Search

The Sokoman Formation is a ca. 100-m-thick succession of interbedded iron formation and fine-grained siliciclastics deposited at 1.88 Ga. Accumulation occurred on a dynamic paleoshelf where oxygen stratification, coastal upwelling of hydrothermally derived Fe and Si, microbial processes, tide and storm currents, diagenesis, and low-grade prehnite–pumpellyite metamorphism controlled lithofacies character and produced complex associations of multigenerational chert, hematite, magnetite, greenalite, stilpnomelane and Fe carbonate. Hematite-rich facies were deposited along suboxic segments of the coastline where photosynthetic oxygen oases impinged on the seafloor. Hematitic, cross-stratified grainstones were formed by winnowing and reworking of freshly precipitated Fe-(oxyhydr)oxide and opal-A by waves and currents into subaqueous dunes. Magnetite-rich facies contain varying proportions of greenalite and stilpnomelane and record deposition in anoxic middle shelf environments beneath an oxygen chemocline. Minor negative Ce anomalies in hematitic facies, but prominent positive Ce and Eu anomalies and high LREE/HREE ratios in magnetite-rich facies imply the existence of a weakly oxygenated surface ocean above anoxic bottom waters. The Fe isotopic composition of 31 whole rock (−0.46 ⩽ δ 56 Fe ⩽ 0.47‰) and 21 magnetite samples (−0.29 ⩽ δ 56 Fe ⩽ 0.22‰) from suboxic and anoxic lithofacies was controlled primarily by the physical oceanography of the paleoshelf. Despite low-grade metamorphism recorded by the δ 18 O values of paragenetically related quartz and magnetite, the Sokoman Formation preserves a robust primary Fe isotopic signal. Coastal upwelling is interpreted to have affected the isotopic equilibria between Fe 2+ aq and Fe-(oxyhydr)oxide in open marine versus coastal environments, which controlled the Fe isotopic composition of lithofacies. Unlike previous work that focuses on microbial and abiotic fractionation processes with little regard for paleoenvironment, our work demonstrates that depositional setting is paramount in governing the Fe isotopic composition of iron formations irrespective of what Fe-bearing minerals precipitated.

Black limestone clasts are present across the globe at modern and ancient subaerial exposure surfaces, and in intertidal and supratidal environments. These problematic clasts are particularly abundant at multiple subaerial exposure surfaces across southern Australia. The genesis of such exceptionally preserved clasts can be interpreted through universal fabrics across wide geographic regions and geologic ages. Regardless of location or age, all south Australian blackened clasts studied have a similar microstructure that is dominated by porous micritic laminae and chambered tubules. Such laminae are interpreted to represent calcified root cells and occur in two different varieties: 1) micrite-walled tubules with hollow centers that are thought to represent calcification in the rhizosphere or outer surfaces of the root, and 2) tubules consistent with calcification of both the medulla and cortex, which preserved medulla, cortex, and parenchymatic cells along with xylem vessels. Preserved root fabrics are present only in the blackest clasts and are consequently thought to be the main mechanism driving limestone blackening. The dark coloration in southern Australian clasts is derived from the impregnation of partially decayed terrestrial organic substances. These organic substances were trapped in the cellular structures of roots as they calcified and were subsequently prevented from further decay and oxidation. A threshold of approximately 7-weight % organic carbon is needed in the insoluble residue to produce the distinct black coloration in limestone. This process occurs at the base of slightly organic-rich humic soil profiles where they interact with underlying carbonate bedrock (B–C soil horizon). Roots both physically and chemically (through acid secretion) corrode the underlying limestone and provide the calcium bicarbonate needed in the root calcification process. Specific semiarid flora have the ability to calcify their cellular structures, which could explain why blackened clasts occur at only some unconformities. This result means that the formation of blackened clasts via root calcification and entrapment of organics relies upon specific conditions of semiarid plants and climate along with organic soils. Such conditions would be restricted to a specific climate regime and when combined with the new proposed blackening mechanism herein could help explain their limited distribution and occurrences worldwide.

Both uranous and uranyl minerals are present in the Centennial unconformity-type U deposit situated in the SW Athabasca Basin, Canada. At least two generations of uraninite are present (disseminated and massive), often strongly altered to coffinite, followed by minor fibrous coffinite forming in veins. Uranyl minerals, mostly uranophane with minor haiweeite precipitating in veins and in hand sample-scaled breccias, are present in significant amounts ( ca . 5% of the ore) not observed elsewhere in the Athabasca Basin. The multi-stage evolution of the Centennial deposit is explained by the existence of high permeability conduits, promoting the recurrent circulation of P-rich fluids that remobilized U locally. Stable isotopes composition of uranophane (δD of ca . –130 ‰ and δ 18 O of ca . 6 ‰) suggests that recent oxidizing meteoric fluids penetrated to the unconformity (at a depth of ca . 800 m) via the major Dufferin lake fault.

The Bahamian Archipelago is one of the few locations in the world where ooid formation is actively occurring. Ooid cortices from six locations in the region were incrementally dissolved and analyzed for 14C, δ18O, δ13C, Mg/Ca, and Sr/Ca ratios. Ooids were examined under SEM after each step in the incremental analyses to characterize the nature of dissolution. Radiocarbon dating indicates that ooids presently at the sediment–water interface began forming across the Archipelago between 1000 and 2800 yr BP and continue to form today. The ooids have the same pattern of microboring alteration across the region. The surface and outer cortex of the ooids are punctuated with unfilled microborings, whereas the inner cortex contains two morphologies of aragonite cement filling the microborings. The two morphologies of cement form in association with two different species of cyanobacteria, one is Solentia sp. the other is interpreted to be Hyella sp. The chemistry of ooids from across the region is remarkably similar. δ18O and δ13C values for all samples vary directly, having a slope of approximately 1. The outer cortex has low δ18O and δ13C values of −3.4‰ and 0.2‰ respectively, whereas the δ18O and δ13C values of the inner cortex are high with values of 1.9‰ and 6.8‰ respectively. The presence of aragonite cement in microborings in the inner cortex increases the overall isotopic composition of both oxygen and carbon in the ooid, causing it to appear close to equilibrium with seawater. The isotopic variation in δ18O and δ13C within the cortex can be characterized as a mixing line between the low values in the unaltered ooid laminae and the aragonite cement in the microborings. The most exterior portion of the ooid has very high Mg/Ca values and is interpreted as an amorphous calcium carbonate (ACC) coating. There are two other phases in the cortex, both being aragonite. The outer cortex has a higher Mg/Ca ratio and lower Sr/Ca ratio than the inner cortex. This difference in chemistry is a result of the presence of aragonite cement in the inner cortex. Stable isotopic and trace element results coupled with SEM investigations indicate that microbes do not play a role in ooid formation, but instead alter the texture and chemistry of ooids after they have formed. A new model of ooid formation is proposed whereby a veneer of ACC precipitates on an ooid while it is at the sediment–water interface (the active phase). This veneer of ACC later recrystallizes to aragonite needles and a new cortex layer is formed (the stationary phase). Observations from this study lead to a deeper understanding of the chemical processes involved in ooid genesis, which allow a better understanding of paleoenvironments hosting ooid formation.

Laser-ablation high-resolution inductively coupled plasma mass spectrometry (LA-HR-ICP-MS) is a rapid, accurate, and inexpensive technique for making in situ U-Pb isotopic measurements of uraninite and davidite. The advantages of this method include: (1) mineral separation and chemical digestion are not required; (2) measurements on complex samples are feasible because significant isobaric interferences can be resolved; and (3) accurate U-Pb and 207Pb/206Pb dates on 10–25 μm spots can be obtained rapidly. The LA-HR-ICP-MS method is applied to U oxide minerals from four deposits and prospects from northern Australia, and the new dates are compared to previously published conventional thermal ionization mass spectroscopy (TIMS) dates, and to known ages of geologically important events. These comparisons permit us to assess elemental fractionation of U and Pb for uraninite and davidite of the new method, relative to zircon, as well as its geochronological accuracy and precision. U-Pb apparent ages measured previously agree well with our measurements for El Sherena, Palette, and Mt. Isa. Additionally, the upper-intercept 207Pb/206Pb dates for Adelaide River (701 ± 190 Ma, 1σ) and Palette (841 ± 94 Ma, 1σ) uraninite, measured here, are similar to those previously obtained for Palette (730 Ma), Nabarlek (920 Ma), and Koongarra (870 Ma), and the upper-intercept date for El Sherena uraninite (1573 ± 160 Ma) is within error of that previously determined for Ranger (1550 ± 15 and 1472 ± 40 Ma). Such apparent-age agreement for uraninite (and similarly for davidite) indicates that U and Pb fractionations are within error of that for zircon, whereas the inherent imprecision of our dates and their associated MSWD values greater than 2.5 probably indicate that multiple resetting events affected our samples. Analytically, these results demonstrate that the LA-HR-ICP-MS technique provides excellent spatial resolution while also removing argide, phosphide, sulfide, and halide interferences that can otherwise lead to erroneous data when using quadrupole-ICP-MS. Geologically, the individual 207Pb/206Pb and upper-intercept U-Pb dates of uraninite from Adelaide River and Palette are ca. 800 Ma, possibly reflecting recrystallization of uraninite during the break-up of Rodinia.

The Pierina high-sulfidation epithermal Au–Ag deposit (9°26.5′S; 77°35′W) was emplaced in the Middle Miocene into a hypabyssal-to-extrusive “pumice-tuff” and an underlying, older, dacitic flow-dome complex, both of which are cut by hydrothermal breccias and small dacitic domes. Stage I advanced argillic alteration generated a core of vuggy silica, focused in the tuff, and surrounded successively by zones of quartz–alunite, dickite±kaolinite±pyrophyllite, and illite–montmorillonite±kaolinite. Laser-ablation ICP-MS analysis of the sulfide minerals of the succeeding, Stage II, Cu (–Pb, Bi, Sb, Zn, As)–barite mineralization, largely confined to the vuggy-silica zone, reveals that both Au and Ag were introduced at that time. This assemblage was almost entirely obliterated during Stage III, when oxidation by low-temperature meteoric waters generated botryoidal hematite–goethite assemblages, which are now the main precious-metal hosts. Stage IV barite–acanthite mineralization shows limited temporal overlap with Stage III, but dominantly overprints the hematite–goethite assemblage. The deposit incorporates alunite exhibiting a wide range of modes of occurrence, grain size, and morphology. Disseminated alunite dominates Stage I alteration, which replaced phenocrysts and fragments, and locally hosts corroded, ≤40 μm alumino-phosphate-sulfate (APS) inclusions. The alunite shows limited Na substitution [molar Na/(Na+K)=

A method for in situ U–Pb isotopic analyses by secondary ion mass spectrometry (SIMS) has been developed for uranium minerals with a range of chemical compositions. This method combines the advantages of conventional U–Pb dating (i.e., use of concordia) and in situ analysis, and therefore is ideally suited for the study of chemically complex and fine-grained uranium oxides associated with uranium deposits. An ion-yield normalizing coefficient (� SIMS) that accounts for variation in relative ion-yields with chemical composition of the mineral of interest was calculated using uraninite standards that cover a range of U

The Gambier Limestone, a bryozoan-rich, cool-water carbonate of Late Eocene to Early Miocene age in the Otway basin of southeastern Australia, is about one-third variably dolomitized limestone and dolostone. Individual dolomite crystals range from 10 to 500 µm and are of three types: (1) silt-size crystals (4-64 µm diameters) having a brightly luminescent core (10-20 µm) of nearly stoichiometric dolomite with 1 mol% MnCO3 and SiO2 but no Fe, surrounded by a thin, dull or alternating dark and bright CL cortex that is nonstoichiometric with 44-46 mol % MgCO3 but sharp to diffuse zoning in Mn and Fe, (2) sand-size crystals, which also have a brightly luminescent and stoichiometric core and dark and bright non-stoichiometric thin inner cortex, but are mostly a thick outer cortex (at ca. > 50 µm diameter) that is nonluminescent and nonstoichiometric with some Fe but no Mn, and (3) identical sand-size crystals with a variably leached core and inner cortex that is either a void or a void filled with late calcite cement. These three crystal populations represent two distinct dolomites and their alteration product. The smallest of the silt-size crystals, which are rarely preserved, are also the cores of the larger crystals upon which grew an inner and outer cortex of somewhat different composition. The 18O values of dolomite rhombs (-1.6 to 2.9 ) correlate positively with 13C values (0.7 to 3.0), with the lowest values from silt-size or small sand-size crystals and the highest values from large sand-size crystals with leached, unfilled cores. Low and variable 18O and 13C values in conjunction with high and variable Mn and Fe contents in both the core and inner cortex of all crystals are consistent with formation from a mixture of brackish water and seawater. Higher values for the outer cortex in sand-size crystals are typical of those expected for precipitation from normal or near-normal seawater. Low Sr, Mn, and Fe contents in the largest crystals, in conjunction with fluid inclusions of seawater salinity in the outer cortex, are also consistent with the bulk of the large crystals having precipitated from seawater. Using the Sr-isotope evolution curve for Cenozoic seawater, Sr-isotope ratios of the dolomites can be divided into four distinct groups, which correspond to times of major transgressions. This Cenozoic example, in which dolomite petrography and crystal chemistry can be explained in terms of simple transgression-regression, offers a simple sedimentological explanation for many similar dolomites where processes of formation cannot be so easily constrained.

The Oligo-Miocene Gambier Limestone is a poorly lithified, fine-grained, bryozoan-rich grainstone/wackestone and marl. Allochems in these rocks, which are typical of many modern and ancient cool-water carbonates, were originally calcite (LMC), Mg-calcite (IMC and HMC) and minor aragonite, but most are now neomorphosed to LMC. With the exception of calcretes, most limestones have < 5% cement, 13C and 18O values from 0 to +1.5o/oo, and preserved 87Sr/86Sr ratios that indicate that neomorphism occurred in seawater rather than meteoric or evolved pore waters. Marine diagenesis of the limestones was facilitated by their slow accumulation rates, which were ca. 15 mm/ky based on Sr-isotope stratigraphy, relatively fine grain size, and high porosities which locally reach 50%. The Sr and Mg contents of grainstones are significantly lower than in the original sediments, whereas a less porous marl has higher Sr and Mg contents. The grainstones have about 300 ppm Sr and 6000 ppm Mg, consistent with slow abiotic precipitation of calcite under near-equilibrium conditions with seawater, which is the thermodynamic drive for this diagenesis. The higher Sr and Mg content of the marl results from variable preservation of original allochems of biotic calcite. The results from this study and others on modern cool-water carbonates indicate that the extent of diagenesis varies with lithology, but much neomorphism occurred in seawater, with minimal effects from meteoric waters. As such, the chemical characteristics of seawater may be effectively preserved in some ancient cool-water carbonates.

The Star and Island Lake plutons of the Central metavolcanic belt, La Ronge domain, host a variety of barren and auriferous quartz veins in northeast-trending shear zones (040 degrees 80 degrees NW). Shear zones are coincident with competency contrasts in the plutons, which may be attributed to the northeast-trending boundary of igneous phase changes, or the presence of northeast-trending dikes. Veins range from 30 cm to 2 m wide, sup to hundreds of meters long, and exhibit a discontinuous pinch and swell nature common to most shear zone-hosted mesothermal quartz veins. Barren veins are paragenetically simple, quartz being the dominant hydrothermal mineral with sparse microcline of variable origin. Muscovite, a common gangue mineral in auriferous veins, is rare in barren veins; in barren veins its occurrence is correlated with slightly elevated gold values.Quartz microtextures indicate incremental vein emplacement and deformation in a dynamic environment. Dynamic recrystallization (subgrain formation > recovery rate) is the dominant ductile deformation mechanism. Macro-and microfractures, preserved as veins and planar arrays of fluid inclusions, are a result of episodic brittle failure. The cyclic nature of these events suggests in turn fluid pressure cycling in the brittle-ductile transition.Regionally, veins have a bimodal distribution of delta 18 O quartz values with barren and auriferous veins occupying both populations. Excepting the Jasper mine, on both a regional and hand sample scale, there is no shift in isotopic composition of vein quartz with increased ductile deformation. In fact the majority of the veins are remarkably homogeneous with respect to delta 18 O quartz. Petrographically, fluid inclusions in barren veins are identical to those described in auriferous veins and exhibit temperatures of total homogenization similar to those of inclusions in auriferous veins. Compositionaly, fluid inclusions in barren veins have similar wt percent NaCl and bulk CO 2 as auriferous veins; however, an important distinction is that auriferous veins may have up to 30 mole percent CH 4 , a phase absent in barren veins. Similarities between auriferous and barren veins such as field relationships, microstructures, stable isotopes, and fluid inclusions are interpreted to indicate a broad temporal contemporanity of auriferous and barren fluids. Distinct mineral paragenesis and CH 4 content may represent different redox states of barren and auriferous fluids. Lack of reduced carbon species in barren veins may reflect an oxidizing fluid with few reduced sulfur ligands capable of transporting gold.

A chemical separation method of Hf from rocks is presented. A procedure involving three stages of ion-exchange columns and sulfuric acid as an eluant produces consistent Zr- and Ti-free Hf separations. The methods described are well suited for high-Ti samples, such as mafic rocks. In addition, the method provides a very efficient means for Hf separation from zircons.

A three-component, diffusion-limited, open-system exchange model for pyroxene, olivine and fluid is presented that reproduces the range of oxygen isotope compositions of coexisting silicates from mantle-derived samples. Closed-system exchange cannot reproduce observed disequilibrium fractionations. Measured oxygen self-diffusion data were used to constrain fluid-silicate reaction rates. The magnitudes of fluid-mineral fractionations for typical mantle fluids are presented and show that equilibrated fluid will always be more 18O-rich than coexisting silicates. Three conditions are necessary to generate the disequilibrium pyroxene-olivine oxygen isotope fractionations observed in the mantle dataset. There must be at least an order of magnitude difference between the fluid-pyroxene and fluid-olivine reaction rates, fluid and solid phases must be out of equilibrium prior to reaction, and fluid must be moving several orders of magnitude faster than it is reacting with the silicates. Assuming initial isotopic equilibrium between pyroxene and olivine and reasonable porosities, the following constraints apply. If pyroxene is reacting faster than olivine, to be expected when pyroxene and olivine grain sizes are subequal, a fluid flow rate of at least 5 × 103 times the pyroxene/fluid reaction rate is needed to generate the most extreme negative pyroxene-olivine fractionations seen in natural samples. If olivine reacts faster, the case when its effective grain size is 0.2 cm/y at 1200°C and >3 cm/y at 1400°C are needed when pyroxene is the faster reactant. Flow rates >1 cm/y at 1200°C and >15 cm/y at 1400°C are necessary in the olivine dominated system. Regardless of which silicate reacts faster, large amounts of fluid are required, with fluid/rock ratios approaching unity, implying a fluid-focusing mechanism operating in the mantle source region of the xenoliths. Measurable, negative, disequilibrium pyroxene-olivine 18O fractionations are very short-lived at mantle conditions, surviving

Equilibrium oxygen isotopic fractionation factors between coexisting olivine and enstatite at mantle temperatures and pressures, 1000–1400°C and ≥ 13 kbar, were determined from a series of silicate-witherite oxygen isotope exchange experiments. Silicate-witherite fractionations ranged from − 2%0 at 1000°C to − 1%0 at 1400°C, and showed a systematic jump in magnitude at 1200°C of ~0.5%0 which remains to be explained. Enstatite-witherite fractionations varied nonlinearly with respect to inverse temperature squared. The high-temperature pyroxene-witherite data suggest enstatite undergoes a reversible structural change as it approaches an incongruent breakdown reaction to olivine, quartz, and sanbornite, which may account for the observed nonlinearity. A least-squares fit to the low-temperature equilibrium enstatite-olivine fractionation data forced through the origin has the functional form 1000 In αen-olT(K) = (1.41 ± 0.43) × 106/T2−(0.0 ± 0.24) for temperatures greater than 1000°C ( ± 2σ errors). At equilibrium under mantle conditions, the oxygen isotopic composition of enstatite is always heavier than, or equal to, that of coexisting olivine.

Oligocene to mid-Miocene, deep-shelf, bryozoan-rich limestones across southern Australia are variably altered to gray to orange, Ca-rich, medium-crystalline, sucrosic dolomite. Degree of replacement ranges from scattered rhombs to complete dolostone units several tens of meters thick and many kilometers in areal extent. The locale and timing of dolomitization are tightly constrained to shallow burial and mid- to late Miocene. Dolostone varies from friable to dense and well lithified. Dolomitization is fabric specific; muddy sediments are preferentially replaced; calcite bryozoans and brachiopods form biomolds. Geochemistry suggests that dolomitization was predominantly by seawater-limestone interaction, but admixing of continental possibly meteoric groundwater is required by data on stable and radiogenic isotopes and trace elements. Sr isotopes confirm a mid-Miocene age for the dolomite, if precipitated from seawater with no inherited limestone values. Dolomite crystals have undergone variable degrees of dissolution, and meteoric calcite cement has locally filled the resultant rhombic voids. Dissolution began in the crystal cores, implying some sort of metastability, and expanded outward until, in some ins ances, the whole dolomite crystal was dissolved, leaving dolomolds in otherwise unaffected limestone. This dolomite occurrence, well constrained by regional geology, shows that widespread, porous and permeable, fabric-destructive, medium-crystalline, sucrosic dolostone can form in the shallow subsurface soon after sediment deposition. Such dolomite, however, may be metastable in the presence of variably mixed continental-marine waters, and newly formed crystals can undergo dedolomitization soon after precipitation.

The Proterozoic Star Lake pluton in northern Saskatchewan, Canada, hosts several auriferous quartz veins that were deposited along an array of northeasterly striking shear zones. Gold mineralization was fracture controlled, and the main stage of deposition occurred during late fracturing episodes within the shear zones. The majority of the fluid inclusions trapped within the vein quartz are of secondary origin but are compositionally similar to those assigned as primary inclusions. The aqueous and CO 2 -bearing fluids (5-60 mole % CO 2 ) are of low to moderate salinity (1-17 wt % NaCl equiv) and low to moderate bulk density (< or =0.8- approximately 1.03 g/cc). The fluids in the inclusions have a wide range of homogenization temperatures from 200 degrees to 500 degrees C and, in conjunction with oxygen isotope equilibration temperatures of 400 degrees to 500 degrees C for most of the vein minerals, indicate trapping pressures for most fluids of 3 to 5.5 kbars. Fluid unmixing occurred within the veins, particularly among secondary fluid inclusions along microfractures. Fluid immiscibility facilitated through pressure release within shear zones may have been a major factor in gold precipitation wherein CO 2 -phase separation acted to destabilize gold complexes and subsequently led to gold precipitation.Quartz from auriferous veins has delta 18 O values ranging from 10 to 11.6 per mil, whereas barren quartz veins yield higher delta 18 O values of 12.9 to 13.4 per mil, indicating derivation from a distinct source. Calculated delta 18 O and delta D values for the mineralizing fluids range from approximately 6 to 8, and approximately -80 to -40 per mil, respectively, suggesting derivation from a metamorphic source or, at least, a system involving low water/rock ratios. The delta 34 S values of sulfides from the veins have a restricted range of 2 to 3.3 per mil, consonant with derivation from a uniform source. The delta 13 C values of calcite range from -11.2 to -5.6 per mil and thus are less distinctive as to the origin of carbon. Rb-Sr isotope systematics of tourmaline and feldspar in the veins indicate that the Au veins were deposited at 5 to 100 Ma after crystallization of the Star Lake pluton and contemporaneous with the waning stages of Hudsonian metamorphism.

Elemental concentrations in tree-rings from red and white oak trees at six sites across Southern Ontario, Canada, were assessed to determine whether they passively record changes in geochemical cycling in the presence of environmental stress. Periods of stress were defined as sustained periods with elevated δ 13 C values in tree-rings relative to atmospheric CO 2 during the same period. In some trees, nutrient concentrations (Ca, Mg, Mn) were erratic during historic periods of stress while chemically similar non-nutrients (Ba, Sr) and the anthropogenic pollutant Pb were not. Tree-ring concentrations of Ca and Sr were related to bedrock type and leachable concentrations in the soil. In contrast, tree-ring concentrations of Mg were not related to bedrock type, although Mg concentration in the soil leachate was. Tree-ring Mn, Ba and Pb concentrations were not related to bedrock type or soil concentrations, but were inversely related to soil pH. Erratic behavior of nutrient elements during historic periods of stress suggests that some nutrient concentrations in the environment were not always passively recorded by tree-rings.

Kyser, O'Neil, and Carmichael (1981, 1982) measured theδ18O values of coexisting minerals from peridotite nodules in alkali basalts and kimberlites, interpreting the nodules as equilibrium assemblages. Using Ca-Mg-Fe element-partition geothermometric data, they proposed an empirical18O/16O geothermometer: T(°C)=1,151−173Δ−68Δ2, whereΔ is the per mil pyroxene-olivine fractionation. However, this geothermometer has an unusual “crossover” at 1,150 °C, and in contrast to what might be expected during closed-system equilibrium exchange, the most abundant mineral in the nodules (olivine) shows a much greater range inδ18O (+4.4 to +7.5) than the much less abundant pyroxene (all 50 pyroxene analyses from spinel peridotites lie within the interval +5.3 to +6.5). Onδ18O-olivinevs. δ18O-pyroxene diagrams, the mantle nodules exhibit data arrays that cut across theΔ18O=zero line. These arrays strongly resemble the non-equilibrium quartzfeldspar and feldspar-pyroxeneδ18O arrays that we now know are diagnostic of hydrothermally altered plutonic igneous rocks. Thus, we have re-interpreted the Kyser et al. data as non-equilibrium phenomena, casting doubt on their empirical geothermometer. The peridotite nodules appear to have been open systems that underwent metasomatic exchange with an external, oxygen-bearing fluid (CO2, magma, H2O, etc.); during this event, the relatively inert pyroxenes exchanged at a much slower rate than did the coexisting olivines and spinels, in agreement with available exchange-rate and diffusion measurements on these minerals. This accounts for the correlation betweenΔ18O pyroxene-olivine and the whole-rockδ18O of the peridotites, which is a major difficulty with the equilibrium interpretation.

Measurements have been made of the oxygen isotope and chemical composition of glass and phenocrysts in lavas and coexisting minerals in mantle nodules. Temperatures of formation of these assemblages have been estimated from various chemical thermometers and range from 855° to 1,300° C. The permil fractionations between coexisting orthopyroxene and clinopyroxene in the lavas and nodules are all near zero. The fractionations between pyroxene and olivine vary from +1.2 to −1.4 and are a smooth function of temperature over the entire range. This function is given by T(° C)=1151-173Δ (px-d)-68Δ2(px-d) and has an uncertainty of ±60° (2σ). At temperatures above 1,150° C, olivine in the nodules becomes more18O-rich than coexisting clinopyroxene, orthopyroxene, and plagioclase. In combination with the experimental work of Muehlenbachs and Kushiro (1974), the olivine-pyroxene fractionations indicate that olivine also becomes substantially more18O-rich than basaltic liquids above 1,200° C. Geothermometers based on the oxygen isotope equilibration of basaltic liquid with olivine, pyroxene, and plagioclase are presented.

Low bromine concentrations, radiometric ages, textural relations and large-scale dissolution and collapse features in Devonian potash deposits of the Elk Point Basin in southern Saskatchewan indicate that diagenetic processes and late influxes of water have variably recrystallized the salts. Lack of correlation between homogenization temperature (Th), δD or δ18O values of fluid inclusions in the salt with stratigraphic position further indicates that the sequence is not primary. The stable isotopic compositions of fluid inclusion waters, including those in chevron-texture halites, are not those expected for primary crystallization waters and, in conjunction with Th, indicate recrystallization occurred at temperatures of 35–70°C from water originating in formations above the Prairie Evaporite Formation. Using Th as an estimate of the depth of burial and stable isotopes as indicators of the source of fluids, three major recrystallization events related to the evolution of the Elk Point Basin can be recognized.

The Little Rocky Mountains, Montana, are a result of doming of Archean basement gneisses and Phanerozoic sedimentary rocks produced by the emplacement of a quartz monzonite to syenite intrusive complex of early Tertiary age. The intrusive rocks have whole-rock delta 18 O values near 8 per mil, variable initial epsilon Nd values from -11 to -19, and initial 87 Sr/ 86 Sr ratios from 0.7050 to 0.7061 all of which are compatible with derivation from lower crustal rocks having variable compositions. The Zortman and Landusky mining areas in the Little Rocky Mountains produce Au (and Ag) from fracture systems filled with clay minerals and low concentrations of Au-bearing pyrite within the intrusive rocks. K-Ar ages of illites associated with the Au mineralization are near 60 m.y. and are similar to K-Ar and fission-track ages of the intrusive rocks themselves. The delta D and delta 18 O values of these illites indicate that they, and the Au-bearing pyrite, formed at temperatures near 250 degrees C from meteoric fluids having delta 18 O values near 1 per mil and delta D values near -80 per mil. This Au event is characterized by illites having much higher initial 87 Sr/ 86 Sr ratios (as high as 0.7784) than the intrusive rocks which host the fracture system, suggesting that Sr, and by inference the Au, most likely was derived from Precambrian basement gneisses.After the gold was emplaced, later fluids released the gold by oxidizing the pyrite; they then formed kaolinite and hydrothermal fluorite and dolomite. The latter two are characterized by 87 Sr/ 86 Sr ratios near 0.708, similar to those of the intrusive rocks and Phanerozoic carbonates. Low-salinity fluid inclusions in fluorite homogenize at 100 degrees to 200 degrees C and have delta 18 O values near -10 per mil and delta D values near -110 per mil, consistent with those of Tertiary ground waters. The delta 34 S values of pyrite do not correlate with gold contents, and their wide variation from -11.4 to +6.3 per mil cannot be used to distinguish among igneous, sedimentary, and metamorphic sources.