Biotites and associated minerals as markers of magmatic fractionation and deuteric equilibration in granites

Analyses of 80 biotite, alkali feldspar, oligoclase, hornblende, Fe-Ti oxide separates from the coarse-grained granites of a late-hercynian epizonal diapir, the Ploumanac'h complex, Brittany, show that these minerals display a regular concentric cryptic layering related to fractional crystallization. The Ca, Mg, Ba, Sr, Cr, V content of minerals decreases as the Na, Fe, and Rb content increases. Biotites become more dioctahedral towards the outer residual syenogranite, with a correlative K deficiency. Trioctahedral biotites from the inner accumulative monzogranite are secondarily oxidized with a gain of Fe³⁺ and a loss of OH. This alteration id due to the percolation of exsolved fluids rich in H₂O and containing a small amount of CO₂, F, S, Cl. During this autometamorphic stage, trace elements like Rb, Sr are completely redistributed on the scale of hand specimens, with a restricted range of partition coefficients between biotite, perthite and oligoclase. This equilibration occurred at a temperature about 550 ° C and a fluid pressure about 1,000 bars, with f _H2_O probably less than 500 bars. A later stage of fluid circulation along fractures brings up a slight Li metasomatism. Biotites are a sensitive marker of both magmatic and postmagmatic stages of subsolvus or ‘wet’ plutonites.     

Infrared study of the pyrolysis products of sporopollenin and lignite

The effect of pyrolysis at increasing temperature on sporopollenin, lignite and sporopollenin oxidized at 200°C has been investigated using measured infrared band absorption coefficients.Oxidation of sporopollenin in air at 200°C is marked by a decrease in the content of saturated hydrocarbon chains and a strong increase in the concentration of carboxylic acid groups.Pyrolysis of a thick bed of sporopollenin at increasing temperatures leads to the removal of a large proportion of oxygenated functions, before the removal of hydrocarbons. For lignite and oxidized sporopollenin, the loss of both types of functional groups extends over a broader temperature range. Reorganization of the carbonaceous residue at high temperature is hindered if a sufficiently low content of oxygenated functions, carbonyl and carboxyl as well as hydroxyl and ether groups, is not reached before the elimination of hydrocarbons.     

Ukinrek Maars, Alaska, II. Deposits and formation of the 1977 craters

The initial phase of the eruption forming Ukinrek Maars during March and April 1977 were explosions from the site of West Maar. These were mainly phreatomagmatic and initially transitional to strombolian. Activity at West Maar ceased after three days upon the initiation of the East Maar. The crater quickly grew by strong phreatomagmatic explosions. During the first phases of phreatomagmatic activity at East Maar, large exotic blocks derived from a subsurface till were ejected. Ballistic studies indicate muzzle velocities for these blocks of 80–90 m s⁻¹.Phreatomagmatic explosions ejected both juvenile and non-juvenile material which formed a low rim of ejecta (< 26 mhigh) around the crater and a localized, coarse, wellsorted (σ_φ = 1−1.5) juvenile and lithic fall deposit. Other fine ash beds, interstratified with the coarse beds, are more poorly sorted (σ_φ = 2−3) and are interpreted as fallout of wet, cohesive ash from probably milder phases of activity in the crater. Minor base surge activity damaged trees and deposited fine ash, including layers plastered on vertical surfaces. Viscous basalt lava appeared in the center of the East Maar crater almost immediately and a lava dome gradually grew in the crater despite phreatomagmatic eruptions adjacent to it.The development of these maars appears to be mainly controlled by gradual collapse of crater and conduit walls, and blasting-out of the slumped debris by phreatomagmatic explosions when rising magma contacted groundwater beneath the regional water table and a local perched aquifer.Ballistic analysis on the ejected blocks indicates a maximum muzzle velocity of 100–150 m s^-1, values similar to those obtained from other ballistic studies on maar ejecta.     

La Notion de Migration en Paléontologie

Zusammenfassung Gemessen mit dem Maßstab der geologischen Zeiten, dauern die Wanderungen, wie sie die Biologen studieren, nur fast einen Augenblick; demgegenüber aber erstrecken sich die Verschiebungen der Fauna und Flora, wie sie die geologischen Studien festlegen, über Millionen von Jahren. Diese Verschiebungen sind statistische Erscheinungen aus dem Gebiet der Biogeographie und stellen das Ergebnis von zahlreichen biologischen Bewegungen dar. Die Verfasser untersuchen die Wanderungen der Säugetiere (stete Entwicklung durch erwachsene Typen), die Wanderungen der Pflanzen (unstete Entwicklung durch Keime), die Wanderungen der wirbellosen Seetiere, die man in die eine oder in die andere der vorhergehenden Abteilungen einreihen kann. Einige neue Ausdrücke werden vorgeschlagen.     

Determination of predevelopment denudation rates of an agricultural watershed (Cayaguás River, Puerto Rico) using in-situ-produced Be in river-borne quartz

Accurate estimates of watershed denudation absent anthropogenic effects are required to develop strategies for mitigating accelerated physical erosion resulting from human activities, to model global geochemical cycles, and to examine interactions among climate, weathering, and uplift. We present a simple approach to estimate predevelopment denudation rates using in-situ-produced cosmogenic ¹⁰Be in fluvial sediments. Denudation processes in an agricultural watershed (Cayaguás River Basin, Puerto Rico) and a matched undisturbed watershed (Icacos River Basin) were compared using ¹⁰Be concentrations in quartz for various size fractions of bed material. The coarse fractions in both watersheds bear the imprint of long subsurface residence times. Fine material from old shallow soils contributes little, however, to the present-day sediment output of the Cayaguás. This confirms the recent and presumably anthropogenic origin of the modern high denudation rate in the Cayaguás Basin and suggests that pre-agricultural erosional conditions were comparable to those of the present-day Icacos.     

Where Can Seismic Anisotropy Be Detected in the Earth’s Mantle? In Boundary Layers...

—During the last 30 years, considerable evidence of seismic anisotropy has accumulated demonstrating that it is present at all scales, but not in all depth ranges. We detail which conditions are necessary to detect large-scale seismic anisotropy. Firstly, minerals must display a strong anisotropy at the microscopic scale, and/or the medium must be finely layered. Secondly, the relative orientations of symmetry axes in the different crystals must not counteract in destroying the intrinsic anisotropy of each mineral, and there must be efficient mechanisms of orientation of minerals and aggregates. Finally, the strain field must be coherent at large scale in order to preserve long wavelength anisotropy. Part of shallow anisotropy can be related to the past strain field (frozen-in anisotropy), however the deep anisotropy is due to the present strain field. All these conditions are fulfilled only in boundary layers of convective mantle.¶We review in this paper, the seismic data sets which provide insight into the location at depth of large-scale anisotropy from the D"-layer up to the lithosphere. In addition to the well-documented seismic anisotropy in the lithosphere and asthenosphere, there is new evidence of seismic anisotropy in the upper (400–660 km) and lower (660–900 km) transition zones and in the D"-layer. Nonetheless the bulk of the lower mantle seems close to isotropy. If we assume the hypothesis that seismic anisotropy is associated with boundary layers in convective systems, these observations strongly suggest that the transition zone is a boundary layer which makes the pasage of matter between the upper and the lower mantle difficult. However, this general statement does not rule out flow circulation between the upper and lower mantles. Finally, the geophysical, mineral physics and geological applications are briefly reviewed. An intercomparison between surface wave anisotropy and body-wave anisotropy data sets is presented. We discuss the scientific potential of seismic anisotropy and how it makes it possible to gain more insight into continental root, deformation and geodynamics processes.     

87Rb/87Sr study of diogenites

The five diogenites, Johnstown, Roda, Ellemeet, Shalka and Tatahouine, give scattered data in the⁸⁷Rb/⁸⁶Sr,⁸⁷Sr/⁸⁶Sr diagram. This can result from a disturbance which occurred later than 4.45 Ga ago. However, it is shown that if samples of sufficient size were analyzed, there meteorites could plot on the eucrite isochron and are thereby in agreement with a genetic relation between eucrites, howardites and diogenites. The age of eucrite differentiation from diogenites has been computed using data from the two families yielding an age of 4.47±0.1Ga(2σ) (λ=1.42×10^?11a^?1), the initial⁸⁷Sr/⁸⁶Sr ratio being BABI.     

87Sr/86Sr ratios in hydrothermal waters and deposits from the East Pacific Rise at 21°N

⁸⁷Sr/⁸⁶Sr ratios of three hydrothermal waters collected on the East Pacific Rise at 21°N define a mixing line between seawater and a hydrothermal end-member at 0.7030 which is derived by seawater-basalt interaction at ca. 350°C and water/rock ratio of about 1.5. Sr concentrations are not affected in the process while Mg uptake from seawater is almost complete. Up to2/3 of this hydrothermal component is involved in anhydrite precipitation while the Sr isotopic ratio in sulfides (chalcopyrite + sphalerite) cannot be distinguished from that of sulfate. It is estimated that ca. 1 × 10¹⁰ moles of strontium are yearly cycled in the hydrothermal systems of mid-oceanic ridges, thereby affecting the⁸⁷Sr/⁸⁶Sr budget of seawater. Mass balance between river runoff, limestone precipitation and ridge basalt alteration suggests that the⁸⁷Sr/⁸⁶Sr ratios of the river runoff are in the range 0.7097–0.7113, and are largely dominated by limestone alteration.     

230Th-238U radioactive disequilibria in tholeiites from the FAMOUS zone (Mid-Atlantic Ridge, 36°50′N): Th and Sr isotopic geochemistry

We analyzed, U, Th and²³⁰Th/²³²Th activity ratios for a few tholeiites from the Mid-Atlantic Ridge FAMOUS zone at 36°50′N. The results show a fairly wide scatter for both Th/U and (²³⁰Th/²³²Th) ratios. Seawater contamination appears to be responsible for this scatter and, for the uranium, produces an increase in content yielding a (²³⁴U/²³⁸U) ratio greater than 1 and, for the Th, an increase of the (²³⁰Th/²³²Th) ratio which is a very sensitive indicator for contamination. Also, the latter often is selective: U, Th and Sr are not affected in the same manner.When discarding all data for contaminated samples, the FAMOUS zone appears to be very homogeneous with aTh/U ratio value of 3.05 and a (²³⁰Th/²³²Th) ratio value of 1.24. Comparison with other active volcanic areas reveals a negative correlation between (²³⁰Th/²³²Th) and⁸⁷Sr/⁸⁶Sr ratios for present lavas which is indicative of a consistency in Th-U and Rb-Sr fractionation in the source regions of these magmas. The Th isotopic geochemistry can thus provide useful information for the study of present volcanism, information as valuable as that from Sr, Pb or Nd isotopes.