Acid Rain and Below-Cloud Scavenging in South-Western China

Major urban areas in south-western China exhibit unique air pollutionproblems due to increasing use of high sulphur-content fuels in an environmentof unfavourable topography and climate. Ambient levels of sulphur dioxideexceed the air quality objectives, and this gas is the major precursor of acidrain. Cloudwater chemistry studies are reported for urban, suburban andcountryside locations, during the period 1985–1989. Although cloudwateracidity was found to increase towards the cloud base, the acidity was muchgreater for rainwater samples collected simultaneously, and was morepronounced in urban rather than neighbouring suburban or countryside regions.The main contribution to the acidity arises from below-cloud scavenging of gasand aerosol and model calculations are able to simulate this behaviour.     

One-dimensional models of groundwater flow, sediment thermal history and petroleum generation within continental rift basins

We investigate the effects of convective heat transfer on the thermal history of sediments and petroleum formation within continental rift basins using one-dimensional mathematical modelling. The transport equations used in this study to describe vertical groundwater flow and conductive/convective heat transfer are solved by the finite element method. Sediment thermal history is quantitatively represented using first-order rate kinetic expressions for kerogen degradation and an empirical fanning Arrhenius model for apatite fission track annealing. Petroleum generation is also represented in the model by a suite of first-order rate kinetic expressions. The analysis provides insights into how pore fluid circulation patterns are preserved in the rock record as anomalies in palaeogeothermometric data within continental rifts. Parameters varied in the numerical experiments include the ratio of conductive to convective heat transfer (thermal Peclet number; Pe) and the composition of the disseminated organic matter in the sediment (type II and III kerogen). Quantitative results indicate that vertical groundwater flow rates on the order of a mm/yr cause a change in computed vitrinite reflectance of the rocks and a shift in the depth to oil generation by as much as 3000 m. Differences in thermal gradients between recharge and discharge areas (Pe= 0.6) also change the width of the zone of oil generation by a factor of two. Even more dramatic, however, are the large changes in predicted apatite fission track length distributions and model ages between recharge and discharge areas. For example, a sediment package buried to a depth of 2400 m over 200 Myr within the groundwater recharge column had a fission track length distribution with a computed mean and standard deviation of 12.83 μm and 0.77 μm, respectively. The fission track model age for this sediment package was 209 Ma. The same sediment package in the discharge area has a distribution with a mean track length of 5.68 μm, a standard deviation of 3.37 μm, and a fission track model age of 2.6 Ma. Transient groundwater flow simulations, in which fluid circulation ceases after a period of time within the rift basin, are also presented to illustrate how disturbances in palaeogeothermometric parameters are preserved on geological time-scales. Vitrinite reflectance profiles require about 10 Myr to return to conductive conditions within groundwater recharge areas while the convective disturbances are preserved indefinitely along the discharge column, as long as further subsidence does not occur. Ancient groundwater flow systems are preserved as anomalies in computed apatite fission track model ages and distributions much longer after groundwater flow stops, relative to organic-based geothermometers. Significant differences exist in model ages between recharge (145 Ma) and discharge (90 Ma) areas 200 Myr after flow has ceased. However, calculated fission track histogram distributions are virtually identical in recharge and discharge areas after about 50 Myr. Our study suggests that ancient groundwater flow systems can be detected by comparing thermochronometric data between suspected recharge and discharge areas within continental rifts. Vitrinite reflectance profiles, observed offsets in the depth to the onset of petroleum generation, and apatite fission track annealing studies are all well suited for detecting groundwater flow systems which have been relatively long lived (10⁷ years). Apatite fission track age data are probably best suited for identifying ancient groundwater flow systems within rifts long (>200 Myr) after flow ceases.     

Maintaining the Rural Landscape

Cultural landscapes require maintenance. The example of hedging in South Island, New Zealand, is used to explore the factors which generate perennial crises of maintenance, and ultimately affect the sustainability of a productive rural landscape. This logic of maintenance is applicable to other forms of landscape investment in drains, buildings, sown pastures and soils.     

SEISMIC INVERSION BY A RMS BORN APPROXIMATION IN THE SPACE-TIME DOMAIN1

A new approximate method to calculate the space-time acoustic wave motion generated by an impulsive point source in a horizontally layered configuration is presented. The configuration consists of a stack of fluid layers between two acoustic half-spaces where the source and the receiver are located in the upper half-space. A distorted-wave Born approximation is introduced; the important feature of the method is the assumption of a background medium with vertical varying root-mean-square acoustic wave speed. A closed-form expression for the scattered field in space and time as a function of the contrast parameters is deduced. The result agrees closely with rigorously calculated synthetic seismograms. In the inverse scheme the wave speed and mass density can be reconstructed within a single trace. Results of the inversion scheme applied to synthetic data are shown.     

The Cr-minerals of Outokumpu--Their Chemistry and Significance

The massive Cu–Co–Zn sulfide ores at Outokumpu inFinnish Karelia are unusual among Cyprus type deposits by virtueof the high Cr content exhibited by both the ores and the hostrocks. Throughout both the ore horizons and the metasedimentaryhost rock, bulk-rock Cr content averages 0.2 per cent Cr₂O₃.The high Cr contents result in an unusual degree of Cr substitutionin a number of silicate and oxide phases. Cr₂O₃ analyses presentedhere include values up to 3.74 per cent in tremolite, 3.65 percent in diopside, 26.90 per cent in uvarovitic garnet, 15.37per cent in epidote, 24.63 per cent in muscovite, 8.56 per centin biotite, 5.18 per cent in chlorite, 2.99 per cent in staurolite,and 72.65 per cent in chromite. Cr contents of these mineralscan be extremely variable even on the thin-section scale. Thehigh level of Cr in the metasediments, together with that ofNi, reflects their enhanced concentration in the original sedimentsdue to presumed derivation from a Cr-rich komatiite-bearingsubstrate. High levels of both elements in the stock works indicatethat they may have been partly re-distributed by the hydrothermalore-forming fluids. The variability of Cr content within individualsilicate grains suggests that they overgrew small-scale variationsin sedimentary Cr content sufficiently rapidly to preclude diffusivesmoothing of µ_Cr gradients. This suggests that while Crmay be mobile in hydrothermal systems, it is immobile undermetamorphic conditions.     

The M?gantic Unstrusive Complex, Qu?bec: a Study of the Derivation of Silica-Oversaturated Anorogenic Magmas of Alkaline Affinity

The Cretaceous M?gantic intrusive complex of southern Qu?beccontains early noritic gabbrodiorites which represent cumulatesfrom crustally contaminated hawaiite to syenite magmas. Wholerock and mineral chemistry, as well as textural evidence, indicatethat post-cumulus recrystallization and reaction were important,and most of the amphibole and biotite are thought to have formedin this way. A younger plutonic quartz-syenite ringdyke maynot be cogenetic with the gabbro-diorites sice it lacks orthopyroxene.It may, however, be cogenetic with basaltic to riebeckite granitedykes. Fractionation of olivine, plagioclase, aluminous clinopyroxene,and minor Ti-magnetite from critically undersaturated alkalibasaltic magmas generated hawaiitic magmas. The developmentof quartzbearing mugearitic and syenitic residua from the hawaiitescan best be modelled by fractionation of amphibole, plagioclase,olivine, oxides, and apatite. Attempts to model fractionationusing observed phenocrysts (including clinopyroxene) were unsuccessful.Amphibole fractionation is interpreted to have taken place througha reaction with still-porous, higher-temperature cumulates onthe walls of the magma chamber. The plutonic syenites probablyrepresent alkali feldspar cumulates from the residual syeniticmelts. Magnesian calc-alkaline lamprophyres exhibit olivineto phlogopite reaction textures, are enriched in Cr, Ni, K,Rb, Nb, Y, Zr, and Si relative to the basaltic dykes, yet havesimilar incompatible element ratios. Their relation to the basaltsis problematical. The late biotite-granite core to the complexis identical to typical White Mountain granites and may haveformed as an anatectic cap on rising, fractionating, mantlederivedmagmas.     

The Effect of Postcumulus Reactions on Composition of Chrome-spinels from the Jimberlana Intrusion

Chromites in olivine adcumulates, mesocumulates and orthocumulatesfrom drill core of the Jimberlana intrusion have been analysedand related to the cumulate type and to the nature of the surroundingsilicate mineral. Chromites in adcumulates and mesocumulatesshow a restricted range of composition and are high in Mg, Aland Cr. The orthocumulate chromites vary in composition fromthat found in adcumulates to chromites which are much higherin Fe and Ti and with a higher Fe^3?: Fe^2? ratio. The chromitesin orthocumulates vary in composition depending upon the natureof the enclosing silicate mineral. This is believed to reflectthe ability of the enclosing mineral to protect the originalcumulus chromite from reaction with the intercumulus liquid.Thus chromite within early bronzite oikocrysts was protectedfrom reaction whereas that in plagioclase and phlogopite wasprotected at a much later stage and has a higher Fe and Ti contentChromite within olivine appears to have been able to equilibratewith intercumulus liquid until late in the magmatic historyexcept where the olivine enclosing chromite has itself beensurrounded by bronzite. It is suggested that chromite can exchangeelements with intercumulus liquid through the olivine. Thereare two possibilities; either elements such as Cr, Al, Ti andFe^{3 ?} were able to diffuse through the olivine structure orthe apparently enclosed chromite crystals were able to maintaindirect contact with the melt along fine fractures produced bythe differential thermal contraction of olivine and chromite. The average diameters of chromite crystals within orthocumulatebronzite and olivine are 28 and 20 microns respectively whereaschromites in plagioclase and phlogopite have average diametersof 48 and 56 microns. There is no obvious correlation betweenthe size of the chromite and their composition for grains foundwithin a particular silicate. Chromites of every size have beenable to equilibrate with the liquid unless they were protectedfrom reaction. Nucleation of reaction products played an important role indetermining the final composition of any particular chromitecrystal. The significance of reaction and nucleation on a localscale of millimetres is considered with respect to the majorsilicates and to the location of the last liquid. It is suggestedthat the last liquid tended to concentrate in pockets of reactantcrystals, where product crystals failed to nucleate until latein the magmatic history. It is estimated that in rocks withan orthocumulate texture, the intercumulus liquid crystallizedover a temperature range as large as 300 ?C and that it becamesignificantly more oxidizing near the solidus temperature.     

An Example of Consequent Mantle Metasomatism in Peridotite Inclusions from Nunivak Island, Alaska

Many of the coarse-grained peridotite inclusions in basanitesfrom Nunivak Island, Alaska, contain amphibole and a smallerfraction also contain phlogopite and apatite. All of these peridotiteshave light REE/heavy REE abundance ratios greater than chondritesand many have abundances of K, Rb, Sr, Ba and light REE whichexceed estimates for primitive mantle. On the basis of mineraltextures and compositions we infer that the clinopyroxene, amphibole,phlogopite and apatite equilibrated with a metasomatic fluid.Isotopic (Sr and Nd) ratios and parent-daughter abundance datafor the coarse-grained peridotites constrain the age of themetasomatism to be less than 200 million years. Associated amphibole pyroxenite inclusions are not metasomatized;these inclusions probably formed as crystal segregates froman alkalic magma. Both pyroxenites and coarse-grained peridotitesare isotopically similar to basalts from Nunivak Island. Usingthese data, we propose a model in which the metasomatized peridotiteswere wallrocks located adjacent to the pyroxenites, and thatmetasomatism of these peridotites was caused by the infiltrationof a residual silicate melt or volatile-rich fluid derived fromthe parental magma of the pyroxenites; i.e. the metasomatismwas a consequence of basaltic magmatism. Furthermore, the parentalmagma of the pyroxenites was probably petrogenetically relatedto the Nunivak volcanism. REE modelling of fluids in equilibriumwith clinopyroxenes from the coarse-grained peridotites is consistentwith this model.     

Longitudinal transport of turbidity currents—–a model study of Horgen events

Experiments were carried out in a 10 × 6 m basin to simulate turbidity currents generated by the Horgen Slumping Events of 1875. The conditions for kinematic similarity were satisfied and the experiments gave further insight into the mechanics of transport triggered by the Horgen slumps. The experimental turbidity currents laid down thick deposits on a subaqueous fan, and thin sheets of turbidite on the floor of the elongate basin through longitudinal transport, comparable with the simulated deposits in Lake Zurich. It is concluded that longitudinal transport is a general phenomenon of turbidity currents.