phlogopite and quartz lamellae in diamond bearing diopside from marbles of the kokchetav massif kazakhstan exsolution or replacement reaction exsolution lamellae of pyroxene in garnet grt coesite in titanite and omphacite from uhpm terranes are widely accepted as products of decompression however interpretation of oriented lamellae of phyllosilicates framework silicates and oxides as a product of decompression of pyroxene is very often under debate results are presented here of fib tem feg emp and synchrotron assisted infrared ir spectroscopy studies of phlogopite phlog and phlogopite quartz qtz lamellae in diamond bearing clinopyroxene cpx from ultra high pressure uhp marble these techniques allowed collection of three dimensional information from the grain boundaries of both the single phlogopite two phase lamellae phlogopite quartz and fluid inclusions inside of diamond included in k rich cpx and understanding their relationships and mechanisms of formation the cpx grains contain in their cores lamellae i which are represented by topotactically oriented extremely thin lamellae of phlogopite that generally are two units cell wide but locally can be seen to be somewhat broader and microdiamond the core composition is ca sub 0 94 k sub 0 04 na sub 0 02 al sub 0 06 fe sub 0 08 mg sub 0 88 si sub 1 98 al sub 0 02 o sub 6 00 fluid inclusions rich in k and si are recognized in the core of the cpx having no visible connections to the lamellae i lamellar ii inclusions consist of micron size single laths of phlogopite and lens like quartzmore or slightly elongated phlogopite quartz intergrowths all are situated in the rim zone of the cpx the composition of the rim is ca sub 0 95 fe sub 0 03 na sub 0 02 al sub 0 05 fe sub 0 05 mg sub 0 90 si sub 2 o sub 6 and the rim contains more ca mg than the core with no k there such chemical tests support our microstructural observations and conclusion that the phlogopite lamellae i are exsolved from the k rich cpx precursor during decompression it is assumed that cpx precursor was also enriched in h sub 2 o because diamond included in the core of this cpx contains fluid inclusions the synchrotron ir spectra of such diamond record the presence of oh sup stretching and h sub 2 o bending motion regions lamellar ii inclusions are interpreted as forming partly because of modification of the lamellae i in the presence of fluid enriched in k fe and si during deformation of the host diopside the latter is probably related to the shallower stage of exhumation of the uhp marble this study emphasizes that in each case to understand the mechanism of lamellar inclusion formation more detailed studies are needed combining both compositional structural and three dimensional textural features of lamellar inclusions and their host less

intercalibration of astronomical and radioisotopic time the 40ar 39ar radioisotopic dating technique is one of the most precise and versatile methods available for dating events in earth s history but the accuracy of this method is limited by the accuracy with which the ages of neutron fluence monitors dating standards are known calibrating the ages of standards by conventional means has proved difficult and contentious the emerging astronomically calibrated geomagnetic polarity time scale apts offers a means to calibrate the ages of 40ar 39ar dating standards that is independent of absolute isotopic abundance measurements seven published 40ar 39ar dates for polarity transitions nominally ranging from 0 78 to 3 40 ma are based on the fish canyon sanidine standard and can be compared with apts predictions solving the 40ar 39ar age equation for the age of the fish canyon sanidine that produces coincidence with the apts age for each of these seven reversals yields mutually indistinguishable estimates ranging from 27 78 to 28 09 ma with an inverse variance weighted mean of 27 95 0 18 ma normalized residuals are minimized at an age of 27 92 ma indicating the robustness of the solution

the messinian event on the margin of the mediterranean sea in the nice area southern france abstract seismic reflection profiles have been used to map the messinian erosion surface and associated basin margin deposits on the continental margin off nice a palaeovalley extends seaward from the incised var palaeovalley mapped on land the lower part of this valley is filled by an acoustically incoherent unit with a flat top and steep slope into the basin this unit is interpreted as a coarse grained deltaic deposit that accumulated synchronously with the upper messinian evaporites in a ponded lake basin two submersible dives on strategic outcrops supported the interpretation of the seismic profiles we examined and sampled outcrops of probably early tertiary sandstones underlying the messinian erosion surface and of a fining up predominantly conglomeratic sequence that fills messinian channels on the continental slope and probably overlies the deltaic deposits these conglomerates pass up into early pliocene marls and have turbidite sedimentary structures these are prodeltaic deposits formed during the transgression at the end of the messinian based on seismic facies mapping sampling and submersible observations a palaeogeographic reconstruction of the area is proposed a basal erosion surface including the var palaeovalley represents extreme dessication of the western mediterranean and was followed by accumulation of the main messinian salt following this a lake was ponded in the ligurian basin with accumulation of evaporites and shales a gilbert type delta prograded into the basin from the var palaeovalley relative sea lake level lowering is estimated to be about 1200 m at this time there has been little faulting on the margin since the early pliocene transgression but there is estimated to have been about 800 m of flexural subsidence due to both thermal subsidence and water loading

coesite bearing eclogites from the dabie mountains central china petrogenesis p t paths and implications for regional tectonics coesite and kyanite bearing eclogites are abundant in the southern part of the dabie mountains southern dabie terrane sdt two types of eclogites from the sdt types iii and iv were selected for detailed paragenetic study type iii eclogites most abundant in the northern part of the sdt occur as blocks in gneisses and marble and contain eclogitic assemblages of omphacite garnet phengite epidote coesite kyanite carbonate rutile ilmenite these minerals exhibit weak compositional zoning and contain few mineral inclusions type iv eclogites mostly in the southern part of the sdt occur as coherent layers interbedded with gneisses and amphibolites and have assemblages of omphacite garnet glaucophane kyanite epidote phengite quartz rutile ilmenite garnets of type iv eclogites exhibit a prograde compositional zoning and have mineral inclusions of paragonite phengite epidote quartz and rutile in the core and omphacite barroisite and mg katophorite in the rim prograd

messinian erosional and salinity crises view from the provence basin gulf of lions western mediterranean abstract though the late miocene messinian salinity crisis has been intensely researched along the circum mediterranean basins few studies have focused on the central part of the mediterranean basin and especially the pre salt deposits to improve our knowledge of the messinian events it is imperative to better understand this domain in this study we provide a more complete understanding of this central domain in the provence basin we were able to recognize a thick marine detrital series up to 1000 m derived from the messinian subaerial erosion which is partly prolongated in the distal part by b a thick unit of deep marine deposits up to 800 m prior to the evaporites c a thick presumed alternation of detritals and evaporites 1500 m below the mobile halite and d a two step transgression at the end of the messinian spatially we document the eroded shelf to the deep basin and from the western to the eastern parts of the gulf of lions and temporally we extend the interpretations from the early deposition of detritic sediments to the final sea level rise the results provide a new basis for discussion not only for the development of the messinian salinity crisis but also for the reconstruction of the subsidence history of the provence basin

reconciling astrochronological and 40ar 39ar ages for the matuyama brunhes boundary and late matuyama chron when five matuyama brunhes m b boundary records from the north atlantic are placed on isotope age models produced by correlation of the 18o record directly or indirectly to an ice volume model the m b boundary lies consistently at the young end of marine isotope stage 19 with a mean age for the midpoint of the reversal of 773 1 ka standard deviation 0 4 kyr 7 kyr younger than the presently accepted astrochronological age for this polarity reversal 780 781 ka two recently proposed revisions of the age of the 40ar 39ar fish canyon sanidine fcs standard to 28 201 0 046 ma and 28 305 0 036 ma would adjust 40ar 39ar ages applicable to the m b boundary and other reversals and excursions back to 1 2 ma to ages older than the new astrochronological ages by 8 24 kyr the variables used to construct the ice volume models cannot account for the discrepancy the fcs standard age that best fits the astrochronological ages is 27 93 ma which is within the uncertainty associated with the commonly used value of 28 02 0 16 ma but younger than the recently proposed fcs ages the edc2 and edc3 age models in the dome c antarctic ice core yield ages of 771 7 ka and 766 4 ka respectively for the 10be flux peak that denotes the paleointensity minimum at the reversal boundary implying that the edc2 rather than edc3 age model is consistent with the observations from marine sediments at least close to the m b boundary

alpe arami a peridotite massif from depths of more than 300 kilometers the abundance of fetio 3 and chromite precipitates in olivine of the alpe arami peridotite massif switzerland requires a much higher solubility for highly charged cations than is found in mantle xenoliths from depths of 250 kilometers three previously unknown crystal structures of fetio 3 were identified that indicate that the originally exsolved phase was the high pressure perovskite polymorph of ilmenite implying a minimum depth of origin of 300 kilometers these observations coupled with the unique lattice preferred orientation of olivine further suggest that the original composition of olivine may reflect that of a precursor phase perhaps wadsleyite that is stable only at depths greater than 400 kilometers

a refined astronomically calibrated 40ar 39ar age for fish canyon sanidine abstract intercalibration between the astronomical and radio isotopic dating methods provides a means to improving accuracy and reducing uncertainty of an integrated multi chronometer geologic timescale here we report a high precision 40 ar 39 ar age for the fish canyon sanidine fcs neutron fluence monitor by multi collector noble gas mass spectrometry through cross calibration with a1 tephra sanidines a1ts of the direct astronomically tuned faneromeni section crete the astronomically intercalibrated 40 ar 39 ar age of fcs of 28 172 0 028 ma 2 external errors is within the uncertainty of but more precise 0 10 than the previous 40 ar 39 ar age determined by intercalibration with astronomically tuned tephras from the melilla basin morocco using this proposed age for fcs combined with measurements using the a1ts as the neutron fluence monitor a weighted mean bishop tuff 40 ar 39 ar sanidine age of 0 7674 0 0022 ma 2 external errors is indistinguishable from the id tims u pb zircon age 0 7671 0 0019 ma the consistency between the astronomically calibrated 40 ar 39 ar sanidine age and u pb zircon age for this quaternary unit suggests that agreement between these two radio isotopic dating techniques is now achievable at better than 0 3 2 in the youngest part of geologic time

regional isostatic response to messinian salinity crisis events abstract the salinity crisis of the mediterranean during messinian time was one of the most dramatic episodes of oceanic change of the past 20 or so million years resulting in the deposition of kilometer thick evaporitic sequences a large and rapid drawdown of the mediterranean water level caused erosion and deposition of non marine sediments in a large lago mare basin both the surface loading by the lower messinian evaporites and the removal of the water load resulted in isostatic flexural rebound that significantly affected river canyons and topographic slopes we use flexure models to quantitatively predict possible signatures of these events and verify these expectations at well studied margins the highly irregular shape of the reconstructed basin calls for a three dimensional model near basin margins plate bending effects are most pronounced which is why flexure is particularly important for a relatively narrow basin like the mediterranean we focus on one specific sea level scenario for the messinian salinity crisis where most of the evaporite load was deposited during a sea level highstand followed by a rapid desiccation evaporite loading at current sea level is expected to cause subsidence of the deep basins by hundreds of meters and simultaneous uplift of continental parts of the margins differential uplift may lead to significant slope angle changes and thus gravity flows the relative scarcity of lower evaporite sequences along the margins may be a result of these phenomena normal faulting of lower evaporite and older sediments and rocks is expected on the margins desiccation enhances erosion of the freshly exposed continental shelf and slope subsidence and riverbed sedimentation occurs on the continental margins and significant uplift towards the basin center reverse faulting is predicted at the margins finally regional isostatic uplift following zanclean flooding is predicted to destabilize margin slope deposits and to cause marginal uplift river down cutting and normal faulting

supersilicic clinopyroxene and silica exsolution in uhpm eclogite and pelitic gneiss from the kokchetav massif kazakhstan abundant exsolved quartz rods occur in matrix clinopyroxene of eclogite from the kokchetav massif kazakhstan these rocks are diamond grade ultrahigh pressure uhp metamorphic rocks that recrystallized at p 6 gpa and t 1000 c zircon is an excellent container which effectively protects peak uhp metamorphic phases from retrogression therefore to ascertain the pre exsolution composition of the clinopyroxene we analyzed clinopyroxene inclusions in zircon of the eclogite and a diamond bearing biotite gneiss clinopyroxene in zircon has an excess of fe 3 vi al over iv al na k and calculated cation totals significantly less than 4 0 per six o atoms the stoichiometry of these pyroxenes can be reconciled if the ca eskola end member ca 0 5 0 5 alsi 2 o 6 is considered the zircon hosted clinopyroxene in the eclogite contains up to 9 6 mol of the ca eskola component and in the biotite gneiss up to 18 2 mol whereas the matrix clinopyroxene contains much less 1 3 mol on average recalculation of the composition of the matrix clinopyroxene prior to exsolution of quartz rods yields 6 8 mol ca eskola component which is consistent with the composition of the clinopyroxene inclusions in zircon we conclude that the ca eskola component in the peak metamorphic clinopyroxenes broke down by a retrograde reaction 2 ca 0 5 square 0 5 alsi 2 o 6 rightarrow caal 2 sio 6 3 sio 2 resulting in exsolution of the quartz rods in the matrix clinopyroxene subducted crustal and supracrustal rocks form the ca eskola clinopyroxene at high pressures and temperatures the vacancy containing clinopyroxene may have an important bearing on the physico chemical properties of the subducted slab at upper mantle depth

an intercalibration study of the fish canyon sanidine and biotite 40ar 39ar standards and some comments on the age of the fish canyon tuff the age of the fish canyon tuff fct has been the subject of intense study due to its potential significance as a geochronological test to establish age concordance between the k ar and u pb isotopic systems minerals from the fct notably sanidine and biotite are in widespread use as 40ar 39ar fluence monitors standards and have been employed as both primary and intercalibrated standards in attempting to constrain the true age of fct the distinction between primary vs intercalibrated ages is important as only the primary age determinations are valid an investigation of both fish canyon sanidine fcs and fish canyon biotite fct 3 was undertaken to assess the chemical and isotopic homogeneity of the minerals on a grain to grain scale electron microprobe results show that fcs is chemically homogeneous verifying previous studies although there are some very small zones of minor k depletion fct 3 biotite commonly contains numerous and small mineral inclusions 40ar 39ar experiments further show that fcs is remarkably homogeneous confirming previous observations whereas fct 3 shows much greater isotopic variability due to contamination from the mineral inclusions by setting the age of fcs equal to 27 98 0 15 ma 2 as intercalibrated to the u pb system by villeneuve et al 2000 the resultant intercalibration factor between fcs and fct 3 is 1 005 0 017 yielding an age of 28 13 0 47 ma 2 for fct 3 although the significant intragrain chemical and isotopic variability precludes its use as an effective 40ar 39ar fluence monitor the true age of the fct is problematic a survey of all primary ages for fct shows that mean k ar ages for plagioclase hornblende biotite and sanidine are consistently younger than mean u pb ages from titanite and zircon in conjunction with recently published petrological results which indicate that the fct contains material from multiple crystallization events this significant discrepancy between the k ar and u pb data sets demonstrates that the fish canyon tuff cannot serve as a golden standard between the k ar and u pb systems

new insights into the mechanism for chloritization of biotite using polytype analysis near atomic resolution transmission electron microscopy was used to investigate the chloritization mechanisms of biotite in a granitic rock and to reveal polytypic details of the resulting chlorite comparison of stacking sequences in 2 m 1 and longer period biotite polytypes with sequences in areas containing chlorite layers revealed that typically two biotite layers transform to one chlorite layer losing two potassium interlayer sheets and two tetrahedral sheets in some cases more than two biotite layers are replaced by one chlorite layer less commonly a potassium interlayer sheet is replaced by a brucite like sheet one biotite layer is transformed to one serpentine layer via loss of a potassium interlayer and a tetrahedral sheet in places based on the relative frequency of the two chlorite layers to one biotite layer vs one biotite layer to one chlorite layer mechanisms the net result of chloritization is a considerable volume decrease along c near atomic resolution images recorded down 010 310 or formula 1 revealed that the chlorite polytype in the biotite chlorite interstratifications is predominantly ii bb however mixtures of i bb i ab ii ab and ii bb also occur the aa stacking sequences were never found chlorite polytypes may be determined in part by the chloritization mechanisms and in part by relief of local shear by a 3 displacements 1 embed tex math 1 gif

clinoenstatite in alpe arami peridotite additional evidence of very high pressure observations by transmission electron microscopy show that lamellae of clinoenstatite are present in diopside grains of the alpe arami garnet lherzolite of the swiss alps the simplest interpretation of the orientation crystallography and microstructures of the lamellae and the phase relationships in this system is that the lamellae originally exsolved as the high pressure c centered form of clinoenstatite these results imply that the rocks were exhumed from a minimum depth of 250 kilometers before or during continental collision

ultra deep subduction of yematan eclogite in the north qaidam uhp belt nw china evidence from phengite exsolution in omphacite phengite exsolution in omphacite from the yematan eclogite north qaidam uhp belt nw china is described mineralogical investigations show that the precursor omphacite in the yematan eclogite contains up to 1 16 wt k2o and 10 000 ppm h2o experimental studies document this omphacite to be formed at pressures higher than 6 gpa at 900 c the pressure temperature conditions of 3 68 gpa and 892 c for phengite exsolution in omphacite associated with garnet during the exhumation were obtained by using grt omp ph geothermobarometer we conclude that omphacitic clinopyroxene in subducted eclogites may act as a robust medium to transport h2o and potassium deep into the interior of the earth this study suggests that the yematan eclogite in north qaidam uhp metamorphic belt nw china may have been subducted and exhumed from depths of more than 200 km

thermal and compaction processes in a young rifted basin containing evaporites gulf of lions france the gulf of lions northwestern mediterranean is a young oligocene miocene rifted basin characterized by the presence of a thick evaporitic sequence observed data surface heat flow well temperatures geopressures below the evaporites crustal thinning are used to model 1 the basin forming mechanisms and 2 the intrasedimentary processes that influence the temperature history and thus the maturation of source rocks a two dimensional finite difference basin model temispack simulates the sedimentation compaction and overpressures related to the low permeability of the post rift sequence the heat transfer from the rifted lithosphere into the sediments and the level of maturation by using a kinetic approach the classic passive margin model which relates the history of heat flow and the rate of subsidence to a uniform thinning factor yields subsidence and heat flow predictions that agree with observations the recently accelerated subsidence observed in the deep margin is probably an effect of recent regional compression rather than the consequence of alternative rifting mechanisms the history of maturation of organic matter was influenced by the variation of the regional heat flow due to the rifting the effect of sedimentation or blanketing and the variability of the lithologies and porosities the undercompaction of sediments underlying the evaporites is accounted for by the model which suggests that hydraulic fracturing is taking place at present in the overpressured sediments fluid flow associated with the compaction disequilibrium has negligible thermal effect undercompacted shales below the evaporites have lower thermal diffusivity and thus higher temperatures which has a significant effect on maturation

the 10a phase a high pressure expandable sheet silicate stable during subduction of hydrated lithosphere h2o storage and release in deep subducting lithosphere is controlled by complex reaction suites involving a variety of hydrous phases as a result of its relatively large thermal stability and intermediate composition the 10a o phase mg3si4o10 oh 2wnh2o has been regarded as a relevant h2o reservoir in a wide range of rock compositions and mineral assemblages high pressure syntheses of the 10a o phase were carried out at 6 7 gpa and 650 c under fluidsaturated conditions in a walker type multi anvil apparatus from 5 min to 430 h x ray powder diffraction of large platy hexagonal crystals of the 10a o phase up to 100 wm were indexed on the basis of a trioctahedral type structure long term run products s 110 h reveal sensitivity of the 10a o phase to treatment with acetone leading to the appearance of diffractions at greater d spacings 10 2 11 6 a o with respect to the basal peak of the 10a o phase 9 64 10 07 a o this swelling behavior is strongly related to synthesis run duration the raman spectrum of the 10a o phase at frequencies less than 800 cm 31 shows a strong similarity to talc in the si o stretching region 800 1100 cm 31 the 10a o phase exhibits three modes 909 992 and 1058 cm 31 as compared to two in talc the bending mode of water x2 i s found at 1593 cm 31 in the oh stretching region peaks at 3593 3622 and 3668 cm 31 were observed the acetone treated sample shows a c h stretching mode at 2923 cm 31 while the double bond cno signal is absent the swelling behavior of the 10a o phase is interpreted as due to intercalation of acetone with pre existing interlayer water the efficiency of this process is dependent on the amount of the interlayer water which in turn depends on run duration the relation between the response to acetone treatment and run duration is therefore interpreted as a time dependent hydration of the 10a o phase the fractions transformed from non expandable to expandable fractions was fitted to the avrami empirical law which suggests that kinetics are mainly controlled by diffusion rather than phase boundary reactions the ability to accommodate variable amounts of h2o makes the 10a o phase a major h2o sink whenever a hydrous phase such as chlorite and serpentine breaks down during prograde transformations in the subducted lithosphere under h2o saturated conditions a fully hydrated 10a o phase occurs when h2o undersaturated conditions prevail a h2o deficient 10a o phase incorporates the volatile component available the exchange capacity of interlayer molecules in the 10a o phase structure opens new scenarios on the control of fluid compositions escaping from subducted

a critical re evaluation of the miocene pliocene boundary as defined in the mediterranean abstract a critical re evaluation of the miocene pliocene m p boundary as defined informally in the mediterranean is presented we show that ages reported earlier for this boundary ranging from 4 81 to 4 93 ma do not imply any diachroneity of the basal zanclean lithohorizon which reflects the pliocene flooding of the mediterranean and defines the m p boundary these ages most likely represent an artefact resulting from intraformational variations in sedimentation rate and inconsistent interpretation of coluor variations in terms of sedimentary cycles the best estimate for the age of this boundary arrives at 4 86 ma if the conventional geomagnetic polarity time scale gpts of berggren et al 1 is used and at 5 16 ma if the new gpts of cande and kent 2 is used an age of 5 32 ma is obtained if the astronomically calibrated time scale of hilgen 3 is adhered to because of its presumably global paleoclimatological significance the m p boundary as currently defined in the mediterranean is preferred to a redefinition of this boundary at the gilbert chron 5 boundary in a continous open marine sequence outside the mediterranean in addition this redefinition would lead to unnecessary confusion in global chronostratigraphy

messinian pre evaporite sapropels and precession induced oscillations in western mediterranean climate abstract cyclical fluctuations in planktic foraminiferal assemblages have been recognized in the pre evaporitic messinian in a marginal basin of the western mediterranean the fluctuations coincide with a dominantly precession controlled sedimentary cyclicity sapropels during sapropel deposition high planktic foraminiferal diversities are indicative of relatively stable marine conditions while during homogeneous marl deposition low diversities seem to indicate the presence of unfavourable more saline surface water conditions the dominance of a precession related signal indicates that regional climate oscillations rather than obliquity related glacio eustatically controlled influxes of atlantic and or mediterranean waters are responsible for the faunal fluctuations and sedimentary cyclicity our scenario links the persistence of normal marine conditions during sapropel formation with increased rainfall and run off along the western mediterranean at times that perihelion occurred in northern hemisphere summer less favourable highly saline surface water conditions prevailed during periods of drier climate induced by opposite precessional extremes the cyclical oceanographic fluctuations could also have governed periodic reef growth along the margins

low pressure and low temperature k bearing kosmochloric diopside from the osayama serpentinite melange sw japan kosmochloric diopside with high k content up to 0 56 wt 0 026 k atom per formula unit was discovered from the osayama serpentinite melange in the chugoku mountains sw japan k bearing clinopyroxene fills microcracks 5 150 m in wide together with uvarovite within albite vein of a tremolite rock compositions of analyzed clinopyroxene consist mainly ofkosmochlor augite 92 98 mol ko 1 9 3 8 aug 5 6 7 6 components and minor amounts of jadeite 0 6 mol aegirine 0 5 mol ca tschermak 0 3 mol and k kosmochlor 0 2 mol although the k content in clinopyroxene is also variable and heterogeneous even in a single vein clinopyroxene with higher k content occurs in ko rich part higher magnification secondary electron images confirmed that exsolution and inclusion are essentially absent in the analyzed clinopyroxenes the good negative correlation between cr na k and ca mg fe 2 indicates the cr incorporation into the octahedral site furthermore k correlates with na and cr indicating a simultaneous enrichment of k for na and cr during pyroxene growth textual relations and parageneses and compositions of minerals suggest that the k cpx precipitated together with uvarovite in brittle microcracks directly from a ca and cr rich hydrothermal fluid at approximately p 4 gpa environment permits to host relatively large k cation into the clinopyroxene structure our finding indicates that the incorporation of k into the kosmochlor diopside series solid solution with at least 0 2 cr cation p f u is possible even at low p conditions

precise k ar 40ar 39ar rb sr and u pb mineral ages from the 27 5 ma fish canyon tuff reference standard the accuracy of ages measured using the 40ar 39ar technique is affected by uncertainties in the age of radiation fluence monitor minerals at present there is lack of agreement about the ages of certain minerals used as fluence monitors the accuracy of the age of a standard may be improved if the age can be measured using different decay schemes this has been done by measuring ages on minerals from the oligocene fish canyon tuff fct using the k ar 40ar 39ar rb sr and u pb methods k ar and 40ar 39ar total fusion ages of sanidine biotite and hornblende yielded a mean age of 27 57 0 36 ma the weighted mean 40ar 39ar plateau age of sanidine and biotite is 27 57 0 18 ma a biotite feldspar rb sr isochron yielded an age of 27 44 0 16 ma the u pb data for zircon are complex because of the presence of precambrian zircons and inheritance of radiogenic pb zircons with 207pb 235u 0 4 yielded a discordia line with a lower concordia intercept of 27 52 0 09 ma evaluation of the combined data suggests that the best age for fct is 27 51 ma

stability of phlogopite in granitic melts an experimental investigation water saturated and water undersaturated experiments ah2o 1 0 and 0 5 were performed in the temperature range 780 1040 c at 2 and 5 kbar in order to determine the upper thermal stability of phlogopite in granitic melts starting compositions were a subaluminous mixtures of 20 wt synthetic phlogopite and 80 wt synthetic anhydrous haplogranitic glass b peraluminous mixtures normative corundum 4 of 20 wt synthetic phlogopite and 80 wt synthetic anhydrous peraluminous haplogranitic glass the molar quartz albite orthoclase ratio of the glasses of the 2 kbar runs was 35 39 26 and that of the 5 kbar runs 30 42 28 in the subaluminous system phlogopite is stable up to 820 c at ah2o 1 0 and up to 780 c at ah2o 0 5 at higher temperatures it is replaced by enstatite in the peraluminous system phlogopite has a remarkably higher thermal stability up to 1000 c at 5 kbar and ah2o 1 0 and there is a temperature interval of 80 c at ah2o 1 0 and 90 100 c at ah2o 0 5 between the first appearance of enstatite and the disappearance of phlogopite in the peraluminous system phlogopite is a solid solution ss of phlogopite muscovite talc and eastonite components the crystalline product of the phlogopitess breakdown reaction is an aluminous enstatite the mgo content of the melt depends on the normative corundum content of the starting material and the run temperature it is independent of pressure in the subaluminous system the mgo content ranges between 0 05 and 0 3 wt in the temperature interval 780 880 c at both investigated water activities the mgo content of the peraluminous melts at ah2o 1 0 ranges between 0 4 and 1 7 wt and at ah2o 0 5 between 0 2 and 1 4 wt in the temperature range 780 980 c

coesite bearing eclogite from the dabie mountains in central china coesite and coesite pseudomorphs are recognized in eclogite from the dabie mountains a collision zone between the sino korean and yangtze cratons in central china this third known occurrence of coesite in deep crustal rocks is within an archean gneiss terrane and is the first locality where coesite has been identified as an inclusion in both omphacite and garnet crystals at the same location coesite bearing eclogite is estimated to form at pressures of 28 kbar and temperatures from 600 to 710 c the dabie mountains eclogite underwent retrograde metamorphism to amphibolite facies after its formation the preservation of coesite in eclogite is found to be strongly controlled by the pressure temperature time path that coesite bearing eclogite followed during its uplift and retrograde metamorphism

precipitation of pyroxenes and mg2sio4 from majoritic garnet simulation of peridotite exhumation from great depth our experimental simulations of the exhumation path of mantle peridotites show that high temperature 1400 c decompression of lherzolite from 14 to 13 and 12 gpa results in exsolution of interstitial blebs of diopside and mg2sio4 wadsleyite lamellae from majoritic garnet at lower pressures from 8 to 5 gpa at t 1400 c only enstatite exsolves as blebs at garnet boundaries continuous high temperature decompression from 14 to 7 gpa produces zoned majoritic garnet containing blebs of exsolved pyroxenes inside garnet rims no intracrystalline precipitation of pyroxene was observed in garnet although such lamellae are found in some natural garnet peridotites the explanation appears to be the three orders of magnitude difference in grain size between experimental and natural specimens our data suggest that mg2sio4 and diopside exsolutions reflect the deepest point of the exhumation path of garnet peridotites whereas enstatite precipitation may be restricted to garnets with less majoritic component at shallower depths

the argus multicollector noble gas mass spectrometer performance for 40ar 39ar geochronology we describe a new high sensitivity multicollector noble gas mass spectrometer argus that has been specifically designed for simultaneous collection of all ar isotopes and hence is ideally suited for 40ar 39ar geochronology the instrument uses a detector housing that holds five faraday collectors one collector is equipped with a 1011 ohm resistor for 40ar and the remaining four are fitted with 1012 ohm resistors for 36ar to 39ar argus has a mass resolution of 225 250 and a measured sensitivity of 7 10 14 mol v at 200 a trap current during the course of a 1 month run cycle 145 air calibrations yielded a weighted average 40ar 36ar of 300 67 0 07 2 eight data points rejected and mean square weighted deviation 1 18 the ages of three mineral standards taylor creek rhyolite sanidine heidelberg biotite and alder creek sanidine are within error of the accepted ages and uncertainties for two are improvements on previously published data argus decreases analytical time while allowing more measurements to be made at higher precision compared to standard peak jumping single collector mass spectrometers

the late messinian salinity crisis and late miocene tectonism interaction and consequences on the physiography and post rift evolution of the gulf of lions margin the interpretation of rhone maritime seismic profiles and the accurate mapping of all the structural features affecting the miocene cover made it possible for us to develop herein a new structural model for the top of the eroded miocene of the gulf of lions platform late miocene extensional faults exerted a considerable structural control on the messinian erosion in particular mapping of transfer zones has shown that these structures controlled retrogressive erosion of the platform this late miocene extension could have been caused by gravity driven destabilization of the platform associated with base level lowering at the onset of the messinian salinity crisis

calibration of ga1550 biotite standard for k ar and 40ar 39ar dating abstract the mineral separate ga1550 biotite has become an international standard for k ar and 40 ar 39 ar dating studies although it was prepared as an intralaboratory standard at anu to monitor tracer depletion from a gas pipette it is one of a small number of samples that has been calibrated against 38 ar tracers some of which had been mixed with known amounts of atmospheric argon so that a so called primary calibration has been performed by measuring ga1550 biotite against additional tracers from the same batch we have determined the radiogenic argon content of this sample as 1 342 0 007 10 9 mol g and together with the measured k content of 7 645 0 050 weight percent we derive a best estimate for the k ar age as 98 5 0 5 ma where the error is derived from averaging the ages determined relative to the 38 ar tracer

comment on the joint determination of 40k decay constants and 40ar 40k for the fish canyon sanidine standard and improved accuracy for 40ar 39ar geochronology by paul r renne et al 2010

unusual exsolution phenomena in omphacite and partial replacement of phengite by phlogopite kyanite in an eclogite from the erzgebirge in the erzgebirge crystalline complex eclogites occur in three different high pressure hp units 1 2 and 3 recording contrasting pressure p temperature t conditions eclogites from hp unit 1 experienced peak metamorphic conditions in the coesite stability field at about 33 kbar 850 c commonly these eclogites from hp unit 1 are all very similar with an eclogitic peak assemblage of omphacite garnet coesite k feldspar rarely accompanied by kyanite and omphacites systematically deviating from a stoichiometric composition in contrast an eclogite recently found near blumenau is mineralogically and geochemically different from the typical eclogites of hp unit 1 this unusual eclogite reveals the eclogitic equilibrium assemblage omphacite garnet coesite phengite phlogopite kyanite and yields metamorphic peak conditions of 870 c and 29 kbar there is clear textural evidence of the formation of phlogopite and kyanite under partial consumption of phengite and garnet moreover the omphacite is stoichiometric and contains abundant exsolution lamellae the thickest of which were identified as quartz by the electron microprobe the finer lamellae were studied by transmission electron microscopy tem oligoclase was identified as an exsolution phase other lamellae proved to consist of k white mica also interpreted as exsolution prior to exsolution the omphacite composition must have been cation deficient as that of the other common hp unit 1 eclogites these non stoichiometric compositions are ascribed to partial substitution by the ca eskola pyroxene component which calculates to an average of 8 mol for omphacite in hp unit 1 eclogites according to experiments this substitution becomes significant at p 30 kbar exsolution of k white mica may indicate hydroxyl defects in the original omphacite also favoured by high pressure oligoclase and k white mica exsolution from ca eskola rich clinopyroxene has not previously been reported the omphacite has a disordered c2 c structure and in just one case very small a few tens of nanometres antiphase domains resulting from the c2 c to p2 n transformation are present these features may indicate a brief thermal history and rapid tectonic processes

direct comparison of astronomical and 40ar 39ar ages of ash beds potential implications for the age of mineral dating standards we present first results of 40ar 39ar multiple single fusion datings on biotites and feldspars from two volcanic ash beds found on crete gavdos and koufonisi greece preferred 40ar 39ar ages calculated against tcr sanidine with an age of 27 92 ma intercalibrated to an age of 28 09 0 10 ma 1 for fct 3 biotite and an age of 24 99 0 07 ma for dra sanidine are slightly but consistently younger than astronomical ages obtained independently for the same ash beds the best fit to the astronomical ages is obtained when the age of tcr sanidine is increased slightly to 27 98 0 19 ma the age of fct 3 biotite to 28 15 0 19 ma and the age of dra sanidine to 25 05 0 17 ma the ages for the standards arrive slightly younger if the 40ar 39ar age of 6 936 0 006 ma 1 for a single pure sanidine separate of the lower ash dated astronomically at 6 941 ma is considered most reliable

sodic amphibole exsolutions in garnet from garnet peridotite north qaidam uhpm belt nw china implications for ultradeep origin and hydroxyl defects in mantle garnets two types of amphibole exsolution lamellae were discovered in garnet from a few garnet peridotites from the north qaidam ultrahigh pressure metamorphic belt northern tibet nw china the amphibole lamellae are strictly oriented in four directions corresponding to the isometric form 111 i e octahedron planes of garnet observations by transmission electron microscopy tem show that amphibole lamellae are topotaxtially concordant with the host garnet and possess 9 8 to 10 a lattice c spacing electron microprobe analysis reveals that these exsolved amphibole lamellae are high in na and ti the tem observations and recalculated compositions indicate that the oriented amphibole lamellae exsolved from original supersilicic majorites with high concentrations of na2o 0 3 wt and hydroxyl up to 1000 ppm by weight these results imply that the host garnet peridotites were formed at depths greater than 200 km and that garnet can be an important reservoir of water at such depths in the mantle

hydroxyl defects in garnets from mantle xenoliths in kimberlites of the siberian platform a suite of more than 200 garnet single crystals extracted from 150 xenoliths covering the whole range of types of garnet parageneses in mantle xenoliths so far known from kimberlites of the siberian platform and collected from nearly all the kimberlite pipes known in that tectonic unit as well as some garnets found as inclusions in diamonds and olivine megacrysts from such kimberlites were studied by means of electron microprobe analysis and single crystal ir absorption spectroscopy in the voh vibrational range in search of the occurrence energy and intensity of the voh bands of hydroxyl defects in such garnets and its potential use in an elucidation of the nature of the fluid phase in the mantle beneath the siberian platform the voh single crystal spectra show either one or a combination of two or more of the following major voh bands i 3645 3662 cm 1 ii 3561 3583 cm 1 iii 3515 3527 cm 1 and minor bands ia 3623 3631 cm 1 iia 3593 3607 cm 1 the type of combination of such bands in the spectrum of a specific garnet depends on the type of the rock series of the host xenolith mg mg ca ca mg fe or alkremite on the xenolith type as well as on the chemical composition of the respective garnet nearly all garnets contain band systems i and ii band system iii occurs in ti rich garnets with wt tio2 ca 0 4 from xenoliths of the mg ca and mg fe series only the voh spectra do not correspond to those of oh defects in synthetic pyropes or natural ultra high pressure garnets from diamondiferous metamorphics there were no indications of voh from inclusions of other minerals within the selected 60 60 m measuring areas in the garnets the voh spectra of pyrope knorringite and pyrope knorringite uvarovite rich garnets included in diamonds do not show band systems i to iii instead they exhibit one weak broad band voh 200 460 cm 1 near 3570 cm 1 a result that was also obtained on pyrope knorringite rich garnets extracted from two olivine megacrysts the quantitative evaluation on the basis of relevant existing calibrational data bell et al 1995 of the sum of integral intensities of all voh bonds of the garnets studied yielded a wide range of water concentrations within the set of the different garnets between values below the detection limit of our single crystal ir method near 2 10 4 wt up to 163 10 4 wt the water contents vary in a complex manner in garnets from different xenolith types obviously depending on a large number of constraints inherent in the crystal chemistry as well as the formation conditions of the garnets during the crystallization of their mantle host rocks secondary alteration effects during uplift of the kimberlite play if any only a minor role despite the very complex pattern of the water contents of the garnets preventing an evaluation of a straightforward correlation between water contents of the garnets and the composition of the mantle s fluid phase during garnet formation at least two general conclusions could be drawn 1 the wide variation of water contents in garnets is not indicative of regional or local differences in the composition of the mantle s fluid phase 2 garnets formed in the high pressure high temperature diamond pyrope facies invariably contain significantly lower amounts of water than garnets formed under the conditions of the graphite pyrope facies this latter result 2 may point to significantly lower fh2o and fo2 in the former as compared to the latter facies

cation deficient phlogopitic mica exsolution in diopside from garnet peridotite in sulu china article i nfo deep subduction and return to the surface of sediments and other continental rocks is now well known forming terranes of ultrahigh pressure metamorphism uhpm a common constituent of these terranes is garnet peridotite mantle rock that has been brought to the surface along with the returning subducted material microstructures of these peridotites especially precipitates within their constituent minerals offer promise of providing information not only about their depth of origin but also about the subduction environment here we report exsolution lamellae of cation deficient phlogopitic mica within diopside of a drill core from the uhpm terrane of sulu china detailed analysis by transmission electron microscopy tem shows these precipitates to be phlogopitic mica that is highly complex both structurally and compositionally with a significant component of the 10 a phase a talc like phase stable only at quite high pressures several mica polytypes are present the simplest one 1m being the most cation deficient the presence of a significant 30 10 a phase component in the mica suggests a means of expanding the 10 a phase bridge for subduction of h2o to deep mantle depths published by elsevier b v

depositional environments of the mediterranean lower evaporites of the messinian salinity crisis constraints from quantitative analyses abstract we use simple quantitative analyses to evaluate controversial water level scenarios for the mediterranean lower evaporites of the messinian salinity crisis our results indicate that a shallow water scenario for the lower gypsum units with mediterranean water level lower than the sill at gibraltar would imply unrealistic salt thicknesses on the order of 3 km some outflow to the open ocean must have persisted implying that the mediterranean was a deep water basin during lower gypsum formation since glacio eustatic fluctuations do not seem to have had a major influence on lower gypsum deposits mediterranean water level was even substantially higher than the gibraltar sill our analyses furthermore show that precessional changes in the freshwater budget may explain the observed cyclic lithological changes of gypsum and non evaporitic sediments potential precipitation of gypsum in the deep mediterranean basins would have critically depended on the availability of oxygen and thus on the stratification of the water column finally our results indicate that the deep mediterranean halite units could have been deposited under shallow conditions assuming that they correspond to the 70 kyr time interval between glacials tg12 and tg14 when mediterranean outflow to the atlantic was blocked

wa1ms a 2 61 ga muscovite standard for 40ar 39ar dating abstract the 40ar 39ar dating technique requires the use of neutron fluence monitors standards to allow calculation of the age of a sample precise calibration of these standards is crucial to obtaining accurate ages and decreasing the uncertainties associated with 40ar 39ar dates few fully intercalibrated 40ar 39ar standards with a sufficient total fusion grain to grain reproducibility are currently in use in the argon community for precambrian samples only hb3gr hornblende 1 08 ga yields sufficient grain to grain reproducibility and has an appropriate age for acceptable argon isotopic ratio measurements here we present chemical and intercalibration results for a new 2 61 ga standard wa1ms is a muscovite extracted from an archaean shear zone in the lake johnston greenstone belt western australia in situ trace element analysis by ela icpms revealed consistent k contents subtle zonation and intra grain and grain to grain heterogeneities in rb sr ti and fe but a lack of mineral inclusions wa1ms has been investigated over 3 irradiations ranging from 25 to 40 h in two reactors with several disc positions and three grains sizes and has been calibrated against fcs and ga1550 and hb3gr overall we carried out 48 total fusion and 4 step heating experiments of wa1ms crystals flat age spectra and average f value 40ar 39ark relative standard deviations ranging from of 0 43 to 0 60 p 0 15 0 83 for 47 48 analyses demonstrate the reproducibility of wa1ms and its suitability as a reliable 40ar 39ar standard we calculated r wa1ms fcs 205 59 0 25 r wa1ms ga1550 57 25 0 06 and r wa1ms hb3gr 3 9713 0 014 all with p 0 14 allowing direct comparison between wa1ms and any standards in used in the community provided that they have been calibrated against any of the three standards used in the calibration and regardless of the age adopted for each of these standards the recently revised decay constant values and standard ages proposed by renne et al 2011 yield a weighted mean age of 2614 2 1 5 ma 0 055 p 0 8 1 for wa1ms against the three standards when calibrated solely against fcs in order to minimize correlated errors between fcs vs ga1550 and hb3gr we calculate a preferred age of 2613 0 2 4 ma 0 09 which is recommended when using this standard additionally this corresponds to a 40ar 40k value of 0 3375 0 0057 10 1 for wa1ms which is independent of the value of the decay constant used for calculation and allows recalculation of the age of wa1ms for any preferred set of decay constants and standard ages using the constants proposed by steiger and jager 1977 directly calculated from the three r values yields an age of 2598 2 3 5 ma corresponding to an identical although less precise 40ar 40k value of 0 3375 0 0086 10 1 benefits of this standard include 1 an increased relative precision per crystal analyzed relative to other standards for proterozoic and archaean samples 2 no post irradiation waiting time due to the absence of ca no need to wait for 37arca decay and 3 loading of standard grains that are optically different from the unknown e g pyroxene or hornblende to avoid standard unknown mixing wa1ms is freely available to the 40ar 39ar community

self mixing magma in the ruiz peak rhyodacite new mexico usa a mechanism explaining the formation of long period polytypes of mica abstract the rhyodacite of ruiz peak volcano new mexico usa is an exceptional rock because it contains both long period and short period polytypes of mica our petrographic study shows that this rhyodacite is characterized by numerous disequilibrium textures of phenocrysts mica amphibole clinopyroxene olivine and plagioclase contained within both dark grey and reddish coloured groundmass the presence of two groundmasses as well as of disequilibrium textures reaction rims resorption dendritic skeletal morphologies etc suggests a complex magmatic history these two types of groundmass are not due to a mixing of magmas but result from a degassing process during the magma ascent in the conduit the disequilibrium textures are interpreted to be the result of small short lived convection cells in the magmatic chamber which may allow crystal crystal crystal spiral and spiral spiral interactions to occur leading to the formation of long period polytypes of mica for the first time the relationships between the crystallographic features of mica and the host rock formation are underlined in this study it follows that long period polytypes of mica can be considered markers of the complex history of magmas

messinian subaerial erosion of the proto nile delta abstract the evaporitic drawdown and subsequent dessication of the mediterranean basins during the messinian salinity crisis has been confirmed by detailed seismic mapping of the ensuing erosion surface in the nile delta region the basal messinian unconformity determined by seismic analysis and borehole correlation extends from almost sea level in the southern delta area to depths of at least 5 km in the nile cone the onlap of the contemporaneously deposited fluvio deltaic clastics of the qawasim formation occurs at between 2 5 to 4 km below present gradual initial drawdown of the mediterranean sea with occasional sudden sporadic falls is implied by the presence of entrenched meanders and impaired rejuvenation terraces in the nile canyon formed as the proto nile river cut down into the pre existing tortonian delta complex the former tortonian delta slope became an extensive linear erosional front with over 3 km of vertical relief the uppermost levels of which were protected by tabular oligocene basalt flows preserved as mesas and buttes the lower slopes were the site of coalescing piedmont fans fed by upper slope wadi systems the piedmonts merged into the qawasim fluvio deltaic deposits of the proto nile fluvial flood plain the subcrop beneath the basal messinian unconformity reveals a remarkable diversity of rock type each with its own distinctive control on the geomorphology and geometry of the ensuing messinian erosional surface fracture patterns within the oligocene basalts produced a trellis drainage network which became later captured by a dendritic system developed on less resistant prodelta shales of the former tortonian delta slope close study of seismic profiles clearly demonstrates that the basal messinian erosional disconformity can be traced northwards to depths of at least 2 5 3 0 km before becoming affected by later pliocene faulting this validates a deep dessication model for the evolution of the messinian evaporites

calibrating the duration and timing of the messinian salinity crisis in the mediterranean linked tectonoclimatic signals in thrust top basins of sicily the messinian 9salinity crisis9 which affected the mediterranean represents one of the most dramatic examples of base level fluctuation known in the geological record an amplitude of perhaps 2 km within a stage with a duration of less than 2 ma deposits within the caltanissetta basin of central sicily are used to calibrate the duration and timing of these fluctuations two successions of evaporites termed 9first cycle9 and 9second cycle9 are separated by an inter regional unconformity the first cycle is regionally regressive while the second is transgressive chronostratigraphic calibration of these deposits has been provided by a linked magnetostratigraphic structural and sedimentological study the regression was protracted the earliest evaporites in our study accumulated early in chron c3ar pre 6 88 ma and the youngest accumulated late in chron c3an post 6 0 ma during this interval the basinward shift in coastline was 70 km and in vertical section implies a relative fall in sea level at 0 3 0 4 m ka 1 lowstand probably finally occurred at 5 8 5 5 ma transgression marked by accumulation of the 9second cycle9 deposits which all record reversed magnetizations c3r apparently occurred far more rapidly 200 ka prior to the return to 9normal9 marine conditions in the central mediterranean late in chron c3r local rates of tectonic deformation are relatively slow within the thrust belt which underlies the caltanissetta basin therefore it is likely that the timing and rates of the messinian 9salinity crisis9 on sicily are generally applicable to other basins in the region and help to underpin rates of climate change within this part of the neogene

chronology causes and progression of the messinian salinity crisis the messinian salinity crisis is widely regarded as one of the most dramatic episodes of oceanic change of the past 20 or so million years 1 3 earliest explanations were that extremely thick evaporites were deposited in a deep and desiccated mediterranean basin that had been repeatedly isolated from the atlantic ocean1 2 but elucidation of the causes of the isolation whether driven largely by glacio eustatic or tectonic processes have been hampered by the absence of an accurate time frame here we present an astronomically calibrated chronology for the mediterranean messinian age based on an integrated high resolution stratigraphy and tuning of sedimentary cycle patterns to variations in the earth s orbital parameters we show that the onset of the messinian salinity crisis is synchronous over the entire mediterranean basin dated at 5 96 0 02 million years ago isolation from the atlantic ocean was established between 5 59 and 5 33 million years ago causing a large fall in mediterranean water level followed by erosion 5 59 5 50 million years ago and deposition 5 50 5 33 million years ago of non marine sediments in a large lago mare lake sea basin cyclic evaporite deposition is almost entirely related to circum mediterranean climate changes driven by changes in the earth s precession and not to obliquity induced glacio eustatic sea level changes we argue in favour of a dominantly tectonic origin for the messinian salinity crisis although its exact timing may well have been controlled by the 400 kyr component of the earth s eccentricity cycle

biopyribole evolution during tremolite synthesis from dolomite and quartz in co2 h2o fluid a series of experiments with and without sample retreatment was performed on a starting mixture of dolomite plus quartz at 0 5 gpa and 600 c in a co 2 h 2 o fluid with a mole fraction of co 2 of 0 2 for durations up to 582 h the initial reaction of dolomite and quartz led to rapid formation of talc and calcite instead of tremolite and calcite by the intended reaction 5 dolomite 8 quartz h 2 o tremolite 3 calcite 7 co 2 with continued treatment the talc calcite quartz assemblage gradually reacts to form increasing amounts of tremolite and diopside with the eventual loss of quartz and nearly complete loss of talc the detailed structure of the amphibole and the nature of the mg enrichment were revealed using aem analysis of individual amphibole crystals the most abundant defects are triple chain chain multiplicity faults cmfs which appear as isolated lamellae with single periodicity in short duration experiments and increasingly as groups of lamellae with variable multiplicity and periodicity in longer duration experiments and especially in a long duration experiment without retreatment in this latter experiment a calcian clinojimthompsonite domain extending 15 unit cells along the b axis was observed the sample after 582 h and five retreatments shows on average 3 5 true solid solution with mg expressed as the mg cummingtonite mc component after correction for the presence of cmfs this sample is thought to most closely approach the equilibrium composition for the amphibole the sample after 250 h with three retreatments has about 10 whereas that after 582 h without retreatment has 0 6 mc component the latter having a relatively high density of cmfs this study affirms the importance that precursor non amphibole biopyriboles play in the formation of tremolitic amphibole

tristan da cunha constraining eruptive behavior using the 40ar 39ar dating technique the historically active volcanic ocean island of tristan da cunha exhibits a complex and dynamic history with numerous often compositionally distinct parasitic centers punctuating the large edifi ce to date the temporal relationship between differing styles of activity has been unclear we have applied high precision 40ar 39ar dating to 15 carefully selected samples from tristan da cunha to ascertain spatio temporal relationships of recent volcanism explore episodicity and establish if the most recent summit activity post dated eruptions from parasitic centers lower on the fl anks this has yielded a new suite of reliable holocene ages with the youngest dated deposit at 3 1 ka 1 a recent fl ow at the summit was constrained to 5 1 ka 1 confi rming that summit and parasitic activity on the volcano s fl anks overlap in time the oldest dated deposits were 118 4 ka 1 from a parasitic cone in the southern sector and 81 10 ka 1 from one of the lowest sub aerial shield forming lava fl ows in the northern sector large scale sector collapse is bracketed between 34 1 ka and 26 5 ka 1 via dating of the youngest headwall lava fl ow and oldest sub aerial scarp fi lling deposits no systematic relationship between the new temporal framework vent location and eruptive compositions was found although magmatic fl ux has been inferred to be relatively low tristan da cunha is capable of relatively frequent eruptions from a wide variety of vent locations across a broad range of compositions

clinopyroxenite from the sulu ultrahigh pressure terrane eastern china origin and evolution of garnet exsolution in clinopyroxene the rizhao garnet clinopyroxenite occurs as small mantle slices faulted into subducted continental crust in the sulu ultrahigh pressure uhp terrane china two representative garnet clinopyroxenites with exsolution microstructures were studied 1 a massive ilmenite rich specimen composed of coarse grained clinopyroxene containing abundant exsolution rods of grossular rich garnet 20 vol ilmenite 3 4 vol in a fine grained matrix of clinopyroxene garnet ilmenite and 2 megacrystic garnet bearing sample characterized by garnet containing inclusions of clinopyroxene and ilmenite the clinopyroxene inclusions show exsolution rods and blebs with variable amounts of garnet 1 15 vol and minor ilmenite both exsolved and matrix garnets have similar grossular rich compositions the aggregate composition of clinopyroxene garnet ilmenite intergrowth is similar to the whole rock composition we propose that the parental phase of exsolved garnet ilmenite lamellae and clinopyroxene host was either a homogeneous clinopyroxene hypothesis a or a majoritic garnet hypothesis b that experienced three discrete evolution stages if the parental phase was clinopyroxene abo3 with an si deficiency in the b site the rizhao garnet clinopyroxenite could have an initial assemblage clinopyroxene grt ilm formed at near solidus conditions 1400 c at 5 gpa in the upper mantle the second stage is defined by garnet exsolution from the primary clinopyroxene involving decreasing temperature and or increasing pressure related to continental subduction coexisting clinopyroxene host and garnet exsolution grtexs recrystallized at temperatures of 900 c at an assumed minimum p of 5 gpa the third stage is represented by recrystallization of exsolution phases to form the fine grained matrix of cpx grt ilm at 700 c and 3 gpa during the early exhumation of the uhp terrane the speculative hypothesis b implies that the proposed majoritic garnet was from the mantle transition zone 450 km depth this suggestion remains to be tested

not all moissanites are created equal new constraints on moissanite from metamorphic rocks of bulgaria abstract terrestrial moissanite sic is widely reported as an ultra high pressure mineral occurring in kimberlites diamonds and ultramafic mafic rocks of mantle origin however the conditions of crystallization remain largely unknown moreover dozens of sic occurrences have been reported from continental crust sources such as granitoids andesite dacite volcanic rocks and their breccia metasomatic and metamorphic rocks and even limestones the validity of many of these reports is still debated primarily due to possible contaminations from the widespread use of synthetic sic abrasives in samples preparation indeed reports of well documented in situ occurrences of moissanite in association with co existing minerals are still scarce the only condition of moissanite formation that is agreed upon is that extremely reducing media are required e g 4 5 6 log units below the iron wustite buffer here we report the new occurrence of moissanite that was found in situ within the garnet staurolite mica schists of topolovgrad metamorphic group of triassic age in southern bulgaria the 10 300 m moissanite crystals are situated within 0 1 1 2 mm isolated clusters filled with amorphous carbon and nanocrystalline graphite most of moissanite crystals are 15r rhombohedral and 6h hexagonal polytypes and one prismatic crystal found within them exhibits unusual concentric polytypical zoning with core 15r intermediate zone 6h and rim 3c cubic experimental data show that this type of polytypical zonation is likely due to a decrease in temperature or and pressure and changes in si c ratio indeed amphibolite facies metamorphism 500 580 c garnet staurolite zone followed by a subsequent cooling during the retrograde stage of green schist facies metamorphism 400 500 c could have provided a change in temperature the sic containing clusters exhibit evidence that they are pre metamorphic and we hypothesize that their protolith was a black shale material likely rich in carbon hydrocarbon and terrigenous silica the latter served as a source of isolated chemically reduced media which is required for sic formation other concepts to explain moissanite occurrences in metasedimentary rocks are also discussed importantly our findings show that the formation conditions of moissanite are likely more variable than previously recognized

erosional processes and paleo environmental changes in the western gulf of lions sw france during the messinian salinity crisis abstract current interpretation of the messinian salinity crisis msc involves partial desiccation of the mediterranean sea coupled with the deposition of thick evaporites in the deep basins new sets of seismic reflection profiles in the western part of the gulf of lions confirm the basinward extension of the messinian erosion and enable the mapping of distinctive seismic markers indicating the messinian erosional surface or messinian unconformity the basin margin detrital deposits and the deep evaporite sequence the geometrical relationship between these three elements and their relationship to the paleogeography of the margin during the msc provide new information about the evolution of the study area during the messinian the messinian erosional surface mes commonly correlated with the desiccation phase and the deposition of deep evaporites during the apogee of the event is generally interpreted as a subaerial feature in the gulf of lions it is a complex diachronic polygenic erosional surface observed at the base of the prograding plio quaternary sequence beneath the shelf and slope it extends downslope beneath the deep basin upper evaporites and the salt and possibly correlates conformably with the base of the so called deep lower evaporites the whole morphology of the mes reflects a buried drainage pattern supporting the interpretation of fluvial erosion driven by a substantial drop in sea level our results also suggest that large submarine gravity flows occurred prior to any significant accumulation of salt in the basin and prior to the upper evaporites consequently interbedded clastic deposits may partly account for the parallel reflectors of the lower evaporites since river erosion persisted throughout the msc the salt and upper evaporite units may also contain a large amount of detrital sediments the good quality of the new seismic data clearly reveals fan shaped messinian deposits in the downstream part of the main messinian valleys i e the nile var and spanish rivers the depositional scenarios generally involve a substantial sea level fall coupled with deltaic prodeltaic accumulations a chaotic seismic unit unit d filling messinian lows and extending beneath the salt within the study area is interpreted as a messinian clastic unit we propose a polyphase scenario of detrital fan deposition involving pre syn and post salt deposition in subaqueous subaerial environments in the gulf of lions a late miocene tectonic phase that affected the western shelf also played an important role in controlling a the pattern of the messinian fluvial network b the location of maximum erosion on the shelf and c the location of the detrital fan depocentre downslope

characterization of the rhyolite of bodie hills and 40ar 39ar intercalibration with ar mineral standards abstract the rhyolite of bodie hills california is characterized compositionally and the geochronology of selected phases is studied sanidine bhs from the rhyolite is well suited as a 40ar 39ar reference material with high k ca and radiogenic yield intercalibration with ga1550 biotite from the dromedary igneous complex new south wales australia yields an age of 9 7946 0 0031 ma for bhs relative to an age of 98 79 0 54 ma for ga1550 and a calibration factor rbhs ga1550 of 0 096719 0 000032 bhs is also intercalibrated with sanidines of the taylor creek rhyolite tcs rtcs bhs of 2 90874 0 00067 fish canyon tuff fcs rfcs bhs of 2 88339 0 00088 and rhyolite of alder creek acs racs bhs of 0 12028 0 00024 these calibration factors yield ages of 28 344 0 011 ma 28 099 0 013 ma and 1 1809 0 0024 ma for tcs fcs and acs respectively relative to ga1550 full propagation of errors increases these uncertainties and that of bhs to 0 9 of their ages calibration of bhs using the astronomically tuned age of fcs determined by kuiper et al 2008 yields an age of 9 8295 0 0036 ma stepwise heating of bhs reveals the same small progressive increase in age across the age spectrum reported for fcs acs and other potassium feldspars e g foland and xu 1990 phillips et al 2017 this increase is consistent with mass fractionation of argon during step heating and favors use of single step fusion ages of all sanidines for monitor and calibration purposes zircons from the rhyolite of bodie hills are strongly zoned in u and th and u pb geochronologic analyses suggest multiple generations of zircon growth the youngest ages indicate a crystallization age of 9 97 0 08 ma 2 arguably similar to the 40ar 39ar results when residence time is considered but previous episodes of zircon growth began at least 400 ka prior to eruption precise u th he thermochronology of these zircons is difficult because crystal to crystal variations in the magnitude and complexity of their u th zoning complicates accurate alpha ejection calculations replicate conventional single crystal u th he ages are more widely dispersed than predicted by analytical uncertainties but laser ablation u th he ages are more reproducible with an inverse variance weighted mean of 9 71 0 58 ma

offshore evidence of polyphase erosion in the valencia basin northwestern mediterranean scenario for the messinian salinity crisis the messinian salinity crisis at the end of the miocene strongly affected the physiography of the mediterranean margins and in particular that of the valencia basin and its margins the valencia basin which is an aborted tertiary rift related to the opening of the oceanic provencal basin shows evidence of two major messinian erosion surfaces a basal erosion surface bes and a top erosion surface tes bracketing the messinian evaporite succession the basin s margins show only a single erosion surface here termed the margins erosion surface mes in examining the geometrical relationships of these erosion surfaces with the deep basin facies deposited in the abyssal provencal basin during the messinian salinity crisis we have been able to establish a chronology for the messinian in the valencia basin briefly the basal erosion surface extends the margins erosion surface into the central valencia basin and records an early and important sea level fall at the beginning of the messinian salinity crisis the miocene basin series were incised by canyons while the exposed margins underwent substantial subaerial erosion the basin s detrital products linked to this basal erosion grade laterally into the massive messinian salt layer of the provencal basin the overlying upper evaporites unit appears to be widely transgressive beyond the edge of the messinian salt and onlaps the basal erosion surface in the valencia basin this disposition suggests a progressive sea level rise during the deposition of the upper evaporites possibly due to the rapid accumulation of thousands of metres of shallow water evaporite facies in the provencal basin the top erosion surface at the top of the upper evaporites shows a network of incisions and we suggest that this final erosion was

precise determination of the isotopic composition of potassium application to terrestrial rocks and lunar soils we detail a method for the precise and accurate determination of isotopic variations in the 41k39k ratio with a precision of 0 5 2 m purified potassium is chemically extracted from rocks soils minerals or solutions by ion exchange chromatography complete chemical yields 99 8 are achieved in order to avoid laboratory induced isotopic fractionations the purified potassium is converted to a glass by melting with barium borate flux and the resultant bead is mounted for ion probe analysis the sims method utilized by the ion probe produces extremely stable k ion beams with no measurable temporal variability in the isotope ratio the instrumental fractionation is steady at about 4 and is corrected for by measurement of a standard the measurement of gravimetrically prepared isotopic standards indicates that the method is accurate at the stated level of precision and free of egregious errors analysis of terrestrial samples including peridotite basalts granites carbonatite biotite schists and seawater indicate the complete absence of isotopic variations in 41 1k among terrestrial materials at the 0 5 o level application to lunar soils and a regolith breccia confirms previously observed large isotopic fractionation effects garner et al 1975a church et al 1976 some lunar soils e g 14163 are shown to have large sample heterogeneity 7 o while others e g soils and a regolith breccia at several apollo 15 sites station 79 are homogeneous at the level of analytical precision the presence of potassium isotopic effects in bulk soils up to 12 7 in this study with magnitudes comparable to the rayleigh fractionation factor 25 indicates that volatility during micrometeorite impact melting played a large role in lunar regolith formation as much as 15 of the regolith potassium has been lost from the moon through the tenuous lunar atmosphere

a quantitative analysis of the desiccation and re filling of the mediterranean during the messinian salinity crisis abstract notwithstanding the great deal of attention that the messinian evaporites of the mediterranean region have received from an observational point of view there is to date no consensus as to their mechanism of formation we aim to contribute to the investigation through a quantitative analysis of the processes of desiccation and re filling these processes are thought to have played a role in particular during the deposition of the upper part of the evaporite sequence we calculate the evolution of sea level and average salinity based on both the present day geometry and a paleogeographic reconstruction and assess the sensitivity to variations in the freshwater budget our results support previous inferences that desiccation and re filling are fast desiccation occurs on a time scale of 3 8 kyr re filling probably even faster equilibrium sea levels imply that most water has gone from the western basin while a significant water column remains in the eastern basin whether or not the eastern basin reaches the level of halite saturation depends critically on in particular the freshwater budget the fast rate of desiccation and re filling imply that temporal differences in the onset of salt precipitation between western and eastern basin and between marginal basins and basin centres are below the resolution of astronomical dating also when atlantic sea level periodically varied from below to above the level of the intervening sill the mediterranean basin will have responded with repeated desiccation and re filling fast re filling is found to require only a small connection to the atlantic ocean this in combination with the previous results suggests the mediterranean is unlikely to attain stable intermediate water levels

seeing a mountain in a grain of garnet the collision of continents which results in mountain formation takes place through subduction in which one slab of crust is pushed below another the rock that is subducted is put under high pressure and new mineral phases can form as discussed in the perspective by liou et al a report by darling et al in the same issue p 91 1 describes a low pressure mineral cristobalite that was found in high pressure rocks understanding how this low pressure form of silicate was created and preserved may lead to better understanding of the process of subduction and how deep material is brought to the surface 1 lookup doi 10 1126 science 276 5309 91

how did the messinian salinity crisis end the cause of the desiccation of the mediterranean sea during the messinian salinity crisis has been widely debated but its re flooding remains poorly investigated interpretations generally involve tectonic collapse of the strait of gibraltar or global sea level rise or even a combination of both the dramatic sea level fall in the mediterranean has induced deep fluvial incision all around the desiccated basin we investigate erosion dynamics related to this base level drop by using the numerical simulator eros we show that intense regressive erosion develops inevitably in the gibraltar area eventually inducing the piracy of the atlantic waters by an eastward flowing stream and the subsequent re flooding of the mediterranean

oriented kokchetavite compound rods in clinopyroxene of kokchetav ultrahigh pressure rocks abstract two microdiamond bearing samples a dolomite marble and a garnet clinopyroxene rock from the kokchetav ultrahigh pressure metamorphic terrane were selected in the present study to explore the possible origin of kalsi3o8 rod inclusions oriented along the c axis of clinopyroxene host the kalsi3o8 rod inclusions at clinopyroxene cores where k2o content is high in the range of 0 5 1 0 wt are mostly fine grained with a rod width less than 1 m aem studies showed that the kalsi3o8 phase in most rod inclusions is kokchetavite k feldspar is present only in a few cases probably the result of phase transformation recrystallization from kokchetavite during rock exhumation electron diffractions further showed that kokchetavite rods are oriented parallel to clinopyroxene 0 0 1 direction and they exhibit the same epitaxial relation with the clinopyroxene host in both samples with the al si o4 tetrahedra chains along the hexagonal a axis of kokchetavite parallel to the single sio4 chain along the c axis of clinopyroxene i e 1 2 1 0 ko 0 0 1 cpx and 0 0 0 1 ko 1 0 0 cpx it is interesting to note that kokchetavite is always in association with phengite tremolite cristobalite si rich al k ca bearing low crystallinity phase si ca cl as phase calcite apatite lollingite feas2 forming compound rods furthermore all these phases are also present within submicron scale polyphase inclusion pockets in garnet within garnet clinopyroxene rock sample these kokchetavite compound rods are therefore most likely to have resulted from melt fluid clinopyroxene interactions leading to epitaxial deposition rather than exsolution sensu stricto from the clinopyroxene host the suggested melt fluid would have an external and or an internal origin related to rock partial melting involving phengite breakdown discrete phlogopite and phengite needle like inclusions with a needle width less than 1 m as well as phlogopite phengite and kokchetavite mica intergrowth needles are also not uncommon in clinopyroxene cores there are specific crystallographic orientation relationships among phases these micas might have formed earlier than kokchetavite and have exsolved from clinopyroxene host although the mass balance issue during exsolution and the temporal relation between phlogopite and phengite needles remain to be settled clinopyroxene rims generally contain mica needles only domains near fractures with coarse grained kokchetavite compound rods or with coarse grained phlogopite quartz needles are not uncommon these clinopyroxene rims domains are low in k2o

choking the mediterranean to dehydration the messinian salinity crisis strait uplift due to isostasy played a pivotal role in the development and maintenance of a desiccated mediterranean basin during the late miocene messinian salinity crisis new three dimensional flexure models for the western mediterranean suggest that most of the giant evaporite body was deposited before sea level lowering any mechanism that subsequently caused a mediterranean sea level lowering by a few hundred meters during several thousand years sufficed to cause isostatic uplift and closure of the gateways thus choking the mediterranean to desiccation due to this isostatic feedback multiple desiccation and reflooding phases driven by milankovitch cycles are unlikely when the atlantic connection had been lost and sea level had dropped a sizeable regional uplift of the strait of gibraltar developed that presented a formidable obstacle to reflooding rollback and steepening of the gibraltar slab may have played a key role at the end of the miocene by dynamically lowering the sill region thus reinstating the oceanic connection

precise age determinations and petrogenetic studies using the kca method high precision mass spectrometric determination of calcium isotope ratios allows the 40k 40ca radioactive decay to be used for dating a much broader range of geologic materials than is suggested by previous work 40ca42ca is used to monitor enrichments in 40ca and can be measured to 0 01 2 using an exponential mass discrimination correction russell et al 1978 and large ion currents the earth s mantle has such a low kca 0 01 that it has retained primordial 40ca42ca 151 016 0 011 normalized to 42ca44ca 0 31221 as determined by measurements on two meteorites pyroxene from an ultramafic nodule metabasalt and carbonatite 40ca42ca ratios can be conveniently expressed relative to this value as ca in units of 10 4 to test the method for age dating a mineral isochron has been obtained on a sample of pikes peak granite which has been shown to have concordant k ar rb sr and u pb ages plagioclase k feldspar biotite and whole rock yield an age of 1041 32 m y 2 in agreement with previous age determinations k 0 5543 b y 1 k 0 8952 40k 0 01167 the initial 40ca42ca of 151 024 0 016 ca 0 5 1 0 indicates that assimilation of high k ca crust was insufficient to affect calcium isotopes measurements on two mica granite from eastern nevada indicate that the magma sources had k ca 1 similar to intermediate composition crustal rocks these results show that the k ca system can be used as a precise geochromometer for common felsic igneous and metamorphic rocks and may prove applicable to sedimentary rocks containing authigenic k minerals the relatively short half life of 40k the non volatile daughter and the fact that potassium and calcium are stoichiometric constituents of many minerals make the k ca system complementary to other dating methods and potentially applicable to a variety of geologic problems

oriented multiphase needles in garnet from ultrahigh temperature granulites connecticut u s a this study investigates distinctive oriented needle shaped multiphase inclusions found within cores of garnets from felsic granulites of the upper member of the bigelow brook formation and the brimfield schist northeastern connecticut u s a the rocks crop out in the southern end of the central maine terrane and thus are a part of the acadian neoacadian orogen the typical mineral assemblage is garnet sillimanite k feldspar plagioclase quartz cordierite biotite spinel the sillimanite is commonly pseudomorphous after kyanite the multiphase needle inclusions and associated exsolved needles of rutile and ilmenite are mostly oriented parallel to of garnet the multiphase needles contain various combinations of quartz micas chlorite rutile graphite a siliceous compositionally variable phase phase cv zn spinel apatite zircon and rare ilmenite we hypothesize that they represent inclusions of fluid that adhered to exsolving ti fe oxide needles mostly rutile or extended along zones of weakness in garnet this requires that multiphase needle formation occurred in response to cooling and or decompression the needles ultimately decrepitated during retrogression we note that micaceous needle shaped multiphase inclusions are rarely described the closest analogs of which we are aware are found in uhp garnets of the erzgebirge perchuk 2008 the brimfield schist in this area underwent ultrahigh temperature metamorphism uhtm of 1000 c at a minimum pressure of 1 gpa ague et al 2013 here we provide new temperature estimates for the adjacent upper member of the bigelow brook formation ternary feldspar reintegration using the activity model of benisek et al 2004 and zr in rutile thermometry tomkins et al 2007 give average temperatures of 990 and 1010 c respectively at 1 gpa for this unit therefore the recently discovered uht zone in the brimfield schist of northeastern connecticut extends to the east to include the upper member of the bigelow brook formation consequently the first confirmed regional uht locality in the united states is larger than initially recognized and is at least 25 km long and 5 10 km wide the oriented elongate multiphase inclusions and petrographically obvious oriented ti fe oxide needles may be useful indicators of extreme temperature and or pressure rocks in other field areas

response to the comment by w h schwarz et al on joint determination of 40k decay constants and 40ar 40k for the fish canyon sanidine standard and improved accuracy for 40ar 39ar geochronology by p r renne et al 2010

experimental and geologic evaluation of monazite u th he thermochronometry catnip sill catalina core complex tucson az abstract monazite is a petrologically important and analytically promising target for u th he thermochronology previous studies have reported highly variable he diffusion results from monazite from a single sample and demonstrated that composition can significantly affect he diffusion parameters in this study we performed incremental heating of single monazite grains to experimentally determine the 4he diffusion properties of reference monazite 554 that occurs within a peraluminous two mica granite from the catnip sill within the catalina core complex arizona assuming that the grain size defines the diffusion geometry the six experiments yielded e a values of 212 to 238 5 kj mol 1 1 and d o values of 15 7 to 103 cm 2 s 1 with one value of 784 cm 2 s 1 monazite u th he data from five grains yielded closure temperatures of 291 to 262 c c 15 c and ages of 23 8 20 3 1 2 2 ma the weighted mean age is 21 8 0 73 mswd 1 83 n 5 and the weighted mean closure temperature is 282 6 c mswd 0 96 n 5 1 we tested the accuracy of these results by comparing our monazite thermochronology data with monazite th pb depth profiling results the 40ar 39ar thermal history for the catnip sill constrained using coexisting muscovite biotite and k feldspar and published regional zircon and apatite fission track results the monazite th pb data indicate emplacement of the catnip sill at 45 ma the 40ar 39ar muscovite and biotite data indicate cooling from 460 to 350 c from 27 to 26 ma k feldspar mdd modeling suggests cooling from 360 to 240 c from 26 to 24 ma zircon fission track data indicate cooling through 250 c between 29 and 20 ma additional cooling through 110 c is recorded by apatite fission track ages of 19 16 ma because the monazite thermochronology results are reproducible and consistent with the thermal history constrained by the other chronometers our results 1 confirm the accuracy of the 4he diffusion kinetics from monazite 554 and 2 demonstrate that monazite can be an effective u th he thermochronometer provided that he diffusion properties are either measured or reasonably assumed for each sample

alternate interpretation of the messinian salinity crisis controversy resolved a high resolution chronology of the mediterranean messinian salinity crisis is proposed two types of evaporite deposition may be distinguished those in marginal areas vs those in basinal ones their diachroneity is deduced from the stratigraphic relationships linking these evaporites to a major messinian erosional surface a two step model is proposed for the evolution of the salinity crisis through time during the first phase from 5 75 to 5 60 ma the deposition of marginal evaporites took place in response to a modest sea level fall in the second interval from 5 60 to 5 32 ma the mediterranean basin became isolated during this later period the deposition of basinal evaporites and the cutting of the mediterranean canyons took place

calcium amphibole exsolution lamellae in chromite from the semail ophiolite evidence for a high pressure origin abstract we report here for the first time aluminous ca amphibole exsolution lamellae in an anhydrous chromite host of podiform chromitite from the nakhl massif of the semail ophiolite transmission electron microscopy observations show that the ca amphibole lamellae have a crystallographic topotaxy and d spacing relationship with the host chromite i e 103 am 11 8 chr 020 am 2 2 0 chr and d060am d220chr that indicate an exsolution origin the presence of ca amphibole exsolution implies the incorporation of si4 and ca2 cations and hydroxyl in the host precursor chromite the reconstructed precursor chromite had sio2 concentrations of 0 33 0 58 wt suggesting upwelling from a minimum pressure of 8 gpa depth of 250 km hydroxyl groups occupy octahedral vacancies which can be introduced in high pressure chromite by substituting 4 cr3 by 3 si4 and a vacancy during upwelling vacancies decrease when si4 cations exsolve stepwise from chromite it leads to a coupled decrease of oh occupied vacancies and exsolution of ca amphibole lamellae at pressures below 3 gpa our results provide new evidence for a deep origin of the chromitite bearing mantle section of the semail ophiolite

ultradeep greater than 300 kilometers ultramafic upper mantle xenoliths geophysical discontinuities in earth s upper mantle and experimental data predict the structural transformation of pyroxene to garnet and the solid state dissolution of pyroxene into garnet with increasing depth these predictions are indirectly verified by omphacitic pyroxene exsolution in pyropic garnet bearing xenoliths from a diamondiferous kimberlite conditions for silicon in octahedral sites in the original garnets are met at pressures greater than 130 kilobars placing the origin of these xenoliths at depths of 300 to 400 kilometers these ultradeep xenoliths support the theory that the 400 km seismic discontinuity is marked by a transition from peridotite to eclogite

perspectives on 40ar 39ar dating isotopic dating of geological samples using the k ar method and its variant the ar ar dating technique provides ages that in favourable circumstances are precise and accurate to within 1 limiting factors include the accuracy of the decay constants for k and the age of neutron fluence monitor minerals for rapidly cooled igneous rocks a k ar or ar ar age normally will give a good estimate of the age of eruption but for slowly cooled igneous rocks or metamorphic rocks an age measured on a sample is likely to be a cooling age as ar ar ratios can now be determined often to better than 0 1 the main limitation on accuracy relates to how well the k decay constants are known better determination of the and decays of k the basis for the decay constants is suggested at the present time rather than adoption of new decay constants linked to another decay scheme until improved independently measured decay constants for k become available there may be some circumstances where the newly proposed decay constants need to be used especially when comparing ages on volcanic igneous rocks with those measured in another system

40ar 39ar dating into the historical realm calibration against pliny the younger laser incremental heating of sanidine from the pumice deposited by the plinian eruption of vesuvius in 79 a d yielded a40ar 39ar isochron age of 1925 94 years ago close agreement with the gregorian calendar based age of 1918 years ago demonstrates that the 40ar 39ar method can be reliably extended into the temporal range of recorded history excess 40ar is present in the sanidine in concentrations that would cause significant errors if ignored in dating holocene samples

implications of salt sediment interactions on the architecture of the gulf of lions deep water sedimentary systems western mediterranean sea we integrated structural analysis of multichannel seismic data and isopach maps of the quaternary sedimentary cover to evaluate how salt tectonics has influenced the stratigraphic evolution offshore the gulf of lions the interplay between salt tectonics and sedimentation occurs at two scales at local scale listric normal faults create bathymetric relief able to deviate sediment transport axes with implications on depocenters configuration at depocenter scale overburden vertical movements due to salt evacuation or salt diapiric rise create sub basins providing accommodation space for clastic deposition these mechanisms have significant implications on the architecture and the internal organization of turbidite systems offshore the gulf of lions

mediterranean messinian salinity crisis constraints from a coeval marginal basin sorbas southeastern spain abstract the extent timing and effects of late miocene desiccation and evaporite deposition in the mediterranean sea remain controversial marginal basins containing messinian 5 3 7 1 ma sequences now exposed onland supplement information from the deep mediterranean but have evaporites of a variety of ages some of these messinian evaporites pre date and others post date deep desiccation none appears to be exactly coeval with those of the deep mediterranean in the betic cordillera of southern spain basins e g granada basin far from the present day mediterranean lost connection with the mediterranean due to uplift 1 ma before the salinity crisis and have marine evaporites of early not late messinian age in contrast basins such as the sorbas basin in southeastern spain adjacent to the present day mediterranean record events directly reflecting the course of the salinity crisis the sorbas basin retained marine connection with the western mediterranean up until evaporative drawdown in the mid messinian and was rapidly reflooded immediately afterwards the sorbas messinian sequence is up to 320 m thick it is complete and normal marine except for an incised basinwide erosion surface overlain by a 130 m thick gypsum deposit we propose that this surface marks the interval of deep desiccation of the adjacent mediterranean basin erosion occurred shortly after 5 9 ma and the irregular unconformity has relief of at least 240 m confirming mediterranean drawdown significantly below the atlantic level sorbas gypsum deposits overlie the unconformity and are intercalated with marls estimated at 5 5 ma containing normal marine messinian biotas these gypsum deposits therefore post date deep desiccation and formed in a silled basin immediately following marine reflooding they mark the end of the salinity crisis and are overlain by 70 m of normal marine sediments containing unreworked foraminifers of latest messinian age this succession demonstrates that the salinity crisis occurred faster and was completed sooner than previously believed it took place within 0 4 ma time scale of berggren et al 1995b is 0 2 ma on the time scale of baksi 1993 and was completed before the end of the messinian this corrects previous views that the salinity crisis continued until the early pliocene there is no evidence in the sorbas basin for brackish lago mare conditions and the normal marine biotas that directly precede and follow drawdown rebut earlier suggestions of prolonged salinity effects