Dehydration Melting and Water-Saturated Melting of Basaltic and Andesitic Greenstones and Amphibolites at 1, 3, and 6. 9 kb

The melting relations of five metamorphosed basalts and andesites(greenstones and amphibolites), collected from the late JurassicSmartville arc complex of California, were investigated experimentallyat 800–1000? C and 1, 3, and 6. 9 kb. Dehydration-melting(no water added) experiments contained only the water structurallybound in metamorphic minerals (largely amphiboles). They yieldedmildly peraluminous to metaluminous granodioritic to trondhjemiticmelts (Na/K is a function of starting composition) similar inmajor element composition to silicic rocks in modern oceanicarcs. The dehydration melts are water-undersaturated, with,and coexist with the anhydrous residual solid (restite) assemblageplagioclase + orthopyroxene + clinopyroxene + magnetite ? ilmen-ite,with plagioclase constituting 50% of the restite mode. In thedehydration-melting experiments at 3 kb the onset of meltingoccurred between 850 and 900 ? C, as amphibole and quartz brokedown to yield pyroxenes plus melt. Total pressure is greaterthan in the dehydration-melting experiments and has little effecton melt composition or phase relations. In the water-saturated (water added, so that experiments, meltsformed at 3 kb and above are strongly peraluminous, rich inCa and poor in Fe, Mg, Ti, and K. Their compositions are unlikethose of most silicic igneous rocks. These melts coexist withthe amphibole-rich, plagioclase-poor restite assemblage amphibole+ magnetite ? clinopyroxene ? plagioclase ? ilmenite. The highlyaluminous nature of the melts and the plagioclase-poor natureof the restite both reflect the substantial contribution ofplagioclase (along with quartz) to melts in high-pressure water-saturatedsystems. Water pressure equals P_toul in the water-saturatedexperiments and has a profound effect on both melt compositionand phase relations. At 1 kb, the water-saturated experimentsyielded melt and mineral products with some characteristicsof the dehydration-melting experiments (no amphibole at highT), and some characteristics of the 3-kb, water-saturated experiments(amphibole plus melt coexisting at lower T, elevated Al, loweredFe). As pressure is increased from 3 to 6. 9 kb, the stabilityfields of both plagioclase and clinopyroxene decrease relativeto amphibole and the Al contents of the melts increase. These experiments have important implications for the petrogenesisof low-K silicic rocks in arcs. First, dehydration melting isa viable mechanism for the formation of these rocks; water-saturatedmelting is not. Second, because of the influence of rock compositionon melt composition, low-grade metamorphic and hydrothermalprocesses that alter the alkali contents and Na/ K in arc basementterranes may have a direct impact on the petrogenesis of silicicmagmas in arcs, particularly the formation of extremely low-Ktrondhjemites. Third, the experiments predict that anhydrous,pyroxene- and plagioclase-rich ‘granulitic’ restiteassemblages should develop as a result of partial melting inarc terranes. Such assemblages occur in at least two deeplyeroded arc complexes.     

Reaction Between Ultramafic Rock and Fractionating Basaltic Magma II. Experimental Investigation of Reaction Between Olivine Tholeiite and Harzburgite at 1150-1050?C and 5 kb

We present results of experiments on mixtures of olivine tholeiiteand mantle harzburgite, at 5 kb and 1050–1150?C, underconditions of controlled hydrogen fugacity. The basalt end-memberwas Kilauea 1921 olivine tholeiite+3 wt.% H₂O, and the harzburgiteend-member was a mixture of olivine and orthopyroxene mineralseparates made from a mantle-derived lherzolite xenolith. Theexperiments on mixtures of basalt and harzburgite difl not reachequilibrium in runs ranging from 12 to 200 h duration. Relativelylarge concentration gradients persisted in both liquid and solidphases in mixed samples, whereas ‘control’ samplescontaining only basalt were reasonably homogeneous and wereprobably close to equilibrium. Compositions of solid phases produced, measured by electronmicroprobe, show a regular increase in Mg/(Mg+Fe) with increasingproportion of harzburgite at constant temperature, but olivineand clinopyroxene in mixed samples were not in Fe-Mg exchangeequilibrium. Modes measured for each sample show that the fractionof liquid relative to the amount of basalt in the sample wasconstant at constant temperature, and independent of bulk composition:reaction between 1921 basalt and harzburgite does not changethe mass of liquid in the system. Average experimental liquidcompositions for each sample were obtained by mass balance.Using Kds defined by the ‘control’ sample for eachtemperature, and mass balance constraints, phase assemblages(solid- and liquid-phase compositions and proportions) werecalculated for all mixtures. Whether samples included harzburgite or not, all average experimentalliquid compositions, and all predicted liquid compositions,for samples run at 1050?C, are high-alumina basalts by the definitionof Kuno (1960). By the criteria of Irvine & Baragar (1971),all but two average experimental liquid compositions in basalt-harzburgitemixtures, and all predicted liquid compositions in basalt-harzburgitemixtures, are calc-alkaline basalts and basaltic andesites,whereas liquids in samples containing only basalt are tholeiiticbasalts. Combined crystallization and reaction with harzburgitein the upper mantle will produce calc-alkaline derivative liquidsfrom an olivine tholeiite liquid under conditions of temperature,pressure, water and oxygen fugacity, and initial bulk compositionwhich would produce a tholeiitic liquid line of descent by crystallizationin a closed system. ^*Present address: Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543Present address: Grant Institute of Geology, University of Edinburgh, West Mains Road, Edinburgh EH9 3JW, UK     

Anorthositic Rocks of the Duluth Complex: Examples of Rocks Formed from Plagioclase Crystal Mush

Anorthositic rocks compose 35–40% of the Middle Proterozoic(Keweenawan; 1?1 Ga) Duluth Complex—a large, compositemafic body in northeastern Minnesota that was intruded beneatha comagmatic volcanic edifice during the formation of the Midcontinentrift system. Anorthositic rocks, of which six general lithologictypes occur in one area of the complex, are common in an earlyseries of intrusions. They are characterized on a local scale(meters to kilometers) by nonstratiform distribution of rocktypes, variably oriented plagioclase lamination, and compositeintrusive relationships. Variably zoned, subhedral plagioclaseof nearly constant average An (60) makes up 82–98% ofthe anorthositic rocks. Other phases include granular to poikiliticolivine (Fo_66–38), poikilitic clinopyrox-ene (En'_73–37),subpoikilitic Fe-Ti oxides, and various late-stage and secondaryminerals. Whole-rock compositions of anorthositic rocks are modelled bymass balance to consist of three components: cumulus plagioclase(70–95 wt.%), minor cumulus olivine (0–5%), anda gabbroic postcumulus assemblage (5–27%) representinga trapped liquid. The postcumulus assemblage has textural andcompositional characteristics which are consistent with crystallizationfrom basaltic magma ranging from moderately evolved olivinetholeiite to highly evolved tholeiite (mg=60–25). Sympatheticvariations of mg in plagioclase and in mafic minerals suggestthat cumulus plagioclase, though constant in An, was in approximateequilibrium with the variety of basaltic magma compositionswhich produced the postcumulus assemblages. Standard models of mafic cumulate formation by fractional crystallizationof basaltic magmas in Duluth Complex chambers, although ableto explain the petrogenesis of younger stratiform troctoliticto gabbroic intrusions, are inadequate to account for the field,petrographic, and geochemical characteristics of the anorthositicrocks. Rather, we suggest an origin by multiple intrusions ofplagioclase crystal mushes—basaltic magmas charged withas much as 60% intratelluric plagioclase. The high concentrationsof cumulus plagioclase (70–95%) estimated to compose theanorthositic rocks may reflect expulsion of some of the transportingmagma during emplacement or early postcumulus crystallizationof only plagioclase from evolved hyperfeldspathic magma. Althoughthe evolved compositions of anorthositic rocks require significantfractionation of mafic minerals, geophysical evidence indicatesthat ultramafic rocks are, as exposure implies, rare in theDuluth Complex and implies that plagioclase crystal mushes werederived from deeper staging chambers. This is consistent withinterpretations of olivine habit and plagioclase zoning. Moreover,plagioclase could have been segregated from coprecipitatingmafic phases in such lower crustal chambers because of the buoyancyof plagioclase in basaltic magmas at high pressure. The geochemicaleffects of plagioclase suspension in basaltic magmas are consistentwith observed compositions of cumulus plagioclase in the anorthositicrocks and with the geochemical characteristics of many comagmaticbasalts. The petrogenesis of the anorthositic rocks and theoverall evolution of Keweenawan magmas can be related to thedynamics of intracontinental rift formation.     

Experimental Phase Relations of Water- and Sulfur-Saturated Arc Magmas and the 1982 Eruptions of El Chich?n Volcano

The equilibrium phase relations of two volcanic rocks from thesubduction-related Mexican Volcanic Belt have been determinedwith an argon-pressurized internally heated vessel. One rockis the trachyandesite erupted from El Chich?n Volcano in 1982;the other is a primitive basalt erupted from Jorullo Volcanoin 1759. A simplified synthetic equivalent to the trachyandesitewas also investigated in lesser detail. All charges were saturatedwith hydrous vapor and a sulfur-bearing mineral. Temperatureranged from 800 to 1000?C, pressure from 1 to 4 kb, and f_o2was controlled by four different solid oxygen buffers in a doublegold capsule configuration: fayalite-magnetite-quartz (FMQ),Ni-NiO (NNO), manganosite-hausmanite (MNH), and magnetite-hematite(MTH). Pyrrhotite was the only sulfur-bearing mineral observed in chargesbuffered under FMQ and NNO, whereas anhydrite crystallized underthe more oxidizing MNH or MTH; both of these observations areconsistent with those of earlier workers. With increasing temperatureand pressure, SiO₂ and K₂O decreased in the experimental melts,whereas Al₂O₃ and CaO increased. Sulfur solubility in silicatemelts was low (<0?1 wt% equivalent SO^t₃) for pyrrhotite-saturatedcharges, but significantly greater (to 1?3 wt.% SO^t₃) when anhydritewas present. Sulfur solubility in anhydrite-saturated meltsshowed strong positive dependence on both temperature and P_vapor. Sulfur amounted to some 2?5 wt.% (SO^t₃) of the total ejectaduring the 1982 El Chich?n eruptions, and the original magmaticsulfur content was in the range 1?25–2–5 wt% SO^t₃.Extrapolations of experimental temperature and pressure dependencesfor sulfur solubility indicate that such concentrations couldbe contained in a hydrous, oxidized, basaltic parent melt generatedunder Benioff zone conditions. During ascent through the uppermantle and crust, the sulfur solubility limit of the melt wouldcontinuously decrease; in response, most of the sulfur wouldbe transferred from the melt to anhydrite crystals and a separategas phase. Trachyandesite pumices erupted from El Chich?n in1982 contained both pyrrhotite and anhydrite at a temperatureof 800?C. The composition of the natural pyrrhotite yieldedan f_o2 estimate 1 log unit above the NNO buffer. Based on compositionalvariations in the experimental melts with temperature and pressure,the composition of the matrix glass in the 1982 pumices indicatesequilibration of the magmatic liquid at about P_total=P_vapor=2kb just before eruption. At that time, sulfur in El Chich?ntrachyandesite was about equally partitioned between anhydritemicrophenocrysts and some 20 vol.% gas phase in which H₂S wasprobably the dominant sulfur-bearing species. The melt thencontained only 0?05 wt.% SO^t₃, consistent with experimentalsolubility limits at 800?C and P_vapor=2 kb.     

Selective transport of heavy minerals by shoaling waves

A model to account for the occurrence of heavy mineral concentrations as discrete lamina on beaches is presented. The model is based on the shoreward variation in the distribution of bottom fluid velocities due to shoaling, progressive, gravity, water waves. This variation causes certain portions of the sediment population to be transported shoreward at a faster rate than other portions. On moderate energy shorelines composed of sand-sized sediment the selectively transported portion may include the relatively hard-to-move heavy mineral fraction. Wave tank experiments with artificially prepared sediment populations provide empirical support for the proposed model. Under certain wave conditions it was found that the heavy mineral fraction of the sediment population moved shoreward at a faster rate than did the light mineral fraction.     

Source,timing, frequency and flux of ice‐rafted detritus to the Northeast Atlantic margin, 30–12 ka: testing the Heinrich precursor hypothesis

Haapaniemi, A.I., Scourse, J.D., Peck, V.L., Kennedy, H., Kennedy, P., Hemming, S.R., Furze, M.F.A., Pieńkowski, A.J., Austin, W.E.N., Walden, J., Wadsworth, E. & Hall, I.R. 2010: Source, timing, frequency and flux of ice‐rafted detritus to the Northeast Atlantic margin, 30–12 ka: testing the Heinrich precursor hypothesis. Boreas, Vol. 39, pp. 576–591. 10.1111/j.1502‐3885.2010.00141.x. ISSN 0300‐9483. Increased fluxes of ice‐rafted detritus (IRD) from European ice sheets have been documented some 1000–1500 years before the arrival of Laurentide Ice Sheet (LIS)‐sourced IRD during Heinrich (H) events. These early fluxes have become known as ‘precursor events’, and it has been suggested that they have mechanistic significance in the propagation of H events. Here we present a re‐analysis of one of the main cores used to generate the precursor concept, OMEX‐2K from the Goban Spur covering the last 30 ka, in order to identify whether the British–Irish Ice Sheet (BIIS) IRD fluxes occur only as precursors before H layers. IRD characterization and planktonic foraminiferal δ¹⁸O measurements constrained by a new age model have enabled the generation of a continuous record of IRD sources, timing, frequency and flux, and of local contemporary hydrographic conditions. The evidence indicates that BIIS IRD precursors are not uniquely, or mechanistically, linked to H events, but are part of the pervasive millennial‐scale cyclicity. Our results support an LIS source for the IRD comprising H layers, but the ambient glacial sections are dominated by assemblages typical of the Irish Sea Ice Stream. Light isotope excursions associated with H events are interpreted as resulting from the melting of the BIIS, with ice‐sheet destabilization attributed to eustatic jumps generated by LIS discharge during H events. This positive‐feedback mechanism probably caused similar responses in all circum‐Atlantic ice‐sheet margins, and the resulting gross freshwater flux contributed to the perturbation of the Atlantic Meridional Overturning Circulation during H events.     

A METHODOLOGY FOR THE RAPID EVALUATION OF GROUNDWATER RESOURCES,SAO PAULO STATE,BRAZIL

ABSTRACT

Reservoirs are of necessity always built on the basis of incomplete hydrological information which introduces uncertainty into their design and operation. Since the advent of the electronic digital computer attempts have been made to reduce the uncertainty in hydrological design and reservoir management by the use of synthetic hydrology and simulation. It has been found by simulation that the expected benefits from a proposed reservoir system are often a function of the stochastic process selected for the synthetic hydrology, as well as depending upon the magnitude, and choice of driving parameters (commonly, the mean, variance, lag one serial correlation and Hurst's ‘h’). It is suggested that hydrological records are often two short and most statistical tests too weak for the hydrologist to be able to pick ‘the correct’ synthetic hydrological world with any reasonable degree of certainty. However, it would appear that for many problems and places that there is sufficient hydrological data for the hydrologist to assign probabilities to various prior distributions, and to optimize reservoir management and design by Bayesian decision theory.     

Last Glacial Maximum equilibrium-line altitude trends and precipitation patterns in the Sangre de Cristo Mountains, southern Colorado, USA

Precipitation patterns during the Last Glacial Maximum (LGM) in the Rocky Mountains varied due to the influence of the continental ice sheets and pluvial lakes. However, no constraints have been placed on potential changes of southeasterly Gulf of Mexico-derived moisture that today contributes considerable precipitation to the easternmost ranges of the southern and middle Rocky Mountains. The Sangre de Cristo Mountains of southern Colorado are ideally situated to assess the relative importance of westerly and southeasterly-derived moisture during the LGM. Based on reconstructions of 30 palaeoglaciers in the Sangre de Cristo Mountains, we find that LGM equilibrium-line altitudes (ELAs) on the east side of the range were systematically 100–200 m lower than ELAs on the west side. The observed ELA pattern is strikingly similar to modern precipitation patterns in the study area, suggesting that southeasterly-derived precipitation had a significant influence on the mass balances of LGM glaciers.     

C) United Nations

Synopsis

An inexpensive continuously recording turbidity transducer on two different types of mounting has been developed at the University of Natal, and installed at two stations in the Umgeni River system. The geometry and components of the transducers are detailed, and a sampler used to calibrate the transducers is also described. A portable turbidity meter has also been developed.