<p>Cosmogenic ³²Si (t_1/2 &sim; 140 yrs) was used in a novel way to constrain the quantity of reactive Si storage and early diagenetic reactions of Si in the highly mobile deltaic sediments along the coast of French Guiana, representative of deposits along the ~1600 km Amazon&ndash;Guianas coastline downdrift of the Amazon delta. A sequential leach was developed to extract and purify SiO₂ from different operational pools in large samples of surface sediments (0&ndash;10cm). This methodology, a hot 1% Na₂CO₃ leach followed by a hot 4M NaOH leach, was adapted from the existing leaches widely used to estimate biogenic silica (bSi) content in marine sediments, and ultimately to constrain the global oceanic Si budget. ³²Si activity was determined in each pool via its daughter product ³²P. Results from several sites in coastal mudbanks near Kourou and Sinnamary indicate no detectable ³²Si activity in the bSi fraction, whereas ³²Si was detected in the Si-NaOH fraction after removal of bSi. The lack of detectable activity in the 1% Na₂CO₃ leach and its detection in the NaOH fraction (0.4&ndash;2.5 dpm) indicate that the method widely used to determine bSi content recovers only a minor fraction of the originally deposited reactive bSi in these deposits. The results are consistent with rapid alteration of biogenic silica and clay authigenesis or reverse weathering. They also demonstrate that the current estimate of biogenic silica storage in tropical deltaic sediments is significantly underestimated. Assuming an initial diatom specific activity range of &sim;5&ndash;40 dpm/kg SiO₂, the ³²Si activity in the NaOH fraction corresponds to a reactive Si storage of &sim;150&ndash;18,000 &micro;mol Si/g sediment. This magnitude is more consistent with estimates of reactive Si (&Sigma;Si_hr) storage in the Amazon delta based on modified operational leach techniques that target poorly crystalline clays and with diagenetic modeling of pore water K⁺, F^&minus;, and Si(OH)₄, though these modified leaches also appear to underestimate the amount of reactive Si stored along this system. To directly confirm whether these modified operational extractions underestimate reactive Si storage, a sequential extraction methodology was also developed to first isolate ³²Si activity in the &Sigma;Si_hr fraction (0.1N HCl followed by 1% Na₂CO₃) and then extract any remaining ³²Si from the residual fraction using 4M NaOH. </p><p> Sediment from 2 stations in the Gulf of Papua, Papua New Guinea, 1 station in the northern Gulf of Mexico near the Southwest Pass, and 1 station in Long Island Sound (Smithtown Bay) were also extracted for ³²Si in the bSi fraction as well as the residual fraction after removal of bSi. Bulk ³²Si activities in the residual fractions in the Gulf of Papua (0.5&ndash;0.7 dpm/kg sediment) were used to extrapolate Si storage in the outer topset and forset of the clinoform delta. </p><p> ³²Si activity was detected in the both the bSi (0.21 &plusmn; 0.04 dpm/kg sediment) and the residual fraction (0.44 &plusmn; 0.08 dpm/kg sediment) from the site in the Gulf of Mexico. A Si burial rate using the ³²Si activity in the bSi fraction (assuming an activity of 15dpm/kg in starting Si materials) of 0.004Tmol/y was calculated over approximately 5000 km2 of the delta, whereas the burial rate calculated using the Si content in this same fraction from a classic bSi leach, was &sim;0.006Tmol/y. Adding the Si burial rate using the ³²Si activity in the residual fraction (0.008Tmol/y) yielded a total storage per year of 0.012Tmol Si, &sim;10% of the total Si inputs (dissolved and amorphous Si) from the Mississippi-Atchafalaya river system. ³²Si activity was also detected in the residual fraction (0.53 &plusmn; 0.08 dpm/kg sediment) after removal of &Sigma;Si_hr and using this activity yielded similar calculated rates of Si burial (&sim;0.01 Tmol/y). </p><p> In Smithtown Bay, Long Island Sound, ³²Si activity was also detected in both the bSi (0.15 &plusmn; 0.05 dpm/kg sediment) and the residual (0.4 &plusmn; 0.2 dpm/kg sediment) fractions from the site in Smithtown Bay, Long Island Sound, yielding a total Si storage estimate (assuming an activity of 15 dpm/kg in starting Si materials) of 1.6 &times; 10^&minus;3 Tmol/y over the entire Sound, comparable to estimates of Si storage calculated using the Si content in the classic bSi (1.1 &times; 10^&minus;3 Tmol/y) and the classic &Sigma;Si_hr (2.2 &times; 10^&minus;3 Tmol/y) leaches. It appears that reverse weathering is an important sink of Si in these deposits and that classic bSi or &Sigma;Si_hr leaches can underestimate Si storage in these system by two to four-fold. (Abstract shortened by UMI.)</p>

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Isotopic and trace element data on igneous rocks in nine multiphase magmatic complexes of Laramide age in Arizona place constraints on their petrogenesis and on the factors leading to the formation of porphyry copper deposits. The igneous rocks form a data array from ∊Nd(T) and Srₒ values of 0 and 0.704, to -14 and >0.710, respectively. Isotopic compositions indicate that early, intermediate volcanic rocks retained a mantle component whereas later intrusions were derived predominantly from Precambrian lower crust. The REE display temporally systematic behavior. Progressively younger igneous rocks in a district show a decreasing concentration of REE which is more pronounced for the HREE than for the LREE; they acquire greater upward concavity in their HREE profiles; and the Eu anomaly steadily becomes less negative. An increasing role for hornblende is indicated, either in the residuum of melting or as a fractionating phase. The evolving REE and isotopic behavior parallels the progression from barren, to subproductive, to productive intrusions. The geochemical behavior can be understood in the broader context of magmagenesis at the Laramide convergent margin. Early in the Laramide, the crust was cool and brittle, thereby allowing magmas formed in the mantle wedge as a consequence of volatile loss from the descending slab to ascend to high crustal levels. As the crust warmed the ascent of mantle-derived magmas was arrested in the lower crust where they induced anatexis in Precambrian crust. Three related models can account for the systematic REE behavior, crustal anatexis, and the timing of Laramide metallogenesis: (1) metasomatism of the lower crust, (2) progressively greater assimilation of hydrous crust by mantle-derived melts, and (3) migration of the anatectic zone into more hydrous rocks at higher crustal levels. Each process would allow melting to continue in confined columns of crust as well as provide increasingly volatile-rich magmas that were necessary for melts to evolve fluids capable of forming large porphyry copper deposits. The ultimate ability of a melt to form a porphyry copper deposit may, therefore, depend on characteristics obtained either in its crustal source region or during its passage through the crust.

The activities of aqueous entities in fluids from active hydrothermal systems can provide detailed fluid-mineral equilibria information. Numerical integration of the equations describing homogeneous equilibria as a function of temperature and pressure is the best way to determine ion activities at elevated pressure-temperature conditions. Activities of most cations decrease with increasing temperature, but the activity of the hydrogen ion may either increase or decrease. The dominant control over changes in ion activity ratios as temperature and pressure increase is the evolution of H⁺. Changes in the activity of the hydrogen ion depend on equilibrium constants for reactions that produce or consume H⁺, the measured pH, and the concentration of elements such as chlorine and sulfur in the fluid. pH and ion to hydrogen activity ratios calculated at elevated temperatures display a wide range of behavior; they may be sensitive to the initial pH, insensitive to the initial pH, or a unique function of the initial pH. When pH is sensitive to the initial pH, uncertainties in the fluids composition are magnified at high temperature and pressure and quantitative information regarding fluid-mineral equilibria cannot be retrieved from the fluid. Although changes in activity ratios for a given fluid may be insensitive to the initial pH, fluid composition must be precisely determined for fluids where activity ratios are independent of temperature and pressure to precisely determine the temperature at which a given assemblage equilibrates. Activity ratios in acid sulfate-chloride solutions are a strong function of temperature and pressure and are sensitive to the initial fluid composition. Therefore, quantitative information cannot be obtained from these fluids. Activity ratios computed for mid-ocean ridge hydrothermal vent fluids and alkali chloride solutions are not sensitive to the initial pH, and can provide quantitative information regarding subsurface fluid-mineral equilibria. Analysis of mid-ocean ridge fluids in the H₂SO₄-H₂S-FeO-H₂O-HCl system suggests these fluids are only slightly undersaturated with anhydrite at vent conditions. Heating these fluids will move them into the magnetite stability field, while cooling moves them into the pyrite and pyrrhotite stability fields.

An array of stratigraphic, chemical, isotopic, and mineralogical evidence indicates that an impact terminated the Cretaceous Period. The 180-km-diameter Chicxulub crater, which now lies buried on the Yucatan peninsula of Mexico, was probably formed by the impact. The impactor was probably a long-period comet. Shock devolatization of the thick carbonate/evaporite sequence impacted at Chicxulub probably led to a severe and long-lasting greenhouse warming and a prompt pulse of sulfuric acid rain. The fallout of crater ejecta formed two layers: a lower layer which varies in thickness following a power-law relation based on distance from the Chicxulub crater and an upper, globally-distributed, uniformly ∼3-mm-thick layer. The upper layer probably represents the fallout of condensates and entrained solid and liquid particles which were distributed globally by the impact fireball. The lower layer consists of brecciated rock and impact melt near the crater and largely altered tektites far from the crater. The clasts of this layer were probably ballistically transported. The Raton, New Mexico K/T boundary section preserves the fireball and ejecta layers in a coal-free nonmarine environment. Siderophile, chalcophile, and lithophile trace element anomalies occur similar to those found at marine K/T boundary localities. Soot occurs peaking in the 3-mm-thick fireball layer and the immediately overlying 3 mm of sediment, implying prompt burning of the Cretaceous forests. The Brazos River, Texas continental-shelf K/T sections preserve coarse boundary sediments which were probably produced by impact waves. Siderophile and chalcophile trace-element anomalies occur suggesting that the fireball layer and possibly part of the ejecta layer are interbedded with the coarse boundary sediments. The Beloc, Haiti deep-sea K/T sections preserve a thick ejecta sequence including altered and unaltered tektites and shocked minerals capped by the fireball layer. The thick K/T ejecta preserved at this and other nearby K/T localities require a source crater of Chicxulub's size and location. The composition of the tektites and shocked grains require an impact into recently extracted continental crust with a carbonate/evaporite component as found at the Chicxulub crater.