multiple fluid flow events and the formation of saddle dolomite case studies from the middle devonian of the western canada sedimentary basin abstract in western canada sedimentary basin wcsb large scale and focused fluid flow that caused hydrothermal dolomitization have been suggested with different timing and intensity in this study we conducted a petrographic and geochemical comparison between the middle devonian sulphur point and slave point carbonates from northwestern alberta the results demonstrate the presence of both an early fluid flow event associated with the late devonian to mississippian antler orogeny as well as a later event coincident with the late cretaceous to early tertiary laramide orogeny early fluid flow event is characterized by high salinity fluids and high temperatures and oxygen isotopic values of marine or slightly enriched values as demonstrated in saddle dolomite from the slave point formation in contrast later fluids that caused the precipitation of saddle dolomite in the sulphur point formation are characterized by having slightly saline values comparable homogenization temperatures but more enriched 18 o values and slightly depleted 13 c signatures geochemical data indicate that this later hydrothermal fluid was mixture of middle devonian brines and radiogenic basement fluids

in vitro growth of iron sulphide chimneys possible culture chambers for origin of life experiments iron monosulphide globules and tubes grown in the laboratory have similar morphologies to the fossil pyrite botryoids and chimneys found in the silvermines exhalative sedimentary ore body of carboniferous age in ireland we envisage analogous fine structures growing at hot springs 100 200 c in the earliest oceans as having provided the culture chambers and flow reactors for life to originate by phosphorylation and growth of organic molecules on the iron sulphide surfaces such sulphide structures grown in the laboratory could be used in origin of life experiments

petroleum geology of the puguang sour gas field in the sichuan basin sw china abstract the puguang sour gas field in northeastern sichuan basin is the largest gas discovery in the paleozoic marine strata in china with at least 9 trillion cubic feet tcf of proven gas reserves it is one of the many anticlinal structures formed during the indo sinian orogeny with gas being trapped in a composite structure lithology pool the presence of the upper permian coal measures lower silurian and lower cambrian marine shales provided excellent hydrocarbon source rocks the lower triassic and upper permian oolitic dolomites serve as high quality reservoir rocks with 10 25 porosity and 1 1000 md permeability largely due to diagenetically produced secondary porosity the lower and middle triassic evaporate beds have acted as the direct top seal for the large lower triassic upper permian gas accumulation the thick upper triassic and jurassic mudstone beds regionally cap the underlying marine carbonate strata the deep seated basement faults acted as vertical hydrocarbon migration conduits during faulting and became effective flow barriers when the faulting ceased the deep burial of a paleo oil accumulation in reservoirs rich in sulfate resulted in extensive thermochemical sulfate reduction tsr this together with the new gas injection and structural reconfiguration transformed a paleo oil pool to a large sour gas pool

sandstone vs carbonate petroleum reservoirs a global perspective on porosity depth and porosity permeability relationships plots are presented comparing average porosity vs depth for 30 122 siliciclastic petroleum reservoirs and 10 481 carbonate petroleum reservoirs covering all petroleum producing countries except canada however separate plots cover 5534 siliciclastic and 2830 carbonate reservoirs of the alberta basin in canada average permeability vs average porosity is shown for the non canadian reservoirs key similarities and differences between sandstones and carbonates are noted and implications are discussed regarding the dominant factors controlling reservoir quality in each lithology trends of steadily decreasing median and maximum porosity with increasing depth reflect burial diagenetic porosity loss in response to increasing thermal exposure with depth these trends seem inconsistent with the suggestions that both sandstones and carbonates commonly increase in porosity by dissolution during deeper burial carbonate reservoirs have lower values of median and maximum porosity for a given burial depth probably because of greater chemical reactivity of carbonate minerals relative to quartz and the resulting lower resistance to chemical compaction and associated cementation relative paucity of low porosity 0 8 siliciclastic reservoirs at all depths compared with carbonates may reflect the more common occurrence of fractures in carbonates and the effectiveness of these fractures for facilitating economic flow rates in low porosity rock overall carbonate reservoirs do not have lower permeability for a given porosity compared with sandstones but do have strikingly lower proportions of both high porosity and high permeability values the data presented can serve as a general guide for the distribution of reservoir quality that can reasonably be expected in exploration wells drilled to any given depth in the absence of detailed geologic information such as burial and thermal history

magnesium isotopic composition of the deep continental crust to constrain the behavior of mg isotopes during deep crustal processes and the mg isotopic composition of the middle and lower continental crust 30 composite samples from high grade metamorphic terranes and 18 granulite xenoliths were investigated the composites derive from eight different high grade metamorphic terranes in the two largest archean cratons of china including 13 ttg gneisses 5 amphibolites 4 felsic 4 intermediate and 4 mafic granulites they have variable bulk compositions with sio 2 ranging from 45 7 to 72 5 representative of the middle crust beneath eastern china the 26 mg values of these samples vary from 0 40 to 0 12 reflecting heterogeneity of their protoliths which could involve upper crustal sediments the granulite xenoliths from the cenozoic hannuoba basalts also have a diversity of compositions with mgo ranging from 2 95 to 20 2 these xenoliths equilibrated under high temperatures of 800 950 c corresponding to depths of the lower continental crust 30 km they yield a large 26 mg variation of 0 76 to 0 24 the light mg isotopic compositions likely result from interactions with isotopically light metamorphic fluids probably carbonate fluids together with previously reported data the average 26 mg values of the middle and lower continental crusts are estimated to be 0 21 0 07 and 0 26 0 06 respectively the bulk continental crust is estimated to have an average 26 mg of 0 24 0 07 which is similar to the average of the mantle the large mg isotopic variation in the continental crust reflects the combination of several processes such as continental weathering involvement of supracrustal materials in the deep crust and fluid metasomatism

extraterrestrial iridium sediment accumulation and the habitability of the early earth s surface we have measured the concentrations of iridium and platinum in ancient metasediments from akilia island southern west greenland to obtain the first direct constraints on the extraterrestrial mass flux at the earth s surface before 3 8 gyr the lunar impact record indicates this was a time of heavy bombardment the average lunar cratering rate between 4 15 and 3 8 gyr was elevated 150 times the modern value hence heightened ir and pt abundances are possible in sediments from this time however the observed concentrations of ir and pt in the akilia metasediments are extremely low in most samples ir 3 ppt and pt 20 ppt in contrast the average modern crustal abundances of ir and pt are 50 ppt and 2 ppb respectively the very low concentrations observed in the akilia metasediments cherts and banded iron formations can be reconciled with the lunar record by quantifying the effects of sedimentation rate on the efficiency with which the extraterrestrial mass flux is sampled the observations are explained if rapidly accumulating sediments sampled stochastic bombardment by an impactor population governed by a power law mass distribution such that exogenous ir and pt are concentrated in rare horizons the required sedimentation rates are geologically plausible this explanation has two consequences first evidence of massive impact events may be rare in sediments older than 3 8 gyr despite heavy bombardment second conditions at the earth s surface may have been conducive for extensive accumulation of sedimentary sequences and for microbial life during most of the period 4 15 3 8 gyr it is possible that the earth s surface hosted microbial habitats during most of this time if microbes living in deep sea or deep crustal environments survived photic zone vaporizing impacts that would have occurred every 30 myr hence evidence of life is expected even in the earliest sedimentary sequences

construction of the cambrian and ordovician seawater 87sr 86sr curve abstract the estimated path of seawater 87 sr 86 sr during the cambrian and ordovician is based on 172 analyses the sample suite is largely from locations in oklahoma new york tennessee and texas where rocks of this age are widely exposed and well studied the oldest reliable samples examined in this study are of middle cambrian age and yield an 87 sr 86 sr near 0 70888 sw 19 the estimated ratio rises to near 0 70915 sw 8 in the lower upper cambrian the highest value during the phanerozoic and probably for all geologic time the estimated path drifts lower to near 0 70903 sw 4 at the cambrian ordovician boundary this further declines to near 0 70875 sw 32 at the end of the ibexian and reaches a value near 0 70821 sw 86 at the whiterockian mohawkian boundary the decline in the seawater 87 sr 86 sr continues into later mohawkian time to flatten abruptly near 0 70782 sw 125 this ratio is essentially constant through the latest ordovician the curve rises near to a ordovician silurian boundary value poorly constrained from our data near 0 70784 sw 123 the early paleozoic seawater curve shows the alternating dominance of continental and oceanic sources of sr continental sr dominated seawater from the beginning of the cambrian into late cambrian from that time as the cambrian transgression proceeded and radiogenic crystalline rocks were covered oceanic sr began to dominate sr from low ratio oceanic sources dominated through early mohawkian time an abrupt balance is struck between oceanic and continental sources to end the decline in the late ordovician and maintained until near the end of the ordovician the general curve shape follows the pattern suggested by proposed sealevel changes the rise and fall of seawater 87 sr 86 sr during the cambrian and ordovician characterized seawater variation during all the paleozoic

compositional variations of olivine from the jinchuan ni cu sulfide deposit western china implications for ore genesis the jinchuan ni cu sulfide deposit is hosted by an elongated olivine rich ultramafic body that is divided by subvertical strike slip faults into three segments central eastern and western the central segment is characterized by concentric enrichments of cumulus olivine crystals and interstitial sulfides pyrrhotite pentlandite chalcopyrite intergrowth whereas the eastern and western segments are characterized by an increase of sulfides toward the lower contacts in all segments sulfides are concentrated at the expense of intercumulus silicates olivine re crystallization is found to be associated with actinolite alteration in some samples the compositional variations of primary olivine from the sulfide poor samples can be explained by a small degree of olivine crystallization 50 modal and sulfide averaging 5 wt in the rocks are consistent with the interpretation that the jinchuan deposit was formed by olivine and sulfide laden magma successively ascending through a conduit to a higher now eroded level sulfide enrichment toward the center in the central segment and toward the lower contact in the eastern and western segments may have in part resulted from flow differentiation and gravitational settling during magma ascent respectively

submarine hot springs and the origin of life the discovery of hydrothermal vents at oceanic ridge crests and the appreciation of their importance in the element balance of the oceans is one of the main recent advances in marine geochemistry1 it is likely that vents were present in the oceans of the primitive earth because the process of hydrothermal circulation probably began early in the earth s history2 here we examine the popular hypothesis3 7 that life arose in these vents this proposal however is based on a number of misunderstandings concerning the organic chemistry involved an example is the suggestion that organic compounds were destroyed on the surface of the early earth by the impact of asteroids and comets but at the same time assuming that organic syntheses can occur in hydrothermal vents7 the high temperatures in the vents would not allow synthesis of organic compounds but would decompose them unless the exposure time at vent temperatures was short8 11 even if the essential organic molecules were available in the hot hydrothermal waters the subsequent steps of polymerization and the conversion of these polymers into the first organisms would not occur as the vent waters were quenched to the colder temperatures of the primitive oceans

hydrothermal petroleum genesis migration and deposition in guaymas basin gulf of california hydrothermal activity and seabed mounds have been explored in guaymas basin by the deep sea drilling project dsdp piston coring dredging and diving with the deep submersible research vessel dsrv alvin sedimentary organic matter derived primarily from immature degraded microbial detritus is easily converted to petroleum under the hydrothermal regime these petroleums are mature and migrate in the fluids and by diffusion to the seabed the fluid migration is aided by near critical aqueous solution and supercritical carbon dioxide and hydrocarbons petroleum compositions vary from condensates to naphthenic to waxy all with significant amounts of asphaltenes and hydrothermal products such as olefins and toxic polynuclear aromatic hydrocarbons pah the heavy ends condense at the seabed depositing mainly as a cement with the sulfides and other minerals and to a lesser extent as entrapped oil and crystalline wax in vugs and conduits of the mounds the pah are high temperature resynthesis aromatiza

multiple s isotopes zircon hf isotopes whole rock sr nd isotopes and spatial variations of pge tenors in the jinchuan ni cu pge deposit nw china previous geochemical data for the jinchuan ni cu platinum group elements pge deposit the single largest magmatic sulfide deposit in the world are derived primarily from the upper parts of the deposit this paper reports new pge and s hf sr nd isotope data for the lower parts of the deposit that have become accessible for sampling by ongoing underground mining activity new pge data from this study together with previous results indicate that pge tenors in the bulk sulfide ores of the deposit increase eastward except for two fault offset ore zones which occur together within the western part of the deposit generally these two ore zones show depletions in ipge ir ru rh but not in ppge pt pd and cu and more fractionated olivine and cr spinel compositions than the rest of the deposit these differences can be explained by a more evolved parental magma for the ipge depleted ore zones the eastward increase of pge tenors in the rest of the deposit can be explained by upgrading of preexisting sulfide liquid in a subhorizontal conduit by a new surge of magma moving through the conduit from west to east which took place before the formation of the ipge depleted ore zones the jinchuan ultramafic rocks are characterized by elevated initial 87sr 86sr ratios from 0 7077 to 0 7093 negative e nd values from 9 2 to 10 5 and zircon e hf values from 4 to 7 these data indicate up to 20 of crustal contamination in the jinchuan magma four of nine multiple sulfur isotope analyses for the jinchuan deposit show anomalous 33s values varying from 0 12 to 2 67 these results together with elevated 34s values 2 for some of the samples analyzed previously by other researchers indicate the involvement of external sulfur from archean and proterozoic sedimentary rocks modeling results based on our olivine data and magma compositions estimated previously by other researchers indicate that fractional crystallization did not play a major role in triggering sulfide saturation in the jinchuan magma and that the parental magma experienced previous sulfide segregation during ascent or in staging chambers

origin of the inconsistent apparent re os ages of the jinchuan ni cu sulfide ore deposit china post segregation diffusion of os abstract apparent re os ages of some magmatic sulfide ore deposits are older than the zircon and baddeleyite u pb ages which are interpreted as the formation age of the host intrusions the jinchuan ni cu pge deposit of china the world s third largest is such a case we report apparent re os isochron ages of 1117 67 ma 1074 120 ma and 867 75 ma with initial 187os 188os ratios of 0 120 0 012 0 162 0 017 and 0 235 0 027 for disseminated ores sulfides from the disseminated ores and massive ores from jinchuan respectively using these data and re os ages from the literature we find that the oldest apparent re os age and lowest initial os isotope ratio are from disseminated ores which contain small amounts of sulfide minerals the highest initial os isotope ratios and youngest apparent re os ages consistent with the zircon and baddeleyite u pb ages are from massive ores containing 90 100 modal sulfide and net textured ores with about 25 modal sulfides yield apparent re os ages and initial os ratios intermediate between those of the disseminated and massive ores because os diffusion between sulfides is inhibited by the intervening silicates even at high temperatures re equilibration did not occur in the disseminated ore and the samples retained the os ratios of the contaminated magma leading to geologically meaningless ages that are older than the formation age of the rocks while os bearing sulfide minerals and magnetite show low closure temperatures of os diffusion and the sulfide minerals in the massive ore are closely connected with each other facilitating fast diffusion of os re equilibration of os was achieved during cooling of the ore from about 850 c after the segregation to about 400 c thus an age corresponding to the formation time and an elevated initial os ratio were yielded by the massive ore os isotopes in the net textured ore behave in the way intermediate between the disseminated and massive ores pb isotope data support the os results disseminated ores have heterogeneous pb isotope ratios whereas pb in the massive ores is more uniform consistent with pb isotopic equilibration in the massive ores but not in the disseminated ores

flow rates and reaction rates in the galapagos rise spreading center hydrothermal system as inferred from 228ra 226ra in vesicomyid clam shells abstract the 228ra 226ra ratios in a previously dated vesicomyid clam shell were used to determine that seawater was in contact with mid oceanic ridge basalt glass for 22 45 years prior to arrival to the surface at 350 c at the galapagos rise spreading center the minimum rate of reaction for the 45 year sojourn time based on a water rock ratio of 2 8 derived from 226ra concentrations is 8 g of basalt altered per kg of seawater per year

structural lithological and geochemical constraints on the dynamic magma plumbing system of the jinchuan ni cu sulfide deposit nw china eastern and western portions of the jinchuan ultramafic intrusion have previously been interpreted as dismembered segments of a single elongate intrusion by late faults however the different stratigraphic sequences of the two portions indicate that they are originally two separate intrusions referred to as eastern and western intrusions in this study the eastern intrusion is characterized by a concentric distribution of rock types with a core of sulfide dunite enveloped by lherzolite whereas the western intrusion is composed of the upper and lower units interpreted as magmatic mega cycles with regular variations in lithology and chemistry in the western intrusion the upper unit consists of fine grained dunite lherzolite and pyroxenite from its base to its top the mgo contents decrease upward from the dunites 42 45 wt to the lherzolites 36 41 wt while al2o3 and incompatible elements increase upward in contrast the lower unit consists of coarse grained dunites and lherzolites containing 37 40 and 28 35 wt mgo respectively sharp contacts between the upper and lower units and fine grained dunite xenoliths at the top of the lower unit indicate that the lower unit intruded along the base of the upper unit disseminated and net textured sulfides primarily occur in the lower unit and comprise the no 24 ore body very low s contents 100 ppm of the wall rocks at jinchuan indicate that they were not the source of s causing sulfide immiscibility sulfide segregation more likely occurred in deep seated magma chambers and sulfides were deposited in the western intrusion when sulfide bearing magmas passed through the intrusion in contrast the eastern intrusion was formed by injections of sulfide free and sulfide bearing olivine crystal mushes respectively from another deep seated staging magma chamber the eastern and western intrusions and the deep seated magma chambers comprise a complicated magma plumbing system at jinchuan normal faults played a significant role in the formation of the magma plumbing system and provided pathways for the magmas

organic geochemistry of the 9 6 km bertha rogers no 1 well oklahoma abstract organic geochemical analyses of fine grained rocks from the 9 590 km bertha rogers no 1 well have been carried out total organic carbon soxhlet extraction and silica gel chromatography c15 saturated and aromatic hydrocarbon gas chromatography and mass spectrometry pyrolysis kerogen analysis x ray diffraction and visual kerogen analysis rocks ranged in age from permian to ordovician the well has an estimated bottom hole temperature of 225 c some data from this study are inconsistent with conventional theories concerning the generation and thermal destruction of hydrocarbons for example appreciable amounts of c15 gas condensate like hydrocarbons are present in very old rocks currently at temperatures where current theory predicts that only methane and graphite should remain also substantial amounts of pyrolyzable c15 hydrocarbons remain on the kerogen in these deeply buried paleozoic rocks this suggests at least in somes cases that temperatures much higher than those predicted by current theory are required for generation and thermal destruction of hydrocarbons the data from this well also suggest that original composition of organic matter and environment of deposition may have a much stronger influence on the organic geochemical characteristics of fine grained sediments than has previously been ascribed to them the results from this well from other deep hot wells in which temperatures exceed 200 c and from laboratory experiments suggest that some of the basic concepts of the generation and maturation of petroleum hydrocarbons may be in error and perhaps should be reexamined

mesogenetic dissolution of the middle ordovician limestone in the tahe oilfield of tarim basin nw china abstract the mesogenetic dissolution is well developed in the middle ordovician yijianfang formation o2yj limestone and the dissolution pores are very important for petroleum accumulation in the south slope area of the tahe oilfield which lies in the north of the tarim basin northwestern china mottled dotted or laminar dissolution can be observed in the o2yj limestone under microscope the grains lime matrix and all stages of calcite cements including oil inclusion bearing blocky calcite cements can all be found dissolved ubiquitously the stylolites in the limestone were enlarged and rounded because of dissolution some dolomite rhombs precipitated along stylolites in burial environment were found dissolved as well the dissolution of the blocky calcite cements and dolomite rhombs and the enlarging of stylolites demonstrate that the dissolution took place in the mesogenetic environment concentration of trace elements including rees of the eroded part of the o2yj limestone is intermediate between that of the uneroded part and that of the underlying lower ordovician limestone hydrocarbon source rocks both 13cpdb and 18opdb values of the eroded part are less than those of the uneroded part respectively the geochemical characteristics indicate that the eroding fluids are hydrocarbon bearing fluids coming from the underlying hydrocarbon source rocks

seismic stratigraphy and geological evolution of the cenozoic cool water eucla platform great australian bight seismic images of the continental margin in the western great australian bight reveal the internal anatomy of a long lived cenozoic cool water carbonate shelf the cenozoic succession is divisible into seven seismic sequences that reflect four depositional phases phase a paleocene middle eocene is a progradational siliciclastic wedge deposited in a depositional sag and represents lowstand and transgressive sedimentation phase b middle eocene earliest middle miocene contains cool water ramp carbonates with biogenic mounds eocene oligocene overlain by a warm water flat topped platform rimmed by the early middle miocene little barrier reef coeval deep water carbonate deposition formed a multi lobed sediment apron this phase was terminated by gentle uplift and tilting throughout southern australia in the late middle miocene phase c late miocene early pliocene represents cool water lowstand wedge and ramp deposition and contains numerous biogenic mounds in the youngest sequence this phase is terminated by an unconformity attributed to marine erosion phase d pliocene quaternary is a thick succession of cool water carbonates with spectacular clinoform ramp geometry that forms most of the modern outer shelf and contains large deep water biogenic mounds this platform the first large cool water carbonate shelf imaged by high quality seismic reflection data demonstrates the interaction between regional tectonic and local and global paleo oceanographic processes as australia drifted northward during the cenozoic the great australian bight moved from high to middle latitudes and the regional oceanographic regime remained cool water largely because of coeval development of the antarctic circumpolar current in the evolving southern ocean short episodes of warm water deposition probably reflect incursions of a proto leeuwin current from the indian ocean whereas growth of the miocene coral algal little barrier reef resulted from a short term global increase in sea surface temperatures this platform dominated by stacked carbonate ramps is most similar to the west florida shelf but contains many more biogenic mounds the western great australian bight carbonate platform is an excellent modern analog for the mesoscale structure of cool water platforms in the older geologic record

dolomite reservoirs porosity evolution and reservoir characteristics systematic analyses of the published record of dolomite reservoirs worldwide reveal that the majority of hydrocarbon producing dolomite reservoirs occurs in 1 peritidal dominated carbonate 2 subtidal carbonate associated with evaporitic tidal flat lagoon 3 subtidal carbonate associated with basinal evaporite and 4 nonevaporitic carbonate sequence associated with topographic high unconformity platform margin buildup or fault fracture reservoir characteristics vary greatly from one dolomite type to another depending upon the original sediment fabric the mechanism by which dolomite was formed and the extent to which early formed dolomite was modified by postdolomitization diagenetic processes e g karstification fracturing and burial corrosion this paper discusses the origin of dolomite porosity and demonstrates the porosity evolution and reservoir characteristics of different dolomite types

drilling the solid earth global geodynamic cycles and earth evolution the physical and chemical evolution of the earth is driven by geodynamic cycles that are global in scale operating over 4 57 ga of earth s history some processes are truly cyclic e g the wilson cycle while others are irreversible e g core formation heat and mass transfer between the lowermost mantle e g core mantle boundary and the surface drives these global geodynamic processes subduction of lithospheric plates transfers cool fractionated material into the lower mantle and leads indirectly to the formation of new oceanic lithosphere while the rise of thermochemical plumes recycles the remnants of these plates back to the surface driven by heat transfer across the core mantle boundary these global geodynamic cycles are responsible for hotspot volcanism the formation of continental crust collisional orogenies continental rifting subduction zone processes arcs accretionary prisms and ore deposits each of these presents opportunities for investigation by continental scientific drilling in addition these cycles affect other processes that are targets of continental scientific drilling the origin and evolution of life and an oxygenated atmosphere the impact of large volcanic eruptions on climate and geological hazards such as earthquakes and volcanic eruptions in this paper we present the scientific rationale for continental scientific drilling to study global geodynamic processes review past successes in this realm that were sponsored in part by icdp and suggest potential new targets for drilling campaigns that focus on solid earth evolution this paper builds on discussions at the 2013 icdp science meeting on the future of continental scientific drilling held in potsdam in november 2013

simple kinetic models of petroleum formation part ii oil gas cracking kinetic schemes describing oil gas cracking are a required component of models which predict the masses compositions and phases of petroleum expelled from source rocks an additional application is in predicting the temperature zone over which reservoired oil deposits will be degraded to gas this simple first order kinetic model calibrated on closed system laboratory pyrolysis of 16 different source rock samples describes the bulk conversion of oil c6 molecular range to gas c1 5 molecular range cracking rates vary markedly between samples depending on the saturate to aromatic ratio of the generated oil the initial hydrogen index hl0 of the sample taken as an indirect measure of the saturate to aromatic ratio of the oil it generates correlates strongly with optimized rate constants for oil cracking in individual samples thus cracking rates are predictable using a simple routinely performed geochemical screening measurement source rocks with high hl0 tend to generate saturate rich oils with a high mean activation energy and a tight distribution of bond energies which crack relatively slowly over a high but relatively narrow temperature range source rocks with low hl0 generate aromatic rich oils with a low mean activation energy and a broad distribution of bond energies which crack relatively rapidly over a lower but relatively wide temperature range this is an important contributing factor in the observed gas proneness of type iii source rocks with low hl0 the projected cracking window defined limits at 10 90 of initial oil degraded reference heating rate 2 c ma 1 varies from 155 to 205 c in very high quality source rocks hl0 600 mg g c 1 to 115 205 c in very poor quality source rocks hl0 50 mg g c 1 an order of magnitude increase decrease in heating rate elevates depresses these temperature windows by ca 15 c the extrapolations to the geological subsurface are subject to a confidence limit no better than 7 c extrapolation to the reservoir environment requires caution because kerogen an important potential catalyst and hydrogen donor allowing 100 cracking efficiency in the source rock is absent furthermore fractionation during expulsion causes saturate enrichment of expelled ultimately reservoired oils thus in reservoir cracking should be at least as slow as cracking in good quality source rocks and coke or pyrobitumen formation is needed to conserve the hydrogen balance a case history based on deep petroleum pools of the uk central north sea confirms these projections demonstrating no positive evidence of significant in reservoir cracking at temperatures as least as high as 174 c and perhaps even 195 c previous studies have overestimated the importance of in reservoir cracking future studies aiming to predict petroleum composition in traps should concentrate on understanding the charge composition expelled from the kitchen

carbonate porosity creation by mesogenetic dissolution reality or illusion many authors have proposed that significant volumes of porosity are created by deep burial dissolution in carbonate reservoirs we argue however that this model is unsupported by empirical data and violates important chemical constraints on mass transport because of the ubiquitous presence and rapid kinetics of dissolution of carbonate minerals the mesogenetic pore waters in sedimentary basins can be expected to be always saturated and buffered by carbonates providing little opportunity for the preservation of significantly undersaturated water chemistry during upward flow even if the initial generation of such undersaturated pore water could occur a review of the literature where this model has been advanced reveals a consistent lack of quantitative treatment in consequence the presumption of mesogenetic dissolution producing a net increase in secondary porosity should not be used in the prediction of carbonate reservoir quality

petroleum reservoir porosity versus depth influence of geological age average porosity values for the producing zones of oil and gas fields worldwide are examined as a function of the present depth for sandstone and carbonate lithologies divided into 10 groupings by reservoir depositional age precambrian silurian to pliocene pleistocene the wide variations in average reservoir porosity within each depth range reflect the extreme ranges in porosity controlling factors such as depositional facies early

sulfur isotope variations within the platreef ni cu pge deposit genetic implications for the origin of sulfide mineralization the platreef the world s third largest platinum group element pge deposit is a 10 400 m thick pyroxenitic unit at the base of the northern limb of the mafic bushveld complex hosting pge mineralization in association with base metal sulfides the sulfide mineralization is thought to have been either largely magmatic in origin with contamination by assimilation of local floor rocks considered an ore modifying process or a product of assimilation of country rock s and silicic contamination which caused s saturation in the magma with the contamination being fundamental to the ore forming process we have performed an extensive and detailed sulfur isotope study of the platreef which indicates that magmatic signatures 34s 0 to 2 are preserved in early formed sulfide droplets within the platreef pyroxenites in the area from sandsloot to witrivier these values are comparable to sulfide inclusions in diamonds in the nearby klipspringer kimberlite and are considered to have a primary mantle origin there is no indication of any significant external s in any of the primary sulfides in the sandsloot witrivier area however later sulfides found in calc silicate floor rocks at sandsloot and zwartfontein and in xenoliths of calc silicate throughout the section indicate an input of country rock s anhydrite bearing horizons in the malmani dolomites may have exchanged s with magmatic platreef sulfide during platreef intrusion imparting a heavier s isotope signature in the fluid affected rocks than those in the early sulfides the archean basement although containing minor amounts of sulfide with a negative 34s value is not a significant contributor to the s budget of the platreef previous studies have indicated that in areas where the sedimentary floor rocks contain appreciable sulfides rather than sulfates such as at turfspruit the platreef sulfides are extensively contaminated with country rock s assimilation of pyrite bearing shales in the turfspruit area has locally upgraded the s content of the platreef and given the basal platreef sulfides a heavier isotopic signature sulfur saturation in the platreef magma took place before contamination probably in a staging chamber prior to intrusion a major pulse of magma entrained the preformed pge rich sulfides and was injected to form the platreef where assimilation of country rock sulfides upgraded the s content on a strictly local scale and hydrothermal leaching introduced s from country rock sulfates into later stage sulfides again on a local scale south of zwartfontein it is proposed that the platreef was intruded into sediments of the transvaal supergroup which formed the floor and the roof of the platreef north of zwartfontein the platreef intruded the boundary between the malmani supergroup sediments and the archean basement this could have potentially mineralized the roof calc silicates north of zwartfontein in a manner similar to the floor in the sandsloot area the later intrusion of the main zone magma formed a magmatic unconformity on top of the cold platreef and entrained large rafts of mineralized calc silicate for tens of kilometers north of the last footwall outcrop of calc silicate

magnesium isotopic evidence for chemical disequilibrium among cumulus minerals in layered mafic intrusion abstract magnesium isotopic compositions of olivine clinopyroxene and ilmenite from the baima intrusion sw china for the first time are investigated to constrain the magnitude and mechanisms of mg isotope fractionation among cumulus minerals in layered mafic intrusions and to evaluate their geological implications olivine and clinopyroxene have limited mg isotope variations with 26 mg ranging from 0 33 to 0 05 and from 0 29 to 0 13 respectively similar to those of mantle xenolithic peridotites by contrast ilmenites display extremely large mg isotopic variation with 26 mg ranging from 0 50 to 1 90 the large inter mineral fractionations of mg isotopes between ilmenite and silicates may reflect both equilibrium and kinetic processes a few ilmenites have lighter mg isotopic compositions than coexisting silicates and contain high mgo contents without compositional zoning indicating equilibrium fractionation the implication is that the light mg isotopic compositions of lunar high ti basalts may result from an isotopically light source enriched in cumulate ilmenites on the other hand most ilmenites have heavy mg isotopic compositions coupled with high mgo concentration and chemical zoning which can be quantitatively modeled by kinetic mg isotope fractionations induced by subsolidus mg fe exchange between ilmenite and ferromagnesian silicates during the cooling of the baima intrusion the extensive occurrence of kinetic mg isotope fractionation in ilmenites implies the possibility of widespread compositional disequilibrium among igneous minerals in magma chambers consequently disequilibrium effects need to be considered in studies of basaltic magma evolution magma chamber processes and magmatic fe ti oxide ore genesis

formation of the jinchuan ultramafic intrusion and the world s third largest ni cu sulfide deposit associated with the 825 ma south china mantle plume we report here shrimp u pb geochronological geochemical and nd isotopic data for the jinchuan ultramafic intrusion gansu province china which hosts the world s third largest magmatic ni cu sulfide deposits u pb baddeleyite analyses yield an age of 812 26 ma for the ultramafic intrusion this age is indistinguishable within analytical uncertainties from the u pb zircon ages of 827 8 ma and 828 3 ma for the sulfide bearing ultramafic rocks and the dolerite dykes that cut the ultramafic intrusion respectively these u pb dating results show beyond doubt that the jinchuan ultramafic intrusion and associated ni cu sulfide deposit were formed at 825 ma rather than 1500 ma as has been widely believed the ultramafic rocks exhibit large negative nd t values 8 9 to 12 0 that decrease with increasing la sm suggesting that their parental magmas were derived from a long term enriched lithospheric mantle and experienced crustal contamination mineralogical petrological and geochemical data all indicate that the jinchuan intrusion was generated by melting of the enriched lithospheric mantle heated by an anomalously hot plume the u pb ages of 825 ma for igneous baddeleyites and zircons and 900 880 ma for inherited zircons in the jinchuan mafic ultramafic rocks are comparable with those in the qaidam block and qilian belt the western extension of the qinling belt that was likely derived from northern yangtze craton the jinchuan ni cu sulfide bearing intrusion along with coeval regional plume related mafic dykes and tholeiites and mafic ultramafic complexes with associated v ti and ni cu pge mineralization is interpreted to be genetically related to the 825 ma south china mantle plume

field relationships and geochemical constraints on the emplacement of the jinchuan intrusion and its ni cu pge sulfide deposit gansu china field mapping and petrological geochemical investigation of the jinchuan intrusion in north central china clarifies how the intrusion was emplaced and provides a new model that explains how its large and rich ni cuplatinoid deposits may have formed the intrusion was emplaced into high grade gneisses and marbles along a disconformity at the base of an overlying cover sequence indicating that it was emplaced as a sill not a nearvertical dike as previously proposed after emplacement the intrusion was rotated to its present orientation and deformed and metamorphosed under greenschist facies conditions relative enrichment of incompatible trace elements coupled with negative u th and nb ta anomalies in all samples from the intrusion provide evidence that the parental magma assimilated granitoid rocks in the lower crust the presence of abundant marble xenoliths now decarbonatized to diopside rich skarns and chemical indices such as high cao sio2 indicate that the magma assimilated carbonate on reaching its present site this contamination may be linked to the formation of the ni cu platinoid ores we propose that the assimilation of carbonate rich fluids increased the oxygen fugacity of the magma and led to the segregation of metal rich sulfides

mg sr nd isotopic constraints on the genesis of the giant jinchuan ni cu pge sulfide deposit nw china abstract magmatic sulfide deposits are principal resources of nickel and platinum group elements pge in the world these deposits form as the result of the concentration of immiscible sulfide liquid droplets segregated from mafic or ultramafic magma sulfide saturation in the magmas is commonly linked to contamination by the continental crust the jinchuan ni cu pge deposit is the largest single magmatic sulfide deposit in the world it is hosted by a small mafic ultramafic intrusion in the north china craton the jinchuan mafic ultramafic intrusion intruded paleoproterozoic marbles gneisses and granitic migmatites this paper reports the magnesium strontium and neodymium isotopic compositions of the intrusion and the country rocks olivine pyroxene and whole rocks of the lherzolites from the jinchuan intrusion have 26 mg values ranging from 0 40 to 0 26 with a weighted mean value of 0 31 0 06 2sd which is slightly lighter than expected in a mantle derived magma the marble country rocks have relatively light 26 mg values varying from 2 00 to 1 17 with a weighted mean value of 1 53 0 02 2sd the variations are attributed to the interaction between basaltic magma and marble country rocks as the principal mechanism which generated the low mg isotopic signatures in the jinchuan deposit the 87sr 86sr i and end values of the lherzolites vary from 0 706615 to 0 714612 and from 8 29 to 6 13 respectively the marble country rocks have relatively high 87sr 86sr i ratios 0 713662 to 0 740069 and low end values 25 43 to 12 41 the negative nb anomalies in all samples from the intrusion indicate that the parental magma underwent crustal contamination mg sr nd isotopic mixing calculations indicate that the jinchuan intrusion underwent 10 20 bulk contamination involving the lower continental crust followed by

mineralogic and stable isotope studies of hydrothermal alteration at the jinchuan ni cu deposit china the jinchuan intrusion hosts one of the largest magmatic ni cu sulfide deposits in the world net textured and disseminated pyrrhotite pentlandite and chalcopyrite are found in olivine rich rocks 60 80 vol that occur as a northwest southeast striking dike olivine in the intrusion has been variably converted to mixtures of serpentine and fine grained magnetite interstitial pyroxene and minor plagioclase have been altered to mixtures of chlorite amphibole epidote clinozoisite na rich feldspar and lesser amounts of calcite although the interstitial net texture of sulfide minerals surrounding olivine grains is preserved up to 30 vol percent of sulfide minerals are replaced by magnetite serpentine and chlorite an external origin for mg and si needed for the isovolumetric replacement of sulfides cannot be discounted but a local origin related to olivine and pyroxene alteration is favored serpentine and chlorite may be enriched in cu and ni but the enrichment can account for less than 1 percent of the metals lost during the replacement of sulfide minerals by silicates and oxides some of the metals and sulfur that were removed from the net textured sulfide assemblages have been redeposited on a centimeter scale in fine ribbons and veins within serpentine chlorite and amphibole sulfur isotope values of the pyrrhotite pentlandite chalcopyrite assemblage at jinchuan fall in the range of 2 to 8 per mil with over 80 percent of the values between 2 and 2 per mil immediate country rocks in the jinchuan area are precambrian granitoids schists and marbles with sulfur concentrations less than 100 ppm although the 34 s values of the main sulfide assemblage at jinchuan are in the range that is characteristic of mantle derived sulfur assimilation of sulfur from unexposed country rocks cannot yet be discounted secondary pyrite occurs in veins and stringers and rarely as a replacement of pyrrhotite and is characterized by 34 s values between 7 and 27 per mil the strongly negative 34 s values of secondary pyrite are thought to be a result of oxidation of primary magmatic sulfide assemblages and partial reduction of aqueous sulfate oxygen and hydrogen isotope values of silicate minerals suggest that at least two fluids have been involved in the hydrothermal alteration at jinchuan the 18 o values of plagioclase amphibole and serpentine all suggest the involvement of a fluid with a 18 o value between 2 and 5 per mil at 300 to 400 c the d values of amphibole and one population of serpentine suggest a d value for the fluid in the range of 45 to 63 per mil these values are consistent with the involvement of evolved seawater or a metamorphic fluid in the early stages of alteration the possible involvement of evolved seawater in hydrothermal alteration is consistent with a rift setting for the emplacement of the jinchuan intrusion however the wide range in serpentine d values computed d of the fluid 45 to 103 suggest either that serpentinization involved a mixture of evolved seawater and or metamorphic water and meteoric water or more likely that serpentine continued to exchange hydrogen isotopes with late stage meteoric waters after it formed

sulfide saturation in mafic magmas is external sulfur required for magmatic ni cu pge ore genesis the importance of externally derived sulfur in the genesis of sulfide rich magmatic ni cu platinum group element pge deposits remains a key yet unresolved issue calculations utilizing a variety of mafic magma types indicate that in general sulfide saturation by fractional crystallization occurs after ni has been depleted due to olivine crystallization cu and pge rich layers may form during relatively later stages of closed system crystallization but unless the collection of the cotectic proportion of immiscible sulfide is extremely efficient the mass of sulfide is too small to produce an economic deposit we show that there are numerous processes that may lead to early sulfide saturation in mafic ultramafic magmas contamination of mantle derived magmas by siliceous country rocks or their partial melts will lower the sulfur content needed to induce sulfide liquid saturation typically by amounts ranging from 200 to 700 ppm the mixing of magmas particularly if the result is to lower the liquidus temperature of the mixed magma may also lower the sulfur content needed to attain sulfide saturation by similar amounts an increase in magma fo2 related to the addition of volatiles such as h2o and co2 is less effective in decreasing the sulfur concentration needed to achieve sulfide liquid saturation contamination processes that lead to an increase in the activity of sio2 in the melt and hence may promote orthopyroxene rather than olivine crystallization aid in generating relative ni enrichment in remaining liquid as a result of the lower dni mineral melt value of orthopyroxene relative to olivine although contamination and magma mixing may produce early sulfide saturation without the addition of externally derived sulfur ni rich sulfide deposits can form in such cases only from large volume open systems where the efficiency of sulfide collection is high no matter what the liquidus minerals may be without the addition of country rock derived sulfur the mass of sulfide necessary to generate economic ni cu pge concentrations requires efficient sulfide collection from large but not necessarily unrealistic volumes of magma small deposits 4 30 mt of sulfide may form from the collection of cotectic proportions of sulfide from less than 50 km3 of magma larger deposits such as those at noril sk could involve more than 200 km3 of magma this volume of magma is not unreasonable particularly in rift plume related settings despite such possibilities sulfur isotope data clearly indicate that externally derived sulfur has been involved in the formation of many large deposits and that collection of mantle derived sulfide in sufficient quantities to produce orebodies is a rare process we propose that magmatic ni cu pge sulfide ore formation normally requires significant sulfide supersaturation and that the addition of sulfur derived from xenoliths is the most viable mechanism for producing sulfide well above the cotectic proportion in mafic ultramafic magmas

formation mechanism of deep cambrian dolomite reservoirs in the tarim basin northwestern china abstract well ts1 which is located in the north uplift of the tarim basin northwestern china has a total depth of 8408 m and is the deepest petroleum well in the world the well penetrates a high quality upper cambrian dolomite reservoir from 6884 m to 8408 m the porosity of this dolomite increases with increasing depth reaching 9 1 at a depth of 8407 56 m a large number of dissolution pores are present in the core retrieved from well ts1 some of the pores are partially filled with coarse dolomite crystals which display saddle shapes and undulose extinction the pore filling dolomites have high concentrations of fe and mn averaging 3158 1 and 172 5 ppm respectively the concentration of ba is up to 4000 3 ppm the oxygen isotope composition of the pore filling dolomites is relatively light and the 18 o values range from 10 9 to 5 1 the 87 sr 86 sr values are relatively high ranging from 0 709361 to 0 709975 the mineralogical and geochemical features demonstrate that the pore filling dolomites were precipitated from hydrothermal fluids that significantly altered the deep cambrian dolomite reservoirs during upward migration along faults and fractures in the permian according to petrologic geochemical and seismic features the dolomite reservoirs were first dissolved by meteoric water during or shortly after deposition and were then significantly altered by hydrothermal fluids during burial a porosity variation model was proposed for the deep dolomite reservoirs that were influenced by hydrothermal fluids the model of increasing porosity with increasing depth in well ts1 indicates that there may be high quality dolomite reservoirs in the deep strata of basins where hydrothermal fluids are active the small amount of liquid hydrocarbon obtained from cores at a depth of more than 8400 m demonstrates that the deep dolomite reservoir is still suitable for hydrocarbon accumulation due to an unusually low geothermal gradient of approximately 20 c km the deep strata in basins with similar geological parameters may therefore be potential exploration targets elsewhere in the world

hydrothermal dolomite a product of poor definition and imagination abstract the latest dolomite bandwagon is the hydrothermal dolomite model in its present form this bandwagon is doomed or at least very much overstated for at least two reasons 1 there are several definitions of hydrothermal and hardly any author specifies which one s he is using 2 very few of the dolomites hitherto called hydrothermal have been demonstrated to be hydrothermal according to any definition except the worst as presently applied the term hydrothermal dolomite is confusing and or meaningless we suggest to use white s geol soc amer bull 68 1957 1637 time honored definition of hydrothermal as aqueous solutions that are warm or hot relative to its surrounding environment with no genetic implications regarding the fluid source hence a dolomite should be called hydrothermal only if it can be demonstrated to have formed at a higher than ambient temperature regardless of fluid source or drive furthermore this definition does not carry a lower or upper temperature limit even a dolomite formed at 40 c could be hydrothermal by extension dolomites formed at temperatures lower than ambient are not hydrothermal even if they formed at a rather high temperature for example groundwater may penetrate a rock sequence through a highly permeable pathway such that it is heated to 150 c at a depth where the surrounding rock has a temperature of 250 c we suggest to call dolomite formed from this water hydrofrigid dolomite formed in or near thermal equilibrium with the surrounding rocks may be called geothermal furthermore not all saddle dolomite formation requires advection fluid flow to transport mg saddle dolomite can be formed in at least three ways i e from advection local redistribution of older dolomite during stylolitization and as a by product of thermochemical sulfate reduction in a closed or semi closed system only the first and the last of these three possibilities have a chance of being hydrothermal almost all dolomites and dolostones in the western canada sedimentary basin have recently been re interpreted as hydrothermal applying the rationale outlined above reveals however that this basin contains very little hydrothermal dolomite rather most dolomites in this basin and almost all dolostones south of the peace river arch are geothermal and or the proof of a hydrothermal origin has not been made this has important implications beyond the various case studies at hand as attempts to tie dolomitization to orogenic events become moot at least in the southern part of the basin

the onset and early evolution of life the tension between co 2 dissolved at relatively high atmospheric pressure in the hadean ocean and h 2 generated as ocean water oxidized ferrous iron during convection in the oceanic crust was resolved by the onset of life we suggest that this chemosynthetic life emerged within hydrothermal mounds produced by alkaline solutions of moderate temperature in the relative safety of the deep ocean fl oor exothermic reaction between hydrothermal h 2 hcoo and ch 3 s with co 2 was catalyzed in inorganic membranes near the mound s surface by mackinawite fes nanocrysts and ready made clusters corresponding to the greigite fe 5 nis 8 structure such clusters were precursors to the active centers e g the c cluster fe 4 nis 5 of a metalloenzyme that today catalyzes acetate synthesis viz the bifunctional dehydrogenase enzyme acs codh the water and some of the acetate h 3 c coo produced in this way were exhaled into the ocean together as fl uid waste glycine h 3 n ch 2 coo and other amino acids as well as tiny quantities of rna generated in the same milieu were trapped within tiny iron sulfi de cavities energy from the acetate reaction augmented by a proton gradient operating through the membrane was spent polymerizing glycine and other amino acids into short peptides upon the phosphorylated mineral surface in turn these peptides sequestered and thereby protected the catalytically and electrochemically active pyrophosphate and iron nickel sulfi de clusters from dissolution or crystallization intervention of rna as a polymerizing agent for amino acids also led to an adventitious though crude process of regulating metabolism a process that was also to provide genetic information to offspring the fl uxes of energy and nutrient available in the hydrothermal mound commensurate with the requirements of life encouraged differentiation of the fi rst microbes into two separate domains at the bifurcation the bacteria were to specialize in acetogenesis and the archaea into methanogenesis representatives of both these domains left the mound by way of the ocean fl oor and crust to colonize the deep biosphere once life had emerged and evolved to the extent of being able to reduce nitrogen for use in peptides and nucleic acids light could have been used directly as an energy source for biosynthesis certain bacteria may have been able to do this where protected from hard uv by a thin coating of chemical sediment produced at a subaerial hot spring operating in an obducted and uplifted portion of the deep biosphere

strontium neodymium and lead isotope variations of authigenic and silicate sediment components from the late cenozoic arctic ocean implications for sediment provenance and the source of trace metals in seawater proven ance changes of silicate sediment deposited during the late cenozoic 5 0 ma on the alpha ridge central arctic ocean are determined from variations in strontium lead and neodymium isotope compositions whereas strontium and lead isotope compositions are relatively invariant from 5 to 1 7 ma end values start to increase at 3 ma subsequently s7sr 86sr and 2 6pb z 4pb ratios progressively increase and end values progressively decrease from 1 7 ma to the present day from these isotope variations three different endmember compositions for sediment source regions are defined the two endmember compositions defined by sediment that was deposited by sea ice from 5 to 1 7 ma are consistent with a significant component being derived from the east siberian shelf the progressive change in isotope compositions from 1 7 to 0 ma correlates with an increase in coarse detritus deposited by icebergs over this time period the isotope data are consistent with a progressive increase in the proportion of sediment that was deposited in the central arctic ocean since 1 7 ma having been derived from the northern canada or queen elizabeth island region changes in the strontium neodymium and lead isotope compositions of arctic seawater are determined by analyzing the oxide fractions of fe mn micronodules strontium isotope compositions of the microno dule oxide fractions are similar to published late cenozoic seawater data and indicate that 97 of the sr is seawater derived but minute contributions of sr from silicate detritus prohibit using the hydrogenous fractions for chronostratigraphic purposes neodymium and lead isotope compositions of the micronodule oxide fractions which reflect those of arctic seawater follow the isotope variations of the silicate components throughout the late cenozoic these relations indicate that river water can not be the primary source of rare earth elements and pb to the dissolved reservoir in arctic seawater and that ice rafted detritus by dissolution or exchange processes is an important source of trace elements to seawater in ice covered oceans copyright 1997 elsevier science ltd

thermal stability of amino acids in siliceous ooze under alkaline hydrothermal conditions abstract hydrothermal systems have been suggested as being favourable environments for the origin and evolution of life on the primitive earth to test this hypothesis it is necessary to investigate under hydrothermal conditions the behaviour of biomolecules such as amino acids aas a major component of organisms in contrast to submarine hydrothermal systems hot springs in the rift valley in eastern africa on the thick continental crust often have a high ph alkaline conditions as a result of enrichment via na 2 co 3 and volatile gases we reacted siliceous ooze with an aqueous solution of nacl and na 2 co 3 at elevated temperature 100 300 c to evaluate the thermal stability of the aas under alkaline hydrothermal conditions the aas in the sediment in peptide form were released to the liquid phase and decomposed through hydrolysis of peptide bonds comparison of the results with those from similar experiments using the same sediment sample under neutral conditions reveals that the rates of decomposition of the aas are significantly inhibited under alkaline conditions moreover aas remained in both the solid and liquid phases even after heating at 300 c for 240 h our results indicate that aas are more thermally stable in alkaline solution indicating that these hydrothermal conditions are more favourable for the evolution of primitive life on the thick continental crust they also imply that alkaline hydrothermal systems on other planets might be plausible places for extraterrestrial life

comments regarding hydrothermal dolomitization and porosity development in the paper formation mechanism of deep cambrian dolomite reservoirs in the tarim basin northwestern china by zhu et al 2015

detrital zircon constraint on the timing of amalgamation between alxa and ordos with exploration implications for jinchuan type ni cu ore deposit in china abstract the jinchuan ni cu deposit which is the largest single magmatic sulfide ore deposit in the world is hosted by a mafic ultramafic intrusion in the alxa block northwestern china the timing of amalgamation between the alxa block and the adjacent ordos block which is important for precambrian reconstruction of north china craton and the search for another jinchuan type ore deposit is contentious the age distribution patterns of detrital zircon crystals with precambrian ages from the devonian and older sedimentary rocks of the alxa and ordos blocks are similar this supports the traditional view that the alxa and ordos blocks were amalgamated in the paleoproterozoic along the 1 95 ga khondalite belt long before the emplacement of the 830 ma jinchuan intrusion the u pb lu hf isotope data of comagmatic zircon crystals from the jinchuan intrusion together with its whole rock sm nd isotopes and trace element compositions given previously by other researchers indicate that the jinchuan ore bearing intrusion is the product of continental rift related basaltic magmatism which took place during the early stage of rodinia breakup based on these observations we conclude that in addition to the alxa block the adjacent regions in the north china craton such as the ordos basin the khondalite belt and the yinshan terrane are most promising for new discoveries of jinchuan type ni cu ore deposits

open or closed geochemical systems during diagenesis in sedimentary basins constraints on mass transfer during diagenesis and the prediction of porosity in sandstone and carbonate reservoirs descriptions of mineralogy and textural relationships in sandstones and limestones have been used to establish a sequence of diagenetic events epigenesis involving mineral dissolution and precipitation which have been interpreted to have occurred during the burial history published epigenetic sequences commonly imply a geochemically open system with very significant changes in the bulk chemical composition of the sediments during burial near surface diagenetic reactions may be open involving significant changes in the sediment composition and formation of secondary porosity caused by high pore water flow rates of meteoric water or reactions with sea water near the sea floor calculations show that the bulk chemical composition of the sediments below the reach of high pore water flow rates of meteoric water or hydrothermal convection should remain nearly constant during progressive burial because of limited pore water flow mass transport between shales and sandstones is also limited because the pore water is in most cases buffered by the same minerals so that the concentration gradients are low recent studies show that silica released from clay mineral reactions in mudstones has been precipitated locally as small quartz crystals and not exported to adjacent sandstones if the geochemical constraints for mass transfer during burial diagenetic reactions are accepted the chemical reactions involved in diagenesis can be written as balanced equations this offers the possibility to make predictions about reservoir quality based on assumptions about primary sediment composition related to facies and provenance large scale changes in the bulk composition of sandstones and mudstones during burial diagenesis have been suggested but because such changes cannot be explained chemically and physically no predictions can be made burial diagenetic processes are in most cases not episodic but occur as slow adjustments to increased stress and temperature driving the sediments toward increased mechanical and thermodynamic stability as a result the porosity of a single lithology must decrease during progressive burial but each lithology has a different porosity curve this article discusses quantitative calculations and estimates that show clearly that burial diagenesis must represent geochemically nearly closed systems where mineral dissolution and precipitation must be balanced this provides a theoretical basis for the modeling and prediction of reservoir quality

hydrothermal dolomite reservoirs in a fault system and the factors controlling reservoir formation a case study of lower paleozoic carbonate reservoirs in the gucheng area tarim basin abstract the widespread hydrothermal activity in the lower paleozoic carbonate reservoirs of the tarim basin has received much attention the discovery of hydrothermal dolomite reservoirs indicates that hydrothermal alteration may have a positive effect on reservoir development however the factors controlling hydrothermal dolomite reservoirs and their pore development characteristics are difficult to recognize and interpret based on the lithology petrology pore fabrics and geochemical characteristics of the lower paleozoic carbonate reservoirs in the gucheng area this research discloses that the origin of the carbonate reservoirs is related to alteration by hydrothermal brine further hydrothermal dolomite reservoirs are strongly heterogeneous in different fault zone structures this research divides the fault structures into fault cores type 1 and damage zones types 2 and 3 type 1 is characterized by many breccias cemented by saddle dolomite locally developed vugs or fractures in the cores low density 1 7 2 8 g cm3 and resistance 22 4 1075 in the log response and shows abundant black spotted vugs and sinusoidal fractures in formation microscanner images fmi due to poor connectivity and high heterogeneity permeability 3 24 to 0 01 md average 0 59 md porosity 5 1 to 0 7 average 1 8 type 1 hydrothermal dolomite reservoirs show both positive and negative hydrothermal fluid contributions in the damage zones especially in the areas near fault cores type 2 is rich in fracture pore systems in both the cores and fmi and is characterized by serrated fluctuations in acoustic waves and densities 2 4 2 8 g cm3 type2 performs well in terms of physical properties permeability 8 06 to 0 011 md average 0 837 md porosity 6 6 to 0 65 average 2 5 indicating potentially favorable areas for the development of hydrothermal dolomite reservoirs at the distal end of a damage zone or in poorly developed areas type 3 features filling by hydrothermal products or higher densification permeability 0 77 to 0 0003 md average 0 125 md porosity 1 6 to 0 2 average 0 2 the well logging response of type3 which has a density of around 2 8 g cm3 and flat acoustic wave curves is similar to that of the host rock the differences in the development of hydrothermal dolomite reservoirs among diverse fault zones suggest that the damage zone type 2 may be a potential area for hydrothermal dolomite reservoirs in the fault system this study illustrates a model in which the hydrothermal fluid dissolves host rock in the proximal area and precipitates it at the distal end of the damage zone the hydrothermal dolomite reservoir characteristics of different fault zones are also further interpreted and described therefore different locations within a fault zone need to be considered and distinguished in studies of the factors controlling hydrothermal dolomite reservoirs in fault systems this view may provide new ideas regarding whether hydrothermal activity in the tarim basin plays a positive role in the formation of reservoirs and offers new suggestions for the exploration of hydrothermal dolomite reservoirs in fault systems

water in the martian interior evidence for terrestrial morb mantle like volatile contents from hydroxyl rich apatite in olivine phyric shergottite nwa 6234 abstract understanding the distribution and behavior of volatiles in martian magmas and the corresponding mantle provides crucial constraints on planetary properties such as mantle melting temperatures and melt production as well as the availability of magmatic fluids for any potential occurrences of life current evaluations of the pre eruptive volatile concentrations of martian magmas are based on analyses of hydrous minerals in martian meteorites such as apatite because they record the volatile content oh f cl and the halogen and water fugacities of their parental magmas estimates for the pre eruptive volatile contents of martian basaltic magmas range from nearly anhydrous to 2 wt h2o however these magmas may have potentially degassed upon eruption and or emplacement losing their magmatic water while retaining some halogens here we report apatite compositional data from the martian meteorite nwa 6234 an intermediate olivine phyric shergottite and calculate f2 cl2 h2o fugacity ratios for its parental magma apatites in nwa 6234 contain the highest observed oh f content among martian meteorites and importantly have volatile fugacity ratios similar to terrestrial basalts this suggests that the actual content of fluorine chlorine and water of the martian mantle parental to snc meteorites are higher than previously thought and are similar to the volatile content of the terrestrial mid ocean ridge basalt source region

geochemistry zircon u pb dating and hf isotopies composition of paleozoic granitoids in jinchuan nw china constraints on their petrogenesis source characteristics and tectonic implication abstract granitoids are widely distributed in jinchuan at the southwestern margin of the north china plate which is also an important area of mineral deposits the research subject of this article are two paleozoic granitoids a cataclastic syenogranite and a granodiorite porphyry this study presents whole rock geochemistry and zircon u pb hf isotope data for the two granitoids to determine their petrogenesis source characteristics and tectonic significance the cataclastic syenogranite is characterized by metaluminous composition with high potassium and la n yb n from 39 to 48 the composition with strong negative eu anomalies and zircon saturation temperatures t zr from 947 to 1072 c classify this intrusion as an a type granite the granodiorite porphyry is metaluminous with high sodium sub alkaline la n yb n ratios from 27 to 32 these i type intrusions have no eu anomalies and t zr ranges from 818 to 845 c both the cataclastic syenogranite and granodiorite porphyry show enrichment of lree and lile and depletion of hree and hfse except hf and zr using single zircon la icp ms u pb dating the emplacement age of the cataclastic syenogranite and granodiorite porphyry are determined at 433 4 3 7 ma and 361 7 4 6 ma respectively zircons from the cataclastic syenogranits have uniform negative e hf t values 11 0 5 to 9 0 5 implying the involvement of an old palaeoproterozoic crustal source in magma genesis the zircons from the granodiorite porphyry have e hf t values that range from 8 1 0 to 10 0 6 suggesting heterogeneous source materials involving both juvenile and ancient crust reworked crustal components based on the geological significance of granites at the southwestern margin of the north china plate the closure of the north qilian ocean occurred at 444 ma geochemical features suggest that the cataclastic syenogranite and granodiorite porphyry formed in an intraplate extensional and compressional setting respectively hence after a period of extensional post collisional intraplate evolution strong compressive forces affected the area

metamorphic degassing of carbonates in the contact aureole of the aguablanca cu ni pge deposit spain analysis of magmatic and sedimentary rocks of several large igneous provinces has demonstrated that the release of gas during plutonic metamorphic processes may be linked to global climate change and mass extinctions aguablanca one of the largest cu ni pge deposits in europe formed during the variscan orogeny when a mafic magma intruded limestones and shales creating a contact aureole composed of marble skarn and hornfels our petrological and geochemical investigation of the aureole provides evidence that a combination of the two processes led to the formation of the ore deposit the assimilation of terrigenous sediments supplied s to the magma while the assimilation of carbonates changed the oxygen fugacity and decreased the solubility of sulfur in the magma the metamorphic assemblages in the contact aureole are directly related to heterogeneity of the protolith and particularly to the original proportions of calcite and clay we modeled carbon dioxide degassing during contact metamorphism and showed that pure limestone is relatively unproductive because of its high reaction temperature the presence of clay however leads to the formation of calc silicates and significantly enhances co2 degassing our estimations suggest that degassing of the aguablanca contact aureole released about 74 8 mt of co2 a relatively low volume that we attribute to the composition of the host rock mainly a pure limestone a far larger volume of carbon dioxide was emitted by the contact metamorphism of dolostones in the contact aureole of panzhihua part of emeishan large igneous province sw china we propose that the level of emission of carbon dioxide depends strongly on the nature of the protolith and has to be considered when predicting environmental impact during the emplacement of large igneous provinces

inter mineral mg isotope fractionation during hydrothermal ultramafic rock alteration implications for the global mg cycle abstract both riverine and ocean waters are enriched in 24 mg compared to the homogeneous chondritic mg isotopic composition of the earth s mantle requiring a fractionation step that is generally attributed to low temperature continental crust weathering here we present new observations that indicate that this 24 mg enrichment of surface waters may originate from mg isotope fractionation during the hydrothermal alteration of primary silicate minerals mineral separates of hydrothermally altered ultramafic rocks were collected from three different localities in norway coexisting olivine and serpentine exhibit invariant mg isotope ratios suggesting that serpentinization does not fractionate mg isotopes in contrast carbonation results in significant inter mineral mg isotope fractionation between the antigorite magnesite and talc the carbonation of the natural samples is constrained by o isotope thermometry at 275 c and hence closes the temperature gap between previous investigations of the natural distribution of mg isotopes during surface weathering and magmatic processes the precursor antigorite has an isotopic composition of 26 mg dsm 3 0 11 0 05 whereas the talc is enriched in 26 mg with mean mg 26 0 17 0 08 and the magnesite is depleted in 26 mg with mean mg 26 0 95 0 15 as carbonate minerals dissolve faster than silicate minerals the chemical weathering of carbonated ultramafic and by analogy mafic rocks on the continents will yield isotopically lighter mg to natural surface waters consistent with field observations moreover the mg fractionation observed in this study suggests that sub seafloor hydrothermal carbonation may be a significant contribution to the mg isotopic composition of ocean water

origin and reservoir characteristics of upper ordovician trenton black river hydrothermal dolomite reservoirs in new york in the past decade more than 20 new natural gas fields have been discovered in laterally discontinuous dolomites of the upper ordovician black river group in south central new york the dolomites form around basement rooted wrench faults that are detectable on seismic data most fields occur in and around elongate fault bounded structural lows interpreted to be negative flower structures away from these faults the formation is composed of impermeable limestone and forms the lateral seal for the reservoirs in most cases the faults die out within the overlying trenton limestone and utica shale most porosity occurs in saddle dolomite coated vugs breccias and fractured zones matrix porosity is uncommon in the black river cores described for this study the patchy distribution around basement rooted faults and geochemical and fluid inclusion analyses supports a fault related hydrothermal origin for the saddle and matrix dolomites this play went for many years without detection because of its unconventional structural setting i e structural lows versus highs using the appropriate integrated structural stratigraphic diagenetic model more hydrothermal dolomite natural gas reservoirs are likely to be discovered in the black river of new york and in carbonates around the world

submarine hot springs and the origin of life commentaire sur l article de miller s l et bada j l paru dans nature 1988 334 609 611

deep sea smokers windows to a subsurface biosphere since the discovery of hyperthermophilic microbial activity in hydrothermal fluids recovered from smoker vents on the east pacific rise the widely accepted upper temperature limit for life based on pure culture data has risen from below the boiling point of water at atmospheric pressure to approximately 115 degrees c many microbiologists seem willing to speculate that the maximum may be closer to 150 degrees c we have postulated not only higher temperatures than these under deep sea hydrostatic pressures but also the existence of a biosphere subsurface to accessible seafloor vents new geochemical information from the endeavour segment of the juan de fuca ridge indicative of subsurface organic material caused us to re examine both the literature on hyperthermophilic microorganisms cultured from deep sea smoker environments and recent results of microbial sampling efforts at actively discharging smokers on the endeavour segment here we offer the case for a subsurface biosphere based on an interdisciplinary view of microbial and geochemical analyses of endeavour smoker fluids a case in keeping with rapidly evolving geophysical understanding of organic stability under deep sea hydrothermal conditions

the tazhong hybrid petroleum system tarim basin china abstract the tazhong petroleum system is geographically located in the central tarim basin including the tazhong uplift the north slope and the southern area of the manjiaer depression the total thickness of early palaeozoic to cenozoic sediments reaches 6500 9500 m the manjiaer depression and the north slope are the two major petroleum source areas with three sets of source rocks cambrian black shales and ordovician dark carbonates of the manjiaer depression and ordovician argillaceous limestones of the north slope there are three sets of reservoir cap rock combinations in this petroleum system the main periods for oil generation from these source rocks occurred during the middle silurian to early devonian late carboniferous to early triassic and tertiary there were three corresponding phases of petroleum migration well developed thrust faults and unconformity surfaces in this area provided the main pathways for petroleum migration as some of the faults cut through all the lower palaeozoic strata the oil in any given reservoir rock may be actually a mixture of oils from different sources complicating the origin and properties of the hydrocarbons in the reservoir and forming a hybrid petroleum system the formation of a typical oil and gas pool in this petroleum system may be attributed to one or all of three petroleum events which were closely associated with the regional tectonics the first event occurred in the early palaeozoic tectonic cycle and the generated oil accumulated in lower silurian sandstone the second event occurred in the late palaeozoic tectonic cycle and formed light oil pools mainly in lower silurian and ordovician reservoirs the third event took place during the cenozoic tectonic cycle and formed normal crude oils the oil and gas generated from different sources in different phases may co exist with one another in a reservoir rock however the commercial oil and gas pools are related to the cenozoic and late palaeozoic events

modelling of thermal convection in sedimentary basins and its relevance to diagenetic reactions abstract mathematical calculations of thermal convection have been carried out using a porous three player model to simulate pore water flow in a sedimentary basin with layers of different permeabilities the calculated flow lines demonstrate that even very thin layers eg around salt domes and igneous or hydrothermal intrusions analyses of formation waters from sedimentary basins like the north sea often show evidence of a crude stratification of the pore water with respect to salinity and isotopic composition this is evidence suggesting that large scale convection or other types of mixing does not take place and positive salinity depth gradient may help to physically stabilize the formation water the fact that many of the shallow reservoir rocks from the north sea have formation water with low salinity and negative 18 o values suggest that this is modified meteoric water and that migration of petroleum from deeper parts of the basin occurred as a separate phase along restricted migration pathways and was not associated with a high flux of pore water in the absence of thermal convection the total pore water flux through sandstones will be rather small except locally where we may have focused compactional flow through small cross sections diagenetic reactions will therefore normally be relatively isochemical during deeper burial

the violent environment of the origin of life progress and uncertainties dating of terrestrial fossils and returned lunar samples reveals that the origin of life on earth occurred not in a quiescent peaceful environment but rather in a violent impact ridden one this realization has important consequences on the one hand sufficiently large and fast impactors can erode planetary atmospheres and the very largest of these may have sterilized the surface of the earth in this regard deep sea hydrothermal vents become especially interesting for the history of early life as they provide an environment protected against all but the greatest impact devastation at the same time impactors would have been delivering key biogenic elements such as carbon and nitrogen to earth s surface and with much greater difficulty intact organic molecules as well estimates of the various sources of prebiotic organics suggest that the heavy bombardment either produced or delivered quantities of organics comparable to those produced by other energy sources however substantial uncertainties exist after reviewing the current understanding of the role of the heavy bombardment in the origins of life a number of remaining key uncertainties are considered and attempts are made to both quantify their magnitude and point to means of resolving them