Monazite solubility and dissolution kinetics: implications for the thorium and light rare earth chemistry of felsic magmas

A series of monazite dissolution experiments was conducted in a hydrous (1–6 wt.%) granitic melt at 8 kbar over the temperature range 1,000–1,400° C. A polished cube of monazite was immersed in a natural obsidian melt and allowed to partially dissolve. Electron microprobe traverses perpendicular to the crystal-melt interface revealed concentration gradients in the LREEs and P. Diffusivities of the LREEs and P were calculated from these profiles, yielding the following Arrhenius relations for the LREEs: D=0.23 exp(–60.1 kcal mol^–1/RT) at 6% water D=2.30×10⁷ exp(–122.1 kcal mol^–1/RT) at 1% water These results demonstrate the importance of dissolved water on REE diffusion. Phosphorus diffusivities are nearly identical to those of the rare-earths, suggesting that P diffusion charge-compensates REE diffusion. The concentration of LREEs required for monazite saturation in these melts is given by the level of dissolved LREEs at the crystal-melt interface. These values also show a dependence on dissolved water, with LREE_sat=60 ppm at 6% H₂O when extrapolated down to 700° C, and LREE_sat=30 ppm at 1% H₂O. Calculated dissolution rates based on the above parameters indicate that minute (<30 m diameter) monazite crystals will be readily digested by an enclosing anatectic magma under reasonable geologic conditions (i.e., T=700–800° C and >2% H₂O), whereas larger (> 50 m) crystals will likely be residual over the duration of an anatectic event. The low solubility of monazite in this melt suggests that the LREE depletion observed in some felsic differentiation suites may be the result of monazite crystallization. Limited experimental and geochemical/petrologic evidence indicates that compositional changes in the melt accompanying differentiation decrease the solubility of monazite drastically. Kinetic and chemical constraints may also lead to localized monazite saturation and inclusion in major phases or even other accessories. Variations in the REE composition of monazite from different parageneses probably reflects the REE pattern of the parent melt, and may be due to gradational differences in the stability of individual or subgroup REE-complexes as a function of melt composition. Particularly important in this regard seems to be the lime+alkali/alumina balance of the melt and its volatile content.     

The Minnesota long-term temperature record

A remarkable climatological record exists in the heart of the North American interior that deserves recognition and exploitation. The objective of this paper is to bring this unique record that commenced in October, 1819, at Ft. Snelling, Minn., to the attention of the climate change community.Minnesota Agricultural Experiment Station, University of Minnesota, Scientific Journal Series Paper No. 13975.     

Statistical modeling of historic shore erosion rates on the Chesapeake Bay in Maryland

Few strong relationships exist along the Chesapeake Bay shoreline between the historic erosion rate and the distribution of any of several coastal parameters which were defined and tested using traditional regression and discriminant analysis procedures To develop a simple predictive equation for shore erosion that could be used by coastal managers, the entire Chesapeake Bay shoreline was partitioned into naturally occurring reaches 2–5 km in length, and the historic erosion rate on each reach was modelled as a function of five variables (a) shoreline type, (b) “100-year” storm surge height, (c) mean tide range, (d) wave climate, and (e) potential littoral drift rate The statistical analysis yielded a multiple correlation coefficient (r ²) of 30 8%, discriminant analysis showed only the first two variables listed above are useful predictors (i e, statistically significant) of historic erosion rates A 95-mile portion of the same bay shoreline in Queen Anne’s and Talbot counties was then partitioned into shorter reach lengths (1/2–2km) and more variables were included The multiple correlation coefficient (r ²) improved slightly to 32 9%, but only shoreline type and potential littoral drift rate were found to be useful predictors of historic erosion rates Curiously, the ability to model statistically the historic shore erosion rate is best on those reaches already substantially protected by structures For Queen Anne’s and Talbot counties, the multiple regression coefficient improved to 61 5% when only reaches 1/2–2km in length protected by structures were considered.     

Jointly determined focal parameters along a fault in California's northern coast range

The authors recently determined seismic parameters of earthquakes located along a lineation of microearthquakes in the Coast Range Province of northern California (Dehlinger and Bolt, 1984). The lineation closely follows the mapped Bartlett Springs fault zone, which is considered to be the source of seismic activity. This fault strikes N40°W for about 40 km and the mapped fault zone consists of a 2–3 km-wide belt of melange. Earthquake hypocenters indicate that this belt dips steeply to the northeast, with shocks occurring along multiple dislocation planes that together extend downward to depths of about 12 km.Focal mechanisms and associated stress fields were determined for 22 of the best recorded recent shocks along the earthquake lineation. Analyses of these shocks provided a comparison between individually and jointly determined focal mechanisms and of the directions of the corresponding stress axes, for identical shocks along the entire fault length. Thirteen of the shocks exhibited right-lateral, San Andreas type of source motions. Average values of these 13 individually determined focal parameters are within the standard deviation of the same set of jointly determined values. It is thus verified that the probability model algorithm for joint parameters determinations developed by Brillinger et al. (1980) yields reliable values of focal parameters. Moreover, the jointly determined parameters appear to be more reliable than the average-determined ones. The focal mechanisms and associated stress fields along the entire length of the Bartlett Springs fault zone are consistent with shearing motions between the North American and Pacific lithospheric plates that produce displacements along the San Andreas transform fault.     

Interstitial Sn2+ in synthetic and natural cassiterite crystals

Evidence for the presence of Sn²⁺ in an octahedral interstitial site in synthetic and natural cassiterite (SnO₂) is presented. The absorption and relative absorption spectral peaks measured are similar to ones found in Sn²⁺-doped KI by absorption spectrophotometry and Sn²⁺-doped soda-silica glass by reflection spectroscopy. The estimated quantity of interstitial Sn²⁺ present is found by calculating that needed to balance the uncompensated M³⁺ impurity in substitutional Sn⁴⁺ sites where M³⁺ is mainly Al³⁺ and Fe³⁺. Estimates of the oscillator strengths of three Sn²⁺ bands detected by absorption spectrophotometry in the synthetic crystal are given. The interstitial Sn²⁺ content in synthetic and natural cassiterite is not affected by heating,⁶⁰Co gamma irradiation, or UV light treatment with a high pressure xenon-mercury lamp.     

Lead and strontium isotopic evidence for crustal interaction and compositional zonation in the source regions of Pleistocene basaltic and rhyolitic magmas of the Coso volcanic field,California

The isotopic compositions of Pb and Sr in Pleistocene basalt, high-silica rhyolite, and andesitic inclusions in rhyolite of the Coso volcanic field indicate that these rocks were derived from different levels of compositionally zoned magmatic systems. The 2 earliest rhyolites probably were tapped from short-lived silicic reservoirs, in contrast to the other 36 rhyolite domes and lava flows which the isotopic data suggest may have been leaked from the top of a single, long-lived magmatic system. Most Coso basalts show isotopic, geochemical, and mineralogic evidence of interaction with crustal rocks, but one analyzed flow has isotopic ratios that may represent mantle values (⁸⁷Sr/⁸⁶Sr=0.7036,²⁰⁶Pb/²⁰⁴Pb=19.05,²⁰⁷Pb/²⁰⁴Pb=15.62,²⁰⁸Pb/²⁰⁴Pb= 38.63). The (initial) isotopic composition of typical rhyolite (⁸⁷Sr/⁸⁶Sr=0.7053,²⁰⁶Pb/²⁰⁴Pb=19.29,²⁰⁷Pb/²⁰⁴Pb= 15.68,²⁰⁸Pb/²⁰⁴Pb=39.00) is representative of the middle or upper crust. Andesitic inclusions in the rhyolites are evidently samples of hybrid magmas from the silicic/mafic interface in vertically zoned magma reservoirs. Silicic end-member compositions inferred for these mixed magmas, however, are not those of erupted rhyolite but reflect the zonation within the silicic part of the magma reservoir. The compositional contrast at the interface between mafic and silicic parts of these systems apparently was greater for the earlier, smaller reservoirs.     

Growth rates of manganese-rich nodules at MANOP Site H (Eastern North Pacific)

Estimates of the growth rates of ferromanganese nodules at MANOP Site H (6°33′N, 92°49′W) lie between 7 and 1200 mm/my. The most probable range (estimated from nodule size distributions, nodule chemistry, accumulation rates of authigenic elements and U-series modeling) is 30–300 mm/my. Such rates are much faster than measured values for most pelagic nodules. The distributions of U-series and stable elements suggest that a typical nodule consists of two compositionally distinct non-detrital components, one rich in Mn, U and Sb, and the other in Fe, Co, REE, Ni, Cu and Th. When the uncorrected U-series data, which do not show simple decay patterns with depth, are normalized to the relative proportion of the Fe-rich component, the data yield well-constrained growth rates of 55 mm/my for a nodule top and 200 mm/my for its bottom. The component enriched in Mn, which is most abundant on nodule bottoms, appears to accrete much faster than the component enriched in Fe.     

Stable isotope geochemistry of acid mine drainage: Experimental oxidation of pyrite

Sulfate and water from experiments in which pyrite was oxidized at a pH of 2.0 were analyzed for sulfur and oxygen stable isotopes. Experiments were conducted under both aerobic and anaerobic sterile conditions, as well as under aerobic conditions in the presence of Thiobacillus ferrooxidans, to elucidate the pathways of oxidation. Oxygen isotope fractionation between SO^2?₄ and H₂O varied from +4.0 %. (anaerobic, sterile) to + 18.0 %. (aerobic, with T. ferrooxidans.). The oxygen isotope composition of dissolved oxygen utilized in both chemical and microbially-mediated oxidation was also determined (+11.4 %., by T. ferrooxidans; +18.4 %., chemical). Contributions of water-derived oxygen and dissolved oxygen to the sulfate produced in the oxidation of pyrite could thus be estimated. Water-derived oxygen constituted from 23 to ~ 100 percent of the oxygen in the sulfate produced in the experiments, and this closely approximates the range of contribution in natural acid mine drainage. Oxidation of sulfides in anaerobic, water-saturated environments occurs primarily by chemical oxidation pathways, whereas oxidation of sulfides in well-aerated, unsaturated zone environments occurs dominantly by microbially mediated pathways.     

Resource conflict,policy change and practice in Canadian fisheries management

The Canadian marine fisheries are presently in the midst of a grave crisis. On both the Atlantic and Pacific coasts, fish stocks have been depleted through overfishing. Fish habitats are being damaged or permanently destroyed through industrial development. Gross overcapacity exists in both the primary and secondary sectors of the fishing industry, which is primarily attributable to the common property status of the resource. Many fishermen and processors have been or are in states of near bankruptcy as increasing energy costs, high product inventories and market weakness have crippled operations. A lack of alternative employment opportunities for many participants coupled with a high regional and community dependence on depressed resource-based industries has confounded attempts by government to introduce measures leading to more rational fisheries management. The outcome has been a strategy characterized as ‘incremental ad hocery’. Major inquiries were conducted into both the Atlantic and Pacific fisheries in the early 1980s. Conflict between and among the federal and provincial governments, various sectors of the commercial fishery, Indian and recreational usergroups, fishery interests and other industries, as well as Canada and other nations, has made it difficult to implement needed reforms. Stock and habitat maintainence and improvement; user regulations, coupled with tenure rights for fishermen; and marketing research and promotion are accepted as key components of any long-run fishery management strategy. In the short-run, the need is to develop approaches that are implementable in the context of diverse conflicting and vested interests.