diffusion kinetics of fe2 and mg in aluminous spinel experimental determination and applications the diffusion coefficients of fe2 and mg in aluminous spinel at 20 kb 950 to 1325 c and at 30 kb 1125 c have been determined via diffusion couple experiments and numerical modeling of the induced diffusion profiles the oxygen fugacity fo2 was constrained by graphite encapsulating materials the retrieved self diffusion coefficients of fe2 and mg at 20 kb 950 to 1325 c fit well the arrhenian relation d d0exp q rt where q is the activation energy with d0 fe 1 8 2 8 10 5 d0 mg 1 9 1 4 10 5 cm2 s q fe 198 19 and q mg 202 8 kj mol comparison with the data at 30 kb suggests an activation volume of 5 cm3 mol from analysis of compositional zoning in natural olivine spinel assemblages in ultramafic rocks previous reports concluded that d fe mg in spinel with cr cr al 0 5 is 10 times that in olivine the diffusion data in spinel and olivine have been applied to the problems of preservation of mg isotopic inhomogeneity in spinel within the plagioclase olivine inclusions in allende meteorite and cooling rates of terrestrial ultramafic rocks

thin section and s e m textural criteria for the recognition of cement dissolution porosity in sandstones cement porosity relationships are described from the lower triassic sherwood sandstone group and the middle jurassic ravenscar group in the united kingdom calcite cemented sandstones display a variety of replacement textures with preferential replacement of grains and of overgrowth faces with high free surface energy dolomite and siderite cemented sandstones display similar textures but replacement is less specific and euhedral overgrowth surfaces are commonly embayed by carbonates examination of the more porous sandstones with the scanning electron microscope reveals a range of pitting and embayment textures in authigenic overgrowths and in detrital grains these range from small v shaped notches and pits through regular and irregular shaped embayments into large depressions these textures appear to be morphologically similar to the quartz surfaces seen in thin sections of carbonate cemented sandstones and are interpreted to have been formed by the dissolution of pore filling and grain replacive authigenic carbonates this is confirmed by examination of experimentally exhumed overgrowth surfaces from carbonate cemented sandstones these textures indicate that part of the intergranular porosity in these sediments is secondary in origin and has been generated by the dissolution of carbonate cements the identification of such textures may lead to a more confident interpretation of the nature of intergranular porosity in the subsurface

trace element and stable isotope characteristics of algoma type sulfidic banded iron formations from the wutai complex central north china craton abstract the major archive of iron deposits in china occur within precambrian banded irons formations bifs here we investigate the wutai complex that represents an archean to paleoproterozoic subduction related arc system between the ordos and qianhuai micro blocks of the central north china craton ncc we report the carbon and oxygen isotopes of calcites from carbonate rich bifs and in situ trace elements la icp ms and sulfur isotopes ion probe of pyrites from sulfidic bifs and carbonate samples with an aim to characterize the nature and genesis of the iron deposits associated with these rocks the 13cv pdb 18ov pdb and 18ov smow of calcites range from 2 4 to 1 1 20 4 to 19 3 and 9 9 to 11 respectively displaying archean greenstone belt and marine carbonate settings pyrite from the bifs and carbonates are enriched ni as se te and pb their 34scdt values range from 8 8 to 7 9 with a peak at 0 0 comparing with the features of typical algoma type bifs our study provides insights into the depositional environment of the bifs in the wutai complex

sandstone diagenesis and reservoir quality prediction models myths and reality models and concepts of sandstone diagenesis developed over the past two decades are currently employed with variable success to predict reservoir quality in hydrocarbon exploration not all of these are equally supported by quantitative data observations and rigorous hypothesis testing simple plots of sandstone porosity versus extrinsic parameters such as current subsurface depth or temperature are commonly extrapolated but rarely yield accurate predictions for lithified sandstones calibrated numerical models that simulate compaction and quartz cementation when linked to basin models have proven successful in predicting sandstone porosity and permeability where sufficient analog information regarding sandstone texture composition and quartz surface area is available analysis of global regional and local data sets indicates the following regarding contemporary diagenetic models used to predict reservoir quality 1 the effectiveness of grain coatings on quartz grains e g chlorite microquartz as an inhibitor of quartz cementation is supported by abundant empirical data and recent experimental results 2 vertical effective stress although a fundamental factor in compaction cannot be used alone as an accurate predictor of porosity for lithified sandstones 3 secondary porosity related to dissolution of framework grains and or cements is most commonly volumetrically minor 2 exceptions are rare and not easily predicted with current models 4 the hypothesis and widely held belief that hydrocarbon pore fluids suppress porosity loss due to quartz cementation is not supported by detailed data and does not represent a viable predictive model 5 heat flow perturbations associated with allochthonous salt bodies can result in suppressed thermal exposure thereby slowing the rate of quartz cementation in some subsalt sands

crystal growth and disequilibrium distribution of oxygen isotopes in an igneous ca al rich inclusion from the allende carbonaceous chondrite abstract ts34 is a type b1 ca al rich inclusion cai from the allende cv3 chondrite consisting of spinel melilite ti al rich clinopyroxene fassaite and minor anorthite in an igneous texture oxygen and magnesium isotopic compositions were measured by secondary ion mass spectrometry in spots of known chemical composition in all major minerals in ts34 using the sequence of formation from dynamic crystallization experiments and from chemical compositions of melilite and fassaite the oxygen isotopic evolution of the cai melt was established oxygen isotopic compositions of the constituent minerals plot along the carbonaceous chondrite anhydrous mineral line the spinel grains are uniformly 16o rich 17o 22 7 1 7 2sd while the melilite grains are uniformly 16o poor 17o 2 8 1 8 irrespective of their akermanite content and thus their relative time of crystallization the fassaite crystals exhibit growth zoning overprinting poorly developed sector zoning they generally grow from ti rich to ti poor compositions the fassaite crystals also show continuous variations in 17o along the inferred directions of crystal growth from 16o poor 17o 3 to 16o rich 17o 23 covering the full range of oxygen isotopic compositions observed in ts34 the early crystallized 16o poor fassaite and the melilite are in oxygen isotope equilibrium and formed simultaneously the correlation of oxygen isotopic compositions with ti content in the fassaite imply that the oxygen isotopic composition of the cai melt evolved from 16o poor to 16o rich during fassaite crystallization presumably due to oxygen isotope exchange with a surrounding 16o rich nebular gas formation of spinel the liquidus phase in melts of this composition predates crystallization of all other phases so its 16o rich composition is a relic of an earlier stage anorthite exhibits oxygen isotopic compositions between 17o 2 and 9 within the range of those of fassaite indicating co crystallization of these two minerals during the earliest to intermediate stage of fassaite growth the melilite and fassaite yield an 26al 26mg mineral isochron with an initial value of 26al 27al 0 5 003 0 075 10 5 corresponding to a relative age of 0 05 0 02 myr from the canonical al mg age of cais these data demonstrate that both 16o rich and 16o poor reservoirs existed in the solar nebula at least 0 05 myr after the birth of the solar system

the formation of boundary clinopyroxenes and associated glass veins in type b1 cais we used focused ion beam thin section preparation and scanning transmission electron microscopy fib stem to examine the interfacial region between spinel and host melilite for spinel grains in type b1 inclusions from the allende and leoville carbonaceous chondrites boundary clinopyroxenes decorating spinel surfaces have compositions similar to those of coarser clinopyroxenes from the same region of the inclusion suggesting little movement after formation host melilite displays no anomalous compositions near the interface and late stage minerals are not observed suggesting that boundary pyroxenes did not form by crystallization of residual liquid allende spinels display either direct spinel melilite contact or an intervening boundary clinopyroxene between the two phases spinel melilite interfacial regions in a leoville b1 are more complex with boundary clinopyroxene as observed in allende but also variable amounts of glass secondary calcite perovskite and an mg al oh rich and ca si poor crystalline phase that may be a layered double hydrate one possible scenario of formation for the glass veins is that open system alteration of melilite produced a porous hydrated aggregate of mg carpholite or sudoite aluminous diopside that was shock melted and quenched to a glass the hydrated crystalline phase we observed may have been a shocked remnant of the precursor phase assemblage but is more likely to have formed later by alteration of the glass in the mantle boundary clinopyroxenes may have been crystallized from ti rich liquids formed by the direct dissolution of perovskite and an associated sc zr rich phase or as a reaction product between dissolving perovskite and liquid in the core any perovskite and associated ti enriched liquids that may have originally been present disappeared before the growth of boundary clinopyroxene and the observed boundary clinopyroxene may have nucleated and grown from the liquid along with the larger core clinopyroxene

an extraordinary example of photokarren in a sandstone cave cueva charles brewer chimanta plateau venezuela biogeomorphology on a small scale a distinctive suite of small scale erosional forms that are oriented towards the light occur close to the entrance of cueva charles brewer a large cave in a sandstone tepui in se venezuela these are the third example of photokarren ever studied in the world the other two being from borneo and ireland they are the only photokarren ever described from sandstone and the only example from a non carbonate environment the host rock is a poorly lithified unit of the precambrian quartz arenite of the roraima supergroup the forms are all oriented towards the light at 30 regardless of rock surface orientation the primary negative erosional form is the tube coalescence of tubes results in the positive remnant forms of rods pinnacles and cones the final stage is a bumpy wavy surface of degraded cones the size of the features varies with erosion rate and details of the form vary with development stage the main population averages 4 4 cm in depth with 55 of the surface eroded this is divided into 10 tubes 70 rods 10 cones 5 linear valley and 5 wavy lowland the micro ecosystem includes many bacteria diatoms red algae green algae liverworts and oribatid mites but surprisingly no cyanobacteria the presence of a surface biofilm inside the forms but not on the remnant rock surface and in the non degraded forms the direct relationship of biomass with depth suggests that biological activity is the dominant control on development in addition direct bacterial corrosion was noted these same features occur to varying extents in the photokarren of borneo and ireland and the model for development that we present provides a unifying theory for all photokarren this study also includes the first published petrographic analysis of uppermost unit of the mataui formation

solution weathering rate and origin of karst landforms and caves in the quartzite of auyan tepui gran sabana venezuela abstract the paper reports the results of sio 2 analyses in the aonda cave system located on the auyan tepui one of the widest table mountains of the gran sabana south venezuela characterised by karst landforms developed in siliceous rock chemical analyses underline the very low concentration of sio 2 of the surface water percolation and cave drip waters have a sio 2 concentration of about 1 mg l the mean silica load of the cave stream is 184 mg s mainly derived from surface solution removal in the allogenic recharge area in the aonda cave system the mean sio 2 dissolved load is 40 mg s in part from surface solution 15 and mainly from underground processes 85 the low solubility of sio 2 in slightly acidic water implies the importance of the time factor in the formation of cave systems with the present dissolution rate about 10 ma would be necessary to form the known karst system this estimation can be significant only if we assume that climate has been stable in the last few tens of millions of years furthermore this age can be taken as a minimum estimate while according to the geomorphic evolution of the area the origin of the aonda cave system could be reasonably dated back to at least 20 30 ma that is to the oligocene

underground erosion and sand removal from a sandstone tableland stolowe mountains sw poland abstract models of geomorphic evolution of tablelands vary as far as the patterns of cliff line degradation and hence scarp retreat are concerned among surface processes rock slope failures from caprock cliffs often induced by preferential seepage from rock faces are emphasized in this paper evidence is provided that mechanical removal of sand from inside the caprock through underground erosion may also play an important role in the long term evolution of cliffed escarpments in sandstone tablelands more than 140 sandy cones and nearly 80 sand sheets emerging from fissures in the sandstone cap were mapped and measured along six cliff sections of variable length in the stolowe mountains tableland in sw poland the density of sandy accumulation forms varies along the cliff length with cones preferentially associated with intact cliffs and sand sheets being more common along degraded cliff sections further redistribution of sand downslope results in widespread sandy covers below cliff lines which extend for a few hundred metres and are 1 3 m thick as demonstrated by electrical resistivity tomography sand removal from caprock accounts for progressive non catastrophic degradation of tableland margins through gradual subsidence of blocky compartments and the development of ruiniform relief behind the cliff lines thus it may be an important component of long term evolution of stepped relief in sedimentary terrains

a hybrid model to evaluate subsurface chemical weathering and fracture karstification in quartz sandstone abstract a hybrid numerical finite differences and semi analytical empirical model has been developed to evaluate the spatial and time scale of chemical weathering along a fracture in quartz sandstones the model is based on the diffusion transport occurring from the bedrock mass toward the fracture walls driven by the sio2 concentration gradient between water in the bedrock pores high silica content and water flowing in the fracture low silica content because of molecular diffusion intergranular water becomes undersaturated with respect to silica this promotes dissolution of quartz at grain boundaries and results in a porosity profile decreasing from the fracture walls toward the rock interior bedrock individual grains are released and gradually removed by the water flowing into the fracture when a critical value of porosity is reached grain release threshold grt this process drastically increases the fracture enlargement rate the model outcomes establish a minimum time of 90 ka to reach the grt this time is independent of length aperture and dip of the fracture but is controlled by parameters such as the initial fracture aperture water temperature quartz grain size and initial bedrock porosity it is attested that the water flowing in the fracture remains undersaturated with respect to silica over very long timescales in the order of 105 years and over very long flow paths in turn this suggests that the extremely slow reaction between quartz and water is the key factor for the formation of subterranean karst like features in quartz sandstone otherwise silica saturation would be reached after short distances and deep weathering in this lithology would be prevented finally the model outcomes were compared to field data from the gran sabana caves venezuela and other quartz sandstone weathering landforms elsewhere showing that dissolution diffusion in the rock matrix is a reliable mechanism for the formation of these peculiar karst like features

algoma type banded iron formation bif abitibi greenstone belt quebec canada abstract a typical algoma type banded iron formation bif occurs in orvilliers montgolfier and aloigny townships in the abitibi greenstone belt quebec canada the bif is composed of millimeter to decimeter thick beds of alternating fine grained dark gray to black well laminated magnetite rich and or hematite beds and quartz feldspar metasedimentary graywacke beds the bif is well defined by magnetic anomalies these bif layers are commonly associated with decimeter to meter thick horizons of metasedimentary rocks and mafic to intermediate volcanic rocks which are locally crosscut by dikes of felsic or mafic intrusive rocks and as well narrow dikes of lamprophyre the upper and lower contacts of the bif are gradational with the adjacent graywacke all geological units in the area are metamorphosed to the greenschist facies of regional metamorphism magnetite is mainly associated with subordinate amounts of hematite quartz na rich plagioclase and muscovite the fine grained magnetite content is composed of 77 to 89 of the principal iron oxide minerals present the magnetite occurs as disseminated idiomorphic to sub idiomorphic small crystals which average 20 m 5 m in size hematite is the second most abundant iron oxide mineral although less abundant red jasper occurs in cherty horizons with strongly folded fragments and within fault zones this particular algoma type iron formation stratigraphically extends more than 36 km along strike it dips sub vertically with a true width from 120 m to 600 m the origin of the bif is closely linked to regionally extensive submarine hydrothermal activity associated with the emplacement of volcanic and related subvolcanic rocks in an archean greenstone belt

the lithobiontic ecological niche with special reference to microorganisms abstract revised terminology is proposed that describes the ecological niches of microorganisms within hard mineral substrates organisms attached to the external surfaces of the rock are termed epiliths while those in the interior of the rock are all termed endoliths the latter are called chasmoendoliths if they inhabit fissures in rocks cryptoendoliths if they dwell within structural cavities and euendoliths if they actively penetrate calcareous substrates

re evaluation of rock core logging for the prediction of preferred orientations of karst in the kuala lumpur limestone formation based on the relationship between the geological structures and river patterns the preliminary prediction of the karstic features in the kuala lumpur limestone formation based on the existing geological map appears to be directed between n030 to 070 and n120 to 150 the factual evidence of this occurrence is obtained from investigation in the field first using the two sites exposed for construction of the smart tunnel under kuala lumpur and second based on the smart tunnel project rock cores recovery a much detailed prediction was made by re evaluating the tcr rqd and scr values the reassessment of these values enables the drilled ground to be classified into four different qualities of rock mass good quality limestone moderate quality limestone weathered limestone and fully developed void like karst according to this new classification system the tunnel appears to align generally in the heavily karstified limestone in the northern section and massive good quality limestone in the southern section of the study area which is represented as rose diagrams the evidence for this analysis is given and the methods used for these studies are explained

deep karst conduits flooding and sinkholes lessons for the aggregates industry abstract limestone aggregate quarries in deeply penetrating karst terrain are often at considerable risk of artesian inflow from groundwater or surface water channeled through the karstic aquifer the inflow occurs through what are likely to be complex conduits that penetrate hundreds of feet into bedrock rates of inflow can exceed the operation s pumping capabilities proving to be uneconomic to manage over the long term over time inflow rates can increase dramatically as turbulent flow through the conduit erodes its soft residual clay rich fill one recent investigation observed an inflow rate of more than 40 000 gpm from a surface water source floodwater persistently laden with sediment is an indicator of conduit washout and implies increasing inflow rates over time conduits carrying floodwater can exist in a variety of forms along deeply penetrating geologic faults joints or following the path of preferentially eroded bedding preferential structural deformation along faults or bedding can enhance dissolution during subsequent interaction with groundwater the resulting conduit may be a complex combination of many geological features making the exploration and remediation of the pathway difficult sinkholes at the site can occur within several contexts pre existing subsidence structures can reactivate and subside further forming new collapse sinkholes within soil directly overlying the conduit cover collapse sinkhole development can be a direct result of increasing downward groundwater velocities and subsurface erosion associated with the enlargement of a conduit normal operation events such as a quarry blast can also provide a significant new linkage between the groundwater and the quarry allowing rapid drainage of the groundwater reservoir with such drainage and erosion of karst fill sinkholes will develop over localized water table depressions most significantly over enhanced permeability zones associated with fractures paradoxically although the rise in quarry water level will lead to regional reduction in the hydraulic gradients on local scales drainage of the groundwater reservoir increases gradients and leads to the development of cover collapse sinkholes recommended methods for preliminary site investigation can include a detailed review of geological literature and drilling logs to compile a conceptual model of the site a fracture trace analysis with em geophysics can confirm the locations of major faults and fractures fingerprinting of the various water sources to the quarry and the water in the quarry is an inexpensive and effective means of identifying the source and likely direction of the groundwater and surface water flow automated geophysical equipment on the market for performing rapid resistivity and microgravity surveys speeds up the site screening process during reconnaissance exploration for deep structure it is recommended that mine planning fully incorporate this information so that quarry operators can take proactive measures to avoid catastrophic and costly flooding events

a geomorphological strategy for conducting environmental impact assessments in karst areas in their efforts to protect regional groundwater supplies governmental agencies are increasingly requiring studies of karst areas and their features in areas where tracer tests or geophysics are not required funded or otherwise feasible geomorphological methods remain as the primary tool for assessing karst this study proposes a geomorphologically based environmental impact assessment strategy for karst areas while it is supported with results from a study of the karstic edwards aquifer recharge zone on the camp bullis military training installation tx usa it is based on the study of several karst areas and is generalized to accommodate and be fine tuned for regional variations biological and other resource issues can also be assessed with this strategy the assessment identifies environmentally sensitive features and areas as is often required to meet regulatory directives in karst areas with relatively small features excavation is a key tool for accurate assessment although the results of this study will help to better manage karst areas proper management must be done on a regional scale the highly permeable nature of karst precludes adequate management solely on a feature by feature basis studies on the relationship of water quality to impervious cover show adverse environmental impacts significantly increase when impervious cover exceeds 15 of a surface watershed the camp bullis study finds similar impacts in its groundwater drainage basin supporting the argument of 15 impervious cover as a regionally effective means of also protecting karst aquifers when coupled with protection of critical areas identified by field surveys

a petrologic and ion microprobe study of a vigarano type b refractory inclusion evolution by multiple stages of alteration and melting abstract usnm 1623 8 is a type b vigarano inclusion 6 mm in diameter consisting of melilite ak12 72 fassaite 17 23 a12o3 5 17 tio2 anorthite and spinel secondary alteration is negligible and consists of diopside and sparse nepheline melilite is normally zoned and contains up to 0 35 wt na2o higher than generally seen in melilite from other refractory inclusions a strong correlation between akermanite content and na2o in 1623 8 melilite suggests that the na was present during igneous crystallization rather than introduced later by diffusion anorthite in 1623 8 is coarse grained but commonly has an irregular border with neighboring melilite crystals many spinel grains enclosed within anorthite have selvages of melilite around them which in places are continuous with melilite exterior to the anorthite the anorthite appears to be replacing melilite ion microprobe analyses of anorthite in 1623 8 show that there is no detectable excess 26mg in nine anorthite crystals that have 27 al 24 mg as high as 970 implying an upper limit of 26 al 27 al 0 7 one crystal was found that has excess 26mg consistent with 26 al 27 al 0 3 10 6 melilite in the outer portion of the inclusion 27 al 24 mg 2 7 11 5 contains excess 26mg that considered together with spinel and pyroxene defines an isochron consistent with the canonical early solar system value of 26 al 27 al 0 4 5 10 5 melilite near an anorthite 27 al 24 mg 1 6 5 9 has 26mg values in excess of those predicted from 26 al 27 al 0 4 5 10 5 presumably caused by redistribution of radiogenic 26mg from adjacent anorthite the high na2o inferred to have been present in the melt suggests that the precursor solids were na bearing such as for example refractory material having an allende like overprint of feldspathoid bearing secondary alteration the melting event must have been brief in order that not all of the na was vaporized during melting the isotopic composition of the anorthite relative to melilite indicates a subsequent much later after the decay of most or all 26a1 episode of partial melting or solidstate recrystallization that did not affect the al rich melilite but reset most of the anorthite and some mg rich melilite near anorthite because the inclusion is both rimmed and slightly altered this late resetting event appears to have been a nebular one an assumption of homogeneously distributed 26a1 in the regions of the nebula where the inclusion formed and was later modified implies a several millionyear interval between condensation of the precursors and final melting of 1623 8

source and genesis of sulphate and phosphate sulphate minerals in a quartz sandstone cave environment gypsum caso4 2h2o alunite kal3 so4 2 oh 6 and rare phosphate sulphate sanjuanite al2 po4 so4 oh 9 h2o and rossiantonite al3 po4 so4 2 oh 2 h2o 14 have recently been identified as secondary mineral deposits in different quartz sandstone caves in the gran sabana region venezuela due to the extended time scale required for speleogenesis in the hard and barely soluble quartz sandstone lithology these caves are considered to be as old as 20 to 30 my the study of these peculiar secondary mineral deposits potentially reveals important insights for understanding the interaction between deep superficial and atmospheric processes over thousands to perhaps millions of years in this study chemical and petrographic analyses of potential host rock sources sulphur and oxygen isotope ratios and meteorological hydrological and geographical data are used to investigate the origin of sulphates and phospho sulphates the results suggest that the deposition of sulphates in these caves is not linked to the quartz sandstone host rock rather these mineral deposits originate from an external atmospheric sulphate source with potential contributions of marine non sea salt sulphates terrestrial dimethyl sulphide and microbially reduced h2s from the forests or peatbogs within the watershed air currents within the caves are the most plausible means of transport for aerosols driving the accumulation of sulphates and other secondary minerals in specific locations moreover the studied sulphate minerals often co occur with silica speleothems of biological origin although this association would suggest a possible biogenic origin for the sulphates as well direct evidence proving that microbes are involved in their formation is absent nonetheless this study demonstrates that these quartz sandstone caves accumulate and preserve allogenic sulphates playing a yet unrecognized role in the sulphur cycle of tropical environments

escarpment retreat in sedimentary tablelands and cuesta landscapes landforms mechanisms and patterns abstract although the general concept that escarpments in layered rocks retreat through time was offered more than 150 years ago recognizing the exact mechanisms patterns and rates of retreat remains a challenge in this paper we provide the state of the art of the theme of escarpment retreat reviewing both the classic contributions often forgotten but important case studies and the latest research in separate parts of the paper we discuss 1 terminology applied to the geomorphology of escarpments 2 the main assumptions and pathways of long term evolution of escarpments including conceptual models presented so far 3 processes involved in cliff retreat mainly weathering mass movements and erosion 4 the role of spring erosion and fluvial processes leading to fluvial dissection and development of canyons present in many tablelands 5 the possible role of climate change in escarpment evolution 6 rates of escarpment retreat in conclusions we summarize the major advances in our understanding of escarpment evolution of the last two three decades such as the recognition of the key role of emerging groundwater in non uniform escarpment retreat of an important role of silica dissolution and underground removal of dissolved solids increasing appreciation of rock properties in governing the patterns of escarpment retreat the proposal that cliffs do not necessarily retreat via catastrophic rock slope failures but may disintegrate in situ and the first attempts to date escarpment retreat using cosmogenic isotopes finally the remaining gaps and possible research priorities for the future are identified

a global review of solutional weathering forms on quartz sandstones abstract solutional landforms in limestone have been described for over a hundred years but landforms of similar morphology on highly siliceous sandstones and quartzites have also been identified in a wide variety of environments and generally termed pseudokarst these include large bedrock pinnacles and towers caves corridors grikes solution basins and runnels and even silica speleothems quartzites and quartz sandstones have been held to be amongst the most chemically resistant of rocks but the similarity both in morphology and genetic process of many landforms developed from them to features of known solutional origin on limestone has prompted some authors to refer to these quartzose landforms as true karst the most detailed studies of quartzose karst landforms have been in present day tropical regions or areas believed to have been tropical in the geologically recent past this concentration of research in hot wet areas allied with the long held assertion of the insolubility of silica especially quartz has led to a belief that tropical climatic conditions are necessary for karstic solution of these rocks however the existence of quartzose karst landforms in temperate and even sub polar latitudes especially where there is no evidence of prior tropical conditions suggests that the requirement of tropical weathering is no longer tenable the reports of these quartzose solutional landforms are widely scattered through the geomorphological and geological literature but a comprehensive world wide review of the range of solutional landforms on quartzose rocks has not previously been published because of the increasing awareness in this karst type such a summary is sorely overdue

hydrothermal experiments on alteration of ca al rich inclusions cais in carbonaceous chondrites implication for aqueous alteration in parent asteroids carbonaceous chondrites contain ca al rich inclusions cais which consist mainly of refractory minerals including gehlenite spinel perovskite and hibonite which are predicted to be the first phases formed from early solar nebula on the basis of thermodynamic calculations however many cais contain secondary minerals such as nepheline calcite and phyllosilicates the formation process of secondary minerals in cais is still controversial whether they were produced by the reaction with solar nebula or by aqueous alteration in parent asteroids we performed hydrothermal experiments on gehlenite spinel and diopside which are common in cais in carbonaceous chondrites to study aqueous alteration in parent asteroids mixtures of gehlenite with sio2 and or al2o3 and those of sio2 with al2o3 were also employed to study products of these systems which have a close relationship to crystallization of secondary minerals in cais in fluid in parent asteroids hydrogrossular calcite and amorphous al si were obtained from gehlenite by hydrothermal experiments whereas both spinel and diopside escaped alteration suggesting that gehlenite is the weakest mineral among these refractory minerals against alteration this result is consistent with observation of cais analcime nepheline hydrate and tobermorite were obtained from the runs for mixtures of gehlenite with sio2 whereas the products from mixtures of al2o3 with sio2 were analcime nepheline hydrate and hydrosodalite these results suggest that the alteration process of gehlenite is as follows 1 gehlenite was dissolved by fluid in chondrite parent asteroids to release ca al and si from the surface 2 co32 in fluid reacted with ca to crystallize calcite 3 na al and si in fluid crystallized nepheline hydrate hydrosodalite and or analcime 4 ca al and si in fluid crystallized hydrogrossular without co32 and excess ca and si crystallized ca si hydrate such as tobermorite heating experiments on several minerals obtained through hydrothermal experiments were also performed to study thermal metamorphism in parent asteroids hydrogrossular was converted into hydrogrossular with a lower h2o content and analcime and nepheline hydrate were converted into nepheline tobermorite is known to be converted into wollastonite with increasing temperature the results of experimental studies suggest that most secondary minerals found in cais in carbonaceous chondrites were produced from gehlenite by aqueous alteration and the subsequent thermal metamorphism in parent asteroids

structural textural geochemical and isotopic signatures of synglaciogenic neoproterozoic banded iron formations bifs at bafq mining district bmd central iran the possible ediacaran missing link of bifs in tethyan metallogeny abstract the neoproterozoic magnetite apatite hematite pyrolusite jaspilite deposits in the bafq mining district bmd contain more than 1 7 gt ores with an average grade of 50 wt fe and 0 01 to 7 78 wt p and were probably formed between 635 and 547 ma in a riftogenic felsic submarine exhalative sequence of the esfordi formation the ore zones occur in proximal zone of magnetite rich albitized rhyolite keratophyres interdistal zone of rhyolitic tuff tuffaceous sediments and distal zone of pyrolusite jaspilite these sequences are covered by the diamictites and cap carbonates the bifs are linked to the altered rhyolites rhyodacites jaspilites and diamictites and contain magnetite hematite and apatite the presence of spriggina dickinsonia medusites and persimedusites chahgazensis sennewald and kruger 1979 hahn and pflug mccall 2006 in the kushk shale member of the esfordi formation conforms to the australian fauna of the ediacaran period 635 540 this relative age is supported by some reliable pb isotopic data 635 547 ma on galena monazite and apatite huckriede et al 1962 torab 2008 stosch et al 2011 the most frequent structures textures in the ore zones include felsic autobrecciation massive colloidal banded detrital and glaciogenic the bifs are highlighted by high values of lree fractionation and no significant eu and ce anomalies the ores show high values of fe 2 o 3 14 60 and sio 2 4 34 and low contents of al 3 32 cr 21 48 ppm co 42 82 ppm ni 125 ppm v 868 ppm and ti 0 13 similar to those of the ediacaran rapitan bifs the cap carbonates show depletion in 13 c with a range from 0 43 to 6 6 per mil and then return to near excursion of about 2 97 in the lower cambrian carbonates these are followed by 18 o values which range from 6 64 to 11 86 the presence of jaspilites diamictites c and o isotopic signatures ree patterns and immobile element relationships highlights a glaciogenic bif importantly the glaciogenic structures textures and jaspilites do not support the misconception of the previously proposed magmatic carbonatitic and metasomatic hydrothermal iocg kiruna ore deposits

porosity reduction by pressure solution a theoretical model for quartz arenites pressure solution and the subsequent precipitation of the dissolved material may play an important role in the lithification of sandstones and limestones this paper presents a one dimensional model for a well sorted quartz arenite sequence undergoing diagenesis the sedimentary sequence is treated as a saturated porous medium in which pressure solution occurs at individual grain contacts quartz dissolved at grain contacts precipitates on free surfaces of adjacent grains this solution and precipitation process reduces porosity and forces the migration of pore fluid permeability decreases with porosity hindering fluid migration and excess fluid pressures are generated distributions of porosity pore pressure and fluid flow rates are given for several sedimentation rates and thermal gradients complete reduction of the primary intergranular porosity was accomplished in all cases examined by a depth of 3 5 km

anatomy of biologically mediated opal speleothems in the world s largest sandstone cave cueva charles brewer chimanta plateau venezuela abstract siliceous speleothems can be formed in sandstone caves recently opal biospeleothems have been found in the world s largest cave in precambrian sandstones on the chimanta tepui in venezuela the speleothems although reminiscent of normal stalactites and stalagmites from limestone caves are in fact large microbialites more than a dozen forms were distinguished but they share a common structure and origin they consist of two main types 1 fine laminated columnar stromatolite formed by silicified filamentous microbes either heterotrophic filamentous bacteria or cyanobacteria and 2 a porous peloidal stromatolite formed by nostoc type cyanobacteria the first type usually forms the central part and the second type the outer part of speleothems fungal hyphae metazoan and plant remains also subordinately contribute to speleothem construction the speleothems occur out of the reach of flowing water the main source of silica is the condensed cave moisture which is the main dissolution reprecipitation agent speleothems which originated by encrustation of spider threads are unique

timing of deposition and tectonothermal events of banded iron formations in the anshan benxi area liaoning province china evidence from shrimp u pb zircon geochronology of the wall rocks abstract most of the algoma type bifs and associated volcanic suites in the north china craton ncc have undergone high grade metamorphism during the 2 5 ga tectonothermal event isotopic systems in high grade metamorphic rocks are often reset thereby losing the earlier records whereas zircons in low grade metamorphic rocks are potential candidates to evaluate earlier thermal events detailed textural relationship and internal structures of zircons and their age data for eight wall rock samples with metamorphic grades from greenschist to upper amphibolite facies of the algoma type bifs in the anshan benxi area are used to constrain the age of protolith zircons in the greenschist to lower amphibolite facies are generally metamorphism resistant whereas those in the upper amphibolite facies rocks show variable recrystallization shrimp zircon u pb dating of magmatic zircons in six samples constrains the peak of bif deposition at 2 56 2 53 ga which is consistent with 2 5 ga crustal growth in the ncc in addition the greenschist facies metamorphic rocks in the anshan area also record an event of volcanic activity at 2 72 2 65 ga we consider that low grade metamorphic wall rocks were not significantly affected by the 2 5 ga tectonothermal event and the 2 72 2 65 ga age group records another important magmatic event corresponding to 2 7 ga crustal growth the metamorphic zircons from these rocks record a thermal event during 1 88 1 84 ga

petrologic and almg isotopic clues to the accretion of two refractory inclusions onto the leoville parent body one was hot the other wasn t abstract two type b refractory inclusions found within several centimeters of each other in the foliated leoville meteorite differ greatly in their styles of deformation textures and mg al isotopic characteristics one is a subspheroidal type b1 with textures similar to those observed previously in many type b1s from other cv3 meteorites and whose deformation fracturing some granulation has been almost entirely brittle in nature its isotopic composition shows 26 mg 24 mg correlated with 27 al 24 mg which although disturbed indicates a lower limit for initial 26 al 27 al of 4 9 10 5 the other inclusion is a highly elongate l w 6 object whose long axis lies in the plane of foliation of the leoville meteorite it has a predominantly metamorphic texture but relict islands of an earlier and more normal type b precursor are preserved in its core the isotopic compositions of the two textural variants in this inclusion are different anorthite in the metamorphic region mostly showing little or no excess 26mg 26 al 27 al 0 6 that would be attributable to in situ decay of 26al whereas anorthite in the relict islands shows well resolved excess 26mg that gives a lower limit for initial 26 al 27 al of 4 6 10 5 the effects of shock metamorphism on the leoville meteorite are clearly seen in both inclusions including brittle deformation and local shock melting but are not responsible for the major textural differences between the two inclusions the most likely origin for the deformed and recrystallized features of the elongate inclusion is that it accreted onto the leoville parent body while still very hot and deformed into its present shape at the instant it impacted if the resetting of the al mg system occurred during this event it must have taken place at least 3 3 million years after formation of the precursor inclusion the possibility that other deformed leoville components accreted while hot should not be discounted

an updated global review of solutional weathering processes and forms in quartz sandstones and quartzites abstract quartz is considered one of the less soluble minerals of the earth s crust and thus hardly affected by chemical weathering despite this for more than forty years it is clear that the formation of caves and peculiar solutional weathering dominated landforms in quartz rich lithologies is common and shares several similarities to the well known karstic ones in carbonate rocks in the last thirty years great strides have been made in furthering our knowledge of the distribution of these forms around the world and the geochemical processes involved these studies have clearly shown that solutional weathering is a fundamental process acting through intergranular dissolution of quartz increasing the rock porosity and decreasing the rock strength to erosion this process has been described in the concepts of arenization and phantomization and the widespread evidences of the fundamental role of quartz solution in landform genesis has even developed to the extent of several geomorphologists reassessing the definition of the term karst and its application to these peculiar lithologies nonetheless the process is complicated by several factors related both to environmental conditions water chemistry and availability as well as to the compositional and textural characters of the lithology presence of clays iron hydroxides carbonate cement etc all these aspects have to be taken carefully in consideration in order to understand if solution is a dominant or accessory process in the landscape evolution in this review the state of knowledge on the relevant chemical processes weathering mechanisms and speleogenesisk involved in the surface and underground karstification and clear examples of quartz solution and solutional landforms from different world locations are outlined and discussed

contexte structural et mineralisations auriferes des gites casa berardi abitibi quebec the casa berardi gold deposits are located 180 km north of rouyn noranda within archean rocks of the abitibi subprovince the casa berardi deformation zone cbdz which contains these deposits is

constraints on the nature of the kapuskasing structural zone from the study of proterozoic dyke swarms petrochemical paleomagnetic and structural studies on early proterozoic dyke swarms show that the rocks of the kapuskasing structural zone ksz central superior province were upthrust about 10 15 km along the ivanhoe lake fault after intrusion of 2 04 ga kapuskasing dykes this uplift was part of a more widespread deformation in flanking terranes that involved sinistral motion along north north northwest trending faults dextral displacement along east northeast northeast trending faults and a distortion of the 2 45 ga matachewan swarm to the west and northwest of the ivanhoe lake fault the most spectacular demonstration that the ksz is largely a product of proterozoic deformation is that matachewan dykes change their magnetic polarity on crossing the zone a consequence of remanence acquisition at deep crustal levels being delayed until after a reversal of the earth s magnetic field and prior to uplift of the dykes matachewan and kapuskasing dykes within the amphibolite to granulite grade rocks o

spatial sampling considerations and their applications to characterizing fractured rock and karst systems assessing the existence location and mapping of geologic anomalies such as fractures and cavities within soil and rock are common to both engineering and hydrologic project needs there are numerous measurement techniques available to detect map and characterize fractures and cavities a summary of the common methods available for fracture and cavity detection and mapping are presented along with their spatial sampling capabilities the remote sensing and geophysical methods are but a small part of this list but provide a wide range of tools to attack the problem they include some of the most effective methods for locating and mapping fractures and cavities and cover the complete range of spatial sampling from regional to no more than the surface of a borehole wall an understanding of all of the tools available and their spatial sampling limitations is a necessary part of planning and carrying out an effective fracture cavity investigation

comment on sandstone caves on venezuelan tepuis return to pseudokarst by r aubrecht t lanczos m gregor j schlogl b smida p liscak ch brewer carias l vlcek geomorphology 132 2011 351 365 in the recent work of aubrecht et al 2011 the presence of unlithified or poorly lithified beds of sands in the quartz sandstone stratigraphic succession is proposed as a key factor for speleogenesis in the venezuelan tepuis in this comment we observe that in the cited work the geologic history of the region in terms of sedimentation environment diagenesis and low grade burial metamorphism has not been considered furthermore the peculiar pillar flow columns that aubrecht et al describe as a proof of the unlithification are lacking in many other different cave systems in the same area four critical points are discussed the burial metamorphism of the mataui formation the significance of the schmidt hammer measurements the cave morphologies and the role of sio2 dissolution finally we suggest that weathering in its wider significance is probably the triggering process in speleogenesis and there is no need to invoke a differential diagenesis of the sandstone beds

karstic problems in the construction of milwaukee s deep tunnels a critical component of milwaukee s 2 8 billion water pollution abatement program is a 31 2 km inline storage system comprising three deep tunnel sections that were bored between 1984 and 1993 at a depth of 80 100 m within silurian aged niagara dolostone construction of these deep tunnels proved more difficult and expensive than estimated because the karstic nature of the dolostone particularly its hydrology had not been fully appreciated rock collapse subsidence and groundwater intrusion necessitated remedial grouting and lining of about 45 of the tunnels costing some 50 million above estimates and delaying completion by 9 months tunnel performance since completion continues to be controversial

replacement textures in cai and implications regarding planetary metamorphism textural and chemical features of five coarse grained calcium aluminum rich inclusions from the allende meteorite indicate that some of the melilite in these inclusions was formed by a secondary metamorphic event and not by primary crystallization from a melt or by a sequential nebular condensation process these inclusions contain embayed pyroxene surrounded by melilite physically separated pyroxene crystals are often in optical continuity indicating that they were once part of larger single crystals that have been partly replaced by melilite other evidences of metamorphism include reaction textures between melilite and spinel and metamorphic textures such as kink band like features lobate sutured grain boundaries and 120 triple points this type of metamorphic process requires the addition of ca which we propose came from calcite or by introduction of a fluid phase we believe that the most likely environment for this metamorphic process is on a small planetary body and not in the solar nebula the results of this study are compatible with oxygen isotopic heterogeneities within cai and provide a mechanism for producing lower temperature alteration phases and the rim phases found in these inclusions we conclude that planetary processes must thus be considered in the formation history of cai and that it is necessary to reconsider the classification system of these objects in light of the replacement process proposed here

origin of zoned fine grained inclusions in the allende meteorite many fine grained ca al rich inclusions in the allende meteorite show a consistent pattern of mineralogic and textural zonation the center zone a consists mostly of spinel nepheline al diopside and salite with minor amounts of olivine sodalite grossular anorthite perovskite and ilmenite in most inclusions these minerals occur as loosely packed discrete anhedral to subhedral grains with only minor intergrowth between phases in the others zone a is made of 5 to 30 m sized spinel centered multi rimmed objects and of finer grained loosely packed interstitial material the intermediate zone b is characterized by the presence of hedenbergite and andradite in addition to the minerals which occur in zone a partially filling voids one type of outer zone c consists mostly of intergrown olivine and nepheline with small ilmenite grains enclosed the other type of zone c consists of symplectic texture with spinel rimmed by nepheline or anorthite rimmed by al diopside the textural and mineralogical similarity of zone a except for the rimmed spinel objects to altered areas of coarsegrained melilite rich cais suggests that a similar open system alteration affected the fine grained inclusions and that zone a s contained melilite before being completely altered to anorthite grossular nepheline and sodalite by comparing the mineralogies before and after the alteration some ca must have evaporated from and si fe na and cl condensed onto the inclusions under conditions of low temperatures 1000 k and or at ho ratios lower than the cosmic abundance ca is more volatile than si due to the formation of gaseous ca hydroxides the thermodynamic stability of minerals in zones a and b also indicates alteration at temperatures less than 1000 k the mode of occurrence of hedenbergite and andradite in zone b implies that they are condensates deposited on the edge of zone a it is likely that the ca migrating out of the center of the inclusions and the si and fe migrating inward from the surrounding nebular gas coprecipitated according to the thermodynamic stability of these minerals zone c on the other hand most likely existed even before alteration surrounding what is now zone a however the mineralogy of zone c appears to have changed by alteration the zone c consisting of olivine nepheline and ilmenite was probably a spinel rich belt mantling the inclusion

an archean fold thrust belt in the northwestern abitibi greenstone belt structural and seismic evidence an integration of structural field data and lithoprobe seismic reflection line 28 in the northwestern abitibi greenstone belt agb reveals a crustal scale south to southwest vergent thrusting event that developed in sequence above a shallowly 15 north dipping sole thrust at a mid crustal level seismic reflector geometry above this decollement suggests a mid crust 6 20 km depth dominated by low angle thrusts with smooth trajectory ramps and culmination folds or antiformal stacks similar to the structural style of neighbouring high grade plutonic gneissic opatica and sedimentary pontiac subprovinces in contrast low to high angle east west trending thrusts at the upper crust greenstone belt level 6 9 km depth are interpreted to be listric they occur in two fault systems the chicobi and taibi that resemble imbricate fan systems the contrasting structural geometry of the upper and mid crust is interpreted as variations in level through the thrust stack and resembles paleozoic mountain

the microstructure of minerals in coarse grained ca al rich inclusions from the allende meteorite abstract the microstructure and microchemistry of minerals in ca al rich coarse grained inclusions cai from the allende meteorite have been investigated with transmission electron microscopy tem spinels contain only low to moderate dislocation densities and are characterized by a ubiquitous fine black spotty texture believed to originate from a slightly non stoichiometric composition ti al pyroxenes are relatively featureless but contain veins of secondary phases apparently deposited in unhealed cracks chromite has been identified in the veins suggesting transport of oxidized iron during alteration melilites exhibit the greatest variety of microstructures and are the most heavily altered phase in cai high dislocation densities are common and crystals exhibit considerable internal strain indicating that they have not been annealed alteration occurs both as veins along cracks and in fronts extending across several grains plagioclase is commonly twinned but dislocations are rare the size and morphology of antiphase domains suggest a high temperature of formation with significant low temperature annealing submicron pyroxene precipitates are a ubiquitous and unusual feature of allende plagioclase whose properties are most consistent with prolonged slow cooling and equilibration after plagioclase crystallization the precipitates appear to be sufficiently abundant to contain the majority of mg present in plagioclase but do not easily account for na and ti abundances wollastonite needles from a cavity in a fluffy type a inclusion exhibit the growth habits and relatively perfect external surfaces indicative of direct condensation from a vapor alteration products are predominantly crystalline and alteration of melilite appears to have proceeded primarily by solid state diffusion at a temperature of approximately 920 k overall the tem observations suggest that cai formed under near equilibrium conditions characterized by slow cooling and that solid state bulk diffusion was the major process affecting their post crystallization history

reply to the comment on sandstone caves on venezuelan tepuis return to pseudokarst abstract in aubrecht et al 2011 we brought new data showing that the arenitic caves in the venezuelan tepuis did not necessarily originate through quartz cement dissolution arenization but the main portion of the caves originated because of poor lithification prior to erosion an additional important process is lateritization which is responsible for at least 30 of the caves volume sauro et al 2012 in their comment support the arenization theory quartz cement dissolution and provide several arguments against some points of our research their main objections and arguments relate to the following topics 1 the validity of the arenization theory 2 the origin and importance of the finger flow pillars 3 the importance of the schmidt hammer measurements and 4 metamorphism of the mataui formation in our reply we present further documentation that 1 the quartz cement dissolution theory of cave evolution was far less documented petrographically than the new theory presented by us although the presence of quartz dissolution is evident there is no current evidence that it is widespread or that it plays a trigger role many of the presented examples of corrosion on quartz grains could have been caused by local alkalization in addition the hydrogeochemical data presented in the comment to support the arenization theory do not in fact contradict our speleogenetic interpretations 2 finger flow pillars are speleogenetic indicators and not speleogenetic factors their absence in some caves provides no evidence against our theory 3 schmidt hammer measurements only objectivize the data on contrasting hardness in the mataui formation beds they do not say anything about the origin of this difference 4 the arguments of sauro et al 2012 involve the metamorphic overprint of the mataui formation shown by the presence of pyrophyllite and quartz mobilizations because quartz mobilization also occurs in hypergenic conditions it may be neglected as a metamorphic indicator pyrophyllite is mentioned as a metamorphic indicator in the literature anchizone to epizone but it does not explain why there is still a vast quantity of kaolinite that did not undergo this reaction in the soft arenitic beds there is an evidence that pyrophyllite may originate at lower temperatures some phases of the alumino silicate mineral phases existing at atmospheric pressure and 25 c temperature show that h 4 sio 4 concentration is a critical factor in kaolinite pyrophyllite transition an increase in this concentration can theoretically cause transformation of kaolinite to pyrophyllite without increased temperature or pressure

karst aquifer evolution in a changing water table environment 1 a vertical cross section through a karst aquifer is modeled by means of the finite element method to study the evolution of fractures and flow in the aquifer the karst aquifer receives a constant recharge along the top boundary by precipitation and drains toward a resurgence assumed to be the base level in a valley flow is allowed both in the permeable rock matrix and the fracture network and the fractures are enlarged with time by chemical dissolution hence during the early evolution of the karst aquifer the conductivity increases over several orders of magnitude and the initially high water table drops to a steady state base level niveau as a consequence fractures above the final water table change from phreatic to vadose flow conditions a systematic parameter study is carried out to investigate the aquifer evolution over a wide range of parameters such as recharge rate initial fracture width and density and initial calcium concentration the numerical models cover a wide range of drainage patterns from phreatic water table caves to deep bathyphreatic caves to vadose river caves the models suggest that a single theoretical approach is capable of explaining most common cave passage patterns

the formation of rims on calcium aluminum rich inclusions step i flash heating abstract wark lovering rims of six calcium aluminum rich inclusions cais representing the main cai types and groups in allende efremovka and vigarano were microsurgically separated and analysed by neutron activation analysis naa all the rims have similar 4x enrichments relative to the interiors of highly refractory lithophile and siderophile elements the naa results are confirmed by ion microprobe and scanning electron microscope sem analyses of rim perovskites and rim metal grains less refractory eu yb v sr ca and ni are less enriched in the rims the refractory element patterns in the rims parallel the patterns in the outer parts of the cais in particular the rims on type b1 cais have the igneously fractionated rare earth element ree pattern of the melilite mantle below the rim and not the ree pattern of the bulk cai proving that the refractory elements in the rims were derived from the outer mantle and were not condensates onto the cais the refractory elements were enriched in an al2o3 rich residue 50 m thick after the most volatile 80 of the outermost 200 m of each cai had been volatilized including much mg si and ca some volatilization occurred below the rim and created refractory partial melts that crystallized hibonite and gehlenitic melilite the required flash heating probably exceeded 2000 c but for only a few seconds in order to melt only the outer cai and to unselectively volatilize slow diffusing o isotopes which show no mass fractionation in the rim the volatilization did however produce heavy mass fractionated mg in rims in some cais this was later obscured when normal mg diffused in from accreted olivine grains at relatively high temperature not the lower temperature meteorite metamorphism and created the 50 m set of monomineralic rim layers of pyroxene melilite and spinel

a mixed seawater and hydrothermal origin of superior type banded iron formation bif hosted kouambo iron deposit palaeoproterozoic nyong series southwestern cameroon constraints from petrography and geochemistry abstract the kouambo iron deposit contains banded iron formations bifs and is located in the northwestern margin of the congo craton the bifs are hosted in palaeoproterozoic nyong series a dominantly metasedimentary formations which were metamorphosed into greenschist to granulite facies the kouambo bifs are medium to coarse grained banded rocks consisting of alternation of si rich and fe rich mesobands and belong to oxide facies iron formations geochemistry analyses reveal that these iron formations are composed of 96 wt fe2o3 and sio2 and have low concentrations of al2o3 tio2 and trace hfse suggesting chemical precipitates of silica and iron moreover these bifs have low concentrations of al2o3 tio2 and trace hfses high field strength elements e g zr hf ta pb and th suggesting that terrigenous detrital materials contributed insignificantly to the sedimentation the post archean australian shale paas normalized ree y patterns display seawater like profile minor lree depletion and hree enrichment positive y anomalies however they display positive eu and negative ce anomalies and low y ho ratio average 29 which suggest the influence of the hydrothermal fluids the weak positive eu eu paas ratio average 1 5 associated with the low v 17 5 ppm co 6 1 ppm and ni 27 5 ppm contents similar to other superior type bifs worldwide are consistent with the deposition of the kouambo bifs in continental marginal sea or back arc basin environment in summary the kouambo bifs show a seawater like ree y signature however the positive eu anomalies and reduced y ho ratios relative to seawater indicates a possible mixing with hydrothermal fluids 0 5

sandstone caves on venezuelan tepuis return to pseudokarst abstract venezuelan table mountains tepuis host the largest arenite caves in the world the most frequently used explanation of their origin so far was the arenization theory involving dissolution of quartz cement around the sand grains and subsequent removing of the released grains by water new research in the two largest arenite cave systems churi tepui system in chimanta massif and ojos de cristal system in roraima tepui showed that quartz dissolution plays only a minor role in their speleogenesis arenites forming the tepuis are not only quartzites but they display a wide range of lithification and breakdown including also loose sands and sandstones speleogenetic processes are mostly concentrated on the beds of unlithified sands which escaped from diagenesis by being sealed by the surrounding perfectly lithified quartzites only the so called finger flow pillars testify to confined diagenetic fluids which flowed in narrow channels leaving the surrounding arenite uncemented another factor which influenced the cave forming processes by about 30 was lateritization it affects beds formed of arkosic sandstones and greywackes which show strong dissolution of micas feldspars and clay minerals turning then to laterite barro rojo the main prerequisite to rank caves among karst phenomena is dissolution as the dissolution of silicate minerals other than quartz appears to play not only a volumetrically important role but even a trigger role these arenitic caves may be ranked as karst

small scale modelling of cementation by descending silica bearing fluids explanation of the origin of arenitic caves in south american tepuis abstract geoscientific research was performed on south american table mountains tepuis and in their sandstone cave systems to explain speleogenesis in these poorly soluble rocks two theories were introduced a arenization theory implying selective weathering of quartz along grain boundaries and releasing of sand grains b selective lithification theory implying cementation by descending silica bearing fluid flow the latter theory presumes that the descending fluid flow becomes unstable on the interface between two layers with different porosity and splits to separate flow channels so called finger flow the arenites outside these channels remain uncemented to verify the latter theory small scale modelling was performed using layered sands and sodium silicate solution fine to medium sand was used 0 08 0 5 mm along with a coarse sand fraction 0 5 1 5 mm the sands were layered and compacted in a transparent plastic boxes three liters of sodium silicate solution so called water glass were left to drip for several hours to the top of the sediment the fine grained layers were perfectly laterally impregnated whereas the descending fluid flows split to fingers in the coarse grained layers due their higher hydraulic conductivity this small scale laboratory simulation mimics the real diagenesis by descending silica bearing fluids and matches the real phenomena observed on the tepuis the resulting cemented constructions closely mimic many geomorphological features observed on tepuis and inside their caves e g finger flow pillars overhangs imperfectly formed aborted pillars in forms of hummocks hanging from ceilings locally also thicker central pillars that originated by merging of smaller fluid flow channels the modelling showed that selective lithification theory can explain most of the geomorphological aspects related to the speleogenesis in tepuis

u pb geochronology of the eastern abitibi subprovince part 1 chibougamau matagami joutel region u pb zircon titanite and baddeleyite ages are reported for 20 volcanic and plutonic rock units in the chibougamau matagami and joutel areas of the northern abitibi belt pretectonic magmatism occurred in three main pulses volcanic rocks of the obatagamau formation in the chibougamau area were erupted at 2759 ma following a possible hiatus in igneous activity volcanism accompanied by the emplacement of layered mafic intrusions occurred in the chibougamau matagami and joutel areas in the period of 2730 2722 ma in the chibougamau area a slightly younger period of plutonism between 2720 and 2712 ma is thought to be comagmatic with felsic volcanic rocks in the uppermost part of the roy group syntectonic intrusive rocks in the northern abitibi belt range in age from 2701 to 2693 ma these ages are broadly comparable to ages obtained for syntectonic plutons elsewhere in the abitibi belt suggesting that tectonism associated with the kenoran orogeny occurred at approximately the same time throughout

an integrated model for transitional pressure solution in sandstones abstract a key aspect of the deformation of sedimentary rocks during diagenesis is pressure solution however it is often not clear why some rocks of a given mineralogy depth and pressure exhibit a great deal of pressure solution while very similar rocks under similar conditions do not to address such difficulties which we believe arise from the natural non linear physico chemical processes a quantitative model for pressure solution applied to sandstones has been developed this model subsumes various factors 1 pressure temperature and burial rate 2 grain size shape and packing of the mineral grains 3 the variability of water film thickness and coefficient of diffusion between any two grains in contact as a function of stress across the contact pore fluid pressure salinity ph and temperature in this study we distinguish several types of surface sites over quartz grains in a sandstone with corresponding differences in chemical potential thus allowing the diffusive transfer of matter from one site to another the contact between two grains is divided into two parts the actual true contact between two grains is a site of water film diffusion and the inclusions inside the contact the free face inclusion contacts are sites of free face pressure solution on the pore surface site precipitation occurs it is shown that a kinetics transition from a reaction limited deformation to a diffusion limited deformation occurs in the upper crust a geometric transition occurs too due to the textural evolution of the rock with deformation the main goal of this paper is to show the dynamics of the shifting importance of actual and free face inclusion contacts using a unified model including both features the result is a predictive model for porosity variations with depth due to pressure solution in sandstones

crystallization sequences of ca al rich inclusions from allende an experimental study the equilibrium crystallization sequence at 1 atmosphere in air of a melt corresponding in composition to the average composition of type b ca al rich inclusions from the allende meteorite is spinel 1550 c melilite 1400 c ak22 anorthite 1260 c ti al rich clinopyroxene 1230 c ti fassaite the melilite becomes increasingly akermanitic with decreasing temperature the pyroxene is similar in composition to fassaites from type b inclusions preliminary results suggest that the crystallization sequence is similar at oxygen fugacities near the iron wustite buffer the results of these experiments have been integrated with available phase equilibrium data in the system cao mgo al 2o 3 sio 2tio 2 and a phase diagram for predicting the crystallization sequences of liquids with compositions of coarse grained ca al rich inclusions has been developed available bulk compositions of coarse grained inclusions form a well defined trend in terms of major elements extending from type a and bl inclusions near the spinel melilite join to more pyroxene rich type b2 inclusions the trend deviates from the expected sequence of solid condensates from a nebular gas at p 10 3 atm if pure diopside is assumed to be the clinopyroxene that condenses the type a b1 end of the trend is similar in composition to calculated equilibrium condensates at 1202 1227 c and the trend as a whole parallels the sequence of condensates expected from diopside condensation at 1170 c the trend is consistent to first order with the condensation of solid ti rich fassaite in place of pure diopside at higher temperatures than those at which pure diopside is predicted to condense partially molten condensates may be likely in this case or if the nebular pressure is higher than 10 3 atm

geomorphology of the gran sabana guayana shield southeastern venezuela abstract the forms of the gran sabana have been developed mainly on rocks of the gently folded precambrian roraima group a sequence of quartzites and siliceous conglomerates with intercalated jasper chert arkose siltstone and shale locally intruded by diabase dikes and sills two planation surfaces are preserved on the gran sabana the auyan tepui 2000 2900 m elevation and the wonken 900 1200 m surfaces with a speculative mesozoic initial age they are still extending table mountains dominate the landscape they comprise pediments foothills precipitous scarps hundreds of meters high a rim and a flattish summit the table mountain topography is controlled by joint systems water percolates along joints and bedding planes dissolves the siliceous cement the quartzite disaggregates and large blocks are isolated and collapse accumulating at the foot of the scarps and on top of the table mountain summits the percolating water forms large and intricate cave systems which frequently emerge in the scarp zone as high waterfalls these mark the lowest level of erosion of the table mountains karstic structures are prominent in the summit zone the evolution of the table mountain topography has been controlled by a combination of tectonic structural stratigraphic climatic and temporal factors

quartz sandstone peak forest landforms of zhangjiajie geopark northwest hunan province china pattern constraints and comparison the zhangjiajie sandstone peak forest geopark in northwest hunan province china is a comprehensive geopark containing many spectacular quartz sandstone landforms limestone karst landscapes and various other important geoheritage resources it is listed as a unesco world geopark and is also part of the world heritage wulingyuan scenic and historic interest area for its important landscape features many of the sandstone landforms particularly the vast number of thin pillars or spires are very unusual and serve as the core landscapes of the geopark but zhangjiajie displays a diverse range of landform types exhibiting spectacular patterns and regular distributions in this paper the geomorphic traits distribution pattern and constraints of the sandstone landforms of the zhangjiajie geopark are examined our study indicates that in the outcropping areas the sandstones display four distinctive levels from 300 to 1 000 m above sea level and these extend clearly from the highest sandstone plateau platform to the center of the valleys the high sandstone platforms developed close to a flat high level erosional surface and subsequent erosion into this plateau has resulted in successively lower levels of landforms that transition gradually from peak walls peak clusters peak forests and peak pillars to remnant peaks in the lower valley bottoms the form and distribution of the zhangjiajie sandstone landforms are primarily dominated by the geological setting particularly the presence of brittle structures fractures and joint sets trending nnw ene and ne triggered by the episodic tectonic movements major streams and escarpments frequently occur along these structural directions while some of the peak walls peak clusters and peak forests have their longer elongated axes corresponding to ne or nnw directions with an increased density of peak forms at the intersection of these fractures and joints the geometry of the diverse sandstone landforms is also influenced to a certain degree by the climatic water system distribution lithologic properties biological process meteorological features and denudation processes the suite of quartz sandstone landforms in zhangjiajie can be compared with other sandstone landscapes regionally and our interpretation of the sandstone peak forest formation processes offers a significant contribution to the study of topographic features and the geomorphic evolution of sandstone landscapes

u pb zircon geochronology of the selbaie cu zn ag au mine abitibi subprovince canada the selbaie mine is a large tonnage low grade cu zn ag au deposit with both stratiform and epithermal characteristics mineralization comprises lower grade disseminated zn ag ore and pyritic siliceous sinter of the a1 zone at the surface and higher grade cu zn ag au quartz sulfide lenses of the a2 zone below and transitional with the a1 zone and of the b zone 1 km to the west the copper rich quartz sulfide lenses comprise networks of subvertical quartz sulfide veins vein arrays and breccia they have irregular locally gradational boundaries and variable attitudes with dips from 25 o to 90os the mineralization is within a volcanic stratigraphic succession of heterolithic breccias and felsic and intermediate tuffs and flows which are cut by mafic and felsic intrusions and felsite dikes the felsic volcanic rocks and a tonalitic marginal phase from the adjacent brouillan pluton are calc alkalic and have geochemical signatures that support a cogenetic relationship the mineralization and mine stratigraphic succession are truncated by the brouillan faults a series of subparallel west directed predominantly brittle thrusts with the brouillan pluton on the upper plate above the mine stratigraphic succession galenas from stratiform and vein ore have uniform lead isotope compositions of 2o6pb 2o4pb 13 184 to 13 211 2o7pb 2o4pb 14s351 to 14 369 and 2ospb 2o4pb 33s039 to 33 100 with model values of 8 1 8 2 and model k values of 4 1 to 4 2 n 5 among the most primitive of abitibi volcanic associated massive sulfide depositss they have model ages that range from 2 71 to 2 68 ga a u pb zircon age of 2729 ma for a sericite altered calc alkalic quartz porphyry flow 150 m up section from stratiform mineralization closely approximates the age for this mineralization type a u pb zircon age for a felsite dike that cuts postore east west foliation is 2726 3 ma slightly younger but overlapping at the 20 level with the age for the quartz feldspar porphyry flow indicating that the copper rich quartz sulfide lenses are essentially coeval with the lower grade stratiform mineralization a sample of sheared chloritic granodiorite taken from one of the brouillan faults near the margin of the brouillan pluton has a complex zircon opulation six concordant anal ses of abraded multigrain fractions individual grains or grain tips 20 p 206 s

geochemistry of surface and subsurface waters in quartz sandstones significance for the geomorphic evolution of tepui table mountains gran sabana venezuela summary in situ measurements of discharge ph electric conductivity ec temperature and sio2 content have been carried out during five expeditions in the last 20 years on the summit plateaus inside caves and along the rivers of the surrounding lowlands of three tepui massifs in venezuela auyan roraima and chimanta additionally detailed chemical analyses were performed on waters sampled in a newly discovered extensive quartz sandstone cave system on the auyan tepui rock samples of the quartz sandstone bedrock from different locations have been analysed to obtain their chemical composition with a wavelength dispersive x ray fluorescence spectrometer these data show that the majority of silica present in surface and subsurface water comes from dissolution of quartz and only in minor amount from hydrolysis of other silicate minerals probably the presence of a hardened crust of iron hydroxides limits the dissolution of silica on the top surface of tepuis dissolution in the subsurface instead is more significant and causes in the long term the arenisation of the quartz sandstone and its subsequent removal by mechanical erosion on the other hand waters flowing on the arkosic rock outcropping on the lowland below the tepuis obtain their high dissolved silica content mainly from hydrolysis of silicates the morphological evolution of these table mountains appears thus to be controlled mainly by the underground weathering of the quartz sandstone with the opening of deep fractures grietas and the collapse of large underground horizontal cave systems scarp retreat instead seems to be related to the higher weathering rate of the more arkosic formations underlying the quartz sandstones

the effect of surface texture on the determination of the surface hardness of rock using the schmidt hammer the schmidt hammer is employed by geomorphologists as a convenient field instrument for measuring the hardness of rock surfaces field trials indicate that the readings obtained are a function of not only the hardness of the surfaces but also their texture smooth planar surfaces give much higher readings than rough or irregular surfaces this makes interpretation of the results particularly difficult when the hammer is used to measure the hardness of natural rock surfaces that have been roughened by weathering

genesis of giant sinkholes and caves in the quartz sandstone of sarisarinama tepui venezuela abstract quartz sandstone of the sarisarinama massif in venezuela hosts the world biggest collapse dolines in quartz rich lithologies with volumes up to some millions of cubic meters due to extremely complex logistics required to reach the massif the genesis of these depressions and of the underlying caves has never been studied in detail the lack of field campaigns and extended data has fostered a decade long scientific debate on whether their origin was due to epigenic or hypogenic processes this study integrates petrological structural and hydrochemical observations including analyses of silica concentration ph conductivity of surface and cave waters ec to investigate the speleogenetic processes acting underground petrographic and compositional analyses of the host rock mataui formation show that in the sarisarinama region quartz sandstones are regularly characterized by clay interlayers with significant content of pyrophyllite and kaolinite and minor amount of iron hydroxides compared to surface waters subsurface infiltration along vertical fractures and fault planes show enrichment in silica higher ph and lower ec confirming that chemical weathering is effective underground provoking intergranular silica dissolution along structural discontinuities the weathering of the clay and iron hydroxide interlayers guides the speleogenesis weakening specific stratigraphic levels and causing the collapse and fragmentation of the more resistant quartz sandstone strata the initial void created by piping of the loose sand released by quartz sandstone weathering can migrate upwards by means of roof and wall breakdown this chain of events eventually triggers a collapse at the surface which generates a circular or squared sinkhole the weathering acts mainly along the dominant fracture networks showing a clear guidance by regional tectonics these speleogenetic controls rule out the hypothesis of a hypogenic origin of the simas suggesting a primary role of long term epigenic chemical weathering and mechanical erosion guided by joints weak clay and iron hydroxide interlayers followed by subsequent massive collapses

depositional gaps in abitibi greenstone belt stratigraphy a key to exploration for syngenetic mineralization models of greenstone belt development are crucial for exploration allochthonous models predict belts to be a collage of unrelated fragments whereas autochthonous models allow for prediction of syngenetic mineral deposits within specific stratigraphic intervals superior province greenstone belts consist of mainly volcanic units unconformably overlain by largely sedimentary timiskaming style assemblages and field and geochronological data indicate that the abitibi greenstone belt developed autochthonously we describe major revisions to stratigraphy of the abitibi greenstone belt and the implications of an autochthonous development of the volcanic stratigraphy for exploration for syngenetic mineralization the abitibi greenstone belt is subdivided into seven discrete volcanic stratigraphic episodes on the basis of groupings of numerous u pb zircon ages of pre 2750 2750 to 2735 2734 to 2724 2723 to 2720 2719 to 2711 2710 to 2704 and 2704 to 2695 ma we present revised lithotectonic and or stratigraphic nomenclature using these time intervals including 1 isotopic inheritance in younger episodes which indicates that the older episodes 2750 2735 and 2734 2724 ma had greater extent than is presently seen 2 dikes feeding younger volcanic episodes 2706 ma cutting older volcanic units 2734 2724 ma and 3 2710 to 2704 ma mafic to ultramafic sills intruding the 2719 to 2711 ma episode changes to the nomenclature include the identification of pre 2750 ma volcanic episode supracrustal fragments in the northern and southern abitibi greenstone belt and subdivision of the 2719 to 2711 ma 2710 to 2704 ma and 2704 to 2695 ma episodes into lower and upper parts we present the results of this lithostratigraphic subdivision as the first geochronologically constrained stratigraphic and or lithotectonic map of the abitibi greenstone belt many of the volcanic episodes are intercalated with and capped by a relatively thin sedimentary interface zone dominated by chemical sedimentary rocks stratigraphic and geochronological analysis of these zones indicates discontinuous deposition with localized gaps of 2 to 27 m y between volcanic episodes the zones consist of up to 200 m of iron formation chert breccia heterolithic debris flows of volcanic provenance sandstone and or argillite and conglomerate modeling of the time required for deposition of the volcanic units based on rates of magma production in modern arc and plume environments is on the order of 103 to 104 years whereas the time interval between basalt rhyolite cycles is 106 years the sedimentary interface zones are therefore interpreted as condensed sections zones with very low rates of sedimentation in a basinal setting or zones with negligible rates of sedimentation marked by silicification of existing rock types the sedimentary interface zones are therefore considered submarine correlative conformities disconformities or unconformities separating the equivalent of group level stratigraphic and lithotectonic units the unconformity bounded stratigraphic model provides a new regional to deposit scale interpretive model for use in exploration for syngenetic mineralization

crystallization of calcium aluminum rich inclusions experimental studies on the effects of repeated heating events the observed textures and chemistry of ca al rich inclusions cais are presumed to be the culmination of a series of repeated heating and cooling events in the early history of the solar nebula we have examined the effects of these heating cooling cycles experimentally on a bulk composition representing an average type b ca al rich inclusion composition we have tested the effect of the nature of the starting material although the most recent and or highest temperature event prior to incorporation into the parent body dominates the texture and chemistry of the cai prior events also affect the phase compositions and textures we have determined that heating precursor grains to about 1275 c prior to the final melting event increases the likelihood of anorthite crystallization in subsequent higher temperature events and a prior high temperature even that produced dendritic melilite results in melilite that shows evidence of rapid crystallization in subsequent lower temperature events prior low temperature pre crystallization events produce final ran products with pyroxene compositions similar to type b ca al rich inclusions and the glass residual liquid composition is more anorthitic than any other experiments to date the addition of pt powder to the starting material appears to enhance the ability of anorthite to nucleate from this composition

a once molten coarse grained ca rich inclusion in allende a compact spheroidal type b inclusion in allende contains melilite laths that project radially inward from the inclusion edge which show interference growth textures the combined textural and chemical features of this object cannot be explained by independent vapor solid condensation of grains in space followed by random aggregation of these grains into an inclusion rather it probably formed from a once molten droplet that crystallized in response to radiative cooling from its outer surface the crystallization sequence in this and another similar inclusion in which oxygen isotopes have been measured is melilite spinel anorthite fassaite this sequence supports the idea that oxygen isotopic heterogeneities in coarse grained inclusions were formed after complete solidification of these objects by partial exchange with a less16o rich gas and not during or before a melting event

the petrogenesis of type b1 ca al rich inclusions the spinel perspective minor element variations in mgal 2o 4 spinel from the type b1 calcium aluminum rich inclusion cai allende ts 34 confirm earlier studies in showing correlations between the minor element chemistry of spinels with their location within the inclusion and with the chemistry of host silicate phases these correlations result from a combination of crystallization of a liquid produced by re melting event s and local re equilibration during subsolidus reheating the correlation of the ti and v in spinel inclusions with the ti and v in the adjacent host clinopyroxene can be qualitatively explained by spinel and clinopyroxene crystallization prior to melilite following a partial melting event there are however difficulties in quantitative modeling of the observed trends and it is easier to explain the ti correlation in terms of complete re equilibration the correlation of v in spinel inclusions with that in the adjacent host clinopyroxene also cannot be quantitatively modeled by fractional crystallization of the liquid produced by re melting but it can be explained by partial re equilibration the distinct v and ti concentrations in spinel inclusions in melilite from the edge regions of the cai are best explained as being affected by only a minor degree of re equilibration the center melilites and included spinels formed during crystallization of the liquid produced by re melting while the edge melilites and included spinels are primary the oxygen isotope compositions of ts 34 spinels are uniformly 16 o rich regardless of the host silicate phase or its location within the inclusion similar to other type b1 cais clinopyroxene is 16 o rich but melilite is relatively 16 o poor these data require that the oxygen isotope exchange in ts 34 melilite occurred subsequent to the last re melting event

parameters influencing porosity in sandstones a model for sandstone porosity prediction thirteen parameters were reviewed for their influence on porosity in sandstone core material during compaction in basins of average geothermal gradients the first order parameters are age time of burial detrital quartz content maximum depth of burial and sorting overpressure may slow the compaction process depending on onset of overpressure during burial approximately 2 primary porosity may be preserved for every 1 000 psi 6 9 mpa above hydrostatic pressure depth related temperature and median grain size were found to be only second order parameters on the basis of 428 cases function coefficients were calculated for the relationship between porosity and the first order parameters this function permits porosity prediction with a high degree of accuracy th equation is valid for sandstones older than 3 ma with little or no cement no leaching depth of burial in excess of 500 m and little or no shear stresses secondary porosity is excluded and estimations will therefore be conservative where leaching occurred due to interrelationship of compaction and cementation the porosity reduction by cement is usually only a small fraction of the total reduction the function can therefore also be applied to little cemented sandstones

minor element partitioning and sector zoning in synthetic and meteoritic anorthite regular geometric apparently crystallographically controlled sector zoning has been produced in synthetic anorthite crystallized from a liquid of type b ca al rich inclusion cai composition cathodoluminescence cl provides a rapid method for observing the sectors sharp discontinuities in mg concentration always accompany sharp changes in cl intensity at sector boundaries although cl intensity anti correlates with mg concentration there is not a well defined quantitative relationship and mg concentration steps range from as little as 5 to almost a factor of 2 for different cl boundaries when measurable na also tends to anti correlate with cl brightness in a given crystal and thus correlates with mg the partition coefficients for mg and na can vary by up to a factor of 2 depending on the particular sector and thus are not constant for cai anorthite which formed by fractional crystallization in contrast ti either ti 3 or ti 4 concentration is not a function of sector zoning and thus does not affect the cl intensity the sector zoning and correlated chemical variations for synthetic anorthite are similar to those in natural anorthite from type b cais the cl intensity boundary sharpness and minor element zoning in synthetic anorthite are not affected by annealing at 850 c for three weeks overall our observations suggest that the observed cl patterns and minor element zoning in type b anorthite are igneous in origin and have not been modified by subsolidus reequilibration or alteration this leads to the conclusion that the 26 mg 26 al systematics in cais based almost entirely on anorthite have not been modified by subsolidus reequilibration or alteration

evidence of inception horizons in karst conduit networks this paper outlines the conclusion from an analysis of 18 large cave systems around the world comprising more than 1500 km of conduits the 3d geometry of complex cave systems have been analysed in relation to their geological context as well as their hydrogeological boundary conditions the methodology allowed for the first time statistical evidence of the inception horizon concept thereby it confirms that the development of karst conduits under phreatic conditions is strongly related to a restricted number of so called inception horizons an inception horizon is a part of a rock succession that is particularly susceptible to the development of karst conduits because of physical lithological or chemical deviation from the predominant carbonate facies within the limestone sequence it passively or actively favours the localisation of dissolutional voids

zoning patterns of fe and v in spinel from a type b ca al rich inclusion constraints on subsolidus thermal history we obtained two dimensional concentration maps for the minor elements fe and v in 21 spinel crystals in the allende type b1 inclusion ts 34 with a 4 5 m resolution locally high concentrations of fe occur along at least one edge of the spinels and decrease toward the center of the grains enrichment in v can also occur along edges or at corners in general there is no overall correlation of the fe and v distributions but in local regions of two grains the v and fe distributions are correlated strongly suggesting a local source for both elements in these two grains opaque assemblages are present that appear to locally control the v distributions this coupled with previous work suggests that prior to alteration ts 34 contained v rich metal oxidation of this metal during alteration can account for the edge corner v enrichments but provide only minor feo contributions explaining the overall lack of correlation between fe and v most of the feo appears to have been externally introduced along spinel boundaries during alteration these alteration phases served as sources for diffusion of feo into spinel feo distributions in spinel lead to a mean attenuation length of 8 m and using literature diffusion coefficients in isothermal and exponential cooling approximations for peak temperatures in the range 600 700 c this leads to a time scale for calcium aluminum rich inclusion cai alteration in the range of decades to centuries

a model of the early evolution of karst aquifers in limestone in the dimensions of length and depth a new model of the early evolution of limestone karst aquifers in the dimensions of length and depth is presented in its initial state the aquifer consists of a rock massive with evenly spaced fractures of about 50 mm aperture widths with an hydraulic conductivity of 10 27 ms 21 in addition to this a coarser network of prominent fractures with aperture widths of several 100 mm is also present boundary conditions of constant recharge 450 mm year or constant head from the input of allogenic streams are imposed first the position of the water table in the aquifer is calculated then dissolutional widening during a time step in all the fractures below the water table is found by use of the well known nonlinear dissolution kinetics of limestone this is iterated and the position of the water table as well as the fracture widths are found as a function of time in the case of constant recharge to a karst plateau the water table in any case drops to base level and conduits there propagate from the spring headwards if constant head conditions are valid the position of the water table remains almost stable and conduits propagate along the water table from the input towards the spring there is competition between conduit evolution along prominent fractures and along tight fissures close to the water table in any case under constant head conditions one of these pathways wins and early karst evolution is terminated by a breakthrough event with an explosive increase of the flow through the aquifer until constant head conditions break down depending on the boundary conditions of constant head or constant recharge or a combination of both it is possible to describe models of cave genesis which have been derived from field evidence such as the water table models of swinnerton geol soc am bull 34 1932 662 and rhoades and sinacori j geol 49 1941 785 as well as the four state model by ford and ewers can j earth sci 15 1978 1783 q 2001 elsevier science b v all rights reserved

the origin and significance of reverse zoning in melilite from allende type b inclusions in many type b allende inclusions melilite is reversely zoned over restricted portions of each crystal textural relationships and the results of dynamic crystallization experiments suggest that the reverselyzoned intervals in these type melilites result from the co precipitation of melilite with clinopyroxene from a melt prior to the onset of anorthite precipitation when clinopyroxene begins to precipitate the al mg ratio of the melt rises causing the crystallizing melilite to become more gehlenitic an effect which is negated by crystallization of anorthite because the equilibrium crystallization sequence in these liquids is anorthite before pyroxene melilite reverse zoning can occur only when anorthite nucleation is suppressed relative to pyroxene this has been achieved in our experiments at cooling rates as low as 0 5 c hour our experiments further indicate however that reverse zoning does not form at cooling rates 50 c hour probably because the clinopyroxene becomes too al rich to drive up the al mg ratio of the liquid type inclusions with reversely zoned melilites must have cooled at rates greater than those at which anorthite begins to crystallize before clinopyroxene but 50 c hour such rates are far too slow for the type droplets to have cooled by radiation into a nebular gas but are much faster than the cooling rate of the solar nebula itself one possibility is that type b s formed in local hot regions within the nebula where their cooling rate was equal to that of their surrounding gas other possibilities are that their cooling rates reflect their movement along nebular temperature gradients or the influence of a heat source the sun or viscous drag on inclusions as they moved through the nebular gas are potential candidates for such heat sources