the late devensian 22 000 bp irish sea basin the sedimentary record of a collapsed ice sheet margin the late devensian 20 ka bp glacial geology of the irish sea basin 4000 km2 is an event stratigraphy recording the entry of marine waters into a glacio isostatically depressed basin and the rapid retreat of the irish sea glacier as a tidewater ice margin marine limits occur up to 140 m o d across much of the central basin the ice margin was uncoupled from its bed exposing a subglacially scoured topography to glaciomarine processes the irish sea glacier was a major drainage conduit of the last british ice sheet calving of the marine ice margin resulted in fast flow surging of ice streams recorded by drumlin fields around the northern basin margin and tunnel valleys rapid evacuation of the basin may have stranded large areas of dead ice in peripheral zones e g cheshire shropshire lowlands and initiated the collapse of the ice sheet thick wedges of ice contact glaciomarine sediments were deposited during ice retreat as morainal bank complexes by successive tidewater ice margins stabilized at pinning points around the irish sea coast where morainal banks occur on the seaward side of drumlin swarms there is a clear sequential relationship between rapid ice loss from calving ice margins the development of fast flowing ice streams drumlinization and the pumping of subglacial sediment to tidewater raised delta complexes are locally associated with marine limits along the high relief coastal margins of wales east central ireland and the lake district associated valley infill complexes record downslope resedimentation of heterogenous sediments into the marine environment during ice retreat co eval offshore deposits are represented by well stratified glaciomarine complexes that infill a subglacially scoured topography that shows networks of tunnel valleys glaciomarine mud drapes occur well to the south of the maximum limit of grounded ice in the basin e g north devon scilly islands southern ireland the age of these distal sediments previously mapped as pre devensian tills is constrained by amino acid ratios basin rebound following deglaciation was rapid with over 100 m recovery in 3 ka and was followed by a low marine still stand peat accumulating in offshore areas now as much as 55 m below sea level has been drowned by the postglacial eustatic rise in sea level the glacio sedimentary model identified in this paper involving rapid ice retreat and related sedimentation triggered by rising relative sea level suggests that isotatic downwarping is an important mechanism for deglaciating continental shelves

high pressure phases in shock induced melt veins of the umbarger l6 chondrite constraints of shock pressure we report a previously undocumented set of high pressure minerals in shock induced melt veins of the umbarger l6 chondrite high pressure minerals were identified with transmission electron microscopy tem using selected area electron diffraction and energy dispersive x ray spectroscopy ringwoodite fa30 akimotoite en11fs89 and augite en42wo33fs25 were found in the silicate matrix of the melt vein representing the crystallization from a silicate melt during the shock pulse ringwoodite fa27 and hollandite structured plagioclase were also found as polycrystalline aggregates in the melt vein representing solid state transformation or melting with subsequent crystallization of entrained host rock fragments in the vein in addition fe2sio4 spinel fa66 fa99 and stishovite crystallized from a feo sio2 rich zone in the melt vein which formed by shock melting of feo sio2 rich material that had been altered and metasomatized before shock based on the pressure stabilities of the high pressure minerals ringwoodite akimotoite and ca clinopyroxene the melt vein crystallized at approximately 18 gpa the fe2sio4 spinel stishovite assemblage in the feo sio2 rich melts is consistent with crystallization of the melt vein matrix at the pressure up to 18 gpa the crystallization pressure of 18 gpa is much lower than the 4590 gpa pressure one would conclude from the s6 shock effects in melt veins stffler et al 1991 and somewhat less than the 25 30 gpa inferred from s5 shock effects schmitt 2000 found in the bulk rock

frictional melting processes and the generation of shock veins in terrestrial impact structures evidence from the steen river impact structure alberta canada abstract shock produced melt within crystalline basement rocks of the steen river impact structure sris are observed as thin 1 510 m wide interlocking networks of dark veins which cut across and displace host rock minerals solid state phase transformations such as ferro pargasite to an almandine andradite majorite garnet and amorphization of quartz and feldspar are observed in zones adjacent to comparatively wider 50 500 m sections of the shock veins shock pressure estimates based on the coupled substitution of na ti4 and si4 for divalent cations al3 and cr3 in garnet 14 19 gpa and the pressure required for plagioclase ab62 83 amorphization at elevated temperature 14 20 gpa are not appreciably different from those recorded by deformation effects observed in non veined regions of the bulk rock 14 20 gpa this spatial distribution is the result of an elevated temperature gradient experienced by host rock minerals in contact with larger volumes of impact generated melt and large deviatoric stresses experienced by minerals along vein margins micrometer size equant crystals of almandine pyrope majorite garnet define the shock vein matrix consistent with rapid quench 100 200 ms at 7 5 10 gpa crystallization of the vein occurred during a 0 1 0 15 s shock pressure pulse majoritic garnet formed during shock compression by solid state transformation of pargasite along shock vein margins is observed in tem bright field images as nanometer size gouge particles produced at strain rates in the supersonic field 106 108 these crystals are embedded in vesiculated glass and this texture is interpreted as continued movement and heating along slip planes during pressure release the deformation of high pressure minerals formed during shock compression may be the first evidence of oscillatory slip in natural shock veins which accounts for the production of friction melt via shear when little or no appreciable displacement is observed our observations of the mineralogy chemistry and microtextures of shock veins within crystalline rocks of the sris allow us to propose a model for shock vein formation by shear induced friction melting during shock compression

a uranium thorium dated late eemian or early midlandian organic deposit from near kilfenora between spa and fenit co kerry ireland this paper discusses the dating of stratigraphically important quaternary sequences from a site near fenit co kerry which have been the subject of debate since they were first described by mitchell in 1970 the overall stratigraphy of the quaternary deposits have been investigated and detailed analyses of the organic material carried out pollen from biogenic sediments have been analysed and samples of peat dated using the uranium thorium disequilibrium method the pollen assemblages match no others previously recorded in ireland and appear to represent a cool temperate phase following the last interglacial the uranium thorium dates of between 114000 and 123000 yr bp indicate that the deposit dates from oxygen isotope stage 5 possibly post dating the last temperate stage the eemian stage interglacial oxygen isotope substage 5e the dating of this deposit and the realisation that it is not penultimate temperate stage gortian in age invalidates much recent speculation on the age of the gortian interglacial

the distinct morphological and petrological features of shock melt veins in the suizhou l6 chondrite the morphology and petrology of distinct melt veins in the suizhou l6 chondrite have been investigated using scanning electron microscopy electron microprobe analyses and raman spectroscopy synchrotron energy dispersive diffraction and transmission electron microscopy it is found that the melt veins in the suizhou meteorite morphologically are the simplest straightest and thinnest among all shock veins known from meteorites at first glance these veins look like fine fractures but petrologically they are solid melt veins of chondritic composition and consist of fully crystalline materials of two distinct lithological assemblages with no glassy material remaining the suizhou melt veins contain the most abundant high pressure mineral species when compared with all other veins known in chondrites thus these veins in suizhou are classified as shock veins all rock forming and almost all accessory minerals in the suizhou shock veins have been transformed to their high pressure polymorphs and no fragments of the precursor minerals remain in the veins among the 11 high pressure mineral phases identified in the suizhou veins three are new high pressure minerals namely tuite after whitlockite xieite and the cf phase after chromite on the basis of transformation of plagioclase into maskelynite it is estimated that the suizhou meteorite experienced shock pressures andmore shock temperatures up to 22 gpa and 1000 c respectively shearing and friction along shock veins raised the temperature up to 1900 2000 c and the pressure up to 24 gpa within the veins hence phase transition and crystallization of high pressure minerals took place only in the suizhou shock veins fast cooling of the extremely thin shock veins is regarded as the main reason that up to 11 shock induced high pressure mineral phases could be preserved in these veins less

early diagenesis of shallow water periplatform carbonate sediments leeward margin great bahama bank ocean drilling program leg 166 mineralogic petrographic and geochemical analyses of sediments recovered from two leg 166 ocean drilling program cores on the western slope of great bahama bank 308 m and 437 m water depth are used to characterize early marine diagenesis of these shallow water periplatform carbonates the most pronounced diagenetic products are well lithified intervals found almost exclusively in glacial lowstand deposits and interpreted to have formed at or near the seafloor i e hardgrounds hardground cements are composed of high mg calcite 14 mol mgco 3 and exhibit textures typically associated with seafloor cementation geochemically hardgrounds are characterized by increased 18 o and mg contents and decreased 13 c sr and na contents relative to their less lithified counterparts despite being deposited in shallow waters that are supersaturated with the common carbonate minerals it is clear that these sediments are also undergoing shallow subsurface diagenesis calculation of saturation states shows that pore waters become undersaturated with aragonite within the upper 10 m at both sites dissolution and likely recrystallization of metastable carbonates is manifested by increases in interstitial water sr and sr ca profiles with depth we infer that the reduction in mineral saturation states and subsequent dissolution are being driven by the oxidation of organic matter in this fe poor carbonate system precipitation of burial diagenetic phases is indicated by the down core appearance of dolomite and corresponding decrease in interstitial water mg and the presence of low mg calcite cements observed in scanning electron microscope photomicrographs

groundwater flow beneath a hypersaline pond cluett key florida bay florida copyright c 1997 elsevier science b v all rights reserved florida bay carbonate mud islands are known to be sites of holocene diagenesis including dolomitization and the hydrology is an essential component of this process on cluett key a typical mud island in western florida bay groundwater salinities range to 145 beneath a shallow ephemeral pond which occupies 70 of the island a sharp hydrostratigraphic boundary separates low permeability unconsolidated carbonate mud from underlying highly permeable pleistocene limestone we report the results of a 2 year project aimed at determining the rates patterns and driving force of groundwater flow beneath the island water level measurements are used to demonstrate the presence of a large hydraulic head drive caused by the difference in elevations of the pond and florida bay and enhanced by the higher density of groundwaters in the carbonate mud compared with groundwater in the limestone the hydraulic head drive is essentially vertical because florida bay water levels are transmitted with little attenuation to the limestone underlying the island distributions of groundwater density and pore pressures are consistent with vertical groundwater flow based on an estimated vertical hydraulic conductivity of approximately 5 10 3 m day 1 vertical interstitial velocities are on the order of 25 cm year 1 with a residence time in the carbonate mud of approximately 15 years this velocity is very similar to that calculated independently from tritium concentrations in pore waters both horizontal and vertical density gradients exist in the carbonate mud these density variations induce circulations owing to vorticity and may lead to the formation of instability plumes reflux but dynamical scaling suggests that these motions are much slower than those induced by the dominant hydraulic drive buoyancy effects may however be dominant on other lower islands in florida bay where the hydraulic head drive is much smaller than on cluett key diffusion may blur sharp gradients in salt ca 2 or mg 2 but is not an important vehicle for wholesale movement of these species through the island cluett key shares some similarities with holocene carbonate atoll islands the two layer hydrostratigraphy and transmission of tidal signals under the island in contrast to atoll islands however groundwater flow on cluett key is governed by transmission of the mean sea level to the base of the holocene basal tidal fluctuations induce little mixing in the low permeability holocene sediments in addition the groundwater on cluett key is dominantly oceanic not meteoric and is consequently denser than underlying groundwater 1997 elsevier science b v

deglacial warming of the northeastern atlantic ocean correlation with the paleoclimatic evolution of the european continent abstract isotopic micropaleontologic and pollen analyses of deep sea cores from the bay of biscay and the northeastern atlantic ocean show that the deglacial warming of this oceanic area was closely correlated with the paleoclimatic evolution of the adjacent european continent temperatures were at least as warm as those of today in the bay of biscay between 13 300 and 11 000 b p coinciding with the combined bolling allerod warm continental events a major spread of polar water occurred between 11 000 and 10 000 b p during this event which coincides with the younger dryas continental cold event marine temperatures were almost as low as those of the last glacial maximum the final deglacial warming of the norteastern atlatntic ocean occurred during the following 3000 yr

fractional crystallization of olivine melt inclusion in shock induced chondritic melt vein abstract the formation of ringwoodite wadsleyite and majorite from their parental low pressure polymorphs in melt veins in chondritic meteorites is usually interpreted to be the result of shock induced solid state phase transformation formation and survival of individual mineral melt enclaves in the chondritic high pressure melt was not considered a viable possibility we report evidence for melting of individual large olivine fragments entrained in melt veins their survival as melt enclaves in the chondritic melts and their subsequent fractional crystallization at high pressures and temperatures the fractionally crystallized olivine melt enclaves appear to be ubiquitous in chondrites in contrast ca poor pyroxene fragments in the same veins and ca poor pyroxene in chondrules entrained do not show any sign of melting texture and compositions of olivine fragments are indicative of fractional crystallization from individual olivine melts alone fragments of original unzoned olivine fa 24 26 melted and melts subsequently fractionally crystallized to mg rich wadsleyite fa 6 10 and mg poor ringwoodite fa 28 33 with a compositional gap of 26 mol fayalite in contrast compositions of ringwoodite and wadsleyite that emerged from solid sate phase transformations are identical to that of parental olivine thus erasing any source of enigma the olivine monomineralic melts barely show any signs of mixing with the chondritic liquid prior to or during their individual fractional crystallization our findings demonstrate that the formation of high pressure minerals during shock events in asteroids also results from melting and fractional crystallization from some individual mineral melts that barely mixed with the chondritic melt host a mechanism previously not recognized or accepted

metal silicate interaction in quenched shock induced melt of the tenham l6 chondrite abstract the metal silicate microstructures in the shock induced melt pockets of the tenham l6 chondrite have been investigated by analytical transmission electron microscopy the melt areas formed under high pressure high temperature dynamic shock conditions consist of spherical fe ni metal iron sulfide globules embedded in a silicate glass matrix showing that the melt was quenched at high cooling rate the fe ni fraction in the globules is two phase composed of a bcc phase 5 wt ni and an fcc phase 49 wt ni indicating that fractional crystallisation of the metal occurred during the fast cooling the metal fraction also contains appreciable amounts of non siderophile elements mostly si mg and o suggesting that these elements were trapped in the metal either as alloying components or as tiny silicate or oxide inclusions in the iron sulfide fraction the na content is high 3 wt suggesting chalcophile behaviour for na during the shock event the composition of the silicate glass reflects non equilibrium melting of several silicate phases olivine pyroxene and plagioclase moreover the feo content is high compared to the feo contents of the unmelted silicates some fe redistribution took place between metal and silicate liquids during the shock event the silicate glass also contains tiny iron sulfide precipitates which most probably originated by exsolution during quench suggesting that the molten silicate retained significant amounts of s dissolved at high temperature and high pressure based on these observations we suggest that non equilibrium phenomena may be important in determining the compositions of metal and silicate reservoirs during their differentiation

transformation textures mechanisms of formation of high pressure minerals in shock melt veins of l6 chondrites and pressure temperature conditions of the shock events the high pressure polymorphs of olivine pyroxene and plagioclase in or adjacent to shock melt veins smvs in two l6 chondrites sahara 98222 and yamato 74445 were investigated to clarify the related transformation mechanisms and to estimate the pressure temperature conditions of the shock events wadsleyite and jadeite were identified in sahara 98222 wadsleyite ringwoodite majorite akimotoite jadeite and lingunite naalsi3o8 hollandite were identified in yamato 74445 wadsleyite nucleated along the grain boundaries and fractures of original olivine the nucleation and growth of ringwoodite occurred along the grain boundaries of original olivine and as intracrystalline ringwoodite lamellae within original olivine the nucleation and growth of majorite took place along the grain boundaries or fractures in original enstatite jadeite containing assemblages have complicated textures containing particle like stringer like and polycrystalline like phases coexistence of lingunite and jadeite containing assemblages shows a vein like texture we discuss these transformation mechanisms based on our textural observations and chemical composition analyses the shock pressure and temperature conditions in the smvs of these meteorites were also estimated based on the mineral assemblages in the smvs and in comparison with static high pressure experimental results as follows 13 16 gpa 1900 c for sahara 98222 and 17 24 gpa 2100 c for yamato 74445

ahrensite γ fe 2 sio 4 a new shock metamorphic mineral from the tissint meteorite implications for the tissint shock event on mars ahrensite ima 2013 028 fe 2sio 4 is the natural fe analog of the silicate spinel ringwoodite mg 2sio 4 it occurs in the tissint martian meteorite where it forms through the transformation of the fayalite rich rims of olivine megacrysts or fe rich microphenocrysts in contact with shock melt pockets the typical sequence of phase assemblages traversing across a tissint melt pocket into olivine is quenched melt or fayalite pigeonite intergrowth bridgmanite wustite ahrensite and or ringwoodite highly deformed olivine nanocrystalline ringwoodite deformed olivine we report the first comprehensive set of crystallographic spectroscopic and quantitative chemical analysis of type ahrensite and show that concentrations of ferric iron and inversion in the type material of this newly approved mineral are negligible we also report the occurrence of nanocrystalline ringwoodite in strained olivine and establish correlations between grain size and distance from melt pockets the ahrensite and ringwoodite crystals show no preferred orientation consistent with random nucleation and incoherent growth within a highly strained matrix of olivine grain sizes of ahrensite immediately adjacent to melt pockets are consistent with growth during a shock of moderate duration 1 10 ms

high pressure phases in a shock induced melt vein of the tenham l6 chondrite constraints on shock pressure and duration abstract the microtexture and mineralogy of a 580 m wide melt vein in the tenham l6 chondrite were investigated using field emission scanning electron microscopy and transmission electron microscopy to better understand the shock conditions the melt vein consists of a matrix of silicate plus metal sulfide grains that crystallized from immiscible melts and sub rounded fragments of the host chondrite that have been entrained in the melt and transformed to polycrystalline high pressure silicates the melt vein matrix contains two distinct textures and mineral assemblages corresponding to the vein edge and interior the 30 m wide vein edge consists of vitrified silicate perovskite ringwoodite akimotoite majorite with minor metal sulfide the 520 m wide vein interior consists of majorite magnesiowustite with irregular metal sulfide blebs although these mineral assemblages are distinctly different the pressure stabilities of both assemblages are consistent with crystallization from similar pressure conditions the melt vein edge crystallized at about 23 25 gpa and the vein interior crystallized at about 21 25 gpa this relatively narrow pressure range suggests that the melt vein either crystallized at a constant equilibrium shock pressure of 25 gpa or during a relatively slow pressure release using a finite element heat transfer program to model the thermal history of this melt vein during shock we estimate that the time required to quench this 580 m wide vein was 40 ms because the entire vein contains high pressure minerals that crystallized from the melt the shock pressure duration was at least 40 ms using a synthetic hugoniot for tenham and assuming that the sample experienced a peak shock pressure of 25 gpa near the impact site we estimate that the tenham parent body experienced an impact with collision velocity 2 km s based on a one dimensional planar impact model we estimate that the projectile size was 150 m in thickness

controls on sonic velocity in carbonates compressional and shear wave velocities v p and v s of 210 minicores of carbonates from different areas and ages were measured under variable confining and pore fluid pressures the lithologies of the samples range from unconsolidated carbonate mud to completely lithified limestones the velocity measurements enable us to relate velocity variations in carbonates to factors such as mineralogy porosity pore types and density and to quantify the velocity effects of compaction and other diagenetic alterations

cenozoic dolomites of carbonate islands their attributes and origin abstract dolomites found on and below carbonate islands atolls and oceanic platforms provide useful insights into the origin of dolomite insights that may not be attainable from the study of more ancient cratonic dolomites for this reason they have been the subject of study for decades a critical mass of case studies now exist and some significant conclusions can be drawn from the cumulative data in most cases the association with an island is causal and not genetic nevertheless these dolomite occurrences are referred to herein as island dolomites one type of dolomite on carbonate islands is penecontemporaneous dolomite a phase that forms while the host sediment is in its original depositional setting island examples are holocene in age occur in holocene sediments and originate as a direct precipitate from either normal or evaporated seawater these are microcrystalline poorly ordered ca rich 18o enriched and geochemically unstable phases that are susceptible to recrystallization in the setting in which they formed post depositional dolomite is the other type of dolomite associated with carbonate islands this type of dolomite replaces older precursors and forms as cement distinctive characteristics include a dominance of fabric preserving texture pore lining cement rims that may exhibit micron scale banding with low mg calcite and formation in association with precursor dissolution sr isotopic dating indicates that all examples are neogene or quaternary in age those formed during the middle miocene through pliocene are massive laterally continuous and often multigenerational in contrast younger examples tend to be localized partial replacements of a single generation sr isotope ages also suggest global synchroneity in many dolomitization events which suggests a connection between dolomitization global eustacy and or global climatic factors geochemical attributes of post depositional island dolomites are ca enrichment positive 18o and 13c low sr contents 150 300 ppm and low fe global similarity in petrography and geochemistry of replacive island dolomites argues for a similar origin inferred origins however depend primarily on how 18o and sr data are interpreted values for 18o and dsr must be assumed due to uncertainties in oxygen isotope fractionation and sr partitioning there is no consensus or uniformity in those assumptions thus interpretations can vary and be biased to a desired result covariant trends in 13c and 18o some negative 13c values and high sr all favor a mixing zone origin but examples with these attributes are few lack of covariance in the isotopes mean 18o of 2 0 to 3 5 and low sr future studies should standardize the type of data collected and the analytical techniques employed multiple geochemical attributes should be measured on microsampled components and quantitative modeling should be employed in order to constraint interpretations as much as possible also needed are a better understanding of the kinetic processes that form these dolomites more careful assessment of their recrystallization status and an improved understanding of dsr and 18o at low temperature

natural occurrence of fe2sio4 spinel in the shocked umbarger l6 chondrite here we report the first natural occurrence of fe 2 sio 4 spinel in a shock induced melt pocket of the umbarger l6 chondrite optical microscopy scanning electron microscopy electron microprobe analysis and analytical transmission electron microscopy were used to examine the sample fe 2 sio 4 spinel was identified by tem using selected area electron diffraction and energy dispersive x ray spectroscopy the symmetry of the diffraction patterns the ratios of d spacings and interplanar angles are consistent with the spinel structure however the cell parameter of fe 2 sio 4 spinel 8 52 a calculated from d spacing data is 3 5 larger than that of synthetic fe 2 sio 4 spinel 8 235 a chemical analyses of the spinel show olivine stoichiometry with fe fe mg ratios ranging from 0 62 to 0 99 fe 2 sio 4 spinel and stishovite occur within feo sio 2 rich zones in the melt pocket surrounded by sio 2 rich glass and fe rich phyllosilicates fe 2 sio 4 spinel plus stishovite also occur with other high pressure minerals in the melt pocket ringwoodite akimotoite augite and hollandite structured plagioclase we infer that the fe 2 sio 4 spinel crystallized from a zone of feo sio 2 rich melt within the shock induced melt pocket two models for feo sio 2 rich melt are discussed it was either a residual melt after crystallization of mgo rich silicates in a chondritic melt pocket or it was produced by shock melting of feo sio 2 rich material

ca rich majorite derived from high temperature melt and thermally stressed hornblende in shock veins of crustal rocks from the ries impact crater germany shock veins up to 1 1 mm thick were found within non porous lithic clasts from suevite breccia of the nordlinger ries impact structure these veins were studied by optical microscopy in transmitted and reflected light and by scanning electron microscopy in shocked amphibolites two types of ca rich majorite occur within and adjacent to the veins the first type crystallized from shock induced melts within the veins si contents of these majorites suggest dynamic pressure of 15 17 gpa implying minimum temperatures in the range of 2 150 2 230 c the second type of majorite was formed adjacent to the shock veins within pargasitic hornblende this majorite contains significant amounts of h2o 0 7 0 9 wt based on the textural setting the shrinkage cracks and the chemical compositions of both phases a solid state mechanism is deduced for the hornblende to majorite phase transition both genetic types of ca rich majorite are described for the first time from a terrestrial impact crater along with stishovite majorite constitutes the second silicate mineral displaying sixfold coordination of si at ries using micro raman spectroscopy jadeite coesite and jadeite grossular were identified within local melt glasses of alkali feldspar and plagioclase composition respectively stishovite aggregates produced by solid state reaction along with shock induced high pressure melt glasses of almandine composition were also detected in shock veins of a garnet cordierite sillimanite restite the quenched homogeneous almandine glasses point to melting temperatures of more than 2 500 c for the veins our findings demonstrate that terrestrial shock veins can give valuable information on shock induced mineral transformations and transient high pressures of host rocks during a natural impact event

high pressure polymorphs in yamato 790729 l6 chondrite and their significance for collisional conditions shock pressure recorded in yamato y 790729 classified as l6 type ordinary chondrite was evaluated based on high pressure polymorph assemblages and cathodoluminescence cl spectra of maskelynite the host rock of y 790729 consists mainly of olivine low ca pyroxene plagioclase metallic fe ni and iron sulfide with minor amounts of phosphate and chromite a shock melt vein was observed in the hostrock ringwoodite majorite akimotoite lingunite tuite and xieite occurred in and around the shock melt vein the shock pressure in the shock melt vein is about 14 23 gpa based on the phase equilibrium diagrams of high pressure polymorphs some plagioclase portions in the host rock occurred as maskelynite sixteen different cl spectra of maskelynite portions were deconvolved using three assigned emission components centered at 2 95 3 26 and 3 88 ev the intensity of emission component at 2 95 ev was selected as a calibrated barometer to estimate shock pressure and the results indicate pressures of about 11 19 gpa the difference in pressure between the shock melt vein and host rock might suggest heterogeneous shock conditions assuming an average shock pressure of 18 gpa the impact velocity of the parent body of y 790729 is calculated to be 1 90 km s 1 the parent body would be at least 10 km in size based on the incoherent formation mechanism of ringwoodite in y 790729

diagenetic processes in holocene carbonate sediments florida bay mudbanks and islands the sedimentology mineralogy and pore fluid chemistry of seven cores from the holocene sediments of florida bay were studied to determine the physical processes and diagenetic reactions affecting the sediments the cores were taken in a transect from a shallow mudbank onto a small adjacent island jimmy key steady state models of pore fluid chemistry are used to estimate the rates of various reactions in the mudbank sediments little carbonate mineral diagenesis is taking place no change in sediment mineralogy is detectable and pore water profiles of ca2 mg2 and sr2 show only minor variation chloride concentrations indicate substantial biological mixing of seawater from the bay into the sediments in one of the cores pore water analyses of sulphate and alkalinity show only a low degree of sulphate depletion and a decreasing extent of sulphate reduction downcore models of sulphate reduction in the mudbank show that there is substantial chemical exchange between the sediment pore fluids and water from the bay probably as a result of bio irrigation the sulphate and alkalinity data also suggest that the underlying pleistocene rocks contain water of near normal seawater composition stratigraphic analysis and 13c analyses of the organic carbon in the sediments of the island cores show that the sediments were primarily deposited in a subtidal mudbank setting only the upper 20 30 cm is supratidal in origin nevertheless island formation had a significant effect on pore fluid chemistry and the types of diagenetic reactions throughout the sediment column chloride in the sediment pore fluids is more than twice the normal seawater concentrations over most of the depth of the cores the constant elevated chloride concentrations indicate that hypersaline fluids which formed in ponds on the island are advected downward through the sediments models of the chloride profiles yield an estimate of 2 5 cm yr 1 as a minimum advective velocity changes in pore water chemistry with depth are interpreted as indicating the following sequence of reactions 1 minor high mg calcite dissolution and low mg calcite precipitation from 0 to 35 cm 2 ca or mg sulphate dissolution and low mg calcite precipitation from 5 to 35 cm 3 dolomite or magnesite precipitation together with sulphate reduction from 35 to 55 cm and 4 little reaction below 55 cm in addition one or more as yet unidentified reactions must be taking place from 5 to 55 cm depth as an imbalance in possible sources and sinks of alkalinity is observed the imbalance could be explained if chloride is not completely conservative despite the pore fluid chemical evidence for diagenetic reactions involving carbonate minerals no changes in sediment mineralogy were detected in x ray diffraction analyses probably because of the comparatively young age of the island

shock induced p t conditions and formation mechanism of akimotoite pyroxene glass assemblages in the grove mountains grv 052082 l6 meteorite akimotoite mg fe sio 3 ilmenite was encountered in shock induced melt veins of grove mountains grv 052082 a highly equilibrated low iron ordinary chondritic meteorite l6 coexistence of ringwoodite majorite and majorite pyrope solid solution indicates the shock pressure at 18 23 gpa and temperature of 2000 2300 c during the natural dynamic event most low ca pyroxene clasts entrained in the melt veins have been partially or entirely transformed into akimotoite pyroxene glass assemblages which contain micrometer sized areas with various brightness in the backscattered electron images different from the chemically homogeneous grains in the host rock fs 20 5 21 3 the transmission electron microscopy study of a focused ion beam fib slice from the heterogeneous areas shows that the assemblages are composed of feo depleted and heterogeneous akimotoite fs 6 19 crystals 100 nm up to 400 nm in size scattered in feo enriched and relatively homogeneous pyroxene glass fs 31 39 all analyses of the akimotoite pyroxene glass assemblages plot on a fractionation line in feo mgo diagram with the host rock pyroxene at the middle between the compositions of feo depleted akimotoite and the feo enriched pyroxene glass these observations are different from previous reports of almost identical compositions of akimotoite bridgmanite mg fe sio 3 perovskite or pyroxene glass to the host rock pyroxene chen et al 2004 ferroir et al 2008 ohtani et al 2004 tomioka and fujino 1997 which is consistent with solid state transformation from pyroxene to akimotoite and preexisting bridgmanite that could be vitrified the observed fractionation trend and the granular shapes of akimotoite suggest crystallization from liquid produced by shock melting of the host rock pyroxene and the pyroxene glass matrix was probably quenched from the residual melt however this interpretation is inconsistent with the static experiments that expect crystallization of majorite mg fe sio 3 garnet instead of akimotoite from pyroxene liquid sawamoto 1987 our discovery raises the issue on formation mechanisms of the high pressure polymorphs of pyroxene and places additional constraints on the post shock high pressure and high temperature conditions of asteroids

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

geochemical indicators of depositional and early diagenetic facies in holocene carbonate muds and their preservation potential during stabilisation holocene carbonate muds 63 m fraction and macro organic matter were collected from marine subtidal mudbanks and supralittoral mud islands of florida bay and freshwater marshes of the florida everglades u s a geochemistry sedimentology and mineralogy were used to determine whether carbonate mud formation in specific marine and freshwater environments was geochemically distinct and to assess the importance of early diagenetic reactions on mud chemistry and mineralogy significant variations were found in the relative amounts of carbonate minerals not just between the marine and freshwater environments but also within florida bay in particular mg calcite is enriched in crane key muds by 10 relative to nearby cross bank the mg calcite appears to precipitate in the surface living cyanobacterial mats of the key and seems to be linked to mg chelation by the cyanobacterial organic matter this mg calcite has a distinct 13c around 4 6 lighter than mg calcite derived from thalassia epibionts in the cross bank muds dolomite was found just below the cyanobacterial mats forming in the only part of the core where reactive organic carbon has been completely removed by sulphate reducing bacteria the 13c values of this dolomite at least 4 are consistent with previously published 13c values of dissolved inorganic carbon in pore waters at this horizon which support dolomite formation in situ mg supply for dolomite formation may be from dissolution of the overlying cyanobacterial mat mg calcite the various types of organic matter have distinctive 13c values ranging from 25 mangrove and sawgrass to 10 cyanobacterial mats and 8 thalassia seagrass values intermediate between these end members record mixing of the organic matter types which shows particularly the time when the bank muds became dominated by thalassia organic matter i e were relatively removed from mangrove vegetation five holocene mud suites bank key cyanobacterial mats mangrove swamp and freshwater everglades swamp show distinct chemical signatures in their mean 13c 18o mgco3 and sr values na distinguishes between seawater and non seawater influenced environments but fe mn and p are mainly controlled by non carbonate materials comparison with published stable isotope and elemental data for pleistocene limestones in the south florida area suggests that only the facies diagnostic signatures of 13c and mg will be resolvable once the muds have been stabilised to micrite alteration of the 18o and sr values during stabilisation will probably be more severe to the extent that original facies variations are destroyed

majoritic garnet grains within shock induced melt veins in amphibolites from the ries impact crater suggest ultrahigh crystallization pressures between 18 and 9 gpa shock induced melt veins in amphibolites from the nordlinger ries often have chemical compositions that are similar to bulk rock i e basaltic but there are other veins that are confined to chlorite rich cracks that formed before the impact and these are poor in ca and na majoritic garnets within the shock veins show a broad chemical variation between three endmembers 1 text viii text m 2 3 text vi text al 2 text iv text sio 4 3 normal garnet grt 2 text viii text m 2 3 text vi text m 2 text si ti text iv text sio 4 3 majorite maj and 3 text viii text na text m 2 2 text vi text si ti text al text iv text sio 4 3 na majorite50grt50 whereby m2 mg2 fe2 mn2 ca2 in particular we observed a broad variation in vi si ti which ranges from 0 12 to 0 58 cations per formula unit cpfu all these majoritic garnets crystallized during shock pressure release at different ultrahigh pressures those with high vi si ti 0 36 0 58 cpfu formed at high pressures and temperatures from amphibole rich melts while majoritic garnets with lower vi si ti of 0 12 0 27 cpfu formed at lower pressures and temperatures from chlorite rich melts furthermore majoritic garnets with intermediate values of vi si ti 0 24 0 39 crystallized from melts with intermediate contents of ca and na to the best of our knowledge the morb type ca na rich majoritic garnets with maximum contents of 2 99 wt na2o and calculated crystallisation pressures of 16 18 gpa are the most extreme representatives ever found in terrestrial shocked materials at the ries the duration of the initial contact and compression stage at the central location of impact is estimated to only 0 1 s we used a 200 m thick shock induced vein in a moderately shocked amphibolite to model its pressure temperature time p t t path the graphic model manifests a peak temperature of 2600 c for the vein continuum pressure lasting for 0 02 s a quench duration of 0 02 s and a shock pulse of 0 038 s the small difference between the continuum pressure and the pressure of majoritic garnet crystallization underlines the usefulness of applying crystallisation pressures of majoritic garnets from metabasites for calculation of dynamic shock pressures of host rocks majoritic garnets of chlorite provenance however are not suitable for the determination of continuum pressure since they crystallized relatively late during shock release an extraordinary glass and majorite bearing amphibole fragment in a shock vein of one amphibolite documents the whole unloading path

a shock produced mg fe sio3 glass in the suizhou meteorite ovoid grains consisting of glass of stoichiometric mg fe sio3 composition that is intimately associated with majorite were identified in the shock veins of the suizhou meteorite the glass is surrounded by a thick rim of polycrystalline majorite and is identical in composition to the parental low ca pyroxene and majorite these ovoid grains are surrounded by a fine grained matrix composed of majorite pyrope garnet ringwoodite magnesiowstite metal and troilite this study strongly suggests that some precursor pyroxene grains inside the shock veins were transformed to perovskite within the pyroxene due to a relatively low temperature while at the rim region pyroxene grains transformed to majorite due to a higher temperature after pressure release perovskite vitrified at post shock temperature the existence of vitrified perovskite indicates that the peak pressure in the shock veins exceeds 23 gpa the post shock temperature in the meteorite could have been above 477 c this study indicates that the occurrence of high pressure minerals in the shock veins could not be used as a ubiquitous criterion for evaluating the shock stage of meteorites

shock metamorphism of ordinary chondrites a revised petrographic classification of progressive stages of shock metamorphism of ordinary chondrites is proposed six stages of shock s1 to s6 are defined based on shock effects in olivine and plagioclase as recognized by thin section microscopy the characteristic shock effects of each shock stage are s1 unshocked sharp optical extinction of olivine s2 very weakly shocked undulatory extinction of olivine s3 weakly shocked planar fractures in olivine s4 moderately shocked mosaicism in olivine s5 strongly shocked isotropization of plagioclase maskelynite and planar deformation features in olivine and s6 very strongly shocked recrystallization of olivine sometimes combined with phase transformations ringwoodite and or phases produced by dissociation reactions s6 effects are always restricted to regions adjacent to melted portions of a sample which is otherwise only strongly shocked in stages s3 to s6 localized melting results from stress and temperature peaks which locally deviate from the equilibration shock pressure due to differences in shock impedance these melting effects are a opaque melt veins shock veins b melt pockets with interconnecting melt veins c melt dikes and d troilite metal deposits in fractures based on a critical evaluation of data from shock recovery experiments a shock pressure calibration for the six shock stages is proposed which defines the s1 s2 s2 s3 s3 s4 s4 s5 and s5 s6 transitions at 5 5 10 15 20 30 35 and 45 55 gpa respectively whole rock melting and formation of impact melt rocks or melt breccias occurs at about 75 90 gpa the symbol for the shock stage may be used in combination with the symbol for the petrologic type to abbreviate the complete classification of a chondrite e g h5 s3 we propose this new shock classification and pressure calibration system to replace previous systems which are out of date with respect to the pressure calibration nominally restricted to l chondrites and based on incomplete and in part illdefined sets of shock effects we have classified seventy six ordinary chondrites using the new classification system and conclude the following 1 1 shock effects and the sequence of progressively increasing degrees of shock metamorphism are very similar in h l and ll groups differences in the frequency distribution of shock stages are relatively minor e g l chondrites appear to have the largest fraction with stages s5 and s6 this suggests that the collisional histories of the h l and ll parent bodies were similar 2 2 petrologic type 3 chondrites are deficient in stages s4 to s6 and with increasing petrologic type the frequency of stages s4 to s6 increases we suggest that the more porous and volatile rich type 3 chondrites are subject to melting at a lower shock pressure than the nonporous chondrites of higher petrologic type volatiles trapped in pores cause shock induced dispersal of the shocked and melted material into small particles which are not expected to survive as meteorites 3 3 stage s3 is the most abundant in nearly all petrologic types 4 4 at shock pressures in excess of about 35 gpa s5 and s6 4he and 40ar are almost completely lost pressures below 10 gpa s1 and s2 do not cause noble gas losses

jadeite formation in shocked ordinary chondrites abstract albitic feldspar in shocked ordinary chondrites yamato 791384 l6 and yamato 75100 h6 and albite recovered from static high pressure and high temperature synthetic experiments kubo et al 2010 were investigated with a transmission electron microscope tem subsequent to a conventional micro raman spectroscopy analysis to clarify albite dissociation reaction under high pressure and high temperature condition when jadeite forms from albite sio 2 phase as a residual phase of albite dissociation reaction should accompany jadeite from the stoichiometry however albitic feldspar in and adjacent to shock melt veins of the shocked chondrites dissociates into jadeite residual amorphous or poorly crystallized material having varied chemical compositions between jadeite and sio 2 phase tem observations of albitic feldspar in the shocked chondrites and albite recovered from the static high pressure and high temperature synthetic experiments show that jadeite crystallization is initiated by grain refinement of albite or albitic feldspar nucleation occurs along grain boundaries or at triple junctions of the fine grained albite crystal assemblage jadeite crystal starts to grow from the nucleus through grain boundary diffusion considering pressure condition recorded in the shock melt veins of the shocked chondrites stishovite is the most likely as a residual sio 2 phase accompanying jadeite high pressure and high temperature condition induced by a dynamic event is very short stishovite would be hardy formed through a dynamic event due to sluggish nucleation rate of stishovite compared with that of jadeite thus leading to induce heterogeneous and incomplete albite dissociation reaction albite dissociates into jadeite residual amorphous material

shock features in iron nickel metal and troilite of l group ordinary chondrites shock metamorphic features in opaque minerals feni metal and troilite of 22 l chondrites have been studied petrographically and geochemically in an attempt to establish a connection between the present silicate based shock classification scheme stoffler et al 1991 and the peak shock and postshock thermal history recorded in these minerals unshocked to weakly shocked s1 s3 l chondrites contain feni metal and troilite that display textures related to normal slow cooling they may also contain rare disequilibrium shock features which suggest localized departures from equilibrium shock conditions above shock stage s3 selected melting of feni metal and troilite produces melt droplets whose composition and abundance correspond to the maximum equilibrium shock state achieved by the sample at these higher shock levels the abundance of other shock induced features such as polycrystalline kamacite sheared and fizzed troilite coarse grained pearlitic plessite polycrystalline troilite and polymineralic melt veins serve as textural criteria that can be used to establish peak shock conditions minimum postshock temperatures obtained from analyses of plessite components show a systematic increase in temperature with an increase in shock stage thereby providing additional information about the postshock thermal histories of l chondrites at the highest shock levels recorded in l chondrites s6 and above melting and chemical homogenization of feni metal produces flattened ni profiles that may partially to completely obscure any evidence for an earlier slow cooling history all of these features serve as aids for shock classifying l chondrites as well as for quantifying minimum peak temperatures that resulted during shock metamorphism

from olivine to ringwoodite a tem study of a complex process the study of shock metamorphism of olivine might help to constrain impact events in the history of meteorites although shock features in olivine are well known so far there are processes that are not yet completely understood in shock veins olivine clasts with a complex structure with a ringwoodite rim and a dense network of lamellae of unidentified nature in the core have been reported in the literature a highly shocked s5 6 l6 meteorite asuka 09584 which was recently collected in antarctica by a belgian japanese joint expedition contains this type of shocked olivine clasts and has been therefore selected for detailed investigations of these features by transmission electron microscopy tem petrographic geochemical and crystallographic studies showed that the rim of these shocked clasts consists of an aggregate of nanocrystals of ringwoodite with lower mg fe ratio than the unshocked olivine the clast s core consists of an aggregate of iso oriented grains of olivine and wadsleyite with higher mg fe ratio than the unshocked olivine this aggregate is crosscut by veinlets of nanocrystals of olivine with extremely low mg fe ratio the formation of the ringwoodite rim is likely due to solid state diffusion controlled transformation from olivine under high temperature conditions the aggregate of iso oriented olivine and wadsleyite crystals is interpreted to have formed also by a solid state process likely by coherent intracrystalline nucleation following the compression shock release is believed to have caused opening of cracks and fractures in olivine and formation of olivine melt which has lately crystallized under postshock equilibrium pressure conditions as olivine

a method for the determination of organic carbon in calcareous marine sediments abstract methods for the determination of organic and inorganic carbon in marine sediments are discussed the suggested method consists of determination of organic carbon in a chn analyzer microcombustion at 1100 c after acidification of the sample with 1 n hydrochloric acid for removal of inorganic carbon a correction factor for weight increase after acidification due to conversion of carbonate to chloride and to hydration is applied our method as a c v of 1 7 organic carbon cannot be determined by subtracting separately measured inorganic carbon from the total carbon content

quaternary deposits and glacial stratigraphy in ireland quaternary stratigraphic studies in ireland have been based largely on lithostratigraphy the division of complex sedimentary sequences into the traditional framework of munsterian and midlandian events has created a series of stratigraphic scenarios which cannot be fully substantiated either by biostratigraphy or by facies basin analysis many of the current problems of glacial stratigraphy in ireland therefore arise directly from the field procedures used and the frameworks used to subdivide sedimentary sequences these problems are centred on the selection of type sites the subdivision of sedimentary sequences into unnatural divisions the concepts of direct and indirect stratigraphy classical models of climatically determined ice advance retreat cycles and the genetic inferences of the term till the conceptual basis of various stratigraphic schemes has changed little since the work of farrington 1934 1966 it is argued that a more realistic approach to description and assessment of glacigenic sediments should be based on basinal facies analysis and must include a documentation of the composition and geometry of three dimensional facies variations palaeoenvironmental interpretations of complex glacigenic sequences especially in coastal locations should take account of the influences of marine ice sheets on the sedimentary record in the past most sequences were interpreted in terms of glacioterrestrial models a large body of sedimentary evidence now indicates that shallow glaciomarine environments occurred along the margins of the last ice sheet during its maximum and subsequent retreat phases the locations and patterns of associated icemarginal systems indicate that the magnitude of and patterns of crustal deformation were greater and more complex than formerly realised thus the primary control on sea level variation at successive ice sheet margins was largely a function of glacioisostatic disequilibrium although complex glacial and glacially related sequences are known it is argued here that most of the multiple glacigenic sequences in ireland can be accommodated within the midlandian cold stage the basal units from certain sequences and erratic fans have been attributed to events earlier than the midlandian however it is difficult to compare these traces of earlier events from different sectors of the island due to poor dating correlation and the fact that direct and indirect stratigraphy is not adequately defined or separated the age of the gortian style interglacial sequences the implications of their floristic composition and their relationships to drift stratigraphy remains a major problem in stratigraphic constructions of the irish quaternary the stratigraphic significance of many till units must remain largely unknown until these matters are resolved at present it is not clear if the gortian sequences represent more than one interglacial a series of exposures in ulster provide evidence for major environmental changes during the midlandian cold stage these include an early stadial an early forest biozone 48 1 ka bp an early herb biozone 41 5 ka bp a middle interstadial phase of cold but non glacial aspect 30 5 ka bp a late stadial max 24 22 ka bp and a late glacial climatic oscillation the late midlandian stadial comprises a major glaciation that almost covered the island and a later more restricted phase when drumlins formed in the northern and western parts of the island max 17 ka bp

the pressures and temperatures of meteorite impact evidence from micro raman mapping of mineral phases in the strongly shocked taiban ordinary chondrite taiban is a heavily shocked l6 chondrite showing opaque melt veins raman spectroscopy was used to characterize the high pressure silicate assemblages in a thin section crossed by a shock created 4 mm wide melt vein raman spectra using different excitation wavelengths allowed identification of mineral phases such as olivine wadsleyite ringwoodite high ca clinopyroxene majorite pyrope jadeite maskelynite and lingunite olivine is fe depleted in contact with the ringwoodite which suggests chemical fractionation during a solid state olivine ringwoodite transformation raman imaging revealed a close correlation between the blue ringwoodite color and the peak observed at 877 cm 1 this signal shows strong near resonance raman enhancement when measured with near ir excitation lines 785 and 830 nm close to the optical absorption bands of the ringwoodite we propose that the blue color of the ringwoodite is due to a small amount of iron in fourfold coordination inside the spinel structure and that yields the observed spectral features in differently colored ringwoodite under the formation conditions of the studied silicate pocket all enstatite transformed to a majorite pyrope solid solution whereas the high ca clinopyroxene likely remained unchanged maskelynite grains in the margins of the pocket often contain lingunite or are totally transformed to jadeite based on static high pressure results the mineral assemblages in the pocket suggest peak pressure in the 17 20 gpa range with maximum temperature t max in the range 1850 1900 k as the formation conditions for this taiban chondrite during shock

shock metamorphism of elephant moraine a79001 implications for olivine ringwoodite transformation and the complex thermal history of heavily shocked martian meteorites abstract lithology a of martian meteorite elephant moraine eet a79001 contains fragments entrained within a 100 m thick shear induced shock vein these fragments the shock vein matrix and walls of olivine along the vein as well as shock deformation and transformation in rock forming minerals in the bulk rock were investigated using scanning electron microscopy the electron microprobe and raman spectroscopy the presence of ringwoodite the spinel structured high pressure mg fe 2 sio 4 polymorph has been confirmed in eeta79001 for the first time ringwoodite occurs within and around the shock vein exhibiting granular and lamellar textures in both textures ringwoodite consists of 500 nm size distinct grains ringwoodite lamellae are 115 nm to 1 3 m wide planar fractures in olivine provided sites for heterogeneous nucleation of ringwoodite analyses performed on the largest grains 1 m show that ringwoodite is consistently higher in iron fa 27 4 32 4 relative to surrounding olivine fa 25 1 267 7 implying that there was fe mg exchange during their transformation and therefore their growth was diffusion controlled in the shock environment diffusion takes place dynamically i e with concurrent deformation and grain size reduction this results in enhanced diffusion rates 10 8 m 2 s over nm m distances shock deformation in host rock minerals including strong mosaicism pervasive fracturing polysynthetic twinning pyroxene only extensive shock melting local transformation of olivine to ringwoodite and complete transformation of plagioclase to maskelynite in the bulk rock indicate that eeta79001 was strongly shocked the short shock duration 0 01 s combined with a complex thermal history resulted in crystallization of the 100 m thick shock vein in eeta79001 during the pressure release and partial back transformation of ringwoodite to olivine based on the pressure stabilities of clinopyroxene ringwoodite crystallization at the shock vein margin began at 18 gpa olivine and clinopyroxene crystallized at

back transformation of high pressure minerals in shocked chondrites low pressure mineral evidence for strong shock post shock annealing of meteorites can destroy their shock induced features particularly high pressure minerals and complicate the estimation of impact pressure temperature conditions however distinguishing post shock annealing features from thermal metamorphism effects can be practically difficult here we report results from mbale a highly shocked l chondrite to investigate the mechanisms kinetics and identification criteria for post shock annealing of high pressure signatures olivine fragments within shock melt veins in mbale occur as chemically heterogeneous nanocrystalline aggregates that contain trace wadsleyite and ringwoodite their strong variation in fayalite content provides evidence of iron partitioning during transformation of olivine to wadsleyite followed by back transformation to olivine after decompression experimental studies of transformation kinetics show that wadsleyite transforms to olivine in seconds at temperatures above 1200 k and in hours at temperatures between 900 and 1200 k thermal models of shock melt cooling show that shock veins in mbale cooled to 1200 k in 1 s the shock pulse must have been shorter than 1 s to provide the high temperature conditions for post shock back transformation of wadsleyite many highly shocked l chondrites which have abundant high pressure minerals must have experienced relatively long shock durations combined with rapid cooling of shock melt regions to preserve high pressure phases the most highly shocked samples such as impact melt breccias lack high pressure phases because of post shock back transformations

current understanding of pleistocene temperate stages in ireland abstract research on irish pleistocene temperate stages has been dominated by investigation of deposits with very similar floristic records collectively termed the gortian and traditionally correlated with the continental holsteinian recently detailed investigation of interglacial material from cork harbour suggested the gortian group may contain floristically similar deposits of different ages the pollen record had gortian holsteinian characteristics and dating by measurement of uranium thorium disequilibrium and amino acid racemization suggested that the deposits were younger than stage 11 correlated with the holsteinian by reille and de beaulieu reille de beaulieu quat res 44 1995 205 the suggestion that there may have been more than one holsteinian style temperate stage in europe might explain the floristic similarity of gortian and holsteinian deposits and the apparent younger age of the cork deposits the lack of a last interglacial stratotype is the most outstanding problem in irish quaternary studies the pollen content of sediments at fenit co kerry dated to ca 118 ka bp suggested they may represent a cool temperate phase at the end of the last interglacial recently investigated sub drumlin organic rich deposits at knocknacran co monaghan are also possibly last interglacial in age and these are discussed in detail here

estimating shock pressures based on high pressure minerals in shock induced melt veins of l chondrites here we report the transmission electron microscopy tem observations of the mineral assemblages and textures in shock induced melt veins from seven l chondrites of shock stages ranging from s3 to s6 the mineral assemblages combined with phase equilibrium data are used to constrain the crystallization pressures which can be used to constrain shock pressure in some cases thick melt veins in the tenhaml6 chondrite contain majorite and magnesiowstite in the center and ringwoodite akimotoite vitrified silicate perovskite and majorite in the edge of the vein indicating crystallization pressure of 25 gpa however very thin melt veins 5 30 m wide in tenham contain glass olivine clinopyroxene and ringwoodite suggesting crystallization during transient low pressure excursions as the shock pressure equilibrated to a continuum level melt veins of umbarger include ringwoodite akimotoite and clinopyroxene in the vein matrix and fe2sio4 spinel and stishovite in sio2 feo rich melt indicating a crystallization pressure of 18 gpa the silicate melt veins in roy contain majorite plus ringwoodite indicating pressure of 20 gpa melt veins of ramsdorf and nakhon pathon contain olivine and clinoenstatite indicating pressure of less than 15 gpa melt veins of kunashak and la lande include albite and olivine indicating crystallization at less than 2 5 gpa based upon the assemblages observed crystallization of shock veins can occur before during or after pressure release when the assemblage consists of high pressure minerals and that assemblage is constant across a larger melt vein or pocket the crystallization pressure represents the equilibrium shock pressure

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

freshwater organic deposits and stratified sediments between early and late midlandian devensian till sheets at aghnadarragh county antrim northern ireland the following events have been identified from a complex sequence of midlandian devensian sediments recently exposed at aghnadarragh county antrim 1 a lower till interpreted as early midlandian was deposited by a major lowland ice sheet which moved south eastwards from central ulster across the lough neagh depression 2 deglacial conditions were followed by a periglacial phase characterised by ice wedge growth and sedimentation by gravelly debris flows the latter contain the oldest known remains of mammuthus primigenius in ireland 3 an early midlandian interstadial is represented by a woody detritus peat with evidence of betula pinuos and picea woodlands and a rich beetle fauna wood from this horizon has been shown to be beyond the range of radiocarbon dating 48 180 bp 4 interstadial conditions were succeeded by a cold non glacial phase dominated by in channel gravelly flows and deposition of organic muds which contain plant and insect fossils these horizons are older than 46 850 bp 5 an upper non drumlinised till was deposited during the late midlandian by a major lowland ice sheet which moved generally eastwards across the lough neagh lowlands from central ulster this glaciation probably reached its maximum at ca 20 24 ka 6 drumlin formation occurred in the lough neagh lowlands towards the end of the late midlandian the limiting moraines are dated to 17 ka the lower till at aghnadarragh is the first positive record of a major lowland ice sheet in ireland during the early part of the last cold stage the early midlandian interstadial peats have not been documented elsewhere in ireland and correlate broadly with the chelford interstadial complex of the english midlands related exposures elsewhere in ulster confirm that the middle part of the last cold stage was free of major lowland ice masses but deposits of this period are absent from aghnadarragh

coupling between sulfur recycling and syndepositional carbonate dissolution evidence from oxygen and sulfur isotope composition of pore water sulfate south florida platform u s a sulfur cycling in fe poor organic rich shelf carbonates known to have rapid rates of so4 2 reduction remains poorly studied despite the volumetric significance of shelf deposits in modern and ancient carbon budgets we investigated sulfur cycling in modern carbonates of the florida platform from end member depositional environments muddy sands from the atlantic reef tract and finer grained mudbank and island flank deposits from florida bay relations between pore water chemistry so4 2 co2 ca 2 cl and oxygen and sulfur stable isotope compositions of so4 2 require direct coupling between sulfur redox cycling and syndepositional carbonate dissolution oxygen isotope compositions of pore water sulfate were remarkably shifted away from the established value for marine so4 2 9 5 despite near normal so4 2 cl ratios chemical evolution was least in reef tract pore waters and greatest in florida bay relative to overlying seawater mudbank sediments exhibited sulfate depletion with 18oso4 and 34sso4 values both increasing by about 7 more bioturbated island flank sediments colonized by thalassia grass had a 5 increase in 18oso4 variable 34sso4 values 17 7 to 23 3 and exceptionally high ca 2 cl ratios the large excess of ca 2 up to 1 7 mm requires a much larger acid source than the amounts derived from utilization of dissolved o2 0 3 mm and small degrees of net so4 2 reduction 0 5 mm reduced a conceptual model was constructed using chemical and isotopic data on natural pore waters and on sulfate isotope fractionation factors obtained from sediment incubation experiments the model outputs show that pore water compositions can be explained by a redox cycle where microbial so4 2 reduction is followed by very efficient h2s oxidation thus maintaining virtually invariant so4 2 cl ratios the enhanced o2 transport may be driven by associated marine grass rhizome systems and microbial communities established in bioturbated sediments the net result of the cycle is that the rate of sulfide oxidation which is largely balanced by the rate of microbial sulfate reduction is stoichiometrically related to the rate of carbonate dissolution this is consistent with previously reported rates of carbonate dissolution 400 mol cm2 yr and average rates of sulfate reduction 200 mol cm2 yr from the florida platform and a 2 1 stoichiometry

shear induced ringwoodite formation in the martian shergottite dar al gani 670 abstract shock produced melt veins in the martian shergottite dar al gani 670 crosscut large olivine crystals the upper part of one of these crystals appears to be sheared off and displaced along the shock vein from the olivine vein interface small lamellae of ringwoodite grow into the host crystal the 1 3 m wide and up to 20 m long lamellae consist of small bands and blocks and are orientated along specific crystallographic orientations texture and composition i e more fe rich than the host olivine indicate that lamellae formed via incoherent diffusion controlled growth it is suggested that a combination of high particle velocities and shock induced defects lead to enhanced diffusion rates in addition shearing caused grain size reduction allowing rapid fe mg interchange and induced lattice defects serving as nucleation sites for ringwoodite crystallographic orientation of ringwoodite lamellae indicates that during shock deformation the 001 hk 0 slip system was activated in olivine natural high pressure phases in martian meteorite allow to constrain phase transitions taking place in the inaccessible earth s mantle high pressure shear instabilities of olivine at subduction zones in 400 700 km depth are considered being responsible for deep earthquakes at such p t conditions breakdown of olivine results in formation of ringwoodite filled micro anticracks which interact with each other finally leading to catastrophic shear failure our results strongly suggest that shearing itself contributes to a runaway process of enhanced ringwoodite formation and thus reinforces catastrophic material failure that may result in deep earthquakes

dolomitization of holocene shallow marine deposits mediated by sulfate reduction and methanogenesis in normal salinity seawater northern belize abstract dolomite constitutes an average of 12 of the holocene organic rich sediments over a 15 km2 area of the cangrejo shoals mudbank in northern belize although it defines a laterally persistent stratiform body that averages 3 m thick it is present throughout the 7 6 m thick sediment section these transgressive sediments are less than 6400 years old and were deposited in shallow marine environments of normal salinity the dolomite is dominantly cement and average crystal size is 7 m there are no significant correlations among amount of dolomite vs sediment texture mineralogy porosity or mole mgco3 in associated particulate high mg calcite depth or location on the shoals the dolomites are poorly ordered and calcic 39 5 44 5 mole mgco3 with low mean mn 210 ppm and relatively high mean sr 1034 ppm concentrations there is no evidence of recrystallization or geochemical alteration of the dolomite 18o values of the dolomites range from 0 5 to 2 8o oopdb and the mean value 2 1o oo suggests that the dolomite precipitated from normal salinity pore water dolomite 13c values range from 5 2o oo to 11 6o oopdb mean seawater 13c 0 5o oo which suggests dolomitization promoted by both bacterial sulfate reduction and methanogenesis in environments with anoxic pore water dolomitization attending these organodiagenetic reactions apparently was reversible over time and episodic rather than continual precipitation is indicated requisite mg and ca were provided by seawater and by some dissolution of host sediments the most rapid period of dolomitization may have been during early transgression when relatively high sedimentation rates sustained high levels of organodiagenesis and pore water alkalinities

shock events in the solar system the message from minerals in terrestrial planets and asteroids impacts are central to the origin and evolution of planets of the solar system the shapes of craters which can reach up to 1 000 km in diameter on the moon provide critical information on the large scale dynamics of the impact and related shock minerals formed at high pressure and temperature found in shocked terrestrial rocks and meteorites give additional and complementary insights on the shock process at a smaller scale typically from a few micrometers to a few millimeters local flaws in rocks such as voids and mineral interfaces are the preferential sites for the formation of high pressure melts and minerals calculations based on the physics of shocks and the thermodynamics and kinetics of mineral transformations provide orders of magnitude for the duration transient pressure and prevailing temperature conditions of shock events case studies on shocked terrestrial and extraterrestrial materials illustrate the links between these parameters and impact duration many of the high pressure min

transmission electron microscope observation of α β and γ mg fe 2sio4 in shocked meteorites planar defects and polymorphic transitions abstract high pressure polymorphs of olivine mg fe 2sio4 have been observed by transmission electron microscopy in the shocked tenham and catherwood meteorites planar defects are characterized in and polymorphs and their possible role in polymorphic transitions is assessed and discussed in relation with the published literature the stacking fault 1 2 1 01 010 observed in the phase can produce a shear transformation to the phase if it occurs on every other 010 plane conversely the stacking fault 1 2 1 1 2 110 observed in the phase can be transformed to the phase if it is repeated on every other 110 plane and to an intermediate yet unobserved e phase if it is repeated on every 110 plane shear transformation between olivine and spinel could involve the e phase as an intermediate stage

the nature of the l chondrite parent body s disruption as deduced from high pressure phases in the sixiangkou l6 chondrite the disruption of the l chondrite parent body lcpb at similar to 470ma is currently the best documented catastrophic celestial impact event based on the large number of l chondritic materials associated with this event uranium lead u pb dating of apatite and its high pressure decomposition product tuite in the sixiangkou l6 chondrite provides a temporal link to this event the u pb system of phosphates adjacent to shock melt veins was altered to varying degrees and the discordance of the u pb system correlates closely with the extent of apatite decomposition this suggests that the u pb system of apatite could be substantially disturbed by high temperature pulse during shock compression from natural impacts at least on the scale of mineral grains although many l chondrites can be temporally related to the catastrophic lcpb impact event the shock conditions experienced by each individual meteorite vary this could be due to the different geologic settings of these meteorites on their parent body the shock pressure and duration derived from most meteorites may only reflect local shock features rather than the impact conditions although they could provide lower limits to the impact conditions the sixiangkou shock duration similar to 4s estimated from high pressure transformation kinetics provides a lower limit to the high pressure pulse of the lcpb disruption impact combined with available literature data of l chondrites associated with this impact event our results suggest that the lcpb suffered a catastrophic collision with a large projectile with a diameter of at least 18 22km at a low impact velocity 5 6kms 1 this is consistent with astronomical estimates based on the dynamical evolution of l chondritic asteroids

high pressure phases in shock induced melt of the unique highly shocked ll6 chondrite northwest africa 757 northwest africa 757 is unique in the ll chondrite group because of its abundant shock induced melt and high pressure minerals olivine fragments entrained in the melt transform partially and completely into ringwoodite plagioclase and ca phosphate transform to maskelynite lingunite and tuite two distinct shock melt crystallization assemblages were studied by fib tem analysis the first melt assemblage which includes majoritic garnet ringwoodite plus magnetite magnesiowustite crystallized at pressures of 20 25 gpa the other melt assemblage which consists of clinopyroxene and wadsleyite solidified at 15 gpa suggesting a second veining event under lower pressure conditions these shock features are similar to those in s6 l chondrites and indicate that nwa 757 experienced an intense impact event comparable to the impact event that disrupted the l chondrite parent body at 470 ma

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

influence of insolation and glaciation on atmospheric circulation in the north atlantic sector implications of general circulation model experiments for the late quaternary climatology of europe abstract atmospheric general circulation models have been used to simulate the sensitivity of regional climates to late quaternary changes in insolation ice sheets and atmospheric co 2 model results for full glacial conditions 18 ka show the atlantic westerly jet strengthened and shifted south a corresponding southward shift of the icelandic low and a strengthening and northward shift of the subtropical anticyclone sta a glacial anticyclone developed over the european ice sheet and cold dry conditions prevailed over much of europe the full glacial climate anomaly for the north atlantic sector is explained by a combination of low atmospheric co 2 reducing global surface temperatures and circulation changes caused by the ice sheets by the early holocene 9 ka the ice sheets were much reduced while the orbital anomaly was near its maximum producing mid continental summer warming and winter cooling and a reduced latitudinal temperature gradient in the northern hemisphere in both seasons the jet and the icelandic low were displaced to north of their present position producing strong onshore flow and mild wet winters in northern europe while the sta was also shifted northwards producing offshore flow and dry summers in the same region during the late glacial transition the effects of insolation and glaciation were antagonistic for some features of the circulation and synergistic for others the model results suggest palaeoclimatic hypotheses that could be directly tested by comparison with palaeoclimatic data mapped at a synoptic scale

host rock solid state transformation in a shock induced melt vein of tenham l6 chondrite abstract the host rock fragments entrained in a 580 m wide melt vein of the tenham l6 chondrite were investigated using field emission scanning electron microscopy fesem and transmission electron microscopy tem to better understand the solid state transformation mechanisms and the shock conditions the melt vein consists of a matrix of silicate plus metal sulfide that crystallized from immiscible melts and sub rounded host rock fragments that have been entrained in the melt and transformed to polycrystalline high pressure silicates by solid state transformation mechanisms these high pressure phases include ringwoodite low ca majorite clinoenstatite hollandite structured plagioclase and ca rich majorite the ca rich majorite occurs as a symplectitic intergrowth with a ca poor amorphous silicate phase in a 200 m diameter chondrule in the vein this intergrowth seems to be the result of a disproportionate breakdown of a ca rich clinopyroxene precursor into ca rich majorite and femg sio 3 perovskite which subsequently vitrified upon pressure release the tem observations suggest that most solid state transformations in the tenham are reconstructive the transformation of olivine to polycrystalline ringwoodite appears to involve incoherent intracrystalline nucleation and interface controlled growth lamellae in partially transformed olivine are not continuous coherent lamellae but rather lamellae of polycrystalline ringwoodite which is inconsistent with a coherent lamellar transformation mechanism growth rate calculations based on published kinetic data suggest that the time required to grow 1 m ringwoodite crystal is 100 ms at 1600 k suggesting that the minimum shock pulse of approximately 100 ms

the beginning of the ice age in europe and its major subdivisions abstract the time range of the quaternary is discussed in terms of the view that this period is essentially the one in the earth s history in which during certain intervals inland ice developed in the northern hemisphere this process first became evident about 2 3 ma bp since then the quaternary thus defined has witnessed many climatic variations although for practical reasons it is not feasible to assign the status of stage to each distinclty warm or cold phase climatic change is nevertheless the most appropriate basis for world wide correlation it seems possible to use both climatic change and other correlation parameters to arrive at a practicable subdivision of the quaternary period into stages before about 1 ma vegetational succession during interglacials in western europe differed from that seen in later times a possible cause for this divergence is discussed in the later part of the quaternary after about 1 ma two distinct types of warm temperate intervals occurred one associated with high sea levels and an oceanic climate the other showing lower sea levels and a cooler and more continental climate the difference between these two types is shown by comparison of the distribution of abies during these time intervals

shock conditions recorded in nwa 8159 martian augite basalt with implications for the impact cratering history on mars nwa 8159 is an augite rich martian basalt formed by cooling of a relatively evolved ca rich ti poor and lree depleted lava under relatively oxidizing conditions during the early amazonian in addition to its distinct igneous petrogenesis and high fo 2 nwa 8159 is also set apart from most martian shergottites with respect to the low degree of shock metamorphism required to preserve crystalline igneous plagioclase an 50 65 in this study mineral transformations within and adjacent to shock veins in nwa 8159 were investigated using scanning electron microscopy raman spectroscopy and transmission electron microscopy to better constrain the unusal shock history of this meteorite the transformation of olivine to ahrensite fe ringwoodite along shock vein margins and tissintite and coesiteformed from igneous mineral labradorite and silica grains entrained as clasts within shock veins has been documented in this study we report on a previously unidentified mineral assemblage of ca na majoritic garnet sodic clinopyroxene and stishovite crystallized from shock melt this mineral assemblage indicates a crystallization pressure of approximately 16 gpa which is within the range of previous shock pressure estimates for this meteorite 15 23 gpa the presence of a majoritic garnet bearing assemblage throughout veins up to 0 6 mm wide indicates that the sample remained at high pressure throughout the melt vein quench based on thermal models the sample must have remained at high pressure for 100 ms this shock duration is an order of magnitude longer than those experienced by more highly shocked shergottites such as tissint or zagami 30 gpa 10 20 ms and would seem to imply a relatively large impact event recent numerical models demonstrate that a range of shock pressures and durations are realized by rocks within the ejected spall zone of a hypervelocity impact the shock conditions experienced by nwa 8159 therefore do not require an impact event distinct from other shergottites rather our findings suggest that this meteorite originated from near the martian surface at the edge of the impact site the shock history of nwa 8159 provides a picture of mars consistent with that derived from remote observation that of a random cratering process that samples a geologically long lived and complex planet

glaciomarine facies within subglacial tunnel valleys the sedimentary record of glacioisostatic downwarping in the irish sea basin coastal exposures of late pleistocene sediments deposited after 19 000 yr bp near dublin ireland provide a window into the infill of a subglacially cut tunnel valley exposures close to the steeply dipping bedrock wall of the valley show boulder gravels within multi storey u shaped channels cut and filled by subglacial meltwaters driven by a high hydrostatic head gravels are truncated by poorly sorted ice proximal glaciomarine sediments that record the pumping of large volumes of subglacial debris along the tunnel valley to a tidewater ice sheet margin the sedimentary succession is dominated by sediment gravity flow facies comprising interbedded diamict and massive poorly sorted gravel facies interpreted as subaqueous debris flow deposits gravel beds show local inverse and normal coarse tail graded facies recording the restricted development of turbulent flow sediment gravity flow deposits fill broad 2 km shallow 10 m and overlapping channels penetrative deformation structures e g dykes are common at the base of channels the same subglacially eroded topography and glaciomarine infill stratigraphy can be identified on high resolution seismic profiles across nearly 600 km2 of the western irish sea tunnel valleys are argued to have been exposed to glaciomarine processes by the rapid retreat of a calving tidewater ice sheet margin in response to marine flooding caused by glacio isostatic downwarping below the last british ice sheet the facies associations described in this paper comprise an event stratigraphy that may be found on other glaciated continental shelves

a new mineral with an olivine structure and pyroxene composition in the shock induced melt veins of tenham l6 chondrite we report a new mineral that occurs in shock induced melt veins of the tenham l6 chondrite the new mineral identified by transmission electron microscopy tem occurs as acicular nanocrystals in a glassy matrix at the edge of shock induced melt veins that crystallized during rapid quench at high pressure the elongate crystals have aspect ratios up to 25 widths range from 5 to 40 nm and lengths are up to 500 nm energy dispersive x ray spectroscopy eds analyses provide the relative cation abundances that are consistent with a pyroxene like stoichiometry na 0 06 ca 0 02 mg 0 71 fe 0 20 al 0 11 si 0 94 o 3 selected area electron diffraction saed patterns from single crystal and polycrystalline aggregates indicate an olivine structure with refined cell parameters a 4 78 b 10 11 and c 5 94 a and a calculated density of 3 32 g cm 3 synchrotron x ray microdiffraction data are consistent with an olivine structure and provide similar cell parameters a 4 778 b 10 267 c 5 937 a the pyroxene composition represents a large deviation from olivine stoichiometry na 0 08 ca 0 03 mg 0 95 fe 0 26 al 0 15 si 0 25 0 28 2 si 1 o 4 with 0 28 formula units of vacancies 0 11 of na plus ca 2 and 0 25 of si 4 in octahedral sites our observations indicate that a metastable and nonstoichiometric olivine structure can crystallize from a silicate melt during rapid quench trace amounts of such defects may be present in stable olivines in the deep upper mantle

the gortian temperate stage abstract interglacial sediments assigned to the gortian temperate stage are recorded at eleven sites throughout ireland but they remain undated by absolute means however the unique vegetational assemblages found within the gortian appear to belong to the middle pleistocene on biostratigraphic grounds the gortian vegetational records are reviewed and a summary of the possible age of the interglacial is presented along with recent work that has been carried out on stratigraphically important sites within ireland

two distinct assemblages of high pressure liquidus phases in shock veins of the sixiangkou meteorite shock produced complex veins including earlier and later veins are identified in the sixiangkou l6 chondrite the early vein is intersected by the late vein and consists of coarse grained aggregates of ringwoodite majorite and lingunite and fragments of olivine pyroxene plagioclase metal and troilite as well as a fine grained matrix of garnet ringwoodite metal and troilite the late vein mainly consists of a fine grained matrix of garnet magnesiowustite metal and troilite as well as a small amount of coarse grained aggregates the amount of fine grained matrix suggests that the late vein was nearly completely melted whereas the early vein underwent partial melting both fine grained assemblages of garnet plus ringwoodite in the early vein and garnet plus magnesiowustite in the late vein are liquidus phases crystallized from shock induced melt based on our understanding of the liquidus assemblages the late vein experienced a higher pressure and temperature than the early vein

shock induced transition of naalsi3o8 feldspar into a hollandite structure in a l6 chondrite natural naalsi3o8 hollandite was found in the tenham chondrite it occurs as aggregates of extremely fine grains of several tens of nanometers the hollandite phase seems to have formed from host feldspar in a solid state reaction during the shock event it is very sensitive to electron beam irradiation and easily becomes amorphous similar to mgsio3 perovskite contained in the same chondrite the existence of crystalline silicate perovskite and hollandite in tenham indicates that the peak pressure and the post shock temperature were much lower than previously believed

shock melt veins of tenham chondrite as a possible paleomagnetic recorder rock magnetism and high pressure minerals heavily shocked meteorites of shock stages s5 and s6 contain shock induced melt veins smvs smvs might have reset the remanence of an asteroidal metamorphism at the time of giant collisions against a chondrite parent body here we present micropaleomagnetic and petrologic studies of smvs in l6s5 tenham chondrite with 500 m thick black veins enclosing high pressure minerals such as ringwoodite paleomagnetic data show that the high temperature ht 200 c 650 c and high coercivity hc 20 100 mt stable components of smvs formed a cluster even from different portions of smv whereas the stable ht component of surrounding host rock showed a scattered orientation under stereonet projection the host rock hc components form a girdle between the mixing of smvs and unknown overprints tracing the magnetic susceptibility foliation magnetic force microscopy and backscattered electron images confirmed kamacite and taenite assemblages in iron sulfides as remanence carrying minerals in smvs hysteresis data of smvs revealed the presence of single domain sd feni metals with mrs ms 0 1 and hrc hc 2 although these parameters are only applicable to magnetite because the metastable ringwoodite in smvs transforms back to olivine at 188 c for 1000 myr metamorphism or at 900 c for 1 h postshock heating the preservation of ringwoodite suggests that smvs have not experienced either thermal condition the magnetic time temperature relation for sd feni metals suggested that 200 c unblocking temperature corresponds to the storage time of 100 years for kamacite and 4500 myr for taenite at room temperature the difference of ht components discards the possibility of postshock heating therefore the smv s remanence is a characteristic shock induced thermal remanence that has newly been acquired during hypervelocity collision under a cryptic magnetic field

the geochemistry of carbonate diagenesis the past present and future stable carbon and oxygen isotopes 18o and 13c values and trace elements have been applied to the study of diagenesis of carbonate rocks for over 50 years as valuable as these insights have been many problems regarding the interpretation of geochemical signals within mature rocks remain for example while the 18o values of carbonate rocks are dependent both upon the temperature and the 18o value of the fluid and additional information including trace element composition aids in interpreting such signals direct evidence of either the temperature or the composition of the fluids is required such information can be obtained by analysing the 18o value of any fluid inclusions or by measuring the temperature using a method such as the clumped isotope technique such data speak directly to a large number of problems in interpreting the oxygen isotope record including the well known tendency for 18o values of carbonate rocks to decrease with increasing age unlike the 18o 13c values of carbonates are considered to be less influenced by diagenesis and more a reflection of primary changes in the global carbon cycle through time however many studies have not sufficiently emphasized the effects of diagenesis and other post depositional influences on the eventual carbon isotopic composition of the rock with the classic paradigm that the present is the key to the past being frequently ignored finally many additional proxies are poised to contribute to the interpretation of carbonate diagenesis although the study of carbonate diagenesis is at an exciting point with an explosion of new proxies and methods care should be taken to understand both old and new proxies before applying them to the ancient record

effects of shock and martian alteration on tissint hydrogen isotope ratios and water content abstract the tissint meteorite a picritic shergottite fell to earth in morocco on the 18th of july 2011 and is only the fifth martian meteorite witnessed to fall hydrogen isotope ratios and water contents are variable within different minerals in tissint ringwoodite and shock melt pockets contain elevated d h ratios relative to terrestrial values d 761 4224 these high ratios in recrystallized phases indicate significant implantation of hydrogen from the d rich martian atmosphere during shock in contrast although olivine has detectable water abundances 230 485 ppm it exhibits much lower d h ratios d 88 to 150 suggesting this water was not implanted from the martian atmosphere the minimal terrestrial weathering experienced by tissint gives confidence that the olivine hosted water has a martian origin but its high concentration indicates direct inheritance from the parental melt is improbable especially given the low pressure of olivine crystallisation incorporation of a low d crustal fluid or deuteric alteration during crystallisation could explain the relatively high water contents and low d h ratios in tissint olivine

thermal modeling of shock melts in martian meteorites implications for preserving martian atmospheric signatures and crystallization of high pressure minerals from shock melts the distribution of shock melts in four shergottites having both vein and pocket geometry has been defined and the conductive cooling time over the range 2500 c to 900 c calculated isolated 1 mm2 pockets cool in 1 17 s and cooling times increase with pocket area an isolated vein 1 7 mm in northwest africa nwa 4797 cools to 900 c in 4 5 s interference between thermal haloes of closely spaced shock melts decreases the thermal gradient extending cooling times by a factor of 1 4 to 100 this is long enough to allow differential diffusion of ar and xe from the melt small pockets 1 mm2 lose 2 2 ar and 5 2 xe during cooling resulting in a small change in the ar xe ratio of the dissolved gas over that originally trapped with longer cooling times there is significant fractionation of xe from ar and the ar xe ratio increases rapidly the largest pockets show less variation of ar xe and likely preserve the original trapped gas composition considering all of the model calculations even the smallest isolated pockets have cooling times greater than the duration of the pressure pulse i e 0 01 s the crystallization products of these shock melts will be unrelated to the peak shock pressure experienced by the meteorite

akimotoite in the tenham meteorite crystal chemistry and high pressure transformation mechanisms the transformation of pyroxene to its high pressure polymorph akimotoite mgsio3 ilmenite type structure is documented in the shock induced melt veins of the l6 tenham chondrite four textural relationships between pyroxene and akimotoite are observed in former pyroxene grains entrained in the shear melt vein and in pyroxene grains attached to the wall of the melt vein in one of the entrained enstatite grains the transformation to akimotoite is partial one third of the grain is transformed to a polycrystalline aggregate of akimotoite with a scalloped interface with enstatite akimotoite fe0 4mg1 24ca0 07na0 12al0 14 si1 963al0 037 o6 is slightly enriched in ca cao 1 74 al and na with respect to enstatite cao 0 71 narrow bands of polycrystalline akimotoite with three crystallographic orientations and a chemical composition similar to the surrounding enstatite intersect the other part of the grain in a second grain entrained in the shear melt vein tablets of akimotoite are interwoven with a pyroxene glass with a lower na and fe content and a higher mg content compared to the adjacent akimotoite this pyroxene glass is probably the product of mg fe sio3 perovskite amorphization polycrystalline akimotoite is also formed at the expense of enstatite at the vein wall of the shear melt vein in that case akimotoite is also enriched in ca cao 2 85 na nao 1 72 and al al2o3 4 14 all these observations suggest that akimotoite is mainly formed by solid state transformation of former pyroxenes with subsequent diffusion of calcium aluminum and sodium from the chondritic melt of the shear melt vein finally the first complete x ray diffraction pattern of natural akimotoite is presented

majorite olivine high ca pyroxene assemblage in the shock melt veins of pervomaisky l6 chondrite high pressure minerals majorite pyrope garnet and jadeite were found in the pervomaisky l6 ordinary chondrite majorite pyrope 79 mol majorite was observed within the fine grained silicate matrix of a shock melt vein smv coexisting with olivine and high ca pyroxene this is the first report of a garnet olivine high ca pyroxene assemblage that crystallized from the melt in the smv matrix of meteorite p t conditions of the formation of the smv matrix with olivine fragments are 13 5 15 0 gpa and 1750 2150 c the lowest parameters among all known majorite bearing h l chondrites the estimated conditions include the olivine olivine ringwoodite phase boundary and there is a possibility that observed olivine is the result of wadsleyite ringwoodite back transformation during a cooling and decompression stage in the framework of this hypothesis we discuss the problem of survival of the high pressure phases at the post shock stage in the meteorites and propose two possible p t paths 1 the high pressure mineral is transformed to a low pressure one during adiabatic decompression above the critical temperature of direct transformation and 2 quenching below the critical temperature of direct transformation within the stability field of the high pressure phase and further decompression the aggregates with plagioclase composition ab 81 1 an 14 9 or 4 1 occur in host rock fragments near or inside of the smv and have a radial concentric spherulite like microstructure previously described in the novosibirsk meteorite and that is very similar to the texture of tissintite in the tissint martian meteorite it is likely that jadeite is related to crystallization of the smv and could have formed from albitic feldspar plagioclase melt at 13 5 15 0 gpa and 2000 c

heterogeneous mineral assemblages in martian meteorite tissint as a result of a recent small impact event on mars the microtexture and mineralogy of shock melts in the tissint martian meteorite were investigated using scanning electron microscopy raman spectroscopy transmission electron microscopy and synchrotron micro x ray diffraction to understand shock conditions and duration distinct mineral assemblages occur within and adjacent to the shock melts as a function of the thickness and hence cooling history the matrix of thin veins and pockets of shock melt consists of clinopyroxene ringwoo dite stishovite embedded in glass with minor fe sulfide the margins of host rock olivine in contact with the melt as well as entrained olivine fragments are now amorphosed silicate perovskite magnesiowustite or clinopyroxene magnesiowustite the pressure stabilities of these mineral assemblages are 15 gpa and 19 gpa respectively the 200 lm wide margin of a thicker mm size up to 1 4 mm shock melt vein contains clinopyroxene olivine with central regions comprising glass vesicles fe sulfide spheres fragments of host rock within the melt are polycrystalline olivine after olivine and tis sintite glass after plagioclase from these mineral assemblages the crystallization pressure at the vein edge was as high as 14 gpa the interior crystallized at ambient pressure the shock melts in tissint quench crystallized during and after release from the peak shock pressure crystallization pressures and those determined from olivine dissociation therefore represent the minimum shock loading shock deformation in host rock minerals and complete transformation of plagioclase to maskelynite suggest the peak shock pressure experienced by tissint p 29 30 gpa these pressure estimates support our assessment that the peak shock pressure in tissint was significantly higher than the minimum 19 gpa required to transform olivine to silicate perovskite plus magnesiowustite small volumes of shock melt 100 lm quench rapidly 0 01 s whereas thermal equilibration will occur within 1 2 s in larger volumes of melt 1 mm 2 the apparent variation in shock pressure recorded by variable mineral assemblages within and around shock melts in tissint is consistent with a shock pulse on the order of 10 20 ms combined with a longer duration of post shock cooling and complex thermal history this implies that the impact on mars that shocked and ejected tissint at 1 ma was not exceptionally large

origin of the eclogitic clasts with graphite bearing and graphite free lithologies in the northwest africa 801 cr2 chondrite possible origin from a moon sized planetary body inferred from chemistry oxygen isotopes and ree abundances abstract in order to clarify the origin of the eclogitic clasts found in the nwa801 cr2 chondrite kimura et al 2013 especially that of the high pressure and temperature p t condition 3 gpa and 1000 c we conducted ion microprobe analyses of oxygen isotopes and rare earth element ree abundances in the clasts oxygen isotopic compositions of the graphite bearing lithology gbl and graphite free lithology gfl show a slope 0 6 correlation slightly below the cr ch cb chondrites field in the o three isotope diagram with a large variation for the former and almost homogeneous composition for the latter the average ree abundances of the two lithologies show almost unfractionated patterns based on these newly obtained data as well as mineralogical observations bulk chemistry and considerations about diffusion timescales for various elements we discuss in detail the formation history of the clasts consistency of the geothermobarometers used by kimura et al 2013 suggesting equilibration of various elements among different mineral pairs provides a strong constraint for the duration of the high p t condition we suggest that the high p t condition lasted 102 103 years this clearly precludes a shock high pressure hp model and hence strongly supports a static hp model a static hp model requires a moon sized planetary body of 1500 km in radius furthermore it implies two successive violent collisions first at the formation of the large planetary body when the clasts were placed its deep interior and second at the disruption of the large planetary body when the clasts were expelled out of the parent body and later on transported to the accretion region of the cr chondrites we also discuss possible origin of o isotopic variations in gbl and presence absence of graphite in gbl gfl respectively in relation to smelting possibly occurred during the igneous process es which formed the two lithologies finally we present a possible formation scenario of the eclogitic clasts

a large shock vein in l chondrite roosevelt county 106 evidence for a long duration shock pulse on the l chondrite parent body a large shock induced melt vein in l6 ordinary chondrite roosevelt county 106 contains abundant high pressure minerals including olivine enstatite and plagioclase fragments that have been transformed to polycrystalline ringwoodite majorite lingunite and jadeite the host chondrite at the melt vein margins contains olivines that are partially transformed to ringwoodite the quenched silicate melt in the shock veins consists of majoritic garnets up to 25 m in size magnetite maghemite and phyllosilicates the magnetite maghemite and phyllosilicates are the terrestrial alteration products of magnesiowustite and quenched glass this assemblage indicates crystallization of the silicate melt at approximately 20 25 gpa and 2000 c coarse majorite garnets in the centers of shock veins grade into increasingly finer grained dendritic garnets toward the vein margins indicating increasing quench rates toward the margins as a result of thermal conduction to the surrounding chondrite host nanocrystalline boundary zones that contain wadsleyite ringwoodite majorite and magnesiowustite occur along shock vein margins these zones represent rapid quench of a boundary melt that contains less metal sulfide than the bulk shock vein one dimensional finite element heat flow calculations were performed to estimate a quench time of 750 1900 ms for a 1 6 mm thick shock vein because the vein crystallized as a single high pressure assemblage the shock pulse duration was at least as long as the quench time and therefore the sample remained at 20 25 gpa for at least 750 ms this relatively long shock pulse combined with a modest shock pressure implies that this sample came from deep in the l chondrite parent body during a collision with a large impacting body such as the impact event that disrupted the l chondrite parent body 470 myr ago

melting experiments on the enstatite pyrope join at 80 152 kbar melting temperatures of the assemblage pyroxene garnet on the enstatite pyrope join were experimentally determined at 11 different pressures between 80 and 152 kbar with a split sphere anvil apparatus ussa 2000 the compositions of pyroxene garnet and melt coexisting along the solidus were also determined comparison with previous data at lower temperatures revealed a large temperature dependence of the compositions of garnet coexisting with pyroxene it is proposed that the observed variation in the garnet composition is caused by disorder in garnet the obtained alumina contents of orthopyroxene coexisting with garnet on the solidus at 80 110 kbar contributed to an improvement of the garnet peridotite thermobarometry an internally consistent set of thermodynamic parameters was derived which allows calculation of the temperature pressure phase diagram for the enstatite pyrope join at 20 260 kbar

climatic deterioration and the end of the gortian interglacial in sediments from derrynadivva and burren townland near castlebar county mayo ireland pollen and macrofossil analyses of two sequences of organic sediment in western ireland have allowed the recognition of the latter parts of the gortian interglacial a middle pleistocene temperate stage the palaeobotanical information from the two sites identifies the preservation of partial sequences from the gortian interglacial believed to be the equivalent of the hoxnian stage of britain and the holsteinian of europe the pollen and macrofossil data from one of the sites derrynadivva span the middle and latter parts of the temperate stage the results show that fire influenced the vegetation throughout the middle part of the temperate stage and the termination of the organic depositional sequence appears to have been due to environmental deterioration the former is identified through the abundant occurrence of charcoal in the sediments whereas the latter is apparent in the sediment sequence and in the reworking of thermophilous pollen types into the upper part of the deposit the second site burren townland records only part of the later zones of the interglacial but the ericaceous assemblages of the latter part of the gortian interglacial are well developed and well preserved macrofossil analyses and scanning electron microscopy work on the pollen at the latter site has allowed the recognition of rhododendron ponticum adding to previous records of this species which now has a very disjunct european distribution

high pressure polymorphs of magnesian orthopyroxene from a shock vein in the yamato 000047 lherzolitic shergottite abstract we found a simple thin shock vein less than or equal to about 60 m in width and 1 8 mm in length in the poikilitic area in the yamato y 000047 lherzolitic shergottite the shock vein occurs only in magnesian ca poor clinopyroxene which may have transformed from orthopyroxene during the pressure increase at the shock event the shock vein consists of mg0 8 fe0 2 sio3 pyroxene polymorphs such as columnar akimotoite two kinds of pyroxene glasses dendritic akimotoite and framboidal pyroxene glass in the order from the periphery to the center the compositions and textures suggest that columnar akimotoite in the periphery of the shock vein crystallized from solid state phase transition of clinopyoroxene during the cooling of the vein and the remains in the shock vein solidified from shock produced melt the glass includes two kinds of massive glass in the vein and framboidal glass in the vein center the framboidal glass is the most magnesian and may have been vitrified from perovskite crystallized from high pressure melt produced at high temperature 3000 c and high pressure 23 40 gpa dendritic akimotoites in the vein center metastably crystallized from residual shock melt the formation sequences of the constituent phases in the shock vein happen in the following order columnar akimotoites rim glass center glass framboidal glass and dendritic akimotoites the increase of the raman intensity of 660 670 cm 1 in the order of rim glass center glass and framboidal glass suggests that the formation of the pyroxene chain proceeds faster in the vein center than in the vein rim due to its slower cooling the finding of the shock vein consisting merely of high pressure polymorphs of pyroxene akimotoite and framboidal glass vitrified perovskite is the first reported among all martian meteorites