Re–Os isotopic constraints on magma mixing in the Peridotite Zone of the Stillwater Complex, Montana, USA

Chromite separates within the Peridotite Zone of the 2,700 Ma-old Stillwater Complex are characterized by low ¹⁸⁷Re/¹⁸⁸Os (0.009 to 1.74) and relatively high Os concentrations (8.74 to 78.2 ppb). Their calculated initial Os isotopic compositions likely reflect the compositions of the magmas from which they crystallized. The chromites show variable initial Os isotopic compositions (%_Os of +2.0 to +16.4) over the vertical extent of the Peridotite Zone, implicating at least two sources of Os. Both the range of %_Os and values of %_Os decrease upsection. These variations in %_Os were caused by mixing of variable proportions of two magmas having different Os isotopic compositions. One of the magmatic components was a more primitive magma with a nearly chondritic Os isotopic composition. The other magma had a radiogenic Os isotopic composition as a result of assimilation of crust, perhaps of sedimentary rocks beneath the Stillwater Complex. The gradual decrease in the initial %_Os values of the chromite layers with increasing stratigraphic height implies a decreasing relative contribution from the contaminated magma throughout the growth of the Peridotite Zone. Small variations in %_Os between different chromite occurrences within the H multicyclic unit reflects the petrologic requirement that chromite layers crystallize from slightly different proportions of the magmas, compared to chromite from olivine- and orthopyroxene-rich layers.     

Soil erosion in the Falkland Islands: an assessment

Observations and data concerning soil erosion on clay-rich, sand-rich and organic soil materials are reported for the Falkland Islands, South Atlantic, where soil erosion is both widespread and currently active, though rather uneven in its distribution. The most extensively eroded areas are located on coastal sand deposits and where sandy soils occur at inland sites. There is evidence that some erosion is natural and was initiated before the Islands were settled; other erosion is of more recent origin and can be associated with land management practices, principally grazing management, stocking rates, and the frequency and intensity of pasture burning. The impact of soil erosion on the agricultural economy of the Islands appears to be negligible and it is not perceived to be an important environmental issue. This lack of concern probably arises because the on-farm and off-farm costs of erosion have never been determined and there are no data relating to rates of erosion. It is suggested that a research programme be established aimed at monitoring erosion rates, assessing land use practices that initiate erosion, and assessing the costs of erosion. Furthermore, guidelines for soil conservation should be established. If pasture burning is to continue, a recommended code of practice should be drawn up. Given the importance of the sheep industry to the Falkland Islands, it is desirable that the grazing potential be maintained.     

Absolute gravity measurements on the canadian gravity standardization network

Summary Absolute measurements of gravity have been made at 6 locations ranging from Ottawa, Ont., in southern Canada, to Alert, N.W.T., the world's most northerly permanent settlement, as part of a program to provide scale and level for the Canadian Gravity Standardization Network (CGSN). Except at Resolute, N.W.T., CGSN-74 gravity values, upon which our gravity reductions are currently based, agree with the absolute gravity meter results to within about .25µm/s². The scale of our CGSN-80 gravity network, upon which our spring-balance type gravity meter scale constants are derived, agrees with the scale defined by the absolute gravity measurements to within about one part in ten thousand.     

Dominance of Tilapia mossambica,an introduced fish species,in three Puerto Rican estuaries

We compared species presence, abundance, and size characteristics of fish in three brackish, coastal marshes at Humacao, Roosevelt Roads, and Boqueron, Puerto Rico, in February and March 1988. The three marsh ecosystems were similar with respect to the presence of large expanses of open water bordered by emergent vegetation, creeks, and mangroves, and all had some recreational use. We sampled fish using gill nets. Tilapia (Oreochromis) mossambica were the most abundant fish, accounting for 55–79% of the samples at all three marshes. Overall, tilapia were both the largest (North Lagoon) and the smallest (Frontera Creek) at Humacao. Tilapia were most common in open lagoons rather than creeks or bays (except for Mandri Creek), and their distribution seemed unrelated to salinity. Tarpon (Megalops atlantica) were more abundant at low salinities, whereas other fish were more abundant at higher salinities.     

Shipboard measurements of atmospheric dimethylsulfide and hydrogen sulfide in the Caribbean and Gulf of Mexico

Simultaneous shipboard measurements of atmospheric dimethylsulfide and hydrogen sulfide were made on three cruises in the Gulf of Mexico and the Caribbean. The cruise tracks include both oligotrophic and coastal waters and the air masses sampled include both remote marine air and air masses heavily influenced by terrestrial or coastal inputs. Using samples from two north-south Caribbean transects which are thought to represent remote subtropical Atlantic air, mean concentrations of DMS and H₂S were found to be 57 pptv (74 ng S m^-3, =29 pptv, n=48) and 8.5 pptv (11 ng S m^-3, =5.3 pptv, n=36), respectively. The ranges of measured concentrations for all samples were 0–800 pptv DMS and 0–260 pptv H₂S. Elevated concentrations were found in coastal regions and over some shallow waters. Statistical analysis reveals slight nighttime maxima in the concentrations of both DMS and H₂S in the remote marine atmosphere. The diurnal nature of the H₂S data is only apparent after correcting the measurements for interference due to carbonyl sulfide. Calculations using the measured ratio of H₂S to DMS in remote marine air suggest that the oxidation of H₂S contributes only about 11% to the excess (non-seasalt) sulfate in the marine boundary layer.     

THREE-DIMENSIONAL, CROSS-SEMIVARIOGRAM CALCULATIONS FOR HYDROGEOLOGICAL DATA

Abstract. Geostatistics is a powerful tool for the analysis of hydrogeological data, but few well-documented computer programs for performing the necessary calculations have been presented in the technical literature. This is especially true for applications that require either three-dimensional or multivariate analyses. This paper describes a FORTRAN subroutine, VARIO, that can be used to compute experimental direct- and cross-semivariograms from a set of sample data, for any specified direction in one-, two-, or three-dimensional space. The subroutine combines into groups those sample pairs that fall within predetermined angular tolerances of the specified direction. The number of sample pairs used to compute the value of the experimental semivariogram at each value of separation can be specified in four different ways, depending on the nature of the available data. Written in FORTRAN 77, VARIO can be used on any computer that supports a FORTRAN 77 compiler. Source code listing, user instructions, and example input and output data for VARIO are presented.     

The calculation of bulk strain in oblique and inclined balanced sections

The calculation of bulk strain from balanced sections is a technique commonly used in the study of orogenic belts. The validity of a balanced section is often enhanced by controls such as seismic and/or borehole data. The construction of a conventional vertical section perpendicular to orogenic strike (i.e. in the transport direction) may not allow control data to be directly incorporated into the section. The control data can be projected into the plane of the section or, alternatively, the section may be constructed in an orientation such that it contains the available control data. In the case of a vertical section constructed at an oblique angle to the transport direction the bulk strain value obtained is a minimum and can be corrected to the true value of bulk strain in the perpendicular, vertical section by a simple trigonometric calculation. If the section is perpendicular to strike but inclined at an angle to the vertical, the correction of the bulk strain values is mathematically unique to the section. This problem can be overcome by using a digitizing table in conjunction with a simple computer program. The general case of a section both oblique to the strike direction and inclined to the vertical can be resolved by removing the effects of the inclination to produce a vertical, oblique section from which the bulk strain can be obtained and corrected. In realistic geological problems, the oblique section will be a more common problem than the inclined section. In both cases deviations of 10° and less introduce such small errors in the estimates of bulk strain that their effects can be ignored.     

An evaluation of the zircon method of isotopic dating in the Southern Arabian Craton

A zircon study has been made on eleven samples of igneous rocks from the Saudi Arabian Craton. Ages of sized and magnetic fractions of zircon concentrates show variable degrees of discordance which seem to result from a very young disturbance that produces linear arrays in the Concordia plot. Model age calculations based on a statistically and geologically reasonable lower intercept produce very consistent internal relationships. The Pan African Orogeny, considered to be responsible for loss of radiogenic argon and strontium from minerals of many rocks, does not appear to have affected the zircon data, even though uplift had exposed the rocks of the Arabian Shield at that time.Tonalite, granodiorite, and crosscutting leucoadamellite bodies in the southern part of the An Nimas Bathylith yield ages in the time range 820–760 Ma. A narrow time range of 660 to 665 million years was indicated for ages of widely separated and compositionally different intrusive bodies all to the east of the An Nimas Bathylith. This work suggests that the younger end of the age spectrum established from regional K-Ar and Rb-Sr measurements may be underestimated, and that magmatic activity could be more episodic than previously assumed.     

Complexation and reduction as factors in the link between metal ion concentrations and organic matter in the Indian River

Complexation of metal ions by organic matter is frequently considered to play a part in metal ion dissolution in natural waters. A field study of a relatively unperturbed stream, high in organics, associates this with the fraction related to soil organic acids (humic acids). The association might have two origins. The first is complexation. However, well known sequences of complexing tendency do not predict the behaviour. A better theory uses the additional factor of the reducing capacity of dissolved organic matter toward Fe(III) and Mn(IV).