inferred oxygen isotope profile of archaean oceanic crust onverwacht group south africa whole rock oxygen isotope data from a suite of mafic and ultramafic samples from the onverwacht group the basal unit of the archaean greenstone succession of the barberton mountain land south africa show a range of values from 18o 3 to 18o 14 a range which coincides with those of phanerozoic ophiolites and oceanic crust when the samples are arranged in an inferred ophiolitic pseudostratigraphy from basal serpentinized ultramafic cumulates through altered mafic extrusives they have an oxygen isotope distribution profile which is indistinguishable from that of phanerozoic ophiolites the distinctive isotopic profiles and secondary mineral assemblages in phanerozoic ophiolites are caused by hydrothermal interaction between seawater 18o 0 and oceanic crust 18o 5 8 the existence of a similar isotopic and metamorphic profile in the onverwacht group argues strongly that these rocks were hydrothermally altered by an archaean ocean whose isotopic composition was indistinguishable from modern sea water

oxygen and carbon isotope evidence for seawater hydrothermal alteration of the macquarie island ophiolite abstract rocks of the miocene macquarie island ophiolite south of new zealand have oxygen and carbon isotopic compositions comparable to those of seafloor rocks basalt glass and weathered basalts have 18o values at 5 8 6 0 and 7 9 9 5 respectively similar to drilled seafloor rocks including samples from the leg 29 dsdp holes near macquarie island compared to the basalt glass the greenschist to amphibolite facies metaintrusives are depleted in18o 18o 3 2 5 9 similar to dredged seafloor samples whereas the metabasalts are enriched 18o 7 1 9 7 although the gabbros are only slightly altered in thin section they have exchanged oxygen with a hydrothermal fluid to a depth of at least 4 5 km there is an approximate balance between18o depletion and enrichment in the exposed ophiolite section the carbon isotopic composition of calcite in the weathered basalts 13c 1 0 2 0 is similar to those of drilled basalts but the metamorphosed rocks have low 13c values 14 6 to 0 9 these data are compatible with two seawater circulation regimes in the upper regime basalts were weathered by cold seawater in a circulation system with high water rock ratios 1 0 based on calcite compositions weathering temperatures were less than 20 c and the carbon was derived from a predominantly inorganic marine source as previously suggested for the samail ophiolite it is postulated that the lower hydrothermal regime consisted of two coupled parts at the deeper levels seawater circulating at low water rock ratios 0 2 0 3 and high temperatures 300 600 c gave rise to18o depleted gabbro and sheeted dikes via open system exchange reactions during reaction the seawater underwent a shift in oxygen isotopic composition 18o 1 0 5 0 and subsequently caused18o enrichment of the overlying metabasalts in the shallower part of the hydrothermal regime the metabasalts were altered at relatively high water rock ratios 1 0 10 0 and temperatures in the range 200 300 c the relatively low water rock ratios in the hydrothermal regime are supported by the low 13c values of calcite interpreted as evidence of juvenile carbon in contrast to the inorganic marine carbon found in the weathered basalts

sims oxygen isotopes indicate phanerozoic fluids permeated a precambrian gold deposit abstract the reef deposit is an anomalous au cu occurrence in the paleoproterozoic terranes of northern wisconsin better known as host to significant cu zn volcanogenic massive sulfide vms deposits previous work using lead isotopes and fluid inclusions has identified a protracted development of the mineralization from initial formation as the root zone veins of a vms deposit during the penokean orogeny 1 9 1 8 ga with the most recent mineralization remobilization activity associated with late paleozoic mississippi valley type mvt fluid flow haroldson et al 2018a 2018b here we use the oxygen isotope history of the reef deposit to verify and further examine the deposit s protracted development laser fluorination oxygen isotope measurements of primary mineralized quartz veins range in 18o from 6 8 to 10 0 vsmow and a trend is observed of increasing 18o values of parallel vein zones along a 400 m traverse from northwest to southeast likely from a temperature gradient during initial formation temperature estimates for a vms deposit setting 230 to 400 c are consistent with a hydrothermally shifted formation fluid using seawater 18o estimates during the penokean orogeny and evolving to higher 18o by incorporation of magmatic fluids or interaction with local crust in situ oxygen isotope measurement by secondary ion mass spectrometer sims of cross cutting quartz and carbonate range in 18o from 19 4 to 28 4 for quartz 25 3 to 28 1 for dolomite and 9 6 to 29 3 vsmow for calcite high 18o values 19 are measured in a crustiform textured quartz stockwork microveinlet in dolomite observed in late carbonate microveinlets and calcite in settings associated with the late quartz and dolomite and a separate calcite only setting directly linked with gold mineralization remobilization

some petrologic and oxygen isotopic relationships in the amulet mine noranda quebec and their bearing on the origin of archean massive sulfide deposits a concentrically zoned alteration pipe is present at the amulet a cu zn mine in the archean abitibi greenstone belt the pipe consists of a central core zone of so called dalmatianite spotted cordierite anthophyllite rock surrounded by a zone of biotite bearing grid fracture alteration study of the dalmatianite indicates that these rocks experienced two episodes of recrystallization first the original andesite was completely reconstituted during a hydrothermal event producing a quartz chlorite assemblage this transformation was associated with the formation of the massive sulfide lens and presumably took place in a submarine environment later contact metamorphism associated with the intrusion of the nearby lake dufault granodiorite resulted in partial recrystallization of the chlorite and quartz to cordierite and anthophyllite whole rock delta 18 o values decrease from about 6 to 10 in most of the abitibi belt to 5 to 7 in the country rocks surrounding the ore deposit to values as low as 3 6 in the core of the dalmatianite zone this oxygen isotopic zoning must have been produced by the hydrothermal activity because the dehydration reactions associated with contact metamorphism cannot have affected delta 18 o by more than 0 5 per mil assuming an alteration temperature of 300 degrees or 50 degrees c and a water rock ratio greater than two the hydrothermal fluid must have had delta 18 o 0 5 or 1 0 this indicates that the amulet ore deposit formed from a hydrothermal fluid that had a delta 18 o value similar both to modern seawater and to the fluids which formed the phanerozoic massive sulfide deposits of the kuroko and cyprus types the apparent constancy of delta 18 o of seawater during the precambrian and the continued importance of seawater hydrothermal processes are important constraints which must be considered in developing models of the history of the earth

nd and pb isotopic constraints on the origin of the purtuniq ophiolite and early proterozoic cape smith belt northern quebec canada abstract we report pb isotopic compositions for 35 samples and sm nd isotopic data for 44 volcanic rocks from the purtuniq ophiolite the continental povungnituk and oceanic chukotat groups in the cape smith belt pb pb whole rock dating of the sheeted dike complex and cumulate rocks of the ophiolite which are preserved in separate thrust sheets yield identical isochron ages of 1 97 0 04 and 1 97 0 11 ga respectively supporting a contemporaneous origin combined data for layered cumulates sheets dikes and pillow basalts constrain the age of the ophiolite to 1 98 0 03 ga 1 8 0 in good agreement with a previously reported 1 998 ga u pb zircon age in contrast pb isotopic data of the 1 96 1 92 ga continental povungnituk and related transitional continental oceanic chukotat group basalts yield young dates of 1 89 and 1 72 ga respectively which may record tectonothermal events related to continental collision sm nd isotopic data for the ophiolite indicate highly heterogeneous sources nd t 1 0to 6 1 and form a mixing array corresponding to an anomalously old age of 2 2 ga a similar sm nd isotopic array with an age of 2 3 ga is displayed by the data for lree enriched continental povungnituk volcanic rocks and the associated more depleted oceanic basalts of the chukotat group magma contamination by late archean basement can be ruled out and we attribute the excessively old sm nd ages to melting and mixing of two distinct mantle sources the dominant magma source with nd t values of 3 to 5 is similar to the depleted early proterozoic mantle a subordinate mantle component displays nd t values as low as 1 and enrichment in lree relative to the depleted early proterozoic mantle it may have resulted from a relatively young enrichment event that produced a source with high nd sm and moderately radiogenic nd at 2 ga at the other extreme it may reflect a much older event with little source enrichment in lree in this case the strong lree enrichment in some volcanics would be due to lree fractionation during melting and it can be assumed that the isotopically evolved component had been stored outside the depleted mantle for a long time in order to avoid its destruction by convection the isotopic evidence is compatible with a magma origin in a plum pudding type mantle or a heterogeneous highly depleted mantle plume the isotopic heterogeneity in the ophiolite suggests an origin at a slowly spreading ridge in an immature ocean basin

oxygen and carbon isotopic composition of ordovician brachiopods implications for coeval seawater one hundred and two articulate brachiopods and twenty one marine cements from ordovician strata were analyzed for oxygen and carbon isotopes these together with 200 previously published analyses of ordovician brachiopods mostly of upper ordovician age define a general trend of increasing 18o values with decreasing age from a minimum of 10 5 in the earliest ordovician to a maximum of 1 5 at the ordovician silurian boundary the 13c values show a general positive correlation with 18o and increase from 2 5 in the earliest ordovician to as high as 7 at the ordovician silurian boundary the magnitudes of ordovician excursions for both 18o and 13c are similar to those reported previously for the entire phanerozoic different genera of brachiopods from the same stratigraphie level did not yield any large differences in their isotopic compositions suggesting that vital effects did not influence decisively the isotope values of the studied skeletal parts petrographic examination and trace element analyses indicate that brachiopod shells from llandeilo to ashgill strata are well preserved and thus may have retained their primary isotopic signatures the same criteria for the tremadoc arenig and some llanvirn brachiopods suggest partial diagenetic alteration of 18o values in these samples but this does not preclude their utility as a recorder of a primary 13c signal the observed 18o trend for the well preserved portion of the record may reflect progressive cooling during ordovician perhaps complemented by a changing 18o values of seawater the latter could have been a consequence of a net increase in the rate of high temperature water rock interaction relative to its low temperature counterpart as indicated also by the large coeval decrease of marine 87sr86sr superimposed on the above long term 18o trend is a positive excursion of about 2 at the end of the ordovician probably reflecting a large expansion of polar ice caps the magnitude of this excursion indicates a degree of glaciation comparable to that at the height of the quaternary glacial episodes the secular 13c trend may reflect a progressive increase in marine organic productivity and or enhanced organic deposition in the ordovician oceans particularly noticeable at the time of the terminal ordovician glaciation

unified structural representation of the southern california crust and upper mantle we present a new 3d description of crust and upper mantle velocity structure in southern california implemented as a unified structural representation usr the usr is comprised of detailed basin velocity descriptions that are based on tens of thousands of direct velocity vp vs measurements and incorporates the locations and displacement of major fault zones that influence basin structure these basin descriptions were used to developed tomographic models of crust and upper mantle velocity and density structure which were subsequently iterated and improved using 3d waveform adjoint tomography a geotechnical layer gtl based on vs30 measurements and consistent with the underlying velocity descriptions was also developed as an optional model component the resulting model provides a detailed description of the structure of the southern california crust and upper mantle that reflects the complex tectonic history of the region the crust thickens eastward as moho depth varies from 10 to 40 km reflecting the transition from oceanic to continental crust deep sedimentary basins and underlying areas of thin crust reflect neogene extensional tectonics overprinted by transpressional deformation and rapid sediment deposition since the late pliocene to illustrate the impact of this complex structure on strong ground motion forecasting we simulate rupture of a proposed m 7 9 earthquake source in the western transverse ranges the results show distinct basin amplification and focusing of energy that reflects crustal structure described by the usr that is not captured by simpler velocity descriptions we anticipate that the usr will be useful for a broad range of simulation and modeling efforts including strong ground motion forecasting dynamic rupture simulations and fault system modeling the usr is available through the southern california earthquake center scec website http www scec org

the 18o 16o ratio of 2 billion year old seawater inferred from ancient oceanic crust an oxygen isotope profile of the 2 billion year old purtuniq ophiolite overlaps with similar profiles of younger ophiolites and the modern oceanic crust this overlap implies i that there was a similar style of seawater ocean crust interaction during the past 2 billion years ii that the oxygen isotope composition of early proterozoic seawater was similar to the modern value iii that early proterozoic sea floor spreading rates were similar to or greater than average modern rates and iv that early proterozoic carbonate rocks and cherts with low 18o 16o ratios do not reflect global scale 18o depletion of early proterozoic oceans

an oxygen isotopic profile through the upper kilometer of the oceanic crust dsdp hole 504b dsdp hole 504b is the deepest basement hole in the oceanic crust penetrating through a 571 5 m pillow section a 209 m lithologic transition zone and 295 m into a sheeted dike complex an oxygen isotopic profile through the upper crust at site 504 is similar to that in many ophiolite complexes where the extrusive section is enriched in18o relative to unaltered basalts and the dike section is variably depleted and enriched basalts in the pillow section at site 504 have 18o values generally ranging from 6 1 to 8 5 smow mean 7 0 although minor zeolite rich samples range up to 12 7 rocks depleted in18o appear abruptly at 624 m sub basement in the lithologic transition from 100 pillows to 100 dikes coinciding with the appearance of greenschist facies minerals in the rocks whole rock values range to as low as 3 6 but the mean values for the lithologic transition zone and dike section are 5 8 and 5 4 respectively oxygen and carbon isotopic data for secondary vein minerals combined with the whole rock data provide evidence for the former presence of two distinct circulation systems separated by a relatively sharp boundary at the top of the lithologic transition zone the pillow section reacted with seawater at low temperatures near 0 c up to a maximum of around 150 c and relatively high water rock mass ratios 10 100 water rock ratios were greater and conditions were more oxidizing during submarine weathering of the uppermost 320 m than deeper in the pillow section the transition zone and dikes were altered at much higher temperatures up to about 350 c and generally low water rock mass ratios 1 and hydrothermal fluids probably contained mantle derived co2 mixing of axial hydrothermal fluids upwelling through the dike section with cooler seawater circulating in the overlying pillow section resulted in a steep temperature gradient 2 5 c m across a 70 m interval at the top of the lithologic transition zone progressive reaction during axial hydrothermal metamorphism and later off axis alteration led to the formation of albite and ca zeolite rich alteration halos around fractures this enhanced the effects of cooling and18o enrichment of fluids resulting in local increases in 18o of rocks which had been previously depleted in18o during prior axial metamorphism

long lived continent ocean interaction in the early proterozoic ungava orogen northern quebec canada the early proterozoic ungava orogen contains evidence for the development of a longlived 80 m y and presumably wide ocean that was subsequently destroyed during plate convergence leading to arc continent collision a magmatic arc was built on 2 00 ga oceanic crust above a north dipping subduction zone between about 1 90 and 1 86 ga development of a south verging continental thrust belt containing units associated with prior rifting of the archean superior province basement probably records impingement of the upper plate against the underthrust continental margin at ca 1 87 ga subduction of the superior province continental crust is inferred to have led to a reversal in subduction polarity a younger south dipping subduction zone is suggested by a suite of 1 85 1 83 ga quartz diorite to granite plutons that intrude the upper plate oceanic crust and the older arc the arc continent collision resulted in a minimum of 65 km of tectonic overlap of the composite arc9s plutonic core on the autochthonous footwall basement

geotechnical engineering significance of great plains polygonal fault system an extensive polygonal fault system pfs within fine grained upper cretaceous sediments beneath the great plains of north america has implications for geotechnical engineering geological well con

purtuniq ophiolite cape smith belt northern quebec canada a reconstructed section of early proterozoic oceanic crust the crustal part of a dismembered ophiolite is preserved in the structurally highest thrust sheets of the early proterozoic cape smith belt in northern quebec canada the rocks of the ophiolite represent two magmatic suites each with a mantle source that is nd isotopically distinct the older suite comprises pillowed and massive volcanic rocks sheeted dikes dominantly gabbroic layered cumulate rocks and rare plagiogranite sills and dikes in the volcanic sequence this 5 km thick tholeiitic suite is ca 1998 ma and is compositionally and petrographically similar to rocks formed at modern oceanic spreading ridges the younger suite 4 km comprises sheeted mafic dikes and layered ultramafic to mafic cumulate rocks that are similar to tholeiitic rocks found in modern plume generated oceanic islands such as hawaii the composite section is in excess of 9 km although the total thickness of the purtuniq ophiolite may not be typical of that for oceanic crust formed at early proterozoic spreading ridges elsewhere it does suggest that ancient ridge formed crust was at least as thick as that in modern oceans

depth variation of carbon and oxygen isotopes of calcites in archean altered upperoceanic crust implications for the co2 flux from ocean to oceanic crust in the archean abstract middle archean greenstones with mid ocean ridge basalt affinity and overlying bedded chert banded iron formation bif are exposed in the cleaverville area pilbara craton western australia on the basis of the hydrothermal carbonation of these cleaverville greenstones we estimated the potential co2 flux from ocean to oceanic crust and the physical chemical conditions of the subseafloor hydrothermal system for the middle archean the greenstones exhibit various extents of carbonation and the igneous minerals contained in the greenstones are partly or completely replaced by calcite and other secondary minerals the degreeof carbonation correlates with stratigraphy the volume concentration of calcite in greenstones decreaseswith increasing depth below the chert bif horizon our results clearly indicate that the hydrothermal carbonation occurred along the axial zones of a middle archean mid ocean ridge both 13c and 18o values of calcite also change with the depth below the chert bif horizon the 18o value of calcite decreases with increasing depth reflecting increasing temperature while the 13c value of calcite changes from positive to negative stratigraphically downward a model reconstructing the 13c of calcite suggests that the observed isotopic variation reflects the thermal structure and that the carbon source for calcite was derived from seawater these mineralogical geochemical and geological features of calcite provide the amount of co2 in the whole oceanic crust per seafloor unit area 1 2 107 mol m2 based on this value co2 flux from the ocean to the oceanic crust in the middle archean can be estimated to be 1 5 1014 mol yr when the spreading rate m2 yr of archean oceanic crust is assumed to have been three times higher than it is today the estimated co2 flux into the oceanic crust is two orders of magnitude higher than the modern value which points to the significance of sea floor hydrothermal carbonation in the archean carbon cycle

regional 18o depletion of neoproterozoic igneous rocks from avalonia cape breton island and southern new brunswick canada neoproterozoic igneous rocks of the avalonian mira terrane cape breton island and caledonia terrane southern new brunswick have experienced regional 18 o depletion the majority of these rocks have 18 o wr values between 1 and 6 markedly different from igneous rocks of the inboard ganderian terranes which have normal high 18 o wr values of 7 to 12 the 18 o depletion of these avalonian terranes resulted from pervasive hydrothermal alteration all of the igneous rocks contain classic propylitic alteration assemblages of turbid feldspar chlorite epidote sericite and calcite and host multiple generations of veins containing the same phases the 18 o values of the rock forming minerals show strong disequilibrium with qtz fs ranging from 2 5 to 11 0 and kfs plg from 0 5 to 1 0 such values are indicative of post magmatic hydrothermal alteration the hydrothermal alteration occurred at ca 560 550 ma during initial transcurrent rifting of avalonia at the gondwanan margin the development of rift wrench basins with associated bimodal volcanism allowed large volumes of hydrothermal fluids to penetrate and circulate within mira and caledonia terrane crust before its submergence in the early cambrian ca 540 530 ma the hydrothermal fluids were predominantly meteoric although the presence of a significant seawater component cannot be ruled out the almost ubiquitous 18 o depletion exhibited by the neoproterozoic rocks in these avalonian terranes is absent in the associated ganderian terranes suggesting that avalonia remained separate from ganderia until at least the cambrian ordovician this 18 o depletion provides an additional geochemical tool for identification of avalonian crust in other areas of the appalachians and europe

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

the δ18o record of phanerozoic abiotic marine calcite cements monomineralic abiotic marine cements formed in low latitude phanerozoic reefs provide the direction and amplitude of secular variation of delta sup 13 c and delta sup 18 o in marine calcite and defines two end member compositions 580 to 360 my minus 7 to minus 5 per thousand delta sup 18 o sub pdb and 360 to present minus 3 to 0 per thousand delta sup 18 o sub pdb sampling of the devono carboniferous transition 375 320 my at several global sites reveals a rapid change in carbonate isotopic compositions bracketed within fammenian to early visean aged strata a 7 to 15 my time interval this shift corresponds to a 2 offset in mean delta sup 13 c and 3 4 offset in delta sup 18 o the abruptness of such change and its overall correlation with variations in sup 87 sr sup 86 sr delta sup 34 s delta sup 13 c and li al ratios in marine sediments suggests a primary offset in marine water composition

geochemistry of precambrian carbonates iv early paleoproterozoic 2 25 0 25 ga seawater in an effort to narrow down the first and second order variations in isotopic composition of the early paleoproterozoic 2 25 0 25 ga seawater we report a comprehensive study of mineralogy chemistry and isotopic composition of the malmani dolomite transvaal supergroup south africa duck creek dolomite wyloo group australia and bruce limestone member of the espanola formation huronian supergroup canada based on trace element data their diagenetic rank increases in the order duck creek bruce malmani the interpolation of alteration trends to best values yields an estimate of 0 70550 for 87sr86sr comparable to our previously published value of 0 70496 for 13c the measured range of 0 1 5 pdb is similar to that observed for phanerozoic marine carbonates in contrast the best 18o value for dolostones is 5 pdb depleted in 18o relative to phanerozoic counterparts but comparable to the estimates obtained by similar approach for archean facies the isotope geochemistry and mineralogy of bruce limestone member is consistent with the proposition that the sequence may have been deposited in a lacustrine environment possibly related to the recession of the brace glaciers if such interpretation proves unacceptable from a sedimentological standpoint the origin of the unusual isotopic signatures must be sought despite the above trace element interpretation in as yet unspecified post depositional alteration phenomena in contrast to phanerozoic carbonates the early paleoproterozoic as well as the previously described archean counterparts are enriched in mn and fe

symposium on ophiolite genesis and evolution of oceanic lithosphere the recently published special publication no 49 of the geological society of london the geology and tectorlics of the oman region edited by a h f robertson m p searle and a c ries feb 1990 resulted largely from a six year amoco funded project concentrating mainly on the structure strat atigraphy and sedimentology of the platform margin and basin sediments beneath the semail ophiolite this study and more particularly the brgm french project 1982 1990 formed the basis of the ophiolite conference it was held at the sultan qaboos university sultanate of oman 7 18 january 1990 and was the first major international geological conference to be held in oman over 300participants from 27 countries attended since the classic work of glennie et al 1974 and the shell team the semail ophiolite in the oman mountains has been widely regarded as the largest best exposed and most spectacular example of an obducted ophiolite complex anywhere in the world subsequent field projects including the open university oman ophiolite project 1975 1985 summarized by lippard et al 1986 and the university of california usgs project 1977 1979 published in a special issue of journal of geophysical research 1981 vol 86 concentrated mainly on the semail ophiolite complex since 1982 the bureau de recherches geologiques et minieres brgm have remapped large areas of the central and southern mountains and 13 maps at 1 100 ooo scale have been published together with new detailed studies of the semail ophiolite some of the french teams results have been published in a

the implication of the oxygen isotope records in coexisting cherts and phosphates 18 o values have been determined for 53 coexisting chert phosphate pairs from the phanerozoic and precambrian in addition 18 o values were also determined for 18 biogenic phosphates from the phanerozoic the oxygen isotope data for the chert phosphate pairs permit the calculation of the temperatures of formation of the chert phosphate pairs as well as the 18 o of the waters in which they equilibrated in turn the calculated 18 o of the waters permit the calculation of temperatures from the isotopic data of the biogenic phosphates the temperatures for the tertiary and mesozoic samples are compatible with the literature values determined from oxygen isotope data for well preserved calcareous marine skeletal remains the temperatures calculated from the chert phosphate pairs of the paleozoic are usually higher and vary in cycles of about a 150 million year period the early precambrian temperatures are as high as 80 c and decrease with time the use of chert phosphate pairs for temperature determinations are sufficiently promising to warrant further investigations of this type on well preserved chert phosphate or carbonate phosphate pairs

geochemistry of precambrian carbonates v late paleoproterozoic seawater in an effort to obtain better constrained data for the first and second order variations in the isotopic composition of late paleoproterozoic 1 8 0 2 ga seawater we report a study of mineralogy chemistry and isotopic composition of the coronation supergroup 1 9 ga nwt canada and the mcarthur group 1 65 ga nt australia petrologically both carbonate sequences are mostly dolostones the mcarthur population however contains more abundant textural features that attest to the former presence of sulfates and halite and the facies investigated herein probably represent ancient equivalents of modern evaporitic sabkhas and lacustrine playa lakes this is reflected in 18 o enrichment up to 2 9 pdb and high mn fe and sr concentrations nonetheless the available 18 o data indicate that the best estimate for the archean to late paleoproterozoic limestones is about 7 pdb with dolostones some 3 2 heavier the observed 18 o doiomitc calcite is thus within the range of the proposed fractionation factor for a cogenetic mineral pair this together with the consistency of isotopic patterns for sedimentological facies over almost 2 billion years from the late paleoproterozoic to the present suggest that dolomitization was an early diagenetic event and that the 18 o depletion of the archean to late paleoproterozoic carbonates is not an artefact ofpost depositional alteration the 13 c although within the range of other precambrian and phanerozoic carbonates indicates a possible secular evolution from 0 2 pdb at 1 9 ga ago to 1 1 5 at 1 65 ga ago the secular evolution for reflects the 87 sr increase from 0 7031 0 0008 at 3 5 0 1 ga ago through 0 7025 0 0015 at 2 8 0 2 ga 0 7033 at 2 5 ga 0 7050 at 2 25 0 25 ga 0 7047 at 1 91 to 0 706 at 1 65 ga ago