experimental calibration of the aluminum in hornblende geobarometer with application to long valley caldera california volcanic rocks a new geobarometer based on the al content of igneous hornblendes in equilibrium with melt fluid biotite quartz sanidine plagioclase sphene and magnetite or ilmenite has been calibrated experimentally the calibration was performed by equilibrating the required phase assemblage over the pressure range 2 8 kbar at 740 780c and then analyzing euhedral hornblendes in equilibrium with glass melt experiments were performed on natural samples of both volcanic and plutonic rocks earlier empirical calibrations of this geobarometer relied on analyzing natural hornblendes from plutons with the required phase assemblage and inferring pressure from nearby metamorphic country rocks the experimental calibration differs from the empirical calibrations especially above 5 kbar and shows that the al content of hornblendes in equilibrium with the required phase assemblage is greater for a given total pressure than previously thought the geobarometers uncertainty is dramatically reduced the derived equation is p plus minus 0 5 kbar 3 minus 3 46 plus minus 0 24 4 23 plus minus 0 13 al sup t the geobarometer is applied to post bishop tuff volcanic rocks from long valley caldera california and reveals that most rhyodacites in this complex erupted from depths of about 6 km these eruptions occurred over 500 000 yr suggesting that the rhyodacitic magma reservoir beneath longmore valley had reached a steady p depth t state less

magmatic epidote and its petrologic significance epidote is a major magmatic mineral in tonalite and granodiorite in a belt coextensive with the mesozoic accreted terranes between northern california and southeastern alaska textural and chemical evidence indicates that epidote crystallized as a relatively late but magmatic mineral that formed through reaction with hornblende in the presence of a melt phase the observed relations concur with experimental data on crystallization of epidote from synthetic granodiorite at 8 kbar total pressure plutonic rocks bearing magmatic epidote must have formed under moderately high pressures corresponding to lower crustal depths under fairly oxidizing conditions

constraints on the origin of archean trondhjemites based on phase relationships of nuk gneiss with h2o at 15 kbar we report the t x h2o phase relations for the trondhjemitic nuk gneiss which comprises the principal component of the second phase of archean 3 0 2 8 by igneous activity in the godthab region of southwestern greenland a pressure of 15 kbar was chosen to place constraints on possible protoliths for trondhjemitic melts at lower crustal depths under h2o saturated conditions a melting interval of 135 c separates the solidus at 610 c from the liquidus at 745 c h2o saturation at 15 kbar occurs at approximately 15 5 wt h2o the h2o undersaturated liquidus extends along a curved path from 745 c at 15 5 wt h2o to 1100 c at 2 h2o lower h2o contents were not investigated at low h2o contents 7 mafic residues consisting of some combination of hb ga ep and bi are possible and could correspond to amphibolitic source rocks at lower melt h2o contents 5 possible residues consist of na pl qz ga and could correspond to an earlier generation of tonalitic trondhjemitic rocks however such residues would not impart the highly fractionated ree patterns characteristic of archean trondhjemites if a first generation of tonalitic trondhjemitic melts was generated by higher pressure partial fusion of eclogite and emplaced at 55 km depth it would crystallize to an assemblage consisting almost entirely of na pl qz with highly fractionated ree patterns these rocks in turn could be partially melted to yield a second generation of trondhjemites which would inherit the highly fractionated ree patterns because neigher pl nor qz is capable of significantly fractionating hree from lree

hornblende geobarometry of the nelson batholith southeastern british columbia tectonic implications hornblende geobarometry has been applied to granitic rocks of the middle jurassic nelson batholith british columbia locally containing magmatic epidote geobarometry suggests equilibration pressu

paleoecology of an early eocene mammalian fauna from paleosols in the clarks fork basin northwestern wyoming u s a winkler d a 1983 paleoecology of an early eocene mammalian fauna from paleosols in the clarks fork basin northwestern wyoming u s a palaeogeogr palaeoclimmatol palaeoecol 43 261 298 the paleoecology and depositional history of an early eocene vertebrate assemblage dominated by small mammals is analyzed at a richly fossiliferous locality in the willwood formation northwestern wyoming systematic surface sampling using a quadrat net screen washing and quarrying of fresh sediment show that all vertebrate material in these fluvial deposits is contained in dark gray mudstones representing the a horizons of paleosols seven such paleosols occur in 22 m of stratigraphic section the fauna includes 34 species of mammals and also birds reptiles amphibians fishes and gastropods all vertebrate fossil material is fragmented but it shows no evidence of transportation or current sorting surface weathering and possibly carnivore activity are responsible for the fragmentation and for an overabundance of dense and compact skeletal elements detailed surface collections were made to obtain multiple samples from the different bone beds paleosols principle component analysis shows that differences in the type and abundance of fossils from the various collecting areas are related to differences between contained bone beds not differences along single bone beds one paleosol has a red b horizon and six have an orange b horizon the gray over red colored paleosol is faunally distinct from gray over orange paleosols it contains the only specimens of two large archaic mammals and no very small mammalian taxa cluster analysis on the occurrence of vertebrate taxa is used to define subsets of the fauna that may represent animals living in different habitats separate clusters are formed by a group of the most common terrestrial herbivorous mammals and the crocodilian allognathosuchus and by a group of small possibly arboreal herbivores and carnivores the relative abundance of each mammalian species was initially calculated from a sample representing a minimum of 300 individual animals collected from the surface the surface sample is dominated by mammals that are medium sized for the fauna screen washing yields a greater relative abundance of very small mammals than was found on the surface this indicates that the surface sample is biased against small forms corrected relative abundances based on both surface and washing samples show that the fauna is dominated numerically by very small mammals one multituberculate species and several insectivores screen washing of fresh matrix is the best means of sampling a locality for paleoecological purposes

extreme ecosystem instability suppressed tropical dinosaur dominance for 30 million years a major unresolved aspect of the rise of dinosaurs is why early dinosaurs and their relatives were rare and species poor at low paleolatitudes throughout the late triassic period a pattern persisting 30 million years after their origin and 10 15 million years after they became abundant and speciose at higher latitudes new palynological wildfire organic carbon isotope and atmospheric pco2 data from early dinosaur bearing strata of low paleolatitudes in western north america show that large high frequency tightly correlated variations in 13corg and palynomorph ecotypes occurred within a context of elevated and increasing pco2 and pervasive wildfires whereas pseudosuchian archosaur dominated communities were able to persist in these same regions under rapidly fluctuating extreme climatic conditions until the end triassic large bodied fast growing tachymetabolic dinosaurian herbivores requiring greater resources were unable to adapt to unstable high co2 environmental conditions of the late triassic

assessing relative abundances in fossil assemblages abstract the relative abundances of taxa or skeletal elements in a fossil assemblage can provide important information concerning the palaeoecology and taphonomy of the assemblage however these relative abundances must be estimated from samples of the assemblage rather than measured directly the sampling error this produces decreases the accuracy with which relative abundances can be estimated from the fossil record using the multinomial distribution it is possible to place constraints on the accuracy of estimation of relative abundance provided that two out of three key parameters sample size required degree of similarity and confidence level are known applying this methodology to the fossil record it can be shown that in order to be 95 confident the taxon relative abundances of a fossil assemblage lie within 5 of those found in a sample 534 individuals must be collected this methodology enables the assessment of published relative abundance estimates and the development of sampling protocols for future studies

emplacement petrological and magnetic susceptibility characteristics of diverse magmatic epidote bearing granitoid rocks in brazil argentina and chile abstract magmatic epidote mep bearing granitoids from five neoproterozoic tectonostratigraphic terranes in northeastern ne brazil early palaeozoic calc alkalic granitoids in northwestern nw argentina and from three batholiths in chile have been studied the elongated shape of some of these plutons suggests that magmas filled fractures and that dyking was probably the major mechanism of emplacement textures reveal that in many cases epidote underwent partial dissolution by host magma and in these cases may have survived dissolution by relatively rapid upward transport by the host magma in plutons where such a mechanism is not evident unevenly distributed epidote at outcrop scale is armoured by biotite or near solidus k feldspar aggregates which probably grew much faster than epidote dissolution preventing complete resorption of epidote by the melt al in hornblende barometry indicates that in most cases amphibole crystallized at p 5 kbar kyanite bearing thermal aureoles surrounding plutons that intruded low grade metamorphic rocks in ne brazil support pluton emplacement at intermediate to high pressure mep show overall chemical variation from 20 to 30 mol mole percent pistacite ps and can be grouped into two compositional ranges ps20 24 and ps27 30 the highest ps contents are in epidotes of plutons in which hornblende solidified under p 3 0 10 3 si typical of magnetite series granitoids crystallized under higher oxygen fugacity in ne brazil argentina and chile it seems that mep is more common in precambrian to palaeozoic ilmenite series granitoids while its occurrence in magnetite series granitoids is more restricted to mesozoic to tertiary granitoids

epidote phenocrysts in dacitic dikes boulder county colorado epidote ps21 crystallized early as elongate phenocrysts in late cretaceous rhyodacitic dikes in the vicinity of ward boulder county colorado other unusual phenocryst phases are garnet gr17 24 and muscovite in a xenolith containing kyanite corundum biotite and plagioclase magmatic garnet grew as a rim around xenocrystic pyrope rich py37 garnet the xenolith was derived from a granulite facies zone not represented at the present day erosion surface which is composed of upper amphibolite facies cordierite and sillimanite bearing gneisses the dike magmas were fed not from an immediately underlying batholith but from a magma chamber at a depth corresponding to a pressure of 8 13 kilobars phenocrysts cystallized in the temperature range 800 to 700 c under h2o and o2 activities greater than normal for silicic magmas this occurrence shows convincingly not only that epidote can be magmatic but that it is a possible early crystallization phase in silicic magmas

time averaging taphonomy and their impact on paleocommunity reconstruction death assemblages in texas bays the death assemblages in upper copano bay and upper laguna madre texas were compared with the living communities as sampled over a two year period in order to determine how taphonomy and time averaging affect paleocommunity reconstruction the living and death assemblages were compared using taxonomic composition numerical abundance diversity biomass and trophic and habitat proportions taxonomic composition particularly adult taxonomic composition and the biomass of the death assemblage more accurately reflect the characteristics of the living community than do numerical abundance and other community attributes based upon numerical abundance such as diversity taxonomic composition and biomass are less modified by taphonomy and time averaging because they are temporally more persistent characteristics of the community community and paleocommunity attributes are not equivalent because the paleocommunity is described using time averaged data ecological theory must be supplanted by paleoecological theory that takes into account this temporal component

constraints on rates of granitic magma transport from epidote dissolution kinetics epidote is stable only at high pressure in granitic magmas but is found in magmas crystallized at shallow crustal levels this observation suggests qualitatively that magma transport is sufficiently rapid to prevent complete epidote resorption experimental measurements of epidote dissolution showed that the presence of epidote implies that magma transport from the deep crust in these systems was rapid and that the transport was by dikes rather than diapirs

the arsikere granite of southern india magmatism and metamorphism in a previously depleted crust abstract the arsikere granite is a small diapiric potassic pluton intruded into archean gneisses of the western part of the dharwar craton of southern india a rb sr age of 2563 ma indicates that the granite was formed during a period of granulite development at lower crustal levels at least to the south of the dharwar craton schist belts in the craton were metamorphosed to greenschist or amphibolite facies at this time but gneisses underwent no isotopic or other geochemical effects the arsikere granite contains quartz oligoclase highly perthitic alkali feldspar originally precipitated as a single phase f rich biotite plus primary epidote fluorite and f rich muscovite sphene forms large euhedrons which incorporate nearly all of the ti in the rock leaving pure magnetite as the only opaque phase equilibration parameters include temperatures of 700 from feldspars f o 2 10 12 bar biotite and opaques and f oh 10 3 biotite nearby schist belts were metamorphosed at 5 kbar pressure at the same time as granite emplacement if the granite was emplaced at 5 kbar the magma could not have been water saturated perthite an initial 87 sr 86 sr ratio of 0 7021 in the arsikere granite is too low for the magma to have formed by partial melting of rocks with the rb sr ratios of gneisses currently exposed in the craton apparently the magma formed in a lower crust that had been depleted in rb and probably other lithophilic elements several hundred million years earlier at this time the craton also became water poor and inert to further geochemical activity such as resetting isochrons during later metamorphism

local scale proxy evidence for the presence of closed canopy forest in north western borneo in the late pleistocene bones of strategy i bats from the archaeological record of the great cave of niah sarawak abstract the limits of the distribution of closed canopy forest in southeast asia during the glacial cycles of the pleistocene are the subject of continuing debate in this context this paper reports the results of an analysis of a novel line of evidence as a local scale case study 22 dated assemblages of bat bones that were recovered from the archaeological sequences of the great cave of niah in sarawak malaysian borneo were examined by examination of the morphological characteristics of the distal articulation of the humerus seven families and nine genera were identified five species level attributions could be made these assemblages formed a time series of 14 chronological intervals 48 0 35 ky bp ky bp 103 calibrated years before present 1950 incorporating samples from the high glacial interval 26 18 ky bp and last glacial maximum 23 19 ky bp bones of bat taxa that are adapted to forage in the cluttered environments in the interior of closed canopy forest strategy i bats were employed as proxy evidence for the presence of closed canopy forested habitats in the vicinity bones of strategy i bats principally small hipposideros spp were identified in all examined assemblages and intervals in the time series statistical testing indicated no evidence of differences in the absolute frequency of distal humeri from strategy i bats in the pleistocene samples 48 11 8 ky bp or in the entire time series 48 0 35 ky bp and no significant linear trend was found in the time series to suggest an increase of the bones strategy i bat taxa with the onset of the holocene these findings suggest that closed canopy forest habitats have been a persistent component in the landscape around the great cave of niah since 48 ky bp a taphonomic hypothesis is proposed to explain the dichotomy between the findings of this study and the pollen record of the site

statistical testing for paleocommunity recurrence are similar fossil assemblages ever the same abstract observations of the recurrence of similar fossil assemblages through long intervals of geologic time punctuated by rapid changes in both the composition and structure of fossil assemblages has recently resulted in the concept of coordinated stasis to describe this pattern in the behavior of paleocommunities through time coordinated stasis may imply the existence of ecological mechanisms that actively maintain particular community structures and thus have a significant impact on the process of macroevolution however before such mechanisms can be invoked it must be shown that ecological stasis as observed in the fossil record is more significant than the persistence of similar community types due to the repeated reinvasion of recurring habitats from a persistent species pool the null hypothesis for paleocommunity recurrence analysis of the relationship between communities and paleocommunities and an expansion of the ecological hierarchy at the community paleocommunity level allows the creation of a rigorous definition of the entities composing the local paleocommunity the samples collected within a stratigraphic horizon at a single locality paleocommunity groups of samples shown to be statistically identical and paleocommunity type groups of samples that are similar but can be shown to be statistically different these definitions permit the development of the null model for paleocommunity recurrence and establish a base level of variability within a local paleocommunity that permits rigorous statistical comparisons to be made between paleoecological samples at larger temporal and geographic scales paleoecological samples from four marine tongues in the middle pennsylvanian breathitt formation are used to analyze species abundance variability at several spatial and temporal scales and to test for paleocommunity recurrence although recurrence of statistically identical local paleocommunities occurs at single localities and between localities within individual marine units it is usually not detectable between marine tongues suggesting that paleocommunity recurrence in the marine strata of the breathitt formation is the recurrence of paleocommunity types and does not falsify the null hypothesis

a calculated petrogenetic grid for the system k2o feo mgo al2o3 sio2 h2o with particular reference to contact metamorphosed pelites a petrogenetic grid is presented for the system kfmash k o feo mgo a1 0 sio h o including biotite muscovite k feldspar chlorite chloritoid staurolite cordierite garnet ortho amphibole orthopyroxene spinel andalusite sillimanite kyanite quartz and corundum with h o in excess which was calculated using the computer program thermocalc and the powell and holland internally consistent thermodynamic dataset by removing the normal constraint of having quartz in excess both quartz bearing and quartz absent equilibria are shown quartz absent equilibria are particularly relevant at high t and low p conditions because of their common occurrence at these conditions the calculated mineral assemblage and mineral compositional variations in terms of femg and fe mg siall exchange vectors are broadly compatible with observations on natural rocks particularly when non kfmash components are taken into account

tomography of the source area of the 1995 kobe earthquake evidence for fluids at the hypocenter seismic tomography revealed a low seismic velocity 5 and high poisson s ratio 6 anomaly covering about 300 square kilometers at the hypocenter of the 17 january 1995 magnitude 7 2 kobe earthquake in japan this anomaly may be due to an overpressurized fluid filled fractured rock matrix that contributed to the initiation of the kobe earthquake

non biotic controls of observed diversity in the paleontologic record an example from the permo triassic karoo basin of south africa paleodiversity trends through geologic time can be affected by a number of geologic taphonomic and anthropogenic biases that obscure or prejudice paleoecological patterns in the fossil record although much work has concentrated on the relationship between geologic exposure sample size and taxonomic richness i e number of taxa few studies have investigated the potential effects of dataset quality and changing taxonomy we use four different specimen level datasets of permo triassic tetrapods from the karoo basin of south africa to investigate how sample size geologic outcrop dataset quality and taxonomic revision all affect observed estimates of generic richness evenness and relative abundance our results indicate that large scale patterns of richness evenness and abundance such as the effects of the end permian mass extinction are generally robust to these potential biases across the four different datasets in contrast absolute values vary significantly as do finer scale patterns in agreement with past studies taxonomic errors revised taxonomy and new taxa have little effect on patterns of richness and evenness instead the addition of large numbers of new specimens to the dataset has the largest effect on these paleodiversity metrics despite application of sample standardization we conclude that although large specimen datasets hundreds to thousands of specimens are robust to various potential biases and can recover large scale paleobiologic trends they are still affected by many non biotic controls and workers should strive to improve dataset quality and understand the underlying reasons for observed paleodiversity patterns

dead shell assemblages faithfully record living molluscan assemblages at one tree reef abstract reef associated sediments accumulate over time recording the history of biological communities the agreement between live and dead assemblages has been extensively studied because discrepancies between the two can reveal taphonomic bias anthropogenic impact and or a decrease in temporal resolution of dead assemblages to centennial scales i e time averaging given the potential of sedimentary assemblages to provide temporal perspectives on the history of modern communities assessments of live dead agreement in reef mollusk assemblage composition are necessary and timely for understanding rapid environmental change one tree reef otr southern great barrier reef australia has had very little direct anthropogenic influence over the past four decades making it a good system for focusing on taphonomic patterns live n 1335 and dead n 6919 molluscan assemblages were collected from three shallow 6 m carbonate soft sediment lagoons diversity and evenness metrics indicated no significant difference between the live and dead assemblages and dead assemblage rank order abundance explains 82 of that of the live assemblage differences in composition were largely due to sample size the low probability of sampling rare species and to a lesser degree differences in skeletal durability taphonomic factors were responsible for less than 20 of the differences in species composition between live and dead shell assemblages the live molluscan assemblage sampled was similar to the live assemblage sampled 30 years ago given that time averaging is approximately 19 years in the otr lagoon we conclude that the composition of the living assemblage has remained largely unchanged for the last 30 years these findings indicate that dead assemblages preserved in shallow fully carbonate environments primarily reflect the composition of the original source communities making them useful to identify recent changes in coral reef areas where anthropogenic impacts are present

rb sr geochronology of granitoid rocks from the pampean ranges argentina a reconnaissance rb sr geochronological investigation of granitoid rocks from the pampean range provides a framework whereby four culminations of igneous activity are delineated granitoids emplaced prior to 500 ma pre and synkinematic bodies ca 475 ma late kinematic bodies and ca 440 and 340 ma post kinematic bodies a number of the post kinematic granitoids have disturbed sr isotope systematics which is related to the formation of microcline megacrysts the carboniferous granitoids are distinguished by their high initial sr ratios i e 0 710 a feature compatible with an origin involving reworking of upper crust or crustal contamination there appears to be no systematic e w migration of granitoid emplacement during the ca 300 ma evolution of the basement rocks in the pampean ranges