David F. Nava

Bio: David F. Nava is an academic researcher from Goddard Space Flight Center. The author has contributed to research in topics: Reaction rate constant & Chemical kinetics. The author has an hindex of 17, co-authored 53 publications receiving 857 citations.

Ancient and modern volcanic rocks: A trace element model

Abstract: In terms of the relative abundance of K, Rb, Cs, Sr and Ba, modern oceanic and island-arc basalts may be viewed as a gradational family ranging from alkali basalt (enriched in these elements) to low-K-tholeiite, such as ocean-floor basalt (depleted in these elements). In island arcs, the contents of these trace-elements in basalts are proportional to the depth to the seismic zone. Our model for this relationship suggests derivation of alkali basalts from non-depleted mantle by low degrees of partial melting at relatively great depths; the low-K tholeiites are satisfactorily explained as products of high degrees of partial melting of residual or “depleted” mantle at relatively shallow depths. Some of these tholeiites are shown to be contaminated with oceanic sediment, and the usefulness of Cs and Ba concentrations to detect such contamination is established. New trace-element analyses reveal that the basalts of the Archaean (2.7 by) greenstone belts of Canada average 2100 ppm K, 6 ppm Rb, 0.4 ppm Cs, 175 ppm Sr and 70 ppm Ba. These trace-element abundances show that the Archaean basalts are most closely related to the modern low-K tholeiites of island arcs. This analogy leads us to propose that Archaean basalts are derived by high degrees of melting at shallow depths. We find no evidence for the presence of residual or “depleted” mantle during the Archaean. Our model implies a thin lithosphere during the Archaean and we suggest that Archaean volcanics evolved in an environment dominated by multiple thin oceanic plates with arc-trench boundaries and shallow subduction zones.

Temperature dependence of the reaction of nitrogen atoms with methyl radicals

Abstract: The discharge-flow mass spectrometry technique has been used to measure the kinetics of the reaction N + CH{sub 3} {yields} products over the temperature range 200-423 K. The results are as follows (10{sup {minus}11} cm{sup 3} s{sup {minus}1}): k{sub 1}(200 K) = (6.4 {plus minus} 2.1), k{sub 1}(298 K) = (8.5 {plus minus} 2.0), k{sub 1}(363 K) = (14 {plus minus} 3.0), and k{sub 1}(423 K) = (17 {plus minus} 5.0). Quoted uncertainties include statistical (95% confidence) and systematic (15%) errors. Interpreting the temperature dependence is difficult, as there is a possibility that the reaction behaves in a non-Arrhenius manner. Possible causes of this behavior are discussed, and comparisons are made with reactions showing similar properties. The results of this study have implications regarding the formation of HCN in the atmosphere of Titan.

Absolute rate constants for the reaction of atomic hydrogen with ketene from 298 to 500 K

Abstract: Rate constants for the reaction of atomic hydrogen with ketene have been measured at room temperature by two techniques, flash photolysis-resonance fluorescence and discharge flow-resonance fluorescence. The measured values are (6.19 + or - 1.68) x 10 to the -14th and (7.3 + or - 1.3) x 10 to the -14th cu cm/molecule/s, respectively. In addition, rate constants as a function of temperature have been measured over the range 298-500 K using the FP-RF technique. The results are best represented by the Arrhenius expression k = (1.88 + or - 1.12) x 10 to the -11th exp(-1725 + or - 190/T) cu cm/molecule/s, where the indicated errors are at the two standard deviation level.

Apollo 14: Some geochemical aspects

Abstract: Chemical analyses have been obtained for five samples of Apollo 14 regolith fines, three 14230 core samples, 14049 soil clod, 14305 and 14319 breccias, 14310 basalt, and some separated phases The chemical uniformity of these Apollo 14 samples indicates thorough mixing and/or uniform source rocks Basalt 14310 can be matched well in composition by a four-to-one mixture of soil and plagioclase Eu(2+)/Eu(3+) ratios calculated for 14310 pigeonite and plagioclase are similar to those for Apollo 12 and 15 mare-type basalt phases; this indicates similar redox conditions The investigated Apollo 14 samples are chemically similar to Apollo 12 and 15 KREEP as distinct from Apollo 11, 12, and 15, and Luna 16 mare-type basalts A relationship between the two types of basalt, in which mare-basalts would represent fused cumulates, is suggested

Rate constant for the reaction of hydroxyl radical with formaldehyde over the temperature range 228-362 K

Abstract: Absolute rate constants for the reaction OH ? H2CO measured over the temperature range 228-362 K using the flash photolysis-resonance fluorescence technique are given. The results are independent of variations in H2CO concentration, total pressure Ar concentration, and flash intensity (i.e., initial OH concentration). The rate constant is found to be invariant with temperature in this range, the best representation being k sub 1 = (1.05 ? or - 0.11) x 10 to the 11th power cu cm molecule(-1) s(-1) where the error is two standard deviations. This result is compared with previous absolute and relative determinations of k sub 1. The reaction is also discussed from a theoretical point of view.

Mechanism and modeling of nitrogen chemistry in combustion

Abstract: Our current understanding of the mechanisms and rate parameters for the gas-phase reactions of nitrogen compounds that are applicable to combustion-generated air pollution is discussed and illustrated by comparison of results from detailed kinetics calculations with experimental data. In particular, the mechanisms and rate parameters for thermal and prompt NO formation, for fuel nitrogen conversion, for the Thermal De-NOx and RAPRENOx processes, and for NO2 and N2O formation and removal processes are considered. Sensitivity and rate-of-production analyses are applied in the calculations to determine which elementary reactions are of greatest importance in the nitrogen conversion process. Available information on the rate parameters for these important elementary reactions has been surveyed, and recommendations for the rate coefficients for these reactions are provided. The principal areas of uncertainty in nitrogen reaction mechanisms and rate parameters are outlined.

Modeling nitrogen chemistry in combustion

Abstract: Understanding of the chemical processes that govern formation and destruction of nitrogen oxides (NOx) in combustion processes continues to be a challenge. Even though this area has been the subject of extensive research over the last four decades, there are still unresolved issues that may limit the accuracy of engineering calculations and thereby the potential of primary measures for NOx control. In this review our current understanding of the mechanisms that are responsible for combustion-generated nitrogen-containing air pollutants is discussed. The thermochemistry of the relevant nitrogen compounds is updated, using the Active Thermochemical Tables (ATcT) approach. Rate parameters for the key gas-phase reactions of the nitrogen species are surveyed, based on available information from experiments and high-level theory. The mechanisms for thermal and prompt-NO, for fuel-NO, and NO formation via NNH or N2O are discussed, along with the chemistry of NO removal processes such as reburning and Selective Non-Catalytic Reduction of NO. Each subset of the mechanism is evaluated against experimental data and the accuracy of modeling predictions is discussed.

Geochemical magma type discrimination: application to altered and metamorphosed basic igneous rocks

Abstract: Five minor and trace elements, known to be chemically stable during alteration and metamorphism, have been combined in a set of binary diagrams that distinguish fresh tholeiites from alkali basalts. Of the five elements: Ti, P, Zr, Y, Nb, only P shows slight mobility during metamorphism, which is not sufficient to alter greatly the point distribution on the binary diagrams. Using these stable elements altered basaltic rocks: greenstones, spilites and amphibolites may be distinguished in the same way as fresh basalts, and their original magma may be identified as tholeiitic or alkaline basalt. All five elements are readily and rapidly determined, using XRF, thus this method may be applied as a rapid, easy way of discriminating the magma types of altered basaltic rocks. Using this method it can be demonstrated that alkali basalt magma was produced in minor quantities in the Precambrian.

Fine structures of mutually normalized rare-earth patterns of chondrites

Abstract: REE abundances in ten chondrites (nine falls and one find) were determined very accurately by mass-spectrometric stable isotope dilution techniques. All of the chondrites have different relative and absolute REE patterns. Except for Eu and, rarely, for Ce, the REE abundances in chondrites are smoothly fractionated from sample to sample. Notwithstanding differences in the abundances of common REE, four of five L6 chondrites have very similar absolute Eu abundances; their mutually-normalized REE patterns are not curved, but are composed of two rectilinear segments. The Leedey-normalized REE pattern for St. Severin (LL6) is composed of two concave curves. Yonozu's (H4,5) pattern shows negligibly concave curvature on both sides of Eu. Kesen's (H4) pattern is unusual in its overall pattern but also in irregularities for particular elements. The irregularity may be connected with the unusually high vapor pressure of metallic Yb. The REE pattern for the Allende bulk sample shows a discontinuity, presumably reflecting its considerable heterogeneity of composition and structure. It is evident that any pattern of ordinary chondrites cannot be produced from the Allende bulk pattern. A comparison is also made with the results on the chondrite composites previously investigated.

The application of trace elements to the petrogenesis of igneous rocks of granitic composition

Abstract: Although trace element modeling has been used to great advantage for petrogenetic interpretations of basaltic systems, similar studies on igneous rocks of granitic composition have been fewer. In general the mineral/melt distribution coefficients for rare earth elements (REE) in granitic melts are equal to or greater than those for similar minerals in the basaltic system. Thus the effects of these minerals on the REE patterns of granitic melts during partial melting or differentiation are exaggerated as compared to basaltic systems, making detection of residual phases easier. For the K/Rb ratio, if neither a K-feldspar component nor biotitephlogopite is present in the residue, it is difficult to reduce the K/Rb ratio of the melt relative to the parent by a factor of two by either differentiation or partial melting. The petrogenesis of four distinctly different rocks are received: (1) an Archean tonalite presumably derived by partial melting of an Archean tholeiite at mantle depths, leaving a garnet plus clinopyroxene residue; (2) an Archean quartz monzonite presumably derived by partial melting of a short-lived graywacke-argillite sequence at crustal depths; (3) a dacite from Saipan presumably derived by differentiation from a basaltic parent; and (4) a trachyte from Ross Island, Antarctica, presumably derived by differentiation from a basanitoid parent and contaminated by continental crustal components.