rock composition dolomite stoichiometry and rock water reactions in dolomitic carbonate rocks dolomitic carbonate rock units from a variety of phanerozoic localities and depositional environments throughout north america were evaluated with respect to their dolomite content and dolomite composition by x ray diffraction analysis dolomite compositions range from 48 to 57 mole caco 3 with well defined modes at 51 nearly stoichiometric dolomite and 55 distinctly calcian dolomite caco 3 values of percentage of dolomite also exhibit a pronounced albeit less well defined bimodal distribution with modes at 97 dolostones and 20 dolomitic limestones dolomite similar distributions characterize most other published data on phanerozoic dolomitic carbonates these data suggest that two separate processes may lead to two distinct populations of sedimentary dolomite 1 dolomitic limestones originate in diagenetically closed systems during magnesian calcite dissolution calcite and dolomite precipitation reactions and 2 dolostones originate in diagenetically open systems during th

cement stratigraphy of pennsylvanian holder formation sacramento mountains new mexico cyclic strata of the holder formation virgilian new mexico were deposited across a pennsylvanian shelf to basin transition during a time when sea level fluctuated over tens of meters cement stratigraphy studies indicate abundant calcite precipitation from low temperature fresh water within shelf shelf crest and shelf edge marine limestones freshwater cementation occurred during 15 periods of intraformational subaerial exposure these early cements are least abundant in basin and lagoon limestones the distribution of the early cement zones suggests that cementation was controlled by paleotopography stratigraphic position below subaerial exposure surfaces lithology and configuration of the paleoaquifer system distribution of early calcite cements provides new data for interpretation of cycles diagenetic systems and porosity evolution in petroleum reservoirs trace element analyses support a low temperature freshwater origin for the early cements cement stratigraphy studies fluid inclusion analyses and trace element analyses indicate a later cement that occluded remaining limestone porosity precipitated from a sodium and calcium rich brine at a temperature of about 100 c stratigraphic reconstruction dates this cementation as cretaceous or later fluorescent oil filled fluid inclusions were trapped along fractures in the late cement indicating oil migration during or after late stage cementation

a facies model for a quaternary andesitic composite volcano ruapehu new zealand ruapehu composite volcano is a dynamic volcanic sedimentary system characterised by high accumulation rates and by rapid lateral and vertical change in facies four major cone building episodes have occurred over 250 ka from a variety of summit flank and satellite vents eruptive styles include subplinian strombolian phreatomagmatic vulcanian and dome related explosive eruptions and extrusion of lava flows and domes the volcano can be divided into two parts a composite cone of volume 110 km3 surrounded by an equally voluminous ring plain complementary portions of ruapehu s history are preserved in cone forming and ring plain environments cone forming sequences are dominated by sheet and autobrecciated lava flows which seldom reach the ring plain the ring plain is built predominantly from the products of explosive volcanism both the distal primary pyroclastic deposits and the reworked material eroded from the cone much of the material entering the ring plain is transported by lahars either generated directly by eruptions or triggered by the high intensity rain storms which characterise the region ring plain detritus is reworked rapidly by concentrated and hyperconcentrated streams in pulses of rapid aggradation immediately following eruptions and more gradually in the longer intervals between eruptions

phase transition in mgsio 3 perovskite in the earth s lower mantle a new polymorph of mgsio3 more stable than the pbnm perovskite phase has been identified by first principles computations it has the cairo3 structure with cmcm symmetry and consists of sio3 layers intercalated by eightfoldcoordinated mg ions high temperature calculations within the quasiharmonic approximation give a volume change of f1 5 and a clapeyron slope of f7 5f0 3 mpa k at f2750 k and f125 gpa these pressure temperature p t conditions are close to those in which a phase transition in mgsio3 perovskite has been observed by in situ angle dispersive x ray diffraction measurements this transition appears to be associated with the du discontinuity d 2004 elsevier b v all rights reserved

the effect of temperature on the seismic anisotropy of the perovskite and post perovskite polymorphs of mgsio3 using elastic constants determined by ab initio molecular dynamics over a range of temperatures we show that the seismic anisotropy of mgsio3 perovskite is significantly temperature dependent at 90 gpa the direction of greatest shear wave splitting changes from 001 at 0 k to 010 at 3500 k in addition there is no shear wave splitting in the 100 direction at 0 k but a significant amount of splitting is exhibited at 3500 k we have also calculated the first set of high temperature elastic constants of the new post perovskite phase by ab initio molecular dynamics methods we find that in contrast to mgsio3 perovskite temperature has little effect on the acoustic anisotropy of this phase we determine ln vs ln vp for the post perovskite phase and find it to be very low about 0 9 this may be an important parameter for mapping out the post perovskite phase in the lowermost mantle

discriminant function analysis of late quaternary tephras from five volcanoes in new zealand using glass shard major element chemistry abstract the microprobe determined glass shard major element chemistry of tephras derived from five north island new zealand volcanoes mayor island okataina taupo tongariro and mount egmont and younger than ca 20 000 yr b p was subjected to discriminant function analysis four separate approaches were adopted to test the match of the tephras with their known sources 1 an analysis of raw microprobe data 2 an analysis of normalized data 3 an analysis of the data transformed by calculating the log 10 of oxide scores divided arbitrarily by the chlorine content and 4 a repeat of 3 with multivariate outlier scores as determined by principal components analysis deleted all yielded excellent classification functions efficiencies of 91 100 with the eruptives associated with each of the five volcanoes being chemically distinct from one another in each approach the first two canonical discriminant functions accounted for 90 of the variation between groups the removal of multivariate outliers from the data set had only minor effects on the performance of the discriminant function procedures separate discriminant function analysis of the relatively alike taupo and okataina eruptives gave a greater degree of multivariate separation the numerical classifications generated should enable unidentified tephras erupted since ca 20 000 yr b p from the five volcanoes to be provisionally matched with their sources

echinoderm stabilization associated with a paleokarst surface at the mississippian pennsylvanian boundary in tennessee u s a abstract pennington formation limestone deposits in tennessee immediately underlie a paleokarst surface that straddles the mississippian pennsylvanian unconformity vertic paleosols with significant gleying and coal seams overlie the paleokarst surface this study quantifies the fe loss associated with paleosol gleying and subsequent incorporation of fe into echinoderm grains that underwent stabilization echinoderm grains immediately below the paleokarst surface have lower mg and 13c values and higher fe concentrations compared to echinoderm grains meters below the paleokarst surface additionally echinoderm grains have variable 13c and invariant 18o values characteristic of alteration in a meteoric setting paleosols draping the paleokarst surface and coal seams that were deposited immediately above the mississippian pennsylvanian unconformity provided ample organic matter that could have driven heterotrophic microbial activity additionally the paleosols were an ample source of oxidized fe that could have been used as a terminal electron acceptor by an anaerobic microbial community therefore we suggest that fe reduction transferred fe from the paleosol to the limestone during meteoric stabilization of echinoderm grains fluid rock modeling constrains the magnitude of fe transfer that occurred during echinoderm stabilization stabilization can account for only a fraction 8 to 14 of the total quantity of fe lost from the gleyed paleosol thus supporting the premise of continued transfer of fe from the paleosol to limestone during burial diagenesis additionally this study constrains the timescale associated with echinoderm stabilization which was likely 12 ky finally stabilization timescale is consistent with echinoderm alteration associated with organic diagenetic conditions with elevated dissolved inorganic carbon in stabilizing pore fluids that were localized immediately below the paleokarst surface

toward a comprehensive upper quaternary tephra and ignimbrite stratigraphy in new zealand using electron microprobe analysis of glass shards abstract new zealand quaternary marine and terrestrial sequences contain numerous tephras or volcanic ash horizons that are the distal correlatives of voluminous welded ignimbrite sheets erupted from central north island electron microprobe analyses of glass shards from the distal tephras demonstrate their homogeneity and are shown to identify each tephra examined by matching tephras from stratigraphically controlled sequences the first comprehensive tephra stratigraphy spanning from 50 000 to 700 000 yr ago and covering the new zealand region is advanced analyses on glass shards from the unwelded base of ignimbrite sheets are comparable to the distal tephra analyses and allow correlation between ignimbrites and to the distal tephras the better exposed tephra record constrains the number of separate eruptive events and the stratigraphy of the ignimbrites both of which were previously confused by lack of outcrop samples from pumiceous marine sediments were found to contain two or more chemically distinct populations of glass the pumice is in cross bedded sands or sand lenses within conglomerate attesting to a shallow high energy environment where reworking could occur however each glass population could be matched to older known tephras

transformation of echinoid mg calcite skeletons by heating abstract interambularcral plates of echinoid heterocentrotus trigonarius composed of mg calcite 1 14 mol mgco3 were heated in three timed series of experiments at 300 c dried plate fragments and fragments with added water were heated separately in pressurized bombs x ray powder diffractometry unit cell dimensions and phase compositions are used to monitor reaction progress after 10 h heating in the bombs dolomite 43 5 mol mgco3 and mg calcite appear 4 7 mol mgco3 by 20 h all mg calcite 1 is consumed and at 120 h dolomite composition has evolved to 47 mol mgco3 and calcite to 2 mol mgco3 whole plates heated at 300 c in an open muffle furnace develop dolomite 42 mol mgco3 and mg calcite 2 6 mol mgco3 after 10 h and remain compositionally invariant throughout subsequent heating to 620 h limited skeletal water catalyzes the early reaction but escapes from the open furnace and consequently reaction ceases after 10 h the experimentally produced dolomite has relative mg ca ordering of 75 to 79 the stabilization of echinoid mg calcite by heating at 300 c to a mixture of dolomite and calcite occurs through a dissolution precipitation reaction the alteration fabric produced within the stereom consists of irregularly shaped branched dolomite crystals 5 m homoaxially set in a calcite 2 bomb or mg calcite 2 furnace matrix round and tubular pores 1 to 5 m are randomly distributed throughout this fabric the stereom pore system remains intact during furnace heating but is destroyed during heating in bombs the texture of experimentally stabilized echinoid skeletons is different from that of fossil echinoderms that are composed of microrhomic dolomite homoaxially set in a single calcite crystal

effects of the fe3 spin transition on the properties of aluminous perovskite new insights for lower mantle seismic heterogeneities abstract we have measured the effects of the coupled substitution of fe3 and al on the density and compressibility of mantle silicate perovskite pv up to 95 gpa x ray emission spectroscopy and synchrotron mossbauer spectroscopy reveal a rapid increase in the population of low spin fe3 in fe3 al bearing pv over a narrow pressure range near 70 gpa which is in sharp contrast with al free fe3 bearing pv where fe3 undergoes a gradual spin transition and with al free fe2 bearing pv where fe2 does not become low spin at low pressure fe3 and al expand the perovskite lattice however near the pressure range of the abrupt increase in the low spin population the unit cell volume of fe3 al bearing pv becomes similar to that of mg endmember pv while those of al free fe3 bearing pv and al free fe2 bearing pv remain larger throughout the lower mantle consequently pv in al rich systems should have lower density in the shallow lower mantle but similar or greater density than pv in pyrolite in the deep lower mantle affecting the buoyancy and mechanical stability of heterogeneities although the fe3 spin transition in pv is unlikely to cause a seismic discontinuity at mantle temperatures it may result in a large change in bulk sound speed at 1200 1800 km depth such that a vertically extending structure with an elevated amount of fe3 would generate slower and faster anomalies above and below the depth of the spin transition respectively relative to the surrounding mantle this may have important implications for bulk sound speed anomalies observed at similar depths in seismic tomography studies

geological history of the north eastern ring plain of ruapehu volcano new zealand abstract continuous exposures along the upper waikato stream provide new insights into the north eastern ring plain of ruapehu volcano extending the known stratigraphy beyond 22 6 ka time control in the sequence is provided by five rhyolitic tephra units erupted from central north island volcanoes comprising kawakawa okaia omataroa hauparu and rotoehu tephras the sequence is dominated volumetrically by diamictons and fluvial deposits resulting both from volcanic events and periods of instability on the flanks of ruapehu within the sequence are 60 individual andesitic lapilli units derived primarily from ruapehu volcano via mostly sub plinian eruption mechanisms an average eruption rate of more than one lapilli eruption per 1000 years is estimated for the ca 60 ka record the style of deposition on the ring plain changes over time and appears to reflect climate change over the last glacial period in periods of severe climatic conditions during marine 18 o stage 4 porewan stadial and the last glacial maximum of marine 18 o stage 2 ohakean the north eastern ring plain aggraded rapidly with deposition of thick continuous diamicton sequences the other recognized cool period in the southern north island the stadial of late 18 o stage 3 ratan did not appear to induce major aggradation on the north eastern ring plain during periods of mild climate within the last glacial deposition on the north eastern ring plain was dominated by fall accession of either tephra or material reworked from other parts of the ring plain by aeolian processes

biomineralisation in the palaeozoic oceans evidence for simultaneous crystallisation of high and low magnesium calcite by phacopine trilobites abstract the chemical composition and microstructure of the calcite cuticles of eleven species of phacopine trilobites have been investigated by electron beam imaging diffraction and microanalysis and results reveal that the lenses of their schizochroal eyes differed significantly in chemical composition from the rest of the cuticle in vivo apart from the eye lenses most cuticles are inferred to have escaped extensive recrystallisation because their constituent crystals are sub micrometre in size and have a preferred orientation that is consistent between species their current compositions of 1 4 to 2 4 mol mgco3 are likely to be close to original values although as they commonly luminesce and contain detectable manganese and iron some diagenetic alteration has taken place the associated lenses have a microstructure that is suitable for focusing light yet are optically turbid owing to the presence within calcite of micropores and crystals of microdolomite apatite celestite and pyrite the microdolomite indicates that lenses recrystallised from an original high mg calcite composition and this is supported by the presence of nanometre scale modulated microstructures in both the calcite and dolomite these lenses currently contain 1 to 6 mol mgco3 and by comparison with the proportion of magnesium lost from echinoderm stereom in the same thin sections may have contained 7 5 mol mgco3 in vivo in some samples more extensive diagenetic alteration is evidenced by recrystallisation of the cuticle including lenses to coarse equant calcite or enrichment of the cuticle but not necessarily the lenses in magnesium accompanying replacement by a mg fe phyllosilicate the phacopine trilobites had to modify partition coefficients for magnesium considerably in order to grow lenses with contrasting compositions to the rest of their cuticles and such a strong vital effect on biomineralisation suggests that incorporation of magnesium was essential for functioning of their calcite optical systems

petrography geochemistry and origin of burial diagenetic facies siluro devonian helderberg group carbonate rocks central appalachians petrographic and geochemical studies of pore filling cements and replacement products in the helderberg group upper silurian lower devonian central appalachians document the diagenetic history of these rocks shallow ramp skeletal limestones and buildups were partially lithified by marine cements however most pore filling cement formed under shallow 300 m to deep burial 300 4 000 m conditions regional cathodoluminescent zonation patterns in early calcite cements indicate meteoric ground waters were involved in shallow burial cementation early zoned calcite cements consist of updip nonluminescent cements with thin luminescent laminae in limestone or sandstone that pass downdip into subzoned dull cements interlayered bright and dull laminae and then into nonzoned dull cement in basinward limestone the regional distribution stable isotopic compositions and timing of early zoned cements relative to other diagenetic events suggest that all of the early cements formed synchronously from meteoric ground waters that became progressively more reducing as they flowed downdip at least 150 km helderberg sandstone tongues probably were preferential conduits for meteoric gro nd waters because early calcite cements formed only on scattered skeletal grains in sandstones late void filling dull cement formed from deep burial pore fluids at burial depths of 300 to 4 000 m deep burial pore fluids were dilute to saline na ca cl brines from primary fluid inclusion data with stable isotopic compositions that probably were similar to isotopic compositions of formation waters from modern oil fields void filling dull calcite cements occluded nearly all remaining porosity before late paleozoic fracturing and deformation latest diagenetic phases include dolomite fluorite silica and fracture filling dull calcite this study shows that integrated petrographic and geochemical approaches to diagenetic studies are useful for relating various diagenetic phases and fabrics to burial history and paleoaquifer distribution

regional distribution of microdolomite inclusions in mississippian echinoderms from southwestern new mexico abstract crinoid grains from mississippian skeletal packstones and grainstones from the lake valley and kelley formations new mexico have retained most of their original mg which now resides in 1 to 10 m microdolomite inclusions the mean microdolomite content of these crinoids is about 9 3 1 1 95 confidence limit by volumes as determined by petrography the microdolomite contents show regional variations being lowest in the northern part of the study area about 6 and highest about 11 170 km to the south there are no consistent stratigraphic gradients in microdolomite contents the average apparent mg content of the mississippian crinoids is estimated from the 9 3 microdolomite by assuming 50 original intraskeletal porosity this estimate of 10 4 mole mgco3 is lower than the average 14 5 mole of modern low latitude crinoids in spite of this the higher values of mg contents of mississippian crinoids fall within the range of mg contents of modern low latitude crinoids we interpret these data to mean that original mg contents of mississippian crinoids were comparable to those of modern crinoids and that the relatively low mean mg content of mississippian crinoids is due to diagenetic loss of mg we rule out lower temperatures or mg contents of mississippian sea waters and biological differences of modern and mississippian crinoids as causin the low mg contents of mississippian crinoids mg loss from mississippian crinoids is supported by the decrease in microdolomite content toward rims of grains and by the fact that intragranular variation is lower than intergranular variations of microdolomite content on the basis of the above evidence we suggest that on average mississippian crinoids have retained about 75 of their mg the southward increase in mg content of mississippian crinoids we interpret to reflect a greater diagenetic mg loss in the north than in the south rather than a southward increase in sea water temperature or a southward change in taxa this interpretation is supported by the southward decrease in intensity of chemical compaction and by the covariance of mg and 18o contents of crinoids our diagenetic model based on these data and on previous work invokes regional fresh phreatic groundwater systems that formed in response to late mississippian early pennsylvanian emergence of the shelf the groundwaters were recharged in the north and flowed generally southward northern areas contained diagenetically more aggress ve groundwaters which leached more mg from crinoid grains than did groundwaters farther south

late quaternary volcanism in new zealand towards an integrated record using distal airfall tephras in lakes and bogs studies on distal airfall tephra layers preserved in lake sediments and peats in northern new zealand have documented the stratigraphic chronologic and compositional relationships of 46 eruptives aged c 17000 700yr bp which originated from six north island volcanic centres taupo 9 tephras okataina 8 maroa 1 rhyolitic mayor island 2 peralkaline tongariro 11 egmont 15 andesitic sources were distinguished by mineralogy and composition field relations and 14c chronology all known rhyolitic tephra producing eruptions from taupo okataina and maroa volcanoes since c 17000yr bp are represented but only a small proportion of the known tephras erupted from tongariro egmont or mayor island volcanoes is recorded the distal tephras from these latter volcanic centres may thus reflect atypically powerful or oblique eruptions or dispersal by strong winds an improved record of volcanism for the tongariro egmont and mayor island centres might be obtainable from suitable lakes or bogs more proximal to them

sr mg ratios of modern marine calcite empirical indicators of ocean chemistry and precipitation rate holocene biotic and abiotic marine calcite have a similar range of mg contents 0 to 22 and 4 to 21 mol mgco3 respectively yet biotic calcite has sr2 concentrations that are consistently 1250 ppm higher than those of abiotic calcite as in laboratory experiments a positive linear relation is observed between dsr and calcite mg content this produces two distinct linear trends on a plot of sr2 vs mg2 concentrations principal axes of variation for both trends have similar slopes yet distinctly different sr2 concentration intercepts biotic y 0 024x 1298 r2 0 70 abiotic y 0 027x 47 r2 0 77 the similar slopes of these trends reflect the constancy of mgca and srca ratios of modern seawater equations describing the dependence of dsr on calcite mg content are derived from both trends biotic dsr 3 16 x 10t 6 ppm mg 0 169 abiotic dsr 3 52 x 10 6 ppm mg 0 0062 characterization of sr mg trends for holocene materials allows comparison with analogous trends from ancient samples to estimate relative changes in seawater mgca and srca ratios the relatively high sr contents of biotic calcite result from rapid precipitation rates associated with shell accretion in marine organisms calcites precipitated from seawater in laboratory experiments have dsr values that are similar to those of biotic marine calcite suggesting that both precipitate at approximately the same rate our estimates of surface area normalized precipitation rates in planktonic and benthonic foraminifera are comparable to those of seeded ph stat experiments we conclude that the dsr values for biotic and experimental marine calcite are kinetically controlled whereas the lower precipitation rates of abiotic marine calcite yield dsr values that approximate equilibrium conditions experimentally derived equations describing the relation between dsr and calcite precipitation rate indicate that the offset in sr content between biotic and abiotic calcite is the result of abiotic precipitation rates that are two to five orders of magnitude lower than those of biotic precipitates however observations of naturally occurring marine cements suggest that the five order of magnitude offset best represents natural system processes

the postperovskite transition the discovery of the postperovskite ppv transition has profound impact on our understanding of the core mantle boundary cmb region unlike perovskite pv the ppv phase has a layered structure of the sio6 octahedra which may lead to a large contrast in some properties with pv recent studies have proposed unusual properties of ppv such as a large positive clapeyron slope a large sensitivity of the transition depth to iron a decrease in bulk sound speed at the transition and a development of significant lattice preferred orientation many of the proposed properties can provide explanations for the intriguing seismic observations at the cmb region yet significant discrepancies still exist however rapid developments in mineral physics will continue to improve our knowledge on the changes across the ppv transition

high pressure temperature elasticity of fe and al bearing mgsio3 implications for the earth s lower mantle fe and al are two of the most important rock forming elements other than mg si and o their presence in the lower mantle s most abundant minerals mgsio3 bridgmanite mgsio3 post perovskite and mgo periclase alters their elastic properties however knowledge on the thermoelasticity of fe and al bearing mgsio3 bridgmanite and post perovskite is scarce in this study we perform ab initio molecular dynamics to calculate the elastic and seismic properties of pure fe3 and fe2 and al3 bearing mgsio3 perovskite and post perovskite over a wide range of pressures temperatures and fe al compositions our results show that a mineral assemblage resembling pyrolite fits a 1d seismological model well down to at least a few hundred kilometers above the core mantle boundary i e the top of the d region in d a similar composition is still an excellent fit to the average velocities and fairly approximate to the density we also implement polycrystal plasticity with a geodynamic model to predict resulting seismic anisotropy and find post perovskite with predominant 001 slip across all compositions agrees best with seismic observations in the d

spin states and hyperfine interactions of iron incorporated in mgsio3 post perovskite abstract using density functional theory hubbard u dft u calculations we investigate the spin states and nuclear hyperfine interactions of iron incorporated in magnesium silicate mgsio 3 post perovskite ppv a major mineral phase in the earth s d layer where the pressure ranges from 120 to 135 gpa in this pressure range ferrous iron fe 2 substituting for magnesium at the dodecahedral a site remains in the high spin hs state intermediate spin is and low spin ls states are highly unfavorable as to ferric iron fe 3 which substitutes magnesium at the a site and silicon at the octahedral b site to form mg fe si fe o 3 ppv we find the combination of hs fe 3 at the a site and ls fe 3 at the b site the most favorable neither a site nor b site fe 3 undergoes a spin state crossover in the d pressure range the computed iron quadrupole splittings are consistent with those observed in mossbauer spectra the effects of fe 2 and fe 3 on the equation of state of ppv are found nearly identical expanding the unit cell volume while barely affecting the bulk modulus

spin crossover of iron in aluminous mgsio3 perovskite and post perovskite using density functional theory hubbard u dft u calculations we investigate how aluminum affects the spin crossover of iron in mgsio3 perovskite pv and post perovskite ppv the major mineral phases in the earth s lower mantle we find that the presence of aluminum does not change the response of iron spin state to pressure only ferric iron fe3 in the octahedral b site undergoes a crossover from high spin hs to low spin ls state while fe3 in the dodecahedral a site remains in the hs state same as in al free cases however aluminum does significantly affect the placement of fe3 in these mineral phases the most stable atomic configuration has all al3 in the b site and all fe3 in the a site thus in the hs state metastable configurations with ls fe3 in the b site can happen only at high pressures and high temperatures therefore experimental observations of ls fe3 at high pressures in al bearing pv require diffusion of iron from the a site to the b site and should be sensitive to the annealing temperature and schedule in the earth s lower mantle the elastic anomalies accompanying the b site hs ls crossover exhibited in al free pv are likely to be considerably reduced according to the b site fe3 population

controls on regional variability in marine pore water diagenesis below the seafloor in upper jurassic lower cretaceous prodeltaic sandstone and shales scotian basin eastern canada abstract diagenesis in the uppermost jurassic to lower cretaceous deltaic sandstones and shales of the scotian basin is an important control on reservoir quality ferruginous zone sub oxic marine pore water diagenesis controls the initial formation of fe2 silicates that are the precursors of grain rimming chlorite that preserves porosity this study assesses the regional controls on the type of marine pore water diagenesis by studying the sedimentology mineralogy and geochemistry of the retrogradational units and underlying progradational units in parasequences from conventional cores in two wells in different parts of the basin coated grains preserve a record of whether marine pore water diagenesis below the seafloor was dominantly in the ferruginous or sulphidic geochemical zone four types of coated grain were distinguished each with a different mineral paragenesis mineralogical and chemical evidence of ferruginous zone diagenesis includes the presence of diagenetic chlorite and siderite and the correlation of p with fe or ti pyrite and fe calcite are found where the sulphidic zone is more significant than the ferruginous zone ferruginous zone diagenesis was common in low sedimentation rate retrogradational sediments with low organic carbon and in delta front turbidites and river mouth sandstones estuarine tidal flat and prodeltaic facies that are directly supplied by riverine sediments have a lower fe ti ratio than do fully marine shoreface and open shelf facies as a result of input of detrital ilmenite and its alteration products the relative contribution of colloidal iron hydr oxides appears greater in distal low sedimentation rate environments where large changes in sedimentation rate occurred at ravinement surfaces the underlying progradational rocks have evidence of ferruginous zone diagenesis whatever their facies rapid upward migration of the pore water profile resulting from the change in sedimentation rate reduced the time available for mineral products to form in the deeper pore water zones this study has shown that the availability of fe and organic carbon varying in a complex manner in marine deltaic sediments but that the resulting diagenesis by marine pore water can be predicted from facies and paleogeographic setting

mechanisms of al3 incorporation in mgsio3 post perovskite at high pressures abstract aluminum is the fifth most abundant element in the earth s mantle yet its effect on the physical properties of the newly found mgsio3 post perovskite ppv the major mineral of the earth s d layer is not fully known in this paper large scale ab initio simulations based on density functional theory dft within the generalized gradient approximation gga have been carried out in order to investigate the substitution mechanism of al3 into ppv at high pressures we have examined three types of al substitution in ppv 6 25 mol al substitution via a charge coupled mechanism ccm 6 25 mol al substitution via oxygen vacancy mechanism ovm and an oxygen vacancy si free end member mg2al2o5 for both the ccm and ovm five models where the al atoms were put in different positions were simulated at various pressures in the range 10 150 gpa our calculations show that the most favorable mechanism is a charge coupled substitution where al3 replaces the next nearest neighbor cation pairs in the ppv structure the calculated zero pressure bulk modulus of al bearing ppv is 3 15 lower than that of the al free ppv in agreement with previous works we find that the incorporation of al2o3 slightly increases the post perovskite phase transition pressure with the al partition coefficient k 2 67 at 120 gpa and 3000 k

mississippian paleocean chemistry from biotic and abiotic carbonate muleshoe mound lake valley formation new mexico u s a discussion hasiuk and lohmann 2008 present a geochemical study of cement and crinoid remains from the much studied mississippian muleshoe mud mound new mexico u s a they deduce mississippian paleocean chemistry from data on altered cement and crinoids that are assumed to have recrystallized in sea water and retained their original marine chemistry it is surprising in this context that they present data for just one brachiopod as brachiopods are accepted as one of the most reliable marine proxies they do not discuss the interpretation of muleshoe brachiopod data made by wu and chafetz 2002 the latter authors differentiate intermound from mound brachiopods by their delta 13c values higher mound values are thought to be due to seepage of 13c enriched waters produced by methanogenic bacteria the muleshoe mound is interpreted by stanton et al 2000 as being deposited in water 200 m deep in the dysoxic oxygen minimum zone omz consequently can muleshoe carbonate influenced by methane seepage and deposited in the omz be regarded as an appropriate choice of material for an open marine proxy

mollusc and brachiopod skeletal hard parts intricate archives of their marine environment the biogenic carbonate hard parts of fossil bivalves cephalopods and brachiopods are among the most widely exploited marine archives of phanerozoic environmental and climate dynamics research the advent of novel analytical tools has led many workers to explore non traditional geochemical and petrographic proxies and work performed in neighbouring disciplines sheds light on the complex biomineralization strategies applied by these organisms these considerations form a strong motivation to review the potential and problems related to the compilation and interpretation of proxy data from bivalve cephalopod and brachiopod hard parts from the viewpoint of the sedimentologist and palaeoceanographer specific focus is on the complex biomineralization pathways of a given dissolved ion or food particle from its aquatic environment via the digestion and biomineralization apparatus in molluscs and brachiopods and its incorporation into a biomineral given that molluscs and brachiopods do not secrete their hard parts from seawater but rather from their mantle and periostracum this paper evaluates differences and similarities of seawater versus that of body fluids cephalopods bivalves and brachiopods exert a strong biological control on biomineralization that to some degree may buffer their shell geochemistry against secular changes in seawater chemistry disordered amorphous calcium carbonate precursor phases later transformed to crystalline biominerals may be significant in carbonate archive research due to expected geochemical offset relative to the direct precipitation of stable phases a reasonable level of understanding of the related mechanisms is thus crucial for those who use these skeletal hard parts as archives of the palaeo environment the impact of what is commonly referred to as biological factors on the geochemistry of mollusc and brachiopod hard parts is explored for conventional isotope systems such as carbon oxygen strontium and traditionally used element to calcium ratios in particular the often used 13ccarb or the mg ca and sr ca elemental proxies are fraught with problems an interesting new research field represents the analysis calibration and application of non traditional proxies to mollusc and brachiopod hard parts examples include the carbonate clumped isotope 47 approach and the analysis of the isotopes of ca mg n li s or element to ca ratios such as li ca or b ca and rare earth elements based on considerations discussed here a series of do s and don ts in mollusc and brachiopod archive research are proposed and suggestions for future work are presented in essence the suggestions proposed here include experimental work also field experiments making use of recent archive organisms or where possible a reasonable recent analogue in the case of extinct groups moreover the detailed understanding of the architecture of mollusc and brachiopod hard parts and their ultra structures must guide sampling strategies for geochemical analyses where feasible a detailed understanding of the diagenetic pathways and the application of multi proxy and multi archive approaches should form the foundation of fossil carbonate archive research the uncritical compilation of large data sets from various carbonate shelled organisms collected at different locations is not encouraged

the potential of crinoids as u th sm he thermochronometers abstract u th sm he dating of crinoid fossils from late paleozoic rocks from a variety of sites in north america suggest that this material can be used as a useful low t thermochronometer under certain conditions available data suggest that common he is present in crinoids and at the low production rates typical of calcite this common he will be sufficient to produce spurious ages in crinoid samples of any age heating of samples to temperatures in excess of the opening temperature will expel all he common and radiogenic with all subsequent he accumulation being exclusively radiogenic such samples have potential for being robust thermochronometers sensitive to temperatures in the range of 60 110 c without common he analysts can take advantage of the large size of crinoid columnals and produce reliable ages even when young miocene or with low production rates eu 0 1 ppm samples not subjected to post depositional heating above the opening temperature burial to depths of 2 to 4 km depending on geothermal gradient sedimentation rate and residence time at peak t will likely retain common he and be unsuitable for thermochronologic studies

a perovskitic lower mantle inferred from high pressure high temperature sound velocity data determination of the shear wave velocities for silicate perovskite and ferropericlase under the pressure and temperature conditions of the deep lower mantle indicates that perovskite constitutes much more of the lower mantle than predicted by the conventional mantle model and is consistent with the chondritic earth model earth s upper mantle is apparently depleted of silicon when compared with meteorites which are thought to represent the material from which the earth formed this missing silicon problem has provoked intense debate it suggests that the deficit might be balanced by silicon in the core or that the upper mantle is not representative of the entire mantle and that the lower mantle is enriched in silicon murakami et al provide evidence to support the latter case they made laboratory measurements of sound wave transmission through silicate perovskite and ferropericlase minerals at pressures and temperatures matching those of the lower mantle and compared the resulting shear wave values with seismic data that sample the lower mantle they conclude that the mineralogical model that provides the best fit is one in which perovskite constitutes greater than 93 of the lower mantle this suggests that the lower mantle is enriched in silicon in comparison with the upper mantle consistent with the chondritic earth model and that there is limited mass transport between the upper and lower mantle the determination of the chemical composition of earth s lower mantle is a long standing challenge in earth science accurate knowledge of sound velocities in the lower mantle minerals under relevant high pressure high temperature conditions is essential in constraining the mineralogy and chemical composition using seismological observations1 but previous acoustic measurements were limited to a range of low pressures and temperatures here we determine the shear wave velocities for silicate perovskite and ferropericlase under the pressure and temperature conditions of the deep lower mantle using brillouin scattering spectroscopy2 the mineralogical model that provides the best fit to a global seismic velocity profile1 indicates that perovskite constitutes more than 93 per cent by volume of the lower mantle which is a much higher proportion than that predicted by the conventional peridotitic mantle model it suggests that the lower mantle is enriched in silicon relative to the upper mantle which is consistent with the chondritic earth model such chemical stratification implies layered mantle convection with limited mass transport between the upper and the lower mantle

elasticity of mg fe si al o 3 perovskite at high pressure the most abundant mineral on earth has a perovskite crystal structure and a chemistry that is dominated by mgsio3 with the next most abundant cations probably being aluminum and ferric iron the dearth of experimental elasticity data for this chemically complex mineral limits our ability to calculate model seismic velocities for the lower mantle we have calculated the single crystal elastic moduli c ij for mg fe3 si al o 3 perovskite using density functional theory in order to investigate the effect of chemical variations and spin state transitions of the fe3 ions considering the favored coupled substitution of mg2 si4 by fe3 al3 we find that the effect of ferric iron on seismic properties is comparable with the same amount of ferrous iron ferric iron lowers the elastic moduli relative to the al charge coupled substitution substitution of fe3 for al3 giving rise to an fe mg ratio of 6 causes 1 8 lower longitudinal velocity and 2 5 lower shear velocity at ambient pressure and 1 1 lower longitudinal velocity and 1 8 lower shear velocity at 142 gpa the spin state of the iron for this composition has a relatively small effect 0 5 variation on both bulk modulus and shear modulus c 2005 elsevier b v all rights reserved

electronic spin transitions in iron bearing mgsio3 perovskite the electronic spin state of iron in perovskites with the chemical composition mg0 9375fe0 0625sio3 mg0 8750fe0 1250sio3 and mg0 9375fe0 0625si0 9375fe0 0625o3 have been determined at 0 k and various pressures between 40 160 gpa using ab initio methods the results indicate that ferric iron exhibits a wide range of spin transition pressures between about 60 160 gpa while ferrous iron has a much narrower span between about 130 145 gpa in general the lowest spin transition pressures are associated with the most energetically favorable substitution configurations where iron atoms are closest together the effect of spin state on calculated elastic properties is found to be small c 2006 elsevier b v all rights reserved

a multiple parameter approach to andesitic tephra correlation ruapehu volcano new zealand abstract a multi parameter approach was used to correlate andesitic tephras in a complex tephra sequence ranging in age from ca 23 to ca 75 ka on the eastern ring plain of ruapehu volcano north island field properties combined with ferromagnesian mineral assemblages and mineral compositions were required to map and correlate this sequence three tephra units could be identified based on their unique physical appearance but other tephras could not be correlated on this basis alone hornblende and olivine proved to be valuable marker minerals enabling further distinction of two of the marker units recognised by field properties as well as defining two further marker tephras unweathered titanomagnetite crystals present in all of the tephras were subjected to major element analysis by electron microprobe canonical discriminant function analysis dfa of these analyses enabled the grouping and discrimination of tephra units further aiding the identification of defined marker units as well as defining new marker units the titanomagnetite chemistry showed a strong relationship to the ferromagnesian mineralogy showing that the ferromagnesian phenocrysts formed from the same melt or under the same melt conditions prior to eruption of each tephra canonical dfa was also applied to hornblende and olivine mineral analyses to identify further marker beds and to confirm identifications of previously defined units this statistical analysis was found to be invaluable in reducing the large amount of compositional data from this study into a useable form for andesitic tephra correlation and mapping

high resolution 3 d s velocity structure in the d region at the western margin of the pacific llsvp evidence for small scale plumes and paleoslabs abstract although previous tomographic studies found a large low s velocity province llsvp in the lowermost mantle beneath the pacific due to a lack of resolution it remains unclear whether the llsvp consists of clusters of small scale low velocity anomalies or large scale anomalies we recently deployed a seismic array in thailand which provides a dataset with wide azimuthal coverage at the western pacific llsvp we analyze the new dataset using waveform inversion and find high velocity anomalies extending vertically to a height of 400 km above the core mantle boundary cmb beneath the philippine sea and small scale low velocity patches with a diameter of 300 km at the cmb beneath new guinea the locations of the high velocity anomalies are consistent with the past izanagi plate subduction boundary and the low velocity anomalies can be interpreted as a small scale plume cluster hence we conclude that vertical flow upwelling plumes and downwelling of slabs is dominant in the lowermost mantle beneath the western pacific region

structure of magnesian calcite from sea urchins x ray scans and high resolution transmission electron microscopy hrtem imaging indicate that the calcite of sea urchin spines and skeletal plates forms in mosaic crystals both coherent and incoherent boundaries were observed between slightly misaligned crystallites small domains show an ordered superstructure using hrtem and both x ray and electron diffraction data we found that the most appropiate model for this superstructure has ca mg 3 1 and alternating ca and mg layers along c we did not find evidence for the presence of protein molecules within the calcite structure

echinoderm skeletal preservation calcite aragonite seas and the mg ca ratio of phanerozoic oceans abstract the alternation between abiotic calcite and aragonite precipitation in shallow seawater and the oscillation of seawater mg ca ratio throughout the phanerozoic are both popular hypotheses echinoderms with well preserved stereom provide new empirical evidence that supports both these hypotheses analyses are reported from 29 specimens cambrian to eocene age containing ossicles preserved either as mg calcite 3 3 to 8 mole mgco3 or as mixed calcite and dolomite 5 to 12 5 mole mgco3 echinoderms with high mole mgco3 occur in the early cambrian and late carboniferous to triassic low values come from the silurian and jurassic to cretaceous the average composition of echinoderms with preserved stereom does not fall below 3 5 mol mgco3 during the phanerozoic and reaches its highest mean value of 16 0 mol mgco3 today an empirical partition coefficient of 0 03182 for modern tropical echinoids is used to indicate the mg ca ratio of ancient seawater from the mg ca ratio of fossil stereom some assumptions are involved and a likely error is calculated from the mg2 variation of modern echinoids high mean seawater mg ca ratios are calculated for early cambrian 3 3 and the late carboniferous to triassic 2 3 but never reached today s value of 5 2 low mg ca ratios 1 1 are indicated from jurassic to cretaceous echinoderms the 29 echinoderm samples plot close to first order mg ca seawater oscillations derived from geochemical models and mg ca ratios determined from fluid inclusions but considerable discrepancies exist when shorter 106 myr time intervals are considered further data and improved understanding of mg partitioning is required before accurate secular variation in the mg ca of seawater can be determined from echinoderms however they are an underused resource in this context and provide an excellent seawater archive

ab initio elasticity and thermal equation of state of mgsio3 perovskite we have used high temperature ab initio molecular dynamic simulations to study the equation of state of orthorhombic mgsio3 perovskite under lower mantle pressure temperature conditions we have determined the gru neisen parameter q as a function of volume our state of the art simulations accurate to within 10 resolve the long standing controversy on thermal expansion k and gru neisen parameter of mgsio3 perovskite under ambient conditions we find the values for k and q of 1 86u10 35 k 31 and 1 51 respectively in excellent agreement with the latest experimental studies calculated elastic constants and the static equation of state at 0 k agree well with previous simulations we have found no evidence for the high temperature phase transitions of orthorhombic mgsio3 perovskite to cubic or tetragonal phases at mantle temperatures fl 2001 elsevier science b v all rights reserved

diagenesis of fibrous magnesian calcite marine cement implications for the interpretation of δ18o and δ13c values of ancient equivalents fibrous high mg calcite hmc marine cement from the pliocene hope gate formation jamaica occurs as 1 2 mm thick isopachous coatings on skeletal grains in local occurrences of halimeda grainstone and is commonly syntaxially overgrown by clear prismatic low mg calcite lmc interpreted as a mixing zone cement the presence of inclusions at discrete levels in the fibrous marine cement produces internal growth banding wherein clear inclusion free zones alternate with cloudy inclusion rich zones clear zones have uniform mg contents 13 7 0 2 mol mgco3 and stable isotopic compositions 18o 0 8 0 1 13c 3 2 0 1 consistent with a marine origin in contrast inclusion rich zones contain fine scale intergrowths of lmc dolomite and hmc and have carbon and oxygen isotopic compositions that define a linear mixing trend between compositions of inclusion free hmc and prismatic lmc cement overgrowths petrographic observations indicate that the mixing trend defined by isotopic data from inclusion rich zones is related to the addition of large volumes of secondary lmc as cement in primary pore space as intergrowths among fibers of primary hmc marine cement and or as replacements of primary hmc crystallites dolomite is present in inclusion rich zones in volumes generally compatible with amounts predicted to result from the closed system stabilization of hmc to lmc and dolomite in addition isotopic and geochemical data from some areas of inclusion rich zones are consistent with mass balance constraints assuming a closed system which require that bulk isotopic and geochemical compositions are retained these relations suggest that small remnants of hmc in inclusion rich zones of hope gate marine cement have altered to lmc and dolomite while isolated from system pore fluids such that primary marine isotopic signatures were imparted on diagenetic products similar petrographic geochemical and isotopic relations have been noted in ancient occurrences suggesting that 18o and 13c values from some former marine cements should be useful for the determination of isotopic compositions of original marine cements and ultimately for the determination of ocean chemistry in the geologic past

recrystallization in living porcelaneous foraminifera archaias angulatis textural changes without mineralogic alteration abstract mineralogic stabilization of porcelaneous foraminifera has been cited as an example of diagenesis in which chemical change occurs without any textural alteration in contrast our study of living specimens of a common species of this group archaias angulatis indicates a sequence in which textural alteration of the magnesium calcite skeleton occurs without loss of magnesium from the test in samples of a angulatis from florida bay florida skeletal rods 1 2 m long and 0 1 m wide are extensively recrystallized to dense minimicrite 0 05 0 1 m anhedral equant crystals both rods and minimicrite contain 9 11 5 mole mgco3 micritization increases with age of the skeletons and occurs more rapidly in septal and basal walls than in lateral walls and pillars recrystallization could be due to change in the partial pressure of co2 resulting from changing distributional patterns of respiration and photosynthesis of algal symbionts recrystallization within living foraminifera has important implications for studies of fossil diagenesis and biomineralization

deformation crystal preferred orientations and seismic anisotropy in the earth s d layer abstract we use a forward multiscale model that couples atomistic modeling of intracrystalline plasticity mechanisms dislocation glide twinning in mgsio3 post perovskite ppv and periclase mgo at lower mantle pressures and temperatures to polycrystal plasticity simulations to predict crystal preferred orientations cpo development and seismic anisotropy in d we model the cpo evolution in aggregates of 70 ppv and 30 mgo submitted to simple shear axial shortening and along corner flow streamlines which simulate changes in flow orientation similar to those expected at the transition between a downwelling and flow parallel to the core mantle boundary cmb within d or between cmb parallel flow and upwelling at the borders of the large low shear wave velocity provinces llsvp in the lowermost mantle axial shortening results in alignment of ppv 010 axes with the shortening direction simple shear produces ppv cpo with a monoclinic symmetry that rapidly rotates towards parallelism between the dominant 100 010 slip system and the macroscopic shear these predictions differ from mgsio3 post perovskite textures formed in diamond anvil cell experiments but agree with those obtained in simple shear and compression experiments using cairo3 post perovskite development of cpo in ppv and mgo results in seismic anisotropy in d for shear parallel to the cmb at low strain the inclination of scs sdiff and skks fast polarizations and delay times vary depending on the propagation direction at moderate and high shear strains all s waves are polarized nearly horizontally downwelling flow produces sdiff scs and skks fast polarization directions and birefringence that vary gradually as a function of the back azimuth from nearly parallel to inclined by up to 70 to cmb and from null to 5 change in the flow to shear parallel to the cmb results in dispersion of the cpo weakening of the anisotropy and strong azimuthal variation of the s wave splitting up to 250 km from the corner transition from horizontal shear to upwelling also produces weakening of the cpo and complex seismic anisotropy patterns with dominantly inclined fast scs and skks polarizations over most of the upwelling path models that take into account twinning in ppv explain most observations of seismic anisotropy in d but heterogeneity of the flow at scales

taphonomy palaeoecological implications and colouration of cambrian gogiid echinoderms from guizhou province china based on rich material 381 specimens examined from two cambrian echinoderm faunas the early cambrian balang fauna and middle cambrian kaili fauna in guizhou province south china the taphonomy of gogiid echinoderms is described in detail and the preservation of stereomic microstructure and organic remains of cambrian gogiid echinoderms is reported here for the first time taphonomic considerations include entombment patterns decay sequences individual specific diagenetic histories unusual burial postures selective disarticulation patterns and postmortem elongation in particular five categories of gogiid entombment patterns are proposed to describe the multi directional orientations recorded at the burial time of articulated gogiids gogiid bearing slabs of guizhou material primarily 70 display the type 2 entombment pattern articulated gogiids preserved with fan shaped brachioles thus most guizhou gogiids were buried with brachioles preserved in feeding posture during obrution events balang gogiid faunas contain the oldest evidence of palaeoecological interactions among echinoderms and other indigenous taxa in addition to pre burial and post burial decay other potential causes for unusual disarticulation patterns exhibited by the gogiids from the lower cambrian balang formation include pre burial bio disturbance and post burial bioturbation based on ichnogenera including rusophycus and planolites chemical analyses reveal that carbon calcium manganese and iron are the major elements responsible for the variety of colours exhibited by guizhou gogiids three dimensional stereomic microstructure mean stereom pore size 8 4 8 7 m average trabecular thickness 4 5 4 6 m occurs on the external surfaces of thecal plates in two gogiid species stereom preservation in calcite suggests that the dissolution of calcareous echinoderm plates yielding characteristic mouldic preservation is sub recent after lithificaiton and exposure of gogiid bearing marine sedimentary successions on or near the land surface

synthetic seismic anisotropy models within a slab impinging on the core mantle boundary the lowermost few hundreds of kilometres of the earth s mantle are elastically anisotropic seismic velocities vary with direction of propagation and polarization observations of strong seismic anisotropy correlate with regions where subducted slab material is expected in this study we evaluate the hypothesis that crystal preferred orientation cpo in a slab as it impinges on the core mantle boundary is the cause of the observed anisotropy next we determine if fast polarization directions seen by shear waves can be mapped to directions of geodynamic flow this approach is similar to our previous study performed for a 2 d geodynamic model in this study we employ a 3 d geodynamic model with temperature dependent viscosity and kinematic velocity boundary conditions defined at the surface of the earth to create a broad downwelling slab tracers track the deformation that we assume to be accommodated by dislocation creep we evaluate the models for the presence of perovskite or post perovskite and for different main slip systems along which dislocation creep may occur in post perovskite 100 010 and 001 resulting in four different mineralogical models of cpo combining the crystal pole orientations with single crystal elastic constants results in seismically distinguishable models of seismic anisotropy the models are evaluated against published seismic observations by analysing different anisotropic components the radial anisotropy the splitting for sub vertical phases i e azimuthal anisotropy and the splitting for subhorizontal phases the patterns in radial anisotropy confirm our earlier results in 2 d observations of radial anisotropy and splitting in subhorizontal phases are mostly consistent with our models of post perovskite with 010 slip and 001 slip our model of 001 slip predicts stronger splitting than for 010 slip for horizontally propagating phases in all directions the strongest seismic anisotropy in this model occurs where the slab impinges on the core mantle boundary the azimuthal anisotropy pattern for 001 slip shows fast axis directions at the edges of the slab sub parallel to flow directions suggesting horizontal flows may be mapped out in the lowermost mantle using seismic observations

a numerical approach toward the classification of magnetites from tephra in southern alberta high titanium glass encased magnetites associated with intermixed tephras in upper subalpine soils in banff national park are shown to be characteristic of mazama tephra and of regional significance similar magnetites occur in a mazama layer above a paleosol at fish creek alberta and within a sample of mazama ash ua 396 from windy point alberta detailed sampling and analysis of the fish creek tephra indicate that the magnetites do not represent a separate eruptive event or an older ashfall cluster analysis of glass encased magnetites from reference samples of mazama st helens y and bridge river ashes revealed composite grouping of mazama and bridge river magnetites and separate and strong groupings of high tio2 magnetites discriminant analysis identifies mgo tio2 and al2o3 as the most significant chemical discriminants between the various ashes discriminant function analysis identifies the high titanium magnetites as mazama in type and successfully classifies 96 of the tephra derived magnet

stratigraphy chronology styles and dynamics of late quaternary eruptions from taupo volcano new zealand taupo volcano is the southerly of two dormant caldera volcanoes in the rhyolite dominated central portion of the taupo volcanic zone in the north island of new zealand taupo has an average magma output rate of 0 2 m 3 s 1 over the past 65 000 years and is one of the most frequently active and productive rhyolite volcanoes known the structure of the modern inverse volcano was formed largely by caldera collapse associated with the voluminous 22 600 14 c years bp oruanui eruption and has been little modified since except for collapse following the 1850 14 c years bp eruption the products of 28 eruptions labelled psi omega a ldots z all of which post date the oruanui eruption are defined and described here twenty seven of these eruptions are represented by pyroclastic deposits of which three were accompanied by a mappable lava extrusion and one eruption z solely by evidence for a lava extrusion the deposits of seven eruptions b c e s v x and y largely correspond to previously defined tephra formations karapiti poronui opepe waimihia whakaipo mapara and taupo respectively the previously defined motutere and hinemaiaia tephras are reinterpreted to represent the products of 12 eruptions g to r while the remaining eight deposits and one eruption are newly recognized eruption psi occurred at ca 17 200 14 c or 20 500 calibrated years bp and eruption z about 1740 calibrated years bp eruption volumes vary by more than three orders of magnitude between 0 01 and more than 44 km 3 and repose periods by more than two orders of magnitude from ca 20 to 6000 years the eruption deposits reflect great variations in parameters such as volume the dispersal characteristics of the fall deposits the presence or absence of intraeruptive time breaks the formation of pyroclastic flows the degree of magma water interaction the vesiculation state of the magma on fragmentation and the relative proportions of juvenile obsidian versus foreign lithologies in the lithic fractions all but seven fall deposits are plinian in dispersal two y1 and probably w are sub plinian one y5 has been termed ultraplinian while psi and a are too poorly preserved for their dispersal to be assessed the lengths of repose periods in the post oruanui sequence range are not randomly distributed but show self similar properties fractal dimensionality repose intervals r in years of not more than 350 years follow n 53 5r 0 21 and those of not less than 350 years follow n 2096r 0 83 where n is the number of eruptions the shorter repose periods may reflect triggering processes such as regional extension affecting magma bodies during their viable lifetimes while longer repose intervals i e not less than 350 years may reflect an episodicity of major rifting events or the production of magma bodies below the volcano bulk volumes v in km 3 of the eruption products also show self similar properties fractal dimensionality with n 6 17v 0 46 however there are then apparently random relationships between eruption volumes and the preceding or succeeding repose period such that prediction of the time and size of the next eruption is impossible the post oruanui activity at taupo represents noise superimposed on the more uniform longer term activity in the central taupo volcanic zone where large greater than 100 km 3 eruptions such as the oruanui occur at more evenly spaced intervals of one per 40 60 000 years

mgsio3 postperovskite at d conditions the postperovskite transition in mgsio3 at conditions similar to those expected at the d discontinuity of earth s lower mantle offers a paradigm for interpreting the properties of this region despite consistent experimental and theoretical predictions of this phase transformation the complexity of the d region raises questions about its geophysical significance here we report the thermoelastic properties of cmcm postperovskite at appropriate conditions and evidences of its presence in the lowermost mantle these are i the jumps in shear and longitudinal velocities similar to those observed in certain places of the d discontinuity and ii the anticorrelation between shear and bulk velocity anomalies as detected within the d region in addition the increase in shear modulus across the phase transition provides a possible explanation for the reported discrepancy between the calculated shear modulus of postperovskite free aggregates and the seismological counterpart in the lowermost mantle

microstructural evolution during experimental albitization of k rich alkali feldspar crystals of k feldspar c2 m in contact with highly concentrated aqueous nacl solutions at 500 c and 200 mpa are pseudomorphically replaced by high albite c bar 1 as a result of an interface coupled dissolution reprecipitation process the reaction occurs at an extremely sharp reaction front 10 nm and involves the complete breakdown of the initial framework structure this results in the release of tetrahedrally incorporated elements such as fe3 and ti4 and a significant increase in si al disorder across the reaction interface the evolving microstructure is controlled by crystallographic relations between the phases this leads to highly anisotropic sawtooth shaped intergrowths of albite and initial k feldspar resulting in the least structural misfit between the two framework structures as a result the newly formed interfaces appear to be semicoherent and cracks across the reaction fronts even indicate elastic strain the reaction produces 2 distinctive albite types albite 1 and albite 2 both are polycrystalline with albite 2 showing significantly larger subgrain sizes this indicates a secondary coarsening step driven by the reduction in interfacial energy within the polycrystalline replacement product the reaction also produces a highly porous rim however the porosity is not evenly distributed resulting in a porous albite 1 and a non porous albite 2 that mostly surrounds large euhedral pores despite the substantial volume fraction of porosity in albite 1 no significant 3d interconnectivity could be detected making the presence of a pervasive porosity unlikely however the result of coarsening is the continuous modification of the 3d porosity distribution this could potentially provide a mechanism for fluid transport through the replacement rim until textural and chemical equilibration is achieved

equations of state of the high pressure phases of a natural peridotite and implications for the earth s lower mantle abstract to help determine the chemical composition of the earth s mantle we characterised the high pressure mineral assemblage of an undepleted natural peridotite thought to be representative of the earth s upper mantle to 107 gpa using high resolution x ray diffraction at lower mantle conditions the peridotite transforms to the assemblage 76 2 mg0 88fe0 052 fe0 013 al0 12si0 94 o3 orthorhombic perovskite by volume at zero pressure 17 2 mg0 80fe0 20 o magnesiowustite and 7 1 casio3 perovskite the measured room temperature bulk properties of this high pressure assemblage together with a range of estimates of thermal properties of the constituent minerals appear to be inconsistent with seismological constraints on the density and bulk modulus of the lower mantle our results suggest that the lower mantle differs in bulk composition e g richer in iron mg 0 85 from current estimates for the upper mantle requiring some amount of segregation between the upper and lower mantle over geological history

discriminant function analysis of glass chemistry of new zealand and north american tephra deposits major trace and rare earth element analyses of volcanic glass are used separately or in combination for correlating quaternary tephras often by graphical or simple comparative methods we have taken a statistical approach using discriminant function analysis dfa to assess the relative discriminating power of the different elements in volcanic glasses from several tectonovolcanic provinces we found that major oxides are powerful discriminating variables for widespread tephras from the taupo volcanic zone in new zealand and here they can be more discriminating than trace elements a wide selection of tephras from the western united states can also be distinguished on major oxides alone particularly those from cascade range volcanoes for tephras from large intracontinental calderas such as long valley or yellowstone ree and trace elements are more effective at discriminating than major oxides however tephras erupted from the long valley area can be distinguished on major oxide composition by dfa despite their similar chemistry the selection and relative significance of different elements for discriminating tephras depends on the total data set being compared as well as the source volcano and the individual eruptive events caution must be exercised in the nonstatistical selection of compositional data for characterizing tephras dfa is a more powerful and objective tool for the comparison of tephra chemistry

diagenesis and crystal caskets echinoderm mg calcite transformation dry canyon new mexico u s a abstract the stereom of living echinus esculentus skeletons is composed of mg calcite interambulacral plates 6 mole mgco3 spines 4 mole mgco3 and is compared with the stereom of pennsylvanian echinoderms from the holder formation dry canyon some dry canyon echinoderms had their pore system 55 volume filled by magnesian ferroan calcite cement before the stereom transformed type 1 into small 20 m diameter in ossicles that underwent type 2 transformation indicates ionic transfer over tens of microns the margins of some type 2 ossicles set in a clay matrix are fe rich and contain barite crystals the fe2 and ba2 were transferred from the surrounding clay hundreds of microns into these ossicles type 1 transformation was catalyzed by internal water from the stereom the presence of a ferroan calcite crystal casket confined alteration to the skeletal mg calcite type 2 transformation involved both stereom and pore filling mg calcite cement the unstable dolomite crystals formed by both transformation processes were protected from further change by the crystal caskets in which they are encased the transformation of dry canyon echinoderms was probably triggered or accelerated by rising temperature caused by burial the bulk composition of stereom that has undergone type 1 transformation can be used as a proxy for the original major element chemistry of echinoderm skeletons minor amounts of mg calcite cement that occur in dry canon type 1 ossicles indicate the chemistry of porewater close to the sediment water interface fossil echinoderms are preserved in many ways and it is suggested that their textural variations can be used to map the distribution of diagenetic reactions the stereom is destroyed in most fossil echinoderms and even in those where the stereom shape is preserved its internal structure is changed echinoderm mg calcite does not transform with perfect textural preservation

mineralogical stabilization of high magnesium calcite geochemical evidence for intracrystal recrystallization within holocene porcellaneous foraminifera mineralogical stabilization of porcellaneous foraminifera is known to consist of chemical change without textural alteration at any scale however the nature of the alteration process has not been fully reconciled porcellaneous foraminifera from the freshwater diagenetic zone of two small islands in the schooner cays bahamas are in the midst of mg loss and delta 18 o change with no textural alteration at any scale these data indicate that the mineralogical stabilization process or recrystallization is a repetitive intracrystal incongruent dissolution precipitation reaction each recrystallization produces a calcite with a slightly lower mg content than its predecessor the rate of stabilization is dependent on time and hydrologic flux older phreatic zone material is the most altered and younger vadose zone material is the least altered numerical modeling of the chemical diagenesis indicates that the molar water rock ratio of a single recrystallization is mg between 0 0001 and 0 0003 these values are two orders of magnitude less than that generally assigned to calcite precipitation from a large fluid reservoir and they suggest that d mg is dependent on the type and scale of reaction though the nature of that dependency is unknown

paleozoic mg calcite preserved implications for the carboniferous ocean modern mg calcite echinoderm skeletons are thought to lose mg so rapidly that no pre pleistocene ossicles retain their original mg concentrations however carboniferous echinoderm skeletons from the imo formation of arkansas and the holder formation of new mexico are reported herein that are still composed of mg calcite 7 8 to 14 5 mol mgco 3 their sr concentration is also high 620 to 3710 ppm the pore system in these echinoderm skeletons is filled with ferroan calcite or siderite that has not grown in optical continuity with the skeleton the discovery of carboniferous mg calcite confirms an interpretation already made from their diagenetic replacements marine mg calcite has been associated with proposed icehouse conditions and although the carboniferous echinoderm skeletons are compatible with this as they formed during gondwana glaciation they are too few to be of significance also they are organic secretions and so can override secular mineral trends based on inorganic precipitates the carboniferous echinoderm skeletons produced in tropical waters have a concentration of mg similar to values for modern echinoderms the simplest explanation for this similarity is that both precipitated by similar processes from oceans of similar composition this does not support predictions based on mass balance modeling that the mg ca ratio of the carboniferous ocean was half that of today9s the sr content of carboniferous echinoderms is less than that for modern echinoderms leading to a prediction of 6 ppm sr concentration for the carboniferous ocean further examples of mg calcite are required to substantiate these interpretations

high temperature compression of ferropericlase and the effect of temperature on iron spin transition abstract high temperature compression experiments with in situ x ray diffraction of ferropericlase fp with a composition of mg 0 81 fe 0 19 o were made in a laser heated diamond anvil cell to pressures p of 116 gpa at a constant temperature t of 1600 1900 k room temperature experiments with a laser annealing technique were also carried out on the same material anomalous unit cell volume reductions that can be explained by the spin transition of ferrous iron were observed at p 63 96 gpa and 45 63 gpa at t 1600 1900 k and 300 k respectively indicating that the spin transition pressure interval expands with increasing temperature the observed density changes across this spin transition at t 1600 1900 k and 300 k are about 1 6 and 1 0 respectively indicating that the spin transition pressure interval expands with increasing temperature the thermal expansivity of fp is large in the mid lower mantle due to the effect of the spin transition in a peridotitic composition the spin transition in fp increases the rock density by 0 35 at the lowermost mantle conditions calculated densities show that both perovskitic and peridotitic mantle models may explain the prem lower mantle density however the peridotitic lower mantle model requires less assumption to satisfy the prem density and is more self consistent

new distribution coefficient for the incorporation of strontium into dolomite and its implications for the formation of ancient dolomites late tertiary dolomites form the bahamas exhibit a strong inverse correlation between sr and mol mgco sub 3 it is proposed that this relation is a result of a distribution coefficient dependent on the concentration of ca in dolomite because these dolomites precipitated from water with a sr sup 2 ca sup 2 ratio similar to that of seawater a distribution coefficient d sub sr sup dolomite of 0 0118 is calculated for sr in stoichiometric dolomite this coefficient increases by about 0 0039 for every additional mol caco sub 3 in nonstoichiometric dolomite these data have three important implication first the new d sub sr sup dolomite is substantially lower than previously suggested second a relation has been established among the sr sup 2 ca sup 2 ratio of dolomite forming waters the major element composition of dolomites and the sr content of dolomites allowing the determination of the sr sup 2 ca sup 2 ratio of fluids involved in the formation of ancient dolomites third some published data sets on dolomites show a similar relation between sr content and major element composition suggesting that these dolomites may also have formed from seawater other data sets deviate from the correlation defined by little bahama bank dolomites indicating either dolomitization in fluids other than seawater or changes ofmore the sr sup 2 ca sup 2 ratio of seawater through time less

carbon isotope excursions and crinoid dissolution at exposure surfaces in carbonates west texas u s a abstract carbonates from the horseshoe atoll reinecke field west texas u s a show four subaerial exposure events by the presence of rhizoliths alveolar septal fabric and thin calcretes that subdivide the pennsylvanian succession into five sequences vertical profiles of bulk rock 13c analyses show values that are generally 1 pdb but negative shifts of up to 2 occur beneath exposure surfaces the bulk rock 13c values 1 could be explained as a mixture of 63 marine carbonate 13c 4 and 37 pedogenic carbonate 13c 4 while a negative bulk rock excursion of 13c 1 can be explained as a mixture of 37 marine and 63 pedogenic calcite distinctive negative shifts in the 13c profiles do not occur under some exposure surfaces that contain soil generated features and one negative shift occurs that apparently is unrelated to an exposure surface bulk rock 13c values are difficult to interpret because the volume of their various components is unknown furthermore 13c profiles may miss subaerial exposure surfaces certain styles of crinoid alteration may indicate paleo subaerial exposure surfaces fossil crinoids can be preserved in a variety of ways but crinoids with dissolution holes that subsequently either become filled with calcite cement or were crushed during burial are restricted to 2 m below exposure surfaces dissolution of mg calcite crinoids is due to their relatively high mg composition 12 mole mgco3 combined with their location beneath soil capped surfaces from which meteoric waters greatly undersaturated with respect to calcite emerged late stage dissolution removed finely crystalline calcite but crinoids by this time had stabilized to coarsely crystalline calcite and dolomite and thus were unaffected evaluation of other stratigraphic systems is needed to determine whether the use of holey crinoids to identify subaerial exposure surfaces has broad application