotopic composition and mineralogical and geochemical characteristics these granitoids have geochemical similarities with other mesoproterozoic granitoids the world over including enrichments in k 2 o rb ba th u rare earth elements ree nb y zr and depletions in mgo cao sr cr ni v sc relative to typical phanerozoic calc alkaline granitoids the mafic granitoids have mineralogical textural and geochemical features consistent with an origin as cumulates from a more felsic primary magma and the felsic granites likely represent the fractionated residual melt to this accumulation process none of the granitoids from the batholith preserve the primary magma composition which is deduced to be similar to abundant quartz latite volcanic rocks of the region it is estimated by geochemical modeling that about 27 fractionation of observed phenocrysts from a primary quartz latite is required to produce a typical felsic granite 71 sio 2 production of the primary quartz latite magma was likely the result of partial melting of existing paleoproterozoic crust in response to intrusion into the crust of mantle derived magmas the combination of widespread crustal melting contemporaneous granulite facies metamorphism and mantle derived magmatism suggests involvement of large scale tectonothermal processes in the origin of these granitoids the petrogenesis of these rocks and similar occurrences in laurentia and baltica indicates that processes involving reworking of existing paleoproterozoic crust were similar throughout many continental areas during the mesoproterozoic

stable boundary layer parametrizations for local and larger scales stability functions f ri for a very stable boundary layer were studied by experimenting in winter antarctic conditions with a one dimensional 1d numerical model and comparing with the plateau site tower observations the local representations for f ri produced good simulations of the temperature profiles and wind hodographs while the standard monin obukhov formulation indicated too little and the general circulation model gcm type formulations too much vertical mixing next idealised simulations were made for winter night hilly northern finland landscape with a high resolution 2d model using a local f ri at each column realistic quasi steady mesoscale katabatic flows resulted these enhanced vertical mixing both locally and regionally area averaged i e regional surface fluxes were compared with those obtained from simulations using gcm type large scale stability functions over flat land new parametrizations for both local and larger scales are suggested based on the experiments copyright 2009 royal meteorological society

response of idealized hadley circulations to seasonally varying heating 1 the response of hadley circulations to displacements of the latitude of maximum heating is investigated in idealized axisymmetric and eddy permitting models consistent with an earlier study and with theory for the nearly inviscid limit lindzen and hou 1988 the strength of the hadley circulation is sensitive to displacements of heating the winter cell strengthens and summer cell weakens when the maximum heating is displaced off the equator however in conflict with the nearly inviscid limit but consistent with observations of earth s atmosphere the strength of an annually averaged hadley circulation is comparable to the hadley circulation driven by an annually averaged heating the disagreement between these results and the nearly inviscid limit is ascribed to vertical diffusion of momentum and dry static energy in the axisymmetric model and to baroclinic eddy fluxes in the eddy permitting model nonlinear amplification of the annually averaged hadley circulation is only seen near the upper boundary in simulations with a rigid lid near the tropopause suggesting that the amplification is an artifact of the upper boundary condition

oxygen isotope geochemistry of the amphiboles isotope effects of cation substitutions in minerals abstract the occurrence of coexisting amphiboles in rocks and the likelihood of concurrent isotope closure allows equilibrium oxygen isotope fractionations among the amphiboles to be recovered from natural samples oxygen isotope analyses of mineral separates using laser fluorination show that coexisting amphiboles increasingly partition 18o in the order hornblende gedrite 0 9 previously published data for hornblende actinolite glaucophane and garnet show that act hbl 0 2 gln grt 1 and hbl grt 0 thus glaucophane strongly partitions 18o relative to the calcic amphiboles the fractionation between two amphiboles of arbitrary composition can be predicted from the known fractionations for mica endmembers pyroxene endmembers and exchange components such as caal nasi 1 naal camg 1 camg 1 mgfe 1 femn 1 kna 1 kal si 1 and fe3 al 1 applications of the exchange component method reproduce measured amphibole fractionations to within 0 1 to 0 2 whereas other predictive methods cause misfit for typical metamorphic hornblende of 0 5 at 575 c although the isotope effects of cation exchanges may be small at high t they magnify dramatically for minerals formed in surficial diagenetic and low t metamorphic environments different composition clays are predicted to have equilibrium 18o differences of 2 9 if the isotope fractionation can be determined for one mineral endmember then calibrated exchanges allow accurate prediction of the isotope fractionations for intermediate compositions of most ortho ring chain and sheet silicates

time scales of pluton construction at differing crustal levels examples from the mount stuart and tenpeak intrusions north cascades washington deciphering the magmatic history of continental magmatic arcs in general and the growth history of individual intrusions in particular is key to understanding the complex history of magma generation segregation and transport that define the dynamics of crustal growth we utilize high precision u pb geochronology to resolve a detailed magmatic history from two composite intrusions the 2 4 kbar mount stuart batholith and the 7 10 kbar tenpeak pluton emplaced in the cretaceous north cascades arc this temporal framework provides a way to evaluate models of pluton growth that explain common features of intrusions such as concentric compositional zoning and internal magmatic contacts u pb zircon crystallization ages were obtained from 12 samples of the mount stuart batholith and 8 samples of the tenpeak intrusion representing the range of compositional diversity and geographical extent these dates indicate that the mount stuart batholith was constructed over a 5 5 m y time period that was punctuated by four intervals of high magma flux the durations of the high flux periods are short a few hundred thousand years relative to the duration of the batholith the consistent pattern of magmatic fabrics and the lack of distinct contacts in the batholith may be explained by the juxtaposition of melt rich and mush zones with subtle contacts between mineralogically and texturally similar tonalite and time transgressive magma fabrics in contrast the tenpeak intrusion was constructed over a 2 6 m y time period with magma influx distributed throughout the intrusive history and texturally distinct magma bodies the tenpeak intrusion lacks distinct age domains which suggests that any magma reservoir was smaller in size and potentially more ephemeral although the distinct age domains and discrete compositional and textural phases indicate that pluton growth occurred incrementally neither pluton bears resemblance to a purely end member incremental growth model whereby a pluton is constructed from hundreds to thousands of discrete magma pulses that have little if any interaction in particular ages from the youngest domain of the mount stuart batholith indicate that a melt rich magma reservoir of 520 km 3 existed over a 170 90 k y time span

a comprehensive mass flux scheme for cumulus parameterization in large scale models abstract observational studies indicate that a mass flux approach may provide a realistic framework for cumulus parameterization in large scale models but this approach through the introduction of a spectral cloud ensemble leads normally to rather complex schemes in this paper the question is addressed whether much simpler schemes can already provide realistic values of the thermal forcing by convection under various synoptic conditions this is done through verifying such a scheme first on data from field experiments for periods of tropical penetrative convection gate marshall islands tradewind cumuli atex bomex and extratropical organized convection sesame 79 and then in a nwp model the scheme considers a population of clouds where the cloud ensemble is described by a one dimensional bulk model as earlier applied by yanai et al in a diagnostic study of tropical convection cumulus scale downdrafts are included various types of convection are represented i e penetrative convection in c

evaluating the construction and evolution of upper crustal magma reservoirs with coupled u pb zircon geochronology and thermal modeling a case study from the mt capanne pluton elba italy abstract evaluating mechanisms and rates for magma transport and emplacement in the upper crust is important in order to predict the thermal and rheological state of the crust and understand the relationship between plutonism and volcanism u pb geochronology on zircon is commonly used to constrain magma emplacement and storage time in the crust but interpreting complex zircon age populations in terms of in situ crystallization versus crystallization at a deeper level is not trivial this study focuses on the mt capanne pluton in elba italy a well documented example of arc related laccolith emplaced in the upper continental crust previous studies proposed that the mt capanne intrusion was accreted in less than 10 000 yr by distinct and mappable magma pulses here we couple high precision id tims u pb zircon geochronology with numerical thermal simulations to evaluate emplacement rates test different emplacement models inform zircon age interpretations and evaluate the potential for melt storage during construction of the mt capanne pluton our results require that the mt capanne intrusion was built in at least 250 000 yr by multiple magma injections a variety of emplacement scenarios show that melt was preserved for

40ar 39ar and 18o 16o studies of the chegem ash flow caldera and the eldjurta granite cooling of two late pliocene igneous bodies in the greater caucasus mountains russia volcanic and intrusive rocks of the chegem caldera and the nearby eldjurta eldzhurtinskiy granite record a late pliocene episode of silicic magmatism in the north central caucasus mountains surface exposures created by the recent rapid uplift and erosion of the caucasus mountains span a 2 km vertical section of chegem caldera fill and 1150 m of the eldjurta granite cored mineral exploration drillholes in the eldjurta granite extend the sampling to a depth of 4 km the unique sampling range available in these two young igneous bodies affords an excellent opportunity to study their denudation and cooling histories which we examine by means of 40 ar 39 ar and 18 o 16 o measurements on an extensive sample suite total fusion biotite and sanidine ages from eight chegem tuff samples both intracaldera and outflow are analytically indistinguishable with a weighted mean of 2 82 0 02 ma a cross cutting granodiorite porphyry intrusion has a sanidine total fusion age of 2 84 0 03 ma and whole rock incremental heating of a post caldera andesite flow which caps the caldera fill yields an age of 2 82 0 02 ma thus caldera formation and post caldera resurgence and volcanism all occurred within a very short time 50 000 yr biotite total fusion ages of ten eldjurta granite samples including seven from 500 m intervals in the 4 km deep drillhole show a systematic linear decrease in age with depth from 1 90 ma near the roof contact of the granite to 1 56 ma at a depth of 3700 m assuming these ages were set at the same temperature this age depth gradient implies an isotherm migration rate of 13 mm yr between 1 90 and 1 56 ma this migration rate is due to a combination of rapid denudation and downward relaxation of isotherms with cooling rates between 200 and 500 c ma during this period oxygen isotopic compositions of quartz k feldspar plagioclase and biotite from the drillhole samples below the 800 m depth are fairly uniform and record primary igneous 18 o values with little evidence for subsolidus hydrothermal activity however in surface outcrop samples and in the shallowest drillhole sample mineral 18 o values have been lowered by up to 3 by interaction with an external meteoric hydrothermal fluid the primary mineral 18 o values of the eldjurta granite are distinctly higher than the corresponding phenocryst 18 o values in the chegem volcanic rocks indicating that the two bodies evolved as separxate magma batches

oxygen self diffusion in quartz under hydrothermal conditions oxygen self diffusion dox has been monitored in single crystal quartz in the temperature range 515 850 c under hydrothermal conditions in the beta field between 700 and 850 c the data are represented by two linear arrhenius relations for transport parallel and perpendicular to c values for do m2 s 1 are 2 09 10 11 parallel to c and 3 16 10 10 perpendicular to 101 0 values for h kj mol 1 are 138 54 parallel to c and 203 72 perpendicular to 101 0 at 700 c in the total pressure range 11 5 100 mpa dox is independent of water f h2o and oxygen f o2 fugacities between the ni nio and fe3o4 fe2o3 buffers the results are consistent with diffusion via a simple charged vacancy mechanism under an extrinsic point defect regime further experiments are required to confirm the nature of the mobile oxygen defect a key aspect of the results is the observation that at the low water fugacities of the present experiments a hydrogen containing defect appears to play no role in the oxygen transport mechanism this is in contrast to other published sets of data and leads directly to the requirement for detailed interlaboratory comparisons

impact of climate change on the magnetic mineral assemblage in marine sediments from izu rear arc nw pacific ocean over the last 1 myr abstract a rock magnetic study was conducted on upper pleistocene marine sediments from international ocean discovery program expedition 350 site u1437 in order to highlight the paleoenvironmental changes in the nw pacific ocean influenced by the east asian monsoon regime remanent magnetization analyses hysteresis properties first order reversal curves and low temperature magnetic measurements were carried out along with electron microscope observations the results indicate that coarse grained titano magnetite is the dominant magnetic phase in the magnetic mineral assemblage time series analysis supports that this assemblage is modulated by global climate changes over the last 1 myr during the interglacial stages magnetic minerals are more abundant and are dominated by coarse grained titano magnetite of both terrigenous likely from mainland china and volcanic izu arc front japan origin during the glacial stages the magnetic mineral content is lower probably reflecting partial dissolution of ti magnetite and the magnetic assemblage is composed of terrigenous coarse grained titano magnetite and of higher coercivity presumably finer eolian particles likely hematite as a result of the enhancement of the winter monsoon in continental asia the magnetic mineral assemblage reflects a superimposition of volcanic and global climate signals bulk organic geochemical analyses for total and organic carbon as well as nitrogen and sulfur contents confirm a climatic signature in the composition of the sediments with more oxygenated water masses being present during glacial periods additional x ray fluorescence measurements on bulk samples indicate various origins of the sediment particles with both proximal and distal sources

depositional changes during the danian selandian transition in loubieng france zumaia spain and sidi nasseur tunisia insights from and limits of rock magnetism depositional changes are studied in three sections encompassing the danian selandian transition loubieng france zumaia spain and sidi nasseur tunisia using magnetic susceptibility as a proxy additional rock magnetic analyses are used to avoid ambiguous interpretation of magnetic susceptibility the magnetic susceptibility measured on 90 to 270 samples per section is mainly controlled by paramagnetic minerals and linked to detrital input major increases in the detrital input are correlated to the end of the latest danian event a hyperthermal and to the danian selandian boundary in loubieng two gradual increases in magnetic susceptibility within limestones beds precede the major detrital input increases and start synchronously with the beginning of the latest danian event and the onset of haematite deposition around the danian selandian boundary respectively this haematite is suspected to be of primary origin based among other things on low magnetic viscosity values which is used here as an indicator of diagenetic origin in haematite and goethite the red levels where haematite is interpreted to be of primary origin could be linked to the hyperthermal event previously hypothesized for the basal selandian the comparison of the magnetic susceptibility chemo and biostratigraphic data between the three sections highlights the condensed nature of the sedimentation around the danian selandian boundary in the sections of the atlantic realm the lower part of the selandian shows a particularly low sedimentation rate at zumaia compared to loubieng and sidi nasseur the latter displays the most complete record of the three

hadley cell dynamics in a virtually dry snowball earth atmosphere the hadley cell of a virtually dry snowball earth atmosphere under equinox insolation is studied in a comprehensive atmospheric general circulation model in contrast to the hadley cell of modern earth momentum transport by dry convection which is modeled as vertical diffusion of momentum is important in the upper branch of the snowball earth hadley cell in the zonal momentum balance mean meridional advection of mean absolute vorticity is not only balanced by eddies but also by vertical diffusion of zonal momentum vertical diffusion also contributes to the meridional momentum balance by decelerating the hadley cell through downgradient mixing of meridional momentum between its upper and lower branches when vertical diffusion of momentum is suppressed in the upper branch the hadley cell strengthens by a factor of about 2 this is in line with the effect of vertical diffusion in the meridional momentum balance but incontrastwithitseffectinthezonalmomentumbalance neitheraxisymmetrichadleycelltheoriesbasedon angular momentum conservation nor eddy permitting hadley cell theories that neglect vertical diffusion of momentum are applicable to the snowball earth hadley cell because the snowball earth hadley cell is a particular realization of a dry hadley cell these results show that an appropriate description of dry hadley cells should take into account vertical transport of momentum by dry convection

timing of incremental pluton construction and magmatic activity in a back arc setting revealed by id tims u pb and hf isotopes on complex zircon grains abstract the lifetimes and thermal histories of upper crustal plutons are increasingly determined using geochronology but complex growth of datable minerals in magmas impedes simple age interpretations careful field observation helps constrain zircon u pb dates in terms of timing of magma injection for example because relative ages of successive magma pulses must be honored we use id tims u pb zircon geochronology and field geology to construct timescales of incremental pluton assembly in the st jean du doigt sjdd bimodal layered intrusion brittany france field evidence suggests that early pulses were injected into a cold environment with little supersolidus interaction among successive magma pulses later injections occurred in a progressively hotter environment with protracted mafic and felsic magma interaction zircon dates show that early activity ca 347 ma predates the thermally mature episode by about 1 ma which terminated at ca 345 ma dates from samples displaying core rim zircon overgrowths span about 5 ma 351 346 ma which we interpret to represent two distinct crystallization events hf isotopic data from cores and rims are homogeneous precluding zircon inheritance from basement rocks these textures and dates could instead reflect zircon saturation fluctuations at the emplacement depth or antecrystic zircon grains recording pre emplacement magmatic growth

resolved snowball earth clouds abstractrecent general circulation model gcm simulations have challenged the idea that a snowball earth would be nearly entirely cloudless this is important because clouds would provide a strong warming to a high albedo snowball earth gcm results suggest that clouds could lower the threshold co2 needed to deglaciate a snowball by a factor of 10 100 enough to allow consistency with geochemical data here a cloud resolving model is used to investigate cloud and convection behavior in a snowball earth climate the model produces convection that extends vertically to a similar temperature as modern tropical convection this convection produces clouds that resemble stratocumulus clouds under an inversion on modern earth which slowly dissipate by sedimentation of cloud ice there is enough cloud ice for the clouds to be optically thick in the longwave and the resulting cloud radiative forcing is similar to that produced in gcms run in snowball conditions this result is robust to large changes in the clo