direct determination of the timing of sea level change during termination ii an outcrop within the last interglacial terrace on barbados contains corals that grew during the penultimate deglaciation or termination ii we used combined 230th and231pa dating to determine that they grew 135 8 0 8 thousand years ago indicating that sea level was 18 3 meters below present sea level at the time this suggests that sea level had risen to within 20 of its peak last interglacial value by 136 thousand years ago in conflict with milankovitch theory predictions orbital forcing may have played a role in the deglaciation as may have isostatic adjustments due to large ice sheets other corals in the same outcrop grew during oxygen isotope 18o substage 6e indicating that sea level was 38 5 meters below present sea level about 168 0 thousand years ago when compared to the 18o signal in the benthic v19 30 v19 28 record at that time the coral data extend to the previous glacial cycle the conclusion that deep water temperatures were colder during glacial periods

evidence from u th dating against northern hemisphere forcing of the penultimate deglaciation milankovitch proposed that summer insolation at mid latitudes in the northern hemisphere directly causes the ice age climate cycles1 this would imply that times of ice sheet collapse should correspond to peaks in northern hemisphere june insolation but the penultimate deglaciation has proved controversial because june insolation peaks 127 kyr ago whereas several records of past climate suggest that change may have occurred up to 15 kyr earlier2 3 4 5 6 7 8 there is a clear signature of the penultimate deglaciation in marine oxygen isotope records but dating this event which is significantly before the 14c age range has not been possible here we date the penultimate deglaciation in a record from the bahamas using a new u th isochron technique after the necessary corrections for recoil mobility of 234u and 230th and a small age correction for sediment mixing the midpoint age for the penultimate deglaciation is determined to be 135 2 5 kyr ago this age is consistent with some coral based sea level estimates but it is difficult to reconcile with june northern hemisphere insolation as the trigger for the ice age cycles potential alternative driving mechanisms for the ice age cycles that are consistent with such an early date for the penultimate deglaciation are either the variability of the tropical ocean atmosphere system or changes in atmospheric co2 concentration controlled by a process in the southern hemisphere

rates of late quaternary normal faulting in central tibet from u series dating of pedogenic carbonate in displaced fluvial gravel deposits along the main boundary fault of the central tibetan shuang hu graben two well preserved fluvial terrace surfaces are vertically offset by 1 3 m terrace i and 14 8 m terrace ii using thermal ionization mass spectrometry tims we have determined 48 230th 234u 238u ages of small 10 40 mg samples from pedogenic carbonate rinds on clasts in the terrace deposits based on textural microstratigraphic and geochemical criteria we conclude that the u series ages of innermost rind samples provide reliable ages of 16 4 1 9 ka for terrace i and 233 1 9 3 ka for terrace ii this constrains the average rate of vertical displacement along the normal fault to be 0 079 0 011 mm yr during the past 16 kyr and 0 064 0 007 mm yr during the past 233 kyr combining these results with slip rate estimates for other normal faults along the graben margins indicates that the cumulative vertical displacement on all normal faults did not exceed 0 3 mm yr during the late quaternary this new rate from central tibet is distinctly lower than the rate of 1 9 0 6 mm yr inferred for normal faults bounding the yadong gulu graben system in southern tibet consistent with more pronounced and common graben development in southern tibet we therefore suggest that normal faulting in southern tibet is largely controlled by local processes and that the age of its initiation is not a valid proxy for uplift of the tibetan plateau as has been widely assumed our data also place chronologic constraints on the termination of periglacial conditions in central tibet during the last three interglaciations rind formation appears to have been relatively continuous on clasts in terrace i from 16 ka clast rinds from terrace ii in contrast are characterized by three main episodes of rind growth that are widely separated in time the earliest and least well preserved episode started at 233 1 9 3 ka a second episode occurred at 131 8 7 8 ka and a third episode was approximately coeval with rind formation in terrace i after 16 4 1 9 ka in each case the age of onset of carbonate rind accumulation coincides with independently estimated ages of global deglaciation accordingly we conclude that periglacial conditions in the central tibetan plateau terminated synchronously with early stages of global warming following the last three glacial maxima and that fluvial aggradation terrace formation and pedogenic carbonate accumulation in terrace soils ensued rapidly

in situ x ray observations of phase assemblages in peridotite and basalt compositions at lower mantle conditions implications for density of subducted oceanic plate 1 phase relationships in klb 1 peridotite normal type mid oceanic ridge basalt nmorb and k rich basalt compositions were investigated experimentally from 28 to 143 gpa and 2050 to 3000 k a laser heated diamond anvil cell with the synchrotron x ray diffraction was developed to perform in situ observations of samples at simultaneous high pressures and temperatures we confirmed that the peridotite composition crystallized into an assemblage of mg perovskite ca perovskite magnesiowustite at pressures below 97 gpa and that nmorb and k rich basalt compositions both crystallized as mg perovskite ca perovskite stishovite an aluminous phase with a cafe2o4 type structure at pressures below 68 gpa the orthorhombic mg perovskite is stable in natural rock compositions at pressures less than 97 gpa corresponding to about 2200 km depth phase transition of silica phase from stishovite to cacl2 type phase was observed at 78 gpa in nmorb composition moreover we observed that nmorb crystallized in to an assemblage of cairo3 type mg fe sio3 ca perovskite pbo2 type silica an aluminous phase with a calcium cati2o4 type structure at 143 gpa corresponding to the core mantle boundary depth we measured the unit cell volumes of minerals in the nmorb composition at high p t and thereby estimated the density of subducted morb to a depth of 1800 km in the lower mantle in this depth regime morb is denser than model mantle densities e g prem and ak135 the estimated density of morb may contribute to an understanding of dynamics of the subducted slab in the deep lower mantle

miocene jiali faulting and its implications for tibetan tectonic evolution the karakoram jiali fault zone kjfz comprises a series of right lateral shear zones that southerly bound the eastward extrusion of northern tibet relative to india and stable eurasia here we present new 40 ar 39 ar age data from the puqu and parlung faults two easternmost branches of the jiali fault zone which indicate a main phase of the kjfz shearing from v18 to 12 ma thus the tibetan eastward extrusion bounded by principal strike slip fault zones started and was probably most active around the middle miocene an interval marked also by active east west extension in southern tibet the coincidence of these two tectonic events strongly suggests a common causal mechanism which is best explained as oblique convergence between india and asia under the framework of this mechanism the extension in southern tibet is not a proxy for the plateau uplift the kjfz activity was furthermore coincident with right lateral displacements along the gaoligong and sagaing faults in southeast asia this defines a miocene deformation record for the regional dextral accommodation zone that in response to the continuing india asia collision may have accounted for the initiation and prolonged history of clockwise rotation of the tibetan extrusion around the eastern himalayan syntaxis 2002 elsevier science b v all rights reserved

significant late neogene east west extension in northern tibet field mapping in northern tibet reveals that the normal slip along late cenozoic northsouth trending faults is comparable to that estimated for equivalent structures in southern tibet the orientation of fault striations in two north south trending rifts suggests an eastnortheast west northwest direction of extension in northern tibet which in turn implies that northeast striking active faults in northern tibet have significant left slip components initiation of rifting in northern tibet postdates the early oligocene and possibly occurred after 4 ma the broad similarities in the magnitude of slip and the direction of extension for normal faults in both northern and southern tibet imply that the entire plateau has been extending this precludes significant eastward extrusion of north tibet relative to south tibet and requires a regional boundary condition as the cause of east west extension for the entire tibet plateau

thermal convection in a nonhomogeneous aquifer abstract geothermal activity creates destabilizing temperature gradients in groundwater very often stabilizing salinity gradients also exist in the aquifer the combination of temperature and salinity distributions in the aquifer may induce various kinds of hydrodynamic instability such instabilities are possible mainly in thick aquifers which usually consist of various nonhomogeneous formations in this study we investigate the effect of the aquifer nonhomogeneity on the stability of the flow field as well as on heat and salinity transport through the aquifer by galerkin expansion the criteria of the flow field stability were identified and calculated quantitatively it was found that the point of instability is demonstrated by marginal stability or overstability the nonhomogeneity of the aquifer leads to asymmetry of the convection cells

simultaneous determination of melting phase relations of mantle peridotite and mid ocean ridge basalt at the uppermost lower mantle conditions abstract interpretation of melting phase relationships of mantle peridotite and subducted basaltic crust is important for understanding chemical heterogeneity in the earth s interior although numerous studies have conducted melting experiments on peridotite and mid ocean ridge basalt morb and suggested that the solidus temperature of morb is lower than that of peridotite at whole mantle pressure conditions both solidus temperatures overlap within their uncertainties in this study we conducted simultaneous experiments on klb 1 peridotite and normal morb n morb at pressures from 25 gpa to 27 gpa and temperatures from 2398 k to 2673 k to compare the solidus temperatures and their melting phase relations the experimental results show that the solidus temperature of the n morb is nearly identical to the klb 1 peridotite at 25 gpa but lower at 27 gpa in addition we found that the crossover of melt fractions between klb 1 peridotite and n morb occurs at 25 27 gpa these changes are likely to be attributed to the majorite bridgmanite transition of morb this indicates that the dominant melting component may change depending on the location of the uppermost lower mantle our calculation result on the density of partial melts along the mantle geotherm suggests that partial melts of klb 1 peridotite are gravitationally stable around the top of the transition zone whereas partial melts of n morb are gravitationally stable even at the top of lower mantle these results suggest that the distribution of partial melts may be different between klb 1 peridotite and n morb in the deep earth our results may be useful for understanding the fate of partial melts of peridotitic mantle and recycled basaltic crust

mineral chemistry and density of subducted basaltic crust at lower mantle pressures subducted slabs are less dense than the surrounding mantle near the base of the transition zone 660 km depth because of the survival of garnet in former basaltic crust by this depth mantle peridotite has transformed to denser perovskitite1 2 the buoyancy of the former basaltic crust may contribute to the observed accu mulation or horizontal displacement of many slabs at the base of the transition zone3 here we report experimental confirmation of the widely held belief that the basaltic crust of slabs eventually transforms to a dense perovskititic lithology stable in the lower mantle synthetic mid ocean ridge basalt more glass subjected to pressures of 45 80 and 100 gpa in a laser heated diamond anvil cell transforms to an assemblage of aluminous mg fe silicate perovskite non quenchable casio3 perovskite stishovite and a sodic aluminous phase with the ca ferrite structure fig 1 per ovskititic more is about 0 06 g cm 3 more dense than a model lower mantle prem derived from seismological data thus even thermally equilibrated perovskititic slabs should encounter no sig nificant hindrance to subduction and convection in the lower mantle

natural cati2o4 structured fecr2o4 polymorph in the suizhou meteorite and its significance in mantle mineralogy abstract the first natural occurrence of a high pressure polymorph of chromite has been discovered in the shock metamorphosed suizhou meteorite the composition of this high pressure polymorph is identical to that of the precursor chromite the raman spectrum of this polymorph is distinct from that of chromite synchrotron x ray diffraction analysis revealed that this polymorph has an orthorhombic cati2o4 type structure the cell parameters are a 9 462 6 a b 9 562 9 a c 2 916 1 a v 263 8 4 a3 z 4 space group bbmm and the density 5 63 g cm3 the numbers in parentheses are standard deviations on the last significant digits this polymorph is 11 5 denser than chromite the p t conditions for the phase transformation from chromite to the cati2o4 structured polymorph are estimated at 20 23 gpa and 1800 to 2000 c respectively this dense cati2o4 structured fecr2o4 phase could be a host phase for cr al fe mg and mn and other metallic elements in the deep earth

microscale haline convection a proposed mechanism for transport and mixing at the water table region field measurements of chloride profiles in the water table region of a deep aquifer are interpreted in terms of haline convection at the microscale level under natural flow conditions the flow field is characterized by a small peclet number between 6 25 10 2 and 6 25 10 3 so that molecular diffusion dominates over mechanical dispersion it is suggested that haline convection is a major transport and mixing mechanism at the water table region of granular aquifers under natural flow conditions wherever the intervention of man changed the chemical composition of natural replenishment in the studied region it is estimated that the critical density difference which overcomes viscous drag forces is 0 23 0 28 kg m3 this value agrees with the magnitude of the critical free convection parameter modified rayleigh number of earlier models

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

phase transformations in subducted oceanic crust and buoyancy relationships at depths of 600 800 km in the mantle abstract phase transformations in a representative oceanic crust composition morb have been studied at pressures up to 28 gpa at 1200 c and 1500 c using a multi anvil apparatus the assemblage of majorite casio 3 rich perovskite stishovite which is displayed by the morb composition at pressures above 20 gpa was found to transform further into an assemblage containing a new aluminous phase at pressures above 25 gpa this al phase has a complex composition and is also relatively sodium rich its x ray diffraction pattern is similar to that observed for mgal 2 o 4 calcium ferrite type and is tentatively indexed on the basis of an orthorhombic cell yielding a zero pressure density of 3 87 2 g cm 3 majorite garnet was found to remain as the most abundant phase at the highest pressure studied 28 gpa and does not appear likely to transform to perovskite until pressures exceed 29 30 gpa these results imply that the basaltic crust component of subducted oceanic lithosphere would be 0 1 0 2 g cm 3 less dense than surrounding mantle pyrolite between the depths of 660 800 km thereby confirming an earlier hypothesis of anderson moreover the underlying layer of harzburgite would be 0 05 g cm 3 less dense than pyrolite throughout most of this depth interval these buoyancy relationships contribute to a barrier which impedes and may sometimes prevent the entry of slabs into the lower mantle

later pleistocene holocene climate conditions of qinghai xizhang plateau tibet based on carbon and oxygen stable isotopes of zabuye lake sediments abstract we present a time series of carbon and oxygen stable isotope records of the last 30 000 14c years throughout the last glacial postglacial cycle from western qinghai xizhang tibet plateau a 20 m core drilled in the south basin of zabuye salt lake was analyzed for inorganic and organic carbon and total sulfur contents 13c and 18o values of carbonates our results indicate that climatic changes have led to a drastic negative shift of stable isotope ratios at the transition between the last full glacial and the postglacial phase during later pleistocene times 16 2 kyr bp and a rapid positive shift at the transition from pleistocene to holocene 10 6 kyr bp the first shift is marked by the drop of 18ocarb values of about 10 from 2 to 8 and 13ccarb values of about 3 from 5 to 2 the second shift which occurred at the transition from pleistocene to holocene was of similar magnitude but in the opposite direction isotope data combined with total organic and inorganic carbon contents and the lithological composition of the core suggest this lake was an alluvial pre lake environment prior to ca 28 14c kyr bp during ca 28 16 2 14c kyr bp zabuye lake was likely a moderately deep lake with limited outflow the cool and arid glacial climate led the lake level to drop drastically extended residence time overwhelmed the lower temperature and caused a steady increase of 13ccarb and 18ocarb values and total inorganic carbon content in the sediments during ca 16 2 10 6 14c kyr bp this lake probably overflowed and received abundant recharge from melting glaciers when the deglaciation was in its full speed a spike of markedly enhanced 13ccarb and 18ocarb is seen at 11 5 kyr bp probably due to the isotopic effects left behind by the short but severe younger dryas yd event after ca 10 6 14c kyr bp zabuye lake probably closed its surface outflow due to strong desiccation and drastic climate warming the early and middle holocene were characterized by unstable climatic conditions with alternating warmer cooler episodes as indicated by the severe fluctuations of total organic carbon 13c and 18o values a hypersaline salt lake environment was finally formed at zabuye after 5 14c kyr bp when the mirabilite and halite concentrations steadily increased and became the dominant minerals in the sediments severe imbalance of inflow outflow resulted in the drastic increase of total sulfur 13ccarb and 18ocarb values and dominance of halite in the lake since ca 3 8 kyr bp to present

spin transition of ferric iron in the nal phase implications for the seismic heterogeneities of subducted slabs in the lower mantle abstract al rich phases nal new hexagonal aluminous phase and cf calcium ferrite phase are believed to constitute 10 30 wt of subducted mid ocean ridge basalt morb in the earth s lower mantle in order to understand the effects of iron on compressibility and elastic properties of the nal phase we have studied two single crystal samples fe free na1 14mg1 83al4 74si1 23o12 and fe bearing na 0 71 mg 2 05 al 4 62 si 1 16 fe 2 0 09 fe 3 0 17 o 12 using synchrotron nuclear forward scattering nfs and x ray diffraction xrd combined with diamond anvil cells up to 86 gpa at room temperature a pressure induced high spin hs to low spin ls transition of the octahedral fe3 in the fe bearing nal is observed at approximately 30 gpa by nfs compared to the fe free nal the fe bearing nal undergoes a volume reduction of 1 0 1 2 a3 at 33 47 gpa as supported by xrd which is associated with the spin transition of the octahedral fe3 the fits of birch murnaghan equation of state b m eos to p v data yield unit cell volume at zero pressure v 0 183 1 1 a 3 and isothermal bulk modulus k t 0 233 6 gpa with a pressure derivative k t 0 3 7 2 for the fe free nal v 0 hs 184 76 6 a 3 and k t 0 hs 238 1 gpa with k t 0 hs 4 fixed for the fe bearing nal the bulk sound velocities v of the fe free and fe bearing nal phase are approximately 6 larger than those of al fe bearing bridgmanite and calcium silicate perovskite in the lower mantle except for the spin transition region where a notable softening of v with a maximum reduction of 9 4 occurs in the fe bearing nal at 41 gpa considering the high volume proportion of the nal phase in subducted morb the distinct elastic properties of the fe bearing nal phase across the spin transition reported here may provide an alternative plausible explanation for the observed seismic heterogeneities of subducted slabs in the lower mantle at depths below 1200 km

coesite and clinopyroxene exsolution lamellae in chromites in situ ultrahigh pressure evidence from podiform chromitites in the luobusa ophiolite southern tibet abstract we report in situ petrological evidence of deep mantle origin from podiform chromitites in the luobusa ophiolite southern tibet analytical transmission electron microscopy measurements reveal that chromites in podiform chromitites from the luobusa ophiolite have numerous exsolution lamellae of diopsidic clinopyroxene and coesite which indicate an ultrahigh pressure origin of over 3 gpa 100 km deep the presence of these lamellae coupled with abundant micro inclusions of clinopyroxene requires high solubility of sio2 and cao in the host chromite and suggests a precursor of chromite a cafe2o4 structured high pressure polymorph stable at pressures over 12 5 gpa 380 km deep these nano scale observations and geological occurrence indicate that the mantle peridotite under the tibetan mid ocean ridge was transported from the deep mantle at least 100 km probably more than 380 km deep by the mantle convection it implies that the root of mantle upwelling has a much deeper origin than previously believed

modes of free convection in fractured low permeability media 1 abundant data indicate significant fluid and solute fluxes across low permeability shales in the gulf of mexico the precise mechanisms of which are poorly understood in this study we analyze possible modes of intrafracture and interfracture free convection that may occur across fractured low permeability layers such as shales fracture spacing fracture aperture shale thickness and the density gradient across the shale unit are shown to play key roles in the governing fluid and solute transport processes all modes of free convection parallel to the fracture plane perpendicular to the fracture plane and convection between fractures on the larger layer scale are theoretically possible for reasonable hydrogeologic parameters free convection parallel to the fracture plane is shown to be the dominant and most likely mode of free convection requiring only very modest salinity differences for onset to occur least likely is convection perpendicular to the fracture plane the results presented here suggest that free convection may not be uncommon in thick shale sequences such as in the gulf of mexico basin an important consequence of these findings is that analyses that do not consider both interfracture and intrafracture convection modes may significantly underestimate the likelihood of the occurrence of free convection these findings have important implications for the study of free convection and solute transport processes in fractured low permeability media and associated numerical modeling analyses

u series isochron dating a generalized method employing total sample dissolution the age of a sedimentary deposit can be determined by measuring 238u 234u 232th and 230th in two or more samples of the deposit with the requirement that these samples have different u th ratios but an identical 230th232th ratio when deposited this paper presents the theoretical and experimental aspects of the dating method the latter involves techniques for securing samples of varied u th ratios from the same deposit plotting isochrons of the radiochemical data obtained on totally dissolved samples for age analysis and estimating the errors involved although applications to impure carbonates and lacustrine evaporite sediments are given as examples in this paper the method potentially can be generalized for dating a variety of authigenic deposits such as rock varnish peat opal sulfate minerals and marine phosphorites

factors and processes governing the 14c content of carbonate in desert soils epsl elsevier earth and planetary science letters 125 1994 385 405 factors and processes governing the 14c content of carbonate in desert soils ronald amundson a yang wang a oliver chadwick b susan trumbore c leslie mcfadden d eric mcdonald d steven wells e michael deniro f a division of ecosystem sciences 108 hilgard university of california berkeley ca 94720 usa b earth sciences division jet propulsion laboratory pasadena ca 91109 usa c department of geosciences university of california irvine ca 92717 usa d department of earth and planetary sciences university of new mexico albuquerque nm 87131 usa e department of earth sciences university of california riverside ca 92521 usa f department of geological sciences university of california santa barbara ca 93106 usa received 1 september 1993 revision accepted 14 april 1994 abstract a model is presented describing the factors and processes which determine the measured 14c ages of soil calcium carbonate pedogenic carbonate forms in isotopic equilium with soil co 2 carbon dioxide in soils is a mixture of co 2 derived from two biological sources respiration by living plant roots and respiration of microorganisms decomposing soil humus the relative proportion of these two co z sources can greatly affect the initial 14c content of pedogenic carbonate the greater the contribution of humus derived co 2 the greater the initial 14c age of the carbonate mineral for any gwen mixture of co 2 sources the steady state co 2 distribution vs soil depth can be described by a production diffusion model as a soil ages the 14c age of soil humus increases as does the steady state 14c age of soil co 2 and the initial 14c age of any pedogenic carbonate which forms the mean 14c age of a complete pedogenic carbonate coating or nodule will underestimate the true age of the soil carbonate this discrepancy increases the older a soil becomes partial removal of outer and younger carbonate coatings greatly improves the relationship between measured 14c age and true age although the production diffusion model qualitatively explains the 14c age of pedogenic carbonate vs soil depth in many soils other factors such as climate change may contribute to the observed trends particularily in soils older than the holocene 1 introduction r a d i o c a r b o n d a t i n g o f s e d i m e n t a r y d e p o s i t s in d e s e r t s is h i n d e r e d by a lack o f o r g a n i c carbon paradoxically most o f the d e p o s i t s in t h e s e re mk gions a r e b l a n k e t e d with i n o r g a n i c c a r b o n in the f o r m o f p e d o g e n i c c a r b o n a t e since t h e early 1960s 14c ages have b e e n c a l c u l a t e d for t h e s e c a r b o n a t e s 1 fig 1 illustrates t h a t an a p p r o x i m a t e 1 1 r e l a t i o n s h i p exists b e t w e e n 14c ages of soil c a r b o n a t e a n d coexisting o r g a n i c m a t e r i a l s a n d ages d e t e r m i n e d by o t h e r m e a n s see also 2 3 t h e s e r e l a t i o n s h i p s a r e n o t perfect s o m e c a r b o n a t e is y o u n g e r t h a n i n d e p e n d e n t ages sug 0012 821x 94 07 00 1994 elsevier science b v all rights reserved ssdi 0012 821x 94 00097 i

a note on the extent of glaciation throughout the himalaya during the global last glacial maximum abstract quantitative chronologies for the impressive glacial successions that occur throughout the himalaya have until recently been almost totally lacking within the last decade two new techniques have promised to remedy this situation these techniques optically stimulated luminescence and cosmogenic nuclide surface exposure dating enable the age of many glacial features to be determined and have allowed us to study the extent and timing of himalayan glaciation in the late quaternary new data show that the local last glacial maximum lgm occurred during the early part of the last glacial cycle in most areas during marine isotope stage 3 mis 3 mis 3 was a time of increased insolation when the south asian summer monsoon strengthened and penetrated further north into the himalaya the concomitant increased precipitation occurring as snow at high altitudes produced positive glacier mass balances thereby allowing glaciers to advance on the other hand during the global lgm 18 24 ka himalayan glaciation was very restricted in extent generally extending

experimental constraints on the formation of the tibetan podiform chromitites abstract the discovery of diamonds and highly reduced minerals in podiform chromitites which have generally been interpreted as magmatic rocks formed from partial melts of upper mantle peridotites under low pressure conditions has raised many questions about the origin of these enigmatic bodies in order to provide experimental constraints on the formation and emplacement of podiform chromititesin ophiolites we carried out a number of multi anvil experiments in the magnesiochromite sio 2 system at temperatures of 1000 1600 c and pressures of 5 15 gpa the experimental results demonstrate that magnesiochromite is stable up to 14 gpa and decomposed into eskolaite cr 2 o 3 together with a quenchable modified ludwigite structured phase fe mg 2 al cr 2 o 5 at higher pressures thus placing an approximate maximum depth for chromite crystallization and or metamorphism this depth corresponds to the top of the mantle transition zone mtz at 410 km the ludwigite structured post chromite phase has significant implications for understanding phase transformations and cr incorporation partitioning of minerals in the mtz on the basis of our results we propose a multi stage model for the formation of podiform chromitites that incorporates the geochemical textural and mineralogical features of these bodies

electron microscopy of high pressure phases synthesized from natural garnets in a diamond anvil cell implications for the mineralogy of the lower mantle abstract in order to identify the crystalline hosts for aluminium and calcium in the deep mantle the high pressure transformations of natural garnets pyrope grossular and fe rich garnet have been prepared at high pressures 40 and 50 gpa and temperatures between 1000 and 2500 k in a laser heated diamond anvil cell dac the recovered high pressure phases have been studied by analytical transmission electron microscopy atem the host minerals for al and ca appear to depend on temperature and iron content 1 at moderate temperature and without significant amounts of iron in the starting material the main host minerals for ca and al aluminium are casio 3 perovskite and a compound with an unknown structure and a ca mg al 2 sio 6 composition 2 at higher temperature the main host minerals for ca and al are respectively casio 3 perovskite and a new high pressure phase of al 2 sio 5 having the v 3 o 5 structure 3 when the iron content is sufficiently large in the starting material fe fe mg 15 the major host mineral for ca and al is a compound with a hollandite type structure and a ca mg fe al 2 si 2 o 8 composition whatever the temperature the solubility of al in mg perovskite was also studied it appears that this solubility strongly decreases with increasing pressure and slightly increases with temperature such behaviour of al in the deep mantle could lead to the occurrence of a progressive phase change in the lower mantle mineralogical models of the deep mantle are then discussed in the light of the above results

the himalayan arc large scale continental subduction oroclinal bending and back arc spreading abstract palaeomagnetic results from the himalayan arc and southern tibet compared with simulated apparent polar wander paths for the indian plate show a consistent pattern of rotations of the himalayan arc relative to the indian shield varying gradually from 45 clockwise in the northwestern himalaya to slightly counterclockwise in the lhasa region this pattern is consistent with continental underthrusting of greater india beneath the tibetan plateau since the early miocene over at least 650 km at the longitude of western nepal and oroclinal bending since the latest miocene available palaeomagnetic observations support the steady state model for the formation of the himalayan arc with refinements as follows 1 collision between greater india s northern boundary and southern asia occurred at equatorial latitudes with progressive suturing from palaeocene in the northwestern himalaya until early eocene in the eastern himalaya 2 continuing convergence and indentation of greater india into southern asia over about 2000 km up to the early miocene resulted in southeastward extrusion of indochina and 3 neogene counterclockwise rotational underthrusting of greater india along the main central thrust with pliocene quaternary oroclinal bending of the himalayan arc

high pressure transitions in the system mgal2o4 caal2o4 a new hexagonal aluminous phase with implication for the lower mantle abstract phase transitions in mgal 2 o 4 and caal 2 o 4 and in the binary system mgal 2 o 4 caal 2 o 4 were established at high pressures and high temperatures mgal 2 o 4 spinel dissociates to periclase and corundum at 15 16 gpa and 1200 c 1600 c with an equilibrium boundary of p gpa 12 3 0 0023 t c the mixture recombines into a phase with calcium ferrite structure at about 26 5 gpa at 1600 c at 780 c 1400 c caal 2 o 4 crystallizes with the calcium ferrite structure at pressure above 8 9 gpa at lower pressures phases iii and iv are stable on the join mgal 2 o 4 caal 2 o 4 solubility of mgal 2 o 4 component into caal 2 o 4 rich calcium ferrite is limited to about 10 mol at 20 21 gpa and 1200 c and a new phase with composition of mg x ca 1 x al 2 o 4 x 0 66 0 8 was found to be stable at pressure above 15 gpa powder x ray diffraction data show that the new phase possesses hexagonal symmetry moreover it is interpreted that an al rich phase observed in high pressure assemblage of mid ocean ridge basalt morb above 25 gpa by irifune and ringwood irifune t ringwood a e 1993 phase transformations in subducted oceanic crust and buoyancy relationships at depths of 600 800 km in the mantle earth planet sci lett 117 101 110 has the same structure as the hexagonal phase found in this study rather than calcium ferrite structure and another al rich phase synthesized at 24 27 gpa in high pressure experiments from natural pyrope rich garnet starting material is also similar to the hexagonal phase these results suggest that aluminous phase of the hexagonal structure would be one of major constituent minerals of subducted oceanic crust in the upper part of the lower mantle

double diffusive convection in groundwater wells 1 double diffusive convection caused by variations in solute concentration and temperature within a fluid body has previously been studied in connection with oceanic mixing processes astrophysics metallurgy geology and vadose zone hydrology but has not previously been studied in connection with thermohaline transport in a groundwater well this paper considers whether double diffusive convection induced by salinity and or temperature variations is a plausible mechanism for heat and solute transport in a groundwater well this is examined theoretically using rayleigh number stability onset criteria for fluid in a cylinder that considers the cylindrical well geometry and geothermal solute boundary conditions theoretical results suggest that both monotonic and oscillatory double diffusive convection are plausible transport phenomena in groundwater wells these analyses have important implications for hydrogeology because they provide another theoretically plausible mechanism by which heat and solute may mix in a well this previously unaccounted for thermohaline mixing phenomenon may therefore be critical in the interpretation of well hydrogeologic and hydrochemical data

assessment of temperature gradients in multianvil assemblies using spinel layer growth kinetics 1 we present an empirical equation to link the thickness of a mgal2o4 spinel layer growing at the interface between mgo and al2o3 multianvil high pressure assembly pieces to pressure p temperature t and time t extending the recent study of watson et al 2002 to the pressure range 6 16 gpa and temperature range 1673 2273 k for the spinel thickness x we obtain this equation can be used to assess the thermal gradient in any multianvil assembly where mgo and al2o3 filler pieces are in contact at p t conditions within the mgal2o4 spinel stability field as an illustration we show that the central hot spot in which temperatures can be considered constant is close to 1 mm long in a common octahedral multianvil assembly with 8 mm edge length and rhenium furnace while it extends to 3 mm long in an octahedron with 18 mm edge length using a straight graphite heating element in addition we present the results of a spinel growth experiment performed at 2273 k and 15 gpa with 18o enriched mgo which shows that oxygen is mobile at a length scale exceeding that of the spinel layer this finding raises the possibility that under some circumstances growth of spinel and of reaction product layers in other oxide systems might be accomplished by concurrent fluxes of oxygen and cations this mobile oxygen model differs from the more conventional mg al exchange model proposed by watson and price 2002 for the lower pressure experiments and might explain the observed differences in systematics between the high and low pressure data sets

spin transition of ferric iron in the calcium ferrite type aluminous phase we investigated fe free and fe bearing cf phases using nuclear forward scattering and x ray diffraction coupled with diamond anvil cells up to 80 gpa at room temperature octahedral fe3 ions in the fe bearing cf phase undergo a high spin to low spin transition at 25 35 gpa accompanied by a volume reduction of 2 0 and a softening of bulk sound velocity up to 17 6 based on the results of this study and our previous studies both the nal and cf phases which account for 10 30 vol of subducted morb in the lower mantle are predicted to undergo a spin transition of octahedral fe3 at lower mantle pressures spin transitions in these two aluminous phases result in an increase of density of 0 24 and a pronounced softening of bulk sound velocity up to 2 3 for subducted morb at 25 60 gpa and 300 k the anomalous elasticity region expands and moves to 30 75 gpa at 1200 k and the maximum of the v reduction decreases to 1 8 this anomalous elastic behavior of fe bearing aluminous phases across spin transition zones may be relevant in understanding the observed seismic signatures in the lower mantle

clues from a shocked meteorite earth9s crust contains higher abundances of alkali metals than the mantle in the crust feldspars are the key hosts for alkali metals but which minerals are involved in processing alkali metals in the deep earth is not known as akaogi discusses in this perspective a high pressure mineral called hollandite has all the right properties to act as a deep earth alkali host this mineral has been produced in high pressure experiments and has now also been identified in a shocked meteorite by gillet et al page 1633

new insights into southern hemisphere temperature changes from vostok ice cores using deuterium excess correction abstract the combination of both vostok ice core deuterium and deuterium excess histories over the last four climate cycles back to 420 ka bp offers a unique opportunity for reconstructions of vapor source region and local temperature variations tsource and tsite respectively relative to their modern values our study is based on an inversion of ice isotopic composition using a rayleigh based isotopic model which estimates the dependence of vostok precipitation isotopic composition on different climate controls moisture source temperature local deposition temperature and ocean isotopic composition both reconstructed local and source temperatures show no substantial effects from this correction in the sense that none of the rapid or large variations in raw deuterium and deuterium excess profiles can be entirely attributed to a second order climate control our discussion is focused on moisture source temperature and meridional temperature gradient history tsource tsite first using sea surface temperature time series we confirm that moisture source temperature signal recorded in vostok deuterium excess over the last 150 ka fully reflects the obliquity time varying relative contribution of low and high ocean latitudes to vostok precipitation second comparisons of source to site meridional temperature gradient with the vostok sodium record over 420 ka suggest that moisture transport and sea salt transport to the ice sheet are controlled by processes in different latitudes the sea salt being more of a high latitude signal

thermal equation of state of cafe2o4 type mgal2o4 abstract in situ x ray diffraction measurements of cafe 2 o 4 type mgal 2 o 4 have been conducted at pressures up to 42 gpa and temperatures to 2400 k using kawai type multianvil apparatus with sintered diamond anvils additional measurements have also been conducted at pressures to 12 gpa using diamond anvil cell with helium as a pressure medium at room temperature and at temperatures to 836 k at the ambient pressure using a high temperature x ray diffractometer the analysis of room temperature data yielded v 0 240 1 2 a 3 k 0 205 6 gpa and k 0 4 1 3 a fit of the present data to high temperature birch murnaghan equation of state eos yielded k 0 t p 0 030 2 gpa k and 0 a 0 b 0 t with values of a 0 1 96 13 10 5 k 1 and b 0 1 64 24 10 8 k 2 the present data set was also fitted to mie gruneisen debye mgd eos and we obtained 0 1 73 7 q 2 03 37 and 0 1546 104 k density changes of morb have been estimated using the newly obtained thermoelastic parameters assuming that the al rich phase in this composition possesses the cafe 2 o 4 type structure under the lower mantle p t conditions the calculated densities along geotherms for the normal mantle and subducting cold slabs are both significantly higher than those of typical seismological models confirming the conclusion of some recent results on morb by laser heated diamond anvil cell experiments

the compressibility of hexagonal al rich nal phase similarities and differences with calcium ferrite type cf phase with implications for the lower mantle abstract we have determined the equation of state of a hexagonal al rich type nal phase k0 07na0 81ca0 12 1 01 mg1 62fe0 38 2 00 al4 98fe0 10ti0 05si0 88 6 01o12 an aluminum host mineral that is likely to occur in subducted oceanic crusts mid oceanic ridge basaltic morb in the earth s lower mantle angle dispersive x ray powder diffraction experiments were conducted up to 36 gpa at room temperature at the bl04 b2 at spring 8 fitting a third order birch murnaghan equation of state to the data gives a bulk modulus of k 0 214 2 gpa with k 0 3 0 1 a volume v0 184 55 6 a3 and a zero pressure density of 0 4 g cm3 when k0 is fixed at 4 the value of k 0 202 3 0 9 gpa these values are comparable to those for similar anhydrous hexagonal aluminous phases reported previously by ono et al phys chem minerals 29 2002a 527 the density of nal phase is lower than that of other minerals excluding the cf phase co existing in the subducted oceanic crust such as mg perovskite ca perovskite and stishovite we find that the elastic properties of the hexagonal al rich phase do not change despite substantial variations in si al fe mg na and k content in the nal chemical composition

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

equations of state of na k al host phases and implications for morb density in the lower mantle abstract we report equations of state eos of two important silicate phases known to store na k and al at pressures and temperatures prevailing in the earth s lower mantle using laser heated diamond anvil cell we produced the ca ferrite type cf and the new aluminum rich nal phases using different synthetic and natural starting materials at pressures ranging from 23 to 65 gpa unit cell volumes were investigated by means of angle dispersive x ray diffraction at the id30 beamline of the esrf synchrotron ring grenoble france cf phases were synthesized in naalsio4 mgal2o4 and naalsio4 mg fe 2sio4 systems and for a natural mid ocean ridge basalt morb the insertion of mgal2o4 in the naalsio4 based lattice has reduced effects on the cf phase eos whereas that of mg fe 2sio4 is mainly to increase the room pressure volume cf phases most relevant to the earth s mantle show compression behavior comparable to the pure naalsio4 cf phase with bulk moduli values close to k0 190 gpa and k0 4 we also synthesized nal phases in naalsio4 mgal2o4 system and using cana and k rich silicate glasses as starting materials all nal compounds investigated show similar compression behavior except for one anomalously naalsio4 rich composition for nal phase with the most natural conditions relevant composition we refine bulk moduli of k0 182 gpa and k0 6 we then model the density profiles expected for the cf and nal phases using our newly determined eos and the most accurate chemical compositions reported in the literature these two phases appear significantly less dense than a pyrolitic type material at all lower mantle pressures and 300 k however the density profiles calculated for mineral assemblages corresponding to typical morb compositions point out to denser morbs compared to the pyrolite this is due to the high fe content in morb yielding a densest fe rich perovskite that counterbalances the lower densities of the cf and nal phases the density gap is calculated between 0 4 and 4 0 depending on the starting morb composition and pressure

phase transformations of subducted basaltic crust in the upmost lower mantle phase relations in natural mid oceanic ridge basalt morb were investigated to depths of 700 1400 km in the lower mantle using a laser heated diamond anvil cell and synchrotron x ray diffraction xrd the crystalline phases their unit cell volumes and proportions were determined from rietveld refinement of the in situ xrd spectra at the top of the lower mantle mg rich perovskite is the most abundant phase followed by ca rich perovskite and stishovite two al rich phases occur in smaller amounts the ca ferrite type and the new aluminum rich phase or nal this assemblage is stable down to 1150 km approximately at greater depths the new aluminum rich phase disappears and the assemblage of mg rich perovskite ca rich perovskite stishovite and ca ferrite phase is stable up to at least 1400 km in view of the present phase diagram we discuss the nature and relations of the lower mantle al phases in morb composition and their bearing on the density evolution of the oceanic crust

high pressure phase transitions in mgcr2o4 mg2sio4 composition reactions between olivine and chromite with implications for ultrahigh pressure chromitites

implications of shortening in the himalayan fold thrust belt for uplift of the tibetan plateau 1 recent research in the himalayan fold thrust belt provides two new sets of observations that are crucial to understanding the evolution of the himalayan tibetan orogenic system first u pb zircon ages and sm nd isotopic studies demonstrate that 1 greater himalayan medium to high grade metasedimentary rocks are much younger than true indian cratonic basement and 2 these rocks were tectonically mobilized and consolidated with the northern margin of gondwana during early paleozoic orogenic activity these observations require that greater himalayan rocks be treated as supracrustal material in restorations of the himalayan fold thrust belt rather than as indian cratonic basement in turn this implies the existence of greater himalayan lower crust that is not exposed anywhere in the fold thrust belt second a regional compilation of shortening estimates along the himalayan arc from pakistan to sikkim reveals that 1 total minimum shortening in the fold thrust belt is up to 670 km 2 total shortening is greatest in western nepal and northern india near the apex of the himalayan salient and 3 the amount of himalayan shortening is equal to the present width of the tibetan plateau measured in an arc normal direction north of the indus yalu suture zone this information suggests that a slab of greater indian lower crust composed of both indian cratonic lower crust and greater himalayan lower crust with a north south length of 700 km may have been inserted beneath the tibetan crust during the cenozoic orogeny we present a modified version of the crustal underthrusting model for himalayan tibetan orogenesis that integrates surface geological data recent results of mantle tomographic studies and broadband seismic studies of the crust and upper mantle beneath the tibetan plateau previous studies have shown that incremental mesozoic and early cenozoic shortening had probably thickened tibetan crust to 45 55 km before the onset of the main cenozoic orogenic event thus the insertion of a slab of greater indian lower crust with maximum thickness of 20 km tapering northward could explain the cenozoic uplift of the tibetan plateau the need for tibetan crust to stretch laterally as the greater indian lower crust was inserted may explain the widespread east west extension in the southern half of the plateau our reconstruction of the himalayan fold thrust belt suggests that indian cratonic lower crust of presumed mafic composition and high strength should extend approximately halfway across the tibetan plateau to the banggong suture from there northward we predict that the tibetan plateau is underlain by more felsic and therefore weaker lower crust of greater himalayan affinity two slab break off events are predicted by the model the first involved neotethyan oceanic lithosphere that foundered 45 35 ma and the second consisted of greater indian lithosphere most likely composed of greater himalayan material that delaminated and foundered 20 10 ma asthenospheric upwelling associated with the break off events may explain patterns of cenozoic volcanism on the tibetan plateau although the model predicts a northward migrating topographic front due solely to insertion of greater indian lower crust the actual uplift history of the plateau was complicated by early middle tertiary shortening of tibetan crust

holocene left slip rate determined by cosmogenic surface dating on the xidatan segment of the kunlun fault qinghai china cosmogenic dating using in situ sup 26 al and sup 10 be in quartz pebbles from alluvial terrace surfaces constrains the late holocene slip rate on the xidatan segment of the kunlun fault in northeastern tibet two terrace risers offset by 24 3 and 33 4 m having respective ages of 1799 388 and 2914 471 yr imply a slip rate of 12 1 2 6 mm yr the full range of ages obtained le 22 8 k y most of them between 6 7 and 1 4 k y confirm that terrace deposition and incision hence landform evolution are modulated by post glacial climate change coupled with minimum offsets of 9 12 m this slip rate implies that great earthquakes m approximately 8 with a recurrence time of 800 1000 yr rupture the kunlun fault near 94 e

phase stability and elastic properties of the nal and cf phases in the namg2al5sio12 system from first principles new hexagonal aluminous nal phase and orthorhombic calcium ferrite cf type phase are considered to be major mantle components of the mid ocean ridge basalt morb at pressure and temperature conditions in the lower mantle which can potentially host alkali elements with large ionic radii the high pressure stability and elastic properties of both nal and cf phases are therefore of fundamental importance for understanding the fate of subducted morb here we report those properties of the namg2al5sio12 system studied by means of the first principles computation method nal was found to transform to the cf phase at 39 6 gpa accompanied by discontinuities in density 1 8 as well as compressional wave 0 2 shear wave 0 9 and bulk sound 1 0 velocities the property of subducted morb was evaluated using these results and the velocity contrast between pyrolite and morb of 5 was found to be quite comparable to the shear velocity anomaly observed for seismic scatterers at depths from 1100 to 1800 km however since the transformation of the nal to cf phase within morb produces insignificant increases in the seismic velocities it would be seismologically undetectable on the other hand the anisotropy change associated with the phase transition is significant and could be seismically detectable using observations such as shear wave splitting measurements since the cf phase is considerably more anisotropic compared to the nal phase

phase relations and melt compositions in cmas pyrolite h2o system up to 25 gpa abstract phase relations and melt compositions in the cao mgo al 2 o 3 sio 2 pyrolite under hydrous 2 of h 2 o and anhydrous conditions have been determined at 10 25 gpa and temperatures from 1400 to 2400 c at the pressure of 16 and 25 gpa the apparent solidus temperatures for the hydrous system are about 200 and 100 c lower than the solidus temperatures for the dry system respectively the apparent solidus temperature changes drastically from 1600 c at 13 5 gpa to about 1850 c at 16 gpa majorite is a liquidus phase in the hydrous pyrolite from 10 to 25 gpa the liquidus phases in the dry pyrolite are olivine at 10 14 gpa majorite at 14 20 gpa and periclase at pressures above 20 gpa we observed expansion of the stability field of anhydrous phase b and its complex relations with the other phases in the cmas pyrolite system we suppose it is formed not only by incongruent melting of wadsleyite but also by the subsolidus reaction of wadsleyite and majorite in the dry condition compositions of partial melts formed by low degree of melting 10 20 have al 2 o 3 depleted and cao rich compositions up to 22 gpa and enriched in al 2 o 3 5 7 wt and cao at 25 gpa at 10 gpa dry and hydrous melts formed by low degree of melting have high sio 2 48 51 wt and relatively high al 2 o 3 3 5 5 2 wt their compositions are generally consistent with those of aluminum depleted komatiite at 13 22 gpa dry and hydrous melts have low sio 2 44 49 wt and al 2 o 3 1 7 3 0 wt and high cao contents 9 15 wt an essential depression of the apparent solidus temperature in the hydrous deep upper mantle compared to that in the transition zone provides the dehydration melting of the hydrous plume the present results support a model of komatiite genesis by dehydration melting of rising wet plumes at the base of the upper mantle

normal faulting in central tibet since at least 13 5 myr ago tectonic models for the evolution of the tibetan plateau interpret observed east west thinning of the upper crust to be the result of either increased potential energy of elevated crust1 or geodynamic processes that may be unrelated to plateau formation2 3 4 5 6 a key piece of information needed to evaluate these models is the timing of deformation within the plateau the onset of normal faulting has been estimated to have commenced in southern tibet between about 14 myr ago7 and about 8 myr ago8 and in central tibet about 4 myr ago9 here however we report a minimum age of approximately 13 5 myr for the onset of graben formation in central tibet based on mineralization ages determined with rb sr and 40ar 39ar data that post date a major graben bounding normal fault these data along with evidence for prolonged activity of normal faulting in this and other tibetan grabens support models that relate normal faulting to processes occurring beneath the plateau thinning of the upper crust is most plausibly the result of potential energy increases resulting from spatially and temporally heterogeneous changes in thermal structure and density distribution within the crust and upper mantle beneath tibet this is supported by recent geophysical and geological data10 11 12 13 14 15 16 17 which indicate that spatial heterogeneity exists in both the tibetan crust and lithospheric mantle

elasticity of subducted basaltic crust at the lower mantle pressures insights on the nature of deep mantle heterogeneity abstract the origin of the large low velocity structures observed in the lowermost mantle is still unclear to make new insights into deep mantle heterogeneity we performed ab initio modeling of the elasticity of representative petrology with the pyrolitic and basaltic compositions based on the elasticities of the following mineral phases magnesium silicate perovskite post perovskite ferropericlase calcium silicate perovskite stishovite cacl 2 and calcium ferrite titanate type mgal 2 o 4 calculations elucidated that the basalt composite is 2 denser than the pyrolitic one through all of the pressures in the lower mantle in addition the shear v s and bulk sound v velocities were found to be 2 slower and faster in the basalt composition in particular at 30 gpa and 120 gpa respectively while both the pyrolitic and basaltic composites have almost the same compressional wave speed v p thermochemically distinct piles which are thought to be related to accumulated hot and dense subducted crusts would therefore be expected to produce as much as a 5 to 6 low velocity anomaly in the s wave but only 1 in the p wave these suggest that the basaltic pile seems less compatible to the seismological signatures of the large low velocity structures if accompanied by thermal anomalies reaching 1000 to 1500 k

analytical transmission electron microscopy study of a natural morb sample assemblage transformed at high pressure and high temperature natural mid ocean ridge basalt morb samples recovered from diamond anvil cell dac experiments performed between 33 to 89 gpa and 1700 to 2600 k were studied with a transmission electron microscope tem we used the focused ion beam fib lift out technique to prepare the recovered high pressure laser heated samples for tem study observations of tem sections show the presence of five phases for samples transformed at pressures ranging from 33 to 45 gpa al bearing mg perovskite ca silicate perovskite stishovite and two al rich phases the al rich phases were identified by selected area electron diffraction saed patterns and chemical composition analysis and include the new aluminous nal phase with hexagonal structure and the calcium ferrite cf type phase chemical analyses obtained by analytical transmission electron microscopy atem show that mg silicate perovskite is the major host for al with significant amounts also distributed between the cf type and nal phases and less than 1 wt in stishovite beyond pressures of 40 gpa 1100 km depth the al content of mg perovskite and cf type phase increases between 45 and 50 gpa the nal phase disappears this mineralogical change may explain reported seismic anomalies in subduction zones at mid mantle depths

parameterized thermal convection in a layered region and the thermal history of the earth parameterized convective schemes for constant viscosity fluids are used to investigate the influence of layering on the relationship between the rate of heat loss from and heat supply to a number of convecting layers of different viscosity and thicknesses simple analytic expressions are obtained which accurately predict the behavior to a step decrease in the heat supply and show that layering considerably increases the response time the same arguments are applied to a planet consisting of spherical shells they show that the response of the earth s heat loss to the decay of radioactive elements depends strongly on whether or not mantle convection extends from the core to the surface if it does so the surface heat flux is only about 50 greater than the heat generation rate however if upper and lower mantle convection occurs in two separate shells as seismological and geochemical arguments suggest the response time is increased the present surface heat flux is then principally controlled by the thermal structure of the earth at the time it was formed the best estimate of the present ratio of the heat loss to heat generation is about two and this value is consistent with the behavior of an earth model consisting of a two layered mantle overlying a core

high pressure phase transitions of natural chromitite from tibetan ophiolites abstract the podiform chromitites in the tibetan ophiolites have been thought to have experienced metamorphism corresponding to depths at the top of the mantle transition zone mtz however little is known about the phase relations of natural podiform chromitite in the mtz as well as the implications for processes related to formation of the ultra high pressure uhp chromitite an experimental study is presented to constrain the phase relations of natural chromitite at pressures and temperatures of 11 18 gpa and 1400 1600 c the experimental results show that the chromitite transforms into phase assemblages composed of chromite eskolaite garnet for pressures at the base of the upper mantle and chromite eskolaite garnet mc phase at the top of the mtz chromitite is replaced by a cati2o4 structured phase ct phase eskolaite garnet in the middle of the mtz the mc phase has an average composition of mg fe 3 cr al 2 2x 4 3y mg fe xsix y o6 x 0 2 y 0 3 with a cubic space group of fd3m it is suggested that the mc phase is a new silicon bearingmg and cr rich phase with idealized stoichiometry of mg3cr2o6 the observation of the mc phase in a natural rock system is helpful for understanding the incorporation of mg and cr minerals in the mtz and helps to constrain the related geological processes that controlled the formation of the tibetan chromitites

u th isotope systematics from the soreq cave israel and climatic correlations precise 230 th 234 u ages were obtained on thirty one growth laminae in speleothem samples which are self consistent with the detailed layer stratigraphy samples with low 232 th 238 u ratios give ages with analytical uncertainties of 40 years at 2 ky and 1000 years at 90 ky some growth zones with high but variable 232 th 238 u were dated by intermal isochrons this permits the determination of the initial 230 th 232 th assuming equilibrium of 232 th and 238 u series in the source of the high 232 th component this shows initial 230 th 232 th in activity units of from 1 3 to 2 9 the calculated atomic ratios of 232 th 238 u for the high 232 th component range from 1 08 to 2 4 which is well below the average crustal value speleothem materials with high 232 th 238 u are found to exhibit clear correlations of 232 th with si al and fe while 238 u correlates with sr and ba analyses of soreq cave drip waters show that the particulates in the waters have high 232 th concentrations and a 232 th 238 u ratio much lower than that found in the high 232 th component in speleothems but with 230 th 232 th 1 0 to 2 4 we infer that the trapped high th component in speleothems is from particulate matter in water with a large concentration of adsorbed u and not simply from detrital material the speleothems have only small234u excess the initial 234 u 238 u 0 show a range of 1 02 to 1 14 that was found to correlate with age over the past 25 ky the youngest samples have values in the same range as the modern drip waters there appears to be a correlation of 234 u 238 u 0 with the 18 o values there is a drop of 18 o in the time interval 20 to 15 ky which then remains relatively constant to recent times as the high 18 o values have been related to rainfall and associated climatic conditions we suggest that the 234 u 238 u in the speleothem reflects the effects of rainfall and soil weathering conditions on drip water composition and may provide a proxy for climate change