Metabasalts from the Mid-Atlantic Ridge: new insights into hydrothermal systems in slow-spreading crust

An extensive suite of hydrothermally altered rocks were recovered byAlvin and dredging along the MARK [Mid-Atlantic Ridge, south of the Kane Fracture Zone (23–24°N)] where detachment faulting has provided a window into the crustal component of hydrothermal systems. Rocks of basaltic composition are altered to two assemblages with these characteristics: (i) type I: albitic plagioclase (An_02–10)+mixed-layer smectite/chlorite or chlorite±actinolite±quartz±sphene, <10% of the clinopyroxene is altered, and there is no trace metal mobility; (ii) type II: plagioclase (An_10–30)+amphibole (actinolite-magnesio-hornblende) +chlorite+sphene, >20% of the clinopyroxene is altered, and Cu and Zn are leached. The geochemical signature of these alteration types reflects the relative proportion and composition of secondary minerals, and the degree of alteration of primary phases, and does not show simple predictive relationships. Element mobilities indicate that both alteration types formed at low water/rock ratios. The MARK assemblages are typical of the greenschist and transition to the amphibolite facies, and represent two distinct, albeit overlapping, temperature regimes: type I-180 to 300°C and type II-250 to 450°C. By analogy with DSDP/ODP Hole 504B and many ophiolites, the MARK metabasalts were altered within the downwelling limb of a hydrothermal cell and type I and II samples formed in the upper and lower portions of the sheeted like complex, respectively. Episodic magmatic and hydrothermal events at slow-spreading ridges suggest that these observed mineral assemblages represent the cumulative effects of more than one hydrothermal event. Groundmass and vein assemblages in the MARK metabasalts indicate either that alteration conditions did not change during successive hydrothermal events or that these assemblages record only the highest temperature event. Lack of retrograde reactions or overprinting of lower temperature assemblages (e.g., zeolites) suggests that there is a continuum in alteration conditions while crustal segments remain in the ridge axis environment. The type II samples may be representative of thereaction zone where compositions of hydrothermal fluids actively venting at the seafloor today become fixed. This prediction necessitates interaction between hydrothermal fluids and intersertal glass and/or mafic phases, in addition to plagioclase, in order to produce the observed range in vented fluid pH.     

Unit-cell dimensions and tetrahedral-site ordering in synthetic gallium albite (NaGaSi3O8)

The tetrahedral-site order-disorder transformation in gallium albite (NaGaSi₃O₈) has been investigated using Rietveld structure refinement. Study of gallium-substituted albite (in contrast to pure albite [NaAlSi₃O₈]) is facilitated by a relatively rapid order-disorder transformation and the large difference in X-ray scattering efficiencies of gallium and silicon. High albite-structure NaGaSi₃O₈, grown in a Na₂WO₄ flux, was ordered by hydrothermal annealing below 820° C and dry annealing above 820° C, to avoid melting, using a load pressure of approximately 1 kbar. Equilibration of the order-disorder reaction has been verified by three independent reversals of ordering. The transformation between low gallium albite and high gallium albite occurs over the temperature range 890° C 970° C. The gallium content of the T ₁o site increases continuously with decreasing temperature. The gallium contents of the T ₁m and T ₂m sites decrease smoothly with increasing ordering while the gallium content of the T ₂o site decreases, then increases and then decreases again with decreasing temperature. Unit-cell parameters and the triclinic obliquity vary throughout the order-disorder transformation and undergo abrupt changes at 913±3° C and 937±3° C. These abrupt changes correlate with changes in the gallium content of the T ₂o site, the X and Z ordering parameters and the configurational entropy. The order-disorder transformation in gallium-aluminum albite (NaGa_0.5Al_0.5Si₃O₈) occurs in the temperature range 765° C-850° C, at a temperature intermediate to the transformation in albite (50% order at about 680±20° C) and gallium albite.     

Vertical distribution of infauna in sediments of a subestuary of central Chesapeake Bay

The vertical distribution of infauna was quantified in eight strata from 0–35 cm in sand and mud sediments of a lower mesohaline subestuary of Chesapeake Bay. Large numbers of small polychaetes, amphipods, and clams occurred in the upper 5 cm of both sediment types, whereas large clams (Macoma balthica in mud andMya arenaria in sand) extended down to 30 cm and comprised most of the biomass in their respective sediment types. There was extensive overlap of the species inhabiting both sediment types. Vertical stratification within and among species apparently reflected constraints on burrowing depth related to body size rather than resource partitioning among competitors. The maximal sediment penetration of 35 cm, which was exhibited byHeteromastus filiformis, was considerably less than the maximal penetration for deep burrowing species in some marine infaunal communities. Several species which burrowed deeper than 5 cm exhibited significant temporal shifts in their vertical distribution.     

Biogeochemical processes affecting the distribution and vertical transport of hydrocarbon residues in the coastal Mediterranean

Sediments, sediment trap material, dominant surface plankton and collected fecal material were sampled concurrently with surface seawater in a coastal Mediterranean ecosystem and analyzed for non-volatile hydrocarbons and chlorinated hydrocarbons. Results showed consistent partitioning of hydrocarbon classes between dissolved and particulate phases of surface waters which appeared to be related to component solubility and particle availability. Analysis of biological materials showed the biota were important not only in packaging residues into large, fast sinking particles, but also in modifying the composition of components through metabolism and selective incorporation into body tissues and feces. Apparent sedimentation rate was calculated by analysis of ²¹⁰Pb in sediment core samples and used to estimate average deposition rates of organics to the sea/sediment interface. The flux of particles through 100 m, as measured in the trap material in this sampling interval, was sufficient to balance most of the petroleum input to the sediments but accounted for only 17% of the average flux of PCBs to the sediments, and virtually none of the more soluble chlorinated hydrocarbon flux. Vertical transport via large fecal material compared to average background particles was seasonally low corresponding to a seasonal minimum in plankton biomass in late summer. Results show that hydrocarbon residues transported long distances away from input sources are highly modified, pointing to the geochemical significance of physical-chemical partitioning between seawater phases, incorporation into organisms and fecal material and biological/chemical degradation.     

The effect of Al(OH)4 on the dissolution rate of quartz

The influence of Al(OH)₄⁻ on the dissolution rate of quartz at pH 10-13 and 59-89 °C was determined using batch experiments. Al(OH)₄⁻ at concentrations below gibbsite solubility depressed the dissolution rate by as much as 85%, and this effect was greater at lower pH and higher Al(OH)₄⁻ concentration. Dissolution rates increased with increasing temperature; however, the percent decrease in rate due to the presence of Al(OH)₄⁻ was invariant with temperature for a given H⁺ activity and Al(OH)₄⁻ concentration. These data, along with what is known about Al-Si interactions at high pH, are consistent with Al(OH)₄⁻ and Na⁺ co-adsorbing on silanol sites and passivating the surrounding quartz surface. The observed pH dependence, and lack of temperature dependence, of inferred Al(OH)₄⁻ sorption also supports the assumption that the acid-base behavior of the surface silanol groups has only a small temperature dependence in this range. A Langmuir-type adsorption model was used to express the degree of rate depression for a given in situ pH and Al(OH)₄⁻ concentration. Incorporation of the rate data in the absence of aluminate into models that assume a first-order dependence of the rate on the fraction of deprotonated silanol sites was unsuccessful. However, the data are consistent with the hypothesis proposed in the literature that two dissolution mechanisms may be operative in alkaline solutions: nucleophilic attack of water on siloxane bonds catalyzed by the presence of a deprotonated silanol group and OH⁻ attack catalyzed by the presence of a neutral silanol group. The data support the dominance of the second mechanism at higher pH and temperature.     

Geomagnetism in the satellite era

Kathryn A Whaler considers the opportunities and challenges arising from the greatly enhanced geomagnetism data sets now available, in her 2006 Presidential Address.     

Hydroxyl (6/2) airglow emission intensity ratios for rotational temperature determination

OH(6/2) Q1/P1 and R₁/P₁ airglow emission intensity ratios, for rotational states up to j = 4.5, are measured to be lower than implied by transition probabilities published by various authors including Mies, Langhoff et al. and Turnbull and Lowe. Experimentally determined relative values of j transitions yield OH(6/2) rotational temperatures 2 K lower than Langhoff et al., 7 K lower than Mies and 13 K lower than Turnbull and Lowe.     

Distribution of barium and fulvic acid at the mica-solution interface using in-situ X-ray reflectivity

The interfacial structures of the basal surface of muscovite mica in solutions containing (1) 5 × 10⁻³ m BaCl₂, (2) 500 ppm Elliott Soil Fulvic Acid I (ESFA I), (3) 100 ppm Elliott Soil Fulvic Acid II (ESFA II), (4) 100 ppm Pahokee Peat Fulvic Acid I (PPFA), and (5) 5 × 10⁻³ m BaCl₂ and 100 ppm ESFA II were obtained with high resolution in-situ X-ray reflectivity. The derived electron-density profile in BaCl₂ shows two sharp peaks near the mica surface at 1.98(2) and 3.02(4) Å corresponding to the heights of a mixture of Ba²⁺ ions and water molecules adsorbed in ditrigonal cavities and water molecules coordinated to the Ba²⁺ ions, respectively. This pattern indicates that most Ba²⁺ ions are adsorbed on the mica surface as inner-sphere complexes in a partially hydrated form. The amount of Ba²⁺ ions in the ditrigonal cavities compensates more than 90% of the layer charge of the mica surface. The electron-density profiles of the fulvic acids (FAs) adsorbed on the mica surface, in the absence of Ba²⁺, had overall thicknesses of 4.9-10.8 Å and consisted of one broad taller peak near the surface (likely hydrophobic and positively-charged groups) followed by a broad humped pattern (possibly containing negatively-charged functional groups). The total interfacial electron density and thickness of the FA layer increased as the solution FA concentration increased. The sorbed peat FA which has higher ash content showed a higher average electron density than the sorbed soil FA. When the muscovite reacted with a pre-mixed BaCl₂-ESFA II solution, the positions of the two peaks nearest the surface matched those in the BaCl₂ solution. However, the occupancy of the second peak decreased by about 30% implying that the hydration shell of surface-adsorbed Ba²⁺ was partially substituted by FA. The two surface peaks were followed by a broad less electron-dense layer suggesting a sorption mechanism in which Ba²⁺ acts dominantly as a bridging cation between the mica surface and FA. When the muscovite reacted first with FA and subsequently with BaCl₂, more Ba²⁺ could be adsorbed on the FA-coated mica surface. The peak closest to the mica included Ba²⁺ ions adsorbed directly on the mica in an amount similar to that in the BaCl₂ solution but more broadly distributed. A second peak observed within the FA layer suggests that the FA coating provides additional sites for Ba²⁺ sorption. The results indicate that enhanced uptake of heavy metals can occur when an organic coating already exists on a mineral surface.