influences of quaternary climatic changes on processes of soil development on desert loess deposits of the cima volcanic field california summary soils formed in loess are evidence of both relict and buried landscapes developed on pliocene to latest pleistocene basalt flows of the cima volcanic field in the eastern mojave desert california the characteristics of these soils change systematically and as functions of the age and surface morphology of the lava flow four distinct phases of soil development are recognized phase 1 weakly developed soils on flows less than 0 18 m y old phase 2 strongly developed soils with thick argillic horizons on 0 18 0 7 m y old flows phase 3 strongly developed soils with truncated argillic horizons massively impregnated by carbonate on 0 7 to 1 1 m y old flows and phase 4 degraded soils with petrocalcic rubble on pliocene flows a critical aspect of the development of stage 1 soils is the evolution of a vesicular a horizon which profoundly affects the infiltration characteristics of the loess parent materials laboratory studies show that secondary gypsum and possibly other salt accumulation probably occurred during the period of phase 1 soil development slight reddening of the interiors of peds from vesicular a horizons of phase 1 soils and presence of weakly developed b horizons indicates a slight degree of in situ chemical alteration however clay and fe oxide contents of these soils show that these constituents as well as carbonates and soluble salts are incorporated as eolian dust in contrast to phase 1 soils chemical and mineralogical analysis of argillic horizons of phase 2 soils indicate proportionally greater degrees of in situ chemical alteration these data the abundant clay films and the strong reddening in the thick argillic horizons suggest that phase 2 and phase 3 soils formed during long periods of time and periodically were subjected to leaching regimes more intense than those that now exist flow age data and soil stratigraphic evidence also indicate that several major loess deposition events occurred during the past 1 0 m y loess events are attributed to past changes in climate such as the pleistocene to holocene climatic change that periodically caused regional desiccation of pluvial lakes reduction of vegetational density and exposure of loose unconsolidated fine materials during times of warmer interglacial climates precipitation infiltrates to shallower depths than during glacial periods extensive saline playas which developed in the mojave desert during the holocene are a likely source of much of the carbonates and soluble salts that are accumulating at shallow depths both in phase 1 soils and in the formerly noncalcareous nongypsiferous argillic horizons of phase 2 and 3 soils

stable isotope geochemistry and phase equilibria of coesite bearing whiteschists dora maira massif western alps peak metamorphic temperatures for the coesite pyrope bearing whiteschists from the dora maira massif western alps were determined with oxygen isotope thermometry the 18o smow values of the quartz after coesite 18o 8 1 to 8 6 n 6 phengite 6 2 to 6 4 n 3 kyanite 6 1 n 2 garnet 5 5 to 5 8 n 9 ellenbergerite 6 3 n 1 and rutile 3 3 to 3 6 n 3 reflect isotopic equilibrium temperature estimates based on quartz garnet rutile fractionation are 700 750 c minimum pressures are 31 32 kb based on the pressure sensitive reaction pyrope coesite kyanite enstatite in order to stabilize pyrope and coesite by the temperature sensitive dehydration reaction talc kyanite pyrope coesite h2o the a h2o must be reduced to 0 4 0 75 at 700 750 c the reduced a h2o cannot be due to dilution by co2 as pyrope is not stable at x co2 0 02 t 750 c p 30 kb in the absence of a more exotic fluid diluent e g ch4 or n2 a melt phase is required granite solidus temperatures are 680 c 30 kb at a h2o 1 0 and are calculated to be 70 c higher at a h2o 0 7 consistent with this hypothesis kyanite jadeite quartz bands may represent a relict melt phase peak p t f h2o estimates for the whiteschist are 34 2 kb 700 750 c and 0 4 0 75 the oxygen isotope fractionation between quartz 18o 11 6 and garnet 18o 8 7 in the surrounding orthognesiss is identical to that in the coesitebearing unit suggesting that the two units shared a common final metamorphic history hydrogen isotope measurements were made on primary talc and phengite d smow 27 to 32 on secondary talc and chlorite rite after pyrope d 39 to 44 and on the surrounding biotite d 64 and phengite d 44 gneiss all phases appear to be in nearequilibrium the very high d values for the primary hydrous phases is consistent with an initial oceanicderived connate fluid source the fluid source for the retrograde talc chlorite after pyrope may be fluids evolved locally during retrograde melt crystallization the similar d but dissimilar 18o values of the coesite bearing whiteschists and hosting orthogneiss suggest that the two were in hydrogen isotope equilibrium but not oxygen isotope equilibrium the unusual hydrogen and oxygen isotope compositions of the coesite bearing unit can be explained as the result of metasomatism from slab derived fluids at depth

high grade metamorphism and partial melting of basic and intermediate rocks rocks of basic and intermediate bulk composition occur in orogenic terranes from all geological time periods and are thought to represent significant petrological components of the middle and lower continental crust however the former lack of appropriate thermodynamic models for silicate melt amphibole and clinopyroxene that can be applied to such lithologies at high temperature has inhibited effective phase equilibrium modelling of their petrological evolution during amphibolite and granulite facies metamorphism in this work we present phase diagrams calculated in the na2o cao k2o feo mgo al2o3 sio2 h2o tio2 o2 nckfmashto compositional system for a range of natural basic and intermediate bulk compositions for conditions of 2 12 kbar and 600 1050 c using newly parameterized activity composition relationships detailed in a companion paper by green et al in this issue particular attention is given to mid ocean ridge basalt morb and diorite protolith bulk compositions calculated subsolidus mineral assemblages in all basic and intermediate rock types are modally dominated by hornblende and plagioclase with variable proportions of epidote clinopyroxene garnet biotite muscovite quartz titanite or ilmenite present at different pressures the h2o saturated wet solidus has a negative p t slope and occurs between 620 690 c at mid to lower crustal pressures of 5 10 kbar the lowest t melts generated close to the wet solidus are calculated to have granitic major element oxide compositions melting at higher temperature is attributed primarily to multivariate hydrate breakdown reactions involving biotite and or hornblende partial melt compositions calculated at 800 1050 c for morb show good correlation with analysed compositions of experimental glasses produced via hydrate breakdown melting of natural and synthetic basic protoliths with niggli norms indicating that they would crystallize to trondhjemite or tonalite diorite is shown to be significantly more fertile than morb and is calculated to produce high t melts 800 c of granodioritic composition subsolidus and suprasolidus mineral assemblages show no significant variation between different members of the basalt family although the p t conditions at which orthopyroxene stabilizes thus defining the prograde amphibolite granulite transition is strongly dependent on bulk rock oxidation state and water content the petrological effects of open and closed system processes on the mineral assemblages produced during prograde metamorphism and preserved during retrograde metamorphism are also examined via a case study analysis of a natural archean amphibolite from the lewisian complex northwest scotland

modelling phase assemblage diagrams for magnesian metapelites in the system k2o feo mgo al2o3 sio2 h2o geodynamic consequences for the monte rosa nappe western alps magnesian metamorphic rocks with metapelitic mineral assemblage and composition are of great interest in metamorphic petrology for their ability to constrain p t conditions in terranes where metamorphism is not easily visible phase assemblage diagrams for natural and model magnesian metapelites in the system kfmash are presented to document how phase relationships respond to water activity bulk composition pressure and temperature the phase assemblages displayed on these phase diagrams are consistent with natural mineral assemblages occurring in magnesian metapelites it is shown that the equilibrium assemblages at high pressure conditions are very sensitive to a h2o specifically the appearance of the characteristic hp assemblage chloritoid talc phengite quartz with excess h2o in the magnesian metapelites of the monte rosa nappe western alps is due to the reduction of a h2o furthermore the mineral assemblages are determined by the whole rock feo feo mgo ratio and effective al content x a as well as p and t the predicted mineral associations for the low and high x a model bulk compositions of magnesian metapelites at high pressure are not dependent on the x a variations as they show a similar sequence of mineral assemblages above 20 kbar the prograde sequence of assemblages associated with phengite with excess sio2 and h2o for low and high x a bulk compositions of magnesian metapelites is carpholite chlorite chlorite chloritoid chloritoid talc chloritoid talc kyanite talc garnet kyanite garnet kyanite biotite at low to medium p t conditions a low x a stabilises the phengite bearing assemblages associated with chlorite chlorite k feldspar and chlorite biotite while a high x a results in the chlorite phengite bearing assemblages associated with pyrophyllite andalusite kyanite and carpholite a high x a magnesian metapelite with nearly iron free content stabilises the talc kyanite phengite assemblage at moderate to high p t conditions taking into account the effective bulk composition and a h2o involved in the metamorphic history the phase assemblage diagrams presented here may be applied to all magnesian metapelites that have compositions within the system kfmash and therefore may contribute to gaining insights into the metamorphic evolution of terranes as an example the magnesian metapelites of the monte rosa nappe have been investigated and an exhumation path with p t conditions for the western roof of the monte rosa nappe has been derived for the first time the exhumation shows first a near isothermal decompression from the alpine eclogite peak conditions around 24 kbar and 505 c down to approximately 8 kbar and 475 c followed by a second decompression with concomitant cooling

airborne dust and its significance to soils abstract movement of mineral dust by the atmosphere has been underway for ages but efforts to assess its magnitude and significance have come largely in the last century attempts to identify and understand effects of the dust on soils are even more recent mostly in the last 30 years airborne movements have several forms dust is transported by large storms that can be spectacular dust is also moved by smaller pulses and in a rather continuous flux most movement is within continents or from continents to nearby oceans but fine dust fine silt and clay is carried from one continent to another prime sources of the dust are the deserts and semi arid regions of the world for example large quantities of dust are carried out of the sahara every year quantities of dust carried by individual pulses from the small ones to the massive storms extend from a few thousand to 150 million metric tons during the course of a year the total quantities can be large one example is an estimate that on the average 770 million metric tons originating in the western united states are carried 2300 km each year that amount would add roughly 3000 kg ha 1 yr 1 to soils between the rocky mountains and the mississippi river small pulses and the near continuous flux add between 100 and 200 kg ha 1 yr 1 to soils of the eastern united states on the average comparable quantities are being added to soils in other parts of the world as well airborne accessions of dust have a variety of effects in soil development and on soils after they have been formed dust dominates processes of genesis in a few soils affects horizon differentiation appreciably in many more is a primary source of constituents essential to some horizons and layers in a number of soils provides the bulk of the material for silty surface horizons in many soils and can contribute supplies of nutrient elements for plants examples of all such effects are covered in this review first among these effects to be recognized even though not understood were the silty surface layers that failed to match underlying horizons later recognized was airborne dust as a source of calcium carbonate for calcic horizons that the dust could add significant quantities of nutrient elements as for example south of the sahara came into the picture even later the beneficial effects of airborne dust to many soils raises an interesting question is the dust more or less useful in its new location than it was in its source area an adequate answer to that question would provide an improved basis for assessing the effects of wind erosion

coesite bearing eclogites from the dabie mountains central china petrogenesis p t paths and implications for regional tectonics coesite and kyanite bearing eclogites are abundant in the southern part of the dabie mountains southern dabie terrane sdt two types of eclogites from the sdt types iii and iv were selected for detailed paragenetic study type iii eclogites most abundant in the northern part of the sdt occur as blocks in gneisses and marble and contain eclogitic assemblages of omphacite garnet phengite epidote coesite kyanite carbonate rutile ilmenite these minerals exhibit weak compositional zoning and contain few mineral inclusions type iv eclogites mostly in the southern part of the sdt occur as coherent layers interbedded with gneisses and amphibolites and have assemblages of omphacite garnet glaucophane kyanite epidote phengite quartz rutile ilmenite garnets of type iv eclogites exhibit a prograde compositional zoning and have mineral inclusions of paragonite phengite epidote quartz and rutile in the core and omphacite barroisite and mg katophorite in the rim prograd

the pyrope coesite rocks and their country rocks at parigi dora maira massif western alps detailed petrography mineral chemistry and pt path both the coarse and fine grained varieties of the partly coesite bearing pyrope quartzites their interlayered jadeite kyanite rocks and the biotite phengite gneiss country rock common to all of them were subjected to detailed petrographic and textural studies in order to determine the sequence of crystallisation of their mineral constituents which were also studied analytically by microprobe prior to pyrope and coesite growth the mg rich metapelites were talc kyanite chlorite rutile ellenbergerite schists which upon continued prograde metamorphism developed first pyrope megacrysts in silica deficient local environments at the expense of chlorite talc kyanite and subsequently the smaller pyrope crystals with coesite inclusions from reacting talc kyanite based on geobarometrically useful mineral inclusions as well as on experimentally determined phase relations a prograde pt path simplified for water activity 1 is constructed which passes through the approximate pt conditions 16 kbar and 560 c 29 kbar and 720 c and finally up to 37 kbar at about 800 c where the mg rich metapelite was a pyrope coesite rock with phengite kyanite and talc still present during the retrograde path pyrope was altered metasomatically either into phlogopite kyanite quartz or at a later stage to chlorite muscovite quartz both assemblages yield pt constraints the latter about 7 9 kbar 500 600 c the country rock gneisses have also endured high pressures of at least 15 kbar but they provide mostly constraints on the lowest portion of the uplift conditions within the greenschist facies about 5 kbar 450 c microprobe data are presented for the following minerals pyrope ellenbergerite dumortierite unusually mgti rich jadeite vermiculite formed after na phlogopite paragonite and for several generations of phengite chlorite talc phlogopite dravite and glaucophane in the high pressure rocks as well as for biotite chlorite phengites epidote garnet albite and k feldspar in the country rock gneisses an outstanding open problem identified in this study is the preservation of minerals as inclusions within kyanite and pyrope beyond their pt stability limits

neogene cooling and exhumation of upper amphibolite facies whiteschists in the southwest pamir mountains tajikistan abstract the pamir mountains of central asia are the product of accretion events during the paleozoic and mesozoic and have been further impacted by the continental collision between india and asia during the cenozoic deformation continues today in what is now a mountain belt within a continental interior the geology of the pamir is characterized by several arcuate shaped structurally bound regions with a general east west trend the southwest pamirs are noteworthy for the extensive development of whiteschists talc kyanite rocks and other ultramagnesian metamorphic assemblages in the goran series our interpretation of the goran series is that upper amphibolite facies metamorphism at temperatures above 650 c and pressures near 700 mpa kyanite talc kyanite potassium feldspar was followed by decompression to below 400 mpa sillimanite cordierite late stage andalusite is found only at kuhi lal available u pb k ar and rb sr data gave 2130 ma as a minimum age for the amphibolite facies metamorphism our 40ar 39ar cooling ages for biotite phlogopite and hornblende from kuhi lal darai stazh and mulvoj indicate miocene cooling for the goran series biotites from mica schist gave good plateau ages of 8 9 0 3 10 2 0 3 and 18 3 0 2 ma and phlogopite associated with sapphirine forsterite and enstatite gave a 16 9 0 3 ma plateau hornblende from amphibolite gave a plateau age of 22 2 1 3 ma and an age of 17 ma from a saddle shaped release spectrum our 17 22 ma ages are comparable to previously reported 18 23 ma rb sr isochron ages on the pamir shugnan complex of gneissic granite exposed 15 km northeast of the northernmost locality kuhi lal and thus could be related to heating associated with this plutonism our preferred interpretation is that the ages represent regional cooling due to exhumation related to tectonic activity in the miocene

the thermo tectonic evolution of the southern congo craton margin as determined from apatite and muscovite thermochronology abstract the southern irumide belt sib of zambia consists of predominantly mesoproterozoic terranes that record a pervasive tectono metamorphic overprint from collision between the congo and kalahari cratons in the final stages of gondwana amalgamation this study applies multi method thermochronology to samples throughout southern zambia to constrain the post collisional phanerozoic thermo tectonic evolution of the region u pb apatite and 40ar 39ar muscovite data are used to constrain the cooling history of the region following congo kalahari collision and reveal ages of c 550 450 ma variations in the recorded cooling ages are interpreted to relate to localised post tectonic magmatism and the proximity of analysed samples to the congo kalahari suture apatite fission track data are used to constrain the low temperature thermo tectonic evolution of the region and identify mean central ages of c 320 300 210 200 and 120 110 ma thermal modelling of these samples identifies a number of thermal events occurring in the region throughout the phanerozoic carboniferous to permian triassic heating is suggested to relate to the development of karoo rift basins found throughout central africa and constrain the timing of sedimentation in the basin permian to jurassic cooling is identified in a number of samples reflecting exhumation as a result of the mauritanian variscan and gondwanide orogenies subsequent cooling of the majority of samples occurs from the cretaceous and persists until present reflecting exhumation in response to larger scale rifting associated with the break up of gondwana each model reveals a later phase of enhanced cooling beginning at c 30 ma that if not an artefact of modelling corresponds to the development of the east african rift system the obtained thermochronological data elucidate the previously unconstrained thermal evolution of the sib and provides a refined regional framework for constraining the tectonic history of central africa throughout the phanerozoic

aeolian deposition and its effect on soil and vegetation changes on stabilised desert dunes in northern china abstract spatial and temporal patterns of aeolian deposition at shapotou northern china were studied on a chronosequence 0 12 29 and 37 years of stabilised desert dunes aeolian deposition markedly influenced soil and vegetation changes on the dunes the spatial pattern of aeolian deposition was studied using dust traps and measurements of the depth of aeolian accumulation mean deposition from august 1993 to july 1994 was 372 g m 2 and showed a complex spatial pattern determined by prevailing wind direction topography and shrub distribution contrasts between the topographic pattern of aeolian deposition trap data and accumulation thickness of aeolian deposits indicate that the measured pattern of aeolian deposition alone is insufficient to explain the pattern of accumulation an accretionary surface soil or grey sand containing much aeolian derived particles is developing on the stabilised dunes at a mean rate of 1 6 mm year rapid accumulation of aeolian particles significantly changed the surface environment increased moisture retention by the finer grey sand resulted in decreased moisture penetration and subsequent desiccation of the deeper dune sands this has led to vegetation changes with the deep rooted planted xerophytic shrubs declining from 12 to only 3 cover after 37 years of stabilisation the shrubs were replaced by a widespread microphytic crust and later by shallow rooted annual species which increased from 0 to approximately 12 cover over the same period the implications of projected increases in the dustiness of arid regions on the stability of desert dunes and vegetated desert margins are discussed stabilised dunes may become increasingly vulnerable to deflation with the progressive decline in shrub cover

age origin and palaeogeography of the southern irumide belt zambia the southern irumide belt sib records over one and a half billion years c 2000 500 ma of tectonic evolution along the southern congo craton margin to understand this evolution we present u pb lu hf ree zircon and structural data for the sib of zambia which are used to investigate its formation evolution and relationship to the irumide belt of the southern congo craton orthogneiss in the chewore rufunsa and chipata terranes yields ages between c 2040 and 2000 ma this implies the presence of paleoproterozoic basement throughout the sib similar to basement rocks within the irumide belt detrital zircon data from the chipata terrane yield age populations and ehf t values that are equivalent to those for samples from other sib terranes and the irumide belt the similarities between basement units and overlying sedimentary sequences in the sib and irumide belt indicate that the sib formed an integral part of the southern congo margin since the paleoproterozoic rather than accreting to this margin during the late mesoproterozoic subsequent structural deformation in this region occurred as two phases a north south directed compression during the late neoproterozoic to cambrian and a weaker east west directed compression during the phanerozoic supplementary material sample locations and descriptions including descriptions of zircon morphologies and cathodoluminescence characteristics accompanied by images u pb lu hf and ree zircon data including diagrams for all unknowns and reference materials as well as whole rock geochemical data including diagram for granites of the nyimba sinda terrane are available at https doi org 10 6084 m9 figshare c 4386344

co2 injectivity in saline aquifers the impact of non darcy flow phase miscibility and gas compressibility a key aspect of co2 storage is the injection rate into the subsurface which is limited by the pressure at which formation starts to fracture hence it is vital to assess all of the relevant processes that may contribute to the pressure increase in the aquifer during co2 injection building on an existing analytical solution for immiscible and spatially varying non darcy flow this paper presents a mathematical model that accounts for combined effects of non darcy flow phase miscibility and gas compressibility in radial two phase displacements results show that in low permeability formations when co2 is injected at high rates non darcy simulations forecast better displacement efficiency compared to flow under darcy conditions this will have a positive effect on the formation co2 storage capacity this however comes at the cost of increased well pressures more favorable estimations of the pressure buildup are obtained when co2 compressibility is taken into account because reservoir pressures are reduced due to the change in the gas phase properties also non darcy flow results in a significant reduction in halite precipitation in the near well region with a positive effect on co2 injectivity in the examples shown non darcy flow conditions may lead to significantly different pressure and saturation distributions in the near well region with potentially important implications for co2 injectivity

high fo2 metasomatism during whiteschist metamorphism zambezi belt northern zimbabwe the kadunguri whiteschists are a group of talcand kyanitethe kalahari craton it includes the damara belt of namibia the lufilian arc of zambia and the democratic bearing lithologies that occur in the chewore inliers from the zambezi belt of northern zimbabwe they crop out on the southern republic of congo and the zambezi belt of southern zambia and northern zimbabwe it interacts complexly margin of the chewore ophiolite terrane a mesoproterozoic ophiolite and island arc as a 5 km 1 5 km southeasterly dipping semiat its eastern margin with the north south trending east african orogen of northern mozambique and east continuous block and contain the second known natural occurrence of yoderite major element analyses define the whiteschists within africa the zambezi belt is a complex polyorogen recording the relatively simple mfash system major and trace element analyses indicate that the whiteschists originate from the metasomatic major tectonothermal events at 890 880 ma and 520 550 ma both of which predominantly rework older alteration of alkalic ocean island type metabasalts similar to those in the underlying ophiolite terrane synmetamorphic or metasomatic crustal components mesoproterozoic to archaean with the addition of minor juvenile material hanson et al mineral parageneses indicate peak p t conditions of between 13 and 15 kbar at 550 600 c and the highly oxidizing nature of 1988 1993 1994 wilson et al 1993 armstrong et al 1999 within this belt and within basement inliers of all reactions indicates the presence of a high fo2 metasomatic fluid the peak p t conditions require that this synmetamorphic exotic the lufilian arc are a string 150 km long 50 100 km metasomatic fluid was available at depths near 55 km the age of wide of high pressure kyanite bearing meta gabbroic high pressure metamorphism is constrained within the pan african eclogites and whiteschist fragments fig 1 vrana tectonothermal cycle at 550 520 ma tectonometamorphism in barr 1972 vrana et al 1975 cosi et al 1992 johnson the zambezi belt is related to a period of extensive crustal thickening oliver 1998 dirks sithole 1999 john et al 1999 possibly related to amalgamation of gondwanaland 2000 until recently these high pressure moderate temperature fragments have been poorly studied but some are now the focus of detailed investigations e g johnson oliver 1998 oliver et al 1998 dirks sithole 1999

relative permeability and trapping of co2 and water in sandstone rocks at reservoir conditions 1 we report the results of an experimental investigation into the multiphase flow properties of co2 and water in four distinct sandstone rocks a berea sandstone and three reservoir rocks from formations into which co2 injection is either currently taking place or is planned drainage relative permeability and residual gas saturations were measured at 50 c and 9 mpa pore pressure using the steady state method in a horizontal core flooding apparatus with fluid distributions observed using x ray computed tomography absolute permeability capillary pressure curves and petrological studies were performed on each sample relative permeability in the four samples is consistent with general characteristics of drainage in strongly water wet rocks measurements in the berea sample are also consistent with past measurements in berea sandstones using both co2 brine and oil water fluid systems maximum observed saturations and permeabilities are limited by the capillary pressure that can be achieved in the experiment and do not represent endpoint values it is likely that maximum saturations observed in other studies are limited in the same way and there is no indication that low endpoint relative permeabilities are a characteristic of the co2 water system residual trapping in three of the rocks is consistent with trapping in strongly water wet systems and the results from the berea sample are again consistent with observations in past studies this confirms that residual trapping can play a major role in the immobilization of co2 injected into the subsurface in the mt simon sandstone a nonmonotonic relationship between initial and residual co2 saturations is indicative of a rock that is mixed or intermediate wet and further investigations should be performed to establish the wetting properties of illite rich rocks the combined results suggest that the petrophysical properties of the multiphase flow of co2 water through siliciclastic rocks is for the most part typical of a strongly water wet system and that analog fluids and conditions may be used to characterize these properties further investigation is required to identify the wetting properties of illite rich rocks during imbibition processes

the carbonate hosted willemite prospects of the zambezi metamorphic belt zambia zambian willemite zn2sio4 deposits occur in the metasedimentary carbonate rocks of the proterozoic katangan supergroup the most important orebodies are located around kabwe and contain both sulphides and willemite in dolomites of low metamorphic grade the star zinc and excelsior prospects lusaka area discovered in the early 1920s occur in the metamorphic lithotypes of the late proterozoic zambezi supracrustal sequence which were deposited in a transtensional basin formed during the oblique collision of the kalahari and congo cratons the deposits are hosted by the limestone and dolomitic marbles of the cheta and lusaka formations structural analysis indicates that several fracture sets host the deposits which may be genetically related to the pan african mwembeshi dislocation zone a major geotectonic boundary between the lufilian arc and the zambezi belt in both prospects willemite replaces the marbles and is found along joints and fissures with open space filling textures and locally may develop colloform and vuggy fabrics as well silver as well as traces of germanium and cadmium have been detected within the willemite ore and lead or zinc sulphides are scarce or absent calcite locally replaces willemite willemite is associated with specular hematite and franklinite and post dates the zn spinel gahnite in the paragenesis genthelvite zn4be3 sio4 3s occurs as a minor phase in irregular aggregates the willemites from the lusaka area though mn poor show green cathodoluminescence colours and bright green fluorescence in short wave uv as the high temperature willemites in usa thermometric analyses of primary fluid inclusions in willemite yield homogenization temperatures that range from 160 c to 240 c and salinities of 8 16 wt equiv nacl the homogenization temperatures suggest a hypogene hydrothermal origin for the willemite concentrations the geochemistry of fluid inclusion leachates suggests that the hydrothermal fluids were brines derived from highly evaporated seawater precise age constraints are currently lacking for the lusaka area deposits though the deposits are not deformed indicating that they post date the lufilian orogeny 520 ma the possibility of precursor ores exists the gahnite franklinite willemite deposits could have been derived from a metamorphosed primary sulphide or even nonsulphide concentration that has subsequently been completely destroyed however there is no real evidence of such a primary source for the willemite mineral association the lusaka zinc ores may have been produced by an extensive hydrothermal system with fluids discharging along basinal fracture zones controlled by the pre pan african rifting stage a paragenesis similar to that of the lusaka prospects has been proposed to be a vector towards massive sulphide ores in several parts of the world therefore it is possible that these small willemite showings in zambia may be part of a much bigger and still unexplored zinc province

yoderite a mineral with essential ferric iron its lack of occurrence in the system mgo al 2 o 3 sio 2 h 2 o

stability of yoderite in the absence and in the presence of quartz an experimental study in the system mgo al2o3 fe2o3 sio2 h2o

correlation between long term pedogenic caco3 formation rate and modern precipitation in deserts of the american southwest abstract the rates of caco 3 formation in desert soils of the american southwest are highly variable and potentially useful in estimating the age of soil profiles the objective of this study was to examine the correlation between the long term rate of caco 3 formation ltr and modern mean annual precipitation map and temperature mat ltr is not significantly related to temperature but is linearly related to map ltr 0 0150 map 37 r 2 0 924 the slope of the regression is significantly different from 0 0 and is equivalent to a calcium concentration of 6 0 mg liter 1 in rainfall however the intercept term which suggests a lower limit of precipitation for caco 3 formation of 37 mm is not significantly different from 0 0 the ltr map relationship may be useful in estimating the age of soil profiles by the caco 3 method in the desert southwest

a second natural occurrence of yoderite a second example of yoderite has been discovered in whiteschists from the southern chewore hills of northern zimbabwe the mineral is pale green in colour and occurs in an equilibrium assemblage with talc chlorite kyanite dravite hematite there is no quartz present recalculated microprobe analyses give a structural formula of mg2al5 7fe0 3si4o18 oh 2 similar to that obtained for the type locality at mautia hill tanzania i e mg2al5 6fe0 4si4o18 oh 2 textural relationships and relative proportions of minerals suggest that the yoderite was formed by reaction between talc chlorite kyanite and hematite experimental evidence suggests high water pressure metamorphic conditions at temperatures exceeding a reaction curve that extends between 13 kbar at 590 c and 21 kbar at 650 c the yoderite bearing whiteschist is associated with a 1 4 ga dismembered ophiolite it is proposed that this yoderite occurrence is associated with a relict subduction suture zone

the upper thermal stability of chlorite quartz an experimental study in the system mgo al2o3 sio2 h2o experiments up to water pressures of 21 kbar have been undertaken to bracket the reactions chlorite quartz talc kyanite h2o chlorite quartz talc cordierite h2o and talc kyanite quartz cordierite h2o by reversed runs in the system mgo al2o3 sio2 h2o mash these reaction curves intersect at an invariant point ip1 at ph2o 6 4 0 2 kbar and a temperature of 624 4 c the curve of the chlorite quartz breakdown to talc kyanite h2o at water pressures above 6 4 kbar shows a negative dp dt with the slope decreasing with rising pressure whereas the slope of the breakdown curve to talc cordierite h2o at water pressures is clearly positive the composition of the chlorite solid solution reacting with quartz has been estimated to be approximately mg4 85al1 15 al1 15si2 85o10 oh 8 over the entire pressure range investigated the composition of the talc solid solution forming by the breakdown of chlorite quartz appears to be mg2 94al0 06 al0 06si3 94o10 oh 2 at ph2o 2kbar with increasing pressure the al content of talc decreases reaching a value of about 0 06 atoms per formula unit at p h2o 21 kbar as a consequence of the new experimental data the existing phase topologies of the mash system and k2o mash system have been revised for example the invariant point ip1 and the univariant reaction curve kyanite talc h2o chlorite cordierite are stable for this reason the development of medium to high temperature metamorphic rocks compositionally approximating the mash system must be reconsidered the whiteschists from sar e sang afghanistan are treated as an example the application of the present experimental data to metamorphic rocks of more normal composition requires the examination of the influence of further components this leads to the conclusion that the introduction of fe2 into magnesian chlorite extends its stability field in the presence of quartz by 10 15 c in comparison with pure mg chlorite

uranium mobilization by fluids associated with ca na metasomatism a p t t record of fluid rock interactions during pan african metamorphism western zambian copperbelt abstract a dozen uranium occurrences are known in the western zambian copperbelt domes region zambia the host rocks neoproterozoic kyanite talc micaschists have been interpreted as meta evaporites that formed part of an epicontinental sequence affected by upper amphibolite metamorphism during the pan african orogeny ca na metasomatism expressed by the presence of apatite epidote and scapolite is regionally extensive in the katanga and the pre katanga rocks in the domes region the present study focuses on the solwezi dome in the western zambian copperbelt two distinct uranium mineralization events that took place here mark the transition from crustal thickening by tectonic accretion to exhumation of the metamorphic host rocks u pb dating of uraninite and th pb dating of monazite show that the first u mineralization event took place between 550 ma and 530 ma the event is therefore contemporaneous with crustal thickening and on the basis of fluid inclusion analyses can be related to circulation of hypersaline ca rich metamorphic fluids trapped at minimum p t conditions of 460 mpa and 500 c on a regional scale the event is related to an epidote apatite metasomatic assemblage the second mineralization event dated at between 510 and 500 ma by th pb dating of monazite is related to high salinity ca na rich metamorphic fluids and occurred during exhumation of the high grade metamorphic rocks these fluids were trapped at the ductile brittle transition at 150 to 260 mpa and 200 to 300 c a regional scale scapolitization is attributed to these late fluid circulations these fluid rock interactions described at the regional scale are also characterized at the mineral scale alteration reactions assisted by cacl 2 rich fluids indicate that monazite and allanite hosted by the pre katanga rocks potentially acted as u sources for these pan african synmetamorphic uranium mineralizations

deformed batholiths in the pan african zambezi belt zambia age and implications for regional proterozoic tectonics the zambezi belt makes up part of the late proterozoic pan african network of orogenic belts in southern africa in zambia supracrustal rocks within the zambezi belt are in contact with two extensive units of granitic augen gneiss which have previously been interpreted as uplifted masses of remobilized sialic basement field relations and u pb zircon geochronologic data demonstrate that the gneisses are deformed batholiths that intruded the supracrustal sequence during two separate episodes of plutonism at ca 1100 and 820 ma the proterozoic tectonothermal evolution of the zambezi belt thus involved production of voluminous granitic magmas isotopic ages for the batholiths place new constraints on the timing of major depositional and orogenic events within the belt and enable comparisons to be made with other parts of the regional pan african network

similarity solutions for fluid injection into confined aquifers fluid injection into the deep subsurface such as injection of carbon dioxide co 2 into deep saline aquifers often involves two fluid flow in confined geological formations similarity solutions may be derived for these problems by assuming that a sharp interface separates the two fluids by imposing a suitable no flow condition along both the top and bottom boundaries and by including an explicit solution for the pressure distribution in both fluids when the injected fluid is less dense and less viscous than the resident fluid as is the case for co 2 injection into a resident brine gravity override produces a fluid flow system that is captured well by the similarity solutions the similarity solutions may be extended to include slight miscibility between the two fluids as well as compressibility in both of the fluid phases the solutions provide the location of the interface between the two fluids as well as drying fronts that develop within the injected fluid applications to cases of supercritical co 2 injection into deep saline aquifers demonstrate the utility of the solutions and comparisons to solutions from full numerical simulations show the ability to predict the system behaviour

hypersaline fluids generated by high grade metamorphism of evaporites fluid inclusion study of uranium occurrences in the western zambian copperbelt in the pan african lufilian belt western zambian copperbelt uranium mineralizations preferentially scattered in kyanite talc micaschists metamorphosed evaporitic sediments or concentrated along transposed quartz veins provide an opportunity to 1 understand the time space relationship between the ore minerals and the deformation of the host rocks 2 identify the different fluid events associated with specific stages of quartz deformation and 3 characterize the ore fluid geochemistry in terms of fluid origin and fluid rock interactions in the u occurrences studied in lolwa and mitukuluku domes region western zambian copperbelt two mineralizing stages are described the first generation of ore fluids 53 59 wt cacl2 13 15 wt nacl n2 h2 in the gas phase of fluid inclusions circulated during the high temperature quartz recrystallization at 500 700 c this temperature is in agreement with the p t conditions recorded during the crustal thickening related to continental collision at ca 530 ma la icpms analyses show the presence of uranium within this fluid with a concentration mode around 20 ppm the second generation of ore fluid 21 32 wt nacl 19 21 wt cacl2 co2 co in the gas phase of fluid inclusions percolated at lower temperature conditions at the brittle ductile transition between 200 and 300 c this temperature could be related to the exhumation of the high grade metamorphic rocks at ca 500 ma the formation of h2 and co is interpreted as the result of radiolysis in the presence of dissolved uranium in the aqueous phase of these fluid inclusions finally a late fluid 14 16 wt naclequiv circulated in the brittle domain but seems unrelated to u re mobilization event

coesite inclusions and prograde compositional zonation of garnet in whiteschist of the hp uhpm kokchetav massif kazakhstan a record of progressive uhp metamorphism coarse grained whiteschist containing the assemblage garnet kyanite phengite talc quartz coesite is an abundant constituent of the ultrahigh pressure metamorphic uhpm belt in the kulet region of the kokchetav massif of kazakhstan garnet displays prograde compositional zonation with decreasing spessartine and increasing pyrope components from core to rim cores were recrystallized at t 380 c inner to 580 c outer at p 10 kbar garnet ilmenite geothermometry margarite quartz stability and mantles at t 720 760 c and ph20 34 36 kbar coesite graphite stability phengite geobarometer kfmash system reaction equilibria textural evidence indicates that rims grew during decompression and cooling within the qtz stability field silica inclusions quartz and or coesite of various textural types within garnets display a systematic zonal distribution cores contain abundant inclusions of euhedral quartz type 1 inclusions inner mantle regions contain inclusions of polycrystalline quartz pseudomorphs after coesite type 2 with minute dusty micro inclusions of chlorite and more rarely talc and kyanite in their cores intense radial and concentric fractures are well developed in the garnet intermediate mantle regions contain bimineralic inclusions with coesite cores and palisade quartz rims type 3 which are also surrounded by radial fractures subhedral inclusions of pure coesite without quartz overgrowths or radial fractures type 4 occur in the outer part of the mantle garnet rims are silica inclusion free type 1 inclusions in garnet cores represent the low p low t precursor stage to uhpm recrystallization and attest to the persistence of low p assemblages in the coesite stability field coesites in inclusion types 2 3 and 4 are interpreted to have sequentially crystallized by net transfer reaction kyanite talc garnet coesite h2o and were sequestered within the garnet with progressively decreasing amounts of intragranular aqueous fluid during the retrograde evolution of the rock all three inclusion types diverged from the host garnet p t path at the coesite quartz equilibrium and followed a trajectory parallel to the equilibrium boundary resulting in inclusion overpressure coesite in type 2 inclusions suffered rapid intragranular h2o catalysed transformation to quartz and ruptured the host garnet at about 600 c when inclusion p 27 kbar garnet host p 9 kbar instantaneous decompression to the host garnet p t path passed through the kyanite talc chlorite quartz reaction equilibrium resulting in the dusty micro assemblage in inclusion cores type 3 inclusions suffered a lower volumetric proportion transformation to quartz at the coesite quartz equilibrium and finally underwent rupture and decompression when t 400 c facilitating coesite preservation type 4 coesite inclusions are interpreted to have suffered minimal transformation to quartz and proceeded to surface temperature conditions along or near the coesite quartz equilibrium boundary

zircon response to diamond pressure metamorphism in the kokchetav massif ussr ultrahigh pressure metamorphism of the diamond bearing gneisses in the kokchetav massif ussr has been dated at 530 7 ma 2 sgr by shrimp ion microprobe analysis of metamorphic zircons zircon xenocrysts as old as 2000 ma have retained original isotopic compositions through the extreme metamorphic conditions and indicate that the protolith that was buried to diamond forming pressures included early proterozoic components highly variable reset ting of u pb isotopic systems in zircon xenocrysts at the time of metamorphism suggests that diffusive lead loss had commenced but was incomplete a disequilibrium observation consistent with relatively transient exposure of the rocks to extreme crustal depths

evidence for major mass transfer and volume strain during regional metamorphism of pelites systematic examination of published sedimentary and metamorphosed pelite analyses has revealed evidence of significant mass transfer and volume strain during regional metamorphism statistical analysis of the data shows that barrovian zone metamorphism of pelitic schist generally causes increases in the whole rock concentrations of the low solubility elements ti and all the observed increases in ti and al contents as functions of metamorphic grade are almost certainly due to residual enrichment caused by the removal of other more soluble species application of mass balance principles to the petite compositional trends indicates that the average pelite may lose as much as 30 of its mass and volume during progressive metamorphism from subgreenschist to amphibolite facies conditions the bulk of the lost mass is silica not volatiles in addition other elements particularly ca na and k appear to be highly mobile in deep crustal metamorphism contrary to conventional interpretations it is concluded that the regional metamorphism of pelites is not an isochemical process

tectonometamorphic evolution of the chewore inliers partial re equilibration of high grade basement during the pan african orogeny only minor structural reworking during m2 granulite terrane the chewore inliers are isolated outcrops of the zambezi mobile samples within 2 km of the basal thrust margin preserve m1 belt within the mesozoic lower zambezi rift valley in zimbabwe mineral assemblages but these minerals were chemically re equidetailed mapping has recognized four terranes the zambezi librated without recrystallization during m2 at conditions of 5 6 quartzite granulite and ophiolite terranes apart from the 1 5 kbar and 631 100 c granulite terrane samples were ophiolite terrane all are dominated by supracrustal gneisses with totally recrystallized in shear zones at the margin of this terrane concordant granitic orthogneiss units of 1071 8 and 1083 these samples equilibrated at conditions identical to the peak of 8 ma age these terranes experienced low p high t metamorphism m2 at 7 7 1 9 kbar and 590 110 c the re equilibrated m1 terminated by isobaric cooling at 945 34 ma m1 and recrystallized sample sets define two points on the clockwise assemblages of sillimanite spinel garnet garnet orthopyroxene and p t path experienced by the granulite terrane during further two pyroxene mafics are recorded in the granulite terrane and burial and reworking in the pan african orogeny and are consistent conditions of formation were 4 4 1 7 kbar and 800 c m1 with the m2 p t path documented for the other terranes mineral parageneses and associated ductile deformation structures dominate the granulite terrane but m1 mineral parageneses are only preserved as sillimanite spinel inclusions in garnet cores in the other terranes the zambezi quartzite and ophiolite terranes were almost totally recrystallized during reworking in the m2

on the occurrence and boron isotopic composition of tourmaline in ultra high pressure metamorphic rocks abstract the extensive p t stability and the high chemical variability of tourmaline tur together with its common occurrence in metasediments proves its high potential for petrological and isotope geochemical studies on fluid rock interaction in subduction and collision related rocks this paper reviews the occurrence major element chemistry and boron isotopic composition of tur in high and ultrahigh pressure metamorphic uhpm rocks in addition it presents a new discovery of coesite bearing tur schorl from the erzgebirge germany as well as tur dravite related to the retrograde history of coesite and diamond bearing rocks from the erzgebirge and the kokchetav massif kazakhstan the scarce data on worldwide occurrences of u hpm tur reveal a high occupation of the crystallographic x site dominated by na and the possible presence of excess b with little further distinctiveness in its major element composition when compared with tur from medium grade rocks high k 2 o contents in tur are probably not related to uhp growth or equilibration the b isotopic composition of u hpm tur ranges in 11 b from 16 to 1 with many samples in or below the range of continental crust in contrast tur formed during retrograde fluid influx typically shows high 11 b values up to 28 suggesting heavy b fluids infiltrating the exhuming u hpm units coesite inclusions in tur characterized by raman spectroscopy are regarded as the best indicator for its uhp stability supplementary material analytical methods tourmaline compositions and boron isotope values are available at http www geolsoc org uk sup18354

soil properties along a hillslope modified by wind erosion in the ordos plateau semi arid china abstract spatial variations of soil properties at three topographic sites along a hillslope were studied in the ordos plateau of semi arid china soil properties at the three positions of the hillside acted as disconnected values soil particles 0 5 mm in diameter decreased downslope and those between 0 05 and 0 5 mm formed a larger fraction of the soil in the midslope than in the summit or footslope total mass of c n and p in the middle slope soil was the lowest among the soils in the three topographic positions abnormality of carbonate content and of mineralogical components of the soil along the hillslope suggests wind transportation from outside sources this result indicates that the site had undergone several cycles of rapid erosion and deposition

kinetics of the coesite to quartz transformation abstract the survival of coesite in ultrahigh pressure uhp rocks has important implications for the exhumation of subducted crustal rocks we have conducted experiments to study the mechanism and rate of the coesite quartz transformation using polycrystalline coesite aggregates fabricated by devitrifying silica glass cylinders containing 2850 h 10 6 si at 1000 c and 3 6 gpa for 24 h conditions were adjusted following synthesis to transform the samples at 700 1000 c at pressures 190 410 mpa below the quartz coesite equilibrium boundary reaction proceeds via grain boundary nucleation and interface controlled growth with characteristic reaction textures remarkably similar to those seen in natural uhp rocks we infer that the experimental reaction mechanism is identical to that in nature a prerequisite for reliable extrapolation of the rate data growth rates obtained by direct measurement differ by up to two orders of magnitude from those estimated by fitting a rate equation to the transformation time data fitting the rates to turnbull s equation for growth therefore yields two distinct sets of parameters with similar activation energies 242 or 269 kj mol but significantly different pre exponential constants extrapolation based on either set of growth rates suggests that coesite should not be preserved on geologic time scales if it reaches the quartz stability field at temperatures above 375 400 c the survival of coesite has previously been linked to its inclusion in strong phases such as garnet that can sustain a high internal pressure during decompression other factors that may play a crucial role in preservation are low fluid availability possibly even less than that of our nominally dry experiments and the development of transformation stress which inhibits nucleation and growth these issues are discussed in the context of our experiments as well as recent observations from natural rocks

influences of eolian and pedogenic processes on the origin and evolution of desert pavements well developed desert pavements are present above eolian deposits that mantle flows of the cima volcanic field located in the mojave desert california soil stratigraphic data and geochemical data demonstrate that eolian and pedogenic processes play major roles in the evolution of these pavements eolian dust 1 accelerates mechanical fragmentation of flow rock providing the source material for pavements and 2 accumulates slowly below basaltic colluvium in flow depressions eventually promoting development of cumulate soils below the evolving stone pavement an increase in dust flux during the holocene has raised ancient pleistocene pavements as much as 20 cm above the former land surface the results of our studies demonstrate for the first time that most desert pavements do not form by deflation by overland flow or by upward migration of stones through a slowly formed clayey argillic horizon desert pavements are born and maintained at the surface

timing and pt evolution of whiteschist metamorphism in the lufilian arc zambezi belt orogen zambia implications for the assembly of gondwana abstract one of the world s most extensive occurrences of whiteschists talc kyanite schists is located in south central africa with several exposures along a ca 700 km northwest southeast striking zone the metamorphic evolution and age relations of whiteschists and associated rocks from four localities three in the lufilian arc and one in the zambezi belt were investigated in the lufilian arc whiteschists associated garnet amphibolites and biotite kyanite garnet gneisses record peak metamorphic conditions of about documentclass aastex usepackage amsbsy usepackage amsfonts usepackage amssymb usepackage bm usepackage mathrsfs usepackage pifont usepackage stmaryrd usepackage textcomp usepackage portland xspace usepackage amsmath amsxtra usepackage ot2 ot1 fontenc newcommand cyr renewcommand rmdefault wncyr renewcommand sfdefault wncyss renewcommand encodingdefault ot2 normalfont selectfont declaretextfontcommand textcyr cyr pagestyle empty declaremathsizes

thermohydrological conditions and silica redistribution near high level nuclear wastes emplaced in saturated geological formations evaluation of the thermohydrological conditions near high level nuclear waste packages is needed for the design of the waste canister and for overall repository design and performance assessment most available studies in this area have assumed that the hydrologic properties of the host rock are not changed in response to the thermal mechanical or chemical effects caused by waste emplacement however the ramifications of this simplifying assumption have not been substantiated we have studied dissolution and precipitation of silica in liquid saturated hydrothermal flow systems including changes in formation porosity and permeability using numerical simulation we compare predictions of thermohydrological conditions with and without inclusion of silica redistribution effects two cases were studied namely a canister scale problem and a repository wide thermal convection problem and different pore models were employed for the permeable medium fractures with uniform or nonuniform cross sections we find that silica redistribution in water saturated conditions does not have a sizeable effect on host rock and canister temperatures pore pressures or flow velocities

formation dry out from co2 injection into saline aquifers 1 effects of solids precipitation and their mitigation f ormation d ry o ut from co 2 i njection into s aline a quifers p art 1 e ffects of s olids p recipitation and their m itigation karsten pruess 1 and nadja muller 2 earth sciences division lawrence berkeley national laboratory university of california berkeley ca 94720 shell international exploration and production b v exploratory research rijswijk the netherlands abstract injection of co 2 into saline aquifers may cause formation dry out and precipitation of salt near the injection well which may reduce formation porosity permeability and injectivity this paper uses numerical simulation to explore the role of different processes and parameters in the salt precipitation process and to examine injection strategies that could mitigate the effects the main physical mechanisms affecting the dry out and salt precipitation process include 1 displacement of brine away from the injection well by injected co 2 2 dissolution evaporation of brine into the flowing co 2 stream 3 upflow of co 2 due to gravity effects buoyancy 4 backflow of brine towards the injection point due to capillary pressure gradients that oppose the pressure gradient in the co 2 rich gas phase and 5 molecular diffusion of dissolved salt the different mechanisms operate on a range of spatial scales co 2 injection at constant rate into a homogeneous reservoir with uniform initial conditions is simulated in 1 d radial geometry to resolve multi scale processes by taking advantage of the similarity property i e the evolution of system conditions as a function of radial distance r and time t depends only on the similarity variable r 2 t simulations in 2 d vertical cross sections are used to examine the role of gravity effects we find that counterflow of co 2 and brine can greatly increase aqueous phase salinity and can promote substantial salt precipitation even in formations with low dissolved solids salt precipitation can accentuate effects of gravity override we find that injecting a slug of fresh water prior to commencement of co 2 injection can reduce salt precipitation and permeability loss near the injection well

eclogites in the makuti gneisses of zimbabwe implications for the tectonic evolution of the zambezi belt in southern africa the gneisses of the makuti group in north west zimbabwe are characterized by complex geometries that resulted from intense non coaxial deformation in a crustal scale high strain zone that accommodated extensional deformation along the axis of the zambezi belt at c 800 ma within low strain domains in the makuti gneisses undeformed metagabbroic lenses preserve eclogite and granulite facies assemblages which record a part of the metamorphic history that predates pan african events eclogitic rocks can be subdivided into 1 corona textured metagabbros that preserve igneous textures and 2 garnet omphacite rocks in which primary textures are destroyed the lenses of eclogitic rocks are enveloped in a mantle of garnet clinopyroxene hornblende gneiss which is a common rock type in the makuti gneisses the eclogites preserve multi staged domainal symplectic reaction textures that developed progressively as the rocks experienced loading followed by decompression heating in the metagabbros the original clinopyroxene plagioclase and olivine domains acted separately during the peak of metamorphism with plagioclase being replaced by garnet and kyanite and olivine being replaced by orthopyroxene and possibly omphacite the peak assemblage was overprinted by 1 the multi mineralic corona assemblage pargasite orthopyroxene spinel plagioclase replacing garnet kyanite clinopyroxene possibly at c 19 kbar 760 25 c 2 orthopyroxene pargasite plagioclase scapolite coronas replacing orthopyroxene 15 1 5 kbar 750 50 c and 3 moats of orthopyroxene plagioclase replacing garnet 10 1 kbar 760 50 c the garnet omphacite rocks record similar peak conditions 15 1 1 kbar 760 60 c garnet clinopyroxene hornblende plagioclase gneisses envelop the eclogites and record matrix conditions of 11 1 5 kbar at 730 50 c using assemblages that are oriented in the regional fabric these rocks are characterized by decompression heating textures reflecting temperature increases during exhumation of the makuti gneisses the eclogite facies rocks formed during a collisional event prior to 850 ma their formation could be related to a suture zone that developed along the axis of the zambezi belt during the formation of rodinia between 1400 and 850 ma the main deformation metamorphism in the makuti gneisses occurred around 800 ma and involved extension and exhumation of the high p rocks break up of rodinia which experienced a high t metamorphic overprint around 550 500 ma a collisional event associated with the formation of gondwana resulted in renewed burial and metamorphic recrystallization of the makuti gneisses

relict 1 4 ga oceanic crust in the zambezi valley northern zimbabwe evidence for mesoproterozoic supercontinental fragmentation the archean cratons of southern africa are separated from one another by younger orogenic belts it is widely believed that these deformation belts developed entirely within continental crust but in the zambezi belt of northern zimbabwe relict oceanic crust i e ophiolite has now been discovered the zircon age of a plagiogranite dike within the ophiolite 1393 22 ma makes this the oldest dated remnant of oceanic crust in africa presence of the ophiolite is direct evidence for the rifting and or assembly of continental fragments within africa the age of the ophiolite is consistent with its being a remnant of sea floor formed during the episode of continental fragmentation that immediately preceded the grenville orogeny the global event that marked the coalescence of the late proterozoic supercontinent of rodinia

contrasting styles of phanerozoic and precambrian continental collision abstract there are differences in the style of collisional orogens between the phanerozoic and the precambrian most notably the appearance of blueschists and ultrahigh pressure metamorphic uhpm rocks in the geological record since the late neoproterozoic whereas these rocks are absent from older orogens understanding collisional orogenesis in the context of present day values for ambient upper mantle temperature and radiogenic heat production provides a reference from which to extrapolate back to conditions in the precambrian to evaluate differences in the way phanerozoic and precambrian collisional orogens develop a series of experiments was run using a 2 d petrological thermomechanical numerical model in which the collision of spontaneously moving continental plates was simulated for values of ambient upper mantle temperature and radiogenic heat production increasing from those appropriate to the present day thus models of modern collisional orogens involving different modes of exhumation of uhpm rocks were extrapolated back to conditions appropriate for the precambrian based on these experiments an increase of the ambient upper mantle temperature to 80 100 k above the present day value leads to two distinct modes of collision that are different from the modern collision regime and for which the terms truncated hot collision regime strong mafic lower continental crust and two sided hot collision regime weak felsic lower continental crust are proposed some proterozoic orogens record post extension thickening to generate counter clockwise metamorphic p t paths followed by slow close to isobaric retrograde cooling such as occurred in the paleoproterozoic khondalite belt in the north china craton and the late mesoproterozoic early neoproterozoic eastern ghats province part of the eastern ghats belt of peninsular india these orogens have similarities with the truncated hot collision regime in the numerical models assuming subsequent shortening and thickening of the resulting hot lithosphere other proterozoic orogens are characterized by clockwise looping metamorphic p t paths and extensive granite magmatism derived from diverse crustal and subcontinental lithospheric mantle sources these orogens have similarities with the two sided hot collision regime in the numerical models both regimes are associated with shallow slab breakoff that precludes the formation of uhpm rocks the temperature of the ambient upper mantle where this transition in geodynamic regimes occurs corresponds broadly to the neoproterozoic era

diamond inclusions in garnets from metamorphic rocks a new environment for diamond formation diamonds commonly occur in kimberlites lamproites and alluvial sediments derived from these rocks more recently diamonds or their graphite pseudomorphs have been discovered in ultramafic massifs1 and picrites2 here we report the occurrence of diamonds in situ in crustal rocks highly retrograded high pressure metamorphic garnet pyroxene and pyroxene carbonate garnet rocks biotite gneisses and schists from the kokchetav massif northern kazakhstan ussr the diamonds are cubo octahedral averaging 12 m in size and occur in zircons and with euhedral graphite as inclusions in unzoned garnets we believe that the zircon and garnet matrices protected these diamonds from retrogressive transformation to graphite mica rutile titanite clinopyroxene kyanite and zircon also occur as inclusions in garnet often intergrown with the diamonds equilibration relations of inclusions and host garnets indicate that both diamonds and graphite crystallized from a fluid phase under static conditions at pressures of 40 kbar and temperatures 900 1 000 c

whiteschists and orthoamphibole cordierite rocks and the p t t path of the modum complex south norway abstract this paper describes the second known occurrence of whiteschist from norway whiteschists and orthoamphibole cordierite rocks which can both be described in the m f ash system are found in the modum complex together with albitites in contact zones between metagabbros and supracrustals they are formed due to extensive hydrothermal activity bulk rock and mineral chemistry define a bimodal fe mg distribution talc kyanite schists and fe poor orthoamphibole cordierite rocks form a leucocratic group and fe rich orthoamphibole cordierite rocks a melanocratic group the break down of an early orthoamphibole cordierite paragenesis to a later talc kyanite paragenesis indicates a cooling path at isobaric conditions or under increasing pressure the temperature pressure regime is of the order of 600 750 c and 6 8 kb reaction textures with talc albite kyanite forming pseudomorphs after gedrite indicate excess albite these nonisochemical reactions indicate the presence of an active na bearing fluid at deep levels in the crust

variation in soil chemical properties along toposequences in an arid region of the levant abstract detailed characterization of soils and their variation along different topography positions has not been investigated in depth for mediterranean arid regions there is a need to accurately understand the variation and the spatial distribution of soil properties within dry region of the levant such understanding is required to optimize the use and management of scarce land and water resources the objective of this study was to examine the effect of hillslope characteristics on the variation of selected soil chemical properties in an arid mediterranean climate at each of five selected transects four sites were chosen to represent four different topographic positions summit shoulder backslope and toeslope a soil profile was examined at each site and a representative sample from each horizon was withdrawn for chemical analyses the analyses indicated that generally the carbonate contents of the surface horizons decreased from higher to lower positions of the toposequence the carbonate content increased with depth for profiles occupying the lower positions this suggests more intense leaching within soil at lower positions the effect of steepness and curvature on controlling the variation of soil properties was obvious at the summit and shoulder positions leaching process seems to hinder the effect of steepness and curvature for soils at lower positions lower positions receive runoff water and organic matter from upper positions which complicate the relationship between landform shape and organic matter content continuous tillage resulted in lower organic matter contents for soils at lower positions higher ph values were reported for soils down along the transect due to the movement of soil material from upper hillslope positions electrical conductivity exchangeable calcium and potassium contents decreased downslope due to higher moisture accumulation however no obvious relationship was found between the variation of ph or ec in one hand and the variation of steepness and curvature on the other potassium content was variable due to its greater mobility the analyses indicated that variation in the soil cec is governed by two factors the leaching pattern which is controlled by hillslope position and the accumulation of eolian carbonates at the soil surface the distribution of iron oxides and types of clay minerals indicated more weathering in a descending direction and with soil depth which is attributed to higher availability of soil moisture along the same direction these relationships suggested systematic variation of chemical properties along toposequences in this arid environment

the pressure path of solid inclusions in minerals the retention of coesite inclusions during uplift abstract an inclusion model allows calculation of the stresses in and around minerals included in other minerals during changes in pressure and temperature the equations are applied to illustrate cooling and uplift histories of quarzo feldspathic rocks from 500 c and various pressures to ambient conditions even in the absence of pore fluid pressure microfractures may open at external pressures of 200 400 mpa and temperatures of 200 400 c due to differential volume changes of the constituent minerals coesite included in garnet cannot have formed during progressive metamorphism from quartz at lithostatic pressures below the coesite stability field because of differential volume changes coesite inclusions are captured by their host minerals at ultra high pressures and they persist to lower pressures because of the large volume increase occurring at the coesite to quartz transition the pi t path followed by the sio2 inclusion traces the boundary between the stability fields of coesite and quartz until radial fractures develop in the host at low external pressure

present day ultrahigh pressure conditions of coesite inclusions in zircon and garnet evidence from laser raman microspectroscopy coesite the dense high pressure polymorph of quartz occurs as inclusions in mechanically strong minerals in deeply subducted metamorphosed crustal rocks in a number of eurasian collisional orogens it is the primary indicator mineral of ultrahigh pressure p metamorphism whereas some coesite inclusions are untransformed most exhibit partial transformation to palisade quartz and a concomitant increase in volume resulting in rupture and radial fracturing of the host grain coesite can be identified by its diagnostic raman spectrum the strongest band at atmospheric pressure room temperature is at 521 cm 1 laser raman microspectroscopic analyses of coesite inclusions within garnet and zircon in ultrahigh p metamorphic rocks from kazakhstan indonesia and china reveal consistent differences in the raman spectra of 1 partially transformed coesite quartz main raman band at 521 cm 1 and 2 untransformed monomineralic coesite grains main band at 525 526 cm 1 applying the room temperature calibration of pressure dependence of the coesite raman spectrum we conclude that the latter coesite inclusions are subject to a remarkable pressure differential of 19 23 kbar with the host grains and are still undergoing pressure temperature conditions on or close to the quartz coesite equilibrium boundary

partial eclogitisation of gabbroic rocks in a late precambrian subduction zone zambia prograde metamorphism triggered by fluid infiltration gabbros and eclogites occur closely associated in a 200 km long and up to 40 km wide area of the zambezi belt in central zambia this area is interpreted to represent part of a late precambrian suture zone with the mafic rocks being relics of subducted oceanic crust gradual stages of prograde transformation from gabbro to eclogite are preserved by disequilibrium textures of incomplete reactions this resulted in kyanite omphacite bearing assemblages for eclogites that have al poor bulk compositions undeformed eclogites typically preserve features of a former gabbroic texture reflected by replacements of plagioclase and magmatic pyroxene by eclogite facies minerals textures of deformed eclogites range from sheared porphyroclastic to porphyroblastic relics of magmatic pyroxene are common and complete eclogitisation occurred only in millimetre to centimetre scale domains in most of the rocks no evidence for prograde blueschist or amphibolite facies mineral assemblages was found in eclogites in contrast the fine grained intergrowth of omphacite garnet kyanite and quartz which replace former plagioclase or was formed in the pressure shadow of magmatic pyroxene relics indicates that eclogitisation might have affected the gabbroic protoliths directly without any significant intervening metamorphic reactions eclogitisation took place under p t conditions of 630 690 degreesc and 26 28 kbar suggesting a large overstepping 10 kbar of reaction boundaries eclogitisation was initialised and accompanied by a channelised fluid flow resulting in veins with large subhedral grains of omphacite kyanite and garnet the gabbro to eclogite transformation was enhanced by a fluid which allowed the necessary material transport for the dissolution precipitation mechanism that characterises the metamorphic mineral replacements the process of eclogitisation was limited by reaction kinetics and dissolution precipitation rates rather than by the metamorphic p t conditions even though ductile deformation occurred and equilibrium phase boundaries were overstepped the infiltration of fluids was necessary for triggering the gabbro to eclogite transformation