bulk chondrite variability in mass independent magnesium isotope compositions implications for initial solar system 26al 27al and the timing of terrestrial accretion abstract we have determined 26 mgdsm 3 the mass independent variations in 26mg 24mg of primitive bulk meteorites to precisions better than 3 ppm 2se our measurements of samples from 10 different chondrite groups show 26 mgdsm 3 that vary from 5 to 22 ppm our data define an array with a positive slope in a plot of 26 mgdsm 3 against 27al 24mg which can be used to determine 26al 27al 0 i e initial 26al 27al and 26 mgdsm 3 0 i e initial 26 mgdsm 3 on such an isochron plot the best fit of our new measurements combined with literature data implies 26al 27al 0 of 4 67 0 78 10 5 and 26 mgdsm 3 0 of 31 6 5 7 ppm 2se for ordinary and carbonaceous chondrites other than cr chondrites which have anomalously low 26 mgdsm 3 these parameters are within uncertainty of those defined by previous measurements of bulk calcium aluminium rich inclusions cais that set canonical 26al 27al 0 5 10 5 the most straightforward interpretation of all these observations is that differences in the al mg of bulk ordinary and carbonaceous chondrites are dominantly controlled by variable contributions of early formed refractory and major silicate components derived from a common canonical reservoir the 26 mgdsm 3 of enstatite chondrites are slightly more radiogenic 3 ppm at similar al mg to the ordinary chondrites we speculate that this is related to the timing of removal of a refractory component from the source reservoirs of these different meteorite groups the higher 26 mgdsm 3 of the enstatite chondrites suggests later 0 5 ma post cais condensation and loss of this refractory component despite inferred consistency of 26al 27al 0 and 26 mgdsm 3 0 across most chondrite groups some nebular heterogeneity is required to account for the compositions of cr chondrites our preferred interpretation is that the cr source region has lower 26 mgdsm 3 0 as the most appropriate isotopic reference for the earth our new mean enstatite chondrite composition allows us to assess possible ingrowth of 26mg from live 26al during accretion of the earth the earth has 26 mgdsm 3 within uncertainty of enstatite chondrites despite its higher al mg this requires that the terrestrial increase in al mg which we attribute to vapour loss during accretion must have happened 1 5 ma post cai formation in an instantaneous fractionation model

low to medium temperature phase equilibria in a gas of solar composition equilibrium mineral assemblages in a gas of solar composition have been calculated in the temperature and pressure ranges of 423 1023 k and 1 to 10 8 bar respectively the results include the modal percent variation of all condensates and the chemical compositions of ideal and non ideal coexisting solid solutions the major mineral assemblage stable over most of the pressure and temperature field is and fe ni alloys olivine orthopyroxene clinopyroxene plagioclase ilmenite other minerals entering the field are orthoclase at 628 k and troilite fes at 621 k both with no effect of variation in pressure rutile is stable in parts of the high temperature field a typical model percent composition above 621 k is fe ni 2 46 fe ni 40 26 olivine 28 94 orthopyroxene 18 25 clinopyroxene 6 26 plagioclase 3 56 and ilmenite 0 27 t 663 k p 10 4 bar below 621 k there is a marked drop in the average density of the material a small amount of almandine rich garnet appears at 533 k the temperature of appearance decreasing slightly with decreasing pressure at low temperatures biotite tremolite and talc are stable at high pressures while graphite becomes important below 10 7 bar compositions of plagioclase olivine orthopyroxene ilmenite and garnet are found to be potential cosmobarometers and cosmothermometers

archeomagnetic study of seven contemporaneous kilns from murcia spain new archeointensity values have been determined from seven spanish kilns sampled at the same archeological site and dated by archeological constraints as between 1100 and 1200 a d the directions of the characteristic remanent magnetization and paleointensities have been obtained from classical thellier experiments conducted on 69 samples collected from the walls of the kilns thermoremanent magnetization trm anisotropy and the cooling rate effect upon trm intensity acquisition have been investigated in all the samples differences between mean archeointensities per site before and after corrections for cooling rate are between 3 and 17 the study of these seven contemporaneous kilns coming from the same archeological site and treated in the same way in the laboratory allows us to discuss the different factors that can cause the dispersion between sites generally observed in archeointensity studies the results confirm on one hand the need for cooling rate corrections in this kind of study and on the other hand the need to obtain several archeointensity determinations per century in order to construct a reliable archeointensity curve finally the new data provide a robust mean intensity 60 3 3 3 t at the latitude of paris for the middle of the xii century in western europe and constrain better the evolution of the geomagnetic field intensity during medieval times in this area

a multi step model for the origin of e3 enstatite chondrites it appears that the mineralogy and chemical properties of type 3 enstatite chondrites could have been established by fractionation processes removal of a refractory component and depletion of water in the solar nebula and by equilibration with nebular gas at low to intermediate temperatures approximately 700 950 k we describe a model for the origin of type 3 enstatite chondrites that for the first time can simultaneously account for the mineral abundances bulk chemistry and phase compositions of these chondrites by the operation of plausible processes in the solar nebula this model which assumes a representative nebular gas pressure of 10 5 bar entails three steps 1 initial removal of 56 of the equilibrium condensed phases in a system of solar composition at 1270 k 2 an average loss of 80 85 water vapor in the remaining gas and 3 two different closure temperatures for the condensed phases the first step involves a refractory element fractionation and is needed to account for the overall major element composition of enstatite chondrites assuming an initial system with a solar composition the second step water vapor depletion is needed to stabilize si bearing metal oldhamite and niningerite which are characteristic minerals of the enstatite chondrites variations in closure temperatures are suggested by the way in which the bulk chemistry and mineral assemblages of predicted condensates change with temperature and how these parameters correlate with the observations of enstatite chondrites in general most phases in type 3 enstatite chondrites appear to have ceased equilibrating with nebular gas at approximately 900 950 k except for fe metal which continued to partially react with nebular gas to temperatures as low as 700 k

oxygen isotopic compositions and origins of calcium aluminum rich inclusions and chondrules primary minerals in calcium aluminum rich inclusions cais al rich and ferromagnesian chondrules in each chondrite group have 18o values that typically range from 50 to 5 0 neglecting effects due to minor mass fractionations the oxygen isotopic data for each chondrite group and for micrometeorites define lines on the three isotope plot with slopes of 1 01 0 06 and intercepts of 2 1 this suggests that the same kind of nebular process produced the 16o variations among chondrules and cais in all groups chemical and isotopic properties of some cais and chondrules strongly suggest that they formed from solar nebula condensates this is incompatible with the existing two component model for oxygen isotopes in which chondrules and cais were derived from heated and melted 16o rich presolar dust that exchanged oxygen with 16o poor nebular gas some fun cais inclusions with isotope anomalies due to fractionation and unknown nuclear effects have chemical and isotopic compositions indicating they are evaporative residues of presolar material which is incompatible with 16o fractionation during mass independent gas phase reactions in the solar nebula there is only one plausible reason why solar nebula condensates and evaporative residues of presolar materials are both enriched in 16o condensation must have occurred in a nebular region where the oxygen was largely derived from evaporated 16o rich dust a simple model suggests that dust was enriched or gas was depleted relative to cosmic proportions by factors of 10 to 50 prior to condensation for most cais and factors of 1 5 for chondrule precursor material we infer that dust gas fractionation prior to evaporation and condensation was more important in establishing the oxygen isotopic composition of cais and chondrules than any subsequent exchange with nebular gases dust gas fractionation may have occurred near the inner edge of the disk where nebular gases accreted into the protosun and shu and colleagues suggest that cais formed

new archeointensity data from nw argentina 1300 1500 ce abstract a good characterization of the geomagnetic field strength at centennial to millennial time scales in the southern hemisphere is particularly crucial to disentangle the long term evolution of the south atlantic anomaly saa an intriguing geomagnetic feature currently observed at the earth s surface here we present 59 new archeointensities obtained from four well dated groups of potteries with ages ranging between 1300 and 1500 ce and collected in northwest argentina the new data were obtained in accordance with the thellier paleointensity method including partial thermoremanent magnetization ptrm checks and trm anisotropy and cooling rate corrections we have also performed a comparative study of the efficiency of magnetic susceptibility arm and trm anisotropy tensors to correct the trm anisotropy effect upon intensity estimates our results suggest that the magnetic susceptibility tensor systematically underestimates the trm anisotropy effect by 10 30 our new data together with selected archeointensities already published confirm that the decrease of the geomagnetic field intensity in south america started around 1600 1650 ce due to the arrival of the saa

redox conditions in the solar nebula observational experimental and theoretical constraints crystallization experiments on liquids with compositions similar to those of compact type a type b1 and type b2 refractory inclusions were conducted under controlled temperature and fo2 conditions application of the results to the compositions of coexisting ti 3 bearing fassaitic clinopyroxene melilite pairs in natural inclusions shows that if they crystallized at 1509 k they did so at log fo2 19 8 0 9 only slightly below the equilibrium log fo2 of a partially condensed system of solar composition at the same temperature

archeointensity in northeast brazil over the past five centuries abstract this study presents the first archeointensity results from northeast brazil obtained from 14 groups of architectural brick fragments sampled in the city of salvador bahia state 13 s 38 5 w and dated between the middle of the xvith century and the beginning of the xixth century the dating is ascertained by historical documents complemented by archeological constraints yielding in all cases age uncertainties of less than 50 years analyses were carried out using two experimental protocols 1 the zero field in field version of the classical thellier and thellier method as proposed by coe tt zi including partial thermoremanent magnetization ptrm and ptrm tail checks and 2 the triaxe procedure involving continuous high temperature magnetization measurements both trm anisotropy and cooling rate effects were taken into account for the intensity determinations the cooling rate effect was further explored for the tt zi protocol using three increasing slow cooling times 5 h 10 h and 25 h between 450 c and room temperature following archeological constraints the slowest cooling time was retained in our study yielding decreases of the raw intensity values by 4 to 14 for each fragment a mean intensity was computed and retained only when the data obtained from all specimens between 2 and 6 satisfied a coherence test at 5 a total of 57 fragments 183 specimens was considered for the computations of site mean intensity values with derived standard deviations of less than 8 of the corresponding means when separately computed using the two experimental techniques the site mean intensity values always agree to within 5 a good consistency is observed between intensity values of similar or close ages which strengthen their reliability our data principally show a significant and continuous decrease in geomagnetic field intensity in northeast brazil between the first half of the xviith century and the xxth century one result dated to the second half of the xvith century further suggests that the geomagnetic field intensity reached a maximum around 1600 ad this evolution is in good agreement with that expected in the city of salvador from the available global geomagnetic field models however the accuracy of these models appears less well constrained between 1550 ad and 1650 ad

seasonal variations of high latitude field aligned currents inferred from orsted and magsat observations 1 in this paper we report on field aligned currents inferred from high precision three component geomagnetic field observations made on board the danish satellite orsted over polar regions because of a slow drift in local time of the satellite orbit through the noon midnight sector we were able to study the seasonal dependence of the dynamic properties of the dayside and nightside field aligned current systems over the northern and southern hemispheres we find an average over the pole distance between dayside and nightside currents of 32 during summer but 37 during winter and 36 during equinox the decrease in the size of the summer polar cap is caused by a shift of both daytime and nighttime current systems to higher magnetic latitudes for comparison the dawn dusk cross polar distance of the region 1 region 2 field aligned currents has been determined from high precision data observed by magsat a satellite flown in 1979 1980 in a dawn dusk orbit the latter results show that the dawn dusk distance between r1 r2 currents exhibits little seasonal dependence and amounts to 34 for all seasons in both polar caps the seasonal dependence is confirmed for the high latitude field aligned intensities they are larger by a factor of 1 5 1 8 in the sunlit summer polar cap in comparison with the winter hemisphere our results suggest that the r1 r2 and dayside field aligned currents are well balanced between the pairs of downward upward currents for all seasons as well as between hemispheres during equinox we were not able to confirm results reported in earlier studies that the net currents tend to increase with an enhancement of ionospheric conductivity caused by the solar illumination or substorm activity

high paleointensities of the geomagnetic field from thermomagnetic studies on rift valley pillow basalts from the mid atlantic ridge nineteen pillow basalts dredged within the rift valley of the mid atlantic ridge at 36 8on were studied by the thellier stepwise heating method in order to determine the paleointensity of the geomagnetic field when they erupted on to the sea floor previously reported fission track ages are 2 000 to 6 000 years for the youngest rocks mainly olivine basalts and 10 000 to 100 000 years for the others mainly plagioclase basalts and pyroxene basalts all but three pillow basalts meet the conditions commonly considered as indicative of quite reliable paleointensity estimates stability of the direction of nrm during its thermal demagnetization constant ratio of nrm trm natural remanent magnetization to thermoremanent magnetization over 50 or more of the original nrm intensity 80 to 94 for 11 specimens and reproducibility of low temperature partial trm ptrm however strong field thermomagnetic measurements indicate that 11 of these 16 samples display a significant increase in curie temperature 15 to 80oc during the paleointensity experiments below 250oc notwithstanding the linearity of the nrm trm plot in this temperature interval this alteration probably due to low temperature oxidation of the specimens seems typical of young pillow basalts and may result in paleointensity estimates which are too high this result shows that excellence of remanence tests nrmtrm linearity and ptrm stability does not ensure the absence of chemical changes during the thellier experiments and therefore the validity of the paleointensity obtained the assumption that reliable paleointensities are obtained when the curie point increase is 10oc or less led to the selection of five of our specimens all from the youngest group their mean paleointensity is 64 2 20 5 xt standard deviation and the corresponding virtual axial dipole moment vadm is 11 5 3 7 x 1022 a m 2 given the variations of the vadm of the earth s magnetic field over the last 6 000 years as established from archeomagnetic studies our paleointensity results suggest that the latest eruptions on the inner floor of the rift valley at 36 8on occurred 1 500 1 000 years ago

origin of kamacite schreibersite and perryite in metal sulfide nodules of the enstatite chondrite sahara 97072 eh3 perryite fe ni x si p y schreibersite fe ni 3p and kamacite afeni are constituent minerals of the metal sulfide nodules in the sahara 97072 eh3 enstatite chondrite meteorite we have measured concentrations of ni cu ga au ir ru and pd in these minerals with laser ablation inductively coupled plasma mass spectrometry icp ms we also measured their fe ni p si and co concentrations with electron microprobe in kamacite ratios of ru ir pd ir and pd ru cluster around their respective ci values and all elements analyzed plot near the intersection of the equilibrium condensation trajectory versus ni and the respective ci ratios in schreibersite the pd ru ratio is near the ci value and perryite contains significant cu ga and pd we propose that schreibersite and perryite formed separately near the condensation temperatures of p and si in a reduced gas and were incorporated into fe ni alloy upon further cooling sulfidation of fe in kamacite resulted in the formation of additional perryite at the sulfide interface still later transient heating re melted this perryite near the fe fes eutectic temperature during partial melting of the metal sulfide nodules the metal sulfide nodules are pre accretionary objects that retain ci ratios of most siderophile elements although they have experienced transient heating events

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

are there connections between the earth s magnetic field and climate understanding climate change is an active topic of research much of the observed increase in global surface temperature over the past 150 years occurred prior to the 1940s and after the 1980s the main causes invoked are solar variability changes in atmospheric greenhouse gas content or sulfur due to natural or anthropogenic action or internal variability of the coupled ocean atmosphere system magnetism has seldom been invoked and evidence for connections between climate and magnetic field variations have received little attention we review evidence for correlations which could suggest such causal or non causal connections at various time scales recent secular variation 10 100 yr historical and archeomagnetic change 100 5000 yr and excursions and reversals 10 3 10 6 yr and attempt to suggest mechanisms evidence for correlations which invoke milankovic forcing in the core either directly or through changes in ice distribution and moments of inertia of the earth is still tenuous correlation between decadal changes in amplitude of geomagnetic variations of external origin solar irradiance and global temperature is stronger it suggests that solar irradiance could have been a major forcing function of climate until the mid 1980s when anomalous warming becomes apparent the most intriguing feature may be the recently proposed archeomagnetic jerks i e fairly abrupt 100 yr long geomagneticfieldvariationsfoundatirregularintervalsoverthepastfewmillennia usingthearcheologicalrecordfromeuropetothe middle east these seem to correlate with significant climatic events in the eastern north atlantic region a proposed mechanism involves variations in the geometry of the geomagnetic field f i tilt ofthe dipole to lower latitudes resulting in enhanced cosmic ray induced nucleation of clouds no forcing factor be it changes in co2 concentration in the atmosphere or changes in cosmic ray flux modulatedbysolaractivityandgeomagnetism orpossiblyotherfactors canatpresentbeneglectedorshowntobetheoverwhelming single driver of climate change in past centuries intensive data acquisition is required to further probe indications that the earth s and sun s magnetic fields may have significant bearing on climate change at certain time scales 2006 elsevier b v all rights reserved

secular variation of the geomagnetic dipole during the past 2000 years we have constructed a very simple model of a time varying geocentric dipole based on the archeomagnetic records obtained at four widely separated sites on the globe for the past 2 ka the predictions of the model in terms of directional variations have been tested against actual archeomagnetic data from 12 sites distributed over the globe they were also compared with the hongre et al 1998 time varying spherical harmonic model and with the cals7k 2 model by korte et al 2005 we find that the misfits between the predictions of our simple dipole model and the data are equivalent to those of the spherical harmonic models for the european sites and not strikingly larger for the rest of the world many discrepancies can be accounted for by uncertainties inherent to the archeomagnetic records which along with the small number and poor geographical distribution of sites leads us to conclude that the present state of the database does not allow the extraction of secular variations described by terms going beyond degree 2 it appears also that dipole tilt could be responsible for the main part of the secular variation associated with time constants exceeding 102 years in a second step we used the paleointensity records contained in the same database to construct the curve depicting the variations of the true dipole moment the present decrease of the dipole did not begin prior to 1000 years ago and the dipole was actually increasing from 0 until a d 500 the dipole moment of cals7k is lower than the present estimate probably due to large repartition of energy to higher harmonics to minimize the misfit between the inversion and the data the tilt and strength of the dipole can predict the dipole field at any site and were used to derive the contribution of the nondipole field to values of paleointensity at paris during the past 2 ka the results show that the archeomagnetic jerks are associated with various configurations depending on the phase relationship between the nondipolar and dipolar parts of the field

feo rich silicates in the sahara 97159 eh3 enstatite chondrite mineralogy oxygen isotopic compositions and origin we report the mineralogy and oxygen isotopic compositions of feo rich silicates in the sahara 97159 eh3 chondrite this component is referred to as feo rich because it contains substantially more feo than the characteristic feo poor silicates in the highly reduced enstatite meteorites these feo rich silicates are mostly low ca pyroxene fs535 and their compositions suggest an origin under more oxidizing conditions like those for the ordinary chondrites however the mafic silicates in ordinary and carbonaceous chondrites are dominantly olivine and the feo rich silicates in the e chondrites are less commonly olivine the oxygen isotopic compositions of the feo rich silicates are indistinguishable from those of feo poor silicates in sahara 97159 these observations suggest that both the feo rich silicates and the feo poor silicates in eh chondrites formed from the same oxygen reservoir where redox conditions varied widely

mars in the aftermath of a colossal impact abstract the abundance of highly siderophile elements hses inferred for mars mantle from martian meteorites implies a late veneer lv mass addition of 0 8 wt with broadly chondritic composition late accretion to mars by a differentiated ceres sized 1000 km diameter object can account for part of the requisite lv mass and geochronological constraints suggests that this must have occurred no later than ca 4480 ma here we analyze the outcome of the hypothetical lv giant impact to mars with smoothed particle hydrodynamics simulations together with analytical theory results show that in general about 50 of the impactor s metallic core shatters into 10 m fragments that subsequently fragment into sub mm metallic hail at re accretion this returns a promising delivery of hses into martian mantle compared to either a head on and hit and run collision in both cases 10 myr a dense pre noachian co2 atmosphere should lower the escape efficiency of hydrogen by ir emission a more detailed hydrodynamic atmospheric model of this early hydrogen atmosphere is warranted to examine its effect on pre noachian mars

geomagnetic field intensity behavior in the middle east between 3000 bc and 1500 bc 1 an archeointensity study was carried out on 14 sites of syrian baked clay artifacts archeologically dated between 2500 bc and 1600 bc using an experimental protocol involving high temperature magnetization measurements well defined mean intensity values were derived for 13 different sites with three to nine results obtained at the fragment level per site results of similar ages are coherent and the new data set is in good agreement with previous archeointensity results obtained from the same region all together these data allow one to refine the evolution of the geomagnetic field intensity in the middle east during the third and the second millennium bc in particular they show the occurrence of three periods of rather sharp intensity increase at 2600 bc 2200 bc and 1600 bc possibly at the times of climatic cooling in the eastern north atlantic further suggesting a connection between the earth s magnetic field and multi decadal climatic events

lanthanide and actinide chemistry at high c o ratios in the solar nebula abstract chemical equilibrium calculations were performed to study the condensation chemistry of the ree and actinides under the highly reducing conditions which are necessary for the formation of the enstatite chondrites 1 2 our calculations confirm that the ree and actinides condensed into oldhamite cas the major ree and actinide host phase in enstatite chondrites at a carbon oxygen co ratio 1 in an otherwise solar gas five basic types of ree abundance patterns several of which are analogous to ree abundance patterns observed in the ca al rich inclusions cais in carbonaceous chondrites are predicted to occur in meteoritic oldhamites all of the reported ree patterns in oldhamites in enstatite chondrites can be interpreted in terms of our condensation calculations the observed patterns fall into 3 of the 5 predicted categories the reported th and u enrichments and ratios in meteoritic oldhamites are also consistent with predictions of the condensation calculations finally pure ree sulfides are predicted to condense in the 10 6 to 10 9 bar range and may be found in enstatite chondrites if they formed in this pressure range

magnesium and 54cr isotope compositions of carbonaceous chondrite chondrules insights into early disk processes we report on the petrology magnesium isotopes and mass independent 54cr 52cr compositions 54cr of 42 chondrules from cv vigarano and nwa 3118 and cr nwa 6043 nwa 801 and lap 02342 chondrites all sampled chondrules are classified as type ia or type iab have low 27al 24mg ratios 0 04 0 27 and display little or no evidence for secondary alteration processes the cv and cr chondrules show variable 25mg 24mg and 26mg 24mg values corresponding to a range of mass dependent fractionation of 500 ppm parts per million per atomic mass unit this mass dependent mg isotope fractionation is interpreted as reflecting mg isotope heterogeneity of the chondrule precursors and not the result of secondary alteration or volatility controlled processes during chondrule formation the cv and cr chondrule populations studied here are characterized by systematic deficits in the mass independent component of 26mg 26mg relative to the solar value defined by ci chondrites which we interpret as reflecting formation from precursor material with a reduced initial abundance of 26al compared to the canonical 26al 27al of 5 10 5 model initial 26al 27al values of cv and cr chondrules vary from 1 5 4 0 10 6 to 2 2 0 4 10 5 the cv chondrules display significant 54cr variability defining a range of compositions that is comparable to that observed for inner solar system primitive and differentiated meteorites in contrast cr chondrites are characterized by a narrower range of 54cr values restricted to compositions typically observed for bulk carbonaceous chondrites collectively these observations suggest that the cv chondrules formed from precursors that originated in various regions of the protoplanetary disk and were then transported to the accretion region of the cv parent asteroid whereas cr chondrule predominantly formed from precursor with carbonaceous chondrite like 54cr signatures the observed 54cr variability in chondrules from cv and cr chondrites suggest that the matrix and chondrules did not necessarily formed from the same reservoir the coupled 26mg and 54cr systematics of cr chondrules establishes that these objects formed from a thermally unprocessed and 26al poor source reservoir distinct from most inner solar system asteroids and planetary bodies possibly located beyond the orbits of the gas giants in contrast a large fraction of the cv chondrules plot on the inner solar system correlation line indicating that these objects predominantly formed from thermally processed 26al bearing precursor material akin to that of inner solar system solids asteroids and planets

formation of niningerite by silicate sulfidation in eh3 enstatite chondrites unequilibrated eh chondrites contain silica bearing chondrules with abundant niningerite mg fe mn s and troilite fes distinguishing them from the silica bearing chondrules in ordinary and carbonaceous chondrites the conventional explanation for the origin of niningerite and oldhamite cas is that they are condensates from c rich nebular gas however models of condensation from a solar gas with an elevated c o ratio predict mineralogy that is inconsistent with petrographic observations we report petrographic and chemical evidence from 45 silica bearing chondrules from the eh3 chondrites sahara 97072 and alan hills 84170 for formation of niningerite and oldhamite by sulfidation of ferromagnesian silicates the results indicate extensive thermal processing of the chondrules including melting before during and after sulfidation bulk compositions of chondrules exhibiting varied degrees of sulfidation suggest that depletion of mg and enrichment of fe mn and na accompany the reactions sulfidation of feo bearing silicates which formed at oxidizing conditions at least iw 3 can occur with exposure to a h poor c and s rich gaseous reservoir 6 8 fo2 log units below the iw buffer at temperatures high enough for partial melting of silicates physicochemical analysis of mineral reactions inferred from sulfidized chondrules suggests a melted metal sulfide assemblage in a h poor environment is capable of generating sufficient s vapor to drive sulfidation the reaction of silicates with the s gas will result in progressive extraction of fe ca and mg into sulfides with the stoichiometric amounts of silica either reacting with olivine to form enstatite or when olivine is exhausted precipitating as free silica the sulfidation environment drastically increases mg volatility resulting in evaporative loss until saturation in the ambient gas is reached the newly formed mg rich niningerite will tend to reach equilibrium composition by losing mg and gaining fe plus mn from the ambient gas consistent with the observed chemical fractionation in the sulfidized chondrules therefore sulfidation of ferromagnesian silicates can also explain the low bulk mg si ratios of the enstatite chondrites if the sulfidizing gas was lost before accretion of the enstatite chondrite parent body

directional changes of the geomagnetic field in west africa insights from the metallurgical site of korsimoro abstract this work shows the first archeomagnetic directions from western africa measured on 32 iron smelting kilns dated between 650 and 1800 ad the archeological excavation of the vast metallurgical site of korsimoro established the existence of four distinct iron smelting techniques the time frame of each technique could be clearly determined with radiocarbon dating many of the kilns investigated in this study could also be dated individually with residual charcoals found in their inside the results indicate that the inclination of the field changed gradually from shallow normal to shallow reversed during 800 to 1300 ad and then went back to shallow normal around 1600 1700 ad the declination was instead stable around 10 e between 800 and 1400 ad thereafter it started changing towards north this trend correlates well with available secular variation curves from the balkan and from spain and indicates that the field variation in west africa was similar to the one in europe

the zaklodzie enstatite meteorite mineralogy petrology origin and classification the zaklodzie meteorite was found in september 1998 about 40 km west of zamo c in southeast poland macroscopic and microscopic observations in transmitted and reflected light microprobe analyses cathodoluminescence images and x ray diffraction data show that the meteorite is composed of clino and orthoenstatite two generations of feldspars relict olivine forsterite a polymorph of sio2 apparently cristobalite and opaque minerals fe ni alloy kamacite and taenite troilite schreibersite graphite and sulfide mg mn fe s which is probably keilite the texture is fine to inequigranular of cumulate type locally intergranular the mgs fes thermometer indicates that the sulfides crystallized at 580 600 oc thus the zaklodzie meteorite formed by the nearly complete melting of an enstatite chondrite protolith probably at 4 4 ga the process was likely caused by the decay of the 26al nuclide in the planetesimal interior the second stage of its evolution which could have happened at 2 1 ga involved partial re melting of most fusible components probably due to collision with another body the structure composition and origin of the meteorite and its relation to the parent rock indicate that zaklodzie may represent a primitive enstatite achondrite

geomagnetic field for 0 3 ka 1 new data sets for global modeling 1 paleomagnetic and archeomagnetic records are used in both regional and global studies of earth s magnetic field we present a description and assessment of five newly compiled data sets also used in the companion paper by korte et al 2009 to produce a series of time varying spherical harmonic models of the geomagnetic field for the last 3000 years data are drawn from our compilation of lake sediment records and from the online database geomagia50v2 the five selections are available from the earthref digital archive at http earthref org cgi bin erda cgi n 944 data are grouped according to the source of material and we conducted separate assessments of reliability for archeomagnetic artifacts and lava flows the arch3k dat data set and for sediments sed3k dat the overall number of data is 55 greater than in previous compilations constrained data sets were selected using different criteria for each group winnowing of archeological data was based on uncertainties supplied by the original data providers the lake sediment data assessment relied on preassigned age uncertainties and one or more of the following comparisons with archeomagnetic data from the same region regional consistency among several lakes and consistency with global archeomagnetic models we discuss relative merits of a larger unconstrained data set or a smaller possibly more reliable one the constrained data sets eliminate a priori up to 35 of the available data in each case and rely on potentially subjective assessments of data quality given the limited data available our analyses indicate that iterative rejection of a small number 1 1 5 of outlying data during global field modeling is a preferable approach specific regional comparisons among the models and data support the conclusion that korte et al s outlier free cals3k 3 model based on all available measurements from sediments and archeological artifacts currently provides the best global representation of the 0 3 ka field the arch3k 1 model provides a better fit to the denser european archeomagnetic data and may be better in that region

chondrule formation by repeated evaporative melting and condensation in collisional debris clouds around planetesimals a synthesis of previous work leads to a model of chondrule formation that involves periodic melting of dispersed dust in debris clouds that were generated by collisions between chondritic planetesimals i suggest that chondrules formed by the passage of nebular shock waves through these dust clumps which temporarily surrounded disrupted planetesimals type i chondrules formed by more intense evaporative heating of fewer particles in tenuous clumps or at the edges of dense clumps and type ii chondrules formed by less intense evaporative heating of more particles deeper within dense clumps chondrules reaccreted by self gravity into the planetesimals mixing with less heated dust and rock this process of disruption melting and reaccretion could have repeated many times in this way chondrite components of various origins and thermal histories could remain preserved in planetesimals as a distinctive mix of materials for extended periods of time while still allowing for a repetitive melting process that converted some of the planetesimal debris into chondrules i also suggest that during chondrule formation the inner solar nebula gas was evolving by the gradual incorporation and heating of icy bodies depleted in 16 o causing a general increase in gaseous d 17 o with time in most places especially close to the snow line in this model early formed type i chondrules in c chondrites with lower d 17 o values were produced inside the snow line and later formed type i and type ii chondrules in c and o chondrites with higher d 17 o values were created nearer the snow line after it had moved closer to the young sun

new archaeointensity data from italy and geomagnetic field intensity variation in the italian peninsula we present new archaeointensity results from three italian kilns situated at ascoli satriano vagnari and fontanetto po obtained with the thellier modified by coe double heating method these data complement the directional results previously published all sites are dated on the basis of archaeological information and or thermoluminescence dating the results are corrected for the anisotropy of the thermoremanent magnetization and the cooling rate effects the new data are compared with previously published archaeointensity data from italy and nearby countries within 900 km radius from viterbo an initial data set including archaeointesity data mainly coming from italy france switzerland czech republic slovakia hungary greece and bulgaria has been compiled after the application of strict selection criteria the most reliable data have been used for the calculation of a preliminary italian intensity secular variation sv curve for the last 3000 yr the new curve covers the 300 bc 400 ad and 1200 1900 ad periods it is established by means of sliding windows of 200 yr shifted by 100 yr the lack of reliable data for the 1000 200 bc and 400 1200 ad time intervals does not permit the calculation of a continuous curve clearly more high quality archaeointensity data from italy and europe are still needed to draw a robust intensity sv curve for the italian peninsula that could be used for archaeomagnetic dating in combination with the directional data

possible impact of the earth s magnetic field on the history of ancient civilizations we report new archeointensity results from iranian and syrian archeological excavations dated from the second millennium bc these high temperature magnetization data were obtained using a laboratory built triaxial vibrating sample magnetometer together with our previously published archeointensity results from mesopotamia we constructed a rather detailed geomagnetic field intensity variation curve for this region from 3000 bc to 0 bc four potential geomagnetic events archeomagnetic jerks marked by strong intensity increases are observed and appear to be synchronous with cooling episodes in the north atlantic this temporal coincidence strengthens the recent suggestion that the geomagnetic field influences climate change over multi decadal time scales possibly through the modulation of cosmic ray flux interacting with the atmosphere moreover the cooling periods in the north atlantic coincide with episodes of enhanced aridity in the middle east when abrupt societal changes occurred in the eastern mediterranean and mesopotamia although the coincidences discussed in this paper must be considered with caution they lead to the possibility that the geomagnetic field impacted the history of ancient civilizations through climatically driven environmental changes triggering economic social and political instability

archaeointensity determinations from italy new data and the earth s magnetic field strength variation over the past three millennia summary three kilns and a collection of baked bricks from italian archaeological sites have been studied for archaeointensity determination using the thellier method as modified by coe all sites are dated based on archaeological information and their ages range from 500 to 800 ad and 1500 to 1700 ad rock magnetic studies identify magnetite ti magnetite and hematite as the main magnetic minerals and magnetic susceptibility versus temperature shows a good thermal stability of the samples the intensity results have been corrected for anisotropy of the thermoremanent magnetization and cooling rate effects the new data together with 136 previously published results are used to estimate the variation of the earth s magnetic field over the past three millennia the time distribution of the italian absolute intensity data is irregular with the majority of determinations concentrated during the last four centuries while older periods are very poorly covered most of the data come from volcanic rocks and show significant discrepancies all data have been compared with archaeointensity results from europe and regional and global models predictions this work indicates the need to obtain more high quality archaeointensity results in particular for the periods older than 200 bc and between 200 and 1000 ad in order to determine a robust italian intensity secular variation curve that in combination with directional data could be used for archaeomagnetic dating

noble gases in enstatite chondrites ii the trapped component the trapped noble gas record of 57 enstatite chondrites e chondrites has been investigated basically two different gas patterns have been identified dependent on the petrologic type all e chondrites of type 4 to 6 show a mixture of trapped common chondritic rare gases q and a subsolar component range of elemental ratios for e4 6 chondrites 36ar 132xe 582 270 and 36ar 84kr 242 88 e3 chondrites usually contain q gases but also a composition with lower 36ar 132xe and 36ar 84kr ratios which we call sub q 36ar 132xe 37 0 18 0 and 36ar 84kr 41 7 18 1 the presence of either the subsolar or the sub q signature in particular petrologic types cannot be readily explained by parent body metamorphism as postulated for ordinary chondrites we therefore present a different model that can explain the bimodal distribution and composition of trapped heavy noble gases in e chondrites trapped solar noble gases have been observed only in some e3 chondrites about 30 of each group eh3 and el3 chondrites amounting to 9 of all analyzed e chondrites show the solar signature notably only one of those meteorites has been explicitly described as a regolith breccia

high temperature phase equilibria in a solar composition gas abstract using recent additions to thermochemical data on minerals and information on their solid solution behavior new equilibrium phase diagrams have been computed in a system of solar gas composition si al mg ca fe ni ti na k c h o s n in the pressure and temperature ranges of 1 to 10 6 bar and 1153 to 1773 k respectively these calculations show that fe ni alloy condenses before all silicates included here except melilite down to a pressure of 2 10 4 bar below which plagioclase and clinopyroxene condense first orthopyroxene condenses next followed by ilmenite pressure temperature variation of the chemical composition of melilite clinopyroxene orthopyroxene metal alloy and plagioclase may be used for cosmothermometry and cosmobarometry for equilibrium assemblages the major transition from the refractory oxides and melilite the meteorite inclusion assemblage to an assemblage of fe ni alloy olivine plagioclase and pyroxenes planet forming takes place within a narrow interval of pressure and temperature small fluctuations of either pressure or temperature across this narrow region result in drastic changes in types and modes of minerals which may explain the wide mineralogical varieties of meteorites

microwave archaeointensities from peruvian ceramics summary accurate determination of the strength of the ancient geomagnetic field is limited by mineralogical alteration that may occur when samples are heated to produce a thermoremanent magnetization thellier thellier 1959 shaw 1974 by using direct microwave excitation of the magnetic grains we have been able to form a thermoremanent magnetization without significantly heating the bulk sample thus avoiding thermal alteration walton et al 1993 incorporation of this new microwave heating technique with the conventional thellier palaeointensity method has dramatically reduced the scatter of archaeointensity results from peruvian ceramics and clearly defined how the field strength has varied in peru over the past 2000 years the new microwave technique will therefore enhance the feasibility of archaeointensity dating and provide accurate field strength data for geomagnetic modelling

does earth s magnetic field secular variation control centennial climate change abstract we obtained new archeointensity data from french faience potsherds dated from the 17th to 19th century these results further document the occurrence of sharp changes in geomagnetic field secular variation in western europe over the past three millennia the intensity variation curve shows several maxima whose rising parts appear to coincide in time with the occurrence of cooling events documented in this region from natural and historical data this coincidence suggests a causal link between enhanced secular variation of the geomagnetic field and climate change over centennial time scales challenging the role of solar forcing as the sole factor provoking these climatic variations we propose that the archeomagnetic jerks described by gallet et al 1 y gallet a genevey v courtillot on the possible occurrence of archeomagnetic jerks in the geomagnetic field over the past three millennia earth planet sci lett 214 2003 237 242 may engage the mechanism for centennial climate change

archeoint an upgraded compilation of geomagnetic field intensity data for the past ten millennia and its application to the recovery of the past dipole moment 1 this paper presents a compilation of intensity data covering the past 10 millennia archeoint this compilation which upgrades the one of korte et al 2005 contains 3648 data and incorporates additional intensity and directional data sets a large majority of these data 87 were acquired on archeological artifacts and the remaining 13 correspond to data obtained from volcanic products the present compilation also includes important metadata for evaluating the intensity data quality and providing a foundation to guide improved selection criteria we show that 50 of the data set fulfill reasonable reliability standards which take into account the anisotropic nature of most studied objects potsherds the stability of the magnetization and the data dispersion the temporal and geographical distributions of this sub data set are similar to those of the main data set with 72 of the data dated from the past three millennia and 76 obtained from western eurasia approximately half of the selected intensity data are associated with at least an inclination value to constrain the axial and full dipole evolution over the past three millennia requires that we avoid any overrepresentation of the western eurasian data we introduce a first order regional weighting scheme based on the definition of eight widely distributed regions of 30 width within which the selected data are numerous enough the regional curves of virtual axial dipole moments vadm and of mixed vadm virtual dipole moments vdm averaged over sliding windows of 200 years and 500 years testify for strong contributions from either equatorial dipole or nondipole components the computation of global vadm and mixed vadm vdm variation curves assuming an equal weight for each region yields a dipole evolution marked by a distinct minimum around 0 b c a d followed by a maximum around the third fourth century a d a second minimum is present around the eighth century a d this variation pattern is compatible with the one deduced from earlier more

geomagnetic field intensity behavior in south america between 400 ad and 1800 ad first archeointensity results from argentina abstract an absolute archeointensity study in northwest argentina provided 25 independent geomagnetic field lectures supported by 37 radiometric dates between ad 400 and 1800 the mean cooling rate and remanence anisotropy corrected archeointensity values obtained in this study range from 36 5 2 6 to 63 1 8 7 t with corresponding virtual axial dipole moments vadms from 7 4 0 5 to 12 7 1 8 1022 am2 most of the data are concentrated between a relatively narrow interval from 1350 ad to 1550 ad three general features may be detected the time intervals from about ad 1150 to 1350 and 1450 to 1600 are characterized by quite monotonic decay of geomagnetic intensity while some increase is observed from ad 1600 to 1700 the archeointensity decrease from about 17th century seems to be a general characteristic of global geomagnetic field because it was observed at different places worldwide in the absence of reliable climate variation record for south america it is delicate to make any firm conclusions about the relationship between the earth s magnetic field and multi decadal climatic events however it is probably not a coincidence that persistent warm climate detected from patagonia during ad 1200 to 1350 is consistent to geomagnetic intensity decrease revealed in this study

a box model for the paleoceanography of the black sea the evolution of the black sea s salinity after the opening of the bosporus about 7500 years ago is investigated using a simple two box model the model consists of watermass and salt conservation equations and allows for changes in halocline depth the paleoceanographic box model is forced by present day mediterranean inflow and outflow and atmospheric forcings analytic solutions for the evolution of the box volumes are given model salinities reach 90 of their the present day values in both boxes about 2 500 years after the opening of the bosporus the evolution of the salinities is shown to be almost independent of the evolution of the box volumes and the results are compared with the existing paleoceanographic proxy records

mineral equilibrium in fractionated nebular systems we investigated the equilibrium mineral assemblages in chemically fractionated nebular systems using a computer routine that finds the set of minerals and gases which minimizes the gibbs free energy of a system with stipulated elemental abundances diagrams are presented showing the equilibrium mineralogy as a function of temperature 400 2300 k for unfractionated solar material and five fractionated systems the fractionated systems were defined by mixing in various proportions the following four volatility components that solar material can be divided into refractory dust carbonaceous matter ices and h2 gas dust enrichment is seen to increase temperatures of condensation evaporation and the fe2 content of mafic minerals and to permit existence of stable melt phases enrichment of dust and organic matter produces mineral assemblages that are similar in many ways to those of enstatite chondrites but with mafic minerals that are far more reduced fe2 poor than those in primitive enstatite chondrites enrichment of dust organics and ices leads to highly ferrous mineralogies even at the highest temperatures but does not predict the stability of hydrous phases above 450 k

evidence for rapid geomagnetic field intensity variations in western europe over the past 800 years from new french archeointensity data abstract the number of reliable archeointensity determinations obtained from western europe for the past millennium remains limited moreover the large scatter between different datasets available is puzzling the present study analyzed 31 new groups of baked clay ceramic or brick fragments sampled in france 29 groups and in belgium 2 groups these groups contain several fragments collected from different artefacts and are precisely dated principally from historical constraints between the xiiith and the xixth centuries additionally we re evaluated 14 intensity values that we previously obtained from the same time period the fragments were analyzed using two different thermal methods 1 the in field zero field iz or the izzi version of the classical thellier and thellier method and 2 the triaxe protocol that involves high temperature magnetization measurements data were corrected for the anisotropy of thermoremanent magnetization trm and the dependence of trm acquisition on the cooling rate was taken into account in the different protocols archeointensity data obtained on twin specimens sampled from the same fragment and using both experimental techniques generally show a good agreement i e within 5 at the fragment and at the site level all retained site level averaged intensity results 43 of 45 groups have standard deviations of less than 5 t furthermore groups of approximately the same age have very consistent archeointensity altogether the data presented herein recover a detailed and smoothed geomagnetic field intensity variation curve characterized by two peaks in intensity the first during the second half of the xivth century and the second around ad 1600 followed by a significant decreasing trend in intensity during most the xviith and xviiith centuries this evolution does not satisfactorily fit with the expected intensity values for france derived from geomagnetic field models relying on a different evolution of the axial dipole moment our results lead us to propose that the axial dipole moment decreased from ad 1600 to the end of the xviiith century then slightly increased up to ad 1850 before decreasing again to present day