An empirical sapphirine-spinel Mg---Fe exchange thermometer and its application to high grade xenoliths in the Popes Harbour dyke, Nova Scotia, Canada

Spinel-sapphirine-corundum-rutile parageneses in metapelitic xenoliths from the lamprophyric Popes Harbour dyke are enclosed by feldspathic (±rare quartz) haloes that embay aluminosilicates and biotite. These feldspathic haloes contain plagioclase (An_20–40) and/or an alkali or ternary (hypersolvus) feldspar, and show a variety of igneous and devitrification textures, suggesting an anatectic origin. The spinel-bearing parageneses are interpreted as the refractory residue formed by the incongruent melting of biotite, aluminosilicates and associated phases.

Equilibration temperatures of these assemblages are estimated from an empirical sapphirine-spinel Mg---Fe exchange thermometer derived from literature data on both silica-saturated and undersaturated sapphirine granulites. Linear regression (R=0.81) of the calibrant data yields the expression T(°C) = [800 + (228^*InKd)] − 273 where

Kd=(X^sp_Fe/X^sp_Mg)/(X^sa_Fe/X^sa_Mg)

Precision is estimated at ± 100°C, but will likely be less for highly oxidized sapphirine granulites owing to (1) errors in the stoichiometric estimation of X_Fe³⁺ from microprobe data and (2) the formation of magnetite at the expense of spinel or sapphirine, leading to an increase in X_Mg in either or both phases during cooling. Application of this expression to the reduced (graphite-bearing) Popes Harbour xenoliths yields T of 725–795°C. Anatexis is attributed to thermal metamorphism by the lamprophyric magma prior to and/or during entrainment of xenolith material in the dyke. Higher-T assemblages were quenched before the xenoliths attained thermal equilibrium with the magma, consequently prograde reaction textures and compositional zoning patterns are preserved.     

An experimental study of the isotopic enrichment in Ar, Kr, and Xe when trapped in water ice

The isotopic enrichment of argon, krypton, and xenon, when trapped in water ice, was studied experimentally. The isotopes were found to be enriched according to their (m1/m2)1/2 ratio. These enrichment factors could be useful for comparison among the uncertain cosmic or solar isotopic ratios, the hopeful in situ cometary ratio, and those in Earth's atmosphere, in the context of cometary delivery of volatiles to Earth.     

Magma mixing in granitic rocks of the central Sierra Nevada,California

The El Capitan alaskite exposed in the North American Wall, Yosemite National Park, was intruded by two sets of mafic dikes that interacted thermally and chemically with the host alaskite. Comparisons of petrographic and compositional data for these dikes and alaskite with published data for Sierra Nevada plutons lead us to suggest that mafic magmas were important in the generation of the Sierra Nevada batholith. Specifically, we conclude that: (1) intrusion of mafic magmas in the lower crust caused partial melting and generation of alaskite (rhyolitic) magmas; (2) interaction between the mafic and felsic magmas lead to the observed linear variation diagrams for major elements; (3) most mafic inclusions in Sierra Nevada plutons represent chilled pillows of mafic magmas, related by fractional crystallization and granitoid assimilation, that dissolve into their felsic host and contaminate it to intermediate (granodioritic) compositions; (4) vesiculation of hydrous mafic magma upon chilling may allow buoyant mafic inclusions and their disaggregation products to collect beneath a pluton's domed ceiling causing the zoning (mafic margins-to-felsic core) that these plutons exhibit.     

Assessing the adequacy of North American agricultural land resources

This paper provides a prospective analysis of supply/demand conditions for agricultural land in Canada and the U.S.A. according to various growth scenarios and an assessment of the economic and environmental costs associated with expanding and intensifying the cropland base. Future cropland expansion is estimated on the basis of the interrelationship between growth in demand and the productivity of cereal output. The adequacy of the land resource base is assessed by comparing individual projections with expected reserves of cropland adjusted for non-agricultural demands for land. Within the context of these findings the paper proceeds with a general discussion of the man-made, resource and environmental constraints to expanding production and the resulting economic and environmental costs to society.     

SrCa and MgCa ratios in polygenetic carbonate allochems from a Michigan marl lake

Rapid accumulation of CaCO₃ is occurring in Littlefield Lake, a marl lake located in central Michigan. The sediment, which is 95% CaCO₃, primarily consists of eight different genetic groups of carbonate allochems. These include calcite muds, sands, algal oncoids and Chara encrustations, as well as the dominant aragonitic gastropods Valvota tricarinota. Gyraulus deflectus and Amnicola integra. and the dominant aragonitic pelecypod Sphaerium partumeium. Samples of each of these groups were analyzed for Ca, Sr and Mg. Molar $\text{Mg}\text{Ca}$ ratios are primarily controlled by allochem mineralogy, with calcitic forms having $\text{Mg}\text{Ca}$ ratios 5–10 times larger than aragonitic (shelled) forms. The $\text{Sr}\text{Ca}$ ratios are primarily controlled by biochemical fractionation, and are significantly lower than $\text{Sr}\text{Ca}$ ratios of inorganically precipitated aragonite from other settings. Partition coefficients were determined for both Sr and Mg for each carbonate allochem group and, based on comparisons with results reported by other workers, the partition coefficients determined here are generally considered ‘typical’ or representative values for biogeneous freshwater carbonates. An analysis of variance of the data indicates that most genera and species of carbonate-secreting organisms in marl lakes have highly characteristic $\text{Sr}\text{Ca}$ and $\text{Mg}\text{Ca}$ ratios. These ratios can potentially serve as geochemical tracers in future investigations of lacustrine carbonate diagenesis. Both Sr and Mg are influenced by grain size and/or surface area, probably due to the presence of these elements in non-lattice-held (exchangeable) positions.     

Q'gas: a minicomputer-based system to aid in the interpretation of exploration-oriented geochemical data

The past few years have seen an increasing application of computer-based procedures in the processing and interpretation of geochemical data. In many cases this has been carried out by non-geologists using large, general purpose computers remote from the exploration effort. In such situations, appreciation of the geological nature of the problems is often inadequate and in a number of cases misapplication of procedures has resulted. The present paper describes the Q'GAS system which consists of a minicomputer and a series of compatible interactive programs. These programs can be used independently by geologists who have a minimum of experience with computers allowing them the opportunity to carry out personally the processing and interpretation of geochemical data and to ensure that the methodology is based on a firm geological framework. The expense of such a system is relatively low: approximately $25,000 capital costs, and $3500 per year running costs including a maintenance contract. In addition approximately one man-month per year is required for supervision and up-keep.The system has the capability of: (a) selecting subgroups of samples that meet specified criteria (e.g. specific rock type or value ranges); (b) transforming data (addition, subtraction, multiplication, division, logarithms, exponentiation and random number generation); (c) making statistical computations; and (d) producing graphical displays (e.g. histograms, X-Y plots, symbol maps etc.). Flexible diskettes are used for data storage and communication with the system is achieved through a video terminal. Hard-copy output is produced on a small printer. The system presently includes programs for data management, univariate statistics with histograms, correlation analysis, X-Y plots, line printer symbol maps, line printer geochemical profiles, multiple linear regression, discriminant analysis, and R-mode factor analysis. Attention is presently being given to developing programs that use a pen plotter for producing better quality maps and diagrams at any scale.Experience has shown that the simpler programs for construction of maps, graphs, and diagrams can provide an immediate improvement in the quality, thoroughness, and speed of data interpretation as well as significantly reducing the tedium associated with manual methods. The multivariate statistical techniques, as always, require a higher level of expertise and many more man hours if they are to be used successfully.     

Packrat middens from Canyon de Chelly,northeastern Arizona: Paleoecological and archaeological implications

In western North America, pollen data from highland lakes are often used to reconstruct vegetation on the adjacent lowlands. Plant macrofossils and pollen from packrat middens now provide a means to evaluate such reconstructions. On the basis of pollen diagrams from the Chuska Mountains, H. E. Wright, Jr., A. M. Bent, B. S. Hansen, and L. J. Maher, Jr., ((1973), Geological Society of America Bulletin, 84, 1155–1180) arrived at conservative estimates for late Pleistocene depression of highland conifers. In their interpretation, a proposed slight depression of 500 m for lower tree line precluded expansion of Pinus ponderosa into elevations now in desertscrub. Instead, it was suggested that pinyon pine and Artemisia occupied the lowland plateaus. Packrat midden records on either side of the Chuskas fail to verify this model. Early Holocene middens from Chaco Canyon, New Mexico, and a terminal Pleistocene midden from Canyon de Chelly, Arizona, show that blue spruce, limber pine, Douglas fir, dwarf juniper, and Rocky Mountain juniper expanded at least down to 1770 m elevation Neither Colorado pinyon nor ponderosa pine was found as macrofossils in the middens. Artemisia pollen percentages are high in the terminal Pleistocene midden, as they are in the Chuska Mountain pollen sequence, suggesting regional dominance by sagebrush steppe. Of 38 taxa identified, only 3 are shared by middens dated 11,900 and 3120 yr B.P. from Canyon de Chelly, indicating a nearly complete turnover in the flora between the late Pleistocene and late Holocene. Although corn was previously thought to have been introduced to the Colorado plateaus after 2200 yr B.P., the midden dated 3120 yr B.P. contains pollen of corn and other indicators of incipient agriculture.     

The Gault and its Junction with the Woburn Sands in the Leighton Buzzard Area,Bedfordshire and Buckinghamshire

Sections in the Gault and of the Gault-Woburn Sands junction exposed in the Leighton Buzzard area of Bedfordshire are described. These give a much clearer picture of the ammonite zonal stratigraphy than hitherto. The depositional history of the Albian sediments is discussed. The relationship of the Shenley Limestone to the regularis nodule beds in the south of the area is demonstrated, together with the nature of the mixed regularis and kitchini nodule beds seen in the central area. Periods of erosion occurred in post-kitchini mammillatum Zone times, and after renewed sedimentation in the eodentatus Subzone. Overlying these beds are clays of spathi Subzone age which pass laterally into glauconitic marginal loams against a platform of Woburn Sands in the northern part of the area. Upon this platform occur knolls of bedded Silty Beds, capped by Shenley Limestone, against which sediments of high spathi, intermedius and niobe Subzones age thin. The base of the Upper Gault, of high cristatum Subzone age, rests non-sequentially upon an eroded surface of the Lower Gault. Clays of orbignyi Subzone age are overlain by a bed of phosphatic nodules representing the lower part of the varicosum Subzone, which in turn is overlain by thick clays which may in part be of varicosum and auritus Subzones age.     

Seasonal cycles of dissolved constituents in streamwater in two forested catchments in the mid-Atlantic region of the eastern USA

Streamwater discharge and chemistry of two small catchments on Catoctin Mountain in north-central Maryland have been monitored since 1982. Repetitive seasonal cycles in stream-water chemistry have been observed each year, along with seasonal cycles in the volume of stream discharge and in groundwater levels. The hypothesis that the observed streamwater chemical cycles are related to seasonal changes in the hydrological flow paths that contribute to streamflow is examined using a combination of data on groundwater levels, shallow and deep groundwater chemistry, streamwater discharge, streamwater chemistry, soil-water chemistry, and estimates of water residence times. The concentrations of constituents derived from rock weathering, particularly bicarbonate and silica, increase in streamwater during the summer when the water table is below the regolith-bedrock interface and stream discharge consists primarily of deep groundwater from the fractured-bedrock aquifer. Conversely, the concentrations in streamwater of atmospherically derived components, particularly sulfate, increase in winter when the water table is above the regolith-bedrock interface and stream discharge consists primarily of shallow groundwater from the regolith. Tritium and chlorofluorocarbon (CFC) measurements suggest that the groundwater in these systems is young, with a residence time of less than several years. The results of this study have implications for the design of large-scale water-quality monitoring programs.