Whole-rock Values Research Articles

Determination of the ammonium content of potassic rocks and minerals by capacitance manometry: a prelude to the calibration of FTIR microscopes

Previous results for stepped-heating experiments on micas have been used to develop a sealed-ampoule technique for the determination of the ammonium contents of small samples (1–10 mg) of muscovite, biotite and whole-rock garnet-micaschists, by capacitance manometry. Co-existing muscovites and biotites from a garnet-mica-schist, gave the following results: [NH 4 +] Mu = 468 ± 10 ppm ( n = 4) and [NH 4 +] Bi = 1466 ± 50 ppm ( n = 4). [ NH 4 +] Mu [ NH 4 +] Bi = 0.32 ± 0.01 is in the range found previously by others who studied ammonium partitioning between muscovite and biotite by other methods. The δ 15N values of muscovite (+ 15.5 ± 0.9‰) and biotite (+ 15.4 ± 0.7‰) were indistinguishable, suggesting minimal selective partitioning of the nitrogen isotopes between the two minerals. The single whole-rock value of + 16.6‰ was perhaps slightly elevated compared to the mean values of the mineral separates but quite close to some of the individual measurements. The size of sample required for the technique (1–10 mg) is far smaller than those required for previous studies, being much closer to the spatial resolution of microscopes for Fourier transform infrared (FTIR) spectroscopy. Ammonium absorbs strongly in the infrared (notably at 1430 cm −1) and FTIR microscopy is potentially the fastest technique available for the determination of ammonium concentrations. It is hoped that the techniques described herein could be used to prepare a range of inter-laboratory standards which would allow for the calibration of these absorption bands, for all of the major minerals, permitting numerous detailed studies to be undertaken.

Carlin gold deposits, Nevada; origin in a deep zone of mixing between normally pressured and overpressured fluids

Gold mineralization at Carlin is clearly younger than hydrocarbon maturation (pre-Cretaceous) and felsic dike intrusion (Cretaceous), and older than deep oxidation (late Tertiary). Within the episode of gold mineralization, the main gold ore (MGO) stage and late gold ore (EGO) stage are distinguished paragenetically, with a variety of vein and mineralization types in each. MGO stage fluids contained 5 to 10 mole percent CO 2 , appreciable H 2 S, and 3 + or - 1 wt percent NaCl equiv. At least portions of MGO stage mineralization were characterized by two-phase boiling (CO 2 exsolution) at 215 degrees + or - 30 degrees C and 800 + or - 400 bars. In contrast, EGO stage fluids were gas poor with salinities 18 O (sub H 2 O) values of 5 to 9 per mil, whereas EGO stage fluids resembled unevolved meteoric water with delta 18 O (sub H 2 O) values 18 O values shifted from near whole-rock values of 12 + or -3 per mil to around 0 + or -1 per mil as EGO stage fluids flooded the system. Jasperoids also record a large range (9-22%) in delta 18 O (sub H 2 O) values. These data indicate the involvement of two very different fluids in ore deposition. Because MGO and EGO stage features are closely associated spatially with each other and with Au, As, Sb, Hg, and other ore elements, both fluids are believed to have both been present during most stages of ore deposition. At pressures of 80 to 85 percent lithostatic, depths of 3.8 + or - 1.9 km are required to accommodate the 800 + or - 400 bars of pressure recorded in MGO stage fluid inclusions. Carlin, therefore, is not an epithermal or hot spring deposit. Carbon dioxide in gas-rich MGO stage fluids may have originated either directly from buried intrusions or their contact aureoles, or from low-grade regional metamorphism at depth. The water may have been originally meteoric, and Au may be magmatic or derived from leaching of deep metamorphic or sedimentary rocks. Ore deposition appears to have occurred in zones of throttling at a pressure seal between normally pressured and overpressured regimes, where fluids experienced a change from near-lithostatic to hydrostatic conditions. Such pressure seals are common in deep sedimentary basins and may be a key to highly localized gold deposition. Mixing of two fluids and interaction with host rocks along thin permeable bioclastic horizons are believed to have been the major factors in depositing ore.

Oxygen isotope geochemistry of zircon

The high-temperature and small sample size of an I.R. laser system has allowed the first detailed study of oxygen isotope ratios in zircon. Low-magnetism zircons that have grown during metamorphism in the Adirondack Mts., N.Y. preserve primary δ 18O values and low-magnetism igneous zircons are likewise primary, showing no significant affect due to subsequent granulite facies metamorphism. The measured fractionation between zircon and garnet is Δ( Gt-Zrc) = 0.0 ± 0.2‰(1σ) for most low-magnetism zircons in meta-igneous rocks. The consistency of this value indicates equilibration at temperatures of 700–1100°C and little or no change in the equilibrium fractionation over this temperature range. In contrast, detrital low-magnetism zircons in quartzite preserve igneous compositions, up to 4‰ out of equilibrium with host quartz, in spite of granulite facies metamorphism. The oxygen isotope composition of zircon can be linked to UPb ages and can ‘see though’ metamorphism, providing a new tool for deciphering complex igneous, metamorphic and hydrothermal histories. Zircons separated by magnetic susceptibility show a consistent correlation. Low-magnetism zircons have the lowest uranium contents, the most concordant UPb isotopic compositions, and primary δ 18O values. In contrast, high-magnetism zircons are up to 2‰ lower in δ 18O than low-magnetism zircons from the same rock. The resetting of oxygen isotope ratios in high-magnetism zircons is caused by radiation damage which creates microfractures and enhances isotopic exchange. Zircons from the metamorphosed anorthosite-mangerite-charnockite-granite (AMCG) suite of the Adirondacks have previously been dated (1125–1157 Ma) and classified as igneous, metamorphic or disturbed based on their physical and UPb isotopic characteristics. Low-magnetism zircons from the AMCG suite have high, nearly constant values of δ 18O that average 8.1 ± 0.4‰(1σ) for samples ranging from 39 to 75 wt% SiO 2. Only olivine metagabbros have lower average values (6.4‰), consistent with the hypothesis that they represent nearly pristine samples of the anorthosite's parent magma. Whole-rock values of δ 18O are also high in the AMCG suite and increase with SiO 2 content, as predicted for a process of assimilation and fractional crystallization. Taken together, these data suggest that the elevated values of oxygen isotope ratios result from partial melting and contamination involving metasediments in the deep crust, before the crystallization of zircon. More normal values elsewhere in the Grenville Province record deep-seated, pre-1150 Ma regional differences.

Detection of ammonium in geological materials by evolved gas analysis

The association of anomalously high levels of ammonium with both metallic and energy mineral deposits is of potential use in mineral exploration. Conventional geochemical methods for detecting NH4 often provide only whole-rock NH4 values and do not identify the specific minerals hosting NH4. They may also lack sensitivity or be prone to interference. Evolved NH3 analysis is shown to be capable of distinguishing between different NH4-bearing minerals and can detect NH4 values down to 120 ppm in rocks hosting silver-gold vein mineralization. Fully quantitative determination of NH4 by this method is not possible as the NH3 evolved from minerals during heating partially oxidised; however, amounts of evolved NH3 do show a moderate positive linear correlation with NH4 content determined by a modified Kjeldahl method.

Constraints on the formation conditions of iron meteorites based on concentrations and isotopic compositions of nitrogen

A suite of iron meteorites has been analysed for their content and isotopic composition of nitrogen to investigate the behaviour of nitrogen in the iron meteorite parent bodies and any constraints this can place on the origin and formation of these meteorites. Whole-rock samples of iron meteorites yielded a range in δ 15N values from −96 to +156%. Most of the nitrogen is found in the Fe, Ni metal phase, with areas of plessite containing ca. 3 times more nitrogen than kamacite and with a δ 15N value 3%. lower, although occasionally high concentrations (up to ≈40%) of nitrogen can be found in nitrides. The nitrogen isotopic composition of the nitrides, and other phases present, is typically less than ±5%. of that of the whole-rock value. The effect of extreme shock and/or recrystallisation of the metal phase is to cause a loss of nitrogen and a corresponding shift in δ 15N values towards heavier values by up to 50%. due to diffusive loss of nitrogen. In contrast, the range of δ 15N values shown by other members of an individual group with more normal shock features is <11%. For group IIIAB specimens this intragroup variation appears to be due to a slight loss of nitrogen during crystallisation, this mechanism having been taken to extreme conditions during the pallasite formation event. In the non-magmatic IAB group, individual samples display almost no isotopic variation. The nitrogen abundance in irons appears to be closely linked to the concentration of volatile trace elements and, although this may be the result of condensation effects, the high levels of nitrogen abundance may suggest that these levels were set during a re-equilibration event on the parent bodies. On the other hand, the only parameter with which δ 15N values show any systematic covariation are the metallographic cooling rates of the irons, suggesting an indirect link between nitrogen and the internal heat source of the parent bodies. One possibility is that there may have been an injection of 15N-rich nitrogen together with a short-lived radionuclide such as 26Al, or perhaps 60Fe, into the solar nebula prior to formation of the parent bodies.

Fluid inclusion and mineral isotopic compositions (H [sbnd]C [sbnd]O) in eclogitic rocks as tracers of local fluid migration during high-pressure metamorphism

Eclogite facies metagabbros from the Monviso ophiolitic complex (Italian Western Alps) provide a unique opportunity to trace fluid migration processes in a portion of the oceanic crust that has undergone subduction at a depth of > 40 km. We have determined the omphacites δ 18O and the abundance, δD and δ 13C of hydrous and carbonaceous compounds present in whole rocks which are believed to trace the residual phases of what was mobilized in the original rocks during subduction. Prograde dehydration reactions and eclogitization of hydrothermally altered oceanic metagabbros was accompanied by approximately 90% fluid loss. The remaining fluid was trapped as primary water-rich fluid inclusions in omphacite megacrysts that developed at the expense of the precursor magmatic pyroxene. Deformation of the eclogitic rocks resulted in continuous recycling of fluid between mylonites and omphacite veins without further fluid loss from the host ductile shear zone. Oxygen isotopes of omphacite (omp) and hydrogen isotopes of water in the fluid inclusions (FI), analyzed in low-strain rocks, mylonites and undeformed/deformed veins show marked variations, δ 18O omp and δD FI values ranging from +3.0 to +5.3‰ and from −31 to −93‰, respectively. Detailed isotopic analysis of several individual vein-wallrock pairs show that the scale of isotopic equilibration is of the order of one centimeter. Therefore, the eclogitic minerals and fluids filling the veins are concluded to be locally derived. Our results argue against recent models which suggest large-scale mass flushing of isotopically homogeneous fluids during subduction zone metamorphism. On average, the H 2O contents and δD FI value are within the upper mantle range. The carbon has been inherited from the metamorphic transformation of the original carbon present in the oceanic crust and has a mean δ 13C value of −24.2 ± 1.2‰. Two carbonaceous components can be recognized, condensed carbon which represents the major carbon species and carbonate daughter crystals present in fluid inclusions. δ 18O omp values are significantly lower than those reported for mantle minerals and are similar to pyroxene and whole-rock values from hydrothermally altered oceanic crust. It is suggested that the isotopic imprint of the Monviso eclogitic minerals and fluids represents the signature of mid-ocean ridge hydrothermal alteration and subduction zone eclogitization processes. The rocks studied could be the missing link between altered oceanic gabbros and eclogitic xenoliths. In addition, the range in δ 18O omp values recorded in the different microstructural domains covers most of the δ 18O values reported in type A, B and C eclogitic xenoliths, implying that caution must be exercised when using oxygen isotopes as an indicator of the origin of eclogitic xenoliths.

On the significance of crater ages: New ages for Dellen (Sweden) and Araguainha (Brazil)

Samples of a few grams in weight from the impact melt at the Dellen crater (Sweden) and the Araguainha dome (Brazil) were analyzed by Rb-Sr and Sm-Nd techniques. For Dellen, pure labradorite, orthopyroxene, and glass separates from a clast-free impact melt rock with holohyaline ground mass yield an internal Rb-Sr isochron with 89.0 ± 2.7 Ma ( I sr = 0.83571 ±0.00007; 2σ), compared to Ar-Ar plateau ages of 102 ± 3.2 Ma and 109.6 ±2.0 Ma published for total melt rocks. Various whole-rock values not plotting on this Rb-Sr isochron indicate primary variations in 87Sr 86 Sr over the melt sheet. The age of the Araguainha dome (Brazil) is bracketed by the Rb-Sr model age of 243 ± 19 Ma ( I Sr = 0.7226 ± 0.0005), for altered cordierite in the impact melt, and by the internal Rb-Sr isochron age of 449 ± 9 Ma ( I Sr = 0.7140 ± 0.0007), for an uplifted granite (shock stage Ib). In melt rocks from both impact structures 143Nd 144Nd was not homogenized during total rock melting: Sm-Nd data either scatter around the primary isochron (Araguainha) or point to a partial resetting (Dellen). This data array may be caused by tiny refractory REE-rich inclusions in the main mineral phases which dominate the Sm-Nd system of the impact melt rocks. The significant difference between the new isochron age for Dellen and Ar-Ar “ages” show that the data base for analysis of periodicity in cratering record most probably lack precision and accuracy.

Massive sulfide deposits of the Noranda area, Quebec. IV. The Mobrun mine

The Mobrun polymetallic deposit near Rouyn–Noranda comprises two complexes of massive sulfide lenses within mainly felsic volcanic rocks of the Archean Blake River Group. The Main lens contained 3.37 Mt of massive sulfides, with 1989 reserves of 0.95 Mt at 0.81% Cu, 2.44% Zn, 30.3 g/t Ag, and 2.2 g/t Au. The 1100 complex, located ~250 m to the southeast of the Main complex, contains estimated 1989 reserves of 10.4 Mt at 0.76% Cu, 5.43% Zn, 37.4 g/t Ag, and 1.35 g/t Au.Host volcanic rocks of the Main complex are mostly massive, brecciated, and tuffaceous rhyolites. The rhyolites are commonly strongly sheared parallel to lithological contacts, which are locally displaced by high-angle faults. Immobile-element plots such as Y–Zr and Nb–Zr show a separation of rhyolite data into two distinct alteration trends that generally correspond to massive and in situ brecciated rhyolite of the footwall, and tuffaceous rhyolite of the hanging wall. The hanging wall has tholeiitic Zr/Y ratios (3–5), whereas the footwall has mildly calc-alkaline Zr/Y ratios (7–9). Several immobile-element trends indicate that there was a subtle but clear change in rhyolite composition near the time of ore deposition. Identification of chemically distinct footwall and hanging wall rhyolites allows these units to be recognized and traced along strike, even where alteration is strong. Sericitization and silicification extend at least 100 m from the orebody, with local chloritic zones in the upper footwall. Calculated mass changes indicate that the footwall generally has lost silica mass relative to the hanging wall. Alteration zones associated with mineralization have mass gains in FeO + MgO and K2O gains, but mass loss in silica.The 1100 complex, located stratigraphically below the Main complex, is hosted by rhyolite, with one main andesite interval in the footwall. The footwall contains three chemically distinct rhyolite types, all tholeiitic. Hanging-wall rhyolites are, however, mildly calc-alkaline, and thus are chemically comparable to, and correlated with, the footwall of the Main complex. Rhyolites within ~100 m stratigraphically of the Main and 1100 complexes commonly have positively shifted δ18O whole-rock values of 11–13‰. These high values are interpreted as the result of an initial, widespread phase of low-temperature hydrothermal alteration that increased δ18O values by 3–5‰ relative to unaltered rhyolites. Some footwall rhyolites, however, are relatively depleted in 18O, strongly depleted in Ca–Na and depleted in Eu2+. Rhyolites with these chemical features have been overprinted by higher temperature alteration, presumably in localized feeder zones. All four rhyolite types near the 1100 complex are chemically recognizable despite contrasting alteration.The orebodies are interpreted as synvolcanic, based on their occurrence along distinctive volcanic contacts, and the presence of primary sulfide textures where deformation is minor. The chemostratigraphic framework defined for the host rhyolite sequence can be used to trace critical volcanic contacts through lithologically monotonous, strongly altered, and faulted stratigraphy.

Low-18O Scourie dike magmas from the Lewisian complex, northwestern Scotland

Research Article| June 01, 1991 Low-18O Scourie dike magmas from the Lewisian complex, northwestern Scotland Ian Cartwright; Ian Cartwright 1Department of Earth Sciences and VIEPS, Monash University, Clayton, Victoria 3168, Australia Search for other works by this author on: GSW Google Scholar John W. Valley John W. Valley 2Department of Geology and Geophysics, University of Wisconsin, Madison, Wisconsin 53706 Search for other works by this author on: GSW Google Scholar Author and Article Information Ian Cartwright 1Department of Earth Sciences and VIEPS, Monash University, Clayton, Victoria 3168, Australia John W. Valley 2Department of Geology and Geophysics, University of Wisconsin, Madison, Wisconsin 53706 Publisher: Geological Society of America First Online: 02 Jun 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (1991) 19 (6): 578–581. https://doi.org/10.1130/0091-7613(1991)019<0578:LOSDMF>2.3.CO;2 Article history First Online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Ian Cartwright, John W. Valley; Low-18O Scourie dike magmas from the Lewisian complex, northwestern Scotland. Geology 1991;; 19 (6): 578–581. doi: https://doi.org/10.1130/0091-7613(1991)019<0578:LOSDMF>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract The Scourie dike swarm of the Lewisian complex, northwestern Scotland, comprises a dominant suite of quartz tholeiites and lesser volumes of norites, olivine gabbros, and bronzite picrites that were emplaced at depths of >14km in two episodes at 2.0 and 2.4 Ga.Unaltered quartz tholeiite dikes have low and homogeneous whole-rock δ18O values (2.04‰ ±0.14‰ relative to Standard Mean Ocean Water) and concordant mineral δ18O values (plagioclase = 3.1‰ to 4.1‰, clinopyroxene = 1.5‰ to 1.9‰, magnetite = -0.3‰ to 0.6‰). The dikes intrude granulite facies gneisses that have heterogeneous δ18O whole-rock values (5.8‰ to 9.7‰) and show no evidence of 18O depletion. Low-δ18O intrusive igneous rocks are generally believed to have exchanged with high-temperature meteoric or oceanic fluids after emplacement. The heterogeneous high δ18O values of the country rocks suggest that the Lewisian complex has not been pervasively infiltrated by such fluids. It is also unlikely that fluids were channeled along the dikes because: (1) the gneisses immediately adjacent to the dikes have unaltered mineralogies and stable-isotope ratios; (2) the dikes preserve unaltered igneous textures and mineralogies; (3) the dike minerals do not show open-system oxygen-isotope resetting; and (4) surface-derived fluids generally do not penetrate to the depths at which the dikes were intruded. It is more likely that these dikes were emplaced as primary low-δ18O magmas derived from the subcontinental lithosphere. The geochemistry of the Scourie dikes suggests that they have not assimilated large volumes of continental crust, and the best explanation of the data is that the dikes were derived from a low-δ18O mantle source that possibly contained the remains of subducted, hydrothermally altered oceanic crust. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Open System Diagenesis and the Loss of Provenance Signatures: Isotopic and Elemental Data from Sandstones, Frio Formation, South Texas: ABSTRACT

Oligocene Frio sandstones in which detrital feldspars are completely altered typically contain authigenic minerals that exceed 30% of the rock volume. Elemental and isotopic analyses of whole rocks show that authigenesis is accompanied by both gain and loss of material. Subsurface sandstones for elemental and isotopic analysis were sampled from approximately 3,000-14,000 ft (0.9-4.3 km). Over this depth range, dissolution and minor albitization of detrital K-feldspar is accompanied by loss of 2 to 3 wt.% of K{sub 2}O from the whole rock. Loss of about 50% of the initial strontium also correlates with alteration of detrital feldspars. {sup 87}Sr/{sup 86}Sr for whole-rock and silicate fractions change with depth as strontium characteristic of the detrital fraction is replaced by more radiogenic strontium derived from deeper in the basin. Aluminum, titanium, phosphorous, manganese, and barium also exhibit trends compatible with net export, but uncertainties about initial variation remain. Trends for silicon, sodium, iron, boron, and lithium are equivocal. Magnesium gain accompanies precipitation of authigenic chlorite. Whole-rock calcium values are dominated by calcite that includes both detrital and authigenic components. Significant dissolution of the detrital carbonate component does not occur, and calcium in authigenic carbonate exceeds by about five times the calcium releasedmore » during destruction of detrital silicates, thus requiring considerable calcium and CO{sub 2} import. Material transfer on the scale observed suggests that simple burial diagenesis is not a closed-system process for several major elements and introduces a major obstacle to the use of elemental data for provenance interpretation.« less

Oxygen and carbon isotope composition of cold-water Berriedale Limestone (Lower Permian), Tasmania, Australia

The Berriedale Limestone formed at about 80°S paleolatitude and contains many glacial dropstones. It formed during a period of major Gondwana deglaciation. The Berriedale Limestone contains mostly bryozoans, brachiopods and bivalves, with some intraclasts and rare pellets. The faunal diversity is low and the fauna are similar to the modern cold-water foramol faunal assemblage. Micrite, microspar and spar occur as equant to well developed rhombs of calcite. The coarse spar cements are bored and are ruptured by dropstones, indicating submarine origin of low-Mg calcite at water-temperatures of around 3°C. The mixing zone cementation was preceded by erosion of early formed crystals. The eroded crystals occur as inclusions in mixing zone cements. The fauna are characterized by heavy δ 13C and light δ 18O. The whole-rock field of δ 18O- δ 13C falls at the edge of “Normal Marine Limestone” and deviates to lighter δ 18O values (down to −16.7‰ PDB). Lightest δ 18O values ( ∼ −22‰ PDB) of fresh-water sparry calcite cement are similar to those in the Early Permian continental tillites, suggesting that the Permian sea was diluted by isotopically light melt waters. Micrite δ 18O values (−9.2 to −12.6‰ PDB) are within the range of whole-rock values. The δ 18O values of calcite in shales are lighter than limestone values. The δ 18O values of the fauna give an unrealistic range of sea-water temperatures because the fauna have equilibrated with variable amounts of melt waters. However, calculated original δ 18O values of the fauna indicate temperatures < 4°C. The heaviest δ 18O of fauna gives cold temperatures of 9°C (with δ w −2.8‰) and −3°C (with δ w −6‰ ). The lightest values of sparry calcite cements ( −22‰ PDB) indicate that the limestone reacted with cold melt waters. The δ 18O of Permian sea is estimated to be about +1.2‰ and was diluted by melt waters as light as −27‰ SMOW.

Age of the barramundi orogeny in northern Australia by means of ion microprobe and conventional U-Pb zircon studies

U-Pb zircon geochronological studies are used to quantify the tectonostratigraphic history of a major crust-forming event in early Proterozoic terranes of northern Australia. Magmatism and tectonic processes associated with this event, the Barramundi Orogeny, appear similar over very wide areas, about one third of the Australian continent. The Barramundi event marks the deformation, metamorphism and cratonization of the earliest Proterozoic ∼2.0−1.9 Ga basinal successions, and it was followed by and closely linked to extensive orogenic felsic magmatism. The Yaringa Metamorphics are an inlier of paragneiss and schist that pre-dates 1865 ± 3 Ma Leichhardt felsic magmatism and younger basin formation in the Mount Isa Inlier. From an ion microprobe U-Pb zircon study of massive and migmatitic gneiss, the time of Yaringa metamorphism is established at 1890±8 Ma. Recognized younger Proterozoic deformations in the interval 1610−1550 Ma are generally not evident in zircon U-Pb systems, but have totally reset local Rb-Sr whole-rock values. Conventional U-Pb work on multi-grain zircon samples of Yaringa gneiss gives a geologically meaningless age (2090±40 Ma) that is an artifact of complex zircon growth, part of which was initially inherited from a 2.55 to 2.2 Ga provenance, and the other major part of which was formed in situ during the 1890 Ma high-grade metamorphism. Predictably, for simple zircon systems in low-grade metavolcanic térranes, conventional and ion microprobe zircon ages are in close agreement. Dating of volcaniclastic sequences above and below the Barramundi tectonic break in the Pine Creek Inlier brackets the orogeny in this terrane to a similar time frame, 1885–1870 Ma. Some zircons which crystallized at this time form selvedges around older cores whose age of 2100-2000 Ma may be relics of an earlier deep crustal underplating. The uniform chemistry of post-Barramundi felsic magmatism, and the similar tectonic setting of the areally extensive Barramundi event, are mirrored by their temporal coherency. Cycles of geosynclinal deposition, burial, metamorphism and uplift took place during intervals of several million years or less, implying comparable rates for early Proterozoic orogenesis to those established for Tertiary and Quaternary settings.

Oxygen-isotopic composition of basalts from young spreading axes in the eastern Pacific

Oxygen-isotopic composition data are reported for young oceanic basalts from two different tectonic settings in the eastern Pacific: (1) the uppermost basement on the Galapagos Rift (&lt; 1 Ma old), and (2) subbasement depths up to ≈150 m in the Gulf of California (≤ 3.5 Ma old). In the Galapagos area, whole-rock δ18O values exhibit a narrow range of 5.7–6.6‰, consistent with minimal to slight low-temperature alteration. Limited alteration probably reflects a combination of young basement age, low temperatures for solutions in the uppermost basement, and confinement of solutions mainly to discrete fractures. In the Gulf of California, by contrast, high whole-rock values are found at site 474A, near the Baja margin, where the greatest range in δ18O also occurs, from 2.5 to 12.5‰. This indicates considerable variation in both the temperature and degree of alteration at this site, probably a consequence of the heterogeneous basement sequence (intercalated pillow lavas, sills, and wet sediments). Differences in δ18O of up to 6‰ can exist over several metres, implying sharp changes in temperature or local water/rock ratio. The development of low-temperature alteration is more pronounced at site 474A (≈3.5 Ma) than at the several other (younger) crustal locations drilled in the Gulf of California, presumably because of longer exposure to seawater during recession of site 474A from the spreading axis.Analysis of primary magmatic phases at all sites yield 18O fractionations between plagioclase, clinopyroxene, and magnetite consistent with crystallization in a closed system and without high-temperature subsolidus reaction with seawater oxygen (even where sediments are intercalated within the basement). At site 485 in the Gulf of California, a thick basaltic sill yielded the lowest calculated crystallization temperatures (920 ± 50 °C) on the basis of plagioclase–magnetite fractionations; this agrees reasonably with groundmass quartz estimated to have formed at 860 ± 40 °C. Two vein carbonates from the uppermost part of the igneous sequence at this site were formed at temperatures of less than a few tens of degrees.We also report high-precision strontium- and oxygen-isotopic data on mineral separates from four basalts recovered at site 504B (6 Ma old). The most striking feature of these data is very substantial contamination of most mineral phases by seawater Sr, even though the same splits have typical primary magmatic oxygen-isotopic values. Relative to an unaltered mid-ocean ridge basalt 87Sr/86Sr value of 0.70265 ± 0.0001, 87Sr/86Sr ratios of plagioclase range from 0.70270 to 0.70359 (for apparently primary δ18O values of 5.35–5.75‰); for clinopyroxene, 87Sr/86Sr ratios range from 0.70309 to a very enriched value of 0.70695 (δ18O values of 4.97–5.30‰, with one value of 6.05‰). These results indicate that young basalts may have experienced significant interaction with seawater, which is not recorded in terms of oxygen-isotope exchange.

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 in 18O relative to unaltered basalts, and the dike section is variably depleted and enriched. Basalts in the pillow section at Site 504 haveδ 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 in 18O 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 CO 2. 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 and 18O enrichment of fluids, resulting in local increases inδ 18O of rocks which had been previously depleted in 18O during prior axial metamorphism.

Oxygen isotope systematics indicating large-scale circulation of fluids in granitic rocks from southwest Sweden

In the Precambrian basement of SW Sweden two formations of granitic rocks have been strongly recrystallized but have preserved their granitic texture. Most magmatic crystals have been replaced by green-schist-facies mineral assemblages. δ 18O whole-rock values for the granitic rocks vary from +1.7‰ to +8.2‰ (relative to SMOW) and extend below the normal range for granitic rocks (+6 to +10‰). Quartz-K-feldspar pairs show 18 O 16 O fractionations of +5.4‰ and +7.8‰, indicating disturbance of magmatic equilibrium fractionations. It is concluded that the O-isotope redistribution took place during the pervasive recrystallization of the rocks, under the influence of a large-scale flux of water (partly) of meteoric origin, probably in a convective hydrothermal system set up by the intrusion of the granites themselves. Estimation of pre-alteration δ 18O whole-rock values for one of the granitic units suggests a partly S-type origin.

Strontium isotopic composition of some brines from the Precambrian Shield of Canada

Twenty-four groundwater samples from seven operating mines at Sudbury, Yellow-knife and Thompson (Ontario, North West Territories and Manitoba, resp.), all from depths greater than 1 km and ranging in total dissolved solids (TDS) from 1900 to 250,000 mg l −1, were measured for their 87 Sr 86 Ar values. Each geographic location gives a limited range in values and each location is distinct from the others. This is interpreted as the result of extensive water-rock interaction on a local scale. For most of the time, these brines were isolated and only recently have been exposed to surface water as a result of the mining operations. The extent of the isolation is shown by the contrasting isotopic values of two “pockets” of water (0.711 vs. 0.716) located on opposite sides of the same fault system on the North Range at Sudbury. The exchange at all sites probably has continued until the present, as indicated by the close agreement between water and present-day 87 Sr 86 Sr whole-rock values. If so, it suggests that there is no single age for such brines, but it may be possible to date stages in the water's evolution by determining the age of secondary minerals that equilibrated with the water.

Oxygen-isotope geochemistry of Archean granitoid gneisses and related rocks in the English River Subprovince, northwestern Ontario

Whole-rock δ 18O values for Archean granitoid gneisses from the southern domain of the English River Subprovince, northwestern Ontario, increase from an average of +6.5 for 3000 Ma old tonalites in the eastern Lac Seul area to an average of +7.9 for 2750–2800 Ma old tonalites, trondhjemites and granodiorites that occur in the Kenora and eastern Lac Seul areas. Still younger granitoid plutons and dykes (2660-2560 Ma) that intrude the older gneisses also have average δ 18O = +7.9. These values lie towards the low end of the range known for normal granitoid rocks. Oxygen-isotope data for minerals from the granitoid rocks indicate minor isotopic disequilibrium, but no evidence exists for large-scale depletion in 18O subsequent to granitoid formation. Rocks rich in 18O are abundant only in the northern domain of the English River Subprovince, which is dominated by paragneiss and metasedimentary migmatite (δ 18O +9−12). Within the Kenora area of the southern domain, however, a remnant supracrustal sequence characterized by a thin metachert/iron formation unit (δ 18O = +8−10) has been recognized. The low whole-rock oxygen-isotope values indicate that pre-existing 18O-rich supracrustal rocks were not involved in the genesis of the 3000 Ma old gneisses and have made only limited, if any, contribution to the magmas that formed the younger units. This conclusion is supported by the low initial Sr-isotope ratios of these rocks (0.7009-0.7027). The oxygen- and strontium-isotope data require that most of the granitoid rocks in the southern domain of the English River Subprovince comprise juvenile or near-juvenile additions to the continental crust.

A Stable Isotope Study of Carbonate Cements in Sligo Formation: ABSTRACT

The Sligo Formation in Texas is an Early Cretaceous subsurface carbonate sequence representing the upper part of a transgressive cycle. The Sligo carbonates attain a maximum thickness of 305 m at the ancestral shelf edge where the sequence consists of rudistid boundstones and grainstones at depths of 4,570 m. Porosity and permeability within the Sligo are controlled by the abundance of radiaxial fibrous calcite cement and/or coarse equant-calcite-spar cement. Other cements recognizable in thin section are meniscus calcite and clear euhedral dolomite. The equant calcite spar has an average of ^dgr18O relative to PDB of -1.92 and ^dgr13C of 2.80. The radiaxial fibrous calcite has average ^dgr18O of -1.77 and ^dgr13C of 1.61. The average wholerock values are ^dgr18O of -1.52 and ^dgr13C of 2.61. The similarity between these values suggests isotopic homogenization due to diagenesis; however, the equant-calcite spar has a range in ^dgr18O of -4.98 to 0.29 indicating deposition by meteoric waters. The radiaxial fibrous- alcite cement has a narrow range in ^dgr18O of -2.10 to -1.70 consistent with an origin as an early marine replacement cement. Intriguing correlations exist between modal abundances of cement types and their ^dgr18O and ^dgr13C values. These data provide important constraints for models predicting porosity and permeability evolution during carbonate diagenesis and have important implications for hydrocarbon exploration strategies. End_of_Article - Last_Page 628------------

Geochemical, mineralogical, and isotopic data relating to the origin and tectonic setting of the Rossland volcanic rocks, southern British Columbia

Bulk-rock and mineral chemical and isotopic analyses of Rossland volcanic rocks are used to infer the nature of the magma extruded in the Nelson–Rossland area of southern British Columbia during the Early Jurassic. Metamorphism of the volcanic rocks to subgreenschist and greenschist facies precludes use of mobile major and trace elements (e.g., Na, K, and Rb) as petrogenetic indicators. Data on immobile elements (Ti, Zr, and Y) and pyroxene compositions indicate that the volcanic rocks formed in a destructive-margin plate tectonic environment. Present-day 87Sr/86Sr ratios range from 0.70372 to 0.70480 but do not define an isochron. Corrected to Jurassic time, the initial ratios range from 0.70328 to 0.70404. Whole-rock δO18 values range from 7.9 to 11.6%, correlating inversely with metamorphic grade. Clinopyroxene δO18 of 4.8–6.5 is comparable with fresh clinopyroxenes from mafic rocks of mantle origin. In view of the preponderance of basaltic rather than andesitic rock types, and because of the nature of the lithologies within the volcanic rocks and associated sediments, an island-arc setting is indicated. The appearance of primary amphibole in basaltic members of the Rossland suite, and the occurrence of ankaramitic rocks, are thought to indicate a mildly alkalic rather than a subalkalic parent magma. Comparison of the Rossland volcanic rocks with those of recent island arcs, and consideration of the Upper Triassic – Lower Jurassic paleogeography in the Cordillera, suggest the rocks may be related to a localized oceanic basin, their extrusion being associated with faults bounding its western edge.

The genesis of sulfide mineralization in a portion of the Potgietersrus Limb of the Bushveld Complex

In the Potgietersrus limb of the Bushveld Complex, South Africa, sulfide mineralization is associated with the intrusive contact between Bushveld pyroxenitic gabbros and floor rocks consisting of Malmani dolomite, the overlying Penge banded ironstone, and sedimentary rocks of the Pretoria series. Whole-rock values of up to 0.5 and 1.8 weight percent copper and nickel, respectively, were obtained in borehole intersections through the contact zone and values of up to 28 ppm platinum group metals have been reported from the area.Petrographic and isotope geochemical studies were undertaken on a representative suite of rocks. It was shown that the pyroxenitic gabbro in contact with the floor rocks was equivalent in composition to the Merensky Reef. Xenoliths of Malmani dolomite incorporated into the Bushveld rocks reacted with the magma and were metamorphosed to calc-silicate and carbonate silicate hornfelses. Increased sulfide concentrations are found in association with the xenoliths. These rocks, together with the Bushveld pyroxenitic gabbros and recrystallized argillaceous sediments of the Pretoria series and the pre-Bushveld noritic sills, constitute a 120-m-thick mineralized zone known as the Platreef.The results of carbon and oxygen isotope determinations on unaltered Malmani dolomite are consistent with values expected for an evaporitic environment of deposition during the Precambrian. Anhydrite normally associated with evaporitic sequences is likely to have been preserved in the Malmani dolomite when it was incorporated as xenoliths into the Bushveld magma. Sulfur isotope studies on sulfides from the Platreef zone give a range of delta 34 S values of from 6.8 to 9.2 per mil. As the delta 34 S values for primary magmatic Bushveld sulfides fall in the range -0.6 to +3.5 per mil, the isotope geochemistry clearly indicates a contribution of sedimentary-derived sulfur to the original Bushveld magma. Sulfide mineralization in the Platreef is considered, therefore, to have been derived from the combined influence of a sulfur-rich Merensky Reef-type magma and the incorporation of additional sulfur from the Malmani dolomite, either through direct diffusion or in association with a carbonate melt.Between the banded ironstone and the Platreef zone there is a 50-m-thick zone of coarsegrained iron- and sulfide-rich pegmatoidal pyroxenite. Pyrrhotite from these rocks gave delta 34 S values of 2.7 and 8.7 per mil which are within the possible range of magmatic sulfur. These rocks may be related to the intrusive pipes found elsewhere in the Bushveld Complex.