Origin of biotite-apatite-rich enclaves, Achala batholith, Argentina

The Achala batholith of Argentina contains very unusual layered enclaves containing up to 30% apatite and 50% biotite in some layers. This modal mineralogy produces bulk-rock compositions that cannot represent liquids, having as little as 29% SiO₂ and up to 8% P₂O₅. Nor can the enclaves represent metasedimentary xenoliths because: (1) none of the Precambrian wall rocks has these compositions; (2) none of the metasedimentary xenoliths present within the batholith shows any degree of transition to the mica-apatite-rich enclaves; (3) the compositions and textures in the enclaves are inconsistent with metasediments; (4) a geochronological study of zircon from an enclave gives an age of 368 ± 2 Ma, the exact age of zircons in the granitic host rocks. For these reasons, we conclude that the enclaves are neither xenoliths of Precambrian wall rocks nor restite of a Precambrian source. The identical age of the enclave and the host granites, coupled with textural, mineralogical, and bulk-rock characteristics of the enclaves, indicates that the enclaves are magmatic segregations, i.e., cumulates. The F-rich nature of the stubby-shaped apatites and biotites indicates a high F content of the magma parental to the enclaves. We infer that the viscosity of the melt was lowered sufficiently to allow cumulates to form in spite of the granitic composition of the melt. Received: 12 December 1996 / Accepted: 11 August 1997     

137Cs, 60Co and 125I bioaccumulation by seaweeds from the Angra dos Reis nuclear power plant region

As part of a broad research program on the behavior of critical radionuclides to be discharged into the sea by the first Brazilian nuclear powerplant, the uptake accumulation and loss of ¹³⁷Cs, ⁶⁰Co and ¹²⁵I by locally abundant seaweed species was studiedUptake in static 12 liter aquarium experiments reached apparent steady-state in 2–7 days (⁶⁰Co and ¹²⁵I) or 2–3 weeks (¹³⁷Cs). Elimination followed a reverse pattern, being comparatively fast for ¹³⁷Cs and slow for ⁶⁰Co and ¹²⁵I. Dry weight bioaccumulation factors (BFs) were variable, falling in the 10¹ range for ¹³⁷Cs, 10³ for ¹²⁵I and 10³–10⁴ for ⁶⁰Co. Various short-term experiments performed over a 16 month period, showed marked temporal variations of ⁶⁰Co BFs for all species.The results demonstrated that the studied species may play an important role in the transfer of the critical radionuclides through local food webs and can be employed as useful monitors for routine or accidental radionuclide releases.     

A new estimate of the galactic halo mass based on the kinematics of globular clusters

We calculated the energy distribution function for globular clusters in our Galaxy, using the inversion procedure first suggested by Eddington. If the halo mass distribution is of the formM _H=r ^1.21, then the observed data on the velocity dispersion of F-clusters can be explained, resulting an enclosed mass of 4.3×10¹¹ M within a galactocentric radius of 33 kpc.     

A new global landslide dam database (RAGLAD) and analysis utilizing auxiliary global fluvial datasets

To address the current data and understanding knowledge gap in landslide dam inventories related to geomorphological parameters, a new global-scale landslide dam dataset named River Augmented Global Landslide Dams (RAGLAD) was created. RAGLAD is a collection of landslide dam records from multiple data sources published in various languages and many of these records we have been able to precisely geolocate. In total, 779 landslide dam records were compiled from 34 countries/regions. The spatial distribution, time trend, triggers, and geomorphological characteristic of the landslides and catchments where landslide dams formed are summarized. The relationships between geomorphological characteristics for landslides that form river dams are discussed and compared with those of landslides more generally. Additionally, a potential threshold for landslide dam formation is proposed, based on the relationship of landslide volume to river width. Our findings from our analysis of the value of the use of additional fluvial datasets to augment the database parameters indicate that they can be applied as a reliable supplemental data source, when the landslide dam records were accurately and precisely geolocated, although location precision in smaller river catchment areas can result in some uncertainty at this scale. This newly collected and supplemented dataset will allow the analysis and development of new relationships between landslides located near rivers and their actual propensity to block those particular rivers based on their geomorphology.

     

Determination of the mean H<Emphasis Type="SmallCaps">i</Emphasis> absorption of the intergalactic medium

In recent years, the Lyman-α forest in quasar spectra has been used, together with N-body simulations, to determine the underlying matter distribution in the intergalactic medium (IGM). One of the key parameters to be known in order to compare observations and numerical simulations is the mean HI absorption in the IGM. To derive the latter, one has first to fit the quasar continuum. We have observed 20 high redshift and highly luminous QSOs (m _V ≤ 17.5 and 2.40 ≤ z _em ≤ 3.91) at intermediate spectral resolution, with either EMMI (ESO Multi-Mode Instrument) on the ESO-NTT telescope or CARELEC at the OHP (Observatoire de Haute-Provence), and applied different methods of determining the QSO continuum to this QSO sample. We have measured the amount of absorption, known as the flux decrement, DA, in the Lyman-α forest for these different methods and compared the results. In addition, we have compared DA values measured along the same lines of sight observed at high and intermediate spectral resolutions. We discuss the systematics resulting from the use of automatic continuum fitting methods.     

Phase separation,fluid mixing,and origin of the greisens and potassic episyenite associated with the Água Boa pluton,Pitinga tin province,Amazonian Craton,Brazil

Based on petrographical data, three types of greisen have been characterized at the western border of Água Boa pluton: siderophyllite–topaz–quartz greisen (greisen 1), fluorite–phengite–quartz greisen (greisen 2) and quartz–chlorite–phengite greisen (greisen 3). Episyenites were also identified.Two fluids of independent origin interacted with the same protolith – a hornblende-biotite alkali feldspar granite – and produced both the greisens and potassic episyenite: (1) an acid, low-salinity (4–12 wt.% NaCl eq.), F-rich, relatively hot (400–350 °C) reduced aqueous-carbonic fluid (CH₄–H₂O–NaCl–FeCl₂ ± KCl), which by immiscibility gave rise to fluid IA (aqueous) and IC (carbonic); and (2) a lower salinity (2–4 wt.% NaCl eq.) and temperature (200–150 °C) aqueous fluid (H₂O–NaCl), which was responsible for all dilution processes. Fluid 1 seems to have had a magmatic-hydrothermal origin, while fluid 2 is probably surface-derived (meteoric water?). An alkaline, F-poorer and diluted equivalent of fluid IA was interpreted to have caused the episyenitization of the granite host rock as well as the formation of phengite-rich greisen 3. The continuos interaction of this fluid with the potassic episyenite produced a moderate- to high-salinity (20–24 wt.% NaCl eq.), low-temperature (200–100 °C) fluid (H₂O–NaCl–CaCl₂ ± KCl), leading to the formation of chlorite-rich zone of greisen 3 and late silicification of potassic episyenite.In the greisen 1, decreasing F-activity and increasing oxygen fugacity, as the system cooled down, favored the formation of a topaz-rich inner zone, which grades into a siderophyllite-rich zone outwardly. Greisen 2 was formed under more oxidizing conditions by fluids poorer in F than those trapped in the siderophyllite-rich zone.The oxidation of aqueous-carbonic fluid took place at three distinct stages: (i) below the FMQ buffer; (ii) between the FMQ and NNO buffers; and (iii) above the NNO buffer.The dissolution cavities generated during the episyenitization process increased the permeability of the altered rocks, resulting in an increase of the fluid/rock ratios at the potassic episyenite and greisen 3 sites.All these fluids were trapped under pressure conditions of <1.0 kbar, representing shallow crustal levels and are consistent with those that have been estimated for the Pitinga tin–granites.The oxygen fugacity, F-activity gradients and salinity variations that occurred during the cooling of the hydrothermal system, in addition to differences in permeability, were important factors in the formation of distinct greisens. They not only controlled the fluid compositional changes, but also caused the cassiterite and sulfide precipitation at the greisen sites.