Oxygen Isotope Composition Of Carbonates Research Articles

Carbon and oxygen isotope composition of carbonates from the Qaqarssuk Carbonatite Complex, southern West Greenland

Stable isotope data from the Jurassic Qaqarssuk Carbonatite Complex show that carbonatites intruded during the main intrusive event have δ18O-values ranging from +7 to +8.1‰ SMOW and δ13C-values ranging from −3.5 to −3.1‰ PDB.Late-stage søvite veins are enriched in light carbon relative to the main-stage carbonatites, with δ13C-values ranging from −5 to −4‰ PDB. This is interpreted as loss of heavy carbon to a gas phase.Late-stage REE-carbonatites have δ3C-values in the same range as the main-stage carbonatites, but elevated δ3C-values relative to late-stage søvites. The REE-carbonatites have elevated δ18O-values (+7.4 to +9.2‰ SMOW) relative to both main-stage carbonatites and late-stage søvite.Carbonates in metasomatically altered basement and contaminated carbonatite have elevated δ18O-values (+8.2 to +8.8‰ SMOW), probably caused by exchange of oxygen with the basement.Oxygen isotope geothermometry give temperatures in the range 313–608°C, which is low relative to the expected igneous temperatures. These low temperatures are explained as caused by subsolidus reactions such as exsolution and recrystallization which can be observed in the carbonates. There is poor correspondence between oxygen and carbon isotope geothermometry as well as with solvus geothermometry, indicating that the calibration of the isotope geothermometer established in metamorphic carbonate rocks cannot be applied directly to carbonatites.

A ground-water mixing model for the origin of the Imini manganese deposit (Cretaceous) of Morocco

The Imini deposit, just south of the Atlas Mountains of Morocco, consists of three beds of manganese oxide ore in a slightly deformed dolomite unit about 10 m thick, of Cenomanian-Turonian (Late Cretaceous) age. The dolomite host is sugary textured, massive, and permeable; dolomite occurs as nearly euhedral rhombs. Each ore bed is about a meter thick and consists almost entirely of intergrowths of pyrolusite and hollandite-family minerals (BaO up to 11% and PbO up to 28%) with a flattened geode structure. Overlying the ore beds are solution collapse breccias with dolomite clasts, janggunite (Mn,Fe oxide) and dolomite matrix and pyrolusite-hollandite-calcite cement. The ore, solution-collapse breccia, and coarsely recrystallized pure dolomite are coextensive--a zone only 400 to 1,000 m wide but 25 km long and elongate east-northeast-west-southwest.Diagenetic features of the deposit are: recrystallized texture of the dolomite, flattened geode structure of the ore, solution-collapse breccia and associated draped laminae of the insoluble residues, fossil replacement by Mn oxides, and zoned chert nodules with inclusions. A mineralogic paragenesis shown independently in each of these features from older to younger is (1) precursor carbonates, (2) dolomite-janggunite, and (3) hollandite-pyrolusite-chert-calcite.Diagenesis probably occurred during the Late Cretaceous in a mixing zone of fresh and saline ground water, as similar diagenetic sequences without manganese deposits are widely reported to have formed in such environments. Carbon and oxygen isotope compositions of carbonates and oxygen isotope compositions of chert show that an increasing component of fresh meteoric ground water was present as diagenesis proceeded. Falling sea level allowed the mixing zone to occupy various positions within the carbonate unit and convert the primary calcareous carbonate rocks to dolomite with chert nodules.Anoxic seawater is known both worldwide and from the local depositional basin (Atlas gulf) in the Cenomanian-Turonian. This is believed to be the key to manganese deposition, because anoxic conditions greatly enhance manganese solubility. Two specific versions of the mixing-zone model are presented for manganese deposition; one in which a primary sedimentary manganese precipitate formed along margins of the anoxic basin and was recrystallized during diagenesis in the mixing zone, and the other in which anoxic saline ground water brought manganese into the mixing zone, where it precipitated.

Oxygen isotope composition and rate of growth of patella coerulea, monodonta turbinata and M. articulata shells from the western coast of sicily

The oxygen-isotope composition of carbonate from the outermost portions of Patella coerulea, Monodonta turbinata and M. articulata shells has been measured from April 1979 to July 1981, with an interval during the summer of 1980, and correlated with the ambient temperature. Carbonate from M. turbinata has yielded values that, on the whole, reproduce the environmental temperature range acceptably: P. coerula and M. articulata indicate precipitation taking place at temperatures lower than those measured in seawater; this is particularly acceptuated in summer. This pattern does not necessarily exclude shell carbonate from precipitating under equilibrium conditions with seawater; rather it indicates a higher calcification rate in medium—warm periods for M. turbinata (with slower precipitation in winter), and in medium–cold periods for P. coerulea and M. articulata (with slow calcification, and even short interruptions during the warmest spells in summer).

A pollen analytical investigation supported by an 18O-record of a late glacial lake deposit at Grænge (Denmark)

A core from a Late Glacial lake deposit at Graenge on the island of Lolland (Denmark) has been investigated with regard to the composition of the sediment, the palynology, including pollen concentrations and Betula pollen size measurements, and oxygen isotope composition of carbonate. It is concluded that the investigated part of the core covers the upper part of the Earlier Dryas, the Allerød, the Late Dryas, and the early part of the Holocene. Deposits of Earlier Dryas age were laid down subaerially. From the onset of the Allerød and afterwards a lake was present in which precipitation of calcium carbonate took place. The vegetational and oxygen isotope changes are related to changes in the environment. The early part of the Allerød possibly had the highest temperatures of this period but unstable, raw soil conditions and delayed immigration of tree-birch prevented an extensive growth of trees. During the middle part of the Allerød, soil conditions had improved and birch settled in the surroundings. During the Late Dryas the carbonate precipitation in the lake decreased to a minimum and tree-birch almost disappeared, suggesting that the temperature had dropped to, or below, the minimum temperature limit for this plant. The increase of temperature during the upper part of the Late Dryas seems to have taken place already before 10,000 B.P.

Oxygen and carbon isotope compositions of altered carbonates from the western Pacific, core 53.0, Deep Sea Drilling Project

In the lower part of DSDP core 53.0, partly recrystallized carbonate sediments and well recrystallized limestone breccias of Oligo-Miocene age are associated with altered volcanic flows, lithified tuffs, and tuff breccias, suggesting that carbonate alteration was the result of thermal metamorphism. However, the oxygen isotope compositions of these carbonates (−3.4 to +0.6‰ rel. PDB) are not compatible with recrystallization and isotope exchange with sea water at high temperatures. Evaluating the effects of the composition of the water which exchanged with the carbonates and of carbonate-water isotope exchange in closed systems yields the following approximate maximum temperature of recrystallization: limestone breccias, 100°C; calcite veins rimming breccia clasts, 30°C; and unconsolidated sediments overlying the breccias, 20°C. Therefore, the volcanics at site 53.0 must have been emplaced into the primary carbonate sediments at relatively low temperatures. Subsequent carbonate alteration was probably a consequence of chemical changes in ambient pore waters resulting from the submarine weathering of volcanic material.

The isotopic composition of carbonatite and kimberlite carbonates and their bearing on the isotopic composition of deep-seated carbon

New carbon and oxygen isotopic compositions of carbonates from 14 carbonatite and 11 kimberlite occurrences are reported. A review of the available data on the carbon isotopic composition ranges of carbonatite and kimberlite carbonates shows that they are similar and overlap that of diamonds. The mean carbon isotopic composition of carbonates from 22 selected carbonatite complexes (−5.1%., s = ± l.4%. vs PDB) is indistinguishable from that of 13 kimberlite pipes (−4.7%. s = ±1.2%.) as well as that of 60 individual diamond analyses (−5.8%., s = 1.8%.). The oxygen isotopic compositions of kimberlite carbonates, however, are enriched in O 18 by several permil with respect to those of carbonates from the subvolcanic type of carbonatite. The data suggest that not all carbonatite, kimberlite and diamond occurrences have the same average carbon isotopic composition and that significant differences exist between them. Carbon isotopic composition measurements available for the East African Rift system suggest geographic and/or tectonic groupings e.g. carbonate lavas, tuffs and intusive carbonatites associated with the Eastern Rift yield a range of δC 13 values from −5.8 to −7.4%., similar to that of the carbonate rocks associated with the Western Rift volcanism (−5.8 to −7.9%.). In contrast the interrift area encompassing Lakes Victoria, Malawi (Nyasa) and Chilwa, apparently are characterized by carbonatitic carbonates of higher C 13 content (−2.4 to −4.4%.). If carbonatite and kimberlite carbonates as well as diamonds represent deep seated carbon, the mean isotopic composition of this carbon is estimated as −5.2%. and the range is −2 to −8%. The selection of any particular value within this range to be used as a criterion of deep-seated origin is at the moment not warranted. Indeed, the choice of any specific composition for such carbon may be meaningless, as the source of kimberlite, carbonatite and diamond carbon may not be isotopically uniform.

The carbon and oxygen isotopic composition of carbonates from the Oka carbonatite complex, Quebec, Canada

The Oka complex is a carbonatite belonging to the alkaline intrusive suite of the Monteregian petrographic province. The carbon and oxygen isotopic composition of carbonates from this complex were studied and found to vary by 7%. for carbon and about 13%. for oxygen. The mode of the δC 13- δO 18 distribution for the carbonatite lies at δC 13 = − 5% . vs. PDB and δO 18 = 7.4%. vs. SMOW. The isotopic composition variability could not be related to weathering or hydrothermal alteration. The frequency distributions (650 samples) of carbon and oxygen isotopic compositions are highly skewed toward increasing C 13 and O 18 concentrations. The carbon isotopic composition frequency distribution may be interpreted as the result of a Rayleigh fractionation process taking place during the formation of the carbonatite. The variations in O 18 content are greater than would be expected from such a process; part of the increased variability may be due to isotopic exchange with or the incorporation of the intruded gneisses. Relationships between isotopic composition and rock type could be demonstrated in several cases. With respect to sövites, rauhaugites show a distinct, and carbonates from the alkaline ultrabasic rocks a slight enrichment in C 13. The oxygen isotopic compositions of sövites and rauhaugites are consistently lighter than those of carbonates from the alkaline ultrabasic rocks, and the carbonates from fenites and silicate rocks, for which a derivation by ultra-fenitization is suggested on petrographic grounds, show the highest average O 18 concentrations. Small gradients in isotopic composition exist in cross sections through the carbonatite complex.

Carbon and oxygen isotopic composition of carbonates in limestone blocks and related geodes from the “black pozzolans” formation of the Alban Hills

A determination was made of the carbon and oxygen isotopic composition in the carbonates of ejected limestone blocks and related fluorine and boron bearing geodes found in the Corcolle “black pozzolans” from the Alban Hills. In all samples considered, the 18 O 16 O and 13 C 12 C ratios increase from the outside in, the latter more markedly than the former. In particular, the isotopic composition of oxygen always falls within the range of sedimentary and metamorphic limestones, while unusually low δ 13C values are recorded for carbon. The data obtained are interpreted as a result of a complex metamorphic process governed by temperature and by the reactions with volcanic gases and hydrothermal solutions.

Oxygen-18 variations in belemnite guards

Spot measurements of the oxygen isotopic composition of carbonate were carried out on some Jurassic and Cretaceous belemnites. Both the well preserved and the badly preserved specimens show radial variations of the 18O/ 16O ratios.

Carbon and oxygen isotopic composition of carbonates in lavas and ejectites from the Alban Hills, Italy

Carbon and oxygen isotopic analyses have been carried out on carbonates from lavas, ejectites and sedimentary formations in the region of the Alban Hills.

The carbon and oxygen isotopic composition of carbonates from a mica peridotite dike near Dixonville, Pennsylvania

The mica peridotite dike at Dixonville, Pennsylvania, belongs to a belt of ultramafic intrusions in eastern North America. Samples in a cross section through the dike, approximately 14 cm wide at the point of study, have been investigated. The carbon and oxygen isotopic composition of carbonate inclusions, of a 1 to 2 mm thick dolomite vein and of the carbonate contained in the fine-grained matrix of the peridotite were measured. The carbonate inclusions contain the heaviest terrestrial carbon discovered so far, their δC 13 values with respect to the PDB standard range from 12%. to 24.8%.. The carbon and oxygen isotopic composition varies systematically through the cross section of the dike and extremely steep δC 13 gradients exist.