The Liquid Line of Descent of Anhydrous, Mantle-Derived, Tholeiitic Liquids by Fractional and Equilibrium Crystallization--an Experimental Study at 1{middle dot}0 GPa

Two series of anhydrous experiments have been performed in anend-loaded piston cylinder apparatus on a primitive, mantle-derivedtholeiitic basalt at 1·0 GPa pressure and temperaturesin the range 1060–1330°C. The experimental data provideconstraints on phase equilibria, and solid and liquid compositionsalong the liquid line of descent of primary basaltic magmasdifferentiating in storage reservoirs located at the base ofthe continental crust. The first series are equilibrium crystallizationexperiments on a single basaltic bulk composition; the secondseries are fractionation experiments where near-perfect fractionalcrystallization was approached in a stepwise manner using 30°Ctemperature steps and starting compositions corresponding tothe liquid composition of the previous, higher-temperature glasscomposition. Liquids in the fractional crystallization experimentsevolve with progressive SiO₂ increase from basalts to dacites,whereas the liquids in the equilibrium crystallization experimentsremain basaltic and display only a moderate SiO₂ increase accompaniedby more pronounced Al₂O₃ enrichment. The principal phase equilibriacontrols responsible for these contrasting trends are suppressionof the peritectic olivine + liquid = opx reaction and earlierplagioclase saturation in the fractionation experiments comparedwith the equilibrium experiments. Both crystallization processeslead to the formation of large volumes of ultramafic cumulatesrelated to the suppression of plagioclase crystallization relativeto pyroxenes at high pressures. This is in contrast to low-pressurefractionation of tholeiitic liquids, where early plagioclasesaturation leads to the production of troctolites followed by(olivine-) gabbros at an early stage of differentiation. KEY WORDS: liquid line of descent; tholeiitic magmas; equilibrium crystallization; fractional crystallization     

Some Thermal Constraints on Crustal Assimilation during Fractionation of Hydrous, Mantle-derived Magmas with Examples from Central Alpine Batholiths

We provide a model for the fractional crystallization of hydrousmantle-derived magma to form calc-alkaline plutons, based uponmass balance for geological examples of fractionation sequencesin the lower continental crust. This is complemented by a thermalmodel for the heat budget obtained from a projected phase diagramand thermodynamic data. Fractional crystallization (FC) andassimilation–fractional crystallization (AFC) paths havebeen calculated with these models and the mass ratio of assimilationto crystallization as a function of parent magma type and temperature,crustal rock fertility and temperature, and mechanism of assimilation,have been determined. When these results are combined with F(melt fraction) and r (ratio of mass assimilated/crystallized)values evaluated from geochemical data then new information,not available with the methods separately, can be deduced. Thisincludes when and at what depth and temperature in the crustthe assimilation took place, as well as the likely parent magmatype and temperature of the assimilant. Our results are presentedin simple graphical fashion to facilitate future studies thatexamine the evolution of individual calc-alkaline plutons andthe mechanisms of crustal contamination, and to improve meltmodels involving hydrous magma in volcanic arcs and in the lowercontinental crust KEY WORDS: assimilation; hydrous mantle magma; thermal models; fractional crystallization; magma mixing; Alpine batholiths; Adamello; Bergell     

Using performance analysis to identify candidate operations for GIS software parallelization

This paper examines the potential impact of parallel computing on the computational performance of GIS. Performance analyses on existing GIS operations, vector topology creation and interpolation, in commercial GIS products in a serial environment are used to work out the critical computational factors that determine elapsed time performance. Analyses reveal that the extraction of all data necessary to perform a GIS operation at a particular geographical locality dominates the I/O and CPU resource usage. For GIS to benefit from parallel computing, the performance of this task must be addressed.     

Late Quaternary sequence stratigraphy of Lake Malawi (Nyasa), Africa

High resolution seismic data, multichannel seismic data and sediment cores were used to examine the Songwe Sequence, the uppermost of four depositional sequences identifiable on multichannel seismic data from Lake Malawi (Nyasa). The sequence has a maximum thickness of about 115 m in two areas of the lake, but is typically less than 70 m thick over most of the basin. The sequence is distributed along the entire length of the 560 km long lake, and is concentrated in three main depocentres. ¹⁴C age dates from sediment piston cores are extrapolated to provide an age estimate of about 78 000 yr bp for the oldest sediments within the Songwe Sequence. In the North and Central bathymetric basins of the lake, high resolution seismic data indicate a dynamic depositional environment, dominated by turbidity and mass flow deposits. Seismic data from the southern basin show acoustically transparent sediments with relatively low amplitude internal reflections, indicative of pelagic and hemipelagic sedimentation. In many areas the Songwe Sequence is underlain by a pronounced angular unconformity, suggestive of a significant, prolonged, low lake stage prior to deposition of the sequence. Seismic reflectors within the Songwe Sequence can be correlated to younger low lake stages identified from sediment core data. Major late Quaternary low lake level stages in Lake Malawi, interpreted from features identified in the seismic data and sediment core analyses, are tentatively interpreted at 6000 to 10 000 yr bp , 28 000 to >40 000 yr bp , and prior to 78 000 yr bp . Budget calculations indicate mean sediment concentrations from catchment runoff during the period of deposition of the Songwe Sequence to be about 190 mg 1⁻¹, comparable to estimates of modern rainy season discharges from the major river systems. Erosion rates within the drainage basin are estimated to be higher than the African average by a factor of three or more, probably due to the high relief within the Lake Malawi catchment.     

Emplacement and Differentiation of the York Haven Diabase Sheet, Pennsylvania

Many of the high-Ti quartz-normative tholeiitic intrusive sheetsin the early Mesozoic rift basins of the Eastern USA exhibitlateral differentiation from mafic cumulate units, through diabase,to relatively evolved iron-rich rock types. We have investigateda representative example in detail, the York Haven sheet inthe Gettysburg basin of south–central Pennsylvania. Itranges in thickness from 330 m to 675 m, and we have sampledit from base to top along four separate stratigraphic sectionsevenly spaced over the extent of the intrusion. The easternmostsection (York Haven) is entirely basaltic bronzite cumulate(average 15 vol. % bronzite), whereas the westernmost (ReesersSummit) consists of diabase and low-MgO diabase with a middleto upper ‘sandwich zone’ of ferrogabbro. The interveningsections feature rock types transitional between the two end-membersequences. Chemically, the rock series shows a gradual eastto west depletion of compatible elements (Mg, Ca, Ni, and Cr),and enrichment of incompatible elements [Ti, Fe, Na, K, P, Cu,Zr, Th, Ta, Hf, Sb, Cs, As, platinum group elements (PGEs),and rare earth elements (REEs)]. We suggest two main processes for the trends observed in theYork Haven sheet. First, flow differentiation during ascentand lateral injection of the parental magma produced a tongueof basaltic bronzite cumulate that thins from southeast to northwestand passes laterally into diabase, and, at the distal end ofthe intrusion, into low-MgO diabase. Then, in the latter stagesof crystallization, densitydriven hydrothermal fluids transportedincompatible elements westward, into structurally higher partsof the intrusion. Reaction of this residual aqueous fluid withpartly crystallized low-MgO diabase produced a zone of ferrogabbrorich in hydrothermal replacement products (e.g., Cl-amphibole,biotite, ferrohypersthene, and skeletal ilmenite) and precipitates(e.g., quartz, fayalite, Cl-apatite, sulfides, and PGE minerals). ^* Present address: US Geological Survey, Hawaiian Volcano Observatory, Hawaii National Park, Hawaii 96718     

The use of phillipsite in test construction of agglutinated deep-sea benthonic foraminifera*

Agglutinated deep-sea benthonic foraminifera with tests composed of phillipsite are documented and illustrated from a red clay sequence in a Giant Piston Core from the central North Pacific. Associated with the benthonic foraminifera are phillipsite crystal rosettes and aggregates of phiilipsite crystals. The benthonic foraminifera and phillipsite crystals constitute most of the > 38 μm fraction, but have little effect upon the mean size of the red clay sediments.     

Subsolidus and Partial Melting Reactions in the Quartz-excess CaO+MgO+Al2O3+SiO2+H2O System under Water-excess and Water-deficient Conditions to 10 kb: Some Implications for the Origin of Peraluminous Melts from Mafic Rocks

Experimental results up to 10 kb pressure are presented on thestability of amphibole in the quartz-excess CaO+MgO+Al₂O₃ (CMASH)system under H₂O)-excess and H₂O deficient conditions. Amphiboleis stable above the solidus under H₂O-excess conditions whereasunder H₂O-deficient conditions dehydration melting of amphibole-bearingassemblages defines the solidus. The successive appearance ofamphibole, talc, and zoisite with increasing pressure considerablymodifies the plagioclase-pyroxene-garnet-kyanite reactions documentedexperimentally in the CaO+MgO+Al₂O₃+SiO₂ system for gabbro-granulite-eclogitetransitions. Although both clino pyroxene and cordierite (withanorthite+orthopyroxene+quartz) may melt eutectically at oneatmosphere to form diopside-normative and corundum-normativemelts respectively, at higher pressures under H₂O-excess conditionsthe peritectic melting of mafic rock compositions produces corundum-normativeliquids together with either clinopyroxene or amphibole. Dehydrationmelting produces melts which are not corundum-normative. Thesedata are used to discuss the origins and evolution of contrastingbasalt-andesite-dacite-rhyolite volcanic suites and graniticplutons, many of whose silicic variants are corundum-normativein character, such as the Toba luff ignimbrites, Indonesia (Beddoc-Stephenset al., 1983) and I-type granite minimum melts (White &Chappell, 1977). In contrast, it is proposed that for the Cascadesbasalt-andesite-dacite-rhyolite suite the ortho pyroxene-plagioclase-quartzthermal divide was maintained up to rhyolite compositions, therebyprohibiting the derivation of corundum-normative rocks fromdiopside-normative parent magmas. The deduced reaction relations between pyroxenes, amphibole,plagioclase, quartz, and liquid are used to explain the absenceor extreme scarcity of hydrous phases in some hydrous magmas.These phase relations can also explain the development of laterplagioclase overgrowths on resorbed plagioclase cores in graniticintrusives, and the general absence of resorption and overgrowthsin chemically equivalent extrusive rocks. A theoretical analysis of the partial melting of forsterite-bearingassemblages in the CaO+MgO+Al₂O₃+SiO₂+H₂O system shows thatunder H₂O-excess conditions partial melting may generate corundum-normative(but low SiO₂) melts from a peridotite source at shallow depths.     

The upper Hawkesbury River, New South Wales, Australia: a Holocene example of an estuarine bayhead delta

Holocene deposits of the Hawkesbury River estuary, located immediately north of Sydney on the New South Wales coast, record the complex interplay between sediment supply and relative sea-level rise within a deeply incised bedrock-confined valley system. The present day Hawkesbury River is interpreted as a wave-dominated estuarine complex, divisible into two broad facies zones: (i) an outer marine-dominated zone extending 6 km upstream from the estuary mouth that is characterized by a large, subtidal sandy flood-tidal delta. Ocean wave energy is partially dissipated by this flood-tidal delta, so that tidal level fluctuations are the predominant marine mechanism operating further landward; (ii) a river-dominated zone that is 103 km long and characterized by a well developed progradational bayhead delta that includes distributary channels, levees, and overbank deposits. This reach of the Hawkesbury River undergoes minor tidal level fluctuations and low fluvial runoff during baseflow conditions, but experiences strong flood flows during major runoff events. Fluvial deposits of the Hawkesbury River occur upstream of this zone. The focus of this paper is the Hawkesbury River bayhead delta. History of deposition within this delta over the last c. 12 ka is interpreted from six continuous cores located along the upper reaches of the Hawkesbury River. Detailed sedimentological analysis of facies, whole-core X-ray analysis of burrow traces and a chronostratigraphic framework derived from 10 C-14 dates reveal four stages of incised-valley infilling in the study area: (1) before 17 ka BP, a 0–1 m thick deposit of coarse-grained fluvial sand and silt was laid down under falling-to-lowstand sea level conditions; (2) from 17 to 6·5 ka BP, a 5–10 m thick deposit composed of fine-grained fluvial sand and silt, muddy bayhead delta and muddy central-basin deposits developed as the incised valley was flooded during eustatic sea-level rise; (3) during early highstand, between 6·5 and 3 ka BP, a 3–8 m thick bed of interbedded muddy central-basin deposits and sandy river flood deposits, formed in association with maximum flooding and progradation of sandy distributary mouth-bar deposits commenced; (4) since 3 ka BP, fluvial deposits have prograded toward the estuary mouth in distributary mouth-bar, interdistributary-bay and bayhead-delta plain environments to produce a 5–15 m thick progradational to aggradational bayhead-delta deposit. At the mouth of the Hawkesbury estuary subaqueous fluvial sands interfinger with and overlie marine sands. The Hawkesbury River bayhead-delta depositional succession provides an example of the potential for significant variation of facies within the estuarine to fluvial segment of incised-valley systems.     

EARLY INTELLECTUAL INFLUENCES ON D. W. MEINIG: A FORMER STUDENT'S FOND MEMORIES*

ABSTRACT. As an undergraduate and graduate student in the 1940s and a young professor at the University of Utah in the 1950s, D. W. Meinig was influenced by a number of scholars. They included six historians, three geographers, two anthropologists, and two philosophers. I identify the influence of the thirteen scholars on Meinig's major achievements: the culture area model, geography as an art, the historical imperative for geography, cultures and civilizations, and geopolitics and imperialism.     

SPECTRAL ANALYSIS OF GEOMORPHIC TIME SERIES: AUTO-SPECTRUM

The collection of time series data is an essential component in the investigation of earth surface processes. Spectral analysis of these time series can provide an invaluable insight into the behaviour of geophysical processes. Spectral analysis of a single time series produces an auto-spectrum which provides a representation of the amount variance of the time series as a function of frequency. Prior to spectral analysis, the time series should be plotted to identify the presence of any trends in the mean or the variance of the series, and to identify anomalies in the data which should be corrected. To satisfy the assumption of stationarity, any trend (in either the mean or variance) should be removed from the time series. Consequently, the probability density function of the time series should be plotted and compared with the Gaussian distribution. The final stage in preparing the time series for spectral analysis is to apply a taper to reduce spectral leakage and distortion of the auto-spectrum. Following the calculation of the periodogram, spectral estimates should be combined to reduce the variability associated with the estimates and thereby ensure that the autospectrum is more representative. Finally, confidence limits should be constructed around the spectral density function so that statistically significant spectral peaks (or troughs) can be identified.