Triple Oxygen Isotope Composition of Carbonates

This dissertation presents a method of analyzing the triple oxygen isotope compositions of carbonates, presents an empirical calibration of the carbonate-water equilibrium fractionation line, presents a triple oxygen isotope equipped fluid-rock mixing model for carbonates to see-through diagenesis, and applies all these findings to ancient carbonate samples. Using modern carbonates and associate water, the following equations are calculated to describe equilibrium triple oxygen isotope fractionation of carbonates: 1000lnalpha18Occ-wt=2.84x106/T2-2.96 1), Thetacc-wt=-1.39/T+0.5305 2). Using these fractionation equations provides an extremely useful tool to determine whether a carbonate sample is altered or preserves its original isotopic composition. In samples that are altered, a fluid-rock mixing model is used to see-through the diagenesis. Applying these tools to ancient carbonate rocks shows that many samples thought to be pristine are altered and are confusing paleoenvironmental interpretations. This work shows that seawater temperature and isotopic composition is unchanged over the Phanerozoic, an important consideration when reconstruction paleoenvironments

Calibration and application of silica-water triple oxygen isotope thermometry to geothermal systems in Iceland and Chile

Triple oxygen isotope analyses were made on geothermal fluids and precipitates from Chile and Iceland to calibrate the silica-water isotopic fractionation for abiotic silica formation at elevated temperatures and were used to evaluate potential fractionation effects of biogenic vs. abiogenic samples and polymorphism. Coexisting water and amorphous silica precipitated inside the heat exchanger of the Hellisheioi power plant at 60 and 118 degrees C have triple oxygen isotope fractionations in excellent agreement with previous results from analyses of biogenic silica precipitated in cold waters. In contrast to samples from the geothermal plant, natural amorphous silica precipitates and waters formed in active hot springs (T = 63-84 degrees C) in the Puchuldiza geothermal area of northern Chile gave temperature estimates from the silicawater thermometer far lower (37-46 degrees C) than the measured water temperatures. Active silica precipitation was found to only occur at and above the air-water interface on glass slides placed in the hot spring waters for 9 days. The calculated temperatures and visual inspection suggest that precipitation occurred along channel edges when saturation was overstepped by a factor of two. In contrast to the surficial neoformed amorphous silica, subsurface silica samples (>10 cm) have recrystallized to opal-CT and quartz within a sinter mound and these samples preserve isotope temperatures of 82 degrees C and 89 degrees C, in good agreement with the ambient temperatures of the thermal spring conduit system. The delta O-18 values of abiogenic, low temperature silica formed in spring water far from the thermal waters with a measured temperature of 19 degrees C correspond to a silica-water temperature estimate of 20 degrees C. All samples preserved isotope data corresponding to their expected formation temperatures and appear to be in equilibrium in the triple oxygen isotope system. A best-fit theta-T relationship for silica-water using our inorganic silica-water samples is theta = 0.5305 = 1.82(+/- 0.02)/T(K) ; R-2 = 0.998 (where theta(a-b) = ln alpha O-17(a-b)/ln alpha O-18(a-b)). This new equation is indistinguishable from a previous empirical fit by Sharp et al. (2016) based primarily on biogenic silica samples, suggesting that the biogenic and abiogenic samples secreted silica with the same fractionation. Our results show that triple oxygen isotope measurements are robust and can be used to estimate the temperature of formation, the isotopic composition of the formation water, and discern between equilibrium and non-equilibrium processes.United States National Science Foundation Grant DGE-1418062 European Commission 290040 International Geothermal Association (IGA) FONDAP-CONICYT project 15090013 Millennium Nucleus for Metal Tracing Along Subduction (NMTM), MSI Grant NC13006

Calibration of carbonate-water triple oxygen isotope fractionation: Seeing through diagenesis in ancient carbonates

High precision triple oxygen isotope measurements of carbonates can better constrain temperatures and oxygen isotope compositions of seawater through geologic time than 18O/16O measurements alone, but lack of a definitive calibration has hindered progress. In this study, we fluorinated both carbonate and water samples to measure quantitatively the triple oxygen isotope composition of each phase. We compared the oxygen isotope fractionation between carbonate and water for different carbonate materials: calcite synthesized with and without carbonic anhydrase, abiogenic calcite from Devils Hole, and extant biogenic calcite and aragonite of marine origin. We found similar 1000lnα18Occ-wt values for all materials and combined the results with the high temperature experimental data of O'Neil et al. (1969), resulting in the following fractionation equation (T in Kelvins) 1000lnα18Occ-wt=2.84(±0.02)×106T2-2.96(±0.19). The calcite triple oxygen isotope values yielded a θ-T relationship of θcc-wt = –1.39(±0.01)/T + 0.5305 whereas the aragonite triple oxygen isotope values yielded a θ-T relationship of θara-wt = –1.53(±0.02)/T + 0.5305. The calcite-water triple oxygen isotope equilibrium fractionation equation for natural samples is Δ17′Occ-Δ17′Owt=2.84(±0.02)×106T2-2.96(±0.19)-1.39(±0.01)T+0.5305-λ. The combined 1000lnα18O and 1000lnα17O relationships can be used to assess equilibrium in ancient samples and to evaluate potential secular changes in the δ18O value of seawater. Most of the Phanerozoic samples analyzed in this study, which were determined to be pristine in previous studies, have undergone some level of diagenesis. Two samples appear to preserve their original oxygen isotope compositions and suggest a cool ocean with a δ18O value similar to the modern ocean. Using a fluid-rock interaction model, we can “see through” the diagenetic process and estimate the triple oxygen isotope composition of the carbonate prior to alteration. In doing so, we show that for the time intervals and sample locations measured in this study, Phanerozoic oceans had a comparable range of oxygen isotope compositions and temperatures as modern seawater