complex concentric granitoid intrusions in the coastal mobile belt espirito santo brazil the santa angelica pluton an example the brazilian coastal mobile belt in southern espirito santo consists of archean to upper proterozoic units which have been metamorphosed and deformed to a gneiss migmatite and granulite facies rock assemblage during archean and proterozoic tectono thermal events

geology and petrogenesis of the edgecumbe volcanic field se alaska the interaction of basalt and sialic crust the edgecumbe volcanic field is a holocene volcanic province located on kruzof island se alaska exposed within the 260 km2 field are basalt andesitic basalt andesite dacite and rhyodacite the rhyodacites were erupted after the basalts and before the andesites the volcanics which are al rich 14 18 wt and lack an iron enrichment trend range from tholeiites 47 wt sio2 through rhyodacites 72 but a compositional gap of approximately 9 wt separates the dacites and rhyodacites initial 87sr 86sr ranges from 0 70297 in the basalts to 0 70440 in a pyroxene andesite 18o increases across the suite 5 8 to 7 9 plagioclase an32 86 is the dominant phenocryst in all but one lava olivine fo58 86 occurs in the basic lavas 53 wt sio2 but is replaced by orthopyroxene en43 73 and clinopyroxene en31wo41 en48wo40 in the more siliceous volcanics in the basalts and rhyodacites plagioclase is weakly zoned but extreme zoning 30 mole an is characteristic of phenocrysts in the intermediate lavas fractionation of the observed phenocryst assemblages could not have produced the more silicous volcanics instead they were generated by partial melting of intrusive basement 87sr 86sr 0 70487 18o 8 7 9 3 by basaltic magma and subsequent assimilation mass balance calculations show the rhyodacites are almost pure partial melt 5 basaltic component whereas the intermediate lavas contain between 30 and 60 partial melt

microgranitoid enclaves in the felsic looanga monzogranite new england batholith australia pressure quench cumulates abstract sparse microgranitoid enclaves mge in the leucocratic i type looanga monzogranite near bendemeer n s w australia range from microdiorite to micromonzogranite and all have fine to medium grainsize igneous microstructures the enclaves that vary from sio 2 53 to 69 wt are all less silicic than the host monzogranite 71 76 wt although compositionally diverse the enclaves and host monzogranite pluton share a common mineralogy of quartz oligoclase ferro edenitic hornblende iron rich mg 35 biotite fluor apatite and k feldspar except for the core of one double enclave the enclaves have the same 87 sr 86 sr initial ratio as the host pluton a characteristic of the enclaves is high mno mno mgo feo ratios with mno abundances of the more mafic enclaves up to 0 8 wt higher than any common magma the enclaves have a wide range of na 2 o k 2 o ratios 0 5 to 2 8 and in common with the host pluton contain hornblendes with na 2 o k 2 o ratios varying from 1 5 to 2 3 the hornblendes in two enclaves have lower na 2 o k 2 o ratios than their host enclave making it unlikely that the hornblende could have crystallised from a melt of the same composition as these enclaves chemically and mineralogically the more mafic enclaves have characteristics expected of cumulates formed from a magma of similar composition as the host pluton in that they contain the same minerals but are enriched in the near liquidus phases hornblende plagioclase and biotite and depleted in the near solidus phases quartz and k feldspar except for some minor replacement of pyroxene by hornblende the minerals do not show microstructural evidence of being made over from other minerals it is argued that the mineral chemistry of these enclaves is also a primary feature rather than the result of mineralogical equilibration with the host monzogranite magma the two most felsic enclaves are medium grained monzogranites sio 2 68 and 70 wt and are considered to be compositionally little different from the magmas from which they crystallised these two exhibit hydrothermal alteration and are considered to be fragments of an earlier roof phase of the intrusion the quench microstructure and cumulate chemistry of the more mafic enclaves are argued to result from p h 2 o reduction events within the upper parts of the magma chamber due to roof fracture brought on by the pressure increase imparted to the magma by the fluid release from a water saturated magma the sudden reduction in fluid pressure results in an increase in the liquidus and solidus temperatures of the water saturated magma and this rather than a drop in temperature produces quench conditions the reduction in p h 2 o shifts the cotectic compositions in the q ab or system closer to k feldspar and quartz and this and the heat of crystallisation restrict the amount of these two near solidus minerals that crystallise the enclaves form as crystal cluster cumulates around minerals already in the magma and or any other solid substrate available including the magma chamber roof this pressure quench cumulate mechanism explains why the looanga enclaves are mineralogically similar to the host granite and we suggest that this process may be more widely applicable to enclaves in granitic rocks

sediment yields and sources in a welsh upland lake catchment during the past 800 years estimates of suspended sediment yield over the past 800 years have been derived from a whole basin study of lake sediments in llyn geirionydd north wales magnetic and physical sediment properties were used to correlate a suite of cores to a master chronology based on 210pb and 14c dates the sources of sediment within the catchment were identified by comparing the magnetic and heavy metal properties of sediments soils and stream sediments mean suspended sediment yields range from 6 to 18 t km 2 yr 1 with high yields in the periods 1765 1830 and 1903 1985 corresponding to mining activities in the catchment the impact of earlier deforestation agricultural expansion or climatic change on sediment yields is low although there is evidence that agricultural activities increased levels of peak stream discharge afforestation in the 20th century does not appear to have significantly increased sediment yield sediment sources have remained fairly constant through time they appear to be widespread and dominated by stream channel sides rather than point sources except during the mining phases when spoil material has dominated the sediment load sediment loads to the lake today are still dominated by sediment derived from unvegetated spoil heaps

the magnesium isotope δ26mg signature of dolomites abstract dolomite precipitation models and kinetics are debated and complicated due to the complex and temporally fluctuating fluid chemistry and different diagenetic environments using well established isotope systems 18 o 13 c 87 sr 86 sr fluid inclusions and elemental data as well as a detailed sedimentological and petrographic data set we established the precipitation environment and subsequent diagenetic pathways of a series of proterozoic to pleistocene syn depositional marine evaporative sabkha dolomites syn depositional non marine evaporative lacustrine and palustrine dolomites altered marine mixing zone dolomites and late diagenetic hydrothermal dolomites these data form the prerequisite for a systematic investigation of dolomite magnesium isotope ratios 26 mg dol dolomite 26 mg ratios documented here range from 2 49 to 0 45 26 mg mean 1 75 1 08 n 42 the isotopically most depleted end member is represented by earliest diagenetic marine evaporative sabkha dolomites 2 11 0 54 2 n 14 in comparing ancient compositions to modern ones some of the variation is probably due to alteration altered marine 1 41 0 64 2 n 4 and earliest diagenetic lacustrine and palustrine dolomites 1 25 0 86 2 n 14 are less negative than sabkha dolomites but not distinct in composition various hydrothermal dolomites are characterized by a comparatively wide range of 26 mg ratios with values of 1 44 1 33 2 n 10 by using fluid inclusion data and clumped isotope thermometry 47 to represent temperature of precipitation for hydrothermal dolomites there is no correlation between fluid temperature 100 to 180 c and dolomite mg isotope signature r 2 0 14 nor is there a correlation between 26 mg dol and 18 o dol magnesium isotope values of different dolomite types are affected by a complex array of different mg sources and sinks dissolution precipitation and non equilibrium fractionation processes and overprinted during diagenetic resetting further progress on the use of 26 mg dol as a proxy will require new theoretical and experimental data for 26 mg fluid dol that includes dehydration effects of the free mg aquo ion versus fluid temperature in ancient diagenetic systems complex variables must be considered these include fluid chemistry and physical properties mg sources and sinks temporal changes during precipitation and post precipitation processes including open and closed system geochemical exchange with ambient fluids all of these factors complicate the application of 26 mg dol as proxy for their depositional or diagenetic environments nevertheless the data shown here also indicate that 26 mg dol can in principle be interpreted within a detailed framework of understanding

proterozoic anorthosite granite nain batholith structure and intrusion processes in an active lithosphere scale fault zone northern labrador we present field evidence of the structure and evolution of a typical proterozoic massif type anorthosite the anorthosite forms a batholith composed of numerous plutons and ring complexes intruded episodically in two main sequences each producing anorthositic rocks and monzonite ferrodiorite followed by granite the batholith is located where a lithosphere scale fault zone 50 km wide and extending over 1000 km from greenland to labrador intersected an older continental suture intermittent episodes of mainly sinistral transtension along this gardar voisey s bay fault zone were associated with the generation of a number of pulses of magmatism and controlled the rise of magmas to middle and upper crustal levels between 1363 and 1289 ma both the large and small scale structures and magma intrusion processes are similar to those of many cordilleran batholiths with tabular plutons associated with ring dykes emplaced by cauldron subsidence and basin shaped plutons associated with cone sheets these

mineralogical and isotopic evidence for phenocryst matrix disequilibrium in the garner mountain andesite petrographic trace element and isotopic evidence demonstrates that magma mixing preceded the eruption of the garner mountain andesite the flow contains reversely zoned plagioclase phenocrysts and amphibole pseudomorphs composed of plagioclase clinopyroxene orthopyroxene and opaque oxides partially resorbed quartz grains are also present in contrast to the isotopically uniform matrix plagioclase phenocrysts have 87sr 86sr ratios that correlate negatively with matrix sr and positively with matrix rb abundances these observations demonstrate increasing isotopic disequilibrium between the plagioclase and matrix in the more evolved varieties of the flow

influence of sediment source and type on the magnetic susceptibility of fiord and shelf deposits baffin island and baffin bay n w t volume and mass magnetic susceptibility are measured on recent and late quaternary sediments from the fiords and shelf of eastern baffin island baffin bay and the northern labrador sea major changes in hinterland bedrock geology are associated with order of magnitude changes in magnetic susceptibility the fiords that head into the proterozoic foxe fold belt have mass magnetic susceptibilities ms of 2 4 10 7 m3 kg compared with 25 60 10 7 m3 kg in sediments derived from archean gneisses and granites another low ms region is the middle reach of frobisher bay ms studies in cambridge mcbeth and itirbilung fiords included analyses of surface samples and samples from lehigh 3 m and piston 11 m cores lehigh cores had higher ms values on average than the piston cores suggesting a coarsening in texture or an increase in the flux of magnetic minerals during the last 0 3 3 ka an analysis of the ms logs of 17 piston cores indicated that six broad patterns of ms can be recognized based on a

oxygen isotope exchange in a two layer model of oceanic crust abstract a numerical model is presented for the transport and exchange of oxygen isotopes in an idealized two layer oceanic crust basalt carbonate that is saturated with a non advecting pore fluid open to the overlying ocean the model allows for time varying sedimentation rate porosity chemical diffusivity and temperature dependent 18o fractionation factors reaction is modeled as solution precipitation of calcite in the carbonate sediment and as modified solution precipitation basalt dissolves smectite precipitates in the basalt layer generic models using input parameters that are typical of those measured in dsdp odp cores and constant sedimentation rates illustrate the behavior of the system sufficiently well to allow first order effects to be discerned that have important implications for paleotemperature and ocean floor weathering studies the solution precipitation reactions allow oxygen to be exchanged between the low 18o basalt layer and the high 18o carbonate sediment through the medium of the pore fluids this results in 18o increasing upwards in the basalt and decreasing downwards in the sediment pore fluid 18o values decrease with depth from zero to values of 1 to 3 at the basalt sediment interface and to values of 7 to 15 within the basalt because reaction rates are slow relative to diffusion in the pore fluid high frequency 18o variations in the solid are preserved indefinitely but the amplitude of the variations decreases as e rt where r is the reaction rate fraction reacted per unit time the results suggest that the diagenetic shifts of the 18o values of benthic and high latitude planktonic foraminifera samples younger than oligocene age are typically negligible for samples older than 50 ma diagenetic effects are typically significant and generally cause estimates of bottom and high latitude surface ocean temperatures in the cretaceous and early tertiary to be too high on the other hand the model suggests that diagenesis will shift the 18o values of low latitude planktonic foraminifera to higher values and that paleotemperature estimates of low latitude surface waters particularly in the eocene and oligocene may be too low using a synthetic 18o record for dsdp site 167 we demonstrate how numerical models can be used to correct specific measured records for diagenesis this approach could provide a means of extracting paleotemperature information from heavily altered late cretaceous and older samples and for quantifying the subtle effects of diagenesis on the 18o record at all sites

hydrogeochemistry and fractionation pathways of mg isotopes in a continental weathering system lessons from field experiments abstract the potential of magnesium isotope records from cave carbonate archives speleothems has been documented but remains underexplored this is due to the limited knowledge regarding the complex suite of physico chemical and biological disequilibrium fractionation processes affecting meteoric fluids in the soil zone the carbonate hostrock and calcite precipitation in the cave this study presents 26 mg data from a monitored cave in germany bunker cave including rain water 26 mg 0 70 0 14 soil water 26 mg 0 51 0 10 and drip waters 26 mg 1 65 0 08 sampled between november 2009 and may 2011 field precipitation experiments i e calcite precipitated on watch glasses 26 mg 3 56 0 26 may 2006 to june 2010 were found to be of limited use this is because of experimental crystallographic and sampling artefacts conversely variations in soil and drip water 26 mg over time are predominantly related to seasonal variations in water availability and air temperature affecting the subtle weathering ratio between mg bearing clay minerals in the soil here mainly chlorite and montmorillonite and the low mg calcite hostrock bunker cave 26 mg drip water values display a significant dependency on the air temperature outside the cave this is because air temperature influences co 2 levels in the soil and hence rock water interaction for fast drip sites the direct correlation of 26 mg soil water and 26 mg drip water documents a relative short residence time of the fluid in the carbonate aquifer and thus limited isotope equilibration and mixing of different reservoirs this result is encouraging and adds new evidence to the poorly understood hydro geochemistry of carbonate aquifers slow seepage flow drip sites display an annual 26 mg drip water pattern that is geochemically unrelated to that of the soil water further research including laboratory experiments must focus on the complex fractionation between drip water and speleothem calcite mg isotope record

mafic replenishments into floored silicic magma chambers commingling between contemporaneous mafic and felsic magmas is now widely recognized in a broad range of intrusions and intrusive complexes these interactions are important features for two main of reasons 1 the rapidly chilled margins of mafic magma against silicic magma commonly preserve the compositions of mafic liquids and 2 because the mafic magma solidifies rapidly the resulting final configurations of mafic and felsic magmas can provide insights into physical processes and changing viscosity contrasts and rheologies of magmas and felsic crystal mush during crystallization of the mafic magma mingling of contrasted magmas was first recognized in the 1950s wider recognition of interactions between mafic and silicic liquids led to concepts of net veining in the 1960s intramagmatic flows chilled basaltic layers separated by felsic cumulates in the 1970s and in the 1990s to mafic silicic layered intrusions masli which could be as much as a few kilometers thick and more than 100 km2 in area it was quickly appreciated that these masli preserved stratigraphic records of mafic replenishments into silicic magma chambers floored by felsic crystal mush volcanic studies had anticipated the occurrence of this last type of intrusion on the grounds that extensive ponding of basaltic magmas beneath silicic chambers was seen to be essential to keep large silicic systems like yellowstone active for millions of years this paper looks at the history of changing perceptions and interpretations of magma mingling and whether or not sill complexes are distinct from mafic silicic layered intrusions the stratigraphy of mafic silicic layered intrusions records changing magmatic compositions events and processes in a temporal framework comparable to that provided by coeval volcanic rocks as a result careful study of masli has great potential for linking plutonic and volcanic processes and events

terminology for layered intrusions the terminology of igneous cumulates was originally formulated on the concept that the rocks were concentrations of minerals accumulated by crystal settling it is now well established for several major intrusions that plagioclase that had been typed as cumulus should in fact have floated in its parental magmatic liquid nevertheless the terminology has been valuable in the classification of the rocks of these and other intrusions and it has been useful in certain aspects of their genetic interpretation the name cumulate is therefore redefined so that crystal settling is a possible but not essential process in the origins of the rocks to which it is applied the overall nomenclature of layered intrusions is then reviewed and definitions of names are refined and systematized to clarify their applications careful distinction is made between descriptive and genetic names a cumulate is defined as an igneous rock characterized by a framework of touching mineral crystals and grains that evidently were concentrated through fractional crystallization of their parental magmatic liquids a layer is a sheetlike unit of cumulate that is a distinctive entity in its compositional and or textural features a lamina is a thin 2 5 cm sharply defined layer igneous lamination is a general name for planar structure that is pervasive through cumulates on the scale of their grain size layering is the overall structure and fabric manifest through combinations of layers laminae and lamination the difference between layers and layering is essentially the difference between individuals and a population and the terms that describe one are not necessarily appropriate to the description of the other but the fundamental variants of all of layering lamination grading in layers and layer contacts can essentially be described by one or more of the four modifiers grain size modal textural and cryptic where the last refers to cumulus miner al compositions a system for subdividing layered intrusions is proposed in which distinction is made between general divisions ranked as series zones and subzones and prominent units that are called groups members rhythmic units and cyclic units applications to the skaergaard bushveld muskox stillwater and other intrusions are briefly described

assessing the biomineralization processes in the shell layers of modern brachiopods from oxygen isotopic composition and elemental ratios implications for their use as paleoenvironmental proxies abstract fossil brachiopod shells are often used as valuable archives to reconstruct paleoenvironmental conditions in deep time however biomineralization processes can impact their fidelity as geochemical proxies brachiopod shells comprise an outer primary layer a secondary fibrous layer and sometimes a tertiary columnar layer therefore it is essential to assess the potential effects of the biomineralization processes in each of the different shell microstructures of modern brachiopods this study analyses the oxygen isotopic composition together with li ca na ca mg ca and sr ca data at high spatial 20 50 m resolution in seven modern brachiopod species focusing on differences between the primary secondary and tertiary layers in all studied species 18o values of the outer primary layer are consistently out of equilibrium with seawater also this shell layer is enriched in li na mg and sr contrary to the primary layer the innermost secondary layer is near or at oxygen isotopic and elemental equilibrium with ambient seawater the columnar tertiary shell layer if present has the least variable and the heaviest oxygen isotopic composition within the range of equilibrium values with seawater this tertiary layer however is depleted in minor and trace elements relative to the other shell layers thus the tertiary layer is more suitable for oxygen isotopic studies whereas the innermost secondary layer of the most mature parts of the shell is the best target in two layered shells while we do not observe any clear interspecific relationships between mg ca and sr ca ratios on one hand and environmental parameters such as temperature salinity and ph on the other hand there is a positive interspecific relationship between na ca and salinity and a negative interspecific relationship between li ca and temperature suggesting their potential use as proxies of physicochemical parameters of seawater

magnetic and chemical parameters of andic soils and their relation to selected pedogenesis factors in the case of andic soils developed on volcanic rocks permanently humid conditions are essential for andosolisation processes although volcanic rocks are usually rich in highly magnetic iron oxides knowledge about these minerals in andic soils with respect to the soil development is missing the aim of this contribution is to investigate the relationship between magnetic and basic chemical properties of soils developed on volcanic basement and parameters related to the pedogenesis factors the studied sites are located on basaltic lava flows in the eastern part of the french massif central investigated pedons with alu andic sil andic and vitric properties were pedologically described and analysed by a set of magnetic and soil chemical methods magnetic parameters include low field volume specific and mass specific magnetic susceptibility frequency dependent magnetic susceptibility remanent properties including s ratio and coercivities chemical parameters included the phh2o soil organic matter total cation exchange capacity base saturation and content of fe and al extracted in dithionite citrate acid ammonium oxalate and pyrophosphate solutions micro morphology of andosols and parent material was observed by scanning electron microscopy completed with energy dispersive spectrometry the pedogenesis factors are represented by parent rock age annual precipitation and soil thickness our findings suggest that 1 precipitation is the most important pedogenesis factor with a strong relationship to the relative magnetic grain size concentration of ferrimagnetic minerals aluminium dissolved in pyrophosphate and acid ammonium oxalate and content of organic matter 2 parent material age shows a well pronounced relationship with magnetic grain size parameters represented by the frequency dependent susceptibility and ratio of saturation remanent to saturation induced magnetisation mrs ms selective dissolution parameters and phh2o 3 thickness of soil profile shows a link to mrs ms phh2o and content of organic matter

lake catchment based studies of erosion and denudation in the merevale catchment warwickshire u k sediment yields estimated from contemporary stream monitoring of suspended sediment in the merevale forested catchment north warwickshire were compared with the sediment yield record obtained from analysis of lake sediment in the downstream reservoir total sediment volume and mass for nine periods since 1861 were calculated by extrapolation of synchronous levels in 54 cores identified from magnetic susceptibility and other magnetic measurements and by using 210pb and 137cs analysis to provide an absolute sediment chronology sediment yield estimated from the two methods show comparable but low 50 200 kg ha 1 yr 1 levels of sediment loss while suggesting that lake sediments can be a useful means for extending the period over which contemporary monitoring exists the study stresses the need for detailed historical records of land use and climate against which long term sediment yield records can be evaluated

the effect of permafrost vegetation and lithology on mg and si isotope composition of the yenisey river and its tributaries at the end of the spring flood abstract this work focuses on the behavior of the stable mg and si isotope compositions of the largest arctic river the yenisey river and 28 of its major and minor tributaries during the spring flood period samples were collected along a 1500 km latitudinal profile covering a wide range of permafrost lithology and vegetation despite significant contrasts in the main physico geographical climate and lithological parameters of the watersheds the isotope composition of both dissolved mg and si was found to be only weakly influenced by the degree of the permafrost coverage type of vegetation forest vs tundra and lithology granites basalts carbonates or terrigenous rocks this observation is generally consistent with the lack of chemical uptake of mg and si by soil mineral formation and vegetation during the early spring the radiogenic sr isotope composition of the yenisey and its tributaries varied within a narrow range 0 708 87 sr 86 sr 0 711 reflecting the dominance of phanerozoic rock weathering and or atmospheric deposition on these compositions the mg and si isotopic compositions of riverine samples reflect two main processes with distinct isotopic signatures first isotopically heavier mg 26 mg 1 0 0 2 and isotopically lighter si 30 si 1 0 0 25 are added to the waters by river suspended matter dissolution and leaching from vegetation biomass topsoil litter second isotopically lighter mg 26 mg 1 5 to 1 75 and isotopically heavier si 30 si 1 75 2 0 are delivered to the yenisey s tributaries from deep underground water feeding the rivers via taliks this lighter mg and heavier si isotopic composition is interpreted to originate from precambrian dolomite dissolution and aluminosilicate dissolution coupled with authigenic mineral precipitation respectively in deep underground water reservoirs taking account of the isotopic composition evolution over the course of the year established earlier on mono lithological watersheds of the yenisey basin the average annual isotopic signatures of the yenisey river arriving to the arctic ocean are estimated to be 26 mg 1 58 0 30 and 30 si 1 60 0 25 as the yenisey is the largest river feeding the arctic ocean and as it samples a large variety of environments and lithologies these values may be reasonable estimates for the average mg and si isotopic composition of the dissolved riverine flux to the arctic ocean

land use history and sediment flux in a lowland lake catchment groby pool leicestershire uk whole basin sediment records have been used to investigate the impact of land use change and catchment processes on sediment flux to groby pool leicestershire uk over the past c 200 years estimates of sediment yields for the period ad 1945 1989 were calculated using dry mineral mass data from multiple cores up core changes in loss on ignition values were used to correlate cores and these were tied to a master chronology based on 2 10 pb and 137 cs profiles sediment sources within the catchment were identified by comparing the magnetic properties of lake sediment soil and stream samples in addition documentary records were used to establish the pattern of land use change and other activities which might affect material flux to the lake a lakewide change in sediment type is recorded after parliamentary enclosure in 1789 as the focus of arable activity shifted from outside to inside the lake s drainage catchment trends in lake sediment mineral magnetic properties also indicate a slight shift in the source of allochthonous inputs from subsoil to topsoil derived material at that time recent changes in mineral magnetics are thought to reflect the changing supply of subsoil flux resulting from modifications to inlet streams coupled with increased inputs of atmospheric pollutants sediment yield estimates of is t km 2 yr 1 obtained for the period 1945 1989 are similar to other lowland uk sites with similar lake catchment ratios activities such as quarrying road construction and agricultural intensitication appear to have had little or no impact on sediment yields to the lake in recent times this is thought to be due to the interplay between catchment and instream storage a low drainage density and the buffering effect of riparian vegetation

fractional crystallization and magma mixing in the tigalak layered intrusion the nain anorthosite complex labrador the tigalak intrusion is a dominantly dioritic layered body about 80 km2 in area which ranges in composition from norite to granodiorite local areas of the layered rocks display upward fractionation from norite to ferrodiorite periodic reversals of mineral composition trends record the emplacement of less fractionated dioritic magma heterogeneous mixtures of dioritic and granodioritic rocks occur widely in mappable lenses and layers that alternate up section and along the strike with more uniformly layered rocks in these mixtures chilled dioritic pillows occur abundantly in a hybrid cumulate matrix of granodiorite to diorite composition cross cutting granodioritic dikes grade upward into stratigraphically bound lensoid masses of the hybrid cumulates it appears that the hybrid rocks formed as a result of the emplacement of the granodioritic magma through lower cumulates into the dioritic magma chamber and that the dioritic pillows represent chilled bodies of ferich dioritic magma that commingled with cooler granodioritic magma and settled to the floor of the tigalak magma chamber

climate change and human occupations in the lake daihai basin north central china over the last 4500 years a geo archeological perspective abstract high resolution climate variations since the last 4500 years in the monsoonal arid transition zone of north central china were revealed through the integration of proxies from sediment cores in the lake daihai basin human occupations in the lake basin deduced from archeological findings and historical literatures were then incorporated into the climate sequence to demonstrate the patterns of human responses to the climate changes and the recent anthropogenic effects it indicated that 1 climate dominated human environment adaptations prevailed prior to 2700 cal yr bp an amicable climate setting before 4100 cal yr bp would facilitate the growth of the laohushan culture lc in the lake basin while a pronounced deterioration of water thermal condition after that had led to human exodus and the collapse of the lc the reduced human activity in the lake basin indicated at 3800 3500 cal yr bp and a subsequent cultural blank at 3500 2700 cal yr bp were both in response to the climate and lake level fluctuations during 3800 2800 cal yr bp 2 transition to a positive human adaptation was seen at 2700 1100 cal yr bp represented by the exploitation of arable land for cultivation and animal husbandry as the lake contracted 3 an increasing human presence that affected environmental processes became more severe over the last 1100 cal yr bp this was basically due to the ongoing lake shore reclamation for cropping and more recently heavy metals emissions from fossil fuel combustion and local industries

magnetic studies of erosion in a scottish lake catchment 1 core chronology and correlation1 magnetic susceptibility opb 137cs 14c and paleomagnetic secular variation are used to establish the chronology of sedimentation in a set of cores from loch frisa in western scotland the 14c dates obtained are not compatible with the chronology derived from all the other techniques and this is ascribed to inwash of old particulate carbon from the watershed central cores show little evidence of changes in sedimentation rate over the last 150 years whereas marginal cores contain evidence of major increases arising from ploughing and from drainage associated with catchment afforestation since 1935 the results illustrate the value of a multiple core approach to sedimentation and erosion studies even where complex lake morphometry precludes calculation of sediment budgets

magnesium isotopes in permafrost dominated central siberian larch forest watersheds abstract to unravel the mg isotope fractionation pathways within the continuous permafrost zone in the larch deciduous forest of central siberia we measured the mg isotopic composition of two large siberian rivers nizhnaya tunguska and kochechum which flow into the yenisey a small forested stream and the major fluid and solid sources of mg in the watershed atmospheric precipitates surface suprapermafrost flow interstitial soil solutions plant biomass litter and mineral soils the obtained results indicate a significant seasonal variation in riverine water mg isotope signatures during the winter baseflow the mg isotope composition of large rivers is significantly lighter than the source basaltic rocks and the atmospheric depositions these differences support the presence of fluids enriched in lighter mg isotopes such as those affected by the mineral precipitation of secondary silicates or fluids that dissolve sedimentary carbonate rocks during the spring flood and in the summer and fall seasons the river fluid 26 mg values increased by 0 2 0 3 and approached the mg isotope composition of the ground vegetation dwarf shrubs mosses and the soil organic horizon overall the riverine waters were 0 3 0 7 lighter than the unaltered bedrock and the deep minerals soil horizons the mg isotopic compositions of larix gmelinii organs i e stem wood roots and needles exhibit a low variability however an enrichment of 0 2 0 3 in the 26 mg of larch needles in the course of the growing season from june to september can be observed this enrichment most likely demonstrates uptake of isotopically heavier mg by the plant in addition to the progressive thawing of the mineral soil deepening of the active layer of the soil overall the mg isotope approach indicates the important contribution of vegetation larch needles mosses and dwarf shrubs to the riverine mg isotope signature and helps to reveal the contribution of isotopically light carbonate rocks in the large rivers of the central siberian plateau

a 13 000 year record of environmental magnetic variations in the lake and peat deposits from the chandra valley lahaul implications to holocene monsoonal variability in the nw himalaya abstract an 13 000 year old record of in situ peat bog and lacustrine sediments from a post glacial lake in the chandra valley of the lahaul himalaya is studied to generate the record of environmental magnetic signatures in response to indian summer monsoon ism variability in the nw himalaya the chronology of studied chandra peat trench cpt profile is based on 9 ams 14c calibrated dates cal yr bp the piece wise linear regression analysis of age depth model indicates non linear sedimentation with higher rates from 4142 cal yr bp to the present and lower accumulation during 12 880 4142 cal yr bp the mineral magnetism showed three dominant assemblages of magnetic minerals with varying concentrations in peat lake sediment sequence i e 1 mixed ferri and antiferromagnetic minerals between 12 880 and 11 019 cal yr bp 2 dominant antiferromagnetic mineralogy from 11 019 to 3172 cal yr bp and 3 significantly increased concentrations of mixed ferri and antiferromagnetic minerals from 3172 cal yr bp to the present with characteristically increased ferrimagnetic concentrations after 2032 cal yr bp the characteristic increase in antiferromagnetic mineral concentration accompanied by decreased ferrimagnetic concentration from 10 398 to 5770 cal yr bp suggest possible oxidation of magnetite to hematite under warmer climate conditions and corresponds to increased ism intensity during early to mid holocene warm wet climate in the nw himalaya the distinctly increased concentration of antiferromagnetic minerals from 6732 to 5770 cal yr bp indicates mid holocene climate optimum hco in the lahaul himalaya the strengthening and weakening of ism recorded in environmental magnetic signatures during 1260 852 cal yr bp 690 1098 ad and between 852 and 239 cal yr bp 1098 1711 ad corresponds to medieval warm period mwp and little ice age lia events respectively

environmental magnetism principles and applications 1 in environmental magnetism rock and mineral magnetic techniques are used to investigate the formation transportation deposition and postdepositional alterations of magnetic minerals under the influences of a wide range of environmental processes all materials respond in some way to an applied magnetic field and iron bearing minerals are sensitive to a range of environmental processes which makes magnetic measurements extremely useful for detecting signals associated with environmental processes environmental magnetism has grown considerably since the mid 1970s and now contributes to research in the geosciences and in branches of physics chemistry and biology and environmental science including research on climate change pollution iron biomineralization and depositional and diagenetic processes in sediments to name a few applications magnetic parameters are used to routinely scan sediments but interpretation is often difficult and requires understanding of the underlying physics and chemistry thorough examination of magnetic properties and of the environmental processes that give rise to the measured magnetic signal is needed to avoid ambiguities complexities and limitations to interpretations in this review we evaluate environmental magnetic parameters based on theory and empirical results we describe how ambiguities can be resolved by use of combined techniques and demonstrate the power of environmental magnetism in enabling quantitative environmental interpretations we also review recent developments that demonstrate the mutual benefit of environmental magnetism from close collaborations with biology chemistry and physics finally we discuss directions in which environmental magnetism is likely to develop in the future

recent sediment flux and erosional processes in a welsh upland lake catchment based on magnetic susceptibility measurements abstract single sample magnetic susceptibility measurements are used to correlate synchronous levels in 16 cores of dated 210 pb 137 cs recent sediment taken from the deep and oligotrophic llyn peris n wales in order to provide a basis for calculating total sediment and chemical influx through time results show that sediment influx has steadily increased since c 1750 a d to reach peak levels in the period 1966 1976 a d equivalent to a rise in erosion in the catchment from c 5 t km 2 yr 1 to c 42 t km 2 yr 1 comparison of down core susceptibility fluctuations with sediment pollen and organic pigment data indicates that maximum susceptibility values relate to periods of channel erosion and minimum susceptibility values relate to periods of slate debris inwash from spoil tips in the catchment a study of historical records reveals that pre twentieth century peak levels of erosion were due to the effects of extractive industries while twentieth century erosion has been caused by overgrazing increased trampling pressure and heavy construction works in the catchment

magnetism of the huguangyan maar lake sediments southeast china and its paleoenvironmental implications abstract the huguangyan hgy lake a typical maar lake in the southeast of china is sensitive to the evolution of the asian monsoon system including the east asian summer monsoon easm and the indian summer monsoon ism so far the mechanism of how magnetic properties of the hgy lake sediments respond to climate changes has not been well determined in this study we performed a detailed mineral magnetic analysis on the high resolution hgy sediments results revealed that finer grained magnetite superparamagnetic and single domain magnetite mixed with a small amount of goethite and hematite is the dominant magnetic mineral which is consistent with the magnetic properties of the red weathered soils collected from the catchments but differs greatly from that of the aeolian inputs therefore magnetic minerals of the hgy sediments were originated mainly from the lake catchment rather than the wind blown dust inputs considering the high tephra wall relative elevation of 10 20 m and great precipitation surface runoff by rainfall is interpreted as the main process to transport red soils into the lake the magnetic records of hgy sediments revealed that the summer monsoon was enhanced at the early holocene and then declined abruptly at the mid late holocene 5 5 kyr cal bp this pattern strongly mimics the evolution pattern of ism but is inconsistent with the easm thus we suggest that the magnetic records from hgy have the great potential to investigate the evolution of ism

theoretical calculation of equilibrium mg isotope fractionations between minerals and aqueous solutions abstract there are large discrepancies existing in equilibrium mg isotope fractionation calculations and experimental investigations for cases related to mineral vs solution to clarify this confusing issue a newly designed cluster model based quantum chemistry method i e volume variable cluster model vvcm is used to provide equilibrium mg isotope fractionation factors between mg bearing carbonates calcite aragonite dolomite and magnesite amorphous calcium carbonates accs brucite and aqueous species i e mg2 aq mghco3 aq and mgoh aq we find that local configuration sampling of aqueous species is essential to provide precise fractionations between mineral and solution the phonon based periodic boundary method is also used for several minerals and it obtains very similar fractionations with vvcm results our results are very close to those of pinilla et al 2015 although via completely different approaches both of them have included the effect of local configuration disorder however both of them are significantly different from some of experimental results for cases of carbonates vs solutions the existence of various mg bearing species in fluids of experiments the direct incorporation of hydrated mg2 into the solids the mg2 concentration effect and the existence of intermediate precursors e g accs are several possible causes for the mismatches relative to coexisting aqueous mg2 we find that accs will enrich heavy mg isotopes i e 1 45 at 25 c agreeing with previous experimental estimation equilibrium mg isotope fractionation factors between brucite and solutions are also predicted besides we applied vvcm to predict the mg isotope fractionations between high temperature phases i e forsterite diopside enstatite tremolite and spinel the predicted factors are in the order of spinel tremolite diopside enstatite forsterite this study provides a base for understanding the accumulating mg isotope data

historic and contemporary sediment transfer in an upland pennine catchment uk a sediment budget for an upland catchment reservoir system at burnhope reservoir north pennines uk has been developed this provides a framework for quantifying historic and contemporary sediment yields and drainage basin response to disturbance from climate change and human activities in the recent past bathymetric survey core sampling 137cs dating and aerial photographs have been used to assess sediment accumulation in the reservoir the average reservoir sedimentation rate is 1 24 cm yr 1 annual sediment yield 33 3 t km 2 yr 1 10 trap efficiency 92 mean annual reservoir sedimentation over the 67 year period has been estimated at 592 t 10 inputs of suspended sediment from direct catchwater streams account for 54 of sediment supply to the budget best estimate yield of 318 t yr 1 129 while those from actively eroding reservoir shorelines contribute 328 t yr 1 92 sediment yield estimates from stream monitoring and reservoir sedimentation are an order of magnitude lower than those reported from south pennine reservoirs of comparable drainage basin area analysis of historical rainfall series for the catchment shows fluctuations in winter and summer rainfall patterns over the past 62 years from 1976 to 1998 there has been a diverging trend between winter and summer rainfall with a large increase in winter and a gradual decrease in summer totals periods of maximum variation occur during the summer drought events of the late 1970s early 1980s and mid 1990s analysis of the particle size of core sediments highlights abrupt increases in sand sized particles in the top 20 cm of the core based on the 137cs chronology these layers were deposited from the late 1970s onwards and relate to these diverging rainfall records and rapidly fluctuating reservoir levels this provides evidence of potential sediment reworking within the reservoir by rapid water level rise after drought copyright 2008 john wiley sons ltd

mollusc and brachiopod skeletal hard parts intricate archives of their marine environment the biogenic carbonate hard parts of fossil bivalves cephalopods and brachiopods are among the most widely exploited marine archives of phanerozoic environmental and climate dynamics research the advent of novel analytical tools has led many workers to explore non traditional geochemical and petrographic proxies and work performed in neighbouring disciplines sheds light on the complex biomineralization strategies applied by these organisms these considerations form a strong motivation to review the potential and problems related to the compilation and interpretation of proxy data from bivalve cephalopod and brachiopod hard parts from the viewpoint of the sedimentologist and palaeoceanographer specific focus is on the complex biomineralization pathways of a given dissolved ion or food particle from its aquatic environment via the digestion and biomineralization apparatus in molluscs and brachiopods and its incorporation into a biomineral given that molluscs and brachiopods do not secrete their hard parts from seawater but rather from their mantle and periostracum this paper evaluates differences and similarities of seawater versus that of body fluids cephalopods bivalves and brachiopods exert a strong biological control on biomineralization that to some degree may buffer their shell geochemistry against secular changes in seawater chemistry disordered amorphous calcium carbonate precursor phases later transformed to crystalline biominerals may be significant in carbonate archive research due to expected geochemical offset relative to the direct precipitation of stable phases a reasonable level of understanding of the related mechanisms is thus crucial for those who use these skeletal hard parts as archives of the palaeo environment the impact of what is commonly referred to as biological factors on the geochemistry of mollusc and brachiopod hard parts is explored for conventional isotope systems such as carbon oxygen strontium and traditionally used element to calcium ratios in particular the often used 13ccarb or the mg ca and sr ca elemental proxies are fraught with problems an interesting new research field represents the analysis calibration and application of non traditional proxies to mollusc and brachiopod hard parts examples include the carbonate clumped isotope 47 approach and the analysis of the isotopes of ca mg n li s or element to ca ratios such as li ca or b ca and rare earth elements based on considerations discussed here a series of do s and don ts in mollusc and brachiopod archive research are proposed and suggestions for future work are presented in essence the suggestions proposed here include experimental work also field experiments making use of recent archive organisms or where possible a reasonable recent analogue in the case of extinct groups moreover the detailed understanding of the architecture of mollusc and brachiopod hard parts and their ultra structures must guide sampling strategies for geochemical analyses where feasible a detailed understanding of the diagenetic pathways and the application of multi proxy and multi archive approaches should form the foundation of fossil carbonate archive research the uncritical compilation of large data sets from various carbonate shelled organisms collected at different locations is not encouraged

palaeoenvironmental reconstruction of the alluvial landscape of neolithic catalhoyuk central southern turkey the implications for early agriculture and responses to environmental change archaeological discussions of early agriculture have often used the neolithic village of catalhoyuk in central southern turkey as a key example of the restricting effect of environment on agricultural production and organization central to these discussions is the palaeoenvironmental reconstruction of the landscape surrounding the site this paper presents an important new dataset from an intensive coring programme undertaken between 2007 and 2013 in the immediate environs of the site designed to improve significantly the spatial resolution of palaeoenvironmental data using sediment analyses including organic content magnetic susceptibility particle size total carbon and nitrogen contents and carbon isotope analysis coupled with 3d modelling we are able to present a new reconstruction of the palaeotopography and sedimentary environments of the site our findings have major implications for our understanding of neolithic agricultural production and social practice we present four phases of environmental development phase 1 consists of the final phases of regression of palaeolake konya in the later parts of the pleistocene dominated by erosion due to wind and water that created an undulating surface of the marl deposited in the palaeolake phase 2 occurs in the latest pleistocene and early holocene and indicates increased wetness probably characteristic of a humid anabranching channel system in which there are localized pockets of wetter conditions in phase 3a this infilling continues producing a flatter surface and there are fewer pockets being occupied by wetter conditions the fluvial regime shifts from humid to dryland anabranching conditions the earliest period of occupation of the neolithic east mound coincides with this phase phase 3b coincides with the shift of occupation to the west mound in the chalcolithic when there is evidence for a very localized wetter area to the southeast of the west mound but otherwise a continuation of the dryland anabranching system finally phase 4 shows a shift to the pre modern style of fluvial environment modified by channelization this reanalysis demonstrates the importance of extensive spatial sampling as part of geoarchaeological investigations with this new evidence we demonstrate that the landscape was highly variable in time and space with increasingly dry conditions developing from the early holocene onwards in contrast to earlier landscape reconstructions that have presented marshy conditions during the early holocene that impacted agriculture we argue that localized areas of the floodplain would have afforded significant opportunities for agriculture closer to the site in this way the results have important implications for how we understand agricultural practices in the early neolithic

testing the preservation potential of early diagenetic dolomites as geochemical archives early marine diagenetic dolomite is a rather thermodynamically stable carbonate phase and has potential to act as an archive of marine porewater properties however the variety of early to late diagenetic dolomite phases that can coexist within a single sample can result in extensive complexity here the archive potential of early marine dolomites exposed to extreme post depositional processes is tested using various types of analyses including petrography fluid inclusion data stable 13c and 18o isotopes 87sr 86sr ratios and u pb age dating of various dolomite phases in this example a triassic carbonate platform was dissected and overprinted diagenetic temperatures of 50 to 430 c in a strike slip zone in southern spain eight episodes of dolomitization a dolostone cataclasite and late stage meteoric vadose cementation were recognized the following processes were found to be diagenetically relevant i protolith deposition and fabric preservation and marine dolomitization of precursor aragonite and calcite during the middle late triassic ii intermediate burial and formation of zebra saddle dolomite and precipitation of various dolomite cements in a proto atlantic opening stress regime t ca 250 c during the early middle jurassic iii dolomite cement precipitation during early alpine tectonism rapid burial to ca 15 km and high grade anchizone overprint during alpine tectonic evolution in the early eocene to early miocene iv brecciation of dolostones to cataclasite during the onset of the carboneras fault zone activity during the middle miocene and v late stage regression and subsequent meteoric overprint data shown here document that under favourable conditions early diagenetic marine dolomites and their archive data may resist petrographic and geochemical resetting over time intervals of 108 or more years evidence for this preservation includes preserved late triassic seawater 13cdic values and primary fluid inclusion data data also indicate that oversimplified statements based on bulk data from other petrographically complex dolomite archives must be considered with caution

the nain churchill boundary and the nain plutonic suite a regional perspective on the geologic setting of the voisey s bay ni cu co deposit northern labrador is the site of two major lithotectonic elements of eastern canadian shield geology first it encompasses a paleoproterozoic continental suture zone archean rocks of the nain province tectonically juxtaposed against paleoproterozoic and reworked archean rocks of the churchill province the nain churchill suture evolved between 1860 and 1740 ma impressing upon the archean and paleoproterozoic rocks a regional metamorphic and tectonic signature referred to as the torngat orogen an important component of the torngat orogen is the tasiuyak gneiss a paleoproterozoic metasedimentary unit that occupies most of the eastern margin of the churchill province second northern labrador is the site of a world renowned terrane of mesoproterozoic anorogenic igneous rocks the nain plutonic suite that sits astride the suture and acts as a stitching batholith between the nain and churchill provinces the nain plutonic suite comprising amalgamated granitic anorthositic dioritic and troctolitic intrusions largely obscures the suture for nearly half of its total length the suite evolved between 1350 and 1290 ma and is interpreted as a product of magmatism associated with limited intracontinental rifting of the conjoined nain churchill crust above a mantle plume the voisey s bay ni cu co deposit serendipitously discovered in 1993 resides within one of the oldest troctolitic intrusions of the nain plutonic suite a 1334 ma intrusion that transgresses a contact between archean and paleoproterozoic gneisses genetic controls on the setting of mineralization have been built upon the nain churchill boundary as a zone of weakness that was exploited by mantle magmas and have implicated the tasiuyak gneiss as a crustal sulfur source although the archean paleoproterozoic junction at voisey s bay is presently interpreted as representing the nain churchill boundary in reality the provincial affinity of the archean gneisses is equivocal this uncertainty dictates caution in assigning the nain churchill boundary as the convenient transcrustal conduit that allowed the voisey s bay mafic intrusions rapid transit from their mantle staging area modeling the sulfide deposit as being solely a result of sulfur saturation of mafic magma by proximal paleoproterozoic tasiuyak paragneisses is also questionable because other genetic processes may have been responsible for example desilication of gneiss is strongly indicated by aluminous refractory minerals in vestiges of fused tasiuyak gneiss the spatial distribution of the mineralized rock at voisey s bay apparently within the subchamber plumbing system feeding a larger intrusion and within remnants of the larger intrusion itself may serve as a paradigm for similar deposits in the nain area but the preerosional comparative morphology of that larger intrusion relative to the better preserved ones may never be known a thorough understanding of the setting and genesis of the voisey s bay deposit will better facilitate establishing exploration targets in other parts of the nain region in labrador as a whole and in other geologically similar regions of the world

magnetic fingerprint of the sediment load in a meander bend section of the seine river france abstract this study aims to evaluate the potential of magnetic methods to determine the composition of the sediment load in a cross section of an unmanaged meander in the upstream stretch of the seine river marnay sur seine suspended particulate matter spm was collected based on a regular sampling scheme along a cross section of the river at two different depth levels during a low water stage may 2014 and a high water stage february 2015 riverbed sediments rbs were collected during the low water stage and supplementary samples were taken from the outer and inner banks magnetic properties of the dry bulk spm and sieved rbs and bank sediments were analysed after characterizing the main magnetic carrier as magnetite hysteresis parameters were measured giving access to the grain size and the concentration of these magnetite particles the results combined with sedimentary grain size data were compared to the three dimensional velocity profile of the river flow in the rbs where the magnetic grain size is rather uniform the concentration of magnetite is inversely proportional to the mean grain size of the total sediment indicating that magnetite is strongly associated with the fine sedimentary fraction the same pattern is observed in the samples from the outer and inner banks during the low water stage the uniformly fine spm grain size distribution characterizes the wash load the magnetic fraction is also relatively fine within the pseudo single domain range with concentration similar to that of the fine rbs fraction during the high water stage spm samples correspond to mixtures of wash load and resuspended sediment from the bedload and riverbanks here the grain size distribution is heterogeneous across the section showing coarser particles compared to those in the low water stage and more varying magnetite concentrations while the magnetic grain size is like that of the low water stage the magnetite concentration in the high water spm can be modelled based on a mixing of the magnetite concentrations of the different grain size fractions thus quantifying the impact of resuspension in the cross section

a tem and microprobe study of a two perthite alkali gabbro implications for the ternary feldspar system an unusual magmatic three feldspar syenogabbro occurs 3 m inside the contact of the klokken gabbro syenite stock it contains plagioclase mesoperthite and cryptoperthite together with a low temperature symplectite intergrowth textural relationships have been investigated by cathodoluminescence bulk chemistry by microprobe and exsolution microtextures and intracrystalline boundaries by tem the mesoperthite has a bulk composition around or26ab52an22 well outside accepted limits of ternary feldspar solid solution it is a mainly coherent lamellar intergrowth of sodic andesine and orthoclase with incipient mtwinning the cryptoperthite bulk composition around or61ab33an6 is a coherent lamellar intergrowth of orthoclase and sodic low oligoclase the compositions of the exsolved phases have been estimated the meso and cryptoperthite crystals have sharp boundaries with each other

diorites and associated rocks in the anglem complex at the neck northeastern stewart island new zealand an example of magma mingling abstract exposures of the anglem complex cretaceous at the neck northeastern stewart island consist dominantly of diorite microdiorite tonalite and granite dyke rocks include microgranite microdiorite andesite and microgranodiorite a diorite suite composed of hornblende gabbro diorite and quartz bearing diorite forms a complex association of basic to intermediate lithologies with highly variable grainsize the heterogeneous suite formed in part by mingling of diorite and tonalite intrudes the diorite suite enclaves within this suite show highly variable margins from sharp sometimes crenulated and apparently chilled contacts through to diffuse contacts prior to and during intrusion of this heterogeneous magma significant textural modifications of dioritic enclaves occurred and some important controls on this modification process are discussed aspects considered include contrasts between the host melt and enclaves in temperature viscosity chemistry volatile content deformation and recrystallisation a liquid immiscibility model is discounted in favour of the magma mingling model subsequent intrusion of granite was followed by a porphyritic microgranodiorite microdiorite and microgranite dykes intrude all of the above rocks with the exception of the microgranodiorite coexisting microdiorite microgranite and composite microdiorite microgranite dykes in the one dyke swarm also provide compelling evidence for magma mingling

a mineral magnetic study of holocene sediment yields and deposition patterns in the llyn geirionydd catchment north wales mineral magnetic investigations of sediment cores and catchment materials from llyn geirionydd north wales form the basis of a study of holocene sediment yields and deposition patterns catchment soils and bedrock materials are highly variable in magnetic concentration in contrast the lake sediments are more homogenous in magnetic concentration magnetic hysteresis properties of catchment materials bulk sediments and magnetic extracts demonstrate that the occurrence of authigenic greigite fe3s4 in early holocene sediments is responsible for differences between the magnetic characteristics of sediments older than 5000 bp and the more recent sediments and catchment materials a whole lake stratigraphy based on core correlation using magnetic measurements pollen stratigraphy and 14c age determinations indicates sediment yields which increase almost threefold from the early holocene to the period from c ad 500 1850

impact of amorphous precursor phases on magnesium isotope signatures of mg calcite abstract various marine calcifiers form exoskeletons via an amorphous calcium carbonate acc precursor phase and magnesium plays an important role in the temporary stabilization of this metastable phase thus the use of mg isotope ratios of marine biogenic carbonates as a proxy to reconstruct past seawater chemistry calls for a detailed understanding of the mechanisms controlling mg isotope signatures during the formation and transformation of acc to the final crystalline carbonate mineral for this purpose we have investigated the mg isotope fractionation between ca mg co 3 solids and aqueous fluids at 25 c and ph 8 3 during i the direct precipitation of crystalline mg calcite and ii the formation of mg rich acc mg acc and its transformation to mg calcite the outcome documents that the small mg isotope fractionation between mg acc and reactive fluid mg acc fluid 26 1 0 0 1 is not preserved during the transformation of the accs into mg calcite following a pronounced isotopic shift accompanying the transformation of mg acc into mg calcite 26 mg calcite fluid progressively decreases with reaction progress from 3 0 to 3 6 reflecting both the approach of isotopic equilibrium and the increase of calcite mg content to near 20 mol mg in contrast the crystalline mg calcite precipitated directly from the reacting fluid i e lacking a discernable formation of an amorphous precursor exhibits only small temporal variations in 26 mg calcite fluid which overall is affected by the precipitation kinetics the values found in this study at the onset of mg acc precipitation for mg isotope fractionation between mg acc and the fluid mg acc fluid 26 1 0 and between mg acc and mg 2 aq mg acc mg 2 aq 26 2 0 are consistent with the formation of a hydrated ca nanoporous solid accommodating mg bicarbonate carbonate species in combination with hydrated magnesium this material crossed by percolating channels filled with the reacting fluid easily converts to mg rich calcite via exchange and or dissolution precipitation reactions the results of this study provide new insights on the acquisition of the mg chemical and isotopic signatures by the skeletal marine carbonates precipitated in confined media from strongly supersaturated fluids they also offer new guides for the interpretation of the isotopic signature of these organisms for paleo environmental reconstructions

the rapid resetting of the ca isotopic signatures of calcite at ambient temperature during its congruent dissolution precipitation and at equilibrium abstract this study provides direct experimental evidence of the resetting of the calcium ca isotope signatures of calcite in the presence of an aqueous fluid during its congruent dissolution precipitation and at equilibrium at ambient temperatures over week long timescales batch reactor experiments were performed at 25 c in aqueous nacl solutions air or co2 gas mixtures were bubbled through this fluid to fix ph during congruent calcite dissolution the fluid became enriched in isotopically heavy ca and the ca isotope composition continued to become heavier after the fluid attained bulk chemical equilibrium with the mineral the 44 42ca composition of the fluid was up to 0 8 higher than the dissolving calcite at the end of the dissolution experiments calcite precipitation was provoked by increasing the reactor fluid ph after chemical equilibrium had been attained via dissolution rayleigh isotope fractionation effects were observed immediately after the ph was increased and rapid calcite precipitation occurred however isotopic exchange continued after the system chemically equilibrated eradicating this rayleigh signal taken together these observations 1 confirm dynamic mineral fluid equilibrium i e dissolution and precipitation occur at equal non zero rates at equilibrium and 2 indicate that isotopic compositions of calcite can readily equilibrate even when this mineral is in bulk chemical equilibrium with its coexisting fluid this latter observation suggests the preservation of paleo environmental isotopic signatures in calcite may require a combination of the isolation of the fluid mineral system from external chemical input and or the existence of a yet to be defined calcite dissolution precipitation inhibition mechanism

holocene environmental change in the west african sahel sedimentological and mineral magnetic analyses of lake sediments from jikariya lake northeastern nigeria previous studies have demonstrated long term changes in effective moisture in sub saharan africa here we reconstruct holocene environments using a similar to 7 m lake sediment sequence recovered from the northeastern nigerian sahel and attempt to distinguish basin specific changes from regional climatic variations the sequence was analysed for sedimentological properties mineral magnetism and pollen and dated by cs 137 pb 210 excess and c 14 extremely arid conditions of the terminal pleistocene ended ca 11 500 cal bp calendar years when climate ameliorated and a lake developed until the occurrence of an arid event leading to lake desiccation at similar to 11 200 cal bp following this climate ameliorated and a water body re emerged very wet conditions predominated 11 200 5600 cal bp followed by drought between 5600 and 5500 cal bp and a return to moderate humidity from 5500 to 4000 cal bp after 4000 cal bp a marked deterioration occurred culminating in lake desiccation at ca 800 cal bp after this time the climate remained generally dry and the re emerging lake was shallow comparison of these results with other well dated sequences in the region demonstrates the importance of basin specific influences on the palaeolimnological records in addition to regional climatic controls disentangling these different controls as well as the reconstruction of holocene climate therefore requires a multiple basin approach copyright c 2008 john wiley sons ltd

evidence for cirque glaciation in the colorado front range during the younger dryas chronozone abstract late pleistocene glacial chronologies developed for the front range mountains of colorado include two or more cirque glacier advances locally known as the satanta peak advances sediment cores were recovered from sky pond an alpine lake 3320 m located less than 100 m downvalley from a moraine that exhibits late pleistocene to early holocene relative age features and appears to correlate to the satanta peak deposits one of the cores penetrated 0 5 m of basal diamict and recovered 3 3 m of overlying sediments that are predominantly gyttja an accelerator mass spectrometry ams age of 12 040 60 yr14c b p was obtained from directly above the basal diamict and is similar to other reported ages for cirque deglaciation in the front range mountains the lower portion of the gyttja contains an interval of clastic sediments that show characteristics consistent with glacial activity in alpine catchments radiocarbon ages obtained from below and near the upper contact of this clastic interval are 11 070 50 and 9970 8014c yr b p respectively an additional ams age of 10 410 9014c yr b p was obtained from within the clastic interval in a second core the most likely source for this interval of clastic sediments is a moraine situated directly upvalley from sky pond and consequently it appears that the deposition of this moraine was coeval with the european younger dryas event 11 000 10 00014c yr b p similarities in soil development weathering features and altitude between this moraine and the type satanta peak moraines suggest that the moraines are correlative these findings are in agreement with a growing body of evidence that suggests a relatively minor advance of alpine glaciers occurred in the north american rockies during the younger dryas chron

further paleomagnetic results from pleistocene sediments in northern canada supporting the existence of stationary secular variation sources abstract our recently published results from pleistocene sediments in northwestern canada provided no compelling evidence for the so called blake event but did yield an interesting secular variation pattern interpreted as the signature of one or more stationary sources near the core mantle boundary new data from a site about 1 km away confirm this interpretation and thus support the division of the non dipole field into drifting and standing parts in the geological as well as the bistoric past measurements of the initial susceptibility of all 694 samples in this study and the previous one greatly facilitated stratigraphic correlation between the four 6 m sections involved

impact of diagenetic alteration on brachiopod shell magnesium isotope δ26mg signatures experimental versus field data due to their thermodynamically stable low mg calcite mineralogy the shells of brachiopods are often counted among the most reliable archives of the physicochemical conditions that occurred during the phanerozoic in marine waters consequently traditional and non traditional isotope and elemental proxy data from brachiopod valves have been analyzed in numerous studies and results obtained have been placed in context with ancient seawater properties this paper tests the sensitivity of brachiopod shell magnesium isotope 26mg data to diagenetic alteration we apply a dual approach by i performing hydrothermal alteration experiments using meteoric marine and burial reactive fluids and ii comparing these data to naturally altered ancient brachiopod shells the degree of alteration of individual shells is assessed by a combination of fluorescence and cathodoluminescence microscopy the absence of luminescence might indicate both well preserved shell material but also the secondary enrichment of quenching elements such as iron along diagenetic pathways complementary oxygen isotope data provide insight into the question of open versus closed system behavior of brachiopod shells brachiopod shell magnesium isotope values respond to differential fluid temperature chemistry and experiment durations the patterns observed are complicated by the interplay of kinetic and thermodynamic patterns and the presence of variable amounts of water soluble and water insoluble organic matter within these biominerals generally the range in bulk 26mg from experimentally altered 1 52 and that of bulk samples from ancient diagenetically altered brachiopod valves 1 53 exceed the geochemical variability of 26mgbrachiopod bulk values of most recent specimens 1 26 in the lower and upper range more 26mg enriched 0 8 and more 26mg depleted 0 7 values respectively are found in altered shells in comparison to unaltered ones the data shown here are considered significant for those aiming to reconstruct palaeoenvironmental parameters based on brachiopod archives consequently we propose tentative guidelines for magnesium isotope research applied to ancient carbonates

geochemistry of the nain massif anorthosite labrador magma diversity in five intrusions abstract the petrography and major and trace element concentrations of the anorthositic bird lake massif blm susie brook slab sbs lister massif lm port man vers run pmr and kikkertavak area kik in the nain plutonic suite nps labrador delineate the mineralogical and composition ranges of these large intrusions from these a variety of magmas and staging histories can be inferred the blm and sbs are mainly noritic anorthosites of the western region exposed along tikkoatokhakh bay tik tilting and stretching deformation distinguish sbs from blm but the two units show no mineralogical or chemical differences the kik and pmr intrusions are mainly troctolitic to noritic anorthosites of the eastern region and they contain more mafic components than the tik bodies chemically all the anorthositic rocks are high in ba sr and the ratio k rb and low in zr rb and rb sr the three plutonic bodies blm sbs and lm along tik are similar and can be clearly distinguished from kik and pmr intrusions by element concentrations and ratios diagrams of element pairs show that each intrusion or group has its own evolutionary trend and variable style indicating that they represent independent magma batches rather than fractionally related volumes of the same magma ree patterns show light ree enrichment and heavy ree depletion and strong positive eu anomalies correlated with cumulus plagioclase in chemical comparison with adjacent plutons one pmr chilled leucotroctolite after subtraction of 18 cumulus plagioclase is similar to the kiglapait magma ki one melatroctolite in kik is close to the comb layered melatroctolite of the snowflake zone in the hettasch intrusion hi and the average leucotroctolite and leuconorite in kik are similar to the leucotroctolite unit and the upper leuconorite respectively at paul island pi therefore the magmas from pmr and kik are similar to those of wellknown troctolitic bodies and their feldspar rich cumulates kik differs from pmr in having somewhat higher an and xmg values all these lines of evidence show that at least three types of magma existed in the study area sodic noritic magma accumulating plagioclase in tik troctolitic magma in kik and a magma similar to but more evolved than kik in pmr the methods used also illustrate the feasibility of characterizing magma compositions from diverse types of cumulates regional differences in modal and chemical composition occur in the nps noritic compositions with pale plagioclase are located in the western and southern regions and troctolitic compositions with darker plagioclase are located in the eastern region a reasonably accurate boundary can now be drawn between the two regions which differ in the nature of their transport from the mantle in comparison with a primitive mantle composition high ba pb k sr ti and low rb nb zr in the nain anorthositic rocks are distinctive these anomalies and the isotopic results of other investigators are consistent with mantle source enrichment followed by variable contamination during magma ponding at the base of the depleted crust

towards a better understanding of magnesium isotope ratios from marine skeletal carbonates abstract this study presents magnesium stable isotope compositions of various biogenic carbonates of several marine calcifying organisms and an algae species seawater samples collected from the western dutch wadden sea and reference materials the aim of this study is to explore the influence of mineralogy taxonomy and environmental factors e g seawater isotopic composition temperature salinity on magnesium isotopic 26mg ratios of skeletal carbonates using high precision multi collector inductively coupled plasma mass spectrometry we observed that the magnesium isotopic composition of seawater from the semi enclosed dutch wadden sea is identical to that of open marine seawater we further found that a considerable component of the observed variability in 26mg values of marine skeletal carbonates can be attributed to differences in mineralogy furthermore magnesium isotope fractionation is species dependent with all skeletal carbonates being isotopically lighter than seawater while 26mg values of skeletal aragonite and high magnesium calcite of coralline red algae indicate the absence or negligibility of metabolic influences the 26mg values of echinoids brachiopods and bivalves likely result from a taxon specific level of control on mg isotope incorporation during biocalcification moreover no resolvable salinity and temperature effect were observed for coralline red algae and echinoids in contrast mg isotope data of bivalves yield ambiguous results which require further validation the data presented here point to a limited use of mg isotopes as temperature proxy but highlight the method s potential as tracer of seawater chemistry through earth s history

environmental magnetism past present and future environmental magnetism involves the application of rock and mineral magnetic techniques to situations in which the transport deposition or transformation of magnetic grains is influenced by environmental processes in the atmosphere hydrosphere and lithosphere the first explicit description of environmental magnetism as a distinct field was in 1980 since that time environmental magnetism has become a broad field that is finding application in an ever increasing array of scientific disciplines in this review of the present state of environmental magnetic studies we divide the field into three broad but arbitrary categories the first involves the use of mineral magnetic assemblages in the geological record to study physical processes in depositional environments this category includes the correlation of sediment cores using magnetic susceptibility measurements studies of geomagnetic field behavior the analysis of depositional and postdepositional mechanical processes that affect sediments and the examination of magnetic parameters that might represent proxies for paleoclimatic variation the second category encompasses studies of the processes responsible for variations in the magnetic minerals brought into a sedimentary environment these provenance investigations include studies of changes in catchment derived sediment in lakes fluctuations in contributions from terrigenous aeolian and glaciogenic components in deep sea sediments and the origin of atmospheric particulates the final category addresses in situ changes and transformations of magnetic minerals in sedimentary environments including pedogenesis authigenetic diagenetic formation of ferrimagnetic phases dissolution of magnetic minerals due to reductive diagenesis and contributions of biomagnetism to sedimentary magnetism because environmental magnetism can address problems in so many disciplines and because many of these problems may be inaccessible to other techniques it is likely that the scope of environmental magnetism will continue to expand rapidly environmental magnetism is capable of providing important data for studies of global environmental change climatic processes and the impact of humans on the environment all of which are major research initiatives in the international scientific community these factors suggest that environmental magnetism has a bright and diverse future

sediment tracing in a reservoir catchment system using a magnetic mixing model magnetic mixing models are developed for specific susceptibility and a demagnetisation parameter which demonstrate how the magnetic properties of soils are altered by mixing these phenomena are used to interpret the composition of reservoir sediments in terms of the relative proportions of two types of source materials top soil and sub soil which are eroded from the catchment area and deposited in the reservoir the model provides a black box approach to the reservoir catchment system dependent on four simplifying assumptions which are the subject of current research application of the model to an afforested catchment suggests a marked change in the types of erosion processes associated with the maturation of the forestry plantation over a period of 50 yr

magnesium isotope fractionation during low mg calcite precipitation in a limestone cave field study and experiments abstract the chemical and isotopic composition of speleothem calcite and particularly that of stalagmites and flowstones is increasingly exploited as an archive of past environmental change in continental settings despite intensive research including modelling and novel approaches speleothem data remain difficult to interpret a possible way foreword is to apply a multi proxy approach including non conventional isotope systems for the first time we here present a complete analytical dataset of magnesium isotopes 26 mg from a monitored cave in nw germany bunker cave the data set includes 26 mg values of loess derived soil above the cave 1 0 0 5 soil water 1 2 0 5 the carbonate hostrock 3 8 0 5 dripwater in the cave 1 8 0 2 speleothem low mg calcite stalactites stalagmites 4 3 0 6 cave loam 0 6 0 1 and runoff water 1 8 0 1 in the cave respectively magnesium isotope fractionation processes during weathering and interaction between soil cover hostrock and solute bearing soil water are non trivial and depend on a number of variables including solution residence times dissolution rates adsorption effects and potential neo formation of solids in the regolith and the carbonate aquifer apparent mg isotope fractionation between dripwater and speleothem low mg calcite is about 1000ln mg cc mg aq 2 4 a similar mg isotope fractionation 1000ln mg cc mg aq 2 1 is obtained by abiogenic precipitation experiments carried out at aqueous mg ca ratios and temperatures close to cave conditions accordingly 26 mg discrimination during low mg calcite formation in caves is highly related to inorganic fractionation effects which may comprise dehydration of mg 2 prior to incorporation into calcite surface entrapment of light isotopes and reaction kinetics relevance of kinetics is supported by a significant negative correlation of mg isotope fractionation with the precipitation rate for inorganic precipitation experiments

a combined radiometric and mineral magnetic approach to recent geochronology in lakes affected by catchment disturbance and sediment redistribution abstract as a result of empirical testing of 210 pb dating models in a wide variety of lakes we conclude that there are many situations where despite major changes in sedimentation rates as a result of lake or catchment disturbance the crs constant flux model provides a sound basis for establishing recent chronology factors which may complicate or vitiate the assumptions of the model include short water residence time bioturbation for which we propose a modified dating model discontinuous or disturbed sedimentation sediment resuspension and shifts in sediment source in the case of the last three processes we illustrate ways in which the problems can be identified and evaluated using mineral magnetic measurements we propose an evaluative approach to 210 pb dating projects with emphasis on independent dating evidence on tests of internal consistency and on the use of data from several correlatable cores

the geochemistry of carbonate diagenesis the past present and future stable carbon and oxygen isotopes 18o and 13c values and trace elements have been applied to the study of diagenesis of carbonate rocks for over 50 years as valuable as these insights have been many problems regarding the interpretation of geochemical signals within mature rocks remain for example while the 18o values of carbonate rocks are dependent both upon the temperature and the 18o value of the fluid and additional information including trace element composition aids in interpreting such signals direct evidence of either the temperature or the composition of the fluids is required such information can be obtained by analysing the 18o value of any fluid inclusions or by measuring the temperature using a method such as the clumped isotope technique such data speak directly to a large number of problems in interpreting the oxygen isotope record including the well known tendency for 18o values of carbonate rocks to decrease with increasing age unlike the 18o 13c values of carbonates are considered to be less influenced by diagenesis and more a reflection of primary changes in the global carbon cycle through time however many studies have not sufficiently emphasized the effects of diagenesis and other post depositional influences on the eventual carbon isotopic composition of the rock with the classic paradigm that the present is the key to the past being frequently ignored finally many additional proxies are poised to contribute to the interpretation of carbonate diagenesis although the study of carbonate diagenesis is at an exciting point with an explosion of new proxies and methods care should be taken to understand both old and new proxies before applying them to the ancient record

precise magnesium isotope measurements in core top planktic and benthic foraminifera 1 this study presents a new methodology to obtain highly precise measurements 0 1 of magnesium isotope ratios in very small samples of foraminiferal carbonate 40 50 g here this technique is used to examine mg isotopic variation among different species of core top foraminifera over a range of different ambient conditions despite the high degree of temperature control on the abundance of elemental mg incorporated into foraminiferal tests analyses of surface dwelling globigerinoides ruber and g sacculifer from five cores with associated sea surface temperatures ranging from 20 to 31 c show no significant temperature dependent variations in their mg isotope ratios analyses of different size fractions of g sacculifer show an increase in mg ca with test size but no variation of mg isotope ratio in all nine planktic and benthic species were analyzed all show identical mg isotope ratios with a mean of 26mg 4 72 apart from small differences in three species namely o universa g sacculifer which are both 0 4 lighter than the average and p obliquiloculata which is 0 4 heavier these results highlight the constancy of foraminiferal mg isotope ratios despite changing environmental conditions which dominate mg ca variation and arguably affect ca isotope fractionation this is an important observation which needs to be included in any model of foraminiferal calcification the insusceptibility of 26mg values to external parameters makes mg isotopes ideally suited to constraining past variations in the mg isotope budget of the oceans and the information this carries about the history of oceanic dolomitization continental weathering and hydrothermal behavior

constraining magnesium cycling in marine sediments using magnesium isotopes abstract magnesium concentrations in deep sea sediment pore fluids typically decrease down core due to net precipitation of dolomite or clay minerals in the sediments or underlying crust to better characterize and differentiate these processes we have measured magnesium isotopes in pore fluids and sediment samples from ocean drilling program sites 1082 1086 1012 984 1219 and 925 that span a range of oceanographic settings at all sites magnesium concentrations decrease with depth at sites where diagenetic reactions are dominated by the respiration of organic carbon pore fluid 26mg values increase with depth by as much as 2 because carbonates preferentially incorporate 24mg low 26mg the increase in pore fluid 26mg values at these sites is consistent with the removal of magnesium in mg carbonate dolomite in contrast at sites where the respiration of organic carbon is not important and or weatherable minerals are abundant pore fluid 26mg values decrease with depth by up to 2 the decline in pore fluid 26mg at these sites is consistent with a magnesium sink that is isotopically enriched relative to the pore fluid the identity of this enriched magnesium sink is likely clay minerals using a simple 1d diffusion advection reaction model of pore fluid magnesium we estimate rates of net magnesium uptake removal and associated net magnesium isotope fractionation factors for sources and sinks at all sites independent estimates of magnesium isotope fractionation during dolomite precipitation from measured 26mg values of dolomite samples from sites 1082 and 1012 are very similar to modeled net fractionation factors at these sites suggesting that local exchange of magnesium between sediment and pore fluid at these sites can be neglected our results indicate that the magnesium incorporated in dolomite is 2 0 2 7 depleted in 26mg relative to the precipitating fluid assuming local exchange of magnesium is minor at the rest of the studied sites our results suggest that magnesium incorporated into clay minerals is enriched in 26mg by 0 to 1 25 relative to the precipitating fluid this work demonstrates the utility of magnesium isotopes as a tracer for magnesium sources sinks in low temperature aqueous systems

tracing movements of topsoil by magnetic measurements two case studies an array of magnetic measurements is used to trace the occurrence of magnetically enhanced topsoil in two different environmental settings the first is a cultivated slope section where the results confirm that movements of topsoil have occurred but that identification of specific erosional processes is problematic the second is a lake catchment system where measurements on soils and sediments allow a partial reconstruction of soil movements from different sources over the last 200 y the study highlights both the need to analyse the magnetic mineralogies of different particle size fractions and the difficulties that may be encountered in trying to discriminate between topsoil and subsoil material on the basis of demagnetization parameters alone

blanket peat erosion in a mid wales catchment during two drought years the rate of blanket peat erosion was measured at an upland site in central wales during the 1983 1984 drought years erosion pins a peat surface sediment trap and sediment sampling in the effluent stream were used to estimate the rate of peat surface recession and the rate of organic sediment loss from the catchment an overall rate of surface recession of 16 mm y 1 on exposed peat faces was recorded this differed between faces of different aspects with the greatest recession on southwest faces eroding peat surfaces exhibited maximum recession during the summer but the peat surface sediment trap indicated that the highest rates of sediment loss from peat faces due to rain wash occurred during the autumn and early winter stream sediment sampling showed that the yield of organic sediment from the catchment was 34 4 t km 2 yr 1 with greatest losses also during the autumn and early winter the evidence suggested that the surface recession as measured on erosion pins included a wastage or shrinkage component which possibly accounted for as much as 80 per cent of the apparent loss direct and circumstantial evidence suggested that peat wastage during the summer months was the most important agent of surface recession in the study period which encompassed the two dry summers desiccation provided available sediment during the autumn but organic sediment supply became limited as the winter progressed despite the occurrence of frost heave

rupture and inflation of a basic magma chamber by silicic liquid the prevailing view among volcanic geologists is that the introduction of basic magma into a silicic magma chamber may trigger explosive silicic volcanism1 and lead to the eruption of mixed magmas2 although the patterns of basic and silicic eruptions in some areas may support an input of basic magma1 just before silicic eruption evidence from the study of the erupted rocks is generally equivocal many occurrences are as readily explained by injection of silicic into basic magma although this latter sequence is a more efficient way to produce contemporaneous magma mixing and explosive silicic volcanism it appears to have been largely overlooked this report describes evidence in the newark island layered intrusion for the rapid emplacement of large volumes of silicic magma into a basic magma chamber and suggests that such events in the uppermost crust may also be important in triggering explosive silicic volcanism and commingling of magmas just before eruption

misinterpreting proxy data for paleoclimate signals a comment on shukla et al 2020 shukla et al explored paleoclimatic signals from a 8 m thick profile of a moraine dammed lake in the central himalaya exposed due to lake burst from a flash flood in 2013 the main objective of t

inter mineral mg isotope fractionation during hydrothermal ultramafic rock alteration implications for the global mg cycle abstract both riverine and ocean waters are enriched in 24 mg compared to the homogeneous chondritic mg isotopic composition of the earth s mantle requiring a fractionation step that is generally attributed to low temperature continental crust weathering here we present new observations that indicate that this 24 mg enrichment of surface waters may originate from mg isotope fractionation during the hydrothermal alteration of primary silicate minerals mineral separates of hydrothermally altered ultramafic rocks were collected from three different localities in norway coexisting olivine and serpentine exhibit invariant mg isotope ratios suggesting that serpentinization does not fractionate mg isotopes in contrast carbonation results in significant inter mineral mg isotope fractionation between the antigorite magnesite and talc the carbonation of the natural samples is constrained by o isotope thermometry at 275 c and hence closes the temperature gap between previous investigations of the natural distribution of mg isotopes during surface weathering and magmatic processes the precursor antigorite has an isotopic composition of 26 mg dsm 3 0 11 0 05 whereas the talc is enriched in 26 mg with mean mg 26 0 17 0 08 and the magnesite is depleted in 26 mg with mean mg 26 0 95 0 15 as carbonate minerals dissolve faster than silicate minerals the chemical weathering of carbonated ultramafic and by analogy mafic rocks on the continents will yield isotopically lighter mg to natural surface waters consistent with field observations moreover the mg fractionation observed in this study suggests that sub seafloor hydrothermal carbonation may be a significant contribution to the mg isotopic composition of ocean water

magnetic separation and evaluation of magnetization of brazilian soils from different parent materials abstract ten selected brazilian soils representative of oxisols ultisols alfisols and inceptisols and derived from gneiss basalt sandstone diabase and itabirite were sampled and examined to provide insight into the behavior of their magnetic minerals the objective was to measure the magnetization of the sand silt and clay fractions and to submit the clay fraction to variable magnetic fields to obtain the magnetization curves and to study their magnetic behavior as related to their parent material lithology the separation procedure using a magnetic separator was done in two different ways dry separation for the sand and wet separation for the silt and clay fractions x ray diffraction patterns of magnetic and nonmagnetic fractions were obtained and the segregation of ferrimagnetic minerals were shown the magnetization s of the total and the magnetic and nonmagnetic soil fractions was evaluated using a special analytical balance the s values obtained varied from 0 03 to 17 72 j t 1 kg 1 for sand 0 05 to 8 91 j t 1 kg 1 for silt and 0 05 to 2 54 j t 1 kg 1 for the clay fractions the results from the magnetically concentrated fractions varied in the same fashion but reached higher values magnetization curves showed spontaneous magnetization ms values of 0 6 to 3 35 j t 1 kg 1 high field slopes ranging from 0 17 to 0 77 j t 2 kg 1 and clearly the influence of the parent material lithology on the nature of the soil magnetic behavior magnetic measurements suggested pathways for ferrimagnetic minerals transformations in soil systems

geochemical constraints on the petrogenesis of the kamiskotia gabbroic complex and related basalts western abitibi subprovince ontario canada barrie c t gorton m p naldrett a j and hart t r 1991 geochemical constraints on the petrogenesis of the kamiskotia gabbroic complex and related basalts western abitibi subprovince ontario canada precambrian res 50 173 199 the kamiskotia gabbroic complex kgc is a large 170 km 2 late archean synvolcanic intrusion located in the western abitibi subprovince canada it is divided into four stratigraphic units a lower zone lz comprised of adcumulus and mesocumulus peridotites troctolites and gabbro norites mg number 86 73 a middle zone mz comprised of mesocumulus gabbro norites mg number 74 60 mesocumulus and orthocumulus gabbro norites and ferroan gabbro norites of the upper zone uz mg number 64 37 which are overlain and partly along strike with granophyric rocks of intermediate to felsic composition the cumulates are characterized by fiat ree patterns with lan ybr 0 4 2 6 and range from 0 6 2 chondrites for olivine bearing lz adcumulates to 10 25 chondrites for upper uz meso to orthocumulates europium anomalies are strongly positive for the lz and and diminish upsection to the upper uz which has slight negative eu anomalies chilled rocks are found in mixed magma and contact chill outcrops and have a mg number of 54 58 lan ybn 0 8 1 2 and slight negative eu anomalies phase relationships and mass balance calculations using the ree indicate that lz cumulates may be derived from the average chill composition similarly upper uz cumulates may be derived from evolved basalts which overlie the kgc assimilation fractional crystallization modeling indicates that the evolved basalts may be derived from the average chill composition by 70 80 fractionation at low pressures or lower degrees of fractionation accompanied by recharge of the average chill composition into the system the basalt compositions do not reflect significant assimilation of nearby felsic rocks or average archean crust during fractionation primitive kgc liquids are estimated by adding an average olivine bearing adcumulate composition to the kgc chill composition in equal proportions to obtain a liquid with an mg number of 70 the ree content of such a liquid is consistent with a derivation by 34 partial melting of a chemically primitive mantle neodymium isotope ratios for kgc cumulates mineral separates and overlying volcanic rocks plot on a 2710 ma isochron with nd t 2 5 0 8 in close agreement with u pb zircon ages for the kgc and kamiskotia felsic volcanic rocks at 2707 2 ma and 2705 2 ma respectively the initial value is identical to the isotopically depleted morb iike abitibi mantle signature at 2 7 ga and provides supporting evidence for fractionation dominated supracrustal processes with no detectable assimilation of an isotopically enriched crustal component kamiskotia mafic magmatism is similar to more recent rift related systems represented by the skaergaard intrusion east greenland and differentiated tholeiitic volcanic fields in eastern iceland and along the galapagos spreading center

mingling in mafic magma chambers replenished by light felsic inputs fluid dynamical experiments abstract we investigate by means of laboratory analogues the fluid dynamics of magma hybridisation caused by the intrusion of felsic magma into a mafic magma chamber the input felsic magma is modelled by a cold low density fluid and the resident mafic magma by a layer of fluid wax close to its solidification point the cold rising input drives convection and solidification in the resident fluid and the solidification causes significant perturbations to an otherwise simple input flow greatly enhancing mixing and mingling between the two model magmas the dimensionless parameters controlling the evolution of the system are determined and different morphologies of the solid mass fit into clearly defined fields in the parameter space the most important implication of the experiments is that the freezing of mafic chambers due to felsic intrusion may give rise to intense hybridization of large volumes of magma this potentially important mechanism for hybridization has to date received little attention in the literature

experimental calibration of mg isotope fractionation between dolomite and aqueous solution and its geological implications abstract hydrothermal experiments at 220 160 and 130 c were performed to calibrate the mg isotope fractionation factor between dolomite and aqueous mg hydrothermal experiments included synthesis of dolomite using different starting materials as well as exchange experiments that used poorly ordered proto dolomite the morphology of synthesized dolomite was dependent on starting mineralogy suggesting that dolomite was synthesized by different pathways hydrothermally synthesized dolomite was initially fine grained disordered or poorly ordered dolomite that with time recrystallized to coarser grained ordered dolomite isotopic exchange was monitored using 87 sr 86 sr ratios and 25 mg tracers and these indicated near complete isotope exchange between dolomite and aqueous solutions at the end of most hydrothermal experiments the mg isotope fractionation factor between dolomite and aqueous solution obtained from synthesis and exchange experiments converged with time and was independent of dolomite morphology suggesting attainment of isotopic equilibrium combining results from synthesis and exchange experiments the temperature dependent mg isotope fractionation factor for ordered dolomite is 26 mg dolo aq 0 1554 0 0096 10 6 t 2 where t is in kelvin in contrast poorly ordered dolomite has a 26 mg dolo aq fractionation factor that is up to 0 25 lower than that of ordered dolomite and this is attributed to longer mg o bonds in imperfectly ordered dolomite the experimentally calibrated 26 mg dolo aq fractionation factors lie between those calculated by schauble 2011 and rustad et al 2010 the 26 mg dolo aq fractionation factor extrapolated to lower temperatures using the 26 mg t function of this study matches the 26 mg dolo aq fractionation factor obtained by modeling of mg isotope compositions of odp drill core samples this study shows that significant mg isotope fractionation occurs during dolomite precipitation these results collectively demonstrate that mg isotopes in dolomite are a useful tool for studying mg global cycling and dolomitization

magnesium isotope fractionation during precipitation of inorganic calcite under laboratory conditions abstract the mg isotope composition of biogenic and inorganic carbonate bears on paleoclimate and paleooceanography studies because of the potential for constraining temperatures so called vital effects and marine mg fluxes previous work has shown that marine organisms produce a wide range of mg isotope compositions that are species dependent where 26 24mgcarb sol fractionations vary from 1 to 5 e g hippler et al 2009 gca constraining mg isotope fractionation during inorganic carbonate precipitation is important because this serves as a baseline with which to compare biogenic samples as well as constrain mg cycling in natural environments we report mg isotope fractionation factors between mg bearing calcite and aqueous mg mg ca molar ratio between 3 1 and 13 1 from 20 free drift and one chemo stat experiment conducted at temperatures between 4 c and 45 c for solutions buffered at p co 2 between 0 038 and 3 pure caco3 seed crystals were used to promote the heterogeneous growth of carbonate from solution and to minimize kinetic isotope effects associated with nucleation and rapid precipitation from strongly super saturated solutions under these conditions calcite overgrowths that contained 0 8 14 9 mol mgco3 precipitated on the seed crystals the measured 26mg 24mg fractionation factors between mg calcite and solution 26mgcal sol are modestly correlated with temperature changing from 2 70 at 4 c to 2 22 at 45 c the fractionation factors are not correlated with experimental conditions chemo stat vs free drift mg content of the overgrowth p co 2 or the mg ca ratio of the solution the temperature dependence of the mg isotope fractionation is 26mgcal sol 0 158 0 051 106 t2 0 74 0 56 where t is temperature in kelvin fractionation of mg isotopes in calcite is much less sensitive to temperature than oxygen isotope fractionation which limits its application as a geothermometer in contrast the mg isotope fractionations for biogenically precipitated mg calcite vary greatly suggesting its potential to discern vital effects in natural samples finally the relatively small temperature effect on mg isotope fractionation greatly simplifies use of mg isotopes in modern or ancient marine systems to constrain mg fluxes including continental weathering

conservative behavior of mg isotopes in massive dolostones from diagenesis to hydrothermal reworking abstract mg isotopes in syndepositional dolomite have been suggested to be a potential proxy for understanding seawater chemistry however it is argued that the 26mg values of dolomite could be complicated by the effects of early diagenesis and later hydrothermal activities further investigations into the behaviors of mg isotopes in dolomitization systems are needed to resolve this controversy in the present study we investigated early ordovician dolostones from the tarim basin including diagenetically altered dolomites hydrothermally altered dolomites and well preserved dolostones that precipitated in the slope margin and interior of a carbonate platform different types of dolomites bulk dolostone and limestone were sampled by microdrilling for analyses of c o mg isotope compositions and ree concentrations the dolostone 13c values match those of coeval seawater and the ree distribution patterns in the dolostones are comparable with those in the limestones indicating that the dolostones originated from syndiagenetic dolomitization the 26mg values of the various syndiagenetic dolomites that formed in the slope margin and interior of the carbonate platform are similar averaging approximately 2 06 0 20 no stratigraphic variability in the dolomite mg isotopes can be discerned which implies that the mg isotope compositions of the porewater were homogeneous during massive dolomitization and remained in equilibrium with seawater additionally the 26mg values of the altered dolostone do not show correlations with diagenetic and hydrothermal signals demonstrating that dolomite mg isotopes are insensitive to postdepositional alteration given these facts we propose that mg isotopes in dolostones have conservative behaviors during diagenesis and late stage hydrothermal reworking

magnesium isotope fractionation between brucite mg oh 2 and mg aqueous species implications for silicate weathering and biogeochemical processes article history brucite with its octahedral structure has a lattice configuration that is similar to the mg bearing octahedral layers in phyllosilicates understanding stable mg isotope fractionation between brucite and aqueous solution therefore bears on interpretation of mg isotope data in natural weathering systems in this study we experimentally determined mg isotope fractionation between brucite and two mg aqueous species the free mg aquo ion mg oh2 6 2 and edta bonded mg mg edta 2 results from recrystallization and brucite synthesis experiments suggest mild preferential partitioning of light mg isotopes into brucite compared to mg aquo ions at low temperatures where measured 26 mg brucite mg 2

medium term effects of afforestation on sediment dynamics in a water supply catchment a mineral magnetic interpretation of reservoir deposits in the macclesfield forest n w england short term increases in stream sediment loads during pre afforestation ground preparation have often been followed by a return to low sediment loads however in the macclesfield forest in the southern pennines comparison of the magnetic characteristics of soils and downstream reservoir deposits suggest that significant changes in the processes of soil erosion occur later in the cycle of forest management at the time of canopy closure a preliminary magnetic mixing model indicates that the proportion of reservoir sediments derived from the subsoil below 0 3 m depth has increased from 30 to 70 per cent between 1929 and 1981 changes in hydraulic stresses subsequent to road construction and forest development in association with unstable pleistocene deposits may provide an explanation for these events