global distribution of large lunar craters implications for resurfacing and impactor populations by using high resolution altimetric measurements of the moon we produced a catalog of all impact craters 20 kilometers in diameter on the lunar surface and analyzed their distribution and population characteristics the most densely cratered portion of the highlands reached a state of saturation equilibrium large impact events such as orientale basin locally modified the prebasin crater population to 2 basin radii from the basin center basins such as imbrium orientale and nectaris which are important stratigraphic markers in lunar history are temporally distinguishable on the basis of crater statistics the characteristics of pre and postmare crater populations support the hypothesis that there were two populations of impactors in early solar system history and that the transition occurred near the time of the orientale basin event

caloris impact basin exterior geomorphology stratigraphy morphometry radial sculpture and smooth plains deposits the mariner 10 flybys revealed the eastern third of the caloris basin and demonstrated its importance as a chronostratigraphic marker for the geological history of mercury uncertain after that mission were the size of the basin the full distribution of materials deposited or modified during the impact process and the stratigraphic and age relationships between caloris and surrounding terrain images obtained during messenger s january 2008 flyby of mercury revealed that the caloris basin is 15 larger than previously estimated and moderately elliptical 1525 by 1315 km basin related sculpture and secondary craters are dispersed widely in areas surrounding the basin confirming the widespread significance of this event as a stratigraphic marker units mapped around caloris on the basis of mariner 10 data are generally recognized in regions observed for the first time by messenger including most facies of the caloris group the caloris montes odin formation and van eyck formation the only unit without obvious exposures in messenger images is the nervo formation which was originally interpreted as impact melt or fallback ejecta new measurements of the size frequency distribution of impact craters both within the caloris basin and on surrounding units particularly the odin formation support the hypothesis that the vast majority of circum caloris plains postdate the basin and are likely volcanic the messenger observations re emphasize the importance of understanding the exterior stratigraphy of the basin in order to utilize the caloris event as a discrete time horizon in the geological history of mercury

paleomagnetic determination of emplacement temperatures of pyroclastic deposits an under utilized tool paleomagnetic data from lithic clasts collected from mt st helens usa volcan lascar chile volcan de colima mexico and vesuvius italy have been used to determine the emplacement temperature of pyroclastic deposits at these localities and to highlight the usefulness of the paleomagnetic method for determining emplacement temperatures at mt st helens the temperature of the deposits tdep at three sites from the june 12 1980 eruption was found to be 532 c 509 c and 510 570 c respectively one site emplaced on july 22 1980 was emplaced at 577 c these new paleomagnetic temperatures are in good agreement with previously published direct temperature measurements and paleomagnetic estimates lithic clasts from pyroclastic deposits from the 1993 eruption of lascar were fully remagnetized above the respective curie temperatures which yielded a minimum tdep of 397 c samples were also collected from deposits thought to be pyroclastics from the 1913 2004 and 2005 eruptions of colima at colima the sampled clasts were emplaced cold this is consistent with the sampled clasts being from lahar deposits which are common in the area and illustrates the usefulness of the paleomagnetic method for distinguishing different types of deposit tdep of the lower section of the lithic rich pyroclastic flow lrpf from the 472 a d deposits of vesuvius was 280 340 c this is in agreement with other recently published paleomagnetic measurements in contrast the upper section of the lrpf was emplaced at higher temperatures with tdep 520 c this temperature difference is inferred to be the result of different sources of lithic clasts between the upper and lower sections with the upper section containing a greater proportion of vent derived material that was initially hot our studies of four historical pyroclastic deposits demonstrates the usefulness of paleomagnetism for emplacement temperature estimation

mercury cratering record viewed from messenger s first flyby morphologies and size frequency distributions of impact craters on mercury imaged during messenger s first flyby elucidate the planet s geological history plains interior to the caloris basin displaying color and albedo contrasts have comparable crater densities and therefore similar ages smooth plains exterior to caloris exhibit a crater density 40 less than on interior plains and are thus volcanic and not caloris impact ejecta the size distribution of smooth plains craters matches that of lunar craters postdating the late heavy bombardment implying that the plains formed no earlier than 3 8 billion years ago ga at diameters less than or equal to 8 to 10 kilometers secondary impact craters on mercury are more abundant than primaries this transition diameter is much larger than that on the moon or mars a low density of craters on the peak ring basin raditladi implies that it may be younger than 1 ga

high pressure phase relations and thermodynamic properties of caal4si2o11 cas phase abstract phase relations in caal4si2o11 were examined at 12 23 gpa and 1000 1800 c by multianvil experiments a three phase mixture of grossular kyanite and corundum is stable below about 13 gpa at 1000 1800 c at higher pressure and at temperature below about 1200 c a mixture of grossular stishovite and corundum is stable indicating the decomposition of kyanite above about 1200 c caal4si2o11 cas phase is stable at pressure higher than about 13 gpa the triple point is placed at 14 7 gpa and 1280 c the equilibrium boundary of formation of cas phase from the mixture of grossular kyanite and corundum has a small negative slope and that from the mixture of grossular stishovite and corundum has a strongly negative slope while the decomposition boundary of kyanite has a small positive slope enthalpies of the transitions were measured by high temperature drop solution calorimetry the enthalpy of formation of caal4si2o11 cas phase from the mixture of grossular kyanite and corundum was 139 5 15 6 kj mol and that from the mixture of grossular stishovite and corundum was 94 2 15 4 kj mol the transition boundaries calculated using the measured enthalpy data were consistent with those determined by the high pressure experiments the boundaries in this study are placed about 3 gpa higher in pressure and about 200 c lower in temperature than those by zhai and ito zhai s ito e 2008 phase relations of caal4si2o11 at high pressure and high temperature with implications for subducted continental crust into the deep mantle phys earth planet inter 167 161 167 combining the thermodynamic data measured in this study with those in the literature dissociation boundary of cas phase into a mixture of ca perovskite corundum and stishovite and that of grossular into ca perovskite plus corundum were calculated to further constrain the stability field of cas phase the result suggests that the stability of cas phase would be limited at the bottom of transition zone and top of the lower mantle when sediments are subducted into the deep mantle it is also suggested that cas phase may be stable at the depth of the upper part of the lower mantle when partial melting of basalt occurs at the depth

experimental and textural investigation of welding effects of compaction sintering and vapor phase crystallization in the rhyolitic rattlesnake tuff abstract the abrupt changes in character of variably welded pyroclastic deposits have invited decades of investigation and classification we conducted two series of experiments using ash from the nonwelded base of the rhyolitic rattlesnake tuff of oregon usa to examine conditions of welding one series of experiments was conducted at atmospheric pressure 1 at in a muffle furnace with variable run times and temperature and another series was conducted at 5 mpa and 600 c in a cold seal apparatus with variable run times and water contents we compared the results to a suite of incipiently to densely welded natural samples of the rattlesnake tuff experiments at 1 at required a temperature above 900 c to produce welding which is in excess of the estimated pre eruptive magmatic temperature of the tuff the experiments also yielded globular clast textures unlike the natural tuff during the cold seal experiments the gold sample capsules collapsed in response to sample densification textures and densities that closely mimic the natural suite were produced at 5 mpa 600 c and 0 4 wt h2o over run durations of hours to 2 days clast deformation and development of foliation in 2 week runs were greater than in natural samples both more and less water reduced the degree of welding at otherwise constant run conditions for 5 mpa experiments changes in the degree of foliation of shards and of axial ratios of bubble shards and non bubble mainly platy shards are consistent with early densification related to compaction and partial rotation of shards into a foliation subsequent densification was associated with viscous deformation as indicated by more sintered contacts and deformation of shards sintering local fusion of shard shard contacts was increasingly important with longer run times higher temperatures and greater pressures during runs with high water concentrations sintering was rare and adhesion between clasts was dominated by precipitation of sublimates in pore spaces a few tenths wt h2o in the rhyolite glass promote the development of welding by sharp reduction of glass viscosity large amounts of water inhibit welding by creating surface sublimates that interfere with sintering and may exert fluid pressure counter to lithostatic load if sintering and vapor phase sublimates seal permeability in the tuff

enstatite jadeite join and its role in the earth s mantle phase relations on the enstatite jadeite join were experimentally determined at solidus temperatures and 90 152 kb and at 1400 2050 c 175 219 kb with a split sphere anvil apparatus ussa 2000 new findings include immiscibility in garnet and determination of the stability of naalsio4 calcium ferrite structure with stishovite a thermodynamic model for the enstatite jadeite join was developed to calculate a complete phase diagram for the join at 500 2500 c and 0 270 kb the results indicate that the two major discontinuities in the earth s mantle at 400 and 670 km depths could correspond respectively to the formation and the breakdown of garnet with a pyroxene composition a model for a chondritic upper mantle is proposed in which large scale chemical and mineral layering was produced by fractionation of liquidus phases in a magma ocean solidification was completed at 400 km depth by crystallization of sodium enriched residual melts which produced a pyroxene layer at 300 400 km depths

cooling rates of plinian fall and pyroclastic flow deposits in the bishop tuff inferences from water speciation in quartz hosted glass inclusions cooling rates inferred from water speciation in rhyolitic glass inclusions in quartz phenocrysts from the bishop tuff california vary over more than 5 orders of magnitude from about 10 to 10 4 c s 1 many of the cooling rates are within the previously published experimentally calibrated range of this technique but the slowest rates 10 4 c s 1 are extrapolated such extrapolation suggests cooling rates as low as 10 8 c s 1 0 3 c year 1 for the interiors of thick pyroclastic flow deposits the temperatures at which oh and molecular h2o species would be in equilibrium range from 400 c for glass inclusions from plinian fall deposits to 250 c for the most slowly cooled glassy inclusions from pyroclastic flow deposits notwithstanding appreciable uncertainties cooling rates correlate with depositional process fall vs flow deposition and distance above the cold ground or below overlying hot pyroclastic flow deposits plinian inclusions within 2 5 m of the cold pre bishop ground cooled at 1 10 c s 1 consistent with thermal modeling of small pumice clasts falling through cold air very slow inferred cooling rates for pyroclastic flow deposits 10 6 to 10 8 c s 1 also agree with thermal conduction models faster cooling rates 10 1 to 10 4 c s 1 for inclusions in the lowermost 0 5 m of pyroclastic flow deposits reflect the effect of quenching against the cold ground at blind spring hill east of long valley caldera intermediate cooling rates 10 3 to 10 6 c s 1 are observed in plinian deposits overlain by and partially interbedded with pyroclastic flow deposits indicating that reheating of rapidly cooled plinian inclusions can reset water speciation melt inclusions in pyroclastic flow deposits are quenched to glass only if the deposit is relatively thin or has a relatively low emplacement temperature in relatively thick or hot deposits inclusions may be quenched to glass at the base of the deposit where it is more rapidly cooled by contact with the pre eruption ground

high pressure minerals in shocked meteorites heavily shocked meteorites contain various types of high pressure polymorphs of major minerals olivine pyroxene feldspar and quartz and accessory minerals chromite and ca phosphate these high pressure minerals are micron to submicron sized and occur within and in the vicinity of shock induced melt veins and melt pockets in chondrites and lunar howardite eucrite diogenite hed and martian meteorites their occurrence suggests two types of formation mechanisms 1 solid state high pressure transformation of the host rock minerals into monomineralic polycrystalline aggregates and 2 crystallization of chondritic or monomineralic melts under high pressure based on experimentally determined phase relations their formation pressures are limited to the pressure range up to 25 gpa textural crystallographic and chemical characteristics of high pressure minerals provide clues about the impact events of meteorite parent bodies including their size and mutual collision velocities and about the mineralogy of deep planetary interiors the aim of this article is to review and summarize the findings on natural high pressure minerals in shocked meteorites that have been reported over the past 50 years

formation of a metastable hollandite phase from amorphous plagioclase a possible origin of lingunite in shocked chondritic meteorites abstract we conducted high pressure experiments in plagioclase with different anorthite contents at 18 27 gpa and 25 1750 c using both a laser heated diamond anvil cell and a kawai type multi anvil apparatus to clarify the formation conditions of the hollandite phase in shocked chondritic and martian meteorites lingunite naalsi3o8 rich hollandite was found first to crystallize from amorphous oligoclase as a metastable phase before decomposing into the final stable state this process might account for the origin of lingunite found along with maskelynite in shocked chondritic meteorites metastable lingunite appeared at 20 24 gpa and 1100 1300 c in laboratory tests lasting tens of minutes however it might also form at the higher temperatures and shorter time periods of shock events in contrast the hollandite phase was not observed during any stage of crystallization when using albite or labradorite as starting materials the formation process of ca na hollandite in the labradorite composition found in martian shergottites remains unresolved the orthoclase contents of the hollandite phase both in shocked meteorites 2 4 8 2 mol and our oligoclase sample 3 9 mol are relatively high compared to the albite and labradorite samples 0 6 and 1 9 mol respectively this might critically affect the crystallization kinetics of hollandite phase

high pressure transformations in naalsio4 and thermodynamic properties of jadeite nepheline and calcium ferrite type phase abstract phase transitions in naalsio 4 were examined at 3 5 27 gpa and 950 1600 c using a multianvil type and a bridgman anvil type high pressure apparatus the results show that naalsio 4 nepheline first dissociates into an assemblage of naalsi 2 o 6 jadeite and naalo 2 with distorted zincite structure at about 4 gpa at about 7 gpa this assemblage changes to jadeite plus naalo 2 with distorted rocksalt structure jadeite and naalo 2 react to form an naalsio 4 phase with calcium ferrite structure at 18 20 gpa and this boundary has a negative slope enthalpies of nepheline and calcium ferrite type naalsio 4 jadeite and and naalo 2 were measured by high temperature oxide melt solution calorimetry the enthalpy for transition of naalsio 4 nepheline to jadeite plus naalo 2 was 6 2 4 4 kj mol and that for the transition in naalo 2 was 23 6 2 4 kj mol at 298 k the enthalpy of formation of naalsio 4 calcium ferrite from jadeite plus naalo 2 was 79 6 6 9 kj mol at 298 k the calculated boundaries of these transitions from the measured enthalpy data are generally consistent with those by experimental determination considering the errors of the measured enthalpies and thermophysical properties of the relevant phases and the uncertainty of pressure in the high pressure experiments the calculated boundary for dissociation of jadeite to calcium ferrite plus stishovite has a positive slope the present calculation of the transition boundaries involving naalsio 4 calcium ferrite may suggest that configurational entropy of calcium ferrite possibly due to al si disorder could contribute to the slopes of the boundaries

magnetic properties and emplacement of the bishop tuff california anisotropy of magnetic susceptibility ams and characteristic remanence were measured for 45 sites in the 0 76 ma bishop tuff eastern california thirty three sites were sampled in three stratigraphic sections two in owens gorge south of long valley caldera and the third in the adobe lobe north of long valley the remaining 12 sites are widely distributed but of limited stratigraphic extent weakly indurated highly porous to dense welded ash flow tuffs were sampled saturation magnetization vs temperature experiments indicate two principal iron oxide phases low ti magnetites with 525 570 c curie temperatures and maghemite with 610 640 c curie temperatures af demagnetization spectra of isothermal remanent magnetizations are indicative of magnetite maghemite predominantly in the multidomain to pseudo single domain size ranges remeasurement of ams after application of saturating direct fields indicates that randomly oriented single domain grains are also present the degree of anisotropy is only a few percent typical of tuffs the ams ellipsoids are oblate with kmin axes normal to subhorizontal foliation and kmax axes regionally aligned with published source vents for 12 of 16 locality means kmax axes plunge sourceward confirming previous observations regarding flow sense topographic control on flow emplacement is indicated by the distribution of tuff deposits and by flow directions inferred from kmax axes deposition east of the benton range occurred by flow around the south end of the range and through two gaps benton notch and chidago gap flow down mammoth pass of the sierra nevada is also evident at least some of the adobe lobe in the northeast flowed around the west end of glass mountain eastward flow directions in the upper owens gorge and southeast directions in the lower owens gorge are parallel to the present canyon suggesting that the present drainage has been established along the pre bishop paleodrainage characteristic remanence directions from 45 sites 267 samples yield an overall mean of d 348 i 53 for the bishop tuff a correlation is found in two of the three profiles between density and remanence inclination a mean remanence direction based on 13 localities together with data from uncompacted xenoliths and data from the ash fall tuff at lake tecopa is d 353 i 54 k 172 95 2 9 n 15

phase relations for caal2si2o8 anorthite composition in the system caoal2o3sio2 at 14 gpa abstract multi anvil experiments at 14 gpa and 1500 c reveal that caal 2 si 2 o 8 anorthite composition undergoes subsolidus disproportionation into grossular ca 3 al 2 si 3 o 12 stishovite sio 2 and a calcium aluminosilicate phase of composition caal 4 si 2 o 11 this last phase cas was first synthesised by irifune et al 1994 earth planet sci lett 126 351 368 in a continental crust composition at p t conditions appropriate for the transition zone in the system cao al 2 o 3 sio 2 a new three phase assemblage grossular stishovite cas encountered at 14gpa replaces the grossular stishovite kyanite assemblage known to be stable at around 10gpa because of an univariant reaction between grossular and kyanite that yields stishovite and cas by 1600 c another univariant reaction between grossular and stishovite produces cas plus liquid schreinemakers analysis suggests that given ca al rich lithologies the cas phase should be stable at transition zone pressures

emplacement temperature resolution and age determination of cerro colorado tuff ring by paleomagnetic analysis el pinacate volcanic field sonora mexico abstract the lithic in the cerro colorado tephra deposits is dominated by clasts from shallow country rocks for a diatreme not well mixed or by a combination of material that reflects the relative proportions of country rock in a well mixed diatreme the clasts in a tephra ring bed might be ideal to use the paleomagnetic study to determine the emplacement temperature we report emplacement temperature ranges of pyroclastic lithic fragments from the cerro colorado tuff ring in the el pinacate volcanic field sonora northwestern mexico paleomagnetic analyses particularly the thermoremanent magnetization procedure was carried out in 38 small cores drilled from the 10 to 30 cm in size lithic fragments it estimates the emplacement temperature ranges of the juvenile basaltic cauliflower bombs and not juvenile diverse blocks produced in the phreatomagmatic eruption titanomagnetite and titanohematite are the main magnetic carriers along with titanomaghemite the 22 emplacement temperatures determined in this study are grouped in three temperature ranges low 310 370 c medium 400 460 c and high 500 c the low and medium temperature samples show two or more components while the high temperature samples have in general only one component this wide range of emplacement temperature could be related to the composite thermal history of the pyroclastic material incorporated in the water saturated pyroclastic flow deposit the comparison of the mean direction of the secondary magnetization component with the secular variation curve of the global model sha dif 14 suggests an age of 3915 59 yr bp

paleomagnetism and paleothermometry of the sydney basin 1 thermoviscous and chemical overprinting of the milton monzonite the early triassic 245 ma milton monzonite of the sydney basin australia has four distinct components of natural remanent magnetization nrm with only slightly overlapping ranges of unblocking temperatures the low temperature lt component the first to be thermally demagnetized is thought to be a late cretaceous 100 ma thermoviscous overprint acquired in slow cooling during uplift the high temperature ht component the second to be demagnetized is probably the primary thermoremanent magnetization trm of the milton intrusion but could possibly be a jurassic overprint lt and ht are usually carried by magnetite and occasionally by pyrrhotite samples from nine sites have a further nrm component which unblocks at higher temperatures than ht but below the magnetite curie temperature of 580 c this component is argued to be a chemical remanent magnetization crm because of its discrete range of high unblocking temperatures above those of the thermal components ht and lt and is called crm1 crm1 has almost the same direction as lt and is likely carried by authigenic magnetite produced during uplift 100 ma samples from five sites have a fourth nrm component with a direction resembling that of ht but carried by hematite this fourth component could be a primary trm but is more likely a crm and is therefore called crm2 the ht crm2 mean direction is d 50 i 75 5 defining a paleopole at 16 s 172 e the ht crm2 paleopole falls near 150 ma on the australian apparent polar wander path but is a considerable distance from paleopoles of permian and early triassic age there is no known tectonic or other remagnetizing event in the sydney basin around 150 ma for this reason we propose that the ht crm2 paleopole defines a new triassic segment of the australian polar wander path the lt crm1 mean direction is d 348 i 79 with a paleopole falling at 56 s 1580 e near 100 ma on the polar wander path this age is consistent with uplift and cooling related to initial rifting of the tasman sea

magnetic grain size variations through an ash flow sheet influence on magnetic properties and implications for cooling history rock magnetic studies of tuffs are essential to the interpretation of paleomagnetic data derived from such rocks provide a basis for interpretation of aeromagnetic data over volcanic terranes and yield insights into the depositional and cooling histories of ash flow sheets a rhyolitic ash flow sheet the miocene aged tiva canyon member of the paintbrush tuff contains both titanomagnetite phenocrysts present in the magma prior to eruption and cubic fe oxide microcrystals that grew after emplacement systematic variations in the quantity and magnetic grain size of the microcrystals produce large variations in magnetic properties through a section of the ash flow sheet penetrated in a borehole on the nevada test site natural remanent magnetization varies from less than 1 10 4 to more than 8 10 4 a m3 kg 1 and in phase magnetic susceptibility varies from less than 1 10 6 to more than 10 10 6 m3 kg 1 the microcrystals which include both magnetite and maghemite have curie points and maximum unblocking temperatures between 580 c and 640 c rock magnetic data including in phase and quadrature magnetic susceptibilities as well as hysteresis parameters demonstrate that these microcrystals are of superparamagnetic and single domain sizes titanomagnetite phenocrysts are the dominant remanence carriers in the central 50 m of the section whereas microcrystals are important contributors to remanent magnetization and magnetic susceptibility in two 15 m thick zones at the top and bottom within these zones the size of microcrystals decreases both toward the quenched margins and toward the interior of the sheet the decrease in microcrystal size toward the interior of the sheet is interpreted to indicate the presence of a cooling break possibly represented by a concentration of pumice laboratory heating produces changes in magnetic properties which vary complexly from sample to sample such changes include 1 decrease in saturation magnetization 2 both increase and decrease in in phase and quadrature magnetic susceptibilities and 3 shifts in coercivity spectra the complex sample to sample variation in response to heating is attributed to differences in initial grain size distributions

does crater saturation equilibrium occur in the solar system abstract similarity is found in crater densities on the most heavily cratered surfaces throughout the solar system this is hypothesized to result from a steady state saturation equilibrium being approached or achieved by cratering processes this hypothesis conflicts with some recent interpretations however it accounts for 1 a similarity in maximum relative crater density below certain theoretically predicted values on all heavily cratered surfaces 2 a leveling off at this same relative density among 100 m scale secondary craters in populations on lunar maria and other sparsely cratered lunar surfaces 3 the approximate uniformity of maximum relative densities on saturn satellites in spite of dramatic variations predicted from nonsaturation models assuming heliocentric impactors the lunar frontside upland crater population sometimes described as a well preserved production function useful for interpreting other planetary surfaces is found not to be a production function it was modified by intercrater plains formed at least partly by early upland basaltic lava flooding recently confirmed spectrophotometrically consistent with this counts in pure uplands those lacking intercrater plains match the proposed saturation equilibrium density variations among large d 64 km crater populations are found but these may involve several hypothesized mechanisms that rapidly obliterate large craters especially on icy surfaces recent models in which different populations of interplanetary bodies hit different planets need further appraisal

curie temperatures of synthetic titanomagnetites in the fe ti o system effects of composition crystal chemistry and thermomagnetic methods 1 the present study is aimed at improving the calibration of the compositional dependence of the curie temperature tc of titanomagnetite tmt on the basis of temperature dependent magnetic susceptibility c t curves measured on synthetic tmts in the fe ti o system in order to assess the possible influence of high temperature cation vacancies onto the tc values we have synthesized two types of assemblages in subsolidus conditions at 1 bar 1100c and 1300c under controlled oxygen fugacity conditions tmts synthesized in equilibrium with ilmenite hematitess ilmss are expected to have the highest vacancy concentrations those in equilibrium with wustite wus the lowest the composition and homogeneity of the synthetic tmts were carefully checked with a scanning electron microscope sem and an electron microprobe emp tc was determined from c t curves using a kappabridge and for comparison from ms t curves measured with a variable field translation balance our data set shows systematically higher tc values for tmt coexisting with ilmss than for tmt coexisting with wus most c t curves are nonreversible whereby the largest dtc 40 k concern tmt ilmss o f intermediate compositions synthesized at 1300c nonreversibility is interpreted as reflecting cation reordering in tmt during the high temperature c t measurements tc values obtained from ms t curves are higher than those obtained from the c t curves whereby the difference regularly increases up to 40 k with increasing ti content up to xusp 0 6 our new calibration curves are suitable to retrieve tmt compositions in basalts that were rapidly cooled and not oxidized by deuteric or hydrothermal fluids citation lattard d r engelmann a kontny and u sauerzapf 2006 curie temperatures of synthetic titanomagnetites in the fe ti o system effects of composition crystal chemistry and thermomagnetic methods j geophys res 111 b12s28

paleomagnetic estimates of temperatures reached in contact metamorphism a paleomagnetic study of some silurian devonian lavas from the lorne plateau scotland locally reheated by the intrusion of small tertiary dikes has shown that a laboratory equivalent of the actual peak temperature reached in the reheating can be accurately determined theoretically derived corrections are applied to the experimentally determined temperatures to obtain an estimate of the actual peak temperatures reached in a series of samples across the contact zones these corrected experimental profiles can be compared with peak temperature profiles calculated from heat conduction models the two profiles fit well when the temperatures of the lavas immediately prior to the intrusion are taken into account ambient temperature estimates of 75 25 c and 150 25 c have been obtained for the lavas before intrusion by a normally magnetized dike and a reversely magnetized dike respectively

an experimental study of the intensity and stability of thermoremanent magnetization acquired by synthetic monodomain titanomagnetite substituted by aluminium summary titanomagnetites of composition fe2 4 al ti0 6o4 and fe2 6 al ti0 4o4 0 0 1 and 0 2 in both cases were prepared in the monodomain state by pulverization of sintered synthetic material in low fields the thermoremanence trm was found to be linear with inducing field and of high enough intensity to account for typical natural remanent magnetizations of fresh submarine basalts the higher field trm acquisition curves follow the neel model curve for an assemblage of non interacting identical particles in a general way only the differences being due to interactions or the range of particle blocking temperatures and volumes or other features of the samples not included in the model the unblocking temperatures of low field trm lie in a narrow range below the curie point the low field trm is very resistant to alternating field demagnetization and provides a very striking illustration of the strength of the trm mechanism in preserving a stable record of a weak magnetic field the result of a lowrie fuller test on the material is consistent with the monodomain state

iron titanium oxide minerals in block and ash flow deposits implications for lava dome oxidation processes abstract the ikeshiro pyroclastic flow deposit at yufu volcano kyushu in japan consists of typical block and ash flows generated by collapse of the ikeshiro lava dome erupted ca 2000 years ago lava clasts in the ikeshiro pyroclastic flow deposit were previously found to consist to two types of rock sample with different magnetic mineral assemblages established by rock magnetic experiments saito et al 2003 j volcanol geotherm res 126 127 142 we examined iron titanium oxides in samples from the ikeshiro pyroclastic flow deposit with an optical microscope and with an electron microprobe analyzer as a result samples were classified into two types with different iron titanium oxide mineral assemblages type a oxides are characterized by homogeneous titanomagnetite and titanohematite type b oxides are exsolved and composed of two or three phases ti poor titanomagnetite titanohematite pseudobrookite and rutile the reconstituted compositions of type b oxides show the same fe ti ratio as type a oxides this indicates that type b oxides are produced by oxidation of type a oxides type a oxides yield an equilibrium temperature of about 800 850 c at an oxygen fugacity of nno 2 using a two oxide geothermobarometer this indicates that deuteric oxidation in the lava dome separated samples of each type type a rocks originated in oxygen poor parts of the lava dome with temperatures of about 800 850 c while type b rocks originated in oxygen rich parts of the lava dome type a oxides remained unoxidized while type b oxides oxidized and were transformed into complex grains by intense deuteric oxidation the collapse of the lava dome generated the ikeshiro pyroclastic flow all oxides were quenched from about 800 850 c and preserved their compositions by rapid cooling

constraints on cooling rates and permeabilities of pumice in an explosive eruption jet from colour and magnetic mineralogy abstract in explosive volcanic eruptions vesicular magma droplets produced by fragmentation are propelled into the atmosphere where they are chilled to form pumices the thermal history of droplets and the permeability of their internal bubble networks determine how much they are deformed in the eruption jet and hence what information pumices record about the state of the magma at fragmentation we study these aspects of the minoan plinian eruption of santorini volcano by quantifying the rate of oxidation reactions that took place when air entered the hot magma fragments in our experiments white minoan pumices were heated for minutes to hours between 600 and 850 c either in air or in an atmosphere with an oxygen fugacity at the ni nio buffer pumices were unchanged by heating at ni nio those heated in air often became pink to dark pink depending on heating time and their curie temperatures as determined by magnetic susceptibility measurements increased we use oxidation rates deduced from these experiments in conjunction with calculations of the rate of conductive cooling and of the rate at which air can enter a pumice to constrain the conditions experienced by pumices during the eruption natural minoan pumices less than about 5 cm in radius are white whereas larger ones often have white rims and pink interiors with curie temperatures higher than those of white material we infer that small pumices were cooled before being oxidized and that oxidation of the interiors of large clasts mostly took place during flight at temperatures within a few tens of degrees of magmatic values white rims of large pumices despite being permeable were cooled before oxidation could occur permeability developed in the liquid state but did not develop early enough with respect to cooling or was not large enough to allow extreme oxidation we give measurements of pumice permeabilities that should be close to magmatic values

the plinian eruptions of 1912 at novarupta katmai national park alaska the three day eruption at novarupta in 1912 consisted of three discrete episodes episode i began with plinian dispersal of rhyolitic fallout layer a and contemporaneous emplacement of rhyolitic ignimbrites and associated proximal veneers the plinian column was sustained throughout most of the interval of ash flow generation in spite of progressive increases in the proportions of dacitic and andesitic ejecta at the expense of rhyolite accordingly plinian layer b which fell in unbroken continuity with purely rhyolitic layer a is zoned from 99 to 15 rhyolite and accumulated synchronously with emplacement of the correspondingly zoned ash flow sequence in mageik creek and the valley of ten thousand smokes vtts only the andesiterichest flow units that cap the flow sequence lack a widespread fallout equivalent indicating that ignimbrite emplacement barely outlasted the plinian phase on near vent ridges the passing ash flows left proximal ignimbrite veneers that share the compositional zonation of their valley filling equivalents but exhibit evidence for turbulent deposition and recurrent scour episode ii began after a break of a few hours and was dominated by plinian dispersal of dacitic layers c and d punctuated by minor proximal intraplinian flows and surges after another break dacitic layers f and g resulted from a third plinian episode iii intercalated with these proximally are thin intraplinian ignimbrites and several andesite rich fall flow layers both cd and fg were ejected from an inner vent 400 m wide nested within that of episode i into which the rhyolitic lava dome novarupta was still later extruded two finer grained ash layers settled from composite regional dust clouds layer e which accumulated during the d f hiatus includes a contribution from small contemporaneous ash flows and layer h settled after the main eruption was over both are distinct layers in and near the vtts but distally they merge with cd and fg respectively they are largely dacitic but include rhyolitic shards that erupted during episode i and were kept aloft by atmospheric turbulence published models yield column heights of 23 26 km for a 22 25 km for cd and 17 23 km for fg and peak mass eruption rates of 0 7 1x108 0 6 2x108 and 0 2 0 4x108 kg s 1 respectively fallout volumes adjusted to reflect calculated redistribution of rhyolitic glass shards are 8 8 km3 4 8 km3 and 3 4 km3 for episodes i ii and iii microprobe analyses of glass show that as much as 0 4 km3 of rhyolitic glass shards from eruptive episode i fell with cde and 1 1 km3 with fgh most of the rhyolitic ash in the dacitic fallout layers fell far downwind se of the vent near the rhyolite dominated ignimbrite however nearly all of layers e and h are dacitic showing that the downwind rhyolitic ash is of co plinian rather than co ignimbrite origin

the evolution of mercury s crust a global perspective from messenger mapping the distribution and extent of major terrain types on a planet s surface helps to constrain the origin and evolution of its crust together messenger and mariner 10 observations of mercury now provide a near global look at the planet revealing lateral and vertical heterogeneities in the color and thus composition of mercury s crust smooth plains cover approximately 40 of the surface and evidence for the volcanic origin of large expanses of plains suggests that a substantial portion of the crust originated volcanically a low reflectance relatively blue component affects at least 15 of the surface and is concentrated in crater and basin ejecta its spectral characteristics and likely origin at depth are consistent with its apparent excavation from a lower crust or upper mantle enriched in iron and titanium bearing oxides

geology of the caloris basin mercury a view from messenger the caloris basin the youngest known large impact basin on mercury is revealed in messenger images to be modified by volcanism and deformation in a manner distinct from that of lunar impact basins the morphology and spatial distribution of basin materials themselves closely match lunar counterparts evidence for a volcanic origin of the basin s interior plains includes embayed craters on the basin floor and diffuse deposits surrounding rimless depressions interpreted to be of pyroclastic origin unlike lunar maria the volcanic plains in caloris are higher in albedo than surrounding basin materials and lack spectral evidence for ferrous iron bearing silicates tectonic landforms contractional wrinkle ridges and extensional troughs have distributions and age relations different from their counterparts in and around lunar basins indicating a different stress history

ejection of martian meteorites we investigated the transfer of meteorites from mars to earth with a combined mineralogical and numerical approach we used quantitative shock pressure barometry and thermodynamic calculations of post shock temperatures to constrain the pressure temperature conditions for the ejection of martian meteorites the results show that shock pressures allowing the ejection of martian meteorites range from 5 to 55 gpa with corresponding post shock temperature elevations of 10 to about 1000 c with respect to shock pressures and post shock temperatures an ejection of potentially viable organisms in martian surface rocks seems possible a calculation of the cooling time in space for the most highly shocked martian meteorite allan hills alh 77005 was performed and yielded a best fit for a post shock temperature of 1000 c and a meteoroid size of 0 4 to 0 6 m the final burial depths of the sub volcanic to volcanic martian rocks as indicated by textures and mineral compositions of meteorites are in good agreement with the postulated size of the potential source region for martian meteorites during the impact of a small projectile 200 m as defined by numerical modeling artemieva and ivanov 2004 a comparison of shock pressures and ejection and terrestrial ages indicates that on average highly shocked fragments reach earth crossing orbits faster than weakly shocked fragments if climatic changes on mars have a significant influence on the atmospheric pressure they could account for the increase of recorded ejection events of martian meteorites in the last 5 ma

shock induced deformation of shergottites shock pressures and perturbations of magmatic ages on mars shergottites and chassignites practiced major deformation effects whose nature magnitude and relevance were controversially evaluated and disputatively debated our studies of many shocked shergottites present contrary to numerous previous reports ample evidence for pervasive shock induced melting amounting of at least 23 vol of the shergottite consisting of maskelynite and pyrrhotite partial melting of pyroxene titanomagnetite ilmenite and finding of several high pressure polymorphs and pressure induced dissociation reactions our results cast considerable doubt on using the refractive index ri or cathodoluminescence cl spectra of maskelynite in estimating the magnitudes of peak shock pressure in both shergottites and ordinary chondrites ri of maskelynite was set after quenching of the feldspar liquid before decompression to maskelynite glass followed by glass relaxation after decompression at the closure temperature of relaxation the ri procedure widely practiced in the past 38 years revealed unrealistic very high pressure estimates discrepant with the high pressure mineral inventory in shocked shergottites and ordinary chondrites and with results obtained by robust laboratory static experiments shergottites contain the silica high pressure polymorphs the scrutinyite structured polymorph seifertite a monoclinic ultra dense polymorph of silica with zro2 structure stishovite a dense liquidus assemblage consisting of stishovite na hexa aluminosilicate na cas and both k lingunite and ca lingunite applying individual high pressure silica polymorphs alone like stishovite to estimate the equilibrium shock pressure is inadequate due to the considerable shift of their nominal upper pressure bounds intrinsically induced by spatially variable absorptions of minor oxides like al2o3 na2o feo mgo and tio2 this practice revealed variable pressure estimates even within the same shergottite subjected to the same peak shock pressure occurrence of na cas stishovite lack of the naalsio4 ca ferrite structured polymorph or jadeite indicates that the peak shock pressures barely exceeded 22 gpa we present convincing and ample evidence refuting the claim that the shock induced high pressure inventory in shergottites and ordinary chondrites are disequilibrium assemblages resulted from local pressure spikes in excess of 80 gpa and during the decompression stage such scenario calls for a series of incomplete and quenched retrograde reactions starting with the crystallization of mg silicate perovskite magnesiowustite if the claimed peak shock pressure was in excess of 80 gpa this would be followed by replacement of this pair by majorite pyrope ss magnesiowustite or akimotoite magnesiowustite below 23 gpa and 2000 degrees c polycrystalline ringwoodite above 16 gpa respectively and finally replacement by polycrystalline olivine below 16 gpa such incomplete retrograde reactions were never encountered in any shergottite chassignite or shocked ordinary chondrite so far olivine ringwoodite phase transformation in the l6 y 791384 commences with the coherent mechanism producing ringwoodite lamellae with their 111 planes parallel to the 100 of olivine followed by the incoherent mechanism due to build up of strain in the parental olivine this is in accord with the olivine ringwoodite settings produced in static laboratory experiments in a multi anvil device olivine ringwoodite phase transitions were also encountered in comparable settings in the shergottite nwa 1068 application of experimentally obtained kinetic parameters of the olivine ringwoodite phase transitions reveals possible duration of the natural dynamic events up to few seconds thus unambiguously refuting the claimed disequilibrium decompression mechanism the shock induced pervasive melting of labradorite pyrrhotite titanomagnetite ilmenite and partial melting of clinopyroxene strongly suggests shock induced partial to complete resetting of the ar ar rb sr sm nd re os u pb and lu hf radiometric systems this also casts considerable doubt on the radiometric ages shorter than 575 ma reported in the past 38 years to allegedly be the igneous crystallization ages these short ages probably resulted from partial or total shock induced age resetting c 2012 elsevier ltd all rights reserved

characterizing the superparamagnetic grain distribution f v hk by thermal fluctuation tomography 1 in 1965 d j dunlop showed that the joint distribution of particle volumes and microcoercivities f v hk0 can be determined for magnetically monomineralic thermally stable single domain ssd ensembles by taking advantage of the joint temperature and field dependence of relaxation time we have developed a procedure that follows dunlop s strategy to obtain f v hk0 for ensembles containing both superparamagnetic and ssd grains based on backfield remanence curves measured over a range of temperatures each point on the derivative curves represents the integrated contribution from grains that lie along a corresponding blocking contour on the neel plot a suitable set of such line integral samples can be used to reconstruct the f v hk0 distribution using the methods of tomographic imaging samples of the basal tiva canyon tuff have narrow size distributions of elongate ti poor titanomagnetite tomographic inversion of the low temperature backfield spectra yield sharply peaked f v hk0 distributions from which we calculate modal grain dimensions in good agreement with those observed by transmission electron microscopy analysis of synthetic samples containing bimodal populations clearly distinguishes the two modes because our simplified forward calculations incompletely account for the effects of orientation distribution the width of the coercivity distribution at each temperature is underestimated and consequently the inverse calculations yield grain distributions that are overly broad frequency and temperature dependent susceptibilities calculated for the inverted f v hk0 distributions accord fairly well with measured susceptibilities for the weakly interacting tiva canyon samples less well for a moderately interacting paleosol specimen and poorly for a strongly interacting ferrofluid

high pressure phase relations in the system caal4si2o11 naal3si3o11 with implication for na rich cas phase in shocked martian meteorites abstract high pressure phase relations in the system naal 3 si 3 o 11 caal 4 si 2 o 11 were examined at 13 23 gpa and 1600 1900 c using a multianvil apparatus a ca aluminosilicate with caal 4 si 2 o 11 composition designated cas phase is stable above about 13 gpa at 1600 c in the system naal 3 si 3 o 11 caal 4 si 2 o 11 the cas phase dissolving naal 3 si 3 o 11 component coexists with jadeite corundum and stishovite below 22 gpa above which the cas phase coexists with na rich calcium ferrite corundum and stishovite at 1600 c the solubility of naal 3 si 3 o 11 component in the cas solid solution increases with increasing pressure up to about 50 mol at about 22 gpa above which the solubility decreases with pressure the maximum solubility of naal 3 si 3 o 11 component in the cas phase increases with temperature up to around 70 mol at 1900 c at 22 gpa the dissociation of naalsi 2 o 6 jadeite to naalsio 4 calcium ferrite plus stishovite occurs at about 22 gpa lattice parameters of the cas phase with the hexagonal ba ferrite structure change with increase of the naal 3 si 3 o 11 component a axis decreases and c axis slightly increases resulting in decrease of molar volume enthalpies of the cas solid solutions were measured by high temperature drop solution calorimetry techniques the results show that enthalpy of hypothetical naal 3 si 3 o 11 cas phase is much higher than the mixture of naalsi 2 o 6 jadeite corundum and stishovite and is close to that of the mixture of naalsio 4 calcium ferrite corundum and stishovite when we adopt the na ca ratio of 75 25 of the natural na rich cas phase in a shocked martian meteorite zagami the phase relations determined above suggest that the natural cas phase crystallized from melt at pressure around 22 gpa and temperature close to or higher than 2000 2200 c the inferred p t conditions are consistent with those estimated using other high pressure minerals in the shocked meteorite

high pbehavior of anorthite composition and some phase relations of the cao al2o3 sio2 system to the lower mantle of the earth and their geophysical implications 1 multianvil experiments with long experimental durations have been made with the anorthite composition caal2si2o8at pressure temperature p t conditions of 14 25 gpa and 1400 2400 c at subsolidus conditions these experiments demonstrated three phase assemblages grossular gr kyanite ky stishovite st at 14 gpa gr calcium alumino silicate phase cas st at 18 gpa and cas casio3 perovskite capv st at above 20 gpa which are related by the reactions gr ky cas st and gr st cas capv following the method of schreinemakers we combined our data with the literature data to deduce ap tphase diagram for a portion of the cao al2o3 sio2system at subsolidus conditions which subsequently helped to solve some long lasting discrepancies in the high pbehavior of the compositions of anorthite and grossular the crystal chemistry of the cas and capv solid solutions was examined and new substitution mechanisms were firmly established along the solidus the melting reaction at 14 gpa is peritectic while that at 22 gpa is eutectic for both pressures st is the first phase to melt out and the melt is generally andesitic for the an composition its density starts to be significantly higher than the density of pyrolite at 2 5 gpa a much lower pressure than that for the or ab or qtz composition 7 5 10 gpa so that the an enriched continental crust material should readily plunge into the upper mantle

phase relations in the system kalsi3o8 naalsi3o8 at high pressure high temperature conditions and their implication for the petrogenesis of lingunite abstract multi anvil experiments have been made with the composition k0 2na0 8 alsi3o8 at 14 25 gpa and 1400 2400 c these experiments suggest that at subsolidus conditions the stable phase assemblages are hollandite i jadeite stishovite at pressures up to 20 gpa hollandite ii jadeite stishovite at pressures between 20 and 24 gpa and hollandite ii calcium ferrite type naalsio4 stishovite at pressures 24 gpa where hollandite i and hollandite ii are distinguished by their chemical compositions and phase diagram topology the maximum naalsi3o8 in hollandite occurs at 22 gpa and its fraction is positively correlated to temperature attaining a value of 51 mol at 2200 c at supersolidus conditions the solidus and liquidus phases around 14 gpa are hollandite i and jadeite respectively at 22 gpa however the solidus phase is jadeite while the liquidus phase is either stishovite or hollandite ii with a very low fraction of naalsi3o8 these experimental results therefore suggest that lingunite with the hollandite structure and 90 mol naalsi3o8 found in some meteorites is a disequilibrium product it is similarly proposed that the phase with the structure of jadeite but the composition of plagioclase found along with lingunite is also a disequilibrium phase

u pb isotopic systematics of shock loaded and annealed baddeleyite implications for crystallization ages of martian meteorite shergottites abstract shock recovery and annealing experiments on basalt baddeleyite mixtures were undertaken to evaluate shock effects on u pb isotopic systematics of baddeleyite shock pressures up to 57 gpa caused fracturing of constituent phases mosaicism of olivine maskelynitization of plagioclase and melting but the phase transition from monoclinic baddeleyite structure to high pressure temperature polymorphs of zro 2 was not confirmed the u pb isotopic systems of the shock loaded baddeleyite did not show a large scale isotopic disturbance the samples shock recovered from 47 gpa were then employed for annealing experiments at 1000 or 1300 c indicating that the basalt baddeleyite mixture was almost totally melted except olivine and baddeleyite fine grained euhedral zircon crystallized from the melt was observed around the relict baddeleyite in the sample annealed at 1300 c for 1 h the u pb isotopic systems of baddeleyite showed isotopic disturbances many data points for the samples annealed at 1000 c plotted above the concordia both radiogenic lead loss uranium gain and radiogenic lead gain uranium loss were observed in the baddeleyite annealed at 1300 c complete radiogenic lead loss due to shock metamorphism and subsequent annealing was not observed in the shock loaded annealed baddeleyites studied here these results confirm that the u pb isotopic systematics of baddeleyite are durable for shock metamorphism since shergottites still preserve fe mg and or ca zonings in major constituent phases i e pyroxene and olivine the shock effects observed in martian baddeleyites seem to be less intense compared to that under the present experimental conditions an implication is that the u pb systems of baddeleyite in shergottites will provide crystallization ages of martian magmatic rocks

environmental magnetism principles and applications 1 in environmental magnetism rock and mineral magnetic techniques are used to investigate the formation transportation deposition and postdepositional alterations of magnetic minerals under the influences of a wide range of environmental processes all materials respond in some way to an applied magnetic field and iron bearing minerals are sensitive to a range of environmental processes which makes magnetic measurements extremely useful for detecting signals associated with environmental processes environmental magnetism has grown considerably since the mid 1970s and now contributes to research in the geosciences and in branches of physics chemistry and biology and environmental science including research on climate change pollution iron biomineralization and depositional and diagenetic processes in sediments to name a few applications magnetic parameters are used to routinely scan sediments but interpretation is often difficult and requires understanding of the underlying physics and chemistry thorough examination of magnetic properties and of the environmental processes that give rise to the measured magnetic signal is needed to avoid ambiguities complexities and limitations to interpretations in this review we evaluate environmental magnetic parameters based on theory and empirical results we describe how ambiguities can be resolved by use of combined techniques and demonstrate the power of environmental magnetism in enabling quantitative environmental interpretations we also review recent developments that demonstrate the mutual benefit of environmental magnetism from close collaborations with biology chemistry and physics finally we discuss directions in which environmental magnetism is likely to develop in the future

planetary cratering 2 studies of saturation equilibrium a realistic computer model has been developed to display images of imaginary cratered surfaces taking into account empirically measured input size distributions of primary and secondary craters ejecta blanket morphology including feathering with distance obliteration due to ejecta from outside the imaged area lighting effects etc the model allows us to track surface evolution of morphology as new craters are added using the model as well as lunar photos we have studied the approach to saturation equilibrium defined as a condition when no further proportionate increase in crater density occurs as input cratering increases we find that an identifiable saturation equilibrium occurs close to a level previously identified for this state hartmann 1984 typically fluctuating around a crater density from 0 4 to 2 times that level this result is fairly robust vis a vis the range of model parameters we have chosen flooding basin ejecta blankets and other obliterative effects can introduce structure and oscillations within this range even after saturation equilibrium is achieved these findings may constrain or revise certain earlier interpretations of satellite and planet surface evolution and impactor populations which were predicated on the assumed absence of saturation equilibrium in our fourth experimental run we found that suppression of sandblasting by subresolution impacts allows the smallest secondaries to rise above the saturation equilibrium line a result that might be relevant to a similar situation on gaspra and perhaps some other asteroids

anatomy of black veins in zagami clues to the formation of high pressure phases abstract black melt veins in the martian basaltic meteorite zagami were investigated using analytical transmission electron microscopy with the goal to constrain the mechanisms and ptt conditions that result in the formation of high pressure phases therein our study reveals the existence of a number of high pressure minerals stishovite k and na ca rich hollandites akimotoite silicate ilmenite as well as amorphous grains of silicate perovskite composition crystals of a ferro magnesian silicate titanite with a low ca content have been identified for the first time the veins formed by rapid shear melting and solidified in extremely short times as a consequence the vein melt could not homogeneously mix and it partially preserves the chemical heterogeneity of precursor minerals accounting for the large diversity of high pressure minerals most of the tiny phases directly crystallized from the melt under high pressure at very high temperatures silicate perovskite probably amorphized upon decompression and evolved at its grain boundaries by solid state diffusion toward the lower pressure assemblage wustite stishovite not only do these observations help to understand the formation of high pressure phases in shock events but they also provide clues to the mineralogical complexity that can be expected in planetary mantles

experimental determination of coexisting iron titanium oxides in the systems fetialo fetialmgo fetialmno and fetialmgmno at 800 and 900 c 1 4 kbar and relatively high oxygen fugacity a synthetic low melting rhyolite composition containing tio2 and iron oxide with further separate additions of mgo mno and mgo mno was used in hydrothermal experiments to crystallize ilm hem and usp mt solid solutions at 800 and 900 c under redox conditions slightly below nickel nickel oxide nno to approx 3 log 10 f text o 2 units above the nno oxygen buffer these experiments provide calibration of the feti oxide thermometer oxygen barometer at conditions of temperature and oxygen fugacity poorly covered by previous equilibrium experiments isotherms for our data in roozeboom diagrams of projected usp vs ilm show a change in slope at 60 ilm consistent with the second order transition from feti ordered ilm to feti disordered ilm hem this feature of the system accounts for some but not all of the differences from earlier thermodynamic calibrations of the thermobarometer in rhyolite containing 1 0 wt mgo 0 8 wt mno or mgo mno usp mt crystallized with up to 14 of aluminate components and ilm hem crystallized with up to 13 geikielite component and 17 pyrophanite component relative to the fetialo system these components displace the ferrite components in usp mt and the hematite component in ilm hem as a result projected contents of ulvospinel and ilmenite are increased these changes are attributed to increased non ideality along joins from end member hematite and magnetite to their respective mg and mn bearing titanate and aluminate end members the compositional shifts are most pronounced in ilm hem in the range ilm50 80 a solvus region where the chemical potentials of the hematite and ilmenite components are nearly independent of composition the solvus gap widens with addition of mg and even further with mn the bacon hirschmann correlation of mg mn in usp mt and coexisting ilm hem is displaced toward increasing mg mn in ilmenite with passage from ordered ilmenite to disordered hematite orthopyroxene and biotite crystallized in experiments with added mgo and mgo mno their x fe varies with log 10 f text o 2 and t consistent with equilibria among ferrosilite annite and ferrite components and the chemical potentials of sio2 and orthoclase in the liquid experimental equilibration rates increased in the order opx bt ilm hem usp mag

evolution of the rembrandt impact basin on mercury messenger s second mercury flyby revealed a 715 kilometer diameter impact basin the second largest well preserved basin scale impact structure known on the planet the rembrandt basin is comparable in age to the caloris basin is partially flooded by volcanic plains and displays a unique wheel and spoke like pattern of basin radial and basin concentric wrinkle ridges and graben stratigraphic relations indicate a multistaged infilling and deformational history involving successive or overlapping phases of contractional and extensional deformation the youngest deformation of the basin involved the formation of a 1000 kilometer long lobate scarp a product of the global cooling and contraction of mercury

subduction of continental crust and terrigenous and pelagic sediments an experimental study abstract an important role for the recycling of terrigenous sediments in the mantle was proposed by armstrong 1 2 aspects of this hypothesis have been tested via an experimental study of phase equilibria in a composition similar to that of average upper continental crust over the pressure interval 6 24 gpa this composition is sufficiently close to those of the principal argillaceous and siliceous classes of pelagic sediments to use the results to interpret the behaviour of these latter sediments in addition to terrigenous sediments during subduction the subsolidus phase relationships displayed by lithologies derived from these materials are complex with major roles being played by k hollandite garnet na clinopyroxene and stishovite continentally derived lithologies achieve densities similar to or greater than surrounding mantle at depths greater than 200 km the geodynamical implications of these buoyancy relationships are discussed a reconnaissance study has also been made to assess the melting and element partitioning behaviours in the subducted continental crust lithologies during partial melting at relatively low pressures 5 10 gpa orthoclase wadeite and k hollandite are eliminated near the solidus whereas the stability field of na clinopyroxene extends to temperatures well above the solidus liquids formed by small to moderate degrees of partial melting of subducted continentally derived lithologies in this pressure interval have high k na ratios and high sio2 contents at higher pressures 16 24 gpa the stability fields of k hollandite and stishovite extend towards the liquidus whereas sodium bearing phases are eliminated much closer to the solidus resultant partial melts consequently possess relatively low k na ratios and sio2 contents lead possesses a high crystal liquid partition coefficient in k hollandite whereas uranium is excluded the broad stability field of k hollandite during partial melting of these lithologies at pressures above 15 gpa therefore has the capacity to cause fractionation of lead from uranium the petrological and geochemical implications of these results are discussed in the context of the partial melting of terrigenous materials during subduction and the interactions of the partial melts with the surrounding mantle

high pressure phase relations in the composition of albite naalsi3o8 constrained by an ab initio and quasi harmonic debye model and their implications abstract phase relations in the composition of albite naalsi3o8 at pressures up to 40 gpa were constrained by a theoretical method that combines the ab initio calculation and quasi harmonic debye model first the p t dependence of the thermodynamic potentials of the individual phase stishovite sio2 st the calcium ferrite type naalsio4 cf jadeite naalsi2o6 jd or the hypothetical hollandite structured naalsi3o8 na holl was derived our results are generally in consistent agreement with available experimental data and previous theoretical predictions second the gibbs free energy of the hypothetical na holl phase was compared with that of the phase assemblage jd st js or cf 2st cs our results show that the na holl phase is not a thermodynamically stable phase over the studied p t conditions of 0 40 gpa and 100 600 k which rules it out as a possible intermediate phase along the transition path from the js phase assemblage to cs phase assemblage our calculations have predicted that the js phase assemblage transforms into the cs phase assemblage at about 33 6 gpa at 0 k and the clayperon slope of this phase transition is about 0 014 gpa k this study implies that lingunite na holl found in some meteorites is not possibly a thermodynamically stable high p phase and the cf phase probably plays an important role in maintaining the sodium budget and hosting the large ion lithophile elements in the deep interior of the earth

geomagnetic paleointensity in historical pyroclastic density currents testing the effects of emplacement temperature and postemplacement alteration thellier type paleointensity experiments were conducted on welded ash matrix or pumice from the 1912 novarupta nv and 1980 mt st helens msh pyroclastic density currents pdcs with the intention of evaluating their suitability for geomagnetic paleointensity studies pdcs are common worldwide but can have complicated thermal and alteration histories we attempt to address the role that emplacement temperature and postemplacement hydrothermal alteration may play in nonideal paleointensity behavior of pdcs results demonstrate two types of nonideal behavior unstable remanence in multidomain md titanomagnetite and nonideal behavior linked to fumarolic and vapor phase alteration emplacement temperature indirectly influences msh results by controlling the fraction of homogenous md versus oxyexsolved pseudo single domain titanomagnetite nv samples are more directly influenced by vapor phase alteration the majority of nv samples show distinct two slope behavior in the natural remanent magnetization partial thermal remanent magnetization plots we interpret this to arise from a thermo chemical remanent magnetization associated with vapor phase alteration and samples with high water content 0 75 loss on ignition generate paleointensities that deviate most strongly from the true value we find that pdcs can be productively used for paleointensity but that as with all paleointensity studies care should be taken in identifying potential postemplacement alteration below the curie temperature and that large welded flows may be more alteration prone one advantage in using pdcs is that they typically have greater areal spatial exposure than a basalt flow allowing for more extensive sampling and better assessment of errors and uncertainty

low temperature magnetic behavior of titanomagnetites titanium substitution has a significant effect on the temperature dependence 5 300 k of low temperature saturation remanence and of initial susceptibility in synthetic and natural titanomagnetite fe3 xtixo4 0 x 0 4 and electronic and lattice relaxation phenomena over the entire titanomagnetite compositional range these features produce diagnostic magnetic behavior useful for titanomagnetite identification in natural samples moreover the apparent similarities between the thermal decay of low temperature remanence of titanomagnetite with pyrrhotite pure magnetite and superparamagnetic phases can lead to complications in interpreting low temperature remanence data when titanomagnetite is present

rapid surficial oxidation of synthetic fe ti oxides at high temperature observations and consequences for magnetic measurements 1 synthetic polycrystalline samples of fe ti oxides titanomagnetite tmtss ilmenite hematitess ilmss pseudobrookitess psbss are very sensitive to changes in the redox conditions at high temperatures either during synthesis experiments or during thermomagnetic measurements for instance exposure to air for a few seconds at the end of a synthesis run at 1300 c of a tmtss ilmss sample produces surficial oxidation down to a depth of some 100 m this oxidation zone is well visible on backscattered electron images of polished sections through the sample pellet it is characterized by so called trellis oxyexsolution textures i e fine lamellae of ilmss within the tmtss crystals and lamellae of psbss within the ilmss crystals in this oxidation zone the newly grown ilmss lamellae and the surrounding tmtss are more fe rich than the original crystals the presence of trellis textures in the crystals of both coexisting phases tmtss and ilmss show that only short scaled elemental transport within the crystals was involved and that equilibrium was not attained even though the oxidation zone is very narrow the imprint of the new tmtsscompositions is well recognizable in temperature dependent magnetic susceptibility curves in temperature dependent saturation magnetization ms t curves however the contribution of more fe rich tmtss from the oxidation zone can be easily overseen however surficial oxidation of tmtss does occur during ms t measurements with a variable field translation balance apparently in relation with insufficient ar flowing around the sample further examples of rapid surficial oxidation of fe ti oxide samples are also discussed

cooling rates of pyroclastic deposits inferred from mineral magnetic investigations a case study from the pleistocene mýtina maar czech republic tephra layers of the m tina maar czech republic contain ferrimagnetic mg al rich titanomagnetite which is suggested to originate from a fractionated alkaline co 2 rich lithospheric mantle melt we investigated the magnetic mineralogy and curie temperature t c from tephra deposits of two drill cores 9 m depth t c calculated 208 14 c from chemical composition fe 2 0 8 mg 0 5 fe 3 1 1 al 0 3 ti 0 3 o 4 is in accordance with tc retrieved from cooling curves of temperature dependent magnetic susceptibility measurements 195 232 c however thermomagnetic curves are irreversible either with lower type i or higher type ii tc in the heating curve all curves show transition temperatures above ca 390 c indicating maghemitization we interpret the irreversibility of tc t c in terms of different degrees of cation ordering overprinted or masked by different degrees of maghemitization which is a low temperature phenomenon negative t c indicates that original deposited titanomagnetite has cooled faster and therefore has stored a lower degree of cation ordering compared to heating cooling rate of 11 c min in the kappabridge type ii with positive t c indicates higher degree of cation ordering and therefore slower cooling rate the central part of this deposit shows most severe maghemitization indicating rather wet emplacement we therefore suggest different eruption styles for deposition of type i pyroclastics with more phreatomagmatic and type ii pyroclastics with more phreato strombolian eruption styles our study is a new approach to discriminate different cooling histories in maar deposits using the curie temperature of titanomagnetite we suggest that this method has the potential to discriminate different emplacement modes resulting from different eruption styles

curie temperatures of titanomagnetite in ignimbrites effects of emplacement temperatures cooling rates exsolution and cation ordering pumices ashes and tuffs from mt st helens and from novarupta contain two principal forms of titanomagnetite homogeneous grains with curie temperatures in the range 350 500 c and oxyexsolved grains with similar bulk composition containing ilmenite lamellae and having curie temperatures above 500 c thermomagnetic analyses and isothermal annealing experiments in combination with stratigraphic settings and thermal models show that emplacement temperatures and cooling history may have affected the relative proportions of homogeneous and exsolved grains and have clearly had a strong influence on the curie temperature of the homogeneous phase the exsolved grains are most common where emplacement temperatures exceeded 600 c and in laboratory experiments heating to over 600 c in air causes the homogeneous titanomagnetites to oxyexsolve rapidly where emplacement temperatures were lower curie temperatures of the homogeneous grains are systematically related to overburden thickness and cooling timescales and thermomagnetic curves are generally irreversible with lower curie temperatures measured during cooling but little or no change is observed in room temperature susceptibility we interpret this irreversible behavior as reflecting variations in the degree of cation ordering in the titanomagnetites although we cannot conclusively rule out an alternative interpretation involving fine scale subsolvus unmixing short range ordering within the octahedral sites may play a key role in the observed phenomena changes in the curie temperature have important implications for the acquisition stabilization and retention of natural remanence and may in some cases enable quantification of the emplacement temperatures or cooling rates of volcanic units containing homogeneous titanomagnetites

a new natural high pressure na ca hexaluminosilicate caxna1 x al3 xsi3 xo11 in shocked martian meteorites a ca na hexaluminosilicate whose ca end member was previously synthesized in numerous high pressure experiments has been identified by raman spectroscopy in heavily shocked martian meteorites this mineral has a structural formula close to caxna1 x al3 xsi3 xo11 and is similar to the calcium aluminum silicate phase previously synthesized in high pressure experiments performed on anorthite and rocks of basaltic composition this new mineral occurs in shock melt pockets in two distinct settings and is intimately intergrown with sio2 stishovite the first setting encountered in zagami consists of idiomorphic equant crystals overgrown by acicular stishovite that crystallized from a melt of labradorite composition the second setting contains the na ca hexaluminosilicate phase intergrown with stishovite and hollandite and was formed during partial melting at high pressures the mineralogical association na ca hexaluminosilicate stishovite was observed in shock melt pockets which have distinct bulk compositions in seven martian shergottites this new mineral represents after majorite the second natural occurrence of a silicate mineral with silicon in both four and six coordination the assemblage stishovite na ca hexalummosilicate sets constraints on the pressure and temperature conditions that prevailed during shock in some of the studied meteorites the na ca hexaluminosilicate mineral is a potential carrier of al and na during subduction of oceanic crust in the lower mantle of the earth c 2003 elsevier b v all rights reserved

magnetism and magnetic mineralogy of ash flow tuffs from yucca mountain nevada the magnetic susceptibility and remanent magnetization of an ash flow sheet are profoundly influenced by cooling history after emplacement spatial variations in these properties therefore provide a potentially useful tool for geological studies maxima and minima in measured along profiles at yucca mountain nevada identify persistent magnetic marker horizons within vitric portions of the tiva canyon and underlying topopah spring members of the paintbrush tuff the correlatable variations of with vertical position in these units exist over the 8 km of strike examined the observed stratigraphic changes in magnetic properties reflect variations in amounts and mineralogy of fe ti oxide phenocrysts and the presence shape size and mineralogy of magnetic fe oxide microcrystals that precipitated at high temperature after emplacement of each sheet vitrophyre near the top of the topopah spring member has a distinctive maximum in due to titanomagnetite phenocrysts susceptibility in the lower portion of the overlying tiva canyon member reaches maximum values of about 1 3 10 2 si near the base of the member beneath the columnarly jointed vitrophyre due to the presence of abundant superparamagnetic cubic fe oxide these elongate microcrystals are several tens of nm long and are typically deformed and tapered up section in the tiva canyon member the susceptibility decreases going to a minimum within the columnar vitrophyre at this stratigraphic level the precipitated fe oxides are larger and carry intense nrm of up to 30 a m is only 3 10 3 the whisker like microcrystals of fe oxide which typically nucleated on what appears to be chain silicate grew as needles up to 25 m in length the size variations of the precipitated fe oxides which were established using transmission electron microscopy tem and petrographic observation are consistent both with variations in magnetic susceptibility measured at the outcrop and with variations in the intensity of remanent magnetization several interpretations of the shape anisotropy of the precipitated feoxide are possible including growth by a dislocation mechanism additionally the observed elongation of precipitated microcrystals is consistent with theoretical predictions for growth in a uniaxial stress field susceptibility variations as established at the outcrop as well as in the borehole offer a potentially useful tool for stratigraphic correlation of ash flow sheets

the superparamagnetism of yucca mountain tuff yucca mountain tuff contains small titanomagnetite grains with narrow size distributions in the superparamagnetic range schlinger et al 1988 magnetic measurements on three samples comprising hysteresis loops at low and ambient temperatures acquisition and demagnetization of isothermal remanent magnetization thermal demagnetization of the saturation remanence and of a low temperature thermoremanence and frequency and temperature dependent susceptibilities allow evaluation of the magnetic properties in terms of neel s 1949 single domain theory precise grain volume distributions have been obtained by applying the blocking volume concept to thermal demagnetization results in contrast an attempt to derive mean particle volumes by fitting a langevin curve to the room temperature magnetization curves fails probably because the precondition for the langevin function kv kt 1 is not met it is only for the sample with the smallest grains and in weak fields 20 mt that a langevin fit provides a reasonable volume estimate there is good agreement between the experimental results and the calculated frequency and temperature dependence of susceptibility thus verifying that neel s theory is sufficient for the magnetic description of single domain assemblages spanning the superparamagnetic stable single domain boundary however some deviations between modeled and measured susceptibilities exist and the physical causes may include size dependent anisotropy nonuniform magnetizations and also an uncertain preexponential time constant 0 while 0 10 11 s gives the best fit for the sample with the largest grains 0 10 9 s is more reasonable for the others thus 0 may indeed be size and temperature dependent as predicted by brown 1959 the commonly cited parameter fd frequency dependence of susceptibility reaches 30 at room temperature rt for one sample with a blocking temperatures just below rt while fd 0 at rt for a superparamagnetic sample with smaller grains these results thus exemplify that fd is not limited to 15 as a number of studies suggest and that fd 0 must not be taken to imply the absence of superparamagnetic grains

phase relations of harzburgite and morb up to the uppermost lower mantle conditions precise comparison with pyrolite by multisample cell high pressure experiments with implication to dynamics of subducted slabs

phase transition processes of olivine in the shocked martian meteorite tissint clues to origin of ringwoodite bridgmanite and magnesiowustite bearing assemblages abstract tissint is a heavily shocked olivine phyric shergottite we investigated and documented several phase transformations and dissociation phenomena of olivine adjacent to and in many shock induced melt pockets of tissint olivine grains entrained in the melt pockets dissociated to equigranular magnesiowustite bridgmanite latter now vitrified with approaching to the boundaries between the melt pockets and host rocks of tissint some olivine grains entrained in the melt pockets are incompletely dissociated spherulitic magnesiowustite bridgmanite assemblages occur in the incompletely dissociated olivine several residual olivine domains are retained at the center of spherulitic texture with further approaching to the boundaries between the melt pockets and host rocks some olivine grains entrained in the melt pockets are segmented with a cellular like texture presumably induced by defect arrangement many fine grained magnesiowustite occurs around the individual cells of the segmented olivine grains considering the temperature gradient in the melt pockets and the arrangement of dissociation and segmentation textures olivine dissociation reaction was presumably initiated by the segmentation of olivine subsequently followed by magnesiowustite crystallization around the individual cells of the segmented olivine and by bridgmanite formation subsequently spherulitic magnesiowustite bridgmanite assemblages occur around residual olivine domain and finally olivine completely dissociates to equigranular magnesiowustite bridgmanite assemblage olivine grains adjacent to the melt pockets transform to randomly oriented polycrystalline ringwoodite assemblages in rare case ahrensite occurs in the more iron rich portions of the olivine grains with increasing distance from the melt pockets sets of ringwoodite lamellae occur in increasing abundance into the interior of the olivine grains the lamellae consist of almost crystallographically oriented polycrystalline ringwoodite assemblages the phase transitions from olivine to ringwoodite were presumably promoted by interface controlled incoherent mechanism the ringwoodite would be metastable phase which formed in the stability field where olivine dissociates into magnesiowustite bridgmanite

176lu 176hf and 147sm 143nd ages of the martian shergottites evaluation of the shock resetting hypothesis through diffusion kinetic experiments and modeling and petrological observations abstract we address the age controversy of the shergottite suite of martian meteorites 200 ma dates determined by 176lu 176hf 147sm 143nd and various other decay systems as opposed to a 4 ga whole rock pb pb date by calculating the timescales needed to reset hf and nd isotopes in clinopyroxene which is the primary host of rare earth elements amongst the minerals used to compose the 176lu 176hf and 147sm 143nd isochrons at the peak and post shock p t conditions likely to have been experienced by the shergottites it is concluded that impact heating is highly unlikely to have significantly reset the 176lu 176hf and or 147sm 143nd mineral isochrons of these meteorites this conclusion is bolstered by the nature of measured concentration profiles of cr and fe across olivine maskelynite boundaries and ti profiles across clinopyroxene maskelynite melt interfaces in the shergottite rbt 04262 the lack of any evidence of diffusion in the measured concentration profiles coupled with the qualitative incompatibility of the measured crystal melt fractionation trends at these interfaces with the nature of fractionation expected from equilibrium partitioning provides strong evidence that no substantial chemical exchange took place between the solid and melt phases during peak shock p t conditions furthermore we find that the compositional profiles of major elements in coexisting augite and pigeonite are homogenous and reflect a magmatic temperature of 1143 9 c we argue that the pyroxene compositions and consequently the 176lu 176hf and 147sm 143nd geochronometers were highly unlikely to have been reset by the thermal effect of a shock event x ray mapping of a selected section of rbt 04262 shows no evidence of enhanced grain boundary diffusion we thus conclude that the 200 ma dates recorded by the 176lu 176hf and 147sm 143nd geochronometers most likely record the crystallization age of the shergottites

the global population of large craters on mercury and comparison with the moon 1 we have compiled a near global catalog of impact craters on mercury for diameters 20 km from images obtained during the flybys of the mariner 10 and messenger spacecraft the observed variations in crater density suggest that the smooth plains within and around the caloris basin are the most prominent contiguous comparatively young regions on mercury no other comparably large area appears as young in more heavily cratered terrain even the most densely cratered regions on mercury are deficient in craters between 20 km and 100 km in diameter compared with the moon a result that extends an observation made from mariner 10 images of a smaller fraction of mercury s surface this deficit is interpreted to reflect crustal resurfacing of mercury early in its history most likely by volcanic processes for craters larger than 100 km in diameter the density of craters on mercury in a given size range is similar to that of the moon because such larger craters are less easily removed by volcanic resurfacing we interpret this similarity to be the result of impact saturation effects on both bodies

formation of end member naalsi3o8 hollandite type structure lingunite in diamond anvil cell abstract the existence of end member naalsi 3 o 8 hollandite type structure lingunite as a result of high pressure phase transition of a natural albite is investigated between 15 and 30 gpa at 2000 c using laser heated diamond anvil cells x ray analysis of the quenched samples showed that albite transforms to jadeite plus stishovite at 17 gpa and to a hollandite type structure between 19 and 23 gpa with further decomposition to naalsio 4 calcium ferrite type structure plus stishovite at higher pressures the formation of hollandite type structure in the limited range of pressure at high temperatures may elucidate the conditions of meteoritic impacts where this mineral has been formed naturally calcium ferrite type structure however is the only stable dissociation product of albite under the deep lower mantle conditions

depth of the ash flow deposit in the valley of ten thousand smokes katmai national park alaska in anticipation of a drill hole seismic refraction and gravity profiles acquired more than 20 years ago have been reinterpreted in light of new density data to determine the thickness of the ash flow deposit in the valley of the thousand smokes vtts the data show the ash flow is at least 170m thick and most likely welded in its lower portion its average density is 1800 kg m 3 seismic velocities range from 0 6 to 1 km sec for the deposit s unwelded 30 to 50m thick top section and 1 8 to 2 5 km sec for the lower welded section the data confirm the first realistic estimate of the volume of the ash flow of about 11 km by curtis 1968 based on a geomorphic analysis of stream profiles

testing the independence of partial thermoremanent magnetizations of single domain and multidomain grains implications for paleointensity determination 1 the thellier method of paleointensity determination requires that partial thermoremanent magnetizations ptrms be additive mutually independent and reciprocal in their thermal blocking and unblocking we tested the independence law by thermally demagnetizing a sum of orthogonal ptrms produced by replacing part of an original trm tc t0 h by ptrm t t0 h with h rotated 90 between the trm and the ptrm tc curie temperature t0 room temperature the composite remanence simulates thermal remagnetization in nature for single domain grains thermal demagnetization resolved the orthogonal ptrms cleanly two remagnetization temperatures were tried t 400 c and 550 c the latter gave a better test because the two ptrms were almost equal directions were recovered to within 5 for three of the four ptrms trajectories on vector diagrams intersected at exactly 400 c or 550 c and the arai plots were linear and yielded the correct field intensity for both ptrms however multidomain magnetite violated the independence law a remagnetization temperature t 400 c produced almost equal ptrms but in thermal cleaning because of overlap of their unblocking spectra they no longer appeared orthogonal each was displaced 13 15 toward the direction of the other as judged by quasi linear segments on a vector diagram the angle between the fields that produced the ptrms thus appeared to be 60 not 90 arai plots for both ptrms were strongly curved and yielded no usable paleointensity estimate phenomenological modeling correctly predicted the main results of the independence experiments

geochronology of the martian meteorite zagami revealed by u pb ion probe dating of accessory minerals abstract the precise chronology of geological events on mars is hampered by the lack of absolute ages for the martian timescale and the significant uncertainties that result from the extrapolation of the lunar timescale to mars hartmann and neukum 2001 martian meteorites represent the only samples of mars currently available attempts to identify source craters for the meteorites have thus far proven inconclusive hamilton et al 2003 lang et al 2009 mouginis mark and boyce 2012 precluding their use in constraining the absolute martian timescale the majority of the known martian meteorites are basalts shergottites all dated shergottites have mineral separate rb sr or sm nd ages of borg et al 2005 here we report a 238u 206pb age of 182 7 6 9 ma by ion microprobe analysis of baddeleyite zro2 grains in the zagami shergottite there is no correlation between discordancy and baddeleyite grain location relative to shock metamorphism mineral petrography demonstrates that baddeleyite is the result of late stage igneous crystallization and raman spectroscopy shows that baddeleyite has not been transformed by shock into preservable high pressure polymorphs supported by an age of 153 81 ma for phosphate grains obtained using the same method we conclude that zagami crystallized at 180 ma in agreement with previous results from mineral separate geochronology therefore the shergottites represent igneous rocks preferentially ejected from young terrains on mars in a small number of ejection events

paleothermometry of the sydney basin evidence from overprinting of magnetizations of late permian and mesozoic rocks and from the rank of permian coals and mesozoic phytoclasts coal particles suggests that surface rocks in the sydney basin eastern australia have been raised to temperatures of the order of 200 sup 0 c or higher as vitrinite reflectance an index of coal rank or coalification is postulated to vary predictably with temperature and time estimates of the paleotemperatures in the sydney basin based on observed vitrinite reflectance measurements can be made in conjunction with reasonable assumptions about the tectonic and thermal histories of the basin these estimates give maximum paleotemperatures of present day surface rocks in the range 60 249 sup 0 c depending on factors such as location in the basin the thickness of the sediment eroded and the maximum paleogeothermal gradient higher coal rank and consequently larger eroded thicknesses and paleogeothermal gradients occur along the eastern edge of the basin and may be related to seafloor spreading in the tasman sea on the basin s eastern margin a theory of thermal activation of magnetization entailing the dependence of magnetic viscosity on the size distribution of the magnetic grains is used to obtain an independent estimate of the maximum paleotemperaturesmore in the sydney basin this estimate places the maximum paleotemperature in the range 250 300 sup 0 c along the coastal region both coalification and thermal activation of magnetization models provide strong evidence of elevated paleotemperatures which in places exceed 200 sup 0 c and the loss of sediment thicknesses in excess of 1 km due to erosion less

fabrics facies and flow through a large volume ignimbrite pampa de oxaya chile the miocene oxaya formation exposed along the western andean slope in northern chile represents one of the largest ignimbrite provinces on earth in this study magnetic fabric data were acquired from a 1 km long core drilled vertically through a single cooling unit of the oxaya formation the ca 22 ma cardones ignimbrite samples for magnetic analysis were obtained every 20 m from the fine grained matrix of the core detailed measurements of the variation in bulk magnetic properties including natural remanent magnetization nrm susceptibility and anisotropy of magnetic susceptibility ams were used to monitor changes in magnetic mineralogy as well as changes in the strength and orientation of the magnetic fabric throughout the flow ams ellipsoid orientation and shape reflect rapid deposition from a concentrated granular fluidized flow and constrain both the location of the source caldera for this catastrophic eruption and processes of transport and deposition in this large volume ignimbrite after utilizing the magnetic remanence to correct for rotation about the core axis well grouped and imbricated petrofabric orientations reveal a well defined sw 247 transport direction down a proto western andean slope indicating syn or post welding flow and confirming the deformed lauca caldera as the likely source of the eruption systematic variations in fabric shape t and intensity p with depth reveal a predominately oblate fabric toward the top and base of the flow and predominately prolate fabrics in the center these vertical changes in fabric reveal that this massive apparently homogeneous deposit has a systematic layering this layering reflects depth dependent temperature variations temporal changes in the flow boundary zone during deposition and post depositional processes

mercury s geochronology revised by applying model production function to mariner 10 data geological implications 1 model production function mpf chronology uses dynamic models of the main belt asteroids mbas and near earth objects neos to derive the impactor flux to a target body this is converted into the crater size frequency distribution for a specific planetary surface and calibrated using the radiometric ages of different regions of the moon s surface this new approach has been applied to the crater counts on mariner 10 images of the highlands and of several large impact basins on mercury mpf estimates for the plains show younger ages than those of previous chronologies assuming a variable uppermost layering of the hermean crust the age of the caloris interior plains may be as young as 3 59 ga in agreement with messenger results that imply that long term volcanism overcame contractional tectonics the mpf chronology also suggests a variable projectile flux through time coherent with the mbas for ancient periods and then gradually comparable also to the neos

multicomponent cubic oxide exsolution in synthetic basalts temperature dependence and implications for magnetic properties 1 although the compositional unmixing of cubic structured iron oxides has profound effects on the magnetic properties of rocks that contain them a basic understanding of the kinetics and thermodynamics of this process has not been achieved in experimental studies due to sluggish reaction rates in binary oxide phases exploiting the fact that many natural fe oxides contain multiple additional cations including ti mg and al we perform novel forward laboratory experiments in which cubic cubic phase exsolution proceeds from initially homogeneous multicomponent oxides a variety of fe ti mg al cubic iron oxides were nucleated and grown in synthetic multicomponent basalt under different fo2 environments and annealed at temperatures ranging from 590 790 c for up to 88 days fine scale lamellar intergrowths of fe ti al mg oxides interpreted to represent cubic phase exsolution were observed in seven samples one that was synthesized and annealed at approximately constant fo2 the quartz fayalite magnetite or qfm buffer and six that were synthesized at very oxidizing conditions qfm 6 log units and then annealed at moderately oxidizing qfm conditions results demonstrate that the consolute temperature of the multicomponent system is significantly higher than anneal temperatures and curie temperatures suggesting that samples that undergo this type of exsolution can carry a total thermal remanent magnetization exsolved samples are characterized by a dramatic increase in magnetization and coercivity and a shift in curie temperature s confirming predictions that this type of exsolution exerts strong control on the strength and stability of magnetization

phase relations of caal4si2o11 at high pressure and high temperature with implications for subducted continental crust into the deep mantle abstract phase relations of the cas phase caal4si2o11 have been investigated at pressures of 10 23 gpa and at temperatures of 1000 1600 c by means of the quench method using the kawai type high pressure apparatus an assemblage of grossular gr corundum cor kyanite ky is stable up to about 10 gpa and at the temperatures examined the assemblage converts into the cas phase at 1600 c and higher pressures below 1450 c however the assemblage first changes to that of gr cor stishovite st corresponding to the dissociation of ky into cor st and then converts to the cas phase with increasing pressure the gr cor st cas boundary has a negative dp dt slope and the gr ky cor cas boundary has a near horizontal slope the triple point at which gr cor ky st and the cas phase coexist is located at around 11 5 gpa and 1450 c stability of the cas phase up to 23 gpa and 1600 c indicates that the cas phase is an important host of al and ca in the continental crust subducted into deep mantle comparison of the densities between the pyrolitic mantle and the subducted continental crust indicates that the continental crust provides a plunging force to the slab subducted into the upper mantle and transition zone whereas the continental crust in turn gives a buoyancy in the lower mantle as the consequence of the substantial changes in mineral assemblage in both the continental crust and pyrolitic mantle through the 660 km discontinuity the formation of the cas phase plays an important role in changing the density of the subducting continental crust

time temperature relations for the remagnetization of pyrrhotite fe7s8 and their use in estimating paleotemperatures abstract paleotemperature controls the maturation of coal and hydrocarbons in sedimentary basins and is also important in determining paleogeothermal gradient and hence tectonic style in exhumed metamorphic terrains one method of estimating paleotemperature analyses the partial remagnetization of a rock due to heating in thick volcanic or sedimentary sequences over subcrustal heat sources such as plumes or at convergent plate margins the overprinted natural remanent magnetization nrm of a rock records both the age and the paleotemperature of remagnetization but a temperature correction from laboratory to geological time scales is required using theoretical time temperature relations time temperature relations are well known for magnetite fe 3 o 4 but are reported here for the first time for pyrrhotite fe 7 s 8 another common nrm carrier data for each mineral separately yield independent estimates of paleotemperature if geologically reasonable estimates of heating time can be made paleotemperature can be estimated without geological input if data for both minerals are combined together with the age of remagnetization determined from the paleomagnetic pole of the nrm overprint these paleotemperature estimates can be used to infer the history of heating and uplift following burial as a test case we examine thermally acquired nrm overprints carried by pyrrhotite fe 7 s 8 and magnetite fe 3 o 4 in the milton monzonite of southeastern australia these overprints record a heating event about 100 ma ago probably thermal doming prior to rifting of the tasman sea that upgraded coal rank in the sydney basin extrapolating from laboratory to geological times using the new time temperature contours for pyrrhotite we estimate that the presently exposed sydney basin in the vicinity of the milton monzonite was remagnetized by heating to 165 30 c for 100 ka assuming a paleogeothermal gradient of 70 c km appropriate for young or incipient rifts the depth of burial at the time of remagnetization is estimated to have been 2 3 0 4 km this figure is in excellent agreement with independent estimates based on reflectance data for the coal accessory mineral vitrinite

eclogitic minerals in a shocked basaltic meteorite analytical transmission electron microscopy atem was used to characterize mineral parageneses occurring in melt veins of shock origin in the martian meteorite zagami we discovered a new ultra high pressure eclogite facies mineral assemblage consisting of omphacite stishovite and kalsi3o8 hollandite these tiny 200 300 nm high pressure minerals are embedded in silicate glass which forms the groundmass of the shock veins the microtexture of the veins and zoning of the clinopyroxene indicate the formation of minerals by crystallization from a high pressure melt using calibration data from shock and multi anvil experiments we infer that frictional melting occurred in veins at minimum temperatures of 2400 2500 c and at a pressure of ca 30 gpa subsequently the high pressure phases crystallized during decompression in the pressure interval from 25 5 gpa

growth of planetary crusts the planets and satellites of the solar system show much diversity but most have formed crusts which differ substantially from their bulk compositions three principal types of crust may be broadly distinguished primary formed after accretional heating e g lunar highlands secondary formed following partial melting in planetary mantles e g lunar maria terrestrial oceanic crust and tertiary formed by processing of secondary crusts of which the continental crust of the earth is the only identifiable example crustal growth before 3800 m y was complicated by the heavy planetesimal bombardment active erosion on the earth is likely to have destroyed the brecciated rubble resulting from over 200 mare orientale scale events in this period the mercurian crust is probably primary perhaps analogous to the lunar highlands in contrast the observable martian and venusian crusts are secondary dominated by basaltic volcanism the icy crusts of the satellites of the outer planets are mixtures of both primary and secondary crusts with complex histories ganymede is of special interest in displaying a secondary grooved terrain water ice crust which has split an older more heavily cratered possibly primary crust slight expansion has resulted probably from polymorphic transitions from high density ice viii to low density ice i the primary lunar crust 12 of lunar volume grew in 108 years secondary crusts derived by partial melting of mantles grow at a much slower rate total production of terrestrial oceanic crust over 4000 m y is only 2 of planetary volume whereas terrestrial continental crustal growth is even slower producing over the same period a volume only about 0 33 that of the earth crustal growth on the other planets appears to be an irreversible process without evidence of recycling no evidence for massive planetary expansion as required by expanding earth hypotheses is apparent on the other planets or satellites of the solar system

effects of titanomagnetite reordering processes on thermal demagnetization and paleointensity experiments titanomagnetite fe3 xtixo4 0 x 1 is a common naturally occurring magnetic mineral critical to many paleomagnetic studies underlying most interpretations is the assumption that lacking chemical alteration curie temperature tc remains constant however recent work has demonstrated that tc of many natural titanomagnetites varies strongly as a function of thermal history independent of chemical alteration this is inferred to arise from reordering of cations and or vacancies in the crystal structure and changes occur at temperatures and times relevant to standard paleomagnetic thermal treatments because changes take place at t tclose will have their original tb spectrum truncated at t tclose predicted behavior during thellier type paleointensity experiments results in only modest deviations in nrm lost or ptrm gained from the non reordering case much larger deviations are predicted for ptrm checks compared to paleointensity results from titanomagnetite bearing pyroclastic deposits modeled non ideal behavior occurs in the same temperature intervals but is much more systematic reordering is likely one contributing factor to failure of paleointensity experiments this article is protected by copyright all rights reserved

paleointensity estimates from ignimbrites an evaluation of the bishop tuff 1 ash flow tuffs or ignimbrites typically contain fine grained magnetite spanning the superparamagnetic to single domain size range that should be suitable for estimating geomagnetic field intensity however ignimbrites may have a remanence of thermal and chemical origin as a result of the complex magnetic mineralogy and variations in the thermal and alteration history we examined three stratigraphic sections through the 0 76 ma bishop tuff where independent information on postemplacement cooling and alteration is available as a test of the suitability of ignimbrites for paleointensity studies thermomagnetic curves suggest that low ti titanomagnetite tc 560 c 580 c is the dominant phase with a minor contribution from a higher tc phase s significant remanence unblocking above 580 c suggests that maghemite and or titano maghemite is an important contributor to the remanence in most samples we obtained successful paleofield estimates from remanence unblocked between 440 c and 580 c for 46 of 89 specimens 15 sites at two of three total localities these specimens represent a range of degrees of welding and have variable alteration histories and yet provide a consistent paleofield estimate of 43 0 t 3 2 equivalent to a vadm of 7 8 1022 am2 the most densely welded sections of the tuff have emplacement temperatures inferred to be as high as 660 c suggesting that the remanence may be primarily thermal in origin though a contribution from thermochemical remanence cannot be excluded these results suggest that ignimbrites may constitute a viable material for reliable paleointensity determinations

paleomagnetism and paleothermometry of the sydney basin 2 origin of anomalously high unblocking temperatures the milton monzonite of southeastern australia was thermoviscously remagnetized as a result of cretaceous burial and uplift thermal demagnetization separates the low unblocking temperature lt overprint from the high unblocking temperature ht primary remanence with a relatively sharp junction between lt and ht components in vector projections for single domain grains the junction temperature tl between two such vectors corresponds to the maximum blocking temperature tr reactivated in nature apart from a correction for the difference between natural and laboratory timescales however measured tl values are distributed over an implausibly wide range 250 c for burial remagnetization of an untilted intrusion like the milton monzonite furthermore many tl values are anomalously high compared to the predictions of single domain theory multidomain grains are the cause of these anomalies samples pretreated before thermal demagnetization by zero field cycling to liquid nitrogen temperature so as to erase multidomain remanence and isolate single domain remanence do have the theoretically expected tl values in these samples realistic remagnetization time and temperature tr tr conditions in nature are predicted using the t t contours of pullaiah et al 1975 the anomalously high tl values before low temperature treatment are due to multidomain grains which carry 50 of the lt overprint the lt thermal demagnetization curve in samples dominated by multidomain grains is quasi exponential in shape with a high temperature tail extending almost to the curie point as predicted by multidomain theory these high lt unblocking temperatures which are much greater than plausible remagnetization temperatures reached in nature overlap and mask the lower part of the ht unblocking temperature spectrum driving up tl values and leading to inflated estimates of tr although multidomain remanence is a sufficient explanation of anomalously high unblocking temperatures of thermoviscous overprints in the milton monzonite chemical overprinting may be a factor in other lithologies and tectonic settings