The breakdown of potassium feldspar at high water pressures

The equilibrium position of the reaction between sanidine and water to form “sanidine hydrate” has been determined by reversal experiments on well characterised synthetic starting materials in a piston cylinder apparatus. The reaction was found to lie between four reversed brackets of 2.35 and 2.50 GPa at 450 °C, 2.40 and 2.59 GPa at 550 °C, 2.67 and 2.74 GPa at 650 °C, and 2.70 and 2.72 GPa at 680 °C. Infrared spectroscopy showed that the dominant water species in sanidine hydrate was structural H₂O. The minimum quantity of this structural H₂O, measured by thermogravimetric analysis, varied between 4.42 and 5.85 wt% over the pressure range of 2.7 to 3.2 GPa and the temperature range of 450 to 680 °C. Systematic variation in water content with pressure and temperature was not clearly established. The maximum value was below 6.07 wt%, the equivalent of 1 molecule of H₂O per formula unit. The water could be removed entirely by heating at atmospheric pressure to produce a metastable, anhydrous, hexagonal KAlSi₃O₈ phase (“hexasanidine”) implying that the structural H₂O content of sanidine hydrate can vary. The unit cell parameters for sanidine hydrate, measured by powder X-ray diffraction, were a = 0.53366 (±0.00022) nm and c = 0.77141 (±0.00052) nm, and those for hexasanidine were a = 0.52893 (±0.00016) nm and c = 0.78185 (±0.00036) nm. The behaviour and properties of sanidine hydrate appear to be analogous to those of the hydrate phase cymrite in the equivalent barium system. The occurrence of sanidine hydrate in the Earth would be limited to high pressure but very low temperature conditions and hence it could be a potential reservoir for water in cold subduction zones. However, sanidine hydrate would probably be constrained to granitic rock compositions at these pressures and temperatures. Received: 6 May 1997 / Accepted: 2 October 1997     

Noble gases in the earth's mantle

The abundances and isotopic compositions of noble gases in two samples from ultramafic xenoliths in alkali basalt, a young kaersutitic amphibole separated from a peridotite xenolith from Dish Hill, California and an ancient whole-rock lherzolite xenolith from Baja California, are reported and compared with the results of analyses on other mantle samples. In addition to previously recognized excesses of ³He and ¹²⁹Xe, our results indicate that ambient gases in the mantle show a general enrichment of the lighter-mass nonradiogenic isotopes of Ar, Kr and Xe, and Ar with ⁴⁰Ar/³⁶Ar = 3 · 10².     

Model systems for anatexis of pelitic rocks

Subsolidus and melting reactions involving calcic plagioclase in pelitic assemblages in the K-Na-Ca model system occur at higher temperatures than their K-Na counterparts. For the most calcic plagioclase compositions observed in high-grade pelitic rocks (An₂₅-An₄₀) the equilibria are rarely extended by more than 30 ° C above those in KA1O₂-NaAlO₂-Al₂O₃-SiO₂-H₂O, although all discontinuities in facies inferred for the K-Na system are continuously displaced when they involve Ca-bearing plagioclase. The maximum pressure-temperature overlap between muscovite dehydration and initial melting reactions occurs in the pressure range of 4–6 kbar between about 640 ° and 720 ° C. This provides optimum conditions for anatectic melt generation in felsic rocks of the appropriate compositions progressively metamorphosed in kyanite-sillimanite facies series. Progressive regional metamorphism at pressures of 2–4 kbar, corresponding to andalusite-sillimanite facies series, shows little overlap between muscovite dehydration and initial melting reactions. Consequently anatectic melt generation in andalusite-sillimanite facies series would require the participation of biotite in dehydration-melting reactions. Felsic intrusive rock in andalusite-sillimanite terranes could have risen upward from their anatectic sites in high grade kyanite-sillimanite facies series at depth. Many andalusite-sillimanite facies series terranes culminating in migmatites could represent upward movement of kyanite-sillimanite facies series rocks to shallower depths with uplift rates faster than cooling rates.     

A reinterpretation of the entrainment process in some laboratory experiments

Pollard et al. (1973) proposed a closure hypothesis for the entrainment process in the stress-mixed layer of a stratified fluid. Some laboratory experiments suitable for testing the hypothesis were carried out by Kantha et al. (1977) and by Kato and Phillips (1969). In the early period of each experiment, before side-wall friction becomes important, these experiments support the closure hypothesis, with a critical Froude number close to 1, perhaps F = 1.2 ± 0.2.     

Turbulence Measurements above a Pine Forest

Eddy fluxes of momentum, sensible and latent heat, and turbulence spectra measured over the Thetford Forest during 10 days in the Spring of 1973 are described. The measured total heat flux (H + λE) for 122 20-min periods agreed closely on average with independent estimates from an energy balance method. There was evidence that the energy balance data gave small systematic overestimates of available energy during the hours before noon, compensated by slight underestimates for the remainder of the day. A comparison of measured wind speeds and friction velocities in neutral stability confirmed the validity of the aerodynamic method for estimating momentum fluxes at heights of a few roughness lengths above the canopy. In stable conditions the log-linear wind profile $U=(u_{?t}/k)({? ln}((z-d)/z_{0})+←pha(z-d-z_{0})/L)$ with α = 3.4 ± 0.4 provided a good fit to the data. Spectra in unstable conditions were generally more sharply peaked than those measured by other workers over smoother terrain: differences were less marked in the case of vertical velocity in stable conditions. Temperature spectra in these stable conditions showed high energy at relatively low wavenumbers, and wT cospectra showed a cospectral gap; both of these results were associated with an intermittent sawtooth structure in the temperature fluctuations.     

Pollution records from sediments of three lakes in New York State

Sediment core segments from Sylvan Lake, Lake Champlain and Lake Canadarago were dated radiometrically with ²¹⁰Pb and ¹³⁷Cs. Their respective sedimentation rates were determined to be 0.11, 0.14 and 0.52 g cm^?2 yr^?1. For the two lakes of lower sedimentation the variations of selected elemental abundances as function of depth were analyzed. Two groupings were found: Al, K, Ti, Rb and Zr were correlated among themselves but reflected different variations in the input of terrigenous erosion material to the lakes. The Cu, Zn and Pb correlated among themselves showed similar depth dependence with increasing concentrations toward the top which can be attributed to cultural pollution. Recent ‘excess’ fluxes to the sediments above the natural contribution by clastic material were derived for the location of the cores, which for Cu, Zn and Pb amounted to 3.8, 24 and 16 μg cm^?2 yr^?1 respectively for Sylvan Lake and 4.9, 20 and 16 μg cm^?2 yr^?1 for Lake Champlain. The corresponding ²¹⁰Pb flux was 3.3 and 2.3 dpm cm^?2 yr^?1, respectively for the two lakes.Approximate residence times in the water column were obtained for trace metals at the Lake Champlain location. Short residence times estimated for Pb (< 0.15 yr) and Cu (< 0.4 yr) indicate fast removal, whereas those for Zn (1.0 ± 0.3 yr) and Cr (2.0 ± 0.5 yr) appeared to be dominated by the water residence time.     

Stable isotope studies of fluid inclusions in speleothems and their paleoclimatic significance

Fluid inclusions found trapped in speleothems (cave deposited travertine) are interpreted as samples of seepage water from which enclosing calcium carbonate was deposited. The inclusions are assumed to have preserved their D/H ratios since the time of deposition. Initial ¹⁸O/¹⁶O ratios can be inferred from δD because rain- and snow-derived seepage waters fall on the meteoric water line (δD = 8δ¹⁸O + 10). Estimates of temperature of deposition of the carbonate can be calculated from inclusion D/H ratios and δ¹⁸O of enclosing calcite in Pleistocene speleothems. For most speleothems investigated (0–200,000 yr old) δ¹⁸O of calcite appears to have decreased with increasing temperature of deposition indicating that the dominant cause of climate-dependent change in δ¹⁸O of calcite was the change in K_cw, the isotope fractionation equilibrium constant, with temperature; δ¹⁸O of meteoric precipitation generally increased with increasing temperature, but not sufficiently to compensate for the decrease in K_cw.     

Oldest Atlantic seafloor

Basalt recovered beneath Jurassic sediments in the western Atlantic at Deep Sea Drilling Project sites 100 and 105 of leg 11 has petrographic features characteristic of water-quenched basalt extruded along modern ocean ridges. Site 100 basalt appears to represent two or three massive cooling units, and an extrusive emplacement is probable. Site 105 basalt is less altered and appears to be a compositionally homogeneous pillow lava sequence related to a single eruptive episode.Although the leg 11 basalts are much more closely related in time to the Triassic lavas and intrusives of eastern continental North America, their geochemical features are closely comparable to those of modern Mid-Atlantic Ridge basalts unrelated to postulated mantle plume activity. Projection of leg 11 sites back along accepted spreading flow lines to their presumed points of origin shows that these origins are also outside the influence of modern plume activity. Thus, these oldest Atlantic seafloor basalts provide no information on the time of initiation of these plumes. The Triassic continental diabases show north to south compositional variations in Rb, Ba, La, and Sr which lie within the range of plume-related basalt on the Mid-Atlantic Ridge (20 °–40 °N). This suggests that these diabases had mantle sources similar in composition to those beneath the present Mid-Atlantic Ridge. Plumes related to deep mantle sources may have contributed to the LIL-element enrichment in the Triassic diabase and may alos have been instrumental in initiating the rifting of the North Atlantic. Systematically high values for K and Sr⁸⁷/Sr⁸⁶ in the Triassic diabases may reflect superimposed effects of crustal contamination in the Triassic magmas.Contribution Number 3953 from the Woods Hole Oceanographic Institution