platinum group mineralization in the tertiary igneous province new data from mull and skye scottish inner hebrides uk in recent years platinum group mineral deposits have been reported from several of the tertiary igneous complexes in east greenland these intrusions form part of the same igneous province as the tertiary igneous centres in northwest scotland and recent work confirmed the presence of abundant platinum group minerals in the rum central complex in this paper we report for the first time the presence of abundant and diverse platinum group minerals from the tertiary ben buie intrusion on mull and the cuillin complex of skye inner hebrides scotland the platinum group minerals in the ben buie intrusion are associated with chromitite layers in ultramafic cumulates the grains are small 1 2 m locally very abundant and dominated by pd bi te sb phases in the peridotite series of the cuillin complex the platinum group minerals are dominated by laurite commonly enclosed within chromite or silicate a combined orthomagmatic hydromagmatic origin is proposed for the mineralization however the style and type of mineralization differs between each of the igneous centres platinum group element mineralization is a ubiquitous feature of the tertiary igneous province in northwest scotland and in greenland and is intimately linked to the regional tectonic setting

the j m platinum palladium reef of the stillwater complex montana ii origin by double diffusive convective magma mixing and implications for the bushveld complex sequences of cumulates form concurrently by downdip accretion from a column of liquid layers that are primarily separated by diffusive interfaces the low cumulates grow in advance of the upper cumulates in accord with their higher crystallization temperatures an important observation is that mixing of the two parent liquids lowers their crystallization temperatures and consequently should produce unsaturated hybrids that could dissolve earlier cumulates the repetition of cyclic units above the j m reef is explained by having periodic additional injections of dense magma beneath the subsiding column of crystallizing liquid layers neither the j m reef nor the bushveld ore zones should ever have been continuous across the floor of their respective magma chambers and for bushveld the circumstances resulted in zones that are actually minable modified journal abstract

description of arid geomorphic features using landsat tm data and ground truth information wadi fatima kingdom of saudi arabia abstract this paper presents a geomorphic interpretation mapped on the basis of a landsat thematic mapper tm 5 satellite image this image was digitally enhanced to improve interpretability further the resulting image was used as a color coding for the interpretation map this enables a larger user community to take advantage of it for different tasks a field campaign gave additional informaton of the main characteristics of wadi fatima besides a thorough description of the geomorphological features the significance of the structural history for the geomorphological development is given the satellite interpretation map of wadi fatima is enclosed in the appendix

zircon trace element chemistry at sub micrometer resolution for tarawera volcano new zealand and implications for rhyolite magma evolution zoned crystals can be important recorders of magmatic processes in space and time however in most situations the temporal dimension is difficult to quantify here we have employed secondary ion mass spectrometry depth profiling to excavate parallel pits into non polished crystal faces of zircon to obtain 5 m resolution u th disequilibrium ages one pit that can be correlated with trace element zoning at sub m resolution derived from a second pit data from 17 crystals representing each of the four rhyolite eruptions of tarawera volcano an intra caldera edifice within the okataina volcanic centre reveal diverse zircon growth conditions over time most crystals display rimward depletions in zr hf and ti broadly consistent with cooling and crystallization however a significant fraction of crystals lacks these patterns and displays rimward trace element variations consistent with isothermal or prograde crystallization oscillatory zonation patterns in y th and u are superimposed on the zr hf and ti trends despite the limited number of crystals analyzed in this way the striking lack of ubiquitous trace element zoning patterns in crystals from the same hand sample implies that fractional crystallization upon cooling was punctuated by magma recharge and crystal mixing affecting different parts of the magma reservoir by combining data from all crystals a systematic change to more heterogeneous trace element abundances is revealed by zircon crystal domains 100 ka prior to caldera formation and is best explained by the post caldera system consisting of small isolated melt pockets that evolved independently an important conclusion is that the zircon cargo in volcanic rocks reflects thermally and compositionally divergent processes that act near simultaneously in a magma storage region and not exclusively the conditions in the eruptible magma

chronology and dynamics of a large silicic magmatic system central taupo volcanic zone new zealand the central taupo volcanic zone in new zealand is a region of intense quaternary silicic volcanism accompanying rapid extension of continental crust at least 34 caldera forming ignimbrite eruptions have produced a complex sequence of relatively short lived nested and or overlapping volcanic centers over 1 6 m y silicic volcanism at taupo is similar to the yellowstone system in size longevity thermal flux and magma output rate however taupo contrasts with yellowstone in the exceptionally high frequency but small size of caldera forming eruptions this contrast reflects the thin rifted nature of the crust which precludes the development of long term magmatic cycles at taupo 11 refs 4 figs 1 tab

roundness of sand grains of longitudinal dunes in saudi arabia abstract thirty nine sand samples were collected from two longitudinal dunes of different size with their interdunes and an alluvial fan at wadi ash shumaysi in the western region of saudi arabia roundness of sand samples 1 50 and 2 50 fractions was measured sands of the interdunes are better rounded than those of the adjacent dunes dune sands tend to be less rounded with increasing dune height dunes with higher and steeper windward slopes have more pronounced sand grain variation than those with gentle slopes this is attributed to selective sorting by the wind and easier transport of less rounded grains over dunes the source sediments are considered to be the major factors controlling the roundness characteristics of aeolian sediments of the dunes and their interdunes this study confirms that increasing roundness is associated with increase in grain size lastly the shape of dune sands is not influenced by dune form

geochemical and isotopic zoning patterns of plagioclase megacrysts in gabbroic dykes from the gardar province south greenland implications for crystallisation processes in anorthositic magmas chemical and sr isotopic zoning patterns in plagioclase megacrysts from gabbroic dykes in the gardar province can be used to elucidate magma chamber and emplacement processes the megacrysts occur either as single crystals or assembled as anorthosite xenoliths the size of the megacrysts varies from 1 cm to 1 m they consist of a large core with variable zonation an58 39 and a relatively small 600 m normally zoned rim an62 27 the contact between core and rim is sharp and marked by a sharp increase in anorthite content which can reach 11 mol an this gap is interpreted as having formed during dyke emplacement due to a sudden pressure release some of the megacryst cores show a fairly constant composition whereas others exhibit an unusual wavy oscillatory zoning which has not been reported elsewhere to our knowledge the oscillatory zoning has wavelengths of up to 2 500 m and a maximum amplitude of 7 mol an it is interpreted as reflecting movements of the crystals in the magma reservoir the sr isotopic composition of one crystal shows a radiogenic inner core 87sr 86sr i 0 7044 and a less radiogenic outer core 87sr 86sr i 0 7039 0 7036 the lack of a significant change between outer core and rim 87sr 86sr i 0 7037 is consistent with formation of the more an rich rim due to pressure release variations in the core may be related to movements of the crystal and or magma mixing a trace element profile across a megacryst shows a small increase in sr and small decreases in ba and la contents of the recalculated melt composition across the core rim boundary whereas p ce nd and eu remain constant melt compositional changes upon emplacement are therefore considered to be of minor importance constant ratios of incompatible trace elements in the megacryst cores indicate a dominant influence of a lower crustal source on trace element budgets

factors affecting the nature and rate of dust production from natural dune sands this paper evaluates the influence of natural sand particle characteristics on the amount and particle size distributions of dust produced by aeolian abrasion it contrasts with previous studies of aeolian abrasion by conducting experiments using i whole sand samples as opposed to selected size fractions ii natural mature dune sands rather than artificial or freshly crushed material and iii weathered sands that have acquired a superficial clay coating instead of grains with clean surfaces whilst previous research has found clear positive relationships between particle size sorting roundness and the amount of dust produced by aeolian abrasion the relationships determined in this study show some variation according to the geomorphological context from which the original samples were obtained the most important factor affecting the amount and particle size characteristics of the dust produced was the presence of a clay coating on the grain surface that is removed by the abrasion process the dust produced by this mechanism had a modal size of 2 5 m and material 10 m comprised up to 90 of the particles produced

evaluation and application of the ion microprobe in the strontium isotope geochemistry of carbonates abstract a modified aei im20 ion microprobe has been used to measure 87 sr 86 sr ratios in carbonates a suite of carbonates with varying major elements ca mg fe mn was studied at low and high m m 3000 mass resolution to determine the types and intensities of molecular species isobaric with sr peaks sr data collected at low mass resolution must be corrected for ca 2 and camgo species rb sr ratios are extremely low and correction for 87 rb is not required 87 sr usable sr isotopic data may be obtained from calcite given sr 400ppm and for sr 5000 ppm a precision of 1 0 0007 in 87 sr 86 sr 2 mean can be achieved under optimum conditions the corrections for ca 2 and camgo are smaller than the within run precision in calcite but in dolomite the correction for camgo is 1 5 mass fractionation corrections to 87 sr 86 sr based on 86 sr 88 sr 0 1194 are typically 8 to 10 good agreement between ion probe and solid source mass spectrometer results was found for calcites of known sr isotopic composition st4a 400ppm sr average ion probe 87 sr 86 sr 0 7267 0 0015 solid source 87 sr 86 sr 0 72680 0 00008 14 jcg36 6000ppm sr average ion probe 87 sr 86 sr 0 7056 0 0009 solid source 87 sr 86 sr 0 70588 0 00009 16 jcg44 6000ppm sr average ion probe 87 sr 86 sr 0 7057 0 0006 solid source 87 sr 86 sr 0 70540 0 00008 16 the ability of the ion microprobe to measure 87 sr 86 sr for 10 m spots in calcite was used 1 to measure variation in 87 sr 86 sr of 0 01 on a centimetre scale in a hydrothermally altered basalt from the isle of skye northwestern scotland and 2 to determine the sr isotopic composition of tiny 87 sr 86 sr

chronological study of oxygen isotope composition for the solar protoplanetary disk recorded in a fluffy type a cai from vigarano abstract fluffy type a ca al rich inclusions cais containing reversely zoned melilite crystals are suggested to be aggregates of direct condensates from solar nebular gas we conducted an investigation of 26 al 26 mg systematics of a fluffy type a cai from vigarano named v2 01 with known oxygen isotopic distributions of reversely zoned melilite crystals we also conducted oxygen isotope measurements of coexisting minerals two of six reversely zoned melilite crystals show continuous variations in magnesium isotopic composition with 25 mg decreasing along the inferred direction of crystal growth which supports the idea that they originated through condensation petrography suggests that the constituent minerals of v2 01 formed in the following order first spinel and fassaite enclosed by melilite then reversely zoned melilite crystals and spinel and diopside in the wark lovering rim the spinel enclosed by melilite has 16 o rich compositions 17 o 24 and on an al mg evolutionary diagram plots along model isochron with an initial value of 26 al 27 al 0 5 6 0 2 10 5 the fassaite enclosed by melilite crystals shows variable oxygen isotopic compositions 17 o 12 and 17 and plots on an isochron with 26 al 27 al 0 5 6 0 2 10 5 the oxygen isotopic compositions of reversely zoned melilite showed continuous variations in 17 o along the inferred direction of crystal growth suggesting that surrounding nebular gas during the formation of the reversely zoned melilite changed from 16 o poor 17 o values larger than 10 to 16 o rich 17 o 25 the six reversely zoned melilite crystals show indistinguishable initial 26 al 27 al values with an average 26 al 27 al 0 of 4 7 0 3 10 5 which is clearly distinguishable from the value of enclosed spinel and fassaite indicating a younger formation age than the enclosed spinel and fassaite the spinel and diopside from the wark lovering rim show 16 o rich compositions 17 o 23 with 26 al 27 al 0 4 5 0 4 10 5 the values of 26 al 27 al 0 are consistent with the formation sequence inferred from petrography the formation period for the v2 01 cai is estimated to be 0 18 0 07 myr from the difference in initial 26 al 27 al values these data suggest that the oxygen isotopic composition of solar nebular gas surrounding the cai changed from 16 o rich to 16 o poor and back to 16 o rich during the first 0 2 myr of solar system formation

new insights into the processes controlling compositional zoning in plagioclase abstract although plagioclase is the most abundant mineral in crustal rocks the factors that control the magnitude of compositional zoning in plagioclase remain poorly constrained the composition of magmatic plagioclase depends upon physical parameters such as temperature t and total pressure p as well as the full range of compositional controls including the water content of the melt wt h 2 o the changes in these physical parameters can be quantified using available numerical phase equilibria models for various differentiation scenarios and the magnitude of these effects on plagioclase zoning can be calculated the role of temperature total pressure and water content on plagioclase zoning has been experimentally investigated previously in a pure albite anorthite ab an system in this study we use the melts algorithm to quantify the effects of p t and water content of the melt on compositional zoning in plagioclase in a basaltic system our first two simulations are designed to evaluate the magnitude of compositional changes that may be attributable to specific system changes exclusive of assimilation and recharge the first scenario involves a two step crystallization process in the first step magma rises from a deeper to a shallower chamber while crystallizing plagioclase at successively lower pressures in the second step the magma pools in the shallower magma chamber and continues to crystallize isobarically if these two steps are repeated the result is the development of reverse normal and oscillatory zoning in plagioclase the second scenario involves plagioclase zoning in a large convecting magma chamber where plagioclase crystallizes as the magma convects resulting in oscillatory zoning the third scenario involves a two step process where crystallization is interrupted by multiple recharge events in this case magma crystallizes isobarically as a new batch of primitive magma is injected periodically to the magma chamber and crystallization continues after mixing during the repetition of these two steps plagioclase develops normal and reverse zoning based on our results we interpret zoning in plagioclase in terms of the partitioning of na 2 o and cao between the melt and plagioclase and the changes in the total volume during dissolution reactions of albite vab dissolution and anorthite van dissolution components for the first two scenarios our calculations show that as pressure decreases vab dissolution and van dissolution decrease while as pressure increases both of them increase vab dissolution always being larger than van dissolution controls the zoning in plagioclase by compensating the larger volume change in the melt either by albite component dissolving into its oxides or by oxides reacting to produce albite component therefore at a given temperature with decreasing pressure na 2 o partitions increasingly into the melt compared to cao making the coexisting plagioclase more an rich thus ascent of magma from a deeper to a shallower chamber isentropically or convection of a magma in a large chamber polybaricaly results in the development of reverse zoning in plagioclase the magnitude of the effects is on the order of 3 mol an per kbar for both decompression driven crystallization and isothermal polybaric convection

the cumulus and post cumulus evolution of chrome spinels in ultrabasic layered intrusions evidence from the cuillin igneous complex isle of skye scotland chrome spinels from the layered peridotilte series of the unmetamorphosed anorogenic 60 ma cuillin igneous complex isle of skye display a wide variety of compositions cumulus within seams chrome spinels from the lowest exposed portion of the peridotite series exhibit features indicative of textural equilibrium are rich in al and mg and have low values of the ratio cr cr al cumulus chrome spinels from higher up in the series are different from these particularly textural disequilibrium is evident intercumulus plagioclase and olivine are present and the chrome spinels are rich in cr fe and ti with high values of the ratio cr cr al intercumulus dispersed chrome spinels tend towards anhedral forms and define enrichment trends towards fe both fe2 and fe3 with decreasing mg cr and al and towards al with decreasing fe2 and cr and increasing mg individual crystals are completely homogeneous and are devoid of reaction rims the observed textural characteristics and compositional data of the chrome spinels documented here suggest that the semi quantitative peritectic reaction aluminous chrome spinel melt plagioclase olivine chromian chrome spinel is responsible for the observed parageneses and that both the environment of crystallization eumulus or intercumulus and the role of plagioclase olivine crystallization are critical parameters for this geochemical trend in spinels within upper crustal magmatic systems the effects of pyroxene crystallization on the development of this geochemical trend are also considered this investigation highlights the need to consider the role of post cumulus mineral melt reactions and their influences upon the final compositions of major oxide and silicate phases within layered intrusions

carbonate assimilation at merapi volcano java indonesia insights from crystal isotope stratigraphy recent basaltic andesite lavas from merapi volcano contain abundant complexly zoned plagioclase phenocrysts analysed here for their petrographic textures major element composition and sr isotope composition anorthite an content in individual crystals can vary by as much as 55 mol an

sr isotope disequilibrium in columbia river flood basalts evidence for rapid shallow level open system processes geochemical variability among columbia river basalt group flood lavas has been attributed to two different origins derivation from heterogeneous mantle and modification of mantle derived magmas by open system processes involving continental crust we present in situ analyses of sr isotopes from core to rim transects of plagioclase phenocrysts and groundmass from each major columbia river basalt group formation and show that plagioclase crystals are usually internally zoned in 87 sr 86 sr and are in 87 sr 86 sr disequilibrium with their host groundmass these data unequivocally demonstrate that columbia river basalt magmas regardless of the nature of mantle sources were modified by opensystem processes operating at crustal depths one dimensional diffusion modeling indicates that observed isotopic heterogeneities cannot have existed at magmatic temperatures for more than a few years or decades in general results indicate that these flood basalt magmas were erupted while still in the process of assembly a typical columbia river flood basalt magma melt plus phenocrysts therefore only attains its final geochemical identity just before or during eruption a fact that is generally obscured when evaluating conventional whole rock isotope analyses

insights into silicic melt generation using plagioclase quartz and melt inclusions from the caldera forming rotoiti eruption taupo volcanic zone new zealand the rotoiti 120 km3 and earthquake flat 10 km3 eruptions occurred in close succession from the okataina volcanic centre at 50 ka while accessory mineral geochronology points to long periods of crystallization prior to eruption 104 105 years and separate thermal histories for the magmas little was known about the rates and processes of the final melt production and eruption crystal zoning patterns in plagioclase and quartz reveal the thermal and compositional history of the magmatic system leading up to the eruption the dominant modal phase plagioclase displays considerable within crystal zonation an37 74 40 227 ppm mgo 45 227 ppm tio2 416 910 ppm sr and 168 1164 ppm ba resorption horizons in the crystals are marked by sharp increases 10 30 in sr mgo and xan that reflect changes in melt composition and are consistent with open system processes melt inclusions display further evidence for open system behaviour some are depleted in sr and ba relative to accompanying matrix glass not consistent with crystallization of modal assemblage mi also display a wide range in xh2o that is consistent with volatile fluxing quartz cl images reveal zoning that is truncated by resorption and accompanied by abrupt increases in ti concentration 30 80 ppm that reflect temperature increases 50 110 c diffusion across these resorption horizons is restricted to zones of 20 m suggesting most crystallization within the magma occurred in 2000 years these episodes are brief compared to the longevity 104 105 year of the crystal mush zones all textural and compositional features observed within the quartz and plagioclase crystals are best explained by periodic mafic intrusions repeatedly melting parts of a crystal rich zone and recharging the system with silicic melt these periodic influxes of silicic melt would have accumulated to form the large volume of magma that fed the caldera forming rotoiti eruption

inter and intracrystalline isotopic disequilibria techniques and applications in the last 20 years we have seen a significant expansion of techniques related to geochemical and isotopic microsampling of materials in the earth sciences from constraining pre eruptive histories of flood basalt magmas to identifying the natal rivers of origin of anadromous fishes these techniques have had significant impacts in a wide variety of scientific fields nowhere has the impact been greater than in identifying the sources processes and timing of processes involved in igneous magmatic systems both technique refinements and the development of new technologies have aided in advancing microsampling applications thus allowing for a better understanding of the sources and mechanisms responsible for changing geochemical and isotopic signatures in natural systems in this chapter we focus on the techniques and technologies associated with radiogenic isotope microsampling and review applications of these techniques as utilized in scientific investigations isotope microsamping is a logical extension of earlier studies that evaluated individual components of magmas and magma systems including melts and minerals from the use of petrographic microscopes and the later introduction of the electron microprobe the focus on internal chemical variations in melts and minerals is critical to assessing the petrogenetic histories of igneous rocks even today these technologies are used to ensure that further trace element and isotopic analyses are undertaken in a textural and major element context for trace elements and isotopes early studies e g cortini and van calsteren 1985 confirmed variations in the melt and mineral components of many igneous rocks but focused on mineral or glass separates potential information associated with isotopic variations retained by individual crystals or internal variations within individual crystals was lost later studies such as geist et al 1988 focused on isotope variations within single megacrysts to constrain mixing scenarios between basaltic andesitic and rhyolitic magmas and davidson et al 1990 used

26al 26mg chronology and oxygen isotope distributions of multiple melting for a type c cai from allende abstract disequilibrium oxygen isotopic distributions of ca al rich inclusions cais correspond to multiple melting events in the solar nebula 26al 26mg systematics may be applicable for age differences among such melting events we have carried out a coordinated study of detailed petrographic observations and in situ oxygen and magnesium isotope measurements for a type c cai ek1 04 2 from the allende cv3 meteorite to determine the melting events and their ages the cai consists mainly of spinel anorthite olivine and pyroxene and has a core and mantle structure petrography of the core suggests that the crystallization sequence of the core minerals is from spinel anorthite olivine and to pyroxene the mantle has the same mineral assemblage as the core and shows incomplete melting and solidification textures oxygen isotopic compositions of the minerals are distributed along the carbonaceous chondrite anhydrous mineral ccam line 18o 44 to 9 which indicates to preserve a chemical disequilibrium status in the cai spinel shows a 16o rich signature 18o 43 while anorthite is 16o poor 18o 8 olivine and pyroxene in the core have the same oxygen isotopic composition 18o 15 which indicates their equilibrium olivine and pyroxene in the mantle have variable oxygen isotopic compositions and are slightly depleted in 16o 18o 13 to 4 compared with the same minerals in the core the 26al 26mg systematics is consistent with the disequilibrium status observed according to the petrography and oxygen isotopes spinel is plotted on a line of 26al 27al 0 3 5 0 2 10 5 anorthite is plotted on a line of 1 5 10 7 and olivine and pyroxene in the core are plotted on a line of 1 7 10 6 plots of olivine and pyroxene in the mantle are scattered below the isochron of these minerals in the core this study indicates that the ek1 04 2 type c cai underwent multiple heating events after the formation of its cai precursor the precursor cai was formed 0 4 myr after the formation of the solar system defined by canonical cai formation at least 1 6 myr after the precursor cai formation the cai was partially melted and the melt exchanged oxygen isotopes with surrounding 16o poor nebular gas 16o poor olivine and pyroxene in the core crystallized from the melt subsequently al rich chondrules accreted onto the cai and the cai experienced partial melting again and recrystallized to form the mantle the oxygen and magnesium isotopes in anorthite were redistributed during thermal metamorphism in the allende parent body our study reveals that the cai had been retained in the solar nebula for at least 1 6 myr and underwent multiple melting events in the nebula and oxygen and 26al 26mg systematics has been partially disturbed depending on crystal sizes by metamorphism on the parent body

distinct sulfur saturation histories within the palaeogene magilligan sill northern ireland implications for ni cu platinum group element mineralisation in the north atlantic igneous province the 60 m thick magilligan sill is part of the british palaeogene igneous province in the north atlantic the sill comprises layers of dolerite and olivine gabbro and it intrudes a thick sequence

ion microprobe sr isotope analysis of carbonates with about 5μm spatial resolution an example from an ayu otolith abstract a high lateral resolution secondary ion mass spectrometer nanosims ns50 ion microprobe has been used to measure 87sr 86sr ratios in natural caco3 samples a 2 na o primary beam was used to sputter a 5 7 m diameter crater on the sample surface and secondary positive ions were extracted for mass analysis using an accelerating voltage of 8 kv and a mattauch herzog geometry the multi collector system was adjusted to detect 43ca by a detector em 2 80 ca 2 em 3 86sr em 4 and 87sr em 4b ions at the same time then the magnetic field was scanned for the em 4 to detect 85rb 86sr and 87sr while the em 4b can measure 86sr 87sr and 88sr respectively a mass resolution of 3600 at 10 peak height was attained with a flat peak top while the sensitivity of sr was about 10 cps na ppm an aragonite sample coral skeleton standard jcp 1 was used as a reference for sr isotope ratio calibration repeated analyses of the jcp 1 show that the 87sr 86sr ratio agrees well with the seawater signature within a precision of 0 3 at 2 after the series of corrections such as the ca dimer 87rb and a mass bias estimated by the 88sr 86sr ratio the method was applied to an otolith ear stone from ayu plecoglossus altivelis altivelis collected from the yodo river japan although experimental errors are relatively large up to 3 at 2 the ratios of the core region are higher than the seawater signature while more distal values agree well with seawater the very outermost part again shows the relatively higher 87sr 86sr ratios the spatial variation of 87sr 86sr ratios was consistent with amphidromous migration of the fish namely born in the lake and grown in the coastal sea and finally collected in a river

repeated recharge assimilation and hybridization in magmas erupted from el chichon as recorded by plagioclase and amphibole phenocrysts abstract compositional and isotopic zoning patterns in plagioclase and amphibole phenocrysts from el chichon record multiple cycles of country rock assimilation magma injection hybridization and mixing laser ablation icp ms and electron microprobe analyses of plagioclase crystals from 7 eruptions spanning 3100 years reveal four types of zoning these compositional and isotopic zones are often associated with textural changes observed in the crystals in thin section e g sieved or patchy regions amphiboles are frequently zoned in al and si and in two magmas may have clinopyroxene rims interestingly most plagioclase show multiple and repeated zoning patterns moreover all magmas contain all zoning patterns and textures and crystals with substantially different sequences of zones occur within mm of one another the most reasonable explanation for the origin of these textures is a frequently recharged chamber plagioclase zones with increasing anorthite contents an and decreasing 87sr 86sr ratios record injection by a hotter possibly wetter and more primitive magma lower 87sr 86sr ratio zones with decreasing an and increasing 87sr 86sr ratios record assimilation of country rock and or hybridization of the host and injected magmas injection of hot magma may provide the heat for country rock assimilation changes in an without corresponding changes in 87sr 86sr ratio likely record slight variations in pressure or temperature during crystallization or the far field thermal effects of magma injection variations in 87sr 86sr ratio unaccompanied by anzoning record assimilation of country rock amphibole zoning patterns also record periodic heating events amphibole with clinopyroxene rims record episodes where the magma was heated beyond the amphibole stability field bulk compositional homogeneity and the juxtaposition of many crystals with disparate zoning patterns in single pumice require the magmatic system to be well mixed strontium diffusion rates indicate that the plagioclase zoning patterns cannot have persisted at magmatic pressures and temperatures for more than 500 years thus cycles of injection and assimilation occur on timescales equal to or shorter than the eruption recurrence interval long term compositional and isotopic homogeneity indicate that there is a balance between recharge assimilation and crystallization

roundness parameters of quartz grains of recent aeolian sand deposits in kuwait abstract roundness properties of quartz grains from various types of recent aeolian deposits in kuwait were examined the results confirmed that rounded and well rounded grains are relatively rare the investigated quartz grains were found to be mainly subangular and subrounded although no difference was noted in the roundness properties of the various types of aeolian deposits significant variation was recorded among the size fractions grain size distribution and the maturity of source sediments are considered to be the main factors controlling the roundness properties of recent aeolian sediments

sr and pb isotope micro analysis of plagioclase crystals from skye lavas an insight into open system processes in a flood basalt province crystals in many magmatic rocks are heterogeneous in terms of their isotope composition detailed crystal isotope stratigraphy cis studies have shown that crystals act as reliable recorders of their magma source composition and of the pathways of magma interaction and successfully identify the end members involved in magmatic systems in this paper we have analysed cores and rims and single plagioclase crystals from samples of the skye volcanic centre part of the british palaeogene igneous province the isotope analyses reveal that plagioclase crystals from different volcanic samples through thelava succession in skye skye main lava series preshal more basalts and big feldspar lavas have larger sr and pb isotopic ranges than the isotopic ranges found in hundreds of whole rock analyses the large sr and pb isotopic variation in plagioclase crystals in skye is due to variable degrees of crustal contamination of the parental magmas during fractional crystallization and during the ascent of such magmas to the surface the isotope variation of plagioclase crystals in the early lavas skye main lava series reflects assimilation fractional crystallization processes dominated by the lower crust whereas plagioclase crystals from later erupted magmas big feldspar lavas and preshal more basalts show assimilation of upper crust superimposed on previous lower crustal contamination processes the variability in sr and pb isotopes documented in different crystals within the same rocks shows that the crystals have been aggregated in magmas from different sites of storage and differentiation during the ascent of the magmas to the surface

platinum group mineralization in the rum layered intrusion scottish hebrides uk abundant and diverse platinum group minerals occur in the rum layered intrusion inner hebrides scotland platinum group minerals are associated with chromite rich horizons in both the eastern and western layered series and are enclosed by ni and cu rich sulphides or silicates the most common phases are pd cu alloys pt fe alloys laurite ru ir os s2 sperrylite ptas2 and pt pd bearing bismuthides tellurides and antimonides the mineralization on rum is due to 1 initial concentration in the parental magma linked to a high degree of melting associated with mantle plume activity and 2 localized concentration due to magma mixing and subsequent crystallization within an open system magma chamber

combining csd and isotopic microanalysis magma supply and mixing processes at stromboli volcano aeolian islands italy integrating isotopic microanalysis with other analytical techniques creates powerful new methodologies for understanding the evolution of rock samples at the sub grain scale here we present crystal size distribution csd data for a 26 000 year old sample from stromboli volcano and accompanying isotopic microanalysis of the phenocrysts a technique called the icsd plot is introduced which given stated assumptions allows the integration of both sets of data to generate timelines of isotopic evolution through the volcanic system the combined approach is powerful allowing investigation of the magma supply mixing crystallisation and contamination processes prior to eruption of a volcanic sample for stromboli volcano the combined analysis suggests that the change in magma type following a cone collapse took roughly five years to complete similar to the timescale of changes seen in recent decades

multiple rhyolite magmas and basalt injection in the 17 7 ka rerewhakaaitu eruption episode from tarawera volcanic complex new zealand abstract the 17 7 ka rerewhakaaitu eruption episode volume 5 km3 dre rhyolite magma was the second of five major episodes that have built the tarawera volcanic complex in the okataina volcanic centre over the past 22 kyr the rerewhakaaitu episode produced a widespread tephra fall deposit associated proximal pyroclastic flow deposits and voluminous rhyolite lava extrusions two different rhyolite magmas t1 and t2 were simultaneously erupted from the main vent area throughout much of the eruption episode t1 magma was a crystal poor orthopyroxene hornblende rhyolite that is highly evolved whole rock sio2 77 wt with a moderate temperature 760 c based on fe ti oxides t2 is a crystal rich biotite hornblende rhyolite that is less evolved sio2 75 wt with a fe ti oxide temperature of 700 c ejecta from the simultaneous and sequential eruption of these two magmas include some pumice clasts with mixed hybrid and mingled glass compositions and crystal populations a third rhyolite magma t3 was extruded from another vent 3 km distant to form an apparently contemporaneous lava dome t3 was the least evolved sio2 74 wt and hottest 820 c of the three magmas saturation pressures calculated using dissolved h2o and co2 contents of melt inclusions in quartz crystals indicate that t2 magma stagnated and crystallised at about 12 km depth while small quartz crystals in t1 magma grew during ascent through 8 km depths some t1 and t2 rhyolite clasts contain vesicular brown blebs with widely variable andesite to rhyolite glass compositions accompanied by olivine clinopyroxene and calcic plagioclase crystals that are interpreted as xenocrysts derived from injected basalt temperatures over 1000 c estimated from pyroxene phase equilibria in these clasts reflect intrusion of the more mafic magma which is now identified as the priming and triggering mechanism for three of the four post 22 ka tarawera rhyolite eruption episodes however the rhyolite magma bodies and conduits modelled for each episode have considerable differences in characteristics and geometry our preferred model for the rerewhakaaitu episode is that eruptions occurred from three laterally and vertically isolated rhyolite magma bodies that were initially primed and triggered by basalt intrusion during a regional rifting event the ascending hotter and less viscous t1 rhyolite magma intersected and further invigorated a stagnant pond of cooler denser and more viscous t2 magma and lubricated its transport to the surface

concentration of platinum group elements by magmatic fluids in layered intrusions evidence leading to a model which supports the formation of pt pd reefs in a layered intrusion by the action of magmatic fluids is presented as well as some of the constraints imposed on such a model the stillwater complex is used as an example of this model within the stillwater complex no laterally continuous sulfide enriched cumulates occur above the lowermost parts of the ultramafic series until one reaches the platiniferous j m reef this sulfide poor stratigraphic interval beneath the j m reef is characterized by unusually cl rich apatite although reported sulfide modes and whole rock sulfur contents in the cumulates below the j m reef are approximately an order of magnitude lower than that expected for a sulfide saturated magma sulfide saturation prior to the formation of the j m reef is suggested by relatively high pd and pt concentrations reported in the cumulates below the j m reef as well as by the presence of sulfide as inclusions in silicate and oxide cumulus minerals hence it is suggested that the footwall cumulates to the j m reef were originally sulfide saturated but later lost sulfur to a fluid phase exsolving from inter cumulus liquid sporadic sulfide bearing pegmatoids in some of these rocks are additional evidence for late loss of sulfur these observations as well as the association of the j m reef at the first significant stratigraphic break in the cl cl f ratio of apatite and the relatively high solubility of pt and pd as aqueous cl complexes are consistent with the formation of the j m reef by exsolution of cl and volatile rich fluids from footwall intercumulus liquids which leached and transported ore components both the pge and s upward to the level of the reef using published whole rock platinum group element pge concentrations for the ultramafic and lower banded series the cumulates below the j m reef must have been enriched by a factor of at most 1 24 for pt and 1 75 for pd if these rocks were to be the source for the metals in the j m reef model raleigh fractionation calculations suggest that the cumulates beneath the j m reef could have had the required metal concentrations if during the crystallization of a parent magma containing no more than 15 ppb of either metal the average bulk solid liquid distribution coefficients for pt and pd were no more than 5 these values are consistent with assumption of sulfide saturation prior to the formation of the j m reef during solidification of intercumulus liquid the separation and loss of a cl rich fluid will deplete the residual assemblage of s and pge if 0 1 to 0 5 wt percent fluid containing 5 percent s evolved during solidification of the footwall cumulates the amount of fluid expected to be exsolved by the crystallization of approximately 2 to 10 percent fluid saturated intercumulus liquid the fluid liquid distribution coefficients for pt and pd would need to be almost as large as the sulfide liquid silicate liquid distribution coefficients to remove significant amounts of pt and pd much smaller distribution coefficients are needed if sulfide is completely lost to fluid during degassing the latter possibility is consistent with the low s contents of the footwall cumulates beneath the j m reef numerical models of the degassing of intercumulus liquids suggest that thick cumulus sequences can act as chromatographic columns to separate the pge and s during the degassing of intercumulus silicate liquids the pge being enriched at a sulfide dissolution front as upward migrating sulfide undersaturated fluids resorb cumulus sulfides zones of pge and s enrichment can occur where degassing fronts encounter stratigraphic physical chemical discontinuities such as changes in the fluid saturation temperature of the intercumulus silicate liquids

magma evolution and open system processes at shiveluch volcano insights from phenocryst zoning phenocryst zoning patterns are used to identify open system magmatic processes in the products of the 2001 eruption of shiveluch volcano kamchatka the lavas and pumices studied are hornblende plagioclase andesites with average pre eruptive temperatures of 840c and fo2 of 1 5 2 1 log units above nickel nickel oxide nno plagioclase zoning includes oscillatory and patchy zonation and sieve textures hornblendes are commonly unzoned but some show simple multiple or patchy zoning apatite microphenocrysts display normal and reverse zoning of sulphur the textural similarity of patchy hornblende and plagioclase together with ba sr systematics in patchy plagioclase indicate that the cores of these crystals were derived from cumulate material plagioclase liquid equilibria suggest that the patchy texture develops by resorption during h2o undersaturated decompression when h2o saturated crystallization recommences at lower pressure reduced ph2o results in lower xan in plagioclase causing more al rich hornblende to crystallize plagioclase cores with diffuse oscillatory zoning and unzoned hornblende crystals probably represent a population of crystals resident in the magma chamber for long periods of time in contrast oscillatory zoning in the rims of plagioclase phenocrysts may reflect eruption dynamics during decompression crystallization increasing fe al in oscillatory zoned rims suggests oxidation as a result of degassing of h2o during decompression a general lack of textural overlap between phenocryst types suggests that different phenocryst populations were spatially or temporally isolated during crystallization we present evidence that the host andesite has mixed with both more felsic and more mafic magmas olivine and orthopyroxene xenocrysts with reaction or overgrowth rims and strong normal zoning indicate mixing with basalt sieve textured plagioclase resulted from mixing of a more felsic magma with the host andesite the mineralogy and mineral compositions of a mafic andesite enclave are identical to those of the host magma which implies efficient thermal quenching and thus small volumes of intruding magma mixing of this magma with the host andesite results in phenocryst zoning because of differences in dissolved volatile contents we suggest that small magma pulses differentiated at depth and ascended intermittently into the growing magma chamber producing incremental variations in whole rock compositions

fingerprinting feldspar phenocrysts using crystal isotopic composition stratigraphy implications for crystal transfer and magma mingling in s type granites microsampling of cm scale feldspar crystals within an s type granite from the lachlan fold belt of southeastern australia has revealed complex internal sr and nd isotopic variations the observed isotopic zonations are in part interpreted as recording feldspar crystallisation in a dynamically mixing magma system the isotopic composition of which was varying in response to the influx of more mafic and isotopically more mantle like magmas the latter stages of which are now represented in modified form by microgranular enclaves similar core to rim isotopic variations in feldspar megacrysts from a microgranular enclave and the adjacent host granite strongly suggest megacrysts in the enclave were transferred from the granitic magma during crystallisation feldspar rims have higher 87sr 86sri and lower nd i than adjacent whole rock analyses but match those of mineral separates from the surrounding enclave matrix this suggests that the final stages of megacryst growth occurred in the presence of a component that had previously interacted with a high 87sr 86sr low nd i component such as metasedimentary wall rocks isotopic heterogeneities are also presererved within different mineral phases in the enclave matrix suggesting that differing phases grew at differing stages of equilibration between the enclave magma and its host granitic magma our results reveal major isotopic heterogeneities on a single crystal and also inter mineral scale in a pluton which shows well constrained evidence for magma mingling these results indicate the suitability of feldspars as recorders of isotopic change in magmatic systems even those which have cooled slowly in the plutonic environment and suggest that much heterogeneity in plutonic systems may be overlooked on a whole rock scale

mineral scale sr isotope constraints on magma evolution and chamber dynamics in the rum layered intrusion scotland sr isotopic zoning within single plagioclase crystals from rocks from unit 9 of the rum layered intrusion is used to infer events during crystal growth in a magma undergoing contamination the 87sr 86sr diversity among minerals and between cores and rims of plagioclase crystals increases as the boundary between unit 9 and the overlying unit 10 peridotite is approached models of near solidus interaction of the cumulate with a fluid or melt or large scale textural re equilibration cannot easily account for the systematic differences in 87sr 86sr between small crystals and the rims of larger crystals

multiple plagioclase crystal populations identified by crystal size distribution and in situ chemical data implications for timescales of magma chamber processes associated with the 1915 eruption of lassen peak ca products of the 1915 lassen peak eruption reveal evidence for a magma recharge magma mixing event that may have catalyzed the eruption and from which four compositional members were identified light dacite black dacite andesitic inclusion and dark andesite crystal size distribution textural and in situ chemical major and trace element and sr isotope data for plagioclase from these compositional products define three crystal populations that have distinct origins phenocrysts long axis40 5 mm that typically have core an contents between 34 and 36 mol microphenocrysts long axis between 0 1 and 0 5 mm that have core an contents of 66 69 and microlites long axis50 1mm with variable an core contents from 64 to 52 phenocrysts are interpreted to form in an isolated dacitic magma chamber that experienced slow cooling based on textural compositional and isotopic data for the magma represented by the dacitic component magma recharge was not an important process until just prior to the 1915 eruption average residence times for phenocrysts are in the range of centuries to millennia microphenocrysts formed in a hybrid layer that resulted from mixing between endmember reservoir dacite and recharge magma of basaltic andesite composition high thermal contrast between the two end member magmas led to relatively high degrees of undercooling which resulted in faster crystal growth rates and acicular and swallowtail crystal habits some plagioclase phenocrysts from the dacitic chamber were incorporated into the hybrid layer and underwent dissolution precipitation seen in both crystal textures and rim compositions average microphenocryst residence times are of the order of months microlites may have formed in response to decompression and or syn eruptive degassing as magma ascended from the chamber through the volcanic conduit chemical distinctions in plagioclase microlite an contents reveal that melt of the dark andesite was more mafic than the melt of the other three compositions we suggest that mixing of an intruding basaltic andesite and reservoir dacite before magma began ascending in the conduit allowed formation of a compositionally distinct microlite population melt in the other three products was more evolved because it had undergone differentiation during the months following initial mixing as a consequence melt and microlites among these three products have similar compositions the results of this study highlight the integrated use of crystal size distribution textural and in situ chemical data in identifying distinct crystal populations and linking these populations to the thermal and chemical characteristics of complex magma bodies

the ion microprobe as a tool for obtaining strontium isotopes in magmatic plagioclase a case study at okataina volcanic centre new zealand abstract we investigated the potential of multi collector secondary ion mass spectrometry mc sims as a tool for obtaining sr isotopic compositions in plagioclase a ubiquitous mineral in igneous rocks that serves as a recorder of crystallization history mc sims allows for high spatial resolution analysis 12 m in this study of isotopes and therefore improves the temporal scale at which fluctuations in crystallization conditions can be recognized ultimately improving our understanding of rates of magmatic processes plagioclase crystals from two young rhyolitic deposits from two major eruptive complexes tarawera and haroharo of the okataina volcanic centre in new zealand were analysed results were corrected for matrix effects using linear modelling of mc sims data versus an contents as well as 87sr 86sr ratios acquired via laser ablation inductively coupled plasma mass spectrometry la mc icp ms corrected mc sims sr isotopic ratios had an average 2 uncertainty of 0 0008 per spot and were homogeneous in okataina plagioclase at high spatial resolutions average la mc icp ms 87sr 86sr ratios of plagioclase from both intra caldera volcanic complexes tarawera 87sr 86sr 0 7056 and haroharo 87sr 86sr 0 7054 suggest similar magma sources and similar assimilation and fractional crystallization processes for the two complexes overall homogeneity of plagioclase excluding relict cores indicates no significant changes in contributions i e crustal assimilation mafic influx to the system during the majority of plagioclase crystal growth furthermore lack of 87sr 86sr ratio fluctuations in plagioclase rims suggest interaction between the resident silicic magma and the intruding mafic magma that triggered the eruptions was largely limited to volatile and heat transfer using appropriate standards and analysis this mc sims method can be used to detect short lived open system events in magma reservoirs where differences in 87sr 86sr isotopic ratios are significant

strontium diffusion kinetics in plagioclase feldspars abstract a pronounced systematic variation by a factor of 104 in strontium tracer diffusion kinetics has been measured in four feldspar specimens that span the range of plagioclase compositions the experiments were carried out in air using isotopically enriched 86sr and analyzed with an ion microprobe the arrhenius relations for strontium diffusion are plagioclase ab d 0 m 2 s q kj g atom sr temperature range c albite 98 1 05e 279 550 1080 oligoclase 73 7 4e 7 261 750 1100 labradorite 36 1 1e 6 295 800 1300 anorthite 4 5 7e 9 267 900 1300 no difference was found within the measurement uncertainty between these results and those for albite under hydrothermal conditions at one kbar 100 mpa strontium diffusion rates parallel to and normal to the albite twin lamellae are the same there is no effect on the strontium diffusion rate in albite between an oxygen partial pressure of 0 2 bar air and 10 19 bar graphite buffer a relation is developed that permits calculation of the strontium diffusion arrhenius parameters for any normal plagioclase composition the large variation in diffusion rate with composition has a major effect on the ease of strontium isotope exchange at high temperatures between plagioclases and coexisting phases in rocks which may prove useful in determining thermal histories

platinum group element chromitite associations in the bushveld complex new and recently published platinum group element data of chromite rich layers in the bushveld complex are evaluated chondrite normalized platinum group element plots of these data show that trends for chromitite layers differ from that of the merensky reef the data furthermore indicate remarkable consistencies in compositional trends for many kilometers along strike of individual layers suggesting that the conditions of formation of chromitite layers and their associated platinum group element mineralization were very uniform over large areas of the magma chamber significant differences in the proportions of platinum group elements within a particular layer are only evident on a regional scale the platinum group minerals of the middle group of chromitite layers in the marikana area of the western bushveld are mainly sulfides and arsenides laurite is the most common platinum group mineral and is mostly enclosed by chromite a variety of other platinum group minerals occur in close association with base metal sulfides extremely low bulk sulfur content in the chromitite layers is ascribed to a postcumulus loss of sulfur postcumulus loss of sulfur and the existence of laurite enclosed within chromite casts doubt on the validity of any petrogenetic models for platinum mineralization in chromitite rich layers based on partition coefficients and the r factor

contrasting punctuated zircon growth in two syn erupted rhyolite magmas from tarawera volcano insights to crystal diversity in magmatic systems abstract two mineralogically and chemically distinct rhyolite magmas t1 and t3 were syn erupted from the same conduit system during the 21 9 ka basalt intrusion triggered okareka eruption from tarawera volcano new zealand high spatial resolution u th disequilibrium dating of zircon crystals at the 3 5 m scale reveals a protracted yet discontinuous zircon crystallization history within the magmatic system both magma types contain zircon whose interiors predate the eruption by up to 200 ka the dominant age peak in the t1 magma is 30 ka with subordinate peaks at 45 75 and 100 ka whereas the t3 magma has a dominant zircon interior age peak at 90 ka with smaller modes at 35 and 150 ka these patterns are consistent with isolated pockets of crystallization throughout the evolution of the system crystal rim analyses yield ages ranging from within error of the eruption age to at least 90 ka prior to eruption highlighting that zircon crystallization frequently stalled long before the eruption continuous depth profiling from crystal rims inward demonstrates protracted growth histories for individual crystals up to 100 ka that were punctuated by asynchronous hiatuses of up to 30 ka in duration disparate zircon growth histories can result from localized thermal perturbations caused by mafic intrusions into a silicic reservoir the crystal age heterogeneity at hand sample scale requires considerable crystal transport and mixing we propose that crystal mixing was achieved through buoyancy instabilities caused by mafic magma flow through crystal mush a terminal pre eruptive rejuvenation event was capable of mobilizing voluminous melts that erupted but was too short

magma mixing recharge and eruption histories recorded in plagioclase phenocrysts from el chichon volcano mexico operative throughout the 200 ky history of the el chichon magma consistent core to rim decreases of sr sr ratios and coincident system increases in sr concentrations in plagioclase phenocrysts of varying size 1 cm to 2 mm are reported from samples of the 1982 and pre 1982 200 ka eruptions of el chichon volcano maximum sr sr ratios of 0 7054 significantly higher than the wholerock isotopic ratios 0 7040 0 7045 are found in the cores

magma mixing recorded by sr isotopes of plagioclase from dacites of the quaternary tengchong volcanic field se tibetan plateau abstract the tengchong volcanic field east of the burma arc in sw china comprises numerous quaternary volcanoes the volcanic rocks can be grouped into four units numbered 1 4 from oldest to youngest units 1 3 and 4 are composed of trachybasalt basaltic trachyandesite and trachyandesite respectively whereas unit 2 consists of hornblende dacite primary minerals in the dacite include plagioclase clinopyroxene amphibole and magnetite all set in a glassy groundmass some of the dacites are moderately to highly weathered and show effects of low temperature alteration with loss on ignition loi up to 9 6 wt and positive correlations of loi with al2o3 but negative correlations with sio2 cao and na2o fresh dacites loi plagioclase in the fresh dacite has relatively high sr contents and 87sr 86sr ratios ranging from 0 7050 to 07125 except for one value of 0 7516 whereas plagioclase in the altered dacite has low sr contents and highly variable sr isotopic compositions 0 7039 0 7138 indistinguishable from those of the fresh rocks both an values and sr isotopic compositions of the plagioclase indicate mixing of mantle and crustally derived magmas we therefore propose that mantle derived basaltic magma caused partial melting of the lower middle crust and that mixing of the mafic and felsic magmas produced the dacite in staging magma chambers the dacites contain minerals crystallized from both the mafic and felsic magmas as well as a few xenocrysts plucked from the country rocks

the ad1315 tarawera and waiotapu eruptions new zealand contemporaneous rhyolite and hydrothermal eruptions driven by an arrested basalt dike system a series of large hydrothermal eruptions occurred across the waiotapu geothermal field at about the same prehistoric time as the ad1315 kaharoa rhyolite magmatic eruptions from tarawera volcano vents 10 20 km distant triggering of the waiotapu hydrothermal eruptions was previously attributed to displacement of the adjacent ngapouri fault the kaharoa rhyolite eruptions are now recognised as primed and triggered by multiple basalt intrusions beneath the tarawera volcano a 1000 t day pulse of co2 gas is recorded by alteration mineralogy and fluid inclusions in drill core samples from waiotapu geothermal wells this co2 pulse is most readily sourced from basalt intruded at depth and although not precisely dated it appears to be associated with the waiotapu hydrothermal eruptions we infer that the hydrothermal eruptions at waiotapu were primed by intrusion of the same arrested basalt dike system that drove the rhyolite eruptions at tarawera this dike system was likely similar at depth to the dike that generated basalt eruptions from a 17 km long fissure that formed across the tarawera region in ad1886 fault ruptures that occurred in the waiotapu area in association with both the ad1886 and ad1315 eruptions are considered to be a result rather than a cause of the dike intrusion processes

plagioclase residence times at two island arc volcanoes kameni islands santorini and soufriere st vincent determined by sr diffusion systematics the diffusive relaxation of trace element profiles in plagioclase phenocrysts may provide important constraints on magma residence times in crustal magma chambers initial trace element profiles in plagioclase phenocrysts are governed by variations in the concentration of a trace element in the melt and by the plagioclase melt partition coefficient trace element diffusion will subsequently act to modify this initial profile and given enough time produce a profile that is in equilibrium with the anorthite variations within the crystal we argue that the trace element partition coefficient da b between two parts a and b of a plagioclase crystal of variable anorthite content is equal to the ratio of their crystal liquid partition coefficients and that the equilibrium profile of the crystal can be calculated the time required to establish diffusive equilibrium is dependent on the wavelength and amplitude of the initial trace element concentration range and on the diffusivity of the trace element in plagioclase strontium plagioclase melt partition coefficients and diffusivities are calculated for a range of magmatic temperatures and plagioclase compositions a one dimensional diffusion model is developed that describes the diffusive destruction of oscillatory trace element zoning with time and allows the calculation of upper limits for plagioclase crystal residence times in a magma reservoir the model is tested using major and trace element concentrations measured along crystal traverses of plagioclase phenocrysts from the kameni island dacites santorini and from the 1979 soufriere andesite st vincent three out of eight plagioclase phenocrysts have sr concentration profiles that are not in diffusive equilibrium for these the diffusion model is employed to calculate maximum crystal residence times from incomplete diffusive equilibration of trace element zoning in plagioclase maximum crystal residence times range from 100 to 450 years this is in good agreement with estimates from crystal size distribution and from ra th disequilibrium studies for the kameni islands for soufriere however such short residence times are incompatible with u th mineral errorchron data that suggest residence times of 40 ka in a thermally buffered magma reservoir to reconcile these apparently different ages we invoke a more complicated magmatic history for soufriere where an initially buffered magma reservoir is disturbed by magma mixing and suffers limited additional crystal fractionation prior to eruption

sr isotope zoning in plagioclase from parinacota volcano northern chile quantifying magma mixing and crustal contamination we present analyses of sr isotope zoning by microdrilling and thermal ionization mass spectrometry in plagioclase crystals from parinacota volcano central volcanic zone northern chile which were analysed for major and minor element zoning in a previous study although the isotopic range of the bulk rock samples is small at this volcano 0 7067 0 7070 except for one flow of mafic andesite at 0 7061 significant variations are seen 0 70649 0 70700 within and between plagioclase crystals a general negative correlation is observed between sr isotope composition and sr concentration in the liquid in equilibrium with each plagioclase zone as calculated from chemical zoning data and partition coefficients additional scatter is superimposed on this general trend indicating a decoupling between isotopic and chemical variations for sr in one dacite sample a detailed isotopic profile shows increasing contamination during crystal growth except for an abrupt decrease correlated with a dissolution surface and interpreted as a recharge event we apply energy constrained recharge assimilation and fractional crystallization modelling to the melt evolution recorded in the chemical and isotopic zoning in this crystal results suggest 20 assimilation of the local wall rock gneiss at high initial temperatures the isotopic data confirm the involvement of two contrasting mafic magmas which are sampled at flank cinder cone vents one lower ajata has a low sr content with high 87sr 86sr the other upper ajata has a high sr content with lower 87sr 86sr in some samples from parinacota the isotopic composition of plagioclase crystal rims or groundmass crystals is significantly higher than that of the high 87sr 86sr mafic magma in others where chemical zoning profiles suggest that recharge was from the low 87sr 86sr magma the 87sr 86sr of the groundmass and crystal rims is higher than expected this indicates either additional parent magmas to the two previously identified or further crustal assimilation either at lower crustal depths before crystallization of plagioclase or just after the last recharge our results illustrate the complexity of magma crust interaction beneath parinacota which is likely to be representative of many other central andean volcanoes formed on thick crust such complex interactions can be revealed by combined study of chemical and isotopic zoning in plagioclase in a textural petrographic context despite a small whole rock isotopic range the distinct contamination patterns of various samples suggest an important role for the geometry location and evolution of the magma plumbing system and in general variations of the thermal and compositional structure of the crust underneath the volcano

recharge in volcanic systems evidence from isotope profiles of phenocrysts strontium isotope ratios measured from core to rim across plagioclase feldspar crystals can be used to monitor changes in the isotope composition of the magma from which they grew in samples from three magma systems from convergent margin volcanoes sudden changes in major element composition petrographic features and strontium isotope composition were found to correspond to discrete magmatic events most likely repeated recharge of more mafic magma with lower ratios of strontium 87 to strontium 86 into a crustally contaminated magma

geology and geochemistry of intrusions and flood basalts of the noril sk region ussr with implications for the origin of the ni cu ores geochemical studies indicate that five principal magma types characterize the 3 8 km vertical thickness of flood basalt of the noril9sk region from base to top these are 1 a fractionated type with alkalic affiliations ivakinsky and syverminsky 2 an ni rich suite including picritic basalt gudchichinsky 3 a primitive but ni depleted suite with fiat rare earth element ree profiles and also including picritic basalts tuklonsky 4 a light ree enriched crustally contaminated suite lower nadezhdinsky nd 1 above 4 there is a transitional suite upper nadezhdinsky nd 2 and morongovsky mr 1 and mr 2 which is overlain by the fifth type a second primitive suite without picrites mokulaevsky similar but not identical in its chemistry to the tuklonsky type this is followed by tuffs and basalts of the kharayelakhsky kumginsky and samoyedsky suites which are largely composed of similar uncontaminated tholeiitic magma the latter three suites have not been studied by us intrusions of the region fall into five main groups 1 alkalic intrusions 2 ti rich dolerite dikes 3 dolerite sills and dikes 4 differentiated bodies not related to ore junctions and 5 differentiated bodies related to ore junctions the ni cu ores are associated with ni and cr rich type 5 olivine rich intrusions with flat ree profiles that are geochemicaly similar to the mokulaevsky magma light ree enriched ni and cr poor olivine rich type 5 intrusions with uneconomic mineralization occur closely associated with the strongly mineralized variants and resemble geochemically the basalts transitional between the lower nadezhdinsky and upper morongovsky they are cut by the strongly mineralized bodies indicating that the latter are of late morongovsky to mokulaevsky age the mineralized intrusions are associated with about 1 5 to 1 10 of their mass in sulfides this is a factor of 25 to 50 more than could have been carried in solution and deposited by in situ liquation from the magma of these intrusions the platinum group element pge concentrations in the ores require that the sulfides giving rise to them equilibrated with 15 to 200 times more magma than is represented by the intrusions much of the ore is very massive and crosscuts the host intrusions the sequence of igneous activity and mineralization in the immediate vicinity of noril9sk and talnakh from the base of the nadezhdinsky upward is thus interpreted to be 1 extrusion of highly crustally contaminated ni and cu poor basalt 2 extrusion of a series of basalt flows each progressively less contaminated and less ni and cu depleted than the preceding one accompanied by intrusion of contaminated ni depleted magma with weak mineralization 3 extrusion of uncontaminated ni and cu rich basalt accompanied by intrusion of uncontaminated ni rich magma with strong mineralization followed very closely by introduction of associated massive ore and 4 continued extrusion of ni and cu rich basalt it is proposed that crustal contamination of the original ni and cu rich magma occurred at the top of a vertically extensive fault controlled magma chamber and caused segregation of immiscible sulfides with a low r factor these depleted this magma in chalcophile elements and settled deeper in the chamber as they settled they reacted with less contaminated magma scavenging additional chalcophile elements they finally came to rest near the base of the chamber to form a zone of sulfide and thus chalcophile element enriched magma eruption and in some instances intrusion of progressively stratigraphically lower less contaminated and thus less depleted layers in the chamber gave rise to the observed sequence of basalts and intrusions the answer to the strong enrichment in ni cu and pge associated with the ore bearing intrusions is that these metals came from the magma that is now represented by the depleted basalts

primary magmas at the volcanic front of the ne japan arc coeval eruption of crustal low k tholeiitic and mantle derived medium k calc alkaline basalts at azuma volcano it has long been thought that the low k tholeiitic th and medium k calc alkaline ca lavas in the nejapan arc were produced by fractional crystallization from mantle derived basalt magmas and that the latter formed from mixing of mafic and felsic magmas both derived from a common primaryth basalt through fractionation an alternative view was recently proposed on the basis of sr isotope microanalysis of plagioclase phenocrysts from the zao volcano suggesting that 1 the low k th basaltic andesites formed by melting of lower crustal amphibolite and that 2 the medium k ca basalts to andesites formed by mixing of mantle derived basalt and crustal th melts to investigate further the origin of the primary low k th and medium k ca basalts we investigated basalts and andesites from azuma volcano azuma is a quaternary eruption center at the volcanic front in the ne japan arc that has erupted two types of basalt 1 radiogenic sr sr sr1 4 0 7058 0 7062 low k th basalt lavas without evidence of magma mixing and assimilation 2 unradiogenic sr sr sr1 4 0 7039 0 7041 medium k ca basalt lavas with subtle evidence for magma mixing associated intermediate lavas are voluminous and are all 3 mildly radiogenic sr sr sr1 4 0 7044 0 7055 medium k andesites all of which have ca affinities with evidence for rigorous magma mixing but no crustal assimilation the low k th basalt has an isotopic composition similar to that of crustal granitoids beneath azuma and has a composition indicating that it potentially formed from a high degree lower crustal amphibolite melt the medium kca basalt has a basaltic groundmass with mg rich olivine fo89 and calcic plagioclase phenocrysts an90 and the most unradiogenic sr sr sr1 4 0 7037 0 7038 suggesting that it originated from a primary mantle melt major and trace element microanalysis of the basaltic groundmass indicates that the primary magma composition is close to high k we conclude that the mantle derived basalt at azuma is the result of a highto medium k magma that was later mixed with a low k th basalt melt from the amphibolitic lower crust to form medium k ca basalts and andesites this supports the view of a lower crustal origin of the low kth basalts and simultaneously requires a reappraisal of the origin of the across arc variation in k contents of the mantle derived primary arc basalts as the highto medium k ca basalt is geochemically fairly similar to the high k rear arc basalt in the ne japan arc

basalt triggering of the c ad 1305 kaharoa rhyolite eruption tarawera volcanic complex new zealand abstract the ad 1305 kaharoa eruption episode of tarawera volcanic complex was the largest 4 km3 magma eruption in new zealand in the last 1000 years high silica 76 sio2 rhyolite was erupted from seven vents along an 8 km linear zone to form pumiceous pyroclastic deposits and lava domes initial vent clearing explosions were followed by a series of plinian pumice eruptions that spread tephra downwind and pyroclastic density current deposits over the volcano slopes the early pyroclastic eruptions were followed by extrusion of a dome in the summit vent and the migration of further plinian activity to adjacent vents the episode finished with the extrusion of three summit lava domes accompanied by block and ash flows two main types of rhyolite were erupted 1 a low zr low sr high rb type comprising the early plinian pyroclastics and 2 a high zr high sr low rb type comprising the late eruptives including the summit domes basaltic material basalt and basaltic andesite is commonly present as free clasts within the kaharoa pyroclastic deposits along with rare clasts of granodiorite diorite gabbro and olivine clinopyroxenite the basaltic clasts are often veneered by kaharoa rhyolite basaltic material is also common as inclusions within rhyolite pumices along with olivine and augite xenocrysts derived from the basalt the basaltic clasts and inclusions contain complexly zoned plagioclase and corroded olivine commonly with a reaction rim consistent with disequilibrium caused by magma mixing some basaltic clasts and inclusions are hornblende free others contain groundmass hornblende plumes of brown glass often extend from hornblende bearing basaltic inclusions into the surrounding rhyolite these plumes demonstrate magma mixing as does the presence of rhyolitic glass and quartz xenocrysts in some basaltic inclusions most basaltic inclusions have crenulate boundaries suggesting they were fluidal when coming into contact with the rhyolite magma diorite gabbro and olivine clinopyroxenite clasts have mineralogy similar to the basalts commonly contain patches of fine grained basaltic material and are considered co magmatic with the basalts intrusion of basalt magma into the base of the rhyolite magma chamber appears to have occurred for some time before the kaharoa eruptions began allowing considerable mixing with the rhyolite magma transfer of water to some basalt inclusions and formation of hornblende in their groundmass conversely the hornblende free basalt inclusions did not have time to react with the rhyolite magma and must have been intruded shortly before the start of the eruption intrusion of this hornblende free basalt appears to have triggered the kaharoa eruption after the magma chamber had been primed by the earlier intrusions

geochemical evolution of the tertiary mull volcano western scotland the early tertiary mull volcano western scotland is one of the most dissected and best exposed igneous complexes of the north atlantic province the new and published geochemical data enable us to chart the magmatic evolution of the mull volcano from the oldest lavas through the intrusive rocks of three overlapping igneous centres to the youngest dykes in this study we identify four successive magma types within the remnant volcano the earliest type the mull plateau group comprises mildly alkaline basaltic rocks with steep chondrite normalized rare earth element ree patterns this type is succeeded within the lava succession and dyke swarm by the coire gorm magma type with essentially flat chondrite normalized ree patterns a third magma type represented within the lava and dykes the central mull tholeiites is more depleted in incompatible trace elements than the preceding types and has flat to lree depleted chondrite normalized patterns the major intrusions and cone sheets of mull centre 1 and early centre 2 belong to this magma type midway through the igneous activity associated with centre 2 the magma type changed to become more alkalic and more enriched in incompatible trace elements this magma type the late mull type is found to persist through the cone sheets and major intrusions of centre 3 to the youngest dykes these changes in magma composition were related to variations in the mantle source and depth of partial melting beneath mull and or differences in the efficiency of melt pooling before ascent through the lithosphere with the exception of the early staffa magma sub type part of the mull plateau group the location of magma chambers in which the bulk of contamination occurred changed with time from deep lower crustal lewisian gneiss to shallow upper crustal moine schist intermediate members of the plateau group and the late mull magma type are enriched in fe ti and p relative to the central mull tholeiites we attribute this difference to the more alkalic nature of these suites lower fo2 and the formation of fe3 p complexes in the magma the intermediate rocks were important in magma mixing processes with two types of mixing identified on mull 1 cryptic mixing between basalts and low fe intermediate magmas typified by lavas and early basic cone sheets of the central mull tholeiite magma type 2 observable mingling between rhyolitic magmas and high fe intermediate magmas of the late mull type shown by the mixed magma bodies of the glen more and loch ba ring dykes the main factor in determining which type of mixing occurred appears to have been the density contrast between the various magmas it may safely be maintained that mull includes the most complicated igneous centre as yet accorded detailed examination anywhere in the world bailey et al 1924 memoir of the geological survey of great britain scotland hmso edinburgh 1924

in situ sr isotopic analysis by laser ablation we demonstrate that the sr isotopic compositions of geological materials can be measured in situ through laser ablation using an icp multiple collector double focusing magnetic sector mass spectrometer mc icpms this provides rapid texturally sensitive high precision 0 004 sr isotopic analysis without the need for chemical preparation of samples the system used employs a q switched nd yag infrared laser the laser ablated material is carried by ar flow to an icp torch the resulting ions are extracted from the plasma and focused with d c quadrupole lenses and an electrostatic analyzer prior to entering a magnetic sector analyzer and collection in an array of faraday collectors a feldspar megacryst with independently known 87sr86sr of 0 703117 13 and a modern gastropod shell with independently known 87sr86sr of 0 709170 10 were used evaluate the method both have 2000 ppm sr laser power was adjusted to give an average 88sr of 2 10 11 amps during analysis to replicate the beam size during typical thermal ionization mass spectrometry tims within run precisions in 87sr86sr of 4 10 5 compare favorably with the precision from tims thirteen analyses of the cameroon feldspar megacryst yielded a weighted average 87sr86sr of 0 703106 22 in perfect agreement with the tims measurement similarly 22 analyses of the modern gastropod shell gave a weighted average of 0 709182 22 identical to the known value no memory effect was observed when switching from the cameroon feldspar sample run for several hours to the modern shell material the power of this technique is illustrated with a preliminary study of systematic differences in 87sr86sr of 0 0004 between two feldspar crystals of a long valley basalt one feldspar has identical 87sr86sr to the host basalt confirming the lack of any matrix effect with this method

sr isotope disequilibrium during differentiation of the bandelier tuff constraints on the crystallization of a large rhyolitic magma chamber the 1 61 ma otowi member of the bandelier tuff jemez mountains volcanic field new mexico is a caldera forming high silica rhyolitic ignimbrite with a precursor fallout deposit the otowi member has high rb sr ratios the 87 rb 86 sr ratios from sanidines and glasses range from 14 to 570 most sanidines from glomerocrysts in the ignimbrite have 87 sr 86 sr i 0 7052 0 7056 whereas the ratios from glasses from glomerocrysts range from 0 7052 to 0 7079 quartz phenocrysts containing glass inclusions from both the initial fallout and the ignimbrite are markedly more radiogenic at 87 sr 86 sr i 0 7105 0 7113 despite having much lower rb sr ratios than glomerocryst glasses these relations require that the inclusion glasses are more contaminated with proterozoic country rock than are glomerocryst glasses textural isotopic and trace element data support a model in which crystals grow in a boundary layer with the most inclusion rich quartz grains closest to the magma country rock contact phenocrysts and glomerocrysts represent fragments from different zones of the chamber9s crystalline carapace disseminated throughout the magma prior to eruption an important implication of these results is that glass inclusions do not necessarily represent precursor magma compositions hence extrapolation of measured volatile contents of inclusion glasses to the entire volume of an erupted magma should be approached with caution

recording the transition from flare up to steady state arc magmatism at the purico chascon volcanic complex northern chile abstract the long term evolution of continental magmatic arcs is episodic where a few transient events of high magmatic flux or flare ups punctuate the low flux magmatism or steady state that makes up most of the arc history how this duality manifests in terms of differences in crustal architecture magma dynamics and chemistry and the time scale over which transitions occur is poorly known herein we use multiscale geochemical and isotopic characteristics coupled with geothermobarometry at the purico chascon volcanic complex pcvc in the central andes to identify a transition from flare up to steady state arc magmatism over 800 kyr during which significant changes in upper crustal magmatic dynamics are recorded the pcvc is one of the youngest volcanic centers related to a 10 1 ma ignimbrite flare up in the altiplano puna volcanic complex of the central andes activity at the pcvc initiated 0 98 0 03 ma with the eruption of a large 80 100 km3 crystal rich dacite ignimbrite high restricted 87sr 86sr isotope ratios between 0 7085 and 0 7090 in the bulk rock and plagioclase crystals from the purico ignimbrite combined with mineral chemistry and phase relationships indicate the dacite magma accumulated and evolved at relatively low temperatures around 800 850 c in the upper crust at 4 8 km depth minor andesite pumice erupted late in the ignimbrite sequence records a second higher temperature 965 c higher pressure environment 17 20 km but with similar restricted radiogenic bulk rock 87sr 86sr 0 7089 0 7091 to the dacites the compositional and isotopic characteristics of the purico ignimbrite implicate an extensive zone of upper crustal mixing assimilation storage and homogenization mash between 30 and 4 km beneath the pcvc 1 ma the final eruptions at the pcvc 0 18 0 02 ma suggest a change in the magmatic architecture beneath the pcvc these eruptions produced three small the textural and compositional characteristics of the pcvc attest to two distinct stages in its history at 1 ma the system was broadly homogeneous and dominantly dacitic recording extensive upper crustal magmatism by 0 2 ma the pcvc had transitioned to a more compositionally heterogeneous smaller volume mixed dacite to basaltic andesite system coinciding with the appearance of less enriched baseline compositions the evolution of pcvc is a microcosm of the central andean arc in this region where from 10 to 1 ma upper crustal mash processes resulted in the production and eruption of large volumes of homogeneous crystal rich dacite during a regional ignimbrite flare up since 1 ma decreasing explosivity smaller eruptive volumes increasing heterogeneity and the emergence of less isotopically enriched basaltic andesite to dacite composite volcanoes signal a return to steady state arc volcanism we posit that the transition from flare up to steady state captured at the pcvc tracks the waning of the arc scale thermal engine high magmatic fluxes during the flare up would lead to elevated geothermal gradients and efficient crustal processing leading to a dominantly crustal magmatism feeding the large volume purico ignimbrite this upper crustal mash zone would act as an efficient filter to any parental compositions precluding them from the eruption record as magmatic flux and thermal energy wanes crustal isotherms would relax leading to greater thermal contrast between parental magmas upper crust and remnant felsic magmas stored in the upper crust these changes are manifested in the preservation of textural and compositional heterogeneity and the survival of less isotopically enriched magmas in the upper crust the chemical imprint of these arc scale changes in magma dynamics is recorded at all scales from bulk rock to intra crystalline the distinct magma dynamics and chemical signatures of the two modes of arc magmatism detailed here should provide a model for investigations of mature continental arc evolution through time and space

mineral textures and zoning as evidence for open system processes investigations of mineral textures and zoning as evidence for open system processes during magmatic evolution have always been a centerpiece of petrological studies and have provided some of the best evidence for magma mixing and crustal contamination for many decades e g milch 1905 kuno 1936 eichelberger 1975 sato 1975 anderson 1976 in fact evidence from mineral studies was instrumental in the acceptance of magma mixing as important petrological process ever since it was initially proposed by bunsen 1851 in recent years mineral studies are invigorated by the development of high precision high resolution analytical instruments and techniques through which textural information and compositional data are combined e g jerram and davidson 2007 as minerals respond texturally and compositionally to changing magmatic environments they preserve in their crystal growth stratigraphy a wealth of information regarding their past history of magmatic processes and compositions cf ginibre et al 2007 on the other hand magmatic liquids melts are snapshots of current magmatic states and provide less direct evidence of the processes responsible for their evolution in addition liquids may crystallize thereby destroying the direct evidence they provide with an appropriate set of observations and measurements we can correlate textures with mineral compositions and thus produce a richer composite picture of magmatic evolution than compositional data of minerals alone this contribution reviews the zoning and textures of minerals frequently encountered in volcanic rocks many of which apply equally to plutonic rocks and the interpretations ascribed to these features in terms of open system magmatic processes first i will address the analytical tools that are used to acquire textural and compositional data followed by a brief review of open system processes and a summary of the relevant textures from that point on i will address some mineral specific aspects followed by a discussion

the triple group and the platinova gold and palladium reefs in the skaergaard intrusion stratigraphic and petrographic relations this paper describes the setting structure and textural relations of the platinova reefs in the skaergaard intrusion the reefs comprise a strata bound occurrence of au pge estimated at 91 million tons of rock with 1 8 ppm au and similar amounts of pd the precious metals occur in two major zones a lower major pd reef which is concordant with the modal layering and followed by at least ten minor pd reefs also concordant and an au concentration which fringes the pd reefs outward and upward through the triple group units the major pd concentration and the au concentration are separated in a systematic way across the intrusion near the center of the intrusion the au concentration occurs some 60 m higher in the stratigraphy than the main pd reef but the separation decreases gradually outward being only a few meters near the margins the dominant precious metal minerals are small grains of cu fe au pd pt alloys which typically are included in interstitial sulfides bornite digenite and smaller amounts of chalcopyrite the textural relations suggest a close genetic association between the sulfides and au pge minerals an association which is confirmed by whole rock compositional data our observations demonstrate that the platinova reefs formed after significant concentration of the au pge and s in the residual magma during fractional crystallization before it became saturated with respect to a sulfide fe cu s o liquid this sulfide liquid acted as a collector of au pge and settled into the layered series cumulates we propose that the structure of the deposit evolved through fractional crystallization of the au pge bearing sulfide liquid as it was filter pressed along with the interstitial silicate melt upward into the overlying unconsolidated cumulates the fractional crystallization resulted in an evolution of the au pge bearing phases with progressive fractionation from pd rich alloys at the base of the deposit to au rich alloys at the top we argue that the sulfides evolved to their present extreme cu rich fe poor composition during oxidation desulfurization reactions in the same event which oxidized the iron titanium oxides of the intrusion this oxidation desulfurization probably had no overall effect on the distribution of au and pge

deposition and generation of multiple widespread fall units from the c ad 1314 kaharoa rhyolitic eruption tarawera new zealand the c ad 1314 kaharoa eruption from tarawera volcano north island new zealand was a complex long duration multi episode rhyolitic event up to 13 discrete sub plinian to plinian style phases occurred in two stages from at least two different vents along an 8 km long fissure the early explosive stage produced seven pumice lapilli fall units dispersed solely to the southeast of the volcano while a later stage produced six fall units dispersed almost entirely to the north and northwest the duration of fallout for these 13 lapilli units is estimated to have totaled about 1 5 days the presence of massive textured ash layers between many of the lapilli fall units suggests that longer duration pyroclastic density current producing phases and more quiescent fallout intervals occurred between the intense explosive phases of the eruption total duration of ash fallout 7 5 days the ash deposition extended the minimum duration of the explosive episodes to at least 9 days based on settling rates of the ash possibly the explosive phase of eruptive activity lasted for 2 weeks including some brief hiatuses and pyroclastic density current deposition and was followed by 4 years of mainly lava dome forming activity at least 15 km3 7 3 km3 dense rock equivalent dre of tephra was erupted during the main explosive episodes and at least 9 1 km3 dre during the whole eruption more than doubling the previous estimate this may be an underestimate by up to 50 of the fallout volume eruption column heights for the two most widely dispersed fall units are estimated to have ranged from 25 28 km above the vents and durations of individual lapilli fall unit forming phases ranged from 1 5 to 4 h but the ash producing phases lasted much longer individually up to 30 h and represent a longer lived shut down to each eruptive episode time averaged mass discharge rates are estimated to have varied by more than one order of magnitude during the main plinian style phases from 107 to 108 kg s however the explosive phases were unsteady as indicated by fluctuating grain size with stratigraphic height in the lapilli fall units and common capping ash beds the north northwesterly dispersal of the later stage fall units and the possible duration of 4 days for these episodes indicate an unusual period of southerly to southeasterly winds therefore the main explosive part of the kaharoa eruption is most likely to have occurred during the austral winter to spring months the kaharoa eruption style is also shared by the three previous rhyolite dominated eruptions that have built tarawera volcano over the past 22 000 years knowledge of the widespread pumice and ash fallout from the kaharoa eruption is important in quantifying the hazard from this type of extended duration explosive activity and has implications for future similar rhyolitic activity in new zealand

chronometry and speedometry of magmatic processes using chemical diffusion in olivine plagioclase and pyroxenes the magmatic processes that fuel volcanism crustal growth ore formation and discharge of volcanic gases and aerosols to the atmosphere occur across a range of timescales from millions of years to just a few seconds for example the production of new oceanic crust at mid ocean ridges is a near continuous process that can operate in any one ocean basin on timescales of more than 100 m y however the driving force for such processes is the spreading of the ocean plates that happens on a cm yr timescale at the other end of the spectrum explosive volcanic eruptions involve the ascent and fragmentation of magma at velocities of the order of 100 m s such that the journey from a magma chamber to an ash cloud may take place in a matter of minutes in this case the driving force is the rapid expansion of magmatic gas in response to changes in pressure at intermediate timescales magmatic processes may give rise to hydrothermal ore deposits on timescales of less than a million years for an individual deposit while growth of giant granite batholiths may require piecemeal assembly of magma batches on timescales of a few million years although each of these processes has a characteristic time averaged timescale on which it operates this is typically the end result of one or more natural processes that operate on much shorter timescales for example mid ocean ridges do not extrude magma continuously onto the ocean floor mineralising fluids do not discharge continuously through the shallow crust and granitic magmas do not dribble continuously into evolving batholithic chambers in some cases it is the long term timescales that are important for example the spreading rate of ocean basins in others it is the short term timescales that are important for example the episodic growth of lava domes at active volcanoes although

time scales of magmatic processes from modeling the zoning patterns of crystals the advent of polarized light microscopy in the middle of the 19th century allowed mineralogists and petrologists interested in igneous rocks to recognize the widespread occurrence of fine scale heterogeneities in the optical properties of minerals e g see young 2003 for details the interpretation of mineral zoning patterns as archives of magmatic processes has been with us for some time e g larsen et al 1938 tomkeieff 1939 the development of the electron microprobe in the 1960 s allowed mineral zoning profiles to be quantitatively analyzed and modeled e g bottinga et al 1966 moore and evans 1967 enhanced textural observations permitted recognition of many kinds of detailed structures during the growth and dissolution of magmatic minerals fig 1 e g anderson 1983 pearce and kolisnik 1990 and these processes were also explored using experimental and numerical models e g albarede and bottinga 1972 lofgren 1972 kirkpatrick et al 1976 loomis 1982 major element zoning patterns are routinely measured and with the arrival of the ion microprobe the identification of heterogeneous distribution of trace elements opened the window for more realistic and sophisticated scenarios and models fig 1 e g kohn et al 1989 blundy and shimizu 1991 singer et al 1995 we now measure abundances of naturally occurring isotopes at the scale of tens of micrometers e g davidson et al 2007a ramos and tepley 2008 and this allows in situ dating of crystals e g cooper and reid 2008 figure 1 a nomarski dic image of a plagioclase crystal from a gabbroic xenolith of volcan san pedro costa et al 2002 the crystal shows a well defined core and a rim separated by an oscillatory zoned interval ion microprobe analysis pits and the trace of the electron microprobe traverse between the pits are evident b major

age of the baula pge mineralization orissa india and its implications concerning the singhbhum archaean nucleus abstract two types of platinum group element pge mineralization are identified in the mafic facies of the baula complex intruding the gabbro anorthosite unit of the singhbhum archaean nucleus the first pt dominated is magmatic the pgm are included in the magmatic silicates of a gabbro norite intrusion the bangur gabbro cutting the complex s ultramafic facies and are found preferentially where the gabbro incorporates clasts of dunite and chromitite near the gabbro ultramafics contact the second pd dominated is hydrothermal the pgm are associated with hydrous silicates located in the gabbro matrix of a magmatic breccia in which the blocks are from the baula ultramafic facies the originating fluids for the second type of mineralization were exsolved from the magma and concentrated pge and volatile elements we see no evidence of remobilization or external fluid contamination a structural study has demonstrated that a the breccia was provoked by the intrusion of the upper part of the bangur gabbro and b the breccia matrix corresponds to the bangur gabbro consequently both types of pge mineralization are associated with the same magmatic event a zircon age of 3122 5 ma has been obtained for the main bangur gabbro intrusion and similar ages 3123 7 and 3119 6 ma have been obtained on the pge mineralized gabbro matrix of the breccia this supports the hypothesis of a genetic link between the bangur gabbro with its pt dominated pge and the breccia with its pd dominated pge giving a mean weighted age of 3121 3 ma for the mineralization sm nd data on eight gabbro samples of the baula complex yielded a similar age of 3205 280 ma the dating of these events places them close to the broad scale stabilization of the singhbhum archaean nucleus estimated at 3 1 ga and defines one of the oldest pge mineralizing events ever recorded the bangur gabbro shows many affinities with the gabbro anorthosite unit of the singhbhum archaean nucleus in terms of age of emplacement and isotopic composition it may be therefore that the bangur gabbro actually forms part of the gabbro anorthosite unit and that a fundamental control of its two types of pge mineralization is the presence of ultramafic clasts

isotope and chemical microsampling constraints on the history of an s type rhyolite san vincenzo tuscany italy fine scale heterogeneities in strontium isotope ratios 40ar 39ar ages and chemical com sition have been determined for individual mineral grains and enclaves in the rhyolites of san vincenzo italy the rhyolites are peraluminous with phenocrysts of alkali feldspar piagioclase biotite and cordierite and have previously been divided into two groups on the basis of whole rock major element chemistry group b lavas are distinguished from group a in having higher mgo and cao contents as well as containing two pyroxenes and chilled latite enclaves the group b lavas show textural evidence for d sequilib um such as resorption and sieve zones in feldspars laser fusion 40ar 39ar dating of individual alkali feldspar crystals suggests that the eruption of these rhyolites took place 4 38 0 04 ma though small amounts of excess argon are present in subhedral and cloudy crystals the groundmass in the group a sample has a higher initial r7sr 86sr 0 725 than that of the group b 0 7 13 the sr isotope compositions of chemically and texturally characterized individual grains reveal marked disequilibrium between minerals and the host glass as well as within grain zonation the feldspars and coarse grained biotites have initial 87sr 8 sr that are intermediate between those of the group a and b glasses with alkali feldspar generally having higher initial 87sr 86sr ratios than plagioclase chilled latite enclaves which are present in only the group b lavas have relatively low initial 8 sr 86sr ratios in the range 0 7082 0 7088 the mineral chemistry and isotope data indicate that restite does not form a significant portion of the crystalline assemblage and therefore cannot be the source of the strontium isotope dis equilibrium xenocrysts mixed into the rhyolite from a mafic magma were identified as clinopyroxene megacrysts clinopyroxene orthopyroxene clots orthopyroxene plagioclase clots and plagioclase with extensive sieve textures these grains all have relatively low initial 87sr 86sr ratios other grains that may be xenocrysts are cordierites with cores containing inclusions of biotite spinel and sillimanite and alkali feldspars with excess 4 ar the strontium isotope di quilib um in phenocrysts of plagioclase alkali feldspar and biotite is attributed to a changing melt chemistry that was too rapid for strontium diffusive equilibration a mode1 for the san vincenzo magma chamber is proposed in which the sr isotope evolution of the group b rhyolites is due to magma mixing of a crustal melt with latite magma containing unra diogenic strontium the variable initial strontium isotope compositions of group a phenocrysts may be due to assimilation or crystallization along the interface between mingled group a and b magmas

decoupled crystallization and eruption histories of the rhyolite magmatic system at tarawera volcano revealed by zircon ages and growth rates we obtained u th disequilibrium age data on zircons from each of the four rhyolite eruptions that built tarawera volcano in the last 22 ka within the okataina volcanic center ovc caldera new zealand secondary ion mass spectrometry analyses on unpolished euhedral crystal faces that lack resorption features show that crystal growth variously terminated from near eruption age to 100 ka prior to eruption age depth profiling of crystals reveals long periods of continuous 34 ka and discontinuous growth 90 ka growth hiatuses of up to 40 ka duration occur but do not all relate to obvious resorption surfaces age differences up to similar magnitude are found on opposing faces of some crystals suggesting episodes of partial exposure to melts these features are best explained by periodic complete or partial sub solidus storage and or inclusion in larger crystal phases followed by rapid liberation prior to eruption this is supported by high abundances of u and th 500 2 000 ppm in some zircons consistent with periods of high crystallinity 70 in the magmatic system based on crystal melt partitioning contemporaneous but contrasting rim ward trends of these elements within crystals even in the same lava hand sample require synchronous growth in separate melt bodies and little connectivity within the system but also significant crystal transport and mixing prior to eruption many crystals record continuity of growth through the preceding 60 ka ovc caldera collapse and subsequent eruptions from tarawera this demonstrates a decoupling between eruption triggers such as shallow crustal extension and mafic intrusion and the crystallization state of the ovc silicic magmatic system the data highlights the need to distinguish between the time for accumulation of eruptible magma and the long term magma residence time based on the age of crystals with high closure temperatures when assessing the potential for catastrophic eruptions

magmatic interactions as recorded in plagioclase phenocrysts of chaos crags lassen volcanic center california the silicic lava domes of chaos crags in lassen volcanic national and mingling of this material into the silicic host these processes park contain a suite of variably quenched hybrid basaltic andesite are commonplace in some orogenic magma systems and may be magmatic inclusions the inclusions represent thorough mixing elucidated by isotopic microsampling and analysis of the plagioclases between rhyodacite and basalt recharge liquids accompanied by some crystallizing from them mechanical disaggregation of the inclusions resulting in crystals mixing into the rhyodacite host preserved by quenching on dome emplacement sr sr ratios 0 7037 0 7038 of the inclusions are distinctly lower than those of the host rhyodacite

magma recharge contamination and residence times revealed by in situ laser ablation isotopic analysis of feldspar in volcanic rocks we illustrate a rapid and effective method for determining variations in 87sr 86sr across single plagioclase feldspar crystals using laser ablation interfaced to a multicollector inductively coupled plasma source mass spectrometer the speed of analysis proves to be a particular advantage permitting representative populations of crystals to be characterized or enabling reconnaissance analysis of rocks to test for mineral scale isotopic heterogeneity our results from magmatic crystals in lavas from el chichon volcano confirm trends previously obtained by the more time consuming microdrilling method for the first time with confidence the laser data allow us to associate abrupt changes in 87sr 86sr with distinct petrographic discontinuities such as dissolution surfaces in the crystal because the isotopic variations within crystals would be expected to equilibrate diffusively with time the width over which abrupt changes in 87sr 86sr occur can also be used to constrain timescales at which the crystal has been held at magmatic temperatures

strontium isotope disequilibrium of plagioclase in the upper critical zone of the bushveld complex evidence for mixing of crystal slurries we report in situ sr isotope data for plagioclase of the bushveld complex we found disequilibrium sr isotopic compositions on several scales 1 between cores and rims of plagioclase grains in the merensky pyroxenite the bastard anorthosite and the ug1 unit and its noritic footwall 2 between cores of different plagioclase grains within thin sections of anorthosite and pyroxenite of the merensky unit the footwall anorthosite of the merensky reef and the footwall norite of the ug1 chromitite the data are consistent with a model of co accumulation of cumulus plagioclase grains that had crystallized from different magmas followed by late stage overgrowth of the cumulus grains in a residual liquid derived from a different level of the compacting cumulate pile we propose that the rocks formed through slumping of semi consolidated crystal slurries at the top of the critical zone during subsidence of the center of the intrusion slumping led to sorting of crystals based on density differences resulting in a layered interval of pyroxenites norites and anorthosites

contrasting plagioclase textures and geochemistry in response to magma dynamics in an intra caldera rhyolite system okataina volcano abstract the changing magmatic dynamics of the rhyolite caldera volcano okataina volcanic centre new zealand is revealed in plagioclase growth histories crystals from the 0 7 ka kaharoa eruption are characterized by resorbed cores displaying a cellular texture of high an 40 zones partially replaced by low an 50 20 elevated an fe mg sr and ti follow the resorption surface and display rimward depletion trends accompanied by ba and ree enrichment the zonation is consistent with fractional crystallization and cooling the cores display wide trace element diversity pointing to crystallization in a variety of melts before transport and mixing into a common magma where the rims grew plagioclase from the 36 ka hauparu eruption display several regrowth zones separated by resorption surfaces which surround small resorbed cores with a spongy cellular texture of variable an content an 40 50 the crystals display step wise regrowth of successively higher an fe mg and ti content consistent with progressive mafic recharge two crystal groups are distinguished by trace element chemistry indicating growth in separate melts and co occurrence via magma mingling for plagioclase in both eruption deposits partition coefficients d estimated from crystal rim groundmass glass analyses produce melt compositions similar to the array of rock and glass compositions erupted and are consistent with the processes of fractional crystallization and recharge however d values estimated from some published formulations based on an content and temperature produce unrealistic melts the contrasting zoning patterns in plagioclase correspond to the evolutionary history of magmatism at okataina emptying of the magma reservoir following caldera eruption at 46 ka reduced barriers to mafic magma ascent this is recorded by the frequent resorption and recharge episodes in hauparu crystals subsequent redevelopment of a more silicic reservoir zone post 26 ka dampened thermal and mass perturbations resulting in simpler growth histories of the kaharoa crystals the plagioclase lack features associated with rapid decompression events that are common in andesite systems this reflects the rapid ascent of the rhyolite magmas and lack of precursory eruptions that could decompressed the system

silicic recharge of multiple rhyolite magmas by basaltic intrusion during the 22 6 ka okareka eruption episode new zealand abstract deposits of the 22 6 ka okareka eruption episode from tarawera volcanic complex record the sequential and simultaneous eruption of three discrete rhyolite magmas following a silicic recharge event related to basaltic intrusion the episode started with basaltic eruption 0 01 km3 magma and rapidly changed to a plinian eruption involving a moderate temperature 750 c cummingtonite bearing rhyolite magma t1 with a volume of 0 3 km3 hybrid basalt rhyolite clasts demonstrate direct basaltic intrusion that helped trigger the eruption crystals shards and lapilli of two other rhyolite magmas then joined the eruption sequence they comprise a cooler 720 c crystal rich biotite hornblende rhyolite magma t2 0 3 km3 and a hotter 780 c crystal poor pyroxene hornblende rhyolite magma t3 4 5 km3 all mid to late stage ash units contain various mixtures of t1 t2 and t3 components with a general increase in abundance of t3 and rapid decline of t1 with time about 4 km3 of t3 magma was extruded as lavas at the end of the episode contrasts in melt composition crystal and volatile contents and temperatures influenced viscosity and miscibility and thus limited pre eruption mixing of the rhyolite magmas the eruption sequence and the restricted direct basaltic intrusion into only one magma t1 is consistent with the rhyolites occupying separate melt pods within a large crystal mush zone melt crystal equilibria and volatile contents in melt inclusions indicate temporary magma storage depths of