Reactive Infiltration of MORB-Eclogite-Derived Carbonated Silicate Melt into Fertile Peridotite at 3GPa and Genesis of Alkalic Magmas

We performed experiments between two different carbonated eclogite-derived melts and lherzolite at 1375°C and 3 GPa by varying the reacting melt fraction from 8 to 50 wt %. The two starting melt compositions were (1) alkalic basalt with 11·7 wt % dissolved CO2 (ABC), (2) basaltic andesite with 2·6 wt % dissolved CO2 (BAC). The starting melts were mixed homogeneously with peridotite to simulate porous reactive infiltration of melt in the Earth’s mantle. All the experiments produced an assemblage of melt + orthopyroxene + clinopyroxene + garnet ± olivine; olivine was absent for a reacting melt fraction of 50 wt % for ABC and 40 wt % for BAC. Basanitic ABC evolved to melilitites (on a CO2-free basis, SiO2 ∼27–39 wt %, TiO2 ∼2·8–6·3 wt %, Al2O3 ∼4·1–9·1 wt %, FeO* ∼11–16 wt %, MgO ∼17–21 wt %, CaO ∼13–21 wt %, Na2O ∼4–7 wt %, CO2 ∼10–25 wt %) upon melt–rock reaction and the degree of alkalinity of the reacted melts is positively correlated with melt–rock ratio. On the other hand, reacted melts derived from BAC (on a CO2-free basis SiO2 ∼42–53 wt %, TiO2 ∼6·4–8·7 wt %, Al2O3 ∼10·5–12·3 wt %, FeO* ∼6·5–10·5 wt %, MgO ∼7·9–15·4 wt %, CaO ∼7·3–10·3 wt %, Na2O ∼3·4–4 wt %, CO2 ∼6·2–11·7 wt %) increase in alkalinity with decreasing melt–rock ratio. We demonstrate that owing to the presence of only 0·65 wt % of CO2 in the bulk melt–rock mixture (corresponding to 25 wt % BAC + lherzolite mixture), nephelinitic-basanite melts can be generated by partial reactive crystallization of basaltic andesite as opposed to basanites produced in volatile-free conditions. Post 20% olivine fractionation, the reacted melts derived from ABC at low to intermediate melt–rock ratios match with 20–40% of the population of natural nephelinites and melilitites in terms of SiO2 and CaO/Al2O3, 60–80% in terms of TiO2, Al2O3 and FeO, and <20% in terms of CaO and Na2O. The reacted melts from BAC, at intermediate melt–rock ratios, are excellent matches for some of the Mg-rich (MgO >15 wt %) natural nephelinites in terms of SiO2, Al2O3, FeO*, CaO, Na2O and CaO/Al2O3. Not only can these reacted melts erupt by themselves, they can also act as metasomatizing agents in the Earth’s mantle. Our study suggests that a combination of subducted, silica-saturated crust–peridotite interaction and the presence of CO2 in the mantle source region are sufficient to produce a large range of primitive alkalic basalts. Also, mantle potential temperatures of 1330–1350°C appear sufficient to produce high-MgO, primitive basanite–nephelinite if carbonated eclogite melt and peridotite interaction is taken into account

Mercury Isotope Fractionation by Environmental Transport and Transformation Processes

Mercury is a toxic metal with well known health risks, but uncertainties regarding its environmental fate remain. Analytical tools capable of distinguishing small variations in mercury isotope composition have recently become available and there is considerable interest in applying these to help improve understanding of mercury's complex biogeochemical cycle, and to identify specific sources to remediate. In this dissertation, mercury isotope fractionation by three environmental transport and transformation processes - mercury diffusion through a polymer, thermal decomposition of HgS(s), and mercury diffusion through air - are investigated. Clear understanding of processes that affect mercury isotopes, such as these, is needed to ensure field scale isotopic data are interpreted correctly.A new analytical method for measuring mercury isotopes with high precision was developed to pursue the work described here. In this method, both mercury and thallium (for instrumental mass bias corrections) are introduced to a multi-collector inductively coupled plasma mass spectrometer (MC-ICP-MS) as a liquid aerosol. The addition of cysteine to liquid samples effectively controlled mercury memory effects. A purge and trap sample preparation technique, using KMnO4 or HOCl as mercury oxidants, was used in this work to prepare mercury in a common matrix. The long-term reproducibility of the method was approximately 0.3 / for δ202Hg, which is similar to other contemporary methods.Mercury diffusion through a polymer was found to have a very large isotope effect. This effect was determined by measuring Hg0 that permeated PVC tubing and matching this with models of the rate and isotopic composition of this gas. The isotope fractionation factor for this process, α202 = 1.00288±0.00040, is the largest factor yet determined for mercury near ambient conditions. This fractionation factor represents the relative diffusion coefficients of 198Hg and 202Hg in the polymer.There have been recent observations of mercury isotope variations at mercury mines that were speculated to have resulted from heating of mercury ores. In experiments described here, thermal decomposition of HgS(s) did not result in bulk isotope fractionation of the remaining HgS(s). This was evaluated by heating HgS(s) particles in an argon gas flow for different periods of time and measuring the mass and isotopic composition of remaining HgS(s). A model of congruent evaporation from a solid explained this lack of bulk isotope fractionation well. This model indicates that, while changes in the isotopic composition of a thin surface layer are possible, isotopic changes of the bulk material are very small.Mercury diffusion in air was found to have a large isotope effect that can be predicted by kinetic gas theory using only the molecular masses of mercury isotopes and air. This effect was determined by observing mercury remaining in a well mixed reservoir that was depleted by diffusion through a set of hypodermic needles. The ratio of 198Hg to 202Hg diffusion coefficients in air was determined to be 1.00125±0.00011. Kinetic theory predicts this ratio to be 1.00126. The fractionation factor of this fundamental and common environmental process is similar to the larger isotope fractionation factors documented previously.The determination of mercury isotopic variations with new analytical tools offers a promising approach for examining mercury in the environment. Interpretations of field measurements will need to be guided by mechanistic understanding developed under controlled conditions. The work described in this dissertation enables better understanding of mercury isotope fractionation by environmental transport and transformation processes that lead to isotopic variations throughout the environment. These isotopic differences suggest not only a means of interpreting environmental transport and transformation processes but also determining the dominant sources of mercury where there have been multiple releases

Assessing Science Inquiry using MDP Goal Detectors

Complex cognitive tasks, such as science inquiry, often involve a sequence of goals, each of which is pursuedthrough a sequence of actions. Effective assessment of inquiry performance requires identification of these student goals.Markov decision processes (MDPs) have been used to infer goals and beliefs over a single directed sequence of actions (Bakeret al., 2009), but multi-goal complex systems are computationally prohibitive to model. This research investigates the useof targeted MDPs as goal detectors, embedded within a larger hidden Markov model (HMM) that accounts for the transitionbetween goals. This multi-layer approach allows the MDP state spaces to remain small while modeling complex cognition.Because canonical HMM estimation is complicated by the dynamic nature of MDPs, in which action probabilities depend oncontext, we explore several different estimation methods. The approach is applied to log-file data of test-taker interactions witha simulation-based science inquiry assessment

Liquid immiscibility in the join NaAlSi_3O_8-Na_2CO_3-H_2O and its bearing on the genesis of carbonatites

Phase relationships in the join NaAlSi_3O_8-Na_2CO_3-H_2O through the quinany system Na_2O-Al_2O_3-SiO_2-CO_2-H_2O were studied experimentally at 1 kb as part of a series of studies aimed at elucidating the relationships between alkaline igneous rocks and their associated carbonatites. The phases encountered are albite, cancrinite, sodium carbonate, a silicate-rich liquid, an Na_2CO_3-rich liquid, and vapor. A liquid miscibility gap between the two liquid phases is intersected by this join over a wide range of compositions at temperatures in excess of 725°C; the compositions of the liquids approach each other with increasing H_2O content at constant temperature. The minimum temperature of the vapor-saturated liquidus decreases continuously with increasing H_2O content; it lies at 865°C for a composition of 81 wt percent NaAlSi_3O_8, 19 wt percent Na_2CO_3 with no H_2O present, and at 645°C for an anhydrous composition of 80 wt percent NaAlSi_3O_8, 20 wt percent Na_2CO_3 with 50 wt percent H_2O present. The minimum temperature on the solidus decreases from 685°C with 5 wt percent H_2O present to 590°C with 75 percent H_20 present for an anhydrous composition 75 wt percent NaAlSi_3O_8 + 25 wt percent Na_2CO_3. Albite is the main silicate phase al low H_2O contents, while cancrinite is the main silicate phase at high H_2O contents. The three fluid phases which coexist in this simplified system are: (1) an undersaturated alkaline silicate liquid, (2) an alkaline carbonate liquid containing only a small amount of dissolved silicate, and (3) a vapor phase with a composition varying between H_2O and CO_2, and containing Na_2O and SiO_2 in solution. These fluid phases can be compared with, respectively, (1) nepheline or ijolite magmas, (2) carbonatite melts, and (3) fenitizing solutions, which together form complexes of alkaline igneous rocks and associated carbonatites. It is proposed that processes of fractional crystallization in a carbonated alkalic magma, combined with vapor transport, can result in the formation of these three coexisting fluid phases

Melting Relationships in the System NaAlSi_3O_8-NaF-H_2O to 4 Kilobars Pressure

The phase relationships in the systems NaF-H_2O and NaAlSi_3O_8-NaF-H_2O were determined between 600° and 900° C. at pressures up to 4 kb., and those in the anhydrous system NaAlSi_3O_8-NaF were determined at 1 atm. The phases encountered were albite, villiaumite (NaF), liquid, and vapor. The liquid quenches to a glass containing skeletal NaF crystals. Primary villiaumite is readily distinguished from "quench" NaF. Albite crystals coexisting with liquid are several times larger than subsolidus crystals. The binary reaction villiaumite + vapor ↔ liquid occurs at 860° ± 7° C. at 1 kb., and the binary reaction albite + villiaumite ↔ liquid occurs at 860° ± 5° C. at 1 atm. The beginning of melting in the ternary system is the reaction albite + villiaumite + vapor ↔ liquid; this occurs at 753° ± 5° C. at 0.5 kb., at 688° ± 5° C. at 1 kb., at 640° ± 5° C. at 2 kb., at 630° ± 5° C. at 3 kb., and at 600° ± 5° C. at 4 kb. The composition of the univariant liquid in the system NaF-H_2O at 1 kb. is approximately 80 wt. per cent NaF and 20 wt. per cent H_2O; the composition of the univariant liquid in the anhydrous system is approximately 84 wt. per cent NaAlSi_3O_8 and 16 wt. per cent NaF. The composition of the univariant liquid in the ternary system varies with pressure. At 1 kb. the composition expressed in terms of the anhydrous components is approximately 86 wt. per cent NaAlSi_3O_8 and 14 wt. per cent NaF, and the water content is about 30 wt. per cent; at 4 kb. it is approximately 75 wt. per cent NaAlSi_3O_8 and 25 wt. per cent NaF, and with a water content of about 45 wt. per cent. Critical conditions are probably reached at a pressure not far above 4 kb. The solubility of the solids in the vapor phase at 4 kb. is about 40 wt. per cent. These results indicate that small variations in NaF content of a silicate magma can produce large variations in the water content of residual magmas and large variations in the amount of water required to saturate the magma. NaF causes residual magmas to persist to significantly lower temperatures than the final consolidation temperature if H_2O were the only dissolved volatile component

Modifying Cognitive Load Component Survey for K-12 STEM Testing

Test-taker’s capability to answer questions is influenced by available cognitive resources for problem solving. Due tothe limited working memory capacity, excessive cognitive load for interpreting instruction would impact test-taker’s construct-relevant process and test validity. Especially in STEM assessment where multimedia and interactive design are widely used,test-takers can easily get overwhelmed by a large amount of visual or audio information. Testing materials should be designed tominimize the unnecessary cognitive load in order to increase cognitive resources for problem solving in the task. The CognitiveLoad Component Survey is one of the first self-report measurements distinguishing different types of cognitive load: intrinsiccognitive load, extraneous cognitive load, and germane cognitive load. We report modifications of this survey to fit into K-12educational assessment, results of measuring cognitive loads in a simulation-rich science assessment, and implications to usethis survey for future assessment development