isotopic and elemental evidence for meteoric alteration of a pennsylvanian phylloid algal mound holder formation new mexico u s a the wrong figure 5 was printed with this article the correct figure follows fig 5 thin section photomicrographs of the characteristic lithologies in yucca mound a and b leopard

structural controls on sulfide mobilization at the high grade flying fox ni cu pge sulfide deposit forrestania greenstone belt western australia the flying fox komatiite hosted ni deposit is located in the forrestania greenstone belt of the yilgarn craton in western australia the host volcanosedimentary succession has been metamorphosed to amphibolite facies and deformation has sheared and dismembered the orebody creating 11 distinct ore shoots the ore shoots are hosted at the base of a komatiite sequence structurally overlying deformed metasedimentary rocks the ni sulfide orebody has undergone six phases of deformation 1 d 1areg e w compression resulted in tilting of the stratigraphy 2 d 1bff continued e w compression resulted in coaxial flattening along the footwall sedimentary rock sulfide komatiite contact and the basalt hanging wall sedimentary rock contact 3 d 2ff ene wsw compression resulted in noncoaxial shearing of the d 1bff structures 4 d 3ff ese wnw compression resulted in shallow angle thrusts 5 d 4ff during e w extension normal ductile brittle faults formed by the pooling and dilation of granitic magma along the f 2 structural contact between the footwall sedimentary and komatiite rocks and 6 d 5ff intrusion of a proterozoic dolerite dike during n s extension accompanied by late stage brittle normal faulting during deformation the ni sulfides reverted to monosulfide solid solution mss and were mechanically relocated along the footwall sedimentary rock komatiite contact offset into the footwall sedimentary rocks and entrained in the granitic magma creating atypical granite hosted sulfides the ni sulfides were mobilized up to 5 m away from the ultramafic rocks during the d 1 and d 2 events however subsequent dismemberment of the original orebody into separate ore shoots along flat lying faults during the d 3 event led to offsets of up to 300 m the detailed structural analysis of the flying fox deposit has shown that restoration of the lithostratigraphy along known faults can be used to define the volcanic stratigraphy including primary ni ore horizons this information can be used as a guide to the location of concealed extensions to known ore shoots or completely unknown orebodies at depth or along strike

effects of grand cycle cessation on the diagenesis of upper cambrian carbonate deposits in the southern appalachians u s a abstract the vertical transition from the mainly subtidal alternating shale and carbonate units or grand cycles of the conasauga group middle to upper cambrian to the peritidal dolostone of the knox group upper cambrian to lower ordovician marks a major change in the early paleozoic passive margin sedimentation of the southern appalachians the grand cycles represent an interplay between intrashelf shale basin and carbonate platform deposition the end of grand cycle deposition occurred in response to carbonate platform progradation over the infilled intrashelf basin and is associated with a prominent change in diagenetic patterns observed in the uppermost conasauga group strata the maynardville formation the maynardville records upward shallowing from subtidal shale and limestone into peritidal dolostone this change from a subtidal into a peritidal depositional regime influenced the early diagenesis of the maynardville and in conjunction with the changes in the regional facies distribution at the end of grand cycle deposition extended its influence upon the burial diagenesis of these deposits the subtidal deposits of the maynardville contain a variety of calcite cements dolomite is not abundant in these deposits where it occurs mainly as a fine crystalline ferroan phase associated with argillaceous layers and pressure dissolution features the local source for this dolomite during burial was provided by expulsion of pore fluids from interbedded shale deposits diagenesis of clay minerals and pressure dissolution of carbonates ferruginous coatings on hardgrounds in the subtidal deposits provided an additional local source of iron for the formation of replacement saddle dolomite the subtidal deposits also contain saddle dolomite cement associated with mississippi valley type mvt minerals which indicate the involvement of externally derived fluids during late burial diagenesis of these strata the peritidal deposits on the other hand are extensively dolomitized fine crystalline dolomite most likely represents early diagenetic replacement of tidal flat carbonate sediment coarse crystalline replacement dolomite formed during burial by recrystallization of early dolomite and dolomitization by warm burial brines fenestrae desiccation cracks and evaporite dissolution voids are occluded with dolomite cement which is commonly complexly zoned zoned dolomite precipitated during shallow to intermediate burial and is postdated by saddle dolomite cement saddle dolomite in pore centers dissolutional voids and tectonic fractures formed during late burial from warm fluids associated with the migration of mvt brines the effects of changes in the depositional setting on diagenesis are reflected in similar early diagenesis between the subtidal maynardville and the underlying conasauga group carbonate deposits and between the peritidal deposits of the upper maynardville and the rest of the overlying knox group the depositional environments and early diagenesis also affected the burial diagenesis the changes in regional facies distribution caused by carbonate platform progradation at the end of grand cycle deposition resulted in a decrease in the proportion of siliciclastic components this decrease was accompanied by a decline in the amount of ferroan carbonate in the peritidal deposits the maynardville is conformably overlain by carbonate deposits of the knox group this allowed the maynardville knox succession to behave as a single hydrologic unit for the migration of burial fluids in response to the formation of the ordovician foreland sevier basin to the southeast during the conversion from a passive into a convergent continental margin the nolichucky shale separates the maynardville knox succession from the carbonate deposits of the underlying conasauga group which were in contrast affected by the migration of burial fluids from the cambrian intrashelf shale basin to the west

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

paleoaquifer and deep burial related cements defined by regional cathodoluminescent patterns middle ordovician carbonates virginia middle ordovician ramp carbonates virginia were deposited in a subsiding foreland basin bordered on the southeast by tectonic highlands ramp carbonates were lithified in part by turbid marine cements but major cementation was by nonferroan clear rim and equant cements zoned defined by cathodoluminescence clear cements consist of nonluminescent oldest bright and dull youngest cements the zonation relates to increasingly reducing conditions of pore waters zoned cements in peritidal beds best developed in southeastern belts have complex zonations pendant to pore rimming fabrics and are associated with crystal silt which abuts all cement zones solutional cavities and erosional surfaces which locally truncate dull cement these cements are meteori vadose to shallow phreatic cements in northwestern exposures of peritidal beds are dominated by non zoned dull cement which lacks abundant evidence of early near surface precipitation major cementation of subtidal facies occurred under burial conditions burial cements best developed in southeastern belts have a simple zonation reflecting progressive burial up to 7 5 km 4 5 mi of the carbonate ramp shallow burial nonluminescent cement formed from oxidizing meteoric waters which expelled anoxic connate marine waters these meteoric waters were carried by aquifers from tectonic upland recharge areas on the southeastern basin margin bright cement formed under more reducing conditions following stagnation of the paleoaquifer with burial and possibly was precipitated at depths of 2 to 3 km 1 to 2 mi burial cements in northwestern exposures of subtidal beds are dominated by dull cement initial generations of which precipitated from downdip portions of aquif rs deeper burial dull calcite and ferroan dolomite cements largely formed at burial depths of 2 to 3 km 1 to 2 mi and temperatures of 75 to 135 c 165 to 275 f or more associated with hydrocarbon formation emplacement during the late devonian to mississippian latest clear dull cement fills tectonic fractures and was emplaced during late paleozoic alleghenian deformation probably at temperatures of 200 to 300 c 390 to 575 f and depths of 5 to 7 km 3 to 4 5 mi deeper burial diagenesis appears to be genetically linked to late paleozoic mississippi valley type mineralization in the southern appalachians zoned peritidal and burial cements are confined mainly to southeastern portions of the ramp where cementation was influenced by meteoric waters shed from tectonic uplands on the southeast and carried northwest by paleoaquifers northwestern portions of the ramp were influenced very little by upland sourced meteoric waters and nonzoned dull cements precipitated from relatively reducing waters the distribution of nonluminescent cement in middle ordovician subtidal facies defines the regional distribution of oxidizing portions of the paleoaquifer system such incursion of upland sourced oxidizing meteoric waters into ramp carbonates should be a common feature of foreland basin carbonates deposited adjacent to tectonic uplands furthermore foreland basin carbonates that undergo progressive burial should show a simple nonluminescent to bright to dull sequence of cement zones the close association of zoned cements and regional uplands in the middle ordovician sequence indicates the importance of assessing regional geologic relationships environmental parameters geologic history and tectonics in understanding regional cementation patterns and cementation processes of anci nt carbonate platforms

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

petrographic and geochemical evidence for origin of paleospeleothems new mexico implications for the application of fluid inclusions to studies of diagenesis abstract solution pockets in the mississippian lake valley formation of new mexico occur below the sub pennsylvanian unconformity large masses of columnar calcite spar form within the solution pockets the calcite shows fine growth banding stalagmitic and stalaclitic fabrics coarse detrital inclusions and truncation of crystal terminations indicating a vadose zone or speleothem origin for the spar the calcite is twinned and cross cut by both stylolites and forroan calcite filled fractures indicating deeper burial after precipitation near the surface the mean 13c of 7 29 and 18o of 3 06 pdb of the speleothem calcite defines the light carbon end member of an inverted j shaped curve of isotopic compositions of pre pennsylvanian components in the host limestone the isotopic relationship between speleothems and pre pennsylvanian components suggests a pre pennsylvanian age for the speleothems the isotopic composition and low magnesium content of speleothem calcite reflects precipitation from meteoric water altered little by stabilization of components of the host rock primary fluid inclusions are either one phase all liquid which indicate calcite precipitation below about 40 50 c or two phase with variable ratios of vapor to liquid the two phase inclusions formed by high temperature reequilibration of low temperature fluid inclusions and entrapment of air and water in the vadose zone the petrographic characteristics and one atmosphere pressure of some of the fluid inclusions suggest precipitation of speleothems in the vadose zone some secondary fluid inclusions are two phase with rather consistent ratios of vapor to liquid these inclusions indicate a late history of relatively high temperatures of the primary fluid inclusions 74 of the two phase population have been refilled with brines of deeper burial and 26 retain fresh water for the one phase fluid inclusions 100 are fresh water therefore the one phase all liquid inclusions in partially reequilibrated low temperature fluid inclusion populations should be viewed as reliable indicators of temperature and salinity of cement precipitation in this and other diagenetic studies

nonmonotonic variation of seawater 87 sr 86 sr across the ivorian chadian boundary mississippian osagean evidence from marine cements within the irish waulsortian limestone abstract detailed analysis of compositionally unaltered marine fibrous cements mfc from a single core through the mississippian irish waulsortian limestone indicates that the variation of seawater 87sr 86sr is nonmonotonic across the ivorian chadian boundary this nonmonotonic variation has not been recognized by previous studies furthermore marine cements yielded 87sr 86sr ratios lower than previously reported values for the ivorian chadian osagean marine fibrous cements are interpreted to be compositionally unaltered on the basis of nonluminescent character and stable isotope c o composition comparable to previous estimates of mississippian marine calcite the isotope chemistry c o sr and cathodoluminescent character of the marine fibrous cements therefore remained intact during their conversion from high mg calcite to low mg calcite microdolomite a conversion that probably took place in marine water during precipitation of zone 1 calcite cement the oldest non mfc cement high stratigraphic resolution was obtained by restricting the sample set to a single core 429 m long thereby eliminating chronostratigraphic correlation errors the mfc cements are pre arundian based on previous study and are interpreted to be in stratigraphic order and to be approximately the same age as the host limestones the core is estimated to represent about 9 8 million years of waulsortian limestone deposition 87sr 86sr ratios of pristine mfc samples range from a high of 0 707908 in the early ivorian to a low of 0 707650 in the late ivorian and mid chadian with an early chadian maximum at 0 707800 the maximum rate of change in seawater 87sr 86sr is 0 00012 ma comparable in magnitude to tertiary values our data document the presence of fine scale seawater 87sr 86sr modulations for the ivorian chadian in contrast to the previously published monotonic seawater 87sr 86sr curve for this interval and emphasize the importance of well characterized intraformational isotopic baselines

sector zoning in calcite cement crystals implications for trace element distributions in carbonates abstract compositional sector zoning in calcite cement crystals from a sedimentary limestone is represented by differences of mg content of over 1300 ppm or 95 and of sr and mn contents of up to 400 ppm or roughly 50 and 80 respectively between crystallographically nonequivalent time synchronous growth sectors 10 1 1 sectors are enriched with both mg and mn relative to 10 1 4 sectors and other sectors show both greater and lesser enrichment this systematic zoning pattern requires effective distribution coefficients for these trace elements to have been different for growth on different faces differential growth rate effects for nonequivalent faces are shown to be unimportant relative to differential growth face element preferences surface crystal structure and differing coordination environments are regarded as the principal factors influencing sector related elemental partitioning differences in calcite and dolomite

nature and distribution of diagenetic phases and petrophysical properties of carbonates the mississippian madison formation bighorn basin wyoming usa abstract the purpose of this study is to characterize factors controlling the nature and distribution of the diagenetic phases that gave rise to carbonate reservoirs to do so a pluridisciplinary approach was carried out integrating sedimentologic and diagenetic studies on the madison formation lower carboniferous 357 340 my which is a carbonate reservoir in subsurface the madison formation outcropping in the bighorn basin wyoming usa is a 340 m thick carbonate series composed of four to seven 3rd order depositional sequences s1 to s7 depending of the palaeogeographic location the first three sequences s1 to s3 were deposited under arid and warm conditions during tournaisian times which favoured high accumulation of carbonates leading to a morphological change from a quite flat ramp s1 to s2 to a wide platform s3 it also probably favoured the early calcite cementation isopachous and syntaxial calcite cements of the subtidal deposits and the early dolomitization d1 of the supratidal to intertidal sabkha ones in addition the very flat profile occurring during s3 was also responsible for the postponed dolomitization d2 of s1 s2 due to reflux of brines at various degrees depending on the palaeogeographic location the deposition of s4 to s7 under humid conditions during visean times were associated with 1 a decrease of the carbonate accumulation and of the dolomitization 2 dissolution processes at micro pores network and macro scales karst collapse breccia and calcite cementation c1 the spatial distribution of all these first diagenetic phases acts as a controlling factor on the distribution of the later burial diagenetic phases thus the mesogenetic calcite cements are mostly observed in the secondary porosity created by dissolution karsts collapse breccias or dolomitization the burial diagenesis of the madison formation was characterized by 1 a dolomitization phase d3 in proximal parts of the platform leading to an increase of porosity and permeability and 2 calcite cementations c2 c3 in the distal parts of the platform leading to a decrease of these properties the madison formation provides a good spatial representation of sedimentary and diagenetic heterogeneities that may occur in the carbonate reservoirs due to various palaeogeographic locations climates burial history and structures

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

calcite cement stratigraphy and cementation history of the burlington keokuk formation mississippian illinois and missouri abstract iron content and cathodoluminescence define six regionally extensive calcite cement zones over 100 000 sq km of study area in the burlington keokuk formation of illinois and missouri the regional cement stratigraphy is composed of mappable zones analogous to lithostratigraphic units which represent diagenetic episodes during which calcite with distinct cathodoluminescent properties precipitated in a given time interval relative to other diagenetic events such as cement fracturing and calcite dissolution individual cement zones may not be strictly isochronous throughout the study area but are not strongly diachronous two different but related cathodoluminescent sequences have been identified within pre pennsylvanian cements in the burlington keokuk formation 1 within lower strata early nonferroan cement consists of both nonluminescent and luminescent calcite zones ii iv 2 within upper strata early nonferroan cement consists of entirely luminescent calcite zone ii petrographic relationships indicate that zone ii is time equivalent with zones ii iv in lower burlington keokuk strata the occurrence of cement zones ii iv encased in pennsylvanian shale and quartz sand within pre pennsylvanian karst cavities constrains the age of early nonferroan calcite as pre pennsylvanian in ll cases pre pennsylvanian cements are followed by ferroan dull to moderately luminescing calcite zone v which in turn is overgrown by nonferroan moderate to brightly luminescing calcite zone vi cross cutting relations of burlington keokuk syntaxial calcite with mississippi valley type mineralization phases constrains the minimum age of cement zones v and vi as early permian pre pennsylvanian cements formed in a shallow meteoric aquifer characterized by vertically segregated chemical facies pre pennsylvanian cements within lower strata precipitated from waters of varying eh while those in upper strata formed from reduced waters post mississippian pre early permian cements precipitated under less than 1 km of overburden from reducing meteoric waters or subsurface brines

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

glacioeustasy meteoric diagenesis and the carbon cycle during the middle carboniferous middle carboniferous carbonates in the western u s have undergone pleistocene bahamas style meteoric diagenesis that may be associated with expanding late paleozoic ice sheets fourteen stratigraphic sections from carbonate platforms illustrate the regional distribution and variable intensity of physical and chemical diagenesis just below the middle carboniferous unconformity these sections contain top negative carbon isotope excursions that terminate in regional exposure surfaces that are associated with some combination of karst towers desiccation cracks fabric destructive recrystallization or extensive root systems the timing of the diagenesis is synchronous with similarly scaled top negative carbon isotope excursions observed by others in england kazakhstan and china the mass flux of negative carbon required to generate similar isotopic profiles across the areal extent of middle carboniferous platform carbonates is a significant component of the global carbon cycle we present a simple carbon box model to illustrate that the 13c of dissolved inorganic carbon in the ocean could be elevated by 1 4 as isotopically light carbon from the weathering of terrestrial organic matter reacts with exposed platforms before reaching the ocean and atmosphere these results represent an improvement on global biogeochemical models that have struggled to provide a congruent solution to the high 13c of the late paleozoic icehouse

regional cementation from unconformity recharged aquifer and burial fluids mississippian newman limestone kentucky abstract the mississippian newman limestone in kentucky was studied to assess the effects of unconformities within and above the sequence on calcite cementation early cements are low in fe and show a nonluminescent to dull to nonluminescent zonation early nonluminescent cement mainly formed in updip areas from oxidizing meteoric waters in a regional paleoaquifer that formed during regional regression in the late mississippian during late mississippian transgression waters became reducing to form early dull cement up to 600 ppm mn up to 2 700 ppm fe but averaging 570 ppm the aquifer underwent later recharge during mississippian pennsylvanian regression to form a second nonluminescent cement this event continued into the pennsylvanian when the sequence underwent increasing burial earl cements have 13o values that become lighter downdip perhaps reflecting a downdip temperature increase due to burial 13c values are lightest in the north postulated recharge area and reflect input of organically derived co2 in waters there most calcite cementation appears to have been associated with two post newman unconformities which generated regional aquifers and reflects a shift to a wetter climate in late mississippian pennsylvanian time early cement abundance decreases away from postulated recharge areas later iron rich burial cements up to 2 200 ppm mn up to 11 000 ppm fe fill remaining pore spaces compaction induced fractures spalled cement rims and tectonic fractures lined with saddle dolomite and pyrite they formed above 60 c at depths of 1 to 3 km 13o values are compatible with derivation by pressure solution of preexisting calcites in isotopically heavy waters highly modified by burial diagenesis light 13c values reflect isotopically light organic carbon in the deltaic waters entering the subsurface or being expelled from shales or buffering of waters by light c early calcite cements the 13c calcite cements become heavier downdip by equilibration of waters with host limestone newman sediments may have been subjected to late postcementation migration of warm fluids based on secondary fluid inclusions these late stage fluids most likely were chemically complex warm brines 7 17 equiv wt naci 50 to 160 c that resided in the formation for sufficiently short times that conodont cai values were not affected

δ18o values 87sr86sr and sr mg ratios of late devonian abiotic marine calcite implications for the composition of ancient seawater abstract late devonian frasnian abiotic marine calcite has been microsampled and analyzed for 87 sr 86 sr ratios 18o and 13c values and minor element concentrations portions of marine cement crystals from the alberta and canning basins have escaped diagenetic alteration and preserve original marine 18o values 4 8 0 5 pdb 13c values 2 0 to 3 0 pdb 87 sr 86 sr ratios 0 70805 3 and sr mg weight ratios 0 04 to 0 05 marine 87 sr 86 sr ratios are globally consistent and can be correlated within the alberta basin and among the alberta canning and williston basins correlation of isotopic and chemical data strengthen the conclusion that marine cements from the leduc formation preserve original marine 18o values which are 3 to 4 lower than those of modern marine cements these low 18o values are best explained by precipitation from 18o depleted seawater and not by elevated seawater temperature or diagenetic alteration for comparison with devonian data analogous data were collected from holocene mg calcite and aragonite marine cements from enewetak atoll marshall islands mg calcite and aragonite marine cements are in isotopic equilibrium with ambient seawater and mg calcite cements are homogeneous with respect to sr and mg contents empirically derived homogeneous distribution coefficients for mg and sr in modern abiotic mg calcite from enewetak atoll are 0 034 and 0 15 respectively an equation describing the dependence of dsr on mg content was based on a compilation of sr and mg data from holocene abiotic marine calcite dsr 3 52 10 6 ppm mg 6 20 10 3 unlike that derived from experimental data this sr mg relation is consistent over a range of 4 to 20 mol mgco3 and may represent precipitation phenomena which are minimally controlled by kinetic effects comparison of sr and mg contents of analogous devonian and holocene marine cements suggests that the mg ca ratio of late devonian seawater was significantly lower and that the sr ca ratio was significantly higher than that of modern seawater

the distribution of 3 0 ga and 2 7 ga volcanic episodes in the yilgarn craton of western australia abstract the results of a conventional zircon u pb investigation of the ages of felsic volcanic rocks in a number of greenstone belts in the yilgarn craton of western australia indicate that volcanism took place during two distinct episodes at ca 3 0 and ca 2 7 ga each episode spans a time period of about 75 ma and the two ages of volcanism are spatially separated 3 0 ga volcanic rocks are found in greenstone belts in the northern part of the western gneiss terrain and in the murchison province whereas 2 7 ga volcanic rocks are present in the southern part of the western gneiss terrain and in the southern cross and eastern goldfields provinces there is no evidence of inheritance in the zircon u pb systems which would support an older crustal origin for these rocks present results together with previous studies of the ages of granites and gneisses suggest that the volcanic episodes were accompanied by broadly contemporaneous granite plutonism it is suggested that the evolution of the yilgarn craton took place in three main stages the first was the development of a stable 3 4 ga granitic nucleus which incorporated earlier generations of granitoid a 3 7 ga layered complex and possibly 4 2 ga primitive crust this was followed by the two separate granite greenstone episodes at ca 3 0 and ca 2 7 ga involving the addition of significant amounts of new crust each episode was characterised by the accretion of extensive granite greenstone terranes at the margins of the stable craton the intrusion of granite plutons into the evolving craton and the reworking of older cratonic rocks to form granites and gneisses with inherited isotopic systems

the physical volcanology of archaean komatiite sequences from forrestania southern cross province western australia abstract the archaean supracrustal sequence at forrestania contains at least five komatiitic belts which can be traced along strike for distances in excess of 30 km lithologies range from olivine cumulates which crystallised at the interface between substrate and flowing lava to spinifex textured rocks which underwent rapid crystallisation within ponded lava lobes characteristic associations of different rock types are interpreted in terms of variations in eruption rate which presumably reflect proximity to the vent or to a major lava river thick sequences of olivine adcumulate to mesocumulate crystallised in a dynamic environment where lava flow was continuous and eruptive velocities were high sheeted bodies of olivine cumulate formed from voluminous turbulent flow of lava over relatively refractory substrates such as basalt and oxide facies banded iron formation ribbon shaped olivine cumulate bodies formed in an equally dynamic environment within major lava channels where the substrate was less refractory and consisted of quartz rich sedimentary rocks which were susceptible to thermal erosion distributary lava channels in more distal settings are represented by thinner ribbon like accumulations of olivine mesocumulate to orthocumulate unusual poikilitic pyroxene bearing rocks are present at the base of some lava channels and appear to represent the crystallisation product of a komatiitic lava which has been contaminated by cherty sedimentary rocks pyroxene and plagioclase bearing cumulates are present at the top of other channel sequences and reflect late stage ponding and in situ differentiation of komatiitic lava within the channel the channel facies rocks are flanked by sequences comprising multiple thin spinifex textured flow units which formed from the episodic overflow of lava from the distributary channels similar rocks also occur stratigraphically above many of the thick olivine cumulate bodies and reflect episodic emplacement of thin cooling units during the waning stages of the eruptive cycle

chromium isotopic homogeneity between the moon the earth and enstatite chondrites abstract among the elements exhibiting non mass dependent isotopic variations in meteorites chromium cr has been central in arguing for an isotopic homogeneity between the earth and the moon thus questioning physical models of moon formation however the cr isotopic composition of the moon relies on two samples only which define an average value that is slightly different from the terrestrial standard here by determining the cr isotopic composition of 17 lunar 9 terrestrial and 5 enstatite chondrite samples we re assess the isotopic similarity between these different planetary bodies and provide the first robust estimate for the moon in average terrestrial and enstatite samples show similar e 54 cr on the other hand lunar samples show variables excesses of 53cr and 54cr compared to terrestrial and enstatite chondrites samples with correlated e 53 cr and e 54 cr per 10 000 deviation of the 53cr 52cr and 54cr 52cr ratios normalized to the 50cr 52cr ratio from the nist srm 3112a cr standard unlike previous suggestions we show for the first time that cosmic irradiation can affect significantly the cr isotopic composition of lunar materials moreover we also suggest that rather than spallation reactions neutron capture effects are the dominant process controlling the cr isotope composition of lunar igneous rocks this is supported by the correlation between e 53 cr and e 54 cr and 150sm 152sm ratios after correction of these effects the average e 54 cr of the moon is indistinguishable from the terrestrial and enstatite chondrite materials reinforcing the idea of an earth moon enstatite chondrite system homogeneity this is compatible with the most recent scenarios of moon formation suggesting an efficient physical homogenization after a high energy impact on a fast spinning earth and or with an impactor originating from the same reservoir in the inner proto planetary disk as the earth and enstatite chondrites and having similar composition

aureole deformation by flattening around a diapir during in situ ballooning the cannibal creek granite the cannibal creek granite a foliated pluton within a strongly deformed aureole shows several diagnostic features of a diapir the intensity of the aureole deformation diminished away from the contact the aureole foliation is continuous with that of the granite and forms closed elliptical trend lines and the contact metamorphic minerals are synkinematic with regard to the aureole foliation the pluton deformed its aureole in three stages involving initially brittle then ductile and finally brittle deformation styles initial intrusion of a low strength magma stoped kinked and flexed aside its overburden crystallization of the magma prevented further rise and it ballooned and deformed its aureole evidently entirely by ductile bulk shortening perpendicular to the contact this the major stage of aureole deformation records no evidence of vertical movement of the pluton or of progressive simple shear along its margin during this stage the magma may have been theologically a high strength bing

kinetic control of morphology composition and mineralogy of abiotic sedimentary carbonates abstract conventional thinking has long held that the abiotic precipitation of calcium carbonate occurs with a causal relationship between fluid my ca ratios and crystal morphology crystal composition and carbonate mineralogy resulting in the formation of meteoric equant low magnesium calcite and marine acicular high magnesium calcite and aragonite problematically calcites with varying amounts of incorporated magnesium occur either as equant or acicular crystals and aragonite may coexist with calcite in either environment commonly however a systematic relation exists between crystal morphology composition mineralogy and rates of reactant supply to growing crystal surfaces for example equant rather than acicular crystals of calcite form in modern deep and or cold marine meteoric phreatic and deep burial settings where the degree of carbonate saturation and or rates of fluid flow are low in areas of higher saturation and or fluid flow such as in warm shallow marine and meteoric vadose environments acicular calcite may predominate this relation is also seen in systems in which aragonite and calcite form in intimate association aragonite precipitation is favored when rates of reactant supply are high calcite forms when rates are low such relations suggest that crystal morphology composition and mineralogy are controlled by the kinetics of surface nucleation and the amount of reactants principally carbonate ions at growth sites precipitating phases are the ones which can best accommodate such excess reactants ambient mg ca ratios only indirectly control the nature of inorganically precipitated carbonate phases

chronostratigraphic significance of cathodoluminescence zoning in syntaxial cement mississippian lake valley formation new mexico abstract echinoderm syntax ial cement crystals have been collected at several stratigraphic levels within the lake valley formation which is overlain by a major pre pennsylvanian subaerial exposure surface the crystals were microsampled along growth bands and yielded high resolution elemental and isotopic information that record fluid evolution during their growth although cement crystals show little variation in cathodoluminescence character and bulk chemistry throughout the regional extent of the formation intracrystalline patterns in minor element and stable isotope chemistry allow for the identification of several settings of cementation including the marine phreatic marine meteoric mixing and meteoric phreatic zones when placed in a regional stratigraphic context crystal growth records enable reconstruction of the temporal and spatial extent of these diagenetic environments isotopic petrographic and stratigraphic constraints indicate that cementation was related to two temporally distinct meteoric systems most lake valley formation syntaxial cement formed in the marine phreatic and marine meteoric mixing zones during the earliest phase of cementation of this cement in excess of 60 formed in the marine phreatic zone and lowermost marine meteoric mixing zone smaller volumes precipitated from progressively fresher mixtures of marine and meteoric fluid during the gradual expansion of a freshwater lens that developed in response to pre pennsylvanian lowering of sea level meteoric phreatic fluids were relatively unimportant during this episode of cementation in contrast cement of entirely meteoric phreatic origin is associated with a later meteoric system but is found only in the northern part of the study area and at stratigraphic levels immediately adjacent to the pre pennsylvanian unconforrnity our results indicate that cements in the upper and lower parts of the lake valley formation are genetically and temporally unrelated thus the cement cathodoluminescence stratigraphy which has been previously applied to the lake valley can not be valid over the full extent of the formation this has wider implications our demonstration that diverse and temporally distinct diagenetic settings can produce identical cl characteristics shows that caution should be employed when using cement cl stratigraphy alone to extend diagenetic interpretations over large geographic regions or through substantial stratigraphic successions

discrimination of multiple episodes of meteoric diagenesis in a kimmeridgian reefal complex north iberian range spain abstract stratigraphic relations detailed petrography and fine scale geochemical and isotopic analysis of diagenetic phases formed within kimmeridgian age reefal carbonates of the torrecilla en cameros formation of northern iberian ranges in spain indicate a complex history of alteration in marine meteoric burial and uplift related settings the reefal succession is separated from the overlying tithonian berriasian fluvial and lacustrine continental deposits by a single unconformity which is marked by brecciation and karstification nevertheless a record of three distinct stages of alteration associated with this unconformity is preserved in the succession of calcite cements present within the reefal carbonates recognition of temporally distinct episodes of meteoric diagenesis associated with a single unconformity is atypical in carbonate rocks but this study illustrates how such complex systems can be deconvoluted the first episode of subaerial exposure and meteoric alteration resulted in neomorphism of marine allochems and precipitation of a first generation of nonferroan clear calcite nfc cement following local faulting and brecciation a second nfc precipitated throughout the reefal unit although similar in petrographic character this phase of cementation is distinct in its isotopic composition reflecting changes in the regional climate of the iberian peninsula during the early tithonian another phase of meteoric alteration of the reefal unit is recorded by renewed corrosion including dissolution of preexisting calcite cements and precipitation of prismatic calcite prior to the deposition of tithonian berriasian lacustrine and continental sediments these continental units in turn record yet another episode of alteration by meteoric waters as nfc cements formed within intragranular and dissolution porosity in the lacustrine limestones notably these cements are also distinct on the basis of their petrographic and geochemical character following subsidence and burial during aptian to late cretaceous times migration of regionally derived fluids perhaps in response to the onset of tectonic deformation to the north led to cementation of saddle ankerite and ferroan calcite throughout the sedimentary sequence alteration of these burial related diagenetic phases has subsequently taken place in response to the regional uplift during the tertiary this final episode of meteoric alteration is indicated by replacement of the ferroan calcite and ankerite by a cloudy nonferroan calcite

low magnesian calcite marine cement in middle ordovician hardgrounds from kirkfield ontario abstract middle ordovician trentonian biosparites from the bobcaygeon and verulam formations near lake simcoe ontario exhibit numerous features demonstrating lithification near the sediment water interface these include planar erosion surfaces which truncate shells and intraclasts undercut hummocks on upper surfaces vertical borings which cross cut fossil debris and early cement and a diverse fauna of encrusting organisms which grew attached to lithified surfaces a marine origin for early cement is indicated by the development of hardgrounds within a transgressire regionally onlapping sequence the occurrence of dozens of such units within normal marine sediments and a lack of features recording early oxidation and or partial dissolution during exposure to meteoric water marine cement from four of these units examined in thin section by sem and by cathodoluminescence consists of clear zoned syntaxial overgrowths on echinoderm debris and equant spar on trilobite bryozoan and brachiopod fragments as such this cement 1 is morphologically identical to meteoric phreatic low magnesian calcite cement from calcitized pleistocene sequences 2 is interpreted as having been precipitated as low magnesian calcite 3 is anomalous in habit and inferred composition in comparison with holocene marine cement and 4 demonstrates that generalizations which relate cement morphology and composition to environmental water chemistry in modern diagenetic systems may not be valid when applied to more ancient carbonate sequences

granite magma formation transport and emplacement in the earth s crust the origin of granites was once a question solely for petrologists and geochemists but in recent years a consensus has emerged that recognizes the essential role of deformation in the segregation transport and emplacement of silica rich melts in the continental crust accepted petrological models are being questioned either because they require unrealistic rheological behaviours of rocks and magmas or because they do not satisfactorily explain the available structural or geophysical data provided flow is continuous mechanical considerations suggest that far from being geologically sluggish granite magmatism is a rapid dynamic process operating at timescales of or 100 000 years irrespective of tectonic setting

stable isotope geochemistry of alpine ophiolites a window to ocean floor hydrothermal alteration and constraints on fluid rock interaction during high pressure metamorphism the subduction of hydrated oceanic lithosphere potentially transports large volumes of water into the upper mantle however despite its potential importance fluid rock interaction during high pressure metamorphism is relatively poorly understood the stable isotope and major element geochemistry of pennine ophiolite rocks from italy and switzerland that were metamorphosed at high pressures are similar to that of unmetamorphosed ophiolites suggesting that they interacted with little pervasive fluid during high pressure metamorphism cover sediments also have oxygen isotope ratios within the expected range of their protoliths in the rocks that escaped late greenschist facies retrogression different styles of sub ocean floor alteration may be identified using oxygen isotopes petrology and major or trace element geochemistry within the basalts zones that have undergone high and low temperature sub ocean floor alteration as well as relatively unaltered rocks can be distinguished serpentinites have 18o and 2h values that suggest that they were formed by hydration on or below the ocean floor the development of high pressure metamorphic mineralogies in metagabbros occurred preferentially in zones that underwent sub ocean floor alteration and which contained hydrated fine grained reactive assemblages given that the transformation of blueschist facies metabasic rocks to eclogite facies assemblages involves the breakdown of hydrous minerals e g lawsonite zoisite and glaucophane and will thus liberate considerable volumes of fluids metamorphic fluid flow must have been strongly channelled high pressure quartz calcite omphacite glaucophane titanoclinohumite veins that cut the ophiolite rocks represent one possible channel however stable isotope and major element data suggest that they were not formed from large volumes of exotic fluids fluids were more likely channelled along faults and shear zones that were active during high pressure metamorphism such strong fluid channelling may cause fluids to migrate toward the accretionary wedge especially along the slab mantle interface which is probably a major shear zone this may preclude all but a small fraction of the fluids entering the mantle wedge to flux melting additionally because fluids probably interact with relatively small volumes of rock in the channels they cannot scavenge elements from the subducting slab efficiently

quantitative studies of compaction in mississippian skeletal limestones new mexico abstract quantitative study of compaction in mississippian coarse grained skeletal grainstones and cement rich packstones over a region of about 25 000 km2 of southwestern new mexico has shown that intergranular compaction was a major process of porosity destruction in more than 90 percent of all coarse grainstones and packstones having been deposited with at least 42 percent intergranular porosity these calcarenites now contain about 27 percent intergranular space now occupied by cement plus mud they therefore have lost about 38 percent of their intergranular porosity by intergranular compaction the remainder having been lost by cementation regionally compaction intensity is greatest in the north with southern measured sections on average containing about 32 percent and orthernmost sections about 20 percent intergranular space point count data on grain contacts show that intergranular pressure solution increases with compaction intensity and is most abundant in the north it was the main process affecting intense intergranular compaction and was the reason for compaction intensity being greatest in the north bryozoan deformation and grain rearrangement although poorly quantified were also important contributors to intergranular compaction whereas mechanical breakage was of negligible importance of the factors that potentially controlled compaction intensity chemistry of paleogroundwaters and overburden were probably the most important whereas bryozoan content terrigenous clay content and depositional textures may have been of secondary importance and tectonic stress was probably of no importance we propose a model in which a major control on intergranular pressure solution intensity was the degree of saturation with respect to calcite in pre pennsylvanian groundwater systems these groundwaters flowed from north to south with the main recharge region in the north groundwaters near and in the recharge regions in the north more commonly were undersaturated or had lower degrees of saturation with respect to calcite compared to groundwaters in downflow south regions this promoted greater chemical compaction in the north even under low to moderate overburden tens to hundreds of meters maximum of about 1000 meters

geochemical imprint of meteoric diagenesis in holocene ooid sands schooner cays bahamas correlation of calcite cement geochemistry with extant groundwaters abstract the geochemistry of meteoric calcite cements and their parent waters on three small holocene ooid sand islands in the schooner cays bahamas is explicated in terms of observed hydrologic and hydrochemical processes dissolved mg is derived soley from admixed seawater or aerosols the mg contents of the cements 0 4 to 3 0 mole are thus a salinity indicator and suggest cementation in waters composed of 16 mmoles liter per yr variations in mg and sr contents across cement mosaics reflect long term temporal fluctuations in the amou t of aragonite to calcite alteration and incursions of brackish waters the pore water oxygen reservoir is water buffered and derived from meteoric rainfall and seawater mixing but is seasonally enriched in 18o by evaporation the 18o values of the cements 3 4 to 5 0 pdb reflect the relative proportions of cements formed during and immediately after the rainy season versus those formed during the dry season the pore water carbon is derived from organic respiration atmospheric carbon and dissolved aragonite organic respiration dominates the 18c of the vadose cements 7 8 to 1 1 pdb and the 13c of phreatic groundwaters 2 2 to 14 0 pdb in contrast 13c enriched phreatic cements 1 2 to 3 7 pdb reflect carbon from dissolved aragonite and are not in carbon isotopic equilibrium with the present groundwaters this case example reveals the complex geochemical signal that can result from even a very simple diagenetic history such as a single phase of meteoric diagenesis acting on a purely aragonitic sediment

deformation and remobilization in some western australian nickel ores abstract volcanic associated nickel deposits in the archaean greenstone belts of western australia show a variety of features related to post depositional deformation and medium to high grade regional metamorphism deformation and accompanying remobilization have resulted in changes to the gross structure shape and orientation of ore shoots and have led to the development of separate offset ores tectonic breccia ores and irregular sulphide stringers physical remobilization of ore has been mainly on a mesoscopic scale up to 150 m and largely by mechanical processes but with some fluid assisted and stress induced chemical remobilization of sulphur and possibly copper most of the present ore fabrics and textures are the result of combined deformation metamorphically induced phase changes and recrystallization massive and matrix ores show well developed tectonite fabrics defined by mineralogical layering preferred orientation of pyrrhotite and alignment of silicate and spinel inclusions layering in the ores is probably the result of both strain controlled exsolution from high temperature phases and mechanical segregation of different minerals within polyminerallic aggregates during deformation observational and experimental data suggest that sulphide deformation was dominantly by dislocation type mechanisms kinking and twinning at medium temperatures 300 550 c with diffusional mechanisms becoming important at high temperature 500 c dynamic recrystallization during deformation probably accounts for the annealed state of individual mineral grains within most ores except those deformed at a late stage

evolution of the archaean granite greenstone terranes of the eastern goldfields western australia shrimp upb zircon constraints abstract thirty five high precision shrimp u pb zircon dates with an average 95 confidence error of 6 ma have been obtained from the granite greenstone terranes in the southern part of the eastern goldfields of western australia these dates establish the timing of major deformation metamorphism and granite intrusion events and test inferred stratigraphic relationships within the greenstones these new data reveal that felsic volcanic rocks were erupted throughout the southern part of the eastern goldfields between c 2713 and c 2672 ma analytically indistinguishable dates of c 2705 ma were obtained for felsic volcanic rocks interbedded with komatiitic lavas at three sites dates on felsic volcanic rocks within tectonostratigraphic packages that include komatiites are also consistent with an inferred eruption age for these komatiites of c 2705 ma komatiitic lavas in the southern part of the eastern goldfields were probably emplaced during a single voluminous eruption that resulted in the flooding of a vast area of the late archaean landscape filling the topographic lows around older felsic volcanic vents the stratigraphic complexity within tectonostratigraphic domains is attributed to the existence of an undulating topography during the eruption of the mafic and ultramafic lavas and to the rapid and localized deposition of felsic volcanic rocks close to a number of isolated volcanic centres prior to and after the komatiite lava eruption event between c 2685 and c 2675 ma early pre d2 intrusive equivalents of the felsic volcanic rocks were emplaced as thick granite sheets into the base of the greenstone sequences regional east west compression between c 2675 and c 2657 ma reactivated early extensional structures and resulted in the stacking of the greenstones this was accompanied by further intrusion of granite within developing antiforms burial of greenstone slices by stacking and the effect of hydrothermal fluids emanating from both the granites and buried greenstones resulted in regional metamorphism to greenschist facies prior to c 2640 ma monzogranite and granodiorite emplaced near the western southern and eastern margins of the greenstones were deformed and metamorphosed to upper amphibolite facies and converted into gneisses during d2 and d3 compression events movement along large scale transcurrent d3 shear zones had ceased by c 2635 ma remnants of a c 2958 to c 2930 ma greenstone sequence preserved near norseman and ravensthorpe in the southeastern yilgarn craton and xenocryst zircons ranging in age from c 2910 2730 ma in the c 2713 2672 ma felsic volcanic rocks indicate that the exposed greenstone sequences were deposited on continental basement the eastern goldfields granite greenstone terranes were formed by the closure of a series of narrow back arc rift basins that developed along a continental margin and above an active subduction zone located to the east of the rift basins

textural evidence for remobilization in metamorphic environments remobilization of stratabound massive sulphide deposits involves both dislocation flow mechanical remobilization and fluid state processes chemical remobilization textural support for such remobilization is presented at micro meso and macroscales microscale evidence establishes the degree the amount of a deposit affected of remobilization likewise mesoscale textures establish the degree and may provide mesoscale models of macroscale remobilization processes but still do not establish the extent the distance over which translocation may have occurred of remobilization where macrotexture indicates a departure from an original conformable relationship gross remobilization is suggested and may be the consequence of mechanical and or chemical processes the case for gross mechanical and or chemical remobilization is often difficult to sustain because alternative hypotheses e g hydrothermal replacement may be compatible with the observed relationships remobilization of a conformable ore layer may involve internal redistribution during which sulphides have been modified by dislocation flow and or fluid state processes within the layer constraints or external where the layer constraints have been substantially transgressed during dislocation flow and or fluid phase remobilization if mesoscale textures indicate remobilization of the external type then macroscale gross remobilization remains a possibility subject to substantiation

isotopic and elemental evidence for meteoric alteration of a pennsylvanian phylloid algal mound holder formation new mexico u s ameteoric diagenesis of a pennsylvanian phylloid algal mound new mexico

calcite cementation in the upper floridan aquifer a modern example for confined aquifer cementation models detailed petrographic study of the oligocene suwannee limestone a principal unit of the upper floridan aquifer system indicates that all the calcite cements in the suwannee limestone are precompaction in origin most are clearly associated with either early mineralogical stabilization or oligocene subaerial exposure features none of the cements are in isotopic 18 o and 13 c or trace element fe mn and sr equilibrium with the extant aquifer waters the cements therefore formed prior to the onset of regionally confined freshwater aquifer conditions these rocks have been basically inert with respect to calcite cementation during the past 5 to 10 m y while they have been part of the regional confined aquifer using this modern aquifer as an example it is likely that confined paleoaquifer cementation has been overestimated as an explanation for regional cementation patterns in ancient limestones

porosity preservation in the upper smackover jurassic carbonate grainstone walker creek field arkansas response of paleophreatic lenses to burial processes reply a section of the arco bodcaw no 1 core from the upper smackover jurassic of southern arkansas was studied in detail in an effort to understand the control of porosity petrographic isotopic atomic absorption and cathodoluminescence data on whole rocks and components were generated on 24 samples from a 115 foot length of core the core is composed almost entirely of accretionary and cryptocrystalline grains and cement porosity is unrelated to grain type size or sorting on a sample to sample basis the porous intervals are interpreted to have been stablized to calcite under fresh water phreatic conditions the porous intervals approach fresh water values of 5 0 per thousand delta 18 o 1 5 mol percent mg and 300 ppm sr and are further characterized by luminescent oolites and relatively slight solution compaction effects the nonporous intervals represent relict vadose and subphreatic environments which stabilized after burial the nonporous intervals approach burial values of 2 8 per thousand delta 18 o 2 5 mol percent mg and 600 ppm sr and are further characterized by nonluminescent grains and intense solution compaction data indicate that oolites were initially composed of a metastable mineralogy cryptocrystalline grains were composed of a more stable mineralogy and algal grains were composed of a mixture of mineralogies oolites in the vadose and subphreatic environments were not fully stabilized under fresh water conditions but vadose grains carried with them a light delta 13 c soil gas signal after burial these grains were fully stabilized under post fresh water burial conditions observed porosity distribution is readily explained in terms of solution compaction and cementation grains with metastable mineralogy in relict vadose and subphreatic intervals were heavily solution compacted at relatively shallow depths before being converted to stable calcite intense solution compaction destroyed pore space in these intervals dolomite cementation seems linked with the compaction process the stablized mineralogy of the phreatic lens together with associated meteoric cements resisted solution compaction and thereby preserved pore space

microstratigraphy of calcite cements in pliocene cool water limestones new zealand relationship to sea level burial and exhumation events

trace element and isotope geochemistry of zoned calcite cements lake valley formation mississippian new mexico insights from water rock interaction modelling abstract regionally extensive pre pennsylvanian meteoric zoned calcite cements from the mississippian age lake valley formation new mexico comprise three major zones zones 1 2 and 3 each with a distinct isotope and trace element geochemistry zone 1 has the highest mg 18o and 13c contents zone 2 intermediate values and greatest mn and fe and zone 3 has the lowest trace element 18o and 13c values covariation of c and o isotopes for the three zones define an inverted l shaped trend and covariations of mg mn and mg fe define l shaped trends all suggestive of rock evolution curves resulting from water rock interaction these covariant trends were the impetus for quantitative mass balance modelling of water rock interaction iterative mass balance modelling of covariations of stable isotopes c and o and trace elements mg mn fe of these zoned cements implies that water rock interaction progressively decreased water rock ratios increased during precipitation cementing waters for the oldest cement zone zone 1 resulted from water rock ratios in the range of tens to a few hundred whereas the youngest cement zone 3 resulted from water rock ratios in the 100 s to 1000 s the modelling also implies that differences in geochemistry of the cement zones cannot be explained solely by changes in water rock ratios but require greater 18o and greater ca and mg concentrations in input waters that evolved to zone 1 precipitational waters than those for zones 2 and 3 zones 2 and 3 can reasonably be modelled using input waters with the same isotopic compositions the same ca concentration and for some zone 2 cement the same mg concentrations the minimum mg ca ratios in input waters is constrained by the lower mg plateau in mg mn space and is about the same for all three zones about 1 25 the maximum mg ca ratios about 1 20 occurred in zone 2 input waters mn fe covariations also show an l shaped trend a trend whch fits neither a simple water rock interaction model nor a simple eh model a model is proposed in which eh and limited mn sources determine the amount of fe and mn available in solution to partake in water rock interaction water rock interaction modelling provides one useful approach to quantitatively interpreting isotopic and trace element data from diagenetic carbonates to gain a clearer understanding of evolution of diagenetic fluids

early cementation during marine meteoric fluid mixing mississippian lake valley formation new mexico abstract pre pennsylvanian low magnesium calcite cements from the mississippian lake valley formation of south central new mexico comprise three major cathodoluminescent zones previous work by meyers and lohmann 1985 suggested that interzonal isotopic and minor element variation resulted from spatial and temporal exchange between meteoric water and marine carbonate within a single meteoric system that the carbonate for cementation was derived from dissolution of metastable carbonate grains and that any contribution from marine water was insignificant isotopic analysis of the first two cathodoluminescent zones using detailed micron scale sampling methods revealed a record of simultaneous variation in 18o and 13c values at the intrazonal scale that is incompatible with compositional change anticipated from the evolution of meteoric fluids in response to progressive water rock interaction rather isotopic data in conjunction with stratigraphic and petrographic constraints suggest that regionally extensive mixing between marine and meteoric water along the basal margin of the phreatic lens is the process responsible for the cementation of much of this platform succession although trends of progressive 18o and 13c depletion in cement composition are compatible with precipitation from a fluid that evolved from a marine to a meteoric dominated composition low mg and sr concentrations are problematic in the context of experimentally derived partition coefficients for calcite because of compelling evidence indicating a mixing zone origin for these cements however we attribute low mg and sr contents to temperature and or pco2 effects or to differences in conditions under which calcite is precipitated

diagenetic history of pipe creek jr reef silurian north central indiana u s a abstract calcite cements in the silurian ludlovian pipe creek jr reef north central indiana are compositionally zoned with characteristic minor element concentrations and stable isotope signatures and were precipitated in different diagenetic environments superposition and crosscutting relationships allow us to group cement zones and to relate them to the sequence stratigraphic evolution of the reef pipe creek jr reef grew in normal marine waters with the reef top high greater than 50 m above the platform floor flank facies are volumetrically important and are preserved largely as limestone in contrast to most dolomitized silurian reefs in the midcontinent syndepositional marine cements fill primary porosity and synsedimentary fractures and are interlayered with marine internal sediment now low magnesium calcite their isotopic compositions are similar to those of depositional grains and cements estimated to have precipitated from ludlovian sea waters depositional porosity was reduced by 75 by the precipitation of these syndepositional cements which stabilized the steeply dipping flank slope postdepositional clear calcite cements are interpreted as shallow phreatic and burial cements on the basis of their relationship to periods of karstification and fracturing shallow phreatic cements with concentric cathodoluminescent cl zonation precipitated in primary pores and are postdated by fractures and caves filled with middle devonian sandstone cl zonal boundaries are sharp and some near a major stratigraphic unconformity show evidence of dissolution the volumetric abundance of the individual cl zones varies in the reef indicating a complex superposition of waters of varying chemistry and rock water interaction that are probably related to relative sea level changes this important aspect of the reef stratigraphy is recorded only by the diagenetic succession because evidence of earlier sea level changes is removed by a major later regional unconformity burial cements are the youngest diagenetic feature recognized and they rest conformably or unconformably over older cements they exhibit both concentric cl zonation and sectoral zoning they are ferroan to nonferroan and they contain thin sulfide zones along growth band boundaries their isotopic compositions do not overlap with shallow phreatic or marine cement values degraded oil postdates burial cements and is composed of the same sterane class as the devonian age antrim shale the probable source rock this source contrasts with that of reef reservoirs in the michigan basin where silurian strata are commonly the hydrocarbon source

stable isotope signatures associated with palaeosols pennsylvanian holder formation new mexico the holder formation pennsylvanian virgilian of southern new mexico usa consists of limestones interbedded with siliciclastics it was deposited during times of glacio eustatic sea level change and was exposed subaerially during multiple sea level lowstands microcomponents and whole rock samples of limestones were analysed for 13c and 18o values to examine the method of whole rock isotopic analysis for detecting subaerial exposure events and to determine the diagenetic processes acting during subaerial exposure whole rock isotopic shifts are not consistently present across petrographically identified subaerial exposure surfaces apparently whole rock isotopic shifts do not result from wholesale replacement of the host sediment during soil formation however the isotopic shifts are present in calcareous soil precipitated microcomponents such as rhizoliths laminated crusts and soil precipitated cements the components are heterogeneous in isotopic composition but converge on a meteoric calcite line at about 18o 5 5 these microcomponents are heterogeneous in distribution and may either dominate or be a minor constituent of the whole rock at a single stratigraphic horizon without petrographic selection of palaeosol components the detection of whole rock isotopic shifts may depend on the selection or chance sampling of a rock containing abundant microcomponents precipitated in a soil environment only minor whole rock isotopic shifts come from those rocks bearing no evidence of exposure and bearing lithological characteristics suggesting subaerial exposure was unlikely

cambrian sea water preserved as inclusions in marine low magnesium calcite cement the existence of temporal changes in the chemical composition of the oceans which could provide constraints on the potential variability of the ocean atmosphere system remains an open question assessments of the chemistry of ancient oceans have relied largely on analysis of marine precipitates generally carbonate and evaporite minerals1 these studies suggest that whereas marine salinity has remained relatively stable over phanerozoic time1 magnesium calcium and sulphate concentrations of ancient oceans and co2 partial pressure of ancient atmospheres may have changed2 4 the ratios of isotopes of carbon oxygen sulphur and strontium also appear to have varied3 5 6 here we present analyses of primary one phase fluid inclusions in cambrian and ordovician marine cements which appear to represent aliquots of early palaeozoic oceans the cements have trace element stable isotope and strontium isotope contents that are consistent with their having been precipitated in a cambrian ordovician marine environment and the fluids have marine salinities as these apparently primary cements are low magnesium calcite unlike the predominantly high magnesium calcite and aragonite of today s carbonate precipitates the chemistry of the cambrian ocean atmosphere system seems to have been different from that of today

petrogenesis of cenozoic temperate water calcarenites south australia a model for meteoric shallow burial diagenesis of shallow water calcite sediments abstract the gambler limestone is one of several extensive shallow water shelf carbonates of eocene to miocene age exposed along the southern margin of australia it is a muddy to grainy bryozoan calcarenite with accessory benthonic foraminifers and echinoids the sediments originally composed almost entirely of calcite or mg calcite have been in vadose and phreatic environments for over 10 ma yet are virtually unlithified the only cements of any consequence are epitaxial on echinoids numerous karst features dolines caves speleothems and surface karren attest to prolonged residence in the meteoric zone the gambler limestone is presently one of the best fresh water aquuifers in australia most flow occurs through intergranular pores in sediments with over 30 porosity cementation is by minor intergranular pressure solution which has developed under overburden of less than 100 m the overlying naracoorte limestone miocene a calcarenite of warmer water aspect contains numerous aragonite molds and is well cemented we propose that such cool water limestones are a better model for the meteoric diagenesis of calcite sediments of all ages than are aragonite rich tropical sediments it is probable that many similar early and middle paleozoic calcite limestones may have been in the meteoric zone for prolonged periods yet contain little or no petrographic or geochemical record of such exposure

mg fractionation in crustose coralline algae geochemical biological and sedimentological implications of secular variation in the mg ca ratio of seawater the mg ca ratio of seawater has varied significantly throughout the phanerozoic eon primarily as a function of the rate of ocean crust production specimens of the crustose coralline alga neogoniolithon sp were grown in artificial seawaters encompassing the range of mg ca ratios shown to have existed throughout the phanerozoic significantly the coralline algae s skeletal mg ca ratio varied in lockstep with the mg ca ratio of the artificial seawater specimens grown in seawater treatments formulated with identical mg ca ratios but differing absolute concentrations of mg and ca exhibited no significant differences in skeletal mg ca ratios thereby emphasizing the importance of the ambient mg ca ratio and not the absolute concentration of mg in determining the mg ca ratio of coralline algal calcite specimens grown in seawater of the lowest molar mg ca ratio mmg ca 1 0 actually changed their skeletal mineralogy from high mg skeletal mmg ca 0 04 to low mg calcite skeletal mmg ca 0 04 suggesting that ancient calcitic red algae which exhibit morphologies and modes of calcification comparable to neogoniolithon sp would have produced low mg calcite from the middle cambrian to middle mississippian and during the middle to late cretaceous when oceanic mmg ca approached unity by influencing the original mg content of carbonate facies in which these algae have been ubiquitous this condition has significant implications for the geochemistry and diagenesis of algal limestones throughout most of the phanerozoic the crustose coralline algae s precipitation of high mg calcite from seawater that favors the abiotic precipitation of aragonite indicates that these algae dictate the precipitation of the calcitic polymorph of caco3 however the algae s nearly abiotic pattern of mg fractionation in their skeletal calcite suggests that their biomineralogical control is limited to polymorph specification and is generally ineffectual in the regulation of skeletal mg incorporation therefore the mg ca ratio of well preserved fossils of crustose coralline algae when corrected for the effect of seawater temperature may be an archive of oceanic mg ca throughout the phanerozoic magnesium fractionation algorithms that model algal skeletal mg ca as a function of seawater mg ca and temperature are presented herein the results of this study support the empirical fossil evidence that secular variation of oceanic mg ca has caused the mineralogy and skeletal chemistry of many calcifying marine organisms to change significantly over geologic time

brachiopods as indicators of original isotopic compositions in some paleozoic limestones the original stable isotopic composition of paleozoic marine carbonate rocks is important to establish a baseline for evaluating diagenetic alteration and providing a reliable data base for modeling exogenic geochemical cycles of the elements we have compared the 18 o and 13 c values in the nonluminescent portions of brachiopods with similar published data of whole brachiopods other whole fossils and estimates of original isotopic composition of marine cements the oxygen and carbon isotopic records for each of these paleozoic components show generally good internal consistency significant discrepancies however were noted between results on the different components isotopic and trace element data presented in this study suggest that permo carboniferous whole brachiopods are little altered chemically nonetheless a more reliable but not fool proof approach for obtaining pristine isotopic values is microsampling of least altered regions based on cathodoluminescence in conjunction with staining techniques and elemental analyses the carbon and oxygen isotopic compositions of all components decrease in 18 o and 13 c in geologically older material neither temporal trend is monotonic however oxygen isotopic results suggest an important positive shift in 18 o of about 2 during the early carboniferous whereas the carbon isotope trend shows a major positive shift 2 3 during the mid carboniferous

smnd geochronology of greenstone belts in the yilgarn block western australia abstract sm nd geochronology has been used to date 3 widely spaced metavolcanic greenstone belts in the yilgarn block the measured isochron ages are 2 78 0 03 ga initial nd 2 5 0 3 for greenstones at kanowna in the eastern goldfields province 3 05 0 10 ga initial nd 0 9 0 7 for diemals marda in the southern cross province and 2 98 0 12 ga initial nd 0 7 1 2 for the warriedar fold belt in the murchison province these data show that substantial crustal components of age 3 0 ga exist in the southern cross and murchison provinces and support published evidence that the eastern goldfields province contains no crustal material 2 8 ga these ages are all significantly younger than ages commonly observed in the western gneiss terrain and so support the suggestion of a progressive crustal age trend across the yilgarn block the measured age values depend strongly on the inclusion of data for felsic components of the greenstone belts data for mafic and ultramafic units allow that eruption in the 3 localities could have been contemporaneous at 2 8 ga the age range of metavolcanic rocks within each province is possibly comparable to the maximum possible age difference between study areas 0 2 ga so the documented lithological variations between the greenstone successions of the 3 provinces cannot be attributed to depositional age differences leaching experiments on altered warriedar samples indicate that both alteration and isotopic homogenisation occurred early in the evolution of the system probably prior to the 2 6 ga regional metamorphic event the initial nd values do not conform to previously suggested mantle depletion curves when considered with published data for greenstone belts they suggest complex mantle heterogeneity with end diverging over time to both positive and negative values

geochemistry of brachiopods oxygen and carbon isotopic records of paleozoic oceans abstract combined trace element and isotope studies of 319 brachiopods covering the ordovician to permian time span show that 13c and 18o in well preserved specimens varied during the paleozoic the overall 13c secular trend is in accord with the previously published observations but its details are obscured by vital isotopic fractionation effects at generic level nonetheless the results suggest that the negative correlation between marine 13ccarbonate and 34ssulphate deteriorates at time scales of 106 years due to the long residence time and thus slow response of so42 in the ocean for oxygen isotopes all devonian and older specimens have 18o of 4 while the well preserved permian samples have near present day 18o of about 1 pdb this isotopic dichotomy is probably not due to post depositional phenomena salinity or biogenic fractionation effects this leaves open the perennial arguments for a change in 18 o 16 o of sea water versus warmer ancient oceans the present data are difficult to explain solely by the temperature alternative the coincidence of the proposed shift in 18o with the large late paleozoic changes in marine 87 sr 86 sr 13 c 12 c 34 s 32 s and sea level stands argues for a tectonic cause and for a change in 18 o 16 o of sea water although such explanation is difficult to reconcile with global balance considerations and with isotopic patterns observed in alteration products of ancient basalts and ophiolites whatever the precise cause or combination of causes the implications for tectonism and or paleoclimatology are of first order significance

the role of fluids in the metamorphism of komatiites agnew nickel deposit western australia the agnew nickel sulfide deposit is spatially associated with a lenticular body of ultramafic rocks which shows a concentric zonation in metamorphic mineralogy olivine tremolite chlorite cummingtonite enstatite assemblages occur at the margin of the ultramafic lens giving way to olivine anthophyllite olivine talc and olivine antigorite assemblages successively inwards these rocks are interpreted as having crystallized from komatiitic lavas and exhibit a spectrum of compositions from those of original flow tops to pure olivine adcumulates the relative modal abundances of metamorphic olivine tremolite and chlorite reflect original proportions of cumulus olivine and komatiite liquid in the protolith peak metamorphic conditions are estimated at 550 c based on garnet biotite thermometry at a maximum pressure of 3 kb this temperature falls within the narrow range over which metamorphic olivine may co exist with enstatite anthophyllite talc or antigorite depending upon the fugacity of water in the metamorphic fluid the observed mineralogical zonation is therefore attributed to infiltration by co2 rich fluids generated by decarbonation of talc carbonate rocks formed during pre metamorphic marginal alteration of the ultramafic lens metamorphic fluids were essentially binary mixtures of water and co2 with minor h2s having a maximum partial pressure less than 1 percent of total pressure enstatite bearing assemblages formed in the presence of co2 rich fluids at fluid rock volume ratios close to one while anthophyllite talc and antigorite bearing assemblages formed in the presence of progressively more water rich fluids at progressively lower fluid rock ratios

basin scale thermal and fluid flow histories revealed by carbonate clumped isotopes δ47 middle jurassic carbonates of the paris basin depocentre the reconstruction of past diagenetic conditions in sedimentary basins is often under constrained this results from both the analytical challenge of performing the required analyses on the minute sample amounts available from diagenetic mineral phases and the lack of tracers for some of the diagenetic parameters the carbonate clumped isotopes thermometry 47 opens new perspectives for unravelling the temperatures of diagenetic phases together with the source of their parent fluids two parameters that are otherwise impossible to constrain in the absence of exploitable fluid inclusions here is reported the study of a large number of sedimentary and diagenetic carbonate phases from middle jurassic reservoirs of the paris basin depocentre by combining detailed petrographic observations with a large number of 47 data n 45 on a well documented paragenetic sequence including calcite and dolomite burial cements the data reveal carbonate crystallization at temperatures between 29 c and 98 c from fluids with 18owater values between 7 and 2 in response to the progressive burial and uplift of the paris basin throughout 165 myr of basin evolution coupled to the time temperature evolution previously estimated from thermal maturity modelling these temperatures allow determining the timing of four successive cementation episodes the overall dataset indicates a history of complex water mixing with a significant contribution of hypersaline waters from the triassic aquifers migrated upward along faults during the cretaceous subsidence of the basin subsequent large scale infiltrations of meteoric waters induced a dilution of these pre existing brines in response to the paris basin uplift in the tertiary overall the data presented here allow proposing an integrated approach to characterize the cementation events affecting the studied carbonate reservoir units based on temperature oxygen isotope composition and salinity of the parent fluids as well as on petrographic grounds this article is protected by copyright all rights reserved

the isotopic record of ocean chemistry and diagenesis preserved in non luminescent brachiopods from mississippian carbonate rocks illinois and missouri fossil brachiopod shells are commonly used as a faithful record of the chemical and isotopic composition of the ancient ocean water in which they grew owing to their stable mineralogy and their presence in marine strata throughout phanerozoic time the utility of these fossils for preserving ocean water compositions is evaluated through analyses of multiple specimens that grew in contemporaneous sea water at individual stratigraphic horizons in the mississippian age burlington keokuk formation of mid continent north america significant variations in sr c and o isotopic compositions are observed in contemporaneous non luminescent shells indicating that apparently pristine brachiopod shell material has been altered by post depositional processes of diagenesis the least altered compositions at each horizon are used to estimate secular variations of 87 sr 86 sr in early mississippian ocean water the estimated rates of change of ocean water 87 sr 86 sr are comparable in magnitude to the rapid sr isotopic changes documented for cenozoic ocean water for which high frequency glacial cycles are inferred as a driving mechanism the rigorous assessment of diagenetic effects on the isotopic signatures of marine phases is requisite for the advancement of high resolution reconstructions of paleo ocean chemistry

intracrystalline alteration of low magnesian calcite cement in the devonian pillara formation canning basin western australia abstract intracrystalline alteration is present in approximately 25 of the low mg calcite cement throughout the lowermost 160 m of the middle to upper devonian pillara formation in the emanuel range canning basin western australia a younger luminescently and chemically distinct calcite phase disrupts primary zonation in older cement and results in irregular mottled cathodoluminescence cl the younger calcite is always in optical continuity with host calcite continuous records of mg fe mn and sr abundances were measured across concentrically zoned non cl and yellow cl cement altered by orange cl calcite using a new microprobe method designed for high spatial resolution and rapid collection of minor element abundances in calcite cement minor element composition of the crosscutting orange cl phase designated as cement viii differs significantly from that of the older zoned cement designated as cement via mg abundance in cement via is variable ranging from 700 to 4600 ppm with a mean of 2760 ppm whereas that of cement viii is relatively constant at about 400 ppm mn in the yellow cl zones of cement via is variable 400 7720 ppm with a mean of 4025 ppm mn in cement viii is relatively constant at 1420 ppm fe and sr were not detected in cement via abundances of fe and sr are highly variable in cement viii fe ranges from low mg calcite traditionally is regarded as a stable carbonate phase with little tendency toward intracrystalline alteration of chemistry or microstructure significant elemental variations measured over small scale intervals within time equivalent zones in cement via indicate that some parts of the crystal have different chemical potential which could provide the thermodynamic drive for intracrystalline alteration alteration could take place either by a one step iterative process of recrystallization or by a two step dissolution cementation process lack of micro collapse features in host calcite in combination with lack of minor element memory between host and secondary calcite support alteration by recrystallization at a high water rock ratio however most alteration of ceme t in the emanuel range probably originated by dissolution cementation on the basis of the combination of 1 lack of minor element memory effect between host and secondary calcite and 2 rare occurrences of concentrically zoned secondary calcite in which the zonation is parallel to the contact with the host cement creation of intracrystalline porosity by partial dissolution of metastable parts of low mg calcite could provide conduits for fluid and element transport that have been unrecognized in modeling porosity changes and fluid movement through carbonate strata

cathodoluminescence in quaternary carbonate deposits abstract the cathodoluminescent oscillatory and sectoral growth zones common in crystals formed in ancient limestone successions in a variety of putative environments appear to be rare or absent from recent and pleistocene marine carbonate sequences the factors controlling cathodoluminescence and reasons for this disparity are examined the cathodoluminescent zones in the cements of ancient rocks have been interpreted as responses to variations in the redox potential of formative pore waters during crystal growth although similar cathodoluminescent behaviour is recorded from some deposits including travertines and quaternary speleothems formed in what are thought to have been strongly oxidizing environments the apparent absence of cathodoluminescence in the most recent and pleistocene marine deposits that presumably reflect deposition and diagenesis in environments that are also characteristically oxidized therefore seems anomalous the controlling influences on cathodoluminescence are reviewed together with evidence relating to observations of pleistocene marine deposits and likely conditions of formation but where it is present the mechanism s for its development remain elusive

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

geochemistry and crystal morphology of aragonite cements of mixing zone origin barbados west indies abstract isopachous aragonite ray cement arc lines large rugs 5 cm through a 4 m interval recovered in core samples from a borehole drilled along the southeast coast of barbados the vuggy cement interval is 12 16 m below core top and most of the interval is below the modern water table 13 m the top of the interval corresponds to the top of the 194 ka coral reef terrace which is unconformably overlain by the 122 ka terrace arc was precipitated after a period of meteoric diagenesis during which vuggy porosity was produced arc shows variable ultrastructures including 1 loosely packed randomly oriented rays the 100 arc has uncharacteristically low 13c values relative to predicted and reported values for marine aragonite cement likewise the 18o values are lower than both reported and most predicted o18o values for marine aragonite stable isotope compositions of this cement fall along a mixing line between aragonite equilibrium compositions calculated for meteoric and marine end member fluids strontium concentrations are greater than in most known marine aragonite this suggests precipitation from fluids with an elevated sr ca ratio such fluids are found in modern barbados mixed waters whose elevated sr ca ratios result from updip meteoric dis olution of aragonite magnesium concentrations are slightly higher and more variable than in reported pleistocene marine aragonite sodium concentrations are lower than expected marine values and covary with changes in 13c and 18o values isotope and minor element geochemistry of these cements suggests precipitation from mixed meteoric marine waters fluid mixing models for modern barbados waters suggest precipitation from mixed fluids ranging from 50 to 75 seawater precipitation of aragonite from mixed waters may be facilitated by degassing of co2 charged mixed waters within large vugs and by mg ca ratios sufficiently elevated to inhibit precipitation of calcite

isotope signatures associated with early meteoric diagenesis the environments in which carbonate diagenesis proceeds have been documented in previous studies of holocene and late pleistocene sediments and limestones on barbados west indies variations in the carbon and oxygen isotopic composition of limestones produced during early freshwater diagenesis have been observed in this study to occur in specific patterns six potentially useful patterns emerge when one views stable isotope data within a stratigraphic framework 1 the subaerial exposure surface is characterized by strongly 12c enriched limestones 13c compositions of underlying limestones grow progressively heavier with increasing depth 2 the subaerial exposure surface may also be marked by slight 18o enrichment 3 an abrupt shift in 18o values may differentiate sediments above the exposure surface from those below 4 sediments altered in the marine meteoric mixing zone may be characterized by positive covariance between their 18o and 13c compositions 5 the vadose phreatic boundary may be marked by a sharp increase in 13c values in the seaward portions of a fresh groundwater system and 6 samples contemporaneously altered in a single fresh groundwater system within an areally restricted region should display a narrow range of 18o and a wide range of 13c compositions analysis of samples from five palaeozoic and mesozoic formations which contained petrographic evidence of early freshwater diagenesis showed that isotope patterns similar to those observed in barbados limestones have been preserved in rocks as old as mississippian these isotope patterns could prove to be useful for identifying diagenetically induced porosity trends in carbonate rocks they might be used to identify limestones diagenetically altered in meteoric environments to identify mixing zone cements and dolomites and to trace the regional and vertical distributions of early meteoric groundwater systems in ancient carbonate formations

pluton emplacement along an active ductile thrust zone piute mountains southeastern california interaction between deformational and solidification processes plutons that are emplaced synchronously with regional deformation offer the opportunity to understand the partitioning of deformation and metamorphism in time and space during orogeny the 85 ma east piute pluton of the eastern mojave desert is an unusually clear example of a syntectonic pluton it was emplaced during thrusting in the southern cordillera combined petrologic and structutal studies lead to empirical models for the dynamic interaction of magmatic and deformational processes during granitoid emplacement and final crystallization trace and major element and isotopic data suggest that closed system differentiation of a single magma batch was responsible for compositional variation granodiorite to leucocratic granite correlation between degree of chemical evolution of compositional units and intrusive sequence suggests that discrete melt segregation events punctuated fractional crystallization modeling of trace element data suggests segregation of monzogranite melts after 50 70 crystallization of the magma and segregation of leucocratic granite after about 80 crystallization deformation promoted segregation of melt fractions at discrete times during crystallization during final ascent and after the magma had crystallized sufficiently 50 70 to attain shear strength monzogranite liquid was incompletely segregated along the active thrust zone at the southern margin of the pluton at essentially the present crustal level and after about 80 crystallization the remaining leucocratic granite melts were more effectively segregated and migrated into thrust related extensional openings including tension gashes undeformed pegmatite and aplite dike arrays indicate that final crystallization coincided with final increments of shear zone movement solid state deformational fabrics and brittle synthetic thrusts that offset the pluton margin record sharp temperature decline during and after crystallization because plutons crystallize rapidly they can be used to estimate strain rates during orogeny thermal modeling suggests that the transition from rheological liquid to solid in the east piute pluton occurred during an interval of about 10 4 yr as final increments of ductile deformation coincided with final crystallization of evolved compositional units total strain recorded by fabric in the pluton apparently accumulated during this interval shear strains estimated from the s c fabric in the granodiorite unit and rotation of pegmatite filled vein arrays are on the order of 1 thus strain rates in the shear zone were on the order of 10 12 sec during crystallization of the pluton this corresponds to displacements of tens of centimeters per year these high strain rates suggest punctuated melt enhanced movement episodes during contractional orogeny

structural controls on nickel sulphide ore shoots in archaean komatiite kambalda wa the volcanic trough controversy revisited abstract structural evaluation of the extensive exploration database for the kambalda tramways area archaean yilgarn craton western australia is addressing the long outstanding issue of whether the trough structure embayments that partly confine the nickel sulphide nis ore shoots are volcanic or tectonic in origin the trough structures are complex linear domains associated with talc carbonate alteration overprinting thickened komatiite overlying footwall basalt two end member trough structure types are recognised 1 re entrant trough structures and 2 open trough structures the re entrant trough structures are narrower at the top than at the base strongly asymmetric in cross section restricted in distribution to kambalda dome and transgress and truncate ore the open trough structures are wider at the top than at the base weakly asymmetric in cross section prevalent outside kambalda dome and are mineralised along their lateral margins in addition to trough associated ore other nis ore shoots overlie near planar komatiite basalt contacts and are overlain by serpentinite with relict igneous textures all these features indicate the possibility that the trough structures reflect structural folding thickening and alteration at rock contacts marking different competency during regional fold thrusting consequently nis exploration strategies should consider structural and metamorphic features as well as volcanologic and stratigraphic features recognition of folding and thrusting patterns related to major competency contrasts in favourable komatiite basalt stratigraphy near the crest of regional anticlines and domes would aid exploration success

13c enriched carbonate in mississippian mud mounds alamogordo member lake valley formation sacramento mountains new mexico u s a abstract the origin of the carbonate mud constituting paleozoic mid shelf and deeper water mud mounds has long been debated mississippian mud mounds and their laterally adjacent coeval inter mound strata in the lake valley formation are well exposed along cliff faces in south central new mexico the mounds originated on erosional surfaces on a gently sloping ramp in 110 to 250 m water depth carbonate muds that constitute the inter mound strata mud mounds and syndepositional neptunian dikes within the mounds are low mg calcite microspars with similar petrographic cathodoluminescent and fluorescent characteristics carbon and oxygen stable isotope analyses demonstrate that the microspars constituting the inter mound mound and neptunian dikes have average 13c values of 2 6 0 2 3 4 0 6 and 4 8 0 3 pdb respectively additionally well preserved brachiopods from the inter mound strata and from muleshoe mud mound have 13c values markedly different from each other but similar to their surrounding microspars i e brachiopod s 2 7 and 3 9 pdb respectively the stable isotope distributions within each environment similarity of values between microspar and brachiopod data from the same depositional environments as well as agreement of 13c values with other investigations of coeval strata support the conclusion that the measured 13c values are essentially primary values it is envisioned that 13c enriched fluids sourced by bacterial fermentation in the subjacent strata rose into the mounds some through neptunian dikes and influenced the precipitation of the microspars in the mounds and dikes near the sediment water interface these fluids mixed with normal marine waters thus precipitating the microspars or their precursors which have high 13c values that characterize the mud mounds the inter mound microspars and inter mound brachiopods have the lowest 13c values and are in agreement with worldwide coeval marine calcites that is the inter mound microspars formed under normal marine conditions the high 13c values measured in all of the mound related samples indicate that all mounds received a contribution of 13c enriched waters this relationship indicates that microspar precipitation in the mounds was associated with seepage of methanogenic fluids from the underlying strata

geochemistry of komatiites from forrestania southern cross province western australia evidence for crustal contamination the 2 9 ga old forrestania greenstone belt is intermediate in character between typical 3 0 ga and 2 7 ga belts in containing both al depleted and al undepleted komatiites the extensive nickel exploration database on the belt provides a unique opportunity to examine geochemical variations in the context of volcanological evolution within each of the six komatiitic units in the greenstone belt there is a transition from komatiites 18 28 wt mgo in the liquid to high and low magnesium komatiitic basalts 5 18 wt mgo with stratigraphic height and much of the chemical variance within the komatiitic suite can be explained in terms of the addition or fractionation of olivine there are however aspects of the incompatible element signatures which must be attributed to other factors the more primitive komatiites from the lower ultramafic units are al depleted al2o3 tio2 10 5 13 0 and depleted in both light and heavy ree lasmn 0 5 gdybn 1 5 komatiites from the uppermost ultramafic units are variably less depleted in al al2o3tio2 16 5 20 5 this is interpreted to reflect decreasing retention of garnet in the source region of the forrestania komatiites over time and possibly a decrease in the depth of melting within the al depleted suite there are also anomalous enrichments in sio2 feo tio2 and zr which correlate with regional variations in the composition of substrate metasedimentary rocks banded iron formation versus felsic tuff coupled with field evidence for trough structures at the base of lava channel sequences these chemical anomalies provide strong evidence for supracrustal thermal erosion by komatiite lavas the forrestania data indicate that 2 9 ga old komatiites were generated over a range of melting conditions spanning those typical of older and younger archaean sequences and that contamination by supracrustal thermal erosion can influence komatiite chemistry on a regional scale