meteoric diagenesis of plio pleistocene limestones at enewetak atoll abstract petrographic lithologic mineralogic and minor elemental data from plio pleistocene carbonates at enewetak atoll indicate that these sediments have been calcitized in the near surface meteoric diagenetic environment shallow 100 m mostly pleistocene sediments remain aragonite rich with a few discrete meter scale calcite rich intervals developed either at pleistocene soil zones or in thin 5 m paleophreatic lenses deeper 100 to 180 m mostly pliocene sediments are characterized by pervasive calcitization of aragonite extensive dissolution with well developed moldic porosity and the occurrence of isopachous bladed and pore filling equant calcite cements pervasively calcitized pliocene limestones exhibit a narrow range in 18o value 1 0 53 a wide range in 13c value 1 1 63 and low mean sr 0 02 wt and mg 0 25 wt contents these data are most consistent with diagenetic alteration of plio pleistocene carbonates by meteoric phreatic calcitization although in some intervals calcitization in the mixed meteoric marine environment may have also occurred petrographic data document the superposition of subaerial exposure surfaces a d paleophreatic lenses in the subsurface of enewetak sr isotope data and stratigraphic forward modeling are consistent with the multiphase meteoric phreatic calcitization of pliocene limestones in response to subaerial exposure shortly after sediment deposition and during pleistocene sea level lowstands quantitative water rock interaction modeling of whole rock chemical trends suggests that calcitization occurred in an open diagenetic system at water rock w r volume ratios of 103 to 105 proxy indicators of climate e g retention of aragonite at depth and the development of caliche at subaerial exposure surfaces are consistent with an arid to semi arid plio pleistocene climate moreover a limited supply of meteoric waters in the vadose environment despite vertically and temporally extensive paleovadose environments is consistent with limited calcitization of the pleistocene sequence alternatively previous estimates of the rates of vadose calcitization may be too high to be applicable at enewetak stratigraphic shortening of the plio pleistocene sequence in response to carbonate dissolution during atoll emergence is not consistent with available data

magmatic evolution of a volcano studied by 230th 238u disequilibrium and trace elements systematics the etna case age determinations of several lava flows from etna through 230th 238u disequilibrium internal isochrons yield a precise chronology of the volcano s history for the last 200 000 years and emphasize the main episodes in the formation of this huge complex strato volcano study of 230th232th 0 initial ratios of lavas together with their trace element compositions yields a consistent model of magmatic evolution implying the existence for 200 000 years of a deep reservoir of alkalic magma periodically mixed with magmas of tholeiitic affinity these short periods of mixing appear to be related to the formation of the large calderas of etna in addition to these processes affecting the deep reservoir fractional crystallization also occurred in more superficial levels of the volcanic edifice thereby yielding several series of differentiation of relatively short duration as for its geochemistry etna s volcanism is of oceanic type but with its own characteristics and in a peculiar geodynamic context at the edge of the african plate

kinematic considerations for mantle mixing recent experimental and computational studies tal and computational mixing studies in highly controlled show that islands u mixedrcgions that donot interactwith flows show that even seemingly inconsequential errors in the surrounding regions are ubiquitous features in chaotically computed velocity field of 3 d or time dependent flows can advecting fluids such is ands uitenaturall account for the geochemically inferred coexistence of apparently distinct long lived geochemical heterogeneity with relatively homo geneous regions of an actively convecting mantle these results also indicate that mixing patterns the set of islands and folds characterizing the large scale material advection are sensitive to small variations in the rheology of the fluid therefore interpretation of numerical simulations of mantle transport and mixing is less straightforward than currently supposed computational studies of analytic flow solutions with systematically introduced and controlled errors indi cate that mantle simulations are unlikely to accurately com pute individual trajectories for even moderate time but that tmjectory ensembles can be accuntely computed for long time significantly computations also indicate that mixing and transport resultsmay not evolve smoothly with increased rheological realism

stable isotopic signature of a palaeoaquifer mississippian alamogordo member limestones sacramento mountains new mexico usa nine stratigraphic sections each 5 m thick were sampled from the alamogordo member limestones of the lake valley formation sacramento mountains new mexico usa four stratigraphic sections consist entirely of lime mudstone and wackestone whereas the other five sections have a prominent layer of crinoidal packstone about 1 m thick at their base stable isotopic analyses reveal that the lime muds in the sections with basal packstone layers show a downward decrease in 1 8 o and constant 1 3 c values whereas those in the sections solely composed of lime mudstone and wackestone have in general relatively uniform 1 8 o and 1 3 c values the diagenesis of the alamogordo member limestones was previously believed to have been governed by the downward percolation of meteoric water from a regional pre pennsylvanian exposure surface 100 m above this unit however the uniform 1 3 c and downward decrease in 1 8 o values in the lime muds in the sections with basal packstones indicate that the meteoric water ascended within the alamogordo member rather than descended from the overlying exposure surface this indicates that the basal packstones were probably a conduit for meteoric water this is further supported indirectly by the relatively uniform 1 8 o and 1 3 c values of the lime mud in the sections without basal packstones the implications are that the oxygen isotopic gradients may be used to identify palaeoaquifers flow directions within these aquifers and that meteoric diagenesis below an exposure surface could be governed by flow through a palaeoaquifer

diagenetic origin of carbon and oxygen isotope compositions of permian triassic boundary strata bulk carbonate 13 c and 18 o compositions of profiles across permian triassic p t boundary sections in china italy austria and iran show wide varieties of trends the 13 c depletions occur in all sections and range from 2 to 8 pdb in magnitude these excursions take place over intervals ranging from less than 0 1 to more than 40 m the 18 o values may increase or decrease toward the p t boundary but decrease sharply by 2 9 pdb at or above the boundary cross plots of 13 c and 18 o values from all sections show positive covariance wide differences in magnitudes trends and position of the excursions relative to the boundary as well as the covariance patterns suggest that p t boundary 18 o and 13 c values are partially or entirely diagenetic in origin formed in association with exposure surfaces this interpretation implies that p t boundary sections studied till date were subaerially exposed before during and after the mass extinction resulting in the removal of strata containing key information about the extinction mechanism this inference is consistent with the paleontological studies that have shown the presence of gaps at the boundary and further supported by the sharp lithologic changes observed at virtually all p t boundary sections subaerial exposures are documented by detailed sedimentologic and isotopic studies from central tethyan sections in abadeh and shah reza in iran proposed p t boundary extinction models are based on isotopic values that are diagenetic in origin and stratigraphic sections that are incomplete leading to extinction mechanisms with little physical supporting evidence

multistage magma mixing revealed in phenocryst zoning of the yunokuchi pumice akagi volcano japan detailed study of the texture and chemical zoning of phenocrysts in the compositional and thermal histories at the sites a plinian fall deposit yunokuchi pumice from akagi volcano where they grew only recently have such studies been japan permits reconstruction of the subvolcanic magma chamber undertaken focusing on magma chambers as the sites hypersthene phenocrysts are complexly zoned some have spongy of phenocryst formation tait 1988 de silva 1989 cellular textures whereas others exhibit simple reverse zoning augite umino iwano 1992 fodor et al 1993 nakada et phenocrysts have texturally similar cores but al and ti vs mgal 1994 the magma chambers depicted by these number zoning patterns are correlatable with hypersthene cores studies are mainly composed of phenocryst poor magma most plagioclase phenocrysts have a uniform calcic core enclosed surrounded by a comparatively thin crystallizing boundby a normally zoned rim others have variable an contents and ary layer phenocrysts are interpreted to have crysmottled textures these characteristics and melt glass compositions tallized either within the main body of magma or in suggest a complex multistage mixing of low t 940 960 c situ within the boundary layer highly silicic and high t 980 1060 c less silicic magmas combination of geophysical and petrological data stratigraphic variations of the phenocryst types show that the earlier together with fluid dynamical simulations of magmatic erupted magma inherited phenocrysts from more high t magma processes beneath mid oceanic ridges has shown that a components we propose the presence of an inhomogeneous mushy mush of crystals and interstitial melt 25 55 vol chamber filled with a highly silicic magma which was recharged crystals probably makes up the majority of the magma periodically by less silicic magmas the large viscosity contrast and chamber and only a small volume exists as a meltdensity balance of the new magma relative to the mush controlled enriched lens e g sinton detrick 1992 nicolas et al the efficiency of mixing and the position of emplacement of the high1993 barth et al 1994 likewise it is highly probable t magma in the mush tapping the chamber first expelled the that magma chambers beneath island arc volcanoes fluidal mixed magma followed by progressive erosion of the mush largely consist of a mush of crystals and interstitial melt as the eruption continued recent studies on arc volcanic products have presented evidence for the existence of such mushy magma chambers these include an estimation of very high crystallinity

diagenesis and sea level change lessons from moruroa french polynesia an inclined borehole through the margin of moruroa atoll french polynesia in which the ages of units are constrained by radiometric dates provides a unique opportunity to examine the relationship between sea level change and diagenetic history the oldest rocks encountered are dated at 355 840 yr bp and the overlying succession records five cycles of deposition and emergence culminating in the deposition of the holocene sequence successive intervals of emergence provided the driving force for diagenetic change promoting the through flow of chemically aggressive fresh waters that were at times either pervasive or confined within relatively isolated pathways on a millimetre to metre scale these locally varying flows and flow rates defined restricted zones characterized by differences in water chemistry and diagenetic signatures in contrast to some sequences elsewhere the upper part of the moruroa atoll succession is typified by alternations of intervals in which either several generations of cement growth are represented or only one or two cements are present three distinct diagenetic signatures can be recognized as follows i fresh water phreatic zones are reflected in neomorphism and the deposition of cements that occluded porosity thus preventing the deposition of later cements and conferring relative stability ii vadose zones are characterized by more complex diagenetic sequences reflecting multiple phases of dissolution and cement growth attesting to the relative duration of exposure and the conditions of ground water flow and iii deep in the core sequences reflect lowstand deposits that suffered sub sea diagenesis generating pores in which multi layered marine cements apparently formed at more frequent intervals than those subject to subaerial processes all cements appear to be of narrowly constrained ages these results highlight the contentious issue of whether the distributions of former diagenetic environments can be related to specific sea level stands and erosion surfaces it seems that in the moruroa core some cycles of sea level change are reflected in distinctive packages of diagenetic features that have not been overprinted by cement growths during later sea level stands

magma ascent rates from amphibole breakdown an experimental study applied to the 1980 1986 mount st helens eruptions recent 1980 1986 mount st helens dacites contain the phenocryst assemblage plagioclase amphibole low ca pyroxene magnetite ilmenite and rare high ca pyroxene which indicates that they all originated from an 8 km deep reservoir at 900 20 c with xh2o 0 67 in fluid according to experimental data iron titanium oxide phenocryst compositions indicate that all post may 18 dacitic magmas erupted at 900 20 c except for the final lava extrusion in october 1986 the magma reservoir may have cooled to 866 c by october 1986 amphiboles in the post may 18 1980 magma contain one or more amphibole populations characterized by reaction rims of different thicknesses the development of the amphibole reaction rims in these rocks is a response to water loss from the coexisting melt during an approximately adiabatic ascent from a deep reservoir constant p and t and isothermal decompression experiments show that during a 900 c constant rate decompression from 8 km to the surface no reaction rim develops on amphibole in 4 days a 10 m rim develops in 10 days and a 35 m rim develops in 20 days these experimental data and histograms of rim widths in 1980 1986 mount st helens dacites show that post may 18 eruptions are composed in large part of magma represented by a population of thin rimmed amphiboles magma which ascended from the deep 8 km reservoir in 6 to 10 days the remainder of each sample consists of magma containing amphiboles with reaction rims ranging from 14 to 60 m magma which apparently spent from 8 to 25 days along the conduit margins before being mixed thoroughly millimeter scale into the erupting magma the mixing in a viscous slowly ascending dacite may be enhanced by its flow through partially crystallized magma emplaced earlier and by the evolution and loss of a large vesicle population the experimental calibration of amphibole reaction rim width versus decompression time yields average ascent velocities for post may 18 dacites of about 15 30 m h for magma represented by the thick rimmed amphiboles and from 35 to 50 m hr for magma represented by the thin rimmed crystals an ascent rate of 66 m h is indicated for the may 18 1980 eruption which contains amphiboles with no reaction rims the volume of endogenous dome growth which preceded extrusion of magma newly derived from the deep source region suggests that the effective conduit volume beneath mount st helens in 1981 1982 was equivalent to a cylinder 8 km long and 8 9 m in radius

heterogeneous diagenetic patterns in the pleistocene ironshore formation of grand cayman british west indies abstract the ironshore formation on grand cayman consists of six unconformity bounded units a to f that developed in response to repeated transgressive regressive cycles during the middle to late pleistocene corals and matrices in limestones from the rogers wreck point rwp offshore george town gt and western onshore wo areas are characterized by complex diagenetic fabrics that reflect marine bioerosion micrite envelopes internal sediments fibrous high mg calcite acicular aragonite isopachous prismatic calcite cements freshwater phreatic circumgranular cements even random blocky calcite cements and vadose meniscus calcite cements blocky calcite cements diagenesis throughout these limestones the matrices have undergone more meteoric diagenetic alteration than the corals overall however no systematic stratigraphic patterns exist to the distribution of these diagenetic fabrics and it is generally impossible to link the different phases of diagenesis with specific unconformities that cap each unit these heterogenous patterns of diagenetic features can be attributed to many intrinsic and extrinsic factors including the original compositions of the different components in the limestones the porosity and permeability of the substrate the nature of the diagenetic fluids climate and the duration of exposure during each lowstand integration of available data however indicates that maximum diagenesis took place during the lowstands denoted by the unconformities at the top of unit c marine isotope stage 7 and unit d marine isotope stage 5e when long periods of exposure were accompanied by wet climates with high annual rainfalls

trace element and stable c o and radiogenic sr isotope geochemistry of stromatolitic carbonate rocks of the mesoarchaean pongola supergroup implications for seawater composition abstract the mesoarchaean pongola supergroup hosts one of the oldest well preserved stromatolitic carbonate successions major and trace element geochemistry along with c o and sr isotopic analyses of three stratigraphic sections of stromatolitic carbonate rocks were undertaken in order to infer the depositional environment and to unravel the chemical characteristics of contemporaneous seawater the carbonate rocks consist mainly of dolomite and to a lesser extent ankerite their variable sr concentrations of 95 to 450 mg kg are interpreted as resulting from early to late dolomitization of metastable carbonates aragonite and or high mg calcite facies control and calcitization during meteoric diagenesis 18o values range from 19 6 to 12 0 vpdb and reflect recrystallization of carbonates caused by high water rock interactions during diagenesis and metamorphism a 13c value of 2 5 vpdb was measured in carbonate samples least affected by alteration and we suggest that it approximates the composition of dissolved inorganic carbon in coeval seawater the samples display variable 87sr 86sr isotopic ratios from as low as 0 708 to 0 777 these ratios are considerably higher than those expected in archaean open ocean water masses buffered by hydrothermal interaction with the mantle we interpret the highly radiogenic sr isotopic ratios in some carbonate samples to reflect isotopic exchange with radiogenic sr from siliciclastic volcaniclastic components in the samples and fluids from clay rich layers intercalated with the carbonates the least radiogenic sr isotopic ratio may approximate the mixed isotopic composition of seawater in the epicontinental basin and of riverine freshwater draining the kaapvaal craton the dolomitic carbonate samples show shale normalized rare earth element and yttrium distribution patterns that resemble those of modern ocean water with positive lasn gdsn and ysn anomalies superchondritic y ho ratios a small negative cesn anomaly and depleted light rare earth elements relative to the heavy rare earth elements these anomalies are not as pronounced as expected for an open marine setting which together with the sr isotopic results support a more restricted setting with riverine input and fluctuations in marine ingressions a convex upward shale normalized rare earth element and yttrium pattern is probably a result of several factors including organic matter complexation and element fractionation during estuarine mixing the small negative cesn anomaly may suggest small amounts of free oxygen in shallow waters due to local oxygenic microbial activity

response of carboniferous permian mid oceanic seamount capping buildup to global cooling and sea level change akiyoshi japan abstract we discuss the response of a carboniferous permian mid oceanic atoll type carbonate buildup to global cooling and sea level fluctuation during gondwana glaciation our examination focused upon the stratigraphic distribution of emergence indicative diagenetic features as a proxy for sea level falls and we correlated their timing with hiatuses indicated by fusuline biostratigraphic discontinuities and the demise of warm adapted metazoan reef builders we examined the pennsylvanian moscovian to lower permian asselian shallow marine limestone succession ca 500 m thick of the akiyoshi limestone group southwest japan reconstructed as a mid oceanic seamount capping buildup in the panthalassan ocean the examined succession is dominated by atoll lagoon facies with subordinate coral chaetetes patch reefs and calci microbial mounds we have recognized several stratigraphic intervals less than 60 m thick in the examined succession in which emergence indicative diagenetic facies are concentrated these facies include the microcodium structure pendant cements recrystallization and algal casts each of the intervals occurs immediately below a fusuline zone boundary and represents a sea level fall event which lasted presumably 1 ma or less possibly reflecting the sea level fluctuation of third order cycles the emergence event in late kasimovian is the most prominent in the series of moscovian through asselian emergence events and resulted in a dissolution unconformity indicated by a fusuline biostratigraphic discontinuity we infer that many intercalations of less freshwater altered limestone in these stratigraphic intervals resulted from short lived emergence resubmergence episodes possibly due to the high frequency glacio eustyaic sea level fluctuation we also showed that the demise of warm adapted metazoan reef builders of corals and chaetetes happened nearly synchronously with the culmination of gondwana glaciation we hypothesize that the demise of these warm adapted reef builders is a biotic response of the akiyoshi buildup to the severe and prolonged global cooling during the pennsylvanian to early permian icehouse period the akiyoshi metazoan reef builders were replaced by calci microbes phylloid algae and calci sponges that exhibit an increasing significance during the lower to middle permian our results reveal the distinct sedimentary and biotic responses of a panthalassan atoll to global cooling and sea level fluctuation during gondwana glaciation in the southern hemisphere we conclude that carboniferous permian mid oceanic atoll stratigraphy has significant archival potential for recognizing the global palaeoclimatic change

unconformities and porosity development in carbonate strata ideas from a hedberg conference the purpose of this paper is to highlight ideas from a hedberg conference concerning detection of unconformities and associated porosity in carbonate strata many types of information including cores and outcrops seismic data eustatic sea level curves wireline logs biostratigraphic data stable isotope trends cycle stacking patterns and tectonic and basin evolution models can be used to predict and or detect subaerial unconformities all methods of detecting subaerial exposure with these types of information have limits and pitfalls detailed analysis of the stratigraphy sedimentology and diagenetic history of a rock sequence from outcrop or core data is considered the most reliable means of detecting subaerial exposure though an integrated approach using all a ailable information is recommended the relationship between subaerial exposure and subsurface porosity in carbonates is not simple or easily predicted subsurface porosity is the product of many factors operating during deposition subaerial exposure and burial several concepts should be recognized when evaluating porosity associated with subaerial unconformities 1 carbonate diagenesis during subaerial exposure rearranges pore networks but usually does not increase the total porosity near surface dissolution can simply lower a topographic surface without increasing porosity within the host limestone in many cases total porosity in carbonates under unconformities is actually reduced during subaerial exposure 2 permeability is likely to change more than porosity during subaerial exposure increasing in some c ses but decreasing in others 3 pore systems evolve with time during subaerial exposure short periods of subaerial exposure 10 000 400 000 yr are often associated with greater porosity than long intervals of subaerial exposure 1 20 million yr prolonged subaerial exposure may change permeability less than porosity because high permeability karst related conduits can form quickly and persist for millions of years 4 many carbonates subjected to subaerial exposure have little or no porosity in the deep subsurface because compaction cementation and stratal collapse reduce porosity during burial 5 unconformity related diagenesis may enhance reservoir potential by creating pore systems that are resistant to compaction during deeper burial 6 vugs caverns and breccias can fo m in the deep subsurface because of dissolution by basinal fluids independent of subaerial exposure 7 lithologic changes at unconformities like shales overlying carbonates can influence the flow of subsurface fluids during end page 857 deeper burial resulting in deep burial dissolution localized along unconformities if subaerial exposure and exposure surfaces are integrated into a sequence stratigraphic paleogeographic and climatic framework systematic and hopefully predictable patterns will emerge however we do not fully understand the complex interplay between the factors critical to creating and preserving porosity in carbonates additional research is needed to quantify critical processes and products so we can reliably predict porosity associated with subaerial exposure during exploration in frontier basins and on a smaller scale during reservoir analysis

the mixing of magmas in plutonic and volcanic environments analogies and differences abstract magma mixing processes have been widely recognized in both the plutonic and volcanic environments but the quality and quantity of information that can be extracted from the two environments are substantially different understanding the advantages and limits associated with the study of plutonic and volcanic rocks is essential to establish precise methodological approaches to build the most complete conceptual model by merging information from these two complementary igneous environments in this work we review magmatic interaction processes in the plutonic and volcanic environments by considering several aspects of these geological phenomena in particular we first briefly report on the structural and geochemical evidence for magma mixing in both plutonic and volcanic rocks with the aim to provide a general picture of this natural phenomenon successively we discuss some recent results about magma mixing achieved using the concepts from chaos theory and discuss their potential impact on magma differentiation finally we attempt to build a general picture of this igneous process by merging present day information from both the plutonic and volcanic environments it emerges from the general picture that the time spent by the magmatic system in the molten or partially molten state is the crucial factor for the preservation of the fingerprints of magma mixing in the two environments we propose a conceptual model that may be useful to understand what kind of information we can obtain from volcanic and plutonic rocks and ultimately to maximize our knowledge about magma mixing

the record of pliocene sea level change at enewetak atoll abstract detailed seismic stratigraphy lithostratigraphy and chemostratigraphy indicate that atoll wide subaerial exposure surfaces major disconformities developed during major sea level lowstands form prominent seismic reflectors and are coincident with biostratigraphic breaks in the plio pleistocene on enewetak atoll sea level models based on the stratigraphic position and age of major disconformities suggest a maximum sea level highstand elevation of 36 m above present sea level and a maximum sea level lowstand elevation of 63 m below present sea level for the pliocene

the magma mixing origin of mantled feldspars the key to mantled feldspar genesis is epitaxial nucleation of plagioclase on k feldspar or k feldspar on plagioclase once this nucleation takes place there is a relatively straightforward process of crystal growth yielding rapakivi and antirapikivi textures the most common mantling is plagioclase on k feldspar which occurs in both volcanic and plutonic environments in the volcanic environment the morphology of the plagioclase overgrowth typically is dendritic though in subvolcanic and shallow plutonic environments dendritic growth is followed by a more or less continuous non cellular shell of plagioclase in the plutonic environment early stages of plagioclase overgrowth also tend to be dendritic although with coarser grained characteristics dendritic morphology is thus a common denominator in rapakivi genesis since growth of dendritic plagioclase is clearly related to marked undercooling in silicate melt systems its occurrence in many volcanic rocks is to be expected equivalent quenching in the plutonic environment requires a cooling mechanism independent of conductive heat transfer to wallrock and also independent of effective cooling related to sudden loss of volatile phases that could only occur late in the crystallization of most magmas and therefore after much dendritic plagioclase had already formed internal quenching of portions of magma systems must occur if mafic magma is abruptly mixed with felsic magma such magma mixing yields a heterogeneous system at first one that is in a drastic state of disequilibrium and tending to force nucleation of one feldspar type on the surface of another resulting in epitaxial crystallization of dendritic plagioclase on k feldspar mantling of one feldspar type by another during magma mixing is paralleled by dendritic growth zones in coexisting plagioclase crystals

low pressure crystallization paths of h2o saturated basaltic hawaiitic melts from mt etna implications for open system degassing of basaltic volcanoes melt inclusions indicate that the basaltic hawaiitic magmas from mt etna sicily contained up to 2 3 wt h2o dissolved in the melt prior to eruption the effect of h2o degassing during magma ascent has been experimentally constrained between 1135 and 1009 c for ph2o ptotal varying from 800 to 270 bars the starting material was a primitive hawaiitic lava sample mgo 7 1 wt representative of the less evolved lava emitted at mt etna experiments were conducted in tzm pressure vessels with ag70pd30 capsules in order to minimize the feo loss at temperatures of 1135 1090 c ph2o 800 bars with ninio and fmq buffers olivine fo83 80 is the liquidus phase in equilibrium with a residual hawaiitic melt mg 0 60 0 57 cao al2o3 0 82 salitic pyroxene begins to crystallize at 1075 c plagioclase at 1025 c and at 1009 c the magma is 33 5 crystallized with olivine ca rich pyroxene and plagioclase 16 60 24 at ph2o 270 bars fmq buffer olivine fo79 3 80 and salitic pyroxene are the main liquidus phases between 1100 and 1090 c they are in equilibrium with hawaiitic melts mg 0 51 cao al2o3 0 73 at 1070 c the experimental charges are highly crystallized 51 5 with olivine fo70 salite and plagioclase an78 4 76 3 in 14 44 42 relative proportions increasing the water content of hawaiitic basaltic magmas expands the stability field of the olivine relative to the other phases lowers the crystallization temperatures of pyroxene and plagioclase and results in the development of a more an rich plagioclase when compared to the natural samples these results support a model of early and moderate crystallization of olivine from etnean primitive hawaiitic magmas containing close to 2 5 wt h2o with nno oxygen buffering conditions at low pressure ph2o ptotal we propose a model of crystallization driven by decompression and water degassing during the emplacement of magma in the volcanic pile itself and possibly concomitant with the opening of fractures

quaternary atoll development new insights from the two dimensional stratigraphic forward modelling of mururoa island central pacific ocean knowledge about the quaternary evolution of mid ocean atolls comes mainly from drilling and field observations carried out on a number of pacific carbonate islands however little is known about the early to mid pleistocene atoll development history especially at margin and foreslope settings using previous field and subsurface data from mururoa atoll and a process based modelling software dionisos a two dimensional forward stratigraphic model of atoll development is proposed for the past 1 8 million years myr observational data from vertical to inclined coring seismic and bathymetric surveys indicate that from approximately 0 45 to 0 40 million years before present ma carbonate deposition at mururoa atoll resulted in a series of mostly prograding reef units the model is first constrained at the base by the shape and topography of the pre quaternary basement a number of sensitivity tests were performed to define the respective influence of variant parameters the best fit development scenario that accounts for the overall geometry and stratigraphic architecture of the quaternary sediment packages is obtained by using the sea level curve by miller et al 2005 uniform subsidence rate of 105 m myr 1 and carbonate production rates gradually increasing from 0 50 to 8 mm yr 1 between 1 80 ma and the present additional controlling parameters include subaerial erosion at a constant rate of 0 25 m kyr wave energy and sediment transport processes the stratigraphic forward model predicts a succession of three distinct types of carbonate systems that have developed since the earliest pleistocene toe of slope systems from 1 80 ma to about 0 80 ma open platform systems from 0 80 ma to 0 50 ma and framework reef systems from about 0 50 ma to the present the development of these different systems is most likely to be controlled by climate and changes in sea level cycles during the low amplitude 41 kyr cycle periods of the earliest pleistocene ambient conditions were not conducive to framework reef growth shallow water carbonate sedimentation was dominantly gravity driven operating along the platform foreslopes only during the mid pleistocene climate transition narrow open platform units have developed at the upper parts of the pre quaternary basement flanks with the onset of the high amplitude 100 kyr sea level modes and climate restoration reef frameworks started to be generated these models from mururoa agree with a number of previous studies suggesting that most of the true framework reefs were not initiated prior to 0 50 ma mururoa atoll is demonstrated to be a robust analogue for providing more realistic interpretations of the development history of pacific atolls further modelling with three dimensional dionisos could generate better predictions by taking into account hydrodynamic and transport parameters more accurately

late cenozoic sea level and the rise of modern rimmed atolls this paper is not subject to u s copyright the definitive version was published in palaeogeography palaeoclimatology palaeoecology 451 2016 73 83 doi 10 1016 j palaeo 2016 03 018

small scale disequilibrium in a magmatic inclusion and its more silicic host basaltic andesite inclusions and their host dacite from the purico chascon complex in northern chile are isotopically distinct textural characteristics of the inclusions are typical of those resulting from magma mingling serial sectioning across the interface of an inclusion and its host dacite complemented by microdrill sampling and detailed microprobe work has enabled us to examine the scale of mixing and chemical isotopic and trace element disequilibrium the results of this work show that 1 the composition of the inclusion is relatively homogeneous 2 the dacite host is generally higher in 87sr 86sr and lower in 143nd 144nd than the enclave but it is heterogeneous on a small scale and probably a hybrid 3 the isotopic composition in the marginal zone apparently on both host and inclusion sides of the weakly chilled interface actually shows the highest 87sr 86sr and lowest 143nd 144nd 4 large plagioclase crystals in the inclusions and host are xenocrystic with higher 87sr 86sr than any of the other samples these observations are reconciled with a model of magma evolution in a crustal magma chamber in such a scenario the mafic magma is overlain by a cap of rhyolite a partial melt of the dacitic ignimbrites which now underlie the purico chascon complex overturn of such a magma system gives rise to a hybrid dacite containing discrete mafic inclusions

coral reef response to quaternary sea level and environmental changes state of the science there is considerable uncertainty about future sea level projections due to the current poor understanding of the vulnerability and sensitivity of ice sheets to global warming fossil coral reefs represent a unique archive of sea level and associated environmental changes that extend far beyond instrumental records here recent advances in this field are summarized focusing on periods of earth history typified by rapid sea level rise and short term climate excursions sea level and reef response records from the last deglaciation and periods characterized by higher sea level stands and short term and low magnitude sea level oscillations such as the mid to late holocene and the last interglacial are synthesised special emphasis is given to understanding how coral reefs have responded to these changes in terms of reef development architecture and composition finally seven major challenges and future research directions are identified

laser interferometry study of oscillatory zoning in plagioclase the record of magma mixing and phenocryst recycling in calc alkaline magma chambers iztaccihuatl volcano mexico

revisiting the quaternary development history of the western new caledonian shelf system from ramp to barrier reef abstract the present paper is based on the study of a new core tenia 3 151 20 m long and revisitation of previous core data from the western new caledonian barrier reef based on variations in lithology and stable isotope signatures and the occurrence of unconformity surfaces the tenia 3 sequence is subdivided into ten units regarded as deposited during interglacial high sea levels five detrital and two framework facies are delineated using their biological and textural attributes including coral growth forms and assemblages these facies are interpreted as representative of outer to inner carbonate shelf ramp to barrier reef environments variations in the facies within each stratigraphic unit occur mainly as vertical transitions from lower energy and probably deeper to higher energy probably shallower settings the succession of depositional events is reconstructed using all available age criteria including lithostratigraphy magnetostratigraphy uranium series dating and nannofossil biostratigraphy shallow water coral communities appear to have been developed as soon as carbonate deposition started at around 1 2 million years ma but the initiation of typical coral reef tracts is likely to have occurred not prior to 0 40 million years during marine isotope stage mis 11 carbonate deposition previous to mis 11 is considered to have operated along an open shelf margin i e a ramp or non rimmed platform based on the re examination of the core dataset previously acquired from the new caledonian reefs the coral communities dominated by massive forms mainly poritids are assumed to have remained stable in composition over the past 1 4 ma similarly as indicated by a preliminary study of pollen assemblages in the core the composition of terrestrial flora dominated by podocarpaceae and araucariaceae has not apparently suffered significant change over the past 1 ma and reflects climate conditions cooler and drier than today mean subsidence rates are derived tentatively over the past 1 ma from the stratigraphic position at present reached by the top of depositional units mean rates of 0 08 mm yr 1 for the western new caledonian margin are consistent with past sea level estimates the quaternary depositional history of the new caledonian shelf margin appears to have been controlled by both the amplitude of sea level changes and by differing vertical tectonic movements comparison between the depositional shelf histories from western new caledonia north eastern australia southern belize and the ryukyu islands supports the assumption that coral populations over the early and mid pleistocene developed as non reefal communities along ramp systems not forming continuous outer shelf reef rims at a global scale quaternary reef rimmed platforms are likely to have initiated after the mid pleistocene climate transition from marine isotope stage 11 at about 0 4 ma

chaotic dynamics and fractals in magmatic interaction processes a different approach to the interpretation of mafic microgranular enclaves abstract fractals have been used for describing a wide range of natural processes however applications of fractal geometry to the study of magma mingling mixing processes is almost completely lacking in spite of the fact that such an approach could give new insights into the nature of magmatic interactions fractal analyses of two geochemically linked groups of mafic microgranular enclaves mme from the sithonia plutonic complex northern greece have been performed because of the difficulties involved in using contours of enclaves to estimate their fractal dimensions a different approach is used in this paper fractal dimension is estimated using distribution maps of chemical elements inside enclaves the results obtained indicate that a clear correlation between fractal dimensions of enclaves and geochemical evidences of interaction exists and hence the two groups of enclaves reflect different degrees of interaction from a dynamical point of view stretching and folding processes between two magmas with different rheologies are considered representative of magmatic interaction this dynamic system can easily generate chaotic patterns recognized by the presence of fractal structures the contemporaneous occurrence of well mixed regions together with poorly mixed regions in the same system is thought to be similar to the host enclave system found in many granitoid plutons in particular mme are believed to represent islands of partially mixed mafic fluid subjected to different degrees of interaction with the host granitoid whereas host rocks are believed to represent mixed regions where the mixing process acted more efficiently producing almost completely hybrids it is shown that fractal geometry and chaotic dynamics can be considered suitable techniques in studying complex petrological phenomena and they can represent helpful methods in the development of petrogenetic models

meteoric diagenesis in pleistocene reef limestones of xisha islands china abstract this paper focuses on a borehole xichen 1 well drilled on the chenhang island xisha islands in the south china sea mineralogical petrographic stable isotopic and minor element data from the holocene to pleistocene interval 0 179 m in the xichen 1 well are discussed in detail the 400 m long core is divisible into four mineralogical facies a high mg calcitic aragonite facies 0 16 91 m holocene an aragonitic low mg calcite facies 16 91 30 60 m late pleistocene a low mg calcitic facies 30 60 179 m middle early pleistocene and a low mg calcitic and dolomitic facies 179 400 m early pleistocene late miocene the holocene section has much higher whole rock 18o and 13c values and mg and sr content than the non dolomitized pleistocene limestones 16 91 179 m the 16 91 165 m interval is characterized by a relatively invariant oxygen isotopic composition and very heterogeneous carbon isotopic composition between 165 and 179 m there is a positively correlated increase of whole rock 18o and 18c with depth and mg content also shows a gradual increase with depth petrographic data demonstrate that the pleistocene reef sequence has been extensively affected by meteoric waters we conclude that the late pleistocene section 16 91 30 60 m and the middle early pleistocene section 30 60 165 m have suffered incomplete and complete meteoric diagenesis respectively and that the early pleistocene interval 165 179 m was diagenetically altered in a meteoric marine mixing environment

petrology of the younger andesites and dacites of iztaccihuatl volcano mexico i disequilibrium phenocryst assemblages as indicators of magma chamber processes

was the ediacaran shuram excursion a globally synchronized early diagenetic event insights from methane derived authigenic carbonates in the uppermost doushantuo formation south china abstract the ediacaran period is characterized by the most profound negative carbon isotope 13c excursion in earth history the shuram excursion various hypotheses including the massive oxidation of dissolved organic carbon doc in the oceans the weathering of terrestrial organic carbon or the release and oxidation of methane hydrates and or expelled petroleum from the subsurface have been proposed as sources of the 13c depleted carbon more recently it has been suggested that global scale precipitation of early authigenic carbonates driven by anaerobic microbial metabolism in unconsolidated sediments may have caused the shuram excursion but empirical evidence is lacking here we present a comprehensive analysis of a shuram associated interval from the uppermost doushantuo formation in south china our study reveals petrographic evidence of methane derived authigenic calcite formed as early diagenetic cements and nodules that are remarkably depleted in 13c suggesting a buildup of alkalinity in pore fluids through the anaerobic oxidation of methane aom and systematically depleted in 18o relative to co occurring dolomite early authigenesis of these minerals is likely to be driven by increased microbial sulfate reduction triggered by enhanced continental weathering in the context of a marked rise in atmospheric oxygen levels in light of the finding of methane derived authigenic carbonates at zhongling and based on our basin scale stratigraphic correlation we hypothesize that the marked 13c and 18o depletion including their co variation noted worldwide in the shuram excursion may reflect an episode of authigenesis occurring within a sulfate methane transition zone smtz if true the shuram excursion was then a global biogeochemical response to enhanced seawater sulfate concentration in the ediacaran ocean driven by the neoproterozoic oxidation of surface environments this paleo oceanographic transition may have therefore paved the way for subsequent evolution and diversification of animals our study highlights the significance of an integrated approach that combines petrography mineralogy and texture specific micro drilling geochemistry in chemostratigraphic studies

tourmaline nodules from capo bianco aplite elba island italy an example of diffusion limited aggregation growth in a magmatic system the morphology of tourmaline nodules occurring in the capo bianco aplite elba island italy is studied outcrop features indicate that tourmaline nodules are the product of magmatic crystallization as they are aligned along flow fields developed within the magmatic hosting mass mesoscopic observations indicate that nodule morphologies are very variable from rounded to dendritic morphometric analyses show that tourmaline nodules are fractals and that fractal dimension quantifies their degree of irregularity numerical simulations of nodule growth are performed by using a diffusion limited aggregation process the presence in natural samples of nodules with different morphologies is explained by considering a chaotic magmatic system characterized by a complex interplay between growth rate in different dynamical regions latent heat of crystallization and local convection dynamics it is suggested that higher growth rates correspond to growth of tourmaline nodules in dynamical regions where the transfer of nutrients is very efficient in such conditions the latent heat released by the growing nodule is high inducing strong local convection dynamics destabilizing the nodule interface and promoting the formation of dendritic morphologies on the contrary the growth of nodules in dynamical regions characterized by weak transfer of nutrients is inhibited leading to weak local convection dynamics and consequently to the formation of rounded morphologies

kinematic significance of morphological structures generated by mixing of magmas a case study from salina island southern italy morphological features of mixing mingling structures of heterogeneous juveniles from the 13 ka old upper pollara eruption salina island southern italy are studied these heterogeneous rocks result from the mixing between an andesitic and a rhyolitic magma concentration patterns generated by magma mixing are analyzed on digital images and results have been compared with those obtained from numerical simulations of mixing processes coupling chaotic advection and chemical diffusion results show that the upper pollara rocks were produced by mixing 24 49 of andesite and 76 51 of rhyolite multifractal analysis of the mixing structures is performed and the mixing intensity i e the degree of interaction between the two end member magmas has been deduced from the analysis of the singularity spectrum results show that narrower is the multifractal spectrum the higher is the degree of homogenization of magmas reynolds number re during mixing has been estimated from the geometrical analysis of mixing structures by using experimentally defined relationships between shape parameters and re results show that re is between ca 500 and 7000 there is a positive correlation between the estimated initial percentage of mafic magma in the different analyzed samples and re in agreement with the observation that the higher is the percentage of the mafic lower viscosity magma the higher is the turbulence of the mixing system in addition our analysis reveals an unexpected inverse relationship between the calculated re and the degree of magma homogeneity suggesting that energy dissipation may have had a major role in the controlling mixing process because dissipation is inversely proportional to the mixing efficiency results suggest that mixing processes between the andesitic and rhyolitic magmas mainly developed in the conduit it is also suggested that mixing occurred in a shear layer type or pipe type flow

mineral compositional and textural evidence for magma mingling in the saraykent volcanics abstract upper cretaceous paleocene calc alkaline dacitic and rhyodacitic lava flows and domes are exposed in the saraykent yozgat region within central anatolian crystalline complex cacc saraykent volcanics are generally plagioclase hornblende biotite titanomagnetite quartz augite phyric subrounded vesicular magmatic enclaves ranging from a few millimeters to a few centimeters in size are present in the saraykent dacites the enclaves are mainly holocrystalline and have similar mineral assemblage to the host dacites almost all phases in saraykent volcanics exhibit varying degrees of disequilibrium features indicative of magma mingling plagioclase phenocrysts display sieved and normal type populations a wide range in rim compositions and oscillatory zoning maximum core to rim compositional change in sieved rhyodacite plagioclases is up to 24 mol increase in an some plagioclase core compositions are unusually ca rich they range from an27 to an75 for the saraykent dacites and from an30 to an66 for the saraykent rhyodacites hornblende and biotite phenocrysts have reaction rims indicating disequilibrium crystallization with magma both hornblende and biotite crystals show reverse zoning in terms of decreasing mg mg fe quartz phenocrysts are corroded and embayed presence of normally and reversely zoned augites in the same sample and high ti high al augites more comparable with basaltic compositions are other lines of evidence for disequilibrium the enclave hornblendes are both edenitic and pargasitic the latter being more typical of mafic compositions the enclave biotite crystals have higher mg mg fe2 ratios than the host dacite biotite crystals and display normal reverse and oscillatory zoning plagioclase core compositions vary between an44 and an55 in the enclaves although the development of sieve texture in plagioclase and reaction rims on mafic minerals may be attributed to decompression this cannot explain the observed compositional change and normal type of plagioclase in the same sample similarly a cognate i e fractionation origin for the enclaves can be suggested due to their mineralogical similarity to the host dacite however other criteria such as oscillatory zoning and unusually calcic cores of plagioclases heterogeneity of plagioclase rims resorbed and embayed phenocrysts and augite compositions indicating a more mafic magma are enough to support the suggestion that magma mingling was a viable process in the generation of the saraykent volcanics

fractal analysis of enclaves as a new tool for estimating rheological properties of magmas during mixing the case of montana reventada tenerife canary islands the volcanic unit of montana reventada on the island of tenerife canary islands spain is an example of magma mingling and mixing in which the eruptive process was triggered by an intrusion of basanite into a phonolite magma chamber the eruption started with emplacement of a basanitic scoria deposit followed by emplacement of a phonolitic lava flow characterized by the presence of mafic enclaves these enclaves represent approximately 1 of the outcrop and are basanitic phono tephritic and tephri phonolitic in composition the morphology of each enclave is different varying from rounded to complex finger like structures usually with cuspate terminations in this study we quantified textural heterogeneity related to the enclaves generated by the mixing process and thus provided a new perspective on the 1100 ad montana reventada eruption the textural study was performed by use of fractal geometry methods and the results show that the logarithm of the viscosity ratio between the phonolitic magma and the enclaves ranges between 0 39 and 0 81 with a mode at 0 49 this enables us to infer the water content is 2 2 5 wt for the phonolitic magma and 1 5 2 wt for the basanitic magma and the enclaves

strange attractors in magmas evidence from lava flows abstract magma mixing structures from three different lava flows salina vulcano and lesbos are studied in order to assess the possible chaotic origin of magma mixing processes structures are analysed using a new technique based on image analysis procedures that extract time series that are representative of the relative change in composition through the structures these time series are then used to reconstruct the attractors underlying the magma mixing process and to calculate the fractal dimension of the attractors results show that attractors exist and possess fractional dimensions this evidence suggests that the mixing of magmas is a chaotic process governed by a low number of degrees of freedom in addition fractal dimension analyses allows us to discriminate between different regimes of mixing in the three lava flows in particular our analyses suggest that the lava flow of salina underwent more turbulent mixing than the lava flows of lesbos and vulcano

does the global stratigraphic reproducibility of δ13c in neoproterozoic carbonates require a marine origin a pliocene pleistocene comparison the large and systematic negative shifts in the 13 c values 12 of carbonate dominated rocks that preceded neoproterozoic glacial successions have been interpreted to record a dramatic series of global environmental and evolutionary events these values are widely considered to be marine rather than diagenetic in origin because stratigraphic patterns of change are systematic and reproducible from basin to basin distinct in magnitude and associated with recognizable stratigraphic markers such as glacial deposits in contrast diagenetic systems are commonly considered to have a more local and stochastic influence on 13 c values cores taken in quaternary carbonate platform sediments however reveal a curious similarity in magnitude thickness and core to core reproducibility where diagenetic alteration has occurred in response to sea level fall sea level changes produced similar 13 c and 18 o stratigraphic records at globally disparate locations which are unrelated to the global marine 13 c signal and bear no relation to the global carbon cycle by analogy with the pliocene pleistocene we propose that spatial reproducibility of 13 c in some neoproterozoic successions might be attributed to causes other than secular variation of the global carbon cycle including diagenesis this observation does not negate the stratigraphic utility of the carbon isotopic values only the origin of the signal

insights into magmatic evolution and recharge history in capraia volcano italy from chemical and isotopic zoning in plagioclase phenocrysts plagioclase phenocrysts in dacites from the high k calc alkaline capraia volcano were investigated for major trace element and sr isotope variations in order to gain better insight into the proposed open system behaviour of the volcano repeated dissolution zones in plagioclases from the early erupted monte castello mc and later monte rucitello mr dacitic units are associated with a sharp increase in an content and variation in isotopic compositions 87sr 86sr 0 70872 0 71004 which argue for repeated magma mixing in particular petrographic and mineralogical evidence suggest that recharge with hot basaltic magma occurred repeatedly during the lifetime of the capraia volcanic system we attempted to correlate shared crystallization and resorption histories using chemical mol an fe and sr isotopic zoning in plagioclase in this correlation framework the observed mismatch between chemical and isotopic data in some phenocrysts is interpreted to reflect either growth of phenocrysts in a separate magma chamber or compositional zoning within a single magma chamber a minor role of crustal contamination is inferred instead the intra crystal variations in isotopic composition in plagioclase reflect the existence of distinct components that may have thoroughly mixed at the end of the mc plagioclase growth history the dominant radiogenic component is similar to lamproites occurring in tuscany recharge with this component occurred throughout the genesis of the mr dacites but was restricted to the early growth history of the mc dacites the mr plagioclases are inferred to have crystallized from separate more differentiated batches of magma that were likely to have evolved at the top of a zoned magma reservoir decrease in the an content after most dissolution zones in all phenocrysts and low temperature estimates from zircon saturation thermometry 800 c also emphasize the importance of crystal fractionation during the evolution of the volcano in agreement with the occurrence of a significant amount of rhyolitic material thus it is inferred that a rather large magma chamber may have existed during the first cycle of eruptive activity

diagenesis in limestone dolostone successions after 1 million years of rapid sea level fluctuations a case study from grand cayman british west indies abstract meteoric diagenesis in young marine carbonate sediments has commonly been linked to fluctuations in quaternary glacio eustatic sea levels the extent to which these sea level changes are recorded in these carbonate successions however remains questionable this is amply demonstrated by the diagenetic record found in the limestones and dolostones of the cayman formation miocene on the cayman islands on the eastern part of grand cayman dolomitization that ceased by 1 million years ago created an architecture whereby the limestones in the central part of the island were surrounded by dolostones in coastal areas of the island since then the upper 90 m of the cayman formation has been repeatedly cycled through many different marine and meteoric diagenetic zones as large rapid eustatic oscillations in sea level affected the island the records of these diagenetic cycles in the dolostones and limestones are however different and impossible to match to the cyclic changes in sea level in the peripheral dolostones post dolomitization diagenetic features are sparse in contrast the limestones in the interior of the island exhibit a wider variety of meteoric diagenetic features including extensive dissolution and calcite cementation the dolostones have low porosity

dry climate near the western pacific warm pool pleistocene caliches of the nansha islands south china sea abstract the nansha spratly islands are located in the middle of the south china sea at about 10 n near the northwestern margin of the western pacific warm pool wpwp a borehole was drilled at an atoll at nansha and cores were taken the upper 165 4 m of the borehole consists entirely of limestone of reef facies and lagoon facies which had been influenced by extensive meteoric diagenesis petrographic and geochemical studies have identified at least four subaerial exposure surfaces ses in the pleistocene carbonate sequence these ses are thought to have resulted from global sea level changes and characterized by caliche formation when developed in limestone caliches typically form in areas where annual precipitation ranges from 500 mm up to about 1200 mm compared with 1800 2200 mm of annual rainfall of the present day nansha islands the nansha caliche therefore indicates the existence of several dry climate episodes during pleistocene sea level lowstands lower rainfall and higher evaporation during such dry conditions may explain the higher sea surface salinities reported elsewhere in the south china sea the nansha caliche may also indicate reduced extent of the wpwp and southerly shift of the intertropical convergence zone during major sea level lowstands

stable isotope and elemental trends in pleistocene sabkha dolomites descending meteoric water vs sulfate reduction abstract a 55 m thick pleistocene dolomite section was sampled in 33 borings in the arabian gulf offshore of al jubayl saudi arabia the regional setting suite of sedimentary structures abundance of evaporites crystals nodules and chickenwire horizons and geochemistry of the dolomite are all consistent with accumulation as part of a series of vertically stacked sabkha depositional complexes analyses of the ordering stoichiometry and overall stable isotope composition indicate that after dolomitization the sediment underwent diagenesis in contact with meteoric waters stable isotope analyses of 385 samples have demonstrated that within this sequence there are four correlatable horizons that display marked excursions to low 13c values some shifts exceed 6 between vertically adjacent samples the 18o values show a similar but less pronounced shift to low values at the same stratigraphic horizons the samples with low 13c and 18o values have a greater admixture of fine grained siliciclastics are light gray i e reducing conditions and have higher concentrations of fe mn and zn than other parts of the deposit these diagenetic attributes especially the marked change in 13c values may be due to either diagenesis due to the descent of meteoric water beneath a soil horizon or diagenesis associated with bacterially mediated sulfate reduction processes sulfate reduction is the favored hypothesis it is envisioned that after the initial dolomitization diagenesis associated with bacterially mediated sulfate reduction of laterally extensive microbial mats within the top parts of sabkha sequences produced the correlatable horizons of low 13c and 18o values

impact of long term climate change and sea level fluctuation on mississippian to permian mid oceanic atoll sedimentation akiyoshi limestone group japan abstract to understand the impacts of long term climatic episodes and sea level changes on mid oceanic atoll sedimentation the stratigraphic changes of litho and bio facies and stratigraphic levels of freshwater diagenetic alteration and biostratigraphic discontinuity of the akiyoshi limestone group upper visean to upper capitanian were reviewed this group records approximately 70 million years of atoll sedimentation in a mid oceanic realm of the panthalassa ocean which developed nearly synchronously with the late paleozoic gondwana glaciation the entire succession of the akiyoshi limestone group was divided into four facies associations skeletal oolitic grainstone muddy limestone muddy limestone skeletal grainstone and reefal limestone associations skeletal oolitic grainstone and reefal limestone associations upper visean to bashkirian are characterized by an abundance of oolite and crinoidal grainstone and reef facies of warm adapted metazoan reef builders corals chaetetes and bryozoans respectively characteristics of skeletal oolitic grainstone and reefal limestone associations indicate their deposition in a warm climate with elevated sea level muddy limestone association moscovian to kasimovian records the demise of the warm adapted metazoan reef builders and frequent emergence events of the buildup many geochemical data support the significant drop in temperature during moscovian through kasimovian time these depositional and biotic events of muddy limestone association indicate that its sedimentation was strongly affected by cool climate and generally low sea level muddy limestone skeletal grainstone association is characterized by an upward increasing significance of calcimicrobes calcareous algae and calcisponges the succession records less frequent emergence events than that of muddy limestone association ii these characters reflect a warming climate and sea level rise in gzhelian to capitanian time the climate episodes and sea level changes interpreted from the akiyoshi limestone group generally correspond to the global trend of mississippian to permian long term climatic change and sea level fluctuation related to the major glacial events of gondwana glaciation the late visean to bashkirian warm climate and elevated sea levels moscovian to kasimovian cooling and sea level lowering and gzhelian to capitanian warming and sea level rise recorded in the akiyoshi buildup are comparable to the global climate model and sea level fluctuation which contain early to middle mississippian pre glacial warm climate and highstands middle to late pennsylvanian cooling and related glacio eustatic sea level fluctuation at the culmination of gondwana glaciation and early to middle permian global warming and sea level rise due to the retreat of gondwana ice sheets akiyoshi atoll sedimentation was most significantly affected pennsylvanian global cooling and related sea level lowering but their impact was short lived the onset of the cooling and frequent emergence events in the akiyoshi buildup occurred delayed approximately 10 millions years later than that of comparable episodes on the pangean shelves the post glacial warming in the akiyoshi buildup started at least several millions of years earlier than in epicontinental seas of the pangea this phenomenon may be due to the palaeoposition of the akiyoshi buildup in a mid oceanic realm of the panthalassa ocean

magnesium and strontium systematics in tropical speleothems from the western pacific abstract we present bulk average sr and mg data for 13 speleothems from different locations in the western equatorial pacific guam solomon islands and vanuatu these data plot on a single straight line in a graph of ln sr ca vs ln mg ca with a slope of 0 9 a 22 000 year record of sr and mg in one of these samples from guam also plots with the same slope suggesting that the process partitioning mg and sr within the guam speleothem is the same as the one that partitions mg and sr between the different pacific speleothems we rule out temperature growth rate detrital phases and sea spray as likely mechanisms for this correlation we construct mathematical models of limestone diagenesis and show that this cannot explain the slope of the correlation our favoured explanation for the correlation is calcite water interaction in the form of prior calcite precipitation pcp and or incongruent calcite dissolution icd we present a formal mathematical model of pcp and prove that the slope of a graph of ln sr ca and ln mg ca is given by kd sr 1 kd mg 1 a similar equation is derived for icd in a companion paper sinclair 2011 using published values for kdsr and kdmg this slope is calculated to be 0 88 in excellent agreement with our observations because the slope is independent of solution and host limestone composition it can be used to diagnose calcite water interaction in individual cave water studies where host rock composition is unknown or in speleothems for which no field drip water data or host rock geochemistry exists approximately half of published drip and speleothem mg sr studies plot within error of this slope we hypothesize that the overall trend in our pacific speleothem data results from the individual dripwater systems evolving from a roughly similar initial rock composition set by late quaternary reef limestone in the guam speleothem the broad peak in mg and sr centred on the early mid holocene reflects a change in hydrology rainfall with the most likely scenario being that dry conditions prevailed on guam at this time

the great barrier reef a 700 000 year diagenetic history a deep borehole through ribbon reef 5 in the great barrier reef off north eastern australia has identified a variety of cements including epitaxial radial prismatic and spherular aragonite together with blocky prismatic and fibrous calcite these cements are discontinuously arranged within the sequence that consists predominantly of grainstones but locally includes clotted muddy and filamentous textures that may be of microbial origin calcite cements vary in morphology with groups of crystals that include acute scalenohedral rhombohedral and flattened concordant terminations these show varying densities of inclusions that locally define growth zones and in some terminations divide in the manner of split crystals to form fibrous fringes morphological changes in calcite are inferred to reflect changes in water chemistry and crystal growth rates at the time of growth allied to their relationship to the palaeo water table and linked in turn to changes in sea level neomorphism and dissolution are widespread and variations in the severity of both imply response to the degree of undersaturation of pore waters that at times were probably balanced within very narrow limits a total of 10 depositional units are identified those units at the base of the borehole reflect deposition and diagenesis within a marine environment the influence of meteoric waters indicated by stable isotopes is first apparent at the top of unit 1 and in unit 2 184 to 155 m below sea floor petrographic evidence of vadose conditions appears at the tops of unit 3 131 to 99 m below sea floor units 4 to 8 all deposited under marine conditions provide isotopic evidence of meteoric or mixing zone waters and petrographic indicators of vadose conditions typically at the top of the units evidence indicates that in unit 5 the water table was mobile and units 6a 6b 7 and 8 all characterized by ultraviolet fluorescent cements are capped by sub aerial erosion surfaces unit 9 the holocene reflects the recent re establishment of marine conditions the extent of alteration of the entire sequence reflects the substantial and pervasive influence of meteoric waters this effect is interpreted as a result of a greater rainfall and river flow from the mainland during early and late stages of interstadial periods the study reflects progress in the ability to recognize the diagenetic signal generated by sea level change however whereas the isotopic results reflect the changing relationships between vadose and phreatic zones in groundwater systems beneath successive emergent surfaces their correspondence with petrographic features is expressed only weakly and commonly lacks the systematic sequential overprinting implied by the distribution of cathodoluminescent zones of cements in many ancient limestones

analysis and simulation of magma mixing processes in 3d abstract magma mixing structures from the lava flow of lesbos greece are analyzed in three dimensions using a technique that starting from the serial sections of rock cubes allows the reconstruction of the spatial distribution of magmas inside rocks two main kinds of coexisting structures are observed i active regions ar in which magmas mix intimately generating wide contact surfaces and ii coherent regions cr of more mafic magma that have a globular shape and do not show large deformations the intensity of mingling is quantified by calculating both the interfacial area ia between interacting magmas and the fractal dimension of the reconstructed structures results show that the fractal dimension is linearly correlated with the logarithm of interfacial area allowing discrimination among different intensities of mingling the process of mingling of magmas is simulated using a three dimensional chaotic dynamical system consisting of stretching and folding processes the intensity of mingling is measured by calculating the interfacial area between interacting magmas and the fractal dimension as for natural magma mixing structures results suggest that as in the natural case the fractal dimension is linearly correlated with the logarithm of the interfacial area allowing to conclude that magma mixing can be regarded as a chaotic process since chemical exchange and physical dispersion of one magma inside another by stretching and folding are closely related we performed coupled numerical simulations of chaotic advection and chemical diffusion in three dimensions our analysis reveals the occurrence in the same system of active mixing regions and coherent regions analogous to those observed in nature we will show that the dynamic processes are able to generate magmas with wide spatial heterogeneity related to the occurrence of magmatic enclaves inside host rocks in both plutonic and volcanic environments

behavior of carbonate associated sulfate during meteoric diagenesis and implications for the sulfur isotope paleoproxy abstract although carbonate associated sulfate cas is used widely as a proxy for the sulfur isotope composition of ancient seawater little is known about the effects of diagenesis on retention of primary 34 s signals our case study of the key largo limestone pleistocene florida is the first systematic assessment of the impact of meteoric diagenesis on cas properties geochemical and petrographic data show that meteoric diagenesis has affected the exposed coralline facies to varying degrees yielding differences now expressed as sharp reaction fronts between primary and secondary carbonate minerals within individual coral heads specifically analyses across high resolution transects in the key largo limestone show that concentrations of strontium and sodium decrease across the recrystallization front from original aragonite to meteoric low magnesium calcite by factors of roughly 5 and 10 respectively predictably 18 o values decrease across these same fronts the 13 c relationships are more complex with the most depleted values observed in the latest formed calcite such trends likely reflect carbon isotope buffering capacity that decreased as reaction progressed as well as protracted development of soil profiles and the associated terrestrial biomass and thus depleted 13 c during sea level lowstand conversely 34 s values of cas vary within a narrow buffered range from 20 6 to 22 6 compared to 20 8 22 0 of coeval pleistocene seawater across the same mineralogical transition despite sulfate concentrations that drop in the diagenetic calcite by an average factor of 12 collectively these data point to robust preservation of primary 34 s for carbonates that have experienced intense meteoric diagenesis which is encouraging news for those using the isotopic composition of cas as a paleoceanographic proxy at the same time the vulnerability of cas concentrations to diagenetic resetting is clear

coral reefs and sea level change abstract coral reefs provide significant evidence for former sea level positions because of their geological preservation and suitability for dating interpretation of this evidence presumes an understanding of reef geomorphology modern reef organism distributions and environmental factors influencing them fossil reef terraces formed during the last interglacial marine oxygen isotope mis substage 5e 128 116 ka are prevalent on many tropical shorelines and there has been ongoing debate as to the height reached by sea level during that highstand observations from numerous last interglacial sites suggest that sea level was at least 3 m above present sea level implying less extensive icesheets than at present an elevation of 6 m has commonly been adopted when correcting tectonically active sites for uplift recent compilations suggest elevations up to 8 9 m but incorporate few observations from reefs where the last interglacial is found below sea level oscillation of sea level during mis 5e has been interpreted from several sites with recent studies inferring rapid rise of several metres at the end of the interglacial these interpretations are at the limits to the precision with which corals can currently be dated and their palaeo water depths inferred it is not surprising that constraining last interglacial sea level changes within uncertainties of less than 1 2 m remains controversial considering sea level variations recognised between reef sites in the holocene and observed geographical variation in isostatic or flexural adjustments fossil coral reefs on uplifting margins also provide clear evidence for mis substages 5c and 5a and those on huon peninsula indicate fluctuations related to heinrich events mis 3 interpretations show considerable variability between sites with still greater uncertainties about sea level timing and elevation during previous interglacials future study of extensive sequences of fossil reefs preserved on rapidly subsiding margins could address these uncertainties submerged reefs have already yielded important information about sea level rise during the last deglaciation coring around barbados and tahiti as well as on the huon peninsula has produced a broadly consistent picture of ice melt reflecting eustatic change since the last glacial maximum these studies have shown the sensitivity of reefs to rapid sea level rise associated with meltwater pulses with some reefs drowning while others back stepped integrated ocean drilling program iodp expeditions to tahiti and recently the great barrier reef extended these records but details of timing nature and impact of deglacial meltwater pulses remain elusive studies of holocene reefs have indicated different growth strategies some kept up with sea level while others caught up when sea level decelerated holocene sea level appears to have experienced a gradual rise up to present across the caribbean providing accommodation space for reefs to accrete vertically whereas in the indo pacific sea level has been near its present level since 7 ka with many reef flats emergent following a slight fall of sea level caused by ocean siphoning microatolls on reef flats provide perhaps the clearest evidence of past sea level position but in their absence novel biological or other sea level indicators are required to better constrain palaeo water depths there is an urgent need for further research from additional key reef locations not only to decipher processes driving past sea level change and its geographical variability but also to better understand how coral reefs will respond in the context of future sea level rise

sr concentrations and isotope ratios as tracers of ground water circulation in carbonate platforms examples from san salvador island and long island bahamas abstract the depth to which seawater and fresh water circulate through modern carbonate platforms may be estimated with 87 sr 86 sr isotope ratios of dissolved sr 2 that is enriched through carbonate mineral dissolution and recrystallization in 23 water samples from onshore san salvador island and long island bahamas carbonate mineral dissolution and aragonite to calcite transformations elevate sr 2 concentrations to twice seawater values in water with near seawater salinity and to about 130 times the expected value for seawater that has been mixed with fresh water carbonate mineral dissolution enriches ca 2 concentrations to around 30 times seawater concentrations only in the mixed waters water with seawater salinity has approximately seawater ca 2 concentrations assuming two end member mixing between seawater sr 2 and mineral derived sr 2 model estimates indicate that mineral derived sr 2 of 19 samples have 87 sr 86 sr ratios equivalent to modern seawater within error of the measurement indicating alteration of shallow buried late pleistocene to holocene carbonate minerals four samples have mineral derived sr 2 with 87 sr 86 sr ratios lower than modern seawater value these low ratios reflect alteration of carbonate minerals that were deposited around 1 mybp although the measured 87 sr 86 sr values could reflect ages as great as 4 6 ma considering the analytical uncertainty of the measurements these estimated ages are likely to be minimum values because alteration of modern carbonate minerals at the surface would provide an unknown but probably large amount of sr 2 with modern seawater isotope signatures thereby overprinting any low 87 sr 86 sr ratios of non modern mineral derived sr 2 three of the four samples with low 87 sr 86 sr ratios have high salinities and were collected from the interior of the islands they reflect seawater flow paths at least tens of meters deep may link the ocean to water several kilometers inland the fourth sample is from the fresh water lens below a pleistocene beach ridge 125 ka only 100 m from the shore line this sample suggests the lens may be thicker than expected based on estimates of recharge hydraulic conductivity and size of the ridge

controls on early diagenetic overprinting in icehouse carbonates insights from modeling hydrological zone residence times using carb3d abstract icehouse carbonate diagenesis is complex with carbonate sequences being exposed to multiple meteoric hydrozones vadose zone freshwater lens mixing zone on multiple occasions a forward modeling approach using carb3d has enabled quantitative exploration of the nature of this diagenetic overprinting by explicit simulation of meteoric hydrozone residence times during evolution of an isolated carbonate platform a sensitivity analysis was performed encompassing the fundamental controls on hydrozone residence time frequency and magnitude of sea level variation subsidence rate and climate the latter via its effect on freshwater lens thickness and rate of dissolutional lowering of the land surface residence time within all meteoric hydrozones is characterized by a high degree of lateral continuity vertical continuity is much more limited with significant variations on a fourth order sequence scale across the platform below sequence boundaries prolonged exposure to the vadose zone results from multiple missed beats when the platform remains exposed at relative sea level maxima overprinting gives total meteoric residence times significantly in excess of the duration of exposure of the overlying sequence boundary there is a hierarchy of eustatically driven overprint cycles with higher frequency cyclicity modifying the primary signal generated by lower frequency cycles fifth order cycles thus have only a minor impact on hydrozone residence times whilst third order seismic scale cycles are a critical control and could be used as a first order approximation of the amount of diagenetic overprinting subsidence has a strong control on residence time with greater overprinting at lower subsidence rates but in high subsidence regimes sediment packages move below the zone of meteoric water influence sufficiently rapidly that residence time can be minimal high land surface dissolution rates which result from a wetter climate remove a large proportion of the subaerially exposed sediment and reduce vadose zone residence time higher effective recharge can increase the thickness of freshwater lens and mixing zones which increases residence times significantly residence times in the vadose zone can be directly linked to a sequence stratigraphic framework but for the freshwater lens and the mixing zone this is possible only with a well constrained relative sea level history