\(^{182}Hf-^{182}W\) chronometry is considered the most powerful tool to determine the formation timescale of the terrestrial planets. However, previous work employed oversimplified accretion and core formation models. The accretion and core formation models presented here for the \(^{182}W \) isotopic evolution in the mantles of the accreting Earth and Mars, can incorporate the core formation conditions constrained by siderophile element geochemistry and can be successfully applied to constrain the formation timescale of Earth and Mars. Elemental abundance analyses of the Allende meteorite and two martian meteorites lead to new estimates of core-mantle concentration ratios of Si, V, Cr and Mn for Earth and two distinct mantle Hf/W ratios for Mars respectively, and provide better constraints on the models. It is concluded that formation of the proto-Earth (\(\sim87\%\) of its present mass) has to complete rapidly in \(10.7 \pm 2.5 Myr\) after the onset of the Solar vSystem for a late \((\ge 52 Myr)\) Moon-forming giant impact. The mean time of Mars’ accretion is determined to be \(3.6 \pm 0.1 Myr\), meaning that Mars accretes to 95% of its present mass in \(10.8 \pm 0.3 Myr\) after the formation of the Solar System. Therefore, Mars is not a planetary embryo, and Mars and proto-Earth may be formed on a similar timescale if a late Moon-forming giant impact is assumed. In contrast, if the Moon formed early at \(\sim30 Myr\) then it takes about 3 times longer to form the protoEarth compared to Mars. A stochastic mantle stirring and sampling model was developed to investigate the evolution of W isotope heterogeneities in the mantles of Earth and Mars after accretion and core formation. Our results confirm the mantle stirring rate of \(\sim 500 Myr\) constrained by the long-lived isotope systems in Earth and suggest that the mantle stirring rate in Mars is much slower \((\sim 2 Ga)\). A new concept is developed: the core formation memory of a siderophile element. Siderophile elements are shown to have different capabilities in recording core formation history, a very important fact to consider in any core formation modeling. / Earth and Planetary Sciences

The Asian summer monsoon is one of the largest and most influential of the Earth's climate systems, both in terms of human and climate system interactions. Previous research has focused on interannual and millennial variability of the monsoon. Little is known about century-scale fluctuations. Significant threats to human health and economies could result from unanticipated extremes in monsoon behavior on this timescale, however. Using geochemical proxies, I developed a record of century-scale water-balance fluctuations during the Holocene for Ahung Co (Lake) in Tibet. Conditions became drier at ∼7200 and ∼3700 calendar years BP. The event at ∼3700 calendar years BP coincides with the fall of the Indus civilization, which some researchers believe was caused by a decrease in precipitation. This is the first water-balance record from Tibet to have an age model free from reservoir effects. The age model revealed a gap in sedimentation at the top of the core. Other lake records should be reexamined for similar gaps. To determine the cause of water-balance fluctuations in Ahung Co, I used meteorological observations and evaporation models to reconstruct water fluxes to and from the lake over the past eight years. Water-balance fluctuations reflect changes in summer monsoon strength. Water fluxes during the summer are an order of magnitude greater than during other seasons. In addition to controlling precipitation, the summer monsoon controls evaporation through changes in cloudiness, humidity and temperature. These findings are characteristic of monsoon regions and should be relevant to other lakes on the plateau. I compiled paleoclimate records from the East Asian and Southwest Asian monsoon regions to determine the timing and possible causes of abrupt climate change. Unlike previous studies, I used an objective, statistical analysis to identify abrupt events. Events occurred at ∼11,500 calendar years BP, ∼4,500-5,000 calendar years BP and 1300 AD. The middle Holocene event contradicts the notion of a gradual decrease in monsoon strength through the Holocene. The timing of events suggests connections with the North Atlantic and other climate systems in the tropics.

Amorphous iron-aluminium oxides are ubiquitous in the environment and control the availability of heavy metals (Pb, Cu). Oxide physicochemical properties are affected by the proportion of Fe and Al, the Fe/Al ratio. Five amorphous oxides containing Fe and/or Al were synthesised: "Fe" (only Fe); "3FeAl" (3 moles Fe per mole Al, Fe/Al = 3); "FeAl" (Fe/Al = 1); "Fe3Al" (Fe/Al = 0.3); "Al" (only Al). Kaolinite clay was also investigated along with kaolinite to which Fe-Al oxides had been added. / The effect of Fe/Al ratio on these physicochemical properties was studied: (1) Specific surface area---EGME retention; (2) pH-dependent cation and anion exchange capacity---Ca(NO3)2 saturation; (3) Particle size density---sieving, pipette methods; (4) Surface charge density---potentiometric titrations. / The Fe/Al ratio influenced the specific surface area: Fe and 3FeAl had significantly greater surface areas than Fe3Al and Al; FeAl exhibited the maximum surface area. The cation exchange capacity increased rapidly above pH 5, with Fe > 3FeAl > Al > Fe3Al > FeAl at pH 6 to 7. Surface charge density of low Fe/Al oxides was more sensitive to pH changes implying ≡AlOH surface hydroxyl groups are more easily protonated and de-protonated than ≡FeOH species. An open (porous) structure of loosely linked small particles was proposed with SO42- anions forming bridges between Al3+ and Fe3+ ions during oxide precipitation while inhibiting extensive crystal growth. / Pb and Cu retention was measured using 24 hour batch suspension tests with varying pH and contaminant concentrations (0.5 to 25 mmol/L). Higher Fe/Al ratio oxides retained more Cu and Pb at all pH values. Cu retention (0.5 to 5 mmol/L) and Pb retention (all concentrations) followed Fe > 3FeA > FeAl > Fe3Al > Al but at 25 mmol Cu/L, only Fe accumulated more Cu. However, low Fe/Al ratio oxides preferentially sorbed Cu over Pb from multi-contaminant solutions. Adsorption rather than bulk precipitation was the dominant accumulation mechanism. Geochemical modelling suggested surface precipitation occurred at higher contaminant concentrations. / Adding oxides to kaolinite significantly increased the clay's specific surface area, surface charge density and contaminant retention capacity; higher Fe/Al ratio oxide-amended clays retained more Pb and Cu. / Amorphous Fe-Al oxides substantially enhance heavy metal accumulation by soils with higher Fe/Al ratio oxides being particularly effective.

Alkali-rich pyroxenite and sovitic carbonatite intruded amphibolite grade metamorphic rocks of the Bancroft terrane, Grenville province, in the Eganville-Pembroke region, Ontario, between 1090 and 1038 Ma ago. Intrusive field relationships, metastable feldspar mineralogy and oxygen isotope geothermometry indicate temperatures of emplacement above 650$ sp circ$C and locally rapid cooling rates. Clinopyroxene compositions from pyroxenites, carbonatites and fenites show identical sodium and iron enrichment trends. REE and C and O isotope ratios in carbonate dikes are typical of carbonatites. The presence in nearby carbonate metasediments of diopside- and apatite-rich skarn zones resulting from regional metamorphism explains the confused state of literature concerning the intrusive suite. Previously recognized resetting of radiogenic isotope systems may have resulted from a regional fenitization event associated with the emplacement of this suite.

Oxidation of dissolved arsenite was studied in 0.05M and 0.7M NaCl aqueous solutions. The experimental conditions included a range of pH (3 to 9), presence and absence of ambient light, presence and absence of oxygen, and presence and absence of amorphous and crystalline Mn and Fe oxyhydroxides at (Mn - Ox and Fe-Ox) constant solid: solution ratios of 0.0002. All experiments were conducted at a temperature of 23 +/- 4°C and initial arsenite concentration of approximately 100 ppb. Arsenic speciation was determined both in the aqueous phase and in the combined solid and aqueous phases. / Aqueous arsenite oxidation by oxygen alone is slow. Light, pH, and ionic strength had no effect on the oxidation rate of arsenite by dissolved oxygen. In the presence of manganese- and iron oxyhydroxides (Mn-Ox and Fe-Ox), the oxidation of arsenite is rapid: within 15 minutes, more than 80% of the initial As(III) was oxidized to As(V) by crystalline and amorphous Fe-Ox, and by amorphous Mn-Ox. In the presence of crystalline Mn-Ox, more than 20% of the initial As(III) was oxidized within the first 5 minutes of the experiment. Dissolved oxygen was not essential for the arsenite oxidation by the metal oxides. (Abstract shortened by UMI.)

The mass flow event resulting from the 1996 flood in the Saguenay region buried the mercury-contaminated indigenous sediments at the head of the Saguenay Fjord under up to 50 cm of postglacial deltaic sediments. This study investigated whether this deposit served as an efficient geochemical barrier to the remobilization of mercury and its availability to the benthic organisms. The vertical distributions of total mercury and methyl mercury (MeHg) in the sediments and pore waters were measured in box cores recovered at three stations along the main axis of the Saguenay Fjord and in the Baie des Ha!Ha!, in successive years between 1996 and 2002. The total solid mercury profile time-series shows that most of the mercury remobilized from the contaminated, indigenous sediments was trapped below or slightly above the former sediment-water interface. Strong correlations with acid-volatile sulphide profiles and extractions of pyrite-associated mercury indicate that most of this mercury was co-precipitated with authigenic iron sulphides. The mercury that was not sequestered by iron sulphides, diffused into the flood layer where it was scavenged by organic matter or methylated. Mercury sequestration at SAG-05 occurred within the older indigenous sediments, in contrast to SAG-09 where it occurred at or above the original sediment-water interface. The sediments are richer in organic matter, more reducing and, thus, establishment of suboxic conditions and precipitation of authigenic iron sulphides occurred more rapidly. The abundance of mercury at the former sediment-water interface and the low dissolved SigmaH2S concentrations, buffered by acid-volatile sulphide precipitation, both favored mercury methylation. A strong correlation between the distribution of acid-volatile sulphides and methyl mercury in the sediment also reveal that the former may serve as a sink for the latter. Throughout the sediment cores, sediment-water partitioning of MeHg as well as Hg(II) is controlled in great part by the residual organic matter content of the sediment.

Abstract Not Available.

This thesis presents a general kinetic model and its application to feldspar dissolution. In the framework of Monte Carlo methods, the model simulates the stochastic processes of feldspar dissolution by incorporating bond breakage, bond formation, surface diffusion, and the detachment and attachment of various mineral-forming ions. Quantum mechanical calculations are implemented to quantify the energies required to break and form Si-O-Si and Si-O-Al bonds. Empirical interatomic potential modeling is used to examine the relaxation of the feldspar's surface structure from the ideal bulk structure. The kinetic model incorporates the proper crystallographic surface structures, emphasizing the essential role of dynamics of neighboring sites on the crystal surface. The application of the general kinetic model to feldspar dissolution elucidates the kinetic patterns observed in experimental observations and improves our fundamental understanding of feldspar dissolution. In addition, we use the model with the appropriate change in parameterization to investigate the morphological evolution of a (001) surface of barite etched in pure water. The generation of two sets of oppositely-oriented triangular etch pits in consecutive atomic layers validates the model's efficiency. It is the first stochastic treatment out of numerous studies concerning barite dissolution to address this unique pit morphology. We further extend the model from abiotic water-mineral interactions and apply it to microbe-water-rock interactions in order to explore this subject as a complementary approach.

The chemical composition of groundwater is controlled largely by subsurface mineral/fluid interactions. Experimental measurement of plagioclase dissolution rates provides a critical means of understanding the relationship between groundwater composition and reservoir characteristics. This thesis focuses on two areas in which understanding of crystal dissolution is poor, i.e., dissolution kinetics as a function of undersaturation under near-equilibrium conditions, and the relationship between dissolution rate, mechanism, crystallography and surface reactivity. We employ a powerful approach: quantification of crystal surface normal retreat rates using vertical scanning interferometry (VSI), which provides direct observation of changes in the mineral surface due to dissolution reactions. This thesis presents results from three experimental studies during which we measured the dissolution rates of albite and anorthite single-crystal, cleavage surfaces as a function of solution saturation state during flow-through experiments over a wide range of temperatures (25--200°C). In addition, we identified specific dissolution mechanisms on reacted crystal surfaces using atomic force microscopy (AFM) and scanning electron microscopy (SEM). The first study compared the dissolution rates of a fine-grained albite powder with those of two albite cleavage surfaces [(010) and (001)]. The dissolution rates of fine-grained albite powders are substantially enhanced compared to those prevailing on large single crystal cleavage surfaces. The second study compared the dissolution rates on previously dissolved albite surfaces exhibiting etch pits and pristine surfaces lacking dissolution features. Experimental results document an up to 2 orders of magnitude difference in dissolution rate between the differently pretreated surfaces, which are dominated by different dissolution mechanisms. The rate difference, which persists over a range of solution saturation state, indicates that the dissolution mechanisms obey different Gibbs free energy difference (DeltaG) dependencies. We propose that the rate gap is the direct consequence of a kinetic bifurcation in dissolution rate and mechanism as DeltaG varies. The existence of the kinetic bifurcation indicates that a single, continuous function describing the relationship between dissolution rate and DeltaG is insufficient. The third study focused on the dissolution kinetics of anorthite with respect to DeltaG and surface reactivity. Our results indicate that dissolution experiments conducted with mineral powders have limited relevance to natural systems.

No description available.