T. Kurtis Kyser

Evidence from mantle xenoliths for relatively thin (<100 km) continental lithosphere below the Phanerozoic crust of southernmost South America

Abstract Garnet peridotite xenoliths in the Quaternary Pali-Aike alkali olivine basalts of southernmost South America are samples of the deeper portion of continental lithosphere formed by accretion along the western margin of Gondwanaland during the Phanerozoic. Core compositions of minerals in garnet peridotites indicate temperatures of 970 to 1160°C between 1.9 and 2.4 GPa, constraining a geothermal gradient which suggests a lithospheric thickness of approximately 100 km below this region. Previously, this lithosphere may have been heated and thinned to ≤80 km during the Jurassic break-up of Gondwanaland, when widespread mafic and silicic volcanism occurred in association with extension in southern South America. Subsequent cooling, by up to >175°C, and thickening, by about 20 km, of the lithosphere is reflected in low-temperature ( 970°C) peridotites by textural evidence for the transformation of spinel to garnet. A recent heating event, which probably occurred in conjunction with modal metasomatism related to the genesis of the Pali-Aike alkali olivine basalts, has again thinned the lithosphere to 970°C) peridotites as chemical zonation of pyroxenes involving a rimward increase in Ca, and by kelyphitic rims around garnet. The majority of moderate- and high-temperature (>970°C) xenoliths are petrochemically similar to the asthenospheric source of mid-oceanic ridge basalts: fertile (>20% modal clinopyroxene and garnet), Fe-rich garnet lherzolite with major element composition similar to estimates of primitive mantle, but large-ion-lithophile and light-rare-earth element depletion relative to heavy-rare-earth elements, and with Sr, Nd, Pb, Os, and O isotopic compositions similar to MORB. In contrast, infertile, Mg-rich spinel harzburgite is predominant among low-temperature (

Brachiopod δ18O values do reflect ambient oceanography: Lacepede Shelf, southern Australia

Although commonly used as proxies for attributes of ancient ocean waters, the δ 18 O values of brachiopods from modern seas are little studied. To evaluate the utility of brachiopods as recorders of regional oceanography, modern shells from the Lacepede Shelf (25 000 km 2 ) of southern Australia were analyzed for δ 18 O, and the results were compared to the values of ambient seawater. Southern Ocean waters cover this area of extensive cool-water carbonate deposition, but there are distinct sectors of seasonal upwelling and lesser fluvial outflow. δ 18 O values of brachiopods across the environmental spectrum from 40 to 300 m water depth are in general isotopic equilibrium with surrounding seawater. Nevertheless, δ 18 O values from individual sample sites vary as much as 0.60‰. The area of cold-water upwelling in particular is clearly delimited by a group of high δ 18 O values. The range of values across this one shelf, on the order of 2.5‰, is similar to the range of values postulated on the basis of similar results for secular changes in many ancient oceans.

Glendonites in Neoproterozoic low-latitude, interglacial, sedimentary rocks, northwest Canada: Insights into the Cryogenian ocean and Precambrian cold-water carbonates

Stellate crystals of ferroan dolomite in neritic siliciclastic and carbonate sedimentary rocks between Sturtian and Marinoan glaciations in the Mackenzie Mountains are interpreted as replaced glendonites. These pseudomorphs after ikaite indicate that shallow seawater at that time was near freezing. Stromatolites verify that paleoenvironments were in the photic zone and physical sedimentary structures such as hummocky cross-bedding confirm that the seafloor was repeatedly disturbed by storms. Glendonites within these low-latitude, continental shelf to coastal sedimentary deposits imply that global ocean water during much of Cryogenian time was likely very cold. Such an ocean would easily have cooled to yield widespread sea ice and, through positive feedback, growth of low-latitude continental glaciers. In this situation gas hydrates could have formed in shallow-water, cold shelf sediment, but would have been particularly sensitive to destabilization as a result of sea-level change. Co-occurrence of pisolites and glendonites in these rocks additionally implies that some ooids and pisoids might have been, unlike Phanerozoic equivalents, characteristic of cold-water sediments.

Mobile Pb-isotopes in Proterozoic sedimentary basins as guides for exploration of uranium deposits

Lead isotope ratios and associated trace element concentrations (U, Th and Pb) extracted by partial-leaching with 2% nitric acid from Proterozoic sandstones and basement rocks reveal much about the fluid evolution of sedimentary basins hosting unconformity-type uranium deposits. In addition, these techniques have great potential as a guide for exploration of uranium and other types of deposits in basins of any age. Isotope ratios of Pb in Proterozoic sandstones from basins known to contain high-grade uranium deposits are radiogenic at key geological localities and settings distal to known mineralization and particularly in altered zones proximal to mineralization. Sandstones completely cemented by quartz overgrowths typically have non-radiogenic Pb isotope ratios, indicating early closure of porosity and isolation of these rocks from later fluid events. Alternatively, the unconformity served as both a source of uranium and radiogenic Pb as well as an avenue for late-stage (<250–900 Ma) fluid flow. The mafic volcanic units, which are relatively reducing lithologies and therefore have removed uranium from basinal brines, have uranium-supported radiogenic Pb isotope ratios. Comparison of 238U/206Pb and 206Pb/204Pb ratios is useful in determining the timing and nature of U and Pb migration before, during and after mineralization in these basins. This comparison can be used to delineate the presence of radiogenic Pb isotope ratios that are not internally supported by uranium and thorium in rocks, eventually providing the explorationist with geochemical vectors that point toward sites of high potential for economic uranium mineralization.

Sulfur isotope systematics and platinum group element behavior in REE-enriched metasomatic fluids: A study of mantle xenoliths from Dish Hill, California, USA

The sulfur isotopic compositions, trace and rare earth element (REE), and platinum group element (PGE) contents were determined for a suite of anhydrous spinel lherzolite xenoliths and their metasomatized equivalents from Dish Hill, California, USA, to evaluate the mobility of these elements in metasomatic fluids. Anhydrous xenoliths have REE patterns that vary from being slightly LREE-depleted to flat, as well as variable enrichments of LILE (Ba and Sr) and HFSE (Th and Nb) that indicate they were affected by cryptic metasomatism. The formation of amphibole in xenoliths (modal metasomatism) is accompanied by pronounced enrichments in HFSEs and REES, particularly LREEs. With the exception of a single xenolith with slightly elevated PGE contents, the metasomatic fluids did not produce any appreciable changes in the platinum group element (PGE) contents, suggesting these elements are relatively immobile under these mantle conditions. Typical concentrations of PGEs in xenoliths from Dish Hill are 0.8 ppb Os, 1 ppb Ir, 0.6 ppb Rh, 2.5 ppb Pt, 1.8 ppb Pd, and 1.7 ppb Au. Modal metasomatism has resulted in a variable increase in sulfur content of the xenoliths, from a value of <10 ppm for anhydrous xenoliths, to between 25 and 120 ppm for amphibole-rich xenoliths. The δ34S values of sulfides in anhydrous xenoliths from Dish Hill are near +7, which are unusually high for the mantle and indicate that the lithospheric mantle under California was most likely affected by subduction of crustal sulfur. Sulfides in amphibole-bearing xenoliths have variable but generally lower δ34S values of 3 to 8.7, suggesting that some portion of the metasomatic sulfur was derived from an asthenospheric mantle source with a more primordial sulfur isotopic composition.

Shallow Burial Dolomitization and Dedolomitization of Cenozoic Cool-Water Limestones, Southern Australia: Geochemistry and Origin

ABSTRACT 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.

Measurement of U-Pb ages of uraninite and davidite by laser ablation-HR-ICP-MS

Abstract 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.

Fabrication of solid calibration standards by a sol–gel process and use in laser ablation ICPMS

A method for the fabrication of solid multi-element calibration standards by a sol–gel process, suitable for laser ablation ICPMS, is described. The addition of an analyte (Se) and an internal standard (S) to a normal sol–gel method does not impair the production of the glass-like discs (xerogels). Heterogeneity of Se and S concentrations in the xerogels is less than that of NIST SRM 610 and 612 glass standards. Small differences in slopes of calibrations based on sulfide standards and those based on xerogel standards reflect differential matrix effects. The calculated S contents in the NIST SRM 610 (634 ± 151 µg g−1) and 612 (133 ± 17 µg g−1), using the xerogels as standards, are comparable to available S concentration data. Multiple element standards of Se, S and transition metals are hampered by the formation of inclusions resulting in visible heterogeneity, limiting the concentrations of transition metals with Se and S in the xerogels. The xerogels are potential standards for a variety of elements in glasses, minerals and other materials

Isotopic evidence for microbial activity during supergene oxidation of a high-sulfidation epithermal Au-Ag deposit

The light stable isotope chemistry of barite and acanthite in the Pierina Au-Ag deposit, Peru, provides strong support for microbial involvement at the economically critical supergene oxidation stage. Early hypogene barite yields δ34S values of 23.6‰ to 28.5‰ and δ18O values of 5.8‰ to 10.9‰, and was precipitated with the original precious metal-enriched sulfide assemblage. In contrast, late barite, deposited during formation of supergene goethite and hematite, the main ore-hosts, yields δ34S and δ18O values of 1.4‰ to 14.2‰ and −2.8‰ to 4.7‰, correlated mutually and with the δ34S values of coprecipitated acanthite (δ34S = 0.4‰ to 3.9‰). Low δ34S values of barite and acanthite record sulfide mineral oxidation by meteoric waters. Increasing δ34S and δ18O values of barite and δ34S values of acanthite indicate enrichment of the sulfate reservoir in 34S and 18O, reflecting the preferential utilization of light isotopes by microbes during aqueous sulfate reduction. Continued sulfate reduction locally reduced supergene fluids, remobilizing iron and locally destroying the goethite-hematite assemblage. Precious metals were released but reacted with sulfide in the reduced waters to form Au-rich acanthite.

Application of molar element ratio analysis of lag talus composite samples to the exploration for iron oxide-copper-gold mineralization : Mantoverde area, northern Chile

A molar element ratio analysis based on whole-rock, multi-element geochemical data for regional lag talus composite samples is evaluated as an exploratory method for iron oxide–copper–gold mineralization (IOCG) in hyperarid settings. This study is focused in the now hyperarid Mantoverde area, III Región of northern Chile (latitude 26°01′ to 26°53′S), and comprises the Andean Coastal Cordillera and Central Valley. The area contains numerous structurally controlled iron oxide-Cu-(Au) prospects and deposits hosted by calc-alkaline volcanic and plutonic rocks of Jurassic–Cretaceous age. Comparison with data for outcrop samples indicates that lag talus composite samples reflect the composition of bedrock in terms of major and selected trace elements. As with outcrop samples, zirconium is the most conserved element in the talus samples, and is therefore used as common denominator in different molar ratios. A molar element ratio analysis using geochemical data from talus composite samples indicates that rocks associated with mineralization have been K-enriched and Na-depleted. Consequently, gradients in the K/Al and Na/Al molar ratios are useful targeting parameters. Similarly, a lithogeochemical alteration index, recently defined by the authors, quantifies the degree of hydrothermal alteration of host rocks and can be used to both target potential anomalous sectors (strongly altered rocks) and delimit barren areas. It is evident that lag talus composite samples constitute a reliable and effective sampling medium in regional exploration programmes for IOCG deposits in hyperarid settings.